It is important to understand the damage process of optical coatings subjected to nanosecond laser ablation. However, investigating the high power laser interaction with optical coatings in a short time and space resolution is challenging. The dynamic evolution of materials ejection and layer peeling-off processes during damage of HfO2/SiO2-based anti-reflection coatings induced by 1064 nm pulsed laser were systematically examined by employing the time-resolved shadowgraph technique. Therefore, materials response imaging was performed by shadowgraphy to better understand the interaction between coatings and laser beam in air and vacuum con-ditions. The coatings damage in vacuum was not influenced by the plasma plume, which showed markedly different responses to the laser-coatings interaction. By comparing the form of damage crater and layer peeling -off through SEM analysis and step profiler, it was found that more laser energy dissipated into the target with the decrease in ambient air pressure. For the reverse damage process in air and vacuum, materials ejection by the continuous impact of the laser energy was observed and the forms of materials ejection and damaged craters were the same. Investigating the materials response under different air pressure conditions provided valuable insights into the damage mechanisms during the nanosecond laser-induced damage of optical coatings.

Ta2O5/SiO2 mixed film is a very promising material for the preparation of new optical and optoelectronic devices, but there are few reports on its etching characteristics. In this paper, Ta2O5/SiO2 mixed films with various proportions of Ta2O5 were prepared by ion-beam sputtering deposition. CHF3-based reactive ion etching (RIE) was used to etch Ta2O5/SiO2 mixed films. The etching profiles of Ta2O5/SiO2 mixed films were observed by using a field-emission scanning electron microscope (SEM). The RIE etch rates were investigated as a function of the Ta2O5/SiO2 mixture ratio, RIE power, chamber pressure and etching gas ratio. It is found that the etch rate of Ta2O5/SiO2 mixed films increase with an increase of RIE power and chamber pressure, and decrease with an increase of Ta2O5 composition in the Ta2O5/SiO2 mixed films. Moreover, it is also found that as the proportion of F-based gas increases, the etching rate of the Ta2O5/SiO2 mixed film first increases and then saturates. These results would be of importance for the fabrication of optical and optoelectronic devices based on Ta2O5/SiO2 mixed films.

High-repetition rate laser-induced damage of Ta2O5:SiO2 coatings was investigated at 355 nm wavelength and a repetition rate of 30 kHz. Laser-induced damage thresholds of coatings with different mixture ratios were measured. The relationships between laser-induced damage threshold and the material band gap and defect absorption were analyzed. Laser-induced damage threshold decreased with increase in number of laser pulses owing to enhancement of the absorption. Enhancement of absorption is attributed to the increase in density of mid-state defects induced by the picosecond laser pulses. The relationship between mid-state defects and the material mixture ratio was also analyzed.

Vacuum-air-shift causes trouble to conventional electron beam evaporation (EBE) coatings. This study compares the protection effect of ion assistance and the ALD (atomic layer deposition) Al2O3 capping layer. The vacuum-air-shift was reproduced on an EBE Ta2O5 single-layer film, a Ta2O5/SiO2 double-layer coating, and a high-reflectance mirror. Similar samples were prepared by EBE assisted with ion beam bombardment. Part of the as-deposited coatings were capped by ALD Al2O3 film with a thickness of about 100 nm. The vacuum-air-shift of the samples was tested with an in-situ spectrophotometer. The cross-section of the coatings was imaged by a scanning electron microscope. The ion assistance was found to reduce the vacuum-air-shift to a limit extent, while the ALD capping film was found to eliminate the vacuum-air-shift. The mechanism of the two protection techniques was analyzed, and their usage was compared and discussed. The ALD capping technique may be used in high precision EBE coatings with convenience and low cost.

Functionally graded layers (FGLs) with quasi-continuous interfaces are deposited by ion-beam sputtering (IBS). The laser-induced damage threshold (LIDT) of FGLs is more than twice that of film-substrate interface. Basic time-resolved microscope system is designed to capture the ultrafast dynamics process of nanosecond laserinduced damage in FGLs and film-substrate interface, respectively. Materials ejections and damages craters at the film-substrate interface are observed by focused ion beam (FIB) microscope. The primary sources leading to low LIDTs are the nano-precursors distributed among the subsurface. The nano-precursors formed by the metastable chemical structures (FeOx and CeSixOy) are agglomerated and grown during migration process because of sputtered atomic collision motion. High normalized electric field intensity enhancement in the subsurface region (about 275 nm) is considered to be an important factor that promotes absorption of the nanoprecursors. The experiment first provides a sufficient experimental observation of the laser-induced damage process inside films which deepens people's understanding of nano-precusors and lays a solid basis for solving the damage of film-substrate interface.

Laser damage threshold of optical components is an important indicator to measure the ability of components to resist laser damage. A low-absorption film is plated on the surface of the reaction sintered SiC substrate produced by ion beam sputtering method, and the fundamental frequency reflection efficiency is over 99.8%. A set of 1060 nm continuous laser damage threshold testing system has been established. After loading the SiC-based film sample with a continuous wave laser with a power density of 30 kW/cm2 for 30 s, it was found that the surface temperature rise of the SiC-based film was less than 2 K. the changes of temperature field and thermal stress on the surface of the SiC mirror when the continuous laser loading power and loading time by finite element method. The results are basically consistent with the experimental data. The experimental results verify that the thermal stress and thermal melting effect caused by high-power continuous laser loading are the mechanisms for the damage of mirrors, which provides an idea for improving the laser damage threshold of optical components.

Nanosecond pulse laser-induced damage is associated with nodular defects and nano-precursors in hafnium oxide (HfO2)/silica (SiO2) multilayer films. In this study, the dynamic evolution of material ejection and layer peelingoff in HfO2/SiO2 thin-film beam splitters used for third harmonic separation were examined by time-resolved shadowgraph technique under the fundamental frequency of 1064 nm (1 omega) and third harmonics 355 nm (3 omega) laser beam, respectively. The initial material ejection and subsequent peeling-off of layers were studied to clarify most thin films damage events caused by 1 omega and 3 omega lasers. The peeling-off of films under 1 omega laser was mainly caused by thermomechanical effect. In contrast, the peeling-off of films under 3 omega laser was mainly affected by laser-induced mechanical damage. The different transmission forms of layers peeled off from the films revealed the influence of mechanical strength and thermal strength on damage progression. The Taylor-Sedov theoretical model was used to analyze the propagation characteristic shock waves after nanosecond laser irradiation of films. In summary, identification of propagation forms and speed of peeling-off layers provides helpful insights into the damage mechanisms during nanosecond laser-induced damage of optical thin films.

GaN thin films have been directly grown on monolayer MoS2 by plasma-enhanced atomic layer deposition at 260 degrees C using triethylgallium as the gallium precursor along with Ar/N-2/H-2 plasma for the first time. Mechanically exfoliated coalesced films of mono-MoS2 are characterized by Raman spectroscopy before and after the deposition. GaN shows a polycrystalline wurtzite structure through X-ray diffraction, whereas high-resolution transmission electron microscopy reveals that GaN has crystallized at the interface region with mono-MoS2 along the [0001] direction, exhibiting well-ordered lattice and sharp interface. This type of film growth on MoS2 template here is the evidence of the Stranski-Krastanov mode. In terms of the dependence of the growth rate and the surface morphologies on ALD cycles, we further concluded that the GaN growth can be divided into four phases, giving a deep insight into the growth mechanism of hybrid GaN/MoS2 system.

We demonstrate that a low-temperature GaN insertion layer could significantly improve the surface morphology of non-polar a-plane GaN.The two key factors in improving the surface morphology of non-polar a-plane GaN are growth temperature and growth time of the GaN insertion layer.The root-mean-square roughness of a-plane GaN is reduced by 75％ compared to the sample without the GaN insertion layer.Meanwhile,the GaN insertion layer is also beneficial for improving crystal quality.This work provides a simple and effective method to improve the surface morphology of non-polar a-plane GaN.

The influence of excessive H-2 flow during barrier growth on optical and electrical properties of InGaN light-emitting diodes (LEDs) are investigated in this study. The room temperature photoluminescence of LEDs decays with excessive H-2 treatment. Temperature-dependent photoluminescence (TDPL) reveals an increase of the density and a decrease of the activation energy of deep non-radiative recombination centers in the H-2 treated LEDs. The external quantum efficiency (EQE) of the LEDs suffers from excessive H-2 treatment. The leakage current on the reverse and forward sides of the LEDs are reduced significantly when treated with H-2, which may be due to the suppressed Poole-Frenkel effect.

Nonpolar (11–20) a-plane GaN films with AlN nucleation layer were grown on (10–12) r-plane sapphire substrate by metal organic chemical vapor deposition (MOCVD). The crystalline and surface qualities of a-plane GaN were found to closely depend on the growth conditions of AlN nucleation layer. With decreasing AlN growth temperature, the AlN grains became larger and sparser, which significantly reduced the defects density of a-plane GaN films. The growth time of the low temperature AlN layer was further optimized, and a-plane GaN films with reduced anisotropy in the crystalline quality, surface morphology and in-plane strains were achieved. It was found that the lateral growth lengths along different directions of GaN could be modulated by the growth time of AlN nucleation layer, thus changing the anisotropy of a-plane GaN films.

X-ray diffraction patterns are widely used to calculate the periodicity fluctuation of multiple quantum wells (MQWs). Nonetheless, we find the commonly used formula sometimes cannot give an exact linear relationship between the full width at half maximum and the satellite peak orders. Here, based on the formula, we deduce a more accurate formula to characterize the periodicity fluctuations in InGaN/GaN MQWs. The revised formula has higher linearity correlation degree and the calculated interface roughness coincides well with X-ray reflectivity measurement results.

Nonpolar a-plane GaN films were directly grown on titanium patterned sapphire substrates (Ti-PSS) by metalorganic chemical vapor deposition (MOCVD). It is demonstrated that the GaN film grown on Ti-PSS has superior surface quality and less surface pits than that on flat sapphire substrate. More importantly, the anisotropic behavior of the X-ray rocking curve-full width at half maximum values for the on-axis reflections were significantly improved. It is found that, the increased mosaic block size along m-direction could be the main reason for the reduction in the crystalline quality anisotropy. This work provides a simple and effective method to improve the crystal quality of non-polar GaN films.

In this work, high-quality a-plane GaN was obtained by direct growth on a stripe-patterned sapphire substrate.

The characteristics of a-plane GaN films directly grown on silicon dioxide (SiO2) hole-array patterned r-sapphire substrates (HPSS) were investigated in this work.

We demonstrate the use of in situ SiNx preferentially deposited on the etched defect pits as a mask to block the propagation of threading dislocations (TDs). Etch pits are generated on the GaN template using the wet etching technique. The SiNx layer is then deposited on etch pits, followed by a regrown GaN layer. It turns out that a 60% reduction in TD density of the GaN epilayer is obtained compared with the conventional GaN growth method. The significant improvement is more attributed to the prevention effect by covering etch pits rather than the merging effect by reorienting the dislocation propagation.

We report the fabrication of 4-inch nano patterned wafer by two-beam laser interference lithography and analyze the uniformity in detail. The profile of the dots array with a period of 800 nm divided into five regions is characterized by a scanning electron microscope. The average size in each region ranges from 270 nm to 320 nm, and the deviation is almost 4%, which is approaching the applicable value of 3% in the industrial process. We simulate the two-beam laser interference lithography system with MATLAB software and then calculate the distribution of light intensity around the 4 inch area. The experimental data fit very well with the calculated results. Analysis of the experimental data and calculated data indicates that laser beam quality and space filter play important roles in achieving a periodical nanoscale pattern with high uniformity and large area. There is the potential to obtain more practical applications.

Absorption and carrier transport behavior plays an important role in the light-to-electricity conversion process, which is difficult to characterize. Here we develop a method to visualize such a conversion process in the InGaN/GaN multi-quantum wells embedded in a p–n junction. Under non-resonant absorption conditions, a photocurrent was generated and the photoluminescence intensity decayed by more than 70% when the p–n junction out-circuit was switched from open to short. However, when the excitation photon energy decreased to the resonant absorption edge, the photocurrent dropped drastically and the photoluminescence under open and short circuit conditions showed similar intensity. These results indicate that the escaping of the photo-generated carriers from the quantum wells is closely related to the excitation photon energy.

Rectifying characteristics induced by the polarization fields are achieved in the GaN/graded-Al-x Ga1-xN/GaN double heterojunction structure (DHS). By grading AlxGa1-xN from x = 0.4(0.3) to 0.1, the DHS displays a better conductivity for smaller reverse bias than for forward bias voltages (reverse rectifying behavior) which is opposite to p-n junction rectifying characteristics. The mechanism of reverse rectifying behavior is illustrated via calculating the energy band structures of the samples. The band gap narrowing caused by decreasing Al composition could compensate the for the band tilt due to the polarization effect in AlxGa1-xN barriers, thus lowering the barrier height for electron transport from top to bottom. The reverse rectifying behavior could be enhanced by increasing the Al content and the thickness of the multi-layer graded AlxGa1-xN barriers. This work gives a better understanding of the mechanism of carrier transport in a DHS and makes it possible to realize novel GaN-based heterojunction transistors.

The green light emitting diodes (LEDs) have lower quantum efficiency than LEDs with other emission wavelengths in the visible spectrum. In this research, a novel quantum well structure was designed to improve the electroluminescence (EL) of green InGaN-based LEDs. Compared with the conventional quantum well structure, the novel structure LED gained 2.14 times light out power (LOP) at 20-mA current injection, narrower FWHM and lower blue-shift at different current injection conditions.