Sharp Luminescence Lines Research Articles

Resonant sharp hot free-exciton luminescence in 6H- and 4H-SiC due to inhibited exciton-phonon interaction

Experimentally observed sharp luminescence lines from hot free-exciton recombination in high-purity 6H- and 4H-SiC are presented. The phenomenon is explained in terms of inhibition of the exciton-phonon scattering, prohibited for excitons created resonantly near the bottom of the lowest exciton band at low temperatures. This gives rise to the hot, sharp luminescence. The model is in agreement with the observed quenching of the hot luminescence at higher temperatures (>5 K) and in more highly doped samples, as well as with the dispersion of the exciton band obtained from the measured electron and hole effective masses.

Exciton–erbium energy transfer in Si nanocrystal-doped SiO 2

Silicon nanocrystals were formed in SiO 2 using Si ion implantation followed by thermal annealing. The nanocrystal-doped SiO 2 layer was implanted with Er to peak concentrations ranging from 0.015 to 1.8 at.%. Upon 458 nm excitation, a broad nanocrystal-related luminescence spectrum centered around 750 nm and two sharp Er luminescence lines at 982 and 1536 nm are observed. By measuring the temperature-dependent intensities and luminescence dynamics at a fixed Er concentration, and by measuring the Er concentration dependence of the nanocrystal and Er photoluminescence intensity, the nanocrystal excitation rate, the Er excitation and decay rate, and the Er saturation with pump power we conclude that: (1) the Er is excited by excitons recombining within Si nanocrystals through a strong coupling mechanism; (2) the exciton–Er energy transfer rate is >10 6 s −1; (3) the exciton–Er energy transfer efficiency is >60 %; (4) each nanocrystal can have at most ∼1–2 excited Er ions in its vicinity, which is attributed to either an Auger de-excitation or a pair-induced quenching mechanism; (5) at a typical nanocrystal concentration of 10 19 cm −3, the maximum optical gain at 1.54 μm of an Er-doped waveguide amplifier based on Si nanocrystal-doped SiO 2 is ∼0.6 dB cm −1; (6) the effective Er excitation cross-section using this nanocrystal sensitization scheme is σ eff≈10 −15 cm 2 at 458 nm, which is a factor 10 5–10 6 larger than the cross-section for direct optical pumping of Er. This enables the fabrication of an Er-doped nanocrystal waveguide amplifier that can be pumped using a white light source.

Progress in Growth and Physics of Nitride-Based Quantum Dots

Our recent progress in growth and optical properties of GaN-based quantum dot (QD) structures is reviewed. After discussing the impact of GaN-based QDs on threshold current characteristics, we have shown InGaN self-assembled QDs on a GaN epitaxial layer with average diameter as small as 8.4 nm and strong photoluminescence emission from the QDs at room temperature. Furthermore, light emission from individual QDs with sharp luminescence line was detected by single dot spectroscopy. Using these growth results, we fabricated a laser structure with InGaN QDs embedded in the active layer. A clear threshold was observed in the dependence of the emission intensity on the excitation energy at room temperature under optical excitation. Finally, growth of InGaN QDs grown by selective growth is also demonstrated.

Confined Multiexciton States of GaAs/AlGaAs Quantum Dots Grown on a (411)A GaAs Surface

We have investigated the micro photoluminescence (micro-PL) spectra of a single GaAs/AlGaAs quantum dot (QD) grown on a (411)A GaAs substrate, and observed sharp luminescence lines. These luminescence lines are classified into three types in terms of their excitation-power dependence. For one of them the intensity increases linearly with excitation power density, and the others that appear at the lower and higher energy sides of the linear one exhibit superlinear dependence, The difference in the excitation-power dependence reflects the number of excitons created in a QD, namely, the linear dependence originates from the lowest state of a confined exciton, and the others from biexcitonic and multiple exciton states.

Exciton–erbium interactions in Si nanocrystal-doped SiO2

The presence of silicon nanocrystals in Er doped SiO2 can enhance the effective Er optical absorption cross section by several orders of magnitude due to a strong coupling between quantum confined excitons and Er. This article studies the fundamental processes that determine the potential of Si nanocrystals as sensitizers for use in Er doped waveguide amplifiers or lasers. Silicon nanocrystals were formed in SiO2 using Si ion implantation and thermal annealing. The nanocrystal-doped SiO2 layer was implanted with different doses of Er, resulting in Er peak concentrations in the range 0.015–1.8 at. %. All samples show a broad nanocrystal-related luminescence spectrum centered around 800 nm and a sharp Er luminescence line at 1536 nm. By varying the Er concentration and measuring the nanocrystal and Er photoluminescence intensity, the nanocrystal excitation rate, the Er excitation and decay rate, and the Er saturation with pump power, we conclude that: (a) the maximum amount of Er that can be excited via exciton recombination in Si nanocrystals is 1–2 Er ions per nanocrystal, (b) the Er concentration limit can be explained by two different mechanisms occurring at high pump power, namely Auger de-excitation and pair-induced quenching, (c) the excitable Er ions are most likely located in an SiO2-like environment, and have a luminescence efficiency <18%, and (d) at a typical nanocrystal concentration of 1019 cm−3, the maximum optical gain at 1.54 μm of an Er-doped waveguide amplifier based on Si nanocrystal-doped SiO2 is ∼0.6 dB/cm.

Observation of resonant tunneling through single self-assembled InAs quantum dots using electrophotoluminescence spectroscopy

The resonant tunneling through single InAs quantum dots embedded in an n-GaAs/i-Al0.38Ga0.62As/n-GaAs diode has been studied by using microscopic electrophotoluminescence spectroscopy. Many sharp luminescence lines which originated from single quantum dots were observed by injecting resonant electrons from the emitter to the dots. Bias dependence of a single luminescence line was investigated. The peak intensity showed triangular dependence which was similar to the current–voltage characteristics of electron resonant tunneling from three dimension to zero dimension. When the bias voltage was increased, the peak energy slightly shifted to a lower energy indicating the existence of Stark effect, and the linewidth slightly increased. The higher the luminescence energy was, the broader the linewidth was. This result agrees with the calculated resonant level width. The lifetime of resonant states was estimated to be 2.4–27 ps for luminescence linewidth of 250–22 μeV.

Strong exciton-erbium coupling in Si nanocrystal-doped SiO2

Silicon nanocrystals were formed in SiO2 using Si ion implantation followed by thermal annealing. The nanocrystal-doped SiO2 layer was implanted with Er to a peak concentration of 1.8 at. %. Upon 458 nm excitation the sample shows a broad nanocrystal-related luminescence spectrum centered around 750 nm and two sharp Er luminescence lines at 982 and 1536 nm. By measuring the excitation spectra of these features as well as the temperature-dependent intensities and luminescence dynamics we conclude that (a) the Er is excited by excitons recombining within Si nanocrystals through a strong coupling mechanism, (b) the Er excitation process at room temperature occurs at a submicrosecond time scale, (c) excitons excite Er with an efficiency >55%, and (d) each nanocrystal can have at most ∼1 excited Er ion in its vicinity.

Thermal Shift and Broadening of Spectral Lines in Neodymium Doped Garnets

ABSTRACTThermal shift and broadening of sharp luminescence lines in three Nd-doped laser garnet crystals, GGG, GSGG, and CYMGG, are investigated in a temperature range of 78 to 600K. These lines are due to the 4F3/2 → 4I9/2 (∼0.94μ) and 4F3/2 → 4I11/2 (∼lμ) transitions. Line widths and positions at different temperatures are reported for the lines of RI→Yl, RI→Y2, R2→Y3, RI→Z5 in GGG:Nd, RI→Y1, RI→Y2, R2→Y1, R2→Y3, RI→Y6, R2→Y6, RI→Z5 in GSGG:Nd,Cr, and R2→Y1, RI→Z5 in CYMGG:Nd,Cr. We observed that all the lines shift to longer wavelengths (red shift) with increasing temperature except for the R1,2 →Z5 lines that shift to shorter wavelength (blue shift) in all the three garnet crystals. For the widths, all the lines become broader with increasing temperature. It was also observed that the line width in the germanium garnet CYMGG is much broader than in GGG and GSGG.

Observation of many-body effects in the excitonic spectra of semiconductor quantum wires

We report the observation of a blue shift of very sharp luminescence lines in GaAs/AlGaAs quantum wires at increasing excitation density. These sharp lines correspond to recombination of one-dimensional excitons localized in local potential minima due to interface disorder. The evolution of the fine-structured photoluminescence spectra with increasing exciton density is interpreted in terms of many-body effects which lead to an increase of the exciton self-energy and to the formation of excitonic complexes

Effects of Si-doping on luminescence properties of InxGa1−xN epitaxial layers

Recombination dynamics of intense near-UV luminescence from InxGa1−xN (x⩽0.1) ternary alloy epitaxial layers, especially from Si-doped ternary alloy layers, has been studied by means of time-resolved spectroscopy. The intensity of the near-UV luminescence from Si-doped layers was approximately one order of magnitude stronger than that from undoped layers. Furthermore, an additional sharp luminescence line was observed under high-density excitation only from the Si-doped layers. The effects of Si-doping on the optical properties of InxGa1−xN layers are discussed.

Magnetic field effects on luminescence and excitation spectra in β-ZnP 2

Magnetic field effects on the triplet exciton are investigated in β-ZnP 2. The sharp luminescence line of the 1s triplet exciton weakly allowed for the polarization E ∥ b splits into two or three lines with g-value of 4.0 according to the direction of the applied magnetic field H ∥ b or H ∥ c and also to the polarization E ∥ b or E ∥ c for which the luminescence is observed. These results are explained simply by a group theory.

Absorption and luminescence spectroscopy of Zn 2SiO 4 willemite crystals doped with Co 2+

The polarized absorption spectra of Zn 2SiO 4 willemite crystals doped with Co 2+ consist of three band systems in the near-infrared and visible spectra region centered at 3800, 7000, and 17000 cm −1, respectively. In the tetrahedral approximation they are assigned to the d → d transitions 4A 2 → 4T 2 , 4T 1( 4F), and 4T 1( 4P), respectively. The crystal field parameter 10 Dq is 4000 cm −1, and the Racah parameters B and C are 740 and 3330 cm −1, respectively. From the fine structure in the origin region of the 4A 2 → 4T 2 absorption band it follows that the 4A 2 ground-state splittings of Co 2+ occupying the two crystallographically inequivalent Zn 2+ sites of C 1 symmetry are 13 and 15 cm −1, respectively. Only a weak emission originating at 15335 cm −1 is observed upon 4A 2 → 4T 1( 4P) photoexcitation. It is assigned to the 2E → 4A 2 sharp-line luminescence of Co 2+ located in a minority site. No luminescence is observed from regularly incorporated Co 2+ ions.

Visible and infrared (1.54 μm) emission from Er-lmplanted porous Si for photonic applications

This paper explores the challenges faced in developing efficient room temperature porous Si-based light emitting diodes. We experimentally demonstrate that porous Si is an excellent host material for erbium ions to emit strong, room temperature, sharp-line luminescence at 1.54 μm. A comparison of photoluminescence data for erbium implanted samples of bulk Si, porous Si, and quartz indicate that quantum confinement likely enhances the erbium IR luminescence efficiency. Due to the 650 to 850°C annealing, it is unlikely that the environment of erbium in our samples is amorphous Si.

Energy-Transfer Processes in Oxygen-Codoped GaAs:Er

AbstractGaAs grown by metalorganic chemical vapor deposition and codoped with Er and oxygen shows a simple spectrum with sharp luminescence lines due predominantly to one type of Er center. This center is identified as an Er atom substituting a Ga site, coupled with two oxygen atoms. Photoluminescence measurements under host excitation and photoluminescence excitation measurements that directly excite the 4f-shell of the Er3+ ion indicate that this center is excited much more efficiently than other Er centers simultaneously present in the same sample. A possible energy-transfer mechanism and its implications are discussed.

X-ray photoelectron spectroscopy and optoelectrical properties of low-concentration erbium-doped GaSb layers grown from Sb-rich solutions by liquid-phase epitaxy

We have attempted to grow low hole-concentration GaSb layers by introducing the rare-earth element Er into Sb-rich solutions by liquid-phase epitaxy. The x-ray photoelectron spectra suggest that the strong affinity of Er will lead to interactions between the Er and residual group-VI impurities (e.g., O, S, and Te) in the growth melt for efficient Er gettering and between the Er and the low electronegativity of group-V element Sb for the formation of stable chalcogenides. The carrier concentration of GaSb layers can be lowered due to the Er gettering and the suppression of complex acceptor defects. Intense sharp luminescence lines of free-exciton and excitons bound to neutral acceptors dominate the low-temperature photoluminescence spectra. The higher breakdown voltage exhibited in the Er-doped GaSb mesa diodes is due to the reduction of carrier concentration in the Er-doped GaSb layers.

Comment on: room-temperature line emission at 1.2 μm from chromium-activated barium ortho-titanate (J. Lumin. 59 (1994) 113)

The sharp line luminescence around 1.2 μm reported in [J. Lumin. 59 (1994) 113] for chromium doped Ba 2TiO 4 is shown to be due to a Mn 5+ impurity. Cr 4+ doped Ba 2TiO 4 exhibits a broad band luminescence at 20 K with a lifetime τ ⩽ 7 μs.

Transition-Metal Impurity Luminescence in GaAs and its Application to Material Characterization

AbstractA number of sharp characteristic luminescence lines has been observed for GaAs doped with 3d transition-metal impurities in the near-infrared region, the origin being attributed to the zero-phonon intracenter transitions between the energy levels of the metal ions split by the crystal field of the GaAs lattice. It is also known that these luminescence lines are very sensitive to the surrounding field of the transition-metal impurities. The luminescence of Cr-doped GaAs has been most extensively studied, the spectrum revealing a very sharp luminescence line at 0.839 eV. In this paper we review the results on the successful applications of this Cr-related luminescence line to characterization of in-depth profiles of arsenic vacancy in thermally annealed GaAs, local strain field in In-doped GaAs and interface stress at heterostructures grown on Cr-doped GaAs substrate.

Absorption and luminescence spectroscopy of ferrate (VI) doped into crystals of K 2MO 4 (M = S, Se, Cr, Mo)

The absorption spectra of the ferrate (VI) ion (FeO 2- 4) in K 2MO 4 (M = S, Se, Cr, Mo) host lattices consist of a series of relatively weak bands at low energy, which can be assigned to transitions within the partially filled 3d shell and some intense bands at higher energy, which are assigned to ligand-to-metal charge transfer transitions (LMCT). In the near-infrared (NIR) region sharp lines are observed belonging to the spin-forbidden spin-flip transitions 3A 2 → 1E and 3A 2 → 1A 1. The lowest excited state is the 1E state, serving as initial state for 1E → 3A 2 sharp-line luminescence at around 6200 cm -1. Another luminescence is observed centered at 9000 cm -1, which is assigned to the 3T 2 → 3A 2 transition. It is rather broad and three orders of magnitude weaker than the 1E luminescence at 30K as a result of efficient non-radiative relaxation processes to the 1E state. The temperature dependence of the total intensity and the lifetime of the 1E → 3A 2 luminescence is understood within a complex scheme of radiative and non-radiative processes.

High resolution photoluminescence studies of acceptor bound excitons in CDS/CDTE heterostructure under magnetic field.

CdS layers have been grown by CVD onto (111) CdTe substracts. The layers exhibit high quality hexagonal structure; sharp luminescence lines identified with radiative recombination of excitons bound to donors and acceptors were detected. In particular two novel transitions at 2.5458 eV and 2.5462 eV could be seen in the PL spectrum at 4K. In bulk CdS only one line occurs at 2.5459 eV and has been associated with the recombination of an exciton bound to a neutral acceptor. In order to identify the origin of the two novel lines the effects of temperature and of magnetic field, applied parallel and perpendicularly to the hexagonal axis, have been investigated using high resolution photoluminescence spectroscopy. The results are quantitatively analysed in terms of transitions occuring from electronic states of the acceptor bound exciton complex (A 0,X) and we show that they can be understood if the two lines occur at two different acceptor centres.

Diamond luminescence: resolved donor-acceptor pair recombination lines

Cathodoluminescence and photoluminescence were analysed in diamond films prepared by microwave plasma assisted chemical vapour deposition. Results are presented for various diamond films differing in doping and crystalline structure. In the energy range 2.67−2.07eV, a series of 13 sharp luminescence lines was observed at temperatures below 200 K. In a tentative interpretation this 2.67 eV system is attributed to donor-acceptor pair recombination. The Coulomb energy depends on the discrete values for the pair separation in the expected manner, including a Bohr radius correction for the three closest pairs. Likely candidates for the pairs are the nitrogen donor and the boron acceptor. The presence of nitrogen is evident from the luminescence of the 2.16, 2.81 and 3.19 eV nitrogen centres. Boron is present by doping or contamination. Vibronic sidebands from the nearest neighbour pair were observed and analysed. Characteristic local vibrational mode frequencies of nitrogen and the boron isotopes are found, providing further support for the proposed model.