AVS 64th International Symposium & Exhibition
    Electronic Materials and Photonics Division Wednesday Sessions
       Session EM-WeM

Paper EM-WeM10
On the Abnormality in Mobility of ZnO Thin Film Transistors Based on Sol-Gel Deposited Channel Layers

Wednesday, November 1, 2017, 11:00 am, Room 14

Session: Charge Transport in Disordered Materials
Presenter: Vahid Mirkhani, Auburn University
Authors: V. Mirkhani, Auburn University
K. Yapabandara, Auburn University
S. Wang, Auburn University
M.P. Khanal, Auburn University
S. Uprety, Auburn University
M.H. Sk, Qatar University, Qatar
A. Ahyi, Auburn University
M.C. Hamilton, Auburn University
M. Park, Auburn University
Correspondent: Click to Email
Extensive research has been conducted on zinc oxide (ZnO) thin film transistors (TFTs) for the last couple of decades. Solution processes such as sol-gel are popular fabrication methods among researchers due to their simplicity and economical aspects. We have observed an abnormality in transconductance and mobility curve of the TFT with a channel layer prepared through sol gel spin coating process. Transconductance and mobility curves, determined from transfer characteristics, exhibit two peaks, instead of an expected single peak. We have rationalized the phenomenon by pondering the ZnO-ZnO interfaces. During the ZnO thin film deposition process, the spin coating and calcination steps are repeated multiple times in order to achieve a desirable layer thickness. When a layer is deposited, prior to the second spin coating step, the vacancies on the layer surface (comprised of grain boundaries of different grains) act as active sites, adsorbing molecules from the ambient. The adsorption of gasses such as oxygen and water molecules on ZnO structure surfaces has been studied for several decades; although the detailed kinetics and chemical reactions are debatable. Moisture and oxygen may be chemisorbed on the surface by receiving an electron and form a trapped negative ion on the surface, forming a depletion region at the surface of the layer. Oxygen may capture an electron and react with oxygen vacancies (VO and VO+), in order to form chemisorbed ions such as O–, O2– or O2–, whereas H2O may form OH– on the surface. Potential barriers created at the interlayer interfaces are wider and higher than the regular barriers created at the grain boundaries in the bulk. The interlayer interface depletion layers formed by negative ions will not be affected by the annealing process due to their stability at room temperature. Thus, the initial ZnO-ambient interface (prior to the next layer-deposition process) evolves into the ZnO-ZnO interlayer interface. The significance of the depletion layers at the ZnO-ZnO interfaces becomes evident when the carriers are not able to drift across these interfaces at gate voltages smaller than a critical voltage (VC). As the gate voltage increases, the width of the depletion regions decrease and carriers are able to tunnel through the barriers with ease and utilize all the deposited layers as a single channel and hence, an increase in the transconductance and mobility and later, a second peak. It is proposed that the first peak is related to the formation of the channel in the top layer and the second peak is assigned to all the deposited layers acting as a single-layer-channel.