ADMITTANCE MEASUREMENTS OF MAGNETIC FREEZEOUT IN N(-)-TYPE GAAS

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, E(I), on magnetic field B. The capacitor studied has a substrate doping N(S) approximately 7 X 10(15) cm-3, which is about one-half the doping for the metal-insulator transition in n-type GaAs. For B greater than or similar to 6 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 E(I) 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 E(I) = alphamu(B)B, where mu(B) is the Bohr magneton and alpha 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.