Francis Chi-Chung Ling

Thermal evolution of defects in undoped zinc oxide grown by pulsed laser deposition

Undoped ZnO films are grown by pulsed laser deposition on c-plane sapphire with different oxygen pressures. Thermal evolutions of defects in the ZnO films are studied by secondary ion mass spectroscopy (SIMS), Raman spectroscopy, and positron annihilation spectroscopy (PAS), and with the electrical properties characterized by the room temperature Hall measurement. Oxygen deficient defect related Raman lines 560 cm−1 and 584 cm−1 are identified and their origins are discussed. Thermal annealing induces extensive Zn out-diffusion at the ZnO/sapphire interface and leaves out Zn-vacancy in the ZnO film. Two types of Zn-vacancy related defects with different microstructures are identified in the films. One of them dominates in the samples grown without oxygen. Annealing the sample grown without oxygen or growing the samples in oxygen would favor the Zn-vacancy with another microstructure, and this Zn-vacancy defect persists after 1100 °C annealing.

The Zn-vacancy related green luminescence and donor–acceptor pair emission in ZnO grown by pulsed laser deposition

A low temperature (10 K) photoluminescence study shows that green luminescence (GL) peaking at 2.47 eV and near band edge (NBE) emission at 3.23 eV are introduced in undoped ZnO grown by pulsed laser deposition (PLD) after 900 °C annealing. The NBE emission exhibiting blue shift with increasing temperature is assigned to the transitions of the donor–acceptor-pair (DAP)/free-electron-to-acceptor (FA). Positron annihilation spectroscopy (PAS) study shows that the introduction of the GL is correlated with the formation of the Zn vacancy-related defect (VZn). Comparing the transition energies of VZn obtained by the previous first principles calculation [Janotti and Van de Walle, Phys. Rev. B: Condens. Matter Mater. Phys., 2007, 76, 165202], the GL is associated with the transition from the conduction band to the e(−/2−) state of VZn and the DAP/FA emission involves the acceptor level e(0/−) of VZn.

Nature of red luminescence band in research-grade ZnO single crystals: A “self-activated” configurational transition

By implanting Zn+ ions into research-grade intentionally undoped ZnO single crystal for facilitating Zn interstitials (Zni) and O vacancies (VO) which is revealed by precise X-Ray diffraction rocking curves, we observe an apparent broad red luminescence band with a nearly perfect Gaussian lineshape. This red luminescence band has the zero phonon line at ∼2.4 eV and shows distinctive lattice temperature dependence which is well interpreted with the configurational coordinate model. It also shows a low “kick out” thermal energy and small thermal quenching energy. A “self-activated” optical transition between a shallow donor and the defect center of Zni-VO complex or VZnVO di-vacancies is proposed to be responsible for the red luminescence band. Accompanied with the optical transition, large lattice relaxation simultaneously occurs around the center, as indicated by the generation of multiphonons.

Role of multivalent Cu, oxygen vacancies and CuO nanophase in the ferromagnetic properties of ZnO: Cu thin films

Comprehensive microstructural, electronic and magnetic analyses have been carried out on ZnO:Cu thin films grown by pulsed laser deposition on c-plane sapphire under different oxygen partial pressures. Detailed X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high resolution transmission electron microscopy (HRTEM) analyses reveal that increase in oxygen growth pressure degrades the epitaxy of ZnO:Cu thin films due to inclusion of nanosize CuO in the ZnO host lattice. HRTEM and magnetization studies suggest that thin film quality plays a less effective role in governing the magnetic properties of these samples. Instead, room temperature ferromagnetism (FM) of these ZnO:Cu thin film samples are highly tunable by the simultaneous presence of CuO nanophases and multivalent Cu and concentrations, which are in strong contest with each other. For low oxygen partial pressure grown sample, the effective network is the main contributor to the observed FM and is in competition with CuO nanophases only when there is a relatively low concentration with a dominant Cu2+ oxidation state. For vacuum grown samples containing high concentration and Cu1+ as dominant oxidation state, the network becomes less effective and a CuO nanophase (4–5 nm) is the dominent FM supplier. The extrinsic FM in the vacuum grown sample, which is the best epitaxial quality sample, is further confirmed by the zero field cooled (ZFC) and field cooled (FC) magnetization protocols.

A study of the T2 defect and the emission properties of the E3 deep level in annealed melt grown ZnO single crystals

We report on the space charge spectroscopy studies performed on thermally treated melt-grown single crystal ZnO. The samples were annealed in different ambients at 700 °C and also in oxygen ambient at different temperatures. A shallow donor with a thermal activation enthalpy of 27 meV was observed in the as-received samples by capacitance-temperature, CT scans. After annealing the samples, an increase in the shallow donor concentrations was observed. For the annealed samples, E27 could not be detected and a new shallow donor with a thermal activation enthalpy of 35 meV was detected. For samples annealed above 650 °C, an increase in acceptor concentration was observed which affected the low temperature capacitance. Deep level transient spectroscopy revealed the presence of five deep level defects, E1, E2, E3, E4, and E5 in the as-received samples. Annealing of the samples at 650 °C removes the E4 and E5 deep level defects, while E2 also anneals-out at temperatures above 800 °C. After annealing at 700 °C, t...

Sb-related defects in Sb-doped ZnO thin film grown by pulsed laser deposition

Sb-doped ZnO films were fabricated on c-plane sapphire using the pulsed laser deposition method and characterized by Hall effect measurement, X-ray photoelectron spectroscopy, X-ray diffraction, photoluminescence, and positron annihilation spectroscopy. Systematic studies on the growth conditions with different Sb composition, oxygen pressure, and post-growth annealing were conducted. If the Sb doping concentration is lower than the threshold ∼8 × 1020 cm−3, the as-grown films grown with an appropriate oxygen pressure could be n∼4 × 1020 cm−3. The shallow donor was attributed to the SbZn related defect. Annealing these samples led to the formation of the SbZn-2VZn shallow acceptor which subsequently compensated for the free carrier. For samples with Sb concentration exceeding the threshold, the yielded as-grown samples were highly resistive. X-ray diffraction results showed that the Sb dopant occupied the O site rather than the Zn site as the Sb doping exceeded the threshold, whereas the SbO related deep ...

Band offset and an ultra-fast response UV-VIS photodetector in γ-In2Se3/p-Si heterojunction heterostructures

High-quality γ-In2Se3 thin films and a γ-In2Se3/p-Si heterojunction were prepared using pulse laser deposition (PLD). The band offset of this heterojunction was studied by XPS and the band structure was found to be type II structure. The valence band offset (ΔEv) and the conduction band offset (ΔEc) of the heterojunction were determined to be 1.2 ± 0.1 eV and 0.27 ± 0.1 eV, respectively. The γ-In2Se3/p-Si heterojunction photodetector has high responsivity under UV to visible light illumination. The heterojunction exhibits highly stable photodetection characteristics with an ultrafast response/recovery time of 15/366 μs. The ultrafast response time was attributed to type II structure band alignment, which was good for the separation of electron–hole pairs and it can quickly reduce recombination. These excellent properties make γ-In2Se3/p-Si heterojunctions a promising candidate for photodetector applications.

Zn-vacancy related defects in ZnO grown by pulsed laser deposition

Undoped and Ga-doped ZnO (002) films were grown c-sapphire using the pulsed laser deposition (PLD) method. Znvacancy related defects in the films were studied by different positron annihilation spectroscopy (PAS). These included Doppler broadening spectroscopy (DBS) employing a continuous monenergetic positron beam, and positron lifetime spectroscopy using a pulsed monoenergetic positron beam attached to an electron linear accelerator. Two kinds of Znvacancy related defects namely a monovacancy and a divacancy were identified in the films. In as-grown undoped samples grown with relatively low oxygen pressure P(O2)≤1.3 Pa, monovacancy is the dominant Zn-vacancy related defect. Annealing these samples at 900 oC induced Zn out-diffusion into the substrate and converted the monovacancy to divacancy. For the undoped samples grown with high P(O2)=5 Pa irrespective of the annealing temperature and the as-grown degenerate Ga-doped sample (n=1020 cm-3), divacancy is the dominant Zn-vacancy related defect. The clustering of vacancy will be discussed.

Surface localization of the Er-related optical active centers in Er doped zinc oxide films

Er-doped ZnO films were grown on c-plane sapphire by employing the pulsed laser deposition method. In accordance with the previously reported literature studies, post-growth annealing was required to activate the optical emission originated from the intra-shell transitions of the Er atoms. Importantly, the present systematic studies revealed that the thermal activation of the optical activity is due to the atomistic rearrangements occurring only in the vicinity of the film surface. The processes exhibit a distinct signature of changing oxygen coordination with the Er atom, as observed by the x-ray photoemission study.Er-doped ZnO films were grown on c-plane sapphire by employing the pulsed laser deposition method. In accordance with the previously reported literature studies, post-growth annealing was required to activate the optical emission originated from the intra-shell transitions of the Er atoms. Importantly, the present systematic studies revealed that the thermal activation of the optical activity is due to the atomistic rearrangements occurring only in the vicinity of the film surface. The processes exhibit a distinct signature of changing oxygen coordination with the Er atom, as observed by the x-ray photoemission study.

Zn-vacancy related defects identified in ZnO films grown by pulsed laser deposition

Undoped and Cu-doped ZnO grown on sapphire using pulsed laser deposition (PLD) were studied by positron annihilation spectroscopy (PAS), photoluminescence (PL), high-resolution transmission electron microscopy (HRTEM) and secondary ion mass spectroscopy (SIMS). In the undoped samples, two kinds of VZn-related defects, namely VZn1 and VZn2 are identified. VZn1 was identified in as-grown samples grown at relatively low substrate (~300 °C). After annealing at 900 °C, VZn-2, the green luminescence (GL) peaking at 2.47 eV and the near band edge (NBE) emission at 3.23 eV in the low temperature photoluminescence (LT-PL) were simultaneously introduced. Another kind of VZn-related defect is identified in the Cu-doped ZnO sample, and is tentatively assigned to the VZn decorated with the Cu.