The hexagonal pyramid appropriate for the description of hexagonal polytypes of SiC. A –face belongs to the family of planes whereas an –face belongs to the family of planes.
Cross-sectional SEM image of a vicinal (0001) -type (nominally doped at ) 4H SiC sample photoelectrochemically etched at in aqueous HF. The uv light intensity was . The etch planes are tilted relative to the front surface (normal ) to approximately follow the 8° off-cut of the initial substrate. Perforating triangular pores are arranged in layers about thick.
Plan-view SEM image of a 4H SiC sample, cut 8° off the axis towards , anodized to obtain the triangular porous morphology. About of material was removed by RIE prior to imaging. The exposed channels apparently propagate preferably along directions.
Sequence of cross-sectional SEM images of a thick triangular morphology porous 4H SiC sample. In the sequence, (a) corresponds to the top of the film and (e) to the bottom. The distance from the front surface and the corresponding porosity are (a) , , (b) , , (c) , , (d) , , (e) , .
Cross-sectional SEM image of a 4H SiC sample whose vicinal carbon face was exposed to the photoelectrochemical etching. Planarity characteristic to the porous (0001) silicon-face samples is not observed, although the average pore size is similar.
Plan view of a oriented photoelectrochemically etched 4H SiC sample. Triangles on the surface are about the same size as the triangles seen on cross-sectional SEM images of the vicinal (0001) samples (see Fig. 2.
Cross-sectional SEM image of a porous 4H SiC sample. The particular surface shown in the image is a plane. When this picture is rotated 90° clockwise, it resembles the cross-sectional image of a vicinal (0001) sample (see Fig. 2).
Cross-sectional SEM image of a porous 4H SiC sample. The surface shown in the image is a basal plane. Compare the wormy character of the pore channels with Fig. 3.
The variation of the electrostatic potential at the semiconductor/electrolyte interface caused by the difference in the corresponding electrochemical potentials [see Eq. (1)]. The schematic corresponds to zero applied bias.
The absolute value of current density vs applied bias dependence of an -type 4H SiC nonporous electrode in 5% aqueous HF taken in the dark. The intercept is used in Eq. (3) to estimate the width of the space charge region. The intercept is taken at at which the current changes sign indicating the transition from the cathodic to the anodic regime.
The current density vs applied bias dependence of an -type 4H SiC nonporous electrode in 5% aqueous HF under uv illumination. The light intensity is about . The solid line is the fit to Eq. (12).
Current density vs time for a vicinal (0001) 4H -type SiC sample photoelectrochemically etched at for . The sample area is . Integration of the curve provides the total charge transferred during the course of the reaction leading to the estimate of porosity .
One-dimensional schematic drawing of the positively biased interpore spacing: (a) band bending for the initial stage of pore formation, (b) band bending during an advanced stage of the semiconductor dissolution. The Fermi level in the semiconductor is pinned to surface states (ss). cb stands for surface band, and vb for valence band.
The porosity as a function of depth from the top of the sample. The porous film was obtained in the run shown in Fig. 11. The values of porosity are extracted from digital analysis of the SEM images. The solid line is a fit to Eq. (15) which gives for the average porosity.
Schematic of the projection of the 4H SiC crystal lattice as viewed from a direction. The triangle represents a cross section of a pore channel typically seen in photoelectrochemically etched highly doped 4H SiC. The planes comprising the walls of the triangular channel are indicated. Open circles represent silicon atoms and filled circles represent carbon atoms. is the angle between the (0001) and the planes.
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