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  • Public defence: 2019-02-27 13:15 Planck, Fysikhuset, Linköping
    Karhu, Robin
    Linköping University, Department of Physics, Chemistry and Biology, Semiconductor Materials. Linköping University, Faculty of Science & Engineering.
    CVD growth of SiC for high-power and high-frequency applications2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    Silicon Carbide (SiC) is a wide bandgap semiconductor that has attracted a lot of interest for electronic applications due to its high thermal conductivity, high saturation electron drift velocity and high critical electric field strength. In recent years commercial SiC devices have started to make their way into high and medium voltage applications.

    Despite the advancements in SiC growth over the years, several issues remain. One of these issues is that the bulk grown SiC wafers are not suitable for electronic applications due to the high background doping and high density of basal plane dislocations (BPD). Due to these problems SiC for electronic devices must be grown by homoepitaxy. The epitaxial growth is performed in chemical vapor deposition (CVD) reactors. In this work, growth has been performed in a horizontal hot-wall CVD (HWCVD) reactor. In these reactors it is possible to produce high-quality SiC epitaxial layers within a wide range of doping, both n- and p-type.

    SiC is a well-known example of polytypism, where the different polytypes exist as different stacking sequences of the Si-C bilayers. Polytypism makes polytype stability a problem during growth of SiC. To maintain polytype stability during homoepitaxy of the hexagonal polytypes the substrates are usually cut so that the angle between the surface normal and the c-axis is a few degrees, typically 4 or 8°. The off-cut creates a high density of micro-steps at the surface. These steps allow for the replication of the substrates polytype into the growing epitaxial layer, the growth will take place in a step-flow manner. However, there are some drawbacks with step-flow growth. One is that BPDs can replicate from the substrate into the epitaxial layer. Another problem is that 4H-SiC is often used as a substrate for growth of GaN epitaxial layers. The epitaxial growth of GaN has been developed on on-axis substrates (surface normal coincides with c-axis), so epitaxial 4H-SiC layers grown on off-axis substrates cannot be used as substrates for GaN epitaxial growth.

    In efforts to solve the problems with off-axis homoepitaxy of 4H-SiC, on-axis homoepitaxy has been developed. In this work, further development of wafer-scale on-axis homoepitaxy has been made. This development has been made on a Si-face of 4H-SiC substrates. The advances include highly resistive epilayers grown on on-axis substrates. In this thesis the ability to control the surface morphology of epitaxial layers grown on on-axis homoepitaxy is demonstrated. This work also includes growth of isotopically enriched 4H-SiC on on-axis substrates, this has been done to increase the thermal conductivity of the grown epitaxial layers.

    In (paper 1) on-axis homoepitaxy of 4H-SiC has been developed on 100 mm diameter substrates. This paper also contains comparisons between different precursors. In (paper 2) we have further developed on-axis homoepitaxy on 100 mm diameter wafers, by doping the epitaxial layers with vanadium. The vanadium doping of the epitaxial layers makes the layers highly resistive and thus suitable to use as a substrate for III-nitride growth. In (paper 3) we developed a method to control the surface morphology and reduce the as-grown surface roughness in samples grown on on-axis substrates. In (paper 4) we have increased the thermal conductivity of 4H-SiC epitaxial layers by growing the layers using isotopically enriched precursors. In (paper 5) we have investigated the role chlorine have in homoepitaxial growth of 4H-SiC. In (paper 6) we have investigated the charge carrier lifetime in as-grown samples and traced variations in lifetime to structural defects in the substrate. In (paper 7) we have investigated the formation mechanism of a morphological defect in homoepitaxial grown 4H-SiC.

    List of papers
    1. The Role of Chlorine during High Growth Rate Epitaxy
    Open this publication in new window or tab >>The Role of Chlorine during High Growth Rate Epitaxy
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    2015 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 821-823, p. 141-144Article in journal (Refereed) Published
    Abstract [en]

    The influence of chlorine has been investigated for high growth rates of 4H-SiC epilayers on 4o off-cut substrates. Samples were grown at a growth rate of approximately 50 and 100 μm/h and various Cl/Si ratios. The growth rate, net doping concentration and charge carrier lifetime have been studied as a function of Cl/Si ratio. This study shows some indications that a high Cl concentration in the growth cell leads to less availability of Si during the growth process.

    Place, publisher, year, edition, pages
    Pfaffikon, Switzerland: Scientific.Net, 2015
    Keywords
    Chemical Vapor Deposition (CVD), Chlorine, Doping, Epitaxy, High Growth Rate
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-123951 (URN)10.4028/www.scientific.net/MSF.821-823.141 (DOI)
    Conference
    European Conference on Silicon Carbide & Related Materials, Grenoble, France, 21-25 September 2014
    Available from: 2016-01-14 Created: 2016-01-14 Last updated: 2019-02-14Bibliographically approved
    2. Long Charge Carrier Lifetime in As-Grown 4H-SiC Epilayer
    Open this publication in new window or tab >>Long Charge Carrier Lifetime in As-Grown 4H-SiC Epilayer
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    2016 (English)In: Materials Science Forum, ISSN 0255-5476, E-ISSN 1662-9752, Vol. 858, p. 125-128Article in journal (Refereed) Published
    Abstract [en]

    Over 150 μm thick epilayers of 4H-SiC with long carrier lifetime have been grown with a chlorinated growth process. The carrier lifetime have been determined by time resolved photoluminescence (TRPL), the lifetime varies a lot between different areas of the sample. This study investigates the origins of lifetime variations in different regions using deep level transient spectroscopy (DLTS), low temperature photoluminescence (LTPL) and a combination of KOH etching and optical microscopy. From optical microscope images it is shown that the area with the shortest carrier lifetime corresponds to an area with high density of structural defects.

    Place, publisher, year, edition, pages
    Trans Tech Publications, 2016
    Keywords
    Carrier Lifetime, Chemical Vapor Deposition (CVD), Chlorine, Epitaxy
    National Category
    Condensed Matter Physics
    Identifiers
    urn:nbn:se:liu:diva-154468 (URN)10.4028/www.scientific.net/MSF.858.125 (DOI)
    Available from: 2019-02-13 Created: 2019-02-13 Last updated: 2019-02-14
  • Public defence: 2019-03-01 13:00 I:101, Hus I, Linköping
    Halvarsson Lundkvist, Agneta
    Linköping University, Department of Behavioural Sciences and Learning, Education and Sociology. Linköping University, Faculty of Arts and Sciences.
    Learning Dynamics of Workplace Development Programmes: Studies in Swedish national programmes2019Doctoral thesis, comprehensive summary (Other academic)
    Abstract [en]

    The thesis focuses on workplace development programmes (WPDPs) that operationalize national policies on workplace development. WPDPs are time-limited and they provide support to organizations that aim to improve their operational performance or employees’ work conditions. The support that such programmes provide to organizations consists of competence-development activities provided through networks, courses or hands-on coaching. The supported organizations aimed at changing work practices to increase their operational performance.

    Departing from a workplace-learning perspective, the overall aim of the thesis was to contribute to knowledge about learning in workplace development programmes and the supported organizations when realizing policies on workplace development. A qualitative multiple-case study design was used and a total of 115 interviews and notes from meetings are included in the data material. Four substudies made up the empirical base.

    The findings revealed that realizing policies on workplace development required continuous learning among stakeholders at different levels of the WPDPs. Thus, conditions that enabled learning were important throughout the whole WPDP, which was seen as a large, complex social learning system. Learning between different levels of the WPDP was especially important, which required stakeholder representatives with the appropriate qualifications or characteristics that enabled them to take on roles as brokers between the stakeholders involved in realizing the policy. The overall conclusion pointed towards the dynamics of realizing policies of workplace learning, which cannot be achieved by a one-size-fitsall model for learning.

    The findings imply that funders and other authorities that make policies on workplace development through WPDPs should scrutinize how learning among stakeholders that enter into partnership to operate WPDPs is to be facilitated. Linear plans that are not complemented with careful explanations of how such learning is to be facilitated may constitute warning signs. Furthermore, the findings imply that organizations looking for external support to develop the innovative capabilities of employees must be careful in choosing their support. A WPDP may consume the resources of an organization without providing any benefit, if it does not provide the appropriate support. On the other hand, a WPDP may be a great source for learning, particularly if it includes support in designing the change effort, and help in developing an internal support infrastructure that will continue supporting workplace development after the programme ends.

    List of papers
    1. Program steering by learning
    Open this publication in new window or tab >>Program steering by learning
    2015 (English)In: Sustainable development in organizations: studies on innovative practices / [ed] Mattias Elg, Per-Erik Ellström, Magnus Klofsten, Malin Tillmar, Cheltenham: Edward Elgar Publishing, 2015, p. 77-92Chapter in book (Refereed)
    Place, publisher, year, edition, pages
    Cheltenham: Edward Elgar Publishing, 2015
    Keywords
    Organisationsförändringar
    National Category
    Economics and Business
    Identifiers
    urn:nbn:se:liu:diva-125518 (URN)000374407200006 ()978-17-8471-688-2 (ISBN)
    Available from: 2016-02-25 Created: 2016-02-25 Last updated: 2019-02-13Bibliographically approved
    2. The role of brokers in a workplace development programme targeting SMEs
    Open this publication in new window or tab >>The role of brokers in a workplace development programme targeting SMEs
    2018 (English)In: European Journal of Training and Development, ISSN 2046-9012, E-ISSN 2046-9020Article in journal (Refereed) Epub ahead of print
    Abstract [en]

    Purpose

    The purpose of this paper is to investigate the development of a workplace development programme (WPDP) targeting small and medium sized manufacturing enterprises (SMEs) by focussing on the people who acted as brokers providing cross-boundary connections between its collaborating partners.

    Design/methodology/approach

    The material, from interviews with 32 individuals and 11 meetings, was analysed in a boundary-crossing framework, which provided tools to reveal how the roles of brokers at different levels (operative, strategic and national) of the WPDP affected its development.

    Findings

    The findings indicate that cross-boundary connections were made by persons who acted as brokers within and between different levels of the WPDP. The brokers who provided cross-boundary connections between different levels of the WPDP were found to play the most important role for the prompt development of the WPDP.

    Originality/value

    Apart from unique empirical material depicting the development of a collaborative venture between national and regional stakeholders of the manufacturing industry, the value of this study is the attention given to the people behind the policymaking of publicly funded national WPDPs, revealing the complex business of developing policy-driven competence development activities to employees in SMEs.

    Place, publisher, year, edition, pages
    Emerald Group Publishing Limited, 2018
    Keywords
    SME, Manufacturing industry, Programme development, Broker, Cross-boundary connections, Workplace development programme
    National Category
    Learning
    Identifiers
    urn:nbn:se:liu:diva-151603 (URN)10.1108/EJTD-04-2018-0037 (DOI)2-s2.0-85053333619 (Scopus ID)
    Available from: 2018-09-27 Created: 2018-09-27 Last updated: 2019-02-13Bibliographically approved
    3. Learning conditions for continuous improvement in a public service organization
    Open this publication in new window or tab >>Learning conditions for continuous improvement in a public service organization
    2018 (English)In: Journal of Workplace Learning, ISSN 1366-5626, E-ISSN 1758-7859Article in journal (Refereed) Epub ahead of print
    Abstract [en]

    Purpose – This study focuses on a transformation effort in a social welfare department of a Swedish municipality where continuous improvement, which is a Lean principle, was introduced in employees’everyday work via a workplace development programme (WPDP). The aim of this paper is to explore the conditions (internal and external) that enabled or constrained employee learning during the introduction of continuous improvement into employees’ everyday work in a WPDP-supported social welfare department.

    Design/methodology/approach – This case study is based mainly on 22 semi-structured interviews with individuals holding different positions in the department and overarching municipality.

    Findings – The findings show that multiple and emerging conditions, both internal and external, shaped a predominantly restrictive learning environment during the introduction of continuous improvement into the social welfare department. The major conditions identified were related to the initial implementation and top management’s steering and monitoring of the “Lean investment”, activities and support provided by the WPDP, activities and support provided by the internal Lean support team and first-line managers’ abilities to facilitate employee learning.

    Originality/value – Apart from unique empirical material depicting an effort towards change under conditions far from favourable for employee learning, the value of this study lies in the attention given to the external dynamics that drive change in line with the concept of new public management in public service organizations, including a WPDP that supported the social welfare department.

    Place, publisher, year, edition, pages
    Emerald Group Publishing Limited, 2018
    Keywords
    Organizational change, learning environment, public service organization, workplace development programmes
    National Category
    Learning
    Identifiers
    urn:nbn:se:liu:diva-152661 (URN)10.1108/JWL-03-2018-0049 (DOI)
    Available from: 2018-11-12 Created: 2018-11-12 Last updated: 2019-02-13Bibliographically approved
    4. Conditions for Employee Learning and Innovation: Interweaving Competence Development Activities Provided by a Workplace Development Programme with Everyday Work Activities in SMEs
    Open this publication in new window or tab >>Conditions for Employee Learning and Innovation: Interweaving Competence Development Activities Provided by a Workplace Development Programme with Everyday Work Activities in SMEs
    2018 (English)In: Vocations and Learning, ISSN 1874-785X, E-ISSN 1874-7868, Vol. 11, no 1, p. 45-63Article in journal (Refereed) Published
    Abstract [en]

    The aim of this article is to investigate how the formal competence development activities provided by the Production Leap, a workplace development programme (WPDP), were interwoven with everyday work activities and to identify the conditions that enabled learning and employee-driven innovation that contributed to production improvement, in small and medium-sized enterprises (SMEs) in Sweden. The study adopts a qualitative case approach and draws on evidence from research conducted in four manufacturing SMEs that participated in this Swedish WPDP. Funded by EU authorities, WPDPs provide competence development activities to SMEs in order to boost their production capabilities and/or promote innovation. The findings reveal that the competence development activities provided by the programme triggered learning in everyday work activities and fostered the development of different approaches to employee-driven innovation in the enterprises. The conclusion is that it is essential to consider that employee-driven innovations may take different forms and involve functions that can support innovative learning that goes beyond minor adjustments to the existing standards of production. Moreover, employee-driven innovation may impose new demands on management leadership skills. The findings provide important guidance for future WPDPs, for vocational education and training or university activities that are customised to SME contexts to promote production capabilities, and for SMEs that aim to strengthen employee-driven innovation.

    Place, publisher, year, edition, pages
    Dordrecht: Springer Netherlands, 2018
    National Category
    Learning
    Identifiers
    urn:nbn:se:liu:diva-137495 (URN)10.1007/s12186-017-9179-6 (DOI)000426597000003 ()
    Funder
    VINNOVA, 2013-02430
    Note

    Funding agencies: Swedish Agency for Innovation Systems (VINNOVA) [Vinnova 2013-02430]

    Available from: 2017-05-18 Created: 2017-05-18 Last updated: 2019-02-13Bibliographically approved