Materials Science Forum Vols. 527-529

Abstract: A selective atmospheric pressure chemical vapor deposition (APCVD) process has been developed to deposit porous polycrystalline silicon carbide (poly-SiC) thin films containing a high density of through-pores measuring 50 to 70 nm in diameter. The selective deposition process involves the formation of poly-SiC films on patterned SiO2/polysilicon thin film multilayers using a carbonization-based 3C-SiC growth process. This technique capitalizes on significant differences in the nucleation of poly-SiC on SiO2 and polysilicon surfaces in order to form mechanically-durable, chemically-stable, and well anchored porous structures, thus offering a simple and potentially more versatile alternative to direct electrochemical etching.

Abstract: We have investigated the growth of SiC, following a modified sol-gel process, which not only allows the realization of 3D photonic bandgap materials but also is useful for various SiC applications like templates in medicine or filters in harsh environment. Depending on the sol-gel annealing procedure one obtains macro-porous SiC, amorphous SiC, or from nano to micrometer-sized 3C-SiC single crystals. At low annealing temperatures preferably nanowires are grown. Via various sol-gel-annealing procedures we are able to prepare single crystals with sizes ranging from several nm up to several 100 %m, while the resulting polytype only depends on the annealing temperature available. Not only for photonic applications useable procedures for doping with shallow level donors and acceptors as well as with deep level defects are essential. We show that controlled doping is possible either during the sol-gel preparation or via the gas phase during the following annealing procedure.

Abstract: SiC nanopowder has been formed using an original technological approach based on grinding of bulk porous SiC nanostructures. The initial porous SiC nanostructures were obtained by anodization of n+-type 4H-SiC substrate in HF/Ethanol solution under UV illumination. Large single SiC nanoparticles (~ 30 nm in diameter) constituting the nanopowder have a porous structure which can be clearly visible. On the other hand, small single SiC nanoparticles (~ 4 nm in diameter) exhibit a clear crystalline structure. A broad and very intense luminescence band (400 – 900 nm) provided from the nanopowder corresponds to the radiative processes involving nanoparticle surface states. A smaller photoluminescence peak centred at 358 nm may correspond to radiative recombination of the photogenerated excitons confined in the individual and spatially separated 4HSiC nanoparticles.

Abstract: In this paper, we report a novel route to synthesize nano-sized cubic silicon carbide (3CSiC) powder by a chemical vapor deposition (CVD) technique in a resistance-heated furnace. The nanoparticles were deposited on the relatively cold region of a hot-wall quartz reactor. Hexamethyldisilane (HMDS) was used as the source material for both silicon and carbon. The presence of crystalline 3C-SiC was identified using powder x-ray diffraction (XRD) technique. From the XRD data, the crystallite size was also estimated to be in the range of nanometers (nm). A clear evidence of the particle size (~ 10 - 30 nm) was obtained by transmission electron microscopy (TEM). Selected area electron diffraction (SAED) was carried out on the nanoparticle assembly. The ring shaped pattern is a clear indication of polycrystalline particle formation. High resolution TEM (HRTEM) of nanoparticles was performed to study the crystal structure in detail. The nanoparticles were also characterized by Raman spectroscopy at room temperature. Finally, the influence of the growth parameters is also reported in the present study.

Abstract: We demonstrate the fabrication and the electrical transport properties of single crystalline 3C silicon carbide nanowires (SiC NWs). The growth of SiC NWs was carried out in a chemical vapor deposition (CVD) furnace. Methyltrichlorosilane (MTS, CH3SiCl3) was chosen as a source precursor. SiC NWs had diameters of less than 100 nm and lengths of several μm. For electrical transport measurements, as-gown SiC NWs were prepared on a highly doped silicon wafer, pre-patterned by a photo-lithography process, with a 400 nm thick SiO2 layer. Source and drain electrodes were defined by e-beam lithography (EBL). Prior to the metal deposition (Ti/Au : 40 nm/70 nm) by thermal evaporation, the native oxide on SiC NWs was removed by buffered HF. The estimated mobility of carriers is 15 cm2/(Vs) for a source-drain voltage (VSD) of 0.02 V. It is very low compared to that expected in bulk and/or thin film 3C-SiC. The electrical measurements from nanowire-based field effect transistor (FET) structures illustrate that SiC NWs are weak n-type semiconductor. We have also demonstrated a powerful technique, a standard UV photo-lithography process, for fabrication of SiC nanowires instead of using EBL process.

Abstract: The major objective of our studies was the thermodynamic analysis of the nSiС+SiO2 system and revealing potentialities for the implementation of the SiC gas-phase transport conditions. As a result of thermodynamic scanning of the chemical activity of nSiС+SiO2 system the conditions for implementing the SiC gas-phase transport were found out within a wide temperature range. It was found out that the basic process in the gas-phase transport of silicon carbide is: SiCs + SiOg→ 2Sig + СОg SiC evaporation at T2 2Sig + СОg → SiCnanowhiskers + SiOg SiC deposition at T1 Sequential evaporation and deposition of silicon carbide result in the growth of SiC crystals from a gas phase. The processes of SiC gas-phase transport and deposition were experimentally realized. Synthesized were SiC nanocrystals over 300 μm long, ~ 300 nm in diameter that forms a three-dimensional subskeleton inside the carbon skeleton.

Abstract: A brief survey is given of some recent progress regarding ion implantation processing and related effects in 4H- and 6H-SiC. Four topics are discussed; an empirical ion range distribution simulator, dynamic defect annealing during implantation, formation of highly p+-doped layers, and deactivation of N donors by ion-induced defects.

Abstract: The annealing behavior of the N+ implantation-induced defects in 4H-SiC(0001) has been investigated by means of Rutherford backscattering spectrometry in the annealing temperature range from 200 to 1000 oC. The samples are multiple-implanted by N+ ions with energy range from 15 to 120 keV at a total dose of 2.4 x 1015 /cm2. Three annealing stages are observed by isochronal annealing; first stage from 200 to 400 oC, second stage from 400 to 600 oC and third stage from 600 to 1000 oC. The 80 percent of the N+ implantation-induced defects are annealed out at the temperature above 600 oC. The annealing mechanism of the defects in each stage is discussed.

Abstract: We have performed nitrogen and phosphorus co-implants at room temperature to obtain high n-type carrier concentration layers in SiC. An inductive heating RTA furnace has been used for the activation annealing. The influence of the temperature ramp parameters such as rise/decrease temperature speed and intermediate annealing steps on the dopant activation rate and surface morphology have been investigated. A reduction of the temperature ramp slope reduces the surface roughness by 50%. Inclusion of a pre-activation annealing step at low temperatures (1300°C) further reduces the surface roughness. However, the use of slower ramps or an intermediate annealing step during ramp up reduces the free carrier concentration. The faster the ramp up, the higher the activation rate and the resulting doping. We also demonstrate that the inclusion of a postactivation annealing at intermediate temperatures (1150°C) reduces significantly the surface roughness. In addition, the use of this post-annealing treatment does not degrade the activation rate nor the carrier Hall mobility, and activation rates close to 100% have been obtained.