Narrow Photoluminescence Research Articles

Excitation density dependence of the photoluminescence from CdxHg1-xTe multiple quantum wells

A study of the photoluminescence from a four-period CdxHg1-xTe multiple quantum well structure at 11 K as a function of excitation density is presented. High-resolution X-ray diffraction and transmission electron microscopy revealed that the quantum well structure is of high quality. This was supported by the narrow photoluminescence peak originating in the ground state electron - heavy hole transition, with a full width at half maximum of only 7.4 meV for an excitation density of 1.3 W/cm2. When the excitation density was increased from 1.3 to 23.4 W/cm2, the peak position was shifted toward higher energy by 2.6 meV and the full width at half maximum increased from 7.4 to 10.9 meV.

Surface Passivation and Photoluminescence of Mn-Doped ZnS Nanocrystals

Enhanced photoluminescence (PL) is reported from Mn-doped ZnS nanocrystals (NCs) capped with ZnS (ZnS:Mn/ZnS core-shell NCs) and thioglycolic acid (TGA) (ZnS:Mn/ZnS core-shell NCs dispersed in an alkaline TGA solution). The NCs were prepared using a reverse micelle route. Comparing with initial ZnS:Mn core NCs, the ZnS:Mn/ZnS core-shell NCs exhibit much stronger orange PL (~580 nm). This is presumably the result of effective passivation of quenching ZnS:Mn NCs surface states by a pure ZnS shell. As for TGA-capped ZnS:Mn/ZnS core-shell NCs, the parallel decrease of a defect-related emission of ZnS is associated with the formation of a shell surface layer of TGA-Zn complexes. In summary, the combination of ZnS shells with TGA ligands was demonstrated to yield ZnS:Mn NCs with narrow size distribution and intense PL.

Effects of organic adsorbent on the photoluminescence of porous silicon microcavities

The effects of organic absorbent on the refractive index and photoluminescence of porous silicon microcavities were theoretically investigated by using the Bruggeman effective medium approximation. Experimentally， porous silicon microcavities were fabricated by computer controlled electrochemical etching， and then interacted with the molecules of oil vapor from a rotary pump. We found that the narrow photoluminescence spectra of porous silicon microcavities were sensitive to the adsorption and de-adsorption of the oil vapor molecules， which leads to a 71nm red-shift and the intensity variation in photoluminescence spectrum. The experimental results are qualitatively consistent with the theoretical estimate.

Polyfluorene nano-rings and nano-dots on mica surfaces: evaporation-induced polymer self-assembly and photoluminescence properties of the assemblies

A facile formation of nano-ring architectures of poly(9,9-di-n-decylfluorene) (PF10) on mica surfaces in a room temperature atmosphere was accomplished by evaporation-induced self-assembly from a dilute chloroform solution saturated with water (∼0.1%). AFM investigation verified that isolated rings having diameters within a narrow distribution of around 150 nm are reproducibly formed on mica by simple drop-cast and evaporation procedures. Wormlike assemblies and/or irregular dots were predominantly observed when untreated and dehydrated chloroform solutions of PF10 were subjected to the same conditions, respectively. Water droplets generated from dissolved water during evaporation were assumed to act as a dynamic template for the ring formation. Photoluminescence (PL) measurements at 77 K indicated that the PL properties of the PF10 rings were very different from those of the PF10 dots. Very narrow PL bands from the rings implied that the PF10 molecules exist in a planar conformation and have well-ordered interchain packing. Broader PL bands from the dots suggested a disordered glassy phase. Further investigation of a series of PFs containing shorter and longer alkyl side-groups, varying from n-hexyl to n-octadecyl, suggested a marked side-chain dependency for the ring assembly formation ability.

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)

Variation of the morphology of strained AlGaInAs quantum wells with substrate orientation

Abstract Strain-compensated AlGaInAs quantum wells/barriers were grown by OMVPE on InP substrates of different orientations: (1 0 0), (1 0 0) 3° miscut toward (1 1 1)A, (5 1 1)A and (5 1 1)B, (3 1 1)A and (3 1 1)B. Smooth surfaces and narrow photoluminescence lines were obtained on (1 0 0) and (3 1 1)B, even for the highest compressive strain tested, +2%. Under high growth rate, (5 1 1)A was found to yield the roughest surfaces and widest photoluminescence lines. When growth pauses were inserted in the quantum wells, (5 1 1)A, (5 1 1)B, and (1 0 0) 3° miscut toward (1 1 1)A showed very rough surfaces.

Fast photoluminescence dynamics in ZnO thin films under high-density excitation conditions

We have investigated the photoluminescence (PL) dynamics of crystalline ZnO thin films prepared by an rf-magnetron sputtering method under high-density excitation conditions at 10 K using an optical-Kerr gating (OKG) method. In the time-resolved PL spectra, we clearly observed the change of the PL properties with increasing time delay. In the initial time region up to about 5 ps, a broad PL band appears at the low-energy side of the free exciton band, which is considered to be due to the electron–hole plasma (EHP) accompanied with a band-gap renormalization. After the broad PL band vanishes, a narrow PL band originating from exciton–exciton scattering, the so-called P emission, appears; namely, its peak energy reaches the energy lower than the free exciton energy by the exciton binding energy. Thus, the time-resolved PL spectra clearly demonstrate the dynamical change from the EHP emission to the P emission at the cross-point time of about 5 ps, which corresponds to the dynamical phase transition from the EHP state to the exciton state.

Formation characteristics of shape-engineered InAs/InAlGaAs quantum dots grown on InP substrates

This article reports the structural and optical properties of shape-engineered InAs/InAlGaAs quantum dots (QDs) on InP substrates formed using an alternate growth method (AGQDs), where a thin InAs layer (0.8 or 1 monolayer) and a thin InAlGaAs layer (1 monolayer) were deposited alternately. For five-stacked AGQD layers, a double-peak feature was observed in the photoluminescence (PL) spectra. The two peaks were related to two different QD branches as confirmed by excitation-power-dependent PL. Observation of a double-peak feature in the PL spectra indicated that the growth conditions for the AGQD layers were not optimized. A slight increase in growth temperature resulted in the merging of the double-peak feature to a single peak with a relatively narrow PL linewidth.

Amplified Spontaneous Emissions from π-Conjugated Dye Doped Polymer Film with Possible Dye Ordering

Smooth polymer films doped with π-conjugated organic dyes, formed around a multimode silica fiber under the influence of capillary force, exhibited very narrow photoluminescence peaks upon laser excitation as compared with the fluorescence in solution. The peak position showed no dependence on the excitation wavelength, indicating that it could not be explained by a whispering gallery mode that should be observed in a medium with broad optical gain. The peaks can be explained by amplified spontaneous emissions (ASE) because the intensity depended linearly on the excitation intensity with a threshold. When the dye-doped film was formed on the portion of a fiber with the clad etched out and was excited by the linearly polarized laser propagating inside the fiber core, we observed ASE peaks not of the dyes but of the optical fiber itself, suggesting the possibility that the dyes were oriented with their transition moment pointing parallel to the film surface, therefore incapable to be photoexcited. These fiber ASE peaks shifted to longer wavelength when we varied the excitation wavelength to shorter wavelength, which clearly ruled out the possibility of silicate Raman scattering as the origin.

Structural and UV photoluminescence properties of single crystalline SnO 2 films grown on α - Al 2O 3 (0001) by MOCVD

High-quality single crystalline SnO 2 films have been deposited on α-Al 2O 3 (0001) substrates by metalorganic chemical vapor deposition (MOCVD). The structural and photoluminescence (PL) properties of the SnO 2 films were investigated. The prepared samples were epitaxial single crystalline films with the rutile structure of pure SnO 2. A single and sharp ultraviolet (UV) PL peak near 331 nm was observed at room temperature (RT). At a temperature of 13 K, two other narrow PL peaks located at 369 and 375 nm as well as a broad feeble peak near 500 nm were observed. The corresponding PL mechanisms were investigated.

Metal organic chemical vapor deposition and investigation of ZnO thin films grown on sapphire

A new type of large area metal organic chemical vapor deposition (MOCVD) system for the growth of high quality and large size ZnO materials is introduced. Materials properties of the un-doped, n- and p-doped ZnO epi-films grown on sapphire substrates by this MOCVD system are studied by various techniques, including high resolution X-ray diffraction (XRD), UV–Visible optical transmission (OT), photoluminescence (PL) and photoluminescence excitation (PLE), synchrotron radiation X-ray photoelectron spectroscopy (SR-XPS). The wurtzite (w) ZnO crystal structures grown with primary (0002) orientation were identified. Results have shown the high crystalline quality of MOCVD-grown ZnO films, indicated by the narrow XRD, PL and Raman line widths, strong PL signals, sharp OT edge and smooth surface. In particular, high p-type carrier concentration of > 10 17 cm − 3 have been achieved besides the good n-type doping in ZnO.

Evidence of quantum dot‐like nano‐objects in InGaN quantum wells provided by narrow photoluminescence spectra from localized exciton

AbstractWe report on narrow photoluminescence (PL) spectra obtained from spatially localized excitons in InGaN quantum wells (QW). These PL lines (less than 1 meV wide) are clearly detected in QWs on several buffer structures and substrates with the micro‐PL technique in low temperature regions. A narrow PL spectrum is one of the characteristics of an exciton confined in a quantum dot (QD). Our results directly confirm that QD‐like nano‐objects exist in InGaN QWs. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

열처리 분위기에 따른 유로퓸 실리케이트 박막의 특성 변화

We investigate the influence of the ambient gas during thermal annealing on the photoluminescence (PL) properties of europium-silicate thin films. The films were fabricated on substrates by using a radio-frequency magnetron sputtering method and subsequent rapid thermal annealing (RTA). The mechanism for the formation of the europium silicates during the annealing process was investigated by using X-ray diffraction (XRD) spectroscopy, Auger electron spectroscopy (AES) and transmission electron microscopy (TEM). A series of narrow PL spectra from <TEX>$Eu^{3+}$</TEX> ions was observed from the film annealed in <TEX>$O_2$</TEX> ambient. Broad PL spectra associated with <TEX>$Eu^{2+}$</TEX> ions, with a maximum intensity at 600 nm and a FWHM of 110 nm, were observed from the thin film annealed at <TEX>$1000^{\circ}C$</TEX> in <TEX>$N_2$</TEX> ambient.

Closely stacking growth of highly uniform InAs quantum dots on self-formed GaAs nanoholes

Uniform InAs quantum dots (QDs) were closely stacked on self-formed GaAs nanoholes by molecular beam epitaxy (MBE) using Stranski–Krastanov (SK) growth mode. GaAs nanoholes were spontaneously formed just above the embedded InAs QDs by thermal annealing. Since surface migration length of In adatoms is longer than a spatial distance between neighboring nanoholes, InAs islands were randomly formed on the nanoholes. As InAs coverage increased, uniformity of the stacked InAs QDs improved by a size-limiting effect due to {1 3 6} facet formation. The closely stacked InAs QDs with uniform size were strongly coupled with underlying InAs QDs through the nanoholes and revealed extremely narrow photoluminescence (PL) spectra with about 15 meV in linewidth.

Molecular beam epitaxial growth and characteristics of ultra-low threshold 1.45 μm metamorphic InAs quantum dot lasers on GaAs

We report the molecular beam epitaxial growth and characteristics of 1.45 μm metamorphic InAs quantum dot lasers grown on GaAs. By detailed investigation of the growth kinetics of the metamorphic quantum dot heterostructures, we have achieved high-quality 1.45 μm metamorphic quantum dot layers that exhibit intense and narrow photoluminescence linewidths (∼30 meV) at room temperature. Utilizing the techniques of p-doping and tunnel injection, we have also realized high-performance 1.45 μm lasers that exhibit ultra-low threshold current density (⩽70 A/cm 2), very high temperature stability ( T 0=556 K) in the temperature range of 263–305 K, and large frequency response ( f −3dB=8 GHz).

InGaAs quantum dots grown with As 4 and As 2 sources using molecular beam epitaxy

InGaAs/GaAs quantum dot (QD) structures are grown with As 4 and As 2 sources using molecular beam epitaxy (MBE). In 0.3Ga 0.7As and In 0.4Ga 0.6As QDs grown with an As 4 source have anisotropic shape, which is elongated to [ 1 1 ¯ 0 ] direction, while QDs grown with an As 2 source have isotropic shape. These results are thought to result from the difference in the surface structures under As 4 and As 2 sources. QDs grown with an As 4 source have narrower photoluminescence (PL) linewidths and a shorter wavelength emission than those grown with an As 2 source.

High-density, uniform InSb∕GaSb quantum dots emitting in the midinfrared region

The authors have developed a multistep molecular-beam epitaxy growth technique which allows them to grow InSb quantum dots with high structural perfection and high density. This technique consists in the deposition at a very low temperature followed by a properly designed annealing step. Fully strained InSb∕GaSb quantum dots with a density exceeding 7×1010cm−2 and lateral sizes in the 20–30nm range have been obtained. Narrow photoluminescence emission is obtained around 3.5μm up to room temperature.

Temperature-dependent bimodal size evolution of InAs quantum dots on vicinal GaAs(100) substrates

The effects of growth temperature on the bimodal size distribution of InAs quantum dots on vicinal GaAs(100) substrates grown by metal organic chemical vapor deposition are studied. An abnormal trend of the bimodal size evolution on temperature is observed. With the increase of the growth temperature, while the density of the large dots decreases continually, that of the small dots first grows larger when temperature was below 520°C, and then exhibits a sudden decrease at 535°C. The trend is explained by taking into account the presence of multiatomic steps on the substrates. Photoluminescence (PL) studies show that quantum dots on vicinal substrates have a narrower PL linewidth, a longer emission wavelength, and a larger PL intensity than those of the dots with exact substrates.

Ultraviolet photoluminescence from Gd-implanted AlN epilayers

Deep ultraviolet emission from gadolinium (Gd)-implanted AlN thin films has been observed using photoluminescence (PL) spectroscopy. The AlN epilayers were ion implanted with Gd to a total dose of ∼6×1014cm−2. Using the output at 197nm from a quadrupled Ti:sapphire laser, narrow PL emission was observed at 318nm, characteristic of the trivalent Gd ion. A broader emission band, also centered at 318nm, was measured with excitation at 263nm. The PL emission intensity decreased by less than a factor of 3 over the sample temperature range of 10–300K and decay transients were of the order of nanoseconds.

Study of InAs/GaAs quantum dots grown by MOVPE under the safer growth conditions

InAs quantum dots (QDs) have been formed on GaAs (001) substrate by metal-organic vapor phase epitaxy (MOVPE) under the safer growth conditions: using tertiarybutylarsine (TBA) to replace AsH3 as the arsenic source and replacing hydrogen by pure nitrogen as the carrier gas. Effects of growth conditions on the QD formation have been investigated. It is observed that the wetting layer is stabilized with some material being transferred to form the QDs due to the strain relaxation process during the QD formation. Dot size dispersion becomes broader when the post-growth interruption is more than 20 s. Compared with normal one-step grown QDs, dot density increases greatly by 213% after employing two-step deposition for QD growth. This is explained by considering the indium-flux-dependent nucleation density at step 1 and kinetically self-limiting growth at step 2. The two photoluminescence (PL) emission peaks, 1.203 μm and 1.094 μm, from the two-step grown QDs are attributed to E1–HH1 and E1–LH1 transitions of the QDs, respectively. The measured results agree well with those received by an 8 k·p theoretical calculation. The narrow PL linewidth of ~50 nm shows high quality of the QDs. This paves the way to develop safer MOVPE process, using TBA/N2 instead of AsH3/H2, to grow QDs for device application.