A STUDY ON CHARACTERISTICS OF ZINC OXIDE THIN FILM TRANSISTORS FABRICATED BY ATOMIC LAYER DEPOSITION

Abstract

Thin film transistors (TFTs) are used as switching devices in active matrix liquid crystal displays and image sensors. These devices are generally made with amorphous and polycrystalline Si. Recently, the application of Si-based TFTs has been expanded to next-generation display devices such as transparent circuits and flexible displays. However, in these new applications, Si-based TFTs have the disadvantages of light sensitivity, light degradation, low mobility, and opacity. One solution is to use an oxide semiconductor as a channel layer in the TFT. Oxide semiconductors exhibit higher mobility and transparency than amorphous Si, and lower deposition temperatures than polycrystalline Si. It has been suggested that ZnO be used as the channel layer in transparent TFTs. ZnO is a transparent semiconductor with a wurzite structure (a = 3.25 ?, and c = 5.12 ?) and a bandgap of 3.4 eV. ZnO-based TFTs are particularly interesting because of their potential applications in photodetecting devices, biosensors, light-emitting diodes, laser diodes, and flat panel displays. There have been several reports on the fabrication of ZnO-based TFTs by means of radio frequency magnetron sputtering, pulsed laser deposition, chemical vapor deposition, and a chemical solution process. However, these deposition methods require high process temperatures or an additional annealing step to obtain good transistor properties such as high mobility and low operation voltage. High temperature processes are, however, not suitable for flexible substrates or organic dielectric films. Therefore, in this study, we introduce an atomic layer deposition (ALD) method because of its several advantages over other techniques such as large area capability and good growth control in terms of homogeneity, composition, and thickness. Moreover, the ALD process can be carried out at very low temperatures. These characteristics are essential to a wide variety of applications requiring large area uniformity and a low thermal budget process. However, ZnO films grown by ALD exhibit a high carrier concentration (~1018 cm-3), which is not suitable for TFT applications. Therefore, it is necessary to adequately control the carrier concentration in order to build a high quality ZnO TFT. In this study, the carrier concentration of the channel layer was controlled by modifying the processing temperature and channel layer thickness. The characteristics of the ZnO thin films were examined at different deposition temperatures and channel layer thicknesses. Furthermore, ZnO TFTs were fabricated at various temperatures and the channel layer thicknesses were varied from 25 to 55 nm. Their electrical properties were evaluated to determine the proper process temperature ranges and channel layer thicknesses for the creation of active channel layers.