Chunyang Kong

Tunable zinc interstitial related defects in ZnMgO and ZnCdO films

We report tunable band gap of ZnO thin films grown on quartz substrates by radio frequency magnetron sputtering. The zinc interstitial (Zni) defects in ZnO films were investigated by X-ray diffraction, Raman scattering, Auger spectra, first-principle calculations, and Hall measurement. Undoped ZnO film exhibits an anomalous Raman mode at 275 cm−1. We first report that 275 cm−1 mode also can be observed in ZnO films alloyed with Mg and Cd, whose Raman intensities, interestingly, decrease and increase with increasing Mg and Cd alloying content, respectively. Combined with the previous investigations, it is deduced that 275 cm−1 mode is attributed to Zni related defects, which is demonstrated by our further experiment and theoretical calculation. Consequently, the concentration of Zni related defects in ZnO can be tuned by alloying Mg and Cd impurity, which gives rise to different conductivity in ZnO films. These investigations help to further understand the controversial origin of the additional Raman mode ...

A theoretical calculation of the piezoresistivity and magnetoresistivity in p-type semiconducting diamond films

A theoretical study of the piezoresistivity and magnetoresistivity in p-type heteroepitaxial diamond films is presented, based on the Fuchs-Sondheimer thin-film theory and the valence band split-off model. The Boltzmann transport equation in the relaxation time approximation was solved and a mixed scattering by lattice vibrations, ionized impurities and surfaces was considered. The analytical expressions for the piezoresistive and magnetoresistive effects has been developed in a parallel-connection resistance model for the light-hole, the heavy-hole and the split-off bands. The calculated results are discussed and compared with experimental data. It is found that the self-spin interaction of the heavy holes could be related to stress. Moreover, a possible mechanism is presented to explain the fact that the magnetoresistivity of the p-type heteroepitaxial diamond films is greater than that of the diamond bulk.

EFFECT OF THICKNESS ON THE PROPERTIES OF In-DOPED ZnO THIN FILMS PREPARED BY RF MAGNETRON SPUTTERING

In-doped zinc oxide (ZnO:In) thin films with thickness from 157 nm to 592 nm have been deposited on glass substrates by radio frequency (RF) magnetron sputtering. The effect of the film thickness on the structural, electrical and optical properties of ZnO:In thin films has been investigated. It is found that the films are hexagonal wurtzite structure with c-axis perpendicular to the substrate, and with increasing thickness, the crystallinity, the grains size and the conductivity of the films increases, but the strains along c-axis and the transmittance decrease. The decrease of the resistivity in a thicker film is attributed to the slight increase of the carrier concentration and the significant increase of Hall mobility. The transmittance of all the films is over 80% in the visible region (400–800 nm) and the band gap decrease with the increase of film thickness. The film with the thickness of around 303 nm has the resistivity of 6.07 × 10-3 Ω⋅cm and the transmittance of 90% in the visible range. Based on the good conductivity and high transmittance, the ZnO:In films prepared by magnetron sputtering can be regarded as a potential transparent electrode.

The formation mechanism and stability of p-type N-doped Zn-rich ZnO films

Nitrogen-doped Zn-rich ZnO films [ZnO:(Zn, N)] were deposited on quartz substrates using a radio-frequency (RF) magnetron sputtering and ion implantation technique. Hall-effect measurements confirmed that a p-type ZnO:(Zn, N) film with a hole concentration of ~1016 cm−3, which exhibits significantly higher stability than p-type ZnO:N film prepared under non-Zn-rich conditions, is obtained by optimized post-annealing condition. With the help of X-ray photoelectron spectroscopy, Auger electron spectroscopy, Raman spectroscopy (Raman), ultraviolet and visible spectrophotometer and first-principles calculations, it is found that a certain concentration of zinc interstitial (Zni) donor defects which easily bond to substitutional nitrogen (NO) to form defect complexes (denoted as Zni@NO) were observed in the p-type ZnO:(Zn, N) film. Further theoretical and experimental investigations indicate that the relatively stable p-type conductivity of ZnO:(Zn, N) film is attributed to the formation of passive complex (Zni–2NO), which can form an impurity band (IBM) above the valence band maximum, resulting in a decrease in the acceptor ionization energy and an improvement in the stability of p-type ZnO:(Zn, N) film. This p-type formation mechanism is consistent with donor–acceptor co-doping method. Nevertheless, the p-type performance of the ZnO:(Zn, N) film would still gradually decline over time. The remaining interstitial nitrogen atoms (Ni) in p-type film is easy trapped by the acceptor NO to form a dual-donor defect (N2)O, which is one of possible important factors for the eventual instability of p-type ZnO:(Zn, N) films.

UV PHOTODETECTORS OF c-BN FILMS

Cubic boron nitride films (c-BN) were prepared by sputtering of a hexagonal BN sintered target. The films obtained were characterized by scanning electron microscopy, Infrared absorption spectra and UV-visible transmission spectra. The UV photodetetor devices of c-BN films were fabricated by photolithographic techniques. The devices exhibited a large photocurrent when exposed to light with a wavelength less than 223nm. The results obtained also show that the photoconductive properties closely depend on the deposition conditions. The photoconductive gain for the sample deposited at r. f. power of 300W was greater than that of the sample prepared at r. f. Power of 100W. This can be ascribed to that the carrier lifetime and mobility of the films have been improved.

Growth and characterization of indium doped zinc oxide films sputtered from powder targets

Indium doped ZnO films were grown on quartz glass substrates by radio frequency magnetron sputtering from powder targets. Indium content in the targets varied from 1at% to 9at%. In doping on the structure, optical and electrical properties of ZnO thin films were studied. X-ray diffraction shows that all the films are hexagonal wurtzite with c-axis perpendicular to the substrates. There is a positive strain in the films and it increases with indium content. All the films show a high transmittance of 86% in the visible light region. Undoped ZnO thin film exhibits a high transmittance in the near infrared region. The transmittance of indium doped ZnO thin films decreases sharply in the near infrared region, and a cut-off wavelength can be found. The lowest resistivity of 4.3×10-4 Ω·cm and the highest carrier concentration of 1.86×1021 cm-3 can be obtained from ZnO thin films with an indium content of 5at% in the target.

Fabrication and characterization of p-type In–N codoped ZnMgO films

Indium (In) and nitrogen (N) codoped ZnMgO films (ZnMgO:In–N) were fabricated on quartz substrates by radio frequency magnetron sputtering and ion-implantation technique. p-ZnMgO:In–N films were successfully achieved after post-implantation annealing at an appropriate temperature ranging from 570 to 590 °C. X-ray diffraction (XRD) indicates that severe damage in films is introduced by N ion implantation and the damaged lattice can be partially recovered after post-implantation annealing. The analysis of Raman spectroscopy and X-ray photoelectron spectroscopy (XPS) demonstrates that post-implantation annealing can promote a reduction of donor type zinc interstitials (Zni) and the formation of InZn+2NO acceptor complex, which mainly contribute to the realization of p-type ZnMgO:In–N films.

Magnetic properties and spin state transition of La0.7Ca0.3(Co1−yGay)O3

Magnetic properties and spin state transfer in Ga doping perovskite cobaltites La0.7Ca0.3(Co1−yGay)O3 (y=0, 0.1, 0.2, 0.3, and 0.4) are carefully investigated in this work. On Ga doping, the Curie temperature sharply decreases and the magnetization is greatly reduced, indicating Ga doping breaks the ferromagnetic interaction in the system. For y≥0.2, the ferromagnetic and antiferromagnetic transitions have been observed in thermal magnetization measurement whereas the dominated antiferromagnetic interaction is confirmed by the negative Curie–Weiss temperatures. Evaluation of the effective moment of Co ions seems to imply that Ga doping stablizes Co-ion high spin state.

THE FORM EFFECTS OF MAGNETORESISTIVITY IN p-TYPE DIAMOND FILMS

The geometric form effects of magnetoresistivity (MR) from chemical vapor deposited p-type diamond films were investigated. The MR patterns (width/length ratio = 1, 2, 3, 4, 5, 6) were produced on p-type diamond films by photolithography and ion etching in oxygen plasma. The experimental results showed that the MR in diamond films with the strip structure changed with width–length ratio at magnetic field intensity of 3 T. The changes of MR strongly depended on the geometric form of the samples. With the width–length ratio increasing from 1 to 6, the MR increases from 0.08 to 1.0. It was almost changing linearly with the increase of width-to-length ratio. The geometric form effect is closely related to the Hall effect. A calculation formula of form effect of MR was presented.

ELECTRICAL AND MAGNETIC PROPERTIES OF NANOCYSTALLINE DIAMOND FILMS

The magneto resistive effect in nanocystalline diamond films was investigated at room temperature. The nanocystalline diamond films on silicon were deposited by hot filament chemical vapor deposition. The experimental results showed that a striking magneto resistive effect in p-type doped nanocystalline diamond films was observed. The relative changes in the resistivity of the films were about 0.3 at room temperature under the magnetic field of 5T, and increased with decreasing the geometrical size of the devices. It was found that the magneto resistive effect in the nanocystalline diamond films was less than that of epitaxial diamond films. This was ascribed to a large number of grain boundaries. The results are discussed in detail.