3D distributed Bragg reflectors in porous Si

Three-dimensional (3D) distributed Bragg reflectors, which reflect all incident wavelengths, can be fabricated with micrometer dimensions in porous silicon, resulting in white reflective surfaces when viewed over a wide angular range. Large area arrays containing many individual micrometer-size pixellated reflectors that can be tuned to reflect a narrow or wide range of wavelengths are designed to appear either as constant or changing reflective images to the naked eye. This opens avenues in controlling the reflection of light in all directions for applications in wide-angle displays, broadband reflective surfaces for resonant white light emission from semiconductor nanocrystals, and three-dimensional microcavities. Figure 1 shows the effect on DBR porous layers across an ion irradiated line for two fluences. Consider the change of the shape of the DBR layers with depth. The boundary tilt angle () between the unirradiated and irradiated silicon surface changes with fluence and with anodization depth, allowing tilted surfaces with a flat top (left) or no flat top (right) to be produced.

FIG. 1. Cross-section SEM images of deeply anodized DBR pixels, formed by irradiating 6 um wide lines with fluences of (a) to (c) 2x1015 /cm2 and (d) to (f) 4x1015 /cm2 and. In (a) and (d) 20 pairs of high/low porosity layers were anodized to a first stage DBR. In (b) and (e) the first stage DBR layers were removed and the same anodization process repeated to give a second stage. In (c) and (f), the same process was repeated.

Figure 2 demonstrates how white light illumination is reflected normal to the wafer surface from different stages and geometries of individual 3D DBR pixels. Those in the lower and upper rows were irradiated with fluences of 1x1015/cm2 and 2x1015/cm2 respectively, with varying sizes. In Figs. 2a–d, DBRs were produced. Under normal illumination, Fig. 2b, the top, flat DBR pixel surfaces reflect red or blue, depending on the fluence, and the tilted boundaries reflect nothing. With additional inclined illumination, Fig. 2c, the boundaries in the lower row reflect green light, i.e., blue-shifted compared to the top surface, consistent with Bragg reflection.

FIG. 2. Plan view SEM image of deeply anodized DBR pixels. Optical reflection images of this structure under (b) normal and (c) additional inclined illumination from two directions at 45°. (d) Circular DBR pixels under normal and inclined illumination from one direction at 45°.

Now consider many identical or differing pixels producing a large reflective pattern. A 3 mm wide Dragon was fabricated as a pixelated DBR. Under normal illumination, the background of the Dragon appears yellow in Fig. 3a, owing to light reflected from the gaps between the irradiated lines. The flat, central DBR regions of the irradiated lines reflect little light since irradiation blue-shifts the reflected wavelengths beyond the visible range and hence the dragon appears black in Fig. 3a. Under inclined illumination, the pattern is almost uniformly white, Fig. 3b, with wavelengths across the full visible spectrum reflected. This demonstrates that white light is reflected from pixelated DBRs.

FIG. 3. Optical images showing light reflected at normal incidence, when illuminated with white light at normal incidence 90°, inclined illumination at (b) 60° and(c) 20° to the wafer surface in the horizontal plane.

Another large-area DBR structure was designed to reflect a pattern, which changes with viewing angle. An alphanumeric pattern was produced with each segment irradiated to selectively reflect light only at (i) normal or (ii) over a wide range of viewing angles, or (iii) only over off-normal angles. After producing an array of suitably pixelated DBRs, Fig. 4 shows the numerals 9, 5, and 7 reflected with good contrast between those regions which change from “bright” to a “dark” when viewed at progressively shallower angles.

FIG. 4. Optical images of reflected light from a patterned 3D DBR alphanumeric display, for progressively inclined white light illumination from (a) to (c). Three different types of patterning and fluences were used in regions (i) to (iii) to create differing reflective behaviors. The cross section profile of the resultant DBRs produced in each case is shown in (c).

For more details, see: D. Mangaiyarkarasi, M. B. H. Breese, and Y. S. Ow. Fabrication of three dimensional porous silicon distributed Bragg reflectors. APPLIED PHYSICS LETTERS 93, 221905 (2008)