Properties of 4H-SiC by Sublimation Close Space Technique
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INTRODUCTION SiC is suitable for high power devices because of its wide-bandgap and high thermal conductivity. To make a SiC device having high breakdown voltage, thick epitaxial layers are needed. In conventional CVD method ( in which silane and propane are normally used), epitaxial growth rate is about 3 uim/h . From industrial point of view, rapidly grown thick epilayers are required. Sublimation epitaxy ( CST : close space technique) has been demonstrated to grow thick epilayers at higher growth rate [1-51. A close space technique has two advantages compared to other growth methods. One is that the growth apparatus has a simple configuration. The other is the ability to keep the
growth system pure since only two materials are needed, one for the source and the other for the substrate. This growth method had been applied to grow various semiconductors such as Ge [6]. This technique has also been applied to SiC [7,81. It is basically the same as conventional sublimation method [9]. A merit of this configuration is that it minimizes the area of graphite wall in the crucible. Normally in the conventional sublimation method unwanted carbon species comes from the wide area of the graphite wall , and then the quality of epilayers degrades. In CST, unwanted C species are minimized and sublimed vapor is transferred to the substrate in quasi-thermal equilibrium condition. We have previously studied homoepitaxy of 4H-SiC by CST[5]. The surface morphology was observed by optical microscope in the Nomarsky mode and AFM ( Atomic Force Microscope). Crystallinity and purity of the epilayer were characterized by Raman spectroscopy. Smooth layers without step-bunching were obtained at a pressure lower than 40 Torr. The surface morphology of the epilayers was mainly influenced by the off-angle of the substrate and the ambient pressure.
EXPERIMENT 3C-SiC polycrystalline plates of semiconductor grade (MUHSIC:Admap Ltd. ) were used as SiC source material while 4H-SiC (Si-face, C-face) substrates of commercially 179 Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
obtained
wafers inclined 80 off (0001) towards were used as substrates. Size of the
substrate was 5 mm x 5 mm. In order to have an accurate spacer height between source and substrate, a graphite spacer with a square opening size of 4 mm x 4 mm was used. The source and substrate were set in a graphite crucible. A crucible with outer diameter of 34 mm, inner diameter of 24 mm and 33 mm long including cap was used. The crucible was thermally
shielded by graphite foam. A vertical quartz tube cooled by an air-fan was used as the reaction tube. The temperatures
of top and bottom of the crucible were measured
simultaneously by two-color pyrometers(Chino IR-AQ). The experiments were carried out in Ar atmosphere. The crucible in the reaction tube was heated by an rf generator at a frequency of about 45kHz. Our typical growth conditions were as follows: growth temperature of source material (Tg) 1900-2100'C, temperature gradient ( grad T ) 3.5kC/mm, growt
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