Electrochemical and Solid-State Letters, 15 (4) H133-H135 (2012) H133 Thermal Evolution of Band Edge States in ZnO Film as a Function of Annealing Ambient Atmosphere

RF-sputtered ZnO films were annealed under various annealing ambient atmospheres, including a vacuum, air, and water vapor. The physical and electrical properties of ZnO films annealed in various ambient atmospheres, were studied as a function of annealing temperature. The carrier concentration was dramatically increased, and the mobility was decreased when the films were annealed in a vacuum or water vapor. Even though the annealing ambient atmosphere and temperature were different, the preferred orientation and crystallization of the annealed ZnO films are maintained. However, two distinct band edge states below the conduction band, observed by spectroscopic ellipsometry measurement, undergo a thermal change as a function of annealing ambient atmosphere and these changes are correlated to changes in carrier concentration and mobility. © 2012 The Electrochemical Society. [DOI: 10.1149/2.005205esl] All rights reserved. Manuscript submitted October 13, 2011; revised manuscript received January 16, 2012. Published February 7, 2012 N-type ZnO material with wide-bandgap (∼3.3 eV) has been investigated for a wide range of applications, such as UV light emitters, spintronic devices, transparent high-power electronics, surface acoustic wave devices, piezoelectronic transducers, gas and biological sensors, and solar cells. 1, 2 In particular, ZnO-based thin film transistors (TFTs) are very attractive in flexible and transparent electronics because the devices show the moderate hall mobility (>1 cm 2 /V.s) even on room temperature and the ZnO semiconductors is transparent in the visible range. Recently, several researchers have reported TFTs application using stoichiometry ZnO semiconductor by various deposition methods. 6 Unfortunately, the impurities doping into ZnO films have induced in high cost and complication for doping elements. The ion implantation also produces the collision damage into ZnO matrix, which can generate unnecessary and uncontrollable defect states. 7 In this study, we examine the effects of simple thermal treatments under various annealing ambient atmospheres on ZnO films, in terms of physical, and electrical properties. Moreover, physical origins of change in electrical properties of ZnO films are investigated and are analyzed by the evolution of band edge states as a function of thermally driven energy. Experimental P-type Si wafers with thermally grown SiO 2 (100 nm) were used as substrates onto which ZnO films (50 nm) were deposited without the substrate heating by a radio frequency (RF) sputtering system. The RF power and process pressure were set to 75 W and 10 mTorr, respectively, which was controlled by an Ar gas flow rate of 50 sccm. After that, ZnO films were annealed by a furnace system with a quartz tube and quartz loading bar. The annealing duration was 1 hour and the temperature was at 150 • C, and 300 • C. In order to examine the effects of the post-annealing, the annealing ambient atmosphere was maintained for air, vacuum, and water vapor conditions. The annealing in the ambient atmosphere of air was performed on the day had z E-mail: [email protected] humidity of ∼38% The vacuum ambient atmosphere was controlled by a rotary pump to maintain a pressure below 5 × 10 −2 Torr. The water vapor ambient atmosphere was produced using a water bubbler system with a N 2 gas flow of 100 sccm. Water vapor annealing was carried out at 10mtorr using a rotary pump and throat valve. In order to the electrical properties such as carrier concentration and hall mobility of ZnO films, Hall measurements were carried out using van der Pauw configuration with the permanent magnet of 0.55 Tesla at room temperature. The crystallization and its orientation of the annealed ZnO films were observed by transmission electron microscopy (TEM),and X-ray diffraction (XRD). The surface morphology was investigated by using atomic force microscopy (AFM). The detailed electronic structures, related to changes in bandgap and band edge state below the conduction band, were analyzed by spectroscopic ellipsometry (SE). Results and Discussion • C, the carrier concentration shows small variations under the value of ∼10 15 cm −3 and the mobility represents the similar value, regardless of annealing ambient atmosphere. However, at the annealing temperatures over 300 • C, the carrier concentration was dramatically increased to ∼10 19 cm −3 and the mobility was decreased to ∼10 cm 2 V −1 s −1 in the ambient atmosphere of a vacuum and water vapor, which is reasonable value comparing to the previous report

Controlling resistive switching behavior in the solution processed SiO2-x device by the insertion of TiO2 nanoparticles

The resistive switching behavior of the solution processed SiOx device was investigated by inserting TiO2 nanoparticles (NPs). Compared to the pristine SiOx device, the TiO2 NPs inserted SiOx (SiOx@TiO2 NPs) device achieves outstanding switching characteristics, namely a higher ratio of SET/RESET, lower operating voltages, improved cycle-to-cycle variability, faster switching speed, and multiple-RESET states. Density functional theory calculation (DFT) and circuit breaker simulation (CB) were used to detail the origin of the outstanding switching characteristic of the SiOx@TiO2 NPs. The improvement in resistive switching is mainly based on the difference in formation/rupture of the conductive path in the SiO2 and SiO2@TiO2 NPs devices. In particular, the reduction of resistance and lower switching voltage of TiO2 NPs control the formation and rupture of the conductive path to achieve more abrupt switching between SET/RESET with higher on/off ratio. This method of combined DFT calculation and CB offers a promising approach for high-performance non-volatile memory applications

Low temperature activation of amorphous In-Ga-Zn-O semiconductors using microwave and e-beam radiation, and the associated thin film transistor properties

In-Ga-Zn-O (IGZO) films deposited by sputtering process generally require thermal annealing above 300°C to achieve satisfactory semiconductor properties. In this work, microwave and e-beam radiation are adopted at room temperature as alternative activation methods. Thin film transistors (TFTs) based on IGZO semiconductors that have been subjected to microwave and e-beam processes exhibit electrical properties similar to those of thermally annealed devices. However spectroscopic ellipsometry analyses indicate that e-beam radiation may have caused structural damage in IGZO, thus compromising the device stability under bias stress

Effects of Embedded TiO2−x Nanoparticles on Triboelectric Nanogenerator Performance

Triboelectric nanogenerators (TENGs) are used as self-power sources for various types of devices by converting external waves, wind, or other mechanical energies into electric power. However, obtaining a high-output performance is still of major concern for many applications. In this study, to enhance the output performance of polydimethylsiloxane (PDMS)-based TENGs, highly dielectric TiO2−x nanoparticles (NPs) were embedded as a function of weight ratio. TiO2−x NPs embedded in PDMS at 5% showed the highest output voltage and current. The improved output performance at 5% is strongly related to the change of oxygen vacancies on the PDMS surface, as well as the increased dielectric constant. Specifically, oxygen vacancies in the oxide nanoparticles are electrically positive charges, which is an important factor that can contribute to the exchange and trapping of electrons when driving a TENG. However, in TiO2−x NPs containing over 5%, the output performance was significantly degraded because of the increased leakage characteristics of the PDMS layer due to TiO2−x NPs aggregation, which formed an electron path

Quantitative analysis of defect states in InGaZnO within 2 eV below the conduction band via photo-induced current transient spectroscopy

Abstract This work investigates the function of the oxygen partial pressure in photo-induced current measurement of extended defect properties related to the distribution and quantity of defect states in electronic structures. The Fermi level was adjusted by applying a negative gate bias in the TFT structure, and the measurable range of activation energy was extended to < 2.0 eV. Calculations based on density functional theory are used to investigate the changes in defect characteristics and the role of defects at shallow and deep levels as a function of oxygen partial pressure. Device characteristics, such as mobility and threshold voltage shift under a negative gate bias, showed a linear correlation with the ratio of shallow level to deep level defect density. Shallow level and deep level defects are organically related, and both defects must be considered when understanding device characteristics

Enhanced efficiency in lead-free bismuth iodide with post treatment based on a hole-conductor-free perovskite solar cell

Despite the excellent merits of lead perovskite solar cells, their instability and toxicity still present a bottleneck for practical applications. Bismuth perovskite has emerged as a candidate for photovoltaic (PV) applications, because it not only has a low toxicity but is also stable in air. However, the power conversion efficiency (PCE) remains an unsolved problem. We performed band gap tuning experiments to improve the efficiency. The absorption of ABi3I10 structure films was extended within the visible region, and the optical band gap was decreased considerably compared to that for Cs3Bi2I9. Furthermore, we explained the correlation between the structure and the optical properties via a first-principles study. A device employing CsBi3I10 as a photoactive layer exhibits a PCE of 1.51% and an excellent ambient stability over 30 days