Forbidden Energy Band Gap Research Articles

Structural, compositional and optical properties of gallium selenide thin films doped with cadmium

AbstractPolycrystalline cadmium doped gallium selenide thin films were obtained by the thermal co‐evaporation of GaSe crystals and Cd grains onto glass substrates. The structural, compositional and optical properties of these films have been investigated by means of X‐ray diffraction, energy dispersive X‐ray analysis and UV‐visible spectroscopy techniques, respectively. Particularly, the elemental analysis, the crystalline nature, the energy band gap, the refractive index, the dispersion energy and static dielectric constant have been identified. The absorption coefficient spectral analysis in the sharp absorption region revealed a direct forbidden energy band gap of 1.22 eV. The cadmium doping has caused a significant decrease in the values of the energy band gap and in all the dispersive optical parameters, as well. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Boron implantation effects in CdS thin films grown by chemical synthesis

CdS thin films grown on ITO/glass substrates by using chemical bath (CB) were boron-implanted employing 100 keV beam of boron ions (B +) with fluences in the range 1.0×10 15–1.0×10 16 ions/cm 2. The B doping was successfully carried out, as was proved by the major carrier density introduced in the range 0.8×10 18–5.4×10 18 cm −3, which was calculated from thermo power measurements. Raman spectroscopy results support the assumption that triply ionized boron (B 3+) enters into the CdS lattice occupying Cd 2+ sites, which create shallow donor levels in the forbidden energy band gap.

Growth of CdS:Cu Nanocrystals by Chemical Synthesis

CdS:Cu nanocrystalline films were prepared by chemical bath on glass substrates at a deposition temperature of . Different Cu-doping levels were obtained by changing the volume of the Cu-reagent-solution into the CdS growing solution. X-ray diffraction (XRD) and optical absorption (OA) measurements were carried out to characterize the material. From the XRD patterns it is concluded that grains in undoped films grow in the wurtzite hexagonal phase, and in Cu-doped films they grow in the zinc blende cubic phase. The average grain size, located in the range , was calculated by employing Scherrer’s formula. From OA spectra the forbidden energy bandgap was determined by using the relation, where α is the OA-coefficient and the photon energy. Due to the doping, shifts to lower values depending on the impurity level of the film. Furthermore, the dependence of with radius size and interplanar distances of the lattice is discussed. Gibbs free energy calculation for the Cu doping CdS process is also included.

Cd(S(1 − x) + CO3(x)) thin films by chemical synthesis

A microcrystalline mixture of cadmium carbonate (CdCO3) and cadmium sulfide (CdS) were grown in the thin film format onto glass substrates by means of chemical bath. The temperature of the bath (T d ) was selected in the interval 23–80∘C. At low temperatures, CdCO3 is the compound predominant in the layers. At high temperatures CdS is the compound deposited on the substrate. At intermediate T d -values a mixture of both materials are present, i.e., the gradual transition from an insulator (CdCO3) to a semiconductor (CdS) growth occurs when T d increases. Physical properties of films were studied by means of X-ray diffraction and optical absorption. The forbidden energy band gap of direct electronic transitions (E g ) was calculated by applying the α2 ∝ (hν − E g ) relation to the optical absorption spectra.

Photonics: practically there?

Abstract Materials that contain a photonic band gap have the potential to manipulate light with remarkable precision. Successful fabrication of such structures, known as photonic crystals, has fueled interest in a whole host of novel optical devices, ranging from miniature lasers and all-optical circuits to smart textiles and biomedical transport systems. Growing confidence that ‘the time is right’ to realize the new technology’s commercial potential has been demonstrated by the emergence of numerous start-up companies, all bidding to bring such innovative products to market. The photonics revolution, proponents say, is just around the corner. Strange things happen to light when it passes through photonic crystals. A significant variation in refractive index between the material’s periodic lattice structure and its substrate traps transmitted photons in either one area or the other, creating distinct ‘allowed’ and ‘forbidden’ energy regions. Light with wavelengths equivalent to the forbidden region, the so-called photonic bandgap, is stopped from passing further. Wavelengths from the rest of the electromagnetic spectrum, on the other hand, are free to continue their passage through the material unhindered. In effect, the material is able to halt the passage of light just as the periodic potential of semiconductors, such as silicon, bars electrons from occupying the forbidden energy bandgap.

Optical and luminescence properties of complex lead oxides

Reflectivity, luminescence, and excitation spectra as well as nanosecond luminescence decay kinetics were measured for PbSO/sub 4/ and PbCO/sub 3/ (as well as CaCO/sub 3/:Pb) by photon excitation between 4 and 35 eV. These measurements were compared to those of PbWO/sub 4/ in order to establish the role of lead states in these compounds, which are potentially interesting as scintillators. New luminescence bands were observed in PbSO/sub 4/ at 340 nm and in PbCO/sub 3/ at 320 nm. Luminescence properties are discussed, assuming the lead parentage of electronic states in the vicinity of the forbidden energy bandgap.

Conduction and valence band offsets in GaAlAs/GaAs/GaAlAs quantum wells from photo-luminescence and deep level transient spectroscopy under hydrostatic pressure

It is shown that deep level transient spectroscopy at low temperatures and under hydrostatic pressure on MBE grown GaAlAs/GaAs/GaAlAs quantum wells using Au Schottky barrier diodes combined with photo-luminescence (PL) measurements provides a powerful tool for determining the conduction and valence band offsets if the well is considered as a ‘giant’ trap having characteristics very different from a normal trap. The conduction and valence band offsets have been determined to be 72 and 28% of GaAs/GaAlAs forbidden energy band gap difference as determined by PL measurements on samples with varying well thickness for the same GaAlAs compositions of the binding layers.

LSO-Ce fluorescence spectra and kinetics for UV, VUV and X-ray excitation

LSO-Ce fluorescence emission and excitation spectra and decay kinetics have been measured for UV, VUV and X-ray excitation at RT and 80 K. The features of the fluorescence excitation spectra of two types of cerium centres in the region 3 to 6 eV are analysed in the assumption of competitative absorption between them. It is shown that the centres can have similar absorption bands. Forbidden energy bandgap for LSO is evaluated to be not less than 6.5 eV.

Electronic structure of pristine and sodium-doped cyano-substituted poly(2,5-dihexyloxy-p-phenylenevinylene): A combined experimental and theoretical study

The electronic structure of cyano-substituted poly(2,5-dihexyloxy-p-phenylene-vinylene), or CN-PPV, has been studied in both pristine and doped states. Ultraviolet photoelectron spectroscopy (UPS) and x-ray photoelectron spectroscopy (XPS), as well as optical absorption spectroscopy have been carried out under ultrahigh vacuum (UHV) conditions, and the results have been interpreted with the help of quantum-chemical calculations. For the pristine polymer, the addition of cyano groups to the vinylene units does not affect the width of the π-bands closest to the Fermi level; however, the positions of the flat parts of the upper π-bands are shifted by approximately 0.4 eV towards higher binding energies relative to the Fermi energy, as compared with the corresponding bands of other alkoxy-substituted poly(p-phenylenevinylene)s. On the other hand, there are only marginal differences in the optical absorption spectra; the interband absorption onset is comparable to the values for alkoxy-substituted poly(p-phenylenevinylene)s. In the case of sodium doping, it is found experimentally that at saturation doping, there is about one sodium ion per phenylene vinylene unit; in that situation, two new states appear in the previously forbidden energy bandgap, which are consistent with the formation of bipolaron bands. These results are similar to those obtained for sodium-doping of poly(p-phenylenevinylene) (PPV). The peak-to-peak splitting of the bipolaron peaks in CN-PPV is 1.05 eV, compared with about 2.0 eV for sodium-doped PPV at saturation doping; this difference is related to the pinning of some of the transferred charges to the cyano vinylene groups and the phenylene rings that they are conjugated to in CN-PPV, causing a stronger confinement of the bipolaron charge carriers.

Calculated forbidden band gap in periodic protein models indicating them to be insulators.

THE idea of electron and hole transport between distant sites in biological macromolecules, especially in fibrous proteins1 prompted quantum chemists to calculate the forbidden energy band gap in simple periodic protein models2–7. Conductivity measurements of several proteins under various conditions indicated a band gap or activation energy (E in the expression for the conductivity σ = σ0 exp(– E/2kT) as function of the temperature T) of the order of 3 eV (refs 8, 9), a value in agreement with the only theoretical energy band gap available at that time (ref. 2). Therefore, the intrinsic mechanism of semiconduction along hydrogen bridges seemed to be well established in 1960. Later, however, both experimentally10–13 and theoretically the question arose whether the conduction mechanism is really intrinsic. The more, refined all-valence-electron quantum chemical calculations5–7 indicated larger gaps (values in the range 6.1–16.7 eV were reported) than obtained in experiments. Most interestingly the calculation of Morokuma5 at complete-neglect-of-differential-overlap level showed that in a polyglycine chain the bandgap was not too sensitive to the changes of the chain's conformation: in conformers with appreciable H-bonding the gap found was slightly larger than in the β-sheet conformation. This latter finding pointed out the very small influence of H-bonds on the forbidden gap of periodic protein models. These semi-empirical calculation methods were designed for molecular purposes and there is still little known of their applicability to the calculation of the gap in crystals and polymers14,19. Thus we describe here use of the more reliable non-empirical (‘ab initio’) crystal orbital method15 to calculate the band gap in a most simple polypeptide model β-polyglycine.

On the theory of double injection in solids with traps non-uniformly distributed in energy and in space

A theoretical analysis based on (a) planar double injection and (b) filamentary double injection is presented for the current-voltage characteristics of a solid with traps non-uniformly distributed in the forbidden energy band gap and in space inside it.