Showing papers on "Cathodoluminescence published in 1998"

Direct Evidence that Dislocations are Non-Radiative Recombination Centers in GaN

Abstract: Plan-view transmission electron microscopy (TEM) and cathodoluminescence (CL) images were taken for the same sample at exactly the same location in n-type GaN grown on sapphire substrate by metalorganic chemical vapor deposition (MOCVD). There was a clear one to one correspondence between the dark spots observed in CL images and the dislocations in TEM foils, indicating that the dislocations are non-radiative recombination centers. The hole diffusion length in n-type GaN was estimated to be neighboring 50 nm by comparing the diameters of the dark spots in thick samples used for CL and samples that were thinned for TEM observation. The efficiency of light emission is high as long as the minority carrier diffusion length is shorter than the dislocation spacing.

Structural origin of V-defects and correlation with localized excitonic centers in InGaN/GaN multiple quantum wells

Abstract: In the growth of InGaN/GaN multiple quantum well (MQW) structures, a novel defect (called the “V-defect”) initiates at threading dislocations in one of the first quantum wells in a MQW stack. This defect is common to almost all InGaN MQW heterostructures. The nature of the V-defect was evaluated using transmission electron microscopy (TEM), scanning TEM (STEM), and low-temperature cathodoluminescence (CL) on a series of In0.20Ga0.80N/GaN MQW samples. The structure of the V-defect includes buried side-wall quantum wells (on the {1011} planes) and an open hexagonal inverted pyramid which is defined by the six {1011} planes. Thus, in cross section this defect appears as an open “V”. The formation of the V-defect is kinetically controlled by reduced Ga incorporation on the pyramid walls ({1011} planes). The V-defect is correlated with the localized excitonic recombination centers that give rise to a long-wavelength shoulder in photoluminescence (PL) and CL spectra. This long-wavelength shoulder has the fol...

Heteroepitaxy of ZnO on GaN and its implications for fabrication of hybrid optoelectronic devices

Abstract: ZnO thin films have been grown heteroepitaxially on epi-GaN/sapphire (0001) substrates. Rutherford backscattering spectroscopy, ion channeling, and high resolution transmission electron microscopy studies revealed high-quality epitaxial growth of ZnO on GaN with an atomically sharp interface. The x-ray diffraction and ion channeling measurements indicate near perfect alignment of the ZnO epilayers on GaN as compared to those grown directly on sapphire (0001). Low-temperature cathodoluminescence studies also indicate high optical quality of these films presumably due to the close lattice match and stacking order between ZnO and GaN. Lattice-matched epitaxy and good luminescence properties of ZnO/GaN heterostructures are thus promising for ultraviolet lasers. These heterostructures demonstrate the feasibility of integrating hybrid ZnO/GaN optoelectronic devices.

Deep energy levels in CdTe and CdZnTe

Abstract: The deep levels present in semiconducting CdTe and semi-insulating CdTe:Cl and Cd0.8Zn0.2Te have been investigated by means of cathodoluminescence, deep level transient spectroscopy (DLTS), photo-induced current transient spectroscopy, and photo-DLTS. The latter two methods, which can be applied to semi-insulating materials, allow to characterize the deep traps located up to midgap and can determine whether they are hole or electron traps. We have identified 12 different traps, some common to all the investigated samples, some peculiar to one of them. A comparison of the results obtained from the various materials is given and the status of defect models is reviewed.

Luminescence of solids

Abstract: Theories of Luminescence. Experimental techniques. Photoluminescence. Cathodoluminescence. Ionoluminescence. Electroluminescence. Thermoluminescence. Lyoluminescence. Sonluminescence. Mechanoluminescence. Bioluminenscence and Chemiluminescence. Index.

Exciton localization in InGaN quantum well devices

Abstract: Emission mechanisms of a device-quality quantum well (QW) structure and bulk three dimensional (3D) InGaN materials grown on sapphire substrates without any epitaxial lateral overgrown GaN base layers were investigated. The InxGa1−xN layers showed various degrees of in-plane spatial potential (band gap) inhomogeneity, which is due to a compositional fluctuation or a few monolayers thickness fluctuation. The degree of fluctuation changed remarkably around a nominal InN molar fraction x=0.2, which changes to nearly 0.08–0.1 for the strained InxGa1−xN. This potential fluctuation induces localized energy states both in the QW and 3D InGaN, showing a large Stokes-like shift. The spontaneous emission from undoped InGaN single QW light-emitting diodes (LEDs), undoped 3D LEDs, and multiple QW (MQW) laser diode (LD) wafers was assigned as being due to the recombination of excitons localized at the potential minima, whose lateral size was determined by cathodoluminescence mapping to vary from less than 60 to 300 nm...

Growth kinetics and optical properties of self-organized GaN quantum dots

Abstract: Self-organized GaN islands of nanometric scale were fabricated by controlling the Stranski–Krastanov growth mode of GaN deposited by molecular beam epitaxy on AlN. Evidence for ripening of dots under vacuum has been observed, resulting in changes in dot size distribution. We also show that in superlattice samples, consisting of multiple layers of GaN islands separated by AlN, the GaN islands are vertically correlated provided that the AlN layer thickness remains small enough. The luminescence peak of GaN dots is blueshifted with respect to bulk emission and its intensity does not vary with temperature, both effects demonstrating the strongly zero-dimensional character of these nanostructures.

Direct observation of the core structures of threading dislocations in GaN

Abstract: Here we present the first direct observation of the atomic structure of threading dislocation cores in hexagonal GaN. Using atomic-resolution Z-contrast imaging, dislocations with edge character are found to exhibit an eight-fold ring core. The central column in the core of a pure edge dislocation has the same configuration as one row of dimers on the {10-10} surface. Following recent theoretical work, it is proposed that edge dislocations do not have deep defect states in the band gap, and do not contribute to cathodoluminescence dislocation contrast. On the other hand, both mixed and pure screw dislocations are found to have a full core, and full screw dislocation cores were calculated to have states in the gap.

Local probe techniques for luminescence studies of low-dimensional semiconductor structures

Abstract: With the rapid development of technologies for the fabrication of, as well as applications of low-dimensional structures, the demands on characterization techniques increase. Spatial resolution is especially crucial, where techniques for probing the properties of very small volumes, in the extreme case quantum structures, are essential. In this article we review the state-of-the-art in local probe techniques for studying the properties of nanostructures, concentrating on methods involving monitoring the properties related to photon emission. These techniques are sensitive enough to reveal the electronic structure of low-dimensional semiconductor structures and are, therefore, able to give detailed information about the geometrical structure, including fabrication-related inhomogeneities within an ensemble of structures. The local luminescence probe techniques discussed in this review article can be divided into four categories according to the excitation source: (i) spatially localized microphotoluminescence spectroscopy using either strong focusing or masking; (ii) near-field optical microscopy to reach below the diffraction limitation of far-field optics, by either exciting, detecting, or both exciting and detecting in the near field; (iii) cathodoluminescence using focused energetic electrons in an electron microscope; and (iv) scanning tunneling luminescence, using low-energy electrons injected or extracted from the tip of a scanning tunneling microscope.

Luminescence behavior of pulsed laser deposited Eu:Y2O3 thin film phosphors on sapphire substrates

Abstract: Europium-doped yttrium oxide (Eu:Y2O3) luminescent thin films have been grown in situ on single crystal (0001) sapphire substrates using a pulsed laser deposition technique. The films grown under different deposition conditions have been characterized using microstructural and luminescent measurements. The photoluminescence (PL) and cathodoluminescence (CL) brightness data obtained from the Eu:Y2O3 films grown under optimized conditions have indicated that sapphire is a promising substrate for the growth of high quality Eu:Y2O3 thin film red phosphor. The success in the fabrication of Eu:Y2O3 films with high PL and CL brightness is attributed to favorable optical properties (low absorption of and low refractive index for red light) of the substrate material and improved growth of grains with unidirectional orientation on (0001) sapphire substrates.

Role of Dislocation in InGaN Phase Separation.

Abstract: The role of dislocation for luminescence in InGaN grown on sapphire substrate by metal organic chemical vapor deposition (MOCVD) method was investigated by cathodoluminescence (CL) and atomic force microscopy (AFM). The CL emission area and dark spots between InGaN and GaN layers in InGaN/GaN single quantum well (SQW) and multiple quantum well (MQW) structures showed completely one to one correspondence. These results indicate that dislocations in InGaN work as non-radiative recombination centers. Furthermore it was confirmed that the phase separation in InGaN is caused by spiral growth due to mixed dislocations, and such a growth mechanism is discussed.

Deep centers and their spatial distribution in undoped GaN films grown by organometallic vapor phase epitaxy

Abstract: Deep traps in undoped n-GaN layers grown by organometallic vapor phase epitaxy on sapphire substrates were studied by temperature dependent conductivity, photoinduced current transient spectroscopy (PICTS), thermally stimulated current, electron beam induced current (EBIC), and band edge cathodoluminescence (CL) methods. Presence of electron traps with energy levels 0.1–0.2 eV below the conduction band and hole traps with energy levels of about 0.25, 0.5, and 0.85 eV above the valence band edge was detected. CL and EBIC measurements show that the deep recombination centers in GaN are distributed inhomogeneously with well defined cellular pattern. Both carrier lifetime and luminescence intensity are enhanced at cell walls indicating lower density of recombination centers. However, the density of main hole trap (0.85 eV) is enhanced in these regions as determined by local PICTS measurements. Photoconductivity in many GaN samples exhibits very long decay times at temperatures between 100 and 300 K. The effec...

Cathodoluminescence of ZnO Films

Abstract: The cathodoluminescence of ZnO films deposited on Si substrates by reactive dc sputtering is investigated. Apart from the characteristic green emission band (522 nm) of ZnO, an ultraviolet (392 nm) band and a blue (430-460 nm) band have been observed. The x-ray diffraction and cathodoluminescent spectra reveal the dependences of the luminescence on the preparation conditions and crystallinity of the ZnO films.

Cathodoluminescence microcharacterization of the defect structure of irradiated hydrated and anhydrous fused silicon dioxide

Abstract: The irradiation-sensitive defect structure of pure anhydrous and hydrated fused amorphous silicon dioxide $(a\ensuremath{-}{\mathrm{SiO}}_{2})$ has been investigated using cathodoluminescence (CL) microanalysis. Irradiation of ${\mathrm{SiO}}_{2}$ by a continuous stationary electron beam results in a subsurface, trapped-charge-induced electric field that causes the electromigration of mobile charged defect species within the volume of irradiated specimen. CL emissions, observed between 300--900 nm at specimen temperatures between 5 and 295 K are identified with particular defect centers including the nonbridging oxygen-hole centers with strained bond and/or nonbridging hydroxyl precursors (hydrated specimen only), the self-trapped exciton, oxygen-deficient centers such as the neutral oxygen vacancy and/or the twofold coordinated silicon defect, and the charge-compensated substitutional aluminum center.

Low voltage cathodoluminescence properties of blue emitting SrGa2S4:Ce3+ and ZnS:Ag,Cl phosphors

Abstract: The low voltage properties of ZnS:Ag and SrGa2S4:Ce3+ have been investigated for applications in field emission displays. It was observed that although ZnS:Ag,Cl has slightly better chromaticity than SrGa2S4:Ce3+, the high luminous efficiency, fast decay time, and better saturation behavior of the thiogallate potentially make it a very important blue phosphor.

Strong excitonic recombination radiation from homoepitaxial diamond thin films at room temperature

Abstract: We have observed strong emission lines at 5.27 and 5.12 eV at room temperature in cathodoluminescence spectra from homoepitaxial diamond films grown by step-flow mode prepared with microwave plasma assisted chemical vapor deposition. The temperature dependence of integrated intensity of the emission lines indicates that they are attributed to free-exciton recombination radiation associated with a transverse optical phonon and its replica. These results have demonstrated the superior crystalline quality of the homoepitaxial diamond films grown by the step-flow mode.

The incorporation of arsenic in GaN by metalorganic chemical vapor deposition

Abstract: We report on the successful incorporation of arsenic (As) in GaN during metalorganic chemical vapor deposition (MOCVD). A characteristic room-temperature luminescence band centered around 2.6 eV (480 nm), similar to the peak position of the As ion-implanted GaN, is found to be related to the As impurity in the MOCVD grown GaN:As films. The arsenic incorporation efficiency as a function of experimental conditions and structure is presented. Temperature- and power-dependent cathodoluminescence measurements have been performed to help establish the nature of the As-related peak.

Near band-edge transition in aluminum nitride thin films grown by metal organic chemical vapor deposition

Abstract: Cathodoluminescence measurements were performed for carbon doped and undoped aluminum nitride thin films in the temperature range from liquid helium to room temperature. The AlN films were grown on three different substrates: 6H–SiC, 4H–SiC, and sapphire. From these samples, a strong luminescence peak surrounded by two weaker peaks in the near band-edge region, near 6 eV, was observed. For AlN on sapphire, this near band-edge transition can be further resolved into three peaks at 6.11, 5.92, and 5.82 eV. These peaks are believed to be due to exciton recombination. The effects of substrate materials and carbon doping on the exciton peak were discussed. The temperature dependence of the peak position and line width of this transition was also studied. The temperature coefficient of the band-gap energy is estimated to be 0.51 meV/K.

Monte Carlo modeling of cathodoluminescence generation using electron energy loss curves

Abstract: This work demonstrates the validity of approximating cathodoluminescence generation throughout the electron interaction volume by the total electron energy loss profile. The energy loss profiles in multilayer specimens were accurately calculated using the Monte Carlo simulation CASINO. Resolution of cathodoluminescence images can be estimated from the electron beam spot diameter, the electron penetration range, and the minority carrier diffusion length.

Selective Area Growth of GaN on Si Substrate Using SiO 2 Mask by Metalorganic Vapor Phase Epitaxy

Abstract: Selective area growth (SAG) of GaN on (111) Si substrate was studied using AlGaN as an intermediate layer. A hexagonal dot with a (0001) plane on the top and of a 5 µm or a submicron size was obtained using a patterned dot structure of silicon dioxide (SiO2) mask. The facet structure revealed that the axis of hexagonal GaN is parallel to the axis of the Si substrate. The cathodoluminescence (CL) spectrum at 133 K exhibited a strong near-band-edge emission band for the submicron dots, which suggests excellent crystallinity. Epitaxial lateral overgrowth (ELO) of GaN on the Si substrate is demonstrated.

Optical Characterization of Lateral Epitaxial Overgrown GaN Layers

Abstract: The optical properties of homoepitaxial GaN layers deposited by organometallic vapor phase epitaxy on stripe-patterned GaN films on 6H-SiC substrates have been investigated. Analysis of the spatially-resolved Raman scattering spectra indicate an improvement in material quality of the overgrown region. Room-temperature color cathodoluminescence imaging and low-temperature photoluminescence measurements indicate that a donor and an acceptor, different from those detected in the underlying GaN/AlN/SiC substrate, have been incorporated in the epitaxial layer. Detailed photoluminescence studies of the near-band-edge emission strongly suggest that Si is the additional donor detected in the homoepitaxial GaN layer. Its occurrence, along with that of an acceptor-related defect which is primarily found in the laterally overgrown region, is discussed.

Investigation of Ce-doped silicates for low voltage field emission displays

Abstract: The feasibility of using Ce-doped silicates as an alternative to the P22 blue phosphor (ZnS:Ag) was investigated for low voltage field emission displays (FEDs). Silicates of various composition were evaluated based on their chromaticity, intrinsic efficiency, and brightness saturation behavior. The influence of silicate composition and particle morphology on the cathodoluminescence properties was also assessed. Saturation measurements indicated that the high saturation resistance of the Ce-doped samples can yield better performance than ZnS:Ag when operating at low voltages. The silicates are also attractive for their stability in a FED environment.

Growth and characterization of Eu:Y{sub 2}O{sub 3} thin-film phosphors on silicon and diamond-coated silicon substrates

Abstract: Europium-activated yttrium oxide (Eu:Y{sub 2}O{sub 3}) phosphor films have been grown in situ on (100) bare silicon and diamond-coated silicon substrates using a pulsed laser deposition technique. Diamond-coated silicon substrates were prepared by hot filament chemical vapor deposition onto (100) silicon wafers. Measurements of photoluminescence and cathodoluminescence properties of Eu:Y{sub 2}O{sub 3} films have shown that the films grown on diamond-coated silicon substrates are brighter than the films grown on bare silicon substrates under identical deposition conditions. The improved brightness of the Eu:Y{sub 2}O{sub 3} films on diamond-coated silicon substrates is attributed to reduced internal reflection, low photon energy absorption by substrate, and enhanced scattering of incident beam with lattice. All these effects are primarily brought about by the presence of a rough diamond interfacial layer between the phosphor films and substrates.

GaN epitaxial lateral overgrowth and optical characterization

Abstract: We demonstrate the epitaxial lateral overgrowth (ELO) of GaN from narrow stripes with triangular cross sections by atmospheric pressure metal organic chemical vapor deposition, and characterize the optical properties of these stripes at each stage of the growth using spatially resolved cathodoluminescence spectroscopy, wavelength imaging, and line scans. An improvement of the optical quality of the GaN materials grown by the ELO technique is clearly shown by the appearance of a free exciton peak, the enhancement of bandedge emission, and the weakening of the yellow emission.