Elangovan Elamurugu

Metal/metal-oxide nanocoatings on black silicon nanograss for enhanced solar absorption and photochemical activity

In this work, we present experimental results showing optical absorption enhancement of silicon wafer through etching and metal/metal-oxide nanolayers deposition. Black silicon nanograss were fabricated from single crystalline silicon by reactive ion etching, and ZnO, Pt, and CeO2 nanolayers were deposited through atomic layer deposition as well as magnetron sputtering. The resulting structure exhibits less than 8% reflection over broadband solar spectrum. The fabricated structures are analyzed by scanning electron microscope, focused ion beam milling slice and view and transverse electron microscope sample preparation. The results are compared to finite difference time domain simulations based on the actual fabricated structures. A study of the influence of various parameters on the geometry of the fabricated micro and nanostructures and the corresponding change in optical properties is also presented. Applications of such highly absorbing metamaterials to solar photocatalysis is discussed.

Metal oxide thin film transistor based sensing

Metal oxide based thin film transistors (TFT) have shown strong improvement of their electric characteristics with n-type devices being gradually introduced commercially in display technology. Following on this success path, potential applications of such TFTs to Internet of Things (IoT) chemical sensing enablement are discussed, with a focus on the effect of polyols on molybdenum doped indium oxide and zinc oxide fully transparent TFTs.

Characterization of molybdenum doped indium oxide/aluminum doped zinc oxide thin film stacks for optoelectronic applications

Multilayer (ML) thin films, based on indium molybdenum oxide (IMO) and aluminum zinc oxide (AZO), having different stacking were deposited using RF sputtering at room temperature (RT). The total-layer thickness of the MLs ranges between 93 nm and 98 nm. The deposited films were characterized by their structural, electrical, microstructural, and optical properties. X-ray diffraction (XRD) peaks obtained at 2θ of around 30.6° and 34.27° are matched with cubic-In2O3 (222) and hexagonal-ZnO (002), respectively. The MLs have both nano-crystalline and polycrystalline structures depending on the layer properties. A conspicuous feature of XRD analysis is the absence of diffraction peak from 50 nm thick IMO layer when it is stacked below 50 nm thick AZO, whereas it appears significantly when the stacking is reversed to place IMO above AZO layer. Hall measurements confirmed that the deposited MLs are n- type conducting and the electrical properties are varied as a function of layer properties. The deposited MLs show high shortwavelength infrared transmittance (SWIRT) even at 3300 nm, which is ranging as high as 75 % - 90 %. Overall, the MLs show high transmittance in the entire Vis-SWIR region. The optical band gap (Eg) calculated using the absorption coefficient (α) and photon energy (hν) of the deposited MLs is ranging between 3.19 eV and 3.56 eV, depending on the layer properties. Selected as- deposited films were annealed in open air at 400 °C for 1 h; the transmittance of annealed films was improved but their electrical properties deteriorated. Atomic force microscopy (AFM) analysis shows that the root-mean-square (RMS) roughness of the MLs ranges between 0.8 nm and 1.5 nm.

Erbium-doped ZnO and TiO2 nanolaminates for photonic

In this work, we study the crystal morphology, electron transport and photo-electroluminescence of erbium-doped nanolaminates of ZnO, TiO2 and SiO2, deposited by RF-sputtering, with and without an annealing step. The effect of the nanolaminate on the interface roughness, erbium distribution on the laminate and its correlation to the photo- and electroluminescence (visible and infrared domains) is presented. A discussion on potential nanoscale electrical excitation pathways of active erbium species is also presented.

Fluorine and oxygen plasma influence on nanoparticle formation and aggregation in metal oxide thin film transistors

Despite recent advances in metal oxide thin-film transistor technology, there are no foundry processes available yet for large-scale deployment of metal oxide electronics and photonics, in a similar way as found for silicon based electronics and photonics. One of the biggest challenges of the metal oxide platform is the stability of the fabricated devices. Also, there is wide dispersion on the measured specifications of fabricated TFT, from lot-to-lot and from different research groups. This can be partially explained by the importance of the deposition method and its parameters, which determine thin film microstructure and thus its electrical properties. Furthermore, substrate pretreatment is an important factor, as it may act as a template for material growth. Not so often mentioned, plasma processes can also affect the morphology of deposited films on further deposition steps, such as inducing nanoparticle formation, which strongly impact the conduction mechanism in the channel layer of the TFT. In this study, molybdenum doped indium oxide is sputtered onto ALD deposited HfO2 with or without pattering, and etched by RIE chlorine based processing. Nanoparticle formation is observed when photoresist is removed by oxygen plasma ashing. HfO2 etching in CF4/Ar plasma prior to resist stripping in oxygen plasma promotes the aggregation of nanoparticles into nanosized branched structures. Such nanostructuring is absent when oxygen plasma steps are replaced by chemical wet processing with acetone. Finally, in order to understand the electronic transport effect of the nanoparticles on metal oxide thin film transistors, TFT have been fabricated and electrically characterized.

Photocurrent enhancement in nanocoatings of cerium oxide and platinum on black silicon

Black silicon is surface modification by reactive ion etching that creates a forest of silicon micro-spikes and increases surface area of the sample. When the spikes’ height exceed an optical wavelength, light is trapped on the surface through multiple pathway scattering, increasing the optical absorption of visible and near infrared radiation. Cerium oxide (CeO2) is believed to have good photoactivity, and finds many applications including photoelectrolysis. However, the large band gap limits the efficiency of the water splitting process. We suggest black silicon surfaces as substrates for CeO2 sputter coating to increase photon-material interaction. An additional catalytic layer of platinum is deposited to create highly energetic electrons as a result of plasmonic resonance and enhances incident photon to current efficiency (IPCE). The difference of surface current for laser on and off condition is found to be 32 times higher in a nanolayered coated black silicon sample as compared to flat silicon. The resistance of flat silicon substrate was 11 Ω for laser-off state, decreasing to 9 Ω when the laser was turned on. On the other hand, the black silicon substrate sample had a higher resistance of 70 Ω in dark which decreased to 1.5 Ω for laser on state.

Impact of glycerol on zinc-oxide-based thin film transistors with indium molybdenum oxide transparent electrodes

We report the fabrication of thin film transistors with ZnO channel and indium molybdenum oxide electrodes by sputtering. The fabricated transistors were then exposed to glycerol. We observe a temporary change in device performance after immersion of the FET in glycerol. Control structures without channel material are also used for demonstrating that the effect of saturation current increase is not due to glycerol alone as sugar alcohol is a low conductive medium. Various electrical and optical parameters are extracted. The presented results are useful for further integration of photonics and electronics in sensing applications

Fully transparent thin film transistors based on zinc oxide channel layer and molybdenum doped indium oxide electrodes

In this work we report the fabrication of thin film transistors (TFT) with zinc oxide channel and molybdenum doped indium oxide (IMO) electrodes, achieved by room temperature sputtering. A set of devices was fabricated, with varying channel width and length from 5μm to 300μm. Output and transfer characteristics were then extracted to study the performance of thin film transistors, namely threshold voltage and saturation current, enabling to determine optimal fabrication process parameters. Optical transmission in the UV-VIS-IR are also reported.