Optical properties of silicon-based materials: A comparison of porous and spark-processed silicon

Abstract Crystalline silicon is the principal semiconducting material and will doubtless continue to be the mainstay in the electronics industry for the foreseeable future. However, silicon is lacking the properties necessary to emit light efficiently and thus cannot be employed for optically active or optoelectronic applications. The poor optoelectronic behavior is caused by the indirect band gap that obstructs radiative transitions and restricts them to the infrared part of the spectrum. The discovery of an intense, room-temperature luminescence in the visible spectral range from porous silicon layers, prepared by electrochemical etching, thus attracted enormous attention. In addition to chemical etching, spark processing was also shown to generate a silicon-based substance that strongly photoluminesces in several visible bands at room temperature. Despite all research efforts, a vigorous debate is still ongoing about the best description for the complex phenomena of light emission in these materials, mainly focusing on quantum size effects, interface states, and specific chemical agents. This review provides a comparative overview of the preparation, structure, and optical phenomena found in porous and spark-processed Si, while emphasizing the lesser known method of spark processing. It is shown that por-Si and sp-Si display similarities in their micro- and macrostructure, can be excited by similar means, and emit light having similar properties. Nonetheless, the underlying recombination principles are probably very dissimilar. Emphasis is also given to the question, to what comparative extent can visible luminescence from both materials be traced back to known properties for hydrogenated amorphous silicon, silicon quantum dots, and silicon polymers?