(Invited) SiC Growth Parameter Evolution Utilizing Infrared Thermal Imaging Towards Realization of Extremely Low Dislocation Bulk SiC

Abstract

The NASA Glenn Research Center (GRC) has employed infrared thermal imaging technology in a hot wall chemical vapor deposition (CVD) system for thermal measurement of silicon carbide (SiC) crystals mounted vertically in the gas stream. The use of infrared thermal imaging technology revealed deficiencies in growth parameters that had previously not been observable utilizing traditional noncontact monitoring techniques. GRC has been investigating the lateral expansion (⊥ to (0001)) of pseudo SiC fiber seed crystals using a custom designed horizontal flow hot wall CVD system in proof of concept research, for a radically new bulk growth process. This bulk growth process would, ideally produce SiC boules with just one single centrally located screw dislocation (SD). Dislocations in general have a negative impact on SiC devices and device fabrication. Current commercially available SiC substrates have a dislocation density of ~1000 cm2. A key requirement of this process is the lateral homoepitaxial growth expansion of a SiC fiber without the introduction of additional defects. This requires maintaining growth temperatures along the length of the SiC fiber. In the absence of purpose grown fiber like seed crystals, pseudo seed crystals were created by mechanically dicing thin slivers from a-plane and m-plane oriented boule slices. The slivers (~17000 x 2000 x 400) µm’s were subsequently subjected to gaseous etching in hydrogen to produce needle like seed crystals with fiber like dimensions with a width of ~100 µm’s. The seed crystals were oriented vertically in a gas stream of the horizontal CVD system. Traditional optical pyrometry coupled with a small CCTV aligned along the horizontal axis was used to be provide real time thermal and optical data of the CVD. Due to the extremely small cross-sectional area of the seed crystals, the thermal radiation was insufficient to be measured with optical pyrometry and was ineffective in resolving the temperature of the vertical slivers. Only the temperature of the carrier holding the seed crystals could be resolved. A commercially available infrared thermal imaging system equipped with an extra-long working distance (530mm) lens replaced the pyrometer and CCTV. The thermal imager requires that the target be only 3 x 3 pixels to obtain accurate temperatures. When coupled with long working distance optics the temperature of the seed crystals could be resolved. A thermal gradient that was previously unobserved was now viewable. Growth parameters were adjusted to compensate for the gradient preventing nucleation of 3C-SiC at the tip of the seed crystal. A growth rate (⊥ to (0001)) of ~80 μm/hr was achieved. Total lateral growth was > 3.5mm. Initial analysis by synchrotron white beam x-ray topography (SWBXT) confirms, that the fiber growth was homoepitaxial, matching the polytype of the respective underlying region of the seed sliver. Although the as grown SiC contained a high density of basal plane dislocations (BPD’s) it appears that the BPD’s originated in the pseudo seed crystal without additional BPD’s introduced during the growth.