Welcome to Functional Electronic Materials
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.
On the origin of strong photoluminescence polarization in GaNP nanowires
S. Filippov, S. Sukrittanon, Y. Kuang, C. W. Tu, Per O. Å. Persson, W. M. Chen, and I. A. Buyanova
|Published in Nano Lett. (2014). dx.doi.org/10.1021/nl502281p|
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.
Zinc-Vacancy–Donor Complex: A Crucial Compensating Acceptor in ZnO
J. E. Stehr, K. M. Johansen, T. S. Bjørheim, L. Vines, B. G. Svensson, W. M. Chen, and I. A. Buyanova
|Published as a Letter in Phys. Rev. Applied 2, 021001 (2014)|
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.
Trap-Assisted Recombination via Integer Charge Transfer States in Organic Bulk Heterojunction Photovoltaics
Q. Bao, O. Sandberg, D. Dagnelund, S. Sandén, S. Braun, H. Aarnio, X. Liu, W. M. Chen, R. Österbacka and M. Fahlman
|Published in Adv. Funct. Mater. (2014). doi: 10.1002/adfm.201401513|
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.
Energy Upconversion in GaP/GaNP Core/Shell Nanowires for Enhanced Near-Infrared Light Harvesting
A. Dobrovolsky, S. Sukrittanon, Y. Kuang, C.W. Tu, W. M. Chen, and I. A. Buyanova
|Published in Small. doi: 10.1002/smll.201401342 (2014)|
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.
Turning ZnO into an Efficient Energy Upconversion Material by Defect Engineering
Jan E. Stehr, Shula L. Chen, Nandanapalli Koteeswara Reddy, Charles W. Tu, Weimin M. Chen and Irina A. Buyanova
|Published in Advanced Functional Materials DOI: 10.1002/adfm.201400220|
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.
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
|Published in Nature Materials 13, 190–194 (2014) doi:10.1038/nmat3824|
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.
Efficient room-temperature nuclear spin hyperpolarization of defect atom in a semiconductor
Yuttapoom Puttisong, Xingjun Wang, Irina A. Buyanova, L. Geelhaar, H. Riechert, A. J. Ptak, C. W. Tu and Weimin. M. Chen
|Published in Nature Communications DOI: 10.1038/ncomms2776|
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.
Room-temperature electron spin amplifier based on Ga(In)NAs alloys
Yuttapoom Puttisong, Irina A. Buyanova, A. J. Ptak, C. W. Tu, L. Geelhaar, H. Riechert and Weimin M. Chen
|Published in Advanced Materials DOI: 10.1002/adma.201202597|
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.
Handbook of Spintronic Semiconductors
edited by Weimin M Chen and Irina A Buyanova
Published in Spring 2010 by Pan Stanford Publishing. 400 pages (approx.) ISBN 978-981-4267-36-6More...
Room-temperature defect-engineered spin filter based on a non-magnetic semiconductor
Xingjun J. Wang, Irina A. Buyanova, F. Zhao, D. Lagarde, A. Balocchi, X. Marie, C. W. Tu, J. C. Harmand and Weimin M. Chen
Published in Nature Materials 8, 198 (2009).
free parallel publication.
En översikt på svenska finns i LiUs nyhets-arkiv.
Effect of Ni-coating on ZnO nanowires: A raman scattering study
Stanislav Filippov, Xingjun Wang, M. Devika, N. Koteeswara Reddy, C. W. Tu, Weimin. M. Chen and Irina. A. Buyanova.
|Published in J. Appl. Phys. 113, 214302 (2013)|
Optical properties of GaP/GaNP core/shell nanowires: A temperature-dependent study
Alexander Dobrovolsky, Shula Chen, Yanjin Kuang,Supanee Sukrittanon, Charles W Tu, Weimin M Chen and irina Buyanova
|Published in Nanoscale Research Letters 2013,8:239|
Dynamics of donor bound excitons in ZnO
Shula Chen, Weimin M. Chen and Irina A. Buyanova
|Published in Appl. Phys. Lett. 102, 121103 (2013)|
Defects in N, O and N, Zn implanted ZnO bulk crystals
|Published in J. Appl. Phys. 113, 103509 (2013)|
Effect of hyperfine-induced spin mixing on the defect-enabled spin blockade and spin filtering in GaNAs
Yuttapoom Puttisong, Xingjun Wang, Irina A. Buyanova and Weimin M. Chen
|Published in Phys. Rev. B 87, 125202 (2013)|
Optically detected magnetic resonance studies of point defects in quaternary GaNAsP epilayers grown by vapor phase epitaxy
|Published in Appl. Phys. Lett. 102, 021910 (2013)|
Zeeman splitting and dynamics of an isoelectronic bound exciton near the band edge of ZnO
The first detailed study of an exciton bound to an isoelectronic center
Shula Chen, Weimin M. Chen and Irina A. Buyanova
|Published in Phys. Rev. B 86, 235205 (2012)|
Defect properties of ZnO nanowires revealed from an optically detected magnetic resonance study
Jan E. Stehr, Shula Chen, Stanislav Filippov, M. Devika, N. Koteeswara Reddy, C. W. Tu, Weimin M. Chen and Irina A. Buyanova
|Published in Nanotechnology 24, 015701 (2013)|
Sub-millisecond dynamic nuclear spin hyperpolarization in a semiconductor: A case study from PIn antisite in InP
Xingjun Wang, Irina A. Buyanova and Weimin Chen
|Published in Phys. Rev. B 86, 205202 (2012)|
Mechanism for radiative recombination and defect properties of GaP/GaNP core/shell nanowires
|Published in Appl. Phys. Lett. 101, 163106 (2012)|
Evidence for coupling between exciton emissions and surface plasmon in Ni-coated ZnO nanowires
Qejun Ren, Stanislav Filippov, Shula Chen, M. Devika, N. Koteeswara Reddy, C. W. Tu, Weimin M. Chen and Irina A. Buyanova
|Published in Nanotechnology 23 425201 (2012)|
Surface plasmons are used to enhance light emission from ZnO/transition metal core/shell nanowires
Efficient upconversion of photoluminescence via two-photon absorption in bulk and nanorod ZnO
Shula Chen, Jan Stehr, N. Koteeswara Reddy, C. W. Tu, Weimin M. Chen and Irina A. Buyanova
|Published in Applied Physics B: Lasers and Optics 2012, DOI: 10.1007/s00340-012-5138-y|
Deeper understanding of different up-conversion mechanisms in ZnO
Long delays of light in ZnO caused by exciton-polariton propagation
Shula chen, Weimin M. Chen and Irina A. Buyanova
|Published in Physica Status Solidi (b) 249, 1307 (2012)|
This article has been selected as Editor Choice in Physica Status Solidi b : basic solid state physics with a back cover page of this journal in volume 249, issue 7, July 2012Abstract...
Antiferromagnetic interaction in coupled CdSe/ZnMnSe quantum dot structures
Daniel Dagnelund, Q. J. Ren, Irina A. Buyanova, A. Murayama, and Weimin M. Chen
|Published in Applied Physics Letters 101, 052405 (2012)|
Uncover the exact mechanism for magnetic coupling in QD structures
Temperature dependence of dynamic nuclear polarization and its effect on electron spin relaxation and dephasing in InAs/GaAs quantum dots
Jan Beyer, Yuttapoom Puttisong, Irina A. Buyanova, S. Suraprapapich, C. W. Tu, and Weimin M. Chen
|Published in Applied Physics Letters 100, 143105 (2012)|
Towards identification of the dominant electron spin relaxation process at elevated temperatures
The Hanle effect and electron spin polarization in InAs/GaAs quantum dots up to room temperature
Jan Beyer , Irina A. Buyanova , S. Suraprapapich , C. W. Tu and Weimin M Chen
|Published in Nanotechnology 23, 135705 (2012)|
What determine spin polarization at room temperature
Effects of a longitudinal magnetic field on spin injection and detection in InAs/GaAs quantum dot structures
Jan Beyer , P. H. Wang , Irina A. Buyanova , S. Suraprapapich , C. W. Tu and Weimin M. Chen
|Published in Journal of Physics: Condensed Matter 24, 145304 (2012)|
Revealing the factors limiting spin injection and spin detection efficiency
This paper has been selected as an invited IOP journal highlights article in 2012
Effects of P implantation and post-implantation annealing on defect formation in ZnO
Xingjun Wang, Weimin M. Chen, F. Ren, S. Pearton, and Irina A. Buyanova
|Published in Journal of Applied Physics 111, 043520 (2012)|
Identification of dominant defects and their role in materials properties
Efficient room-temperature spin detector based on GaNAs
Yuttapoom Puttisong, Irina A. Buyanova, L. Geelhaar, H. Riechert, C. W. Tu, and Weimin M. Chen
|Published in Journal of Applied Physics 111, 07C303 (2012)|
Most efficient room-temperature spin detector to date
Effects of hydrogenation on non-radiative defects in GaNP and GaNAs alloys: An optically detected magnetic resonance study
Daniel Dagnelund, I. P. Vorona, G. Nosenko, Xingjun Wang, C. W. Tu, H. Yonezu, A. Polimeni, M. Capizzi, Weimin M. Chen, and Irina A Buyanova
|Published in Journal of Applied Physics 111, 023501 (2012)|
Remarkable and surprising difference revealed between GaNP and GaNAs
Responsible for this page:
Stanislav Filippov, Yuttapoom Puttisong
Last updated: 09/03/14