| 1. |
- Bergström, Erik, 1976-
(författare)
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Supporting Information Security Management : Developing a Method for Information Classification
- 2020
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- In the highly digitalised world in which we live today, information and information systems have become critical assets to organisations, and hence need to be safeguarded accordingly. In order to implement and work with information security in a structured way, an Information Security Management System (ISMS) can be implemented. Asset management is a central activity in ISMS that aims at identifying, assigning ownership and adding protection to information assets. One activity within asset management is information classification that has the objective to ensure that the information receives an appropriate level of protection in accordance with its importance to the organisation. Information classification is a well-known practice for all kinds of organisations, both in the private and public sector, and is included in different variants in standards such as ISO/IEC 27002, COBIT and NIST-SP800.However, information classification has received little attention from academia, and many organisations are struggling with the implementation. The reasons behind why it is problematic, and how to address such issues, are largely unknown. Furthermore, existing approaches, described in, for example, standards and national recommendations, do not provide a coherent and systematic approach to information classification. The short descriptions in standards, and literature alike, leave out essential aspects needed for many organisations to adopt and implement information classification. There is, for instance, a lack of detailed descriptions regarding (1) procedures and concepts, (2) how to tailor the approach for different situations, (3) a framework that structures and guides the classification, (4) what roles should be involved in the classification, and (5) how information with different granularity is handled.This thesis aims to increase the applicability of information classification by developing a method for information classification in ISMS that draws from established standards and practice. In order to address this aim, a Design Science Research (DSR) study was performed in three cycles. A wide range of data was collected, including a series of interviews with experts and novices on information classification, a survey, most of the Swedish public sector information classification policies, and observations. There are three main contributions made by this thesis (1) the identification of issues and enablers for information classification, (2) the design principles underpinning the development of a method for information classification, and (3) the method for information classification itself. Contributions have also been made to the context around information classification, such as, for example, 20 practical suggestions for how to meet documented challenges in practice.
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| 2. |
- Johansson, Fredrik
(författare)
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Evaluating the performance of TEWA systems
- 2010
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- It is in military engagements the task of the air defense to protect valuable assets such as air bases from being destroyed by hostile aircrafts and missiles. In order to fulfill this mission, the defenders are equipped with sensors and firing units. To infer whether a target is hostile and threatening or not is far from a trivial task. This is dealt with in a threat evaluation process, in which the targets are ranked based upon their estimated level of threat posed to the defended assets. Once the degree of threat has been estimated, the problem of weapon allocation comes into the picture. Given that a number of threatening targets have been identified, the defenders need to decide on whether any firing units shall be allocated to the targets, and if so, which firing unit to engage which target. To complicate matters, the outcomes of such engagements are usually stochastic. Moreover, there are often tight time constraints on how fast the threat evaluation and weapon allocation processes need to be executed. There are already today a large number of threat evaluation and weapon allocation (TEWA) systems in use, i.e. decision support systems aiding military decision makers with the threat evaluation and weapon allocation processes. However, despite the critical role of such systems, it is not clear how to evaluate the performance of the systems and their algorithms. Hence, the work in thesis is focused on the development and evaluation of TEWA systems, and the algorithms for threat evaluation and weapon allocation being part of such systems. A number of algorithms for threat evaluation and static weapon allocation are suggested and implemented, and testbeds for facilitating the evaluation of these are developed. Experimental results show that the use of particle swarm optimization is suitable for real-time target-based weapon allocation in situations involving up to approximately ten targets and ten firing units, while it for larger problem sizes gives better results to make use of an enhanced greedy maximum marginal return algorithm, or a genetic algorithm seeded with the solution returned by the greedy algorithm.
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| 3. |
- Karlsson, Louise
(författare)
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P-glycoprotein and chiral antidepressant drugs : Pharmacokinetic, pharmacogenetic and toxicological aspects
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The blood-brain barrier (BBB) is formed by the capillary endothelial cells, joined together by tight junctions, with transporter proteins. BBB acts to regulate the brain concentrations of substances including many drugs. Transport across the cells is necessary for a drug to ensure that the drug reaches the site of action and transport proteins such as P-glycoprotein (P-gp; ABCB1) can limit the entrance into various tissues, including the brain.Molecules that are not superimposable on their mirror images and thus exist in two enantiomeric forms (enantiomers) are said to be chiral. A racemic compound is one composed of a 50:50 mixture of two enantiomers, S- and R-enantiomers. Two examples of frequently prescribed racemic drugs are the chiral antidepressants venlafaxine (VEN) and citalopram (CIT). The enantiomers of VEN possess different pharmacodynamic profiles where the R-enantiomer is a potent inhibitor of both serotonin and noradrenaline reuptake (SNRI), while the S-enantiomer is more selective in inhibiting serotonin reuptake (SSRI). The SSRI effect of CIT resides in the S-enantiomer, whereas the R-enantiomer is considered to be therapeutically inactive, or even that it counteracts the effects. The S-enantiomer of CIT is now available as a separate SSRI (escitalopram, EsCIT). VEN and CIT are also among the most commonly found drugs in forensic autopsy cases.Few previous studies have examined a possible enantioselective activity of P-gp. Thus, the general aim of this thesis was to study the enantiomeric distribution of chiral antidepressant drugs, focusing on the role of P-gp in the BBB. For this purpose, a mouse model disrupted of the genes coding for P-gp (abcb1ab (-/-) mice) was used. Brain and serum concentrations of the enantiomers of VEN and CIT, and their major metabolites, were compared to the corresponding wild-type mice (abcb1ab (+/+) mice). The open-field locomotor and rearing activities were examined after chronic VEN administration. In addition to the animal studies, genetic and toxicological aspects of P-gp were studied in a forensic autopsy material, where intoxication cases were compared with cases that were not related to intoxications.The brain to serum concentration ratios for VEN, CIT and EsCIT differed between knockout mice and wild-type mice, with 2-3 fold higher brain concentrations in mice with no expression of P-gp. Hence, all studied drugs, and their major metabolites, were substrates for P-gp. There was no evidence for a stereoselective P-gp mediated transport. The P-gp substrate properties were reflected in the open-field behavior test where the knockout mice displayed increased center activity compared with wild-type mice following chronic VEN exposure. The genotype distribution of ABCB1 SNPs C1236T, G2677T and C3435T in VEN positive cases was significantly (or borderline) different between the intoxication cases and the non-intoxication cases. This difference in genotype distribution was not observed for the CIT positive cases.To conclude, the present work has led to an increased knowledge about how the enantiomers of VEN and CIT are affected by the BBB transporter P-gp. Using an animal model, VEN and CIT have proved to be actively transported out of the brain by P-gp and no difference was observed for the enantiomers with regard to P-gp transport. Further, the ABCB1 genotype distribution was different in intoxication cases compared with non-intoxication cases. Taken together, these findings offer the possibility that the expression of P-gp in humans may be a contributing factor for limited treatment response and increased risk of side-effects following antidepressant drug treatment.
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| 4. |
- Kentson, Magnus
(författare)
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Fatigue and Peripheral Muscle Dysfunction: Studies on Vitamin D Status, Muscle Metabolism and Systemic Inflammation in Patients with COPD : Aspects of COPD severity beyond FEV1 and exacerbations
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- BackgroundThe severity of Chronic Obstructive Pulmonary Disease (COPD) is usually described in terms of forced expiratory volume in one second (FEV1) and number of exacerbations. However, COPD is a complex disease with different ways of expression, involving pulmonary symptoms, extra pulmonal manifestations and comorbidities, which altogether affect the patient by contributing to reduced functional capacity, increased shortness of breath, reduced health-related quality of life and increased mortality. Systemic inflammation is common in COPD and can potentially constitute a link between the lungs and other organs. The aim of this thesis was to broaden the aspects of COPD severity beyond FEV1 and exacerbations by studying fatigue, the role of vitamin D, nutritional factors, systemic inflammation and peripheral muscle function in patients with COPD. Methods and ResultsIn paper I, we included 101 patients with COPD, and 34 control subjects. Assessment of experience of fatigue, functional limitation due to fatigue, and the relationships to physiological, psychological and situational variables and quality of life (QoL) were evaluated. We found that experience of fatigue was highly prevalent (72% versus 56% in control subjects) and a troublesome symptom in COPD. Patients with COPD and fatigue had lower lung function, shorter walking distance, more dyspnoea, anxiety and depressive symptoms and poorer health status compared to patients without fatigue (all p < 0.01). Several contributing factors were identified to experience of fatigue and functional limitations of fatigue with dyspnoea, depressive symptoms and insomnia as the most prominent factors. No clear association with systemic inflammation was found. Paper II evaluated vitamin D status in 66 patients with advanced COPD (28 with long-term oxygen therapy (LTOT)) and 47 control subjects. 25-hydroxyvitamin 25(OH)D were deter-mined in early fall in a short period of seven weeks. Questionnaires about COPD symptoms, general health, lifestyle, dietary habits and QoL were answered. Lung function tests and blood sampling including systemic inflammatory markers, carotenoids and protein carbonylation (PC) were assessed. The peak annual 25(OH)D of COPD patients was significantly lower than in the control subjects, but there was no significant difference between COPD patients with and without LTOT. Among vitamin D-deficient COPD patients, 25(OH)D correlated positively with lung function, blood oxygenation, food portion size, Mediterranean Diet Score and Ultra-violet Score and negatively with dyspnoea and DOSE-index, a composite index for COPD se-verity. Ongoing vitamin D supplementation was the single most important intervention to maintain 25(OH)D levels <50 nmol/L. In paper III, we evaluated in the same cohort as paper II oxidative damage and levels of carotenoids. Patients with COPD (±LTOT) did not demonstrate increased oxidative damage. Com-pared with the control group, levels of several carotenoids were significantly lower in COPD, and the diet contained significantly less fruit and vegetables. Lycopene correlated positively with saturation and lutein correlated positively with some inflammatory markers but negatively with IL-6, an important marker for systemic inflammation. The study highlights the importance of dietary factors in COPD. In paper IV, 32 patients with COPD answered questionnaires, and were subjected to lung function tests and blood analysis including systemic inflammatory markers. Magnetic resonance imaging (MRI) for analysis of whole-body and thigh muscle composition was performed. Bioenergetics in the resting thigh muscle, (PCr/Pi ratio), were analysed using 31phosphorus magnetic resonance spectroscopy (31P-MRS). We found that adverse muscle composition was common in the COPD group. Clinical characteristics reflecting COPD severity were all associated with a raise of the PCr/Pi ratio in the thigh muscle. Increased MFIa correlated positively to systemic inflammatory markers, negative to physical activity and PCr/Pi ratio. We compared the COPD group with a virtual control group from UK Biobank (n= 3200). ConclusionsSevere COPD is much more than airway obstruction and exacerbations. The presence of fatigue is associated, as well as vitamin D status and nutritional factors, with important clinical out-comes reflecting COPD severity. Adverse muscle composition is common in COPD and there seems to be a link between systemic inflammation, muscle fat infiltration and bioenergetics.
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| 5. |
- Kolkowska, Ella
(författare)
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A method for analyzing value-based compliance in systems security
- 2013
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Aim: The aim of this thesis is to design a method that supports analysis of different values that come into play in compliance and non compliance situations within information systems security (ISS). The thesis addresses the problem of lack of ISS compliance methods that support systematic analysis of compliant and non-compliant behaviours as well as the reasons for these behaviours. The problem is addressed by designing a method that supports analysis of different values that come into play in compliance and non compliance situations in ISS. The method is called Value Based Compliance method (VBC method).Research questions: The main research question of the thesis is: How should a method for analysis of different values that come in play in compliance and non-compliance situations within ISS be designed? This research question is answered by answering three sub-questions: 1) What values and goals (perspective) should the VBC method realize? 2) What underpinning design principles should the VBC method build on? 3) How should the VBC method be constructed to realize the VBC perspective and to incorporate the design principles?Research method: Design Science Research (DSR) was chosen as a research approach in this thesis. DSR prescribe how to carry on a design process of an artefact with preserved rigor and relevance. The approach is both useful in order to solve real life problems and theoretically ground suchproblems. The VBC method is informed by a number of kernel theories and based on current knowledge in ISS compliance literature. The method is also empirically tested in three different contexts, during six DSR cycles.Contributions: The three main contributions from the thesis are: the VBC perspective, the design principles and the VBC method. The VBC perspective is in line with a social view on ISS’s role in organisation. This perspective is realized in the VBC method by analysing values and value conflicts that come in play in compliance and non-compliance situations. Thus this study contributes to the field of ISS by designing a method that realizes the social view on ISS’s role in an organisation. The five design principles for a VBC method is the second contribution. The design theory with the five empirically tested design principles may be the point of departure for development of other compliance methods focusing on analysis of values and value conflicts that come into play in relation to ISS compliance. The design principles contribute also to the ISS compliance field by 1) extending compliance analysis with consideration of the different rationalities (values and goals) 2) acknowledging the difference between rational and non-rational ISS actions and 3) emphasizing the importance of finding articulated as well as unarticulated ISS actions. Finally, the VBC method itself contributes to the ISS compliance research and practice by offering a formalized, theoretically and empirically grounded method for systematic analysis of compliance and non-compliance situations as well as rationalities that come into play in these situations.
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| 6. |
- Lindskog, Carin, 1965-
(författare)
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Tensions experienced and balancing strategies used in Agile Software Development environments
- 2022
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- To achieve efficiency in rapidly changing environments, working methods that promote change and flexibility are needed. Thus, the Agile ways of working (i.e. Agile values, principles, and frameworks) are today the most common approach to developing software and something that is currently spreading to many other industries and organizations outside of the traditional IT sector. However, it is challenging to combine the business-driven need for predictability and planning with the Agile ways of working, e.g. changeability, self-management and step-by-step development and delivery; therefore, several tensions can arise at the team and organizational levels. Tensions can, for example, be experienced if the team members’ experiences are rooted in traditional project environments with stable processes and predefined requirements based on detailed planning. This dissertation aims to contribute to our knowledge of Agile Software Development (ASD) by examining the contradictions and tensions in ASD environments, as well as how to balance and react to these tensions. The dissertation responds to the great need for conceptual development in the field of Information Systems. The target groups are both researchers and practitioners.The research reported on in this dissertation is based on one conceptual study and two empirical studies. The studies reported on in four appended papers jointly contribute toward answering three research questions (RQs):RQ 1: What is the nature of the tensions experienced in ASD environments?RQ 2: What are the sources of the tensions experienced in ASD environments?RQ 3: What ambidextrous strategies can be used to both balance and react to the tensions found in ASD environments?This dissertation brings together concepts from academic domains of knowledge (organizational theories of [paradoxical] tensions and ambidexterity and Activity Theory derived from socio-psychological theory) in order to provide a new insight into the complexity of ASD.This dissertation contributes by discussing and visualizing how patterns of contradictions, categorized tensions and ambidextrous (i.e. the ability to manage two seemingly contradictory activities) strategies have been compiled and connected. This dissertation also contributes by describing how three models that can be used to develop the concept of “shared mental models”, which is so important to team-based learning.
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| 7. |
- Norell, Daniel, 1973-
(författare)
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Taming the Erratic : Representation and materialization in post-digital architectural design
- 2016
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- This thesis investigates materialization and representation in contemporary architectural design practice. Due to cultural and technological shifts, the act of design is no longer squarely located in the abstract realms of drawings or digital geometries. Computer aided manufacturing, simulation and scanning offer new design opportunities that are located in the transfer between representation and material. This has given rise to a post-digital model of practice and thought, in which ‘real’ and discrete chunks of matter are incorporated at the earliest stages of design.The thesis is practice-based, and spans in scope from design to technology to theory. The design work included explores materialization and representation from a particular point of view. In addition, it suggests a methodological approach to design, and explores the theoretical implications in this approach. These implications are addressed in two connected research questions: How can material processes, whether real or simulated, turn transfers between geometry and materialized objects into productive design opportunities? And how might material simulation alter the ways in which representations are conceptualized and used by architects? In parallel with practice-based work, the thesis suggests a theoretical framework for current issues of representation and materialization in architecture. This framework draws from the recent history of the digital turn in architecture as well as from recent design research work and theory in a post-digital turn.This thesis makes contributions in three main areas. Through the design work Erratic, it makes a visceral case for how the use of material simulation might open up new ways of harnessing material agency. It positions simulation in the field of architecture in-between established polarities such as geometry vs. matter, virtual vs. real and drawing vs. mock-up. It discusses the conceptual difference between design based on geometry and design based on discrete pieces of material. Finally, it proposes that form in architecture increasingly can be conceptualized as ‘chunks,’ as opposed to reduced descriptions of geometry.
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| 8. |
- Abdellah, Tebani, et al.
(författare)
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Integration of molecular profiles in a longitudinal wellness profiling cohort.
- 2020
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Ingår i: Nature communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- An important aspect of precision medicine is to probe the stability in molecular profiles among healthy individuals over time. Here, we sample a longitudinal wellness cohort with 100 healthy individuals and analyze blood molecular profiles including proteomics, transcriptomics, lipidomics, metabolomics, autoantibodies andimmune cell profiling, complementedwith gut microbiota composition and routine clinical chemistry. Overall, our results show high variation between individuals across different molecular readouts, while the intra-individual baseline variation is low. The analyses show that each individual has a unique and stable plasma protein profile throughout the study period and that many individuals also show distinct profiles with regards to the other omics datasets, with strong underlying connections between the blood proteome and the clinical chemistry parameters. In conclusion, the results support an individual-based definition of health and show that comprehensive omics profiling in a longitudinal manner is a path forward for precision medicine.
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| 9. |
- Ask, Andreas, 1977-
(författare)
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The Role of Enterprise Architecture in Local eGovernment Adoption
- 2012
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Licentiatavhandling (övrigt vetenskapligt/konstnärligt)abstract
- eFörvaltning ses som ett medel för att göra kommunalt förvaltningsarbete mer effektivt och interoperabelt. Dock begränsas kommuner av oförmåga att uppnå det vilket försvårar genomförandet av deras uppdrag. En orsak till varför införandet av eFörvaltning inte ger de systematiska fördelar som efterfrågas beror på att införandet av informations och kommunikations teknologier (IKT) inte är tillräckligt. Förändringen som krävs kan inte ske över en natt utan är en gradvis förändring över tid som även kräver nya sätt att arbeta på. Kommuner behöver hantera sin eFörvaltningsutveckling på ett mer strukturerat sätt för att öka möjligheterna att förverkliga de fördelarna som sägs kunna erhållas. Enterprise Architecture (EA) har på senare tid kommit att ses som en möjlig lösning för att komma tillrätta med denna problematik. Genom att arbeta mot en EA skapas möjligheten att brygga IKT system med affärsprocesser och därigenom leda till effektivitet och interoperabilitet. Dock är EA inom offentlig sektor ifrågasatt. Forskare menar att EA saknar tydlig definierad omfattning och begreppsvärld, vilket gör användningen av EA inom förvaltningsarbete svår. De omfattande logiska EA ramverk som finns idag saknar innehåll som är specifikt relaterade till offentlig sektor. Vari det blir problematiskt att arbeta mot en EA inom offentlig sektor.Denna licentiatavhandling lägger följande forskningsfråga: ”Vilken roll har EA för införandet av eFörvaltning inom Svenska kommuner?” Samt följande underfrågor ”Vilka förutsättningar finns för att använda EA vid eFörvaltningsutveckling?” ”Vad har Örebro kommun uppnått genom att arbete mot en EA i deras eFörvaltningsutveckling?” Och ”Vilka problem har Örebro kommun upplevt av att arbeta mot en EA vid eFörvaltningsutvecklingen?”. En longitudinell fallstudie av ett eFörvaltningsutvecklingsprojekt i Örebrokommun genomfördes mellan 2007 och 2009 med syfte att utröna dessa frågor och för att skapa insikt om EAs roll för eFörvaltningsutveckling. Förutsättningar, resultat och upplevda problem vid eFörvaltningsutvecklingen studerades. I anslutning till detta empiriska arbete genomfördes även en jämförandestudie av eFörvaltningsmål såsom de anges i officiella statliga dokument inom så väl Sverige som i EU gentemot förmodade fördelarna med EA som diskuteras i samtida EA-litteratur.Studien visar att EA som fenomen ses som en förutsättning för att lyckas med eFörvaltning. 7 viktiga aspekter gällande förutsättningen för att lyckas med kommuners eFörvaltningsutveckling identifierades: Skillnaden mellan administrativa och politiska ansvaret, Politiskt mandat, Politisk timing, Resursfördelning, Samordning under NPM, Leverantörsberoende, Val av standard och bäst praxis. För att förbättra möjligheten att röna framgång i eFörvaltningsutveckling behöver dessa aspekter aktualiseras och hanteras. New Public Management (NPM) som styrmodell utgör ett strukturellt hinder för eFörvaltningsutveckling som förhindrar en mer explicit användning av EA-ramverk, vilket påverkar möjligheten att uppnå uppsatta mål negativt. Trots den strukturella problematik som föreligger, kan stöd erhållas genom att arbeta utifrån ett ”EA-tänk”. Ett ”EAtänk” kan här ses som ett medel för att påbörja förändringen mot en EA utan att för den delen explicit använda sig av ett EA-ramverk, eller -metod. Dock är det viktigt skapa en förståelse för att ”EA-tänk” som fenomen i sin tur ger upphov till strukturer och således kan leda till problem som även de måste hanteras för att eFörvaltningsutveckling skall lyckas. eFörvaltning, NPM och ett ”EA-tänk” skapar en strukturell triad där olika strukturella egenskaper i vissa fall sammanfaller med varandra vari de kan stödja kommuner i dess arbete och således bidra till positiv utveckling. Medan det i andra fall leder till negativ utveckling där administrationen upplever problem att ta till sig av de rekommendationer eFörvaltningsprojektet kan tänkas ge till organisationen som helhet. Paradoxalt nog skapar denna strukturella triad en situation där kommuner måste arbete runt sig själv för att komma framåt. Normer och befintliga strukturer i kommuner förhindrar effektiv samarbete både internt mellan olika förvaltningar och externt gentemot andra kommuner och landsting vilket leder till fragmentariska framsteg mot projektmålen och i slutänden även kommunens övergripande mål gällande eFörvaltningsutveckling. Givet de i många fall motstridiga strukturella egenskaper som finns mellan NPM och eFörvaltning innebär att det initiala arbetet mot EA i eFörvaltningsutveckling är ett ytterst komplext fenomen. För att öka möjligheten att eFörvaltningsutvecklingen blir lyckad krävs det att kommuner känner till och hantera den negativa inverkan NPM har på eFörvaltningsutveckling. Samtidigt behöver de känna till och hanterar problematiken som uppstår av att arbeta mot en EA genom ett ”EA-tänk”, Samt att som studien visat, EA ger inget stöd för mer politiseras mål, vari kommuner behöver annat sätt och andra strategier för sådant arbete. Detta är viktigt att beakta, speciellt med tanke på att mer politiserade mål i mångt och mycket är de mål som kommer premieras av politiker. Av just den enkla orsaken att politiker inte blir omvalda på grund av “effektiva IKT integrerade interna processer” utan blir omvalda utifrån förbättringar som är synliga och som gagnar medborgare positivt.Avhandlingen bidrar till forsknings genom att öka förståelse för och behovet av att hantera både positiva och negativa faktorer som påverkar möjligheten att lyckas med eFörvaltningsutveckling inom kommunalt arbete. Vilket inkluderar de 7 identifierade kritiska faktorer måste aktualiseras och hanteras för att öka möjligheten att lyckas. Den negativa effekt NPM har på eFörvaltningsutveckling och hur ett ”EA-tänk” kan leda till positiv utveckling, även om det som denna studie visat inte kan bidra till mer politiserade mål. Vad gäller denna licentiatavhandlings bidrag till praktiken så bidrar den med att belysa den problematiska situation som föreligger av att institutionaliserade strukturer i många fall verkar som ett hinder mot förändring. Studien möjliggör även till att kommuner kan skapa en förståelse för sin egna eFörvaltningsutveckling och möjligtvis kan verka för att inte uppleva samma problematik som identifierats i Örebros eFörvaltningsutveckling samt visa på hur en kommun kan arbeta för att hantera den problematik som inte kan förhindras alternativ som uppstår trots denna kunskap.
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| 10. |
- Eriksson, Martin
(författare)
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Photoluminescence Characteristics of III-Nitride Quantum Dots and Films
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- III-Nitride semiconductors are very promising in both electronics and optical devices. The ability of the III-Nitride semiconductors as light emitters to span the electromagnetic spectrum from deep ultraviolet light, through the entire visible region, and into the infrared part of the spectrum, is a very important feature, making this material very important in the field of light emitting devices. In fact, the blue emission from Indium Gallium Nitride (InGaN), which was awarded the 2014 Nobel Prize in Physics, is the basis of the common and important white light emitting diode (LED).Quantum dots (QDs) have properties that make them very interesting for light emitting devices for a range of different applications, such as the possibility of increasing device efficiency. The spectrally well-defined emission from QDs also allows accurate color reproduction and high-performance communication devices. The small size of QDs, combined with selective area growth allows for an improved display resolution. By control of the polarization direction of QDs, they can be used in more efficient displays as well as in traditional communication devices. The possibility of sending out entangled photon pairs is another QD property of importance for quantum key distribution used for secure communication.QDs can hold different exciton complexes, such as the neutral single exciton, consisting of one electron and one hole, and the biexciton, consisting of two excitons. The integrated PL intensity of the biexciton exhibits a quadratic dependence with respect to the excitation power, as compared to the linear power dependence of the neutral single exciton. The lifetime of the neutral exciton is 880 ps, whereas the biexciton, consisting of twice the number of charge carriers and lacks a dark state, has a considerably shorter lifetime of only 500 ps. The ratio of the lifetimes is an indication that the size of the QD is in the order of the exciton Bohr radius of the InGaN crystal making up these QDs in the InGaN QW.A large part of the studies of this thesis has been focused on InGaN QDs on top of hexagonal Gallium Nitride (GaN) pyramids, selectively grown by Metal Organic Chemical Vapor Deposition (MOCVD). On top of the GaN pyramids, an InGaN layer and a GaN capping layer were grown. From structural and optical investigations, InGaN QDs have been characterized as growing on (0001) facets on truncated GaN pyramids. These QDs exhibit both narrow photoluminescence linewidths and are linearly polarized in directions following the symmetry of the pyramids.In this work, the neutral single exciton, and the more rare negatively charged exciton, have been investigated. At low excitation power, the integrated intensity of the PL peak of the neutral exciton increases linearly with the excitation power. The negatively charged exciton, on the other hand, exhibits a quadratic power dependence, just like that of the biexciton. Upon increasing the temperature, the power dependence of the negatively charged exciton changes to linear, just like the neutral exciton. This change in power dependence is explained in terms of electrons in potential traps close to the QD escaping by thermal excitation, leading to a surplus of electrons in the vicinity of the QD. Consequently, only a single exciton needs to be created by photoexcitation in order to form a negatively charged exciton, while the extra electron is supplied to the QD by thermal excitation.Upon a close inspection of the PL of the neutral exciton, a splitting of the peak of just below 0.4 meV is revealed. There is an observed competition in the integrated intensity between these two peaks, similar to that between an exciton and a biexciton. The high energy peak of this split exciton emission is explained in terms of a remotely charged exciton. This exciton state consists of a neutral single exciton in the QD with an extra electron or hole in close vicinity of the QD, which screens the built-in field in the QD.The InGaN QDs are very small; estimated to be on the order of the exciton Bohr radius of the InGaN crystal, or even smaller. The lifetimes of the neutral exciton and the negatively charged exciton are approximately 320 ps and 130 ps, respectively. The ratio of the lifetimes supports the claim of the QD size being on the order of the exciton Bohr radius or smaller, as is further supported by power dependence results. Under the assumption of a spherical QD, theoretical calculations predict an emission energy shift of 0.7 meV, for a peak at 3.09 eV, due to the built-in field for a QD with a diameter of 1.3 nm, in agreement with the experimental observations.Studying the InGaN QD PL from neutral and charged excitons at elevated temperatures (4 K to 166 K) has revealed that the QDs are surrounded by potential fluctuations that trap charge carriers with an energy of around 20 meV, to be compared with the exciton trapping energy in the QDs of approximately 50 meV. The confinement of electrons close to the QD is predicted to be smaller than for holes, which accounts for the negative charge of the charged exciton, and for the higher probability of capturing free electrons. We have estimated the lifetimes of free electrons and holes in the GaN barrier to be 45 ps and 60 ps, in consistence with excitons forming quickly in the barrier upon photoexcitation and that free electrons and holes get trapped quickly in local potential traps close to the QDs. This analysis also indicates that there is a probability of 35 % to have an electron in the QD between the photoexcitation pulses, in agreement with a lower than quadratic power dependence of the negatively charged exciton.InN is an attractive material due to its infrared emission, for applications such as light emitters for communication purposes, but it is more difficult to grow with high quality and low doping concentration as compared to GaN. QDs with a higher In-composition or even pure InN is an interesting prospect as being a route towards increased quantum confinement and room temperature device operation. For all optical devices, p-type doping is needed. Even nominally undoped InN samples tend to be heavily n-type doped, causing problems to make pn-junctions as needed for LEDs. In our work, we present Mg-doped p-type InN films, which when further increasing the Mg-concentration revert to n-type conductivity. We have focused on the effect of the Mg-doping on the light emission properties of these films. The low Mg doped InN film is inhomogeneous and is observed to contain areas with n-type conductivity, so called n-type pockets in the otherwise p-type InN film. A higher concentration of Mg results in a higher crystalline quality and the disappearance of the n-type pockets. The high crystalline quality has enabled us to determine the binding energy of the Mg dopants to 64 meV. Upon further increase of the Mg concentration, the film reverts to ntype conductivity. The highly Mg doped sample also exhibits a red-shifted emission with features that are interpreted as originating from Zinc-Blende inclusions in the Wurtzite InN crystal, acting as quantum wells. The Mg doping is an important factor in controlling the conductivity of InN, as well as its light emission properties, and ultimately construct InN-based devices.In summary, in this thesis, both pyramidal InGaN QDs and InGaN QDs in a QW have been investigated. Novel discoveries of exciton complexes in these QD systems have been reported. Knowledge has also been gained about the challenging material InN, including a study of the effect of the Mg-doping concentration on the semiconductor crystalline quality and its light emission properties. The outcome of this thesis enriches the knowledge of the III-Nitride semiconductor community, with the long-term objective to improve the device performance of III-Nitride based light emitting devices.
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