JP3631157B2 - Ultraviolet light emitting diode - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an ultraviolet light emitting diode.
[0002]
[Prior art]
Conventionally, in a light emitting diode using an InGaN mixed crystal, when the emission wavelength is close to the band gap wavelength of 363 nm of GaN, the emission efficiency is remarkably reduced. Therefore, only a light having a wavelength longer than 380 nm can be practically produced. .
[0003]
Ultraviolet light-emitting diodes using AlGaN mixed crystals can emit ultraviolet light with wavelengths shorter than this, so we started using a light source that replaces the i-line of mercury lamps and an excitation light source for titanium oxide photocatalysts with a band gap of about 350 nm. There are many application fields.
[0004]
Up to now, by using AlGaN mixed crystal for the quantum well layer of the quantum well structure as the light emitting layer, it is possible to emit light even at a wavelength shorter than 360 nm, and the short period mixed crystal superlattice made of AlGaN mixed crystal in the cladding layer It has been shown that a low resistance cladding layer can be realized by using a structure.
[0005]
[Problems to be solved by the invention]
Although low-resistance ultraviolet light-emitting diodes with good monochromaticity have been realized with such a technique, there has been a problem that the light emission efficiency is low.
[0006]
Further, when an AlGaN mixed crystal is directly grown on a SiC substrate, it is difficult to reduce the dislocation density from about 10 ⁸ to 10 ⁹ cm ^−2, which causes an increase in reactive current.
[0007]
Further, in a nitride that is difficult to do p-type impurity doping, a p-type contact layer is formed by doping Mg into a GaN layer that is easily doped. For this reason, the light absorption in the contact layer increases for light emission with a wavelength shorter than 360 nm, which is the band gap wavelength of the GaN layer, or a wavelength close to 360 nm (360 nm to 380 nm), and efficient light extraction is possible. There was a problem of being disturbed.
[0008]
Also, in order to suppress this light absorption, when trying to p-type doping a simple AlGaN mixed crystal having a longer band gap wavelength than the emission wavelength, the acceptor level becomes extremely large at 170 meV or more, so the hole concentration is low, It was very difficult to obtain a low resistance contact resistance, and it was difficult to make it transparent.
[0009]
An object of the present invention is to improve the light emission efficiency of an ultraviolet light emitting diode without significantly impairing the monochromaticity and low resistance of the emission spectrum.
[0010]
[Means for Solving the Problems]
In order to solve the above problems, the present invention provides a cladding layer having a short-period superlattice structure having an average composition of Al _x Ga _1-x N (x> 0.1), Al _y Ga _1-y N (x> In the ultraviolet light emitting diode having a quantum well barrier layer made of y> 0) and a quantum well layer made of Al _z Ga _1-z N (y>z> 0), the n-type cladding layer and the quantum well An n-type blocking layer made of n-type Al _{x ′} Ga _{1-x ′} N (x ′ > x + 0.1) and a p-type clad layer between the barrier layer and the quantum well structure made of the quantum well layer And a p-type blocking layer made of p-type Al _{x ″} Ga _{1-x ″} N (x ″ > x + 0.1), the quantum well layer being the only one And the only quantum well layer is unevenly distributed on the p-type blocking layer side. Thus, by providing a current blocking layer having a high Al composition between the cladding layer of the ultraviolet light emitting diode based on AlGaN mixed crystal and the light emitting layer having the quantum well structure, the overflow of carriers is suppressed and the light emission efficiency is improved. Can be improved. In addition, by using a short-period mixed crystal superlattice as a cladding layer, the cladding layer is transparent and low in resistance to the light emitting layer, and light can be extracted with low power and high efficiency. In addition, by making the only quantum well layer unevenly distributed on the p-type blocking layer side, an element having excellent monochromaticity can be realized without the emission spectrum extending to a long wavelength.
The p-type blocking layer may further include a p-type contact layer made of a short-period mixed crystal superlattice made of an AlGaN mixed crystal having two types of Al composition on the opposite side of the quantum well structure. By using the short-period mixed crystal superlattice for the p-type contact layer in this way, the p-type contact layer is transparent and low in resistance to the light emitting layer, and light can be extracted efficiently with low power.
The substrate of the ultraviolet light emitting diode further includes a GaN substrate. By using the GaN substrate in this manner, the internal quantum efficiency of the light emitting layer itself can be improved.
Further, a SiC substrate is used as the substrate of the ultraviolet light emitting diode, and a GaN buffer layer is further provided on the SiC substrate. By using the GaN buffer layer in this way, the internal quantum efficiency of the light emitting layer itself can be improved.
[0019]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. In the drawings described below, components having the same function are denoted by the same reference numerals, and repeated description thereof is omitted.
[0020]
Embodiment 1
FIG. 1A is a diagram showing an ultraviolet light-emitting diode having the blocking layer of Embodiment 1 of the present invention and having a single quantum well layer that is unevenly distributed on the p side of the quantum well structure, and FIG. The figure which shows the ultraviolet light emitting diode of the conventional single quantum well structure which does not have, (C) is a figure which shows the blocking layer of the reference example 1 of this invention, and has a quantum well layer in five layers, (D) It is a figure which shows the ultraviolet light emitting diode which has the blocking layer of the reference example 2 of this invention, and has arrange | positioned the single quantum well layer in the center of the quantum well structure.
[0021]
1 is a quantum well structure, 2 is a quantum well layer of the quantum well structure 1, 3 is a barrier layer (barrier layer) of the quantum well structure 1, 4 is an n-type cladding layer, 5 is a p-type cladding layer, and 6 is an n-type blocking layer Layer 7 is a p-type blocking layer.
[0022]
These elements were produced by forming a nitride semiconductor multilayer structure on the Si surface of the SiC substrate (0001) by using a metal organic chemical vapor deposition (MOVPE) method. First, an n-type Al _x Ga _1-x N (x> 0.1, x≈0.18) layer is formed on an n-type SiC substrate at a growth pressure of 300 Torr (40,000 Pa) with a thickness of about 500 nm and a period of 3 nm. An n-type mixed crystal superlattice structure (SPANL: Al _0.16 Ga _0.84 N / Al _0.2 Ga _0.8 N, average composition Al _0.18 Ga _0.82 N) is grown to a thickness of about 250 nm. The n-type cladding layer 4 is formed, and the n-type Al _{x ′} Ga _{1−x ′} N (x ′> x + 0.1, x′≈0.3) blocking layer 6 has a thickness of about 20 nm and the n-type Al _z Ga. _1-zN (y>z> 0, z≈0.06) quantum well layer 2 and quantum composed of n-type Al _y Ga _1-yN (x>y> 0, y≈0.12) barrier layer 3 well structure 1, p-type _{Al x '' Ga 1-x} '' N (x ''> x + 0.1,, x '' ≒ 0.3) blocking layer 7 having a thickness of about 20 m, p-type mixed crystal superlattice period _{3nm (SPASL: Al 0.16 Ga 0.84} N / Al 0.2 Ga 0.8 N, the average composition _Al 0.18 _{Ga 0.82} N) consisting of The p-type cladding layer 5 was laminated with a thickness of about 250 nm, and the p-type GaN contact layer 4 was laminated with a thickness of about 15 nm.
[0023]
Next, a semitransparent electrode made of Pd (upper layer) / Au (lower layer on the substrate side), which is p-type ohmic of about 200 μm × 200 μm square, is formed on the upper surface of the substrate, and a wiring part having a width of about 10 μm and about 50 μm × 50 μm square A lead electrode having a thickness of 200 nm consisting of a pad portion was formed. Thereafter, a Ti (upper layer) / Au (substrate lower layer) electrode was provided on the back surface of the substrate to produce an ultraviolet light emitting diode.
[0024]
FIG. 2 is a graph showing the results of measuring the injection current dependence of the emission intensity of the elements (A), (B), and (C).
[0025]
As is clear from FIG. 2, the output of the device (B) is saturated at a relatively low current, but since there is no blocking layer, electrons and holes pass through the quantum well structure 1 and electrons This is considered to be due to the fact that the holes reach the p-type cladding layer 5 and the holes reach the n-type cladding layer 4, resulting in a recombination current mainly consisting of non-radiative recombination.
[0026]
In the device (C), the output gradient is small. This is because the number of quantum well layers is large, the injection level is low, and the ratio of radiative recombination is higher than that of non-radiative recombination. This is probably due to the decrease.
[0027]
FIG. 3 is a diagram showing the results of measuring the emission spectra of the device (A) and the device (D).
[0028]
As is clear from FIG. 3, in the element (D) in which the quantum well layer 2 is in the central portion of the quantum well structure 1, the emission spectrum spreads to a long wavelength and the monochromaticity is inferior. This is because a hole diffusion length is short or a deep level due to a band edge discontinuity, for example, a two-dimensional hole gas is formed at the heterointerface between the p-type blocking layer 7 and the barrier layer 3 of the quantum well structure 1. It is thought that this is the reason, but details are unknown. In the present invention, thus, to place the quantum well layer 2, Ru allowed to localize on the p side of the quantum well structure 1. The "to be unevenly distributed to the p-side of the quantum well structure 1" is to arrange the quantum well layer 2 closer to the p-type blocking layer 7 than the center of the quantum well structure 1.
[0029]
FIG. 4 is a diagram showing light output characteristics of an element using a Pd (thickness 2.5 nm) / Au (thickness 2.5 nm) layer as a semitransparent electrode of the element (A).
[0030]
As is apparent from FIG. 4, this device achieves an output of 1 mW at an injection current of 420 mA. The measurement is performed by directly injecting a current into the element chip under the condition that no resin sealing or a reflecting mirror is provided. This is an extremely high output for an AlGaN-based ultraviolet light-emitting diode, which is an order of magnitude higher than the conventional report example. Further, the internal quantum efficiency estimated from the differential efficiency is approximately 10%, and it is possible to further increase the output by applying resin sealing or a reflecting mirror.
[0031]
As described above, in the first _{embodiment, Al x Ga 1-x N} (x> 0.1) layer n-type cladding layer 4, p-type cladding layer _{5, Al y Ga 1-y} N (x >Y> 0) In an ultraviolet light emitting diode in which the layer is a barrier layer 2 having a quantum well structure and the Al _z Ga _1-z N (y>z> 0) layer is a quantum well layer having a quantum well structure 1, an n-type cladding layer 4 and the quantum well structure 1, an n-type Al _{x ′} Ga _{1-x ′} N (x ′> x + 0.1) layer is formed as the n-type blocking layer 6, and the p-type cladding layer 5 and the quantum well structure 1 A p-type Al _{x ″} Ga _{1-x ″} N (x ″> x + 0.1) layer is provided as a p-type blocking layer 7 therebetween. Thus, an n-type, p-type current blocking layer having a high Al composition between the n-type, p-type cladding layers 4 and 5 of the ultraviolet light-emitting diode based on an AlGaN mixed crystal and the light-emitting layer made of the quantum well structure 1. By providing 6, 7, it is possible to suppress the overflow of carriers and improve the light emission efficiency.
[0032]
The n-type and p-type cladding layers 4 and 5 are superlattices having an average composition of Al _x Ga _1-x N (x> 0.1). By using the short-period mixed crystal superlattice for the cladding layers 4 and 5 in this way, the cladding layers 4 and 5 are transparent and have low resistance to the light emitting layer, and light can be extracted efficiently with low power. Become.
[0033]
Moreover, by making the quantum well layer 2 of the quantum well structure 1 unevenly distributed on the p side of the quantum well structure 1, an element having excellent monochromaticity can be realized without the emission spectrum extending to a long wavelength.
[0034]
As the substrate of the ultraviolet light emitting diode, it is desirable to use an n-type GaN substrate having a thickness of 100 μm or more and 1 mm or less. By using the GaN substrate in this manner, the internal quantum efficiency of the light emitting layer itself can be improved.
[0035]
Further, as the p-type contact layer, it is possible to form a layer made of an AlGaN mixed crystal having two types of Al compositions and a short-period mixed crystal superlattice having a period of 1 nm or more and 8 nm or less. By using the short-period mixed crystal superlattice for the p-type contact layer in this way, the p-type contact layer becomes transparent and low in resistance to the light emitting layer, and light can be extracted with low power and high efficiency.
[0036]
Further, by using a GaN substrate having a thickness of 30 μm or more and 1 mm or less as the substrate of the ultraviolet light emitting diode, the internal quantum efficiency of the light emitting layer itself can be improved.
[0037]
Moreover, the internal quantum efficiency of the light emitting layer itself can be improved by using a SiC substrate as the substrate of the ultraviolet light emitting diode and providing a GaN buffer layer having a thickness of 150 μm or more and 1 mm or less.
[0038]
Embodiments 2 and 3
5A and 5B are schematic cross-sectional views showing the structures of Embodiments 2 and 3 of the present invention, respectively.
[0039]
5A and 5B, 10 is a GaN substrate, 11 is a GaN buffer layer grown by the MOVPE method, 13 is an n-type mixed crystal AlGaN layer, and 14 is an n-type short-period superlattice Al _x Ga _1-x. N (x> 0.1, Al composition in combination with 16% and 20%, cycle 3 nm) cladding layer formed of layer 15 includes a blocking layer 6 and the quantum well structure 1 of FIG. 1 (a) structure , 16 is a p-type short-period superlattice Al _x Ga _1-x N (x> 0.1, Al composition is a combination of 16% and 20%, period 3 nm), and 17 is two types of Al A contact layer made of an AlGaN mixed crystal having a composition and made of a p-type short-period mixed crystal superlattice having a period of 1 nm to 8 nm, 18 an n-type ohmic electrode made of Al and Au, and 19 a translucent p made of Pd and Au. Type ohmic electrode In FIG. 5 (B), 20 is a SiC substrate (Si face), 21 is an n-type AlGaN wetting layer formed by high temperature growth. The emission wavelength of the present ultraviolet light emitting diode is 200 nm or more and 380 nm or less.
[0040]
These elements were fabricated by forming a nitride semiconductor multilayer structure on each of the substrates 10 and 20 using the MOVPE method in the same manner as in the first embodiment.
[0041]
FIG. 6 is a diagram showing the results of measuring the injection current dependence of the light emission intensity of the device of FIG. 5A fabricated on the GaN substrate 10.
[0042]
As is apparent from the comparison with FIG. 4, the emission intensity is improved by about one digit. Light that propagates downward from the light-emitting layer (quantum well structure) and is absorbed by the substrate 10, light that is reflected off the air-semiconductor interface on the surface side and is not emitted to the outside, and is shaded by electrode pads and electrode needles When the light that cannot reach the measuring device is taken into consideration, it can be seen that the internal quantum efficiency is close to 100%. In addition, despite the large injection current of 400 mA, the applied voltage is less than 6 V, so that the low dislocation substrate reduces non-radiative recombination and improves luminous efficiency, and p-type short-period mixed crystal It is clear that the superlattice contact layer 17 has high transparency and low resistance.
[0043]
Further, in the device in which the AlGaN layer is directly grown on the SiC substrate as in the first embodiment, the light emission efficiency is lower by one digit or more than the diode using InGaN or GaN as the light emitting layer. Conversely, it is clear that a diode having InGaN or GaN as a light-emitting layer can achieve better internal quantum efficiency of the light-emitting layer than a device shown in FIG. 5A when a GaN substrate is used. Further, in the structure of FIG. 5A, the cladding layers 14 and 16 and the contact layer 17 are transparent to light having a wavelength of 360 nm or more, and also have a low resistance. By applying to a diode having InGaN with a small composition, it is possible to remarkably improve the light emission efficiency in the wavelength range of 360 nm to 380 nm, which has been difficult in the past.
[0044]
As described above, in the second and third embodiments, the emission wavelength of the ultraviolet light emitting diode is 200 nm or more and 380 nm or less, is composed of an AlGaN mixed crystal having two types of Al compositions, and is mixed with a short period of 1 nm or more and 8 nm or less. A p-type contact layer 17 made of a crystal superlattice is provided. By using the short-period mixed crystal superlattice for the p-type contact layer 17 in this way, the p-type contact layer 17 becomes transparent and low in resistance to the light emitting layer, and it is possible to extract light with low power and high efficiency. .
[0045]
The quantum well layer having the quantum well structure contains at least Ga, In, and N, or Ga, In, Al, and N.
[0046]
In addition, by using a GaN substrate having a thickness of 30 μm or more and 1 mm or less as the substrate of the ultraviolet light emitting diode, the internal quantum efficiency of the light emitting layer itself can be improved.
[0047]
Further, by using a SiC substrate as the substrate of the ultraviolet light emitting diode and providing the GaN buffer layer 11 having a thickness of 150 μm or more and 1 mm or less, the internal quantum efficiency of the light emitting layer itself can be improved.
[0048]
Although the present invention has been specifically described above based on the embodiments, the present invention is not limited to the above-described embodiments, and it is needless to say that various modifications can be made without departing from the scope of the invention. For example, the present invention can be applied to an ultraviolet light emitting diode into which In is introduced or B is introduced. Moreover, it is applicable not only to a light emitting diode but also to a laser structure having a light guide layer. Furthermore, it goes without saying that various changes such as the composition of the substrate and nitride can be made.
[0049]
【The invention's effect】
As described above, according to the present invention, it is possible to improve luminous efficiency by providing a blocking layer in an ultraviolet light emitting diode based on an AlGaN mixed crystal. Further, by using a GaN substrate or a GaN buffer layer, the internal quantum efficiency of the light emitting layer itself can be improved. Furthermore, by using the short period mixed crystal superlattice for the p-type cladding layer and the p-type contact layer, the light emitting layer is transparent and has low resistance, and light can be extracted efficiently with low power.
[Brief description of the drawings]
1A is a diagram showing an ultraviolet light-emitting diode according to Embodiment 1 of the present invention, FIG. 1B is a diagram showing a conventional ultraviolet light-emitting diode, and FIG. 1C is an ultraviolet light-emitting diode according to Reference Example 1 of the present invention; (D) is a figure which shows the ultraviolet light emitting diode of the reference example 2 of this invention.
FIGS. 2A and 2B are graphs showing the results of measuring the injection current dependence of the emission intensity of the elements of (A), (B), and (C).
FIG. 3 is a diagram showing the results of measuring the emission spectra of the element (A) and the element (D).
FIG. 4 is a diagram showing light output characteristics of an element using a Pd / Au layer as a translucent electrode of the element of FIG.
5A and 5B are schematic cross-sectional views showing the structures of Embodiments 2 and 3 of the present invention, respectively.
6 is a graph showing the result of measuring the dependence of the light emission intensity on the injection current of the device of FIG. 5A fabricated on a GaN substrate. FIG.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... Quantum well structure, 2 ... Quantum well layer, 3 ... Barrier layer, 4 ... n-type cladding layer, 5 ... p-type cladding layer, 6 ... n-type blocking layer, 7 ... p-type blocking layer, 10 ... GaN substrate, DESCRIPTION OF SYMBOLS 11 ... GaN buffer layer, 13 ... n-type mixed crystal AlGaN layer, 14 ... n-type short period superlattice AlGaN clad layer, 15 ... Structure including blocking layer and quantum well structure, 16 ... p-type short period superlattice AlGaN clad layer 17 ... p-type short period mixed crystal superlattice contact layer, 18 ... n-type Al / Au ohmic electrode, 19 ... translucent p-type Pd / Au ohmic electrode, 20 ... SiC substrate, 21 ... n-type AlGaN wetting layer.

Claims (4)

A cladding layer having a short-period superlattice structure having an average composition of Al _x Ga _1-x N (x>0.1);
A quantum well barrier layer made of Al _y Ga _1-y N (x>y>0);
In an ultraviolet light emitting diode having a quantum well layer made of Al _z Ga _1-z N (y>z> 0),
n made of n-type Al _{x ′} Ga _{1-x ′} N (x ′> x + 0.1) between the n-type cladding layer and the quantum well structure composed of the quantum well barrier layer and the quantum well layer A mold blocking layer;
a p-type blocking layer made of p-type Al _{x ″} Ga _{1-x ″} N (x ″> x + 0.1) between the p-type cladding layer and the quantum well structure;
The quantum well layer is unique,
The ultraviolet light-emitting diode, wherein the only quantum well layer is unevenly distributed on the p-type blocking layer side. The p-type blocking layer further includes a p-type contact layer made of a short-period mixed crystal superlattice made of an AlGaN mixed crystal having two types of Al composition on the opposite side of the quantum well structure. The ultraviolet light emitting diode as described. The ultraviolet light-emitting diode according to claim 1, further comprising a GaN substrate as a substrate of the ultraviolet light-emitting diode. 3. The ultraviolet light emitting diode according to claim 1, wherein a SiC substrate is used as the substrate of the ultraviolet light emitting diode, and a GaN buffer layer is further provided on the SiC substrate.

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