JP3614070B2 - Nitride semiconductor light emitting diode - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a light emitting diode (LED), a laser diode (LD), a solar cell, a light emitting element such as a photosensor, and a nitride semiconductor element (In _X Al _Y Ga _1-XY N, 0.ltoreq.X, 0.ltoreq.Y, X + Y.ltoreq.1), and in particular, relates to a nitride semiconductor light emitting diode that emits light in the ultraviolet region having an emission peak wavelength of 370 nm or less.
[0002]
[Prior art]
In recent years, ultraviolet LEDs have become practical. For example, in Applied Physics, Vol. 68, No. 2 (1999), p152 to p155, a GaN buffer layer, an n-type GaN contact layer (film thickness: 4 μm), an n-type AlGaN cladding layer, undoped on a sapphire substrate A nitride semiconductor device in which an InGaN active layer (In composition is almost zero), a p-type AlGaN cladding layer, and a p-type GaN contact layer (film thickness: 0.12 μm) is described. This UV LED has a light emission output of 5 mW when the emission peak is 371 nm under certain conditions, whereas the n-type and p-type contact layers are GaN when the emission wavelength is shorter than this. Therefore, it is described that self-absorption occurs and the light emission output decreases rapidly. Further, it has been suggested that the self-absorption can be prevented by using AlGaN for the n-type and p-type contact layers in order to prevent the light emission output from decreasing and to reduce the oscillation wavelength.
[0003]
[Problems to be solved by the invention]
However, if the contact layer is grown with AlGaN having a high Al composition ratio to such an extent that self-absorption can be sufficiently prevented, it is difficult to obtain good ohmic contact with the electrode. This is probably due to the fact that AlGaN causes physical inactivation of dopants and the difference in work function with the metal serving as the electrode due to variations in the Fermi level. It may be difficult to obtain ohmic contact. Such a decrease in ohmic contact is large in the p-type contact layer.
In addition, as described above, doping the contact layer with a large amount of impurities is almost proportional to the amount of impurities in order to improve the point that the inclusion of Al composition causes dopant inactivation and makes it difficult to obtain ohmic contact. Since impurity levels are formed, self-absorption increases, and the light emission output may decrease due to a decrease in crystallinity.
[0004]
Furthermore, it is conceivable that the self-absorption is relatively lowered by reducing the film thickness by using GaN as the contact layer. However, if the film thickness of the p-type contact layer is reduced, for example, the device characteristics become insufficient. If the film thickness of the n-type contact layer is reduced, the operation becomes complicated and the exposure becomes difficult when the n-type contact layer is exposed.
[0005]
As described above, in the light emitting diode having a light emission peak wavelength of 370 nm or less in the prior art, self-absorption occurs in the contact layer and the light emission output decreases. However, in order to prevent self-absorption by GaN, the contact layer is simply formed as AlGaN. As a result of the growth, the ohmic contact is lowered, and when the impurity doping amount is increased in order to obtain the ohmic contact, self-absorption occurs due to the formation of impurity levels and the increase of crystal defects. Therefore, it is desirable to improve the ohmic contact while preventing self-absorption.
[0006]
SUMMARY OF THE INVENTION An object of the present invention is to provide a nitride semiconductor light emitting diode having an emission peak wavelength of 370 nm or less that can obtain good ohmic contact and suppress self-absorption and improve emission output.
[0007]
[Means for Solving the Problems]
That is, the present invention can achieve the object of the present invention by the following configurations (1) to (6).
(1) In a nitride semiconductor device having at least an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer on a substrate,
The active layer is composed of a nitride semiconductor layer having an emission peak wavelength of 370 nm or less,
As the p-type nitride semiconductor layer, the p-type contact layer in contact with the p-electrode has Al on the side in contact with the p-electrode at a high concentration of p-type impurities. _a Ga _1-a A first p-type contact layer containing N (0 ≦ a <0.05) and a p-type impurity in contact with the first p-type contact layer on the active layer side of the first p-type contact layer At a lower concentration than the first p-type contact layer, and the Al composition ratio is higher than that of the first p-type contact layer. _b Ga _1-b And a second p-type contact layer containing N (0 <b <0.1),
Furthermore, as the n-type nitride semiconductor layer, an n-type contact layer in contact with the n-electrode is made of Al. _d Ga _1-d A nitride semiconductor light-emitting diode comprising N (0 <d <0.1).
(2) The first p-type contact layer in contact with the p-electrode has a p-type impurity concentration of 1 × 10 ¹⁹ ~ 1x10 ²² / Cm ³ The nitride semiconductor light-emitting diode according to (1), which is contained.
(3) The second p-type contact layer contains 1 × 10 p-type impurities. ²⁰ / Cm ³ The nitride semiconductor light-emitting diode according to (1) or (2), which is contained below.
(4) The thickness of the first p-type contact layer in the p-type contact layer is 100 to 500 angstroms, and the thickness of the second p-type contact layer is 400 to 2000 angstroms. The nitride semiconductor light emitting diode according to (1).
(5) The n-type contact layer contains 1 × 10 n-type impurities. ¹⁷ ~ 1x10 ¹⁹ / Cm ³ The nitride semiconductor light-emitting diode according to (4), which is contained.
(6) Between the active layer and the n-type contact layer, Al _e Ga _1-e A first nitride semiconductor layer containing N (0 <e <0.3), and an Al layer between the active layer and the p-type contact layer. _f Ga _1-f The nitride semiconductor light-emitting diode according to (1), further including a second nitride semiconductor layer containing N (0 <f <0.4).
[0008]
That is, according to the present invention, the n-type contact layer is made of AlGaN having a specific Al composition ratio, and the p-type contact layer is formed of a first p-type contact layer having a high p-type impurity concentration and a small Al composition ratio, and p By forming the first p-type contact layer having a low p-type impurity concentration and a high Al composition ratio, and forming the first contact layer having a high p-type impurity concentration on the side in contact with the p-electrode, a good ohmic resistance can be obtained. Contact can be obtained, self-absorption can be suppressed, and a nitride semiconductor light-emitting diode having an emission peak wavelength with a favorable emission output of 370 nm or less can be obtained.
Here, in the present invention, the high or low p-type impurity concentration in the p-type contact layer and the high or low Al composition ratio mean that the first p-type contact layer and the second p-type contact layer constitute the p-type contact layer. The relative relationship in the p-type contact layer is shown.
[0009]
As described above, the present inventor conducted various studies on the point that when the Al composition is simply included to prevent self-absorption as described above, the ohmic contact is reduced due to the inactivation of impurities and the like. The type contact layer is made of AlGaN having a specific Al composition ratio, and the p-type contact layer has a two-layer structure of a layer for obtaining ohmic contact and a layer for securing a film thickness to maintain device characteristics. Thought.
Then, the present inventor has the first p-type contact layer in contact with the p-electrode having a high p-type impurity concentration and a low Al composition ratio as described above, and the second p-type contact having a low Al composition ratio and a low Al composition ratio. By forming a p-type contact layer with an n-type contact layer and combining it with an n-type contact layer having a specific Al composition ratio, nitrides with good ohmic contact and self-absorption can be prevented Achieved to obtain a semiconductor light emitting diode.
[0010]
Further, in the present invention, the first p-type contact layer contains p-type impurities at 1 × 10 5. ¹⁹ ~ 1x10 ²² / Cm ³ , Preferably 5 × 10 ²⁰ ~ 5x10 ²¹ / Cm ³ When it contains, it is preferable at the point which obtains a favorable ohmic contact.
Furthermore, in the present invention, the second p-type contact layer contains 1 × 10 p-type impurities. ²⁰ / Cm ³ Or less, preferably 5 × 10 ¹⁸ ~ 5x10 ¹⁹ / Cm ³ When it is contained, it is preferable in that self-absorption can be prevented even if the thickness of the p-type contact layer is increased in order to maintain device characteristics.
[0011]
Furthermore, in the present invention, when the thickness of the first p-type contact layer in the p-type contact layer is 100 to 500 angstroms, preferably 150 to 300 angstroms, the film thickness can be increased even if the p-type impurity concentration is increased. If it is set thin, self-absorption due to impurity levels can be made relatively small, ohmic contact with the p-electrode can be made good, and the thickness of the second p-type contact layer is 400 to 2000. When the thickness is angstrom, preferably 800 to 1200 angstrom, self-absorption can be prevented and device characteristics can be maintained.
[0012]
Further, according to the present invention, the n-type contact layer containing Al has an n-type impurity concentration of 1 × 10 5. ¹⁷ ~ 1x10 ¹⁹ / Cm ³ , Preferably 1 × 10 ¹⁸ ~ 1x10 ¹⁹ / Cm ³ If present, it is preferable in that self-absorption is prevented, ohmic contact is maintained, and light emission output is improved.
As described above, when the Al composition ratio of the n-type contact layer and the n-type impurity concentration are specified and combined, it is preferable in terms of preventing ohmic contact and cracking and improving the light emission output as in the case of the p-type contact layer. .
[0013]
Furthermore, the present invention provides an Al layer between the active layer and the n-type contact layer. _e Ga _1-e A first nitride semiconductor layer containing N (0 <e <0.3), and an Al layer between the active layer and the p-type contact layer. _f Ga _1-f When the second nitride semiconductor layer containing N (0 <f <0.4) is included, it is preferable from the viewpoint of improving the light emission output because the confinement of carriers in the active layer can be improved.
Further, when the Al composition ratio of each of the first nitride semiconductor layer and the second nitride semiconductor layer is within the above range, and the Al composition ratio and impurity concentration of the contact layer are combined, the prevention of crack generation and ohmic resistance can be achieved. The contact can be made favorable, and this is preferable in terms of improvement in light emission output.
Since the first nitride semiconductor layer and the second nitride semiconductor layer have a function as a cladding layer, in the present invention, hereinafter, the first nitride semiconductor layer is referred to as an n-type cladding layer, and the second The nitride semiconductor layer is a p-type cladding layer. However, it is not limited to this.
[0014]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, the present invention will be described in more detail with reference to FIG. FIG. 1 is a schematic cross-sectional view of a nitride semiconductor device according to an embodiment of the present invention.
In FIG. 1, a buffer layer 2 and Al are formed on a substrate 1. _d Ga _1-d N-type contact layer 3 containing N (0 <d <0.1), Al _e Ga _1-e N-type cladding layer 4 comprising N (0 <e <0.3), In _g Ga _1-g Active layer 5 of N (0 ≦ g <0.1), Al _f Ga _1-f P-type cladding layer 6 containing N (0 <f <0.4), Al _b Ga _1-b A second p-type contact layer 7b containing N (0 <b <0.1), Al _a Ga _1-a A nitride semiconductor device having a light emission peak wavelength of 370 nm or less, which is obtained by laminating and growing a p-type contact layer 7 composed of a first p-type contact layer 7a containing N (0 ≦ a <0.05). Has been. An n-electrode is formed on the n-type contact layer 3, and a p-electrode is formed in contact with the first p-type contact layer 7 a of the p-type contact layer 7.
First, the n-type contact layer 3 and the p-type contact layer 7 of the present invention will be described.
[0015]
[N-type contact layer 3]
In the present invention, the n-type contact layer 3 is at least Al. _d Ga _1-d A nitride semiconductor layer containing N (0 <d <0.1, preferably 0.01 <d <0.05). When the Al composition ratio is in the above range, it is preferable in terms of crystallinity and ohmic contact as well as preventing self-absorption.
Furthermore, the n-type contact layer 3 contains n-type impurities at 1 × 10. ¹⁷ ~ 1x10 ¹⁹ / Cm ³ , Preferably 1 × 10 ¹⁸ ~ 1x10 ¹⁹ / Cm ³ When it contains in the density | concentration, it is preferable at the point of maintenance of ohmic contact, prevention of crack generation, and maintenance of crystallinity. Thus, combining the Al composition ratio constituting the n-type contact layer and the n-type impurity concentration is preferable in terms of preventing self-absorption and preventing ohmic contact and cracking.
Although it does not specifically limit as an n-type impurity, For example, Si, Ge etc. are mentioned, Preferably it is Si.
Although the film thickness of the n-type contact layer 3 is not specifically limited, 0.1-20 micrometers is preferable, More preferably, it is 1-10 micrometers. When the film thickness is within this range, it is preferable in terms of crystallinity (as a base) near the interface (for example, near the interface with the n-type cladding layer) and a decrease in resistivity.
[0016]
[P-type contact layer 7]
In the present invention, the p-type contact layer 7 includes a first p-type contact layer 7a having a high p-type impurity concentration in contact with the p-electrode and a low Al composition ratio, and a first p-type contact layer 7a having a low p-type impurity concentration and a high Al composition ratio. 2 p-type contact layers 7b.
Thus, by forming the p-type contact layer 7 with two types of layers having different p-type impurity concentrations and Al composition ratios, a good ohmic contact can be obtained with the first p-type contact layer 7a. When the p-type contact layer is adjusted to a thickness that can maintain the device characteristics, even when the thickness of the second p-type contact layer 7b is increased, self-absorption can be prevented because the Al composition ratio is high. . That is, in the p-type contact layer 7, the first p-type contact layer 7a for obtaining ohmic contact and the second p-type for securing the film thickness to such an extent that the element characteristics can be maintained while preventing self-absorption. It consists of two layers with the contact layer 7b.
Hereinafter, the first p-type contact layer 7a and the second p-type contact layer 7b will be described.
The first p-type contact layer 7a has a higher impurity concentration and a lower Al composition ratio than the second p-type contact layer 7b.
[0017]
As such a first p-type contact layer 7a, Al is used. _a Ga _1-a A nitride semiconductor layer containing N (0 ≦ a <0.05, preferably 0 <a <0.01) can be given. When the Al composition ratio is in the above range, it is preferable that even when the p-type impurity concentration is high, impurity inactivation can be prevented and good ohmic contact with the p-electrode can be obtained. In addition, it is preferable that the first p-type contact layer 7a contains an Al composition within the above range in terms of preventing self-absorption, and also in terms of crystallinity. The Al composition ratio of the first p-type contact layer 7a is adjusted to be lower than the Al composition ratio of the second p-type contact layer 7b within the above range.
[0018]
The p-type impurity concentration of the first p-type contact layer 7a is not particularly limited, but is preferably such that good ohmic contact with the p-electrode can be obtained. For example, specifically, 1 × 10 ¹⁹ ~ 1x10 ²² / Cm ³ , Preferably 5 × 10 ²⁰ ~ 5x10 ²¹ / Cm ³ It is. When the p-type impurity concentration is in the above range, ohmic contact can be favorably obtained, which is preferable. The p-type impurity concentration of the first p-type contact layer 7a is adjusted to be higher than the impurity concentration of the second p-type contact layer 7b within the above range.
Further, the film thickness of the first p-type contact layer 7a is not particularly limited, but may be a film thickness that allows good ohmic contact with the p-electrode. For example, specifically, the film thickness is 100 to 500 angstroms. , Preferably 150 to 300 angstroms. With such a film thickness, even if the Al composition ratio is lowered, the film thickness is relatively thin, so that self-absorption can be prevented, and furthermore, a high ohmic contact is obtained by containing a high concentration of p-type impurities. preferable.
[0019]
Next, as the second p-type contact layer 7b, Al _b Ga _1-b It is a nitride semiconductor layer containing N (0 <b <0.1, preferably 0.01 <b <0.05). When the Al composition ratio is in the above range, it is preferable from the viewpoint of preventing self-absorption as in the case of the n-type contact layer, and more preferable from the viewpoint of crystallinity and ohmic contact.
The Al composition ratio of the second p-type contact layer 7b is adjusted to be higher than that of the first p-type contact layer 7a within the above range.
The p-type impurity concentration of the second p-type contact layer 7b is desirably contained to such an extent that the second p-type contact layer 7b exhibits p-type. For example, specifically, 1 × 10 ²⁰ / Cm ³ Or less, preferably 5 × 10 ¹⁸ ~ 5x10 ¹⁹ / Cm ³ It is. A p-type impurity concentration in the above range is preferable in terms of preventing self-absorption due to formation of impurity levels. The p-type impurity concentration of the second p-type contact layer 7b is adjusted to be lower than that of the first p-type contact layer 7a within the above range.
[0020]
The thickness of the second p-type contact layer 7b is not particularly limited, but the second p-type contact layer 7b is 400 to 1200 angstrom, preferably 800 to 1200 angstrom. It is preferable that the film thickness of the second p-type contact layer 7b be in the above range in that the film thickness can be adjusted to maintain the element characteristics while preventing self-absorption.
Although it does not specifically limit as a p-type impurity contained in the p-type contact layer 7, For example, Preferably Mg is mentioned.
[0021]
Here, as described above, the first p-type contact layer 7a and the second p-type contact layer 7b constituting the p-type contact layer 7 have portions where the impurity concentration and Al composition ratio overlap. However, it is preferable that the first p-type contact layer 7a is adjusted so that the impurity concentration is higher and the Al composition ratio is lower than the second p-type contact layer 7b within the ranges described. Effects can be obtained.
If the p-type contact layer 7 is formed of only a layer having a low Al composition ratio or only a layer having a high impurity concentration, the ohmic contact is good, but the prevention of self-absorption is not satisfactory enough. It becomes difficult to obtain both the ohmic contact and the prevention of self-absorption.
Further, the p-type contact layer 7 can obtain a good ohmic contact with the first p-type contact layer 7a having a high p-type impurity concentration and a small Al composition ratio, and a second p-type impurity concentration is low and the Al composition ratio is high The p-type contact layer 7b prevents self-absorption and maintains the device characteristics so that the ohmic contact and self-absorption can be prevented by the respective layers. Then, the first p-type contact layer 7a and the second p-type contact layer 7b act synergistically to obtain a device having a good light emission output by preventing good ohmic contact and self-absorption. it can.
Further, in the current nitride semiconductor, it is more difficult to form p-type than n-type, and it is important to prevent good ohmic contact on the p-side and self-absorption on the p-side by p-side emission in terms of device structure. .
[0022]
Furthermore, the Al composition ratio of the n-type contact layer 3 is specified as described above, preferably the impurity concentration is specified in addition to the Al composition ratio, and in addition, the p-type contact layer 7 has a p-type impurity concentration. Combining the formation of the first p-type contact layer 7a having a high Al composition ratio and the second p-type contact layer 7b having a low p-type impurity concentration and a high Al composition ratio causes a reduction in ohmic contact. Self-absorption can be prevented more favorably, and it is more preferable in terms of improving the light emission output.
[0023]
Further, other layers constituting the element will be described below.
[Substrate 1]
In the present invention, as the substrate 1, sapphire whose main surface is a sapphire C-plane, R-plane or A-plane, or other spinel (MgA1 ₂ O ₄ In addition to an insulating substrate such as SiC), a semiconductor substrate such as SiC (including 6H, 4H, and 3C), Si, ZnO, GaAs, and GaN can be used.
[0024]
[Buffer layer 2]
In the present invention, the buffer layer 2 includes Ga _h Al _1-h A nitride semiconductor composed of N (where h is in the range of 0 <h ≦ 1). Preferably, the composition with a smaller proportion of Al exhibits a marked improvement in crystallinity, more preferably a buffer layer 2 composed of GaN. Is mentioned.
The film thickness of the buffer layer 2 is adjusted to a range of 0.002 to 0.5 μm, preferably 0.005 to 0.2 μm, and more preferably 0.01 to 0.02 μm. When the film thickness of the buffer layer 2 is in the above range, the crystal morphology of the nitride semiconductor becomes good, and the crystallinity of the nitride semiconductor grown on the buffer layer 2 is improved.
The growth temperature of the buffer layer 2 is 200-900 degreeC, Preferably it adjusts in the range of 400-800 degreeC. When the growth temperature is in the above range, favorable polycrystals are obtained, and the crystallinity of the nitride semiconductor grown on the buffer layer 2 as seed crystals is preferable.
The buffer layer 2 grown at such a low temperature may be omitted depending on the type of substrate, the growth method, and the like.
[0025]
[N-type contact layer 3]
A nitride semiconductor comprising AlGaN containing the n-type impurity.
[0026]
[N-type cladding layer 4]
In the present invention, the n-type cladding layer 4 has a composition larger than the band gap energy of the active layer 5 and is not particularly limited as long as carriers can be confined in the active layer 5. _e Ga _1-e N (0 <e <0.3, preferably 0.1 <e <0.2). When the n-type cladding layer is made of such AlGaN, it is preferable in terms of confinement of carriers in the active layer.
The thickness of the n-type cladding layer is not particularly limited, but is preferably 0.01 to 0.1 μm, more preferably 0.03 to 0.06 μm.
The n-type impurity concentration of the n-type cladding layer is not particularly limited, but is preferably 1 × 10. ¹⁷ ~ 1x10 ²⁰ / Cm ³ And more preferably 1 × 10 ¹⁸ ~ 1x10 ¹⁹ / Cm ³ It is. The impurity concentration within this range is preferable in terms of resistivity and crystallinity.
[0027]
The n-type cladding layer may be a multilayer film (including a superlattice structure) in addition to the single layer as described above. In the case of a multilayer film layer, the above Al _e Ga _1-e A multilayer film layer composed of N and a nitride semiconductor layer having a smaller band gap energy may be used. For example, a layer having a smaller band gap energy may include In _k Ga _1-k N (0 ≦ k <1), Al _j Ga _1-j N (0 ≦ j <1, e> j). The thickness of each layer forming the multilayer film layer is not particularly limited, but in the case of a superlattice structure, the thickness of one layer is 100 angstroms or less, preferably 70 angstroms or less, more preferably 10 to 40 angstroms. In the case of a single layer that does not form a structure, it can be a layer having the above composition.
In addition, when the n-type cladding layer is a multilayer film composed of a layer having a large band gap energy and a layer having a low band gap energy, at least one of the layer having a large band gap energy and the layer having a small band gap energy is doped with an n type impurity. You may let them. In addition, when doping both the layer having a large band gap energy and the layer having a small band gap energy, the doping amount may be the same or different.
[0028]
[Active layer 5]
In the present invention, the active layer 5 includes a nitride semiconductor having a composition such that the emission peak wavelength is 370 nm or less. Preferably In _g Ga _1-g N (0 ≦ g <0.1) nitride semiconductors may be mentioned. The In composition ratio of the active layer is decreased as the emission peak wavelength becomes shorter, but the In composition ratio is almost zero.
Although it does not specifically limit as a film thickness of an active layer, The film thickness of the grade by which a quantum effect is acquired is mentioned, for example, Preferably it is 0.001-0.01 micrometer, More preferably, it is 0.003-0.007 micrometer. is there. A film thickness in the above range is preferable in terms of light emission output.
In addition to the single quantum well structure as described above, the active layer includes the In layer. _g Ga _1-g A multi-quantum well structure may be used in which N is a well layer and a barrier layer having a composition having a larger band gap energy than the well layer.
The active layer may be doped with impurities.
[0029]
In addition, the adjustment of the In composition ratio of the active layer is not particularly limited as long as the emission peak wavelength is 370 nm or less, and the specific value is obtained, for example, from the following theoretical value calculation formula. Can be cited as an approximate value. However, since the wavelength obtained by actually emitting light forms a quantum level having a quantum well structure, the energy (Eλ) of the wavelength becomes larger than the band gap energy (Eg) of InGaN. There is a tendency to shift to a shorter wavelength side than the required emission wavelength.
[0030]
[Calculation formula of theoretical value]
Eg = (1-χ) 3.40 + 1.95χ-Bχ (1-χ)
Wavelength (nm) = 1240 / Eg
Eg: Band gap energy of InGaN well layer
χ: In composition ratio
3.40 (eV): Band gap energy of GaN
1.95 (eV): InN band gap energy
B: Shows the bowing parameter, 1-6 eV. The Boeing parameter fluctuates in this way in recent studies, from SIMS analysis and the like, which was conventionally assumed to be 1 eV on the assumption that the crystal is not distorted. This is because it has become clear that the degree of distortion differs depending on the above and the like, and becomes 1 eV or more.
[0031]
Although there is a slight difference between the oscillation wavelength considered from the specific In composition ratio obtained from SIMS analysis of the well layer as described above and the oscillation wavelength when actually oscillated, the actual oscillation wavelength is The wavelength is adjusted to a desired wavelength.
[0032]
[P-type cladding layer 6]
In the present invention, the p-type cladding layer 6 is not particularly limited as long as it has a composition larger than the band gap energy of the active layer 5 and can confine carriers in the active layer 5. _f Ga _1-f N (0 <f <0.4, preferably 0.15 <f <0.3). When the p-type cladding layer is made of such AlGaN, it is preferable in terms of confinement of carriers in the active layer.
The film thickness of the p-type cladding layer is not particularly limited, but is preferably 0.01 to 0.15 μm, more preferably 0.04 to 0.08 μm.
The p-type impurity concentration of the p-type cladding layer is not particularly limited, but preferably 1 × 10. ¹⁸ ~ 1x10 ²¹ / Cm ³ And more preferably 1 × 10 ¹⁹ ~ 5x10 ²⁰ / Cm ³ It is. A p-type impurity concentration in the above range is preferable in terms of reducing bulk resistance without reducing crystallinity.
[0033]
In addition to the single layer as described above, the p-type cladding layer may be a multilayer film layer (including a superlattice structure). In the case of a multilayer film layer, the above Al _f Ga _1-f A multilayer film layer composed of N and a nitride semiconductor layer having a lower band gap energy may be used. _k Ga _1-k N (0 ≦ k <1), Al _j Ga _1-j N (0 ≦ j <1, f> j). The thickness of each layer forming the multilayer film layer is not particularly limited, but in the case of a superlattice structure, the thickness of one layer is 100 angstroms or less, preferably 70 angstroms or less, more preferably 10 to 40 angstroms. In the case of a single layer that does not form a structure, it can be a layer having the above composition.
In addition, when the p-type cladding layer is a multilayer film composed of a layer having a large band gap energy and a layer having a small band gap energy, at least one of the layer having a large band gap energy and the layer having a small band gap energy is doped with a p-type impurity. You may let them. In addition, when doping both a layer having a large band gap energy and a layer having a small band gap energy, the doping amount may be the same or different.
[0034]
[P-type contact layer 7]
A nitride semiconductor comprising AlGaN containing the p-type impurity.
[0035]
In the present invention, various materials can be used for the p electrode and the n electrode, which are appropriately selected from known electrode materials and the like. Specific examples of the electrode include those described in Examples described later.
Further, the present invention has been described with reference to FIG. 1 as an embodiment of the element structure. However, the emission peak wavelength is 370 nm or less, and the above-described specific n-type contact layer and p-type contact layer of the present invention are used. If desired, the effects of the present invention can be obtained. In addition to FIG. 1, other layers may be formed in order to improve device characteristics such as electrostatic withstand voltage, forward voltage, and life characteristics.
[0036]
In addition, the element of the present invention is subjected to an annealing treatment in order to make the p-side layer p-type to have a low resistance. As an annealing treatment, as described in Japanese Patent No. 2540791, after a gallium nitride compound semiconductor doped with a p-type impurity is grown by a vapor phase growth method, the atmosphere is substantially free of hydrogen. There is a method in which a heat treatment is performed at a temperature of 400 ° C. or higher, and hydrogen is extracted from a gallium nitride compound semiconductor doped with a p-type impurity to obtain a p-type.
[0037]
【Example】
Hereinafter, the present invention will be described in more detail with reference to examples which are embodiments of the present invention. However, the present invention is not limited to this.
[0038]
[Example 1]
In Example 1, the nitride semiconductor device of FIG. 1 is produced.
(Substrate 1)
The surface of the substrate 1 made of sapphire (C surface) is cleaned at 1050 ° C. in a hydrogen atmosphere in a reaction vessel.
[0039]
(Buffer layer 2)
Subsequently, a buffer layer 2 made of GaN is grown on the substrate 1 at a film thickness of about 200 Å using ammonia and TMG (trimethylgallium) at 510 ° C. in a hydrogen atmosphere.
[0040]
(N-type contact layer 3)
Next, at 1050 ° C., TMG, TMA (trimethylaluminum), ammonia, silane (SiH ₄ ) And Si is 5 × 10 ¹⁸ / Cm ³ Doped n-type Al _0.04 Ga _0.96 An n-type contact layer 3 made of N is grown to a thickness of 4 μm.
[0041]
(N-type cladding layer 4)
Next, using TMG, TMA, ammonia, and silane at 1050 ° C., Si was 5 × 10 ¹⁷ / Cm ³ Doped n-type Al _0.18 Ga _0.82 An n-type cladding layer 4 made of N is formed with a thickness of 400 angstroms.
[0042]
(Active layer 5)
Next, an active layer made of undoped InGaN is grown to a thickness of 55 Å using TMI, TMG, and ammonia at 700 ° C. in a nitrogen atmosphere. The In composition ratio is so small (almost zero) that it cannot be measured.
[0043]
(P-type cladding layer 6)
Next, in a hydrogen atmosphere at 1050 ° C., TMG, TMA, ammonia, Cp ₂ Mg (cyclopentadienylmagnesium) is used and Mg is 1 × 10 ²⁰ / Cm ³ Doped Al _0.2 Ga _0.8 A p-type cladding layer 6 made of N is grown to a thickness of 600 angstroms.
[0044]
(P-type contact layer 7)
Subsequently, on the p-type cladding layer 6, TMG, TMA, ammonia, Cp ₂ Using Mg, Mg is 1 × 10 ¹⁹ / Cm ³ Doped Al _0.04 Ga _0.96 A second p-type contact layer 7b made of N is grown to a thickness of 0.1 μm, and then the gas flow rate is adjusted to make Mg 2 × 10 6. ²¹ / Cm ³ Doped Al _0.01 Ga _0.99 A second p-type contact layer 7b made of N is grown to a thickness of 0.02 μm.
[0045]
After completion of the growth, the wafer is annealed at 700 ° C. in a nitrogen atmosphere in a nitrogen atmosphere to further reduce the resistance of the p-type layer. Then, a mask having a predetermined shape is formed, and etching is performed from the p-type contact layer side with an RIE (reactive ion etching) apparatus to expose the surface of the n-type contact layer 3 as shown in FIG.
[0046]
After the etching, a translucent p-electrode 8 containing Ni and Au having a thickness of 200 angstroms is formed on almost the entire surface of the first p-type contact layer 7a of the p-type contact layer 7 which is the uppermost layer, and on the p-electrode 8 A p-pad electrode 10 made of Au for bonding is formed to a thickness of 0.2 μm. On the other hand, an n-electrode 9 containing W and Al is formed on the surface of the n-type contact layer 3 exposed by etching. Finally, to protect the surface of the p-electrode 8, SiO ₂ After the insulating film is formed, the wafer is separated by scribing to form a 350 μm square LED element.
[0047]
This LED element has an emission peak wavelength of 370 nm at a forward voltage of 20 mA, Vf of 3.8 V, and an output of 2.0 mW.
The light extraction efficiency of the LED of Example 1 is approximately 2.5 times that of the conventional n-type and p-type contact layers that do not contain Al.
As described above, according to the above element structure, ohmic contact is improved, so that Vf equivalent to the conventional one can be maintained, and further, self-absorption can be prevented, so that the light emission output can be improved.
[0048]
【The invention's effect】
As described above, according to the present invention, the n-type contact layer and the p-type contact layer are composed of Al composition ratio and impurity concentration, and the p-type contact layer is composed of two layers. It is possible to provide a nitride semiconductor device which prevents absorption and improves light extraction efficiency and has a good light emission output of 370 nm or less.
Furthermore, in the present invention, the Mg concentration of the n-type and p-type contact layers and the combination with a specific cladding layer make it possible to prevent cracks and the like in addition to good ohmic contact and self-absorption prevention. Light emission output can be obtained.
[Brief description of the drawings]
FIG. 1 is a schematic cross-sectional view of an LED according to an embodiment of the present invention.
[Explanation of symbols]
1 ... Board
2 ... Buffer layer
3 ... n-type contact layer
4 ... n-type cladding layer
5 ... Active layer
6 ... p-type cladding layer
7 ... p-type contact layer
7b. Second p-type contact layer
7a: first p-type contact layer
8 ... p electrode
9 ... n electrode
10 ... Pad electrode

Claims (6)

In a nitride semiconductor device having at least an n-type nitride semiconductor layer, an active layer, and a p-type nitride semiconductor layer on a substrate,
The active layer is composed of a nitride semiconductor layer having an emission peak wavelength of 370 nm or less,
As the p-type nitride semiconductor layer, a p-type contact layer in contact with the p electrode has Al _a Ga _1-a N (0 ≦ a <0.05) containing p-type impurities at a high concentration on the side in contact with the p electrode. And a first p-type contact layer in contact with the first p-type contact layer on the active layer side of the first p-type contact layer, and p-type impurities are introduced from the first p-type contact layer. And a second p-type contact layer containing Al _b Ga _1-b N (0 <b <0.1) which is contained at a low concentration and has an Al composition ratio higher than that of the first p-type contact layer. Being
Further, as the n-type nitride semiconductor layer, the n-type contact layer in contact with the n-electrode comprises Al _d Ga _1-d N (0 <d <0.1), and nitride semiconductor light emission diode. 2. The nitride semiconductor light-emitting diode according to claim 1, wherein the first p-type contact layer in contact with the p-electrode contains 1 × 10 ¹⁹ to 1 × 10 ²² / cm ^{3 of} a p-type impurity. . ^3. The nitride semiconductor light emitting diode according to claim 1, wherein the second p-type contact layer contains a p-type impurity of 1 × 10 ²⁰ / cm ³ or less. The thickness of the first p-type contact layer in the p-type contact layer is 100 to 500 angstroms, and the thickness of the second p-type contact layer is 400 to 2000 angstroms. Item 14. The nitride semiconductor light-emitting diode according to Item 1. The nitride semiconductor light-emitting diode according to claim 4, wherein the n-type contact layer contains 1 × 10 ¹⁷ to 1 × 10 ¹⁹ / cm ^{3 of} an n-type impurity. A first nitride semiconductor layer including Al _e Ga _1-e N (0 <e <0.3) between the active layer and the n-type contact layer; 2. The nitride according to claim 1, further comprising a second nitride semiconductor layer containing Al _f Ga _1-f N (0 <f <0.4) between the p-type contact layer and the p-type contact layer. Semiconductor light emitting diode.

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