The Influence of ZnO Layer Thickness on the Performance and Electrical Bias Stress Instality in ZnO Thin Film Transistors

University of Buea supported the first author during the writing of this manuscript Open access articleThin Film Transistors (TFTs) are the active elements for future large area electronic applications, in which low cost, low temperature processes and optical transparency are required. Zinc oxide (ZnO) thin film transistors (TFTs) on SiO2/n+-Si substrate are fabricated with the channel thicknesses ranging from 20 nm to 60 nm. It is found that both the performance and gate bias stress related instabilities of the ZnO TFTs fabricated were influenced by the thickness of ZnO active channel layer. The effective mobility was found to improve with increasing ZnO thickness by up to an order in magnitude within the thickness range investigated (20 – 60 nm). However, thinner films were found to exhibit greater stability in threshold voltage and turn-on voltage shifts with respect to both positive and negative gate bias stress. It was also observed that both the turn on voltage (Von) and the threshold voltage (VT) decrease with increasing channel thickness. Moreover, the variations in subthreshold slope (S) with ZnO thickness as well as variations in VT and Von suggest a possible dependence of trap states in the ZnO on the ZnO thickness. This is further correlated by the dependence of VT and Von instabilities with gate bias stress

Behavioral modeling of zinc-oxide, thin-film, field-effect transistors and the design of pixel driver, analog amplifier, and low-noise RF amplifier circuits

Zinc-oxide (ZnO) is of great interest due to transparent properties, high breakdown voltages, and low cost. Behavioral modeling is presented in this dissertation to model ZnO thin-film field-effect transistor (FET) drain current versus gate-source overdrive voltage. Initial findings show that in “strong inversion,” saturation, the drain current equation reveals a quartic-law dependency on gate-source overdrive voltage instead of square-law dependency seen in complementary metal-oxide semiconductor (CMOS) with no mobility reduction effects. This is postulated to result from the ZnO mobility showing a square-law increase with gate-source overdrive voltage. A “strong inversion,” saturation model having ±1.6% deviation from measured data is created in verilog-A to simulate and design circuits. Circuits include a fabricated and measured pixel driver circuit sinking 28 µA of current while only having a gate area of 20 µm2. This ZnO thin-film FET pixel driver is believed to have the highest current density reported at the time of this writing. Also, the first known ZnO thin-film FET analog amplifier is analytically designed for a gain of 3 V/V at 10 kHz while drawing only 8 µA of supply current. Finally, the first known ZnO thin-film FET low-noise RF amplifier is designed, utilizing scattering parameters measured at the Air Force Research Laboratory on a device with minimum channel length of 1.25 µm. This amplifier has a small-signal gain of 12.6 dB at 13.56 MHz, and a current drain of 268.4 mA at a drain voltage of 13 V

Zinc Oxide Thin Film Transistors: Advances, Challenges and Future Trends

This paper presents a review on recent developments and future trends in zinc oxide thin film transistors (ZnO TFTs) together with challenges involved in this technology. It highlights ZnO TFT as next generation choice over other available thin film transistor technology namely a – Si: H (amorphous hydrogenated silicon), poly-Si (polycrystalline silicon) and OTFT (organic thin film transistor). This paper also provides a comparative analysis of various TFTs on the basis of performance parameters. Effect of high –k dielectrics, grain boundaries, trap densities, and threshold voltage shift on the performance of ZnO TFT is also explained

Solution processed metal oxide microelectronics: from materials to devices

Owing to their many interesting characteristics, the application of metal oxide based electronics has been growing at a considerable rate for the past ten years. High performance, optical transparency, chemical stability and suitability toward low cost deposition methods make them well suited to a number of new and interesting application areas which conventional materials such as silicon, or more recently organic materials, are unable to satisfy.The work presented in this thesis is focussed on the optimisation of high performance metal oxide based electronics combined with use of spray pyrolysis, as a low cost deposition method. The findings presented here are split into three main areas, starting with an initial discussion on the physical and electronic properties of films deposited by spray pyrolysis. The results demonstrate a number of deposition criteria that aid in the optimisation and fabrication of high performance zinc oxide (ZnO) based thin-film transistors (TFTs) with charge carrier mobilities as high a 20 cm2/Vs. Solution processed gallium oxide TFTs with charge carrier mobilities of ~0.5 cm2/Vs are also demonstrated, highlighting the flexibility of the deposition method. The second part of the work explores the use of facile chemical doping methods suitable for spray pyrolysed ZnO based TFTs. By blending different precursor materials in solution prior to deposition, it has been possible to adjust certain material characteristics, and in turn device performance. Through the addition of lithium it has been possible alter the films grain structure, leading to significantly improved charge carrier mobilities as high as ~54 cm2/Vs. Additionally the inclusion of beryllium during film deposition has been demonstrated to control TFT threshold voltages, leading to improved integrated circuit performance. The final segment of work demonstrates the flexibility of spray pyrolysis through the deposition of a number of high-k dielectric materials. These high performance dielectrics are integrated into the fabrication of TFTs already benefiting from the findings of the previously discussed work, leading to highly optimised low-voltage TFTs. The performance of these devices represent some of best currently available from solution processed ZnO TFTs with charge carrier mobilities as high as 85 cm2/Vs operating at 3.5 V.Open Acces

Zinc Oxide Thin Film Transistors: Advances, Challenges and Future Trends

This paper presents a review on recent developments and future trends in zinc oxide thin film transistors (ZnO TFTs) together with challenges involved in this technology. It highlights ZnO TFT as next generation choice over other available thin film transistor technology namely a – Si: H (amorphous hydrogenated silicon), poly-Si (polycrystalline silicon) and OTFT (organic thin film transistor). This paper also provides a comparative analysis of various TFTs on the basis of performance parameters.  Effect of high –k dielectrics, grain boundaries, trap densities, and threshold voltage shift on the performance of ZnO TFT is also explained

Thin-film transistors fabricated using sputter deposition of zno

Development of thin film transistors (TFTs) with conventional channel layer materials, such as amorphous silicon (a-Si) and polysilicon (poly-Si), has been extensively investigated. A-Si TFT currently serves the large flat panel industry; however advanced display products are demanding better TFT performance because of the associated low electron mobility of a-Si. This has motivated interest in semiconducting metal oxides, such as Zinc Oxide (ZnO), for TFT backplanes. This work involves the fabrication and characterization of TFTs using ZnO deposited by sputtering. An overview of the process details and results from recently fabricated TFTs following a full-factorial designed experiment will be presented. Material characterization and analysis of electrical results will be described. The investigated process variables were the gate dielectric and ZnO sputtering process parameters including power density and oxygen partial pressure. Electrical results showed clear differences in treatment combinations, with certain I-V characteristics demonstrating superior performance to preliminary work. A study of device stability will also be discussed

High-performance Zinc Oxide Thin-Film Transistors For Large Area Electronics

The increasing demand for high performance electronics that can be fabricated onto large area substrates employing low manufacturing cost techniques in recent years has fuelled the development of novel semiconductor materials such as organics and metal oxides, with tailored physical characteristics that are absent in their traditional inorganic counterparts such as silicon. Metal oxide semiconductors, in particular, are highly attractive for implementation into thin-film transistors because of their high charge carrier mobility, optical transparency, excellent chemical stability, mechanical stress tolerance and processing versatility. This thesis focuses on the development of high performance transistors based on zinc oxide (ZnO) semiconducting films grown by spray pyrolysis (SP), a low cost and highly scalable method that has never been used before for the manufacturing of oxide-based thin-film transistors. The physical properties of as-grown ZnO films have been studied using a range of techniques. Despite the simplicity of SP, as-fabricated transistors exhibit electrical characteristics comparable to those obtained from ZnO devices produced using highly sophisticated deposition processes. In particular, electron mobility up to 25 cm2/Vs has been achieved in transistors based on pristine ZnO films grown at 400 °C onto Si/SiO2 substrates utilising aluminium source-drain (S-D) electrodes. A strong dependence of the saturation mobility on the work function of S-D electrodes and the transistor channel length (L) has been established. Short channel transistors are found to exhibit improved performance as compared to long channel ones. This was attributed to grain boundary effects that tend to dominate charge transport in devices with L < 40 μm. High mobility, low operating voltage (<1.5 V) ZnO transistors have also been developed and characterised. This was achieved through the combination of SP, for the deposition of ZnO, and thermally stable solution-processed self-assembling monolayer gate dielectrics. Detailed study of the temperature dependence of the operating characteristics of ZnO transistors revealed a thermally activated electron transport process that was described by invoking the multiple trapping and release model. Importantly, ZnO transistors fabricated by SP are found to exhibit highly stable operating characteristics with a shelf lifetime of several months. The simple SPbased fabrication paradigm demonstrated in this thesis expands the possibilities for the development of advanced simple as well as multi-component oxide semiconductors far beyond those accessible by traditional deposition methods such as sputtering. Furthermore, it offers unprecedented processing scalability hence making it attractive for the manufacturing of future ubiquitous oxide electronics

Electrical Characterization of Thin-Film Transistors Based on Solution-Processed Metal Oxides

This chapter provides a brief introduction to thin-film transistors (TFTs) based on transparent semiconducting metal oxides (SMOs) with a focus on solution-processed devices. The electrical properties of TFTs comprising different active layer compositions (zinc oxide, aluminum-doped zinc oxide and indium-zinc oxide) produced by spin-coating and spray-pyrolysis deposition are presented and compared. The electrical performance of TFTs is evaluated from parameters as the saturation mobility (μsat), the TFT threshold voltage (Vth) and the on/off current (Ion/Ioff) ratio to demonstrate the dependence on the composition of the device-active layer and on ambient characterization conditions (exposure to UV radiation and to air)

Nanowire Zinc Oxide MOSFET Pressure Sensor

Fabrication and characterization of a new kind of pressure sensor using self-assembly Zinc Oxide (ZnO) nanowires on top of the gate of a Metal-Oxide-Semiconductor Field-Effect Transistor (MOSFET) is presented. Self-assembly ZnO nanowires were fabricated with a diameter of 80 nm and 800 nm height (80:8 aspect ratio) on top of the gate of the MOSFET. The sensor showed a 110% response in the drain current due to pressure, even with the expected piezoresistive response of the silicon device removed from the measurement. The pressure sensor was fabricated through low temperature bottom up ultrahigh aspect ratio ZnO nanowire growth using anodic alumina oxide (AAO) templates. The pressure sensor has two main components: MOSFET and ZnO nanowires. Silicon Dioxide growth, photolithography, dopant diffusion, and aluminum metallization were used to fabricate a basic MOSFET. In the other hand, a combination of aluminum anodization, alumina barrier layer removal, ZnO atomic layer deposition (ALD), and wet etching for nanowire release were optimized to fabricate the sensor on a silicon wafer. The ZnO nanowire fabrication sequence presented is at low temperature making it compatible with CMOS technology

Atomic Layer Deposited Zinc Tin Oxide Channel for Amorphous Oxide Thin Film Transistors

Bottom-gate thin film transistors with amorphous zinc tin oxide channels were grown by atomic layer deposition (ALD). The films maintained their amorphous character up to temperatures over 500 $^{\circ}$C. The highest field effect mobility was ~13 $cm^2/V^.s$ with on-to-off ratios of drain current ~10$^9$-10$^{10}$. The lowest subthreshold swing of 0.27 V/decade was observed with thermal oxide as a gate insulator. The channel layers grown at 170 $^{\circ}$C showed better transistor properties than those grown at 120 $^{\circ}$C. Channels with higher zinc to tin ratio (~3-4) also performed better than ones with lower ratios (~1-3).Physic