Showing papers on "Cathodoluminescence published in 2001"

Photoluminescence and cathodoluminescence studies of stoichiometric and oxygen-deficient ZnO films

Abstract: Photoluminescence and cathodoluminescence (CL) spectra of stoichiometric and oxygen-deficient ZnO films grown on sapphire were examined. It was found that the intensities of the green and yellow emissions depend on the width of the free-carrier depletion region at the particle surface; the thinner the width, the larger the intensity. Experimental results and spectral analyses suggest that the mechanism responsible for the green (yellow) emission is the recombination of a delocalized electron close to the conduction band with a deeply trapped hole in the single ionized oxygen vacancy Vo+ (the single negatively charged interstitial oxygen ion Oi−) center in the particle.

High throughput fabrication of transition-metal-doped epitaxial ZnO thin films: A series of oxide-diluted magnetic semiconductors and their properties

Abstract: Combinatorial laser molecular-beam epitaxy method was employed to fabricate epitaxial ZnO thin films doped with all the 3d transition metal (TM) ions in a high throughput fashion The solubility behavior of TM ions was discussed from the viewpoints of the ionic radius and valence state The magneto-optical responses coincident with absorption spectra were observed for Mn- and Co-doped samples Cathodoluminescence spectra were studied for Cr-, Mn-, Fe-, and Co-doped samples, among which Cr-doped ZnO showed two sharp peaks at 297 eV and 371 eV, respectively, at the expense of the exciton emission peak of pure ZnO at 325 eV Different magnetoresistance behavior was observed for the samples codoped with n-type carriers Ferromagnetism was not observed for Cr- to Cu-doped samples down to 3 K

Edge and screw dislocations as nonradiative centers in InGaN/GaN quantum well luminescence

Abstract: Transmission electron microscopy (TEM) and scanning electron microscope cathodoluminescence (CL) have been used to determine the influence of edge and screw dislocations on the light emitting properties of InxGa1−xN quantum wells TEM is used to locate and identify the nature of dislocations CL on the same samples is used to determine the spatial variation of the luminescence A direct correlation of CL maps with TEM has been established, showing that threading edge dislocations act as nonradiative recombination centers with an associated minority carrier diffusion length of 200 nm Threading dislocations of screw and mixed type were found to be associated with surface pits which were also nonradiative in the quantum well (QW) emission, but owing to the absence of QW growth on the pit facets The contributions of edge and screw/mixed dislocations to the reduction of the QW emission are quantified, and the wider significance of these results is discussed

Compositional pulling effects in InxGa1-x N/GaN layers: A combined depth-resolved cathodoluminescence and Rutherford backscattering/channeling study

Abstract: A depth-resolved study of the optical and structural properties of wurtzite InGaN/GaN bilayers grown by metallorganic chemical vapor deposition on sapphire substrates is reported. Depth-resolved cathodoluminescence (CL) and Rutherford backscattering spectrometry (RBS) were used to gain an insight into the compositional profile of a 75-nm thick InGaN epilayer in the direction of growth. CL acquired at increasing electron energies reveals a peak shift of about 25 meV to the blue when the electron beam energy is increased from 0.5 to \ensuremath{\sim}7 keV, and shows a small shift to lower energies between \ensuremath{\sim}7 and 9 keV. For higher accelerating voltages the emission energy peak remains constant. This behavior can be well accounted for by a linear variation of In content over depth. Such an interpretation conforms to the In/Ga profile derived from RBS, where a linear decrease of the In mole fraction from the near surface (\ensuremath{\sim}0.20) down to the near GaN/InGaN interface (\ensuremath{\sim}0.14) region fits the random spectra very well. Furthermore, by measuring the tetragonal distortion at different depths, using RBS/channeling, it is shown that regions of higher In content also appear to be more relaxed. This result suggests that strain hinders the incorporation of In atoms in the InGaN lattice, and is the driving force for the compositional pulling effect in InGaN films.

Observation of 4H–SiC to 3C–SiC polytypic transformation during oxidation

Abstract: We have observed the formation of single and multiple stacking faults that sometimes give rise to 3C–SiC bands in a highly doped n-type 4H–SiC epilayer following dry thermal oxidation. Transmission electron microscopy following oxidation revealed single stacking faults and bands of 3C–SiC in a 4H–SiC matrix within the 4H–SiC epilayer. These bands, parallel to the (0001) basal plane, were not detected in unoxidized control samples. In addition to the 3.22 eV peak of 4H–SiC, Cathodoluminescence spectroscopy at 300 K after oxidation revealed a spectral peak at 2.5 eV photon energy that was not present in the sample prior to oxidation. The polytypic transformation is tentatively attributed to the motion of Shockley partial dislocations on parallel (0001) slip planes. The generation and motion of these partials may have been induced by stresses caused either by the heavy doping of the epilayer or nucleation from defect.

Alkoxy sol-gel derived Y3−xAl5O12:Tbx thin films as efficient cathodoluminescent phosphors

Abstract: Thin-film Y3−xAl5O12:Tbx3+ (YAG:Tb) phosphor derived from a sol-gel chemistry is analyzed by x-ray diffraction, scanning electron microscopy, photoluminescence (PL) and cathodoluminescence (CL). The metal alkoxides organic precursors were chosen as the starting materials to form the sol-gel. This liquid sol-gel was spin coated on sapphire and silicon substrates to form the uniform thin films, then crystallized by annealing. The PL intensity of the crystallized film at 545 nm green emission was 15 times higher than that of the as-coated noncrystalline film. CL measurements show that luminance and efficiency are comparable to the films deposited by other techniques.

Bright blue electroluminescence from an InGaN/GaN multiquantum-well diode on Si(111): Impact of an AlGaN/GaN multilayer

Abstract: We present an electroluminescence test structure which consists of an InGaN/GaN multiquantum well as active region on the top of an AlGaN/GaN multilayer grown by metalorganic vapor phase epitaxy on Si(111) substrate. The integral room-temperature electroluminescence spectrum reveals a peak emission wavelength of 467 nm and a significantly higher brightness than an identical reference structure on sapphire substrate. In microelectroluminescence imaging, two emission peaks at 465 and 476 nm can be separated originating from locally different areas of the diode. Cathodoluminescence measurements in cross section and high-resolution x-ray diffraction measurements show that the structure is less strained than a sample without the AlGaN/GaN multilayer. The AlGaN/GaN multiple layer sequence which has a total thickness of 1.5 μm causes lattice relaxation during growth after a thickness of around 0.9 μm as directly visualized by cathodoluminescence line scans across the diode.

Visible emission from AlN doped with Eu and Tb ions

Abstract: We report the observation of visible cathodoluminescence (CL) from AlN thin films grown on sapphire (0001) substrate by molecular beam epitaxy and doped by implantation with Eu3+ and Tb3+ ions. The strongest rare earth (RE) CL was observed from samples annealed at 1100 °C for 0.5 h in N2 ambient. The sharp characteristic emission lines corresponding to Eu3+ and Tb3+ intra-4fn shell transitions are resolved in the spectral range from 350 to 900 nm. The CL spectra were recorded over 1–16 keV electron energy in the temperature range of 8–330 K. The depth resolved CL spectral analysis gives the luminescence surface a dead layer thickness of ∼16 nm for implanted AlN samples. We observed several different recombination centers luminescing in the 286–480 nm spectral region due to the presence of structural defects and oxygen impurities. The time resolved spectra and the CL kinetics were studied. The decay times for 5D0→7F2 (Eu3+), 5D3→7F5 (Tb3+), and 5D4→7F6 (Tb3+) transitions at 300 K are ∼0.4, ∼0.9, and ∼0.4 m...

Nanoscale luminescence spectroscopy of defects at buried interfaces and ultrathin films

Abstract: Low energy electron-excited nano-luminescence (LEEN) spectroscopy provides electronic band gap, confined state, and deep level trap information from semiconductor surfaces and interfaces on a nanometer scale. Correlation of luminescence features with their spatial location inside a growth structure—either depth wise or laterally—also provides information on the physical origin and growth dependence of the electronically active defects that form. LEEN spectroscopy of localized states illustrates this approach for a representative set of III–V nitride interfaces, including metal-GaN Schottky barriers, GaN/InGaN quantum wells, GaN ultrathin films, AlGaN/GaN pseudomorphic heterostructures across a single growth wafer, and GaN/Al2O3 interfaces. In each case, electronic properties are sensitive to the chemical composition, bonding, and atomic structures near interfaces and in turn to the specifics of the growth process.

Cathodoluminescence of crystalline and amorphous SiO2 and GeO2

Abstract: Cathodoluminescence (CL) and its temperature-dose behaviour are presented for different crystalline and amorphous modifications of SiO 2 and GeO 2 as well as for Ge-doped SiO 2 layers. The crystalline samples include four-fold coordinated Si and Ge in hexagonal quartz and quartz-like crystals, respectively, as well six-fold coordinated atoms in tetragonal rutile-like crystals. The detected luminescence bands, in general, are attributed to three optical active luminescence centres: the two-fold coordinated silicon (=Si:) and germanium (=Ge:) centre, respectively, the non-bridging oxygen hole centre (NBOHC) and the self trapped exciton (STE). The first ones, the oxygen deficient centres (ODC), are especially developed in both, in the tetragonal crystal rutile-like modifications as well as in glassy states. The huge violet luminescence in Ge-implanted SiO 2 -layers is attributed to the two-fold coordinated Ge in the silica matrix.

Impact of Internal Electric Field and Localization Effect on Quantum Well Excitons in AlGaN/GaN/InGaN Light Emitting Diodes

Abstract: Strained In x Ga 1-x N quantum wells (QWs) on thick GaN base layers were investigated to verify the importance of localized QW excitons in their spontaneous emission mechanisms. A strength of the internal piezoelectric field (F PZ ) across the QW increases with increasing x up to 1.4 MV/cm for x = 0.25, since the in-plain strain increases. For the QWs with the well thickness L greater than 3 nm, F PZ dominates the emission peak energy due to the quantum-confined Stark effect. Absorption spectra of both hexagonal and cubic InGaN QWs exhibited a broad band-tail regardless of the presence of F PZ normal to the QW plane. The luminescence peak energy of the 3 nm thick QWs was higher than the bandgap energy of the unstrained bulk crystal for x < 0.15, showing that doping of Si in barriers or injection of carriers effectively screens the field. The emission lifetime increased with increasing monitoring wavelength, Also, a temperature-induced change in the luminescence peak energy decreased with increasing x. The real-space variation of the luminescence peak energy was confirmed by the spatially resolved monochromatic cathodoluminescence mapping method. The localization depth increases with increasing x, The carrier localization is confirmed to originate from the effective bandgap inhomogeneity due to a fluctuation of the local InN mole fraction, which is enhanced by the large and composition-dependent bowing parameter of InGaN material.

Effect of a microstructure on the formation of self-assembled laser cavities in polycrystalline ZnO

Abstract: The optical properties of polycrystalline ZnO have been studied to elucidate the occurrence of random laser action. The spatially-resolved refractive index has been mapped out by using the scanning electron energy loss spectroscopy across the grain boundary. It is observed that the refractive index decreases gradually when the probe beam is approaching to the grain boundary. A thin reflective layer of ∼10 nm is found to form in the vicinity of the grain boundary, which assists the optical scattering. The photon scattering factor of the reflective layer has been determined and is shown to correlate well with the results of the coherent backscattering method. Together with the cathodoluminescence studies, it is suggested that the overall structure, which includes the grain and grain boundary, determines the laser action in ZnO.

Charge-transfer nature in luminescence of YNbO4 : Bi blue phosphor

Abstract: Bismuth doped yttrium niobate (YNbO4:Bi) is a potential blue phosphor for field emission displays application. It is observed after introducing Bi ions that cathodoluminescence efficiency of YNbO4:Bi becomes lower than that of YNbO4. From the calculations of critical distance (RC) of energy transfer and Huang–Rhys parameter (SHR) of YNbO4:Bi, it is found that the energy transfer by a dipolar-type interaction is unlikely compared with that of a short-range interaction possible in the luminescence of YNbO4:Bi. Thus, it is believed that the luminescence property of YNbO4:Bi is mainly affected by the host lattice YNbO4 having self-luminescent NbO43− complex. By time-resolved spectroscopy, it is found that luminescence characteristics of activator Bi3+ in YNbO4:Bi shows a charge-transfer behavior.

Cathodoluminescence study of apatite crystals

Abstract: Cathodoluminescence (CL) spectrometry represents a promising technique for the analysis of trace-element concentrations and distributions in minerals. However, a higher precision and a standardization of the recording conditions are required to use CL spectral data quantitatively. A significant step towards a more quantitative treatment of CL spectra is presented in this study. A procedure to correct the spectra for the various efficiencies as a function of the wavelength of the CL detector is proposed using low-pressure mercury-vapor and quartz-iodine lamps. CL spectra presented in this study are thus corrected for the system response. Apatite CL spectra, which are commonly composed of two broad bands centered at 3.5 and 2.2 eV, are deconvoluted to isolate component bands and determine their areas. The crystallographic control by prismatic or basal sections of apatite on spectral intensities is significant and only prismatic sections should be used. Signal decrease associated with electron bombardment (electron beam aging) is exponential and appears drastic in the first hundred seconds but continues even after 15 minutes of beam bombardment. All observed CL bands could be correlated with a specific activator [rare earth elements (REE) or manganese]. The 3.5 eV band is composed of three bands at 3.59 eV, 3.29 eV, and 2.87 eV. Ion microprobe results and comparison between CL and photoluminescence data support Ce 3+ activation for the origin of these bands. The relationship between CL band intensity and REE concentration measured by ion microprobe analysis demonstrates that CL also can provide semi-quantitative data for Gd 3+ , Ce 3+ , Dy 3+ , and Sm 3+ when recording conditions are strictly controlled.

Luminescent characteristics of blue-emitting Sr2B5O9Cl: Eu thin-film phosphors

Abstract: Thin films of Sr2B5O9Cl: Eu were deposited on glass plates using spray pyrolysis of aqueous solutions. Films exhibited blue cathodoluminescence and photoluminescence, indicating the films contained Eu2+ rather than Eu3+. A luminance of 76 cd/m2 at 5 kV was obtained for annealed films on glass substrates. The chromaticity coordinates were x=0.162 and y=0.035 with a dominant wavelength of 450 nm and a 95% color purity.

Depth profiling of ZnO thin films by cathodoluminescence

Abstract: The depth-resolved luminescence of the ZnO epilayer has been studied by using cathodoluminescence (CL) at room temperature. Other than the usual excitonic and deep level emissions, two peaks at 3.13 and 2.57 eV have been observed and are attributed to the defects. The variation of all the emission peaks has been examined as a function of accelerating voltage. The decrease of near-band edge emissions with depth is due to the internal absorption caused by the pronounced band tail. The deep level emission, however, is shown to increase with increasing depth. We have modeled the CL spectra with the consideration of internal absorption and determine the profile of the Urbach parameter, EUrbach, to study the structural imperfection at different depths. A strong dependence between the intensity ratios of the defect emissions to the excitonic emission and the imperfection of material has been found.

Crack‐Free InGaN/GaN Light Emitters on Si(111)

Abstract: We present MOCVD-grown crack-free GaN based light emitters on Si with a layer thickness of 3.6 μm. The crack free layer is grown on a thin predeposited GaN layer with fields defined by a Si x N y mask. In X-ray diffraction measurements a reduction in stress is observed for the patterned sample as compared to a similar unstructured sample. Lateral growth occurs at the edges of the fields with strongly differing growth rates for perpendicular directions. The impact of the facet type on the growth rate and impurity incorporation is observed by scanning electron microscopy and cathodoluminescence measurements. In electroluminescence the diode shows a bright blue emission at 421 nm.

Eu +3 and Cr +3 doping for red cathodoluminescence in ZnGa 2 O 4

Abstract: Cathodoluminescence emission spectra and photoluminescence (PL) excitation spectra were used to evaluate Eu+3 and Cr+3 as activators for red luminescence in ZnGa2O4. In the ZnGa2O4:Eu materials red emission from Eu+3 and blue intrinsic emission were observed. The blue intrinsic emission increased relative the Eu+3 emission with increasing current density and is attributed to preferential current saturation of the Eu+3 activators. In addition, PL excitation measurements revealed that the inefficient energy transfer from the ZnGa2O4 host to the Eu+3 is due to poor spectral overlap between the ZnGa2O4 emission and the Eu+3 absorption. Cr-doping resulted in a saturated red-color, and no host emission was observed over the entire current density regime investigated. The PL excitation of the ZnGa2O4:Cr revealed good overlap between the ZnGa2O4 host and the Cr+3 absorption. Efficient energy transfer to the Cr+3 activators occurs via multipolar interactions.

Luminescence of epitaxial GaN laterally overgrown on (0001) sapphire substrate: Spectroscopic characterization and dislocation contrasts

Abstract: Photoluminescence and cathodoluminescence spectra are recorded on epitaxial GaN laterally overgrown on (0001) sapphire. Photon recycling, which influences the position of the near band edge transition, is evidenced in cathodoluminescence (CL) spectra by changing the accelerating voltage. CL monochromatic images recorded at different wavelengths show that dislocations act as efficient nonradiative recombination centers, and that they are not responsible for the yellow band.

Cathodoluminescence study of diluted magnetic semiconductor quantum well/micromagnet hybrid structures

Abstract: Cathodoluminescence (CL) was studied in hybrid structures consisting of a diluted magnetic semiconductor (DMS) Cd1−xMnxTe (x=0.06 and 0.09) quantum well buried 300 A below the surface on which Fe islands with micrometric dimensions were deposited. The CL at T=10 K collected from areas far away from the Fe island was consistent with the photoluminescence spectra obtained prior to Fe deposition as were the raster scans and spot excited CL spectra taken in nonmagnetized structures close to the Fe islands. After a magnetization at a magnetic field of 3 T, the CL peak related to DMS quantum well (QW) shifts by up to 4 meV to lower energy only when the exciting beam is focused close to edges of an island. The observed shifts are interpreted as due to a fringe field, affecting the DMS QW, of magnetic domains formed in the Fe islands. The experiments prove a feasibility of the concept of usage of the fringe fields to achieve further confinement of excitons in submicron DMS/ferromagnet hybrid structures.

A study of luminescence centers related to copper and oxygen in ZnSe

Abstract: The effects of deviation from stoichiometry and the copper and oxygen concentrations on cathodoluminescence spectra were studied in ZnSe condensates obtained by chemical vapor deposition and doped with Cu during growth. The results were supplemented with the study of the microstructure and microcomposition using scanning electron microscopy, measurements of electrical conductivity, and calculations of the equilibrium between the native point defects. It is shown that three types of Cu-related centers are always accompanied with self-activated centers that include oxygen at the Se lattice site (OSe). The paired centers SA(I)-Cu(I), SAL(II)-Cu(II), and III-Cu(III) are typical of all II-VI compounds. All observed I-III Cu-containing centers are associative. Models of emission centers are suggested. A change of the emission type I-III is related to the recharging of the same group of defects that include OSe, Zn(Cu), and V Zn. New phenomena that occurred in the region of I-III bands and were related to the profound purification of the material were observed.

Correlation between morphology and cathodoluminescence in porous GaP

Abstract: Porous layers fabricated by anodic etching of n-GaP substrates in a sulfuric acid solution were studied by electron microscopy and cathodoluminescence (CL) microanalysis. The morphology of porous layers was found to depend strongly upon the anodization conditions. When the etching process starts at the initial surface, “catacomb-like” pores and current-line oriented pores are introduced at low and high anodic current densities, respectively. After the initial development of either kind of pore, further anodization at the current density of about 1 mA/cm2 favors the propagation of pores along 〈111〉 crystallographic directions. The spatial and spectral distribution of CL in bulk and porous samples is presented. A comparative analysis of the secondary electron and panchromatic CL images evidenced a porosity induced increase in the emission efficiency.