Silicon Carbide Growth by High Temperature CVD techniques

• Silicon Carbide (SiC) is a wide band gap semiconductor, which already W. Shockley in 1959 expected to replace silicon owing to its outstanding figures of merit for high-power, high-frequency and high-temperature electronics.Since 1978, there has been a continuous progress in SiC crystal growth, nowadays essentially based on the seeded sublimation technique enabling the realisation of substrates with increasing diameter and quality. This has triggered a strong scientific and technological interest in SiC. On-going research and development in the international community are progressing fast towards an increasing commercial importance of SiC electronics. This in turn increases the demands on growth techniques, because most of the remaining identified limitations for industrial applications originate from the material quality and cost.The main contribution of the present work relates to the development and investigation of two growth techniques, which can simply be described as chemical vapour deposition at high temperatures. The first one, carried out between 1650 and 1850 °C in a vertical hotwall, or chimney reactor, is shown to enable one order of magnitude higher growth rates than conventional chemical vapour deposition (CVD) systems, while maintaining acomparable material quality. The second one, carried out at 2000-2300 °C in an inverted vertical reactor, allows growth rates ranging between 0.3 and 0.8 mm/h, which are of interest for crystal growth applications.The thesis is divided into seven papers.In paper I, we introduce a fast SiC epitaxy process which is carried out in a chimney CVD reactor adapted to growth temperatures ranging from 1650 to 1850 °C. The epitaxial growth of 4H-SiC, with rates of 10 to 50 μm/h, is analysed in terms of gas flow dynamics, rate determining factors as precursor supply and growth temperature. The deposition uniformity is shown to be essentially mass transport limited, whereas the exponential growth rate dependence with temperature is related to gas phase reactions limiting steps. The relation between the growth conditions and the material quality, such as morphology, purity, and carrier lifetime is investigated in paper II. Epitaxial layers grown at 25 μm/h are shown to enable fabrication of high voltage Schottky rectifiers with breakdown voltages as high as 3.8 kV. The incorporation of the nitrogen n-type dopant is studied in paper III, bothin the high doping range (n ∼1018 cm-3) and in the high purity range (n∼1014 to 1015 cm-3), as a function of growth parameters. Accurate control of the doping is shown to be possible both by standard means as C/Si control, but also by the growth pressure and temperature.In paper IV we investigate the influence of surface morphology and structural defects on the reverse performance of kV-class Schottky rectifiers. Only diodes processed on areas with low density of screw dislocations are reported to block voltages of 2 kV and more. In paper V, the structural defects present in low doped (1015 cm-3 range), thick (30-40 μm) CVD grown epilayers are investigated using synchrotron X-ray diffraction topography; the observed defects are shown to be either replicated from the substrate or introduced during the epitaxial growth process. Substrate replicated threading edge dislocations aligned within small angle boundaries are shown to relate to local decreases of the carrier lifetime measured on as-grown epilayers. Network of dislocations lines are also found and may becaused by a doping induced misfit between the epilayer and the conductive substrate.In paper VI and VII we describe the development of a high growth rate process which may be an attractive alternative to Physical Vapour Transport (PVT) growth techniques for controlled growth of high purity, high quality, SiC crystals.In paper VI, we investigate the growth process, described as High Temperature Chemical Vapour Deposition (HTCVD). The conditions leading to growth of both 6H and 4H-SiC crystals with rates ranging from 0.3 to 0.8 mm/h are described. The growth rate limiting steps as process temperature, temperature gradient, precursor supply are analysed, together with gas phase nucleation phenomena. The growth of 6H and 4H-SiC crystals with thickness ranging up to 7 mm and diameter up to 40 mm is reported.4H-SiC crystal growth by HTCVD is investigated in paper VII. The conditions leading to a stable growth of the 4H polytype are addressed together with structural properties such a micropipe defect density. Owing to the high purity of the HTCVD technique, growth of high-resistivity 4H-SiC (p > 109 Ω.cm at room temperature) is shown to be achieved without the need of intentional compensation.

Ämnesord

NATURVETENSKAP  -- Fysik -- Den kondenserade materiens fysik (hsv//swe)

NATURAL SCIENCES  -- Physical Sciences -- Condensed Matter Physics (hsv//eng)

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