Exciton spin relaxation in GaN observed by spin grating experiment

Abstract

The authors studied the exciton spin relaxation of bulk GaN by creating spin polarization gratings using degenerate four-wave mixing spectroscopy. The spectrally resolved analysis achieved with this technique facilitated the direct evaluation of spin polarizations in the individual excitons ͑A and B excitons͒. The spin polarizations for each exciton decay very quickly ͑ s ϳ 1 ps͒ at low temperatures. Moreover the s is faster than the dephasing time T 2 throughout the measured temperature range, suggesting the existence of fast intrinsic spin relaxation processes, which can be attributed to a large exchange constant characterized in GaN. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2430402͔Because of the technological importance of spin-based electronics, so-called spintronics, 1 considerable effort has recently been devoted to understanding the optically induced spin polarization in GaN and related alloys, both experimentally 2-5 and theoretically. 6-9 A noteworthy issue is the significant variation in the spin relaxation time obtained in previous experiments; in Ref. 2, a long electron spin coherence of up to 20 ns was observed in Si-doped GaN by using time-resolved Faraday rotation measurements, while short ͑subpicosecond͒ excitonic-spin relaxation times were observed in a GaN epilayer by using spin-dependent transient reflectivity change measurements. 3 Although theoretical work has suggested a significant influence of the sample quality on the spin dynamics in semiconductors, 7 the mechanism that causes the difference between these two results is still unclear.In this work, we studied exciton spin relaxation in bulk GaN by using spectrally resolved four-wave mixing ͑SR-FWM͒ measurements. Strikingly fast spin relaxations, which were even faster than the dephasing times T 2 , were observed for each exciton resonance at low temperatures. The relaxation time in an A exciton ͑X A ͒ decreases with increasing temperature, and then reaches the experimental resolution limit above 100 K. The results can be understood in terms of an excitonic effect in GaN. Figure 1͑a͒ is a schematic illustration of the experiment. In order to evaluate the spin lifetime in the spectral domain, we employed a three-pulse FWM measurement. 10-12 A frequency doubled, mode-locked Ti:sapphire laser which produced a pulse width of ϳ200 fs ͑spectral width of ϳ13 meV͒ at a repetition rate of 76 MHz was employed as an excitation light source. Two simultaneous excitation pulses and one probe pulse with a controlled time delay were focused at the same point on the sample surface using a lens ͑f = 200 mm͒. These three pulses have almost the same power of about 0.3 mW. The pump pair consisting of crosslinear ͑colinear͒ polarizations of wave vectors k 1 and k 2 forms an exciton spin polarization ͑population͒ grating ͓Fig. 1͑b͔͒ which mainly decays via the exciton spin relaxation ͑recombination͒ process. 13 A third probe pulse of wave vector k 3 diffracts off this grating to produce the FWM signal in the k 4 = k 3 ϯ k 2 ± k 1 direction, which ...

Study on dephasing dynamics of Exciton Fine Structure in GaN

“…It is important to note that the dephasing times obtained above are much faster than the exciton lifetimes evaluated from the transient pump-probe and the time-resolved photoluminescence measurements, but are still longer than the spin relaxation time [14]. This excludes the scattering-induced relaxation mechanisms because in this case the dephasing time is limited by the spin relaxation time [15]. The dominant dephasing process is thus the intrinsic spin-flip processes of GaN excitons, which are possibly caused by the large exchange interaction.…”

Dislocation-enhanced spin relaxation in wurtzite GaN

“…[1] and reference therein), few experimental data can be found in the literature concerning the experimental investigation of spin physics in nitride compounds [2][3][4][5][6]. Their weak spin-orbit coupling and large exciton-binding energy (respectively, $17 and $26 meV in bulk GaN) make them promising candidates for spintronic applications, allowing the control and manipulation of the exciton spin, even at high temperature.…”

Four‐wave‐mixing study of exciton fine structure in GaN

“…We provided some profound insights on the dephasing dynamics of EFS by using a spectrally-resolved four-wave-mixing (SR-FWM) technique [5,6]. The coherent spin dynamics of bulk GaN exciton was also investigated by using a transient spin grating measurement [7]. There, the spectrally-resolved transients show extremely fast decay within a few ps, which can be explained by the efficient spin-flip process between electron and hole due to the large exchange constant of GaN.…”