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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.

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.

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.

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.

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.

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.

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.

The quantum efficiency (QE) in HgCdTe photovoltaic pixel arrays employing a photon-trapping (PT) structure realized with a periodic array of pillars intended to provide broadband operation was investigated. It was found that the QE depends heavily on the passivation of the pillar surface. This is due to the presence of large fixed positive charge on the surface of pillars passivated with anodic oxide. A three-dimensional numerical simulation model was used to study the effect of the surface charge density and surface recombination velocity on the exterior of the pillars. Then, the QE of this structure was evaluated subject to different surface conditions. It was found that alone the surface charge density or surface recombination is detrimental to the QE but that the QE is recovered when both phenomena are present. Subsequently, the crosstalk was analyzed and the superior performance of the PT structure was demonstrated by evaluating the modulation transfer function.

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

This work presents an in-depth investigation of the properties of complexes composed of hydrogen, silicon or oxygen with carbon, which are major unintentional impurities in undoped GaN. This manuscript is a complement to our previous work on carbon--carbon and carbon-vacancy complexes. We have employed a first-principles method using Heyd-Scuseria-Ernzerhof hybrid functionals within the framework of generalized Kohn-Sham density functional theory. Two H--C, four Si--C and five O--C complexes in different charge states have been considered. After full geometry relaxations, formation energies, binding energies and both thermal and optical transition levels were obtained. The calculated energy levels have been systematically compared with the experimentally observed carbon related trap levels. Furthermore, we computed vibrational frequencies for selected defect complexes and defect concentrations were estimated in the low, mid and high carbon doping scenarios considering two different cases where electrically active defects: (a) only carbon and vacancies and (b) not only carbon and vacancies but also hydrogen, silicon and oxygen. We confirmed that $\mathrm{C_N}$ is a dominant acceptor in GaN. In addition to it, substantial amount of $\mathrm{Si_{Ga}-C_N}$ complex exists in a neutral form. This complex is a likely candidate for unknown form of carbon observed in undoped $n$-type GaN., Comment: 19 pages, 23 figures, 8 tables

The Gesub1-x/subSnsubx/suballoy is a promising material for optoelectronic applications. It offers a tunable wavelength in the infrared (IR) spectrum and high compatibility with complementary metal-oxide-semiconductor (CMOS) technology. However, difficulties in growing device quality Gesub1-x/subSnsubx/subfilms has left the potentiality of this material unexplored. Recent advances in technological processes have renewed the interest toward this material paving the way to potential applications. In this work, we perform a numerical investigation on absorption coefficient, radiative recombination rate, and Auger recombination properties of intrinsic and doped Gesub1-x/subSnsubx/subfor application in the extended-short wavelength infrared and medium wavelength infrared spectrum ranges. We apply a Green's function based model to the Gesub1-x/subSnsubx/subfull electronic band structure determined through an empirical pseudopotential method and determine the dominant recombination mechanism between radiative and Auger processes over a wide range of injection levels.

Infrared (IR) photon detectors must be cryogenically cooled to provide the highest possible performance, usually to temperatures at or below ~ 150K. Such low operating temperatures (Top) impose very stringent requirements on cryogenic coolers. As such, there is a constant push in the industry to engineer new detector architectures that operate at higher temperatures, so called higher operating temperature (HOT) detectors. The ultimate goal for HOT detectors is room temperature operation. While this is not currently possibly for photon detectors, significant increases in Top are nonetheless beneficial in terms of reduced size, weight, power and cost (SWAP-C). The most common HgCdTe IR detector architecture is the P+n heterostructure photodiode (where a capital letter indicates a wide band gap relative to the active layer or “AL”). A variant of this architecture, the P+N−n−N−N heterostructure photodiode, should have a near identical photo-response to the P+n heterostructure, but with significantly lower dark diffusion current. The P+N−n−N−N heterostructure utilizes a very low doped AL, surrounded on both sides by wide-gap layers. The low doping in the AL, allows the AL to be fully depleted, which drastically reduces the Auger recombination rate in that layer. Minimizing the Auger recombination rate reduces the intrinsic dark diffusion current, thereby increasing Top. Note when we use the term “recombination rate” for photodiodes, we are actually referring to the net generation and recombination of minority carriers (and corresponding dark currents) by the Auger process. For these benefits to be realized, these devices must be intrinsically limited and well passivated. The focus of this proceeding is on studying the fundamental physics of the intrinsic dark currents in ideal P+N−n−N−N heterostructures, namely Auger recombination. Due to the complexity of these devices, specifically the presence of multiple heterojunctions, numerical device modeling techniques must be utilized to predict and understand the device operation, as analytical models do not exist for heterojunction devices.

Carbon is a readily incorporated impurity during growth in GaN and is known for deteriorating the electronic properties of devices. We study the diffusion of carbon in GaN employing Density Functional Theory. We compute the migration barriers of carbon interstitials and we propose possible mechanisms of diffusion in the wurtzite GaN crystal. The Minimum Energy Paths (MEP) and the migration barriers are obtained using the Nudged Elastic Band (NEB) method as well as the Dimer method. Apart from the neutral carbon interstitial, we also study the diffusion of carbon in the most relevant charged states of 1−, 1+ and 2+. Our results indicate that carbon interstitials are mobile at growth temperatures.

Carbon (C) is a common and important impurity in GaN. C doping is employed to obtain semi-insulating buffer layers, which are used for high electron mobility transistors. However, increasing C concentration means the increase of C-related trap levels in GaN, which causes detrimental effects to the device performance. These unexpected C-related trap levels are observed in a number of experiments, but the physical origins of these levels are still unknown. We performed first-principles calculations within density functional theory using hybrid density functionals. Our focuses were on the C-related complexes, about which very little is known so far. From the formation energies of various C-related complexes we obtained thermal and optical activation energies of the trap levels and directly compared them with the experimental values in order to identify the physical origins of the experimentally observed trap levels.

Using Density Functional Theory we have investigated the migration of oxygen vacancies and hydrogen impurities in the oxide β-Ga 2 O 3 . For our studies we used the Nudged Elastic Band (NEB) method in order to determine the Minimum Energy Paths (MEP) and the migration barriers. Neutral and doubly charged oxygen vacancies are considered to be the most common intrinsic defects in Ga 2 O 3 . In addition, hydrogen is a common background impurity. Our results indicate that these defects are mobile at growth temperatures.

Indium gallium arsenide (In 1−x Ga x As) is an ideal material choice for short wave infrared (SWIR) imaging due to its low dark current and excellent collection efficiency. By increasing the indium composition from 53% to 83%, it is possible to decrease the energy gap from 0.74 eV to 0.47 eV and consequently increase the cutoff wavelength from 1.7 μm to 2.63 μm for extended short wavelength (ESWIR) sensing. In this work, we apply our well-established numerical modeling methodology to the ESWIR InGaAs system to determine the intrinsic performance of pixel detectors. Furthermore, we investigate the effects of different buffer/cap materials. To accomplish this, we have developed composition-dependent models for In 1−x Ga x As, In 1−x Al x As, and InAs 1−y P y . Using a Green’s function formalism, we calculate the intrinsic recombination coefficients (Auger, radiative) to model the diffusion-limited behavior of the absorbing layer under ideal conditions. Our simulations indicate that, for a given total thickness of the buffer and absorbing layer, structures utilizing a linearly graded small-gap InGaAs buffer will produce two orders of magnitude more dark current than those with a wide gap, such as InAlAs or InAsP. Furthermore, when compared with experimental results for ESWIR photodiodes and arrays, we estimate that there is still a 1.5x magnitude of reduction in dark current before reaching diffusion-limited behavior.

We are developing a numerical simulation model for back-illuminated nBn detectors. This model is based on the simultaneous numerical solution of the carrier continuity equations and Poisson equation on a three-dimensional finite-element grid. This paper reports our initial results for one-dimensional simulations of a back-illuminated nBn device with an InAs absorber layer and an AlAsSb barrier layer. These results include spatial profiles for the energy bands and the hole concentration profiles for a wide range of bias voltages, temperatures, and illumination fluxes. These results help to illustrate the underlying physics of the nBn device. In particular, these results show the influence of the doping type and concentration in the barrier layer on the dark current and the photocurrent, and demonstrate that the quantum efficiency at zero bias voltage can be degraded significantly by barrier layer doping concentrations that are too high.

2-D numerical simulation is employed to assess a number of possible design approaches aimed at optimizing the internal quantum efficiency (IQE) of ZnO-based light-emitting diodes (LEDs) grown along the c-axis. First, the relative performance of similar ZnO-based and GaN-based LED structures is compared and discussed. Second, the effects on IQE of thickness, doping, and alloy composition of the MgZnO electron blocking layer (EBL) is studied in order to maximize the carrier confinement in the active region. The optimum number of quantum wells is also addressed, and different strategies for barrier doping are considered, showing that, if the EBL is doped p-type, a similar doping in the barriers is not required to compensate for the spontaneous and piezoelectric interface charges and to enhance hole transport. Different choices of the geometrical and doping parameters of the n-type access region are considered, and the impact of different values of the electron mobility is determined. Finally, the analysis of a ZnO/BeZnO LED structure suggests that the incorporation of BeZnO layers does not provide significant advantages.

Band-to-band Auger recombination mechanisms in HgCdTe are investigated as functions of temperature in the small modulation limit by using realistic electronic structures obtained by empirical pseudopotential calculations and their corresponding wavevector-dependent dielectric functions. The calculated Auger lifetimes are compared with the semiempirical Beattie-Landsberg-Blakemore (BLB) model, which has been extensively employed to reproduce experimental data. The Auger-1 lifetime can be fitted well to the BLB model with a constant overlap integral |F 1 F 2| = 0.16, near the lower limit of the range reported in the literature. The role of the Auger-7 process in p-type HgCdTe is also investigated and the ratio γ between the intrinsic Auger-7 and Auger-1 lifetimes is found to be about 10.

We analytically establish an anomalous transverse flow of heat in 8-\textit{Pmmn} borophene, one of the several two-dimensional (2D) allotropes of Boron (B). The dispersion of this allotrope contains a pair of anisotropic and tilted Dirac cones which are gapped by placing the 2D \textit{B} sheet under an intense circularly-polarized illumination. A gap in the Dirac dispersion leads to a finite Berry curvature and connected anomalous Hall effects. In the case of thermoelectrics, this manifests as a heat current perpendicular to the temperature gradient - the thermal Hall effect. A quantitative calculation of the attendant thermal Hall conductivity reveals dependence on the intrinsic anisotropy and tilt of the Dirac cone. Further, by estimating the longitudinal thermal conductivity using the Weidemann-Franz law, we also outline steps to compute the thermal Hall angle that gauges the generation efficiency of such transverse heat processes. Finally, we touch upon the idea of thermal rectification wherein the direction of flow of the anomalous heat reverses through a simple switch of the polarization of incident light and is of interest in thermal logic circuits., 5 pages, 3 figures

p+np+ CMOS Si LED structures were modeled in order to investigate the effect of various depletion layer profiles and defect engineering on the photonic transitions in the 1.4-2.8 eV, 450-750 nm regime. Modeling shows that by utilizing a short linear increasing E-field in the p+n reverse-biased junction with a gradient of approximately 5 × 105 V cm-1· ?m-1, and injecting carriers from an adjacent p+n junction, increased localized optical yield by a factor 50-100. A number of device designs were realized using CMOS 0.35 ?m technology. The device design involves normal CMOS design and processing procedures with no excessive microdimensioning. The current devices operated in the 6-8 V, 1 ?A-2 mA regime, and yield emission intensities of up to 100 nW ?m-2. The current emission levels are about three orders higher than the low-frequency detectability limit of Si CMOS p-n detectors of corresponding area, which make diverse electro-optical applications such as MOEMS devices, and diverse optical signal processing and wave-guiding and the development of ?smart chips? feasible in standard CMOS integrated circuitry.

As a preparatory step toward establishing reliable numerical design tools for ZnO-based optoelectronic devices, we have reassessed the available information on material parameters relevant for the simulation of light-emitting diodes (LEDs) with active regions including ZnO, MgZnO, and BeZnO layers. The impact of different approximations for the electronic structure and the interface polarization charge on the optical properties of bulk ZnO and ZnO/MgZnO quantum wells has been evaluated, and a consistent set of parameters has been used not only for systematic comparison of ZnO/MgZnO and ZnO/BeZnO single quantum well structures but also for the first simulation of a realistic ZnO/BeZnO multiple quantum well LED.

We report on the modulation transfer function (MTF) in short-wave infrared indium gallium arsenide (InGaAs) on indium phosphide (InP) planar photodetector arrays. Our two-dimensional numerical method consists of optical simulations using the finite-difference time domain method and drift-diffusion simulations using the finite-element method. This parametric study investigates MTF dependence on pitch, the addition of refractive microlenses, the thickness of the InGaAs absorber, and the doping concentration of the InGaAs absorber. A focus is placed on the connection between the lateral diffusion of photogenerated holes in InGaAs and the MTF. It is found that the MTF of small-pitch arrays exhibit sub-ideal behavior due to pixel cross-talk resulting from a long minority carrier diffusion length. By incorporating monolithic microlenses with the InP substrate, the MTF response is improved for all pitches investigated, particularly for spatial frequencies near the respective cutoff frequencies. We also find a strong dependence of the MTF on the thickness and doping concentration in the absorbing region. Trends in dark current and quantum efficiency are reported.

We discuss, through numerical device simulation, a number of possible design approaches intended for optimizing the internal quantum efficiency (IQE) of light-emitting diodes based on InGaN quantum wells (QWs) grown along the c-axis emitting in the near-ultraviolet region. We study the effects on IQE of thickness, doping, and alloy composition of the electron and hole blocking layers in order to maximize the confinement of both carrier species in the active region. We discuss the selection of the number of QWs to be employed in the active region and their optimum width, and we show the comparatively minor effects of the thickness of the barrier layers. We also compare different strategies for barrier doping, confirming that a p-type doping in all barriers helps to compensate the spontaneous and piezoelectric surface charges and to enhance hole transport. Finally, we evaluate the impact of Auger recombination on IQE and its role in the experimentally observed efficiency droop. Whenever possible, we suggest practical design criteria and provide technologically feasible sets of design parameters.

Dislocations are known to influence the electrical and optical properties of long-wavelength infrared (LWIR) HgCdTe detectors and have been shown to limit the performance of arrays fabricated on heteroepitaxial substrates. To help better understand dislocations in HgCdTe, a new method for preparing HgCdTe diagnostic epitaxial single-crystal samples by chemically removing the supporting CdZnTe substrate has been developed. Using this new sample preparation technique, the behavior of misfit and threading dislocations in HgCdTe epitaxial layers has been investigated by using a defect etch to reveal the dislocations present in the thin HgCdTe films. In most cases etch pits on the surface of the film are spatially correlated with etch pits on the bottom of the HgCdTe film. The small displacements of the related etch pits were used to obtain crystallographic information concerning the paths followed by threading dislocations on allowed slip planes in the HgCdTe crystal. In addition, transmission electron microscopy (TEM) is used to obtain more specific information regarding the Burgers vector of the dislocation. While this new sample preparation technique is useful for studying dislocations in HgCdTe epitaxial layers, it can also be used to study stress from ohmic contacts and passivation layers. The technique can be used for both liquid-phase epitaxy (LPE)- and molecular-beam epitaxy (MBE)-grown HgCdTe on CdZnTe substrates.

The potential applications of germanium and its alloys in infrared silicon-based photonics have led to a renewed interest in their optical properties. In this letter, we report on the numerical determination of Auger coefficients at T = 300 K for relaxed and biaxially strained germanium. We use a Green's function based model that takes into account all relevant direct and phonon-assisted processes and perform calculations up to a strain level corresponding to the transition from indirect to direct energy gap. We have considered excess carrier concentrations ranging from 1016 cm−3 to 5 × 1019 cm−3. For use in device level simulations, we also provide fitting formulas for the calculated electron and hole Auger coefficients as functions of carrier density.

The heat capacity of conduction electrons obtained from the Sommerfeld expansion is shown to be tunable via the Rashba and Dresselhaus spin-orbit coupling parameters. Using an AlInSb/InSb/AlInSb as a representative heterostructure with alterable well and asymmetric barrier regions, the heat capacity is found to be higher for the spin-down electrons and suffers a reduction for wider wells. A further lowering is obtained through the application of an uniaxial strain. Finally, we suggest a method to determine the spin lifetimes for spins relaxing via the D'yakonov-Perel' mechanism from experimental estimates of thermodynamic potentials such as the Helmholtz free energy and the heat capacity., Comment: 4 pages; 3 figures

Understanding the radiative and non-radiative properties of semiconductor materials is a prerequisite for optimizing the performance of existing light emitters and detectors and for developing new device architectures based on novel materials. Due to the ever increasing complexity of novel semiconductor systems and their relative technological immaturity, it is essential to have design tools and simulation strategies that include the details of the microscopic physics and their dependence on the macroscopic (continuum) variables in the macroscopic device models. Towards this end, we have developed a robust full-band structure based approach that can be used to study the intrinsic material radiative and non-radiative properties and evaluate the same characteristics of low-dimensional device structures. A parallel effort is being carried out to model the effect of substrate driven stress/strain and material quality (dislocations and defects) on microscopic quantities such as non-radiative recombination rate. Using this modeling approach, we have extensively studied the radiative and non-radiative properties of both elemental (Si and Ge) and compound semiconductors (HgCdTe, InGaAs, InAsSb and InGaN). In this work we outline the details of the modelling approach, specifically the challenges and advantages related to the use of the full-band description of the material electronic structure. We will present a detailed comparison of the radiative and Auger recombination rates as a function of temperature and doping for HgCdTe and InAsSb that are two important materials for infrared detectors and emitters. Furthermore we will discuss the role of non-radiatiave Auger recombination processes in explaining the performance of light emitter diodes. Finally we will present the extension of the model to low dimensional structures employed in a number of light emitter and detector structures.

A key design feature of P+-on-n HgCdTe detectors is the depth of the p-type region. Normally, homojunction architectures are utilized where the p-type region extends into the narrow-gap absorber layer. This facilitates the collection of photo-carriers from the absorber layer to the contact; however, this may result in excess generation-recombination (G-R) current if defects are present. Alternatively, properly adopting a heterojunction architecture confines the p-type region (and the majority of the electric field) solely to the wide-gap layer. Junction placement is critical since the detector performance is now dependent on the following sensitivity parameters: p-type region depth, doping, valence band offset, lifetime and detector bias. Understanding the parameter dependence near the hetero-metallurgical interface where the compositional grading occurs and the doping is varied as either a Gaussian or error function is vital to device design. Numerical modeling is now essential to properly engineer the electric field in the device to suppress G-R current while accounting for the aforementioned sensitivity parameters. The simulations reveal that through proper device design the p-type region can be confined to the wide-gap layer, reducing G-R related dark current, without significantly reducing the quantum efficiency at the operating bias V = -0.100V.

Emission spectra of avalanching n(+)p junctions manufactured in a standard CMOS technology with no process modifications were measured over a broad photon energy spectrum ranging from 0.8 eV to 2.8 eV at various temperatures. The temperature coefficients of the emission rates at different photon energies were determined. Below a photon energy of 1.35 eV the temperature coefficient of emission was positive, and above 1.35 eV the temperature coefficient was negative. Two narrowband emissions were also identified from the temperature characterization, namely an enhanced positive temperature coefficient at 1.15 eV photon energy, and an enhanced negative temperature coefficient at 2.0 eV. Device simulations and Monte Carlo simulations were used to interpret the results.

What has turned into highly complex and somewhat misunderstood efficiency loss mechanisms occurring in light-emitting diodes (LEDs) based on the InN-GaN material system at high injection levels are discussed. Suggestions are made as to the dominant mechanism(s) in an open forum format as well as pointing out some of the shortcomings of the methodologies used and premises forwarded. It is unequivocally known that increased junction temperature would cause a reduction in radiative power due to mainly the reduction in radiative recombination efficiency. Another obvious mechanism is the asymmetry in doping in wide bandgap semiconductors, such as GaN, wherein the hole concentration lags well behind that of electrons in the active region. Because an electron and a hole are required for radiative recombination, the radiative efficiency cannot keep up with increasing carrier injection due to progressively lagging hole population. This results in either electron escape without radiative recombination or electron accumulation, which in turn changes the internal bias of the device, manifested as reduced internal forward bias, which reduces the rate of increase in light intensity. Some of the reports ascribe the efficiency loss at high injection levels to Auger recombination (mainly through indirect and recently reportedly direct deductions) as the main and or the only source of efficiency loss by in many cases simply relying on the temperature and injection independent (not well taken) A, B, C coefficients to fit a third order polynomial to the efficiency vs. injection current. As for the direct deduction, the spectroscopic analysis of Auger kicked hot electrons as they traverse through the G and L bands before being emitted into the vacuum by means of cesiated surface challenges the existing theories and some experiments regarding carrier scattering and G-L separation. Despite just a few reports to the contrary, the bulk of the resonant optical emission experiments do not support the Auger argument as being the main cause. In parallel, there exists a body of theoretical and experimental reports for electron overflow of ballistic/quasi-ballistic electrons traversing the active region to p-GaN, escaping recombination altogether in the active region. In fact and from the get go, the LED industry ubiquitously employed, and continues to do so, an (Al,In)GaN electron-blocking layer (EBL) to prevent electron escape for improved light output that in and of itself would more than suggest that the electron escape (overflow) does indeed occur. The only adverse effect of EBL is that it impedes hole injection due to the valence band offset between the p-type (Al,In)GaN EBL and p-GaN and also generates piezoelectric (if not lattice matched) and differential spontaneous polarization induced fields that pull down the conduction band edge at the interface reducing EBL's effectiveness. To at least reduce the aforementioned aggravating factors to some extent, the electron overflow and the associated efficiency loss can be reduced substantially (particulars of which depend on the active layer design) by inserting a stair-case electron injector (SEI) with a step-like indium composition to act as an ``electron cooler'' or by linearly graded cooler in some form or another prior to the active region. Use of multiple layered heterostructures for the active layers also plays the role of electron cooler, albeit not necessarily in the most optimum fashion. As if oblivious to the raging issues alluded to above, the LED industry has been moving along very successfully with a 2-prong approach. In one, dubbed the ``high voltage LED'', a set of LEDs (most likely configured in the form of a full-wave bridge rectifier) operating at low currents, where the efficiency is at its maximum, is used, which also has the added benefit of much reduced power supply complexity and weight. The other is the continual improvement of layer quality and optimum active layer design, taking technological parameters into consideration, which at the time of writing sported 63% wall-plug efficiency at an injection current of 350 mA for \sim 1.1 mm \times 1.1 mm LEDs, which translates to approximately 75% efficiency once the voltage (about 3 V) and phosphor conversion efficiencies are taken into account. Assuming the same extraction and external quantum efficiencies, one obtains about 86% for each. It would not be an exaggeration to conclude that mid 90% internal quantum efficiencies are very likely in play here, which means that the purported inherent problems dominating the discussion are practically reduced to an academic exercise. Despite aggravating factors involving holes and hetero-barriers, commercial LEDs almost invariably use multilayer heterostructure designs for the active layers, loosely termed as multiple quantum wells, presumably to circumvent technological challenges and the ramifications of the polarization induced field even though double heterojunction varieties could be preferable from the point of view of hole transport. Eventually, the real limiting factors are the extent of hole supply (can be mitigated by increased hole concentration, which is well known and continually explored) and the proverbial thermal wall (can be mitigated by increased efficiency and efficient heat removal) of course.

An electrical and thermal study of submicron GaN multi-finger permeable base transistors (PBTs) is presented. Carrier transport in the intrinsic finger is studied with drift-diffusion and energy balance models, validated against ensemble Monte Carlo simulation; device performances are optimized through an aggressive geometrical scaling. Non-isothermal analysis of the complete multi-finger structure demonstrates the need for a mobility model whose temperature dependence is not confined to low-field parameters. The potential of GaN PBTs for microwave power applications is finally investigated by evaluating the relevant figures of merit.

Multicolor infrared (IR) focal planes are required for high-performance sensor applications. These sensors will require multicolor focal plane arrays (FPAs) that will cover various wavelengths of interest in mid wavelength infrared/long wavelength infrared (MWIR/LWIR) and long wavelength infrared/very long wavelength infrared (LWIR/VLWIR) bands. There has been significant progress in HgCdTe detector technology for multicolor MWIR/LWIR and LWIR/VLWIR FPAs.1–3 Two-color IR FPAs eliminate the complexity of multiple single-color IR FPAs and provide a significant reduction of weight and power in simpler, reliable, and affordable systems. The complexity of a multicolor IR detector MWIR/LWIR makes the device optimization by trial and error not only impractical but also merely impossible. Too many different geometrical and physical variables need to be considered at the same time. Additionally, material characteristics are only relatively controllable and depend on the process repeatability. In this context, the ability of performing “simulation experiments” where only one or a few parameters are carefully controlled is paramount for a quantum improvement of a new generation of multicolor detectors for various applications.

We investigate the various cation substitutional dopants in Ga2O3 for the possibility of p–type conductivity using density functional theory. Our calculations include both standard density functional theory and hybrid functional calculations. We demonstrate that all the investigated dopants result in deep acceptor levels, not able to contribute to the p–type conductivity of Ga2O3. In light of these results, we compare our findings with other wide bandgap oxides and reexamine previous experiments on zinc doping in Ga2O3.

A generalized numerical model for the phonon-assisted optical interband transition based on the Green's function formalism was developed and implemented to investigate optical processes in germanium and silicon media intended for on-chip light emitter and laser applications. High-fidelity full band structures obtained from the empirical pseudopotential method, self-energies, and the corresponding spectral density functions for the phonon-perturbed electron and holes have been computed numerically as a function of strain, temperature, and doping level. Validation has been carried out by showing the model's ability to accurately reproduce the measured temperature dependent absorption coefficient data for both germanium and silicon. Absorption coefficients, radiative recombination rates of germanium and silicon active media were investigated with different biaxial tensile strain, doping concentrations and injection conditions. Furthermore, when the model is employed to compute the optical gain in strained germanium, we find that the use of tensile strain and high injection are the preferable approaches to obtain population inversion. At the same time, strong absorption from the spin-orbit to the heavy-hole band limits the maximum injection density that can be applied. Finally, when applied to study silicon, the proposed model also successfully reproduces the experimentally observed radiative recombination peak due to the two-phonon process.

The high field hole transport in 4H-SiC has been studied using a full band Monte Carlo (MC) simulation model that includes band to band tunneling and allows mixing of the band states during carrier ...

Various forms of carbon based complexes in GaN are studied with first-principles calculations employing Heyd-Scuseria-Ernzerhof hybrid functional within the framework of density functional theory. We consider carbon complexes made of the combinations of single impurities, i.e. $\mathrm{C_N-C_{Ga}}$, $\mathrm{C_I-C_N}$ and $\mathrm{C_I-C_{Ga}}$, where $\mathrm{C_N}$, $\mathrm{C_{Ga}}$ and $\mathrm{C_I}$ denote C substituting nitrogen, C substituting gallium and interstitial C, respectively, and of neighboring gallium/nitrogen vacancies ($\mathrm{V_{Ga}}$/$\mathrm{V_N}$), i.e. $\mathrm{C_N-V_{Ga}}$ and $\mathrm{C_{Ga}-V_N}$. Formation energies are computed for all these configurations with different charge states after full geometry optimizations. From our calculated formation energies, thermodynamic transition levels are evaluated, which are related to the thermal activation energies observed in experimental techniques such as deep level transient spectroscopy. Furthermore, the lattice relaxation energies (Franck-Condon shift) are computed to obtain optical activation energies, which are observed in experimental techniques such as deep level optical spectroscopy. We compare our calculated values of activation energies with the energies of experimentally observed C-related trap levels and identify the physical origins of these traps, which are unknown before., Comment: 17 pages, 15 figures, 12 tables. Replacement for published version

We have numerically analyzed the electromagnetic and electrical characteristics of InAsSb nBn infrared detectors employing a photon-trapping (PT) structure realized with a periodic array of pyramids intended to provide broadband operation. The three-dimensional numerical simulation model was verified by comparing the simulated dark current and quantum efficiency to experimental data. Then, the power and flexibility of the nBn PT design was used to engineer spectrally filtering PT structures. That is, detectors that have a predetermined spectral response to be more sensitive in certain spectral ranges and less sensitive in others.

Processing improvements have facilitated manufacturing reduced pixel dimensions for lattice-matched InGaAs on InP short-wave infrared detectors. Due to its technological maturity, this material system continues to garner attention for low-light level imaging applications. With pixel dimensions smaller than minority carrier diffusion lengths, optimizing array performance by reducing crosstalk from lateral carrier diffusion remains an important design issue. Analytical models, however, have provided limited insight on underlying mechanisms limiting device performance in the conventional planar double heterointerface device. Quantitative modeling provides tools to investigate performance sensitivities and their underlying mechanisms. In this work we develop a three-dimensional numerical simulation for dense P+n In0.53Ga0.47As on InP photo detector focal plane arrays using a conventional planar, back-illuminated structure. We evaluate optical generation with finite-difference time-domain analysis, and model carrier transport in a drift diffusion analysis simultaneously solving the carrier continuity and Poisson equations. Using this model we investigate modulation transfer function variations with pixel pitch and diffused junction geometries for small dimension arrays. By accounting for carrier diffusion effects, these results should provide a benchmark against which to evaluate modulation transfer function contributions from other effects, such as crosstalk attributable to photon recycling.

We have studied the performance of deep-UV avalanche photodetectors based on AlGaN alloys. We have evaluated the carrier multiplication gains and excess noise factors using a model based on recurrence equations and activated ionization coefficients derived from a full-band Monte Carlo model. We find that for devices with Al 0.4 Ga 0.6 N the excess noise factor increases linearly with the gain. When Al 0.6 Ga 0.4 N is used, the excess noise factor is significantly reduced. Finally, from the analysis of the possible device configurations, electric field distribution and its magnitude, we find that the fabrication of these devices will be challenging and alternative approaches should be considered.