Showing papers on "Cathodoluminescence published in 1992"

On the ‘‘band‐A’’ emission and boron related luminescence in diamond

Abstract: Cathodoluminescence spectra of undoped and boron‐doped diamond films are reported and are compared with each other. The dominant spectra feature of these samples is the broad luminescence bands centered around 2.83 and 2.32 eV in undoped and relatively heavily doped samples, respectively. Our result indicates that the commonly observed 2.83 eV band (often called by the name ‘‘band‐A’’) is neither related to boron nor to donor‐acceptor pairs in diamond. From all of the available data, we conclude that the 2.32 eV band is due to boron related centers and the 2.83 eV band is due to dislocations in diamond.

Carrier capture and quantum confinement in GaAs/AlGaAs quantum wire lasers grown on V‐grooved substrates

Abstract: Carrier capture mechanisms in semiconductor quantum wire (QWR) lasers grown by organometallic chemical vapor deposition on nonplanar substrates were investigated by cathodoluminescence and photoluminescence (PL) spectroscopy. Efficient carrier capture into the QWRs via adjacent quantum wells is manifested by a complete transfer of luminescence intensity from the well‐ to the wire‐spectral lines at temperatures above ∼100 K. In addition, higher QWR subbands separated by 19 meV are observed in the PL spectra, in agreement with the calculated subband spacing. The quantum well assisted carrier capture in these wires is important for the efficient room temperature operation of lasers and other optoelectronic devices based on QWRs.

Interfacial properties of very thin GaInAs/InP quantum well structures grown by metalorganic vapor phase epitaxy

Abstract: GaInAs/InP single quantum well structures with thicknesses below 5 nm were grown by metalorganic vapor phase epitaxy at reduced pressure. The sharpness of the heterojunctions in this III/V system is found to be strongly dependent on the applied gas switching sequence between the growth of the two materials caused by As carry over after GaInAs and by group V atom exchange at the surface during a hydride stabilized growth interruption. The photoluminescence properties can be improved by adding intermediate monolayers of InAsP between InP and GaInAs and GaInAsP between GaInAs and InP. The photoluminescence of very thin quantum wells is split into multiplets due to the formation of growth islands at the interface. Size and lateral distribution of these islands have been observed directly by cathodoluminescence analysis. On the other hand, transmission electron microscopy measurements show that the interfaces within the growth island regions are not atomically smooth but of a certain roughness. Small microisla...

Microscopic thickness variation of macroscopically uniform quantum wells

Abstract: Recently, GaAs/AlxGa1−xAs quantum wells were grown, which display remarkable uniformity, over macroscopic distances, in the splitting of their photoluminescence lines. The negligible variation of the peak splitting was interpreted as being consistent with a number of possible interface configurations, the simplest of which was abrupt, atomically smooth interfaces with discrete thickness changes of integer multiples of monolayers. However, here, further analysis of the splittings show them to be slightly below one monolayer, supporting a modification of the ‘‘atomically smooth island’’ picture to one of microroughness, defined as that having a length scale below the exciton diameter (≊15 nm), superimposed on islands larger than the exciton diameter. In addition, by extending these measurements to the ‘‘mesoscopic’’ regime by a high resolution cathodoluminescence technique, we directly show that these microrough islands can have sizes smaller than the spatial resolution of our scanning cathodoluminescence m...

The measurement of deep level states caused by misfit dislocations in InGaAs/GaAs grown on patterned GaAs substrates

Abstract: Rectangular Schottky diodes were fabricated on In0.06Ga0.94As grown by organometallic vapor phase epitaxy on GaAs substrates patterned with mesas. The density of α and β misfit dislocations at the strained‐layer interface changed with the size of the rectangular mesas. Since all mesas (four sizes and two orientations) are processed simultaneously, all other defect concentrations are expected to remain constant in each diode. Scanning cathodoluminescence showed that the misfit dislocation density varied linearly with rectangle size. Deep‐level transient spectroscopy showed that an n‐type majority‐carrier trap is present at 0.58 eV below the conduction band with a concentration that increases with increasing α‐type misfit dislocation density. The β misfit dislocation density had no influence the deep level spectra, indicating that this trap is related to the cores of only α‐type misfit dislocations. The capture rate trend corroborates the view that the trap is associated with the dislocation cores and not with isolated defects. Calculations indicate that the trap concentration is comparable to the concentration expected if all of the dislocation core atoms are electrically active.

Luminescence characteristics of quantum wires grown by organometallic chemical vapor deposition on nonplanar substrates

Abstract: Luminescence properties of GaAs/AlGaAs quantum wire (QWR) heterostructures grown by organometallic chemical vapor deposition on V‐grooved substrates are reported A model of the crescent‐shaped wires yields parabolic QWR potential wells with subbands separated by 217, 39, and 167 meV for electrons, heavy holes, and light holes and effective width of 16 nm for the ground electron state Spectrally and spatially resolved cathodoluminescence images reveal highly uniform emission from the QWR regions Photoluminescence excitation spectra exhibit enhanced absorption at the QWR subbands, with subband separations in good agreement with the model

Temperature‐dependent chemical and electronic structure of reconstructed GaAs (100) surfaces

Abstract: Low‐energy electron diffraction, soft x‐ray photoemission, cathodoluminescence (CL), and Auger electron spectroscopies have been performed to investigate the geometric, chemical, and electronic properties of GaAs (100) surfaces as a function of annealing temperature and surface reconstruction. These measurements indicate gradual changes in surface geometry, composition, deep level CL features, and Fermi‐level (EF) position with increasing temperature of surface preparation. In contrast, it was observed that pronounced changes in the surface ionization potential and work function between different surface reconstructions. For most of the desorption temperatures and surface reconstructions, the secondary electron emission exhibits characteristic double onsets, possibly due to the existence of differently reconstructed patches on the surface. The implications of these variations in the surface chemical and electronic structure of GaAs (100) surfaces on their metal contact properties. It was concluded that (a...

Cathodoluminescence investigation of impurities and defects in single crystal diamond grown by the combustion‐flame method

Abstract: Cathodoluminescence (CL) in a transmission electron microscope has been used to investigate impurities and defects in single crystals in diamond films grown by the combustion‐flame method and correlate them with the microstructure. Nitrogen was present in a number of different impurity defect forms; band A emission was correlated with dislocations while the defect giving rise to the 575 nm vibronic band was the only defect observed in fault‐free crystal. Several other nitrogen‐related defects were also observed to a lesser extent. The spatial distribution of these defects varied greatly and residual stresses in the 0.7–1.8 GPa range were estimated. In one single crystal, the relative intensities of the bands in the CL spectra acquired at various locations suggest that dislocation density, though variable, is highest at the nucleation end.

Cathodoluminescence imaging and spectroscopy of dislocations in Si and Si1−xGex alloys

Abstract: Dislocations in float‐zone Si which has been plastically deformed and deliberately copper contaminated, and misfit dislocations in a relaxed Si1−xGex alloy layer grown on a Si substrate by molecular beam epitaxy, have been investigated by monochromatic and panchromatic cathodoluminescence imaging and by cathodoluminescence spectroscopy. The measurements show that the D3 and D4 luminescence features originate on the slip lines in plastically deformed Si and at the misfit dislocations in the Si1−xGex alloy layer whereas the D1 and D2 bands are dominant between the slip lines and the misfit dislocations.

Cathodoluminescence of natural zircons

Abstract: Cathodoluminescence (CL) measurements made on natural zircon crystals are reported for the temperature range 45-300 K. Different combinations of excitation conditions used at various temperatures, coupled with information on the decay lifetimes of the emission features provided by modulation of the electron beam, enable identification of excited state transitions occurring within rare-earth impurities. A broad-band feature at 385 nm is noted, in common with other luminescence studies using different forms of excitation, and is attributed to an intrinsic decay process.

Optical emission properties of semi‐insulating GaAs grown at low temperatures by molecular beam epitaxy

Abstract: We have used cathodoluminescence (CL) and photoluminescence spectroscopy to observe deep‐level states in GaAs grown at low‐substrate temperatures by molecular beam epitaxy (LT GaAs) and the evolution of these states upon annealing. The as‐grown material shows intense deep‐level emissions which can be associated with an excess concentration of arsenic, mostly present as As‐antisite and As‐interstitial defects. These emissions subside with annealing for a few minutes at temperatures above 450 °C. CL measurements clearly show a dramatically reduced concentration of traps in the post‐growth 600 °C annealed material. Additional measurements carried out on As/GaAs systems indicate a high surface‐recombination velocity for these interfaces. These results account for a diminished role of electronic point defects in controlling the insulative behavior of LT GaAs and strongly supports a ‘‘buried’’ Schottky barrier model, which involves ultrafast recombination of carriers at surfaces of embedded arsenic clusters for...

Cathodoluminescence emission spectra of trivalent europium-doped yttrium oxysulphide

Abstract: Europium-doped yttrium oxysulphide phosphor (Y2O2S:Eu3+) was prepared. Y2O2S:Eu3+ was found to have a hexagonal structure with lattice parametersa=0.3776 nm andc=0.6538 nm. The Cathodoluminescence emission spectra of Y2O2S:Eu3+ were recorded between 400 and 635 nm at both 77 and 293 K. Most of the numerous emission lines were assigned on the basis of known energy levels of Eu3+ ions, crystal field splittings, and selection rules. More attention was paid to the high-intensity emission lines in the spectral red region, because this phosphor can be used as an efficient red-emitting phosphor in colour televisions. The highly intense lines at 611.3, 616.9 and 626.5 nm are attributed to the transitions from5 D 0 to one of the Stark components of the7 F 2 level. The other transitions from5 D 1 5 D 2 and5 D 3 to7 F J (J=0, 1–5) were found and assigned.

Facet modulation selective epitaxy–a technique for quantum-well wire doublet fabrication

Abstract: The technique of facet modulation selective epitaxy and its application to quantum-well wire doublet fabrication are described. Successful fabrication of wire doublets in the AlxGa1–xAs material system is achieved. The smallest wire fabricated has a crescent cross section less than 140 A thick and less than 1400 A wide. Backscattered electron images, transmission electron micrographs, cathodoluminescence spectra, and spectrally resolved cathodoluminescence images of the wire doublets are presented.

Lateral spreading of focused ion-beam-induced damage

Abstract: We study the lateral spreading of implantation‐induced damage and measure the position dependence of the cathodoluminescence intensity of GaAs/AlAs heterostructures patterned by a focused Ga+ ion beam. Two luminescence lines, one from a buried AlGaAs/GaAs quantum well and the other from a deeper lying AlAs/GaAs short period superlattice are detected. Implantation doses in the range 1012–1015 cm−2 are investigated. We find that the lateral spreading of implantation induced damage considerably exceeds the implanted region in the case of the quantum well (50 nm below the surface), but is well limited to the implanted region in the case of the superlattice (250 nm below the surface). Micro‐Raman measurements allow us to locally probe the degree of crystallinity at a certain point of the sample.

Cathodoluminescence and transmission electron microscopy study of dark line defects in thick In0.2Ga0.8As/GaAs multiple quantum wells

Abstract: TEM and cathodoluminescence (CL) imaging and spectroscopy have been performed on In(0.2)Ga(0.8)As/GaAs MQW structures. Cross-sectional and plan-view TEM demonstrates that misfit dislocations (MDs) are confined to the MQW-to-GaAs interfacial regions. The observed large variation in the exciton luminescence intensity is interpreted as due to the presence of nonradiative recombination centers spread homogeneously in the MQW region away from interface MDs. These nonradiative recombination centers compete with exciton and midgap radiative centers at wavelengths of 950 nm and 1000-1600 nm, respectively, resulting in spatiallty correlated dark line defects for all CL imaging wavelengths.

The influence of codopants and fabrication conditions on germanium defects in optical fiber preforms

Abstract: The GeE' defect concentration in MCVD germanosilicate preforms, as measured by cathodoluminescence profiles, is dramatically reduced by the addition of phosphorus as a codopant. The GeE' concentration is increased by using higher deposition temperatures and oxygen deficient conditions. >

Fluctuation in shell composition in Nautilus (Cephalopoda, Mollusca): evidence from cathodoluminescence

Abstract: Nautilusr pompilius and N. macromphalus shells show, under cathodoluminescence microscopy. a zoned luminescence of yellow to green for the former and blue to blue-green for the latter. Cathodoluminescence results from fluctuations in the amount of manganese present in the aragonite. There is a relationship between this phenomenon and growth lines. Luminescence intensity increases with ontogeny. Variations in the metabolic activity appear to be linked with the manganese content of the shell. Environmental factors may have an effect by way of their repercussion on the metabolic activity of Nautilus. The Mn2+ amount may be influenced by the physico-chemical pattern of the surrounding water but it appears also to be controlled at the species level.

High quality In0.48Ga0.52P grown by gas source molecular beam epitaxy

Abstract: We report the growth of high quality In0.48Ga0.52P lattice matched to GaAs substrates by gas source molecular beam epitaxy. A full width at half‐maximum (FWHM) of the (004) double crystal x‐ray diffraction peak as small as 19 arc sec was measured from a 1.8 μm thick epilayer. Such line width is the narrowest reported thus far for any growth technique. Room temperature cathodoluminescence shows a band gap of ∼1.92 eV and the narrowest measured FWHM is 29 meV. Electron mobilities as high as 3200 and 25 700 cm2/V s have been obtained from Hall measurements at 300 and 77 K, respectively, which are comparable with the highest values ever reported.

Lateral band‐gap patterning and carrier confinement in InGaAs/GaAs quantum wells grown by molecular beam epitaxy on nonplanar dot patterns

Abstract: Two‐dimensional (2D) lateral band‐gap patterning and carrier confinement were observed in InGaAs/GaAs patterned quantum‐well (QWL) dots grown by molecular beam epitaxy on prepatterned, nonplanar GaAs substrates. Growth of the strained QWL layers on craters of 0.2–5 μm diameters etched onto the substrate results in 2D lateral potential wells formed at the inside and outside regions of the craters due to preferential migration of Ga and In adatoms. Evidence for lateral carrier confinement and efficient radiative recombination in the resulting dot‐ and ring‐shaped potential wells was provided by low‐temperature cathodoluminescence spectroscopy and imaging. The results of this study indicate the feasibility of producing damage‐free quantum dot and quantum ring heterostructures by growth of QWLs on patterned, nonplanar substrates.

A simulation model for cathodoluminescence in the scanning electron microscope

Abstract: An improved three-dimensional model for simulating cathodoluminescence (CL) in a semiconductor under electron-beam irradiation is described. The Monte Carlo method is used to simulate electron-beam-semiconductor interaction while F. Berz and H.K. Kuiken's (1976) formulation is used to obtain the excess carrier distribution. Optical losses of photons both within the semiconductor and at the semiconductor-air interface are also accounted for in this model. This model has been used to simulate the CL intensity as a function of electron-beam voltage, beam incidence angle, surface recombination velocity, diffusion length, absorption coefficient, and surface dead-layer thickness. The radiation patterns over the top face of a specimen with flat geometry are also simulated. >