Characterization of deep centers in undoped semi-insulating GaAs substrates by normalized thermally stimulated current spectroscopy: Comparison of 100 and 150 mm wafers

A well-established characterization method for investigating deep traps in semi-insulating (SI) GaAs is thermally stimulated current (TSC) spectroscopy; however, TSC is not considered to be a quantitative technique because it involves carrier mobility, lifetime, and geometric factors, which are either unknown or poorly known. In this paper, we first show how to quantify a TSC spectrum, by normalizing with infrared (hv = 1.13 eV) photocurrent, and then apply this method (called NTSC) to study the lateral uniformity of the main deep centers across the diameters of undoped SI GaAs wafers. The wafers used in the study include both the standard 100 mm sizes and the new 150 mm variations, and are grown by both the low and high pressure liquid encapsulated Czochralski techniques. The results reveal that the 150 mm wafers have a worse NTSC uniformity for the main traps and a higher degree of compensation, as compared these parameters for the 100 mm wafers. In addition, nonuniformities related to the electric field effects on both the TSC spectrum and the low temperature photocurrent are found in the 150 mm wafer grown by the low pressure technique.     

二氧化硅与硅界面及其对能带轮廓及MOSFET栅遂穿电流影响的理论研究

利用第一原理对双键及桥氧两种二氧化硅与硅界面模型进行了理论研究。结果表明双键模型的界面转变区宽度较大。这种差别会导致MOSFET栅漏电的不同。遂穿电流的计算表明界面双键模型结构有较大的栅漏电。     

DIRECT SOLUTION BY 2-D AND 3-D FINITE ELEMENT METHOD ON FORWARD PROBLEM OF LOW FREQUENCY CURRENT FIELDS IN INHOMOGENEOUS MEDIA

The paper adopts finite element method to analyze the forward problem of low-frequency current fields in inhomogeneous media. Firstly, the direct solution of 2-D and 3-D scalar potential is given. Secondly, the technique of covering finite elements for problems with movement has been presented; namely, when the place of testing point moved, the meshing data will be produced automatically to avoid re-meshing and distortion of the mesh. Thirdly the free and prescribed potential method is used to make the finite element coefficient matrices. Then this paper provides the result of a validity test obtained by simulating the laterolog-3 logging, compared with the numerical model-matching method. Finally, the MLL response is calculated.     

The effect of ethylene glycol/citric acid molar ratio in the initial precursor of TiO2 nanoparticle paste synthesized by a polymerizable complex method on the photovoltaic properties of dye-sensitized solar cells

A polymerizable complex method, also known as a Pechini method, was employed to synthesize titanium-sol (Ti-sol) as a matrix for TiO₂ nanoparticle paste, suitable for fabrication of semiconducting mesoporous TiO₂ layer as a photoanode of dye-sensitized solar cells (DSSCs). The purpose of the present work was to investigate the effect of ethylene glycol (EG)/citric acid (CA) molar ratio (Z), in the initial Ti-sol precursor, on the photovoltaic properties of DSSCs. From viscosity (µ) measurement and Fourier transform infrared (FTIR) spectrum of Ti-sols it was revealed that the amount of polyester in the sol decreases with increasing Z. The higher polyester content in the Ti-sols with lower Z ratios led to their higher surface tension (γ) and as a result the higher contact angle (α). The low wettability of fluorine doped tin oxide (FTO) coated glass with Ti-sol was the main reason of micro-cracking of TiO₂ layers after sintering. This effect was significant for lower Z ratios. Micro-cracks increase the back electron–hole recombination rate. Also, at higher Z ratio, the back electron–hole recombination rate increased, which was due to the lower Ti⁴⁺ ions in the Ti-sol precursor and poor interconnection between TiO₂ nanoparticles. Therefore, the maximum short circuit current density (I_sc) and the maximum conversion efficiency (η) were obtained for Z=4. Fill factor (FF) decreased with increasing Z. But, open circuit voltage (V_oc) was nearly independent of Z.