Nucleation, Epitaxial Growth, and Characterization of Beta-Silicon Carbide Thin Films on Silicon by Rapid Thermal Chemical Vapor Deposition
The principal objective of this dissertation is to study monocrystalline epitaxial SiC thin film nucleation and growth on Si by rapid thermal vapor deposition (RTCVD). In addition to the actual growth process development, this has also included the characterization of the as-grown SiC films in terms of crystal structure, morphology, chemical composition, and electrical properties, as well as the fabrication of simple SiC/Si heterojunction diodes. The (100) Si substrate was first converted to SiC layer by carbonization at elevated temperatures in simple hydrocarbon ambients at both atmospheric and low pressures. The SiC films were evaluated by X-ray diffraction, FT-IR, ellipsometry, SEM, AFM, AES, and TEM analyses. The effects of flow rates, temperature, temperature ramp rate, and pressure were studied. The crystallinity, thickness, and morphology of SiC films were found to be a strong function of the hydrocarbon concentration in the gas stream and the growth pressure. Voids (hollow spaces) have been observed to exist in the Si substrate underneath the SiC film except for one condition: high hydrocarbon concentration in the gas stream under which an ultra-thin (~ 10 nm) void-free single crystal SiC film is formed. High -resolution TEM analysis of this film indicated that five SiC lattice planes aligned with four Si lattice planes. Optimum conditions in terms of crystallinity are: 1300 ^circC, 90 sec, 25-50^ circC/s temperature ramp, 13 sccm C _3H_8, 1.5 lpm H _2. The growth rate is in the range of 0.5 -1 nm/sec. Both X-ray diffraction and TEM analyses indicated that films grown under the optimum conditions were single -crystal epitaxial cubic SiC thin films. In addition, simple hydrocarbon gases such as propane, ethylene, acetylene, and methane showed similar behaviors when reacting with Si except for slight difference in reactivity. The evolution of SiC nucleation on Si was studied by combining the surface analysis capability of SEM and AFM with the excellent ability of RTCVD to control the growth reaction process. A nucleation mechanism which has been proposed to explain the existing experimental observations consists of the following: (i) the initial nucleation density is determined by the precursor concentration in the reaction gas; (ii) the lateral and vertical growth of individual nuclei proceeds by the consumption of Si atoms around their periphery; (iii) Si voids are formed along the SiC/Si interface when nuclei grow large enough to come in contact and, thus, to restrict the supply of Si atoms. The subsequent SiC film growth after carbonization was carried out by the addition of silane into the gas steam. The growth was optimized by varying growth temperature and Si/C ratio in the gas steam. The optimum growth conditions are: 1200^circC, Si to C ratio of 0.4 in the gas stream, and 1.5 lpm H_2 . SiC films have also been grown using organosilane precursors such as diethylsilane and methylsilane. Compared to SiC film growth by silane and propane, the growth by organosilanes was achieved at lower temperatures (700-1000 ^circC), and resulted in a sharp, void-free interface, smooth surface morphologies, and higher growth rates in the range of 2-12 nm/sec. Unintentionally-doped SiC films (~ 2000A) grown on p-type Si by carbonization in C _3H_8 ambient and by growth from methylsilane precursor have been used to fabricate SiC/Si heterojunction diodes. Ni deposited by sputtering and annealed in Ar ambient serves as the ohmic contact to SiC film. I-V and C-V were used to characterize the SiC/Si diodes. Diodes made on SiC films by carbonization in C_3H_8 and by growth from methylsilane both showed an ideality factor of close to unity and a reverse breakdown voltage of larger than 150 volts.
- Pub Date:
- SILICON CARBIDE;
- Engineering: Electronics and Electrical; Engineering: Materials Science; Physics: Electricity and Magnetism