Materials Science Forum Vols. 527-529

Abstract: The move towards commercialization of SiC based devices places increasing demands on the quality of the substrate material. While the industry has steadily decreased the micropipe (MP) levels in commercial SiC substrates over the past years, the achievement of wafers that are entirely free of MPs marks an important milestone in commercialization of SiC based devices. We present the results of a study for controlling the nucleation and propagation of MP defects in SiC ingots grown via PVT. Our studies confirm that during bulk growth of SiC, foreign polytype nucleation such as 3C-polytype occurs at the initial stages of growth (nucleation period) and/or during subsequent growth in the presence of facets. Results in this investigation suggest that polytype instability during crystal growth adversely impacts the MP density. Based on this key concept, growth conditions for nucleation and growth stages were optimized. These conditions were subsequently implemented in an innovative PVT growth environment to achieve a growth technique with highly effective polytype control. Under continuously modulated growth conditions, MPs induced by seed material and/or formed during the growth were eliminated. 2-inch and 3-inch diameter MP-free (zero MP density) conducting 4H-SiC ingots were obtained.

Abstract: Over the past year, II-VI has transitioned from 2” to 3” commercial SiC substrates. Large-diameter semi-insulating 6H-SiC and n-type 4H-SiC single crystals are grown using the Advanced PVT growth process. Expansion of boule diameter from 2 to 3 and up to 4.25 inches has been carried out using a specially designed growth technique. Stable semi-insulating properties in 6H-SiC are achieved by precise vanadium compensation. The technique of compensation is optimized to produce a controlled and spatially uniform distribution of vanadium and high and spatially uniform electrical resistivity reaching 10 10 – 1011
·cm. N-type 3-inch 4H-SiC crystals are grown using doping with nitrogen, and 3-inch 4H-SiC substrates show uniform resistivity of about 0.018
·cm. The best quality semiinsulating (SI) 3” 6H-SiC substrates demonstrate micropipe density of 3 cm-2, and n-type 3” 4H-SiC substrates - about 1 cm-2. X-ray rocking curve topography of the produced 3” SiC substrates is used for evaluation of their crystal quality.

Abstract: The effects of growth conditions, diffusion barrier coatings, and hot zone materials on B incorporation in 6H-SiC crystals grown by physical vapor transport (PVT) were evaluated. Development of high purity source material with a B concentration less than 1.8x1015 atoms/cm3, was critical to the growth of boules with a B concentration less than 3.0x1016 atoms/cm3. Application of refractory metal carbide coatings to commercial graphite to serve as boron diffusion barriers and the use of very high purity pyrolytic graphite components ultimately led to the growth of SiC boules with boron concentrations as low as 2.4x1015 atoms/cm3. The effect of growth temperature and pressure were closely examined over a range from 2100°C to 2300°C and 5 to 13.5 Torr. This range of growth conditions and growth rates had no effect on B incorporation. Attempts to alter the gas phase stoichiometry through addition of hydrogen gas to the growth environment also had no impact on B incorporation. These results are explained by considering site competition effects and the ability of B to diffuse through the graphite growth cell components.

Abstract: For undoped 6H-SiC boules grown by physical vapor transport the variations of resistivity, of the type and density of deep electron and hole traps, and of the concentration of nitrogen and boron were studied as a function of position in the cross section normal to the growth axis and along the growth direction. It was observed that the concentrations of all deep electron and hole traps decreased when moving from seed to tail of the boule and from the center to the edge of the wafers. Modeling of the growth process suggests that the C/Si ratio increases in a similar fashion and could be responsible for observed changes. We also discuss the implications of such stoichiometry changes on compensation mechanisms rendering the crystals semi-insulating and on electrical uniformity of SI-SiC wafers.

Abstract: The variation of nitrogen doping concentration was systematically investigated with respect to the amount of silicon powder added to the SiC powder for growing n-type 6H-SiC single crystal by the sublimation method. To change intentionally the Si content in the SiC powder, 0wt% to 2wt% of a silicon powder was added to first-thermal treated SiC powder and the mixed powder was treated again at 1800oC for 3 hours to eliminate excess free-metallic silicon. Nitrogen doped 6H-SiC single crystals were grown by using 2nd-thermal treatment SiC powder at fixed N2/(Ar + N2) (3%). The nitrogen doping concentration of 6H-SiC crystals increased with increasing Si content in the SiC powder. In this work, we could identify that the additional silicon powder in SiC powder plays a role in the enhancement of nitrogen doping in 6H-SiC crystals grown by the sublimation method.

Abstract: A detailed understanding of the incorporation of N2 gas during PVT growth of SiC is required to achieve high performance, low resistivity n+ SiC substrates necessary for power device applications. In this report, nitrogen incorporation is investigated for growth of 4H SiC crystals from 2” to 3” diameter in conditions ranging from unintentionally doped to low resistivity (0.015
- cm). For a wafer in a particular boule a resistivity uniformity of ± 5% is typical although the uniformity decreases when the wafer orientation is cut off axis from the bulk growth direction. Within a boule growth, the nitrogen incorporation is found to be a function of growth time. As growth continues, the resistivity of wafers cut further from the seed increases. A typical 3” on axis sliced wafer has a within wafer resistivity uniformity of 5% compared with an average seed to tail variation of 10%. Due to the axial resistivity gradient the within wafer resistivity uniformity of off axis sliced wafers is 8%. These axial and radial gradients are thought to be a function of the changing C/Si ratio during growth. Nitrogen incorporation as a function of PVT geometry, N2 partial pressure, and growth temperature are investigated and discussed. In particular, nitrogen incorporation is found to depend on the crucible size and nitrogen partial pressure, but is not strongly dependent on the absolute growth temperature, for growth temperature ranging over 150°C. Modeling of PVT growth shows the axial resistivity gradient can be linked with a change in the C/Si ratio versus time. Trends and N2 gas incorporation behavior will be discussed using resistivity mapping, SIMS, and Hall effect data.

Abstract: The development of the Continuous Feed Physical Vapour Transport (CF-PVT) process requires a perfect control of each phenomenon in the growth cell. Along this line, the present paper gives some inputs on the CF-PVT mass transfer regimes with respect to the process parameters, both from qualitative and quantitative viewpoints. For example, two boundary cases have been evidenced depending on the temperature. At low temperature, the growth is limited by the sublimation step between the source and the seed. In this case, the CF-PVT process can be roughly assimilated to the classical seeded sublimation technique. At high temperature, the process is limited by the feeding step, i.e. the CVD deposition and infiltration on the lower part of the source. Measurements are correlated to in-situ X-ray imaging. The ability of the X-ray imaging to in-situ qualify and quantify the mass transfer is discussed.

Abstract: Bulk crystals and epitaxial layers of 6H SiC have been grown and their surface morphologies have been investigated. Seeded sublimation has been employed to obtain bulk 6H SiC crystals whereas a silicon tetrachloride-propane based chemical vapor deposition (CVD) was used for growing epitaxial layers. The hot-zones were designed using numerical simulation. Growth rates up to 200 μm/hr could be achieved in the CVD process. A new growth-assisted hydrogen etching was developed to reveal the distribution of the micropipes present in the substrate. Morphological features were studied using Nomarski, atomic force microscopy (AFM), and scanning electron microscopy (SEM), and the structural quality was evaluated using synchrotron X-ray topography.

Abstract: The transfer by wafer-bonding of single-crystalline SiC thin films to a polycrystalline SiC support to obtain a “quasi-wafer” is an attractive way for lowering the cost of silicon carbide wafers. Such a process needs high quality polycrystalline substrates, with controlled and high-level bulk properties (thermal conductivity, electrical resistivity) and with very low surface roughness and surface bowing. Currently, polycrystalline SiC wafers which are available are siliconized SiC or CVD processed SiC wafers. Siliconized ceramic wafers are very heterogeneous (mixture of 3C, 6H, 15R and silicon), while CVD ones are of better quality (homogeneous and textured 3C). However neither the siliconized SiC nor the CVD SiC can be CMP polished with low roughness over large dimension. In this paper, wafers with large and textured grains (> 1cm) are processed and characterized. The polishing of such structures is studied and optimized to obtain low surface roughness. To meet these requirements high temperature processes used for single crystal growth were selected. Structural investigations performed on the grown ingots showed an important influence of the used seed since no preferential crystallographic orientation was observed during the growth. The final polishing quality was of high level but step heights were observed between grains.

Abstract: We have studied the impact of the chemical nature of additional gases fed into the modified physical vapor transport (M-PVT) growth cell. In particular experiments were carried out using helium, argon, nitrogen and propane in the growth setup. Numerical modeling was used to address the underlying physical and chemical effects that impact the global temperature field. It is found that chemical decomposition of complex gases plays a secondary role as heat source or sink. However, temperature variations related to varying gas compositions fed to the systems are primarily induced by changes of the graphite foam isolation properties.