FAR-INFRARED PHOTOCONDUCTIVITY AS A PROBE FOR NON-EQUILIBRIUM PHASE TRANSITIONS IN n-GaAs

In high purity n-GaAs at low temperatures impact ionization breakdown occurs at an electric field of a few volts per cm resulting in a rapid increase of the current at a critical electric field strength [1,2]. The transition from the low conducting state, where almost all electrons are bound to shallow donors, to the high conducting state, where almost all donors are ionized, can be considered as a phase transition in an open system far from thermal equilibrium [3,4]. The steady state concentration of electrons n corresponds to the order parameter; and the kinetic coefficient of Impact ionization X is the control parameter comparable to the temperature in equilibrium phase transformations. The single electron excitation probability X s due to thermal excitation takes the place of an external force conjugate to the order parameter. In this paper we report on measurements of the cyclotron resonance induced far-infrared photoconductivity in n-GaAs as a function of the electric bias field. Close to the threshold field a pronounced peak of the photosignal is observed. The experiments are discussed by means of a simple generation-recombination model showing that the photoconductivity probes the generalized susceptibility X = dn/d-Xs of the non-equilibrium phase transition. In the limiting case of vanishing thermal excitation, X approaches a Curie-Weiss law diverging for a critical value X c of the control parameter. The experiments were carried out on high purity epitaxial layers mounted in a metallic light pipe and