Showing papers on "Cathodoluminescence published in 1982"

Properties of AlxGa1−xN films prepared by reactive molecular beam epitaxy

Abstract: Single‐crystal films of the solid solution AlxGa1−xN of the entire composition range have been fabricated on sapphire and silicon substrates by reactive molecular beam epitaxy (MBE) at 700 °C. The properties of the films have been studied by the reflection high energy electron diffraction technique, x‐ray diffraction, and electrical and optical measurements. The lattice constant of the film is not a linear function of the composition, and the fundamental absorption edge also shows nonlinear dependence on the composition. The narrow intense peak of the cathodoluminescence concerned with the band to band or shallow impurity band transition, varies from 3.4 to 6 eV with the composition, which suggests the feasibility of AlxGa1−xN films grown by reactive MBE for optical devices in the ultraviolet spectral region.

Low‐dislocation‐density GaAs epilayers grown on Ge‐coated Si substrates by means of lateral epitaxial overgrowth

Abstract: Single‐crystal GaAs layers have been obtained by means of lateral epitaxial overgrowth seeded within stripe openings in a SiO2 mask over GaAs layers grown on Ge‐coated Si substrates. Transmission electron microscope and scanning cathodoluminescence studies indicate that the laterally overgrown GaAs layers have a dislocation density of less than 104 cm−2, compared to 107–108 cm−2 for the GaAs layers grown directly on the Ge/Si substrates. Initial experiments indicate that the electrical properties of the laterally overgrown layers are comparable to those of conventional GaAs epilayers grown on single‐crystal GaAs substrates.

The segregation of impurities and the self‐compensation problem in II‐VI compounds

Abstract: The electrical and optical properties of ZnTe or CdTe crystals are generally inhomogeneous after annealing. This is shown to be due to the redistribution of preexisting impurities. Indeed, after growth, these crystals contain some excess tellurium which must precipitate since the solidus line has a retrograde shape. The liquid Te droplets are then able to purify the surrounding materials by a solid‐liquid segregation mechanism. On annealing, the tellurium excess can disappear more or less rapidly: the Te precipitates and the small inclusions shrink progressively while the impurities are released into the crystal. This mechanism explains the bright dots observed in the scanning electron microscope (cathodoluminescence mode) and due to the high lithium concentration. Under other conditions, dark dots are obtained and the role of copper is suspected. This model is discussed on the basis of the available data (segregation and diffusion coefficients). Further confirmation is obtained from direct chemical analysis of the crystal and of the inclusions itself. The consequences of the segregation of impurities on the interpretation of the self‐compensation problem are analyzed. A fully satisfactory interpretation of conductivity variation with zinc pressure previously attributed to the zinc vacancy, can be derived with the single hypothesis that acceptor and donor impurities in sufficient amount are stored in the crystal. The dominant role of residual impurities in the close control of the II‐VI materials is particularly emphasized.

Role of Dislocations in Semi-Insulation Mechanism in Undoped LEC GaAs Crystal

Abstract: For the undoped semi-insulating LEC GaAs crystal, microscopic inhomogeneity was investigated on the fixed position by using cathodoluminescence (CL), secondary ion mass spectrometry (SIMS), X-ray topography (XRT) and scanning leakage current measurement (IL). Microscale fluctuations observed in these measurements were attributed to the cellular dislocation structures. It was suggested that the impurity gettering effect of dislocation plays an important role in the semi-insulation mechanism in GaAs crystal. Carriers are inactive in the boundary region of cell structures but are active in the inner region of cell structures.

Interference effects in luminescence studies of thin films

Abstract: Interference effects, including multiple-beam and wide-angle, associated with luminescence from within a thin film are described. A simple geometrical model is used to calculate the s- and p-polarized luminescent light assuming electric-dipole radiation. The luminescence exhibits fringes when measured both as a function of the film thickness and as a function of the wavelength of the light. In the latter case the fringes can also show a beating effect. The model is applied to several experimental examples of cathodoluminescence in SiO(2) and an example of photoluminescence in a-Si.

Effects of Cd‐vapor and Te‐vapor heat treatments on the luminescence of solution‐grown CdTe:In

Abstract: We have employed cathodoluminescence at 80–300 K in the first comprehensive study of the effects of Cd‐vapor or Te‐vapor heat treatments on the luminescence of solution‐grown CdTe:In. The broad 1.4‐eV band present in as‐grown material is weakened by Te firing and typically enhanced by Cd firing. These results do not support earlier connections between this luminescence and the VCd‐InCd complexes predicted by defect chemistry calculations to be dominant in CdTe:In. Alternatives to straightforward interpretation are discussed for both experiments and defect modeling. The effects of the heat treatments on the injection‐level dependence, frequency response, and temperature dependence of the 1.4‐eV luminescence are described. This luminescence arises from localized transitions within compact complexes in our as‐grown material, but different species of complexes or competing transition mechanisms are involved in heat‐treated material. The edge emission, present in both as‐grown and fired material, peaks near 1.57 eV at 80 K and thus is close to the band‐gap energy of CdTe. However, frequency‐response data reveal anomalous energy‐storage processes which can slow the edge‐emission kinetics to the microsecond regime following Cd firing. Another surprising result is the strong coupling in both kinetics and injection‐level dependence between the edge emission and a sharp extrinsic band near 1.54 eV which is prominent in Te‐fired CdTe.

Cathodoluminescence from ‘giant’ platelets, and of the 2·526 eV vibronic system, in type Ia diamonds

Abstract: The spectrum of the yellow cathodoluminescence emitted by ‘giant’ platelets in diamond is shown to be a featureless broad band superimposed on the shortwavelength tail of the well-known infrared luminescence from platelets. ‘Giant’ platelets are definitely not associated with the H3 system as has been suggested earlier. Also, strongly polarized emission in a vibronic band with a zero-phonon line at 490·7 nm (2·526 eV) has been observed to originate at regions of diamond that are plastically deformed. The 490·7 nm system has also been observed in volume distributions in growth bands.

Effect of electron beam on the cathodoluminescence from indented MgO in the scanning electron microscope

Abstract: The effect of an electron beam on the cathodoluminescence from indented MgO single crystals has been studied in the scanning electron microscope. It has been found that the luminescence intensity and spectrum vary with irradiation time. The behavior is attributed to changes in impurities and vacancies defects.

Cathodoluminescence evaluation of dark spot defects in InP/InGaAsP light‐emitting diodes

Abstract: In this study, the formation of dark spot defects (DSD’s) in InP/InGaAsP light‐emitting diodes (LED’s) is evaluated by cathodoluminescence imaging and energy dispersive x‐ray spectroscopy (EDS). Defects resulting in DSD’s are shown to be located in either the p‐InGaAsP contact layer, the p‐InP confining layer, or the InGaAsP active layer. The presence of gold was not detected at the DSD’s using EDS. However, gold was found in the form of submicron‐sized inclusions in the contact layer and confining layer of cylindrically lapped wafers using EDS. Our results strongly suggest that the migration of gold from the p contact during device processing and aging results in the formation of DSD’s in InP/InGaAsP LED’s.

Single‐crystal GaAs films on amorphous substrates by the CLEFT process

Abstract: By using the CLEFT process, epitaxial GaAs films can be grown on a reusable GaAs single‐crystal substrate and transferred to amorphous substrates. An essential step in forming the epitaxial films is lateral growth seeded from openings in a mask on the single‐crystal substrate. The lateral growth process has been studied as a function of growth conditions and crystal orientation, and optimized to give continuous films with specular surfaces. Very few defects are found in the films by cathodoluminescence and transmission electron microscopy. As an initial demonstration of film quality, solar cells having efficiencies as high as 17% at AM1 and an area of (1)/(2) cm2 have been fabricated. The CLEFT process is advantageous for device applications because amorphous or polycrystalline substrates can be much less expensive than single‐crystal substrates, and also because they can have electrical and optical properties leading to improved device performance.

Degraded InGaAsP/InP double heterostructure laser observation with electron probe microanalyzer

Abstract: Dark spots associated with degradation in InGaAsP/InP double heterostructure lasers were examined by electron probe microanalyzer, cathodoluminescence micrograph, and sequential selective etching procedures. It has been clarified that these dark spots penetrate from the p‐cladding layer to the n‐cladding layer and have no direct relation with InP substrate defects. Segregation of As and Ga in an active layer geometrically correlates with these dark spots. Dark spot generation and segregation were seen to develop with degradation induced in the laser operation.

Cathodoluminescence at frictional damage in MgO single crystals

Abstract: The frictional damage in single-crystal MgO was studied using the cathodoluminescent mode of a scanning microscope as well as selected-area electron channelling pattern analysis. With a hemispherical diamond slider, the distorted structured layer beneath the surface was noted, the cathodoluminescence of which was quenched, presumably due to the high density of defects. This non-luminescent layer was encased in the plastically deformed luminescent zone, which extended downwards to where there was a (0 0 1) cleavage crack. It was also found that with a larger ball slider, luminescent slip lines on both {1 1 0}90 and {1 1 0}45 were pronounced inside the track after a single to-and-fro traversal, and successive traversals were able to generate a non-luminescent, distorted structured layer inside the track.

Investigation of impurity variations by cathodoluminescence imaging: Application to GaSb:Te

Abstract: Crystals grown by the Czochralski technique frequently exhibit periodic variations of impurity concentration. These growth striations are microscopic in size and provide detailed information on the growth conditions. Although these striations are easily revealed by chemical etching, quantitative changes in carrier concentration are not easily determined. In this letter, the cathodoluminescence (CL), efficiency of GaSb:Te crystals is presented as a function of carrier concentration. The periodic variations in CL intensity, spatially correlated with growth striations revealed by an etchant, corresponds to ∼20% changes in carrier concentration. Finally, within the resolution of the CL technique, no evidence of a second phase is found in GaSb crystals doped with Te to the solubility limit.

Excitonic luminescence and edge emission in AgGaS2 crystals

Abstract: The low temperature cathodoluminescence spectra of AgGaS2 are investigated in this work. In the purest single crystals (sample A1) at 4.2 K, the luminescence spectra are dominated by free excitons, by exciton bound to impurities, and by an edge emission peaked at 2.590 eV. Our study shows the presence of an acceptor level at 110 meV which should be responsible of the bound exciton at 2.689 eV as well as of the edge emission. In other samples, we only observe the free exciton at 2.700 eV and a green broad emission band at 2.470 eV.

Cathodoluminescence studies of postrange defect introduction from ion implantation in CdSe

Abstract: Cathodoluminescence measurements in conjunction with layer removal by chemical etching show that shallow 200‐keV Xe+ ion implantations cause significant postrange defect introduction in CdSe. Strong spectral changes are present at depths an order of magnitude beyond the calculated projected ion range (∼500 A). There is significant residual damage at depths greater than a micron in some instances. Implantation at 80 K yields deeper and more severe 300‐ K stable postrange defect introduction than does implantation at 300 K. One component of the postrange damage enhances nonradiative (<1 eV) recombination. The introduction of, or increased prominence in, deep‐center luminescence near 1.4 eV is tentatively associated with cadmium vacancy (VCd) introduction in the postrange zone. The simultaneous strong growth in the prominence of luminescence near 1.7 eV is explained by the novel interpretation that incumbent interstitial alkali impurities (Na, Li) are reacting with ion‐implantation‐induced VCd to enhance the...

Nonradiative regions in GaInAsP/InP double heterostructure laser material: correlation with dislocation clusters in the substrates

Abstract: The results of an examination of GaInAsP/InP laser material grown by liquid phase epitaxy using transmission cathodoluminescence, polarised infra-red microscopy and electrochemical etching techniques are reported. It is shown that dislocation clusters in the InP substrate give rise to non-radiative regions in the active layer.

Cathodoluminescence and optical absorption in deformed MgO single crystals

Abstract: Optical absorption at 220 nm (5.7 eV) and cathodoluminescence emission at 410 nm (3.0 eV) have been measured in deformed MgO single crystals with different geometries. Whereas previous studies indicate that a luminescence band at 430 nm (2.9 eV) is due to photoexcitation of the absorption band at 220 nm, there appears to be no direct correlation between the cathodoluminescence and the absorption band.