99 research outputs found
Growth of single crystalline GaN from chlorine-free gas phase
No full textThis work is focused on the development of GaN bulk growth to overcome some limits of the established HVPE method. Evaporated Ga is transported by a carrier gas to the substrate where it reacts with atomic nitrogen from the ammonia decomposition to form GaN at temperatures of 1000-1200 degrees C and pressures of 200-800 hPa. To prove the methods potential, I man thick polycrystalline layers were deposited on the substrate holder with rates up to 50 mu mh(-1). Numerical simulations of temperature-, flow- and concentration field of the gas species were used to optimize crucible geometry and growth regime. It was experimentally found that in addition to temperature, total pressure and ammonia flow other parameters like source composition; crucible material and flow conditions near the substrate may affect seeding, growth rate and layer quality. First epitaxial layers up to 280 mu m thickness were successfully grown on different bulk and template substrates
Growth of GaN crystals from chlorine-free gas phase
No full textThe aim of this research is the development of an efficient chlorine-free growth process for GaN bulk crystals. Ga evaporated from a heated melt is transported to the substrate where it reacts with reactive nitrogen generated from the decomposition of ammonia to form gallium nitride. Numerical simulation was used to optimise the set-up geometry and growth regime. It was observed that growth rate and layer quality are influenced by typical growth parameters such as temperature, total pressure and ammonia flow, as well as by source composition, crucible material and flow conditions in the growth area. Initially, mm-sized crystallites were deposited on the seed holder with growth rates well above 10 mu m/h, which proved the potential of the method. Recently, the work was focused on the deposition on various single crystalline substrates. Epitaxial layers up to a bulk-like thickness of 280 mu m were achieved on GaN-sapphire templates. (C) 2007 Elsevier B.V. All rights reserved
HVPE GaN substrates: growth and characterization
No full textGaN substrates with low dislocation densities were prepared by halide vapor-phase epitaxy (HVPE) on c-plane sapphire and by means of a post-growth laser-induced lift-off or natural stress-induced (self-) separation process. The HVPE growth on InGaN/GaN buffer layers and subsequent self-separation method was seen as advantageous, in comparison with the laser-induced lift-off one, in terms of lower cost and better crystalline quality of the GaN material obtained. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)</p
Aligned AlN nanowires by self-organized vapor-solid growth
No full textHighly oriented AlN single crystal nanowires with aspect ratio up to 600, diameter in the range of 40-500 nm, and 100 mu m lengths, have been synthesized via a vapor-solid growth mechanism. The results were obtained at 1750 degrees C and 850 mbar nitrogen pressure on vicinal SiC substrates pretreated by SiC sublimation epitaxy in order to attain distinguishable terraces. It was found that the nanowires change in thickness after they have reached a critical length, and this fact contributes to an understanding of the growth mechanism of AlN nanowires. The nanowires are hexagonally shaped and perfectly aligned along the [0001] direction with a small tilt given by the substrate vicinality. Under nitrogen excess a preferential growth along the c-axis of the wurtzite structure takes place while below some critical value of nitrogen pressure the growth mode switches to lateral. The AlN nanowires are shown to have a dislocation free wurtzite crystal structure. Some possible applications are discussed.</p
Effect of sun exposure on vitamin D and skin cancer
No full textUvod: Čezmerna izpostavljenost ultravijoličnim žarkom poškoduje kožne celice in povečuje tveganje za nastanek kožnega raka. Kljub temu je sončna svetloba ključna za sintezo vitamina D, ki je bistven za naše zdravje. Zaradi tega je pomemben uravnotežen pristop med varno izpostavljenostjo soncu in zaščito pred škodljivimi učinki ultravijoličnega sevanja. Namen: Namen diplomskega dela je s pomočjo pregleda literature raziskati potrebno količino vitamina D, ob upoštevanju primerne izpostavljenosti soncu za zmanjšanje tveganja kožnega raka. Metode dela: Za izdelavo diplomskega dela smo naredili pregled strokovne in znanstvene literature v časovnem razponu od leta 2014 do 2024. Iskanje je potekalo v naslednjih bibliografskih zbirkah: Medline (PubMed), CINAHL Ultimate, Trip medical datebase in prek iskalnika Google učenjak. Iskali smo literaturo v angleškem in slovenskem jeziku. Rezultati: S pridobljenimi rezultati in pregledom literature smo ugotovili, kolikšen je optimalen čas izpostavljenosti sončni svetlobi, ki je za posameznika varen pri pridobivanju vitamina D. Predstavili smo dejavnike, ki vplivajo na potreben čas izpostavitve soncu, kot so: pigmentacija kože, količina izpostavljene kože, geografska lega, letni čas in uporaba kreme za sončenje. Razprava in zaključek: Primeren čas dnevne izpostavljenosti za sintezo vitamina D je približno 10–15 min, saj je pogojen tudi z drugimi dejavniki. Glede izpostavljenosti sončni svetlobi je pomembno poudariti koncept »malo in pogosto«. Takšno vedenje omogoča varno in zadostno sintezo vitamina D pri večini posameznikov, obenem pa zmanjšuje nevarnost za poškodbo kože. Posebno pozornost pri izobraževanju in preventivnih dejavnostih bi bilo treba nameniti mladim ter osebam iz nižjih socialno-ekonomskih okolij, saj so lahko bolj dovzetni za neprimerno izpostavljanje soncu. Pomembno je, da kot zdravstveni delavci spodbujamo uporabo sodobnih aplikacij in pripomočkov, ki lahko dokazano spremenijo naše vedenje in hkrati preprečijo neželene posledice.Introduction: Overexposure to ultraviolet rays damages skin cells and increases the risk of skin cancer. However, sunlight is essential for the synthesis of vitamin D, which is essential for our health. For this reason, a balanced approach between safe sun exposure and protection against the harmful effects of ultraviolet radiation is important. Purpose: The aim of diploma is to investigate, through a literature review, the amount of vitamin D needed to reduce the risk of skin cancer, taking into account adequate sun exposure. Methods: The descriptive method was used to write the diploma, reviewing professional and scientific literature, in the time from 2014 to 2024. The search was conducted in the following bibliographic collections: Medline (PubMed), CINAHL Ultimate, Trip medical database and through the Google Scholar search engine. We searched for literature is in English and Slovenian. Results: With the obtained results and a review of literature, we determined the optimal time of exposure to sunlight that is safe for an individual to obtain vitamin D. We presented the factors that affect the required time to the sun as suchskin pigmentation, amount of skin exposed, geographical location, season and use of sunscreen. Discussion and conclusion: The appropriate time of daily exposure for the synthesis of vitamin D is about 10-15 minutes, as it is also conditioned by other factors. Regarding exposure to sunlight, it is important to emphasize the concept of "little and often". Such behavior enables safe and sufficient synthesis of vitamin D in most individuals, while at the same time reducing the risk of skin damage. Special attention in education and preventive activities should be given to young peopleand people from lower socio-economic backgrounds, as they may be more susceptible to inappropriate exposure to the sun. It is important that, as healthcare professionals, we encourage the use of modern applications and devices that can be proven to change our behavior and at the same time prevent unwanted consequences
The role of carbon in transport processes during PVT growth of bulk GaN
No full textThick GaN layers were deposited by the reaction of gallium with ammonia. The Physical Vapour Transport (PVT) process with NH(3) addition is characterized by moderate NH(3) flow rates, pressures, and consumption of reactants, by reduced parasitic growth and no NH(4)Cl formation. Carbon addition to the ammonia results in growth of thicker layers at increased transport rate and in single crystals even at Ga super saturations, which would normally be too high for single crystalline growth. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Reduction of the dislocation density in HVPE-grown GaN epi-layers by an in situ SiNx treatment
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Investigations of the growth conditions for GaN-bulk crystals grown by the sublimation technique
No full textSpontaneous nucleation of GaN and the growth of thick polycrystalline layers by the sublimation technique in nitrogen atmosphere have been studied as a function of growth environment and conditions. Average growth rates higher than 10 microns per hour were realised by reaction of gallium, evaporated from an elemental source, with nitrogen from ammonia gas. The growth temperature was in the range of 1000 1300 degrees C; the pressure was varied between 200 - 800 hPa and the ammonia flow between 50-500 sccm min(-1). It was found that the growth rate only moderately decreases during a long-term experiment when using an optimised set of growth parameters. Furthermore, a strong influence of the growth environment on the growth rate and impurity content was found. (c) 2007 WELEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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