Surface Reconstruction and Morphology of Hydrogen Sulfide Treated GaAs (001) Substrate
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not provided commonly. To obtain a high quality ZnSe layer on GaAs, it is very important to control the surface condition of the GaAs substrates. As one of the solutions, a two chamber molecular beam epitaxy (MBE) system is used to grow a GaAs epitaxial buffer layer and to control the surface stoichiometry. Recently, it was reported that (2 x 4) As-stabilized GaAs surface followed by pre-irradiation of Zn brought layer-by-layer growth of ZnSe and very high quality ZnSe epilayers[1]. As other approaches, surface treatment techniques such as ammonium-sulfide treatment[2J and hydrogen plasma cleaning[3] have been proposed. These techniques, if they contribute to high quality ZnSe, are more handy and convenient, and will be applicable to regrowth of II-VI on patterned III-V layers for future novel opto-electronic integrated devices. Recently, we proposed in situ surface treatment of GaAs substrates by using hydrogen sulfide (H2S) for the MBE growth of ZnSe[4]. In order for the development of this new technique, it is very important to investigate relation between the surface structure (reconstruction, morphology, etc.) and the treatment conditions. In this paper, surface reconstruction and morphology of H2 S treated GaAs substrates are investigated by in situ reflection high energy electron diffraction (RHEED) and ex situ atomic force microscopy (AFM) in terms of annealing temperature and H2S irradiation sequence. In addition, 15 Mat. Res. Soc. Symp. Proc. Vol. 448 ©1997 Materials Research Society
successful application of this technique to MI3E growth of ZnSe-based semiconductors, including a new tensile-strained quamitum well structure, is also mentioned. EXPERIMENT Substrates used in the experiments were Zn-doped p+-GaAs (001) just oriented wafers. The substrates were prepared by the standard cleaning and etching procedures. Then, they were mounted onto a molybdenum holder by indium welding, and loaded into the exchanging chamber immediately. The main chamber is pumped by a diffusion pump with a Vacuum Generators CCT-150 liquid nitrogen trap. It is equipped with gas-cells and RHEED system. The background pressure is lower than 1 x 10-9 Torr after chamber baking. H2 S gas (99.99% purity) was introduced through the gas cell whose temperature was kept at 100'C to avoid condensation of the gas. At this temperature, H2S does not readily thermally decompose. Therefore the species imnpinging onto the sample surface are uncracked H2S molecules. The flow rate of H2S was controlled by a mass flow controller. Beam pressure was measured by a movable ion gauge at the substrate position. The substrate surface was characterized by in situ RHEED (10 keV) and ex situ atomic force microscope (AFM) observations. In order for the AFM observation, the substrate heater and H2S irradiation were switched off when a desirable surface was obtained, then after cooling down to 300'C the sample was loaded to the exchanging chamber and transferred to the AFM system in the air as soon as possible. It is thought that the surface morphology is ba
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