Synthesis and Characterization of Silicon Nanowire Arrays for Photovoltaic Applications

Open Access
Author:
Eichfeld, Sarah M
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
June 05, 2009
Committee Members:
  • Joan Marie Redwing, Dissertation Advisor
  • Joan Marie Redwing, Committee Chair
  • Suzanne E Mohney, Committee Member
  • Thomas E Mallouk, Committee Member
  • Christopher Muhlstein, Committee Member
Keywords:
  • SiNWs
  • nanowire solar cells
  • SiNW growth
Abstract:
Due to rising energy costs and the growing demand for renewable energy, silicon nanowire arrays have become of interest for solar cells. Radial p-n junction silicon nanowire arrays allow for the decoupling of the directions of light absorption and carrier collection, which allows for the possibility of increased efficiencies. Prior device modeling studies have demonstrated that increased carrier collection can be obtained in radial p-n junction nanowires when the wire radius is approximately equal to the minority carrier diffusion length in the material. Consequently, radial p-n junction silicon nanowires are anticipated to enable increased efficiency in solar cells fabricated with less pure and therefore potentially lower cost silicon. Fabrication of these structures on low cost substrates such as glass would then enable a further cost reduction. The overall objective of this thesis was the development of processes for the fabrication of radial p-n silicon nanowires (SiNWs) using bottom-up nanowire growth techniques on silicon and glass substrates. Vapor-liquid-solid (VLS) growth was carried out on Si(111) substrates using SiCl4 as the silicon precursor. Growth conditions including temperature, PSiCl4, PH2, and position were investigated to determine the optimum growth conditions for epitaxially oriented silicon nanowire arrays. The experiments revealed that the growth rate of the silicon nanowires exhibits a maximum as a function of PSiCl4 and PH2. Gas phase equilibrium calculations were used in conjunction with a mass transport model to explain the experimental data. The modeling results demonstrate a similar maximum in the mass of solid silicon predicted to form as a function of PSiCl4 and PH2, which results from a change in the gas phase concentration of SiHxCly and SiClx species. This results in a shift in the process from growth to etching with increasing PSiCl4. In general, for the atmospheric pressure conditions employed in this study, growth at higher temperatures >1000°C and higher SiCl4 concentrations gave the best results. TEM analysis of silicon nanowires grown at different SiCl4 concentrations revealed no distinct differences in the structural properties of the SiNWs. Doping of the SiNWs using trimethylboron (TMB) was also examined. Gated I-V measurements demonstrated that the SiNWs exhibited p-type behavior. Wires doped with a TMB/SiCl4 ratio of 0.1 had a resistivity in the range of 10-3 Ω-cm. However, wires with a TMB/SiCl4 ratio of 2x10-3 exhibited a resistivity of 103 Ω-cm. The growth of silicon nanowire arrays on anodized alumina (AAO)-coated glass substrates was also investigated. Glass will not hold up to the high temperatures required for Si nanowire growth with SiCl4 so SiH4 was used as the Si precursor instead. Initial studies were carried out to measure the resistivity of p-type and n-type silicon nanowires grown in free-standing AAO membranes. A series of nanowire samples were grown in which the doping and the nanowire length inside the membrane were varied. Circular metal contacts were deposited on the top surface of the membranes and the resistance of the nanowire arrays was measured. The measured resistance versus nanowire length was plotted and the nanowire resistivity was extracted from the slope. The resistivity of the silicon nanowires grown in the AAO membranes was then compared to the resistivity of silicon nanowires grown on Si and measured using single wire four-point measurements. It was determined that the undoped silicon nanowires grown in AAO have a lower resistivity compared to nanowires grown on Si substrates. This indicates the presence of an unintentional acceptor. The resistivity of the silicon nanowires was found to change as the dopant/SiH4 ratio was varied during growth. The growth and doping conditions developed from this study were then used to fabricate p-type SiNW arrays on the AAO coated glass substrates. The final investigation in this thesis focused on the development of a process for radial coating of an n-type Si layer on the p-type Si nanowires. While prior studies demonstrated the fabrication of polycrystalline n-type Si shell layers on Si nanowires, an epitaxial n-type Si shell layer is ultimately of interest to obtain a high quality p-n interface. Initial n-type Si thin film deposition studies were carried out on sapphire substrates using SiH4 as the silicon precursor to investigate the effect of growth conditions on thickness uniformity, growth rate and doping level. High growth temperatures (>900oC) are generally desired for achieving epitaxial growth; however, gas phase depletion of the SiH4 source along the length of the reactor resulted in poor thickness uniformity. To improve the uniformity, the substrate was shifted closer to the gas inlet at higher temperatures (950°C) and the total flow of gas through the reactor was increased to 200 sccm. A series of n-type doping experiments were also carried out. Hall measurements indicated n-type behavior and four-point measurements yielded a change in resistivity based on the PH3/SiH4 ratio. Pre-coating sample preparation was determined to be important for achieving a high quality Si shell layer. Since Au can diffuse down the sides of the nanowire during sample cooldown after growth, the Au tips were etched away prior to shell layer deposition. The effect of deposition temperature on the structural properties of the shell layer deposited on the VLS grown SiNWs was investigated. TEM revealed that the n-type Si shells were polycrystalline at low temperatures (650oC) but were single crystal at 950°C. SiNW samples grown on glass were also coated; however, due to the temperature constraints, the maximum temperature used was 650oC and therefore the n-type Si shells were polycrystalline. SiNW arrays grown using SiCl4 on Si(111) substrates were grown with a high degree of orientation and average growth rates of 3-4 µm/min at temperatures of 1050°C. Modeling results indicated the nanowire growth was limited by mass transport. SiNW arrays were also grown using SiH4 on AAO coated glass. Epitaxial n-type Si regrowth was demonstrated on SiNW arrays grown on Si(111) with SiCl4. The n-type Si deposition was carried out using SiH4 at 950°C and a total pressure of 3 Torr. SiNWs grown on AAO coated glass substrates were also radially coated with n-type Si; however, due to temperature limitations the shell was polycrystalline. Future work on this project could include a study on the doping of SiNWs grown using SiCl4. Device measurements using the radial p-n junction SiNW arrays fabricated could also be carried out. Finally, a study on the effects of sample preparation, nanowire, and n-type shell doping could be correlated to the device measurements.