University Ph.D. Oral Examination - Alex R. Guichard - Tuesday, 10/28, CISX Auditorium

Alex Guichard arguicha at stanford.edu
Mon Oct 20 13:36:46 PDT 2008


Growth and Optical Properties of CMOS-compatible Silicon Nanowires

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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