Adil A. Chahboun

Abstract: In this paper, the study of the formation of ZnMnO nanocrystals (NCs) embedded in an alumina matrix starting from ZnMnO/Al2O3 multilayers grown on a Si substrate by pulsed laser deposition with subsequent annealing is presented. The correlation between the morphology of grown NCs with the ratio R of the thickness of the ZnMnO layer to that of the Al2O3 layer in the as-grown multilayer structure was observed. The results show the possibility of controlling the strain, size, and distribution of grown NCs by setting the ratio R.

Abstract: In this work, SiGe/SiO2 multi-layer (ML) films with layer thickness in the range of a few nanometers were successfully fabricated by conventional RF-magnetron sputtering at 350ºC. The influence of the annealing treatment on SiGe nanocrystals (NCs) formation and crystalline properties were investigated by Raman spectroscopy and grazing incidence X-ray diffraction (GIXRD). Our results show that the ML structure obtained on the as-grown samples is preserved with well organized SiGe NCs within 5-nm-thick layers after performing a rapid thermal annealing (RTA) at 800ºC for 30 min as shown by Rutherford backscattering spectroscopy (RBS) and grazing incidence small angle X-ray scattering (GISAXS). The effects of the RTA treatment on the ML structure have been investigated by high resolution transmission electron microscopy (HRTEM). A memory effect of the SiGe NCs on metal-oxide-semiconductor (MOS) structure has been demonstrated.

Abstract: In this work, we report on the synthesis of Ge nanocrystals (NCs) by pulsed laser deposition (PLD) at room temperature (RT) in an argon atmosphere without any further annealing process. Our results show that functional thin films of crystalline Ge nanoparticles with spherical shapes can be obtained by PLD directly on alumina layers deposited on n-doped Si (100) substrates. In addition, we also demonstrate that a uniform size distribution of NCs with an average diameter of about 3 nm and a density of 2.3 x 1011 cm-2 can be obtained by optimizing a shadow mask set-up where a solid disk is introduced between the target and the substrate. Charge/discharge effects in Ge NCs deposited on a high-k amorphous alumina layer are also evidenced by conductive atomic force microscopy, which makes them suitable for memory applications.

Abstract: Thin Zn0.98Mn0.02O films were grown by pulsed laser deposition on glass substrates under oxygen pressure. The structural properties were studied by X-rays diffraction and Raman techniques, while the conductivity was characterized by Hall effect. The oxygen pressure during the growth seems to govern the structural and the electrical properties of the thin Zn0.98Mn0.02 films. In fact, the micron size grain and the resistivity of the Zn0.98Mn0.02O increase with the partial oxygen pressure. However, no evident effect was observed on the magnetic behavior. Electronic structure calculations were performed and magnetic moment carried by Mn atom was computed as well.

Abstract: In this work, we investigate the structural properties of Ge quantum dot lattices in amorphous silica matrix by low-temperature magnetron sputtering deposition of (GeþSiO2)/SiO2 multilayers. The dependence of quantum dot shape, size, separation, and arrangement type on the Ge-rich (GeþSiO2) layer thickness is studied. We show that the quantum dots are elongated along the growth direction, perpendicular to the multilayer surface. The size of the quantum dots and their separation along the growth direction can be tuned by changing the Ge-rich layer thickness. The average value of the quantum dots size along the lateral (in-plane) direction along with their lateral separation is not affected by the thickness of the Ge-rich layer. However, the thickness of the Ge-rich layer significantly affects the quantum dot ordering. In addition, we investigate the dependence of the multilayer average atomic composition and also the quantum dot crystalline quality on the deposition parameters. 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.3702776]

Abstract: Thin Zn0.98Co0.02O films were grown by pulsed laser deposition at different oxygen partial pressure (PO2) and its influence on their structural and electrical properties was investigated. Raman and photoluminescence studies revealed that zinc interstitial defects significantly decreased with increase of PO2. Complex impedance spectroscopy has made to elucidate conduction mechanism and electronic relaxation process in Zn0.98Co0.02O films. Resistivity/impedance in the films grown at 0.1 mbar decreased as the temperature increases while the films grown at 0.01 and 0.001 mbar have shown opposite trend. The change in resistivity/impedance with temperature in the films grown at low and high PO2 is attributed to annihilation of defects and thermal activation of free carriers respectively. The relaxation time in the films grown at low PO2 increases with the temperature. It exhibits an exponential dependence on the inverse temperature with three different slopes. The corresponding energies estimated from Arrhenius type relation are very close to the energies for electronic relaxation of zinc interstitials, zinc antisities and oxygen vacancies respectively. Impedance analysis and current-voltage characteristics suggest that the resistivity of Zn0.98Co0.02O films is mainly due to bulk effect of the films.

Abstract: In this work, we report on the electrical characterization of crystalline Ge nanoparticles (NPs) produced by pulsed laser deposition (PLD) at room temperature (RT) in Ar gas inert atmosphere using a shadowed off-axis deposition geometry. Our results show that functional thin films of crystalline Ge NPs embedded between thin alumina films can be obtained on p-type Si (100) substrates following a low temperature and short rapid thermal annealing (RTA) treatment. Metal–oxide–semiconductor (MOS) structures with and without Ge NPs embedded in the alumina were prepared for the electrical measurements. The results indicate a strong memory effect at relatively low programming voltages (±4V) due to the presence of Ge NPs.

Abstract: Si1-xGex nanocrystals (NCs), embedded in Al2O3 matrix, were fabricated on Si (100) substrates by RF-magnetron sputtering technique with following annealing procedure at 800ºC, in nitrogen atmosphere. The presence of Si1-xGex NCs was confirmed by grazing incidence X-ray diffraction (GIXRD), grazing incidence small angle X-ray scattering (GISAXS) and Raman spectroscopy. The influence of the growth conditions on the structural properties and composition of Si1-xGex NCs inside the alumina matrix was analyzed. Optimal conditions to grow Si1-xGex (x~0.8) NCs sized between 3 and 4 nm in Al2O3 matrix were established.

Abstract: Here we present an investigation of (Ge+SiO2)/SiO2 multilayers deposited by magnetron sputtering and subsequently annealed at different temperatures. The structural properties were investigated by transmission electron microscopy, grazing incidence small angles X-ray scattering, Rutherford Backscattering spectrometry, Raman and X-ray photoelectron spectroscopy. We show a formation of self-assembled Ge clusters during the deposition at 250 oC. The clusters are ordered in a three-dimensional lattice, they have very small sizes (about 3 nm) and narrow size distribution. The crystallization of the clusters was achieved at annealing temperature of 700 ºC.

Abstract: Ge nanocrystals (NCs) embedded in Al2O3 were grown by RF-sputtering. X-ray diffraction, high resolution transmission electron microscopy, and Raman spectroscopy techniques were used to characterize the stresses on the NCs. While small NCs (< 10 nm) have been observed to be spherical and fully relaxed in the matrix, the larger ones (> 17 nm) demonstrated a compressive stress effect. This could be linked to the crystal structure of the adjacent Al2O3 matrix.

Abstract: Germanium (Ge) nanocrystals (NCs) embedded in alumina thin films were produced by deposition on silicon (111) substrates using radio-frequency (RF) magnetron sputtering. By changing growth condition and annealing parameters samples with large and small Ge-NCs were produced. The average size of NCs in the sample with arger NCs was estimated to be 7.2, 30.0 and 7.6 nm, and 5.0, 7.0 and 4.8 nm for the sample with smaller NCs, according to Xray diffraction (XRD), Raman and high resolution transmission electron microscopy (HRTEM) results, respectively. Both, XRD and Raman peak positions of the larger NCs are shifted to higher angles and larger wave numbers in relation to the Ge bulk values, whereas the Raman peak was red-shifted for the smaller NCs indicating phonon confinement. HRTEM shows twinned structures, which is an indication of relaxation. Strain evaluation of the bigger NCs gave values < 0.5 % which is within the estimated error of the evaluation technique.

Abstract: We have investigated the structural properties of Si1-xGex nanocrystals formed in an amorphous SiO2 matrix by magnetron sputtering deposition. The influence of deposition parameters on nanocrystals size, shape, arrangement and internal structure was examined by X-ray diffraction, Raman spectroscopy, grazing incidence small angle X-ray scattering, and high resolution transmission electron microscopy. We found conditions for the formation of spherical Si1-xGex nanocrystals with average sizes between 3 to 13 nm, uniformly distributed in the matrix. In addition we have shown the influence of deposition parameters on average nanocrystal size and Ge content x

Abstract: Zn1-xMnxO (x= 0.01, 0.02, 0.05, 0.1, and 0.15) powders were prepared by solid-state reaction processes. The structural and magnetic properties of the powders were investigated. X-ray diffraction (XRD) analysis of the samples indicates the presence of wurtzite crystal structure similar to pure ZnO and absence of any other secondary phase till 5 % Mn doping. This suggests that doped Mn2+ ions substitute Zn2+ sites. However, Raman results revealed the presence of secondary phase even at very low Mn doping (2%), which demonstrates that Mn does not incorporate completely the ZnO lattice. The powders exhibit ferromagnetism at low temperatures and paramagnetic at room temperature. Transition from ferromagnetic to paramagnetic is highlighted by measurements at 230 K.

Abstract: In this paper, Ge/Al2O3 multilayer systems were grown by pulsed laser ablation. The grown samples were annealed at 900ºC to encourage the formation of Ge nanocrystals. RBS and TEM confirmed the presence of a multilayer system. GISAXS demonstrates the formation of Ge nanoclusters formed between alumina layers. Room temperature I-V measurements showed weak carrier trapping in the system. This was explained by the leakage caused by Ge diffusion through the multilayer.

Abstract: CdTe-doped SiO2 thin films were produced by RF-magnetron co-sputtering technique. Presence of CdTe nanocrystals inside the silica matrix was confirmed by Raman 13 spectroscopy and grazing incidence X-ray diffraction. The samples demonstrate size dependent photoluminescence. Temperature dependence photoluminescence 15 measurements were carried out in the temperature range 15–295 K and revealed energy-emission thermal stability. This feature can be usefully applied in a production 17 of the light-emitting diodes with wavelength stability in a wide temperature range.

Abstract: We report on the structural and electrical properties of Mn-doped ZnO/Al2O3 nanostructures produced by the pulsed laser deposition technique. Grazing incidence small angle X-rays scattering (GISAXS) and Rutherford Backscattering Spectrometry revealed the multilayered structure in as-deposited samples. Annealing of the nanostructures was shown to promote the formation of nanocrystals embedded in Al2O3 matrix, as was evidenced by GISAXS and high resolution transmission microscopy. Particle-induced X-ray emission analysis showed a doping of 8 at. % Mn in ZnO. Grazing incidence X-ray diffraction and Raman spectroscopy demonstrated that the nanocrystals have the pure wurtzite ZnMnO crystalline phase of the NCs. Resonant Raman scattering displayed an increase of intensity of the 1LO mode as well as broadening of the 2LO mode related to the size effect. Capacitance-voltage measurements showed carrier’s retention with a voltage shift higher than those reported for similar systems.

Abstract: Mn- doped ZnO/TiO2 multilayer nanostructures were grown on Si(100) substrates by the pulsed laser deposi-tion technique. Post growing annealing in oxygen or ni-trogen atmosphere was performed to improve the crystal-linity of the nanostructures. The crystalline structure and morphology of the nanostructures were characterized by Raman scattering, wide angle X-ray scattering, and graz-ing incidence small angle X-ray scattering. Rutherford backscattering and particle induced X-ray emission analysis were performed to study the composition.

Abstract: In this work post-growth annealing effect on CdSe/SiO2 thin films grown by rf-magnetron co-sputtering technique was investigated. Annealed samples were characterised by Raman scattering, grazing incidence X-ray diffraction and room temperature photoluminescence. The nanocrystals (NCs) size changed from 1.5 to 5 nm by varying the annealing temperature from 550 to 400 °C. Evaporation of Se at high temperatures was invoked to explain this phenomenon. Fowler–Nordheim tunnelling mechanism was found to be responsible for carrier transport for samples with bigger CdSe NCs sizes, and which showed better photovoltaic properties

Abstract: In this work, CdSe nanocrystals (NCs) embedded in SiO2 matrix were grown by radio frequency (RF)-sputtering technique. X-ray technique was used to characterise the structural properties of the system. The NC's size was estimated to be around 4±1 nm in diameter. The temperature dependence of the photoluminescence from the CdSe/SiO2 system showed carriers thermal exchange between the NCs and deep defects in the matrix. The evolution of the excitonic energy emission with temperature is about 10 meV in the temperature range of 15K-295 K. This weak shift was explained by thermal mismatch between the matrix and the NCs.

Abstract: SiGe quantum dots in a dielectric matrix show numerous interesting properties, which are very relevant for applications in different nano-based devices. Homogeneous sizes and uniform spatial distribution of quantum dots in the host matrix are very important parameters for their potential applications. Here we present a study of structural properties of SiGe quantum dots formed in amorphous silica matrix by magnetron sputtering technique. We investigate deposition conditions leading to the formation of dense and uniformly sized quantum dots, distributed homogeneously in the matrix. A detailed analysis based on grazing-incidence small-angle x-ray scattering revealed the dependences of quantum dot sizes, size distributions, spatial arrangement and concentration of quantum dots in the matrix, as well as the Si:Ge content on the deposition conditions

Abstract: CdSe nanocrystals (NCs) embedded in a solid SiO2 matrix were fabricated by RF-sputtering technique. Raman and photoluminescence spectroscopy illustrated the NCs size dependent confinement effect. The CdSe NCs charging effects were electrically characterized by means of capacitance–voltage measurements. A memory effect was demonstrated through memory window measurements.

Abstract: Ge NCs have attracted considerable attention because of their potential applications in nonvolatile memory and integrated optoelectronics. A number of groups have already proposed integrate flash memories based on Ge NCs embedded SiO2 matrix. Since Al2O3 presents a high dielectric constant comparatively to SiO2, it is a good candidate to replace silica in flash memory systems, and therefore improve their performances. Moreover, Al2O3 presents good mechanical properties, and supports high temperature, which leads it to be an ideal material for Si processing conditions. However, a few studies have been reported on Ge NCs embedded in Al2O3 matrix.

Abstract: Charging effects in CdSe nanocrystals embedded in SiO2 matrix fabricated by rf magnetron co-sputtering technique were electrically characterized by means of capacitance–voltage (C–V) combined with current–voltage (I–V). The presence of CdSe nanocrystals was demonstrated by X-ray diffraction technique. The average size of nanocrystals was found to be approximately 3 nm. The carriers transport in the CdSe/SiO2 structure was shown to be a combination of Fowler–Nordheim tunnelling and Poole–Frenkel mechanisms. A memory effect was demonstrated and a retention time was measured.

Abstract: CdSe nanocrystals (NCs) embedded in SiO2 thin films were prepared using RF-magnetron co-sputtering. The average NC size was estimated to be 18 nm. The dark and photocurrent temporal dependences have been measured as a function of the magnitude of applied voltage (50–150 V). Annealing the samples seems to improve the photoconductivity (~10−12 Ω−1) that increases with the film thickness and slightly changes under the bias voltage. Furthermore, the photovoltage measurements showed that a concentration of CdSe in the range of 27 mol% leads to the generation of a photovoltaic signal up to 5 V at 400 µW cm−2. These results demonstrate the potential of silica films with embedded CdSe NCs for photovoltaic applications.

Abstract: SiO2 thin films doped with Ge nanocrystals (NCs) were grown using the RF-sputtering technique. X-ray diffraction studies revealed a diamond structure for Ge NCs. The presence of Ge NCs in the grown films was also confirmed by Raman spectroscopy. Photoluminescence spectroscopy of the samples revealed an emission band at 2.07 eV, which is tentatively attributed to defects located at the Ge-matrix interface. This was found to be quite sensitive to variations of local matrix composition, induced by the annealing process.

Abstract: In this paper we report on the optical and structural properties of CdTe nanoparticles embedded in silica matrix grown by the radio-frequency (RF) magnetron sputtering technique with subsequent annealing in vacuum. Optical transmittance, Raman scattering, X-ray photoelectron spectroscopy (XPS), grazing incidence X-ray diffraction (GIXRD) and photoluminescence (PL) were used to study the grown samples. GIXRD shows the presence of the cubic phase of CdTe nanocrystals (NCs) in the silica matrix, with size ranging from 2.5 nm to 10.5 nm. Raman spectroscopy illustrates features linked to LO phonons in the CdTe structure. The absorption spectra of annealed samples demonstrate shoulders on the absorption in the range 650-700 nm, in agreement with the PL emission observed at 650-725 nm. However, the blue shift observed in the NCs bandgap in relationship with their size was below the expected level. We attribute this feature to the graded interface between the NCs and the matrix, which causes a weak confinement of carriers in CdTe NCs.

Abstract: Ge nanocrystals (NCs) embedded in aluminum oxide were grown by RF-magnetron sputtering. Raman, high resolution transmission electron microscopy (HRTEM), selected area diffraction (SAD), and X-ray diffraction (XRD) techniques confirmed good cristallinity of the NCs from samples annealed at 800 °C. The average NC size was estimated to be around 7 nm. Photoluminescence (PL) measurements show an emission related to the NCs. The temperature dependence of the PL confirms the confinement phenomenon in the Ge NCs.

Abstract: The possibility of controlling the photoluminescence (PL) intensity and its temperature dependence by means of in-growth and postgrowth technological procedures has been demonstrated for InAs/GaAs self-assembled quantum dots (QDs) embedded in an InGaAs quantum well (QW). The improvement of the QD emission at room temperature (RT), achieved due to a treatment with tetrachloromethane used during the growth, is explained by the reduction of the point defect concentration in the capping layer. It is shown that the PL quenching at RT appears again if the samples are irradiated with protons, above a certain dose. These findings are accounted for by the variations in the quasi-Fermi level position of the minority carriers, which are related to the concentration of trapping centers in the GaAs matrix and have been calculated using a photocarrier statistical model including both radiative and nonradiative recombination channels. By taking into consideration the temperature dependent distribution of the majority and minority carriers between the QDs, embedding QW and GaAs barriers, our calculated results for the PL intensity reproduce very well the experimentally observed trends.

Abstract: We report the effect of defects that appear during capping layer growth or are introduced intentionally, on the photoluminescence (PL) properties of InAs strained quantum dots. A treatment of the samples with CCl4 during the growth leads to the reduction of native defect concentration in the GaAs matrix. This is shown to improve the emission intensity and to remove the quenching effect at room temperature. Some samples were subjected to proton irradiation in order to generate more defects in the matrix. The comparison between the emission from as-grown samples and irradiated ones shows a decrease of the PL intensity with the irradiation dose. However, no change in the activation energy for carrier's thermal escape was observed. The deterioration of the emission properties with the increasing irradiation dose is probably related to traps generated by the irradiation in the matrix, which reduce the overall concentration of the photocarriers.

Abstract: In this paper, we have studied the evolution of the photoluminescence (PL) emission from CdS quantum dots embedded in silica glass with time. An increase of PL emission with time was observed due the passivation of non-radiative recombination centres. Annealing of the samples in different environments (oxygen, hydrogen, and air) seems to reduce the PL emission, due to the activation of non-radiative defect states.

Abstract: We report results that witness the possibility of controlling the temperature (T) dependence of the photoluminescence (PL) from self-assembled InAs/GaAs quantum dots (QDs) overgrown by an InxGa1–xAs layer forming a quantum well. A growth treatment using tetrachloromethane eliminates the quenching of the PL intensity at room temperature. A reduction in the concentration of defects in the GaAs matrix and the corresponding increase of the radiative lifetime of the photocarriers are invoked to explain this effect. A simple model analyzing the behavior of the quasi-Fermi level in the QD heterostructure confirms our explanation.

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Abstract: Ballistic electron and hole emission microscopies, have been used to probe Au/ZnSe/GaAs and Au/ZnSe/BeTe/GaAs (001) heterostructures as a function of the ZnSe/BeTe interface stoichiometry. Ballistic electron and hole transport into n- and p-type ZnSe epilayers, respectively, were observed with spectra that conformed well to theoretical behavior. The measured average voltage thresholds +1.25 (p type) and –1.41 V (n type) were in good agreement with previously observed values for the Au/ZnSe Schottky barrier heights and consistent with the value of the ZnSe direct band gap (2.67 eV). However, the ballistic transport into the BeTe conduction bands required to measure the ZnSe/BeTe conduction band offset was not confirmed. Time-dependent and spatial variations of the voltage thresholds were observed for p- and n-type diodes, with and without BeTe epilayers, and were presumed to be linked to switching Fermi level pinning positions from varying Au/ZnSe interface states densities.

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Abstract: The Au/n-Si(100) contact has been studied using reverse ballistic electron emission microscopy and spectroscopy. Two types of localised collector currents have been observed; one, positive corresponding to electron injection into Si, and the other, negative, associated with hole injection into the semiconductor. The comparative trial of BEEM and reverse BEEM images from the same area shows this difference to be linked to the interface structure. Effects of surface roughness on the observed contrasts are also discussed.

Abstract: In this article, ballistic electron emission microscopy ~BEEM! induced modifications on Au/Si and Au/SiO2 /Si contacts are presented. BEEM current can be locally enhanced or reduced in a controlled manner. These observations are attributed to tip induced modifications on the gold surface. According to Au thickness, x-ray reflectivity experiments show different surface evolutions correlated to the size variations of the modifications introduced as a function of time.

Abstract: Ballistic Electron Emission Microscopy (BEEM) has been used to characterise the Au/n-ZnSe contact. A mean statistical BEEM threshold of 1.63eV is in good agreement with literature. Metal - Insulator- Semiconductor (MIS) structures are invoked to explain the Schottky barrier height dispersion and the observed shift of BEEM thresholds to higher values.

Abstract: Ballistic electron emission microscopy ~BEEM! has been used to investigate the Au/n-ZnSe contact at high voltage. A statistical barrier height value of 1.6360.05 eV is obtained. The metal–insulator– semiconductor structure is invoked to explain domains of low electron transmission. Features appear in BEEM spectra at higher voltages and can be attributed to the density of empty states in the semiconductor. Impact ionization effects are observed when the electron kinetic energy exceeds the band-gap energy.

Abstract: The Au/n-Si(100) contact has been studied using reverse ballistic electron emission microscopy and spectroscopy. Two types of localized collector currents have been observed: one, positive corresponding to electron injection into Si, and the other, negative, associated with hole injection into the semiconductor. The comparative trial of BEEM and reverse BEEM images from the same area shows this difference to be linked to the interface structure. Effects of surface roughness on the observed contrasts are also discussed.

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