Browsing by Author "Dhaka, Veer"

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  • GaAs nanowires grown on Al-doped ZnO buffer layer
    (2013) Haggren, Tuomas; Pyymaki Perros, Alexander; Dhaka, Veer; Huhtio, Teppo; Jussila, Henri; Jiang, Hua; Ruoho, Mikko; Kakko, Joona-Pekko; Kauppinen, Esko; Lipsanen, Harri
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report a pathway to grow GaAs nanowires on a variety of substrates using a combination of atomic layer deposition and metallo-organic vapor phase epitaxy (MOVPE). GaAs nanowires were grown via MOVPE at 430–540 °C on an atomic-layer-deposited Al:ZnO buffer layer. The resulting nanowires were affected only by the properties of the buffer layer, allowing nanowire growth on a number of substrates that withstand ∼400 °C. The growth occurred in two phases: initial in-plane growth and subsequent out-plane growth. The nanowires grown exhibited a strong photoluminescence signal both at room temperature and at 12 K. The 12 K photoluminescence peak was at 1.47 eV, which was attributed to Zn autodoping from the buffer layer. The crystal structure was zincblende plagued with either twin planes or diagonal defect planes, which were related to perturbations in the seed particle during the growth. The used method combines substrates with variable properties to nanowire growth on a transparent and conductive Al:ZnO buffer layer.
  • Selective-Area Metalorganic Vapour Phase Epitaxy of Gallium Arsenide Nanowires
    (2013-03-18) Kakko, Joona-Pekko
     Sähkötekniikan korkeakoulu | Master's thesis
    III--V -puolijohteiden nanolangat ovat varteenotettava alusta seuraavan sukupolven optoelektroniikan ja nanofotoniikan sovelluksille. Suurin haaste näiden nanolankojen laajalle käytölle on niiden kasvattaminen pystysuoraan ennalta määrättyihin paikkoihin. Tässä työssä tutkittiin GaAs-nanolankojen selektiivistä kasvattamista metallo-orgaanisella kaasufaasiepitaksialla. Pystysuoria, haluttuun paikkaan sijoitettuja GaAs-nanolankoja saatiin onnistuneesti kasvatettua (111)-kidesuuntaisille pii- ja GaAs-alustakiteille. Nanolankojen paikka ja halkaisija määritettiin elektronisuihkulitografialla. GaAs-nanolankoja tutkittiin elektronimikroskopialla, sähköisillä mittauksilla ja mikrofotoluminenssimittauksilla (µ-PL). GaAs-alustakiteelle kasvatetuista GaAs-nanolangoista mitattiin lävistäjät ja pituudet ja luotiin niille usean selittäjän lineaariset regressiomallit. Samanlainen tilastollinen malli luotiin piialustakiteelle kasvatettujen GaAs-nanolankojen tuotolle. Havaittiin, että GaAs-alustakiteelle kasvatetut nanolangat ovat enimmäkseen wurtsiittia, mikä voitiin päätellä yksittäisen nanolangan µ-PL-mittauksista (77 K). Yksittäisille nanolangoille tehdyissä sähköisissä mittauksissa havaittiin ohmisen kontaktin sijasta Schottky-käyttäytyminen, mikä antaa aihetta jatkotutkimukselle. Piille kasvatettujen nanolankojen havaittiin sisältävän paljon pinousvikoja ja niiden hilarakenteen todettiin koostuvan sekä sinkkivälkkeestä että wurtsiitista. Nanolankojen PL-intensiteettiä onnistuttiin parantamaan jopa satakertaiseksi passivoimalla ne AlGaAs-kuorilla sekä pienentämällä nanolankojen keskinäistä etäisyyttä ja kuvioidun aukon kokoa. Työssä saavutetut tulokset voivat osoittautua hyödyllisiksi nanolankasovelluksia suunniteltaessa.
  • Growth of gallium arsenide nanowires on silicon
    (2011) Haggrén, Tuomas
     School of Electrical Engineering | Master's thesis
    In this thesis, gallium arsenide nanowires (NW) were grown on silicon substrates by atmospheric pressure metal organic vapor phase epitaxy. NWs offer a myriad of novel applications and solutions in a variety of different fields, including optoelectronics. Epitaxial NWs with good optical properties are required in many of the possible applications. The aim of this work was to grow NWs epitaxial with a good yield, optimize the growth parameters in the used system, and to passivate the surface of the NWs by core-shell structures and atomic layer deposition (ALD) in order to improve their photoluminescence (PL). The used shell materials were AlGaAs and GaAsP, and the ALD coating was aluminum nitride. The NW yield and morphology were characterized with scanning electron microscopy, their crystal structure and quality with transmission electron microscopy, chemical composition with energy-dispersive X-ray spectroscopy and optical properties with PL measurements. The results showed that the NW s can be grown with a good yield within an appropriate range of temperature, molar ratio, and total molar flow. The passivation methods resulted in drastically improved PL intensity. Also unexpectedly large redshift with AlGaAs shells was observed.
  • Hybrid GaAs nanowire-polymer device on glass: Al-doped ZnO (AZO) as transparent conductive oxide for nanowire based photovoltaic applications
    (2020-10-15) Khayrudinov, Vladislav; Mäntynen, Henrik; Dhaka, Veer; Pyymaki Perros, Alexander; Haggren, Tuomas; Jussila, Henri; Lipsanen, Harri
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Al-doped ZnO (AZO) is pursued as an alternative low-cost transparent conductive oxide (TCO) to expensive ITO. Atomic layer deposition grown AZO films showing resistivity of 5 x 10-3 Ωcm and transmittance > 85% in the visible region are reported. Au-assisted GaAs nanowires are grown directly on an optimized AZO coated glass and a GaAs nanowire-polymer hybrid device on glass is demonstrated which confirms that the as-grown GaAs nanowires form a perfect ohmic contact to AZO film. The device shows that AZO can be used as transparent electrode as well as low-cost growth platform for GaAs NWs. Finally, a simple device idea is proposed to fabricate optically transparent GaAs nanowire based solar cells on low-cost glass.
  • III–V nanowires on black silicon and low-temperature growth of self-catalyzed rectangular InAs NWs
    (2018-04-23) Haggren, Tuomas; Khayrudinov, Vladislav; Dhaka, Veer; Jiang, Hua; Shah, Syed; Kim, Maria; Lipsanen, Harri
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report the use of black silicon (bSi) as a growth platform for III–V nanowires (NWs), which enables low reflectance over a broad wavelength range as well as fabrication of optoelectronic devices by metalorganic vapor phase epitaxy. In addition, a new isolated growth regime is reported for self-catalyzed InAs NWs at record-low temperatures of 280 °C–365 °C, where consistently rectangular [-211]-oriented NWs are obtained. The bSi substrate is shown to support the growth of additionally GaAs and InP NWs, as well as heterostructured NWs. As seed particles, both ex-situ deposited Au nanoparticles and in-situ deposited In droplets are shown feasible. Particularly the InAs NWs with low band gap energy are used to extend low-reflectivity wavelength region into infrared, where the bSi alone remains transparent. Finally, a fabricated prototype device confirms the potential of III–V NWs combined with bSi for optoelectronic devices. Our results highlight the promise of III–V NWs on bSi for enhancing optoelectronic device performance on the low-cost Si substrates, and we believe that the new low-temperature NW growth regime advances the understanding and capabilities of NW growth.
  • Nanowire network–based multifunctional all-optical logic gates
    (2018-07-27) Yang, He; Khayrudinov, Vladislav; Dhaka, Veer; Jiang, Hua; Autere, Anton; Jussila, Henri; Lipsanen, Harri; Sun, Zhipei
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    All-optical nanoscale logic components are highly desired for various applications because light may enable logic functions to be performed extremely quickly without the generation of heat and cross-talk. All-optical computing at nanoscale is therefore a promising alternative but requires the development of a complete toolbox capable of various logic functionalities. We demonstrate nanoscale all-optical switches by exploiting the polarization-dependent light emission property of crossbar InP and AlGaAs nanowire networks. These networks can perform various logic operations, such as AND, OR, NAND, and NOR binary logic functions. Furthermore, on the basis of these logic operations, our networks successfully enable all-optical arithmetic binary calculations, such as n-bit addition, to be conducted. Our results underscore the promise of assembled semiconductor nanowire networks as a building block of on-chip all-optical logic components for future nanophotonics.
  • Nonlinear microscopy using cylindrical vector beams: Applications to three-dimensional imaging of nanostructures
    (2017-05-29) Bautista, Godofredo; Kakko, Joona Pekko; Dhaka, Veer; Zang, Xiaorun; Karvonen, Lasse; Jiang, Hua; Kauppinen, Esko; Lipsanen, Harri; Kauranen, Martti
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    The three-dimensional (3D) optical fields that arise from the focusing of cylindrical vector beams (CVB) with radial and azimuthal polarizations provide new sources of contrast for optical microscopy of nano-objects. So far, these demonstrations have been restricted to two-dimensional transversal scanning, i.e., along the focal plane of interest, or use of point-like objects, i.e., single molecules and nanoparticles. Here, we demonstrate the first application of CVBs for 3D imaging of 3D nano-objects. This technique is done by acquiring 3D image scans of the second-harmonic generation signal from vertically-aligned semiconductor nanowires, whose second-order response is primarily driven by the longitudinal electric field, i.e., the field component along the nanowire axis. Our technique provides a new way to study individual nano-objects in three dimensions through the unique combination of nonlinear microscopy and CVBs.
  • Protective capping and surface passivation of III-V nanowires by atomic layer deposition
    (2016-01-01) Dhaka, Veer; Perros, Alexander; Naureen, Shagufta; Shahid, Naeem; Jiang, Hua; Kakko, Joona-Pekko; Haggren, Tuomas; Kauppinen, Esko; Srinivasan, Anand; Lipsanen, Harri
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    Low temperature (∼200 °C) grown atomic layer deposition (ALD) films of AlN, TiN, Al2O3, GaN, and TiO2 were tested for protective capping and surface passivation of bottom-up grown III-V (GaAs and InP) nanowires (NWs), and top-down fabricated InP nanopillars. For as-grown GaAs NWs, only the AlN material passivated the GaAs surface as measured by photoluminescence (PL) at low temperatures (15K), and the best passivation was achieved with a few monolayer thick (2Å) film. For InP NWs, the best passivation (∼2x enhancement in room-temperature PL) was achieved with a capping of 2nm thick Al2O3. All other ALD capping layers resulted in a de-passivation effect and possible damage to the InP surface. Top-down fabricated InP nanopillars show similar passivation effects as InP NWs. In particular, capping with a 2 nm thick Al2O3 layer increased the carrier decay time from 251 ps (as-etched nanopillars) to about 525 ps. Tests after six months ageing reveal that the capped nanostructures retain their optical properties. Overall, capping of GaAs and InP NWs with high-k dielectrics AlN and Al2O3 provides moderate surface passivation as well as long term protection from oxidation and environmental attack.
  • Site-specific growth of oriented ZnO nanocrystal arrays
    (2019-01-01) Bai, Rekha; Pandya, Dinesh; Chaudhary, Sujeet; Dhaka, Veer; Khayrudinov, Vladislav; Lemettinen, Jori; Kauppinen, Christoffer; Lipsanen, Harri
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We report on the growth of ZnO nanocrystals having a hexagonal, prismatic shape, sized 700 nm × 600 nm, on bare indium tin oxide (ITO) substrates. The growth is induced by a low ion flux and involves a low-temperature electrodeposition technique. Further, vertically aligned periodic nanocrystal (NC) growth is engineered at predefined positions on polymer-coated ITO substrates patterned with ordered pores. The vertical alignment of ZnO NCs along the c-axis is achieved via ion-by-ion nucleation-controlled growth for patterned pores of size ≈600 nm; however, many-coupled branched NCs with hexagonal shape are formed when a patterned pore size of ≈200 nm is used. X-ray diffraction data is in agreement with the observed morphology. A mechanism is proposed to interpret the observed site-specific oriented/branched growth that is correlated to the pore size. As ordered NC arrays have the potential to generate new collective properties different from single NCs, our first demonstration of a cost effective and facile fabrication process opens up new possibilities for devices with versatile functionalities.
  • Strong surface passivation of GaAs nanowires with ultrathin InP and GaP capping layers
    (2014) Haggren, Tuomas; Jiang, Hua; Kakko, Joona-Pekko; Huhtio, Teppo; Dhaka, Veer; Kauppinen, Esko I.; Lipsanen, Harri
     School of Electrical Engineering | A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä
    We demonstrate efficient surface passivation of GaAs nanowires using ultrathin in-situ grown epitaxial InP and GaP capping layers, with metallo-organic vapor phase epitaxy as the growth system. The passivation increased photoluminescence intensity by three orders of magnitude compared to unpassivated nanowires, and the effect remained strong after a month of storage in air. Effective passivation was acquired over a wide range of growth temperatures, although the highest studied temperatures caused additional detrimental effects such as etching and GaAsP formation. The capping layer thickness was in the order of few monolayers. Therefore, the impact on any other properties of the nanowires besides the surface states was minuscule. As a simple and effective method the studied capping layers offer an excellent way for nanowire passivation.
  • Towards single nanowire solar cell based on novel radial p-n junction
    (2017-05-08) Khayrudinov, Vladislav
     Sähkötekniikan korkeakoulu | Master's thesis
    One of the most promising applications, the 1D semiconductor nanowires (NWs) offer, is the development of next generation solar cells at low-cost and higher efficiency. Group III–V semiconductor NWs, especially GaAs and InP, are the ideal semiconductor materials to build such devices on low-cost platforms. The aim of this work was to fabricate and study the fundamental working mechanisms of single nanowire solar cell and photodetector based on novel radial p-n junction in a core-shell geometry. The Au-assisted GaAs NWs were grown and doped in situ using metalorganic vapour phase epitaxy (MOVPE). In this study, a unique lithography-free technique was employed on an ensemble of NWs to isolate the core (p-type) from the shell (n-type) on the growth substrate. Electron beam lithography (EBL) was used to make metal contacts to the single core-shell NWs. In that respect, specific contact schemes were developed to realize ohmic contacts between the metal electrodes and the NWs. Electrical measurements of single nanowire device revealed perfect I-V behaviour confirming the formation of radial p-n junction diode. For characterization of NWs, scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) and photocurrent spectroscopies were employed. Furthermore, surface passivation of NWs was identified as one of the key issues in functioning of the photovoltaic device as no photo response was detected without the same. The surface passivation of GaAs NWs with higher band gap AlGaAs layers resulted in dramatic enhancement of the PL intensity, and the photocurrent measurements of the device revealed a broad photo response in the visible spectrum range of the light, indicating a successful working of a single nanowire as a photodetector. However, although a functioning radial p-n junction was demonstrated, no solar response was detected from the single nanowire structure because of yet many unaddressed challenges such as high contact resistance, doping optimisation, absence of intrinsic region and unoptimised geometry. In summary, a single nanowire based photodetector was successfully demonstrated, and a full fabrication process was studied and developed for making progress towards realising the functional single nanowire based radial p-n junction solar cells in the future.
  • Wafer-scale self-organized InP nanopillars with controlled orientation for photovoltaic devices
    (2015) Sanatinia, Reza; Berrier, Audrey; Dhaka, Veer; Perros, Alexander; Huhtio, Teppo; Lipsanen, Harri; Anand, Srinivasan
     A1 Alkuperäisartikkeli tieteellisessä aikakauslehdessä