Enhanced spin lifetime in semiconductors with applied electric fields

Abstract

AThe GaAsP/GaAs superlattice (SL) structure has been widely recognized as the most efficient spin polarized electron source with 90°0 maximum polarization and more than 1% quantum efficiency [1]. The main spin depolarization mechanisms in these structures are: interband absorption smearing due to bandedge fluctuations, hole scattering between the heavy hole (HH) and light hole (LH) states that causes a broadening of the LH band, spin precession due to an effective magnetic field generated by the lack of crystal inversion symmetry and spin orbit coupling i.e. the Dyakonov-Perel mechanism (DP) and electronhole scattering. The last 2 mechanisms are material related and they take place during the transport of electrons in the photocathode active region. By decreasing the transport time of the electrons in the lOOnm cathode active region, all depolarization mechanisms are suppressed and the total number of scattering events also decreases. Based on the scattering rates in GaAs as a function of energy, it is possible to minimize the total number of scattering events when electrons have energies in the 0.05-0.1eV region. Electrons can acquire such energies inside the active region during the presence of accelerating fields of 0.1-V/lOOnm. For such fields, the cathode area can be completely depleted when the doping concentration of the active region is _1017cm-3.A metal grid was deposited on lOOnm GaAs based with different structure and doping concentrations. The grid formed a Schottky contact with the semiconductor film. I-V curves, quantum efficiency (QE) and spin polarization measurements were performed for various samples. The results are presented in this paper. 2 -0 4 02 00 0 4 05 0 Bias Vcotage (V) Bi'S Volt tiage (V) Figure 1. Spin polarization enhancement due to applied electric field on 1017cm-3 p-doped lOOnm (a)GaAs/GaAsP sample with Re grid and (b) GaAs sample with W grid. The spin polarization of the photogenerated electrons was measured when circularly polarized laser light was illuminating the samples.Further improvements in observing the bias effect on the polarization can be achieved by fabricating more stable Schottky contacts at the cathode surface and using samples with high polarization such as strained GaAsP/GaAs superlattices.