JP3434162B2 - 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) or an LD (laser diode), a light receiving element such as a solar cell or an optical sensor, or a nitride semiconductor used for a transistor, an integrated circuit or the like. (InX
AlYGa1-X-YN, 0≤X, 0≤Y, X + Y≤1) element.

[0002]

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

Further, the present applicant recently announced a laser oscillation of 410 nm at room temperature in pulse current using this material (for example, Jpn. J. Appl. Phys. Vol35 (1996).
L74-76). This laser element has a pulse current (pulse width 2μ
s, pulse period 2 ms), threshold current 610 mA, threshold current density 8.7 kA / cm 2, threshold voltage 21 V 41
The laser oscillation of 0 nm is shown.

For example, in an LED element having a double hetero structure having InGaN as an active layer, the active layer has an n-type and p-type clad layer made of AlGaN and an n-type made of GaN.
It is sandwiched between p-type contact layers (see, for example, JP-A-8-83929). The n-type layers such as the n-type contact layer and the n-side cladding layer are doped with n-type impurities such as Si and Ge, and the p-type layers such as the p-side contact layer and the p-side cladding layer are Mg, Zn and the like. P-type impurities are doped. Usually, in the case of such a structure, the n electrode is formed.
The carrier concentration of the p-side contact layer on which the mold contact layer and the p-electrode are formed is the same as or higher than that of the clad layer in contact with each contact layer. That is, it was usual that the n + layer having a high carrier concentration, the n− layer having a low carrier concentration, the active layer, the p− layer having a low carrier concentration, and the p + layer having a high carrier concentration were laminated in this order from the substrate. . (Although it does not have a double hetero structure, see, for example, JP-A-6-151963 and JP-A-6-151964)

[0005]

Certainly, when the carrier concentration of the contact layer in contact with the electrode is large, the contact resistance with the electrode material is small, and good ohmic characteristics are likely to be obtained. When the ohmic contact between the electrode and the contact layer is improved, the Vf (forward voltage) of the LED and the threshold current of the LD are likely to decrease. However, the nitride semiconductor is a material having many crystal defects, and if n-type impurities or p-type impurities are doped at a high concentration in order to obtain a high carrier concentration in such a material, the crystallinity deteriorates and the device itself becomes poor. The output tends to decrease.

Therefore, the present invention has been made in view of the above circumstances, and an object thereof is to provide an element made of a nitride semiconductor which has better crystallinity, high output and high efficiency. In particular, it is to realize a laser element that continuously oscillates with a low threshold current and a highly efficient LED element.

[0007]

With respect to nitride semiconductor devices such as LDs and LEDs, we have newly found that the above problems can be solved by improving the n-type layer grown on the substrate. Invented the invention. That is, the nitride semiconductor device according to claim 1 of the present invention is GaN
A substrate and a first layer made of undoped GaN with a film thickness of 0.1 μm or more, which is not doped with n-type impurities, are provided on the GaN substrate, and n-type impurities are further provided on the first layer. ^{1 × 10 17 / cm 3 ~1} × 10 21 / cm 3 doped film thickness becomes from the following nitride semiconductor 4μm or 0.2 [mu] m, and having a second layer negative electrode is formed .

A nitride semiconductor device according to a second aspect of the present invention is a GaN substrate, and an n-type impurity is doped on the GaN substrate, and the n-type impurity concentration is small and the film thickness is 0.1 μm or more. And a first layer of GaN, further comprising a nitride semiconductor having a higher n-type impurity concentration than the first layer and having a film thickness of 0.2 μm or more.
It is characterized in that it has a second layer having a thickness of less than or equal to μm, and a negative electrode is formed on the second layer.

In the first and second aspects, the substrate and the first
Does not necessarily indicate that the layer of the second layer is in contact with the layer of the second layer, and it may be formed between the substrate and the first layer, or between the first layer and the second layer. Even if the nitride semiconductor layer is inserted, it is within the scope of the present invention.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a schematic cross-sectional view showing one structure of the nitride semiconductor device of the present invention, specifically LE.
The structure of a D element is shown. As a basic structure, a low-temperature growth buffer layer 11 made of, for example, non-doped GaN, for example, a low carrier concentration first layer 1 made of non-doped GaN is provided on a substrate 10 made of, for example, sapphire.
2, a high carrier concentration second layer 13 made of, for example, Si-doped GaN, an active layer 14 made of InGaN having a single quantum well structure, a p-side clad layer 15 made of Mg-doped AlGaN, for example Mg-doped GaN. It has a structure in which the p-side contact layer 16 is sequentially stacked. A light-transmitting positive electrode 17 (hereinafter, the positive electrode is referred to as a p-electrode) is formed on almost the entire surface of the uppermost p-side contact layer 16, and a pad electrode 18 for bonding is formed on the surface of the p-electrode 17. Has been formed. In the device of the present invention, the n-electrode 19 has a low n-type impurity concentration, or is grown on the first layer 12 that is not doped with n-type impurities. Formed on the surface of the second layer 13. That is,
The second layer 13 acts as an n-side contact layer as a current injection layer.

On the other hand, the first layer 12 having a low impurity concentration
Is not as a contact layer where the negative electrode is formed,
It acts as a base layer for growing the second layer, which acts as a contact layer. If it is attempted to form the n-side contact layer, which becomes the current injection layer, with a film thickness of several μm or more and a single nitride semiconductor layer with a high carrier concentration as in the conventional case, it is necessary to grow a layer with a high n-type impurity concentration. There is. A thick film layer having a high impurity concentration tends to have poor crystallinity. Therefore, even if another nitride semiconductor such as an active layer is grown on the layer having poor crystallinity, the crystal defects are taken over by the other layer, and the crystallinity cannot be improved. Therefore, in the present invention, first, the first layer having a low impurity concentration and good crystallinity is grown before growing the second layer to be a contact layer, so that the second layer having a high carrier concentration and good crystallinity is grown. To grow the layers of. Generally, the carrier concentration of the first layer that does not contain n-type impurities or has a low n-type impurity concentration tends to be lower than that of the second layer.

In the present invention, examples of the n-type impurity doped in the first layer and the second layer include Si, Ge and S.
Examples of the periodic table group IV elements such as n, C, and Ti are mentioned. Among them, Si and Ge are commonly used for doping a nitride semiconductor to adjust carrier concentration, resistivity, and the like. Further, in the case of a nitride semiconductor layer, it tends to show n-type even in non-doped state (without being doped with impurities) due to nitrogen vacancies formed in the semiconductor layer, but when the crystallinity is improved, a high resistance layer with a small carrier concentration is obtained. There is a possibility that
Therefore, the conductivity type of the first layer of the present invention is not specified.

The n-type impurity concentration of the first layer may be lower than that of the second layer, but most preferably, the n-type impurity is not doped (hereinafter referred to as non-doped). This is because a non-doped nitride semiconductor having the best crystallinity can be obtained. In the case of the present invention, the impurity concentration of the second layer is more important, and the range is 1 × 10 1.
It is desirable to adjust to a range of 7 / cm3 to 1 × 1021 / cm3, and more preferably 1 × 1018 / cm3 to 1 × 1019 / cm3. If it is smaller than 1 × 1017 / cm3, it becomes difficult to obtain a preferable ohmic contact with the material of the n-electrode, so that it is not possible to expect a decrease in threshold current and voltage in the laser device.
If it is larger than 1 × 10 2 / cm 3, the leak current of the element itself increases and the crystallinity deteriorates, so that the life of the element tends to be shortened.

When the first layer is doped with an n-type impurity, a layer having a smaller carrier concentration can be formed by making the amount of impurities smaller than that of the second layer. Further, an n-type impurity having a low activation rate (that is, the carrier concentration does not increase so much even if the impurity is doped) may be doped. However, in the present invention, it is preferable to form the first layer in a non-doped state because it is preferable to form the first layer without doping impurities so that the first layer has better crystallinity.

Now, the buffer layer 11 will be described. The buffer layer 11 is a nitride semiconductor layer that is grown at a temperature lower than the growth temperature of the first layer before growing the first layer with a thickness of usually less than 0.1 μm. Specific examples include non-doped GaN, AlN, and AlGaN layers.
This layer is a layer grown to improve the crystallinity of the first layer, and when a buffer layer is grown on the substrate,
It has a function of relaxing lattice mismatch between the substrate and the nitride semiconductor. Since this buffer layer is usually a layer containing polycrystals, it is almost impossible to measure the carrier concentration, or even if it can be measured, for example, 1 × 1021 /
It is a very large layer with cm3 or more and very low mobility. Therefore, in the present invention, a film thickness of 0.1 μm grown on a substrate or with a single composition before growing the first layer.
Below low temperature growth buffer layer is not included in the first layer of the present invention.

Further, at least one of the first layer and the second layer may be a strained superlattice layer in which nitride semiconductor layers having a film thickness of 100 Å or less and different in composition are laminated. it can. When a superlattice layer is formed, this layer has a superlattice structure, which dramatically improves the crystallinity of the nitride semiconductor layer and reduces the threshold current. That is, the film thickness of each nitride semiconductor layer forming the superlattice layer is set to 100 angstroms or less, and the film thickness is set to the elastic strain limit or less. When the thickness of the nitride semiconductor layer forming the superlattice layer is set to be equal to or less than the elastic strain limit in this way, it becomes difficult for fine cracks and crystal defects to enter the crystal, and a nitride semiconductor with good crystallinity is grown. it can. Therefore, even if another nitride semiconductor layer is grown on this superlattice layer, the crystallinity of the other nitride semiconductor layer is improved because the superlattice layer has good crystallinity. Therefore, since crystal defects are reduced and crystallinity is improved in the entire nitride semiconductor, the threshold current is reduced and the life of the laser device is improved.

The nitride semiconductor layers forming the superlattice layer may be made of nitride semiconductors having different compositions,
The band gap energies may be different or the same. For example, the first layer (A
When the layer) is composed of InXGa1-XN (0 ≦ X ≦ 1) and the next layer (B layer) is composed of AlYGa1-YN (0 <Y ≦ 1), the band gap energy of the B layer is always higher than that of the A layer. However, the A layer is formed of InXGa1-XN (0 ≦
X ≦ 1), and the B layer is InZAl1-ZN (0 <Z ≦
According to the constitution of 1), the A layer and the B layer have different compositions but may have the same band gap energy. In addition, the A layer is made of AlYGa1-YN (0≤Y≤1), and B
If the layer is made of InZAl1-ZN (0 <Z ≦ 1),
Similarly, the first layer and the second layer may have different compositions but have the same band gap energy. The superlattice layer of the present invention may have the same composition but different bandgap energies.

Preferably, the nitride semiconductors of the A layer and the B layer forming the superlattice layer have different bandgap energies, and the average bandgap energy of the nitride semiconductor forming the superlattice layer is activated. It is desirable to adjust so that it is larger than the layer. Preferably, one layer is made of InXGa1-XN (0 ≦ X ≦ 1),
The other layer is AlYGa1-YN (0≤Y≤1, X ≠ Y
= 0), a superlattice layer having good crystallinity can be formed. Further, AlGaN has a property that cracks easily occur during crystal growth. Therefore, if the A layer constituting the superlattice layer is a nitride semiconductor layer containing no Al and having a film thickness of 100 angstroms or less, it acts as a buffer layer when the other B layer made of the nitride semiconductor containing Al is grown. However, the B layer is less likely to be cracked. Therefore, even if superlattice layers are laminated, a crack-free superlattice can be formed, so that the crystallinity is improved and the life of the device is improved. This also uses one layer of InXGa1-X
N (0 ≦ X ≦ 1) and the other layer is AlYGa1-
This is an advantage when YN (0≤Y≤1, X ≠ Y = 0).

The thickness of each nitride semiconductor layer constituting the superlattice layer is 100 angstroms or less, and more preferably 7 Å.
It is adjusted to 0 angstroms or less, most preferably 10 angstroms or more and 40 angstroms or less. If it is thicker than 100 Å, the film thickness exceeds the elastic strain limit, and minute cracks or crystal defects are likely to occur in the film. The lower limit of the film thickness of the well layer and the barrier layer is not particularly limited as long as it is one atomic layer or more.
It is desirable to adjust it to 0 angstrom or more. However, when the first layer having a large film thickness is composed of a superlattice layer, it is 70 angstroms or less, and when the second layer having a small film thickness and containing n-type impurities is a superlattice layer, it is 40 angstroms. The following is desirable.

When the bandgap energies of the nitride semiconductor layers forming the superlattice layer are different from each other, the n-type impurity is doped in a layer having a larger bandgap energy or a layer having a smaller bandgap energy is undoped. The threshold voltage and the threshold current tend to decrease when the n-type impurity is doped into the one having a larger band gap energy.

Importantly, the first layer is grown thicker than the second layer, the first layer having a thickness of 0.1 μm or more, more preferably 0.5 μm or more, most preferably 1 μm or more, 2
It is desirable to adjust it to 0 μm or less. The first layer is 0.
If the thickness is less than 1 μm, the second layer having a high impurity concentration must be grown thick and the second layer as a contact layer must be grown.
There is a tendency that improvement in the crystallinity of the layer cannot be expected so much. If the thickness is more than 20 μm, the first layer itself tends to have many crystal defects. Further, as an advantage of growing the first layer thickly, there is an improvement in heat dissipation. That is, when a laser element is manufactured, heat is likely to spread in the first layer, which improves the life of the laser element. Further, the leaked light of the laser light spreads in the first layer, and it becomes easy to obtain a laser light having an elliptical shape.

On the other hand, the second layer has a thickness of 0.2 μm or more and 4 μm or more.
The following adjustments are desirable. If it is thinner than 0.2,
When forming the negative electrode later, it is difficult to control the etching rate so as to expose the second layer.
If it is more than m, crystallinity tends to deteriorate due to the influence of impurities. This is the same when the first layer and the second layer are composed of superlattice layers. Needless to say, when the first layer and the second layer are composed of superlattice layers, the layer thicknesses of the entire superlattice layers are indicated. First layer is superlattice 20
Stacking more than μm is very troublesome and unsuitable for the manufacturing process. However, when the second layer is formed of a superlattice layer, it may be formed to have a film thickness of 4 μm or more, but it is very time-consuming to grow it with a thick film similarly to the first layer.

[0023]

The present invention will be described in detail in the following examples. Figure 2
FIG. 3 is a schematic cross-sectional view showing the structure of a laser device according to an embodiment of the present invention, showing the structure when the device is cut in a direction perpendicular to the resonance direction of laser light. The device of the present invention will be described below with reference to this drawing. Note that the general formula InXAlYGa1-X-YN shown in the present specification merely indicates the composition ratio of the nitride semiconductor, and even if different layers are represented by the same general formula, the X value of those layers is shown. , Y values, etc. do not match.

Example 1 A substrate 20 made of sapphire (C surface) was set in a reaction vessel, the inside of the vessel was sufficiently replaced with hydrogen, and the temperature of the substrate was raised to 1050 ° C. while flowing hydrogen. Clean the substrate. In addition to the sapphire C surface on the substrate 20,
In addition to sapphire having R-face and A-face as main faces, other insulating substrates such as spinel (MgA12O4), SiC
(Including 6H, 4H, 3C), ZnS, ZnO, GaA
A semiconductor substrate such as s or GaN can be used.

(Buffer Layer 21) Subsequently, the temperature is lowered to 510 ° C., hydrogen is used as a carrier gas, ammonia and TMG (trimethylgallium) are used as source gases, and a buffer layer 2 made of GaN is formed on the substrate 1.
Is grown to a film thickness of about 200 Å. The buffer layer 20, AlN, GaN, AlGaN, etc. is 900
It can be formed at a temperature of not higher than 0.1 ° C. and a thickness of less than 0.1 μm, preferably several tens to several hundreds of angstroms. This buffer layer is formed in order to mitigate the lattice constant irregularity between the substrate and the nitride semiconductor, but it may be omitted depending on the growth method of the nitride semiconductor, the type of the substrate, and the like.

(First Layer 22) After growing the buffer layer 20, only TMG is stopped and the temperature is raised to 10
Raise to 50 ° C. When the temperature reaches 1050 ° C., TMG and ammonia gas are also used as the source gas, and the first layer is made of non-doped GaN with a carrier concentration of 1 × 10 18 / cm 3.
Layer 22 is grown to a thickness of 5 μm. The first layer is In
XAlYGa1-X-YN (0≤X, 0≤Y, X + Y≤1)
The composition is not particularly limited.

(Second Layer 23) Next, a 1 μm thick second layer 23 made of n-type GaN doped with 1 × 10 19 / cm 3 of Si using TMG, ammonia, and silane gas as an impurity gas at 1050 ° C. Grow thick. The carrier concentration of the second layer 23 was 1 × 1019 / cm3, which was the same as the doping amount. In particular, since an n-type impurity having a high activation rate such as Si can obtain a carrier concentration almost the same as the amount of the doped impurity, the n-type nitride semiconductor doped with Si will be referred to as the Si-doped amount in the following description. Therefore, it is assumed that the carrier concentration is shown. The composition of the second layer is also InXAlYGa1-X-YN.
(0 ≦ X, 0 ≦ Y, X + Y ≦ 1), and the composition thereof is not particularly limited, and the first layer 22 and the second layer 23 may be composed of nitride semiconductors having different compositions. .

(Crack prevention layer 24) Next, the temperature is set to 800 ° C. and TMG and TM are added to the source gas.
In 0.1 doped with Si (1 × 10 19 / cm 3) using I (trimethylindium), ammonia, and silane gas
A crack prevention layer 24 of Ga0.9N is grown to a film thickness of 500 angstrom. This crack prevention layer 1
0 is an n-type nitride semiconductor containing In, preferably InG
Growing with aN can prevent cracks from entering the nitride semiconductor layer containing Al. This crack prevention layer is 100 angstroms or more,
It is preferable to grow the film with a thickness of 0.5 μm or less. 1
If it is thinner than 00 angstrom, it is difficult to act as a crack preventive as described above, and if it is thicker than 0.5 μm,
The crystals themselves tend to turn black. The crack prevention layer 24 may be omitted depending on the conditions such as the growth method and the growth apparatus, and particularly when the second layer 23 has a superlattice structure.

(N-side clad layer 25) Next, the temperature is set to 1050 ° C., TMA (trimethylaluminum), TMG, NH3, and SiH4 are used as source gases, and n-type Al0.25 doped with Si at 1 × 10 19 / cm 3 is used.
An n-side cladding layer 25 made of Ga0.75N is grown to a film thickness of 0.5 μm. The n-side clad layer 25 acts as a carrier confinement layer and an optical confinement layer, and it is desirable to grow a nitride semiconductor containing Al, preferably AlGaN, 100 angstroms or more, 2 μm or less, and more preferably 500 angstroms or more, 1
A clad layer having good crystallinity can be formed by growing the layer with a thickness of less than μm.

(N-Side Light Guide Layer 26) Subsequently, 0.2 μ of an n-side light guide layer 26 made of n-type GaN doped with Si at 1 × 10 19 / cm 3 at 1050 ° C.
Grow with a film thickness of m. This n-side light guide layer 26 acts as a light guide layer of an active layer, and it is desirable to grow GaN and InGaN, and it is usually grown to a film thickness of 100 angstroms to 5 μm, and more preferably 200 angstroms to 1 μm. desirable. The n-side light guide layer may be non-doped.

(Active layer 27) Next, the active layer 27 is grown by using TMG, TMI, ammonia and silane gas as source gases. The temperature of the active layer 27 is kept at 800 ° C., and Si is initially 8 × 10 18 / cm 2.
25 well layers of In0.2Ga0.8N doped with 3
It is grown to a film thickness of angstrom. Next, Si is changed to 8 × 101 at the same temperature only by changing the TMI molar ratio.
A barrier layer of 8 / cm3 doped In0.01Ga0.95N is grown to a thickness of 50 Angstroms. This operation is repeated twice to finally form a multiple quantum well structure in which well layers are stacked. Impurities with which the active layer is doped may be doped into both the well layer and the barrier layer as in this embodiment, or may be doped into either one. The threshold value tends to decrease when the n-type impurity is doped.

(P-side cap layer 28) Next, the temperature is raised to 1050 ° C., and TMG, TMA, ammonia, and Cp 2 Mg (cyclopentadienyl magnesium) are used. × 1020 / cm3 Doped Al0.1
A p-side cap layer 28 made of Ga0.9N is grown to a film thickness of 300 angstrom. This p-side cap layer 2
8 is preferably p-type, but since it is thin, it may be i-type in which carriers are compensated by doping with n-type impurities. The thickness of the p-side cap layer 28 is adjusted to 0.1 μm or less, more preferably 500 angstroms or less, and most preferably 300 angstroms or less. 0.1μ
This is because when the film is grown to have a film thickness larger than m, cracks are likely to occur in the p-side cap layer 28, and a nitride semiconductor layer having good crystallinity is difficult to grow. Also, carriers cannot pass through this energy barrier due to the tunnel effect. If the AlGaN having a higher Al composition ratio is formed thinner, the LD element is likely to oscillate. For example, if the Y value is AlYGa1-YN of 0.2 or more, it is desirable to adjust it to 500 angstroms or less. p-side cap layer 28
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 29) Subsequently, at 1050 ° C., a p-side light guide layer 26 made of GaN doped with Mg at 1 × 10 20 / cm 3 is 0.2 μm.
To grow. Like the n-side light guide layer 26, it is desirable that the p-side light guide layer 29 act as an optical guide layer of an active layer to grow GaN and InGaN. Usually, 100 angstrom to 5 μm, and more preferably 200 angstrom to. It is desirable to grow the film with a thickness of 1 μm. Although the p-side light guide layer is doped with p-type impurities, it may be composed of a non-doped nitride semiconductor.

(P-side clad layer 30) Subsequently, at 1050 ° C., a p-side clad layer 30 made of Al 0.25 Ga 0.75 N doped with 1 × 10 20 / cm 3 of Mg is grown to a thickness of 0.5 μm. Like the n-side clad layer 25, this layer acts as a carrier confinement layer and an optical confinement layer, and it is desirable to grow a nitride semiconductor containing Al, preferably AlGaN, and 100 angstroms or more and 2 μm or less, and more preferably 500
When grown to a thickness of angstrom or more and 1 μm or less, a clad layer having good crystallinity can be grown.

In the case of the semiconductor element having the double hetero structure having the active layer having the quantum well layer as in this embodiment, the film thickness 0 which is in contact with the active layer 27 and has a band gap energy larger than that of the active layer is obtained. A cap layer made of a nitride semiconductor having a thickness of 1 μm or less, preferably a p-side cap layer 28 made of a nitride semiconductor containing Al is provided, and the p-side cap layer is located farther from the active layer than the p-side cap layer 28 is. P with a smaller bandgap energy than 28
A side light guide layer 29 is provided, and a nitride semiconductor having a band gap larger than that of the p side light guide layer 29, preferably Al, is provided at a position farther from the active layer than the p side light guide layer 29.
It is very preferable to provide the p-side clad layer 30 made of a nitride semiconductor containing a. Moreover, since the thickness of the p-side cap layer 28 is set as thin as 0.1 μm or less, it does not act as a carrier barrier, and holes injected from the p layer pass through the cap layer 28 due to the tunnel effect. LDs that can be recombined efficiently in the active layer
Output is improved. In other words, the injected carriers have a large band gap energy in the cap layer 28, so that even if the temperature of the semiconductor element rises or the injection current density increases, the carriers do not overflow into the active layer, and the cap layer 28 does not overflow. Since it is blocked by, the carriers are accumulated in the active layer and it is possible to efficiently emit light.

(P-side contact layer 31) Finally, Mg is deposited on the p-side cladding layer 30 at 1050 ° C.
A p-side contact layer 31 made of GaN doped with 2 × 10 20 / cm 3 is grown to a film thickness of 150 Å. The p-side contact layer 31 is a p-type InXAlYG
a1-X-YN (0≤X, 0≤Y, X + Y≤1), preferably Mg-doped GaN, the most preferable ohmic contact with the p-electrode 32 is obtained. The material of the p-electrode 32 that can obtain a preferable ohmic contact with the p-side contact layer is, for example, Ni, Pd,
Examples thereof include Ag and Ni / Au. Furthermore, p
The thickness of the side contact layer 31 is preferably adjusted to 500 angstroms or less, more preferably 300 angstroms or less, and most preferably 200 angstroms or less. This is because the resistivity is further reduced by adjusting the film thickness of the p-type nitride semiconductor layer having a high resistivity to 500 angstroms or less, so that the current and voltage at the threshold value are reduced. In addition, the amount of hydrogen removed from the p-type layer tends to increase, and the resistivity tends to decrease. further,
The effect of thinning the contact layer 31 is as follows. For example, a p-side clad layer made of p-type AlGaN is formed in contact with a p-side contact layer made of p-type GaN having a thickness of more than 500 Å, and the impurity concentration of the clad layer is the same as that of the contact layer. When the carrier concentration is the same, if the film thickness of the p-side contact layer is smaller than 500 angstroms, the carriers on the cladding layer side are likely to move to the contact layer side, and the carrier concentration on the p-side contact layer tends to increase. It is in. Therefore, if an electrode is formed on the contact layer having a high carrier concentration, good ohmic contact can be obtained.

After the reaction is completed, the temperature is lowered to room temperature, and the wafer is further heated to 700 ° C. in a reaction vessel in a nitrogen atmosphere.
Annealing is performed at 0 ° C. to further reduce the resistance of the p-type impurity-doped layer.

After the annealing, the wafer was taken out from the reaction vessel and, as shown in FIG.
The side contact layer 31 and the p-side cladding layer 30 are etched into a ridge shape having a stripe width of 4 μm. In particular, by forming a layer of a nitride semiconductor layer or more containing Al above the active layer into a ridge shape, the light emission of the active layer is concentrated in the lower part of the ridge, the transverse mode is easily unified, and the threshold value is lowered. Cheap. After forming the ridge, a mask is formed on the surface of the ridge, and as shown in FIG. 2, the surface of the second layer 23 on which the n-electrode 33 is to be formed is exposed by making the stripe ridge symmetrical. As a material of the n-electrode 33, a metal or an alloy such as Al, Ti, W, Cu, Zn, Sn, or In can obtain a preferable ohmic property.

Next, p electrodes 32 made of Ni and Au are formed in stripes on the surface of the p-side contact layer 31. On the other hand, an n electrode 33 made of Ti and Al is formed on almost the entire surface of the stripe-shaped second layer 23. It should be noted that the almost entire surface is 8
An area of 0% or more. In this way, the second layer 23 is exposed symmetrically with respect to the p-electrode 32, and the second layer 2 is exposed.
Providing the n electrode on almost the entire surface of 3 is also very advantageous in reducing the threshold value.

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 20 on the side where the nitride semiconductor is not formed is lapped by using the diamond abrasive. The thickness of the substrate is 50 μm. After lapping, the substrate surface is mirror-finished by polishing with 1 μm with a finer polishing agent.

After polishing the substrate, scribe the polishing surface 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. SiO2 and TiO on the cavity surface
A dielectric superlattice made of 2 was formed, and finally the bar was cut in a direction parallel to the p-electrode 32 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 bonded, and laser oscillation was attempted at room temperature. At room temperature, the threshold current density was 3.0 kA / cm2. At a threshold voltage of 4.5 V, continuous oscillation with an oscillation wavelength of 405 nm was confirmed, and a life of 30 hours or more was shown.

Example 2 In Example 1, when the second layer 23 was grown, Si was added to 1 ×.
A layer of Al0.1Ga0.9N doped with 1019 / cm3 is grown to 20 angstroms, and then a layer of n-type GaN doped with the same amount of Si is grown to 20 angstroms. Then, this operation was repeated 200 times, and the total film thickness of the carrier concentration was 1.times.10.sup.19 / cm.sup.3.
A second layer 23 consisting of a 8 μm superlattice layer is formed.

Next, without growing the crack prevention layer 24, the n-side cladding layer 25 is grown directly on the second layer 23 in the same manner as in Example 1, and thereafter the laser of FIG. Nitride semiconductors are laminated so as to form a device structure.

After the growth, the ridge is formed, and then the second layer 2 is formed.
The surface of 3 is exposed by etching. The second layer 2
On the surface of No. 3, a well layer made of Si-doped GaN was exposed. After that, an electrode was formed in the same manner as in Example 1 to obtain a laser element, and the threshold current density was 2.8 k at room temperature.
Continuous oscillation with an oscillation wavelength of 405 nm was confirmed at A / cm 2 and a threshold voltage of 4.3 V, and a life of 40 hours or more was shown.

Example 3 In Example 2, when the second layer 23 was grown, Si was 2 ×.
A layer of 1019 / cm3 doped Al0.1Ga0.9N is grown to 30 Å, followed by a layer of undoped GaN to 30 Å.
This operation was repeated 200 times to obtain a total film thickness of 1.2μ.
A second layer 23 of m superlattice layers is formed. After that, when a laser device was manufactured in the same manner as in Example 2, continuous oscillation with an oscillation wavelength of 405 nm was confirmed with a threshold current density of 2.7 kA / cm 2, a threshold voltage of 4.1 V, and a lifetime of 50 hours or more was shown.

As described above, when the superlattice layer is used as the second layer 23 and the n-electrode is formed, the threshold voltage tends to decrease. It is speculated that this may have had a similar effect to HEMT. For example, in a superlattice layer in which a nitride semiconductor layer having a large bandgap doped with an n-type impurity and a non-doped nitride semiconductor layer having a small bandgap are stacked, a layer doped with an n-type impurity and a non-doped layer At the heterojunction interface with, the barrier layer side is depleted,
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 formed on the side with a small band gap, electrons are not scattered by impurities when traveling, so that the mobility of electrons in the superlattice is increased and the resistivity is lowered. Therefore, if the superlattice is used as a contact layer at the time of electrode formation, it is presumed that the mobility is increased and the voltage of the device is lowered, but the details are unknown.

Example 4 In Example 1, when the first layer 22 was grown, a well layer made of non-doped n-type GaN was formed to have a thickness of 40 Å.
Next, a barrier layer made of non-doped n-type Al0.1Ga0.9N is grown to 60 Å. This operation was repeated 200 times, and the average carrier concentration was 5 × 1017 /
A first layer 22 made of a superlattice layer having a total film thickness of 2 μm and having a cm 3 is formed.

Then, in the same manner as in Example 1, 1 × 1 of Si was added.
Second layer 2 made of n-type GaN doped with 019 / cm3
3 was grown to a film thickness of 1 μm, a crack prevention layer 24 was grown thereon, and then a laser element was manufactured in the same manner as in Example 1. As a result, a laser element having substantially the same characteristics as in Example 2 was obtained. I was able to make it.

[Embodiment 5] In Embodiment 1, Si was added in an amount of 1 × 10 5 when growing the first layer.
A laser device was manufactured in the same manner as in Example 1 except that n-type GaN doped with 18 / cm3 was used, and the threshold current density was 3.1 kA / cm2 and the threshold voltage was 4.
At 6 V, continuous oscillation with an oscillation wavelength of 405 nm was confirmed, and 2
It showed a life of 5 hours or more.

[Embodiment 6] This embodiment will be described based on the LED element shown in FIG. After growing the buffer layer 11 made of non-doped GaN at 600 ° C. on the substrate 10 made of sapphire in the same manner as in Example 1, a carrier concentration of 1 × 1 was formed on the buffer layer 11.
A first layer 12 of 018 / cm3 of undoped n-type GaN was grown to 4 μm, and then Si was added at 1 × 1019 / cm3.
A second layer 13 of doped n-type GaN is grown to 1 μm.

Next, an active layer 14 made of In0.4Ga0.6N and having a film thickness of 30 angstrom and having a single quantum well structure is formed.
And a p-side cladding layer 15 made of Mg-doped p-type Al0.2Ga0.9N doped with Mg of 5 × 1019 / cm3 is grown to 0.5 μm, and Mg
A p-side contact layer 16 made of Mg-doped p-type GaN doped with 1019 / cm3 is grown to 0.2 μm.

After the growth, the wafer is taken out of the reaction container, annealed in the same manner as in Example 1, and then etched from the p-side contact layer 16 side to form the n-electrode 19 on the surface of the second layer 13. Expose. A translucent p-electrode 17 made of Ni-Au having a film thickness of 200 angstrom is formed on almost the entire surface of the p-side contact layer 16 as the uppermost layer, and the pad electrode 1 made of Au is formed on the p-electrode 17.
8 is formed. Ti-Al is also formed on the exposed surface of the second layer.
Then, the n-electrode 19 is formed.

When the wafer on which the electrodes were formed as described above was separated into chips of 350 μm square and used as an LED element, green emission of 520 nm was emitted at If 20 mA, and Vf was 3.1 V. On the other hand, the first layer and the second layer are formed of a single Si-doped GaN (Si: 1 × 1).
Vf of LED element composed of 019 / cm3) is 3.4V
Met.

[0054]

As described above, according to the present invention,
When a second layer doped with an n-type impurity is formed on the first layer made of an undoped nitride semiconductor and a negative electrode is formed on the second layer, the crystallinity is good and the carrier concentration is high. Since the high second layer can be formed, the threshold current and the voltage are lowered, and an extremely efficient element can be realized. Further, by applying the element of the present invention to a laser element, a laser element having a low threshold current and a low threshold voltage and continuously oscillating at room temperature can be obtained. Obtaining such a laser device is very significant as a light source for optical communication such as CVD and optical fibers. Furthermore, the present invention can be applied to other optical devices such as LEDs and light receiving elements using nitride semiconductors. For example, when the present invention is applied to an LED element, it is possible to obtain an extremely highly efficient LED with a reduced Vf (forward voltage).

[Brief description of drawings]

FIG. 1 is a schematic cross-sectional view showing the structure of an LED element according to an embodiment of the present invention.

FIG. 2 is a schematic sectional view showing the structure of a laser device according to another embodiment of the present invention.

[Explanation of symbols]

Substrate Buffer layers 12, 22, ... First layer 13, 23 ... 2nd layer 14, 27 ... Active layer 15, 30 ... P-side cladding layer 16, 31 ... P-side contact layer 17, 32 ... P-electrode 19, 33 ... N-electrode

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Kazuyuki Butterfly               491, Oka, Kaminaka-cho, Anan City, Tokushima Prefecture 100 Nichia               Chemical Industry Co., Ltd. (72) Inventor Shuji Nakamura               491, Oka, Kaminaka-cho, Anan City, Tokushima Prefecture 100 Nichia               Chemical Industry Co., Ltd.                (56) References Japanese Patent Laid-Open No. 10-215034 (JP, A)                 JP-A-9-293897 (JP, A)                 Japanese Patent Laid-Open No. 7-302929 (JP, A)                 JP-A-7-321374 (JP, A)                 JP-A-7-94784 (JP, A)

Claims (2)

(57) [Claims] 1. A GaN substrate, and a first layer made of undoped GaN not doped with n-type impurities and having a film thickness of 0.1 μm or more on the GaN substrate.
Of n-type impurities on the layer of 1 × 10 ¹⁷ / cm ³ to 1 × 1.
0 ²¹ / cm ³ Doped film thickness 0.2 μm or more 4 μm
The second nitride semiconductor which is formed of a negative electrode and has a negative electrode
A semiconductor device having the following layers. 2. A GaN substrate, and a GaN substrate, and a first layer of GaN doped with an n-type impurity and having a low concentration of the n-type impurity and having a film thickness of 0.1 μm or more. On the first layer, a nitride semiconductor having an n-type impurity concentration higher than that of the first layer and having a film thickness of 0.2 μm or more 4
A nitride semiconductor device comprising a second layer having a thickness of not more than μm, and a negative electrode being formed on the second layer.