The optical damage performance of electrically conductive gallium nitride (GaN) and indium tin oxide (ITO) films is addressed using large area, high power laser beam exposures at 1064 nm sub-bandgap wavelength. Analysis of the laser damage process assumes that onset of damage (threshold) is determined by the absorption and heating of a nanoscale region of a characteristic size reaching a critical temperature. This model is used to rationalize semi-quantitatively the pulse width scaling of the damage threshold from picosecond to nanosecond timescales, along with the pulse width dependence of the damage threshold probability derived by fitting large beam damage density data. Multi-shot exposures were used to address lifetime performance degradation described by an empirical expression based on the single exposure damage model. A damage threshold degradation of at least 50% was observed for both materials. Overall, the GaN films tested had 5-10 × higher optical damage thresholds than the ITO films tested for comparable transmission and electrical conductivity. The route to optically robust, large aperture transparent electrodes and power optoelectronics may thus involve use of next generation widegap semiconductors such as GaN.
© 2016 Optical Society of America
The optical laser damage performance of widegap semiconductors under peak fluence operating conditions and lifetime exposures can limit optoelectronic devices used in high rep rate, high power lasers , in power laser diodes , in photoconductive switches for high power radiation generation [3, 4], in Pockels cell for optical switching and amplification , and in optical light valves for beam shaping technologies . Transparent electrode development to date has mostly focused on improving light transmission and film electrical conductivity to minimize losses [7, 8]. However, there is an emerging need to also increase the optical damage performance of large aperture transparent conductive and photoconductive films to support increasingly more powerful power optoelectronics operating at high switching rates and laser intensities. In particular, illuminated semiconductor materials can be key failure points in those devices because of free carrier and light absorbing defects, even in wide bandgap materials exposed to sub-bandgap photon energies . When subjected to repeated pulse laser exposures, the “optical incubation” of widegap semiconductors significantly lowers their damage fluence threshold , which is their most relevant lifetime performance criterion. On the other hand, single pulse exposure allows probing the fundamental mechanisms of light absorption resulting in damage initiation or degradation. In crystalline semiconductors, nanoscale damage precursors are related to optically active crystal defects [11–13]. These defects mostly exist due to processing or growth conditions and are therefore responsible for extrinsic damage, but attribution of laser damage to specific defects remains elusive. Therefore, a better understanding and characterization of defects contribution to optical damage would help push semiconductors damage threshold up to their intrinsic limits when exposed to sub-bandgap photon energies. That limit is determined by multiphoton or avalanche ionization from exposure to high laser intensities . However, in practice, that limit is never achieved on the scale of a device area because intentional or un-intentional defects in semiconductors are a source of light absorption [15, 16].
In this letter, we focus on the optical damage performance of two important semiconducting materials that can currently be scaled to large aperture transparent electrodes or photoconductive crystals. The first, Tin-doped Indium Oxide (ITO), is widely used due to its wide bandgap (Eg = 4.0 eV), high light transmission, and high electrical conductivity. ITO is typically coated by physical vapor deposition as a thin polycrystalline layer with grain sizes on the order of the film thickness. ITO represents thus a baseline semiconductor material for comparison to other widegap conductive materials used as transparent electrodes. The second is doped Gallium Nitride (GaN), a next generation semiconductor that is increasingly used in power electronics because of its wideband gap (Eg ~3.4eV), thermal stability, high mobility, and high electric field breakdown . GaN is typically grown epitaxially by chemical vapor deposition above ~1300K on top of an Aluminum Nitride (AlN) buffer layer on sapphire substrates, which results in a significant number of dislocation defects near that lattice mismatched interface.
ITO and GaN laser damage morphologies from single and lifetime laser pulse exposures were addressed recently contrasting their laser damage modes and mechanisms. Degenerately doped ITO with ~1020-1021 cm−3 free carrier density, Ne, exhibits bulk thermal degradation beginning with film thermomechanical cracking, melting, and evaporation as the beam fluence is increased. For such ITO films, free carrier absorption is the dominant damage mechanism involving rapid free electron-phonon relaxations and lattice thermalization on time scales ~1-10 ps. In contrast, for GaN films with Ne ~1019 cm−3 laser damage occurs by highly localized absorption from interfacial defects and clusters  resulting in explosive material ejection and formation of pits . Most laser damage and lifetime studies on GaN have focused on damage well above threshold for the purposes of laser processing of GaN in the sub-ps pulse width regime [19, 20]. Therefore, in a fluence regime that is irrelevant to optoelectronic devices. Similarly, ITO laser damage studies have focused on laser patterning of ITO films above threshold [21–24]. Single pulse laser damage studies of ITO were performed for a limited set of pulse conditions, pulse lengths, or on limited areas [21, 25–28]. An analysis that is based on large test areas with a high power laser, over a range of pulse lengths, that includes lifetime performance tests has been missing. Furthermore, we include analysis on two semiconductor model systems with radically different absorption mechanisms, in part, to emphasize the generality of the physical processes involved. Such tests can ultimately help rationalize the optical damage performance of a broad range of semiconductors under practical conditions.
This study addresses the fluence dependent spatial density of GaN damage for a range of fluence energies and pulse lengths, from which a damage threshold probability distribution is derived along with a number density of absorbing defects, Nd. The damage threshold probability distribution can be related to a distribution of the size of the absorbing regions and their absorption coefficient, α , which is specific to a material or process. For the ITO used, such distribution does not apply since the film damages in bulk and not at discrete absorption sites, yet, a characteristic absorption size can still be applied to model ITO films damage. The temporal scaling of the damage threshold is described, along with the lifetime trends using a simple energy balance model for absorbing defect clusters or regions of absorption within the films. This model assumes specifically that the onset of apparent damage (i.e., threshold) is determined by reaching a critical temperature, Tc, from nanoscale absorbing regions of a characteristic size [29, 30]. The lifetime performance damage thresholds are analyzed by introducing an empirical relation describing material “fatigue” using an incubation coefficient, S, that captures the material defect accumulation from repeated pulse exposure cycles. That relation is still based on a physical model of the single pulse threshold allowing, in principle, extension of lifetime optical damage performance models to a range of materials and optical device conditions. A stable device operating fluence, F(∞), is also extracted. In general, thus, the analysis of the optical damage performance presented here on GaN and ITO are likely applicable to other widegap semiconductors.
2. Experimental method
Optical damage performance and laser damage threshold values of ITO and GaN film samples were determined using laser pulse exposures at normal incidence and a λL = 1064 nm (Ev = 1.16 eV) corresponding to the Nd:YAG laser fundamental wavelength. Damage threshold values are determined based on observations of film transformation by optical microscopy post-exposure. The cumulative damage probability curves, g, were obtained by exposing 10 sites to a given fluence, and counting the number, n, of damaged sites such that g(τp, F) = n/10 using a laser system with a Gaussian beam of ϕ = 0.65 mm diameter (1/e2) of various pulse widths . Lifetime tests were performed with the same laser at 10 Hz rep rates (τp = 9.5 ns) up to N = 105 exposures (power stability ± 3%) on commercially available ITO and GaN samples from various sources. Picosecond pulse lengths damage threshold values were determined using another laser systems with Gaussian beams ϕ ~0.15 mm dia. (1/e2) described elsewhere . Single large beam exposures up to ~1 cm2 and fluences, F, up to ~45 J/cm2 were carried out using a high power laser system with beam diagnostics described previously . The resulting fluence and damage site maps were registered to derive damage density curves, ρ(F) , such that a significant number of sparse absorbing defects on GaN samples could be probed at pulse lengths of τp = 9.5, 3, and 0.3 ns. All pulse temporal profiles used were Gaussian (FHWM). In general, damage trends with fluence were very similar between samples, although lifetime performance could vary by orders of magnitude, even within samples of the same batch, indicating limited control in the production of optically robust samples, especially for GaN films. ITO films with thickness, d, ranging from 10 to 100 nm on five mm thick glass substrates were used as-received with Ne on the order of 1020-1021 cm−3 and mobilities, µe ~-10 cm2(Vs)−1 as determined from Hall Effect measurements. Intentionally doped (Silicon) GaN films with thickness ranging from 2 to 5 µm included a 300 nm AlN buffer layer on top of the 500 µm thick sapphire wafers.
3. Results and discussion
3.1 Laser damage morphology
GaN samples were used as-received and as grown with Ne ~1019 cm−3 and µe~-100 cm2(Vs)−1. Sheet resistances, Rs, ranged from 10 to 100 Ohm/sq for both GaN and ITO samples, and total transmission values of ~75-90% were comparable at 1064 nm as determined from transmission spectroscopy. Thicker GaN films had higher mobility values but much lower Ne than the ITO films by a factor of ~10, resulting in Rs values that were comparable for GaN and ITO samples given the scaling of Rs ~1/Neµed. Still, laser energy absorption scales as ~exp[-(Ne/µe)d], hence free carrier absorption induced significant bulk heating in ITO films. In contrast, in GaN films with much lower Ne, localized defect driven absorption was the dominant damage mechanism .
The differences in laser damage morphology are clearly visible between ITO and GaN films in Figs. 1(a) and 1(b), respectively. The corresponding laser beam fluence maps are included in Figs. 1(c) and 1(d). In the ITO film case, the extent of bulk ablation and evaporation film removal is, as expected, directly related to the “hot” areas of the damage fluence map. In areas of high beam fluences complete removal down to the substrate was observed, while in others some film remained within the lower fluence areas. In GaN film, the number density of µ-sized damage pits increased with the local beam fluence.
3.2 Damage threshold probability distribution
The range of fluences accessible for a given exposure and pulse length is determined by the inherent beam spatial contrast (Figs. 1(c) and 1(d)). This beam contrast is exploited here to derive the ρ(F) data set shown in Fig. 2(a) for τp = 9.5, 3, and 0.3 ns. Due to the variability in GaN samples characteristics and the limited area available for large beam tests on 54 mm round samples, only a few pulse lengths and fluence ranges could be probed down to a minimum of 300 ps for large beam tests. The ρ(F) data was fitted assuming a single Rician damage threshold probability distribution of absorbing GaN defect clusters with shape parameters (σ, ν), f(F |σ, ν). The Rician distribution is similar to a Gaussian which fits laser damage data well [25, 34]. However, unlike a Gaussian, a Rician has the benefit of vanishing at zero fluence. The product of the cumulative f and Nd defect density gives the damage density for F, with Nd, σ, and ν as fitting parametersFig. 2(a)). This Nd variability occurs for at least two reasons. The first may involve variability in defect distributions within the film area. The second maybe related to pulse length conditions discussed below that produce a number of apparent damage events depending on the size of the absorbing region. Notably, some GaN samples damaged at much lower fluences (<1 J/cm2) with Nd up to 106/cm2 (data not shown). Such samples were always associated with much higher Y-band UV photoluminescence emitted from interfacial areas near the film-substrate region, potentially due to emissions and absorption from carbon complex impurities [18, 35, 36]. In the results shown in Fig. 2(a) some areas of the samples were exposed to fluences < 5 J/cm2 near the edge of the beam (Fig. 1(d)) but no damage was apparent either due to a lack of damage detection sensitivity, or to the presence of fewer such defects within the areas addressed by the beam at these fluences. The damage probability distributions derived from the fit using |Eq. (1), P = f(F |σ, ν), get narrower for shorter pulses (Fig. 2(b)). This narrowing of the distribution is consistent with laser damage of KDP crystals, where peak absorption was predicted to depend on inclusion size [12, 37, 38]. Figure 2(c) illustrates this point as follows. A large absorbing volume of radius, R, is exposed to either a short (τp1) or a long pulse (τp2) resulting in thermal diffusion lengths, L1 and L2, that are both much smaller than R. In this case, most of the temperature increase is determined by the total energy absorbed given approximately by the absorption cross section, beam intensity, and pulse duration. Thus, for a given beam intensity, a critical damage temperature Tc is reached for the longer pulse but not for the shorter pulse for which less energy is accumulated. In Fig. 2(d), when R is significantly smaller than L, such as for L2, the heat affected zone ~R + L becomes too large for the energy absorbed to drive the temperature up to Tc. In that case, Tc is only reached for the shorter pulse where the heat generation remains confined to the absorption region when L1 <<R. The corresponding temperature rise is tracked as T1and T2 in Figs. 2(c) and 2(d) for pulse durations τp1, and τp2 respectively. This interpretation gives a rational for the experimentally derived damage probability distribution, essentially by stating that different sized absorbers in the distribution determine the threshold for different pulse lengths, hence the range of Nd observed for different τp. A more rigorous approach would include the Mie scattering contribution, however, there’s still no index data for the absorbing defects regions to derive, a priori, a shape for P = f(F) based on a size distribution of R.
3.3 Damage threshold pulse scaling
Following characterization of the spatial scaling of laser damage above, the temporal pulse scaling is addressed next. Threshold intensities as a function of pulse lengths from ps to ns are shown for both ITO and GaN in Fig. 3. Remarkably, although the experimental threshold intensity values are much lower for ITO than GaN due to their different absorption mechanisms, both follow similar non-monotonic trends. This suggests that similar heat induced failure processes are occurring for laser damaged ITO and GaN. Based on an approximation of the heat flow equation solution  that assumes a uniform temperature rise, ΔT, within an absorbing region with the same thermal properties as its surrounding, the threshold temperature Tc can be predicted from18, 19, 39]. Film absorption was measured by transmission and ellipsometry spectroscopy for ITO giving αITO = 5300 cm−1, and a similar value was assumed for GaN defect absorption, αGaN = 5000 cm−1. The choice of Tc in ΔT, becomes problematic since the threshold optical failure may involve reaching a different Tc associated with specific thermally activated process, including thermomechanical failure, point defects formation, melting, explosive boiling and, perhaps, complex dynamic effects where positive feedback loops (ex. bandgap collapse) can drive absorption to the point of catastrophic optical damage . We assume here that the onset of damage is determined at Tc at the melting point of ITO Tm = 1800K, and GaN at Tm = 2800K. The selection of Tm as a Tc is partially justified because catastrophic optical damage in other III-V semiconductors were found to involve local melting from heating as a result of non-radiative recombination initiated at defects . Furthermore, our own imaging reveals melting in region of laser damage in both ITO and GaN .
Fixed thermal material properties from published sources were used for both ITO and GaN in Eq. (3) K = 11 W/m.K, D = 4.6 × 10−6 m2/s and K = 128W/m.K, D = 4.3 × 10−5 m2/s for ITO and GaN, respectively. Using the melting point as a criterion for optical threshold failure, the pulse length dependent threshold curve predictions from Eq. (3) agree with the upper limit of the data for both ITO and GaN films as represented by the solid lines in Fig. 3. Lower bound values represented by dashed lines were obtained by using a lower Tc = 1000K for ITO and a Tc = 1500K for GaN, which may be related to a thermomechanical stress threshold for ITO , or a thermal runaway in GaN [13, 40–43]. At damage onset the affected region in ITO films often appeared dark with a dense network of fractures without apparent grain melting . For GaN, such darkening was not apparent and explosive eruptions occurred at damage onset. Interestingly still, this model approximately captures the threshold pulse scaling behavior from picosecond to nanosecond time scales and, qualitatively at least, describe the laser damage of both ITO and GaN by simple consideration of the energy flux balance confined to a discrete laser light absorbing region. Similar results were noted for dielectric materials based on a thermomechanical stress criterion . Photon absorption by free carriers occurs on sub-ps time scales, but the ensuing electron-lattice thermalization occurs over ps or longer times, while the damage processes takes nanoseconds or microseconds to evolve [14, 44]. Although we have not used sub-ps pulse exposures, our model suggests that full thermalization and permanent onset of damage for semiconductors extends down to at least 1 ps laser pulse excitation, with thermally activated transformations near the melting point as a critical temperature. This is consistent with previous results on dielectrics . Finally, from Eq. (3) the Fth ~τ1/2 pulse scaling rule arises naturally and can be applied to the optical damage performance of both ITO and GaN as a good approximation for ps to ns pulse widths exposures.
3.4 Lifetime optical damage threshold
Following measurements and analysis of single pulse scaling thresholds, we determined the lifetime optical damage threshold for 5µm thick Si doped GaN film on 0.3 µm AlN buffer layer and sapphire substrate, and for a typical 10 nm thick ITO film on glass. Damage probability curves for single and multiple exposures, N, up to 105 cycles are shown in Fig. 4 for the same pulse width (τp = 9.5 ns). The ITO sample damaged at much lower fluences than the GaN sample by a factor of 5-10 × reflecting the bulk thermal degradation from high Ne absorption in ITO films and its lower thermal stability. The probability curves are steep, with increases from 0% to 100% damage probability from increases in fluence of 0.25 to 0.5 J/cm2 for ITO and for 1-2 J/cm2 for GaN, suggesting that for both materials the local laser pulse absorption is relatively uniform within the areas tested by the beam. The incubation effect is apparent for both samples as the threshold is reduced by 50% for GaN and 70% for ITO when going from single pulse exposure to N = 105 exposures. For ITO films, a visible darkening of the film precedes gross degradation related to the formation of thermomechanical crack formation that increases laser light absorption . In GaN films, no such transformation occurs prior to gross optical failure, thus the origins of this incubation remains unclear. It is reasonable to assume that the same critical temperature determines the damage threshold during incubation, except that an increase in laser absorption over multiple exposures reduces the fluence needed to produce apparent film damage. Still, a weakening of the material may lower that critical temperature as well.
The single pulse GaN damage probability curve determined from the ρ(F) curve fitting (Fig. 2(a)) is shown in Fig. 4(a) as a dashed line. The expected threshold damage where the probability curves begin to increase (near Fth) with F agrees well with the data. However these curves under-predict the extent of damage above Fth. This discrepancy maybe related to the use of different samples with different Nd defect densities, or to the scatter of data used for the fitting of ρ(F) data.Eq. (3), a larger S indicates faster degradation with N, and Fth(∞) is the fluence below which no damage occurs. For the GaN film, S = 0.09, while for ITO S = 0.007 corresponding to a faster initial degradation for GaN than ITO, while F(∞) = 4.4 J/cm2 is higher for GaN than for the ITO film F(∞) = 0.53 J/cm2.
The absorbers heating model presented here was applied to describe the experimental GaN and ITO films pulse-width damage threshold scaling, damage threshold probability distribution, and the lifetime threshold degradation. This model can describe the optical damage performance of conductive widegap semiconductors based on their thermal and optical properties, along with the laser parameters, and a characteristic absorption length scale. Transparent, conductive, or photoconductive electrodes based on GaN with low free carrier, low defects, and high mobility represent a promising route for improving the optical damage performance of emerging high power optoelectronic devices.
U.S. Department of Energy (DOE) (DE-AC52-07NA27344); Lawrence Livermore National Laboratory (LLNL); Laboratory Directed Research and Development grant (15-ERD-057)
This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under contract DE-AC52-07NA27344 within the LDRD program.
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