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 (1) formed from Zn-doped GaN, whose dislocation density is less than or equal to 5.0 × 10 7 cm -2 is, a buffer layer (2) adjacent to the freestanding substrate (1); 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) a diffusion-suppressing layer formed from Al-doped GaN with a thickness of 20 nm to 200 nm and an Al concentration of 5×10 18 cm -3 up to 1×10 21 cm -3 is and 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.

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 elements (see, for example, JP 5 039 813 B2). 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, with the ease of diffusion depending on the crystallinity of the GaN crystal (see, for example, 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).Furthermore, it is known that a high-resistance layer doped with iron (Fe), which is a transition metal element, is formed on a substrate, and furthermore, an intermediate layer with a strong Fe uptake effect is formed between the high-resistance layer and an electron transition layer, which prevents Fe from being absorbed into the electron transition layer (see, for example, JP 2013 - 74 211 A).

[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, 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, pp. 1-5; 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, pp. 2413-2417; 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, pp. 103-108).

[0011] When the GaN film is epitaxially grown on the semi-insulating GaN single-crystal substrate doped with the transition metal element or the group 2 element to form an epitaxial substrate for the semiconductor elements, a problem arises: an acceptor element such as Fe, Mg, and Zn diffuses into the GaN film and acts as an electron trap within the film, consequently causing a current breakdown phenomenon (see, for example, JP 2010 - 171 416 A). JP 2010 - 171 416 A discloses that the readily diffused acceptor element, such as Fe and Mg, causes a current breakdown.

[0012] Further state of the art is also known from DE 10 2012 020 978 A1 and JP 2011 - 68 548 A. Brief description

[0013] 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.

[0014] According to the present invention, an epitaxial substrate for semiconductor elements comprises: a semi-insulating, freestanding substrate formed from Zn-doped GaN, the dislocation density of which is less than or equal to 5.0×10 7 cm -2 is; a buffer layer adjacent to the freestanding substrate; a channel layer adjacent to the buffer layer; and a barrier layer provided on an opposite side of the buffer layer with an intervening channel layer, wherein the buffer layer is a diffusion-suppressing layer formed of Al-doped GaN with a thickness of 20 nm to 200 nm and an Al concentration of 5×10 18 cm -3 up to 1×10 21 cm -3is and suppresses diffusion of Zn from the free-standing substrate into the channel layer, resulting in a concentration of Zn in the channel layer that is less than or equal to 1×10 16 cm -3 is.

[0015] According to a further aspect of the present invention, a semiconductor element comprises: a semi-insulating, freestanding substrate formed from Zn-doped GaN, the dislocation density of which is less than or equal to 5.0×10 7 cm -2is; a buffer layer adjacent to the freestanding substrate; a channel layer adjacent to the buffer layer; a barrier layer provided on the opposite side of the buffer layer with an intervening channel layer; and a gate electrode, a source electrode, and a drain electrode provided on the barrier layer, wherein the buffer layer is a diffusion-suppressing layer formed from Al-doped GaN with a thickness of 20 nm to 200 nm and an Al concentration of 5 × 10 18 cm -3 up to 1×10 21 cm -3 is and suppresses diffusion of Zn from the free-standing substrate into the channel layer, resulting in a concentration of Zn in the channel layer that is less than or equal to 1×10 16 cm -3 is.

[0016] 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 formed from Zn-doped GaN, the dislocation density of which is less than or equal to 5.0×10 7 cm -2 is; b) a buffer layer formation step of forming a buffer layer adjacent to the free-standing 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 lying between them, wherein in the buffer layer formation step the buffer layer is a diffusion-suppressing layer formed from Al-doped GaN with a thickness of 20 nm to 200 nm and an Al concentration of 5 × 10 18 cm -3 up to 1×1021 cm -3 is formed and 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.

[0017] 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.

[0018] 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 Al element in an epitaxial substrate forming a HEMT element of sample No. 1-4. Fig. Figure 3 is a drawing illustrating the concentration profile of the Zn element and the Al element in an epitaxial substrate forming a HEMT element of sample No. 1-1. Description of embodiment(s)

[0019] 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). <Kurzbeschreibung von Epitaxialsubstrat und HEMT-Element>

[0020] 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.

[0021] 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.

[0022] The freestanding substrate 1 is a GaN substrate, which is covered with 1×10 18 cm-3 or is more Zn doped and has a (0001) plane orientation and a resistivity greater than or equal to 1×10 2 The dislocation density of the free-standing substrate 1 is preferably less than or equal to 5 × 10⁻⁶ Ωcm at room temperature and possesses semi-insulating properties. With regard to suppressing diffusion of Zn into the channel layer 3, the dislocation density of the substrate 1 is preferably less than or equal to 5 × 10⁻⁶. 7 cm -2 Although the size of the freestanding substrate 1 is not particularly limited, considering, for example, ease of handling (e.g., gripping and moving), the freestanding substrate 1 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.

[0023] 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.

[0024] The buffer layer 2 is a layer with a thickness of 10 to 1000 nm formed (adjacent to) a main surface of the freestanding substrate 1. In the present embodiment, the buffer layer 2 is formed at a temperature that is essentially the same as the formation temperature of the channel layer 3 and the barrier layer 4, unlike a so-called low-temperature buffer layer formed at a low temperature below 800 °C.

[0025] 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 Al in a concentration of 1 × 10⁻⁶ 18 cm-3 up to 5×10 21 cm -3 layer formed with doped GaN. In the above case, the diffusion of Zn from the freestanding substrate 1 into the channel layer 3 is preferably suppressed and, accordingly, the current breakdown is preferably suppressed in the HEMT element 20 produced using the epitaxial substrate 10.

[0026] The buffer layer 2 is preferably designed to have a thickness of 20 to 200 nm. The buffer layer 2 is preferably made of Al in a concentration of 5×10 18 cm -3 up to 1×10 21 cm -3 doped GaN is formed. In these cases, the current breakdown in the HEMT element 20 produced using the epitaxial substrate 10 is further suppressed.

[0027] Although it is also possible to make the thickness of buffer layer 2 greater than 1000 nm and the Al concentration of buffer layer 2 higher than 5×10 21 cm -3In these cases, a crack may occur in a surface of the epitaxial substrate 10 (a surface of the barrier layer 4).

[0028] If the thickness of buffer layer 2 is less than 10 nm and the Al concentration of buffer layer 2 is less than 5×10 17 cm -3 If the effect of suppressing the diffusion of Zn is not sufficiently achieved and consequently the current breakdown is not sufficiently suppressed, then such conditions are not preferable.

[0029] 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.

[0030] 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.

[0031] In any case, as a preferred example, the channel layer 3 is made of GaN and the barrier layer 4 is made of AlGaN (Al x Ga 1-x N, 0 <x<1) oder InAlN (In y Al 1-y N, 0 <y<1) gebildet. Jedoch ist eine Kombination der Kanalschicht 3 und der Sperrschicht 4 nicht darauf beschränkt.

[0032] 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 is formed from Al-doped GaN, channel layer 3 from GaN, and barrier layer 4 from AlGaN, 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 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.

[0033] 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.

[0034] 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. <Verfahren zum Herstellen des Epitaxialsubstrats und des HEMT-Elements> (Production of the freestanding substrate)

[0035] First, a procedure for producing the freestanding substrate 1 using the flux method is described.

[0036] 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 a 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 by 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.

[0037] 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.

[0038] 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.

[0039] 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.

[0040] 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.

[0041] 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.

[0042] 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 1 formed with doped GaN (Zn-doped, freestanding GaN single crystal substrate).

[0043] The dislocation density of the freestanding substrate 1 can be controlled by changing the thickness of the Zn-doped GaN single-crystal layer formed in the flux-GaN template. This exploits the fact that a thicker Zn-doped GaN single-crystal layer results in a region with a lower dislocation density. Accordingly, the freestanding substrate 1 can be made to have a dislocation density of less than or equal to 5 × 10⁻⁶. 7 cm -2as described above, also obtained by appropriately defining a formation thickness and a grinding amount after laser removal processing of the Zn-doped GaN single crystal layer. (Preparation of the epitaxial substrate)

[0044] 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 arranged on a heating element provided in the reactor within the MOCVD oven. The formation temperature is the heating element temperature.

[0045] 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 sources 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. Buffer layer 2: Formation temperature = 900 to 1200 °C; Reactor pressure = 5 to 30 kPa; Carrier gas = hydrogen; Gas ratio of group 15 to group 13 = 5000 to 20000; Gas ratio of Al source gas to group 13 source gas = 0.00002 to 0.1. 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. 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. Barrier layer 4 (in the event that it is made of InAlN): 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 the HEMT element)

[0046] The HEMT element 20 utilizing the epitaxial substrate 10 can be manufactured by applying a publicly known method.

[0047] For example, after element separation processing to remove part of an edge 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.

[0048] 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.

[0049] 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.

[0050] 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.

[0051] 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.

[0052] The HEMT element 20 is obtained through the processes described above. (Effect of the buffer layer)

[0053] 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 GaN dopants and is designed to act as a 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. More specifically, the buffer layer 2 is doped with Al at a concentration of 1×10 18 cm -3 up to 5×10 21 cm -3 doped GaN layer.

[0054] If buffer layer 2 is not doped with Al 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 the Zn acting as the acceptor element acts like an electrostatic trap, a current breakdown phenomenon occurs in the HEMT element 20.

[0055] 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 through the creation of the buffer layer 2 from the GaN layer doped with Al 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.

[0056] 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 (Experimental Example 1)

[0057] After fabricating the freestanding, Zn-doped GaN single-crystal substrate, seven types of epitaxial substrates were prepared using the freestanding substrate as the base substrate under the same conditions, except for varying the thickness of the buffer layer. Furthermore, the HEMT element was fabricated using each epitaxial substrate. The following are common samples No. 1-1 to No. 1-7 for the seven types of epitaxial substrates and the HEMT elements fabricated using each of the epitaxial substrates. [Production of the Zn-doped GaN single crystal substrate using the flux method]

[0058] 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, thereby obtaining a MOCVD-GaN template that can be used as a seed crystal substrate.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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 / cm². 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.

[0063] 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.

[0064] 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]

[0065] Subsequently, an epitaxial substrate was prepared using the MOCVD process. Specifically, an Al-doped GaN layer as a buffer layer, a GaN layer as a channel layer, and an AlGaN layer as a barrier layer were layered onto each Zn-doped GaN substrate in that order, under the following condition. In the following description, a gas ratio of group 15 to group 13 is a ratio (molar ratio) of a feed quantity from a group 15 (N) source to a feed quantity from group 13 (Ga, Al) sources.

[0066] Al-doped GaN buffer layer: Formation temperature = 1050 °C; Reactor pressure = 5 kPa; Gas ratio of group 15 to group 13 = 15000; Gas ratio of Al source gas to group 13 source gas = 0.001; Thickness = 0, 10, 20, 100, 200, 1000 or 2000 nm.

[0067] GaN channel layer: Formation temperature = 1050 °C; Reactor pressure = 100 kPa; Gas ratio of group 15 to group 13 = 2000; Thickness = 1000 nm.

[0068] 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.

[0069] The epitaxial substrate containing the Al-doped GaN buffer layer with a thickness of 0 nm means that the GaN channel layer is formed directly on the Zn-doped GaN substrate without forming the Al-doped GaN buffer layer. The formation condition of the Al-doped GaN buffer layer is based on the assumption that the Al concentration in the buffer layer is 5 × 10 19 cm -3 amounts.

[0070] After each layer had formed under the conditions 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]

[0071] The HEMT element was then fabricated using each epitaxial substrate. 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.

[0072] 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.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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.

[0077] The seven types of HEMT elements were obtained through the processes described above. When these HEMT elements were examined under a differential interference microscope, the occurrence of a crack in a surface of the epitaxial substrate (i.e., a surface of the barrier layer) was confirmed only in the HEMT element of sample no. 1-7. [RTEM investigation of the HEMT element]

[0078] An examination using a scanning transmission electron microscope (RTEM) was performed on the HEMT element of sample no. 1-4, and based on the results of this examination, a thread dislocation density of the Zn-doped GaN substrate was determined to be 2×10 6 cm -2 The dislocation density of the Zn-doped GaN substrate of the other samples, which were prepared under the same conditions, is also estimated to be essentially the same as that of sample no. 1-4.

[0079] The filament dislocation density was calculated based on a total number of dislocations confirmed in each viewing field when observing the freestanding substrate in a multitude of viewing fields. [SIMS investigation of the HEMT element]

[0080] An elemental analysis in one depth direction was performed on the epitaxial substrate of each HEMT element using SIMS (secondary ion mass spectrometry), and a concentration profile of the Zn element and the Al element was obtained.

[0081] Fig. Figure 2 is a drawing illustrating the concentration profile of the Zn element and the Al element in the epitaxial substrate forming the HEMT element of sample No. 1-4. Fig. Figure 3 is a drawing illustrating the concentration profile of the Zn element and the Al element in the epitaxial substrate forming the HEMT element of sample No. 1-1.

[0082] The concentration profile in Fig. 2 shows the following aspects. (1) The GaN substrate is saturated with the Zn element in a high concentration (1×10 19 cm -3 ) endowed. (2) The Al concentration of the Al-doped GaN buffer layer is 5×10 19 cm -3 . (3) The concentration of the Zn element, which is high in part from the interface between the buffer layer and the GaN substrate towards one side of the substrate, decreases rapidly in the buffer layer, and also gradually decreases in the channel layer, dropping to 5×10 15 cm -3 , which is a detection limit (background level) of Zn in SIMS measurement.

[0083] The aspects of these points (1) to (3) were also found in the HEMT elements of samples No. 1-2 to No. 1-6, except that the lower limit of the concentration value of the Zn element in the channel layer in the HEMT element of sample No. 1-2 was 8×10 15 cm -3 This means that in the HEMT element of samples No. 1-2 to No. 1-6, the diffusion of the Zn element with which the GaN substrate was doped into the channel layer is suppressed.

[0084] In contrast, the concentration profile in Fig. 3 the following aspects. (4) The GaN substrate is saturated with the Zn element in a high concentration (1×10 19 cm -3 ) endowed. (5) The concentration of the Zn element in the channel layer gradually decreases, however, the degree of decrease is small compared to that in the HEMT element of sample No. 1-4, and even in the vicinity of the barrier layer the concentration of the Zn element is higher than or equal to 8×10 16 cm -3 , that is, by an order of magnitude higher than that of the HEMT element of sample no. 1-4.

[0085] The aspects of points (4) and (5) mean that in the HEMT element of sample No. 1-1, the Zn element with which the GaN substrate was doped diffuses into the channel layer. The result described above means that the diffusion of Zn from the substrate into the channel layer is suppressed by providing the Al-doped GaN buffer layer between the Zn-doped GaN substrate and the channel layer; that is, the Al-doped GaN buffer layer acts as the diffusion-suppressing layer. [Investigation of the electrical properties of the HEMT element]

[0086] The drain-current-drain voltage characteristics (Id-Vd characteristics) of the HEMT elements of samples No. 1-1 to No. 1-6 were 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 was used as an index for investigating current breakdown. pulse in pulse mode to the drain current Id DC In DC mode, when the drain voltage Vd = 5 V and the gate voltage Vg = 2 V are applied (= Id pulse / Id DC The equation (0 ≤ R ≤ 1) was introduced, and the ratio R was obtained for each HEMT element. If the value of R is greater than or equal to 0.7, it can be determined that the current breakdown in the HEMT element is low.

[0088] Table 1 lists the Al concentration of the buffer layer and the Zn concentration of the channel layer (derived from the concentration profile), along with the value R, the thickness of the buffer layer, and the presence / absence of a crack in the surface of the epitaxial substrate (referred to as a "film crack" in Table 1) for each sample of Experiment 1. The Al and Zn concentration values ​​were taken from a central portion of the target layer in the thickness direction (the same applies to subsequent experiments). Table 1 also indicates whether each sample is the example or a comparison example. [Table 1] Sample No. Buffer layer thickness (nm) Al concentration of the buffer layer (cm³) -3 ) Zn concentration of the channel layer (cm³) -3 ) Memory gap present Value R Example / Comparative example Experiment 1 1-1 0 - 8,0×10 16 No 0,25 comparative example 1-2 10 5,0×10 19 8,0×10 15 No 0,71 Example 1-3 20 5,0×10 19 5,0×10 15 (B.G.L) No 0,80 Example 1-4 100 5,0×10 19 5,0×10 15 (B.G.L) No 0,90 Example 1-5 200 5,0×10 19 5,0×10 15 (B.G.L) No 0,92 Example 1-6 1000 5,0×10 19 5,0×10 15 (B.G.L) No 0,77 Example 1-7 2000 5,0×10 19 - Yes - comparative example

[0089] As shown in Table 1, in the HEMT element of sample No. 1-1, which has no buffer layer, the Zn concentration of the channel layer is 8×10 16 cm -3 , which is higher than 1×10 16 cm -3is, and the value R is only 0.25.

[0090] In contrast, in the HEMT elements of samples No. 1-2 to 1-6, which each have a buffer layer with a thickness of 10 to 1000 nm, the Zn concentration of the channel layer is less than or equal to 1×10 16 cm -3 and the value R is greater than or equal to 0.70. This means that the current breakdown in the HEMT elements of samples No. 1-2 to 1-6 can be assumed to be low.

[0091] In particular, in the HEMT elements of samples No. 1-3 to No. 1-5, which each have a buffer layer with a thickness of 20 to 200 nm, the Zn concentration of the channel layer rises to approximately 5×10 15 cm -3Back to the detection limit for SIMS, where R is greater than or equal to 0.80. BGL in Table 1 means that the Zn concentration is at a background level (the same applies to Table 2 and Table 3). This means that the current breakdown in the HEMT elements of samples 1-3 to 1-5 is expected to be particularly low. (Experimental example 2)

[0092] After fabricating the freestanding, Zn-doped GaN single-crystal substrate under conditions and using procedures comparable to those in Experiment 1, seven types of epitaxial substrates were prepared using the freestanding substrate as the base substrate. The fabrication conditions in the above case were the same as in Experiment 1, except that the ratio of Al source gas to group 13 source gas during the formation of the Al-doped GaN buffer layer was varied to control the Al concentration. Specifically, the ratio of Al source gas to group 13 source gas during buffer layer formation was varied between seven values: 0.00001, 0.00002, 0.0001, 0.001, 0.02, 0.1, and 0.2. The buffer layer thickness was set to 100 nm.

[0093] Since the manufacturing conditions of the Zn-doped GaN substrate are the same as those in experimental example 1, it is estimated that its dislocation density is essentially the same as that of sample No. 1-4.

[0094] Furthermore, the HEMT element was fabricated using each epitaxial substrate. Samples 2-1 through 2-7 are used below to represent the seven types of epitaxial substrates and the HEMT elements fabricated using each of them. The epitaxial substrate and HEMT element of sample 2-4 are the same as the epitaxial substrate and HEMT element, respectively, of sample 1-4 in Experiment 1.

[0095] The following procedures were performed on the seven types of HEMT elements obtained: examination under a differential interference microscope, elemental analysis in the depth direction using SIMS, calculation of the Al concentration of the buffer layer and the Zn concentration of the channel layer based on the resulting concentration profile, and calculation of the value R based on the Id-Vd characteristic curve using a semiconductor parameter analyzer, all in a manner comparable to Experiment 1. However, the calculation of the Zn concentration, the investigation of the Id-Vd characteristic curve, and the calculation of the value R were not performed on the HEMT element of sample no. 2-7, in which the presence of a crack in the surface of the epitaxial substrate (i.e., the surface of the barrier layer) had been confirmed as a result of examination under a differential interference microscope.

[0096] Table 2 lists the Al concentration of the buffer layer, the Zn concentration of the channel layer, and the value R, along with the thickness of the buffer layer and the presence / absence of the crack in the surface of the epitaxial substrate for each sample of test example 2. Table 2 also indicates what each sample falls under: the example or a comparison example. [Table 2] Sample No. Buffer layer thickness (nm) Al concentration of the buffer layer (cm³) -3 ) Zn concentration of the channel layer (cm³) -3 ) Memory gap present Value R Example / Comparative example Experiment 2 2-1 100 5,0×10 17 7,0×10 16 No 0,40 comparative example 2-2 100 1,0×10 18 1,0×10 16 No 0,70 Example 2-3 100 5,0×10 18 5,0×10 15 (B.G.L) No 0,85 Example 2-4(=1-4) 100 5,0×10 19 5,0×10 15 (B.G.L) No 0,90 Example 2-5 100 10×10 21 5,0×10 15 (B.G.L) No 0,91 Example 2-6 100 5,0×10 21 9,0×10 15 No 0,75 Example 2-7 100 1,0×10 22 - Yes - comparative example

[0097] As shown in Table 2, in the HEMT element of sample No. 2-1, where the Al concentration in the buffer layer is equal to 5.0×10 17 cm -3 is the Zn concentration of the channel layer 7×10 16 cm -3 , which is higher than 1×10 16 cm -3 is, and the value R is only 0.40.

[0098] In contrast, in the HEMT elements of samples No. 2-2 to No. 2-6, where the Al concentration in the buffer layer is 1.0×10 18cm -3 up to 5.0×10 21 cm -3 The Zn concentration of the channel layer is less than or equal to 1×10 16 cm -3 and the value R is greater than or equal to 0.70. This means that the current breakdown in the HEMT elements of samples No. 2-2 to No. 2-6 can be assumed to be low.

[0099] Especially in the HEMT elements of samples No. 2-3 to No. 2-5, in which the Al concentration in the buffer layer is 5.0×10 18 cm -3 up to 1.0×10 21 cm -3 If the Zn concentration in the channel layer is approximately 5×10, it will rise to about 5×10. 15 cm -3 Returning to the detection limit for SIMS, the value R is greater than or equal to 0.85. This means that the current breakdown in the HEMT elements of samples No. 2-3 to No. 2-5 is likely to be particularly low. (Experimental example 3)

[0100] After fabricating the freestanding, Zn-doped GaN single crystal substrate using procedures identical to those in Experiment 1, three types of epitaxial substrates were prepared using the freestanding substrate as the base substrate, and the HEMT elements were fabricated using each epitaxial substrate.

[0101] However, when fabricating each freestanding, Zn-doped GaN single-crystal substrate, the growth time for forming the GaN single-crystal layer using the flux process was varied, thus varying the thickness of the GaN single-crystal layer formed on the surface of the MOCVD GaN template. This was intended to achieve the freestanding, Zn-doped GaN single-crystal substrates with different dislocation densities. Specifically, the thickness of the GaN single-crystal layer was varied by varying the holding time (maintaining a heating temperature of 850 °C) between three values: 100 hours, 70 hours, and 40 hours, and between three values: 1000 µm, 600 µm, and 200 µm.

[0102] The fabrication conditions of the epitaxial substrate and the HEMT element were the same as those used in the fabrication of the epitaxial substrate according to sample no. 1-4. For example, when forming the Al-doped GaN buffer layer, the gas ratio of Al source gas to Group 13 source gas was set to 0.001, resulting in an Al concentration of 5.0 × 10 19 cm -3 The value was [missing value], and the thickness of the buffer layer was set to 100 nm.

[0103] In the following, common samples No. 3-1 to No. 3-3 are used for the three types of epitaxial substrates and the HEMT elements produced using each of the epitaxial substrates. However, the epitaxial substrate and the HEMT element of sample No. 3-1 are the same as the epitaxial substrate and the HEMT element of sample No. 1-4, respectively, in experimental example 1.

[0104] The three types of HEMT elements obtained were examined under the differential interference microscope, the filament dislocation density of the Zn-doped GaN substrate was investigated based on the RTEM observation result, the element analysis in the depth direction using SIMS, the calculation of the Al concentration of the buffer layer and the Zn concentration of the channel layer based on the concentration profile obtained thereby, and the calculation of the value R based on the investigation result of the Id-Vd characteristic curve using the semiconductor parameter analyzer in a manner comparable to experimental example 1.

[0105] Table 3 lists the dislocation density of the freestanding, Zn-doped GaN single-crystal substrate (referred to in Table 3 as "GaN substrate"), the Zn concentration of the channel layer, and the value R, along with the presence / absence of the crack in the surface of the epitaxial substrate, for each sample of Experimental Example 3. Table 3 also indicates whether each sample is the example or a comparison example. [Table 3] Sample No. Dislocation density of the GaN substrate (cm³) -2 ) Zn concentration of the channel layer (cm³) -3 ) Film break present Value R Example / Comparative example Experiment 3 3-1(=1-4) 2,0×10 16 5,0×10 15 (B.G.L) No 0,90 Example 3-2 5,0×10 7 1,0×10 16 No 0,70 Example 3-3 1,0×10 8 9,0×10 16 No 0,22 comparative example

[0106] As shown in Table 3, in the HEMT element of sample No. 3-3, in which the dislocation density of the Zn-doped, freestanding GaN single crystal substrate is equal to 1.0×10 8 cm -2 is the Zn concentration of the channel layer 9×10 1e cm -3 , which is higher than 1×10 16 cm -3 is, and the value of R is only 0.22.

[0107] In contrast, in the HEMT elements of samples No. 3-1 and No. 3-2, in which the dislocation density of the Zn-doped, freestanding GaN single crystal substrate is less than or equal to 5.0×10 7 cm -2 is, the Zn concentration of the channel layer is less than or equal to 1×10 16 cm -3 and the value R is greater than or equal to 0.70. This means that the current breakdown in the HEMT elements of samples No. 3-1 and No. 3-2 can be assumed to be low.

[0108] In particular, in the HEMT element of sample no. 3-1, in which the dislocation density of the freestanding, Zn-doped GaN single crystal substrate is 2.0×10 6 cm -2 If the Zn concentration in the channel layer is approximately 5×10, it will rise to about 5×10. 15 cm -3Returning to the detection limit for SIMS, the value R is 0.90. This means that the current breakdown in the HEMT element of sample no. 3-1 is likely to be particularly low. (Conclusion from experimental examples 1 to 3)

[0109] The following is confirmed by the results of the experimental examples 1 to 3 described above.

[0110] In the case where the HEMT element is exposed through layers of the channel layer and the barrier layer to the freestanding, Zn-doped GaN single-crystal substrate, which contains the Zn element in a high concentration greater than or equal to 1×10 18 cm -3 When doped, the freestanding substrate is created such that its dislocation density is less than or equal to 5.0×10 7 cm 2 The Al-doped GaN buffer layer will have an Al concentration between 1×10 18 cm -3 and 5×10 21 cm-3 The channel layer is formed on the freestanding substrate to have a thickness of 10 to 1000 nm, and then the channel layer is formed on it, and consequently the diffusion of Zn from the freestanding substrate into the channel layer can be preferentially suppressed.

[0111] Specifically, the Zn concentration in the canal layer can be reduced to 1×10 16 cm -3 or be reduced less. In the HEMT element in which the diffusion of Zn is preferentially suppressed, the occurrence of current breakdown is preferentially suppressed.

[0112] Especially if the thickness of the buffer layer is between 20 and 200 nm or if the Al concentration of the buffer layer is between 5×10 18 cm -3 and 1x10 21 cm -3 If the situation is such that the occurrence of the power outage is further suppressed.

Claims

[1] Epitaxial substrate (10) for semiconductor elements (20), comprising: a semi-insulating, freestanding substrate (1) formed from Zn-doped GaN, whose dislocation density is less than or equal to 5.0 × 10 7 cm -2 is, a buffer layer (2) adjacent to the freestanding substrate (1); 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) a diffusion-suppressing layer formed from Al-doped GaN with a thickness of 20 nm to 200 nm and an Al concentration of 5×10 18 cm -3 up to 1×10 21 cm -3 is and 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 channel layer (3) is formed from GaN and the barrier layer (4) is formed from AlGaN. [3] Semiconductor element (20), containing: a semi-insulating, freestanding substrate (1) formed from Zn-doped GaN, whose dislocation density is less than or equal to 5.0 × 10 7 cm -2 is, a buffer layer (2) adjacent to the freestanding substrate (1); 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) a diffusion-suppressing layer formed from Al-doped GaN with a thickness of 20 nm to 200 nm and an Al concentration of 5×10 18 cm -3 up to 1×10 21 cm -3 is and 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. [4] Semiconductor element (20) according to claim 3, wherein the channel layer (3) is formed from GaN and the barrier layer (4) is formed from AlGaN. [5] Method for producing an epitaxial substrate (10) for semiconductor elements (20), comprising: a) a production step of the production of a semi-insulating, freestanding substrate (1) formed from Zn-doped GaN, whose dislocation density is less than or equal to 5.0×10 7 cm -2 is; 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 in between, wherein in the buffer layer formation step the buffer layer (2) is formed as a diffusion-suppressing layer formed from Al-doped GaN, which has a thickness of 20 nm to 200 nm and an Al concentration of 5×10 18 cm -3 up to 1×10 21 cm -3 has, and suppresses diffusion of Zn from the freestanding substrate (1) into the channel layer (3), so that the concentration of Zn is less than or equal to 1×10 16 cm -3 is. [6] Method for producing the epitaxial substrate (10) for the semiconductor elements (20) according to claim 5, wherein the channel layer (3) is formed from GaN and the barrier layer (4) is formed from AlGaN. [7] Method for producing the epitaxial substrate (10) for the semiconductor elements (20) according to claim 5 or claim 6, wherein the freestanding substrate (1) is produced by means of a flux process.

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