Epitasxial Growth of ZnSe on S-Passivated GaAs (100) Substrate
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structure. During the initial stage of growth of ZnSe on GaAs, the dissociated Se is free to move on the surface to react with the substrate atoms. From the heats of formation for Ga 2Se3, GaSe and GaAs that are -85, -39 and -17 kcal/mol, respectively, the formation of (Ga, Se) compounds at the interface is favorable. It has also been reported [4] that ZnSe shows a tendency for 3-D growth mode on a Se-exposed GaAs (100) substrate, which is very undesirable for epitaxial growth. The procedure of high temperature annealing of GaAs substrate before epitaxial growth inevitably results in an As-deficient surface, which promotes the formation of (Ga, Se) interfacial compounds. The above problem could be partially solved by passivating the GaAs substrate with sulfur atoms. A monolayer of sulfur atoms bonded to the substrate atoms is easily obtained by dipping in Na2S.9H20 or (NH4 )2 Sx solution. Recently we found a new sulfur-containing solution 52 C12 , which is extremely efficient in removing the oxides of GaAs and leaves a layer of sulfur bonded to both Ga and As atoms [5]. The advantage of using 52 C12 over NaS.9H 20 or (NH4 ) 2 Sx is the free of oxygen, which is the major ingredient causing the failure of S passivation. In this work we employ Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) to study the S2C12-passivated GaAs (100) and use the passivated substrate for ZnSe growth by hot wall epitaxy (HWE). The observed improvement of the crystalline quality of the epilayer as verified by Raman scattering measurements is interpreted as the saturation of the Ga, As dangling bonds by a monolayer of sulfur atoms. 335 Mat. Res. Soc. Symp. Proc. Vol. 326. ©1994 Materials Research Society
II. EXPERIMENTAL Unintentionally doped ZnSe layers were grown along the [100] azimuth by HWE on n-type GaAs(100) wafers with a doping concentration of 1018 cm-3 . The substrates were first ultrasonically cleaned in trichlorethylene, acetone and methanol sequentially, followed by a blowing-dry by nitrogen gas. Then, one sample was immersed in $2 C12 for 5s, rinsed in CCI 4 briefly and in flowing de-ionized (DI) water repetitively. Another sample was immersed in 60'C (NH 4)2Sx for 30 min., and rinsed in DI water. The sample for reference was dipped in 5H12SO 4: 111202: 1H20 (refer as 5:1:1) at room temperature for 60s. Finally, all the samples were blown dry by nitrogen gas before being loaded into the chamber. AES and XPS measurements were carried out on an ESCALAB-5 multifunctional electron spectrometer with a base pressure of 5x10-10 Torr. The samples were loaded into the chamber at the same time, and the measurements began about 10 hours after the loading The Auger signals excited by a 3 keV, 3 jLA primary electron beam and the photoelectrons excited by the Al ka Xray source were collected by a hemispherical analyzer. Prior to ZnSe deposition, the S-passivated samples were heated at 350'C for 10 min, while the 5:1:1 as-etched one was heated up to 580'C to remove the surface oxides. 6-N powdered ZnSe was used
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