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1783.Full.Pdf Pure circular polarization electroluminescence at room temperature with spin-polarized light-emitting diodes Nozomi Nishizawaa, Kazuhiro Nishibayashia, and Hiro Munekataa,1 aLaboratory for Future Interdisciplinary Research of Science and Technology, Tokyo Institute of Technology, 4259-J3-15 Nagatsuta, Midori-ku, Yokohama 226-8503, Japan Edited by J. M. D. Coey, Trinity College Dublin, Dublin, Ireland, and approved January 3, 2017 (received for review June 17, 2016) We report the room-temperature electroluminescence (EL) with resonator, pure-CP lasing was demonstrated by the optical pump- nearly pure circular polarization (CP) from GaAs-based spin- ing up to RT (21–24). On the other hand, the CP lasing achieved by polarized light-emitting diodes (spin-LEDs). External magnetic the electrical pumping has been PCP ∼ 0.23 at 50 K with a direct 2 fields are not used during device operation. There are two small current of J ∼ 2.8 kA/cm and B ∼ 2.0 T in InGaAs-based QWs ∼ schemes in the tested spin-LEDs: first, the stripe-laser-like structure (25), PCP 0.55 at 230 K with a pulsed current of presumably ∼ 2 ∼ that helps intensify the EL light at the cleaved side walls below the J 18 kA/cm and B 2.0 T in InGaAs-based quantum dots (26), ∼ μ ∼ spin injector Fe slab, and second, the crystalline AlOx spin-tunnel and PCP 0.28 at RT with pump energy of 87 JandB 0.35 T in barrier that ensures electrically stable device operation. The purity GaN nanorods filled with Fe3O4 nanoparticles (27). of CP is depressively low in the low current density (J) region, Here, we report the unforeseen appearance of nearly pure CP electroluminescence, P = 0.95 at RT from the edge-emission- whereas it increases steeply and reaches close to the pure CP when CP type spin-LED structure without the application of an external J > 100 A/cm2. There, either right- or left-handed CP component is field, which was less investigated in the past (28, 29). Our results significantly suppressed depending on the direction of magnetization suggest the appearance of some nonlinear effect in the regime of of the spin injector. Spin-dependent reabsorption, spin-induced bire- moderately high current density (J > 100 A/cm2),andleadustothe fringence, and optical spin-axis conversion are suggested to account opportunity of studying the RT operation of, at least, semiconductor- for the observed experimental results. based spin-photonic devices. Shown in Fig. 1A is a schematic cross-section, the cleaved GaAs SCIENCES spintronics | circular polarization | semiconductors | nonlinear effect | (110) side wall, of the tested LED chips. They consist of a poly- APPLIED PHYSICAL spin injection crystalline Fe in-plane spin injector, a crystalline γ-like AlOx tunnel barrier (30), and an epitaxial AlGaAs/GaAs/AlGaAs double het- s well represented by the giant magnetoresistance, tunneling erostructure (DH) (31). A magnetization vector of the spin injector magnetoresistance, and spin-transfer-torque magnetic ran- is controlled either parallel or antiparallel to the GaAs [110] axis by A the technical magnetization. Spin polarization of electrons, P = dom access memory, spintronics research based on spin trans- + − + − e (n − n )/(n + n ), with n the electron density at the Fermi level, is port in magnetic metals has been contributing significantly in the ∼ progress of electronics through the advancement in recording bit assumedtobePe 0.4 in Fe (32). At the stage of initial injection, 70% of electron spins point toward the GaAs [110] axis that is density and low-power memory retention (1, 2). Proposal of the superposition of the two primary crystal axes [100] and [010], spin-current modulation with an electric field (3) and invention − – whereas the remaining 30% have the opposite [1ð Þ 10] spin axis. of diluted magnetic III V semiconductors (4) have opened the Spin/charge transport takes place vertically toward the [001] axis. opportunity of introducing spin degree of freedom in semi- The DH wafer was designed by the authors in view of (i) not conductor technology (5). After those works, light-induced severely reducing the spin polarization during carrier transport magnetism (6), electric-field-controlled magnetism (7), spin qubits across the upper n-AlGaAs–clad layer and (ii) avoiding large in semiconductors (8, 9), spin-polarized light-emitting diodes (spin- optical loss due to the top spin-injector metal. Taking account of LEDs) (10, 11), and spin-metal–oxide–semiconductor field-effect transistor (12) were either demonstrated or proposed, which have Significance caused an impact on the metal-based spintronics and applied physics that is not small. However, works that assure the room- Most of the experiments on the spin manipulation in semicon- temperature (RT) operation of those semiconductor-based devices ductors, the principal materials in modern electronic and photonic have not been accomplished to date. devices, were carried out at cryogenic temperatures and high Concerning spin-LEDs, studies on a spin injector consisting of magnetic fields because thermal energy tends to randomize spin a ferromagnetic metal (FmM) and a tunnel barrier (TB) (13–15) information in the semiconductor that is nonmagnetic. Here, we succeeded those using semiconductor-based spin injectors (10, 11). The idea of the FmM-TB injector is to take advantage of report very surprising experimental results of pure circular po- spin-polarized carriers at RT with FmM and to simultaneously larization electroluminescence at room temperature with no ex- suppress with the TB the backward flow of unpolarized carriers ternal magnetic fields. The results are obtained by electrically that is unavoidable in the diffusive transport (16, 17). Many injecting moderately high density of spins into semiconductor works have been carried out with the FmM-TB injector since double heterostructures, the structures that were invented in then. Among those, the highest circular polarization (CP) value, connection with semiconductor lasers one-half century ago. The + − + − + − results suggest the appearance of some spin-dependent nonlinear PCP ≡ {I(σ ) − I(σ )}/{I(σ ) + I(σ )}, with I(σ ) and I(σ ) the intensity of right- and left-handed EL component, respectively, processes that lead to recovering and even enhancing the in- was PCP ∼ 0.3 ∼ 0.35 at RT in the external magnetic flux of B = jected, initial spin information in semiconductors. 0.8 T, which was achieved in the context of studying the spin- filtering effect of the MgO TB (18, 19). Most of the past works Author contributions: N.N. and H.M. designed research; N.N. performed research; N.N., regarding spin-LED were carried out under the vertical ar- K.N., and H.M. analyzed data; and N.N. and H.M. wrote the paper. rangement with low J ranging from 0.1 to 1 A/cm2 and forcing The authors declare no conflict of interest. spins aligned vertically by applying out-of-plane external mag- This article is a PNAS Direct Submission. netic fields (20). With vertical-cavity surface-emitting laser 1To whom correspondence should be addressed. Email: [email protected]. structure incorporating the quantum wells (QWs) (21–23) or This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. thick active layer (24) together with a means of a vertical optical 1073/pnas.1609839114/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1609839114 PNAS | February 21, 2017 | vol. 114 | no. 8 | 1783–1788 Downloaded by guest on September 28, 2021 the active layer. Moreover, the active layer is intentionally doped p-type to reduce the radiative recombination lifetime (37) and suppress the contribution of nonradiative recombination. Note that initially injected [110] (or [1ð−Þ 10]) spins are dispersed into the spins of other orthogonal orientations ([11ð−Þ 0] and [001]) as a result of spin relaxation. Radiative recombination of these spins is observed as the linearly polarized emission: namely, the helicity-independent background, in the CP-resolved EL de- tection system whose optical axis is along the [110] axis (Fig. 2A). The DH part was grown on a p-GaAs (001) wafer using a metal-organic vapor phase epitaxy reactor at Optowell Co., Ltd. to ensure high optical quality that the radiative recombination dominates at RT. A 1-nm-thick crystalline γ-like AlOx tunnel barrier was then grown by the authors using a molecular beam epitaxy chamber (30) The density of interface states at the AlOx/ 11 −2 −1 GaAs interface has been found to be Dit ∼ 3 × 10 cm ·eV (38), which is far less than that at the amorphous AlOx/GaAs interface. This was followed by the fabrication of 100-nm-thick, 40-μm-wide Au (20 nm)/Ti (5 nm)/Fe(100 nm) spin-injector stripes on top of the tunnel barrier using a separate e-beam evaporator and standard photolithography. Finally, the wafer was thermally annealed at 230 °C for 60 min in the nitrogen gas atmosphere, and then cleaved into 1.1 × 2.0-mm rectangle chips. The resultant aspect ratio of the spin injector is 1:25. The long side of the injector, the easy axis, is parallel to the short side of the chip, which is the GaAs [110] axis. They were mounted on a copper block for the EL experiments. EL spectra obtained from the control experiment using the chip without an Fe layer are shown in Fig. 1B, together with photoluminescence (PL) spectrum obtained by the surface Fig. 1. (A) A schematic cross section of spin-LEDs. (B) EL and PL spectra obtained from the control chip consisting of Au/Ti/γ-like AlOx/DHs. Solid lines + represent EL spectra with three different current densities, J = 100 (σ , red; − and σ , black), 75 (green) and 50 (blue) A/cm2. The range of applied voltage is between 2–4 V.
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