Sublimation growth of an in-situ-deposited layer in SiC chemical vapor deposition on 4H-SiC(1 1 2¯ 0)
Introduction
The demand for high-power SiC devices with thick active layers has fueled the development of deposition techniques capable of high growth rate epitaxy. Liquid phase epitaxy [1], [2] and sublimation epitaxy [3] have been reported with growth rates >100 μm/h; however, both of these techniques have been hindered by the lack of high-purity sources and the inability to achieve low background impurity concentrations [4]. Chemical vapor deposition (CVD) (or vapor phase epitaxy) is the most widely researched growth process employed to meet the material demands for SiC-based high-power electronics. The most successful modifications are horizontal [5] and vertical [6], [7] (or chimney-style) hot-wall CVD. While growth rates to 20 μm/h have been reported with the former [8], the bottom-up gas flow configuration of the latter is an inherent advantage [9], [10] that has enabled reproducible growth rates to 60 μm/h [11].
The reaction chamber in a hot-wall CVD system is comprised of a dense graphite susceptor (cylindrical or rectangular) that is inductively heated and enveloped in graphite foam insulation. This configuration enables a high heating efficiency and facilitates high growth temperatures—both are beneficial for obtaining high growth rates. Hot-wall CVD is performed at temperatures ⩾1500 °C. A modified version of this technique, namely high-temperature CVD, has enabled growth rates up to 500 μm/h at 2300 °C [10]. Hydrogen is the most common diluent; under these extreme process conditions it reacts with (1) the SiC substrate and (2) the graphite parts and/or the protective silicon carbide coating on these parts.
The reaction between the hydrogen and the SiC substrate is commonly referred to as hydrogen etching and is used routinely as a surface treatment step prior to SiC epitaxial growth. Several research groups have performed thermodynamic calculations that predict the reaction pathway for hydrogen etching [12], [13], [14]. It has been proposed that SiC etches in flowing hydrogen via the evaporation of silicon and the formation of hydrocarbons [12], [13]. Further, Kumagawa et al. [13] suggested the SiC surface dissociates initially into liquid silicon and solid carbon, and the former subsequently vaporizes. However, Hallin et al. [15], Burk and Rowland [16], and Powell et al. [17] have observed silicon droplets after hydrogen etching. The work of Hartman et al. [14] predicts that the formation of atomic hydrogen is necessary for the etching process to proceed thermodynamically. Based on the calculations by Hartman et al., [14] the reaction between atomic hydrogen and the silicon and/or the carbon resulting from the dissociation of SiC produces Si(g) or SiH4(g) and/or CH4(g). The same authors contend that free silicon could remain on the surface in the form of droplets at low etching temperatures (1400–1500 °C) due to, for example, the decreased concentration of atomic hydrogen and slower reaction kinetics [14].
In a similar manner, hydrogen can also react with the SiC coating on graphite parts within the growth chamber. The coating is used as a barrier to prevent impurities within graphite from incorporating into SiC epitaxial layers [18]. Theoretical calculations by Raback et al. [19] show that at experimental conditions similar to those employed in this work (i.e., total pressure ∼10−2 atm and T⩽1600 °C) the H2-SiC reaction will create a primarily carbon-rich atmosphere. The three byproducts with the highest partial pressures were determined to be C2H2(g), CH4(g), and SiH(g) [19].
These Si- and C-containing byproducts can significantly influence the growth ambient and can serve as constituents for the in-situ re-deposition of SiC. In prior work, we showed that films grown on “in-situ-deposited” 4H-SiC(1 1 2¯ 0) were epitaxial and exhibited polytype replication and regions with reduced defect densities [20]. In the research presented herein, a critical analysis of SiC CVD has revealed the presence of significant SiC coating decomposition (termed sublimation) in hot-wall epitaxy. The latter results in the growth of SiC thin films in flowing hydrogen. The kinetic factors associated with this phenomenon are examined, and the influence that these layers have on the subsequent growth is also described.
Section snippets
Experimental procedure
Homoepitaxial growth of the SiC films was achieved using a vertical, hot-wall CVD system containing a sample holder assembly heated by radiation from an inductively coupled susceptor. These two parts were machined from semiconductor-grade graphite, purified in a Cl-containing atmosphere and coated with SiC prior to insertion into the CVD system. Porous graphite insulation located above the holder and around the susceptor acted as a radiation shield and thermal barrier, respectively, and
Identification and growth of in-situ-deposited SiC
One way in which the presence of in-situ-deposited SiC was identified was by monitoring the thickness of the EL as a function of increasing growth time. The thickness achieved using PR1 at 1450 °C, a C/Si ratio of 1.0 and flow rates of the hydrogen diluent of 590 and 1000 sccm increased linearly from a non-zero intercept, as shown in Fig. 2. The slope of the trendline corresponds to a growth rate of 509 nm/h. Extrapolation of the trendline to zero growth time indicates the presence of a
Summary
Two process routes have been employed to critically analyze the growth of 4H-SiC(1 1 2¯ 0) films on 4H-SiC(1 1 2¯ 0) substrates as a function of temperature in a vertical, hot-wall CVD system. Route (1) was performed with reactant gases; the activation energy was 3.72 eV/atom (359 kJ/mol). The only source of reactants in route (2) was the decomposition of the SiC coating from several parts within the growth chamber. Activation energies of 5.37 eV/atom (518 kJ/mol) and 5.64 eV/atom (544 kJ/mol), consistent
Acknowledgements
R.F. Davis was supported in part by the Kobe Steel, Ltd. University Professorship. The authors acknowledge Cree, Inc. for the SiC wafers used in this research.
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