ABSTRACT
Gate-voltage manipulation of electron spins in semiconductor heterostructures is the subject of intensive study with regard to semiconductor spintronic devices such as spin transistors, spin light-emitting diodes, and quantum computers [1, 2].The general expression for the structure inversion asymmetry (SIA) Rashba coupling parameter in GaN/AlGaN heterostructures was obtained in Chapter 2. Basically, the spin-orbit coupling parameter aR decreases as the bandgap increases. Thus, the spin splitting in narrow bandgap semiconductors like InGaAs is expected to be larger than that in larger bandgap materials. However, in a wide bandgap III-nitride heterostructure, the spin splitting, since it is approximately proportional to an average electric field in the growth direction, is affected by the in-built electric field. It is known that lattice polarization strongly affects the performance of GaN-based electronic and optoelectronic devices [3, 4].In addition, the spin splitting depends not only on the Rashba parameter aRbut also on the Fermi level in degenerate conduction or the valence band. That is why strong polarization doping effect in III-nitrides, that is the increase in carrier density in the channel due to internal electric fields, may enhance overall spin splitting making it comparable to that found in narrow-gap III-V structures. One more Wide Bandgap Semiconductor Spintronics Vladimir Litvinov Copyright © 2016 Pan Stanford Publishing Pte. Ltd. ISBN 978-981-4669-70-2 (Hardcover), 978-981-4669-71-9 (eBook) www.panstanford.com
reason to take III-nitrides into consideration is the experimental data on the narrow bandgap in InN (0.69 eV) [5] that might include InGaN alloys in the spintronic material family. Besides, InGaN quantum well can be grown as topological insulator (see Chapter 7) that is the class of materials which reveals strong coupling between spin and transport characteristics [6].In this chapter we discuss the spintronic capabilities of wurtzite III-nitride heterostructures and quantum wells, namely, the Rashba spin-orbit coupling parameter and conduction-band spin splitting, determine their sensitivity to gate voltage, and establish the relation between the polarization field and electron spin splitting. Since GaN-InN-AIN is the pyroelectric material family, spontaneous and piezoelectrical polarizations may affect the electrically driven magnetism of confined electrons. In Section 3.1 we relate the internal built-in electric field to the interface electrical polarization caused by spontaneous and lattice-mismatch-induced piezoelectric contributions. Section 3.2 deals with the high-electron-mobility transistor (HEMT) structure, where polarization-induced doping results in a high density of two-dimensional (2D) electrons. Rashba interaction in polarization-doped heterostructure is explored in Section 3.3. Section 3.4 deals with structurally symmetric quantum wells whose conduction band profile is distorted by polarization fields.
