Processing, Structure, and Transistor Performance: Combustion versus Pulsed Laser Growth of Amorphous Oxides

Abstract

Solution-phase growth of amorphous oxide semiconducting films will likely provide significant advances toward the realization of affordable, flexible, transparent electronics. However, solution processing methods are still under development, and the bulk of amorphous oxide studies investigate films grown via physical vapor deposition (PVD) methods. To leverage the existing knowledge base on amorphous oxides, this study directly compares one of the most promising solution processing techniques, combustion synthesis (CS; spin and spray methodologies), with a more established PVD growth technique, pulsed laser deposition (PLD). The roles of processing technique and composition are understood by coupling structural studies, including analysis of local bonding and density, with the existing literature of PVD amorphous oxide films. This work represents the first example of X-ray absorption spectroscopy analysis of spray combustion processing (Spray-CS) a-oxide films. Trends in film structure and transistor performance are compared in the amorphous In-Ga-O (a-IGO) system, across a broad In-Ga composition space, as calibrated by X-ray fluorescence. Semiconducting layers deposited by either CS or PLD are used as the channel layer in thin film transistors (TFTs) fabricated on SiO2/Si substrates. A drop in TFT saturation mobility, with increasing Ga content, for PLD and spin-coated combustion processing (Spin-CS) devices is understood to be the disruption of carrier mobility imposed by the disparate local structure around Ga, as opposed to around In. In contrast, saturation mobility for Spray-CS films is dominated by the processing method and cannot be tuned with composition. TFTs with PLD-derived channel layers exhibit the highest saturation mobility, 42 cm(2) V-1 s(-1) (maximum), which may be linked to their high density. Similar to mobility trends, TFT on-voltage rises with Ga content for Spin-CS and PLD-derived channel layers. A rise in on voltage is associated with decreasing carrier concentration induced by the dilution of low-oxygen-coordinated In centers, the source of carriers. As with mobility, on-voltage for Spray-CS derived TFTs is not well tuned with composition but is consistently near zero. Local structure studies reveal that the Spray-CS growth technique results in high In-O coordination levels, which explains the low carrier concentrations and resultant on voltage behavior. This work investigates, for the first time, the structural phase stability of solution-processed oxides through in situ glancing incidence X-ray diffraction annealing studies. Thermal phase stability, which informs processing parameters and device stability, is shown to increase with Ga content for all films.