Luminescence imaging of photoelectron spin precession during drift in a p-type GaAs microfabricated Hall bar

Abstract

Using a microfabricated, p-type GaAs Hall bar, is it shown that the combined application of co-planar electric and magnetic fields enables the observation at 50 K of spatial oscillations of the photoluminescence circular polarization due to the precession of drifting spin-polarized photoelectrons. Observation of these oscillations as a function of electric field E gives a direct measurement of the minority carrier drift mobility and reveals that, for E = 800 V/cm, spin coherence is preserved over a length as large as 25µm. PACS numbers:In the context of future active spintronic devices, the diffusion [1, 2] and drift [3][4][5][6][7][8] of spins in semiconductors have been investigated by numerous authors. N-type GaAs seems particularly promising because of the weak spin relaxation [9], but p-type material cannot be overlooked since in proposed bi-polar spintronic devices [10], it is the minority carrier (electron) spin that determines the common base current gain. While in p-type material charge transport has been investigated [11][12][13], very few studies have considered spin transport [14,15]. Timeresolved investigations have been used [16] but these investigations do not provide a direct spatial imaging. On the other hand, continuous wave (CW) imaging of spin transport, using luminescence [17], or Kerr microscopy [1] gives diffusion and drift lengths from which it is possible to obtain µτ for the charge transport where µ is the mobility and τ is the lifetime, or µτ s for spin transport where τ s is the spin lifetime. Consequently, CW determination of mobilities or of diffusion constants requires separate determinations of lifetimes [2].In the present work, we investigate spin transport in p-type GaAs using a CW technique in which precession in a magnetic field tranverse to the light excitation acts to effectively time-resolve the experiment so that the relevant parameters for spin drift transport can be determined. As shown in Fig. 1, in the same way as performed elsewhere using Kerr microscopy [18,19], we use a microfabricated Hall bar, fabricated from a 3 µm -thick p-type GaAs sample (acceptor doping range 10 18 cm −3 ), with the long axis along which the electric field is applied parallel to the [110] crystallographic direction [20]. The maximum value of of the electric field used here (800 V/cm) is well below that required to saturate the drift velocity at low temperatures [21]. The photoelectrons are generated by a tightly-focussed 12 µW CW excitation by a laser at 1.59 eV, so that transport is not affected by ambipolar effects [14,20], or by Pauli blockade [15,22]. During electron drift in the electric field E, their spin precesses in a magnetic field B (0.23 T) applied in the sample plane. Spin precession results in coherent spatial oscilla- tions of the spin density, which are observed by monitoring the degree of circular polarization of the luminesence in the direction parallel to the photo-excitation wavevector. Qualitatively, the spatial period l of the oscillations is related to the photoelectron...