Online Monitoring of PVT SiC Bulk Crystal Growth Using Digital X-Ray Imaging
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digitized x-ray images of the graphite crucible taken during the growth we have (i) monitored the evolution of the SiC crystal and (ii) investigated the degradation of the SiC source powder (i.e. graphitization) with increasing process time. In this paper we will introduce our new on-line xray imaging method of SiC growth and we will discuss its feasibility by showing several shots of the high temperature SIC vapor growth process. EXPERIMENT 6H and 4H SiC bulk single crystals have been prepared by the PVT technique in an inductively heated graphite crucible at elevated temperatures of T = 2100'C ... 2300'C (figure 1). The temperature has been monitored at the seed end and at the source end of the crucible using optical pyrometers. Argon has been used as carrier gas at a system pressure of p = 5mbar ... 40mbar. As a specialty of our setup, the induction coil can be moved electrically in vertical direction giving rise to a wide range of geometrical configurations and the possibility of an instationary growth process with increasing growth time. As seeds we have used (0001) oriented 30mm ... 40mm SiC crystals grown by PVT in our laboratory. The micropipe density varied between 200cm" ... 500cm-2. At defined times nitrogen was added for 10min to the argon carrier gas in order to introduce doping striations revealing the shape of the growth interface [3]. The source material (SiC powder) was synthesized from elemental Si and C. A typical growth run can be divided in three parts: (i) Heating up is performed under an 259 Mat. Res. Soc. Symp. Proc. Vol. 572 © 1999 Materials Research Society
PVT setup
pyoneter
A
Wlop
PAr coil position
x-ray tube
x-ray detector •Tbottorn
pYeter Figure 1. PVT setup for the growth of SiC bulk crystals. The inductively heated graphite crucible (loaded with a SiC seed crystal (top) and a SiC powder source (bottom)) is imbedded in a graphite like insulation inside a quartz ampoule. Using a x-ray tube and an image plate based detector and scanner, shots of the ongoing processes inside the graphite crucible were taken online during the growth process.
argon atmosphere (p = 800mbar). (ii) At the beginning of the growth (defined as Oh) the argon pressure is lowered to 5mbar ... 40 mbar giving rise to a physical vapor transport of various SiC species from the SiC source to the colder SiC seed. (iii) At the end of the growth run (typically after 72h) the inductive heating is switched off. In order to visualize the ongoing processes inside the graphite crucible during growth we have applied a x-ray imaging system (x-ray tube, image plate detector and scanner) to the PVT setup (figure 1). During x-ray exposure by the x-ray tube an image of the graphite crucible interior (SiC seed/crystal and SiC source material) is transferred to the x-ray detector. While the x-ray source is a conventional x-ray tube for medical care (y-energy = 60keV, I=7mA), a high speed, high resolution and high dynamic range x-ray detector based on image plates and digital recording was used. An overview on the physics of im
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