Photoemission of spin-polarized electrons from GaAs

The spin polarization of electrons photoemitted from (110) GaAs by irradiating with circlarly polarized light of energy $1.5<\mathrm{}\hslash \omega <3.6\mathrm{}$ eV was measured by Mott scattering. The GaAs surface was treated with cesium and oxygen to obtain a negative electron affinity (NEA). The spectrum of spin polarization $P\left(\hslash \omega \right)$ exhibits a peak ($P=40\%$) at threshold arising from transitions at $\Gamma$ and positive ($P=8\%$) and negative ($P=-8\%$) peaks at 3.0 and 3.2 eV, respectively, arising from transitions at $L \left(\Lambda \right)$. Anomalous behavior, consisting of a depolarization at threshold and an increase and shift in the peak polarization to 54% at 1.7 eV, is attributed to a small positive electron affinity (PEA) characteristic of some samples. Restriction of the photoelectron emission angle by the PEA leads directly to the anomalously high $P$. Results of calculations show that $P$ cannot be increased above 50% for emission arising from transitions at $\Gamma$ in NEA GaAs. Our detailed interpretation of the spectra indicates how spin-polarized photoemission can be used to study the spin-dependent aspects of electronic structure. The outstanding qualities of NEA GaAs as a source of spin-polarized electrons are discussed and compared with other sources.