Electron spin dynamics in GaN

Abstract

This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.Gallium nitride is a promising material system for spintronics, offering long spin relaxation times and prospects for room-temperature ferromagnetism. We review the electron spin dynamics in bulk GaN. Time-resolved magneto-optical studies of both the wurtzite and the cubic phase of GaN show the dominance of Dyakonov-Perel (DP) relaxation for free conduction band electrons. Spin relaxation in the wurtzite phase is characterized by an intrinsic spin relaxation anisotropy and the limitation of spin lifetimes by a strong Rashba term. Spin lifetimes are strongly enhanced in cubic GaN, where only a weak Dresselhaus term contributes to DP relaxation. Ion-implanted wurtzite GaN shows a strong increase of electron spin lifetimes for increasing implantation dose, caused by increasing localization of carriers. The spin dynamics of conduction band electrons in Gd-implanted GaN as a candidate for a room-temperature ferromagnetic semiconductor is also only governed by localization effects and does not show signs of an efficient exchange coupling between the electrons and the magnetic Gd ions.1 Introduction Spintronics as the novel spin-based semiconductor electronics has been an intense and fruitful field of research in the last years, leading to the discovery and detailed understanding of numerous spin effects [1][2][3][4][5]. In addition, a plethora of possible applications including, e.g., the famous spin transistor [6] or spin optoelectronic devices [7][8][9], has been proposed. However, for the realization of the proposed concepts challenging requirements are imposed on the underlying semiconductor materials, like, e.g., very long spin lifetimes or ferromagnetic behavior with Curie temperatures above room temperature. These challenges have triggered intense research on optimized structures and new materials. In particular, GaN has attracted strong interest as it promises to combine fascinating properties, ranging from weak spin-orbit coupling due to its light constituting elements over high-temperature ferromagnetism upon doping with rare earth ions [10] up to its good optoelectronic properties [11] and its well established use in the semiconductor industry [12]. In addition, GaN is a highly instructive model system for the influence of crystal symmetry on the electron spin dynamics as besides the thermodynamically stable wurtzite phase also the metastable cubic phase can be prepared [13]. While the spin dynamics in the dilute nitrides GaNAs and GaInNAs [14][15][16][17][18], and of holes [19] and especially excitons in GaN [20][21][22][23]

Density Dependence of Electron Spin Relaxation Time in GaA

“…The electron density dependence of the spin relaxation time at relatively low electron densities is similar to the experimental result reported by Oertel et al [9] (the black circles in Figure 2). On the other hand, some experimental results [20]- [22] showed that the spin relaxation time increases with increasing carrier density and then decreases after reaching a maximum value at approximately ) can be corrected if the electron-electron interaction is considered because the spin relaxation time (rate) by the electron-electron interaction decreases (increases) with increasing electron density. Figure 5 shows the temperature dependence of the relaxation relaxation times for different electron densities.…”

Strain dependent electron spin dynamics in bulk cubic GaN

“…73,74 Furthermore, thermally activated carrier scattering between the cubic and the hexagonal GaN phases could lead to strongly enhanced spin relaxation due to the very fast spin relaxation in the polar hexagonal GaN. [27][28][29][30] Generally, several other spin relaxation mechanisms will also contribute to spin relaxation in c-GaN. Elliott-Yafet 75 (EY) relaxation as well as relaxation due to the Bir-Aronov-Pikus 76 (BAP) mechanism as other spin relaxation mechanisms for mobile electrons 47 are, however, expected to contribute only weakly to spin relaxation.…”

“…Especially GaN is expected to offer long spin lifetimes due to its combination of weak SOC and large band gap. 25 Slow spin relaxation was indeed observed in the metastable cubic phase of GaN (c-GaN) with its high symmetry, 26,27 while the thermodynamically favored hexagonal phase of GaN (h-GaN) shows fast spin relaxation due to its lower symmetry. [28][29][30] The experimental spin relaxation times in c-GaN are, however, still substantially shorter than theoretically predicted.…”

“…31 Unintentional strain fields caused by microstrain variations [32][33][34] or by a small h-GaN content 35 were discussed as a possible reason for the observed discrepancy. 27 The impact of strain fields on spin relaxation in c-GaN has, however, not been studied so far. Here, we investigate the electron spin dynamics in bulk c-GaN under variable uniaxial strain by time-resolved magneto-optical Kerr-rotation (TRKR) spectroscopy.…”

Electron spin dynamics in mesoscopic GaN nanowires

“…For NWs with a wurtzite (WZ) structure, contributions due to the inversion asymmetry of the underlying bulk material and due to interface effects are expected. 8 We will start by discussing the effective magnetic field X wz ðkÞ for bulk wurtzite GaN, which is distinctively different from the effective magnetic field for semiconductors with a zincblende structure, 28 as it includes a term linear in k (Rashba term) 29 and a cubic k 3 -term due to the wurtzite bulk inversion asymmetry, 30…”