We report optical studies in undoped GaAsN epilayers and InGaAsN quantum wells, which show that a strong electron spin polarization can persist at room temperature. Introducing less than 1% of nitrogen in the binary (GaAs) or ternary (InGaAs) alloy increases the spin relaxation time at T=300 K by a factor greater than 20 in as-grown material.

The spin properties have been investigated by cw and time-resolved photoluminescence (PL) experiments. The PL circular polarization degree can reach values up to 35-40% and this giant value persists within 2 ns, which exceeds the photoelectron lifetime by more than an order of magnitude [1-3]. Moreover, the application of a transverse magnetic field (~8 kG) or the change of photoexcitation polarization from circular to linear both result in a decrease of the PL decay time by a factor of 2.

We explain all these effects by a spin-dependent recombination process of free conduction electrons on deep paramagnetic centers. Since the capture of a free electron in the same spin state as the one of a localized electron is not efficient [4], the dynamic spin polarization of paramagnetic centers arises. This results in an increase of the average spin polarization of free electrons. The free-electron density is then entirely controlled by the spin relaxation in the conduction band. As the same spin relaxation drives the difference of spin-up and spin-down free electrons their giant spin polarization is constant in time.

[1] A.Y. Egorov et al., JAP 98, 13539 (2005)

[2] V.K. Kalevich et al., JETP Lett. 82, 455 (2005)

[3] L. Lombez et al., APL 87, 252115 (2005)

[4] C. Weisbuch et al., Solid State Com. 14, 141 (1974)

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1. Structural and optical properties of GaPN(As) and InGaPN layers grown by MBE