Crystallographic plane-orientation dependent atomic force microscopy-based local oxidation of silicon carbide
© Ahn et al; licensee Springer. 2011
Received: 10 October 2010
Accepted: 18 March 2011
Published: 18 March 2011
The effect of crystalline plane orientations of Silicon carbide (SiC) (a-, m-, and c-planes) on the local oxidation on 4H-SiC using atomic force microscopy (AFM) was investigated. It has been found that the AFM-based local oxidation (AFM-LO) rate on SiC is closely correlated to the atomic planar density values of different crystalline planes (a-plane, 7.45 cm-2; c-plane, 12.17 cm-2; and m-plane, 6.44 cm-2). Specifically, at room temperature and under about 40% humidity with a scan speed of 0.5 μm/s, the height of oxides on a- and m-planes 4H-SiC is 6.5 and 13 nm, respectively, whereas the height of oxides on the c-plane increased up to 30 nm. In addition, the results of AFM-LO with thermally grown oxides on the different plane orientations in SiC are compared.
Silicon carbide (SiC) is a well-known wide band gap semiconductor material, which exhibits high values of thermal conductivities, critical fields, and chemical inertness. However, there have been challenges in processing SiC into device applications, since the electric characteristics and yield ratio of SiC-based devices are hampered by micro-pipes and stacking faults. Typical SiC wafers have dislocation densities in the range of 103-105 cm-2, and in order to prevent this problem, extensive studies on bulk growths, thermal oxidations, and etching properties have been conducted on various crystalline planes in SiC [1–4].
Three different sets of 4H-SiC samples were prepared with corresponding different plane orientations of a- (N D: 5.9 × 1018 cm-2), c- (N D: 9.6 × 1018 cm-2), and m- (N D: 9.3 × 1018 cm-2) planes. AFM (Bruker AXS Inc.)-based local oxidation was performed using the contact mode, whereas the topographic AFM measurement was performed in the non-contact mode AFM. Si cantilevers with a spring constant of 48 N/m, a resonance frequency of 190 kHz, and a radius of 5 nm were used to analyze the morphology of surfaces. For the AFM-LO, Pt/Ir-coated Si conductive tips with radii of 50 nm were used. The spring constant and the resonance frequency were set at 3 N/m and 70 kHz, respectively. The temperature and the humidity of the atmosphere were controlled at 27°C (± 2°C) and 40% (± 5%), respectively, during the AFM-LO process. A dc bias was applied between the cantilever and the substrate for the local oxidation. The electrical field was then created between the native oxide layer and the substrate, which caused the oxyanions (OH-) to drift through the oxide film [14–16]. In the case of SiC, the reactions in the AFM-LO were described by the following chemical reactions. In the anode (sample surface), the oxidation takes place as follows: SiC + 2H2O + 4h+ → SiO2 + 4H+ + C4+, SiC + 3/2O2 + 4h+ → SiO2 + CO↑. The oxyanions (OH-) contribute to the formation of the oxide patterns in the surface, while hydrogen generation occurs at the tip (cathode) to complete the electrochemical reaction, 2H+(aq) + 2e- → H2. The local oxide patterns were formed on n-type a-, m-, and c-planes of c-face 4H-SiC with a doping concentration of 1019 cm-3.
Results and discussion
In general, it is difficult to form oxide patterns on SiC using AFM-LO because of both physical hardness and chemical inactivity. The binding energy of a Si-C bond (451.5 kJ/mol) is higher than that of a Si-Si bond (325 kJ/mol), and thus the reactions of oxyanions (OH-) into a Si-C bond require a higher activation energy. The removal of carbon atoms in the forms of CO or CO2 species also requires additional energy. The simulation examination that contains the 2D electric field distribution between the tip and both Si and SiC substrates to optimize the doping concentration of materials and the direction of applied bias in oxide formation was carried out. We used ATLAS simulator by Silvaco Inc. to design the tip and semiconductor (SiC or Si) structure with a 10-nm-thick oxide layer and the doping concentration for the semiconductor was varied in the range between 1015 and 1019 cm-2.
Oxidation rates of AFM-LO and thermal oxidation, as well as theoretical planar atomic density at three different plane orientations of 4H-SiC orientations with doping concentration profiles of 4H-SiC
Oxide height (nm)
Thermal oxidation (nm)
Planar atomic density (atoms/cm2)
Local oxidation (nm)
Doping concentration (×1018 cm-2)
In conclusion, the effects of crystalline plane orientations of a-, m-, and c-planes on the AFM-LO of 4H-SiC wafers were investigated. It has been shown that the AFM-LO oxide heights of a-plane and m-plane 4H-SiC are lower than that of c-plane due mainly to the difference of planar density. It has clearly been shown that the AFM-LO rate on c-plane 4H-SiC is significantly higher than the other plane orientations, which can be correlated to the areal density of the first layer for the different surfaces as well as the doping concentration. The oxide height decreases as the scan speed increases, which suggests that a longer anodization time resulted in increased oxidation rates.
atomic force microscopy
AFM-based local oxidation
This study was supported by the "System IC2010" project and "Survey of high efficiency power devices and inverter system for power grid" project of Korea Ministry of Knowledge Economy, by the National Research Foundation of Korea Grant funded by the Korean Government 2010-0011022, and by a Research Grant from Kwangwoon University in 2011.
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