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| 2. |
- Syväjärvi, Mikael, 1968-, et al.
(författare)
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A surface study of wet etched AlGaN epilayers grown by hot-wall MOCVD
- 2007
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Ingår i: Journal of Crystal Growth, Vol. 300. - : Elsevier BV. - 0022-0248. ; , s. 242-245
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Konferensbidrag (refereegranskat)abstract
- Epitaxial layers of AlGaN were grown by hot-wall MOCVD and their surfaces wet chemically etched with phosphorous acid. The as-grown surfaces and the development of the etched surfaces after 10 and 20 min of etching were studied with atomic force microscopy (AFM) and CL. In the as-grown layers growth features may be resolved while the RMS is as low as 1.4 Å in a scan area of 2×2 μm. Surfaces etched for 10 min had developed etch pits and a low RMS roughness of 7 Å indicating a uniform quality of the layers. Micrometer scale hexagonal features were observed after 20 min of etching. In some cases a deep hexagonal etch pit is observed in the centre of the hexagonal feature with a 30° rotation to each other, suggesting that the origin is substrate-induced defects. © 2006 Elsevier B.V. All rights reserved.
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| 3. |
- Syväjärvi, Mikael, et al.
(författare)
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Fast epitaxy by PVT of SiC in hydrogen atmosphere
- 2005
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 275:1-2, s. e1103-e1107
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Tidskriftsartikel (refereegranskat)abstract
- Epitaxial growth in hydrogen atmosphere has been studied in relation to sublimation epitaxial growth. A new type of features with a hexagonal shape are observed in the layers grown in hydrogen atmosphere. The morphological details of the features have been studied with optical microscopy and atomic force microscopy. An interactive relation of the defect appearance with the step flow growth mode seems to be present. The results are compared with growth in vacuum, argon, and helium conditions. The possible influence of thermal component to a reactive one in hydrogen etching is discussed.
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| 4. |
- Syväjärvi, Mikael, 1968-, et al.
(författare)
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Formation of ferromagnetic SiC : Mn phases
- 2005
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Ingår i: Materials Science Forum, Vols. 483-485. ; , s. 241-244
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Konferensbidrag (refereegranskat)abstract
- Ferromagnetic phases in as-grown SiC have been studied. An interpretation about the formation based on details of the phase appearance in the layers from optical microscopy, AFM, and TEM investigations is related to the growth. Some phases were found to have a nucleation at the edge of the phase and detailed TEM investigations show that the phases have an increased grain density at the edge while the main part of the phase is monocrystalline.
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| 5. |
- Yakimova, Rositsa, 1942-, et al.
(författare)
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Novel material concepts of transducers for chemical and biosensors
- 2007
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Ingår i: Biosensors & bioelectronics. - : Elsevier BV. - 0956-5663 .- 1873-4235. ; 22:12, s. 2780-2785
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Tidskriftsartikel (refereegranskat)abstract
- The objectives of this work are to contribute to the knowledge about physical and chemical properties of WBG semiconductors, such as ZnO and GaN towards development of advanced bio- and chemical sensors. For the semiconductors, growth techniques typically yielding single crystal material are applied. Thin epitaxial quality films of ZnO and GaN are fabricated on SiC or sapphire substrates. An emphasis is given to ZnO due to the interesting combination of the semiconductor and oxide properties. Surface bio-functionalization of ZnO is performed by APTES, MPA or MP-TMS molecules. We have compared some of the results to (hydroxylated) GaN surfaces functionalized by MP-TMS. The covalent attachment of the self-assembled biomolecular layers has been proven by XPS analysis. For complementary electrical characterization impedance spectroscopy measurements were performed. The results are intended to serve the realization of bioelectronic transducer devices based on SiC or GaN transistors with a ZnO gate layer. To take advantage of the catalytic properties of ZnO, initial prototypes of chemical sensors for gas sensing are processed on ZnO deposited either on SiC or on sapphire and they are further tested for the response to reducing or oxidizing gas ambient. The sensor devices show sensitivity to oxygen in the surface resistivity mode while a Pt Schottky contact ZnO/SiC device responds to reducing gases. These results are compared to published results on Pt/GaN Schottky diodes. © 2007 Elsevier B.V. All rights reserved.
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| 6. |
- Yakimova, Rositsa, 1942-, et al.
(författare)
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Surface functionalization and biomedical applications based on SiC
- 2007
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Ingår i: Journal of Physics D. - 0022-3727 .- 1361-6463. ; 40:20, s. 6435-6442
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Tidskriftsartikel (refereegranskat)abstract
- The search for materials and systems, capable of operating long term under physiological conditions, has been a strategy for many research groups during the past years. Silicon carbide (SiC) is a material, which can meet the demands due to its high biocompatibility, high inertness to biological tissues and to aggressive environment, and the possibility to make all types of electronic devices. This paper reviews progress in biomedical and biosensor related research on SiC. For example, less biofouling and platelet aggregation when exposed to blood is taken advantage of in a variety of medical implantable materials while the robust semiconducting properties can be explored in surface functionalized bioelectronic devices. © 2007 IOP Publishing Ltd.
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| 7. |
- Yazdi, Gholamreza, 1966-, et al.
(författare)
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Aligned AlN nanowires and microrods by self-patterning
- 2007
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 90:12, s. 123103-
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Tidskriftsartikel (refereegranskat)abstract
- Self-patterned AlN microrods and nanowires were grown on 4H-SiC substrate by a physical vapor transport method. AlN hexagonal pyramids were found to be nucleation sites for the evolution of the observed morphological forms. The average diameter and length of the nanowires are about 200 nm and 90 µm, respectively. The density of microrods corresponds to the concentration of the pyramids, while the nanowires are less compact. Low-temperature cathodoluminescence spectra of microrods show band gap emission of AlN at 208 nm, which confirms that they are AlN single crystals. A formation mechanism of the AlN structures is suggested.
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| 8. |
- Yazdi, Gholamreza, 1966-, et al.
(författare)
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Employing discontinuous and continuous growth modes for preparation of AlN nanostructures on SiC substrates
- 2007
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Ingår i: ECSCRM 2006, Newcastle, UK. - : Trans Tech Publications, Switzerland. ; , s. 1031-1034
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Konferensbidrag (refereegranskat)abstract
- In this report we present results on growth and characterization of AlN wires and thinfilms on SiC substrates. We have employed PVT technique in close space geometry for AlNdeposition on SiC off oriented substrates, most of which were prepared to have scratch-free smoothas-grown surface by SiC sublimation epitaxy. By manipulating the surface kinetics we have beenable to determine growth conditions yielding discontinuous or continuous morphologiescorresponding to nanowires and thin films, respectively. A particular feature of the latterexperiments is the fast temperature ramp up at the growth initiation. The AlN surface morphologywas characterized by optical, AFM and XRD tools, which showed good crystal quality independentof the growth mode.
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| 9. |
- Yazdi, Gholamreza, 1966-, et al.
(författare)
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Fabrication of free-standing AlN crystals by controlled microrod growth
- 2008
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 300:5, s. 935-939
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Tidskriftsartikel (refereegranskat)abstract
- The aim of this study was to propose a growth procedure for preparation of crack-free thick aluminum nitride (AlN) layers that can be easily separated from the substrate. The overall process is based on the physical vapor transport method employing a seed and a source material. In this case, the substrate is an epitaxial 4H-SiC layer and the growth of AlN is initiated at etch pits formed during the ramp up time prior to establishing growth temperature. Development of hexagonal pyramids on which arrays of microrods are formed is the core of the growth procedure. Free-standing wafers having 10 mm diameter and about 120 μm thick have been fabricated.
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| 10. |
- Yazdi, Gholamreza, 1966-, et al.
(författare)
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Formation of needle-like and columnar structures of AlN
- 2007
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Ingår i: Journal of Crystal Growth. - : Elsevier BV. - 0022-0248 .- 1873-5002. ; 300:1, s. 130-135
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Tidskriftsartikel (refereegranskat)abstract
- The present study focused on understanding the formation of needle-like and columnar structures by investigating the initial nucleation of aluminium nitride (AlN) on SiC substrates with SEM, AFM, and XRD. The grown AlN consisted of high concentration (8×104 cm−2) hexagonal hillocks (HHs) that originate from threading dislocations in the substrate. The KOH etching technique has been used to examine the origin and formation process of HHs and defect reduction in the grown AlN crystals. A model is introduced to explain the AlN HH formation. The SEM result shows that the AlN columnar structure was formed by merging of needles, which are grown exactly on completed AlN HHs, followed by a lateral growth.
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