Publications (50)83.48 Total impact

Article: Tightbinding analysis of the electronic structure of dilute bismide alloys of GaP and GaAs
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ABSTRACT: We develop an atomistic, nearestneighbor sp3s* tightbinding Hamiltonian to investigate the electronic structure of dilute bismide alloys of GaP and GaAs. Using this model we calculate that the incorporation of dilute concentrations of Bi in GaP introduces Birelated defect states in the band gap, which interact with the host matrix valence band edge via a Bi composition dependent band anticrossing (BAC) interaction. By extending this analysis to GaBiAs we demonstrate that the observed strong variation of the band gap Eg and spinorbitsplitting (SO) energy with Bi composition can be well explained in terms of a BAC interaction between the extended states of the GaAs valence band edge and highly localized Birelated defect states lying in the valence band, with the change in Eg also having a significant contribution from a conventional alloy reduction in the conduction band edge energy. Our calculated values of Eg and SO are in good agreement with experiment throughout the investigated composition range x less than 13%. In particular, our calculations reproduce the experimentally observed crossover to an Eg < SO regime at approximately 10.5% Bi composition in bulk GaBiAs. Recent xray spectroscopy measurements have indicated the presence of Bi pairs and clusters even for Bi compositions as low as 2%. We include a systematic study of different Bi nearestneighbor environments in the alloy to achieve a quantitative understanding of the effect of Bi pairing and clustering on the GaBiAs electronic structure.Physical review. B, Condensed matter 11/2011; · 3.66 Impact Factor 
Conference Paper: Tight binding analysis of the electronic structure of dilute bismide and nitride alloys of GaAs
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ABSTRACT: We use a nearestneighbour sp<sup>3</sup>s* tightbinding Hamiltonian to investigate the electronic structure of GaAsbased dilute bismide and bismidenitride alloys. We show that the observed strong variation of the band gap (E<sub>g</sub>) and spinorbit splitting energy (Δ<sub>SO</sub>) with Bi composition in GaBi<sub>x</sub>As<sub>1x</sub> is due primarily to a bandanticrossing interaction between the extended states of the host matrix valence band maximum and highly localised Birelated defect states lying in the valence band, with the change in E<sub>g</sub> also having a significant contribution from a conventional alloy reduction in the conduction band edge energy. We calculate a crossover to an E<sub>g</sub> <; Δ<sub>SO</sub> regime at approximately 10.5% Bi composition in bulk GaBi<sub>x</sub>As<sub>1x</sub>, in agreement with recent experimental studies of GaBi<sub>x</sub>As<sub>1x</sub> epilayers grown on GaAs. Finally, we present calculations which show that the effects of N and of Bi are largely independent of each other in random GaBi<sub>x</sub>N<sub>y</sub>As<sub>1xy</sub> alloys, of relevance for future high efficiency photonic devices.Transparent Optical Networks (ICTON), 2011 13th International Conference on; 07/2011  [Show abstract] [Hide abstract]
ABSTRACT: Photomodulated reflectance (PR) spectra of (B,Ga,In)As epilayers reveal unusual changes of the fundamental band gap PR line shape with temperature and hydrostatic pressure. We show that these changes arise because temperature variation or hydrostatic pressure shifts the conduction band edge (CBE) into resonance with boronrelated cluster states. The resulting line shape changes are described by a level repulsion model which yields states of mixed character with an exchange of oscillator strengths. This model is corroborated by theoretical calculations which show a finite density of boron cluster states above the CBE at room temperature, with appropriate symmetry to couple to the CBE state.Physical review. B, Condensed matter 01/2011; 83. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: We investigate the magnetotransport properties of ntype (B,Ga,In)As alloys and an ntype GaAs reference sample under hydrostatic pressure up to 16 kbar in the temperature range from 1.6 to 300 K. The free carrier concentration and the mobility of the reference sample are almost independent of the applied hydrostatic pressure. In contrast, the free carrier concentration and the mobility in B0.027Ga0.913In0.06As alloys drop by orders of magnitude over the accessible pressure range. The observations can be explained by assuming that a boronrelated density of localized states exists in the vicinity of the conduction band edge of the alloy. The analysis of the pressureinduced changes of the free carrier concentration yields an image of the boronrelated density of localized states with characteristic features that are found to be in good agreement with theoretical calculations using a linear combination of isolated states model. Our results provide strong evidence that boronrelated states act as isovalent traps in (B,Ga,In)As alloys.Physical review. B, Condensed matter 01/2011; 83(3). · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The replacement of As by N in GaAs introduces several pertubations, including both a change in potential at the N site and also long range strain relaxation in the crystal. It can be very useful to describe this perturbation in terms of the change in the Hamiltonian, ΔH, due to the introduction of N. Using planewave approaches, ΔH treats explicitly all atomic displacements in the structure. By contrast, a tightbinding approach allows a much simpler analysis. We illustrate the benefits of the tightbinding method by considering Ga(P,As:Bi) and GaNAs. We show that the tightbinding method provides a clear and quantitative explanation for many of the unusual electronic properties of these and related alloys, including the nonmonotonic variation of electron effective mass, me, and gyromagnetic ratio, ge, in GaNAs.Journal of Physics Conference Series 08/2010; 242(1):012002.  [Show abstract] [Hide abstract]
ABSTRACT: The bandanticrossing (BAC) model has been widely applied to analyse the electronic structure of dilute nitride IIIVN alloys such as GaNxAs1−x. The BAC model describes the strong band gap bowing observed at low N composition in GaNxAs1−x in terms of an interaction between the GaAs host matrix conduction band edge and a higher lying band of localized N resonant states. In practice, replacing As by N introduces a range of Nrelated defect levels, associated with isolated N atoms, N–N pairs and larger clusters of N atoms. We show that the effect of such defect levels on the alloy conduction band structure is strongly dependent on the relative energy of the defect levels and the host conduction band edge. We first consider GaNxAs1−x, where we show that the unexpectedly large electron effective mass and gyromagnetic ratio, and their nonmonotonic variation with x, are due to hybridization between the conduction band edge and specific nitrogen states close to the band edge. The Nrelated defect levels lie below the conduction band edge in GaNxP1−x. We must therefore explicitly treat the interaction between the higher lying GaP host Γ conduction band minimum and defect states associated with a random distribution of N atoms in order to obtain a good description of the lowest conduction states in disordered GaPN alloys. Turning to other alloys, Nrelated defect levels should generally lie well above the conduction band minimum in InNSb, with the band dispersion of InNSb then well described by a twolevel BAC model. Both InP and InAs are intermediate between InSb and GaAs. By contrast, we calculate that Nrelated defect levels lie close to the conduction band minimum in GaNSb, and will therefore strongly perturb the lowest conduction states in this alloy. Overall, we conclude that the BAC model provides a good qualitative explanation of the electronic properties of dilute nitride alloys, but that it is in many cases necessary to include the details of the distribution of Nrelated defect levels to obtain a quantitative understanding of the conduction band structure in dilute nitride alloys.Semiconductor Science and Technology 02/2009; 24(3):033001. · 1.92 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: III—N—As as well as III—N—P materials have been successfully employed in optoelectronic devices. Nitrogen impurities in the dilute range have been investigated in the indirect gap semiconductor GaP since the 1960s, where they were used to produce green lightemitting diodes. In recent years new developments in molecular beam epitaxy and metalorganic vapour phase epitaxy growth methods have made it possible to introduce up to a few percent of nitrogen into direct gap GaAs and indirect gap GaP. GaInNAs with N concentrations of about 1% has been used as the active material in vertical cavity surface emitting laser (VCSEL) devices operating at telecommunication wavelengths. However, GaAs:N and GaP:N differ considerably in terms of their electronic structure. In GaNAs the nitrogen impurity states in the doping regime lie resonantly in the conduction band. With increasing Ncontent a strong redshift of the GaAslike fundamental band gap (referred to as E −) occurs. It is accompanied by the formation of an Ninduced E + band, which blueshifts with increasing N. This repulsion behaviour of E − and E + can be well parameterized by a simple twolevel bandanticrossing model, which forms the basis of the 10 band k·p model successfully employed for describing the electronic states of III—N—As layers in laser structures in the vicinity of the GaAslike E − band gap. The situation in GaP is somewhat different because the N levels in the doping regime are situated in the band gap close to the X conduction band states (corresponding to the indirect gap), i.e. well below the Г conductionband states of GaP (corresponding to the direct gap). In other words, the order of the N states and the Г conduction band states is reversed. With increasing N content, the lowest conduction band must then evolve from the Nlike states in GaNP. The question then arises as to whether the bandanticrossing model yields a good description of the lowest conduction band E − in this case. If so, the lowest band gap will in a twolevel bandanticrossing model acquire a large Гlike density of states comparable to a direct semiconductor, which is a prerequisite for employing GaNP based heterostructures in the active region of laser devices. In this review, we compare the electronic structure of GaNAs and GaNP, demonstrating that, in GaNP, the Г character in the energy range of the N localized states is spread over a broad variety of transitions. This situation cannot be properly parameterized by the simple bandanticrossing model and indicates that, in contrast to GaNAs, the lowest conduction band states are not suitable to promote laser action in GaNP alloys.11/2008: pages 343367; 
Article: Evolution of N defect states and optical transitions in ordered and disordered GaP1−xNx alloys
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ABSTRACT: We show using an sp3s* tightbinding model that the band anticrossing (BAC) model describes well the evolution of the lowest Nrelated conduction states in ordered GaP1−xNx alloys, including the evolution of the Γ character with increasing x. We obtain a good description of the lowest conduction states in disordered GaPN structures by explicitly treating the interaction between the GaP host Γ conduction band minimum and defect states associated with a random distribution of N atoms. We find a very similar value for the total Γ character mixed into the N levels in the ordered and disordered cases, but a wider distribution of states with Γ character in the disordered case. We show that the band gap reduction with increasing composition is dominated by the increasing formation of N cluster states. Overall key features of the band structure can be well described using a modified BAC model which explicitly includes the broad distribution of N levels in disordered GaPN alloys.Journal of Physics Condensed Matter 06/2008; 20(29):295211. · 2.22 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The puzzling electronic properties of GaAsN have been investigated through the compositional dependence of two highly sensitive band structure parameters: the electron effective mass, me, and the electron gyromagnetic factor, ge. In the N concentration range from 0% to 0.7%, both me and ge show a highly nonlinear dependence on N composition that can be explained in terms of alternating on and offresonance conditions between the redshifting conduction band edge and specific energypinned N cluster states. Furthermore, the electronic properties of the material are studied under hydrostatic pressure, P. This allows tuning in a same sample the interaction between extended and localized states and disclosing a hierarchy between different nitrogen complexes as regards the extent of the perturbation these complexes exert on the electronic properties of the GaAs host crystal. (© 2008 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)Physica Status Solidi (A) Applications and Materials 01/2008; 205(1):107  113. · 1.53 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: Previous experiments on BxGa1–xAs containing a few percent boron show a dramatic increase in electron effective mass, m*e, similar to that observed in many GaNxAs1–x samples. By contrast, there is a nearlinear blueshift of the energy gap, which can be conventionally described using the virtual crystal approximation. We use a tightbinding model to show that isolated B atoms have little effect either on the band gap or lowest conduction band dispersion in BxGa1–xAs. By contrast, B pairs and clusters introduce defect levels close to the conduction band edge (CBE) which, through a weak bandanticrossing (BAC) interaction, significantly reduce the band dispersion in and around the Γ point, thus accounting for the strong increase in m*e and reduction in mobility observed in these alloys. Calculations show that replacing gallium by aluminium shifts the CBE upwards, leading to a large density of Brelated states in the energy gap. By contrast, indium shifts the band edge downwards, leading eventually to a band edge m*e close to that predicted by the virtual crystal approximation. (© 2008 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)physica status solidi (c) 01/2008; 5(2):454  459.  [Show abstract] [Hide abstract]
ABSTRACT: The dependence of the optical band gap of InNxSb1x and GaNxSb1x on nitrogen content has been calculated using an sp(3)s(*) tightbinding Hamiltonian in large supercell calculations that explicitly include the effects of allowing a random distribution of nitrogen atoms. We calculate for InNSb that the energy levels associated with nitrogen complexes consisting of two or three atoms mostly lie above the InNSb conduction band edge (CBE). The steady reduction in the band gap of InNSb with increasing N composition is then well described using a twolevel bandanticrossing model. The calculated effective mass of electrons in InNSb also smoothly decreases with composition, as predicted by the bandanticrossing model, due to the steady closure of the band gap and weak mixing with the N states. For GaNSb the situation is dramatic: the band gap and optical properties are shown to be strongly affected and highly sensitive to the distribution of the nitrogen atoms. We find that there is a wide distribution of N levels lying close to and below the CBE. The higherlying N states push the CBE down in energy, as in GaAs, but the large number of lowerenergy N states mix in strongly with the conduction band edge states, severely disrupting the band edge dispersion in GaNSb.Physical review. B, Condensed matter 01/2008; · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The dependence of the optical band gap of InNxSb1−x and GaNxSb1−x on nitrogen content has been calculated using an sp3s* tightbinding Hamiltonian in large supercell calculations that explicitly include the effects of allowing a random distribution of nitrogen atoms. We calculate for InNSb that the energy levels associated with nitrogen complexes consisting of two or three atoms mostly lie above the InNSb conduction band edge (CBE). The steady reduction in the band gap of InNSb with increasing N composition is then well described using a twolevel bandanticrossing model. The calculated effective mass of electrons in InNSb also smoothly decreases with composition, as predicted by the bandanticrossing model, due to the steady closure of the band gap and weak mixing with the N states. For GaNSb the situation is dramatic: the band gap and optical properties are shown to be strongly affected and highly sensitive to the distribution of the nitrogen atoms. We find that there is a wide distribution of N levels lying close to and below the CBE. The higherlying N states push the CBE down in energy, as in GaAs, but the large number of lowerenergy N states mix in strongly with the conduction band edge states, severely disrupting the band edge dispersion in GaNSb.Phys. Rev. B. 01/2008; 77(16).  [Show abstract] [Hide abstract]
ABSTRACT: In GaAs1xNx, the band gap energy decreases very rapidly with x and the electron effective mass shows a quite unusual compositional dependence characterized by a sudden doubling for x congruent to 0.1%. In this work, we investigate the origin of this behavior by photoluminescence measurements under hydrostatic pressure in asgrown and hydrogenated GaAs0.9989N0.0011 samples. First, we show that two nitrogen pair states emitting at 1.488 and 1.508 eV contribute mainly, but to a different extent, in determining the steep increase in the electron mass observed for x congruent to 0.1%. Tightbinding supercell calculations assign the 1.488 eV levels to isolated N pairs and the 1.508 eV states to N pairs perturbed by a nearby N atom, in disagreement with previous attributions but consistent with the electron mass data. Second, photoluminescence at high hydrostatic pressure discloses that these N pair states show quite different rates of passivation by hydrogen. By combining these findings with the calculated lattice energies associated with each N complex, we conclude that strain relaxation is a key mechanism driving the interaction of hydrogen with N atoms in GaAs1xNx.Physical Review B 01/2008; 77:155213. · 3.66 Impact Factor 
Article: Band Structure of InSbN and GaSbN
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ABSTRACT: We use a tightbinding Hamiltonian to investigate the variation of energy gap with nitrogen (N) composition in InSbN and GaSbN, including the effect on the energy gap due to a random configuration of N atoms. We find that the assumed distribution of N atoms does not significantly affect the calculated energy gap in InSbN. By contrast, the electronic properties of GaSbN are strongly dependent on the assumed N distribution, with Nrelated defect levels strongly perturbing the lowest conduction band states and energy gap.01/2008;  [Show abstract] [Hide abstract]
ABSTRACT: The conduction band structure of BxGa1−xAs has a nearlinear blueshift of the energy gap, which can be described using the virtual crystal approximation, but a dramatic increase in the band edge effective mass me* at low B composition, similar to that observed in GaNxAs1−x. We use a tightbinding model to show that isolated B atoms have little effect either on the band gap or lowest conduction band dispersion in BxGa1−xAs. In contrast, B pairs and clusters introduce defect levels close to the conduction band edge, which, through a weak bandanticrossing interaction, significantly reduce the band dispersion in and around the Γ point, thus accounting for the strong increase in me* and reduction in mobility observed in these alloys.Phys. Rev. B. 08/2007; 76(7).  [Show abstract] [Hide abstract]
ABSTRACT: The influence of nitrogen cluster states on the conduction band (CB) structure of GaAs1−xNx is probed by measuring the effective mass and gyromagnetic ratio of electrons for x < 0.7%. An unusual compositional dependence of these two important CB parameters is found. Such behaviors are well reproduced by a modified k⋅p model taking into account a non‐monotonic loss of Γ character of the CB minimum due to multiple crossings between the red‐shifting conduction band edge and N cluster states. As well, sudden variations in the electron mass can be externally induced by applying a hydrostatic pressure, which brings the upward moving CB edge into interaction with N states, which at ambient pressure are resonant with the GaAs1−xNx CB continuum. © 2007 American Institute of PhysicsAIP Conference Proceedings. 04/2007; 893(1):157158. 
Article: Effect of localized B and N states on the magnetotransport of (B,Ga,In)As and (Ga,In)(N,As)
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ABSTRACT: Magnetotransport properties of n and ptype (B,Ga,In)As and (Ga,In)(N,As) were studied in the temperature range from 2 to 300 K and in magnetic fields up to 10 T and at hydrostatic pressures up to 16 kbar. The magnetotransport in (B,Ga,In)As and (Ga,In)(N,As) is very similar. Ptype samples show normal semiconductor behaviour whereas the electron transport in both alloys is strongly affected by the interaction of the free carriers with the density of states of localized B and N impurity states, respectively. (© 2007 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)physica status solidi (b) 01/2007; 244(1):431436. · 1.49 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: GaNxAs1–x and Ga1–yInyNxAs1–x are the most prominent members of a novel class of nonamalgamation type semiconductor alloys where a fraction x of the anions of the host (e.g., GaAs or Ga1–yInyAs) is replaced by N isovalent impurity atoms. The localized Nstates in GaNxAs1–x and Ga1–yInyNxAs1–x form a series of discrete energy levels (e.g., isolated Nstate, Npairs and higher order Ncluster states) resonant with the conduction band of the host. The effect of the alloying with nitrogen on the bandstructure of GaNxAs1–x and Ga1–yInyNxAs1–x can be well parameterized using a bandanticrossing (BAC) model, namely, assuming a level repulsion between an effective Nstate and the conduction bandedge state. The dependence of several physical properties on nitrogen incorporation can be predicted qualitatively in the framework of this model, e.g., a tremendous increase of the electron effective mass in GaNxAsx with increasing x, a huge cross section for scattering of electrons by N impurities in electronic transport, etc. Most of these predictions can be tested and verified by performing hydrostatic pressure experiments which, within the picture of the BAC model, allow one to tune continuously the energy difference between the hostlike conduction band edge and the effective Nlevel within one and the same specimen. Several examples of this kind will be discussed. Furthermore, we will demonstrate the limitations of the BAC model in the case of GaNxP1–x and also of GaNxAs1–x. In particular, we will show several examples where the description by a single effective Nstate fails and that the multiplicity of the Nstates needs to be taken into account. Again hydrostatic pressure experiments prove to be a useful and suitable tool for revealing the effects due to Ncluster states. (© 2007 WILEYVCH Verlag GmbH & Co. KGaA, Weinheim)physica status solidi (b) 01/2007; · 1.49 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: The effective gyromagnetic factor of electrons, g(e)(*), has been determined by Zeeman splitting measurements in a large number of GaAs1xNx/GaAs (x < 0.7%) samples. Upon N incorporation, g(e)(*) shows first a sign reversal with respect to that of GaAs, then increases abruptly for a nitrogen concentration of order of 0.04%, and finally displays a nonmonotonic dependence on composition for higher x values. This behavior is well reproduced by a modified k center dot p model taking into account a nonmonotonic loss of Gamma character of the conduction band minimum due to multiple crossings between the redshifting conduction band edge and Ncluster states.Physical Review B 12/2006; 74(24):245202. · 3.66 Impact Factor  [Show abstract] [Hide abstract]
ABSTRACT: A tightbinding model of the electronic structure of substitutional nitrogen in GaAs, together with a variational description of quasilocalized nitrogeninduced electronic states near the conduction band edge, is used to calculate the nitrogenrelated alloy scattering of conduction band electrons in the dilute nitride alloy, GaNxAs1−x. The electron mobility in the nondegenerate and degenerate doping regimes is calculated for bulk and quantum well geometries from the energydependent scattering rate using the Boltzmann transport equation in the relaxationtime approximation. Nitrogen cluster states are found to dominate the scattering near the conduction band edge and play a crucial role in limiting the electron mobility. In the experimentally relevant regime of degenerate doping and at nitrogen concentrations of 1 to 2%, the roomtemperature mobility is found to be limited to values less than 300 cm2(V s)−1, in agreement with experimental measurements.Physical review. B, Condensed matter 01/2006; 74(3). · 3.66 Impact Factor
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842  Citations  
83.48  Total Impact Points  
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Institutions

2005–2011

Tyndall National Institute
Corcaigh, Munster, Ireland


2003–2004

University College Cork
 Department of Physics
Cork, M, Ireland


1999–2003

University of Surrey
 Department of Physics
Guildford, ENG, United Kingdom
