Far infrared reflection spectroscopy of layered semiconductors

Abstract

The optical properties of a selection of layered semiconductor samples have been studied in the far-infrared by reflection dispersive Fourier transform spectroscopy (DFTS) and attenuated total reflection (ATR) spectroscopy in DFTS the reference mirror in a two-beam Michelson interferometer is replaced with the sample to obtain a dispersive interferogram. This technique allows both the amplitude and phase to be measured directly. The amplitude and phase reflectivities of a variety of epitaxial layers (including a GaAs/AlAs superlattice)on GaAs substrates are presented. The measurements on the epitaxial layers are in good agreement with theoretical results obtained using standard multilayer opticalcalculations.ATR spectroscopy, which permits the study of nonradiative transitions, has been used for the first time tostudy surface phonon-polar itons on a GaAs/AlxGa-xAs multiple quantum well (MQW) structure, as well as a CdTe epitaxial layer deposited on a GaAs substrate. Both P- and S-polarised measurements have been made with a far infrared FT spectrometer fitted with an ATR stage at the output port. In the MQW sample, the experimental results are compared with theoretical calculations derived by treating the MQW structure as an effective uniaxial dielectric medium. The measured and theoretical results are in excellent agreement both qualitatively and quantitatively. Surface phonon-polaritons are observed in P-polarisation but only bulk modes are seen in S-polarisation as predicted by theoretical calculation. For the CdTe epitaxial sample, surface phonon-polaritons are observed by ATR in P-polarisation in the CdTe reststrahl region. In S-polarisation, a sharp guided wave is observed in the GaAs reststrahl region. The results for both polarisations are in good agreement with calculations.