Threading Dislocation Lines Research Articles

Threading dislocation lines and their impact on bulk cubic boron nitride’s low-field electron transport response

Building upon Chilleri et al.’s [Solid State Commun. 352 (2022) 114776] relaxation-time approximation based low-field electron drift mobility formalism, we incorporate treatment of threading dislocation related scattering into this analytical framework. Then, for a reasonable range of threading dislocation line density selections, representative of boron nitride’s zinc-blende phase, the role that threading dislocation lines play in shaping the low-field electron transport response of this material is examined. The six scattering processes considered in this analysis include the ionized impurity, polar optical phonon, acoustic deformation potential, piezoelectric, inter-valley, and threading dislocation line related scattering processes. The impact of threading dislocations is assessed through evaluations of the scattering rates, the fractional scattering contributions, and the low-field electron drift mobility contributions associated with the various scattering processes. The dependence of the total low-field electron drift mobility on the threading dislocation line density is also examined. We conclude the analysis with a comparison with the results of experiment. Threading dislocation lines are found to be a significant contributor to bulk zinc-blende boron nitride’s low-field electron transport response.

Threading dislocation lines within indium nitride versus gallium nitride: The implications of different dominant dislocation line charge screening mechanisms

Within n-type gallium nitride, the threading dislocation line defect sites act to trap free electrons from the bulk of this semiconductor material, thereby reducing the overall free electron concentration. In contrast, within n-type indium nitride, the threading dislocation line defect sites donate electrons, thereby contributing to the large unintentional free electron concentration observed for this semiconductor. In the present work, using an elementary model for the energy levels associated with the threading dislocation line defect sites, some of the implications of the different electron acceptor/donor characteristics of the threading dislocation line defect sites within n-type gallium nitride are contrasted with those within n-type indium nitride. Fundamentally different dominant screening mechanisms are found for the different materials, the screening within n-type gallium nitride being provided by the bulk ionized donors, the screening within n-type indium nitride being provided by the free carrier screening. We also explicitly derive and contrast the occupation statistics of the threading dislocation line defect sites within these two semiconductor materials.

Impact of Si doping on dislocation behavior in MOVPE-grown AlN on high-temperature annealed AlN buffer layers

In this work, we compare the defect structure in unintentionally doped and Si-doped AlN layers grown by metalorganic vapor phase epitaxy (MOVPE) on high-temperature annealed (HTA) sputtered AlN templates on sapphire substrates. Since the HTA process leads to a reduction of the in-plane lattice constant of the AlN layers, further homoepitaxial overgrowth results in compressively strained AlN layers. With increasing MOVPE-AlN layer thickness, strain relaxation takes place mostly by formation of dislocation half-loops of an irregular shape, which accumulate at the homoepitaxial MOVPE-AlN/HTA-AlN interface. We suggest that these dislocations nucleate at the layer surface and move down to the homoepitaxial interface at high temperatures. The formation of these irregular and hardly controllable defects can be avoided by introduction of Si-doping into the MOVPE-AlN layers. Si-doping enlarges the inclination of threading dislocation lines stemming from the HTA-AlN template, producing an alternative mechanism for strain relaxation.

The Impact of AlN Templates on Strain Relaxation Mechanisms during the MOVPE Growth of UVB‐LED Structures

AbstractStrain relaxation mechanisms in AlGaN based light emitting diodes emitting in the ultraviolet B spectral range (UVB‐LEDs) grown on different AlN/sapphire templates are analyzed by combining in situ reflectivity and curvature data with transmission electron microscopy. In particular, the impact of dislocation density, surface morphology, and lattice constant of the AlN/sapphire templates is studied. For nonannealed AlN/templates with threading dislocation densities (TDDs) of 4 × 109 and 3 × 109 cm−2 and different surface morphologies strain relaxation takes place mostly by conventional ways, such as inclination of threading dislocation lines and formation of horizontal dislocation bands. In contrast, a TDD reduction down to 1 × 109 cm−2 as well as a reduction of the lattice constant of high temperature annealed AlN template leads to drastic changes in the structure of subsequently grown AlGaN layers, e.g., to transformation to helical dislocations and enhanced surface enlargement by formation of macrofacets. For the growth of strongly compressively strained AlGaN layers for UVB‐LEDs the relaxation mechanism is strongly influenced by the absolute values of TDD and the lattice constant of the AlN templates and is less influenced by their surface morphology.

AlN growth on nano-patterned sapphire: A route for cost efficient pseudo substrates for deep UV LEDs

C-plane-oriented sapphire substrates that were patterned on the nanoscale were overgrown by AlN using metal-organic vapor phase epitaxy. The occurrence of undesired misaligned AlN growth was detected. We found that this misaligned growth can be overcome by a proper choice of growth temperature and V/III ratio. Up to 8 μm thick c-plane-oriented AlN with a coalesced surface was obtained. An effective dislocation reduction was found due to bending of threading dislocation lines toward free surfaces during lateral growth. The distribution of crystal defects suggests that step bunching in AlN is accompanied by dislocation accumulation. Furthermore, nearly defect-free AlN crystallites with a hexagonal shape and a size of about 2 μm were observed. Schematic cross-section representation of AlN grown on nano-patterned sapphire. Different AlN crystal orientations (arrows) and dislocations (solid black lines) are indicated.

Curvature and bow of bulk GaN substrates

We investigate the bow of free standing (0001) oriented hydride vapor phase epitaxy grown GaN substrates and demonstrate that their curvature is consistent with a compressive to tensile stress gradient (bottom to top) present in the substrates. The origin of the stress gradient and the curvature is attributed to the correlated inclination of edge threading dislocation (TD) lines away from the [0001] direction. A model is proposed and a relation is derived for bulk GaN substrate curvature dependence on the inclination angle and the density of TDs. The model is used to analyze the curvature for commercially available GaN substrates as determined by high resolution x-ray diffraction. The results show a close correlation between the experimentally determined parameters and those predicted from theoretical model.

Electron mobility limited by scattering from threading dislocation lines within gallium nitride

Theoretical as well as experimental studies in the literature suggest that defect sites associated with the threading dislocation lines within [Formula: see text]-type gallium nitride (GaN) act to trap free electrons from the bulk of this semiconductor material. As a result, the core of the threading dislocation lines become negatively charged. The charge accumulated along the core of a threading dislocation line should be screened by a charge of opposite polarity and equal in absolute value per unit length along the dislocation line. In the present work, we model this screened charge buildup along the threading dislocation lines by two concentric space-charge cylinders. Quantum mechanical theory of scattering in cylindrical coordinates is then employed in order to numerically compute the electron mobility limited by scattering from the charged threading dislocation lines. The dependence of the computed electron mobility on the dislocation line density and on the amount of charge accumulated per unit length along the core of the dislocation lines is also investigated in this work. Our computed electron mobility results are compared with results from existing calculations of the GaN dislocation scattering limited electron mobility in the literature.

A transition in the nature of the occupancy of the dislocation lines within n-type wurtzite gallium nitride

Within the framework of the model of Read [Philos. Mag. 45, 775 (1954)], we examine the occupancy of the dislocation lines within n-type wurtzite gallium nitride. In particular, we examine the transition that occurs as the bulk doping concentration is increased, from the depletion limit to the non-depletion limit. We note that an abrupt transitional bulk doping concentration can be defined. The dependence of this transitional bulk doping concentration on the dislocation line density is then determined. We note that existing theoretical results on the occupation statistics of the threading dislocation lines within wurtzite gallium nitride also exhibit such a transition. Since these theoretical results assume different structures for the core of the threading dislocation lines, we conclude that this transition between the depletion and non-depletion domains should be a universal feature, holding true irrespective of the particular structure being assumed for the core of the threading dislocation lines. Conditions under which this abrupt transition smoothens are also discussed.

Growth and Band Gap Control of Corundum-Structured α-(AlGa)2O3 Thin Films on Sapphire by Spray-Assisted Mist Chemical Vapor Deposition

Following the previous achievement of highly crystalline α-Ga2O3 thin films on c-plane sapphire, the growth of corundum-structured α-(AlxGa1-x)2O3 was examined aiming at the future application of α-(AlxGa1-x)2O3/Ga2O3 heterostructures to power devices and other functional devices. The results show the control of x and band gap up to 0.81 and 7.8 eV, respectively, maintaining the dominant corundum structure. The transmission electron microscope observation suggested the formation of the crystallographically good interface of α-(AlxGa1-x)2O3/Ga2O3 without the severe generation of threading dislocation lines from the interface.

The occupancy of the threading dislocation lines within n-type gallium nitride: Recent progress

ABSTRACTWithin the framework of a grand partition function formalism, we examine the occupancy of the dangling bond dislocation defect sites and the VGa-ON dislocation defect sites within uncompensated n-type gallium nitride. The sensitivity of these results to variations in the unoccupied dislocation defect energy level is examined. We find that the VGa-ON dislocation defect sites’ greater capacity to store charge plays a large role in influencing the results, i.e., greater free electron and bulk donor concentrations are required in order to fully saturate the threading dislocation lines with charge.

Electron mobility limited by scattering from screened positively charged dislocation lines within indium nitride

In the present work, we address the open question of the contribution from threading dislocations to the problem of unintentional n-type conductivity exhibited by indium nitride through an examination of the effect that positively charged dislocation lines have on the transverse electron mobility within this material. Assuming that the threading dislocation lines within indium nitride act as a source for free electrons, the screening associated with the positively charged threading dislocation lines is evaluated. The impact this screening has on the dislocation limited electron mobility within this material is then considered. Our results indicate that one of the implications of attributing a donor character to the threading dislocation lines within indium nitride would be a strong non-uniformity in the free electron concentration in the plane of growth of this semiconductor. This contrasts dramatically with the case of gallium nitride.

Effect of Al0.06Ga0.94N/GaN Strained-Layer Superlattices Cladding Underlayer to InGaN-Based Multi-Quantum Well Grown on Si(111) Substrate with AlN/GaN Intermediate Layer

We report effect of the insertion of Al0.06Ga0.94N/GaN strained-layer superlattices (SLSs) cladding underlayer to InGaN-based multi-quantum well (MQW) structure grown on Si(111) substrate with AlN/GaN intermediate layer. The Al0.06Ga0.94N/GaN SLS underlayer improves emission wavelength uniformity and shows a narrower emission full-width at half-maximum (FWHM) than that of conventional GaN underlayer. A Gaussian fitting was performed to photoluminescence (PL) spectra to obtain emission wavelength behavior and integrated intensity of the peak energy. A higher MQW internal quantum efficiency (ηiqe) of 29.4% is obtained in sample with Al0.06Ga0.94N/GaN SLS underlayer than that of 20.6% in GaN underlayer. Transmission electron microscopy (TEM) analysis indicates that the Al0.06Ga0.94N/GaN SLS layer abruptly bends threading dislocation lines thus reduces threading dislocation density to the MQW. Reciprocal space mapping (RSM) suggests that compressive strain in the GaN portion of Al0.06Ga0.94N/GaN SLS forces the abrupt inclination of the threading dislocations.

Effect of surface treatment of GaN based light emitting diode wafers on the leakage current of light emitting diode devices

To form low-resistance Ohmic contact to p-type GaN, InGaN/GaN multiple quantum well light emitting diode wafers are treated with boiled aqua regia prior to Ni/Au (5 nm/5 nm) film deposition. The surface morphology of wafers and the current–voltage characteristics of fabricated light emitting diode devices are investigated. It is shown that surface treatment with boiled aqua regia could effectively remove oxide from the surface of the p-GaN layer, and reveal defect-pits whose density is almost the same as the screw dislocation density estimated by x-ray rocking curve measurement. It suggests that the metal atoms of the Ni/Au transparent electrode of light emitting diode devices may diffuse into the p-GaN layer along threading dislocation lines and form additional leakage current channels. Therefore, the surface treatment time with boiled aqua regia should not be too long so as to avoid the increase of threading dislocation-induced leakage current and the degradation of electrical properties of light emitting diodes.

Optical analysis of dislocation‐related physical processes in GaN‐based epilayers

AbstractIn this paper, recent progresses in optical analysis of dislocation‐related physical properties in GaN‐based epilayers are surveyed with a brief review. The influence of dislocations on both near‐band edge emission and yellow luminescence (YL) is examined either in a statistical way as a function of dislocation density or focused on individual dislocation lines with a high spatial resolution. Threading dislocations may introduce non‐radiative recombination centers and enhance YL, but their effects are affected by the structural and chemical environment. The minority carrier diffusion length may be dependent on either dislocation density or impurity doping as confirmed by the result of photovoltaic spectra. The in situ optical monitoring of the strain evolution process is employed during GaN heteroepitaxy using an AlN interlayer. A typical transition of strain from compression to tension is observed and its correlation with the reduction and inclination of threading dislocation lines is revealed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Interaction between dislocations density and carrier concentration of gallium nitride layers

We investigated the problem of oxygen-dislocations interaction and its influence on carrier concentration in GaN layers. We achieved the samples with various dislocation density. Then we checked carrier concentration by means of Hall measurements. The samples with a higher EPD were characterized by a higher electron concentration. We assumed that oxygen diffuses along the threading dislocation lines, acts as a shallow donor and influences unintentional doping. Hence, the more dislocations present in GaN layers, the higher carrier concentration.

Cracking of III-nitride layers with strain gradients

Experimental results are demonstrated for the cracking of nominally compressed AlyGa1−yN layers grown on AlxGa1−xN buffer layers with smaller lattice constants (y<x). The authors present a theoretical analysis showing that the inclination of pure edge threading dislocation lines effectively reduces the compressive stress, causing relaxation, and after reaching a certain thickness begins to generate the tensile stress gradient responsible for cracking. The critical layer thickness for crack nucleation in such a gradient elastic field has been found. The results of the modeling are in good agreement with experimental observations of crack onset in nominally compressed nitride layers.

Stress relaxation in mismatched layers due to threading dislocation inclination

A recently observed mechanism of elastic stress relaxation in mismatched layers is discussed. The relaxation is achieved by the inclination of pure edge threading dislocation lines with respect to the layer surface normal. The relaxation is not assisted by dislocation glide but rather is caused by the “effective climb” of edge dislocations. The effective dislocation climb may result from the film growth and it is not necessarily related to bulk diffusion processes. The contribution of the dislocation inclination to strain relaxation has been formulated and the energy release due to the dislocation inclination in mismatched stressed layers has been determined. This mechanism explains recently observed relaxation of compressive stresses in the (0001) growth of AlxGa1−xN layers.

Scattering of electrons at threading dislocations in GaN

A model to explain the observed low transverse mobility in GaN by scattering of electrons at charged dislocation lines is proposed. Filled traps along threading dislocation lines act as Coulomb scattering centers. The statistics of trap occupancy at different doping levels are investigated. The theoretical transverse mobility from Coulomb scattering at charged traps is compared to experimental data. Due to the repulsive potential around the charged dislocation lines, electron transport parallel to the dislocations is unaffected by the scattering at charged dislocation lines.

Threading dislocation lines in two-sided flux-array decorations

Two-sided flux decoration experiments indicate that threading dislocation lines ~TDL’s !, which cross the entire film, are sometimes trapped in metastable states. We calculate the elastic energy associated with the meanderings of a TDL. The TDL behaves as an anisotropic and dispersive string with thermal fluctuations largely along its Burger’s vector. These fluctuations also modify the structure factor of the vortex solid. Both effects can, in principle, be used to estimate the elastic moduli of the material. @S0163-1829~97!05842-6# Recent two-sided flux decoration experiments have proven an effective technique to visualize and correlate the positions of individual flux lines on the two sides of Bi2Sr2CaCu2O8 ~BSCCO! thin superconductor films. 1,2 This material belongs to the class of high-Tc superconductors ~HTSC’s !, whose properties have drawn considerable attention in the last few years. 3 Due to disorder and thermal fluctuations, the lattice of rigid lines, representing the ideal behavior of the vortices in clean conventional type-II superconductors, is distorted. Flux decoration allows one to quantify the wandering of the lines as they pass through the sample. The resulting decoration patterns also include different topological defects, such as grain boundaries and dislocations, which in most cases thread the entire film. Decoration experiments are typically carried out by cooling the sample in a small magnetic field. In this process, vortices rearrange themselves from a liquidlike state at high temperatures, to an increasingly ordered structure, until they freeze at a characteristic temperature. 4,5 Thus, the observed patterns do not represent equilibrium configurations of lines at the low temperature where decoration is performed, but metastable configurations formed at this higher freezing temperature. The ordering process upon reducing temperature requires the removal of various topological defects from the liquid state: Dislocation loops in the bulk of the sample can shrink, while threading dislocation lines ~TDL’s ! that cross the film may annihilate in pairs, or glide to the edges. However, the decoration images still show TDL’s in the lattice of flux lines. The concentration of defects is actually quite low at the highest applied magnetic fields H ~around 25G!, but increases as H is lowered ~i.e., at smaller vortex densities !. Given the high-energy cost of such defects, it is most likely that they are metastable remnants of the liquid state. ~Metastable TDL’s are also formed during the growth of some solid films. 6 ! Generally, a good correspondence in the position of individual vortices and topological defects is observed as they cross the sample. Nevertheless, differences at the scale of a few lattice constants occur, which indicate the wandering of the lines. Motivated by these observations, we calculate the extra energy cost associated with the deviations of a TDL from a straight line conformation. The meandering TDL behaves like an elastic string with a dispersive line tension which depends logarithmically on the wave vector of the distortion. By comparing the experimental data with our results for mean-square fluctuations of a TDL, it is in principle possible to estimate the elastic moduli of the vortex lattice. Hence, this analysis is complementary to that of the hydrodynamic model of a liquid of flux lines, used so far to quantify these coefficients. 10 On the other hand, the presence of even a single fluctuating TDL considerably modifies the density correlation functions measured in the decoration experiments. The contribution of the fluctuating TDL to the longwavelength structure factor is also anisotropic and involves the shear modulus, making it a good candidate for the determination of this coefficient. In the usual experimental setup, the magnetic field H is oriented along to the z axis, perpendicular to the CuO planes of the superconductor. The displacements of the flux lines from a perfect triangular lattice at a point ( r,z), are described in the continuum elastic limit, by a two-dimensional ~2D! vector field u(r,z). The corresponding elastic free-energy cost is

High-resolution X-ray diffraction study of highly mismatched III–V heterostructures by analysis of the layer Bragg peak width

Recently, a method has been proposed in order to measure the threading dislocation density in epitaxial layers, based on X-ray diffraction and the analysis of the layer Bragg peak width β. According to this model, β is broadened by the rotation of the lattice planes and the modification of the lattice parameter near the threading dislocation lines. In the present work, this method is applied on GaAs/InAs and InAs/GaAs heterostructures. Different geometrical conditions and reflections, including (004), (115), (335), (444) and (117), have been investigated, showing a strong difference in β when, for a fixed reflection, the grazing incidence or grazing emergence geometry is chosen. Reciprocal lattice maps on the same samples demonstrate that this effect cannot be explained unless some degree of correlation between strain and tilt is assumed. The experimental findings are explained in term of lateral non-uniformity of the layer due to the misfit dislocation distribution. Further, the first results of a study about the effects of the layer roughness on β, performed on a InGaAs/GaAs sample, are presented.