N. Srinivasa Rao

Swift heavy-ion modification of semiconductor heterostructures

Ion-beam modification of material properties is of great interest from a research and industrial application point of view. In particular, modifications and characterizations of semiconductor heterostructures of nanoscale thickness have been investigated in great detail due to their potential applications in electronic and optoelectronic device applications. In this article, we review recent work in this field and present some of our results on irradiation studies of partially relaxed InGaAs/GaAs heterostructures, both thickness and composition dependence. Some of the recent results on GaN/Sapphire samples have also been discussed. We characterized these structures using high-resolution X-ray diffraction (XRD), Raman spectroscopy and RBS/Channelling. The defects densities have been obtained from high-resolution XRD and RBS/Channelling studies which are complemented by Raman results. The in-plane strain measured from XRD has been utilized to obtain the defects densities, which are confirmed by energy dependence of the dechannelling parameter in RBS/Channelling.

Ion beam-induced modification of structural and optical properties of MgTiO3 nanocrystalline thin films

ABSTRACT MgTiO3 thin films were deposited on an amorphous silicon dioxide substrate by the RF magnetron sputtering method by varying the argon and oxygen mixing percentage. These pristine thin films were subjected to thermal annealing at a temperature of 700°C and subsequently two sets of crystalline thin films were irradiated using 100 MeV Au8+ ions at a fixed fluence of 2 × 1013 ions/cm2. XRD spectra reveal that the crystallite size reduces from 21 to 15 nm and crystallinity also decreases drastically after irradiation. AFM images show significant variation in the surface morphology upon irradiation. Optical studies suggest that the band gap as well as the refractive index are both reduced due to SHI irradiation. Ion beam induced modification of the structural, morphological and optical properties of MgTiO3 thin films and the possible mechanisms responsible for variation of the properties of MgTiO3 due to swift heavy ion beam irradiation have been discussed in detail.

Synthesis and tailoring of GaN nanocrystals at room temperature by RF magnetron sputtering

We report here the synthesis of hexagonal GaN nanocrystals on p-silicon substrates by Radio-Frequency (RF) magnetron sputtering without substrate heating or by post-deposition annealing treatment. GaN nanocrystals are synthesized and tailored as a function of RF power at constant Ar and N2 flow rates (working pressure). The observed reduction in grain sizes as a function of RF power has been correlated with an increase in compressive strain. The effect of RF power on crystallite orientation has been determined by diffraction intensities using the degree of c-axis orientation. Atomic force microscopy shows uniform lateral nanocrystal sizes with spherical in shape by insignificant difference in Root-Mean-Square (RMS) roughness values for all the deposited thin films. Transmission electron microscope and field emission-scanning electron microscope studies have been performed to understand the surface morphologies and grain sizes. Thus, tailoring the size of the nanocrystals has been discussed by correlating RF powers, working pressures and cathode voltages on lattice vibrations.

SHI irradiation induced effects in functionalized MWCNTs

Multi-walled carbon nanotubes (MWCNTs) have attracted extensive attention globally due to their applications in modern nanotechnology. It is very important to study the effects on these MWCNTs under swift heavy ion (SHI) irradiation since ion irradiation has been recognized as one of the best tools for nanostructuring of materials, in general. Here, we present the effects of 80 MeV Ni ions with a fluence ranging from 3×1012 to 3×1013 ions/cm2 on functionalized MWCNT mats. The properties of pristine and irradiated samples were studied using X-ray diffraction, Raman spectroscopy and transmission electron microscopy. The effects of SHI irradiation on these samples are discussed in detail.

Ion beam-mixing effects in nearly lattice-matched AlInN/GaN heterostructures by swift heavy ion irradiation

Ion beam-induced intermixing is of great interest and has been observed in various systems such as metal–semiconductor interfaces. However, similar effects in III-nitrides have not been studied in any detail, yet. In the present study, swift heavy ion-induced intermixing of nearly lattice-matched Al(1−x) In x N/GaN heterostructures have been investigated using Rutherford backscattering spectrometry and high-resolution X-ray diffraction techniques. Inter-diffusion of Ga and In elements across the Al(1−x) In x N/GaN interface and the consequent formation of a new mixed quaternary alloy (AlGaInN) layer have been observed. The influence of electronic energy loss of SHI on intermixing effects has been studied.

Characterization Of GeO2 Nanocrystals Prepared By Microwave Annealing

GeOx films have been deposited on silicon substrate using RF magnetron sputtering. The as‐deposited samples were annealed at 900 °C using microwave annealing. All the samples were subsequently characterized by X‐ray diffraction (XRD) to observe the GeO2 nanocrystal formation. Raman spectroscopy and Transmission electron microscopy (TEM) measurements were also carried out to confirm the presence of the nanocrystals. The film topography was studied by atomic force microscopy (AFM).

Synthesis and characterization of nc-Ge embedded in SiO2/Si matrix

We have prepared germanium nanoparticles embedded in SiO2 matrix by atom beam co-sputtering (ABS) of Ge+SiO2 on Si substrate. The as-deposited films were annealed at various temperatures in Ar+H2 atmosphere and irradiated with various energies with fixed fluence. The pristine and irradiated samples were characterized by Raman, X-ray diffraction and atomic force microscopy (AFM). Rutherford back scattering (RBS) was used to quantify the concentration of Ge in the SiO2 matrix and the film thickness. Raman studies of the films indicate the formation of Ge crystallites as a result of swift heavy ion (SHI) irradiation. Moreover, the crystalline nature of Ge improves with an increase in energy. Glancing angle X-ray diffraction and Raman results also confirm the presence of Ge crystallites in the irradiated samples. Similarly, 400 keV Ge+ ions implanted into silicon substrate at higher fluence at 573 K have been irradiated with 100 MeV Au8+ions at RT. These irradiated implanted samples were subsequently characterized by XRD and Raman to understand the crystallization behavior. We also studied the surface morphology of a high-energy irradiated sample by AFM. The irradiation results were compared with those obtained by thermal annealing in ABS. The basic mechanism for crystallization induced by SHI in these films has been investigated.

Ion-beam-treated strained AlGaN/GaN multi-quantum wells: HAADF-STEM, HRTEM, Raman and HRXRD characterizations

Ion beams are extensively used for the modification and analysis of a wide range of materials. However, basic mechanisms of defect propagation with the effect of different electronic energy losses are not understood yet and are essential to improve device performance. In the present study, AlGaN/GaN multi-quantum wells (MQWs), which were grown on sapphire by metal organic chemical vapor deposition, have been subjected to swift heavy ion irradiation at a fixed fluence as a function of type of projectile and its energy to understand the defect formation. Irradiated materials have been compared with the as-deposited ones to see the effects on composition, periodicity and strain. The composition gradient as a function of S e has been investigated with analytical and high-angle annular dark field scanning transmission electron microscopy (TEM) and the results have been further confirmed by high-resolution X-ray diffraction. The interface quality has been analyzed with aberration-corrected high-resolution TEM images. Effects of electronic energy loss on Raman modes have been investigated. Finally, the effects of ion beams on MQWs interfaces are discussed.

SHI Effects On Ge+SiO2 Composite Films Prepared By RF Sputtering

Ge+SiO2 composite films were deposited on Silicon substrate using RF magnetron sputtering. The as‐deposited samples were irradiated with 150 MeV Ag+12 ions at a fixed fluence of 3×1013 ions/cm2. These samples were subsequently characterized by X‐ray diffraction (XRD) and Raman spectroscopy to understand the crystallization behavior. Formation of Ge nanocrystal in amorphous silicon dioxide film was studied using transmission electron microscopy (TEM). We also studied the surface morphology of these high energy irradiated samples by Atomic Force Microscopy (AFM). The basic mechanism for ion beam induced crystallization in these films has been discussed.