Hierarchical Porous Patterns of n-type 6H–SiC Crystals via Photo-electrochemical Etching

Hierarchical porous patterns have been fabricated on the C face, Si face, and cross section of n-type 6H–SiC crystal via photo-electrochemical etching using HF/C₂H₅OH and HF/H₂O₂ as electrolytes. The porous layer displayed multiple and multiscale microstructures on different faces, including stalactite-like, sponge-like and dendritic porous structures on C face, echinoid micro-patterns on Si face, and columnar and keel-shaped micro-patterns on the cross section. The formation of hierarchical porous pattern is ascribed to the dynamic competition balance between the electrochemical oxidation rate and the oxide removal rate. It was found that increasing the ionic strength of the electrolyte can obviously disturb the surface morphology of the porous SiC during the photo-electrochemical etching. Possible mechanisms for selective etching were further discussed.

Source:Journal of Materials Science & Technology

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Defect evolution during growth of SiC crystals

SiC single crystals grown by sublimation exhibit relatively high dislocation densities and often contain a network of slightly misoriented grains. In order to understand the evolution of dislocation structures and grain boundaries during growth, we studied wafers sliced from different parts of the sublimation-grown SiC single crystals. The wafers have been characterized using imaging with crossed-polarizing filters, etching in molten KOH, optical microscopy and X-ray rocking curves. It was found that in the growth direction from the seed towards the boule dome the dislocation density decreases and the crystal quality as determined by X-ray diffraction measurements improves, while the cross-polarizer image contrast becomes more pronounced. The observed trends are discussed.

Source: Journal of Crystal Growth

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Low indium-doped LEC GaAs crystal growth employing thermal stress analysis

Recently, dislocation-free LEC GaAs crystals have been able to be grown by indium doping at the 10²⁰ cm^-3level. However, striations and In inclusions are still problems in In-doped LEC GaAs. We have overcome these problems by reducing the doped In concentration. The ingots have been grown so that the calculated stress value in any part of the ingot may not exceed 0.07 kg/mm², which was the critical resolved shear stress estimated by the thermal stress analysis. Employing this technique under the same growth conditions as those for the low dislocation, (4–5) x 10³ cm^-2 etch pit density, undoped GaAs crystal, dislocation-free 2 inch diameter wafers have been obtained at (1–4) x 10¹⁹ cm^-3 In doping level.

Source:Journal of Crystal Growth

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Two-channel IR gas sensor with two detectors based on LiTaO3 single-crystal wafer

Sc:Ce:LiTaO₃ crystals were grown by using the Czochralski method. The two-wave coupling experiments and transmitted facula distortion method were used to measuring the photorefractive properties and the optical damage resistance, respectively. It was found that the photorefractive response speed of Sc:Ce:LiTaO₃ was four times faster than that of Ce:LiTaO₃ and the resistance ability to optical damage was two orders higher than that of Ce:LiTaO₃. It could be attributed to the Ce ions losing their electron acceptor properties, which result in the increase of photoconductivity. In this paper, site occupation mechanism of impurities was also discussed to explain the increased of photoconductivity.

Source:Xiangli Liu

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Investigations on growth and two-wavelength holographic storage properties varied with RuO2 codoping in Fe:LiNbO3 crystals

A series of Ru:Fe:LiNbO₃ crystals with fixed concentration of Fe₂O₃(0.1 wt%) and varied RuO₂ were grown by the Czochralski technique via the optimum growth condition. After growth, the crystals were cut to wafers and then oxidized to Nb₂O₅ powder. The X-ray diffraction and UV–vis spectra were measured to analyze the defect structure of the crystals. The blue photorefractive properties of Ru:Fe:LiNbO₃ crystals were investigated in a two-wave mixing experiment at the wavelength of 476 nm. The results showed that Ru:Fe:LiNbO₃ crystals with high concentration of RuO₂ are promising candidates for holographic storage. The two-wavelength nonvolatile holographic storage experiment was carried out with sample 6.

Source:Journal of Crystal Growth

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Electrical properties and electrical field in depletion layer for CZT crystals

Current—voltage (I—V) and capacitance—voltage (C—V) characteristics of Au/p-CZT contacts with different surface treatments on cadmium zinc telluride (CZT) wafer's surface were measured with Agilent 4339B high resistance meter and Agilent 4294A precision impedance analyzer, respectively. The Schottky barrier height was 0.85±0.05, 0.96±0.05 eV for non-passivated and passivated CZT crystals by I—V measurement. By C—V measurement, the Schottky barrier height was 1.39±0.05, 1.51±0.05 eV for non-passivated and passivated CZT crystals. The results show that the passivation treatment can increase the barrier height of the Au/p-CZT contact and decrease the leakage current. The main reason is that the higher barrier height of Au/p-CZT contacts can decrease the possibility for electrons to pass through the native T_eO₂ film. Most of the applied voltage appears on the depleted layer and there is only a negligible voltage drops across the nearly undepleted region. Furthermore, the electric field in the depleted layer is not uniform and can be calculated by the depletion approximation. The maximum electric field of CZT crystals is E_ml=133 V/cm at x=0 for non-passivated CZT crystal and E_m2=55 V/cm for passivated CZT crystal, respectively.

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