Download files
Abstract
This thesis presents insights into the electrical properties of doped silicon nanocrystals (ncSi) for 3rd generation photovoltaic solar cells. While investigating doping of PECVD ncSi-based PV devices (Chapter 3), it has become clear that commonly used techniques to dope and characterise these ncSi films are not effective. In view of this, specially designed structures for Capacitance-Voltage (CV) measurements
and pulsed laser annealing techniques have been developed to characterise the doping effects and improve doping efficiency, respectively.
Two first-author journal papers have been published based on the work presented in Chapter 4 and Chapter 5 of this thesis. Chapter 4 presents the research about CV measurements and evaluations of electrically active boron doping concentrations in ncSi. The work about improved dopants (Boron and Phosphorus) activation in ncSi by pulsed KrF laser is mainly included in Chapter 5. The characterization and fabrication techniques also presented in this thesis have assisted other journal publications on the research of doped ncSi material, ncSi-based photovoltaic devices and renewable energy applications.
The two most important contributions of this thesis are:
1. The demonstration of an inverted MOS structure to measure doping using CV measurements and
2. The improvement of the conductivity of ncSi films using a pulsed KrF excimer laser after the crystal growth.
This thesis starts with the investigation of plasma-enhanced chemical vapour deposition (PECVD) for fabricating silicon nanocrystals along with boron (B) and phosphorus (P) doping. The material properties of non-stoichiometric silicon oxide before and after annealing and the B/P doped ncSi are investigated. After the demonstration of ncSi preparation and doping by PECVD, ncSi-based photovoltaic devices were eventually fabricated and analysed. The devices showed diode I-V characteristics and an open-circuit voltage of 230 mV was achieved. However, the parameters extracted from electrical measurements indicated severe limitations due to low carrier transport and strong nonradiative recombination. These limiting factors can be attributed to the ineffective doping in ncSi, which is a general problem for nanostructured semiconductor materials.
To investigate the doping in ncSi with high resistivity, we proposed an inverted MOS structure for CV measurements. Numerical CV modelling is developed to quantify the electrical properties such as doping concentration, doping type and interface trap density distribution. We investigated highly resistive boron doped ncSi films, which unexpectedly show a high doping concentration. The saturation of doping and the low doping efficiency, less than 5%, are observed and discussed. The corresponding low effective mobility is attributed to a strong scattering of excess impurities and defects.
Lastly, as a means to improve the electrical quality of these films, we demonstrated that a pulsed KrF excimer laser (λ=248 nm, τ=22 ns) can be used as a post-furnace annealing method to greatly increase the electrically active doping concentration in ncSi, which potentially can reduce the extremely high impurity density currently used for doping ncSi. We propose that the increase in free carrier concentration after the laser treatment is the result of interstitial P/B dopants activation, which is initially inside the ncSi. Evidence of mobility-limited carrier transport and degenerate doping in the ncSi are measured with temperature-dependent conductivity, which further reveal the carrier-conduction mechanism in doped ncSi.