Picosecond spin relaxations of acceptor‐bound exciton and A‐band free exciton in wurtzite GaN

Abstract

The spin relaxation process of acceptor-bound excitons in wurtzite GaN is observed by spin-dependent pump and probe reflectance measurement with subpicosecond time resolution. The time evolutions measured at 15-50 K have a single exponential component corresponding to the electron spin relaxation time of 1.40-1.14 ps. These spin relaxation times are slightly longer than those of the A-band free excitons of 0.47-0.25 ps in GaN at 150-225 K. The spin relaxation time is found to be proportional to T -0.175 , where T is the temperature. This weak temperature dependence indicates that the main spin relaxation mechanism is the Bir-Aronov-Pikus process. 1 Introduction Spin dynamics in semiconductors has been investigated with great interest, not only for fundamental physics but also for the application of spin-dependent optical nonlinearity such as spinmemory, spin-transistor and spin-switch devices. The picoseconds spin relaxation process in semiconductors became observable in the 1990s, using time-resolved spin-dependent pump and probe measurement [1] and time-resolved photoluminescence (PL) measurement with a streak camera system [2,3]. These experimental results revealed that the spin relaxation process decays rapidly in a few or a few ten picoseconds [1,4]. The application of this fast spin relaxation to ultrafast switching devices has been attempted [5][6][7]. In addition, because of the recent discovery of the considerable effect of spin polarization on vertical-cavity surface-emitting lasers [8], information on spin behavior has become important in the development of optical devices.The III-nitride-based semiconductors have been intensively investigated owing to strong interest in their application to optical devices such as the blue laser [9]. Investigations on spin dynamics in the GaN system are expected to yield information on the applicability of spin-related phenomena in the GaN system [10][11][12][13][14]. We have reported spin relaxation times of A-band free exciton in wurtzite GaN of 0.47-0.25 ps at 150-225K [11]. In cubic GaN, we have observed a long spin relaxation time of over 5 ns at 15 K [14]. The spin relaxation time is strongly affected by the band structure and the exciton state. In this paper, we report the observation of the spin relaxation process of acceptor-bound excitons in wurtzite GaN using time-resolved spin-dependent pump and probe reflectance measurement with subpicosecond time resolution.A shallow neutral acceptor-bound exciton (ABE) has a two-hole state derived from two holes in the topmost valence band [15]. Only one such state with J = 0 is allowed by the Pauli principle in wurtzite semiconductors. The additional electron in the ABE state then contributes to its unpaired spin; thus, this