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- Dahlström, Örjan, 1973-, et al.
(författare)
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Does retrieval strategy disruption cause general and specific collaborative inhibition?
- 2011
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Ingår i: Memory. - : Taylor and Francis. - 0965-8211 .- 1464-0686. ; 19:2, s. 140-154
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of the experiment on collaborative memory was to investigate if the collaborative inhibition is due to collaborating pair's disruption of each others' retrieval strategies (the retrieval strategy disruption hypothesis, RSD). The participants' (N=36) task was to recall a list of 60 words individually and collaboratively. Retrieval strategies were manipulated by presenting word lists organised either by categories or by country of origin and adoption of retrieval strategies were examined by the adjusted ratio of clustering score. Half of the dyads received word lists organised by the same strategy and half of the dyads received word lists organised by different strategies. The results revealed a main effect of collaboration, i.e., collaborative recalled items were significantly fewer than the sum of the non-redundant individually recalled items. Both conditions (same strategies vs different strategies) suffered to the same extent from collaboration, which did not support the RSD hypothesis. However, focusing on words recalled individually but not collaboratively, dyads with different strategies, as predicted by the RSD, forgot more items during collaboration than did dyads with the same strategy. Additional results suggest that collaborative forgetting is mainly manifested by forgetting of non-overlapping items (as measured by individual recalls).
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| 2. |
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| 3. |
- Dahlström, Örjan, 1973-, et al.
(författare)
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The applied value of collaborative memory research in aging – Some critical comments
- 2013
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Ingår i: Journal of Applied Research in Memory and Cognition. - : Elsevier. - 2211-3681 .- 2211-369X. ; 2:2, s. 122-123
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Tidskriftsartikel (övrigt vetenskapligt/konstnärligt)abstract
- The article by Blumen, Rajaram, and Henkel (2013) raises some very interesting research topics. Using the aging population as the prime example, they also provide general recommendations for future research in the area of collaborative memory; ‘it's time to become more applied’, and we appreciate such a suggestion.The article spans many subfields and for obvious reasons, it is not possible to consider every potential issue in this field in one single article. In addition, there are several issues that could be either extended or added. We will in this commentary focus on issues we consider important for the understanding of the current literature, and we will add some from our own research.
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| 4. |
- Danielsson, Örjan, 1973-, et al.
(författare)
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Growth rate predictions of chemical vapor deposited silicon carbide epitaxial layers
- 2002
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Ingår i: Journal of Crystal Growth. - : Elsevier. - 0022-0248 .- 1873-5002. ; 243:1, s. 170-184
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Tidskriftsartikel (refereegranskat)abstract
- Complete 3D simulations of a silicon carbide chemical vapor deposition (CVD) reactor, including inductive heating and fluid dynamics as well as gas phase and surface chemistry, have been performed. For the validation of simulated results, growth was conducted in a horizontal hot-wall CVD reactor operating at 1600°C, using SiH4 and C3H8 as precursor gases. Simulations were performed for an experimental hot-wall CVD reactor, but the results are applicable to any reactor configuration since no adjustable parameters were used to fit experimental data. The simulated results obtained are in very good agreement with experimental values. It is shown that including etching and parasitic growth on all reactor walls exposed to the gas greatly improves the accuracy of the simulations. © 2002 Elsevier Science B.V. All rights reserved.
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| 5. |
- Danielsson, Örjan, 1973-, et al.
(författare)
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Shortcomings of CVD modeling of SiC today
- 2013
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Ingår i: Theoretical Chemistry accounts. - : Springer Berlin/Heidelberg. - 1432-881X .- 1432-2234. ; 132:11, s. 1398-
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Tidskriftsartikel (refereegranskat)abstract
- The active, epitaxial layers of silicon carbide (SiC) devices are grown by chemical vapor deposition (CVD), at temperatures above 1,600 °C, using silane and light hydrocarbons as precursors, diluted in hydrogen. A better understanding of the epitaxial growth process of SiC by CVD is crucial to improve CVD tools and optimize growth conditions. Through computational fluid dynamic (CFD) simulations, the process may be studied in great detail, giving insight to both flow characteristics, temperature gradients and distributions, and gas mixture composition and species concentrations throughout the whole CVD reactor. In this paper, some of the important parts where improvements are very much needed for accurate CFD simulations of the SiC CVD process to be accomplished are pointed out. First, the thermochemical properties of 30 species that are thought to be part of the gas-phase chemistry in the SiC CVD process are calculated by means of quantum-chemical computations based on ab initio theory and density functional theory. It is shown that completely different results are obtained in the CFD simulations, depending on which data are used for some molecules, and that this may lead to erroneous conclusions of the importance of certain species. Second, three different models for the gas-phase chemistry are compared, using three different hydrocarbon precursors. It is shown that the predicted gas-phase composition varies largely, depending on which model is used. Third, the surface reactions leading to the actual deposition are discussed. We suggest that hydrocarbon molecules in fact have a much higher surface reactivity with the SiC surface than previously accepted values.
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| 6. |
- Danielsson, Örjan, 1973-
(författare)
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Simulations of Silicon Carbide Chemical Vapor Deposition
- 2002
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Most of the modern electronics technology is based on the semiconducting material silicon. The increasing demands for smaller electronic devices with improved performance at lower costs drive the conventional silicon technology to its limits. To meet the requirements from the industry and to explore new application areas, other materials and fabrication methods must be used. For devices operating at high powers, high temperatures and high frequencies, the so-called wide bandgap semiconductors can be used with great success. Silicon carbide (SiC) and III-nitrides are wide bandgap materials that have gained increased interest in recent years. One important technique in manufacturing of electronic devices is chemical vapor deposition (CVD), by which thin layers can be deposited. These layers may have different electrical properties, depending on the choice of material and doping. Generally in CVD, a reactive gas mixture flows through a heated reactor chamber, where the substrates are placed. Complex chemical reactions take place in the gas and on the substrate surface, leading to many intermediate species and by-products, and eventually to the desired deposition. For the growth of device quality material it is important to be able to control the properties of the grown layers. These properties generally depend on the growth conditions in the reaction chamber, and on the chemistry of the deposition process. So far, empirical trial-and-error methods have been employed in the development of growth processes. Due to the lack of basic understanding of the governing physical processes, progress is costly and time consuming. Improving and optimizing the CVD process, as well as improving the fundamental understanding of the whole process is of great importance when good quality material should be produced. For this, computer simulations of the relevant physical and chemical phenomena can provide the necessary tools. This thesis focuses on computer simulations of the CVD process, in particular CVD of SiC. Simulations can be used not only as a tool for optimizing growth processes and reactor designs, they can also give information about physical phenomena that are difficult to measure, such as the gas-phase composition or the flow paths inside the reactor.Heating of the CVD susceptor is a central part of the process. For the growth of high quality SiC a relatively high temperature must be used. A convenient method for heating to high temperatures is by induction. A low resistive material, such as graphite, is placed inside a coil, which is given an alternating current. The graphite is then heated by the induced currents due to ohmic resistance. In this thesis the temperature distribution inside a CVD reactor, and how it is influenced by changes in coil frequency, power input to the coil and graphite thickness, is investigated. It is shown that by changing the placement and shape of the coil and by using insulation material correctly, a more uniform temperature distribution can be obtained.A model for the growth of SiC is used to predict growth rates at various process parameters. A number of possible factors influencing the growth rate are investigated using this model. The importance of including thermal diffusion and the effect of etching by hydrogen is shown, and the effect of parasitic growth investigated. Simulations show a mass transport limited growth, as seen from experiments.An improved susceptor design with an up-lifted substrate holder plate is investigated and compared to a conventional hot-wall reactor and to a cold-wall reactor. It is shown that stress induced by thermal gradients through the substrate is significantly reduced in the hot-wall reactor, and that stress due to backside growth can be diminished using the new design. Positive side effects are that slightly higher growth rates can be achieved, and that the growth temperature can be slightly lowered in the new susceptor.The doping incorporation behavior is thoroughly investigated experimentally for intentional doping with nitrogen and aluminum. The doping incorporation on both faces of SiC, as well as on two different polytypes is investigated. Equilibrium calculations are preformed, giving possible candidates for species responsible for the doping incorporation. To predict nitrogen doping concentrations, a simplified quantitative model is developed and applied to a large number of process parameters. It is seen that the same species as predicted by equilibrium calculations are produced, but the reactions producing these species are relatively slow, so that the highest concentrations are at the outlet of the reactor. It is thus concluded that N2 must be the major specie responsible for the nitrogen incorporation in SiC.For the growth of III-nitrides, ammonia is often used to give the nitrogen needed. It is well known that ammonia forms a solid adduct with the metalorganic gas, which is used as the source for the group III elements. It would thus be beneficial to use some other gas instead of ammonia. Since purity is of great importance, N2 gas would be the preferred choice. However, N2 is a very stable molecule and difficult to crack, even at high temperatures. It is shown that hydrogen can help in cracking nitrogen, and that growth of III-nitrides can be performed using N2 as the nitrogen-bearing gas, by only small changes to a conventional hot-wall CVD reactor.
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| 8. |
- Huang, Jing-Jia, 1990-
(författare)
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Surface-Controlled Chemical Vapor Deposition of Silicon Carbide
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Polycrystalline cubic silicon carbide, 3C-SiC, has long been investigated in the field of hard coating materials. The typical synthesis method for 3C-SiC coatings is thermal chemical vapor deposition (CVD) using either multicomponent precursors, e.g. methyltrichlorosilane, or a combination of single component precursors, e.g. silane and propane. In this thesis, the fabrication of polycrystalline SiC coatings has been explored from the new aspects on the basis of thermal CVD utilizing silicon tetrachloride (SiCl4) and various hydrocarbons, i.e. toluene (C7H8), methane (CH4) and ethylene (C2H4) as the precursors. The goal of this thesis is to control the surface chemistry in the SiCl4-based SiC CVD and has been accomplished by the following three different approaches: In the first approach to control the surface chemistry of SiC CVD, the difference in the adsorption energy of aromatic and aliphatic hydrocarbons on different SiC crystal planes was utilized. Under identical deposition conditions, a highly <111>-oriented 3C-SiC coating was deposited using C7H8 as the carbon precursor, whereas using CH4 resulted in a randomly oriented 3C-SiC. The results from quantum chemical calculation showed that the active film forming carbon species, i.e. C6H6 in the C7H8 process and CH3 in both C7H8 and CH4 processes, behaved differently when they adsorbed on the 3C-SiC (111) and (110) planes. CH3 is strongly chemisorbed on both planes, while C6H6 is chemisorbed on the (111) plane, but only physiosorbed on the other. The significant difference in the adsorption energy of CH3 and C6H6 on the (111) and (110) planes therefore explains the resulting highly <111>-oriented 3C-SiC from the C7H8 process. Furthermore, the ability to deposit 3C-SiC coatings with alternating highly <111>- and randomly oriented layers by merely switching the carbon precursor between C7H8 and CH4 or C2H4 in a single CVD deposition has further proven that the effect of aromatic hydrocarbons on the preferred growth orientation of 3C-SiC was controlled primarily by the surface chemistry. The second approach to the surface-controlled SiC CVD was based on the reduction of surface reaction probability (β) for conformal film growth via low-temperature, low-pressure CVD, which was originally proposed by Abelson and Girolami. Their strategies in reducing β, including lowering the temperature and increasing the precursor partial pressure, were successfully adapted to the SiC CVD growth using SiCl4 and C2H4 as the precursors in this thesis, where an elevated temperature and a moderate pressure were used. Moreover, the addition of Cl species as a growth inhibitor to the process further reduced the β, leading to a superconformal SiC growth. The third approach employed in this thesis for the SiC growth was pulsed CVD. Instead of a continuous and simultaneous SiCl4 and C2H4 flow, the precursors were pulsed alternately into the chamber with each precursor pulse being separated by a H2 purge. In this precursor delivery mode, the gas phase reactions between SiCl4 and C2H4 were avoided and hence the SiC growth was mostly controlled by the surface chemistry. Altering the pulse durations of the precursors led to a variation of growth per cycle (GPC), which was explained by a two-step mechanism. During the SiCl4 pulse, a thin layer of Si is deposited, which is carburized by carbon species produced during the C2H4 pulse. Additionally, the separation of precursor pulses should lead to a large increase in the surface coverage of Cl species, further enhancing the inhibition effect and resulting in a superconformal SiC growth. By using this approach, superconformal SiC coatings were achieved at temperatures where conventional CVD only yielded nonconformal SiC coatings. The observed decline in coating conformality with an elongated purge implied that more surface Cl species were replaced by H during the H2 purge and consequently the inhibition effect was diminished.
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