JPH1092749A - Method of forming nitride compound semiconductor layer - Google Patents

Abstract

PROBLEM TO BE SOLVED: To promote the generation of a grown island, having a flat surface and grown of an In-containing nitride compound semiconductor having low reactivity and high decomposability by reducing the volumetric ratio of a gas of a large thermal conductivity to a gas of a small thermal conductivity with the lapse of time along with increase in the thickness of the nitride compound semiconductor layer. SOLUTION: In vapor growth of a nitride compound semiconductor layer, a gas creating a growth atmosphere is not a gas material but a gas-type used as a diluent gas of a carrier gas or a doping gas. In a growth atmosphere made by mixing a gas having a volume V0 and a gas having a smaller thermal conductivity and a volume V, the volumetric mixing ratio γ of both gases is calculated from V/V0 . In the case where a mixed gas of hydrogen having a relatively large thermal conductivity and argon having a relatively small thermal conductivity is used as a growth atmosphere, at the time of forming the nitride compound semiconductor layer, a numerical value given in accordance with an intended ultimate thickness is used as an initial value, and V/V0 is reduced with the lapse of time along with increase in the thickness.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for forming a nitride compound semiconductor layer, and more particularly to a method for forming a nitride compound semiconductor layer having a low pit (pore) density and an excellent surface state.

[0002]

2. Description of the Related Art An environment-resistant field effect transistor (FET) or a short-wavelength visible light emitting diode (LE) such as blue is used.
D) and other nitride compound semiconductor devices have the general formula Al _{x
Ga y In z N (x +} y + z = 1,0 ≦ x, y, z ≦
It is configured using the nitride compound semiconductor layer represented by 1). Generally, it is formed on a crystal substrate made of α-alumina single crystal (sapphire), silicon carbide (SiC), zinc oxide (ZnO), or the like. Alternatively, it is also formed on an epitaxial growth layer made of a nitride compound semiconductor material.
In the formation of a nitride compound semiconductor layer by a vapor phase growth method such as an organometallic thermal decomposition (MOCVD) method, a hydrogen (H ₂ ) gas is used as a carrier gas in a growth atmosphere mainly composed of hydrogen. It is customary to carry out this. This hydrogen generated by thermal decomposition of the raw material such as an organic metal compound, and compounds with fragments, such as unwanted methyl group to the growth (-CH _3), to create a highly volatile substance easily transported out of the growth system This is because it is considered to be convenient (JP-A-60-175412). Further, it is said that when a nitride compound semiconductor layer is grown in the presence of hydrogen gas, the morphology of the layer surface tends to be improved (see Journal of Crystal Growth (J. Cry).
stal Growth), 68 (1984), 16
3).

[0003] The formation (growth) temperature of a nitride compound semiconductor layer is generally high, for example, sapphire (0001) plane (C
Surface) The film forming temperature for forming gallium nitride (GaN) on a substrate generally exceeds about 1000 ° C. Al _x Ga
_For mixed crystals containing aluminum (Al), such as _1-xN (0 <x ≦ 1), the optimal formation temperature is further from about 50 ° C.
About 100 ° C. Under such a growth environment that requires a formation temperature exceeding 1000 ° C., so-called “winding” of a growth source gas or a hydrogen carrier gas at a high temperature occurs remarkably. "Winding" is mainly caused by the fact that a gas received under a high temperature environment increases its pressure.
This "winding" occurs remarkably in the growth environment, particularly near the substrate surface where the temperature is high. Due to this "rolling up", the source gas does not sufficiently reach the surface of the substrate or the surface of the layer to be deposited as an underlayer, and rolls upward, so that a nitride compound semiconductor layer having an intended thickness cannot be obtained. Or, a problem such that film formation is hardly achieved occurs. That is, a defect generally experienced in forming a nitride compound semiconductor layer under a growth atmosphere consisting of hydrogen alone is that the density of growth islands generated on the surface of the deposit is low, and as a result, the surface of the growth layer becomes rough. (Japanese Patent Application Laid-Open No. Sho 60-175412)
No.).

FIGS. 1 to 3 are schematic diagrams showing a process of forming a gallium nitride (GaN) layer in a growth atmosphere consisting only of hydrogen in a time-series manner. FIG. 1 shows the (0001) plane (C
FIG. 3 schematically shows an initial stage of growth of a nitride compound semiconductor layer in which a growth island (103) made of gallium nitride starts to be generated centering on a growth nucleus (102) on a sapphire substrate (101). On the sapphire C-plane substrate,
Hexagonal pillar-shaped growth islands (103) are many and scattered. FIG. 2 shows that the growth of the growing island is more advanced and the growing island (10
3) shows the stage where it has developed vertically and horizontally, and coalescence has begun between some growing islands. Furthermore, at the stage of aging, the coalescence of the growth islands (103) is repeated, and a layer having a flat surface is formed in a region where the growth islands are closely bonded to each other.
However, in the growth in a hydrogen atmosphere, the growth in the vertical direction (height direction) occurs more selectively and preferentially than in the horizontal direction (horizontal direction), so that the horizontal width of the columnar crystal is greatly increased. The opportunity for growing islands to coalesce is not very large. If the integration and fusion between the growth islands (103) are incomplete, gaps (104) are formed between the growth islands, and this gap (10
Finally, pits (pores) (1) surrounded by growing islands
05) (see FIG. 3). In view of the cause of the generation of such pits, a method of forming (growing) a larger growth island or increasing the density of the growth island from the initial stage of layer formation, or a method of forming the growth island in the horizontal direction (horizontal direction). It is suggested that if the formation method promotes dimensional growth, the gap between the growth islands is narrowed, and the generation of pits can be suppressed.

[0005] In order to suppress the "winding" in the high temperature region near the surface of the deposit, which also affects the surface state of the nitride compound semiconductor layer, conventionally, the specific gravity of the gas is set higher than that of hydrogen and hydrogen. A vapor phase growth technique using a mixed gas containing an inert gas such as nitrogen (N ₂ ) as a carrier gas is disclosed (for example, Journal of Electronic Materials (J. Electr)).
on. Mater. ), Vol. 14 , no. 5 (19
85), 633-644, etc.). The intent of this prior art is to suppress the “winding” of hydrogen gas or source gas as a carrier gas by allowing molecules having a specific gravity larger than hydrogen to be present in the growth system. Is what it is. Combination of hydrogen and nitrogen (Japanese Patent Laid-Open No. 4-164895, U.S. Pat.
In addition to the above, JP-A-60-175 discloses a method for vapor-phase growth of a nitride compound semiconductor layer using a mixed gas of hydrogen and argon (Ar) as a growth atmosphere.
412).

However, a common carrier gas, such as hydrogen,
Hydrogen is a method of forming a nitride compound semiconductor in a growth atmosphere in which a gas such as nitrogen or argon having different specific gravity (molecular weight) or thermal conductivity is mixed. Then, it is not known that the shape and the form of the growth island change even. Below, general MOCVD
In the method of forming a nitride compound semiconductor layer in the vapor phase growth method, the influence of the volume mixing ratio of hydrogen and argon on the shape of the growth island and, consequently, the crystal growth mode was determined by the present inventor. The outline is based on the result of forming the (GaN) layer. Here, the volume mixing ratio (γ) of hydrogen (H ₂ ) and argon (Ar) is the ratio of the volume (flow rate) of Ar to the volume (flow rate) of H ₂ constituting the growth atmosphere, that is, γ.
= Is an Ar / H _2. FIG. 4 schematically shows the surface state of a GaN growth layer formed directly at 1000 ° C. on a (0001) -sapphire (C-plane) substrate and having a layer thickness of about 5 μm.
The flow rates of hydrogen and argon constituting the growth atmosphere are respectively
3.0 l / min and 0.1 l / min. Therefore, γ is about 0.03, which is equivalent to all growth in an atmosphere of hydrogen. The flow rate of ammonia (NH ₃ ) as a nitrogen (N) source was 6.0 liter / min. As a gallium (Ga) source, trimethylgallium ((CH ₃ ) ₃ Ga) is used, and its flow rate is N (nitrogen).
/ Ga (gallium) ratio, so-called V / III ratio is about 10 ⁴
The ratio was set to be an extremely general ratio. Although the formation temperature and the total amount of the source gas and the gas for creating the growth atmosphere to be introduced into the growth system are general, as shown in FIG. 106) and a crystal (107) having pyramidal projections are mixed. Due to the mixture of the pyramidal crystals (107), the surface of the growth layer is rough with severe irregularities. The protruding growth grains (107) make the top plate flat.
Well developed above. Crystal body with flat top plate (106)
Is approximately 2.2 μm. FIG. 5 is a schematic view of the surface of the GaN growth layer after the layer formation has been further advanced under the above formation conditions. Crystals that flatten the top plate (10
6) Pyramidal crystal grains (107) further develop on the top,
The pyramidal crystal (107) is predominantly present in almost the entire surface. For this reason, the growth layer surface becomes messy having pyramid-shaped projections. The gap (108) between the pyramid-shaped projections is rarely buried with an increase in the layer thickness, and conversely, becomes a pit having a shape that increases its depth.

The total amount of hydrogen and argon is the same as above,
FIG. 6 schematically shows the surface state of a gallium nitride growth layer formed by changing the flow rates of hydrogen and argon to 2.0 liters / minute and 3.8 liters / minute, respectively, and setting the other growth conditions to be the same as above. Shown in In this case, γ is 1.9. Under this condition, since many crystals (106) having a flat top plate exist on the surface, pits (105) due to incomplete coalescence of the crystals remain, but many areas exist. Has a flat surface. Due to the increased formation time, the layer thickness of the crystal (106) does not increase significantly, but rather the lateral growth predominates and develops into a planar expanded crystal. The planar expanded crystals coalesce and coalesce, resulting in a flat growth layer surface. That is, in order to obtain a growth layer having excellent surface flatness, it is fundamental to efficiently form such a crystal having a flat top plate.

[0008] The growth atmosphere not only affects the shape of the crystal, but also promotes the volatilization of the growth layer depending on the gas species constituting the growth atmosphere. The extent to which the growth layer is volatilized, of course, depends on the material that makes up the layer,
In particular, it occurs remarkably in a nitride compound semiconductor containing indium such as indium nitride (InN) or gallium indium nitride (GaInN). This is mainly due to the ease of sublimation of indium nitride, which decomposes (sublimates) at a relatively low temperature of about 600 ° C. (edited by the Japan Society for the Promotion of Industry, New Materials Technology Committee, “Compound semiconductor device” ( 197
(Issued September 15, 2013), p. 397). Further, the decomposition temperature (hydrogen desorption temperature) of indium hydride (InH ₃ ) is set to 80 ° C., and the decomposition temperature (GaH) of hydrides of other group III elements such as gallium (Ga) and aluminum (Al) is set. ₃ = 140 ° C., AlH ₃ = 150 ° C.) (IA SHEKA et al., “THE
CHEMISTRYOF GALLLIUM ", ELS
EVIER PUB. , 1966, p. 24). When the growth atmosphere is composed of a large amount of hydrogen, it is easily conceived that such a readily decomposable indium hydride will be formed, and if the decomposition of the indium-containing nitride compound semiconductor layer proceeds more, is expected. The composition of the growth atmosphere should be examined not only from the above-mentioned fluid viewpoint at a high temperature but also from the viewpoint of reaction inertness for avoiding a bad chemical reaction with the growth layer.

Gallium indium nitride is currently in practical use
Typical indium-containing nitride compound semiconductors offered
Body. Gallium indium nitride is, for example, blue, blue
A light-emitting diode (L) that emits short-wavelength light such as green or green
ED). Light emission of LED etc.
In the device, the light-emitting layer is an important active layer that has the function of emitting light.
The surface state and crystallinity of the light emitting layer are determined by light emission intensity and the like.
It is a main factor that determines the superiority of the characteristics of the light emitting device.
Therefore, indium nitride and gallium indium nitride
Much attention has been paid to the formation. example
For example, from the viewpoint of the growth atmosphere, the hydride (hy
of indium nitride by the dry method (VPE method)
Phase growth is performed in a nitrogen atmosphere (J.
Crystal Growth,144(1994),
15-19). In addition, molecular beam epitaxial vapor phase epitaxy
Method (MBE method) in a helium (He) atmosphere
Is trying to grow gallium indium
(Appl. Phys. Lett.,66
(13) (1995), 1632-1634). Only
The gas constituting the growth atmosphere is changed from hydrogen to nitrogen or
Simply changing to an active gas will result in surface conditions, especially surface
Indium-containing nitride compound semiconductor with excellent surface flatness
Well-known fact that growth of the growth layer is not always achieved
It is. For example, metal organic pyrolysis vapor deposition (so-called M
Gallium indium nitride (Ga) by OCVD_x 
In_1-x N: 0 <x <1), the growth rate is low.
In the case of an aluminum composition ratio, it is preferable to provide a flat layer surface.
Even if it is a growth atmosphere composition condition, x is approximately
Gallium nitride with a high indium composition ratio exceeding 0.2
The growth of the indium and indium layers is not necessarily superior to surface flatness.
Growth layers were not always provided. That is, nitriding
In the growth of gallium and indium, the composition of the growth atmosphere
Naru makes appropriate changes in view of indium composition ratio (x)
It should have been. In particular, indium-containing nitrided
The surface state of the compound semiconductor layer is slightly
Subtle changes in response to changes in growth layer formation temperature
(The above-mentioned J. Crystal Growth,144
(1994), pp. 15-19), formation temperature of surface state
Poor chemical reactivity that can slow the dependence on the formation
Good surface morphology with stable temperature fluctuations
It has been desired to present a configuration of a growth atmosphere that imparts the following. I
However, indium composition ratio (x) or formation temperature
The idea of changing the composition of the growth atmosphere according to
Not suitable for indium composition ratio (x)
In the growth atmosphere, the indium-containing nitride compound semiconductor layer
The method of forming is not disclosed. Conventional Inn
Sufficient optimization has been achieved in relation to the composition ratio of
In the case of a forming method that is not performed, characteristics of light emitting elements such as LEDs are not used.
Is insufficient to stably obtain a light-emitting layer that improves the
there were.

[0010]

The mere use of a growth atmosphere composed of a mixed gas does not always form a nitride compound semiconductor growth layer having surface flatness. The growth layer having a flat surface is obtained as a result of planarly assembling crystals (elements providing surface flatness) that flatten the top plate. Therefore, the thickness (height) of the element determines the layer thickness of the nitride compound semiconductor layer that can maintain the flatness of the surface. However, the height of the elementary element obviously changes with γ. In other words, when obtaining a nitride compound semiconductor layer having surface flatness, it suggests that an appropriate γ corresponding to the intended thickness of the grown layer exists. That is, in the prior art, since γ is not optimized in accordance with the desired thickness of the nitride compound semiconductor layer, projection-like crystals other than elementary elements that provide surface flatness, etc. Grow into a messy surface.

For simplicity, regardless of the gas species constituting the growth atmosphere, in the vapor phase growth of the nitride compound semiconductor layer,
It has been a general method to perform growth under the same growth atmosphere regardless of the type of the growth layer. Another common technique is to change the gas species constituting the growth atmosphere or the mixing ratio of the gases constituting the growth atmosphere during the growth of gallium indium nitride, for example. In any case, it is customary in the conventional growth technique that the composition of the growth atmosphere is kept constant from the start to the end of the growth of the nitride compound semiconductor layer. That is, although means for changing the composition of the growth atmosphere for each growth layer is known,
No means is employed for changing the composition of the growth atmosphere over time in accordance with the progress of layer growth. As for the method of forming the nitride compound semiconductor layer, there is no disclosure of a method of forming γ by changing γ according to the intended thickness of the nitride compound semiconductor layer. The present invention provides a vapor phase growth method for obtaining a nitride compound semiconductor layer having a small pit density and a flat surface by means for optimizing γ based on a desired thickness of the nitride compound semiconductor layer. . In addition, the present invention, in addition to efficiently promoting the generation of flat growth islands, which are elementary elements for forming a flat surface, has low reactivity with the growth layer surface and easily decomposes indium. A growth atmosphere suitable for growing a nitride compound semiconductor is presented.

[0012]

Means for Solving the Problems The present inventor efficiently forms the above-mentioned elementary elements (growth islands having a flat surface) essential for promoting the flattening of the surface of the growth layer of the nitride compound semiconductor layer. Has found that it is necessary to create a temperature environment in which the temperature upward from the surface of the sediment changes rapidly according to the layer thickness. In addition, such a temperature environment is in a growth atmosphere composed of a mixture of gases having different thermal conductivities,
By changing the mixing ratio of different gases over time according to the increase in the layer thickness, it has been found that a method of changing the thermal conductivity of the gas constituting the growth atmosphere over time is effective. This has led to the present invention. That is, the present invention relates to a method for forming a nitride compound semiconductor layer in a growth atmosphere composed of two gases having different thermal conductivities, wherein the volume of the gas having a large thermal conductivity (V ₀ ),
The ratio of the volume (V) of the gas that makes the thermal conductivity smaller (γ =
(V / V ₀ ) is provided with a method for forming a nitride compound semiconductor layer in a growth atmosphere in which the V / V ₀ decreases with time as the layer thickness of the nitride compound semiconductor layer increases. In particular, the layer thickness is set to t in a growth atmosphere composed of hydrogen and argon.
In the method of forming a nitride compound semiconductor layer (unit: μm), a ratio of a volume (V) of argon (Ar) that makes the thermal conductivity smaller than hydrogen to a volume (V ₀ ) of hydrogen ( Volume mixing ratio: γ) is decreased with time from an initial value (γ ₀ ) in the range of 0.2 / t or more and 6.0 / t or less in accordance with an increase in the thickness of the semiconductor layer. A nitride compound semiconductor layer is formed in a growth atmosphere.
Further, in the present invention, the formation temperature (T: unit ° C) is set to 600 ° C or more and 900 ° C or less, and indium (I
n) When forming an indium-containing nitride compound semiconductor layer having a layer thickness t (unit: μm) in which the atomic content ratio (x: unit%) is 2% or more and 50% or less, the initial value (γ ₀ ) is 0.2 · x · (1 / t) · (T / 550) or more 1.8 ·
The present invention is characterized in that the indium-containing nitride compound semiconductor layer is formed to have a value of x · (1 / t) · (T / 550) or less.

[0013]

DETAILED DESCRIPTION OF THE INVENTION nitride compound semiconductor layer of the present invention have the general formula _{Al x Ga y In z N (} x + y + z = 1,
0 ≦ x, y, in addition to the _{z ≦ 1), Al x Ga} y In z N a
_{P 1-a (0 <a} ≦ 1) and _{Al x Ga y In z N b} As
_In addition to nitrogen represented by _1-b (0 <b ≦ 1), a nitride compound semiconductor containing a Group V element such as phosphorus (P) or arsenic (As) or the like is included. These nitride compound semiconductor layers can be formed on a well-known substrate for forming a nitride compound semiconductor such as sapphire. The substrate is made of a metal material such as hafnium (Hf) or a face-centered cubic lattice structure of gallium arsenide (GaAs) or gallium phosphide (GaP).
A group V compound semiconductor crystal or an elemental (single) semiconductor crystal such as silicon (Si) can also be used. Regardless of the conductivity type of any semiconductor crystal substrate, the plane orientation and off-cut (mis-orientation angle) of the crystal plane forming the substrate surface
The specifications such as may be appropriately selected in consideration of the growth method and growth conditions of the low-temperature buffer layer. 0.5% lattice mismatch with GaN
L that is a composite oxide of lithium (Li) and gallium (Ga) or Li and aluminum (Al)
i ₂ GaO ₃ or Li ₂ AlO ₃ can also be used as the substrate.

Table 1 shows the thermal conductivity at 0 ° C. of the main gases used for creating a growth atmosphere in vapor phase growth (for example, “Rika Kagaku” (1984, Maruzen Co., Ltd.). ) Issuance), p. 476.) Examples of the composition of the growth atmosphere gas obtained by mixing gases having different thermal conductivity include well-known hydrogen-argon, hydrogen-nitrogen (N ₂ ), and hydrogen-nitrogen (N ₂ ). Helium (He), etc. In addition, hydrogen-
A mixture of various gases such as argon-nitrogen is also a growth atmosphere gas.

[0015]

[Table 1]

In the vapor phase growth of the nitride compound semiconductor layer,
Ammonia, which is frequently used as a nitrogen source, is a gas having a thermal conductivity different from that of hydrogen, argon, or the like, but a source gas such as ammonia is not included in the gas constituting the growth atmosphere. The gas that creates the growth atmosphere is not a source gas used for film formation in the vapor phase growth method, but a gas species generally used as a carrier gas or a diluting gas for a doping gas. In forming a growth atmosphere from two gases having different thermal conductivity, γ represents a volume mixing ratio of both gases.
That is, γ is a growth atmosphere in which a gas whose volume is V ₀ and a gas whose thermal conductivity is smaller than that and whose volume is V are mixed, and is calculated from the following equation (1). γ = V / V ₀ Formula (1) Consisting of three or more kinds of gases (n is an integer and n ≧ 1) having different thermal conductivity (k) and volume (V _n ). Γ is determined using the volume value (V ₀ ) of the gas that maximizes k and the weighted average value of k for other gases. The volume mixing ratio (γ) of the gas constituting the growth atmosphere is also the flow ratio of the gas introduced into the growth system to create the growth atmosphere. As an example, hydrogen supplied at a flow rate of 5 liters per minute,
Similarly, γ in a growth atmosphere composed of nitrogen at a flow rate of 10 liters per minute is 2.00.

FIG. 7 is a calculation result showing the heat distribution above the sapphire substrate surface when the flow rate of hydrogen is 5.8 liter / min. In this case, since the growth atmosphere is composed of hydrogen alone, γ = 0 is calculated from the above equation (1).
According to the calculation result when the temperature of the surface of the substrate (101) is assumed to be 1000 ° C., a high-temperature region of 900 ° C. or more is obtained even at a height of about 5 mm from the substrate surface. In a growth atmosphere consisting only of hydrogen having a relatively large thermal conductivity, the rate of change in temperature with respect to the distance from the surface of the deposit is small. That is, the width of the high-temperature isothermal region (the distance above the substrate) is large. Under such a condition that the high-temperature band is wide and the temperature distribution is such that growth is suspended, the nitride compound semiconductor such as hexagonal gallium nitride (GaN) has a c-crystal axis rather than a horizontal direction (horizontal direction). It grows preferentially in the direction (vertical direction). That is, most of the raw material supplied to the region where the layer can be grown near the surface of the deposit is consumed in the direction of increasing the height of the growing island. The growing islands grown in such an environment are columnar crystals having a substantially hexagonal horizontal direction and a bottom area substantially constant in the height direction, and are close to each other because the lateral growth does not significantly occur. Since the chances of the columnar crystals being fused and connected to each other are small, as a result, a situation where the columnar crystals are scattered on the surface of the deposit and become turbulent is caused (see FIG. 1). When the growth islands that become columnar crystals are scattered and large gaps between the growth islands are large, horizontal growth does not occur enough to fill the voids, resulting in a continuous surface with irregularities on the surface. This results in a nitride compound semiconductor layer lacking in properties. That is, in the method for forming a nitride compound semiconductor layer under a growth atmosphere in which γ is extremely or unnecessarily small with respect to an intended film thickness, a layer lacking in continuity having poor surface flatness and smoothness is obtained. Easy to form.

On the other hand, for example, when a hydrogen-argon mixed gas of γ = 1.00 is used as a growth atmosphere, the substrate (10
The temperature change in the upper part of 1) becomes sharper than that in the atmosphere of hydrogen alone, and the isothermal width becomes narrower. In other words, the band of the isothermal temperature is narrowed,
The temperature drops rapidly. In the transition region from the high-temperature region to the low-temperature region, which is higher from the surface of the sediment, growth is no longer inhibited, and growth in the vertical direction (height direction) of the growth island is suppressed due to the narrow width of the high-temperature zone. Is done. If the temperature region required for growth is narrowed, the thickness (height) of the obtained growth island is limited to a small value. If the raw material supply to the high-temperature zone where growth island formation is allowed is constant, the volume of growth islands formed will be almost the same, so growth islands whose growth in the height direction has been suppressed are horizontal. In the direction (horizontal direction), and the bottom area becomes larger. If the growth in the lateral direction is promoted, the chances of neighboring growing islands fusing each other increase, so that pits (pores) generated due to imperfect fusion of the growing islands to each other will increase. A reduction in density results. That is, it is convenient to form a nitride compound semiconductor layer having a flat and smooth surface.

In spite of the relatively large intended thickness of the nitride compound semiconductor layer, improperly large γ also causes
It interferes with the formation of a flat surface. The problem in this case is γ
This is caused by unnecessary reduction of the temperature region width of the temperature required for growth based on the above-mentioned mechanism due to excessively large size. Because γ is excessive, the temperature change above the surface of the deposit becomes excessively sharp. This means that a high-temperature region essential for forming a growth island for flattening the surface, which is an element for forming a flat surface, has not sufficiently extended to the surface of the deposit. Under such a growth environment in a temperature distribution state in which the high-temperature region is reduced, many growth islands that grow two-dimensionally in the horizontal direction are generated. For example, a mode in which the C plane of gallium nitride expands in a layered manner in a direction parallel to the (0001) plane (C plane) of sapphire, zinc oxide (ZnO) crystal, or the like is a simple example.
In other words, the temperature environment in which the high-temperature region is narrowed is advantageous for promoting the two-dimensional growth of the growth island. However, immediately above this high-temperature region, the temperature suddenly drops, so that when the growth of the growing island progresses and the surface of the growing island enters the temperature transition region, the surface of the growing island inhibits the formation of a flat surface. Pyramidal (pyramid) projections occur. Therefore,
In the initial stage of the growth in which the growth island is formed, two-dimensional growth of the growth island can be favorably progressed, but the surface of the growth island is reduced due to the reduced high temperature region where the growth is brought about. Deviates from the region relatively early, and in the latter half of the growth, a large number of growth islands having projections on the surface are formed. The connection of such growth islands whose surfaces are covered with protrusions does not lead to a growth layer having a smooth and flat surface. Therefore, in order to efficiently generate elemental elements that provide a growth layer having a smooth and flat surface, a growth area corresponding to the height of the growth island, that is, the thickness of the growth layer having a flat surface is required. The temperature must be isothermally high.

The significance of adjusting γ is that when forming the nitride compound semiconductor layer, the width of the high temperature zone above the layer where the growth is proceeding and where the growth is brought about is increased by the layer thickness. I have to respond. In the present invention, a numerical value given according to an intended final layer thickness is an initial value of γ (γ
_{As 0} ), the volume mixing ratio (γ: γ> 0) is reduced with the initial value (γ ₀ ) with time as the layer thickness increases.
The intended final layer thickness is, for example, a laminated body in which each of the laminated body constituent layers having different layer thicknesses is stacked, and does not indicate the overall thickness of the laminated body, but each laminated body. It refers to the thickness of the constituent layers. The _value γ ₀ of the present invention gives an optimum value as an upper limit value of the mixing ratio corresponding to the layer thickness, which is suitable for causing formation of a growth island having a flat surface resulting in a flat and smooth continuous film. In the formation of a nitride compound semiconductor layer in a growth atmosphere composed of a mixed gas of hydrogen and argon, γ ₀ has an optimum range according to the relational expression (2) with the intended layer thickness (t: unit μm). Is shown. That is, 0.2 / t ≦ γ ₀ ≦ 6.0 / t (2) This value is obtained from a growth experiment in the process of forming a layered gallium nitride layer by connecting growth islands made of gallium nitride. Although it is the experimental value obtained, it is valid regardless of the type of the constituent material of the growing island, that is, the type of the nitride compound semiconductor. Indium (I
As long as n) is not contained in such a large amount as to be recognized as a constituent element, γ ₀ does not change for formation of a nitride compound semiconductor in a hydrogen-argon growth atmosphere. Optimal γ
The value of ₀ is approximately 700 ° C. unless a nitride compound semiconductor containing a large amount of indium is formed.
In a wide temperature range of about 1200 ° C., there is almost no dependence on the temperature of the layer. Further, although there are some differences in the shape such as the size of the growth island depending on the growth apparatus, it is recognized that the growth mode depending on γ is common. If γ ₀ is greater than 6.0 / t, the high temperature zone that causes growth is inappropriately narrowed,
As mentioned above, the growing island surface that is growing is immediately
As it enters the low temperature zone, pyramid-like projections are generated on the surface of the growing island. Conversely, if a value smaller than 0.2 / t is set as the initial value (γ ₀ ), the high temperature range that causes growth is unnecessarily expanded, and thus the hexagonal column shape preferentially extended in the vertical direction as described above. This causes a state in which the growth islands are isolated and scattered, which is a problem in forming a layer having a flat surface.

The present invention is not limited to the case where the growth atmosphere is composed of hydrogen and argon. For example, even if the nitride compound semiconductor layer is formed in a growth atmosphere composed of hydrogen and nitrogen,
The utility of setting the initial value of is demonstrated. When a mixed gas of hydrogen and nitrogen is used as the growth atmosphere, an appropriate γ ₀ ′ for the layer thickness (t) is approximated by the following equation (3) based on the equation (2). γ ₀ ′ = γ ₀ · (k _N2 / k _Ar ) Equation (3) Here, k _Ar represents the thermal conductivity of argon, and k _N2 represents the thermal conductivity of nitrogen. By setting γ to the initial value (γ ₀ ) proposed by the present invention in the initial stage of generation of the growth island, appropriate narrowing of the high-temperature region near the surface of the sediment according to the intended layer thickness is achieved. Comes from an early stage of growth. This modest narrowing of the temperature promotes the two-dimensional lateral development of the growing islands, and has the advantage that the formation of a flat surface growth layer already from the initial stage of growth is achieved with high efficiency. .
If the growth islands having a flat surface can be formed at a high density, the chances of the coalescence of the growth islands are increased, which is effective in stably forming a smooth film with few pits that is continuous in a plane.

In a growth environment created so that the growth rate is higher than the sublimation rate, the height of the growth island corresponding to the layer thickness is determined by the growth time (the supply of the source gas to the growth reaction system is continued. Time). If γ is constant,
The temperature distribution near the surface of the deposited layer does not change, and the band width of the high-temperature region is maintained at a substantially constant width based on the surface of the deposited layer before the start of growth. Therefore, as the number of growing islands increases, the probability that the surface of the growing island protrudes from the high temperature range in which growth is allowed and penetrates into a lower temperature region increases. Unless the temperature transition between the high temperature region where the growth is achieved and the low temperature region is steep, the above-mentioned projections are generated. This pattern is schematically shown in FIG. Therefore, in the present invention, by decreasing γ with time from γ ₀ as the height of the growing island increases, the tip (surface) of the growing island, whose height increases every moment as the growth time elapses, has a low temperature. Avoid entering the area. That is, even if the height of the growing island increases with time, the distance between the tip of the growing island and the low-temperature region is sufficiently maintained so that the tip (surface) of the growing island remains sufficiently in the high-temperature region. To γ
The bandwidth of the high temperature region is extended with the decrease of FIG. 9 shows that the growing island (106) has developed (129) and has a height (1) over time.
Even if the value of γ is decreased over time, the high-temperature growth temperature region (131) where the progress of growth is brought about is extended upward (132), so that the growth island (106) It is a diagram schematically showing a state in which a tip portion (surface) (133) stays in the high-temperature region (131).

FIG. 10 shows an example of a change with time of γ. gamma linearly (13 for forming time than the initial value gamma ₀ layer
4) Non-linearly (135) or stepwise (13)
6) It is allowed to change. Regardless of the method of changing γ, the value remains lower than the initial value (γ ₀ ) as the layer thickness increases (as the formation time elapses). Specifically, for example, the flow rate of the mixed gas for creating the growth atmosphere can be continuously changed by continuously changing the flow rate using a control instrument such as a mass flow meter (abbreviation: MFC). In the formation of the nitride compound semiconductor layer for decreasing γ with time as in the present invention, it is more preferable that a buffer layer is provided as an underlayer for deposition. In combination with the action of promoting two-dimensional growth in the lateral direction, it is advantageous for obtaining a continuous growth layer having a flat surface. On a crystalline substrate in obtaining a laminate structure composed of several layers thick nitride compound semiconductor layer, under the growth atmosphere of the nitride compound semiconductor layer constituting layers the layer thickness gamma ₀ and gamma corresponding to Form. For example, γ ₀ and γ corresponding to the layer thickness of the first constituent layer constituting the laminate
After forming the first laminate constituting layer under the growth atmosphere of
This corresponds to a method in which ₀ and γ are changed to numerical values suitable for the second constituent layer to form the second laminated body constituent layer. However,
The definition of γ ₀ and γ in the present invention need not be dared to be applied to the growth of a so-called low-temperature buffer layer deposited on a substrate at a low temperature of, for example, about 500 ° C. About 700 ° C to about 1000 ° C
In the growth of the nitride compound semiconductor layer at a low temperature of less than 1, the slow change of the temperature above the substrate (deposit) and the influence of thermal convection generated in a high-temperature environment on the shape of the growth island are not necessarily great. Because.

In the formation of a nitride compound semiconductor layer containing indium, the present invention defines the formation temperature and the range of the content ratio of indium atoms, and then sets the optimum initial value γ.
₀ is disclosed. Examples of the indium-containing nitride compound semiconductor include a ternary mixed crystal such as gallium nitride-indium (GaInN) and an aluminum nitride-indium mixed crystal, or an aluminum nitride-gallium-indium quaternary mixed crystal. Forming temperature (T: unit ° C) is 600 ° C or more and 900
It is limited to the range of not more than ° C. At temperatures below 600 ° C,
Irrespective of the content ratio (x: unit%) of indium (In) atoms to the amount of mixed group III atoms such as gallium (Ga), it is very important to stably obtain an indium-containing nitride compound layer having an excellent surface state. Difficult. In general, the surface has a rough surface in which spherical, hemispherical or island-shaped growth hills are densely packed, which is not equivalent to the provision of a nitride compound semiconductor device. On the other hand, if the formation temperature exceeds 900 ° C., it is difficult to form the indium-containing nitride compound semiconductor layer mainly due to the sublimability of the indium compound. Therefore, in the present invention, γ ₀ is specified in a temperature range practical for forming an indium-containing nitride compound semiconductor. In addition, the indium content ratio (x) is limited to a practical range of 2% to 50%, and an optimum value of γ ₀ is presented. The formation of a nitride compound semiconductor layer having a high indium concentration such as indium nitride (InN) crystal, for example, having an indium content ratio generally exceeding 50% is originally difficult to form sufficiently stably when γ is specified. This is because there is almost no practical utility for defining γ.
For forming a nitride compound semiconductor having a low indium content in which the indium content is less than 2%, γ ₀ represented by the above formula (2) can be applied.

The formation temperature (T: unit ° C.) is 600 ° C. or more and 9
00 ° C or lower, and indium (I
n) When forming an indium-containing nitride compound semiconductor layer having a layer thickness t (unit: μm) in which the atomic content ratio (x: unit%) is 2% or more and 50% or less, specifically, an initial value (Γ ₀ ) is a preferable γ _{0 represented} by the above formula (2).
May be set within the range represented by the following equation (4) based on the range of That is, 0.2 · x · (T / 550) · (1 / t) ≦ γ ₀ ≦ 6.0 · x · (T / 550) · (1 / t) Equation (4) For example, 830 ° C. (T = 830), the layer thickness is 0.1 μm when the content ratio of indium atoms is 8% (x = 8).
m (t = 0.1) indium-containing gallium nitride layer (G
The range of γ ₀ suitable for the formation of a _0.92 In _0.08 N) is 24.
1 ≦ γ ₀ ≦ 724.3. γ ₀ increases t, that is,
The reason why the larger the intended final layer thickness is, the smaller the layer thickness is for the above-mentioned reason (see [0022]). What is specific to the growth of the nitride layer containing indium is that the higher the content ratio (x) of indium atoms and the forming temperature, the larger γ ₀ must be. That is, γ ₀ needs to be increased in proportion to the indium atom content ratio (x). Also, γ ₀
When the formation temperature of the indium-containing nitride layer is T (° C.), it is necessary to multiply by the value given by the ratio T / 550. In the equation T / 550 representing the ratio, the coefficient (= 1
/ 550) is experimentally obtained from the results of a forming experiment repeatedly and eagerly performed by the present inventors. For example, gallium nitride with an indium composition ratio (x) of 0.06.
Taking an example of formation of an indium mixed crystal (Ga _0.94 In _0.06 N) in a hydrogen-argon growth atmosphere, the higher the formation temperature is, the larger γ ₀ is, that is, the lower the thermal conductivity is compared to hydrogen. Means that the mixture ratio of argon and hydrogen to be mixed is increased. Similarly, γ ₀ increases as the indium composition ratio (x) increases. That is, the mixing ratio of argon, which makes the thermal conductivity smaller than that of hydrogen, is made larger. This allows
By increasing the layer formation temperature and increasing the indium composition ratio, the effect of suppressing sublimation due to the surface reaction with hydrogen, which is further promoted, is also simultaneously achieved. However,
It should be noted that in forming the indium-containing nitride layer,
When the same layer is formed in a growth atmosphere in which argon is dominant, it is observed that the incorporation rate of indium sometimes fluctuates to a high ratio. In other words, a region that exceeds the target content ratio of indium atoms tends to be generated. In addition, the formation probability of the indium-containing nitride layer including the region where the content ratio of indium atoms is abnormally high varies. Therefore, in the present invention, in order to stably form an indium-containing nitride layer having an intended content ratio of indium atoms, the upper limit of γ ₀ allowed by the above formula (4) is further defined as follows: In particular, γ ₀ shown in the equation (5) of
It is disclosed as a preferred range. 0.2 · x · (T / 550) · (1 / t) ≦ γ ₀ ≦ 1.8 · x · (T / 550) (1 / t) Equation (5) γ described in Equation (5) _The coefficient (= 1.8) in the formula term defining the upper limit value of ₀ can also maintain the surface morphology of the indium-containing nitride layer in a good state, and has a desired content ratio of indium atoms. It was determined by experiments as giving an upper limit value for stably obtaining an indium-containing nitride layer. Therefore, 830 ° C
(T = 830), the layer thickness is 0.1 μm (t = 0.1) when the content ratio of indium atoms is 8% (x = 8).
Indium containing gallium nitride layer (Ga _0.92 In
_0.08 N) to be formed at the example above, and in particular, the range of preferred gamma ₀ becomes 24.1 ≦ γ ₀ ≦ 217.3. Even in the formation of the indium-containing nitride semiconductor layer, there is no change in decreasing γ with time as the layer thickness increases from the initial value (γ ₀ ).

The growth atmosphere according to the present invention can be used for forming a nitride compound semiconductor layer by a metal organic chemical vapor deposition (MOCVD) method, a halogen type or a hydride type vapor phase growth (VPE) method. . MOCV
In the method D, the growth atmosphere of the present invention can be used regardless of the normal pressure method or the reduced pressure method. As shown in the above formula (2), in the hydrogen-argon growth atmosphere, there is a preferable range of γ due to a difference in a film formation environment condition of the nitride compound semiconductor layer such as a growth pressure. This is because γ is somewhat different.

[0027]

The significance of regulating γ in accordance with the intended thickness of the nitride compound semiconductor layer will be summarized. As mentioned above,
γ changes the temperature distribution above the surface of the deposit. For example, an increase in γ causes a rapid temperature change, and narrows the width of a high-temperature zone required for growing a nitride compound semiconductor. The width of the high-temperature zone corresponds to the thickness of the nitride compound semiconductor layer. That is, the adjustment of γ has an effect of creating a thermal environment in the region above the surface of the deposit according to the intended thickness of the nitride compound semiconductor layer.

[0028]

【Example】

(Example 1) An undoped gallium nitride (GaN) layer having a film thickness of 3 μm was obtained by using sapphire having a plane orientation of (0001) (C plane) as a substrate (101), and increasing γ by increasing growth time ( Increase in layer thickness),
It marks the method of forming while changing from an initial value gamma _0. The mirror-polished surface of the substrate (101) was subjected to a cleaning treatment to promote uniform film formation. First, the substrate (101) was degreased with an organic solvent such as acetone, washed with water, and then acid washed with a commercially available aqueous solution of high-purity ammonium fluoride for the semiconductor industry. After washing with ultrapure water, it was dried by heating with an infrared lamp.

The cleaned sapphire substrate (101) was transported at room temperature into a growth furnace (110) in a normal pressure type MOCVD growth apparatus. FIG. 11 schematically shows the MOCVD growth apparatus. The inside of the growth furnace (110) was once evacuated to a vacuum of about 10 ^-3 Torr by a rotary pump (111). After about 20 minutes, the on-off valve (117) provided in the middle of the evacuation pipe (116) connected to the vacuum pump (111) was closed, and the evacuation operation of the furnace (110) was stopped. Thereafter, argon (Ar) purified by a molecular sieve (molecular sieve) adsorption method was introduced into the furnace (110) by flowing through a 316L stainless steel pipe (112). Thereby, the inside of the furnace (110) was returned to the atmospheric pressure, and thereafter, the pressure in the furnace at the time of film formation was kept substantially at the atmospheric pressure. Then, silver (Ag) -palladium (P
d) Hydrogen gas purified to high purity by a permeable membrane method was supplied through a pipe (113). The flow rates of the hydrogen gas and the argon gas are determined by the respective pipes ((112) and (113)).
Was precisely controlled by a general electronic mass flow controller (abbreviation: MFC) ((114) and (115)) which was individually installed in the middle of the above.

The flow rate of the hydrogen gas was measured using an electronic mass flow meter (11).
It controlled to 8.00 liter / min by 5). Thus, a growth atmosphere consisting only of hydrogen was created. Next, the temperature of the substrate (101) was raised from room temperature to 430 ° C. by a resistance heating method. While maintaining the substrate (101) at the same temperature, 2
After elapse of 5 minutes, ammonia (NH ₃ ) gas as a raw material of nitrogen (N) is passed through the pipe (118), and the surface of the substrate (101) placed in the growth furnace (110) is discharged. Started to supply. As the ammonia source (119), liquefied ammonia stored in an aluminum alloy cylinder was used.

Five minutes after the start of the supply of ammonia gas, hydrogen containing trimethylgallium ((CH ₃ ) ₃ Ga), which is a raw material (126) of gallium (Ga) vaporized by bubbling (foaming) operation Gas is supplied to the piping (1
20) and then supplied near the surface of the substrate (101). The flow rate of ammonia gas (119) is
8) It was adjusted and controlled to 1.00 liter / min by an electronic mass flow meter (121) dedicated to ammonia gas provided on the way. The temperature of a stainless steel bubbling container containing commercially available trimethylgallium for the semiconductor industry was kept at 13 ° C. in an electronically controlled thermostat, and the flow rate of hydrogen gas for bubbling was 20 milliliters (ml) / min. Hydrogen gas containing trimethylgallium as the gallium raw material (126) was continuously supplied into the growth furnace (110) for 20 minutes. Thus, a gallium nitride (GaN) thin film buffer layer having a thickness of about 7 nm was grown.

After the growth of the thin film buffer layer made of gallium nitride (GaN) is completed, a growth atmosphere (110) for growing a hydrogen gas and a hydrogen gas containing trimethylgallium has created a growth atmosphere.
Supply was temporarily interrupted. Almost simultaneously growth reactor (110)
Argon gas was introduced therein. The flow rate of the argon gas was determined to be 6.00 liter / by an electronic mass flow meter (114).
Minutes. The flow rate of ammonia gas is gallium nitride (Ga
N) It was 1.00 liter / min as in the growth of the thin film buffer layer. A substrate having a gallium nitride thin film buffer layer deposited on its surface in an atmosphere consisting of argon gas and ammonia gas (1
01) was raised to 1000 ° C.

After the temperature of the substrate (101) is stabilized at 1000 ° C., the flow rate of the argon gas is reduced to 2.88 liter / min by the electronic mass flow meter (114), and the flow rate is simultaneously measured by the flow meter (115). The supply of hydrogen gas into the growth furnace (110) adjusted to 4.00 l / min was restarted. This created a hydrogen-argon mixed atmosphere with γ ₀ of 0.72. Almost at the same time when the supply of the hydrogen gas to the growth furnace (110) was restarted, the supply flow rate of the ammonia gas into the furnace (110) was increased to 6.00 l / min. After 5 minutes from the start of the supply of the ammonia gas, 20 milliliters per minute (m) containing trimethylgallium vapor as the gallium (Ga) source (126) is used.
l) / minute of bubbling hydrogen gas in a furnace (110)
Supply started inside. For 90 minutes from this point, γ
Was finally reduced to 0.45 at a rate of 0.03 / min from 0.72 of the initial value (γ ₀ ). This change in γ was obtained by reducing the flow rate of argon with an electronic mass flow meter (114) from an initial 2.88 l / min to 1.80 l / min with a flow meter (114). The flow rate of hydrogen was maintained at 4.00 l / min for 90 minutes. The supply of hydrogen gas (bubbling gas) containing the gallium raw material (126) was continued for 90 minutes under such a flow rate condition to grow a gallium nitride (GaN) layer. The reason why the growth time was set to 90 minutes was that a gallium nitride growth layer having a layer thickness of 3 μm was formed.

Trimethylgallium ((CH ₃ ) ₃ Ga)
2 minutes after the supply to the growth system was stopped, the heating of the substrate (101) having the laminated structure in which the undoped GaN growth layer was formed on the GaN thin film buffer layer was stopped, and cooling was started. The flow rates of hydrogen and argon, which created the growth atmosphere, were not changed until the temperature of the substrate (101) dropped to about 400 ° C. After cooling to near room temperature, supply of hydrogen and argon into the growth system was also stopped. Thereafter, the open / close valve (117) attached to the vacuum exhaust pipe (116) is switched from the closed state to the open state, and the inside of the growth furnace (110) is rotated by the rotary pump (111) until the degree of vacuum reaches about 10 ^-3 Torr. After evacuating and maintaining the same degree of vacuum for 10 minutes, argon gas was introduced into the growth furnace (110) through the pipe (112) to purge the furnace and return to atmospheric pressure. . This evacuation / purge operation was repeated three times, and finally the growth furnace (110) was filled with argon gas. Thereafter, the substrate (101) was taken out of the growth furnace (110).

Observation by a general scanning electron microscope (abbreviated as SEM) shows that the surface of the growth layer has no pyramidal projections,
It was smooth and flat. Gallium nitride (Ga) formed on the entire surface of the substrate (101) having a diameter of 2 inches
N) The pits on the surface of the growth layer have
Hardly observed, the periphery of the substrate (approximately 5
mm edge area). It was recognized that the pit density at the periphery was extremely reduced to about 3 to 5 pits / cm ² .

(Comparative Example 1) The formation conditions and operating conditions other than γ were the same as those in Example 1, and 3 μm undoped Ga was used.
An N layer was formed. The flow rate of hydrogen, which is the main constituent of the growth atmosphere, was 6.5 liters per minute, and γ (= Ar / H ₂ )
Is not changed over time during the layer formation for 90 minutes,
It was fixed at 0.05. This value does not satisfy the proper range of γ shown in the expression (2) in the text, and is too small for the intended GaN layer thickness of 3 μm. According to the invention, a reasonable minimum value of γ for a layer thickness of 3 μm is about 0.06. FIG. 12 is a schematic diagram showing a surface state of gallium nitride (GaN) grown in a growth atmosphere in which γ is too small. On the surface of the (0001) sapphire substrate, many GaN columnar crystals (109) having a substantially hexagonal cross section are generated. This columnar crystal (109)
Although the height varies, it is about 3 μm on average.
Thus, if γ is too small with respect to the film thickness, the columnar crystals (109) will be in a state of disorder, and the gap between the columnar crystals will be a source of pits. Was formed.

(Embodiment 2) Using the atmospheric pressure MOCVD growth apparatus described in Embodiment 1 above (see FIG. 11), silicon (Si) is deposited on a gallium nitride (GaN) buffer layer on a sapphire C-plane substrate. ) -Doped n-type aluminum-gallium nitride (AlGaN) mixed crystal layer and silicon (Si) and zinc (Zn) _-doped gallium indium nitride (Ga _0.92
In _0.08 N) layer was formed. The growth atmosphere, growth temperature, growth time, and other formation conditions when forming the buffer layer, and the operation conditions involved in forming the layer were set to be the same as those in Example 1.
The thickness of the buffer layer was 7 nm, almost the same as in Example 1.

After the growth of the gallium nitride buffer layer, the temperature of the substrate (101) was raised to 11 according to the procedure and conditions described in Example 1.
The temperature was raised to 00 ° C. After the temperature was raised and until the growth was started, measures such as gas distribution according to Example 1 were taken. Thereafter, an aluminum-gallium mixed crystal (Al _0.01 Ga _0.92 N) having a silicon-doped aluminum composition ratio of 0.01 and a layer thickness of 4 μm is formed on the gallium nitride buffer layer by atmospheric pressure MOCVD. It was formed by a method. Al _0.01 Ga _0.99
N layer, the flow rate with respect to hydrogen 3.00 l / min, and the gamma ₀ which is a mixture of argon to the flow rate 3.00 l / min was formed in the growth atmosphere to 1.00.
Trimethylaluminum ((CH ₃ ) ₃ Al) was used as the aluminum (Al) raw material (127). This aluminum raw material (127) is also a gallium (Ga) raw material (12).
As in the case of trimethylgallium used in (6), hydrogen gas bubbled from the aluminum source (127) was supplied through a pipe (12).
2) and introduced into the growth furnace (110). The stainless steel container containing trimethylaluminum was kept at 20 ° C. in an electronic thermostat, and the flow rate of hydrogen for bubbling it was set at 40 ml / min. As a silicon doping raw material (124), disilane (Si ₂ H) diluted with hydrogen gas to a volume concentration of 5 ppm is used.
₆ ) used. The flow rate of the disilane doping gas was controlled at 10 liter / min by a mass flow meter. As for the other source gases, trimethyl gallium (Ga source (126)) and ammonia (N (nitrogen) source) (119) were used as in Example 1. Same as 1. While supplying the raw material gas into the growth furnace (110) for exactly 100 minutes,
As the flow rate of the hydrogen gas which constitutes the growth atmosphere is constant, after 20 minutes from the time of the growth of a supply amount of the argon gas Al _{0. 01} Ga _0.99 N mixed crystal layer, an electronic mass flowmeter (11
In step 4), γ was decreased stepwise by 0.05 at intervals of 20 minutes. Repeat this operation four times and
When the growth of the Al _0.01 Ga _0.99 N mixed crystal layer for 00 minutes was completed, γ dropped to 0.80. Growth furnace formed of Al _{0. 01} Ga _0.99 N layer of silicon has been doped group III source gas and silicon doping gas (124) (11
The operation was terminated by interrupting the supply to the inside of 0). The supply of the ammonia gas (119) into the growth furnace (110) was continued thereafter without changing the flow rate.

Two minutes after the completion of the formation of the aluminum-gallium mixed crystal, the flow rate of hydrogen gas was set to 0.1 liter / minute by a mass flow meter (115), and the flow rate of argon was changed to a mass flow meter (115). 115), it was instantaneously changed to 5.9 l / min. This created a hydrogen-argon growth atmosphere with γ ₀ of 59.0. Thereafter, the growth temperature was reduced from 1100 ° C. to 830 ° C. in about 5 minutes.
Gallium nitride with a content ratio of indium atoms of 8%
Since the target layer thickness of the indium layer (In _0.08 Ga _0.92 N) was 0.1 μm, γ ₀ was changed to 2.00.
After the flow rates of the respective gases constituting the growth atmosphere are stabilized, the hydrogen gas accompanying the vapor of the gallium source (126) and the indium source (128) is individually piped ((122) and (12)).
3)) and supplied into the growth furnace (110). The indium source (128) used a cyclopentadienyl indium to the valency monohydric _{(C 5 H 5 In (I} )). By maintaining the stainless steel container storing the indium raw material (128) at a constant temperature of 60 ° C., the content of cyclopentadienyl indium as the content was set to sublimate. The flow rate of hydrogen gas for transporting the indium raw material was set to 120 milliliters (ml) / min. By supplying the group III element growth reactor (110), the n-type In _0.08 Ga on the Al _0.01 Ga _0.99 N layer is supplied.
The formation of the _0.92 N layer was started. Ammonia gas (119)
Was controlled and maintained at 6.00 liter (l) / min by a mass flow meter (121). Further, in order to form an In _0.08 Ga _0.92 N layer doped with both silicon and zinc, the above-mentioned disilane gas (124) is passed through a p-type doping gas (125) through an n-type doping gas flow pipe (124) in advance. Dimethyl zinc ((CH ₃ )
Had been introduced ₂ Zn) gas to the reactor (110).
The introduction amount of both doping gases is approximately 2 × 10 ¹⁸ c
m ⁻³ was set. Group III source gas, ammonia gas (Group V source gas) (119) and n-type and p-type gases
Supply of doping gas into the growth furnace (110) by 30
An n-type In _0.08 Ga _0.92 N layer doped with both silicon and zinc having a layer thickness of 0.1 μm was formed continuously for a further minute.

During the formation of the indium-containing gallium nitride (In _0.08 Ga _0.92 N) layer for 30 minutes, γ decreased linearly from the initial value (γ ₀ ) of 59.0 to 44.0. That is, γ was reduced at a rate of 0.5 / min. is maintained at a constant rate of 0.1 liter / min while maintaining the supply rate of argon gas at a constant rate of 5.9 to 50 ml / min to 4.4 liter / min.
Decreased by linearly decreasing in minutes.

The indium-containing gallium nitride (In)
The formation of a _0.08 Ga _0.92 N) layer is performed by using an indium source (128).
And the supply of the gallium source (126) into the growth furnace (110) was stopped. After the supply of the group III source gas to the growth furnace was stopped, the ammonia gas (11
9) The mass flow meter (121) was supplied to the growth furnace (110) from 6.0 L / min to 1.0 L / min.
Changed instantly. Thereafter, the temperature of the substrate (101) was cooled at a rate of about 20 ° C. to about 600 ° C., and then naturally cooled. When the substrate temperature dropped to about 450 ° C., the supply of the ammonia gas into the growth furnace was stopped. After confirming that the substrate temperature was near room temperature, the supply of hydrogen gas to the growth furnace was stopped, and only the gas supplied to the growth furnace was argon gas. After repeating the evacuation operation of the growth furnace and the purging operation with argon gas several times, the formed laminate was taken out of the growth furnace.

The surface of the laminate was found to be smooth and flat by SEM observation. In the measurement of the surface roughness with a commercially available laser interference type roughness meter, the surface smoothness is rms.
(Root mean square value of absolute value of roughness) 1.9 nm
Met. On the surface of the laminate, the presence of pits having substantially hexagonal openings was hardly recognized. The surface condition of the lower Al _0.08 Ga _0.92 N layer exposed by the plasma etching technique was observed by SEM. The surface of the layer was smooth and flat with almost no pits.

(Comparative Example 2) A laminate similar to that of Example 2 was constructed using γ outside the range of the present invention. γ is set to 0.10 for an n-type aluminum-gallium nitride layer (Al _0.01 Ga _0.99 N) doped with silicon having a layer thickness of 1 μm, and silicon (Si) and zinc (Zn) having a layer thickness of 0.1 μm. ) Was set to 1.00 for the n-type indium-containing gallium nitride layer (In _0.08 Ga _0.92 N). In the growth atmosphere in which γ is 0.10, the flow rate of hydrogen gas is 5.45 l / min, and the flow rate is 0.
It was created by mixing argon gas at 55 l / min. The growth atmosphere with γ of 1.00 was created by mixing hydrogen and argon gas at flow rates of 3.0 L / min. Although γ is changed in the same manner as described in Example 2 according to the thickness of each nitride compound semiconductor layer constituting the stacked body, γ ₀ does not satisfy the requirements of the present invention. These laminated body constituent layers were formed by using gallium nitride (Ga) formed under the same conditions as those of the second embodiment.
N) Formed on buffer layer. FIG. 13 shows a silicon-doped n-type Al
_0.01 Ga _0.99 N layer and n doped together with silicon and zinc
The surface state of a nitride compound semiconductor laminated body composed of an In _0.08 Ga _0.92 N layer is schematically shown. Many pits whose openings were substantially hexagonal were observed on the surface. The density of pits is 10 ⁴ to 1 by counting observation with a general optical microscope.
0 was ⁵ pieces / cm ^2. Due to the presence of the large number of pits, the surface flatness was impaired. Also, γ
A part of the In _0.08 Ga _0.92 N layer was lost due to sublimation because ₀ was inappropriately small, that is, because hydrogen was present in excess.

[0044]

According to the present invention, a nitride semiconductor layer having excellent surface morphology can be obtained.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a time series of a process in which gallium nitride crystals are interconnected, and particularly shows an initial process of growing a gallium nitride semiconductor layer in which the crystals are generated.

FIG. 2 is a schematic diagram showing a time series of a process in which gallium nitride crystals are connected to each other, and particularly shows a process in which the crystals are united with each other.

FIG. 3 is a schematic diagram showing in chronological order a process in which gallium nitride crystals are connected to each other. In particular, a situation in which pits are generated due to incomplete coalescence in the process of combining crystals. It shows.

FIG. 4 is a schematic view of a surface state of a gallium nitride layer grown with an inappropriately small γ.

FIG. 5 is a schematic diagram of a surface state of a gallium nitride layer when growth proceeds in a state where γ is inappropriately too small.

FIG. 6 is a schematic diagram showing a surface state of a gallium nitride layer grown with an appropriate γ.

FIG. 7: Calculated assuming a hydrogen flow rate of 5.8 l / min and a temperature of the sapphire substrate surface of 1000 ° C.
FIG. 3 is a diagram showing a temperature distribution just above a substrate surface.

FIG. 8 illustrates a pattern in which a projection is generated on a flat surface of a growth island because the surface of the growth island protrudes from a high temperature range in which growth is allowed and penetrates into a lower temperature region as the growth island increases. FIG.

FIG. 9: Even if a growing island develops and increases its height,
FIG. 4 is a schematic diagram for explaining a state in which the tip (surface) of a growth island remains in a high-temperature region by decreasing and changing the volume mixing ratio (γ) of gas constituting a growth atmosphere with time.

FIG. 10 is a diagram showing an example of a change mode when the volume mixing ratio (γ) is reduced from an initial value (γ ₀ ).

FIG. 11 is a schematic configuration diagram of a MOCVD apparatus according to an embodiment.

FIG. 12 is a schematic plan view showing a surface state of a gallium nitride layer grown in an inappropriately small γ state in Comparative Example 1.

FIG. 13 is a schematic diagram showing the surface state of an n-type gallium indium nitride layer grown in an inappropriately small γ state in Comparative Example 2.

[Explanation of symbols]

(101) Substrate (102) Growth nucleus (103) Hexagonal columnar growth island (104) Gap between growth islands (105) Pits (pores) (106) Growth island with flat top surface (107) (108) Growth island with pyramidal projections (108) Gap on growth layer surface (109) GaN columnar crystal developed in substantially hexagonal columnar shape (110) MOCVD growth furnace (111) Rotary pump for vacuum evacuation (112) Argon gas Stainless steel pipe for flowing gas (113) Stainless steel pipe for flowing hydrogen gas (114) Electronic mass flow controller for argon gas (M
FC) (115) Electronic mass flow controller for hydrogen gas (MF
C) (116) Vacuum exhaust piping for growth furnace (117) Opening / closing valve for vacuum exhaust piping (118) Stainless steel piping for flowing nitrogen raw material (119) Nitrogen raw material (ammonia source (liquefied ammonia)) (120) Gallium (Ga) ) Stainless steel pipe for flowing raw material (121) Electronic mass flow controller for nitrogen raw material (ammonia gas) (122) Stainless steel pipe for flowing aluminum (Al) raw material (123) Stainless steel for flowing indium (In) raw material Pipe made of (124) Pipe for flowing n-type impurities (125) Pipe for flowing p-type impurities (126) Gallium source (127) Aluminum source (128) Indium source (129) Growing island with increased height (130) ) Height of growing island (131) High temperature zone where growth occurs (132) Reduction of volume mixing ratio (γ) Straight line indicating over time be linearly decreased from the front end portion of the extended high temperature region (133) growth island growth is effected (surface) (134) volume mixing ratio (gamma) of the initial value (gamma ₀₎ Ri (135) A curve showing that the volume mixing ratio (γ) decreases non-linearly with time from the initial value (γ ₀ ). (136) The volume mixing ratio (γ) is stepped with time from the initial value (γ ₀ ). Curve that shows a global decrease

Claims (3)

[Claims] 1. A method for forming a nitride compound semiconductor layer in a growth atmosphere composed of a mixed gas of hydrogen and an inert element gas, wherein a volume of the inert element gas with respect to a volume of hydrogen (V ₀ ) A method for forming a nitride compound semiconductor layer, wherein the ratio (V / V ₀ ) of (V) is decreased with an increase in the thickness of the nitride compound semiconductor layer. 2. A mixed gas is composed of hydrogen and argon, the thickness of a grown layer is t (unit: μm), and the volume of hydrogen (V
₀ ), the ratio γ of the volume (V) of argon (Ar) to the initial value γ _{0 in} the range of 0.2 / t or more and 6.0 / t or less.
2. The method according to claim 1, wherein the thickness is reduced with an increase in the thickness of the growth layer. 3. The formation temperature (T: unit ° C.) is set to 600 ° C. or more and 900 ° C. or less, and indium (I)
n) The initial value γ of the ratio of the volume of argon to the volume of hydrogen when the content ratio of atoms (x: unit%) is 2% or more and 50% or less and the thickness of the grown layer is t (unit: μm).
₀ is equal to or more than 0.2 · x · (1 / t) · (T / 550).
3. The method for forming a nitride compound semiconductor layer according to claim 2, wherein the thickness is not more than 8.x. (1 / t). (T / 550).