Wide Gap Materials Research Articles

Charge propagation dynamics at trapping centers that induce the luminescence of rare-earth dopants in wide-gap materials

For rare-earth doped semiconductors, charge propagation from semiconductors to rare-earths is essential for excitation of optically active dopants. However, the qualitative model that has been widely accepted to describe this process is based upon indirect evidence; in this model, it is believed that trapping and subsequent recombination of the charges at some rare-earth-related defect excite the dopants. In this work, we observe the sequential process directly, and quantify the trapped charge density and relaxation frequency using a photoelectric measurement technique for samarium-doped titanium dioxide with intense visible luminescence under ultraviolet light.

Low-temperature investigation of electronic excitations in wide-gap materials doped with RE3+ or Cr3+ ions

The processes of photon multiplication in insulators have been considered. The luminescence of Tb3+ ions (5 D 3 → 7 F J , 5 D 4 → 7 F J transitions) upon intracenter excitation, the optical excitation of oxyanions, or the formation of separated electrons and holes has been studied for CaSO4 doped with Tb3+ and Na+ ions at 6–9 K. An increase in Tb3+ concentration from 0.2 to 4 at % and transition from single Tb3+-Na+ states to centers that contain two or three terbium ions leads to the redistribution of the luminescence intensities in favor of the 5 D 4 → 7 F J transitions and increase in their efficiency due to the possibility of the cooperative 5 D 3 → 5 D 4 and 7 F 6 → 7 F J transitions and the 4f 75d 1 → 5 D 3 and 7 F 6 → 5 D 4 transitions in the two- and three-terbium centers. Based on the example of MgO single crystals with highly mobile excitons, holes, and electrons, the migration of free excitons and holes toward Cr3+ ions in the crystal bulk and their exit from the bulk to the surface have been revealed at 9 K. Surface losses limit the luminescence quantum yield of MgO:Cr3+, CaSO4:Tb3+, and many other materials.

Ballistic electron emission from quantum-sized nanosilicon diode and its applications

A quantum confinement effect renders silicon a functional wide-gap material with useful functions. For instance, a diode based on nanocrystalline silicon (nc-Si) exhibits characteristic quasi-ballistic emission effects in various media. As means for physical excitation and probing, the applicability to parallel electron beam lithography and high-sensitivity image-pickup has been demonstrated in vacuum. The energetic electron incidence into air and Xe ambient induces negative ion generation by electron attachment into oxygen molecules and vacuum ultraviolet light emission by internal excitation of Xe molecules, respectively. Another effect is that the nc-Si ballistic emitter can supply highly reducing electrons into aqueous and metal-salt solutions without the use of counter electrodes. This is an attractive process that will be applicable to hydrogen generation and thin metal film deposition.

Stimulated Emission from Wide-Gap Semiconductors

Stimulated emission and optical gain problems are reviewed on the basis of analysis of highly photoexcited III-Nitride and ZnO wide-gap materials. Spontaneous and stimulated emission along with optical gain of these materials in UV region is analyzed. The possible mechanisms of inverted population in In-containing materials and ZnO semiconductors are discussed involving mechanisms of dense electron-hole plasma, carrier (exciton) localization, as well as various interaction processes in dense exciton gas.

Calculations of quasi-particle spectra of semiconductors under pressure

Different approximations in calculations of electronic quasi-particle states in semiconductors are compared and evaluated with respect to their validity in predictions of optical properties. The quasi-particle self-consistent GW (QSGW) approach yields values of the band gaps which are close to experiments and represents a significant improvement over single-shot GW calculations using local density approximation (LDA) start wavefunctions. The QSGW approximation is compared to LDA bands for a wide-gap material (CuAlO 2 ) and materials with very small gaps, PbX (X=S, Se, and Te). For wide-gap materials QSGW overestimates the gaps by 0.3―0.8 eV, an error which is ascribed to the omission of vertex corrections. This is confirmed by calculations of excitonic effects, by solving the Bethe-Salpeter equation. The LDA error in predicting the binding energy of the Cu-3d states is examined and the QSGW and LDA + U approximations are compared. For PbX the spinorbit coupling is included, and it is shown that although LDA gives a reasonable magnitude of the gap at L, only QSGW predicts the correct order of the L + 6 and L ― 6 states and thus the correct sign (negative) of the gap pressure coefficient. The pressure-induced gap closure leads to linear (Dirac-type) band dispersions around the L point.

Franck–Hertz effect in cathodo- and photoluminescence of wide-gap materials

Abstract A brief overview of previously obtained and novel data on the manifestations of an analogue of Franck–Hertz effect in photo- and cathodoluminescence of wide-gap inorganic materials is presented. On the example of NaCl:Tl + and MgO:Cr 3+ single crystals, the excitation processes of the luminescence of 6 s 2 Tl + ions and 3 d 3 Cr 3+ ions by 5–15 keV electrons or 5–20 eV photons at 6–420 K have been studied. The rapid processes of the direct energy transfer to Tl + by hot conduction electrons or to Cr 3+ centers by hot electrons and/or hot valence holes have been separated from rapid excitonic and more inertial electron–hole processes.

ダイヤモンドナノ粒子分散膜の熱・電気特性制御と応用

There is an increasing interest in applications of wide-gap materials to high-temperature and high-power electronics. It is necessary to radiate extra heat efficiently through an aluminum heat sink coated with a heat-resistant material with high thermal conductivity. Diamond has the highest thermal conductivity in all materials, which at room-temperature is above 1000 W/mK. However, at high temperatures, diamond films show low adhesion to aluminum substrates due to a large difference in thermal expansion coefficient between diamond and aluminum.

ZnO-MgZnO Quantum-Dot Semiconductor Optical Amplifiers

Abstract: ZnO quantum dot (QD) semiconductor optical amplifier (SOA) is studied theoretically using non-Markovian gain model. SOA performance is then studied by rate equations model. These QD-SOAs have shown to have High gain, low noise figure, and polarization insensitivity. This article appraised patent and patent application related to semiconductor optical amplifier. Keywords: Quantum dot (QD), ZnO, semiconductor optical amplifier (SOA), non-Markovian gain, small-signal gain, rate equations, ground state (GS), excited state (ES), noise figure. 1. INTRODUCTION Zinc oxide (ZnO) has been identified as a promising material for optoelectronics applications ranging from ultra-violet (UV) to red wavelengths due to its large fundamental direct bandgap, high thermal conductivity, high luminous efficiency and mechanical and chemical robustness. ZnO also has an advantage of a large exciton binding energy about 60meV, which assures more efficient excitonic emis-sion at higher temperatures [1, 2]. Various wide-gap semi-conducting materials have been adopted to construct ZnO-related heterostructures. Mg

Thermoelectric Power Under Strong Magnetic Field in Quantum Dots and Quantized Superlattices: Simplified Theory and Relative Comparison

In this paper, we study the thermoelectric power under strong magnetic field (TPSM) in quantum dots (QDs) of nonlinear optical, III-V, II-VI, GaP, Ge, Te, Graphite, PtSb2, zerogap, Lead Germanium Telluride, GaSb, stressed materials, Bismuth, IV-VI, II-V, Zinc and Cadmium diphosphides, Bi2Te3 and Antimony respectively. The TPSM in III-V, II-VI, IV-VI, HgTe/CdTe quantum well superlattices with graded interfaces and effective mass superlattices of the same materials together with the quantum dots of aforementioned superlattices have also been investigated in this context on the basis of respective carrier dispersion laws. It has been found that the TPSM for the said quantum dots oscillates with increasing thickness and decreases with increasing electron concentration in various manners and oscillates with film thickness, inverse quantizing magnetic field and impurity concentration for all types of superlattices with two entirely different signatures of quantization as appropriate in respective cases of the aforementioned quantized structures. The well known expression of the TPSM for wide-gap materials has been obtained as special case for our generalized analysis under certain limiting condition, and this compatibility is an indirect test of our generalized formalism. Besides, we have suggested the experimental method of determining the carrier contribution to elastic constants for nanostructured materials having arbitrary dispersion laws.

Luminescent protection against radiation damage in wide-gap materials

In the wide-gap metal oxides, with E g lower than the formation energy of a pair of Frenkel defects, hot recombination of nonrelaxed electrons and holes cause the creation of defects under the conditions of high-density excitation. Novel manifestations of the fast direct energy transfer by hot electrons to impurity centers (e.g., in Al 2O 3:Sc 3+ and Al 2O 3:Gd 3+) or to intrinsic defects (F and F + centers induced by the previous irradiation of Al 2O 3 with swift heavy ions) have been revealed using the methods of low-temperature cathodo- and photoluminescence. The contribution of the solid-state analogue of the Franck–Hertz effect as well as of the multiplication processes of electronic excitations to the suppression of hot recombination and the increase of radiation resistance of dielectrics has been analysed.

First-principles calculations of x-ray absorption near edge structure and energy loss nearedge structure: present and future

Computational methods for theoretical x-ray absorption near edge structure (XANES)and energy loss near edge structure (ELNES) are classified into a few groups.Depending on the absorption (or excitation) edge, required accuracy and desiredinformation, one needs to select the most suitable method. In this paper, afterproviding a map of available computational methods, some examples of first-principlescalculations of XANES/ELNES for selected wide gap materials are given together withreferences. For ZnO, for example, experimental spectra at three edges, Zn K,L3, and O K, including their orientation dependence, are well reproduced by the supercellcalculations with a core hole. Good agreement between theoretical and experimentalspectra of ZnO alloys can also be seen. Theoretical fingerprints are satisfactorily obtainedin this way. However, there are remaining issues beyond ‘good agreements’ which need tobe solved in the future.

Cathodoluminescence of Actinides in Wide Gap Materials

AbstractSingle crystals and polycrystalline samples of durable actinide host phases such as garnet, zircon, and Ti-pyrochlore doped with varying amounts of U, Pu, Np and Am, were studied by cathodoluminescence (CL) spectroscopy. The following characteristic bands were identified: uranyl-ion [UO2]2+– 495 nm in garnet and pyrochlore; [UO4]2− – 710 nm in garnet; and Am3+ – in garnet and zircon at 550 nm, 614 nm, 740÷750 nm.

Einstein Relation in <I>n</I>-Channel Inversion Layers of Nonlinear Optical, Optoelectronic and Related Materials: Simplified Theory, Relative Comparison and Suggestion for an Experimental Determination

In this paper, we study the Einstein relation for the diffusivity to mobility ratio (DMR) in n-channel inversion layers of non-linear optical materials on the basis of a newly formulated electron dispersion relation by considering their special properties within the frame work of k.p formalism. The results for the n-channel inversion layers of III-V, ternary and quaternary materials form a special case of our generalized analysis. The DMR for n-channel inversion layers of II-VI, IV-VI and stressed materials has been investigated by formulating the respective 2D electron dispersion laws. It has been found, taking n-channel inversion layers of CdGeAs2, Cd(3)AS(2), InAs, InSb, Hg1-xCdxTe, In1-xGaxAsyP1-y lattice matched to InP, CdS, PbTe, PbSnTe, Pb1-xSnxSe and stressed InSb as examples, that the DMR increases with the increasing surface electric field with different numerical values and the nature of the variations are totally band structure dependent. The well-known expression of the DMR for wide gap materials has been obtained as a special case under certain limiting conditions and this compatibility is an indirect test for our generalized formalism. Besides, an experimental method of determining the 2D DMR for n-channel inversion layers having arbitrary dispersion laws has been suggested.

The carrier contribution to the elastic constants in III–V, ternary and quaternary materials in the presence of light waves: Simplified theory, relative comparison and a suggestion for experimental determination

In this paper, we have studied the electronic contribution to the elastic constants for III–V, ternary and quaternary materials in the presence of light waves on the basis of newly formulated electron statistics. It has been found taking n-InAs, n-InSb, n-Hg 1− x Cd x Te and n-In 1− x Ga x As y P 1− y lattice matched to InP, as examples that the elastic constants increase with increasing electron concentration, intensity and wavelength in various manners. The strong dependence of the elastic constants on both the light intensity and wavelength reflects the direct signature of the light waves which is in contrast as compared with the corresponding bulk specimens in the absence of photo-excitation. The well-known results for degenerate wide gap materials in the absence of light waves have been obtained as a special case under certain limiting conditions and this compatibility is the indirect test of our generalized formalism. In this context, we have suggested the experimental method of determining the carrier contribution to the elastic constants for materials having arbitrary carrier energy spectra and our results find six important applications in the regime of photon-assisted transport in modern optoelectronic devices.

4D Electron Diffraction Reveals Correlated Unidirectional Behavior in Zinc Oxide Nanowires

The confined electronic structure of nanoscale materials has increasingly been shown to induce behavior quite distinct from that of bulk analogs. Direct atomic-scale visualization of nanowires of zinc oxide was achieved through their unique pancake-type diffraction by using four-dimensional (4D) ultrafast electron crystallography. After electronic excitation of this wide-gap photonic material, the wires were found to exhibit colossal expansions, two orders of magnitude higher than that expected at thermal equilibrium; the expansion is highly anisotropic, a quasi-one-dimensional behavior, and is facilitated by the induced antibonding character. By reducing the density of nanowires, the expansions reach even larger values and occur at shorter times, suggesting a decrease of the structural constraint in transient atomic motions. This unanticipated ultrafast carrier-driven expansion highlights the optoelectronic consequences of nanoscale morphologies.

Inertia of the bound-electron Kerr-type optical nonlinearity in transparent solids

The inertia of the Kerr-type nonlinear-optical response of bound electrons in solids is analyzed. It is shown that, in the case of wide-gap materials, away from the mid-gap frequency Eg/2ħ, Eg being the band gap, the inertia of this response becomes noticeable only for attosecond pulse features. However, near Eg/2ħ, where the inertia of bound-electron Kerr-type optical nonlinearity is enhanced by a strong dispersion of two-photon absorption and where the nonlinear refraction is especially strong, light pulses as long as tens and even hundreds of femtoseconds are expected to experience self-steepening and an asymmetric spectral broadening due to the retarded Kerr effect.

Photon emission induced by elastic exciton-carrier scattering in semiconductor quantum wells

We present a study of the elastic exciton-electron (X-e−) and exciton-hole (X-h) scattering processes in semiconductor quantum wells, including fermion exchange effects. The balance between the exciton and the free carrier populations within the electron-hole plasma is discussed in terms of ionization degree in the nondegenerate regime. Assuming a two-dimensional Coulomb potential statically screened by the free carrier gas, we apply the variable phase method to obtain the excitonic wavefunctions, which we use to calculate the 1s exciton-free carrier matrix elements that describe the scattering of excitons into the light cone where they can radiatively recombine. The photon emission rates due to the carrierassisted exciton recombination in semiconductor quantum-wells (QWs) at room temperature and in a low density regime are obtained from Fermi’s golden rule, and studied for mid-gap and wide-gap materials. The quantitative comparison of the direct and exchange terms of the scattering matrix elements shows that fermion exchange is the dominant mechanism of the exciton-carrier scattering process. This is confirmed by our analysis of the rates of photon emission induced by electron-assisted and hole-assisted exciton recombinations.

Specific features of use of wide-gap semi-insulating materials for recording of nuclear radiation

Operation of detectors in strong radiation fields or an imperfect fabrication technology gives rise to a considerable amount of structural defects in materials. Using a p +-v-n + detector structure in the unconventional forward-bias mode has been considered as a way to minimize the space-charge accumulation caused by long-term capture of carriers to deep levels. The efficiency of carrier transport and the signal amplitude spectrum were studied as functions of the applied voltage and time of signal shaping by the electronic circuit for the example of a CVD-grown 4H-SiC film containing up to 2.5 × 1017 cm−3 of primary knocked-out atoms. The detector was tested with 5.4 MeV α particles in the temperature range 20–140°C. It was demonstrated that, despite the transport of only one-fourth of the whole amount of charge, the main line of the spectrum has a high energy resolution (7%) for the given conditions.

Influence of light on the Debye screening length in ultrathin films of optoelectronic materials

In this paper, we study the Debye screening length (DSL) in ultrathin films of optoelectronic materials in the presence of light waves. The solution of the Boltzmann transport equation on the basis of the newly formulated electron dispersion laws will introduce new physical ideas and experimental findings under different external conditions. It has been found, taking ultrathin films of n-Hg 1− x Cd x Te, as an example, that the respective two-dimensional (2D) DSL in the aforementioned materials exhibits decreasing quantum step dependence with the increasing film thickness, surface electron concentration, light intensity and wavelength, respectively, with different numerical values. The nature of the variations is totally band structure dependent which is influenced by the presence of the different energy band constants. The strong dependence of the 2D DSL on both the light intensity and the wavelength reflects the direct signature of the light waves. The well-known result for the 2D DSL for nondegenerate wide gap materials in the absence of any field has been obtained as a special case of the present analysis under certain limiting conditions and this compatibility is the indirect test of our generalized formalism. Besides, we have suggested an experimental method of determining the 2D DSL in ultrathin materials in the presence of light waves having arbitrary dispersion laws.

Influence of Light on the Einstein Relation in Ultra-Thin Films of III–V, Ternary and Quaternary Materials: Simplified Theory, Relative Comparison and Suggestion for Experimental Determination

In this paper, we study the Einstein relation for the diffusivity-mobility ratio (DMR) in ultra-thin films of III-V, ternary and quaternary materials in the presence of light waves, whose unperturbed energy band structures are defined by the three-band and two band models of Kane together with parabolic energy bands. The solution of the Boltzmann transport equation on the basis of the newly formulated electron dispersion laws will introduce new physical ideas and experimental findings under different external conditions. It has been found, taking ultrathin films of n-InAs, n-InSb, n-Hg1-xCdxTe,n-In1-xGaxAsyP1-y lattice matched to InP, as examples, that the respective DMR in the aforementioned materials exhibits decreasing quantum step dependence with the increasing film thickness, decreasing electron statistics, increasing light intensity and wavelength, with different numerical values. The DMR decreases with increasing alloy composition and the nature of the variations are totally band structure dependent which is influenced by the presence of the different energy band constants. The strong dependence of the DMR on both the light intensity and the wavelength reflects the direct signature of the light waves which is in contrast as compared with the corresponding ultrathin films of the said materials in the absence of photo-excitation. The well-known result for the DMR for degenerate wide gap materials in the absence of any field has been obtained as a special case of the present analysis under certain limiting conditions and this compatibility is the indirect test of our generalized formalism. Besides, we have suggested an experimental method of determining the DMR in ultrathin materials in the presence of light waves having arbitrary dispersion laws.