Thin film silicon on low temperature glass substrates is currently seen as the best path toreduce the $/W cost of photovoltaic (PV) modules. However, producing thin film polysilicon, on glass, is an ongoing research challenge. Laser crystallisation of a-Si is one of the possible methods. Typically excimer (XMR) lasers are used for laser crystallisation. This thesis introduces the copper vapour laser (CVL) as a viable alternative for thin film photovoltaic applications. The CVL, like the XMR, is a high powered, pulsed laser. However, the CVL has higher pulse rates (4-20kHz), better beam quality and a visible wavelength output (578 & 511nm). Preliminary experiments, using 600K-heated silicon-on-quartz samples, confirmed that CVL crystallisation can produce area weighted average grain size of 0.1-0.15??m, which is comparable to results reported for XMR??? s. Importantly, the CVL results used thicker films (1??m), which is more applicable to thin photovoltaic devices that need 1-10??m of silicon to be viable. The CVL??? s longer wavelength and therefore longer penetration depth (1/alpha) are proffered as the main reason for this result. Extensive laser-thermal modelling highlighted further opportunities specific to CVL crystallisation. Through-the-glass doublesided irradiation was shown in simulations to reduce thermal gradients, which would enhance crystal growth. The simulations also produced deeper melts at lower surface temperatures, reducing the thermal stress on the sample. Subsequent experiments, using silicon-on-glass, confirmed the benefit of through-the-glass doublesided irradiation by maintaining grain sizes without the usual need for substrate heating. Furthermore, Raman analysis showed that doublesided crystallisation achieved full depth crystallisation, unlike single side irradiation which produced partial crystallisation. A new mode of crystallisation, stepwise crystallisation, was also postulated whereby a series of CVL pulses could be used to incrementally increase the crystallisation depth into the silicon. Simulations confirmed the theoretical basis of the concept, with HeNe Raman spectroscopy and analysis of surface grain sizes providing indirect experimental support. The CVL??? s ability to crystallise thicker films more directly applicable to photovoltaic devices secures its viability as an alternative laser for photovoltaic applications. The through-the-glass doublesided irradiation and the stepwise crystallisation provide additional potential for increased process flexibility over XMR???s.

Thin film silicon on low temperature glass substrates is currently seen as the best path toreduce the $/W cost of photovoltaic (PV) modules. However, producing thin film polysilicon, on glass, is an ongoing research challenge. Laser crystallisation of a-Si is one of the possible methods. Typically excimer (XMR) lasers are used for laser crystallisation. This thesis introduces the copper vapour laser (CVL) as a viable alternative for thin film photovoltaic applications. The CVL, like the XMR, is a high powered, pulsed laser. However, the CVL has higher pulse rates (4-20kHz), better beam quality and a visible wavelength output (578 & 511nm). Preliminary experiments, using 600K-heated silicon-on-quartz samples, confirmed that CVL crystallisation can produce area weighted average grain size of 0.1-0.15??m, which is comparable to results reported for XMR??? s. Importantly, the CVL results used thicker films (1??m), which is more applicable to thin photovoltaic devices that need 1-10??m of silicon to be viable. The CVL??? s longer wavelength and therefore longer penetration depth (1/alpha) are proffered as the main reason for this result. Extensive laser-thermal modelling highlighted further opportunities specific to CVL crystallisation. Through-the-glass doublesided irradiation was shown in simulations to reduce thermal gradients, which would enhance crystal growth. The simulations also produced deeper melts at lower surface temperatures, reducing the thermal stress on the sample. Subsequent experiments, using silicon-on-glass, confirmed the benefit of through-the-glass doublesided irradiation by maintaining grain sizes without the usual need for substrate heating. Furthermore, Raman analysis showed that doublesided crystallisation achieved full depth crystallisation, unlike single side irradiation which produced partial crystallisation. A new mode of crystallisation, stepwise crystallisation, was also postulated whereby a series of CVL pulses could be used to incrementally increase the crystallisation depth into the silicon. Simulations confirmed the theoretical basis of the concept, with HeNe Raman spectroscopy and analysis of surface grain sizes providing indirect experimental support. The CVL??? s ability to crystallise thicker films more directly applicable to photovoltaic devices secures its viability as an alternative laser for photovoltaic applications. The through-the-glass doublesided irradiation and the stepwise crystallisation provide additional potential for increased process flexibility over XMR???s.

Copper vapour laser (CVL) pumped high repetition rate narrow bandwidth dye laser is an important source of tunable radiation. It finds numerous applications in spectroscopic investigations and selective material processing like atomic vapour laser isotope separation (AVLIS). Being wavelength selective in these applications stability of the output wavelength and bandwidth are extremely important. The stability of these parameters depend upon the refractive index fluctuation of the dye medium, due to pump beam induced temperature gradients, dye solution flow, and mechanical stability of optical components. Precise measurement of wavelength and bandwidth of a dye laser and control over parameters governing the variations are important for any stable dye laser system. In this thesis, details of investigations carried out on a Rhodamine 6G dye laser for obtaining stable wavelength and output power are presented. Parameters that affect the stability were identified, monitored and put on close loop control to achieve the desired stability. Pump beam i.e. CVL optical power, dye flow rate and dye solution temperature are mainly these parameters. CVL power is mainly a function of input electrical power and pressure of the buffer gas inside the tube. To monitor and regulate these parameters, different sensors and actuators were selected and interfaced with a master slave topology based data acquisition and control system. The DAQ and control system is designed around a micro controller card based on advanced CPU P80552 and has on chip 8 channel 10 bit multiplexed analog input, 16 TTL digital inputs and 16 digital outputs. It works as slave and PC as master. Following closed loops were designed and incorporated to maintain a stable output: a. Average output of CVL was maintained constant by regulating the electric input power through closed loop control. b. The buffer gas pressure was monitored with a semiconductor pressure sensor and was regulated using pulse width modulation. c. Temperature of the dye solution was monitored with PT100 and was controlled using proportional controller. d. Flow rate of dye solution was controlled using a variable frequency drive (VFD) for the dye circulation pump. e. The dye laser wavelength was monitored by using a high resolution spectrograph and pixel position of the peak from CCD image obtained from spectrograph is used for feedback correction using a pico motor. In the present work with application of the above-mentioned input power and pressure loops, a stable output of CVL, is achieved. Variations in power and pulse width of CVL are got limited to within 2%, from 10% when CVL system was working unregulated. This control system does the line regulations and corrects the input electrical power if variations in discharge current occur due to pressure variation. Every dye cell has limits on flow rate because of its geometry. With flow and temperature control dye cell was characterized to work with lower linewidth. VFD (variable frequency drive) is used for flow regulation. Finally active control on set wavelength was also achieved with resolution of 0.01nm accuracy. Measurement of wavelength was done with 0.3 m, 0.054 nm resolution spectrograph. Closed loop pico motor with 30 nm per step linear resolution was used for wavelength control. The thesis is organized in four chapters. First chapter presents a brief introduction to high repetition rate CVL pumped dye laser, operation of a CVL and parameters affecting the dye laser stability and their control schemes. Literature survey in this chapter is focused on different control mechanisms used with such lasers. Second chapter describes the laser system and interfacing of data acquisition system used for experimental setup. Closed loop controls for different parameters are described in this chapter. It also describes the software algorithms developed for this work. Third chapter presents experimental results and analysis with discussion on performance of the control loops. Finally the conclusion is given and few suggestions are made for further work.