Papers by Author: Bharat Krishnan

Abstract: A method was developed for growing SiC nanowires without depositing a metal catalyst on the targeted surfaces prior to the CVD growth. The proposed method utilizes in-situ vapor-phase catalyst delivery via sublimation of the catalyst from a metal source placed in the hot zone of the CVD reactor, followed by condensation of the catalyst-rich vapor on the bare substrate surface to form the catalyst nanoparticles. The vapor-phase catalyst delivery and the resulting nanowire density was found to be influenced by both the gas flow rate and the catalyst diffusion through the boundary layer above the catalyst source. The origin of undesirable bushes of nanowires and the role of the C/Si ratio were established.

Abstract: Growth of SiC nanowires (NWs) on monocrystalline 4H-SiC substrates was conducted to investigate a possibility of NW alignment and polytype control. The growth directions of the NWs on the top surfaces and the vertical sidewalls of 4H-SiC mesas having different crystallographic orientations were investigated. The majority of the NWs crystallize in the 3C polytype with the growth axis. Six orientations of the 3C NWs axis with respect to the substrate were obtained simultaneously when growing on the (0001) plane. In contrast, no more than two NW axis orientations coexisted when growing on a particular mesa sidewall. Growth on a particular {10-10} plane resulted in only one NW axis orientation, giving well-aligned NWs.

Abstract: Vanadium doping from SiCl4 source during epitaxial growth with chlorinated C and Si precursors was investigated as a mean of achieving compensated and semi-insulating epitaxial 4H-SiC layers for device applications. Thin epilayers were grown at 1450°C with a growth rate of ~6 μm/h. Experiments at 1600°C resulted in the growth rates ranging from 60 to 90 µm/h producing epilayers with thickness above 30 µm. V concentrations up to about 10¹⁷cm^-3 were found safe for achieving defect-free epilayer surface morphology, however certain degradation of the crystalline quality was detected by XRD at V concentrations as low as 3-5x10¹⁵ cm^-3. Controllable compensation of nitrogen donors with V acceptors provided low-doped and semi-insulating epitaxial layers. Mesa isolated PiN diodes with V-acceptor-compensated n- epilayers used as drift regions showed qualitatively normal forward- and reverse-bias behavior.

Abstract: A reduced growth pressure (down to 10 Torr) was employed for the low-temperature chloro-carbon epitaxial growth. More than two times lower H₂ flow rate became possible. The optimal input H₂/Si and C/Si ratios were also lower. A significant reduction of the net free donor concentration resulted from the use of the low pressure, delivering partially compensated epilayers with the net free donor concentration below 7x10¹³ cm^-3. Deep levels were characterized in the low-temperature epilayers for the first time. No Z_1/2 or EH_6/7 centers could be detected by DLTS. No strong D₁ photoluminescence signature was observed. The high purity of the obtained epitaxial layers made it possible to use the low-temperature chloro-carbon epitaxial growth to fabricate drift regions of Schottky diodes for the first time. Promising values of the reverse breakdown voltage and the leakage current were obtained from the fabricated devices.

Abstract: In this work, the benefits of the low-temperature halo-carbon epitaxial growth at 1300oC to form anodes of 4H-SiC PiN diodes were investigated. Regular-temperature epitaxial growth was used to form an 8.6 μm-thick n-type drift region with net donor concentration of 6.45x1015 cm-3. Trimethylaluminum doping, in situ during blanket low-temperature halo-carbon epitaxial growth, was used to form heavily doped p-type layers. Forward I-V characteristics measured from diodes having different anode areas indicated that the new epitaxial growth technique provides anodes with low values of the series resistance, even without contact annealing. At room temperature, a 100 μm-diameter diode had a forward voltage of 3.75 V at 1000A/cm² before annealing and 3.23 V after annealing for 2 min at 750°C. The reverse breakdown voltage was more than 680 V (on average) in the devices without edge termination or surface passivation.

Abstract: Growth of SiC nanowires on commercial 4H-SiC substrates by chemical vapor deposition is reported. The main objective was to explore a possibility of reproducing the substrate polytype in order to obtain SiC NWs specifically composed of the hexagonal 4H-SiC polytype. The growth experiments were conducted in a hot-wall CVD reactor with H2 as the carrier gas, SiCl4 as the silicon source, and CH3Cl as the carbon source. Vapor-liquid-solid (VLS) growth mode was enabled by using metal nano-particle on the surface of the 4H-SiC substrates. Formation of nanowires or bigger nano-cones was achieved depending on the temperature and the metal catalyst used. Only SiC phase with no presence of Si was confirmed by X-ray diffraction for the growth temperatures down to 1050°C. The low temperature photoluminescence spectra measured on as-grown NWs showed clear 4H-SiC nitrogen bound excitons in some of the samples, particularly when in-situ N2 doping was used. The density of stacking faults detected by TEM strongly depended on the growth conditions.

Abstract: Chlorinated silicon precursor SiCl4 was investigated as an alternative to SiH4 with HCl addition as a source of additional chlorine in order to suppress the homogeneous nucleation during the low-temperature epitaxial growth at 1300°C. The homogeneous nucleation in the gas phase was further reduced compared to SiH4+HCl growth. The process window for obtaining good epilayer morphology during the CH3Cl/SiCl4 growth was found to correspond to Si supply-limited mode. At lower values of C/Si ratio formation of Si-rich polycrystalline islands/droplets took place. At high C/Si ratio, formation of polycrystalline SiC was the source of morphology degradation. The process window became increasingly narrower at higher Rg, which limited the possibility of significantly increasing Rg at such low growth temperatures. Generation of triangular defects became significant at Rg above 5-6 μm/hr, even when a nearly-optimal value of C/Si ratio was used. Similar experiments were conducted using C3H8, a more traditional precursor, instead of the halo-carbon precursor CH3Cl. While a similar growth rate could be achieved for the same SiCl4 flow rate, much lower values of the C/Si ratio were required. The morphology with C3H8 was worse within the process window. The C/Si process window for the C3H8/SiCl4 growth was much narrower compared to the CH3Cl/SiCl4 growth, and the window essentially disappeared at Rg > 3 4 μm/hr.

Abstract: Thick 4H-SiC epitaxial layers have been grown using a combination of two chlorinated precursors silicon tetrachloride (SiCl4) and chloromethane (CH3Cl) at 16000C. Growth rates up to 100 m/hr have been demonstrated. The use of chloro-silane precursor eliminated the problem of homogenous nucleation of Si in the gas phase, which was significant in CH3Cl/SiH4 growth. Much higher values of Si/H2 and C/H2 ratios without morphology degradation were made possible by increasing the growth temperature from 1300 to 1600°C. Results of photoluminescence and high-resolution X-ray diffraction and time-resolved PL were used to evaluate the quality of the epitaxial layers. The crystalline quality and the growth rate achieved so far offer a promise of exceeding the state of the arts results achieved with more traditional hydro-carbon precursors.

Abstract: Low-temperature halo-carbon homoepitaxial growth is suitable for selective epitaxial growth of 4H-SiC using SiO2 mask. A possibility of achieving high values of doping in combination with the selective growth makes it an alternative to ion implantation for selective doping in SiC. In this work, TMA doping in situ during a blanket low-temperature epitaxial growth was utilized to produce heavily Al doped SiC layers for Ohmic contact formation to p-type SiC. Nearly featureless epilayer morphology with Al atomic concentration exceeding 3x1020 cm-3 was obtained after growth at 13000C with the growth rate of 1.5 µm/hr. Ni TLM contacts with a thin adhesion layer of Ti were formed. The as-deposited metal contacts were almost completely Ohmic even before annealing. The specific contact resistance of 2x10-2 Ohm-cm2 and 6x10-5 Ohms-cm2 was achieved without and with contact annealing respectively. The resistivity of the epitaxial layers better than 0.01 Ohm cm was measured for Al atomic concentration of 2.7x1020 cm-3.

Abstract: In this work, the local-loading effect and its influence on the growth rate enhancement and the growth rate non-homogeneity is investigated during the halo-carbon low-temperature selective epitaxial growth (LTSEG) using an SiO2 mask. The average growth rate during the LTSEG can be more than three-times higher than in blanket epitaxy at the same growth conditions. Both the size of the LTSEG seed windows and the surrounding area covered with the mask determine the growth rate non-homogeneity. A model for predicting the growth rate distribution is suggested.