Polarization-dependent Photoluminescence Excitation Research Articles

Static Rashba Effect by Surface Reconstruction and Photon Recycling in the Dynamic Indirect Gap of APbBr3 (A = Cs, CH3NH3) Single Crystals.

Recently, halide perovskites have gained significant attention from the perspective of efficient spintronics owing to the Rashba effect. This effect occurs as a consequence of strong spin-orbit coupling under a noncentrosymmetric environment, which can be dynamic and/or static. However, there exist intense debates on the origin of broken inversion symmetry since the halide perovskites typically crystallize into a centrosymmetric structure. In order to clarify the issue, we examine both dynamic and static effects in the all-inorganic CsPbBr3 and organic-inorganic CH3NH3PbBr3 (MAPbBr3) perovskite single crystals by employing temperature- and polarization-dependent photoluminescence excitation spectroscopy. The perovskite single crystals manifest the dynamic effect by photon recycling in the indirect Rashba gap, causing dual peaks in the photoluminescence. However, the effect vanishes in CsPbBr3 at low temperatures (<50 K) accompanied by a striking color change of the crystal, arising presumably from lower degrees of freedom for inversion symmetry breaking associated with the thermal motion of the spherical Cs cation compared with the polar MA cation in MAPbBr3. We also show that the static Rashba effect occurs only in MAPbBr3 below 90 K, presumably due to surface reconstruction via MA-cation ordering, which likely extends across a few layers from the crystal surface to the interior. We further demonstrate that this static Rashba effect can be completely suppressed upon surface treatment with polymethyl methacrylate (PMMA) coating. We believe that our results provide a rationale for the Rashba effects in halide perovskites.

Electronic and optical properties of nonpolara-plane GaN quantum wells

In this paper we present a detailed study of the electronic band structure of a series of nonpolar $a$-plane GaN/AlGaN multiple quantum wells (QWs) of varying well width using complementary results from x-ray diffraction, polarization-dependent photoluminescence excitation spectroscopy, and $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ theory. When excited with unpolarized light, excitonic transitions involving different electron subbands are resolved in the excitation spectra. For linearly polarized $(E\ensuremath{\perp}c,E\ensuremath{\parallel}c)$ excitation, these are shown to consist of overlapping transitions involving different hole subbands. These results are then analyzed in detail using strain data determined by the x-ray diffraction measurements in combination with the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ theory to calculate the bulk band structure and the relative oscillator strength of an $a$-plane GaN film under strain. The results are compared with those of an unstrained $c$-plane film. This analysis reveals that the experimentally observed polarization anisotropy can be attributed to anisotropic strain in the $c$ plane. Based on the $\mathbf{k}\ensuremath{\cdot}\mathbf{p}$ Hamiltonian, we apply an effective mass approximation, taking into account strain and nonparabolicity effects, to calculate the single-particle states and energies for the different quantum wells. The possible influence of the weak spin-orbit coupling on the results is studied in detail. Starting from the single-particle energies and including excitonic binding energies, the band edge optical transitions are calculated and successfully compared to the experimental data. Our analysis gives an estimate for the conduction- to valence-band offset ratio of 45:55 for nonpolar GaN/AlGaN QW structures. Additionally, our study also allows us to investigate the magnitude of the crystal-field splitting and spin-orbit coupling in GaN systems.

Optical characteristics of m-plane InGaN/GaN multiple quantum well grown on LiAlO 2 (1 0 0) by MOVPE

We investigate the optical properties of m-plane InGaN/GaN multiple quantum well grown on LiAlO 2 substrate by metal organic vapor phase epitaxy. Polarization-dependent photoluminescence and polarization-dependent photoluminescence excitation measurements have been performed at low temperature to study the optical absorption and emission characteristics. The main emission band possesses large polarization anisotropy which may be attributed to the anisotropic biaxial strain. We found the optical emission is not influenced by the polarization-induced electric field from the excitation-dependent photoluminescence measurements. From our results, we attribute the low-temperature emission band around 3.2 eV to interband transition in the quantum well. Besides, the mechanism of the main emission band is associated with interband transition and subsequent carrier localization. The realization of good-quality non-polar GaN-based devices can then be expected in near future.

Antiferromagnetic phase transition in a single MnTe monolayer

We investigate a quantum-well sample containing a monolayer of MnTe in its center. The exciton Zeeman splitting is measured as a function of temperature by polarization-dependent photoluminescence excitation experiments. The dependence of the inverse Zeeman splitting shows a kink at a critical temperature of 50 K. Simultaneously, a peak in the linewidth of the e1–hh1 transition is observed at the same temperature. The critical temperature agrees very well with the predicted Nèel temperature obtained within the mean-field approximation for a single monolayer MnTe. After annealing the sample, the Zeeman splitting drastically increases and no critical phenomena are observed anymore.

Interband optical transition spectra in GaAs quantum wires with rectangular cross sections

Interband optical transition spectra of rectangular GaAs quantum wires (QWR's) of various cross-sectional sizes are experimentally and theoretically studied. High-quality GaAs QWR's with lateral sizes below 20 nm are formed in AlAs trench structures with (110) vertical sidewalls by using metal-organic chemical vapor deposition. Polarization-dependent photoluminescence excitation (PLE) spectra in the QWR's clearly exhibit absorption peaks corresponding to optical transitions between quantized one-dimensional conduction and valence subbands. It is found that transition strengths and polarization anisotropies in the lowest- and higher-energy PLE peaks significantly vary, depending on the cross-sectional shape of the rectangular wires. The polarization-dependent interband transition matrix elements and the detailed absorption spectra are calculated by a multiband effective-mass theory considering heavy-hole and light-hole subband mixing. The theoretical results clarify the physical origin of observed PLE peaks and explain the strong dependence of interband transition properties on the cross-sectional ratio of QWR's.

Reduction of etching damage in the fabrication of quantum wires

Quantum wire structures were fabricated by patterning quantum well samples with electron beam lithography and various wet chemical etching procedures. Wire structures with 800 Å wire width were achieved by wet etching in NH 4OHH 2O 2H 2O (20:7:973). These samples were characterized by scanning electron microscopy, photoluminescence (PL), and polarization-dependent photoluminescence excitation (PLE) measurements. The PL spectra show significantly stronger peaks than that taken from an unetched quantum well sample. A wire width of 400 Å was estimated from the blue shift of PL peaks. A 22% anisotropy was observed from polarization-dependent PLE spectra, further indicating the existence of two-dimensional quantum confinement.

Direct epitaxial growth of (AlGa)As/GaAs quantum wires by orientation-dependent metalorganic vapour phase epitaxy

The direct growth of (AlGa)As/GaAs quantum wire structures using metalorganic vapour phase epitaxy (MOVPE) is investigated. The present concept takes advantage of the different growth rates of GaAs and (AlGa)As on (100) and {111} planes to transfer a trapezoidal GaAs starting structure with dimensions in the μm-range into a quantum wire structure in the nm-range. For stripe dimensions below 500 nm significant changes in growth behaviour (enhanced growth rates and for special cases also the formation of other crystallographic facets) are observed by using scanning electron microscopy (SEM) studies. Based on these findings (AlGa)As/GaAs quantum wire structures are realized with lateral structure sizes of 30 nm as proved by SEM. The luminescence properties of these structures, which are determined by standard and polarization-dependent photoluminescence excitation spectroscopy, are presented and discussed in comparison to the properties of quantum well structures.