Dislocations, twins, and cracks in In1–xGaxP/(001) GaAs heteroepitaxial layers

Abstract

The occurrence of dislocations, twins, and cracks in In1–xGaxP (0.42 ≦ x ≦ 0.75), i.e. layers grown under compression and tension on (001) GaAs substrates of different misorientation (0°, 2°, 6°) towards [010], is studied for layers of various thicknesses (1 to 2.7 μm). On the surface of layers grown under tension corrugations due to preferred slip on (111) and (111) of the [110] zone can be observed. Additionally, fracture occurs at higher misfit strain, but in [110] direction, i.e. the cleavage plane is (110). The well-defined surface corrugations parallel to [110] correlate to twin lamellae located on inclined {111} slip planes. Twin growth proceeds by nucleation and propagation of 90° Shockley partial dislocations from a surface-near region towards the layer-substrate interface, leaving a less mobile 30° partial behind (in the case of tension). It seems that Maree's conception of spontaneous half loop nucleation at the free surface of a growing layer with critical thickness is favoured. In layers grown under compression the same model is used to explain the formation of dislocation networks. In this case the front segment of half loops generated at the free layer surface consists of a leading 30° Shockley partial and the 90° one trailing behind. Because of the lower mobility (higher friction force) of the first one, the twin formation is suppressed. From simple crystallographic arguments (InGaP belongs to the space group F 43m) it is concluded that in layers grown under tension partial dislocations with excess In/Ga atoms in their core can either be generated or be moved more easily than dislocations with excess P atoms. In layers grown under compression, on the contrary, partials with excess phosphorus in their core can be either generated or be moved more easily than partials with excess In/Ga atoms. Using the model of spontaneous half loop nucleation at the growing surface the demarcation line between the dislocation- and twin-free growth and that where defects occur is calculated and determined experimentally for deposition temperatures between 525 to 750°C. Moreover, the fracture toughness and precursor crack length for layers grown under tensional stress conditions are estimated using a modified Griffith equation and are KIC = 0.696 MPa m1/2 and lc = 216 nm (at 720 °C). Twin formation begins at 720 °C if a critical strain of ϵc = 3.1 × 10−3 and, therefore, a critical shear stress of τc = 87.5 MPa is exceeded.