Decrease In Optical Energy Gap Research Articles

Structural, optical, and electrical properties of bulk single crystals of InAsxSb(1−x) grown by rotatory Bridgman method

Radially-homogeneous and single-phase InAsxSb(1−x) crystals, up to 5.0 at. % As concentration, have been grown using the rotatory Bridgman method. Single crystallinity has been confirmed by x-ray and electron diffraction studies. Infrared transmission spectra show a continuous decrease in optical energy gap with the increase of arsenic content in InSb. The measured values of mobility and carrier density at room temperature (for x=.05) are 5.6×104 cm2/V s and 2.04×1016 cm−3, respectively.

Annealing effects on the structural and optical characteristics of electron beam deposited Ge-Se-Bi thin films

Measurements of absorbance and transmittance in Ge20Se80-xBix (0 ≤ x ≤ 10 at %) chalcogenide thin films in the visible range at room temperature were carried out. The dependence of the optical absorption on the photon energy is described by the relation αhv=B(hv-E0)2. It was found that the optical energy gap E0 decreases gradually from 1.93 to 1.205 eV with increasing Bi content up to 10 at %. The rate of the change decreases with increasing Bi content. The composition dependence of the optical energy gap is discussed on the basis of the concentration of covalent bonds formed in the chalcogenide film. The decrease of optical energy gap with increasing Bi content is related to the increase of Bi–Se bonds and the decrease of Se-Se bonds. The effect of thermal annealing for different periods of time on the behavior of optical absorption of the as-deposited films was investigated. The optical gap increases with increasing annealing time. The rate of change decreases with increasing annealing time, then E0 reaches a steady state. The increase in the values of the optical gap of the amorphous films with heat treatment is interpreted in terms of the density of states model. The structure of the as-prepared and thermally annealed films were investigated using transmission electron microscopy, energy dispersive analysis and X-ray diffraction. It was confirmed that the as-prepared films were in the amorphous state. Phase separation was observed after thermal annealing. The separated crystalline phases were identified.

Optical absorption spectra studies for amorphous Cu2O-Bi2O3 glass system

Infrared absorption spectra were measured in the spectral range 4000–200 cm−1 for the Cu2O-Bi2O3 glass system. Strong bands were observed around 436–477, 600–632, 700–715, 810–875, 975–1000, 1550–1610 and 3225–3510 cm−1 which are due to harmonics of the Bi-O-Bi stretching vibration, Cu-O stretching vibrations, O-Bi-O stretching vibrations, O-Bi-O bending vibrations, Bi-O stretching vibrations, free H2O normal-mode bending vibrations and free H2O molecules or OH− ions, respectively. Quantitative justification of these absorption bands shows that the values of the experimental wave number for most recorded absorption bands agree well with the theoretical ones. The optical absorption spectra were recorded for the same glass system in the spectral range 300–700 Nm. These records show that the absorption edge has a tail extending towards lower energies. The edge shifts towards lower energies with increasing Cu2O content. This shift is mostly related to the structural rearrangement and the relative concentrations of the glass basic units. By increasing the Cu2O content, the optical energy gap decreases, while the width of the localized states increases.

Disorder and absorption edges in ion-irradiated hydrogenated amorphous carbon films.

The optical properties of ion-irradiated hydrogenated amorphous carbon films in the low and medium absorption coefficient region (1--${10}^{5}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$) were investigated as functions of the hydrogen atomic fraction (5--50 H at. %) by using a photothermal deflection spectroscopy technique. The data are interpreted using Urbach and Tauc theories for the different spectral regions. It was found that an increase in hydrogen content induces an overall increase of order in the film, leading to a sharpening of the band edge slope, in agreement with the known tendency of trigonal carbon to cluster in ordered graphitic domains. Correspondingly a decrease of the Tauc optical energy gap (from 1.6 to 0.2 eV) was observed due to the formation of weakly bonded \ensuremath{\pi} electron states in these materials. The data are compared with those reported by several authors on a-Si and a-Si:H samples.

Electrical and optical properties of chalcogenide amorphous semiconductors modified with Ni

The electrical and optical properties of the chalcogenide semiconductor (Se 32Te 32As 4Ge 32) 100− xNi xitx have been studied. As the Ni concentration is increased the electrical dc conductivity is drastically increased and variable range hopping conduction becomes dominant even above room temperature. The optical energy gap decreases with the Ni concentration from 1.18–0.95 eV. Ni-atoms in the chalcogenide semiconductor donate free electrons which occupy the gap state. This occupation causes the shift of the Fermi level toward the conduction band. It is an effect of this shift that the thermal activation energy is decreased. The decrease in optical energy gap is independent of the shift of the Fermi level and is ascribable to the appearance of the additional level located at 0.95 eV above the top of the valence band. This level originates from the 3d-level of the Ni-atom.

Descriptions of outer d electrons in thiospinels

The constraints [1] of symmetry imposed by the spinel structure and[2] of internal consistency within the periodic table are used to construct phenomenological energy diagrams for the transition-metal thiospinels. It is found that band narrowing via intra-atomic exchange is sufficient to localize the d electrons to a cation only if there are at least three unpaired spins at the cation. A-site Co 2+ ions may have collective d electrons, but exhibit a spontaneous atomic moment in Co [Cr 2]S 4. The compounds A[ Cr 2] S 4, where A = Zn, Cd, Mn and Fe, contain localized d electrons, although the antiparalled-spin electron per Fe 2+ ion is just localized. It is argued that the compounds Cu[ B 2]S 4, B = V, Cr, Rh and 1 2(Rh+Cr) , have one hole per molecule in primarily d-like, collective orbitais that are primarily Cu-S in character. Where the B cations have no spontaneous atomic moment ( B ≠ Cr), the compounds are superconducting at lowest temperatures. Cu[Ti 2]S 4 and Cu[Co 2]S 4, though metallic and without spontaneous magnetism, exhibit no superconducting transition above 0.05°K. Cu[Ti 2]S 4 and probably Cu[Co 2]S 4 are distinguished by a lack of (or less than one per molecule) holes in the d-like, collective Cu-S states. Cu 2+ + Cr 3+ is argued to be more stable than Cu + + Cr 4+ in the chalcogenide spinels, as it is known to be in Cu[Cr 2]O 4. The low Curie constant of CuCrRhSe 4, as compared with CuCrTiS 4, is attributed to a temperature-dependent molecular field, which is primarily due to mobile charge carriers in the selenide, rather than to Cr 4+ excited states at high temperatures. The decrease in optical energy gap below T c in CdCr 2Se 4, in contrast to its increase in CdCr 2S 4, is attributed to high-spin and low-spin Cr 2+ ions of approximately the same energy.