Paper EM-ThP6
Electrical Properties of Atomic Layer Deposited ZnO Thin Film Transistor with Various Channel Layer Thickness

Thursday, October 23, 2008, 6:00 pm, Room Hall D

Wide band gap II-VI compound semiconductors have attracted much attention because of many potential applications for optoelectronic devices. Among these, ZnO thin films have been particularly interesting due to their potential applications in optoelectric devices, such as light-emitting diodes, laser diodes, and transparent thin film transistor. Especially, ZnO-based thin film transistors (TFTs) have been intensively studied for flexible electronics, replacing conventional amorphous-Si TFT. Recently, there have been several reports on the fabrication of ZnO TFTs by means of rf magnetron sputtering, pulsed laser deposition, and chemical solution process. These methods need high process temperature or additional annealing process to obtain good transistor properties such as high mobility, low operation voltage. However, high temperature process is not suitable to apply for flexible substrates or organic dielectrics. Hence, we choose atomic layer deposition (ALD) method because ALD offers several advantages over the other techniques such as large area capability and good growth control in terms of homogeneity, composition, and thickness. Moreover, ALD process can be performed at low temperature. These merits are fundamental for a wide variety of applications requiring low thermal budgets. However, atomic layer deposited ZnO films exhibit high carrier concentration (n ~ 10¹⁸ cm^-3). The high channel carrier concentration in the oxide semiconductors is not suitable for TFT applications. Therefore, it is necessary to control the carrier concentration in order to build good quality ZnO-TFT. In this study, we varied thickness of ZnO in order to find optimal condition for thin film transistor. ZnO film on SiO₂/p-type Si substrate was deposited by ALD method using diethylzinc (DEZn) and H2O at 110 °C and the thickness of ZnO film were varied from 40 nm to 70 nm as a function of ALD cycles. The crystallographic orientation of the ZnO films was determined by an X-ray diffractometer (XRD) with Cu Ka radiation and the film morphology was analyzed by Atomic force microscopy (AFM) measurements. Device characterization was carried out at room temperature in the dark using an Agilent B1500A Semiconductor Analyzer. As a result, Ion/Ioff ratio was observed in 10²~10⁶, saturation mobility was observed in 0.02 ~ 1.4cm²/Vsec.