This dissertation explores strategies for improving photolvoltaic efficiency and reducing cost using femtosecond-laser processing methods including surface texturing and hyperdoping. Our investigations focus on two aspects: 1) texturing the silicon surface to create efficient light-trapping for thin silicon solar cells, and 2) understanding the mechanism of hyperdoping to control the doping profiles for fabricating efficient intermediate band materials. / Engineering and Applied Sciences

Hyperdoping of Si with deep-level impurities has attracted renewed interest for its unique physical properties, such as the broad sub-bandgap absorption in the infrared wavelength at room temperature. In this thesis, tellurium (Te) hyperdoped Si has been prepared by ion implantation and pulsed laser melting with the Te doping concentration several orders of magnitude above the solid solubility limit. The structural, electrical and optical properties were systematically investigated. A strong room-temperature broadband infrared absorption down to 0.048 eV (25 μm) is observed in the resulting Te-hyperdoped Si layers. The room-temperature operation of a mid-infrared photodetector is demonstrated based on Te-hyperdoped Si. In addition, an impurity-induced insulator-to-metal transition in Te-hyperdoped Si has been identified via the electrical transport measurements. Besides, the electron concentration in Te-hyperdoped Si layers is approaching 1021 cm-3 and does not show saturation. Combining density functional calculations and Rutherford backscattering/channeling measurements, the microscopic mechanism for yielding the outstanding physical properties listed above has been unveiled. The Te-dimer complex sitting on adjacent Si lattice sites has the smallest formation energy and is thus the preferred configuration at high doping concentration. Those substitutional Te are effective donors, leading to the non-saturating carrier concentration as well as to the insulator to metal transition. Finally, a comprehensive study regarding the thermal stability has been performed and the Te-hyperdoped Si layers exhibits thermal stability up to 400 °C with a duration of at least 10 minutes. Therefore, Te-hyperdoped Si opens two perspectives for opto/micro-electronics. One is realization of broadband infrared photodetection at room temperature by using only Si materials, which may be integrated into on-chip Si-based photonic systems. The second is to achieve ultra-high n-type carrier concentrations for nano-electronics by forming Te-dimer dopants, which can overcome the saturation problem of conventional shallow n-type dopants.