Light-induced metal-insulator transition in n-GaAs/AlGaAs heterostructure: Acoustic methods of study

• Published in: Semiconductors (Woodbury, N.Y.) N.Y.), 2006-12, Vol.40 (12), p.1415-1422
• Main Authors: Drichko, I. L., D’yakonov, A. M., Smirnov, I. Yu, Toropov, A. I.
• Format: Article
• Language: English
• Subjects: ALUMINIUM ARSENIDES; DROPLETS; GALLIUM ARSENIDES; ILLUMINANCE; MAGNETIC FIELDS; MATERIALS SCIENCE; SURFACE AREA; SURFACES
• ISSN: 1063-7826; 1090-6479
• DOI: 10.1134/S1063782606120086

An n-GaAs/AlCaAs heterostructure 'underdoped' with Si, with the dark conductance of the 2D channel at T = 4.2 K lower than 10{sup -8} {omega}{sup -1}, is studied. By successive illumination with a LED, the conductance of the structure could be raised by five orders of magnitude, up to {approx}10{sup -3} {omega}{sup -1}, which allowed studies of the metal-insulator transition (MIT) in the same sample at the same temperature. A new method of MIT studies using acoustoelectric effects is proposed. These effects have been measured at T = 4.2 K under successive illumination of the sample without a magnetic field and in a field up to 6 T. The real, {sigma}{sub 1}, and imaginary, {sigma}{sub 2}, parts of the high-frequency (HF) conductance {sigma}{sup hf} = {sigma}{sub 1} - i{sigma}{sub 2} and their ratio {sigma}{sub 1}/{sigma}{sub 2} was determined. The percolation mechanism of MIT has been established. It is found that, up to {sigma}{sub 1} {approx} 10{sup -7} {omega}{sup -1}, the system is in the insulating state, and electrons are localized at the minima of the random potential. In this situation, the HF hopping conductivity mechanism dominates and is characterized by the relation {sigma}{sub 2} {>=} {sigma}{sub 1}. As the electron concentration increases, electron droplets become larger, and HF conductivity arises within these droplets. The conduction mechanism becomes mixed: the conduction by delocalized electrons within metallic droplets appears in parallel with hopping. As the conductance further increases, above 10{sup -5} {omega}{sup -1}, metallic droplets fill the entire conducting surface, and a metallic state with {sigma}{sub 2} = 0 appears. A curve demonstrating the dependence of the relative part of the surface area occupied by metallic droplets on the conductance of the 2D channel is plotted.