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In order to study the influence of interstitial point defects on the optical properties of Ga 0.75 Al 0.25 N, three Ga 0.75 Al 0.25 N bulk models with Ga, Al, and N interstitial defects were built. And the band structure, dielectric function, complex refractive index, absorption coefficient, reflectivity, and loss function of pure Ga 0.75 Al 0.25 N and the Ga 0.75 Al 0.25 N with interstitial defects were calculated based on first principles. Results show that the band gap values of Ga 0.75 Al 0.25 N with interstitial defects are smaller than that of the pure Ga 0.75 Al 0.25 N. And Ga 0.75 Al 0.25 N crystals with Ga and Al interstitial defects show n-type property, meanwhile Ga 0.75 Al 0.25 N crystal with N interstitial defect show p-type property. The static dielectric constants of crystals with defects are much bigger than that of the pure one. And the metal reflective regions of the crystals shift to lower energy because of the interstitial defects. [ABSTRACT FROM AUTHOR]

As advanced semiconductor materials, GaN, AlN, and GaAlN alloys are widely used in optoelectronic devices. In order to research the optoelectronic properties of GaN, AlN, and Ga 1− x Al x N with different Al component, models of GaN, Ga 0.875 Al 0.125 N, Ga 0.750 Al 0.250 N, Ga 0.625 Al 0.375 N, Ga 0.500 Al 0.500 N, and AlN were built, then the atomic structure, band structure, density of states, Mulliken charge distribution, and optical properties of the six crystals were calculated based on first principle calculations. Results show that the lattice parameters decrease while the band gap increases with the increase of Al component. The GaN, AlN, and GaAlN alloys are all semiconductors with direct band gap. The global transfer index increases with the increase of Al component. When the Al component is bigger and bigger, the absorption peak shifts to higher energy, and the threshold wavelength of Ga 1− x Al x N decreases. [ABSTRACT FROM AUTHOR]

In order to study the nonpolar Ga 0.75 Al 0.25 N surfaces theoretically, models of Ga 0.75 Al 0.25 N(1,0,−1,0) and (1,1,−2,0) surfaces are built. Surface morphology, surface energy, work function, band structure, density of state, and optical properties of the two nonpolar surfaces are calculated using first principle based on density function theory (DFT). Results show that the N atoms and Ga(Al) atoms are not in the same height for both Ga 0.75 Al 0.25 N(1,0,−1,0) and (1,1,−2,0) surfaces. The Ga N and Al N bonds rotate with the N atoms outwards, and the length of Ga N bond and Al N bond decrease compared with that of bulk. The surface energy values of Ga 0.75 Al 0.25 N(1,0,−1,0) and (1,1,−2,0) surfaces are 1.902 J/m 2 and 2.014 J/m 2 respectively. The work function values of Ga 0.75 Al 0.25 N(1,0,−1,0) and (1,1,−2,0) surfaces are 4.025 eV and 4.007 eV respectively. The absorption coefficients of nonpolar surfaces are lower than that of bulk Ga 0.75 Al 0.25 N. This work provides a theoretical basis for the design and preparation of Ga 1− x Al x N photocathodes. [ABSTRACT FROM AUTHOR]

In order to design the optimal component structure of transmission-mode (t-mode) Ga 1− x Al x N photocathode, the optical properties and quantum efficiency of Ga 1− x Al x N photocathodes are simulated. Based on thin film principle, optical model of t-mode Ga 1− x Al x N photocathodes is built. And the quantum efficiency formula is put forward. Results show that Ga 1− x Al x N photocathodes can satisfy the need of detectors with “solar blind” property when the Al component is bigger than 0.375. There is an optimal thickness of Ga 1− x Al x N layer to get highest quantum efficiency, and the optimal thickness is 0.3 μm. There is close relation between absorptivity and quantum efficiency, which is in good agreement with the “three-step” model. This work gives a reference for the experimental research on the Ga 1− x Al x N photocathodes. [ABSTRACT FROM AUTHOR]

Highlights: [•] Optoelectronic properties of Mg doped and Mg–H doped GaAlN were calculated. [•] The formation energy of Mg–H complex is smaller than that of Mg impurity. [•] Mg doping makes the crystal turn into p-type degeneracy semiconductor. [•] Mg doping enhances the absorption coefficient at the range of 1.05–3.47eV. [ABSTRACT FROM AUTHOR]

Abstract: In order to compare optical properties of Wurtzite and zinc-blende Ga0.75Al0.25N and lay a foundation for preparation of Ga0.75Al0.25N photocathodes, absorption coefficient, complex refractive index, dielectric function, reflectivity and loss function of Ga0.75Al0.25N in two forms are calculated using first principle based on density functional theory. Results show that zinc-blende Ga0.75Al0.25N owns smaller band gap than Wurtzite Ga0.75Al0.25N, and its conduction band is more broad. The optical properties differences of two structures mainly occur at the range of 8.6–26.0eV. The peaks of imaginary dielectric function and absorption curves are at higher energy point for zinc-blende Ga0.75Al0.25N. The highest absorption of Wurtzite Ga0.75Al0.25N is 351386.171/cm at 11.05eV, which is smaller than zinc-blende of 437809.895/cm at 13.84eV. The average reflectivity of Wurtzite Ga0.75Al0.25N is lower than that of zinc-blende Ga0.75Al0.25N. The results are conductive for designing component structures of Ga0.75Al0.25N photocathodes. [Copyright &y& Elsevier]

Highlights: [•] We employ first principle to calculate two types of Ga0.75Al0.25N (001) reconstruction surfaces. [•] The work function values of (2×2) and c(2×2) reconstruction surfaces are 3.084eV and 3.431eV, respectively. [•] The Ga0.75Al0.25N c(2×2) reconstruction surface is more stable than the Ga0.75Al0.25N (2×2) reconstruction surface. [•] The absorption coefficients of the surfaces are smaller than that of the bulk. [ABSTRACT FROM AUTHOR]

Highlights: [•] We employ first principle to calculate systems of Cs adsorption on Ga(Mg)0.75Al0.25N (0001) surface. [•] Cs adsorption on Ga(Mg)0.75Al0.25N (0001) surface process is an exothermic chemical process, and the adsorption system is stable. [•] Cs adsorption on Ga(Mg)0.75Al0.25N (0001) surface reduces the work function of the surface and forms the structure of p-type bulk with n-type surface. [•] The absorption coefficient of Cs adsorption system attenuates seriously compared with the clean surface. [ABSTRACT FROM AUTHOR]

Abstract: The band structure, density of states and optical properties of zinc-blende GaN are calculated systematically by using the first-principles plane-wave pseudopotential method, based on the density function theory. Results show that zinc-blende GaN is a typical direct band gap semiconductor. The band gap is 1.496eV, the valence bands are attributed to Ga3d, N2s and N2p electronic states, and the conduction bands are attributed to Ga4s and Ga4p electronic states. The electrical transport properties and types of carriers of GaN are attributed to N2p and Ga4s electronic states near the Fermi level. The static dielectric constant ɛ1(0)=3.9065eV, the refractive index , and the maximum peak of absorption coefficient is 354655.6cm−1. The dielectric function, refractive index, absorption spectra, reflective spectra, optical conductivity and energy loss function are calculated using band structure and density of states. This provides a theoretical basis for the design and application of GaN opto-electronic materials. [Copyright &y& Elsevier]

Abstract: In order to well study the influence of the thickness and doping concentration on optical properties of transmission-mode GaAs photocathode, three exponential-doping and one uniform-doping photocathode modules were prepared by molecular beam epitaxy with a structure of glass/Si3N4/Ga1− x Al x As/GaAs. By use of the spectrophotometer, the optical properties were separately measured including the reflectivity and transmissivity curves. Based on thin film optical principles, the optical properties and their integral values are calculated by matrix formula for the four-layer photocathode module. The result shows that the antireflection and window layers affect the peak and valley of the optical property curves and the active layer influences the absorptivity values of the transmission-mode cathode modules. The photocathode module has high absorptivity within the response waveband when the optimal module has the Si3N4 antireflection layer of 0.1μm, the Ga1− x Al x As window layer of more than 0.4μm, and the GaAs active layer of 1.5μm–2μm and low average doping concentration. [Copyright &y& Elsevier]

In 0.53 Ga 0.47 As is a new near-infrared material for negative electron affinity photocathodes. Geometry optimization combined generalized gradient approximation based on the density functional theory is adopted to calculate the bulk In 0.53 Ga 0.47 As. Lattice constant of bulk In 0.53 Ga 0.47 As after geometry optimization is compared with the theoretical one based on the Vigard law, which proves the reliability of the parameters of the geometry optimization. The energy bands, electronic structures and optical properties of the bulk ternary In 0.53 Ga 0.47 As for near-infrared photocathode are analyzed. Electron transition way which is closely related with the photoemission is analyzed from the energy bands and the density of electrons. The dielectric function and the optical absorption coefficient are made clear based on the partial density of states. It is found that optical absorption coefficient of bulk In 0.53 Ga 0.47 As is bigger than that of GaAs in the near-infrared band, which is in keeping with the experiments. The analysis provides a theoretical foundation to design a better near-infrared negative electron affinity photocathode from the point of microscopic atoms. [ABSTRACT FROM AUTHOR]

Abstract: In order to compare the structures of GaAs photocathodes grown by molecular beam epitaxy (MBE) and metal–organic chemical vapour deposition (MOCVD), four wideband response exponential doping photocathodes were prepared. Their reflectivity and transmittivity were measured by the spectrophotometer, and three thin layer thicknesses were fitted based on the matrix theory in Thin-film Optics. The comparison of the results indicated that for the GaAs photocathode grown by MBE, only one amendatory layer with the low Al component should be added between the Ga1−x Al x As window layer and the GaAs active layer in order to the higher fitting accuracy. Opposite occurs for the MOCVD samples. In the case of accurately controlling the layered doping concentration, the material grown by MBE is the optimal, while that grown by MOCVD is suit for the exponential doping situation. These results are available for the material growth and the module preparation of the varied doping transmission-mode GaAs photocathodes. [Copyright &y& Elsevier]

Abstract: Based on first-principles plane-wave pseudopotential method, the band structure, density of state, E-Mulliken population and optical properties of vacancy defect Ga0.5Al0.5As are calculated. The influence of As vacancy on Ga electronic state is more obvious than on Al electronic state, the influence of Ga vacancy on As electronic state is more obvious than Al vacancy. The destruction caused by vacancy defects to Ga As sp3 hybridized orbital is more serious than Al As sp3 hybridized orbital. The influence of vacancies on the bonding structure, E-Mulliken population and charges are limited to the atoms nearby the vacancies. Ga vacancy and Al vacancy act as acceptor while As vacancy acts as donor. Ga0.46875Al0.5As owns most carries concentration and Ga0.5Al0.5As0.96875 owns lest carries concentration. The influence of vacancy on optical properties is mainly in low energy range (0–12.5eV) and is negligible in higher energy range. Influence of Ga vacancy and Al vacancy on optical properties is rather similar, which are obvious different from the influence of As vacancy on optical properties. [Copyright &y& Elsevier]

Abstract: Using the first-principles plane-wave pseudo-potential method based on density function theory (DFT), the electronic structure and optical properties of Ga0.5Al0.5As (001), (011) and (111) surfaces are calculated. Result shows that (001) surface is reconstructed, (011) surface is not reconstructed but wrinkled, (111) surface is only relaxed. (111) is the most stable surface. (001) surface owns the lowest work function. Absorption coefficient and reflectivity of these surfaces are smaller than bulk, the transmittance of the surfaces are larger than the bulk, which is helpful for the incident light to excite photoelectrons. The decrease of the absorption coefficient and reflectivity at (001) surface are the largest. Calculation of electronic structure and optical properties predict that the (001) surface should have the strongest photoemission. [Copyright &y& Elsevier]