Browsing by Author "Raju, Ramesh"
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Item Atomic layer deposition of Zr-sandwiched ZnO thin films for transparent thermoelectrics(IOP Publishing Ltd., 2023-01-15) Koskinen, Tomi; Volin, Ulrika; Tossi, Camilla; Raju, Ramesh; Tittonen, Ilkka; Department of Electronics and Nanoengineering; Ilkka Tittonen Group; Department of Electronics and NanoengineeringAtomic layer deposited (ALD) transparent thermoelectric materials enable the introduction of energy harvesting and sensing devices onto surfaces of various shapes and sizes in imperceptible manner. Amongst these materials, ZnO has shown promising results in terms of both thermoelectric and optical characteristics. The thermoelectric performance of ZnO can be further optimized by introducing extrinsic doping, to the realization of which ALD provides excellent control. Here, we explore the effects of sandwiching of ZrO2 layers with ZnO on glass substrates. The room-temperature thermoelectric power factor is maximised at 116μW m−1 K−2 with samples containing a 2% nominal percentage of ZrO2. The addition of ZrO2 layers is further shown to reduce the thermal conductivity, resulting in a 20.2% decrease from the undoped ZnO at 2% doping. Our results contribute to increasing the understanding of the effects of Zr inclusion in structural properties and growth of ALD ZnO, as well as the thermal and thermoelectric properties of Zr-doped ZnO films in general.Item Characterisation of GaN based LEDs grown on unpatterned and patterned sapphire(2019-03-11) Chen, Wen; Raju, Ramesh; Kim, Iurii; Sähkötekniikan korkeakoulu; Sopanen, MarkkuGallium nitride light-emitting diodes (GaN LEDs) grown by metal organic vapour phase epitaxy (MOVPE) on sapphire, patterned sapphire substrate (PSS), silicon, and silicon on insulator (SOI) were characterized in this thesis. High resolution X-ray diffraction (HRXRD), atomic force microscopy (AFM), and photoluminescence (PL) spectroscopy were used to characterize the crystal quality, surface morphology and light extraction, respectively. ω − 2θ HRXRD scans were used to take the (0002) full width at half maximum of the GaN layers to calculate the threading dislocation (TD) density. Surface roughness and uniformity was measured by using AFM in semi-contact mode. The multiquantum-well (MQW) structure was characterized by PL spectroscopy, with a laser with an emission wavelength of 415 nm that was directed on the sample. The structure grown on PSS resulted in the most intense luminescence peak and had the lowest TD density with the sapphire sample, although reports of it being lower compared to sapphire have been published. The AFM measurements exhibited a high density of hexagonal pinholes in the PSS, which may be due to poor coalescence at the beginning of the growth.Item Effects of indium flow rate on the structural, morphological, optical and electrical properties of InGaN layers grown by metal organic chemical vapour deposition(Elsevier Science, 2019-11-30) Prabakaran, K.; Raju, Ramesh; Arivazhagan, P.; Jayasakthi, M.; Sanjay, S.; Surender, S.; Pradeep, S.; Balaji, M.; Baskar, K.; Department of Electronics and Nanoengineering; Markku Sopanen Group; Anna University; Indian Institute of Technology Madras; University of MadrasInGaN/GaN heterostructures were grown on c-plane sapphire substrates using metal organic chemical vapour deposition by varying the trimethylindium flow rate as 7, 10 and 14 μmol/min. The structural, morphological, optical and electrical properties of InGaN layers were investigated. Crystalline quality, dislocation densities comprising of screw and edge types in InGaN and GaN layer have been analyzed using High-Resolution X-ray Diffractometer (HRXRD). The composition of Indium (In) in the InGaN layers was estimated around 8–10% which was found to be dependent on the In flow rate. The strain between InGaN and underlying GaN layer have been analyzed through reciprocal space mapping studies along the (1 0–1 5) plane in InGaN/GaN heterostructures. The features of V and trench defects were observed using scanning electron microscopy and atomic force microscopy respectively. The V and trench defect density has been correlated with the pre-existing threading dislocation density estimated using HRXRD measurements. Also the trench defects were observed to be a coalescence of V defects in InGaN layers. The photoluminescence results showcased the band edge emission peaks at three different points (primary flat, centre, and Edge). These peak variations were found to be red shifted in all three points. This may be due to the fluctuations in the Indium composition and its corresponding V and trench defects respectively. The Hall measurements exhibit an alteration in the semiconducting behavior with respect to V and trench defect surrounded InGaN layers. And, it also emphasizes that the compressive strain in underlying GaN can lead to the high sheet concentration compared to the tensile strain in the underlying GaN layer. It clearly suggests that the V and trench defect surrounded InGaN layers are the suitable material for next generation optoelectronics applications.Item Enhancement of visible light photodetector performance for ultrafast switching using flower shaped gallium nitride nanostructures(Elsevier Limited, 2021-03-15) Sankaranarayanan, Sanjay; Kandasamy, Prabakaran; Raju, Ramesh; Gengan, Saravanan; Krishnan, Baskar; Koneru Lakshmaiah Education Foundation; Hunan University; Ilkka Tittonen Group; Anna University; Department of Electronics and NanoengineeringWell-defined Dahlia type gallium nitride flowers (GaNFs) have been synthesized on c-plane sapphire substrates at different growth conditions using chemical vapour deposition system. The growth was carried out without any catalyst medium for favouring the nucleation process. Crystal structure of the GaNFs was obtained using x-ray diffractometer (XRD). Scanning electron microscopy (SEM) revealed the morphology of GaNFs. The elemental traces and compositions of the samples were obtained using energy dispersive x-ray (EDX) spectroscopy and x-ray photoelectron spectroscopy (XPS). The optical properties of the samples were analysed using cathodoluminescence (CL) spectroscopy and Raman spectroscopy. From the current-voltage (I-V) response, it was observed that there are no interface and surface related inhomogeneities in the samples. The fabricated photodetector showcased good device performance due to high carrier density in the sample.Item Fabrication of gallium nitride and nitrogen doped single layer graphene hybrid heterostructures for high performance photodetectors(Nature Publishing Group, 2020-09-02) Sankaranarayanan, Sanjay; Kandasamy, Prabakaran; Raju, Ramesh; Krishnan, Baskar; Department of Electronics and Nanoengineering; Markku Sopanen Group; Ilkka Tittonen Group; Koneru Lakshmaiah Education Foundation; Hunan University; Anna UniversityGallium nitride (GaN) was epitaxially grown on nitrogen doped single layer graphene (N-SLG) substrates using chemical vapour deposition (CVD) technique. The results obtained using x-ray diffractometer (XRD) revealed the hexagonal crystal structure of GaN. Photoluminescence (PL) spectroscopy, energy dispersive x-ray (EDX) spectroscopy and x-ray photoelectron (XPS) spectroscopy revealed traces of oxygen, carbon and nitrogen occurring either as contamination or as an effect of doping during the GaN growth process. In addition, PL revealed a weak yellow luminescence peak in all the samples due to the presence of N-SLG. From the obtained results it was evident that, presence of N-SLG underneath GaN helped in improving the material properties. It was seen from the current–voltage (I–V) response that the barrier height estimated is in good agreement with the Schottky–Mott model, while the ideality factor is close to unity, emphasizing that there are no surface and interface related inhomogeneity in the samples. The photodetector fabricated with this material exhibit high device performances in terms of carrier mobility, sensitivity, responsivity and detectivity. The hall measurement values clearly portray that, the GaN thus grown possess high electron contents which was beneficial in attaining extraordinary device performance.Item Fast formation of thin TiOx layer on titanium surface enabling a broadband light capture and fast charge carrier transfer(Elsevier Science B.V., 2023-10-01) Hou, Xuelan; Zhang, Hang; Raju, Ramesh; Li, Yongdan; Lund, Peter D.; Department of Applied Physics; Department of Electronics and Nanoengineering; Department of Chemical and Metallurgical Engineering; Department of Applied Physics; Department of Electronics and Nanoengineering; Department of Chemical and Metallurgical EngineeringImproving light absorption and conversion ability is essential to improving the performance of photoelectrochemical (PEC) cells in converting and storing solar radiation into chemical energy. As part of this, realizing broadband light absorption in the wavelength of 250–2500 nm is promising to improve solar energy utilization. Previous studies have typically relied on noble metal functional materials to realize visible light harvesting over broadband light capture in PEC water splitting (WS) cells. Here, we report an electrochemical oxidation method to prepare thin TiOx layer on a Ti-foil surface. A nanostructured film is formed within 10–60 s with a tunable absorption range between 250 and 2500 nm, going beyond the 420 nm absorption edge of TiO2. In addition to superior light capture ability, the film shows a 10-fold enhancement in the charge carrier transfer rate under AM 1.5 G light compared to that measured under dark conditions. The method proposed is simple, fast, and low-cost, and it has potential for large-area PEC cells with promoted charge carriers’ transfer.Item Hafnium Aluminate–Polymer Bilayer Dielectrics for Organic Light-Emitting Transistors (OLETs)(American Chemical Society, 2024-02-27) Gallegos Rosas, Katherine; Myllymäki, Pia; Saarniheimo, Mikael; Sneck, Sami; Raju, Ramesh; Soldano, Caterina; Department of Electronics and Nanoengineering; Caterina Soldano Group; Ilkka Tittonen Group; Beneq OyOrganic light-emitting transistors are thin-film transistors capable of generating and sensing light under appropriate bias conditions. Achieving a low-voltage operation while maintaining high efficiency can be obtained by using high-mobility semiconductors, highly efficient emissive layers, and high-capacitance dielectrics. We report on the fabrication, dielectric characterization, and implementation of (organic/inorganic) bilayer dielectric stacks in green organic light-emitting transistors. Our dielectric stack includes a nanoscale hafnium aluminate layer fabricated by atomic layer deposition and a thin layer of a polymer dielectric. We found that the hafnium aluminate layer is amorphous in nature and highly transparent in all the visible range. The bilayer stack showed around 10 times higher capacitance per unit area than our polymer reference dielectric. When used as a dielectric in organic light-emitting transistors, this stack enabled low leakage current and operation below 20 V, with a threshold around 6 V and similar efficiencies. Thus, engineering the dielectric layer can enable the tuning of the device working conditions and performances while keeping robustness and negligible leakage values. Thus, these findings open the way to use nanoscale organic/inorganic bilayer dielectrics to enable low-bias, low-power consumption optoelectronic devices of relevance for next-generation electronics applications.Item MOVPE growth of GaN on patterned 6-inch Si wafer(Institute of Physics Publishing, 2020-04-20) Kim, Iurii; Holmi, Joonas; Raju, Ramesh; Haapalinna, Atte; Suihkonen, Sami; Department of Electronics and Nanoengineering; Markku Sopanen Group; Harri Lipsanen Group; Ilkka Tittonen Group; Okmetic OyWe demonstrate that thicker layers can be achieved in gallium nitride (GaN) epitaxy by using a patterned silicon (Si) substrate compared to a planar Si substrate. GaN films were grown by metalorganic vapour-phase epitaxy on 6-inch Si (111) substrates patterned with arrays of squares with various corner shapes, height and lateral dimensions. Stress spatial distributions in the GaN pattern units were mapped out using confocal Raman spectroscopy. It was found that the corner shapes have an effect on the uniformity of the stress distribution. Patterns with round corners were found to have more uniform stress distribution than those with sharp corners. The largest crack-free square size for a 1.5 μm thick GaN film is 500 × 500 μm2.Item Optical amplification by surface-plasmon-resonant Au grating substrates: Monolayer MoS2 with 170-fold second harmonic generation and 3-fold (off-resonance) Raman scattering(Academic Press Inc., 2021-12) Holmi, Joonas T.; Raju, Ramesh; Ylönen, Jonas; Subramaniyam, Nagarajan; Lipsanen, Harri; Department of Electronics and Nanoengineering; Harri Lipsanen Group; Ilkka Tittonen Group; Department of Electronics and NanoengineeringNanoplasmonics is a potential game-changer in the development of next-generation on-chip photonic devices and computers, owing to the geometrically controlled and amplified linear and nonlinear optical processes. For instance, it resolves the limited light-matter interaction of the unique two-dimensional (2D) crystalline materials like semiconducting monolayer molybdenum disulfide (1L-MoS2). Metal grating (MG) substrates excel at this because their surface plasmons (SPs) can lead to stark field confinement near the surface. This work studies optical amplification of 1L-MoS2 on the gold (Au) MG substrate, which was designed to operate in a glycerol environment with SP resonance (SPR) at 850 nm excitation wavelength. Its design was verified by simulated and experimental reflectances, and topographically inspected by atomic force microscopy (AFM). Two advanced imaging modalities, second harmonic generation (SHG) and confocal Raman microscopy (CRM) were used to evaluate its 170-fold SHG on- and 3-fold CRM off-resonance optical amplifications, respectively. Some MoS2-to-grating adhesion issues due to trapped liquid showed as image nonuniformities. Possible improvements to limitations like surface roughness were also discussed. These Au MG substrates can boost conventional linear and nonlinear backscattering microscopies because they are tunable in the visible and near-infrared range by selecting geometry, metal, and environment.Item Single-step chemical vapour deposition of anti-pyramid MoS2/WS2vertical heterostructures(ROYAL SOC CHEMISTRY, 2021-02-28) Bai, Xueyin; Li, Shisheng; Das, Susobhan; Du, Luojun; Dai, Yunyun; Yao, Lide; Raju, Ramesh; Du, Mingde; Lipsanen, Harri; Sun, Zhipei; Department of Electronics and Nanoengineering; Department of Applied Physics; Centre of Excellence in Quantum Technology, QTF; Zhipei Sun Group; Ilkka Tittonen Group; Harri Lipsanen Group; National Institute for Materials ScienceVan der Waals heterostructures are the fundamental building blocks of electronic and optoelectronic devices. Here we report that, through a single-step chemical vapour deposition (CVD) process, high-quality vertical bilayer MoS2/WS2 heterostructures with a grain size up to ∼60 μm can be synthesized from molten salt precursors, Na2MoO4 and Na2WO4. Instead of normal pyramid vertical heterostructures grown by CVD, this method synthesizes an anti-pyramid MoS2/WS2 structure, which is characterized by Raman, photoluminescence and second harmonic generation microscopy. Our facile CVD strategy for synthesizing anti-pyramid structures unveils a new synthesis route for the products of two-dimensional heterostructures and their devices for application.Item Surface microscopy techniques for organic devices - A study of surface potentials(2024-08-19) Ehgartner, Henri; Raju, Ramesh; Sähkötekniikan korkeakoulu; Soldano, CaterinaWhen designing high-performance organic electronic devices, numerous parameters need to be considered and optimized. One such parameter is the surface potential and work function of a material, which can be measured using Kelvin probe force microscopy (KPFM). This thesis uses amplitude modulated KPFM to study the surface potentials of an organic field-effect transistor (OFET) that uses C8BTBT as the organic semiconductor material for the channel. In this thesis I find an average potential difference between the electrode (Ag) and the channel (C8BTBT) to be 667 mV. However, the surface potential difference varies throughout the potential image. This varying potential has a gradual trend and abrupt changes, which is speculated to be a result of the semi-automated atomic force microscopy (AFM) imaging that is used in amplitude modulated KPFM.
