Far infrared spectroscopy of shallow donors in gaas/algaas multiple-quantum-well structures

Far infrared (FIR) spectroscopy has been demonstrated to be a powerful tool for studies of the electronic properties of semiconductors, particularly in investigation and characterization of shallow impurities. Recent advances in growth techniques have permitted the fabrication of repeated semiconductor heterostructures (quantum wells and superlattices) with interface abruptness on an atomic scale. For the case of Quantum Wells, simple considerations indicate that the electronic states of shallow impurities in such confining structures depend strongly on the width of the wells and/or the impurity location along the growth direction. In the present paper, recent experimental work on shallow donor impurities in GaAs/AlGaAs multiple-quantum-well (MQW) structures is reviewed. Absorption and photoconductivity studies were carried out with a FIR Fourier Transform Spectrometer in magnetic fields up to 9T. MQW samples nominally doped with Si donors in the well centers and at the well edges have been investigated for QW widths between 80Å and 450Å. Results are generally in very good agreement with recent theoretical calculations. Possible use of such measurements to determine the impurity distribution along the growth direction is discussed.