Surface Structure of Sulfur Coated GaAs
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SURFACE STRUCTURE OF SULFUR COATED GaAs YOSHIHISA FUJISAKI AND SHIGE GOTO Hitachi Ltd., Central Research Laboratory, 1-280 Higashikoigakubo, Tokyo 185, Japan
Kokubunji,
ABSTRACT Surface structure of (NH4 ) 2 S treated GaAs. is investigated using PL (PhotoLuminescence), XPS (X-ray Photoelectron Spectroscopy) and RHEED (Reflection of High Energy Electron beam Diffraction). The data taken with these techniques show the strong dependence upon the crystal orientations coming from the stabilities of chemical bonds of Ga-S and As-S on GaAs crystals. The greater enhancement of PL intensity, the clearer RHEED patterns and the smaller amount of oxides on (111)A than (111)B implies the realization of a more stable structure composed mainly of the Ga-S chemical bond. INTRODUCTION Recently, sulfur passivation of GaAs crystals has received a great attention since it improves the quality of GaAs surfaces. It is thought to be attributed to the termination of dangling bonds on the surface by sulfur atoms. However, the detailed surface structure has not been clarified enough. Actually, two models have been proposed on the termination of free bonds [1],[2]. In this study, the surface structure of sulfur coated GaAs is investigated by changing the crystal orientations. Vacuu: : 5E-10 (lorr) nll '1 V 19.fl 599lyzer : 2 (E.V) Secan : 5010 (SEC)NI
EXPERIMENTS Photoluminescence measurements were performed at 4.2K using a 514.5nm light source with 500mk'/cm2 lasing intensity. The XPS analyses were carried out with Al and Mg Ka X-ray sources under the background pressure of 2x10- 1 0 torr. The RHEED observations were performed with 3OKeV electron beams. Just before the RHEED observations, specimens were heated up to 150dC to stick them to a holder using indium solder. In every experiment, specimens were exposed to the air before they were inserted into the analysis chambers. The specimens used in this study were (100), (111)A, and (111)B oriented GaAs crystals. They were, at first, etched chemically with 4H 2 SO 4 /H 2 0 2 solution for a few seconds. Sulfur treatment was performed using (NH4 ) 2 S solution for 20-30 hours at room temperature and 10 minutes at 50t. In order to evaporate the thick amorphous sulfur layer on the crystal, specimens were then heated for 30 minutes at 390t in pure hydrogen atmosphere.
500
132G
(
Fll,)
131? 111I4DIW|; [ liPWf (.Vj
Fig. I XPS spectrmnl ;irroitill As 2,p '32. Si gn lls f ritiu a|t1 1i,,,,l 3e2 i gher ,,ri(rgy t|111l1 As 2p'q/' corrcspord to As 2 O.S.
Mat. Res. Soc. Symp. Proc. Vol. 163. ' 1990 Materials Research Society
1018
RESULTS AND DISCUSSIONS The XPS spectra around As 2 P3/2 peak are shown in Figure 1. The main peaks near 1327eV correspond to As 2 P3/2 coming from the As atoms in the bulk GaAs crystal. The peak near 1329eV which is clearly found in the spectrum of a reference specimen originates from As2 03 . This peak is clearly found only for the reference specimen, but are not so obvious for the (NH4 ) 2 S treated specimens. This is due to the etching ability of (NH4 ) 2 S for b
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