S.H. Xu

A sulfur passivation for GaAs surface by an organic molecular, CH3CSNH2 treatment

A sulfur passivation method for GaAs, CH3CSNH2 treatment has been developed. It is quite effective for removing the surface oxide layer and forming the sulfide passivation layer on GaAs surface. The enhancements of the PL intensity reveal the reduction of the surface recombination velocity and the reduction of density of defect states by this treatment. The synchrotron radiation photoemission spectroscopy measurements show that sulfur atoms bond both Ga and As atoms. After being annealed, a stable sulfur passivation layer is terminated on the surface due to the As2S3 component react with GaAs into the GaS component.

Investigation of neutralized (NH4)2S solution passivation of GaAs (100) surfaces

Synchrotron radiation photoelectron spectroscopy combined with scanning electron microscopy (SEM) and gravimetry has been used to study GaAs (100) surfaces treated with a neutralized (NH4)2S solution. Compared to the conventional basic (NH4)2S 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 (NH4)2S solution. Gravimetric data show that the etching rate of GaAs in the neutralized (NH4)2S solution is about 15% slower than that in the conventional (NH4)2S solution. From SEM observation, fewer etching pits with smaller sizes were found on the neutralized (NH4)2S-treated GaAs surface.

Measurements and calculations of the valence band offsets of SiOx/ZnS(111) and SiOx/CdTe(111) heterojunctions

SiOx(x>1.5) overlayers have been in situ grown on ZnS (111) and CdTe (111) substrates. Synchrotron radiation photoemission spectroscopy has been used to measure the electronic structures and band lineups of these two heterojunctions. The valence band offsets of SiOx/ZnS(111) and SiOx/CdTe(111) derived from the measurements are 2.8±0.2 and 4.7±0.2 eV, respectively. Harrison’s “tight binding” theory is extended into the theoretical estimation of the band lineups of SiO2 related heterojunctions. The agreement between the experimental and theoretical results is good. Our result also explains the positive role of SiO2 layers in ZnS-based thin film electroluminescence devices.

Studies of Mg overlayer on GaAs(100) surface treated by CH3CSNH2

Abstract A new sulfur passivation method for GaAs, CH 3 CSNH 2 treatment, has been developed. By Synchrotron Radiation Photoemission Spectroscopy(SRPES), the chemical states and electronic aspects of the passivated surfaces are investigated. It is found that the oxide layer of the GaAs is effectively removed, and Sulfur atoms bond both to Ga and As atom at room temperature. In addition, the behavior of Mg deposited on the passivated surfaces with and without annealing has been investigated. It is found that Ga atoms can be exchanged from GaS bond, and diffuse into Mg overlayer, but the sulfur atoms remain at the interfaces. At higher Mg coverage, a MgGa alloy may be formed due to excess Mg atoms reacting with Ga atoms at the overlayer.

Study of the Na - Si(111) 3 1 interface using core-level photoemission spectroscopy

The Na - Si(111) and Na - Si(111) interfaces with various coverages of Na have been investigated using low-energy electron diffraction Auger electron spectroscopy and photoemission spectroscopy (PES) with a synchrotron radiation source. In decomposing the PES peak from the Si 2p core level into the surface and bulk components, the relative shifts and intensities of the surface components as well as the band bending for the interface are consistent with the structure model proposed by Monch. The evolution of the band bending from about 1 monolayer to thick Na coverage corresponds to the theory of the metal-induced gap states. The final Schottky barrier height is determined to be eV and does not depend on whether a thicker Na layer is deposited on an ordered Na - Si(111) surface or a disordered thin Na layer on Si(111).

Commissioning and operation of the beamline for photoelectron spectroscopy in NSRL

A grazing incidence spherical grating monochromator (SGM) for photoelectron spectroscopy has been set up in NSRL. It covers the energy range 10–1000 eV. The primary results of commissioning and operation of the beamline have been described. The resolution power could be obtained from 500 to 1000 (E/ΔE) with 50 μm of slit opening in a wide range of photon energy. The improvement of the beamline performance is continuing.

PHOTOEMISSION STUDY OF THE GADOLINIUM/GaAs(100) INTERFACE WITH SYNCHROTRON RADIATION

Soft-X-ray photoemission spectroscopy was used to characterize the Gd/GaAs(100)-interface formation at room temperature. At low Gd coverage (<1 A), the interface is near-abrupt, because no evidence of reaction is observed. With increasing Gd coverage, photoemission signals from chemically reacted product at the interface are observed, causing some intermixing between the overlayer and the substrate. For As atoms, they remain near the interface and have little diffusion. Ga atoms, however, are not kept near the interface, and they can diffuse into the Gd overlayer and segregate onto the surface instead. From the observed variations with metal coverage of binding energies and relative intensities of photoemission signals from the reacted layer, a profile of the interface structure is proposed, and some parameters (decaying length, segregation density and solution density, etc.) have been obtained. The results show that the deposition of Gd onto the GaAs(100) surface induces limited substrate disruption except for some diffusion and segregation of Ga atoms into the metal overlayer. This paper demonstrates that the disruption and epitaxial growth are not mutually exclusive in the Gd/GaAs(100) system.

Synchrotron radiation photoemission study of Ge/ZnS(111) heterojunction

Abstract The band lineup at the Ge/ZnS(111) interface has been studied by synchroton radiation photoemission spectroscopy. The experimental results indicate that the deposition of Ge films by evaporation is a two-dimensional growth at the initial stage. With the growing of Ge overlayer, surface S atoms diffuse into Ge overlayer and react with Ge atoms at the interface. We derive the valence band offsets of 1.94 ± 0.1 eV and 2.23 ± 0.1 eV for Ge/ZnS heterojunctions grown at 200°C and room temperature, respectively. The results are in good agreement with theoretical predictions.

Photoemission studies of Mg deposition on sulfurized GaSb(100) surface

Abstract Synchrotron Radiation Photoemission Spectroscopy (SRPES) has been used to investigate the chemical states and electronic states of a [NH 4 ] 2 S x treated GaSb(100) surface. We have found that the oxides of Ga and Sb are removed and the sulfides of Ga and Sb are formed on the surface. After sulfurized GaSb(100) was annealed, the SbS bond is broken to form elemental Sb, while the GaS bonds terminate the surface; these results imply that ammonia sulfide has a passivating role for GaSb. At room temperature (RT), deposited Mg on passivated surface has been also investigated. It is found that Ga atoms can be exchanged by Mg atoms and diffuse into Mg overlayer. Moreover, the Schottky barrier height of the Mg overlayer on the sulfurized GaSb surface was determined to be about 0.3eV.

K promoted oxidation and nitridation on InP(100) surface: A soft-X-ray photoemission study

Abstract The effects of exposure of molecular nitrogen and oxygen on p-type InP(100) surfaces modified by potassium overlayers were studied by core level and valence band (VB) photoemission using synchrotron radiation. On the K pre-covered surface, the potassium coverage enhanced the nitrogen and oxygen sticking coefficients dramatically. As far as the process of nitrogen adsorption is concerned, nitrogen atoms react mainly with P atoms rather than not react directly with indium atoms. Two kinds of nitride complexes, InPX x and InPN x + y were formed at the K-precovered InP(100) surface. In case of oxygen adsorption, O may bond with K and produce the peroxides O 2 2− and superoxides O 2 − . Following this a few kinds of phosphate phases, In(PO 4 ) x , were formed on the surface. In comparison with InP(110) surface, we found that the oxidation and nitridation promotion for the InP(100) surface was much stronger. The reasons may be the number of surface defects as well as the polarity of the InP(100) surface.