Papers by Keyword: Off-Axis

Abstract: To achieve high quality SiC growth on Si substrate, it is essential to get a smooth Si surface without forming SiC and graphitic islands during the surface cleaning and before the carbonisation process. In this paper, a novel in-situ surface cleaning method designed for the hetero-epitaxial growth of SiC on Si substrate is developed using a custom-made low-pressure chemical vapour deposition reactor. The results indicate that the combination of ramping in oxygen and subsequent flowing of SiH₄ avoids the contamination of Si, enables the oxide layer to be removed smoothly, and subsequently creates a smooth Si surface with regular atomic steps. SiC grown on off-axis Si has better crystallinity and significantly smaller roughness than that grown on on-axis Si.

Abstract: The off-axis solution growth of 4H-SiC was studied focusing on the morphological instabilities by using conventional TSSG technique. The morphology depends strongly on the crystalline polarity, and that on Si surface can be characterized by wandering while that on C surface is characterized by strong step-bunching. By raising the temperature gradient, step bunching on Si surface is considerably suppressed which can be consistent to the constitutional super cooling scheme. However, C surface exhibits strong step bunching as the temperature gradient increase. These behaviors can be explained by the difference in Ehrlich-Schwoebel barrier and diffusion behavior of adatoms.

Abstract: This paper describes the application of loose-abrasive processes to the manufacture of 1.4 meter, off-axis aspheric, hexagonal mirror-segments. These are prototypes for the 39 m European Extremely Large Telescope (E-ELT). The application of active forces to correct the overall form of segments in the telescope, means that the overall form-accuracy achieved in polishing can be less critical than for a non-active mirror. However, it is a requirement that the base-radii and conic constants of mirror-segments are very closely matched, so that the combined image is not degraded. This means that abrasive processes have to operate with respect to an absolute rather than relative datum. Furthermore, there are stringent requirements on mid-spatial frequency defects on segment surfaces, and on edge-roll. These control stray-light, and ultimately detectability of faint astronomical targets. We describe the CNC abrasive techniques we have developed in response. We then demonstrate the success of the approach, which represents the first time ever that segments have been processed entirely in the hexagonal shape:- a milestone in loose-abrasive processing. Finally, we address up-scale for the unprecedented number of segments required for the E-ELT build-phase.

Abstract: The losses of the off-axis rotary optical fiber communication system were derived from optical fiber coupling, three kinds of misalignments between optical fiber collimators (Axial separation Z₀, lateral offset X₀, angular tilting θ), incomplete alignment of optical fiber collimator during rotation and system tremble caused by high speed rotating. Some measures were taken to reduce the loss. The thermally expanded core fiber collimator cut down the influence of axial separation and angular tilting. The position of the optical fiber collimator on the flange was adjusted and infrared right angle prisms were installed to reduce the losses during rotation of the system. In addition, improving the precision and optimizing device of mechanical structure can increase the stability of the whole experiment platform and decrease the losses.

Abstract: In laser beam cutting and laser piercing process, the machining speed and quality are very sensitive to the flow of assist gas and laser beam focusing position. In order not only to improve the cutting speed and the removal capability of the molten material and to save the consumption of the gas flow in laser beam cutting, but also to realize high speed piercing of high aspect-ratio holes, a magnetic-levitated (maglev) lens driving actuator was proposed and fabricated. The actuator can drive the lens to achieve real-time positioning control of the relative radial displacement between the lens axis and the assist gas jet nozzle axis (off-axis control) in radial directions in a range of ±1mm within 1.5 μm of tracking error and bandwidths more than 150Hz, and to achieve real-time positioning control of laser beam focusing point in axial direction in a range of ±5mm within 3 μm of tracking error and bandwidth more than 100Hz.

Abstract: Advanced finite element analysis (FEA) is carried out to simulate the off-axis crushing process of glass/epoxy twill/weave composite box structures. The FE results were compared to relevant experimental result and it was shown that the applied model is capable to predict the damage behaviour and energy absorption of composite box aero-structures under off-axis loading condition.

Abstract: We investigated electrical properties of 4H-SiC trench metal-oxide-semiconductor field-effect transistors (MOSFETs) fabricated on (000_,1) C-face substrates with various off-axis angles. Off-axis angles and directions are 4o, 8o, and 15o towards [__,1120] and 8o towards [1_,100] directions. Most trench MOSFETs showed good on-state performance. Peculiar characteristics that field-effect mobility was 103 cm2/Vs in spite of a relatively high acceptor concentration of 1 × 1017 cm−3 in the channel region were observed for trench MOSFET on 15o-off substrates. From crystallographic analysis, this face is (11_,20) with 15o off towards [000_,1] direction. We can expect that this face has quite good MOS interface properties.

Abstract: Design of off-axis system is a hot and difficult topic in optical design area. The wide field of view(WFOV) off-axis system this paper involved always have the drawbacks of complicated large size and great weight. This paper introduces an application of free-form surface(FFS) in WFOV off-axis optical system design, analyzes FFS optics’ material and fabrication methods. The system uses one FFS lens and one doublet, image quality meets the design specification, and characterizes by simple structure and lightweight.

Abstract: Solution growth on off-axis 4H-SiC sublimation substrate as a buffer layer for the subsequent CVD epitaxial growth was investigated. Dislocation conversion and propagation from the substrate to the CVD epitaxial layer through the solution grown buffer layer was inspected by molten KOH etch pit observation. Effective dislocation conversion from BPD to TED as an effect of the buffer layer insertion with no drastic change in the total EPD was confirmed.

Abstract: Chemical vapor deposition of (111) 3C-SiC on (110) Si substrate was carried out, and the effect of the substrate off-axis introduced on (110) Si substrate for suppressing the twin formation in 3C-SiC hetero-epitaxial layers was investigated. From the growth on hemispherically polished (110) Si substrate, it was found that the off-axis toward the [001] Si axis had a noble effect for suppressing the twin formation, while the off-axis toward the [110] Si axis was ineffective. The growth of single 3C-SiC crystal containing few double positioning boundaries, which are related with the twin formation, was demonstrated on the (110) Si substrate 3° off-axis toward the [001] Si axis. Transmission electron microscopic observation revealed that double positioning boundaries on the (110) Si substrate off-axis toward the [001] Si axis were nearly eliminated within the initial a few hundreds nano meter in thickness.