Dislocations In Wurtzite GaN Research Articles

First-principles investigation of a-line Shockley partial dislocations in wurtzite GaN: core reconstruction and electronic structure

Αtomistic simulations based on density functional theory were performed to investigate Shockley partial dislocations, parallel to the <11–20> direction, in wurtzite gallium nitride. The cores of both 30° and 90° Shockley partial dislocations, their possible reconstructions and their electronic structures were analyzed and compared systematically. Shockley partial dislocations were found to have a variety of core structures while exhibiting different bonding states like: deformed Ga–N bonds, Ga–Ga and N–N homo-nuclear bonds, as well as Ga– and N–dangling bonds. We have demonstrated that the core reconstruction of these dislocations is not always energetically favourable in wurtzite GaN, as it is the case in elemental semiconductors. It results from interplay between the energy gain from eliminating dangling and forming homo-nuclear bonds and the energy excess from the induced strain of hetero-nuclear Ga–N bonds within the core. Depending on their core configurations, Shockley partial dislocations are expected to have different electronic behaviours in wurtzite GaN, as they can act either as sources of parasitic luminescence or non-radiative recombination centres, or give pathways to leakage currents. We have demonstrated that deep states associated with dislocations are not due to the dangling bonds, as generally believed, but rather to Ga–Ga bonds in their cores, and that N–N bonds do not lead to any gap states. These results are of high relevance as guidelines in defect engineering strategies aiming at producing dislocations with a particular electronic behaviour in wurtzite GaN.

Decorated Dislocations against Phonon Propagation for Thermal Management

The impact of decorated dislocations on the effective thermal conductivity of GaN is investigated by means of equilibrium molecular dynamics simulations via the Green-Kubo approach. The formation of "nanowires" by a few atoms of In in the core of dislocations in wurtzite GaN is found to affect the thermal properties of the material, as it leads to a significant decrease of the thermal conductivity, along with an enhancement of its anisotropic character. The thermal conductivity of In-decorated dislocations is compared to the ones of pristine GaN, InN, and random and ordered InxGa1-xN alloy, to examine the impact of doping. Results are explained by the stress maps, the bonding properties and the phonon density of states of the aforementioned systems. The decorated dislocations engineering is a novel way to tune, among other transport properties, the effective thermal conductivity of materials at the nanoscale, which can lead to the manufacturing of interesting candidates for thermoelectric or anisotropic thermal dissipation devices.

Core properties and mobility of the basal screw dislocation in wurtzite GaN: a density functional theory study

We have performed first principles simulations, based on density functional theory (DFT), to investigate the core properties of the basal a-type screw dislocation in wurtzite gallium nitride. Our calculations demonstrate that the fully coordinated shuffle core configuration is the most energetically favourable. The calculated electronic structure of the a-type screw dislocation was found to exhibit exclusively shallow gap states which are not associated with any extended metallization. This may explain why a-type screw dislocations are less detrimental to the performance of GaN based electronic devices than c-type screw dislocations.

Energetics and core structure of the undissociated basal screw dislocation in wurtzite GaN

AbstractWe have carried out computer atomistic simulations, based on Density Functional Theory, to investigate energetics and core structure of the undissociated basal screw dislocation in wurtzite GaN. Our calculations enabled us to establish the energetic hierarchy of two stable core configurations. The shuffle core configuration, which is formed by fully coordinated atoms, was demonstrated to be more energetically favorable than the glide configuration, which exhibits sp2‐like hybridization. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Core structures analyses of (a+c)-edge dislocations in wurtzite GaN through atomistic simulations and Peierls–Nabarro model

The core structures and slip characteristics of (a+c)-edge dislocations on pyramidal planes in wurtzite GaN were investigated employing molecular dynamics simulations. Multiple stable core configurations are identified for dislocations along the glide and shuffle planes. The corresponding generalized-stacking-fault energy (GSFE) curves for the glide and shuffle slips are calculated. The GSFE curves, combined with the Peierls–Nabarro model, demonstrate that the shuffle slip is favored over the glide slip given the markedly lower Peierls energy and stress of the shuffle slip. Our findings also indicate that in general slip motions for (a+c)-edge dislocations are only possible at elevated temperature, and the necessity of further studies of thermally activated processes to better understand the dynamics of (a+c) dislocations in GaN.

Electronic structure and metallization effects at threading dislocation cores in GaN

We have carried out a complete and consistent study on core configurations of the three types of threading dislocations in wurtzite GaN. Their atomic and local electronic structures were analyzed and systematically compared, at the level of Density Functional Theory. Screw and mixed threading dislocations were found to introduce both deep and shallow gap states, while most of core configurations of the edge dislocation introduce only shallow states. We demonstrated that the existence of an extended one-dimensional metallization, associated with unoccupied gap states, is a specific feature of threading screw dislocations. The extended metallization along with the high dispersion of the energy gap states are at the origin of the experimentally observed high electric conduction along threading screw dislocations in GaN. The presence of nitrogen vacancies in the core of screw dislocations is predicted to enhance their electric conduction.

Threading screw dislocations in GaN by the Heyd‐Scuseria‐Ernzerhof hybrid functional

AbstractThreading screw dislocations in wurtzite GaN are studied using Heyd‐Scuseria‐Ernzerhof hybrid functional with in generalized Kohn‐Sham scheme. Several types of core structures, both stoichiometric and non‐stoichiometric, are constructed from the unconventional dislocation line position. We computed the formation energies and compared their stabilities systematically. For the open core type structure, the equilibrium diameter is ˜ 4.9 Å. With the intermediate value of Ga chemical potential, the Ga‐filled type dislocation core, where N atoms near the dislocation line are removed and only Ga atoms exist, becomes the most stable structure. This dislocation core does not introduce deep empty states into the band gap, but introduces deep filled states originated from the Ga–Ga wrong bonds of Ga atoms located along the dislocation core. This leads to the band gap narrowing and degrades the electronic properties such as breakdown voltage. (© 2014 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

The 60° basal dislocation in wurtzite GaN: Energetics, electronic and core structures

We have carried out computer atomistic simulations, based on an efficient density functional based tight binding method, to investigate the core configurations of the 60° basal dislocation in GaN wurtzite. Our energetic calculations demonstrate that the glide configuration with N polarity is the most energetically favorable over both the glide and the shuffle sets in nitrogen-rich growth conditions. However, in the case of gallium-rich growth conditions, the shuffle configuration with gallium polarity becomes the most favorable. Otherwise, we found that all the four configurations of the 60° basal dislocation introduces both shallow and deep states but the glide configuration with N polarity, which introduce only shallow states.

Dissociation of the 60° basal dislocation in wurtzite GaN

AbstractWe have carried out atomistic calculations, using a self‐consistent tight‐binding method based on density functional theory, to determine formation energies of different core configurations of the perfect 60° and 90°, 30° Shockley partial dislocations in wurtzite GaN. Our analysis of different combinations of core configurations concluded that a 60° dislocation with a Ga polarity dissociates into a 90° partial dislocation with a single period and a 30° partial dislocation with a double period structure. Otherwise, a N polarity 60° dislocation dissociates into a 90° partial dislocation with a double period and a 30° partial dislocation with a single period structure (© 2013 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Dislocations and stacking faults in hexagonal GaN

AbstractWe present experimental and theoretical results on extended defects of hexagonal GaN grown by metal organic vapour deposition (MOCVD). Transmission electron microscopy (TEM) measurements indicate the presence of 3 nm wide type I1 stacking faults (SFs) related to the MOCVD growth, and 2–4 nm SFs of unidentified type related to the ion implantation. We simulated infinite SFs of different types I1, I2 and I3. First principles calculations were used to model Shockley partial dislocations, the core structure of the dislocations and intrinsic SFs. We estimate defect level positions and the formation energy of the infinite SFs. We also present results of the calculations and available experimental data on finite size SFs bounded by partial dislocations in wurtzite GaN. Calculation of the infinite SFs revealed shallow levels, but no deep levels. In the SFs bound by dislocations there are deep filled levels in the range of 0.4–0.8 eV from the valence band maximum (VBM) located at the atoms of the 90° dislocations. We also calculated the segregation of the C, n‐dopant, Si or O and the p‐dopant, Mg, to the dislocations and SFs. The effect of the segregation is found to be stronger for the SFs with partials. The results of the calculations are correlated with the experimental data on GaN obtained from high resolution TEM and Hall measurements. We suggest mechanisms of the formation of the SFs I1 and I2 after ion implantation and high temperature anneal and explain the difficulties of p‐type doping with the formation of the extended defects.

Dislocation core investigation by geometric phase analysis and the dislocation density tensor

The dislocation density tensor can be employed to ‘localize’ the dislocation core. We present a method for obtaining experimentally the dislocation core location and size from high resolution transmission electron microscopy observations using geometrical phase analysis. Experimental results are then compared with the results of atomistic simulations, and we also evaluate the applicability of continuum elasticity and its modifications for describing these results. The analysis is performed for partial dislocations in wurtzite GaN. Higher-order gradient elasticity theory is found to be in good agreement with the experimental observations.

Atomic core configurations of the [formula omitted]-screw basal dislocation in wurtzite GaN

The results of atomistic calculations based on an ab initio tight-binding method are reported in this work for four configurations of the perfect a ⇒ -screw basal dislocation in wurtzite GaN: pure shuffle, pure glide and two mixed shuffle–glide. Configurations with pure character (shuffle or glide) are found to be stable, whereas those with mixed shuffle–glide character are found to be transition-like configurations. Further, the calculations predict that the pure glide configuration, containing threefold coordinated atoms with an sp 2 hybridization, to be the most energetically favourable.

Energetics of the 30∘ Shockley partial dislocation in wurtzite GaN

Abstract In the present work, we have investigated the relative energy of different core configurations of the 30∘ Shockley partial dislocation in wurtzite GaN. By using a modified Stillinger–Weber potential, we have carried out large scale calculations on models containing many thousands of atoms. Both glide and shuffle configurations have been considered within the two core polarities (Ga, N). Similarly to what was reported for conventional semiconductors, our calculations showed that the reconstructed glide configurations are energetically favoured over the shuffle ones.

Ab‐initio tight‐binding study of the core structures of the c ‐edge dislocation in wurtzite GaN

AbstractIn the framework of ab‐initio tight‐binding methods, we modelled and studied the atomic and electronic core structures of the c‐edge basal dislocation in wurtzite GaN. We found this dislocation to have four core configurations displaying 5/8/5‐ or 4/8‐atom rings structures. All these configurations induce unfilled deep states in the top half of band gap. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Mixed partial dislocation core structure in GaN by high resolution electron microscopy

AbstractThe core structures of a 1/6 [2$\bar 2$03] mixed partial dislocation in wurtzite GaN have been investigated using a combination of high resolution transmission electron microscopy, circuit mapping, and image simulation of relaxed models. HRTEM simulated images of relaxed atomic models, derived by energetic calculations with a modified Stillinger–Weber‐type empirical interatomic potential, were calculated and compared to the experimental images. Among twenty‐four stable core configurations the 12‐ and 10‐atom rings satisfied the experimental contrast. A pattern registration procedure was used for the matching of simulated and experimental HRTEM images. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

New core configurations of the c‐edge dislocation in wurtzite GaN

AbstractIn the framework of empirical potentials (Modified Stillinger‐Weber), we modelled and studied the energy and the core structure of the c‐edge basal dislocation in wurtzite GaN. Further to the core configurations already known, we report two new ones without wrong bonds. Our calculations show that the configurations containing wrong bonds are the most energetically favourable in both glide and shuffle sets. (© 2006 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Local electronic structure of threading screw dislocation in wurtzite GaN

The atomic and electronic structure of the full-core threading screw dislocation in wurzite GaN has been investigated using a self-consistent density-functional tight-binding calculation. It is shown that the atoms are severely strained in a hexagonal atomic core, and an extra charge transfer of 0.12 e occurs at the core atoms from Ga to N, in addition to the typical charge transfer of 0.56 e for bulk GaN. Filled and unfilled gap states are found to be spread over the entire band gap. The p states of the core N-atom mostly contribute to the tail states of valence and conduction bands, whereas the deep levels are heavily localized at the Ga and N core atoms. The coexistence of acceptor and donor gap states in the vicinity of the screw dislocations could be an origin of the leakage currents observed in GaN-based devices.

Structure and Energy of the 90° Partial Dislocations in Wurtzite-GaN

90 Shockley partial dislocations in GaN are investigated by first-principles calculations. This work is focussed on the electrical properties of dislocation cores, and on investigating the electrical fields around these defects. The band structure analysis shows that both the and core partials possess a midgap state. The -core dislocations give rise to a donor level Ev +0:87 eV that might explain the absorption peak at 2.4 eV revealed by energy loss spectroscopy measurements. The acceptor level Ev + 1:11 eV localized at the -core dislocations might contribute to the yellow luminescence. These dislocations experience a substantial charge polarization along the [0001] growth axis. In addition, we show that these dislocations tend to charge in a high stress field.

Atomic structure and energy of threading screw dislocations in wurtzite GaN

Atomic structure and energy of the screw dislocation b = <000c> in the full core configuration has been investigated with a self-consistent density functional tight binding calculation. A 288-atom cluster was used and the dangling bonds saturated with pseudo-hydrogen. The line energy is comparable to the values obtained using a supercell. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Atomic structures and energies of partial dislocations in wurtzite GaN

The atomic structures of $1∕6⟨20\overline{2}3⟩$ partial dislocations delineating the ${\mathrm{I}}_{1}$ intrinsic basal stacking fault in wurtzite $\mathrm{GaN}$ are modelled using an empirical interatomic potential in combination with anisotropic elasticity calculations. Twelve stable configurations are obtained for each polarity, and their core radii, energies, and atomic configurations are given. The $5∕7$-atom ring core in which the atoms are tetrahedrally coordinated is found energetically favorable among the edge dislocation configurations. For the mixed type partials, $5∕7$- and 12-atom rings are obtained as low-energy cores, but none of them is found to comprise only tetrahedrally coordinated atoms. Each of them is found energetically favorable under distinct structural conditions.