Deposits of clastic carbonate-dominated (calciclastic) sedimentary slope systems in the rock record have been identified mostly as linearly-consistent carbonate apron deposits, even though most ancient clastic carbonate slope deposits fit the submarine fan systems better. Calciclastic submarine fans are consequently rarely described and are poorly understood. Subsequently, very little is known especially in mud-dominated calciclastic submarine fan systems. Presented in this study are a sedimentological core and petrographic characterisation of samples from eleven boreholes from the Lower Carboniferous of Bowland Basin (Northwest England) that reveals a >250 m thick calciturbidite complex deposited in a calciclastic submarine fan setting. Seven facies are recognised from core and thin section characterisation and are grouped into three carbonate turbidite sequences. They include: 1) Calciturbidites, comprising mostly of highto low-density, wavy-laminated bioclast-rich facies; 2) low-density densite mudstones which are characterised by planar laminated and unlaminated muddominated facies; and 3) Calcidebrites which are muddy or hyper-concentrated debrisflow deposits occurring as poorly-sorted, chaotic, mud-supported floatstones. These

Phd by thesis

Silicon carbide (SiC) has a range of useful physical, mechanical and electronic properties that make it a promising material for next-generation electronic devices. Careful consideration of the thermal conditions in which SiC [0001] is grown has resulted in improvements in crystal diameter and quality: the quantity of macroscopic defects such as hollow core dislocations (micropipes), inclusions, small-angle boundaries and long-range lattice warp has been reduced. But some macroscopic defects (about 1-10 cm(-2)) and a large density of elementary dislocations (approximately 10(4) cm(-2)), such as edge, basal plane and screw dislocations, remain within the crystal, and have so far prevented the realization of high-efficiency, reliable electronic devices in SiC (refs 12-16). Here we report a method, inspired by the dislocation structure of SiC grown perpendicular to the c-axis (a-face growth), to reduce the number of dislocations in SiC single crystals by two to three orders of magnitude, rendering them virtually dislocation-free. These substrates will promote the development of high-power SiC devices and reduce energy losses of the resulting electrical systems.

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Ultrahigh-quality silicon carbide single crystals

A full-potential band structure calculation, within the local density approximation to the density functional theory, has been performed for the polytypes 3C, 2H, 4H, and 6H of SiC. The calculated effective electron masses are found to be in very good agreement with experimental values. The electron-optical phonon coupling has been estimated and the polaron masses are calculated to be 3%–13% larger than the corresponding bare masses. The effective electron masses of the second lowest conduction band minima are also presented and the calculated energy difference between the two lowest minima in 4H–SiC is only 0.12 eV. The lowest conduction band in 6H–SiC is found to be very flat and to have a double-well-like minimum along the ML line. The top of the valence bands has been parametrized according to the k⋅p approximation, whereupon the effective hole masses have been determined. The spin-orbit interaction was found to have a strong influence on the value of the effective hole masses. Furthermore, total and partial densities of states are presented.

Relativistic band structure calculation of cubic and hexagonal SiC polytypes

SiC materials are currently metamorphosing from research and development into a market driven manufacturing product. SiC substrates are currently used as the base for a large fraction of the world production of green, blue, and ultraviolet light-emitting diodes (LEDs). Emerging markets for SiC homoepitaxy include high-power switching devices and microwave devices for S and X band. Applications for heteroepitaxial GaN-based structures on SiC substrates include LEDs and microwave devices. In this paper we review the properties of SiC, assess the current status of substrate and epitaxial growth, and outline our expectations for SiC in the future.

SiC materials-progress, status, and potential roadblocks

Applicability of GaN in unipolar and bipolar devices for high-power electronic applications is evaluated with respect to similar devices based on other materials. Specific resistance is used as a measure of unipolar performance. In order to evaluate bipolar performance, 700 and 6000 V p-i-n diodes based on Si, 6H-SiC, and GaN are compared with respect to forward conduction and reverse recovery performance at room temperature and high-temperature conditions. It is shown that GaN is advantageous not only for high voltage unipolar applications, but also for bipolar applications. An empirical closed-form expression is presented to predict the avalanche breakdown voltage of wide band-gap semiconductors. Formulation of the expression is based on an approximation of the impact ionization coefficient in terms of seventh power of the electric field.

Performance evaluation of high-power wide band-gap semiconductor rectifiers

A growth process has been investigated for the epitaxial growth of silicon carbide. The technique can simply be described as chemical vapor deposition (CVD) at high temperatures, hence the name high temperature CVD (HTCVD). The growth process however, differs greatly from that of the CVD process due to the significant sublimation and etch rates at the extreme growth temperatures (1800–2300°C). The grown rates obtained with the HTCVD are in the order of several tens of μm/h to 0.5 mm/h. The purity and crystallinity of the growth layers are outstanding showing strong free exciton related photoluminescence.

High temperature chemical vapor deposition of SiC

Two high temperature CVD techniques, respectively optimised for epitaxial and crystal growth, are presented. A chimney reactor has been developed for fast epitaxy, carried out at 1700‐1900°C, with growth rates ranging from 10 to 25 m mh 1 , and a material quality close to conventional CVD processes. The growth of 4H-SiC epilayers with low n-type doping (10 14 ‐10 15 cm 3 ) and carrier lifetimes up to 0.4 ms is described, while the feasibility of high voltage Schottky rectifiers (1.8 kV) is demonstrated. On the other side, developments of the stagnant flow HTCVD process, where growth is carried out at 2000‐2300°C, are shown to enable growth rates ranging from 0.3 up to 0.8 mm h 1 . The main characteristics of HTCVD grown SiC crystals (up to nearly 7 mm thick) are described. © 1999 Elsevier Science S.A. All rights reserved.

High temperature CVD growth of SiC

Growth-related structural defects in seeded sublimation-grown SiC

Growth of 6H and 4H-SiC crystals via sublimation technique has been studied in a temperature range of 2300–2450°C. The growth rate strongly depends on temperature distribution, growth pressure, and source to seed distance. The influence of the seed quality on the polytype stability and defect occurrence has been studied. 6H–SiC crystals up to 35 mm in diameter and 20 mm in length have been grown with low micropipe density, 50 cm −2 . Micropipe free areas up to 1 cm 2 have been obtained under optimized growth conditions. The analysis of the results obtained with 4H–SiC growth has shown that this polytype has more narrow window of growth parameters which requires more strict control especially at the initial stage of growth. The seed quality plays an important role for the stability of the polytype and for defect control in the growing crystal.

Seeded sublimation growth of 6H and 4H-SiC crystals

SiC boules were grown by the modified Lely method. Macro-defects in the boules and wafers cut from them were studied by means of optical microscopy and chemical etching. The influence of the quality, the surface orientation and attachment of the seed crystal on secondary evaporation, domain and micropipe formation was studied. The results obtained are discussed in terms of defect development at different stages of the crystal growth and under different growth conditions.

A. Vehanen

Papers

High temperature chemical vapor deposition of SiC

High temperature CVD growth of SiC

Growth-related structural defects in seeded sublimation-grown SiC

Seeded sublimation growth of 6H and 4H-SiC crystals

Defect origin and development in sublimation grown SiC boules