High optical quality III-V semiconductor nanopillar arrays fabricated by combining self-assembly and top down approaches pave way for a greener energy
Nanopillars/wires have been investigated in different semiconductor materials for efficient optoelectronic components such as light emitting diodes (LEDs) and solar cells in which they could bring distinct advantages such as light trapping and efficient carrier collection. Several methods including both bottom-up and top-down approaches are being pursued to fabricate nanopillar/nanowire arrays. However, for device applications cost effectiveness, efficiency and good process control are key issues. Colloidal or nanosphere lithography (NSL) has emerged as an effective and inexpensive method for large area nanopatterning where self-assembled colloidal nanoparticles act as etch masks. An appropriate anisotropic etching technique and chemistry can be then used to etch the desired material(s). It is also possible to vary size and spatial density/filling of the etched nanopillars which are important parameters that determine their optical properties (e.g., reflectance, transmission, and absorption). An advantage with top-down approaches is that wafer-scale single crystalline semiconductor materials including heterostructures can easily be obtained from established epitaxial growth technologies. For solar cells, direct band-gap III-V materials like GaAs and InP offer the advantage of effcient light absorption in very thin layers.
In a recent work, reported in the journal Nano Letters (dx.doi.org/10.1021/nl202628m; 2011, 11, 4805–4811) a team of researchers at ADOPT have addressed the above issues and have demonstrate the fabrication of dense and uniform arrays of high optical quality InP-based nanopillars. The pillars, both single and arrays, fabricated from epitaxially grown InP and InP/GaInAsP/InP quantum well structures show excellent photoluminescence (PL) even at room temperature, depict remarkably good quality of the fabricated nanopillars. It was also shown that the structures naturally have good anti- reflection property which is one of the important aspect of efficient light management in solar cells. The measured PL line widths of the InP nanopillars, single and arrays, are comparable to the as-grown InP wafer indicating the high quality of the fabricated pillars. In addition, the researchers have developed a stamping technique to transfer nano-objects onto arbitrary substrates which has implications for other applications. With this procedure arbitrary spatial densities of “free” nanopillars could be obtained, enabling studies of single nanopillars. The use of a sacrificial (here, InGaAs) layer is also demonstrated as an effective way for releasing the pillars. This method can be used to lift-off the nanopillar layer after suitably embedding them, for next generation nanostructured solar cells.
Text by: Anand Srinivasan