Microwave Spectroscopy on Two Dimensional Electron/Hole Gases

Abstract

We develop new techniques to explore how electrons and holes in 2D semiconductors behave under microwave radiation in a low temperature regime. To observe cyclotron resonance (CR) in GaAs/AlGaAs quantum wells, thermal methods with the sample and thermometer sealed in a vacuum can in a 3He environment and reflection spectroscopy via power sensing are developed. This configuration has high sensitivity and can detect CR with only 10nW of microwave power. It obviates the need for sampling thinning/wedging which is required in conventional transmission spectroscopy and hence preserves sample quality. With these advantages, we are able to detect narrow CR peaks with a small full width at half maximum (FWHM). Carrier effective masses are measured in various two dimensional electron gases (2DEGs) and two dimensional hole gases (2DHGs). Transport scattering time and single particle relaxation time are two important time scales that can be extracted from this measurement. The ratio of these times can be used as an indication of scattering angle scale limited by carrier mobility. The most exciting feature of this measurement is that a multi-photon phenomenon is observed for the first time in this system other than when using photoresistance measurements. This is made possible due to the use of millimeter waves, as opposed to the more traditional terahertz and infrared regimes used in earlier configurations. We further investigate the density dependence of the observable order of the multi-photon transitions and find that it is more prominent in higher density samples. By simultaneously measuring microwave reflectance and electrical resistance, we find that the plasmon-coupled CR mode is only present in the optical signal, not in the electrical. We attribute this discrepancy to the difference in the ability to pick up the corresponding signal in the scattering process. Though experimental data is very convincing, theoretical explanation is still needed to account for these phenomena.

We also analyzed the photoresistance of the InAs/GaSb inverted bilayer. This system is known for its marvelous helical edge state in the quantum spin Hall effect (QSHE). By tuning the back gate, the Fermi level can be positioned in the bulk gap where the conductivity only comes from the edge state. This is verified by taking measurements on a Corbino disk where the edge state is shunted. We find that the photocurrent is prohibited in the charge neutral point (CNP) due to the lack of spin flipping mechanisms. A finite magnetic field provides a source for this process from which we see an enhancement of the photocurrent. Further effort in observing gap opening and resonance with the Zeeman energy needs to be made to understand this interesting system. Therefore, better wafer quality is strongly desired.