Browsing by Author "Haggrén, Tuomas"

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Doping of Vapor–Liquid–Solid Grown Gallium Arsenide Nanowires
(2016-04-04) Mäntynen, Henrik
Sähkötekniikan korkeakoulu | Master's thesis
GaAs and other group III–V compound semiconductor nanowires are extensively researched and hold promise for several application areas like electronics, optoelectronics and photovoltaics. Most of the applications require electrical impurity doping for which accurate control and measurements remain challenging. The aim of this work was to grow and characterize doped GaAs nanowires in order to forward the development of the growth process used by the Nanotechnology group. The nanowires were grown via the vapor–liquid–solid mechanism in a metalorganic vapor phase epitaxy device. Doping was done during the growth by using zinc and tin for p-type and n-type doping, respectively. The grown nanowires were inspected with scanning electron microscope and micro-Raman measurements. The doping levels (carrier concentrations) were estimated with electrical measurements, for which a metal contacting scheme employing optical lithography was investigated. Doped nanowires were found to suffer from increased structural defect density and tapering to varying degrees compared to undoped reference nanowires. Aligning metal contacts and nanowires proved to be an issue and the investigated scheme hence did not work as well as expected. However, initial carrier concentration values were obtained for the current process. Based on the obtained results, additional measurements and growth process optimization were recommended.
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.
Kaasujen konsentraation mittaaminen puolijohteisilla nanolanka-antureilla
(2015-05-05) Turunen, Mikko
Sähkötekniikan korkeakoulu | Bachelor's thesis
Nanowire technology for optoelectronic applications
(2016) Haggrén, Tuomas
School of Electrical Engineering | Doctoral dissertation (article-based)
Nanowires (NWs) have enormous potential for a number of future applications, especially in the field of optoelectronics. This thesis advances key areas in optoelectronic nanowire device fabrication. The studied key areas focus on the reduction of fabrication costs and on improving the performance of complete devices, both of which are essential for practical applications. Ordinary window glass is showed to be a feasible growth platform for GaAs NWs, which additionally produces high crystalline and optical quality NWs. These improvements are attributed to impurities originating from the glass. Another low-cost pathway for NW production studied here was Al-doped ZnO buffer layers, which are transparent and conductive. These layers could be deposited on nearly any substrate, and NWs could be subsequently grown on top irrespective of the underlying material. Surface states are detrimental especially for GaAs NW device performance, for which different passivation methods are presented. Widely used AlGaAs shells are shown to alter the optical processes in the NWs, and Al segregation is shown to occur in the shell. Also a novel passivation method is presented using ultrathin InP or GaP capping layers, which provides strong passivation while causing minimal side- effects. Novel fabrication method is presented for large-area position-controlled NW growth. This method utilizes laser interference lithography that is fast and can be performed with relatively simple equipment. In addition, fabrication of dualtype NWs on a single substrate is presented. This method exploits two different NW growth modes and allows the growth of dissimilar NWs side-by-side. The dualtype NWs can enhance light trapping in solar cells and photodetectors, or expand the emitted wavelength range in light-emitting diodes. Another critical step in NW device fabrication is the contact isolation. This work presents a facile and lithography-free method to create shell-substrate isolated core-shell nanowires via spin-on-glass deposition and NW regrowth techniques. The method allows easier production of core-shell NW devices e.g. when NWs are grown directly on electrodes. NW isolation was additionally performed via encapsulation with parylene-C. This process provides antireflection coating for the NWs as well as electrical isolation for electrical contacts, and is suitable for various situations where current isolation methods are suboptimal.
Photodegradation of surface passivated GaAs nanowires
(2020-04-23) Alekseev, P. A.; Smirnov, A. N.; Davydov, V. Yu; Haggrén, Tuomas; Lipsanen, Harri; Dunaevskiy, M. S.; Berkovits, V. L.
A4 Artikkeli konferenssijulkaisussa
Efficiency of in situ AlGaAs and GaP and ex situ nitride surface passivation of p+ GaAs nanowires was studied. The efficiency was estimated by comparing of the photoluminescence intensity of the passivated nanowires with the unpassivated nanowire. The AlGaAs and nitride passivation lead to the increasing of the PL intensity by three orders of magnitude while the GaP passivation increases PL intensity only by one order. Photodegradation of the passivated NWs under intensive laser illumination was observed. AlGaAs, GaP and nitride passivated NWs photodegrade after one-minute exposure under laser power densities of 500, 300 and 30 kW/cm2, respectively.
Planarization of Gallium Arsenide Nanowires
(2014-10-20) Kilpi, Olli-Pekka
Sähkötekniikan korkeakoulu | Master's thesis
In this work gallium arsenide nanowire ensembles were planarized and contacted. These group III-V semiconductor nanowires possess many interesting properties for optoelectronic and electronic devices. However, their reliable integration to functional devices remains one of the major challenges. Vertical integration of the nanowires can be done by fabricating an insulation layer between the top of the nanowires and the substrate before deposition of metal contacts. The aim of this work was to find optimal filling materials for the planarization of different kinds of nanowire structures. Nanowire samples with different diameter and length were used. The aim was to find a material, which creates a void and crack free insulation layer without destroying the nanowires. H-PDMS, parylene and three different polymers, SOG 111, SOG B512 and Accuflo from Honeywell, were used. Structures were characterized by scanning electron microscope, current-voltage and optical measurements. The results showed that B512 and Accuflo were good insulation layers, while other filling materials possessed cracks and voids. In addition, Accuflo destroyed several nanowires, thus only B512 was observed to be a reliable planarization material. Insulation of B512 was confirmed by current-voltage measurements.