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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.

Defects in Advanced Electronic Materials and Novel Low-Dimensional Structures

edited by Jan Stehr Irina Buyanova Weimin Chen

Published in June 2018 by Woodhead Publishing. 306 pages (approx.) ISBN 978-008-1020-53-1


Measurements of Strain and Bandgap of Coherently Epitaxially Grown Wurtzite InAsP–InP Core–Shell Nanowires

D. J. O. Göransson, M. T. Borgström , Y. Q. Huang, M. E. Messing, D. Hessman, I. A. Buyanova, W. M. Chen, and H. Q. Xu

We report on experimental determination of the strain and bandgap of InAsP in epitaxially grown InAsP–InP core–shell nanowires. The core–shell nanowires are grown via metal–organic vapor phase epitaxy. The as-grown nanowires are characterized by transmission electron microscopy, X-ray diffraction, micro-photoluminescence (μPL) spectroscopy, and micro-Raman (μ-Raman) spectroscopy measurements. We observe that the core–shell nanowires are of wurtzite (WZ) crystal phase and are coherently strained with the core and the shell having the same number of atomic planes in each nanowire. We determine the predominantly uniaxial strains formed in the core–shell nanowires along the nanowire growth axis and demonstrate that the strains can be described using an analytical expression. The bandgap energies in the strained WZ InAsP core materials are extracted from the μPL measurements of individual core–shell nanowires. The coherently strained core–shell nanowires demonstrated in this work offer the potentials for use in constructing novel optoelectronic devices and for development of piezoelectric photovoltaic devices.

Near-Infrared Lasing at 1 μm from a Dilute-Nitride-Based Multishell Nanowire

Shula Chen, Mitsuki Yukimune, Ryo Fujiwara, Fumitaro Ishikawa, Weimin M. Chen, and Irina A. Buyanova

A coherent photon source emitting at near-infrared (NIR) wavelengths is at the heart of a wide variety of applications ranging from telecommunications and optical gas sensing to biological imaging and metrology. NIR-emitting semiconductor nanowires (NWs), acting both as a miniaturized optical resonator and as a photonic gain medium, are among the best-suited nanomaterials to achieve such goals. In this study, we demonstrate the NIR lasing at 1 μm from GaAs/GaNAs/GaAs core/shell/cap dilute nitride nanowires with only 2.5% nitrogen. The achieved lasing is characterized by an S-shape pump-power dependence and narrowing of the emission line width. Through examining the lasing performance from a set of different single NWs, a threshold gain, gth, of 4100–4800 cm–1, was derived with a spontaneous emission coupling factor, β, up to 0.8, which demonstrates the great potential of such nanophotonic material. The lasing mode was found to arise from the fundamental HE11a mode of the Fabry–Perot cavity from a single NW, exhibiting optical polarization along the NW axis. Based on temperature dependence of the lasing emission, a high characteristic temperature, T0, of 160 (±10) K is estimated. Our results, therefore, demonstrate a promising alternative route to achieve room-temperature NIR NW lasers thanks to the excellent alloy tunability and superior optical performance of such dilute nitride materials.

Design rules for minimizing voltage losses in high-efficiency organic solar cells

Qian D, Zheng Z, Yao H, Tress W, Hopper TR, Chen S, Li S, Liu J, Chen S, Zhang J, Liu XK, Gao B, Ouyang L, Jin Y, Pozina G, Buyanova IA, Chen WM, Inganäs O, Coropceanu V, Bredas JL, Yan H, Hou J, Zhang F, Bakulin AA, Gao F.

The open-circuit voltage of organic solar cells is usually lower than the values achieved in inorganic or perovskite photovoltaic devices with comparable bandgaps. Energy losses during charge separation at the donor–acceptor interface and non-radiative recombination are among the main causes of such voltage losses. Here we combine spectroscopic and quantum-chemistry approaches to identify key rules for minimizing voltage losses: (1) a low energy offset between donor and acceptor molecular states and (2) high photoluminescence yield of the low-gap material in the blend. Following these rules, we present a range of existing and new donor–acceptor systems that combine efficient photocurrent generation with electroluminescence yield up to 0.03%, leading to non-radiative voltage losses as small as 0.21 V. This study provides a rationale to explain and further improve the performance of recently demonstrated high-open-circuit-voltage organic solar cells.

Room-temperature polarized spin-photon interface based on a semiconductor nanodisk-in-nanopillar structure driven by few defects

Shula Chen, Yuqing Huang, Dennis Visser, Srinivasan Anand, Irina A. Buyanova, Weimin M. Chen 

Owing to their superior optical properties, semiconductor nanopillars/nanowires in one-dimensional (1D) geometry are building blocks for nano-photonics. They also hold potential for efficient polarized spin-light conversion in future spin nano-photonics. Unfortunately, spin generation in 1D systems so far remains inefficient at room temperature. Here we propose an approach that can significantly enhance the radiative efficiency of the electrons with the desired spin while suppressing that with the unwanted spin, which simultaneously ensures strong spin and light polarization. We demonstrate high optical polarization of 20%, inferring high electron spin polarization up to 60% at room temperature in a 1D system based on a GaNAs nanodisk-in-GaAs nanopillar structure, facilitated by spin-dependent recombination via merely 2–3 defects in each nanodisk. Our approach points to a promising direction for realization of an interface for efficient spin-photon quantum information transfer at room temperature—a key element for future spin-photonic applications.

Spin injection and helicity control of surface spin photocurrent in a three dimensional topological insulator

Yuqing Huang, Y. X. Song, S. M. Wang, Irina A. Buyanova, Weimin M. Chen

A three-dimensional (3D) topological insulator (TI) is a unique quantum phase of matter with exotic physical properties and promising spintronic applications. However, surface spin current in a common 3D TI remains difficult to control and the out-of-plane spin texture is largely unexplored. Here, by means of surface spin photocurrent in Bi2Te3 TI devices driven by circular polarized light, we identify the subtle effect of the spin texture of the topological surface state including the hexagonal warping term on the surface current. By exploring the out-of-plane spin texture, we demonstrate spin injection from GaAs to TI and its significant contribution to the surface current, which can be manipulated by an external magnetic field. These discoveries pave the way to not only intriguing new physics but also enriched spin functionalities by integrating TI with conventional semiconductors, such that spin-enabled optoelectronic devices may be fabricated in such hybrid structures.

Dilute Nitride Nanowire Lasers Based on a GaAs/GaNAs Core/Shell Structure

Shula Chen, Mattias Jansson, Jan E. Stehr, Yuqing Huang, Fumitaro Ishikawa, Weimin M. Chen, Irina A. Buyanova

Nanowire (NW) lasers operating in the near-infrared spectral range are of significant technological importance for applications in telecommunications, sensing and medical diagnostics. So far lasing within this spectral range has been achieved using GaAs/AlGaAs, GaAs/GaAsP and InGaAs/GaAs core/shell NWs. Another promising III-V material, not yet explored in its lasing capacity, is the dilute nitride GaNAs. In this work we demonstrate, for the first time, optically pumped lasing from the GaNAs shell of a single GaAs/GaNAs core/shell NW. The characteristic ‘S’-shaped pump power dependence of the lasing intensity, with the concomitant line width narrowing, is observed, which yields a threshold gain, g_th, of 3300 cm-1 and a spontaneous emission coupling factor, β, of 0.045. The dominant lasing peak is identified to arise from the HE21b cavity mode, as determined from its pronounced emission polarization along the NW axis combined with theoretical calculations of lasing threshold for guided modes inside the nanowire. Even without intentional passivation of the NW surface, the lasing emission can be sustained up to 150 K. This is facilitated by the improved surface quality due to nitrogen incorporation, which partly suppresses the surface-related non-radiative recombination centers via nitridation. Our work therefore represents the first step towards development of room temperature infrared NW lasers based on dilute nitrides with extended tunability in the lasing wavelength.

Strongly polarized quantum-dot-like light emitters embedded in GaAs/GaNAs core/shell nanowires

Stanisalv Filippov, Mattias Jansson, Jan E. Stehr, Justinas Palisaitis, Per O. Å. Persson, Fumitaro Ishikawa, Weimin M. Chen, Irina A. Buyanova

Recent developments in fabrication techniques and extensive investigations of the physical properties of III–V semiconductor nanowires (NWs), such as GaAs NWs, have demonstrated their potential for a multitude of advanced electronic and photonics applications. Alloying of GaAs with nitrogen can further enhance the performance and extend the device functionality via intentional defects and heterostructure engineering in GaNAs and GaAs/GaNAs coaxial NWs. In this work, it is shown that incorporation of nitrogen in GaAs NWs leads to formation of three-dimensional confining potentials caused by short-range fluctuations in the nitrogen composition, which are superimposed on long-range alloy disorder. The resulting localized states exhibit a quantum-dot like electronic structure, forming optically active states in the GaNAs shell. By directly correlating the structural and optical properties of individual NWs, it is also shown that formation of the localized states is efficient in pure zinc-blende wires and is further facilitated by structural polymorphism. The light emission from these localized states is found to be spectrally narrow (∼50–130 μeV) and is highly polarized (up to 100%) with the preferable polarization direction orthogonal to the NW axis, suggesting a preferential orientation of the localization potential. These properties of self-assembled nano-emitters embedded in the GaNAs-based nanowire structures may be attractive for potential optoelectronic applications.

Understanding and optimizing spin injection in self-assembled InAs/GaAs quantum-dot molecular structures

Yuqing Huang, Yuttapoom Puttisong, Irina A. Buyanova, Weimin M. Chen

Semiconductor quantum-dot (QD) structures are promising for spintronic applications owing to their strong quenching of spin relaxation processes that are promoted by carrier and exciton motions. Unfortunately, the spin injection efficiency in such nanostructures is very low and the exact physical mechanism of the spin loss is still not fully understood. Here, we show that exciton spin injection in self-assembled InAs/GaAs QDs and QD molecular structures (QMSs) is dominated by localized excitons confined within the QD-like regions of the wetting layer (WL) and GaAs barrier layer that immediately surround the QDs and QMSs. These localized excitons in fact lack the commonly believed 2D and 3D character with an extended wavefunction. We attribute the microscopic origin of the severe spin loss observed during spin injection to a sizable anisotropic exchange interaction (AEI) of the localized excitons in the WL and GaAs barrier layer, which has so far been overlooked. We determined that the AEI of the injected excitons and, thus, the efficiency of the spin injection processes are correlated with the overall geometric symmetry of the QMSs. This symmetry largely defines the anisotropy of the confinement potential of the localized excitons in the surrounding WL and GaAs barrier. These results pave the way for a better understanding of spin injection processes and the microscopic origin of spin loss in QD structures. Furthermore, they provide a useful guideline to significantly improve spin injection efficiency by optimizing the lateral arrangement of QMSs and overcome a major challenge in spintronic device applications utilizing semiconductor QDs.

Fabry–Perot Microcavity Modes in Single GaP/GaNP Core/Shell Nanowires

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

Semiconductor nanowires (NWs) are attracting increasing interest as nanobuilding blocks for optoelectronics and photonics. A novel material system that is highly suitable for these applications are GaNP NWs. In this article, we show that individual GaP/GaNP core/shell nanowires (NWs) grown by molecular beam epitaxy on Si substrates can act as Fabry-Perot (FP) microcavities. This conclusion is based on results of microphotoluminescence (μ-PL) measurements performed on individual NWs, which reveal periodic undulations of the PL intensity that follow an expected pattern of FP cavity modes. The cavity is concluded to be formed along the NW axis with the end facets acting as reflecting mirrors. The formation of the FP modes is shown to be facilitated by an increasing index contrast with the surrounding media. Spectral dependence of the group refractive index is also determined for the studied NWs. The observation of the FP microcavity modes in the GaP/GaNP core/shell NWs can be considered as a first step toward achieving lasing in this quasidirect bandgap semiconductor in the NW geometry.

Effects of Polytypism on Optical Properties and Band Structure of Individual Ga(N)P Nanowires from Correlative Spatially Resolved Structural and Optical Studies

A. Dobrovolsky, P. O. Å. Persson, S. Sukrittanon, Y. Kuang, C. W. Tu, W.  M. Chen, I. A. Buyanova

III–V semiconductor nanowires (NWs) have gained significant interest as building blocks in novel nanoscale devices. The one-dimensional (1D) nanostructure architecture allows one to extend band structure engineering beyond quantum confinement effects by utilizing formation of different crystal phases that are thermodynamically unfavorable in bulk materials. It is therefore of crucial importance to understand the influence of variations in the NWs crystal structure on their fundamental physical properties. In this work we investigate effects of structural polytypism on the optical properties of gallium phosphide and GaP/GaNP core/shell NW structures by a correlative investigation on the structural and optical properties of individual NWs. The former is monitored by transmission electron microscopy, whereas the latter is studied via cathodoluminescence (CL) mapping. It is found that structural defects, such as rotational twins in zinc blende (ZB) GaNP, have detrimental effects on light emission intensity at low temperatures by promoting nonradiative recombination processes. On the other hand, formation of the wurtzite (WZ) phase does not notably affect the CL intensity neither in GaP nor in the GaNP alloy. This suggests that zone folding in WZ GaP does not enhance its radiative efficiency, consistent with theoretical predictions. We also show that the change in the lattice structure have negligible effects on the bandgap energies of the GaNP alloys, at least within the range of the investigated nitrogen compositions of <2%. Both WZ and ZB GaNP are found to have a significantly higher efficiency of radiative recombination as compared with that in parental GaP, promising for potential applications of GaNP NWs as efficient nanoscale light emitters within the desirable amber-red spectral range.

Exciton Fine-Structure Splitting in Self-Assembled Lateral InAs/GaAs Quantum-Dot Molecular Structures

Stanislav Fillipov,  Yuttapoom Puttisong,  Yuqing Huang,  Irina A. Buyanova,  Suwaree Suraprapapich,  Charles W. Tu and Weimin M. Chen

Fine-structure splitting (FSS) of excitons in semiconductor nanostructures is a key parameter that has significant implications in photon entanglement and polarization conversion between electron spins and photons, relevant to quantum information technology and spintronics. Here, we investigate exciton FSS in self-organized lateral InAs/GaAs quantum-dot molecular structures (QMSs) including laterally-aligned double quantum dots (DQDs), quantum-dot clusters (QCs) and quantum rings (QRs), by employing polarization-resolved micro-photoluminescence (µPL) spectroscopy. We find a clear trend in FSS between the studied QMSs depending on their geometric arrangements, from a large FSS in the DQDs to a smaller FSS in the QCs and QRs. This trend is accompanied by a corresponding difference in the optical polarization directions of the excitons between these QMSs, namely, the bright-exciton lines are linearly polarized preferably along or perpendicular to the [11 ̅0] crystallographic axis in the DQDs that also defines the alignment direction of the two constituting QDs, whereas in the QCs and QRs the polarization directions are randomly oriented. We attribute the observed trend in the FSS to a significant reduction of the asymmetry in the lateral confinement potential of the excitons in the QRs and QCs as compared with the DQDs, as a result of a compensation between the effects of lateral shape anisotropy and piezoelectric field. Our work demonstrates that FSS strongly depends on the geometric arrangements of the QMSs, which effectively tune the degree of the compensation effects and are capable of reducing FSS even in a strained QD system to a limit similar to strain-free QDs. This approach provides a pathway in obtaining high-symmetry quantum emitters desirable for realizing photon entanglement and spintronic devices based on such nanostructures, utilizing an uninterrupted epitaxial growth procedure without special requirements for lattice-matched materials combinations, specific substrate orientations and nanolithography.

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

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

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.

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

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

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

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

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.

Semi-metallic polymers

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

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

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

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 978-981-4267-36-6


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


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Last updated: 04/09/19