Showing papers on "Photoluminescence published in 1995"

Light-Induced Redox Reactions in Nanocrystalline Systems

Abstract: A review with 156 refs. on interfacial electron transfer reactions in colloidal semiconductor solns. and thin films and their application for solar light energy conversion and photocatalytic water purifn. Some of the topics discussed include; optical and electronic properties of colloidal semiconductor particles, quantum size effects in the photoluminescence of colloidal semiconductors, light-induced charge sepn., dynamics of interfacial charge transfer processes, properties and prepn. of nanocryst. semiconductor electrodes, energetics and operations of the nanoporous solar cell.

Growth and optical properties of nanometer‐scale GaAs and InAs whiskers

Abstract: The growth process, crystal structure, and optical properties of ultrathin GaAs and InAs wires (whiskers) as thin as 15–40 nm and about 2 μm long are reviewed and discussed. Experimental results for growing whiskers using Au as a growth catalyst during metalorganic vapor phase epitaxy (MOVPE) and the shape and growth direction of whiskers provide new insight into growth control of GaAs and InAs whiskers. The crystal structure of whiskers, Au behavior during MOVPE, and their growth mechanism are reviewed and discussed on the basis of transmission electron microscopic analysis. The photoluminescence spectra of GaAs wires are compared with those of a GaAs epitaxial layer, and the effect of surface treatment on the luminescence peak energy shift is discussed. The time dependent photoluminescence of GaAs wires is also discussed. The application of GaAs whiskers to light emitting devices is reviewed because a semiconductor wire structure employing quantum size effects is a very important element of electronic and optical devices.

Visible photoluminescence from nanocrystallite Ge embedded in a glassy SiO 2 matrix: Evidence in support of the quantum-confinement mechanism

Abstract: Nanocrystallite Ge (nc-Ge) embedded in a glassy ${\mathrm{SiO}}_{2}$ matrix is fabricated and examined by x-ray photoelectron spectrometry, Raman spectrometry, and high-resolution transmission-electron microscopy. The precipitation and growth of nc-Ge are found to be related to a thermodynamical reduction of ${\mathrm{GeO}}_{2}$, the diffusion of Si atoms from the Si substrate into the glassy matrix, and an aggregation of small-sized nc-Ge. The size inhomogeneity can be precisely controlled by a double annealing, and the average size can be changed in the range of 2\char21{}6 nm. Broadband photoluminescence (PL) spectra are observed in the visible wavelength range at room temperature, and they exhibit pronounced blueshifts of the peak energies and broadening of the PL spectra, which can be correlated to the change in the size. The PL excitation spectra show a Stokes energy smaller than 0.1 eV and dependence on the measurement energy. The visible PL also shows a strong correlation to the presence and actual condition (i.e., size) of nc-Ge. Possible origins of the visible PL such as a quantum-confinement model in quantum dots, the presence of luminescent centers (Ge:E') in silica glass, or a structural transition of nc-Ge are discussed. The present experimental data are concluded to be more consistent with a quantum-confinement model than with the other models.

Ga2O3 films for electronic and optoelectronic applications

Abstract: Properties of Ga2O3 thin films deposited by electron‐beam evaporation from a high‐purity single‐crystal Gd3Ga5O12 source are reported. As‐deposited Ga2O3 films are amorphous, stoichiometric, and homogeneous. Excellent uniformity in thickness and refractive index was obtained over a 2 in. wafer. The films maintain their integrity during annealing up to 800 and 1200 °C on GaAs and Si substrates, respectively. Optical properties including refractive index (n=1.84–1.88 at 980 nm wavelength) and band gap (4.4 eV) are close or identical, respectively, to Ga2O3 bulk properties. Reflectivities as low as 10−5 for Ga2O3/GaAs structures and a small absorption coefficient (≊100 cm−1 at 980 nm) were measured. Dielectric properties include a static dielectric constant between 9.9 and 10.2, which is identical to bulk Ga2O3, and electric breakdown fields up to 3.6 MV/cm. The Ga2O3/GaAs interface demonstrated a significantly higher photoluminescence intensity and thus a lower surface recombination velocity as compared to ...

Synthesis and Characterization of InP, GaP, and GaInP2 Quantum Dots

Abstract: Quantum dots of InP, GaP and GaInP{sub 2} with diameters ranging from 20-65 {Angstrom} were synthesized as well-crystallized nanoparticles with bulk zinc blende structure. The high sample quality of the InP and GaP QDs results in excitonic features in the absorption spectra. The GaP and GaInP{sub 2} QD colloids exhibited very intense (quantum yields of 15-25%) visible photoluminescence at room temperature. The photoluminescence for InP preparations showed two emission bands: one band in the visible at the band edge of the QD (50 nm Stokes shift), and a second band above 800 nm. The near-IR PL is attributed to deep traps, presumably phosphorus vacancies on the QD surface. This band disappears after activation of particles by controlled addition of etchant. In that case very intense band-edge emission (quantum yield 30% at room temperature and 60% at 10 K) was obtained. The QDs were characterized by TEM, SAXS, AFM, powder x-ray diffraction, steady state optical absorption and photoluminescence spectroscopy, ps to ns transient photoluminescence spectroscopy, and fs to ps pump-probe absorption (i.e., hole-burning) spectroscopy. Results will also be reported on efforts to produce ordered arrays of InP QDs.

Optical investigations of defects in Cd1-xZnxTe.

Abstract: We investigated the optical properties of defects in CdTe and ${\mathrm{Cd}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Zn}}_{\mathit{x}}$Te (0x1). Residual impurities give rise to specific far infrared absorptions, while metal vacancy-donor complexes (A centers), identified by optically detected magnetic resonance, are characterized by their near infrared (1.4 eV) photoluminescence (PL) properties. The specific zero-phonon-line positions and phonon couplings are worked out for these complexes involving different group-VII (F, Cl, Br, In) or group-III (In, Al) donors. In addition to the A center PL two emission bands are found at 1.135 and at 1.145 eV. The temperature dependences of the PL show that the 1.145 eV luminescence follows the temperature dependence of the band gap, while the energy position of the 1.135 eV band emission shifts to higher energies with increasing temperature. The A-center PL and the luminescence bands at 1.1 eV are investigated throughout the complete alloy composition range form x=0 to 1. The A center and the 1.135 eV band were found to follow the band-gap shift from CdTe to ZnTe, whereas the 1.145 eV luminescence keeps its emission energy constant.

Size, shape, and composition of luminescent species in oxidized Si nanocrystals and H-passivated porous Si

Abstract: Near-edge and extended x-ray-absorption fine-structure measurements from a wide variety of oxidized Si nanocrystals and H-passivated porous Si samples, combined with electron microscopy, ir absorption, forward recoil scattering, and luminescence emission data, provide a consistent structural picture of the species responsible for the luminescence observed in these systems. For porous Si samples whose luminescence wavelengths peak in the visible region, i.e., at {lt}700 nm, their mass-weighted-average structures are determined here to be particles (not wires) whose short-range character is crystalline and whose dimensions---typically {lt}15 A---are significantly smaller than previously reported or proposed. Results are also presented which demonstrate that the observed visible luminescence is not related to either a photo-oxidized Si species in porous Si or an interfacial suboxide species in the Si nanocrystals. The structural and compositional findings reported here depend only on sample luminescence behavior, not on how the luminescent particles are produced, and thus have general implications in assigning quantum confinement as the mechanism responsible for the visible luminescence observed in both nanocrystalline and porous silicon.

Radiative recombination in type-ii gasb/gaas quantum dots

Abstract: Strained GaSb quantum dots having a staggered band lineup (type II) are formed in a GaAs matrix using molecular beam epitaxy. The dots are growing in a self‐organized way on a GaAs(100) surface upon deposition of 1.2 nm GaSb followed by a GaAs cap layer. Plan‐view transmission electron microscopy studies reveal well developed rectangular‐shaped GaSb islands with a lateral extension of ∼20 nm. Intense photoluminescence (PL) is observed at an energy lower than the GaSb wetting layer luminescence. This line is attributed to radiative recombination of 0D holes located in the GaSb dots and electrons located in the surrounding regions. The GaSb quantum dot PL dominates the spectrum up to high excitation densities and up to room temperature.

Room‐temperature visible luminescence from silicon nanocrystals in silicon implanted SiO2 layers

Abstract: We report the observation of visible‐light emission at room temperature from high fluence (0.3–3×1017 cm−2) Si+ implanted thermal SiO2 layers grown on silicon substrates. Significant blue‐light emission and an intense broad luminescent band with a peak beyond 750 nm are observed after annealing at high temperature (T≥1000 °C). The red‐light emission, present only in the highest fluence implant, is attributed to the luminescence emitted from silicon nanocrystals produced by silicon precipitation. The presence of silicon nanocrystals is confirmed by transmission electron microscopy. Significant blue‐light emission is visible after thermal annealing in the 1×1017 cm−2 fluence implant. The peak position shifts from 490 to 540 nm by increasing the annealing cycles temperature.

Green luminescence from copper doped zinc sulphide quantum particles

Abstract: Free‐standing powder of zinc sulphide quantum particles has been synthesized using a chemical route. X‐ray diffraction analysis shows that the diameter of the particles is ∼21±2 A which is smaller than the Bohr exciton diameter for zinc sulphide. UV absorption shows an excitonic peak centered at ∼300 nm corresponding to an energy gap of 4.1±0.1 eV. These particles show a luminescence band at ∼424 nm. The quantum particles could be doped with copper during synthesis without altering the UV absorption or x‐ray diffraction pattern. However, doping shifted the luminescence to 480 nm, green wavelength in the visible region.

Optical properties of rutile near its fundamental band gap.

Abstract: We report low-temperature absorption, time-integrated photoluminescence, and resonant-Raman-scattering spectra near the fundamental band gap of ${\mathrm{TiO}}_{2}$ single crystals. The photoluminescence spectrum comprises a first peak at \ensuremath{\Elzxh}\ensuremath{\omega}=3.031 eV, followed by several peaks at lower energies. A polarization study of the emission spectrum indicates that the highest energy peak corresponds to 2${\mathit{p}}_{\mathit{x}\mathit{y}}$ dipole-allowed second-class excitonic transitions while the lower-energy peaks are phonon replicas of the 1s quadrupolar exciton. This result is corroborated by time-resolved photoluminescence measurements. The near-band-gap optical response of ${\mathrm{TiO}}_{2}$ is thus controlled by two distinct exciton states. The Raman-scattering intensity is found to increase slowly for excitation energies in the range 2.7--3 eV. This indicates that the Raman cross-section enhancement is dominated by virtual transitions involving the first dipole-allowed direct gap at 4.2 eV.

Island formation during growth of Ge on Si(100): A study using photoluminescence spectroscopy

Abstract: We present a photoluminescence (PL) study on the growth mode changeover during growth of Ge on Si(100) substrates. Intense PL signals originating from both the flat Ge layer and the three‐dimensional (3D) Ge islands are observed from Si/Ge/Si quantum wells with various Ge coverage. The onset of the 3D island formation is determined to be 3.7 monolayers (ML). It is also found that the 3D islands grow with only 3.0 ML of the flat Ge layer retained. This implies that only the 3.0 ML Ge is thermodynamically stable on Si(100) and hence corresponds to the ‘‘equilibrium’’ critical thickness.

Temperature dependence of interband transitions in GaN grown by metalorganic chemical vapor deposition

Abstract: The interband transitions in single‐crystal GaN films grown by metalorganic chemical vapor deposition (MOCVD) have been studied as a function of temperature (15≤T≤300 K) by reflectance and photoluminescence measurements. At low temperatures, well‐resolved spectral features corresponding to the GaN band structure were observed. The energies of the excitonic interband ΓV9−ΓC7,ΓV7 (upper band)−ΓC7 and ΓV7(lower band)−ΓC7 transitions are found to be 3.485, 3.493, and 3.518 eV at 15 K, respectively, for the MOCVD GaN. The spectral features are broadened and shift to lower energy as temperature increases. At room temperature (300 K), the ΓV9−ΓC7and ΓV7 (upper band) −ΓC7 transition energies of this wide band‐gap material are determined to be 3.420 and 3.428 eV, respectively. The temperature dependence of these two transitions have been determined using the Varshni empirical relation. Our results yield E0(T)=3.486–8.32×10−4 T2/(835.6+T) eV for the ΓV9−ΓC7 transition and E0(T)=3.494–10.9×10−4 T2/(1194.6+T) eV for ...

1.3 μm photoluminescence from InGaAs quantum dots on GaAs

Abstract: We use molecular beam epitaxy to grown coherently strained InGaAs islands on (100) GaAs substrates. The islands show room‐temperature photoluminescence at 1.3 μm with a full width at half‐maximum of only 28 meV. The integrated photoluminescence intensity is comparable to that of a quantum well. The islands are formed by depositing 22 monolayers of In0.3Ga0.7As with alternating beams of In, Ga, and As2. Atomic force microscopy measurements show that the islands are ellipsoidal sections with an average peak height of 24 nm. The intersection of the islands with the (100) plane is an ellipse whose major axis is along [011] and has a mean length of 54 nm, and whose minor axis is along [011] and has a mean length of 36 nm. The islands form a dense array with an areal coverage of about 40%.

Luminescence quenching by OH groups in highly Er-doped phosphate glasses

Abstract: Highly (up to 4 mol% Er2O3) Er-doped phosphate bulk glasses have been prepared by common glass-melting techniques. Afterwards, a heat treatment was performed on the as-melted samples. The photoluminescence lifetime of Er ions for the 4I132–4I152 transition increases substantially, typically from 3 ms up to 7 ms for a sample doped with 2 mol% Er2O3, due to the heat treatment. The increase of the lifetime is ascribed to a decrease in concentration of hydroxyl groups incorporated in the glass, which is confirmed by IR absorption spectroscopic measurements. The photoluminescence peak intensity also increases because of drying by a factor of 3 to 7 depending on glass composition. Based on electric dipole-dipole interaction theory, the luminescence concentration quenching mechanism by hydroxyl groups is modelled. The model predicts that more than half of the hydroxyl groups in the glass is coupled to Er ions. The influence of the glass structure and role of Al3+ on the Er3+ luminescence is studied by infrared spectroscopy.

Measuring and Modifying the Spontaneous Emission Rate of Erbium near an Interface

Abstract: Spontaneous emission is interpreted as a consequence of interaction between matter and electromagnetic radiation. Previous experiments, using for instance Rydberg atoms or semiconductor structures, have demonstrated that spontaneous emission can be influenced in cavities or near mirrors [1 ‐ 7]. In this Letter we will show an extremely simple way to modify the spontaneous emission rate, merely by bringing liquid films with certain refractive index in contact with a silica glass surface which is locally doped with luminescent Er 31 ions. Erbium shows clear photoluminescence (PL) around 1.54 mm [8], an important wavelength in optical telecommunication [9]. The theoretical description of spontaneous emission is usually based on concepts from quantum electrodynamics such as vacuum fluctuations and a creation and annihilation formalism. The inclusion of a dielectric interface requires special attention [10]. In this case, the variation in the spontaneous emission rate can be accounted for by the local classical density of states (DOS), which appears in Fermi’s golden rule. In this Letter, a straightforward calculation of the local DOS is performed, given the Fresnel equations for transmission and refraction at a dielectric interface. The analysis describes our data on the modification of the spontaneous emission rate as a function of refractive index, and serves to determine for the first time the radiative lifetime of Er in silicate glass, a parameter of great importance for Er-doped optical gain materials in telecommunication technology. Two samples of bulk sodalime silicate glass (refractive index n0 › 1.5) were implanted with 500 keV Er ions. Sample A was covered with a 120 nm thick Al film as a stopping layer, and sample B was uncovered during implantation. After implantation, the Al layer was etched off, and a thermal anneal at 512 ‐ C was performed. Erbium depth profiles for both samples, as determined using 2 MeV 4 He 1 Rutherford backscattering spectrometry (RBS), are shown in Fig. 1. The open data points for sample A show a profile peaked at the glass surface, with a half-width of 70 nm. The measured profile is a convolution of the detection resolution and the actual profile which is discontinuous at the surface. The drawn line shows the deconvoluted profile, which has a surface concentration of , 0.25 at. %. The solid data points in Fig. 1 show a Gaussian profile for sample B, centered at 150 nm depth from the surface, where the Er concentration is 0.17 at. %. The full width at half maximum is 100 nm. PL spectroscopy was carried out at room temperature. The 514.5 nm line of an Ar-ion laser was used to excite the Er, and the luminescence was spectrally analyzed with a monochromator and detected with a liquid-nitrogen cooled Ge detector. PL decay measurements were performed after exciting with a 1.5 ms pulse, using a digital averaging oscilloscope. Further details on sample preparation and PL measurements can be found in Ref. [11]. Various liquids, with refractive indices ranging from 1.3 to 1.7, and thickness in the order of a mm, were brought in contact with the sample surface on the front, while the luminescence signal was collected on the back side (see inset in Fig. 2).

Photoluminescence and electroluminescence of SiGe dots fabricated by island growth

Abstract: We present a study of photo‐ and electroluminescence of SiGe dots buried in Si and compare them with structures containing smooth SiGe layers. The SiGe dot structures were fabricated by low‐pressure chemical vapor deposition using the Stranski–Krastanov growth mode (island growth). We show that the localization of excitons in the dots leads to an increase of the luminescence efficiency at low excitation compared to smooth SiGe layers (e.g., quantum wells). At higher excitation the efficiency decreases which is attributed to nonradiative Auger recombination processes in the dots.

Properties of the yellow luminescence in undoped GaN epitaxial layers

Abstract: Photoluminescence, time-integrated, time-resolved, and photoluminescence excitation spectroscopy have been employed to study the 2.2 eV (``yellow'') emission in undoped GaN epitaxial layers. It is best described by a recombination model involving shallow donors and deep donors of probably intrinsic origin. Optically detected magnetic resonance reveals the participation of the shallow donor based on the analysis of the g value and Lorentzian line shape.

Mechanism of yellow luminescence in GaN

Abstract: We investigated the pressure behavior of yellow luminescence in bulk crystals and epitaxial layers of GaN. This photoluminescence band exhibits a blueshift of 30±2 meV/GPa for pressures up to about 20 GPa. For higher pressure we observe the saturation of the position of this luminescence. Both effects are consistent with the mechanism of yellow luminescence caused by electron recombination between the shallow donor (or conduction band) and a deep gap state of donor or acceptor character.

Efficient interchain photoluminescence in a high-electron-affinity conjugated polymer.

Abstract: We report measurements of photoluminescence efficiency and time dependence in a high-electron-affinity cyano-substituted derivative of poly(p-phenylenevinylene). We find that the luminescence is much longer lived in solid films than in solutions and interpret this result as emission in the film being from an interchain excitation. These results suggest that intermolecular interactions are an important consideration in the design of highly luminescent conjugated polymers.

Optical spectroscopy of opal matrices with CdS embedded in its pores: Quantum confinement and photonic band gap effects

Abstract: The spectra of transmission and reflection of synthetic opal which has 3-dimensional periodic structure were measured at different orientations of incident beam relative to the sample facets. It is shown that opal behaves as «semi-metallic» photonic band gap (PBG) material in the vicinity of photon energy 2.3 eV. The synthesis of CdS microcrystals embedded in the pores of opal was made for the first time in an attempt to form a system of quantum dots. Optical spectra (reflection and transmission, photoluminescence and Raman scattering) were studied. The results demonstrate good crystallinity of microcrystals embedded in opal matrix and exhibit well-pronounced quantum confinement effects in fundamental edge absorption spectra. The spectral overlap of the PBG of opal with electronic band gap of many of II–VI semiconductors seems to make opal/semiconductor system a promising media for experimental studies of such PBG-related effects as inhibition of spontaneous emission, microcavity polariton, etc.

Radiative states in type-II GaSb/GaAs quantum wells.

Abstract: We have studied optical properties of staggered band line-up (type-II) heterostructures based on strained GaSb sheets in a GaAs matrix. The giant valence-band offset characteristic to this heterojunction leads to an effective localization of holes in ultrathin GaSb layers. An intense photoluminescence (PL) line caused by radiative recombination of localized holes with electrons located in the nearby GaAs regions is observed. The separation of nonequilibrium electrons and holes in real space results in a dipole layer and, thus, in the formation of quantum wells for electrons in the vicinity of the GaSb layer. The luminescence maximum shifts towards higher photon energies with rising excitation density reflecting the increase in the electron quantization energy. A bimolecular recombination mechanism is revealed in PL and in time-resolved PL studies. In the case of pseudomorphic monolayer-thick GaSb layers, the radiative exciton ground state does not exist. Accordingly, small absorption coefficients and a featureless behavior of the band-to-band calorimetric absoprtion spectrum are found in the vicinity of ${\mathit{k}}_{\mathit{x},}$y=0. Remarkable enhancement of the absorption coefficient with a characteristic onset is observed for heavy holes with ${\mathit{k}}_{\mathit{x},}$yg0. Radiative states in the continuum of heavy-hole subbands are revealed also in temperature-dependent PL studies. The experimentally measured onset energies point out the importance of GaSb heavy- and light-hole mixing effects. We demonstrate intense luminescence from staggered band line-up GaSb-GaAs heterostructures up to room temperature.

The erbium‐impurity interaction and its effects on the 1.54 μm luminescence of Er3+ in crystalline silicon

Abstract: We have studied the effect of erbium‐impurity interactions on the 1.54 μm luminescence of Er3+ in crystalline Si. Float‐zone and Czochralski‐grown (100) oriented Si wafers were implanted with Er at a total dose of ∼1×1015/cm2. Some samples were also coimplanted with O, C, and F to realize uniform concentrations (up to 1020/cm3) of these impurities in the Er‐doped region. Samples were analyzed by photoluminescence spectroscopy (PL) and electron paramagnetic resonance (EPR). Deep‐level transient spectroscopy (DLTS) was also performed on p‐n diodes implanted with Er at a dose of 6×1011/cm2 and codoped with impurities at a constant concentration of 1×1018/cm3. It was found that impurity codoping reduces the temperature quenching of the PL yield and that this reduction is more marked when the impurity concentration is increased. An EPR spectrum of sharp, anisotropic, lines is obtained for the sample codoped with 1020 O/cm3 but no clear EPR signal is observed without this codoping. The spectrum for the magnetic...

Exciton Coupling Effects in the Absorption and Photoluminescence of Sexithiophene Derivatives

Abstract: published in Advance ACS Abstracts, May 1, 1995. du Markcha1 Joffre, 92002 Nantene, France. 0022-365419512099-9155$09.00/0 SCHEME 1: Chemical Structure of Sexithiophene Derivatives 6T DH6T ClOH21 ClOH21 DD6T have been widely discussed in the literature, and particularly in the case of sexithiophene (6T), very little work has been carried out on the optical characteristics of 6T, and the energetic diagram of this compound is still an object of debate. From photoluminescence measurements under one- and two-photon excitation on a polycrystalline thin film of 6T, an electronic level ordering has been proposed by Taliani et al. in which it is suggested that the lowest Ag exciton level lies at 898 cm-’ above the lowest one-photon-allowed lB, exciton leveL4 Athouel et al. have reported that the n-n* transition in p-sexiphenyl is affected by orientation of these molecules in a thin film.5 Hamano et al. have fabricated molecular oriented thin films of 6T by organic molecular beam deposition.

Electron and hole energy levels in InAs self‐assembled quantum dots

Abstract: Capacitance spectroscopy is used to determine the allowed energy levels for electrons and holes in InAs self‐assembled quantum dots embedded in GaAs. Using this technique, the relative energy of the electron and hole states is measured with respect to their respective energy band minima in the GaAs. This allows the construction of an energy level diagram for these quantum dots which correlates well with previously observed photoluminescence data. By tuning the device geometry, a fine structure in the electron ground state is revealed and attributed to Coulomb charging effects.

Spatially Resolved Visible Luminescence of Self-Assembled Semiconductor Quantum Dots

Abstract: Ensembles of defect-free InAIAs islands of ultrasmall dimensions embedded in AIGaAs have been grown by molecular beam epitaxy. Cathodoluminescence was used to directly image the spatial distribution of the quantum dots by mapping their luminescence and to spectrally resolve very sharp peaks from small groups of dots, thus providing experimental verification for the discrete density of states in a zero-dimensional quantum structure. Visible luminescence is produced by different nominal compositions of In x AI( 1– x ) As-AI y Ga (1– y ) As.

Infrared to visible upconversion in Er3+‐doped‐lead‐germanate glass: Effects of Er3+ ion concentration

Abstract: The upconversion mechanism of Er3+ ions has been studied for lead‐germanate glasses containing Er2O3 concentrations from 0.1 mol % to 2 mol %. Intense green emission was observed at room temperature due to 4S3/2→4I15/2 transition excited by a cw near‐infrared laser beam at 797 nm. This green emission shows a similar intensity for samples with different Er3+ ion concentrations. A weak blue emission of 410 nm originating from the 2H9/2→4I15/2 transition was also observed. This blue emission and a red emission from the 4F9/2 level increase with the increase in Er3+ ion concentration. The bright green emission is attributed to the excited level absorption while the blue emission is due to a third step excitation where energy transfer between excited ions owing to their Coulomb interaction plays a key role.