Huimin Weng

Deep-level defects in n-type 6H silicon carbide induced by He implantation

Defects in He-implanted n-type 6H–SiC samples have been studied with deep-level transient spectroscopy. A deep-level defect was identified by an intensity with a logarithmical dependence on the filling pulse width, which is characteristic of dislocation defects. Combined with information extracted from positron-annihilation spectroscopic measurements, this defect was associated with the defect vacancy bound to a dislocation. Defect levels at 0.38∕0.44eV (E1∕E2), 0.50, 0.53, and 0.64∕0.75eV (Z1∕Z2) were also induced by He implantation. Annealing studies on these samples were also performed and the results were compared with those obtained from e−-irradiated (0.3 and 1.7MeV) and neutron-irradiated n-type 6H–SiC samples. The E1∕E2 and the Z1∕Z2 signals found in the He-implanted sample are more thermally stable than those found in the electron-irradiated or the neutron-irradiated samples.

Annealing studies of Au/GaAs and Al/GaAs interfaces using a variable energy positron beam

Abstract Various thicknesses of Au and Al have been evaporated onto semi-insulating GaAs(110) to form Schottky contacts, which have been subject to different annealing conditions. A variable energy positron beam has been used to probe the metal-semiconductor interfaces by observing the Doppler broadening of the annihilation radiation. The data are well fitted by a three-layer model, namely the metal layer, the interface region and the GaAs bulk. The interfacial region is attributed to the intermixing of the atoms and the formation of new phases occurring at the interface. For the Au/GaAs samples, the interfacial width was observed to increase with increasing annealing temperature and this is attributed to the increase of elemental intermixing at the interface. From the trend of the Doppler broadening signal on annealed samples, we infer that the processes of increasing interfacial order (decreasing S) (such as grain enlargement and vacancy removal) are in competition with the process of decreasing interfacial order (increasing S) (such as vacancy creation through atomic interdiffusion). For Al/GaAs, the interfacial width remained effectively constant up to the annealing temperature of 400°C. From the Doppler broadening signal, the annealing on the interfacial order was found to be metal thickness dependent. This observation has tentatively been explained by the above two competition processes and the grain boundary diffusion of atoms at the Al/GaAs interface.

Monovacancies and Na substitutional defects in PbTe: theoretical positron annihilation study

The electron structures of cubic PbTe and substitutional defects are investigated using the state of the art pseudo-potential plane wave method, in the framework of the density functional theory within the generalized gradient approximation. The structural dependent positron annihilation in cubic PbTe was calculated and the positron lifetimes of the bulk, the Pb mono-vacancy, the Te mono-vacancy and the Na atom substituting on Pb were calculated respectively. The study provides evidence that positrons can distinguish between mono-vacancies and in situ substitutional Na doping. The positron lifetime values were found to be too similar to be distinguished. But calculations of the positron annihilation momentum distribution spectra did show clear differences between the two types of defect and may be exploited to characterize substitutional defects.

An apparatus used to make 22Na sources for use in low-energy positron beams

Abstract The 22 Na nuclide, having a relatively long half-life (2.6 year) and high branching ratio for positron emission (90%), is often the preferred nuclide for producing low-energy positron beams. Suitable commercial 22 Na sources are, however, relatively expensive and are sometimes subject to delay in supply. To circumvent such problems in the long-term maintenance of our positron beam a radioactive source deposition apparatus, capable of preparing up to 20 mCi 22 Na sources from aqueous solution, has been designed and constructed. The 22 NaCl solution is deposited onto a platinum substrate, dried and remotely capped with 3 μm of titanium. The apparatus has been safely employed to produce both 10 mCi and 20 mCi 22 Na sources both of which are now in use in our magnetically guided positron beam.

Positron beam profiling study of the metal-GaAs interface

Abstract Properties of metal contacts on III–V semiconductors are dependent on the electronegativity of the metal. Generally metals have been classified into three categories according to their behaviour upon heat treatment. Au, Ni and W are the three metals belonging to different categories with the descending order of electronegativity. In this study a low energy positron beam has been used to investigate as-grown Au GaAs (SI), Ni GaAs (SI) and W GaAs (SI) samples with different thicknesses of metal overlayers. The method that has been employed is that of monitoring the Doppler broadening of annihilation radiation through S parameter measurements as a function of the beam energy. The S - E data were analysed by the program VEPFIT with different models and different implantation profiles. It is found that the interfacial information able to be extracted from the fitting is limited by present uncertainties in the implantation profile.

CHARACTERIZATION OF Ni-RICH INTERMETALLIC COMPOUND Ni3Al FILMS WITH SLOW POSITRON BEAM

Ni-rich intermetallic compound Ni3Al films were deposited using the dual-target magnetron co-sputtering method. The grain-boundary and grain-size characteristics of the as-deposited and post-annealed Ni3Al films were investigated using a slow positron beam. The results demonstrate a distinct change of S-parameter and positron effective diffusion length Leff values as samples were annealed at different ambient atmospheres and temperatures. It shows that the positron trapping center is mainly associated with intergranular vacancies, and changes to either the vacancy cluster size or the grain size have been invoked to explain the changes of the values of the S-parameter and Leff. Ni-rich oxygen-Ni3Al films were also studied. All films were identified using X-ray diffraction and the results confirm the conclusion obtained by the slow positron beam.

Defect Characterization of 6H-SiC Studied by Slow Positron Beam

The defect formation and annealing behavior in as-grown and electron-irradiated 6H-SiC wafers were investigated by variable-energy slow positron beam. For the n-type as-grown samples, it was found that annealing decreased the defect concentration due to recombination with interstitial, and when it was annealed at 1400°C for 30 min in vacuum, a 20 nm thick Si layer was found on the top of SiC substrate, which is a direct proof of the Si atom diffusing to the surface when annealed at the high temperature stages. During the high temperature annealing stage, we found an obvious surface effect occurred that induced the higher S parameter close to the surface. This may be caused by the diffusion of the Si atoms to the surface during annealing. After 10 MeV electron irradiation of the n-type 6H-SiC, the positron effective diffusion length decreased from 86.2 nm to 39.1 nm. This shows that there are some defects created in n-type 6H-SiC. But in the p-type 6H-SiC irradiated by 10 MeV electrons, the change is very small. This may be because of the opposite charge of the vacancy defects. The same annealing behavior as that of as-grown 6H-SiC samples was also observed for the 1.8 MeV electron-irradiated 6H-SiC samples except that after being annealed at 300 °C, its defect concentration increased. This may be explained as the generation of carbon vacancies, due to either the recombination between divacancies and silicon interstitial, or the charge of the charge states.

Positron beam study of low-temperature-grown GaAs with aluminum delta layers

Abstract We report on the use of a slow-positron beam to examine aluminium delta layers in GaAs grown at low substrate temperature (250°C) by molecular beam epitaxy (LTMBE). It is now established that LTMBE-GaAs contains an excess of arsenic which causes an increase in the lattice parameter. After annealing, this arsenic is redistributed and forms precipitates resulting in the relaxation of the lattice. Previous positron beam studies have shown that the as-grown material has a large concentration of gallium vacancies, and after annealing the S -parameter increases above the as-grown value indicating that vacancy clusters have formed. A region depleted of arsenic precipitates has been shown to form near to aluminium delta layers, and this work is the first to study the vacancy distribution associated with this depletion region. We observe that the as-grown material has a peak value of the normalized S -parameter that is ∼3.5% higher than the substrate, which is much larger than that for a single layer LT-GaAs structure (0.8–1.5%). After annealing in the range 600–800°C the S -parameter collapses down to the substrate value, which again is opposite to the LT-GaAs case and indicates that few precipitates have formed. We correlate these findings with SIMS and TEM data and propose a mechanism involving compositional disordering due to the aluminium layers.