G. Venugopal Rao

Study of defects and strain relaxation in GaAs/InxGa1−xAs/GaAs heterostructures using photoluminescence, positron annihilation, and x-ray diffraction

Strain relaxation and consequent generation of defects in GaAs/InxGa1−xAs/GaAs heterostructures with x=0.05–0.3 prepared by metalorganic chemical vapor deposition have been studied using photoluminescence, positron beam, and x-ray diffraction techniques. Photoluminescence studies have indicated peak shifts and broadening in the spectra as the In concentration is increased. Broadening is attributed to defect generation, caused by In substitution beyond the critical limit. Depth resolved defect-sensitive S-parameter measurements, using a low energy positron beam, exhibit an increase in the S parameter over a depth range corresponding to the In substituted layers as the In concentration is increased. The results are suggestive of the production of open volume defects like misfit dislocations in the In substituted layer. A simplified analysis of positron beam data shows that a 5% In sample is defect-free, indicating that it is pseudomorphic to the substrate. A 10% In sample is in the transition region, while ...

Evidence for excess vacancy defects in the Pd–Si system: positron annihilation, x-ray diffraction and Auger electron spectroscopy study

The transformation of Pd/Si to Pd2Si/Si is investigated using depth-resolved positron annihilation, x-ray diffraction and Auger electron spectroscopy studies. The observed defect-sensitive positron S-parameter value of 1.022–1.054 indicates the existence of divacancies across the silicide/silicon interface and Si substrate region. Our experimental observation of vacancy defects is consistent with the model proposed for excess vacancy generation across the interface consequent to Si diffusion.

Study of inter-diffusion and defect evolution in thin film Al/Ge bilayers using SIMS and positron beam

Depth resolved investigations of defects and their evolution upon inter-diffusion have been carried out on Al/Ge thin film bilayers, using concurrent measurements of positron beam based Doppler broadening lineshape and secondary ion mass spectrometry (SIMS) profiles. Measurements have been made on the as-grown film and films annealed at various temperatures from 370 to 670 K. SIMS studies establish the occurrence of extensive intermixing of the Al and Ge layers beyond 370 K. It is found that there is an asymmetric diffusion across the interface, with Ge being the dominant diffusing species. The intermixed Al/Ge region formed in the near-surface layers of the sample annealed at 670 K shows Ge precipitation, as confirmed by electron microscopy and SIMS imaging. Analysis of the Doppler broadening line shape variation, together with the SIMS concentration profiles, indicates increased production of small vacancy clusters at the Al/Ge interface around 520 K, brought about by the enhanced diffusion of Ge. The interfacial vacancy structure is inferred to be a tri-vacancy cluster.

Positron beam studies of argon irradiated CdS thin films

Depth profiling studies of defects in CdS thin films using a low energy positron beam are reported. CdS films of 1 μm thickness on a glass substrate have been grown using the chemical bath deposition method and irradiated with 140 keV Ar ions to doses of 5×1014 and 5×1016 cm−2, respectively. The Doppler broadening line shape S parameter in as-grown and irradiated films has been monitored as a function of positron beam energy Ep. S vs Ep curves have been analyzed, using a positron diffusion model in terms of annihilations at surface, epithermal and bulk states. The S parameter at the surface (Ss) is found to be significantly higher than that in the bulk. This is understood as due to positron trapping at a large concentration of Cd vacancies in the surface region, which act as acceptors. The observed reduction of Ss with increasing Ar ion dose seems to indicate the formation of antisite defects in the surface region, brought about by irradiation. At sample depths corresponding to the peak damage layers, the...

Studies on Ge/CeO2 thin film system using positron beam and Raman spectroscopy

Abstract Positron beam and Raman spectroscopic studies are reported on a Ge/CeO2 thin film system grown on a Si substrate. The variation of the Doppler line shape S-parameter, as a function of positron beam energy, shows a pronounced maximum corresponding to the Ge-region of as-grown film, while no equivalent feature is seen in the sample annealed at 773 K. The Raman spectrum of the as-grown film exhibits a broad band, as opposed to a sharp peak observed for the annealed sample. Based on the correlated evidence from the two measurements, it is concluded that the Ge layer in the as-grown sample is amorphous, containing a large concentration of structural vacancies. Isochronal annealing measurements of the lineshape parameter in the temperature range of 300 to 773 K indicate a rather continuous transformation of amorphous Ge to the crystalline state.