ICMCTF2012 Session C2-3/F4-3: Thin Films for Photovoltaics and Active Devices: Synthesis and Characterization

Abstract─

In this work, the influence of copper on amorphous type Indium-Gallium-Zinc-Oxide (a-IGZO) thin-film transistor’s (TFTs) transfer curve is studied. The source/drain of a-IGZO TFTs are made in the structures of Cu / Ti and Ti/Al/Ti. The I_D-V_G curves of those TFTs are compared and the results show that the copper deteriorates the performance of the TFTs. It is attributed to the presence of the copper in the channel region of the device, which is verified by SIMS analysis. A Cu-dipping experiment is conducted by dipping devices into the solution of CuSO₄ and the deteriorated I_D-V_G curves are also observed.The simulation of IV curve's degradation is realized through ATLAS device simulator produced by Silvaco, Inc which helps us understand that what kind of trap Cu ions fromed in IGZO during the conventional BCE process used in a-Si TFT.

The bias and environment dependence on the light-accelerated instability of amorphous indium-gallium-zinc-oxide thin film transistors is examined in this study. The experiment result shows the electrical characteristic degradation of devices is not monotonously relying on the charge trapping mechanism for different negative gate bias under illumination. It is also implicated the adsorbent gas species upon surrounding environment (atmosphere, oxygen, moisture and vacuum). During negative gate bias under illumination in oxygen or atmosphere ambient, the negative shift in electrical characteristic is suppressed comparing to the result in vacuum. Thus, a physical model is proposed for transiting dominant mechanisms from photon-created carrier trapping mechanism to adsorbed/desorbed gases phenomenon.

N₂O plasma treatment suppressed the temperature-dependent sub-threshold leakage current of amorphous indium-gallium-zinc-oxide thin film transistors (a-IGZO TFTs). The transfer curve exhibits abnormal sub-threshold leakage current at high temperature. The abnormal electrical properties are explained by the energy band diagrams at both forward and reverse sweep. Above 400K, the hole could be generated from trap-assisted transition and drift to the source side that induced the source barrier lowering. The source side barrier lowering enhances electrons injection from the source to channel and causes an apparent sub-threshold leakage current. This phenomenon only appears in the device without N₂O plas ma treatment, but not in the device with N₂O plasma treatment, which is experimentally verified. The results suggested that the density of states for a-IGZO with N₂O plasma treatment is much lower than that for without plasma treatment. The N₂O plasma treatment repairs the defect to suppressed temperature-dependent sub-threshold leakage current.

To fully understand the complex electrical and optical properties of an organic light emitting device, it has been critical to develop new tools to probe working devices, in this way we can capture the photophysics of excitons generated by charge recombination and their subsequent decay. Most important is the role of the 'non-emissive' triplet excitons which are the dominant exciton thus created. In my talk I will describe the various time resolved laser spectroscopies and electro-optic techniques we use and the key insights into the photophysics of OLEDs that we have discovered which have changed the way we understand these devices and point to new materials and device architectures for ever higher efficiencies. I shall focus on the role of triplet fusion, the process of light generation from two annihilating triplet excitons and how the triplet exciton population can generate between 15 to 37 % of the total electroluminescence output of a device without the use of phosphorescent dopants. This has very important ramifications for device lifetime as blue phosphorescent dopants place severe limitations of achievable lifetime and device performance.

Thin molybdenum films are commonly used as back electrodes in Cu(In,Ga)Se₂ thin film solar cells and as contact material for thin film transistors/liquid crystal displays. For these applications, high electrical conductivity of the films is essential. As it follows from the classical Drude theory of the electrical conductivity in metals, all microstructure defects are in principle acting as scattering centres for electrons. Consequently, the microstructure defects increase always the electrical resistance of the material, but their “scattering cross sections” for electrons are supposed to differ strongly. In our study, we quantified the effect of point defects, dislocations and grain boundaries on the electrical resistivity of molybdenum films having a constant thickness of 500 nm.

The kind and density of the microstructure defects in the Mo films were modified by applying different physical vapour deposition methods (DC magnetron sputtering, pulsed DC magnetron sputtering and RF magnetron sputtering) and by depositing the thin films at different deposition temperatures (room temperature, 150°C, 250°C and 350°C). As expected, the electrical resistivity of the Mo films, which was measured using a 4-point probe, decreased with increasing substrate temperature. Furthermore, the highest resistivity was observed in RF magnetron sputtered samples, where also the effect of the substrate temperature was most pronounced.

The microstructure of the thin films was characterised by using a combination of glancing angle X-ray diffraction (GAXRD), X-ray reflectivity (XRR), XRD pole figure measurement, scanning electron microscopy with electron back scattered diffraction (SEM/EBSD) and transmission electron microscopy (TEM). GAXRD revealed residual stresses, stress-free lattice parameters, crystallite size and microstrain in the molybdenum films. XRR together with SEM and TEM gave information about the morphology of the films; this information was complemented by texture measurements performed using XRD and EBSD. The stress-free lattice parameters were used as a measure of the density of point defects, the microstrain as a measure of the dislocation density and the crystallite and grain size as a measure of the distance between the grain boundaries. The kind of the grain boundaries was deduced from the mutual orientation of crystallites. It was concluded that the point defects have the highest impact on the electrical conductivity of physical vapour deposited molybdenum thin films, followed by dislocations, and grain and crystallite boundaries.

In this study, a two-step process involving sol-gel and microwave hydrothermal techniques are used to synthesis a novel TiO₂ structure, so called as TiO₂ beads. It exhibits the required characteristics for photocataysis such as large surface area and well anatase crystallinity. Various synthesis parameters were investigated to evaluate the proper processing window of TiO₂ beads such as the complex amount in the sol-gel process, temperature and heating time in the microwave hydrothermal process. It was found that microwave hydrothermal techniques can much reduce the process time and the size of TiO₂ nanocrystalline. The texture and morphology of obtained nanoporous TiO₂ beads were examined using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The crystalline phase was analyzed using x-ray diffractometer (XRD) and Raman spectroscopy. The surface chemical bonding state was examined using x-ray photoelectron spectroscopy (XPS). The specific surface, pore diameter and pore volume of beaded TiO₂ was examined using BET. UV-visible diffuse reflectance spectra were achieved using a UV-visible spectrophotometer. The photocatalytic reactivity was obtained by degradation the Methylene blue.