JP5744784B2 - Manufacturing method of nitride semiconductor epitaxial wafer - Google Patents

Description

  The present invention relates to a nitride semiconductor epitaxial wafer and a method for manufacturing the same.

  Nitride compound semiconductors composed of group III elements and nitrogen of indium, gallium, and aluminum can emit light in the most wavelength region from the ultraviolet region to the visible light region by controlling the composition ratio of group III elements. Development is progressing as a material for an innovative high-efficiency light-emitting device, and it has been put to practical use. In addition, since nitride semiconductors have a high saturation electron velocity and a high breakdown voltage, they have been put into practical use as electronic device materials that realize high efficiency and high output in a high frequency region.

  As a conventional nitride semiconductor field effect transistor, one manufactured using an epitaxial wafer in which an AlN layer and a GaN layer are stacked on a single crystal substrate is known (see, for example, Patent Documents 1 and 2).

  When a GaN layer is formed on an AlN layer, island-like crystal growth occurs in the early stage of growth due to lattice mismatch between the AlN crystal and the GaN crystal. During this island-like crystal growth, a facet surface that easily takes in impurities is formed. Is done. Since the band gap of the GaN crystal is about 3.5 eV, and becomes easily conductive when impurities such as silicon are mixed, silicon released from impurities in the early stage of growth of the GaN layer, particularly from the metal organic chemical vapor deposition furnace jig, By taking in, the initial growth portion of the GaN layer, that is, the region close to the boundary with the AlN layer becomes conductive. For this reason, in a transistor manufactured using an epitaxial wafer in which an AlN layer and a GaN layer are stacked on a single crystal substrate, there is a possibility that a leakage current is generated in a region near the boundary between the GaN layer and the AlN layer.

  Thus, in the transistors disclosed in Patent Documents 1 and 2, Fe is doped into a partial region of the GaN layer on the AlN layer side to increase the electrical resistance and prevent leakage current. According to this method, free electrons generated in the initial growth portion of the GaN layer due to silicon contamination are compensated for or offset by intentionally doped Fe, and the initial growth portion of the GaN layer that can be considered as a leak current generation location. Is increased in resistance.

JP 2008-251966 A JP 2010-182872 A

Patent Documents 1 and ₂ disclose that ferrocene (Cp ₂ Fe: biscyclopentadienyl iron) is used for Fe doping. By using ferrocene as an Fe raw material, the Fe doping concentration can be controlled independently of the epitaxial growth conditions of the GaN crystal. Therefore, ferrocene is widely used for Fe doping for the purpose of suppressing leakage current. .

  However, when ferrocene is supplied from the raw material cylinder to the metal-organic vapor phase growth reactor, it has a property of easily adhering to an intermediate pipe or the wall of the reactor. Therefore, if the supply of ferrocene is started simultaneously with the start of the growth of the GaN layer, the ferrocene starts to adhere to the wall surface first and is consumed until it reaches the substrate surface. Immediately after the start of growth, Fe is not doped at the target concentration. For this reason, the leakage current is the main generation point, and the region on the AlN layer side of the GaN layer to be increased in resistance by doping Fe cannot be doped with a sufficient concentration of Fe, and low resistance GaN Become a layer. Therefore, there is a possibility that an inter-element leakage current in the transistor or a drain leakage current at the time of pinch-off occurs.

  Therefore, one of the objects of the present invention is to provide a nitride semiconductor epitaxial wafer having an Fe-containing nitride semiconductor layer that can effectively suppress a leakage current, and a method for manufacturing the same.

  (1) According to one aspect of the present invention, in order to achieve the above object, a substrate, an AlN layer formed on the substrate, a first nitride semiconductor layer formed on the AlN layer, The AlN layer has a Fe-containing region in a part on the first nitride semiconductor layer side, and the first nitride semiconductor layer has a Fe-containing region in at least a part on the AlN layer side. A nitride semiconductor epitaxial wafer is provided.

(2) In the nitride semiconductor epitaxial wafer, an Fe concentration of at least a part of the Fe-containing region of the first nitride semiconductor layer on the AlN layer side is 1 × 10 ¹⁷ cm ³ or more and 1 × 10 ^19. It is preferable that it is cm ³ or less.

(3) The nitride semiconductor epitaxial wafer includes a second nitride semiconductor layer having an electron affinity smaller than that of the first nitride semiconductor layer on the first nitride semiconductor layer, A two-dimensional electron gas exists in a region on the second nitride semiconductor layer side in the nitride semiconductor layer, and the Fe concentration in the region in which the two-dimensional electron gas exists in the first nitride semiconductor layer is 1 × 10 ¹⁶ cm ⁻³ or less may be used.

  (4) According to another aspect of the present invention, an AlN crystal is epitaxially grown on a substrate to form an AlN layer, and a first nitride semiconductor crystal is epitaxially grown on the AlN layer. Forming the nitride semiconductor layer, and starting the supply of ferrocene onto the substrate after the start of the growth of the AlN crystal and before the end of the growth of the AlN crystal, and the first nitride By terminating the supply of the ferrocene after the start of the growth of the semiconductor crystal and before the end of the growth of the first nitride semiconductor crystal, at least a part of the AlN layer on the first nitride semiconductor layer side is obtained. There is provided a method for manufacturing a nitride semiconductor epitaxial wafer, in which Fe is doped into the region and at least a part of the first nitride semiconductor layer on the AlN layer side.

(5) In the method for manufacturing a nitride semiconductor epitaxial wafer, a doping concentration of Fe in at least a part of the first nitride semiconductor layer on the AlN layer side is 1 × 10 ¹⁷ cm ³ or more, 1 × 10 It is preferably ¹⁹ cm ³ or less.

  (6) In the method for manufacturing a nitride semiconductor epitaxial wafer, it is preferable that the growth of the first nitride semiconductor crystal is started within 30 seconds after the growth of the AlN crystal is completed.

  (7) In the method for manufacturing a nitride semiconductor epitaxial wafer, the substrate temperature and environmental pressure of the substrate during the growth of the Fe-doped region of the AlN layer, and the substrate during the growth of the first nitride semiconductor crystal It is preferred that the substrate temperature and ambient pressure are equal.

(8) In the method for manufacturing a nitride semiconductor epitaxial wafer, a step of forming a second nitride semiconductor layer having an electron affinity smaller than that of the first nitride semiconductor layer on the first nitride semiconductor layer. A two-dimensional electron gas is present in a region on the second nitride semiconductor layer side in the first nitride semiconductor layer, and the two-dimensional electron gas is present in the first nitride semiconductor layer The Fe concentration in the region to be performed may be 1 × 10 ¹⁶ cm ⁻³ or less.

  According to one aspect of the present invention, it is possible to provide a nitride semiconductor epitaxial wafer having an Fe-containing nitride semiconductor layer capable of effectively suppressing leakage current and a method for manufacturing the same.

FIG. 1 is a vertical sectional view of a nitride semiconductor epitaxial wafer according to an embodiment. FIGS. 2A to 2D are cross-sectional views showing manufacturing processes of the nitride semiconductor epitaxial wafer according to the embodiment. FIG. 3 shows an example of the relationship between the crystal growth interruption time from the end of the growth of the AlN crystal to the start of the growth of the first nitride semiconductor crystal and the inter-element leakage current in the nitride semiconductor epitaxial wafer. It is a graph. FIG. 4A is a graph showing an example of the relationship between the voltage between element electrodes and the leakage current between elements in the nitride semiconductor epitaxial wafer according to the present embodiment. FIG. 4B is a graph showing an example of the relationship between the voltage between element electrodes and the leakage current between elements in the nitride semiconductor epitaxial wafer according to the comparative example.

Embodiment
(Structure of nitride semiconductor epitaxial wafer)
FIG. 1 is a vertical sectional view of a nitride semiconductor epitaxial wafer according to an embodiment. The nitride semiconductor epitaxial wafer 10 includes a substrate 11, an AlN layer 12 on the substrate 11, a first nitride semiconductor layer 13 on the AlN layer 12, and a second nitride semiconductor layer on the first nitride semiconductor layer 13. 14 is included.

  The single crystal substrate 11 is, for example, a semi-insulating SiC substrate having a polytype of 4H or 6H. The first nitride semiconductor layer 13 is, for example, a GaN layer. The second nitride semiconductor layer 14 is a nitride semiconductor layer having a lower electron affinity than the first nitride semiconductor layer 13. For example, when the first nitride semiconductor layer 13 is a GaN layer, an AlGaN layer can be used as the second nitride semiconductor layer 14.

  When a nitride semiconductor field effect transistor is manufactured using the nitride semiconductor epitaxial wafer 10, the AlN layer 12 is a buffer layer, the first nitride semiconductor layer 13 is a buffer layer and an electron transit layer, and the second nitride semiconductor. Each of the layers 14 functions as a barrier layer.

  A partial region on the first nitride semiconductor layer 13 side of the AlN layer 12 is a first region 12a as an Fe-containing region of the AlN layer 12 containing Fe that is intentionally doped. A region of the AlN layer 12 that is not doped with Fe is defined as a second region 12b.

A partial region on the AlN layer 12 side of the first nitride semiconductor layer 13 is a first region 13a containing intentionally doped Fe. The Fe concentration in the first region 13a is 1 × 10 ¹⁷ cm ³ or more and 1 × 10 ¹⁹ cm ³ or less. For this reason, the first region 13 a has a sufficient electric resistance to suppress the occurrence of leakage current in the nitride semiconductor transistor manufactured using the nitride semiconductor epitaxial wafer 10. If the Fe concentration in the first region 13a is less than 1 × 10 ¹⁷ cm ³ , the leakage current cannot be effectively suppressed, and if it exceeds 1 × 10 ¹⁹ cm ³ , the first nitride semiconductor Morphological degradation of the layer 13 may occur.

  A region of the first nitride semiconductor layer 13 that is not intentionally doped with Fe is defined as a second region 13b. When the second region 13b includes unintentionally doped Fe, the region including the first region 13a and the region containing Fe in the second region 13b is the first nitride semiconductor layer. 13 Fe-containing regions. That is, the first region 13a is a part of the Fe-containing region of the first nitride semiconductor layer 13 on the AlN layer 12 side.

  In a region near the interface between the first nitride semiconductor layer 13 (second region 13b) and the second nitride semiconductor layer 14, the first nitride semiconductor layer 13 and the second nitride semiconductor layer 14 are provided. There is a two-dimensional electron gas generated by the piezo effect in the second nitride semiconductor layer 14 due to the difference in the lattice constants. A region where the two-dimensional electron gas exists in the first nitride semiconductor layer 13 is defined as a two-dimensional electron gas region 13e.

When ferrocene is used as the Fe raw material for forming the first region 13a, the ferrocene adheres to the pipe or the wall of the reaction furnace, and gradually desorbs from the wall after a while by purging with hydrogen gas or nitrogen gas. This adhesion and desorption to the wall like the behavior of ferrocene is called the memory effect. For this reason, after closing the raw material cylinder to stop the supply of ferrocene, the desorbed ferrocene may reach the surface of the substrate 11 and unintentional Fe doping may occur. Due to this memory effect, Fe may be unintentionally doped in the two-dimensional electron gas region 13e of the first nitride semiconductor layer 13, but the Fe concentration in the two-dimensional electron gas region 13e is 1 ×. 10 ¹⁶ cm ⁻³ or less. For this reason, the electrons of the two-dimensional electron gas are offset by Fe, and the output of the transistor manufactured using the nitride semiconductor epitaxial wafer 10 is hardly lowered.

(Nitride semiconductor epitaxial wafer manufacturing process)
FIGS. 2A to 2D are cross-sectional views showing manufacturing processes of the nitride semiconductor epitaxial wafer according to the embodiment.

  First, as shown in FIG. 2A, an AlN crystal is epitaxially grown on the substrate 11 to form a second region 12b of the AlN layer 12.

  Next, as shown in FIG. 2B, while an AlN crystal is grown, supply of an Fe raw material such as ferrocene onto the substrate 11 is started, and Fe is doped into the AlN crystal. As a result, the first region 12a of the AlN layer 12 is formed.

  When ferrocene is used as the Fe raw material, ferrocene adheres to the pipe or the wall of the reaction furnace, and the arrival to the surface of the substrate 11 is delayed. Therefore, the supply amount to the surface of the substrate 11 is not stable immediately after the start of supply. However, the supply to the surface of the substrate 11 at the end of the growth of the AlN crystal so that the supply amount to the surface of the substrate 11 at the start of the growth of the first nitride semiconductor crystal of the first nitride semiconductor layer 13 is stabilized. It is preferred that the amount is substantially stable. For this purpose, the film thickness and growth rate of the first region 12a of the AlN layer 12 may be set as appropriate so as to obtain the time required until the supply amount of ferrocene is stabilized.

  The reason for starting the supply of the Fe raw material after starting the growth of the AlN crystal is as follows. If the supply of the Fe raw material is started before the growth of the AlN crystal starts, only the Fe raw material adheres to the surface of the substrate 11 and decomposes, and an Fe thin film is formed on the substrate 11. Since the Fe thin film has a very high conductivity, in a transistor manufactured using the nitride semiconductor epitaxial wafer 10, a leak current is generated.

  In the present embodiment, since the supply of the Fe raw material is started after the growth of the AlN crystal is started, Fe is taken into the AlN layer 12 as a dopant. The incorporated Fe forms a deep level in the AlN layer 12 and does not affect the insulation of the region. Further, since the lattice constant and the surface shape of the AlN layer 12 are not significantly changed by Fe doping, the function of the AlN layer 12 as a buffer portion is not affected.

  Next, as shown in FIG. 2C, the growth of the first nitride semiconductor crystal such as a GaN crystal is started on the AlN layer 12 while the supply of the Fe raw material is continued. The first nitride semiconductor crystal grows while being doped with Fe, and the first region 13a of the first nitride semiconductor layer 13 is formed. Here, it is preferable that the substrate temperature and the environmental pressure (pressure in the reactor) of the substrate 11 during the growth of the first nitride semiconductor crystal be equal to the substrate temperature and the environmental pressure during the growth of the Fe-doped AlN crystal. This is because if the substrate temperature or the environmental pressure is changed, a stabilization waiting time (a time during which the first nitride semiconductor crystal is not grown) is required for the substrate temperature and the environmental pressure to stabilize after the change. This is because the Fe raw material continuously supplied by the memory effect during the stabilization waiting time adheres to the surface of the AlN layer 12 and decomposes, and a thin film of Fe is formed on the AlN layer 12. Since the Fe thin film has a very high conductivity, in a transistor manufactured using the nitride semiconductor epitaxial wafer 10, a leak current is generated. In addition, during the growth of the second region 12b of the AlN layer 12 and the growth of the first region 12a, the Fe raw material does not adhere to the wall surface even if a stabilization waiting time is provided. There is no worry of being formed.

Even when ferrocene is used as the Fe raw material, since the Fe raw material is supplied before the first nitride semiconductor crystal starts growing, the Fe raw material substrate 11 at the start of the growth of the first nitride semiconductor crystal. The amount supplied upward is stable, and Fe of a target concentration (1 × 10 ¹⁷ cm ³ or more and 1 × 10 ¹⁹ cm ³ or less) is doped into the first nitride semiconductor crystal.

  Further, it is preferable to start the growth of the first nitride semiconductor crystal as soon as possible after the growth of the AlN crystal, that is, after the formation of the first region 12a of the AlN layer 12 is completed. When the growth start of the first nitride semiconductor crystal is delayed, the Fe raw material that is continuously supplied by the memory effect from the end of the growth of the AlN crystal to the start of the growth of the first nitride semiconductor crystal This is because it adheres to the surface of the AlN layer 12 and decomposes, and a thin film of Fe is formed on the AlN layer 12 to become a leak current generation location.

FIG. 3 shows the interruption time of the crystal growth from the end of the growth of the AlN crystal (the first region 12a of the AlN layer 12) to the start of the growth of the first nitride semiconductor crystal, and the nitride semiconductor epitaxial wafer 10. It is a graph which shows an example of the relationship with the leakage current between elements in FIG. The horizontal axis in FIG. 3 represents the interruption time of crystal growth, and the vertical axis represents the magnitude of inter-element leakage current. Here, in the nitride semiconductor epitaxial wafer 10 according to the example of FIG. 3, the first nitride semiconductor layer, the second nitride semiconductor layer, the Fe raw material, and the voltage between the device electrodes are the GaN layer and the AlGaN layer, respectively. , Ferrocene, and 20V. In this case, when the interruption time of the crystal growth is 30 seconds or less, the inter-element leakage current is about 1 × 10 ⁻⁸ A / mm, and it can be seen that almost no generation occurs.

  Next, as shown in FIG. 2 (d), the supply of the Fe raw material is stopped and the growth of the nitride semiconductor crystal is continued to form the second region 13 b of the first nitride semiconductor layer 13. Thereby, the first nitride semiconductor layer 13 having the first region 13a on the lower side is formed. In order to prevent the Fe thin film from being formed between the first region 13a and the second region 13b due to the memory effect, the growth conditions of the substrate temperature and the environmental pressure are set to the same conditions, so that the film thickness exceeds 30 seconds. There is no need to provide an interruption time (stabilization waiting time) for crystal growth.

When ferrocene is used as the Fe raw material, the ferrocene adhering to the walls of the piping and the reactor is gradually desorbed, and the desorbed ferrocene is the surface of the substrate 11 even after the raw material cylinder is closed to stop the supply of ferrocene. In some cases, unintended Fe doping may be performed. For this reason, unintended Fe doping may be performed on the second region 13b of the first nitride semiconductor layer 13 in some cases. However, in the present embodiment, since Fe doping can be efficiently performed on the first region 13a, without excessively increasing the film thickness of the second region 13b formed after the supply of ferrocene is stopped, The Fe concentration in the upper portion of the second region 13b (the portion that becomes the two-dimensional electron gas region 13e) can be suppressed to 1 × 10 ¹⁶ cm ⁻³ or less. For this reason, it is possible to avoid the problem of cost increase due to the thickening of the second region 13b.

  Thereafter, a second nitride semiconductor crystal such as an AlGaN crystal is epitaxially grown on the first nitride semiconductor layer 13 to form the second nitride semiconductor layer 14, and the nitride semiconductor epitaxial wafer 10 shown in FIG. Get.

(Effect of embodiment)
According to the present embodiment, the supply of the Fe raw material is started during the growth of the AlN crystal, that is, before the growth of the first nitride semiconductor crystal, so that the first nitride semiconductor can be used regardless of the type of the Fe raw material. The supply amount of the Fe raw material to the substrate surface at the start of crystal growth is stabilized, and the first region 13a having the desired Fe concentration can be formed on the AlN layer 12 side of the first nitride semiconductor layer 13. Thereby, the leak current between elements in the nitride semiconductor field effect transistor manufactured using the nitride semiconductor epitaxial wafer 10 of the present embodiment or the drain leak current at the time of pinch-off can be suppressed.

In addition, since the supply amount of the Fe raw material to the substrate surface at the start of the growth of the first nitride semiconductor crystal is stable, an Fe raw material that easily adheres to piping such as ferrocene or the wall of the reactor is used. Even if it exists, it is not necessary to supply an excessive amount of Fe raw material in order to form the first region 13a having a target Fe concentration (from the start of the growth of the first nitride semiconductor crystal (GaN layer)). In order to dope Fe at a concentration, there is no need to increase the supply amount of the Fe raw material in consideration of the consumption due to the adhesion to the wall surface, and thus the amount of adhesion to the wall surface is suppressed and the desorption amount from the wall surface is also reduced. Can be suppressed. For this reason, it is not necessary to grow the second region 13b of the first nitride semiconductor layer 13 thickly after the Fe raw material supply is stopped (the upper portion of the second region 13b (the portion that becomes the two-dimensional electron gas region 13e)). The film thickness of the second region 13b required until the Fe concentration becomes 1 × 10 ¹⁶ cm ⁻³ or less can be reduced), and the yield of the high-power transistor obtained from the nitride semiconductor epitaxial wafer can be improved and the first nitride can be obtained. An excellent effect of cost reduction associated with the thinning of the semiconductor layer 13 is obtained.

  As an example, a detailed manufacturing process of the nitride semiconductor epitaxial wafer 10 is shown below.

  First, a semi-insulating SiC substrate having a polytype of 4H or 6H is prepared as the substrate 11.

Next, the substrate 11 is introduced into a MOVPE (Metal Organic Vapor Phase Epitaxy) apparatus, and heat treatment is performed at a substrate temperature of 1,175 ° C. for 5 minutes in an H ₂ / N ₂ mixed gas flow atmosphere not containing NH ₃ . The substrate 11 is cleaned by this heat treatment.

Next, ammonia gas is introduced into the reactor of the MOVPE apparatus for 25 seconds under the condition of 0.5 ≦ H ₂ / NH ₃ ratio ≦ 4. By this ammonia gas flow, desorption of nitrogen atoms is prevented in the subsequent process of forming the AlN layer 12, and the quality of the AlN crystal is improved.

  Next, a part of the AlN layer 12 having an average film thickness of 6 nm is formed using ammonia gas and TMA (Tri Methyl Aluminum) as raw materials. This portion becomes the second region 12 b of the AlN layer 12. The growth conditions of the second region 12b are as follows: the V / III ratio, which is the molar ratio of the group V source gas to the group III source gas, is 3,000, the growth rate is 0.10 nm / sec, and the substrate temperature is 1,175. It was made to become ° C.

  Next, the substrate 11 in the MOVPE apparatus is cooled to a substrate temperature of 1,000 ° C. Simultaneously with the cooling, the reactor pressure in the reactor may be shifted to a desired pressure that is optimal for the formation of the GaN layer as the first nitride semiconductor layer 13.

  Next, a part of the AlN layer 12 having an average film thickness of 4 nm is formed using ammonia gas, TMA, and ferrocene. This portion becomes the first region 12 a of the AlN layer 12. Since this region is formed during the delay of ferrocene supply due to the memory effect, the Fe doping concentration is unstable (the Fe concentration may be zero in the initial growth portion of the first region 12a). However, the AlN crystal originally has a wide band gap exceeding 6.0 eV at room temperature, and is still semi-insulating regardless of the presence or absence of Fe doping. For this reason, since the Fe doping amount in the AlN layer 12 does not affect the electrical characteristics of the transistor, the ferrocene having the same flow rate as the ferrocene that flows when the first region 13a of the next first nitride semiconductor layer 13 is grown. Is preferably flown during the growth of the first region 12a of the AlN layer 12, since the amount of Fe doping in the first region 13a of the first nitride semiconductor layer becomes more stable.

The growth conditions of the first region 12a were such that the V / III ratio was 10,000, the growth rate was 0.02 nm / sec, and the substrate temperature was 1,000 ° C. The ferrocene supply amount was such that the Fe concentration would be 2.0 × 10 ¹⁸ cm ⁻³ when doped as prescribed.

Next, the supply of the TMA to the reactor is terminated, and within 30 seconds after the termination, the substrate temperature of the substrate 11 and the reactor pressure in the reactor remain the same, and the TMG (Tri Methyl Gallium) reactor is supplied to the reactor. Start supplying. Since the supply of ammonia gas continues, the growth of GaN crystals is started by starting the supply of TMG to the reactor, and a part of the GaN layer as the first nitride semiconductor layer 13 is formed. At this time, since the supply of ferrocene continues, this portion becomes the first region 13a of the GaN layer. The first region 13a is formed with a thickness of 300 nm. The growth conditions of the first region 13a are as follows: the V / III ratio is 2,000, the growth rate is 0.40 nm / sec, the substrate temperature is 1,000 ° C., and the Fe concentration is 1.0 × 10 ¹⁸ cm ⁻³ . It was made to become.

Next, the supply of ferrocene is discontinued while the supply of ammonia and TMG is continued. As a result, the second region 13b of the GaN layer is formed. The intentional doping of Fe is not performed in the second region 13b, but ferrocene desorbed from the wall surface due to the memory effect has flowed into the metal organic chemical vapor deposition furnace for a while. In this case, Fe is unintentionally doped. This portion is formed to a thickness of 900 nm. When the two-dimensional electron gas region 13e located above the second region 13b, that is, near the boundary with the AlGaN layer as the second nitride semiconductor layer 14, is formed, it adheres to the wall surface due to the memory effect. Since the ferrocene desorption is almost completed, the Fe concentration due to unintentional doping is reduced to 1 × 10 ¹⁶ cm ⁻³ or less in the two-dimensional electron gas region 13e. The film thickness (T) from the first region 13a to the two-dimensional electron gas region 13e is such that the Fe concentration in the two-dimensional electron gas region 13e is reduced to 1 × 10 ¹⁶ cm ⁻³ or less. The amount of ferrocene adhering to the first nitride semiconductor layer 13 may be determined appropriately in consideration of the amount of ferrocene adhering to the first nitride semiconductor layer 13, and it is not necessary to flow excess ferrocene in the first region 13a in order to dope the desired Fe concentration. (In other words, since the amount of ferrocene desorption after ferrocene supply is stopped can be minimized), this film thickness (T) can be minimized.

  The growth conditions of the second region 13b were such that the V / III ratio was 2,000, the growth rate was 0.40 nm / sec, and the substrate temperature was 1,000 ° C.

  Next, the supply of TMA is resumed while the supply of ammonia and TMG is continued. In some cases, the supply of ammonia is continued while the supply of ammonia is once stopped, and then the supply of TMG and TMA is restarted simultaneously. Thereby, an AlGaN crystal grows and an AlGaN layer as the second nitride semiconductor layer 14 is formed. The thickness of the AlGaN layer needs to be appropriately designed according to the application of the transistor to be manufactured, but here it is formed to a thickness of 30 nm, for example. The growth conditions of the second nitride semiconductor layer 14 were such that the V / III ratio was 1,500, the growth rate was 0.30 nm / sec, and the substrate temperature was 1,000 ° C.

  Through the above steps, the nitride semiconductor epitaxial wafer 10 according to the example is obtained.

(Comparative example)
The nitride semiconductor epitaxial wafer according to the comparative example is the same as the nitride semiconductor epitaxial wafer 10 according to the example, the ferrocene supply start timing (whether or not Fe doping is performed on the AlN layer), and a part of the AlN layer (AlN according to the example). Although the V / III ratio and the growth rate of the portion 12 in which Fe is doped in the layer 12 are different, the conditions for the heat treatment before epitaxial growth are the same. Specifically, an un-AlN layer (film thickness 10 nm, V / III ratio 3,000, growth rate 0.10 nm / sec, substrate temperature 1,175 ° C.), Fe-doped GaN layer (film thickness) on a SiC substrate. 300 nm, V / III ratio 2,000, growth rate 0.40 nm / sec, substrate temperature 1,000 ° C.), un-GaN layer (film thickness 900 nm, V / III ratio 2,000, growth rate 0.40 nm / sec) And a substrate temperature of 1,000 ° C.) and an un-AlGaN layer (film thickness 30 nm, V / III ratio 1,500, growth rate 0.30 nm / sec, substrate temperature 1,000 ° C.). The ferrocene supply amount at the time of forming the Fe-doped GaN layer is the same flow rate as in the example, and the supply amount is such that the Fe concentration becomes 1.0 × 10 ¹⁸ cm ⁻³ when doped as prescribed, The un-GaN layer is unintentionally doped with Fe. Ferrocene supply and GaN crystal growth were started simultaneously, and the AlN layer was not doped with Fe.

  Through the above steps, the nitride semiconductor epitaxial wafer according to the comparative example is obtained.

  Next, the relationship between the inter-electrode voltage and the inter-device leakage current was examined using the nitride semiconductor epitaxial wafers according to the above-described examples and comparative examples. Specifically, a large number of transistors are formed on a wafer, and two-dimensional electron gas regions located between the transistors are separated by increasing the resistance by ion implantation, and particularly between ohmic electrodes (that is, source electrodes) Alternatively, the leakage current (drain leakage current between the drain electrodes) was measured.

  FIG. 4A is a graph showing an example of the relationship between the voltage between element electrodes and the leakage current between elements in the nitride semiconductor epitaxial wafer 10 according to the example. FIG. 4B is a graph showing an example of the relationship between the voltage between element electrodes and the leakage current between elements in the nitride semiconductor epitaxial wafer according to the comparative example. 4A and 4B, the horizontal axis represents the voltage between the element electrodes, and the vertical axis represents the magnitude of the leakage current between the elements.

  As can be seen from FIGS. 4A and 4B, the inter-element leakage current of the nitride semiconductor epitaxial wafer 10 according to the present embodiment is approximately 1 of the inter-element leakage current of the nitride semiconductor epitaxial wafer according to the comparative example. It was confirmed that the leakage current can be effectively suppressed.

  Although the embodiments and examples of the present invention have been described above, the present invention is not limited to the above-described embodiments and examples, and various modifications can be made without departing from the spirit of the invention.

  The embodiments and examples described above do not limit the invention according to the claims. It should be noted that not all combinations of features described in the embodiments and examples are necessarily essential to the means for solving the problems of the invention.

DESCRIPTION OF SYMBOLS 10 Nitride semiconductor epitaxial wafer 11 Substrate 12 AlN layer 13 1st nitride semiconductor layer 13e Two-dimensional electron gas area | region 12a, 13a 1st area | region 14 2nd nitride semiconductor layer

Claims (5)

Forming an AlN layer by epitaxially growing an AlN crystal on a substrate;
Forming a first nitride semiconductor layer by epitaxially growing a first nitride semiconductor crystal on the AlN layer;
Including
After the start of growth of the AlN crystal and before the end of growth of the AlN crystal, supply of ferrocene onto the substrate is started, and after the start of growth of the first nitride semiconductor crystal, and the first nitride the growth before the end of the semiconductor crystal, by ending the supply of the ferrocene, at least the first partial region of the nitride semiconductor layer side of the AlN layer, and before the first nitride semiconductor layer SL Doping Fe into a partial region on the AlN layer side,
A method for manufacturing a nitride semiconductor epitaxial wafer. The doping concentration of Fe into at least a part of the first nitride semiconductor layer on the AlN layer side is 1 × 10 ¹⁷ cm ⁻³ or more and 1 × 10 ¹⁹ cm ⁻³ or less.
The method for producing a nitride semiconductor epitaxial wafer according to claim 1 . The growth of the first nitride semiconductor crystal is started within 30 seconds after the growth of the AlN crystal is finished.
The manufacturing method of the nitride semiconductor epitaxial wafer of Claim 1 or 2 . The substrate temperature and environmental pressure of the substrate during the growth of the Fe doping region of the AlN layer are equal to the substrate temperature and environmental pressure of the substrate during the growth of the first nitride semiconductor crystal,
Any method for manufacturing a nitride semiconductor epitaxial wafer according to one of claims 1-3. Forming a second nitride semiconductor layer having a lower electron affinity than the first nitride semiconductor layer on the first nitride semiconductor layer;
A two-dimensional electron gas exists in a region on the second nitride semiconductor layer side in the first nitride semiconductor layer;
The Fe concentration in the region where the two-dimensional electron gas exists in the first nitride semiconductor layer is 1 × 10 ¹⁶ cm ⁻³ or less.
Any method for manufacturing a nitride semiconductor epitaxial wafer according to one of claims 1-4.

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