MISFIT DISLOCATION FORMATION IN LATTICE-MISMATCHED III-V HETEROSTRUCTURES GROWN BY METAL-ORGANIC VAPOR-PHASE EPITAXY

Misfit dislocation formation in lattice-mismatched III-V heterostructu res both under tensile and under compressive stress has been studied. Layers of GaAs under tensile stress have been grown by metal-organic v apour phase epitaxy on InxGa1-xAs substrates with indium concentration s between 0.1 and 1.1%. Compressively strained InxGa1-xAs layers with indium concentrations between 0.5 and 2.5% have been grown on GaAs sub strates. For the layers under tensile stress an asymmetrical dislocati on pattern has been observed, whereas the compressively strained layer s show a symmetrical dislocation pattern. A model describing the relax ation process by the formation of dissociated hexagonally shaped half- loop dislocations is proposed. A difference in the mobilities of the t wo possible misfit dislocation types is found to be the origin of asym metrical strain relief at low growth temperatures. In layers under ten sile stress the cross slipping of screw dislocations is counteracted b y the shear stress, leading to relaxation in only one direction. In la yers under compressive stress the nucleated misfit dislocations can un dergo cross slipping, resulting in a cross hatched pattern at the surf ace. At higher growth temperatures the dislocation patterns become mor e symmetrical due to the higher dislocation mobilities. Growth hillock s are formed on the surfaces of the layers grown under tensile stress, due to local accumulation of dislocations. This hillock growth is pre vented at higher growth temperatures by the higher mobility of the dis locations. It is also shown that the (mis)orientation of the substrate is revealed by non-parallel groups of dislocation lines observed at t he surface of a relaxed epilayer.