Epitaxial substrate for semiconductor device, semiconductor device, and method of manufacturing epitaxial substrate for semiconductor device

#The present invention relates to an epitaxial substrate that has a multi-layer structure composed of a group III nitride semiconductor, and more particularly, to a multi-layer structured epitaxial substrate for electronic devices and a method of manufacturing the same.

#Nitride semiconductors having high breakdown electric field and high saturation electron velocity have been attracting attention as the next generation of semiconductor materials for high-frequency/high-power devices. For example, a high electron mobility transistor (HEMT) device formed by laminating a barrier layer composed of AlGaN and a channel layer composed of GaN takes advantage of the feature that high-concentration two-dimensional electron gas (2DEG) is generated at a lamination interface (hetero interface) owing to a polarization effect (spontaneous polarization effect and piezo polarization effect) inherent in a nitride material (for example, see “Highly Reliable 250 W High Electron Mobility Transistor Power Amplifier”, Toshihide Kikkawa, Jpn. J. Appl. Phys. 44 (2005), p. 4896).

#As a base substrate of the substrate for HEMT device, for example, a single crystal (heterogeneous single crystal) having a composition different from that of a group III nitride, such as silicon and SiC, is used in some cases. In this case, a buffer layer such as a strained superlattice layer and a low-temperature growth buffer layer is typically formed as an initial growth layer on the base substrate. Therefore, the most basic configuration of a substrate for HEMT device using a base substrate formed of heterogeneous single crystal is obtained by epitaxially forming a barrier layer, a channel layer and a buffer layer on a base substrate. In addition, for the purpose of accelerating spatial confinement of two-dimensional electron gas, a spacer layer having a thickness of approximately 1 nm is provided between the barrier layer and the channel layer in some cases. The spacer layer is composed of, for example, AlN. Moreover, for the purposes of controlling an energy level on the topmost surface of the substrate for HEMT device and improving contact characteristics with an electrode, for example, a cap layer composed of an n-type GaN layer or a superlattice layer is formed on the barrier layer in some cases.

#FIG. 1 is a schematic cross-sectional view schematically showing a configuration of a HEMT device according to a first preferred embodiment;

#FIG. 2 is a graph schematically showing the In compositional distribution of a barrier layer in a depth direction;

#FIG. 3 is a diagram in which the relationship between a two-dimensional electron gas concentration and a composition of the barrier layer is mapped on a ternary phase diagram with three components of InN, AlN and GaN being vertices;

#First Preferred Embodiment

#Configuration of HEMT Device

#FIG. 1 is a schematic cross-sectional view schematically showing the configuration of a HEMT device 10 according to a first preferred embodiment of the present invention. The HEMT device 10 has the configuration in which a substrate 1, a buffer layer 2, a channel layer 3 and a barrier layer 5 are formed by lamination. The buffer layer 2, the channel layer 3 and the barrier layer 5 are preferred examples of layers that are epitaxially formed (details thereof are described below) using the metal organic chemical vapor deposition method (MOCVD method). Hereinbelow, the laminated structure formed by laminating the substrate 1, the buffer layer 2, the channel layer 3 and the barrier layer 5 is also referred to as an epitaxial substrate 10A. Note that ratios of respective layers in FIG. 1 do not reflect actual ones.