Impurity and Stoichiometry Control in Atomic Layer Epitaxy
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IMPURITY AND STOICHIOMETRY CONTROL IN ATOMIC LAYER EPITAXY
H.Yokoyama, K.Ikuta, and N.Inoue NTr LSI Laboratories 3-1, Morinosato Wakamiya, Atsugi-shi, Kanagawa 243-01 Japan
ABSTRACT We investigate the intrinsic point defects in epilayers grown by atomic layer epitaxy (ALE). Ga vacancies and antisite As atoms in the epilayers are detected by photoluminescence spectroscopy. This shows that the ALE epilayer was grown under As-rich conditions. We propose increasing the TMG flux to reduce the number of point defects. With this method, the number of point defects in ALE epil.ayers can be decreased to less than that in conventionally grown epilayers. Moreover, it is-found that these point defects are formed by the incomplete Ga coverage, not by the steric hindrance as previously suggested. The carbon concentration is decreased by one order of magnitude by using nitrogen instead of hydrogen as the carrier gas. As an application of this low defect density, we fabricated a GaAs/AlAs resonant tunneling diode and observed the negative resistance at room temperature. INTRODUCTION Atomic layer epitaxy is very attractive as a layer-by-layer growth technique because it enables us to control the film thickness down to the atomic level and obtain an abrupt interface. This technique was first developed by Suntola in 1974111 using a I1-VI compound. Later, Nishizawa successfully applied this technique to III-V compound semiconductor growth.121 However, there are some serious problems with ALE that have prevented it from becoming a truly practical growth technique. One is the narrowness of the ALE window, which is the range of substrate temperature at which ALE occurs. Another problem is the fact that AlAs, which is indispensable for making heterostructures with GaAs, has not been obtained. The other problems are point defects. There is a possibility that the epilayer does not maintain stoichiometry. The steric hindrance of large MO molecules,r3l and their incomplete adsorption and accelerated desorption may cause incomplete coverage of column-Ill materials. High carbon contamination is also the problem.f 31 Although much progress has been made recently, some problems still remain unsolved. In particular, there have been no reports on non-stoichiometry in the ALE epilayer until recently. This paper investigates the point defects in the ALE epilayer. We discovered the presence of Ga vacancies and As antisites. We discuss the incorporation mechanisms of intrinsic point defects and carbon impurities, and propose methods to reduce these. Finally, we demonstrate negative resistance in GaAs/AlAs resonant tunneling diodes grown with these defect control techniques. EXPERIMENT Growth Conditions The growth apparatus is a conventional vertical-type, low-pressure (70 Toff) metalorganic chemical vapor deposition (MOCVD) reactor.141 The growth was carried out by the alternate supply of trimethylgallium (TMG) or trimethylaluminum (TMA), and 10% arsine diluted in hydrogen. The source gases were supplied for 1-5 s and purged for 3 s. The substrate te
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