Development of Graphene and Silicon Nanowires for Photovoltaic and Field Electron Emission Applications

Abstract

This thesis can be divided into two parts. The first part included Chapter 2 to Chapter 6, which covers the important findings in graphene and its application as solar cell and electron field emitter. The second part of this thesis includes Chapter 7 which discuses about the finding of an innovative technique for the synthesis of silicon nanowires in the manufacturing line of graphene just by switching the gas compositions. Chapter 1 gives an overview of graphene research and explain the motivation of this project. Chapter 8 describes the summary and presents the outlook of the thesis. In chapter 2, fundamental analytical study has been done to understand the interface physics of intimate contact of graphene on both direct and indirect semiconductors. In photovoltaic science, graphene has demonstrated promising applications in dye-sensitized solar cells and organic solar cells. A very limited amount of experimental work has been done to explore graphene in semiconductor devices and in particular, as graphene-based heterojunction solar cells, due to the lack of proper understanding of the interface physics of graphene-on-semiconductors. In this chapter, we discuss analytically the photovoltaic effects upon contacting graphene with different semiconductors such as silicon and gallium arsenide. The Voc is calculated based on the theory developed by S. K. Behura et al. for high level injection (HLI) and by Dubey and Paranjape for low level injection (LLI). HLI in a x semiconductor refers to the concentration of charge carriers which is larger than the doping density i.e., n, p >> ND. The effects of semiconductor doping density and surface recombination velocity on the Voc of both the systems are investigated. The standard metal/semiconductor Schottky model is considered for the calculation of the short-circuit current density. In chapter 3, fabrication of graphene has been tried using micromechanical exfoliation of highly oriented pyrolytic graphite (HOPG) and technology computer aided design (TCAD) 2-D computational study was carried out to use graphene as TCSE in silicon and InGaN solar cells. Physical exfoliation of HOPG is the first technique discovered by A. K. Geim and K. S. Novoselov in 2004 [Noble Prize in Physics, 2010] for the fabrication of high quality graphene films. This thesis aims to develop high quality and large-area graphene films for the photovoltaic and FEE applications. Herein, high quality graphene films have been fabricated using mechanical exfoliation of HOPG. The transferred graphene films on glass substrates were characterized using field emission scanning electron microscopy (FESEM), atomic force miscroscopy (AFM), Raman spectroscopy, UV-Vis, Fourier transform infrared (FT-IR) and Four-probe. A very high intensity ratio of 2D to G-band (≈1.67) and narrow 2D-band full-width at half maximum (≈40 cm-1 ) correspond to the bi-layer graphene (BLG) formation. Finally, the extracted properties of BLG were used as transparent current spreading electrode (TCSE) in silicon (Si) P-N and indium gallium nitride (InGaN) P-I-N junction solar cells. In Chapter 4, combined experimental and theoretical investigations on the heterojunctions of chemically derived graphene with Si have been carried out. The stability study of graphene oxide (GO) and reduced graphene oxide (rGO) in aqueous medium were performed by visual observation and surface charge measurement. The detailed characterizations by FT-IR, UV-Vis and Raman exhibited the formation of rGO with a high optical band gap of 3.6 eV. The rGO was spin-coated on the p-Si substrate for fabrication of a heterojunction device, with the structure of rGO/p-Si. In the fabricated device, incident light was transmitted through the thin rGO film to reach the junction interface, generating photoexciton and thereby a photo-conversion efficiency of 0.02 % was achieved by experimentally and its (rGO/p-Si heterojunction device) theoretical simulation using solar cell capacitance simulation (SCAPS) 1-D tool showed the efficiency of 1.32 %. Such large deviations in efficiency between experiment and theory have been discussed in details. xi In chapter 5, a systematic study of the catalytic CVD growth of graphene on polycrystalline copper (Cu) foil in a low pressure CVD conditions has been presented, aiming to achieve the highest quality with large-scale fabrications, which generally requires comprehensive understanding and effective controlling of the growth process. Herein, fewlayer graphene (FLG) films with large-domain sizes were grown on Cu metal catalyst substrates using a vertical mass-flow hot-filament CVD (HFCVD) reactor, with the intention of commercialization, by optimizing the CVD system and three of the process parameters (i) gas flow compositions, (ii) substrate annealing time and (iii) growth deposition time. The optimized detailed growth process that has been tailored for the synthesis of graphene are as follows: the chamber was first evacuated to 0.1 mTorr, and then the Cu substrate was heated to 1000 °C with a flow of hydrogen at 10 sccm and held for 20 minutes for the annealing and subsequently grain growth of the Cu film. After that, the temperature was kept at 1000 °C with a gas composition of methane: hydrogen (CH4:H2) = 1:50 sccm flowing into the reaction chamber and maintaining a pressure of 300-500 mTorr for the growth of graphene films. After 10 minutes of growth, the chamber was cooled at 25 oC/min under the flow of hydrogen at 10 sccm. These as-synthesized flat graphene films on Cu have shown the room temperature FEE characteristics under vacuum, hence appears to be potential candidate for vacuum electronic device applications. In chapter 6, an attempt has been made to synthesize graphene directly on dielectric substrates using thermal and HFCVD, respectively without any catalyst or special substrate treatment. The fabricated horizontally and vertically oriented graphenes are shown to grow according to Volmer-Weber and Stranski-Krastnov growth mechanisms, respectively. Typical dark current-voltage characteristic of graphene-on-Si (p-type) heterojunction is investigated at room temperature for both types of graphene and an effort has been made to correlate the structure-property relationship. Considering the fact that, field-emission from flat graphene sheets is a challenge due to less number of emission sites, we have synthesized free-standing vertically-oriented FLG films and shown the enhanced field emission characteristics. The ease of large area preparation and the low turn-on field of 22 V/µm in addition to the large field enhancement factor of ≈ 6520 for electron field emission suggest that the vertically-oriented FLGs could be used as a potential edge emitter. In chapter 7, a new process has been developed to grow silicon nanowires (SiNWs) and their growth mechanisms were explored and discussed. This is a novel technique to grow xii SiNWs just by switching the gas compositions used for graphene fabrication by thermal CVD. In this process, SiNWs were synthesized by simply oxidizing and then reducing Si wafers in a high temperature furnace. The process involves hydrogen (H2), in an inert atmosphere, reacts with thermally grown silicon dioxide (SiO2) on Si at 1100 oC enhancing the growth of SiNWs directly on Si wafers. High-resolution transmission electron microscopy studies show that the NWs consists of a crystalline core of approximately 25 nm in diameter and an amorphous oxide shell of approximately 2 nm in thickness, which was also supported by selected area electron diffraction patterns. The synthesized NWs exhibit a high aspect ratio of approximately 167 and room temperature phonon confinement effect. This simple and economical process to synthesize crystalline SiNWs opens up a new way for large scale applications.