Takanari Yasui

Au/ZnSe contacts characterized by ballistic electron emission microscopy

Ballistic electron emission microscopy (BEEM) has been performed on Au/ZnSe (001) diodes prepared in ultra high vacuum. An average barrier height (BH) of 1.37 eV is found for Au/n‐ZnSe in close agreement with previously published values for diodes measured by conventional techniques. The BH distribution is relatively narrow, from 1.32 to 1.43 eV, consistent with cross‐sectional transmission electron microscopy which indicates that the interface is abrupt, and without reaction products. These results differ from those reported for BEEM measurements on chemically etched Au/ZnSe diodes. [R. Coratger et al., Phys. Rev. B. 51, 2357 (1995)].

Role of interface microstructure in rectifying metal/semiconductor contacts: Ballistic electron emission observations correlated to microstructure

Ballistic electron emission microscopy (BEEM) is a technique for the measurement of nanoscopic spatial variations in the barrier height of metal‐semiconductor contacts. We have used BEEM in conjunction with simultaneous scanning tunneling microscopy observations of topography, as well as cross‐sectional and plan‐view transmission electron microscopy, to investigate diode nonidealities and relate them to microstructure. Au/ZnSe and PtSi/Si diodes were examined using these techniques. When analyzed with the parallel conduction model, distributions of barrier heights observed by BEEM in area scans agree well with values measured by conventional techniques and reported in the literature. The wider Au/ZnSe barrier height distribution is thought to be correlated with a rougher interface structure than the PtSi/Si.

Direct observation of nitrogen acceptor passivation in ZnSe by hydrogen plasma

Abstract Passivation effects of nitrogen acceptor in hydrogenated ZnSe are investigated with low-temperature photoluminescence. Nitrogen-doped samples were exposed to a hydrogen plasma produced by an rf plasma cell. The significant PL spectral change indicates that hydrogen neutralizes the activity of nitrogen acceptors in ZnSe. This is the first observation of intentional hydrogen passivation of nitrogen acceptors in ZnSe, which plays an important role in determining the p-type conductivity of ZnSe grown by metalogranic chemical vapor deposition.

Optical properties of ZnS and ZnCdSZnS MQW grown by molecular beam epitaxy on GaAs and CaF2 substrates

Abstract ZnS-based II–VI materials were successfully grown by molecular beam epitaxy (MBE). Epitaxial growth was confirmed on GaAs(100) and CaF 2 (100) substrates by observing reflection high-energy electron diffraction (RHEED) patterns. The low-temperature photoluminescence (PL) spectrum of undoped ZnS grown on both substrates exhibits dominant excitonic emission without any deep emission. Nitrogen (N) doping of ZnS was performed using an RF plasma cell. A new excitonic emission related to N-acceptors was observed around 3.765 eV. A ZnCdS ZnS multiple quantum well (MQW) exhibits a dominant excitonic PL band, and its quantized energy states up to n = 3 were observed in the photoluminescence excitation (PLE) spectrum by monitoring the lower energy side of the PL emission. Oscillatory structures at equidistant energies (about 42 meV) were also observed in the PLE spectrum by monitoring the higher energy side of the PL emission. These peaks are attributed to a multiple-LO-phonon relaxation process of hot excitons.

Indentation-Induced Selective Growth of Carbon Nanotubes

Semiconductor carbon nanotubes (CNTs) have been selectively grown in an indented area formed on Co (8 nm)/Si substrates by catalytic-ethanol chemical vapor deposition (CVD) at 700 to 900 °C for 10 min. The local pressure caused by the indentation process formed metastable Si, inhibited Co/Si combinations, caused Co aggregation, and acted as a catalyst for CNT growth within indents. The CNTs were single-walled nanotubes or bundle of them, and those qualities were uniform between each of the indents, as confirmed by the scanning-electron microscopy (SEM) and Raman spectra.

Anomalous optical absorption in ZnCdSe/ZnSe single quantum wells

Abstract The optical absorption in ZnCdSe/ZnSe single quantum wells was studied by analyzing their reflectance spectra measured by a Fourier transform spectrometer. At lower temperatures, we observed a decrease in the excitonic absorption. This is qualitatively explained using a polariton model with the absorption dominated by phonon scattering. We present a comparison with III–V materials. The result confirms the polariton model of light propagation in quantum wells.

Nanosecond Response Found in Photoexcited Surface Carriers Generated by CoSix Nanoparticles on Si Substrate

We have found photovoltaic carriers generated by CoSix nanoparticles (about 10 nm in radius) fabricated on a Si substrate when the illuminated spot was separated by 1 mm from the anode. The CoSix/Si system provided a sensitivity of 420 mA/W under CW laser excitation at 635 nm with an incident power of 1.8 mW. The carriers response time of <10 ns was observed under pulsed (picosecond) laser excitation. These high-speed carriers were isolated from the Si substrate, as observed from the ~15 ns delayed peak. These results suggest that the carriers photoinduced from the CoSix nanoparticles are surface conductive and have high mobility comparable to that of thermal emission electrons.

Magnetic Circular Dichroic Spectra of CuCl Thin Films

Magnetic circular dichroic (MCD) spectra of 80-A to 1700-A thick CuCl films were measured around the Z 3 exciton region up to 6 tesla (T) at 10 K. The MCD spectra with quantized states of the translational motion of the Z 3 exciton are well explained by Zeeman splitting. The g values of these quantized states are deduced from the best fitting between the MCD spectra and the derivative of transmission spectra. CuCl films from 80 to 290 A thick showed no change in the g value with respect to the film thickness and the quantum number n . The g values of 700- and 1700-A thick CuCl films, which were expected to exhibit bulk characteristics, were found to be identical to those of thinner films. These results indicate that the quantum size effect does not contribute to the relative motion of electrons and holes in excitons for a confinement size exceeding 80 A.

Molecular Beam Epitaxy of ZnSe Films and ZnCdSe/ZnSe Quantum Wells on Cleavage-Induced (110) Surfaces

ZnSe films and ZnCdSe/ZnSe quantum wells were grown for the first time on cleavage-induced GaAs(110) and ZnSe(110) surfaces. When the deposition began, the overgrown surface of the ZnSe film became flat within a few seconds. The optical properties of the ZnSe(110) films and ZnCdSe/ZnSe(110) quantum wells were comparable to those grown on GaAs(001) substrates, indicating the high-quality of these materials. Using a two-step MBE method, high-quality epitaxial materials were grown on a cleavage-induced ZnSe(110) surface. This work demonstrates the possibility of fabricating high-quality quantum wires on a (110) surface obtained by cleaving ZnCdSe/ZnSe(001) superlattices.

Optical polarization measurements of exciton-Zeeman splitting and carrier-spin relaxation in ZnxCd1-xSe/ZnSe multiquantum wells

We investigated exciton-Zeeman splitting and carrier-spin relaxation in ZnxCd1-xSe/ZnSe multi-quantum wells using magnetic circular dichroic (MCD) measurements and photoluminescence polarization (PLP) spectra. We obtained the exciton-g value from the exciton-Zeeman splitting in the MCD spectra. The well width and Cd composition measurements indicate the possibility of controlling the exciton-g value in a nonmagnetic II-VI compound system. We observed the polarization of the higher exciton levels in the PLP spectra as a result of carrier- spin relaxation. The light hole excitons with positive polarization were affected by the strain induced by the Cd composition in the well layer.