Crystal Structure Of Lt Gaas Layers Before And After Annealing
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CRYSTAL STRUCTURE OF LT GaAs LAYERS BEFORE AND AFTER ANNEALING Zuzanna LILIENTAL-WEBER Lawrence Berkeley Laboratory, I Cyclotron Rd., Berkeley, CA 94720 ABSTRACT The structural quality of GaAs layers grown at low temperatures by solid-source and gassource MBE at different growth conditions is described. Dependence on the growth temperature and concentration of As [expressed as As/Ga beam equivalent pressure (BEP)I used for the growth is discussed. A higher growth temperature is required to obtain the same monocrystalline layer thickness with increased BEP The annealing of these layers is associated with the formation of As precipitates. Semicoherent precipitates with lowest formation energies arc formed in the monocrystalline parts of the layers grown with the lowest BEP. Precipitates with higher bormation energies are formed when higher BEP is applied; they are also formed in the vicinity of structural defects. Formation of As precipitates releases strain in the layers. Arsenic precipitates are not formed in annealed ternary (InAlAs) layers despite their semi-insulating properties. The role of As precipitates in semi-insulating properties and the short lifetime of minority carriers in these layers is discussed. INTRODUCTION GaAs layers grown by molecular-beam epitaxy (MBE) at very low temperatures (- 200'C) (LT) have gained considerable interest as buffer layers for GaAs metal-semiconductor field effect transistors (MESFET's) due to high resistivity resulting in excellent device isolation.1, 2 By application of these layers, side gating/back gating can be removed. However, this semiinsulating behavior can be obtained only after annealing. These layers can be applied as well as surface passivation layers. In addition, fast photodetectors can be built based on LT-GaAs, since the minority carrier lifetime in this material is very short (in the range of a few hundred tfec). 3 Earlier studies by electron paramagnetic resonance combined with optical absorption ,4 reveal 1020 cm 3 AsGaantisite defects in as-grown layers, a defect concentration that decreases at least two orders of magnitude after annealing. These layers are grown from As supersaturation and show up to 1.5% excess As, which leads to -0.1 % of expansion of the lattice parameter. This expansion of the lattice parameter disappears after annealing. An important problem of LT-Layers is the breakdown of single crystal growth that is observed at too low temperature and/or too high As/Ga ratio in the layer. Since usually a device structure is grown on top of the LT-layers, their crystalline perfection is very important. This paper reviews recent work on the structure of LT-layers, along with new results of a systematic study of different growth and annealing conditions. GaAs LAYERS GROWN BY MBE It was noticed earlier that the crystalline perfection of the layers is very sensitive to growth parameters, 5 such as growth temperature and As/Ga ratio used for the growth [often called the As/Ga beam equivalent pressure (BEP)]. Some discrepancy in the substrate temperat
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