Low-energy cathodoluminescence microscopy for the characterization of nanostructures

Spatially and spectrally resolved low-energy cathodoluminescence (CL) microscopy was applied to the characterization of nanostructures. CL has the advantage of revealing not only the presence of luminescence centers but also their spatial distribution. The use of electrons as an excitation source allows a direct comparison with other electron-beam techniques. Thus, CL is a powerful method to correlate luminescence with the sample structure and to clarify the origin of the luminescence. However, caution is needed in the quantitative analysis of CL measurements. In this review, the advantages of cathodoluminescence for qualitative analysis and disadvantages for quantitative analysis are presented on the example of nanostructures.     

Local analysis of Eu2+ emission in CaAlSiN3

Abstract

We have investigated the local luminescence properties of Eu-doped CaAlSiN₃ by using low-energy electron beam (e-beam) techniques. The particles yield broad emission centered at 655 nm with a shoulder at higher wavelength under light excitation, and a broad band around 643 nm with a tail at 540 nm under e-beam excitation. Using cathodoluminescence (CL) in a scanning electron microscope (SEM), we have observed small and large particles, which, although with different compositions, exhibit Eu²⁺-related emissions at 645 and 635 nm, respectively. Local CL measurements reveal that the Eu²⁺ emission may actually consist of several bands. In addition to the red broad band, regularly spaced sharp peaks have been occasionally observed. These luminescence variations may originate from a variation in the composition inside CaAlSiN₃.     

Spatial distribution of impurities in ZnO nanotubes characterized by cathodoluminescence

Low-energy cathodoluminescence (CL) imaging and spectroscopy technique was employed to study the impurity distribution in individual ZnO hexagonal nanotubes fabricated by metalorganic chemical vapor deposition on the sapphire (0001) substrate. The CL spectra at 10 K show that acceptor and donor impurities are incorporated in the ZnO nanotubes. CL monochromatic images indicate that the concentration of donor is higher at the bottom part and the distribution of acceptors is more inhomogeneous at the surface of the nanotubes. The non-uniform defects and impurities distributions are explained by unstable growth conditions and contamination from the environment. These results indicate that the low-energy CL is a very powerful method to investigate the inhomogeneity of luminescence properties in the individual nanostructures.     

Cathodoluminescence characterization of rare earth doped composite materials based on porous GaP

Porous GaP layers doped with erbium or europium elements have been obtained by electrochemical etching and further impregnation processes. The thermal treatments for optical activation of rare earth (RE) ions lead to partial oxidation of porous GaP skeleton and a composite material is obtained. The presence of ErPO₄ and EuPO₄ oxide nanophases is detected by X-ray diffraction (XRD) analysis. Visible luminescence from RE ions in the composite material has been investigated by means of the cathodoluminescence (CL) technique in the scanning electron microscope. Intense red and green emission lines characteristic from Er³⁺ and Eu³⁺ ions dominate the CL spectra in the case of parallel and regular nanotubes in the samples. The role of the oxygen content and the detected phases in the luminescence results are discussed.     

Controlling the yellow luminescence intensity from n-GaN during cathodoluminescence

GaN grown on sapphire by hydride vapour phase epitaxy is probed by ion channeling, Raman and cathodoluminescence (CL) spectroscopy. Channeling and Raman spectroscopy indicate that the quality of GaN is very good. Spot mode CL measurement signifies intense near band edge emission as compared with yellow luminescence (YL). During scanning over an area, the YL intensity could be controlled by the electron beam dwell time (DT). The YL intensity decreases with the increase of DT and saturates beyond a threshold value due to overexposure of a given pixel leading to non-radiative emission. But for shorter dwell times repeated excitations of a given pixel increase the intensity of YL. These results may be explained invoking the decay time and density of defects responsible for YL. Control over YL intensity will be useful for assessing low defect concentrations, their origin and also to increase spatial resolution of CL measurements on nanostructures.     

Microstructural investigation into calcium phosphate biomaterials by spatially resolved cathodoluminescence

From cathodoluminescence (CL) investigations of synthetic and natural calcium phosphates it can be concluded that the CL of pure synthetic apatite is mainly characterized by intrinsic luminescence, whereas the luminescence of naturally occurring apatites is frequently activated by trace elements. CL revealed internal structures within plasma‐sprayed hydroxyapatite coatings which were not discernible by SEM‐BSE imaging. However, cathodoluminescence microscopy alone can presently not be used in every case to characterize synthetic calcium phosphate biomaterials because of the dominant intrinsic blue CL emission. In the future, optimum results will likely be achieved by using a combination of CL microscopy and spectroscopy with other spatially resolved analytical methods such as SEM‐BSE, SEM‐CL or micro‐Raman spectroscopy. In the present study, different types of tetracalcium phosphate dental cements could be distinguished due to varying CL colours and CL spectra that are caused by a different content of impurity Mn. These results emphasize the advantages of spectral CL measurements for spatially resolved detection of trace elements in solids.     

Cathodoluminescence microscopy of high T c superconducting YBCO and BSCCO materials

Spectroscopic and imaging cathodoluminescence (CL) microscopy has been used to study high T _c YBCO and BSCCO superconducting thin films and pellets in the scanning electron microscope. The effects of beam parameters, such as voltage, current, and diameter, have been investigated with a view to optimizing the CL signal intensity whilst preventing sample damage. Limiting the CL signal generation volume to within the thin film is important in eliminating any substrate contribution. Areas of strong luminescence have been observed in YBCO and BSCCO pellets as well as BSCCO thin films. At low beam energies, there is some correlation between the CL and secondary electron images. The CL spectra of the strongly luminescent spots differed from those of the poorly luminescencing superconducting phases. CL was able to identify a copper-rich impurity phase in the BSCCO material, and a barium cuprate phase in the YBCO material. It is proposed that the quality of a thin film, with respect to impurities, can be monitored using CL.     

A REBIC and CL study of interfaces in a zinc oxide based varistor

Conductive mode (CM) and cathodoluminescence (CL) microscopy techniques were used to study grain boundary structures in a zinc oxide based varistor, doped with 0.5 mol % Bi₂O₃ and 0.5 mol % Sb₂O₃. By combining these two techniques specific details of the electrical and luminescence properties of individual interfaces could be characterised. CM imaging clearly showed the presence of potential barriers at the grain boundaries. The same grain boundaries were regions of strong CL emission. It is suggested that the dominant CL emission at grain boundaries in this material originates from self-excitation centres at impurities and/or defects within the structure rather than the direct recombination of electron-hole pairs across the forbidden band gap.     

Cathodoluminescence of rare earth implanted Ga2O3 and GeO2 nanostructures

Rare earth (RE) doped gallium oxide and germanium oxide micro- and nanostructures, mostly nanowires, have been obtained and their morphological and optical properties have been characterized. Undoped oxide micro- and nanostructures were grown by a thermal evaporation method and were subsequently doped with gadolinium or europium ions by ion implantation. No significant changes in the morphologies of the nanostructures were observed after ion implantation and thermal annealing. The luminescence emission properties have been studied with cathodoluminescence (CL) in a scanning electron microscope (SEM). Both β-Ga(2)O(3) and GeO(2) structures implanted with Eu show the characteristic red luminescence peak centered at around 610 nm, due to the (5)D(0)-(7)F(2) Eu(3+) intraionic transition. Sharpening of the luminescence peaks after thermal annealing is observed in Eu implanted β-Ga(2)O(3), which is assigned to the lattice recovery. Gd(3+) as-implanted samples do not show rare earth related luminescence. After annealing, optical activation of Gd(3+) is obtained in both matrices and a sharp ultraviolet peak centered at around 315 nm, associated with the Gd(3+) (6)P(7/2)-(8)S(7/2) intraionic transition, is observed. The influence of the Gd ion implantation and the annealing temperature on the gallium oxide broad intrinsic defect band has been analyzed.     

Diffusion synthesis and electronic properties of Fe-doped ZnO

Fe-doped ZnO was successfully fabricated by thermal in-diffusion of Fe into ZnO crystals. X-ray absorption near edge structure (XANES), photoemission and cathodoluminescence (CL) spectroscopy have been combined to examine the Fe diffusion and its effects on the electronic and optical properties of the crystal. Depth-resolved CL demonstrates that Fe in-diffusion occurs to at least 4 μm depth and results in intense green luminescence, whereas the undoped crystal exhibits only the ZnO near-band-edge emission. XANES and valence-band photoemission show that Fe is incorporated as Fe^2+/3+ ions on substitutional Zn sites. The results suggest that the variation in the CL properties is due to a change in the oxygen vacancy charge state as a result of electron transfer from Fe.     

Strong morphological dependence of luminescence efficiency and emission wavelength in hexagonal GaN crystallites directly imaged by scanning cathodoluminescence microscopy

The microstructure and morphology of hexagonal GaN crystallites grown on c-axis sapphire substrates by low pressure chemical vapor deposition is correlated with the luminescence efficiency and emission wavelength. Microscopic variation of local band gap monitored by the luminescence wavelength on the a- and c-planes of hexagonal GaN-crystallites are directly mapped by means of low temperature scanning cathodoluminescence (CL) and CL wavelength imaging (CLWI). Beside minor fluctuations from crystallite to crystallite, the a-planes show pronounced red shift of emission energy of more than 132 meV with respect to the luminescence from the c-planes. The c-plane itself shows additional inhomogeneity on a micron scale. Strongly red shifted luminescence (λ>400 nm) originates from the very center region correlated with a high dislocation density found in TEM. The CL intensity shows a reticulated structure over the c-plane visualizing the local dislocation network.     

Cathodoluminescence study of CdTe crystals doped with Bi and Bi:Yb

Bi doped and Bi and Yb codoped CdTe crystals grown by the Bridgman method have been characterized by cathodoluminescence (CL) in the scanning electron microscope. CL images show a dense network of highly decorated grain boundaries in the Bi doped samples and dopant striations in the codoped crystals, attributed to the presence of Yb. Bi contributes to the appearance of the A luminescence band at 1.43 eV. The influence of Yb dopant on the CL spectra is discussed.     

Study of structural defects limiting the luminescence of InGaN single quantum wells

InGaN quantum well (QW) structures with different thicknesses have been characterised by means of cathodoluminescence (CL) in the scanning electron microscope and transmission electron microscopy (TEM), in order to study the structural defects that limit the device operation. Misfit dislocations appear as non-radiative centres in the CL images and compete with the quantum-well related luminescence. The luminescence red shift with increasing QW thickness has been found to be influenced by composition fluctuations.     

ZnSe<Cu> Luminescence at High Doping Levels

ZnSe was doped with Cu via thermal diffusion from Cu foil in the course of chemical vapor deposition. The Cu distribution over the deposit was studied, and the Cu solubility in ZnSe was determined as a function of temperature. Copper was shown to prevent oxygen incorporation from the gas phase. The microstructure and homogeneity of ZnSe were studied by scanning electron microscopy. The cathodoluminescence (CL) spectra were measured on fracture surfaces in order to elucidate the origin of the Cu-related emission centers. Heavy doping with Cu was shown to gives rise to a long-wavelength CL band, which shifts from 525 to 550 nm as the temperature is raised from 80 to 300 K. The associated changes in oxygen-related self-activated luminescence were correlated with the Cu and O concentrations. To gain more detailed insight into the origin of the green Cu-related emission in p-type ZnSe, we examined its photoexcitation spectrum, temperature-dependent peak position, quenching, and variation with excitation intensity. The conclusion is drawn that the longer wavelength CL bands are due to electron transitions from the Zn_i ^·· donor level to the impurity band. The CL spectrum of heavily doped ZnSe shows no emissions between 800 and 1400 nm, related to the V _Se and V _Zn vacancies.     

Rare earth doped glass–ceramics containing NaLaF4 nanocrystals

Oxyfluoride glasses 16Na₂O–9NaF–5LaF₃–7Al₂O₃–63SiO₂ (mol%) activated with 3% terbium, dysprosium, praseodymium and neodymium fluorides have been prepared and studied by differential thermal analysis, cathodoluminescence, X-ray induced luminescence, X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. We found out that the presence of crystalline phase enhances the X-ray induced luminescence intensity. X-ray induced luminescence is the most intense for the sample activated with terbium and treated at 700 °C, whereas the praseodymium and neodymium activated samples have the fastest decay times.     

Study of the relationship between crystal structure and luminescence in rare-earth-implanted Ga2O3 nanowires during annealing treatments

A systematical analysis of the correlation between the crystalline quality and the luminescence of rare-earth-implanted β-Ga₂O₃ nanostructures with potential applications in visible and ultraviolet photonics is presented. Europium ions led to red emission while gadolinium ions are efficient ultraviolet emitters. Different degrees of lattice recoveries of the nanostructures have been achieved after implantation by rapid thermal annealing treatments carried out at different temperatures. The recovery process has been analyzed by transmission electron microscopy (TEM), high-resolution TEM, and Raman techniques. High-fluence implantation with either of the two rare earth ions induces partial amorphization of the structures. Partial recrystallization of the nanostructures above 500 °C is revealed by Raman analysis. Nearly complete recovery of the crystal structure is obtained in the annealing temperature range 900–1100 °C, coincident with the expected value for bulk Ga₂O₃. Cathodoluminescence and photoluminescence allowed comparison of the Eu³⁺ and Gd³⁺ intraionic luminescence lines after annealing at different temperatures and their correlation with the crystallinity. It has been found that the width of the Eu³⁺ luminescence lines clearly correlates with the width of the Raman peaks, both decreasing with annealing temperature, which shows the possibility of using the luminescence of this rare earth as a probe for lattice disorder. On the other hand, our results suggest that Gd³⁺ lines are much less sensitive to disorder.     

Thin polycrystalline zinc oxide films obtained by oxidation of metallic zinc films

Thin polycrystalline ZnO films were obtained by thermal oxidation of metallic Zn films, thermally deposited on various substrates, such as silica, sapphire and glass, in both air and pure oxygen atmospheres. The quality of the ZnO layers was asserted by Hall effect, cathodoluminescence and atomic force microscopy measurements. Electron concentration of 7.32×10¹² cm⁻³ and mobility of 14.2 cm²/V s with root mean square roughness of 30 nm were obtained for the 900 °C annealed ZnO films in oxygen. Room temperature cathodoluminescence spectra consisted of a narrow near band edge luminescence band and a broad defect-related green band with peak positions at 380 and 500 nm, respectively. ZnO film luminescence properties improved dramatically with the increase of annealing temperature and decrease of O₂ pressure.     

Biotemplating of BaFBr:Eu for X-ray storage phosphor applications

The design of hierarchically patterned novel structures by replicating the cellular tissue of wood has recently attained increasing interest. X-ray storage phosphor BaFBr:Eu²⁺ is manufactured via vacuum assisted repeated infiltration of wood tissue (Pinus sylvestris). A submicrometer precipitate is formed via wet chemical reaction of NH₄F, BaBr₂·2H₂O and EuCl₃·6H₂O in methanol. According to scanning electron microscopy (SEM) and energy dispersive X-ray analysis (EDX), the original wood cell walls are filled with the precipitate and completely transformed into BaFBr struts after sintering at 800 °C. The optical properties of the biomorphous phosphor microstructure are determined by photoluminescence spectroscopy (PL) at room temperature, photo-stimulated luminescence spectroscopy (PSL) and cathodoluminescence spectroscopy (CL) in the SEM. A broadening of the PSL peak is observed and ascribed to the incorporation of calcium impurities present in the pine wood tissue. The potential of biotemplates for generating highly oriented and optically isolated μm- and sub-μm matrix of X-ray storage phosphor material is illustrated.     

Cathodoluminescence characterization of defects in yttrium aluminum garnet doped with Nd

Nominally undoped and Nd-doped yttrium aluminum garnet (YAG) crystals were studied by scanning electron microscopy, electron probe x-ray microanalysis, and cathodoluminescence (CL) microscopy and spectroscopy techniques. The detection limit of CL for Nd in YAG was found to be 10^-6 wt %. The nature of inhomogeneities in the crystals with a high Nd concentration was established. The distribution of background Nd impurity over defect-rich YAG crystals was assessed. A method for the identification of solid-phase inclusions from x-ray microanalysis and CL data is proposed. The effect of the growth technique and impurity concentration on the CL spectrum of YAG is examined.     

Cathodoluminescence and its mapping of flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures

The cathodoluminescence (CL) properties including intensity and distribution of the band to band and defect emission of the flower-like ZnO, ZnO/ZnS core-shell and tube-like ZnS nanostructures have been investigated. It is indicated that the Ultraviolet (UV) emission at 380 nm of the flower-like ZnO nanostructures due to the band to band emission is weaker than their yellow emission at 600 nm induced by interstitial oxygen. Moreover, the UV emission of the ZnO nanorods unevenly distributes from the tip to the end. The UV emission on the tip is stronger than that of others due to the waveguide. On the contrary, the yellow emission at 600 nm is uniform. Furthermore, the UV emission of ZnO has been greatly enhanced and the yellow emission has been inhibited by the formation of ZnO/ZnS core-shell nanostructures in the sulfuration process due to the elimination of interstitial oxygen. However, the polycrystalline tube-like ZnS nanostructures shows the uniform and weak defect emission due to S vacancies.