Toshiaki Ikoma

Coulomb blockade oscillations at room temperature in a Si quantum wire metal-oxide-semiconductor field-effect transistor fabricated by anisotropic etching on a silicon-on-insulator substrate

We have developed a very controllable fabrication process of an extremely narrow (∼10 nm) quantum wire metal‐oxide‐semiconductor field‐effect transistor (MOSFET) on a separation‐by‐implanted‐oxygen (SIMOX) substrate using anisotropic etching and selective oxidation technique. The drain current versus gate voltage characteristics show oscillations caused by Coulomb blockade even at room temperature. The oscillations split into several sharp peaks when the temperature is decreased, indicating that the channel is separated by several serial coupled quantum dots and that the quantum levels of these dots correspond to the observed fine peaks.

Electrochemical Er doping of porous silicon and its room‐temperature luminescence at ∼1.54 μm

We present a new electro‐chemical method for incorporating high concentration Er ions deep into porous silicon layers and its intense photoluminescence at ∼1.54 μm at room temperature. Porous silicon layers prepared by anodic etching of p‐type silicon substrates in HF/H2O are immersed in ErCl3/ethanol solution. Then the negative bias relative to a counter platinum electrode is applied to the samples. Er3+ ions are drawn into fine pores of the porous silicon layers by the electric field. After thermal annealing at ∼1300 °C in an O2/Ar atmosphere, the samples show sharp and intense Er3+‐related photoluminescence at ∼1.54 μm at room temperature upon excitation with an Ar ion laser.

Variation of the midgap electron traps (EL2) in liquid encapsulated Czochralski GaAs

We have characterized the midgap electron trap (so far believed to be EL2) in liquid encapsulated Czochralski GaAs by measuring deep level transient spectroscopy spectra, and found more than two midgap electron traps which can be classified into two groups. A trap belonging to the first group is rather stable in its properties and the other is unstable. The capture cross section of the levels in the latter group varies continuously in depth from the surface and also is changed by thermal annealing. We discuss these characteristics in connection with growth conditions.

Computer-aided analysis of GaAs n-i-n structures with a heavily compensated i-layer

Current-voltage characteristics and space-charge distributions in an n-i-n structure have been numerically analyzed and compared with Lamperts theory. It is found that an effective resistivity in the low-voltage region depends on acceptor and trap densities and the length of an i-layer. The analytical model has been presented to estimate the effective resistivity and the onset voltage for current rise. The back-gating effect also has been analyzed in terms of a separation distance between devices and an acceptor density. To achieve a good isolation between two devices in GaAs ICs, it is suggested that a shallow acceptor density as well as a trap density must be larger than a critical value.

Temperature and energy dependences of capture cross sections at surface states in Si metal‐oxide‐semiconductor diodes measured by deep level transient spectroscopy

A new deep level transient spectroscopy technique is presented to determine capture cross sections at metal‐oxide semiconductor (MOS) surface states. The technique enables us to determine energy and temperature dependences of capture cross sections separately. Applying this method, electron capture cross sections at surface states in Si MOS diodes were measured and found to have strong energy dependence and rather weak temperature dependence. It was also found that there was an effect to increase the emission rate, which may be attributed to barrier lowering at the Si‐SiO2 interface.

Numerical simulation of GaAs MESFET's on the semi-insulting substrate compensated by deep traps

Numerical simulations of GaAs MESFETs are performed in which impurity compensation by deep traps in the semi-insulting substrate is considered. It is found that the higher acceptor density in the substrate results in lower device current due to the formation of a space-charge layer at the channel-substrate interface. It is also found that the drain currents increase continuously with the drain voltage because electrons are injected to fill the traps in the substrate and a current path through the substrate is formed. This substrate current becomes remarkable for shorter gate-length MESFETs on a substrate with lower acceptor and trap densities. It is suggested that, to minimize short-channel effects in GaAs MESFETs, the acceptor density as well as the trap density in the semi-insulating substrate must be high. >

Spectral distributions of photoquenching rate and multimetastable states for midgap electron traps (EL2 family) in GaAs

Photocapacitance transients due to photoquenching have been measured to characterize midgap electron traps (the ‘‘EL2 family’’) in different GaAs wafers. The spectral distributions of photoquenching rates in liquid encapsulated Czochralski, vapor phase epitaxial, and oxygen‐implanted liquid phase epitaxial GaAs were different to each other and also different from that in horizontal Bridgman‐grown GaAs. From these results it is suggested that the EL2 family has multimetastable states and may consist of arsenic‐atom aggregates.

Phase coherence length of electron waves in narrow AlGaAs/GaAs quantum wires fabricated by focused ion beam implantation

The phase coherence length Lφ of electron waves in the one‐dimensional weak localization regime was studied in selectively doped AlGaAs/GaAs quantum wires fabricated by focused ion beam implantation. Estimated Lφ by fitting the modified weak localization theory to the data is ∼1.2 μm at 0.3 K, nine times longer than in the n‐GaAs wires. This difference is well explained by the mobility dependence of Lφ, and shows the advantage of selectively doped structures to obtain long Lφ. Lφ increased with decreasing temperature and saturated below ∼3 K, indicating the existence of temperature‐independent phase breaking mechanisms.

Impact excitation of the erbium‐related 1.54 μm luminescence peak in erbium‐doped InP

The Er‐related 1.54 μm luminescence peak has been observed in erbium‐doped InP layers by impact excitation of Er atoms with energetic carriers accelerated by electric field. Er ions were implanted into n‐type InP and Au/Sn ohmic contacts were formed on top of the surface. The Er‐related sharp peak at 1.543 μm was observed by only applying dc voltages between the electrodes over the temperature range from 77 to 360 K. Neither band‐edge emission nor impurity‐related emission were observed, although they were intense in the photoluminescence spectra of the same sample. The fine structure of the 1.54 μm peak was also different from that of photoluminescence. This 1.54 μm emission was related to erbium atoms excited through collisions with energetic electrons accelerated by the electric field.

Deep levels in gallium arsenide by capacitance methods

The three capacitance methods, i.e., TSCAP, PHCAP, and transient capacitance measurements, are applied to determine electronic properties of deep levels inn-GaAs. In the boat-grown wafer detected are the 0.30 eV electron trap withNT=3.6×1016 cm−3 andSn=2.4×10−15 cm2, and the 0.75 eV electron trap withNT=2.0×1016 cm−3 andSn=1.2×10−14 cm2. In the epitaxial wafer, the 0.45 eV hole trap is detected withNT>1.5×1013 cm−3 andSp=1.4×10−14 cm2 as well as the 0.75 eV electron trap withNT=2.4×1013 cm−3.