Sharp Photoluminescence Lines Research Articles

Exfoliation of epilayers with quantum dots

The technique of exfoliation of II-VI epitaxial layers was developed and applied for structures with CdTe/ZnTe quantum dots, some of them containing single manganese ion. Our demonstration is based on molecular beam epitaxy growth of CdTe, MgTe, and ZnTe buffers on GaAs (100) substrate. Exfoliation was realized thanks to MgTe degradation in water. This allowed to achieve over 1mm2 large, free-standing zinc-blende crystalline structures based on ZnTe, transparent for wavelength over 540nm. The sample exhibit interferences characteristic for a thin film with a thickness of 1.8 µm as anticipated. At low temperature, we observed also sharp photoluminescence lines of quantum dots in the spectral range 540 - 610 nm. The developed technique may be used in the future to investigate transmission through epitaxial structures based on II-IV semiconductors i.e. microcavities, quantum wells, quantum dots or diluted magnetic semiconductors.

Structure and photoluminescence properties of zinc oxide/ytterbium oxide nanocomposites

Nanocomposites based on zinc oxide and ytterbium oxide nanocrystals with Yb atomic content varied from 0.5 up to 6 % were synthesized from alcohol solutions followed by thermal annealing in air. The prepared samples were characterized by means of the X-ray diffraction, transmission electron microscopy and photoluminescence spectroscopy. A sharp photoluminescence line of Yb3+ ions was detected at 983 nm and it was stronger for the samples thermally annealed at 750 °C. The intensity of Yb3+ photoluminescence was found to be slightly depended on Yb concentration and to exhibit a square root dependence on excitation intensity. The experimental results are explained by assuming energy transfer from the photoexcited zinc oxide nanocrystals to ytterbium oxide nanoparticles on their surface. The obtained results indicate possible applications of Yb-doped zinc oxide nanostructures in optoelectronics devices and as infrared biomarkers.

Core–shell carrier and exciton transfer in GaAs/GaNAs coaxial nanowires

Comprehensive studies of GaAs/GaNAs coaxial nanowires grown on Si substrates are carried out by temperature-dependent photoluminescence (PL) and PL excitation, to evaluate effects of the shell formation on carrier recombination. The PL emission from the GaAs core is found to transform into a series of sharp PL lines upon radial growth of the GaNAs shell, pointing toward the formation of localization potentials in the core. This hampers carrier transfer at low temperatures from the core in spite of its wider bandgap. Carrier injection from the core to the optically active shell is found to become thermally activated at T > 60 K, which implies that the localization potentials are rather shallow.

Unifying the low-temperature photoluminescence spectra of carbon nanotubes: the role of acoustic phonon confinement.

At low temperature the photoluminescence of single-wall carbon nanotubes show a large variety of spectral profiles ranging from ultranarrow lines in suspended nanotubes to broad and asymmetrical line shapes that puzzle the current interpretation in terms of exciton-phonon coupling. Here, we present a complete set of photoluminescence profiles in matrix embedded nanotubes including unprecedented narrow emission lines. We demonstrate that the diversity of the low-temperature luminescence profiles in nanotubes originates in tiny modifications of their low-energy acoustic phonon modes. When low-energy modes are locally suppressed, a sharp photoluminescence line as narrow as 0.7meV is restored. Furthermore, multipeak luminescence profiles with specific temperature dependence show the presence of confined phonon modes.

Formation of Eu3+ Luminescent Centers in Eu-Doped ZnO Grown by Sputtering-Assisted Metalorganic Chemical Vapor Deposition

The photoluminescence (PL) and electrical properties of Eu-doped ZnO (ZnO:Eu) grown by sputtering-assisted metalorganic chemical vapor deposition (SA-MOCVD) were investigated. Sharp PL lines due to intra-4f transition in Eu3+ ions were observed when ZnO:Eu was annealed at temperatures (Ta) higher than 500 °C in O2 ambient. In Hall measurements, the as-grown ZnO:Eu showed a high electron density of 3×1019 cm-3, indicating that the doped Eu ions act as a donor in ZnO. The electron density decreased rapidly at Ta≥500 °C and was 2×1017 cm-3 at Ta = 600 °C. These results revealed that a complex consisting of a Eu3+ ion and an interstitial oxygen (Eu3+–Oi complex) is formed by high-temperature annealing and acts as a Eu3+ luminescent center in ZnO.

Enhancement of activation energies of sharp photoluminescence lines for GaInNAs quantum wells due to quantum confinement

It is shown that localized emission from GaInNAs quantum wells (QW) is composed of sharp photoluminescence (PL) lines. Spectral position of these PL lines varies in a very broad range (∼150 meV) but the activation energy of each line is the same within the experimental uncertainty and equals ∼11 meV. This value is higher than in bulk GaInNAs (∼6 meV) and corresponds very well to electron–hole attraction in GaInNAs QW. It means that the source of these sharp PL lines are excitons localized on deep centres, which can recombine radiatively while the thermal energy is smaller than the Coulomb attraction between electrons and holes. Because of this the localized emission composing of sharp PL lines is observed at low temperatures and quenched much faster for GaInNAs layers than GaInNAs/GaAs QWs.

Photoreflectance, photoluminescence, and microphotoluminescence study of optical transitions between delocalized and localized states in GaN0.02As0.98, Ga0.95In0.05N0.02As0.98, and GaN0.02As0.90Sb0.08layers

Optical transitions in as-grown and annealed GaN${}_{0.02}$As${}_{0.98}$, Ga${}_{0.95}$In${}_{0.05}$N${}_{0.02}$As${}_{0.98}$, and GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$ bulklike layers have been studied with photoreflectance (PR), photoluminescence (PL), and microphotoluminescence (\ensuremath{\mu}PL) in a broad range of temperatures. The exciton binding energy and the temperature dependence of the energy gap have been determined from PR measurements. In addition, the alloy-related broadening of optical transitions has been analyzed and compared for the three alloys. The largest alloy inhomogeneities have been observed for GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$. In \ensuremath{\mu}PL spectra of GaN${}_{0.02}$As${}_{0.98}$ and Ga${}_{0.95}$In${}_{0.05}$N${}_{0.02}$As${}_{0.98}$ layers measured at low temperatures and low excitation conditions, sharp PL lines of 100--300-\ensuremath{\mu}eV widths have been clearly observed \ensuremath{\sim}10--20 meV below the energy gap of these alloys. Analyzing the temperature quenching of the sharp PL lines, they have been attributed to the recombination of localized excitons trapped at deep donor (acceptor)-like states. Such lines were not well resolved for GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$ layers. The analysis of thermal quenching of localized exciton emission suggests that for this alloy excitons are localized mostly on alloy content fluctuations. With the increase in the excitation power an additional PL band has been observed in \ensuremath{\mu}PL spectra at the higher-energy side. This band corresponds to the optical transitions between delocalized states which are observed in photoreflectance spectra. The emission of localized excitons without resolved sharp lines has been also observed with macro-PL measurements at low temperatures. In addition, the donor trap--valence band (DT-VB) recombination has been identified for the three alloys in macro-PL spectra. It has been clearly observed that the intensity of DT-VB recombination is the weakest for GaN${}_{0.02}$As${}_{0.90}$Sb${}_{0.08}$ alloys whereas for GaN${}_{0.02}$As${}_{0.98}$ and Ga${}_{0.95}$In${}_{0.05}$N${}_{0.02}$As${}_{0.98}$ alloys it is very similar. The observed differences between the three alloys and the role of postgrowth annealing have been analyzed and discussed in the context of the influence of III-V host on alloy inhomogeneities, formation of native point defects in III-V-N alloys, and their influence on the optical quality of III-V-N alloys.

Single photon emission from impurity centers in AlGaAs epilayers on Ge and Si substrates

We show that the epitaxial growth of thin layers of AlGaAs on Ge and Si substrates allows to obtain single photon sources by exploiting the sparse and unintentional contamination with acceptors of the AlGaAs. Very bright and sharp single photoluminescence lines are observed in confocal microscopy. These lines behave very much as single excitons in quantum dots, but their implementation is by far much easier, since it does not require 3D nucleation. The photon antibunching is demonstrated by time resolved Hanbury Brown and Twiss measurements.

Effects of Thermal Annealing on the Structural and Optical Properties of Carbon-Implanted SiO2

Amorphous carbon (a-C) nanoclusters were synthesized by the implantation of carbon ions (C-) into thermally grown silicon dioxide film (-500 nm thick) on a Si (100) wafer and processed by high temperature thermal annealing. The carbon ions were implanted with an energy of 70 keV at a fluence of 5 x 10(17) atoms/cm2. The implanted samples were annealed at 1100 degrees C for different time periods in a gas mixture of 96% Ar+4% H2. Raman spectroscopy, X-ray photoelectron spectroscopy (XPS) and High Resolution Transmission Electron Microscopy (HRTEM) were used to study the structural properties of both the as-implanted and annealed samples. HRTEM reveals the formation of nanostructures in the annealed samples. The Raman spectroscopy also confirms the formation of carbon nano-clusters in the samples annealed for 10 min, 30 min, 60 min and 90 min. No Raman features originating from the carbon-clusters are observed for the sample annealed further to 120 min, indicating a complete loss of implanted carbon from the SiO2 layer. The loss of the implanted carbon in the 120 min annealed sample from the SiO2 layer was also observed in the XPS depth profile measurements. Room temperature photoluminescence (PL) spectroscopy revealed visible emissions from the samples pointing to carbon ion induced defects as the origin of a broad 2.0-2.4 eV band, and the intrinsic defects in SiO2 as the possible origin of the -2.9 eV bands. In low temperature photoluminescence spectra, two sharp and intense photoluminescence lines at -3.31 eV and -3.34 eV appear for the samples annealed for 90 min and 120 min, whereas no such bands are observed in the samples annealed for 10 min, 30 min, and 60 min. The Si nano-clusters forming at the Si-SiO2 interface could be the origin of these intense peaks.

Mechanism of non-classical light emission from acoustically populated (311)A GaAs quantum wires

We employ surface acoustic waves (SAWs) to control the transfer of photo-generated carriers between interconnected quantum wells and quantum wires (QWRs) grown on pre-patterned (311)A GaAs substrates. Optical studies, carried out under remote acoustic excitation of a single QWR, have shown sharp photoluminescence lines and antibunched photons with tunable emission energy. These features are attributed to recombination of acoustically transported carriers in potential inhomogeneities within the wire. The origin of the photon antibunching is discussed in terms of a ‘bottleneck’ in the number of carriers trapped in the QWR, which restricts the number of recombination events per SAW cycle. We propose a model for antibunching based on the trapping of carriers induced by the SAW piezoelectric field in states at the interface between the GaAs QWR and the AlGaAs barriers. Non-classical light is emitted during the subsequent release of the trapped carriers into the recombination centers within the wire. The spatial distribution of the emitting recombination centers is estimated using time-resolved measurements.

Scanning Fabry-Pérot interferometer with largely tuneable free spectral range for high resolution spectroscopy of single quantum dots

We report on the implementation of a scanning Fabry-Pérot interferometer for photoluminescence spectroscopy investigation. We choose a conveniently small reflectivity of the two planar semitransparent mirrors which, in spite of a moderate cavity finesse, ensures a good mechanical stability over a long time. We also exploit the large tuneability of the cavity length (i.e., of the free spectral range) for changing the spectral resolution over two order of magnitude (from ~300 μeV to ~4 μeV in full width at half maximum). Such a characteristic easily allows to scan both sharp and broad luminescence bands. We test our Fabry-Pérot interferometer on sharp photoluminescence lines resulting from excitonic recombination in self-assembled GaAs quantum dots. We demonstrate the ability of our system to resolve linewidth as small as 4 μeV.

Structural and optical properties of high quality zinc-blende/wurtzite GaAs nanowire heterostructures

The structural and optical properties of three different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time-resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.

Optical Properties of GaAs Quantum Dots Fabricated by Filling of Self-Assembled Nanoholes.

Experimental results of the local droplet etching technique for the self-assembled formation of nanoholes and quantum rings on semiconductor surfaces are discussed. Dependent on the sample design and the process parameters, filling of nanoholes in AlGaAs generates strain-free GaAs quantum dots with either broadband optical emission or sharp photoluminescence (PL) lines. Broadband emission is found for samples with completely filled flat holes, which have a very broad depth distribution. On the other hand, partly filling of deep holes yield highly uniform quantum dots with very sharp PL lines.

ZnO nanocrystals/SiO2 multilayer structures fabricated by RF-magnetron sputtering

Abstract In the present study ZnO nanocrystals embedded in a SiO 2 matrix were successfully produced by means of radio frequency magnetron sputtering deposition using ZnO and SiO 2 targets. Multilayer structures consisting of ZnO/SiO 2 bilayers were deposited on Si substrate wafers either in vacuum or in an oxygen-rich ambience. Subsequently, the structures were heat-treated at different temperatures and in different atmospheres. This procedure resulted in spherical ZnO nanocrystals with a narrow size distribution as determined by transmission electron microscopy. Photoluminescence (PL) properties of the ZnO nanocrystals were studied by means of steady-state luminescence spectroscopy with excitation in the blue-UV spectral range. The PL properties of the samples were observed to depend strongly on the ZnO nanocrystal size. A broad PL band in the visible spectral range is observed and is tentatively ascribed to defects on or in the vicinity of the nanocrystal surfaces. Sharp characteristic PL lines in the UV range were detected in relatively large ZnO nanocrystals. These lines are tentatively attributed to bound exciton and donor–acceptor pair recombinations, and they are similar to the corresponding lines observed in bulk ZnO. The role of surface defects for the PL properties of nanocrystalline ZnO is discussed.

Preparation and photoluminescence of Eu‐doped GaN films by radio frequency magnetron sputtering method

AbstractThe red‐luminescent Eu‐doped GaN film has been prepared on both sapphire and GaN/sapphire substrates by the conventional RF magnetron sputtering method using a compound powder target (GaN + EuN), N2 sputtering gas and subsequent annealing in NH3. The GaN:Eu layers exhibited a red sharp photoluminescence (PL) line at 622 nm due to 5D0–7F2 transition of Eu3+ ions together with series of narrow PL lines: 5D1–7F1 (540 nm), 5D0–7F1 (580–600 nm), and 5D0–7F3 (665 nm). Dependence of Eu‐related PL on the growth condition is described: Eu‐doping level (EuN in target), substrate temperature (Ts), growth time, and annealing temperature (Ta). The optimum conditions for luminescent GaN:Eu are: (i) the Eu of 2 mol% in target, (ii) Ts at 400–450 °C, and (iii) Ta at 1000 °C for 2 hours. Improvements in intensity and reproducibility of Eu‐related PL have been attained by: (i) the increase of growth time, (ii) the use of undoped GaN buffer layer, (iii) the improvement of the target quality, and (iv) the use of the MOVPE grown GaN/Al2O3 template. Preliminary studies of various PL properties are described: the fine structure in PL spectra, PL decay characteristics, temperature dependence and photoluminescence excitation spectrum (PLE). The PL results are discussed in comparison with those in the GaN:Eu grown by other methods in literature. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Highly circular-polarized single photon generation from a single quantum dot at zero magnetic field

Origin of sharp photoluminescence lines observed from an InAlAs quantum dot was identified with the measurements of excitation-power dependences and polarization correlations, together with photon correlation measurements. Single photon emission with high degree of circular polarization (DCP) up to 60% was observed from a positively charged exciton (trion) state in the single quantum dot under non-resonant excitation at zero magnetic field.

Effects of N incorporation on the structural and photoluminescence characteristics of GaSbN/GaSb single quantum wells

The structural and optical properties of GaSbN single quantum wells grown on GaSb substrates by solid source molecular beam epitaxy have been investigated for N concentrations up to 1.5%. The presence of well-resolved and pronounced Pendellosung fringes, dynamical diffraction rods seen in the corresponding reciprocal space map, and triple-axis x-ray full width at half maximum of 10–11 arcsec of the substrate and epilayer peak indicates epilayers of excellent quality with smooth interfaces. Low-temperature photoluminescence (PL) exhibited sharp and discrete N-related PL line features below the GaSb band edge. Their dependence on N concentration as well as measurement temperature and excitation intensity of the PL strongly suggests that these lines correspond to highly localized N pair/cluster states. No significant effect of in situ annealing in Sb ambient on the PL features was observed, while ex situ annealing in N ambient led to the annihilation of these features.

Isotopically Controlled Semiconductors

The following article is an edited transcript based on the David Turnbull Lecture given by Eugene E. Haller (University of California, Berkeley) at the 2005 Materials Research Society Fall Meeting in Boston on November 29, 2005. The David Turnbull Lectureship is awarded to recognize the career of a scientist who has made outstanding contributions to understanding materials phenomena and properties through research, writing, and lecturing, as exemplified by the life work of David Turnbull. Haller was named the 2005 David Turnbull Lecturer for his “pioneering achievements and leadership in establishing the field of isotopically engineered semiconductors; for outstanding contributions to materials growth, doping and diffusion; and for excellence in lecturing, writing, and fostering international collaborations.”The scientific interest, increased availability, and technological promise of highly enriched isotopes have led to a sharp rise in the number of experimental and theoretical studies of isotopically controlled semiconductor crystals. This article reviews results obtained with isotopically controlled semiconductor bulk and thin–film heterostructures. Isotopic composition affects several properties, such as phonon energies, band structure, and lattice constant, in subtle, yet—for their physical understanding–significant ways. Large isotope-related effects are observed for thermal conductivity in local vibrational modes of impurities and after neutron transmutation doping. Spectacularly sharp photo-luminescence lines have been observed in ultrapure, isotopically enriched silicon crystals. Isotope multilayer structures are especially well suited for studies of simultaneous self-and dopant-diffusion. The absence of any chemical, mechanical, or electrical driving forces makes possible the study of an ideal random-walk problem, in which moving atoms go in random directions at random intervals. Isotopically controlled semiconductors may find applications in quantum computing, nanoscience, and spintronics.

Exciton-Phonon Interaction in CuAlS2 Powders

High-resolution photoluminescence (PL) measurement was carried out for copper aluminum disulfide (CuAlS2) powder at 12 K. Several sharp PL lines were observed in the range from 3.580 to 3.320 eV. The emission peaks at photon energies from 3.566 to 3.459 eV were attributed to free-exciton (FE) and bound-excitons (BE). The several weak emissions at below 3.476 eV were clarified to be phonon replicas (PR) by Raman scattering and in the viewpoint of exciton-phonon interaction. We observed the one, two and three-phonon replicas related to E(LO, TO) and B2(LO, TO) vibrational modes in chalcopyrite structure. It was suggested that the strong interaction between excitons and optical phonons took place in obtained CuAlS2 powder.

Optical properties of a single Er center in GaN

Abstract Photoluminescence (PL) spectra near 1.5 μm of Er 3+ implanted into GaN are usually very complex, due to the number of different centers formed depending on Er concentration, annealing conditions as well as residual or intentional impurities. By implanting low Er doses into a high purity material we found it possible to obtain a single optically active center with very sharp PL lines. With use of high resolution resonant optical spectroscopy we were able to determine the Stark splitting of the 4 I 9/2 and 4 I 11/2 energy level and to perform reliable model calculations. We found that the Er 3+ center observed has C 3v site symmetry, i.e., the same as the point group of GaN.