Monochromatic Cathodoluminescence Images Research Articles

Thick AlN Templates By MOCVD for the Thermal Management of III-N Electronics

Ultra-wide bandgap semiconductors have attracted much interest over the last decade. These materials have strong chemical bonding that led to high temperature tolerance, high voltage and power handling capabilities as described by the Baliga figure of merit. Al-rich III-nitride materials are expected to have the best performance over the industry standard Si, and newer options such as SiC and GaN. Continuous improvements dictate the miniaturization of electronic devices, leading to increasingly higher power densities in smaller footprints, leading to heating that causes system failure. Thus, the thermal management is the limiting task for continuous performance scaling. In past we have reported on deep ultraviolet light emitting diodes (LEDs) and AlxGa1-xN (x>0.4) channel Heterojunction Field-Effect Transistors (HFETs) on 2-3 µm thick MOCVD grown high quality AlN/sapphire templates. For template thicknesses over 5 µm either sapphire or AlN patterning is needed to avoid stress related cracking. In this research, we for the first-time report on the direct MOCVD growth and characterization of crack-free, low-impurity AlN templates in excess of 16 μm on basal plane sapphire substrates without doing any external processing. Increasing the thickness improves thermal management and enables an easier sapphire substrate’s laser liftoff for increased DUV light extraction. It would also be used as a heat spreader in power electronics to minimize the operating temperature to maximize power output to industrial loads such as electric vehicles, and large turbines in manufacturing and transportation.All growths were carried out in custom MOCVD reactors with a fast-metalorganic switching manifold using a modified epitaxy procedure in which the Al- and N-precursors (Trimethyl Aluminum and NH3) are pulsed for increased surface mobility. At the initial pulsed growth stage, the growth-temperature and precursor-flow rates are adjusted to yield air-pocketed (voids) rough AlN layers. Most of these high defect materials are located near the AlN/sapphire interface. As the AlN thickness increases in the subsequent growths, the layers turned out smooth and the number of defects is significantly reduced due to dislocation bending/annihilation and cracking is avoided due to strain relief from the voids at the interface. All template growths were carried out at temperatures ~ 1200 - 1300 °C and 40 torr.From X-ray diffraction (XRD), Reciprocal space map (RSM), and Transmission Electron Microscopy (TEM) analysis of these thick AlN templates, we established that they were fully relaxed. Cross-section TEM data clearly shows the presence of voids at ~ 0.4 – 0.9 μm from the AlN/sapphire interface. There is a high density of dislocations present near the void region. Above this region, at least one order of magnitude lower dislocations is observed at the AlN smooth layer region. From the room-temperature Cathodoluminescence (CL) data we see an increase in the band edge emission and a decrease in the defect related long-wavelength signal with increasing AlN template thicknesses. The CL spectra were measured in the edge emission pump geometry on cleaved bars due to the transverse-magnetic polarized nature of the band-edge emission. The monochromatic CL imaging data also confirmed a highly defective region at the AlN/sapphire interface which is also the origin of the long wavelength (below bandgap) CL emission. The Atomic Force Microscopy (AFM) surface scans of the templates showed the surface roughness ~ 0.15 nm to 0.25 nm (for a 5 µm x 5 µm scan). The (102) off axis X-ray linewidths for all these samples ranged from 280 to 330 arc-secs. XRD and Raman experiments confirmed a residual compressive strain in AlN templates compared to the bulk sample. The measured stress ranges from -0.6 GPa to -1.2 GPa for all the templates, reasonable agreement with previous reports. We have found no obvious thickness dependence of the stresses measured from both XRD and Raman, an indication of preserving similar stresses in thick and thin templates.The thermal conductivity of our high-quality 16 µm thick template and the bulk AlN are measured at different temperatures by Time-domain Thermoreflectance (TDTR) technique. The thermal conductivity of the thick AlN at room temperature is 320 Wm-1K-1, or ~10% higher than that of the bulk AlN (~285 W/m-K). At low temperatures (120 K), it exceeds that of bulk AlN by more than 40%. In summary, these templates showed thermal conductivity as high or better than high-cost bulk AlN substrates, demonstrating for the first time that sapphire-based power electronics could be effectively managed thermally. Figure 1

Design of a cathodoluminescence image generator using a Raspberry Pi coupled to a scanning electron microscope.

In this work, an innovative cathodoluminescence (CL) system is coupled to a scanning electron microscope and synchronized with a Raspberry Pi computer integrated with an innovative processing signal. The post-processing signal is based on a Python algorithm that correlates the CL and secondary electron (SE) images with a precise dwell time correction. For CL imaging, the emission signal is collected through an optical fiber and transduced to an electrical signal via a photomultiplier tube (PMT). CL Images are registered in a panchromatic mode and can be filtered using a monochromator connected between the optical fiber and the PMT to produce monochromatic CL images. The designed system has been employed to study ZnO samples prepared by electrical arc discharge and microwave methods. CL images are compared with SE images and chemical elemental mapping images to correlate the emission regions of the sample.

Distribution of Eu2+ and Eu3+ Ions in Hydroxyapatite: A Cathodoluminescence and Raman Study

We present a cathodoluminescence study of the spatial distribution of Eu2+ and Eu3+ dopants in hydroxyapatite powders. The results demonstrate that the distribution of europium ions in the hydroxyapatite lattice depends on their valence state. Monochromatic cathodoluminescence images from prismatic powders show that although the Eu2+ is distributed homogeneously in the entire powder volume, the Eu3+ is present mainly at the powder edges. The luminescence spectrum of the Eu2+ ions displayed a wide and strong blue emission centered at 420 nm, while the luminescence spectrum of the Eu3+ ions displayed several orange-red emissions covering the range from 575 to 725 nm. These emissions correspond to transitions between levels 4f65d1-4f7 (8S7/2) of the Eu2+ ions and 5D0-7FJ levels of the Eu3+ ions. Micro Raman measurements reveal that europium doping generates two phonon signals with frequencies of 555 and 660 cm-1, both of which have not been reported earlier. The powders were synthesized by the combustion synthesis method, maintaining constant the concentration of the europium salt used, and varying the pH of the precursor solutions to modify the concentration ratio of Eu2+ with respect to Eu3+. X-ray photoelectron spectroscopy measurements were used to determine values of 0.32 and 0.55 for the ratio Eu2+/Eu3+ in samples synthesized at pH values of 6 and 4, respectively. Thermal treatments of the samples, at 873 K in an oxygen atmosphere, resulted in a strong quenching of the Eu2+ luminescence due to oxidation of the Eu2+ ions into Eu3+, as well as probable elimination of calcium vacancy defects by annealing.

Highly ordered catalyst-free InGaN/GaN core–shell architecture arrays with expanded active area region

Highly ordered, position-controlled gallium nitride (GaN) nanowire based multiple-quantum-wells (MQWs) core–shell architecture arrays are synthesized by metalorganic chemical vapor deposition (MOCVD). We investigate the possibility of using GaN nanowire arrays as a basal template for the growth of InxGa1−xN/GaN MQWs. The MQWs on three different crystal facets (c-, m-, and semipolar-plane) of GaN nanowire exhibit dissimilar structural properties. The structural characteristics of InGaN/GaN core–shell arrays are inspected by cross-sectional high-resolution transmission electron microscopy (HR-TEM). We also investigate the optical properties of MQW core–shell structure nanoarrays. The luminescent characteristics of InGaN/GaN core–shell structure arrays are determined by photoluminescence (PL) and cathodoluminescence (CL) measurements. The monochromatic CL images clearly show the light emission behavior of InGaN/GaN MQW coaxial structure. Two distinguishable light emission peaks were observed in the GaN nanowire based core–shell structure. The characteristic of light emission mainly depends on the properties of MQWs, which are generated from different crystal facets of GaN. In addition, the light emission intensity shows different behaviors depending on the area of the GaN nanowire m-plane. The results of this study suggest that GaN nanowire arrays can be used as a good alternative basal template for next-generation light-emitting diodes (LEDs).

Cathodoluminescence characterization of ZnO nanorods synthesized by chemical solution and of its conversion to ellipsoidal morphology

Abstract

Effects of an InGaN prelayer on the properties of InGaN/GaN quantum well structures

AbstractIn this paper we report on the effects of including of an InGaN prelayer in an InGaN/GaN multiple quantum well (MQW) structure. Two equivalent 10 period In0.16Ga0.84N MQW structures were studied, one with a 24 nm thick In0.05Ga0.95N prelayer, one without. X‐ray diffraction measurements showed no evidence of strain relaxation in the structure containing the prelayer, while AFM revealed a 15% increase in average diameter of “V”‐defects. Temperature dependent photoluminescence (PL) showed the MQW structure, with the prelayer, to exhibit a higher room temperature internal quantum efficiency than the reference structure. PL excitation spectroscopy revealed an absorption and carrier transfer process associated with the InGaN prelayer, which created an additional recombination pathway resulting in a feature on the high energy side of the PL spectrum. Time‐resolved PL studies showed that this recombination had a weaker temperature sensitivity to that of the primary luminescence feature, and thus may be responsible for the enhanced PL intensity measured at 300 K. Monochromatic cathodoluminescence (CL) images showed the high energy feature originated from the “V”‐defects. It is thus suggested that the InGaN prelayer may provide a transfer mechanism for carriers in the prelayer into the semi‐polar QWs on the “V”‐defect facets. Alternatively the high energy emission may arise from QWs at the base of the stack that have parameters that are significantly different from the rest of the QWs. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Spatially Resolved Cathodoluminescence in the Vicinity of Defects in the High-Efficiency InGaN/GaN Blue Light Emitting Diodes

In addition to the standard 447 nm blue emission from the InGaN/GaN multiple quantum wells, a high-energy shoulder is clearly observed in cathodoluminescence spectra of the high-efficiency InGaN/GaN blue light emitting diodes grown on sapphire substrates by metalorganic chemical vapor deposition. Monochromatic cathodoluminescence images of the samples measured at low temperature reveal a competition between the two emissions in the vicinity of the dislocations. The high-energy emission is dominant at the regions near the dislocation cores, while the blue emission is enhanced around the dislocation edges. The high-energy emission region is considered as a potential barrier that prevents the carriers for the blue emission from nonradiatively recombining at the dislocations.

3D resonator based on luminescence enhanced by both polarized, size-dependent whispering gallery modes and Fabry–Pérot waveguide modes in individual ZnO micro- and nanonails

A 3D resonator based on both polarized, size-dependent whispering gallery (WG) modes and Fabry-Pérot (FP) waveguide modes has been achieved using individual ZnO micro- and nanonails. Monochromatic cathodoluminescence images of the ZnO micro- and nanonails show that enhanced luminescence intensity is focused on two regions, i.e., the profile and the center of the micro- and nanonails. The luminescence from the center region is attributed to the emission enhanced by 1D FP waveguide modes, and that from the hexagonal profile is attributed to the emission enhanced by both polarized, size-dependent 2D WG modes when an individual ZnO nanocolumn is regarded as an optical resonator. Theoretical calculations and computer simulations based on the finite element method (FEM) with perfectly matched layer (PML) boundary conditions are performed, and the corresponding results are close to the experimental phenomena.

Cathodoluminescence Study of Optical Inhomogeneity in Si-Doped AlGaN Epitaxial Layers Grown by Low-Pressure Metalorganic Vapor-Phase Epitaxy

We have investigated the effects of Si doping on the microscopic optical inhomogeneity of Al0.61Ga0.39N epitaxial layers through the combined use of scanning electron microscopy (SEM) and cathodoluminescence (CL). Hexagonal hillocks and corresponding CL inhomogeneity in AlGaN epitaxial layers were observed in SEM and CL images, respectively, and the density of such structures increased with increasing the Si concentration. CL spectra of Si-doped AlGaN epitaxial layers taken at 80 K showed near-band-edge emission around 4.9 eV and an emission line around 4.4 eV. Monochromatic CL images of Si-doped AlGaN at the detection energy of 4.4 eV appeared selectively at the edges of hexagonal hillocks, indicating selective incorporation of Si atoms into defects at grain boundaries.

Strain Relaxation, Defects and Cathodoluminescence of m-Plane ZnO and Zn0.8Mg0.2O Epilayers Grown on γ-LiAlO2 Substrate

The strain relaxation, defects and luminescence properties of nonpolar (100) ZnO and Zn0.8Mg0.2O epilayers grown on γ-LiAlO2 (100) substrates by chemical vapor deposition were investigated by X-ray diffraction, transmission electron microscopy and cathodoluminescence. The X-ray rocking curves of (100) reflection possessed a low FWHM value of 504 and 216 arcsec, respectively, for ZnO and Zn0.8Mg0.2O. Reciprocal space maps indicated that the misfit strain in the [110] direction has been relaxed and resulted in misfit dislocations in high density at the interface with b = , which has been confirmed by TEM. The misfit strain along [0001] was, however, retained. Basal stacking faults, threading dislocations and inversion domains were observed in the ZnO epilayer with densities of 105 cm−1, 109 cm−2 and 104 cm−1, respectively. Moreover, only basal stacking faults and threading dislocations were found in Zn0.8Mg0.2O, having densities of about one order of magnitude lower than those of ZnO. The Zn0.8Mg0.2O epilayers exhibited a near band-edge emission at 3.5 eV whose intensity is about 15 times stronger than that of ZnO at 3.3 eV. Monochromatic cathodoluminescence images acquired at the peak energy revealed that the basal stacking faults showed low emission intensities at room temperature.

Growth and Characterization of Vertically Aligned Nonpolar [11̅00] Orientation ZnO Nanostructures on (100) γ-LiAlO2 Substrate

In this work, we demonstrate the capability of controlling the crystallographic orientation of ZnO nanostructure arrays grown along nonpolar [11̅00] orientation. The Au catalyst was used to initiate and guide the growth of ZnO nanostructures on (100) γ-LiAlO2 substrate. The morphology, structure, and optical properties of the as-synthesized ZnO nanostructures were characterized by field emission scanning electron microscopy, X-ray diffraction, field emission transmission electron microscope, monochromatic cathodoluminescence image, and photoluminescence spectroscopy. The ZnO nanostructure arrays exhibited special shape and were aligned vertically on the (100) γ-LiAlO2 substrate. A cooperative of Au-catalyzed VLS vertebral column growth along [11̅00] orientation, zinc-self-catalyzed VLS growth along [0001], and ZnO VS lateral expansion growth process was proposed for the special ZnO nanostructures.

Effect of defects on the luminescence in semipolar InGaN/GaN quantum wells on planar and patterned m‐plane sapphire substrate

AbstractThe correlation between luminescence and defects was investigated by the structural and optical analyses of semipolar InGaN/GaN quantum wells (QWs) grown on a planar m‐plane sapphire substrate and patterned m‐plane sapphire substrate using metal organic chemical vapor deposition. Scanning electron microscopy images indicated that the semipolar InGaN/GaN QWs grown on the planar substrate had high density of small arrowhead‐like features on the surface, while those grown on the patterned sapphire substrate (PSS) had low density of large arrowhead‐like features. Moreover, for the PSS sample, low defect regions were formed by periodically aligned patterns that reduced defect density, as verified by transmission electron microscopy. The monochromatic cathodoluminescence images revealed that the luminescence of the planar sample was affected by the arrowhead‐like features and high internal defect density, whereas that of the PSS sample was more influenced by the regions with reduced defects, as indicated by bright spotty features. Furthermore, the microphotoluminescence mapping images showed increased In incorporation where the arrowhead‐like features were present. In particular, for the PSS sample, the wavelength of the luminescence from the reduced defect region was approximately 5 nm shorter than that from the region with a high density of internal defects. This result suggests that In incorporation was also increased in the high defect region.

Titanium-assisted growth of silica nanowires: from surface-matched to free-standingmorphologies

We report on an oxide-assisted growth technique for silica nanowires which allows tuningthe growth from surface-matched nanowires to free-standing morphologies based on growthcontrol by Ti in the role of a catalyst and surfactant. Using an adjustable Ticoncentration, we grew silica nanowires with lengths ranging from 100 nm up to severalmillimetres whose defect chemistry was analysed by electron microscopy tools,monochromatic cathodoluminescence imaging and time resolved photoluminescencespectroscopy. The knowledge of the luminescence properties and the related defectoccurrence along with their spatial distribution is pivotal for advancing silicananowire growth in order to realize successful device designs based on self-assembledSi/SiOx nanostructures. We demonstrate a core–shell structure of the grown nanowires with ahighly luminescent 150 nm thick shell and outstandingly fast decaying dynamics (≈1 ns) for glass-like materials. The conjunction of the observed efficient and stableluminescences with their attributed decaying behaviours suggests applications forsilica nanowires such as active and passive optical interconnectors and white lightphosphors. The identification of a time domain difference for the spectral regimefrom 2.3 to 3.3 eV, within the confined spatial dimensions of a single nanowire, isvery promising for future, e.g. data transmission applications, employing silicananowires which exhibit achievable compatibility with commonly applied silicon-basedelectronics. A qualitative growth model based on silica particle diffusion andTi-assisted seed formation is developed for the various types of segregated silicananowires which extends commonly assumed oxide-assisted growth mechanisms.

Bound excitons in ZnO: Structural defect complexes versus shallow impurity centers

ZnO single crystals, epilayers, and nanostructures often exhibit a variety of narrow emission lines in the spectral range between 3.33 and 3.35 eV which are commonly attributed to deeply bound excitons ($Y$ lines). In this work, we present a comprehensive study of the properties of the deeply bound excitons with particular focus on the ${Y}_{0}$ transition at 3.333 eV. The electronic and optical properties of these centers are compared to those of the shallow impurity related exciton binding centers ($I$ lines). In contrast to the shallow donors in ZnO, the deeply bound exciton complexes exhibit a large discrepancy between the thermal activation energy and localization energy of the excitons and cannot be described by an effective mass approach. The different properties between the shallow and deeply bound excitons are also reflected by an exceptionally small coupling of the deep centers to the lattice phonons and a small splitting between their two electron satellite transitions. Based on a multitude of different experimental results including magnetophotoluminescence, magnetoabsorption, excitation spectroscopy (PLE), time resolved photoluminescence (TRPL), and uniaxial pressure measurements, a qualitative defect model is developed which explains all $Y$ lines as radiative recombinations of excitons bound to extended structural defect complexes. These defect complexes introduce additional donor states in ZnO. Furthermore, the spatially localized character of the defect centers is visualized in contrast to the homogeneous distribution of shallow impurity centers by monochromatic cathodoluminescence imaging. A possible relation between the defect bound excitons and the green luminescence band in ZnO is discussed. The optical properties of the defect transitions are compared to similar luminescence lines related to defect and dislocation bound excitons in other II--VI and III--V semiconductors.

Whispering gallery mode enhanced luminescence from an individual ZnO micro- and nanoscaled optical resonator

A whispering gallery mode optical resonator has been achieved using individual ZnO micro- and nanocolumns. Monochromatic cathodoluminescence images of the ZnO micro- and nanocolumns show that enhanced luminescence intensity is located across the profile of the micro- and nanocolumns, and is mainly attributed to the whispering gallery mode (WGM) enhanced emission. The individual hexagonal ZnO micro- and nanocolumns are regarded as an optical resonator structure. Theoretical calculations and computer simulations based on the finite-difference time-domain suggest that the WGMs are generated in the ZnO micro- and nanocolumns, and these results agree closely with the experimental results.

Growth study and impurity characterization of Al xIn 1− xN grown by metal organic chemical vapor deposition

Al x In 1− x N films with 0.14< x<0.24 were grown on GaN templates by metal organic chemical vapor deposition. Growth parameters – temperature, pressure, V/III ratio, growth rate and carrier gas ratio (H 2/N 2) – were explored and the films were characterized by atomic force microscopy, high resolution X-ray diffraction, cathodoluminescence (CL) and secondary mass spectrometry (SIMS). In particular, crystal quality and the surface morphology were sensitive in particular to the growth rate. Uniform emission from a high quality Al 0.82In 0.18N at 335 nm was observed by a monochromatic CL image, suggesting that the layer was compositionally homogeneous. SIMS results showed that the incorporation of carbon in AlInN was stronger at higher growth pressures. However, the carbon incorporation was rather insensitive to other growth parameters such as V/III ratio and carrier gas ratio. In the case of oxygen, its incorporation was not strongly affected by the growth parameters within the range explored in this study.

ZnO nanocone: Application in fabrication of the smallest whispering gallery optical resonator

ZnO semiconductors at the micro- and nanometre scales are attractive in optical, magnetic, and electronic applications because of their particular features and excellent properties. The whispering gallery mode (WGM) is a general and effective type to amplify the intensity of the luminescence emission, and has gained extensive application in lasing and microcavities. In this contribution, we reported that the smallest whispering gallery optical resonator has been achieved in an individual ZnO nanocone whose diameter gradually reduces from bottom to top in the range of 700 to 50 nm. Using the monochromatic cathodoluminescence (CL) equipment attached at a scanning electron microscopy, we observed the alternating patterns of bright and dark rings from the monochromatic CL image of an individual ZnO nanocone, which is attributed to the WGM-like enhanced luminescence emission when the ZnO nanocone is considered as an optical resonator. The smallest mode number of WGM, N=0, was observed in the ZnO nanocone with a radius of 55 nm for the considered light wavelength of 380 nm, and with a radius of 81 nm for the considered light wavelength of 500 nm, respectively. These results showed that the smallest whispering gallery optical resonator from an individual ZnO nanocone has been fabricated. Experiments are in good agreement with both theoretical predictions and computer simulations based on the finite-difference time domain method with perfectly matched layer boundary conditions. These findings provided valuable information for applications of ZnO micro- and nanostructures in optoelectronic devices.

Vertical Growth and Resonator Properties of Hexagonally Shaped Zinc Oxide Nanonails

We synthesized vertically aligned nail-shaped ZnO nanocrystal arrays on silicon substrates via a combination of a carbothermal reduction method and textured ZnO seeding layers that were precoated on silicon substrates by thermally decomposing zinc acetate, and studied their optical properties using cathodoluminescence (CL) and photoluminescence techniques. The ZnO nanonails show a sharp band-gap edge UV emission and a defect-related broad green emission. Monochromatic CL images of an individual ZnO nanonail show variations in spatial distributions of respective CL bands that had different origins. We attribute the spatial variation of CL images to an uneven distribution of luminescent defects and/or a structure-related light out-coupling from hexagonal ZnO nanostructures. The most distinct CL feature from the hexagonal head of an individual ZnO nanonail was the occurrence of a series of distinct resonant peaks within the visible wavelength range. It appeared that the head of a nanonail played the role of a hexagonal cavity so that polarization-dependent whispering gallery modes were stimulated by electron beam excitation.

The structural and optical properties of selectively grown a-plane GaN with LT-GaN and HT-AlN buffer layers

This study examined the effects of HT-AlN and LT-GaN buffer layers on the optical properties, crystallinity and morphology of 3D a-plane GaN structures with InGaN/GaN MQWs. The 3D a-plane GaN on the HT-AlN buffer layer showed a rough surface and various facets, whereas the 3D a-plane GaN on the LT-GaN buffer layer contained mainly {1 0 1¯ 1} facets that were tilted 25° to the c-axis giving a pyramidal shape. Narrower full-width at half maximum and higher intensity of the cathodoluminescence spectra of 3D a-plane GaN with InGaN/GaN MQWs were observed with the LT-GaN buffer layer. The monochromatic cathodoluminescence images at a specific wavelength showed that the LT-GaN buffer layer can effectively improve the region of 3D a-plane GaN with InGaN/GaN MQWs.

Low temperature photoluminescence and cathodoluminescence studies of nonpolar GaN grown using epitaxial lateral overgrowth

Low temperature cathodo- and photoluminescence has been performed on nonpolar a-plane GaN films grown using epitaxial lateral overgrowth. In films overgrown at a low V–III ratio, the emission spectrum is dominated by “yellow” and “blue” luminescence bands, attributed to recombination at point defects or impurities. The intensity of this emission is observed to decrease steadily across the window region along the −c direction, possibly due to asymmetric diffusion of a point defect/impurity species. When overgrown at a higher V–III ratio, the near band edge and basal-plane stacking fault emission intensity increases by orders of magnitude and a donor–acceptor pair band is observed. Using monochromatic cathodoluminescence imaging, the various emission features are correlated with the microstructure of the film. In particular, the peak energy of the basal-plane stacking fault emission is seen to be blueshifted by ∼15 meV in the wing relative to the window region, which may be related to the different strain states in the respective regions.