DA Pair Research Articles

High-resolution photoluminescence study on donor-acceptor pair (DAP) recombination in silicon crystals co-doped with phosphorous and gallium

Before lower purity, lower cost silicon (Si) materials, such as compensated Si, can play a role in the terawatt-level (TW) capacity of photovoltaics, a better understanding of the fundamental properties of impurities in compensated Si is essential. In this work, high-resolution photoluminescence (PL) has been used to study the charge carrier radiative recombination through Donor-Acceptor pairs (DAPs) in phosphorus (P) and gallium (Ga) co-doped Si material grown for solar cell applications. The high spectral resolution of our PL system, 0.06 meV, enables us to overcome hitherto prior issues of overlapping spectral lines, giving access to extremely fine structures associated with DA pair (DAP) recombination. Our results confirm the presence of three broad bands and a discrete line structure related to DAP luminescence. The comparison of the discrete line structure due to DAPs recombination in the PL spectra with the theoretically predicted one allows the accurate determination of the Ga ionization energy. Temperature-dependent PL is then used to understand the thermally-induced changes in the DAP luminescence. In particular, we observe that the radiative recombination channel remains active for distant DAPs up to ∼40 K, unlike that for close-range DAPs for which the radiative channel is quenched after only slight increases in the temperature range 10–25 K. Furthermore, the analysis of the temperature dependent changes in the PL intensity of the broad DAP bands up to ∼200 K is used to derive the ionization energy of P donors in compensated Si material. In light of this important information, the significance of using high resolution PL to analyse spectral features in compensated Si is demonstrated.

Microstructural and light emission properties of ZnSnP2 thin film absorber: Study of native defects

Thin films of ZnSnP2 were successfully grown on p-type silicon (001), sapphire and glass substrate by e-beam evaporation method. The as-deposited films were characterized using scanning and high resolution transmission electron microscopes, reflectance and transmittance spectroscopy and photoluminescence measurement at low temperature. Polycrystalline nature of the films was verified by high resolution transmission electron microscopy. An optical band gap of 1.71 eV was estimated at room temperature. Two broad luminescence bands at 1.529 eV and at 1.634 eV were observed at 15 K. The light emission characteristics of the ZnSnP2 were explained in terms of donor-acceptor pair recombination mechanism. Tin-on-zinc sites and zinc vacancies/zinc-on-tin sites were considered as donor and acceptor due to presence of native defects in the films. A schematic band model based on the experimental finding was suggested to account for the DA pair recombination.

Correlation between the local OH stretching vibration wavenumber and the hydrogen bonding pattern of water in a condensed phase: Quantum chemical approach to analyze the broad OH band

A study of the chemical origin of the broad IR absorption band of the O–H stretching vibration (νOH) of liquid water is mentioned. The study is performed by measuring the local νOH mode of the half-deuterated water (HDO) dissolved in deuterated water (D2O) with an aid of quantum chemical calculation. The band position of a local νOH mode is analyzed as a function of hydrogen (H)-bond coordination pattern, which is taken into account by using a useful index of MOH based on the DA3 pattern method. The DA3 method focuses on a HDO–D2O (donor–accepter; DA) molecular pair, and the hydration about the DA pair is categorized into 36 patterns with respect to the H-bond coordination pattern. The MOH index summarizes the patterns by considering the positive and negative contributions of the hydration to the H-bond in the DA pair. The calculated band position of the H-bonded νOH in the pair is readily grouped by using the MOH index, which can also be used as an index for evaluating H-bonding energy. This paper demonstrates the potential of the DA3 method to analyze the νOH band and H-bonding energy of liquid water and ice by referring to QC calculation results for relatively small water clusters.

Photoluminescence of doped ZnS nanoparticles under hydrostatic pressure

AbstractThe pressure dependence of the photoluminescence from ZnS:Mn2+, ZnS:Cu2+, and ZnS:Eu2+ nanoparticles were investigated under hydrostatic pressure up to 6 GPa at room temperature. Both the orange emission from the 4T1 – 6A1 transition of Mn2+ ions and the blue emission from the DA pair transition in the ZnS host were observed in the Mn‐doped samples. The measured pressure coefficients are –34.3(8) meV/GPa for the Mn‐related emission and –3(3) meV/GPa for the DA band, respectively. The emission corresponding to the 4f65d1 – 4f7 transition of Eu2+ ions and the emission related to the transition from the conduction band of ZnS to the t2 level of Cu2+ ions were observed in the Eu‐ and Cu‐doped samples, respectively. The pressure coefficient of the Eu‐related emission was found to be 24.1(5) meV/GPa, while that of the Cu‐related emission is 63.2(9) meV/GPa. The size dependence of the pressure coefficients for the Mn‐related emission was also investigated. The Mn emission shifts to lower energies with increasing pressure and the shift rate (the absolute value of the pressure coefficient) is larger in the ZnS:Mn2+ nanoparticles than in bulk. Moreover, the absolute pressure coefficient increases with the decrease of the particle size. The pressure coefficients calculated based on the crystal field theory are in agreement with the experimental results. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhanced Emission Induced by FRET from a Long-Lifetime, Low Quantum Yield Donor to a Long-Wavelength, High Quantum Yield Acceptor.

We report observation of high quantum yield, long-lifetime fluorescence from a red dye BO-PRO-3 excited by resonance energy transfer (RET). The acceptor fluorescence was highly enhanced upon binding to the donor-labeled DNA. A ruthenium complex (Ru) was chosen as a donor in this system because of its long fluorescence lifetime. Both donor and acceptor were non-covalently bound to DNA. Emission from the donor-acceptor system (DA) at wavelengths exceeding 600 nm still preserves the long-lifetime component of the Ru donor, retaining average fluorescence lifetimes in the range of 30-50 ns. Despite the low quantum yield of the Ru donor in the absence of acceptor, its overall quantum yield of the DA pair was increased by energy transfer to the higher quantum yield acceptor BO-PRO-3. The wavelength-integrated intensity of donor and acceptor bound to DNA was many-fold greater than the intensity of the donor and acceptor separately bound to DNA. The origin of this effect is due to an efficient energy transfer from the donor, competing with non-radiative depopulation of the donor excited state. The distinctive features of DA complexes can be used in the development of a new class of engineered luminophores that display both long lifetime and long-wavelength emission. Similar DA complexes can be applied as proximity indicators, exhibiting strong fluorescence of adjacently located donors and acceptors over the relatively weak fluorescence of separated donors and acceptors.

Electrostatically driven electronic molecular bistability: Ab initio calculation for a model (LiF)n system

Semiempirical theories predict that some donor–acceptor molecules may exhibit long-lived electronic metastable states. The states result from a cooperative effect of successive electron transfers from several donor to acceptor moieties of the molecule. Calculations within the Mo/ller–Plesset second-order perturbation theory following the unrestricted Hartree–Fock, projected unrestricted Hartree–Fock, and complete active space self-consistent field procedures confirm the effect for the first time at the ab initio level. An equidistant linear chain (DA)n, where the donor (D) and acceptor (A) subunits are the lithium and fluorine atoms, respectively, has been chosen as a model for a molecule with fixed in space D and A substituents. The nearest-neighbor LiF distance is set to be sufficiently large to assure the isolated DA pair has lower energy in the neutral DA state than in the ionic D+A− one, i.e., a single electron transfer to occur requires energy. In the (DA)n system, a single electron transfer from D to the nearest A requires a comparable amount of energy (ΔEn1). It is shown, however, that, due to the cooperative nature of the excitations, the excited state corresponding to m such electron transfers (m>ncrit) may have an excitation energy ΔEnm lower than ΔEn1. Due to this a multiply excited state may be close in energy scale to the nonexcited one, both states separated by energy barrier related to ncrit. The effect has been checked against perturbations that mimic dimerization of the chain and a lateral extension of the D+ and A− charge distribution. It turned out that the cooperative effect is likely to survive these perturbations.

Shallow donor levels of Li-doped ZnSe single crystals

Li-doped Zincselenide (ZnSe) single crystals with various Li concentrations were grown by the sublimation method. Three types of donor–acceptor pair (DAP) bands with their phonon replica are observed in their photoluminescence (PL) spectra at 4.2 K. As the Li concentrations increase, the DAP bands seems to shift to the lower energy side. From the excitation intensity and the temperature dependence of PL spectra, three kinds of constituent donors with the activation energies of 17, 28 and 50 meV are detected. It will be proposed that the origins of donors are Lii (Li interstitials), ClSe (substitutional Cl in Se sites) and complex donors, respectively. These donors become dominant, in turn, to form DA pair recombination as the Li concentrations increase.

Investigation of luminescence properties of GaN single crystals grown on 3C-SiC substrates

Cathodoluminescence (CL) and photoluminescence (PL) investigations of single crystal GaN grown on 3C-SiC(100) substrates by metalorganic vapor-phase epitaxy are reported. The GaN grown on 3C-SiC substrates shows distinct crystal facets. We identified the square facet and inclined facet as cubic GaN and hexagonal GaN, respectively. We studied the luminescence properties from the square facet or from the inclined facets separately by CL. For the cubic GaN, a sharp excitonic transition and a weak DA pair transition were observed from the square facet. On the other hand, an emission line at 3.293eV was obtained from the inclined facets. By studying its temperature dependence, we suggest that the 3.293eV emission is due to the free electron to acceptor transition of hexagonal GaN.

Nitrogen-doped ZnSe(100) films grown by molecular beam epitaxy using a capacitively coupled plasma cell

We have tried nitrogen doping to ZnSe using a capacitively coupled RF plasma cell. The plasma was very stable throughout the doping experiment. The net acceptor concentration (Na − Nd) showed a dip with increasing the nitrogen supply to the cell. The highest Na − Nd obtained was as high as 6 × 1017cm−3. This value seems to be limited by the pumping ability of our system. In the photoluminescence spectra, deep DA pair emissions were dominantly observed in almost all the samples. In the plasma emission spectra, the emissions of N2 radicals were clearly observed, but those of excited N atoms were not observed. The relation between Na − Nd and the plasma emissions is discussed.

Possible charge-transfer modified band structure of organic conductors

The Madelung energies of TTF·TCNQ and NaTCNQ have been calculated including the effect of self-consistent-field screening. Analysis of the energy levels of the DA pair disclosed a remarkable effect of the crystal field on the ionic (D +A −) and neutral (D 0A 0) energy levels. A diagram for the charge-dependent band structure of TTF·TCNQ is presented. It describes correctly the metal-semiconductor transition, the peak of electrical conductivity in this crystal, and explains the role of its one-dimensional structure. Satisfactory estimates of the lattice energy and the charge transfer have been obtained. The semiconducting properties and lattice energy of NaTCNQ are also described by this model.

Molecular beam epitaxy growth of ZnSe on (100)GaAs by compound source and separate source evaporation: A comparative study

ZnSe layers have been grown by MBE by both compound source and separate source evaporation on (100)GaAs substrates. Photoluminescence measurements on layers grown from a compound source showed the material to contain a high concentration of impurities and defects as indicated by the presence of large DA pair recombination and SA emission peaks in the PL spectra. Layers grown from separate sources were grown at various Zn to Se beam pressure ratios ranging from 1 to 30, and with substrate temperatures ranging from 300 to 400 °C. For layers grown at 900 Å/h with a Zn to Se beam pressure ratio of 1:1 the 4.2 K PL spectra were dominated by a Ga-bound exciton at 2.7982 eV the intensity of which was strongly substrate temperature dependent. It was found that the Ga-bound exciton peak could be suppressed by using higher Zn to Se ratios, and at beam pressure ratios >10 the dominant exciton peak was a free-exciton related peak at 2.800 eV. DA pair recombination was not detectable in layers grown from separate sources. Layers grown from separate sources at a higher growth rate (∼0.4 μm/h) contained additional defects indicated by the presence of an Ix peak (exciton bound to a native defect) and a much smaller unidentified peak at 2.49 eV.

ODMR studies of antisite-related luminescence in GaP

By the use of ODMR, we have detected and identified two different antisite defects which contribute to newly-reported infrared luminescence observed in GaP. The antisite proper, P Ga, gives rise to a spin- 1 2 resonance; the associated luminescence originates in the DA pair recombination of an electron trapped at the deep double donor P + Ga with a distant shallow acceptor. An impurity-associated centre of the form P Ga · Y P is observed in a spin-triplet state via a distinct luminescence band which originates in electron-hole recombination localised at the defect.

Luminescence Excitation Spectra and Their Exciton Structures of ZnS Phosphors. I. Mn, (Cu, Al), (Ag, Al) and (Au, Al) Doped Phosphors

Single-ion luminescence due to Mn2+ and donor-acceptor pair luminescence due to (Cu, Al), (Ag, Al) and (Au, Al) centered in hexagonal ZnS have been investigated at 75 K with a major focus on exciton structures in the excitation spectra. Appearance of exciton peaks in Mn2+ luminescence excitation spectra indicates that the isoelectronic Mn center can bind a free exciton. Such an exciton binding is caused by the hole-binding nature of the Mn center. The A1 exciton line is accompanied by an oscillatory structure with constant intervals of 356 cm-1 due to indirect hot exciton creation. Exciton structure in the DA pair excitation spectra varies from ambiguous to distinctive dips with increasing monitored wavelength. Distant DA pairs do not bind free excitons, but close pairs can do so.