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Papers by Joona-Pekko Kakko

Laser & Photonics Reviews, 2016

Nanotechnology, Jan 16, 2016

We report on the synthesis of Au-catalyzed InP nanowires (NWs) on low-cost glass substrates. Ultr... more We report on the synthesis of Au-catalyzed InP nanowires (NWs) on low-cost glass substrates. Ultra-dense and ultra-long (up to ∼250 μm) InP NWs, with an exceptionally high growth rate of ∼25 μm min(-1), were grown directly on glass using metal organic vapor phase epitaxy (MOVPE). Structural properties of InP NWs grown on glass were similar to the ones grown typically on Si substrates showing many structural twin faults but the NWs on glass always exhibited a stronger photoluminescence (PL) intensity at room temperature. The PL measurements of NWs grown on glass reveal two additional prominent impurity related emission peaks at low temperature (10 K). In particular, the strongest unusual emission peak with an activation energy of 23.8 ± 2 meV was observed at 928 nm. Different possibilities including the role of native defects (phosphorus and/or indium vacancies) are discussed but most likely the origin of this PL peak is related to the impurity incorporation from the glass substrate....

Lithuanian Journal of Physics, 2016

Conference on Lasers and Electro-Optics, 2016

Nanotechnology, 2016

A facile and scalable lithography-free technique(5) for the rapid construction of GaAs core-shell... more A facile and scalable lithography-free technique(5) for the rapid construction of GaAs core-shell nanowires incorporating shell isolation from the substrate is reported. The process is based on interrupting NW growth and applying a thin spin-on-glass (SOG) layer to the base of the NWs and resuming core-shell NW growth. NW growth occurred in an atmospheric pressure metalorganic vapour phase epitaxy (MOVPE) system with gold nanoparticles used as catalysts for the vapour-liquid-solid growth. It is shown that NW axial core growth and radial shell growth can be resumed after interruption and even exposure to air. The SOG residues and native oxide layer that forms on the NW surface are shown to prevent or perturb resumption of epitaxial NW growth if not removed. Both HF etching and in situ annealing of the air-exposed NWs in the MOVPE were shown to remove the SOG residues and native oxide layer. While both procedures are shown capable of removing the native oxide and enabling resumption of epitaxial NW growth, in situ annealing produced the best results and allowed construction of pristine core-shell NWs. No growth occurred on SOG and it was observed that axial NW growth was more rapid when a SOG layer covered the substrate. The fabricated p-core/n-shell NWs exhibited diode behaviour upon electrical testing. The isolation of the NW shells from the substrate was confirmed by scanning electron microscopy and electrical measurements. The crystal quality of the regrown core-shell NWs was verified with a high resolution transmission electron microscope. The reported technique potentially provides a pathway using MOVPE for scalable and high-throughput production of shell-substrate isolated core-shell NWs on an industrial scale.

Nano Letters, 2015

A novel method for fabricating dual-type nanowire (NW) arrays is presented. Two growth steps, sel... more A novel method for fabricating dual-type nanowire (NW) arrays is presented. Two growth steps, selective-area epitaxy (SAE) in the first step and vapor− liquid−solid (VLS) in the second step, are used to grow two types of NWs on the same GaAs substrate. Different precursors can be used for the growth steps, resulting in sophisticated compositional control, as demonstrated for sideby-side grown GaAs and InP NWs. It was found that parasitic growth occurs on the NWs already present on the substrate during the second growth step and that the SAE NWs shadow the growth of the VLS NWs. Optical reflectance measurements revealed the dual-type array having improved light trapping properties compared to single-type arrays. Dual-type NW arrays could be practical for thermoelectric generation, photovoltaics and sensing where composition control of side-by-side NWs and complex configurations are beneficial.

Applied Physics Letters, 2014

14th IEEE International Conference on Nanotechnology, 2014

ABSTRACT Triangular lattice GaAs nanowires were fabricated using electron beam lithography define... more ABSTRACT Triangular lattice GaAs nanowires were fabricated using electron beam lithography defined Au arrays and metalorganic vapour phase epitaxy. Reflectance measurements for GaAs NW arrays with varying pitch in a triangular lattice are presented. The measured spectra are compared to the literature data on square lattice NW arrays and selective-area epitaxy grown triangular lattice NW arrays. Reflectance from a triangular lattice NW array is found to have similar characteristics as from a square lattice. Unlike for the square lattice, it is observed that the reflection edge does not depend linearly on the NW diameter with smallest pitches in the triangular lattice and that a Morse potential fit describes the behaviour most closely.

Nano Letters, 2013

We report a new phenomenon related to Alinduced carrier confinement at the interface in core−shel... more We report a new phenomenon related to Alinduced carrier confinement at the interface in core−shell GaAs/Al x Ga 1−x As nanowires grown using metal−organic vapor phase epitaxy with Au as catalyst. All Al x Ga 1−x As shells strongly passivated the GaAs nanowires, but surprisingly the peak photoluminescence (PL) position and the intensity from the core were found to be a strong function of Al composition in the shell at low temperatures. Large and systematic red shifts of up to ∼66 nm and broadening in the PL emission from the GaAs core were observed when the Al composition in the shell exceeded 3%. On the contrary, the phenomenon was observed to be considerably weaker at the room temperature. Cross-sectional transmission electron microscopy reveals Al segregation in the shell along six Al-rich radial bands displaying a 3-fold symmetry. Time-resolved PL measurements suggest the presence of indirect electron−hole transitions at the interface at higher Al composition. We discuss all possibilities including a simple shell−core−shell model using simulations where the density of interface traps increases with the Al content, thus creating a strong local electron confinement. The carrier confinement at the interface is most likely related to Al inhomogeneity and/or Al-induced traps. Our results suggest that a low Al composition in the shell is desirable in order to achieve ideal passivation in GaAs nanowires.

Journal of Applied Physics, 2013

Similar to electron waves, the phonon states in semiconductors can undergo changes induced by ext... more Similar to electron waves, the phonon states in semiconductors can undergo changes induced by external boundaries. However, despite strong scientific and practical importance, conclusive experimental evidence of confined acoustic phonon polarization branches in individual free-standing nanostructures is lacking. Here we report results of Brillouin—Mandelstam light scattering spectroscopy, which reveal multiple (up to ten) confined acoustic phonon polarization branches in GaAs nanowires with a diameter as large as 128 nm, at a length scale that exceeds the grey phonon mean-free path in this material by almost an order-of-magnitude. The dispersion modification and energy scaling with diameter in individual nanowires are in excellent agreement with theory. The phonon confinement effects result in a decrease in the phonon group velocity along the nanowire axis and changes in the phonon density of states. The obtained results can lead to more efficient nanoscale control of acoustic phonons, with benefits for nanoelectronic, thermoelectric and spintronic devices.