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We present a theoretical study of the rapid thermal annealing process and strain on the electronic, optical, and charge dynamical properties of type II GaSb/GaAs quantum dots (QDs). Our theoretical results show a blueshift and the enhancement of interband absorption spectrum because of inter-diffusion processes and the annealed samples. We have identified that increased annealing temperature makes the hole escape times from such QD shorter while the effect of strain on the hole confinement potential becomes weaker. At the same time, in both structures, unstrained and strained QDs, the intra-band absorption in the valence band (IVBA) is redshifted, and its magnitude is decreased after annealing when compared with the as-grown sample. To explain those effects we discuss how the electron–hole wave function overlap varies with thermal treatment and strain and discussed those effects on radiative and thermal processes. Our results explain and are in agreement with the recent experimental observations on similar structures. We have identified that holes could thermally escape out of the type II GaSb/GaAs QD before they are recombined and will contribute to the enhancement in absorption of QD solar cells. The emission and tunneling times of holes were estimated to be $ 10^{-12}$ to $10^{-14}$ s about one order of magnitude faster than the escape times because of radiative recombination processes, $ 10^{-11}$ to $10^{-13}$ s.

A detailed balance calculation reveals an extremely high efficiency of 63.2% for intermediate-band solar cells (IBSCs) under maximum sunlight concentration. However, an actual IBSC device with an efficiency larger than the Shockley-Queisser (SQ) limit has so far not been reported. The main difficulties lie in realizing an efficient sequential two-photon absorption (STPA) which requires a sufficiently long lifetime intermediate state or intermediate band. In this article, we propose the concept of a ratchet type IBSC, utilizing a long lifetime of rare-earth ion luminescence centers in Erbium-doped GaAs. The temperature dependent differential external quantum efficiency reveals a significant STPA contribution originating from the Er3+ luminescence center. All the results were modeled and interpreted by integrating the ratchet effect with up-conversion along with a density functional theory (DFT) simulation. Our work demonstrates that the long lifetime energy-transfer mechanism in Er3+ centers contributes directly to the formation of a ratchet type IB. Intermediate band solar cell is a type of photovoltaic cell which includes additional narrow band states which allow absorption of low energy below-bandgap photons that might otherwise be transmitted from the host material. Here, the authors report a type of ratchet intermediate band solar cell prepared by doping GaAs with erbium and investigate the underlying energy transfer mechanisms.

In order to achieve the highest possible efficiencies, multi-junction solar cells (MJSC) have to have optimally selected parameters. As very complex devices, it is a demanding task finding the optimal set of parameters. One of possible ways to overcome these difficulties is to use genetic algorithm. In the presented model, number of optimizing parameters is 5M+1 for series constrained M-junctions MJSC. Diffusion dark current, radiative and Auger recombinations are accounted together with ASTMG173–03 Global tilted solar spectra and $\mathrm {k}\cdot \mathrm {p}$ absorption. Efficiencies achieved in case of M = 4 are 50.8% and 55.2% when all losses are taken into account and with only radiative recombination, respectively.

Understanding cooling of hot charge carriers in semiconductor quantum dots (QDs) is of fundamental interest and useful to enhance the performance of QDs in photovoltaics. We study electron and hole cooling dynamics in PbSe QDs up to high energies where carrier multiplication occurs. We characterize distinct cooling steps of hot electrons and holes and build up a broadband cooling spectrum for both charge carriers. Cooling of electrons is slower than of holes. At energies near the band gap we find cooling times between successive electronic energy levels in the order of 0.5 ps. We argue that here the large spacing between successive electronic energy levels requires cooling to occur by energy transfer to vibrational modes of ligand molecules or phonon modes associated with the QD surface. At high excess energy the energy loss rate of electrons is 1-5 eV/ps and exceeds 8 eV/ps for holes. Here charge carrier cooling can be understood in terms of emission of LO phonons with a higher density-of-states in the valence band than the conduction band. The complete mapping of the broadband cooling spectrum for both charge carriers in PbSe QDs is a big step toward understanding and controlling the cooling of hot charge carriers in colloidal QDs.

Photovoltaics is amongst the most important technologies for renewable energy sources, and plays a key role in the development of a society with a smaller environmental footprint. Key parameters for solar cells are their energy conversion efficiency, their operating lifetime, and the cost of the energy obtained from a photovoltaic system compared to other sources. The optimization of these aspects involves the exploitation of new materials and development of novel solar cell concepts and designs. Both theoretical modeling and characterization of such devices require a comprehensive view including all scales from the atomic to the macroscopic and industrial scale. The different length scales of the electronic and optical degrees of freedoms specifically lead to an intrinsic need for multiscale simulation, which is accentuated in many advanced photovoltaics concepts including nanostructured regions. Therefore, multiscale modeling has found particular interest in the photovoltaics community, as a tool to advance the field beyond its current limits. In this article, we review the field of multiscale techniques applied to photovoltaics, and we discuss opportunities and remaining challenges. © T. Abu Hamed et al., published by EDP Sciences, 2018., European Cooperation in Science and Technology: MP1406, The authors are grateful for the financial support by the COST Action MP1406 “MultiscaleSolar.”

The efficiency of a solar cell can be substantially increased by opening new energy gaps within the semiconductor band gap. This creates additional optical absorption pathways which can be fully exploited under concentrated sunlight. Here we report a new approach to opening a sizeable energy gap in a single junction GaAs solar cell using an array of small InAs QDs that leads directly to high device open circuit voltage. High resolution imaging of individual QDs provides experimentally obtained dimensions to a quantum mechanical model which can be used to design an optimised QD array. This is then implemented by precisely engineering the shape and size of the QDs resulting in a total area (active area) efficiency of 18.3% (19.7%) at 5 suns concentration. The work demonstrates that only the inclusion of an appropriately designed QD array in a solar cell has the potential to result in ultra-high efficiency under concentration.

Recent efforts to include a quantum dot array within the intrinsic region of a pin GaAs solar cell have focused on minimising the open circuit voltage (Voc) loss relative a control device without quantum dots [1]. Strategies include the addition of strain balancing (e.g. GaP, GaAsP) [2] or high energy (e.g. AlGaAs) barrier layers [3]. In this work we demonstrate a significant improvement of up to 260 mV in Voc by controlling only the size of the quantum dots at the nanometre scale using precise molecular beam epitaxial wafer growth. High resolution scanning transmission electron microscope (STEM) imaging is used to determine the dimensions of individual quantum dots, providing valuable input to a theoretical model. The modelling suggests that the performance improvement is a direct consequence of opening a clear energy gap between the conduction band and the quantum dot ensemble ground state energy e0. With appropriate quantum mechanical design this energy gap can be up to ~90 meV, giving rise to intermediate band behaviour rather than quantum dot solar cell behaviour at room temperature. Current-voltage measurements under air mass 1.5 conditions indicate an efficiency (active area) of 18.4% (19.7%) at 5-suns concentrations. Higher concentration measurements confirm the quality of the material with diode ideality factors as low as 1.16 and Voc ≈ 1 V at 500 suns. References: [1] Y. Okada et al., Appl. Phys. Rev. 2, 021302 (2015), [2] C. G. Bailey et al., Appl. Phys. Lett. 98, 163105 (2011), [3] A. Varghese et al., Nanoscale 8, 7248 (2016).

Controlled reduction of graphene oxide is an alternative and promising method to tune the electronic and optically active energy gap of this two-dimensional material in the energy range of the visible light spectrum. By means of ab initio calculations, based on hybrid density functional theory, that combine the Hartree-Fock method with the generalized gradient approximation (GGA), we investigated the electronic, optical, and radiative recombination properties of partially reduced graphene oxide, modelled as small islands of pristine graphene formed in an infinite sheet of graphene oxide. We predict that tuning of optically active gaps, in the wide range from similar to 6.5 eV to similar to 0.25 eV, followed by the electron radiative transition times in the range from ns to mu s, can be effected by controlling the level of oxidization.

We present a theoretical study of the electronic structure of the wurtzite CuInS2 material. To address reliably some material properties of this new phase we use hybrid density functional theory. Among possible wurtzite polymorphs, we have determined the most stable phase on the basis of total energy minimization. The minimum energy structure exhibits a semiconducting ground state with a band gap of ∼1.3 eV in excellent agreement with experimental data. We use time-dependent density functional theory to compare the optical response of the chalcopyrite and wurtzite phases and to identify the nature of the optically active transitions in the vicinity of the absorption edge. Our analysis indicates that the wurtzite CuInS2 structure is a suitable material for photovoltaic applications.

Coupled semiconductor quantum dot (QD) arrays emerged recently as promising structures for the next generation of high efficiency intermediate band solar cell (IBSC), because of their ability to facilitate the formation of minibands. The quantum coupling effect that exists between states in QDs in an array influences the electronic and optical properties of such structures. So far, great experimental and theoretical efforts have been devoted to study the vertically coupled QD arrays. We present here a method based on multi-band k·p Hamiltonian combined with periodic boundary conditions, applied to predict the electronic and optical properties of InAs/GaAs QDs-based lateral QD arrays. Formation of the intermediate band (IB) in all cases was achieved via delocalisation of the electron ground state (e0). We show that the IB in a laterally coupled QD-IBSC is more robust against external electric field from the solar cell’s pn junction than that in a vertically coupled arrangement. Because of symmetry of the QD array lattice and QD states itself, which are C2v for the zinc blend QDs, the electronic and absorption structures were obtained via sampling throughout the reciprocal space in the first Brillouin zone of QD arrays.

The laser power converters, devices for optical to electrical energy conversion, based on the Vertical Epitaxial Hetero-Structure Architecture (VEHSA) design, has emerged recently as a promising concept that can offer the record open circuit voltage and conversion efficiencies, exceeding 20 V and 60% respectively. We present the heuristical method, based on the genetic algorithm (GA) for the optimization of power conversion efficiency of the VEHSA devices. The method is based on the inverse design paradigm in the materials/device modeling, providing for the bespoke, targeted, the design of the device. As an output our method is giving the optimal p/n junction thicknesses, doping concentrations and the optimal current through the VEHSA device. Those device parameters were optimized with radiative, as well as non-radiative Auger and Shockley-Read-Hall losses taken into account. Our heuristical model also reveal the importance of accurate prediction of the thickness of the first cell in the stack, that deviates significantly from those estimated by light penetration depth and Beer-Lambert law only. When all types of losses are taken into account, our model predicts the maximal efficiency of GaAs based VEHSA to be 69.43 % for a device with one solar cell and 65.7 % for the device with 15 subcells, which is in very good agreement with measured efficiencies of similar devices. We also analyse, and predict the efficiencies of devices based on Al 0.33 Ga 0.67 As ( E g = 2 eV), In 0.34 Ga 0.66 As ( E g = 0.92 eV), and GaSb ( E g = 0.75 eV).

Colloidal quantum dots (CQDs) are promising materials for solar cells because their optoelectronic properties are easily adjusted by control of their size, structure and composition. We present calculations of the band gap and radiative lifetime for varying core diameter and shell thickness of CdSe/CdTe core/shell Type II CQDs using a combination of single particle (2,6)-band k·p and many-electron configuration interaction (CI) Hamiltonians. These calculations are validated by comparison with experimental absorption spectra and photoluminescence decay data. The results are then incorporated into a model of photovoltaic efficiency which demonstrates how the overall performance of a solar cell based on Type II CQDs is affected by changes in the core/shell geometry. The largest effect on photovoltaic efficiency is found to be due to the longer radiative lifetime produced by increasing the shell thickness.

The temperature dependence of the radiative and nonradiative components of the threshold current density of 1.3 mum InAs/GaAs quantum-dot lasers have been analyzed both experimentally and theoretically. It is shown that the weak temperature variation measured for the radiative current density arises because the optical matrix element for excited state transitions is significantly smaller than for the ground state transition. In contrast, nonradiative Auger recombination can have a similar probability for transitions involving excited states as for those involving ground state carriers. The sharp increase in the threshold current density at high temperatures follows the temperature variation of the cubed threshold carrier density confirming that Auger recombination is the dominant recombination mechanism in these devices at room temperature.

We have studied the pressure and temperature dependence of the threshold current density, J th , in 1.3 μm and 1.5 μm GaInNAs edge-emitting lasers. We find that J th is more temperature sensitive for the 1.5 μin devices. From analysis of the spontaneous emission from these devices we find that J th for the 1.3 μm devices is comprised of 50% non-radiative monomolecular recombination, 25% radiative recombination and 25% non-radiative Auger recombination at room temperature. In the 1.5 μm devices however it is composed of 30% monomolecular recombination, 10% radiative recombination and 60% Auger recombination. It is clear that the major difference in J th between the devices is caused by the increased Auger recombination in the 1.5 μm devices. Previously, we found that J th for 1.3 μm GaInNAs lasers increases with pressure due to the increase in Auger recombination brought about by a large increase in the cubed threshold carrier density (n 3 th ). However, for the 1.5 μm devices, we find no significant change in J th with pressures up to 8 kbar. We suggest that this is due to the higher Auger coefficient for the 1.5 μm devices, which compensates for the increase in n 3 th with pressure.

Laterally coupled semiconductor quantum dot (QD) arrays emerged recently as promising structures for the next generation of high efficiency intermediate band solar cell (IBSC).We present here a method based on multi-band k·p Hamiltonian combined with periodic boundary conditions, applied to predict the electronic and optical properties of InAs/GaAs QDs based lateral QD arrays. The absorption coefficients were used to estimate the corresponding photovoltaic conversion efficiencies using the drift-diffusion transportation theory. Special attention was paid on the transition between IB to continuum states in conduction band. By comparing the experimental and theoretical results we have confirmed operation of the InAs/GaAs QD array based solar cell in the intermediate band regime.

We calculate correlated exciton states in type-II core/shell quantum dots (QDs) using a configuration interaction method combined with the k·p theory. We map the 1S1/2(e)1S3/2(h) and 1S1/2(e)2S3/2(h) exciton correlation energy relative to the strong confinement approximation as a function of core radius, shell thickness, and dielectric confinement. The type-II confinement potentials enhance the effect of dielectric confinement which can significantly affect the wave functions and exciton energies in such heterostructures. Dielectric confinement mainly increases the correlation energy for QDs in which the corresponding single-particle hole states are delocalized. We also find that correlation leads to large changes in the optical dipole matrix element, particularly for the lowest CdSe/CdTe QD exciton, in the presence of dielectric confinement. We conclude that dielectric confinement affected the exciton properties in CdSe/CdTe QDs more than in CdTe/CdSe QDs due to the band alignment which encourages holes ...

The controlled and patterned reduction of graphene oxide offers a promising method to tune the electronic and optical properties of the material through a wide range. Using ab initio calculations in which the exact exchange energy from Hartree-Fock theory is combined with the exchange-correlation energy obtained from density functional theory (DFT), we studied the electronic, optical, and radiative recombination properties of reduced graphene oxide (rGO). Our model of rGO is based on epoxy functionalised graphene, within which small regions of pristine graphene are formed by reduction. We predict that the gap can be tuned from similar to 6.85 eV to similar to 0.25 eV in this manner and that the polarization selective absorption properties can be controlled by manipulating the symmetry of these graphene quantum dots. The optically active can therefore be tuned to ranges suitable for use either as the active medium or a transparent conducting oxide (TCO) in photovoltaic solar cells (PVSCs). IEEE Photovoltaic Specialists Conference, IEEE 42nd Photovoltaic Specialist Conference (PVSC), Jun 14-19, 2015, New Orleans, LA

A theoretical study of the electronic and optical properties of the dilute-nitrogen InGaAsN/GaAs quantum dot (QD) structures is presented. The calculations are based on a 10 band k middot p band-anti-crossing Hamiltonian incorporating valence, conduction and nitrogen-induced bands. Numerical results for the model system of capped pyramid-shaped QD with [1 0 1] facets on a thin wetting layer are presented. The analysis shows that the influence of nitrogen induces more confined states in the conduction band (CB) than in equivalent nitrogen-free QDs, reducing the energy of the fundamental optical transition. The better confinement in dilute nitrogen QD is because of both the significantly reduced compressive strain, which was one of the major obstacles for a long-wavelength emission from InAs/GaAs QDs, and the BAC effect. These effects, in conjunction with QD size variation, can be of great benefit for the design of devices emitting at longer wavelengths. Furthermore, in contrast to nitrogen-free QDs, dilute nitrogen QDs exhibit reduced dipole matrix element and larger Coulomb interaction energy. The findings are in good agreement with the reported experimental results on similar structures. With appropriate tailoring of the indium and nitrogen concentration, this system could be a potential candidate for a 1.55 mum emission on GaAs substrate

We present a parallel implementation of the multi-bank k·p code () for calculation of the electronic structure and optical properties of zinc blend structure semiconductor quantum dots. The electronic wave-functions are expanded in a plane wave basis set in a similar way to ab initio calculations. This approach allows one to express the strain tensor components, the piezoelectric field and the arbitrary shape of the embedded quantum dot in the form of coefficients in the Fourier transform, significantly simplifying the implementation. Most of the strain elements can be given in an analytical form, while very complicated quantum dot shapes can be modelled as a linear combination of the Fourier transform of several characteristic shapes: box, cylinder, cone etc. We show that the parallel implementation of the code scales very well up to 512 processors, giving us the memory and processor power to either include more bands, as in the dilute nitrogen quantum dot structures, or to perform calculations on bigger quantum dots/supercells structures keeping the same “cut-off” energy. The program performance is demonstrated on the pyramidal shape InAs/GaAs, dilute nitrogen InGaAsN, and recently emerged volcano-like InAs/GaAs quantum dot systems.

The band-anti-crossing (BAC) model describes the strong band-gap bowing at low N composition x in Ga(In)N x As 1-x in terms of an interaction between the conduction band edge and a higher-lying band of localized nitrogen resonant states. We first present an analytical technique based on the BAC model to calculate electron energies in Ga(In)NAs square quantum well (QW) structures. We then show through detailed comparison with photoreflectance measurements that the BAC model successfully describes ground and excited state interband transition energies in bulk and QW GaNAs samples, both at ambient pressure and as a function of hydrostatic pressure. Turning to the conduction band dispersion, we find that the two-level BAC model is insufficient, and must be modified to give a quantitative understanding of the unexpectedly large electron effective mass values observed in some GaNAs samples, which we attribute to hybridisation between the conduction band edge and nitrogen states close to the band edge. We predict a non-monotonic variation of electron mass with hydrostatic pressure in many GaNAs samples.

The pressure dependence of the components of the recombination current at threshold in 1.3-/spl mu/m GaInNAs single quantum-well lasers is presented using for the first time high-pressure spontaneous emission measurements up to 13 kbar. It is shown that, above 6 kbar, the rapid increase of the threshold current with increasing pressure is associated with the unusual increase of the Auger-related nonradiative recombination current, while the defect-related monomolecular nonradiative recombination current is almost constant. Theoretical calculations show that the increase of the Auger current can be attributed to a large increase in the threshold carrier density with pressure, which is mainly due to the increase in the electron effective mass arising from the enhanced level-anticrossing between the GaInNAs conduction band and the nitrogen level.

We have investigated the temperature and pressure dependence of the threshold current (I/sub th/) of 1.3 /spl mu/m emitting GaInNAs vertical-cavity surface-emitting lasers (VCSELs) and the equivalent edge-emitting laser (EEL) devices employing the same active region. Our measurements show that the VCSEL devices have the peak of the gain spectrum on the high-energy side of the cavity mode energy and hence operate over a wide temperature range. They show particularly promising I/sub th/ temperature insensitivity in the 250-350 K range. We have then used a theoretical model based on a 10-band k.P Hamiltonian and experimentally determined recombination coefficients from EELs to calculate the pressure and temperature dependency of I/sub th/. The results show good agreement between the model and the experimental data, supporting both the validity of the model and the recombination rate parameters. We also show that for both device types, the super-exponential temperature dependency of I/sub th/ at 350 K and above is due largely to Auger recombination.

The band-anti-crossing (BAC) model was originally introduced as a phenomenological method to describe theelectronic structure of GaInNAs. We present here a Greens function model to derive explicitly the BAC model inorderedGa(In)N x As 1 x structures.TheGreensfunctionmodelisbasedonthetight-bindingmethod,whichwehaveusedpreviouslytoconfirmthatNformsaresonantdefectstateabovetheconductionbandedgeinGaAs.Byintro-ducingtheGreensfunctionmodelwederiveexplicitlythattheresonancebecomesdelocalisedandspreadoverseveralenergystatesasx increases,butthatthetwo-levelBACmodelstillgivesanexcellentdescriptionoftheconductionbandedgeinorderedsupercells.Wethenextendthemodeltoshowthattheconventional8-bandk pHamiltonianmustbemodifiedtoincludetwoextraspin-degeneratestates,givinga10-bandmodelforGa(In)N x As 1 x heterostructures. 2002ElsevierScienceLtd.Allrightsreserved. Keywords:GaNAs;k pmethod;Quantumwells;Bandanti-crossing 1.IntroductionWhenasmallfractionofarsenicatomsinGaAsarereplaced by nitrogen the energy gap initially decreasesrapidly,atabout0.1eVper%ofNforx < 0:03[1].Thisbehaviour is markedly different to conventional semi-conductors,andisofinterestbothfromafundamentalperspective and also becauseof its significant potentialdeviceapplications.Thereisthereforeconsiderablead-vantagetodevelopingsimplemodelswhichdescribeandpredictthevariationwithNcomposition,x,ofsuchkeyproperties as the energy gap, the Kane interband mo-mentum matrix element and the band edge effectivemassesinGa(In)N

By measuring the spontaneous emission (SE) from metal organic vapour phase epitaxy (MOVPE) grown ∼1.3 μm GaInNAs/GaAs-based lasers during normal operation, we have quantitatively determined the variation of each of the current paths present in the devices as a function of temperature from 130 to 370 K and compared these with results previously obtained for molecular beam epitaxy (MBE) grown GaInNAs lasers. From the SE measurements we determine how the current, I, close to threshold, varies as a function of carrier density, n, which enables us to separate out the main current paths corresponding to monomolecular (defect-related), radiative or Auger recombination respectively. We find that at room temperature (RT), defect-related recombination contributes ∼360 A/cm2 (MBE) and ∼565 A/cm2 (MOVPE) to the total current density at threshold. Radiative recombination accounts for ∼110 A/cm2 (MBE) and 195 A/cm2 (MOVPE) of Jth with the remaining ∼180 A/cm2 (MBE) and 760 A/cm2 (MOVPE) are due to non-radiative Auger recombination. Our results suggest that a larger threshold carrier density in the MOVPE grown device in comparison to the MBE lasers, can reasonably explain the larger current densities of the different recombination processes at RT. We tentatively associate this with higher optical loss processes in the MOVPE grown material.

Dramatic changes of the electronic band structure occur when incorporating even a small fraction of N into GaAs. One important consequence of the N-incorporation is a strong non-parabolicity of the conduction band of GaN x As 1− x yielding already for x less than 1% a considerable increase in the electron effective mass and a strong variation of the electron effective mass with increasing k -vector. We demonstrate how this N-induced non-parabolic dispersion of the conduction band in Ga(N,As) can be determined by a careful analysis of the interband transitions of Ga(N,As)-based quantum wells as a function of hydrostatic pressure. A series of GaN 0.018 As 0.982 /GaAs wells of various widths was studied by photomodulated reflectance (PR) at 300 K and hydrostatic pressures up to 20 kbar. The PR spectra were fitted using derivative-like line shapes to extract the energy positions of the interband transitions. The transition energies were compared with theoretical values calculated using a 10-band k.p-model including the effects of nitrogen. The good agreement between experiment and theory allows one to extract the valence band offset as well as the conduction band dispersion and hence the change of the effective mass with pressure and energy.

Vertical-cavity surface-emitting lasers with a GaInNAs quantum well active region have been studied as a function of hydrostatic pressure and compared with similar edge-emitting lasers. The variation of threshold currents and lasing energies is studied and compared with a gain model calculated from a 10 band k.p model which includes the N interaction in the conduction band. Good agreement is obtained between experiment and model and indicates that a large Lorentzian broadening parameter of 17.5 meV applies. The gain spectrum is therefore broad and the devices have a large degree of temperature stability and growth error tolerance.

From measurements of the threshold current and lasing energy as a function of pressure in InGaAsP/InP, AlGaInAs/InP and GaInNAs/GaAs based multiple quantum well lasers we determine the relative importance of the monomolecular, radiative and Auger recombination processes. For the InP based devices, we find that a simple combination of radiative and non-radiative Auger recombination can fully explain the pressure dependence of the threshold current where the threshold carrier density is approximately constant as a function of pressure. For the GaInNAs/GaAs devices we observe a large increase in threshold current with pressure. This we show is due to the interaction of the nitrogen level with the conduction band which gives rise to an increased conduction band effective mass resulting in an increase in threshold carrier density of ∼12% over 10 kbar. This large increase in n th increases the monomolecular, radiative and unusually, the Auger recombination current with pressure explaining the large increase in threshold current with pressure.

The pressure dependence of the total threshold current and its respective recombination components in 1.3 μm GaInNAs single quantum-well lasers has been determined by measuring for the first time the window-spontaneous emission under hydrostatic pressures at room temperature. It is shown that, above 6 kbar, the rapid increase of the threshold current with increasing pressure in GaInNAs lasers is related to the unusual increase of the Auger recombination current, while the monomolecular non-radiative current in the total threshold current is almost constant in the pressure range studied. Theoretical calculations show that the unusual increase of the Auger current is due to a large increase in the threshold carrier density with pressure, stronger than the reduction of the Auger coefficient leading to an overall increase in the Auger current.

We describe micro-photomodulated reflectance (PR) measurements on a representative dilute-N InGaNAs/GaAs-based laser device structure designed to emit at 1.3 μm. The quantum well (QW) transition energies obtained from PR are modeled using a realistic 10-band k · p Hamiltonian that includes tight-binding-based energies and coupling parameters for the N-levels. From this we are able to determine accurately the band structure and thus predict some important device properties for this InGaNAs/GaAs-based laser device.

We review how the tight-binding method provides a particularly useful approach to understand the electronic structure of GaInNAs alloys, and use it to derive a modified k?p model for the electronic structure of GaInNAs heterostructures. Using the tight-binding model, we first confirm that N forms a resonant defect level above the conduction band edge in Ga(In)As. We show that the interaction of the resonant N level with the conduction band edge accounts for the strong bandgap bowing observed in GaInNxAs1?x, in agreement with experimental analysis but contrary to some theoretical interpretations. We then use a Green function model to derive explicitly the two-level band-anti-crossing model describing the interaction between the resonant states and the conduction band edge in ordered Ga(In)NxAs1?x. We extend the Green function model to show that the conventional k?p model must be modified to include two extra spin-degenerate nitrogen states, giving a 10-band k?p model to describe the band structure of GaNAs/GaAs and related heterostructures. We describe how this 10-band model provides excellent quantitative agreement with a wide range of experimental data and finally discuss briefly the effects of disorder on the electronic structure in dilute nitride alloys.

The unusual N-induced band formation and band structure of Ga(N, As) and (Ga, In)(N, As) alloys are also reflected in the electronic structure of quantum wells (QWS) and device structures containing these non-amalgamation-type alloys. This review is divided into three parts. The first part deals with band structure aspects of bulk Ga(N, As) and motivates the possibility of a k · p-like parameterization of the band structure in terms of the level repulsion model between the conduction band edge of the host and a localized N-level. The second part presents experimental studies of interband transitions in Ga(N, As)/GaAs and (Ga, In)(N, As)/GaAs QW structures addressing band offsets, electron effective mass changes and an intrinsic mechanism contributing to the blueshift of the (Ga, In)(N, As) band gap on annealing. The observed interband transitions can be well described using a ten-band k · p model based on the level repulsion scheme. The third part deals with (Ga, In)(N, As)-based laser devices. The electronic structure of the active region of vertical-cavity surface-emitting laser and edge-emitter laser structures is studied by modulation spectroscopy. The gain of such structures is measured by optical methods and analysed in terms of a model combining the ten-band k · p description of the band structure and generalized Bloch equations.

By measuring the spontaneous emission (SE) from normally operating /spl sim/1.3-/spl mu/m GaInNAs-GaAs-based lasers we have quantitatively determined the variation of each of the current paths present in the devices as a function of temperature from 130 K to 370 K. From the SE measurements we determine how the current I close to threshold, varies as a function of carrier density n, which enables us to separate out the main current paths corresponding to monomolecular (defect-related), radiative or Auger recombination. We find that defect-related recombination forms /spl sim/55% of the threshold current at room temperature (RT). At RT, radiative recombination accounts for /spl sim/20% of I/sub th/ with the remaining /spl sim/25% being due to nonradiative Auger recombination. Theoretical calculations of the threshold carrier, density as a function of temperature were also performed, using a ten-band k /spl middot/ p Hamiltonian. Together with the experimentally determined defect-related, radiative, and Auger currents we deduce the temperature variation of the respective recombination coefficients (A, B, and C). These are compared with theoretical calculations of the coefficients and good agreement is obtained. Our results suggest that by eliminating the dominant defect-related current path, the threshold current density of these GaInNAs-GaAs-based devices would be approximately halved at RT. Such devices could then have threshold current densities comparable with the best InGaAsP/InP-based lasers with the added advantages provided by the GaAs system that are important for vertical integration.

A method is described for the design and realization of quantum-well (QW) structures optimized in respect to gain in optically pumped intersubband lasers. It relies on finding the optimal smooth potential (QW profile) by employing supersymmetric quantum mechanics, and then using the simulated annealing method to design a suitable QW structure with a small number of layers of different composition, such that it delivers the closest approximate to the optimal smooth profile after the interdiffusion process. This route towards achieving optimal QW profiles should be much easier to apply than a direct modulation of material composition at time of growing the structure.

A method is described for the optimized design of quantum-well structures, with respect to maximizing the stimulated gain in optically pumped intersubband lasers. It relies on applying the supersymmetric quantum mechanics to an initial Hamiltonian. The use of this procedure is demonstrated by designing smoothly graded and stepwise-constant AlxGa1−xAs ternary alloy quantum wells, with the self-consistent potential taken into account.

We use a realistic Hamiltonian to compare the gain characteristics of an ideal InGaAsN/GaAs quantum well laser structure emitting at 1.3 μm with an equivalent N-free InGaAs/GaAs structure. The energy gap of InGaAs is reduced by the addition of N, due to a repulsive interaction between an N resonant band and the conduction band edge. This interaction increases the conduction band edge effective mass and decreases the value of the dipole matrix element linking the conduction and valence band edges. We find that the addition of N reduces the peak gain and differential gain at fixed carrier density, although the gain saturation value and the peak gain as a function of radiative current density are largely unchanged due to the incorporation of N.

A series of GaN x As 1 m x /GaAs quantum well structures with well widths of about 20 nm and x varying between 1% and 3.5% has been grown by metal-organic vapour phase epitaxy. We have studied the evolution of the quantum well states under hydrostatic pressure up to 20 kbar at 300 K by photomodulated reflectance (PR) spectroscopy. The energy positions of the quantum well transitions have been obtained by fitting the PR spectra. The pressure dependence of the allowed heavy-hole transitions e n hh n decreases with increasing n . This directly reflects the strong non-parabolic dispersion of the conduction band originating from the interaction of the N-impurity level with the bands of the GaAs host. The fitted energy positions and their pressure dependence can be well described by a 10 band k.p model. The observed splitting between the lowest light-hole and heavy-hole transitions are in agreement with a type I band alignment.

We present a theoretical method for calculations of exciton and bi-exciton energies in type-II colloidal quantum dots. Our methodology is based on an 8-band k · p Hamiltonian of the zinc-blend structure, which incorporates the effects of spin-orbit interaction, strain between the core and the shell and piezoelectric potentials. Exciton states are found using the configuration interaction (CI) method that explicitly includes the effects of Coulomb interaction, as well as exchange and correlation between many-electron configurations. We pay particular attention to accurate modelling of the electrostatic interaction between quasiparticles. The model includes surface polarization and self-polarization effects due to the large difference in dielectric constants at the boundary of the QD.

We report for the first time a successful fabrication and operation of an InAs/GaAs quantum dot based intermediate band solar cell concentrator photovoltaic (QD-IBSC-CPV) module to the IEC62108 standard with recorded power conversion efficiency of 15.3%. Combining the measured experimental results at Underwriters Laboratory (UL®) licensed testing laboratory with theoretical simulations, we confirmed that the operational characteristics of the QD-IBSC-CPV module are a consequence of the carrier dynamics via the intermediate-band at room temperature.

Despite continued and rapid progress in high performance computing, atomistic level device modelling is still largely out of reach, necessitating the use of quantum mechanical continuum methods, including kcenterdotp perturbation theory. The effective use of such methods requires reliable parameterisation, often obtained from experiment and ab initio calculations. A major limitation of this, the systematic tendency of ab initio density functional theory to underestimate semiconducting material energy band gaps and related properties, can be greatly improved upon by the inclusion of exact exchange, calculated within the Hartree-Fock formalism. We demonstrate that the 14 band kcenterdotp Hamiltonian can be effectively parameterised using this method, at greatly reduced cost in comparison to GW

We present a method for calculating exciton and bi-exciton energies in type-II colloidal quantum dots. Our methodology is based on an 8-band k p Hamiltonian of the zinc- blend structure, which incorporates the effects of spin-orbit interaction, strain between the core and the shell and piezoelectric potentials. Exciton states are found using the configuration interaction (CI) method that explicitly includes the effects of Coulomb interaction, as well as exchange and correlation between many-electron configurations. We pay particular attention to accurate modelling of the electrostatic interaction between quasiparticles. The model includes surface polarization and self-polarization effects due to the large difference in dielectric constants at the boundary of the QD.

Our current understanding of the symmetries of multiband envelope function Hamiltonians for semiconductor quantum dots and their signatures in the energy level structure and wave function shapes is reviewed. We show how symmetry can be used to block-diagonalize the Hamiltonian matrix and consequently strongly reduce the computational effort. A detailed analysis of symmetries of several different model Hamiltonians reveals that the true symmetry of square-based pyramidal quantum dots is captured if either the interface effects are taken into account or additional higher energy bands are included in the multiband Hamiltonian. This indicates that multiband envelope function methods are fully capable of capturing the true atomistic symmetry of quantum dots in contrast to some widespread beliefs. In addition, we show that translational symmetry can be artificially introduced by the numerical method used, such as the plane wave method. Plane wave method introduces artificial quantum dot replica whose charges interact with charges in the real quantum dot and create an additional strain field in the real dot. This issue can be circumvented by the introduction of proper corrections in the procedure for calculation of Coulomb integrals and strain.

The piezoelectric effect in polar semiconductor has seen increased interest in recent years because of the prospect of exploiting semiconducting behavior and piezoelectric response, i.e. generating electric fields in response to pressure, in novel optoelectronic devices with applications as pressure sensors and energy harvesting. In this paper we review the basic concepts and recent findings related to the novel concept of non-linear piezoelectricity, which can be exploited in composite nanostructured materials to increase the piezoelectric response compared to bulk materials. Applications to light emitting diodes and nanowires will also be discussed. We will show how the non-linear theory of piezoelectricity can in some cases lead to opposite predictions compared to the classic linear theory.

The optical characteristics of colloidal quantum dots (QDs) are highly dependent on the physical geometry of the QD (size, shape) as well as composition. These dependencies make such systems attractive for application in novel optical devices, notably for solar cell technology. Empirical electronic structure methods, such as kcenterdotp theory, or empirical pseudopotential theories have successfully reproduced experimentally observed transitions in CdSe and PbSe colloidal QDs. Our approach uses the kcenterdotp method to predict such properties as the electronic structure and dipole transitions of ellipsoidal PbSe/PbS core/shell structure colloidal QDs, as a function of eccentricity. Due to the anisotropy between the longitudinal (z) and transverse (x and y) directions, we present results from elongation along both the x and z directions.

Controlled reduction of graphene oxide (GO) is a promising method to tune the electronic band gap of this two-dimensional material in the energy range of the visible light spectrum. By means of ab initio calculations, based on density functional theory at the generalized gradient approximation level, we investigated electronic properties of partially reduced graphene oxide, modelled as periodic array of small islands of pristine graphene embedded in an infinite sheet of GO. The calculations demonstrated that, by varying the size of the graphene islands from two to eight carbon atoms, it was possible to tune the electronic band gap in a range from 4.38 to 1.31 eV, which is of great importance to the utilization of graphene-based materials in photonic devices. 4th International School and Conference on Photonics, Aug 26-30, 2013, Belgrade, Serbia

The utilisation of graphene structures as photonics materials mandates that an optically active electronic energy gap be formed. Opening of a gap in graphene has been demonstrated by functionalisation with H, F, or O atoms, while experimental observations of graphene oxide have hinted at interesting optical properties, with the potential for absorption of visible light. As such, our analysis is focused on O functionalisation of graphene. We present results from extensive ab initio and hybrid DFT calculations, demonstrating the creation of an optically active gap. 4th International Conference on Theory, Modelling and Computational Methods for Semiconductor Materials and Nanostructures (TMCS), Jan 22-24, 2014, Univ Salford, Manchester, England

A method is described for the optimized design of quantum-well (QW) structures, in respect to maximizing the stimulated gain in optically pumped intersubband lasers. It relies on applying supersymmetric quantum mechanics (SUSYQM) to an initial Hamiltonian, in order to both map one bound state below the spectral range of the initial Hamiltonian, and to generate a parameter-controlled family of isospectral Hamiltonians with the desired energy spectrum. By varying the control parameter, one changes the potential shape and, thus, the values of dipole matrix elements and electron-phonon scattering matrix elements. The use of this procedure is demonstrated by designing smoothly graded and stepwise-constant Al/sub x/Ga/sub 1-x/As ternary alloy QWs, with the self-consistent potential taken into account. Finally, the possibility of employing layer interdiffusion to get optimal smooth potentials is discussed.

Received 17 May 2000; published in the issue dated 15 December 2000 A systematic method is described for the optimized design of quantum-well structures, in respect to maximizing the stimulated gain in optically pumped intersubband lasers. It relies on applying supersymmetric quantum mechanics to an initial Hamiltonian, in order to (i) add one bound state below the spectral range of the initial Hamiltonian, and (ii) to generate a parameter-controlled family of isospectral Hamiltonians with the desired energy spectrum. By changing the value of the control parameter one changes the potential shape and thus the values of dipole matrix elements and electron-phonon scattering matrix elements. The use of this procedure is demonstrated by designing an appropriate smooth or stepwise-constant graded AlxGa1-xAs ternary alloy quantum well.

A systematic procedure is described for the maximization of gain in optically pumped intersubband lasers, via optimal tailoring of the quantum well profile. The procedure relies on using the inverse spectral theory, allowing one to shift the bound states as desired, and additionally to make the isospectral reshaping of the well, with the eventual aim of finding the best combination of those optical dipole and electron-phonon scattering matrix elements which are relevant for the laser gain. Example design is presented for a laser based on the AlxGa1-xAs system, and the band nonparabolicity is accounted for in the final design. (C) 2000 American Institute of Physics. [S0021-8979(00)01511-5].