Donor-acceptor Transitions Research Articles

Optical Properties of Germanium Doped Cubic GaN

ABSTRACTWe report on recent doping experiments of cubic GaN epilayers by Ge and investigate in detail the optical properties by photoluminescence spectroscopy. Plasma-assisted molecular beam epitaxy was used to deposit Ge-doped cubic GaN layers with nominal thicknesses of 600 nm on 3C-SiC(001)/Si(001) substrates. The Ge doping level could be varied by around six orders of magnitude by changing the Ge effusion cell temperature. A maximum free carrier concentration of 3.7×1020 cm-3 was measured in the GaN layers via Hall-effect at room temperature. Low temperature photoluminescence (PL) showed a clear shift of the donor-acceptor emission to higher energies with increasing Ge-doping. Above a Ge concentration of ∼ 2x1018cm-3 the near band edge lines merge to one broad band. From temperature dependent measurements of the observed excitonic and donor-acceptor transitions a donor-energy of ∼ 36 meV could be estimated for Ge.

Optimization of the recipe for the synthesis of CuInS2/ZnS nanocrystals supported by mechanistic considerations

Abstract CuInS2/ZnS (CIS/ZnS) quantum dots (QDs) with high photoluminescence (PL) were synthesized via a facile solvothermal approach. Gaussian deconvolution of PL spectra, transmission electron microscopy, and time-resolved PL spectroscopies were used to characterize the emission properties of the prepared CIS and CIS/ZnS QDs. It was found that the growth of ZnS can reduce the surface defect acting as traps to minimize donor-acceptor emissions, and the contribution of band to donor/acceptor transition becomes a dominating emission with the increase of shell growth time. The blue shift of PL emission wavelength of CIS/ZnS QDs underwent two steps: the dramatic blue shift originated from the decreased fraction donor-acceptor transition due to the reduction of surface defects at the beginning and the subsequently mild blue-shift with the time from the interdiffusion of CIS and ZnS. The effect of trioctylphosphine (TOP) and dodecanethiol (DDT) as ligands during shell growth on the optical properties of QDs were investigated and compared. The PL quantum yield (QY) of CIS core affects the final value of CIS/ZnS QDs, and the higher PL QY is achieved while using CIS core with higher PL QY. Based on the selected ligand DDT, the reaction parameters, such as CIS core reaction time, shell growth time, and Zn/Cu feed molar ratio, were further optimized. CIS/ZnS QDs with high PL QY can be obtained with a Zn/Cu feed molar ratio larger than 4, shell growth time of 30 to 90 min, and shell growth temperature 220°C–240°C, and the maximum value was up to about 80% by adjusting the above-mentioned parameters.

EXCITON RADIATION PECULIARITIES OF RED GALLIUM-PHOSPHIDE LEDS

Electroluminescence spectra of red GaP light-emitting diodes were investigated. Besides the main emission line hν = 1.845 eV, it was detected the additional short-wave component hν = 2.206 eV the existence of which is connected with donor-acceptor transitions between Zn-Sn pairs. It was observed intensity increase of this line at low currents (I 90 mA) due to the thermal effect. The nature of the spectral lines anomalous broadening was discussed.

Annealing effects on Cd0.96Zn0.04Te crystals with Te inclusions probed by photoluminescence spectroscopy

With effectively improved spectral resolution and signal-to-noise ratio, PL features are well resolved in the Te- and Cd-rich CdZnTe crystals and ascribed to the neutral donor-bound exciton (DX), acceptor-bound exciton (AX), A-center, and deep donor–acceptor pair. A detailed analysis indicates that (i) Cd pressure annealing drives the Te inclusions to the surface region of the crystal, destroys the inclusions by excess Cd atoms, and releases abundant donor-like and acceptor-like impurities; (ii) the PL features of the deep energy levels at 1.30–1.55 eV originate in the A-center consisting Cd vacancy and one type shallow donor, and the PL feature at 1.474 eV is the so-called SA luminescence of a donor–acceptor transition between an A-center and another shallow donor. A quantitative description of the recombination scheme is established, and by which the effects of the annealing are clarified.

Ab initio studies of isolated hydrogen vacancies in graphane

We present a density functional study of various hydrogen vacancies located on a single hexagonal ring of graphane (fully hydrogenated graphene) considering the effects of charge states and the position of the Fermi level. We find that uncharged vacancies that lead to a carbon sublattice balance are energetically favorable and are wide band gap systems just like pristine graphane. Vacancies that do create a sublattice imbalance introduce spin polarized states into the band gap, and exhibit a half-metallic behavior with a magnetic moment of 1.00μB per vacancy. The results show the possibility of using vacancies in graphane for novel spin-based applications. When charging such vacancy configurations, the deep donor (+1/0) and deep acceptor (0/−1) transition levels within the band gap are noted. We also note a half-metallic to metallic transition and a significant reduction of the induced magnetic moment due to both negative and positive charge doping.

The properties of gallium oxide thin film grown by pulsed laser deposition

Ga2O3 films were deposited on MgAl6O10(100) substrates by means of pulsed laser deposition (PLD). The influence of oxygen pressure on crystal quality, surface morphology and transmittance were investigated by means of X-ray diffraction (XRD), atomic force microscope (AFM) and spectrophotometer. The results showed that the grain size increased, the surface roughness and FWHM of X-ray rocking curve reduced with the oxygen pressure decreasing. Furthermore, the photoluminescence spectra were recorded as a function of excitation power and temperature. A blue shift of the visible luminescence at higher excitation power was observed, indicating that donor–acceptor transitions were responsible for the visible emissions. The thermal quenching of the blue and green bands corresponded to the activation energies of 0.028eV, 0.037eV and 0.034eV, respectively.

Chemical bath deposited MgxZn1−xS(O) thin films and their photoluminescence properties

A zinc sulfide (ZnS) specimen was intentionally doped with transition metal (Mg-donor) elements using a chemical bath deposition (CBD) technique. Both the un-doped and the magnesium (Mg)-doped ZnS samples were confirmed to have hexagonal wurtzite ZnS crystal structure. The XRD patterns showed no characteristic peak for Mg indicating that the Mg2+ ions had been incorporated into ZnS(O) lattice sites. In contrast to un-doped samples, Mg doping resulted in changes in the morphological features of the spherical clusters which resulted in porous, spongy vermicular structures. The energy band gap of the MgxZn1−xS(O) film was slightly larger than that for the ZnS(O) film. A photoluminescence study revealed that the emissions were near violet–blue–green in color. The emission characteristics consist of two components; emission in the near violet and in visible region. That is the first is between 4160 and 4400Å and the second is at 5190Å, and these are associated with the donor–acceptor transitions with sulfur vacancies as acceptors and the magnesium related defects (trap states) in the samples respectively.

Influence of substrate activation process on structural and optical properties of ZnS thin films

ZnS thin films were deposited on glass substrates by a chemical bath deposition method using a substrate activation process in which aluminum ions become “contaminants” that act as a nucleation center for active components within the deposition solution. The structure and morphology results demonstrate that the films have a ZnS sphalerite crystal structure with a particle size less than 15 nm, and the films consist of small homogeneous grains. The effects of the substrate activation process on the band gap energies and donor-acceptor pair luminescence process were also investigated. A green emission centered at 502 nm was produced due to donor-acceptor transitions from the aluminum acceptor to the ionized and substitution aluminum centers (Al3+).

Dye-Anchoring Functional Groups on the Performance of Dye-Sensitized Solar Cells: Comparison between Alkoxysilyl and Carboxyl Groups

We have compared two dye-anchoring functional groups, alkoxysilyl and carboxyl groups, to investigate their influence on the performance of dye-sensitized solar cells. (Dimethylamino)azobenzene was selected as a chromophore possessing a donor–acceptor transition for the light absorption. Electrochemical and optical measurements were performed for 4-(dimethylamino)azobenzene-4′-carboxylic acid and 4-(dimethylamino)azobenzene-4′-triethoxysilane attached TiO2 films. Electrochemical measurements and DFT calculations indicated almost identical potential energy levels and electron density of HOMO and LUMO states between these two dyes. Solar cell APCE spectra and charge recombination kinetics at the dye/TiO2 interface revealed almost identical charge-transfer rates/yields from and to the dye. The difference was observed on improvement of an open circuit photovoltage (Voc) by 60 mV and on the lifetimes of an electron in the TiO2 conduction band for the dye with the alkoxysilyl functional group compared to the ca...

Suppressing Förster Resonance Energy Transfer between Organic Dyes on a Cosensitized Metal Oxide Surface

Time-resolved fluorescence spectroscopy is used to investigate the efficiency of Förster resonance energy transfer (FRET) between two different types of organic dye sensitizers bound to zirconia nanoparticles. The cosensitization scheme involves using the IR125 dye as the FRET acceptor and either the D149 dye or D35 dye as the FRET donor, with the FRET parameters determined by monitoring the donor dye’s excited state lifetime in the absence and presence of the acceptor dye. The FRET rate is found to be faster for the D149+IR125 pair than for the D35+IR125 pair. From the FRET quantum yields, the Förster distances for the D149+IR125 and D35+IR125 pairs are estimated to be 3.5 and 2.6 nm, respectively. Analysis of the data, in conjunction with time-dependent density functional theory calculations, leads to the conclusion that the variation in the Förster distances is dictated mainly by differences in the donor–acceptor orientation factors. For D149 the rhodanine acetic acid binding group constrains the dye to lie almost parallel to the surface, whereas the cyanoacrylic acid binding group of D35 causes the dye to sit almost perpendicular to the surface. Because IR125 lies parallel to the surface, due to the two sulfonate binding groups, there is better alignment of the transition dipole moments for the D149+IR125 pair than for the D35+IR125 pair. Deliberately choosing dyes with binding motifs that misalign the donor–acceptor transition dipole moments may be an effective strategy for suppressing FRET within dye-sensitized solar cells to enhance their efficiency.

Bismuth-ion doped Cu(In,Ga)Se2 thin films: Preparation, microstructures, and electrical properties

The influence of doping with bismuth ions in the properties of synthesized Cu(In,Ga)Se2 films was investigated via the solution coating process followed by a selenization step. The incorporation of bismuth ions in Cu(In,Ga)Se2 significantly increased the size of the grains in the obtained films. The intermediate compound of bismuth copper oxide was thought to react with selenium vapor to form copper bismuth selenide, which acted as a flux agent at elevated temperatures. The low-temperature photoluminescence spectra of bismuth-containing Cu(In,Ga)Se2 films revealed attenuated intensity of the peak that was attributed to the donor–acceptor pair transition following the incorporation of bismuth. The donor–acceptor transition is caused by the selenium deficiency. Doping with bismuth ions reduced the amounts of the selenium deficiencies in the films. The incorporation of bismuth ions increased the open-circuit voltage and the fill factor of solar cells by increasing the carrier concentration and reducing the resistivity. The efficiency of solar cells increased from 4.37 to 6.29% as bismuth-ion doping f was increased from 0 to 1.0mol%, revealing that the properties of Cu(In,Ga)Se2 absorber layers were effectively improved via the addition of bismuth ions.

Origin of Photoluminescence and XAFS Study of (ZnS)1-x(AgInS2)x Nanocrystals.

Donor-Acceptor transition was previously suggested as a mechanism for luminescence in (ZnS)1-x(AgInS2)x nanocrystals. Here we show the participation of delocalized valence/conduction band in the luminescence. Two emission pathways are observed: Path-1 involves transition between a delocalized state and a localized state exhibiting higher energy and shorter lifetime (∼25 ns) and Path-2 (donor-acceptor) involves two localized defect states exhibiting lower emission energy and longer lifetime (>185 ns). Surprisingly, Path-1 dominates (82% for x = 0.33) for nanocrystals with lower x, in sharp difference with prior assignment. Luminescence peak blue shifts systematically by 0.57 eV with decreasing x because of this large contribution from Path-1. X-ray absorption fine structure (XAFS) study of (ZnS)1-x(AgInS2)x nanocrystals shows larger AgS4 tetrahedra compared with InS4 tetrahedra with Ag-S and In-S bond lengths 2.52 and 2.45 Å respectively, whereas Zn-S bond length is 2.33 Å along with the absence of second nearest-neighbor Zn-S-metal correlation.

Time-resolved and temperature-dependent photoluminescence of ternary and quaternary nanocrystals of CuInS2 with ZnS capping and cation exchange

Time-resolved and temperature-dependent photoluminescence (PL) spectroscopy of ternary compound copper indium disulfide (CuInS2, or CIS) core materials, CIS/ZnS coreshells, and quaternary compound ZnCuInS2 (ZnCIS) revealed their optical properties with spectral, temporal, and thermal characteristics, which were closely linked to surface-related recombination, and shallow or deep defect-related donor-acceptor transitions. The PL peaks of semiconductor nanocrystals (SNCs) with sizes near Bohr radius displayed at ∼775 nm for CIS, ∼605 nm for CIS/ZnS, and ∼611 nm for ZnCIS. The spectral blue shift and spectral narrowing with CIS/ZnS and ZnCIS are assigned to the increased spatial confinement and surface regularity with the etching of core materials. Both the shorter lifetime at surface-trapped states or interface states and the longer lifetime at intrinsic defect-related states of CIS, CIS/ZnS, and ZnCIS SNCs were widely distributed across the entire PL spectral region. The surface or interface-trapped electrons were thermally active even at low temperatures, but the electrons at intrinsic defect-related states were relatively stable, which was attributable to the strong Coulomb energy between the charge carriers.

Temperature dependent donor–acceptor transition of ZnO thin film gas sensor during butane detection

For ZnO thin films excellent hydrogen, carbon monoxide, methane, and butane gas sensing characteristics are reported. First Fourier transformation of the conductance transients together with principal component analyses is found to be very effective in differentiating these combustible gases. At sensor operating temperature in the range 350–380°C, ‘n’ to ‘p’ type carrier reversal in ZnO thin films were observed only during butane sensing. The mechanism, based on the cracking of alkanes (due to the catalytic activity of ZnO), and reaction of the lattice oxygen of ZnO with the highly reactive species (formed after cracking), explained satisfactorily the observed carrier reversal.

Direct observation of Cu, Zn cation disorder in Cu2ZnSnS4 solar cell absorber material using aberration corrected scanning transmission electron microscopy

ABSTRACTChemical analysis of individual atom columns was carried out to determine the crystal structure and local point defect chemistry of Cu2ZnSnS4. Direct evidence for a nanoscale composition inhomogeneity, in the form of Zn enrichment and Cu depletion, was obtained. The lateral size of the composition inhomogeneity was estimated to be between ~1.5 and 5 nm. Photoluminescence confirmed the presence of a broad donor–acceptor transition consistent with the observed cation disorder. Areas of relatively high concentration of ZnCu+ antisite atom donors locally increases the electrostatic potential and gives rise to band bending. Troughs in the conduction band and peaks in the valence band are ‘potential wells’ for electrons and holes, respectively. For a solar cell, these prevent minority carrier electrons from diffusing towards the edge of the space charge region, thereby reducing the carrier separation efficiency as well as reducing the carrier collection efficiency of majority carrier holes. Furthermore, electrons and holes ‘trapped’ within potential wells in close proximity have a high probability of recombining, so that the carrier lifetime is also reduced. High quality Cu2ZnSnS4 crystals free from composition inhomogeneities are therefore required for achieving high efficiency solar cell devices. Copyright © 2012 John Wiley & Sons, Ltd.

Optical signature of Mg-doped GaN: Transfer processes

Mg doping of high quality, metal organic chemical vapor deposition grown GaN films results in distinct traces in their photoluminescence and photoluminescence excitation spectra. We analyze GaN:Mg grown on sapphire substrates and identify two Mg related acceptor states, one additional acceptor state and three donor states that are involved in the donor-acceptor pair band transitions situated at 3.26--3.29 eV in GaN:Mg. The presented determination of the donor-acceptor pair band excitation channels by photoluminescence excitation spectroscopy in conjunction with temperature-dependent photoluminescence measurements results in a direct determination of the donor and acceptor binding, localization, and activation energies, which is put into a broader context based on Haynes's rule. Furthermore, we analyze the biexponential decay dynamics of the photoluminescence signal of the acceptor and donor bound excitons. As all observed lifetimes scale with the localization energy of the donor and acceptor related bound excitons, defect and complex bound excitons can be excluded as their origin. Detailed analysis of the exciton transfer processes in the close energetic vicinity of the GaN band edge reveals excitation via free and bound excitonic channels but also via an excited state as resolved for the deepest localized Mg related acceptor bound exciton. For the two Mg acceptor states, we determine binding energies of 164 $\ifmmode\pm\else\textpm\fi{}$ 5 and 195 $\ifmmode\pm\else\textpm\fi{}$ 5 meV, which is in good agreement with recent density functional theory results. This observation confirms and quantifies the general dual nature of acceptor states in GaN based on the presented analysis of the photoluminescence and photoluminescence excitation spectra.

Solvent-less synthesis of zinc oxide nanostructures from Zn(salen) as precursor and their optical properties

ZnO nanoparticles, 10–20nm in size, were synthesized by heat treatment in air at 500°C for 5h., using [N,N′-bis(salicylaldehydo) ethylene diamine]zinc(II), i.e., Zn(salen), as precursor, which was obtained by a solvent-free solid–solid reaction. Heat-treated products were characterized by X-ray diffraction, Fourier transform infrared spectroscopy, scanning electron microscopy, and transmission electron microscopy. Room temperature photoluminescence spectra of ZnO nanostructures are dominated by green emission attributed to oxygen vacancy related donor–acceptor transition.

A Comparative Study on the Optical and Electrical Properties of Si-Doped Polar and Nonpolar GaN

Si-doped polar (c-plane) and nonpolar (a-plane) GaN layers grown by metal–organic vapor phase epitaxy (MOVPE) were comparatively investigated using photoluminescence (PL) and Hall-effect measurements. While c-plane GaN revealed both band-acceptor and donor–acceptor transitions, the PL spectra for a-plane GaN were related to extended defects such as basal stacking faults (BSFs) and prismatic stacking faults (PSFs). A new emission peak was observed at 3.361 eV in the Si-doped a-plane GaN, which was attributed to Si-doping-induced defects. The temperature-dependent Hall-effect measurements showed that for c-plane GaN, mobility was dominated by optical phonon and ionized impurity scattering at high and low temperature, respectively. Conversely, for a-plane GaN, the scattering mechanism due to dislocations was dominant at all temperatures.

Near-infrared photoluminescence from ZnO

Understanding the defect physics of ZnO is crucial in controlling its properties for various applications. We report the observation of an interesting 1.64 eV near-infrared (NIR) photoluminescence from ZnO and its evolution with annealing temperature. Based on a recent calculation on the transition levels of native point defects of ZnO [A. Janotti and C. G. Van de Walle, Phys. Rev. B 76, 165202 (2007)], the NIR emission can be successfully explained by the donor-acceptor transition between VO and VZn and/or the radiative recombination of shallowly trapped electrons with deeply trapped holes at Oi.

Polaron effects and electric field dependence of the charge carrier mobility in conjugated polymers

Charge transport in conjugated polymers has been investigated using Monte Carlo simulations implemented on top of the Marcus theory for donor-acceptor transition rates. In particular, polaron effects and the dependency of the mobility on the temperature and the applied electric field have been studied. The conclusions are that while the qualitative temperature dependence is similar to that predicted by Miller-Abrahams theory in the Gaussian disorder model (GDM), the electric field dependence is characterized by a crossover into the Marcus inverted region, not present in the GDM. Furthermore, available analytical approximations to describe the electric field dependence of the mobility in Marcus theory fail to fit the simulation data and hence cannot be used to directly draw conclusions about the importance of polaron effects for charge transport in conjugated polymers.