Fast and Ultrafast Pulsed Laser Post-processing of Zinc Oxide Thin-films Deposited on a Thermally Resilient and Thermally Sensitive Substrate

Abstract

This manuscript is split into two projects. The first project is on nanosecond laser annealing of radio-frequency magnetron sputtered zinc oxide thin-films on a silicon dioxide/silicon substrate. The films were sputtered to be approximately 100 nm thick and of a mostly amorphous nature with the composition theorized to be composed of pockets of noncrystalline grains embedded in an amorphous matrix. The samples were then laser annealed using four wavelengths at 1064 nm, 532 nm, 355 nm, and 266 nm over a range of fluences (20-3000 mJ cm−2) with the limiting end corresponding to ZnO ablation for that wavelength. In the case of 1064 nm and 532 nm the fluence range was extended to include near silicon ablation. They were then evaluated using X-ray diffraction (XRD), Rutherford backscattering (RBS), and scanning electron microscopy (SEM). A simulation was also run to investigate peak temperature reached during the lifetime of the pulse for each wavelength and fluence. The X-ray diffraction results showed no positive change in the crystalline nature of our films for wavelengths below the bandgap (1064 and 532 nm). The above bandgap wavelengths (355 and 266 nm) showed very strong crystallization, reaching a highest grain size of approximately 33 nm, a notable grain growth from the 17 nm as-sputtered sample for 355 nm. Rutherford backscattering results displayed an approximate 1:1 oxygen/zinc ratio using the 355 nm wavelength at the highest fluence, similar to the oven annealed set. For 266 nm there was a trend for over-saturation of oxygen in the films, showing a approximately 1.4/1 O/Z ratio. Scanning electron microscopy showed spherical nanostructures of various sizes appearing during irradiation with the 355 nm wavelength, with a relatively smooth and homogeneous surface in between. While the 266 nm wavelength showed both spherical and rope-like nanostructures along with nano-holes pitting the surface. The theoretical results showed that the UV wavelengths and fluences chosen probed a wide range of temperatures from low temperature to near melting for ZnO. The laser annealing took only two minutes compared to the roughly 4+ hour oven annealing process. Overall, notable improvement to the films structural, crystallographic, morphological, and chemical changes were made from laser annealing at a 355 nm wavelength for a fluence range between 160-240 mJ cm−2. This clearly shows that laser annealing at this wavelength and fluence range iiis the preferred alternative for post-processing as opposed to the furnace. Sol-gel spin-coated samples were also laser irradiated in this experiment but no discernible improvements can be seen attributing to the introduction of chemical contaminants inherent to the process. The second project is on femtosecond laser annealing of radio-frequency magnetron sputtered zinc oxide thin-films deposited on a indium tin oxide/polyethylene terephthalate substrate (plastic). The smaller pulse width, in comparison to the nanosecond laser, will be used to induce a more localized, controlled heating to avoid damaging the underlying plastic substrate in the laser annealing process. The ZnO films were sputtered to be approximately 100 nm thick and of the same nature as the last project. To determine what fluence should be used to laser anneal these samples at the ultra-fast timescale, a hybrid two temperature with carrier dynamics model was constructed. This model combines the two temperature model with the 1D heat diffusion model to create a more computational efficient simulation while still creating simulated results agreeable with experimental outcomes. After, this fluence range was then experimentally used to laser anneal our samples at a wavelength of 266 nm using a femtosecond pulsed Ti:Saphire laser. The resulting structural, crystallographic, and morphological characterization was done with X-ray diffraction and atomic force microscopy. It was shown that improvements to the film quality was made using our chosen fluences from X-ray diffraction, showing results comparable to low temperature oven annealing (≈ 300 °C) which cannot be achieved by direct placement of the composite into the furnace. Atomic force microscopy shows a relatively low RMS surface roughness was maintained while producing ordered nano-pillar structures on the film. The results of this project show that the low heat affected zone inherent to ultrafast laser post-processing is a key component of this viable method to treatment of films or devices based on fragile, low-temperature substrates such as plastic which cannot be post-processed in conventional methods like furnace annealing.