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Welcome to Functional Electronic Materials

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In the Division of Functional Electronic Materials, we conduct scientific research on electronic, magnetic and photonic semiconductor materials and nanostructures. The materials systems currently under study include novel spintronic materials, advanced electronic and photonic materials based on wide bandgap semiconductors and highly mismatched semiconductors, and semiconductor nanostructures.

The research is carried out mostly through a close collaboration with many groups worldwide. Our aim is to obtain a better understanding of fundamental physical properties and a good control of materials properties, and to fully explore functionality of the studied materials for applications in future generation micro- and nano-electronics and photonics as well as in potential multifunctional devices and systems.

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Optimizing GaNP Coaxial Nanowires for Efficient Light Emission by Controlling Formation of Surface and Interfacial Defects

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Jan E. Stehr,  Alexander Dobrovolsky,  Supanee Sukrittanon,  Yanjin Kuang,  Charles W. Tu ,Weimin M. Chen, and  Irina A. Buyanova

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Published in Nano Lett., Article ASAP DOI: 10.1021/nl503454s

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We report on identification and control of important nonradiative recombination centers in GaNP coaxial nanowires (NWs) grown on Si substrates in an effort to significantly increase light emitting efficiency of these novel nanostructures promising for a wide variety of optoelectronic and photonic applications. A point defect complex, labeled as DD1 and consisting of a P atom with a neighboring partner aligned along a crystallographic ⟨111⟩ axis, is identified by optically detected magnetic resonance as a dominant nonradiative recombination center that resides mainly on the surface of the NWs and partly at the heterointerfaces. The formation of DD1 is found to be promoted by the presence of nitrogen and can be suppressed by reducing the strain between the core and shell layers, as well as by protecting the optically active shell by an outer passivating shell. Growth modes employed during the NW growth are shown to play a role. On the basis of these results, we identify the GaP/GaNyP1–y/GaNxP1–x (x < y) core/shell/shell NW structure, where the GaNyP1–y inner shell with the highest nitrogen content serves as an active light-emitting layer, as the optimized and promising design for efficient light emitters based on GaNP NWs.

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On the origin of strong photoluminescence polarization in GaNP nanowires

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S. Filippov, S. Sukrittanon, Y. Kuang, C. W. Tu, Per O. Å. Persson, W. M. Chen, and I. A. Buyanova

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Published in Nano Lett. (2014). dx.doi.org/10.1021/nl502281p

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III-V semiconductor nanowires (NWs) have a great potential for applications in a variety of future electronic and photonic devices with enhanced functionality. In this work, we employ polarization resolved micro-photoluminescence (-PL) spectroscopy to study polarization properties of light emissions from individual GaNP and GaP/GaNP core/shell nanowires (NWs) with average diameters ranging between 100 and 350 nm. We show that the near-band-edge emission, which originates from the GaNP regions of the NWs, is strongly polarized (up to 60 % at 150 K) in the direction perpendicular to the NW axis. The polarization anisotropy can be retained up to room temperature. This polarization behavior, which is unusual for zinc blende NWs, is attributed to local strain in the vicinity of the N-related centers participating in the radiative recombination and to preferential alignment of their principal axis along the growth direction. Our findings therefore show that defect engineering via alloying with nitrogen provides an additional degree of freedom to tailor the polarization anisotropy of III-V nanowires, advantageous for their applications as nanoscale emitters of polarized light.

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Zinc-Vacancy–Donor Complex: A Crucial Compensating Acceptor in ZnO

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J. E. Stehr, K. M. Johansen, T. S. Bjørheim, L. Vines, B. G. Svensson, W. M. Chen, and I. A. Buyanova

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Published as a Letter in Phys. Rev. Applied 2, 021001 (2014)

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The aluminum–zinc-vacancy (AlZn−VZn) complex is identified as one of the dominant defects in Al-containing n-type ZnO after electron irradiation at room temperature with energies above 0.8 MeV. The complex is energetically favorable over the isolated VZn, binding more than 90% of the stable VZn’s generated by the irradiation. It acts as a deep acceptor with the (0/−) energy level located at approximately 1 eV above the valence band. Such a complex is concluded to be a defect of crucial and general importance that limits the n-type doping efficiency by complex formation with donors, thereby literally removing the donors, as well as by charge compensation.

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Trap-Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics

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Q. Bao, O. Sandberg, D. Dagnelund, S. Sandén, S. Braun, H. Aarnio, X. Liu, W. M. Chen, R. Österbacka and M. Fahlman

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Published in Adv. Funct. Mater. (2014). doi: 10.1002/adfm.201401513

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Organic photovoltaics are under intense development and significant focus has been placed on tuning the donor ionization potential and acceptor electron affinity to optimize open circuit voltage. Here, it is shown that for a series of regioregular-poly(3-hexylthiophene):fullerene bulk heterojunction (BHJ) organic photovoltaic devices with pinned electrodes, integer charge transfer states present in the dark and created as a consequence of Fermi level equilibrium at BHJ have a profound effect on open circuit voltage. The integer charge transfer state formation causes vacuum level misalignment that yields a roughly constant effective donor ionization potential to acceptor electron affinity energy difference at the donor–acceptor interface, even though there is a large variation in electron affinity for the fullerene series. The large variation in open circuit voltage for the corresponding device series instead is found to be a consequence of trap-assisted recombination via integer charge transfer states. Based on the results, novel design rules for optimizing open circuit voltage and performance of organic bulk heterojunction solar cells are proposed.

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Energy Upconversion in GaP/GaNP Core/Shell Nanowires for Enhanced Near-Infrared Light Harvesting

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A. Dobrovolsky, S. Sukrittanon, Y. Kuang, C.W. Tu, W. M. Chen, and I. A. Buyanova

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Published in Small. doi: 10.1002/smll.201401342 (2014)

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Semiconductor nanowires (NWs) have recently gained increasing interest due to their great potential for photovoltaics. A novel material system based on GaNP NWs is considered to be highly suitable for applications in efficient multi-junction and intermediate band solar cells. This work shows that though the bandgap energies of GaNxP1-x alloys lie within the visible spectral range (i.e., within 540–650 nm for the currently achievable x < 3%), coaxial GaNP NWs grown on Si substrates can also harvest infrared light utilizing energy upconversion. This energy upconversion can be monitored via anti-Stokes near-band-edge photoluminescence (PL) from GaNP, visible even from a single NW. The dominant process responsible for this effect is identified as being due to two-step two-photon absorption (TS-TPA) via a deep level lying at about 1.28 eV above the valence band, based on the measured dependences of the anti-Stokes PL on excitation power and wavelength. The formation of the defect participating in the TS-TPA process is concluded to be promoted by nitrogen incorporation. The revealed defect-mediated TS-TPA process can boost efficiency of harvesting solar energy in GaNP NWs, beneficial for applications of this novel material system in third-generation photovoltaic devices.

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Turning ZnO into an Efficient Energy Upconversion Material by Defect Engineering

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Jan E. Stehr, Shula L. Chen, Nandanapalli Koteeswara Reddy, Charles W. Tu, Weimin M. Chen and Irina A. Buyanova

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Published in Advanced Functional Materials DOI: 10.1002/adfm.201400220

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Photon upconversion materials are attractive for a wide range of applications from medicine, biology, to photonics. Among them, ZnO is of particular interest owing to its outstanding combination of materials and physical properties. Though energy upconversion has been demonstrated in ZnO, the exact physical mechanism is still unknown, preventing control of the processes. Here, defects formed in bulk and nanostructured ZnO synthesized using standard growth techniques play a key role in promoting efficient energy upconversion via two-step two-photon absorption (TS-TPA). From photoluminescence excitation of the anti-Stokes emissions, the threshold energy of the TS-TPA process is determined as being 2.10–2.14 eV in all studied ZnO materials irrespective of the employed growth techniques. This photo-electron paramagnetic resonance studies show that this threshold closely matches the ionization energy of the zinc vacancy (a common grown-in intrinsic defect in ZnO), thereby identifying the zinc vacancy as being the dominant defect responsible for the observed efficient energy upconversion. The upconversion is found to persist even at a low excitation density, making it attractive for photonic and photovoltaic applications.

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Semi-metallic polymers

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Olga Bubnova, Zia Ullah Khan, Hui Wang, Slawomir Braun, Drew R. Evans, Manrico Fabretto, Pejman Hojati-Talemi, Daniel Dagnelund, Jean-Baptiste Arlin, Yves H. Geerts, Simon Desbief,   Dag W. Breiby, Jens W. Andreasen, Roberto Lazzaroni, Weimin M. Chen, Igor Zozoulenko, Mats Fahlman, Peter J. Murphy, Magnus Berggren & Xavier Crispin

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Published in Nature Materials 13, 190–194 (2014) doi:10.1038/nmat3824

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Polymers are lightweight, flexible, solution-processable materials that are promising for low-cost printed electronics as well as for mass-produced and large-area applications. Previous studies demonstrated that they can possess insulating, semiconducting or metallic properties; here we report that polymers can also be semi-metallic. Semi-metals, exemplified by bismuth, graphite and telluride alloys, have no energy bandgap and a very low density of states at the Fermi level. Furthermore, they typically have a higher Seebeck coefficient and lower thermal conductivities compared with metals, thus being suitable for thermoelectric applications.We measure the thermoelectric properties of various poly(3,4-ethylenedioxythiophene) samples, and observe a marked increase in the Seebeck coefficient when the electrical conductivity is enhanced through molecular organization. This initiates the transition from a Fermi glass to a semi-metal. The high Seebeck value, the metallic conductivity at room temperature and the absence of unpaired electron spins makes polymer semi-metals attractive for thermoelectrics and spintronics.

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Media interest:

” Polymers can be semimetals”, Monica Westman Svenselius, LiU news, 2013-12-09.

” Polymerer kan vara halvmetaller”, Monica Westman Svenselius, LiU news, 2013-12-09.

” Polymers can be semimetals”, Phys Org, Dec.9 2013.

” Polymers can behave like insulators, semiconductors and metals -- as well as semimetals”, ScienceDaily, Dec.9 2013.

”Polymers can be semimetals”, Materialstoday, 11 Dec 2013.


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Efficient room-temperature nuclear spin hyperpolarization of defect atom in a semiconductor

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Yuttapoom Puttisong, Xingjun Wang, Irina A. Buyanova, L. Geelhaar, H. Riechert, A. J. Ptak, C. W. Tu and Weimin. M. Chen

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Published in Nature Communications DOI: 10.1038/ncomms2776

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Nuclear spin hyperpolarization is essential to future solid-state quantum computation using nuclear spin qubits and in highly sensitive magnetic resonance imaging. Though efficient dynamic nuclear polarization in semiconductors has been demonstrated at low temperatures for decades, its realization at room temperature is largely lacking. Here we demonstrate that a combined effect of efficient spin-dependent recombination and hyperfine coupling can facilitate strong dynamic nuclear polarization of a defect atom in a semiconductor at room temperature. We provide direct evidence that a sizeable nuclear field (~150 Gauss) and nuclear spin polarization (~15%) sensed by conduction electrons in GaNAs originates from dynamic nuclear polarization of a Ga interstitial defect. We further show that the dynamic nuclear polarization process is remarkably fast and is completed in <5 μs at room temperature. The proposed new concept could pave a way to overcome a major obstacle in achieving strong dynamic nuclear polarization at room temperature, desirable for practical device applications.

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Media interest:

“Ett steg närmare kvantdatorn”, by Monica Westman Svenselius, LiU Nyheter, 2013-04-26

“One step closer to a quantum computer”, by Monica Westman Svenselius, LiU Nyheter, 2013-04-26

“One Step Closer to a Quantum Computer”, Science Daily, April 30 2013

“Success in initializing and reading nuclear spins brings quantum computer a step closer”, PhysOrg, April 30 2013.


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Room-temperature electron spin amplifier based on Ga(In)NAs alloys

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Yuttapoom Puttisong, Irina A. Buyanova, A. J. Ptak, C. W. Tu, L. Geelhaar, H. Riechert and Weimin M. Chen

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Published in Advanced Materials DOI: 10.1002/adma.201202597

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The first experimental demonstration of a spin amplifier at room temperature is presented. An efficient, defect-enabled spin amplifier based on a non-magnetic semiconductor, Ga(In)NAs, is proposed and demonstrated, with a large spin gain (up to 2700% at zero field) for conduction electrons and a high cut-off frequency up to 1 GHz.

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Media interest:

“Viktigt framsteg inom spinntronik”, by Åke Hjelm, LiU Nyheter, Nov.15 2012

“Important advance for spintronics”, by Åke Hjelm, LiU Nyheter, Nov.15 2012

“Första spinnförstärkaren i rumstemperatur”, by Charlotta von Schultz, NyTeknik, 14 november 2012.

”Genombrott: spinnförstärkare i rumstemperatur”, by Jan Tångring, Elektronik Tidningen, Nov.14 2012

“Important Progress for Spintronics: Spin Amplifier Works at Room Temperature”, ScienceDaily (Nov. 16, 2012)

“Important progress for spintronics: A spin amplifier to be used in room temperature”, PhysOrg (Nov.16 201)


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Handbook of Spintronic Semiconductors

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edited by Weimin M Chen and Irina A Buyanova

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Published in Spring 2010 by Pan Stanford Publishing. 400 pages (approx.) ISBN 978-981-4267-36-6

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Room-temperature defect-engineered spin filter based on a non-magnetic semiconductor

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Xingjun J. Wang, Irina A. Buyanova, F. Zhao, D. Lagarde, A. Balocchi, X. Marie, C. W. Tu, J. C. Harmand and Weimin M. Chen

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Published in Nature Materials 8, 198 (2009).

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LiU E-Press offers a free parallel publication.

En översikt på svenska finns i LiUs nyhets-arkiv.

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Media interest:

“Filter gör spinntroniken användbar”, by Hanna Meerveld, Naturvetarna, May 4 2009

“Elektroner spinner i takt i Linköping”, by Anna Wennberg, Elektronik Tidningen, Feb.16 2009

“Stort framsteg för spinntroniken”, by Åke Hjelm, LiU Nyheter, Feb.17 2009

“Great progress for spintronics”, Swedish Research, March 4, 2009


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Responsible for this page: Stanislav Filippov, Yuttapoom Puttisong
Last updated: 12/09/14