Admittance measurements of magnetic freezeout in n - Type GaAs

Abstract

Admittance measurements on an n - type GaAs/AlxGa1-xAs/n+-type GaAs heterostructure (an AlxGa1-xAs capacitor) are used to study the dependence of the activation energy for ac transport, EI, on magnetic field B. The capacitor studied has a substrate doping NS∼7×1015 cm-3, which is about one-half the doping for the metal-insulator transition in n-type GaAs. For B6 T there is a magnetically induced Gray-Brown (GB) dip in capacitance-voltage curves at the flatband voltage. By analogy with thermal freezeout of donors or acceptors, the presence of a GB dip shows that magnetic freezeout occurs. Thermal admittance measurements at fixed bias, fixed magnetic field, and variable temperature and frequency show that EI is proportional to magnetic field. An alternative method, magnetic admittance measurements at fixed bias, fixed temperature, and variable magnetic field and frequency, shows that energy can be expressed as EI=αμBB, where μB is the Bohr magneton and α is temperature dependent. The measurements support a model in which an impurity band in n - type GaAs is split into a lower energy band and a higher energy band. © 1992 The American Physical Society.