Synchrotron radiation photoelectron spectroscopy combined with scanning electron microscopy ͑SEM͒ and gravimetry has been used to study GaAs ͑100͒ surfaces treated with a neutralized ͑NH 4 ͒ 2 S solution. Compared to the conventional basic ͑NH 4 ͒ 2 S solution treatment, a thick Ga sulfide layer and strong Ga-S bond were formed on the GaAs surface after dipping GaAs wafers in a neutralized ͑NH 4 ͒ 2 S solution. Gravimetric data show that the etching rate of GaAs in the neutralized ͑NH 4 ͒ 2 S solution is about 15% slower than that in the conventional ͑NH 4 ͒ 2 S solution. From SEM observation, fewer etching pits with smaller sizes were found on the neutralized ͑NH 4 ͒ 2 S-treated GaAs surface. © 1997 American Institute of Physics. ͓S0003-6951͑97͒01247-3͔Since Sandroff et al. 1 reported that dipping in a sulfide solution could significantly improve the characteristics of GaAs-based devices, ͑NH 4 ͒ 2 S has attracted much attention and been used frequently for passivating GaAs surfaces. [2][3][4][5][6][7][8][9][10] The passivation has proved effective in that, upon dipping, the surface oxide layer can be thoroughly removed and surface Ga and/or As bonds can be saturated by sulfur atoms, as photoemission data show. 2-10 It is also found that satisfactory passivation may be achieved by using a less basic ͑NH 4 ͒ 2 S x rather than the stoichiometric ͑NH 4 ͒ 2 S solution. 6,8 The basicity of the solution may play some role in the surface sulfuration process. In this letter, we report the passivation of the GaAs ͑100͒ surface using a neutralized ͑NH 4 ͒ 2 S solution instead of the basic one. The experimental results obtained show that, compared to the basic one, the neutralized ͑NH 4 ͒ 2 S solution is a moderate etchant to GaAs and that dipping in such a solution may lead to the formation of a thick passivation layer in which S bonds to Ga strongly.The major experimental technique used in the present work is synchrotron radiation photoemission spectroscopy ͑SRPES͒, which reveals the presence of more chemically shifted Ga 3d and As 3d components than those observed previously. 4 In addition, scanning electron microscopy ͑SEM͒ and gravimetry were used to monitor variations in surface morphology and weight of the sample, respectively.SRPES measurements were carried out at the National Synchrotron Radiation Laboratory, Hefei. Semi-insulating GaAs ͑100͒ single-crystal wafers used in the experiments were ultrasonically cleaned in trichloroethylene, acetone, and ethanol in sequence, etched by 5H 2 SO 4 ϩ1H 2 O 2 ϩ1H 2 O solution for 2 min, and then dipped in fresh-prepared neutralized ͑NH 4 ͒ 2 S solution for about 4 h at room temperature. The neutralized ͑NH 4 ͒ 2 S solution was prepared by dropping dilute HCl ͑about 10% v/v͒ in conventional ͑NH 4 ͒ 2 S solution (pHХ11.5) until the pH value reached 7. After being rinsed with deionized water and dried by flowing nitrogen, the samples were transferred into the SRPES chamber with a base pressure of 2ϫ10 Ϫ8 Pa. A photon energy of 90 eV was used in the measurements of the Ga 3d a...

We have developed a mild electrochemical sulfurization technique which can form a very thick sulfide layer on GaAs(100) surface. This sulfide layer is quite stable in air. The photoluminescence spectrum of such anodic sulfurized GaAs surface shows a large intensity enhancement as compared with that of as-etched GaAs samples. No visual intensity decay occurs under the laser beam illumination after the sample has been maintained in air for more than seven months. The structure and composition of the passivation layers are investigated by x-ray photoelectron spectroscopy and the mechanism of layer formation is discussed.

A comparative study of the Raman spectra of ZnSe films grown by molecular beam epitaxy on GaAs(100) substrates passivated by NH4)2)Sx and S2Cl2 solutions is presented. Based on the analysis of the line shape of the first-order longitudinal-optical phonon of ZnSe with spatial correlation model of Raman scattering, it is shown that the ZnSe films grown on the GaAs substrates passivated by S2Cl2 solutions have longer coherence lengths, which indicate that their crystalline qualities are better than those passivated by NH4)2Sx solutions. In addition, the barrier heights of ZnSe/GaAs interfaces for different S passivations have been obtained from the ratios of the intensity of the coupled longitudinal-optical phonon-plasmon mode to that of the longitudinal-optical mode of GaAs Raman peak. The results show that the ZnSe/GaAs samples passivated by S2Cl2 solutions have lower density of interfacial states.

We have developed a new electrochemical passivation method to obtain a quite stable sulfide layer on GaAs surface. This layer is very thick and contains a mixture of Ga, As, S, O and H compounds. The photoluminescence (PL) spectrum of such anodic sulfurized GaAs surface shows big intensity enhancement as compared with that of as-etched GaAs samples; No visual intensity decay occurs under laser beam illumination, which maintains for more than seven months. The structure and composition of the passivation layers are investigated by the X-ray photoelectron spectroscopy and the mechanism of the layer formation is suggested.

No abstract