Lithium Acceptors Research Articles

Broad luminescence from donor-complexed LiZn and NaZn acceptors in ZnO

Zn substitutional lithium (${\mathrm{Li}}_{\text{Zn}}$) and sodium (${\mathrm{Na}}_{\text{Zn}}$) acceptors and their complexes with common donor impurities (${\mathrm{Al}}_{\text{Zn}}, {\mathrm{H}}_{i}$, and ${\mathrm{H}}_{\text{O}}$) in ZnO have been studied using hybrid functional calculations. The results show that the complexes are not exclusively charge neutral, but rather exhibit a thermodynamic ($+/0$) transition level close to the valence band maximum. The positive charge states are associated with a polaronic defect state, similar to those of the well-studied charge-neutral isolated acceptors. This incomplete passivation has profound consequences for the optical properties of the complexes. Indeed, electron transitions from the conduction band minimum to the ($+/0$) transition level of the complexes result in broad luminescence bands that are blueshifted with respect to those originating from the isolated acceptors. Such complexes are proposed as a potential defect origin of the green luminescence observed at the high-energy side of the orange luminescence band (caused by ${\mathrm{Li}}_{\text{Zn}}$) in hydrothermally grown ZnO. This prediction is supported by experimental photoluminescence and secondary ion mass spectrometry data on a hydrothermally grown ZnO sample. We have also explored how the parameters controlling the fraction and screening of exchange in the Heyd-Scuseria-Ernzerhof (HSE) hybrid functional influence the results by comparing two parametrization approaches: (i) the conventional one where the exchange fraction is adjusted to reproduce the experimental band gap, and (ii) tuning both parameters in order to also comply with the generalized Koopmans theorem (gKT). Interestingly, these approaches were found to yield similar results.

The interaction between lithium acceptors and gallium donors in zinc oxide

Diffusion of lithium (Li) in uniformly gallium (Ga)-doped monocrystalline bulk zinc oxide (ZnO) is studied over a wide temperature range (500–1150°C) and is demonstrated to be dictated by the distribution of Ga. Below 800°C, the indiffusion of Li from a Li-doped ZnO sputtered film into n+ single crystalline ZnO yields an abrupt and compensated Li-doped box region with the Li concentration matching the free-electron concentration, in accordance with several previous experimental and theoretical reports. However, experimental observations of Li-diffusion at higher temperatures reveal a dissociative diffusion mechanism for heat treatments up to 1150°C. By employing a reaction-diffusion model that includes both Li and Ga, a dissociation energy of 4.6 eV is obtained from the experimental Li diffusion data. This is in excellent agreement with theoretical results for the dissociation of (LiZnGaZn)0 (4.8 eV) into Lii+ and (GaZnVZn)− and suggests that this neutral and stable acceptor-donor pair prevails in Li- and Ga-doped ZnO.

Manifestation of spin-dependent recombination in afterglow of zinc oxide crystals

Long-lasting afterglow of ZnO single crystals grown by hydrothermal method has been studied using high-frequency (94 GHz) optically detected magnetic resonance (ODMR) at helium temperatures. Giant (up to 30%) afterglow intensity-detected electron paramagnetic resonance signals of shallow donors and deep acceptors, which are lithium atoms substituting for zinc in the ZnO lattice, unambiguously indicate a spin-dependent character of the donor-acceptor recombination. The observed variations in the shape of the afterglow-detected ODMR spectra of lithium acceptors with time passed after irradiation are associated with the difference in the recombination rates of shallow donors with axial and non-axial LiZn centers and with a change in the recombination character with time.

Interplay Between Lithium Bonding and Halogen Bonding in F3CX•••YLi•••NCCN and F3CX•••NCCN•••LiY Complexes (X = Cl, Br; Y = CN, NC)

MP2 calculations with cc-pVTZ basis set were used to analyze intermolecular interactions in F3CX···YLi···NCCN and F3CX···NCCN···LiY triads (X = Cl, Br; Y = CN, NC) which are connected via halogen and lithium bonds. Those complexes with the role of LiY as halogen acceptor and lithium donor show cooperativity with energy values ranging between -1.97 and -2.92 kJ mol -1 . Those complexes with simultaneous role of NCCN as halogen and lithium acceptor are diminutive with energetic effects between 1.24 and 1.86 kJ mol -1 . Results of energy decomposition analysis revealed that the electrostatic interactions are the major source of the attraction in the title complexes. The nuclear quadrupole coupling constant values at the sites of halogen atoms can be regarded as good descriptors to quantify the degree of cooperative/diminutive effects in the title systems.

Raman and Photoluminescence Spectroscopic Detection of Surface-Bound Li+O2− Defect Sites in Li-Doped ZnO Nanocrystals Derived from Molecular Precursors

We present a detailed study of Raman spectroscopy and photoluminescence measurements on Li-doped ZnO nanocrystals with varying lithium concentrations. The samples were prepared starting from molecular precursors at low temperature. The Raman spectra revealed several sharp lines in the range of 100-200 cm(-1), which are attributed to acoustical phonons. In the high-energy range two peaks were observed at 735 cm(-1) and 1090 cm(-1). Excitation-dependent Raman spectroscopy of the 1090 cm(-1) mode revealed resonance enhancement at excitation energies around 2.2 eV. This energy coincides with an emission band in the photoluminescence spectra. The emission is attributed to the deep lithium acceptor and intrinsic point defects such as oxygen vacancies. Based on the combined Raman and PL results, we introduce a model of surface-bound LiO(2) defect sites, that is, the presence of Li(+)O(2)(-) superoxide. Accordingly, the observed Raman peaks at 735 cm(-1) and 1090 cm(-1) are assigned to Li-O and O-O vibrations of LiO(2).

Persistent photoinduced changes in charge states of transition-metal donors in hydrothermally grown ZnO crystals

Electron paramagnetic resonance (EPR) has been used to monitor photoinduced changes in the charge states of deep transition-metal donors (Mn, Fe, Co, and Ni), shallow donors (Al and Ga), and lithium acceptors in a hydrothermally grown ZnO crystal. All of these impurities except the lithium were unintentionally present. Changes in charge states were photoinduced at low temperature using 325nm light. Before illumination, the transition-metal donors were present in divalent and trivalent charge states, and all the lithium acceptors were in the singly ionized charge state. During illumination, neutral lithium acceptors were formed, the Fe3+, Ni3+, and Mn3+ spectra disappeared (i.e., they converted to divalent states), and the Mn2+ and Co2+ spectra increased. When the light was removed, these changes in charge states were persistent and stable at low temperature. The thermal stabilities of the changes were monitored during a pulsed anneal (the sample was warmed in steps to sequentially higher temperatures and returned each time to a lower monitoring temperature). Electrons were released from Fe2+ donors in the 90–120K range, from Ni2+ and Co2+ donors in the 110–140K range, and from Mn2+ donors in the 220–300K range. Thermal activation energies for Fe2+ and Ni2+ are estimated to be 240 and 280meV, respectively.

Acceptor doping in ZnO with group‐I elements

AbstractWe study the interaction of hydrogen donors and Lithium acceptors in bulk ZnO by electron paramagnetic resonance and photoluminescence. With increasing diffusion temperature the number of hydrogen donors increases whereas the number of deep Li acceptors decreases. In the temperature range between 450 and 600 °C a shallow Li related acceptor is formed. The very same defect is found in Li diffused ZnO, and in epitaxial ZnO thin films by in‐situ doping with Li. We suggest a tentative model for the shallow Li acceptor whereby hydrogen would be an essential part of the defect core. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Lithium and sodium acceptors in ZnO

In earlier investigations it was shown that Li and Na produce deep acceptor states which participate in shallow donor to deep acceptor recombinations in the visible spectral region. We will demonstrate that in the case of group-I elements either incorporated by diffusion or in situ during growth, shallow acceptors are introduced also, having binding energies around 300 meV. The corresponding donor–acceptor pair recombinations have weak electron–phonon coupling.

Infrared absorption from OH− ions adjacent to lithium acceptors in hydrothermally grown ZnO

An intense infrared absorption band has been observed in a hydrothermally grown ZnO crystal. At 12K, the band peaks near 3577.3cm−1 and has a half width of 0.40cm−1, and at 300K, the band peaks at 3547cm−1 and has a half width of 41.3cm−1. This absorption band is highly polarized, with its maximum intensity occurring when the electric field of the measuring light is parallel to the c axis of the crystal. Photoinduced electron-paramagnetic-resonance experiments show that the crystal contains lithium acceptors (i.e., lithium ions occupying zinc sites). Lithium and OH− ions are present in the crystal because lithium carbonate, sodium hydroxide, and potassium hydroxide are used as solvents during the hydrothermal growth. In the as-grown crystal, some of the lithium acceptors will have an OH− ion located at an adjacent axial oxygen site (to serve as a passivator), and we assign the 3577.3-cm−1 band observed at 12K to these neutral complexes. Our results illustrate the role of hydrogen as a charge compensator for singly ionized acceptors in ZnO.

Thermal diffusion of lithium acceptors into ZnO crystals

Electron paramagnetic resonance (EPR) has been used to monitor the diffusion of lithium ions into single crystals of ZnO. The in-diffusion occurs when a crystal is embedded in LiF powder and then held in air at temperatures near 750°C for periods of time ranging up to 22 h. These added lithium ions occupy zinc sites and become singly ionized acceptors (because the material is initially n type). A corresponding reduction in the number of neutral shallow donors is observed with EPR. To monitor the lithium acceptors, we temporarily convert them to the EPR-active neutral acceptor state by exposure to laser light (325 nm or 442 nm) at low temperatures. Also, after each diffusion treatment, we monitor the EPR signal of singly ionized copper acceptors and the photo-induced EPR signal of neutral nitrogen acceptors. These nitrogen and copper impurities are initially present in the crystal, at trace levels, and are made observable by the thermal anneals. Infrared-absorption measurements at room temperature in the 2–10 µm region show that the concentration of free carriers decreases as lithium is added to the crystal. After 22 h at 750°C in the LiF powder, the free-carrier absorption is no longer present, and the crystal is semi-insulating.

Luminescence properties of lithium-doped ZnS epitaxial layers grown by MOCVD

Lithium-doped p-type ZnS epitaxial layers have been grown on (0 0 1)-oriented GaAs substrates by low-pressure metalorganic chemical vapor deposition using tertiarybutyllithium as an acceptor dopant source. As-grown samples exhibited electrically high resistive n-type or p-type conductivity. However, all samples exhibited p-type conductivity after annealing in N 2 atmosphere at 550°C. The highest net acceptor concentration ( N A− N D) was estimated to be about 4.1×10 16 cm −3. Photoluminescence (PL) spectra of lithium-doped ZnS epitaxial layers with N A− N D<10 15 cm −3 were dominated by the radiative recombination of excitons bound to a lithium acceptor at 327.6 nm. On the other hand, PL spectra of lithium-doped ZnS epitaxial layers with N A− N D>10 15 cm −3 were dominated by free-to-acceptor emission with LO phonon replicas. From the peak energy of the FA emission, the lithium acceptor ionization energy was estimated to be about 196 meV.

Luminescence and EPR Study of Lithium-Diffused ZnO Crystals

ABSTRACTZinc oxide (ZnO) crystals grown by the seeded chemical vapor transport method have been studied using photoluminescence (PL), thermoluminescence (TL), and electron paramagnetic resonance (EPR) techniques. Lithium acceptors were diffused into the crystals during anneals in LiF powder at temperatures in the 750 to 850°C range. After a lithium diffusion, EPR was used to monitor neutral lithium acceptors and neutral shallow donors, as well as Ni3+, Fe3+, and Cu2+ impurities unintentionally present. Excitonic and deep-level PL emissions were also monitored before and after these diffusions. Two broad overlapping TL emission bands were observed at 117 and 145 K when a Li-diffused crystal was illuminated at 77 K with 325-nm light and then rapidly warmed to room temperature. The two TL bands have the same spectral dependence (the peak in wavelength is 540 nm when the intensity of the light reaches a maximum). These “glow” peaks occur when electrons are thermally released from Ni2+ and Fe2+ ions and recombine with holes at neutral lithium acceptors.

Thermally Stimulated Electron–Hole and Ionic Processes in Zinc Oxide Crystals

The principal results of the investigation of thermally stimulated electron–hole and ionic processes in hydrothermal and gas-phase ZnO single crystals preexcited at low temperatures, based on simultaneous study of photo-EPR and thermoluminescence (TL), are presented. The nature of the traps determining the TL peaks at 17, 24, 40, 53, 90–110, 140–150, and 160–200 K is discussed. In particular, it has been established that the lithium paramagnetic centers (LiZn+–OI) play the role of hole traps in ZnO giving green and red TL in the temperature range 160–200 K and, in the case of association with small-sized donors, also TL in the temperature range 90–110 K. The other traps are electronic in character, and in the presence of acceptor lithium in the crystals, they form yellow-orange TL. Optical quenching of TL has been evaluated, and it has been found that there is a difference ΔE ≈ 0.75 eV between the thermal and optical energies of ionization of lithium acceptors. Irreversible ionic processes associated with the “healing” of cationic vacancies at T ≈ 360–420 K have been revealed.

Optical studies of irradiated crystalline silicon doped with carbon and lithium

Many lithium and carbon related photoluminescence lines are observed in irradiated crystalline silicon doped with lithium and carbon which were not reported previously. The half-time for the thermal decay of the absorption intensities of the G line at 969 meV and the Q line at 1045 meV is less than 15 min, which is smaller than the published data. We believe that the fast decay of the absorption intensities of both centres is due to the switching between different charge states. The radiation damage 4Li-V defect, like the dicarbon centre, can have different charge states. New lithium and carbon related donor or acceptor systems may be created after annealing the irradiated samples at 230 C.

Solid-state multiple-pulse proton NMR studies of doped Poly( p-phenylene)

Recently, it has been found that the conductivity of poly(p-phenylene) (PPP) increases by more than ten orders of magnitude upon doping with certain strong electron donors or acceptors [1]. Few studies of this doping mechanism have been reported [2]. By virtue of its sensitivity to the electronic environment of the nucleus, the solid-state NMR technique has provided important information on similar processes in the doping and conduct ion in polyacetylene [3, 4]. We report studies of the proton resonance in poly(p-phenylene) using multiple-pulse NMR techniques which indicate that these techniques may be useful for probing the environment of such doped conductive polyphenylenes. Many substances have been found to be good dopants for PPP [2], including electron donors such as metal naphthylides and the pure metals and alloys of lithium, sodium, and potassium, and electron acceptors such as arsenic pentafluoride, ant imony pentachloride, fluorosulfonic acid, iodine pentafluoride, and ferric chloride. Our studies include PPP doped with all of these except arsenic pentafluoride, the metal alloys, and lithium. Samples for s tudy by NMR spectroscopy were prepared in NMR tubes and sealed while attached to the vacuum manifold. For liquid dopants, PPP was exposed to the dopant directly. After reaction, excess dopant was removed by evacuation and mild heating, if necessary. Metal dopants were heated to melting to produce the doping reaction. Metal naphthylides were prepared by a standard technique and doping was accomplished as with the liquid dopants, the THF being removed by evacuation. Ferric chloride was dissolved in nitromethane for doping. Typical doping times were in the range of four to seven days. Solid-state proton NMR spectra were obtained using the 8-pulse sequence first demonstrated by Rhim e t al. [5]. The sequence greatly at tenuates dipole-dipole broadening of the resonance line, as shown in Fig. 1 for PPP. As can be seen, the use of the 8-pulse sequence reduces the linewidth by nearly two orders of magnitude. The important point to be made is that the lineshape of the multiple-pulse spectrum does not appear to be dominated by chemical-shift anisotropy. The line is approximately Gaussian

Study of acceptor states in CdTe by donor-acceptor pair excitation luminescence

Excitation of donor-acceptor pair luminescence has been studied in CdTe doped with lithium or chlorine. The excitation spectrum of the lithium acceptor is determined and fitted with the effective mass theory of Baldereschi and Lipari. Revised values of the valence band parameters are deduced: μ = 0.8, δ = 0.054, Ry = 24 meV. The analysis of the 1.45 eV luminescence band in compensated Cl-doped crystals shows the existence of donor-acceptor pair transitions. Three acceptor centers are identified: E A = 89, 111 and 119 meV, and the contribution of a deep donor ( E D > 40 meV) is demonstrated. Besides intracentre type excitation transitions of the 1.45 eV band have been observed in non-compensated chlorine-doped crystals. Thus several recombination channels and distinct acceptor states contribute to the composite 1.45 eV luminescence band.

Excitation spectrum of the lithium acceptor in CdTe and estimation of the valence band parameters

The infrared absorption of lithium doped CdTe shows sharp lines in the energy range 30–80 meV which are attributed to electronic transitions from the ground state to the excited states in the lithium acceptor and LO phonon assisted transitions. This excitation spectrum is analyzed using the theory of Baldereschi and Lipari what leads to a determination of the Luttinger valence band parameters.

Exchange broadened, optically detected ESR spectra for luminescent donor-acceptor pairs in Li doped ZnO

Application of optically detected ESR to the yellow photoluminescence of Li doped ZnO gives ESR spectra for shallow donor - lithium acceptor pairs, showing that at least a fraction of the yellow emission is donor-acceptor (D-A) luminescence. The distribution of separations r DA gives a spectrum of D-A exchange interactions J which broaden the ESR lines, particularly at high microwave power, where pairs with long excited state lifetimes, that is large r DA, are power saturated.

Hydrogen-Multivalent Acceptor Complexes in High-Purity Germanium

Using copper (a fast diffusing, multivalent acceptor impurity in Ge) we have shown that hydrogen can form complexes with multivalent acceptors. Hydrogen is incorporated in the Ge single crystal during its growth from a melt in a H2 atmosphere. By analogy with the interaction between lithium and multivalent acceptors we find two acceptor like complexes: Cu-H and Cu-H2. Using Photothermal Ionization Spectroscopy and Hall effect we assign the following energy levels: Cu-H: EV + 17.5 meV and EV + 170 ± 20 meV and Cu-H2: EV + 17.0 meV. According to our model the Cu-H3 complex is neutral. Consequences for the performance of large volume radiation detectors are discussed.