Wide Gap Materials Research Articles

Multi-axis decomposition of density functional program for strong scaling up to 82,944 nodes on the K computer: Compactly folded 3D-FFT communicators in the 6D torus network

Density functional calculations with a plane-wave basis set are widely used in materials science. Due to recent developments in high-performance computers, the number of nodes equipped in such computers greatly exceeds the number of atoms included in a typical simulation. Thus, it is becoming difficult to perform calculations efficiently even when only a portion of all nodes are used (e.g., 10%). We have developed a multi-axis decomposition scheme in which both G-vectors and band axes are decomposed and 3D-FFT communicators are folded compactly. This proposed scheme retains the inner-most do-loop lengths sufficiently long and restrains the increased MPI communication costs as the number of nodes increases. In an investigation of a wide-gap semiconductor material (SiC), our PHASE/0 DFT code exhibits efficient and strong scaling (up to 82,944 nodes) even for a relatively small system with 3848 atoms, and demonstrates maximum peak performance of 2.25 PFLOPS for a 25,200-atom system despite employing 3D-FFT.

Mechanism for Forming Quantum-Size AlGaN/GaN/InGaN/GaN Heterostructure Layers

The use of silicon in optoelectronic elements and microwave devices is limited by its electrical properties. In this range of application, silicon is replaced by wider-gap materials, e.g., GaN, AlN, InN, and solid solutions based on them. This allows highly efficient devices, including light-emitting diodes and photodetectors operating in a wide radiation spectral range, to be fabricated. In addition, GaN-based materials have been successfully used in designing high-power microwave devices, in particular, transistors with highly mobile electrons operating at high temperatures. We discuss specific features of the technology for fabricating active layers in the AlGaN/GaN/InGaN/GaN heterostructures with the use of metalorganic compounds. The initial substances used are ultra-pure ammonia NH3 and gallium, aluminum, and indium metalorganic compounds in the trimethyl form. The temperature dependence of the epitaxial GaN layer growth rate is investigated. This dependence is shown to be weak in a wide temperature range. The importance of adsorption processes occurring on the growth surfaces is confirmed. Thermodynamical analysis of the regularities of the investigated process is carried out to simulate the process and establish the conditions for forming GaN-based solid solutions. It is shown that during the formation of Ga1 – xInxN solid solutions in the high-temperature region the attainable InN content is not higher than 0.4 mole fractions. As the growth temperature decreases to 600°С, the conditions for incorporating In in the solid solution are noticeably improved and the InN concentration increases to 0.9 mole fractions. It is demonstrated that growing the GaAlN solid solutions in a wide temperature range makes it possible to obtain the AlN content ranging from 0.1 to 0.9 mole fractions. The experimental investigations are confirmed by the calculations. Therefore, when growing the Ga1 – xInxN quantum well layers in the active heterostructural area for commercial blue light-emitting diode chips with an indium content of х = 0.1–0.15, the growth temperature should be reduced. At low temperatures, however, it is difficult to grow epitaxial layers with a high crystal quality.

Electrical Characteristics of Solder-Free SiC Die/Metal Foil/AlN Plate Junctions Fabricated Using Surface Activated Bonding

In conventional power device modules, Si-based dies are attached to metal layers on ceramic plates by using solders. Given that power devices made of widegap materials such as SiC and GaN are going to be launched into market, packaging technologies that exploit their superiority are strongly sought for. Solder-free modules, i.e., modules with dies directly bonded to metal layers on ceramic plates are likely to be ideal since fracture in the solder layer is one of the essential factors that limit the thermal tolerance of conventional power modules. In this work, we fabricated an SiC die/metal/ceramic plate junction by using surface activated bonding (SAB) and examined its current-voltage (I-V) characteristics at elevated temperatures. As ceramics AlN was used since the thermal expansion coefficient of AlN is close to that of SiC. The metal layer on AlN plate was made of an Al foil.We prepared a 635-μm-thick AlN ceramic plate and a 30-μm-thick Al foil. The average roughness (Ra) of the surface in AlN plate was ∼0.5 μm. Ra of the Al foil was 0.06 and 0.19 μm in its mirror and matte surfaces, respectively. The matte surface of Al foil was directly bonded to the AlN plate through a surface activation process using an Ar fast atom beam. In a preparatory study we fabricated an AlN/Al foil/AlN junction by bonding AlN plates to the both sides of an Al foil. We observed the cross section of AlN/Al foil/AlN junction by using scanning electron microscopy. We found that the AlN plate and Al foil were bonded to each other without gaps at the both AlN/Al interfaces although Ra of AlN plates and Al foils was much larger than typical Ra of semiconductor substrates that were bonded using SAB (≤~1 nm). We prepared an SiC epi substrate by growing an n+-SiC buffer layer and an n--SiC epi layer on an n+-4H-SiC (0001) substrate. We formed an ohmic contact on the backside of the epi substrate by evaporating an Al/Ni/Au and annealing at 1000 °C. Then we made a 300-μm-ϕ Ni/Au Schottky contact on the top by evaporation and lift-off. An Al layer was evaporated on the top of ohmic contact so that an SiC Schottky diode die was fabricated. Then the Al surface of ohmic contact of SiC die was bonded to the mirror surface of Al foil on AlN plate using SAB. Note that no intermediate layers such as solder were inserted at bonding interfaces.We measured the I-V characteristics between the Schottky contact of SiC die and the surface of Al foil at temperatures between room temperature and 300 °C. A reverse-bias current was ~10-7 A/cm2 at a reverse-bias voltage up to 6 V for temperatures lower than 250 °C. A larger reverse-bias current (~10-6 A/cm2) was observed in measurement at 300 °C. In the forward characteristics, lower turn-on voltages and larger series resistances were observed for higher temperatures. It is notable that these features are typically observed for Schottky diodes. In addition, no change was observed in appearance of the SiC die/Al foil/AlN junction even after the measurement at 300 ℃. Although the bonding strength in Al foil/AlN interfaces is still unclarified, these results suggest that SAB technologies could be useful for realizing solder-free modules of power devices.Acknowledgment—The Al foil employed in the work was supplied from Toyo Aluminium K.K. Figure 1

Doubly Screened Hybrid Functional: An Accurate First-Principles Approach for Both Narrow- and Wide-Gap Semiconductors.

First-principles prediction of electronic band structures of materials is crucial for rational material design, especially in solar-energy-related materials science. Hybrid functionals that mix the Hartree-Fock exact exchange with local or semilocal density functional approximations have proven to be accurate and efficient alternatives to more sophisticated Green's function-based many-body perturbation theory. The optimal fraction of the exact exchange, previously often treated as an empirical parameter, is closely related to the screening strength of the system under study. From a physical point of view, the screening has two extreme forms: the dielectric screening [1/ϵM] that is dominant in wide-gap materials and the Thomas-Fermi metallic screening [exp(-ζ r) ] that is important in narrow-gap semiconductors. In this work, we have systematically investigated the performances of a nonempirical doubly screened hybrid (DSH) functional that considers both screening mechanisms and found that it excels all other existing hybrid functionals and describes the band gaps of narrow-, medium-, and wide-gap insulating systems with comparably good performances.

Assembly of a Two-Fold Interpenetrated Three-Dimensional Metal–Organic Framework by Using the Template Function of Keggin Anions

A metal–organic framework [Co2(btp)3(GeMo12O40)] (1) (btp = 1,3-bis-(1,2,4-triazol-1-yl)propane) has been constructed by using the template function of the Keggin anions GeMo12O404−. Single-crystal X-ray analysis reveals that the structure exhibits two-fold interpenetrated 3D host metal–organic framework constructed from cobalt(II) and btp linkers and the voids of which are occupied by Keggin anions. The optical band gap of 1 indicates that it is potential wide-gap semiconductive material. The photocatalytic activity of compound 1 in degradation of MB under visible light and UV light irradiation are also investigated.

Comparative study of perovskite-type scintillator materials CsCaI3 and KCaI3 via first-principles calculations

Several members of a large family of perovskite-like halides with a common chemical formula, ABX3 (A = monovalent, B = divalent, and X = halogen ion), are being investigated for their interesting properties and potential technological applications. CsCaI3 and KCaI3 are two such ionic compounds who are of interest in the quest for superior and cost-effective alternatives to NaI or CsI based scintillators. They are the subject of this first-principles based computational study. Both are wide-gap materials having primarily I 5p and Ca 3d characters near the valence and conduction band edges, respectively. Although built from [CaI6] octahedral motifs, structural differences between the two compounds is reflected in anisotropic electron effective mass and distinctive formation and migration of self-trapped holes. We discuss these properties as they relate to scintillation decay and proportional light yield.

Tutorial: Junction spectroscopy techniques and deep-level defects in semiconductors

The term junction spectroscopy embraces a wide range of techniques used to explore the properties of semiconductor materials and semiconductor devices. In this tutorial review, we describe the most widely used junction spectroscopy approaches for characterizing deep-level defects in semiconductors and present some of the early work on which the principles of today's methodology are based. We outline ab-initio calculations of defect properties and give examples of how density functional theory in conjunction with formation energy and marker methods can be used to guide the interpretation of experimental results. We review recombination, generation, and trapping of charge carriers associated with defects. We consider thermally driven emission and capture and describe the techniques of Deep Level Transient Spectroscopy (DLTS), high resolution Laplace DLTS, admittance spectroscopy, and scanning DLTS. For the study of minority carrier related processes and wide gap materials, we consider Minority Carrier Transient Spectroscopy (MCTS), Optical DLTS, and deep level optical transient spectroscopy together with some of their many variants. Capacitance, current, and conductance measurements enable carrier exchange processes associated with the defects to be detected. We explain how these methods are used in order to understand the behaviour of point defects and the determination of charge states and negative-U (Hubbard correlation energy) behaviour. We provide, or reference, examples from a wide range of materials including Si, SiGe, GaAs, GaP, GaN, InGaN, InAlN, and ZnO.

Two B-C-O Compounds: Structural, Mechanical Anisotropy and Electronic Properties under Pressure.

The structural, stability, mechanical, elastic anisotropy and electronic properties of two ternary light element compounds, B2CO2 and B6C2O5, are systematically investigated. The elastic constants and phonon calculations reveal that B2CO2 and B6C2O5 are both mechanically and dynamically stable at ambient pressure, and they can stably exist to a pressure of 20 GPa. Additionally, it is found that B2CO2 and B6C2O5 are wide-gap semiconductor materials with indirect energy gaps of 5.66 and 5.24 eV, respectively. The hardness calculations using the Lyakhov-Oganov model show that B2CO2 is a potential superhard material. Furthermore, the hardness of B6C2O5 is 29.6 GPa, which is relatively softer and more easily machinable compared to the B2CO2 (41.7 GPa). The elastic anisotropy results show that B6C2O5 exhibits a greater anisotropy in the shear modulus, while B2CO2 exhibits a greater anisotropy in Young’s modulus at ambient pressure.

Parametric Amplification of Phase‐Locked Few‐Cycle Pulses and Ultraviolet Harmonics Generation in Solids at High Repetition Rate

AbstractA high‐power femtosecond Yb:fiber system is seeded by a phase‐locked Er:fiber source and drives an ultra‐broadband optical parametric amplifier that operates at 10 MHz repetition rate. The resulting pulses display precise control of the carrier‐envelope phase. Their 8.3 fs temporal duration corresponds to 2.3 optical cycles of the 1100 nm carrier wavelength. Focusing 200 nJ of pulse energy into widegap materials generates optical harmonics up to fifth order. Even in a perturbative regime, strong effects of the carrier‐envelope phase on the emitted spectra are observed.

Microcrystalline-Silicon Solar Cells With Photonic Crystals on the Top Surface

We investigate microcrystalline-silicon (μc-Si) solar cells with photonic crystals on the top surface, which exploit the large-area resonant effect in photonic crystals to enhance light absorption, for the first time. Guidelines for designing the surface photonic crystals are discussed, showing the importance of oblique leakage loss suppression. The influence of direct surface etching of the intrinsic μc-Si layer (or photovoltaic layer) is studied, and it is shown that unbalanced etching of the amorphous and crystalline components of the μc-Si layer deteriorates the electronic performance, in particular, the open-circuit voltage and fill factor. We then introduce photonic crystals under an adequate etching condition, and confirm that the light absorption (or the external quantum efficiency) is successfully enhanced by improving the incident light coupling to the resonant modes. Finally, we fabricate a few-micrometer-thick μc-Si solar cell with a photonic crystal on the top surface and demonstrate an active-area efficiency of 11%, resulting from enhancement of the short-circuit current density, while simultaneously suppressing the deterioration of open-circuit voltage and fill factor. The obtained efficiency is the highest reported for a category of μc-Si solar-cell in which all the n/i/p layers are composed of Si, excluding wide-gap materials such as silicon oxide.

Cathodoluminescnece study of pn‐junctions in widegap materials using cross sectional polishing

Precise doping control is of critical importance in power device fabrication using widegap materials. In this study, we characterize pn‐junctions in GaN and SiC by cathodoluminescence. To improve spatial resolution, the specimens were polished using cross‐sectional polisher. GaN specimens were successfully characterized with line profile of CL spectra across the cross‐section. On the other hand, CL data could not be used straightforwardly for characterization of pn‐junctions in the case of SiC.

Optimization of structural and growth parameters of metamorphic InGaAs photovoltaic converters grown by MOCVD

Metamorphic Ga0.76In0.24As heterostructures for photovoltaic converters are grown by the MOCVD (metal–organic chemical vapor deposition) technique. It is found that, due to the valence-band offset at the p-In0.24Al0.76As/p-In0.24Ga0.76As (wide-gap window/emitter) heterointerface, a potential barrier for holes arises as a result of a low carrier concentration in the wide-gap material. The use of an InAlGaAs solid solution with an Al content lower than 40% makes it possible to raise the hole concentration in the widegap window up ~9 × 1018 cm–3 and completely remove the potential barrier, thereby reducing the series resistance of the device. The parameters of an GaInAs metamorphic buffer layer with a stepwise In content profile are calculated and its epitaxial growth conditions are optimized, which improves carrier collection from the n-GaInAs base region and provides a quantum efficiency of 83% at a wavelength of 1064 nm. Optimization of the metamorphic heterostructure of the photovoltaic converter results in that its conversion efficiency for laser light with a wavelength of 1064 nm is 38.5%.

Nonlinear Interaction of Electronic Excitations Created at High-Densities in Luminescent Materials and Scintillators

Nonlinear interaction of electronic excitations has been shown to play a remarkable role in luminescent material performance. Recently, it has been intensely studied in semiconductors and wide-gap scintillators. The nonlinear interaction is a cause of the nonproportionality of scintillator response to the energy of the absorbed ionizing radiation and can limit its energy resolution, which results in crucial deterioration of the scintillator functionality. The type and degree of nonproportionality are material specific and depend little on impurity content or sample treatment. Theoretical models have been developed, which explain the effect of scintillator response nonproportionality by the interplay between the density and mobility of charge carriers created in a track of ionizing radiation [1-3]. Energy losses in relaxation processes and the final distribution of excited luminescence centres in a material are determined by a number of processes including thermalisation of hot carriers, their recombination, interaction, trapping at defects and impurities and exciton formation. In the present contribution, we demonstrate how the mutual interaction of small-radius Frenkel excitons created at high densities by ionizing radiation or intense laser pulses can reveal itself in the luminescence characteristics of a luminescent material. It is shown that the related phenomena observed in various materials under X or γ rays are similar to those detected under UV-XUV excitation, which allows their theoretical description proceeding from the same principles and their experimental study in substantially better controlled experimental conditions of excitation by ultrashort laser pulses with well-defined timing and spatial parameters. A remarkable effect of saturation of excitonic absorption was discovered under excitation by ultrashort UV pulses from the energy region of phonon-assisted electronic transitions. The changes in the luminescence decay kinetics and various saturation effects in excitonic and impurity luminescence induced by high-density or high-energy excitation are demonstrated and theoretically modelled for several wide-gap materials. The experimental results of this contribution originate mainly from the study of time-resolved luminescence in tungstate crystals of different structures, wolframites CdWO4 and ZnWO4, and homologous scheelites CaWO4, SrWO4, BaWO4, by using femtosecond UV-XUV sources such as free-electron lasers, tuneable femtosecond lasers, high-order harmonic generation systems. It is shown that the emission decay and saturation can be described within the frame of Förster’s dipole-dipole interaction model on the basis of theoretical statements developed in [1]. The importance of studying exciton-exciton interactions is demonstrated by revealing a direct link between the Förster interaction radius and nonproportionality of the material light response. A good correlation between the radius of cation, radius of exciton and radius of Förster interaction is shown for the sequence of homologous scheelites CaWO4, SrWO4, BaWO4.

Harnessing Defect-Tolerance at the Nanoscale: Highly Luminescent Lead Halide Perovskite Nanocrystals in Mesoporous Silica Matrixes.

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as a novel class of bright emitters with pure colors spanning the entire visible spectral range. Contrary to conventional quantum dots, such as CdSe and InP NCs, perovskite NCs feature unusual, defect-tolerant photophysics. Specifically, surface dangling bonds and intrinsic point defects such as vacancies do not form midgap states, known to trap carriers and thereby quench photoluminescence (PL). Accordingly, perovskite NCs need not be electronically surface-passivated (with, for instance, ligands and wider-gap materials) and do not noticeably suffer from photo-oxidation. Novel opportunities for their preparation therefore can be envisaged. Herein, we show that the infiltration of perovskite precursor solutions into the pores of mesoporous silica, followed by drying, leads to the template-assisted formation of perovskite NCs. The most striking outcome of this simple methodology is very bright PL with quantum efficiencies exceeding 50%. This facile strategy can be applied to a large variety of perovskite compounds, hybrid and fully inorganic, with the general formula APbX3, where A is cesium (Cs), methylammonium (MA), or formamidinium (FA), and X is Cl, Br, I or a mixture thereof. The luminescent properties of the resulting templated NCs can be tuned by both quantum size effects as well as composition. Also exhibiting intrinsic haze due to scattering within the composite, such materials may find applications as replacements for conventional phosphors in liquid-crystal television display technologies and in related luminescence down-conversion-based devices.

Low-temperature radiation effects in wide gap materials

Low-temperature radiation effects in wide gap materials

Radiation resistance diagnostics of wide-gap optical materials

Novel approach in the detection of radiation damage created by ion beams in optical materials was demonstrated. Protons of the energy of 100keV and fluence of 1017cm−2 create sufficient amount of crystal lattice defects in the thin surface layer for testing of optical materials needed for future fusion reactors. These structural defects can be detected and analysed using the spectra of cathodoluminescence excited in the irradiated layer by an electron beam with adjustable energy. The method was verified by the enhanced intensity of F-type luminescence that reflects the creation of radiation-induced oxygen vacancies in MgO and Al2O3 crystals. Low radiation resistance of nominally pure (Lu1-xGdx)2SiO5 crystals was demonstrated by almost total suppression of intrinsic luminescence after the same irradiation.

Conditions for beneficial coupling of thermoelectric and photovoltaic devices

Abstract

Fabrication of High-Aspect-Ratio Structural Change Microregions in Silicon Carbide by Femtosecond Bessel Beams

This letter presents the morphology of femtosecond Bessel beams induced high-aspect-ratio structural change regions in bulk silicon carbide. An axion is engaged in transforming Gaussians beams to Bessel beams, which are then focused on the surface or below the surface of the sample by combination of a plano-convex lens and a microscope objective. The sample is scanned by the focused femtosecond Bessel beams at the preset patterns. Through this method, the high-aspect-ratio uniform laser induced structure change regions have been produced and the highest respect ratio can reach 206 with the depth of 330 μm, the width of 1.6 μm in optimized conditions of appropriate focusing position and pulse energy. This result is attributed to uniform energy distribution in the long propagation distance of Bessel beams with nondiffracting. This technique will have great potential applications to make high-aspect-ratio microgrooves in wide-gap and transparent materials.

First-principles study of codoping in lanthanum bromide

Co-doping of Ce-doped LaBr$_3$ with Ba, Ca, or Sr improves the energy resolution that can be achieved by radiation detectors based on these materials. Here, we present a mechanism that rationalizes of this enhancement that on the basis of first principles electronic structure calculations and point defect thermodynamics. It is shown that incorporation of Sr creates neutral $V_\text{Br}$-Sr$_\text{La}$ complexes that can temporarily trap electrons. As a result, Auger quenching of free carriers is reduced, allowing for a more linear, albeit slower, scintillation light yield response. Experimental Stokes shifts can be related to different Ce$_\text{La}$-Sr$_\text{La}$-$V_\text{Br}$ triple complex configurations. Co-doping with other alkaline as well as alkaline earth metals is considered as well. Alkaline elements are found to have extremely small solubilities on the order of 0.1 ppm and below at 1000 K. Among the alkaline earth metals the lighter dopant atoms prefer interstitial-like positions and create strong scattering centers, which has a detrimental impact on carrier mobilities. Only the heavier alkaline earth elements combine matching ionic radii with sufficiently high solubilities. This provides a rationale for the experimental finding that improved scintillator performance is exclusively achieved using Sr, Ca, or Ba. The present mechanism demonstrates that co-doping of wide gap materials can provide an efficient means for managing charge carrier populations under out-of-equilibrium conditions. In the present case dopants are introduced that manipulate not only the concentrations but the electronic properties of intrinsic defects without introducing additional gap levels. This leads to the availability of shallow electron traps that can temporarily localize charge carriers, effectively deactivating carrier-carrier recombination channels. The principles of this ... [continued]

Nondestructive imaging of buried interfaces in SiC and GaN Schottky contacts using scanning internal photoemission microscopy

We demonstrate a nondestructive characterization of buried interfaces in metal/wide-bandgap semiconductor contacts by using scanning internal photoemission microscopy. For Ni/n-SiC contacts annealed at temperatures above 400 °C, a reduction of the Schottky barrier height owing to partial interfacial reaction was visualized. In Au/Ni/n-GaN contacts, upon annealing at 400 °C, thermal degradation from a scratch on the dot was observed. Forward current–voltage curves were reproduced by lowering the Schottky barrier height and the area of the reacted regions by using this method. The present imaging method exploits its nondestructive highly sensitive extinction for characterizing the contacts formed on wide-gap materials.