Room Temperature Cathodoluminescence Research Articles

Band gap narrowing and radiative efficiency of silicon doped GaN

Radiative efficiency, band gap narrowing, and band filling are studied in Si-doped GaN films as a function of carrier concentration (n), using room and low temperature cathodoluminescence (CL). Using the Kane model, a band gap narrowing ΔEg of −(3.6±0.6)×10−8 and −(2.6±0.6)×10−8n1/3 eVn1/3 is obtained for epitaxially strained and relaxed material, respectively. Band-edge CL time response and absolute external photon yield are measured. The internal radiation efficiency is deduced. Its monotonic increase as n increases is explained by the increase in the spontaneous radiative rate with a radiative free carrier band-to-band recombination coefficient B=(1.2±0.3)×10−11 cm3 s−1.

Luminescence properties of AlxGa1–xN(0.4 < x < 0.5)/AlyGa1–yN (0.6 < y ≤ 1) quantum structures grown by gas source molecular beam epitaxy

AbstractWe report structural and optical properties of AlxGa1–xN (0.4 < x < 0.5)/AlyGa1–yN (0.6 < y ≤ 1) quantum structures grown by gas source molecular beam epitaxy with ammonia on (0001) sapphire substrates. The structures are designed for light emission at ∼ 280 nm. The AlxGa1–xN (0.4 < x < 0.5) well material was grown under two dimensional (2D), three dimensional (3D), and (2D+3D) conditions by the varying group‐III/ammonia ratio. The formation of nanoscale islands, or quantum dots (QDs), in the wells grown in 3D and (2D+3D) modes was observed using transmission electron microscopy. Optical properties are investigated using room temperature cathodoluminescence and time‐resolved photoluminescence. Systematic studies allow us to obtain well growth conditions to produce ∼ 60 fold intensity enhancement over purely two‐dimensional structures. Under these conditions, corresponding to deposition ∼ 10 monolayers of well material, we obtain emission at ∼ 280 nm with narrowest line width and longest photoluminescence decay time. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Stress Dependence of Paramagnetic Point Defects in Amorphous Silicon Oxide

Room-temperature red cathodoluminescence (CL) emission (R band) arising from the paramagnetic point-defect population present in amorphous silicon oxide (SiOx) has been characterized with respect to its shift upon applied stress, according to a piezo-spectroscopic (PS) approach. The R band (found at around 630 nm) originates from nonbridging oxygen hole centers (NBOHC; Si-O*) generated in the presence of oxygen-excess sites. It is shown that reliable stress assessments can be obtained in silica glass with a relatively high spatial resolution, provided that appropriate spectroscopic procedures are developed to precisely extract from the CL spectrum the shift upon stress of the R band, isolated from other partly overlapping bands. Macroscopic and microscopic PS calibration procedures are shown to lead to consistent results on silica materials with different chemical characteristics and, thus, with different intrinsic defect populations. In addition, quantitative calibrations of both electron probe size and luminescence emission distribution within the electron probe are given. As an application of the PS technique, the magnitude of the residual stress piled up (mainly due to a thermal expansion mismatch) at a sharp silica/silicon interface has been characterized by taking into account the gradient in defect population developed as a function of distance from the interface. In the Results and Discussion section, brief comments are offered regarding the possible impact of highly spatially resolved stress assessments in silica glass upon the development of new materials and advanced electronic devices.

Cathodoluminescence defect characterization of hydrothermally grown SnO2 nanoparticles

Sn O 2 nanoparticles in the 50–150nm size range were grown by a low temperature hydrothermal process, using SnCl4⋅5H2O as precursor and CH3(CH2)15N(Br)(CH3)3 as stabilizing agent. The as-grown samples were mostly amorphous and their crystallinity improved either by prolonged hydrothermal process or by air annealing at high temperatures. The absence of near-band-edge emission and appearance of a broad visible emission related mainly to oxygen vacancies and crystalline defects were the main characteristics of their room temperature cathodoluminescence (CL) spectra. A luminescent band in the 1.79–1.83eV spectral region was also detected. The intensity of the defect bands reduces both on prolonged hydrothermal treatment and air annealing at high temperatures, indicating a net decrease of defect content on thermal treatments. Panchromatic CL images revealed that most of the defect emissions come from smaller SnO2 nanoparticles.

Formation of ZnS nanoparticles under a Langmuir monolayer of zinc arachidate

The structure and phase composition of ZnS nanoparticles grown under a Langmuir monolayer of zinc arachidate were studied using electron diffraction and high-resolution transmission electron microscopy. Room-temperature cathodoluminescence spectra of obtained samples were measured.

The Origin of Green Emission of ZnO Microcrystallites: Surface-Dependent Light Emission Studied by Cathodoluminescence

Green emission of ZnO microcrystallites has been studied in detail by directly investigating the luminescence from the different crystal surfaces of ZnO microcrystallites by means of room temperature cathodoluminescence (CL). The relative strength of the green emission was found to be strongly dependent on the crystal surfaces. By discussing the atom structures of different crystal surfaces, it is concluded that the green emission mostly comes from the defects on/near the surfaces.

Structure analysis and optical study of In–O–N nanospears

Novel spear-like In–O–N nanostructures have been synthesized on a Si substrate via asimple thermal evaporation process by ammoniating the In-containing reactants at1100 °C. All the In–O–N nanostructures contain a long, thin pole with a diameter ranging from 20to 100 nm and a huge spear-like tip at the end. The structure and morphology, as well asthe crystallinity and composition of the In–O–N nanospears, were characterized by ascanning electron microscope (SEM), a transmission electron microscope (TEM) and theattached energy dispersive system (EDS). TEM analysis confirmed the cubic structure ofIn–O–N nanospears. Room-temperature cathodoluminescence (CL) measurement indicatedthat the In–O–N nanospears have a broad and strong light emission centred at∼454 nm with an unobvious shoulder in the range of 500–600 nm.

Al- and N-polar AlN layers grown on c-plane sapphire substrates by modified flow-modulation MOCVD

We report the growth of N- and Al-polar AlN layers on c-plane sapphire by flow-modulation MOCVD (FM-MOCVD) with some flow sequence modifications. Surface polarities were decided by pre-treatment for sapphire substrates prior to AlN seeding layer growth. Low-temperature nitridation (620 °C) was done for the N-polar samples, and no-nitridation was done for the Al-polar. To avoid strong vapor-phase reaction between TMA and NH 3 and to enhance the surface migration of Al atoms, FM-MOCVD technique was used. The flow sequence was modified for optimization of each N- and Al-polar growth. The surface features were completely different between the N- and Al-polar AlN layers in atomic force microscope images. The former had domed structures and the latter atomic steps. The dislocation densities were counted from plane-view transmission microscope images to be about 3×10 10 cm −2 for the N-polar layer and about 1×10 10 cm −2 for the Al-polar. Secondary-ion mass spectroscopy measurement revealed oxygen- and carbon-incorporation into the layers. Band-edge and impurity-related emissions were observed from only the Al-polar layer by room-temperature cathodoluminescence, whereas the N-polar one did not emit.

Photoluminescence, cathodo‐luminescence and micro‐Raman spectroscopy of as‐grown stacking faults in 4H‐SiC

AbstractWe report the results of combined investigations of as‐grown stacking faults found in a‐thick, undoped, 4H‐SiC epitaxial layer grown by CVD. We have use successively, low temperature photoluminescence (LTPL), room temperature cathodo‐luminescence (RT‐CL), micro‐Raman (µ‐R) and low temperature cathodo‐luminescence (LT‐CL) spectroscopy. From LTPL we find that the defects behave like thin, 2‐dimensional, quantum wells made of 4 bilayers of the 3C polytype embedded in the 4H‐SiC matrix. From RT‐CL, we show that the faults have a triangular shape with a large extension in the basal plane and a maximum emission wavelength centered at about 480 nm at 300 K. This makes them intermediate between the usual (semi‐infinite) QWs and the pure (1‐dimensional) quantum dots. To confirm the 3C character of the SFs, we used µR. Finally, using LT‐CL we scanned across one single (isolated) triangular defect and found that the maximum signal wavelength shifts, depending on the exciting spot position over the defect. To the best of our knowledge, this constitutes the first experimental evidence of a screening of the built‐in electric field when increasing the carrier concentration in the well. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fast, high-efficiency, and homogeneous room-temperature cathodoluminescence of ZnO scintillator thin films on sapphire

Excitonic luminescence in ZnO exhibits subnanosecond lifetimes combined with high efficiency, which makes epitaxial ZnO a promising ultrafast scintillator material for envisaged imaging applications with high data rate. ZnO thin films on sapphire show external ultraviolet electron-photon conversion efficiencies up to 0.42photons∕(keVe−) at room temperature and only minor lateral microscopic cathodoluminescence intensity variations. Peak shifts and occasionally observed double peaks found in cathodoluminescence spectra of epitaxial ZnO films with dependence on the detection geometry, the excitation depth, and the surface morphology are explained by a model based on photon propagation including self-absorption.

Stress dependence of the cathodoluminescence spectrum of N-doped 3C-SiC

The stress dependence of the room-temperature cathodoluminescence spectrum of N-doped cubic silicon carbide has been evaluated in a field-emission-gun scanning electron microscope, using the electron beam as an excitation source for luminescence emission. The electron-stimulated spectrum was dominated by only one broad band centered at about 544nm, with a broad shoulder centered at a slightly lower energy level (≈572nm). The cathodoluminescence spectrum, which was attributed to the four-particle N-bound excitonic transition, arose from substitutional N in the cubic silicon carbide lattice. Using experimentally measured probe response functions and energy shift magnitude collected near the tip of a Vickers indentation microcrack, it was possible to retrieve the actual magnitude of the piezospectroscopic coefficient [i.e., the slope of a linear plot of spectral band shift versus the trace of the stress tensor: Π=0.61±0.02nm∕GPa] of the N-bound exciton (cumulative) band of cubic silicon carbide.

ZnO hexagonal microboxes enclosed only by {0001} facets with epitaxial nanowalls

Hexagonal ZnO microboxes constructed of ZnO {0001} facets with vertical and horizontal single-crystalline nanowalls were synthesized on Si(001) via a self-assembled process by thermal chemical vapor deposition. High-resolution transmission electron microscopy images show that ZnO sheets with nanowalls on top can be assembled at 86° and 62° to each other by twinning and epitaxy, respectively. The room-temperature cathodoluminescence spectrum of the microboxes shows a strong and sharp ultraviolet emission as well as a negligible green emission. The vertical and horizontal nanowalls have potential to be applied as templates for growth of vertically and horizontally aligned nanowires for three-dimensional nanoelectronics.

Stress dependence of the near-band-gap cathodoluminescence spectrum of GaN determined by spatially resolved indentation method

A microscopic procedure has been proposed for evaluating the stress dependence of the (room-temperature) cathodoluminescence (CL) excitonic band emitted from the (0001) crystallographic plane of GaN in a field-emission-gun scanning electron microscope. The room-temperature near-band-gap emission (generally referred to as the excitonic band) mainly consisted of a band arising from free exciton (FX). However, an asymmetric morphology was found for the band, which thus needed to be deconvoluted into the main FX band and a shoulder. The spectral location at intensity maximum of the overall excitonic band under stress-free conditions was observed at room temperature at around 365nm. Experimentally measured spectral shifts were precisely retrieved nearby the tip of a Vickers indentation microcrack, while CL intensity probe response functions were collected at different acceleration voltages at a sharp interface between a GaN film and its sapphire substrate. Based on these assessments, the magnitude of the piezospectroscopic coefficient (i.e., the spectral shift rate versus the trace of a biaxial stress tensor) Π=1.35±0.01nm∕GPa of the excitonic (cumulative) band of GaN could be evaluated. This study not only emphasizes the importance of microscopic piezospectroscopic calibration procedures for precise residual stress assessments in GaN-based devices, but also the need of deconvoluting the electron probe for minimizing the error involved with its finite size.

Blue cathodoluminescence from thulium implanted AlxGa1−xN and InxAl1−xN

Room temperature cathodoluminescence (RTCL) was obtained from Tm implanted AlxGa1-xN with different AlN contents (in the range 0 <= x <= 0.2) and from implanted InxAl1-xN with different InN contents (x = 0.13 and 0.19) close to the lattice match with GaN. The Tm3+ emission spectrum depends critically on the host material. The blue emission from AIxGal-xN:Tm peaks in intensity for an AlN content of x similar to 0.11. The emission is enhanced by up to a factor of 50 times with an increase of annealing temperature from 1000 to 1300 degrees C. The blue emission from In0.13Al0.87N:Tm, annealed at 1200 degrees C, is more than ten times stronger than that from AlxGa1-xN:Tm, x <= 0.2. However, the intensity decreases significantly as the InN fraction increases from 0.13 to 0.19.

Growth of CuCl thin films by magnetron sputtering for ultraviolet optoelectronic applications

Copper (I) chloride (CuCl) is a potential candidate for ultraviolet (UV) optoelectronics due to its close lattice match with Si (mismatch less than 0.4%) and a high UV excitonic emission at room temperature. CuCl thin films were deposited using radio frequency magnetron sputtering technique. The influence of target to substrate distance (dts) and sputtering pressure on the composition, microstructure, and UV emission properties of the films were analyzed. The films deposited with shorter target to substrate spacing (dts=3cm) were found to be nonstoichiometric, and the film stoichiometry improves when the substrate is moved away from the target (dts=4.5 and 6cm). A further increase in the spacing results in poor crystalline quality. The grain interface area increases when the sputtering pressure is increased from 1.1×10−3to1×10−2mbar at dts=6cm. Room temperature cathodoluminescence spectrum shows an intense and sharp UV exciton (Z3) emission at ∼385nm with a full width at half maximum of 16nm for the films deposited at the optimum dts of 6cm and a pressure of 1.1×10−3mbar. A broad deep level emission in the green region (∼515nm) is also observed. The relative intensity of the UV to green emission peaks decreased when the sputtering pressure was increased, consistent with an increase in grain boundary area. The variation in the stoichiometry and the crystallinity are attributed to the change in the intensity and energy of the flux of materials from the target due to the interaction with the background gas molecules.

Aqueous Solution Route to High-Aspect-Ratio Zinc Oxide Nanostructures on Indium Tin Oxide Substrates

High-aspect-ratio ZnO nanowires and nanotubes are formed on indium tin oxide (ITO) substrates using a three-step route at low temperatures. The three steps, including successive ionic layer absorption and reaction (SILAR) deposition of the ZnO seed layer, hydrothermal annealing of the seed layer, and chemical bath deposition (CBD) of the one-dimensional (1D) ZnO nanostructures, are all conducted in aqueous solutions at temperatures below 120 degrees C. Both the hydrothermal annealing of the SILAR seed layer and the low-concentration precursor solution employed in the CBD process are crucial in order to synthesize the uniform and high-aspect-ratio ZnO nanostructures on the ITO substrate. TEM analyses reveal that both the nanowire and the nanotube possess the single-crystal structure and are grown along [001] direction. Room-temperature cathodoluminescence spectrum of the 1D ZnO nanostructures shows a sharp ultraviolet emission at 375 nm and a broad green-band emission.

Fabrication of ZnO microtubes with adjustable nanopores on the walls by the templating ofbutterfly wing scales

ZnO microtubes with adjustable arrayed nanopores on the walls are prepared by usingbutterfly wing scales as natural biotemplates. Flat butterfly wing scales used as templatesrolled into tubes during the calcinations; all the windows on the scales were reserved andformed porous walls. Furthermore the density of the pores on the walls is partiallydetermined by experimental conditions, such as temperature and time of treatment.The nano/microstructures obtained have been investigated by means of x-raydiffraction and field emission scanning electron microscopy. The room temperature(T = 300 K) cathodoluminescence of these unique ZnO microtubes is also studied. The spectrum showsa sharp near band edge emission and a broad deep level emission, which are similar tothose for other previously synthesized ZnO microtubes.

Failure mechanism of AlN nanocaps used to protect rare earth-implanted GaN during high temperature annealing

The structural properties of nanometric AlN caps, grown on GaN to prevent dissociation during high temperature annealing after Eu implantation, have been characterized by scanning electron microscopy and electron probe microanalysis. The caps provide good protection up to annealing temperatures of at least 1300°C, but show localized failure in the form of irregularly shaped holes with a lateral size of 1–2μm which extend through the cap into the GaN layer beneath. Compositional micrographs, obtained using wavelength dispersive x-ray analysis, suggest that these holes form when GaN dissociates and ejects through cracks already present in the as-grown AlN caps due to the large lattice mismatch between the two materials. Implantation damage enhances the formation of the holes during annealing. Simultaneous room temperature cathodoluminescence mapping showed that the Eu luminescence is reduced in N-poor regions. Hence, exposed GaN dissociates first by outdiffusion of nitrogen through AlN cracks, thereby opening a hole in the cap through which Ga subsequently evaporates.

Single-crystalline AlZnO nanowires/nanotubes synthesized at low temperature

Single-crystalline AlZnO nanomaterials were synthesized through a proposed alloy-evaporation deposition method at the low temperature of 550°C by thermal chemical vapor deposition. Transmission electron microscopy images show that AlZnO nanowires, or nanowire/nanotube junction structures, can be synthesized where the Al∕(Al+Zn) atomic ratio is determined to be about 2.5 and 12at.%, respectively, by electron energy loss spectrometry. Room-temperature cathodoluminescence measurements show that the AlZnO nanowires exhibit a strong ultraviolet emission, which shifts to a higher energy from 3.29to3.34eV due to Al incorporation.

Controlled synthesis of CdS nanobelts and the study of their cathodoluminescence

Cadmium sulfide nanobelts were synthesized on an Au-capped Si substrate via chemical vapor deposition by annealing CdS powder at 860 °C. Two types of nanobelts were found. While both types grew towards [101¯0], only one type had Au particle at its tip. Room-temperature cathodoluminescence (CL) was performed on the nanobelts which were prepared between 840 and 1060 °C. Each CL spectrum exhibited two peaks. The maximum band edge emission (BEE) was found at 510 nm in all spectra while that of the deep level emission (DLE) was located between 706 to 756 nm. The ratio of intensity of BEE to that of DLE increased with increasing annealing temperature. The variation of the DLE intensity in the samples was due to the variation of defect density. A blue shift up to 49 nm was detected for DLE as the annealing temperature was increased by 160 °C. Such a shift is due to the presence of more than one type of defects in these CdS nanobelts.