Showing papers on "Cathodoluminescence published in 2002"

Impact of nucleation conditions on the structural and optical properties of M-plane GaN(11̄00) grown on γ-LiAlO2

Abstract: We investigate the structural and optical properties of M-plane GaN(1100) films grown on LiAlO2(100) with nucleation layers grown at high and low temperatures. Samples with a high temperature nucleation layer are found to exhibit a highly anisotropic surface morphology with pronounced corrugation, which basically replicates the surface morphology of the substrate. Photoluminescence spectra of these layers are dominated by a transition at 3.356 eV, which is absent for samples with a low-temperature nucleation layer. In conjunction with scanning electron microscopy, cathodoluminescence maps reveal that this transition predominantly stems from regions below the trenches of the surface corrugation. Transmission electron microscopy shows an abundance of stacking faults within these regions. Excitation-dependent and time-resolved photoluminescence demonstrates the intrinsic character of the 3.356 eV emission, which is thus attributed to excitons bound to stacking faults acting as ultrathin vertical quantum wel...

Growth and characterization of single-crystal GaN nanorods by hydride vapor phase epitaxy

Abstract: Single-crystalline GaN nanorods are formed on a sapphire substrate by hydride vapor phase epitaxy (HVPE). Their structural and optical properties are investigated by x-ray diffraction, scanning and transmission electron microscopy, and cathodoluminescence (CL) techniques. The high density of straight and well-aligned nanorods with a diameter of 80–120 nm formed uniformly over the entire 2 in. sapphire substrate. The x-ray diffraction patterns and transmission electron microscopic images indicate that the formed GaN nanorods are a pure single crystal and preferentially oriented in the c-axis direction. We observed a higher CL peak position of individual GaN nanorods than that of bulk GaN as well as a blueshift of CL peak position with decreasing the diameter of GaN nanorods, which are attributed to quantum confinement effect in one-dimensional GaN nanorods. We demonstrate that the well-aligned, single-crystalline GaN nanorods with high density, high crystal quality, and good spatial uniformity are formed b...

Optical and luminescent properties of undoped and rare-earth-doped Ga2O3 thin films deposited by spray pyrolysis

Abstract: Spray pyrolysis was used to prepare polycrystalline thin films of undoped Ga2O3, as well as those doped separately with Eu3+, Tb3+ and Tm3+. The energy gap values of the as-grown films and those annealed at 600\r{}C and 900\r{}C were found to be 4.75, 4.48 and 4.44 eV, respectively. Films containing Eu3+ and Tb3+ exhibited red and green cathodoluminescence (CL), respectively. The CL of Ga2O3 : Tm mainly showed a broad band in the blue-green region, which resulted from the emission by both the Ga2O3 host and Tm3+. The broad blue-green emission band was divided into three Gaussian peaks at 424 nm (2.92 eV), 497 nm (2.49 eV) and 526 nm (2.36 eV). The CL intensity of undoped Ga2O3 thin films depended on the annealing ambient and temperature, suggesting that it is associated with the presence of oxygen vacancies. Mechanisms responsible for the broad blue-green emission of undoped Ga2O3 thin films were explored.

Molecular-beam epitaxial growth of GaAs and InGaAs/GaAs nanodot arrays using anodic Al2O3 nanohole array template masks

Abstract: Highly ordered arrays of nanosized GaAs-based dots were successfully prepared on GaAs (001) substrates by molecular-beam epitaxy using selected area growth Selected area growth employed alumina nanochannel array (NCA) templates formed by anodic oxidation, bonded to the GaAs substrates Homogeneous GaAs dots, as well as compositionally modulated heterostructures within the nanosized dots, were demonstrated In the latter case, multilayer InGaAs/GaAs heterostructured nanodot arrays were fabricated Dot growth occurred only as defined by the template mask, resulting in a hexagonal lattice of dots with 100 nm period spacing, with dots retaining the circular lateral shape of the pores as determined by the NCA template pore size; dot diameters were adjustable from 45 to 85 nm for a lattice period of 100 nm Cathodoluminescence spectra from an InGaAs/GaAs 10×10 dot array clearly showed an emission peak at 920 nm (5 K), confirming the formation of a high-quality InGaAs/GaAs quantum dot array

In-plane bandgap control in porous GaN through electroless wet chemical etching

Abstract: Nanoporous GaN structures were formed from crystalline GaN on conducting SiC substrate using metal-assisted electroless etching in HF/H2O2. Morphology varies as a function of etch time and solution parameters. The resulting porous GaN (PGaN) displays cathodoluminescence (CL) with two bands blue-shifted from the bulk bandgap energy by 103 meV and 352 meV, respectively. Appearance of the blueshifted emission is correlated with the development of highly anisotropic wire-like structures in the morphology, with the higher energy CL band arising from the portion of the structure with the smallest feature sizes. These observations suggest that the blueshifted emission arises from quantum confinement effects. CL imaging indicates that the blueshifted emission is spatially segregated from the band gap emission. Variations in morphology and light emission properties apparent between Pt-coated and uncoated areas likely arise from hole transport and access of solution reagents to the GaN interface.

Indentation-induced damage in GaN epilayers

Abstract: The mechanical deformation of wurtzite GaN epilayers grown on sapphire substrates is studied by spherical indentation, cross-sectional transmission electron microscopy (XTEM), and scanning cathodoluminescence (CL) monochromatic imaging. CL imaging of indents which exhibit plastic deformation (based on indentation data) shows an observable “footprint” of deformation-produced defects that result in a strong reduction in the intensity of CL emission. Multiple discontinuities are observed during loading when the maximum load is above the elastic-plastic threshold, and such a behavior can be correlated with multiple slip bands revealed by XTEM. No evidence of pressure-induced phase transformations is found from within the mechanically damaged regions using selected-area diffraction patterns. The main deformation mechanism appears to be the nucleation of slip on the basal planes, with dislocations being nucleated on additional planes on further loading. XTEM reveals no cracking or delamination in any of the sam...

Structural and optical properties of InGaN/GaN layers close to the critical layer thickness

Abstract: In this work, we investigate structural and optical properties of metalorganic chemical vapor deposition grown wurtzite InxGa1−xN/GaN epitaxial layers with thicknesses that are close to the critical layer thickness (CLT) for strain relaxation. CLT for InxGa1−xN/GaN structures was calculated as a function of the InN content, x, using the energy balance model proposed by People and Bean [Appl. Phys. Lett. 47, 322 (1985)]. Experimentally determined CLT are in good agreement with these calculations. The occurrence of discontinuous strain relaxation (DSR), when the CLT is exceeded, is revealed in the case of a 120 nm thick In0.19Ga0.89N layer by x-ray reciprocal space mapping of an asymmetrical reflection. The effect of DSR on the luminescence of this layer is clear: The luminescence spectrum shows two peaks centered at ∼2.50 and ∼2.67 eV, respectively. These two components of the luminescence of the sample originate in regions of different strain, as discriminated by depth-resolving cathodoluminescence spectroscopy. DSR leads directly to the emergence of the second, lower-energy, peak. Based on this experimental evidence, it is argued that the appearance of luminescence doublets in InGaN is not evidence of “quantum dotlike In-rich” or “phase separated” regions, as commonly proposed.

Chemical origin of the yellow luminescence in GaN

Abstract: The influence of ion-beam-produced lattice defects as well as H, B, C, N, O, and Si, introduced by ion implantation, on the luminescence properties of wurtzite GaN is studied by cathodoluminescence spectroscopy. Results indicate that intrinsic lattice defects produced by ion bombardment mainly act as nonradiative recombination centers and do not give rise to the yellow luminescence (YL) of GaN. Experimental data unequivocally shows that C is involved in the defect-impurity complex responsible for YL. In addition, C-related complexes appear to act as efficient nonradiative recombination centers. Implantation of H produces a broad luminescent peak which is slightly blueshifted with respect to the C-related YL band in the case of high excitation densities. The position of this H-related YL peak exhibits a blueshift with increasing excitation density. Based on this experimental data and results reported previously, the chemical origin of the YL band is discussed.

High quality ZnO thin films grown by plasma enhanced chemical vapor deposition

Abstract: High quality ZnO thin films have been grown on a Si(100) substrate by plasma enhanced chemical vapor deposition using a zinc organic source [Zn(C2H5)2] and carbon dioxide (CO2) gas mixtures at the low temperature of 180 °C. The dependence of ZnO thin film quality on the gas flow rate ratio of Zn(C2H5)2 to CO2 (GFRRZC) is studied by using x-ray diffraction (XRD), optical absorption (OA) spectra, and cathodoluminescence (CL) spectra. High quality ZnO thin films with a c-axis-oriented wurtzite structure are obtained when the GFRRZC is 0.33. XRD shows that the full width at half maximum of (0002) ZnO located at 34.42° is about 0.2°. At room temperature, a pronounced free exciton absorption peak around 365 nm is clearly observed. Also, a strong free exciton emission without deep level defect emission is observed around 385 nm, and its temperature dependence is studied from the photoluminescence spectra. These observations indicate the formation of a high quality ZnO film. Additionally, nitridation of the Si su...

Contact-induced defect propagation in ZnO

Abstract: Contact-induced damage has been studied in single-crystal (wurtzite) ZnO by cross-sectional transmission electron microscopy (XTEM) and scanning cathodoluminescence (CL) monochromatic imaging. XTEM reveals that the prime deformation mechanism in ZnO is the nucleation of slip on both the basal and pyramidal planes. Some indication of dislocation pinning was observed on the basal slip planes. No evidence of either a phase transformation or cracking was observed by XTEM in samples loaded up to 50 mN with an ∼4.2 μm radius spherical indenter. CL imaging reveals a quenching of near-gap emission by deformation-produced defects. Both XTEM and CL show that this comparatively soft material exhibits extensive deformation damage and that defects can propagate well beyond the deformed volume under contact. Results of this study have significant implications for the extent of contact-induced damage during fabrication of ZnO-based (opto)electronic devices.

Visible emission from ZnO doped with rare-earth ions

Abstract: We report the results of a cathodoluminescence (CL) and photoluminescence (PL) study of ZnO-bulk single crystals and epilayer thin-film samples grown on a sapphire (0001) substrate and doped by implantation with rare-earth ions (RE3+): Pr3+ , Dy3+ , Ho3+ , Er3+ , Tm3+ (bulk crystals, co-doped with Li), Sm3+, Dy3+, and Er3+ (epilayers). The PL and PL excitation (PLE) spectra of polycrystalline ZnO doped with RE3+ ions (Nd3+, Dy3+, Er3+, and Tm3+) and co-doped with Li+, Cl-, and N- ions have also been studied.

Defects induced in fused silica by high fluence ultraviolet laser pulses at 355 nm

Abstract: Point defects induced at the exit surface of optical-grade fused silica by high power (>30 J/cm2) 355 nm laser pulses have been investigated to elucidate the nature of laser damage in optics for use at high peak powers (>2 GW/cm2). Six defects have been identified. Eγ′ and E74′ defects were identified using electron spin resonance spectroscopy. The nonbridging oxygen hole center, a self-trapped exciton, an oxygen-deficient center, and interstitial O2 were identified and spatially resolved in the damage craters using cathodoluminescence microanalysis. The defects are associated with the laser generated shock waves and/or thermal explosion.

Mechanism of enhanced luminescence in InxAlyGa1−x−yN quaternary epilayers

Abstract: We report firm evidence for the underlying mechanism of the enhanced luminescence in InxAlyGa1−x−yN quaternary epilayers. Photoluminescence, Raman scattering, field emission scanning electron microscopy (SEM), energy dispersive x-ray spectrometry (EDS), and cathodoluminescence (CL) measurements have been employed to study the correlation between optical and structural properties in these alloys. The phonon replica structures accompanying luminescence line, InGaN-related phonon modes in Raman spectra, SEM images, element composition analysis by EDS, and localized CL spectra provide the evidence to show that the existence of InGaN-like nanoclusters is responsible for the enhanced luminescence in InxAlyGa1−x−yN quaternary alloys. Our result therefore gives an excellent demonstration showing that because of the existence of nanoclusters a very defective alloy can exhibit a strong emission even at room temperature.

Cathodoluminescence of Ge+, Si+, and O+ implanted SiO2 layers and the role of mobile oxygen in defect transformations

Abstract: Thermally grown SiO 2 layers of thickness d =500 nm have been implanted by Ge + , Si + , and O + ions of energy 350, 150, and 100 keV, respectively, and a uniform implantation dose of D i =5×10 16 ions/cm 2 . Thus the implantation profiles are expected with a concentration maximum of nearly 4 at.% at the half-depth d m ≅250 nm of the SiO 2 layers. After thermal annealing to 900 °C for 1 h in dry nitrogen or vacuum the typical violet luminescence band ( λ =400 nm) of the Ge + implanted centers is increased more than 200-fold and the Ge luminescent center depth profile is shifted from about 250 to 170 nm towards the surface as determined by cathodoluminescence (CL) depth profiling. Implanting oxygen increases the red band ( λ =650 nm) but does not affect the blue band ( λ =460 nm). Silicon surplus increases the amplitude of the blue (B) luminescence, but reduces the amplitude of the red (R) one. Studying the irradiation dose dependence of these blue and red bands we have established defect kinetics in SiO 2 including six main defects and precursors, including the non-bridging oxygen hole center for the red luminescence, the twofold-coordinated silicon as the oxygen deficient center ODC(2) for the blue luminescence and the mobile oxygen as the main transmitter between precursors and the radiation induced defects. The kinetics are described by a set of eight differential equations which predict the dose dependence of the CL.

Carrier recombination at single dislocations in GaN measured by cathodoluminescence in a transmission electron microscope

Abstract: We study radiative and nonradiative recombination at individual dislocations in GaN by cathodoluminescence performed in a transmission electron microscope. The dislocations are produced by indentation of dislocation free single crystals and have a-type Burgers vectors (b=1/3〈1120〉). They are aligned along 〈1120〉 directions in the basal plane. Our direct correlation between structural and optical properties on a microscopic scale yields two main results: (i) 60°-basal plane dislocations show radiative recombination at 2.9 eV; (ii) screw-type basal plane dislocations act as nonradiative recombination centers. We explain the nonradiative recombination by splitting this dislocation into 30° partials that have dangling bonds in the core. The dissociation width of these dislocations is <2 nm.

Potential of cathodoluminescence (CL) microscopy and spectroscopy for the analysis of minerals and materials.

Abstract: The present study summarizes results of cathodoluminescence (CL) microscopy and spectroscopy applied to minerals and materials. CL can be used both in a purely descriptive way to detect and distinguish different minerals or mineral generations by their variable CL colours or as an effective method for spatially resolved analysis of point defects in solids by spectral CL measurements. The cathodoluminescence emission is in all cases either related to lattice defects (e.g. electron defects on broken bonds, vacancies or radiation induced defects) and/or to trace activator ions such as REE2+/3+, Fe3+, Cr3+, Al3+, Mn2+, Pb2+, Cu2+, Sn2+ or uranyl groups. CL spectroscopy is an outstanding method to characterize the degree of purity of materials or to detect trace elements in natural and synthetic minerals. In this way, alterations, diffusion of trace elements or formation of new phases are successfully detectable even in the case of materials with heterogeneous texture and high contents of non-crystalline phases.

Optical Properties of InxGa1—xN with Entire Alloy Composition on InN Buffer Layer Grown by RF‐MBE

Abstract: Optical characterization of In x Ga 1-x N layers was done by photoluminescence, cathodoluminescence and optical absorption measurements. PL emissions less than 1.9 eV were observed from In x Ga 1-x N films with x > 0.53 that showed no phase separation. A bowing parameter of 2.3 eV was obtained from the relationship between PL emission energy and alloy composition. Absorption edge dependence on In composition was similar to the luminescence peak dependence on In composition. These optical properties indicate that the bandgap energy of InN is below 1.0 eV.

Room-temperature intense 320 nm band ultraviolet emission from quaternary InAlGaN-based multiple-quantum wells

Abstract: We demonstrated room-temperature (RT) intense ultraviolet (UV) emission with wavelength in the range of 300–340 nm from Inx1Aly1Ga1−x1−y1N/Inx2Aly2Ga1−x2−y2N multiple-quantum wells (MQWs) fabricated on SiC by metalorganic vapor phase epitaxy. We found that the UV emission is considerably enhanced upon introducing approximately 5% of In into AlGaN. Maximally efficient emission was obtained at 318 nm from the fabricated In0.05Al0.34Ga0.61N/In0.02Al0.60Ga0.38N three-layer MQW when the QW thickness was approximately 1.4 nm. The intensity of 320 nm band emission from the InAlGaN-based MQWs was as strong as that of 410 nm band emission from InGaN-based QWs at RT. We observed emission fluctuations of submicron size in cathode luminescence images of Inx1Aly1Ga1−x1−y1N/Inx2Aly2Ga1−x2−y2N single QW which might be due to In segregation effect. The temperature dependence of photoluminescence emission for InAlGaN-based QWs was greatly improved in comparison with that of GaN- or AlGaN-based QWs.

Micro Epitaxial lateral overgrowth of GaN/sapphire by Metal Organic Vapour Phase Epitaxy

Abstract: GaN/sapphire layers have been grown by Metal Organic Vapour Phase Epitaxy (MOVPE). An amorphous silicon nitride layer is deposited using a SiH4/NH3 mixture prior to the growth of the low temperature GaN buffer layer. Such a process induces a 3D nucleation at the early beginning of the growth, resulting in a kind of maskless ELO process with random opening sizes. This produces a significant decrease of the threading dislocation (TD) density compared to the best GaN/sapphire templates. Ultra Low Dislocation density (ULD) GaN layers were obtained with TD density as low as 7×107cm−2 as measured by atomic force microscopy (AFM), cathodoluminescence and transmission electron microscopy (TEM). Time-resolved photoluminescence experiments show that the lifetime of the A free exciton is principally limited by capture onto residual donors, similar to the situation for nearly dislocation-free homoepitaxial layers.

Exciton spectra of an AlN epitaxial film on (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy

Abstract: Exciton resonance energies in an AlN epilayer on (0001) sapphire substrate grown by low-pressure metalorganic vapor phase epitaxy were determined as a function of temperature by means of optical reflectance (OR) and cathodoluminescence measurements. The OR spectra exhibited distinct reflectance anomalies at the photon energies just above the multiple internal reflection fringes, and the spectral line shape was fitted considering A (Γ7vu→Γ7c) and BC (Γ9v,Γ7vl→Γ7c) exciton transitions. The fitting gave the values of them at 0 K to be 6.211 and 6.266 eV, giving the crystal- field splitting (Δcr) of approximately 55 meV. The AlN film exhibited an excitonic emission even at 300 K, which is due to the small Bohr radius of excitons and large longitudinal optical phonon energies. The Einstein characteristic temperature ΘE was estimated to be 580 K.

Influence of the carrier gas composition on morphology, dislocations, and microscopic luminescence properties of selectively grown GaN by hydride vapor phase epitaxy

Abstract: We have studied the influence of the carrier gas (hydrogen versus nitrogen) on the morphology and defect characteristics of GaN grown by epitaxial lateral overgrowth (ELO) using hydride vapor phase epitaxy (HVPE). Growth was carried out on metalorganic vapor phase epitaxy GaN/sapphire patterned with SiO2 stripes, aligned along the 〈1100〉 GaN direction. The cross sections of the ELO-grown stripes change from trapezoidal to triangular with an increase in hydrogen content in the carrier gas due to a change of the ratio of the growth velocities on the {1122} and (0001) facets. Transmission electron microscopy observations show that while in stripes with trapezoidal morphology, dislocations from the window region still reach the sample surface, for triangular stripes they are bent in the horizontal direction away from the top surface. Cross-sectional cathodoluminescence (CL) microscopy shows basically two distinct regions of luminescence intensity and nature, one showing near-band gap excitonic emission, the...

Band-edge emission of undoped and doped ZnO single crystals at room temperature

Abstract: Band-edge emission of ZnO at around room temperature was investigated by measuring the temperature dependence of cathodoluminescence spectra at 20–300 K. Undoped crystals grown by a vapor transport method and Al-doped crystals by flux method were employed to elucidate the effect of doping on luminescence properties. For the Al-doped crystals, the free-exciton emission was weak through out the temperature range T<300 K. The most intense emission peak of the Al-doped crystal was energetically close to bound exciton annihilation emission. On the other hand, for undoped crystals, it was found that the most intense emission peak at room temperature was at E≈Eg−60 meV and this peak was not assignable to free-exciton annihilation emission. It was also found that this peak is not a reason for the reduction in emission efficiency.

Low Stokes shift in thick and homogeneous InGaN epilayers

Abstract: The optical properties of thick InxGa1−xN layers have been studied using optical absorption and cathodoluminescence techniques. The indium composition x of the layers ranged from 0.03 to 0.17 as determined by Rutherford backscattering measurements. The difference between the band gap and the peak emission energy (Stokes shift) was found to be considerably smaller than reported in the past for these alloys. Monochromatic images show that light emission from most of the film is homogeneous and is associated with a low Stokes shift. A second emission band at longer wavelengths is observed for x⩾0.08. This band originates from indium-rich regions in the vicinity of extended defects, and exhibits a larger Stokes shift. Our observations indicate that it is possible to grow InGaN epilayers with high indium composition, high homogeneity, and lower Stokes shift.