High Pressure Far-Infrared Magneto-Spectroscopy of Impurity and Free Electron States in Silicon-Doped Gallium Arsenide Using a Novel Diamond-Anvil Apparatus.

This thesis contains two parts. In the first part, a novel diamond-anvil cell (DAC) probe for far-infrared (FIR) magneto-spectroscopy in a small-bore magnet is designed and built. A miniature multi-bellows ram is invented to generate amplified force for in situ pressure tuning of the DAC. A constructed 26-mm diameter double-bellows ram is capable of producing 5kN force (scalable to 10kN with four bellows) when driven by ^4He at 4.2K. Signals are enhanced using paraboloidal cones to focus the FIR radiation on the sample and collect the transmitted signal. FIR magneto-transmission down to 80cm ^{-1}, and photoluminescence can be measured on the same sample under the same pressure -field-temperature conditions, independently tunable in the ranges 0-20GPa (in steps as small as 0.05GPa), 0-15T, and 2-300K. In the second part, Si impurity states in nondegenerate GaAs are investigated under pressure via observing the 1s-2p_{+} transition of Si-donors at 4.2K by FIR Fourier transform magneto-spectroscopy (FTMS) and laser magneto-spectroscopy using the novel probe. When DX-centers are never populated under visible illumination, the Si-donors retain their effective-mass nature up to 36.6kbar, above which the 1s-2p_{+ } peak is quenched due to the crossover of 1s(Gamma) and 1s(X) Si-levels. The A_1 localized Si-level remains a high energy resonance for the entire direct-gap regime. When DX-centers are populated by cooling (200K to 4.2K) in the dark under 55kbar, photoionizing the DX-center forms a new Si-center that does not revert to the shallow donor configuration and can not be persistently photoionized at low temperatures. The new center competes with the shallow Si-donors at different Si-sites via the conduction band, and begins to quench the 1s-2p _{+} intensity at ~ 30kbar without anticrossing. Possible defect reactions are proposed to explain the new center. The pressure dependence of the electron effective mass (at {bf k}=0) in GaAs is measured via cyclotron resonance (CR) by FTMS at 17K up to 36.8kbar, above which the CR peak is quenched again due to the 1s(Gamma )-1s(X) crossover. The data are corrected for the band nonparabolicity using a three-band kcdot p calculation. Our results agree well with prior measurements extending only to 17kbar and with the prediction of the three-band kcdotp model.