JP3152152B2 - Compound semiconductor epitaxial wafer - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

FIELD OF THE INVENTION The present invention is grown at low temperatures.
The present invention relates to a compound semiconductor epitaxial wafer provided with a buffer layer made of a group III nitride semiconductor, and more particularly to a compound semiconductor epitaxial wafer provided with a buffer layer made of a group III nitride semiconductor grown at a low temperature, wherein the substrate and the epitaxial layer are lattice-mismatched. The present invention relates to an epitaxial wafer of a group III nitride semiconductor.

[0002]

2. Description of the Related Art Group III nitride semiconductors such as gallium nitride (GaN), aluminum nitride (AlN), indium nitride (InN), and a mixture thereof have been conventionally used in field effect transistors (FETs) and light emitting devices. It is a material for semiconductor devices such as diodes (LEDs). III
Group III nitride semiconductor epitaxial wafer crystal substrates include:
Generally, α-alumina single crystal (sapphire), silicon carbide (SiC), zinc oxide (ZnO) and the like are used. These substrates and the group III nitride semiconductor forming the epitaxial layer have different lattice constants. Therefore, the epitaxial layer of the group III nitride semiconductor generally has a lattice mismatch structure with respect to the substrate.

In the production of a group III nitride semiconductor epitaxial wafer, it is not easy to form a continuous epitaxial thin film having excellent crystallinity on a crystal substrate having a lattice mismatch. A technique of growing a buffer layer and then growing an epitaxial layer is used. This buffer layer is called a low-temperature buffer layer because it is grown at a temperature of 400 ° C. to 800 ° C., which is a low temperature compared to about 1000 ° C. where an epitaxial layer made of a group III nitride is usually grown. The low-temperature buffer layer is provided to alleviate the lattice mismatch between the substrate and the epitaxial layer made of a group III nitride semiconductor and to form a continuous epitaxial thin film.

A low-temperature buffer layer of a group III nitride semiconductor epitaxial wafer is generally made of Al _x Ga ₁ -xN (0 ≦ x ≦ 1) (JP-A-2-229476,
JP-A-4-297523, JP-A-5-41541
And JP-A-6-151962). The film thickness is limited to a narrow range, specifically, 100 to
It is as thin as 500 angstroms (Japanese Unexamined Patent Publication No.
229476). Further, the conventional low-temperature buffer layer is made of an amorphous material.
No. 51962). Alternatively, it is a state in which microcrystals or polycrystals are sparsely scattered in an amorphous state (Japanese Patent Laid-Open No.
-229476). That is, the conventional low-temperature buffer layer is a general low-temperature buffer layer formed at a high temperature exceeding about 1000 ° C.
The buffer layer of the group I nitride single crystal was different not only in growth temperature but also in crystallographic form.

[0005]

An epitaxial layer of a group III nitride semiconductor used in a semiconductor device is formed on this low-temperature buffer layer at a high temperature of about 1000.degree. The conventional amorphous low-temperature buffer layer has a problem in that the low-temperature buffer layer is decomposed due to thermal dissociation in the process of raising the temperature of the epitaxial layer to the growth temperature. Decomposition due to thermal dissociation of the amorphous low-temperature buffer layer occurs because the bonding force between atoms constituting the amorphous is weak. If a part of the low-temperature buffer layer deposited on the crystal substrate disappears, the crystal substrate is exposed in that part. If the surface of the crystal substrate is unevenly exposed as described above, lattice mismatch is caused.
Uniform growth of the group III nitride semiconductor epitaxial thin film is not achieved. That is, in the region where the low-temperature buffer layer remains,
Although the two-dimensional growth of the epitaxial thin film of the group III nitride semiconductor is promoted, in the region where the substrate surface is exposed, the island-like growth nuclei grow only in isolation, and as a result, the coalescence of the growth islands does not occur. Since it is complete, a continuous thin film cannot be obtained. In such a case, in the epitaxial thin film, pits (pores) having substantially hexagonal openings and projections as if substantially flat hexagonal flat plates are stacked are recognized. A semiconductor device manufactured using a group III nitride semiconductor formed of such a discontinuous epitaxial thin film causes abnormal characteristics. For example, LEDs have abnormalities in rectification characteristics and the like.

As described above, conventionally, in a group III nitride semiconductor epitaxial wafer, the non-uniformity of the coating on the substrate surface due to the partial disappearance of the low-temperature buffer layer causes the surface morphology of the epitaxial thin film to be deposited thereon. It was causing deterioration. An object of the present invention is to provide a low-temperature buffer layer that does not disappear at a high temperature, based on knowledge on crystallographic components of a low-temperature buffer layer that is resistant to thermal dissociation at a high temperature, to provide excellent crystallinity and An object of the present invention is to provide a compound semiconductor device having excellent characteristics by growing a continuous epitaxial thin film of a group III nitride semiconductor.

[0007]

The present invention provides a crystal substrate,
A compound semiconductor epitaxial wafer comprising: a low-temperature buffer layer made of a group III nitride semiconductor grown at a low temperature on the crystal substrate; and an epitaxial layer made of a group III nitride semiconductor grown at a high temperature on the low-temperature buffer layer. Wherein the region of the low-temperature buffer layer near the interface with the crystal substrate is a single crystal.

The low-temperature buffer layer according to the present invention is characterized by a crystal structure in a region near an interface with a crystal substrate when grown at a low temperature. That is, the low-temperature buffer layer according to the present invention is characterized in that a single crystal layer is arranged in a region near the interface of the buffer layer with the crystal substrate in a state where the buffer layer is grown at a low temperature. The layer composed of a single crystal does not easily undergo thermal dissociation even in a high temperature environment, unlike the layer composed of an amorphous. This is because, since it is a single crystal, the bonding force between constituent atoms is much stronger than that of an amorphous. Therefore, according to the present invention, in a state where the buffer layer is grown at a low temperature, the single crystal layer is arranged in a region near the interface with the substrate,
By covering the surface of the substrate, the single crystal layer can be left on the substrate surface even under the high temperature environment necessary for depositing the epitaxial layer,
The conventional problem that the substrate surface is partially exposed can be avoided. As described above, the single crystal layer which remains uniformly on the substrate surface even at a high temperature provides an epitaxial thin film having an excellent surface condition.

[0009]

Low-temperature buffer layer according to the embodiment of the present invention have the general formula _{Al x Ga y In z N (} x + y + z = 1,0 ≦
x, y, z ≦ 1). The low-temperature buffer layer composed of the above-mentioned material is deposited on a substrate which is well-known for a Group III nitride semiconductor epitaxial wafer such as sapphire. Examples of the crystal substrate include sapphire, a metal material such as hafnium (Hf), a group III-V compound semiconductor crystal having a face-centered cubic lattice structure such as gallium arsenide (GaAs) or gallium phosphide (GaP), or silicon. Elemental (single) semiconductor crystals such as (Si) can also be used. Alternatively, lithium (Li) and gallium (G) having a lattice mismatch with GaN of less than 0.5% are small.
a) Alternatively, Li ₂ GaO ₃ or Li ₂ AlO ₃ which is an oxide of Li and aluminum (Al) can be used as the substrate. The conductivity type does not matter for a crystalline substrate made of a semiconductor. Further, the specifications such as the plane orientation and off-cut (mis-orientation angle) of the crystal plane forming the substrate surface may be appropriately selected in consideration of the growth method and growth conditions of the low-temperature buffer layer.

The low-temperature buffer layer according to the present invention can be grown by metalorganic compound vapor phase epitaxy (MOCVD). In order to grow the low-temperature buffer layer according to the present invention by the MOCVD method under normal pressure, the growth temperature, the growth rate, and the V / III ratio of the feed material (the group V source gas and the group III source gas supplied to the growth atmosphere) are required. Is preferably set as follows.

The growth temperature of the low-temperature buffer layer according to the present invention generally needs to exceed 400 ° C. But,
When the growth temperature exceeds about 500 ° C., projections are formed on the surface of the low-temperature buffer layer, and the surface state deteriorates. Therefore, although the optimum temperature range slightly varies depending on the apparatus used for growth, the growth temperature of the low-temperature buffer layer is preferably set in the range of 400 ° C. to 480 ° C.

In order to grow the low-temperature buffer layer of the present invention, it is preferable that the growth rate of the buffer layer is 1 nm / min or less. Setting the growth rate to such a small value means keeping the substrate at the growth temperature for a long time. For this reason, migration of the raw material molecules forming the low-temperature buffer layer on the substrate surface is performed.
Is promoted, which increases the chance of rearrangement of the raw material molecules adsorbed on the substrate surface, and is effective in forming a single crystal layer.

Further, the ratio between the group V source gas and the group III source gas supplied to the growth atmosphere, the so-called V / III ratio,
It is preferable to set the value higher than the value used in growing a group III nitride semiconductor by the conventional MOCVD method. The present inventors have conducted intensive studies on the conditions for growing a good low-temperature buffer layer. As a result, as described above, in order to reduce the growth rate of the buffer layer to 1 nm or less per minute, the group III raw material supplied to the growth atmosphere is used. It has been found that, even when the amount of gas is set to a small value, a good low-temperature buffer layer can be stably obtained if the amount of group V gas supplied at the same time is not changed from the conventional value. That is, the V / III ratio should be set to a value exceeding 1.5 × 10 ⁴ in order to obtain a low-temperature buffer layer having a single crystal in the region near the interface with the substrate according to the present invention with sufficient stability. preferable.

By setting the growth conditions as described above, even at a low growth temperature of less than 500 ° C. where an amorphous buffer layer is conventionally formed, the region near the interface with the substrate according to the present invention can be used. It has become possible to obtain a low-temperature buffer layer made of a Group III nitride semiconductor having a single crystal. However, MOC
Since the characteristics of the low-temperature buffer layer made of a group III nitride semiconductor formed by the VD method largely depend on the apparatus used for growth, the above-mentioned growth conditions in the practice of the present invention depend on the growth apparatus used. It is necessary to make appropriate adjustments accordingly. The low-temperature buffer layer according to the present invention can also be formed by vapor phase epitaxy (VPE) or molecular beam epitaxy (MBE) by setting appropriate growth conditions.

Further, in order to uniformly cover the substrate surface with the low-temperature buffer layer, it is necessary to clean the surface of the substrate. On the surface of a sapphire substrate having a large amount of free oxygen and the like on the surface, a low-temperature buffer layer tends to be difficult to grow uniformly in area. For the cleaning treatment of the substrate surface, a known thermal etching method performed in a reducing atmosphere composed of hydrogen or the like, or a wet treatment using an ammonium fluoride aqueous solution, aqua regia, or the like can be used. The cleanliness based on the amount of metal or nonmetal impurities on the surface of the crystal substrate can be evaluated by a surface analysis method such as Auger electron spectroscopy (AES) or secondary ion mass spectrometry (SIMS). The presence or absence of an oxide film on the surface can be evaluated by a reflection electron beam diffraction (RHEED) method or the like.

In the present invention, when the region of the low-temperature buffer layer in the vicinity of the interface with the crystal substrate is a single crystal, the single crystal layer may be only one atomic layer. In order to stably prevent the occurrence, it is preferable that the thickness be at least several atomic layers or more. The number of single crystal layers covering the substrate surface can be determined, for example, by observing a cross section using a transmission electron microscope (TEM). In the low-temperature buffer layer according to the present invention,
That is, in a region where the thickness of the low-temperature buffer layer is larger, polycrystal or amorphous may exist. That is, as the distance from the substrate surface increases, the influence of the regular arrangement based on the single crystal arrangement of the crystal substrate decreases, so that polycrystals and amorphous materials are generated in regions other than the vicinity of the interface of the low-temperature buffer layer. However, these polycrystalline and amorphous forms grow on it III
It does not affect the epitaxial layer of the group III nitride semiconductor.

FIG. 1 schematically shows the structure of the low-temperature buffer layer according to the present invention. In FIG. 1, a low-temperature buffer layer (102) according to the present invention has been grown on a substrate (101).
In the low-temperature buffer layer (102), a region near the interface with the substrate (101) is a single crystal layer (103) made of a single crystal.
The thickness of the single crystal layer (103) is one atomic layer (about 0.5
nm) or more. Preferably it is 2 to 20 nm. The thickness of the single crystal layer (103) can be controlled by appropriately setting the growth time and the like under the above growth conditions. In the low-temperature buffer layer (102), a region having a thickness exceeding the thickness of the single crystal layer, that is, the single crystal layer (10
3) Amorphous (104) and polycrystalline (105) may be formed on the upper surface, but these amorphous and polycrystalline do not affect the epitaxial layer grown thereon. .

[0018]

When a crystalline substrate is exposed to a high temperature environment in order to grow an epitaxial layer on the low temperature buffer layer at a high temperature, most of the conventional low temperature buffer layer mainly composed of amorphous is volatilized. I will. This is because the low-temperature buffer layer is not made of a single crystal, so that the constituent atoms have a weak mutual bonding force and have little resistance to thermal dissociation. On the other hand, according to the present invention, even if the amorphous substance present in the low-temperature buffer layer is thermally dissociated and volatilized, at least one atomic layer composed of a single crystal is arranged at the interface with the substrate. In addition, the substrate surface is prevented from being exposed. This is because a layer formed of a single crystal does not easily cause thermal dissociation unlike an amorphous layer. Therefore, the low-temperature buffer layer according to the present invention can prevent the surface of the crystal substrate from being exposed under a high-temperature environment, and can grow a continuous epitaxial thin film of a group III nitride semiconductor having excellent crystallinity thereon. To

[0019]

【Example】

(Embodiment) A schematic structure of a group III nitride semiconductor epitaxial wafer according to the present invention is shown in FIG. The epitaxial wafer shown in FIG. 2 was grown by the MOCVD method according to the procedure described below.

The low-temperature buffer layer was grown using sapphire having the (0001) C plane as the surface, as the substrate (101). The mirror-polished surface of the substrate (101) was cleaned by the following method before the start of growth to promote the formation of a uniform low-temperature buffer layer. First, degrease with acetone,
After washing with water, washing was performed using a high-purity ammonium fluoride aqueous solution. Then, after washing with ultrapure water, it was dried by heating with an infrared lamp.

The cleaned sapphire substrate (101)
The substrate was placed in a MOCVD growth furnace at room temperature. After the inside of the growth furnace was once evacuated to a vacuum by a rotary pump, high-purity hydrogen gas was flowed into the furnace to form an atmosphere composed of hydrogen gas. The temperature of the substrate (101) was raised from room temperature to 1100 ° C. by a resistance heating method, the substrate (101) was kept at the same temperature for 40 minutes, and subjected to thermal cleaning.

Thereafter, the temperature of the substrate (101) is set to 420 ° C.
Lowered. Hold for about 25 minutes until temperature stabilizes. After that, the supply of ammonia (NH ₃ ) was first started toward the surface of the substrate placed in the growth furnace, and the supply of ammonia was continued for 10 minutes to make the growth environment an atmosphere containing ammonia. Then, trimethylgallium (G
a (CH ₃ ) ₃ ) is started, and the atmospheric pressure MOCV of the low-temperature buffer layer (102) made of GaN on the substrate (101) is started.
Growth by the D method was performed. The supply amount of trimethylgallium into the furnace was 2.0 × 10 ⁻⁶ mol / min. The supply rate of ammonia was 1 liter per minute. The V / III ratio of the raw material in this case was about 2.2 × 10 ⁴ .

Under these conditions, the low-temperature buffer layer (10
The growth of 2) was performed. The layer thickness of the low-temperature buffer layer (102) was 15 nm. That is, the growth rate of the low-temperature buffer layer in this example was 0.75 nm / min. The cross section of the low-temperature buffer layer obtained under the same growth conditions was
As a result, the region having a thickness of about 10 nm from the interface of the low-temperature buffer layer (102) was composed of a single crystal.
The region above it was a region mainly composed of GaN amorphous.

Thereafter, the temperature of the substrate was increased, and the next epitaxial layer was sequentially laminated at 1000 ° C. on the low-temperature buffer layer (102) to form an epitaxial wafer for a light emitting device. (1) Silicon (Si) -doped n-type GaN high-temperature buffer layer (3 μm thick, carrier concentration 2 × 10 ¹⁸ cm ⁻³ )
6) (2) thickness 0.7 μm, carrier concentration about 6 × 10 ¹⁷ cm
N-type Ga doped with ^-3 zinc (Zn) and tin (Sn)
_0.95 In _0.05 N light emitting layer (107) (3) Film thickness 0.6 μm, carrier concentration 2 × 10 ¹⁶ cm ⁻³
P-type Al _0.05 Ga doped with magnesium (Mg)
_0.85 N upper cladding layer (108) (4) Film thickness 0.3 μm, carrier concentration 2 × 10 ¹⁸ cm ⁻³
As a result, on the surface of the contact layer (109), pits (pores) having a substantially hexagonal opening or hexagonal projections are formed on the surface of the contact layer (109). Was hardly recognized, and the surface was flat and smooth.

The epitaxial layers ((106) to (10)
9)), the cross section of the low-temperature buffer layer (102) is
As a result of the observation, when the low-temperature buffer layer (102) is formed at a low temperature, the thickness of the low-temperature buffer layer (102) is about 10 n
Most of the amorphous material existing in the region above
It disappeared after the formation of the epitaxial layer at 0 ° C.
In addition, a spindle-shaped polycrystal (105) was also found in a part of the n-type GaN high-temperature buffer layer (106). This was because the polycrystal contained in the amorphous remained, or It is considered that part of the amorphous was crystallized in a high-temperature environment.

FIGS. 3 and 4 are schematic plan and sectional views, respectively, of a light emitting device manufactured from the above epitaxial wafer. In the above epitaxial wafer, a part of the epitaxial layer is removed by etching in the depth direction using a general photolithography technique or the like, and then the surface of the contact layer (109) and the surface of the n-type GaN high-temperature buffer layer (106) are removed. P-type and n-type electrodes ((111) and (110))
Is formed and then cut into a substantially square to form a light emitting device (LE
D) was prepared. When a forward current was applied to this LED, blue light emission was obtained. The center wavelength of the emission is approximately
It was 440 nm. The emission output measured after molding with a common epoxy resin for semiconductor element sealing is 0.8
It was on the order of milliwatts (mW). In addition, the current-voltage (IV) characteristics of the light-emitting element show normal rectification, and the forward voltage is about 3.6 to 3.8 V when the forward current is 20 milliamperes (mA). Was. The voltage required to flow 10 μA in the reverse direction was 5 V or more.

(Comparative Example) For comparison, an epitaxial wafer of a group III nitride semiconductor was manufactured by changing the conditions for growing the low-temperature buffer layer from those of the above example. That is, in this comparative example, the growth temperature of the low-temperature buffer layer was 390 ° C. The supply amount of Ga (CH ₃ ) ₃ during the growth of the low-temperature buffer layer was 2.9 × 10 ⁻⁵ mol / min, and the supply amount of ammonia was 1 liter per minute. In this case, the V / III ratio is 154
It becomes 0. The growth time of the low-temperature buffer layer was 10 minutes. The growth conditions other than those described above were the same as in the above-described example, and the low-temperature buffer layer was grown. The thickness of the low-temperature buffer layer obtained in this comparative example was about 14 nm. Thereafter, in the same manner as in the above-described embodiment, an epitaxial layer of a group III nitride semiconductor was stacked.

The surface of the low-temperature buffer layer formed under the above conditions had a metallic luster when observed by a differential interference optical microscope. In addition, a general scanning electron microscope (S
During irradiation with a scanning electron beam for observing the surface using EM), as the low-temperature buffer layer having a thickness of about 14 nm in the irradiated area volatilized, the film quality was softer. Cross section TEM
According to the observation by, the low-temperature buffer layer was composed of an amorphous material, and scattered fine polycrystalline particles were also observed.
However, no single crystal layer was present on the substrate surface.
This is believed to be because the growth temperature was lower as compared to the examples described above. Therefore, the low-temperature buffer layer of this comparative example was clearly different from the above-mentioned example.

On the low-temperature buffer layer, an epitaxial layer similar to that of the embodiment was deposited to form an epitaxial wafer for LED. FIG. 5 schematically shows the surface state of the contact layer which is the outermost layer of the epitaxial layer. On the contact layer surface (109), the low-temperature buffer layer disappears during the process of raising the temperature of the substrate to form an epitaxial layer at a high temperature,
The pits (11) frequently generated in the area where the substrate surface is exposed
2) and substantially hexagonal projections (113) were present. When the surface state of each epitaxial layer was observed by sequentially peeling the epitaxial layers, similar pits and projections had already appeared on the GaN high-temperature buffer layer immediately above the low-temperature buffer layer.
Many pits penetrated the epitaxial layer located above the pit, so that the pn junction interface between the n-type light emitting layer and the p-type upper cladding layer was disordered.

In the epitaxial wafer obtained in this comparative example, after removing a part of the epitaxial layer in the depth direction by using a general photolithography technique or the like, the n-type Then, an LED was manufactured by forming a p-type electrode. The LED did not emit light when a forward current of about 20 mA was applied. Further, as shown in FIG. 6, the IV characteristics of the device were close to ohmic characteristics in both the forward and reverse directions, and did not exhibit rectification.

[0031]

According to the present invention, it is possible to obtain an epitaxial wafer having excellent crystallinity and a continuous epitaxial thin film of a group III nitride semiconductor. For this reason,
A compound semiconductor element having excellent electric characteristics can be obtained. Incidentally low-temperature buffer layer according to the present invention, Al _x Ga _y I
_{n z N a P 1-a} (0 <a ≦ 1) or Al _x Ga _y I
_{n z N b As 1-b} (0 <b ≦ 1) represented by, including a Group V element such as arsenic other than nitrogen (As) or phosphorus (P) III
It may be made of a group III nitride semiconductor. Further, the low-temperature buffer layer according to the present invention may be made of zinc oxide (ZnO) or silicon carbide (Si).
C) and other materials other than Group III nitride semiconductors,
The same effect as the present invention can be expected.

[Brief description of the drawings]

FIG. 1 is a view showing a cross section of a low-temperature buffer layer according to the present invention.

FIG. 2 is a schematic cross-sectional view of a compound semiconductor epitaxial wafer according to an embodiment of the present invention.

FIG. 3 shows a compound semiconductor device (L) according to an embodiment of the present invention.
ED).

FIG. 4 shows a compound semiconductor device (L) according to an embodiment of the present invention.
ED) is a sectional view.

FIG. 5 is a diagram showing a surface morphology of an epitaxial wafer according to a comparative example.

FIG. 6 is a diagram showing current-voltage characteristics of a compound semiconductor device (LED) according to a comparative example.

[Explanation of symbols]

 (101) Substrate (102) Low temperature buffer layer (103) Single crystal layer (104) Amorphous (105) Polycrystalline (106) High temperature buffer layer (107) Light emitting layer (108) Upper cladding layer (109) Contact layer ( 110) n-type electrode (111) p-type electrode (112) pit (pore) (113) substantially hexagonal projection

Claims (1)

(57) [Claims] 1. A crystal substrate, a low-temperature buffer layer made of a group III nitride semiconductor grown on the crystal substrate at a low temperature, and an epitaxial layer made of a group III nitride semiconductor grown on the low-temperature buffer layer at a high temperature. Wherein the region near the interface of the low-temperature buffer layer with the crystal substrate is a single crystal.