(Invited) Liquid Electrolyte Contacts to Semiconductor Nanostructures: Probing and Programming Local Defect Behavior

Abstract

Replacing the metal top electrode of a metal-insulator-semiconductor (MIS) or –metal (MIM) junction with a liquid electrolyte contact can be used 1) to study the capacitance and conductance response of semiconductor nanostructures with a conformal liquid contact, and 2) to program, at higher voltages, populations of point defects in an insulator layer, as a means of investigating local resistance switching. This presentation will summarize recent results on both of these topics, beginning with a discussion of electrochemical spectroscopy (EIS) applied to semiconductor-liquid junctions as a means of performing quantitative interface state characterization on micro- and nano-structured samples. Using aqueous KCl solution as a blocking contact to a semiconductor electrode, electrochemical impedance spectroscopy measures the multifrequency capacitance-voltage and conductance-voltage characteristics of planar p-Si (100), planar p-Si0.55Ge0.45 (100), and p-Si crystals with surfaces that are textured to create nanoscale features.1 These substrates are coated with thin atomic layer deposited Al2O3 dielectrics. Analysis of impedance data obtained from the planar samples produces results that are in good agreement with solid-state multi-frequency C-V and g-V data from the same sample, including similar extracted values for density of interface states. Impedance data and the extracted interface trap properties of ALD-Al2O3 passivated silicon microwire arrays for application in solar cells will also be described. Liquid electrolyte contacts have also been used to study resistance switching and its relation to local oxygen deficiency in an initially insulating metal oxide layer.2 Local variations in electrical conductivity in amorphous TiO2 films prepared by atomic layer deposition (ALD) are examined using conductive atomic force microscopy. The local TiO2 conductivity is correlated with changes in oxygen stoichiometry revealed by electron energy loss spectroscopy performed in the electron microscope and by oxygen isotope tracer exchange. Both highly resistive ionic liquids and highly conductive electrolyte solutions are used as liquid top contacts to the TiO2 layers. In this way, the effects of field-driven migration of oxygen ions in and out of the TiO2 film can be distinguished from local accumulation of oxygen vacancies via Joule heating at high programming currents. We find that gating using a resistive ionic liquid contact at room temperature and positive gate bias produces an oxygen deficient ALD-TiO2 film with enhanced electronic conductivity, demonstrating the ease with which field-driven oxygen vacancy formation occurs at low temperature in these thin amorphous layers. However, conductive filaments were formed only when the TiO2 is programmed using a lower-resistivity electrolyte top contact. This indicates the decisive importance of programming current in localization of the point defects required for filamentary conduction. C. Meng, K. Tang, M. Braun, L. Zhang, and P.C. McIntyre, “Electrochemical Impedance Spectroscopy for Quantitative Interface State Characterization of Planar and Nanostructured Semiconductor-Dielectric Interfaces,” Nanotechnol. 28, 415704 (2017). Tang, A.C. Meng, F. Hui, Y. Shi, T. Petach, C. Hitzman, A.L. Koh, D. Goldhaber-Gordon, M. Lanza, and P.C. McIntyre, “Distinguishing Oxygen Vacancy Electromigration and Conductive Filament Formation in TiO2 Resistance Switching Using Liquid Electrolyte Contacts,” Nano Lett. 17, 4390–99 (2017).