F.P. Zhang

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.

The role of S passivation on magnetic properties of Fe overlayers grown on GaAs(100)

We have produced epitaxial Fe overlayers on sulfur-passivated GaAs(100) surfaces by CH3CSNH2 treatment, and investigated the correlation between magnetic properties of the overlayers and surface chemical structure of GaAs(100) surfaces by ferromagnetic resonance and synchrotron radiation photoemission. The surface chemical properties were modified by changing the annealing temperature of the surfaces prior to the growth. The results show that the magnetization of Fe overlayers is crucially determined by the presence of Ga–S chemical bonds and excess As after the anneals. A comparative investigation of the magnetization has been made on both S passivated and clean GaAs(100). It is confirmed that S passivation on the GaAs surface can effectively eliminate the magnetization deficiency previously attributed to interdiffusion of As into the Fe overlayer.

Studies of interface formation between Co with GaAs(100) and S-passivated GaAs(100)

Abstract Interface formation between Co with GaAs(100) and S-passivated GaAs(100) by CH 3 CSNH 2 treatment has been studied with synchrotron radiation photoemission. Strong interface disruption and reaction occurs between the overlayer with GaAs(100) even at low Co coverage (∼0.2 nm), while the reaction is much weaker on S/GaAs(100); a stable interface forms at a coverage of 1 nm and 0.8 nm, respectively. For S-passivated GaAs(100), Ga atoms bonded with S at the surface exchange with Co atoms and cause the formation of Co–S bonding, the amount of As bonded with Co is much less than that on GaAs(100), no segregated As appears at the surface of Co overlayer, in contrast with the case of Co/GaAs(100), indicating that S-passivation on GaAs(100) is an effective way of inhibiting the interdiffusion of As and Ga through the overlayer.

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.

Growth and magnetic properties of Fe overlayer on S-passivated GaAs substrate

Abstract We have produced epitaxial Fe overlayers on S-passivated GaAs(100) surfaces by CH3CSNH2 treatment. The correlation between magnetic properties of the overlayers and surface chemical structure of GaAs(100) surfaces was investigated by changing the annealing temperature of the surface prior to growth. The results show that the magnetization of Fe overlayers is crucially determined by the GaxS chemical bonds and by the presence of excess As after the anneals. A comparative investigation of the magnetization has been performed on both S-passivated and clean GaAs(100). It is confirmed that S-passivation on GaAs surfaces can effectively eliminate the magnetization deficiency previously attributed to interdiffusion of As into the Fe overlayer.

INTERFACE FORMATION AND INTERACTION OF FE OVERLAYER ON S-PASSIVATED GAAS(100)

Abstract We have studied the interface formation and electronic structure of an Fe overlayer deposited on S-passivated GaAs(100). In the first stage of deposition, Fe clusters were formed near S atoms. Compared to Fe/GaAs(100), the sulfur passivation weakens the reaction between As and Fe. It is beneficial to the magnetism at the interface. A magnetic ordering feature could be found at higher coverage due to large exchange splitting.

The chemistry, structure and stability of CH3CSNH2 passivated GaAs(100) surfaces

Abstract An organic sulfide, CH 3 CSNH 2 treated sulfur-passivated GaAs(100), has been studied using synchrotron radiation photo-emission spectroscopy (SRPES), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED). The SRPES and AES measurements show that the treatment removes the GaAs surface oxide layer and forms sulfides of Ga and As on the surface. The thermal stability and surface structure of the passivated samples at different temperatures have also been studied. We found that the surface sulfides are also gradually removed and a clean, ordered and thus Fermi level unpinning surface can finally be achieved. Surface restructuring can be observed from the GaAs(100)–S (2×1) pattern between 260 and 450°C to the (4×1) pattern without S between 460 and 550°C.

An unexpected electronic structure of Gd

Abstract Contradictory results are obtained when Gd is deposited on S–GaP(100) and GaAs(100) substrates, respectively. The Gd4f spectra from Gd/S–GaP show single peaks with binding energy 8.3 eV at a thickness of 0.9 nm. However, the Gd4f spectra from Gd/GaAs evolve from a narrow peak into a two-featured structure with an increment of Gd. At a thickness of 2.33 nm, one feature centers at 10.3 eV binding energy, and another at 8.0 eV. We confidently exclude the contribution of contamination in these unexpected results and a new phase is supposed.

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.

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.