J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

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Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

One-dimensional (1D) metal-oxide nanostructures are ideal systems for exploring a large number of novel phenomena at the nanoscale and investigating size and dimensionality dependence of nanostructure properties for potential applications The construction and integration of photodetectors or optical switches based on such nanostructures with tailored geometries have rapidly advanced in recent years Active 1D nanostructure photodetector elements can be configured either as resistors whose conductions are altered by a charge-transfer process or as field-effect transistors (FET) whose properties can be controlled by applying appropriate potentials onto the gates Functionalizing the structure surfaces offers another avenue for expanding the sensor capabilities This article provides a comprehensive review on the state-of-the-art research activities in the photodetector field It mainly focuses on the metal oxide 1D nanostructures such as ZnO, SnO(2), Cu(2)O, Ga(2)O(3), Fe(2)O(3), In(2)O(3), CdO, CeO(2), and their photoresponses The review begins with a survey of quasi 1D metal-oxide semiconductor nanostructures and the photodetector principle, then shows the recent progresses on several kinds of important metal-oxide nanostructures and their photoresponses and briefly presents some additional prospective metal-oxide 1D nanomaterials Finally, the review is concluded with some perspectives and outlook on the future developments in this area

A comprehensive review of one-dimensional metal-oxide nanostructure photodetectors

Non-metal doping of transition metal oxides for visible-light photocatalysis

Wide band gap semiconductors are essential for today's electronic devices and energy applications because of their high optical transparency, controllable carrier concentration, and tunable electrical conductivity. The most intensively investigated wide band gap semiconductors are transparent conductive oxides (TCOs), such as tin-doped indium oxide (ITO) and amorphous In-Ga-Zn-O (IGZO), used in displays and solar cells, carbides (e.g., SiC) and nitrides (e.g., GaN) used in power electronics, and emerging halides (e.g., γ-CuI) and 2D electronic materials (e.g., graphene) used in various optoelectronic devices. Compared to these prominent materials families, chalcogen-based (Ch = S, Se, Te) wide band gap semiconductors are less heavily investigated but stand out because of their propensity for p-type doping, high mobilities, high valence band positions (i.e., low ionization potentials), and broad applications in electronic devices such as CdTe solar cells. This manuscript provides a review of wide band gap chalcogenide semiconductors. First, we outline general materials design parameters of high performing transparent semiconductors, as well as the theoretical and experimental underpinnings of the corresponding research methods. We proceed to summarize progress in wide band gap (EG > 2 eV) chalcogenide materials-namely, II-VI MCh binaries, CuMCh2 chalcopyrites, Cu3MCh4 sulvanites, mixed-anion layered CuMCh(O,F), and 2D materials-and discuss computational predictions of potential new candidates in this family, highlighting their optical and electrical properties. We finally review applications-for example, photovoltaic and photoelectrochemical solar cells, transistors, and light emitting diodes-that employ wide band gap chalcogenides as either an active or passive layer. By examining, categorizing, and discussing prospective directions in wide band gap chalcogenides, this Review aims to inspire continued research on this emerging class of transparent semiconductors and thereby enable future innovations for optoelectronic devices.

https://escholarship.org/content/qt1xf1k15d/qt1xf1k15d.pdf?t=qof0vs

Wide Band Gap Chalcogenide Semiconductors.

With an ever-increasing number of applications in many advanced fields, gas sensors are becoming indispensable devices in our daily life. Among different types of gas sensors, conductometric metal oxide semiconductor (MOS) gas sensors are found to be the most appealing for advanced applications in the automotive, biomedical, environmental, and safety sectors because of the their high sensitivity, reduced size, and low cost. To improve their sensing characteristics, new metal oxide-based nanostructures have thus been proposed in recent years as sensing materials. In this review, we extensively review gas-sensing properties of core@ shell nanocomposites in which metals as the core and metal oxides as the shell structure, both of nanometer sizes, are assembled into a single metal@metal oxide core–shell. These nanostructures not only combine the properties of both noble metals and metal oxides, but also bring unique synergetic functions in comparison with single-component materials. Up-dated achievements in the synthesis and characterization of metal@metal oxide core–shell nanostructures as well as their use in MOS sensors are here reported with the main objective of providing an overview about their gas-sensing properties.

Metal-core@metal oxide-shell nanomaterials for gas-sensing applications: a review

Physical properties and heterojunction device demonstration of aluminum-doped ZnO thin films synthesized at room ambient via sol–gel method

Thickness-dependent nonlinear absorption behaviors in polycrystalline ZnSe thin films

AgGa0.5In0.5Se2 thin films were deposited onto a quartz substrate by the electron-beam technique. For the investigation of the annealing effect on structural, optical and electrical properties of deposited films, samples were annealed in the temperature range 300–775 °C. The composition analyses of the deposited films carried out by energy dispersive x-ray analysis measurements have shown that the deposited AgGa0.5In0.5Se2 films were indium- and gallium-rich but selenium- and slightly silver-deficient and there was a remarkable change in composition with annealing. As a result of x-ray diffraction measurements, the as-deposited films were found to have an amorphous structure and after annealing at 300 °C a polycrystalline structure with different phases was observed. However, subsequent annealing resulted in the formation of single phase AgGa0.5In0.5Se2 thin film at about 775 °C. The absorption coefficient of the films was determined from the transmission spectra and the band gap values were calculated and found to vary between 1.57 and 2.43 eV following annealing in the temperature range 300–775 °C. The refractive index (n) and extinction coefficient (k) of the films were evaluated by applying the envelope method to the transmission spectra. The spectral distributions of these quantities for both as-deposited and annealed films were determined in detail and it was observed that there has been a remarkable influence of annealing on these quantities. The electrical properties of AgGa0.5In0.5Se2 thin films were also investigated by means of temperature dependent conductivity measurements in the temperature range 100–460 K. The resistivity of the samples depending on the annealing temperature varied between 6.5 × 105 and 16 Ω cm. As a result of the hot-probe method it was observed that the as-deposited films have indicated an n-type behaviour, while all the annealed AgGa0.5In0.5Se2 thin films have shown p-type conduction.

https://iopscience.iop.org/article/10.1088/0022-3727/42/16/165413/pdf

Characterization of AgGa0.5In0.5Se2 thin films deposited by electron-beam technique

TiO2 nanorods (NRs) were synthesized on fluorine-doped tin oxide (FTO) pre-coated glass substrates using hydrothermal growth technique. Scanning electron microscopy studies have revealed the formation of vertically-aligned TiO2 NRs with length of ∼2 µm and diameter of 110–128 nm, homogenously distributed over the substrate surface. 130 nm thick Au contacts using thermal evaporation were deposited on the n-type TiO2 NRs at room temperature for the fabrication of NR-based Schottky-type UV photodetectors. The fabricated Schottky devices functioned as highly sensitive UV photodetectors with a peak responsivity of 134.8 A/W (λ = 350 nm) measured under 3 V reverse bias. (© 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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Au/TiO2 nanorod-based Schottky-type UV photodetectors

Recent progress in the realization of material structures with quantum con-ﬁ nement and high surface to volume ratio in nanoscale interwoven metal and semiconductor building blocks offers a strong potential to build highly func-tional nanodevices. Ultra-sharp tips with distinct material dependent proper-ties of metal and semiconductor exhibit important functionalities in devices including gas ionization sensors, ﬁ eld emission devices, and ion-mobility spectrometry. Herein, a dramatically enhanced ﬁ eld ionization process and a device based on charged particle beams for which the geometrical and surface properties of the constituent semiconductor nanotips are engineered with controlled introduction of metallic impurities to realize close to three orders of magnitude reduction in the ionization electric-ﬁ eld strength are described. Experimentally observed low voltage ﬁ eld ionization phenomenon is explained using the geometrical ﬁ eld enhancement, surface states induced by controlled introduction of metallic impurities, and polarizabilities of gas particles at the nanotips. The nanotips are employed to design ﬁ eld ionization gas sensors whose nanoscale pristine semiconductor tips are controllably decorated with atomic metal impurities to boost the electron tunneling prop-erties under extremely low bias voltages. These devices also outperform their solid-state macroscopic counterparts in terms of simplicity of their construc-tion and higher selectivity.

/pdf/enhanced-field-ionization-enabled-by-metal-induced-surface-4ttigjq3yc.pdf

Hakan Karaagac

Papers

Physical properties and heterojunction device demonstration of aluminum-doped ZnO thin films synthesized at room ambient via sol–gel method

Thickness-dependent nonlinear absorption behaviors in polycrystalline ZnSe thin films

Characterization of AgGa0.5In0.5Se2 thin films deposited by electron-beam technique

Au/TiO2 nanorod-based Schottky-type UV photodetectors

Enhanced Field Ionization Enabled by Metal Induced Surface States on Semiconductor Nanotips