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Core/shell nanowire (NW) heterostructures based on III-V semiconductors and related alloys are attractive for optoelectronic and photonic applications owing to the ability to modify their electronic structure via bandgap and strain engineering. Post-growth thermal annealing of such NWs is often involved during device fabrication and can also be used to improve their optical and transport properties. However, effects of such annealing on alloy disorder and strain in core/shell NWs are not fully understood. In this work we investigate these effects in novel core/shell/shell GaAs/GaNAs/GaAs NWs grown by molecular beam epitaxy on (111) Si substrates. By employing polarization-resolved photoluminescence measurements, we show that annealing (i) improves overall alloy uniformity due to suppressed long-range fluctuations in the N composition; (ii) reduces local strain within N clusters acting as quantum dot emitters; and (iii) leads to partial relaxation of the global strain caused by the lattice mismatch between GaNAs and GaAs. Our results, therefore, underline applicability of such treatment for improving optical quality of NWs from highly-mismatched alloys. They also call for caution when using ex-situ annealing in strain-engineered NW heterostructures. ; Funding Agencies|Swedish Energy AgencySwedish Energy Agency [P40119-1]; Swedish Research CouncilSwedish Research Council [2015-05532]; 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 [2009 00971]; KAKENHI from the Japan Society for the Promotion of ScienceMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of ScienceGrants-in-Aid for Scientific Research (KAKENHI) [16H05970, 23686004]; Kato Foundation for Promotion of Science; Kurata Memorial Hitachi Science and Technology Foundation; Murata Science Foundation; Linkoping University

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 beta, 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 pas sivation 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 nonradiative recombination centers via nitridation. Our work therefore represents the first step toward development of room-temperature infrared NW lasers based on dilute nitrides with extended tunability in the lasing wavelength. ; 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 [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

GaAs-based semiconductors are highly attractive for diverse nonlinear photonic applications, owing to their non-centrosymmetric crystal structure and huge nonlinear optical coefficients. Nanostructured semiconductors, for example, nanowires (NWs), offer rich possibilities to tailor nonlinear optical properties and further enhance photonic device performance. In this study, it is demonstrated highly efficient second-harmonic generation in subwavelength wurtzite (WZ) GaAs NWs, reaching 2.5 × 10−5 W−1, which is about seven times higher than their zincblende counterpart. This enhancement is shown to be predominantly caused by an axial built-in electric field induced by spontaneous polarization in the WZ lattice via electric field-induced second-order nonlinear susceptibility and can be controlled optically and potentially electrically. The findings, therefore, provide an effective strategy for enhancing and manipulating the nonlinear optical response in subwavelength NWs by utilizing lattice engineering. ; Funding: Swedish Research Council European Commission [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 Linköping University [2009 00971]; KAKENHI from Japan Society of Promotion of Science [19H00855, 16H05970]; National Natural Science Foundation of China (NSFC) [12027805, 11991060]

We show by electron spin resonance (ESR) and Raman spectroscopies that the crystal phase transition of the lead-free double-perovskite Cs2AgBiBr6 has a profound symmetry-breaking effect on the high spin states of, for example, a transition-metal ion Fe3+ and the vibrational modes. It lifts their degeneracy when the crystal undergoes the cubic-tetragonal phase transition, splitting the six-fold degenerate S = 5/2 state of Fe3+ to three Kramer doublets and the enharmonic breathing mode T-g of the MBr6 octahedra (M = Ag, Bi, Fe) into E-g + A(g). The magnitudes of both spin and Raman line splitting are shown to directly correlate with the strength of the tetragonal strain field. This work, in turn, demonstrates the power of the ESR and Raman spectroscopies in probing structural phase transitions and in providing in-depth information on the interplay between the structural, spin, and vibrational properties of lead-free double perovskites, a newly emerging and promising class of materials for low-cost and high-efficiency photovoltaics and optoelectronics. ; Funding Agencies|Swedish Research CouncilSwedish Research Council [VR-2017-05285]; Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2019.0082]

In this work, we carry out a comprehensive investigation of structural and optical effects in GaNAsP nanowires (NWs), which are novel materials promising for advanced photovoltaic applications. Despite a significant mismatch in electronegativity between N and As/P atoms, we show that incorporation of nitrogen does not degrade structural quality of the nanowires and the fabricated NW arrays have excellent compositional uniformity among individual wires. From temperature-dependent photoluminescence (PL) measurements, statistical fluctuations of the alloy composition are shown to lead to localization of photoexcited carriers at low temperatures but do not affect material properties at room temperature. According to time-resolved PL measurements, the room-temperature carrier lifetime increases in the GaNAsP NWs as compared with the GaAsP NWs, which indicates reduced nonradiative recombination. Moreover, in spite of the very low N content in the studied NWs (up to 0.16%), their bandgap energy can be tuned by more than 100 meV. This is accompanied by about 30% reduction in the temperature dependence of the bandgap energy. The presented results demonstrate that alloying of GaAsP with nitrogen provides an additional means of design optimization, beneficial for, e.g., NW-based intermediate band solar cells that are highly dependent on the optimum bandgap structure. ; Funding Agencies|Swedish Energy Agency [P40119-1]; Swedish Research Council [2015-0.5532]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; National Science Foundation [DMR-1106369]

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. ; Funding Agencies|Swedish Research Council [621-2010-3815]; U.S. National Science Foundation [DMR-0907652, DMR-1106369]; Royal Government of Thailand

beta-Ga2O3 is a wide bandgap semiconductor that is attractive for various applications, including power electronics and transparent conductive electrodes. Its properties can be strongly affected by transition metal impurities commonly present during the growth such as Cr. In this Letter, we determine the electronic structure of Cr3+ by performing a correlative study of magneto-photoluminescence (magneto-PL) and electron paramagnetic resonance. We unambiguously prove that the so-called R-1 and R-2 PL lines at around 1.79eV originate from an internal transition between the first excited state (E-2) and the (4)A(2) ground state of Cr3+. The center is concluded to have monoclinic local symmetry and exhibits a large zero-field splitting (similar to 147 mu eV) of the ground state, which can be directly measured from the fine structure of the R1 transition. Furthermore, g-values of the first excited state are accurately determined as g(a) = 1.7, g(b) = 1.5, and g(c*) = 2.1. Our results advance our understanding of the electronic structure of Cr in beta-Ga2O3 and provide a spectroscopic signature of this common residual impurity. (C) 2021 Author(s). ; Funding Agencies|Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009 00971]; Interaction of Ionizing Radiation with Matter University Research Alliance (IIRM-URA) - Department of the Defense, Defense Threat Reduction Agency [HDTRA1-20-2-0002]; NSF DMRNational Science Foundation (NSF) [1856662]

We report on optimization of growth conditions of GaAs/GaNAs/GaAs core/shell/shell nanowire (NW) structures emitting at ~1 μm, aiming to increase their light emitting efficiency. A slight change in growth temperature is found to critically affect optical quality of the active GaNAs shell and is shown to result from suppressed formation of non-radiative recombination (NRR) centers under the optimum growth temperature. By employing the optically detected magnetic resonance spectroscopy, we identify gallium vacancies and gallium interstitials as being among the dominant NRR defects. The radiative efficiency of the NWs can be further improved by post-growth annealing at 680 °C, which removes the gallium interstitials. ; Funding agencies: Swedish Energy AgencySwedish Energy Agency [P40119-1]; Swedish Research CouncilSwedish Research Council [2015-05532]; Swedish Foundation for International Cooperation in Research and Higher Education (STINT) [JA2014-5698]; Swedish Government Strategic Res

We show that coating ZnO nanowires (NWs) with a transition metal, such as Ni, can increase the efficiency of light emission at room temperature. Based on detailed structural and optical studies, this enhancement is attributed to energy transfer between near-band-edge emission in ZnO and surface plasmons in the Ni film which leads to an increased rate of the spontaneous emission. It is also shown that the Ni coating leads to an enhanced non-radiative recombination via surface states, which becomes increasingly important at low measurement temperatures and in annealed ZnO/Ni NWs. ; funding agencies|Swedish Research Council|621-2010-3971|World-Class University through NRF||Korea government (MEST)|R31-10026|

Cathodoluminescence (CL) and micro-photoluminescence spectroscopies are employed to investigate effects of structural defects on carrier recombination in GaNAsP nanowires (NWs) grown by molecular beam epitaxy on Si substrates. In the NWs with a low N content of 0.08%, these defects are found to promote non-radiative (NR) recombination, which causes spatial variation of the CL peak position and its intensity. Unexpectedly, these detrimental effects can be suppressed even by a small increase in the nitrogen composition from 0.08% to 0.12%. This is attributed to more efficient trapping of excited carriers/excitons to the localized states promoted by N-induced localization and also the presence of other NR channels At room temperature, the structural defects no longer dominate in carrier recombination even in the NWs with the lower nitrogen content, likely due to increasing importance of other recombination channels. Our work underlines the need in eliminating important thermally activated NR defects, other than the structural defects, for future optoelectronic applications of these NWs. ; Funding Agencies|Linkoping University; Swedish Research Council [2016-05091]; Swedish Energy Agency [P40119-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009 00971]; Swiss National Foundation through the NCCR QSIT

Matched energy level alignment is a key requirement for efficient organic devices such as organic light-emitting diodes, photovoltaics, and field-effect transistors. The effect of thermal stress/annealing on energy level alignment and related properties of the devices are less discussed compared to the extensively explored effect on morphology and corresponding device performance. Here all polymer solar cells (all-PSCs) are employed to study thermal annealing effects on energy level alignment and the corresponding effect on the device properties of the all-PSCs. It is found that optimized energy level alignment can be achieved by thermal annealing. An interface dipole layer at the donor/acceptor interface is introduced by energy level realignment that assists charge generation by reducing geminate recombination so that the voltage loss is dramatically reduced, improving the performance of the all-PSCs. ; Funding Agencies|Knut and Alice Wallenberg Foundation project "Tail of the Sun"; Swedish Research CouncilSwedish Research Council [2016-05498]; Swedish Foundation for Strategic ResearchSwedish Foundation for Strategic Research [SE13-0060]; Swedish Energy AgencySwedish Energy Agency [45411-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO Mat LiU) [2009 00971]

The ultimate goal for singlet fission is that each photo-excited singlet exciton, S1, will result in two triplet excitons with unity yield. However, the singlet fission is now recognized to be complicated, involving bright/dark excited states of different spin multiplicity. Identifying the role of such states is vital to optimize singlet fission yield but difficult due to their elusive spectral signature. Here, we develop an experimental protocol based on a refined magneto-optical probe to access the fast time evolution of various excited states. In diphenylhexatriene crystal, the S1 is found to undergo two competing processes?to form one of the two dark triplet pair intermediates having different exchange energies or to form a bright state, Sx, exhibiting excimer-like delayed photoluminescence. Our result provides a clear picture of a competition event in singlet fission, which is beneficial for the development and tailoring of singlet fission materials with high yield. ; Funding agencies: Swedish Research CouncilSwedish Research CouncilEuropean Commission [VR-2017-05285]; Linkoping University; Knut and Alice Wallenberg Foundation (KAW)Knut & Alice Wallenberg Foundation; Royal Thai Government Scholarship under the Development and Promotion of Science and Technology Talents Project (DPST); EPSRCUK Research & Innovation (UKRI)Engineering & Physical Sciences Research Council (EPSRC) [EP/M005143/1, EP/P027741/1, EP/L011972/1]; Spanish Government MINECO/FEDERSpanish Government [PID2019109555GB-l00]; Basque governmentBasque Government [PIBA19-0004]; CONACYT-MexicoConsejo Nacional de Ciencia y Tecnologia (CONACyT) [591700]

An exclusive advantage of semiconductor spintronics is its potential for opto-spintronics, which will allow integration of spin-based information processing/storage with photon-based information transfer/communications. Unfortunately, progress has so far been severely hampered by the failure to generate nearly fully spin-polarized charge carriers in semiconductors at room temperature. Here we demonstrate successful generation of conduction electron spin polarization exceeding 90% at room temperature without a magnetic field in a non-magnetic all-semiconductor nanostructure, which remains high even up to 110 degrees C. This is accomplished by remote spin filtering of InAs quantum-dot electrons via an adjacent tunnelling-coupled GaNAs spin filter. We further show that the quantum-dot electron spin can be remotely manipulated by spin control in the adjacent spin filter, paving the way for remote spin encoding and writing of quantum memory as well as for remote spin control of spin-photon interfaces. This work demonstrates the feasibility to implement opto-spintronic functionality in common semiconductor nanostructures. An electron spin polarization of 90% is achieved in a non-magnetic nanostructure at room temperature without magnetic field. This is accomplished by remote spin filtering of InAs quantum-dot electrons via an adjacent tunnelling-coupled GaNAs spin filter. ; Funding Agencies|Swedish Research CouncilSwedish Research CouncilEuropean Commission [2015-05532, 2019-04312, 201605091, 2020-04530]; Swedish Foundation for International Cooperation in Research and Higher Education (STINT) [JA2014-5698]; Linkoping University; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; European Research Council, ERC AdG AMETISTEuropean Research Council (ERC) [695116]; Academy of FinlandAcademy of FinlandEuropean Commission [310985, 323989]; Japan Society for the Promotion of Science (JSPS)Ministry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of Science [16H06359, 19H05507]; JSPSMinistry of Education, Culture, Sports, Science and Technology, Japan (MEXT)Japan Society for the Promotion of Science [19K15380]

Lead-free halide double perovskites (A(2)B(I)B(III)X(6)) with attractive optical and electronic features are considered to be a promising candidate to overcome the toxicity and stability issues of lead halide perovskites (APbX(3)). However, their poor absorption profiles limit device performance. Here the absorption band edge of Cs(2)AgBiBr(6)double perovskite to the near-infrared range is significantly broadened by developing doped double perovskites, Cs-2(Ag:Cu)BiBr6. The partial replacement of Ag ions by Cu ions in the crystal lattice is confirmed by the X-ray photoelectron spectroscopy (XPS) and solid-state nuclear magnetic resonance (ssNMR) measurements. Cu doping barely affects the bandgap of Cs2AgBiBr6; instead it introduces subbandgap states with strong absorption to the near-infrared range. More interestingly, the near-infrared absorption can generate band carriers upon excitation, as indicated by the photoconductivity measurement. This work sheds new light on the absorption modulation of halide double perovskites for future efficient optoelectronic devices. ; Funding Agencies|Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2019.0082]; Swedish Energy AgencySwedish Energy Agency [2018-004357]; Grant Agency of the Czech RepublicGrant Agency of the Czech Republic [GA19-05259S]; VR Starting Grant [2019-05279]; Carl Tryggers Stiftelse; Olle Engkvist Byggmastare Stiftelse; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University (Faculty Grant SFO-Mat-LiU) [2009-00971]; China Scholarship Council (CSC)China Scholarship Council; Ministry of Science and Higher Education of the Russian Federation of NUST "MISIS" [K2-2019-001, 211]; Swedish Research Council (VR)Swedish Research Council [2019-05551]; Swedish Research CouncilSwedish Research Council [2016-07213]

Spintronics holds great potential for next-generation high-speed and low-power consumption information technology. Recently, lead halide perovskites (LHPs), which have gained great success in optoelectronics, also show interesting magnetic properties. However, the spin-related properties in LHPs originate from the spin-orbit coupling of Pb, limiting further development of these materials in spintronics. Here, we demonstrate a new generation of halide perovskites, by alloying magnetic elements into optoelectronic double perovskites, which provide rich chemical and structural diversities to host different magnetic elements. In our iron-alloyed double perovskite, Cs2Ag(Bi:Fe)Br-6, Fe3+ replaces Bi3+ and forms FeBr6 clusters that homogenously distribute throughout the double perovskite crystals. We observe a strong temperature-dependent magnetic response at temperatures below 30 K, which is tentatively attributed to a weak ferromagnetic or antiferromagnetic response from localized regions. We anticipate that this work will stimulate future efforts in exploring this simple yet efficient approach to develop new spintronic materials based on lead-free double perovskites. ; Funding Agencies|Knut and Alice Wallenberg FoundationKnut & Alice Wallenberg Foundation [KAW 2019.0082]; Swedish Energy AgencySwedish Energy Agency [2018004357, P43288-1]; Swedish Government Strategic Research Area in Materials Science on Functional Materials at Linkoping University [2009-00971]; Grant Agency of the Czech RepublicGrant Agency of the Czech Republic [GA19-05259S]; China Scholarship Council (CSC)China Scholarship Council; U.S. Department of Energy, Office of Science, Basic Energy Sciences, Materials Sciences and Engineering DivisionUnited States Department of Energy (DOE); European CommunityEuropean Community (EC) [312284]; Czech Ministry of EducationMinistry of Education, Youth & Sports - Czech Republic [LM2015057]; CERIC users grant; International Synchrotron Access Program of the Australian Synchrotron