Monthly Archives: August 2015

Effect of Alumina Buffers on the Stability of Top-Gate Amorphous InGaZnO Thin-Film Transistors on Flexible Substrates

Flexible top-gate amorphous InGaZnO thin-film transistors are fabricated on polyimide substrates. The effect of the alumina buffer layers on the device performance and stability is demonstrated using two types of atomic layer deposition reactant sources: 1) ozone and 2) water. Alumina buffers formed by water reactants have better barrier properties against the ambient than those formed by ozone. Furthermore, less charge trapping at sub-gap density-of-states occurs with higher film density of the buffer layer. Stability characteristics under negative bias temperature stress are enhanced by optimization of the buffer layer formation on flexible substrates.

Performance and Stability Improvements of Back-Channel-Etched Amorphous Indium–Gallium–Zinc Thin-Film-Transistors by CF<sub>4</sub>+O<sub>2</sub> Plasma Treatment

The performance and stability improvement of back-channel-etched amorphous indium–gallium–zinc oxide thin-film transistors (a-IGZO TFTs) by post CF4+O2 plasma treatment is investigated. It is revealed that the metal residue of the wet-etching of source/drain electrodes degrades TFT performance and aggravates the positive threshold voltage ( $V_{mathrm {th}})$ shift under positive gate bias stress. It is demonstrated that the CF4+O2 plasma treatment effectively removes the metal residue and remarkably improves device performance and the $V_{mathrm {th}}$ stability. This improvement is attributed to oxygen vacancy repairing at the back channel interface of the a-IGZO TFT by the O free radicals generated by the O2 plasma.

Small-area DAC using adaptive body bias scheme for AMOLED data driver ICs

A small-area digital-to-analogue converter (DAC) using an adaptive body bias scheme is proposed for active-matrix organic light-emitting diode (AMOLED) data driver ICs. The proposed scheme adjusts the body bias applied to the P-type (PMOS) decoder according to the average picture level (APL) and maintains a low on-resistance of the PMOS decoder at a high APL. This enables reduction of the DAC area by realising the upper-half decoder using only PMOS switches, unlike conventional DACs which use both PMOS and N-type switches. A 20-channel data driver IC is fabricated using a 0.18 µm HVCMOS technology with 1.8 V/18 V devices. The proposed DAC occupies 33% less area than the conventional DAC and the measured differential nonlinearity and integral nonlinearity are +0.09/−0.1 and +0.36/−0.47 LSB, respectively.

Effect of proton irradiation energy on AlGaN/GaN metal-oxide semiconductor high electron mobility transistors

The effects of proton irradiation energy on dc characteristics of AlGaN/GaN metal-oxide semiconductor high electron mobility transistors (MOSHEMTs) using Al2O3 as the gate dielectric were studied. Al2O3/AlGaN/GaN MOSHEMTs were irradiated with a fixed proton dose of 5 × 1015 cm−2 at different energies of 5, 10, or 15 MeV. More degradation of the device dc characteristics was observed for lower irradiation energy due to the larger amount of nonionizing energy loss in the active region of the MOSHEMTs under these conditions. The reductions in saturation current were 95.3%, 68.3%, and 59.8% and reductions in maximum transconductance were 88%, 54.4%, and 40.7% after 5, 10, and 15 MeV proton irradiation, respectively. Both forward and reverse gate leakage current were reduced more than one order of magnitude after irradiation. The carrier removal rates for the irradiation energies employed in this study were in the range of 127–289 cm−1. These are similar to the values reported for conventional metal-gate high-electron mobility transistors under the same conditions and show that the gate dielectric does not affect the response to proton irradiation for these energies.

The effect of annealing temperature on the stability of gallium tin zinc oxide thin film transistors

With the growing need for large area display technology and the push for a faster and cheaper alternative to the current amorphous indium gallium zinc oxide (a-IGZO) as the active channel layer for pixel-driven thin film transistors (TFTs) display applications, gallium tin zinc oxide (GSZO) has shown to be a promising candidate due to the similar electronic configuration of Sn 4+ and In 3+ . In this work TFTs of GSZO sputtered films with only a few atomic % of Ga and Sn have been fabricated. A systematic and detailed comparison has been made of the properties of the GSZO films annealed at two temperatures: 140 °C and 450 °C. The electrical and optical stabilities of the respective devices have been studied to gain more insight into the degradation mechanism and are correlated with the initial TFT performance prior to the application of stress. Post deposition annealing at 450 °C of the films in air was found to lead to a higher atomic concentration of Sn 4+...

Ellipsometry study of process deposition of amorphous Indium Gallium Zinc Oxide sputtered thin films

Publication date: 1 September 2015 Source:Thin Solid Films, Volume 590 Author(s): C. Talagrand, X. Boddaert, D.G. Selmeczi, C. Defranoux, P. Collot This paper reports on an InGaZnO optical study by spectrometric ellipsometry. First of all, the fitting results of different models and different structures are analysed to choose the most appropriate model. The Tauc–Lorentz model is suitable for thickness measurements but a more complex model allows the refractive index and extinction coefficient to be extracted more accurately. Secondly, different InGaZnO process depositions are carried out in order to investigate stability, influence of deposition time and uniformity. Films present satisfactory optical stability over time. InGaZnO optical property evolution as a function of deposition time is related to an increase in temperature. To understand the behaviour of uniformity, mapping measurements are correlated to thin film resistivity. Results show that temperature and resputtering are the two phenomena that affect IGZO uniformity.

Synthesis and electrical characterization of Graphene Oxide films

Publication date: 1 September 2015 Source:Thin Solid Films, Volume 590 Author(s): Muhammad Yasin, T. Tauqeer, Syed M.H. Zaidi, Sait E. San, Asad Mahmood, Muhammet E. Köse, Betul Canimkurbey, Mustafa Okutan In this work, we have synthesized Graphene Oxide (GO) using modified Hummers method and investigated its electrical properties using parallel plate impedance spectroscopic technique. Graphene Oxide films were prepared using drop casting method on Indium Tin Oxide (ITO) coated glass substrate. Atomic force microscopy was used to characterize the films' microstructure and surface topography. Electrical characterization was carried out using LCR meter in frequency regime (100Hz to 10MHz) at different temperatures. AC conductivity σac of the films was observed to be varied with angular frequency, ω as ωS, with S&lt;1. The electrical properties of GO were found to be both frequency and temperature dependent. Analysis showed that GO film contains direct current (DC) and Correlated Barrier Hopping (CBH) conductivity mechanisms at low and high frequency ranges, respectively. Photon absorption and transmittance capability in the visible range and excellent electrical parameters of solution processed Graphene Oxide suggest its suitability for the realization of low cost flexible organic solar cells and organic Thin Film Transistors, respectively.

Local impedance measurement of an electrode/single-pentacene-grain interface by frequency-modulation scanning impedance microscopy

The device performances of organic thin film transistors are often limited by the metal–organic interface because of the disordered molecular layers at the interface and the energy barriers against the carrier injection. It is important to study the local impedance at the interface without being affected by the interface morphology. We combined frequency modulation atomic force microscopy with scanning impedance microscopy (SIM) to sensitively measure the ac responses of the interface to an ac voltage applied across the interface and the dc potential drop at the interface. By using the frequency-modulation SIM (FM-SIM) technique, we characterized the interface impedance of a Pt electrode and a single pentacene grain as a parallel circuit of a contact resistance and a capacitance. We found that the reduction of the contact resistance was caused by the reduction of the energy level mismatch at the interface by the FM-SIM measurements, demonstrating the usefulness of the FM-SIM technique for investigation of the local interface impedance without being affected by its morphology.

Pulsed radio frequency characterisation on 28 nm complementary metal–oxide semiconductor technology

The influence of electrothermal behaviour on radio frequency (RF) performances of 28 nm bulk complementary metal–oxide semiconductor technology is examined. Biased continuous-wave RF and pulsed RF (applying different DC pulse and RF pulse width combinations) characterisations are performed within the 1–30 GHz frequency domain at room temperature and the transit frequency (fT) is extracted at 15 GHz frequency. It has been found that the degradation in fT on I/O devices is about 3 GHz because of the self-heating effect.