Shubhra Gangopadhyay | University of Missouri Columbia (original) (raw)

Papers by Shubhra Gangopadhyay

Hydrogenated amorphous silicon carbide (a-SiC:H) films were prepared by plasma-enhanced chemical ... more Hydrogenated amorphous silicon carbide (a-SiC:H) films were prepared by plasma-enhanced chemical vapor deposition from diethylsilane (C4H12Si) diluted in methane (CH4). The deposition conditions resulted in a highly unstable a-SiC:H film, which reacted with oxygen when exposed to air, forming Si-OH and H-OH bonds and Si-O networks as well. The Rutherford Back Scattering analysis shows 15incorporation. The Fourier transfom infra-red (FT-IR)

Journal of Applied Physics, 2015

ABSTRACT

ECS Transactions, 2013

A facile development of highly efficient platinized titanium dioxide (Pt-TiO2) thin films with un... more A facile development of highly efficient platinized titanium dioxide (Pt-TiO2) thin films with unique one dimensional (1D) structure by using versatile gas phase deposition methods was established. The Pt-TiO2 thin films demonstrated excellent carbon dioxide (CO2) photoreduction efficiency with a maximum quantum efficiency of about 2.41%. Carbon dioxide was selectively converted into methane (CH4), with an optimal CH4 yield of over 1300 µmol/g-cat/hr, that is due to the synergistic effects of high surface area and minimized charge barriers by highly oriented single phase crystallinity of the TiO2 thin films, and efficient electron-hole separation by the ultra-small Pt NPs.

A low pressure, plasma-enhanced chemical vapor deposition (PECVD) system has been built, utilizin... more A low pressure, plasma-enhanced chemical vapor deposition (PECVD) system has been built, utilizing a plasma created by electron cyclotron resonance (ECR) ionization of source gases. The magnetic mirror on the usual, cylindrical geometry ECR system has been replaced by a racetrack, or toroidal, geometry. The toroidal design is employed to yield the high electron temperatures necessary to dissociated large molecules completely and produce the necessary precursors for deposition. The toroidal design allows for greater flexibility in control of the plasma, and the use of current knowledge of toroidal plasmas. This system is designed to maintain a stable plasma at low pressure and low plasma density. The system is being characterized by magnetic field density measurements, and other plasma diagnostic techniques, including langmuir probe measurements made on Argon plasmas. The film quality will be compared to that of films deposited in a conventional linear system under similar conditions. The system is flexible enough to run with virtually any source gas, and is presently set to run with methane for carbon films and with a proprietary liquid source for silicon carbide films.

Plasma enhanced deposition of amorphous Aluminum Nitride (AlN) using Trimethylaluminum, Hydrogen,... more Plasma enhanced deposition of amorphous Aluminum Nitride (AlN) using Trimethylaluminum, Hydrogen, and Nitrogen was performed in a capacitively coupled plasma system. Temperature was varied from 350-550o C, and pressure dependence of the film structure was investigated. Capacitance-voltage (C-V), conductance-voltage (G-V) and current-voltage (I-V) measurements were performed to reveal bulk and interface electrical properties. The electrical properties showed marked dependence on

HfO2 films of varying thickness were deposited on silicon and titanium by reactive electron beam ... more HfO2 films of varying thickness were deposited on silicon and titanium by reactive electron beam evaporation. Metal gates of Titanium were deposited to form MIS and MIM structures. Capacitance-voltage and conductance-voltage measurements were performed to analyze the HfO2/Si interface and HfO2 bulk. For samples having thickness between 30 and 300 Å the C-V curves of the as-deposited samples were marked

High dielectric constant materials that are being aggressively studied for SiO2 replacement in CM... more High dielectric constant materials that are being aggressively studied for SiO2 replacement in CMOS technology have the inherent problem of SiOx formation at the interface, thus reducing the overall gate stack equivalent oxide thickness. We have studied the interface at the onset of deposition using electron diffraction in ultra-high vacuum deposition of HfO2. Various silicon cleaning methods have been shown

Deposition of silver nano-particles (Ag-nps) in a thin aluminum oxide layer resulting in an incre... more Deposition of silver nano-particles (Ag-nps) in a thin aluminum oxide layer resulting in an increased frequency dependent dielectric constant has been investigated for possible applications in nano-FLASH memory and nano-CMOS devices. The design of nano-FLASH memory with embedded Ag-nps in the gate oxide has been analyzed through simulation that demonstrates the charge storage capability of Ag-nps. A large shift in threshold voltage (˜7.5V) was found with a programming gate voltage of only 1 V, indicating very low power consumption by the device. The programming time requirement was found to be less than 10 nano-seconds and is erasable.

Photovoltaic device converts solar energy into electricity by the photovoltaic effect. Efficiency... more Photovoltaic device converts solar energy into electricity by the photovoltaic effect. Efficiency of a photovoltaic device depends on the active material which absorbs light as well as the window layer which transmits light into the active layer. High band gap window layer allows majority of light to transmit from the solar spectrum while active layer has an optimum value for maximum efficiency. We have carried out research on amorphous silicon as an active conversion layer which has a larger band gap and higher light sensitivity than crystalline Si for photovoltaic device, while high band gap zinc oxide (ZnO) can serve as window layer. Betavoltaic device is analogous in principle to a photovoltaic and the difference is that it is specifically designed to convert energy from beta particles rather than photons. Nano crystalline silicon-carbide (SiC) and ZnO are some of the ideal candidates for betavoltaic devices because of its high band gap and radiation hardness. 1,2 Both of them are also sensitive to light, so potentially they can be used to fabricate photovoltaic devices. Present betavoltaic devices based on SiC are not cost effective as SiC requires high temperature process. We were able to achieve SiC films at a low temperature process by converting amorphous SiC into crystalline SiC using aluminum induced crystallization technique. The films were characterized by FTIR, TEM, UV-Vis spectroscopy, Hall measurements and I-V measurements. The p-n junctions are fabricated by depositing p-type SiC layer on n-type substrates. For successful fabrication of betavoltaic device, high quality n-ZnO films are deposited and characterized by Raman spectroscopy, XRD, UV-Vis spectroscopy, Hall measurements, I-V and photoconductivity measurements. ZnO based p-n junctions are fabricated by depositing ZnO (n-type) layer on p-type Si substrate, and the rectifying behaviors were observed. For proof of concept a betavoltaic device was fabricated and tested with beta particle.

ABSTRACT A novel method for producing silver plasmonic gratings for surface plasmon resonance (SP... more ABSTRACT A novel method for producing silver plasmonic gratings for surface plasmon resonance (SPR) based coupling of light has been developed which utilizes soft lithography technique using PDMS stamping of grating from HDDVDs. Further, 20nm wide nano-gaps are formed on the stamp wherein an extreme field concentration occurs leading to enhancements of 118 times with respect to glass. The gratings with nanogaps were used for single molecule studies, with an immobilized layer of tagged DNA molecule. Enhancement with epifluorescence on gratings when compared to total internal reflection microscopy (TIRF) on quartz slides is up to 40 times on nanogaps. Further, single molecule Forster resonance energy transfer imaging used to study the dynamics of DNA performed on the gratings shows intensity enhancement by 10 times on nanogaps in comparison to TIRF on quartz. Finally, in order to improve the reproducibility of the nanogaps, a glancing angle deposition method (GLAD) is coupled with the existing technique in order to form nanogaps. We are thus also able to produce extremely sharp tip regions on these grating structures which further enhance the coupling efficiencies due to a field concentration within these hotspot regions. The layered structure is able to produce height dependent enhancement giving a 3-dimentional view of micromolar concentration of dye on the surface. In conclusion, a method of fabricating plasmonic substrates has been developed which can be utilized for sensing or observation of single molecule interactions using epifluorescence.

We report pentacene-based field effect transistor devices with remarkable improvement in sensitiv... more We report pentacene-based field effect transistor devices with remarkable improvement in sensitivity towards trace nitroaromatic vapors through the incorporation of high density, sub-2 nm metal nanoparticles (NPs). Exploiting the unique electrical properties of these NPs, we have demonstrated a detection limit of 2,4-dinitrotoluene (DNT) vapor to be 56.6 parts per billion while control samples (without any embedded NPs) showed no observable DNT sensitivity. We attribute this remarkable enhancement in sensitivity to the ability of these NPs to function as discrete nodes, participating in charge transfer with adsorbed nitroaromatic molecules. To further improve sensor performance, we reduced the feature size of the channel down to sub-10 nm level by electron beam lithography. A pentacene nanoflower structure with average size of 300 nm was deposited on top of the sub-2nm Au NPs between the nanogap as the tunneling barrier to reduce the tunneling resistance. With this device configuration, the size-dependent Coulomb charging energies of 0.48 eV (for 0.5 nm size of Au NPs) to 0.11 eV (for 1.93 nm of Au NPs) are much larger than thermal energy under 300 K (0.0259 eV), allowing us to observe single electron tunneling behaviors by simple current-voltage (I-V) measurements at room temperature. The single electron interaction between the SET and the target molecules as well as the sensor performance have also been investigated.

A cleaning process resulting in atomically smooth, hydrogen-terminated, silicon surface that woul... more A cleaning process resulting in atomically smooth, hydrogen-terminated, silicon surface that would inhibit formation of native silicon oxide is needed for high-k gate dielectric deposition. Various cleaning methods thus need to be tested in terms of resistance to native oxide formation. Native oxide re-growth is studied as a function of exposure time to atmospheric ambient using ellipsometry. Hafnium dioxide film (k ~23) is deposited on the as-cleaned substrates by electron beam evaporation and subsequently annealed in hydrogen. The difference in the effective oxide thickness re-grown on surfaces treated with the conventional RCA and modified Shiraki cleaning methods, after one-hour exposure, can be as large as 2 Å. This is significant in device applications demanding equivalent oxide thickness less than 20 Å. The degree of hydrogen passivation, surface micro-roughness and organic removal capability are considered to be the main factors that explain the differences between the cleaning methods. Data derived from capacitance-voltage analysis of test capacitors verified the trend observed in the native oxide thickness measurements. An increase of 10~15 % in accumulation capacitance is observed in the samples treated by the new cleaning method.

We report remarkable improvement in sensitivity of pentacene-based field effect transistor device... more We report remarkable improvement in sensitivity of pentacene-based field effect transistor devices towards trace nitro-aromatic explosive vapors through the incorporation of high density, sub-2 nm platinum nanoparticles (NPs) within these structures. Exploiting the unique electronic properties of these NPs, we have demonstrated a detection limit of 56.6 parts per billion of 2,4-dinitrotoluene (DNT) vapor while control samples without any embedded NPs showed no observable sensitivity to DNT vapor. We attribute this remarkable enhancement in sensitivity to the ability of these NPs to function as discrete nodes, participating in the charge transfer with adsorbed nitro-aromatic molecules. V

We present an indium tin oxide/polymethylsilsesquioxane (ITO/PMSSQ) photonic crystal with grating... more We present an indium tin oxide/polymethylsilsesquioxane (ITO/PMSSQ) photonic crystal with grating structure. This topography exhibits multiple coupling resonances in specific regions of the solar spectrum, which can be controlled by tuning the angle and polarization of incident light. The ITO/PMSSQ grating structures are prepared via simple soft lithography procedures using HD-DVDs as templates. The surface topology also includes geometrical singularities such as nanogaps and nanocones, which lead to further enhanced light trapping characteristics as demonstrated by theoretical and experimental results. Together, these singularity-embedded photonic crystal gratings result in extremely concentrated electromagnetic fields at the aforementioned singularities, which could be especially helpful in separating the tightly bonded excitons in organic solar cells and, hence, improve the efficiency significantly.

We present an indium tin oxide/polymethylsilsesquioxane (ITO/PMSSQ) photonic crystal (PC) with gr... more We present an indium tin oxide/polymethylsilsesquioxane (ITO/PMSSQ) photonic crystal (PC) with grating structure. This topography exhibits multiple coupling resonances in specific regions of the solar spectrum, which can be controlled by tuning the angle and polarization of incident light as demonstrated through simulation and experimental results. The ITO/PMSSQ grating structures are prepared by simple soft lithography procedures using commercial HD-DVD and Blu-ray as template molds. The surface topography also includes geometrical singularities such as nanogaps and nanocones, which lead to further enhanced light trapping characteristics. Together, these singularity-embedded photonic crystal gratings result in concentrated electromagnetic fields at the aforementioned singularities, which could be especially helpful in separating the tightly bonded excitons in organic solar cells. Simulation shows an absorption enhancement over the full spectral range of ITO PCs-based solar cells with substantial increases closer to the band edge under s-polarized light.

ABSTRACT Hydrogenated amorphous silicon carbide (a-SiC:H) films were prepared by plasma-enhanced ... more ABSTRACT Hydrogenated amorphous silicon carbide (a-SiC:H) films were prepared by plasma-enhanced chemical vapor deposition from diethylsilane (C4H12Si) diluted in methane (CH4). The deposition conditions resulted in a highly unstable a-SiC:H film, which reacted with oxygen when exposed to air, forming Si-OH and H-OH bonds and Si-O networks as well. The Rutherford Back Scattering analysis shows 15incorporation. The Fourier transfom infra-red (FT-IR) measurement suggests the presence of a-SiC:O:H component and a-C:H moieties throughout the film. The films were then thermally annealed in vacuum at 450C to removed unstable C-H bonds and Si-OH and H-OH groups creating voids in the film. The removal of C-H, Si-OH and H-OH is evident from the FT-IR spectra. During the annealing process, the film also cross-links and formed a stable a-SiC:O:H film. After annealing, the dielectric constant of the films decreased from 4.2 to 2.1 suggesting the presence of voids/pores in the films.

We have used infrared ͑IR͒ absorption spectra to characterize a-C:H and its alloys ͑a-C:H,N,F͒. T... more We have used infrared ͑IR͒ absorption spectra to characterize a-C:H and its alloys ͑a-C:H,N,F͒. The samples were deposited in an rf plasma-enhanced chemical-vapor deposition system with various mixtures of CH 4 , N 2 , and NF 3 gases. IR spectra are analyzed in detail and the peak position assignments are based on the data published in the literature and calculation of the normal mode vibrational frequencies by assuming a simple force field and linear-molecular model. From IR analysis, we found that as the concentration of nitrogen and fluorine increases, the ratio of sp 2 /sp 3 increases, while the hydrogen content decreases. The role of nitrogen is to break the symmetry and activate the olefinic CvC stretching mode. The role of fluorine is to replace hydrogen and to form stronger C-F bond. This explained the improvement of the thermal stability of electrical conductivity of a-C:H,N,F samples. ͓S0163-1829͑97͒04119-2͔

Journal of Applied Physics, 2015

ABSTRACT

ECS Transactions, 2013