The citric acid‐hydrogen peroxide‐water system has been developed for preferential etching of
normalGaAs
through photoresist masks. Etching rates depend strongly on the composition of the solution and on the crystal orientation of
normalGaAs
, with the rates in the order (111)B > (100) > (111)A. Flat‐bottomed holes were obtained for all compositions of the solutions. The solution does not erode photoresist masks, thereby providing preferential etching of
normalGaAs
through such masks.
This paper describes the dependence of the dislocation density reduction effect on the employing position of either thermal cyclic annealing (TCA) or InGaAs-GaAs strained-layer superlattice (SLS) in GaAs-on-Si grown by metalorganic chemical vapor deposition (MOCVD). The dislocation density is reduced to one twenty-fifth of that in as-grown sample by the TCA as the position of TCA becomes farther than about 1.5 µm from the Si surface. The dislocation density is additionally reduced to one third by the SLS as the position of SLS becomes farther than about 2.0 µm. As a result, the dislocation density is reduced to 1.5 × 106 cm-2 by the combined use of TCA and SLS. The dislocation density reduction effect of TCA is determined mainly by the degree of residual stress. That effect of SLS is determined mainly by the degree of additional stress generated by SLS.
We have developed a new solar cell using thin-film silicon supported by a silicon substrate etched in a grid form. The light-trapping structures of this cell have been studied by considering rear surface light reflections, electrical power loss and mechanical strength. High rear reflectance can be obtained by employing a multi-layer rear electrode. The pattern of the rear substrate is designed to provide sufficient mechanical strength and to minimize the electrical power loss, taking account of the current flow path. A conversion efficiency of 14.2% for a practical size of 10×10 cm2 is obtained by applying these calculated parameters using a single-crystal silicon substrate.
This paper describes the study of crack propagation and mechanical fracture in GaAs-on-Si, which are closely related with the residual stress. The crack propagation is often observed as the GaAs thickness exceeds about 3 µm, and the upper limit of the number of cracks increases linearly as the GaAs thickness increases. The cracks propagate from the surface defects, where stress ten times larger than the original residual thermal stress in GaAs-on-Si exists. The mechanical fracture strength (ζ) of the GaAs-on-Si wafer decreases as the GaAs thickness increases, and becomes equal to that of the bulk GaAs at the thickness of about 3 µm due to the concentrated stress near the cracks. The back coating of SiO2 is effective for stress relaxation, and the preliminary result of about 3×108 dyn/cm2 of stress relaxation is obtained.
A new type of silicon solar cell is demonstrated using chemical vapor deposited silicon thin films on a silicon dioxide layer. In order to improve the crystal quality of the thin films, zone-melting recrystallization (ZMR) is applied and grain boundaries of polycrystalline Si films are passivated with H+. It is found that H+ passivation is quite effective for thin film Si solar cells and ZMR conditions to provide dominant (100) orientation is essential for achieving higher conversion efficiency. This (100) nature is also favorable for making effective light confinement scheme with pyramidal textured surface using anisotropic chemical etching. The conversion efficiency as high as 14.2% for a practical size of 10×10 cm2 is achieved. This is the highest for large area thin film polycrystalline Si solar cells so far.
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