Excitonic effects on electron spin orientation and relaxation in wurtzite GaN

Excitonic effects on electron spin orientation and relaxation in wurtzite GaN are investigated with photon-energy-dependent time-resolved Kerr rotation spectroscopy at low temperatures. It is observed that the spin orientation generated with circularly polarized light can be manipulated with photon energy upon the excitation energy being resonated with various fine energy levels in the band structure. Three reversals of the spin orientation and a remarkable reduce of spin relaxation time are obtained, which is caused by the photon-energy dependent transitions from the heavy and light hole bands to the exciton levels and the conduction band edge. A long spin relaxation time of 1.7 ns is attributed to the spin polarization of donor-bound electrons as the formation of donor-bound excitons, while the spin relaxation time of the conduction band electrons is shorter and shows a nonmonotonic dependence on the optical excitation power, revealing the dominant role of the D’yakonov-Perel’ spin relaxation mechanism in wurtzite GaN.

Figure 1

(a) Schematic diagram of the experimental principle. The sign of Kerr rotation $\pm {\theta }_k$ follows from spin orientation $J_z=\pm 1/2$. (b) Photoluminescence spectrum of the sample at 8 K and the excitation power larger than 20 mW. (c) TRKR signals of different excitation energy at 10 K and $B=0.26\mathrm{T}$. (d) Kerr rotation angles at a delay time of 80 ps extracted from the raw data in Fig. 1 and numerical simulations based on the fundamental optical transition.

Figure 2

Schematic diagram of optical transitions under a left circularly polarized light ${\sigma }_{+}$.The inset describes the fast capture of FE.

Figure 3

TRKR signals (in the unit of degree) at the excitation energy of (a) 3.481 eV and (b) 3.501 eV, corresponding to the spin relaxation of donor-bound electrons and CB electrons, respectively.

Figure 4

(a) Carrier density dependence of spin relaxation time and carrier lifetime for CB electrons. (b) Excitation energy dependence of ${\tau }^{\mathrm{tot}}$ and ${\eta }^{CB}$ at $B=0.26\mathrm{T}$ and $n_{\mathrm{exc}}=1.20\times {10}^{16}\mathrm{c}{\mathrm{m}}^{-3}$. Dot splines are guides to the eye.