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-electron g factors in lattice-matched GaAs/Al0.35Ga0.65As and strained-layer In0.11Ga0.89As/GaAs quantum wells is investigated for well widths between 3 and 20 nm. The magnitude, sign, and anisotropy of the g factors were obtained from quantum beats due to Larmor precession of electron spins in time-resolved, polarization-sensitive, pump-probe reflection at 10 K in magnetic fields applied along and at 45 degrees to the growth axis. Slowly varying shifts 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 fixed circular pump polarization and oblique applied fields. These Overhauser shifts confirmed the sign of the g factors and were eliminated by modulation of pump polarization to give precise g factors. For both material systems, variation of the g factor with well width follows qualitatively the dependence on energy, determined by quantum confinement, calculated from three-band k.p theory in the bulk well material. For the lattice-matched system there is excellent quantitative agreement with a full three-band k.p calculation including anisotropy effects of the quantum-well potential. For the strained-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.