Edge Dislocation Density Research Articles

Mosaic Structure of GaN Film Grown on Sapphire Substrate by AP-MOCVD: Impact of Thermal Annealing on the Tilt and Twist Angles

A GaN layer with a thickness of 2 µm was grown on a sapphire substrate using atmospheric pressure metal–organic chemical vapor deposition (AP-MOCVD). Subsequently, the layer was annealed under a nitrogen atmosphere at temperatures ranging from 1000 °C to 1120 °C. High-resolution X-ray diffraction (HRXRD) analysis reveals the impact of thermal annealing on the mosaic structure of the GaN, specifically the tilt and twist variations in four planes: (00.2), (10.3), (10.2), and (10.1). Interestingly, the observed trends suggest a differential effect of annealing on screw and edge dislocation densities. The annealing process reduces the edge and screw dislocation density. Lower values (Dscrew = 1.2 × 108 cm−2; Dedge = 1.6 × 109 cm−2) were obtained for the sample annealed at 1050 °C. Notably, both tilt and twist angles exhibited a minimum at 1050 °C (tilt = 252 arcsecs, and twist = 558 arcsecs), indicating improved crystal quality at this specific temperature. Photoluminescence (PL) spectroscopy further complemented the structural analysis. The intensity and broadening of the yellow band (YL) in the PL spectra progressively increased with the increasing annealing temperature, suggesting the presence of additional defect states. The near band edge PL emission (3.35 and 3.41 eV) variation upon thermal annealing was correlated with the mosaic structure evolution.

Orientation-dependent strain and dislocation in HVPE-grown α -Ga2O3 epilayers on sapphire substrates

Orientation-dependent substrates provide effective platforms for achieving α-Ga2O3 with low dislocation densities, whereas the associated strain and dislocation dynamics have not been fully explored. Herein, we investigated the evolution of growth mode, interfacial strain, and dislocation propagation in the α-Ga2O3 epitaxial layer with various orientations, grown by the halide vapor-phase epitaxy. Strain tensor theory and geometric phase analysis indicate that the m-plane α-Ga2O3 epitaxial layer exhibits the lowest misfit tensile strain, measured at εxx = 1.46% and εyy = 1.81%, resulting in the lowest edge dislocation density. The m-plane lattice exhibits an inclination of 33.60°, while the c-plane lattice is horizontally aligned and the a-plane lattice oriented perpendicularly. The orientation-dependent growth significantly influences stress relaxation through the generation of misfit dislocations, originating from either basal or prismatic slip. Edge dislocations, induced by misfit dislocations, favor the c-axis, remaining well confined within the in-plane interfacial layer of the m-plane α-Ga2O3, leading to reduced low edge dislocation density in the subsequent thick epitaxial layer. These findings shed light on the epitaxial dynamics of α-Ga2O3 heteroepitaxy, paving the way for the development of high-performance power devices.

Growth and performance of n++ GaN cap layer for HEMTs applications

Apart from providing high-quality ohmic contacts to III-N devices, n++ GaN cap layers can eliminate surface-related current collapse effects in high-electron-mobility transistors (HEMTs). Various metal-organic chemical vapor deposition conditions and n-type doping are tested in GaN growth on sapphire by using triethylgallium as a Ga precursor, replacing more conventional trimethylgallium. Consequently, despite relatively low temperature of the growth at 800 °C to facilitate InAlN barrier capping, optimized growth conditions and SiH4 flow provided free electron concentration of 7.2 × 1019 cm−3 and mobility of 103 cm2/Vs. Moreover, electron concentration in the range of 1020 cm−3 is demonstrated by using flow modulation epitaxy. On the other hand, as calibration growths indicated, excessive SiH4 flow alone can lead to high density of edge dislocations and surface undulations without enhancing free electron concentration. 6 nm (Si: 4.2 × 1019 cm−3) and 8 nm thick (Si: 7.2 × 1019 cm−3) n++ GaN cap is implemented in normally-off n++ GaN/InAlN/AlN/GaN HEMTs grown on Si, with a collapse-free performance for the latter case. By calculating energy band and free carrier concentration profiles of the heterostructures it is shown, that free electrons of a sufficiently thick and doped n++ GaN cap can shield the channel from the unstable surface potential.

Defect characterizations of N-rich GaNAs ternary alloys

This paper presents a comprehensive analysis of defects in As-diluted GaN alloys. GaNAs crystal layers with an arsenic content of 1.8, 2.9, 4.1, and 5.5 % are grown on GaN buffer layers using the metalorganic vapour-phase epitaxy (MOVPE) method. Since these alloys have potential in electronics due to the modification of the electronic structure caused by As, it is extremely important to determine their quality. Complementary techniques for the characterization of the alloys are used. Densities of screw and edge dislocations are obtained using X-ray diffraction measurements performed, respectively, from the surface and edge of samples. In turn, a population of vacancies is determined by Doppler broadening variable energy positron annihilation spectroscopy (DB-VEPAS) and variable energy positron lifetime positron annihilation spectroscopy (VEPALS), which provide the distribution of vacancies as a function of depth. An increase in dislocation density is observed accompanied by a rise in the concentration of vacancy agglomerations and reduction in Ga-vacancy-hydrogen associates in the function of As-content.

Temperature-dependent epitaxial evolution of carbon-free corundum α-Ga2O3 on sapphire

Unintentionally doped carbon impurities from organometallic precursors are primary sources of carrier compensation and mobility degradation in wide bandgap semiconductors, leading to lowered performance of power electronic devices. To address this challenge, carbon-free α-Ga2O3 single-crystalline thin films were heteroepitaxially grown on sapphire substrates by using gallium inorganic precursors through a mist chemical vapor deposition technique. Determined through a temperature dependence of growth rates, three distinct growth regimes are identified: the surface reaction limited regime below 480 °C, the mid-temperature mass-transport limited regime (480 °C–530 °C) and the high temperature limited regime related to desorption or phase transition. With an optimized around 530 °C, the densities of screw and edge dislocations are reduced to 7.17 × 106 and 7.60 × 109 cm−2, respectively. Notably, carbon incorporation was eliminated in the α-Ga2O3 grown by inorganic GaCl3, as evidenced by the absence of carbon-related vibrational bands in Raman scattering analysis, while crystalline quality was comparable to that grown with organometallic precursors. The high solubility of GaCl3 in water is expected to enable the rapid growth of high purity α-Ga2O3 with improved electronic transport performances.

Effects of TMAl predose time on the quality of submicron-thick GaN on Si

Submicron-thick (∼350 nm) gallium nitride (GaN) thin films with lower dislocation densities are grown on silicon (Si) substrate by metal organic chemical vapour deposition (MOCVD). Prior to the growth, a trimethylalluminum (TMAl) predose is performed. The effects of different TMAl predose time on the surface morphology and crystalline quality of submicron-thick GaN are investigated in this study. With prolonged TMAl predose time, the surface becomes rougher, while the screw dislocation densities both for GaN and aluminum nitride (AlN) are reduced. The edge dislocation densities tend to decrease first then increase again when the TMAl predose time is elongated from 90 s over 135 s–180 s. From the high resolution transmission electron microscopy (HRTEM), we found the thickness of amorphous layer between Si and AlN decreases with the increase of TMAl predose time. Such an amorphous layer should play an important role in the surface morphology and crystalline quality of submicron-thick GaN. A possible mechanism has also been proposed to elucidate the effects of TMAl predose time.

EQUILIBRIUM OF A NORMALLY LOADED ELASTIC SHALLOW CYLINDRICAL SHELL WITH DISLOCATIONS AND DISCLINATIONS

We consider the problem of the equilibrium of a normally loaded elastic shallow rectangular circular cylindrical shell, which contains sources of internal stress in the form of fields of continuously distributed edge dislocations and wedge disclinations or other sources, for example, thermal ones. Based on the modified system of Karman equations for the equilibrium of an elastic plate with dislocations and disclinations, a system of nonlinear equilibrium equations for the shell was constructed. The resulting system of equations differs from the Karman system of equilibrium equations for an elastic shallow cylindrical shell under the action of a normal load by the presence on the right side of the continuity equation of a nonzero function, called the incompatibility function, which is expressed through the densities of edge dislocations and wedge disclinations. The edges of the shell are freely clamped or hinged. In the absence of internal stress sources, this system transforms into a system of equilibrium equations for a shallow shell, and in the case of an infinitely large radius of curvature (and the presence of internal stress sources) into a modified system of Karman equilibrium equations for a flexible elastic plate with dislocations and disclinations. To solve the system of nonlinear shell equilibrium equations, the Newton – Kantorovich method in combination with the difference method is proposed. For the case of small values of the normal load and small values of the incompatibility function, linearization is carried out with respect to the deflection and stress function. As a result, a linear boundary value problem was obtained in the form of a system of differential equations for the deflection and the stress function, which is solved by the difference method. The solution of the linearized system is used as an initial approximation in the implementation of the Newton – Kantorovich method. Examples of numerical calculations of deflection and stress function for given values of normal load and incompatibility function are given, and corresponding graphs are constructed.

Influence of growth interruption on the morphology and luminescence properties of AlGaN/GaN ultraviolet multi-quantum wells.

The influence of growth interruption on the surface and luminescence properties of AlGaN/GaN ultraviolet multi-quantum wells (UV MQWs) is investigated. It is found that when the well and barrier layers of MQW samples are continuously grown at the same temperature, they have lower edge dislocation density and flatter surface of MQWs compared to samples with interrupted well and barrier growth. Moreover, continuous growth of well and barrier layers is more conducive to improving the luminescence efficiency of MQWs. This phenomenon is attributed to more impurity incorporation induced by the growth interruption, while a continuous growth of well and barrier can reduce surface diffusion and migration processes of atoms, reducing the defects and surface roughness of MQWs. In addition, the continuous growth of well and barrier can better control the reaction between Al and N atoms, avoiding the formation of excessively high Al content AlGaN at the well/barrier interface, thus improving the luminescence of UV MQWs.

AlGaN‐Based Solar‐Blind Ultraviolet Detector with a Response Wavelength of 217 nm

The research of the high Al(x = 0.75) component has always been the focus of the AlGaN solar‐blind ultraviolet (UV) detector. However, due to the lattice and thermal mismatch between the AlGaN and the underlying substrate under existing mainstream heteroepitaxial growth methods, the large density of defects, e.g., point defects, screw dislocations, and edge dislocations, has hindered the performances of AlGaN‐based solar‐blind UV photodetectors. A short superlattice polarization‐induced P‐type doping growth technique is used to fabricate a high‐performance AlGaN‐based back‐illuminated solar‐blind UV p‐i‐n photodetector (PD) fabricated on sapphire substrates. The back‐illuminated AlGaN UV‐PD shows a high external quantum efficiency of 70.2%. The peak responsivity (R) reaches 123 mA W−1 at −5 V with a wavelength of 217 nm. Meanwhile, the dark current density is 2.21 × 10−8 A cm−2. Additionally, the UV/visible rejection ratio for the detectors exceeds four orders of magnitude, and the detectivity (D*) is calculated to be 6.7 × 1012 cm Hz1/2 W−1. The device performance parameters can be attributed to the quality of the epilayer and heterojunctions. This technology provides new ideas for nitride semiconductor materials, further bringing a breakthrough in a wide‐bandgap electronics device.

The microstructural evolution of sputtered ZnO epitaxial films to stress-relaxed nanorods

Epitaxial ZnO thin films and nanorods were grown on c-sapphire substrate by reactive sputtering of Zn in Ar-O2 atmosphere at substrate temperatures ranging from near room temperature to 700 °C. Scanning electron microscopy showed that with increase in substrate temperature, the morphology transformed from columnar films to vertically aligned ZnO nanorods. High resolution x-ray diffraction revealed improvement in crystalline and epitaxial quality with increase in substrate temperature, along with the decrease of edge dislocation density from 5 × 1012 cm−2 to 8 × 1010 cm−2. The films grown near room temperature showed large hydrostatic strain (∼6 × 10−3) and compressive intrinsic biaxial stress (-0.8 GPa), which decreased substantially with increase in substrate temperature, due to the desorption of excess oxygen and reduction in edge dislocation density. Above the substrate temperature of 500 °C, the intrinsic biaxial stress reversed from compressive to mildly tensile in the case of nanorods, due to the intrinsic tensile stress, originating from crystallite coalescence. Raman measurements correlate well with the changes in biaxial stress and confirm the improvements in crystallinity and epitaxial quality of nanorods grown at higher temperatures. Room temperature photoluminescence of the ZnO films showed weak near-band-edge emission, which enhanced drastically in the case of nanorods due to their superior microstructure and epitaxial quality.

Improved crystal quality of AlGaN by Al ion-implantation sapphire substrate

AlGaN as a key material offers a route to high-efficiency optical and electronic devices. However, the heteroepitaxy of AlGaN confronts many challenges, and the crystal quality of AlGaN is still poor. Here, we show an effective method to improve the AlGaN crystal quality through the pretreatment of Al-ion implantation into the sapphire substrate. Our results demonstrate the screw and edge dislocation density of the AlGaN grown on the Al-ion implantation sapphire substrate are both reduced. At an Al-ion dose of 1 × 1012 cm−2, the screw and edge dislocation density are significantly reduced from 8.9 × 107 to 8.2 × 106, and 4.6 × 1010 to 3.3 × 1010 cm−2, respectively. The photoluminescence spectra also indicate enhanced optical performance. This innovative method is effective and simple for reducing the dislocation density of AlGaN epilayers and can be extended to other substrates for AlGaN epitaxy.

Threading dislocation and lattice stress modulation of Si based GaN material with AlPN nucleation layer

An innovative AlPN nucleation layer is proposed for the growth of high quality GaN on Si. AlPN nucleation layer is prepared on Si substrate by metal organic chemical vapor deposition to explore the stress and dislocation adjustment mechanism. It is found that there are stress-related antisite defects in the P-doped lattice. In addition, the incorporation of P makes the dislocation densities of screw dislocations and edge dislocations have different trends, which can realize the independent control of the different types of dislocation densities. A hexagonal-cubic-phase mixed structure model is proposed to analyze and explain the variation trends of dislocation densities and stress. This work not only puts forward a new type of nitride material, which provides an idea for the growth of high-quality GaN on Si. On the other hand, it is of great significance to perfect the independent regulation mechanism of dislocations.

Improvement of the Conformational Stability of 150 mm 4H SiC Wafers

A method for mitigating loss of conformational stability in 150 mm n-type 4H SiC wafers was investigated. Modifications to the physical vapor transport (PVT) process used to grow the parent bulk crystals, combined with post-growth thermal treatment, were examined as means of reducing the internal stresses hypothesized to promote instability. The magnitude of the stresses was analyzed by mechanically thinning sets of wafers produced from each process to determine the critical thickness of stability loss. The average critical thickness was found to be reduced by 13% via growth cell modification, at a reduced level of thermal treatment relative to a control process, with all wafers becoming unstable greater than 30 μm below the minimum recorded production thickness. Assessment of the spatial uniformity of dislocations indicated that lower conformational stability corresponded to elevated densities of basal plane dislocations (BPDs) and threading edge dislocations (TEDs) at the wafer edge relative to the center.

Simulation-aided analysis of ferrite recrystallization behavior of pure iron with different dislocation characters

We investigated the ferrite recrystallization behavior of pure iron with different dislocation characters. Two types of specimens with almost the same dislocation densities and different dislocation characters were prepared. The edge dislocation density in specimen A was larger than that in specimen B, whereas the screw dislocation density in specimen B was larger than that in specimen A. Ferrite recrystallization progressed faster in specimen A than in specimen B. Furthermore, cellular automaton simulation proved that changes in ferrite recrystallization behavior due to the dislocation character are attributed to the difference in the driving force for recrystallization and the activation energy for recovery in each specimen. Thus, we found that the difference in the dislocation character can affect the ferrite recrystallization behavior of pure iron.

Effect of High-Pressure GaN Nucleation Layer on the Performance of AlGaN/GaN HEMTs on Si Substrate.

A high-pressure (HP) GaN nucleation layer (NL) was inserted between AlGaN buffer and an unintentionally doped (UID) GaN layer of an AlGaN/GaN HEMT on Si. The XRD and TEM showed that when the V/III ratio was optimized during the HP-GaN NL growth, the edge dislocation density in the HP-GaN NL layer could be reduced significantly. Experimental results exhibited a lower off-state leakage current, higher maximum ID and Gm (corresponding to 22.5% and 21.7% improvement, respectively), and lower on-state resistance. These results demonstrate that the electrical properties of the AlGaN/GaN HEMT can be improved through the insertion of a HP-GaN NL.

Effects of microalloying with Ag, Li, and Sc on hot-deformed microstructure of Al–Mg–Si–Cu alloys

The effects of microalloying with Ag, Li, and Sc on the hot-extruded microstructures of Al–Mg–Si–Cu alloys were studied. Each microalloying element was observed to affect the hot-deformed microstructure of the alloy, with a retarding effect on its recrystallization; however, the effect of Ag-addition was relatively small. The number fraction of grains with a high Taylor factor and their kernel average misorientation significantly increased when Li and Sc were added to the Al–Mg–Si–Cu alloy, implying a strong anisotropy of slip. It was also found that Li-addition primarily weakened the C component, whereas Sc-addition significantly strengthened the rotated copper (Rt-Cu) component in the deformation texture, and strongly dissipated the recrystallization texture. Additionally, all the microalloying elements slightly increased the dislocation density and fraction of edge dislocations. Thus, a very close dislocation arrangement with a strong screening of dislocations was observed; however, this effect was relatively weak for the Sc-added alloy. The high variation in the microstructural parameters of the Sc-added alloy was understood to depend primarily on the Al3Sc dispersoid and its effect on the other precipitates. In contrast, Ag- and Li- additions did not significantly change the relatively large precipitate structure, which implies that there were changes in the nanoscale precipitates or solute clustering during the hot deformation of Ag- and Li-added alloys.

High-quality AlGaN epitaxial structures and realization of UVC vertical-cavity surface-emitting lasers

AlGaN-based vertical-cavity surface-emitting lasers (VCSELs) have garnered recent interest due to their superior material properties and device benefits. Nevertheless, AlGaN-based VCSELs are extremely difficult to realize due to numerous technical limitations associated with both material epitaxial growth and chip fabrication. This study fabricated a high-quality AlGaN multiple quantum wells (MQWs) structure using epitaxial lateral overgrowth and analyzed it using X-ray diffraction (XRD) and photoluminescence (PL) measurements. With an edge dislocation density (DD) of 109 cm−2, XRD measurements reveal that the AlN template is nearly fully relaxed. The subsequent AlGaN/AlN superlattice (SL) layer is introduced to decrease the edge DD, and the edge DD in the MQWs is ∼108 cm−2. According to PL measurements, the internal quantum efficiency of the MQWs is as high as 62%, and radiative recombination dominated the emission of the MQWs at room temperature. Using these epitaxial wafers, ultraviolet radiation C (UVC) VCSELs were fabricated using various techniques, including laser lift-off (LLO) and chemical mechanical polishing (CMP). The crystallinity of the MQWs was unaffected by sapphire substrate removal using LLO. After removing the sapphire substrate using LLO and CMP, UVC surface-stimulated emission was observed in MQWs. AlGaN-based UVC VCSELs with lasing wavelengths of 275.91, 276.28, and 277.64 nm have been fabricated. The minimum threshold for UVC VCSELs is 0.79 MW cm−2, which is a record low.

RAMIFICATION OF EQUILIBRIA OF A COMPRESSED ELASTIC ORTHOTROPIC PLATE WITH INTERNAL STRESSES

The problem of loss of stability and post-critical behavior of a compressed orthotropic rectangular plate on a nonlinearly elastic base containing continuously distributed fields of dislocations and disclinations and under the influence of a small normal load is considered. The compressive force components are evenly distributed along the edges and act parallel to the main directions of elasticity. The problem is formulated as an analogue of a system of nonlinear Karman equations for an orthotropic plate containing a function called the incompatibility measure, which is expressed in terms of the densities of edge dislocations and wedge disclinations. The system of equations takes into account the small transverse pressure and the reaction of the elastic base in the form of a polynomial of the third degree of deflection. The following boundary conditions are considered: the edges of the plate are freely pinched or movably pivotally supported; two opposite edges of the plate are freely pinched or movably pivotally supported, and the other two are free from loads. The stress function is sought in the form of two components: the stress function caused by the presence of internal sources, determined from the linear boundary value problem, and the stress function caused by the external impact of compressive loads and a nonlinear elastic base, which is determined from the nonlinear boundary value problem. The nonlinear boundary value problem is investigated by the Lyapunov – Schmidt method. To solve the linearized equation from which the critical value of the compressive load is determined, the variational method is used in combination with the difference method. A system of branching equations of the Lyapunov – Schmidt method is constructed, which is investigated by numerical methods. The post-critical behavior of the plate is investigated and asymptotic formulas for new equilibria in the vicinity of the critical load are derived. For various values of compressive loads, the orthotropy parameters of the plate and the intensity parameter of internal stresses, the relations between the values of the base parameters are established, at which its bearing capacity is preserved in the vicinity of the classical value of the critical load.

Growth of N-polar In-rich InAlN by metal organic chemical vapor deposition on on- and off-axis sapphire

Metal organic chemical vapor deposition is used to grow N-polar In-rich InAlN layers directly on on- and off-axis (misoriented by 4° towards a plane) c-plane sapphire substrates. During the InAlN growth, trimethylaluminum, ammonia, and the total flow was kept at 4.74 μmol/min, 3 slm and 10 slm, respectively, while trimethylindium (TMIn) flow was selected between 8.42 and 13.48 μmol/min. All samples were grown at about 706 °C however; TMIn flow of 10.95 μmol/min was also tested at the growth temperature of 686 °C. With increasing the TMIn flow, the In molar fraction increased from 0.55 to 0.69, irrespectively of the substrate miscut. For the moderate TMIn flow and In molar fraction of less than 0.63, density of screw and edge dislocations were from 0.3 to 12 ✕ 109 cm-2 and 5 to 7 ✕ 1010 cm-2, respectively, while decisively lower densities appeared on on-axis structures. On the other hand, InAlN surface RMS roughness was from 1 to 8 nm favouring the off-axis substrate and low TMIn flow. Structural and surface deterioration appeared with the highest TMIn flow of 13.48 μmol/min and In molar fraction of 0.69, which hold also for reduced temperature of the growth. Nevertheless, room temperature photoluminescence full-width at half maximum less than 270 meV appeared for all samples, with maxima between 1.9 and 1.5 eV. In-rich N-polar InAlN grown on on-axis sapphire can be used as a buffer layer in new types of heterostructure devices.

Crystalline Quality and Surface Morphology Improvement of Face-to-Face Annealed MBE-Grown AlN on h-BN

In this study, AlN epilayers were grown by ammonia-assisted molecular beam epitaxy on 3 nm h-BN grown on c-sapphire substrates. Their structural properties were investigated by comparing as-grown and postgrowth annealed layers. The role of annealing on the crystalline quality and surface morphology was studied as a function of AlN thickness and the annealing duration and temperature. Optimum annealing conditions were identified. The results of X-ray diffraction showed that optimization of the annealing recipe led to a significant reduction in the symmetric (0 0 0 2) and skew symmetric (1 0 -1 1) reflections, which was associated with a reduction in edge and mixed threading dislocation densities (TDDs). Furthermore, the impact on the crystalline structure of AlN and its surface was studied, and the results showed a transition from a surface with high roughness to a smoother surface morphology with a significant reduction in roughness. In addition, the annealing duration was increased at 1650 °C to further understand the impact on both AlN and h-BN, and the results showed a diffusion interplay between AlN and h-BN. Finally, an AlN layer was regrown on the top of an annealed template, which led to large terraces with atomic steps and low roughness.