Hexagonal Pits Research Articles

Surface modification of highly oriented pyrolytic graphite by reaction with atomic nitrogen at high temperatures

Dry etching of {0001} basal planes of highly oriented pyrolytic graphite (HOPG) using active nitridation by nitrogen atoms was investigated at low pressures and high temperatures. The etching process produces channels at grain boundaries and pits whose shapes depend on the reaction temperature. For temperatures below 600°C, the majority of pits are nearly circular, with a small fraction of hexagonal pits with rounded edges. For temperatures above 600°C, the pits are almost exclusively hexagonal with straight edges. The Raman spectra of samples etched at 1000°C show the D mode near 1360cm−1, which is absent in pristine HOPG. For deep hexagonal pits that penetrate many graphene layers, neither the surface number density of pits nor the width of pit size distribution changes substantially with the nitridation time, suggesting that these pits are initiated at a fixed number of extended defects intersecting {0001} planes. Shallow pits that penetrate 1–2 graphene layers have a wide size distribution, which suggests that these pits are initiated on pristine graphene surfaces from lattice vacancies continually formed by N atoms. A similar wide size distribution of shallow hexagonal pits is observed in an n-layer graphene sample after N-atom etching.

Si implanted reactivation in GaN grown on sapphire using AlN and oxide cap layers

Gallium nitride (GaN) is a promising material for power electronic devices. Due to GaN sensitivity to high temperature treatments, dopant activation, after ion implant, is one of the major critical steps to be overcome. An annealing cap layer is then mandatory during high temperature treatment to avoid degradations. In this work, cap layers, such as AlN and SiO x , were deposited on Si-implanted N-type GaN. Samples were annealed using both classical (FA) and rapid thermal (RTA) annealing for times ranging from 30 s to 8 h and temperatures from 1000 to 1150 °C. Transmission Electron Microscopy has been done to observe the implanted layer structure. After cap layer removal, samples surface has been investigated through Atomic Force Microscopy measurements. Dopant activity was indirectly evaluated by Specific Contact Resistance (SCR) measurements. This work demonstrates that low SCR value (8.2 × 10 −5 Ω cm 2) with low surface roughness (∼1 nm) can be reached using RTA and an oxide cap layer. However, presence of hexagonal pits in GaN layer is difficult to avoid. Compromise between low SCR with low roughness value and low hexagonal pits density on the GaN surface must be found.

Photo-enhanced chemical etched GaN LED on silicon substrate

In this study, GaN LEDs grown with an intermediate DBR on a Si substrate were chemically etched by 3 M KOH solution under UV light illumination. After 60 min of KOH etching, the hexagonal etch pits and randomized embossments were clearly imaged by SEM and AFM. The etch pits were generated at the threading dislocations, which are common for lattice mismatched growth of GaN on Si. The photoluminescence intensity at 380 nm was enhanced by approximately 21% after PEC etching. The enhanced PL intensity indicated that the generated etch pits and embossments increased the surface area, and effectively increased light scattering effects by randomizing the light rays and increasing the number of scattering events.

Morphology and stability of Au nanoclusters in HOPG nanopits of well-defined depth

Gold nanoparticles with a diameter comprised between 4 and 6 nm are stabilized in nanosized pits of well defined depth in highly oriented pyrolytic graphite (HOPG). These pits are produced by creation of artificial defects, followed by etching under a controlled oxygen atmosphere. At low Au coverage, clusters are found on the edges of the hexagonal pits maximizing the contact to dangling bonds on graphite multisteps. Larger coverage results in Au beads of surprisingly well defined shape and with a constant bead density per unit length. Most remarkable is the stability of these nanostructures under ambient conditions. Temperatures as high as 650 K do not alter the morphology of the gold clusters. Higher temperatures do not lead to a change of the cluster morphology but to catalytically driven etching of the HOPG substrate.

Maskless Fabrication of Nanowells Using Chemically Reactive Colloids

This letter describes the maskless fabrication of nanowells on a silicon substrate using chemically reactive nanoparticles. The amidine-functionalized polystyrene latex (APSL) colloids are adhered onto a silicon wafer, and hydrolysis of the particles' amidine groups generates the ammonium hydroxide etchant locally. The localized release of reactive species and its fast diffusion into the bulk liquid ensure that the silicon etching takes place only under the APSL colloids. Thus, the basal length of the nanowells is precisely controlled by the diameter of the APSL particles. The shape of the nanowells depends on the structure of the substrate: inverted pyramids on silicon (100) and hexagonal pits on silicon (111). The method described here provides an easy, inexpensive, safe, and high-throughput approach for generating nanowells on silicon surfaces. This maskless and simple nanofabrication method will open doors for new applications with locally generated or locally delivered chemistry from nanoparticles.

Formation of wide and atomically flat graphene layers on ultraprecision-figured 4H-SiC(0001) surfaces

Damaged layers and polishing haze exist on substrate surfaces owing to abrasion during figuring processes, which deteriorate surface integrity. We have developed a novel damage-free ultraprecision figuring technique and obtained atomically flat and damage-free SiC(0001) surfaces with a domain size of 300–500 nm. Using ultraprecision-figured 0°-off 4H-SiC(0001) surfaces as substrates, we demonstrated the formation of graphene layers upon them and successfully produced atomically flat monolayer graphene with a domain size of over 300 nm on entire ultraprecision-figured SiC surfaces by annealing in ultra high vacuum without any Si flux. The quality of the graphene layers on the ultraprecision-figured SiC surfaces was markedly higher than that of layers on non-figured SiC surfaces. This indicates that the damaged surface layers with scratches, defects, and so on, enhance roughness on SiC surfaces, and reduce the quality of graphene films. In addition, we found a new phenomenon:step terraces of 500 nm width exhibited erosion at step edges, leading to the creation of hexagonal pits at terraces, while step terraces of 300 nm width exhibited hardly any erosion. A graphitization mechanism was discussed.

Optimization of inductively coupled plasma deep etching of GaN and etching damage analysis

Inductively coupled plasma (ICP) etching of GaN with an etching depth up to 4μm is systemically studied by varying ICP power, RF power and chamber pressure, respectively, which results in etch rates ranging from ∼370nm/min to 900nm/min. The surface morphology and damages of the etched surface are characterized by optical microscope, scanning electron microscope, atomic force microscopy, cathodoluminescence mapping and photoluminescence (PL) spectroscopy. Sub-micrometer-scale hexagonal pits and pillars originating from part of the structural defects within the original GaN layer are observed on the etched surface. The density of these surface features varies with etching conditions. Considerable reduction of PL band-edge emission from the etched GaN surface indicates that high-density non-radiative recombination centers are created by ICP etching. The density of these non-radiative recombination centers is found largely dependent on the degree of physical bombardments, which is a strong function of the RF power applied. Finally, a low-surface-damage etch recipe with high ICP power, low RF power, high chamber pressure is suggested.

Do Sidewall Quantum Wells Exist in GaInN-Based Light-Emitting Diodes?

Sidewall quantum wells in the vicinity of hexagonal V-shaped pits with a reduced thickness and a higher band gap than the regular c-plane quantum wells were suggested as the origin of high luminescence efficiency in GaInN-based light-emitting diodes. In this Brief Note, by transmission electron microscopy combined with energy dispersive x-ray analysis, it was found that there must be no sidewall quantum wells around the V-shaped pits, which are just interference fringes arising from lattice strain.

Etching of GaN by microwave plasma of hydrogen

An etching process for GaN on a sapphire substrate using a microwave plasma of hydrogen has been studied. Scanning electron microscopy observations of the surface morphology show that the etching of GaN with H2 plasma can lead to the formation of etch pits in hexagonal shape. The average size of hexagonal pits is greater than 200 nm. The effects of processing pressure and etching time are demonstrated.

Etching study of dislocations in heavily nitrogen doped SiC crystals

Extensive study of threading dislocations in 4H SiC crystals has been carried out using etching in molten KOH. In contrast to well-defined hexagonal pits formed on lightly doped 4H epilayers, etching of bulk 4H SiC crystals heavily doped with nitrogen produced rounded etch pits with their sizes varying in a wide range. Neither shape nor size of the etch pits in the bulk n+4H crystals could be used to distinguish between threading edge and treading screw dislocations. Data on the density of threading screw dislocations were obtained by counting etch pits on the carbon face of the wafers. Sequential steps of material removal, which included polishing followed by KOH etching, were used to track threading dislocations along the growth direction. It was found that a threading dislocation can produce etch pits of different sizes at different depths in the wafer. Mobility of the front of threading dislocations during growth was assessed by measuring change in the position of the dislocation etch pits upon sequential material removal. Statistical distribution of such displacements in the wafer plane was found to be lognormal. On average, the growth distance of 8 μm corresponded to the change in the etch pit position of about 2 μm. This shows that the front of threading dislocations has significant mobility during SiC sublimation growth, resulting in tilted or curved dislocation lines in the grown crystal.

Formation of Hexagonal Pyramids and Pits on V-/VI-Polar and III-/II-Polar GaN/ZnO Surfaces by Wet Etching

The etching characteristics and evolution of the surface morphology of ZnO and GaN thin films during wet chemical etching were investigated. While the ZnO etch rates using acidic solutions were similar, regardless of etchant or polarity, the etch rate of the Ga-polar GaN surface was dependent on the etchant. N-polar GaN could be etched in alkali-based solutions such as KOH. The surface morphologies of etched ZnO were very similar to those of the GaN surface: hexagonal pyramids and stepped hexagonal pits formed during the etching of O-polar ZnO and N-polar GaN and of Zn-polar ZnO and Ga-polar GaN, respectively. The formation of a hexagonal etch pit on the etched Zn-polar and Ga-polar surfaces was enhanced by surface defects such as pits or threading dislocations. The etching of N-polar GaN by KOH proceeded through the evolutionary stages of hexagonal pyramids: formation, growth, dissociation, and isolation. Ga-polar GaN was etched through the formation, widening, and merging of hexagonal pits.

Surfactant effect of arsenic doping on modification of ZnO (0001) growth kinetics

The effect of arsenic doping on the growth kinetics of ZnO during metalorganic vapor phase epitaxy has been investigated. Arsenic was found to segregate to the growth surface and facilitate layer-by-layer growth. Such surfactant enhances the lateral expansion of the terraces preferential along [1¯1¯20] direction and also reduces the screw lattice distortion. Arsenic is expected to reduce the total surface energy and diffusion barrier of oxygen adatoms, hence producing Zn-rich surface condition on the growth front, in which two-dimensional growth is thermodynamically and kinetically favored. The origin of tiny hexagonal pits formed on the wide terrace is discussed in terms of the modified step-bunching mechanism.

Wet etching and infrared absorption of AlN bulk single crystals

The defects and the lattice perfection of an AlN (0001) single crystal grown by the physical vapor transport (PVT) method were investigated by wet etching, X-ray diffraction (XRD), and infrared absorption, respectively. A regular hexagonal etch pit density (EPD) of about 4000 cm−2 is observed on the (0001) Al surface of an AlN single crystal. The EPD exhibits a line array along the slip direction of the wurtzite structure, indicating a quite large thermal stress born by the crystal in the growth process. The XRD full width at half maximum (FWHM) of the single crystal is 35 arcsec, suggesting a good lattice perfection. Pronounced infrared absorption peaks are observed at wave numbers of 1790, 1850, 2000, and 3000 cm−1, respectively. These absorptions might relate to impurities O, C, Si and their complexes in AlN single crystals.

Identification of SiC polytypes by etched Si-face morphology

The relationship between polytypes and etched Si face morphology of SiC was investigated in this paper. The defect-selective wet-etch technique was used to give an anisotropic etching of SiC. The regular but different shape etching pits were observed on the Si face. Raman spectra were applied to verify the polytypes information of the regions with different etching pits. The experimental results show that the green SiC crystal with perfect hexagonal etching pits is 6H-SiC along the (0 0 0 1) direction and the pistachio region with three long-edges and three short-edges hexagon etching pits belong to 15R-SiC along the (0 0 0 1) direction.

Freestanding AlN single crystals enabled by self-organization of 2H-SiC pyramids on 4H-SiC substrates

A sublimation-recondensation process is presented for high quality AlN (0001) crystals at a high growth rate by employing 4H-SiC substrates with a predeposited epilayer. It is based on the coalescence of well oriented AlN microrods, which evolve from the apex of 2H-SiC pyramids grown out of hexagonal pits formed by thermal etching of the substrate during a temperature ramp up. This process yields stress-free 120-μm-thick AlN single crystals with a dislocation density as low as 2×106cm−2.

Carrier Dynamics in UV InGaN Multiple Quantum Well Inverted Hexagonal Pits

The emission and recombination characteristics of UV or blue light emission from InGaN/GaN quantum well (QW) structures influenced by V-shaped pits have been investigated by near-field and time-resolved photoluminescence measurements. Localization of charge carriers due to the potential barriers caused by the V-shaped pit formation is observed to be modified by thermal excitation. Temperature dependence of recombination dynamics shows evidence of a more complex potential barrier produced by the inverted hexagonal pits embedded within the multiple QWs. The emission from the narrow V-shaped pit QWs shows anomalous temperature dependence behavior that is significantly different from the emission from c-plane QWs. The carrier recombination process in c-plane QWs is significantly longer ~ 5 ns compared to the ~ 1.5 ns in V-shaped pit QWs at low temperatures due to the larger piezoelectric fields in wider wells. At room temperature, the recombination lifetimes are comparable due to increased carrier separation and delocalization within the V-shaped pit QWs.

Morphology and defect of a-GaN grown by metal orgamic chemical vapor deposition

On the substrate of r-sapphires a-GaN films grown by metal orgamic chemical vapor deposition was etched in molten KOH-NaOH for 1.0,1.5 and 2.0 min. Scanning electron microscope, atomic force microscope, X-ray diffraction and cathodoluminescence was used to study its morphology and defect. We find that etching for 1.5 min at the temperature of 400 ℃ is appropriate for a-GaN on sapphires substrate. Different from c-GaN which show hexagonal pits, a-GaN shows parallelogram strips. In the direction of c axis it is easier to be etched. That is because the polarity of a-GaN films is anisotropic which leads to different absorption capacities of OH ions in different directions. We also find hexagonal protuberance on the surface which is associated with threading dislocations.

Structural and optical inhomogeneities of Fe doped GaN grown by hydride vapor phase epitaxy

We present the results of cathodoluminescence experiments on a set of Fe doped GaN samples with Fe concentrations of 5×1017, 1×1018, 1×1019, and 2×1020 cm−3. These specimens were grown by hydride vapor phase epitaxy with different concentrations of Fe. The introduction of Fe is found to promote the formation of structurally inhomogeneous regions of increased donor concentration. We detect a tendency of these regions to form hexagonal pits at the surface. The locally increased carrier concentration leads to enhanced emission from the band edge and the internal T41(G)–A61(S) transition of Fe3+. In these areas, the luminescence forms a finely structured highly symmetric pattern, which is attributed to defect migration along strain-field lines. Fe doping is found to quench the yellow defect luminescence band and to enhance the blue luminescence band due to the lowering of the Fermi level and the formation of point defects, respectively.

Cooperative Function of Ammonium Polyacrylate with Antifreeze Protein Type I

Activity of antifreeze proteins (AFPs) and antifreeze glycoproteins (AFGPs) is often determined by thermal hysteresis, which is the difference between the melting temperature and the nonequilibrium freezing temperature of ice in AF(G)P solutions. In this study, we confirmed that thermal hysteresis of AFP type I is significantly enhanced by a cooperative function of ammonium polyacrylate (NH4PA). Thermal hysteresis of mixtures of AFP type I and NH4PA was much larger than the sum of each thermal hysteresis of AFP type I and NH4PA alone. In mixed solutions of AFP type I and NH4PA in the thermal hysteresis region, hexagonal pyramidal-shaped pits densely formed on ice surfaces close to the basal planes. The experimental results suggest that the cooperative function of NH4PA with AFP type I was caused either by the increase in adsorption sites of AFP type I on ice or by the adsorption of AFP type I aggregates on ice.

Evolution of Surface Morphology by Wet-Etching of ZnO and GaN with Different Polarity

The surface morphologies of ZnO and GaN thin film by wet-chemical etching were investigated. While ZnO was readily etched by acidic solutions regardless of its polarity, GaN was hardly etched by acidic solutions, but etched in alkali-based solutions such as KOH, especially for N-face GaN. Furthermore, we will also discuss the micro-structural evolution of hexagonal pyramids on the O-polar (N-polar) surfaces and hexagonal pits on the Zn-polar (Ga-polar) surfaces of ZnO and GaN, respectively.