Katsunori Makihara

Electrical resistivity and photoluminescence spectrum of layered oxysulfide (LaO)CuS

Abstract On growth conditions the dependence of the electrical resistivity and the photoluminescence (PL) spectrum of a layered oxysulfide (LaO)CuS have been investigated. The electrical resistivity shows semiconducting behavior and its magnitude decreases with the increase in the sintering temperature and time, which is considered to introduce structural defects such as Cu or La vacancies. The PL spectrum consists of six emission bands which are assigned to a direct interband transition and the transitions originating in two kinds of donor and acceptor levels corresponding to defect centers in the band gap. The PL spectra depend on the growth conditions. The introduction of lattice imperfection increases the intensity of the wide emission bands, and the (LaO)CuS sample visually appears to be ‘white’ under UV excitation. The white luminescence is an important property for the application to display back-light.

Self-Assembling Formation of Ni Nanodots on SiO2 Induced by Remote H2 Plasma Treatment and Their Electrical Charging Characteristics

We fabricated nanometer-scale Ni dots and NiSi dots on an ultrathin SiO2 layer using remote H2 plasma and demonstrated the feasibility of remote H2 plasma treatment for controlling the areal density of the dots. 1.8-nm-thick-Ni/SiO2 and Ni/Si-quantum dots (QDs)/SiO2 layer were treated with remote H2 plasma generated by the inductive coupling between an external single-turn antenna and a 60 MHz generator. When a Ni/SiO2 was treated with remote H2 plasma at room temperature, Ni nanodot density could be controlled in the range of 109 to 1012 cm-2 by adjusting the plasma conditions. After the remote H2 plasma treatment of the Ni/Si-QDs, the formation of electrically isolated NiSi dots with an areal density of ~1011 cm-2 was confirmed. These results imply that hydrogen radicals generated in H2 plasma play an important role in improving surface diffusion caused by energy reduction at the Ni/SiO2 interface. The surface potential of the Ni nanodots changes stepwise with the tip bias. This is due to the multistep electron injection into and extraction of Ni nanodots. The minimum tip biases for electron injection into Ni nanodots, NiSi dots and Si-QDs were -0.2, -0.7, and -1.0 V, respectively. This reflected the difference in electron affinity among Ni, NiSi and Si.

Native Oxidation Growth on Ge(111) and (100) Surfaces

We studied the native oxide growth on Ge(100) and (111) surfaces treated by HCl and HF cleaning in clean room air by high-resolution X-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE). The native oxidation of both HCl- and HF-last Ge(100) surfaces exhibited likely layer-by-layer fashion. The native oxide growth of the n-Ge(100) was significantly faster than the p-Ge(100) at the early stage of native oxidation. This can be explained by the formation of an O2- ion through free electron transfer from the Ge to the adsorbed O2 molecules, which induces the surface electric field that can initiate the oxidation. In the case of different crystallographic orientations, the oxide rate of the Ge(100) surface was faster than that of the Ge(111) surface. This might be attributed to larger open space of the Ge(100) surface than that of the Ge(111) surface.

Formation of Metal Silicide Nanodots on Ultrathin SiO2 for Floating Gate Application

We demonstrated a new fabrication method of Pt- and Ni-silicide nanodots with an areal density of the order of ~1011 cm-2 on SiO2 through the process steps of ultrathin metal film deposition on pre-grown Si-QDs and subsequent remote H2 plasma treatments at room temperature. Verification of electrical separation among silicide nanodots was made by measuring surface potential changes due to electron injection and extraction using an AFM/Kelvin probe technique. Photoemission measurements confirm a deeper potential well of silicide nanodots than Si-QDs and a resultant superior charge retention was also verified by surface potential measurements after charging to and discharging. Also, the advantage in many electron storage per silicide nanodot was demonstrated in C-V characteristics of MIS capacitors with silicide nanodots FGs.

Evaluation of valence band top and electron affinity of SiO2 and Si-based semiconductors using X-ray photoelectron spectroscopy

An evaluation method for the energy level of the valence band (VB) top from the vacuum level (VL) for metals, dielectrics, and semiconductors from the results of X-ray photoelectron spectroscopy (XPS) is presented for the accurate determination of the energy band diagram for materials of interest. In this method, the VB top can be determined by the energy difference between the onset of VB signals and the cut-off energy for secondary photoelectrons by considering the X-ray excitation energy (hν). The energy level of the VB top for three kinds of Si-based materials (H-terminated Si, wet-cleaned 4H-SiC, and thermally grown SiO2) has been investigated by XPS under monochromatized Al Kα radiation (hν = 1486.6 eV). We have also demonstrated the determination of the electron affinity for the samples by this measurement technique in combination with the measured and reported energy bandgaps (E g).

Formation of Cobalt and Cobalt-Silicide Nanodots on Ultrathin SiO2 Induced by Remote Hydrogen Plasma

High-density Co nanodots with an areal dot density as high as 2.6 ×1011 cm-2 were formed on thermally grown SiO2 by exposing a ~1.2-nm-thick Co layer to a remote H2 plasma without external heating. Also, Co-silicide nanodots on SiO2 were fabricated by silicidation of pregrown Si nanocrystals on SiO2, in which self-assembling Si nanocrystals by low pressure chemical vapor deposition (LPCVD), ultrathin Co film formation, and remote H2 plasma treatment were conducted sequentially. Electrical separation among nanodots in each of the Co and Co-silicide samples was verified from the changes in surface potential after charge injection using an AFM/Kelvin probe technique. The surface potential changes due to electron charging to Co nanodots and discharging from Co-silicide nanodots occur at a tip bias of 0 V, which are attributed to the work function difference between Co nanodots and Co-silicide nanodots. From the observation by magnetic force microscopy, Co nanodots can be active elements for both spin and charge storage.

Temperature Dependence of Electron Tunneling between Two Dimensional Electron Gas and Si Quantum Dots

Quantum mechanical electron tunneling has potential applications in both science and technology, such as flash memories in modern LSI technologies and electron transport chains in biosystems. Although it is known that one-dimensional quantum electron tunneling lacks temperature dependence, the behavior of electron tunneling between different dimensional systems is still an open question. Here, we investigated the electron tunneling between a two-dimensional electron gas (2DEG) and zero-dimensional Si quantum dots and discovered an unexpected temperature dependence: At high temperature, the gate voltage necessary for electron injection from 2DEG to Si quantum dots becomes markedly small. This unusual tunneling behavior was phenomenologically explained by considering the geometrical matching of wave functions between different dimensional systems. We assumed that electron tunneling would occur within a finite experimental measurement time. Then, the observed electron tunneling is explained only by the contributions of wave packets below the quantum dot with a finite lifetime rather than the ordinary thermal excited states of 2DEG.

Characterization of Electronic Charged States of Impurity Doped Si Quantum Dots Using Atomic Force Microsope/Kelvin Probe Technique

Boron and phosphorous were doped into Si quantum dots (Si-QDs) by pulse injection of 1% B2H6 and PH3 diluted with He, respectively, during the self-assembling formation of Si-QDs from the thermal decomposition of pure SiH4 on a ~4.2-nm-thick SiO2 layer thermally grown on a n+-Si(100) substrate. Electron charging and discharging of both the B- and P-doped Si-QDs were investigated to characterize their charged states by a Kelvin probe technique with a Rh-coated atomic force microscope (AFM) tip. Potential changes due to the extraction of one electron from the B- and P-doped Si-QDs were observed in applying the AFM tip biases of +2.0 and +0.2 V, respectively. At a tip bias of ~1.0 V, potential changes observed in the case of undoped Si-QDs, whose size is almost the same as that of doped Si-QDs, were almost the same as those observed in the case of doped Si-QDs. Changes in the tip bias required for the electron extraction from Si-QDs by doping of B and P atoms are attributable to the difference in the electron emission process such that the emissions of a localized electron from an ionized B acceptor and a conduction electron caused by an ionized P donor need higher and lower tip biases than for the case of emission of a valence electron from undoped Si-QDs, respectively.

Study of electron transport characteristics through self-aligned Si-based quantum dots

This paper discussed about the self-aligned Si-based quantum dots structures with ultrathin oxide interlayer spontaneously formed on thermally-grown SiO2-Si(100) formed by LPCVD, the current-voltage characteristics at room temperature have shown the clear current bump and negative differential conductance and the oscillatory current with a voltage period of ~18mV at around the 1st resonance voltage.

Structural defects effect on ferromagnetism of layered oxysulfide (La1−xCaxO)Cu1−xNixS

Abstract We have studied the effect of structural defects on magnetic properties of (La1−xCaxO)Cu1−xNixS. The magnetization for samples containing less defects shows the magnetism consisting of a diamagnetism and a Pauli paramagnetism. Any ordered phase is not observed down to 2 K . A very weak ferromagnetic component is observed for non-stoichiometric system containing structural defects. These samples exhibit a well-defined hysteresis loop at room temperature. The Curie temperature obtained by the extrapolation of its temperature dependence increases with x. This ferromagnetism may be attributed to Ni and/or S related defects.