G. Gonçalves

Performance and Stability of Low Temperature Transparent Thin-Film Transistors Using Amorphous Multicomponent Dielectrics

High performance transparent thin-film transistors deposited on glass substrates and entirely processed at a low temperature not exceeding 150°C are presented and analyzed in this paper. Besides being based on an amorphous oxide semiconductor, the main innovation of this work relies on the use of sputtered multicomponent oxides as dielectric materials based on mixtures of Ta 2 O 5 with SiO 2 or Al 2 O 3 . These multicomponent dielectrics allow to obtain amorphous structures and low leakage currents while preserving a high dielectric constant. This results in transistors with remarkable electrical properties, such as field-effect mobility exceeding 35 cm 2 V ―1 s ―1 , close to 0 V turn-on voltage, on/off ratio higher than 10 6 , and a subthreshold slope of 0.24 V decade ―1 , obtained with a Ta 2 O 5 :SiO 2 dielectric. When subjected to severe current stress tests, optimized devices show little and reversible variation in their electrical characteristics. The devices presented here have properties comparable to the ones using plasma-enhanced chemical vapor deposited SiO 2 at 400°C, reinforcing the success of this amorphous multicomponent dielectric approach for low temperature, high performance, and transparent electronic circuits.

Room-Temperature Cosputtered HfO2 – Al2O3 Multicomponent Gate Dielectrics

Hafnium oxide-aluminum oxide (HFAIO) dielectric films were cosputtered using HfO 2 and Al 2 O 3 targets, and their properties are studied in comparison with pure HfO 2 films. The X-ray diffraction studies confirmed that the HfO 2 films are nanocrystalline with a monoclinic phase. The as-deposited HfAIO films with a chemical composition of (HfO 2 ) 0 . 86 (AI 2 O 3 ) 0 . 14 are amorphous even after annealing at 500°C. Further, the cosputtered films show a slight reduction in leakage current. The leakage current density may be significantly reduced below 3 × 10 -10 A cm -2 at an electric field of 0.25 MV/cm when applying the proper radio-frequency bias to the substrate.

P-type oxide-based thin film transistors produced at low temperatures

P-type thin-film transistors (TFTs) using room temperature sputtered tin and copper oxide as a transparent oxide semiconductor have been produced on rigid and paper substrates. The SnOx films shows p-type conduction presenting a polycrystalline structure composed with a mixture of tetragonal β-Sn and α-SnOx phases, after annealing at 200 °C. These films exhibit a hole carrier concentration in the range of ≈ 1016-1018 cm-3, electrical resistivity between 101-102 Ωcm, Hall mobility of 4.8 cm2/Vs, optical band gap of 2.8 eV and average transmittance ≈ 85 % (400 to 2000 nm). Concerning copper oxide CuxO thin films they exhibit a polycrystalline structure with a strongest orientation along (111) plane. The CuxO films produced between an oxygen partial pressure of 9 to 75% showed p-type behavior, as it was measured by Hall effect and Seebeck measurements. The bottom gate p-type SnOx TFTs present field-effect mobility above 1.24 cm2/Vs (including the paper p-type oxide TFT) and an on/off modulation ratio of 103 while the CuxO TFTs exhibit a field-effect mobility of 1.3×10-3 cm2/Vs and an on/off ratio of 2×102.

Floating gate memory paper transistor

Reported herein is a nonvolatile n-type floating gate memory paper field-effect transistor, emphasizing the role of the paper structure and properties on the device performance recorded such as in the high capacitance per unit area at low frequencies (>2.5 μFcm-2) and so on the set of high charge retention times achieved (>16000 hours). The device was built via the hybrid integration of natural cellulose fibers, which act simultaneously as substrate and gate dielectric, using amorphous indium zinc and gallium indium zinc oxides respectively for the gate electrode and channel layer. This was complemented by the use of continuous patterned metal layers as source/drain electrodes.

Caracterização elétrica de filmes ultrafinos de PANI/PVS: material potencial para detecção de amônia em galpões de criação avícola

In this work we investigated the optical, electrical and morphological properties of ultrathin PANI/PVS films as active material of ammonia sensor for a poultry house. The layer-by-layer polymer films were deposited onto NiCr interdigitated microelectrode recovered with Au (NiCr/Au) line arrays. The buildup of the multilayers was monitored at each deposition step by UV-VIS spectroscopy while the atomic force microscopy (AFM) was employed to monitor the change in the roughness, in the grain and particle sizes. We observed a significant change in the dc conductivity when the thickness of the films was close to that of the NiCr layer. This effect is attributed to the higher interfacial electrical resistance between NiCr and the polymer film, compared to the resistance of the Au-film interface and polymer bulk. The alternating conductivity of the polymer was typical of solid disordered materials, under the influence of the NiCr electrode at low frequencies. Finally, the electrical sensitivity of the system under the exposure of NH3 reveals PANI/PVS films as a feasible active element for use in poultry house control systems.