Effect of sol stabilizer on the structure and electronic properties of solution-processed ZnO thin films

Abstract

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.

Large‐Area Schottky Junctions between ZnO and Carbon Nanotube Fibres

“…R S gives a low resistivity of ρ ZnO =70 Ωcm, which is in the range of high‐quality solution‐processed ZnO thin films (2.9–138×10 3 Ωcm), although smaller than previously reported values on hydrothermal ZnO (10 3 –10 5 Ωcm) …”

“…With knowledge of G it is possible to calculate μτ =0.04 cm 2 V −2 , which has been a useful parameter when comparing the transport of photogenerated carriers in ZnO materials produced under a range of synthesis conditions . It was calculated as 0.04 cm 2 V −1 , which is in the range observed for solution‐processed ZnO thin films …”

Nanoscale Patterning of Zinc Oxide from Zinc Acetate Using Electron Beam Lithography for the Preparation of Hard Lithographic Masks

“…The improvement in conductivity properties can be attributed to the reduction of carbon content, which has diffused out of the ZnO nanopatterns as carbon monoxide during the annealing process. The film annealed at 400 °C possesses an electrical resistivity of 91.5 Ω cm, and the same order of magnitude of electrical resistivity is obtained for the ZnO deposited by sol–gel spin-coating using zinc acetate. , The I – V curves for the zinc acetate after exposure shows a linear dependence, indicating an Ohmic behavior of the Au-ZnO-Au structure. Ohmic Au/ZnO contacts are frequently observed for air-exposed ZnO surfaces and have been attributed to an accumulation layer induced by OH adsorbates. , …”

High Breakdown Strength Schottky Diodes Made from Electrodeposited ZnO for Power Electronics Applications

“…The associated unintentional n-type doping is consistently reported to be higher than 10 17 cm −3 . 21,22 As a consequence, Schottky barrier contacts formed to solutionprocessed ZnO have, in general, much lower quality with much higher ideality factors (∼8−10) and leakier contacts (on−off ratio <10 4 ).…”

“…Solution-processed ZnO exhibits, in general, much lower quality than vapor-deposited ZnO made under vacuum conditions. , It suffers from higher densities of native defects and grain boundaries as well as higher levels of extrinsic contamination. The associated unintentional n-type doping is consistently reported to be higher than 10 17 cm –3 . , As a consequence, Schottky barrier contacts formed to solution-processed ZnO have, in general, much lower quality with much higher ideality factors (∼8–10) and leakier contacts (on–off ratio <10 4 ).…”