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Carrier transport in type-II superlattice photodetectors is investigated by means of a rigorous nonequilibrium Green's function model based on a physics-based B\"uttiker-probe formalism. Intraband scattering self-energies (carrier-phonon interactions) are computed in the self-consistent Born approximation, while interband self-energies (Shockley-Read-Hall and optical transitions) are included in terms of semiclassical generation-recombination rates, neglecting interband renormalization effects. Current conservation is achieved with an efficient Newton-Raphson algorithm. While carrier transport in infrared detectors is usually understood in terms of quantities (e.g., mobilities and quasi-Fermi-levels) that are admittedly not germane to nonequilibrium Green's function theory, the proposed model provides a quantum-kinetic description of tunneling, miniband transport, hopping, and carrier extraction within a drift-diffusion-friendly framework. The connection with semiclassical theories allows exploration of the possibilities offered by Poisson-Schr\"odinger or localization landscape drift-diffusion approaches.

We present a flexible, full-band, 3-D Monte Carlo (MC) modeling software package for charge transport in semiconductor devices. The numerical model can employ both analytical and full-band descriptions of the band structure simultaneously and uses unstructured meshes for device discretization that can be constructed with commonly available meshing tools. A numerical approach is also included to minimize the self-forces resulting from the unstructured mesh. The scattering rates are computed over a tetrahedral mesh, the refinement capabilities of which provide accurate rates near band minima and maxima. The efficient geometrical device discretization enables us to simulate devices that have traditionally been limited to drift-diffusion approaches due to the computational requirements of MC simulations. To highlight the capabilities of this tool, we present two simulation examples, a conical HgCdTe APD and a silicon trench MOS, both run with a superparticle charge of 1e. In addition to simulating the output characteristics of the trench MOS and the gain and excess noise factor of the APD, we investigate the injection location-dependent time response and corresponding diffusion tail.

Curved focal-plane arrays offer significant advantages compared with their flat counterparts. However, the curving of the material induces strains which alter its optoelectronic properties. A comprehensive framework is presented for the computational investigation of the band gaps of ${\mathrm{InAs}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ alloys under various strain conditions that are relevant for manufacturing curved focal-plane arrays. The framework consists of both standard and hybrid functional density functional theory (DFT) calculations, finite-element analysis (FEA) calculations, and Gaussian process (GP) regression. The DFT calculations are used for investigating the change of the band gap of the material under various strain conditions. This dataset is then used for training a GP model which is utilized to assess the effects of strain across the device, based on the FEA simulations. The results show excellent predictive capabilities for the machine-learning model at a significantly reduced computational cost and are directly transferable to other device designs.

Phase diagram calculations are performed for incoherent and coherent mixtures of an [Formula: see text] (InAsSb) ternary alloy, which is an important material for the applications to infrared detector technology. Our calculations are based on the cluster expansion approach and Monte Carlo simulations combined with first-principles total energy calculations in the framework of density functional theory with Perdew–Burke–Ernzerhof (PBE) and Heyd–Scuseria–Ernzerhof (HSE) exchange-correlation functionals. Because of a lattice mismatch ([Formula: see text]7%) between InAs and InSb, coherency strain plays an important role for the phase stability of the InAsSb alloys. The alloys without the coherency strain (incoherent mixtures) show a miscibility gap with the critical temperature at [Formula: see text]700 K with 42% (45%) Sb concentration in PBE (HSE), which is in good agreement with the experimentally determined equilibrium miscibility gap temperature. The alloys with the coherency strain (coherent mixtures) show several ground states whose structures are short period superlattices along the [201] direction. The critical temperature is [Formula: see text]200 K with 50% Sb concentration in both PBE and HSE, which is reduced by [Formula: see text]500 K compared to that of incoherent mixtures. This reduction of the critical temperature is consistent with the experimental observation where the homogeneous InAsSb alloy continues to grow inside the empirical miscibility gap.

Background The fluid challenge response in surgical patients can be predicted by functional haemodynamic tests. Two tests, the mini-fluid challenge (mini-FC) and end-expiratory occlusion test (EEOT), have been assessed in a few small single-centre studies with conflicting results. In general, functional haemodynamic tests have not performed reliably in predicting fluid responsiveness in patients undergoing laparotomy. Objective This trial is designed to address and compare the reliability of the EEOT and the mini-FC in predicting fluid responsiveness during laparotomy. Design Prospective, multicentre study. Setting Three university hospitals in Italy. Patients A total of 103 adults patients scheduled for elective laparotomy with invasive arterial monitoring. Interventions The study protocol evaluated the changes in the stroke volume index (SVI) 20 s (EEOT20) and 30 s (EEOT30) after an expiratory hold and after a mini-FC of 100 ml over 1 min. Fluid responsiveness required an increase in SVI at least 10% following 4 ml kg-1 of Ringer's solution fluid challenge infused over 10 min. Main outcome measurements Haemodynamic data, including SVI, were obtained from pulse contour analysis. The area under the receiver operating characteristic curves of the tests were compared with assess fluid responsiveness. Results Fluid challenge administration induced an increase in SVI at least 10% in 51.5% of patients. The rate of fluid responsiveness was comparable among the three participant centres (P = 0.10). The area under the receiver operating characteristic curves (95% CI) of the changes in SVI after mini-FC was 0.95 (0.88 to 0.98), sensitivity 98.0% (89.5 to 99.6) and specificity 86.8% (75.1 to 93.4) for a cut-off value of 4% of increase in SVI. This was higher than the SVI changes after EEOT20, 0.67 (0.57 to 0.76) and after EEOT30, 0.73 (0.63 to 0.81). Conclusion In patients undergoing laparotomy the mini-FC reliably predicted fluid responsiveness with high-sensitivity and specificity. The EEOT showed poor discriminative value and cannot be recommended for assessment of fluid responsiveness in this surgical setting. Trial registration NCT03808753.

In this work we investigate carrier transport in tunnel junctions for vertical-cavity surface-emitting lasers by a novel self-consistent simulation framework for semiconductor quantum devices. Based on a Poisson-drift-diffusion foundation, in this approach quantum features are described through a nonequilibrium Green's function formalism. The simulator is validated through a comparison with experimental results.

Theoretical investigations of carrier transport in type-II superlattice detectors have been mostly limited to simplified semiclassical treatments, due to the computational challenges posed by quantum kinetic approaches. For example, interband tunneling in broken-gap configurations calls for a multiband description of the electronic structure, and spatially indirect optical transitions in superlattice absorbers require fully nonlocal carrier-photon self-energies. Moreover, a large number of iterations is needed to achieve self-consistency between Green's functions and self-energies in the presence of strongly localized states not directly accessible from the contacts. We demonstrate an accurate, yet computationally feasible nonequilibrium Green's function model of superlattice detectors by formulating the kinetic equations in terms of problem-matched maximally localized basis functions, numerically generated from few modes representing the main conductive channels of the nanostructure. The contribution of all the remaining modes is folded in an additional self-energy to ensure current conservation. Inspection of spatially and energetically resolved single particle properties offers insight into the complex nature of carrier transport in type-II superlattice detectors, and the connection to semiclassical approaches enables the interpretation of mobility experiments.

First-principles calculations are employed for the study of the band offsets of Al

Capacitance–voltage ( C–V ) profiling is a useful technique for accurate and non-destructive determination of carrier concentrations in semiconductor materials. Recently, this measurement has been applied to the infrared barrier detector to determine the doping densities of the absorber and contact layers. This paper provides three contributions to the development of barrier detectors. First, we develop a physics-based semi-analytical model for computing the C–V characteristics derived from metal–oxide–semiconductor and heterojunction device physics and show that it is in agreement with results obtained from drift-diffusion simulations. Second, we assess the possibility of using the developed model to determine not only the absorber and contact doping densities, but also the doping density and thickness of the barrier layer. Lastly, we use the same drift-diffusion methodology to conduct a comprehensive parametric study of the C–V profile for a variety of absorber n-type doping densities, barrier N- and P-type doping densities, barrier thicknesses, and contact layer n-type doping densities. We also offer an extensive discussion of the role of the various device parameters on shaping the C–V profile. While this paper uses the InAsSb and AlAsSb material system, the analysis can be extended to other materials used to implement barrier devices, such as HgCdTe or superlattices.

A consistent trend in infrared imaging systems is a drive towards smaller pixel pitches in focal plane arrays. In this work, we present an extensive numerical study on how dark current, quantum efficiency, and modulation transfer function are affected when reducing the pixel pitch in SWIR InGaAs pixel arrays. From the results, we propose the introduction of diffusion control junctions into the pixel sub-architecture to lower dark current and improve modulation transfer function, with a minor decrease in specific detectivity.

The band gaps and structural properties of ${\mathrm{InAs}}_{1\ensuremath{-}x}{\mathrm{Sb}}_{x}$ alloys are investigated using both the modified Becke-Johnson exchange potential and hybrid functional calculations. A good agreement between the two approaches is observed for the alloys. The estimated value of $0.85\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ for the bowing parameter enables the use of $\mathrm{InAsSb}$ in long wavelength infrared (LWIR) applications. Furthermore, a lower limit of $0.29\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ for the bowing parameter is obtained for the structures yielding the largest band gaps, demonstrating the strong nonlinearity of the band gap versus the composition for this system.

Using numerical simulations, we investigate the effects of deep traps and deep acceptor levels in magnesium-doped $\mathrm{Ga}\mathrm{N}$ on interface charges in the semiconductor-oxide interface. Specifically, in this work we address two open issues observed in experimental studies on $\mathrm{Ga}\mathrm{N}$ trench metal-oxide-semiconductor field-effect transistors. (i) We investigate the observed clockwise hysteresis in the transfer characteristics and elucidate the underlying physical mechanism causing it. By employing appropriate models for substitutional carbon at nitrogen sites (${C}_{N}$) and nitrogen vacancies (${V}_{N}$), we calculate the hysteresis dependence on the trap concentrations and the measurement sweep duration ${T}_{s}$. We show that ${C}_{N}$ acceptor traps in $p$-$\mathrm{Ga}\mathrm{N}$ are likely responsible for this phenomenon and the largest hysteresis is predicted for a sweep duration of ${T}_{s}\ensuremath{\approx}30\phantom{\rule{0.2em}{0ex}}\mathrm{s}$. (ii) We also address the apparent inconsistency between the experimental and theoretically predicted magnesium-ionization levels and the variations of the measured transfer characteristics, specifically the threshold voltage. We show that the bands bending in the channel area creates a layer in which magnesium is completely ionized. As a result, the magnesium partial ionization does not have an effect and, while the threshold voltage decreases, nor does the breakdown voltage, as observed experimentally. The measured threshold voltage, which is lower than the theoretically predicted value, is caused by fixed and trapped charges at the interface, in agreement with values reported in the literature.

This work investigates carrier transport in tunnel junctions for vertical-cavity surface-emitting lasers (VCSELs). The study is performed with a quantum-corrected semiclassical approach, where tunneling is described rigorously with a nonequilibrium Green's function formalism based on a multiband description of the electronic structure. Validated with experimental results, the proposed approach provides a quantum -kinetic perspective of the tunneling process and paves the way toward a comprehensive theory of VCSELs, bridging the gap between semiclassical and quantum simulations.

We analytically calculate the anomalous transverse electric and thermal currents in massive and tilted Dirac systems, using $ \beta $-borophene as a representative material, and report on conditions under which the corresponding Lorenz number $\left(\mathcal{L}_{an}\right)$ deviates from its classically accepted value $\left(\mathcal{L}_{0}\right)$. The deviations in the high-temperature regime are shown to be an outcome of the quantitative difference in the respective kinetic transport expressions for electric $\left(\sigma\right)$ and thermal $\left(\kappa\right)$ conductivity, and are further weighted through a convolution integral with a non-linearly energy-dependent Berry curvature that naturally arises in a Dirac material. In addition, the tilt and anisotropy of the Dirac system that are amenable to change via external stimulus are found to quantitatively influence $ \mathcal{L}_{an} $. The reported deviations from $\mathcal{L}_{0} $ hold practical utility inasmuch as they allow an independent tuning of $\sigma $ and $ \kappa $, useful in optimizing the output of thermoelectric devices., Comment: 4 pages; 5 figures

This work presents the activities of the Center for Semiconductor Modelling in the area of infrared imaging devices. We outline a methodology that enables the study of large scale infrared detector arrays to quantify their optical and electrical performance. Furthermore, we present an approach to investigate the quantum mechanical transport properties of superlattice based detectors that are an emerging technology with potential applications both in commercial and defense system.

High-electron-mobility transistors (HEMTs) made of wide-band-gap semiconductors have great potential for power electronics and radio-frequency applications. Coupling first-principles calculations with device simulations enables cost-effective semiconductor research and development, including materials exploration and device design. The authors report innovative, comprehensive calculations of electronic transport in wurtzite GaN and AlN and in an (Al,Ga)N/GaN HEMT. The hot-electron energy distributions in the simulated HEMTs can be used to determine the related device degradation, and suggest opportunities for improved designs.

Vacuum Field Effect Transistors (VFETs) are explored using wide-bandgap (III-Nitrides), VFETs avoid high power consumption caused by cathode heating in thermionic emission devices and have small turn-on voltage with potential for millimeter-wave operation. Special attention is placed on emitter design, growth of high-quality materials and evaluation of thermal properties Progress in Si- and III-V-based electronic devices allowed operation at high-frequencies extending well into the millimeter-wave range. Device performance is limited by carrier transport properties and parasitics which have been addressed using different semiconductor materials, shrinking of device dimensions and advanced design approaches, permitting in this way to achieve operation at several hundred GHz. Further performance advances continue to be reported but start reaching a limit. The advances made in nanotechnology together with the possibility of utilizing field- rather than thermionic-emission offer, however, the possibility of revisiting the principle of vacuum tubes. Nanoelectronic devices utilizing such approaches offer the possibility of operating at millimeter-wave frequencies. Materials used for their realization are semiconductors i.e. II-Nitrides that can be processed using well-developed technologies and nanofabrication techniques, providing therefore ways of shrinking dimensions in critical parts of the device such as for example the gap between the emitter and cathode. The paper addresses device approaches for vacuum nanoelectronics using wide-bandgap i.e. III-Nitride semiconductors. The studies rely on appropriate emitter design, growth of high-quality materials and evaluation of thermal properties that can play a critical role in device performance.

Imaging in the extended short-wavelength infrared (eSWIR) spectral band (1.7–3.0 μm) for astronomy applications is an area of significant interest. However, these applications require infrared detectors with extremely low dark current (less than 0.01 electrons per pixel per second for certain applications). In these detectors, sources of dark current that may limit the overall system performance are fundamental and/or defect-related mechanisms. Non-optimized growth/device processing may present material point defects within the HgCdTe bandgap leading to Shockley–Read–Hall dominated dark current. While realizing contributions to the dark current from only fundamental mechanisms should be the goal for attaining optimal device performance, it may not be readily feasible with current technology and/or resources. In this regard, the U.S. Army Research Laboratory performed physics-based, two- and three-dimensional numerical modeling of HgCdTe photovoltaic infrared detectors designed for operation in the eSWIR spectral band. The underlying impetus for this capability and study originates with a desire to reach fundamental performance limits via intelligent device design.

This paper discusses two cases of applying artificial neural networks to the capacitance–voltage characteristics of InAsSb-based barrier infrared detectors. In the first case, we discuss a methodology for training a fully-connected feedforward network to predict the capacitance of the device as a function of the absorber, barrier, and contact doping densities, the barrier thickness, and the applied voltage. We verify the model’s performance with physics-based justification of trends observed in single parameter sweeps, partial dependence plots, and two examples of gradient-based sensitivity analysis. The second case focuses on the development of a convolutional neural network that addresses the inverse problem, where a capacitance–voltage profile is used to predict the architectural properties of the device. The advantage of this approach is a more comprehensive characterization of a device by capacitance–voltage profiling than may be possible with other techniques. Finally, both approaches are material and device agnostic, and can be applied to other semiconductor device characteristics.

The properties of low concentrations of Be as a p-dopant in InAsSb with a composition corresponding to absorption in the long wavelength infrared band were studied. Temperature- and magnetic field-dependent Hall effect data were analyzed with a multi-carrier model allowing extraction of the bulk hole concentration and mobility. The hole density exhibits a weak freeze-out with an activation energy of 3.2 meV. Density functional theory calculations indicate that Be favor the In sites as BeIn with an acceptor binding energy near the valence band maximum. The hole mobility increases monotonically as the temperature is lowered, showing an alloy scattering-limited value of about 1000 cm2 V−1 s−1 at 77 K and plateauing at around 3200 cm2 V−1 s−1 at 20 K. Temperature-dependent photoluminescence was measured up to 200 K and did not indicate any deleterious effects induced by the acceptors. A superlinear bandgap vs temperature behavior is tentatively interpreted as a band-filling effect, which is reduced with added concentrations of acceptors.

First principles calculations are employed for the study of the band offsets of AlxGa1-xN alloys, taking into account their composition and atomic configuration. Specifically, the band offsets are obtained using PBE, PBEsol, HSE, and modified Becke-Johnson calculations, comparing the results and discussing the advantages and disadvantages of each functional. The band alignments are performed using the branch point energies of the materials as their common reference level. HSE calculations predict a valence band offset of 0.9 eV between GaN and AlN. Regarding the alloys, a conduction band edge bowing parameter of 0.55 eV and a practically zero bowing for the valence band edge is predicted on average. The different atomic configurations affect mainly the valence band edges, where deviations from linearity by more than 0.1 eV are observed.

We employ first-principles calculations in the formalism of standard and hybrid density functional theory to study the electronic and structural properties of wurtzite ${\text{Al}}_{x}{\text{Ga}}_{1\ensuremath{-}x}\text{N}$ alloys. We address the discrepancies observed in literature regarding essential electronic properties of these alloys and we investigate the dependence of these properties on the atomic ordering and composition. We show that the bowing parameter is significantly affected by the atomic ordering, ranging from zero to strong downward bowing. The effects of atomic ordering of the alloys on their band offset with respect to the pure phases are also investigated. Finally, using the effective band structure approach, we study the electronic band structure of the random alloys.

We employ first-principles density functional theory to investigate the electronic and structural properties of grain boundaries (GBs) in InAs. In particular, we study the energetics and passivation mechanisms of representative low-$\mathrm{\ensuremath{\Sigma}}$ GBs, including $\mathrm{\ensuremath{\Sigma}}3(111),\mathrm{\ensuremath{\Sigma}}3(112),\mathrm{\ensuremath{\Sigma}}5(120)$, and $\mathrm{\ensuremath{\Sigma}}5(130)$, to establish their relative stability and experimental feasibility. We find that the symmetric-tilt twin-boundary $\mathrm{\ensuremath{\Sigma}}3(111)$ GB is the most stable GB, in excellent agreement with our experimentally characterized GB structures in InAs. In addition to our theoretically predicted GB structures, we systematically study and analyze different configurations of complex multifold experimentally observed InAs GB structures. We discuss the effect of different passivations and doping mechanisms on the electronic properties of the GBs. Understanding the exact nature of the GB electronic structure and stability, as well as their passivation mechanisms is a key step for the further development of InAs based optoelectronic devices on silicon and other heterogeneous large-area substrates.

Understanding the failure mechanisms in high performance detector arrays is critical for meeting the demands for a given application. For SWIR sensing, the detector requires the highest sensitivity possible to operate under photon-starved conditions. Using a 3D drift-diffusion model, we simulate the influence of heterointerface traps on the dark current in In0:53Ga0:47As / InP p-on-n planar heterojunction photodiodes. We calculate how the dark current changes with junction area, and show that it increases linearly with junction radius when the device is limited by surface recombination. An analytical model is developed to understand the geometric dependence of both bulk and surface generation/recombination currents. Finally, insight on mitigation strategies and possible impact on quantum efficiency are both discussed.

This paper presents results from three-dimensional quantitative modeling on dense, moderately doped [ND (NA) = 5 × 1015 cm−3] short-wave infrared (SWIR) p+n and n+p Hg1−xCdxTe double planar heterostructure photodetecting arrays with absorber x = 0.451 and cap x = 0.55. At uniform reverse bias, the competition for minority carriers between closely spaced diodes preserves densities below equilibrium levels throughout the absorber. This carrier suppression has several consequences in addition to suppressing dark current by constraining the minority-carrier gradients at each diode junction. First, the dense arrays maintain volume-average negative net radiative recombination rates (negative luminescence) roughly an order of magnitude larger than comparably biased isolated diodes. Second, the negative excess minority-carrier densities suppress the volume-average net Auger recombination rate by roughly an order of magnitude in dense n-type HgCdTe arrays compared with a single diode. Third, the long minority electron diffusion lengths in the p-type HgCdTe absorber not only suppress lateral diffusion currents, but do so in a manner that provides negative differential resistance. By suppressing intrinsic recombination rates, or lateral diffusion currents, each effect can contribute to increasing R0A products in SWIR HgCdTe dense arrays. These effects should be considered when optimizing device structures for pitch, thickness, feature size, doping, and bias points.

A novel room-temperature negative differential resistance (NDR) effect is proposed, theoretically analyzed, and quantitatively modeled for short-wave infrared (SWIR) HgCdTe photodiode detectors in dense double-layer planar heterostructure arrays with a 2.5 $\mu \text{m}$ cutoff at 300 K. The predicted NDR results from nonequilibrium minority carrier suppression—with associated Auger suppression and negative luminescence—imposed by dense array geometry under uniform reverse bias. Using three-dimensional quantitative modeling, we evaluate representative dark current–voltage characteristics at different array pitch values. The predicted dark current and NDR resulting from structural variations in junction radius are consistent with the analytic dense array lateral diffusion current suppression model. The NDR effect and its relation to geometric parameters should be considered when attempting to minimize dark current in high-temperature SWIR HgCdTe photodiode arrays.

Finite-difference time-domain and finite element method simulations are used to evaluate two-dimensional spot-scan profiles of p-on-n double-layer planar heterostructure (DLPH) detector arrays with abrupt p-type diffusions. The modulation transfer function (MTF) is calculated from the spot-scan profiles. An asymmetric dark and photo current collection mechanism is identified and explained as a result of electric field bunching through the corners of polygonal diffusions in DLPH arrays. The MTF consequences of the asymmetric collection are studied for triangular, square, and hexagonal diffusions in square and hexagonal arrays. We show that the placement and shape of the diffusion relative to the pixel can modify the MTF by several percent. The magnitude of the effect is largest for diffusions with fewer degrees of rotational symmetry.

Future and advanced sensor technologies needed for DoD applications will require more efficient semiconductor materials and devices. Pushing sensor device performance beyond present levels requires a deep understanding of the fundamental limiters. Therefore fundamental research is needed to assure transition of technology from demonstration to system deployment. To address this problem, the Army Research Laboratory (ARL) and Boston University (BU) have come together to create a BU led Consortium for semiconductor Modeling of Materials and Devices (CSM). The Consortium brings together government, academia, and industry in a collaborative fashion to continuously push semiconductor research forward to meet DoD needs. The leveraged attributes of the Consortium include combined broad knowledge base in semiconductor modeling, materials growth and device expertise; sharing of computational resources; project continuity; and extension of the bench. Details regarding the Consortium’s first research topic on understanding vertical transport in Type 2 SL will be discussed.

Defects in semiconductors can play a vital role in the performance of electronic devices, with native defects often dominating the electronic properties of the semiconductor. Understanding the relationship between structural defects and electronic function will be central to the design of new high-performance materials. In particular, it is necessary to quantitatively understand the energy and lifetime of electronic states associated with the defect. Here, we apply first-principles density functional theory (DFT) and many-body perturbation theory within the GW approximation to understand the nature and energy of the defect states associated with a charged nitrogen vacancy on the electronic properties of gallium nitride (GaN), as a model of a well-studied and important wide gap semiconductor grown with defects. We systematically investigate the sources of error associated with the GW approximation and the role of the underlying atomic structure on the predicted defect state energies. Additionally, analysis of the computed electronic density of states (DOS) reveals that there is one occupied defect state 0.2 eV below the valence band maximum and three unoccupied defect states at energy of 0.2--0.4 eV above the conduction band minimum, suggesting that this defect in the +1 charge state will not behave as a carrier trap. Furthermore, we compare the character and energy of the defect state obtained from GW and DFT using the HSE approximate density functional and find excellent agreement. This systematic study provides a more complete understanding of how to obtain quantitative defect energy states in bulk semiconductors.

HgCdTe has been the material of choice for MWIR, and LWIR infrared sensing due to its highly tunable band gap and favorable material properties. However, HgCdTe growth and processing for the ESWIR spectral region is less developed, so alternative materials are actively researched. It is important to compare the fundamental limitations of each material to determine which offers optimal device performance. In this article, we investigate the intrinsic recombination mechanisms of ESWIR materials—InGaAs, GeSn, and HgCdTe—with cutoff wavelength near 2.5μm, and MWIR with cutoff of 5μm. First, using an empirical pseudo-potential model, we calculate the full band structure of each alloy using the virtual crystal approximation, modified to include disorder effects and spin-orbit coupling. We then evaluate the Auger and radiative recombination rates using a Green’s function based model, applied to the full material band structure, yielding intrinsic carrier lifetimes for each given temperature, carrier injection, doping density, and cutoff wavelength. For example, we show that ESWIR HgCdTe has longer carrier lifetimes than InGaAs when strained or relaxed near room temperature, which is advantageous for high operating temperature photodetectors. We perform similar analyses for varying composition GeSn by comparing the calculated lifetimes with InGaAs and HgCdTe. Finally, we compare HgCdTe, InAsSb and GeSn with a cutoff in the MWIR spectral band.

We employ the nudged elastic band and the dimer methods within the standard density functional theory (DFT) formalism to study the migration of the oxygen and gallium vacancies in the monoclinic structure of $\ensuremath{\beta}\ensuremath{-}{\mathrm{Ga}}_{2}{\mathrm{O}}_{3}$. We identify all the first nearest neighbor paths and calculate the migration barriers for the diffusion of the oxygen and gallium vacancies. We also identify the metastable sites of the gallium vacancies which are critical for the diffusion of the gallium atoms. The migration barriers for the diffusion of the gallium vacancies are lower than the migration barriers for oxygen vacancies by 1 eV on average, suggesting that the gallium vacancies are mobile at lower temperatures. Using the calculated migration barriers we estimate the annealing temperature of these defects within the harmonic transition state theory formalism, finding excellent agreement with the observed experimental annealing temperatures. Finally, we suggest the existence of percolation paths which enable the migration of the species without utilizing all the migration paths of the crystal.

Monte Carlo (MC) methods have been shown to capture the ultrafast dynamics of carriers and resulting luminescence in photo-excited GaN. These calculations have assumed a homogeneous material under uniform illumination. We investigate the development of surrogate models to represent the carrier dynamics with fewer degrees of freedom, with the ultimate goal of computing the response of structured materials under inhomogeneous illumination profiles. These models are presently based on the extraction of mean and variance information from single data sets, through the use of fits to s piece-wise linear basis. Two models are then developed, one based on the mean and variance directly, and one on a Gaussian process regression model. New carrier populations with similar statistical profiles are then generated from these models to determine the amount of data reduction that can be obtained.

As material growth and processing have improved, state of the art infrared detector arrays remain limited by material properties and not processing or growth quality. In particular, the dark current can be dominated by diffusion of minority carriers in the quasineutral regions. In this work, we present a unique detector architecture that allows for dark current suppression below the fundamental diffusion limit. We have extensively studied this effect, and report dark current, photocurrent, and quantum efficiency. Finally, we conclude by offering a path to implementing this architecture into existing FPAs.

In this work, we investigate the dynamics of photo-excited carriers in ZnO. Specifically, we study the luminescence spectrum and the effect of temperature on the luminescence rise time. For comparison, experimental time- resolved photo-luminescence studies on ZnO samples are performed. In the theoretical model, interaction with a laser pulse is treated coherently and a generalized Monte Carlo simulation is used to account for scattering processes. The scattering mechanisms included are carrier interactions with polar optical phonons and acoustic phonons, and carrier–carrier Coulomb interactions. We observed a good agreement between the experimental and simulation results for the photo-luminescence spectrum. Furthermore, as the temperature increases, the luminescence rise time decreases, mostly due to the weaker effect of polar optical scattering at lower temperature.

In this work, we develop a method for simulating the modulation transfer function (MTF) of infrared detector arrays, which is based on numerical evaluation of the detector physics. The finite-difference time-domain and finite element methods are used to solve the electromagnetic and electrical equations for the device, respectively. We show how the total MTF can be deconvolved to examine the effects of specific physical processes. We introduce the MTF area difference and use it to quantify the effectiveness of several crosstalk mitigation techniques in improving the system MTF. We then apply our simulation methods to two-thirds generation mercury cadmium telluride (HgCdTe) detector architectures. The methodology is general, can be implemented with commercially available software, has experimentally realizable analogs, and is extendable to other material systems and device designs.