Valence band modification of Cr2O3 by Ni-doping: creating a high figure of merit p-type TCO

p-Type transparent conductors and semiconductors still suffer from remarkably low performance compared to their more widespread n-type counterparts, despite extensive investigation into their development. In this contribution, we present a comparative study on the defect chemistry of potential p-type transparent conducting oxides Mg-doped and Ni-doped Cr 2 O 3 . Conductivities as high as 28 S cm -1 were achieved by Ni-doping. By benchmarking crystallography and spectroscopy characterization against density functional theory calculations, we show that the incorporation of Ni into Cr 2 O 3 contributes to the composition of the valence band, making the formed holes more delocalized, while Mg states do not interact with the valence band in Mg-doped Cr 2 O 3 . Furthermore, it is experimentally proven that Ni has a higher solubility in Cr 2 O 3 than Mg, at least in the highly non-thermodynamic deposition conditions used for these experiments, which directly translates into a higher acceptor concentration. The combination of these two effects means that Ni is a more effective acceptor in Cr 2 O 3 than Mg and explains the improved conductivity observed for the former

Growth of 1T ' MoTe2 by thermally assisted conversion of electrodeposited tellurium films

Molybdenum ditelluride (MoTe2) is a transition metal dichalcogenide (TMD) which has two phases stable under ambient conditions, a semiconducting (2H) and semimetallic (1T') phase. Despite a host of interesting properties and potential applications, MoTe2 is one of the less-studied TMDs, perhaps due its relatively low abundance in nature or challenges associated with its synthesis, such as the toxicity of most precursors. In this report, we describe the fabrication of thin films of phase-pure IT' MoTe2 using predeposited molybdenum and electrodeposited tellurium layers, at the relatively low temperature of 450 C. This method allows control over film geometry and over the tellurium concentration during the conversion. The MoTe2 films are characterized by Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, atomic force microscopy, and electron microscopies. When applied as a catalyst for the hydrogen evolution reaction, the films display promising initial results. The MoTe2 films have a Tafel slope of below 70 mV dec(-1) and compare favorably with other MoTe2 catalysts reported in the literature, especially considering the inherently scalable fabrication method. The variation in electrocatalytic behavior with thickness and morphology of the films is also investigated

Utilising Chromium-Based p-Type Transparent Conducting Oxides in Photovoltaic Devices

This thesis deals with the growth of modern, highly performing p-type transparent conducting oxides and their utilisation in two types of photovoltaic devices. X-ray spectroscopic techniques are used to understand the electronic structure of these materials and the various interfaces they form when used in these devices. The initial chapters provide an overview of the history and current state of the art of transparent conducting oxides in general with specific reference being paid to to the materials used in this work: Cr2O3, Cr2O3:Mg, and CuCrO2. Cr2O3 and Cr2O3:Mg were used as an anode buffer layer in simple organic solar cells, while CuCrO2 and Cr2O3:Mg were used as a transparent p-type layer of a novel silicon heterojunction solar cell. Both Cr2O3 and Cr2O3:Mg anode buffer layers were found to improve the efficiency of organic solar cells by a factor of ~3 compared to cells produced with no buffer layer. All cells produced exhibited an "S\u27\u27-shaped current-voltage curve. To understand why this occurred, band alignment measurements were performed using in-situ high resolution x-ray and ultraviolet photoelectron spectroscopy. An ultra-high vacuum transfer system necessary for the band alignment measurements was built to enable the spectroscopic analysis of samples after growth, prior to being exposed to ambient conditions. Simple n-Si/CuCrO2 solar cells were produced by spray pyrolysis while n-Si/Cr2O3:Mg solar cells were produced by molecular beam epitaxy. Solar cells produced on low resistivity silicon did not demonstrate any power generation, while cells produced on standard resistivity silicon demonstrated lower performance than expected. Band offset measurements were performed on the n-Si/Cr2O3:Mg cells using photoelectron spectroscopy, revealing that an oxide layer present on the silicon surface is the primary factor responsible for their poor solar cell characteristics

Article Quantifying the Performance of P-Type Transparent Conducting Oxides by Experimental Methods

Screening for potential new materials with experimental and theoretical methods has led to the discovery of many promising candidate materials for p-type transparent conducting oxides. It is difficult to reliably assess a good p-type transparent conducting oxide (TCO) from limited information available at an early experimental stage. In this paper we discuss the influence of sample thickness on simple transmission measurements and how the sample thickness can skew the commonly used figure of merit of TCOs and their estimated band gap. We discuss this using copper-deficient CuCrO 2 as an example, as it was already shown to be a good p-type TCO grown at low temperatures. We outline a modified figure of merit reducing thickness-dependent errors, as well as how modern ab initio screening methods can be used to augment experimental methods to assess new materials for potential applications as p-type TCOs, p-channel transparent thin film transistors, and selective contacts in solar cells

Raman spectra of p-type transparent semiconducting Cr<inf>2</inf>O<inf>3</inf>:Mg

Abstract We present an analysis of the Raman spectra of p-type transparent conducting Cr2O3:Mg grown by various techniques including spray pyrolysis, pulsed laser deposition, molecular beam epitaxy and reactive magnetron sputtering. The best performing films show a distinct broad range Raman signature related to defect-induced vibrational modes not seen in stoichiometric, undoped material. Our comparative study demonstrates that Raman spectroscopy can quantify unwanted dopant clustering in the material at high Mg concentrations, while also being sensitive to the Mg incorporation site. By correlating the Raman signature to the electrical properties of the films, growth processes can be optimised to give the best conducting films and the local defect structure for effective p-type doping can be studied

Efficient Resistive Switching and Spike Rate Dependent Plasticity in a New CuCrO2 Memristor for Plausible Neuromorphic Systems

In this work, we introduce a new class of p-type transparent conductive oxide (TCO) CuCrO2 (150 nm) heterogeneously integrated onto FTO/glass for forming free memristor based neuromorphic applications. The fabricated Al/CuCrO2/FTO memristors demonstrate a reliable bipolar resistive switching with an ON/OFF ratio of 1000. The retention of the device was found to be steady even beyond 106 s, which demonstrates its non-volatility. The current-voltage (I-V) characteristics were fitted to evaluate its transport properties and a band-diagram was projected to have a better insight of the device operational principles. To validate the experimental observations, a new model has been developed, and the simulated I-V behavior was analogues to the experimental one. Efforts were then devoted to observe long-term potentiation (LTP) and longterm depression (LTD) utilizing identical but opposite pulses to evaluate the device?s efficacy for synaptic applications. The synaptic behavior was well controlled by the pulse (pulse amplitude and width) variations. The conductance change was found to be symmetric and then saturated, which reflects the popular biological Hebbian rules. Finally, a long-term synaptic modulation has been implemented by establishing the spike rate dependent plasticity (SRDP) rule, which is a part of spiking neural networks and advantageous to mimic the brain?s capability at low power. All the obtained experimental results were systematically corroborated by neural network simulation. Overall, our approach provides a new road map towards the development of TCO based alternative memristors, which can be employed to mimic the synaptic plasticity for energy-efficient bioinspired neuromorphic systems and non-Von Neumann computer architectures