Showing papers on "Cathodoluminescence published in 1996"

Spatial distribution of the luminescence in GaN thin films

Abstract: The spatial dependence of the luminescence intensities at the band edge (364 nm) and at the ‘‘yellow’’ defect‐band (centered at 560 nm) regions for epitaxial GaN films have been studied using cathodoluminescence microscopy at room temperature. The films were grown by metalorganic chemical vapor deposition on (0001) sapphire substrates and were not intentionally doped. Significant nonuniformities in the band‐to‐band and in the yellow band emissions were observed. Yellow luminescence in small crystallites appears to originate from extended defects inside the grains and at low‐angle grain boundaries. The size of band‐to‐band emission sites correlates with low‐angle grain sizes observed by transmission electron microscopy.

Ordered arrays of quantum dots: Formation, electronic spectra, relaxation phenomena, lasing

Abstract: Elastic relaxation on facet edges, renormalization of the surface energy of the facets, and interaction between i&no3 via the strained substrate are the driving forces for self-organization of ordered arrays of uniform coherent three-dimensional is/a& on crystal surfaces. For a (100) surface of a cubic crystal, two-dimensional square lattice of pyramid-like islands (quantum dots) with the periodicity along the directions of the lowest stiffness (OlO) and (OOI) has the minimum energy among different one-dimen- sional and two-dimensional arrays. For the InAs/GaAs(lOO) system, an equilibrium array of dots of the lateral size _ 120-140 A exists in a fixed range of growth parameters. T'he main luminescence peak at 1.1 eV, as well as peaks of excited states coincide in energy with the peaks revealed in the calorimetric absorption spectra regardless of the amount of InAs deposited (2-5 ML). Raman spectra indicate significant strain in InAs dots. The "phonon bottleneck" effect is bypassed via multi-phonon exciton and carrier relaxation. Ultranarrow lines (< 0.15 meV) are observed in cathodoluminescence spectra up to high temperatures. Low threshold current density operation via zero-dimensional states and ultrahigh temperature stability of the threshold current (T, = 450 K) are realized for a quantum dot injection laser. Increase in the gain and significant reduction in the radiative lifetime are possible via the self-organization of vertically-coupled quantum dots (VECODs) arranged in a well ordered artificial three-dimensional tefragonal lattice.

Degradation of zinc sulfide phosphors under electron bombardment

Abstract: ZnS phosphor powders have been subjected to electron bombardment (2 keV, 2 mA/cm2) at a residual gas pressure of 1.2×10−8 Torr and oxygen pressures of 10−6 Torr. Auger electron spectroscopy and cathodoluminescence (CL), both excited by the same electron beam were used to monitor changes in surface state and luminous efficiency during electron bombardment. A direct correlation between the surface reactions and the degradation of CL brightness was observed. Both C and S were depleted from the surface during electron bombardment. The postulated mechanism for the electron stimulated reactions on the phosphor surface is electron beam dissociation of molecular species to atomic species which subsequently react with C to form volatile compounds (COx, CH4, etc.) and with ZnS to form a nonluminescence layer of either ZnO or ZnSO4. The growth in thickness of the nonluminescence surface layer is directly responsible for the degradation in CL brightness.

Identification of optical transitions in cubic and hexagonal GaN by spatially resolved cathodoluminescence.

Abstract: The hexagonal and cubic phases of GaN are characterized by spatially resolved cathodoluminescence (CL) spectra from micrometer-size single crystals with either hexagonal or cubic habits grown by plasma-assisted molecular-beam epitaxy. At 5 K, distinct narrow excitonic lines are found at 3.472 and 3.272 eV for the hexagonal and cubic phase, yielding energy gaps of 3.500 and 3.300 eV, respectively. Detailed temperature-and intensity-dependent CL measurements on cubic GaN crystals enable us to clearly identify the exciton (free: 3.272 eV, bound: 3.263 eV) and the donor-acceptor pair (3.150 eV) transition. Moreover, we determine the donor-band and acceptor-band transition energy for this phase. In addition, phonon replicas of the exciton line and of the donor-acceptor pair transition are observed at 3.185 and 3.064 eV, respectively.

Growth of {100} textured diamond films by the addition of nitrogen

Abstract: Localized {100} fiber textured diamond films were grown by addition of 20–200 ppm nitrogen into the gas phase during hot‐filament chemical‐vapor deposition (CVD). Cathodoluminescence indicates the presence of the nitrogen‐vacancy system in the {100} textured diamond, whereas a blue ‘‘band A’’ luminescence is normally observed in diamond films grown without nitrogen addition. The results demonstrate that the nature of the substrates used for growth has no appreciable influence on the {100} texture, which implies that this fiber texture is obtained by competitive growth and selection of facets. The interaction of nitrogen with the {100} surface is a highly important factor in this process. Homoepitaxial growth shows that the addition of a small amount of nitrogen greatly enhances the growth rate of the {100} faces, making 〈100〉 the fastest growth direction in comparison with the 〈110〉 and 〈111〉 directions. This is attributed to breaking of a part of the dimers on the (2×1) reconstructed {100} surface by nitrogen compounds. The {100} texture in narrow, ring‐shaped areas on diamond layers grown by the flame technique can also be attributed to the occurrence of a certain amount of nitrogen in the gas phase. It is demonstrated that the flame grown polycrystalline diamond layers have morphologies and cathodoluminescence features that are consistent with those observed in the hot‐filament CVD diamond films grown with the addition of nitrogen.

Cathodoluminescence of quartz from sandstones; interpretation of the UV range by determination of trace element distributions and fluid-inclusion P-T-X properties in authigenic quartz

Abstract: Quartz overgrowths from Keuper sandstones of the Paris basin were examined using cathodoluminescence (CL) microscopy and spectroscopy coupled with a scanning electron microscope (SEM). With the use of standard cold CL equipment, it was observed that the emission of authigenic quartz is much less intense than that of detrital quartz grains, but a reversal of intensity was observed with scanned CL in the 200-800 nm range. The main CL emission band of diagenetic quartz is at 330-340 nm in the UV range, with other bands in the visible range. The determination of trace element distributions in authigenic quartz by in situ analyses and by SIMS imagery reveals the correlation of the 330-340 nm emission band with the highest Al and Li contents. The cathodoluminescence emission of diagenetic quartz in the UV range appears to be influenced by the coupled substitution of Al and Li into the crystal. Two explanations are suggested: (1) AI, Li, or both serve as activators, and (2) the incorporation of Al and Li causes lattice defects that lead to an enhancement of the intrinsic luminescence. Trace element analyses and fluid-inclusion studies revealed that quartz overgrowths precipitated from a fluid that partially originated from an Li-enriched primary brine derived from eastern Triassic evaporites of the Paris basin. The specific CL emission band in the UV range can be linked to the diagenetic environment. The chemistry of the fluids appears to be the essential parameter, whereas precipitation temperatures do not influence the occurrence of such emissions.

Manganese‐activated luminescence in ZnGa2O4

Abstract: Manganese‐activated ZnGa2O4, prepared at a sintering temperature of 1375 °C with 0.1% MnO dopant, emitted red and green light with peaks at 666 and 508 nm when excited with a laser source (325 nm). The luminescence centers of red emission were ascribed to Mn4+ ions in the octahedral (B) sites of spinel‐structure zinc gallate. Monitored at these two peaks, the sample exhibited absorption bands at 342 and 464 nm (for red) and 245 and 290 nm (for green) when excited with radiation from a xenon discharge lamp, indicating an excitation process occurring in Mn4+ ions distinct from that of Mn2+ ions (in A sites) which were known to be responsible for the green emission. Cathodoluminescence spectra showed that the integrated intensity of an UV band (peak at 349 nm) increased steadily with beam current while the green light intensity saturated at high current ranges. Several possible saturation mechanisms were discussed. The variation of the intensities versus current was interpreted and formulated based on an ene...

Optical characteristics of p-type GaN films grown by plasma-assisted molecular beam epitaxy

Abstract: Using a molecular beam epitaxy system equipped with an inductively coupled radio frequency nitrogen plasma source, p‐type GaN films were grown on sapphire substrates with no postgrowth treatment. Uniformity of the surface morphology and spatial homogeneity of the luminescence of the films were investigated using scanning electron microscopy and cathodoluminescence (CL) imaging, respectively. By examining the dependence of photoluminescence on the excitation laser power density at 6 and 300 K, three different emissions having different origins were identified. A blue emission at ∼3.25 eV is associated with shallow Mg impurities, while two different lower‐energy emissions at ∼2.43 and ∼2.87 eV are associated with deep Mg complexes. The spatial distributions of the shallow and deep Mg impurities dominating the optical properties of the p‐type GaN films were also examined along the growth direction by low‐ and room‐temperature CL using an electron beam with a range of penetration depths

Cathodoluminescence image of defects and luminescence centers in ZnS/GaAs(100)

Abstract: The spatial distributions of the cathodoluminescence (CL) emissions from thin ZnS films on GaAs(100) have been examined by the low‐temperature CL measurement system combined with a transmission electron microscope (TEM). The correlation between these CL emissions and structural defects were studied by comparing the monochromatic CL images with the TEM images for both plan‐view and cross‐sectional observations. It is found that the neutral acceptor–bound exciton associated emission (A0,X) and the free‐electron‐to‐ionized acceptor transition emission (e,A) are affected by the stacking fault distribution. The localization of the emission due to the deep‐level emission transition near the interface suggest the diffusion of Ga atoms from the GaAs substrate. The characteristic distributions of the CL emission regions can be explained by considering the competitions among the recombination channels of those radiative processes for each type of an excess carrier, an electron, or a hole.

Cathodoluminescence study on dislocations in silicon

Abstract: A study was made on the characteristics of cathodoluminescence (CL) from Si crystals with various configurations of dislocations. D3 and D4 were observed along slip lines, while D1 and D2 were found to be strong in the region where plural slip lines intersect each other. It was shown that not only Lomer–Cottrell dislocations but also jogs act as recombination centers for D1 and D2 luminescence. The spatial distributions of D1 and D2 luminescence were observed to be similar to each other, this was also true of D3 and D4. The effect of hydrogenation on CL distribution was also investigated. Contrary to the reports of other groups, no evidence was obtained on the enhancement of D line luminescence due to metal contamination.

Nature of compensating luminescence centers in Te-diffused and -doped GaSb

Abstract: Diffusion of tellurium in undoped p‐GaSb has been carried out. Using the cathodoluminescence and photoluminescence techniques, the luminescence centers in Te‐diffused samples have been identified and compared with the Te‐doped bulk GaSb. Fundamental differences in the radiative levels are observed between the diffused and the as‐grown doped samples. Evidence for self‐compensating acceptor complexes are seen in diffused samples. With short and moderate diffusion times, a compensating acceptor complex VGaGaSbTeSb is observed. For long diffusion times, the dominant acceptor center has been attributed to the antisite defect GaSb or related complex. The reasons for the formation of various acceptor centers have been discussed.

Comparison of electrical and luminescence data for the A center in CdTe

Abstract: We have investigated the electrical and optical properties of the deep levels responsible for the 1.4–1.5 eV luminescence band usually observed in II–VI compounds. We compared the energy levels found by cathodoluminescence and junction spectroscopy methods for semi‐insulating (CdTe:Cl and Cd0.8Zn0.2Te) and semiconducting samples (undoped CdTe). The techniques utilized were deep level transient spectroscopy (DLTS) on semiconducting samples and photoinduced current transient spectroscopy and photo‐DLTS on high resistivity materials. These last two techniques are complementary and allow the determination of the trap character (donor/acceptor). Three acceptor levels are seen in the electrical transient data at Ev+0.12, 0.14, and 0.16 eV with hole capture cross sections of 2×10−16, 1×10−16, and 4×10−17 cm2, respectively. The lowest level is seen only in Cl doped material corroborating the literature optical and electron spin resonance identification of a level at Ev+0.12 eV as being a VCd+ClTe donor–acceptor p...

MBE growth of cubic GaN on GaAs substrates

Abstract: We review our recent results on plasma-assisted molecular beam epitaxy of cubic GaN films on GaAs(001). The crucial role of the nucleation stage for obtaining epitaxial growth is highlighted by reflection high-energy electron diffraction and transmission electron microscopy. For the subsequent growth, it is found that the surface stoichiometry determines the phase composition of the epilayers as determined by X-ray reciprocal space mapping, Raman and cathodoluminescence spectroscopy. For an in situ control of the surface stoichiometry, we develop a method based on the transitions of the surface reconstructions of cubic (001)GaN. Films grown under conditions optimized by this method are single-phase cubic and exhibit intense excitonic luminescence up to room temperature.

Time‐dependent study of low energy electron beam irradiation of Mg‐doped GaN grown by metalorganic chemical vapor deposition

Abstract: We have investigated the low energy electron beam irradiation (LEEBI) annealing kinetics of Mg‐doped GaN films grown by metalorganic chemical vapor deposition. Our results show that LEEBI annealing at room temperature, monitored by cathodoluminescence spectroscopy as a function of irradiation time, occurs rapidly initially and then proceeds gradually until saturation. We have also demonstrated that LEEBI annealing is effective even at liquid helium temperature, indicating its athermal mechanism. Our observations support the dynamic model involving electronically stimulated dissociation of Mg–H bonds and the escaping and retrapping of atomic hydrogens.

Optical degradation of InGaN/AlGaN light‐emitting diode on sapphire substrate grown by metalorganic chemical vapor deposition

Abstract: We report an optical degradation of an InGaN/AlGaN double‐heterostructure light‐emitting diode (LED) on a sapphire substrate grown by metalorgonic chemical vapor deposition. Electroluminescence, electron‐beam induced current, and cathodoluminescence observations have shown that the degraded InGaN/AlGaN LED exhibits formation and propagation of dark spots and a crescent‐shaped dark patch, which act as nonradiative recombination centers. The values of degradation rate under injected current density of 0.1 kA/cm2 were determined to be 1.1×10−3, 1.9×10−3, and 3.9×10−3h−1 at ambient temperatures of 30, 50, and 80°C, respectively. The activation energy of degradation was also determined to be 0.23 eV.

High‐quality GaInN/GaN multiple quantum wells

Abstract: High‐quality Ga0.92In0.08N–GaN multiple quantum wells structures (MQW) were grown successfully by metalorganic vapor phase epitaxy. Fine multilayer structures with a thickness period of 7–9 nm were detected by secondary ion mass spectroscopy. The dislocation density in the MQW was found to be in the range of 0.5–2×109 cm−2 by transmission electron microscopy. The MQW extremely enhanced by two orders of magnitude, the cathodoluminescence intensity compared with bulk Ga0.91In0.09N.

Scanning tunneling microscopy and tunneling luminescence of the surface of GaN films grown by vapor phase epitaxy

Abstract: We report scanning tunneling microscopy (STM) images of surfaces of GaN films and the observation of luminescence from those films induced by highly spatially localized injection of electrons or holes using STM. This combination of scanning tunneling luminescence with STM for GaN surfaces and the ability to observe both morphology and luminescence in GaN is the first step to investigate possible correlations between surface morphology and optical properties.

Synthesis and Low‐Voltage Characteristics of CaTiO3 : Pr Luminescent Powders

Abstract: The perovskite CaTiO 3 :Pr phosphors which have relatively small bandgaps were synthesized and characterized for their application to field emitter displays. Good crystal powders, homogenous, small (∼1 μm), and spherical, were easily formed without flux. Phosphor preparation parameters such as heat-treatment temperature, time, and activator concentration were optimized for the maximal intensity of photoluminescence. Phosphor cathodoluminescent characteristics were examined by a refractory metal electron gun in a high vacuum chamber. After electrophoretic deposition of phosphors on indium-tin oxide (ITO) glass, then excitation by field emitter arrays, lumen intensity could be obtained up to 10 cd/m 2 with chromaticity of X = 0.625 and Y = 0.297.

Cathodoluminescence and photoinduced current spectroscopy studies of defects in Cd0.8Zn0.2Te.

Abstract: Deep levels in Cd_(1-x)Zn_xTe have not yet been fully characterized and understood, even though this material is very promising for medical and optoelectronic applications. We have investigated p-type semi-insulating Cd_(0.8)Zn_(0.2)Te with cathodoluminescence (CL) and photoinduced current transient spectroscopy (PICTS) methods. PICTS analyses allow detection of deep levels which are not revealed by other current spectroscopy techniques generally used, as they permit scanning of a wider region of the energy gap. Five levels have been detected (0.16, 0.25, 0.57, 0.78, and 1.1 eV) and, by combining the results obtained with the above-mentioned CL techniques, we were able to advance hypotheses on the character (donor or acceptor) and origin of some of these levels. The key role prayed by the 0.78-eV level in controlling the carrier transport properties has also been confirmed.

Applications of a New 206.5-nm Continuous-Wave Laser Source: UV Raman Determination of Protein Secondary Structure and CVD Diamond Material Properties

Abstract: We demonstrate the utility of a new 206.5-nm continuous-wave UV laser excitation source for spectroscopic studies of proteins and CVD diamond. Excitation at 206.5 nm is obtained by intracavity frequency doubling the 413-nm line of a krypton-ion laser. We use this excitation to excite resonance Raman spectra within the π → π amide transition of the protein peptide backbone. The 206.5-nm excitation resonance enhances the protein amide vibrational modes. We use these high signal-to-noise spectral data to determine protein secondary structure. We also demonstrate the utility of this source to excite CVD and gem-quality diamond within its electronic band-gap. The diamond Raman spectra have very high signal-to-noise ratios and show no interfering broad-band luminescence. Excitation within the diamond bandgap also gives rise to narrow photoluminescence peaks from diamond defects. These features have previously been observed only by cathodoluminescence measurements. This new continuous-wave UV source is superior to the previous pulsed sources, because it avoids nonlinear optical phenomena and thermal sample damage; Photoluminescence.

Electron irradiation‐induced changes in the surface topography of silicon dioxide

Abstract: The irradiation of crystalline (α‐SiO2) and amorphous (a‐SiO2) silicon dioxide with a stationary electron beam produces characteristic changes in the surface topography. The development of these changes has been investigated using cathodoluminescence spectroscopy and microscopy, scanning probe (atomic force) microscopy, and scanning electron microscopy. Electron irradiation produces a permanent volume increase on (crystalline) α‐SiO2, while in (amorphous) a‐SiO2 an initial small volume increase is followed by volume loss as irradiation continues. The observed changes are consistent with electromigration of oxygen under the influence of the electric field induced by charge trapping at preexisting or irradiation‐induced defects. Oxygen enrichment may produce expansion of the surface region due to the formation of peroxy linkage defects. In a‐SiO2, charges trapped by defects at grain boundaries produce enhanced electric fields which may result in volume reduction at the surface, when critical field strengths...

InGaAs quantum wires grown by low pressure metalorganic chemical vapor deposition on InP V‐grooves

Abstract: Single InGaAs quantum wires were fabricated by low pressure metal organic chemical vapor deposition on V‐grooved InP substrates. For substrate patterning a new wet chemical etching process that leads to high quality V‐grooves with {111}A facets was used. The growth parameters of the InP buffer layer have a strong impact on the quantum wire formation. Scanning electron microscopy, photoluminescence, and spatially resolved cathodoluminescence experiments have been performed to characterize the structures. The crescent shaped InGaAs quantum wires have dimensions of about 13 nm height and 100 nm width. The wire luminescence is found to be at λ=1575 nm (FWHM=17 meV).

Hollow‐anode plasma source for molecular beam epitaxy of gallium nitride

Abstract: GaN films have been grown by molecular beam epitaxy (MBE) using a hollow‐anode nitrogen plasma source. The source was developed to minimize defect formation as a result of contamination and ion damage. The hollow‐anode discharge is a special form of glow discharge with very small anode area. A positive anode voltage drop of 30–40 V and an increased anode sheath thickness leads to ignition of a relatively dense plasma in front of the anode hole. Driven by the pressure gradient, the ‘‘anode’’ plasma forms a bright plasma jet streaming with supersonic velocity towards the substrate. Films of GaN have been grown on (0001) SiC and (0001) Al2O3 at 600–800 °C. The films were investigated by photoluminescence, cathodoluminescence, x‐ray diffraction, Rutherford backscattering, and particle‐induced x‐ray emission. The film with the highest structural quality had a rocking curve width of 5 arcmin, the lowest reported value for MBE growth to date.

Morphology of luminescent GaN films grown by molecular beam epitaxy

Abstract: GaN thin films were grown by molecular beam epitaxy on sapphire substrates. Scanning electron (SE) and atomic force microscopies reveal that on a typical film an assembly of oriented hexagonal microcrystallites rises above a background of polycrystalline or amorphous material. Cathodoluminescence (CL) spectra of the films feature bright UV exciton peaks and a broad green emission band. We identify the exciton peaks as those of the wurtzite form of GaN. A comparison of SE and CL micrographs of the same sample area shows that the luminescence emanates almost entirely from the hexagonal crystallites.

The luminescence of sapphire subjected to the irradiation of energetic hydrogen and helium ions

Abstract: The luminescence of during and irradiation was measured in the 190 - 820 nm wavelength range. The luminescence evolution with the ion fluence exhibits two behaviours : (i) at low fluence, the amount of centres increases; (ii) at high fluences, these defects are completely (F centres) or partially ( centres) annihilated. This phenomenon results from two concomittant mechanisms : a conversion between F and defects and a destruction of both luminescent species resulting from the radiation-induced damage. By using a simple model we have determined the cross sections associated with creation and annihilation of the centres. The irradiated samples were also investigated by cathodoluminescence and Auger electron spectroscopy. A higher concentration of structural defects and centres is evidenced at the sample area previously irradiated by ions, leading to an unsteady regime of the surface potential under electron excitation.

Stark effect in individual luminescent centers observed by tunneling luminescence

Abstract: We have measured photon emission from individual luminescent states in GaInP/InP heterostructures, containing InP dots, using local injection from a scanning tunneling microscope tip. By changing the tip‐sample bias we are able to see the Stark shift of the emission peaks, as well as the onset of impact ionization. We find that the exciton diffusion length is about 1 μm, while the minority carrier diffusion length is much less in our samples. Below the threshold for impact ionization the excitation is extremely local, limiting the excitation to one or a few quantum dots.

Luminescence studies of GaN grown on GaN and GaN/AlN buffer layers by metalorganic chemical vapor deposition

Abstract: A detailed study of the luminescence properties of GaN layers grown by metalorganic chemical vapor deposition on sapphire substrates with GaN/AlN double buffer layers or GaN single buffer layers was carried out with photoluminescence and cathodoluminescence (CL) spectroscopy. It was discovered that the use of the double buffer layer resulted in an improved surface morphology, but also increased the strain in the samples relative to samples grown on single GaN buffer layers. Free exciton (A exciton), neutral donor‐bound exciton, and acceptor‐bound exciton photoluminescence peaks were observed for GaN films grown on GaN, AlN, and GaN/AlN buffer layers. Acceptor free‐to‐bound luminescence was also observed and the thermal activation energy of the acceptors was measured. From these data we are able to determine the acceptor binding energy, EA, to be 231.5 meV and the donor binding energy ED to be 29.5 meV. An exciton peak, the acceptor free‐to‐bound luminescence, and an unidentified lower energy peak were obs...

Cathodoluminescence study of the spatial distribution of electron‐hole pairs generated by an electron beam in Al0.4Ga0.6As

Abstract: We use the cathodoluminescence mode of a scanning electron microscope to investigate the depth and lateral dependencies of the electron‐hole pairs generation by the electron beam in Al0.4Ga0.6As semiconducting material. A multiquantum well structure acts as a detector to measure the relative number of generated minority carriers by their radiative recombination, allowing a direct assessment of the generation volume in the sample. In contrast to electron‐beam induced current which was used in former studies, the method avoids the effect of carrier diffusion for direct band gap materials. This novel technique can be readily applied to other III‐V and II‐VI semiconductors. The results may be used for the quantitative interpretation of cathodoluminescence and electron‐beam induced current measurements.

Substrate free GaAs photovoltaic cells on Pd-coated silicon with a 20% AM1.5 efficiency

Abstract: Thin-film epitaxial 2/spl times/2.5 mm/sup 2/ GaAs crack free solar cells grown with metal organic vapor phase epitaxy (MOVPE) have been successfully lifted-off from their original GaAs substrate using the epitaxial lift-off (ELO) technique and Van der Waals bonded on Pd-coated silicon substrates without degrading the electrical, optical and structural properties of the devices. A 20% AM1.5 energy conversion efficiency was measured for an anti-reflection coated device, with a fill factor of 0.79, a short-circuit current density of 21.84 mA/cm/sup 2/ with respect to the total area and a V/sub oc/ value of 957 mV at 25/spl deg/C. Electron beam induced current (EBIC) and cathodoluminescence (CL) measurements at room temperature were performed on the devices to study their structural properties. The EBIC signal image evidenced a very good homogeneous material with an EBIC contrast (C=(I/sub max/-I)/I/sub max/) below 1%. The variation of the penetration depth of the electron beam for a detection of the different layers of the cell gave nearly the same EBIC contrast. Only a low density of point like defects, that we assume to be structural defects, was evidenced. The CL image signal in comparison to the EBIC image didn't give any additional results for the existence of strong nonradiative recombination centers. Cross-sectional transmission electron measurements (TEM) of the GaAs-Pd interface showed a very intimate and homogeneous bonding between the grafted layer and its host substrate.