p -type conducting films of α-BuCu2S2 have been deposited onto glass and KBr substrates, yielding a conductivity of 17 S/cm and a Hall mobility of 3.5 cm2/V s. For a 430-nm-thick film, the optical transparency approaches 90% in the visible portion of the spectrum at 650 nm, and a transparency of 40% extends throughout the infrared to the long-wavelength cutoff of the KBr substrate at 23 μm.
Abstract : The objective of this thesis is to contribute to the development of p-type materials for transparent electronics applications. Thin films of ?-BaCu2S2, a p-type semi-transparent semiconductor, are fabricated and characterized. ?-BaCu2S2 has a transmittance of 60% to 80 % in the visible portion of the electromagnetic spectrum. The mobility, conductivity, and carrier concentration of ?-BaCu2S2 are 3.5 cm2/V-s 17 S/cm, and 1019 cm−3, respectively. The potential use of BaCu2S2 in thin-film solar cells is described. A number of p-channel transparent thin-film transistors based on BaCu2S2, NiO, NiO:Li, and CuScO2 are fabricated and characterized. None of these p-TTFTs are operational. The key issues in these transistors are as follows. BaCu2S2 p-TTFTs exhibit excessively large gate leakage current caused by the interaction of BaCu2S2 with the gate insulator. In undoped NiO p-TTFTs and in CuScO2 p-TTFTs, the injected carriers are trapped in the transistor channel layer thin film. CuScO2 p-TTFTs also suffer from gate leakage due to interaction of CuScO2 with the gate insulator. In this work it is found that having Cu containing materials in contact with gate insulators leads to enhanced gate leakage current. In NiO:Li p-TTFTs, the bulk channel layer is too conductive to modulate with the transistor gate; thus, the transistors do not work. Information obtained from the characterization of these p-TTFTs is used to identify and explore important considerations in making a functional p-TTFT. These considerations include efficient injecting contacts to wide-bandgap p-type insulators, and the conductivity of materials used for the transistor channel in p-TTFTs. The topic of injecting contacts to wide-bandgap insulators and the topic of channel layer conductivity are explored and quantified.
Transparent electronics is a nascent technology whose objective is the realization of invisible electronic circuits. Part of the impetus for the development of transparent electronics is the recent availability of p-type transparent conductive oxides (TCOs). With the emergence of p-type TCOs, in addition to conventional n-type TCOs such as indium-tin oxide, tin oxide, and zinc oxide, fabrication of transparent bipolar electronic devices becomes feasible. The first part of this paper reviews TCOs and discusses our work in the development of p-TCOs and alternative TC materials (e.g. sulfides). We have recently invented a novel, n-channel, accumulation-mode transparent thin-film transistor (TTFT). This TTFT is highly transparent, has very little light sensitivity, and exhibits electrical characteristics that appear to be suitable for implementation as a transparent select-transistor in each pixel of an active-matrix liquid-crystal display (AMLCD). Moreover, the processing technology used to fabricate this device is relatively simple and appears to be compatible with inexpensive glass substrate technology. The second part of this paper focuses on TTFTs. If transparent electronics is employed to realize transparent back-plane electronic drivers on transparent substrates, fabrication of a transparent display becomes feasible. The third part of this paper offers an approach for realization of a transparent display.
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