Far-Infrared Spectroscopy of Semiconductors in High Magnetic Fields

Abstract

The parameter (γ) which determines the effect of an applied magnetic field (B) on shallow impurity states is the ratio of the zero point cyclotron energy (l/2ħeB)/m* to the binding energy of the impurity (13.6 m*/mɛ2) where m* is the effective mass and ɛ the dielectric constant. The shallow donors in InSb have been used as a model system for investigating high field effects because of the ability to prepare high-purity samples relatively easily, the low effective mass, and a spherical conduction band which avoids the theoretical complexity of anisotropic bands. The parameter γ is equal to unity at a field of 0.2T and at about this field a metal-insulator phase transition occurs at liquid helium temperatures in high-purity material. At higher fields, sharp transitions between impurity levels can be observed spectroscopically in the far-infrared region of the spectrum and values of γ of about 100 can be achieved in steady fields available in the laboratory [1]. The only situation where such high effective fields are found with completely free electrons is close to the surface of collapsed stars where γ values of 1000 (=108T) can be found near to pulsars [2].