We are investigating conductive gallium nitride films grown on c-plane sapphire for use in a new area of application, high-power optoelectronics. It was found that optically-induced damage in gallium nitride-based transparent conductive thin films occurs at incident laser intensities significantly greater than in conventional metal-oxide based thin films. Furthermore, damage in gallium nitride epi-layers displays a unique morphology consisting of discrete, faceted pits which appear to initiate within fast-grown layers when exposed to high intensity near-infrared laser irradiation. We developed an integrated laser damage system with in-situ diagnostics to probe this damage mode and conducted damage tests of aluminum nitride and gallium nitride/aluminum nitride samples grown under various conditions. Through in-depth analyses using optical microscopy and results from high-throughput damage tests, this paper elucidates some of the prevailing damage processes and design considerations for gallium nitride transparent conductive films important for emerging high-power laser applications.
Transparent conducting films with superior laser damage performance have drawn intense interests toward optoelectronic applications under high energy density environment. In order to make optoelectronic applications with high laser damage performance, a fundamental understanding of damage mechanisms of conducting films is crucial. In this study, we performed laser damage experiments on tin-doped indium oxide films (ITO, Bandgap = 4.0 eV) using a nanosecond (ns) pulse laser (1064 nm) and investigated the underlying physical damage mechanisms. Single ns laser pulse irradiation on ITO films resulted in common thermal degradation features such as melting and evaporation although the laser photon energy (1.03 eV, 1064 nm) was smaller than the bandgap. Dominant laser energy absorption of the ITO film is attributed to free carriers due to degenerate doping. Upon multi-pulse irradiation on the film, damage initiation and growth were observed at lower laser influences, where no apparent damage was formed upon single pulse, suggesting a laser-induced incubation effect.
The ablation of magnetron sputtered metal films on fused silica substrates by a 1053 nm, picosecond class laser was studied as part of a demonstration of its use for in-situ characterization of the laser spot under conditions commonly used at the sample plane for laser machining and damage studies. Film thicknesses were 60 and 120 nm. Depth profiles and SEM images of the ablation sites revealed several striking and unexpected features distinct from those typically observed for ablation of bulk metals. Very sharp thresholds were observed for both partial and complete ablation of the films. Partial film ablation was largely independent of laser fluence with a surface smoothness comparable to that of the unablated surface. Clear evidence of material displacement was seen at the boundary for complete film ablation. These features were common to a number of different metal films including Inconel on commercial neutral density filters, stainless steel, and aluminum. We will present data showing the morphology of the ablation sites on these films as well as a model of the possible physical mechanisms producing the unique features observed.