JP2006313944A - Ultraviolet light emitting element - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an ultraviolet light emitting element having a high output and a long lifetime by optimizing the structure of the element in which InGaN is used as a material of its light emitting layer. <P>SOLUTION: Protrusions and recesses 1S are formed on the surface of a crystal substrate S. A GaN-based crystal layer 2 is vapor-grown on the protrusions and recesses directly or through a GaN-based semiconductor low temperature buffer layer 1. The GaN-based crystal layer is grown until it substantially fills insides of the recesses and planarizes the surface. An InGaN crystal layer having a composition which can generate an ultraviolet ray is grown on the planarized surface as a light emitting layer so as to obtain an ultraviolet light emitting element. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a semiconductor light emitting device, and more particularly to a GaN-based ultraviolet light emitting device using InGaN having a composition capable of emitting ultraviolet light as a light emitting layer.

  In general, in an element using InGaN as a light emitting layer, the proportion of carriers injected into the light emitting layer is trapped in the non-emission center due to localization of carriers due to fluctuations in In composition, resulting in high efficiency. It is explained that luminescence of can be obtained. In a GaN-based light-emitting diode (LED) or GaN-based semiconductor laser (LD), when trying to emit ultraviolet light of 420 nm or less, InGaN (In composition 0.15 or less) is generally used as the material of the light-emitting layer, The structure related to light emission is a single quantum well structure (the so-called DH structure is included because the active layer is thin) or a multiple quantum well structure.

  However, since the ultraviolet light emitting element has a shorter wavelength than the blue / green light emitting element having a light emitting layer having a high In composition, it is necessary to reduce the In composition of the light emitting layer. For this reason, the localization effect due to the above-described fluctuation of the In composition is reduced, the ratio of being captured by the non-radiative recombination centers is increased, and as a result, high output is hindered. Under such circumstances, the reduction of dislocation density that causes non-radiative recombination centers has been actively performed. As a method for reducing the dislocation density, there is an ELO method (lateral growth method), which has achieved higher output and longer life by lowering the dislocation (Jpn. J. Appl. Phys. 39 (2000) pp. L647).

In the GaN-based light emitting device, the light emitting layer (well layer) is sandwiched between clad layers (barrier layers) made of a material having a larger band gap. According to the literature (written by Hiroo Yonezu, published by Engineering Books Co., Ltd., “Optical Communication Device Engineering”, page 72), there is a guideline for setting the band gap difference to “0.3 eV” or more.
From the above background, when InGaN having a composition capable of emitting ultraviolet rays is used for the light emitting layer (well layer), a clad layer sandwiching the light emitting layer in consideration of carrier confinement (in the quantum well structure, not only the cladding layer but also the barrier layer is included) In this case, AlGaN having a large band gap is used.

  Further, when a quantum well structure is formed, the barrier layer needs to be thick enough to generate a tunnel effect, and generally has a thickness of about 3 to 6 nm.

For example, FIG. 3 is a diagram showing an example of a conventional light emitting diode using In _0.05 Ga _0.95 N as a light emitting layer material. The n type GaN contact layer 102 is formed on the crystal substrate S 10 via the buffer layer 101. N-type Al _0.1 Ga _0.9 N cladding layer 103, In _0.05 Ga _0.95 N well layer (light emitting layer) 104, p-type Al _0.2 Ga _0.8 N cladding layer 105, and p-type GaN contact layer 106 are sequentially stacked by crystal growth, This has an element structure in which a lower electrode (usually an n-type electrode) P10 and an upper electrode (usually a p-type electrode) P20 are provided.

  However, the ELO method requires a method of growing a GaN layer as a base, forming a mask layer, and regrowth, and requires a number of times of growth, resulting in a problem that the number of processes is very large. In addition, since the regrowth interface exists, although the dislocation density is reduced, there is a problem that the output is not easily improved.

In addition, when the present inventors examined a conventional device structure in order to further increase the output of an ultraviolet light emitting device in which the material of the light emitting layer is InGaN, the AlGaN layer gives distortion caused by the lattice constant difference to the InGaN light emitting layer. I found out that it was the basis.
In addition, when the barrier layer thickness is reduced in the quantum well structure, Mg diffuses from the p-type layer provided on the light emitting layer to the light emitting layer and forms a non-light emitting center, so that a high output ultraviolet light emitting element cannot be obtained. there were.

  An object of the present invention is to provide an ultraviolet light emitting device with higher output and longer life by solving the above problems and optimizing the structure of the ultraviolet light emitting device using InGaN as the material of the light emitting layer.

The present invention has the following features.
(1) A GaN-based crystal layer is vapor-phase-grown on a crystal substrate whose surface is roughened via a low-temperature buffer layer made of a GaN-based semiconductor, or directly on the GaN-based crystal. An ultraviolet light emitting device characterized by having a semiconductor light emitting device structure in which an InGaN crystal layer having a composition capable of generating ultraviolet rays is grown to be a light emitting layer.
(2) The above (1), wherein the GaN-based crystal layer grown on the crystal substrate through the low-temperature buffer layer is grown while forming a facet structure from the concave bottom surface and convex top surface of the concave and convex portions. UV light emitting element.
(3) The unevenness processed on the surface of the crystal substrate is an unevenness that exhibits a stripe pattern, and the longitudinal direction of the stripe is the <11-20> direction of the GaN crystal grown thereon, or <1-100 > The ultraviolet light-emitting device according to (1) or (2), which is in the direction.
(4) The ultraviolet light-emitting device according to any one of (1) to (3), wherein the light-emitting layer has a quantum well structure including a well layer made of InGaN and a barrier layer made of GaN.
(5) The ultraviolet light-emitting device according to any one of (1) to (4), wherein the layers between the quantum well structure and the low-temperature buffer layer are all made of GaN crystals.
(6) The ultraviolet light-emitting device according to any one of (1) to (5), wherein the barrier layer has a thickness of 6 nm to 30 nm.

  As described above, in a GaN-based ultraviolet light-emitting device, dislocation reduction is achieved by producing a crystal structure on a concavo-convex processed substrate by a single growth, and an n-type cladding layer (also a barrier layer in a quantum well structure) By using GaN as the material, strain can be reduced, and as a preferable aspect in the MQW structure, the thickness of the barrier layer is limited, thereby improving the light emission output of the device and extending the lifetime. .

  The ultraviolet light-emitting element according to the present invention may be an LED, an LD, or the like. Hereinafter, the present invention will be described using the configuration of the LED as an example. Moreover, the structure of the part which concerns on light emission should just be a structure which can light-emit, such as a quantum well structure. In this specification, the quantum well structure refers to an SQW (single quantum well) structure or an MQW (multiple quantum well) structure, and may be a stack of SQW structures.

The GaN-based semiconductor is a compound semiconductor represented by In _X Ga _Y Al _Z N (0 ≦ X ≦ 1, 0 ≦ Y ≦ 1, 0 ≦ Z ≦ 1, X + Y + Z = 1). For example, AlN, GaN AlGaN, InGaN, etc. are mentioned as important compounds.

A case where the structural portion related to light emission has an MQW structure will be described as an example.
FIG. 1 is a diagram showing an example of the structure of a light emitting device according to the present invention.
An unevenness S1 is processed on the surface of the crystal substrate S, and the GaN-based crystal layer 2 is vapor-phase grown on the unevenness S1 via the low-temperature buffer layer 1 made of a GaN-based semiconductor or directly. In the example of the figure, the GaN-based crystal layer 2 is a layer made of GaN crystal. First, the undoped GaN crystal layer 2a fills the concave portion of the concave / convex S1 on the substrate surface and fills the concave / convex portion with a flat surface. In this mode, the n-type GaN layer 2b is grown thereon. Moreover, in the example of the figure, the n-type GaN layer 2b is an n-type contact layer and also serves as a barrier layer of the MQW structure. The MQW structure is grown in the order of an InGaN well layer, a GaN barrier layer, an InGaN well layer, and a GaN barrier layer, followed by a p-type AlGaN cladding layer 4 and a p-type GaN contact layer 5. Furthermore, the n-type electrode P1 and the p-type electrode P2 are formed, and the GaN-based LED capable of emitting ultraviolet light according to the present invention is obtained.

With the above configuration, it is possible to preferably reduce the dislocation of the GaN-based crystal grown on the crystal substrate. In this configuration, low dislocation is achieved by one growth without using an ELO mask layer.
That is, in the ELO method using a mask, after a GaN film is grown on the base, it is once taken out from the growth apparatus to form a mask, and then returned to the growth apparatus and regrown. On the other hand, in the growth method performed by forming irregularities on the crystal substrate, it is not necessary to stop the growth after setting the crystal substrate on which the irregularities have been processed in the growth apparatus, so that there is no regrowth interface. Good crystallinity can be produced.
Furthermore, in the above configuration according to the present invention, since the GaN-based crystal layer is grown without using a mask, there is no problem of crystal quality degradation due to mask decomposition.
As a result of these actions and effects being able to produce a good crystal with few dislocations, the light emission output is remarkably improved. In addition, as a result of the reduction of dislocation density that causes deterioration, the life can be extended.

A method for growing a GaN-based crystal on a crystal substrate will be described.
In this method, irregularities are processed on the surface of the crystal substrate, and a GaN-based crystal is grown in a vapor phase from the irregularities and / or depressions via the GaN-based low-temperature buffer layer. At this time, the recess may be left as a cavity or may be filled with GaN crystal. However, as will be described later, in order to achieve a preferable low dislocation, it grows while forming a facet structure from both the protrusion and the recess. An embodiment in which the recess is substantially filled is preferable.
According to the concavo-convex embedding method while forming the facet structure as described above, the propagation direction of dislocation lines is controlled in the facet structure portion, and it is possible to grow a GaN-based crystal having a low dislocation density on the crystal substrate. This is a growth method unique to the present invention. This growth method unique to the present invention is referred to as “the embedded growth method” and will be described below. Further, the material for embedding the unevenness may be a GaN-based crystal. However, as will be described later, a case where the material is embedded with GaN most preferable as an element will be described as an example.

  In the burying growth method, as shown in FIG. 2A, the unevenness S1 is processed on the surface of the crystal substrate S, and as shown in FIG. As shown in FIG. 2 (c), GaN crystals 21 and 22 are grown from the concave and convex portions, and filled with GaN crystals without forming the concave portions as shown in FIG. 2 (d). In this method, the unevenness is buried and grown. The growth of the GaN crystal at this time is preferably performed while simultaneously forming the facet structure from both the concave and convex portions, but is not limited thereto, and the GaN crystal may be grown only from the convex surface. . It is effective to have a shape that allows crystal growth exclusively from the upper part of the convex part. “Crystal growth is performed exclusively from the upper part” means a state in which crystal growth can be performed predominantly at the apex or top surface of the convex part and in the vicinity thereof, and growth in the concave part may occur in the initial stage of growth. However, it means that the crystal growth of the convex part becomes dominant in the end. If the low dislocation region is formed by lateral growth starting from the upper part of the convex part, the same effect as the ELO requiring a conventional mask is obtained.

Further, by selecting the shape of the projections and depressions and the growth conditions, as a result, the recesses may be filled with GaN crystals without leaving cavities.
In the following description, a case where facet structure growth of a GaN crystal is caused from both a concave surface and a convex surface will be described as the most preferable mode for reducing dislocation.

  In the buried growth method, as shown in FIG. 2A, the facets can be formed from the beginning of the crystal growth by processing the unevenness S1 on the surface of the crystal substrate S in a state where even the buffer layer or the like is not formed. A ground surface is provided in advance. By providing unevenness on the crystal substrate, when performing vapor phase growth of the GaN crystal on this surface, the concave surface and the convex surface partitioned by the mutual step become a unit reference surface on which facet structure growth is generated.

  The crystal substrate used in the burying growth method is a substrate serving as a base for growing a GaN-based crystal, and a buffer layer for lattice matching is not yet formed. As a preferable crystal substrate, sapphire (C plane, A plane, R plane), SiC (6H, 4H, 3C), GaN, AlN, Si, spinel, ZnO, GaAs, NGO, etc. can be used. Other materials may be used as long as the purpose is met. The plane orientation of the substrate is not particularly limited, and may be a just substrate or a substrate with an off angle.

The irregularities processed on the surface of the crystal substrate are irregularities formed by the surface itself. This is different from the unevenness formed by applying a mask layer made of SiO ₂ or the like used in a conventionally known lateral growth method to a flat surface.

  Examples of the unevenness processing method include, for example, a method in which a normal photolithographic technique is used for patterning according to the desired unevenness mode, and etching is performed using the RIE technique to obtain the desired unevenness. Is done.

The concave / convex arrangement pattern used in the embedding growth method is a pattern in which dot-shaped concave portions (or convex portions) are arranged, and linear or curved concave grooves (or convex ridges) are arranged at regular intervals / indefinite intervals. The striped or concentric pattern etc. which were made are mentioned. A pattern in which convex ridges intersect in a lattice shape can be regarded as a pattern in which dot-shaped (square hole-shaped) concave portions are regularly arranged. In addition, examples of the cross-sectional shape of the unevenness include a rectangular (including trapezoidal) wave shape, a triangular wave shape, a sine curve shape, and the like.
Among these various concavo-convex forms, the striped concavo-convex pattern (rectangular cross-sectional shape) in which linear grooves (or ridges) are arranged at regular intervals can simplify the manufacturing process and Manufacture is easy and preferable.

  When the uneven pattern has a stripe shape, the longitudinal direction of the stripe may be arbitrary, but for a GaN-based crystal that grows by embedding the stripe, when the <11-20> direction is used, lateral growth is suppressed, Diagonal facets such as {1-101} planes are easily formed. As a result, the dislocation propagated in the C-axis direction from the substrate side is bent in the lateral direction at the facet surface, and is difficult to propagate upward, which is particularly preferable in that a low dislocation density region can be formed.

  On the other hand, even when the longitudinal direction of the stripe is set to the <1-100> direction, the same effect as described above can be obtained by selecting a growth condition in which a facet surface is easily formed.

  The preferred dimensions when the concavo-convex cross section has a rectangular wave shape as shown in FIG. 2 (a) are as follows. The width W1 of the concave groove is preferably 0.1 μm to 20 μm, particularly preferably 0.5 μm to 10 μm. The width W2 of the convex part is preferably 0.1 μm to 20 μm, particularly preferably 0.5 μm to 10 μm. The amplitude (concave groove depth) d of the unevenness may be 20% or more of the wider one of the concave and convex portions. These dimensions, the pitches calculated from the dimensions, and the like are the same for the unevenness of other cross-sectional shapes.

Next, as shown in FIG. 2B, the GaN-based low-temperature buffer layer 1 is formed in the concave and convex portions.
The material and formation conditions of the GaN-based low-temperature buffer layer may be referred to known techniques. For example, examples of the buffer layer material include GaN, AlN, InN, and the growth temperature is 300 ° C. to 600 ° C. Can be mentioned. The thickness of the buffer layer is preferably 10 nm to 50 nm, particularly preferably 20 nm to 40 nm. When the substrate has a rectangular corrugated cross section, as shown in FIG. 2B, the bottom surface of the concave portion and the top surface of the convex portion are mainly used. It is preferable to form. As the growth apparatus, an apparatus for growing a GaN crystal layer thereon may be used.
When a substrate made of GaN crystal is used as the crystal substrate, the low temperature buffer layer is not essential.

Next, as shown in FIG. 2C, a GaN crystal is grown at a high temperature. By making the concave and convex surfaces into surfaces capable of facet structure growth, as shown in the figure, GaN crystals 21 and 22 exhibiting convex shapes grow from both concave and convex surfaces at the initial stage of growth.
As a result, dislocation lines extending in the C-axis direction from the crystal substrate are bent laterally at the facet plane (the slopes of the GaN-based crystals 21 and 22 shown in FIG. 2C) and do not propagate upward. Thereafter, the crystals 21 and 22 having the respective convex shapes continue to be united with each other as shown by a one-dot chain line in FIG. 2 (c), and further, the growth surface is flattened as shown in FIG. 2 (d). Thus, a GaN crystal layer 2a (= layer 2a in the element of FIG. 1) is obtained. Near the surface of the GaN crystal layer is a low dislocation density region in which dislocation propagation from the substrate is reduced.

  When embedding the irregularities S1 of the crystal substrate S with GaN crystals, from the point of controlling the crystal growth state, as shown in FIG. 1, after embedding the irregularities with an undoped GaN crystal layer 2a without adding impurities, Although it is preferable to grow the n-type GaN crystal layer 2b, the n-type GaN layer may be grown from the beginning on the buffer layer. Further, the n-type GaN layer may be provided separately for the n-type GaN contact layer and the n-type GaN clad layer by changing the carrier concentration.

  In a conventional GaN crystal growth method via a low-temperature buffer layer, a high-temperature GaN film is grown on a flat sapphire C-plane substrate by a MOVPE method or the like via a low-temperature buffer layer such as GaN. When high-temperature GaN is grown on the low-temperature buffer layer, the GaN is coalesced while growing in the lateral direction using the densely aggregated crystals of the buffer layer as growth nuclei, and eventually becomes flat. However, in the conventional method, since the unevenness is not processed on the substrate surface, the growth proceeds so that a stable C surface appears, and thus the surface is flattened. This is because the lateral growth rate is faster than the stable growth rate of the C-plane.

  On the other hand, in the burying growth method, unevenness is processed on the growth surface of the substrate surface to suppress lateral growth, and in addition, by increasing the growth rate in the C-axis direction, oblique growth such as {1-101} Facets can form.

How facet surfaces are formed in a growing GaN crystal can vary in various ways depending on the combination of the width of the recesses and the width of the protrusions, but this facet surface can bend the propagation of dislocations. As shown in FIG. 2C, the preferred embodiment is that the crystal units 21 and 22 grown from the respective unit reference planes are completely both without having a flat portion at the top. It is an aspect of a mountain shape (a roof shape that is long and continuous in a triangular pyramid or mountain range) in which facet surfaces intersect at the top. With such a facet surface, the dislocation lines inherited from the base surface can be bent almost entirely, and the dislocation density directly above the dislocation line can be further reduced.
Note that the facet surface forming region can be controlled not only by combining the widths of the concave and convex portions but also by changing the depth of the concave portions (the height of the convex portions).

In addition, the formation of the facet plane can be controlled by the growth conditions (gas species, growth pressure, growth temperature, etc.) during crystal growth. When NH ₃ partial pressure is low in vacuum growth {1-101
} Faceted facets are easy to come out, and faceted faces are more likely to come out in normal pressure growth compared to reduced pressure.
Further, when the growth temperature is raised, lateral growth is promoted, but when grown at a low temperature, growth in the C-axis direction is faster than lateral growth, and a facet surface is easily formed.
Although it has been shown that the facet shape can be controlled depending on the growth conditions, it may be properly used according to the purpose as long as the effect of the present invention is obtained.

  In the present invention, not only the above-described buried growth method described above, but also a growth method that leaves the recess as a cavity may be used. For example, Japanese Patent Application Laid-Open No. 2000-106455 discloses a method of growing a gallium nitride based semiconductor so as to provide unevenness on a crystal substrate and leave the recessed portion as a cavity. However, in such a growth method, since the concave portion is not filled and left as a hollow portion, the presence of the hollow portion is disadvantageous in releasing heat generated in the light emitting layer to the substrate side, and promotes thermal degradation. There is a problem to do. Further, the propagation of dislocations is not actively controlled, and there is a technical idea of lateral growth that lowers the dislocation only in the region above the recess, and the dislocation propagates in the region above the protrusion. Therefore, it is more preferable to use the burying growth method from the viewpoint of obtaining a more preferable dislocation density reduction effect while solving the above problems.

  As a method for growing a GaN-based crystal layer on a substrate, HVPE, MOVPE, MBE, or the like is preferable. The HVPE method is preferable when forming a thick film, but the MOVPE method or MBE method is preferable when forming a thin film.

Next, the preferable aspect by this invention is demonstrated.
First, in the first preferred embodiment of the present invention, the material of the GaN-based crystal layer 2 formed on the irregularities of the substrate is limited to GaN crystals. On this GaN crystal layer, an MQW structure having an InGaN crystal layer having a composition capable of generating ultraviolet rays as a well layer is formed as a light emitting layer. In other words, the n-type cladding layer is made of GaN, and there is no AlGaN layer between the light emitting layer and the low temperature buffer layer.

  In this embodiment, while using InGaN having a composition capable of generating ultraviolet rays for the light emitting layer, GaN is used as the n-type cladding layer material without using AlGaN, which is conventionally required. In the present invention, it has been found that the hole confinement can be sufficiently achieved even when the n-type cladding layer is GaN with respect to the ultraviolet light emitting layer. This is presumably because the effective mass of holes injected from the p-type layer is heavy, so that the diffusion length is short and it does not reach the n-type cladding layer sufficiently. Therefore, strictly speaking, it can be said that the n-type GaN layer existing as the lower layer of the InGaN light emitting layer in the configuration of the present invention does not correspond to a conventional cladding layer. By removing AlGaN that was present as a cladding layer between the crystal substrate and the light emitting layer and forming a GaN layer, distortion of the InGaN light emitting layer is reduced.

  When the light emitting layer (well layer) is distorted, the well structure is inclined due to the generation of a piezo electric field due to the distortion, and the overlap of wave functions of electrons and holes is reduced. As a result, the probability of recombination of electrons and holes decreases and the light emission output becomes weak. In order to avoid this, an attempt has been made to cancel the piezoelectric field by doping Si into the MQW structure, but this is not a preferable method because it causes a decrease in crystallinity due to doping. As described above, by eliminating the n-type AlGaN layer, high output can be obtained without such fear.

  Combined with the above-described low dislocation using the unevenness of the substrate and the above-described effect of eliminating AlGaN, the InGaN light-emitting layer is low in dislocation and strain is reduced, and the light emission output and the device life are sufficient. improves.

In the second preferred embodiment of the present invention, the material of the barrier layer in the quantum well structure of the light emitting layer is limited to GaN. As a result, the AlGaN layer is eliminated from between the well layer and the low-temperature buffer layer, distortion of the well layer is suppressed, and higher output and longer life are achieved. In the conventional quantum well structure, AlGaN is used for the barrier layer and the cladding layer in consideration of the confinement of carriers in the well layer.
However, these combinations have the following problems because the optimum value of the crystal growth condition differs greatly between AlGaN and InGaN. AlN has a higher melting point than GaN, and InN has a lower melting point than GaN. Therefore, when the optimum temperature is 1000 ° C. for GaN, InGaN is 1000 ° C. or less, preferably about 600 to 800 ° C., and AlGaN is GaN or more. When AlGaN is used for the barrier layer, the optimum crystal growth conditions are not obtained unless the growth temperature of the AlGaN barrier layer and the InGaN well layer is changed, and there is a problem that the crystal quality is lowered. On the other hand, changing the growth temperature results in interruption of the growth, and the well layer, which is a thin film of about 3 nm, has problems such as the thickness fluctuates due to the etching action and crystal defects enter the surface during the growth interruption. . Because of these trade-off relationships, it is difficult to obtain a high quality product by combining the AlGaN barrier layer and the InGaN well layer. In addition, since the barrier layer is made of AlGaN, there is a problem that the well layer is distorted, which hinders high output. Therefore, in the present invention, GaN was used as the material of the barrier layer, and an attempt was made to reduce the above-mentioned trade-off problem, the crystal quality was improved. In addition, when GaN is used as the n-type cladding layer in order to reduce the distortion, the output can be increased by reducing the distortion. Although it was feared that GaN was used as a clad layer, the carrier confinement would be insufficient for InGaN having a composition capable of emitting ultraviolet rays, but it was found that the carrier (particularly, holes) was confined.

Furthermore, in the third preferred embodiment of the present invention, the thickness of the barrier layer in the MQW structure is limited to 6 nm to 30 nm, preferably 8 nm to 30 nm, and particularly preferably 9 nm to 15 nm. The thickness of the barrier layer in the conventional MQW structure is 3 nm to 7 nm.
When the barrier layer is made thick in this way, there is no overlap of wave functions, and the SQW structure is stacked in multiple layers rather than the MQW structure, but sufficiently high output is achieved. If the barrier layer exceeds 30 nm, holes injected from the p-type layer are trapped by dislocation defects that are non-radiative centers existing in the GaN barrier layer before reaching the well layer, and the luminous efficiency decreases. It is not preferable.

  In addition, the thicker barrier layer reduces the damage because the well layer is less susceptible to heat and gas damage when growing the upper layer, and the dopant material (Mg, etc.) from the p-type layer ) Is reduced in the well layer, and further, the strain and the strain on the well layer are reduced.

Example 1
In this example, as shown in FIG. 1, a GaN-based LED having a DH structure was manufactured, and the layer between the light emitting layer and the crystal substrate was made of only GaN.
Stripe patterning with a photoresist on a C-plane sapphire substrate (width 2 μm, period 4 μm, stripe orientation: the longitudinal direction of the stripe is the <11-20> direction for GaN-based crystals growing on the substrate), and an RIE apparatus Then, the substrate was etched to have a square cross section to a depth of 2 μm to obtain a substrate having a striped pattern as shown in FIG. The aspect ratio of the stripe groove cross section at this time was 1.

  After removing the photoresist, the substrate was mounted on the MOVPE apparatus, and the temperature was raised to 1100 ° C. in a hydrogen atmosphere to perform thermal etching. The temperature was lowered to 500 ° C., trimethylgallium (hereinafter TMG) was flown as a group III raw material, and ammonia was flowed as an N raw material to grow a GaN low temperature buffer layer having a thickness of 30 nm. As shown in FIG. 2B, the GaN low-temperature buffer layer was formed only on the upper surface of the convex portion and the bottom surface of the concave portion.

  Subsequently, the temperature is raised to 1000 ° C., TMG and ammonia are flown as raw materials, and after growing the undoped GaN layer 2a on a flat substrate for a time corresponding to 2 μm, the growth temperature is raised to 1050 ° C. The substrate was grown for a time corresponding to 4 μm. When the growth is performed under these conditions, the growth of the GaN layer 2a at this time becomes a ridge shape including a faceted surface with a cross-sectional chevron shape from the top surface of the convex portion and the bottom surface of the concave portion, as shown in FIG. . Subsequent growth temperature changes promote two-dimensional growth and flatten.

  Subsequently, as shown in FIG. 1, an n-type GaN contact layer (cladding layer) 2b, an InGaN well layer with a thickness of 3 nm (emission wavelength is 380 nm, measurement is difficult because the In composition is close to zero), and a GaN thickness of 6 nm A multi-quantum well layer 3 of three periods comprising a barrier layer, a p-type AlGaN cladding layer 4 with a thickness of 30 nm, a p-type GaN contact layer with a thickness of 50 nm are formed in this order to form an ultraviolet LED wafer with an emission wavelength of 380 nm, Formation and element separation were performed to obtain an LED element.

  Each output of the LED element (bare chip state) collected over the entire wafer, wavelength 380 nm, and current of 20 mA was measured.

For comparison, an ultraviolet LED chip (Comparative Example 1) was formed on a sapphire substrate not subjected to uneven processing under the same conditions as described above, and the output was measured.
In addition, an ultraviolet LED chip (Comparative Example 2) is formed on a normal ELO substrate (on which a GaN layer is once formed on a flat sapphire substrate and then a mask layer is formed) under the same conditions as described above. And the output was measured.

  Table 1 shows the results of measuring the average value of the dislocation density in the LED wafer by cathodoluminescence, and the average value of the output, the lifetime in the accelerated test at 80 ° C. and 20 mA (the time to decrease to 80% of the initial output).

  As is apparent from Table 1, in this example, the dislocation density is reduced, and the life and output are increased. As is clear from the results of Comparative Example 2, the dislocation density can be similarly reduced by the ELO method, which is one of the dislocation density reduction methods, but the output is lower than that of this example. This is considered to be a difference in crystallinity due to the presence of the regrowth interface. In addition, since the dislocation density is large on a normal substrate, the output life is also poor compared to the present embodiment.

Example 2
In this example, under the same conditions as in Example 1 except that an n-type Al _0.1 Ga _0.9 N cladding layer was provided between the n-type GaN contact layer 2b and the InGaN well layer in Example 1. An ultraviolet LED chip was formed and its output was measured.
As shown in Table 1 above, the output of the device of Example 1 was 10 mW, whereas the output of the device of this Example was 7 mW. From this result, the output of the device of this example is improved compared to Comparative Examples 1 and 2, but as in Example 1, by eliminating the AlGaN layer between the InGaN well layer and the crystal substrate, It was revealed that the output was further improved.

Example 3
In this example, an experiment was conducted to examine the limited effects related to the thickness of the barrier layer of the MQW structure.
Example 1 except that the thickness of each barrier layer of the MQW structure in Example 1 was set to Sample 1; 3 nm, Sample 2; 6 nm, Sample 3; 10 nm, Sample 4; 15 nm, Sample 5; Similarly, a GaN LED was manufactured. These all belong to the light emitting device according to the present invention.
The output of the ultraviolet LED chip was measured under the same conditions as described above.

The average value of these measurement results is as follows.
Sample 1; 2 mW
Sample 2; 7 mW
Sample 3; 10 mW
Sample 4: 8 mW
Sample 5: 5 mW

Further, as a result of performing photoluminescence measurement on these samples at a low temperature of 4K, light emission from Mg was observed in the vicinity of 3.2 eV in sample 1. This is considered to be a result of Mg diffusing from the p-type layer because the barrier layer is thin.
As is clear from the above results, it was found that the increase in output is further improved when the thickness of the barrier layer is 6 nm to 30 nm.

It is a schematic diagram which shows the structural example of the ultraviolet light emitting element by this invention. Hatching is performed for the purpose of showing the boundary of the region (the same applies to the following figures). In this invention, it is a schematic diagram which shows the method of embedding the unevenness | corrugation of a board | substrate and growing a GaN crystal layer. The In _0.05 Ga _0.95 N was the material of the light emitting layer is a schematic diagram showing an example of a conventional light emitting diode.

Explanation of symbols

S Crystal substrate S1 Concavity and convexity 1 GaN-based semiconductor low-temperature buffer layer 2 GaN-based crystal layer (especially GaN crystal layer)
3 InGaN crystal layer with composition capable of emitting ultraviolet light 4 p-type cladding layer 5 p-type contact layer

Claims (6)

  A GaN-based crystal layer is vapor-phase-grown on a crystal substrate whose surface has been processed with irregularities via a low-temperature buffer layer made of a GaN-based semiconductor or directly, and ultraviolet rays are irradiated on the GaN-based crystal. An ultraviolet light-emitting element having a semiconductor light-emitting element structure in which an InGaN crystal layer having a composition that can be generated is grown to be a light-emitting layer.   2. The ultraviolet light-emitting device according to claim 1, wherein the GaN-based crystal layer grown on the crystal substrate through the low-temperature buffer layer is grown while forming a facet structure from the concave bottom surface and convex top surface of the concave and convex portions. .   The unevenness processed on the surface of the crystal substrate is an unevenness that exhibits a stripe pattern, and the longitudinal direction of the stripe is the <11-20> direction or the <1-100> direction of the GaN crystal that grows thereon. The ultraviolet light-emitting device according to claim 1 or 2.   The ultraviolet light emitting element according to claim 1, wherein the light emitting layer has a quantum well structure including a well layer made of InGaN and a barrier layer made of GaN.   The ultraviolet light-emitting device according to any one of claims 1 to 4, wherein all layers between the quantum well structure and the low-temperature buffer layer are made of GaN crystals.   The ultraviolet light-emitting device according to claim 1, wherein the barrier layer has a thickness of 6 nm to 30 nm.

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