Anisotropy of the electron g factor in lattice-matched and strained-layer III-V quantum wells

The influence of quantum confinement and built-in strain on conduction-elec tron g factors in lattice-matched GaAs/Al0.35Ga0.65As and strained-layer In 0.11Ga0.89As/GaAs quantum wells is investigated for well widths between 3 a nd 20 nm. The magnitude, sign, and anisotropy of the g factors were obtaine d from quantum beats due to Larmor precession of electron spins in time-res olved, polarization-sensitive, pump-probe reflection at 10 K in magnetic fi elds applied along and at 45 degrees to the growth axis. Slowly varying shi fts of precession frequency, due to buildup of nuclear polarization in the samples over similar to 1 h and equivalent to up to 0.5 T, occurred for fix ed circular pump polarization and oblique applied fields. These Overhauser shifts confirmed the sign of the g factors and were eliminated by modulatio n of pump polarization to give precise g factors. For both material systems , variation of the g factor with well width follows qualitatively the depen dence on energy, determined by quantum confinement, calculated from three-b and k.p theory in the bulk well material. For the lattice-matched system th ere is excellent quantitative agreement with a full three-band k.p calculat ion including anisotropy effects of the quantum-well potential. For the str ained-layer system, detailed quantum-well calculations do not exist but k.p theory for epitaxial layers predicts 10 times greater anisotropy for wide wells than we observe. This discrepancy is also apparent in previous, less complete, investigations of strained-layer systems and highlights the need for further theoretical effort.