Department of Physics

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Browsing Department of Physics by Author "Nayak, Arabinda"

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    Characterization of Chemical Vapor Deposition Grown Thin film Semiconductor Nanostructure for Optoelectronic Applications
    Nayak, Arabinda
    This work first represents a detailed high-resolution X-ray reflectivity (XRR) and rocking curve (XRC) analysis of a spontaneous superlattice structure in MOCVD grown AlxGa1-xAs epilayer on (001) GaAs. A model-independent Distorted Wave Born Approximation formalism was applied to the XRR results to obtain the compositional information as a function of depth. A kinematical model was then used successfully to fit the sharp peaks observed in the XRC using the characteristic length and compositional parameters obtained from the XRR simulation. The thermal stability of such superlattice was analyzed with the help of X-ray rocking scans at elevated temperatures. The ordered (spontaneous) structure irreversibly transformed to the disordered one retaining its single-crystalline nature at 900o C. For photo detection, the (Au-Ge) alloy was deposited on the as-grown and the disordered sample to realize an (MSM) photodetector (400-900 nm). Furthermore, a comparative analysis of the two devices was also discussed. In the next part, a symmetric coplanar MSM structure with Nickel as metal contacts was fabricated on quaternary (AlxGa1-x) yIn1-yP epitaxial film on (100) SIGaAs for visible photodetection. The crystallinity and alloy composition (y=0.66 and x=0.67) of the grown material was determined by high-resolution X-ray rocking curve and photoluminescence experiment. The photo-response of the device was studied in the wavelength range of 300-1000 nm which exhibited a peak at 610 nm. The photodetector showed a fast rise time of 91 μs however, the decay characteristics exhibited two channels with decay times of 83 μs and 282 μs, respectively. In our final work, X-ray, SEM, and photoluminescence analysis were employed to understand the band-edge related excitonic lasing features from the vertically oriented ZnO nanowires grown by CVD on sapphire. The nanowires exhibited high degree of crystallinity, strong preferred orientation, and relative inplane alignment having 6-fold hexagonal symmetry of the Wurtzite phase. A 325 nm laser source was employed to optically pump the nanowires. The nanowires showed the onset of lasing below 150 K being prominent at 3.4 K. The threshold pump power was found to be 14.5 mW at 3.4 K. Such nanowires are suitable for lasing in the ultraviolet region of the electromagnetic spectrum below a characteristic temperature of 150 K at a low threshold optical pumping power.
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    Development, Synthesis and Characterization of II-IV-V Chalcopyrite Thin Film for Photovoltaic Device Application
    Nayak, Arabinda
    During last few years much research attention has been devoted for the development and design of new highly efficient economic optoelectronic devices to replace Silicon (Si) based devices for commercial application in various field. The major disadvantage for Si based devices is the indirect electronic bandgap which limits the devices efficiency. This work exhibits the growth, characterization and device application of a novel lead (Pb) free material ZnSnP2 as a replacement of Si. Here, we have successfully grown the ZnSnP2 thin film on p-type silicon (001), sapphire and glass substrate by physical vapor deposition technique for the very first time. It is also evident that during growth the deposition rate plays a significant role to determine the bandgap. At higher growth rate the bandgap of ZnSnP2 thin film decreases due to increasing cationic disorder. Temperature and power dependent photoluminescence properties indicate that the peaks have originated due to donor-acceptor pair related transition. For photodetector application Mg/ZnSnP2/Sn structure is fabricated to operate in the UV-VIS-NIR region of the electromagnetic spectrum. The observed current-voltage characteristics show roll-over like features which was successfully modeled considering two Schottky diodes connected back to back. The maximum values of responsivity, photosensitivity and detectivity are found to be of 29.58 mAW-1, 1.0x102 cmW-1 and 9.42x1010 cmHz1/2W-1 in the forward bias and 0.0076 AW-1, 1.2x102 cm2W- and 15.53x1010 cmHz1/2W-1 in the reverse bias, respectively at illumination of wavelength 850 nm and it offers very fast response speed of 47 μs. The performance of the photodetector is investigated from energy band diagram and to improve it the Sn contact was annealed at 250o C for one hour to reduce barrier height and the respective junctions were treated individually. After annealing the obtained values of the responsivity and detectivity are found to be of 0.2 AW-1 and 4.62x1012 Jones for the Mg/ZnSnP2 and 0.03 AW-1 and 1.62x1011 Jones for the Sn/ZnSnP2 junctions, respectively when they were operated without bias. Under a bias of 2.5 V, these values enhance to 4.7 AW-1 and 1.19x1012 Jones in case of Mg/ZnSnP2 and 12.7 AW-1 and 4.78x1011 Jones for Sn/ZnSnP2 junctions, respectively. The variation of photocurrent is super-linear with the illumination power with the values of the power exponent for the Mg/ZnSnP2 and Sn/ZnSnP2 junctions of 1.75±0.19 and 1.42±0.21, respectively.
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    Study of Electronic Structure, Optical and Magnetic Response of Disordered Solids
    Nayak, Arabinda
    My Ph.D program includes the theoretical studies of the electronic structure, optical and magnetic response of ordered compounds and disordered alloys. We have studied the optical conductivity and dielectric response of disordered, bulk NixPt1−x alloys and disordered, two dimensional SixC1−x. This study is based on the density functional theory (DFT) approximation and the tight binding linear muffin tin orbitals (TB-LMTO) method. Disorder is taken into account by the “Augmented Space Formalism” which allows us to accurately estimate the effect of disorder both homogeneous disorder as well as inhomogeneous disorder related to short-ranged ordering, clustering and segregation. This can be done both in bulk, surfaces and interfaces. We have overcome the defeciency of the local density approximation (LDA) and incorporated the vLB-corrected exchange correlations. Our estimation reasonably, successfully agrees with the initial experimental observations. This encourages me to go forward in this area. The proposed formalism opens up a facile way to band-gap engineer material for optoelectronic application. In parallel, we have generated the electronic structure of disordered Graphene and Graphinic materials with random vacancies and random doping have also been investigated. We have shown how the topology of the Dirac dispersion changes with disorder. We have studied disorder induced lifetime effects in 2D materials. Our study will provide a reference and detailed useful insight for building interesting nano materials. Specifically, I have worked on the magnetization dynamics and excitation spectra of disordered binary FexCo1−x alloys. We have studied the magnon softening for chemically disordered Fe20Co80 alloys in the linear spin wave regime. Our proposed formalism on magnetic response can be extended to study the spin transport, magnetic properties, excitation spectra and spin dynamics of real complex materials, ferromagnetic semiconductors and Heisenberg spin chains in disordered alloys. In this thesis, we have not only been able to study the electronic structure and response function of a few disordered systems but also successfully analyzed and interpreted the experimental results based on our theories.