Material deposition and laser annealing of metal oxide thin films for electronics fabricated at low temperature

摘要

With an aim to investigate methods to realise low thermal-budget fabrication of aluminium doped zinc oxide (AZO) and indium gallium zinc oxide (IGZO) thin films, a dual step fabrication process was studied in this research. Initially, an experimental programme was undertaken to deposit AZO and IGZO films by radio frequency (RF) magnetron sputtering with no external substrate heating and at a wide range of deposition parameters including oxygen to argon ratio, RF power, and sputtering pressure. Thereafter, the samples were subjected to post-depositing annealing in air at ambient temperature utilising the advantages of excimer laser annealing (ELA) with a pulsed krypton fluoride (KrF) excimer laser at different laser fluences and number of pulses. The electrical, structural, compositional, and optical properties of the fabricated samples were systematically investigated as a function of the fabrication (deposition and annealing) conditions. A range of thin film characterisation techniques was used including 4-point probe (4PP), Van der Pauw (VDP), Hall Effect, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Atomic-force microscopy (AFM), Energy-dispersive X-ray spectroscopy (EDX), and optical transmittance and reflectance spectroscopy. Sputter-deposition of AZO and IGZO at room temperature revealed that the electrical properties of the deposited films are profoundly controlled by the deposition conditions applied. Low sputtering pressure of 2 mTorr is desired to obtain the best quality materials. However, high RF power of 180 W (4 W/cm2) is required to produce AZO with enhanced crystallinity, high electron density, and thus low resistivity. While, moderate RF power of 50 W (1.1 W/cm2) is applied to produce amorphous IGZO films with moderate-to-high resistivity suitable for thin film transistors (TFTs). The oxygen to argon ratio is found to have the most significant impact on defining the electrical properties for both AZO and IGZO. The resistivity of IGZO films was dependant on their metallic composition which in turn is controlled by the deposition conditions. TFTs were fabricated on silicon substrates with 40 nm thick IGZO as the active layer deposited at room temperature and different growth conditions. TFT performance was largely affected by the active layer deposition conditions. TFTs with the optimised IGZO, deposited at 50 W and 2 mTorr of 2% oxygen to argon ratio, exhibited a field effect mobility of 0.67 cm2/Vs, an on/off current ratio of 5x105, a turn on voltage of -0.15 V, and a subthreshold swing S of 0.28 V/decade. Upon ELA, AZO showed a resistivity reduction which is shown to result from increasing both the free electron density and mobility. When the optimised as-deposited AZO, 180 nm thick deposited at 180 W and 2 mTorr of 0.2% oxygen to argon ratio, annealed with 5 pulses at 125 mJ/cm2, a 50% resistivity reduction to 5x10-4 Ω.cm was obtained. It was demonstrated that average grain size increase, oxygen related defects decrease, and aluminium activation in doped ZnO are the origin of the AZO resistivity reduction upon ELA. Rapid thermal annealing (RTA) was also examined on AZO; RTA in nitrogen at 300°C for 20s increased the AZO gain size and doping efficiency leading to similar resistivity reduction to that achieved by the optimised ELA. Both ELA and RTA enhanced the AZO visible transmission to > 85 %, while the near infrared transmission was degraded due to higher electron density after annealing. The electro-optical properties of the optimised AZO samples obtained by ELA and RTA, which are very close to those of standard tin doped indium oxide (ITO), demonstrate the viability of AZO as an attractive transparent conducting material for various electronic applications. The potential use of AZO for photovoltaics (PVs) as well as the AZO stability against damp heat exposure were also examined. PVs with optimised ELA and RTA treated AZO samples showed comparable power conversion efficiency (PCE) to that of PVs with high-quality commercial ITO. The damp heat stability of AZO samples was strongly dependant on the fabrication conditions. In regard to IGZO, ELA increased the free electron density and mobility leading to better conductivity, while the amorphous structure is maintained. ELA with single pulse at a low energy density of 30 mJ/cm2 resulted in an improved performance for IGZO TFTs on silicon substrates achieving a field effect mobility of 3.33 cm2/Vs, an on/off current ratio of 3x107, a turn on voltage of +0.35 V, and a subthreshold swing S of 0.27 V/decade. Moreover, ELA was successfully applied to IGZO TFTs on polymer flexible PEN leading to TFTs with enhanced performance. Hence, a combination of RF magnetron sputtering at room temperature and ELA, which are both efficiently applicable to thin films mass production, has been demonstrated to provide a low thermal budget fabrication route for functional materials including AZO, as the most promising substitute to ITO in a wide range of applications, and IGZO as the most attractive material for TFT applications. This combination is an alternative thin film fabrication route to using elevated substrate temperature or post-deposition thermal annealing typically applied in the dominant literature reports, to obtain thin films with suitable characteristics.