Quantum transport and cyclotron resonance study of Ge/SiGe quantum wells in high magnetic fields

Shubnikov-de Haas oscillation and cyclotron resonance were studied for high mobility p-type Ge channels in strained Ge/Si1-xGex quantum wells (well width was 7.5 nm). The samples were grown on Si (001) substrates by gas source MBE. The width of the quantum wells was 7.5 nm. Pulsed high magnetic fields up to 50 T were employed for the measurement. Fine quantum oscillations were observed in rxx, although the quantum Hall plateaux in rxy were slightly deformed due to the pulsed field measurements. Reflecting the quantum effect in the nearly degenerate valence bands, the Fourier transform of the oscillatory spectra consist of several peaks. From the temperature dependence of the oscillation amplitude of a peak at B = 11 T, the effective mass was obtained as 0.17 m0. Cyclotron resonance was measured for two samples with different Ge concentration x which provides different strain. Sample A is the same as the one studied in the transport experiment (x = 0.41), and the sample B has the larger Ge content (x = 0.53). Photon energy was varied in the range between 10 – 17 meV using a FEL of ELBE. Two well-defined resonance peaks were observed in both samples. The effective mass obtained from the peak at the low field side at the lowest photon energy (10.5 meV) is 0.175 m0 and 0.154 m0 for samples A and B, close to the value obtained from the Shubnikov de-Haas experiment. However, the photon energy vs. resonance field Br curves are strongly non-linear in the higher energy. They are super-linear below photon energy of 14.1 meV and a sudden jump of the Br is observed at 16.5 meV, implying the existence of a prominent break of the curves. These results should be analyzed by the calculation of Landau levels in the degenerate valence bands of the quantum wells with strain.