Morphological Evolution and Properties of LPE Grown GaSb, AlGaSb and AlGaAsSb
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the charge preparation cycle increases the incorporation of As in the layer [7]. Thus, the pre-growth dissolution sequence is critical for growing lattice matched layers of AIGaAsSb with high Al content. We have employed two different pre-growth dissolution sequences for the growth of AlGaAsSb. In the first procedure, initially GaAs is dissolved at - 750'C and the melt is cooled down to room temperature. After this, the substrate is loaded in the boat and the other components are added to the melt. The temperature of the system is then raised to - 5VC above the growth temperature and allowed to equilibriate for a period of three hours. In the second method, the charge is prepared by adding Al and GaSb to Ga-melt and by keeping a wafer of GaAs on the top of the melt. The equilibriation is carried out at 5oC above the growth temperature for three hours. The growth experiments were carried out using the supercooling method. The layer compositions have been determined by the Energy Dispersive Analysis of X-rays (EDAX). RESULTS AND DISCUSSION Nucleation morphology of GaSb Our initial studies of growth of GaSb was carried out by the equilibrium cooling technique. Fig. 1(a) shows the photomicrograph of the layer grown on a 70 off (111) misoriented surface from Ga-rich melt at 350 0 C. In general, growth at temperatures below 400'C gave layers with large number of islands, fairly deep undulations and gallium inclusions across the surface similar to that in Fig. 1(a). The poor morphology on highly misoriented substrate is typical for all LPE grown layers [8]. In our case, the problem was probably further enhanced by inadequate wetting and presence of thin oxide film on the substrate due to inefficient reduction of oxides by hydrogen at low temperatures.
FIG. 1. Surface morphologies of GaSb epilayers grown from Ga-rich melts. (a) on 70 off (111) substrates at 350°C, (b) on 7r off (111) substrates at 4500C and (c) on (100) substrates at 550 0C
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Fig. 1(b) shows the morphology of layer grown at 450 0 C. The layer exhibit ripples or usual tide-like pattern, known as terraces. With increase in growth temperature, the width of. the terrace decreases. The temperature at which high quality surfaces are obtained with mirror finish was found to be around 5500C. Furthermore, if the growth was carried out at 5500C on (100) or (110) surface, the quality of the grown layers was even better as shown in Fig. 1(c). The orientation dependence of the layer morphology is understandable as the surface atomic arrangements are different. Usually lamellar growth occurs on the misoriented (111) surfaces [8]. The lammelle are believed to correspond to coalesced low-index growth steps and the shape and spacing of the lamelle are determined by the magnitude of the misorientation from the polar axis. The presence of gallium inclusions in the layers grown at low temperatures can be due to constitutional supercooling or cellular convection in the growth solution [9]. The disappearance of irregularities at high growth temperatures is attributed to the
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