Dependence Of Eg Research Articles

Competition between Electron–Phonon and Spin–Phonon Interaction on the Band Gap and Phonon Spectrum in Magnetic Semiconductors

Using the microscopic s-f model and Green’s function theory, we study the temperature dependence of the band gap energy Eg and the phonon energy ω and damping γ of ferro- and antiferromagnetic semiconductors, i.e., with different signs of the s-f interaction constant I. The band gap is a fundamental quantity which affects various optical, electronic and energy applications of the materials. In the temperature dependence of Eg and the phonon spectrum, there is a kink at the phase transition temperature TC or TN due to the anharmonic spin–phonon interaction (SPI) R. Moreover, the effect of the SPI R and electron–phonon interaction (EPI) A on these properties is discussed. For I>0,R>0, Eg decreases with increasing SPI and EPI, whereas for I<0,R>0, there is a competition; Eg increases with raising the EPI and decreases for enhanced SPI. For R<0, in both cases, the SPI and EPI reduce Eg. The magnetic field dependence of Eg for the two signs of I and R is discussed. The SPI and EPI lead to reducing the energy of the phonon mode ω = 445 cm−1 in EuO (I>0, R<0), whereas ω = 151 cm−1 in EuSe (I>0, R>0) is enhanced with increasing EPI and reduced with SPI. Both the SPI and EPI lead to an increasing of the phonon damping in EuO and EuSe. The results are compared with the existing experimental data.

Copper- and Silver-Containing Heterometallic Iodobismuthates: Features of Thermochromic Behavior.

Nine heterometallic iodobismuthates with the general formula Cat2{[Bi2M2I10}] (M = Cu(I), Ag(I), Cat = organic cation) were synthesized. According to X-ray diffraction data, their crystal structures consisted of {Bi2I10} units interconnected with Cu(I) or Ag(I) atoms through I-bridging ligands, forming one-dimensional polymers. The compounds are thermally stable up to 200 °C. Optical band gaps (Eg), estimated at room temperature via diffuse reflectance measurements, range from 1.81 to 2.03 eV. Thermally induced changes in optical behavior (thermochromism) for compounds 1-9 were recorded, and general correlations were established. The thermal dependence of Eg appears to be close to linear for all studied compounds.

Size- and temperature-control optical direct/indirect band tuning in layered compounds: band gap engineering

The X-ray diffraction (XRD) is studied in thermally evaporated cadmium iodide (CdI2) thin films with various thicknesses (0.4–5.5 μm) and grain sizes (100–160 nm). The grain size, calculated from the XRD, is found to increase with increasing the thickness of the film, while the reflectivity and refractive index decease with increasing the wavelength of the exciting light. The optical absorption spectra show both allowed direct and indirect interband transitions across a fundamental gap in CdI2. It is found that both indirect and direct band gap (Eg) decrease with increasing the thickness of the film and that the indirect Eg is lower than the direct Eg by an amount of about 0.7 eV. The direct Eg is also decreased with increasing both the grain size and temperature. However, the temperature dependence of Eg follows the Varshni relation. Our results highlight the possibility of engineering or tuning the Eg of CdI2 by controlling the thickness of the film, grain size as well as temperature.

Mechanical properties and band gap estimations of stoichiometric GeSe2-As2Se3 glasses

The elastic moduli (Bulk (Ke), Micro-hardness (He), Young (Ye)) and Poisson’s ratio (Pr) of the prepared xGeSe2-(100-x)As2Se3 (0 ≤ x ≤ 100) glasses are estimated using the ultrasonic velocities (Longitudinal (νl) and Transverse wave (νt)) and the density (ρ). The glass transition temperature (Tg) of the glasses has been obtained using the DSC thermograms recorded at a heating rate 10 K/min. As well, many physical and structural features have been obtained theoretically by applying the Chemical Bond Approach (CBA). Theoretical evaluations of optical band-gap (Eg) and the glass-transition temperature (Tg) for the glasses have been studied. It was found that the obtained theoretical values of the parameters Eg and Tg agree with the experimental values within uncertainty < 4%. Also, rigidity parameters such as the heat of atomization (Hs), the mean bond energy (Es), the cohesive energy (CE), and the overall average bond energy (< E > ) have been estimated. The dependence of Eg on the rigidity parameters (Hs, Es, CE, and < E > ) was verified and discussed. Furthermore, the coordination number (CN), lone pair electrons (LP), and crosslinking density (CD) were estimated. Eg values increased from 1.6 to 2.04 eV with increasing the GeSe2 concentration from 0 to 100 at%. So, these materials can be used as optical absorbers in solar cells and sensors.

Optical absorption study of manganese-doped Na 2O-K 2O-P 2O 5 glasses

The mixed alkali phosphate glasses with the general formula 49.95[ xNa 2O-(1 -x)K 2O]- 0.1MnO 2-49.95P 2O 5 with x=0-1mol% were prepared and characterized using the optical absorption technique. The optical absorption spectrum of each glass exhibits a single band in the region around 200-400 nm. From the ultraviolet absorption edges, the optical parameters including optical band gap and the Urbach energy were evaluated. It is interesting to note that these optical parameters are composition-dependent. The optical band gap energy (E g) and the Urbach energy (ΔE) show the presence of a maximum and a minimum respectively around the intermediate composition x=0.5. The non-linearity behavior of the composition dependence of Eg and ΔE is a fingerprint of the mixed-alkali effect in the glasses under study.

Crystal Structure and Band Gap of (MnIn2S4)1–x • (AgIn5S8)x Alloys

The (MnIn2S4)1–x • (AgIn5S8)x alloy single crystals are grown for the first time by the Bridgman method in the entire range of component concentrations. The composition of the single crystals and their crystal structure are determined. It is shown that the alloy crystallizes in the cubic spinel structure. The unit-cell parameter of the single crystals (a) is calculated, and its dependence on the composition parameter (x) is determined. It is established that the parameter a linearly varies with x. From the transmittance spectra in the region of the fundamental absorption edge, the band gap (Eg) of the MnIn2S4 and AgIn5S8 compounds and (MnIn2S4)1–x • (AgIn5S8)x alloys is determined, and the dependence of Eg on the parameter x is constructed. It is found that the band gap Eg nonlinearly varies with x and has a maximum at x = 0.6.

Кристаллическая структура и ширина запрещенной зоны твердых растворов (MnIn-=SUB=-2-=/SUB=-S-=SUB=-4-=/SUB=-)-=SUB=-1-x-=/SUB=-&amp;#183;(AgIn-=SUB=-5-=/SUB=-S-=SUB=-8-=/SUB=-)-=SUB=-x-=/SUB=-

AbstractThe (MnIn_2S_4)_1– x • (AgIn_5S_8)_ x alloy single crystals are grown for the first time by the Bridgman method in the entire range of component concentrations. The composition of the single crystals and their crystal structure are determined. It is shown that the alloy crystallizes in the cubic spinel structure. The unit-cell parameter of the single crystals ( a ) is calculated, and its dependence on the composition parameter ( x ) is determined. It is established that the parameter a linearly varies with x . From the transmittance spectra in the region of the fundamental absorption edge, the band gap ( E _ g ) of the MnIn_2S_4 and AgIn_5S_8 compounds and (MnIn_2S_4)_1– x • (AgIn_5S_8)_ x alloys is determined, and the dependence of Eg on the parameter x is constructed. It is found that the band gap E _ g nonlinearly varies with x and has a maximum at x = 0.6.

Optical transmittance and reflectance studies and evidence of weak electron–phonon interaction in Type-I Ge clathrate Ba8Ga16Ge30

The optical properties of ternary type-I Ge clathrate Ba8Ga16Ge30 are investigated by optical reflectance and transmittance measurements. The refractive index and extinction coefficient are calculated from the reflectance spectrum via the modified Kramers–Kronig analysis method between 0.5 and 3.2 eV. The photon energy dependence of the optical absorption coefficient reveals that Ba8Ga16Ge30 is an indirect band gap semiconductor with a gap energy Eg of 0.69 eV at 5.7 K and 0.66 eV at 285 K. The temperature dependence of Eg can be satisfactorily described by an equation based on the Bose–Einstein statistics model. When compared with those of common elemental, III–V, and II–VI semiconductors, the small temperature coefficient dEg/dT of the Ba8Ga16Ge30 can be considered to represent the weak electron–phonon interaction in the Ba8Ga16Ge30 clathrate.

Solid-state synthesis, thermal stability and optical properties of new scheelite-type Pb1−3x⌷xPr2xWO4 ceramics where ⌷ denotes cationic vacancies

Polycrystalline samples of a new Pb1−3x⌷xPr2xWO4 solid solution with limited homogeneity (0<x≤0.2222) and cationic vacancies (⌷) have been successfully prepared by high-temperature annealing of PbWO4/Pr2(WO4)3 mixtures composed of 40.00mol% or lower amount of praseodymium tungstate. According to the X-ray diffraction analysis the Pb1−3x⌷xPr2xWO4 phases crystallize in the scheelite-type tetragonal symmetry, space group I41/a. Thermal stability of the samples under study strongly depends on praseodymium concentration. Lead tungstate and samples of solid solution under study are insulators and their indirect band gap Eg>3eV. The nonlinear dependence of Eg on the value of x parameter is also observed.

C-atom-induced bandgap modulation in two-dimensional (100) silicon carbon alloys

We experimentally studied the effects of the C atom on bandgap EG modulation in two-dimensional (2D) silicon carbon alloys, Si1−YCY, fabricated by hot C+ ion implantation into the (100) SOI substrate in a wide range of Y (4 × 10−5 ≤ Y ≤ 0.13), in comparison with the characteristics of 3D silicon carbide (SiC). X-ray photoelectron spectroscopy (XPS) and UV-Raman analysis confirm the Si–C, C–C, and Si–Si bonds in the 2D-Si1−YCY layer. The photoluminescence (PL) method shows that the EG and PL intensity IPL of 2D-Si1−YCY drastically increase with increasing Y for high Y (≥0.005), and thus we demonstrated a high EG of 2.5 eV and a visible wavelength λPL less than 500 nm. Even for low Y (<10−3), IPL of 2D-Si1−YCY also increases with increasing Y, owing to the compressive strain of the 2D-Si1−YCY layer caused by the C atoms, but the Y dependence of EG is very small. EG of 2D-Si1−YCY can be controlled by changing Y. Thus, the 2D-Si1−YCY technique is very promising for new EG engineering of future high-performance CMOS and Si photonics.

Band gap modulation of Si-C binary core/shell nanowires by composition and ratio

Core/shell nanowires (CSNWs) composed of Si, C, and SiC are promising systems for optoelectronic devices. Through computational investigations, we find that the band gaps (Eg) of these nanowires can be controlled not only by changing their composition, but also by adjusting the core/shell thickness ratio. For Si/SiC or SiC/C CSNWs with a fixed total number of layers, the dependence of Eg on the core/shell thickness ratio shows a bowing effect. Eg can be tuned from a few eV all the way to zero. These investigations provide direction for designing optoelectronic devices based on Earth-abundant elements.

High temperature optical absorption edge of CdTe single crystal

The optical absorption edge of bulk CdTe single crystal was measured by infrared transmission under saturated Cd pressure in the temperature interval 295–1223 K. The absorption coefficient was directly determined up to the value of 100 cm−1. For higher values, it was estimated by extrapolating the spectra according to the Urbach exponential rule. It was observed that the common temperature-independent intersection of extrapolated Urbach absorption edge, the “Urbach focus,” does not exist in CdTe. The temperature dependence of band-gap energy Eg defined by Eg(300 K) = 1.518 eV and dEg/dT = − 4.4 × 10−4 eV/K was established, postulating linear temperature dependence of Eg by fitting the temperature dependent absorption coefficient at the band edge αg(T) = 6600 – 4T (K) (cm−1).

The apparent activation energy and dynamic fragility of ancient ambers

According to the literature, different types of materials have different glass transition temperature (Tg) dependences of apparent activation energy (Eg) and dynamic fragility (m). In previous work we found that for different ambers, there were different glass transition temperatures. These same samples provide an opportunity to study the Tg dependence of Eg and m for amber, which has not been reported previously in the literature. In this work, nine pieces of amber from different locations and having different ages were investigated by differential scanning calorimetry. Six cooling rates were used to provide the different thermal histories, and the corresponding limiting fictive temperatures were determined using Moynihan's area matching method. From the cooling rate dependence of the limiting fictive temperature both the apparent activation energy and dynamic fragility were calculated. We find that as glass transition temperature increases, both the apparent activation energy and dynamic fragility increase. The Tg dependence of m for amber shows a similar trend with temperature as do the metallic glass formers and compares favorably with the m–Tg dependence of other aromatic polymers.

Thermal analysis and physical properties of Bi–Se–Te chalcogenide glasses

The present work reported the effect of variation of bismuth content on the glass transition, thermal stability and crystallization kinetics of Bix(Se20Te80)100−x (0⩽x⩽12) glasses. Measurements of the heat flow as a function of temperatures and different heating rates (β) for Bix(Se20Te80)100−x glasses were carried out by using differential scanning calorimetry (DSC). Both the heating rate and Bi content are found to affect the values of the characteristic temperatures such as the glass transition temperature (Tg), the peak temperature of crystallization (Tp) and the melting temperature (Tm). From the heating rate dependence of Tg and Tp, values of the activation energy for glass transition (Eg) and for crystallization (Ec) were deduced. Afify’s method was used to derive the values of Ec without any dependence on the kinetic exponent (n). The compositional dependence of Eg and Ec was discussed in terms of the chemical bond approach. Based on the values of Tg, Tp, Tm and Ec the thermal stability criteria (ΔT, Hr, Hg, S, K(T) and Kr(T)) of the studied glasses was evaluated. It was found that, increasing the Bi additions leads the decrease of the glass thermal stability i.e. the glass without any Bi content is the most stable one.

Disorder effects on electronic bandgap and transport in graphene-nanomesh-based structures

Using atomistic quantum simulation based on a tight binding model, we investigate the formation of electronic bandgap Eg of graphene nanomesh (GNM) lattices and the transport characteristics of GNM-based electronic devices (single potential barrier structure and p-n junction) including the atomic edge disorder of holes. We find that the sensitivity of Eg to the lattice symmetry (i.e., the lattice orientation and the hole shape) is significantly suppressed in the presence of disorder. In the case of strong disorder, the dependence of Eg on the neck width fits well with the scaling rule observed in experiments [Liang et al., Nano Lett. 10, 2454 (2010)]. Considering the transport characteristics of GNM-based structures, we demonstrate that the use of finite GNM sections in the devices can efficiently improve their electrical performance (i.e., high ON/OFF current ratio, good current saturation, and negative differential conductance behaviors). Additionally, if the length of GNM sections is suitably chosen, the detrimental effects of disorder on transport can be avoided to a large extent. Our study provides a good explanation of the available experimental data on GNM energy gap and should be helpful for further investigations of GNM-based devices.

Temperature dependence of direct transition energies in [formula omitted]-[formula omitted] epitaxial films on Si(111) substrate

Temperature dependence of direct transition energies (Eg) was investigated in β-FeSi2 epitaxial films on Si(111) substrate. The lattice volume of the epitaxial films was reduced as the annealing temperature (Ta) increased. In photoreflectance measurements, the samples annealed at higher Ta showed a larger temperature dependence of Eg. These results revealed that the temperature dependence of Eg depended on the lattice deformation by the thermal annealing. The fact supports the band gap modifications by the lattice deformation.

Spectro-Structural Characterization of Chalcogenide Films Containing Cd, Te and Se

Chalcogenide films find many applications in electronics as memory device, laser writer & xerography. To tailor the property to the requirement, various compositions of Cd, Te and Se have been deposited and characterized optically. Films have been deposited by thermal evaporation as well as by electron beam evaporation. Thermal analysis such as TGA, DTA and DSC has been carried out before deposition to ensure the composition. Dependence of Eg on the composition has been justified from (T% data) Transmittance measurements. Drastic changes in optical property due to annealing in the range 200°C to 400°C have been investigated.

Chemical trends of the band gaps of idealized crystal of semiconducting silicon clathrates, M8Si38A8 (M = Na, K, Rb, Cs; A = Ga, Al, In), predicted by first-principle pseudopotential calculations

We have calculated the band structures of Si clathrate, M8Si38Ga8 (M=Na, K, Rb, and Cs), using the density-functional theory under the generalized gradient corrected local density approximation, where M is the encapsulated guest alkali atom. They are found to be indirect semiconductors with the calculated gaps (Eg) from 0.45 to 0.89eV, which should be compared to the calculated gap of 0.65eV of crystalline Si with the diamond structure. The gaps become wider with the promotion to the heavier guest alkali atoms and the reasons of gap widening are discussed using the calculated dependence of Eg on the cell-volume of guest-free silicon clathrate (Si46). Effect of the substitutional elements in the clathrate framework (Al and In in place of Ga) was also discussed.

Band Gap Energy in Silicon

The band gap energy, Eg in silicon was found by exploiting the linear relationship between the temperature and voltage for the constant current in the temperature range of 275 K to 333 K. Within the precision of our experiment, the results obtained are in good agreement with the known value energy gap in silicon. The temperature dependence of Eg for silicon has also been studied.

Band Gap Energy in Silicon

The band gap energy Eg in silicon was found by exploiting the linear relationship between the temperature and voltage for the constant current in the temperature range of 275 K to 333 K. Within the precision of our experiment, the results obtained are in good agreement with the known value energy gap in silicon. The temperature dependence of Eg for silicon has also been studied.