Isochronal annealing studies of n-type 6H-SiC with positron lifetime spectroscopy

n-type 6H silicon carbide has been studied using positron lifetime spectroscopy with isochronal annealing temperatures of 400, 650, 900, 1200, and 1400 °C. In the as-grown sample, we have identified the VSi vacancy, the VCVSi divacancy, and probably the VC vacancy. The silicon vacancy and the carbon vacancy were found to anneal out in the temperature range 400-650 °C. The VCVSi divacancy was found to persist at an annealing temperature of 1400 °C.published_or_final_versio

Defect study of Zn-doped p-type gallium antimonide using positron lifetime spectroscopy

Defects in p-type Zn-doped liquid-encapsulated Czochralski - grown GaSb were studied by the positron lifetime technique. The lifetime measurements were performed on the as-grown sample at temperature varying from 15 K to 297 K. A positron trapping center having a characteristic lifetime of 317 ps was identified as the neutral VGa-related defect. Its concentration in the as-grown sample was found to be in the range of 1017-1018 cm-3. At an annealing temperature of 300 °C, the VGa-related defect began annealing out and a new defect capable of trapping positrons was formed. This newly formed defect, having a lifetime value of 379 ps, is attributed to a vacancy - Zn-defect complex. This defect started annealing out at a temperature of 580 °C. A positron shallow trap having binding energy and concentration of 75 meV and 1018 cm-3, respectively, was also observed in the as-grown sample. This shallow trap is attributed to positrons forming hydrogenlike Rydberg states with the ionized dopant acceptor Zn.published_or_final_versio

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 increase of EL2+ neutralization occurring for biases above 10 V. At still higher fields ∼10 kV cm-1 (biases>50 V), there appears to be an additional threshold for more complete EL2+ neutralization adjacent to the contact. The present study clearly demonstrates the often overlooked necessity of catering for built-in electric fields in positron diffusivity studies of III-V semiconductors where surface midgap Fermi-level pinning is common. © 1997 American Institute of Physics.published_or_final_versio

Deep level transient spectroscopy study of particle irradiation induced defects in n-6H-SiC (Abstract)

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Electric-field distribution in Au–semi-insulating GaAs contact investigated by positron-lifetime technique

Positron-lifetime spectroscopy has been used to investigate the electric-field distribution occurring at the Au–semi-insulating GaAs interface. Positrons implanted from a 22Na source and drifted back to the interface are detected through their characteristic lifetime at interface traps. The relative intensity of this fraction of interface-trapped positrons reveals that the field strength in the depletion region saturates at applied biases above 50 V, an observation that cannot be reconciled with a simple depletion approximation model. The data, are, however, shown to be fully consistent with recent direct electric-field measurements and the theoretical model proposed by McGregor et al. [J. Appl. Phys. 75, 7910 (1994)] of an enhanced EL2+ electron-capture cross section above a critical electric field that causes a dramatic reduction of the depletion region’s net charge density. Two theoretically derived electric field profiles, together with an experimentally based profile, are used to estimate a positron mobility of ∼95±35 cm2 V-1 s-1 under the saturation field. This value is higher than previous experiments would suggest, and reasons for this effect are discussed.published_or_final_versio

EL2 deep donor state in semi-insulating GaAs revealed by frequency dependent positron mobility measurements

Positron mobility measurements carried out on semi-insulating GaAs, using the Doppler shift in annihilation radiation technique, show a sharp transition from a high mobility value ∼120 cm2 V-1 s-1 to a lower value ∼45 V-1 s-1 just below room temperature. The temperature of the transition is found to be dependent on the frequency of the applied AC bias. We show that this effect is an artifact due to the thermal ionization of the EL2 deep donor state, which in its ionized state forms a positive space charge that causes the positron to experience large electric fields. This observation suggests a new positron annihilation-based deep-level transient technique applicable to semi-insulating materials. © 1994 The American Physical Society.published_or_final_versio

Influence of gaseous annealing environment on the properties of indium-tin-oxide thin films

The influence of postannealing in different gaseous environments on the optical properties of indiu-tin-oxide (ITO) thin films deposited on glass substrates using e-beam evaporation has been systematically investigated. It is found that the annealing conditions affect the optical and electrical properties of the films. Atomic force microscopy, x-ray diffraction, and x-ray photoemission spectroscopy (XPS) were employed to obtain information on the chemical state and crystallization of the films. These data suggest that the chemical states and surface morphology of the ITO film are strongly influenced by the gaseous environment during the annealing process. The XPS data indicate that the observed variations in the optical transmittance can be explained by oxygen incorporation into the film, decomposition of the indium oxide phases, as well as the removal of metallic In. © 2005 American Institute of Physics.published_or_final_versio

EL2 deep-level transient study in semi-insulating GaAs using positron-lifetime spectroscopy

Positron lifetime measurements performed on Au/GaAs samples at room temperature with an applied square-wave ac bias show a frequency dependent interlace related lifetime intensity that peaks around 0.4 Hz. The observation is explained by the ionization of the deep-donor level EL2 to EL2+ in the GaAs region adjacent to the Au/GaAs interface, causing a transient electric field to be experienced by positrons drifting towards the interface. Without resorting to temperature scanning or any Arrhenius plot the EL2 donor level is found to be located 0.80±0.01±0.05 eV below the conduction-band minimum, where the first error estimate is statistical and the second systematic. The result suggests positron annihilation may, in some instances, act as an alternative to capacitance transient spectroscopies in characterizing deep levels in both semiconductors and semi-insulators.published_or_final_versio

Nature of the acceptor responsible for p-type conduction in liquid encapsulated Czochralski-grown undoped gallium antimonide

The acceptors in undoped liquid encapsulated Czochralski (LEC)-grown GaSb were investigated using temperature dependent Hall (TDH) and positron lifetime spectroscopy measurements. TDH measurements were performed on nonirradiated and electron irradiated undoped samples. The 34 meV acceptor was found to be the important one responsible for the p-type conduction in nonirradiated and the e--irradiated undoped materials annealed at temperatures up to 500°C. This acceptor was not related to any VGa-related defect detected and was most likely the GaSb antisite.published_or_final_versio

Vacancies in electron irradiated 6H silicon carbide studied by positron annihilation spectroscopy

Paper no. R3.19Positron lifetime spectroscopy was employed to study the as-electron-irradiated (10 MeV, 1×10 18 cm -2) n-type 6H silicon carbide sample in the measuring temperature range of 15 K to 294 K. Isochronal annealing studies were also performed up to the temperature of 1373 K by carrying out the room temperature positron lifetime measurement. Negatively charged carbon vacancies and V cV si divacancy were identified as the major vacancy type defects induced by the electron irradiation process. The concentration of the V cV si divacancy was found to decrease dramatically after the 1973 K annealing.published_or_final_versio