Solution-processed ZnO thin films are attractive as active materials in thin film transistors (TFTs) for low-cost electronic device applications. However, the lack of true enhancement mode operation, low mobility, and unreliability in transistor characteristics due to the high density of traps and other defects present challenges in using such TFTs in circuits. We demonstrate in this report that the electrical characteristics of such TFTs can be improved by source injection barriers. Asymmetrical Schottky source metal-oxide-semiconductor field-effect transistors (MOSFETs) have been fabricated by utilizing heavily doped solution-processed ZnO as the active layer. n(+)-ZnO was obtained by using triethylamine as the stabilizer in the solution process instead of the more commonly used monoethanolamine. Au was chosen for source metallization to create a Schottky contact to the ZnO and an Al ohmic contact was chosen as the drain. Voltage applied to the gate induced field emission through the Schottky barrier and allowed modulation of the drain current by varying the width of the barrier. By operating the asymmetrical MOSFET when the Schottky contact is reverse biased, effective control over the transistor characteristics was obtained.
ZnO is an increasingly important wide bandgap semiconductor for optoelectronic applications. Solution processing provides a facile and inexpensive method to form ZnO thin films with high throughput. The sol stabilizer used in the solution processing of ZnO functions variously as a sol homogenizer, chelating agent, wettability improver and capping agent. In spite of its obvious importance in influencing ZnO film properties, a restricted set of short chain alkaline sol stabilizers have been used in prior reports. We examined the effect of six different sol stabilizers, including acidic and longer chain species, along with a recipe without any stabilizer, on the grain size, crystallographic texture, and resistivity of solution processed ZnO films on thermal oxide-coated silicon substrates, and found large variations in the structural and electrical properties as a consequence of the choice of sol stabilizer. We found that ZnO films formed using oleic acid as the sol stabilizer possessed a strong (002) preferred orientation with a Lotgering factor as high as 0.86. The key insight we obtained is that the sol stabilizer strongly influences the film surface area and activation energy barrier for inter-grain transport. We comprehensively studied the steady state and transient behavior of ZnO films deposited using different stabilizers and compared their lifetime and mobility-lifetime products. When exposed to illumination, the conductivity of the deposited films increased by several orders of magnitude. This is attributed to the trapping of the nonequilibrium holes by the surface adsorbed oxide species, which produces equivalent number of excess electrons in the conduction band. Impedance spectroscopy and C-V measurements were performed to calculate the doping of the ZnO thin films. ZnO thin film transistors were also fabricated and the effects of the sol stabilizer on the different parameters of the TFT like mobility and threshold voltage were investigated.
Solution processing (SP) is a cheap, simple and high-throughput method for the fabrication of ZnO thin film transistors (TFTs). Lack of enhancement mode operation, poor crystallinity, traps, and poor control of the carrier concentration are some of the disadvantages of this method. The high intrinsic electron concentration of SP-ZnO makes saturation of TFTs non-trivial. We report on Schottky barrier thin film transistors (SB-TFT). By biasing the source Schottky contact in reverse bias, a depletion region is formed around the source contact hence depleting the region from the free charge carriers which produces the saturation of the device. The effect of the Schottky contact is illustrated by comparing the operation of SB-TFTs with that of conventional TFTs.
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