Thermal Oxidation of Silicon Carbide Substrates

Thermal oxidation was used to remove the subsurface damage of silicon carbide (SiC) surfaces. The anisotrow of oxidation and the composition of oxide layers on Si and C faces were analyzed. Regular pits were observed on the surface after the removal of the oxide layers, which were detrimental to the growth of high quality epitaxial layers. The thickness and composition of the oxide layers were characterized by Rutherford backscat-tering spectrometry (RBS) and X-ray photoelectron spectroscopy (XPS), respectively. Epitaxial growth was performed in a metal organic chemical vapor deposition (MOCVD) system. The substrate surface morphol-ogy after removing the oxide layer and gallium nitride (GaN) epilayer surface were observed by atomic force microscopy (AFM). The results showed that the GaN epilayer grown on the oxidized substrates was superior to that on the unoxidized substrates.     

Selective area growth of GaN nanowires using metalorganic chemical vapor deposition on nano-patterned Si(111) formed by the etching of nano-sized Au droplets

We report on the selective area growth of GaN nanowires (NWs) on nano-patterned Si(111) substrates by metalorganic chemical vapor deposition. The nano-patterns were fabricated by the oxidation of Si followed by the etching process of Au nano-droplets. The size of formed nano-pattern on Si(111) substrate was corresponding to the size of Au nano-droplet, and the diameter of GaN NWs grown was similar to the diameter of fabricated nano-pattern. The interesting phenomenon of using the nano-patterned Si(111) substrates is the formation of very clear substrate surface even after the growth of GaN NWs. However, in the case of GaN NWs grown using Au nano-droplets, there was several nanoparticles including GaN bulk grains on the Si(111) substrates. The smooth surface morphology of nano-patterned Si(111) substrates was attributed to the presence of SiO₂ layer which prevents the formation of unnecessary GaN particles during the GaN NW growth. Therefore, we believe that nano-patterning method of Si(111) which was obtained by the oxidation of Si(111) substrate and subsequent Au etching process can be utilized to grow high-quality GaN NWs and its related nano-device applications.     

射频等离子体分子束外延GaN的极性研究

对采用射频等离子体分子束外延（RF-plasma MBE)生长得到的GaN进行极性研究。由于镓极性（Ga-polar)比氮极性（N－polar)有更好的化学稳定性，通过比较RF－plasma MBE生长得到的不同GaN样品对光辅助湿法刻蚀的稳定性，发现缓冲层生长条件对GaN外延层的极性有着重要影响：较高缓冲层生长温度得到的GaN外延层表现为N－polar，较低缓冲层生长温度得到的GaN外延层表现为Ga-polar。     

Formation of GaN nanostructures with various morphologies by photo-assisted electroless chemical etching

Interaction of GaN crystal faces with chemicals is crucial to understand why various nanostructures are formed during the etching process. We have prepared GaN nanostructures by a photo-assisted electroless chemical etching method in solutions containing KOH and K₂S₂O₈. Morphology nanostructure GaN layers grown by molecular beam epitaxy (MBE) and hydride vapor phase epitaxy (HVPE) were studied. For the GaN layers grown by MBE, the etching reaction process starts at grain boundaries and dislocation domains on the surface and inverted hexagonal pyramids are eventually formed. For the GaN layers grown by HVPE, scattered etch pits with well-defined hexagonal facets are observed after the etching process.     

Lateral epitaxial overgrowth of GaN films on patterned sapphire substrates fabricated by wet chemical etching

A promising technique of lateral epitaxial overgrowth, namely CantiBridge epitaxy, is developed and demonstrated in order to reduce the threading dislocation density in GaN films. Using metalorganic chemical vapor deposition, the GaN films are grown on patterned sapphire fabricated by wet chemical etching, instead of traditional dry etching. The image of atomic force microscopy shows that the threading dislocations in CantiBridge-epitaxy GaN are reduced sharply, which makes a promising to realize the high-performance GaN-based optoelectronic devices.     

Investigation of sub-nm ALD aluminum oxide films by plasma assisted etch-through

A new technique, called “plasma defect etching” (PDE), is proposed for studying the continuity of ultra-thin layers. The PDE technique utilizes the extremely high selectivity in the deep reactive ion etching (DRIE) process, thus achieving visualization of the defects in the layer, because etching of substrate happens only through voids and microholes of the layer. The etch profile generally reproduces the non-continuous structure of the layer. This PDE technique was applied for the investigation of thin, sub-nm aluminum oxide films grown on silicon wafers by atomic layer deposition (ALD) technique. Silicon substrate was etched by SF₆ at cryogenic temperatures in an inductively coupled plasma (ICP) reactor, exploiting the extremely high ratio of silicon/aluminum oxide etch rates in fluorine plasmas. The surface morphology was characterized by scanning electron microscopy (SEM) and atomic force microscopy (AFM). The PDE method shows that in the case of water as an oxidation precursor, separate islands of aluminum oxide form during the five first ALD cycles. On the other hand, the use of ozone precursor helps to oxidize silicon surface and facilitates growth of a uniform layer.     

Achieve 2-inch-diameter homogeneous GaN films on sapphire substrates by pulsed laser deposition

The 2-inch-diameter homogeneous GaN films have been epitaxially grown on sapphire substrates by pulsed laser deposition (PLD) technique with optimized laser rastering and PLD growth conditions. The as-grown GaN films are characterized by in situ reflection high-energy electron diffraction, white-light interferometry, scanning electron microscopy, atomic force microscopy (AFM), grazing incidence angle X-ray reflectivity, reciprocal space mappings, and micro-Raman spectroscopy for surface morphologies and structural properties. The as-grown 2-inch-diameter single-crystalline GaN films exhibit excellent thickness uniformity with a root-mean-square (RMS) inhomogeneity less than 3.4 % and very smooth surface with a RMS roughness less than 1.3 nm measured by AFM. There is a maximum of 1.2 nm thick interfacial layer existing between the as-grown GaN films and sapphire substrates, and the as-grown 310 nm thick GaN films are almost fully relaxed only with an in-plane compressive strain of 0.044 %. This work demonstrates a possibility for achieving high-quality large-scale GaN films with uniform thickness and atomically abrupt interface by PLD, and is of great interest for the commercial development of GaN-based optoelectronic devices.     

Fabrication of 380 nm ultra violet light emitting diodes on nano-patterned n-type GaN substrate

380 nm ultraviolet (UV) light emitting diodes (LEDs) were grown on patterned n-type GaN substrate (PNS) with silicon dioxide (SiO2) nano pattern to improve the light output efficiency. Wet etched self assembled indium tin oxide (ITO) nano clusters serves as dry etching mask for converting the SiO2 layer grown on n-GaN template into SiO2 nano patterns by inductively coupled plasma etching. Three different diameter of ITO such as 200, 250 and 300 nm were used for SiO2 nano pattern fabrication. PNS is obtained by n-GaN regrowth on SiO2 nano patterns and UV LEDs were grown on PNS template by MOCVD. Enhanced light output intensity was observed by employing SiO2 nano patterns on n-GaN. Among different PNS UV LEDs, LED grown on PNS with 300 nm ITO diameter showed enhancement in light output intensity by 2.1 times compared to the reference LED without PNS.     

GaN single crystals grown on HVPE seeds in alkaline supercritical ammonia

Ammonothermal growth (synthesis in supercritical (sc) ammonia fluid) has the promise of producing large low defect gallium nitride crystals through the application of techniques similar to those used in hydrothermal growth. Retrograde solubility of GaN greater than 5% by weight using group I amides as mineralizers is demonstrated in high nickel content autoclaves at pressures of one to three kilobars and temperatures between 300 to 600°C. The above conditions were optimized to grow single-crystal GaN at rates up to 40 μm per day on one cm² seeds. Gallium nitride Hydride Vapor Phase Epitaxy (HVPE) seeds are placed in the higher temperature zone below the nutrient basket employing the same configurations used in reverse gradient hydrothermal growth of berlinite (AlPO₄). GaN single crystals grown by the ammonothermal technique were characterized by X-ray diffraction, photoluminescence, scanning electron microscopy (SEM), atomic force microscopy (AFM), and chemical etching. The nitrogen-terminated face tends to exhibita flatter surface morphology than the gallium-terminated face, which is made up of a series of hexagonal columns. Major impurities in the crystal include potassium from the mineralizer, metals from the autoclave, and oxygen. The nitrogen-terminated face incorporated a lower level of metallic impurities in comparison with the gallium-terminated face. Finally, several process phenomena such as ammonia decomposition, parasitic nucleation of GaN on the autoclave walls, impurity incorporation, and defect generation in single-crystal GaN layers grown on HVPE seeds are identified and their possible mechanisms are discussed.     

Polarity conversion of hydride vapor phase epitaxy growing GaN via growth interruption modulation

GaN films were deposited by hydride vapor phase epitaxy with and without adopting growth interruption modulation (GIM). The surface morphologies of these samples were observed by atomic force microscopy. After chemical etching, the surface morphology of GaN film grown directly on sapphire changed greatly and turned out to be N-polarity, which could be changed into Ga-polarity when growing via interruption modulation. Photoluminescence spectra showed that optical property of Ga-polar film was better than that of N-polar film. High resolution X-ray diffraction revealed the high crystalline quality GaN film grown by using the GIM method. The polarity conversion was the result of surface reconstruction when adopting GIM method.     

Control of curvature in highly compliant probe cantilevers during carbon nanotube growth

Direct growth of a sharp carbon nanotube (CNT) probe on a very thin and highly flexible cantilever by plasma-enhanced chemical vapor deposition (PECVD) is desirable for atomic force microscopy (AFM) of nanoscale features on soft or fragile materials. Plasma-induced surface stresses in such fabrication processes, however, tend to cause serious bending of these cantilevers, which makes the CNT probe unsuitable for AFM measurements. Here, we report a new tunable CNT growth technique that controls cantilever bending during deposition, thereby enabling the creation of either flat or deliberately curved AFM cantilevers containing a CNT probe. By introducing hydrogen gas to the (acetylene + ammonia) feed gas during CNT growth and adjusting the ammonia to hydrogen flow ratio, the cantilever surface stress can be altered from compressive to tensile stress, and in doing so controlling the degree of cantilever bending. The CNT probes grown under these conditions have high aspect ratios and are robust. Contact-mode imaging has been demonstrated using these probe tips. Such CNT probes can be useful for bio-imaging involving DNA and other delicate biological features in a liquid environment.     

Dynamics of native oxide growth on CdTe and CdZnTe X-ray and gamma-ray detectors

We studied the growth of the surface oxide layer on four different CdTe and CdZnTe X-ray and gamma-ray detector-grade samples using spectroscopic ellipsometry. We observed gradual oxidization of CdTe and CdZnTe after chemical etching in bromine solutions. From X-ray photoelectron spectroscopy measurements, we found that the oxide consists only of oxygen bound to tellurium. We applied a refined theoretical model of the surface layer to evaluate the spectroscopic ellipsometry measurements. In this way we studied the dynamics and growth rate of the oxide layer within a month after chemical etching of the samples. We observed two phases in the evolution of the oxide layer on all studied samples. A rapid growth was visible within five days after the chemical treatment followed by semi-saturation and a decrease in the growth rate after the first week. After one month all the samples showed an oxide layer about 3 nm thick. The oxide thickness was correlated with leakage current degradation with time after surface preparation.     

Improved light extraction from large-area vertical light-emitting diodes with deep hole-patterns using nanosphere lithography

The authors report on an improved light extraction method from large-area vertical light emitting diodes (VLEDs) with deep hole-patterns fabricated using nanosphere lithography. In order to produce the ordered deep-hole patterns on the n-type GaN surface, a 150 nm thick Ni dot mask formed via a lift-off process of the Ni coated onto a 500 nm diameter polystylene bead array was employed to enable deep etching. Three VLEDs-one as a reference with no patterns, and two with periodic 360 nm diameter hole patterns, one with 1.0 microm and the other with 1.5 microm depths on the n-type GaN surface, were prepared for comparison. The light output power measured for the VLEDs with the hole-patterns increased by 4.13 and 4.86 times, respectively, as compared to the reference VLED. These enhancements are attributed to the multiple scatterings of the light from the sidewall of the hole-patterns and to the increased surface area to which the light can approach. The higher light output power obtained for the VLEDs with the deep hole patterns might be due to a photon reabsorption reduction within the n-GaN layer.     

Actuation of polypyrrole nanowires

Nanoscale actuators are essential components of the NEMS (nanoelectromechanical systems) and nanorobots of the future, and are expected to become a major area of development within nanotechnology. This paper demonstrates for the first time that individual polypyrrole (PPy) nanowires with diameters under 100 nm exhibit actuation behavior, and therefore can potentially be used for constructing nanoscale actuators. PPy is an electroactive polymer which can change volume on the basis of its oxidation state. PPy-based macroscale and microscale actuators have been demonstrated, but their nanoscale counterparts have not been realized until now. The research reported here answers positively the fundamental question of whether PPy wires still exhibit useful volume changes at the nanoscale. Nanowires with a 50?nm diameter and a length of approximately 6?μm, are fabricated by chemical polymerization using track-etched polycarbonate membranes as templates. Their actuation response as a function of oxidation state is investigated by electrochemical AFM (atomic force microscopy). An estimate of the minimum actuation force is made, based on the displacement of the AFM cantilever.     

Enhanced wet etching of patterned GaN with ion implantation

We present the enhanced wet etching of GaN epilayer implanted with Au+ ion. Patterned GaN with 2 microm-wide sink-like strips was achieved by using 500 keV Au+ ion implantation and KOH etching. The Dependence of etching depth on etching time for the implantation at different ion fluences was investigated. The experiment showed that the damaged GaN area could be almost etched out at high ion fluence, and the etching depth could exceed the project range of incident 500 keV Au+ ion. The etch pits could be observed at the bottom of the etched area. The -400 nm depth etching could be achieved with high fluence implantation after a long etching time, and the edge of etched area could remain clear until the etching process had passed 40 min. As-deposited SiO2 spheres were used to mask the GaN sample in implantation process to investigate the etching effect. -70 nm wave of the GaN surface was observed. The results of our experiments may suggest an approach to the fabricating of GaN devices.     

Structural and optical characteristics of porous GaN generated by electroless chemical etching

This article reports the structural and optical properties of porous GaN prepared by Pt assisted electroless chemical etching. The images of scanning electron microscopy revealed that the density of the pores increased with etching duration, however, the etching duration has no significant effect on the size and shape of the pores. The atomic force microscopy measurements exhibited that the surface roughness was increased with etching durations; however, for long etching duration, the increase of the surface roughness became insignificant. Raman spectra showed that the absence of two forbidden modes i.e. A₁(TO) and E₁(TO) in the as-grown sample was observed in some porous samples. Optical transmission measurements revealed that the increase of pore density would lead to the reduction of light transmission. The studies showed that the structural and optical properties of the GaN could be influenced by the porosity.     

The growth behavior of GaN film on Si(111) substrate by an ion-beam assisted evaporation process

Highly oriented GaN thin films were grown on Si(111) substrate using an ion beam assisted evaporation method. Nitrogen ions, with a kinetic energy of about 40 eV, was supplied by a Kaufman ion source; and Ga vapor was supplied by thermal evaporation. The surface morphology of the nucleation layer, and the crystalline properties of 200–300 nm thick GaN epi-layer were investigated by atomic force microscopy, transmission electron microscopy, and X-ray diffraction. Film grown under a Ga-rich flux condition produced film growth behavior of large islands of hexagonal configuration. Crystallinity on such film, however, was of poorer quality than other films with smaller islands, grown under high nitrogen ion flux conditions. The full width at half-maximum of (0002) diffraction peak was measured at 52 arcminutes for the GaN epilayer single-stepwise grown at 660°C. Ion-enhanced decomposition occurred, causing no film formation at substrate temperatures above 710°C. Additionally, the effect of predeposition of a buffer layer on GaN crystallinity was investigated for surface roughness. AFM measurement revealed that the GaN buffer layer grown on Si(111) showed smooth surface under the relatively N₂⁺-sufficient condition. The introduction of thin GaN buffer layer, grown at 600°C under N₂⁺-sufficient condition, worked on reducing the lattice-mismatch stress and in-plane misorientation of grains, and thus enhancing the crystallinity of the two-stepwise grown GaN epi-layer. Characteristic behavior of GaN epi-layers, single or two stepwise grown on Si(111), show a type of granular (columnar) epitaxy.     

Removal of a protective coating on Al by ion etching for high reflectance in the far ultraviolet

The effect of ion etching on the reflectance of Al coatings in the far ultraviolet is investigated. Ion etching of an overlayer grown on Al was performed by applying 100-300 eV Ar(+) ions using an ion gun. Ion etching was employed to remove the oxide naturally grown on an Al film that had been in contact with atmosphere. Ion etching was also used to remove part or all of the protective MgF(2) film on Al. The reflectance at 121.6 nm, H Lyman alpha line of the overlayer-removed Al surface was monitored after protecting it with a MgF(2) layer. Ion etching on both types of coatings resulted in an excellent reflectance value at 121.6 nm, whereas a reflectance loss was observed at longer wavelengths.     

Structural and photoluminescence study of thin GaN film grown on silicon substrate by metalorganic chemical vapor deposition

GaN epitaxial layer was grown on Si(111) substrate by metalorganic chemical vapor deposition (MOCVD). The structure consists of 50 nm thick high-temperature grown AlN buffer layer, 150 nm thick AlGaN layer, 30 nm low-temperature grown AlN layer, 300 nm GaN layer, 50 nm AlGaN superlattice layer, followed by 100 nm GaN epitaxial layer. The low-temperature AlN interlayer and AlGaN superlattice layer were inserted as the defect-blocking layers in the MOCVD grown sample to eliminate the dislocations and improve the structural and optical properties of the GaN layer. The dislocation density at the top surface was decreased to ∼ 2.8 × 10⁹/cm². The optical quality was considerably improved. The photoluminescence emission at 3.42-3.45 eV is attributed to the recombination of free hole-to-donor electron. The observed 3.30 eV emission peak is assigned to be donor-acceptor transition with two longitudinal optical phonon side bands. The relationship of the peak energy and the temperature is discussed.     

Influence of Treatment Conditions on the Chemical Oxidative Activity of H2SO4/H2O2 Mixtures for Modulating the Topography of Titanium

Host‐tissue integration of medical implants is governed by their surface properties. The capacity to rationally design the surface physico‐chemical cues of implantable materials is thus a fundamental prerequisite to confer enhanced biocompatibility. Our previous work demonstrated that different cellular processes are elicited by the nanotexture generated on titanium (cpTi) and Ti6Al4V alloy by chemical oxidation with a H₂SO₄/H₂O₂ mixture. Here, we illustrate that by varying the etching parameters such as temperature, concentration, and treatment time, we can create a variety of surface features on titanium which are expected to impact its biological response. The modified submicron and nanotextured surfaces were characterized by scanning electron (SEM) and atomic force (AFM) microscopies. Contact angle measurements revealed the higher hydrophilicity of the modified surfaces compared to untreated samples and Fourier transform infrared spectroscopy (FT‐IR) established that the etching generated a TiO₂ layer with a thickness in the 40–60 nm range.