The minority carrier lifetime is a valuable parameter for determining the electrical properties of semiconductors. It is particularly useful in the fabrication of solar cells as minority carrier lifetime is directly related to device efficiency. Pump-probe techniques, in which a pump laser of photon energy above the bandgap energy of the material is used to excite free carrier populations while a sub-bandgap probe laser is used to monitor the change in excess carrier density have been demonstrated to be an effective, non-contact, method to measure the minority carrier lifetime, particularly well-suited for use as an in-line measurement apparatus for the solar cell manufacturing processes In this thesis a non-contact, optical, pump-probe method has been used to determine the minority carrier lifetime of float zone and Czochralski grown <100> silicon samples through measurements of modulated free carrier absorption. The equivalence between measurements performed using the transmitted part of the probe beam and those made with the reflected part have been demonstrated on bare silicon wafers, as well as measurement of effective minority carrier lifetime of a completed solar cell, demonstrating the ability of this technique to measure the effective minority carrier lifetime at any stage of the solar cell manufacturing process. / Thesis / Master of Applied Science (MASc)

A novel experimental technique has been developed for measuring the light trapping enhancement and the carrier recombination lifetime in silicon wafers. The technique is based on the pump/probe modulated free carrier absorption (MFCA) method, where the probe beam, attenuated by generated free carriers, carries information about the effective lifetime and the average light path enhancement in a textured silicon wafer. For the first time, a reflection mode MFCA technique is presented where the reflected part of the probe beam is used to perform measurements, while the conventional technique is based on measurements of the transmitted part of the probe beam. A theoretical model is presented to explain the behavior of the light beam in double-side polished and double-side textured silicon wafers. The model yields good agreement with the experimental results and explains the difference in the amplitudes of the reflected and transmitted signals. The results of the experimental measurements of the light path enhancement in a double-side textured sample are analyzed and the reasons for their deviation from the Lambertian limit are discussed. This work presents new applications of the MFCA technique and shows how it can be used for the simultaneous determination of more than one crucial characteristic of silicon solar cells. / Thesis / Master of Applied Science (MASc)

In this work, homojunction interfacial workfunction internal photoemission (HIWIP) detectors based on GaAs, and heterojunction interfacial workfunction internal photoemission (HEIWIP) detectors based mainly on the Gallium Arsenide/Aluminum Gallium Arsenide material system are presented. Design principles of HIWIP and HEIWIP detectors, such as free carrier absorption, photocarrier generation, photoemission, and responsivity, are discussed in detail. Results of p-type HIWIPs based on GaAs material are presented. Homojunction detectors based on p-type GaAs were found to limit their operating wavelength range. This is mainly due to band depletion arising through carrier transitions from the heavy/light hole bands to the split off band. Designing n-type GaAs HIWIP detectors is difficult as it is strenuous to control their workfunction. Heterojunction detectors based on Gallium Arsenide/Aluminum Gallium Arsenide material system will allow tuning their threshold wavelength by adjusting the alloy composition of the Aluminum Gallium Arsenide/Gallium Arsenide barrier, while keeping a fixed doping density in the emitter. The detectors covered in this work operate from 1 to 128 micron (300 to 2.3 THz). Enhancement of detector response using resonance cavity architecture is demonstrated. Threshold wavelength extension of HEIWIPs by varying the Al composition of the barrier was investigated. The threshold limit of approximately 3.3 THz (92 micron), due to a practical Al fraction limit of approximately 0.005, can be overcome by replacing GaAs emitters in Gallium Arsenide/Aluminum Gallium Arsenide HEIWIPs with Aluminum Gallium Arsenide/Gallium Arsenide emitters. As the initial step, terahertz absorption for 1 micron-thick Be-doped Aluminum Gallium Arsenide epilayers (with different Al fraction and doping density) grown on GaAs substrates was measured. The absorption probability of the epilayers was derived from these absorption measurements. Based on the terahertz absorption results, an Aluminum Gallium Arsenide/Gallium Arsenide HEIWIP detector was designed and the extension of threshold frequency (f0) to 2.3 THz was successfully demonstrated. In a different study, switching in Gallium Arsenide/Aluminum Gallium Arsenide heterostructures from a tunneling dominated low conductance branch to a thermal emission dominated high conductance branch was investigated. This bistability leads to neuron-like voltage pulses observed in some heterostructure devices. The bias field that initiates the switching was determined from an iterative method that uses feedback information, such as carrier drift velocity and electron temperature, from hot carrier transport. The bias voltage needed to switch the device was found to decrease with the increasing device temperature.