Ran Gu

Temperature-dependent photoluminescence of ZnO films codoped with tellurium and nitrogen

The photoluminescence spectra as well as their temperature dependence of the tellurium and nitrogen (Te-N) codoped ZnO films have been investigated in detail. Explicit evidences of the emissions related to two acceptors [A1: the NO-Zn-Te subunits and A2: the conventional N ions substituting on oxygen sites (NO)] have been found. The acceptor activation energy level of the A1 (∼118–124 meV) is much shallower than that of the A2 (∼224–225 meV) indicating that the A1 should be mainly responsible for the room-temperature p-type nature of the codoped samples. Meanwhile, the acceptor activation energy level of A1 shows a slight decrease (∼6 meV) as the Te atomic concentration increases in the codoped samples implying that the actual form of the A1 may be a mixture of the NO-Zn-nTe (n = 1, 2, 3, 4). More incorporation of the Te ions into N-doped ZnO films not only makes the acceptor energy level shallower but also improves the crystalline quality and results in the efficiently suppressed native donorlike defects...

Spin‐polarized tunneling between ferromagnetic films

Using a tunneling Hamiltonian including spin‐flip effect, we derive general expressions for spin‐polarized tunneling conductances as well as magnetoresistance (MR) in ferromagnetic/ insulator/ferromagnetic junctions. The spin‐dependent density of states due to exchange splitting is responsible for the large negative MR, and the spin‐flip tunneling is found to diminish the magnitude of MR. Qualitative agreement is obtained between our theoretical results and recent experimental data.

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.

Mutually beneficial doping of tellurium and nitrogen in ZnO films grown by metal-organic chemical vapor deposition

The advantages of tellurium-nitrogen (Te-N) codoping are investigated in ZnO films grown by metal-organic chemical vapor deposition. Te incorporation gives aid in enhancing the N solubility by lowering its formation energy while N addition helps to stabilize the substituted Te on O sites with the phase-segregated ZnTe crystallites suppressed by forming the N-Zn-Te structures in the N-doped ZnTexO1−x. Carbon related impurities, commonly existing in N-doped ZnO and acting as compensating centers for holes, are fully eliminated by the Te-N codoping. The codoping technique also lowers the energy level of the NO acceptors and leads to realizing N-doped ZnTexO1−x films with holes as major carriers.

Thermal pretreatment of sapphire substrates prior to ZnO buffer layer growth

This research was supported by the State Key Program for Basic Research of China under Grant No. 2011CB302003, National Natural Science Foundation of China (Nos. 61025020, 60990312, and 61274058), Basic Research Program of Jiangsu Province (BK2011437), and the Priority Academic Program Development of Jiangsu Higher Education Institutions.

Temperature-dependent exciton-related transition energies mediated by carrier concentrations in unintentionally Al-doped ZnO films

The authors reported on a carrier-concentration mediation of exciton-related radiative transition energies in Al-doped ZnO films utilizing temperature-dependent (TD) photoluminescence and TD Hall-effect characterizations. The transition energies of free and donor bound excitons consistently change with the measured TD carrier concentrations. Such a carrier-concentration mediation effect can be well described from the view of heavy-doping-induced free-carrier screening and band gap renormalization effects. This study gives an important development to the currently known optical properties of ZnO materials.

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.

Thermal treatment induced change of diluted oxygen doped ZnTe films grown by metal-organic chemical vapor deposition

In this paper, the authors report the growth of diluted oxygen doped ZnTe films (ZnTe:O) by metal-organic chemical vapor deposition (MOCVD). The effect of a post thermal annealing on the properties of the highly mismatched films has been investigated. It is found that the in-situ doping leads to an effective incorporation of oxygen into ZnTe films with different occupation configurations, either on Zn or on Te site. The subsequent annealing process in a vacuum ambient leads to an enhancement of the oxygen incorporation into the ZnTe:O films due to the diffusion of the residual oxygen, while the annealing with the same as-grown sample covered on top of the surface (denoted as “face-to-face” annealing in the text) is beneficial to the improvement of the film quality with manifest intermediate band emission at around 1.9 eV as revealed by the low-temperature photoluminescence. This study indicates that the mass-productive MOCVD technique may be suitable for the growth of highly mismatched ZnTe:O films for th...

Spin and orbital excitations in undoped manganites

We develop a theory for spin and orbital excitations in undoped manganites to account for the spin and orbital orderings observed experimentally. It is found that the anisotropy of the magnetic structure is closely related to the orbital ordering, and the Jahn-Teller effect stabilizes the orbital ordering. The phase diagram and the low-energy excitation spectra for both spin and orbital orderings are obtained. The calculated critical temperatures can be quantitatively comparable to the experimental data.

Transport for currents at angle to the plane of layers in magnetic multilayers

The conductivity in magnetic multilayers for currents at an angle to the plane of the layers (CAP) is related to those for currents parallel (CIP) and perpendicular (CPP) to the plane of the layers. Extra spin-mixing effects have been considered in the calculation for CAP conductivity.