Self-organization in porous 6H-SiC
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H. Arwin Department of Physics and Measurements Technology, Laboratory of Applied Optics, Linko¨ping University, SE-58183, Sweden (Received 28 February 2000; accepted 15 June 2000)
Pores in porous 6H–SiC were found to propagate first nearly parallel with the basal plane and gradually change direction and align with the c axis. As a consequence, well-defined columnar pores were formed. It was shown that the rate of change of propagation directions was influenced by the etching parameters, such as hydrofluoric acid concentration and current density. Larger currents resulted in formation of larger pores. Pore sizes were found to increase with depth due to a decrease of the acid concentration. In addition, due to chemical etching effects, larger pore sizes were obtained close to the sample surface.
SiC is a promising candidate material for high-power, high-temperature, high-frequency, and optical device applications. These are mainly due to the high-saturation electron drift velocity, thermal conductivity, and breakdown field of the material.1 A spin off from SiC materials research results from the high surface area material obtained by electrochemical dissolution of the crystalline bulk material in hydrofluoric acid (HF) solutions.2–5 The resulting structures may find applications in disciplines such as sensor technologies, considering the superior mechanical, thermal, and electronic properties, as well as long time stability of the materials compared with other porous structures such as porous silicon (PS).6 In addition, creating quantum wires through the etching process resulting in an increase of the band gap compared with the bulk material may provide future optoelectronics application possibilities. Strong photo- and electroluminescence above the band gap and in the blue and ultraviolet (UV) part of the spectrum has also been reported.7,8 Therefore, an accurate understanding of the influence of the etching conditions on the morphological properties of the material is required. In a recent report we emphasized the anisotropy in pore propagation in nitrogen-doped 4H–SiC.9 Pores were found to initially propagate nearly parallel to the sample surface and gradually change direction and follow planes such as (1104), (1103), and (1102). A similar phenomenon is also observed in nitrogen-doped 6H–SiC, as will be shown below. In ad-
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J. Mater. Res., Vol. 15, No. 9, Sep 2000 Downloaded: 13 Mar 2015
dition, variations in pore morphology with depth and its dependence on the anodization conditions (i.e., current density and HF-concentration) will be discussed. Porous SiC (PSC) samples were prepared using n-type 6H–SiC wafers from CREE Research, Inc., Durham, NC with a nitrogen-doping level of approximately 4.3 × 1018 cm−3. The 3.5° off-cut is neglected in the following discussions. Photoassisted electrochemical etching was performed on the polished silicon face of the samples using a 24-W UV light source in a mixtu
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