JP3478090B2 - Nitride semiconductor device - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting element such as an LED (light emitting diode), an LD (laser diode), or a super luminescent diode (SLD), a light receiving element such as a solar cell or an optical sensor, or a transistor. Nitride semiconductors used in electronic devices such as power devices (In _X Al _Y Ga _1-XY N, 0 ≦ X, 0
It relates to an element using ≦ Y, X + Y ≦ 1). Note that the general formulas In _X Ga _1-X N and Al _Y used in this specification are used.
Ga _{1 -Y} N and the like merely represent the composition formula of the nitride semiconductor layer, and even if different layers are represented by the same composition formula, the X value and the Y value of those layers are the same. It does not indicate that

[0002]

2. Description of the Related Art Nitride semiconductors have recently been put into practical use as materials for high-brightness blue LEDs and pure green LEDs in full-color LED displays, traffic signals and the like. The LED used in these various devices has a single quantum well structure (SQW: Sing) having a well layer made of InGaN.
le-Quantum-Well) or multiple quantum well structure (MQ
An active layer of W: Multi-Quantum-Well) has a double hetero structure sandwiched between an n-type nitride semiconductor layer and a p-type nitride semiconductor layer. Wavelengths such as blue and green are InGaN
It is determined by increasing or decreasing the In composition ratio of the well layer.

Further, the present applicant recently announced the world's first laser oscillation of 410 nm at room temperature under a pulse current using this material (for example, Jpn.J.Appl.Phys.35 (1996) L).
74, Jpn.J.Appl.Phys.35 (1996) L217 etc.}. This laser device has a double hetero structure having an active layer of a multi-quantum well structure using a well layer made of InGaN, and a threshold current 610 under the condition of a pulse width of 2 μs and a pulse period of 2 ms.
Oscillation at 410 nm with a mA, threshold current density of 8.7 kA / cm ² , is shown. Furthermore, we have developed an improved laser device in Appl.P.
hys.Lett.69 (1996) 1477. This laser device has a structure in which a ridge stripe is formed on a part of a p-type nitride semiconductor layer, has a pulse width of 1 μs, a pulse period of 1 ms, a duty ratio of 0.1%, and a threshold current of 1%.
Oscillation of 410 mA at 87 mA, threshold current density of 3 kA / cm ² , is shown. And we also succeeded in continuous oscillation at room temperature for the first time and announced. {For example, Nikkei Electronics December 2, 1996 Technical Bulletin, Appl.Phys.Lett.69 (1
996) 3034, Appl. Phys. Lett. 69 (1996) 4056, etc.}, this laser device has a threshold current density of 3.6 kA /
cm ² , threshold voltage 5.5 V, 1.5 mW output, 2
7 hours continuous oscillation is shown.

[0004]

The blue and green LEDs made of a nitride semiconductor have a forward current (If) of 20 mA and a forward voltage (Vf) of 3.4V to 3.6V.
Since it is higher than the red LED made of an As-based semiconductor by 2 V or more, further reduction of Vf is desired. Also, L
In D, the current and voltage at the threshold are still high, and in order to continuously oscillate at room temperature for a long time, it is necessary to realize an element with higher efficiency such that the threshold current and voltage are lowered.

If the threshold voltage of the laser element can be lowered, if the technique is applied to the LED element,
A decrease in Vf of the device can be expected. Therefore, an object of the present invention is to realize continuous oscillation for a long time by reducing the current and voltage at the threshold of the LD element mainly made of a nitride semiconductor.

[0006]

According to the present invention, in a nitride semiconductor device, any one or more semiconductor layers other than the active layer can have a strained superlattice structure to improve the crystallinity of the semiconductor layer. It was completed by finding that the electric resistance of the semiconductor layer can be lowered. That is, in the first nitride semiconductor device of the present invention, the active layer has a nitride semiconductor layer on the n-conductive side (hereinafter, referred to as the n-side) and a nitride semiconductor on the p-conductive side (hereinafter, referred to as the p-side). A nitride semiconductor device formed between the first and second nitride semiconductor layers, the first and second nitride semiconductor layers being located at a position apart from or in contact with the active layer in the nitride semiconductor layer on the n-conductive side. An n-side strained superlattice layer formed by laminating a semiconductor layer, wherein the first nitride semiconductor layer has a bandgap energy larger than that of the second nitride semiconductor layer and the second nitride semiconductor layer. And a higher n-type impurity concentration, the n-type impurity concentration of a portion of the first nitride semiconductor layer that is closer to the second nitride semiconductor layer is separated from the second nitride semiconductor layer. It is characterized in that it is smaller than the portion.
Here, in this specification, the p-conductive side refers to the nitride semiconductor layer between the active layer and the positive electrode (p-electrode), and the n-conductive side is opposite to the p-conductive side with the active layer sandwiched therebetween. It refers to the nitride semiconductor layer on the side. Needless to say, the order of stacking the first nitride semiconductor layer and the second nitride semiconductor layer does not matter.

The second nitride semiconductor device of the present invention is a nitride semiconductor device having an active layer formed between an n-conductive side nitride semiconductor layer and a p-conductive side nitride semiconductor layer. In the nitride semiconductor layer on the n-conductive side,
At a position apart from or in contact with the active layer, the first
And an n-side strained superlattice layer formed by stacking a second nitride semiconductor layer and the second nitride semiconductor layer, wherein the first nitride semiconductor layer is the second nitride semiconductor layer.
Has a bandgap energy larger than that of the nitride semiconductor layer and an n-type impurity concentration smaller than that of the second nitride semiconductor layer, and is close to the first nitride semiconductor layer in the second nitride semiconductor layer. It is characterized in that the n-type impurity concentration of the portion to be formed is made smaller than that of the portion separated from the first nitride semiconductor layer.

The first aspect of the present invention configured as described above
Since the second nitride semiconductor element can reduce the electric resistance of the nitride semiconductor layer formed of the superlattice layer, the resistance of the nitride semiconductor layer on the n-conductive side as a whole can be reduced.
Further, in the nitride semiconductor device according to the present invention, the impurity concentration of the first nitride semiconductor layer having a large bandgap energy in the superlattice layer is compared with the impurity concentration of the second nitride semiconductor layer having a small bandgap energy. Therefore, it may be increased or decreased.

As in the first nitride semiconductor device according to the present invention, when the impurity concentration of the first nitride semiconductor layer is made higher than that of the second nitride semiconductor layer, carriers are The first with a large bandgap energy
Can be generated in the second nitride semiconductor layer and injected into the second nitride semiconductor layer having a small band gap energy, and the injected carriers can be injected into the second nitride semiconductor layer having a low impurity concentration and a high mobility.
Since it can be moved in the nitride semiconductor layer, the electric resistance of the superlattice layer can be reduced. When the impurity concentration of the first nitride semiconductor layer is made higher than that of the second nitride semiconductor layer, in the first nitride semiconductor device, the first nitride of the superlattice layer is formed. In the semiconductor layer,
It is preferable that the n-type or p-type impurity concentration of a portion adjacent to the second nitride semiconductor layer (hereinafter referred to as an adjacent portion) be smaller than that of a portion distant from the second nitride semiconductor layer. As a result, carriers moving in the second nitride semiconductor layer can be prevented from being scattered by impurities in the adjacent portion, the mobility of the second nitride semiconductor layer can be further increased, and the superlattice layer The electric resistance can be further reduced.

When the impurity concentration of the first nitride semiconductor layer is made smaller than that of the second nitride semiconductor layer as in the second nitride semiconductor device according to the present invention, In the second nitride semiconductor layer, the first
It is preferable that the n-type impurity concentration of the portion close to the nitride semiconductor layer is lower than that of the portion distant from the first nitride semiconductor layer.

Further, in a third nitride semiconductor device of the present invention, the active layer is formed between the n-conductive side nitride semiconductor layer and the p-conductive side nitride semiconductor layer. In the nitride semiconductor layer on the p-conductive side,
At a position apart from or in contact with the active layer, a third
And a fourth nitride semiconductor layer are stacked to form a p-side strained superlattice layer, and the third nitride semiconductor layer is the fourth nitride semiconductor layer.
Has a bandgap energy larger than that of the nitride semiconductor layer and a p-type impurity concentration larger than that of the fourth nitride semiconductor layer, and is closer to the fourth nitride semiconductor layer in the third nitride semiconductor layer. It is characterized in that the p-type impurity concentration of the portion to be formed is made smaller than that of the portion separated from the fourth nitride semiconductor layer.

Further, in a fourth nitride semiconductor device of the present invention, the active layer is formed between the n-conductive side nitride semiconductor layer and the p-conductive side nitride semiconductor layer. In the nitride semiconductor layer on the p-conductive side,
At a position apart from or in contact with the active layer, a third
And a fourth nitride semiconductor layer are stacked to form a p-side strained superlattice layer, and the third nitride semiconductor layer is the fourth nitride semiconductor layer.
Has a bandgap energy larger than that of the nitride semiconductor layer and a p-type impurity concentration smaller than that of the fourth nitride semiconductor layer, and is close to the third nitride semiconductor layer in the fourth nitride semiconductor layer. It is characterized in that the p-type impurity concentration of the portion to be formed is made smaller than that of the portion away from the third nitride semiconductor layer. In this specification, the n-side strained superlattice layer and the p-side strained superlattice layer are collectively referred to as a superlattice layer. It goes without saying that the order of stacking the third nitride semiconductor layer and the fourth nitride semiconductor layer does not matter.

The third aspect of the present invention configured as described above
Since the fourth nitride semiconductor device can reduce the electric resistance of the nitride semiconductor layer formed of the superlattice layer, the resistance of the nitride semiconductor layer on the p-conductive side as a whole can be reduced.
Further, in the nitride semiconductor device according to the present invention, the impurity concentration of the third nitride semiconductor layer having a large bandgap energy in the superlattice layer is compared with the impurity concentration of the fourth nitride semiconductor layer having a small bandgap energy. Therefore, it may be increased or decreased.

As in the third nitride semiconductor device according to the present invention, when the impurity concentration of the third nitride semiconductor layer is made higher than the impurity concentration of the fourth nitride semiconductor layer, carriers are generated. Third with a large band gap energy
Can be injected into the fourth nitride semiconductor layer having a small bandgap energy, and the injected carriers can be injected into the fourth nitride semiconductor layer having a low impurity concentration and a high mobility.
Since it can be moved in the nitride semiconductor layer, the electric resistance of the superlattice layer can be reduced. In addition, when the impurity concentration of the third nitride semiconductor layer is made higher than that of the fourth nitride semiconductor layer, in the first nitride semiconductor layer of the superlattice layer, a fourth nitride semiconductor layer is formed. It is preferable to make the p-type impurity concentration of a portion close to the semiconductor layer (hereinafter, referred to as a close portion) smaller than that of a portion distant from the fourth nitride semiconductor layer. As a result, carriers moving in the fourth nitride semiconductor layer can be prevented from being scattered by impurities in the adjacent portion, the mobility of the fourth nitride semiconductor layer can be further increased, and the superlattice layer The electric resistance can be further reduced.

Further, like the fourth nitride semiconductor device according to the present invention, when the impurity concentration of the third nitride semiconductor layer is made smaller than the impurity concentration of the fourth nitride semiconductor layer, In the fourth nitride semiconductor layer, the third nitride
It is preferable that the p-type impurity concentration of the portion close to the nitride semiconductor layer is lower than that of the portion away from the third nitride semiconductor layer.

Further, in a fifth nitride semiconductor device according to the present invention, the active layer is formed between the n-conductive side nitride semiconductor layer and the p-conductive side nitride semiconductor layer. In the device, the nitride semiconductor layer on the n-conductive side is separated from the active layer or is in contact with the active layer.
An n-side strained superlattice layer formed by stacking a first and a second nitride semiconductor layer, wherein the first nitride semiconductor layer has a bandgap energy larger than that of the second nitride semiconductor layer; An n-type impurity concentration higher than that of the second nitride semiconductor layer, and an n-type impurity concentration of a portion of the first nitride semiconductor layer adjacent to the second nitride semiconductor layer is set to the second In the nitride semiconductor layer on the p-conductive side,
At a position apart from or in contact with the active layer, a third
And a fourth nitride semiconductor layer are stacked to form a p-side strained superlattice layer, and the third nitride semiconductor layer is the fourth nitride semiconductor layer.
Has a bandgap energy larger than that of the nitride semiconductor layer and a p-type impurity concentration smaller than that of the fourth nitride semiconductor layer, and is close to the third nitride semiconductor layer in the fourth nitride semiconductor layer. It is characterized in that the p-type impurity concentration of the portion to be formed is made smaller than that of the portion away from the third nitride semiconductor layer.

In the sixth nitride semiconductor device according to the present invention, the active layer is formed between the n-conductive side nitride semiconductor layer and the p-conductive side nitride semiconductor layer. In the element, the nitride semiconductor layer on the n-conducting side is provided at a position distant from or in contact with the active layer,
An n-side strained superlattice layer formed by stacking a first and a second nitride semiconductor layer, wherein the first nitride semiconductor layer has a bandgap energy larger than that of the second nitride semiconductor layer; An n-type impurity concentration lower than that of the second nitride semiconductor layer, and an n-type impurity concentration of a portion of the second nitride semiconductor layer adjacent to the first nitride semiconductor layer is set to the first nitride semiconductor layer. In the nitride semiconductor layer on the p-conductive side,
At a position apart from or in contact with the active layer, a third
And a fourth nitride semiconductor layer are stacked to form a p-side strained superlattice layer, and the third nitride semiconductor layer is the fourth nitride semiconductor layer.
Has a bandgap energy larger than that of the nitride semiconductor layer and a p-type impurity concentration larger than that of the fourth nitride semiconductor layer, and is closer to the fourth nitride semiconductor layer in the third nitride semiconductor layer. It is characterized in that the p-type impurity concentration of the portion to be formed is made smaller than that of the portion separated from the fourth nitride semiconductor layer.

In the first and fifth nitride semiconductor devices according to the present invention, the n-type impurity concentration of the first nitride semiconductor layer is in the range of 1 × 10 ¹⁷ / cm ³ to 1 × 10 ²⁰ / cm ³ . And the n-type impurity concentration of the second nitride semiconductor layer is 1 × 10
It is preferably ¹⁹ / cm ³ or less. That is, when the first nitride semiconductor layer having a large band gap energy is heavily doped with n-type impurities, the n-type impurity concentration of the first nitride semiconductor layer is 1 × 10 ¹⁷ / cm ³ to 1 × 10.
^In the range of ²⁰ / cm ³ and the n-type impurity concentration of the second nitride semiconductor layer is lower than that of the first nitride semiconductor layer and is 1 or less.
× is preferably set to ^{10 19} / cm ³ or less. The n-type impurity concentration of the second nitride semiconductor layer having a small band gap energy is more preferably 1 × 10 ¹⁸ / cm ³ or less, and further preferably 1 × 10 ¹⁷ / cm ³ or less. That is, from the viewpoint of increasing the mobility of the second nitride semiconductor layer, the smaller the n-type impurity concentration of the second nitride semiconductor layer, the better, and the second nitride semiconductor layer is undoped. The layer, that is, the state in which impurities are not intentionally doped is most desirable.

A seventh nitride semiconductor device according to the present invention is
A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the active layer in the nitride semiconductor layer on the n-conductive side is the active layer. An n-side strained superlattice layer formed by stacking first and second nitride semiconductor layers is provided at a position apart from or in contact with the layer, and the first nitride semiconductor layer is the first nitride semiconductor layer. The second nitride semiconductor layer has a bandgap energy larger than that of the second nitride semiconductor layer and an n-type impurity concentration smaller than that of the second nitride semiconductor layer.

An eighth nitride semiconductor device according to the present invention is
A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the active layer in the nitride semiconductor layer on the n-conductive side is the active layer. An n-side strained superlattice layer formed by stacking first and second nitride semiconductor layers is provided at a position apart from or in contact with the layer, and the first nitride semiconductor layer is the first nitride semiconductor layer. Second nitride semiconductor layer has a bandgap energy larger than that of the second nitride semiconductor layer and an n-type impurity concentration smaller than that of the second nitride semiconductor layer, and the nitride semiconductor layer on the p-conduction side is located at a position distant from the active layer, Alternatively, it is characterized by having a p-side strained superlattice layer formed by laminating third and fourth nitride semiconductor layers having different bandgap energies and different p-type impurity concentrations at a contact position.

Further, in the second, seventh and eighth nitride semiconductor devices according to the present invention, that is, the impurity concentration of the first nitride semiconductor layer is compared with the impurity concentration of the second nitride semiconductor layer. And the n-type impurity concentration of the first nitride semiconductor layer is 1 × 10 ¹⁹ / cm ³ or less,
The n-type impurity concentration of the second nitride semiconductor layer is 1 × 10
It is preferably in the range of ¹⁷ / cm ³ to 1 × 10 ²⁰ / cm ³ . The first nitride semiconductor layer is preferably 1 × 10 ¹⁸ / cm ³ or less, more preferably 1 × 10 ^18.
17 / cm ³ or less, most preferably undoped
ope), that is, the state in which impurities are not intentionally doped is most desirable.

Further, in the first, second, fifth to seventh nitride semiconductor devices, in order to form a superlattice layer having good crystallinity, the first nitride semiconductor layer is formed. Al _Y Ga _1-Y N (0 <, which can grow a layer having a relatively large energy band gap and good crystallinity
Y <1) and can grow the second nitride semiconductor layer having a relatively small energy band gap and good crystallinity. In _X Ga _1-X N (0 ≦ X <
It is preferably formed in 1). In addition, the first, second,
In the fifth to seventh nitride semiconductor devices, it is further preferable that in the superlattice layer, the second nitride semiconductor layer is made of GaN. As a result, the first nitride semiconductor layer (Al _Y Ga ₁ -YN) and the second nitride semiconductor layer (GaN) can be grown in the same atmosphere, so that the superlattice layer It is extremely advantageous in manufacturing. Also,
In the first, second, fifth to seventh nitride semiconductor devices,
In the superlattice layer, the first nitride semiconductor layer is Al _X
Ga _1-X N is formed by (0 <X <1), the second nitride semiconductor layer is _{Al Y Ga 1-Y N (} 0 <Y <1, X>
It can also be formed of Y). Further, in the first, second, fifth to seventh nitride semiconductor devices, the first nitride semiconductor layer or the second nitride semiconductor layer is not doped with an n-type impurity. Is more preferable.

In the third or sixth nitride semiconductor device, that is, when the impurity concentration of the third nitride semiconductor layer is made higher than that of the fourth nitride semiconductor layer, , The p-type impurity concentration of the third nitride semiconductor layer having a large band gap energy is 1 × 10 ¹⁸
/ Cm ³ to 1 × 10 ²¹ / cm ³ , and the p-type impurity concentration of the fourth nitride semiconductor layer is lower than the impurity concentration of the third nitride semiconductor layer and 1 × 10 ²⁰ / cm ^3. The following setting is more preferable. Note that the fourth nitride semiconductor layer having a small band gap energy is 1 × 10 ¹⁹ /
cm ³ or less is more preferable, and 1 × 10 ¹⁸ / cm
It is more preferably ³ or less. That is, from the viewpoint of increasing the mobility of the fourth nitride semiconductor layer, the smaller the p-type impurity concentration of the fourth nitride semiconductor layer, the better. The state in which impurities are not intentionally doped is most desirable.

In the fourth and fifth nitride semiconductor devices, that is, when the impurity concentration of the third nitride semiconductor layer is made lower than that of the fourth nitride semiconductor layer, The p-type impurity concentration of the third nitride semiconductor layer is 1 × 10 ²⁰ / cm ³ or less, and the p-type impurity concentration of the fourth nitride semiconductor layer is 1 × 10 ¹⁸ / cm ³ to 1 × 1.
It is preferably in the range of 0 ²¹ / cm ³ . The third nitride semiconductor layer is 1 × 10 ¹⁹ / cm ³ or less, more preferably 1 × 10 ¹⁸ / cm ³ or less, most preferably undope, that is, no impurities are intentionally doped. State is most desirable.

In the third to sixth nitride semiconductor devices, in order to form a superlattice layer having good crystallinity, the third nitride semiconductor layer has a relatively large energy band gap and is crystalline. And a fourth layer formed of Al _Y Ga _1-YN (0 <Y <1) capable of growing a good layer.
Of the nitride semiconductor layer of In _X Ga _1-X N (0 ≦ X <1)
Is preferably formed. More preferably, the fourth nitride semiconductor layer is made of GaN. As a result, the third nitride semiconductor layer (Al _Y Ga _1-YN ) is formed.
And the fourth nitride semiconductor layer (GaN) can be grown in the same atmosphere, which is extremely advantageous in manufacturing the superlattice layer. In addition, in the third to sixth nitride semiconductor devices, the third nitride semiconductor layer is Al _X Ga.
_1-X N (0 <X <1), and the fourth nitride semiconductor layer may be formed of Al _Y Ga _1-Y N (0 <Y <1, X> Y). Further, in the third to sixth nitride semiconductor elements, it is preferable that the third nitride semiconductor layer or the fourth nitride semiconductor layer is not doped with p-type impurities.

A ninth nitride semiconductor device according to the present invention is
A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the active layer in the nitride semiconductor layer on the n-conductive side is the active layer. An n-side strained superlattice layer formed by stacking first and second nitride semiconductor layers is provided at a position apart from or in contact with the layer, and the first nitride semiconductor layer is the first nitride semiconductor layer. Second nitride semiconductor layer has a bandgap energy larger than that of the second nitride semiconductor layer and an n-type impurity concentration larger than that of the second nitride semiconductor layer, and in the first nitride semiconductor layer, The n-type impurity concentration of the adjacent portion is made smaller than that of the portion distant from the second nitride semiconductor layer, and the nitride semiconductor layer on the p-conductive side is located at a position distant from or in contact with the active layer. The third and fourth nitride semiconductor layers at the positions The third nitride semiconductor layer has a stacked p-side strained superlattice layer, and the third nitride semiconductor layer has a bandgap energy larger than that of the fourth nitride semiconductor layer and a p-type larger than that of the fourth nitride semiconductor layer. An impurity concentration and a p-type impurity concentration of a portion of the third nitride semiconductor layer adjacent to the fourth nitride semiconductor layer is compared with a portion of the third nitride semiconductor layer distant from the fourth nitride semiconductor layer. It is characterized by being made smaller. In the ninth nitride semiconductor device according to the present invention, the n-type impurity concentration of the first nitride semiconductor layer is in the range of 1 × 10 ¹⁷ / cm ³ to 1 × 10 ²⁰ / cm ³ , N-type impurity concentration of the nitride semiconductor layer 2 is 1
X10 ¹⁹ / cm ³ or less, and the p-type impurity concentration of the third nitride semiconductor layer is 1 × 10 ¹⁸ / cm ³ to 1 × 1.
It is preferable that the concentration is in the range of 0 ²¹ / cm ³ and the p-type impurity concentration of the fourth nitride semiconductor layer is 1 × 10 ²⁰ / cm ³ or less.

A tenth nitride semiconductor device according to the present invention is a nitride semiconductor device in which an active layer is formed between an n-conductive side nitride semiconductor layer and a p-conductive side nitride semiconductor layer. Therefore, in the nitride semiconductor layer on the n-conductive side,
At a position apart from or in contact with the active layer, the first
And an n-side strained superlattice layer formed by stacking a second nitride semiconductor layer and the second nitride semiconductor layer, wherein the first nitride semiconductor layer is the second nitride semiconductor layer.
A band gap energy larger than that of the nitride semiconductor layer and an n-type impurity concentration larger than that of the second nitride semiconductor layer, and a position separated from the active layer in the nitride semiconductor layer on the p-conductive side, or A p-side strained superlattice layer formed by stacking a third and a fourth nitride semiconductor layer is provided at a contact position, and the third nitride semiconductor layer is formed from the fourth nitride semiconductor layer. It has a large band gap energy and a p-type impurity concentration smaller than that of the fourth nitride semiconductor layer. In a tenth nitride semiconductor device according to the present invention, the n-type impurity concentration of the first nitride semiconductor layer is in the range of 1 × 10 ¹⁷ / cm ³ to 1 × 10 ²⁰ / cm ³ , The nitride semiconductor layer has an n-type impurity concentration of 1 × 10 ¹⁹ / cm ³ or less and the third nitride semiconductor layer has a p-type impurity concentration of 1 × 10 ²⁰ / cm ³ or less. The p-type impurity concentration of the fourth nitride semiconductor layer is preferably in the range of 1 × 10 ¹⁸ / cm ³ to 1 × 10 ²¹ / cm ³ .

An eleventh nitride semiconductor device according to the present invention is a nitride semiconductor device having an active layer formed between an n-conductive side nitride semiconductor layer and a p-conductive side nitride semiconductor layer. Therefore, in the nitride semiconductor layer on the n-conductive side,
At a position apart from or in contact with the active layer, the first
And an n-side strained superlattice layer formed by stacking a second nitride semiconductor layer and the second nitride semiconductor layer, wherein the first nitride semiconductor layer is the second nitride semiconductor layer.
Of the nitride semiconductor layer having a bandgap energy larger than that of the second nitride semiconductor layer and an n-type impurity concentration smaller than that of the second nitride semiconductor layer, and a position separated from the active layer in the nitride semiconductor layer on the p-conductive side, or A p-side strained superlattice layer formed by stacking a third and a fourth nitride semiconductor layer is provided at a contact position, and the third nitride semiconductor layer is formed from the fourth nitride semiconductor layer. It has a large band gap energy and a p-type impurity concentration higher than that of the fourth nitride semiconductor layer. In the eleventh nitride semiconductor device according to the present invention, the n-type impurity concentration of the first nitride semiconductor layer is 1 × 10 ¹⁹ / cm ³ or less,
The n-type impurity concentration of the nitride semiconductor layer is 1 × 10 ¹⁷ / cm
³ to 1 × 10 ²⁰ / cm ³ and the p-type impurity concentration of the third nitride semiconductor layer is 1 × 10 ¹⁸ / cm ^3.
It is preferable that the p-type impurity concentration of the fourth nitride semiconductor layer is 1 × 10 ²⁰ / cm ³ or less, in the range of 1 × 10 ²¹ / cm ³ .

A twelfth nitride semiconductor device according to the present invention is a nitride semiconductor device having an active layer formed between an n-conductive side nitride semiconductor layer and a p-conductive side nitride semiconductor layer. Therefore, in the nitride semiconductor layer on the n-conductive side,
At a position apart from or in contact with the active layer, the first
And an n-side strained superlattice layer formed by stacking a second nitride semiconductor layer and the second nitride semiconductor layer, wherein the first nitride semiconductor layer is the second nitride semiconductor layer.
In the nitride semiconductor layer on the p-conducting side having a bandgap energy larger than that of the nitride semiconductor layer and an n-type impurity concentration smaller than that of the second nitride semiconductor layer, or at a position apart from the active layer. Has a p-side strained superlattice layer formed by stacking a third and a fourth nitride semiconductor layer, and the third nitride semiconductor layer is larger than the fourth nitride semiconductor layer. It has a bandgap energy and a p-type impurity concentration lower than that of the fourth nitride semiconductor layer. In a twelfth nitride semiconductor device according to the present invention, the n-type impurity concentration of the first nitride semiconductor layer is 1 × 10 ¹⁹ / cm ³ or less, and the n-type impurity concentration of the second nitride semiconductor layer is Impurity concentration is 1 × 10 ¹⁷ / cm ³
To 1 × 10 ²⁰ / cm ³ and the p-type impurity concentration of the third nitride semiconductor layer is 1 × 10 ²⁰ / cm ³ or less, and the fourth nitride semiconductor layer has a p-type impurity concentration of 1 × 10 ²⁰ / cm ³ or less. The p-type impurity concentration is preferably in the range of 1 × 10 ¹⁸ / cm ³ to 1 × 10 ²¹ / cm ³ .

In the ninth to twelfth nitride semiconductor devices according to the present invention, in the n-side strained superlattice layer, the first nitride semiconductor layer is Al _Y Ga _1-Y N (0 <Y <1. ), And the second nitride semiconductor layer is In _X Ga _1-X.
N (0 ≦ X <1), and in the p-side strained superlattice layer, the third nitride semiconductor layer is Al _Y Ga.
_1-Y formed by N (0 <Y <1) , the fourth nitride semiconductor layer may be formed of _{In X Ga 1-X N (} 0 ≦ X <1). In that case, in the ninth to twelfth nitride semiconductor devices according to the present invention, it is more preferable that the second and fourth nitride semiconductor layers are each made of GaN.

In the ninth to twelfth nitride semiconductor devices according to the present invention, in the n-side strained superlattice layer, the first nitride semiconductor layer is Al _X Ga _{1 -X} N (0 <X <1. ), And the second nitride semiconductor layer is Al _Y Ga _1-Y.
N (0 <Y <1, X> Y), and in the p-side strained superlattice layer, the third nitride semiconductor layer is Al _X Ga.
Formed by _{1- X N (0 <X <} 1), the fourth nitride semiconductor layer may be formed by _{Al Y Ga 1-Y N (} 0 <Y <1, X> Y). Furthermore, in the ninth to twelfth nitride semiconductor devices according to the present invention, in the third nitride semiconductor device, the first nitride semiconductor layer or the second nitride semiconductor layer contains an n-type impurity. It is preferably an undoped layer that is not doped, and the third nitride semiconductor layer or the fourth nitride semiconductor layer is preferably an undoped layer that is not doped with p-type impurities.

Further, in the first to twelfth nitride semiconductor devices according to the present invention, it is preferable that the active layer includes an InGaN layer, and it is further preferable that the InGaN layer is a quantum well layer. The active layer may have a single quantum well structure or a multiple quantum well structure.

In the above-described nitride semiconductor device according to the present invention, the n-side strained superlattice layer is a buffer layer formed in contact with the substrate, n is a photoelectric conversion device such as a light emitting device or a light receiving device. An n-side contact layer on which an electrode is formed,
It is formed as at least one layer of the n-side cladding layer for confining carriers and the n-side optical guide layer for guiding the light emission of the active layer. Further, in the nitride semiconductor device according to the present invention, the p-side strained superlattice layer is a p-side contact layer on which a p-electrode is formed, a p-side clad layer as carrier confinement, and a p-type for guiding light emitted from the active layer. It is formed as at least one layer of the side light guide layers.

A nitride semiconductor device according to one aspect of the present invention is a laser oscillation device in which the active layer is located between a p-side cladding layer and an n-side cladding layer, and the active layer is a p-side cladding layer. At least one of the n-side cladding layers is the n-side strained superlattice layer or the p-side strained superlattice layer. This makes it possible to construct a laser oscillator having a low threshold current.

Further, in the laser oscillator, a nitride semiconductor containing In or GaN is formed between at least one of the p-side cladding layer and the active layer or between the p-side cladding layer and the active layer. , The impurity concentration is 1 × 10 ¹⁹ /
It is preferable that a light guide layer having a size of cm ³ or less is formed. Since this light guide layer has a low absorptance of the light generated in the active layer, it is possible to realize a laser device capable of oscillating with a low gain without extinction of light emission of the active layer. In the present invention, the impurity concentration of the light guide layer is more preferably 1 × 10 ¹⁸ / cm ³ or less, further preferably 1 × 10 ¹⁷ / cm ³ or less, in order to reduce the light absorption rate. Undoped is most preferred.
The light guide layer may have a superlattice structure.

Further, between the optical guide layer and the active layer, a nitride semiconductor having a bandgap energy larger than that of the well layer of the active layer and the optical guide layer and having a film thickness of 0.1 μm or less. Is preferably formed, and the impurity concentration of the cap layer is preferably set to 1 × 10 ¹⁸ / cm ³ or more. In this way, by forming the cap layer having a large band gap energy, the leak current can be reduced. It is more effective if the light guide layer and the cap layer are formed on the p-conductive side nitride semiconductor layer side.

Further, in the present invention, in the first to twelfth nitride semiconductor devices, a nitride semiconductor layer is grown on a heterogeneous substrate made of a material different from the nitride semiconductor, and the grown nitride semiconductor layer is formed. A nitride made of a nitride semiconductor formed on the upper surface of the nitride semiconductor layer so as to partially expose the surface of the nitride semiconductor layer, and then grown to cover the protective film from the exposed nitride semiconductor layer. It is preferably formed on a semiconductor substrate. Thereby, each layer of the first to twelfth nitride semiconductor elements can be formed with good crystallinity, so that the nitride semiconductor element having excellent characteristics can be formed. In the present invention, the heterogeneous substrate and the protective film may be removed before or after the element growth, leaving the nitride semiconductor layer on which the nitride semiconductor element is formed (or to be formed) as the substrate.

[0038]

1 is a schematic cross-sectional view showing the structure of a nitride semiconductor device according to one embodiment of the present invention.
The nitride semiconductor device of this embodiment is an electrode stripe type laser device having an active layer end face as a resonance surface (henceforth, simply referred to as the laser device of the embodiment), and FIG.
Shows a schematic cross-section when the element is cut in a direction perpendicular to the resonance direction of laser light. Hereinafter, an embodiment of the present invention will be described with reference to FIG.

First, in FIG. 1, each reference numeral indicates the following. 10 is a heterogeneous substrate made of a material different from the nitride semiconductor, for example, sapphire, spinel, SiC, Si,
For example, a GaN substrate having a film thickness of 10 μm or more grown on a substrate made of a material such as GaAs or ZnO is shown. The heterogeneous substrate may be removed after the GaN substrate 10 is formed as shown in FIG. 1, or may be used without being removed as shown in Examples described later (FIG. 4). 11 is Si
A buffer layer made of doped n-type GaN and an n-side contact layer are also shown. 12 is located at a position away from the active layer, for example, Si-doped n-type Al having a film thickness of 40 angstrom.
_0.2 Ga _0.8 N (first nitride semiconductor layer) and film thickness 4
Undo Ga of 0 angstrom
1 shows an n-side clad layer having a superlattice structure in which 100 N layers (second nitride semiconductor layers) are alternately laminated. Thirteen
Is between the n-side cladding layer 12 and the active layer 14,
An n-side guide layer made of, for example, undoped GaN having a bandgap energy smaller than that of Al _0.2 Ga _0.8 N of the n-side cladding layer 12 is shown. 14 is three well layers of In _0.2 Ga _0.8 N having a film thickness of 30 Å, and In _0.05 G having a film thickness of 30 Å, which has a bandgap energy larger than that of the well layers.
5 shows an active layer of a multi-quantum well structure in which a total of 5 layers of two barrier layers made of a _0.95 N are alternately laminated. 15
Is larger than the bandgap energy of the well layer of the active layer 14 and larger than the bandgap energy of the p-side optical guide layer 16, for example, Mg-doped p-type Al _0.3.
3 shows a p-side cap layer made of Ga _0.7 N. The band gap energy of the p-side cap layer 15 is preferably larger than that of the p-side cladding layer 17 of the superlattice structure, which has a smaller band gap energy (fourth nitride semiconductor layer). 16 is located between the p-side clad layer 17 and the active layer 14,
3 shows a p-side guide layer made of, for example, undoped GaN having a bandgap energy smaller than that of Al _0.2 Ga _0.8 N. 17 is a position apart from the active layer, for example, Mg-doped p having a film thickness of 40 angstrom.
Type Al _0.2 Ga _0.8 N and an undoped GaN layer having a film thickness of 40 angstroms are alternately formed into 10 layers.
The p-side clad layer having a superlattice structure formed by stacking 0 layers is shown. 18 is Al _0.2 Ga of the p-side cladding layer 17.
₃ shows a p-side contact layer made of, for example, Mg-doped GaN having a bandgap energy smaller than _0.8 N.

As described above, in the laser device according to the embodiment of the present invention, the above-mentioned nitride semiconductor layers 11 are formed on the GaN substrate 10.
To 18 are laminated, a stripe ridge is formed in the nitride semiconductor layer above the p-side cladding layer 17, and the p-electrode 21 is formed on almost the entire surface of the p-side contact layer 18 on the ridge outermost surface. Has been formed. On the other hand, the n-electrode 23 is formed on the surface of the n-side buffer layer 11 exposed by being etched from the upper part of the nitride semiconductor layer. In the present embodiment, the n-electrode 23 is formed on the surface of the n-side buffer layer 11, but since the GaN substrate 10 is used as the substrate, the portion where the n-electrode is formed is etched up to the GaN substrate 10. It is also possible to have a structure in which the surface of the GaN substrate 10 is exposed, an n electrode is formed on the exposed surface of the GaN substrate 10, and a p electrode and an n electrode are provided on the same surface side. An insulating film 25 made of, for example, SiO ₂ is provided on the surface of the nitride semiconductor exposed between the n-electrode 23 and the p-electrode 21. A pad electrode 22 and an n pad electrode 24 are provided. As described above, in the present specification, the nitride semiconductor layer between the active layer and the p-electrode, regardless of the conductivity type of the nitride semiconductor layer,
Collectively referred to as the p-side nitride semiconductor layer, its active layer and Ga
The nitride semiconductor layer between the N substrate 10 and the N substrate 10 is generically called an n-side nitride semiconductor layer.

In the laser device according to the embodiment of the present invention, as shown in FIG.
In the n-side nitride semiconductor layer below the active layer 14, the first nitride semiconductor layer having a large bandgap energy and the first nitride semiconductor layer having a large band gap energy at a position apart from the active layer 14 A second nitride semiconductor layer having a small band gap energy is laminated, and an n-side cladding layer 12 having a superlattice structure having different impurity concentrations is provided. The film thickness of the first nitride semiconductor layer and the second nitride semiconductor layer forming the superlattice layer is 100 angstroms or less,
More preferably, the film thickness is adjusted to 70 angstroms or less, and most preferably 10 to 40 angstroms.
If it is thicker than 100 angstroms, the first nitride semiconductor layer and the second nitride semiconductor layer have a film thickness equal to or larger than the elastic strain limit, and minute cracks or crystal defects are likely to occur in the film. In the present invention, the lower limit of the film thickness of the first nitride semiconductor layer and the second nitride semiconductor layer is not particularly limited as long as it is 1 atomic layer or more.
Most preferably, it is 0 angstrom or more. Further, the first nitride semiconductor layer is a nitride semiconductor containing at least Al,
It is preferable to grow Al _X Ga _1-X N (0 <X ≦ 1). On the other hand, the second nitride semiconductor may be any nitride semiconductor having a bandgap energy smaller than that of the first nitride semiconductor, but is preferably Al _Y Ga _1-Y N (0 ≦ Y < 1, X> Y), In
_A binary mixed crystal or ternary mixed crystal nitride semiconductor such as _Z Ga _{1 -Z} N (0 ≦ Z <1) is easily grown and a crystallinity is easily obtained. Among them, the first is particularly preferable.
Is a nitride semiconductor of Al _X Ga containing no In or Ga.
_1-X N (0 <X <1), and the second nitride semiconductor is A
does not contain a _{l In Z Ga 1-Z N} (0 ≦ Z <1) and,
Above all, in order to obtain a superlattice with excellent crystallinity, the Al mixed crystal ratio
(Y value) 0.3 or less of Al _X Ga _1-X N (0 <X ≦ 0.
Most preferred is a combination of 3) and GaN.

Al _X Ga _1-X N (0 <X <1)
When the first nitride semiconductor is formed by using and the second nitride semiconductor is formed by using GaN, it has the following excellent manufacturing advantages. That is, Al _X Ga _1-X N (0 <X <by the metal organic gas layer growth method (MOCVD).
In forming the 1) layer and the GaN layer, both layers can be grown in the same H ₂ atmosphere. Therefore, Al _X Ga _1-X N (0 <X <
The superlattice layer can be formed by alternately growing the 1) layer and the GaN layer. This is an extremely great advantage in manufacturing a superlattice layer that needs to be formed by laminating several tens to several hundreds of layers.

When forming a clad layer as an optical confinement layer and a carrier confinement layer, it is necessary to grow a nitride semiconductor having a bandgap energy larger than that of the well layer of the active layer. The nitride semiconductor layer having a large bandgap energy is a nitride semiconductor having a high Al mixed crystal ratio. In the past, when a nitride semiconductor having a high Al mixed crystal ratio was grown in a thick film, cracks were likely to occur, so crystal growth was extremely difficult. However, when the superlattice layer is formed as in the present invention, even if the AlGaN layer as the first nitride semiconductor layer forming the superlattice layer is a layer having a slightly higher Al mixed crystal ratio, the film thickness is equal to or less than the elastic critical film thickness. As it is grown, it is hard to crack. Therefore, in the present invention, A
Since a layer having a high mixed crystal ratio can be grown with good crystallinity, a clad layer having a high optical confinement and carrier confinement effect can be formed, and the threshold voltage of a laser device and the Vf (forward voltage) of an LED device are lowered. be able to.

Further, in the laser device according to the embodiment of the present invention, the first nitride semiconductor layer and the second nitride semiconductor layer of the n-side cladding layer 12 are made to have different n-type impurity concentrations from each other. Set. This is what is called modulation doping, and the n-type impurity concentration of one layer is reduced,
When the other layer is doped with a high concentration, preferably in a state where impurities are not doped (undoped), the threshold voltage, Vf, etc. can be lowered. This is because the presence of a layer having a low impurity concentration in the superlattice layer increases the mobility of the layer, and the presence of a layer having a high impurity concentration at the same time causes the superlattice to remain at a high carrier concentration. Due to the ability to form layers. In other words, since a layer having a low impurity concentration and a high mobility and a layer having a high impurity concentration and a high carrier concentration are present at the same time, a layer having a high carrier concentration and a high mobility serves as a clad layer. , Vf is estimated to decrease.

When the nitride semiconductor layer having a large band gap energy is heavily doped with impurities, the modulation doping produces a two-dimensional electron gas between the high impurity concentration layer and the low impurity concentration layer. It is estimated that the resistivity decreases due to the influence of electron gas. For example, in a superlattice layer in which a nitride semiconductor layer having a large bandgap doped with an n-type impurity and an undoped nitride semiconductor layer having a small bandgap are stacked, a layer doped with an n-type impurity and an undoped layer At the heterojunction interface with and, the barrier layer side is depleted, and electrons (two-dimensional electron gas) accumulate at the interface before and after the thickness on the layer side with a small band gap. Since this two-dimensional electron gas is created on the side with a smaller band gap, it is not scattered by impurities when electrons travel,
The mobility of electrons in the superlattice increases and the resistivity decreases.
It is assumed that the modulation doping on the p-side is also influenced by the two-dimensional hole gas. In the case of p-layer, AlGaN is Ga
The resistivity is higher than N. So for AlGaN, p
It is speculated that the threshold value tends to decrease when the device is manufactured because the resistivity of the superlattice layer decreases due to the decrease of the resistivity due to a large amount of doping of the type impurities.

On the other hand, when the nitride semiconductor layer having a small band gap energy is heavily doped with impurities, it is presumed that the following effects are obtained. For example AlGaN
When the layers and the GaN layer are doped with the same amount of Mg, AlGa
In the N layer, the depth of the acceptor level of Mg is large and the activation rate is small. On the other hand, the depth of the acceptor level of the GaN layer is shallower than that of the AlGaN layer, and the activation rate of Mg is high. For example, even if Mg is doped at 1 × 10 ²⁰ / cm ³ Ga
N has a carrier concentration of about 1 × 10 ¹⁸ / cm ³ , whereas AlGaN has a carrier concentration of only about 1 × 10 ¹⁷ / cm ³ . Therefore, in the present invention, AlGa
A superlattice having a high carrier concentration can be obtained by forming a superlattice with N / GaN and doping a GaN layer having a high carrier concentration with more impurities. Moreover, since the superlattice is used, the carriers move in the AlGaN layer having a low impurity concentration due to the tunnel effect, so that the carriers are not substantially affected by the AlGaN layer, and the AlGaN layer functions as a cladding layer having a high band gap energy. Therefore, even if the nitride semiconductor layer having the smaller bandgap energy is doped with a large amount of impurities, it is very effective in reducing the threshold values of the laser element and the LED element. Although this description has been given of the example in which the superlattice is formed on the p-type layer side, the same effect can be obtained when the superlattice is formed on the n-layer side.

When the first nitride semiconductor layer having a large band gap energy is heavily doped with n-type impurities, the preferable doping amount of the first nitride semiconductor layer is 1
It is adjusted to a range of × 10 ¹⁷ / cm ³ to 1 × 10 ²⁰ / cm ³ , and more preferably 1 × 10 ¹⁸ / cm ^{3 to} 5 × 10 ¹⁹ / cm ³ . When the amount is less than 1 × 10 17 ^/ cm ^3, the difference between the second nitride semiconductor layer becomes small, there is a large layer obtained less likely the carrier concentration, and 1 × 10 ²⁰
If it is more than / cm ³ , the leak current of the device itself tends to increase. On the other hand, the n-type impurity concentration of the second nitride semiconductor layer should be lower than that of the first nitride semiconductor layer, preferably 1/10 or less. Most preferably, undoped provides the layer with the highest mobility, but since the film thickness is thin, there is n-type impurity diffused from the first nitride semiconductor side, and the amount thereof is 1 × 10 ^19.
/ Cm ³ or less is desirable. Si, G as n-type impurities
Elements of Group IVB and Group VIB of the periodic table such as e, Se, S and O are selected, and preferably Si, Ge and S are n-type impurities. This action is obtained by doping the first nitride semiconductor layer having a large band gap energy with a small amount of n-type impurities,
The same applies when the second nitride semiconductor layer having a small band gap energy is heavily doped with n-type impurities.

Further, in the laser device of the embodiment of the present invention, in the p-side nitride semiconductor layer above the active layer 14 shown in FIG. 1, the band gap energy is large at a position apart from the active layer 14. A p-side clad having a superlattice structure in which a third nitride semiconductor layer and a fourth nitride semiconductor layer having a bandgap energy smaller than that of the third nitride semiconductor layer are stacked, and the impurity concentrations of which are different from each other. Layer 17
have. The film thicknesses of the third and fourth nitride semiconductor layers forming the superlattice layer of the p-side clad layer 17 are 100 angstroms or less, more preferably 70 angstroms or less, and most preferably the same as the n-side clad layer 12. Adjust to a film thickness of 10-40 Å. Similarly, the third nitride semiconductor layer is a nitride semiconductor containing at least Al, preferably Al _X Ga _1-X N (0 <X ≦.
1) is preferably grown, and the fourth nitride semiconductor is preferably Al _Y Ga _1-Y N (0 ≦ Y <1, X>
_{Y), In Z Ga 1-} Z N (0 2 mixed crystal such as ≦ Z ≦ 1), it is desirable to grow the nitride semiconductor ternary mixed crystal.

When the p-side clad layer 17 has a superlattice structure, the action of the superlattice structure on the laser element is the same as the action of the n-side clad layer 12, but in addition to the case of further formation on the n-layer side. It has the following effects. That is, the p-type nitride semiconductor usually has a resistivity higher by two digits or more than the n-type nitride semiconductor. Therefore, by forming the superlattice layer on the p-layer side, the effect of lowering the threshold voltage remarkably appears. More specifically, it is known that a nitride semiconductor is a semiconductor in which p-type crystals are very difficult to obtain. A technique for removing hydrogen by annealing a nitride semiconductor layer doped with a p-type impurity to obtain a p-type crystal is known (Japanese Patent No. 2540791). However, even if the p-type is obtained, its resistivity is several Ω · cm or more. Therefore, by forming the p-type layer as a superlattice layer, the crystallinity is improved and the resistivity is reduced by one digit or more, so that the threshold voltage can be lowered.

The third nitride semiconductor layer and the fourth nitride semiconductor layer of the p-side clad layer 17 have different p-type impurity concentrations, one of which has a large impurity concentration and the other of which has a higher impurity concentration. Make it smaller. Similar to the n-side clad layer 12,
If the p-type impurity concentration of the third nitride semiconductor layer having a large bandgap energy is increased and the fourth p-type impurity concentration of a small bandgap energy is decreased, preferably undoped, the threshold voltage, Vf, etc. are set. Can be reduced. The reverse configuration is also possible.
That is, the p-type impurity concentration of the third nitride semiconductor layer having a large bandgap energy may be reduced, and the p-type impurity concentration of the fourth nitride semiconductor layer having a small bandgap energy may be increased. The reason is as described above.

The preferable doping amount to the third nitride semiconductor layer is 1 × 10 ¹⁸ / cm ³ to 1 × 10 ²¹ / cm ³ ,
More preferably 1 × 10 ¹⁹ / cm ^{3 to} 5 × 10 ²⁰ /
Adjust to the cm ³ range. 1 When it is less than × 10 18 ^/ cm ^3, likewise the difference between the fourth nitride semiconductor layer becomes small, similarly located in the layer higher is obtained less likely the carrier concentration, and 1 × 10 ²¹ / If it is more than cm ³ , the crystallinity tends to deteriorate. On the other hand, the p-type impurity concentration of the fourth nitride semiconductor layer should be lower than that of the third nitride semiconductor layer, preferably 1/10 or less.
In order to obtain the layer having the highest mobility, the undoped layer is most preferable. In reality, since the film thickness is thin,
It is considered that there is a p-type impurity diffused from the nitride semiconductor side, but the amount thereof is preferably 1 × 10 ²⁰ / cm ³ or less in order to obtain good results in the present invention.
As the p-type impurity, an element of Group IIA or IIB of the periodic table such as Mg, Zn, Ca or Be is selected, and preferably M
Let g, Ca, etc. be p-type impurities. This action is the same when the third nitride semiconductor layer having a large band gap energy is lightly doped with p-type impurities and the fourth nitride semiconductor layer having a small band gap energy is heavily doped with p-type impurities. is there.

Furthermore, in the nitride semiconductor layer forming the superlattice, the layers doped with impurities at a high concentration each have a central portion of the semiconductor layer (the second nitride semiconductor layer or the fourth nitride semiconductor layer) in the thickness direction. The impurity concentration is high in a position away from the nitride semiconductor layer, and the impurity concentration is low (preferably undoped) in the vicinity of both ends (a portion close to the second nitride semiconductor layer or the fourth nitride semiconductor layer). It is desirable to do so. More specifically, for example, AlGaN doped with Si as an n-type impurity and undoped G
When a superlattice layer is formed with the aN layer, AlGaN is Si
Since it is doped with, the electron is emitted to the conduction band as a donor, but the electron falls to the conduction band of GaN having a low potential. Since the GaN crystal is not doped with donor impurities, carriers are not scattered by the impurities. Therefore, the electrons can easily move in the GaN crystal, and the electron mobility is substantially increased. This is similar to the effect of the two-dimensional electron gas described above, and the substantial mobility in the lateral direction of electrons becomes high and the resistivity becomes small. Furthermore, in AlGaN having a large band gap energy, GaN
The effect can be further enhanced if the central region relatively distant from the layer is heavily doped with n-type impurities. That is, among the electrons moving in GaN, the electrons passing through the portion close to the AlGaN layer receive some scattering of n-type impurity ions (Si in this case) in the portion of the AlGaN layer close to the GaN layer. However, if the portion of the AlGaN layer close to the GaN layer is undoped as described above,
Since electrons passing through a portion close to the AlGaN layer are less likely to be scattered by Si, the mobility of the undoped GaN layer is further improved. Although the action is slightly different, a similar effect can be obtained when the superlattice is composed of the third nitride semiconductor layer on the p-layer side and the fourth nitride semiconductor layer, and the third nitride having a large band gap energy can be obtained. It is desirable that the central region of the semiconductor layer be heavily doped with p-type impurities to reduce the portion close to the fourth nitride semiconductor layer or be undoped. On the other hand, a layer in which a nitride semiconductor layer having a small bandgap energy is heavily doped with an n-type impurity can be configured to have the above impurity concentration. The effect tends to be small.

While the n-side clad layer 12 and the p-side clad layer 17 are described above as superlattice layers, the superlattice layer is also used as the contact layer in the present invention. The side light guide layer 13, the p-side cap layer 15, the p-side light guide layer 16, the p-side contact layer 18, and the like can have a superlattice structure. That is, any layer apart from the active layer, a layer in contact with the active layer, or any layer can be a superlattice layer. In particular, the n-side buffer layer 1 in which the n-electrode is formed
If 1 is a superlattice, an effect similar to the HEMT is likely to appear.

Further, in the laser device of the embodiment of the present invention, as shown in FIG. 1, impurities (n-type impurities in this case) are present between the n-side cladding layer 12 made of a superlattice layer and the active layer 14. An n-side light guide layer 13 having a concentration adjusted to 1 × 10 ¹⁹ / cm ³ or less is formed. Even if the n-side light guide layer 13 is undoped, n-type impurities may diffuse and enter from other layers. However, in the present invention, a doping amount of 1 × 10 ¹⁹ / cm ³ or less is used. if there is,
It operates as a light guide layer and does not impair the effects of the present invention. However, in the present invention, the impurity concentration of the n-side light guide layer 13 is preferably 1 × 10 ¹⁸ / cm ³ or less, more preferably 1 × 10 ¹⁷ / cm ³ or less, and undoped. Most preferred. Further, it is desirable that the n-side light guide layer is made of a nitride semiconductor containing In or GaN.

Further, in the laser device of the embodiment, the impurity (p-type impurity in this case) concentration is 1 × 10 ¹⁹ between the p-side cladding layer 17 made of a superlattice layer and the active layer 14.
The p-side light guide layer 16 adjusted to / cm ³ or less is formed. In the present invention, the impurity concentration of the p-side guide layer 16 may be 1 × 10 ¹⁹ / cm ³ or less, but the preferable impurity concentration is 1 × 10 ¹⁸ / cm ³ or less, and most preferably undoped. In the case of a nitride semiconductor, when it is undoped, it usually exhibits n-type conductivity, but in the present invention, the conductivity type of the p-side guide layer 16 may be either n or p. Regardless of p
It is called a side light guide layer. In addition, in practice, p-type impurities may diffuse from other layers and enter the p-side light guide layer 16. It is desirable that the p-side optical guide layer is also made of a nitride semiconductor containing In or GaN.

The reason why it is preferable to allow an undoped nitride semiconductor to exist between the active layer and the cladding layer is as follows. That is, in the case of a nitride semiconductor, the emission of the active layer is usually 360 to 520 nm, especially 380 to 450 n.
It is designed for m. An undoped nitride semiconductor has a lower absorptance of light having the above wavelength than a nitride semiconductor doped with an n-type impurity or a p-type impurity. Therefore, an undoped nitride semiconductor, an active layer that emits light,
By sandwiching it between the cladding layer as a light confinement layer and the extinction of light emission from the active layer, it is possible to realize a laser element that oscillates with a low gain and reduce the threshold voltage. This effect can be confirmed when the impurity concentration of the light guide layer is 1 × 10 ¹⁹ / cm ³ or less.

Therefore, as a preferable combination of the present invention, a clad layer having a superlattice structure in which impurities are modulation-doped is provided at a position apart from the active layer, and the impurity concentration is between the clad layer and the active layer. Is a light emitting device having a low guide layer, preferably an undoped guide layer.

As a further preferred embodiment, in the light emitting device of the present invention, a band larger than the band gap energy between the well layer of the active layer and the interface of the p-side guide layer 16 is provided between the p-side guide layer 16 and the active layer 14. P made of a nitride semiconductor having a gap energy and a film thickness of 0.1 μm or less
The side cap layer 15 is formed, and the impurity concentration of the p side cap layer is adjusted to 1 × 10 ¹⁸ / cm ³ or more. The thickness of the p-type cap layer 15 is adjusted to 0.1 μm or less, more preferably 500 angstroms or less, and most preferably 300 angstroms or less.
This is because if the film is grown to a thickness greater than 0.1 μm, cracks are likely to occur in the p-type cap layer 15 and a nitride semiconductor layer with good crystallinity is difficult to grow. In this way, a layer with a large band gap energy is in contact with the active layer,
By forming a thin film having a thickness of 0.1 μm or less, the leak current of the light emitting element tends to decrease. by this,
Electrons injected from the n-layer side are accumulated in the active layer due to the energy barrier of the cap layer, and the probability of recombination of electrons and holes is increased, so that the output of the device itself can be improved. . Further, the impurity concentration is 1 × 10 ¹⁸
/ Cm ³ or more needs to be adjusted. This cap layer is a layer having a relatively high Al mixed crystal ratio, and a layer having a high Al mixed crystal ratio is likely to have high resistance. For this reason, unless the carrier concentration is increased by doping impurities to lower the resistivity, this layer becomes an i layer having a high resistance, and a p-i-n structure is formed, resulting in poor current-voltage characteristics. It tends to become The cap layer on the p side may be formed on the n side. When it is formed on the n-side, it may or may not be doped with an n-type impurity.

In the laser device of the embodiment configured as described above, since the n-side cladding layer 12 and the p-side cladding layer 17 have a superlattice structure, the n-side cladding layer 12 and the p-side cladding layer 17 are formed. The electric resistance can be lowered, the threshold voltage can be lowered, and laser oscillation can be performed for a long time. Further, in the laser device of this embodiment, the n-side cladding layer 12
In addition to forming the p-side clad layer 17 with a superlattice structure, various means can be taken as described above to further reduce the threshold voltage.

In the above embodiment, the n-side cladding layer 12
Although the p-side clad layer 17 and the p-side clad layer 17 have a superlattice structure, the present invention is not limited to this, and either the n-side clad layer 12 or the p-side clad layer 17 may have a superlattice structure. Even with the above configuration, the threshold voltage can be lowered as compared with the conventional example.

Further, in the embodiment, the n-side cladding layer 12
Although the p-side clad layer 17 and the p-side clad layer 17 have a superlattice structure, the present invention is not limited to this, and any one of p-side and n-side nitride semiconductor layers other than the n-side clad layer 12 and the p-side clad layer 17 The above may be a superlattice structure. Even with the above configuration, the threshold voltage can be lowered as compared with the conventional example.

In the above embodiments, the n-side cladding layer 12 and the p-side cladding layer 17 in the laser device have a superlattice structure, but the present invention is not limited to this, and other nitrides such as light emitting diodes (LEDs) are used. It goes without saying that it can be applied to semiconductor elements. By configuring as above,
In the light emitting diode, Vf (forward voltage) can be lowered.

[0063]

Embodiments of the present invention will be described in detail below with reference to FIGS. FIG. 2 is a perspective view showing the shape of the laser device of FIG.

Example 1 A GaN substrate 10 is prepared by growing a single crystal of GaN with a film thickness of 50 μm on a substrate of sapphire (C plane) via a buffer layer of GaN. This Ga
Set the N substrate 10 in the reaction vessel and set the temperature to 1050 ° C.
And hydrogen as a carrier gas, ammonia and TMG (trimethylgallium) as a source gas, and silane gas as an impurity gas.
N-side buffer layer 1 made of ¹⁸ / cm ³ -doped GaN
1 is grown to a film thickness of 4 μm. This buffer layer also functions as a contact layer for forming the n-electrode when the light emitting device having the structure as shown in FIG. 1 is manufactured. Further, the n-side buffer layer is a buffer layer grown at a high temperature, and is formed on a substrate made of a material different from the nitride semiconductor material such as sapphire, SiC, and spinel.
At a low temperature of 0 ° C. or lower, GaN, AlN, etc. are
It is distinguished from a buffer layer which is directly grown with a film thickness of less than μm.

[0065] (n-side cladding layer 12 = superlattice layer) Subsequently, TMA (trimethyl aluminum) at 1050 ° C., TMG, ammonia, using a silane gas, n-type and 1 × ^{10 19} / cm ³ doped with Si _{Al 0. 2} Ga
_A first layer of _0.8 N is grown to a thickness of 40 Å, then silane gas and TMA are stopped, and a second layer of undoped GaN is grown to a thickness of 40 Å. Then, a superlattice layer is formed such that the first layer + the second layer + the first layer + the second layer + ... 100 layers are alternately laminated, and the superlattice has a total thickness of 0.8 μm. The n-side cladding layer 12 is grown.

(N-side light guide layer 13) Subsequently, the silane gas is stopped, and undoped G at 1050.degree.
The n-side light guide layer 13 made of aN is grown to a film thickness of 0.1 μm. This n-side light guide layer acts as a light guide layer of the active layer, and it is desirable to grow GaN and InGaN, and it is desirable to grow it to a film thickness of usually 100 angstrom to 5 μm, and more preferably 200 angstrom to 1 μm. . This layer can also be an undoped superlattice layer. When forming a superlattice layer, the bandgap energy is larger than that of the active layer and smaller than that of Al _0.2 Ga _0.8 N of the n-side cladding layer.

(Active layer 14) Next, the active layer 14 is grown by using TMG, TMI, and ammonia as source gases. The temperature of the active layer 14 is 800 ° C.
Then, a well layer made of undoped In _0.2 Ga _0.8 N is grown to have a film thickness of 25 Å.
Then, a barrier layer made of undoped In _0.01 Ga _0.95 N is grown to a film thickness of 50 Å at the same temperature only by changing the TMI molar ratio. This operation is repeated twice, and finally an active layer of a multiple quantum well structure (MQW) having a total film thickness of 175 angstroms in which well layers are laminated is grown. The active layer may be undoped as in this embodiment, or may be doped with n-type impurities and / or p-type impurities. The impurities may be doped in both the well layer and the barrier layer, or may be doped in either one.

(P-side cap layer 15) Next, the temperature is raised to 1050 ° C., TMG, TMA, ammonia, and Cp 2 Mg (cyclopentadienyl magnesium) are used, and the band gap energy is larger than that of the p-side optical guide layer 16. , A p-side cap layer 17 made of p-type Al _0.3 Ga _0.7 N doped with Mg at 1 × 10 ²⁰ / cm ³ is grown to a film thickness of 300 Å.
This p-type cap layer 15 has a thickness of 0.1 μm as described above.
The film is formed with a wound thickness of m or less, and the lower limit of the film thickness is not particularly limited, but it is desirable to form the film with a film thickness of 10 angstroms or more.

(P-side light guide layer 16) Subsequently, Cp2Mg and TMA were stopped, and a p-side light guide layer 16 made of undoped GaN having a bandgap energy smaller than that of the p-side cap layer 15 at 1050 ° C. was formed to 0.1 μm. To grow. This layer acts as a light guide layer for the active layer and, like the n-type light guide layer 13, G layer
It is desirable to grow with aN or InGaN. In addition,
The p-side optical guide layer may be a superlattice layer made of an undoped nitride semiconductor or an impurity-doped nitride semiconductor. When the superlattice layer is used, the bandgap energy is larger than that of the well layer of the active layer and smaller than that of Al _0.2 Ga _0.8 N of the p-side cladding layer.

(P-side clad layer 17) Subsequently, a third layer of p-type Al _0.2 Ga _0.8 N doped with Mg at 1 × 10 ²⁰ / cm ³ at 1050 ° C. was formed into 4 layers.
The film is grown to a film thickness of 0 angstrom, then only TMA is stopped, and a fourth layer made of undoped GaN is grown to a film thickness of 40 angstrom. Then, this operation is repeated 100 times to form the p-side cladding layer 17 made of a superlattice layer having a total film thickness of 0.8 μm.

(P-side contact layer 18) Finally, at 1050 ° C., on the p-side cladding layer 17, M
A p-side contact layer 18 made of p-type GaN doped with g of 2 × 10 ²⁰ / cm ³ is grown to a film thickness of 150 Å. The p-side contact layer 18 is a p-type In _X A
1 _Y Ga _1-X-Y N (0 ≦ X, 0 ≦ Y, X + Y ≦ 1), preferably Mg-doped GaN
Then, the most preferable ohmic contact with the p electrode 21 can be obtained. In addition, a nitride semiconductor having a small bandgap energy is used as a p-side contact layer in contact with the p-side clad layer 17 having a superlattice structure containing p-type Al _Y Ga _1-YN , and the film thickness is reduced to 500 angstroms or less. Therefore, the carrier concentration of the p-side contact layer 18 is substantially increased, a desired ohmic contact with the p-electrode is obtained, and the threshold current and voltage of the device are reduced.

The wafer on which the nitride semiconductor was grown as described above was placed in a reaction vessel under a nitrogen atmosphere at 700
Annealing is performed at 0 ° C. to further reduce the resistance of the p-type impurity-doped layer.

After the annealing, the wafer is taken out of the reaction container and, as shown in FIG. 1, the uppermost p-side contact layer 18 and the p-side clad layer 17 are etched by an RIE device to have a stripe width of 4 μm. It has a ridge shape. As described above, by forming the layer above the active layer into a striped ridge shape, the light emission of the active layer is concentrated under the striped ridge, and the threshold value is lowered. In particular, the p-side cladding layer 17 made of a superlattice layer
It is preferable that the above layers have a ridge shape.

Next, a mask is formed on the ridge surface and RIE is performed.
Etching is performed to expose the surface of the n-side buffer layer 11. The exposed n-side buffer layer 11 also functions as a contact layer for forming the n-electrode 23.
Although the n-side buffer layer 11 is used as the contact layer in FIG. 1, the GaN substrate 10 may be etched to use the exposed GaN substrate 10 as the contact layer.

Next, p electrodes 21 made of Ni and Au are formed in stripes on the ridge outermost surface of the p-side contact layer 18. Examples of materials for the p-side electrode 21 that can obtain a preferable ohmic contact with the p-side contact layer include Ni, Pt, and P.
Examples thereof include d, Ni / Au, Pt / Au, Pd / Au and the like.

On the other hand, the n-electrode 23 made of Ti and Al is formed in a stripe shape on the surface of the n-side buffer layer 11 exposed previously. As a material of the n-side buffer layer 11 or the n-electrode 23 that can obtain a preferable ohmic property with the GaN substrate 10, a metal or alloy such as Al, Ti, W, Cu, Zn, Sn, or In is preferable.

Next, as shown in FIG.
Si is formed on the surface of the nitride semiconductor layer exposed between the electrodes 23.
An insulating film 25 made of O ₂ is formed, and the p pad electrode 22 electrically connected to the p electrode 21 through the insulating film 25,
And the n pad electrode 24 is formed. This p-pad electrode 2
2 has a function of substantially expanding the surface area of the p-electrode 21 so that the p-electrode side can be wire-bonded or die-bonded. On the other hand, the n pad electrode 24 is the n electrode 23.
It has the effect of preventing the peeling of.

As described above, the wafer on which the n-electrode and the p-electrode are formed is transferred to the polishing apparatus, and the sapphire substrate on the side on which the nitride semiconductor is not formed is lapped by using the diamond abrasive to remove the sapphire. The thickness of the substrate is 70
μm. After lapping, 1μ with finer abrasive
m polishing is performed to make the surface of the substrate a mirror surface, and the entire surface is metallized with Au / Sn.

After that, scribe the Au / Sn side,
Cleavage is performed in a bar shape in a direction perpendicular to the striped electrode, and a resonator is formed on the cleaved surface. SiO ₂ and TiO on the cavity surface
_A dielectric multilayer film of 2 was formed, and finally the bar was cut in a direction parallel to the p electrode to obtain a laser chip. Next, when the chip was placed face up (the substrate and the heat sink faced each other) on the heat sink, each electrode was wire-bonded, and laser oscillation was attempted at room temperature. At room temperature, the threshold current density was 2.0 kA / cm ² ,
At a threshold voltage of 4.0 V, continuous oscillation with an oscillation wavelength of 405 nm was confirmed, and a life of 1000 hours or more was shown.

[Embodiment 2] FIG. 3 is a schematic cross-sectional view showing the structure of a laser device according to another embodiment of the present invention. The figure at the time of cutting is shown. The second embodiment will be described below with reference to this drawing. still,
In FIG. 3, the same components as those in FIGS. 1 and 2 are designated by the same reference numerals.

G on a substrate made of sapphire (C-plane)
5 × 10 ¹⁸ Si is added through the buffer layer made of aN.
A GaN substrate 10 is prepared in which a single crystal of cm ³ -doped GaN is grown to a film thickness of 150 μm. This GaN
The n-side buffer layer 11 is grown on the substrate 10 in the same manner as in the first embodiment.

(Crack Prevention Layer 19) After growing the n-side buffer layer 11, the temperature is set to 800 ° C., TMG, TMI, and ammonia are used as a source gas, and silane gas is used as an impurity gas, and Si is doped at 5 × 10 ¹⁸ / cm ³ The crack prevention layer 19 made of In _0.1 Ga _0.9 N is grown to a film thickness of 500 Å. The crack prevention layer 19 can prevent cracks from entering the Al-containing nitride semiconductor layer by growing an n-type nitride semiconductor containing In, preferably InGaN. The crack prevention layer is preferably grown to a film thickness of 100 angstroms or more and 0.5 μm or less. When the thickness is less than 100 Å, it is difficult to act as a crack preventive as described above, and when the thickness is more than 0.5 μm, the crystal itself tends to turn black.

After the crack prevention layer 19 is grown, the n-side cladding layer 12 made of a modulation-doped superlattice and the undoped n-side optical guide layer 13 are grown in the same manner as in the first embodiment.

(N-side Cap Layer 20) Next, using TMG, TMA, ammonia, and silane gas, n-type Al doped with Si at 5 × 10 ¹⁸ / cm ³ and having a bandgap energy larger than that of the n-side optical guide layer 13. _An n-side cap layer 20 made of _0.3 Ga _0.7 N is grown to a film thickness of 300 Å.

Thereafter, the active layers 14 and p are formed in the same manner as in the first embodiment.
Side cap layer 15, undoped p-side optical guide layer 16, p-side cladding layer 17 made of modulation-doped superlattice 17, p
The side contact layer 18 is grown.

After the growth of the nitride semiconductor layer, annealing is similarly performed to further reduce the resistance of the p-type impurity-doped layer, and after the annealing, the uppermost p-side contact layer 18 is formed as shown in FIG. The p-side cladding layer 17 is etched to form a ridge having a stripe width of 4 μm.

After the ridge is formed, a p-electrode 21 made of Ni / Au is formed in a stripe shape on the outermost surface of the ridge of the p-side contact layer 18, and a nitride semiconductor layer other than the p-electrode 21 is made of SiO _2. The insulating film 25 is formed, and the p pad electrode 22 electrically connected to the p electrode 21 through the insulating film 25 is formed.

As described above, the wafer on which the p electrode is formed is transferred to the polishing apparatus, the sapphire substrate is removed by polishing, and the surface of the GaN substrate 10 is exposed. An n electrode 23 made of Ti / Al is formed on almost the entire exposed GaN substrate surface.

After the electrodes are formed, the GaN substrate is cleaved at the M plane (the surface corresponding to the side surface of the hexagonal column when the nitride semiconductor is approximated by a hexagonal crystal system), and the cleavage plane has a dielectric material made of SiO ₂ and TiO _2. A multilayer film is formed, and finally the bar is cut in a direction parallel to the p electrode to obtain a laser element. This laser element also showed continuous oscillation at room temperature, and exhibited characteristics substantially equivalent to those of Example 1.

Example 3 In Example 1, after the n-side buffer layer 11 is grown, the crack prevention layer 19 is grown in the same manner as in Example 2. Then, on the crack prevention layer, Si was added at 1 × 10 ¹⁹ / cm ³
An n-side cladding layer 12 consisting of a single doped Al _0.3 Ga _0.7 N layer is grown to a thickness of 0.4 μm. After that, when a laser element was manufactured in the same manner as in Example 1, the laser oscillation was also exhibited at room temperature, but the life was slightly shorter than that of the laser element in Example 1.

[Example 4] In Example 1, Mg was grown at the time of growing the p-side cladding layer 17.
Al _0.3 Ga _0.7 doped with 1 × 10 ²⁰ / cm ³
A laser device was manufactured in the same manner as in Example 1 except that the N layer single layer was grown to a film thickness of 0.4 μm. The laser device also showed laser oscillation at room temperature. A little shorter than.

[Example 5] In Example 1, Al was doped with Si at 1 x 10 ¹⁸ / cm ³ without forming the n-side cladding layer 12 in a superlattice structure.
_0.2 Ga _0.8 N layer 0.4 μm. Similarly, the p-side clad layer is also not made to have a superlattice structure and Mg of 1 × 10 6 is used.
²⁰ / cm ³ doped Al _0.2 Ga _0.8 N layer 0.4
μm. Instead, the n-side optical guide layer 13 is an undoped In _0.01 Ga _0.99 N layer 30 angstrom, and the GaN layer 30 doped with Si at 1 × 10 ¹⁷ / cm ^{3 is used.}
A superlattice structure having a total film thickness of 0.12 μm is formed by stacking angstroms, and the p-side optical guide layer 16 is undoped In
_0.01 Ga _0.99 N layer 30 Å, M
A laser device was manufactured in the same manner as in Example 1 except that a superlattice structure having a total film thickness of 0.12 μm was obtained by stacking 30 g of GaN layer doped with 1 × 10 ¹⁷ / cm ^{3 of} g. Although laser oscillation was shown, the life was slightly shorter than that of the laser device of Example 1.

[Sixth Embodiment] In the first embodiment, when the n-side buffer layer 11 is formed,
The undoped GaN layer has a thickness of 30 angstroms, and Si is 1 × 10 ¹⁹ / cm ³ -doped Al _0.05 Ga.
A superlattice layer having a total film thickness of 1.2 μm is formed by stacking _0.95 N layers with a thickness of 30 Å. Thereafter, in the same manner as in Example 1, an upper layer is grown from the n-side cladding layer 12 to form a laser device. However, when forming the n-electrode, the surface exposed by etching is in the middle of the 1.2 μm superlattice layer, and the n-electrode is formed on the superlattice layer. This laser element also continuously oscillates at room temperature, and the threshold value is set to that in Example 1.
It was slightly decreased as compared with that of No. 1, and the life was 1000 hours or more.

[Embodiment 7] FIG. 4 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention. The seventh embodiment will be described below with reference to this drawing.

As in Example 1, 2 inches φ, (000
1) 500 on the sapphire substrate 30 whose main surface is the C surface
A GaN buffer layer (not shown) is formed at 20 ° C.
After growing to a thickness of 0 Å, the temperature is raised to 10
The undoped GaN layer 31 is grown to a thickness of 5 μm at 50 ° C. The film thickness to be grown is not limited to 5 μm, and it is desirable that the film thickness is made thicker than the buffer layer and adjusted to 10 μm or less.
Substrates include sapphire, SiC, ZnO, spinel, G
A substrate made of a material different from the nitride semiconductor, which is known for growing a nitride semiconductor such as aAs, can be used.

Next, after the growth of the undoped GaN layer 31,
The wafer is taken out of the reaction vessel, a stripe-shaped photomask is formed on the surface of the GaN layer 31, and the CVD is performed.
Depending on the apparatus, a protective film 32 made of SiO _{2 having a} stripe width of 20 μm and a stripe interval (window portion) of 5 μm is formed to a thickness of 0.1 μm.
It is formed with a film thickness of. FIG. 4 is a schematic cross-sectional view showing a partial wafer structure when cut in a direction perpendicular to the long axis direction of the stripe. The shape of the protective film is a stripe shape,
It may have any shape such as a dot shape or a grid shape, but crystal defects are larger when the area of the protective film is larger than the exposed portion of the undoped GaN layer 31, that is, the portion (window portion) where the protective film is not formed. It is easy to grow the GaN substrate 10 with a small amount. As the material of the protective film, for example, silicon oxide (SiO 2
_X ), silicon nitride (Si _X N _Y ), titanium oxide (TiO 2
_{X 2} ), oxides and nitrides such as zirconium oxide (ZrO _X ), multilayer films of these, and metals having a melting point of 1200 ° C. or higher can be used. These protective film materials withstand the growth temperature of the nitride semiconductor of 600 ° C. to 1100 ° C., and have the property that the nitride semiconductor does not grow on the surface or hardly grows.

After forming the protective film 32, the wafer is set in the reaction vessel again, and a GaN layer to be the GaN substrate 10 made of undoped GaN is grown to a thickness of 10 μm at 1050 ° C. The preferable growth film thickness of the GaN layer to be grown varies depending on the film thickness and size of the protective film 32 formed previously, but in the lateral direction (vertical to the thickness direction) above the protective film so as to cover the surface of the protective film 32. Direction) and grow to a sufficient thickness. As described above, when the GaN substrate 10 is grown on the surface of the protective film 32 having the property that the nitride semiconductor is hard to grow, the GaN layer is first grown on the protective film 32. Does not grow, and the GaN layer is selectively grown on the undoped GaN layer 31 in the window. Then, when the GaN layer is continuously grown, G
A state in which the aN layer grows in the lateral direction and covers the protective film 32, and the GaN layers grown from the adjacent windows are connected to each other so that the GaN layer grows on the protective film 32. Becomes That is, the protective film 32 is formed on the GaN layer 31.
The GaN layer is grown laterally via the. Here, the important thing is that the G grown on the sapphire substrate 30.
It is the number of crystal defects in the aN layer 31 and the GaN substrate 10 grown on the protective film 32. That is, due to the mismatch of lattice constants between the heterogeneous substrate and the nitride semiconductor, a large number of crystal defects are generated in the nitride semiconductor grown on the heterogeneous substrate, and these crystal defects are sequentially formed in the upper layer. It is transmitted to the surface during the growth of semiconductors.
On the other hand, as in Example 7, the GaN substrate 10 laterally grown on the protective film 32 was not directly grown on the heterogeneous substrate, but the GaN layer grown from the adjacent window was
The number of crystal defects is much smaller than that of the crystal grown directly from the heterogeneous substrate because the crystal defects are connected during the growth by lateral growth on the protective film 32. Therefore,
A partially formed protective film is formed on a nitride semiconductor layer grown on a heterogeneous substrate, and a GaN layer laterally grown on the protective film is used as a substrate. As compared with the GaN substrate of Example 1, a GaN substrate with far fewer crystal defects can be obtained. Actually, the crystal defects of the undoped GaN layer 31 are 10 ¹⁰ / cm ² or more, but the crystal defects of the GaN substrate 10 by the method of this Example 7 are 10 ⁶ / cm ^2.
It can be reduced to below cm ² .

After the GaN substrate 10 is formed as described above, Si is deposited on the GaN substrate in the same manner as in Example 1.
After growing an n-side buffer layer of GaN doped with × 10 ¹⁸ / cm ³ and the contact layer 11 with a thickness of 5 μm, Si was added in an amount of 5 × 10 ¹⁸ / cm ^{3 in the same} manner as in Example 2.
A crack prevention layer 19 made of doped In _0.1 Ga _0.9 N is grown to a film thickness of 500 Å. The crack prevention layer 19 may be omitted.

(In the central part, n of a superlattice structure with a high impurity concentration
Side clad layer 12) Next, at 1050 ° C., using TMG and ammonia gas,
By growing the undoped GaN layer to a film thickness of 20 Å, a second nitride semiconductor layer having a small band gap energy is formed. Then at the same temperature,
TMA was added to grow an undoped Al _0.1 Ga _0.9 N layer to 5 angstroms, and then silane gas was added to add Si to 1 × 10 ¹⁹ / cm ³ -doped Al _0.1 Ga.
_{After the 0.9} N layer is grown to a thickness of 20 Å, the Si is stopped and the undoped Al _0.1 Ga _0.9 N layer is further grown to a thickness of 5 Å to obtain a large band gap energy. Thickness 30 μm
To form a first nitride semiconductor layer. After that, in the same way,
The second nitride semiconductor layer and the first nitride semiconductor layer are alternately and repeatedly formed. In Example 7, the second nitride semiconductor layer and the first nitride semiconductor layer were laminated so as to each have 120 layers, and the n-side cladding layer 12 having a superlattice structure and having a thickness of 0.6 μm was formed. To form.

Next, in the same manner as in Example 1, the n-side light guide layer 13, the active layer 14, the p-side cap layer 15, and the p-side light guide layer 16 are sequentially grown.

(The central portion of the superlattice structure with a high impurity concentration is p
Side cladding layer 17) Next, at 1050 ° C., using TMG and ammonia gas,
An undoped GaN layer is grown to a film thickness of 20 Å to form a fourth nitride semiconductor layer having a small band gap energy. Then at the same temperature,
TMA was added to grow an undoped Al _0.1 Ga _0.9 N layer to 5 angstroms, and then Cp ₂ Mg was added and Mg was doped with 1 × 10 ²⁰ / cm ³ -doped Al _0.1 Ga.
After growing a _0.9 N layer to a thickness of 20 Å, Cp ₂ Mg is stopped and undoped Al _0.1 Ga
_{A 0.9} N layer is further grown to a film thickness of 5 Å to form a third nitride semiconductor layer having a large band gap energy and a thickness of 30 μm. Thereafter, similarly, the fourth nitride semiconductor layer and the third nitride semiconductor layer are alternately and repeatedly formed. In Example 7, the fourth nitride semiconductor layer and the third nitride semiconductor layer were laminated so as to each have 120 layers, and the n-side cladding layer 17 having a superlattice structure and having a thickness of 0.6 μm was formed. To form.

Finally, after growing the p-side contact layer 18 in the same manner as in Example 1, the wafer is taken out of the reaction container, annealed, and then etched to form layers above the p-side cladding layer 17. A striped ridge shape is used.

Next, as shown in FIG. 4, the ridge is etched symmetrically to form the n electrode 23.
The n-electrode 23 is formed by exposing the surface of the side buffer layer, while the p-electrode 21 is also formed on the ridge outermost surface of the p-side contact layer 18.
Are formed in stripes. After that, when a laser element was manufactured in the same manner as in Example 1, the threshold value, the current density, and the voltage were reduced by about 10% as compared with those in Example 1,
The continuous oscillation lifetime at a wavelength of 405 nm showed a lifetime of 2000 hours or more. This is largely due to the improvement in crystallinity of the nitride semiconductor due to the use of the GaN substrate 10 having few crystal defects. In FIG. 4, Ga
When the N substrate 10 is grown to have a film thickness of, for example, 80 μm or more, the heterogeneous substrate 30 to the protective film 32 can be removed.

[Embodiment 8] In Embodiment 7, when the n-side cladding layer 12 is grown, the central portion is not made to have a high impurity concentration, and a normal undoped G is used.
20 angstrom aN layer and 1 x 10 ¹⁹ Si
/ Cm ³ Doped Al _0.1 Ga _0.9 N layer of 20 angstrom is laminated to form a superlattice structure having a _total film thickness of 0.6 μm.

On the other hand, when the p-side cladding layer 17 is grown, the undoped GaN is not formed in the central portion without a high impurity concentration.
Layer is 20 Å and Mg is 1 × 10 ²⁰ / cm
^A laser device was produced in the same manner as in Example 7 except that a ^3- doped Al _0.1 Ga _0.9 N layer was stacked in a thickness of 20 Å to form a superlattice structure having a _total film thickness of 0.6 μm. Although the threshold value was slightly lower than that of Example 7, the life was almost the same, that is, 2000 hours or more.

Example 9 In Example 7, when the n-side cladding layer 12 was grown, two GaN layers doped with Si at 1 × 10 ¹⁹ / cm ³ were used.
5 Å and undoped Al _0.1 Ga
Alternating layers of _0.9 N layers and 25 Angstroms,
The superlattice structure has a total film thickness of 0.6 μm. On the other hand, when the p-side clad layer 17 is grown, Mg is added at 1 × 10 ²⁰ / cm.
Same as Example 7 except that 25 angstroms of ^3- doped GaN layers and 25 angstroms of undoped Al _0.1 Ga _0.9 N layers are alternately laminated to form a superlattice structure having a _total film thickness of 0.6 μm. When a laser element was manufactured as described above, a laser element having characteristics and life substantially equivalent to those of Example 7 was obtained.

[Example 10] In Example 7, when the n-side cladding layer 12 was grown, two GaN layers doped with Si at 1 x 10 ¹⁹ / cm ³ were used.
5 angstroms and 25 angstroms of Al _0.1 Ga _0.9 N layers doped with 1 × 10 ¹⁷ / cm ^{3 of} Si are alternately laminated to form a superlattice structure having a _total film thickness of 0.6 μm. On the other hand, when growing the p-side cladding layer 17, M
25 angstroms of a GaN layer doped with 1 × 10 ²⁰ / cm ^{3 of} g and 25 angstroms of an Al _0.1 Ga _0.9 N layer doped with 1 × 10 ¹⁸ / cm ^{3 of} Mg were alternately laminated. A laser device was manufactured in the same manner as in Example 7 except that the superlattice structure having a film thickness of 0.6 μm was used.
A laser device having characteristics and life substantially equivalent to those of Example 7 was obtained.

Example 11 In Example 7, Al _0.1 G doped with Si at 1 × 10 ¹⁹ / cm ³ was used without forming the n-side cladding layer with a superlattice structure.
An a _0.9 N layer is grown to a film thickness of 0.6 μm. on the other hand,
When growing the p-side clad layer 17, Mg was added at 1 × 10
25 angstroms of ²⁰ / cm ³ doped GaN layer and 1 × 10 ¹⁸ / cm ³ doped Al _0.1 Ga
Alternating layers of _0.9 N layers and 25 Angstroms,
When a laser device was manufactured in the same manner as in Example 7 except that the superlattice structure having a total film thickness of 0.6 μm was used, the threshold value was slightly increased as compared with Example 7, but the lifetime was also 1000 hours or more. .

[Embodiment 12] In Embodiment 7, the impurity concentration in the superlattice of the n-side cladding layer and the p-side cladding layer is a normal modulation doping (the central portion is not high concentration, but is substantially uniform in the layer), When growing the n-side buffer layer 11, Si was added at 1 × 10 ¹⁹ / cm 3.
^A laser device was manufactured in the same manner as in Example 7, except that 50 angstroms of ^3- doped Al _0.05 Ga _0.95 N layers and 50 angstroms of undoped GaN layers were alternately grown to form a superlattice layer having a total film thickness of 2 μm. As a result, the threshold value was slightly lower than that of Example 7, and the life was 3000 hours or more.

[Example 13] In Example 7, the n-side cladding layer 12 was undoped G.
aN layer 20 angstrom and Si 1 × 10 ¹⁹ /
A superlattice structure having a _total film thickness of 0.6 μm is formed by stacking cm ³ -doped Al _0.1 Ga _0.9 N layer 20 angstrom. The next n-side optical guide layer 13 is made of Si at 1 × 10 ¹⁹ /
A cm ³ -doped GaN layer 25 Å and an undoped Al _0.05 Ga _0.95 N layer 25 Å are alternately grown to form a superlattice structure with a total film thickness of 0.1 μm.

On the other hand, the p-side light guide layer also contains Mg at 1 × 10 5.
^{A 19} Å / cm ³ -doped GaN layer 25 Å and an undoped Al _0.05 Ga _0.95 N layer 25 Å are alternately grown to form a superlattice structure with a total film thickness of 0.1 μm. Next, the p-side clad layer 17 is made to have an undoped GaN layer of 20 angstrom and Mg of 1 × 10.
²⁰ / cm ³ doped Al _0.1 Ga _0.9 N layer 20
Total film thickness of 0.6μ, alternately laminated with Angstrom
A laser device was manufactured in the same manner except that the superlattice structure of m was used. As a result, the threshold value was slightly lower than that of Example 7, and the lifetime was 3000 hours or more.

[Embodiment 14] Like Embodiment 7, Embodiment 14 is a laser device constituted by using the GaN substrate 10. That is, the laser device of Example 14 is configured by forming the following semiconductor layers on the GaN substrate 10 configured similarly to Example 7. First, 1 × 10 ¹⁸ / c of Si was formed on the GaN substrate 10.
An n-side contact layer (n-side second nitride semiconductor layer) made of n-type GaN doped with m ³ or more is grown to a thickness of 2 μm. It should be noted that this layer is composed of undoped GaN and Si.
Alternatively, a superlattice layer made of Al _X Ga _1-X N (0 <X ≦ 0.4) may be used.

Next, after growing the n-side contact layer,
The temperature is set to 800 ° C, and TMG, TM are used in a nitrogen atmosphere.
I, ammonia, silane gas, Si 5 × 10 ¹⁸ /
A crack prevention layer made of In _0.1 Ga _0.9 N doped with cm ³ is grown to a film thickness of 500 Å. This crack prevention layer can prevent cracks from forming in a nitride semiconductor layer containing Al to be grown later by growing the n type nitride semiconductor containing In, preferably InGaN. The crack prevention layer is preferably grown to a film thickness of 100 angstroms or more and 0.5 μm or less. When the thickness is less than 100 Å, it is difficult to act as a crack preventive as described above, and when the thickness is more than 0.5 μm, the crystal itself tends to turn black.

Then, at 1050 ° C., TMA, TMG, ammonia, and silane gas were used to remove Si at 1 × 10 ¹⁹ / c.
The m ³ -doped n-type Al _0.2 Ga _0.8 N layer is 40 Å, and the undoped GaN layer is 4 Å.
A total film thickness of 0.8 μ was obtained by growing the film with a thickness of 0 Å and alternately stacking 100 layers of these layers.
An n-side clad layer made of m superlattice is grown.

Then, undoped Al _0.05 Ga
_An n-side light guide layer made of _0.95 N is grown to a film thickness of 0.1 μm. This layer acts as a light guide layer for guiding the light of the active layer and may be doped with n-type impurities in addition to undoped. Further, this layer may be a superlattice layer made of GaN and AlGaN.

Next, undoped In _0.01 Ga
_An active layer of _0.99 N is grown to a thickness of 400 Å.

Next, 1 × 10 ¹⁹ Mg, which has a larger band cap energy than the p-side optical guide layer formed later, is added.
/ Cm ³ -doped p-type Al _0.2 Ga _0.8 N is formed into a p-side cap layer having a film thickness of 300 Å.

Next, a p-side optical guide layer made of Al _0.01 Ga _0.99 N having a band-cap energy smaller than that of the p-side cap layer is grown to a film thickness of 0.1 μm. This layer acts as a light guide layer for the active layer. The p-side optical guide layer may be a superlattice layer made of an undoped nitride semiconductor. When a superlattice layer is used, the layer with the larger bandcap energy (barrier layer) has a larger bandcap energy than the active layer.
It is made smaller than the p-side clad layer.

Subsequently, a p-type Al _0.2 Ga _0.8 N layer doped with Mg at 1 × 10 ¹⁹ / cm ³ and 40 angstroms of p-type Al _0.2 Ga _0.8 N layers and 40 angstroms of undoped GaN were alternately grown to have a total film thickness of 0. A p-side cladding layer having a superlattice layer structure of 8 μm is grown.

Finally, 1 Mg is deposited on the p-side cladding layer.
A p-side contact layer made of p-type GaN doped with × 10 ²⁰ / cm ³ is grown to a thickness of 150 Å. Particularly in the case of a laser device, a nitride semiconductor having a small band cap energy is used as a p-side contact layer in contact with a p-side clad layer having a superlattice structure containing AlGaN.
Since the film thickness is made as thin as 500 angstroms or less, the carrier concentration of the p-side contact layer is substantially increased, a preferable ohmic contact with the p-electrode is obtained, and the threshold current and voltage of the device tend to be lowered. .

The wafer on which the nitride semiconductor is grown as described above is annealed at a predetermined temperature to further reduce the resistance of the p-type impurity-doped layer, and then the wafer is taken out from the reaction container and is subjected to RIE. P at the top
The side contact layer and the p-side cladding layer are etched to form a ridge shape having a stripe width of 4 μm.
As described above, by forming the layer above the active layer into a striped ridge shape, the light emission of the active layer is concentrated under the striped ridge, and the threshold value is lowered,
In particular, it is preferable that the p-side cladding layer and the layers formed of the superlattice layer or more have a ridge shape.

Next, a mask is formed on the ridge surface and RIE is performed.
Etching is performed to expose the surface of the n-side contact layer, and n-electrodes made of Ti and Al are formed in stripes. On the other hand, p-electrodes made of Ni and Au are formed in stripes on the outermost surface of the ridge of the p-side contact layer. p-type G
Examples of the electrode material capable of obtaining a ohmic contact with the aN layer include Ni, Pt, Pd, Ni / Au, Pt / A.
u, Pd / Au, etc. can be mentioned. Al is used as an electrode material capable of obtaining a preferable ohmic contact with n-type GaN.
Examples of the metal or alloy include Ti, W, Cu, Zn, Sn, In and the like.

Next, an insulating film made of SiO ₂ is formed on the surface of the nitride semiconductor layer exposed between the p electrode and the n electrode, and the p electrode electrically connected to the p electrode through this insulating film. A pad electrode is formed. The p-pad electrode substantially enlarges the surface area of the p-electrode so that the p-electrode side can be wire-bonded or die-bonded.

As described above, the wafer on which the n-electrode and the p-electrode are formed is transferred to the polishing apparatus, and the sapphire substrate on the side on which the nitride semiconductor is not formed is lapped by using the diamond abrasive to remove the sapphire. The thickness of the substrate is 70
μm. After lapping, 1μ with finer abrasive
m polishing is performed to make the surface of the substrate a mirror surface, and the entire surface is metallized with Au / Sn.

After that, the Au / Sn side is scraped,
Bars are cleaved in the direction perpendicular to the striped electrodes to form a resonator on the cleaved surface. SiO ₂ and TiO are formed on the resonator surface.
_A dielectric multilayer film made of 2 is formed, and finally the bar is cut in the direction parallel to the p electrode to obtain a laser chip. Next, when the chip was placed face up (the substrate and the heat sink faced each other) on the heat sink, each electrode was wire-bonded, and laser oscillation was attempted at room temperature. At room temperature, the threshold current density was 2.0 kA / c
At m ² and a threshold voltage of 4.0 V, continuous oscillation with an oscillation wavelength of 368 nm was confirmed, and the life was 1000 hours or more.

[0126]

As described above, since the present invention has the cladding layer made of the superlattice layer in which the impurities are modulation-doped, the threshold voltage is lowered and the laser device capable of continuous oscillation for a long time is provided. Can be realized. Further, this laser element can realize a good laser element having a high characteristic temperature. Characteristic temperature is the threshold current density due to temperature change exp
In proportion to (T / T ₀ ), {T: operating temperature (K), T ₀ :
Characteristic temperature (K)}. The larger T ₀ is, the lower the threshold current density of the LD is, and the LD operates stably even at a high temperature.
For example, in the laser device of Example 1 of the present invention, T ₀ is 15
There is 0K or more. This value indicates that the temperature characteristics of the LD are extremely excellent. Therefore, by using the laser device of the present invention as a writing light source and a reading light source,
Unprecedented capacity can be achieved, and its industrial utility value is extremely high.

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of a laser device according to one embodiment of the present invention.

FIG. 2 is a perspective view of the laser device shown in FIG.

FIG. 3 is a schematic cross-sectional view showing the structure of a laser device of Example 2 according to the present invention.

FIG. 4 is a schematic cross-sectional view showing the structure of a laser device of Example 7 according to the present invention.

[Explanation of symbols]

10 ... GaN substrate, 11 ... n-side buffer layer, 12 ... Superlattice structure n-side cladding layer, 13 ... n-side guide layer, 14 ... Active layer, 15 ... p-side cap layer, 16 ... p-side guide layer, 17 ... p-side clad layer of superlattice structure, 18 ... p-side contact layer, 19: crack prevention layer, 20 ... n-side cap layer, 21 ... p-electrode, 22 ... p pad electrode, 23 ... n electrode, 24 ... n pad electrode, 25 ... Insulating film.

─────────────────────────────────────────────────── --- Continuation of the front page (56) Reference JP-A-9-298341 (JP, A) JP-A-8-250810 (JP, A) JP-A-5-110139 (JP, A) JP-A-8- 23124 (JP, A) JP-A-8-228048 (JP, A) International Publication 97/011518 (WO, A1) Jpn. J. Appl. Phys. L ett. Part 2, 35 [1B] (1996), p. L74-L76 (58) Fields investigated (Int.Cl. ⁷ , DB name) H01S 5/00-5/50

Claims (41)

(57) [Claims] 1. A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the nitride semiconductor on the n-conductive side is formed. The layer has an n-side strained superlattice layer formed by stacking a first nitride semiconductor layer and a second nitride semiconductor layer at a position apart from or in contact with the active layer, the first nitride semiconductor The layer has a bandgap energy larger than that of the second nitride semiconductor layer and an n-type impurity concentration larger than that of the second nitride semiconductor layer, and in the first nitride semiconductor layer, the second A nitride semiconductor device, wherein an n-type impurity concentration in a portion close to the nitride semiconductor layer is made smaller than that in a portion distant from the second nitride semiconductor layer. 2. A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the nitride semiconductor on the n-conductive side is formed. The layer has an n-side strained superlattice layer formed by stacking a first nitride semiconductor layer and a second nitride semiconductor layer at a position apart from or in contact with the active layer, the first nitride semiconductor The layer has a bandgap energy larger than that of the second nitride semiconductor layer and an n-type impurity concentration smaller than that of the second nitride semiconductor layer, and in the second nitride semiconductor layer, the first A nitride semiconductor device, wherein an n-type impurity concentration in a portion close to the nitride semiconductor layer is made smaller than that in a portion distant from the first nitride semiconductor layer. 3. A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the nitride semiconductor on the p-conductive side is formed. A p-side strained superlattice layer formed by stacking a third nitride semiconductor layer and a fourth nitride semiconductor layer at a position apart from or in contact with the active layer, the third nitride semiconductor The layer has a bandgap energy larger than that of the fourth nitride semiconductor layer and a p-type impurity concentration larger than that of the fourth nitride semiconductor layer, and in the third nitride semiconductor layer, the fourth A nitride semiconductor device characterized in that a p-type impurity concentration in a portion close to the nitride semiconductor layer is made smaller than that in a portion far from the fourth nitride semiconductor layer. 4. A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, the nitride semiconductor on the p-conductive side. A p-side strained superlattice layer formed by stacking a third nitride semiconductor layer and a fourth nitride semiconductor layer at a position apart from or in contact with the active layer, the third nitride semiconductor The layer has a bandgap energy larger than that of the fourth nitride semiconductor layer and a p-type impurity concentration smaller than that of the fourth nitride semiconductor layer, and in the fourth nitride semiconductor layer, the third layer A nitride semiconductor device characterized in that a p-type impurity concentration in a portion close to the nitride semiconductor layer is made smaller than that in a portion far from the third nitride semiconductor layer. 5. A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, the nitride semiconductor on the n-conductive side. The layer has an n-side strained superlattice layer formed by stacking a first nitride semiconductor layer and a second nitride semiconductor layer at a position apart from or in contact with the active layer, the first nitride semiconductor The layer has a bandgap energy larger than that of the second nitride semiconductor layer and an n-type impurity concentration larger than that of the second nitride semiconductor layer, and in the first nitride semiconductor layer, the second The n-type impurity concentration of a portion close to the nitride semiconductor layer is made smaller than that of a portion distant from the second nitride semiconductor layer, and the p-conductive side nitride semiconductor layer is separated from the active layer. At the position or the contact position, the 3rd and 4th A p-side strained superlattice layer formed by laminating a nitride semiconductor layer, wherein the third nitride semiconductor layer has a bandgap energy larger than that of the fourth nitride semiconductor layer and the fourth nitride semiconductor layer. And a p-type impurity concentration lower than that of the layer, and a p-type impurity concentration of a portion of the fourth nitride semiconductor layer adjacent to the third nitride semiconductor layer is separated from the third nitride semiconductor layer. A nitride semiconductor device characterized in that it is made smaller than the portion that has been formed. 6. A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the nitride semiconductor on the n-conductive side is formed. The layer has an n-side strained superlattice layer formed by stacking a first nitride semiconductor layer and a second nitride semiconductor layer at a position apart from or in contact with the active layer, the first nitride semiconductor The layer has a bandgap energy larger than that of the second nitride semiconductor layer and an n-type impurity concentration smaller than that of the second nitride semiconductor layer, and in the second nitride semiconductor layer, the first The n-type impurity concentration in the portion close to the nitride semiconductor layer is made smaller than that in the portion distant from the first nitride semiconductor layer, and in the nitride semiconductor layer on the p-conductive side, the n-type impurity concentration is separated from the active layer. At the position or the contact position, the 3rd and 4th A p-side strained superlattice layer formed by laminating a nitride semiconductor layer, wherein the third nitride semiconductor layer has a bandgap energy larger than that of the fourth nitride semiconductor layer and the fourth nitride semiconductor layer. A p-type impurity concentration higher than that of the layer, and the p-type impurity concentration of a portion of the third nitride semiconductor layer adjacent to the fourth nitride semiconductor layer is separated from the fourth nitride semiconductor layer. A nitride semiconductor device characterized in that it is made smaller than the portion that has been formed. 7. The n-type impurity concentration of the first nitride semiconductor layer is in the range of 1 × 10 ¹⁷ / cm ³ to 1 × 10 ²⁰ / cm ³ , and the n-type impurity concentration of the second nitride semiconductor layer is Concentration is 1x
The nitride semiconductor device according to claim 1 or 5, which has a density of 10 ¹⁹ / cm ³ or less. 8. A nitride semiconductor device in which an active layer is formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the nitride semiconductor on the n-conductive side is formed. The layer has an n-side strained superlattice layer formed by stacking a first nitride semiconductor layer and a second nitride semiconductor layer at a position apart from or in contact with the active layer, the first nitride semiconductor The layer has a bandgap energy higher than that of the second nitride semiconductor layer and an n-type impurity concentration lower than that of the second nitride semiconductor layer. 9. A nitride semiconductor device in which an active layer is formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the nitride semiconductor on the n-conductive side is formed. In the layer, an n-side strained superlattice layer formed by stacking a first nitride semiconductor layer and a second nitride semiconductor layer is provided at a position apart from or in contact with the active layer, and the first nitride semiconductor The layer has a bandgap energy larger than that of the second nitride semiconductor layer and the second nitride semiconductor layer.
N-type impurity concentration lower than that of the nitride semiconductor layer, and the band gap energies of the p-conductive side nitride semiconductor layer are different from each other or are in contact with the active layer. P formed by stacking third and fourth nitride semiconductor layers having different type impurity concentrations
A nitride semiconductor device having a side strained superlattice layer. 10. The n-type impurity concentration of the first nitride semiconductor layer is 1 × 10 ¹⁹ / cm ³ or less, and the n-type impurity concentration of the second nitride semiconductor layer is 1 × 10 ¹⁷ / cm 3. ^3-
The nitride semiconductor device according to a any one of claims 2, 8 and 9 is the range of 1 × ^{10 20} / cm ^3. 11. The first nitride semiconductor layer is Al _Y G
a _1-Y consists N (0 <Y <1) , the second nitride semiconductor layer is _{In X Ga 1-X N (} 0 ≦ X <1) consisting essentially of claims 1, 2, 5-10 The nitride semiconductor device according to any one of the above. 12. The nitride semiconductor device according to claim 11, wherein the second nitride semiconductor layer is made of GaN. 13. The first nitride semiconductor layer is Al _X G
a _1-X N (0 <X <1), and the second nitride semiconductor layer is Al _Y Ga _1-Y N (0 <Y <1, X> Y).
The nitride semiconductor device according to claim 1, comprising: 14. The method according to claim 1, wherein the first nitride semiconductor layer or the second nitride semiconductor layer is not doped with an n-type impurity. The nitride semiconductor device described. 15. The p-type impurity concentration of the fourth nitride semiconductor layer is in the range of 1 × 10 ¹⁸ / cm ³ to 1 × 10 ²¹ / cm ³ of the ^third nitride semiconductor layer. Concentration is 1
The nitride semiconductor device according to claim 3 or 6, which has a density of × 10 ²⁰ / cm ³ or less. 16. The p-type impurity concentration of the third nitride semiconductor layer is 1 × 10 ²⁰ / cm ³ or less, and the p-type impurity concentration of the fourth nitride semiconductor layer is 1 × 10 ¹⁸ / cm ³ ~ 1x
The nitride semiconductor device according to claim 4 or 5, wherein the range is 10 ²¹ / cm ³ . 17. The third nitride semiconductor layer is Al _Y G
a _1-Y N (0 <Y <1), and the fourth nitride semiconductor layer is In _X Ga _1-X N (0 ≦ X <1). 17. The nitride semiconductor device according to any one of 16. 18. The nitride semiconductor device according to claim 17, wherein the fourth nitride semiconductor layer is made of GaN. 19. The third nitride semiconductor layer is Al _X G
_{a 1-X N (0 <} X <1) consists, said fourth semiconductor layer is _{Al Y Ga 1-Y N (} 0 <Y <1, X> Y)
The nitride semiconductor device according to any one of claims 3 to 6, 9, 15, and 16 comprising: 20. The third nitride semiconductor layer or the fourth nitride semiconductor layer is not doped with a p-type impurity, and the third nitride semiconductor layer or the fourth nitride semiconductor layer is not doped with a p-type impurity.
The nitride semiconductor device according to item 4. 21. A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the nitride semiconductor on the n-conductive side is formed. In the layer, an n-side strained superlattice layer formed by stacking a first nitride semiconductor layer and a second nitride semiconductor layer is provided at a position apart from or in contact with the active layer, and the first nitride semiconductor The layer has a bandgap energy larger than that of the second nitride semiconductor layer and the second nitride semiconductor layer.
An n-type impurity concentration higher than the nitride semiconductor layer of
In the first nitride semiconductor layer, the n-type impurity concentration of a portion close to the second nitride semiconductor layer is made smaller than that of a portion distant from the second nitride semiconductor layer, and the p conductivity is reduced. The nitride semiconductor layer on the side has a p-side strained superlattice layer formed by laminating a third nitride semiconductor layer and a fourth nitride semiconductor layer at a position apart from or in contact with the active layer. The third nitride semiconductor layer has a bandgap energy larger than that of the fourth nitride semiconductor layer
P-type impurity concentration higher than the nitride semiconductor layer of
In the third nitride semiconductor layer, a p-type impurity concentration in a portion close to the fourth nitride semiconductor layer is made smaller than that in a portion distant from the fourth nitride semiconductor layer. Nitride semiconductor device. 22. The n-type impurity concentration of the second nitride semiconductor layer is such that the n-type impurity concentration of the first nitride semiconductor layer is in the range of 1 × 10 ¹⁷ / cm ³ to 1 × 10 ²⁰ / cm ^3. The impurity concentration is 1 × 10 ¹⁹ / cm ³ or less, and the p-type impurity concentration of the third nitride semiconductor layer is 1 × 10 9.
¹⁸ / cm ³ to 1 × 10 ²¹ / cm ³ , and the p-type impurity concentration of the fourth nitride semiconductor layer is 1 × 10 ^20.
22. The nitride semiconductor device according to claim 21, which has a density of not more than / cm ³ . 23. A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the nitride semiconductor on the n-conductive side is formed. In the layer, an n-side strained superlattice layer formed by stacking a first nitride semiconductor layer and a second nitride semiconductor layer is provided at a position apart from or in contact with the active layer, and the first nitride semiconductor The layer has a bandgap energy larger than that of the second nitride semiconductor layer and the second nitride semiconductor layer.
And a n-type impurity concentration higher than that of the nitride semiconductor layer, the third and fourth nitride semiconductors are provided at a position distant from or in contact with the active layer in the p-conductive side nitride semiconductor layer. And a p-side strained superlattice layer formed by stacking layers, the third nitride semiconductor layer having a bandgap energy larger than that of the fourth nitride semiconductor layer and the fourth nitride semiconductor layer having a bandgap energy larger than that of the fourth nitride semiconductor layer.
And a p-type impurity concentration lower than that of the nitride semiconductor layer. 24. A range of the n-type impurity concentration of the first nitride semiconductor layer is ^{1 × 10 1 7 / cm 3} ~1 × 10 20 / cm 3, n of the second nitride semiconductor layer The type impurity concentration is 1 × 10 ¹⁹ / cm ³ or less, and the p-type impurity concentration of the third nitride semiconductor layer is 1 × 10 9.
24. The nitride semiconductor device according to claim 23, which is ²⁰ / cm ³ or less, and the p-type impurity concentration of the fourth nitride semiconductor layer is in the range of 1 × 10 ¹⁸ / cm ³ to 1 × 10 ²¹ / cm ³ . 25. A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the nitride semiconductor on the n-conductive side is formed. In the layer, an n-side strained superlattice layer formed by stacking a first nitride semiconductor layer and a second nitride semiconductor layer is provided at a position apart from or in contact with the active layer, and the first nitride semiconductor The layer has a bandgap energy larger than that of the second nitride semiconductor layer and the second nitride semiconductor layer.
N-type impurity concentration lower than that of the nitride semiconductor layer, and the third and fourth nitride semiconductors are provided at positions apart from or in contact with the active layer in the nitride semiconductor layer on the p-conductive side. And a p-side strained superlattice layer formed by stacking layers, the third nitride semiconductor layer having a bandgap energy larger than that of the fourth nitride semiconductor layer and the fourth nitride semiconductor layer having a bandgap energy larger than that of the fourth nitride semiconductor layer.
And a p-type impurity concentration higher than that of the nitride semiconductor layer. 26. The n-type impurity concentration of the first nitride semiconductor layer is 1 × 10 ¹⁹ / cm ³ or less, and the n-type impurity concentration of the second nitride semiconductor layer is 1 × 10 ¹⁷ / cm ^3. cm ³
˜1 × 10 ²⁰ / cm ³ , and the p-type impurity concentration of the third nitride semiconductor layer is 1 × 10 3.
¹⁸ / cm ³ to 1 × 10 ²¹ / cm ³ , and the p-type impurity concentration of the fourth nitride semiconductor layer is 1 × 10 ^20.
26. The nitride semiconductor device according to claim 25, which has a density of not more than / cm ³ . 27. A nitride semiconductor device having an active layer formed between a nitride semiconductor layer on the n-conductive side and a nitride semiconductor layer on the p-conductive side, wherein the nitride semiconductor on the n-conductive side is formed. In the layer, an n-side strained superlattice layer formed by stacking a first nitride semiconductor layer and a second nitride semiconductor layer is provided at a position apart from or in contact with the active layer, and the first nitride semiconductor The layer has a bandgap energy larger than that of the second nitride semiconductor layer and the second nitride semiconductor layer.
The n-type impurity concentration lower than that of the nitride semiconductor layer, and third and fourth nitride semiconductor layers at positions apart from or in contact with the active layer in the nitride semiconductor layer on the p-conductive side. And a p-side strained superlattice layer formed by laminating the third nitride semiconductor layer, the third nitride semiconductor layer having a bandgap energy larger than that of the fourth nitride semiconductor layer, and the fourth nitride semiconductor layer having a larger bandgap energy.
And a p-type impurity concentration lower than that of the nitride semiconductor layer. 28. The n-type impurity concentration of the first nitride semiconductor layer is 1 × 10 ¹⁹ / cm ³ or less, and the n-type impurity concentration of the second nitride semiconductor layer is 1 × 10 ¹⁷ / cm ^3. cm ³
˜1 × 10 ²⁰ / cm ³ , and the p-type impurity concentration of the third nitride semiconductor layer is 1 × 10 3.
28. The nitride semiconductor device according to claim 27, wherein the concentration is ²⁰ / cm ³ or less, and the p-type impurity concentration of the fourth nitride semiconductor layer is in the range of 1 × 10 ¹⁸ / cm ³ to 1 × 10 ²¹ / cm ³ . 29. In the n-side strained superlattice layer, the first nitride semiconductor layer is Al _Y Ga _1-Y N (0 <Y <
1), and the second nitride semiconductor layer is made of In _X Ga.
_1−X N (0 ≦ X <1), and in the p-side strained superlattice layer, the third nitride semiconductor layer is made of Al _Y Ga _{1 -Y} N (0 <Y <1), The fourth nitride semiconductor layer is formed of In _X Ga _1-X N (0 ≦ X
The nitride semiconductor device according to any one of claims 21 to 28, which comprises <1). 30. The nitride semiconductor device according to claim 29, wherein each of the second and fourth nitride semiconductor layers is made of GaN. 31. In the n-side strained superlattice layer, the first nitride semiconductor layer is Al _X Ga _1-X N (0 <X <.
1), wherein the second nitride semiconductor layer is Al _Y Ga
_1-Y N (0 <Y <1, X> Y), and in the p-side strained superlattice layer, the third nitride semiconductor layer is Al _X Ga _1-X N (0 <X <1). And the fourth nitride semiconductor layer is Al _Y Ga _1-Y N (0 <Y
<1. X> Y), The nitride semiconductor device according to any one of claims 21 to 28. 32. The first nitride semiconductor layer or the second nitride semiconductor layer is an undoped layer that is not doped with an n-type impurity. Nitride semiconductor device. 33. The third nitride semiconductor layer or the fourth nitride semiconductor layer is an undoped layer that is not doped with p-type impurities. Nitride semiconductor device. 34. The nitride semiconductor device according to claim 1, wherein the active layer includes an InGaN layer. 35. The nitride semiconductor device according to claim 34, wherein the InGaN layer is a quantum well layer. 36. The nitride semiconductor device is a laser oscillation device in which the active layer is located between a p-side cladding layer and an n-side cladding layer, and the active layer is located between the p-side cladding layer and the n-side cladding layer. 35. The nitride semiconductor device according to claim 1, wherein at least one of the n-side strained superlattice layer and the p-side strained superlattice layer. 37. The nitride semiconductor device is a laser oscillation device, wherein In is added to at least one of the p-side clad layer and the active layer or between the p-side clad layer and the active layer. Containing nitride semiconductor or GaN, with an impurity concentration of 1 × 1
37. The nitride semiconductor device according to claim 36, wherein an optical guide layer having a density of 0 ¹⁹ / cm ³ or less is formed. 38. The nitride semiconductor device according to claim 37, wherein the light guide layer has a superlattice structure. 39. A bandgap energy larger than those of the well layer of the active layer and the optical guide layer and having a film thickness of 0.1 μm between the optical guide layer and the active layer.
38. The nitride semiconductor device according to claim 36 or 37, wherein the following cap layer made of a nitride semiconductor is formed, and the impurity concentration of the cap layer is 1 × 10 ¹⁸ / cm ³ or more. 40. The nitride semiconductor device according to claim 39, wherein the light guide layer and the cap layer are formed on the p-conductive side nitride semiconductor layer side. 41. A nitride semiconductor layer is grown on a heterogeneous substrate made of a material different from that of the nitride semiconductor, and the surface of the nitride semiconductor layer is partially exposed on the grown nitride semiconductor layer. 41. After forming a protective film as described above, on a nitride semiconductor substrate made of a nitride semiconductor layer grown from the exposed nitride semiconductor layer so as to cover the protective film, any one of claims 1 to 40. A nitride semiconductor device formed by forming the nitride semiconductor device according to any one of the above.