Porous silicon distributed Bragg reflectors

The refractive index of porous Si is lower than for bulk Si: it is inversely proportional to the anodization etch current density. A distributed Bragg reflector (DBR) selectively reflects a band of incident wavelengths and is easily formed in highly doped p-type Si by periodically lowering and raising the etch current density, resulting in a sequence of porous layers with alternating high and low refractive index, Figure 1 shows SEMs of DBR lines created with different proton fluences. The wafer was etched with an alternating high/low current density for 4 seconds per layer, with a total of 15 bilayers formed. The etch rate is progressively slowed by larger irradiation fluences, resulting in thinner porous layers that reflect shorter incident wavelengths.

Figure 1. SEMs of DBR lines created with proton fluences of (a) 5x1014/cm2, (b) 1x1015/cm2, (c) 2.5x1015/cm2, (d) 7.5x1015/cm2. The lines are 4 um wide, the linewidth is arrowed in (a) where it is least obvious.

Fig. 2a shows an optical image of a 500 × 500 μm2 region irradiated with different overlaid scan patterns, with different fluences. Each fluence produces a different reflected color when illuminated with white light, with red/orange colors corresponding to areas of lowest fluence. The potential of this approach to form patterned arrays of color pixels and lines for display applications is shown in Fig. 2b, where vertical lines, each 10 μm wide, were irradiated to form alternating red-green-blue stripes. Figs. 2c,d shows examples of selectively patterning color reflective areas in which two dragon images appear in different colors, corresponding to different irradiated fluences in each area.

Fig. 2. Examples of patterned DBRs, showing optical reflection images of (a) the painting La Musique by Henri Matisse, created by irradiating a 500 × 500 μm2 area. (b) Colour pixels of 10x10 μm area, irradiated and anodized to form alternating colours. (c), (d) 500 × 500 μm2 areas showing two dragon images corresponding to different irradiated fluences in each area. In each case the sample was illuminated with white light.