Y.F. Hu

Saturated electric field effect at semi-insulating GaAs-metal junctions studied with a low energy positron beam

The interfacial electric field established under different reverse bias conditions in Au and Ni on semi-insulating GaAs junctions has been studied by means of a low energy positron beam. The technique used is that of monitoring the positron drift to the interface through changes in the annihilation radiation lineshape as a function of incident positron beam energy at different reverse biases. The data show a small but clear electric field drift of positrons towards the interface that increases more rapidly at low voltages (less than 50 V) which at higher biases tends towards saturation. This confirmation of electric field saturation adds further weight to the picture of an electric field enhanced electron capture cross section for the ionized EL2 defect. Electric field values extracted from the data are compared with results from other techniques and suggest that enhanced electron capture is already occurring at the relatively low built-in fields (∼1 kV cm−1) found at the unbiased junction, with a rapid i...

Depth profiling of vacancy-type defects in homogenous and multilayered a-Si films by positron beam Doppler broadening

Positron annihilation measurements have been carried out on a-Si:H thin films deposited by plasma enhanced chemical vapour deposition at a high rate and on pin/pin double-junction diodes prepared conventionally by means of the variable-energy positron beam Doppler-broadening technique. The depth profiles of microvoids in films grown under different conditions have been determined. We found a smaller void fraction in the surface region of all films compared to the interior. The depth profiles of microvoid-like defects in the a-Si:H films were extracted by use of the VEPFIT programme. Variable-energy positron-annihilation spectroscopy results on the diode show that the interfaces are of good quality except for the Au/n interface, consistent with the diode characteristic.

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.

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.

Probing of microvoids in high-rate deposited a-Si : H thin films by variable energy positron annihilation spectroscopy

In this paper, positron annihilation measurements have been carried out on a-Si : H thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) at high and low rates by means of the variable energy positron beam Doppler-broadening technique. The depth profiles of microvoids in the films grown under different conditions have been determined. We found a smaller void fraction in the surface region of all films compared to the bulk, and a smaller void fraction in low rate than in high growth rate films. By plotting S and W parameters in the (S, W) plane, we have shown that the vacancies in all of the high-rate and low-rate deposited intrinsic samples, and in differently doped low-rate samples are of the same nature, although there appears to be a higher density of defects in the boron than phosphorus doped films. The depth profiles of the microvoid-like defects in the a-Si : H films are extracted by use of the VEPFIT program.

Positron studies of arsenic precipitation in low-temperature GaAs grown by molecular beam epitaxy

Gallium arsenide grown at low substrate temperature by molecular beam epitaxy has been studied using a variable energy slow positron beam. As-grown LT-GaAs was found to have a higher concentration of vacancy-related defects (/spl sim/10/sup 17/ cm/sup -3/) than the semi-insulating substrate. After annealing at 600/spl deg/C, the positron S parameter results suggest the formation of clusters which we associate with arsenic precipitation. The lowering of the S parameter at the surface was thought to be due to oxygen and this was confirmed by XPS measurements. For the first time, we have examined aluminium delta-layers using a positron beam and found that the Al-layers can be resolved to depths of at least 1700 /spl Aring/ by this method. The lowering of the S parameter after annealing would suggest that the Al forms Al/sub x/Ga/sub 1-x/As, and that the presence of the Al layers may inhibit the diffusion of arsenic, thereby reducing the formation of vacancy-defects.

Study of microvoids in high-rate a-Si:H using positron annihilation

In this paper, the authors have carried out the positron annihilation measurement on high-rate and low-rate a-Si:H thin films deposited by PECVD. By means of the slow positron beam Doppler-broadening technique, the depth profiles of microvoids in a-Si:H have been determined. They have also studied the vacancy-type defect in the surface region in high-rate grown a-Si:H, making comparison between high-rate and low-rate a-Si:H. By plotting S and W parameters in the (S, W) plane, they have shown that the vacancies in all of the high-rate and low-rate deposited intrinsic samples, and in differently doped low-rate samples are of the same nature.