Alex R. Guichard
Department of Materials Science and Engineering
Advisor: Professor Mark L. Brongersma
Tuesday, October 28th, 2008
1:30 PM (Refreshments served at 1:15 PM)
CIS-X Auditorium
Silicon is the dominant semiconductor in both the microelectronics and photovoltaic industries. The main reasons for its success can be traced back to its excellent materials and electronic properties. In contrast, its indirect bandgap makes bulk Si a quite uninteresting optical material. In the early 1990s the discovery of efficient room temperature light emission from electrochemically etched porous Si and subsequent reports of optical gain from Si nanocrystals (Si-nc) early this decade resulted in an explosion of research in this area; these events sparked the dream of realizing Si-based light sources, optoelectronic circuitry, and possibly even a laser. By now, this research has provided a significant understanding of the fundamental optical properties of Si nanostructures. Despite the rapid advancements, an efficient electrically-pumped light source based on these materials does not yet exist. This is in part due to their inefficient charge injection and transport properties. Moreover, the growth processes for Si nanostructures are not yet fully CMOS compatible.
In this presentation, I discuss a potential alternative material system for to porous Si and Si-nc: Si nanowires (SiNWs). I will illustrate the use of CMOS compatible fabrication techniques such as chemical vapor deposition (CVD), lithographic patterning, and thermal oxidation to generate Si NWs with diameters as small as 3 nm. At these diameters, quantum mechanical phenomena substantially modify the electronic and optical properties of the NWs. Photoluminescence (PL) measurements, demonstrate that their emission wavelength can be tuned by precisely controlling the crystalline Si NW diameter, as determined by dark field and high-resolution transmission electron microscopy. The PL decay lifetimes of these NWs are on the order of 50 µs, which suggest the PL is originating from confined excitons in the indirect bandgap Si cores. For solar cell and laser applications, we also quantify undesirable, non-radiative Auger recombination (AR) processes in the NWs. It was found that AR is about 2 orders of magnitude slower than in Si –ncs, which have been a serious contender for a Si-based laser. Although these results are promising, single NW studies reveal the need for better passivation strategies before efficient NW light sources can be realized.
A second potential application for SiNWs is as a building block for low-cost, thin film, Si-based photovoltaics (PV). The market for thin-film PV, particularly organic thin-film PV, exists because it offers a potential cost reduction versus bulk, crystalline-Si-based PV. However, many thin film technologies, while possessing superior optical absorption properties compared to crystalline Si (c-Si), suffer from poor electronic transport properties. Here, I present a new hybrid organic/inorganic PV design that combines the excellent optical properties of highly absorptive organic dye molecules and the useful electronic properties of high-mobility crystalline SiNWs. In the proposed cell, light is first absorbed in the dye and via Förster energy transfer electron-hole pairs are generated in the SiNW. The charges can be extracted from the Si NWs by generating a p-n junction in the wires and contacts at both ends. Here, I investigate the feasibility of such a device by performing photocurrent spectroscopy on individual dye-coated, lightly-doped Si NWs. An approximately twofold increase in the photocurrent is obtained in the wavelength range corresponding to the dye's absorption band, indeed suggesting the possibility to use dyes to boost the efficiency of weakly absorbing Si structures. These results could pave the way for new low-cost, Si-based solar cell designs that leverage the strengths of the Si PV and microelectronics industries.
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