Synchrotron radiation photoelectron spectroscopy has been used to investigate III-V phosphide GaP and InP ͑100͒ surfaces treated with a neutralized (NH 4 ͒ 2 S solution. Compared to the conventional basic ͑NH 4 ͒ 2 S solution treatment, a thick sulfide layer with P-S bond and strong GaS ͑In-S͒ bond of high thermal stability is formed on the neutralized ͑NH 4 ͒ 2 S-treated GaP ͑InP͒ ͑100͒ surfaces. The possible passivation mechanisms of the two ͑NH 4 ͒ 2 S solutions to III-V phosphide surfaces are also discussed.

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...

The time-dependent fields experienced by a nucleus as it passes through a crystal can cause transitions to excited nuclear states. The transition rate is especially large when the speed of the nucleus satisfies a resonance condition. Previous estimates of the nuclear excitation rates have been based on symmetry arguments and experimental data for related processes. We present here a calculation of the nuclear excitation rate which does not rely on these simplifying approximations.Vsing nonrelativistic quantum mechanics, the excitation cross sections of highly stripped nuclei channeled along the Si (110) crystal axis have been calculated. The results are smaller than those given by previous estimates by several orders of magnitude. We note that there are similarly large discrepancies between different theoretical and experimental results for other phenomena involving coupled electronic and nuclear transitions. The reason for these disagreements is not clear. PACS number(s): 23.20.Lv, 23.20.Nx, 32.90.+a

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