JP2006179861A - Semiconductor epitaxial wafer and field effect transistor - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a semiconductor epitaxial wafer that prevents a dielectric layer from being formed in a buffer layer caused by the contamination of a conductive impurity and has high characteristics in a semiconductor epitaxial wafer having the buffer layer on a substrate and a FET containing an HEMT using the same. <P>SOLUTION: The semiconductor epitaxial wafer 300 has a structure in which the buffer layer 320 is formed as configured by a first AlN buffer layer 321 including an undoped AlN of thickness of 25 nm, a first GaN buffer layer 322 including an undoped GaN of thickness of 50 nm, a second AlN buffer layer 323 including undoped AlN of thickness of 300 nm, and a second GaN buffer layer 324 including an undoped GaN of thickness of 1,500 nm sequentially from the bottom on a substrate 310 including sapphire. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a field effect transistor including a semiconductor epitaxial wafer and a high electron mobility transistor using the same.

  In a field effect transistor (hereinafter referred to as FET) including a high electron mobility transistor (hereinafter referred to as HEMT), a current flowing between a source electrode and a drain electrode is controlled by spreading of a depletion layer from the gate electrode.

Among such FETs, a GaN buffer layer or a double-structure buffer layer composed of GaN and AlGaN is formed on a sapphire substrate or a silicon carbide (SiC) substrate, and a gallium nitride (GaN) system is formed on the buffer layer. For example, Patent Documents 1 and 2 report that an epitaxial layer is formed by growing the epitaxial layer.
JP 2001-102564 A JP 2002-50758 A

However, when manufacturing an epitaxial wafer in which an epitaxial layer made of GaN is grown, the technology for cleaning the interface between the epitaxial layer and the substrate is not well established, and one of the source gases of GaN As a result, the high purity of ammonia (NH ₃ ) gas, which is, cannot be obtained, the conductive impurities are easily mixed into the epitaxial layer.

  For this reason, even when a GaN buffer layer is formed on a substrate, conductive impurities are mixed in the buffer layer, and the buffer layer is required to have higher insulation than other layers. Nevertheless, the buffer layer has high conductivity substantially equal to that of the channel layer particularly in the portion close to the substrate, which causes the depletion layer to hardly spread from the gate electrode.

  In addition, it is difficult to obtain an electronic device such as an FET having good characteristics because a current flows through the portion having high conductivity (hereinafter referred to as a conductive layer).

  Even in the FETs described in Patent Documents 1 and 2, it is considered that sufficient characteristics are not obtained for the above reasons.

  Accordingly, it is an object of the present invention to prevent a semiconductor epitaxial wafer having a buffer layer on a substrate from forming a conductive layer in the buffer layer due to mixing of conductive impurities and to have a high characteristic. And it is providing the FET containing HEMT using the same.

  In order to achieve the above object, the present invention provides a semiconductor epitaxial wafer having a buffer layer on a substrate, wherein the buffer layer includes a first GaN buffer layer, an AlN buffer layer, and a gallium nitride buffer layer in order on the substrate. A semiconductor epitaxial wafer characterized by comprising the structure formed in (1) above is provided.

  In order to achieve the above object, the present invention provides a semiconductor epitaxial wafer having a buffer layer on a substrate, wherein the buffer layer includes a first AlN buffer layer, a first GaN buffer layer, a second AlN buffer layer, Provided is a semiconductor epitaxial wafer comprising a structure in which a gallium nitride buffer layer is sequentially formed on the substrate.

  In order to achieve the above object, the present invention includes a field effect transistor comprising a channel layer, an electron supply layer, a source electrode, a gate electrode, and a drain electrode on the semiconductor epitaxial wafer of the present invention. I will provide a.

  According to the present invention, in a semiconductor epitaxial wafer having a buffer layer on a substrate, it is possible to prevent a conductive layer from being formed in the buffer layer due to mixing of conductive impurities, and to provide a semiconductor epitaxial wafer having high characteristics and the same. It is possible to provide an FET including a HEMT using the above.

[First Embodiment]
The structure of the transistor according to the first embodiment of the present invention will be described separately for a semiconductor epitaxial wafer structure and a transistor structure.

(Structure of semiconductor epitaxial wafer)
FIG. 1 is a cross-sectional view of a semiconductor epitaxial wafer according to the first embodiment.

  A semiconductor epitaxial wafer 100 includes a first GaN buffer layer 121 made of undoped GaN having a thickness of 50 nm, an AlN buffer layer 122 made of undoped AlN having a thickness of 300 nm, and a thickness on a substrate 110 made of sapphire or SiC. The buffer layer 120 includes a second GaN buffer layer 123 (gallium nitride buffer layer) made of undoped GaN having a thickness of 1500 nm.

  Here, the epitaxial growth of the semiconductor epitaxial wafer 100 is performed by a metal organic chemical vapor deposition (MOVPE) method. In this MOVPE method, for example, trimethylgallium (TMG) is used as a gallium source, trimethylaluminum (TMA) is used as an aluminum source, ammonia gas is used as a nitrogen source, and hydrogen is used as a carrier gas.

  The epitaxial growth of the semiconductor epitaxial wafer 100 is performed, for example, by placing the semiconductor epitaxial wafer 100 in a heater-heated depressurization furnace with the surface facing up to the ceiling and setting the pressure in the furnace to 13332 Pa (100 Torr). Do it.

  The growth temperature of the first GaN buffer layer 121 is 300 to 800 ° C., and the thickness can be changed within the range of 10 to 100 nm. This is because crystals having a good surface state cannot be obtained under conditions outside this range.

  The growth temperature of the second GaN buffer layer 123 is 300 to 800 ° C., and the thickness can be changed in the range of 500 to 2000 nm. For example, when the thickness is less than 500 nm, sufficient two-dimensional electron gas characteristics (sheet carrier concentration and electron mobility) cannot be obtained in HEMT. When the thickness exceeds 2000 nm, free carriers are generated due to residual impurities in the buffer layer. This is because the resistance of the buffer layer is lowered.

In the first embodiment, the gallium nitride buffer layer is described as being a GaN buffer layer (second GaN buffer layer 123). However, this material is not limited to GaN, and Al if _x in _y composition range of _{Ga 1-x-y N (} 0 ≦ x <0.1,0 ≦ y <0.3), since it does not adversely affect the 2-dimensional electron gas characteristics of the HEMT, which composition Ratio material may also be used.

  The growth temperature of the AlN buffer layer 122 is 950 ° C. to 1300 ° C., and the thickness is 200 nm or more. This is because when the temperature is lower than 950 ° C., it becomes difficult for AlN to grow laterally (lateral direction), the surface becomes rough, and when the temperature exceeds 1300 ° C., the underlying first GaN buffer layer 121 evaporates and GaN This is because the effect of the buffer layer is lost. It is more desirable that the temperature is 950 ° C to 1200 ° C.

The effect of the GaN buffer layer is that the first GaN buffer layer 121 is formed on the substrate 110 and the AlN buffer layer 122 layer is formed on the substrate 110 rather than directly growing the AlN buffer layer 122 layer on the substrate 110 to be thick. By forming the crystal, a crystal having a better surface state can be obtained. The GaN buffer layer 121 also has a function of lowering the dislocation density of the AlN buffer layer 122 layer. When this is less than 1 × 10 ⁸ cm ⁻² , there is a problem that it is difficult to increase the resistance (the resistance decreases). Therefore, the thickness of the AlN buffer layer 122 is set to 200 nm or more so that the dislocation density is 1 × 10 ⁸ cm ⁻² or more.

(Transistor structure)
FIG. 2 is a cross-sectional view of a HEMT that is a transistor according to the first embodiment.

  The HEMT 200 includes a first GaN buffer layer 121 made of undoped GaN having a thickness of 50 nm, an AlN buffer layer 122 made of undoped AlN having a thickness of 300 nm, and a 2000 nm thick material on a substrate 110 made of sapphire or SiC. On the epitaxial wafer 100 formed with the buffer layer 120 composed of the second GaN buffer layer 123 made of undoped GaN, a channel layer 201 made of undoped GaN having a thickness of 100 nm and a thickness of 25 nm are formed in order from the bottom. A carrier supply layer 202 made of n-type AlGaN is formed. A cap layer 203 and a gate electrode 204 made of n-type GaN having a thickness of 2 nm are formed on the carrier supply layer 202, and a source electrode 205 and a drain electrode 206 are formed on the cap layer 203. Structure formed A.

  Here, the epitaxial growth of the HEMT 200 is performed by the MOVPE method under the same conditions as in the case of the semiconductor epitaxial wafer 100. In this MOVPE method, for example, TMG is used as a gallium material, TMA is used as an aluminum material, ammonia gas is used as a nitrogen material, hydrogen is used as a carrier gas, and monosilane is used as an n-type dopant.

(Effects of the first embodiment)
According to the first embodiment, in the semiconductor epitaxial wafer 100 and the HEMT 200, the AlN buffer layer 122 has a thickness of 200 nm or more while having a GaN buffer layer that causes a decrease in the dislocation density of the buffer layer. The dislocation density can be set to 1 × 10 ⁸ cm ⁻² or more to prevent the formation of a conductive layer.

[Second Embodiment]
The structure of the transistor according to the second embodiment of the present invention will be described separately for a semiconductor epitaxial wafer structure and a transistor structure.

(Structure of semiconductor epitaxial wafer)
FIG. 3 is a cross-sectional view of a semiconductor epitaxial wafer according to the second embodiment.

  The semiconductor epitaxial wafer 300 includes a first AlN buffer layer 321 made of undoped AlN having a thickness of 25 nm and a first GaN buffer layer 322 made of undoped GaN having a thickness of 50 nm on a substrate 310 made of sapphire in order from the bottom. A buffer layer 320 composed of a second AlN buffer layer 323 made of undoped AlN having a thickness of 300 nm and a second GaN buffer layer 324 (gallium nitride-based buffer layer) made of undoped GaN having a thickness of 1500 nm was formed. It has a structure. The difference in configuration from the semiconductor epitaxial wafer 100 according to the first embodiment is that an AlN layer (first AlN buffer layer 321) is formed between the substrate and the first GaN layer.

  Since the epitaxial growth method and conditions of this semiconductor epitaxial wafer 300 are the same as those of the semiconductor epitaxial wafer 100 according to the first embodiment, description thereof is omitted.

  The growth temperature and the thickness of the first GaN buffer layer 322, the second GaN buffer layer 324, and the second AlN buffer layer 323 are the same as those of the first GaN buffer layer according to the first embodiment. 121, the second GaN buffer layer 123, and the AlN buffer layer 122.

  Here, the growth temperature of the first AlN buffer layer 321 is 950 ° C. to 1300 ° C., and its thickness can be changed between 10 and 50 nm. The first AlN buffer layer 321 serves to prevent a conductive layer from being formed in the buffer layer 320 due to impurities attached to the substrate surface, but if the thickness is too large, the first GaN buffer layer 321 is formed thereon. Since a surface state that may be stacked with the layer 322 cannot be obtained, the thickness is set to 10 to 50 nm. It is more desirable that the temperature is 950 ° C to 1200 ° C.

(Transistor structure)
FIG. 4 is a cross-sectional view of a HEMT that is a transistor according to the second embodiment.

  The HEMT 400 includes a first AlN buffer layer 321 made of undoped AlN having a thickness of 25 nm, a first GaN buffer layer 322 made of undoped GaN having a thickness of 50 nm, and a thickness on a substrate 310 made of sapphire or SiC. On the epitaxial wafer 300 formed by forming a buffer layer 320 composed of a second AlN buffer layer 323 made of undoped AlN having a thickness of 300 nm and a second GaN buffer layer 324 made of undoped GaN having a thickness of 1500 nm, A channel layer 401 made of undoped GaN with a thickness of 100 nm and a carrier supply layer 402 made of n-type AlGaN with a thickness of 25 nm are formed in order from the bottom, and a cap made of n-type GaN with a thickness of 2 nm is formed on the carrier supply layer 402 Layer 403 and gate electrode 404; It has a structure in which the source electrode 405 and drain electrode 406 is formed on the cap layer 403.

  Here, the epitaxial growth method and conditions of the HEMT 400 are the same as those of the HEMT 200 according to the first embodiment, and thus the description thereof is omitted.

(Effect of the second embodiment)
According to the second embodiment, the semiconductor epitaxial wafer 300 and the HEMT 400 include the first AlN buffer layer 321, so that compared to the semiconductor epitaxial wafer 100 and the HEMT 200 according to the first embodiment, It is possible to more reliably prevent the conductive layer from being formed in the buffer layer 320.

  EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited thereto.

In Example 1 of the present invention, the thickness of the AlN buffer layer 122 in the semiconductor epitaxial wafer 100 according to the first embodiment was changed to 100 nm, 200 nm, and 300 nm, and their conductivity (current) The measurement was carried out with the value, the unit being A / mm), and the dislocation density (unit is cm ⁻² ).

  Here, the epitaxial growth of the semiconductor epitaxial wafer 100 was performed by a metal organic chemical vapor deposition (MOVPE) method. In this MOVPE method, trimethylgallium (TMG) was used as a gallium source, trimethylaluminum (TMA) was used as an aluminum source, ammonia gas was used as a nitrogen source, and hydrogen was used as a carrier gas.

  The epitaxial growth of the semiconductor epitaxial wafer 100 is performed by placing the semiconductor epitaxial wafer 100 in a heater-heated vacuum furnace with the surface facing up to the ceiling, and setting the pressure in the furnace to 13332 Pa (100 Torr). It was.

  FIG. 5 is a cross-sectional view of the measuring element according to the first embodiment. The measurement element 500 is obtained by providing measurement electrodes 501 and 502 on the second GaN buffer layer 123 of the semiconductor epitaxial wafer 100. A voltage of 10 V was applied to the measurement electrodes 501 and 502, and the magnitude of the current flowing at that time was measured.

FIG. 6 is a graph showing the measurement results. When the thickness of the AlN buffer layer 122 is 100 nm, 200 nm, and 300 nm, the current values are about 5 × 10 ⁻⁷ A / mm, 5 × 10 ⁻⁸ A / mm, and 1 × 10 ⁻⁸ A / mm, respectively. The dislocation densities were about 5 × 10 ⁷ cm ⁻² , 1 × 10 ⁸ cm ⁻² , and 5 × 10 ⁸ cm ⁻² , respectively.

From the above results, it was found that the best value was obtained when the thickness of the AlN buffer layer 122 was 300 nm. Further, when the thickness of the AlN buffer layer 122 is 200 nm or more, the dislocation density of the buffer layer 122 is about 1 × 10 ⁸ cm ⁻² or more, and the formation of a conductive layer in the buffer layer 120 is prevented. It has been found that the object of the present invention is achieved.

(Comparative example)
FIG. 9 is a cross-sectional view of a conventional measuring element of a semiconductor epitaxial wafer. The measurement element 700 includes a first GaN buffer layer 731 having a thickness of 50 nm grown on a sapphire substrate 720 in order from the bottom at a growth temperature of 500 ° C., and a first GaN buffer layer having a thickness of 2000 nm grown at a growth temperature of 1050 ° C. Measurement electrodes 701 and 702 are provided on a semiconductor epitaxial wafer 710 having a structure in which a buffer layer 730 composed of two GaN buffer layers 732 is formed. The buffer layer epitaxial growth method and the raw materials used are the same as in Example 1.

As a result of applying a voltage of 10 V to the measurement electrodes 701 and 702 and measuring the magnitude of the current flowing at that time, a very large value of 1 × 10 ⁻¹ A / mm was obtained compared to Example 1. .

  In Example 2 of the present invention, the thickness of the first AlN buffer layer 321 in the semiconductor epitaxial wafer 300 according to the second embodiment was changed to 0, 25 nm, 50 nm, and 75 nm. The conductivity (indicated by current value, unit is A / mm) was measured.

  Here, the epitaxial growth of the semiconductor epitaxial wafer 300 was performed by a metal organic chemical vapor deposition (MOVPE) method. In this MOVPE method, trimethylgallium (TMG) was used as a gallium source, trimethylaluminum (TMA) was used as an aluminum source, ammonia gas was used as a nitrogen source, and hydrogen was used as a carrier gas.

  The epitaxial growth of the semiconductor epitaxial wafer 300 is performed by placing the semiconductor epitaxial wafer 100 in a heater-heated vacuum furnace with the surface facing up to the ceiling, and setting the pressure in the furnace to 13332 Pa (100 Torr). It was.

  FIG. 7 is a sectional view of the measuring element according to the second embodiment. The measurement element 800 is obtained by providing measurement electrodes 801 and 802 on the second GaN buffer layer 324 of the semiconductor epitaxial wafer 300. A voltage of 10 V was applied to the measurement electrodes 801 and 802, and the magnitude of the current flowing at that time was measured.

FIG. 8 is a graph showing the measurement results. When the thickness of the first AlN buffer layer 321 is 0 nm, 25 nm, 50 nm, and 75 nm, the current values are about 2 × 10 ⁻⁸ A / mm, 1 × 10 ⁻⁹ A / mm, and 2 × 10 ⁻⁹ A, respectively. / Mm, 1 × 10 ⁻⁷ A / mm.

  From the above results, it was found that the best value was obtained when the thickness of the first AlN buffer layer 321 was 25 nm. It was also found that when the thickness of the first AlN buffer layer 321 is 15 to 55 nm, particularly 20 to 50 nm, the current value is lower than when the first AlN buffer layer 321 is not present.

1 is a cross-sectional view of a semiconductor epitaxial wafer according to a first embodiment of the present invention. It is sectional drawing of HEMT which is a transistor concerning the 1st Embodiment of this invention. It is sectional drawing of the semiconductor epitaxial wafer which concerns on the 2nd Embodiment of this invention. It is sectional drawing of HEMT which is a transistor concerning the 2nd Embodiment of this invention. It is sectional drawing of the element for a measurement which concerns on Example 1 of this invention. It is a graph showing the measurement result which concerns on Example 1 of this invention. It is sectional drawing of the element for a measurement which concerns on Example 2 of this invention. It is a graph showing the measurement result which concerns on Example 2 of this invention. It is sectional drawing of the measuring element of the conventional semiconductor epitaxial wafer.

Explanation of symbols

100, 300 Semiconductor epitaxial wafer 110, 310 Substrate 120, 320 Buffer layer 121, 322 First GaN buffer layer 122 AlN buffer layer 123, 324 Second GaN buffer layer 200, 400 HEMT
201, 401 Channel layer 202, 402 Carrier supply layer 203, 403 Cap layer 204, 404 Gate electrode 205, 405 Source electrode 206, 406 Drain electrode 321 First AlN buffer layer 323 Second AlN buffer layer 500 Measuring element 501 , 502, 801, 802 Measuring electrodes

Claims (8)

In a semiconductor epitaxial wafer having a buffer layer on a substrate,
The buffer layer has a structure in which a first GaN buffer layer, an AlN buffer layer, and a gallium nitride-based buffer layer are sequentially formed on the substrate. In a semiconductor epitaxial wafer having a buffer layer on a substrate,
The buffer layer has a structure in which a first AlN buffer layer, a first GaN buffer layer, a second AlN buffer layer, and a gallium nitride buffer layer are sequentially formed on the substrate. Wafer.   The semiconductor epitaxial wafer according to claim 1, wherein the gallium nitride-based buffer layer is a second GaN buffer layer.   The growth temperature of the AlN buffer layer or the first and second AlN buffer layers is 950 to 1300 ° C., and the growth temperature of the first and second GaN buffer layers is 300 to 800 ° C. The semiconductor epitaxial wafer according to claim 3.   The thickness of the first AlN buffer layer is 10 to 50 nm, the thickness of the first GaN buffer layer is 10 to 100 nm, and the thickness of the AlN buffer layer or the second AlN buffer layer is 200 nm or more. The semiconductor epitaxial wafer according to claim 1, wherein the gallium nitride buffer layer has a thickness of 500 to 2000 nm.   The semiconductor epitaxial wafer according to claim 1, wherein the substrate is a sapphire substrate or a silicon carbide substrate. The semiconductor epitaxial wafer according to claim 1, wherein a dislocation density in the AlN buffer layer or the second AlN buffer layer is 1 × 10 ⁸ cm ⁻² or more.   A field effect transistor comprising a channel layer, an electron supply layer, a source electrode, a gate electrode, and a drain electrode on the semiconductor epitaxial wafer according to claim 1.

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