Inclination Of Dislocations Research Articles

10.6% external quantum efficiency germicidal UV LEDs grown on thin highly conductive n-AlGaN

We report on the material challenges of the growth of highly conductive n-AlGaN in germicidal ultraviolet light emitting diodes (GUV LEDs), with the degradation of the surface morphology of thick highly doped n-AlGaN due to the Si anti-surfactant effect. Threading dislocation inclination, increasing relaxation, and eventual cracking were observed with epitaxial n-AlGaN films thicker than 400 nm, along with an increasing Ga composition with the same metalorganic flows. With the optimization of the n-AlGaN conductivity in previous works, thin n-AlGaN films with high conductivity along with a smoothing superlattice were incorporated in GUV LED devices, resulting in LEDs with 285 nm electroluminescence, a low forward voltage of 4.2 V with a peak external quantum efficiency (EQE) of 10.6% and a peak wall-plug efficiency of 8.6% below 1 A/cm2, and an EQE of 5.5% at 20 A/cm2.

Growth mode modulation and crystalline quality improvement of highly relaxed n-Al0.6Ga0.4N on high-temperature-annealing AlN/sapphire template via SiH4-pretreatment

High-quality AlGaN-based film is the key to attain efficient deep ultraviolet emitters. In this study, we developed a silane-pretreatment method to modulate the growth mode of n-Al0.6Ga0.4N on high-temperature-annealing AlN/sapphire template (HTA-AlN) and improve the crystalline quality. It’s found that pretreating the surface of HTA-AlN template by SiH4 could disturb the pseudo-crystal epitaxy of n-Al0.6Ga0.4N and prompt the 3D growth at the AlGaN/AlN interface. The stress of n-Al0.6Ga0.4N was thus released rapidly by fresh-born dislocations. During the following growth, the growth mode of n-Al0.6Ga0.4N with SiH4-pretreatment transformed quickly from 3D to 2D, and the stress was released gradually because of the dislocation inclination. Besides, the threading dislocation density of 6 µm-thick n-Al0.6Ga0.4N with SiH4-pretreatment was reduced to 8.6 × 108 cm−2, which was one third of that without SiH4-pretreatments. This work paves the way for preparation and improvement of the DUV emitters.

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 management and AlN crystal quality improvement with an alternating V/III ratio AlN superlattice

We report the metal-organic chemical vapor deposition growth of high-quality AlN on sapphire enabled by an alternating V/III ratio AlN superlattice. We demonstrated that the insertion of an alternating V/III ratio AlN superlattice facilitates the relaxation of tensile stress during growth and assists the annihilation of threading dislocations. Dislocation inclination was proposed to play a major role in the stress relaxation and dislocation reduction, which not only provides an effective misfit-dislocation component but also increases the dislocation reaction probability. By this method, crack-free 3.25 μm-thick AlN films were grown on flat sapphire substrate (FSS) and nano-patterned sapphire substrate (NPSS) at 1180 °C, and the full width at half maximum of (002)/(102) rocking curves was 204/408 and 152/323 arcsec for such AlN/FSS and AlN/NPSS templates.

Controlled nucleation time for improving aluminum nitride growth

In this work, aluminum nitride (AlN) nucleation was varied at 30, 60 and 120 s as an attempt to reduce the surface roughness and the dislocation density of the overgrown AlN layer. By increasing the nucleation time up to 60 s, the surface became smoother and the dislocation density decreased. With 60 s of nucleation, the dislocation density can be as low as 8.63 × 108 cm -2. Under this condition, the formation of 3D growth improved. This, in turn, favored more dislocation inclination in the AlN layer. However, the 3D growth disappeared by prolonged nucleation for 120 s. As a result, no further improvement of the AlN layer was observed.

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.

Inclination of screw dislocations on the performance of homoepitaxial GaN based UV photodetectors

We analyze the dislocation-assisted charge carrier transport and effective photocurrent generation in the photodetection devices. Specifically, the impact of screw dislocations on the performance of GaN based metal–semiconductor-metal ultraviolet photodetector is investigated. The experiments reveal that reducing screw dislocations had a strong impact on dark current (~3x decrement) of the devices as well as on the photo-generated current (~20x enhancement) upon illumination (ultraviolet, 325 nm). The responsivity of the photodetection device has been increased from 85.05 mA/W to 130.17 mA/W with decreasing dislocation density. Because, higher dislocations created leakage paths that develop trap states which enhances the possibility of recombination process of photo-generated electron-hole pairs leading to lower charge collection. Further, the external quantum efficiency increases from 32.51% to 49.76% by reducing dislocation density. The work in this study proposes that reduction of defects/dislocations will be an effective approach to enrich the III-nitride semiconductor system for advancement in optoelectronic devices.

Vacancy-engineering-induced dislocation inclination in III-nitrides on Si substrates

The incorporation of point defects into semiconductors could substantially tailor their optical and electrical properties as well as the spin-based quantum properties. In terms of structural properties, however, efforts have seldom been devoted to the relevant aspects. Herein, we propose point defects engineering by intentionally introduced vacancies to improve the structural properties. GaN-on-Si are selected as a paradigm to demonstrate the applicability of this approach. By tuning the growth stoichiometry, nonequilibrium Ga vacancies are intentionally introduced and absorbed by dislocation cores, which leads to dislocation inclination and annihilation in GaN. In addition, this dislocation inclination can proceed without relaxing the compressive lattice stress. These together enable high quality GaN thick layers on Si substrates with dislocation density of $1.6\ifmmode\times\else\texttimes\fi{}{10}^{8}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}2}$ and a record electron mobility of $1090\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{2}/\mathrm{V}\phantom{\rule{0.16em}{0ex}}\mathrm{s}$ at a carrier density of $1.3\phantom{\rule{0.16em}{0ex}}\ifmmode\times\else\texttimes\fi{}\phantom{\rule{0.16em}{0ex}}{10}^{16}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. With these advances, a quasivertical GaN Schottky barrier diode with the lowest specific on-resistance of $0.95\phantom{\rule{0.16em}{0ex}}\mathrm{m}\mathrm{\ensuremath{\Omega}}/\mathrm{c}{\mathrm{m}}^{2}$ and highest on/off ratio of ${10}^{10}$ on Si substrates is demonstrated. These results demonstrate the promise of point defect engineering as a strategy to improve the structural properties and pave the way for high-performance III-nitride based electronic and optoelectronic devices on Si platforms.

Anterior Retropharyngeal Cage Distraction and Fixation for Basilar Invagination: "The Wedge Technique".

ObjectiveSurgery is indicated for basilar invagination (BI) in symptomatic patients. In many patients, symptoms and signs occur due to an upward-migrated and malaligned odontoid with fixed or mobile atlantoaxial instability. Posterior distraction and fixation of the atlantoaxial joints has evolved to become the standard of care, but has some inherent morbidity. In this study, we propose that the unilateral anterior submandibular retropharyngeal approach with customized wedge-shaped titanium cages inserted into both atlantoaxial joints and anterior atlantoaxial fixation with a plate screw construct is a safer and easier option in many cases of BI. MethodsFrom February 2014 to February 2019, 52 patients (age range, 15–78 years; 40 males and 12 females) with symptomatic BI with atlantoaxial dislocation and minimal sagittal facetal inclination and only mild Chiari malformation without syringomyelia were offered anterior submandibular retropharyngeal atlantoaxial distraction and fixation surgery. ResultsNeurological improvement occurred in 80% of patients, while the neurological status of 20% remained unchanged. No patients worsened, and no major complications or mortality was observed. ConclusionIn properly selected cases of symptomatic BI, anterior wedge cage distraction with anterior atlantoaxial fixation is a safe and simple option.

Analysis of strain and dislocation evolution during MOCVD growth of an AlGaN/GaN power high-electron-mobility transistor structure

We present the results of a comprehensive analysis of GaN-on-Si based HEMT epi-wafers grown by metal-organic chemical vapor deposition (MOCVD) in a production-scale reactor. An AlGaN/AlN superlattice was used as the buffer layer. Detailed characterization was combined with process modeling by STREEM-AlGaN software. Comparative analysis of modeling results, characterization data, and in situ curvature measurements allows the study of the evolution of structural properties of the epi-wafer during growth. The initial compressive mean stress in the superlattice gradually decreases during starting period of the growth and then becomes almost constant. The filtering of the dislocations is more effective in the bottom part of the SL, as both experiment and modeling demonstrate large inclination of dislocations in AlGaN layers of the superlattice, while the predicted dislocation density decreases due to annihilation. Proposed buffer layer and growth recipe resulted in final reduction of the dislocation density to ∼2 · 108 cm−2 with good structural uniformity over 6″ wafers and a residual bow below 50 μm.

Impact of metalorganic vapor phase epitaxy growth conditions on compressive strain relaxation in polar III-nitride heterostructures

A novel approach to estimating the critical thicknesses (CTs) of compressively strained III-nitride layers is suggested, based on a quasi-thermodynamic growth model and accounted for the effect of material decomposition during dislocation half-loop formation on the CT value. The approach provides good quantitative agreement with available data on CTs of MOVPE-grown InGaN/GaN and AlGaN/AlN epilayers. The extremely large CTs observed for high Al-content AlGaN alloys grown on bulk AlN substrates may be attributed, in particular, to the dominant AlGaN decomposition mechanism, producing group-III metallic liquid and gaseous nitrogen. The suggested approach may also be helpful for analysis of threading dislocation inclination in compressively strained layers and applicable to studying point defect formation in semiconductors and its dependence on growth conditions.

Coordinated stress management and dislocation control in GaN growth on Si (111) substrates by using a carbon nanotube mask.

A novel method is proposed to realize coordinated stress management and dislocation control in GaN growth on Si (111) substrates. This method is facilely implemented by using a unique carbon nanotube (CNT) mask with a nanoscale diameter and a high fill factor. It is found that most dislocations are directly blocked by high-density nanovoids formed around CNTs within 150 nm from the GaN/AlN interface. In this way, without dislocation inclination, the threading dislocation density (TDD) of the GaN epilayer is dramatically reduced. Thus the compressive stress relaxation due to dislocation inclination during GaN growth is significantly decreased and more compressive stress can be preserved. In addition, the high-density nanovoids also contribute to decreasing the thermal tensile stress during cooling down. As a result, coordinated stress management and dislocation control are realized and a 3.5 μm-thick crack-free GaN epilayer with a homogeneously low TDD of 2.51 × 108 cm-2 is obtained. With the facile fabrication of the CNT mask and a simple epitaxy procedure, the quality of the GaN epilayer grown on Si substrates is greatly improved, which extends the application of the nanomaterial in the GaN epitaxy on the Si substrate and demonstrates great application potential for high-performance electronic devices.

Characterization of dislocations in GaN layer grown on 4-inch Si(111) with AlGaN/AlN strained layer superlattices

Dislocations in a GaN layer grown on 4-in. Si(111) with AlGaN/AlN strained layer superlattices using a horizontal metal–organic chemical vapor deposition system were characterized by transmission electron microscopy and scanning transmission electron microscopy. Pure screw dislocations were not found in the observed area but mixed and edge dislocations were found. The dislocation density in the GaN layer decreased from the bottom (∼2 × 1010 cm−2) to the top (∼6 × 109 cm−2). Some dislocations were inclined from the c-axis, and half-loop dislocations were observed in the GaN layer. Plan-view weak-beam dark-field analysis indicated that the dislocation inclination was caused by climb and glide motions.

A Comparative Evaluation of Primary Total Hip Arthroplasty via Lateral and Posterolateral Approaches

Background: Total hip arthroplasty (THA) is regarded as the most successful and common surgical approach in orthopedic surgeries. Several surgical approaches have been described for THA to introduce minimally invasive surgical exposures. Posterolateral and lateral approaches are traditional and still most commonly used approaches for primary THA. In deciding which approach to use, consideration of complications, and clinical and radiological outcomes, using one approach or another should be evaluated. Objectives: The purpose of this study was to compare the early postoperative outcomes of posterolateral and lateral surgical approaches. Patients and Methods: The current study was conducted on a group of 134 primary hip arthroplasty of 120 patients who had undergone THA surgery at Shafa Orthopedic Hospital in Tehran, Iran, from March 2011 to October 2014. The lateral approach was used in 79 hips and, posterolateral approach was used in 55 hip based on surgeon’s preference. Two groups were assessed considering the following outcomes: clinical evaluation including Harris hip score (HHS), blood loss, blood transfusion, hemoglobin level, infections, deep vein thrombosis (DVT) and dislocation, as well as radiological evaluation including the proximal femur fracture, limb length discrepancy and cup inclination angle. The follow-up for both groups was at 6, 12, 24, 36 and 48 weeks after the surgery. Results: There was no significant difference between the two approaches regarding demographic characteristics, HHS, blood loss, transfusion, hemoglobin level, dislocation and cup inclination angle. However, there was a significant difference in the incidences of infections, DVT, proximal femur fracture and discrepancy of limb length between the two approaches. Conclusions: The results of this study indicate that both lateral and posterolateral approaches offer similar early clinical outcomes and some different radiological outcomes.

High mobility AlGaN/GaN heterostructures grown on Si substrates using a large lattice-mismatch induced stress control technology

A large lattice-mismatch induced stress control technology with a low Al content AlGaN layer has been used to grow high quality GaN layers on 4-in. Si substrates. The use of this technology allows for high mobility AlGaN/GaN heterostructures with electron mobility of 2040 cm2/(V·s) at sheet charge density of 8.4 × 1012 cm−2. Strain relaxation and dislocation evolution mechanisms have been investigated. It is demonstrated that the large lattice mismatch between the low Al content AlGaN layer and AlN buffer layer could effectively promote the edge dislocation inclination with relatively large bend angles and therefore significantly reduce the dislocation density in the GaN epilayer. Our results show a great potential for fabrication of low-cost and high performance GaN-on-Si power devices.

Defect analysis in AlGaN layers on AlN templates obtained by epitaxial lateral overgrowth

The defect distribution in thick AlN layers obtained by epitaxial lateral overgrowth (ELO-AlN) has been analyzed as a function of the miscut direction of the patterned sapphire substrate. A 0.25° miscut toward the sapphire a-plane leads to formation of smooth ELO-AlN layers containing vertical coalescence grain boundaries and exhibiting an almost homogeneous threading dislocation (TD) distribution with a TD density ranging from 5×108cm−2 to 8×108cm−2. In contrast, a 0.25° miscut toward the sapphire m-plane results in formation of periodically arranged macrosteps on the surface of the coalesced ELO-AlN layers as well as formation of inclined coalescence grain boundaries leading to an inhomogeneous TD distribution. A subsequent AlxGa1−xN deposition onto ELO-AlN template with surface macrosteps leads to Ga enrichment on the step sidewalls and, for lower Al-contents (e.g. x=0.5), even to additional defect formation. For higher Al contents (e.g. x=0.8) no additional threading dislocations are formed in the AlGaN layers and the observed TD density corresponds to that of the ELO-AlN template: less than 108cm−2 in the wing regions and from 6×108cm−2 to 9×108cm−2 above the ridges. Compressive strain during growth of Al0.8Ga0.2N on ELO-AlN tends to be compensated by threading dislocation inclination. However, due to the low TD densities the inclination angles are more than 3 times larger than those observed in Al0.8Ga0.2N layers on planar AlN/sapphire templates.

The influence of buffer layer coalescence on stress evolution in GaN grown on ion implanted AlN/Si(111) substrates

The effect of AlN buffer layer morphology on the evolution of growth stress in GaN epilayers deposited by metalorganic chemical vapor deposition on N+ ion-implanted AlN/Si(111) substrates was investigated. AlN buffer layers were grown using either a continuous or pulsed source flow process which altered the grain size and extent of coalescence of the films. In situ stress measurements revealed that substrate implantation reduced the initial compressive stress in the GaN epilayers likely due to a decoupling of the AlN lattice from the underlying crystalline Si substrate. The buffer layer morphology was found to significantly alter the influence of ion-implantation on the film properties. GaN epilayers grown on ion-implanted AlN/Si(111) substrates prepared with the pulsed conditions exhibited a 63% decrease in threading dislocation (TD) density compared to unimplanted substrates. In addition, these films were observed to grow under a low overall stress compared to the other samples which exhibited a more typical compressive to tensile stress transition during growth. The low overall growth stress of the GaN grown on the implanted pulsed AlN/Si(111) was explained in terms of a reduced strain gradient from dislocation inclination.

Effects of Silicon Doping and Threading Dislocation Density on Stress Evolution in AlGaN Films

ABSTRACTIn-situ wafer curvature measurements were used to study the effect of Si doping on intrinsic growth stress during the metalorganic chemical vapor deposition (MOCVD) growth of AlxGa1-xN (x=0-0.62) layers on SiC substrates. Post-growth transmission electron microscopy (TEM) characterization was used to correlate measured changes in stress with changes in film microstructure. Si doping was found to result in the inclination of edge-type threading dislocations (TDs) in AlxGa1-xN which resulted in a relaxation of compressive stress and generation of tensile stress. The experimentally measured stress gradient was similar to that predicted by an effective climb model. Dislocation inclination resulted in a reduction in the TD density for Si-doped layers compared to undoped AlxGa1-xN likely due to increased opportunities for dislocation interaction and annihilation. The TD density, which increased with increasing Al-fraction, was found to significantly alter the stress gradients in the films. Film stress was also observed to play a role in TD inclination. In undoped AlxGa1-xN, TD inclination was observed only when the film grew under a compressive stress while in Si-doped AlxGa1-xN, TD inclination was observed independent of the sign or magnitude of the film stress. Si dopants are believed to alter the concentration of surface vacancies which gives rise to dislocation jog via a surface-mediated climb mechanism.

On the strain in n-type GaN

It was demonstrated that Ge has the same effect as Si on the strain evolution in n-type GaN as measured by x-ray diffraction. Dislocation inclination, which causes tensile strain in n-type GaN, was clearly observed by transmission electron microscopy where Ge doping was introduced during epitaxial growth. This result is explained by the Fermi level effect model that indicates dislocation inclination due to the climbing process through Ga vacancies. Therefore, there is no dependence of dislocation inclination on dopant species.

Strain in Si doped GaN and the Fermi level effect

Using high resolution x-ray diffraction and Hall effect measurements, we found that the tensile strain caused by dislocation inclination in Si doped GaN became immeasurable when carbon codoping was used to compensate the free carriers. This result suggested that the tensile strain is related to free carrier concentration instead of Si concentration. Such an effect could be explained by the Fermi level effect on the surface-mediated dislocation climb governed by Ga vacancies, whose concentration is strongly influenced by the Fermi level position. This phenomenon is possibly similar to the well-known Fermi level effect in GaAs and GaP systems.