Haruhiro Oigawa

Universal Passivation Effect of (NH4)2Sx Treatment on the Surface of III-V Compound Semiconductors

The effectiveness of (NH4)2Sx treatment on the (100) surface of GaP, (Al, Ga)As, InP and InAs was studied in comparison to that on GaAs by means of Auger electron spectroscopy (AES) and reflection high-energy electron diffraction (RHEED). It was concluded that the existence of sulfur atoms bonded to semiconductors prevents the adsorption of oxygen. This phenomenon brings about the metal-dependent Schottky barrier fabricated on the (NH4)2Sx-treated surfaces, implying the reduction in the interface state density. The structure and effect of the (NH4)2Sx-treated surface of III-V compounds are qualitatively the same.

The Effect of (NH4)2S Treatment on the Interface Characteristics of GaAs MIS Structures

MIS capacitors prepared on the (NH4)2S-treated GaAs substrate showed a marked reduction in the density of the dominant pinning levels near 0.6 eV below the conduction band. The annealing effect on the interface characteristics was also investigated. Analyses by means of secondary ion mass spectroscopy (SIMS) and Auger electron spectroscopy (AES) indicate that sulfur atoms at the interface stabilize the oxygen-free GaAs surface both electronically and thermally.

Synchrotron radiation photoemission analysis for (NH4)2Sx-treated GaAs

The chemistry of the (NH4)2Sx‐treated n‐GaAs (100) surfaces has been studied using synchrotron radiation photoemission spectroscopy. Ga 3d, As 3d, and S 2p photoemission spectra are measured before and after annealing in vacuum with a photon energy of about 210 eV, where S 2p core level spectra can be sensitively detected. It is found that Ga‐S, As‐S, and S‐S bonds are formed on the as‐treated GaAs surfaces, and that stable Ga‐S bonds become dominant after annealing at 360 °C for 10 min in vacuum. The thickness of the surface sulfide layer is reduced from about 0.5 to 0.3 nm by annealing. The surface Fermi‐ level position of the as‐treated surfaces is determined to be about 0.8 eV below the conduction band minimum, which is about 0.1 eV closer to the valence band maximum than that of the untreated surfaces. A Fermi‐level shift of 0.3 eV toward a flat band condition is also observed after annealing. It is found that the Ga‐S bonding plays an important role in passivating GaAs surfaces.

Metal-Dependent Schottky Barrier Height with the (NH4)2Sx-Treated GaAs

Schottky barriers have been prepared on (NH4)2Sx-treated GaAs. The barrier height was observed to change remarkably with the kind of metals, which is predicted in the case of low interface state density. We found that the interface trap density was reduced to 9.8×1012 cm-2eV-1 by the treatment from 6.5×1013 cm-2eV-1 for the untreated one.

Studies on an (NH4)2Sx-Treated GaAs Surface Using AES, LEELS and RHEED

Surface properties of (NH4)2Sx-treated GaAs (100), (111)Ga and ()As planes were studied by means of Auger electron spectroscopy (AES), low-energy electron energy loss spectroscopy (LEELS) and reflection high-energy electron diffraction (RHEED). We found that oxide-free and sulfur-terminated GaAs surfaces produced by the (NH4)2Sx treatment provided the reduction of interface state density. Furthermore, comparison of various planes revealed that (i) sulfur atoms could combine with both Ga and As and (ii) bonding between Ga and S was stronger than that between As and S.

Reflection high‐energy electron diffraction and x‐ray photoelectron spectroscopic study on (NH4)2Sx‐treated GaAs (100) surfaces

(NH4)2Sx‐treated GaAs (100) surfaces were heated in an ultrahigh vacuum. Reflection high‐energy electron diffraction (RHEED) and Ga, As, S, and O x‐ray photoelectron spectroscopic (XPS) changes were observed. The sulfide‐treated surface showed a streaky 1×1 RHEED pattern without heating. A 2×1 RHEED pattern appeared during heating to 260 and 420 °C. At these temperatures, the S XPS peak was still observed. The 2×1 pattern is thought to be S induced. On the (NH4)2Sx‐treated surface, no oxidized As XPS signal was observed. Moreover, the O XPS peak disappeared rapidly during the heating above 260 °C. These results suggest that the 2×1 S structure caused the GaAs (100) surface passivation.

Bonding states of chemisorbed sulfur atoms on GaAs

Abstract The chemistry of the S/GaAs system is studied using synchrotron radiation photoemission spectroscopy. Monolayer-order sulfur atoms are successfully chemisorbed on clean n-GaAs (001) surfaces at room temperature by using an Ag/AgI/Ag 2 S/Pt electrochemical cell, which generates an atomic sulfur flux. Photoemission spectra of core levels are measured with a photon energy of about 210 eV before and after annealing at 360°C for 10 min in vacuum. Ga 3d, As 3d, and S2p spectra indicate that Ga-S and As-S bonds are formed on the as-chemisorbed GaAs surfaces at room temperature, and that Ga-S bonds become dominant after annealing at 360°C. These results are the same as for the (NH 4 ) 2 S x -treated n-GaAs surfaces. It is found that the Ga-S bonding formation is the key for passivating GaAs surfaces for both sulfur-chemisorbed and (NH 4 ) 2 S x -treated GaAs.

Evidence for the passivation effect in (NH4)2Sx‐treated GaAs observed by slow positrons

We applied slow positrons to the as‐etched GaAs and/or the (NH4)2Sx‐treated GaAs. The results show that a thin oxide film is easily formed on the surface of as‐etched GaAs as soon as the etched surface is exposed to air for several minutes before the measurement. On the other hand, the monolayer of chemisorbed sulfur atoms in the (NH4)2 Sx‐treated GaAs is effective in protecting the clean surface from the adsorption of oxygen atoms. The mean diffusion length of positrons is not affected by the conditions of the surface treatment. This implies that the centers for the trapping of a positron are not created below the free surface by those treatments.

Probing subpicosecond dynamics using pulsed laser combined scanning tunneling microscopy

Time-resolved tunneling current measurement in the subpicosecond range was realized by ultrashort-pulse laser combined scanning tunneling microscopy, using the shaken-pulse-pair method. A low-temperature-grown GaNxAs1−x(x=0.36%) sample exhibited two ultrafast transient processes in the time-resolved tunnel current signal, whose lifetimes were determined to be 0.653±0.025 and 55.1±5.0ps. These values are of the same order of magnitude as those measured in the conventional pump–probe reflectivity measurement.

Hetero-epitaxy of layered compound semiconductor GaSe onto GaAs surfaces for very effective passivation of nanometer structures

Abstract Growth of layered III-VI compound semiconductor GaSe on a GaAs(111)B substrate has been investigated for the purpose of an effective passivation of the GaAs surface. Although GaSe and GaAs have completely different lattice structures, it has been found that a good GaSe film having its own lattice constant grows epitaxially. The growth proceeds by weak Van der Waals forces between each layer of GaSe so that the lattice matching condition is drastically relaxed. The photoluminescence intensity of the GaAs(111)B surface covered by the GaSe film is larger than that of an as-etched surface, even after the sample has been left in air for a month. The decay of the photoluminescence intensity of the GaSe-covered surface has been found to be slower than that of the sulfur-passivated surface, indicating its excellent ability for the passivation. This seems to result from the stability of covering GaSe film in addition to the proper termination of dangling bonds by selenium atoms at the GaSe/GaAs interface.