Showing papers on "Cathodoluminescence published in 1989"

On the mechanisms of strain release in molecular‐beam‐epitaxy‐grown InxGa1−xAs/GaAs single heterostructures

Abstract: Inx Ga1−x As/GaAs single heterostructures have been grown by molecular‐beam epitaxy with different growing rates and In molar fractions. Indium composition, layer thickness, and residual strain have been measured mainly by Rutherford backscattering/channeling spectrometry and the results on selected samples compared with the results of other techniques like Auger electron spectroscopy and single‐ and double‐crystal x‐ray diffraction. Cathodoluminescence, x‐ray topography, transmission electron microscopy, and ion dechanneling have been employed to observe dislocations and to characterize their nature and density. While the onset of misfit dislocations has been found to agree with the predictions of the equilibrium theory, the strain release has been found to be much lower than predicted and the results are compared with the available metastability or nucleation models. Present results are in best agreement with nucleation models. Moreover, annealing experiments show that these heterostructures are at (or ...

Evaluation of Advanced Semiconductor Materials by Electron Microscopy

Abstract: These proceedings comprise papers reviewing technical progress as well as applications in the areas of: electron microscopy; electron diffraction; energy loss microanalysis; cathodoluminescence; Schottky Barriers; interfaces; analysis of local strains; surface microscopy; and defects in heteroepitax

Intrinsic and extrinsic cathodoluminescence from single-crystal diamonds grown by chemical vapour deposition

Abstract: The authors have analysed the cathodoluminescence spectra from single-crystal diamonds grown by the decomposition of a methane-hydrogen mixture in a microwave plasma. Crystals grown using methane concentrations of

Determination of nonradiative surface layer thickness in quantum dots etched from single quantum well GaAs/AlGaAs

Abstract: Low‐temperature cathodoluminescence spectroscopy was used to investigate the luminescence efficiency of reactive ion etched quantum dots, varying in diameter from 200 μm down to 60 nm. The luminescence efficiency was found to be degraded both with decreasing nanostructure size and with increasing etch depth. A solution to the standard model for diffusion and recombination was applied to the data to determine the surface recombination velocity S. We found that for dots smaller than the diffusion length, the standard diffusion model becomes insensitive to the value of S and fails to predict that there is a size of dot in which the luminescence is completely extinguished. To understand qualitatively the luminescence degradation in etched nanostructures we describe a damage layer thickness ξ. The value of ξ determines the smallest quantum structure that will still emit light. We show that ξ increases with increasing etch depth and is therefore dependent on etching conditions.

Indium adatom migration during molecular beam epitaxial growth of strained InGaAs/GaAs single quantum wells

Abstract: Indium composition variations in strained InGaAs/GaAs quantum wells grown on nonplanar substrates by molecular beam epitaxy have been analyzed by spatially and spectrally resolved low‐temperature cathodoluminescence. For our growth conditions, the In adatom migration length on (100) facets has been determined to be ∼25 μm. A maximum relative increase of In incorporation of ≂6% on (100) ridges is observed and is found to be strain independent (In composition) for quantum wells nominally 35 and 70 A thick with In composition of 0.10–0.22. Significantly asymmetric indium adatom migration is observed between adjacent (100) facets for ridges and grooves formed with (111)A and (311)A multifaceted sidewalls, indicating that multifaceting kinetically inhibits adatom migration. For structures designed for one‐step growth of index‐guided injection lasers with built‐in nonabsorbing waveguides, we show that differences greater than 80 meV in the effective band gap of a 70 A quantum well can be achieved between the ga...

Optical centres related to nitrogen, vacancies and interstitials in polycrystalline diamond films grown by plasma-assisted chemical vapour deposition

Abstract: The authors have used cathodoluminescence spectroscopy to examine point defects present in polycrystalline diamond films grown by the decomposition of a methane-hydrogen mixture in a microwave plasma. For films grown with low methane concentrations (0.3%) the bright-blue luminescence is predominantly due to donor-accepted pair recombination, together with some weak zero-phonon lines. As the CH4 concentration in the gas mixture is increased the cathodoluminescence spectra from the resulting films contain a number of additional zero-phonon lines. Some of these are unique to this type of diamond; other lines at 2.156 eV, 2.807 eV, 3.188 eV and 4.582 eV are associated with optical centres that have been studied in diamonds produced by high-pressure synthesis, and indicate that the diamond films contain carbon interstitials, nitrogen-vacancy and nitrogen-interstitial centres. The widths of the zero-phonon lines in the diamond films suggest that the material is heavily strained.

Cathodoluminescence studies of isotope shifts associated with localised vibrational modes in synthetic diamond

Abstract: Sharp structure between 2.60 and 2.64 eV observed in the cathodoluminescence spectra of synthetic diamond, following radiation damage and annealing at 300 degrees C, has been identified as a series of local-mode phonon replicas of the zero-phonon line at 2.807 eV. By examining the isotope shifts of these replicas in diamonds grown using 13C (and containing natural nitrogen) compared with diamonds synthesised from natural carbon and doped with 15N, the authors conclude that the optical centre contains one nitrogen atom in association with the interstitial produced by the radiation damage. The local-mode structure is very similar to that observed for the system with a zero-phonon line at 3.188 eV, which has also been shown to be a single nitrogen plus interstitial centre. Coupling of the 2.807 eV electronic transition to lattice modes is, however, much weaker than for the 3.188 eV transition. In diamonds which have not been thermally annealed the intensities of these vibronic bands, initially zero, are observed to increase with increasing time under the influence of the electron beam-a phenomenon observed previously for the 2.156 eV optical centre. The absence of localised vibrational modes, the response of the zero-phonon line to random strain and the greatly increased luminescence intensity following annealing at 800 degrees C lead us to propose that the 2.156 eV centre involves single nitrogen with one or more vacancies.

Direct determination of the ambipolar diffusion length in GaAs/AlGaAs heterostructures by cathodoluminescence

Abstract: A new technique for determining carrier diffusion lengths by cathodoluminescence measurements is presented. The technique is extremely accurate and can be applied to a variety of structures. Ambipolar diffusion lengths are determined for GaAs quantum well material, bulk GaAs, Al0.21Ga0.79As, and Al0.37Ga0.63As. A large increase in the diffusion length is found for Al0.37Ga0.63As and is attributed to an order of magnitude increase in lifetime.

Ga adatom migration over a nonplanar substrate during molecular beam epitaxial growth of GaAs/AlGaAs heterostructures

Abstract: Thickness variations in GaAs/AlGaAs quantum wells grown on patterned substrates by molecular beam epitaxy have been analyzed by spatially and spectrally resolved low‐temperature cathodoluminescence. For the lower and upper (100) facets joined by an angled (311)A facet, relative increases in quantum well thicknesses up to ≂6% and 20% are observed, respectively, in the vicinity of the intersection of the facets. Following an exponential behavior, the Ga adatom migration length is found to be in the range of 1–2 μm on both the lower and upper (100) facets and is independent of quantum well thickness. This migration length is orders of magnitude greater than previously reported for Ga adatoms during molecular beam epitaxy growth.

Refinement of Zinc Silicate Phosphor Synthesis

Abstract: Zinc silicates are important in CRTs and lamp phosphors. Previous work has shown that a sol‐gel‐like method (termed CGUPS) can produce uniform zinc silicate phosphor particles whose surface layer possesses an appropriate activator concentration gradient and exhibits efficient cathodoluminescence (CL). This work further investigates this synthesis method in detail and verifies quantitatively the processing parameters for yielding optimal phosphors. Phosphors are evaluated as a function of starting silicic acid particle size ranges and activator (Mn) and coactivator (As) concentrations. Our results indicate that the CGUPS synthesis is not very sensitive to the processing parameters within their initially defined ranges. This study also demonstrates that the CL efficiency of phosphors does depend on particle size and is maximum for 4–7 and 7–12 μm size range with 10 keV electron excitation. For optimal CL efficiency and persistance, the Mn concentration is found to be 0.3% in terms of Mn/Zn ratio and the As/Mn ratio is 36%.

The polarised adsorption and cathodoluminescence associated with the 1.40 eV centre in synthetic diamond

Abstract: Synthetic diamonds that are grown using nickel or a nickel alloy as the solvent-catalyst contain 1.40 eV centres which are strongly segregated in the (111) growth sectors. Absorption and cathodoluminescence measurements made using polarising optics show that the dipole moment of the centre is preferentially aligned with the (111) growth planes. The absorption and luminescence systems have a zero-phonon doublet at 1.4008 and 1.4035 eV, and, surprisingly, the high-energy component exhibits a much higher degree of polarisation than the low-energy line. Polarisation-dependent structure is also observed in the vibronic absorption and luminescence bands. The zero-phonon splitting is due to a splitting of the electronic ground state, and a speculative model is proposed that may form a basis for understanding the different polarisation behaviours of the two lines.

Luminescence spectroscopy of Mn2+ centres in rock-forming carbonates

Abstract: Cathodoluminescence emission spectra and luminescence excitation spectra of Mn2+ in calcites, dolomites, magnesites, a manganocalcite and a rhodochrosite have been measured at both room temperature and 77 K. Ligand-field parameters have been calculated from the excitation spectra which reflect the size of the metal cation site in which the Mn2+ ion resides. The technique of luminescence excitation spectra has enabled the absorption profile of Mn2+ in Mg and Ca sites in dolomite to be separately determined. The variation of the distribution of Mn2+ between Mg and Ca sites in dolomites is demonstrated and discussed in terms of a simple model. It is concluded that the distribution of Mn2+ in dolomites may well provide information about its conditions of formation and/or subsequent alteration.

Implantation enhanced interdiffusion in GaAs/GaAlAs quantum structures

Abstract: A high‐energy (up to 150 keV) Ga+ focused ion beam is used to implant quantum well structures and interdiffuse GaAs/GaAlAs heterojunctions thus creating quantum wires and boxes. We investigate the optical properties of these structures using low‐temperature cathodoluminescence. Wires as wide as 800 A have been observed 2000 A below the surface. We study the optical damage and the interdiffusion process as a function of the implantation parameters (ion energy ion dose) and as a function of the rapid thermal annealing time. A universal correlation between the optical damage and the interdiffusion length has been found. The optimum process conditions are proposed.

Cathodoluminescence measurement of an orientation dependent aluminum concentration in AlxGa1−xAs epilayers grown by molecular beam epitaxy on a nonplanar substrate

Abstract: Cathodoluminescence scanning electron microscopy is used to study AlxGa1−x As epilayers grown on a nonplanar substrate by molecular beam epitaxy. Grooves parallel to the [011] direction were etched in an undoped GaAs substrate. Growth on such grooves proceeds on particular facet planes. We find that the aluminum concentration in the epilayers is dependent on the facet orientation, changing by as much as 35% from the value in the unpatterned areas. The transition in the aluminum concentration at a boundary between two facets is observed to be very abrupt.

Molecular beam epitaxy of GaAs/AlGaAs quantum wells on channeled substrates

Abstract: GaAs/AlGaAs quantum wells (QWs) were grown by molecular beam epitaxy on GaAs (100) substrates patterned with ridges and grooves in the [011] direction. Low‐temperature cathodoluminescence was used to measure the Al fraction and QW thickness on top of the ridges and grooves as a function of ridge and groove width. Surface diffusion during growth depletes Ga from the side facets while increasing the incorporation of Ga on the (100) sections of ridges and grooves. The QW thickness on top of a ridge grown at 710 °C increases from 72 to 95 A, and the Al fraction x decreases from x=0.33 to x=0.29 as the ridge width is narrowed from 30 to 4 μm. Graded refractive index separate confinement heterostructure lasers with nominally 70 A QWs and Al0.2Ga0.8As barriers were grown on patterned substrates at 695 and 725 °C. Lasers fabricated on the overgrown 4‐μm‐wide ridges have a 20 meV decrease in emission energy compared to laser diodes on 30 μm ridges.

Carbon reduction in GaAs films grown by laser‐assisted metalorganic molecular beam epitaxy

Abstract: Secondary‐ion mass spectroscopy and cathodoluminescence (CL) studies were carried out on GaAs films grown by Ar+ laser‐assisted metalorganic molecular beam epitaxy. The Ar+ laser irradiation leads to the formation of a 400‐μm‐diam spot. In the growth temperature range 425–500 °C, the carbon concentration within the spot is maintained at 1017 cm−3, while that in the area not irradiated by the laser increases from 1017 to 1019 cm−3. The process of decomposition of the triethylgallium molecules and the mechanism by which the carbon concentration is maintained by the laser irradiation are discussed in detail. Low‐temperature CL spectra revealed that the CL signal intensity in the selectively grown spot was some ten times greater than that in the area not irradiated by the laser.

Misfit dislocations in pseudomorphic In0.23Ga0.77As/GaAs quantum wells: Influence on lifetime and diffusion of excess excitons

Abstract: The recombination dynamics of excess charge carriers around misfit dislocations in strained layer GaAs/In0.23Ga0.77As/GaAs quantum wells are directly imaged with spectrally and time‐resolved infrared cathodoluminescence imaging with subnanosecond time resolution. This unique experimental approach for the first time allows the imaging of excitonic lifetime around dislocations in such quantum wells. A strong reduction is observed. The quenching by more than two orders of magnitude of the quantum efficiency upon an increase of the line dislocation density from 4×103 to 5×106 m−1 is explained quantitatively by a diffusion model. The critical layer thickness is determined to agree well with the ‘‘mechanical equilibrium of forces’’ model.

Cathodoluminescence system for use in a scanning electron microscope including means for controlling optical fiber aperture

Abstract: An improved cathodoluminescence light collection system for use in electron microscopes provides an optical fiber, the facet or aperture of which is positioned adjacent the sample. The cathodoluminescence light, collected by the optical fiber in this manner, may be used to provide spectrally resolved cathodoluminescence images of the sample as well as local cathodoluminescence spectra of the type available in prior art cathodoluminescence light collection systems, but without incurring the numerous disadvantages of such prior art light collection systems. The present invention is relatively inexpensive, far easier to maintain because it does not require use of a mirror and, it is more compact in size thereby making it compatible with the physical limitations of more electron microscopes.

Cathodoluminescence system for a scanning electron microscope using an optical fiber for light collection

Abstract: We describe a novel light collection system for cathodoluminescence scanning electron microscopy. Cathodoluminescence emitted from the sample surface enters directly into the facet of an optical fiber, which is held less than a millimeter away from the sample to optimize the collection efficiency. The fiber is small enough that it has a minimal effect on access to the sample by other detection apparatus. Three axis positioning of the fiber is accomplished with motorized translation stages located in the sample chamber. Techniques for generating cathodoluminescence images and local spectra are discussed. The techniques are applied to oval defects in molecular beam epitaxially grown GaAs/AlGaAs epilayers, and the results are presented.

Electroluminescent Display Materials

Abstract: Luminescence is the emission of light, other than thermal incandescence, as a result of external stimulation of a material. A material that shows efficient luminescence is called a phosphor. Numerous phosphors have been developed that are important emissive-type electronic display materials for cathode ray tubes, vacuum fluorescent display devices, color plasma display panels, electroluminescent display panels, and, as a matter of course, for fluorescent lamps. Types of luminescence are distinguished by the source of energy input. Photoluminescence involves excitation by optical radiation, cathodoluminescence originates in excitation by electron beam (cathode ray), and e1ectroluminescence is excitation by electric field or current. Luminescence is a subject of deep interest in the field of materials science and for practical application. We are mainly concerned with electroluminescence, especially high-field assisted e1ectro­ luminescence, that is, the light emission obtained by passing electrical excitation energy through a material under an applied high electric field. The main reason for the intensive attention being paid to electro­ luminescent devices, especially to thin film ac electroluminescent devices, is the feasibility of realizing high quality flat panel displays. Thin film ac electroluminescent devices have a multilayered structure of thin films of transparent electrode, insulator, phosphor, and metal electrode. All these are important materials in fabricating an electroluminescent device, and offer numerous topics for research in areas such as electron transport processes and mechanisms of excitation of luminescent centers. In view of the rapid advance in physics and technologies in recent years, it is not possible to give a comprehensive detailed description of

Cathodoluminescence study of substrate offset effects on interface step structures of quantum wells

Abstract: The interface structures of single quantum wells grown on vicinal (100) substrates with a controlled offset 0.2° are studied by cathodoluminescence microscopy. Bright and dark stripe patterns are periodically observed in monochromatic images of the single quantum wells and the stripes replicate stair‐like stepped interfaces. It is further shown that the observed terrace width is several times larger than the average interstep distance of monolayer steps calculated from the offset angle and that the step height is not one monolayer. Based on the results, a new interface structure model has been proposed that takes the offset angle into account. Finally, it is shown that employing growth interruption during epitaxial growth as an interface smoothing technique has limitations.The interface structures of single quantum wells grown on vicinal (100) substrates with a controlled offset 0.2° are studied by cathodoluminescence microscopy. Bright and dark stripe patterns are periodically observed in monochromatic images of the single quantum wells and the stripes replicate stair‐like stepped interfaces. It is further shown that the observed terrace width is several times larger than the average interstep distance of monolayer steps calculated from the offset angle and that the step height is not one monolayer. Based on the results, a new interface structure model has been proposed that takes the offset angle into account. Finally, it is shown that employing growth interruption during epitaxial growth as an interface smoothing technique has limitations.

Nonradiative Damage Measured by Cathodoluminescence in Etched Multiple Quantum Well GaAs/AlGaAs Quantum Dots

Abstract: We report a study of nonradiative surface recombination in etched GaAs quantum well structures. Low‐temperature cathodoluminescence was used to measure the relative luminescence efficiencies of etched quantum dots as a function of size, etch depth and etching conditions, and quantum well width. The relationship between etching damage and quantum well width was determined by using three samples, each consisting of three quantum wells of 2, 4, and 9 nm thickness, with the placement of the wells relative to the surface varied systematically. Arrays of quantum dots which ranged in size from 5 μm down to 40 nm were produced by electron beam lithography and reactive ion etching or ion beam assisted etching. The nonradiative surface damage produced by the etching process degrades the luminescence efficiency in quantum dots smaller than 1 μm in diameter. We have determined that etching processes which use argon gas increase the nonradiative surface layer thickness compared to etching processes which use xenon. We...

Study of CdTe(111)B epilayers grown by molecular-beam epitaxy

Abstract: The growth of high‐quality Hg1−xCdxTe(111)B epitaxial layers involves the growth of a CdTe(111)B buffer layer, whether the growth is performed onto related or foreign substrates. CdTe(111)B layers have been investigated using the x‐ray double‐crystal rocking curve and the cathodoluminescence imaging techniques. The optimization of the growth conditions for CdTe(111)B layers is presented for a high‐quality homoepitaxy. The heteroepitaxy of CdTe(111)B on GaAs(100) substrates is then discussed, in the case of GaAs substrates oriented (100), misoriented by 2° towards [110], and misoriented by 5° towards [111]. The results indicate that a 2° misorientation leads to CdTe(333) peaks with full width at half‐maximum up to four times narrower then either of the other orientations tested. Furthermore, only threading dislocations were visible on this orientation by cathodoluminescence imaging.

Cathodoluminescence and electrical properties of CuAlS2

Abstract: Etude de l'effet d'un recuit thermique, en atmospheres variables (zinc ou cadmium/aluminium) sur les proprietes electriques et de luminescence de monocristaux de CuAlS 2

Optical characterization of stress in narrow gaas stripes on patterned si substrates

Abstract: Photoluminescence (PL) and spatially resolved cathodoluminescence (CL) techniques are used to characterize the stress in narrow GaAs stripes grown on Si platforms. Since no overgrowth occurs over the recess edges, the GaAs stripes are grown unconstrained along one dimension. A duplication of the optical transitions is found in the PL spectrum from a region containing embedded GaAs and stripes. The peaks in the high‐energy shoulders of the PL spectrum are identified by CL measurements with high spatial resolution as the luminescence contribution of the GaAs stripes. They are submitted to lower internal stress values. A study on the geometrical dependence of the strain regime shows that a nonuniform biaxial strain field with a dominant longitudinal component is present in the stripes. The uniform biaxial strain, found in GaAs on Si (001), is present at stripe intersections.