Siva Prasad Kotamraju

Effect of HCl addition on gas-phase and surface reactions during homoepitaxial growth of SiC at low temperatures

A complex influence of HCl addition on gas-phase and surface reactions during the low-temperature halo-carbon homoepitaxial growth of 4H-SiC was investigated. The addition of HCl was employed to reduce the undesirable effects of homogeneous gas-phase nucleation leading to formation of silicon clusters in the gas phase. It was established that dissociation of silicon clusters by HCl is efficient even at untraditionally low homoepitaxial growth temperature below 1300 °C. The information about the spatial distribution of this dissociation process along the gas flow direction was obtained. It was established that the influence of HCl is more complicated than the simple model suggesting that the enhanced dissociation of silicon clusters in the gas phase leads to an additional supply of silicon species for the epitaxial growth. While the growth rate does significantly increase at least for some HCl flow rates, complex changes in the effective silicon-to-carbon ratio in the growth zone of the reactor indicate th...

Electrical and Optical Properties of High-Purity Epilayers Grown by the Low-Temperature Chloro-Carbon Growth Method

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 Z1/2 or EH6/7 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.

Epitaxial Growth of 4H-SiC with High Growth Rate Using CH3Cl and SiCl4 Chlorinated Growth Precursors

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.

Aluminum Doping by Low-Temperature Homoepitaxial Growth for Ni Ohmic Contacts to p-Type 4H-SiC

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.

Use of Vanadium Doping for Compensated and Semi-Insulating SiC Epitaxial Layers for SiC Device Applications

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.

Use of SiCl4 as Silicon Precursor for Low-Temperature Halo-Carbon Epitaxial Growth of 4H-SiC

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.

Low-Temperature Homoepitaxial Growth with SiCl4 Precursor Compared to HCl Assisted SiH4-based Growth

Chlorinated silicon precursor SiCl4 was investigated as a source of additional chlorine instead of or in combination with HCl during the low temperature (13000C) halo-carbon epitaxial growth. No Si cluster cloud was visible inside the hot-wall susceptor indicating negligible homogeneous gas-phase nucleation. The growth rate was significantly enhanced compared to the SiH4-case, but was relatively close to the SiH4+HCl case. Similar to the SiH4+HCl growth, the increase of the growth rate caused by suppressed cluster formation was less significant than expected. The depletion of the growth species by vigorous polycrystalline deposition upstream of the hot zone, which was earlier reported for the SiH4+HCl growth, was also significant in the SiCl4-based growth. Closer to the growth zone, carbon species also get incorporated in the polycrystalline deposits.

Silicon carbide nanowires grown on 4H-SiC substrates by chemical vapor deposition

In this work, SiC nanowires (NWs) were grown by chemical vapor deposition (CVD) on commercial 4H-SiC substrates. The growth was conducted in an inductively heated hot wall CVD reactor traditionally used for homoepitaxy of 4H-SiC, operating at 150 Torr with H 2 as the carrier gas. The growth experiments utilized the precursor chemistry that previously enabled the so-called low-temperature homoepitaxial growth of SiC – SiCl 4 as the silicon precursor and CH 3 Cl as the carbon precursor. Vapor-liquid-solid (VLS) growth mode was employed. Two metal catalysts Au and Ni were used for NW growth in a wide range of growth temperatures from below 1050 0 C to above 1300 0 C. It was established that high precursor flow rates favor the regular epitaxial growth (though disturbed by the presence of the islands of the metal catalyst) at temperatures above 1200 0 C. Reduction of the precursor flow rates and the growth temperature caused formation of micro-needles and eventually NWs. NW diameters in the range from below 10 to 100 nm were observed using scanning electron microscopy. Only SiC phase with no presence of Si, even for the growth temperatures down to 1050 0 C, was confirmed by X-ray diffraction.

Factors Influencing the Growth Rate, Doping, and Surface Morphology of the Low-Temperature Halo-Carbon Homoepitaxial Growth of 4H SiC with HCl Additive

In this work, a possibility to further suppress silicon vapor condensation and formation of Si clusters in order to improve the growth rate and morphology during the low-temperature halo-carbon epitaxial growth of 4H-SiC was investigated. While a pronounced dissociating of Si clusters was clearly demonstrated, the enhancement of the growth rate and morphology was less significant then expected. In addition, the homogeneity of the growth rate and doping along the gas flow direction indicated that a significant and non-equal depletion of Si and C growth species takes place at sufficiently high HCl supply. HCl flow-dependent formation of polycrystalline Si and SiC deposits in the upstream portion of the hot zone was shown to be the source of this depletion.

Role of Cl/Si Ratio in the Low-Temperature Chloro-Carbon Epitaxial Growth of SiC

In order to understand the influence of the Cl/Si ratio on the morphology of the low-temperature chloro-carbon epitaxial growth, HCl was added during the SiCl4/CH3Cl growth at 1300°C. Use of higher Cl/Si ratio allowed only modest improvements of the growth rate without morphology degradation, which did not go far beyond what has been achieved previously by optimizing the value of the input C/Si ratio. On the other hand, when the epitaxial growth process operated at too low or too high values of the input C/Si ratio, i.e., outside of the window of good epilayer morphology, any additional increase of the Cl/Si ratio caused improvement of the epilayer morphology. It was established that this improvement was due to a change of the effective C/Si ratio towards its intermediate values, which corresponded to more favorable growth conditions.