Surface Pit Density Research Articles

High-Quality SiC Crystal Growth by Temperature Gradient Control at Initial Growth Stage

A modified process condition has been proposed for the growth of high-quality 6-inch 4H-SiC single crystal. Temperature gradient (dT[°C] = Tbottom-Tupper) was controlled by changing coil position in order to investigate the effect of the temperature gradient on the SiC crystal quality. SiC ingot surface and etch pit density (EPD) of etched SiC wafer were investigated according to different dT conditions at the initial stage of SiC crystal growth. The surface of SiC crystal ingots grown with different dT for 10h were observed by OM and etched SiC wafers were prepared from SiC crystal ingots after main growth step for 100h. Different dT conditions in the initial growth stage resulted in dramatically different surface images and the crystal quality evaluated by EPD.

Improving Optical and Electrical Characteristics of GaN Films via 3D Island to 2D Growth Mode Transition Using Molecular Beam Epitaxy

Molecular beam epitaxy (MBE) is demonstrated as an excellent growth technique for growing a low-defect GaN channel layer, which is crucial for controlling vertical leakage current and improving breakdown voltage (BV) in GaN-based high-electron mobility transistors (HEMTs). The 3D islands to 2D growth mode transition approach was induced by modulating substrate growth temperature (Tsub), displaying an overall improvement in film quality. A comprehensive investigation was conducted into the effects of Tsub on surface morphologies, crystal quality, and the optical and electrical properties of GaN films. Optimal results were achieved with a strain-relaxed GaN film grown at 690 °C, exhibiting significantly improved surface characteristics (root-mean-square roughness, Rq = 0.3 nm) and impressively reduced edge dislocations. However, the film with the smoothest surface roughness, attributed to the effect of the Ga-rich condition, possessed a high surface pit density, negatively affecting optical and electrical properties. A reduction in defect-related yellow emission further confirmed the enhanced crystalline quality of MBE GaN films. The optimized GaN film demonstrated outstanding electrical properties with a BV of ~1450 V, surpassing that of MOCVD GaN (~1180 V). This research significantly contributes to the advancement of MBE GaN-based high electron mobility transistor (HEMT) applications by ensuring outstanding reliability.

Realization of cathodoluminescence in the 180 nm spectral range by suppressing thermal stress in mist chemical vapor deposition of rocksalt-structured MgZnO films

Rocksalt-structured (RS) Mg x Zn1−x O films with x = 0.65–1.0 were grown on MgO (100) substrate using the mist CVD method. A comparative study of the RS-Mg0.92Zn0.08O films grown under slow and rapid-cooling rates apparently showed simultaneous reductions in the surface pit density, FWHM values for the X-ray diffraction peak, and defect-related cathodoluminescence (CL) for the film grown under the slow-cooling rate. CL spectra for the RS-Mg x Zn1−x O films grown under the slow-cooling rate eventually showed near-band-edge emission peaks in the 180–190 nm spectral range for MgO molar fraction x ≥ 0.92 at RT.

N-type conducting AlInN/GaN distributed Bragg reflectors with AlGaN graded layers

We obtained a 40-pair Si-doped n-type conducting AlInN/GaN distributed Bragg reflector (DBR) with a low surface pit density, 3.0 × 106 cm−2, by introducing 5 nm Si-doped Al0.39Ga0.61N graded layers grown at high temperature, 1150 °C. A combination of a 0.6 nm GaN cap layer on AlInN and a subsequent thermal cleaning during a temperature increase process up to 1150 °C for the following AlGaN graded layer growth was effective for a suppression of pit/threading dislocation generations at the interfaces of the AlInN layers and the AlGaN graded layers in the DBRs without any additional cleaning processes. We also found that an initial AlN mole fraction of 0.39 in the graded AlGaN layers provided the lowest vertical resistance of the Si-doped AlInN/GaN DBRs with the Si-doped AlGaN graded layers, suggesting that Al0.39Ga0.61N provides the lowest potential spike in the conduction band of the interface with Al0.82In0.18N among AlGaN alloys.

V-Pits-Induced Photoresponse Enhancement in AlGaN UV-B Photodetectors on Si (111)

We demonstrate the influence of surface terminated V-pits in tuning dark current and spectral responsivity of Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub> N-based UV-B photodetectors with metal-semiconductor-metal geometry on Si (111) substrate. We show that the V-pit morphological defects contribute to a large internal gain in these photodetectors, thereby leading to a substantial enhancement in external quantum efficiency (EQE) at relatively low applied biases. For photodetectors fabricated on metal organic chemical vapor deposition grown Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.25</sub> Ga <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0.75</sub> N epilayers with a surface pit density of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">8</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , an EQE of 100% was measured at a meager bias of 1.7 V, which increased significantly with bias. The EQE, photo-to-dark current ratio, and UV-to-visible rejection ratio measured 5 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> %, 1.2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> , and 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sup> , respectively, at 5 V. The evidence of localized enhancement of photoresponse at the surface terminations of V-pits is exemplified by UV-assisted conductive atomic force microscopy. Temperature-dependent carrier transport analysis under dark and UV illumination revealed cumulative contributions of pit-induced thermionic field emission and hole-trapping-induced gain to the observed large EQE. This work presents the highest value of responsivity for III-nitride UV-B detectors at a given bias.

The influence of AlN nucleation layer on RF transmission loss of GaN buffer on high resistivity Si (111) substrate

A sufficiently low transmission loss in radio frequency (RF) is one of the critical requirements for GaN-on-Si RF devices to achieve high performance. We have systematically studied the mechanism and effect of the AlN nucleation layer on the RF loss of the GaN-on-Si device buffer stack. Our results show that the RF loss is strongly influenced by the growth parameters of the AlN nucleation layer during epitaxial process. It is observed that the AlN nucleation layer grown at a low thermal budget with a low density of deep surface pits can efficiently reduce the AlN/Si interface loss by suppressing the conductive channel at AlN/Si interface which is governed largely by the thermal diffusion of Al and Ga into the Si substrate. By optimizing the growth process of the AlN nucleation layer, the RF loss of the GaN-on-Si device buffer can be dramatically reduced by up to ∼40%.

Etching Mechanisms in III-V Semiconductors: Electrochemical Etching of Indium Phosphide

Anodization of n-InP electrodes was carried out over a range of temperatures and KOH concentrations. Scanning electron microscopy showed <111>A aligned pore growth, with decreasing pore width as the temperature was increased. This variation in pore width is explained in terms of the relative rates of electrochemical reaction and hole diffusion and supports the three-step model proposed earlier. As temperature was increased, both the areal density and width of surface pits decreased, resulting in a large increase in the current density through the pits. This explains a corresponding observed decrease in porous layer thickness: smaller pits sustain mass transport for a shorter time before precipitation of etch products blocks the pores. In galvanostatic experiments, radius of curvature at pore tips increased with current density but pore width decreased. As KOH concentration was increased, both pore width and layer thickness decreased to minima at ~9 mol dm-3 after which they again increased. A transition from porous layer formation to planar etching was observed below 2 mol dm-3 KOH. All of these trends are explained by the three-step model and mass transport effects in surface pits.

Optimizing reactive ion etching to remove sub-surface polishing damage on diamond

Low defect smooth substrates are essential to achieve high quality diamond epitaxial growth and high performance devices. The optimization of the Ar/O2/CF4 reactive ion etching (RIE) plasma treatment for diamond substrate smoothing and its effectiveness to remove subsurface polishing damage are characterized. An O2/CF4 RIE process and the effect of different process parameters (inductively coupled plasma, platen power, and pressure) were initially examined. This process, however, still produced a detrimental effect to surface roughness, with etch pits across the surface of the sample. The addition of argon to the process achieved near-zero surface pit density and reduced roughness by 20%–44% after 6 and 10 μm etching. Iterative high-resolution X-ray diffraction measurements provided a nondestructive tool to examine the effectiveness of polishing damage removal and in this case reduced after removal of 6 μm of material from the surface of the diamond substrate with the smoothing treatment.

Surface texture evaluation of additively manufactured metallic cellular scaffolds for acetabular implants using X-ray computed tomography

The surface topography of acetabular implants plays a key role in providing cell attachment and proliferation. The measurement and characterisation of the surface texture of the cellular scaffold layer on the acetabular cup are very difficult due to the 3D nature of scaffold geometry. It is proposed to use X-ray computed tomography (XCT) to measure the surface texture of an electron beam melting-produced titanium acetabular cup. The surface texture of its cellular scaffold is evaluated using the newly developed 3D surface texture parameters, which allows surface characterisation on 3D triangular mesh surfaces. Four commonly used height parameters, i.e. the arithmetical mean height Sa, the root mean square height Sq, the skewness Ssk and the kurtosis Sku, are calculated from surface patches extracted from the XCT scanned triangular mesh surface. In addition, the surface peak density and pit density, which are more related to cell communication and proliferation, are estimated based on the 3D watershed segmentation. The Wolf pruning with an empirical threshold 12 µm is used to control the over-segmentation.

Anodic Formation of Nanoporous Indium Phosphide in KOH Electrolytes: Effects of Temperature and Concentration

Anodization of n-InP electrodes was carried out over a range of temperatures and KOH concentrations. Scanning electron microscopy showed <111>A aligned pore growth with pore width decreasing as the temperature was increased. This variation is explained in terms of the relative rates of electrochemical reaction and hole diffusion and supports the three-step model proposed earlier. As temperature is increased, both the areal density and width of surface pits decrease resulting in a large increase in the current density through the pits. This explains an observed decrease in porous layer thickness: pits sustain mass transport at the necessary rate for a shorter time before precipitation of etch products blocks the pores. As the concentration of KOH is increased, both pore width and layer thickness decrease to minima at ∼9 mol dm−3 after which they again increase. This variation of pore width is also explained by the three-step model and the variation in layer thickness is explained by mass transport effects. Layer porosity follows a similar trend to pore width, further supporting the three-step model. A transition from porous layer formation to planar etching is observed below 2 mol dm−3 KOH, and this is also explained by the three-step model.

Influence of the Al content of the AlGaN buffer layer in AlGaN/GaN high‐electron‐mobility transistor structures on a Si substrate

We investigated the effect of the Al content in the AlGaN buffer layer on the initial AlN nucleation layer, for AlGaN/GaN high‐electron‐mobility transistors (HEMT), on a Si substrate. Reducing the Al content in the AlGaN layer decreased the surface pit density of the AlGaN layer, and increased the leakage current of the AlGaN/AlN/Si structures, but the crystal quality of the AlGaN layer was not changed by the Al content. The dislocation density of the GaN layer and the two‐dimensional electron gas characteristics of the HEMT structures, were almost the same for the AlGaN buffer layer with differing Al content. However, the vertical‐direction breakdown voltage (VDBV) was decreased at an Al content of 0.760 compared with other HEMT structures. In addition, the fluctuations of SLS layer were observed at an Al content of 0.760. The warpage and cracking of the HEMT structures were minimized at an Al content of 0.558. Based on these results, the VDBV of HEMT structures are not correlated with the VDBV of the AlGaN buffer layer. However, the VDBV of HEMT structures is affected by the surface pit density of the AlGaN buffer layer. In addition, the warpage of HEMTs on a Si substrate, can be controlled by the Al content of the AlGaN buffer layer.

The Relationship between Surface Pits Density and Growth Parameters during the Epitaxial Growth of 4H-SiC

In this study, different parameters of 4H-SiC epitaxial growth were used to investigate the influence on surface pits density. It was found that the density of surface pits can be reduced significantly at lower C/Si ratio condition but doping uniformity became worse simultaneously. The background doping was higher than 2E15 cm-3 when C/Si ratio was lower than 1.0. Influences of growth temperature and growth rate are also discussed. The lower surface pits density 4H-SiC epilayer with good uniformity (s/mean below 2%) can be realized during optimal condition.

Composition, structure and morphology of Al1−xInxN thin films grown on Al1−yGayN templates with different GaN contents

Four nominally Al0.85In0.15N thin films with different strains were grown simultaneously on Al1−yGayN (y = 1, 0.93, 0.87 and 0.69) templates by metal organic chemical vapour deposition. The nominal InN content of ~15% was chosen to achieve close lattice matching of Al1−xInxN with the templates of intermediate GaN molar fractions, a small tensile strain for growth on GaN, and compressive strain for the template with the lowest GaN fraction. The film deposited on GaN reveals the highest structural quality, the lowest surface roughness and a homogeneous composition with depth. For growth on the Al1−yGayN ternary templates, the film roughness and the surface pit density both increase with decreasing GaN content, in line with the roughening of the growth templates themselves. Detailed study indicates that the structural and morphological qualities of the templates influence not only the quality of the Al1−xInxN films but also their composition profile. Results suggest that surface roughness of the template and growth on the inclined facets lead to compositional gradients due to a decreased InN incorporation on these facets and to the formation of V-pits.

Effect of growth temperature on the epitaxial growth of ZnO on GaN by ALD

We report on the epitaxial growth of ZnO on GaN template by atomic layer deposition (ALD). Diethylzinc (DEZn) and water vapour (H2O) were used as precursors. The structure and the quality of the grown ZnO layers were studied with scanning electron microscope (SEM), X-ray diffraction (XRD), photoluminescence (PL) measurements and positron annihilation spectroscopy. The ZnO films were confirmed epitaxial, and the film quality was found to improve with increasing deposition temperature in the vicinity of the threshold temperature of two dimensional growth. We conclude that high quality ZnO thin films can be grown by ALD. Interestingly only separate Zn-vacancies were observed in the films, although ZnO thin films typically contain fairly high density of surface pits and vacancy clusters.

Effects of Mg-doped AlN/AlGaN superlattices on properties of p-GaN contact layer and performance of deep ultraviolet light emitting diodes

Mg-doped AlN/AlGaN superlattice (Mg-SL) and Mg-doped AlGaN epilayers have been investigated in the 284 nm deep ultraviolet (DUV) light emitting diodes (LEDs) as electron blocking layers. It was found that the use of Mg-SL improved the material quality of the p-GaN contact layer, as evidenced in the decreased density of surface pits and improved surface morphology and crystalline quality. The performance of the DUV LEDs fabricated using Mg-SL was significantly improved, as manifested by enhanced light intensity and output power, and reduced turn-on voltage. The improved performance is attributed to the enhanced blocking of electron overflow, and enhanced hole injection.

Effect of Deoxidation Pretreatment on the Corrosion Inhibition Provided by a Trivalent Chromium Process (TCP) Conversion Coating on AA2024-T3

Aluminum alloy surfaces are typically (i) degreased and (ii) deoxidized and or desmutted to expose a clean and low-oxygen surface in preparation for the application of a pretreatment conversion coating. We hypothesized that the corrosion inhibition provided by one type of conversion coating, trivalent chromium process (TCP), would depend on the deoxidation chemistry and treatment time due to effects on the specimen's surface cleanliness and surface roughness, pit density, and pit depth. To this end, the effect of treatment time in an acidic fluoride deoxidizer/desmutter on the corrosion inhibition provided by a TCP coating on AA2024-T3 was studied. Longer treatment times of 6 and 12 min produced greater surface roughness, pit density and pit depth than did shorter times of 0.5 and 2 min. As a consequence, the TCP coating formed less completely and with more defects over the former surfaces. Greater corrosion inhibition was found on the less damaged specimens as evidenced by decreased anodic and cathodic currents in polarization curves, increased polarization resistances, Rp, and more positive breakdown or pitting potentials, Epit, in a choride-containing electrolyte solution. Deoxidation in 0.1 M NaOH was also investigated because of the more uniform aluminum dissolution that occurs in this medium. These specimen surfaces remained relatively smooth with the TCP coating providing the greatest level of corrosion inhibition, as evidenced by a larger Rp and more positive Epit. This is attributed to reduced surface roughing and pitting as well as the formation of an inert oxide on the exposed copper intermetallics.

Effect of Current Density on Pore Formation in n-InP in KOH

InP samples which were galvanostatically anodised in 5 mol dm-3 KOH exhibited crystallographic pore etching at current densities ranging from 1 mA cm-2 to 20 mA cm-2. Surprisingly, the total charge at growth termination (~1.8 C cm-2) and the corresponding layer thickness (~3.0 µm) were independent of current density in experiments at continuous constant current. Surface pit density was proportional to current density, indicating that the average current per pit was 1.7 pA, independent of current density. The influence of mass transport on the termination of porous layer growth was demonstrated in an experiment at low current density where the current was interrupted a number of times before layer growth ended. At low current densities, pores had sharp tips, triangular cross-sections and large pore widths. At higher current densities, both the pore tips and the pore cross-sections became more rounded while the pore width decreased. These variations are explained in terms of the effect that different rates of hole supply (at different current densities) can have on the rate of indium vacancy formation on the three {111}A faces that make up the pore tip.

Detection of surface carbon and hydrocarbons in hot spot regions of niobium superconducting rf cavities by Raman spectroscopy

Raman microscopy/spectroscopy measurements are presented on high purity niobium (Nb) samples, including pieces from hot spot regions of a tested superconducting rf cavity that exhibit a high density of etch pits. Measured spectra are compared with density functional theory calculations of Raman-active, vibrational modes of possible surface Nb-O and Nb-H complexes. The Raman spectra inside particularly rough pits in all Nb samples show clear differences from surrounding areas, exhibiting enhanced intensity and sharp peaks. While some of the sharp peaks are consistent with calculated NbH and ${\mathrm{NbH}}_{2}$ modes, there is better overall agreement with C-H modes in chain-type hydrocarbons. Other spectra reveal two broader peaks attributed to amorphous carbon. Niobium foils annealed to $&gt;2000\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ in high vacuum develop identical Raman peaks when subjected to cold working. Regions with enhanced C and O have also been found by SEM/EDX spectroscopy in the hot spot samples and cold-worked foils, corroborating the Raman results. Such regions with high concentrations of impurities are expected to suppress the local superconductivity and this may explain the correlation between hot spots in superconducting rf (SRF) cavities and the observation of a high density of surface pits. The origin of localized high carbon and hydrocarbon regions is unclear at present but it is suggested that particular processing steps in SRF cavity fabrication may be responsible.

Compatibility of Biofuel/Diesel Blends on Storage Tank Material

Non- edible oils from the plant seeds are the most promising alternative fuel for diesel engines as they are renewable, environment friendly, non-toxic, biodegradable, no sulphur and aromatics, favourable heating value and higher cetane number. But their inherently high viscosity compared to fossil diesel is undesirable for diesel engines. Lowering the viscosity can be achieved by breaking the triglyceride molecules and separating the fatty acid molecules from the glycerine molecules - transesterification. Biodiesel is more susceptible to oxidative attack than diesel fuel. The rate of oxidation is determined by the degree of unsaturation in the fatty acid alkyl ester chain, water content and environmental factors such as temperature, nature of storage container and air exposure during storage. Acceptable storage tank materials include mild steel, stainless steel, fluorinated polyethylene and polypropylene. Biodiesel has a solvent effect which releases the deposits accumulated on tank walls leading to corrosion. In the present investigation, the compatibility of biodiesel from Pongamia pinnata (PBD) and Jatropha curcus (JBD) and their different blends with commercial diesel (5%, 10% and 20%) with mild steel has been studied by mass loss method for a period of 100 h. 3% NaCl was used to depict water contamination. Though negligible corrosion rates were observed in both the biodiesels, corrosion rate in JBD was found to be higher. The surface topography of the metal samples after the exposure period was analyzed by scanning electron microscopy. The surface profile, pit distribution and pit density of the metal coupons were determined by laser profilometry. The wettability studies also supported the non corrosive nature of the biodiesel used.

Modification of dislocation behavior in GaN overgrown on engineered AlN film-on-bulk Si substrate

The changes that the AlN buffer and Si substrate undergo at each stage of our substrate engineering process, previously shown to lead to a simultaneous and substantial reduction in film crack density and dislocation density in overgrown GaN, are presented. Evidence of ion-implantation assisted grain reorientation for AlN islands coupled with physical isolation from the bulk Si substrate prove to be the dominating driving forces. This is further emphasized with x-ray diffraction analysis that demonstrates a reduction in the in-plane lattice constant of AlN from 3.148 Å to 3.113 Å and a relative change in rotation of AlN islands by 0.135° with regard to the Si substrate after substrate engineering. Misfit dislocations at the AlN-Si interface and disorder that is normally associated with formation of amorphous SiNx at this interface are considered to be two of the major contributors to dislocation nucleation within overgrown GaN. Following our technique, the disappearance of disorder at the AlN-Si interface is observed. Extensive ellipsometry and transmission electron microscopy suggests that larger AlN islands with a smoother surface morphology could further reduce the dislocation density below that previously reported. A 1.2 μm GaN layer deposited on an AlN buffer with larger islands and smoother morphology exhibits a 14× reduction in surface pit density after undergoing the ion-implantation assisted substrate modification technique.