Weng Hui-Min

Dependence of intrinsic defects in ZnO films on oxygen fraction studied by positron annihilation

Defects in ZnO films grown by radio-frequency reactive magnetron sputtering under variable ratios between oxygen and argon gas have been investigated by using the monoenergetic positron beam technique. The dominate intrinsic defects in these ZnO samples are O vacancies (V-O) and Zn interstitials (Zn-i) when the oxygen fraction in the O-2/Ar feed gas does not exceed 70% in the processing chamber. On the other hand, zinc vacancies are preponderant in the ZnO Elms fabricated in richer oxygen environment. The concentration of zinc vacancies increases with the increasing (2) fraction. For the oxygen fraction 85%, the number of zinc vacancies that could trap positrons will be smaller. It is speculated that some unknown defects could shield zinc vacancies. The concentration of zinc vacancies in the ZnO films varies with the oxygen fraction in the growth chamber, which is in agreement with the results of photoluminescence spectra.

Positron lifetime studies on 8 MeV electron-irradiated n-type 6H silicon carbide

The positron lifetime technique was employed to study vacancy-type defects in 8 MeV electron-irradiated n-type 6H silicon carbide. A long-lifetime component having a characteristic lifetime of 223–232 ps was observed in the irradiated sample and was attributed to the VCVSi divacancy. Other positron traps, which dominated at low temperatures, were observed to compete with the VCVSi for trapping positrons. A positron trapping model involving a positron shallow trap, a negatively charged monovacancy and the VCVSi divacancy was found to give a good description of the temperature-dependent positron lifetime data of the 1200 ◦ C annealed sample. The identity of the monovacancy could not be unambiguously determined, but its lifetime was found to be in the range 160–172 ps.

Realization of the Three-Qubit Toffoli Gate in Molecules *

We present the experimental realization of this gate with a solution of chlorostyrene molecules. Our method does not depend heavily on the two-qubit controlled operation, which used to serve as the basic quantum operation in quantum computing. At present, we use transition operator that can be applied to all qubits in one operation. It appears that no experimental realization has yet been reported up to now regarding the implementation of quantum Toffoli gate using transition pulse on three-qubit nuclear magnetic resonance quantum computers. In addition, our method is experimentally convenient to be extended to more qubits.

Evolution of native point defects in ZnO bulk probed by positron annihilation spectroscopy

This paper studies the evolution of native point defects with temperature in ZnO single crystals by positron lifetime and coincidence Doppler broadening (CDB) spectroscopy, combined with the calculated results of positron lifetime and electron momentum distribution. The calculated and experimental results of the positron lifetime in ZnO bulk ensure the presence of zinc monovacancy, and zinc monovacancy concentration begins to decrease above 600 °C annealing treatment. CDB is an effective method to distinguish the elemental species, here we combine this technique with calculated electron momentum distribution to determine the oxygen vacancies, which do not trap positrons due to their positive charge. The CDB spectra show that oxygen vacancies do not appear until 600 °C annealing treatment, and increase with the increase of annealing temperature. This study supports the idea that green luminescence has a close relation with oxygen vacancies.

Study of Coincidence Doppler Broadening in Carbon Nanotubes

Coincidence Doppler broadening measurements of positron annihilation for multi-walled carbon nanotubes, double-walled carbon nanotubes, single-walled carbon nanotubes and graphite were performed. The ratio curves of the Doppler broadening for these samples to silicon were obtained. It is shown that there are distinct peaks at the position of 10×10−3m0c for both carbon nanotubes and graphite, however the amplitudes of the peaks are not the same. We have the opinion that these peaks arise from the annihilation of positron with the 2s and 2p electron of carbon element.

SIMION simulation of a slow pulsed positron beam

A new slow pulsed positron beam, including a positron source, a moderator, a chopper, a pre-buncher, a main-buncher and a sample chamber, etc, has been installed and tested. It is necessary to simulate the acceleration, transportation and space focusing of positrons to meet the needs of beam debugging and further positron annihilation experiments. The result from SIMION simulations shows that the radius of the focused positron beam is less than 5 mm, which is further confirmed in our practical debugging process.

Influence of Dopants in ZnO Films on Defects

The influence of dopants in ZnO films on defects is investigated by slow positron annihilation technique. The results show S that parameters meet SAl > Sun > SAg for Al-doped ZnO films, undoped and Ag-doped ZnO films. Zinc vacancies are found in all ZnO films with different dopants. According to S parameter and the same defect type, it can be induced that the zinc vacancy concentration is the highest in the Al-doped ZnO film, and it is the least in the Ag-doped ZnO film. When Al atoms are doped in the ZnO films grown on silicon substrates, Zn vacancies increase as compared to the undoped and Ag-doped ZnO films. The dopant concentration could determine the position of Fermi level in materials, while defect formation energy of zinc vacancy strongly depends on the position of Fermi level, so its concentration varies with dopant element and dopant concentration.

Simulation of time bunching for a pulsed positron beam

Simulate anneal arithmetic has been used to settle the problem of time bunching on a pulsed slow-positron beam device. This paper has searched for the parameters of the device in a large scope and achieved the time resolution within 150ps at the target with accelerating voltage in a range of 0.5–30kV.

Vacancy in 6H-Silicon carbide studied by slow positron beam

The defect changes in 6H-SiC after annealing and 10MeV electron irradiation have been studied by using a variable-energy positron beam. It was found that after annealing, the defect concentration in n-type 6H-SiC decreased due to recombination with interstitials. When the sample was annealed at 1400°C for 30min in vacuum, a 20-nm thickness Si layer was found on the top of the SiC substrate, this is a direct proof of the Si atoms diffusing to surface when annealed at high temperature stages. After 10MeV electron irradiation, for n-type 6H-SiC, the S parameter increased from 0.4739 to 0.4822, and the relative positron-trapping rate was about 27.878 times of the origin sample, this shows that there are some defects created in n-type 6H-SiC. For p-type 6H-SiC, it is very unclear, this may be because of the opposite charge of vacancy defects.

Low Energy Positron Beam Apparatus for Near-surface Studies and Its Application

A low-energy positron beam apparatus has been designed and constructed. The beam can deliver a current of 4×104 e+ /s to a target when a 7.3mCi 22Na source is used. The energy range available at present is between 25eV and 16keV. We have applied it to study initially on the dynamic process of WSix film formation and on the distribution defects produced by N+ ion in nickel.