Mitsuo Kawashima

Layer‐by‐layer sublimation observed by reflection high‐energy electron diffraction intensity oscillation in a molecular beam epitaxy system

Reflection high‐energy electron diffraction (RHEED) intensity oscillation during sublimation of GaAs is observed at high substrate temperatures above 690 °C. This post‐growth RHEED intensity oscillation suggests that the sublimation occurs layer by layer. One period of this oscillation precisely corresponds to sublimation of one monolayer. Aluminum arsenide acts as a sublimation stopper. The sublimation rate was measured accurately as a function of substrate temperature.

Chlorine-related photoluminescence lines in high-resistivity Cl-doped CdTe

Abstract High-resistivity Cl-doped CdTe crystals grown by the gradient-freeze method were studied by high-resolution photoluminescence (PL) and electrical measurements. A sharp PL line ascribed to a Cl-related defect was observed at 1.5903 eV in all of the high-resistivity crystals doped with different amounts of chlorine. A broad line around 1.586 eV was clearly observed in highly doped crystals. The intensities of these lines increased with the amount of chlorine. Annealing under Cd-saturated atmosphere eliminated the 1.5903 eV line and intensified the neutral donor-bound exciton line together with its two-electron transition. The high-resistivity crystals were also converted to n-type with low resistivity by the Cd annealing. These results show that the 1.5903 eV line is a key emission line for the high-resistivity, and suggest that the line is associated with Cd-vacancy/chlorine complex, probably in the form of V CD -Cl.

Carrier drift mobilities and PL spectra of high resistivity Cadmium Telluride

Abstract Time-of-flight (TOF) measurements and photoluminescence (PL) properties of Cl-doped semi-insulating CdTe, grown by the gradient freeze (GF) method are described. By choosing appropriate growth conditions, drift mobilities as high as 1100 and 80 cm 2 /V·s for electrons and holes respectively were obtained, which are comparable to those grown by the traveling heater method (THM). From a comparison of PL and TOF results, correlations between the intensity of PL line W (at 1.587 eV) and the drift mobilities of both electrons and holes have been found. The relations of the PL lines W and G (at 1.591 eV) are analyzed by a compensating defect model composed of (V Cd Cl Te ) and (V Cd 2Cl Te ).

ZnSe homo-epitaxial growth by molecular beam epitaxy

Abstract High-quality undoped ZnSe homo-epitaxial growth by molecular beam epitaxy was demonstrated. A chemical etchant H 2 O 2 , NH 4 OH and H 2 O, combined with a new thermal cleaning process, has been found promising for obtaining good initial ZnSe substrate surfaces necessary for high-quality homo-epitaxial growth by molecular beam epitaxy. The photoluminescence (PL) result is compared to that of a hetero-epitaxial layer on GaAs (001) substrate. A free excitonic PL emission line has been distinctly observed on the homo-epitaxial ZnSe layer grown on (001) substrate. A dominant emission line I 2 in the 4.2 K PL spectrum of the homo-epitaxial ZnSe layer has been ascribed to In donors introduced by the In solder used to adhere the substrate to the molybdenum holder.

Annealing behavior of bound exciton lines in high quality CdTe

Annealing behavior of shallow donor‐bound and acceptor‐bound exciton lines in CdTe crystals grown by the Bridgman method was investigated by high‐resoluton photoluminescence measurements at 4.2 K. The intensity of the neutral acceptor‐bound exciton line (A0X) at 1.5896 eV, which was commonly observed in p‐type CdTe, drastically decreased by annealing under Cd saturated atmosphere. By further subsequent annealing under Te saturated atmosphere, the A0X line recovered again. On the other hand, the intensity of the neutral donor‐bound exciton line increased by annealing under Cd saturated atmosphere and decreased by the further annealing under Te saturated atmosphere. It was found that this change in the photoluminescence lines by the annealing process was reversible. These results strongly suggest that the A0X line at 1.5896 eV can be ascribed to the recombination of excitons trapped at Cd‐vacancy/donor‐impurity complexes.

Correlation between electrical and photoluminescence measurements in high‐quality p‐type CdTe

High‐quality p‐type CdTe crystals grown by the gradient freeze method were characterized by the radial resistivity distribution, Hall‐effect, and photoluminescence (PL) measurements. These measurements were performed over the crystal ingot as a function of position along the growth direction. The resistivity and the carrier concentration monotonically varied from 200 to 20 Ω cm and from 1×1014 to 1×1015 cm−3 along the growth direction, respectively. All photoluminescence spectra measured have well‐resolved spectra. It was found that both the resistivity and the carrier concentration strongly correlate to the photoluminescence intensity ratios of the neutral donor‐bound to the neutral acceptor‐bound exciton lines. The Hall mobility slightly decreased along the growth direction, and this result corresponded to the broadening of one longitudinal‐optic‐phonon replica of free excitons in the PL spectra. This good correspondence between the electrical properties and the PL spectra has provided useful means for ...

Photoluminescence and annealing behavior of Ga-doped CdTe crystals

Ga-doped CdTe crystals grown by the vertical gradient freeze method were studied by measuring high-resolution photoluminescence (PL) and electrical properties. Comparisons were made for as-grown and annealed samples under Cd saturation. The as-grown samples grown from slightly Te rich melt were of high resistivity (108 Ω cm) although gallium on the Te site alone in CdTe acts usually as a shallow donor. The Cd-annealed samples, on the other hand, were n-type with low resistivity. A new prominent PL line at 1.5841 eV was found in the high-resistivity as-grown crystals. Furthermore, the 1.5841 eV line completely disappeared after Cd annealing. These results suggest that the luminescence center related to the 1.5841 eV line is a Cd-vacancy/gallium associated and is responsible for high resistivity. This is direct evidence to explain the high-resistivity mechanism due to so-called self-compensation in CdTe(Ga). The two-electron transitions in Ga donors measured by PL are also reported.

Growth of ZnSe on Ge(100) substrates by molecular‐beam epitaxy

Epitaxial growth of ZnSe on Ge(100) substrates by molecular‐beam epitaxy was investigated and compared with the simultaneous growth on GaAs(100). Single crystalline ZnSe on Ge was obtained with a flat and smooth surface at a temperature of higher than 370 °C and flux ratio JSe/JZn of around 2.0. The crystal quality, as revealed by double‐crystal x‐ray rocking curves, was not superior to that grown on GaAs, despite a better lattice parameter match with the substrate. This is due to the fact that the grown layer contains antiphase domains and inclines against the Ge substrate towards 〈011〉 by about 0.1°.

Planar doping of p-type ZnSe layers with lithium grown by molecular beam epitaxy

Abstract High quality p-type ZnSe layers with hole concentrations of 3 × 10 17 − 1.4 × 10 18 cm -3 have been grown using a planar doping technique employing lithium during molecular beam epitaxy (MBE). The Li-doping is investigated by reflection high energy electron diffraction (RHEED). It is found that islands of Li 2 Se are formed on the growing ZnSe surface in the case of heavy Li doping. Secondary ion mass spectroscopy (SIMS) and photoluminescence (PL) are used to characterize the layers grown. SIMS analysis shows an abrupt change in Li concentration at the interface between highly doped (mid 10 19 cm -3 ) and undoped layers. This is the first report of the successful suppression of Li diffusion. PL spectra at 4.2 K show dominant acceptor-bound exciton emission (I 1 ). The line shape of I 1 is noticeably broadened with a tail towards the low energy side. This may be explained in terms of a Stark effect on bound excitons arising from charged impurities.

Evaluation of directly bonded silicon wafer interface by the magic mirror method

Abstract The magic mirror method was used to evaluate directly bonded Si wafers. An unbonded region at the silicon-to-silicon interface was detected as a bubble in an optical image by this method. The bubbles at the interface generate a locally convex distortion at the surface and this convex surface is visualized as a dark region using the magic mirror method. A comparison of the resulting effect was made with transmission X-ray topography and ultrasonic flaw detection, and good correspondence was found among the three methods, especially between the magic mirror method and transmission X-ray topography. We applied this method to the observation of changes of interface bubbles before and after thermal annealing.