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I would like to truly thank my advisor Dr. Mulugeta Bekele for the continuous guidance and follow up he has been making both in the course work and in doing this thesis. My greatest thanks goes to the Kebe family (Solomon, Tesfaye, and Sr. Felekech), Feleke Getachew, Sr. Degefech H/Yesus and Belay Legesse for the financial and unfolding moral support they made to me without which everything would have ended a daydream. In addition, I would like to thank my parents and all my friends, specially Birhanu Aragie and Wondesen Ayalew, for all kinds of encouragement and support I get from them. I would like to extend my thanks to Dr. Tilahun Tesfaye of the Addis Ababa education bureau for devoting his time to share me his computer skills. It gives me great pleasure to acknowledge the Department of Physics and the School of Graduate Studies,Addis Ababa University, for all the support I get during my study; and the International Programme for Physical Sciences (IPPS), Uppsala University, Sweden for the computer facilities they provide to our group. And God, you are always near, and all the strength came. Thank you