In–Ga–Zn–O MESFET with transparent amorphous Ru–Si–O Schottky barrier

Rectifying transparent amorphous Ru–Si–O Schottky contacts to In–Ga–Zn–O have been fabricated by means of reactive sputtering without any annealing processes nor semiconductor surface treatments. The ideality factor, effective Schottky barrier height and rectification ratio are equal to 1.6, 0.9 eV and 10⁵ A/A, respectively. Ru–Si–O/In–Ga–Zn–O Schottky barriers were employed as gate electrodes for In–Ga–Zn–O metal–semiconductor field‐effect transistors (MESFETs). MESFET devices exhibiting on‐to‐off current ratio at the level of 10³ A/A in a voltage range of 2 V, with subthreshold swing equal to 420 mV/dec were demonstrated. A channel mobility of 7.36 cm²/V s was achieved. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Vortex pinning properties of –Ba–Cu–O and –Ba–Cu–O superconducting bulks

We have studied the effect of a small amount of Y-site substitution by La or Pr ions on the vortex pinning in the Y–Ba–Cu–O system. (Y_1-xLa_x)–Ba–Cu–O and (Y_1-xPr_x)–Ba–Cu–O bulks were fabricated by the melt-textured growth, in which x was varied from 0 to 0.01. The critical current density J_c at 77 K is improved in magnetic fields parallel to the c-axis above 2–4.5 T and the corresponding irreversibility field, H_irr, shifts to the higher value in both bulks.     

Local structure around Zn and Ga in solution‐processed In–Ga–Zn–O and implications for electronic properties

We study by X‐ray absorption spectroscopy the local structure around Zn and Ga in solution‐processed In–Ga–Zn–O thin films as a function of thermal annealing. Zn and Ga environments are amorphous up to 450 °C. At 200 °C and 450 °C, the Ga atoms are in a β‐Ga₂O₃ like structure, mostly tetrahedral gallium oxide phase. Above 300 °C, the Zn atoms are in a tetrahedral ZnO phase for atoms inside the nanoclusters. The observed formation of the inorganic structure above 300 °C may be correlated to the rise of the mobility for IGZO TFTs. The Zn atoms localized at the nanocluster boundary are undercoordinated with O. Such ZnO cluster boundary could be responsible for electronic defect levels. Such defect levels were put in evidence in the upper half of the band gap. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Single and networked CuO nanowires for highly sensitive p‐type semiconductor gas sensor applications

Development of high‐performance p‐type semiconductor based gas sensors exhibiting fast‐response/recovery times with ultra‐high response are of major importance for gas sensing applications. Recent reports demonstrated the excellent properties of p‐type semiconducting oxide for various practical applications, especially for selective oxidation of volatile organic compounds (VOCs). In this work, sensors based on CuO nanowire (NW) networks have been successfully fabricated via a simple thermal oxidation process on pre‐patterned Au/Cr pads. Our investigation demonstrates high impact of the process temperature on aspect ratio and density of copper oxide NWs. An optimal temperature for growth of thin and densely packed NWs was found to be at 425 °C. The fabricated sensors demonstrated ultra‐high gas response by a factor of 313 to ethanol vapour (100 ppm) at an operating temperature of 250 °C. High stability and repeatability of these sensors indicate the efficiency of p‐type oxide based gas sensors for selective detection of VOCs. A high‐performance nanodevice was fabricated in a FIB‐SEM system using a single CuO NW, demonstrating an ethanol response of 202 and rapid response and recovery of ～198 ms at room temperature. The involved gas sensing mechanism of CuO NW networks has been described. We consider that the presented results will be of a great interest for the development of higher‐performance p‐type semiconductor based sensors and bottom‐up nanotechnologies. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Selective fabrication of n‐ and p‐type SnO films without doping

Undoped n‐ and p‐type tin monoxide (SnO) films have been selectively fabricated by pulsed laser deposition with a Sn target and careful control of oxygen partial pressure. The films are epitaxially grown in optimal growth conditions on yttria‐stabilized zirconia (YSZ) substrates with out‐of‐plane and in‐plane orientation relationships of (001)_SnO//(001)_YSZ and [110]_SnO//[100]_YSZ, respectively. Both Seebeck and Hall measurements show consistent results on the carrier types of the films. The electron Hall mobility is approximately 11 cm²/Vs at room temperature and the carrier activation energy is 0.14 eV for the n‐type film. The growth at increased oxygen partial pressure yields p‐type films, demonstrating the selective fabrication of both n‐ and p‐type SnO films without doping. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

High‐pressure high‐temperature synthesis of Rh2O3‐II‐type In2O3 polymorph

The metastability of the bixbyite‐ and corundum‐type In₂O₃ polymorphs up to 33 GPa (at room temperature) is shown. While compressed (in diamond anvil cells) and laser‐heated, both polymorphs undergo a phase transition to the Rh₂O₃‐II‐type structure (space group Pbcn, No. 60). The direct transition from bixbyite to Rh₂O₃‐II structure has not yet been observed for any other oxide.

     

One decade of fully transparent oxide thin‐film transistors: fabrication,performance and stability

We review the history of fully transparent oxide thin‐film transistors. Their performance and stability increased during the past ten years of their existence, thus enabling the design of novel applications in transparent electronics. However, certain disadvantages of the well established leading technology of metal–insulator–semiconductor field‐effect transistors (MISFETs), adapted from the silicon‐based complementary metal–oxide–semiconductor (CMOS) and thin‐film transistor technology, may be overcome by alternative transistor designs like metal–semiconductor field‐effect transistors (MESFETs). We compare the stability of published transparent MISFET with our transparent MESFET (TMESFET) technology against bias stress, towards illumination, at elevated temperatures and long‐term stability.

     

Effect of III‐nitride polarization on VOC in p–i–n and MQW solar cells

We performed detailed studies of the effect of polarization on III‐nitride solar cells. Spontaneous and piezoelectric polarizations were assessed to determine their impacts upon the open circuit voltages (V_OC) in p–i(InGaN)–n and multi‐quantum well (MQW) solar cells. We found that the spontaneous polarization in Ga‐polar p–i–n solar cells strongly modifies energy band structures and corresponding electric fields in a way that degrades V_OC compared to non‐polar p–i–n structures. In contrast, we found that piezoelectric polarization in Ga‐polar MQW structures does not have a large influence on V_OC compared to non‐polar MQW structures. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Scanning photocurrent and PL imaging of a frozen polymer p–i–n junction

Polymer light‐emitting electrochemical cells (LECs) are two‐terminal, solid state devices with a mixed ionic/electronic conductor as the active layer. Once activated by a DC voltage or current, a doping‐induced homojunction dictates the electrical and optical response of the LEC, making it highly unique and attractive among organic devices. However, the depletion width, a fundamental parameter of any semiconductor homojunction, has never been determined experimentally for a static LEC junction. In this study, we apply spatially resolved photocurrent and photoluminescence (PL) scanning to an extremely large planar LEC that had been turned on to emit strongly then subsequently frozen. These concerted scanning and imaging studies depict a p–i–n junction structure in which the peak built‐in electric field lies at the interface between the intrinsic region and the p‐doped region. The corresponding 18 μm depletion width is very small compared to the 700 μm interelectrode spacing.

     

Interplay of spin–orbit coupling and superconducting correlations in germanium telluride thin films

There is much current interest in combining superconductivity and spin–orbit coupling in order to induce the topological superconductor phase and associated Majorana‐like quasiparticles which hold great promise towards fault‐tolerant quantum computing. Experimentally these effects have been combined by the proximity‐coupling of super‐conducting leads and high spin–orbit materials such as InSb and InAs, or by controlled Cu‐doping of topological insu‐lators such as Bi₂Se₃. However, for practical purposes, a single‐phase material which intrinsically displays both these effects is highly desirable. Here we demonstrate coexisting superconducting correlations and spin–orbit coupling in molecular‐beam‐epitaxy‐grown thin films of GeTe. The former is evidenced by a precipitous low‐temperature drop in the electrical resistivity which is quelled by a magnetic field, and the latter manifests as a weak antilocalisation (WAL) cusp in the magnetotransport. Our studies reveal several other intriguing features such as the presence of two‐dimensional rather than bulk transport channels below 2 K, possible signatures of topological superconductivity, and unexpected hysteresis in the magnetotransport. Our work demonstrates GeTe to be a potential host of topological SC and Majorana‐like excitations, and to be a versatile platform to develop quantum information device architectures. (© 2016 The Authors. Phys. Status Solidi RRL published by WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

High mobility In2O3:H transparent conductive oxides prepared by atomic layer deposition and solid phase crystallization

The preparation of high‐quality In₂O₃:H, as transparent conductive oxide (TCO), is demonstrated at low temperatures. Amorphous In₂O₃:H films were deposited by atomic layer deposition at 100 °C, after which they underwent solid phase crystallization by a short anneal at 200 °C. TEM analysis has shown that this approach can yield films with a lateral grain size of a few hundred nm, resulting in electron mobility values as high as 138 cm²/V s at a device‐relevant carrier density of 1.8 × 10²⁰ cm^–3. Due to the extremely high electron mobility, the crystallized films simultaneously exhibit a very low resistivity (0.27 mΩ cm) and a negligible free carrier absorption. In conjunction with the low temperature processing, this renders these films ideal candidates for front TCO layers in for example silicon heterojunction solar cells and other sensitive optoelectronic applications. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Synthesis and thermochemical study of quinoxaline‐N‐oxides: enthalpies of dissociation of the N–O bond

The synthesis of three new quinoxaline mono‐N‐oxides derivatives, namely, 2‐tert‐butoxycarbonyl‐3‐methylquinoxaline‐N‐oxide, 2‐phenylcarbamoyl‐3‐ethylquinoxaline‐N‐oxide, and 2‐carbamoyl‐3‐methylquinoxaline‐N‐oxide, from their corresponding 1,4‐di‐N‐oxides is reported. Samples of these compounds were used for a thermochemical study, which allowed derivation of their gaseous standard molar enthalpies of formation, , from their enthalpies of formation in the condensed phase, , determined by static bomb combustion calorimetry, and from their enthalpies of sublimation, , determined by Calvet microcalorimetry. Finally, combining the for the quinoxaline‐N‐oxides derived in this work with literature values for the corresponding 1,4‐di‐N‐oxides and atomic oxygen, the bond dissociation enthalpies for cleavage of the first N?O bond in the di‐N‐oxides, DH₁(N–O), were obtained and compared with existing data. Copyright © 2011 John Wiley & Sons, Ltd.     

Densification of sol–gel silica thin films induced by hard X‐rays generated by synchrotron radiation

In this article the effects induced by exposure of sol–gel thin films to hard X‐rays have been studied. Thin films of silica and hybrid organic–inorganic silica have been prepared via dip‐coating and the materials were exposed immediately after preparation to an intense source of light of several keV generated by a synchrotron source. The samples were exposed to increasing doses and the effects of the radiation have been evaluated by Fourier transform infrared spectroscopy, spectroscopic ellipsometry and atomic force microscopy. The X‐ray beam induces a significant densification on the silica films without producing any degradation such as cracks, flaws or delamination at the interface. The densification is accompanied by a decrease in thickness and an increase in refractive index both in the pure silica and in the hybrid films. The effect on the hybrid material is to induce densification through reaction of silanol groups but also removal of the organic groups, which are covalently bonded to silicon via Si—C bonds. At the highest exposure dose the removal of the organic groups is complete and the film becomes pure silica. Hard X‐rays can be used as an efficient and direct writing tool to pattern coating layers of different types of compositions.     

P‐type tin‐oxide thin film transistors for blue‐light detection application

We reported the characteristics of p‐type tin‐oxide (SnO) thin film transistors (TFTs) upon illumination with visible light. Our p‐type TFT device using the SnO film as the active channel layer exhibits high sensitivity toward the blue‐light with a high light/dark read current ratio (I_light/I_dark) of 8.2 × 10³ at a very low driven voltage of <3 V. Since sensing of blue‐light radiation is very critical to our eyes, the proposed p‐type SnO TFTs with high sensitivity toward the blue‐light show great potential for future blue‐light detection applications.

     

Optical and electrical properties of BaSnO3 and In2O3 mixed transparent conductive films deposited by filtered cathodic vacuum arc technique at room temperature

For the crystalline temperature of BaSnO$_{3}$ (BTO) was above 650 ℃, the transparent conductive BTO-based films were always deposited above this temperature on epitaxy substrates by pulsed laser deposition or molecular beam epitaxy till now which limited there application in low temperature device process. In the article, the microstructure, optical and electrical of BTO and In$_{2}$O$_{3}$ mixed transparent conductive BaInSnO$_x$ (BITO) film deposited by filtered cathodic vacuum arc technique (FCVA) on glass substrate at room temperature were firstly reported. The BITO film with thickness of 300 nm had mainly In$_{2}$O$_{3}$ polycrystalline phase, and minor polycrystalline BTO phase with (001), (011), (111), (002), (222) crystal faces which were first deposited at room temperature on amorphous glass. The transmittance was 70%-80% in the visible light region with linear refractive index of 1.94 and extinction coefficient of 0.004 at 550-nm wavelength. The basic optical properties included the real and imaginary parts, high frequency dielectric constants, the absorption coefficient, the Urbach energy, the indirect and direct band gaps, the oscillator and dispersion energies, the static refractive index and dielectric constant, the average oscillator wavelength, oscillator length strength, the linear and the third-order nonlinear optical susceptibilities, and the nonlinear refractive index were all calculated. The film was the n-type conductor with sheet resistance of 704.7 $\Omega /\Box $, resistivity of 0.02 $\Omega \cdot$cm, mobility of 18.9 cm$^{2}$/V$\cdot$s, and carrier electron concentration of $1.6\times 10^{19}$ cm$^{-3}$ at room temperature. The results suggested that the BITO film deposited by FCVA had potential application in transparent conductive films-based low temperature device process.     

Growth of GaN on sapphire via low‐temperature deposited buffer layer and realization of p‐type GaN by Mg doping followed by low‐energy electron beam irradiation (Nobel Lecture)

This is a personal history of one of the Japanese researchers engaged in developing a method for growing GaN on a sapphire substrate, paving the way for the realization of smart television and display systems using blue LEDs. The most important work was done in the mid to late 1980s. The background to the author's work and the process by which the technology enabling the growth of GaN and the realization of p‐type GaN was established are reviewed. ***

     

Probing the ferroelectric phase transition in sol–gel‐derived polycrystalline bismuth ferrite thin films

Polycrystalline BiFeO₃ (BFO) thin films were successfully grown on Pt/Ti/SiO₂/Si(100) and SrTiO₃ (STO) (100) substrates using the chemical solution deposition (CSD) technique. X‐ray diffraction (XRD) patterns indicate the polycrystalline nature of the films with rhombohedrally distorted perovskite crystal structure. Differential thermal analysis (DTA) was performed on the sol–gel‐derived powder to countercheck the crystal structure, ferroelectric (FE) to paraelectric (PE) phase transition, and melting point of bismuth ferrite. We observed a significant exothermic peak at 840 °C in DTA graphs, which corresponds to an FE–PE phase transition. Raman spectroscopy studies were carried out on BFO thin films prepared on both the substrates over a wide range of temperature. The room‐temperature unpolarized Raman spectra of BFO thin films indicate the presence of 13 Raman active modes, of which five strong modes were in the low‐wavenumber region and eight weak Raman active modes above 250 cm⁻¹. We observed slight shifts in the lower wavenumbers towards lower values with increase in temperature. The temperature‐dependent Raman spectra indicate a complete disappearance of all Raman active modes at 840 °C corresponding to the FE–PE phase transitions. There is no evidence of soft mode phonons. Copyright © 2008 John Wiley & Sons, Ltd.     

Photoelectrochemical properties of texture‐controlled nanostructured α‐Fe2O3 thin films prepared by AACVD

Nanostructured α‐Fe₂O₃ thin film electrodes were deposited by aerosol‐assisted chemical vapour deposition (AACVD) for photoelectrochemical (PEC) water splitting on conducting glass substrates using 0.1 M methanolic solution of Fe(acac)₃. The XRD analysis confirmed that the films are highly crystalline α‐Fe₂O₃ and free from other iron oxide phases. The highly reproducible electrodes have an optical bandgap of ～2.15 eV and exhibit anodic photocurrent. The current–voltage characterization of the electrodes reveals that the photocurrent density strongly depended on the film morphology and deposition temperature. Scanning electron microscopy (SEM) analysis showed a change in the surface morphology with the change in deposition temperature. The films deposited at 450 °C have nanoporous structures which provide a maximum electrode/electrolyte interface. The maximum photocurrent density of 455 µA/cm² was achieved at 0.25 V vs. Ag/AgCl/3M KCl (～1.23 V vs. RHE) and the incident photon to electron conversion efficiency (IPCE) was 23.6% at 350 nm for the electrode deposited at 450 °C. (© 2014 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Dzyaloshinskii–Moriya interactions and magnetic texture in Fe films deposited on transition‐metal dichalcogenides

The magnetic properties of materials based on two‐dimensional transition‐metal dichalcogenides (TMDC), namely bulk Fe_1/4TaS₂ compound as well as TMDC monolayers with deposited Fe films, have been investigated by means of first‐principles DFT calculations. Changing the structure and the composition of these two‐dimensional systems resulted in considerable variations of their physical properties. For the considered systems the Dzyaloshinskii– Moriya (DM) interaction has been determined and used for the subsequent investigation of their magnetic structure using Monte Carlo simulations. Rather strong DM interactions as well as large | D ₀₁|J₀₁ ratios have been obtained in some of these materials, which can lead to the formation of skyrmionic structures varying with the strength of the applied external magnetic field. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

On the application of a single‐crystal κ‐diffractometer and a CCD area detector for studies of thin films

A multipurpose six‐axis κ‐diffractometer, together with the brilliance of the ESRF light source and a CCD area detector, has been explored for studying epitaxial relations and crystallinity in thin film systems. The geometrical flexibility of the six‐axis goniometer allows measurement of a large volume in reciprocal space, providing an in‐depth understanding of sample crystal relationships. By a set of examples of LaAlO₃ thin films deposited by the atomic layer deposition technique, the possibilities of the set‐up are presented. A fast panoramic scan provides determination of the crystal orientation matrices, prior to more thorough inspection of single Bragg nodes. Such information, in addition to a broadening analysis of families of single reflections, is shown to correlate well with the crystallinity, crystallite size, strain and epitaxial relationships in the thin films. The proposed set‐up offers fast and easy sample mounting and alignment, along with crucial information on key features of the thin film structures.