JP2020070196A - Method for growing nitride semiconductor layer - Google Patents

Abstract

To grow a nitride semiconductor layer having higher crystal quality on a heterogeneous substrate.SOLUTION: A method for growing a nitride semiconductor layer comprises: the first step S101 of crystal-growing a buffer layer consisting of a nitride semiconductor on a substrate (a heterogeneous substrate) formed of a crystal different from the nitride semiconductor; and a second step S102 of epitaxially growing a semiconductor layer consisting of a nitride semiconductor on the buffer layer. In the first step S101, the buffer layer is crystal-grown in a thickness without depositing the group III element of the nitride semiconductor constituting the buffer layer on the growth surface of the buffer layer in the temperature conditions of decomposing the nitride semiconductor constituting the buffer layer. In the second step, the semiconductor layer is epitaxially grown in the temperature conditions of decomposing the nitride semiconductor constituting the semiconductor layer.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a method for growing a nitride semiconductor layer, in which a semiconductor layer made of a nitride semiconductor is grown on a substrate made of a crystal different from a nitride semiconductor such as sapphire.

  Conventionally, nitride semiconductors such as InN have been widely used as semiconductor materials for light emitting diodes in the blue and short wavelength regions. Nitride semiconductors are grown using metal organic vapor phase epitaxy (MOVPE) method, molecular beam epitaxy (MBE) method, etc., but sapphire substrate, Si substrate, SiC substrate, etc. which are not nitride semiconductors are used as growth substrates. Different types of substrates are often used.

  Here, it is known that if a nitride semiconductor such as InN is directly grown on these different substrates, a good nitride semiconductor layer cannot be obtained. Therefore, various techniques have been developed to grow a nitride semiconductor layer having high crystal quality. For example, an InN buffer layer having a thickness of about 30 nm is grown at a low growth temperature of 300 ° C., and an InN layer is grown on the low-temperature grown InN layer at a high growth temperature of about 550 ° C. in a range where metal In is not deposited. There is a method (two-step growth method) (see Non-Patent Document 1). By this two-step growth method, as shown in FIG. 9, the InN layer 203 grown at high temperature is formed on the c-plane sapphire substrate 201 via the InN buffer layer 202 grown at low temperature.

  However, it is reported that the InN buffer layer 202 grown on the sapphire substrate 201 at 300 ° C. is flat, but the growth temperature is low, so that sufficient crystal quality cannot be obtained. On the other hand, InN decomposes at a growth temperature condition of 550 ° C. or higher. Therefore, when an InN thin film is grown at a growth temperature of 550 ° C. or higher, metal In is deposited on the growth surface. Therefore, in order to obtain a thick InN layer, it is desirable to grow the InN layer at a temperature as high as possible within a temperature range where metal In is not precipitated (see Non-Patent Document 1).

  Further, in the above-mentioned technique, although the low-temperature buffer layer is flat, the crystallinity is not sufficient, and there is a report example in which the InN layer grown at a relatively high temperature is used as the buffer layer (see Non-Patent Document 2). According to this report, as shown in FIG. 10, an InN buffer layer 302 is grown on a c-plane sapphire substrate 301 at a relatively high growth temperature of 520 ° C., and then the two-step growth method described above is used. An InN buffer layer 303 grown at a relatively low temperature is formed, and an InN layer 304 having a thickness of about 400 nm is grown on the InN buffer layer 303.

  The InN buffer layer 302 grown at a relatively high temperature of 520 ° C. has a thickness of about 10 nm. The growth temperature of the thick InN layer 304 having a thickness of about 400 nm is 550 ° C., which is higher than the growth temperature of the InN buffer layer 302 grown on the c-plane sapphire substrate 301. The growth temperature of the InN buffer layer 302 grown on the c-plane sapphire substrate 301 is lower than the temperature at which InN decomposes, and is within the growth temperature at which metal In does not precipitate on the growth surface.

Meisai et al., “InN and InGaN crystal growth and evaluation of structure and properties”, Applied Physics, Vol. 72, No. 5, pp. 565-571, 2003. Tomohiro Yamaguchi et al., “Realization of High Quality InN Film by Introducing High Temperature InN Buffer Layer”, Journal of Japan Society for Crystal Growth, Volume 30, No. 3, p. 95, 2003.

  As described above, in the conventional technique, the growth temperature of the buffer layer is lower than the temperature at which high crystallinity is obtained even by the method of Non-Patent Document 2. Therefore, the conventional technique has a problem that a nitride semiconductor layer having higher crystal quality cannot be grown on a heterogeneous substrate.

  The present invention has been made to solve the above problems, and an object thereof is to allow a nitride semiconductor layer having higher crystal quality to grow on a heterogeneous substrate.

  The method for growing a nitride semiconductor layer according to the present invention comprises a first step of crystal-growing a buffer layer made of a nitride semiconductor on a substrate made of a crystal different from that of the nitride semiconductor, and a nitriding step on the buffer layer. A second step of epitaxially growing a semiconductor layer made of a physical semiconductor, and in the first step, the buffer layer is formed on the growth surface of the buffer layer under temperature conditions in which the nitride semiconductor forming the buffer layer is decomposed. The buffer layer is crystal-grown to a thickness within a range in which the group III element of the nitride semiconductor does not precipitate, and in the second step, the semiconductor layer is epitaxially grown under a temperature condition within a range in which the nitride semiconductor constituting the semiconductor layer is not decomposed.

  In the above-described method for growing a nitride semiconductor layer, the buffer layer may be made of InN, and the temperature condition in the first step may be 550 ° C. or higher. In this case, in the first step, the buffer layer may be grown to a thickness of 30 nm or less.

  In the above-described method for growing a nitride semiconductor layer, the substrate may be made of hexagonal crystal, and the surface of the substrate may be c-plane.

  In the nitride semiconductor layer growth method, the substrate may be a sapphire substrate.

  In the method of growing a nitride semiconductor layer, the semiconductor layer may be made of InN.

  As described above, according to the present invention, the thickness of the group III element of the nitride semiconductor is not deposited on the growth surface of the buffer layer on the heterogeneous substrate under the temperature condition of the range where the nitride semiconductor is decomposed. Since the buffer layer is crystal-grown, the excellent effect that the nitride semiconductor layer having higher crystal quality can be grown on the heterogeneous substrate is obtained.

FIG. 1 is a flowchart illustrating a method for growing a nitride semiconductor layer according to an embodiment of the present invention. FIG. 2 is a sectional view showing a layer structure of a nitride semiconductor layer formed by the method for growing a nitride semiconductor layer according to the embodiment. FIG. 3 is a photograph showing a result of observing the surface of the InN layer having a thickness of 200 nm manufactured in the example with a scanning electron microscope. In FIG. 4, the ω scan spectrum of X-ray diffraction was measured for the InN layer under each condition manufactured in the example. It is a characteristic view which shows the relationship between the half value width of the peak of InN layer in these (omega) scan spectra, and growth temperature. FIG. 5 shows an X-ray diffraction spectrum (2θ-ω scan) of the semiconductor layer of the present invention manufactured in the example and the InN layer (comparative example) directly grown on the c-plane sapphire substrate at the growth temperature of 600 ° C. FIG. 5 is a characteristic diagram showing a comparison of FIG. FIG. 6 is a characteristic diagram showing the relationship between the thickness of the InN buffer layer grown at a high temperature of 600 ° C. and the electron concentration of the semiconductor layer made of InN. FIG. 7 is a characteristic diagram showing the relationship between the thickness of the InN buffer layer grown at a high temperature of 600 ° C. and the mobility of electrons in the semiconductor layer made of InN. FIG. 8 is a characteristic diagram showing the relationship between the growth temperature of the high-temperature InN buffer layer and the mobility of electrons in the InN layer grown thereon. FIG. 9 is a cross-sectional view showing the layer structure of the nitride semiconductor layer formed by the two-step growth method. FIG. 10 is a cross-sectional view showing the layer structure of the nitride semiconductor layer reported in Non-Patent Document 2.

  Hereinafter, a method for growing a nitride semiconductor layer according to the embodiment of the present invention will be described with reference to FIG.

  First, in a first step S101, a buffer layer made of a nitride semiconductor is crystal-grown on a substrate (heterogeneous substrate) made of a crystal different from the nitride semiconductor. The substrate may be made of, for example, a hexagonal crystal such as a sapphire substrate, and the surface (growth surface) may be the c-plane. In the first step S101, under a temperature condition in a range in which the nitride semiconductor forming the buffer layer is decomposed, a thickness within a range in which the group III element of the nitride semiconductor forming the buffer layer is not deposited on the growth surface of the buffer layer. Crystal growth of the buffer layer. For example, the buffer layer may be made of InN, and the temperature condition may be 550 ° C. or higher in the first step. The buffer layer may be grown to a thickness of 30 nm or less.

  Next, in a second step S102, a semiconductor layer made of a nitride semiconductor is epitaxially grown on the buffer layer. In the second step, the semiconductor layer is epitaxially grown under a temperature condition in which the nitride semiconductor forming the semiconductor layer is not decomposed. For example, when the semiconductor layer is composed of InN, the growth temperature condition of less than 550 ° C. can be used in the second step. According to the method for growing a nitride semiconductor layer in the embodiment, a high crystal quality is obtained by forming a buffer layer made of a nitride semiconductor on a heterogeneous substrate at a high temperature capable of growing with a higher crystal quality. The semiconductor layer can be epitaxially grown in a state in which high crystal quality is maintained without using a too high growth temperature.

  By the method for growing a nitride semiconductor layer in the above-described embodiment, as shown in FIG. 2, the buffer layer 102 made of a nitride semiconductor is crystal-grown on the heterogeneous substrate 101 made of sapphire or the like, and the buffer layer 102 is formed. A state in which the semiconductor layer 103 made of a nitride semiconductor is epitaxially grown thereon is obtained.

  In the conventional two-step growth method, a nitride semiconductor layer is grown at a higher temperature on a thin nitride semiconductor buffer layer grown at a low temperature, so that a high-temperature heat treatment is applied to the buffer layer and the growth is continued. Forming a high crystalline quality growth surface for nitride semiconductor thin films. Thus, the crystallinity of the nitride semiconductor layer is improved by growing the nitride semiconductor layer on the basis of the growth surface of high crystal quality obtained by growing the nitride semiconductor layer at a high temperature.

  On the other hand, in the present invention, the surface of the buffer layer (growth surface) is formed with high crystal quality by thinly crystallizing the nitride semiconductor at high temperature to form the buffer layer.

  Here, for example, InN decomposes at 550 ° C. or higher, and therefore when InN is formed to be thick as a buffer layer at a high temperature, metal In is precipitated on the growth surface and island-like growth occurs, so that the flatness of the surface deteriorates. .. Therefore, in the conventional technique, the buffer layer was grown with InN at a temperature below 550 ° C. at which InN was not decomposed.

  However, even on a heterogeneous substrate, in the InN layer (buffer layer) grown at 550 ° C. or higher, the full width at half maximum of the ω scan spectrum of X-ray diffraction becomes extremely narrow, as described later. This indicates that the crystal orientation of the InN layer itself grown at a high temperature of 550 ° C. or higher at which decomposition starts is significantly improved.

  Further, in the present invention, the thickness of the InN layer (buffer layer) grown at a high temperature is reduced to 30 nm or less to suppress the precipitation of metal In on the growth surface.

  As a result, according to the present invention, a thick semiconductor layer made of a nitride semiconductor can be epitaxially grown with high crystal quality on the buffer layer formed on the different type substrate.

[Example]
Hereinafter, it demonstrates in detail using an Example. First, an InN layer was directly grown on a c-plane sapphire substrate using the plasma MBE method. In the growth of the InN layer, a heated In substrate was irradiated with an In molecular beam from the In cell, and nitrogen gas activated by plasma was supplied to the substrate surface. The flow rate of nitrogen gas was 2 sccm and the plasma power was 500 W. Further, the thickness of the InN layer was fixed to 200 nm, the growth temperature was changed to 200 ° C., 300 ° C., 400 ° C., 500 ° C., 600 ° C., and the InN layer was directly grown at each of these growth temperatures. Note that sccm is a unit of flow rate, and indicates that a fluid at 0 ° C. and 1013 hPa flows by 1 cm ³ per minute.

  Scanning electron microscope images (SEM images) of the surfaces of these InN layers are shown in FIG. Since the thickness of the grown layer is as thick as 200 nm, the flatness is deteriorated and unevenness is formed on the growth surface of the InN layer depending on the temperature conditions. As the growth temperature increases, the unevenness increases and the surface flatness deteriorates.

  Further, the ω scan spectrum of X-ray diffraction was measured for the InN layer under each condition. FIG. 4 shows the relationship between the half-value width of the peak of the InN layer in these ω scan spectra and the growth temperature. The half-width decreases sharply as the growth temperature rises. In particular, under the condition of the growth temperature of 600 ° C., the full width at half maximum is extremely narrow. Further, under the condition of the growth temperature of 600 ° C., since the diameter of the crystal is large, it is considered that it can be used as the crystal nucleus of the layer of the nitride semiconductor formed thereon.

  Based on the above, a thin buffer layer made of InN grown at a high temperature of 600 ° C. was used, and a semiconductor layer made of InN was epitaxially grown thereon at a growth temperature of 300 ° C. to a thickness of 200 nm. An X-ray diffraction spectrum (2θ−ω scan) of the semiconductor layer made of InN thus formed (epitaxial growth) and the InN layer (comparative example) directly grown on the c-plane sapphire substrate at a growth temperature of 600 ° C. 5) is shown in FIG. The buffer layer had a thickness of 30 nm.

  As described above, when InN is grown thick at 600 ° C. which exceeds the decomposition temperature of InN, the flatness of the surface of the grown InN deteriorates. Therefore, the thickness 30 nm of the InN buffer layer grown at 600 ° C. and high temperature indicates the thickness in the range when grown at the growth temperature at which the flatness of the surface of the InN buffer layer is not deteriorated.

  Similar to the conventional report example, in the InN layer of the comparative example directly grown on the c-plane sapphire substrate at the growth temperature of 600 ° C., a peak due to the precipitation of metal In was observed as shown in FIG. 5B. It was On the other hand, in the semiconductor layer made of InN according to the present invention, precipitation of metal In was not observed, as shown in FIG. The reason may be that the thickness of the InN buffer layer grown at 600 ° C. is reduced to 30 nm. Therefore, in the present invention, the thickness of the buffer layer made of InN that grows at a high temperature is preferably 30 nm or less.

  Next, an Al / Au electrode is formed on each of the InN semiconductor layer of the embodiment and the InN layer of the comparative example by a vapor deposition method or the like, and the Hall effect measurement is performed by using the van der Pauw method. I went. In the semiconductor layer made of InN according to the embodiment, the measurement is performed on a plurality of samples having different buffer layer thicknesses formed thereunder. Note that both the semiconductor layer made of InN in the embodiment and the InN layer in the comparative example exhibited n-type conduction.

  FIG. 6 shows the relationship between the thickness of the InN buffer layer grown at a high temperature of 600 ° C. and the electron concentration of the semiconductor layer of InN. The electron concentration of the comparative example in which the InN layer was directly grown on the c-plane sapphire substrate without using the high temperature buffer layer is plotted with the thickness of the buffer layer being 0 nm. As shown in FIG. 6, the electron concentration is almost constant regardless of the presence or absence of the buffer layer.

  FIG. 7 shows the relationship between the thickness of the InN buffer layer grown at a high temperature of 600 ° C. and the electron mobility in the semiconductor layer of InN. The electron mobility in the comparative example in which the InN layer was directly grown on the c-plane sapphire substrate without using the high temperature buffer layer is plotted with the thickness of the buffer layer being 0 nm. By using the InN buffer layer grown at high temperature, which is a feature of the present invention, the electron mobility is increased 10 times. As described above, according to the present invention, the crystal quality of the InN layer can be dramatically improved.

  FIG. 8 shows the relationship between the growth temperature of the high temperature InN buffer layer and the mobility of electrons in the InN layer grown thereon. The buffer layer had a thickness of 30 nm, the InN layer had a thickness of 200 nm, and the mobility was measured by changing the growth temperature of the buffer layer. By setting the growth temperature of the buffer layer made of InN to 550 ° C. or higher, it can be seen that the InN layer formed thereon has a high electron mobility. Therefore, the growth temperature of the InN buffer layer is preferably 550 ° C. or higher.

  Although InN has been described as an example of the nitride semiconductor in the above description, the present invention is not limited to this, and similar effects can be expected even if the nitride semiconductor is GaN, AlGaN, or InGaN. However, when GaN, AlGaN, and InGaN are grown subsequently to the InN buffer layer, the temperature condition may be suitable for the growth of each nitride semiconductor, and the growth temperature of the buffer layer is not lower than the growth temperature of the buffer layer in the epitaxial growth of the semiconductor layer. Good.

  Further, in the above-described embodiments, the example of the c-plane sapphire substrate is shown as a different type of substrate different from the nitride semiconductor, but when a sapphire substrate such as a-plane or m-plane, or a Si substrate or a SiC substrate is used. However, the same effect can be expected. Further, in the above-described embodiments, the thickness of the semiconductor layer made of InN is set to 200 nm, but the thickness is not limited to this, and the layer thickness of the semiconductor layer formed on the buffer layer may be, for example, 200 nm or more. Needless to say.

  As described above, according to the present invention, on a heterogeneous substrate, under a temperature condition of a range in which a nitride semiconductor decomposes, a group III element of the nitride semiconductor is not deposited on the growth surface of the buffer layer. Since the buffer layer is crystal-grown to have a thickness, the nitride semiconductor layer having higher crystal quality can be grown on the heterogeneous substrate. In this way, a nitride semiconductor layer of high crystal quality can be obtained, and it can be expected to improve the characteristics of a device manufactured using this layer.

  The present invention is not limited to the embodiments described above, and many modifications and combinations can be implemented by a person having ordinary knowledge in the field within the technical idea of the present invention. That is clear.

  101 ... Heterogeneous substrate, 102 ... Buffer layer, 103 ... Semiconductor layer.

Claims (6)

A first step of crystal-growing a buffer layer made of a nitride semiconductor on a substrate made of a crystal different from that of the nitride semiconductor;
A second step of epitaxially growing a semiconductor layer made of a nitride semiconductor on the buffer layer,
In the first step, under a temperature condition in which the nitride semiconductor forming the buffer layer is decomposed, a group III element of the nitride semiconductor forming the buffer layer is not deposited on the growth surface of the buffer layer. Crystal growth of the buffer layer to a thickness,
In the second step, the nitride semiconductor layer growth method is characterized in that the semiconductor layer is epitaxially grown under a temperature condition in a range in which the nitride semiconductor forming the semiconductor layer is not decomposed. The method for growing a nitride semiconductor layer according to claim 1,
The buffer layer is composed of InN,
In the first step, the method for growing a nitride semiconductor layer is characterized in that the temperature condition is 550 ° C. or higher. The method for growing a nitride semiconductor layer according to claim 2,
In the first step, the buffer layer is grown to a thickness of 30 nm or less. A method for growing a nitride semiconductor layer, comprising: The method for growing a nitride semiconductor layer according to any one of claims 1 to 3,
The method for growing a nitride semiconductor layer, wherein the substrate is composed of hexagonal crystals, and the surface of the substrate is a c-plane. The method for growing a nitride semiconductor layer according to any one of claims 1 to 4,
The method for growing a nitride semiconductor layer, wherein the substrate is a sapphire substrate. The method for growing a nitride semiconductor layer according to any one of claims 1 to 5,
The method for growing a nitride semiconductor layer, wherein the semiconductor layer is composed of InN.