Toshiyuki Ninomiya

Tunneling process in AlAs/GaAs double quantum wells studied by photoluminescence

The tunneling process of photogenerated carriers in AlAs/GaAs double quantum well structures in which two wells have different widths (60 and 80 A) was studied by steady‐state and time‐resolved photoluminescence spectra. The tunneling process was found to play an important role for a barrier thickness narrower than about 40 A. The tunneling rate was determined as 2×1010 s−1 for a 30‐A barrier. The quantum‐mechanical penetration depth of the wave function into an AlAs barrier was estimated as 6 A from the barrier width dependence of luminescence intensity ratio between the two wells. The tunneling rate and penetration depth are consistent with a simple envelope function approximation with no Γ‐X mixing.

Ionic Conduction in Zone‐Refined KCl

Ionic conductivity in the extrinsic range of temperature (250° to 400°C) was measured for specimens taken from a single‐crystalline ingot of KCl subjected to zone purification. The effect of dislocation density was studied in order to clarify the nature of ionic conduction in these highly purified crystals. Conductivity increased by a factor of 10 in the purest crystals by increasing dislocation density from 106 to 107 cm−2. Conductivity was decreased in the impurer crystals by increasing the density of dislocations. Comparing the results with predictions from charged dislocation theory, the value of 0.7 ev is obtained for the formation energy of positive ion vacancies.

Luminescence and optically detected ESR in a-As2S3

Abstract Luminescence with decay time of about 10 nsec and the Stokes-shift of about 0.84 eV was found in a-As2S3 by the excitation at the absorption edge at 4.2 K. The peak position of the luminescence depended on the excitation energy and the luminescence remembered the polarization of light of the excitation. The experimental data can be explained by assuming that the luminescence and the absorption edge are due to localized excitons. Photo-structural change is discussed according to the exciton model. In addition, ESR of centers of the luminescence observed by Kolomiets et al. was optically detected.

Luminescence with short decay time in arsenic chalcogenide glasses

Abstract Luminescence with decay time of about 10 nsec or shorter was found in a−As2S3 at 4.2 K on the high energy side of the luminescence observed by Kolomiets et al. From the dependence of the emission spectra on the excitation energy, it is interpreted as being due to the recombination of localized excitons.

INTERRELATION BETWEEN GLASS TRANSITION AND REVERSIBLE PHOTOSTRUCTURAL CHANGE IN a-As2S3

Abstract The optical absorption edge has been observed to make a parallel shift to lower energy in a -As 2 S 3 as the glass transition temperature, T g , increases. The structural difference corresponding to different T g and reversible photostructural change give the same linear relation between the edge shift and the volume change. The saturated position of the edge after long time illumination or annealing is determined only by the condition of last treatment and is independent of previous history. These results suggest that both structural changes are of the same kind and that in the glass there exists an equilibrium state under illumination, if crystallization is avoided.

Dislocation Damping in Semiconductors and Abrupt versus Smooth Kinks

A previous dragging point model of dislocation mobility is modified and applied to the dislocation damping data in Si and Ge. The motion of built‐in kinks along a dislocation is considered to be responsible for the damping. In contrast to the abrupt kink model, the free motion of the kinks is impeded by the dragging points. The predictions of this model are compared with those made on the basis of the abrupt kink model, with regard to (a) damping, (b) estimated kink width, and (c) dislocation mobility. The dragging‐point model seems to give a more consistent picture, although direct experimental verification of the presence of dragging points is not yet available. An improved table of derived parameters for the dragging‐point theory of dislocation mobility is given based on additional information from damping measurements in Ge.

Transport properties of photoexcited carriers in a fibonacci superlattice

Abstract We have investigated the electronic structure and the perpendicular transport properties of photoexcited carriers in a GaAs/AlAs Fibonacci superlattice (SL) with an enlarged well (EW) by means of photoluminescence excitation (PLE) spectroscopy and picosecond luminescence measurements. The PLE spectrum of the SL emission shows the structure corresponding to the subbands obtained in the Fibonacci system by splitting of the level of n =1 in the isolated well. Difference in the PLE spectrum between the SL and EW emission also shows that the only very few carriers generated in the lowest subbands in the Fibonacci SL reach the EW at 4 K, and comparison with results for a random and a periodic SLs suggests that the degree of localization in the Fibonacci SL is intermediate among the three systems. It is also found from the temporal evolution behaviors of the photoluminescence that about a half of the carriers created in higher energy states travel to EW within 50 ps and the phonon-assisted transport of the other half at 50 K in the Fibonacci and the periodic SLs is faster than that in the random SL.

Photoluminescence Decay in Amorphous As2S3

The decay of the intensity of the luminescence in a-As2S3 with the energies of 1.16 and 1.57 eV was measured from 10-8 sec to 2×10-3 sec at 4.2 K. The shape of the decay curves shows three kinds of luminescence whose intensity decays according to the empirical decay function (f(t)=tn-1exp -(t/τ)n) with effective decay times (τ) of 2×10-8, 2×10-6 and 2×10-4 sec. Each luminescence process is discussed in this paper.

Change in optical anisotropy of localized states associated with reversible photostructural change in a-As2S3 films

Abstract In annealed As 2 S 3 films, band-gap illumination induces red-shift of optical absorption edge and increase of polarization memory of photoluminescence. In the case of illumination with linearly polarized light, anisotropy is induced in both. From dependence of the edge shift, the anisotropy of the absorption edge and the increase of polarization memory on the illumination conditions (temperature, photon energy and intensity), it was found that there was a strong correlation between the latter two but the edge shift had no correlation with the latters. The latter two reflect the increase of the optical anisotropy of the localized states and the reorientation of the anisotropy axes to the direction perpendicular to the polarization vector of the illumination light, but the bulk change in the structure gives the edge shift.

Time dependence of the polarization of the luminescence in chalcogenide glasses

Abstract Time dependence of the polarization of the luminescence with polarized exciting light and its temperature dependence were observed in chalcogenide glasses. The experimental data can be analyzed with a model based on localized excitons in a fluctuating potential due to random structure of the glasses.