Ichiro Shiono

Evaluation of the CIGS precursor layer deposited by CuGa-NaF sputtering target and its effect for Na distribution

The purpose of this study is to understand the relationship between the precursor layer deposited by CuGa-NaF sputtering target and selenidization process. The addition of Na was carried out by using the CuGa-NaF sputtering target. In the CuGa-NaF precursor film, the presence of Na-Ga-F compound was observed. The desorption of fluoride gas from the Na-Ga-F compound occurred, and it accompanied the migration of Ga element. The desorption of fluoride gas enhanced the reactivity with the Se gas in the selenization process. On the other hand, the presence of Na and the migration of Ga impeded the interdiffusion of In and Ga. The decomposition temperature of H2Se gas and the temperature of formation of CIGS were important for uniformity of composition in selenization process. By optimizing the CuGa-NaF film thickness and the multilayer film structure, a CIGS film with a graded composition of In/Ga and the uniformly incorporation Na was obtained.

Relationship Between Graded Layer Structures and Defects in Silicon-Germanium Virtual Substrates

Silicon-germanium virtual substrates have been synthesized by low-pressure chemical vapor deposition. We obtained threading dislocation densities ranging from 10 5 to 10 6 cm −2 , surface roughness ranging from 1.5 to 4 nm, and also cross-hatch pattern densities, depending on the grading rate and top layer germanium composition. For the typical sample, which has a linear-graded structure with a grading rate of 20%/[µm, and germanium composition of 30 % at the top layer, we obtained dislocation densities of about 10 6 cm −2 and root mean squared surface roughness of about 3 nm. The obtained dislocation densities are equivalent with the virtual substrates synthesized by ultra-high vacuum system. On the other hand the surface roughness is superior to the typical reported values. In this study three kinds of structures, i.e. linear-graded, stepwise, and graded-step structures, were considered. We found the defects are effectively reduced by introduction of an optimum number of steps in the graded layer.