共查询到10条相似文献,搜索用时 125 毫秒
1.
Alfred Seeger 《Applied Physics A: Materials Science & Processing》1975,7(4):257-263
The paper explores the consequences of metastable self-trapping of positrons for the temperature variation of positron diffusion coefficients and trapping rates. It is shown that the diffusion coefficient is expected to decrease with increasing temperatures from a value determined by the scattering of positron Bloch waves by acoustic phonons to a much lower one governed by a hopping process. The temperature variation of the rate of trapping by small defects such as vacancies is more complicated since only at high temperatures can we expect it to be limited by the positron diffusion coefficient. At low temperatures the observed magnitude of the rate of trapping at monovacancies can only be understood if the trapping rate is controlled by the rate of capture of positrons that have arrived at the trap. 相似文献
2.
C.C. Ling A.H. Deng S. Fung C.D. Beling 《Applied Physics A: Materials Science & Processing》2000,70(1):33-38
Positron lifetime spectroscopy has been employed to study the as-grown n-type 1.2᎒18 cm-3 N-doped and p-type 1.8᎒18 cm-3 Al-doped 6H-silicon carbide in the temperature range 10 K-300 K. For the p-type material, a positron trapping site, which has a lifetime of 225ᆟ ps, was found and is attributed to positron annihilating from the VSiVC divacancy-related defect having a neutral charge. For the case of the n-type material, a positron trapping centre having a lifetime of 200Nj ps, attributed to a VSi-related defect, and a shallow trap were observed. The shallow trap, having binding energy of l8 meV was attributed to Rydberg-like states resulting from positron binding with a negative ion. 相似文献
3.
W. Fuhs U. Holzhauer F. W. Richter 《Applied Physics A: Materials Science & Processing》1980,22(4):415-419
The positron lifetime in electron-irradiated undoped and doped silicon crystals is studied as a function of temperature between
90 and 300 K. We show that the temperature dependence of the two lifetime components does not arise from the escape, but from
the trapping rate at defects. The temperature dependences of the capture cross sections are deduced. It is concluded that
in undoped crystals the positrons interact with negatively charged and neutral defects, probably divacancies and vacancy-oxygen
complexes, respectively. In strongly P-doped crystals positron trapping occurs preferably in negatively charged centers. 相似文献
4.
Measurements of the positron lifetime and Doppler-broadened annihilation-radiation have been performed in electron-irradiated GaAs. The positron lifetime at the irradiation induced defects was 0.250 ns at 300 K. The defect clustering stage was found to occur at around 520–620 K, and the coarsening and annealing stage is believed to be above 620 K. Similar annealing stages were also observed in GaAs lightly doped with Si (0.2×1018 cm–3). Both the lifetime and the S-parameter in the irradiated GaAs were found to decrease with temperature from 300 K to 100 K, suggesting the coexistence of shallow traps in electron irradiated GaAs. 相似文献
5.
分别研究了823 K淬火处理和20%形变量的Al-4%Ag低温下Ag析出物对正电子的捕获行为的变化。采用正电子湮没寿命谱(PALS)技术和符合多普勒展宽能谱(CDBS)在温度范围10~293 K内对其进行表征。多普勒展宽能谱结果表明2种样品中均存在Ag析出物。正电子寿命谱的解谱结果中的各组分给出了Ag析出物随测量温度的变化规律。在170 ~273 K之间,正电子湮没行为具有较强的温度依赖性。但对于两个具有不同类型缺陷的样品,在低于170 K时观察到样品中Ag析出物捕获正电子能力出现了差异。随着测量温度的降低,淬火样品中的Ag析出物的正电子寿命和强度基本不变。在低于170 K的测量中,形变样品中的Ag析出物对正电子的捕获能力仍旧存在着较强的温度依赖性,但是变化幅度在逐渐减弱。当测量温度提升到室温(273~293 K),越来越多的正电子从Ag析出物中逃逸,逐渐回到自由状态或被其他深陷阱所捕获,失去了对温度的依赖性。 相似文献
6.
The trapping rate of positrons at dislocations in metals, and its temperature dependence, are calculated. Two different trapping processes, with the excess energy absorbed in either electron-hole pair formation or by phonon creation, are considered and the former is found to be the most important. An extension of the theory to include depletion of the positron density around the dislocations in a diffusion approximation is included. The trapping is found to be transition limited if the temperature is low or the trap potential shallow. At room temperature diffusion is important for deep traps. 相似文献
7.
Positron annihilation measurements were performed in austenitic Fe59Ni25Cr16 alloy containing vacancy-type defects. Lifetime data were recorded between 4K and 400K. The positron trapping process was studied as a function of temperature and cluster size. The smaller defects characterized by a lifetime of 260 ps yield an unchanged trapping rate during temperature scanning. However, the trapping phenomenon is strongly temperature dependent for the larger size defects, with a typical lifetime of 500 ps. 相似文献
8.
K. Petersen N. Thrane G. Trumpy R. W. Hendricks 《Applied Physics A: Materials Science & Processing》1976,10(1):85-90
We have measured the lifetimes of positrons in an aluminum single crystal which was irradiated to a fast neutron fluence of 1.5·1021 n/cm2 (>0.18 MeV) at 50°C. These irradiation conditions produced 4.2·1014 voids/cm3 with a mean diameter of 330 Å, as determined by both small-angle x-ray scattering and transmission electron microscopy. The positron lifetime spectra were resolved into three lifetime components of 100, 300, and 500 ps. The short lifetime component is a result of fast trapping of positrons by the voids; the long lifetime components result from annihilations within the voids. The intensity of the long lifetime components increases with temperature in the range 80 to 300 K and supports the model of a positron state at the void surface. The positron diffusion coefficient appears to have aT 1/2-dependence. A magnetic quenching experiment shows no indication of positronium formation. Finally, an isochronal heat treating sequence shows that the voids anneal out between 200 and 300°C, and that the lifetime spectrum after annealing is described by a single component of 170 ps, the observed lifetime in unirradiated aluminum. 相似文献
9.
K. Petersen N. Thrane G. Trumpy R. W. Hendricks 《Applied physics. B, Lasers and optics》1976,10(1):85-90
We have measured the lifetimes of positrons in an aluminum single crystal which was irradiated to a fast neutron fluence of
1.5·1021 n/cm2 (>0.18 MeV) at 50°C. These irradiation conditions produced 4.2·1014 voids/cm3 with a mean diameter of 330 ?, as determined by both small-angle x-ray scattering and transmission electron microscopy. The
positron lifetime spectra were resolved into three lifetime components of 100, 300, and 500 ps. The short lifetime component
is a result of fast trapping of positrons by the voids; the long lifetime components result from annihilations within the
voids. The intensity of the long lifetime components increases with temperature in the range 80 to 300 K and supports the
model of a positron state at the void surface. The positron diffusion coefficient appears to have aT
1/2-dependence. A magnetic quenching experiment shows no indication of positronium formation. Finally, an isochronal heat treating
sequence shows that the voids anneal out between 200 and 300°C, and that the lifetime spectrum after annealing is described
by a single component of 170 ps, the observed lifetime in unirradiated aluminum.
Research sponsored in part by the U.S.Atomic Energy Commission under contract with the Union Carbide Corporation. 相似文献
10.
Positron annihilation spectroscopy was applied to investigate the nature and thermal behavior of defects induced by Cu diffusion in Zn-doped p-type GaAs crystals. Cu atoms were intentionally introduced in the GaAs lattice through thermally activated diffusion from a thin Cu capping layer at 1100 °C under defined arsenic vapor pressure. During isochronal annealing of the obtained Cu-diffused GaAs in the temperature range of 450?850 K, vacancy clusters were found to form, grow and finally disappear. We found that annealing at 650 K triggers the formation of divacancies, whereas further increasing in the annealing temperature up to 750 K leads to the formation of divacancy-copper complexes. The observations suggest that the formation of these vacancy-like defects in GaAs is related to the out-diffusion of Cu. Two kinds of acceptors are detected with a concentration of about 1016 ? 1017 cm?3, negative ions and arsenic vacancy copper complexes. Transmission electron microscopy showed the presence of voids and Cu precipitates which are not observed by positron measurements. The positron binding energy to shallow traps is estimated using the positron trapping model. Coincidence Doppler broadening spectroscopy showed the presence of Cu in the immediate vicinity of the detected vacancies. Theoretical calculations suggested that the detected defect is V Ga V As-2CuGa. 相似文献