JPH10125608A - Compound-semiconductor epitaxial wafer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a III group nitride growing layer which exhibits a good crystallizability and good surface conditions which lead to the uniformity of device properties in a compound-semiconductor epitaxial wafer, comprised of the III group nitride layer which is grown on a buffer layer. SOLUTION: A compound-semiconductor epitaxial wafer is comprises of a buffer layer 102, made of III group nitride semiconductor laminated ion the surface of a crystal substrate and an epitaxial layer made of III group nitride semiconductor which is laminated on the buffer layer 102. In this case, the buffer layer 102 is made of a group 113 of single crystal particles which are in the state of as-grown, adhered to each other on their winding sides, and whose tops 115 are approximately flay or curved. The single crystal particles also adhere to reach other with an angle made by laminating directions of the single crystal surface (orientation), equal to or less than 30 deg..

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a compound semiconductor epitaxial wafer having an epitaxial layer made of a group III nitride semiconductor, and more particularly to a compound semiconductor epitaxial wafer having a buffer layer made of an aggregate of single crystal grains of a group III nitride semiconductor. The present invention relates to a compound semiconductor epitaxial wafer.

[0002]

2. Description of the Related Art Conventionally, in a compound semiconductor epitaxial wafer having an epitaxial layer of a group III nitride semiconductor used for a blue LED, sapphire (α-Al
₂ O ₃ single crystal) Al on the substrate _{X Ga Y In 1-XY N} ( provided that when forming an epitaxial layer consisting of 0 ≦ X + Y ≦ 1,0 ≦ X ≦ 1,0 ≦ Y ≦ l), a sapphire substrate In general, a buffer layer (buffer layer) made of Al _a Ga _1-a N (0 ≦ a ≦ 1) is provided thereon (Japanese Unexamined Patent Publication No.
229476). Sapphire as substrate and II
Since there is a lattice mismatch with the epitaxial layer of the group I nitride semiconductor (“Journal of the Japan Society for Crystal Growth”, Vol. 1).
5 (1988), p. 334), the buffer layer is provided mainly for the purpose of reducing lattice mismatch between the substrate and the epitaxial layer. In other words,
The buffer layer is formed on a sapphire substrate by a lattice-mismatch type II.
In forming a Group I nitride semiconductor epitaxial layer, it plays an important role crystallographically, such as reducing deterioration of the surface state or crystal quality of the epitaxial layer due to lattice mismatch.

Hitherto, it has been proposed that the crystal form to be provided in the buffer layer must be “amorphous” (“Journal of the Japan Society for Crystal Growth”, Vol. 15 , No. 3 & No. 3) .
4, 74-82). Furthermore, even though it is an "amorphous" film,
For example, a film in which single crystal grains are mixed therein has been regarded as a preferable buffer layer (Japanese Patent Laid-Open No. 22947/1990).
No. 6). FIG. 1 is an example of a schematic cross-sectional view of a buffer layer having a configuration that has been considered preferable in the related art. The main constituent of the conventional buffer layer (102) is an amorphous body (104), and single crystal grains (105) are sparsely scattered therein. The conventional amorphous buffer layer does not have a large amount of single crystal grains mixed in the amorphous film so that the substrate surface is sufficiently covered with the single crystal grains.

[0004] The amorphous buffer layer is made of Al _x Ga _Y In.
_1-XY N (0 ≦ X + Y ≦ 1, 0 ≦ X ≦ 1, 0 ≦ Y ≦ l)
The formation of a functional layer, such as an active layer, which contributes to the manifestation of device characteristics is performed at a high temperature of about 1000 ° C. or higher, whereas the film is formed at a lower temperature. For example,
Conventionally, a temperature for growing a buffer layer made of amorphous is 200 ° C. or 400 ° C. to 900 ° C.
(Japanese Unexamined Patent Publication Nos.
Or JP-A-7-3121350) or 4
A range from 00 ° C to 800 ° C (see JP-A-6-151962) is disclosed. These days, only 50
A relatively low temperature in the conventionally disclosed temperature range of around 0 ° C. is adopted as the growth temperature of the buffer layer (Applied Physics Letters (App)
l. Phys. Lett. , 68 , no. 20 (199
6), pages 2867-2869). Further, the thickness of the conventional amorphous buffer layer is set to be as thin as about 10 nm to 50 nm (see Japanese Patent Application Laid-Open No. 2-229476).

Looking at the bonding condition at the interface between the conventional amorphous buffer layer configured as described above and a crystal substrate such as sapphire, the surface of the crystal substrate (see (101) in FIG. 1) can be seen. 1 (103)) is mostly covered with an amorphous body (see (104) in FIG. 1). Only when the substrate surface is ideally and sufficiently covered with the amorphous film as described above, the film (buffer layer) becomes the underlayer, and the two-dimensional (planar) surface of the growth layer grown at a high temperature is formed. Na) Growth has been thought to be promoted. FIG. 2 schematically shows a growth pattern conventionally imagined when a group III nitride semiconductor layer is formed at a high temperature on a low temperature buffer layer. The growth island (106) generated by using the single crystal grains (105) scattered in the amorphous film as growth nuclei grows two-dimensionally (in a planar manner) using the amorphous film covering the substrate (101) as a base. I do. As the growth progresses, the growth island (106) having a substantially flat upper portion is mutually connected to the adjacent growth island (106), and eventually becomes a layer. FIG. 3 shows that the amorphous film covering the substrate (101) remains throughout even at the time of film formation at a high temperature and acts as an underlayer, whereby the bonding between the growing islands progresses and the surface becomes flat. FIG. 3 schematically shows an ideal process for forming a layer.

However, in practice, in order to form a group III nitride semiconductor layer on an amorphous buffer layer at a high temperature, the buffer layer is exposed when the amorphous buffer layer is exposed to a high temperature film formation environment. The constituent amorphous body disappears due to sublimation or the like. FIG. 4 schematically shows a change in the state of the buffer layer when shifting to a high-temperature film formation environment. In the region (108) where the amorphous body has disappeared by sublimation or the like, the surface of the crystal substrate (101) is exposed. Some of the slightly remaining amorphous body (104) may become crystallized grains (109). Many of the single crystal grains (105) remain as they are to provide growth nuclei. The generation and growth pattern of a growth island at the time of film formation at a high temperature in the case where a region where the substrate surface is exposed due to the disappearance of the amorphous buffer layer will be described with reference to FIG. Growth island (106) growing with single crystal grain (105) as nucleus
In the region (108) where the crystal substrate is exposed, single crystal grains do not exist and there is no growth nucleus, so the degree of growth of the growth island is small in the first place. Also, in this region (108), the absence of the amorphous body makes it difficult for the growth islands to grow two-dimensionally in the lateral direction, and the growth islands are interconnected enough to provide a continuous film. Does not expand laterally. Rather, the growing islands (106) tend to grow columnar in the vertical (vertical) direction. That is, the growth rate in the vertical direction is higher than that in the horizontal direction, and the growth of columnar crystals is promoted. The vertical direction is the c-axis direction of the GaN crystal lattice in the case of a growth island made of GaN on the sapphire C plane, and the horizontal direction is the a-axis direction of the crystal lattice. On the other hand, in the region where the underlayer is conveniently left, the growth island (106) generated with the single crystal grain (105) as a nucleus grows in the lateral direction in a manner of two-dimensional growth. That is, the growth rate in the horizontal direction is higher than that in the vertical direction. If the growth rate in the lateral direction is high, the bottom surface of the growth island (the contact surface with the substrate) is expanded and the chance of bonding to each other is increased. Therefore, in the region where the buffer layer remains, the surface is flat and continuous. A layer having a texture is easily formed.

As described above, whether or not the crystal substrate surface is covered with the amorphous buffer layer is determined by determining whether the crystal substrate is grown on the buffer layer.
This leads to uneven growth of the group III nitride semiconductor layer. This uneven growth mode not only results in deterioration of the surface state of the group III nitride semiconductor layer, but also hinders acquisition of desired electrical characteristics. FIG. 6 is a cross-sectional view of an epitaxial layer formed of a group III nitride semiconductor layer grown at a high temperature on a buffer layer made of an amorphous material. In particular, FIG. It is a cross section schematic diagram. This epitaxial layer (110) comprises an n-type buffer layer (102) and a p-type first epitaxial layer (1).
11) and an n-type second epitaxial layer (112), and includes a pn junction constituted by the first and second epitaxial layers. Pit (107)
Buffer layer (102) deposited on crystalline substrate (101)
Occur almost corresponding to the region (108) in which the amorphous body constituting the element has disappeared. In the region (108), the lateral growth slows down, and the growth islands composed of columnar crystals are generated as described above. The columnar crystals are isolated and scattered from each other, and therefore have a small chance of coalescing each other, and remain as voids (gap) between the columnar crystals. This void is considered to be the cause of generating pits on the growth layer surface. In such an epitaxial wafer, the buffer layer (10) is formed at the pit (107).
2) and the second epitaxial layer (112) may be joined structurally and electrically. Since the second epitaxial layer (112) and the buffer layer (102) are layers having the same conductivity type, electrical conduction occurs between both layers. In a region where the disappearance of the amorphous film is conveniently avoided, a desired pn junction can be formed with the first and second epitaxial layers. However, in a region where the amorphous film occupying most of the area of the substrate has disappeared, a normal pn junction cannot be provided due to an electric short circuit. That is, non-uniform bonding characteristics are provided. FIG. 7 is an example of a current-voltage characteristic of a pn junction type group III nitride semiconductor light emitting diode (LED) manufactured from an epitaxial layer containing many pits. Since the above-described configuration that causes an electrical short circuit in a region where pits are dense is formed, normal current-voltage characteristics showing rectification cannot be obtained. As described above, in the conventional buffer layer mainly composed of an amorphous body, there is a problem that disappearance of the amorphous buffer layer adversely affects both crystallographically and electrically. .

[0008]

A problem associated with the conventional compound semiconductor epitaxial wafer having a buffer layer made of amorphous is that the amorphous body, which is the main constituent of the buffer layer in a high-temperature environment, has a problem. The surface of the layer to be deposited (substrate) was exposed due to the disappearance. Therefore, by forming a buffer layer that can sufficiently cover the surface of the layer to be deposited by mainly forming components that do not easily disappear even in a high-temperature film-forming environment, the conventional amorphous buffer layer It is considered that the problem can be solved. The amorphous body easily disappears because the bonds between the atoms constituting the amorphous body are weak. Therefore, the buffer layer is required to have a configuration that has a strong bonding force that does not easily cause a bond break even in consideration of thermal energy received under a high-temperature environment. It is well known that a single crystal is one of the crystal forms including a larger bonding force between constituent atoms than an amorphous structure. However, although it is known that a group III nitride semiconductor buffer layer made of a single crystal can be obtained only when the growth temperature is set to a temperature exceeding 900 ° C., a buffer (buffer) layer made of a single crystal is conventionally used. For example, it has been considered that such a material is not suitable as a buffer layer because it does not provide good surface morphology (Japanese Patent Laid-Open No. 4-2970).
No. 23 and JP-A-7-312350).

[0009]

Means for Solving the Problems The present inventors have made intensive studies as a material for a buffer layer covering the substrate surface in view of the superior thermal resistance of a single crystal, and as a result, a conventional single crystal has been obtained. Even under the conditions of the growth temperature, which are not considered, single crystal grains can be easily formed by precisely adjusting the other growth conditions, and if the shape and aggregation form of the single crystal grains are defined, the conventional amorphous state can be obtained. The present inventors have found that a buffer layer composed of an aggregate of single crystal grains excellent in the effect of eliminating the disadvantages associated with the buffer layer composed of a single crystal grain can be formed, and the present invention has been accomplished. That is, the present invention relates to
In a compound semiconductor epitaxial wafer composed of a buffer layer composed of a group III nitride semiconductor and an epitaxial layer composed of a group III nitride semiconductor laminated on the buffer layer, the buffer layer is formed from an aggregate of single crystal grains. Become
The single crystal grains constituting the buffer layer are as-grown.
In the n-state, the side surfaces joined to each other are joined as a detour, and the top plate at the top is formed substantially flat or curved. The present invention is particularly characterized in that the single crystal grains constituting the buffer layer have an as-grown state in which the angle difference between the lamination directions is within 30 degrees.

[0010]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS First, a material constituting a buffer layer according to the present invention will be described. The buffer layer is made of, for example, aluminum (Al), gallium (Ga), indium (I
n) or a group III nitride semiconductor containing nitrogen and at least one group III element such as boron (B).
Examples of the group III nitride semiconductor include gallium nitride (Ga
N), binary compounds such as aluminum nitride (AlN) and indium nitride (InN). In addition, aluminum nitride / gallium mixed crystal (Al _x Ga _{1 -x} N: 0 <x <1
(Hereinafter the same)), gallium nitride-indium mixed crystal (G
a _x In _{1- x} N), aluminum nitride indium mixed crystal _{(Al x In 1-x N} ) 3 -element mixed crystal or aluminum nitride gallium indium such as _{(Al a Ga b In c N} : 0 ≦
a, b, c ≦ 1, a + b + c = 1) and the like. B _d Al _e Ga _f In _g containing boron
N (0 <d ≦ 1, 0 ≦ e, f, g ≦ 1, d + e + f + g
= 1), the buffer layer can be formed. Furthermore, arsenic (element symbol: As) and phosphorus (element symbol: P) other than nitrogen element
The low-temperature buffer layer according to the present invention can also be composed of a group III nitride semiconductor containing a group V element such as. The group III nitride semiconductors containing a plurality of group V elements include the chemical formulas GaNAs, GaNP, AlNAs, AlNP, In
(III) · N _m · V _1-m ((III
) Represents a Group III element. V represents a Group V element other than nitrogen, and 0 <m <1. ), One of the second
There are ternary crystals containing a Group I element and two Group V elements.
Also, AlGaNAs, AlGaNP, GaInNA
s, GaInNP, BGaNAs, BGaNP, BAl
NAs, BAlNP, BInNAs, BInNP, Al
InNAs, AlInNP, AlGaInNAs, Al
GaInNP, GaNPAs, AlNPAs, BGaN
The buffer layer can also be composed of a quaternary or multi-elemental crystal composed of a plurality of Group III elements such as AsP, InNPAs, and AlGaInNPAs and a plurality of Group V elements such as nitrogen, arsenic, and phosphorus.

In the present invention, the buffer layer is formed by an aggregate of single crystal grains made of the above-described group III nitride semiconductor material. The reason why the buffer layer is formed from single crystal grains is that the bonding force between atoms forming the crystal is stronger than that of the amorphous. The loss of the buffer layer at a high temperature as described above can be prevented by the strength of the bonding force. That is,
In the present invention, the exposure of the substrate surface due to the disappearance of the buffer layer is prevented by making the components of the buffer layer single crystal grains different from the conventional amorphous.

According to the present invention, the single crystal grains constituting the buffer layer are subjected to a temperature range from about 600 ° C. to about 750 ° C., which is a particularly limited temperature range from the temperature range conventionally used for growing the buffer layer. And as a result of precise control of growth conditions
-Obtained in a grown state. The buffer layer composed of the aggregate of single crystal grains thus formed hardly disappears even at a high temperature, and is superior to a conventional amorphous buffer layer. In addition, it deposits on the buffer layer
When judging the superiority or inferiority from the viewpoint of the surface morphology of the group III nitride semiconductor layer, a better surface morphology is provided as compared with a conventional case where an epitaxial layer is formed on a single crystal buffer layer obtained at a high temperature. The cause of this difference in surface morphology cannot be determined,
It is presumed that there is a difference in the arrangement of the single crystal lattices constituting the single crystal grains. For example, a case where a buffer layer made of gallium nitride (GaN) that is lattice-mismatched with a substrate is formed on a sapphire substrate is taken as an example. In forming a buffer layer at a relatively low temperature, the theory of Frank-van der Merve developed from the concept of pseudophorhism (Takayoshi Hashiguchi and Toshinobu Chikaku, "Thin Film and Surface Phenomena"
(Asakura Shoten, 4th edition published on December 15, 1972), 13
As taught in (1), the spacing between the single crystal lattices is an intermediate value between sapphire and GaN at the interface between the substrate and the buffer layer, and the spacing between the single crystal lattices gradually changes toward the lattice spacing of GaN above the buffer. On the surface side of the layer, the lattice spacing may be restored to that of GaN. On the other hand, in film formation at a relatively high temperature, gallium (Ga) atoms or nitrogen (N) atoms receive a large amount of thermal energy even if lattice mismatch with the substrate exists, so that the film cannot be formed at the interface with the crystal substrate. However, there is a possibility that a film is formed with an original lattice spacing of GaN from the initial stage of film formation. Such a single-crystal buffer layer having a lattice spacing close to the original lattice spacing of GaN is considered to be superior as an underlayer at first glance because the lattice spacing substantially coincides when, for example, a GaN layer is provided thereon. , Abruptly changes at the interface, and contains a large amount of misfit dislocations and the like inside. It is presumed that the growth layer formed on the single crystal buffer layer formed at a high temperature becomes inferior in surface morphology due to the existence and propagation of the large number of crystal defects.

As described above, the reason why the surface condition is affected by the conditions for forming the single crystal grains constituting the buffer layer will be examined from a microscopic viewpoint of the shape of the single crystal grains and the coalesced form. FIG. 8 shows an as-grown on (0001) sapphire surface at a growth temperature of 620 ° C. according to the present invention.
A gallium nitride buffer layer (102) composed of an aggregate of single crystal grains that are already single crystals in a grown state
An undoped gallium nitride layer (1
17) Cross section TEM of epitaxial layer grown
It is a schematic diagram of an image. Many of the single crystal grains (113) constituting the buffer layer (102) have a columnar shape in which the horizontal width changes in the vertical direction on the substrate surface. Single crystal grains (11
Since the side surface (114) of (3) changes to a curved surface, the joint bonding surface (114) of the single crystal grains (113) is often curved. The cross-sectional ridge of the top plate (115) at the top of each single crystal grain (113) is not pyramidal,
Rather, it has a gradually changing curved surface, that is, a substantially flat or curved surface. On the surface of the gallium nitride growth layer provided on the buffer layer having such a configuration, pits (pores) whose openings are substantially hexagonal and whose cross-sectional area decreases in the depth direction from the surface are opposite to pits. Substantially no hexagonal thin plates are stacked and projecting from the surface. On the other hand, the shapes of single crystal grains according to the prior art, which are formed at a growth temperature of about 800 ° C. or higher to form a gallium nitride buffer layer, and the state of their bonding are significantly different from the above case. . FIG. 9 shows an example in which an as-grown gallium nitride buffer layer (102) grown at a growth temperature of 920 ° C. is a single crystal.
It is a schematic diagram of the cross-sectional TEM image of the epitaxial layer which grows the undoped gallium nitride layer (117) at 00 degreeC. In the same figure, a dislocation image appearing as a black line image on the cross-sectional TEM image is also shown. In the figure, the buffer layer (102) is still composed of single crystal grains (113). However, single crystal grains (113)
Is different from the above case in that the cross-sectional area hardly changes in the height direction from the substrate surface, and the cross-section is a prism having a uniform width to some extent. Moreover, the outer peripheral side surface (114) linearly extends vertically upward substantially perpendicular to the substrate surface. The RHEED image obtained from this buffer layer was a spot pattern, indicating that it was a single crystal layer. That is, a buffer layer obtained at a growth temperature exceeding 900 ° C. (see Japanese Patent Application Laid-Open No. 7-312350) in which a single-crystal buffer layer that does not function as a buffer layer in the prior art is formed is certainly a single layer. Although it was a crystal layer, the shape of the single crystal grains constituting the layer was clearly different from that of the buffer layer grown at a lower temperature of around 600 ° C. Further, the top of each single crystal grain (113) was pyramidal, and the ridge line of the surface on the top was also formed of a straight line. As if corresponding to the occurrence of the pyramidal projections, projections composed of an overlayer of hexagonal plate-like crystals appeared on the surface of the growth layer on the buffer layer. Furthermore, it should be noted that dislocations (11) generated near the coalescence interface of the single crystal grains (113)
6) develops substantially linearly and penetrates to the surface of the gallium nitride growth layer (117) on the buffer layer (102). In addition, dislocations (116) are formed in the growth layer (1).
It was confirmed that pits (pores) were formed in the region reaching the surface of 17). That is, conventionally, the buffer layer made of a single crystal is not considered to provide a favorable result with respect to the surface state because the single crystal columns constituting the single crystal layer have a pyramidal shape having a straight and clear outline. This is due to the problem of the shape of the columnar crystal and the large number of dislocations extending vertically upward originating from the bonding surface formed by bonding these single crystal columns to each other. Therefore, in the present invention, even though the buffer layer is composed of single crystal grains, the buffer layer is not composed of an aggregate of prismatic (obelisk-like) single crystal grains having a clear outline, but the single crystal grains are mutually formed. The buffer layer is composed of single crystal grains having a configuration in which the side surfaces are joined as a detour and the density of dislocations propagating linearly in the upward direction is reduced.

Further, the buffer layer comprising an aggregate of single crystal grains according to the present invention is characterized in that the mutual orientation of each single crystal grain constituting the aggregate is defined. Here, the orientation refers to the orientation of the crystal of each single crystal grain that combines with each other to form an aggregate. For example, the orientation of each single crystal grain above the crystal substrate surface can be known from the arrangement direction of the lattice plane (crystal layer) of the single crystal constituting the single crystal grain. The orientation of the crystal layer is determined by, for example, a cross-sectional TEM ( T transmi) which is a kind of crystal analysis method using a transmission electron microscope.
ssion E lectron M icroscop
It can be easily measured by observing the lattice (plane) image by the method e). FIG. 10 shows a case where ammonia (NH ₃ ) is used as a nitrogen (N) source and trimethylgallium ((CH ₃ ) ₃ G
a) is used as a gallium (Ga) source, the growth temperature is set to 600 ° C., and the film thickness is about 5 nm.
aN) A schematic view of a cross-sectional TEM image of the buffer layer. The buffer layer is mainly composed of single crystal grains (113) whose vertical cross section has a spindle shape. When observing the lattice image inside each spindle-shaped single crystal grain (113), the direction in which the crystal planes (lattice planes) are overlaid (119), the so-called lamination orientation of the crystal planes, is not necessarily relative to the surface of the sapphire substrate (101). You can see that they are not unified. That is, the direction (119) in which the lattice planes overlap indicates the orientation orientation according to the present invention, and the difference in the orientation indicates the lattice plane (grid) indicating the direction (119) in which the lattice planes overlap. The angle at which the perpendiculars of the image intersect ((121); hereinafter, referred to as the intersection angle) is quantitatively indicated. If this angle is 0 degree, it is determined that the orientation directions between the single crystal grains are the same, that is, they are coincident. The intersection angle in the present invention is represented by an average value of the measured intersection angles. The shape of the single crystal grains shown in FIG. 10 is merely an example, and the shape, density, and the like of the single crystal grains vary depending on the growth temperature, the supply ratio of the raw materials, and the flow rate of the carrier (carrier) gas. is there.

In the present invention, the above-mentioned intersection angle is defined and the intersection angle is set to 30 degrees or less. The reason for defining the crossing angle in a narrow range within 30 degrees is that the film is formed on the buffer layer.
This is for promoting single crystallization of the group III nitride semiconductor layer. The degree of single crystallization is known from the magnitude of the half width of the diffraction pattern measured by, for example, a two-crystal X-ray diffraction method (so-called XRC method). FIG. 17 shows the relationship between the crossing angle of gallium nitride single crystal grains constituting the low-temperature buffer layer and the half-width of the diffraction pattern of the gallium nitride growth layer grown at a high temperature of 1100 ° C. on the low-temperature buffer layer. The larger the half width of the diffraction pattern, the lower the degree of single crystallization of the group III nitride semiconductor layer formed on the buffer layer. As shown in FIG. 17, if there is no particularly large crossing angle between the single crystal grains constituting the low-temperature buffer layer and the average crossing angle is within 30 degrees in the as-grown state, the growth layer formed at a high temperature The half-width of the diffraction pattern measured by the XRC method for is small. In other words, in this case, since the group III nitride semiconductor layer is generally formed at a high temperature near its melting point, a crystal grain boundary generated by a slight difference in orientation has an extremely small interface between the buffer layer and the high temperature growth layer. Although it may remain in a nearby region, it is considered that the difference in orientation completely disappears as the layer thickness of the growth layer increases, and the orientation becomes substantially uniform. Thus, a single-crystal high-temperature growth layer can be easily obtained. On the other hand, when the average crossing angle becomes larger than 30 degrees as a boundary, the half width of the X-ray diffraction pattern sharply increases. This is because the crossing angle between the single crystal grains constituting the low-temperature buffer layer is large, and the orientation of the growth layers that grow with the single crystal grains as nuclei is partially remarkably different from each other. It is also interpreted that the uniformity of orientation due to the fusion of grain boundaries does not reach the entire growth layer. Generally, a region where the crossing angle exceeds 30 degrees is in a region where the crossing angle is within 30 degrees, and when the crossing angle exceeds approximately 20% to approximately 30% of the entire region, it is difficult to form the entire growth layer as a single crystal layer.

There is no particular limitation on the growth method for directly depositing the buffer layer made of a group III nitride semiconductor on a crystal substrate. A normal pressure or reduced pressure MOCVD growth method or a VPE growth method can be used. In addition, molecular beam epitaxy (MBE) and chemical beam epitaxy (CBE)
A growth method such as a method can also be used. The MBE method includes a gas source (abbreviation: GS) MBE and an organic metal (MO) MBE, depending on the form of the raw material to be used. In growing a group III nitride semiconductor layer by these growth methods, for example, there is no limitation on a gallium, aluminum, indium or nitrogen raw material.

In the present invention, the buffer layer is composed of an aggregate of single crystal grains in which single crystal grains made of the above-described group III nitride semiconductor are arranged so as to cover the substrate surface. In any of the above-mentioned growth methods, gallium nitride (Ga)
Taking N) as an example, in order to form a buffer layer from single crystal grains covering the substrate surface, first, the growth temperature is set to about 400
It is necessary to be higher than ° C. The shape of the obtained single crystal grain changes depending on the growth temperature. About 400 ° C to about 500
In a relatively low temperature range of slightly lower than ℃, the shape of the single crystal grains varies depending on the supply ratio of the raw material (the so-called V / III ratio described later). It is tinged. Over 500 ° C and approx. 600-7
In the range of 50 ° C., the single crystal grains have a prismatic shape in which the horizontal cross section is a polygon such as a hexagon and the top plate at the top is substantially flat. Furthermore, when the growth temperature exceeds 750 ° C., which is a high growth temperature, the prismatic single crystal grains have a top plate portion covered with pyramidal projections. The pyramid-shaped projections cause projections to be formed on the growth layer deposited on the buffer layer, and thus are not preferable for obtaining a growth layer having excellent surface flatness. Judging from the good surface morphology of the growth layer, the shape of the single crystal grain is desirably substantially spherical, spindle-shaped, or columnar with a flat top plate. In particular, a prismatic single crystal grain whose top plate is made substantially flat is preferable because it provides a smooth and flat growth layer having an excellent surface condition. Thus, the growth temperature of the gallium nitride buffer layer is about 60
0 ° C to 750 ° C is preferred.

As described above, in the case of gallium nitride, single crystal grains are formed even at a low temperature range of about 400 ° C. to less than 500 ° C. In a relatively low temperature range, a V / III ratio of approximately 10 ³ or less can provide substantially spherical particles, and a V / III ratio of approximately 1.5 × 10 ⁴ or more can provide spindle-shaped single crystal particles. However, such a buffer layer obtained at a relatively low temperature has a single crystal layer in which single crystal grains are buried in a single crystal layer covering the surface of the deposit and a mixture of single crystal grains. On the other hand, within a preferable growth temperature range of about 600 ° C. to about 750 ° C., the surface of a deposit such as a crystal substrate can be uniformly covered with single crystal grains having a uniform shape. That is, the buffer layer can be formed from an aggregate of single crystal grains having a single structure. Single crystal grains have been known to function as growth nuclei to promote film formation ("Journal of the Japan Society for Crystal Growth",
Vol. 15 (1988), 334). Therefore, configuring the buffer layer from single crystal grains having a single morphology means that the growth nuclei are homogenized, and the homogenization of the growth can be achieved. Homogenization of the growth nuclei is one of the advantages brought by keeping the growth temperature in this range. FIG. 11 shows a C-plane sapphire substrate (10
1) A schematic diagram of a cross-sectional TEM image of a gallium nitride buffer layer according to the present invention formed thereon. As shown in the figure, the buffer layer (102) is made up of an aggregate of prismatic single crystal grains (113) having a top plate having a substantially flat top and a substantially hexagonal horizontal cross section. At this growth temperature, the lateral width of the prismatic single crystal grains (113) is approximately 15
It is in the range of nanometer (nm) ± 5 nm. The width changes mainly depending on the growth temperature, and tends to increase as the growth temperature is increased. The height of the prismatic single crystal grains as well as the width tends to increase as the growth temperature is increased, but in any case, the entire surface of the substrate (deposit) is replaced with the prismatic single crystal grains. There is no. In other words, a state is created in which the substrate surface is covered with an aggregate of single crystal grains serving as growth nuclei.

The above preferred growth temperature range is for gallium nitride. When the buffer layer is made of, for example, aluminum nitride (AlN), a preferable growth temperature slightly shifts to a higher temperature side and becomes about 650 ° C. to about 800 ° C. In the case of forming a buffer layer made of a group III nitride mixed crystal semiconductor containing aluminum (Al) and gallium as constituent elements, a preferable growth temperature tends to shift to a higher temperature side as the composition ratio of aluminum increases. Conversely, in the case where the buffer layer contains indium (In), for example, gallium nitride doped (doped) with indium, the optimum growth temperature is on the low temperature side in accordance with the increase of the indium doping concentration. Deviate. For example, if the doping concentration of indium is about 10 ²⁰ c
If m ⁻³ , a preferable growth temperature is approximately 500 ° C. to 6 ° C.
00 ° C.

Further, in order to make the orientation of the single crystal grains uniform,
It is necessary to precisely control the V / III ratio and store it under limited conditions. What is V / III ratio?
Is the supply ratio of the group V element raw material to the group III element raw material supplied. In growing a gallium nitride (GaN) layer, general ammonia (NH ₃ ) and trimethylgallium (TMG: (CH ₃ )) are used as raw materials for both constituent elements.
Taking an organic gallium compound such as ₃ Ga) as an example, V
The / III ratio is expressed as the ratio of the amount of ammonia to the amount of trimethylgallium supplied to the growth system. For example, C
When a buffer layer made of gallium nitride is obtained by using plane ((0001) plane) sapphire as a substrate, if the V / III ratio (ammonia / TMG supply ratio) is approximately 1.5 × 10 ⁴ , the orientation of each single crystal grain is This has the effect of making the sex uniform. V / I
The cause of achieving uniform orientation by increasing the II ratio, that is, by increasing the concentration of the nitrogen source in the growth system relative to the concentration of the group III element, is still unknown.
It has not been fully elucidated. However, it is a fact that under the condition of an excessive amount of nitrogen material, two-dimensional growth in the horizontal (horizontal) direction of gallium nitride with single crystal grains as growth nuclei prevails. It is presumed that the area occupied by crystal planes having the same arrangement orientation is eventually expanded. That is, by the lateral growth becoming dominant, the mode of the atomic arrangement having the same orientation spreads and expands two-dimensionally, so that a growth environment in which a lattice plane having the same orientation is easily formed is formed. It is presumed to be created.

The above-mentioned orientation is a definition of the orientation in the vertical upward direction with respect to the surface of a substrate or an object to be deposited such as an epitaxial growth layer. The aggregate of single crystal grains constituting the buffer layer according to the present invention is It is desirable that hexagonal columnar single crystal grains are arranged in a uniform direction even on a plane. In short, in the case of hexagonal columnar single crystal grains having a hexagonal horizontal cross section, if the single crystal grains are arranged so that one side surface of the hexagonal column is parallel to each other, they are also oriented in a plane. Will be complete. In addition to simply arranging one side surface in parallel, if the side surfaces are closely bonded to each other so as to eliminate the gap between the parallel side surfaces, high-temperature growth caused by the presence of this space will occur. The generation of pits (pores) on the layer surface is suppressed. Under the above preferable growth conditions, the hexagonal columnar single crystal grains that make the top plate portion of the top formed substantially flat are parallel to the side surfaces of the hexagonal single crystal columns or at least an angle between the side surfaces within 5 degrees. As a result, a buffer layer is obtained in which the single crystal grains are arranged almost in parallel, and there is almost no gap between the side faces, and the single crystal grains are closely assembled with each other.

[0022]

A buffer layer made of a group III nitride semiconductor laminated on the surface of a crystal substrate, and a buffer layer laminated on the buffer layer
In a compound semiconductor epitaxial wafer composed of an epitaxial layer composed of a group III nitride semiconductor, the buffer layer is composed of an aggregate of single crystal grains, and the single crystal grains constituting the buffer layer are in an as-grown state. The side surfaces joined to each other are joined as a detour, and the top plate at the top is formed substantially flat or curved, and in particular, the single crystal grains constituting the buffer layer are as-grown.
By setting the angle difference between the lamination directions within 30 degrees in the state, the surface state of the growth layer deposited on the buffer layer is improved, and the deterioration of the crystal quality is reduced.

[0023]

【Example】

(Example 1) An example in which a gallium nitride (GaN) buffer layer is formed by a normal pressure MOCVD growth method will be described.
First, the buffer layer according to the present invention will be described. The board has
C-plane sapphire with a plane orientation of (0001) was used. The substrate had a circular shape with a diameter of 50 mm and a thickness of about 330 μm. This substrate was sliced from an ingot grown by the CZ method, subjected to lapping and mirror polishing, and then annealed (annealed and heat-treated) in a hydrogen stream. Ammonium hydroxide /
The mixture was treated with a mixed solution of aqueous hydrogen peroxide / ultrapure water at room temperature for about 2 minutes with stirring. After the treatment, water was removed, and the substrate surface was further heated and dried with an infrared lamp.

Thereafter, the substrate was placed almost at the center of a resistance heating type circular heater having a built-in ceramic heater placed in a stainless steel growth furnace. After the mounting, the inside of the reaction furnace was purified by an adsorption method using argon gas having a dew point of about minus (−) 90 ° C. and a gauge pressure of about 1 kg / cm ² at a flow rate of about 5 liters per minute. It was ventilated (purge: minute). afterwards,
The supply of argon gas to the reaction furnace was stopped, and the inside of the reaction furnace was evacuated by a general oil rotary vane vacuum pump using Fomblin oil. The degree of vacuum is about 3 × 10 ^-3 Torr
The air was evacuated to about r and kept at that vacuum for about 10 minutes. Then, after purifying in a reaction furnace using a palladium-silver alloy thin film transmission method, high-purity hydrogen gas having a dew point of about minus (−) 95 ° C., which was further purified by a cryogenic adsorption method, was passed through the reaction furnace. The pressure in the furnace was returned to the atmospheric pressure. After returning to atmospheric pressure, the supply of hydrogen to the reactor was continued for about 10 minutes at a flow rate of about 3 liters per minute.

Thereafter, as the flow rate of hydrogen supplied to the reactor was increased from 3 liters per minute to 9 liters per minute,
The ceramic heating element was energized to raise the temperature of the substrate on the heater from around room temperature to 450 ° C. as a first step to preheat the substrate. After maintaining the temperature at 450 ° C. for about 20 minutes, the temperature was raised to 600 ° C. in a hydrogen stream as a second step. The substrate was kept at the same temperature for 20 minutes after the temperature transient (overshoot) when the temperature reached the growth temperature was settled, to prepare for the growth of the buffer layer.

After a lapse of 20 minutes, ammonia (NH ₃ ) gas obtained by vaporizing liquefied ammonia contained in an aluminum alloy cylinder having an internal volume of about 10 liters is supplied to an electronic mass flow meter (MFC: MFC). ) And precisely supplied to the reactor at a flow rate of 1.00 liter per minute. One minute after the supply of ammonia gas as a nitrogen (N) supply material to the reaction furnace was started, trimethylgallium ((CH
₃ ) ₃ Ga) is supplied to the reactor, and gallium nitride (Ga)
N) The growth of the buffer layer was started. Trimethylgallium was supplied to the reaction furnace through the above-described high-purity hydrogen gas bubbling operation of the same substance contained in a stainless steel bubbler (foaming) vessel. This bubbler container is minus (-) 10 ° C ± 0.5 by an electronically controlled thermostat.
C. was maintained. The flow rate of the hydrogen bubbling gas to the liquid trimethylgallium maintained at the same temperature was precisely controlled to 3 milliliters per minute (ml) by MFC. The V / III ratio calculated from the above ammonia flow rate and the bubbling conditions of trimethylgallium is 2 × 10 ⁴ . The supply of the source gas was continued at the growth temperature and the V / III ratio for 20 minutes to form a gallium nitride buffer layer having a thickness of about 68 nm.

In order to investigate the cross-sectional structure of the gallium nitride buffer layer grown under the above conditions on a sapphire substrate, a strip of the wafer was subjected to general ion-thinning using argon ions.
It was thinned by the method. After the thin layer, the electron beam acceleration voltage was set to 200 kV (K) using a general transmission electron microscope.
As V), the fine structure of the cross section of the buffer layer was observed by a transmission bright field method. Observation with this electron microscope is shown in Example 1
It was shown that the buffer layer according to the above was composed of single crystal grains whose lateral width was changed in the vertical direction. These single crystal grains were densely bonded to each other firmly without generating a gap, and were arranged so as to cover almost the entire surface of the sapphire substrate. The difference in height between the single crystal grains is the height of the single crystal grains, that is, the thickness of the buffer layer (about 68 n).
m) was about 3 nm or less. Further, the magnification at the time of microstructure observation with an electron microscope was increased, and a lattice image of a single crystal grain was taken. As a result, the lattice images observed inside each single crystal grain were arranged almost parallel to the vertical direction of the substrate surface. Further, it was recognized that adjacent single crystal grains were joined so that the lattice planes constituting each single crystal grain were substantially parallel. At the very edge of the wafer, it was determined that the lattice images were not completely parallel between adjacent single crystal grains and had an intersection angle of about 1 to about 2 degrees. However, the number of single crystal grains joined at an intersection angle of the lattice plane of about 1 degree to about 2 degrees was less than about 5%. That is, according to the conditions described in Example 1, a single crystal grain according to the present invention, which has a uniform orientation and is composed of an aggregate of single crystal grains joined so that lattice planes are substantially parallel to each other. It has been shown that a buffer layer consisting of can be provided in an as-grown state.

(Embodiment 2) About 1000 to 15 from atmospheric pressure
A buffer layer made of a mixed crystal of aluminum and gallium nitride (AlGaN) was formed by a reduced pressure MOCVD growth method using a water column of 00 mm (mm H ₂ O) and a slightly reduced growth environment. The substrate has a plane orientation (00
01) C-plane sapphire was used. The substrate was a rectangular substrate having a length of about 25 mm and a width of about 10 mm grown by a ribbon crystal growth method, and a thickness of about 350 μm. This substrate was cut from a ribbon-shaped pulled crystal, subjected to lapping, end face processing and mirror polishing, and then annealed (annealed heat treatment) in a hydrogen stream at a temperature exceeding 1000 ° C. The surface of the substrate was treated with a phosphoric acid solution at about 100 ° C. for about 2 minutes while stirring. After the substrate surface treatment, the substrate was sufficiently washed with ultrapure water having a specific resistance of about 18 megaohm (MΩ). After the water was removed, the substrate surface was irradiated with infrared rays, heated and dried.

Thereafter, the sapphire substrate surface-treated in accordance with the above-described procedure is placed substantially at the center of a substrate mounting table (susceptor) horizontally disposed in a square growth furnace made of high-purity quartz for use in the semiconductor industry. Was placed. After the mounting, the inside of the reactor was ventilated (purged) with nitrogen (N ₂ ) gas having a dew point of about minus (−) 85 ° C. purified by an adsorption method. That is, a nitrogen gas whose gauge pressure was adjusted to about 0.7 kg / cm ² was purged at a flow rate of about 3 liters per minute for about 5 minutes through a clean pressure reducing valve in which a gas contact portion on the inner surface was mirror-polished. . Thereafter, the supply of the nitrogen purge gas to the reaction furnace was stopped, and the inside of the reaction furnace was evacuated by a general vane type rotary vacuum pump. Evacuate until the degree of vacuum reaches about 5 × 10 ^-3 Torr (Torr).
Hold for minutes. Thereafter, conduction to a vacuum evacuation system for evacuating the reactor to vacuum was cut off, the nitrogen gas was introduced into the reactor, and the reactor was returned to atmospheric pressure. For a while, after flowing nitrogen gas into the reaction furnace, the gas flowing through the reaction furnace is purified from nitrogen by a palladium-silver alloy thin film transmission method, and then the dew point purified by the cryogenic adsorption method is reduced to about minus ( −) Switching to high-purity hydrogen gas at 98 ° C. After the gas was switched, the supply of hydrogen to the reactor was continued at a flow rate of about 3 liters per minute for about another 20 minutes.

Thereafter, as the flow rate of hydrogen supplied to the reactor was increased from 3 liters per minute to 9 liters per minute,
The pressure in the reactor was set to be about 1000 millimeters water column (mm H ₂ O) lower than atmospheric pressure. The degree of pressure reduction in the reactor is adjusted by opening and closing the opening control valve (butterfly valve) provided in the middle of the piping leading to the ammonia abatement system equipped with a mechanism for sucking the gas inside the reactor by a blower from the reactor. It was done by doing. This pressure adjustment was automatically controlled by inputting an electrical signal corresponding to the sensed pressure output from a piezoelectric (piezo) type precision pressure gauge to a valve body rotation mechanism for adjusting the opening of the opening adjustment valve. . Next, high-frequency power (maximum frequency = 13.56 MHz) was applied to a water-cooled high-frequency induction coil composed of oxygen-free copper disposed around the quartz reaction tube. Accordingly, the temperature of the substrate on the mounting table is set to be about 4 degrees as the first step from near room temperature.
The temperature was raised to 50 ° C., and the substrate was preheated. After holding at 450 ° C. for about 20 minutes, the second step is to run 68
The temperature was raised to a growth temperature of 0 ° C. It took about 5 minutes to reach 680 ° C. from the substrate preheating temperature of 450 ° C. The substrate was held at the same growth temperature for 20 minutes to prepare for growth of the buffer layer.

After a lapse of 20 minutes, ammonia (NH ₃ ) gas obtained by vaporizing liquefied ammonia contained in an aluminum alloy cylinder having an internal volume of about 10 liters is supplied to an electronic mass flow meter (MFC: MFC). ), And precisely controlled to a flow rate of 3.00 liters per minute to feed the reactor. One minute after the supply of ammonia gas as a nitrogen (N) supply material to the reaction furnace was started, trimethylgallium ((CH
_{3) 3} Ga) and aluminum (Al) trimethylaluminum was feedstock ((CH _{3) 3} Al) was supplied to the reaction furnace, aluminum gallium nitride (AlGa
N) The growth of the mixed crystal buffer layer was started. Trimethylgallium was supplied to the reaction furnace through the above-described high-purity hydrogen gas bubbling operation of the same substance contained in a stainless steel bubbler (foaming) vessel. This bubbler container was kept at 0 ° C. ± 0.2 ° C. by an electronically controlled thermostat. The flow rate of the hydrogen bubbling gas to the liquid trimethylgallium maintained at the same temperature was precisely controlled to 8 milliliters per minute (ml) by MFC. Like trimethylgallium, trimethylaluminum was stored in a stainless steel bubbler container, and the container was kept at a constant temperature of 17 ° C. by an electronically controlled constant temperature bath. Trimethylaluminum was also added to the growth system via bubbling operation with high purity hydrogen gas. The flow rate of the hydrogen bubbling gas was 3.6 ml / min. While keeping the supply ratios of the gallium and aluminum raw materials constant, the supply of the raw material gases and the like to the growth system is continued for 20 minutes, the aluminum composition ratio is made 6%, and the layer thickness is 72 nanometers (nm).
To obtain a buffer layer composed of an Al _0.06 Ga _0.94 N mixed crystal.

The wafer strip was subjected to general ion-shinning using argon ions.
g) After thinning by the method, the electron beam accelerating voltage is set to 200 kilovolts (KV) by a general transmission electron microscope.
The fine structure of the cross section of the buffer layer was observed by a transmission bright field method. FIG. 12 schematically shows a transmission electron microscope image of the cross section. This buffer layer (102) has a cross-sectional width (122).
Is changed in the upward direction from the substrate surface (11)
3). The top plate portion (115) at the top of the single crystal grain (113) is not in the shape of a pyramid, but rather is a gentle curved surface. There are no pyramidal projections on the surface,
It had a smooth curved surface. In addition, since the outline (ridge line) of the side surface of each single crystal grain is not a clear straight line but a curved surface, the contour of the bonding side surface of the single crystal grain is curved. Further, from the lattice plane image (120) showing the orientation of the lattice planes constituting the single crystal grains (113), the crystal lattice planes of the single crystal grains (113) are arranged almost parallel to the interface (103) with the substrate. It had been. It was recognized that lattice plane images (120) were arranged almost in parallel between adjacent single crystal grains (113). It is rare that the crossing angle between single crystal grains exceeds about 1 degree. That is, the second embodiment
According to the conditions described in (1), it is composed of an aluminum nitride-gallium mixed crystal according to the present invention having a uniform orientation, comprising an aggregate of single crystal grains joined so that lattice planes are substantially parallel to each other. Buffer layer made of single crystal grains is as-grown
n states could be provided.

Example 3 A buffer layer composed of single crystal grains of gallium nitride (GaN) doped with indium (In) was formed by a normal pressure MOCVD growth method. As the substrate, sapphire subjected to the same shape, plane orientation, thickness and heat treatment as described in Example 1 was used. Preparation for forming a buffer layer composed of indium-doped gallium nitride single crystal grains was also performed in the same procedure as described in Example 1.

Thereafter, a gallium nitride buffer layer was formed in accordance with the procedure for growing a gallium nitride buffer layer described in Example 1. At that time, the conditions described in Example 1 were changed in two points: (1) growth temperature and (2) additional operation of indium doping. The growth temperature was determined in Example 1 in consideration of the formation of a buffer layer containing indium.
550 ° C. lower by 50 ° C. than that of The indium doping was performed by supplying a hydrogen bubbling gas carrying trimethylgallium to the growth system and simultaneously adding an indium doping source gas into the growth system. As a doping source of indium, cyclopentadienylindium (C ₅ H ₅ I
n) was used. This indium doping source was also housed in a small stainless steel evaporation container like the gallium and aluminum raw material supply source, and the evaporation container was kept at 60 ± 1 ° C. by an electronic thermostat. High-purity hydrogen gas for transporting the sublimated indium source was circulated at a flow rate of 120 ml / min in the evaporation container. If the flow rate fluctuates, the amount of indium conveyed into the growth system fluctuates, so the flow rate was precisely controlled by MFC. The flow rate of ammonia was precisely controlled to 3.00 liters per minute by MFC as in Example 1. Therefore, the V / III ratio was about 1.9 × 10 ⁴ . By maintaining these flow conditions for 20 minutes, a buffer layer having a thickness of about 60 nm was obtained.

After stopping the supply of the gallium supply material and the indium doping material to the growth system and terminating the growth of the indium-doped gallium nitride buffer layer, approximately one minute later, the sapphire substrate is cooled. Then, the supply of power to the resistance heater was stopped. After the temperature is reduced, a chromel / alumel alloy thermocouple (commonly known as JIS-K thermocouple) is disposed just below the center of the heater where the substrate is placed.
When the temperature indicated by the above became approximately 500 ° C., the supply of ammonia gas to the growth system was stopped. When the temperature of the sapphire substrate became close to room temperature, the gas supplied to the reaction furnace was replaced with argon gas instead of hydrogen. Thereafter, the inside of the reaction furnace was alternately evacuated and purged with argon gas for returning to atmospheric pressure several times alternately, and finally, argon gas was circulated. In this state, the substrate mounting table was transported through a water-cooled dustproof square gate valve to an interlock chamber attached to the reaction furnace. After closing the square gate valve for isolating the interlock chamber and the reaction furnace, the evacuation of the interlock chamber and the purging of argon gas were repeated several times, and finally the chamber was filled with argon gas. In this state, an epitaxial wafer composed of a sapphire substrate and an indium-doped gallium nitride buffer layer was taken out of the room from the substrate mounting table in the interlock chamber.

The atomic concentration of indium in the indium-doped gallium nitride buffer layer formed by the above growth is about 6 × 10 by SIMS (secondary ion mass spectrometry).
It was determined to be ¹⁸ cm ^-3 . On the photoluminescence (PL) spectrum measured at room temperature (near 291 K (Kelvin), liquid nitrogen temperature (77 K), and lower temperature of 10 K, the appearance of the emission peak showing the band edge emission of the gallium-indium mixed crystal is as follows. I was not able to admit. Observation by transmission electron microscopy similar to that described above showed that the indium-doped gallium nitride buffer layer was an aggregate in which single crystal grains were densely packed without gaps. The lattice arrangement between the single crystal grains constituting the aggregate was also substantially parallel. That is, no particular difference was observed in the parallelism of the lattice arrangement from the buffer layers made of gallium nitride and the mixed crystal of aluminum nitride and gallium described in Examples 1 and 2. The surface of the buffer layer composed of the top plate portion at the top of the single crystal grain had a gentle curved surface as in the case of the aluminum / gallium nitride buffer layer. However, as a result of observing a cross-sectional bright-field image of a region near the interface between the substrate and the buffer layer under a higher magnification of 20 × 10 ⁴ times, the lateral width was increased in the region near the interface, and the width was increased vertically from the substrate surface. The presence of single crystal grains which tended to reduce the width was observed. The appearance of single crystal grains existing near the interface was slightly different from the undoped gallium nitride buffer layer described in Example 1.

(Comparative Example 1) In Comparative Example 1, the effect of the buffer layer made of a single crystal to improve the surface condition of the growth layer will be described in comparison with the case of a buffer layer that is not a single crystal. A buffer layer made of gallium nitride was formed under the same growth conditions and growth operation as in Example 1 except that the substrate temperature was 380 ° C. Since the growth temperature was lowered, the obtained layer thickness was reduced to 2.5 nm even if the growth time was the same as in Example 1. The crystallinity of this buffer layer was evaluated by the RHEED method and the cross-sectional TEM method. From observation of a transmission electron beam diffraction image of a fine structure constituting the buffer layer by a cross-sectional TEM method, the components of the buffer layer according to Comparative Example 1 were roughly classified into three types. One is to provide a point-like diffraction pattern, the other is to provide a pattern in which a diffraction spot and a diffraction ring coexist, and the other is a component to provide a pattern consisting of only a diffraction ring. That is, from the diffraction pattern, one component is a single crystal grain, the other is a polycrystal, and the third component is an amorphous body. On the other hand, only the diffraction ring appeared in the RHEED pattern obtained from the surface of the buffer layer. Therefore, although this buffer layer contains a single crystal grain or a polycrystal as a constituent element, it is mainly composed of an amorphous body as a whole. The single crystal grains or polycrystals were scattered in the so-called amorphous material, and were far from being closely bonded to each other and densely packed. Observation of the lattice arrangement between single crystal grains under a high-resolution mode of a transmission electron microscope revealed that the lattice arrangement was not uniform. For example, the lattices constituting a single crystal grain are arranged almost in parallel to the substrate surface, and in other single crystal grains, the lattice plane has an angle of about 30 degrees counterclockwise with respect to the substrate surface (horizontal plane). Were held in parallel with each other. From this, it was considered that this buffer layer was the same as the conventional buffer layer, judging from the main constituent elements and the crystal form (Japanese Patent Laid-Open No. 2-2 / 1990).
No. 29476, JP-A-4-297023, JP-A-6-297
No. 151962 and JP-A-7-3121350). Furthermore, while using an amorphous body as a main component,
In a partial region of the (0001) -sapphire substrate, the surface was exposed without being covered with the amorphous body, and almost the entire surface of the substrate was not uniformly and densely covered with single crystal grains.

It should be additionally noted that the temperature was lower than that of Comparative Example 1 by 20.
At a temperature between 0 ° C. and a little over 300 ° C., even a buffer layer made of polycrystal is not formed as far as the inventor has tried,
Only liquid material was deposited. This substance is a gallium droplet or metal gallium amide (Ga (NH ₂ ) ₃ ) which is supposed to be formed at a temperature of about 100 ° C. or more by the reaction of metal gallium and ammonia.
(IA.SHEKA, IS.CHAUS and
T. T. MITYUREVA, "THE CHEMIS
TRY OF GALLIUM ", Chapter
6, 133 pages (ELSEVIER PUNBLISHI
NGCOMPANY, 1966), or alternatively containing carbon (C), hydrogen (H), gallium (Ga) and nitrogen (N) formed by the reaction of trimethylgallium with ammonia. N] polymer (JE
electrochem. Soc. , Vol. 118 , N
o. 11 (1971), 1864. ) Could not be determined. Regardless of the substance generated at such a low temperature, it has been difficult to form even a polycrystalline group III nitride semiconductor layer at a low temperature of 300 ° C. or lower.

On the buffer layer grown at 380 ° C., an undoped gallium nitride growth layer was formed at 1100 ° C. under exactly the same growth conditions and operation as in Example 4 described below. However, the surface of the growth layer was not smooth as obtained in Example 4, and many gaps and projections appeared. FIG. 13 is a cross-sectional TEM image that clearly shows the causal relationship between the generation of pits and protrusions and the deterioration of the conventional amorphous buffer layer when exposed to a high-temperature film-forming environment. Most of the amorphous body, which is the main component of the buffer layer, disappeared when the temperature was raised to a high temperature for growing the gallium nitride growth layer.
A very small portion is as as on the (0001) -sapphire substrate surface.
-It remained without changing the crystal form at the time of the growth.
The tendency that the remaining amorphous body (104) was single-crystallized due to an increase in the substrate temperature was not clearly recognized. A fine single crystal or a polycrystalline particle (123) in which several single crystal grains were aggregated remained randomly. The constituents of the buffer layer (102) are slightly remaining amorphous material (10
4) and polycrystalline particles (123). as-gro
Although the amorphous body was present in the wn state, the region (108) where the surface of the substrate was exposed because the amorphous body disappeared when the temperature was raised to the growth temperature was c perpendicular to the C-plane sapphire substrate.
High temperature (1100 ° C) with growth in the crystal axis direction predominant
In the gallium nitride growth layer (117) grown in step (1), pyramid-shaped single crystal columns (118) were erected. Single crystal pillar (11
8) are not closely united with each other, are isolated and scattered,
Voids (pits) (107) existed between the side walls (129) of the single crystal columns (118). The pit (1) is formed in the growth layer (117) corresponding to the position where the void (107) exists.
07) had occurred. In addition, the top portion (131) of the pyramid constituting the top plate portion (130) of the single crystal column (118) protrudes and is exposed, and the growth layer (117) has protrusions (125).
Will occur. As described above, when the buffer layer composed of the elements that induce the generation of gaps or protrusions is used as the underlayer, it has not been possible to obtain a high-temperature growth layer having an excellent surface condition. In summary, as long as the buffer layer having the conventional structure is used as the underlayer, it is difficult to form an epitaxial layer having a growth layer having a superior surface state as compared with the case of the above-described Example 1.

Example 4 In Example 4, a blue light emitting diode (LED) was formed from a group III nitride semiconductor epitaxial wafer provided with a buffer layer according to the present invention. First, formation of an epitaxial wafer for LED use will be described. As the substrate, a sapphire crystal having both front and rear surfaces mirror-polished and subjected to an annealing treatment in a hydrogen stream for removing impurities such as oxygen contained in a crushed layer generated in the slicing and polishing steps, was used. The substrate thickness was about 85 μm when mirror polishing was performed on both front and back surfaces. The main reason for using a substrate polished on both sides is that uniform contact with the surface of a heating element such as a heater is achieved by improving the surface roughness accuracy of the surface. The substrate surface is a C-plane having a plane orientation of (0001). After immersing the substrate in an aqueous solution of ammonium fluoride at room temperature for about 1 minute,
It was washed with ultrapure water purified using an ion exchange resin permeation method. After washing with water, water was removed by centrifugal force using an ordinary spinner, and the surface was dried by irradiating infrared rays.

The center of the dried sapphire substrate on a substrate mounting table (susceptor) housed in a stainless steel reaction vessel of material number 304 equipped with a gas nozzle made by processing a purity grade quartz material for semiconductor industry and having a gas nozzle. Part. Thereafter, argon gas, which was vaporized from liquefied argon and purified by molecular sieve (molecular sieve) adsorption, was passed through the gas nozzle at a flow rate of 5 liters per minute and supplied into the reaction furnace. Thereby, the argon gas was preferentially supplied to the substrate surface and the inside of the reaction furnace was purged. The supply of argon gas to the reactor was continued for about 20 minutes. The susceptor on which the sapphire substrate is mounted is an interlock type spare chamber (chamber) equipped with an exhaust / purge function for preventing outside air (atmosphere) from entering the reactor, and repeatedly evacuating / purging in advance. And then introduced into the reactor. For this reason, from the determination that the amount of outside air mixed into the reactor is extremely small,
After the purge operation in the reaction furnace with argon gas for 20 minutes was completed, the temperature of the sapphire substrate was immediately increased by replacing the argon gas with hydrogen gas.

The flow rate of hydrogen gas flowing into the reaction furnace through the gas nozzle was reduced to 1 / min by a metal seal type MFC.
While precisely controlling to 0.0 liter, the sapphire substrate is passed through a susceptor made of high-purity graphite,
It was heated mainly by heat transfer by a circular resistance heating type heater. The sapphire substrate was not preliminarily heated to 450 ° C. without raising the temperature to the growth temperature of 650 ° C. all at once. 45
After maintaining at 0 ° C. for 15 minutes, the temperature was raised to 650 ° C. 65
About 18 minutes after the temperature transient immediately after the temperature reached 0 ° C. was recovered, ammonia gas was introduced into a 316L stainless steel tube having a material diameter of の inch to introduce hydrogen gas into the reactor. . Exactly two minutes after the start of the addition of the ammonia hydrogen gas, trimethylgallium as a gallium supply material was added to the hydrogen gas in the same manner as the ammonia. The use form and supply form of ammonia and trimethylgallium are as described in Example 1. MFC with a maximum flow rate of 10 liters / minute and a minimum controllable flow rate of 2% of the maximum flow rate of ammonia
Precisely controlled to 2.80 liters per minute. On the other hand, trimethylgallium was supplied at a maximum flow rate of 10 ml / min by bubbling hydrogen gas at a flow rate of 3.00 ml / min by an MFC having the same control performance. V / III ratio is 1.2 × 1
0 ⁴ after. The flow rates of the source gas and the hydrogen gas as the carrier gas are continuously maintained for 20 minutes, and
An undoped gallium nitride buffer layer having a thickness of 4 nm was obtained.

Observation of the undoped gallium nitride buffer layer obtained under the same conditions by a cross-sectional TEM method shows that the buffer layer has a width of about 50 nm and the top plate gradually changes not into a pyramid but into a flat or curved shape. It is shown that it is composed of an aggregate of single crystal grains as a smooth surface. These single crystal grains are densely densely packed with no gap therebetween, and the bonding surface (bonding interface) between the single crystal grains extends linearly vertically above the (0001) -sapphire substrate surface. Instead, it was curved. The transmission diffraction image of each single crystal grain obtained by irradiating each single crystal grain constituting the buffer layer with an electron beam whose diameter is narrowed down in the high resolution mode is a spot (point) pattern. From this, it became clear that each single crystal grain was a gallium nitride single crystal. The RHEED image of the surface of the buffer layer at the center corresponding to the center of the sapphire substrate having a diameter of 2 inches was composed of a circular spot pattern. In the RHEED image from the peripheral portion of the buffer layer corresponding to the peripheral region of the substrate, a spot slightly shifted from a circle and somewhat elliptical was observed in the diffraction spot of the low Miller index surface, but the entire buffer layer was a single crystal. It has been proven. That is, the gallium nitride buffer layer of the present embodiment is an as-gro
In the wn state, a gallium nitride single crystal grain has already been formed into a single crystal layer densely assembled with each other. Further, observation of the lattice image at high magnification revealed that the single crystal grains were coalesced with the lattice planes substantially parallel to each other. The intersection angle between the lattice planes (see the text) was less than 0.5 degree (°) at the maximum.

The growth of the buffer layer composed of gallium nitride single crystal grains was terminated by stopping the supply of the hydrogen bubbling gas containing trimethylgallium to the hydrogen carrier gas. After temporarily stopping the addition of the gallium raw material supply source into the growth system, the flow rate of the hydrogen carrier gas was reduced to 1 / min.
It was reduced from 0.0 liters to half of 5.00 liters per minute. At the same time, argon gas was added into the growth system at a flow rate of 5.00 liters per minute, and the total amount of carrier gas was maintained at 10.0 liters per minute. At the same time, the temperature of the stainless steel bubbler container containing trimethylgallium was changed from 0 ° C. to 13 ° C. by an electronic thermostat.

After waiting for a while until the flow rate fluctuation accompanying the operation of changing the flow rate of both carrier gases was recovered, the electric power applied to the heater was increased to raise the temperature of the substrate to 1100 ° C. A platinum-platinum-rhodium thermocouple (J) placed immediately below the heater surface in contact with a susceptor, which is an indicator of the substrate temperature
When the temperature reading based on the thermoelectromotive force of the IS standard R thermocouple reached 1100 ° C., the flow rate of ammonia gas was increased from 2.80 liters per minute to 4.00 liters per minute. . Exactly 5 minutes after the indicated value of the substrate temperature and the ammonia flow rate with the increase in the flow rate were stabilized, hydrogen gas bubbled with trimethylgallium was added again into the growth system. Thus, the formation of the undoped gallium nitride growth layer was started. Since the undoped gallium nitride growth layer is a layer grown at a high temperature exceeding 1000 ° C., it is also referred to as a high-temperature buffer layer or a clad layer in view of its effect on the light emission mechanism, in contrast to the buffer layer grown at a relatively low temperature. Is done. In forming the undoped gallium nitride growth layer, the piping system (line) for supplying trimethylgallium was changed. This trimethylgallium supply line is equipped with an MFC with a maximum flow rate of 30 ml / min. The controllable maximum flow rate of the provided MFC is different from the trimethylgallium supply line used for forming the buffer layer. The flow rate of the bubbling hydrogen gas was controlled to 20 ml / min by the MFC in this supply line, and a hydrogen bubbling gas accompanied by trimethylgallium was mixed with a hydrogen / argon carrier gas and added to the growth system. By maintaining the above-described ammonia flow rate, bubbling flow rate of trimethylgallium, and flow rate of the carrier gas, the supply of the raw material gas was continued for 90 minutes to form an undoped gallium nitride growth layer having a layer thickness of about 3.5 μm. Since the undoped gallium nitride growth layer formed under such conditions is formed using the buffer layer composed of the single crystal grains as a base, the RH
It was a single crystal layer having uniform orientation without generating a diffused halo elliptical diffraction spot on the EED pattern.
Further, the layer has an n-type conductivity type, and according to the Hall effect measurement using the step (step) etching method, the carrier concentration gradually decreases from the interface side with the buffer layer toward the surface of the growth layer. Was. The carrier concentration on the surface of the undoped gallium nitride growth layer was measured to be about 1.1 × 10 ¹⁷ cm ⁻³ . The surface of the growth layer was excellent in surface state in which projections and pits (pores) having a substantially hexagonal planar shape were hardly observed.

Hydrogen bubble with trimethylgallium
By stopping the addition of
After completing the formation of the doped gallium nitride growth layer,
Maintain the composition and flow rate of monia and carrier gas
To lower the temperature of the substrate to 760 ° C. 760 ° C
Ten minutes after reaching, trimethylgallium is introduced into the growth system.
Was started to be added. Trimethyl gully
The gallium nitride buffer was added to the growth system.
The piping system for trimethylgallium used during the growth of the
used. Stainless steel containing trimethylgallium
The container is kept at 0 ° C by an electronic thermostat and bubbling
The flow rate of hydrogen for use was 2.00 ml per minute. bird
Simultaneously with the addition of methylgallium to the growth system,
Clopentadienyl indium (cyclopenta)
hydrogen gas-containing growth system (dienyl indium)
Was added. Cyclopentadienyl having monovalent valence
Indium (C _Five H_Five In) adhered to the inner wall
Tenle steel cylinder container is 65 ° C in an electronic thermostat
Held. Sublimated ins introduced into this cylinder
The flow rate of hydrogen gas accompanying the source of dimium is per minute by MFC
It was 120 ml. Retention temperature of each group III raw material
Calculate based on vapor pressure or sublimation pressure value in degrees
And the concentration ratio of the source gas supplied into the growth system,
The gas phase composition ratio called by those in the field is about 0.20.
Was. At the same time as adding the Group III raw material to the growth system,
Concentration of about 100 ppm (parts per million by volume) of diethyl
Zinc ((C_Two H_Five )_TwoHigh purity hydrogen gas containing Zn)
3 min / min from manganese steel high-pressure cylinder containing diluent gas
It was supplied at a flow rate of 0 ml. Ammonia
The flow rate was 4.00 liters per minute, each at 5.00 liters per minute.
Growth composed of liter flow of hydrogen and argon
Hydrogen accompanying group III raw material under the above flow conditions in the atmosphere
The addition of gas to the growth system was continued for 15 minutes. Diethyl
Lead ((C_Two H_Five )_Two The flow rate of hydrogen gas containing Zn) is 15
Precisely controlled and adjusted by MFC so that it is constant over a minute
Was. Thus, indium having a layer thickness of about 50 nm is obtained.
Is about 6.4% and zinc (Zn) is
To obtain an indium-containing gallium nitride layer.

The indium-containing gallium nitride layer obtained under such conditions exhibited an n-type even when doped with Group II zinc which is supposed to act as an acceptor. According to SIMS analysis, zinc was found to be distributed almost uniformly in the indium-containing gallium nitride layer. This indium-containing gallium nitride layer was subjected to optical evaluation to be used as an active layer exhibiting a light emitting function, that is, a light emitting layer. Helium-cadmium (He) having a wavelength of 325 nm is applied to the indium-containing gallium nitride layer having the above thickness.
-Cd) Upon irradiation with laser light, blue photoluminescence (PL) emission with a peak wavelength of about 445 nm appeared at room temperature. On the other hand, no band edge emission of the mixed crystal, which should have appeared if a gallium nitride-indium mixed crystal was formed by the mixed crystal formation by addition of indium, was not recognized. Under the same flow conditions as above, III
In the indium-containing gallium nitride layer in which the layer thickness obtained by extending the supply time of the group element to the growth system is larger, the wavelength of the blue emission spectrum is smaller than that in the case where the layer thickness is small (thin), It shifted to the shorter wavelength side. Layer thickness is about 200n
When the thickness was increased to about m, the resulting PL spectrum showed a blue emission spectrum having a center wavelength of about 420 to 430 nm together with a band edge emission of gallium nitride having a wavelength of about 360 nm. No band edge emission of the gallium nitride / indium mixed crystal, which would appear if the mixed crystal was formed, could be confirmed. Regardless of whether or not band-edge emission is confirmed, the PL emission wavelength obtained under the same growth conditions strongly depends on the thickness of the indium-containing gallium nitride layer. Conversely, when the layer thickness was reduced, the main peak appearing in the PL spectrum was presumed to shift to the longer wavelength side, but what became more prominent due to the thinning was the difference in surface morphology. As the layer thickness is reduced, hemispheric projections (hillocks) are more likely to be generated on the layer surface. It is unclear whether this spherical protrusion is caused by a droplet of a group III element generated at the interface between the gallium nitride growth layer and the indium-doped gallium nitride layer or other causes. is there. Regardless of the cause, the surface has few hemispherical hillocks and excellent visibility.
In the present embodiment, which is intended to form a light emitting layer that emits blue light near 0 nm, the thickness of the layer is set to 5 as described above.
It was set to 0 nm.

The growth of the indium-doped gallium nitride layer is as follows:
The process was terminated by stopping the addition of cyclopentadienylindium into the growth system. After the supply of the indium doping source was stopped, the addition of the gallium supply material to the growth system was continued for exactly 1 minute while the substrate temperature was kept at 760 ° C. and the flow rate of the ammonia gas was unchanged. Thereafter, with the addition of the gallium raw material supply source to the growth system stopped, the substrate temperature was raised again to 1100 ° C. without changing the ammonia flow rate. 760 ° C to 11
To the temperature of 00 ° C., the voltage applied to the ceramic heater element whose resistance constituting the heater for heating the substrate was about 6 ohms (Ω) was instantaneously increased and the temperature was raised within 10 minutes. Immediately after the temperature reached 1100 ° C., hydrogen gas accompanying trimethylgallium was added to the growth system by the supply pipe system of trimethylgallium used for forming the gallium nitride growth layer. At the same time, hydrogen gas accompanying trimethylaluminum was added to the growth system to start growth of an aluminum nitride-gallium mixed crystal layer. In the initial stage of the growth of the mixed crystal layer, a mixed ratio of aluminum nitride and gallium (Al _0.20 Ga
_0.80 N). This adjustment was carried out by adjusting the temperature of the stainless steel bubbler container containing trimethylaluminum by an electronic thermostat and by precisely controlling the flow rate of bubbling hydrogen flowing in the bubbler container by MFC. Until the layer thickness is 80nm
The flow rate condition of gallium for providing an aluminum nitride-gallium mixed crystal layer with an aluminum composition ratio of 0.20 and bubbling hydrogen gas accompanying the aluminum raw material was maintained. Thereafter, while maintaining a constant flow rate of the hydrogen gas accompanying the gallium raw material added to the growth system, the flow rate of the bubbling hydrogen gas accompanying the aluminum raw material added to the growth system is gradually reduced by the MFC, and the final flow rate is reduced. Specifically, the addition to the growth system was stopped. Exactly 3 minutes were consumed from the time when the flow rate of the hydrogen bubbling gas for supplying the aluminum raw material was started to be reduced until the time when the addition to the growth system was stopped. This 3
In a minute, the aluminum composition gradually decreased, and finally the thickness of the transition region layer for the aluminum composition having a surface of gallium nitride was 50 nm. That is, the total thickness of the aluminum nitride-gallium mixed crystal layer including the transition region layer having the aluminum composition was 130 nm.

The formation of the aluminum nitride-gallium mixed crystal layer is terminated by stopping the addition of the gallium and aluminum raw materials to the growth system, and after a lapse of one minute, the power supplied to the heater is cut off to start cooling the substrate. did. During cooling, the amount of cooling water flowing into the hollow portion of the double tube constituting the periphery of the stainless steel reactor was intentionally increased, and the outer wall of the reaction tube was blown with a small blower to forcibly cool.
When the temperature of the substrate was forcibly cooled to 600 ° C., the addition of ammonia gas to the growth system was stopped, and only hydrogen and argon constituting the growth atmosphere were supplied to the reactor. Further, the supply of hydrogen gas into the reaction furnace was stopped when the substrate temperature became close to room temperature due to the progress of forced cooling, so that only argon was supplied into the reaction furnace. Thereafter, the evacuation of the reactor and the purging operation with argon gas were alternately repeated twice each. Finally, the inside of the reaction furnace was evacuated, and after the operation of returning to the atmospheric pressure with the argon gas in the preliminary chamber, the substrate was taken out of the reaction furnace.

No peculiar morphology such as protrusions or pits was observed on the outermost surface of the epitaxial layer, and the surface was excellent in smoothness. This was presumed to be because although some hemispherical projections were present on the surface of the light-emitting layer, flattening gradually progressed with the lamination on the same layer.
In addition, the R of the outermost aluminum nitride-gallium mixed crystal layer
From the HEED (reflected electron beam diffraction) pattern, it was analyzed that the layer was a single crystal and, of course, a single crystal layer having a small stacking fault density and excellent crystallinity. FIG. 14 is a cross-sectional TEM image of the epitaxial layer formed by the above process.
(Transmission electron microscope) It is a schematic diagram of a bright field image. As shown in the figure, the portion corresponding to the gallium nitride buffer layer (102) was imaged as a dark image. This is because most of the crystal defects (126) mainly caused by dislocations and stacking faults, which are presumed to occur mainly due to lattice mismatch with the sapphire substrate (101),
2) shows that it is absorbed in. Also,
Although some dislocations that are not absorbed by the buffer layer and propagate to the upper gallium nitride growth layer (117) are also observed, the majority of the dislocations form a loop in the middle of the gallium nitride growth layer (117) and become horizontal. It was to run in the direction and escape. Further, most of the line images presumed to be mainly caused by stacking faults disappeared in the middle of the gallium nitride growth layer (117). For this reason, the epitaxial layer formed by the above-mentioned process has excellent crystallinity with reduced dislocations penetrating to the indium-doped gallium nitride light-emitting layer and the aluminum nitride-gallium mixed crystal layer deposited on the gallium nitride growth layer. It became. Threading dislocations that reach the outermost surface of the epitaxial layer cause pits to be generated on the surface. Therefore, a growth layer excellent in the surface state with few pits like the aluminum nitride-gallium mixed crystal layer can be obtained because it does not easily disappear like an amorphous body even in a high temperature environment, And a buffer layer composed of single crystal grains bonded to each other with a strong cohesive force provided by a uniform lattice arrangement, absorbs dislocations, and leads to threading dislocations. Was interpreted as having a function of reducing

Next, magnesium (Mg) ions were implanted in multiple stages toward the surface of the growth layer deposited on the sapphire substrate. More specifically, the multi-stage ion implantation of magnesium was performed by a solid source method using high-purity magnesium as a magnesium ion source using an ion implantation apparatus having a maximum acceleration voltage of 200 kV. There is no problem if the substrate is appropriately heated at the time of ion implantation, but here, the implantation was performed without intentionally heating the substrate. The first step for implanting magnesium ions deepest from the surface of the aluminum nitride-gallium mixed crystal layer is to set the acceleration voltage to 160 kilovolts (KV) and the dose to 4.0 × 10 ¹³ cm ^−2. Carried out. Subsequently, a second-stage region into which magnesium was implanted was formed on the surface side. In the second injection, the acceleration voltage is set to 1
The test was performed at a pressure of 30 kV and a dose of 2.0 × 10 ¹³ cm ⁻² . In a region where the aluminum composition ratio in the vicinity of the surface of the aluminum nitride-gallium mixed crystal layer gradually decreases toward the surface side, the third region is formed under the conditions of an acceleration voltage of 100 kV and a dose of 2.0 × 10 ¹³ cm ⁻² . The second stage ion implantation was performed. The reason why the implantation is performed by changing the acceleration voltage is to control the depth from the surface at which the concentration of magnesium atoms is maximized, that is, the projection range. The dose is changed in order to obtain a concentration distribution of magnesium atoms that is almost flat in the depth direction by adjusting the maximum attained concentration (peak concentration) in each projection range.

After the ion implantation, titanium nitride (Ti)
N) A thin film was formed. Titanium nitride thin film is normal CVD
, MOCVD, high frequency or electron cyclotron excited plasma CVD, high frequency (microwave) sputtering, etc., but in this embodiment, a reactive ion sputtering method in a nitrogen atmosphere using a simple titanium nitride target is used. Formed. The film thickness is about 100
In nm, the resistivity was on the order of 10 milliohms (mΩ). This titanium nitride thin film is used as a surface protective film of an aluminum nitride / gallium mixed crystal layer during annealing (heat treatment) at a high temperature to activate the ion-implanted atoms, and also as an electrode of an LED described later. belongs to.

The titanium nitride thin film is used to form aluminum nitride.
After the surface of the gallium mixed crystal layer was protected, the epitaxial layer was annealed in a horizontal heat treatment furnace at a temperature of 1080 ° C. for 30 minutes in a nitrogen atmosphere. As a result, the carrier concentration of the aluminum-gallium nitride mixed crystal layer, which was n-type in the as-grown state, was about 2
It was converted to a p-type layer of × 10 ¹⁷ cm ^-3 . The carrier concentration was higher in the transition region of the aluminum composition on the surface side where the aluminum composition was reduced, as compared with the region where the aluminum composition ratio was constant at 0.20.

A known patterning technique for the above-mentioned epitaxial wafer having a pn junction comprising an n-type indium-containing gallium nitride light-emitting layer and an aluminum-gallium nitride mixed crystal layer formed into a p-type by activating ion-implanted atoms, Processing technology such as plasma etching using an argon / methane (CH ₄ ) / hydrogen mixed gas is applied to
An ED was prepared. The p-type electrode (anode) was a titanium nitride thin film also serving as a protective film for ion implantation as described above. A titanium (Ti) layer having a thickness of about 8
A rectangular bonding pad portion was formed by attaching the film to a thickness of 00 nm. One n-type electrode (cathode) was formed by vacuum-depositing aluminum on the n-type gallium nitride growth layer partially exposed by etching.

About 12,000 LED chips of about 350 μm square formed on almost the entire surface of a substrate having a diameter of 2 inches, electrical element characteristics, particularly pn junction characteristics, were inspected by a usual probing method. FIG. 15 shows a typical current-voltage (I) of the LED according to the present embodiment obtained by the sampling inspection.
-V) The characteristic is that normal rectification is obtained. A "threshold" voltage that causes a sharp increase in forward current, i.e.,
The forward voltage (generally represented by the symbol Vf) was distributed between 3.70 volts and 3.95 volts for more than 90% of the devices tested. The average forward voltage value of about 3000 LEDs subjected to the sampling inspection was 3.8 volts. LEDs having a forward voltage of more than 4 volts were mostly present at the very periphery of the epitaxial layer (the periphery of the substrate), but the forward voltage of the LED located at the center of the epitaxial layer (the central region of the substrate) was almost zero. All were within the above voltage range. When the allowable current was 10 microamps (μA), the reverse breakdown voltage (reverse voltage) exceeded 5V. For example, some LEDs have a low withstand voltage as low as 3 to 4 volts in the reverse voltage (often represented by the symbol V), but are limited to the periphery of the epitaxial layer (corresponding to the periphery of the substrate). Was.
The presence of such a low Vr, low breakdown voltage LED in the central region of the epitaxial layer was rare. That is, an LED having normal pn junction characteristics over almost the entire area of the epitaxial layer except for the peripheral portion was manufactured. If the surface state of the epitaxial layer constituting the epitaxial layer, especially the constituting layer constituting the pn junction, has a large number of gaps and pits, the element operating current may be short-circuited through the pits. It is a well-known fact that it is difficult to obtain good pn junction characteristics that exhibit normal rectification. According to the above inspection results, according to the present invention, both crystallinity and surface state are excellent II
It has clearly been shown that a Group I nitride semiconductor layer can be used to form an epitaxial layer for LED applications.

Next, a forward current was passed through the LED to evaluate the light emission characteristics. When the forward current was 20 mA, the emission center wavelength was about 445 nm, and the emission color was observed to be blue. This emission center wavelength has a forward current value of 5
It is about 443 nm at mA and 440 nm at 50 mA.
Although the wavelength slightly shifts to the shorter wavelength side due to the increase in the forward current value, no particularly remarkable shift of the emission center wavelength due to the forward current value was observed. The in-plane distribution of the emission wavelength is small. For example, when the forward current is 20 mA at room temperature, the uniformity is such that almost all (> 90%) of the LEDs to be inspected are accommodated in the range of 445 ± 2 nm. there were. Further, in addition to the blue light emission of this center wavelength, the wavelength is set to about
Emission in the near ultraviolet region of 90 nm was also observed. When the forward current was 50 mA or less, the intensity of the emission spectrum in the near ultraviolet region did not exceed the intensity of the blue emission spectrum. The emission center wavelength is the measurement temperature (LED
), And the amount of shift of the emission center wavelength is only ± even if the measurement temperature is lowered from room temperature to the liquid helium (He) temperature of 4.2 K (Kelvin).
It was about 2 nm. The characteristics of the LED obtained from the group III nitride semiconductor epitaxial wafer provided with the buffer layer according to the present invention include, in addition to the uniformity of the emission center wavelength, for example, a wavelength of about 445 nm measured at room temperature with a forward voltage of 20 mA. Uniformity of the emission characteristics such as the half-value width (so-called abbreviated FWHM value) of the main emission spectrum.

(Comparative Example 2) In order to compare the electrical characteristics and the light emitting characteristics, a group III nitride semiconductor epitaxial wafer provided with a buffer layer having a different form from that of Example 4
An ED was prepared. The growth conditions of the epitaxial layers other than the buffer layer and the growth procedure and operation for forming those epitaxial layers were the same as those in Example 4 except for the p-type method.
The content was exactly the same as described.

The buffer layer in Comparative Example 2 was grown at a temperature of 750 ° C. on a sapphire C-plane substrate in the same manner as in Example 4. During growth, trimethylgallium kept at 13 ° C. by an electronic thermostat was supplied to the growth reaction system by bubbling hydrogen gas at 5 ml / min. The supply amount of the ammonia (concentration 100%) gas to the growth reaction system was 0.7 liter per minute. The V / III ratio is therefore 2.3 ×
When compared with 10 ³ and Example 4, it was about 1/4. Only hydrogen was used as a carrier gas, and the flow rate was 8 liters per minute. The growth atmosphere consisted only of hydrogen. The addition of hydrogen bubbling gas accompanying trimethylgallium to the growth reaction system was continued for 40 minutes to obtain a gallium nitride buffer layer having a layer thickness of 110 nm.

Observation of the undoped gallium nitride buffer layer obtained under the same conditions by a cross-sectional TEM method reveals that this buffer layer is already in an as-grown state and is an aggregate of single crystal grains whose top plate has become a pyramid. It was shown to be composed of These single crystal grains are densely and densely packed without gaps, and the bonding surface (joining interface) between the crystals is (0001)
-Extending linearly vertically above the sapphire substrate surface; That is, the grain interface due to the coalescence of the single crystal grains was developed almost vertically above the substrate surface. Further, observation of the lattice image at high magnification revealed that the single crystal grains were coalesced with the lattice planes substantially parallel to each other. The angle of intersection between the lattice planes was not uniform, and was found to reach a maximum of 25 degrees (°). It is not clear why such a coalescing mode in which the crossing angle increases with an increase in the growth temperature of the buffer layer tends to appear. In addition, the cause of the presence of single crystal grains having different lattice plane arrangement directions from the others is, for example, the lattice strain generated between the sapphire crystal substrate and the gallium nitride buffer layer, which has a lattice mismatch relationship in the first place. It is guessed that it increases as the growth temperature of the buffer layer increases, or that the mechanical strain introduced due to the difference in thermal expansion coefficient between the two increases with the increase in the growth temperature. Not. The buffer layer in this comparative example is still the same as the aggregate of single crystal grains as in Example 4, but the cross-sectional shape of each single crystal grain constituting the aggregate is
The mode of development of the intersection angles of the lattice planes and the coalesced planes (grain boundaries) of the single crystal grains were clearly different from those of the buffer layer described in Example 4.

On this buffer layer, an undoped n-type gallium nitride layer, an n-type indium-containing gallium nitride light emitting layer and a p-type aluminum nitride
Gallium mixed crystal layers were sequentially formed. A p-type aluminum-gallium nitride mixed crystal is formed at the time of its growth by bis-cyclopentadienyl magnesium (bis-cyclopentadad).
eneylmagnesium: ((CH ₃ ) C ₅ H
₄ ) ₂ Mg) was obtained by a conventional method of adding magnesium as a doping source and then annealing at 800 ° C. for 20 minutes in an argon gas atmosphere. The obtained p-type carrier concentration is 3 × the atomic concentration of the doped magnesium.
Against 10 ²⁰ cm ^-3, was only about 5 × 10 ¹⁶ cm ^-3. Compared to the activation rate of the implanted magnesium ions described in Example 4 as a p-type carrier, about 1/4
Efficiency was low. Incidentally, when magnesium was ion-implanted into the undoped aluminum-gallium nitride mixed crystal layer formed with the same lamination structure on the buffer layer of this comparative example as in Example 4, the concentration of the implanted magnesium atoms was 10 ²⁰ cm ⁻ Despite exceeding ³ ,
The electrically activated p-type carrier was also as small as about 4 × 10 ¹⁶ cm ⁻³ .

A large number of pyramidal projections were present on the outermost surface of this epitaxial layer. These pyramids had a hexagonal bottom surface, and had a shape substantially similar to the pyramids forming the tops of the single crystal grains. Since aluminum nitride / gallium does not exist between the side walls of the pyramidal projections, the n-type underlying aluminum nitride / gallium layer is formed in the region where there are projections whose bottom surfaces do not adhere to each other. The surface of the indium-containing gallium nitride light-emitting layer was exposed. In the surface region where no pyramidal projections existed, it was recognized that pits were generated in a large amount as the planar density exceeded 10 ⁴ cm ⁻² . FIG. 16 is a schematic view of a cross-sectional TEM (transmission electron microscope) bright-field image of the epitaxial layer according to this comparative example. As shown in the figure,
The single crystal grains (113) constituting the gallium nitride buffer layer (102) were densely assembled without substantially changing the crystal morphology even after being exposed to a high growth temperature, and covered the substrate surface. . Further, the boundary side surface (114) where the single crystal grains (113) were united with each other had developed linearly in the vertical direction from the surface of the substrate (101). The boundary side (11
Dislocations (116) were generated originating from 4), and were observed to extend upward with respect to the surface of the substrate (101). This dislocation (116) penetrates into the undoped gallium nitride growth layer (117) deposited on the buffer layer (102), and furthermore, the indium-containing gallium nitride light emitting layer (127) and the uppermost layer of the epitaxial layer on the same layer. It reached the surface of the aluminum nitride-gallium mixed crystal layer (128) as the surface layer. The pits were present corresponding to the regions where the dislocations (116) penetrating these epitaxial layer constituent layers reached the surface of the aluminum nitride / gallium mixed crystal layer (128). From this, it was found that pits were generated due to dislocations propagating from the buffer layer through the epitaxial layer constituting layer. Therefore, even if the buffer layer is composed of single crystal grains, the buffer layer composed of single crystal grains assembled in a coalesced form that facilitates the propagation of dislocations to the upper epitaxial layer constituent layer has a surface state. It was not preferable because it resulted in poor quality.

The electrodes were formed using the materials and methods described in Example 4 to form a square LED chip having a side of about 350 μm. A sampling inspection of the bonding characteristics was performed on about 3000 LED chips by probing (probe method) using a normal automatic prober. Measured I-
Most of the V characteristics have no rectifying property in which a current flows between the positive and negative electrodes almost in proportion to the voltage regardless of the polarity of the applied voltage as shown in FIG. That is, it was shown that a normal pn junction providing rectification was not formed in the path through which current flowed. Rather, the IV characteristic is due to the ohmic (o
hmic) suggested the formation of contacts. As described above, the surface state of the epitaxial layer constituting layer obtained in this comparative example is poor, and the p-type aluminum nitride-gallium mixed crystal layer of the outermost layer lacks continuity due to the occurrence of projections and pits. It became. In such a region, an n-type semiconductor layer such as a light-emitting layer below the p-type aluminum nitride-gallium mixed crystal layer was exposed. This causes a situation in which the electrode material disposed on the surface of the outermost layer directly contacts the n-type light emitting layer or the like. Therefore, the abnormal current-voltage characteristic as shown in FIG. 7 is caused by the fact that the electrode laid for the p-type layer on the surface of the outermost layer comes into contact with the n-type layer, so that the n-type This has been explained as being caused by the state where only the semiconductor layer is interposed. On the other hand, a few LEDs exhibiting normal bonding characteristics
Emitted blue light having a center wavelength of about 445 nm. The forward voltage was also about 3.8 to 4.0 volts (V). However, since there are few chips exhibiting good pn junction characteristics, LEDs exhibiting normal light emission characteristics accounted for about 10% of the total number of samples subjected to the sampling inspection. In summary, although the buffer layer is composed of the same aggregate of single crystal grains, the buffer layer composed of the aggregate of single crystal grains having no parallelism in shape or lattice plane according to the present invention has a good growth layer. It cannot provide an element having a proper surface condition or a normal bonding characteristic.

[0063]

According to the present invention, formation of a group III nitride semiconductor epitaxial layer having excellent surface condition and crystallinity is achieved. As a result, excellent surface state and crystallinity III
For example, an element having uniform characteristics can be obtained from an epitaxial wafer for use in a light emitting element, which is composed of a group III nitride semiconductor epitaxial layer.

[Brief description of the drawings]

FIG. 1 is a schematic sectional view showing components of a conventional buffer layer made of amorphous.

FIG. 2 is an imaginary view showing a pattern in which growth islands are generated and developed with single crystal grains scattered in a buffer layer as nuclei.

FIG. 3 is a schematic diagram showing a pattern in which growing islands generated on a buffer layer are united with each other to form a layer.

FIG. 4 is a schematic view showing a pattern in which when a buffer layer according to a conventional technique is exposed to a high-temperature growth environment, an amorphous body disappears and a surface of a substrate is exposed.

FIG. 5 is a schematic view showing a pattern of actual occurrence and growth of a growth island at the time of high-temperature film formation when a substrate surface is exposed due to disappearance of an amorphous body constituting a buffer layer.

FIG. 6 is a schematic cross-sectional view of an epitaxial layer of a compound semiconductor epitaxial wafer in a region where pits are densely formed on the surface.

FIG. 7 is an example of a current-voltage characteristic of a pn junction type group III nitride semiconductor light emitting device manufactured from an epitaxial layer including many pits (gaps).

FIG. 8 is a schematic diagram showing a cross-sectional TEM (transmission electron microscope) image of the gallium nitride buffer layer according to the present invention, which is composed of single crystal grains grown at a growth temperature of 620 ° C.

FIG. 9 is a schematic diagram showing a cross-sectional TEM image of a gallium nitride buffer layer grown at a temperature (920 ° C.) at which a single crystal layer is conventionally formed.

FIG. 10 is a cross section T of a gallium nitride buffer layer grown at 600 ° C. by an ammonia / trimethylgallium reaction system.
It is a schematic diagram of an EM image.

FIG. 11 is a schematic diagram of a cross-sectional TEM image of a gallium nitride buffer layer grown at 650 ° C. on a sapphire C-plane substrate.

FIG. 12 is a schematic view of a cross-sectional TEM image of an aluminum nitride-gallium mixed crystal buffer layer according to Example 2.

FIG. 13 is a cross-sectional TEM image of an epitaxial layer showing a causal relationship between a change in an amorphous buffer layer and the occurrence of pits and protrusions according to the conventional technique when exposed to a high-temperature film-forming environment.
It is a schematic diagram of an image.

FIG. 14 is a schematic view showing a bright-field cross-sectional TEM image of a group III nitride semiconductor epitaxial layer including a buffer layer made of a single crystal grain according to the present invention described in Example 4;

FIG. 15 shows a typical current-voltage characteristic (I) of a light emitting device (LED) composed of a group III nitride semiconductor epitaxial layer provided with a buffer layer made of a single crystal grain according to the present invention and described in Example 4; -V) is a diagram showing characteristics.

FIG. 16 is a schematic diagram showing a cross-sectional TEM image of an epitaxial layer including a buffer layer according to Comparative Example 2.

FIG. 17 is a diagram showing the relationship between the intersection angle of gallium nitride single crystal grains constituting the buffer layer and the half-value width of the diffraction pattern of the gallium nitride growth layer grown on the buffer layer.

[Explanation of symbols]

(101) Substrate (102) Buffer layer (103) Substrate / buffer layer interface (104) Amorphous substance (105) Single crystal grain (106) Growth island (107) Void (pit, pore) (108) Amorphous (109) A region where the amorphous body is crystallized when the amorphous body is exposed to a high-temperature environment. (110) An epitaxial layer composed of a multi-layered group III nitride semiconductor layer. 111) First epitaxial layer constituting layer having p-type conductivity (112) Second epitaxial layer constituting layer having n-type conductivity (113) Single crystal grains constituting buffer layer (114) Buffer layer Outer peripheral side surface of the single crystal grain constituting (115) Top plate part on top of single crystal grain constituting buffer layer (116) Dislocation (117) Gallium nitride growth layer formed at high temperature (118) Single crystal (119) Direction in which lattice planes are layered (direction perpendicular to lattice plane image) (120) Grid plane image captured by cross-sectional TEM method (121) Angle at which perpendiculars to lattice plane image intersect each other (intersection angle) ( 122) Width of single crystal grains constituting buffer layer (123) Polycrystal which is one component of conventional buffer layer (125) Projection appearing on gallium nitride based growth layer surface (126) Mainly stacking fault (127) Gallium nitride high-temperature growth layer containing indium (In) (128) Aluminum nitride-gallium mixed crystal high-temperature growth layer (129) Side wall of single crystal column (130) Top plate portion of single crystal column (131) The top of the pyramid

Claims (2)

[Claims] 1. A compound semiconductor epitaxial wafer comprising: a buffer layer composed of a group III nitride semiconductor laminated on a surface of a crystal substrate; and an epitaxial layer composed of a group III nitride semiconductor laminated on the buffer layer. The buffer layer is composed of an aggregate of single crystal grains, and the single crystal grains forming the buffer layer are bonded together in a detour shape at a side surface that is joined to each other in an as-grown state, and a top plate portion at the top is formed. A compound semiconductor epitaxial wafer having a substantially flat or curved surface. 2. The compound semiconductor epitaxial wafer according to claim 1, wherein the single crystal grains constituting the buffer layer have an angle difference of a stacking direction of each other within 30 degrees in an as-grown state.