ANISOTROPIC ELECTRICAL-CONDUCTION IN GAAS/IN0.2GA0.8AS/AL0.3GA0.7AS STRAINED HETEROSTRUCTURES BEYOND THE CRITICAL LAYER THICKNESS

The anisotropic conduction of GaAs/In0.2Ga0.8As/Al0.3Ga0.7As inverted high-electron-mobility transistor (HEMT) structures has been investigated. The heterostructures were grown by molecular-beam epitaxy on (100) GaAs substrates. The thickness of the pseudomorphic layer was increased stepwise (150-300 angstrom) beyond the critical layer thickness as determined by the appearance of misfit dislocations. These mixed 60-degrees dislocations surrounded by depletion regions were observed as straight dark lines in cathodoluminescence. The measured resistance R(s) was higher in the [011BAR] direction than in the perpendicular [011] direction. At T = 30 K the conduction ratio of these two directions exceeded 10(5) in the 300-angstrom-thick layer. The magnitude and anisotropy of R(s) was correlated with the anisotropic dislocation patterns resulting from the preferential generation of the alpha dislocations parallel-to [011] as compared to the orthogonal beta dislocations parallel-to [011BAR]. In both directions R(s) depended exponentially on the number of dark lines perpendicular to the probing current. Simultaneously, the functional form of the temperature-dependent R(s)(T) strongly varied with layer thickness. The thin, still elastically strained layers showed the usual behavior of HEMT structures. For the thicker layers a completely different temperature dependence was gradually developing, eventually leading to an exponential increase of R(s) with inverse temperature between 300 and 100 K. Below this range R(s)(1/T) changed more slowly and leveled off at 30 K. All these features are convincingly explained by a model assuming that the electrons can surmount the insulating depletion barriers in the conducting channel by a thermally induced tunneling mechanism.