Stress Effects in p-Type AlGaN/GaN Heterostructures
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Stress Effects in p-Type AlGaN/GaN Heterostructures Agustinus Sutandi, P. Paul Ruden, and Kevin F. Brennan1 Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, MN 55455, U.S.A. 1 School of Electrical and Computer Engineering, Georgia Tech, Atlanta, GA 30332, U.S.A. ABSTRACT The physics of bulk wurtzite-structure III-nitride materials and of III-nitride heterostructures includes many phenomena that can be modulated by the application of stress. In particular, ptype material is expected to display a rich variety of piezo-resistive and piezo-optic effects that originate from the stress-induced modulation of lattice polarization charges, of valence band energies, and of bulk, surface, and interface defect states in the band gap. Here we focus on the expected effects of in-plane uniaxial on p-channel AlGaN/GaN heterostructures grown along the hexagonal axis on sapphire substrates. The valence band structure in the channel region is calculated self-consistently in the framework of a six-band Rashba-Sheka-Pikus (RSP) Hamiltonian. Stress-effects are included (in linear elastic theory) through deformation potentials and through the modulation of interfacial polarization charges associated with the piezoelectric nature of the constituent materials.
INTRODUCTION Group III-nitride semiconductors have become the materials of choice for optoelectronic devices operating in the short wavelength, visible and ultraviolet bands. In addition, these materials show considerable promise for the fabrication of high-frequency, high-power electronic devices and perhaps for sensors that need to operate in caustic environments. One of the distinguishing features of the system of materials that is comprised of GaN, AlN, InN, and their alloys, is its suitability for the growth of epitaxial heterostructures, such as those needed for heterostructure lasers and heterostructure transistors (for example: see [1]). While all III-nitride materials of interest appear to have the wurtzite crystal structure (point group symmetry C6v) as their most stable modification, the lattice constants are different, particularly for indium containing compounds. Consequently, epitaxial heterostructures composed of these materials include layers with built-in strain. If the heterointerfaces are perpendicular to the principal (hexagonal) crystal axis, the built-in biaxial strain does not lower the crystal symmetry. This is the situation examined in detail in the following sections. Residual, biaxial strain may be present throughout the epitaxial structure due to fact that the III-nitrides are typically grown on foreign substrates. The most popular substrate at present is sapphire, cut perpendicular to its principal symmetry axis. Although this is only a threefold axis (the sapphire point group symmetry is C3v), the hexagonal axis of the III-nitride epilayers aligns with it. However, the crystal axes in the basal plane are rotated by π/6. Strain affects the electronic properties of III-nitride heterostructures in multiple ways. First
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