Shichen Su

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.

Post-growth annealing study of heavily Ga-doped zinc oxide grown by radio frequency magnetron sputtering

Heavily doped Ga-doped ZnO (resistivity ?8???10?4???cm and optical transmittance 96%) were fabricated by radio frequency magnetron sputtering. Post-growth annealing studies in H2?Ar and pure Ar atmospheres were conducted. No noticeable thermal-induced change in the Ga depth profile or in the Ga chemical state was found in both atmospheres up to 600??C. The Burstein?Moss effect was exhibited in the effective optical band gap, and the cathodoluminescence (CL) photon energy of the near band edge emission. CL emission related to VZn persisted after the 600??C annealing. The thermal evolutions of the carrier concentrations were nearly identical for both annealing atmospheres. The carrier mobility increased upon annealing in H2/Ar and remained effectively unchanged in pure Ar. The observed changes in carrier concentration and mobility were tentatively associated with VZn, or other compensating defects, which have concentrations as high as 1019?1020?cm?3.

Band alignment of n-SnO2/p-GaN hetero-junction studied by x-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to study band offsets at the n-SnO2/p-GaN hetero-junction. The valence-band offset (�E V) and the conduction-band offset (�E c) are determined to be 0.97 ± 0.2 eV and 0.77 ± 0.2 eV respectively, indicating that the n-SnO2/p-GaN hetero-junction has a type II band alignment.

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 ...

Fabrication, Structural Characterization and Optical Properties of Oversized ZnO Microwires

In this paper, large-sized ZnO microwires (MWs) were successfully synthesized without using any catalyst, following a simple chemical vapor deposition (CVD) method. Highly crystalline ZnO MWs of several hundred micrometers length having diameter in the range of 10–40μm were obtained. The synthesized MWs showed a hexagonal cross-sectional morphology and the longest MW was found to be 1mm long. The structural and optical properties of an individual large sized ZnO MWs were studied. Temperature-dependent photoluminescence (PL) spectra showed emissions for free excitons (FX) and donor-bound excitons (D0X) and decomposition of the D0X into FX was observed beyond 140K. The whispering gallery mode (WGM) lasing in an individual ZnO MW was achieved and also confirmed by two-dimensional finite difference time domain (2D-FDTD) method. These oversized ZnO MWs may find potential applications as WGM resonant cavities and optical waveguides.

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.

Photoluminescence and lasing characteristics of single nonpolar GaN microwires

Nonpolar a-axial GaN MWs were fabricated on a patterned Si substrate via metal–organic chemical vapor deposition (MOCVD) without the assistance of any catalyst. The temperature-dependent photoluminescence (PL) properties of a single GaN MW were discussed comprehensively. Below the temperature of 90 K, the neutral donor-bound exciton (D0X) line dominates in the spectrum, while the free–exciton transition dominates at temperatures above 90 K. The optical properties of GaN MWs exhibit a multiple-mode-stimulated-amplified emission with a peak around 375 nm and a corresponding lasing threshold of about 120 kW cm−2. In addition, the lasing characteristics of GaN MWs were explored by the finite-difference time-domain (FDTD) method.

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.

Reversible Tuning of Ferromagnetism and Resistive Switching in ZnO/Cu Thin Films

Systematic magnetic, electronic, and electrical studies on the Cu0.04Zn0.96O/Ga0.01Zn0.99O cell structure grown on (001) sapphire by the pulsed laser deposition technique show that the Cu multivalent (CuM+) ions modulate magnetic and resistive states of the cells. The magnetic moment is found to be reduced by ∼30% during the high resistance state (HRS) to low resistance state (LRS) switching. X-ray photoelectron spectroscopy results reveals an increase of the Cu+/Cu2+ oxidation state ratio (which has been determined by the relative positions of the Fermi level and the Cu acceptor level) during the HRS to LRS transition. This decreases the effective spin-polarized Cu2+–Vö–Cu+ channels and thus the magnetic moment. A conduction mechanism involving the formation of conductive filaments from the coupling of the CuM+ ions and Vö has been suggested.