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The structural properties of lattice-matched InAlAs/InP layers grown by molecular beam epitaxy have been studied using atomic force microscopy, scanning electron microscopy and micro-photoluminescence spectroscopy. The formation of the surface pits with lateral sizes in the micron range and a depth of about 2 ÷ 10 nm has been detected. The InP substrate annealing temperature and value of InAlAs alloy composition deviation from the lattice-matched InxAl1−xAs/InP case (x = 0.52) control the density of pits ranging from 5 × 105 cm−2 ÷ 108 cm−2. The pit sizes are controlled by the InAlAs layer thickness and growth temperature. The correlation between the surface pits and threading dislocations has been detected. Moreover, the InAlAs surface is characterized by composition inhomogeneity with a magnitude of 0.7% with the cluster lateral sizes and density close to these parameters for surface pits. The experimental data allow us to suggest a model where the formation of surface pits and composition clusters is caused by the influence of a local strain field in the threading dislocation core vicinity on In adatoms incorporating kinetic. [ABSTRACT FROM AUTHOR]

Heteroepitaxial growth of high Al-content AlGaN often results in a high density of threading dislocations and surface hexagonal hillocks, which degrade the performance and reliability of AlGaN-based UVC light emitting diodes (LEDs). In this study, the degradation mechanism and impurity/defect behavior of UVC LEDs in relation to the hexagonal hillocks have been studied in detail. It was found that the early degradation of UVC LEDs is primarily caused by electron leakage. The prominent contribution of the hillock edges to the electron leakage is unambiguously evidenced by the transmission electron microscopy measurements, time-of-flight secondary ion mass spectrometry, and conductive atomic force microscopy. Dislocations bunching and segregation of impurities, including C, O, and Si, at the hillock edges are clearly observed, which facilitate the trap-assisted carrier tunneling in the multiple quantum wells and subsequent recombination in the p-AlGaN. This work sheds light on one possible degradation mechanism of AlGaN-based UVC LEDs. [ABSTRACT FROM AUTHOR]

The aim of this paper is to model the effects of threading dislocations on both gate and drain currents of AlGaN/GaN high electron mobility transistors (HEMTs). The fraction of filled traps increases with the threading dislocations, while the trapping effects cause a decrease in drain current and an increase in gate leakage current. To model the drain current drop, the two simplified RC subcircuits with diodes are proposed to capture the charge trapping/detrapping characteristics. The trap voltages Vg_trap and Vd_trap generated by RC networks are fed back into the model to capture the effects of traps on drain current. Considering acceptor-decorated dislocations, we present a novel Poole–Frenkel (PF) model to precisely describe the reverse leakage gate current, which plays a dominant role in the gate leakage current. The proposed model, which uses physical parameters only, is implemented in Verilog-A. It is in excellent agreement with the experimental data. [ABSTRACT FROM AUTHOR]

The major issues confronting the performance of deep-UV (DUV) laser diodes (LDs) are reviewed along with the different approaches aimed at performance improvement. The impact of threading dislocations on the laser threshold current, limitations on heavy n- and p-doping in Al-rich AlGaN alloys, unavoidable electron leakage into the p-layers of (0001) LD structures, implementation of tunnel junctions, and non-uniform hole injection into multiple quantum wells in the active region are discussed. Special attention is paid to the current status of n- and p-type doping and threading dislocation density reduction, both being the factors largely determining the performance of DUV-LDs. It is shown that most of the above problems originate from intrinsic properties of the wide-bandgap AlGaN semiconductors, which emphasizes their fundamental role in the limitation of deep-UV LD performance. Among various remedies, novel promising technological and design approaches, such as high-temperature face-to-face annealing and distributed polarization doping, are discussed. Whenever possible, we provided a comparison between the growth capabilities of MOVPE and MBE techniques to fabricate DUV-LD structures. [ABSTRACT FROM AUTHOR]

Heteroepitaxial growth of high Al-content AlGaN often results in a high density of threading dislocations and surface hexagonal hillocks, which degrade the performance and reliability of AlGaN-based UVC light emitting diodes (LEDs). In this study, the degradation mechanism and impurity/defect behavior of UVC LEDs in relation to the hexagonal hillocks have been studied in detail. It was found that the early degradation of UVC LEDs is primarily caused by electron leakage. The prominent contribution of the hillock edges to the electron leakage is unambiguously evidenced by the transmission electron microscopy measurements, time-of-flight secondary ion mass spectrometry, and conductive atomic force microscopy. Dislocations bunching and segregation of impurities, including C, O, and Si, at the hillock edges are clearly observed, which facilitate the trap-assisted carrier tunneling in the multiple quantum wells and subsequent recombination in the p-AlGaN. This work sheds light on one possible degradation mechanism of AlGaN-based UVC LEDs.

The aim of this paper is to model the effects of threading dislocations on both gate and drain currents of AlGaN/GaN high electron mobility transistors (HEMTs). The fraction of filled traps increases with the threading dislocations, while the trapping effects cause a decrease in drain current and an increase in gate leakage current. To model the drain current drop, the two simplified RC subcircuits with diodes are proposed to capture the charge trapping/detrapping characteristics. The trap voltages Vg_trap and Vd_trap generated by RC networks are fed back into the model to capture the effects of traps on drain current. Considering acceptor-decorated dislocations, we present a novel Poole–Frenkel (PF) model to precisely describe the reverse leakage gate current, which plays a dominant role in the gate leakage current. The proposed model, which uses physical parameters only, is implemented in Verilog-A. It is in excellent agreement with the experimental data.

The major issues confronting the performance of deep-UV (DUV) laser diodes (LDs) are reviewed along with the different approaches aimed at performance improvement. The impact of threading dislocations on the laser threshold current, limitations on heavy n- and p-doping in Al-rich AlGaN alloys, unavoidable electron leakage into the p-layers of (0001) LD structures, implementation of tunnel junctions, and non-uniform hole injection into multiple quantum wells in the active region are discussed. Special attention is paid to the current status of n- and p-type doping and threading dislocation density reduction, both being the factors largely determining the performance of DUV-LDs. It is shown that most of the above problems originate from intrinsic properties of the wide-bandgap AlGaN semiconductors, which emphasizes their fundamental role in the limitation of deep-UV LD performance. Among various remedies, novel promising technological and design approaches, such as high-temperature face-to-face annealing and distributed polarization doping, are discussed. Whenever possible, we provided a comparison between the growth capabilities of MOVPE and MBE techniques to fabricate DUV-LD structures.

The kinematic and isotropic strain hardening was investigated in columnar-grained copper with preferentially oriented nanoscale twins deformed at two different strain rates of 5 × 10−5 and 5 × 10−3 s−1. A significant back stress is caused majorly by threading dislocation pile-up and accumulation at the grain boundaries and is strain rate independent. The isotropic hardening associated with an increment in the local effective stress stems from dislocation storage at twin boundaries, and a high strain rate is beneficial for enhancing isotropic strain hardening and tensile ductility. Impact statement Long-range back stress was detected to develop during plastic deformation of nanotwinned structures, majorly caused by pile-up and accumulation of threading dislocations at grain boundary regions.

In this paper we describe state-of-the-art approaches to the modeling of strain relaxation and dislocation dynamics in InGaAs/GaAs (001) heterostructures. Current approaches are all based on the extension of the original Dodson and Tsao plastic flow model to include compositional grading and multilayers, dislocation interactions, and differential thermal expansion. Important recent break-throughs have greatly enhanced the utility of these modeling approaches in four respects: i) pinning interactions are included in graded and multilayered structures, providing a better description of the limiting strain relaxation as well as the dislocation sidewall gettering; ii) a refined model for dislocation-dislocation interactions including zagging enables a more accurate physical description of compositionally-graded layers and step-graded layers; iii) inclusion of back-and-forth weaving of dislocations provides a better description of dislocation dynamics in structures containing strain reversals, such as strained-layer superlattices or overshoot graded layers; and iv) the compositional dependence of the model kinetic parameters has been elucidated for the InGaAs material system, allowing more accurate modeling of heterostructures with wide variations in composition. We will describe these four key advances and illustrate their applications to heterostructures of practical interest. [ABSTRACT FROM AUTHOR]

Since the invention of dislocation sidewall gettering (DSG) in 2000 the technique has been applied extensively in infrared focal-plane arrays and flat-panel displays. However, development of DSG technology has been guided mostly by empirical trials due to the lack of detailed physical models. Here we demonstrate the application of a dislocation dynamics model to evaluate DSG approaches in both ZnSySe1-y/GaAs (001) and InGaxAs1-x/GaAs (001) heterostructures. We find that the effectiveness of DSG is strongly dependent on composition in both material systems. [ABSTRACT FROM AUTHOR]

Strained-layer superlattices (SLSs) have been used to modify the threading dislocation behavior in metamorphic semiconductor device structures; in some cases they have even been used to block the propagation of threading dislocations and are referred to in these applications as "dislocation filters." However, such applications of SLSs have been impeded by the lack of detailed physical models. Here we present a "zagging and weaving" model for dislocation interactions in multilayers and strained-layer superlattices, and we demonstrate the use of this model to the threading dislocation dynamics in InGaAs/GaAs (001) structures containing SLSs. [ABSTRACT FROM AUTHOR]

Metamorphic semiconductor devices often utilize compositionally-graded buffer layers for the accommodation of the lattice mismatch with controlled threading dislocation density and residual strain. Linear or step-graded buffers have been used extensively in these applications, but there are indications that sublinear, superlinear, S-graded, or overshoot graded structures could offer advantages in the control of defect densities. In this work we compare linear, step-graded, and nonlinear grading approaches in terms of the resulting strain and dislocations density profiles using a state-of-the-art model for strain relaxation and dislocation dynamics. We find that sublinear grading results in lower surface dislocation densities than either linear or superlinear grading approaches. [ABSTRACT FROM AUTHOR]

We conducted a modeling study of the threading dislocation behavior in chirped and unchirped InGaAs/GaAs (001) strained-layer superlattices (SLSs) using a Dodson & Tsao / Kujofsa & Ayers (DTKA) type plastic flow model. Four types of SLSs were investigated: type I was chirped using compositional modulation, type II was chirped using layer thickness modulation, type III was unchirped with alternating layers of InGaAs and GaAs, and type IV was unchirped with alternating layers of InGaAs having two different compositions. Generally the surface and average values of the dislocation density decreased with increasing total thickness. The dependence on top indium composition was more complex, due to dislocation compensation and multiplication effects, but for type II and IV superlattices, the average and surface threading dislocation densities increased in nearly monotonic fashion with top indium composition. Based on these results, the compositionally-modulated chirped (type I) and InGaAs/GaAs unchirped (type III) superlattices appear to be best suited as buffer layers for metamorphic devices, while the chirped superlattices with layer thickness modulation (type II) and InGaAs/InGaAs unchirped (type IV) superlattices appear to be poorly suited for use as buffer layers for devices containing high indium content. [ABSTRACT FROM AUTHOR]

Metamorphic realization of semiconductor devices has become increasingly important due to the great freedom it affords in layer compositions and thicknesses. However, metamorphic growth is often accompanied by the introduction of high densities of threading dislocation defects. This behavior may be understood by using an annihilation and coalescence model for the threading dislocation behavior which is based on the dislocation interaction length Lint. For its application we considered only glide of dislocations, so the interaction length was assumed to be equal to the length of misfit dislocation segments LMD. The length of misfit segments was determined approximately by the Matthews, Mader, and Light model [J. Appl. Phys., 41, 3800 (1970)] for lattice relaxation, and was assumed to be independent of the distance from the interface. Within this framework we have applied the annihilation and coalescence model to chirped semiconductor superlattices to evaluate these superlattices as strainrelaxed buffers for metamorphic devices. In this work we have studied two basic types of InGaAs/GaAs chirped superlattice buffers: type I superlattices are compositionally modulated while type II superlattices are thickness modulated. [ABSTRACT FROM AUTHOR]

Metamorphic semiconductor devices such as high electron mobility transistors (HEMTs), light-emitting diodes (LEDs), laser diodes, and solar cells are grown on mismatched substrates and typically exhibit a high degree of lattice relaxation. In order to minimize the incorporation of threading defects it is common to use a linearly-graded buffer layer to accommodate the mismatch between the substrate and device layers. However, some work has suggested that buffer layers with non-linear grading could offer superior performance in terms of limiting the surface density of threading defects. In this work, we have compared S-graded buffer layers with different orders and thicknesses. To do so we calculated the expected surface threading dislocation density for each buffer design assuming a GaAs (001) substrate. The threading dislocation densities were calculated using the LMD model, in which the coefficient for second-order annihilation and coalescence reactions between threading dislocations is considered to be equal to the length of misfit dislocations. [ABSTRACT FROM AUTHOR]

Several simple models have been developed for the threading dislocation behavior in heteroepitaxial semiconductor materials. Tachikawa and Yamaguchi [Appl. Phys. Lett., 56, 484 (1990)] and Romanov et al. [Appl. Phys. Lett., 69, 3342 (1996)] described models for the annihilation and coalescence of threading dislocations in uniform-composition layers, and Kujofsa et al. [J. Electron. Mater., 41, 2993 (2013)] extended the annihilation and coalescence model to compositionally-graded and multilayered structures by including the misfit dislocation-threading dislocation interactions. However, an important limitation of these previous models is that they involve empirical parameters. The goal of this work is to develop a predictive model for annihilation and coalescence of threading dislocations which is based on the dislocation interaction length Lint. In the first case if only in-plane glide is considered the interaction length is equal to the length of misfit dislocation segments while in the second case glide and climb are considered and the interaction length is a function of the distance from the interface, the length of misfit dislocations, and the density of the misfit dislocations. In either case the interaction length may be calculated using a model for dislocation flow. Knowledge of the dislocation interaction length allows predictive calculations of the threading dislocation densities in metamorphic device structures and is of great practical importance. Here we demonstrate the latter model based on glide and climb. Future work should compare the two models to determine which is more relevant to typical device heterostructures. [ABSTRACT FROM AUTHOR]

The kinematic and isotropic strain hardening was investigated in columnar-grained copper with preferentially oriented nanoscale twins deformed at two different strain rates of 5 × 10−5 and 5 × 10−3 s−1. A significant back stress is caused majorly by threading dislocation pile-up and accumulation at the grain boundaries and is strain rate independent. The isotropic hardening associated with an increment in the local effective stress stems from dislocation storage at twin boundaries, and a high strain rate is beneficial for enhancing isotropic strain hardening and tensile ductility. Impact statement Long-range back stress was detected to develop during plastic deformation of nanotwinned structures, majorly caused by pile-up and accumulation of threading dislocations at grain boundary regions. [ABSTRACT FROM AUTHOR]

In this paper, the advantages of GaN high electron mobility transistors (HEMTs) grown on native GaN over GaN/Si or GaN/sapphire substrates are investigated and correlated with epitaxial material quality. Transmission electron microscopy plan-view and cross-sectional analyses of GaN/GaN reveal dislocation densities below ${1} \times {10}^{{6}}$ cm−2, which is at least three orders of magnitude lower than that of GaN/Si or GaN/sapphire. In the case of GaN/Si, the dislocations not only originate from the substrate/nucleation layer interface, but also the strain relief and isolation buffer stacks are main contributors to the dislocation density. GaN/GaN HEMTs show superior electrical and thermal performance and feature three orders of magnitude lower OFF-state leakage. The current collapse (also referred to as current dispersion or $R_{ \mathrm{\scriptscriptstyle ON}}$ -increase) after stress bias is less than 15% compared with 50% in the case of GaN/Si. A 2% drop of the ON-state current due to self-heating in dc operation when compared with 13% and 16% for GaN/Si and GaN/sapphire, respectively. The GaN/Si thermal performance becomes comparable to that of GaN/GaN only after substrate removal. Therefore, GaN/GaN provides high ON-state current, low OFF-state leakage current, minimal current collapse, and enhanced thermal power dissipation capability at the same time, which can directly be correlated with the absence of high dislocation density. [ABSTRACT FROM AUTHOR]

In order to evaluate various buffer layers for metamorphic devices, threading dislocation densities have been calculated for uniform composition InxGa1−xAs device layers deposited on GaAs (001) substrates with an intermediate graded buffer layer using the LMD model, where LMD is the average length of misfit dislocations. On this basis, we compare the relative effectiveness of buffer layers with linear, exponential, and S-graded compositional profiles. In the case of a 2 μm thick buffer layer linear grading results in higher threading dislocation densities in the device layer compared to either exponential or S-grading. When exponential grading is used, lower threading dislocation densities are obtained with a smaller length constant. In the S-graded case, lower threading dislocation densities result when a smaller standard deviation parameter is used. As the buffer layer thickness is decreased from 2 μm to 0.1 μm all of the above effects are diminished, and the absolute threading dislocation densities increase. [ABSTRACT FROM AUTHOR]

In this work we investigated the dislocation-dependent behavior of Pendellösung fringes from two types of semiconductor heterostructures: a uniform-composition InGaAs epitaxial layer grown on a GaAs (001) substrate with an intermediate step-graded InGaAs buffer, and an InGaAs/InAlAs high electron mobility transistor grown on an InP (001) substrate. Dynamical x-ray diffraction simulations were carried out in the 004, 115,135, and 117 geometry, assuming Cu kα incident radiation, for both structures. The dislocation density strongly affects the intensities and widths of Pendellösung fringes, and we have established quantitative relationships which will allow characterization of the dislocation density. [ABSTRACT FROM AUTHOR]

Micro-nano ultraviolet light sources have recently attracted great attention due to their promising applications in future quantum display, photoelectric detection sensors, biomedical devices, and so on. Design of ultraviolet light sources with smaller size, stronger emission intensity, and higher integration, are the key aspects for the development of high-performance micro-nano ultraviolet devices. Here, we demonstrate that unidirectional threading dislocation arrays in an AlN thin film can emit ultraviolet light with wavelength of ∼ 317 nm. The density of AlN dislocations is ∼ 4 × 1010 cm−2, and the light emission intensity of AlN dislocations is comparable to the intrinsic emission of the AlN thin film. By combining aberration-corrected transmission electron microscopy, atomic-resolution valence electron energy-loss spectroscopy, and first-principles calculations, the atomic and electronic structures and the band gaps of the AlN threading dislocations are determined. It is found that the AlN threading dislocations are comprised of edge, screw, and mixed dislocations. All the AlN dislocations exhibit smaller band gaps compared to the AlN bulk due to the increase of the Al-N bond length at the dislocation cores. The dangling bonds of Al and N at the edge dislocations, Al-Al and N N bonds at the screw dislocations cause the defect states in the band gaps. This study not only has clarified the atomic origin of dislocation nano-optics, but also will encourage the development of microelectronics and optoelectronic devices based on dislocation ultraviolet light emission. [Display omitted] [ABSTRACT FROM AUTHOR]

In this paper, Si-doped GaN template layer and nitride-based multiple quantum well (MQW) light-emitting diodes (LEDs) with conventional GaN buffer layer and GaN buffer layer using SiN treatment were elaborated by metalorganic vapor phase epitaxy (MOVPE). On both kinds of structures, five InGaN/GaN quantum wells were deposited simultaneously in identical growth conditions. GaN template layer defects that influence on the growth of InGaN/GaN MQW LEDs were systematically studied by means of scanning electron microscopy, high-resolution x-ray diffraction, temperature-dependant photoluminescence measurement, and electroluminescence. It is shown that optical properties of InGaN/GaN MQWs depend on the defect density of elaborated templates. Thereafter, we report an enhancement of the emission of blue MQW LEDs using SiN treatment, compared to the MQW LED emissions deposited on a conventional GaN buffer layer. [ABSTRACT FROM AUTHOR]

We have investigated the evolution of the strain and threading dislocation density in metamorphic compositionally and temperature-graded ZnSSe buffer layers. Linear variation in composition in conjunction with temperature grading may allow control over the relaxation process. Previously, we reported the development of a general kinetic model based on dislocation flow, which accounted for the time evolution of the strain relaxation in semiconductor structures under kinetically limited conditions, including interactions of threading and misfit defects. In this work, we studied ZnSSe/GaAs (001) heterostructures with linear compositional grading and a convex-upward (type A), linear (type B) or convex-downward (type C) temperature grading profile. The thermal budget available for relaxation in these types of structures is controlled by the temperature grading profile, made up of combinations of linear ramps and constant-temperature sections. In all cases, the temperature was varied from T (400°C to 600°C) at the substrate interface to T = 300°C at the surface. We also investigated the effect of varying the compositional gradient in the range from 0.18%/ μm to 1.6%/ μm. Structures with higher average temperature (greater thermal budget) and/or higher grading coefficient exhibited greater extent of relaxation and therefore reduced residual strain. Furthermore, controlling the extent of strain relaxation enabled optimization of the dislocation densities in these heterostructures. [ABSTRACT FROM AUTHOR]

In addition to pure threading screw dislocations (TSDs), the presence of threading mixed dislocations (TMDs) (with a component) has been reported both in 4H-SiC axial slices (wafers cut parallel to the growth axis) and in commercial offcut wafers (cut almost perpendicular to the growth axis). In this paper, we first demonstrate a method to quickly distinguish TMDs from TSDs in axial slices via synchrotron white-beam x-ray topography. Since such axial slices are usually not available for commercial purposes, a systematic method is then developed and demonstrated here to unambiguously determine the Burgers vectors of TMDs in 4H-SiC commercial offcut wafers. In this second study, both synchrotron monochromatic-beam x-ray topography and ray-tracing simulation are used. The x-ray topographs were recorded using grazing-incidence geometry. The principle of this method is that the contrast of dislocations on different reflections varies with the relative orientation of Burgers vectors with respect to the diffraction vectors. Measurements confirm that, in a commercial offcut wafer, the majority of the threading dislocations with screw component are mixed-type dislocations. [ABSTRACT FROM AUTHOR]

We have investigated the evolution of strain and threading dislocation density in metamorphic temperature-graded ZnSe buffer layers. Mismatched semiconductor heterostructures may be designed to take advantage of temperature grading to allow control over the relaxation process. To study temperature grading, we have applied a plastic flow model which predicts nonequilibrium strain relaxation as well as misfit and threading dislocation densities by accounting for the time evolution of kinetically limited and equilibrium strain relaxation, thermal activation of glide, and misfit-threading dislocation interactions. We considered ZnSe/GaAs (001) heterostructures comprising a convex-down (type A), linear (type B), and convex-up (type C) temperature grading profile. The thermal budget available for relaxation in these types of structures is controlled by the temperature grading profile, which consists of combinations of linear ramp-down and/or constant temperature growth; the temperature is varied from T (400°C to 600°C) at the substrate interface to T = 300°C at the surface. We show that structures with a higher thermal budget exhibit a greater extent of relaxation (lower strains). At lower thicknesses, the dislocation density is dominated by the extent of relaxation, whereas at greater thicknesses, it is controlled by annihilation and coalescence mechanisms. [ABSTRACT FROM AUTHOR]

High-resistance (HR) GaN with sheet resistance of 1.0 × 10 8 Ω/sq was grown on sapphire substrates using metal organic chemical vapor deposition. Sheet resistance of the GaN film increases 4 orders of magnitude by changing the nucleation layer (NL) annealing atmosphere from H 2 to N 2 . It is observed that the morphology of the NLs strongly depends on the annealing atmosphere. The analysis results based on high-resolution X-ray diffraction (HR-XRD) and etch pit density (EPD) measurements demonstrate that the density of edge-type threading dislocations increases with the proportion of the N 2 in the annealing atmosphere. Photoluminescence (PL) spectra is employed to analyze the optical properties of GaN films. The XRD and PL results indicate the primary compensating mechanism is due to acceptor levels introduced by the increase in edge-type threading dislocations density. It is concluded that the annealing atmosphere of the NL controls sizes and densities of the nucleation islands, which affect electrical properties of GaN epitaxial films through changing the ratio of edge to screw/mixed-type threading dislocations. [ABSTRACT FROM AUTHOR]

In this work, we report on the results of the theoretical analysis of threading dislocation (TD) density reduction in porous III-nitride layers grown in polar orientation. The reaction-kinetics model originally developed for describing TD evolution in growing bulk layers has been expanded to the case of the porous layer. The developed model takes into account TD inclinations under the influence of the pores as well as trapping TDs into the pores. It is demonstrated that both these factors increase the probability of dislocation reactions thus reducing the total density of TDs. The mean pore diameter acts as an effective interaction radius for the reactions among TDs. The model includes the main experimentally observed features of TD evolution in porous III-nitride layers. [ABSTRACT FROM AUTHOR]

The design of metamorphic buffer layers for semiconductor devices with reduced defect densities requires control of lattice relaxation and dislocation dynamics. Graded layers are beneficial for the design of these buffers because they reduce the threading dislocation density by (1) allowing the distribution of the misfit dislocations throughout the buffer layer therefore reducing pinning interactions, and (2) enhancing mobility from the high built-in surface strain which helps to sweep out threading arms. In this work, we considered heterostructures involving a linearly-graded (type A) or step-graded (type B) buffer grown on a GaAs (001) substrate. For each structure type, we studied the equilibrium configuration and the kinetically-limited lattice relaxation and non-equilibrium threading dislocations by utilizing a dislocation dynamics model. In this work, we have also considered heterostructures involving a constant composition ZnSSe device layer grown on top of a GaAs (001) substrate with an intermediate buffer layer of linearly-graded (type C) or step-graded (type D) ZnSSe. For each structure type, we studied the requirements on the thickness and compositional profile in the buffer layer for the elimination of all mobile threading dislocations from the device layer by the dislocation compensation mechanism. [ABSTRACT FROM AUTHOR]

Highlights: [•] We investigated core configurations of all types of threading dislocations in GaN. [•] Their atomic and local electronic structures were at the DFT level. [•] Screw and mixed dislocations were found to introduce both deep and shallow gap states. [•] Most of core configurations of the edge dislocation introduce only shallow states. [•] The existence of an extended metallization is a specific feature of screw dislocations. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the efficiency enhancement of blue InGaN/GaN light-emitting diodes (LEDs) by incorporating a burried air void photonic crystal (BAVPC) layer within the epitaxial structure. As compared with the conventional patterned sapphire substrate (C-PSS) LEDs and flat sapphire substrate LEDs with BAVPC, the fabricated patterned sapphire substrate (PSS) LEDs with BAVPC exhibit the lowest full-width at half-maximum of (002) and (102) diffraction peaks, the highest light output power of 20.6 mW, and the highest external quantum efficiency of 37.4%. Remarkable performance improvement in the PSS LED with BAVPC is attributed to the better epitaxial quality with threading dislocations terminated by the BAVPC and the higher scattering at interface between GaN and air-void. By positioning the BAVPC directly below the multiple quantum wells (MQWs), it would cause the reduction in the number of trapped optical modes. The methodology optically isolates the MQWs from the underlying substrate and increases the optical output power. Moreover, threading dislocations are significantly suppressed using the BAVPC with high air filling fraction of ~ 50%. It is well proposed that this methodology provides a promising alternative to C-PSS LEDs. [ABSTRACT FROM AUTHOR]

Metamorphic semiconductor devices are commonly fabricated with linearly-graded buffer layers, but equilibrium modeling studies suggest that S-graded buffers, following a normal cumulative distribution function, may enable lower threading defect densities. The present work involves a study of threading dislocation density behavior in S-graded ZnSxSe1-x buffer layers for metamorphic devices on mismatched GaAs(001) substrates using a kinetic model for lattice relaxation and misfit-threading dislocation interactions. The results indicate that optimization of an S-graded buffer layer to minimize the surface threading dislocation density requires adjustment of the standard deviation parameter and cannot be achieved by varying the buffer thickness alone. Furthermore, it is possible to tailor the design of the S-graded buffer layer in such a way that the density of mobile threading dislocations at the surface vanishes. Nonetheless, the threading dislocation behavior in these heterostructures is quite complex, and a full understanding of their behavior will require further experimental and modeling studies. [ABSTRACT FROM AUTHOR]

The major issues confronting the performance of deep-UV (DUV) laser diodes (LDs) are reviewed along with the different approaches aimed at performance improvement. The impact of threading dislocations on the laser threshold current, limitations on heavy n- and p-doping in Al-rich AlGaN alloys, unavoidable electron leakage into the p-layers of (0001) LD structures, implementation of tunnel junctions, and non-uniform hole injection into multiple quantum wells in the active region are discussed. Special attention is paid to the current status of n- and p-type doping and threading dislocation density reduction, both being the factors largely determining the performance of DUV-LDs. It is shown that most of the above problems originate from intrinsic properties of the wide-bandgap AlGaN semiconductors, which emphasizes their fundamental role in the limitation of deep-UV LD performance. Among various remedies, novel promising technological and design approaches, such as high-temperature face-to-face annealing and distributed polarization doping, are discussed. Whenever possible, we provided a comparison between the growth capabilities of MOVPE and MBE techniques to fabricate DUV-LD structures.

Over 350 4H-SiC Schottky barrier diodes (SBDs) of varying size are characterized using current–voltage ( I– V) measurements, with some also measured as a function of temperature. Devices display either a characteristic single-barrier height or atypical dual-barrier heights. Device yields are shown to decrease as device area increases. Molten KOH etching is used to highlight defects for analysis by optical microscopy and atomic force microscopy. The I– V characteristics are compared against the defect density. A positive correlation between effective barrier height and effective electrically active area of the SBDs is found. No correlation is found between threading dislocations and ideality factor or barrier height. [ABSTRACT FROM AUTHOR]

Yielding and strain hardening in metallic thin films on substrates are studied using a simple edge dislocation climb model, modified to mimic dislocation processes in passivated, single crystal FCC metal films with a (111) texture. The aim of the modeling is to produce closed-form solutions for the yield strength and rate of strain hardening that can be compared with experiment. The models give a good account of the dependence of the yield strength of passivated gold films on silicon substrates on the film thickness and they are in broad agreement with the experimental observation that plastic flow in passivated metal films is characterized by very high rates of strain hardening. However, these simple models fail to predict the observed decrease in the rate of strain hardening with increasing film thickness, a result that requires more computationally intensive discrete dislocation modeling. [ABSTRACT FROM AUTHOR]