Probing the intrinsic electrical properties of thin organic layers/semiconductor interfaces using an atomic-layer-deposited Al2O3 protective layer
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(a) Schematics of the device structure. The Si(111) surface is functionalized with carboxylic acid-terminated SAM via hydrosilylation. The reaction leads to approximately 50% of the surface Si–H sites replaced with Si–C. On the other end of the SAM, TMA is able to react with COOH and a layer of Al2O3, approximately 6-7 nm thick, is grown on top of the SAM layer in a commercial ALD reactor at 100 °C. Due to the dramatic reduction in leakage current, the capacitance-voltage and conductance voltage measurements can be performed on the two-layered gate stack. (b) XPS spectrum of C1s core level. The carbon signal can be decomposed into three individual peaks: C–C from the carbon chain (pink), C–Si at the SAM/Si interface (blue) and C–O at the Al2O3/SAM interface (green). The orange dashed line gives the overall fit to the experimental data.
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(a) DC leakage currents of the SAM/Si and Al2O3/SAM/Si structures. The voltage is applied to the Si substrate and collected at the top electrode. The leakage current at −1 V is reduced by 6 orders of magnitude due to the addition of an Al2O3 layer. Si is of n type, with a resistivity of 0.6-1.2 Ω cm. (b) Capacitance voltage characteristic of the two layered Al2O3/SAM/Si structures measured using a mercury probe setup. A normal capacitance behavior is observed in the accumulation region. The small frequency dispersion partly comes from series resistance effects. The humps around −0.3 V for low measurement frequencies are due to the Cit response from interface states. Inset: CV measurements directly made on an 18-carbon SAM/Si interface using the mercury probe. Negative capacitances are observed for 1 kHz when Vsub < −0.8 V. Capacitance roll-over happens for both 10 and 100 kHz. There exists large frequency dispersion in accumulation.
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(a) Simulation fit using a quantum mechanical model and the experimental 1 MHz capacitance data after Rs and L0 corrections. The results from mercury probe measurements almost coincide with the theoretical predictions. But the CV curve of the MOS capacitor demonstrates a larger voltage stretch-out. The EOT, Cox, and surface potential are extracted from the fitting. The experimental curves have been shifted to share the same flat band voltage VFB as the simulation result. (b) Conductance loss for a 500 μm diameter MOS capacitor at various substrate voltages. Peaks are clearly resolved. (c) Distribution of interface states with respect to the Si mid gap for two cases: MOS capacitors with Pt gate (circle) and mercury probe (square). Since the Si substrate is of n type, only those states in the upper half of the band gap are observed in conductance-voltage measurements.
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