Temperature-dependent Field-effect Measurements Research Articles

Characterization of optimized properties for InGaZnO semiconductor and thin film transistor by oxygen partial pressure

The InGaZnO thin films with varied the O2 flow ratios were deposited by a magnetron sputtering system. The effect of O2 content on the structure, surface morphology, optical and electrical properties of InGaZnO thin films is studied. Then, we investigate the relationship between bias stress stability and density of states (DOS) of thin film transistors (TFTs) with various oxygen partial pressure InGaZnO (IGZO) as the channel layer. The temperature-dependent field-effect measurements are used to calculate DOS of IGZO TFTs. With the increase of the oxygen partial pressure, the value of DOS decreases from 2.2 × 1016 to 5.5 × 1015 cm-3. Moreover, the positive gate bias stress of IGZO TFTs are investigated. Under positive gate bias stress, the transfer curves of IGZO TFTs with various oxygen partial pressure show an obvious shift, the value of threshold voltage shift () is 5.1, 2.94, 2 and 0.8 V, respectively. The results suggest that the device with a smaller DOS is more stable than others. Therefore, this study offers a brief and accurate method to demonstrate instability mechanism of IGZO TFTs.

Electrical characteristics and density of states of thin-film transistors based on sol-gel derived ZnO channel layers with different annealing temperatures

We report on the fabrication and electrical characterization of bottom gate thin-film transistors (TFTs) based on a sol-gel derived ZnO channel layer. The effect of annealing of ZnO active channel layers on the electrical characteristics of the ZnO TFTs was systematically investigated. Photoluminescence (PL) spectra indicate that the crystal quality of the ZnO improves with increasing annealing temperature. Both the device turn-on voltage (Von) and threshold voltage (VT) shift to a positive voltage with increasing annealing temperature. As the annealing temperature is increased, both the subthreshold slope and the interfacial defect density (Dit) decrease. The field effect mobility (μFET) increases with annealing temperature, peaking at 800 °C and decreases upon further temperature increase. An improvement in transfer and output characteristics was observed with increasing annealing temperature. However, when the annealing temperature reaches 900 °C, the TFTs demonstrate a large degradation in both transfer and output characteristics, which is possibly produced by non-continuous coverage of the film. By using the temperature-dependent field effect measurements, the localized sub-gap density of states (DOSs) for ZnO TFTs with different annealing temperatures were determined. The DOSs for the subthreshold regime decrease with increasing annealing temperature from 600 °C to 800 °C and no substantial change was observed with further temperature increase to 900 °C.

A comparison of density of states between InGaZnO based TFTs and InZnO based TFTs

ABSTRACTWe investigate thermally induced instability of thin film transistors with incorporating sputtered InZnO and InGaZnO as the channel layer at room temperature, using density of states extracted from temperature-dependent field-effect measurements method and falling rates of activation energy. The trends between density of states and stability under temperature stress are entirely consistent with each other in respect of the reduction of total traps with different channel layer. Moreover, the instability for InGaZnO based TFTs and InZnO based TFTs has been investigated under positive gate bias stress, and the device with InGaZnO channel layer shows a slighter positive threshold voltage shift than that of InZnO based device under positive bias stress. This result demonstrates that the stability of the device is closely related with the density of states.

Effect of oxygen partial pressure on the density of states of amorphous InGaZnO thin-film transistors

The thin-film transistors (TFTs) with InGaZnO active layer with different oxygen partial pressures are fabricated by radio frequency sputtering. The influence of the oxygen partial pressure on the density of states (DOS) for InGaZnO-TFT is investigated by using temperature-dependent field-effect measurements. It indicates that the DOS become smaller with increasing oxygen partial pressure. The results are verified by the threshold voltage shift of InGaZnO-TFT with different oxygen partial pressures. The trend of the variation of DOS is consistent with that of the threshold voltage shift for InGaZnO-TFT. Thus, the gate bias instability is attributed to the charge trapping mechanism based on DOS. Therefore, this work offered a brief and accurate method to calculate DOS for demonstrating the bias stability of transistor.

Suppression in the Negative Bias Illumination Instability of ZnSnO Thin-Film Transistors Using Hafnium Doping by Dual-Target Magnetron Cosputtering System

In this paper, the highly improved negative bias illumination stress (NBIS) stability of zinc-tin-oxide (ZTO) thin-film transistors (TFTs) is achieved by Hf doping, using dual-target magnetron cosputtering system. Compared with a large negative threshold voltage shift ( $\vartriangle V_{T})$ of 9 V in pristine ZTO TFT, the Hf-doped ZTO TFTs shows superior stability and device D only shows 1.9 V negative $\vartriangle V_{T}$ under the same NBIS. The enhancement in NBIS stability of Hf-doped ZTO TFTs is attributed to a lower oxygen vacancy concentrations and a fewer interface trap states suppressed by Hf ion. The decrease of oxygen vacancy concentrations and interface trap states are confirmed by X-ray photoelectron spectroscopy measurement and capacitance voltage ( $C$ – $V$ ) measurement, respectively. It is consistent with trap density extracted from temperature-dependent field-effect measurements, which verifies further the factor of the improvement of in NBIS stability of Hf-doped ZTO TFTs.

Effect of hafnium doping on density of states in dual-target magnetron co-sputtering HfZnSnO thin film transistors

This study investigates the effect of hafnium doping on the density of states (DOSs) in HfZnSnO thin film transistors fabricated by dual-target magnetron co-sputtering system. The DOSs is extracted by temperature-dependent field-effect measurements, and they decrease from 1.1 × 1017 to 4.6 × 1016 eV/cm3 with increasing the hafnium concentrations. The behavior of DOSs for the increasing hafnium concentration HfZnSnO thin film transistors can be confirmed by both the reduction of ΔVT under bias stress and the trapping charges calculated by capacitance voltage measurements. It suggests that the reduction in DOSs due to the hafnium doping is closely related with the bias stability and thermal stability.

Origin of the improved stability under negative gate-bias illumination stress in various sputtering power fabricated ZnSnO TFTs

In this work, we report the influence of ZnSnO channel layer sputtering power in the stability of ZnSnO TFTs under negative gate-bias illumination stress (NBIS). The origin of threshold voltage shift results from combined effect of two factors between the little defect-induced trap densities originated from oxygen vacancies and better channel–insulator interface. The kinetic energy of the ions at a proper rf sputtering power is responsible for less defect-induced trap density and better channel–insulator interface. Therefore, the ZnSnO TFT fabricated at 75 W sputtering power shows a better NBIS stability. In addition, the trap density is extracted by temperature-dependent field-effect measurements and it is consistent with the change of stability under NBIS and thermal stress.

Subgap states in p-channel tin monoxide thin-film transistors from temperature-dependent field-effect characteristics

This paper experimentally investigates the subgap density of states (DOS) in p-type tin monoxide (SnO) thin-film transistors (TFTs) for the first time by using temperature-dependent field-effect measurements. As the temperature increases, the turn-on voltage moves in the positive direction, and the off-current and subthreshold slope continuously increase. We found that the conductivity of the SnO TFT obeys the Meyer–Neldel (MN) rule with a characteristic MN parameter of 28.6 eV−1 in the subthreshold region, from which we successfully extracted the subgap DOS by combing the field-effect method and the MN relation. The extracted subgap DOS from fabricated p-type SnO TFTs are exponentially distributed in energy, and exhibit around two orders of magnitude higher values compared to those of the n-type amorphous indium–gallium–zinc oxide TFTs.

Temperature-dependent field-effect measurements method to illustrate the relationship between negative bias illumination stress stability and density of states of InZnO-TFTs with different channel layer thickness

We investigate the stability of thin film transistors incorporating sputtered InZnO as the channel layer under negative bias illumination stress. The transfer characteristic for various active layer thicknesses is shifted toward the negative direction under negative bias illumination stress and the device with thicker channel layer shows a slighter VTH negative shift than another device under negative bias illumination stress. In order to investigate channel layer thickness can have a great effect on the trap density and thus affect the VTH shift caused by charge trapping; we use temperature-dependent field-effect measurements method to accurately calculate their trap density. The results show that thicker InZnO channel layer has fewer DOSs, resulting in the decrease of charge trapping and the decrease of photoexcitation generated electron carrier. So the device with thicker channel layer shows a slighter VTH negative shift than another device under negative bias illumination stress.

Density of States of a-InGaZnO From Temperature-Dependent Field-Effect Studies

Temperature-dependent field-effect measurements were performed on radio-frequency sputtered amorphous In-Ga-Zn-O thin film transistors (TFTs). We studied the effect of temperature on the TFT electrical properties. We observed that the field-effect mobility (mu) increases and the threshold voltage ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">V</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">T</sub> ) shifts negatively with temperature, while the current on-off ratio and subthreshold slope (S) remain almost unchanged. We also observed that the TFT drain current (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D</sub> ) is thermally activated, and the relation between the prefactor (I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">D0</sub> ) and activation energy ( <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> ) obeys the Meyer-Neldel rule. The density of localized gap states (DOS) was then calculated by using a self-consistent method based on the experimentally obtained <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">E</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">a</sub> . The result shows good agreement with the DOS distribution calculated from SPICE simulations.

Determination of the microscopic prefactor of the conductivity of a-Si:H using temperature dependent field-effect measurements

We explain the Meyer-Neldel rule seen in temperature dependent field-effect measurements. This behaviour provides a method to determine the microscopic prefactor of the conductivity of a-Si:H. The accuracy of this method is critically discussed.

A self-consistent analysis of temperature-dependent field-effect measurements in hydrogenated amorphous silicon thin-film transistors

We calculated a more accurate density of states (DOS) profile from field-effect (FE) measurements in hydrogenated amorphous silicon thin-film transistors, taking into account the anomalously changing conductivity prefactor in accordance with the Meyer–Neldel (MN) rule. We present a self-consistent analysis of the density of gap states profile, where the MN rule is, for the first time, properly considered in relation to the nonuniform shift of the Fermi level as induced by the field effect. Moreover, the calculation yields the correct flat-band voltage and the corresponding flat-band activation energy. The determination of conductivity activation energies free from any initial band bending effects is of importance in all types of transport measurements.