Bare Sapphire Research Articles

Optical properties of two-dimensional tin nanosheets epitaxially grown on graphene

Heterostacks formed by combining two-dimensional materials show novel properties which are of great interest for new applications in electronics, photonics and even twistronics, the new emerging field born after the outstanding discoveries on twisted graphene. Here, we report the direct growth of tin nanosheets at the two-dimensional limit via molecular beam epitaxy on chemical vapor deposited graphene on Al2O3(0001). The mutual interaction between the tin nanosheets and graphene is evidenced by structural and chemical investigations. On the one hand, Raman spectroscopy indicates that graphene undergoes compressive strain after the tin growth, while no charge transfer is observed. On the other hand, chemical analysis shows that tin nanosheets interaction with sapphire is mediated by graphene avoiding the tin oxidation occurring in the direct growth on this substrate. Remarkably, optical measurements show that the absorption of tin nanosheets exhibits a graphene-like behavior with a strong absorption in the ultraviolet photon energy range, therein resulting in a different optical response compared to tin nanosheets on bare sapphire. The optical properties of ultra-thin tin films therefore represent an open and flexible playground for the absorption of light in a broad range of the electromagnetic spectrum and technologically relevant applications for photon harvesting and sensors.

Uncooled High Detectivity Mid-Infrared Photoconductor Using HgTe Quantum Dots and Nanoantennas.

Using a metal/insulator/metal (MIM) structure with a gold nanoantenna array made by electron beam lithography, the responsivity of a HgTe colloidal quantum dot film is enhanced in the mid-infrared. Simulations indicate that the spatially averaged peak spectral absorption of an 80 nm film is 60%, enhanced 23-fold compared to that of the same film on a bare sapphire substrate. The field intensity enhancement is focused near the antenna tips, being 20-fold 100 nm away, which represents only 1% of the total area and up to 1000-fold at the tips. The simulated polarized absorption spectra are in good agreement with the experiments, with a strong resonance around 4 μm. A responsivity of 0.6 A/W is obtained at a 1 V bias. Noise measurements separate the 1/f noise from the generation-recombination white noise and give a spatially averaged photoconductive gain of 0.3 at 1 V bias. The spatially averaged peak detectivity is improved 15-fold compared to the same film on a sapphire substrate without an MIM structure. The experimental peak detectivity reaches 9 × 109 Jones at 2650 cm-1 and 80 kHz, decreasing at lower frequencies. The MIM structure also enhances the spatially averaged peak photoluminescence of the CQD film by 16-fold, which is a potential Purcell enhancement. The good agreement between simulations and measurements confirms the viability of lithographically designed nanoantenna structures for vastly improving the performance of mid-IR colloidal quantum dot photoconductors. Further improvements will be possible by matching the optically enhanced and current collection areas.

Effects of applied negative bias voltage on the optical and structural properties of CaF2 coatings deposited by RF magnetron sputtering

Calcium fluoride (CaF2) is a ceramic material exhibiting versatile properties including broadband transparency, lubrication at high temperature, and mechanical stability. In the current research, aimed at improving optically-driven infrared detection devices, CaF2 coatings were deposited on sapphire. One challenge concerning this CaF2/sapphire system is the significant difference between their coefficients of thermal expansion, which can cause adhesion failure. To avoid such failure, we first studied the behavior of thick CaF2 coatings on silicon substrates where adhesion issues are expected to be more pronounced, then employed thin CaF2 coatings, initially on silicon and subsequently on sapphire substrates.Deposition was carried out by radio frequency magnetron sputtering technique in an argon atmosphere at a fixed power of 4.4 W/cm2. Substrate bias voltage was varied from 0 to - 150 V in order to explore voltage impact on properties of the obtained CaF2 coatings.Coatings deposited with low negative bias voltages presented an inhomogeneous surface with a columnar structure that remained firmly attached to the substrate; optical transmission for these coatings was reduced relative to that for bare sapphire. Alternatively, while coatings deposited with high negative bias voltages became dense and uniform, they were also subjected to high surface reactivity causing formation of CaO/Ca(OH)2 phases and defects, as well as delamination that reduced optical transmission. In contrast, only samples deposited with moderate bias voltages exhibited a sufficiently dense, highly adhesive layer, resulting (relative to that for bare sapphire) in improved optical transmission. Thin CaF2 coatings deposited with −100 V improved transmittance (at a wavelength of 1.9 μm) from almost 88.5 %–92 %, representing approximately a 30 % decrease in radiation loss caused by absorbance, scattering, and reflectance effects. Even though the optical values of transmittance for increased wavelength (up to 5 μm) were reduced, these values still surpassed those obtained for sapphire alone.

Centimeter-Transferable III-Nitride Membrane Enabled by Interfacial Adhesion Control for a Flexible Photosensitive Device.

Flexible III-nitride-based optoelectronic devices are crucial for the next-generation foldable/wearable lighting sterilization and sensor working in the ultraviolet (UV) region. However, the strong bonding effect at the epitaxial interface of III-nitride and bare sapphire substrate makes it difficult for epilayer separation and flexible applications. Although the emerging van der Waals epitaxy (vdWE) with graphene insertion layer offers a feasible route for weakening the interfacial adhesion, the intact centimeter-transferable III-nitride membrane still remains challenging. The spontaneous delamination occurs due to the too weak interfacial adhesion of pure vdW force, and on the contrary, the structural damage of graphene by high-temperature hydrogen etching during the III-nitride growth might also cause separation failure. Up to now, the efficient control of vdWE interfacial adhesion is still an on-going research hotspot. Herein, we demonstrate the interfacial adhesion control of III-nitride vdWE by utilizing graded high-temperature nitridation treatment of the graphene insertion layer, which generates defects and N doping in different levels. The corresponding epitaxial modes of pure-vdWE, quasi-vdWE, and mixed epitaxy are achieved according to the interfacial adhesion difference. It reveals that the quasi-vdWE enabled by small graphene defects and proper N doping triggers the low formation energy for AlN nucleation; meanwhile, the proper interfacial adhesion ensures the growth integrality and intact separation of III-nitride membrane in the centimeter scale. The UV resin-assisted bonding technique is proposed for the successful transfer of III-nitride onto a flexible substrate. The flexible photodetector is fabricated by using a graphene monolayer as the photocarrier transport channel, and it achieves a high device yield of 90%, retaining ∼60% of its initial performance after 250 bending cycles. This work offers the promising strategy for controlling vdWE interfacial adhesion, and the separable and transferable III-nitride membrane lays the foundation for advances of future UV foldable and wearable devices.

Graphene-driving strain engineering to enable strain-free epitaxy of AlN film for deep ultraviolet light-emitting diode

The energy-efficient deep ultraviolet (DUV) optoelectronic devices suffer from critical issues associated with the poor quality and large strain of nitride material system caused by the inherent mismatch of heteroepitaxy. In this work, we have prepared the strain-free AlN film with low dislocation density (DD) by graphene (Gr)-driving strain-pre-store engineering and a unique mechanism of strain-relaxation in quasi-van der Waals (QvdW) epitaxy is presented. The DD in AlN epilayer with Gr exhibits an anomalous sawtooth-like evolution during the whole epitaxy process. Gr can help to enable the annihilation of the dislocations originated from the interface between AlN and Gr/sapphire by impelling a lateral two-dimensional growth mode. Remarkably, it can induce AlN epilayer to pre-store sufficient tensile strain during the early growth stage and thus compensate the compressive strain caused by hetero-mismatch. Therefore, the low-strain state of the DUV light-emitting diode (DUV-LED) epitaxial structure is realized on the strain-free AlN template with Gr. Furthermore, the DUV-LED with Gr demonstrate 2.1 times enhancement of light output power and a better stability of luminous wavelength compared to that on bare sapphire. An in-depth understanding of this work reveals diverse beneficial impacts of Gr on nitride growth and provides a novel strategy of relaxing the vital requirements of hetero-mismatch in conventional heteroepitaxy.

α-Gallium Oxide Films on Microcavity-Embedded Sapphire Substrates Grown by Mist Chemical Vapor Deposition for High-Breakdown Voltage Schottky Diodes.

α-Gallium oxide, with its large band gap energy, is a promising material for utilization in power devices. Sapphire, which has the same crystal structure as α-Ga2O3, has been used as a substrate for α-Ga2O3 epitaxial growth. However, lattice and thermal expansion coefficient mismatches generate a high density of threading dislocations (TDs) and cracks in films. Here, we demonstrated the growth of α-Ga2O3 films with reduced TD density and residual stress on microcavity-embedded sapphire substrates (MESS). We fabricated the two types of substrates with microcavities: diameters of 1.5 and 2.2 μm, respectively. We confirmed that round conical-shaped cavities with smaller diameters are beneficial for the lateral overgrowth of α-Ga2O3 crystals with lower TD densities by mist chemical vapor deposition. We could obtain crack-free high-crystallinity α-Ga2O3 films on MESS, while the direct growth on a bare sapphire substrate resulted in an α-Ga2O3 film with a number of cracks. TD densities of α-Ga2O3 films on MESS with 1.5 and 2.2 μm cavities were measured to be 1.77 and 6.47 × 108 cm-2, respectively. Furthermore, cavities in MESS were certified to mitigate the residual stress via the redshifted Raman peaks of α-Ga2O3 films. Finally, we fabricated Schottky diodes based on α-Ga2O3 films grown on MESS with 1.5 and 2.2 μm cavities, which exhibited high breakdown voltages of 679 and 532 V, respectively. This research paves the way to fabricating Schottky diodes with high breakdown voltages based on high-quality α-Ga2O3 films.

Strain relaxation and dislocation annihilation in compositionally graded α-(AlxGa1-x)2O3 layer for high voltage α-Ga2O3 power devices

α-Ga2O3 of the corundum structure and the large bandgap of 5.3 eV has attracted great interest because it can be grown on a sapphire (α-Al2O3) substrate with the same crystal structure. However, the lattice mismatch (4.3%) and the different thermal expansion coefficients between α-Ga2O3 and the sapphire substrate induce crystalline defects and thermal strain, leading to a high density of threading dislocations and degraded electrical properties of the α-Ga2O3 films grown directly on the substrate. Herein, to circumvents these issues, compositionally graded α-(AlxGa1-x)2O3 layers are adopted to reduce threading dislocations for a high quality of epitaxial α-Ga2O3 films. The evolution of strain relaxation and the inclination of threading dislocations in graded α-(AlxGa1-x)2O3 layers are confirmed by reciprocal space mapping (RSM) and transmission electron microscopy (TEM). Through RSM and TEM studies, we confirmed that compressive strain enhances the inclination of dislocations, and therefore, the dislocations merge and annihilate in the graded α-(AlxGa1-x)2O3 layers. Moreover, owing to dislocations annihilation in the graded α-(AlxGa1-x)2O3 layers, the calculated density of threading dislocations in α-Ga2O3 films with a graded α-(AlxGa1-x)2O3 layer is reduced by 64.9% compared with that of α-Ga2O3 films deposited directly grown on a bare sapphire substrate. Furthermore, a fabricated lateral-structure Schottky diodes reveals enhanced breakdown voltages and forward current density due to the improved crystalline quality using the graded α-(AlxGa1-x)2O3 layer. This study provides an attractive approach for obtaining high-quality epitaxial α-Ga2O3 thin films for high voltage power devices.

Strain Relaxation and Dislocation Annihilation in Compositionally Graded α-(Al xGa 1-x) 2O 3 Layer for High Voltage α-Ga 2O 3 Power Devices

α-Ga2O3 of the corundum structure and the large bandgap of 5.3 eV has attracted great interest because it can be grown on a sapphire (α-Al2O3) substrate with the same crystal structure. However, the lattice mismatch (4.3%) between α-Ga2O3 and the sapphire substrate induces a high density of threading dislocations, which act as breakdown leakage paths and lead to deterioration of the crystallinity and electrical properties of the α-Ga2O3 films grown directly on the substrate. Herein, to circumvent this issue compositionally graded α-(AlxGa1-x)2O3 layers are adopted to reduce threading dislocations for a high quality of epitaxial α-Ga2O3 films. The evolution of strain relaxation and the inclination of threading dislocations in graded α-(AlxGa1-x)2O3 layers are confirmed by reciprocal space mapping (RSM) and transmission electron microscopy (TEM). Through RSM and TEM studies, we confirmed that compressive strain enhances the inclination of dislocations and therefore, the dislocations merge and annihilate in the graded α-(AlxGa1-x)2O3 layers. The calculated density of threading dislocations in an α-Ga2O33 films with a graded α-(AlxGa1-x)2O3 buffer layer is reduced by 64.9% compared with that of an α-Ga2O3 films deposited directly grown on a bare sapphire substrate. Furthermore, a fabricated lateral-structure Schottky diode reveals enhanced breakdown voltages and forward current density due to the improved crystalline quality by using the graded α-(AlxGa1-x)2O3 buffer layer. This study provides an attractive approach for obtaining high-quality epitaxial α-Ga2O3 thin films for high voltage power devices.

Controlled epitaxial growth of GaN nanostructures on sapphire (11–20) using laser molecular beam epitaxy for photodetector applications

We have grown various epitaxial GaN nanostructures on sapphire (11–20) substrates by tuning the buffer layer growth conditions in laser molecular beam epitaxy (LMBE) process. The pre-nitridation and buffer layer GaN growth at low temperature (LT) on sapphire (11–20) critically affect the surface morphology and structural properties. Granular GaN thin film (~ 160 nm) was grown on pre-nitridated sapphire whereas nano-column (NC)-GaN was obtained on LT-GaN buffer layer on bare sapphire having a height of ~ 370 nm at the growth temperature of 700 °C. Nano-porous (NP)-GaN was obtained with pore sizes in the range of 70 ~ 110 nm having vertical height of ~ 560 nm under similar growth conditions on LT-GaN buffered pre-nitridated sapphire. In-situ reflection high energy electron diffraction, high-resolution x-ray diffraction and Raman spectroscopy measurements indicated the epitaxial growth of c-axis oriented, wurtzite crystalline GaN nanostructures on sapphire (11–20) substrate with nearly negligible biaxial stress (0.03–0.23 GPa). Further, metal-semiconductor-metal (MSM) ultra-violet (UV) photodetectors were fabricated on epitaxial GaN nanostructures. The photo responsivity studies revealed that the NP-GaN MSM device has a photoresponse of ~ 358 mA/W at an applied bias of 1V. The photo-responsivity of NP-GaN MSM device is higher than that of GaN film (~ 36 mA/W) and NC-GaN (~7 mA/W) which revealed the importance of shape and size of GaN nanostructures on the responsivity of UV-photodetector devices. These results demonstrate the capability of LMBE technique to grow different GaN nanostructures on sapphire (11–20) substrate by tuning buffer layer conditions for their application as UV-photodetectors.

Graphene-induced crystal-healing of AlN film by thermal annealing for deep ultraviolet light-emitting diodes

We achieve stress-free AlN films with low dislocation density by employing Graphene (Gr)-induced crystal-healing via high-temperature annealing (HTA). The N2 plasma-treated Gr effectively facilitates the formation of high-density and small-size AlN grains during the initial quasi-van der Waals epitaxial growth. Thus, it is easier to rotate the inhomogeneous crystal orientation for subsequent AlN epilayers during the HTA process, greatly improving the recrystallization efficiency of AlN. Due to the improved AlN quality, the 283-nm deep ultraviolet light-emitting diode (DUV-LED) on the HT-annealed AlN with Gr shows a significant increase in light output power of 2.3 times at an injection current of 20 mA compared to its counterpart on bare sapphire. This research broadens the applications of Gr in assisting group-III nitride epitaxy and provides a useful strategy for the manufacture of high-quality AlN and high-performance DUV-LEDs.

Improved nucleation of AlN on in situ nitrogen doped graphene for GaN quasi-van der Waals epitaxy

In the present work, improved crystal quality of GaN on a graphene-covered sapphire substrate was achieved compared to GaN grown on a bare sapphire substrate, and the growth mechanism of GaN quasi-van der Waals epitaxy with an AlN nucleation layer was clarified using metal-organic chemical vapor deposition. The in situ N-doping of graphene by an NH3 source during AlN growth was responsible for AlN nucleation. The first-principles calculation indicated that N atoms doped initially at the step edges of graphene and subsequently at its center and AlN nuclei followed the same sequence, which is consistent with experimental observations of nucleation. The lower migration barrier of Al atoms (0.07 eV) on graphene created larger AlN nuclei compared to that on bare sapphire (0.21 eV), leading to improved quality of the upper GaN epilayer with lower defect density. This work offers guidance for precisely controlling the nucleation morphology and density of GaN base materials and thus realizing high-quality epitaxial materials and related high-performance devices by quasi-van der Waals epitaxy.

Morphology and random lasing of (110)-oriented ZnO nanowalls

Morphology and near-band-edge emission properties of ZnO grown on R-plane sapphire are studied. It is shown that preliminarily deposited thin Ag layer catalyzes the growth of vertically aligned labyrinth-like ZnO nanowalls with the (110) crystallographic orientation. Room-temperature photoluminescence of ZnO nanowalls is more than two times stronger than that of ZnO grown on bare sapphire. Random lasing with rather low thresholds is excited in ZnO nanowalls.

Atomic mechanism of strong interactions at the graphene/sapphire interface

For atomically thin two-dimensional materials, interfacial effects may dominate the entire response of devices, because most of the atoms are in the interface/surface. Graphene/sapphire has great application in electronic devices and semiconductor thin-film growth, but the nature of this interface is largely unknown. Here we find that the sapphire surface has a strong interaction with some of the carbon atoms in graphene to form a C-O-Al configuration, indicating that the interface interaction is no longer a simple van der Waals interaction. In addition, the structural relaxation of sapphire near the interface is significantly suppressed and very different from that of a bare sapphire surface. Such an interfacial C-O-Al bond is formed during graphene growth at high temperature. Our study provides valuable insights into understanding the electronic structures of graphene on sapphire and remote control of epitaxy growth of thin films by using a graphene–sapphire substrate.

Experimental investigation of surface adhesion of Titan analog materials: Mitigation by dust-repellent coatings

We report an experimental investigation of organic Titan regolith simulant interactions with transparent surfaces. In the absence of prior triboelectrification of the particles and surface, the area coverage of sprinkled particles increases with increasing substrate surface energy, as expected. Bare sapphire, sapphire coated with indium tin oxide (ITO), and sapphire coated with ITO and fluorosilane collected 20%, 10%, and 5% particles, respectively. Under turbulent conditions that promote triboelectric charging, the adhesion of Titan regolith analogs is enhanced. Bare sapphire occluded by more than 60% in some cases. However, conductive ITO generally reduced the area coverage below 10% when electrically grounded, and the fluorosilane treatment reduced it slightly further. The surface coatings tested, and anti-static measures more generally, therefore provide significant mitigation of optical window contamination.

ZnO nanostructure-assisted growth of (0002)-oriented GaN thin films by laser molecular beam epitaxy

Vertically aligned zinc oxide (ZnO) nanostructures (NS) have been fabricated without any catalyst on Si (100) and sapphire (0002) substrates using high-pressure-assisted pulsed laser deposition technique. It has been observed that the substrate plays a crucial role in deciding the morphology and optical quality of the nanostructures, wherein ZnO NS on Si substrate exhibit better luminescence characteristics as compared to those grown on sapphire substrate. Gallium nitride (GaN) layers are grown using a customized laser molecular beam epitaxy system and the effect of an intermediate layer in the form of vertically aligned ZnO NS is analyzed. A comparative study is performed with GaN layers grown on ZnO NS-coated Si, bare Si, and bare sapphire substrate. The structural and room temperature photoluminescence properties of the GaN layer significantly improve when grown on ZnO-NS/Si substrates as compared to bare sapphire and Si substrate.

InGaN-based light-emitting diodes grown on various aspect ratios of concave nanopattern sapphire substrate

This study presented the optoelectronic properties of InGaN-based light-emitting diodes (LEDs) grown on nanoscale-patterned sapphire substrates (NPSSs) with various aspect ratio (AR) concave nanopatterns. Concave nanopatterns with low ARs of ∼0.5 (LAR-NPSS) and high ARs of 2, 3 and 4 (HAR-NPSS) were achieved using an anodic aluminum oxide template as the dry etching mask. The diffuse transmission (DT) rate of the incident light wavelength at 450 nm increased linearly, from 2.2% for LAR-NPSS to 12.7% for HAR-NPSS with an AR of 4. However, the diffuse reflection (DR) rate for HAR-NPSS did not increase linearly with the increasing AR values. The limit of the DR rate for HAR-NPSS could be attributed to the guiding effect when the incident light entered into the high AR concave nanopattern; most of the incident light could not be reflected (returned to the air), therefore there was no significant contribution to the DR rate. The luminous intensities of LEDs grown on bare sapphire (B-LED), HAR-NPSS (HAR-LED) and LAR-NPSS (LAR-LED) were 39, 38 and 74 mcd, respectively. Interestingly, the lowest brightness of any LED was the HAR-LED, which had higher DT and DR rates. This significant discrepancy could be attributed to a large number of voids occurring at the initial growth stage of the LED epitaxial layer on the HAR-NPSS that caused the emitted photons to remain trapped inside the LED. In addition, increasing the LED epilayer coverage area (i.e. 1.6-fold) with the inclined facets of low AR concave nanopatterns effectively scattered the emitted photons, leading to an increased light emitting intensity of the LED. The experimental results indicated that the specific AR concave nanopattern of an NPSS could increase the LED epitaxial layer coverage area and prevent void formation, which plays a crucial role in achieving high-brightness in InGaN-based LEDs.

High-Brightness Blue Light-Emitting Diodes Enabled by a Directly Grown Graphene Buffer Layer.

Single-crystalline GaN-based light-emitting diodes (LEDs) with high efficiency and long lifetime are the most promising solid-state lighting source compared with conventional incandescent and fluorescent lamps. However, the lattice and thermal mismatch between GaN and sapphire substrate always induces high stress and high density of dislocations and thus degrades the performance of LEDs. Here, the growth of high-quality GaN with low stress and a low density of dislocations on graphene (Gr) buffered sapphire substrate is reported for high-brightness blue LEDs. Gr films are directly grown on sapphire substrate to avoid the tedious transfer process and GaN is grown by metal-organic chemical vapor deposition (MOCVD). The introduced Gr buffer layer greatly releases biaxial stress and reduces the density of dislocations in GaN film and Inx Ga1-x N/GaN multiple quantum well structures. The as-fabricated LED devices therefore deliver much higher light output power compared to that on a bare sapphire substrate, which even outperforms the mature process derived counterpart. The GaN growth on Gr buffered sapphire only requires one-step growth, which largely shortens the MOCVD growth time. This facile strategy may pave a new way for applications of Gr films and bring several disruptive technologies for epitaxial growth of GaN film and its applications in high-brightness LEDs.

Insight on Structure of Water and Ice Next to Graphene Using Surface-Sensitive Spectroscopy.

The water/graphene interface has received considerable attention in the past decade due to its relevance in various potential applications including energy storage, sensing, desalination, and catalysis. Most of our knowledge about the interfacial water structure next to graphene stems from simulations, which use experimentally measured water contact angles (WCAs) on graphene (or graphite) to estimate the water-graphene interaction strength. However, the existence of a wide spectrum of reported WCAs on supported graphene and graphitic surfaces makes it difficult to interpret the water-graphene interactions. Here, we have used surface-sensitive infrared-visible sum frequency generation (SFG) spectroscopy to probe the interfacial water structure next to graphene supported on a sapphire substrate. In addition, the ice nucleation properties of graphene have been explored by performing in situ freezing experiments as graphitic surfaces are considered good ice nucleators. For graphene supported on sapphire, we observed a strong SFG peak associated with highly coordinated, ordered water next to graphene. Similar ordering was not detected next to bare sapphire, implying that the observed ordering of water molecules in the former case is a consequence of the presence of graphene. Our analysis indicates that graphene behaves like a hydrophobic (or negatively charged) surface, leading to enhanced ordering of water molecules. Although liquid water orders next to graphene, the ice formed is proton disordered. This research sheds light on water-graphene interactions relevant in optimizing the performance of graphene in various applications.

A comparative study of InN growth on quartz, silicon, C-sapphire and bulk GaN substrates by RF magnetron sputtering

In this work, we investigate the growth of indium nitride (InN) films on quartz, bulk GaN, sapphire (001) and Si (111) substrates. An InN buffer layer was first deposited on all the substrates, then an InN film was grown on bare substrate and InN buffered substrates. The films were polycrystalline in nature with preferred orientation along (002) plane. Best structural quality was observed on InN buffered Si substrate. The structural properties were explained by calculating the full width at half maximum, crystallite size, micro-strain, and dislocation density. The morphology of the films revealed similar granular features except for bare sapphire substrate which showed cracks and more oxygen percentage. The application of buffer layer increased the surface roughness for quartz and reduced in other cases. The band gap of InN films was determined using UV–visible reflectance spectroscopy. The lowest band gap value was observed for InN buffered quartz substrate.

Chemically-induced solid-state dewetting of thin Au films

We employed the solid state dewetting technique to produce nanoparticles of silver-gold alloy on a sapphire substrate. We deposited a thin gold layer on the substrate with alloy nanoparticles, and studied its thermal stability at low homological temperatures. We demonstrated that a large number of densely spaced holes form at the initial stages of dewetting of the gold layer with nanoparticles. A similar homogeneous gold film deposited on a bare sapphire substrate remained stable under identical annealing conditions, exhibiting the onset of dewetting at higher temperatures, and with a lower number of holes. We attributed the decreased thermal stability of the gold film deposited on the substrate with the silver-gold nanoparticles to accelerated grooving at the grain boundaries and triple junctions in the film. The grooving process is accelerated by the diffusion fluxes of Au atoms driven from the film towards the nanoparticles by the gradient of chemical potential. We developed a quantitative model of this chemically-induced dewetting process, and discussed its applicability for the design of better catalytic systems. Our work demonstrates that the chemical driving forces have to be reckoned with in the analysis of thermal stability of multicomponent thin films.