Zhonghua Xu

Annealing in tellurium-nitrogen co-doped ZnO films: The roles of intrinsic zinc defects

In this article, the authors have conducted an extensive investigation on the roles of intrinsic zinc defects by annealing of a batch of Te-N co-doped ZnO films. The formation and annihilation of Zn interstitial (Zni) clusters have been found in samples with different annealing temperatures. Electrical and Raman measurements have shown that the Zni clusters are a significant compensation source to holes, and the Te co-doping has a notable effect on suppressing the Zni clusters. Meanwhile, shallow acceptors have been identified in photoluminescence spectra. The NO-Zn-Te complex, zinc vacancy (VZn)-NO complex, and VZn clusters are thought to be the candidates as the shallow acceptors. The evolution of shallow acceptors upon annealing temperature have been also studied. The clustering of VZn at high annealing temperature is proposed to be a possible candidate as a stable acceptor in ZnO.

Influence of oxygen precursors and annealing on Fe3O4 films grown on GaN templates by metal organic chemical vapor deposition

O2 and N2O, popular oxygen precursors for oxide films growth, have been employed to grow Fe3O4 films on GaN templates via metal organic chemical vapor deposition (MOCVD). A (111)-oriented Fe3O4 film was preferably deposited when N2O was used as O precursor, while a Fe2O3 film was grown with O2 as O precursor. A high-temperature annealing has caused a phase transition from α-Fe2O3 to Fe3O4 for O2 case, but no obvious change occurred on the Fe3O4 film for N2O case. Thinner Fe3O4 layer was then grown on a GaN template with N2O as O precursor to form ferromagnetic material (FM)/nonmagnetic material (NM) heterostructure, which is critical for the realization of spin injection in GaN based wide band gap semiconductors. The Ga diffusion from the GaN template to the Fe3O4 layer has been controlled at a rather low level possibly due to the employed low-temperature growth, leading to a high-quality FM/NM heterostructure. An obvious enhancement on the Ga diffusion has been observed upon subsequent annealing for the ...

High temperature sodium ordering transition in NaxCoO2 studied by mechanical spectrum

The mechanical spectrum of γ-NaxCoO2 above room temperature was measured using the vibrating reed method at kilohertz frequency. A sodium ordering transition around 427 K was previously deduced by the mechanical spectrum of Na0.66CoO2, which is annealed at 400 °C in the flowing oxygen gas. A step increase in modulus with the decrease in temperature was observed at this transition. Similar results have been observed in as-prepared and annealed Na0.62CoO2 samples around 430 K. However, modulus softening was observed around 385 K in as-prepared Na0.66CoO2, and the step increase in modulus with the decrease in temperature was observed around 345 K. The modulus softening is expected to be from a structural phase transition. This structural phase transition shifts the sodium ordering transition to 345 K in as-prepared Na0.66CoO2 system.

Identification and control of native defects in N-doped ZnO microrods

In this article, the authors have investigated the properties of the popular native defects in nitrogen-doped ZnO microrod samples grown by the chemical vapor transport method. Excellent crystalline quality has been confirmed in the samples. Optical signatures of zinc interstitials and zinc vacancies have been observed by employing Raman and variable temperature photoluminescence. By tuning the flow rate of oxidant (nitrous oxide) during growth, the concentration of zinc interstitials and vacancies can be modified. When the flow rate of the nitrous oxide is high, the zinc interstitials can be suppressed while the zinc vacancy-related shallow acceptors can be enhanced. These are both beneficial to the realization and further enhancement of p-type conductivity in ZnO material. This study provides a good understanding of the properties of the native point defects in nitrogen-doped ZnO microrods and also offers a simple way to control the defects towards p-type direction.

Thermal evolution of zinc interstitial related donors in high-quality NH 3 -doped ZnO films

In this paper, the authors have investigated the thermal evolution of the forms, density, and electrical properties of the zinc interstitial (Zni) related donors in NH3-doped ZnO films via annealing the as-grown sample at different temperatures. The relatively high crystalline quality has eliminated the effect from grain boundaries and thus guaranteed the validity of the study. Generally, in the presence of nitrogen, the main forms of Zni are the Zni-NO complexes and the Zni small clusters. When increasing the annealing temperature to a moderate level (around 700 °C), the dissociation of the Zni and the NO in the Zni-NO complexes would make them partially desorb from the sample. Meanwhile, part of the isolated Zni created from the dissociation would aggregate to form the Zni small clusters (Zni-Zni). When increasing the annealing temperature to 900 °C, the desorption of the Zni-NO complex would continue, but the Zni small clusters are no longer thermally stable. They would decompose into isolated Zni atoms and finally desorb from the sample. As the Zni-NO complexes and the Zni small clusters are both shallow donors, their gradual desorption while increasing the annealing temperature results in a reduced compensation level. Furthermore, using the NH3 as the nitrogen doping source could bring in a complex shallow acceptor in the form of (NH4)Zn. Simultaneously, annealing at high temperatures (900 °C) may result in the clustering of zinc vacancies. Therefore, the current method proposed in this work could be a feasible path to enhancing the p-type doping efficiency in nitrogen-doped ZnO material.

Photo-assisted hysteresis of electronic transport for ZnO nanowire transistors

Recently, ZnO nanowire field effect transistors (FETs) have received renewed interest due to their extraordinary low dimensionality and high sensitivity to external chemical environments and illumination conditions. These prominent properties have promising potential in nanoscale chemical and photo-sensors. In this article, we have fabricated ZnO nanowire FETs and have found hysteresis behavior in their transfer characteristics. The mechanism and dynamics of the hysteresis phenomena have been investigated in detail by varying the sweeping rate and range of the gate bias with and without light irradiation. Significantly, light irradiation is of great importance on charge trapping by regulating adsorption and desorption of oxygen at the interface of ZnO/SiO2. Carriers excited by light irradiation can dramatically promote trapping/detrapping processes. With the assistance of light illumination, we have demonstrated a photon-assisted nonvolatile memory which employs the ZnO nanowire FET. The device exhibits reliable programming/erasing operations and a large on/off ratio. The proposed proto-type memory has thus provided a possible novel path for creating a memory functionality to other low-dimensional material systems.

Identification and tuning of zinc-site nitrogen-related complexes in ZnO material

In this article, the authors have designed an experiment to artificially introduce the recently proposed zinc-site nitrogen complex acceptors. The novel acceptors have been introduced in ZnO film samples grown at an atmosphere with extra-high oxygen partial pressure. The shallowness of the nitrogen complex acceptors has been experimentally verified. The concentration of the nitrogen-related complex acceptors could be tuned by varying the VI/II ratio in gas during growth. The modulation of the carrier concentration has been observed in accordance with the change of the complex concentration. Although theories predict higher formation rate of the complexes as monotonically increasing the oxygen chemical potential, experiments show that oxygen interstitials and/or antisite oxygen would compete the formation of the zinc-site nitrogen acceptors at extra-oxygen-rich gaseous condition.

Optical fingerprints of donors and acceptors in high-quality NH 3 -doped ZnO films

In this article, the authors have studied the optical properties of the donor-like and acceptor-like defects in a batch of NH3-doped ZnO films with high crystalline quality. The donors and acceptors responsible for the low-temperature photoluminescence lines have been clearly revealed. The main form of the shallow donors has been determined as the Zni-NO complex. A few possibilities have been proposed for the shallow acceptors, including the (NH4)Zn, (N2)Zn, and VZn small clusters. The transition within the Frenkel pair (Zni-VZn) should be the origin for the green deep-level emission. The post-growth annealing process could change the amounts of the donors and acceptors. The shallow and deep acceptors tend to form at higher annealing temperatures while the shallow donors could be suppressed simultaneously. Possible mechanisms of how the annealing process affecting the defects formation have been also discussed.

The Luminescent Inhomogeneity and the Distribution of Zinc Vacancy-Related Acceptor-Like Defects in N-Doped ZnO Microrods

Vertically aligned N-doped ZnO microrods with a hexagonal symmetry were fabricated via the chemical vapor transport with abundant N2O as both O and N precursors. We have demonstrated the suppression of the zinc interstitial-related shallow donor defects and have identified the zinc vacancy-related shallow and deep acceptor states by temperature variable photoluminescence in O-rich growth environment. Through spatially resolved cathodoluminescence spectra, we found the luminescent inhomogeneity in the sample with a core-shell structure. The deep acceptor-isolated VZn and the shallow acceptor VZn-related complex or clusters mainly distribute in the shell region.