Fundamental Optical Gap Research Articles

Equal Footing of Thermal Expansion and Electron-Phonon Interaction in the Temperature Dependence of Lead Halide Perovskite Band Gaps.

Lead halide perovskites, which are causing a paradigm shift in photovoltaics, exhibit an atypical temperature dependence of the fundamental gap: it decreases in energy with decreasing temperature. Reports ascribe such a behavior to a strong electron-phonon renormalization of the gap, neglecting contributions from thermal expansion. However, high-pressure experiments performed on the archetypal perovskite MAPbI3 (MA stands for methylammonium) yield a negative pressure coefficient for the gap of the tetragonal room-temperature phase, which speaks against the assumption of negligible thermal expansion effects. Here we show that for MAPbI3 the temperature-induced gap renormalization due to electron-phonon interaction can only account for about 40% of the total energy shift, thus implying thermal expansion to be more if not as important as electron-phonon coupling. Furthermore, this result possesses general validity, holding also for the tetragonal or cubic phase, stable at ambient conditions, of most halide perovskite counterparts.

Families of solitons in Bragg supergratings

We study fundamental optical gap solitons in the model of a fiber Bragg grating (BG), which is subjected to a periodic modulation of the local reflectivity, giving rise to a supergrating. In addition, the local refractive index is also periodically modulated with the same period. It is known that the supergrating opens an infinite system of new bandgaps in the BG’s spectrum. We use a combination of analytical and computational methods to show that each emerging bandgap is filled with gap solitons (GSs), including asymmetric ones and bound states of the GSs. In particular, bifurcations of the GSs created by the supergrating are studied in terms of a geometric analysis.

Electronic band structure calculations for GaxIn1−xASyP1−y alloys lattice matched to InP

A pseudopotential formalism coupled with the virtual crystal approximation are applied to study the effect of compositional disorder upon electronic band structure of cubic GaxIn1−xAsyP1−y quarternary alloys lattice matched to InP. The effects of compositional variations are properly included in the calculations. Very good agreement is obtained between the calculated values and the available experimental data for the lattice–matched alloy to InP. The absorption at the fundamental optical gaps is found to be direct within a whole range of the y composition whatever the lattice-matching to the substrate of interest. The alloy system GaxIn1−xAsyP1−y lattice matched to InP is suggested to be suitable for an efficient light emitting device (ELED) material.

Optical gap and excitation energies of small Ge nanocrystals

Using the density functional theory (DFT) with the hybrid nonlocal exchange correlation functional of Becke and Lee, Yang and Parr (B3LYP), we have calculated the optical gap and the oscillator strengths for several of the lowest, spin and symmetry allowed, electronic transitions of small Ge nanocrystals passivated by hydrogen. The largest nanoparticle has an approximate diameter of 2 nm. Our results show that the optical gap exhibits size dependence (due to quantum confinement) roughly similar to silicon nanoparticles. However, for this range of diameters, there is an indirect-to-direct transition in the spectra of Ge as the size of the nanocrystals decrease. The first allowed excitation (fundamental optical gap) of each germanium nanoparticle has relatively larger oscillator strengths compared to silicon. The diameter of the smallest Ge nanocrystal capable to emit in the visible region of the spectrum, is approximately 1.9 nm, compared to 2.2 nm for silicon nanocrystals.

Ferromagnetism in Fe and Co‐implanted SnO 2 films

Using the ion implantation technique SnO2 thin films sputtered onto glass and fused silica substrates were doped with various concentrations of Fe and Co. The films have been characterized by optical transmission, ellipticity, and susceptibility measurements. The optical data reveal a fundamental optical gap of 3.5 eV and a refractive index of 2.0 to 2.1. While the optical properties vary very little with the substrate, the annealing and the type of dopant, the susceptibility shows striking variations. Thus, the unannealed films deposited onto glass display at 10 K much larger magnetic moments than films on fused silica. While the former films loose rapidly their magnetization with increasing temperature, the latter keep about 70% still at room-temperature. Various models are discussed to account for the observed variations with substrate, temperature, and annealing. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoinduced charge transport in KTaO 3 :Be

We present the first growth of KTaO3:Be (6% at. in the batch) single crystals, together with photoconductivity and thermally stimulated currents studies of this material. It is probably that very small Be2+ ions (ionic radius ∼0.35 A ) mostly substitute K+(1.64 A) and bounded and/or free electronic states formation including polaronic states. As a results Be-doping effect can strongly influence phase instability and electronic properties of KTaO3. It was found that Be2+ doping leads to appreciable photocurrents already at room temperature under irradiation of the samples with photons even at energies just below the fundamental optical gap. Photoconductivity increases strongly at low temperature. Intense thermally stimulated current peaks between 15 and 100 K are probably due to the presence of shallow hole traps and polaronic centres. The obtained results are compared to past observations in pure KTaO3 and KTaO3:Li (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Electroreflectance of hexagonal gallium nitride at the fundamental and E1 spectral regions

We have measured the room-temperature electroreflectance (ER) of hexagonal gallium nitride. Our measurements were obtained at the fundamental optical gap and at the E1 critical point regions. The measurements were performed using front contact ER. In the fundamental region, the ER spectra were found to be of excitonic nature and in the low field regime. In the E1 region, the ER spectra were also in the low field regime, but were fitted with two-dimensional critical points.

Optical excitation of electron-hole pairs in disordered one-dimensional semiconductors

We apply the optimal fluctuation method to the calculation of the optical absorption in disordered one-dimensional semiconductors below the fundamental optical gap. We find that a photon energy exists at which the shape of the optimal fluctuation undergoes a dramatic change, resulting in a different energy dependence of the absorption rate above and below this energy. In the limit when the interaction of an electron and a hole with disorder is stronger than their interaction with each other, we obtain an analytical expression for the optical conductivity. We show that to calculate the absorption rate, it is, in general, necessary to consider a manifold of optimal fluctuations, rather than just a single fluctuation. For an arbitrary ratio of the Coulomb interaction and disorder, the optimal fluctuation is found numerically.

Spectroscopic ellipsometry studies on BN films from IR to vacuum UV energy region

The dielectric function ε(ω) of fully sp3 or sp2 amorphous BN materials, was calculated in the range 0.3–20 eV by applying the appropriate modeling, using one or two Tauc–Lorentz (TL) oscillators, respectively, in the analysis of the measured ε(ω) from 1.5 to 9.5 eV, combining conventional spectroscopic ellipsometry (SE) with synchrotron radiation (SR) SE. The basic characteristics of the electronic transitions occurring in the different types of BN hybridized bonds, such as energy of absorption, broadening, and the fundamental optical gap were deduced. Based on these results we studied composite BN films prepared by magnetron sputtering, and measured by SRSE, and in situ SE in the range 1.5–5.5 eV. The dominant sp2 or sp3 BN character can easily be revealed when the upper limit of the ε(ω) spectrum is confined to 5.5 eV. However, the extension of this limit to 9.5 eV can provide more valuable information on the bonding configuration in BN materials, which are well-correlated to the Fourier Transform IR SE measurements, also presented in this work.

The effect of compositional disorder on electronic band structure in GaxIn1 − xAsySb1 − yalloys lattice matched to GaSb

A pseudopotential formalism coupled with the virtual crystal approximation are applied to study the effect of compositional disorder upon electronic band structure of cubic GaxIn1−xAsySb1−yquarternary alloys lattice matched to GaSb. The effects of compositional variations are properly included in the calculations. Our theoretical results show that the compositional disorder plays an important role in the determination of the energy band structure of GaxIn1−xAsySb1−y/GaSb and that the bowing parameter is dominated by the group V-anion-based sublattice. Moreover, the absorption at the fundamental optical gaps is found to be direct within a whole range of the x composition.

Electrical and structural properties of AlGaN: A comparison with CVD diamond

Abstract The present state of the AlGaN system is discussed in comparison to the known properties of CVD diamond. Growth and structural features of thin films as well as heterostructures are described. The variation of the fundamental optical gap with aluminum content provides the basis for the application of AlGaN in optical devices for the ultraviolet spectral range. The large conduction band offset between GaN and AlN together with reasonable dopability of AlGaN allow the construction of new field effect transistors with promising performance. The use of AlGaN in surface acoustic wave devices and the potentially negative electron affinity of alloys with high Al content are also briefly mentioned.

Photoelectronic transitions in CuGa 2.5In 2.5S 8

Photoluminescent and photoconductive properties of CuGa 2.5In 2.5S 8 in the temperature range 77–300 K are presented. The analysis of the optical response allows us to outline a scheme for the energy levels in the fundamental optical gap. Their role in the photoexcitation processes is also discussed in the framework of the proposed model.

Ab-initio calculation of nonlinear optical properties of GaAs

The linear and nonlinear optical coefficients of GaAs have been calculated with the ab-initio full potential self-consistent pseudofunction energy band method. The computed optical reflectivity compares very well with experiment and previous empirical pseudopotential calculations, thus indicating the accuracy of our energy bands. The nonlinear optical properties also compare well with experiment and do not require a large local field correction. A direct calculation of the local field correction from first principles shows the correction limited to only 5 per cent and thus supports the finding that the local field correction is small. The only empirical adjustment in our calculation is setting the fundamental optical gap of the band structure to the experimental value of 1.52 eV.

Optical Quenching of Photoconductivity in Single-Crystal Stannic Oxide

The optical quenching of photoconductivity in single-crystal specimens of stannic oxide has been investigated as a function of extrinsic wavelength and of temperature in the region where sensitization occurs. Quenching is found in a single band extending from the fundamental optical gap at 4.0 eV down to a cutoff indicative of a 1.3-eV activation energy for the sensitizing centers. The discrepancy between this value and the 0.33-eV value obtained earlier from thermal-quenching measurements suggests that the sensitizing states themselves do not play an active role in the process of thermal quenching, but rather that the thermal quenching arises through the action of the shallow electron trapping states.