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DE-112016005017-B4 - Epitaxial substrate for semiconductor elements, semiconductor element and manufacturing process for epitaxial substrates for semiconductor elements

DE112016005017B4DE 112016005017 B4DE112016005017 B4DE 112016005017B4DE-112016005017-B4

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 , 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) 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 to 1×10 21 cm -3 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) is less than or equal to 1×10 16 cm -3 .

Inventors

  • Mikiya Ichimura
  • Sota Maehara
  • Yoshitaka Kuraoka

Assignees

  • NGK INSULATORS, LTD.

Dates

Publication Date
20260513
Application Date
20161101
Priority Date
20160114

Claims (7)

  1. Epitaxial substrate (10) for semiconductor elements (20), comprising: a semi-insulating, freestanding substrate (1) formed from Zn-doped GaN, the dislocation density of which is less than or equal to 5.0×10 7 cm -2 , 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 an opposite side of the buffer layer (2) with an intervening channel layer (3), wherein the buffer layer (2) 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 to 1×10 21 cm -3 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) is less than or equal to 1×10 16 cm -3 .
  2. epitaxial substrate (10) for the semiconductor elements (20) according Claim 1 , wherein the channel layer (3) is made of GaN and the barrier layer (4) is made of AlGaN.
  3. Semiconductor element (20) comprising: a semi-insulating, freestanding substrate (1) formed from Zn-doped GaN, whose dislocation density is less than or equal to 5.0 × 10⁷ cm⁻² , a buffer layer (2) adjacent to the freestanding substrate (1); a channel layer (3) adjacent to the buffer layer (2); a channel layer (3) on the opposite side of the buffer layer (2) with an intervening channel layer (3) barrier layer (4) as seen; and a gate electrode (7), a source electrode (5) and a drain electrode (6) which are provided on the barrier layer (4), wherein the buffer layer (2) 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 to 1×10 21 cm -3 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) is less than or equal to 1×10 16 cm -3 .
  4. Semiconductor element (20) according to Claim 3 , wherein the channel layer (3) is made of GaN and the barrier layer (4) is made of AlGaN.
  5. Method for producing an epitaxial substrate (10) for semiconductor devices (20), comprising: a) a fabrication step of producing a semi-insulating, freestanding substrate (1) formed from Zn-doped GaN, the dislocation density of which is less than or equal to 5.0 × 10⁷ cm⁻² ; 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 to 1×10 21 cm -3 , and suppresses diffusion of Zn from the freestanding substrate (1) into the channel layer (3), such that a concentration of Zn is less than or equal to 1×10 16 cm -3 .
  6. Method for producing the epitaxial substrate (10) for the semiconductor elements (20) according to Claim 5 , wherein the channel layer (3) is made of GaN and the barrier layer (4) is made of 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.

Description

Technical field 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 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. 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. 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. 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. 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. 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. 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. 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, and the ease of diffusion depends 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, thereby preventing Fe from being incorporated into the electron transition layer (see, for example, JP 2013 - 74 211 A ). 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 ). 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 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, conseque