Microstrip Array Ring FETs with 2D p-Ga2O3 Channels Grown by MOCVD

Gallium oxide (Ga2O3) thin films of various thicknesses were grown on sapphire (0001) substrates by metal organic chemical vapor deposition (MOCVD) using trimethylgallium (TMGa), high purity deionized water, and silane (SiH4) as gallium, oxygen, and silicon precursors, respectively. N2 was used as carrier gas. Hall measurements revealed that films grown with a lower VI/III ratio had a dominant p-type conduction with room temperature mobilities up to 7 cm2/Vs and carrier concentrations up to ~1020 cm−3 for thinner layers. High resolution transmission electron microscopy suggested that the layers were mainly κ phase. Microstrip field-effect transistors (FETs) were fabricated using 2D p-type Ga2O3:Si, channels. They achieved a maximum drain current of 2.19 mA and an on/off ratio as high as ~108. A phenomenological model for the p-type conduction was also presented. As the first demonstration of a p-type Ga2O3, this work represents a significant advance which is state of the art, which would allow the fabrication of p-n junction based devices which could be smaller/thinner and bring both cost (more devices/wafer and less growth time) and operating speed (due to miniaturization) advantages. Moreover, the first scaling down to 2D device channels opens the prospect of faster devices and improved heat evacuation.

Zn2+-Doped TiO2:WO3 Films Prepared by Electrospinning and Sintering: Microstructural Characterization and Electrical Signature to Moisture Sensing

In this work, Zn2+-doped TiO2:WO3 nanostructured films, with different doping levels, were produced by electrospinning followed by sintering, and tested as potential materials for relative humidity (RH) detection. The materials microstructure was investigated by scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy, and X-ray diffraction (XRD). The electrical characterization was performed by electrical impedance spectroscopy in the range of 400 HZ–40 MHZ, at 20 °C. The sensors’ sensitivity to moisture was evaluated from the impedance variations in response to changes in RH (10–100%). The analyses confirmed the interaction of water molecules with the oxides surface, and showed that zinc atoms were incorporated into the titanium vacancies in the crystal lattice. All the studied sensors showed a p- to n-type conduction transition taking place at around 40% RH. The nanocomposite with 2 wt% of dopant presented the best sensitivity to moisture, with an impedance variation of about 1 order of magnitude. The results are discussed in relation to the microstructure and fabrication route.

Reduction of Thermal Conductivity for Icosahedral Al-Cu-Fe Quasicrystal through Heavy Element Substitution

Icosahedral Al-Cu-Fe quasicrystal (QC) shows moderate electrical conductivity and low thermal conductivity, and both p- and n-type conduction can be controlled by tuning the sample composition, making it potentially suited for thermoelectric materials. In this work, we investigated the effect of introducing chemical disorder through heavy element substitution on the thermal conductivity of Al-Cu-Fe QC. We substituted Au and Pt elements for Cu up to 3 at% in a composition of Al63Cu25Fe12, i.e., Al63Cu25−x(Au,Pt)xFe12 (x = 0, 1, 2, 3). The substitutions of Au and Pt for Cu reduced the phonon thermal conductivity at 300 K (κph,300K) by up to 17%. The reduction of κph,300K is attributed to a decrease in the specific heat and phonon relaxation time through heavy element substitution. We found that increasing the Pt content reduced the specific heat at high temperatures, which may be caused by the locked state of phasons. The observed glass-like low values of κph,300K (0.9–1.1 W m−1 K−1 at 300 K) for Al63Cu25−x(Au,Pt)xFe12 are close to the lower limit calculated using the Cahill model.

Structural Changes Induced by Heating in Sputtered NiO and Cr2O3 Thin Films as p-Type Transparent Conductive Electrodes

NiO and Cr2O3 are transition metal oxides with a partially filled d electron band that supports p-type conduction. Both are transparent to the visible light due to optical absorption beginning at wavelengths below 0.4 μm and the creation of holes by metal vacancy defects. The defect and strain effects on the electronic characteristics of these materials need to be established. For this purpose, NiO and Cr2O3 thin films were deposited on unheated glass substrates by reactive DC sputtering from metallic targets. Their structural, morphological, optical and electrical properties were analyzed comparatively in the as-grown conditions (25 °C) and after heating in air at 300 °C or 500 °C. The cubic NiO structure was identified with some tensile strain in the as-grown conditions and compressive strain after heating. Otherwise, the chromium oxide layers were amorphous as grown at 25 °C and crystallized into hexagonal Cr2O3 at 300 °C or above also with compressive strain after heating. Both materials achieved the highest visible transmittance (72%) and analogous electrical conductivity (~10−4 S/cm) by annealing at 500 °C. The as-grown NiO films showed a higher conductivity (2.5 × 10−2 S/cm) but lower transmittance (34%), which were related to more defects causing tensile strain in these samples.

Enhanced Electrical Properties and Stability of P-Type Conduction in ZnO Transparent Semiconductor Thin Films by Co-Doping Ga and N

P-type ZnO transparent semiconductor thin films were prepared on glass substrates by the sol-gel spin-coating process with N doping and Ga–N co-doping. Comparative studies of the microstructural features, optical properties, and electrical characteristics of ZnO, N-doped ZnO (ZnO:N), and Ga–N co-doped ZnO (ZnO:Ga–N) thin films are reported in this paper. Each as-coated sol-gel film was preheated at 300 °C for 10 min in air and then annealed at 500 °C for 1 h in oxygen ambient. X-ray diffraction (XRD) examination confirmed that these ZnO-based thin films had a polycrystalline nature and an entirely wurtzite structure. The incorporation of N and Ga–N into ZnO thin films obviously refined the microstructures, reduced surface roughness, and enhanced the transparency in the visible range. X-ray photoelectron spectroscopy (XPS) analysis confirmed the incorporation of N and Ga–N into the ZnO:N and ZnO:Ga–N thin films, respectively. The room temperature PL spectra exhibited a prominent peak and a broad band, which corresponded to the near-band edge emission and deep-level emission. Hall measurement revealed that the ZnO semiconductor thin films were converted from n-type to p-type after incorporation of N into ZnO nanocrystals, and they had a mean hole concentration of 1.83 × 1015 cm−3 and a mean resistivity of 385.4 Ω·cm. In addition, the Ga–N co-doped ZnO thin film showed good p-type conductivity with a hole concentration approaching 4.0 × 1017 cm−3 and a low resistivity of 5.09 Ω·cm. The Ga–N co-doped thin films showed relatively stable p-type conduction (>three weeks) compared with the N-doped thin films.