IMPROVED SUBSTRATE-TEMPERATURE STABILITY DURING MOLECULAR-BEAM EPITAXY GROWTH USING INDIUM FREE MOUNTING OF SMALL SUBSTRATES OF VARIOUS SHAPES

Large substrate surface temperature decreases are observed during mole cular beam epitaxy growth onto small indium bonded substrates, due to coating of the molybdenum block. These large temperature transients, a long with other difficulties associated with indium bonding (e.g., pot ential substrate surface damage, unwanted indium on the back of the su bstrate) make indium free substrate mounting desirable, however, indiu m free mounting systems have previously been restricted to whole stand ard-sized round wafers. In this study we compare the temperature chara cteristics of new indium free modular substrate mounting blocks, which can accommodate substrates of various sizes (up to 3 inch diameter) a nd shapes, with traditional indium mounting of small substrates. Altho ugh the new modular indium free holders have a large molybdenum surfac e area exposed to the molecular beams, we find that the substrate surf ace temperature transients during growth are reduced to nearly negligi ble levels versus similar growth on indium bonded substrates, due to t he effective thermal isolation of the substrate from the molybdenum. T he real effects of these temperature differences observed by in situ p yrometry were confirmed by ex situ deep level transient spectroscopy ( DLTS) measurements of homoepitaxial GaAs films, which show a two-to-th ree order of magnitude trap density increase in films grown on indium bonded substrates versus films grown on substrates mounted without ind ium bonding. This DLTS result is consistent with a real growth tempera ture difference between the two mounting techniques as was measured in situ by pyrometry. (C) 1996 American Vacuum Society.