Jinsu Yoo

RIE texturing optimization for thin c-Si solar cells in SF6/O2 plasma

Dry etching plasma parameters were optimized for texturing single crystalline thin silicon solar cells and hence for high efficiency. In reactive ion etching (RIE) texturing, a low etch depth (~2 µm) was obtained compared with the etch depth that occurred in the wet chemical texturing process. For the flow ratios (SF6/O2) of 2 and 3, needle-like and cylindrical type structured surfaces were obtained. In the RIE process, the effects of working pressure, flow ratio, and etching time on reflectance and electrical properties of single crystalline silicon solar cells were investigated. The textured c-Si wafer with needle-like structure has good antireflectance behaviour. The interface defect density (Dit) in these textured silicon wafers increased with etching time. But an improvement in the reduction of interface trap density was observed through the annealing effect. Single crystalline solar cells with cylindrical type texture have higher values for open circuit voltage, short circuit current and efficiency in spite of a higher reflectance as compared with those needle-like structures. It is observed that the optimized SF6/O2 based chemistry in the RIE method is more suitable for thin crystalline silicon solar cells instead of the conventional wet texturization processes.

Spectroscopic Ellipsometry and Raman Studies on Sputtered TiO2 Thin Films

Titanium dioxide films were deposited using DC magnetron sputtering technique onto silicon substrates at ambient temperature and at an oxygen partial pressure of 7×10 –5 mbar and sputtering pressure (Ar + O2) of 1×10–3 mbar. The composition of the films, analyzed by Auger Electron Spectroscopy (AES), revealed the stoichiometry with an O and Ti ratio of 2.08. The optical constants of the as-deposited TiO2 thin film were determined by Spectroscopic Ellipsometry in the photon energy range 1.2 to 5.5 eV at room temperature. The measured dielectric-function spectra reveal distinct structures at energies of the E1, E1+D1 and E2 critical points due to interband transitions. The Dielectric constant values were found to be substantially lower than those for the bulk TiO2. The dielectric related optical constants, such as the refractive index, extinction coefficient, absorption coefficient and normal incidence of reflectivity are presented and analyzed. The deposited films were calcinated at 673 and 773 K. The influence of post-deposition calcination on the Raman scattering of the films was studied. The existence of Raman active modes A1g, B1g and Eg corresponding to the Raman shifts are reported in this paper. The improvement of crystallinity of the TiO2 films as shown by the Raman scattering studies has also been reported.

Study on the Silicon Nano-needle Structure for Nano floating Gate Memory Application

In this work, nano-needle structures ate formed to solve problem, related to low density of quantum dots for nano floating gate memory. Such structures ate fabricated and electrical properties` of MIS devices fabricated on the nano-structures are studied. Nano floating gate memory based on quantum dot technologies Is a promising candidate for future non-volatile memory devices. Nano-structure is fabricated by reactive ion etching using and gases in parallel RF plasma reactor. Surface morphology was investigated after etching using scanning electron microscopy Uniform and packed deep nano-needle structure is established under optimized condition. Photoluminescence and capacitance-voltage characteristics were measured in with nano-needle structure of silicon. we have demonstrated that the nano-needle structure can be applicable to non-volatile memory device with increased charge storage capacity over planar structures.

Plasma enhanced chemical vapor deposited SiN layers for large area mc-Si solar cell processing

Summary form only given. Surface passivation plays a crucial role in the fabrication process of high efficiency multi-crystalline silicon (mc-Si) solar cells. Plasma enhanced chemical vapor deposition (PECVD) of silicon nitride (SiN) is a proven technique for obtaining layers that meet the needs of surface passivation and anti-reflection coatings. In addition, the deposition process appears to provoke bulk passivation as well due to diffusion of atomic hydrogen. This bulk passivation is an important advantage of PECVD deposition when compared to the conventional CVD techniques. A further advantage of PECVD is that the process takes place at a relatively low temperature of 300/spl deg/C, keeping the total thermal budget of the cell processing to a minimum. This paper deals with the fabrication of multicrystalline silicon solar cells with PECVD SiN layers combined with high-throughput screen printing and RTF firing. Using this sequence we were able to obtain solar cells with an efficiency of 13.5% for multi crystalline polished Si wafers of size 125 mm square. POCl/sub 3/ doped polished silicon wafers with a junction depth of about 0.5 /spl mu/m and an emitter sheet resistance of 35 /spl Omega//sq were selected for the cell fabrication. The mean cell efficiency, fill factor and Voc for the cells prepared in the present study were found to be 13.5%, 0.76 and 0.602 V respectively.