Shift Of Conduction Band Edge Research Articles

Influence of a TiCl4Post-Treatment on Nanocrystalline TiO2Films in Dye-Sensitized Solar Cells

In this study, the influence of the TiCl(4) post-treatment on nanocrystalline TiO(2) films as electrodes in dye-sensitized solar cells is investigated and compared to nontreated films. As a result of this post-treatment cell efficiencies are improved, due to higher photocurrents. On a microscopic scale TiO(2) particle growth on the order of 1 nm is observed. Despite a corresponding decrease of BET surface area, more dye is adsorbed onto the oxide surface. Although it seems trivial to match this finding with the improved photocurrent, this performance improvement cannot be attributed to higher dye adsorption only. This follows from comparison between incident photon to current conversion efficiency (IPCE) and light absorption characteristics. Since the charge transport properties of the TiO(2) films are already more than sufficient without treatment, the increase in short circuit current density J(SC) cannot be related to improvements in charge transport either. Transient photocurrent measurements indicate a shift in the conduction band edge of the TiO(2) upon TiCl(4) treatment. It is concluded that the main contribution to enhanced current originates from this shift in conduction band edge, resulting in improved charge injection into the TiO(2).

An improved band-anticrossing model – including the positional dependence of nitrogen – for InGaNAs∕GaAs quantum well lasers

The positional dependence of the localized nitrogen (N) within the quantum well is included with the band-anti-crossing model that describes the interaction of the GaInAs conduction band with the localized N defect. It is found that N located at the center of the well interacts more strongly with the InGaAs conduction band than N localized near the edge of the quantum well. Different distributions of N are investigated by studying the conduction band edge shift, energy level splitting, dipole moments, and gain. These quantities are found to be highly dependent upon the position of the N.

Effect of nitrogen on the electronic band structure of group III-N-V alloys

We have studied optical transitions at the \ensuremath{\Gamma} and L points of the Brillouin zone of ${\mathrm{GaN}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ and ${\mathrm{Al}}_{y}{\mathrm{Ga}}_{1\ensuremath{-}y}{\mathrm{N}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ alloys using photomodulation spectroscopy. For ${\mathrm{GaN}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ with N contents between 0% and 2%, the N-induced shift of the conduction-band L minima is found to be only a fraction of the conduction-band edge shift at the \ensuremath{\Gamma} point. The measurements of ${\mathrm{Al}}_{y}{\mathrm{Ga}}_{1\ensuremath{-}y}{\mathrm{N}}_{x}{\mathrm{As}}_{1\ensuremath{-}x}$ further show that there is no correlation between the location of the X conduction-band minima and the observed ${E}_{+}$ and ${E}_{\ensuremath{-}}$ transitions. The results demonstrate that the N-induced interactions between extended \ensuremath{\Gamma}, L, and X conduction-band states do not play a significant role in modification of the conduction-band structure of III-N-V alloys. The N-induced change of the conduction-band structure is predominantly influenced by the anticrossing interaction between the extended states of the \ensuremath{\Gamma} conduction band and the localized states of nitrogen.

Soft X-ray spectroscopy of single sized CdS nanocrystals: size confinement and electronic structure

Soft X-ray spectroscopy of CdS nanocrystals within their crystalline superlattice allows to relate unambiguously, changes of the electronic structure with the particle size and shape which is known from single crystal X-ray diffraction. We find that the valence and conduction band edge shift contribute to the same extent to the total band gap opening of about 2 eV with respect to CdS bulk as the particle size decreases to 10 Å. Taking a finite height of the potential walls into account, we can reproduce these results within an effective mass approximation model.

Optimization of growth conditions for strained Si/Si 1− yC y structures

Preparation of Si 1− y C y /Si(001) and Si/Si 1− x Ge x /Si 1− y C y /Si(001) heterostructures using molecular beam epitaxy (MBE) with C obtained from sublimation of SiC in a high-temperature cell is reported. Accumulation of surface roughness is found to occur during growth of C-rich layers eventually leading to a reduction of the C-induced strain. The roughness can be suppressed at increased growth rates and/or decreased substrate temperature during growth of Si 1−yC y layers. Modulation of the growth temperature has been found advantageous for optimizing both the morphology and photoluminescence properties. Near band-edge luminescence from Si 1− y C y /Si multiple quantum well (MQW) structures has been observed and a conduction band edge shift equal to 63 meV/% C has been deduced from an effective mass calculation.

Red shift of the optical absorption edge of ferromagnetic semiconductors

We propose an improved moment method, which explicitly takes into account quasiparticle damping, for determining the temperature dependence of the conduction band of a ferromagnetic semiconductor. The theory predicts a red shift of the lower conduction band edge, which strongly depends on the band width W of the semiconductor. The experimental data for the red shifts of the Eu-chalcoginides can then be used to derive the width of the 5 dt 2 g -conduction band. We find W = 2.31 eV for EuO, W = 1.40 eV for EuS, and W = 1.15 eV for EuSe, taken at room temperature.

Work functions of the (001) face of the hexaborides of Ba, La, Ce and Sm

From thermonic and field emission retarding potential methods the following order of increasing work function for RB 6-type single crystals was determined: CeB 6(001) ≲ LaB 6(001) < BaB 6(001) < SmB 6(001) Auger electron spectroscopy indicated that low work function surfaces correlated with low B/R ratios; however, energy dependence of the electron reflection coefficient maxima suggests that a conduction band edge shift relative to the Fermi level among the various RB 6(001) crystals largely accounts for the work function variation. Mass spectrometric studies showed only atomic species vaporizing from all the RB 6 crystals investigated. Large variations in the vapor phase B/R ratios and vaporization energies for crystals of the same material were believed to be due to small variations in bulk stoichiometry.

The Electron and Magnon Spectra of Ferrimagnetic Semiconductors

AbstractA calculation of the magnon spectrum is carried out for a degenerate ferrimagnetic semiconductor. The spin‐polarized conduction electrons indirectly effect the exchange between the local magnetic moments. As a result, in the case of antiferromagnetic direct exchange, the acoustic branch becomes harder with increase in the carrier concentration, whereas the optical branch becomes softer, if the signs of the s‐d exchange integrals are the same in both sublattices. With a certain concentration reached, the initial ferromagnetic ordering becomes absolutely unstable with respect to the long‐wave optical magnons. Also a study is made of the electron spectrum at the Néel point. In spite of the absence of the long‐range order, the short‐range order in the paramagnetic proximity still remains. Therefore, the conduction electrons move in such a manner that their spin follows the direction of the local magnetic moment slowly changing in space. This enables the electron to obtain a considerable gain in energy. It follows that a gigantic conduction band edge shift should also occur above the Néel point.