Taketomo Sato

Electrochemical Formation of Uniform and Straight Nano-Pore Arrays on(001)InP Surfaces and Their Photoluminescence Characterizations

Recently, -oriented nanometer-sized pores (nano-pores) without side branches, unlike porous Si, have been realized by our group on (001) n-InP surfaces by means of electrochemical anodization in 1M HCl solution. However, they exhibit large structural nonuniformity including the presence of an irregular top layer, random pore positioning and wavy pore walls. In this study, attempts have been made to improve pore uniformity and to clarify their optical properties. Anodization in 1 M HCl+HNO3 solution realized highly uniform nano-pore arrays consisting of square-shaped straight pores defined by four crystalline (011) facets. This improvement is explained in terms of preferential etching along the vertical -direction at the pore tip due to the slow etching rate along the lateral -direction as well as the uniform supply of reactant species to pore tips realized by the removal of the irregular top layer during the anodization process. The nano-pore arrays show strong blue- and red-shifted photoluminescence emissions at high and low temperatures, respectively. These are assigned as emissions due to the transition between electron and hole quantum states inside the pore walls and that involving a broad surface state continuum at pore wall surfaces, respectively.

Evolution mechanism of nearly pinning-free platinum/n-type indium phosphide interface with a high Schottky barrier height by in situ electrochemical process

Recently, unusually high Schottky barrier heights (SBHs) have been realized by our group on n-type InP and related materials by an in situ electrochemical deposition of Pt. In an attempt to understand the underlying mechanism of the SBH enhancement, this article investigates in detail the evolution process of the metal (Pt, Ni, Co, and Ag)-InP interface during the in situ electrochemical process, using current–voltage, capacitance voltage, deep level transient spectroscopy, x-ray photoelectron spectroscopy, Raman, atomic force microscope, and scanning electron microscope measurements. Pt deposition by the electrochemical process realized an oxide-free, defect-free, stress-free, and nearly pinning-free interface, whereas Pt deposition by conventional electron beam evaporation and sputtering processes as well as Ag, Ni, and Co deposition by the electrochemical process gave rise to stressed and pinned interfaces. The observed large process dependence of SBH can be explained by none of the metal induced gap s...

Current Transport and Capacitance-Voltage Characteristics of GaAs and InP Nanometer-Sized Schottky Contacts Formed by in situ Electrochemical Process

The nanometer-sized Schottky contacts were successively fabricated on n-GaAs and n-InP substrates by the electrochemical process, and their electrical properties were characterized both experimentally and theoretically. From the detailed I-V measurements using a conductive AFM system, it was found that the current transport properties of the nanometer-sized Schottky contacts were strongly dependent on metal workfunction, however showed nonlinear log I-V characteristics with large n value in range of 1.2 - 2.0 which can not be explained by 1D thermionic emission model. From the theoretical analysis using a computer simulation, it was found that this nonlinear characteristics can be explained by the 3D thermionic emission model with a due consideration of the environmental Fermi level pinning. Furthermore, the calculated C-V characteristics showed much smaller movements of the depletion layer with bias underneath the nano-Schottky contacts. These results strongly indicate the importance of controlling the environmental Fermi level pinning to improve the potential controllability of the nano-Schottky contacts.

Unpinning of Fermi level in nanometer-sized Schottky contacts on GaAs and InP

Abstract Transport properties of two types of electrochemically produced nanometer-sized GaAs and InP Schottky contacts were investigated. One is macroscopic contacts containing many nano-dots and the other is isolated single-dot contacts. Macroscopic contacts showed near ideal thermionic emission characteristics with ideality factors close to unity. I – V characteristics of single nano-dot contacts directly measured by a conductive AFM probe showed nonlinear log I – V behavior with large and voltage-dependent ideality factors. The latter was explained by potential profile modification due to Fermi level pinning on surrounding free surfaces. Both types of contacts indicated that Fermi level pinning disappears as the dot size is reduced, indicating that strong Fermi level pinning is not intrinsic to Schottky contacts.

Characterization of electronic states at insulator/(Al)GaN interfaces for improved insulated gate and surface passivation structures of GaN-based transistors

This paper presents a systematic characterization of electronic states at insulators/(Al)GaN interfaces, particularly focusing on insulator/AlGaN/GaN structures. First, we review important results reported for GaN metal–insulator–semiconductor (MIS) structures. SiO2 is an attractive material for MIS transistor applications due to its large bandgap and high chemical stability. In-situ SiNx is effective for improving the operation stability of high electron mobility transistors (HEMTs). Meanwhile, Al2O3/GaN structures have high band offsets and low interface state densities, which are also desirable for insulated gate applications. We have proposed a calculation method for describing capacitance–voltage (C–V) characteristics of HEMT MIS structures for evaluating electronic state properties at the insulator/AlGaN interfaces. To evaluate near-midgap states at insulator/AlGaN interfaces, a photo-assisted C–V technique using photon energies less than the bandgap of GaN has been developed. Using the calculation in conjunction with the photo-assisted C–V technique, we estimate interface state density distributions at the Al2O3/AlGaN interfaces.

Large Schottky barrier heights on indium phosphide-based materials realized by in-situ electrochemical process

Pt Schottky barriers were formed on InP-based materials by a novel in-situ electrochemical process. The electrical characteristics, surfaces and interfaces of the Schottky diodes were investigated by current-voltage (I-V), capacitance-voltage (C-V), deep-level transient spectroscopy (DLTS) and atomic force microscopy (AFM) measurements. The mechanism for increasing the Schottky barrier heights (SBH) was explained in terms of possible ordered interface formation from the viewpoint of the disorder induced gap state (DIGS) model.

Effects of surface states and Si-interlayer based surface passivation on GaAs quantum wires grown by selective molecular beam epitaxy

Effects of surface states and surface passivation on photoluminescence (PL) properties of GaAs quantum wires (QWRs) are investigated. QWR samples were grown on (001) and (111)B substrates by the selective molecular beam epitaxy (MBE) method. For surface passivation, an ultrathin (about 1 nm) Si interface control layer (Si ICL) was grown by MBE as an interlayer. In both of the selectively grown QWRs on (001) and (111)B substrates, the PL intensity reduced exponentially with reduction of their wire-to-surface distance, being coexistent with a more gradual reduction due to carrier supply reduction. The exponential reduction was explained in terms of interaction between surface states and quantum confined states leading to tunneling assisted nonradiative recombination through surface states. Surface passivation by the Si-ICL method almost completely recovered PL intensities not only for QWRs on the (001) substrate, but also for QWRs on the (111)B substrate.

Electrical Properties of Nanometer-Sized Schottky Contacts for Gate Control of III–V Single Electron Devices and Quantum Devices

The electrical properties of nanometer-sized Schottky contacts formed on n-GaAs and n-InP substrates by an in situ electrochemical process were studied both experimentally and theoretically to understand and improve their gate control behavior in single electron devices and quantum devices. From the current–voltage (I–V) measurements using a conductive atomic force microscope (AFM) system, the nano-Schottky contacts showed nonlinear log I–V characteristics with large and voltage-dependent n values which cannot be explained by the 1D thermionic emission model. The behavior was explained by a novel 3D thermionic emission model including 3D potential distribution modified by an environmental Fermi-level pinning. The depletion characteristics were calculated on the basis of the new model including the environmental effects. The results showed small changes of the depletion layer width with a bias underneath the nano-Schottky contacts due to the environmental Fermi-level pinning. Control of Fermi-level pinning is thus crucial to obtain nano devices in the quantum regime that exhibit good behavior.

Growth kinetics and theoretical modeling of selective molecular beam epitaxy for growth of GaAs nanowires on nonplanar (001) and (111)B substrates

The growth kinetics involved in the selective molecular beam epitaxy growth of GaAs quantum wires (QWRs) on mesa-patterned substrates is investigated in detail experimentally, and an attempt is made to model the growth theoretically, using a phenomenological continuum model. Experimentally, ⟨−110⟩-oriented QWRs were grown on (001) and (113)A substrates, and ⟨−1−12⟩-oriented QWRs were grown on (111)B substrates. From a detailed investigation of the growth profiles, it was found that the lateral wire width is determined by facet boundaries (FBs) within AlGaAs layers separating growth regions on top facets from those on side facets of mesa structures. Evolution of FBs during growth was complicated. For computer simulation, measured growth rates of various facets were fitted into a theoretical formula to determine the dependence of a lifetime of adatoms on the slope angle of the growing surface. The continuum model using the slope angle dependent lifetime reproduced the details of the experimentally observed ...

Electrochemical Formation of Self-Assembled InP Nanopore Arrays and Their Use as Templates for Molecular Beam Epitaxy Growth of InGaAs Quantum Wires and Dots

Attempts were made to optimize the parameters of the electrochemical process to form uniform nanopore arrays and utilize them as templates for molecular beam epitaxy (MBE) growth of InP-based quantum wires and quantum dots. Template parameters such as pore depth, diameter and period were strongly dependent on anodization conditions. In particular, in the pulsed anodization mode, the pore depth could be well controlled in the nanometer range by adjusting the number of the applied pulses. InGaAs MBE growth was attempted using the nanopore templates. Growth of InGaAs in pores occurred at a substantial depth of about 20–60 nm. The measured photoluminescence (PL) spectra had a new peak at about 1.2 eV in addition to the PL emission from the InP substrate and that from the InGaAs top layer. The new peak was tentatively assigned to the peak arising from InGaAs quantum wire arrays embedded in InP pores with a possible alloy composition change.