Oxide semiconductor based thin-fi lm transistors (TFTs) are a promising technology for application in large-area electronics. [ 1 ] Despite their relatively short history, oxide TFTs with chargecarrier mobilities exceeding 100 cm 2 V − 1 s − 1 [ 2,3 ] have already been demonstrated, a performance that is superior to that of amorphous silicon (a-Si) and comparable to polycrystalline Si (poly-Si). Unfortunately, these high-mobility oxide TFTs are usually manufactured using costly vacuum-based processing methodologies. [ 1 , 4-8 ] In an effort to address this problem, recent research has been focussing on the development of TFTs using alternative deposition methods based on solution-processable oxide semiconductors. [ 9 -14 ] Whilst progress on solution-processed oxide semiconductors has been rapidly advancing, research efforts towards the development of new dielectrics has been relatively slow, with most of the reported work performed using conventional dielectrics (e.g., SiO 2 ) with few exceptions. [ 9 , 15,16 ] As a result, the majority of oxide transistors reported to date operate at relatively high voltages and hence consume signifi cantly more power. In order to circumvent this bottleneck, recent work has been focusing on the development of low-voltage oxide transistors, including the use of ultra-thin dielectrics, [ 17 ] high -k dielectrics, [ 15 ] and electrolyte gate dielectrics. [ 18 ] Oxide transistors based on high -k dielectrics have received the most attention and a number of high mobility, low-voltage devices have been demonstrated. [ 15 , 17 ] The high -k materials studied to date include transition metal oxides such as Ta 2 O 5 , TiO 2 , [19][20][21][22] ZrO 2 , [23][24][25][26] Al 2 O 3 , [ 27 ] HfO 2 , [ 28 ] and silicates, [ 29 ] as well as ferroelectric materials such as Pb(Zr,Ti) O 3 and (Ba,Sr)TiO 3 . [ 30 , 31 ] Among these, ZrO 2 and HfO 2 are the most extensively studied dielectrics and are widely considered to be excellent candidates because of their relatively high dielectric constants, good thermal stability, and large band gaps. [32][33][34] Despite their attractive properties, however, ZrO 2 and HfO 2 based TFTs are usually realised using stringent and potentially costly manufacturing techniques. [35][36][37][38][39] Here we demonstrate how spray pyrolysis (SP), a simple and large-area-compatible deposition technique, can be used for the processing of high-quality ZrO 2 layers onto glass substrates containing prepatterned indium tin oxide (ITO) electrodes. We demonstrate their use in high-mobility, low-voltage TFTs based on either ZnO or Li-doped ZnO fi lms deposited by SP [10][11][12][13][14] directly onto ITO/ZrO 2 . Optimised TFTs based on ITO/ZrO 2 / Li-ZnO multilayer structures deposited sequentially at substrate temperatures of 400-450 ° C exhibit excellent electrontransport characteristics with operating voltages below 6 V and a maximum electron mobility on the order of 85 cm 2 V − 1 s − 1 . To our knowledge, this is the highest reported mobility value for transistors bas...
Research on solution processible semiconducting materials is rapidly making progress towards the goal of providing viable alternatives to silicon-based technologies for applications where lower-cost manufacturing and new product features such as mechanical flexibility and optical transparency are desired. One family of materials that has been the subject of intense research over the past twenty years is organic semiconductors.[1] Use of organic materials offers the prospect of low manufacturing cost combined with some desirable physical characteristics such as ease of processing and mechanical flexibility. Despite the impressive progress achieved in recent years a number of obstacles, especially poor air-stability, device performance that is insufficient for a variety of applications and device to device variability, have to be overcome before the advantageous manufacturability, and hence the economic benefits associated with organic semiconductors, can be fully exploited.While research in the area of organic materials and devices has been intensifying, a different class of semiconducting materials, namely metal oxide semiconductors (MOxS), has emerged as possible alternative technology.[2] Metal oxides incorporate important qualities that are currently absent from organic-based semiconductors. For instance, they generally exhibit higher carrier mobilities which are already sufficient for use in optical displays, such as current-driven organic light-emitting diode (OLED) based displays. An additional advantage of MOxS relevant to many electronic applications is the superb optical transparency resulting -2 -from their wide bandgap. The latter makes oxide semiconductors particularly interesting for use in transparent electronics [3] as well as in backplanes for the next generation of currentdriven displays. [4,5] For application in see-through electronics, in particular, transparent thin-film transistors (TFTs) with high switching speeds and low power consumption are required.[6] So far the opacity of amorphous silicon and the insufficient performance of organic semiconductors have impeded the development of such devices. In this respect, MOxS materials simultaneously fulfil the requirements for optical transparency and high charge carrier mobility. In addition, they provide excellent chemical stability combined with mechanical robustness. [7] A further advantage associated with MOxS is the diverse range of techniques that can be employed for thin-film deposition.[7] These include, sputtering, [8][9][10] pulsed-laser deposition (PLD), [11] metalorganic chemical vapour deposition (MOCVD), [12,13] as well as solution processing methods such as dip coating, [14] spin coating [15][16][17][18] and spray pyrolysis (SP). [19][20][21] Solution processing, in particular, offers a number of advantages, which are well known from the area of organic electronics, with the most important being the prospect of easy patterning on large area substrates. In most cases, however, control over the morphology of solution processed film...
The properties of metal oxides with high dielectric constant (k) are being extensively studied for use as gate dielectric alternatives to silicon dioxide (SiO2). Despite their attractive properties, these high‐k dielectrics are usually manufactured using costly vacuum‐based techniques. In that respect, recent research has been focused on the development of alternative deposition methods based on solution‐processable metal oxides. Here, the application of the spray pyrolysis (SP) technique for processing high‐quality hafnium oxide (HfO2) gate dielectrics and their implementation in thin film transistors employing spray‐coated zinc oxide (ZnO) semiconducting channels are reported. The films are studied by means of admittance spectroscopy, atomic force microscopy, X‐ray diffraction, UV–Visible absorption spectroscopy, FTIR, spectroscopic ellipsometry, and field‐effect measurements. Analyses reveal polycrystalline HfO2 layers of monoclinic structure that exhibit wide band gap (≈5.7 eV), low roughness (≈0.8 nm), high dielectric constant (k ≈ 18.8), and high breakdown voltage (≈2.7 MV/cm). Thin film transistors based on HfO2/ZnO stacks exhibit excellent electron transport characteristics with low operating voltages (≈6 V), high on/off current modulation ratio (∼107) and electron mobility in excess of 40 cm2 V−1 s−1.
Modern telecommunications rely on the transmission and manipulation of optical signals. Optical amplification plays a vital part in this technology, as all components in a real telecommunications system produce some loss. The two main issues with present amplifiers, which rely on erbium ions in a glass matrix, are the difficulty in integration onto a single substrate and the need of high pump power densities to produce gain. Here we show a potential organic optical amplifier material that demonstrates population inversion when pumped from above using low-power visible light. This system is integrated into an organic light-emitting diode demonstrating that electrical pumping can be achieved. This opens the possibility of direct electrically driven optical amplifiers and optical circuits. Our results provide an alternative approach to producing low-cost integrated optics that is compatible with existing silicon photonics and a different route to an effective integrated optics technology.
We report the application of ambient spray pyrolysis for the deposition of high-k polycrystalline Y2O3 and amorphous Al2O3 dielectrics and their use in low-voltage ZnO thin-film transistors. The films are studied by means of atomic force microscopy, UV-visible absorption spectroscopy, impedance spectroscopy, and field-effect measurements. ZnO transistors based on spray pyrolysed Y2O3 and Al2O3 dielectrics show low leakage currents, and hysteresis-free operation with a maximum electron mobility of 34 cm2/V s and current on/off ratio on the order of 105. This work is a significant step toward high-performance oxide electronics manufactured using simple and scalable processing methods.
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