JPH10290047A - Nitride semiconductor element - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain an element whose crystallinity is good, whose output is high and whose efficiency is high by a method wherein a first layer which is not doped with n-type impurities, which is in a specific film thickness or higher and which is composed of a nitride semiconductor is formed on a substrate and a second layer in which a negative electrode is formed, which is doped with n-type impurities and which is composed of a nitride semiconductor is formed on its upper part. SOLUTION: For example, a low-temperature-growth buffer layer 11 which is composed of undoped GaN, a first low-carrier-concentration layer 12 which is composed of Si-doped GaN, a third high-carrier-concentration layer 13 which is composed of Si-doped GaN, an active layer 14, a p-type clad layer 15 and a p-type contact layer 16 are laminated sequentially on a substrate 10 which is composed of sapphire. An n-electrode 19 is formed on the surface of the second layer 13 whose carrier concentration is large. That is to say, the second layer 13 acts as an n-side contact layer as a current injection layer. It is important that the first layer 12 is grown thicker than the second layer 13 and that the second layer 12 is adjusted to be 0.1 μm or higher, preferably 0.5 to 20 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a light emitting device such as an LED (light emitting diode) or an LD (laser diode), a light receiving device such as a solar cell or an optical sensor, or a nitride semiconductor used for a transistor or an integrated circuit. (In _X
Al _Y Ga _{1 -XYN} , 0 ≦ X, 0 ≦ Y, X + Y ≦ 1)

[0002]

2. Description of the Related Art Nitride semiconductors have just recently been put to practical use in full-color LED displays, traffic signals and the like as materials for high-brightness blue LEDs and pure green LEDs. LEDs used in these various devices include 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 heterostructure. 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 has recently published a laser oscillation of 410 nm at room temperature under pulse current using this material (for example, Jpn. J. Appl. Phys. Vol. 35 (1996)).
L74-76). This laser device uses 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
It shows laser oscillation of 0 nm.

For example, in an LED element having a double hetero structure using InGaN as an active layer, the active layer includes an n-type and p-type cladding layer made of AlGaN, an n-type cladding layer made of GaN,
It is sandwiched between p-type contact layers (for example, see 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. The p-type layers such as the p-side contact layer and the p-side cladding layer include Mg, Zn, and the like. Is doped. Usually, in the case of such a structure, n
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 cladding layer in contact with each contact layer. That is, 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 are usually stacked in this order from the substrate. . (It does not have a double hetero structure, but refer to, 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 increases, the contact resistance with the electrode material decreases, and good ohmic properties can be easily obtained. When the ohmic property between the electrode and the contact layer is improved, Vf (forward voltage) of the LED and threshold current of the LD tend to decrease. However, a nitride semiconductor is a material having many crystal defects, and if such a material is doped with an n-type impurity or a p-type impurity at a high concentration in order to obtain a high carrier concentration, the crystallinity is deteriorated and the element itself is deteriorated. The output tends to decrease.

Accordingly, the present invention has been made in view of such circumstances, and an object of the present invention is to provide an element made of a nitride semiconductor having higher crystallinity, higher output, and higher efficiency. Specifically, it is to realize a laser element that continuously oscillates at a low threshold current and a highly efficient LED element.

[0007]

Means for Solving the Problems We have newly found that the above problems can be solved by improving the n-type layer grown on the substrate for nitride semiconductor devices such as LDs and LEDs. Invented the invention. That is, the nitride semiconductor device of the present invention is composed of two types of embodiments, and the first aspect is that the first type is formed of a nitride semiconductor having a thickness of 0.1 μm or more which is not doped with an n-type impurity on the substrate.
And a second layer made of a nitride semiconductor doped with an n-type impurity on the first layer.
Characterized in that a negative electrode is formed in this layer.

In a second aspect, a first layer made of a nitride semiconductor having a small n-type impurity concentration and having a film thickness of 0.1 μm or more is provided above a substrate, and a first layer is provided above the first layer. Than n
A second layer having a high impurity concentration; and a negative electrode formed on the second layer. In claims 1 and 2, it does not mean that the substrate, the first layer and the second layer are not necessarily formed in contact with each other, but the substrate and the first layer, or the first layer and the second layer. Between two layers,
Even if another nitride semiconductor layer such as a buffer layer is inserted, it is within the scope of the present invention.

Furthermore, the nitride semiconductor device of the present invention
At least one of the first layer and the second layer is a strained superlattice layer (hereinafter, referred to as a superlattice layer) in which nitride semiconductor layers having a thickness of 100 Å or less and having different compositions are stacked. )). The preferred thickness of the nitride semiconductor layer forming the strained superlattice layer is adjusted to 70 Å or less, more preferably 10 Å to 40 Å. By reducing the thickness of the nitride semiconductor layer, the thickness becomes equal to or less than the elastic strain limit. Therefore, crystal defects and cracks in the nitride semiconductor layer are reduced, and the crystallinity is dramatically improved. , And a highly reliable element can be realized.

[0010]

FIG. 1 is a schematic sectional view showing a structure of a nitride semiconductor device according to the present invention.
3 shows the structure of a D element. 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, for example, non-doped GaN is formed on a substrate 10 made of sapphire.
2, a high carrier concentration second layer 13 made of, for example, Si-doped GaN, for example, an active layer 14 made of InGaN having a single quantum well structure, for example, a p-side cladding layer 15 made of Mg-doped AlGaN, made of, for example, Mg-doped GaN It has a structure in which the p-side contact layers 16 are sequentially stacked. A translucent 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. Is formed. In the device of the present invention, the n-electrode 19 has a low n-type impurity concentration, or a n-type impurity-doped carrier concentration grown on the first layer 12 not doped with the n-type impurity. Is formed on the surface of the second layer 13 having a large thickness. That is,
The second layer 13 functions 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 a contact layer where the negative electrode is formed,
It acts as a base layer for growing a second layer that acts as a contact layer. If an n-side contact layer serving as a current injection layer is formed of a single nitride semiconductor layer having a thickness of several μm or more and a high carrier concentration as in the conventional case, it is necessary to grow a layer having a high n-type impurity concentration. There is. A thick film layer having a high impurity concentration tends to have poor crystallinity. For this reason, even if another nitride semiconductor such as an active layer is grown on a layer having poor crystallinity, the crystal defect is inherited by another layer, so that improvement in crystallinity cannot be expected. Therefore, in the present invention, before growing the second layer which is to be a contact layer, the first layer having a low impurity concentration and good crystallinity is grown, so that the second layer having a large carrier concentration and good crystallinity is formed. The layers are grown. Generally, the first layer having no n-type impurity or having a low n-type impurity concentration tends to have a lower carrier concentration than the second layer.

In the present invention, the n-type impurities doped into the first layer and the second layer include, for example, Si, Ge, S
Elements of Group IV of the periodic table, such as n, C and Ti, can be mentioned. Among them, Si and Ge are commonly used for doping a nitride semiconductor to adjust the carrier concentration, the resistivity and the like. In the case of a nitride semiconductor layer, it tends to show n-type even in a non-doped state (in a state where impurities are not doped) due to nitrogen vacancies formed in the semiconductor layer. It is possible 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 is most preferably not doped with an n-type impurity (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 its range is 1 × 10 ¹⁷
/ Cm ³ to 1 × 10 ²¹ / cm ³ , more preferably 1
It is desirable to adjust it to × 10 ¹⁸ / cm ³ to 1 × 10 ¹⁹ / cm ³ . Because ohmic preferably less than 1 × 10 ¹⁷ / cm ³ and the material of the n-electrode is difficult to obtain a threshold current, not be expected decrease in the voltage at the laser element, 1 × 10 ²¹ / cm
^When it is larger than ³ , the leak current of the element itself increases and the crystallinity deteriorates, so that the life of the element tends to be shortened.

In the case where the first layer is doped with an n-type impurity, a layer having a lower carrier concentration can be formed by reducing the amount of impurities compared to the second layer. Further, an n-type impurity having a small 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 that the first layer is formed in a non-doped state, since it is preferable to form the first layer without doping an impurity, since a crystal having better crystallinity can be obtained.

Here, the buffer layer 11 will be described. The buffer layer 11 is a nitride semiconductor layer that is grown at a lower temperature than the growth temperature of the first layer before growing the first layer with a thickness of usually less than 0.1 μm. Specifically, non-doped GaN, AlN, and AlGaN layers can be used.
This layer is grown to improve the crystallinity of the first layer, and when a buffer layer is grown on the substrate,
It has an effect of alleviating lattice mismatch between the substrate and the nitride semiconductor. Since this buffer layer is usually a layer containing polycrystal, it is almost impossible to measure the carrier concentration, or even if it can be measured, for example, 1 × 10 ²¹ / cm 2
^This layer is very large, ³ or more, and has very low mobility. Therefore, in the present invention, a film having a thickness of 0.1 μm grown on the substrate or with a single composition before the first layer is grown.
Low temperature growth buffer layers less than are 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 thickness of 100 Å or less and having different compositions are stacked. it can. When a superlattice layer is formed, the layer has a superlattice structure, and the crystallinity of the nitride semiconductor layer is significantly improved, and the threshold current is reduced. That is, the thickness of each nitride semiconductor layer constituting the superlattice layer is set to 100 angstrom or less, and is set to a thickness equal to or less than the elastic strain limit. When the thickness of the nitride semiconductor layer constituting the superlattice layer is less than the elastic strain limit, fine cracks and crystal defects are less likely to be formed in the crystal, and a nitride semiconductor with good crystallinity is grown. it can. Therefore, even if another nitride semiconductor layer is grown on the superlattice layer, the crystallinity of the superlattice layer is good, so that the crystallinity of the other nitride semiconductor layer is also good. Therefore, crystal defects are reduced and crystallinity is improved in the entire nitride semiconductor, so that the threshold current is reduced and the life of the laser element is improved.

The nitride semiconductor layers constituting the superlattice layer are mutually
What is necessary is just to be composed of nitride semiconductors having different compositions.
Even if the band gap energy is different or the same
I don't care. For example, the first layer (A
Layer) is In_XGa_1-XN (0 ≦ X ≦ 1), next layer
(B layer) is Al_YGa_1-YN (0 <Y ≦ 1)
And the band gap energy of the B layer must be higher than that of the A layer.
Although it is large, the A layer is _XGa_1-XN (0 ≦ X ≦ 1)
And B layer is In_ZAl_1-ZN (0 <Z ≦ 1)
If the composition is different between the layer A and the layer B, the band gap
The energy may be the same. A layer is made of Al_Y
Ga_1-YN (0 ≦ Y ≦ 1), and the B layer is In_ZAl_1-Z
If N (0 <Z ≦ 1), the first layer and the second layer
Layer has a different composition but the same band gap energy.
It could be one. The superlattice layer of the present invention
Even if the composition is different and the band gap energy is the same
good.

Preferably, the nitride semiconductors of the A layer and the B layer constituting the superlattice layer are laminated with different band gap energies, and the average band gap energy of the nitride semiconductor constituting the superlattice layer is activated. It is desirable to adjust so as to be larger than the layer. Preferably one of the layers as _{In X Ga 1-X N (} 0 ≦ X ≦ 1), constituting the other layers in the _{Al Y Ga 1-Y N (} 0 ≦ Y ≦ 1, X ≠ Y = 0) Thereby, a superlattice layer having good crystallinity can be formed. In addition, AlGaN has a property of easily cracking during crystal growth. Therefore, the A layer constituting the superlattice layer is formed of Al having a thickness of 100 Å or less.
When the nitride semiconductor layer does not contain Al, it acts as a buffer layer when growing another B layer made of a nitride semiconductor containing Al, and the B layer is less likely to crack.
Therefore, even if the superlattice layers are stacked, a superlattice without cracks can be formed, so that the crystallinity is improved and the life of the element is improved. Again, one of the layers is In _x Ga _{1 -x} N (0 ≦ X ≦
1) and the other layer is Al _Y Ga _{1 -Y} N (0 ≦ Y ≦
1, X ≠ Y = 0).

The thickness of each nitride semiconductor layer constituting the superlattice layer is 100 Å or less, more preferably 7 Å or less.
The adjustment is made in a range of 0 angstrom or less, most preferably 10 angstrom or more and 40 angstrom or less. If the thickness is larger than 100 Å, the film thickness becomes larger than the elastic strain limit, and a minute crack or a crystal defect tends to easily enter the film. The lower limits of the thicknesses of the well layer and the barrier layer are not particularly limited, and may be at least one atomic layer.
It is desirable to adjust it to 0 angstrom or more. However, when the first layer having a large thickness is formed of a superlattice layer, the thickness is 70 Å or less. When the second layer having a small thickness and containing an n-type impurity is formed as a superlattice layer, the thickness is 40 Å. It is desirable to make the following.

When the bandgap energies of the nitride semiconductor layers forming the superlattice layer are different from each other, the n-type impurity is doped more into the layer having the larger bandgap energy, or the n-type impurity is made non-doped with the smaller bandgap energy. The threshold voltage and the threshold current tend to decrease more easily when the bandgap energy is doped with an n-type impurity.

Importantly, the first layer is grown thicker than the second layer, the first layer being at least 0.1 μm, more preferably at least 0.5 μm, most preferably at least 1 μm,
It is desirable to adjust it to 0 μm or less. If the first layer is 0.
If the thickness is smaller 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.
Tends to be less likely to improve the crystallinity of the layer. If the thickness is more than 20 μm, the first layer itself tends to have many crystal defects. An advantage of growing the first layer thickly is an improvement in heat dissipation. That is, when a laser element is manufactured, heat easily spreads in the first layer, and the life of the laser element is improved. Further, the leakage light of the laser light spreads in the first layer, so that a laser light having a nearly elliptical shape is easily obtained.

On the other hand, the second layer has a thickness of 0.2 μm or more and 4 μm
It is desirable to adjust as follows. 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, the crystallinity tends to deteriorate due to the influence of impurities. This also applies to the case where the first layer and the second layer are composed of superlattice layers. When the first layer and the second layer are composed of the superlattice layer, it is needless to say that the superlattice layer indicates the film thickness of the entire superlattice layer. The first layer is superlattice 20
Stacking more than μm is very time-consuming and unsuitable for the manufacturing process. However, when the second layer is formed of a superlattice layer, it may be formed with a thickness of 4 μm or more, but when it is grown as a thick film like the first layer, it takes much time and effort.

[0023]

The present invention will be described in detail in the following examples. FIG.
FIG. 1 is a schematic cross-sectional view illustrating a structure of a laser device according to an embodiment of the present invention, showing a structure when the device is cut in a direction perpendicular to a resonance direction of laser light. Hereinafter, the device of the present invention will be described based on this drawing. Note that the general formula In _x Al _Y Ga _{1 -XYN} shown in the present specification simply indicates the composition ratio of a nitride semiconductor. For example, even if different layers are represented by the same general formula, the The X and Y values of the layers do not match.

[Example 1] A substrate 20 made of sapphire (C-plane) was set in a reaction vessel, and the inside of the vessel was sufficiently replaced with hydrogen.
The temperature is raised to ° C., and the substrate is cleaned. Substrate 20
In addition to sapphire C surface, sapphire having R surface and A surface as main surfaces, other insulating substrates such as spinel (MgAl ₂ O ₄ ), SiC (including 6H, 4H, 3C),
A semiconductor substrate of ZnS, ZnO, GaAs, GaN, or the like can be used.

(Buffer Layer 21) Subsequently, the temperature is set to 510
The temperature is lowered to ℃, and a buffer layer 2 made of GaN is grown to a thickness of about 200 Å on the substrate 1 using hydrogen as a carrier gas and ammonia and TMG (trimethylgallium) as a source gas. Buffer layer 20, AlN, G
aN, AlGaN, etc. at a temperature of 900 ° C. or less, 0.1
It can be formed with a thickness of less than μm, preferably several tens of angstroms to several hundreds of angstroms. This buffer layer is formed in order to alleviate the lattice constant mismatch between the substrate and the nitride semiconductor, but 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,
Turn off only TMG and raise the temperature to 1050 ° C.
When the temperature reaches 1050 ° C., a first layer 22 of non-doped GaN having a carrier concentration of 1 × 10 ¹⁸ / cm ³ is grown to a thickness of 5 μm using TMG and ammonia gas as the source gas. The first layer is made of In _x Al _Y Ga _{1 -XYN} (0 ≦
X, 0 ≦ Y, X + Y ≦ 1), and the composition is not particularly limited.

(Second Layer 23) Subsequently, at 1050 ° C., T
Using silane gas for MG, ammonia and impurity gas, S
i is 1 × 10¹⁹/cm^ThreeThe first layer of doped n-type GaN
The second layer 23 is grown to a thickness of 1 μm. This second layer
23 has a carrier concentration of 1 × 10 ¹⁹/cm^Three
Met. N-type impurities with high activation rate, especially Si
Has a carrier concentration almost equal to the amount of doped impurities.
In the following description, Si was doped
The n-type nitride semiconductor has a doping amount of Si,
It is assumed that the concentration is indicated. The composition of the second layer is also In.
_XAl_YGa_1-XYConsists of N (0 ≦ X, 0 ≦ Y, X + Y ≦ 1)
The composition of the first layer 2 does not matter.
2. The second layer 23 is composed of nitride semiconductors having different compositions.
May be.

(Crack prevention layer 24)
0 ° C., and using TMG, TMI (trimethylindium), ammonia and silane gas as source gases,
The × 10 ¹⁹ / cm ³ doped crack preventing layer 24 consisting of In0.1Ga0.9N was grown to a film thickness of 500 angstroms. The crack prevention layer 10 is made of an n-type nitride semiconductor containing In, preferably InGaN, so that cracks can be prevented from entering the nitride semiconductor layer containing Al. The crack preventing layer is preferably grown to a thickness of 100 Å or more and 0.5 μm or less. When the thickness is less than 100 Å, it is difficult to function as a crack prevention as described above, and when the thickness is more than 0.5 μm, the crystal itself tends to turn black. The crack prevention layer 24 can be omitted depending on the conditions of the growth method, the growth apparatus, and the like, and can be omitted particularly when the second layer 23 has a super lattice structure.

(N-side cladding layer 25)
0 ° C., and using TMA (trimethylaluminum), TMG, NH ₃ , and SiH ₄ as raw material gases,
¹⁹ / cm ³ doped n-type Al0.25Ga0.75N
The side cladding layer 25 is grown to a thickness of 0.5 μm. The n-side cladding layer 25 functions as a carrier confinement layer and a light confinement layer, and it is desirable to grow a nitride semiconductor containing Al, preferably AlGaN.
By growing the layer at a thickness of angstrom or more and 2 μm or less, more preferably 500 angstrom or more and 1 μm or less, a cladding layer having good crystallinity can be formed.

(N-side light guide layer 26)
An n-side optical guide layer 26 of n-type GaN doped with Si at 1 × 10 ¹⁹ / cm ³ is grown at a temperature of 0.2 ° C. to a thickness of 0.2 μm. The n-side light guide layer 26 functions as a light guide layer of an active layer, and is preferably used to grow GaN or InGaN. desirable. The n-side light guide layer may be non-doped.

(Active Layer 27) Next, TMG is used as a source gas.
The active layer 27 is formed using TMI, ammonia and silane gas.
Let it grow. The active layer 27 maintains the temperature at 800 ° C.
First, Si is 8 × 10 ¹⁸/cm^ThreeIn0.2Ga0 doped with
A well layer of 8N is formed with a thickness of 25 Å.
Lengthen. Next, it is the same except for changing the molar ratio of TMI.
At temperature, 8 × 10 Si¹⁸/cm^ThreeDoped In0.01G
a. A barrier layer made of 0.95N is formed to a thickness of 50 Å.
Grow with. This operation is repeated twice, and finally the well layer
Are stacked to form a multiple quantum well structure. Doping the active layer
Impurities are doped into both the well layer and the barrier layer as in this embodiment.
Or one of them may be doped. Note that
When an n-type impurity is doped, the threshold value tends to decrease.

(P-side cap layer 28)
Raise to 50 ° C, TMG, TMA, ammonia, Cp2M
g (cyclopentadienylmagnesium), the bandgap energy of which is larger than that of the active layer.
X10 ²⁰ / cm ³ doped Al0.1Ga0.9N p
The side cap layer 28 is grown to a thickness of 300 Å. The p-side cap layer 28 is preferably of a p-type, but may be of an i-type in which the carrier is compensated by doping an n-type impurity due to its small thickness. p-side cap layer 2
The film thickness of No. 8 is adjusted to 0.1 μm or less, more preferably 500 Å or less, and most preferably 300 Å or less. This is because if the layer is grown with a thickness greater than 0.1 μm, cracks are easily formed in the p-side cap layer 28, and it is difficult to grow a nitride semiconductor layer having good crystallinity. In addition, carriers cannot pass through the energy barrier due to the tunnel effect. When the composition ratio of Al is larger and the thickness of AlGaN is smaller, the LD element is more likely to oscillate. For example, in the case of Al _Y Ga ₁ -YN having a Y value of 0.2 or more, it is desirable to adjust the value to 500 Å or less. Although the lower limit of the film thickness of the p-side cap layer 28 is not particularly limited, it is preferable that the film be formed with a film thickness of 10 Å or more.

(P-side light guide layer 29)
A p-side light guide layer 26 made of GaN doped with 1 × 10 ²⁰ / cm ^{3 of} Mg is grown at a temperature of 0.2 ° C. to a thickness of 0.2 μm. The p-side light guide layer 29 acts as a light guide layer of an active layer, like the n-side light guide layer 26, and serves as a GaN, I
It is desirable to grow nGaN, preferably to a thickness of 100 Å to 5 μm, more preferably 200 Å to 1 μm. The p-side light guide layer is doped with a p-type impurity, but may be made of a non-doped nitride semiconductor.

(P-side cladding layer 30) Subsequently, at 1050 ° C.
Al.25 Ga0.7 doped with Mg at 1 × 10 ²⁰ / cm ³
A 5N p-side cladding layer 30 is grown to a thickness of 0.5 μm. This layer acts as a carrier confinement layer and a light confinement layer as in the case of the n-side cladding layer 25.
It is desirable to grow a nitride semiconductor containing l, preferably AlGaN, at least 100 Å.
If the growth is made at most μm, more preferably at least 500 Å and at most 1 μm, a cladding layer with good crystallinity can be grown.

In the case of a semiconductor device having a double hetero structure having an active layer having a quantum well layer as in this embodiment, a film thickness 0 having a band gap energy larger than that of the active layer is provided in contact with the active layer 27. A cap layer made of a nitride semiconductor having a thickness of 0.1 μm or less, preferably a p-side cap layer made of a nitride semiconductor containing Al, is provided at a position farther from the active layer than the p-side cap layer. P whose band gap energy is smaller than 28
A light guide layer 29 is provided, and a nitride semiconductor having a larger band gap than 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 cladding layer 30 made of a nitride semiconductor containing In addition, since the thickness of the p-side cap layer 28 is set to a small value of 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 by a tunnel effect. And efficiently recombine in the active layer,
Output is improved. That is, since the injected carriers have a large band gap energy of the cap layer 28, the carriers do not overflow the active layer even if the temperature of the semiconductor element rises or the injected current density increases. , Carriers are accumulated in the active layer, and light can be emitted efficiently.

(P-side contact layer 31) Finally, on the p-side cladding layer 30, Mg was added at 2 × 10 ²⁰ /
A p-side contact layer 31 made of GaN doped with cm ³ is grown to a thickness of 150 Å. The p-side contact layer 31 is a p-type In _x Al _Y Ga _{1 -XYN} (0 ≦ X,
0 ≦ Y, X + Y ≦ 1), and preferably Mg-doped GaN provides the most preferable ohmic contact with the p-electrode 32. In addition, as a material of the p-side contact layer and the p-electrode 32 from which a preferable ohmic is obtained, for example, Ni, Pd, Ag, Ni / Au and the like can be cited. Further, the thickness of the p-side contact layer 31 is 500 angstrom or less, and more preferably 3 angstrom.
It is desirable to adjust the thickness to not more than 00 Å, most preferably to not more than 200 Å. The reason is that by adjusting the thickness of the p-type nitride semiconductor layer having a high resistivity to 500 Å or less, the resistivity further decreases, so that the threshold current and voltage decrease. In addition, the amount of hydrogen removed from the p-type layer increases, and the resistivity tends to decrease. Further, the effects of reducing the thickness of the contact layer 31 include the following. For example, p
The p-side cladding layer made of p-type AlGaN has a thickness of 500
A p-side contact layer made of p-type GaN, which is thicker than Å, is formed in contact with the substrate. If the impurity concentration of the cladding layer and that of the contact layer are the same and the carrier concentration is the same, the thickness of the p-side contact layer is set to 500. When the thickness is smaller than Å, carriers on the cladding layer side easily move to the contact layer side, and the carrier concentration in the p-side contact layer tends to increase. Therefore, when an electrode is formed on a contact layer having a high carrier concentration, a good ohmic can be obtained.

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

After annealing, the wafer was taken out of the reaction vessel and, as shown in FIG.
The side contact layer 31 and the p-side cladding layer 30 are etched to form a ridge shape having a stripe width of 4 μm. In particular, by forming a layer of a nitride semiconductor layer containing Al or higher which is higher than the active layer into a ridge shape, light emission of the active layer is concentrated on the lower portion of the ridge, the transverse mode is easily united, and the threshold value is lowered. Cheap. After the formation of 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 symmetrically with respect to the stripe-shaped ridge. As a material for the n-electrode 33, a metal or alloy such as Al, Ti, W, Cu, Zn, Sn, and In can be used to obtain an ohmic.

Next, on the surface of the p-side contact layer 31, a p-electrode 32 made of Ni and Au is formed in a stripe shape. On the other hand, an n-electrode 33 made of Ti and Al is formed on almost the entire surface of the second layer 23 in a stripe shape. In addition, almost all is 8
It means 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 an n-electrode on almost the entire surface of 3 is also very advantageous in lowering the threshold.

As described above, the wafer on which the n-electrode and the p-electrode are formed is transferred to a polishing apparatus, and the sapphire substrate 20 on which the nitride semiconductor is not formed is wrapped using a diamond abrasive. The thickness of the substrate is 50 μm. After lapping, the substrate surface is mirror-finished by polishing with a finer abrasive at 1 μm.

After polishing the substrate, the polished surface side is scribed,
Cleavage is performed in a bar shape in a direction perpendicular to the stripe-shaped electrodes, and a resonator is formed on the cleavage plane. SiO ₂ and TiO ₂ on the resonator surface
A dielectric superlattice was formed, and finally the bar was cut in a direction parallel to the p-electrode 32 to form a laser chip. Next, the chip was placed on the heat sink face up (in a state where the substrate and the heat sink faced each other), the respective electrodes were bonded, and laser oscillation was attempted at room temperature.
At room temperature, continuous oscillation at an oscillation wavelength of 405 nm was confirmed at a threshold current density of 3.0 kA / cm ² and a threshold voltage of 4.5 V, and a lifetime of 30 hours or more was shown.

Example 2 In Example 1, when growing the second layer 23, Al doped with 1 × 10 ¹⁹ / cm ^{3 of} Si was used.
A layer made of 0.1Ga0.9N is grown at 20 Å, and a layer made of n-type GaN doped with the same amount of Si is grown at 20 Å. This operation was repeated 200 times, and the carrier concentration was 1 × 10
^A second layer 23 made of a superlattice layer having a total film thickness of 0.8 μm and a thickness of ¹⁹ / cm ³ is formed.

Next, the n-side cladding layer 25 is grown directly on the second layer 23 in the same manner as in the first embodiment without growing the crack prevention layer 24, and thereafter the laser shown in FIG. A nitride semiconductor is stacked so as to have an element structure.

After the growth, after forming the ridge, the second layer 2 is formed.
3 is exposed by etching. The second layer 2
A well layer made of Si-doped GaN was exposed on the surface of Sample No. 3. Thereafter, electrodes were formed in the same manner as in Example 1 to form a laser device. At room temperature, the threshold current density was 2.8 k.
At A / cm ² and a threshold voltage of 4.3 V, continuous oscillation with an oscillation wavelength of 405 nm was confirmed, and a lifetime of 40 hours or more was shown.

Example 3 In Example 2, when the second layer 23 was grown, Al doped with 2 × 10 ¹⁹ / cm ^{3 of} Si was used.
A layer made of 0.1Ga0.9N is grown for 30 Å, and a layer made of non-doped GaN is grown for 30 Å. This operation is repeated 200 times to form a second layer 23 made of a superlattice layer having a total film thickness of 1.2 μm. Thereafter, when a laser device was manufactured in the same manner as in Example 2, continuous oscillation at an oscillation wavelength of 405 nm was confirmed at a threshold current density of 2.7 kA / cm ² and a threshold voltage of 4.1 V, and a life of 50 hours or more was shown. .

As described above, when the superlattice layer is the second layer 23 and the n-electrode is formed, the threshold voltage tends to decrease. This is presumed to have an effect similar to HEMT. For example, in a superlattice layer in which a nitride semiconductor layer with a large band gap doped with an n-type impurity and a non-doped nitride semiconductor layer with a small band gap 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 near the thickness on the layer side where the band gap is small. Since the two-dimensional electron gas is formed on the side having the smaller band gap, the electrons are not scattered by impurities when traveling, so that the mobility of electrons in the superlattice increases and the resistivity decreases. Therefore, when the superlattice is used as a contact layer when forming an electrode, it is presumed that the mobility increases and the voltage of the element decreases, but the details are unknown.

[Embodiment 4] In the first embodiment, when the first layer 22 is grown, the well layer made of non-doped n-type GaN is formed to 40 Å, and then the non-doped n-type Al0.
A barrier layer of 1Ga0.9N is grown at 60 Å. This operation is repeated 200 times to form a first layer 22 composed of a superlattice layer having an average carrier concentration of 5 × 10 ¹⁷ / cm ^{3 and} a total film thickness of 2 μm.

Next, in the same manner as in the first embodiment,
Second layer 23 made of 0 ¹⁹ / cm ³ doped n-type GaN
Is grown to a thickness of 1 μm, and a crack prevention layer 2
4 was grown, and thereafter a laser device was manufactured in the same manner as in Example 1. As a result, a laser device having substantially the same characteristics as in Example 2 could be manufactured.

[0049] In Example 5 Example 1, when the first layer growth, 1 × a Si 10 ¹⁸ / cm ³ doped with n-type Ga
A laser device was fabricated in the same manner as in Example 1 except that N was changed to N.
At A / cm ² and a threshold voltage of 4.6 V, continuous oscillation with an oscillation wavelength of 405 nm was confirmed, and a lifetime of 25 hours or more was shown.

[Embodiment 6] This embodiment will be described based on the LED element of FIG. A buffer layer 11 made of non-doped GaN is grown at 600 ° C. on a substrate 10 made of sapphire in the same manner as in the first embodiment.
A first layer 12 of non-doped n-type GaN having a carrier concentration of 1 × 10 ¹⁸ / cm ³ is grown to a thickness of 4 μm on
A second layer 13 made of n-type GaN doped with i at 1 × 10 ¹⁹ / cm ³ is grown to 1 μm.

Next, an active layer 14 having a single quantum well structure of In.sub.0.4 Ga.sub.0.6 N and a thickness of 30 .ANG.
Is grown, and a p-side cladding layer 15 made of Mg-doped p-type Al0.2Ga0.9N doped with Mg at 5 × 10 ¹⁹ / cm ³ is grown to a thickness of 0.5 μm.
A p-side contact layer 16 of Mg doped p-type GaN doped with 10 ¹⁹ / cm ³ is grown to a thickness of 0.2 μm.

After the growth, the wafer is taken out of the reaction vessel, annealed in the same manner as in Example 1, and then etched from the p-side contact layer 16 side to form the surface of the second layer 13 on which the n-electrode 19 is to be formed. To expose. A light-transmissive p-electrode 17 of Ni-Au having a thickness of 200 Å is formed on almost the entire surface of the uppermost p-side contact layer 16, and the pad electrode 1 of Au is formed on the p-electrode 17.
8 is formed. Ti-Al is also applied to the exposed surface of the second layer.
An 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 to obtain LED elements, green light emission of 520 nm was obtained at If mA of 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
0 ¹⁹ / cm ³ ), the Vf of the LED element was 3.4 V.

[0054]

As described above, according to the present invention,
When a second layer doped with an n-type impurity is formed on a first layer made of an increasingly non-doped nitride semiconductor and a negative electrode is formed on the second layer, crystallinity is improved and carrier density is improved. Since a high second layer can be formed, the threshold current and the voltage are reduced, and a very efficient element can be realized. Further, by applying the device of the present invention to a laser device, a short-wavelength laser device having low threshold current and threshold voltage and continuous oscillation 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 fiber. Furthermore, the present invention is also applicable to other optical devices such as LEDs and light receiving elements using a nitride semiconductor. For example, when the present invention is applied to an LED element, it is possible to obtain a highly efficient LED having a reduced Vf (forward voltage).

[Brief description of the drawings]

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

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

[Explanation of symbols]

 10, 20, ..., substrate 11, 21, ..., buffer layer 12, 22, ..., first layer 13, 23, ... second 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

Continuation of the front page (72) Inventor Shuji Nakamura 100, 491-1 Kaminakamachioka, Anan-shi, Tokushima Prefecture Inside Nichia Chemical Industry Co., Ltd.

Claims (3)

[Claims] 1. A first semiconductor device comprising a nitride semiconductor having a thickness of 0.1 μm or more, which is not doped with an n-type impurity, is formed on an upper portion of a substrate.
And a second layer made of a nitride semiconductor doped with an n-type impurity on the first layer.
A nitride semiconductor element, wherein a negative electrode is formed in a layer of: 2. A first layer made of a nitride semiconductor having a small n-type impurity concentration and a thickness of 0.1 μm or more having a lower n-type impurity concentration than the first layer is provided above the substrate. A nitride semiconductor device, comprising: a second layer having a high type impurity concentration; and a negative electrode formed in the second layer. 3. A strained superlattice layer in which at least one of the first layer and the second layer is formed by stacking nitride semiconductor layers having a thickness of 100 Å or less and having different compositions from each other. 3. The method according to claim 1, wherein
3. The nitride semiconductor device according to item 1.