Epitaxial substrate for semiconductor elements, semiconductor element and manufacturing process for epitaxial substrates for semiconductor elements

Abstract

Epitaxial substrate (10) for semiconductor elements (20), containing: a semi-insulating, freestanding substrate formed from Zn-doped GaN (1); a buffer layer (2) adjacent to the freestanding substrate (1), which is a group 13 nitride layer, which in at least a part of the buffer layer (2) in a thickness direction contains C in a concentration greater than or equal to 1×10 18 cm -3 is endowed; a channel layer (3) adjacent to the buffer layer (2); and a barrier layer (4) provided on the opposite side of the buffer layer (2) with an intervening channel layer (3), wherein the buffer layer (2) is a diffusion-suppressing layer which suppresses diffusion of Zn from the freestanding substrate (1) into the channel layer (3), and a concentration of Zn in the channel layer (3) less than or equal to 1×10 16 cm -3is.

Description

Technical field

[0001] The present invention relates to a semiconductor element and in particular to a semiconductor element formed by using a freestanding substrate consisting of semi-insulating GaN. State of the art

[0002] Nitride semiconductors, which exhibit a large direct junction band gap, a high electrical breakdown field strength, and a high saturation electron velocity, have been used as light emission devices such as LEDs or LDs and as semiconductor materials for high-frequency / high-power electronic devices.

[0003] Typical structures of nitride electronic devices include a high-electron-mobility transistor (HEMT) structure, which is formed by layers of AlGaN as a "blocking layer" and GaN as a "channel layer". This structure utilizes a special characteristic: thanks to strong polarization effects inherent in nitride materials (spontaneous polarization and piezopolarization), a highly concentrated, two-dimensional electron gas is generated at an AlGaN / GaN layer interface.

[0004] Nitride electronic devices are typically fabricated using readily available substrates made of various materials such as sapphire, SiC, and Si. However, a problem arises in GaN films grown heteroepitactically on substrates of different materials: due to differences in lattice constant and coefficient of thermal expansion between GaN and the substrates, a large number of defects occur.

[0005] Meanwhile, when the GaN film is grown homoepitactically on a GaN substrate, the defect caused by the difference in lattice constant and coefficient of thermal expansion described above does not occur, but the GaN film exhibits an advantageous crystalline structure.

[0006] Accordingly, when the nitride HEMT structure is fabricated on the GaN substrate, the mobility of the two-dimensional electron gas at the AlGaN / GaN layering interface increases, and consequently an improvement in the properties of a HEMT element (semiconductor element) fabricated using the above structure can be expected.

[0007] However, commercially available GaN substrates produced using hydride gas phase epitaxy (HVPE) typically exhibit n-type conductivity due to an oxygen impurity incorporated into a crystal. This conductive GaN substrate serves as a leakage current path between the source and drain electrodes when the HEMT element is driven at high voltage. Consequently, the semi-insulating GaN substrate is preferentially used for fabricating the HEMT element.

[0008] It is known to be effective to carry out doping with an element such as a transition metal element (for example, Fe) or a group 2 element (for example, Mg), which forms a low acceptor level in the GaN crystal, in order to achieve the semi-insulating GaN substrate.

[0009] It is already known that a high-quality, semi-insulating GaN single-crystal substrate can be obtained upon introduction of the element zinc (Zn) from Group 2 (see, for example, Patent Document 1). An investigation into the diffusion of the Zn element in the GaN crystal has already been carried out, and the diffusion occurs in a high-temperature atmosphere. The ease of diffusion depends on the crystallinity of the GaN crystal (see, for example, Non-Patent Document 4). Furthermore, it is known that a high-resistance layer doped with iron (Fe), a transition metal element, is formed on a substrate, and an intermediate layer with strong Fe uptake is formed between the high-resistance layer and an electron transition layer, thereby preventing Fe from being incorporated into the electron transition layer (see, for example, Patent Document 2).

[0010] Fabrication of the HEMT structure on the semi-insulating GaN substrate or a substrate with the semi-insulating GaN film for the purpose of investigating each property has already been carried out (see, for example, Non-Patent Document 1 to Non-Patent Document 3).

[0011] When the GaN film is epitaxially grown on the semi-insulating GaN single-crystal substrate, which is doped with the transition metal element or the group 2 element, to form an epitaxial substrate for the semiconductor elements, a problem arises in that an acceptor element such as Fe, Mg and Zn diffuses into the GaN film and acts like an electron trap in the film, consequently causing a current breakdown phenomenon.

[0012] Further prior art is also known from patent documents 3 and 4. State-of-the-art documents, patent documents Patent document 1: JP 5 039 813 B2 Patent document 2: JP 2013 - 74 211 A Patentdokument 3: US 2009 / 0 189 190 A1 Patentdokument 4: JP 2011 - 68 548 A Nicht-Patentdokumente

[0013] Nicht-Patentdokument 1: OSHIMURA, Y. [et al.]: AlGaN / GaN heterostructure field-effect transistors on Fe-doped GaN substrates with high breakdown voltage. In: Japanese Journal of Applied Physics, Vol. 50, 2011, No. 8R, Article-No. 084102, S. 1-5.

[0014] Nicht-Patentdokument 2: DESMARIS, V. [et al.]: Comparison of the DC and microwave performance of AlGaN / GaN HEMTs grown on SiC by MOCVD with Fe-doped or unintentionally doped GaN buffer layers. In: IEEE Transactions on Electron Devices, Vol. 53, 2006, No. 9, S. 2413-2417.

[0015] Nicht-Patentdokument 3: AZIZE, M. ; BOUGRIOUA, Z. ; GIBART, P.: Inhibition of interface pollution in AlGaN / GaN HEMT structures regrown on semi-insulating GaN templates. In: Journal of Crystal Growth, Vol. 299, 2007, No. 1, S. 103-108.

[0016] Non-patent document 4: SUSKI, T. [et al.]: Optical activation and diffusivity of ion-implanted Zn acceptors in GaN under high-pressure, high-temperature annealing. In: Journal of Applied Physics, Vol. 84, 1998, No. 2, pp. 1155-1157. Brief description

[0017] The present invention relates to a semiconductor element and in particular to a semiconductor element formed by using a freestanding substrate formed from semi-insulating GaN.

[0018] According to the present invention, an epitaxial substrate for semiconductor elements comprises: a semi-insulating, freestanding substrate formed from Zn-doped GaN; a buffer layer adjacent to the freestanding substrate, which is a group 13 nitride layer containing C in at least a portion of the buffer layer in a thickness direction at a concentration greater than or equal to 1×10 18 cm -3is doped; a channel layer adjacent to the buffer layer; and a barrier layer provided on the opposite side of the buffer layer with an intervening channel layer, wherein the buffer layer is a diffusion-suppressing layer which suppresses diffusion of Zn from the freestanding substrate into the channel layer, and a concentration of Zn in the channel layer is less than or equal to 1×10 16 cm -3 is.

[0019] According to a further aspect of the present invention, a semiconductor element comprises: a semi-insulating, freestanding substrate formed from Zn-doped GaN; a buffer layer adjacent to the freestanding substrate, which is a group 13 nitride layer containing C in at least a portion of the buffer layer in a thickness direction at a concentration greater than or equal to 1×10 18 cm -3is doped; a channel layer adjacent to the buffer layer; and a barrier layer provided on the opposite side of the buffer layer with an intervening channel layer, wherein the buffer layer is a diffusion-suppressing layer which suppresses diffusion of Zn from the freestanding substrate into the channel layer, and a concentration of Zn in the channel layer is less than or equal to 1×10 16 cm -3 is.

[0020] According to yet another aspect of the present invention, a method for producing an epitaxial substrate for semiconductor devices comprises: a) a fabrication step of producing a semi-insulating, freestanding substrate consisting of Zn-doped GaN; b) a buffer layer formation step of forming a buffer layer adjacent to the freestanding substrate; c) a channel layer formation step of forming a channel layer adjacent to the buffer layer; and d) a barrier layer formation step of forming a barrier layer at a position opposite the buffer layer, with the channel layer interposed, wherein in the buffer layer formation step the buffer layer is formed as a diffusion-suppressing layer, which is a group 13 nitride layer containing C in at least a portion of the buffer layer in a thickness direction at a concentration greater than or equal to 1 × 10 18 cm -3is doped, which suppresses diffusion of Zn from the free-standing substrate into the channel layer, so that the concentration of Zn in the channel layer is less than or equal to 1×10 16 cm -3 is.

[0021] According to the present invention, the semiconductor element in which the current breakdown is reduced when using a semi-insulating, freestanding GaN substrate can be achieved.

[0022] Therefore, it is an object of the present invention to provide an epitaxial substrate for semiconductor elements which suppresses the occurrence of current breakdown. Brief description of the drawings Fig. Figure 1 is a drawing which schematically illustrates a cross-sectional structure of a HEMT element 20. Fig.Figure 2 is a drawing illustrating the concentration profile of the Zn element and the C element in an epitaxial substrate according to Example 1. Fig. Figure 3 is a magnified view of the environment of an interface between a GaN buffer layer and a GaN substrate in Fig. 2. Fig. Figure 4 is a drawing illustrating the concentration profile of the Zn element and the C element in an epitaxial substrate according to a comparative example 1. Fig. Figure 5 is a magnified view of the environment of an interface between a C-doped GaN buffer layer and a GaN substrate in Fig. 4. Fig. Figure 6 is a drawing illustrating the concentration profile of the Zn element and the C element from a surface of a barrier layer 4 in a depth direction in an example 7. Fig.Figure 7 is a drawing illustrating a concentration profile of the Zn element and the C element from a surface of a barrier layer 4 in a depth direction and a secondary ion signal profile of the Al element in an example 8. Description of embodiment(s)

[0023] The periodic table group numbers in this description correspond to the explanation of group numbers 1 to 18 in the 1989 revision of the inorganic chemical nomenclature by the International Union of Pure and Applied Chemistry (IUPAC). Group 13, for example, contains aluminum (Al), gallium (Ga), and indium (In); group 14 contains, for example, silicon (Si), germanium (Ge), tin (Sn), and lead (Pb); and group 15 contains, for example, nitrogen (N), phosphorus (P), arsenic (As), and antimony (Sb). Brief description of epitaxial substrate and HEMT element

[0024] Fig.Figure 1 is a drawing which schematically illustrates a cross-sectional structure of a HEMT element 20 as an embodiment of a semiconductor element according to the present invention, which includes an epitaxial substrate 10 as an embodiment of an epitaxial substrate for semiconductor elements according to the present invention.

[0025] The epitaxial substrate 10 contains a free-standing substrate 1, a buffer layer 2, a channel layer 3, and a barrier layer 4. The HEMT device 20 contains a source electrode 5, a drain electrode 6, and a gate electrode 7, which are arranged on the epitaxial substrate 10 (on the barrier layer 4). The ratios of the respective layers in Fig. 1 do not reflect the actual circumstances.

[0026] The freestanding substrate 1 is a GaN substrate, which is covered with 1×10 18 cm -3or is more Zn doped and has a (0001) plane orientation and a resistivity greater than or equal to 1×10 2 The substrate 1 has a specific thermal conductivity (Ω) at room temperature and possesses semi-insulating properties. Although the size of the freestanding substrate 1 is not particularly limited, considering, for example, ease of handling (e.g., grasping and moving), it preferably has a thickness of approximately a few hundred µm to a few mm. The freestanding substrate 1 can be produced, for example, using a flux process.

[0027] The freestanding substrate 1 formed by the flux process is obtained by the following processes, in short: immersion of a seed crystal substrate into a melt containing metallic Ga, metallic Na, metallic Zn, and C (carbon) in a crystal growth vessel (aluminum oxide crucible) which is rotatably mounted horizontally in a pressure vessel; maintenance of a predefined temperature and pressure in the crystal growth vessel while introducing nitrogen gas and rotating the crystal growth vessel horizontally; and then separation of a GaN single crystal, which is formed as a result on the seed crystal substrate, from the seed crystal substrate. A so-called template substrate, in which a GaN thin film is formed on a sapphire substrate by a MOCVD process, can preferably be used as the seed crystal substrate.

[0028] The buffer layer 2 is a layer with a thickness of approximately 50 to 1000 nm, formed (adjacent to) a main surface of the freestanding substrate 1. In the present embodiment, the buffer layer 2, unlike a so-called low-temperature buffer layer formed at a low temperature below 800 °C, is formed at a temperature that is essentially the same as the formation temperature of the channel layer 3 and the barrier layer 4.

[0029] In the epitaxial substrate 10 according to the present embodiment, the buffer layer 2 is provided as a diffusion-suppressing layer to suppress diffusion of Zn, with which the free substrate 1 is doped, into the channel layer 3 and further into the barrier layer 4 on the channel layer 3 during the preparation of the epitaxial substrate 10. The buffer layer 2, as a preferred example, consists of a layer containing C in at least a portion of a region in the thickness direction at a concentration greater than or equal to 1 × 10⁻⁶. 18 cm -3 layer formed by doped group 13 nitride.

[0030] In the above case, buffer layer 2 can be a single layer consisting entirely of C in a concentration greater than or equal to 1×10 18 cm -3is formed with doped group 13 nitride, or it can have a configuration in which at least one layer of a multilayer buffer layer, which is formed from two or more group 13 nitride layers of different compositions, is doped with C in a concentration greater than or equal to 1×10 18 cm -3 is doped. Examples of single-layer buffers include a GaN buffer layer (C-doped GaN layer), which consists entirely of GaN. Examples of multilayer buffers include a configuration in which a C-doped GaN layer is deposited on an Al a Ga 1-a The buffer layer is N-layer (0 < a ≤ 1). Alternatively, buffer layer 2 can be doped with C at a concentration greater than or equal to 1 × 10⁻⁶. 18 cm -3in a region of at least a portion of a composition gradient buffer layer in the thickness direction, which is formed from group 13 nitride containing two or more group 13 elements (for example, Ga and Al), wherein each element has a presence ratio (molar fraction) that changes in the thickness direction. An effect brought about by buffer layer 2 is described below.

[0031] Furthermore, it is applicable that the buffer layer 2 consists of a layer formed from Al-doped GaN (Al-doped GaN layer) or a layer of AlN. Each of these layers also functions as a diffusion-suppressing layer in a manner similar to the C-doped GaN layer.

[0032] Channel layer 3 is a layer formed adjacent to buffer layer 2. Channel layer 3 is formed to have a thickness of approximately 50 to 5000 nm. Barrier layer 4 is a layer located on the opposite side of buffer layer 2, with channel layer 3 interposed. Barrier layer 4 is formed to have a thickness of approximately 2 to 40 nm.

[0033] The barrier layer 4 can be used as in Fig. The interface shown in Figure 1 is formed adjacent to the channel layer 3, and in this case, an interface between them is a heterotransition interface. Alternatively, a spacer layer (not shown) may be provided between the channel layer 3 and the barrier layer 4, and in this case, a region consisting of an interface between the channel layer 3 and the spacer layer and an interface between the barrier layer 4 and the spacer layer is a heterotransition interface region.

[0034] In any case, as a preferred example, the channel layer 3 consists of GaN and the barrier layer 4 consists of AlGaN (Al x Ga 1-x N, 0 <x<1) oder InAlN (In y Al 1-y N, 0 <y<1). Jedoch ist eine Kombination der Kanalschicht 3 und der Sperrschicht 4 nicht darauf beschränkt.

[0035] The formation of buffer layer 2, channel layer 3, and barrier layer 4 is achieved, for example, using the MOCVD process. In a case where buffer layer 2 and channel layer 3 are formed from GaN and barrier layer 4 from AlGaN, the layer formation can be carried out, for example, using the MOCVD process with a publicly known MOCVD furnace capable of introducing organic metal (MO) source gas for Ga and Al (TMG and TMA), ammonia gas, hydrogen gas, and nitrogen gas into a reactor. This is achieved by heating the freestanding substrate 1 arranged in the reactor to a predefined temperature and depositing a GaN crystal and an AlGaN crystal, which are generated successively on the freestanding substrate 1 by a gas-phase reaction between the organic metal source gas corresponding to each layer and the ammonia gas.

[0036] The source electrode 5 and the drain electrode 6 are metal electrodes, each with a thickness of approximately ten to several hundred nanometers. The source electrode 5 and the drain electrode 6 are preferably formed as multilayer electrodes, for example, from Ti / Al / Ni / Au. The source electrode 5 and the drain electrode 6 have ohmic contact with the barrier layer 4. As a preferred example, the source electrode 5 and the drain electrode 6 are formed by a vacuum evaporation process and a photolithography process. Preferably, after forming the electrodes 5 and 6, a heat treatment lasting several tens of seconds in a nitrogen gas atmosphere at a predefined temperature of 650 to 1000 °C is carried out to improve the ohmic contact of these electrodes.

[0037] The gate electrode 7 is a metal electrode with a thickness of approximately ten to several hundred nanometers. The gate electrode 7 is preferably formed as a multilayer electrode, for example, of Ni / Au. The gate electrode 7 has a Schottky contact with the barrier layer 4. The gate electrode 7 is formed, as a preferred example, by a vacuum evaporation process and a photolithography process. Method for producing the epitaxial substrate and the HEMT element. Production of the freestanding substrate.

[0038] First, a procedure for producing the freestanding substrate 1 using the flux method is described.

[0039] First, a c-plane sapphire substrate with a diameter essentially the same as that of the freestanding substrate 1 to be produced is created, and a low-temperature GaN buffer layer is formed on the surface of the c-plane sapphire substrate at a temperature of 450 to 750 °C to a thickness of approximately 10 to 50 nm. Subsequently, a GaN thin film with a thickness of approximately 1 to 10 µm is formed using the MOCVD process at a temperature of 1000 to 1200 °C, resulting in a MOCVD-GaN template that can be used as a seed crystal substrate.

[0040] Then, using a Na-flux process and the MOCVD-GaN template obtained as the seed crystal substrate, a Zn-doped GaN single-crystal layer is formed.

[0041] Specifically, the MOCVD-GaN template is first arranged in an aluminum oxide crucible, and then the aluminum oxide crucible is filled with 10 to 60 g of metallic Ga, 15 to 90 g of metallic Na, 0.1 to 5 g of metallic Zn, and 10 to 500 mg of C.

[0042] The aluminum oxide crucible is placed in a heating oven and heated for approximately 20 to 400 hours at an oven temperature of 800 to 950 °C and an oven pressure of 3 to 5 MPa, and then cooled to room temperature. After cooling, the aluminum oxide crucible is removed from the oven. As a result of the above procedure, a brown GaN single-crystal layer with a thickness of 300 to 3000 µm is deposited on the surface of the MOCVD GaN template.

[0043] The GaN single-crystal layer obtained in this way is ground with diamond abrasive grains to flatten its surface. This yields the flux-GaN template with the GaN single-crystal layer formed on the MOCVD-GaN template. However, the grinding is carried out to such an extent that the total thickness of the nitride layer on the flux-GaN template is sufficiently greater than the desired thickness of the final freestanding substrate 1.

[0044] The seed crystal substrate is then separated from the flux-GaN template by a laser lifting process in which laser light is emitted from one side of the seed crystal substrate to perform scanning at a scanning speed of 0.1 to 100 mm / s. The third harmonic of an Nd:YAG laser with a wavelength of 355 nm is preferably used as the laser light. In the above case, the pulse width can be set to approximately 1 to 1000 ns and the pulse period to approximately 1 to 200 kHz. During laser emission, the laser light is preferably collected to adjust the luminance. Preferably, the laser light is emitted while the flux-GaN template is heated from a side opposite the seed crystal substrate to a temperature of approximately 30 to 600 °C.

[0045] After separating the seed crystal substrate, a grinding process is performed on the surface from which the seed crystal substrate was detached, resulting in a layered structure. This yields the product containing Zn in a concentration greater than or equal to 1 × 10⁻⁶. 18 cm -3 freestanding substrate formed with doped GaN 1. Preparation of the epitaxial substrate

[0046] The fabrication of epitaxial substrate 10 using the MOCVD process is then described. Epitaxial substrate 10 is obtained by layering buffer layer 2, channel layer 3, and barrier layer 4 in that order under the following condition: the free-standing substrate 1 is positioned on a heating element located in the reactor within the MOCVD oven. However, as an example of buffer layer 2, a case is described in which a single GaN buffer layer, a multilayer buffer layer, or a composition gradient buffer layer containing Ga and Al as group 13 elements is formed. The formation temperature is the heating element temperature.

[0047] In the present embodiment, a gas ratio of Group 15 to Group 13 is a ratio (molar ratio) of a supply quantity of ammonia, which is a Group 15 (N) source, to a total supply quantity of trimethylgallium (TMG), trimethylaluminium (TMA), and trimethylindium (TMI), which are Group 13 (Ga, Al, and In) sources. A gas ratio of Al source gas to Group 13 source gas in a case of fabricating barrier layer 4 from AlGaN is a ratio (molar ratio) of a supply quantity of Al source to a supply quantity of entire Group 13 (Ga, Al) sources, and a gas ratio of In source gas to Group 13 source gas in a case of fabricating barrier layer 4 from InAlN is a ratio (molar ratio) of a supply quantity of In source to a supply quantity of entire Group 13 (In, Al) sources. Both are defined in accordance with a composition (an Al mol ratio x or an In composition ratio y) of a desired barrier layer 4.

[0048] Buffer layer 2: Formation temperature = 900 to 1100 °C; Reactor pressure = 5 to 30 kPa; Carrier gas = hydrogen; Gas ratio of group 15 to group 13 = 100 to 4000; Gas ratio of Al source gas to group 13 source gas = 0 (in one case of the GaN buffer layer); Gas ratio of Al source gas to group 13 source gas = in the range of 0 to 1 in accordance with a position in the thickness direction (in a case of the multilayer buffer layer or the composition gradient buffer layer).

[0049] Channel layer 3: Formation temperature = 1000 to 1200 °C; Reactor pressure = 15 to 105 kPa; Carrier gas = hydrogen; Gas ratio of group 15 to group 13 = 1000 to 10000.

[0050] Barrier layer 4 (in the event that it is made of AlGaN): Formation temperature = 1000 to 1200 °C; Reactor pressure = 1 to 30 kPa; Gas ratio of group 15 to group 13 = 5000 to 20000; Carrier gas = hydrogen; Gas ratio of Al source gas to group 13 source gas = 0.1 to 0.4.

[0051] Barrier layer 4 (in the event that it is made of InAIN): Formation temperature = 700 to 900 °C; Reactor pressure = 1 to 30 kPa; Gas ratio of group 15 to group 13 = 2000 to 20000; Carrier gas = nitrogen; Gas ratio of In source gas to group 13 source gas = 0.1 to 0.9. Manufacturing of the HEMT element

[0052] The HEMT element 20 utilizing the epitaxial substrate 10 can be manufactured by applying a publicly known method.

[0053] For example, after element separation processing to remove part of a boundary region between individual elements by etching to approximately 50 to 1000 nm using a photolithography process and a RIE (reactive ion etching) process, a SiO2 film with a thickness of 50 to 500 nm is formed on a surface of the epitaxial substrate 10 (the surface of the barrier layer 4), and then the SiO2 film is removed at locations where the source electrode 5 and the drain electrode 6 are to be formed by etching using the photolithography process, thus obtaining a SiO2 pattern layer.

[0054] A metal pattern made of Ti / Al / Ni / Au is then formed at the locations where the source electrode 5 and the drain electrode 6 are to be created, using vacuum deposition and photolithography. The metal layers preferably have thicknesses of 5 to 50 nm, 40 to 400 nm, 4 to 40 nm, and 20 to 200 nm, respectively, in that order.

[0055] Subsequently, a heat treatment lasting 10 to 1000 seconds is carried out in a nitrogen gas atmosphere at a temperature of 600 to 1000 °C to improve the ohmic contact of the source electrode 5 and the drain electrode 6.

[0056] Then, the SiO2 film is removed from the SiO2 pattern layer at the locations where the gate electrode 7 is to be formed, using the photolithography process.

[0057] Furthermore, a Schottky metal pattern made of Ni / Au is formed at the locations where the gate electrode 7 is to be created by vacuum deposition and photolithography, thereby forming the gate electrode 7. The metal layers preferably have thicknesses of 4 to 40 nm and 20 to 200 nm, respectively.

[0058] The HEMT element 20 is obtained through the processes described above. Effect of the buffer layer

[0059] As described above, in the HEMT element 20 according to the present embodiment, the freestanding substrate 1 consists of Zn in a concentration greater than or equal to 1×10 18 cm -3The buffer layer 2 is formed with doped GaN and is intended to act as the diffusion-suppressing layer to suppress the diffusion of Zn from the free substrate 1 into the channel layer 3 during the preparation of the epitaxial substrate 10. As an example, the buffer layer 2 consists of at least that part of the region in the thickness direction containing C at a concentration greater than or equal to 1 × 10 18 cm -3 layer formed with doped group 13 nitride shown.

[0060] If buffer layer 2 is not doped with C under the concentration condition described above, Zn diffuses from buffer layer 2 into channel layer 3 and further into barrier layer 4. In the above case, since Zn, acting as the acceptor element, acts like an electron trap, a current breakdown phenomenon occurs in HEMT element 20.

[0061] However, in the HEMT element 20 according to the present embodiment, the occurrence of current breakdown is preferably suppressed by the preferential suppression of Zn diffusion from the free substrate 1 by creating the buffer layer 2 from the group 13 nitride layer doped with C under the concentration condition described above. More specifically, if the concentration of Zn in the channel layer is less than or equal to 1 × 10⁻⁶ 16 cm -3 is, the occurrence of the current breakdown in the HEMT element 20 is preferably suppressed.

[0062] Doping with C at a concentration greater than or equal to 1×10 18 cm -3The presence of at least a portion of buffer layer 2 in the thickness direction is based on the fact that the concentration of carbon (C) can be changed by adjusting a formation condition of the group 13 nitride layer, such as the growth temperature, reactor pressure, the gas ratio of group 15 to group 13, and the formation thickness, for example, during the formation of the group 13 nitride layer by the MOCVD process. The carbon with which the crystal layer formed by the MOCVD process is primarily doped originates from the group 13 source, and, for example, the supply of carbon changes with the supply of group 13 gas, and the stability of the carbon element in the group 13 nitride crystal is influenced by temperature and pressure.

[0063] Thus, when the HEMT element 20 is obtained according to the present embodiment, the growth condition used in the formation of the buffer layer 2 is that the doping with C is greater than or equal to 1×10 18 cm -3 at least in the part of the area in the thickness direction, and the growth condition is used in the formation of channel layer 3 that such doping is not carried out, for example the growth condition that the concentration of C in channel layer 3 is two or three or more orders of magnitude lower than that of buffer layer 2.

[0064] As described above, according to the present embodiment, the semiconductor element capable of suppressing the occurrence of current breakdown can be obtained using the semi-insulating, freestanding GaN substrate. Examples: Example 1: Production of the Zn-doped GaN single crystal substrate using the flux method

[0065] A low-temperature GaN buffer layer is formed at a temperature of 550 °C on the surface of a c-plane sapphire substrate with a diameter of 2 inches and a thickness of 0.43 mm, such that it has a thickness of 30 nm, and subsequently a GaN thin film with a thickness of 3 µm is formed by the MOCVD process at a temperature of 1050 °C, resulting in a MOCVD-GaN template that can be used as a seed crystal substrate.

[0066] A Zn-doped GaN single crystal layer is formed using the Na-flux process with the MOCVD-GaN template, which was obtained as the seed crystal substrate.

[0067] Specifically, the MOCVD-GaN template was first arranged in an aluminum oxide crucible, which was then filled with 30 g of metallic Ga, 45 g of metallic Na, 1 g of metallic Zn, and 100 mg of C. The aluminum oxide crucible was placed in a heating oven and heated for approximately 100 hours at an oven temperature of 850 °C and an oven pressure of 4.5 MPa, and then cooled to room temperature. When the aluminum oxide crucible was removed from the oven after cooling, a brown GaN single-crystal layer approximately 1000 µm thick had been deposited on the surface of the MOCVD-GaN template.

[0068] The GaN single-crystal layer obtained in this manner was ground with diamond abrasive grains to flatten its surface, resulting in a total thickness of 900 µm for the nitride layer formed on a substrate. This yielded the flux-GaN template containing the GaN single-crystal layer formed on the MOCVD-GaN template. No cracks were observed in the flux-GaN template upon visual inspection.

[0069] The seed crystal substrate was then separated from the flux-GaN template using a laser lifting technique. Laser light was emitted from one side of the seed crystal substrate to perform scanning at a rate of 30 mm / s. The third harmonic of an Nd:YAG laser with a wavelength of 355 nm was used as the laser light source. The pulse width was set to approximately 30 ns, and the pulse period to approximately 50 kHz. Upon emission, the laser light was collected into a circular beam with a diameter of approximately 20 µm, resulting in a luminous intensity of approximately 1.0 J / s. The laser light was emitted while the flux-GaN template was heated to a temperature of approximately 50 °C from the side opposite the seed crystal substrate.

[0070] After separating the seed crystal substrate, a grinding operation was carried out on a surface from which the seed crystal substrate had been detached, resulting in a layered structure that had been obtained, thereby yielding a Zn-doped freestanding GaN substrate with a total thickness of 430 µm.

[0071] The crystallinity of the Zn-doped GaN substrate obtained was investigated using an X-ray reflection curve. The half-width of the (0002) plane reflection was 120 seconds, and the half-width of the (10-12) plane reflection was 150 seconds, both showing a favorable crystalline structure. Production of the epitaxial substrate by MOCVD process

[0072] Subsequently, an epitaxial substrate was prepared using the MOCVD process. Specifically, a GaN layer as a buffer layer, a GaN layer as a channel layer, and an AlGaN layer as a barrier layer were layered in that order onto the Zn-doped GaN substrate described above, under the following conditions. In this example, a gas ratio of group 15 to group 13 is a ratio (molar ratio) of the feed quantity from a group 15 (N) source to the feed quantity from group 13 (Ga, Al) sources. The condition for forming the GaN buffer layer is that it is confirmed in advance that the doping with C will be carried out at a high concentration, and the condition for forming the GaN channel layer is that it is confirmed in advance that the doping with C will be minimal.

[0073] GaN buffer layer: Formation temperature = 1050 °C; Reactor pressure = 10 kPa; Gas ratio of group 15 to group 13 = 500; Thickness = 1000 nm.

[0074] GaN channel layer: Formation temperature = 1050 °C; Reactor pressure = 100 kPa; Gas ratio of group 15 to group 13 = 2000; Thickness = 1000 nm.

[0075] AlGaN barrier layer: Formation temperature = 1050 °C; Reactor pressure = 5 kPa; Gas ratio of group 15 to group 13 = 12000; Gas ratio of Al source gas to group 13 gas = 0.25; Thickness = 25 nm.

[0076] After the formation of the layers described above, the heating element temperature was reduced to approximately room temperature and the gas inside the reactor was brought back to atmospheric pressure. The epitaxial substrate that had been produced was then removed. Manufacturing of the HEMT element

[0077] Subsequently, the HEMT element 20 was fabricated using the epitaxial substrate 10. The HEMT element was designed to have a gate width of 100 µm, a source-gate distance of 1 µm, a gate-drain distance of 4 µm, and a gate length of 1 µm.

[0078] First, part of a marginal area between individual elements was removed by etching to a depth of approximately 100 nm using the photolithography and RIE processes.

[0079] Then the SiO2 film was formed on the epitaxial substrate to a thickness of 100 nm, and then the SiO2 film was removed at the locations where the source electrode and the drain electrode were to be formed by etching using the photolithography method, thus obtaining a SiO2 pattern layer.

[0080] A metal pattern of Ti / Al / Ni / Au (each with a film thickness of 25 / 200 / 20 / 100 nm) was then formed at the locations where the source and drain electrodes were to be created by vacuum deposition and photolithography, respectively. This resulted in the formation of the source and drain electrodes. Subsequently, a 30-second heat treatment in a nitrogen gas atmosphere at a temperature of 825 °C was performed to improve the ohmic contact between the source and drain electrodes.

[0081] Then, the SiO2 film was removed from the SiO2 pattern layer at the locations where the gate electrode was to be formed, using the photolithography process.

[0082] Furthermore, a Schottky metal pattern made of Ni / Au (each with a film thickness of 20 / 100 nm) was formed at the locations where the gate electrode was to be formed using the vacuum deposition process and the photolithography process, thereby forming the gate electrode.

[0083] The HEMT element was obtained using the processes described above. SIMS study of the HEMT element

[0084] An elemental analysis in the depth direction was performed on the epitaxial substrate of the HEMT element obtained using secondary ion mass spectrometry (SIMS), and each concentration of the Zn element and the C element was investigated.

[0085] Fig. Figure 2 is a drawing illustrating the concentration profile of the Zn element and the C element in the epitaxial substrate according to the present example. Fig.Figure 3 is a magnified view of the environment of an interface between the GaN buffer layer and the GaN substrate in Fig. 2. In Fig. 2 and Fig. The three results presented illustrate the following aspects. (1) The GaN substrate is saturated with the Zn element in a high concentration (3×10 18 cm -3 ) endowed. (2) Since the concentration of the C element in the GaN buffer layer is greater than or equal to 6×10 18 cm -3 The GaN buffer layer contains an area where the carbon element is present in a concentration greater than or equal to 1×10 18 cm -3 contains, and the concentration of the C element in the channel layer is less than or equal to approximately 1×10 16 cm -3 . (3) The concentration of the Zn element, which is high in one part from the interface between the GaN buffer layer and the GaN substrate towards one side of the substrate, drops abruptly in the GaN buffer layer and falls to 5×10 14 cm -3 This is a detection limit for Zn when measured by secondary ion mass spectrometry (SIMS). This means that the diffusion of the Zn element in the channel layer and the barrier layer is suppressed. (4) A logarithmic change ratio of the Zn concentration per µm (a change ratio of a base-10 logarithmic value of the concentration) in a part in which the Zn concentration is 1×10 17 cm -3 The value is -74.9. Investigation of the electrical properties of the HEMT element

[0086] The drain-current-drain voltage characteristic (Id-Vd characteristic) of the HEMT element was investigated in a DC mode and a pulse mode (static drain bias Vdq = 30 V, static gate bias Vgq = -5 V) using a semiconductor parameter analyzer. A pinch-off threshold voltage was Vg = -3 V.

[0087] A ratio R of the drain current Id was used as an index to investigate current breakdown. pulse·Vd = 5 V·Vg = 2 V in pulse mode to the drain current Id DC·Vd = 5 V·Vg = 2 V In DC mode, when the drain voltage Vd = 5 V and the gate voltage Vg = 2 V are applied (= Id pulse·Vd = 5 V·Vg = 2 V / Id DC·Vd = 5 V·Vg = 2 VThe ratio R (0 ≤ R ≤ 1) was introduced. In the case of the HEMT element according to the present example, the ratio R was calculated to be 0.93. If the value of R is greater than or equal to 0.7, it can be determined that the current breakdown is low, and thus the current breakdown in the HEMT element according to the present example is also low. Comparative example 1

[0088] The HEMT element was fabricated under the same conditions as in Example 1, except that the growth condition for the GaN buffer layer differed from Example 1. The condition for forming the GaN buffer layer is that it is confirmed in advance that the amount of doping carbon is small compared to Example 1.

[0089] GaN buffer layer: Formation temperature = 1050 °C; Reactor pressure = 30 kPa; Gas ratio of group 15 to group 13 = 500; Thickness = 300 nm.

[0090] The SIMS measurement was performed on the HEMT element obtained in the same manner as in Example 1, and each concentration of the Zn element and the C element in the epitaxial substrate was investigated.

[0091] Fig. Figure 4 is a drawing illustrating the concentration profile of the Zn element and the C element in the epitaxial substrate according to the present comparative example. Fig. Figure 5 is a magnified view of the environment of an interface between the C-doped GaN buffer layer and the GaN substrate in Fig. 4. In Fig. 4 and Fig. The 5 results shown indicate the following state. (1) The GaN substrate is doped with the Zn element in a high concentration in the same manner as in Example 1. (2) The concentration of the C element in the GaN buffer layer is at most greater than or equal to approximately 2×1017 cm -3 , that is, higher than that in other areas, but lower than that in Example 1, and the concentration of the C element in the GaN channel layer formed under the same conditions as Example 1 is approximately 4×10 16 cm -3 , that is, higher than the one in example 1. (3) The concentration of the Zn element, which is high in the part from the interface between the GaN buffer layer and the GaN substrate towards the substrate side, decreases in the GaN buffer, but the measure of the decrease is small compared to that in Example 1 and even in the GaN channel layer the concentration of the Zn element is greater than or equal to 5×10 16 cm -3 , that is, two orders of magnitude higher than that of example 1. This means that the Zn element is also diffused into the channel layer. (4) A logarithmic change ratio of the Zn concentration per µm (a change ratio of a base-10 logarithmic value of the concentration) in a part where the Zn concentration is 1×10 17 cm -3 The value is -7.3.

[0092] The value R of the HEMT element was calculated as 0.31 under the same conditions as in Example 1, and it was found that the current breakdown in the HEMT element was not sufficiently suppressed according to the present comparative example. Examples 2 to 6, comparative examples 2 to 5

[0093] The HEMT elements were produced under conditions identical to those of Example 1, except that, for example, the growth conditions of the GaN buffer layer (growth temperature, reactor pressure, gas ratio of group 15 to group 13, and formation thickness) were varied. Then, using SIMS measurements, the depth distribution of the Zn and C concentrations in the resulting HEMT element was determined, and the value R was calculated.

[0094] A summary of one result is shown in Table 1 together with the results of Example 1 and Comparison Example 1. [Table 1] Growth condition of the GaN buffer layer Collapse investigation SIMS measurement Growth temperature [°C] Reactor pressure [kPa] Gas ratio of group 15 to group 13 Thickness [nm] Value R C concentration in GaN buffer layer [cm²] -3 ] Logarithmic rate of change of Zn concentration [1 / pm] Zn concentration at 200 nm downward direction from the barrier layer / channel layer interface [cm²] -3 ] Example 1 1050 10 500 1000 0,93 6×10 18 -74,9 5×10 14 Example 2 1050 10 500 300 0,91 6×10 18 -71,3 6×10 14 Example 3 1050 20 500 1000 0,88 1×10 18 -20,0 6×10 14 Example 4 1050 20 500 300 0,85 1×10 18 -25,1 1×10 15 Comparative example 2 1050 30 500 1000 0,51 2×10 17 -8,9 5×10 15 Comparative example 1 1050 30 500 300 0,31 2×10 17 -7,3 5×10 16 Example 5 1050 30 200 1000 0,90 1×10 1 -80,5 6×10 14 Comparative example 3 1050 30 1000 1000 0,29 8×10 16 Not calculable because Zn concentration is not less than or equal to 1×10 17 cm -3 ] is 2×10 17 Comparative example 4 1050 30 2000 1000 0,21 4×10 16 Not calculable because Zn concentration is not less than or equal to 1×10 17 cm -3 ] is 3×10 17 Example 6 950 30 500 1000 0,85 1×10 18 -34,9 7×10 14 Comparative example 5 1150 30 500 1000 0,25 4×10 16 Not calculable because Zn concentration is not less than or equal to 1×10 17 cm -3 ] is 4×10 17

[0095] As shown in Table 1, in each case of Examples 1 to 6, the HEMT element was used under the condition that the concentration of C in the GaN buffer layer was greater than or equal to 1×10 18 cm-3 was produced, the value R was greater than or equal to 0.85 and the concentration of Zn in the GaN channel layer was less than or equal to 1×10 15 cm -3 This result shows that the HEMT element in which the current breakdown was suppressed was obtained in all examples 1 to 6.

[0096] A logarithmic change ratio of the Zn concentration per µm in a part where the Zn concentration is 1×10 17 cm -3 The value was less than or equal to -20.0. This means that the concentration of Zn in all examples 1 to 6 decreases drastically.

[0097] In contrast, in each case of comparison examples 1 to 5, the HEMT element remained under the condition that the concentration of C in the GaN buffer layer was less than 1×10 18 cm -3is, was produced, the value R to a maximum of 0.51 and exceeded the concentration of Zn in the GaN channel layer, except in comparison example 2, 1×10 16 cm -3 This result shows that the current breakdown in the HEMT element is not suppressed according to comparison examples 1 to 5.

[0098] The logarithmic rate of change of the Zn concentration per µm in the part where the Zn concentration is 1×10 17 cm -3 The Zn concentration was -8.9 in comparison example 1 and -7.3 in comparison example 2, however, in comparison examples 3 to 5, the Zn concentration did not fall below 1×10 17 cm -3 , so the logarithmic rate of change could not be calculated. This result shows that in comparative examples 1 to 5, the measure of the decrease in Zn concentration is small compared to that in examples 1 to 6. Example 7

[0099] In the HEMT element according to Example 1 to Example 6, the concentration of Zn drops abruptly; consequently, the concentration of Zn in the channel layer falls below 1×10 16 cm -3 and thereby suppresses the current breakdown, however, as long as the concentration of Zn in the vicinity of the interface (the hetero interface) between the barrier layer and the channel layer in the channel layer is sufficiently below 1×10 16 cm -3 The power collapse can be suppressed in some cases, even if the degree of decline in Zn concentration is small.

[0100] In the present example, the epitaxial substrate 10 and the HEMT element 20 were produced under the same conditions as in Example 1, except that the growth conditions of the buffer layer 2 and the channel layer 3 differed from Example 1 as follows.

[0101] GaN buffer layer: Formation temperature = 1050 °C; Reactor pressure = 10 kPa; Gas ratio of group 15 to group 13 = 500; Thickness = 100 nm.

[0102] GaN channel layer: Formation temperature = 1050 °C; Reactor pressure = 100 kPa; Gas ratio of group 15 to group 13 = 2000; Thickness = 1000 nm.

[0103] Fig. Figure 6 is a drawing illustrating the concentration profile of the Zn and C elements obtained by taking measurements on the HEMT element in the depth direction using SIMS measurements under conditions identical to those of Example 1, starting from the surface (top side) of the barrier layer 4 in the depth direction. A Fig. The result shown in Figure 6 indicates the following state. (1) The GaN substrate is doped with the Zn element in a high concentration. (2) Since the concentration of the C element in the buffer layer is at most 3×10 18 cm -3 The buffer layer contains an area which contains the carbon element in a concentration greater than or equal to 1×10 18 cm -3 contains, and the concentration of the C element in the channel layer is less than or equal to approximately 2×10 16 cm -3 The concentration of the carbon element changes abruptly at the interface between the buffer layer and the channel layer, but changes gradually in a part from the buffer layer towards the GaN substrate. Fig. Figure 6 shows an increase in the concentration of the carbon element in the vicinity of the barrier layer towards the barrier layer. This is caused by collisions with elements adhering to a sample surface. (3) The concentration of the Zn element, which is high in a part from the interface between the buffer layer and the GaN substrate towards one side of the substrate, drops abruptly in the buffer layer, continues to decrease in the channel layer and falls to 5×10 in a region containing the area around the interface (hetero-interface) between the barrier layer and the channel layer. 14 cm -3 , which is the detection limit for Zn in SIMS measurement. This means that the diffusion of the Zn element is suppressed. (4) A logarithmic change ratio of the Zn concentration per µm (a change ratio of a base-10 logarithmic value of the concentration) in a part in which the Zn concentration is 1×10 17 cm -3 The value is -15.0.

[0104] The R value of the HEMT element described above was calculated to be 0.88. This means that current breakdown is also preferentially suppressed in the HEMT element according to the present example. Example 8

[0105] The epitaxial substrate 10 and the HEMT element 20 were produced under conditions identical to those of Example 1, except that the growth conditions for buffer layer 2 and channel layer 3 differed from those in Example 1. During the formation of buffer layer 2 in the above process, the formation conditions are set up in two stages: a first condition and a second condition. Midway through the formation process, the condition is switched from the first to the second condition. This is intended to form buffer layer 2 as a multilayered buffer layer, in which the GaN layer is bonded to the Al a Ga 1-aThe N-layer (0 < a ≤ 1) or the composition gradient buffer layer, in which the presence ratio of Al and Ga differs in the thickness direction, is layered. The first condition here is that the doping of buffer layer 2 with C is not actively carried out, and the second condition is that the doping of buffer layer 2 with C is confirmed. The total thickness of buffer layer 2 was set to 110 nm.

[0106] Buffer layer (first condition): Formation temperature = 1050 °C; Reactor pressure = 5 kPa; Group 13 source gas = Al source and Ga source; Gas ratio of group 15 to group 13 = 2000; Gas ratio of Al source gas to group 13 source gas = 0.03; Growth rate = 1 nm / s; Growth time = 10 s.

[0107] Buffer layer (second condition): Formation temperature = 1050 °C; Reactor pressure = 10 kPa; Group 13 source gas = Ga source; Gas ratio of group 15 to group 13 = 500; Growth rate = 1 nm / s; Growth time = 100 s.

[0108] GaN channel layer: Formation temperature = 1050 °C; Reactor pressure = 100 kPa; Gas ratio of group 15 to group 13 = 2000; Thickness = 900 nm.

[0109] Fig. Figure 7 is a drawing illustrating the concentration profile of the Zn and C elements obtained by performing the measurement on the HEMT element in the depth direction using SIMS measurement under conditions identical to those of Example 1, starting from the surface (top side) of the barrier layer 4 in the depth direction, and a secondary ion signal profile of the Al element in the depth direction (a distribution of the secondary ion count rate of the Al element in the depth direction). A Fig.The result shown in Figure 7 indicates the following state. (1) The GaN substrate is doped with the Zn element in a high concentration. (2) The buffer layer contains an area which contains the C element in a concentration greater than or equal to 1×10 18 cm -3 and smaller than 6×10 18 cm -3 contains, and the concentration of the C element in the channel layer is less than or equal to approximately 1×10 16 cm -3 The concentration of the carbon element changes abruptly at the interface between the buffer layer and the channel layer, but changes gradually in the part from the buffer layer towards the GaN substrate. Fig. Figure 7 shows an increase in the concentration of the carbon element in the vicinity of the barrier layer in the channel layer towards the barrier layer. This is caused by collisions with elements adhering to a sample surface. (3) The concentration of the Zn element, which is high in a part from the interface between the buffer layer and the GaN substrate towards one side of the substrate, drops abruptly in the buffer layer, continues to decrease in the channel layer and falls to 5×10 in a region containing the area around the interface (hetero-interface) between the barrier layer and the channel layer. 14 cm -3 , which is the detection limit for Zn in SIMS measurement. This means that the diffusion of the Zn element is suppressed. (4) A logarithmic change ratio of the Zn concentration per µm (a change ratio of a base-10 logarithmic value of the concentration) in a part in which the Zn concentration is 1×10 17 cm -3 The value is -13.0. (5) The Al element is contained in an area wider than 110 nm, which is a target thickness of the entire buffer layer, and the area also contains part of the GaN substrate. (6) Since the Al element is hardly detected in an area where the concentration of the C element in the buffer layer reaches a maximum, it can be assumed that the buffer layer contains the GaN layer, which is heavily doped with C, and the AlGaN layer, which is weakly doped with C.

[0110] The R value of the HEMT element described above was calculated to be 0.90. This means that current breakdown is also preferentially suppressed in the HEMT element according to the present example. Example 9

[0111] The HEMT element 20 was fabricated under the same conditions as in Example 8, except that the growth conditions for buffer layer 2 and channel layer 3 differed from those in Example 8. Specifically, in this example as well, the formation condition for buffer layer 2 is set up in two stages: a first condition and a second condition. The transition from the first to the second condition occurs midway through the formation process. The total thickness of buffer layer 2 was set to 350 nm.

[0112] Buffer layer (first condition): Formation temperature = 1050 °C; Reactor pressure = 5 kPa; Group 13 source gas = Al source and Ga source; Gas ratio of group 15 to group 13 = 2000; Gas ratio of Al source gas to group 13 source gas = 0.01; Growth rate = 1 nm / s; Growth time = 50 s.

[0113] Buffer layer (second condition): Formation temperature = 1050 °C; Reactor pressure = 10 kPa; Gas ratio of group 15 to group 13 = 500; Growth rate = 1 nm / s; Growth time = 300 s.

[0114] GaN channel layer: Formation temperature = 1050 °C; Reactor pressure = 100 kPa; Gas ratio of group 15 to group 13 = 2000; Thickness = 1700 nm.

[0115] The following aspects are shown by performing SIMS measurements on the HEMT element obtained under conditions identical to those of Example 1, showing the concentration profile of the Zn element and the C element from the surface (top side) of the barrier layer 4 in the depth direction, and a secondary ion signal profile of the Al element in the depth direction. (1) The GaN substrate is saturated with the Zn element in a high concentration (1×10 19 cm -3) endowed. (2) The buffer layer contains an area which contains the C element in a concentration greater than or equal to 1×10 18 cm -3 and smaller than 6×10 18 cm -3 contains, and the concentration of the C element in the channel layer is less than or equal to approximately 1×10 16 cm -3 The concentration of the carbon element changes abruptly at the interface between the buffer layer and the channel layer, but changes gradually in the part from the buffer layer towards the GaN substrate. (3) The concentration of the Zn element, which is high in a part from the interface between the buffer layer and the GaN substrate towards the substrate side, drops abruptly in the buffer layer, continues to decrease in the channel layer and falls to 5×10 in the vicinity of the interface (hetero-interface) between the barrier layer and the channel layer in the channel layer. 14 cm -3, which is the detection limit for Zn in SIMS measurement. This means that the diffusion of the Zn element is suppressed. (4) A logarithmic change ratio of the Zn concentration per µm (a change ratio of a base-10 logarithmic value of the concentration) in a part in which the Zn concentration is 1×10 17 cm -3 The value is -58.3. (5) The Al element is contained in an area wider than 350 nm, which is a target thickness of the entire buffer layer, and the area also contains part of the GaN substrate. (6) Since the Al element is hardly detected in an area where the concentration of the C element in the buffer layer reaches a maximum, it can be assumed that the buffer layer contains the GaN layer, which is heavily doped with C, and the AlGaN layer, which is weakly doped with C.

[0116] The R value of the HEMT element described above was calculated to be 0.92. This means that current breakdown is also preferentially suppressed in the HEMT element according to the present example.

Claims

[1] Epitaxial substrate (10) for semiconductor elements (20), comprising: a semi-insulating, freestanding substrate formed from Zn-doped GaN (1); a buffer layer (2) adjacent to the freestanding substrate (1), which is a group 13 nitride layer, which in at least a part of the buffer layer (2) in a thickness direction contains C in a concentration greater than or equal to 1×10 18 cm -3 is endowed; a channel layer (3) adjacent to the buffer layer (2); and a barrier layer (4) provided on the opposite side of the buffer layer (2) with an intervening channel layer (3), wherein the buffer layer (2) is a diffusion-suppressing layer which suppresses diffusion of Zn from the freestanding substrate (1) into the channel layer (3), and a concentration of Zn in the channel layer (3) less than or equal to 1×10 16 cm -3 is. [2] Epitaxial substrate (10) for the semiconductor elements (20) according to claim 1, wherein the group 13 nitride layer is a GaN layer. [3] Epitaxial substrate (10) for the semiconductor elements (20) according to claim 1, wherein the group 13 nitride layer either a multilayer buffer layer (2) consisting of layers of two or more group 13 nitride layers of different compositions, wherein at least one of the two or more group 13 nitride layers contains C in a concentration greater than or equal to 1×10 18 cm -3 is endowed, is educated, or a composition gradient buffer layer (2) containing group 13 nitride consisting of two or more group 13 elements, each of which has a presence ratio that changes in one thickness direction. [4] Epitaxial substrate (10) for the semiconductor elements (20) according to one of claims 1 to 3, wherein the channel layer (3) is formed from GaN and the barrier layer (4) is formed from AlGaN. [5] Epitaxial substrate (10) for the semiconductor elements (20) according to any one of claims 1 to 4, wherein the thickness of the buffer layer (2) is 50 nm to 1000 nm. [6] Semiconductor element (20) containing: a semi-insulating, freestanding substrate formed from Zn-doped GaN (1); a buffer layer (2) adjacent to the freestanding substrate (1), which is a group 13 nitride layer, which in at least a part of the buffer layer (2) in a thickness direction contains C in a concentration greater than or equal to 1×10 18 cm -3 is endowed; a channel layer (3) adjacent to the buffer layer (2); a barrier layer (4) provided on the opposite side of the buffer layer (2) with an intervening channel layer (3); and a gate electrode (7), a source electrode (5) and a drain electrode (6) which are provided on the junction (4), wherein the buffer layer (2) is a diffusion-suppressing layer which suppresses diffusion of Zn from the freestanding substrate (1) into the channel layer (3), and a concentration of Zn in the channel layer (3) less than or equal to 1×10 16 cm -3 is. [7] Semiconductor element (20) according to claim 6, wherein the group 13 nitride layer is a GaN layer. [8] Semiconductor element (20) according to claim 6, wherein the group 13 nitride layer either a multilayer buffer layer (2) consisting of layers of two or more group 13 nitride layers of different compositions, wherein at least one of the two or more group 13 nitride layers contains C in a concentration greater than or equal to 1×10 18 cm -3 is doped, is formed, or is a composition gradient buffer layer (2) containing two or more group 13 elements, each of which has a presence ratio that changes in one thickness direction, containing group 13 nitride. [9] Semiconductor element (20) according to any one of claims 6 to 8, wherein the channel layer (3) is formed from GaN and the barrier layer (4) is formed from AlGaN. [10] Semiconductor element (20) according to one of claims 6 to 9, wherein the thickness of the buffer layer (2) is 50 nm to 1000 nm. [11] Method for producing an epitaxial substrate (10) for semiconductor elements (20), comprising: a) a manufacturing step of the creation of a semi-insulating, freestanding substrate formed from Zn-doped GaN (1); b) a buffer layer formation step of forming a buffer layer (2) adjacent to the freestanding substrate (1); c) a channel layer formation step of forming a channel layer (3) adjacent to the buffer layer (2); and d) a barrier layer formation step of forming a barrier layer (4) at a position opposite the buffer layer (2), wherein the channel layer (3) lies between, wherein in the buffer layer formation step the buffer layer (2) is a diffusion-suppressing layer, which is a group 13 nitride layer, which in at least a part of the buffer layer (2) in a thickness direction contains C in a concentration greater than or equal to 1×10 18 cm -3is doped, which suppresses diffusion of Zn from the free substrate (1) into the channel layer (3), such that a concentration of Zn in the channel layer (3) is less than or equal to 1×10 16 cm -3 is. [12] Method for producing the epitaxial substrate (10) for the semiconductor elements (20) according to claim 11, wherein the group 13 nitride layer is formed from GaN. [13] Method for producing the epitaxial substrate (10) for the semiconductor elements (20) according to claim 11, wherein the group 13 nitride layer is formed either as a multilayer buffer layer (2) consisting of layers of two or more group 13 nitride layers of different compositions, wherein at least one of the two or more group 13 nitride layers contains C in a concentration greater than or equal to 1×10 18 cm -3 is endowed, or as a composition gradient buffer layer (2) containing group 13 nitride consisting of two or more group 13 elements, each of which has a presence ratio that changes in one thickness direction. [14] Method for producing the epitaxial substrate (10) for the semiconductor elements (20) according to any one of claims 11 to 13, wherein the channel layer (3) is formed from GaN and the barrier layer (4) is formed from AlGaN. [15] Method for producing the epitaxial substrate (10) for the semiconductor elements (20) according to any one of claims 11 to 14, wherein the freestanding substrate (1) is produced by means of a flux process. [16] Method for producing the epitaxial substrate (10) for the semiconductor elements (20) according to one of claims 11 to 15, wherein in the buffer layer formation step the buffer layer (2) is formed in a thickness of 50 nm to 1000 nm.

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