K. Shimakawa

Fatigue effect in luminescence of glow discharge amorphous silicon at low temperatures

Abstract Fatigue in the luminescence was observed in glow discharge amorphous silicon at 4.2 K and 77 K. This fatigue was not recovered by infra-red illumination, but by heating the sample at higher temperatures. These results are interpreted in terms of enhancement of non-radiative recombination associated with dangling bonds created by high optical excitation.

The Meyer–Neldel rule in chalcogenide glasses

The electrical conductivity in chalcogenide glasses is found to obey the Meyer–Neldel rule,σ=σ00eΔE/EMNe−ΔE/kT. The Meyer–Neldel characteristic energy, EMN, lies in the range 25–60 meV with σ00 in the range 10−5–10−15 S cm−1 which seems to be too small for band transport. A surprising correlation between σ00and EMN is also found.

A model for the photostructural changes in amorphous chalcogenides

Prolonged photoirradiation induces a volume expansion and causes a decrease in the optical bandgap (photodarkening) in well annealed amorphous chalcogenides. A microscopic mechanism for the origin of these changes is proposed in terms of the repulsive Coulomb force between layered clusters which can be negatively charged by electron accumulation in the conductionband tails. It is also proposed that the photoinduced interlayer Coulomb repulsion increases the interlayer separation, which is responsible for the volume expansion, and it also induces an in-plane slip motion which causes the reduction in the bandgap.

Transient photodarkening in amorphous chalcogenides

Abstract Illumination of amorphous arsenic chalcogenides with band-gap light induces photodarkening and volume expansion. In addition to metastable changes, which persist after the light is switched off, transient effects occur during and after illumination. We present measurements of these metastable and transient effects in evaporated films of a-As 2 S 3 , a-As 2 Se 3 and a-Se. The results are discussed in terms of a model in which, on photoexcitation, layer- or chain-like clusters of atoms expand and slip relative to each other.

Photoinduced structural changes in obliquely deposited As- and Ge-based amorphous chalcogenides: correlation between changes in thickness and band gap

Abstract The effect of band gap illumination and annealing below the glass transition temperature on the thickness and the optical band gap of As-based (As 2 Se 3 , As 2 S 3 ) and Ge-based (GeSe 2 , GeS 2 ) obliquely deposited chalcogenide films has been studied. It is observed that in the case of arsenic (As)-based glasses, illumination increases the thickness (expansion) and the band gap decreases (darkening), while as for germanium (Ge)-based glasses, both thickness and band gap show an opposite behavior to that of As-based glasses. By annealing the samples, before and/or after illumination, the trends of the changes in thickness and band gap are reversed. A strong correlation between the changes in thickness and band gap has been established for the first time.

Residual photocurrent decay in amorphous chalcogenides

Abstract The long-term photocurrent decay following the steady state photoexcitation was measured in amorphous As 2 Se 3 film as a function of temperature. The data are described empirically by the extended exponential law and are explained by dispersive diffusion-controlled recombination of excess D 0 ; 2D 0 → D + + D − .

Percolation-controlled electronic properties in microcrystalline silicon: effective medium approach

Electronic transport and optical properties of microcrystalline Si prepared by plasma enhanced chemical vapor deposition are discussed in terms of effective medium approximation (EMA). The electrical conductivity and the Hall mobility as a function of volume fraction of crystalline are replicated well by the EMA calculation with a percolation threshold at 33% crystalline volume fraction. The well known excess optical absorption in fundamental absorption region can be also explained by the EMA. The Hall mobility is sublinearly proportional to the size of crystallites when the volume fraction of crystallite is kept the same, which is attributed to a fractal property of microcrystalline system.

Electronic transport in degenerate amorphous oxide semiconductors

The fundamental parameters associated with electronic transport, that is the scattering time, mean free path, effective mass and hence microscopic mobility for free electrons, have been estimated from the free-carrier absorption, Hall effect and dc conductivity in an optically transparent and highly conducting (degenerate) new class of amorphous oxides. No sign anomaly is observed in the Hall effect and the optical conductivity measured at room temperature obeys the classical Drude formula. The dc conductivity is proportional to temperature below room temperature, suggesting that the transport is in a weak-localization regime at these temperatures.

In situ photoexpansion measurements of amorphous As2S3 films: Role of photocarriers

To understand the dynamics of photoinduced volume expansion (PVE) in amorphous chalcogenides, in situ PVE (time evolution of thickness changes during illumination) measurements have been performed. Two distinct behaviors, transient and metastable PVE, have been observed. A strong correlation between the changes in thickness and photocurrent is reported.

The origin of non-Drude terahertz conductivity in nanomaterials

The Drude-Smith (DS) model hitherto has been a well accepted model for the terahertz conductivity of nanomaterials, even though its physical basis is not clear. It is shown that a series sequence of transport involving grains and grain boundaries produces a Lorentzian-type energy loss and dominates the THz conductivity in nanomaterials, which is able to explain both the real and imaginary parts of the conductivity. The present model represents a completely different point of view than the standard Drude-Smith model.