XRD studies on the femtosecond laser ablated single-crystal germanium in air

Ultrashort laser ablation of single-crystal germanium has been performed in air with femtosecond laser pulses (150 fs, 1 kHz) of 810 nm in the laser fluence range of 0.7–35.4 J/cm². Ablation depth dependence on the laser fluence shows that there are two different processes, which are explained in terms of electronic heating process and the optical penetration one. Structure of ablated region is characterized by means of two different XRD techniques. With increasing the laser fluence higher than 10.2 J/cm², the laser-processed region of germanium exhibits poly-crystalline diffraction peaks in a wide-angle (θ/2θ$\mathrm{}$) scan and a split of diffraction peak of (4 0 0) plane in the rocking curve, which are absent in the lower laser fluence. These observations could be explained in terms of structural changes induced by ultrashort laser irradiation at the higher laser fluence.

Source: Optics and Lasers in Engineering

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Study of EDM cutting of single crystal silicon carbide

Electrical discharge machining (EDM) is developing as a new alternative method for slicing single crystal silicon carbide (SiC) ingots into thin wafers. Aiming to improve the performance of EDM slicing of SiC wafers, the fundamental characteristics of EDM of SiC single crystal were experimentally investigated in this paper and compared to those of steel. Furthermore, EDM cutting of SiC ingot by utilizing copper foil electrodes was proposed and its performance was investigated. It is found that the EDM characteristics of SiC are very different from those of steel. The EDM machining rate of SiC is higher and the tool wear ratio is lower compared to those of steel, despite SiC having a higher thermal conductivity and melting point. Thermal cracks caused by the thermal shock of electrical discharges and the Joule heating effect due to the higher electrical resistivity are considered to be the main reasons for the higher material removal rate of SiC. It is concluded that the new EDM cutting method utilizing a foil electrode instead of a wire electrode has potential for slicing SiC wafers in the future.
Highlights

• Big difference was found between wire EDM characteristics of SiC and cold tool steel SKD11.

• Fracture generation may contribute to the material removal process of EDM of SiC.

• A new EDM cutting method utilizing foil electrode was proposed and investigated.

• Short pulse duration was more suitable for EDM SiC.

• Minimum kerf width of around 100 μm was obtained utilizing the foil EDM method.

Source: Precision Engineering

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Space grown semi-insulating gallium arsenide single crystal and its application

Low noise field effect transistors and analogue switch integrated circuits (ICs) have been fabricated in semi-insulating gallium arsenide (SI-GaAs) wafers grown in space by direct ion-implantation. The electrical behaviors of the devices and the ICs have surpassed those fabricated in the terrestrially grown SI-GaAs wafers. The highest gain and the lowest noise of the transistors made from space-grown SI-GaAs wafers are 22.8 dB and 0.78 dB, respectively. The threshold back-gating voltage of the ICs made from space-grown SI-GaAs wafers is better than 8.5 V. The correlation between the characterizations of materials and devices is studied systematically.

Source:Advances in Space Research

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Interaction of voids and nano-ductility in single crystal silicon

This paper investigates void growth and interaction in a single crystal silicon cubic box under hydrostatic tension by performing three-dimensional strain-controlled molecular dynamics simulations. Two types of fracture behaviors are observed: brittle cleavage on void surface and ductile void coalescence in the inter-void ligament. A critical initial inter-void ligament distance is suggested to be the transition criterion for distinguishing the two behaviors. When the distance between the voids is less than the critical initial inter-void ligament distance, the silicon cube tends to fracture via void coalescence. We demonstrate that the nano-ductility of single crystal silicon is due to vacancy diffusion triggered by void surfaces, which is different from that of metals. In addition, the effect of temperature on the nano-ductility is also investigated. Single crystal silicon becomes ductile at high temperature due to the thermal activated vacancy diffusion.

Highlights

► Voids growth and interaction in single crystal silicon cubes are investigated.

► Two fracture behaviors are observed: brittle cleavage and ductile void coalescence.

► The brittle–ductile transition depends on initial inter-void ligament distance.

► Initial inter-void ligament distance increase with void radius increasing.

► Nano-ductility in single crystal Si is due to vacancies diffusion in the ligament.

Source:Computational Materials Science

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Influence of boron on the point defect equilibrium in highly n-doped gallium arsenide single crystals

High n-type conductivity of melt-grown gallium arsenide single crystals is usually achieved by doping with tellurium or silicon. The lower carrier concentration and Hall mobility in silicon-doped crystals is attributed to the formation of acceptor defects, in particular , the isolated gallium vacancy  and the (Si_Ga–V_Ga)^2-${}_{}{\mathrm{}}_{}{}^{\mathrm{}}$ complex. We show that the contamination of the crystals with boron, which is unavoidable in growing techniques using a boron oxide encapsulant, is decisive for the degree of compensation. In highly n-doped gallium arsenide crystals boron is not only incorporated as the isoelectronic defect . Additionally, high concentrations of  and the negatively charged B_As${\mathrm{}}_{}$-donor complex are formed. These acceptors can dominate the equilibrium of point defects depending on the concentration ratio of the n-dopant and boron.

Source: Physica B: Condensed Matter

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Manufacturing detectors for digital X-ray images of melt-grown CdTe and CdZnTe single crystals

In this work, a progress in the high-yield growth of large detector-grade CdTe and CdZnTe single crystals was made; this seems to be a step to the mass production of sensing element of X-ray images. The scheme of obtaining crystal blocks with minimal loss of material was developed, the technology of manufacturing multi-element X-ray detectors based on these blocks was implemented, the detectors obtained were tested, and digitized X-ray images were taken using them.

Source:sciencedirect

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Thermal oxidation of single crystal aluminum nitride — A high resolution transmission electron microscopy study

The impact of the oxidation time on the structures of thermal oxides formed on AlN was determined by high resolution transmission electron microscopy (HRTEM). Oxidation of AlN single crystals was performed for 2 to 6 h at 1000 °C. Oxidation for 2 h produced mostly amorphous oxide layers whereas oxidation for both 4 and 6 h produced partially crystalline oxide layers. The oxide layer thickness varied from 205 to 600 nm for oxidation times of 2 and 6 h respectively. The crystalline oxide was mostly single phase α-Al₂O₃ except at the surface where it was a mixture of γ-Al₂O₃ and α-Al₂O₃. Based on the different structures produced for different oxidation times, we speculate that the oxide formed changes with thickness: first an amorphous oxide, then γ-Al₂O₃, and finally α-Al₂O₃ as the oxide thickness increases. The AlN crystal was nearly defect- and oxygen-free for oxidation at 1000 °C. This could be due to the rapid diffusion of the nitrogen and aluminum interstitials at a high temperature leading to a point-defect equilibrium throughout the nitride. A faceted interface between Al₂O₃ and AlN could be attributed to the surface diffusion to minimize energy.

Source: Materials Letters

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Free nucleation of aluminum nitride single crystals in HPBN crucible by sublimation

Clear and colorless aluminum nitride (AlN) single crystal thin platelets up to 60 mm² were prepared at 2100 °C and 800 Torr in hot-pressed boron nitride (HPBN) crucibles by free nucleation in a graphite furnace. Crystals grown in HPBN crucibles typically form thin platelets with the fastest growth rate (above 400 μm/h) occurring in the c-axis direction. Growth striations frequently run the length of the crystals, probably due to the presence of boron in the growth environment. Raman spectra and X-ray topography reveal that the crystals have good structural quality. Emissions peaks around 4.10 eV, 3.90 eV, and 3.70 eV were observed in the photoluminescence spectrum, suggesting that boron from the boron nitride crucible may incorporate into the AlN crystals as hexagonal boron nitride.

Source:Materials Science and Engineering: B

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Multilayer structures of silicon-suboxide embedded in single crystal silicon

Si/SiOx multilayer structures with ultra-thin silicon-suboxide layers are fabricated with molecular beam epitaxy. The silicon surface is oxidized during growth interruptions at an oxygen pressure between 1.0×10⁻⁷mbar and 8.0×10⁻⁷ mbar. Overgrowth with Si of the oxidized surface is possible for coverages of a few monolayers of O and improves with increasing substrate temperature. X-ray diffraction shows that the silicon layers are single crystalline. Transmission electron microscopy measurements show that the suboxide layers are ~1nm thick, pseudomorph, and exhibit crystalline order throughout the layer. In addition, transmission electron microscopy shows that the oxygen concentration is laterally inhomogeneous. The multilayer structures are thermally very stable, as rapid thermal annealing up to 1000 °C shows no influence on the X-ray diffraction patterns.

Highlights

• Epitaxial growth of ultra-thin silicon-suboxide layers embedded in c-silicon.

• Growth of multilayer structures of Si/SiOx with up to 20 repetitions.

• Simulation method to determine oxygen content of SiOx layers.

• SiOx layers are stable thermally up to 1000 °C for 5 min.

Source:Journal of Crystal Growth

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