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 Å) beyond the critical layer thickness as determined by the appearance of misfit dislocations. These mixed 60° dislocations surrounded by depletion regions were observed as straight dark lines in cathodoluminescence. The measured resistance Rs was higher in the [01̄1] direction than in the perpendicular [011] direction. At T=30 K the conduction ratio of these two directions exceeded 105 in the 300-Å-thick layer. The magnitude and anisotropy of Rs was correlated with the anisotropic dislocation patterns resulting from the preferential generation of the α dislocations ∥ [011] as compared to the orthogonal β dislocations ∥ [01̄1]. In both directions Rs depended exponentially on the number of dark lines perpendicular to the probing current. Simultaneously, the functional form of the temperature-dependent Rs(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 Rs with inverse temperature between 300 and 100 K. Below this range Rs(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.