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[{"authors":["admin"],"categories":null,"content":"Hello, welcome to my personal site! I am a computational materials physicist working as a lecturer in the School of Engineering at London South Bank University. I am interested in how charge carriers interact with defects in crystalline systems, in particular semiconductors. My work focuses on materials used in energy applications and high power microelectronics.\nI am a Corkonian, exiled to the UK. I studied physics at University College Cork, gaining a first class BSc degree in 2004 and, after a few years wandering the wilderness, returned there to work on my PhD under the supervision of Professor Stephen Fahy. We developed a method to probe complex defect states in dilute nitride semiconductors using carrier scattering properties, which involved creating a computational model of a heterostructure device. After graduating in 2011, I moved to the Department of Chemistry at UCL to experience culture shock. I spent 8 years there pretending to be a chemist, working with Professor Richard Catlow, as well as Professors Aron Walsh and David Scanlon, establishing a fruitful Irish comp. chem. network. In October 2019 I was appointed a lecturer at LSBU in the School of Engineering, so my job now involves pretending to be an electronic engineer.\nOutside of research and teaching, I enjoy running, cycling, football, movies, crime novels and exploring the South East of England with my wife and two kids.\n","date":-62135596800,"expirydate":-62135596800,"kind":"taxonomy","lang":"en","lastmod":-62135596800,"objectID":"2525497d367e79493fd32b198b28f040","permalink":"/authors/admin/","publishdate":"0001-01-01T00:00:00Z","relpermalink":"/authors/admin/","section":"authors","summary":"Hello, welcome to my personal site! I am a computational materials physicist working as a lecturer in the School of Engineering at London South Bank University. I am interested in how charge carriers interact with defects in crystalline systems, in particular semiconductors. My work focuses on materials used in energy applications and high power microelectronics.\nI am a Corkonian, exiled to the UK. I studied physics at University College Cork, gaining a first class BSc degree in 2004 and, after a few years wandering the wilderness, returned there to work on my PhD under the supervision of Professor Stephen Fahy.","tags":null,"title":"John Buckeridge","type":"authors"},{"authors":null,"categories":null,"content":"","date":-62135596800,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":-62135596800,"objectID":"b4c7f25d5b7963758e82d0980c48943b","permalink":"/contact/contact/","publishdate":"0001-01-01T00:00:00Z","relpermalink":"/contact/contact/","section":"contact","summary":"","tags":null,"title":"Contact","type":"contact"},{"authors":null,"categories":null,"content":"I now have a PhD position available, starting in early 2020. Come join me at LSBU School of Engineering to investigate the electronic and optical properties of semiconductors using materials modelling techniques! The PhD studentship will involve developing and applying methodology to treat the interactions between electrons, phonons and photons in semiconductors used in energy and microelectronic devices. The aim will be to use state-of-the-art techniques to calculate how electrons and defects interact in a range of different materials.\nMore details are available at this link, where you will find a pdf with the full project description. There are other PhD studentships available at LSBU too, that you can find advertised there, which may be of interest.\n","date":1573049027,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1573049027,"objectID":"31acbe08e6a1eb90b56af90eb61c0d8f","permalink":"/post/nov2019_phd_ad/","publishdate":"2019-11-07T00:03:47+10:00","relpermalink":"/post/nov2019_phd_ad/","section":"post","summary":"Opportunity to carry out research in materials modelling.","tags":null,"title":"PhD position available","type":"post"},{"authors":null,"categories":null,"content":"I have recently read the excellent book Invisible by Philip Ball. It was a fascinating read, packed full of information on myth, magic, philosophy and science. What really struck me was how much science of the 19th century was either driven by, or related to, investigations into the occult. We\u0026rsquo;re talking about the golden era of seances, mediums, mystics and stage magicians. The great discoveries of the late 19th century: radio waves, x-rays and \u0026lsquo;cathode rays\u0026rsquo; (or electrons) all were \u0026lsquo;invisible\u0026rsquo;, and were often thought to be related to apparent psychic forces, ghosts and other paranormal phenomena.\nIt\u0026rsquo;s amazing to look back at it all now with the benefit of hindsight and question how they could have believed this stuff. Particularly interesting was the work of Sir William Crookes, who was either a crank or a dupe (or maybe both, as Ball suggests). This is Crookes, inventor of the famous radiometer, which many will have encountered in secondary school science. It turns out that he invented this device to try measure invisible, psychic forces, and seemed to believe he had been successful.\nI was pleased to learn about the book pictured here, Occult Chemistry by Annie Beasant and C. W. Leadbeater, two theosophists who basically used their imagination to come up with diagrams of atoms, the structure of which were unknown at the time. In some cases, they came up with structures remarkably similar to the common pictures of atomic orbitals we use nowadays, but they also drew some amazingly complex geometrical shapes that are unlike anything I have seen before. I love that they just assumed they could use their minds to \u0026lsquo;discover\u0026rsquo; what atoms looked like! The book also contains a picture of the periodic table from a paper in the Proceedings of the Royal Society, read in 1898 that I quite like:\n\r\r\rInvisible really indicates just how well scientific investigations are embedded within the sociological frameworks in which they operate. We like to think of science as being ultra-rational, beyond influence from mere human biases, but despite our best efforts it rarely is. Although the scientific method is designed to avoid such influences, its failure to do so (or the failure of its implementation, more precisely) is something we need to be aware of. I wonder what driving forces and theories for investigations into the \u0026lsquo;invisible\u0026rsquo; that science is concerned with now, will look as strange in 100 years as those of Crookes et al. in the late 1800s do today? Many-worlds interpretations? Dark energy? ;)\n","date":1572962627,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1572962627,"objectID":"a621c6e13da782fa7da442af406819d0","permalink":"/post/nov2019_occult_chemistry/","publishdate":"2019-11-06T00:03:47+10:00","relpermalink":"/post/nov2019_occult_chemistry/","section":"post","summary":"Weird investigations of late 19th century science.","tags":null,"title":"Occult Chemistry","type":"post"},{"authors":null,"categories":null,"content":"I am the guest editor for a special issue in the journal Crystals on the topic of defects in semiconductors. It\u0026rsquo;s a relatively broad topic, but hopefully will attract a diverse set of papers. Please submit if you have some suitable work as your paper will be in good company and should reach the correct audience! More info can be found on the website.\n","date":1571839427,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1571839427,"objectID":"1450669f3b3872cb402ca5f690fa951d","permalink":"/post/oct2019_guest_editor/","publishdate":"2019-10-24T00:03:47+10:00","relpermalink":"/post/oct2019_guest_editor/","section":"post","summary":"On the topic of defects in semiconductors","tags":null,"title":"Special issue in Crystals","type":"post"},{"authors":null,"categories":null,"content":"I am very interested in developing methods to model carrier-defect interactions, in order to understand carrier dynamics in realistic systems. My PhD thesis focussed on modelling carrier scattering by N-induced defect states in dilute nitride semiconductors, so this research project is one I have been involved with for quite a while. Electrons or holes can be mobile in a system and be scattered by defects, or may become trapped. Moreover, excitations involving electrons trapped at defects result in many measurable phenomena using spectroscopic techniques. Modelling such processes can be achieved using a variety of techniques of differing complexity and accuracy. Finding optimum approaches and developing the methodology is something I am actively working on. It combines all the problems of modelling defects effectively, modelling electronic structure accurately and finding tractable approaches for electron-ion interactions and optoelectronic processes. Fun!\n","date":1570770047,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1570770047,"objectID":"f9f252f9486a0c31c0442ddea26098ee","permalink":"/project/scattering-theory/","publishdate":"2019-10-11T15:00:47+10:00","relpermalink":"/project/scattering-theory/","section":"project","summary":"Developing methodology to understand how defects in semiconductors interact with charge carriers.","tags":null,"title":"Charge Carrier $-$ Defect Interactions","type":"project"},{"authors":null,"categories":null,"content":"I have developed many analysis tools through the course of my research. My language of choice is FORTRAN 90 (yes, I\u0026rsquo;m a dinosaur), but I also use C++, Python and bash scripting.\n","date":1570770047,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1570770047,"objectID":"83a668d2e49edcb69bbeb8ae26540d17","permalink":"/project/coding/","publishdate":"2019-10-11T15:00:47+10:00","relpermalink":"/project/coding/","section":"project","summary":"Building software tools for data analysis.","tags":null,"title":"Code Development","type":"project"},{"authors":null,"categories":null,"content":"","date":1570770047,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1570770047,"objectID":"4f2b80edf9140768e1c5f28db31d647c","permalink":"/project/dilute-nitrides/","publishdate":"2019-10-11T15:00:47+10:00","relpermalink":"/project/dilute-nitrides/","section":"project","summary":"Determining the effects of N and Bi incorporation in arsenides and antimonides.","tags":null,"title":"Dilute Nitride and Bismide Semiconductors","type":"project"},{"authors":null,"categories":null,"content":"","date":1570770047,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1570770047,"objectID":"6ffec1d39082f55626bad4e30b34b593","permalink":"/project/thermoelectrics/","publishdate":"2019-10-11T15:00:47+10:00","relpermalink":"/project/thermoelectrics/","section":"project","summary":"Finding new materials for thermoelectric applications.","tags":null,"title":"Materials for thermoelectrics","type":"project"},{"authors":null,"categories":null,"content":"","date":1570770047,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1570770047,"objectID":"8d5b2e66515281c382f8219bf465b51a","permalink":"/project/sofcs/","publishdate":"2019-10-11T15:00:47+10:00","relpermalink":"/project/sofcs/","section":"project","summary":"Understanding how defects affect charge transport and catalysis in SOFC cathodes.","tags":null,"title":"Solid Oxide Fuel Cell Cathodes","type":"project"},{"authors":null,"categories":null,"content":"","date":1570770047,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1570770047,"objectID":"8f1227eebbb3f3f9a47c4f09d39212cc","permalink":"/project/wide-gap-semiconductors/","publishdate":"2019-10-11T15:00:47+10:00","relpermalink":"/project/wide-gap-semiconductors/","section":"project","summary":"Investigating defects in wide-gap nitrides and oxides.","tags":null,"title":"Wide-Gap Semiconductors","type":"project"},{"authors":null,"categories":null,"content":"After a busy September, full of childcare, getting projects into a fit state for publication, meetings, planning, backing up files and physical labour, I have completed my move to LSBU. I now have an office with a view! And quite a nice view too.\nI have been made very welcome here by all the staff. The location is nice, the students are friendly and there are plenty of opportunities for research collaborations within the School of Engineering. I have a shorter commute, which involves boris-biking from Waterloo. Very pleasant so far!\n","date":1569852227,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1569852227,"objectID":"134392db6699276504a992341f56a0dd","permalink":"/post/oct2019_lsbu_start/","publishdate":"2019-10-01T00:03:47+10:00","relpermalink":"/post/oct2019_lsbu_start/","section":"post","summary":"Getting organised to move to LSBU","tags":null,"title":"Joining London South Bank University as a lecturer","type":"post"},{"authors":null,"categories":null,"content":"This paper describes the algorithm for my code SC-FERMI, which allows you to compute the self-consistent Fermi energy for a system with a set of defects. The operation of the code is demonstrated using the set of intrinsic point defects in GaN calculated using hybrid QM/MM. As those calculated defect levels were a bit controversial at one stage, I showed that the values used are actually quite similar to other calculations in the literature using plane-wave density functional theory. I had fun deriving the defect concentration expressions from statistical mechanics!\n","date":1569852059,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1569852059,"objectID":"2673c41120036557c130b069993dd7ac","permalink":"/publication/sc-fermi/","publishdate":"2019-10-01T00:00:59+10:00","relpermalink":"/publication/sc-fermi/","section":"publication","summary":"A concise procedure to determine the self-consistent Fermi energy and defect and carrier concentrations in an extended crystalline system is presented. It is assumed that the formation enthalpies of a set of variously charged point defects in thermodynamic equilibrium are known, as well as the density of electronic states in the defect-free system. By applying the constraint of overall charge neutrality, the self-consistent Fermi energy is determined using an iterative searching routine. The procedure is incorporated within a Fortran code 'SC-FERMI': the input consists of the defect formation energies, density of sites where they can form, and the degeneracy of each charge state; the material band gap; and the calculated density of states of the pristine system. The output is the self-consistent Fermi energy, the total concentrations of each defect as well as the concentration of its individual charge states, and the free carrier concentrations. Furthermore, the procedure facilitates fixing the concentration of one or more defects and determining the resulting self-consistent Fermi energy and concentrations of other defects (performed using the related code 'FROZEN-SC-FERMI'), thus modelling `frozen-in' defects which may form by kinetic, rather than thermodynamic, processes. One can fix the total concentration or the concentration of a particular charge state; it is also possible to introduce new defects with a fixed concentration, but here the charge state must be specified. The background theory is discussed in some detail, and the operation of the program is demonstrated by some examples.","tags":null,"title":"Equilibrium point defect and charge carrier concentrations in a material determined through calculation of the self-consistent Fermi energy","type":"publication"},{"authors":null,"categories":null,"content":"Very sad to be leaving UCL after so long. I have made many friends here, got to be involved in a lot of great research projects, formed collaborations that I hope will last the rest of my career and enjoyed life in Bloomsbury. Lunch Punch in the gym will be missed.\nI had many leaving dos, as I left Richard\u0026rsquo;s group in 2018 and David\u0026rsquo;s in 2019. Many nice meals were had, as well as beers in the Housman Room and various pubs. The pic is from a meal in the local Greek restaurant when I left Richard\u0026rsquo;s group.\n","date":1569816227,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1569816227,"objectID":"6cc2f1dc71d01b307603f69c9ce868f5","permalink":"/post/sept2019_ucl_leaving/","publishdate":"2019-09-30T14:03:47+10:00","relpermalink":"/post/sept2019_ucl_leaving/","section":"post","summary":"Sad to leave UCL.","tags":null,"title":"Leaving UCL after over 8 years at the Department of Chemistry","type":"post"},{"authors":null,"categories":null,"content":"These calculations were on the \u0026lsquo;back burner\u0026rsquo; for a while, but ended up providing some insights into the electronic properties of GaSb and InSb, two standard semiconducting materials that not many people have looked into using first electronic structure theory calculations. The defect physics of both are investigated, using plane-wave, hybrid DFT, where the hybrid functional is chosen to reproduce only the band gap of each, but it is shown that the bulk properties of each system are reproduced well.\n","date":1563976859,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1563976859,"objectID":"7bc0a84d05d35666b0dedaf06f9619a9","permalink":"/publication/gasb_insb/","publishdate":"2019-07-25T00:00:59+10:00","relpermalink":"/publication/gasb_insb/","section":"publication","summary":"The presence of defects in the narrow-gap semiconductors GaSb and InSb affects their dopability and hence applicability for a range of optoelectronic applications. Here, we report hybrid density functional theory based calculations of the properties of intrinsic point defects in the two systems, including spin orbit coupling effects, which influence strongly their band structures. With the hybrid DFT approach we adopt, we obtain excellent agreement between our calculated band dispersions, structural, elastic and vibrational properties and available measurements. We compute point defect formation energies in both systems, finding that antisite disorder tends to dominate, apart from in GaSb under certain conditions, where cation vacancies can form in significant concentrations. Calculated self-consistent Fermi energies and equilibrium carrier and defect concentrations confirm the intrinsic $n$- and $p$-type behaviour of both materials under anion-rich and anion-poor conditions. Moreover, by computing the compensating defect concentrations due to the presence of ionised donors and acceptors, we explain the observed dopability of GaSb and InSb.","tags":null,"title":"Intrinsic point defects and the $n$- and $p$-type dopability of the narrow gap semiconductors GaSb and InSb","type":"publication"},{"authors":null,"categories":null,"content":"Hmm, what to say about this paper. First of all, a lot of really good work by Zijuan. There is a lot of information here which should be useful for experimentalists in the field. We were met with a lot of criticism for this work, mainly it seems because people in the field doing electronic structure calculations were not familiar with the method. The results, however, when using hybrid functionals similar to those used by others who performed plane-wave DFT calculations, agree reasonably well with other studies. The BB1k functional, which is very rarely used in solid-state studies, provides some intriguing results for the optical absorption and emission properties of defects in GaN.\n","date":1560780059,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1560780059,"objectID":"7a9ba1e6e067c987a6208345fd4cbcfb","permalink":"/publication/gan_defects/","publishdate":"2019-06-18T00:00:59+10:00","relpermalink":"/publication/gan_defects/","section":"publication","summary":"The semiconducting behaviour and optoelectronic response of gallium nitride is governed by point defect processes, which, despite many years of research, remain poorly understood. The key difficulty in the description of the dominant charged defects is determining a consistent position of the corresponding defect levels, which is difficult to derive using standard supercell calculations. In a complementary approach, we take advantage of the embedded cluster methodology that provides direct access to a common zero of the electrostatic potential for all point defects in all charge states. Charged defects polarise a host dielectric material with long-range forces that strongly affect the outcome of defect simulations; to account for the polarisation, we couple embedding with the hybrid quantum mechanical/molecular mechanical approach and investigate the structure, formation and ionisation energies, and equilibrium concentrations of native point defects in wurtzite GaN at a chemically accurate hybrid-density-functional-theory level. N vacancies are the most thermodynamically favourable native defects in GaN, which contribute to the n-type character of as-grown GaN but are not the main source, a result that is consistent with experiment. Our calculations show no native point defects can form thermodynamically stable acceptor states. GaN can be easily doped n-type, but, in equilibrium conditions at moderate temperatures acceptor dopants will be compensated by N vacancies and no significant hole concentrations will be observed, indicating non-equilibrium processes must dominate in p-type GaN. We identify spectroscopic signatures of native defects in the infrared, visible and ultraviolet luminescence ranges and complementary spectroscopies. Crucially, we calculate the effective-mass-like-state levels associated with electrons and holes bound in diffuse orbitals. These levels may be accessible in competition with more strongly-localised states in luminescence processes and allow the attribution of the observed 3.46 and 3.27 eV UV peaks in a broad range of GaN samples to the presence of N vacancies.","tags":null,"title":"Donor and acceptor characteristics of native point defects in GaN","type":"publication"},{"authors":null,"categories":null,"content":"","date":1557792059,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1557792059,"objectID":"be3c701d47bb1d23ab90bf33c0bbe753","permalink":"/talk/quantum-computing/","publishdate":"2019-05-14T10:00:59+10:00","relpermalink":"/talk/quantum-computing/","section":"talk","summary":"Computational materials science has been progessing at a substantial rate over the last few decades. Classical computers have proven suitable to tackle a range of problems in materials science. In the near future, however, we may reach the limits of what is possible with classical computers, particularly taking into account the energy needs of large scale computation. Quantum computers offer an exciting opportunity to meet significant new challenges far beyond the realms of possibility using classical CPUs. In this talk I outline a set of problems that face computational materials scientists, with the aim of forming a dialogue between materials scientists and experts in quantum computation.","tags":null,"title":"Millenial challenges in computational materials science","type":"talk"},{"authors":null,"categories":null,"content":"The paper in which I got stuck in properly with the Questaal code for QS$GW$ calculations! Once you have it up and running, it\u0026rsquo;s an excellent code and the methods produce nice results that should be useful. BAs is of interest because it has been shown to be a very efficient thermal conductor, while being cheaper than diamond and more compatible with common semiconducting materials. It seems to be a tough system to produce good quality single crystals of though.\n","date":1557237659,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1557237659,"objectID":"82fe400c8afb6a0467dedcaeb32a52d0","permalink":"/publication/bas_elec/","publishdate":"2019-05-08T00:00:59+10:00","relpermalink":"/publication/bas_elec/","section":"publication","summary":"We compute the electronic band structure and optical properties of boron arsenide using the relativistic quasiparticle self-consistent $GW$ approach, including electron-hole interactions through solution of the Bethe-Salpeter equation. We also calculate its electronic and optical properties using standard and hybrid density functional theory. We demonstrate that the inclusion of self-consistency and vertex corrections provides substantial improvement in the calculated band features, in particular when comparing our results to previous calculations using the single-shot $GW$ approach and various DFT methods, from which a considerable scatter in the calculated indirect and direct band gaps has been observed. We find that BAs has an indirect gap of 1.674 eV and a direct gap of 3.990 eV, consistent with experiment and other comparable computational studies. Hybrid DFT reproduces the indirect gap well, but provides less accurate values for other band features, including spin-orbit splittings. Our computed Born effective charges and dielectric constants confirm the unusually covalent bonding characteristics of this III-V system.","tags":null,"title":"The electronic band structure and optical properties of boron arsenide","type":"publication"},{"authors":null,"categories":null,"content":"Some very nice computational work here by Ben Williamson, coupled with careful experiments from Ivan Parkin and Claire Carmalt\u0026rsquo;s groups. Co-doping of TiO$_2$ in order to narrow its band gap so that photocatalytic water splitting with visible light would progress more efficiently, is shown to be largely a \u0026lsquo;myth\u0026rsquo;! TiO$_2$ wants to be $n$-type, so co-doping is not going to be efficient due to the thermodynamics.\n","date":1552744859,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1552744859,"objectID":"9baaa471b35c0a790d1c92de7a23cc65","permalink":"/publication/tio2_myth/","publishdate":"2019-03-17T00:00:59+10:00","relpermalink":"/publication/tio2_myth/","section":"publication","summary":"Modification of TiO$_2$ to increase its visible light activity and promote higher performance photocatalytic ability has become a key research goal for materials scientists in the past 2 decades. One of the most popular approaches proposed this as “passivated codoping”, whereby an equal number of donor and acceptor dopants are introduced into the lattice, producing a charge neutral system with a reduced band gap. Using the archetypal codoping pairs of [Nb + N]- and [Ta + N]-doped anatase, we demonstrate using hybrid density functional theory that passivated codoping is not achievable in TiO$_2$. Our results indicate that the natural defect chemistry of the host system (in this case n-type anatase TiO$_2$) is dominant, and so concentration parity of dopant types is not achievable under any thermodynamic growth conditions. The implications of passivated codoping for band gap manipulation in general are discussed.","tags":null,"title":"Dispelling the Myth of Passivated Codoping in TiO$_2$","type":"publication"},{"authors":null,"categories":null,"content":"","date":1546300800,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1546300800,"objectID":"3bf44c81f2a197de01edde90bcd77783","permalink":"/team/","publishdate":"2019-01-01T00:00:00Z","relpermalink":"/team/","section":"","summary":"Welcome!","tags":null,"title":"LAB Page","type":"widget_page"},{"authors":null,"categories":null,"content":"","date":1546300800,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1546300800,"objectID":"9645a5b7879eb8537a2401e8d01687bc","permalink":"/lab/","publishdate":"2019-01-01T00:00:00Z","relpermalink":"/lab/","section":"","summary":"Welcome!","tags":null,"title":"Research Page","type":"widget_page"},{"authors":null,"categories":null,"content":"","date":1538971259,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1538971259,"objectID":"a76385a27e5a6d6892c92f9237d0ae51","permalink":"/talk/tyc-imperial/","publishdate":"2018-10-08T14:00:59+10:00","relpermalink":"/talk/tyc-imperial/","section":"talk","summary":"GaN is an important wide-gap semiconductor that is an essential component in blue light emitting diodes. Many experimental observations, both optical and electronic, such as intrinsic n-type conductivity and a range of luminescence bands found in a majority of samples, have been attributed to different defect structures in the material using first-principles calculations, typically employing the periodic supercell technique to model defects. The attribution often depends on the level of density functional theory applied, with little consistency across a broad range of studies. To overcome the problems inherent in plane-wave-based defect calculations, we employ the hybrid quantum mechanical/molecular mechanical embedded cluster approach, whose main advantages are access to a common reference energy, lack of image-charge interactions and a detailed description of polarisation due to local charges in a crystal, to compute formation and ionisation energies of point defects and defect complexes in GaN. Furthermore, we calculate equilibrium Fermi energies and carrier and defect concentrations at different temperatures. Our results account for the native n-type nature of GaN as well as suggest plausible explanations for observed deep level transient spectroscopy signatures, while we discuss the unresolved problem of the source of hole carriers in p-type GaN. Moreover, by taking into account the balance between carriers bound in compact and diffuse states and considering different absorption and emission processes involving point defects and defect complexes, we explain various photoluminescence peaks observed routinely in doped and nominally undoped samples. In particular, we attribute the 3.46 eV and 3.26 eV ultraviolet emission peaks to nitrogen vacancies binding compact and diffuse holes respectively, and describe the processes related to gallium vacancies that result in observed yellow luminescence. We demonstrate that the competition between these differently bound carrier states is key to understanding the luminescence properties of GaN, a point that has implications for other wide gap semiconductors, including oxides.","tags":null,"title":"Modelling electrons bound at defect sites in wide gap semiconductors","type":"talk"},{"authors":null,"categories":null,"content":"A fun perspective to be part of! It involved reading a lot of interesting older papers on electronic structure and bonding in condensed matter. We show that the concept of oxidation state has survived intact and is still very useful heuristically. There are exceptions, which we discuss, and future challenges.\n","date":1538316059,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1538316059,"objectID":"59b79962dbf98a079bef71b1b869d363","permalink":"/publication/oxidation_states/","publishdate":"2018-10-01T00:00:59+10:00","relpermalink":"/publication/oxidation_states/","section":"publication","summary":"The concepts of oxidation state and atomic charge are entangled in modern materials science. We distinguish between these quantities and consider their fundamental limitations and utility for understanding material properties. We discuss the nature of bonding between atoms and the techniques that have been developed for partitioning electron density. While formal oxidation states help us count electrons (in ions, bonds, lone pairs), variously defined atomic charges are usefully employed in the description of physical processes including dielectric response and electronic spectroscopies. Such partial charges are introduced as quantitative measures in simple mechanistic models of a more complex reality, and therefore may not be comparable or transferable. In contrast, oxidation states are defined to be universal, with deviations constituting exciting challenges as evidenced in mixed-valence compounds, electrides and highly correlated systems. This Perspective covers how these concepts have evolved in recent years, our current understanding and their significance.","tags":null,"title":"Oxidation states and ionicity","type":"publication"},{"authors":null,"categories":null,"content":"","date":1535677259,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1535677259,"objectID":"ac6dc7792290348746a365347cd6937a","permalink":"/talk/sfsb-conference/","publishdate":"2018-08-31T11:00:59+10:00","relpermalink":"/talk/sfsb-conference/","section":"talk","summary":"The recombination of photoexcited carriers with electrons or holes bound to point defects results in many experimentally observed optical phenomena in wide gap systems, such as ultra-violet, yellow, green, and red luminescence in GaN. Such processes are usually explained with reference to a particular defect state and often such states are studied systematically via computational techniques. The balance between different defect states, however, which may be metastable but relevant in the time-scale of optical processes, is frequently omitted from such analysis. Here we study how different configurations of electrons and holes, whether bound to defects in well-localised 'compact' states, or in extended 'diffuse' states, can alter the observed luminescence in GaN. For our calculations we employ the hybrid quantum mechanical/molecular mechanical embedded cluster method, which offers advantages over more commonly-applied supercell-based techniques when modelling defects in wide gap materials. The analysis regarding the balance between compact and diffuse states, however, is not dependent on the computational technique we employ. Our results allow us to account for various photoluminescence peaks observed routinely in doped and nominally undoped GaN samples. In particular, we attribute the 3.46 eV and 3.26 eV ultraviolet emission peaks to nitrogen vacancies binding compact and diffuse holes respectively, and describe processes related to gallium vacancy complexes that result in yellow, green and red luminescence. We demonstrate that the competition between these differently bound carrier states is key to understanding the luminescence properties of GaN, a point that also has implications for wide gap oxides. Indeed, we show that taking into account the diffuse states associated with oxygen vacancies in In2O3, ZnO and SnO2 helps explain the different intrinsic conductivity properties of these transparent conductors.","tags":null,"title":"The competition between compact and diffuse states in wide gap semiconductors","type":"talk"},{"authors":null,"categories":null,"content":"","date":1533250859,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1533250859,"objectID":"1695c330827ff7e5a4168ef8c7f64777","permalink":"/talk/icps-montpellier/","publishdate":"2018-08-03T09:00:59+10:00","relpermalink":"/talk/icps-montpellier/","section":"talk","summary":"GaN is an important wide-gap semiconductor that is an essential component in blue light emitting diodes. Many experimental observations, both optical and electronic, such as intrinsic n-type conductivity and a range of luminescence bands found in a majority of samples, have been attributed to different defect structures in the material using first-principles calculations, typically employing the periodic supercell technique to model defects. The attribution often depends on the level of density functional theory applied, with little consistency across a broad range of studies. To overcome the problems inherent in plane-wave-based defect calculations, we employ the hybrid quantum mechanical/molecular mechanical embedded cluster approach, whose main advantages are access to a common reference energy, lack of image-charge interactions and a detailed description of polarisation due to local charges in a crystal, to compute formation and ionisation energies of point defects and defect complexes in GaN. Furthermore, we calculate equilibrium Fermi energies and carrier and defect concentrations at different temperatures. Our results account for the native n-type nature of GaN as well as suggest plausible explanations for observed deep level transient spectroscopy signatures, while we discuss the unresolved problem of the source of hole carriers in p-type GaN. Moreover, by taking into account the balance between carriers bound in compact and diffuse states and considering different absorption and emission processes involving point defects and defect complexes, we explain various photoluminescence peaks observed routinely in doped and nominally undoped samples. In particular, we attribute the 3.46 eV and 3.26 eV ultraviolet emission peaks to nitrogen vacancies binding compact and diffuse holes respectively, and describe the processes related to gallium vacancies that result in observed yellow luminescence. We demonstrate that the competition between these differently bound carrier states is key to understanding the luminescence properties of GaN, a point that has implications for other wide gap semiconductors, including oxides.","tags":null,"title":"The defect physics of GaN: how the competition between compact and shallow states determines luminescence properties","type":"talk"},{"authors":null,"categories":null,"content":"","date":1515463259,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1515463259,"objectID":"d671829e9f34dd2e0e8ff1d5a3d4db56","permalink":"/talk/thermoelectric-india/","publishdate":"2018-01-09T12:00:59+10:00","relpermalink":"/talk/thermoelectric-india/","section":"talk","summary":"Computational materials science has the potential to lead the search for novel, highly efficient, sustainably sourced, non-toxic materials for thermoelectric applications. Such materials are required for next generation devices to transform global energy generation and conversion. Key to reaching this potential is the development of suitable techniques to be applied in a tractable manner. Here we present our computational design approach to realising new thermoelectric materials. We discuss methodology, and demonstrate a fully ab initio computational scheme to determine accurate figures of merit, including precise calculations of thermal transport properties. The effects of disorder and reduced dimensionality, which are crucial to limiting the thermal conductivity, are key considerations in our materials design programme. Moreover, we calculate the defect properties of the materials of interest, which allows us to determine their n- and p-type characteristics, and pinpoint suitable dopants. We present results on several systems, including a new Bi-containing oxide that has the potential to be a major low-cost alternative to current thermoelectrics. Our work is facilitated by close collaboration with experimentalists, whose results, which confirm our conclusions, we present and discuss.","tags":null,"title":"Computational design of new thermoelectric materials","type":"talk"},{"authors":null,"categories":null,"content":"","date":1470267059,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1470267059,"objectID":"fa215d50c88e85798192e5125cfcbdc9","permalink":"/talk/tyndall-2016/","publishdate":"2016-08-04T09:30:59+10:00","relpermalink":"/talk/tyndall-2016/","section":"talk","summary":"In many modern devices it is necessary to combine semiconducting properties with a wide energy band gap, whether for transparent conducting materials in touch screens, light emitting diodes operating in the blue or UV range, or energy applications such as collector layers in solar cells or photocatalytically active systems for water splitting. With such a combination there is a delicate balance between electronic disorder, as seen in conventional semiconductors, and ionic disorder, more prevalent in insulators. This balance is governed by the defect chemistry of the system, which is challenging to analyse at the atomic scale experimentally, allowing computational approaches to provide key insights. In this presentation I will discuss the hybrid quantum mechanical/molecular mechanical (QM/MM) embedded cluster approach to modelling defects in wide-gap semiconductors. I will discuss the advantages of this approach, such as the lack of periodic image charge interactions and an unambiguous reference energy to facilitate ionisation energy calculations, as well as the disadvantages. I will demonstrate the effectiveness of hybrid QM/MM by discussing band alignment in different polymorphs of TiO$_2$ (a photocatalytic water splitter), p-type doping in GaN (a key component of blue LEDs) and the role of oxygen vacancies in the three most common transparent conducting oxides (In$_2$O$_3$, ZnO and SnO$_2$).","tags":null,"title":"Modelling point defects in wide-gap semiconductors: elucidating the balance between ionic and electronic disorder","type":"talk"},{"authors":null,"categories":null,"content":"A very long paper based on a large proportion of my PhD thesis work. We built a model of a heterostructure device which could be used to probe N-related defect states in dilute nitrides. There are still plans to build these devices in University of Michigan.\n","date":1319983259,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1319983259,"objectID":"1f9fd1e5e53cf186d9cec9a3b4ee1100","permalink":"/publication/thesis_paper/","publishdate":"2011-10-31T00:00:59+10:00","relpermalink":"/publication/thesis_paper/","section":"publication","summary":"Calculations are presented of n-type carrier mobility in a gated, double-quantum-well InGaAs/GaN$_x$As$_{1−x}$ heterostructure (with $x≈0.01$), where transport occurs in the lowest two subbands (one primarily in the InGaAs layer and the other primarily in the dilute nitride layer) and carrier confinement is provided by surrounding AlGaAs layers and gating fields from front and back gates. Such a device allows a controlled interaction between carriers and localized nitrogen cluster states near the conduction band edge in the dilute nitride. Varying the In composition in the GaInAs layer and the gating fields is predicted to alter the Fermi level, relative to the localized nitrogen cluster states, and cause a strong modulation of the n-type mobility. For appropriately designed heterostructures, the mobility is predicted to vary by up to a factor of three when the Fermi level is resonant with nitrogen cluster electronic states. Thus, the mobility measurements in such a heterostructure can be used as a spectroscopic probe of the localized states in the dilute nitride layer.","tags":null,"title":"Mobility in gated GaN$_x$As$_{1−x}$ heterostructures as a probe of nitrogen-related electronic states","type":"publication"},{"authors":null,"categories":null,"content":"","date":1302836459,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1302836459,"objectID":"b4c29208285b8bb3e5ac929afa9491cd","permalink":"/talk/tyc-2011/","publishdate":"2011-04-15T13:00:59+10:00","relpermalink":"/talk/tyc-2011/","section":"talk","summary":"Dilute nitride alloys have attracted considerable attention in recent years. When a small fraction of the As atoms in GaAs or InGaAs are replaced by N, the energy gap of the material decreases rapidly; for example by 150 meV when the N concentration is 1%, making it a promising material for long wavelength (1.3 and 1.5 micron) telecommunications lasers based on a GaAs substrate and for extending the wavelength range of GaAs-based solar cells further into the infrared. However, the addition of these small concentrations of nitrogen to GaAs has also been found to cause a drastic reduction in n-type carrier mobility. The reductions in band gap and carrier mobility are attributed to an interaction between the GaAs conduction band, and a set of localized or quasi-localized defect states associated with the substitution of N on As sites at random throughout the alloy. The drastically low mobility in particular is attributed to the interaction with localized defect states associated with clusters of N atoms, whose energies are near-resonant with the conduction band edge. We calculate methods of probing defects associated with substitutional N in GaAs. A gated, double-quantum-well InGaAs/GaNAs heterostructure device, in which the interaction between conduction band carriers and states associated with N complexes, forming at random throughout the alloy, can be tuned by varying the gating electric fields, is described. The mobility as a function of gate field is calculated, with reductions in mobility occurring when the Fermi level is resonant with the energies of the N cluster states, providing a possible experimental method of probing these states, which are considered to be the primary mobility-limiting factor in the dilute nitrides.","tags":null,"title":"Theoretical study of local defect structures in dilute nitride semiconductors","type":"talk"},{"authors":null,"categories":null,"content":"","date":1230768000,"expirydate":-62135596800,"kind":"page","lang":"en","lastmod":1230768000,"objectID":"1f4cb24553577f6808a73065326a8b9d","permalink":"/blog/","publishdate":"2009-01-01T00:00:00Z","relpermalink":"/blog/","section":"","summary":"News and inane thoughts","tags":null,"title":"Blog","type":"widget_page"}]