JPH08274414A - Nitride semiconductor laser element - Google Patents

Abstract

PURPOSE: To concentrate a current to an active layer by forming a stripe-like positive electrode in contact with a surface of a p-type layer etched to a stripe in almost the same width as a width of the stripe. CONSTITUTION: After a mask is formed to a specified shape on the surface of a p-type contact layer 7, a nitride semiconductor layer is etched in a 10μm- wide stripe by using RIE. After etching, a negative electrode 12 consisting of Ti/Al is formed to a 20μm-wide stripe in an exposed n-type contact layer 3 and a positive electrode 12 consisting of Ni/Au is formed all over the stripe- like P-type contact layer 7. The positive electrode 12 having almost the same width as a stripe width of the p-type contact layer 7 etched to a stripe is formed in a direct contact with the p-type contact layer 7. Thereby, a current is prevented from spreading, so that a current is concentrated to an active layer 5 directly.

Description

Detailed Description of the Invention

[0001]

The present invention relates to a nitride semiconductor (In _X
The present invention relates to a laser device formed of Al _Y Ga _1-XY N, 0 ≦ X, 0 ≦ Y, X + Y ≦ 1), and particularly to an electrode stripe type laser device.

[0002]

2. Description of the Related Art Nitride semiconductors have a band gap of 1.9.
It is 5 eV to 6.0 eV, and since it is a direct transition type material, it has been attracting attention as a material for semiconductor laser devices from ultraviolet to red.

Typical of conventional nitride semiconductor laser devices
A schematic sectional view showing the structure is shown in FIG. This laser element
The child shows an electrode stripe type structure. fundamentally
Is an n-type cladding layer 32 on the surface of the sapphire substrate 31.
The active layer 33 and the p-type clad layer 34 are stacked in this order.
It has a bull hetero structure. p-type clad layer 34, active
Of the conductive layer 33 and part of the n-type cladding layer 32
Is etched into a strip shape so that the horizontal surface of the n-type cladding layer 32 is
Exposed. The strike on the horizontal surface of the n-type cladding layer 32
A negative electrode 41 in the form of a lip is formed, and the p-type crack of the uppermost layer is formed.
The striped positive electrode 42 should be formed also on the cathode layer 34.
It is a loose flip chip system. In addition, Seiden
A current confinement layer is provided between the pole 42 and the p-type cladding layer 34.
Then SiO ₂An insulating layer 35 made of
In the layer 35, a current is concentrated in the active layer 33 to cause oscillation.
It is said to be built.

[0004]

However, as shown by the arrow in FIG. 3, in the conventional laser device, the current confined by the insulating layer 35 is generated in the p-type cladding layer 34 or the active layer 3.
3 spread out, and it was difficult to partially concentrate the current in the active layer 33. Since the current cannot be concentrated, the active layer 33 emits light uniformly, and the LED has the conventional structure.
It was the actual situation that it only showed the characteristics as.

Therefore, the present invention has been made in view of such circumstances, and an object of the present invention is to provide a nitride semiconductor laser device in which a current is concentrated in an active layer to cause laser oscillation. .

[0006]

In the nitride semiconductor laser device of the present invention, a stripe-shaped positive electrode is in contact with the surface of the stripe-shaped p-type layer with a width substantially equal to the stripe width of the p-type layer. It is characterized in that electrodes are formed.

FIG. 1 is a perspective view showing the shape of a laser device according to an embodiment of the present invention, and FIG. 2 is a schematic cross-sectional view obtained by cutting the perspective view of FIG. 1 in a direction perpendicular to a stripe. As shown in these figures, in the laser device of the present invention, a positive electrode having a stripe width substantially equal to the stripe width of a p-layer etched in a stripe shape is formed in direct contact with the p-layer to spread the current. Instead, the current is concentrated directly in the active layer. A preferable stripe width that can concentrate the current in the active layer without spreading the current is 50 μm or less, more preferably 30 μm or less, and most preferably 1
It is 0 μm or less. This is because if the width is larger than 50 μm, the threshold current for laser oscillation becomes high and the laser oscillation tends not to occur. In addition, in the present invention, the substantially same width means that the electrode width is close to the width of the p-type layer within a width of −10%.

FIG. 2 shows a basic structure of a laser device, which is a double hetero structure in which an active layer is sandwiched between n-type and p-type nitride semiconductor layers, and the structure shown in FIG. Is recommended.

In FIG. 2, an n-type contact layer 2, a first n-type clad layer 3, a second n-type clad layer 4, an active layer 5, a first p-type clad layer 6 and a p-type layer are formed on the surface of a substrate 1. This shows a so-called separate confinement type double hetero structure in which the mold contact layer 7 is sequentially laminated. Part of the p-type contact layer 7, the first p-type cladding layer 6, the active layer 5, the second n-type cladding layer 4, the first n-type cladding layer 3 and the n-type contact layer 2 is etched. Striped negative electrode 1
1 and the positive electrode 1 having a width substantially equal to the stripe width of the p-type contact layer 7 is formed on the surface of the stripe-shaped p-type contact layer 7 left by etching.
2 are formed in direct contact with each other.

The substrate 1 includes sapphire (including C-plane, R-plane and A-plane), SiC (including 6H and 4H), Si, Z.
Although nO, GaAs or the like can be used, sapphire or SiC is generally used.

As the n-type contact layer 2, GaN, Al
A semiconductor layer of a binary mixed crystal such as GaN or a ternary mixed crystal having good crystallinity can be obtained. In particular, when GaN is used, a negative electrode material and a favorable ohmic property can be obtained. To make it n-type, the semiconductor layer is doped with donor impurities such as Si, Ge, and S. In addition, a buffer layer of GaN, AlN, or the like may be formed between the substrate 1 and the n-type contact layer 2 in order to relax the lattice constant irregularity.

The next first n-type clad layer 3 forms a nitride semiconductor having a bandgap larger than that of the second n-type clad layer, and in particular, n-type Al doped with the donor impurity.
GaN has good crystallinity, and a semiconductor layer having a large band gap can be obtained.

Next, the second n-type cladding layer 4 forms a nitride semiconductor layer having a band gap larger than that of the active layer 5,
Particularly, n-type InGaN or n-type GaN doped with the donor impurity is preferable. As will be described later, the second n-type cladding layer 4 is preferably InGaN or GaN even in combination with the active layer 5, and the second n-type cladding layer 4 made of InGaN or GaN is used for laser oscillation. Forming is particularly preferred.

The next active layer 5 is made of non-doped InGaN.
Then, between the bands around 635 nm to 365 nm
Luminescence is obtained. Preferably, the molar ratio of indium is
The crystallinity of InGaN, which is less than half that of um, has good crystallinity.
In addition, the laser element has a long life. Also, dozens of active layers
A multi-layer film in which two or more layers are stacked with a thickness of Gstrom, that is,
It may be a multiple quantum well structure. Single quantum well structure, many
In any active layer having a quantum well structure, the active layer is n
Type or p-type may be used, but especially non-doped (no addition)
By setting, the emission between bands with narrow half width and exciton emission
Light, or quantum well level emission is obtained,
It is particularly preferable for realizing an LD element. Single amount of active layer
Child well (SQW: single quantum well) structure or
Multi quantum well (MQW) structure
As a result, a light emitting device having a very high output can be obtained. SQW, M
QW is the emission between quantum levels of undoped InGaN.
Indicates the structure of the active layer from which light can be obtained.
Of the single composition of the conductive layer_XGa_{1 -X}Consists of N (0 ≦ X <1)
In layers_XGa_1-XN film thickness of 100 Å
Less than or equal to Trom, more preferably 70 Angstrom
Strong emission between the quantum levels can be obtained by
It MQW is In with a different composition ratio._XGa_1-XN (this
In the case of X = 0, X = 1 is included)
And In this way, the active layer should be SQW and MQW
Emits light between quantum levels, which is about 365 nm to 660 nm.
Luminescence is obtained. As the thickness of the quantum well layer
As described above, 70 angstroms or less is preferable.
Yes. In the multiple quantum well structure, the well layer is In _XGa_1-XConstructed with N
And the barrier layer is also In_YGa_1-YN (Y <X, in this case
(Including Y = 0) is desirable. Especially preferred
If the well layer and barrier layer are made of InGaN, the same temperature
Therefore, an active layer with good crystallinity can be obtained. barrier
Layer thickness less than 150 Å, more preferred
If it is 120 angstroms or less, high output light emission
The device is obtained. Further, the active layer 5 contains donor impurities and
It may be doped with an acceptor impurity. Impure
The crystallinity of the active layer doped with the same as that of non-doped
Then, if you dope donor impurities,
Compared to this, the emission intensity between bands can be made even stronger.
It Doping of the acceptor impurities results in interband emission.
Peak on the low energy side by about 0.5 eV from the peak wavelength
The wavelength can be increased, but the half-width becomes wider.
Simultaneous doping with acceptor and donor impurities
And the emission intensity of the active layer doped with only acceptor impurities.
The degree can be further increased. Especially acceptor
When implementing an active layer doped with impurities,
The electron type is n-type by simultaneously doping with donor impurities such as Si.
Is preferred. The active layer 5 is, for example, several angstroms.
It can be grown to a film thickness of 0.5 μm to 0.5 μm.

Next, the first p-type cladding layer 6 is the active layer 5
Formed of a nitride semiconductor with a larger band gap than
Particularly preferably, p-type A doped with acceptor impurities
With lGaN, a semiconductor layer having good crystallinity and a large band gap can be obtained. Further, after doping the acceptor impurities, annealing may be performed at 400 ° C. or higher for the purpose of obtaining a p-type having a lower resistance. Acceptor impurities include, for example, Group II elements such as Zn, Mg, and Cd, C (carbon), and the like.

Further, between the first p-type cladding layer 6 and the active layer 5, a second p-layer having a band gap larger than that of the active layer 5 and smaller than that of the first cladding layer 6 is formed.
A mold clad layer may be inserted. The second p-type cladding layer is preferably p-type InGaN doped with an acceptor impurity. Here, in the combination of the second n-type clad layer 4, the active layer 5, and the second p-type clad layer (the second p-type clad layer does not have to grow in particular), the active layer 5 is formed. By reducing the film thickness of the nitride semiconductor layer forming the active layer as a single quantum well structure or a multiple quantum well structure, elastic strain is generated at the interface with the second n-type cladding layer 4. Since a laser device having a strained quantum well structure is realized, laser oscillation becomes easy. In particular, this elastic strain causes the active layer 5 to be InGaN, and the second n-type cladding layer 4 to be an n-type InG having a band gap larger than that of the active layer 6.
It tends to occur when aN or n-type GaN is used.

Next, the p-type contact layer 8 is the same as the first p-type cladding layer 7 and is p-doped with acceptor impurities.
A binary mixed crystal of GaN, p-type AlGaN, or a ternary mixed crystal having a good crystallinity can be obtained. Especially Ga
When it is N, a positive electrode material and a preferable ohmic contact can be obtained.

As means for etching a nitride semiconductor,
There are both dry etching and wet etching methods, but dry etching is preferable to form a stripe having vertical etching end faces. In dry etching, for example, reactive ion etching, ion milling, ion beam assisted etching, focused ion beam etching, or the like can be used.

FIG. 4 is a schematic cross-sectional view showing the structure of a laser device according to another embodiment of the present invention. The difference from the cross-sectional view of FIG. 2 is the side surface of the p-type layer etched in stripes. An insulating film 20 is formed on the p-type layer, and a stripe-shaped positive electrode 12 is formed in contact with the p-type layer so as to have a width substantially equal to the stripe width of the p-type layer.
That is, it is formed over the surface of the insulating film 20 from the mold layer. That is, when wire-bonding to the positive electrode 12, the stripe-shaped positive electrode 12 as shown in FIG. 2 has a narrow width, so that wire-bonding is almost impossible. Therefore, in order to increase the width of the positive electrode 12, an insulating film 20 made of an insulating material such as SiO ₂ is newly formed on the side surface of the p-type layer, and the surface of the insulating film 20 is electrically connected to the p-type layer. The positive electrode 12 that is electrically connected is formed. With the structure as shown in FIG. 1, the electrodes cannot be wire-bonded and the structure is face down. However, with the structure as shown in FIG. 4, a face up structure can be obtained.
The chip size can be reduced.

[0020]

In the laser device of the present invention, the stripe width of the p-type layer is narrowed so that current concentrates on the active layer. That is, by forming a positive electrode having substantially the same stripe width on the p-type layer etched in a stripe shape, the stripe width is narrow even if the current spreads in the p-type layer, and the current density of the active layer is easily increased. Laser oscillation becomes easier. Also, regarding the light emitted from the active layer in the direction parallel to the substrate, the stripe width of the active layer is narrow, so the distance between the nitride semiconductor and the atmosphere with a large difference in refractive index is shortened, and the light is confined in this narrow region. Laser oscillation easily.

Further, the laser device of the present invention does not require a process of forming an insulating layer for current constriction on the surface of the p-type layer as in the conventional case, and therefore the manufacturing process can be shortened.

[0022]

【Example】

Example 1 On a sapphire substrate 1 having a thickness of 350 μm,
200 angstrom buffer layer made of GaN,
The n-type contact layer 2 made of Si-doped n-type GaN
μm, an n-type clad layer 3 made of Si-doped n-type Al0.3Ga0.7N is 0.1 μm, and Si-doped n-type In0.01Ga
The second n-type cladding layer 4 made of 0.99N has a thickness of 500 angstroms, the active layer 5 made of non-doped In0.08Ga0.92N has a thickness of 100 angstroms, and Mg-doped p-type Al.
The p-type cladding layer 6 made of 0.3 Ga0.7 N is 0.1 μm,
A wafer is prepared in which the p-type contact layer 7 made of Mg-doped p-type GaN is sequentially grown to a film thickness of 0.5 μm.

Next, the p-type contact layer 7 of this wafer
After forming a mask on the surface of the substrate in a predetermined shape, the nitride semiconductor layer is removed by RIE (reactive ion etching).
Etching is performed with a stripe width of 0 μm. After etching, a negative electrode 11 made of Ti / Al is formed in a stripe shape with a width of 20 μm on the exposed n-type contact layer 3.
Ni / Au is formed on the entire surface of the striped p-type contact layer 7.
The positive electrode 12 is formed.

Next, the surface of the sapphire substrate 1 on which the nitride semiconductor layer is not formed is polished by a polishing machine to a thickness of 80 μm. After polishing, the polished surface of the sapphire substrate is scribed with a scriber. The scribe direction is a line orthogonal to the stripe electrode, and the other scribe line is parallel to the electrode. After forming the scribe line, the wafer is pressed by a roller and the nitride semiconductor layer surface cleaved in the direction perpendicular to the electrode is used as an optical resonator.
m laser chip.

Next, after masking the surface of the nitride semiconductor layer of the laser chip, a dielectric multilayer film made of SiO ₂ and ZrO ₂ is formed on the cleavage surface by a sputtering device. This dielectric multilayer film has a function of reflecting 90% or more of the wavelength of the active layer.

After the laser chip thus obtained was placed face down (the electrode and the heat sink surface faced each other) on the heat sink, laser oscillation was attempted at room temperature, and a threshold current density of 1.0 kA / Laser oscillation with an oscillation wavelength of 440 nm was confirmed at cm ² or more.

Example 2 A laser device was manufactured in the same manner as in Example 1 except that the stripe width was set to 30 μm when the nitride semiconductor layer was etched. The threshold current density was 3.0 kA / cm at room temperature. Laser oscillation was confirmed at ² or more.

Example 3 A laser device was manufactured in the same manner as in Example 1 except that the stripe width was set to 50 μm when the nitride semiconductor layer was etched. The threshold current density was 5.0 kA at the liquid nitrogen temperature. Laser oscillation was confirmed at / cm ² or more.

Example 4 A laser device was manufactured in the same manner as in Example 1 except that the stripe width was set to 60 μm when etching the nitride semiconductor layer. The threshold current density was 6.0 kA at the liquid nitrogen temperature. Laser oscillation was confirmed at / cm ² or more, but the element was destroyed immediately.

[Fifth Embodiment] In the first embodiment, the active layer 5 is used.
The composition of the well layer is made of non-doped In0.08Ga0.92N with a thickness of 25 angstroms, and the undoped In0.01Ga0.
A barrier layer of 99N is grown to a thickness of 50 Å. This operation was repeated 13 times, and finally well layers were laminated to grow the active layer 6 having a total thickness of 1000 angstroms. After that, when laser oscillation was tried at room temperature in the same manner as in Example 1, the threshold current density was 1.0 kA /
Laser oscillation with an oscillation wavelength of 440 nm was confirmed at cm ² or more.

[Comparative Example] A laser device was manufactured in the same manner as in Example 1 except that the stripe width was set to 100 μm when the nitride semiconductor layer was etched. As a result, no laser oscillation was observed even at the liquid nitrogen temperature and immediately The element has been destroyed.

[0032]

As described above, in the laser device of the present invention, the positive electrode contacting with the same width as the stripe is formed on the surface of the p-type layer etched in the stripe shape, and this electrode is used for direct current flow. Acts as a stenosis. That is p
Since the stripe width is such that the current density rises enough to cause laser oscillation even when the current spreads in the mold layer, light can be easily confined and oscillation occurs at room temperature. Further, unlike the conventional case, it is not necessary to form a current confinement layer on the surface of the p-type layer with an insulator. Therefore, the technique of fine mask alignment at the time of forming the insulator is not required, and the manufacturing yield is improved. In this way, the realization of a laser device in the short wavelength region at room temperature with a nitride semiconductor has dramatically improved the recording density as a light source for writing and a light source for a compact disc,
Its industrial utility value is extremely high.

[Brief description of drawings]

FIG. 1 is a perspective view showing the shape of a laser element according to an embodiment of the present invention.

2 is a schematic cross-sectional view of the laser device of FIG. 1 cut in a direction perpendicular to a stripe.

FIG. 3 is a schematic cross-sectional view showing the structure of a conventional laser element.

FIG. 4 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention.

[Explanation of symbols]

 1 ... Substrate 2 ... N-type contact layer 3 ... First n-type cladding layer 4 ... Second n-type cladding layer 5 ... Active layer 6 ... -First p-type cladding layer 7 --- p-type contact layer 12 --- Positive electrode 11 --- Negative electrode

Claims (2)

[Claims] 1. A nitride semiconductor, wherein a stripe-shaped positive electrode is formed on the surface of a p-type layer etched in a stripe shape and is in contact with the stripe width of the p-type layer. Laser device. 2. The nitride semiconductor laser device according to claim 1, wherein the stripe width is 50 μm or less.