Structural Decomposition at Sn/GaAs(001) Interface
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ABSTRACT Morphological studies of the system Sn/GaAs(001) are required to interpret electronic anomalies of devices which include doped III-V MBE grown thin films. Previous studies focused on the dynamics of Sn on As terminated surfaces. The current study complements those studies on a Ga terminated surface, i.e., conditions which may occur locally for short periods during MBE growth. Sn is deposited on in-situ cleaned Ga rich GaAs(001) surfaces and the evolution of morphological structures is presented as a function of annealing temperature and Sn supersaturation. Three distinct regimes were observed, (i) a low temperature range (T < 5000C) where Sn shows a propensity toward three-dimensional clustering, (ii) an intermediate temperature range (500°C _ T < 6250C) where a transition in the morphology occurs due to strong interactions with the substrate resulting in the formation of characteristic etch-patterns and (iii) a high temperature regime (T __6500C) where the thermal decomposition of GaAs dominates while the deposited Sn desorbs fast. INTRODUCTION Sn incorporation during GaAs epitaxy is of technological interest due to the electronic properties of Sn as a dopant. Previous studies of the system Sn/GaAs(001) [1] showed, however, that Sn co-deposition does not lead to a simple random incorporation into the lattice but that the metal component undergoes a phase separation on the surface resulting in clustering phenomena. These effects have to be studied in more detail before it is possible to optimize the growth conditions for such layers, in particular, if Sn incorporation as a delta-doping layer is intended which requires an interrupted GaAs growth and the deposition of a fraction of a monolayer of Sn on the surface. EXPERIMENT GaAs samples were ex-situ chemically treated to clean the surface and to prepare a well-defined oxide which was then flashed off at 5850C after transfer into the ultra-high vacuum thin film growth system (base pressure < 2 x 1010 mbar). For sample handling the GaAs wafers were mounted with In on Mo backings and heated radiatively. During growth or post-deposition annealing the temperature was held within ± 1 degree. The absolute temperature is controlled by the heater current and the calibration is considered accurate to within ± 15 degree. Sn was deposited from a Knudsen cell with typical rates of 3 x 1014 atoms/min. For post-deposition studies, GaAs(001) surfaces were cooled down to below 1000C after the thermal oxide removal and prior to Sn deposition. Typical annealing temperatures for the present morphological study is in the range from 4500C to 7000 C. The total amount of deposited Sn was varied between 3 and 30 equivalent monolayers coverage to study the effects of coverage on the growth dynamics.
171 Mat. Res. Soc. Symp. Proc. Vol. 399 01996 Materials Research Society
RESULTS Fig. 1 shows several SEM micrographs for post-deposition annealed Sn films on GaAs(001) as a function of annealing temperature and initial Sn coverage. As a function of temperature, three distinct regime
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