The processing of gate-all-around (GAA) Si transistors requires several isolated and vertically stacked nanometer-thick Si sheets or wires. For this purpose, the sacrificial SiGe layers of a SiGe/Si superlattice are etched selectively and laterally. Controlling the quantity of etched SiGe material, i.e., the so-called SiGe cavity depth, is critical for optimal device performance. Unfortunately, this critical dimension can only be measured by time-consuming cross-sectional transmission electron microscopy (TEM), which results in limited statistics and hence insufficient control of the cavity depth across wafers and batches. This paper evaluates the capabilities of micro hard x-ray fluorescence (μHXRF) for the determination of cavity depth as a fast and non-destructive alternative to TEM. As we show, μHXRF provides cavity depth values in excellent agreement with TEM. In addition, two critical advantages of the technique demonstrated here are that, thanks to the very high energy of the incoming and emitted X-rays, the SiGe volume is extracted without requiring any complex model and without any correlation to other geometrical parameters of the complex GAA device.
By using electrical characterization and classical solid state semiconductor device theory, we demonstrate that the open circuit voltage (V oc) in organic solar cells based on non-intentional doped semiconductors is fundamentally limited by the built-in potential (V bi) originated at a donor-acceptor abrupt (p-n ++) heterojunction in case of selective contacts. Our analysis is validated using P3HT:PCBM devices fabricated in our research group. We also demonstrate that such a result can be generalized using data already reported in literature for fullerenebased solar cells. Finally, we show that the dependence of V oc on the device contacts can be understood in terms of the potential barriers formed by the Fermi level alignment of semiconductors at the heterojunction and at the Schottky junctions.
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