Femtosecond Laser-Induced Micro-Structuring of Thin a-Si:H Films
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Femtosecond Laser-Induced Micro-Structuring of Thin a-Si:H Films Barada K. Nayak and Mool C. Gupta1 Applied Research Center, Old Dominion University, Newport News, Virginia 23606, USA ABSTRACT In a-Si:H solar cells, light trapping has been sought as a method to enhance absorption and improve the efficiencies of these devices. Here we present results for surface texturing of 2µm thick a-Si:H films by 800 nm wavelength femtosecond laser. The texture led to a significant enhancement in optical absorption (80%) in visible through near IR (60%). The textured surface was examined by a scanning electron microscope (SEM) and atomic force microscope (AFM). The results indicated the formation of dense spikes with height of 65-75 nm. The samples were completely black after femtosecond laser treatment. INTRODUCTION Ultra short laser pulse induced surface modification has tremendous technological importance. Several groups1-3 have reported work on laser-assisted micro-structuring of silicon including extensive work by Mazur’s group 4-7 at Harvard. They have demonstrated the formation of arrays of sharp, conical microstructures in silicon wafer by femtosecond laser assisted chemical etching. This texturing process drastically changes the optical, morphological and electronic properties of the original silicon wafers. It has been reported that these surface texturing in silicon could result in fabrication of infrared silicon photo-detectors, chemical and biological sensors, and field emission devices. Our research objective has been to improve the efficiency of a-Si:H based thin film solar cells by femtosecond laser induced crystallization technique and we observed that in addition to the formation of sharp spikes, there is also a phase change taking place, converting the a-Si:H in to a poly-crystal phase. The unique features of our work are: 1.) Observation of amorphous to crystalline phase change as well as the formation of micro spikes similar to the reported in bulk silicon was demonstrated 2.) The spikes were observed under ordinary atmospheric condition as opposed to reported work carried in special gaseous environments. 3.) The absorption in these films are significantly high in visible as well as near IR as compared to untreated region. 4.) The phase change that leads to crystallization should lead to better electrical characteristics of the film. 5.) Observation of higher photo current in treated p-i-n cells compared to untreated region. The crystallization and surface texturing as a one step process could greatly enhance the efficiency of thin film solar cells and at the same time keeping the fabrication cost of these devices low. EXPERIMENT The amorphous silicon films of thickness 2 µm were deposited on to SnO2:F coated glass substrates, using low temperature Plasma Enhanced Chemical Vapor Deposition (PECVD) 1
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technique. The a-Si:H film was deposited at a rate of 1 A°/s in hydrogen diluted silane environment at about 200°C. The samples we
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