Growth and characterization of amorphous In-Zn-O and polycrystalline Al doped ZnO transparent conducting oxide thin films and their applications for next-generation optoelectronic devices

Abstract

Transparent conducting oxides (TCO) exhibit high electrical conductivity (>1000S/cm) and high optical transparency (>80%) in the visible to NIR region (300-1100nm). The combination of high electrical conductivity and high optical transparency makes them suitable for numerous applications in liquid crystal displays (LCDs), photovoltaic cells (PV), thin film transistors (TFT), photo-detectors, photo-diodes, and electrochromic devices. Sn-doped In2O3 (ITO) is the industrial accepted TCO due to its remarkable optoelectronic properties. However, the scarcity and high cost of In in ITO is a major challenge. Moreover, the conventional TCOs also suffer from low carrier mobility, high residual stress and high carrier density for next-generation flexible optoelectronic devices. The above background forms the basis to understand the growth kinetics of TCO thin film for their controlled and tunable optoelectronic properties, material performance and the development of high mobility TCO thin films for the next generation optoelectronic device applications. A unique RF superimposed DC sputtering technique was used to tailor the plasma during the sputtering process and to control the kinetics for the growth of amorphous In-Zn-O and Al-doped ZnO thin films. The growth dynamics were controlled to achieve high optoelectronic properties, low surface roughness and low residual stress for flexible organic light emitting diodes. We correlated surface roughness-residual stress-optoelectronic properties of amorphous indium zinc oxide (a-IZO) thin films using a statistical design of experiment (DOE) approach and found a common growth space to achieve a smooth surface in a stress-free and high optoelectronic merit a-IZO thin films. The sputtering power, growth pressure, oxygen partial pressure, and RF/(RF+DC) power ratio were varied in a two-level system with a full factorial design and results were used to deconvolve the complex growth space. We have identified the significant control growth parameters and their possible interactions. These results demonstrated the utility of the DOE approach to multi-parameter optimization, which provides an important tool for the development of flexible TCO thin films for the next-generation flexible optoelectronic device applications. The optimized high mobility a-IZO thin film could also be a potential material to replace amorphous Si for flexible and high performance flexible thin film transistor. However, the a-IZO thin film shows high carrier density which is the main obstacle to utilize it for flexible and transparent thin film transistors. Carrier suppression without deteriorating the carrier mobility is a key challenge in metal like carrier density of a-IZO. We studied the role of oxygen for confining the behavior of boron dopant as a carrier suppressor and as a carrier donor in a-IZO thin films. The carrier density of amorphous IZO thin film was reduced to the order of ~1016 cm-3 with carrier mobility of ~9 cm2/Vs by precisely controlling the boron and oxygen content in a-IZO thin films. The electrical properties revealed that boron requires an extra presence of oxygen to activate its carrier suppressant nature. The post deposition annealing of IZO and boron doped a-IZO (BIZO) thin film in air confirmed the better thermal stability of BIZO over IZO thin film. This study proposes the appropriate use of B as a carrier donor and a carrier suppressor in IZO matrix as a robust, reliable and thermally stable active layer for flexible and stable electronic device applications The high cost and scarcity of indium have made aluminium doped zinc oxide (AZO) an important material which might be used as a replacement for tin-doped indium oxide (ITO). However, the low carrier mobility in AZO thin film is a long-standing issue. The energy and flux of high energetic ions were controlled by RF superimposed DC sputtering process to increase the grain size and suppress grain boundary potential with minimum residual stress in Al-doped ZnO (AZO) thin film. The nanoscale grain boundary potential was mapped using Kelvin Probe Force Microscopy in AZO thin film and it was observed that carrier mobility is controlled not only by grains size but also by grain boundary potential. Beyond the conventional approaches, we have also utilized a seed layer assisted growth of AZO thin film to achieve a two-fold enhancement in carrier mobility. The analysis of defect chemistry and their nanoscale distribution confirms that seed-layer help to achieve low defects in seed layer assisted grown AZO thin film and mainly responsible for enhanced mobility. Finally, the developed TCO thin films in the present work were used for the transparent contact application for flexible gallium oxide (Ga2O3) based solar-blind and MoS2 based UV-Vis-NIR broadband photodetector application. We demonstrated the high-performance of the a-IZO electrode for Ga2O3 based flexible solar-blind photodetector in contrast to a conventional metal electrode. The AZO electrode also showed an enhanced performance of photodetector. Both the devices showed excellent photodetector performance in contrast to their metallic competitor and found their niche to be used in next-generation optoelectronic devices.