The effectiveness of (NH4)2S x 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)2S x -treated surfaces, implying the reduction in the interface state density. The structure and effect of the (NH4)2S x -treated surface of III-V compounds are qualitatively the same.

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 GaN x As 1−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. Smaller and faster are the key words in the progress of current nanoscience and technology. Thus, for further advances, a method of exploring the ultrafast transient dynamics of the local quantum functions in organized small structures is eagerly desired. Ultrashort optical pulse technology in the near-infrared to ultraviolet region has allowed us to observe transient phenomena in the femtosecond range, the optical-monocycle region, 1,2 which, however, has a drawback of a relatively low spatial resolution due to electromagnetic wavelength. On the other hand, scanning tunneling microscopy (STM), although its time resolution is limited by circuit bandwidth ͑ϳ100 kHz͒, enables us to observe spatial dynamics at the atomic level in real space. 3 Therefore, the integration of ultrashort optical technology with STM has been one of the most exciting goals since their invention. [4][5][6][7] Pioneering works were performed by Hamers et al., 4-7 which have attracted the extensive interest of researchers in various fields. However, there remain critical problems which have prevented the achievement of the laser-combined STM measurement, such as the displacement current due to the stray capacitance of the tunneling gap and photoelectrons produced by multiple photoabsorption. [4][5][6][7] In such cases, since a large area is included in the processes, the superior space resolution of STM cannot be utilized. In particular, the thermal expansion of the STM tip by photoillumination causes much large noise in the tunneling current, making the measurement difficult.Here, we show the results of the time-resolved tunneling current measurement in the subpicosecond range, which can advance the development of future research in terms of ultimate temporal and spatial resolutions.A schematic of the measurement system is shown in Fig. 1. We adopted the recently developed shaken-pulse-pairexcited STM (SPPX-STM) method, which realizes highly sensitive measurement free from the thermal expansion effect of the tip and sample. 8 The tunneling junction is directly illuminated by a sequence of laser pulse pairs and average tunneling current, I t ͑t d ͒, is measured as a function of the delay time between the two pulses, t d . To decrease broadband noise, the delay time of the two pulses t d is modulated with a small amplitude ⌬t d at frequency , and the tunneling current is detected by a lock-in amplifier. Since the tunneling current I t responds to the modulation as In the pulse-pair-excited STM measurement, the first laser pulse in each pulse pair acts as the pump pulse to excite and modulate the electronic structure of the sample surface, which might cause d...

Studies of spin dynamics in low-dimensional systems are important from both fundamental and practical points of view. Spin-polarized scanning tunnelling microscopy allows localized spin dynamics to be characterized and plays important roles in nanoscale science and technology. However, nanoscale analysis of the ultrafast dynamics of itinerant magnetism, as well as its localized characteristics, should be pursued to advance further the investigation of quantum dynamics in functional structures of small systems. Here, we demonstrate the optical pump-probe scanning tunnelling microscopy technique, which enables the nanoscale probing of spin dynamics with the temporal resolution corresponding, in principle, to the optical pulse width. Spins are optically oriented using circularly polarized light, and their dynamics are probed by scanning tunnelling microscopy based on the optical pump-probe method. Spin relaxation in a single quantum well with a width of 6 nm was observed with a spatial resolution of ∼ 1 nm. In addition to spin relaxation dynamics, spin precession, which provides an estimation of the Landé g factor, was observed successfully.

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.

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.