Suchergebnisse

We theoretically study helicity-dependent photocurrent in a three-dimensional topological insulator Bi2Te3 under elastic scattering of different symmetries. By exploring spin-selective optical transitions and symmetry-breaking scattering, we are able to address the out-of-plane spin texture of the topological helical surface states and to generate directional, spin-polarization tunable photocurrent that is otherwise forbidden for the original C-3v symmetry of the surface. This can be achieved regardless of the Fermi level, even under the condition when the topological states are inaccessible in dark. This work paves the way to robustly explore the out-of-plane spin texture for harvesting opto-spintronic functionalities of topological insulators. ; Funding Agencies|Linkoping University; Swedish Research CouncilSwedish Research Council [2016-05091]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]

Nanowires (NWs) with embedded zero-dimensional (0D) quantum dots (QDs) have interesting fundamental properties attractive for a variety of applications. The properties of such embedded QDs can be controlled by 0D quantum confinement and also via strain engineering in axial or radial heterostructures of the nanowire system. We evaluate the electronic structure of QDs, which are formed in the Ga(N, As) shell of the GaAs/Ga(N, As) core-shell NWs due to alloy fluctuations. It is found that the principal quantization axis of the studied QDs is primarily oriented along the NW axis, based on the performed polarizationresolved magneto-photoluminescence measurements. We also show that the QDs exhibit a large spectrally dependent variation of the valence band character, which changes from pure heavy-hole states for the low-energy QD emitters to the mixed light-hole heavy-hole states for the QDs emitting at high energies. We ascribe these changes to combined effects of the uniaxial strain caused by the lattice mismatch between the GaAs core and the Ga(N, As) shell, and the local strain/lattice distortion within the short-range fluctuations in the N content. The obtained results underline the importance of the local strain for valence band engineering in hybrid NW structures with embedded QDs. ; Funding Agencies|Swedish Energy Agency [P40119-1]; Swedish Research Council [2015-05532]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; KAKENHI from the Japan Society for the Promotion of Science [16H05970, 23686004]; Kato Foundation for Promotion of Science; Kurata Memorial Hitachi Science and Technology Foundation; Murata Science Foundation

Fano resonance is a fundamental physical process that strongly affects the electronic transport, optical, and vibronic properties of matter. Here, we provide the first experimental demonstration of its profound effect on spin properties in semiconductor nanostructures. We show that electron spin generation in InAs/GaAs quantum-dot structures is completely quenched upon spin injection from adjacent InGaAs wetting layers at the Fano resonance due to coupling of light-hole excitons and the heavy-hole continuum of the interband optical transitions, mediated by an anisotropic exchange interaction. Using a master equation approach, we show that such quenching of spin generation is robust and independent of Fano parameters. This work therefore identifies spin-dependent Fano resonance as a universal spin loss channel in quantum-dot systems with an inherent symmetry-breaking effect. ; Funding Agencies|Swedish Research CouncilSwedish Research CouncilEuropean Commission [2016-05091, 2020-04530]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

Dynamic nuclear polarization (DNP) is well recognized as being important in spintronics and quantum-information processing. DNP gives rise to high nuclear spin polarization that not only can prolong electron-spin lifetime by generating an Overhauser field (OHF), but also has fertilized the idea to explore nuclear spin qubits. In strained quantum-dot structures (QDSs), a nuclear spin is coupled to a strain field via its quadrupole moment. It has been shown that such nuclear quadrupole interaction (NQI) can be used to achieve appreciable DNP and hence electron-spin polarization. Here, we uncover magneto-optical anomalies from a series of laterally arranged (In,Ga)As QDSs that arise from the NQI and DNP in these nanostructures. We find that the principal axis of NQI in symmetry-lowered QDSs significantly deviates from the growth direction, resulting in tilting of OHF with an angle exceeding 37 degrees. The resulting transverse component of OHF is probed with respect to the crystallographic orientations and its influence on the DNP and ensemble spin dephasing is analyzed. We show that a high-symmetry electronic confinement potential for excitons does not guarantee a high-symmetry NQI for nuclei within the same nano-object, thereby calling for correlated optimization in the symmetry of the electronic confinement potential and that of the nuclear spin bath. Our results underline the role of oblique NQI in electron-spin decoherence and depolarization, which has so far largely been overlooked. This work thus sheds light on design rules for engineering the electronic and spin landscape of QDSs for better performance of DNP desirable for applications in spintronics and quantum computation. ; Funding Agencies|Linkoping University; Swedish Research CouncilSwedish Research Council [2016-05091]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]

Semiconductor quantum dots (QDs) acting as single-photon-emitters are potential building blocks for various applications in future quantum information technology. For such applications, a thorough understanding and precise control of charge states and capture/recombination dynamics of the QDs are vital. In this work, we study the dynamics of QDs spontaneously formed in GaNAsP nanowires, belonging to the dilute nitride material system. By using a random population model modified for these highly mismatched materials, we analyze the results from photoluminescence and photon correlation experiments and show a general trend of disparity in positive and negative trion populations and also a strong dependence of the capture/recombination dynamics and QD charge states on its surroundings. Specifically, we show that the presence of hole-trap defects in the proximity to some QDs facilitates formation of negative trions, which also causes a dramatic reduction of the neutral exciton lifetime. These findings underline the importance of proper understanding of the QD capture and recombination processes and demonstrate the possibility to use highly mismatched materials and defects for charge engineering of QDs. ; Funding Agencies|Swedish Research CouncilSwedish Research CouncilEuropean Commission [2019-04312]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]

We propose native oxide AlGaO(x)outer protective layer for GaAs/AlGaAs core-multishell nanowires to provide yearly stable stronger optical and electrical confinement within the nanowire core. We prepared core-multishell NWs consisting of GaAs core, Al0.2Ga0.8As multi-layered barrier layer, and amorphous Al(0.9)Ga(0.1)O(x)outer shell, which was obtained simply by growing Al-rich AlGaAs and exposing the NWs to the ambient air. Photoluminescence from the NWs reveals that the Al(0.9)Ga(0.1)O(x)outer shell provides efficient optical confinement and creates a compressive strain in the interior of the NW that enhances and blueshifts the photoluminescence of the GaAs core. ; Funding Agencies|KAKENHI from Japan Society of Promotion of Science [19H00855, 16H05970]; Canon foundationCanon Foundation; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]

III-V semiconductor nanowires (NWs), such as those based on GaAs, are attractive for advanced optoelectronic and nanophotonic applications. The addition of Bi into GaAs offers a new avenue to enhance the near-infrared device performance and to add new functionalities, by utilizing the remarkable valence band structure and the giant bowing in the bandgap energy. Here, we report that alloying with Bi also induces the formation of optically-active self-assembled nanodisks caused by Bi segregation. They are located in the vicinity to the (112) corners of the GaAsBi shell and are restricted to twin planes. Furthermore, the Bi composition in the disks is found to correlate with their lateral thickness. The higher Bi composition in the disks with respect to the surrounding matrix provides a strong confinement for excitons along the NW axis, giving rise to narrow emission lines (<450 mu eV) with the predominant emission polarization orthogonal to the NW axis. Our findings, therefore, open a new possibility to fabricate self-assembled quantum structures by combining advantages of dilute bismide alloys and lattice engineering in nanowires. ; Funding Agencies|Swedish Research CouncilSwedish Research Council [2019-04312]; Swedish Foundation for International Cooperation in Research and Higher Education (STINT) [JA2014-5698]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; KAKENHI from Japan Society of Promotion of Science [19H00855, 16H05970]

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. ; Funding Agencies|Linkoping University; KTH; Swedish Research Council [2016-05091, 349-2007-8664]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; Swedish Energy Agency [P40119-1]

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. ; Funding Agencies|Linkoping University; Swedish Research Council [621-2011-4254, 2016-05091]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]; Swedish Foundation for Strategic Research [EM11-0002]; Key Program of Natural Science Foundation of China [61334004]; Creative Research Group Project of Natural Science Foundation of China [61321492]; Natural Science Foundation of China [61404153]; Shanghai Pujiang Program [14PJ1410600]

Quantum dots (QDs) formed in semiconductor nanowires (NWs) form a basis for studying interesting quantum phenomena and provide an exciting platform for various device applications, where in-depth understanding of the formation and electronic properties of such QDs is a requirement. In this work we present detailed investigations of structural, electronic, and optical properties of nitrogen-induced self-assembled QDs in novel Ga(N,As,P) NWs. We show that the three-dimensional quantum confinement of the excitons caused by short-range fluctuations in the N content leads to single-photon emission. We demonstrate that valence band character varies between the QD emitters and attribute it to a varying degree of hole localization within the QDs induced by fluctuations in the As/P ratio. These fluctuations in the chemical compositions are also believed to be at least partly responsible for the observed large variation in exciton lifetimes of the QDs, ranging between 0.5 and 10 ns. The rather long lifetimes demonstrate the resilience of the QDs to nonradiative recombination-a merit for efficient single-photon emitters. Furthermore, we show that the principal axes of the formed QDs are oriented radially, normal to the NW side facets, in contrast to the N-induced QDs in GaNAs NWs which are primarily aligned along the NW axis. Our results, therefore, underline the impacts of the host alloy on the QD formation, which could provide an additional degree of freedom for band structure engineering in nanowires. ; Funding Agencies|Linkoping University; Swedish Research CouncilSwedish Research Council [2016-05091]; Swedish Energy AgencySwedish Energy Agency [P40119-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Swiss National Foundation through the NCCR QSIT

Nanowires (NWs) based on ternary GaAsP and quaternary GaNAsP alloys are considered as very promising materials for optoelectronic applications, including in multi-junction and intermediate band solar cells. The efficiency of such devices is expected to be largely controlled by grown-in defects. In this work we use the optically detected magnetic resonance (ODMR) technique combined with photoluminescence measurements to investigate the origin of point defects in Ga(N)AsP NWs grown by molecular beam epitaxy on Si substrates. We identify gallium vacancies, which act as non-radiative recombination centers, as common defects in ternary and quaternary Ga(N)AsP NWs. Furthermore, we show that the presence of N is not strictly necessary for, but promotes, the formation of gallium vacancies in these NWs. ; Funding: Linkoping University; Swedish Research CouncilSwedish Research CouncilEuropean Commission [2019-04312]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]

One-dimensional ZnO nanowires are a promising material system for a wide range of optoelectronic and photonic applications. Utilization of ZnO, however, requires high-quality ZnO with reliable n-type and p-type conductivity, with the latter remaining elusive, so far. In this work we report on effects of N doping via ion implantation on defect formation in ZnO nanowires studied by optically detected paramagnetic resonance (ODMR) spectroscopy complemented by photoluminescence spectroscopy. After N implantation, zinc interstitial shallow donors, which are formed as a result of ion implantation, are observed in addition to effective mass type shallow donors. Additionally, ODMR signals related to oxygen vacancies can be observed. Implantation also causes formation of a new nitrogen related defect center, which acts as an acceptor. The present findings are of importance for understanding impacts of different defects and impurities on electronic properties of nanostructured ZnO and achieving p-type conductivity via nitrogen doping. ; Funding Agencies|Swedish Energy AgencySwedish Energy Agency [43522-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; AForsk Foundation [15-433]

Gallium oxide (β-Ga2O3) is a wide-bandgap compound semiconductor with a bandgap of ∼4.9 eV that is currently considered promising for a wide range of applications ranging from transparent conducting electrodes to UV optoelectronic devices and power electronics. However, all of these applications require a reliable and precise control of electrical and optical properties of the material, which can be largely affected by impurities, such as transition metals commonly present during the growth. In this work, we employ electron paramagnetic resonance (EPR) spectroscopy to obtain EPR signatures of the 3d-transition metals Co2+ and Cu2+ in β-Ga2O3 bulk crystals and powders that were unknown so far. Furthermore, we show that both Co2+ and Cu2+ preferentially reside on the octahedral gallium lattice site. ; Funding agencies: Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]

Raman spectroscopy is employed to characterize structural and phonon properties of GaP/GaNP core/shell nanowires (NWs) grown by molecular beam epitaxy on Si substrates. According to polarization-dependent measurements performed on single NWs, the dominant Raman modes associated with zone-center optical phonons obey selection rules in a zinc-blende lattice, confirming high crystalline quality of the NWs. Two additional modes at 360 and 397 cm(-1) that are specific to the NW architecture are also detected in resonant Raman spectra and are attributed to defect-activated scattering involving zone-edge transverse optical phonons and surface optical phonons, respectively. It is concluded that the formation of the involved defect states are mainly promoted during the NW growth with a high V/III ratio. ; Funding Agencies|Vetenskapsradet (Swedish Research Council) [621-2010-3815]; U.S. National Science Foundation [DMR-0907652, DMR-1106369]; Royal Government of Thailand Scholarship

The effects of Bi incorporation on the recombination process in wurtzite (WZ) GaBiAs nanowires are studied by employing micro-photoluminescence (mu-PL) and time-resolved PL spectroscopies. It is shown that at low temperatures (T amp;lt; 75 K) Bi-induced localization effects cause trapping of excitons within band-tail states, which prolongs their lifetime and suppresses surface nonradiative recombination (SNR). With increasing temperature, the trapped excitons become delocalized and their lifetime rapidly shortens due to facilitated SNR. Furthermore, Bi incorporation in the GaBiAs NW is found to have a minor influence on the surface states responsible for SNR. ; Funding Agencies|Linkoping University; Swedish Research CouncilSwedish Research Council [2016-05091]; Swedish Energy AgencySwedish Energy Agency [P40119-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]