Room temperature spin injection into (110) GaAs quantum wells using Fe/x-AlO_x contacts in the regime of current density comparable to laser oscillation

We investigate the electrical spin injection into (110) GaAs single quantum wells (SQWs) and multiple quantum wells (MQWs) using light-emitting diodes (LEDs) having Fe/crystalline-AlO_x (x-AlO_x) tunnel barrier contacts. A degree of circular polarization (P_c) of 5.0% is obtained for the SQW LED at 4 K with the current density of 1 kA/cm² which is comparable to that for the laser oscillation in vertical-cavity surface-emitting lasers (VCSELs). On the basis of electron spin relaxation time and carrier lifetime in the (110) GaAs SQW measured by time-dependent photoluminescence and the value of P_c = 5.0%, the degree of spin polarization of initially injected electrons (P₀) in the SQW is estimated to be 6.6% at 4 K. By using the MQW LED having a much stronger electroluminescence, a P_c value of 2.6% is obtained at room temperature (RT) with the current density of 1.5 kA/cm². The temperature and current density dependences of P_c are found to be weak in both the SQW and MQW LEDs. The estimated P₀ of 9.3% at RT suggests that the Fe/x-AlO_x contacts can be used for the RT electrical spin injection for spin-controlled VCSELs.