Porous Silicon

Abstract

In 1990 it was proposed that Si when rendered porous can be an efficient light emitting system, as efficient as many III-V semiconductor systems [1]. The reason for this was attributed to the low dimensionality of the surviving Si skeleton. In addition other effects were recognized: (1) the large photon extraction efficiency due to the decrease in the effective refractive index and (2) the confinement of the carriers in regions free of recombination centers. After this very surprising report, a huge number of papers was published trying to understand the optical properties of porous silicon (PS) and the physical mechanism at the heart of the large luminescence efficiency. Soon it was realized that PS is a very fragile and highly reactive material so that many of its properties are age dependent and unstable. Over the years, sizeable progress was made both in terms of improvement of its stability and in term of efficiency of the device. Microelectronics compatibility of PS was demonstrated and integration of driving circuits with a light emitting element was performed [2]. At the same time microcavities with improved light emission properties were demonstrated [3]. At the end of 2000 another paper shocked the silicon community: the report on light amplification or optical gain in silicon nanocrystals [4]. This paper raises big hopes of the development of a silicon laser even though it is not yet clear whether PS can be used as an active material.In this chapter we will try to review the most important results about the exploitation and the understanding of the properties of PS. The chapter is divided into six parts. The first introduces how porous silicon is made. The second is aimed to demonstrate that the structure of PS is a collection of nanometric objects that contribute to the light emission process. The relevant role of the surface chemistry is underlined. The third and fourth parts discuss the essential features of the absorption and luminescence of PS in order to discriminate between the models of the recombination mechanisms. The fifth presents recent progress in the field of electrical properties of PS. The section six is an overview of the application of PS in light emitting diodes.In the last few years various conference proceedings [5], review articles [6] and edited books [7] have been published on PS.

I–V characteristics of structures with porous silicon in electrolyte

“…In our case, the Schottky barrier does not affect the formation of CVC. It is known that diodes with a thick PS layer have ohmic CVC, whereas diodes with a thin PS layer show a rectifying CVC [11,12]. The presence of the rectification means that in our samples the PS layer is thin and the applied voltage mainly drops on the isotype p-p barrier between the single crystalline/porous silicon [11], which led to the rectification.…”