The Development Of High-performance Positron Annihilation Spectrometers And The Study Of Defect And Magnetism In AlN Films

As a bright pearl in the antimatter world,positron(the antiparticle of electron)has been used to investigate the microstructure of materials because of its unusual sensitivity to electrons.Based on positron physics,positron annihilation spectroscopy(PAS)was established gradually after the discovery of positron by Anderson.PAS consists of three basic techniques:positron annihilation lifetime spectroscopy(PALS),Doppler broad-ening spectroscopy(DBS)and angular correlation of annihilation radiation(ACAR).Based on PALS and DBS,AMOC technique has been developed in the last 30 years.PAS technology and material microstructure characterization are the two master lines in the PAS experimental area,they complement each other and develop together.The development of technique has extended PAS to the characterization of more materials;the characterization of new materials puts forward new requirements for spectrometer technology,which promotes the PAS technology to be improved towards higher perfor-mance.In this paper,we study of the development of a high-efficiency AMOC system,the establishment of a new digital PAL spectrometer and the applications of PAS to diluted semiconductors(DMSs),which are based on the two master lines.The main contents are as follows:On the AMOC technique,we have designed and established a high-efficiency???E? AMOC system.The detector selection,geometry configuration,signal coin-cidence mode and data acquisition were optimized.A well-type scintillator was chosen to detect ?-rays of 1.275 MeV for the higher detection efficiency.The size of scin-tillators and the overall detector configuration were determined by Geant4 simulation.For the signal coincidence,software coincidence instead of hardware coincidence was used.In terms of data acquisition,the double-buffered continuous acquisition mode of PCI9820 digitizer was used to improve the sampling efficiency.Finally,an AMOC system with the counting rate of 180 cps and time resolution of about 280 ps was es-tablished successfully.The counting rate was twice as high as that of the traditional one.On the PAL spectroscopy,we have developed a high-resolution digital PAL spec-trometer using a pair of scintillation detectors and a fast digital oscilloscope.The coin-cidence time resolution of BaF2-based H6610 detectors was investigated using a digital oscilloscope.The time responses of detectors have been optimized by tuning the supply voltages,the sampling rate of the oscilloscope,and digital fraction constants,achiev-ing a coincidence time resolution of about 162 ps for the 0.511 MeV annihilation ?-ray pairs and 108 ps for the 60Co cascade ?-rays,respectively.Furthermore,a digital PAL spectrometer composed of two BaF2-based H6610 detectors and an oscilloscope was developed with a time resolution of around 130 ps,much better than most digital PAL spectrometers.For the application of PAS,we applied DBS based on slow positron beam to the characterization of AlN DMSs.C-doped AlN DMS films were prepared via C+ ion im-plantation.Room-temperature ferromagnetism is observed in C+-implanted AlN films with C+ doses of 5×1016(AlN:C5×1016)and 2×1017 cm-2(AlN:C2×1017).AlN:C2×1017 ex-hibits a saturation magnetization of?0.104 emu/g,nearly 1.5 times that of AlN:C5×1016.X-ray diffraction and X-ray photoelectron spectroscopy(XPS)measurements reveal that the implanted C+ions occupy the interstitial lattice sites and substitute at the sites of A1 atoms.XPS and DBS measurements demonstrate the existence of Al-vacancy related defects in the C+-implanted AlN films.First-principles calculations indicate that the ferromagnetism in A1N:C5×1016 and AlN:C2×1017 is mainly originated from de-fect complexes involving interstitial C atoms and Al vacancies,which have the lowest formation energy among AlN:C defects containing C atoms and A1 vacancies.