The last fifteen years have witnessed a phenomenal growth in thin film transistor (TFT) technology, driven by an insatiable demand for larger and larger displays. Here, the TFT is a key building block that provides switching, driving, or read-out functions [1–4] in the active matrix underlying the liquid crystal or organic light emitting diode display. Indeed the flat panel display has been making rapid progress in providing interactive information retrieval as opposed to one-way information delivery.[5, 6] To keep up with this paradigm shift, the display must integrate in situ sensors to detect the external signal and yet maintain the thin and large area form factor, high resolution at low cost. In order to meet these myriad of technological challenges, there has been an active interest in high performance sensors based on the TFT structure, for process compatibility with the switching TFT in active matrix and offer the interactive solution.

An interactive display detects the presence of touch with a finger, hand or external photo-signal. Most interactive displays having touch functionality fall broadly into the following categories; resistive,[7] surface acoustic wave,[8] capacitive,[9, 10] infrared,[11] and photo-sensing [12, 13] depending on the working principle of touch functionality. Each type of touch functionality has its advantages and disadvantages; a summary of these have been discussed in a recent review article.[14] In general, resistive, surface acoustic wave, and surface capacitance touch methods have the intrinsic issue of RC delay,[14] which prevents scaling to large areas. An infrared touch-screen uses an array of bulky components such as XY infrared light emitting diode (LED) sources and photo-detector pairs around the edges of the screen to sense interruption. This arrangement is not only uneconomical, it does not lend itself to on-panel integration for the next generation display. A promising alternative is the photo sensing approach since it has the potential to eliminate RC delay issues, as long as the photo-sensor has high responsivity and high sensing speed enabling high frame rate applications. Considerable research efforts have been devoted to the development of various active semiconductors for photosensors, including organic materials,[15] nano structures,[16] and metal oxides.[17] To ensure stability, controllability, reliability, and process compatibility with the active matrix, the wide bandgap metal oxide semiconductor has merits over organic and other nano-materials based on a bottom-up approach, due to its intrinsically better electrical and optical properties.[18] This provides the device with the desired functionality, extending its adoption in new application areas. In our investigation, we examine photosensors in which the active semiconductor layer is amorphous metal oxide (a-MeO) in conventional bottom-gate TFT configuration. We address the sensitivity of the phototransistor and its process/structural compatibility with the TFT switch in the active matrix. Compared to an amorphous silicon (a-Si) based photo-TFT used as a reference sample in this work, the a-MeO photo-TFT has a much higher photocurrent and responsivity for a broad range of illumination intensities. It also has a high on-current (I on) in the dark state by virtue of its high mobility. These are important requirements for large area interactive displays without restrictions stemming from a low photo sensing margin or low current. The active structure of the a-MeO photo-TFT reported here is composed of a double layer, GaInZnO (GIZO) and InZnO (IZO), suitably optimized for photoresponse, extrinsic quantum efficiency, and stability. Figure 1a shows the cross-sectional transmission electron …