Edge Dislocation Density Research Articles

Study of Defects and Nano-patterned Substrate Regulation Mechanism in AlN Epilayers.

The high crystal quality and low dislocation densities of aluminum nitride (AlN) grown on flat and nano-patterned sapphire substrate that are synthesized by the metal-organic chemical vapor deposition (MOCVD) method are essential for the realization of high-efficiency deep ultraviolet light-emitting diodes. The micro-strains of 0.18 × 10-3 cm-2 for flat substrate AlN and 0.11 × 10-3 cm-2 for nano-patterned substrate AlN are obtained by X-ray diffractometer (XRD). The screw and edge dislocation densities of samples are determined by XRD and transmission electron microscope (TEM), and the results indicate that the nano-patterned substrates are effective in reducing the threading dislocation density. The mechanism of the variation of the threading dislocation in AlN films grown on flat and nano-patterned substrates is investigated comparatively. The etch pit density (EPD) determined by preferential chemical etching is about 1.04 × 108 cm-2 for AlN grown on a nano-patterned substrate, which is slightly smaller than the results obtained by XRD and TEM investigation. Three types of etch pits with different sizes are all revealed on the AlN surface using the hot KOH etching method.

Degradation in Efficiency of InGaN/GaN Multiquantum Well Solar Cells With Rising Temperature

In this article, the authors theoretically and experimentally investigate the mechanism of degradation of InGaN/GaN multi-quantum well (MQW) solar cells. InGaN/GaN MQW solar cells with chips of size <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1\times1$ </tex-math></inline-formula> mm2 were fabricated and characterized within a range of temperature of 50 °C–250 °C to determine the effects of dislocations on their efficiency. The internal intensity of X-ray diffractions was used to construct a 2-D physical model of the density of edge dislocations in the InGaN/GaN MQW that was then simulated in Silvaco. The results show that the relative error in <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$\eta $ </tex-math></inline-formula> between the simulation and the experiment was less than 5% over the examined range of temperature. This shows that an intensified number of nonradiative recombinations owing to high temperature contribute to edge dislocations that eventually degrade the performance of photoelectric devices. The work here provides a theoretical and experimental basis for the growth of high-quality InGaN materials to fabricate highly efficient solar cells.

Graphene‐Assisted Epitaxy of High‐Quality GaN Films on GaN Templates

AbstractGraphene (Gr)‐assisted epitaxy of semiconductors has been demonstrated as a revolutionary strategy for the fabrication of nitride materials and flexible devices. However, till now, most of the GaN epitaxy assisted by Gr is performed on thermally stable foreign substrates because Gr suffers obvious deterioration due to the decomposition of GaN substrate. Thus, large lattice and thermal mismatches still exist and inevitably lead to a high density of dislocations in nitrides. To address this issue, here, the Gr‐assisted epitaxy of high‐quality GaN films on GaN templates is demonstrated by constructing specially designed self‐organized defective (SOD) Gr as a buffer layer. The SOD Gr allows for the spontaneous relaxation of strain in GaN epilayers by weakening the interfacial interactions; and thus, blocks the upward spread of threading dislocations in GaN templates. Therefore, screw and edge dislocation densities in GaN epilayers show reductions of 51% and 62%, respectively, compared to that in homoepitaxy. Based on the GaN film grown on SOD Gr, the as‐fabricated blue light‐emitting diode exhibits a 17% enhancement of light output power and a better emission wavelength stability than that without Gr. This work opens a practical pathway for the growth of high‐quality nitrides and manufacturing of high‐performance nitride‐based devices.

The Mechanisms of AlGaN Device Buffer Layer Growth and Crystalline Quality Improvement: Restraint of Gallium Residues, Mismatch Stress Relief, and Control of Aluminum Atom Migration Length

The mechanisms of AlGaN device buffer layer growth were studied. Gallium residues in the reactor chamber may be harmful to the quality of the AlN strain modulation layer, which eventually worsens the AlGaN buffer layer. By restraining the gallium residues, the crystalline quality of the AlGaN layer is markedly improved. In addition, enhancing stress relief in nucleation and coalescence stages will reduce the edge dislocations induced by strain relaxation in the 2D growth stage. A slower precursor flow rate can promote the stress relief in nucleation and coalescence stages. By comparison, a suitable suppression of Al atoms’ surface migration can decrease surface roughness, which can be realized by increasing the precursor flow rate. Eventually, we obtained a AlGaN buffer layer having both low edge dislocation density and a flat surface using a two-step growth method.

Ultra-dense dislocations stabilized in high entropy oxide ceramics

Dislocations are commonly present and important in metals but their effects have not been fully recognized in oxide ceramics. The large strain energy raised by the rigid ionic/covalent bonding in oxide ceramics leads to dislocations with low density (∼106 mm−2), thermodynamic instability and spatial inhomogeneity. In this paper, we report ultrahigh density (∼109 mm−2) of edge dislocations that are uniformly distributed in oxide ceramics with large compositional complexity. We demonstrate the dislocations are progressively and thermodynamically stabilized with increasing complexity of the composition, in which the entropy gain can compensate the strain energy of dislocations. We also find cracks are deflected and bridged with ∼70% enhancement of fracture toughness in the pyrochlore ceramics with multiple valence cations, due to the interaction with enlarged strain field around the immobile dislocations. This research provides a controllable approach to establish ultra-dense dislocations in oxide ceramics, which may open up another dimension to tune their properties.

Microstructure and mechanical properties of pitch based mixture modified by additions of graphene oxide precursor and TiB2 particles

Pitch based mixture as binder material is crucial component for structure stability and mechanical properties of carbon/graphite electrodes. In this work, coal tar pitch (CTP) mixture was modified by additions of graphene oxide (GO) precursor and fine TiB2 particles. It is found that 1 wt.%GO addition increases the crystallite size, La value and reduce the defects density of (002) planes in CTP mesophase and carbonization products with increased elastic modulus. The additions of 3 or 5 wt.%TiB2 increase the stacking height of (002) planes (Lc) of CTP pyrolysis and carbonization products with increased edge dislocation density and ultimate strain. The combined additions of TiB2 and GO generate a synergistic effect on further improving the mechanical properties of CTP carbonization products with increasing the stacking order of (002) plane, crystallite size (La and Lc) and edge dislocation density. CTP-1GO-5TiB2 shows a compressive strength 194.7 MPa higher than the pure CTP (155.4 MPa). Further findings suggest that a combined mechanism of GO and TiB2 additions can be established through stabilizing C–C bonds while reducing C-O bonds along with chemical component transformations after pyrolysis and carbonization. TiB2 particles play a key role in making the synergistic effect on improving the mechanical strength and reducing the oxidation of C–C bonds with additional increase in La value.

Atomic Mechanism of Strain Alleviation and Dislocation Reduction in Highly Mismatched Remote Heteroepitaxy Using a Graphene Interlayer.

Remote heteroepitaxy is known to yield semiconductor films with better quality. However, the atomic mechanisms in systems with large mismatches are still unclear. Herein, low-strain single-crystalline nitride films are achieved on highly mismatched (∼16.3%) sapphire via graphene-assisted remote heteroepitaxy. Because of a weaker interface potential, the in-plane compressive strain at the interface releases by 30%, and dislocations are prevented. Meanwhile, the lattice distortions in the epilayer disappear when the structure climbs over the atomic steps on substrates because graphene renders the steps smooth. In this way, the density of edge dislocations in as-grown nitride films reduces to the same level as that of the screw dislocations, which is rarely observed in heteroepitaxy. Further, the indium composition in InxGa1-xN/GaN multiquantum wells increases to ∼32%, enabling the fabrication of a yellow light-emitting diode. This study demonstrates the advantages of remote heteroepitaxy for bandgap tuning and opens opportunities for photoelectronic and electronic applications.

Effect of initial condition on the quality of GaN film and AlGaN/GaN heterojunction grown on flat sapphire substrate with ex-situ sputtered AlN by MOCVD

The growth condition of initial medium (870 °C–920 °C) temperature (MT) layer is crucial to the quality of high temperature (HT) GaN layer and AlGaN/GaN heterojunction grown on flat sapphire substrate (FSS) with ex-situ sputtered AlN. The superior wetting of GaN and AlN makes the MT layer tend to directly two-dimensional (2D) growth mode at the initial stage. In this study, it was found that high pressure was effective in lowering Ga adatom diffusion and promoting early three-dimensional (3D) growth, which resulted in reducing (102) X-ray full-width-at-half-maximum as well as edge dislocation density significantly. The lowest X-ray rocking curve full-width-at-half-maximum of (002) and (102) were 35 arcsec and 220 arcsec respectively, which were comparable with GaN layer grown on patterned sapphire substrate (PSS). High quality AlGaN/GaN heterojunction was achieved with sheet electron density of 1.18×1013cm−2, Hall mobility of 1909 cm2/V⋅s and sheet resistance of 336 Ω/□. Controlled growth interruption was carried out to study the evolution mechanism of MT layer growth.

Growth of High Mobility InN Film on Ga‐Polar GaN Substrate by Molecular Beam Epitaxy for Optoelectronic Device Applications

AbstractThe fabrication of high‐speed electronic and communication devices has rapidly grown the demand for high mobility semiconductors. However, their high cost and complex fabrication process make them less attractive for the consumer market and industrial applications. Indium nitride (InN) can be a potential candidate to fulfill industrial requirements due to simple and low‐cost fabrication process as well as unique electronic properties such as narrow direct bandgap and high electron mobility. In this work, 3 µm thick InN epilayer is grown on (0001) gallium nitride (GaN)/Sapphire template under In‐rich conditions with different In/N flux ratios by molecular beam epitaxy. The sharp InN/GaN interface monolayers with the In‐polar growth are observed, which assure the precise control of the growth parameters. The directly probed electron mobility of 3610 cm2 V‐1 s‐1 is measured with an unintentionally doped electron density of 2.24 × 1017 cm‐3. The screw dislocation and edge dislocation densities are calculated to be 2.56 × 108 and 0.92 × 1010 cm‐2, respectively. The step‐flow growth with the average surface roughness of 0.23 nm for 1 × 1 µm2 is confirmed. The high quality and high mobility InN film make it a potential candidate for high‐speed electronic/optoelectronic devices.

Dislocation dynamics in α-Ga2O3 micropillars from selective-area epitaxy to epitaxial lateral overgrowth

Epitaxial lateral overgrowth (ELO) is an effective strategy to achieve metastable phased α-Ga2O3 with low dislocation densities, which is desirable for developing ultralow-loss and ultrahigh power devices, whereas the involved dislocation dynamics have not been fully exploited. In this Letter, we investigated the dislocation propagations and reactions in α-Ga2O3 micropillar arrays selectively grown by halide vapor phase epitaxy technique. Screw dislocations in α-Ga2O3 micropillars grown from the selective area epitaxy (SAE) to ELO mode exhibited an independent character with an average density of 4.5 × 106 cm−2 while the edge dislocation density was reduced to 5.3 × 108 cm−2. During the initial SAE process, the α-Ga2O3 hexagonal pyramid is developed with the observed inversion domains within the pillar cores. The successive epitaxial lateral overgrowth ELO facilitates the formation of inclined facets upon the SiO2 mask. Almost complete filtering of the underlying threading dislocation has been demonstrated in the ELO wings. Strong image forces induced by inclined free surfaces drive the propagation and reaction of threading dislocations until annihilation, which is well described by the dislocation-filtering model during the dynamic geometry transition of micropillars. These findings may pave the way for the success of the heteroepitaxy of low dislocation density α-Ga2O3 toward the development of high-performance power devices.

Fowler-Nordheim tunneling in β-Ga2O3/SrRuO3 Schottky interfaces

Interfaces in heterostructures always emerge as prototype electronic devices with tunable functionality. The fundamental properties of these interfaces can be finely manipulated by epitaxy engineering. Recently, heterostructures based on Ga2O3, an ultra-wide bandgap semiconductor, have been reported for use in high powered device applications. Herein, we will demonstrate a heterostructure of β-Ga2O3/SrRuO3 integrated on c-plane sapphire, where the high density of edge dislocations are evidenced in the heterostructure interfaces. Apart from the dominant Schottky emission mechanism, Fowler-Nordheim tunneling is also revealed by leakage current analysis, which may be ascribed to the edge dislocations at the interfaces. These results boost the basic understanding of ultra-wide bandgap materials and devices.

Growth and crystal phase transformation of ε-Ga2O3 grown on 4H–SiC by MOCVD

Ga2O3 thin films were hetero-epitaxially grown on 4H–SiC substrates by metal-organic chemical vapor deposition (MOCVD) under various growth conditions. The growth process and crystal quality of the ε-Ga2O3 thin films grown on C-face and Si-face of 4H–SiC substrates were compared. The ε-Ga2O3 film grown on a Si-face substrate shows better performance in improving crystal quality compared to the case of growth on a C-face. A ε-Ga2O3 thin film grown at 665 °C with an H2O flow rate of 200 sccm was thermally annealed at 900 °C for 10 min in an oxygen atmosphere for a crystal phase transformation to a β-Ga2O3 thin film. The RMS roughness of the transformed β-Ga2O3 thin film was 9.023 nm while the RMS value of directly grown was 129.1 nm. The estimated screw and edge dislocation density of the ε-Ga2O3 thin film were 1.41 × 1013 cm−2 and 4.92 × 1012 cm−2 respectively. Transformation of the crystal phase from ε-Ga2O3 to β-Ga2O3 can be considered as an effective method in hetero-epitaxial growth of Ga2O3 on SiC substrate.

Hierarchical microstructures enabled excellent low-temperature strength-ductility synergy in bulk pure tungsten

Refractory tungsten (W) is notoriously brittle at room temperature, which restricts its workability and narrows the temperature range of critical applications. Here, we report a multi-scale microstructure modulation strategy to achieve an excellent combination of low-temperature ductility and high strength in bulk pure W. Through fast two-step low temperature sintering of the activated W powders and high-energy-rate forging treatments, unique hierarchical microstructures were constructed in pure W, including lamellar elongated matrix grains, with profuse interior fine sub-grains, and high density of mobile edge and mixed dislocations. At room temperature, the hierarchical structured bulk W exhibits a detectable tensile ductility and an ultimate tensile strength (UTS) of 1.35 GPa. The high strength can be maintained at elevated temperatures, i.e., UTS > 1.0 GPa at 200 °C; it also has a remarkable tensile ductility of 15.3% at this temperature. These properties are significantly better than those reported in bulk pure W and W alloys with second phase particles. The shielding and blunting effects from low-angle grain boundaries and highly mobile dislocations, and the lamellar structure reaped delamination toughening effect are the main mechanisms for the improved low-temperature ductility and strength. This study demonstrates a practical route to achieve attractive low-temperature strength-ductility synergy in bulk W without involving any alloying elements, and it is a feasible and low-cost pathway to design high performance refractory metals and alloys.

A structural analysis of ultrathin barrier (In)AlN/GaN heterostructures for GaN‐based high‐frequency power electronics

Metal–organic chemical vapor deposition (MOCVD) is one of the best growth methods for GaN‐based materials as well‐known. GaN‐based materials with very quality are grown the MOCVD, so we used this growth technique to grow InAlN/GaN and AlN/GaN heterostructures in this study. The structural and surface properties of ultrathin barrier AlN/GaN and InAlN/GaN heterostructures are studied by X‐ray diffraction (XRD) and atomic force microscopy (AFM) measurements. Screw, edge, and total dislocation densities for the grown samples have been calculated by using XRD results. The lowest dislocation density is found to be 1.69 × 108 cm−2 for Sample B with a lattice‐matched In0.17Al0.83N barrier. The crystal quality of the studied samples is determined using (002) symmetric and (102) asymmetric diffractions of the GaN material. In terms of the surface roughness, although reference sample has a lower value as 0.27 nm of root mean square values (RMS), Sample A with 4‐nm AlN barrier layer exhibits the highest rough surface as 1.52 nm of RMS. The structural quality of the studied samples is significantly affected by the barrier layer thickness. The obtained structural properties of the samples are very important for potential applications like high‐electron mobility transistors (HEMTs).

Structural, electrical, and luminescence properties of (0001) ZnO epitaxial layers grown on c-GaN/sapphire templates by pulsed laser deposition technique

A systematic study of growth, structural, electrical, and luminescence properties of zinc oxide (ZnO) layers grown on c-oriented GaN/sapphire templates by the pulsed laser deposition technique is carried out. A thorough high-resolution x-ray diffraction study reveals that c-ZnO films with high crystalline quality can be grown under certain growth conditions. Screw and edge dislocation densities in these films are found to be as low as 7×108 and 3×1010cm−2, respectively. All layers are found to be unintentionally n-type with ∼1019cm−3 electron concentration and mobility as high as ∼50 cm2 V−1 s−1. Temperature and excitation intensity dependent photoluminescence (PL) studies as functions of the growth conditions are carried out to identify the transition processes behind various luminescence features found in these samples. At low temperatures, PL spectra are marked by sharp neutral donor bound excitonic transitions, their phonon replicas, and two broad luminescence bands at 2.2 and 2.9 eV. These broad bands are attributed to transitions from the (2+/0) oxygen vacancy (VO) and (2+/+/0) zinc-interstitial (Zni) levels, respectively, to the valence band. Thermal energy needed to depopulate these defects is found to be 11 and 385 meV, respectively, for the (2+/0) VO and (2+/+/0) Zni levels. Low temperature PL spectra for the samples grown with relatively high oxygen pressures are featured by the Zn-vacancy (VZn) related neutral acceptor bound excitonic transition, its phonon replicas, and a broad band at 2.75 eV. This band diminishes with increasing temperature and, instead, another broad feature appears at ∼2.1 eV. Our study attributes the 2.75 eV band to transition from the conduction band to (0/−) VZn levels and the 2.1 eV feature to the transition between (−/2−) VZn levels and the valence band. It has been found that all the defect related features can be minimized by adjusting the growth conditions.

THE MECHANISM OF THE SHOCK WAVE OF PLASTIC DEFORMATION IN METALS

A problem related to the evolution of the density of edge dislocations based on a system of equations for a dislocation ensemble is considered. Considering that during the evolution of the dislocation ensemble, the perturbations of the total dislocation density are small, an equation is obtained that obeys the evolution of the excess dislocation density (dislocation charge). It is shown that in the resulting equation, the nonlinear term due to the annihilation of dislocations is decisive in the evolution of the dislocation charge, provided that the dimensionless parameter R (similar to the Reynolds number in a viscous medium) is greater than one. Under these conditions, the original equation admits a self-similar solution in the form of a wave front of a dislocation charge I(x,t) running along a certain direction. This solution, however, is physically unstable, due to problematic boundary conditions. Therefore, a more feasible case was analyzed in which the initial evolutionary equation for the dislocation charge is reduced to the Burgers equation. For this equation, for sufficiently large values of R, an asymptotic solution I(x,t) = x/t was obtained in the form of a triangular shock wave, which has an amplitude I(x,t) and a front width dx ~ 1/R at the boundary of the wave x0 (for x &gt; x0, I = 0 must be fulfilled). Estimates show that the propagation of the dislocation charge takes on the character of a shock front (R &gt;&gt; 1) if there are internal prerequisites for such an evolution of the dislocation charge, namely: effective conditions arise for the formation of a sufficient number of clusters of N0 in front of various barriers (it is shown that R ~ N0). Otherwise, the dynamics of the dislocation charge acquires the character of diffusion spreading.

Investigation on the relationship between dislocation type and the annihilation mechanism of GaN-on-Si

In this work, we have found that there is a clear difference between the annihilation of screw dislocation and edge dislocation, and the density of edge dislocations remains almost unchanged during the growth from AlN to GaN. Through the transmission electron microscope (TEM), the internal mechanism of this phenomenon has been explored in this paper, which can be attributed to the failure of AlGaN buffer layer and AlN insertion layer to block the edge dislocation. The explanation proposed in this paper may provide new ideas to further reduce the dislocations in GaN-on-Si, improving the crystal quality.

Dislocation density control of GaN epitaxial film and its photodetector

At present, GaN-based ultraviolet photodetectors (UV PDs) is confronted with the challenge of high dark current and low responsivity. In this work, we designed and prepared high-performance GaN-based UV PDs on the sapphire substrates by the combination of low-temperature pulsed laser deposition (LT-PLD) and high-temperature metal-organic chemical vapor deposition growth (HT-MOCVD). The as-grown GaN film shows high-crystalline with edge dislocation density of 6.50 × 107 cm−2 and screw dislocation density of 1.92 × 108 cm−2, respectively. Furthermore, the optimized electrode structure has been well designed to enhance the carrier transport characteristics in GaN-based UV PDs, which improves the performance of GaN-based UV PDs accordingly. Based on the high-quality GaN film and the optimized electrode structure, high-performance GaN-based UV PDs are fabricated, which reveals the very high responsivity and small dark current of 1.4 A/W and 4.8 nA@5V, respectively. These high-performance GaN-based UV PDs shed light on the potential application in UV warning.

The role of AlN thickness in MOCVD growth of N-polar GaN

The influence of the thickness of aluminum nitride (AlN) on the surface morphology, crystalline quality, and polarity of N-polar gallium nitride (GaN) grown by metal organic chemical vapor deposition (MOCVD) is investigated in this study. With a medium temperature (1000 ℃) AlN buffer layer of 150 nm, a hexagonal-defect-free N-polar GaN thin film with low dislocation densities (4.2 × 108 cm-2 for screw dislocation density and 3.7 × 109 cm-2 for edge dislocation density) is obtained on a 2-in. vicinal sapphire substrate (c off m plane 2°). With the reduction of the AlN thickness to 80 nm, the N-polar GaN surface becomes rough and the crystalline quality is exacerbated. While the thickness of AlN is increased to 200 nm, the polarity of GaN is changed to Ga-polar. The transmission electron microscopy (TEM) results reveal an undulated interface between GaN and AlN with a large density of pyramidal defects which may contribute to the polarity inversion of GaN. The controllable polarity inversion through changing the thickness of AlN provides a promising method for the polarity engineering of nitride-based devices.

The influence of residual GaN on two-step-grown GaN on sapphire

In this paper, it is found that the residual GaN has a great impact on the two-step-grown GaN on sapphire substrates. The samples which are grown with a conventional system baking process present a higher dislocation density than those without the baking process. It is found that, without baking, residual GaN accumulate in the MOCVD reactor, decompose, form the nucleation site on the sapphire substrate during the process of high-temperature surface treatment, and thus influence the initial nucleation process of GaN layer growth. Based on this understanding, a novel growth method to improve the quality and stability of two-step-grown GaN on sapphire is proposed, in which the TMGa and NH3 were added in the high-temperature surface treatment process. With this method, as low as 3.71 × 107 cm−3 and 1.94 × 108 cm−3 for screw and edge dislocation density of GaN epitaxial films were achieved on sapphire.