We analyze the stress distribution in the nonuniformly bent GaN epilayers grown on a sapphire substrate. By using theoretical analysis combined with an analytical formula describing the realistic shape for the wafer bending of GaN epiwafers, we examine the effect of nonuniformity in the wafer bending on the stress-value variation over the entire wafer. We show that the stress on the GaN thin film can deviate by ∼1 MPa from the value obtained by the simple Stoney’s formula that is typically used for the uniformly bent wafer. We also show that the maximum value of the stress linearly increases with the bow difference along the horizontal and vertical directions.
A built-in potential exists in the reflective liquid-crystal-on-silicon (LCoS) microdisplay cell due to the work-function difference between the aluminum and indium-tin-oxide electrodes. As a consequence, the flicker is generated unless the dc offset voltage is applied to compensate the built-in potential. In this paper, we present the experimental result that the dc offset voltage changes with time as the display is operated and the ions are generated in the liquid crystal (LC). To understand the experimental result, we simulate the ion motion in the LCoS cell by considering both the drift and diffusion. We discuss how the ion concentration in the LC affects the screening of the internal electric field in the bulk LC region and subsequently the built-in potential in the LCoS cell. We show that for an ion concentration higher than the value required to fully compensate the initial built-in potential, the polarity of the built-in potential is changed due to the high electric field near electrodes. By matching the experimental and simulation results, we predict how the ion concentration in the LC increases as a function of operation time.
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