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CuInSe 2 (CIS) layers were grown by co-evaporation in a molecular beam epitaxy system onto soda lime glass substrates by using both two-step and three step processes. The physical properties of the layers were investigated using X-ray diffraction (XRD) and optical spectroscopy. The sample atomic composition was assessed by energy dispersive analysis of X-rays. Cu-rich or In-rich CIS thin films were obtained exhibiting strong preferential (112) and (220)/(204) orientations in both cases. We performed thermal annealing at 450 °C under nitrogen, keeping Se overpressure to avoid Se desorption from the layer. The annealed layers all exhibit improved crystalline quality, with reduced stoichiometric discrepancy. The secondary phases like Cu x Se 1 − x or In x Se 1 − x are no more observable by XRD measurements. Regarding the preferential orientation, thermal annealing of Cu-rich CIS layers favours the (112) orientation leading to a more (112) textured layer after annealing, whatever the initial preferential growth orientation was. In opposite, thermal annealing of In-rich samples increases the (220)/(204) texture of the sample.

International audience; Light scattering spectroscopy, mainly developed for Cu(In,Ga)Se2 thin films growth, has been demonstrated to be an efficient in-situ monitoring tool. It consists of illuminating the substrate surface using a light source and collecting the diffused light with a spectrometer. The on-line information allows monitoring the point of stoichiometry of the grown layer. In this work, we propose an alternative approach of this method, replacing the spectrometer by a standard webcam. This system allows to precisely choose the analyzed area on the substrate surface, which may be the whole sample surface or just a specific zone. We have grown CuInSe2 (CIS) thin films by co-evaporation under vacuum using the three-stage process. We have simultaneously performed CIS growth on both Mo-coated and bare glass substrates within the same growth run, while, on purpose, the scattered light was collected from the Mo-coated substrate. The CIS thin films were characterized by X-ray diffraction, atomic force microscopy, Hall effect measurements, absorption spectroscopy and energy dispersive spectroscopy for composition. By interrupting the CIS growth at different stages of the growth process, we have correlated the scattered light features with the Cu-rich/In-rich transitions and we observe the reproducibility of the scattered light spectra structures.

We study the free exciton fine structure and the contribution of propagating excitations in the lower-polariton and upper-polariton branches (LPB and UPB) in the 2 K fluorescence spectrum of GaN on Al 2 O 3 . The latter e⁄ect is observed for both A and B lines. A line-shape fitting of the photoluminescence was made using four Lorentzian functions. The distribution of polaritons in the UPB(A), LPB(B) and UPB(B) are found consistent with a Boltzmann electronic temperature of 29 K when exciting the fluorescence with a He—Cd laser at 325 nm. The longitudinal—transverse splittings can be extracted from the splittings between energies of dips in the PL bands at 3489.4 and 3497.8 meV and the values of the transverse excitons are smaller than 2 meV. ( 1998 Elsevier Science B.V. All rights reserved.

Luminescence, induced absorption and degenerate four-wave mixing experiments are perform on GaN epilayers grown on a sapphire substrate by MOCVD.We measure the nonlinear behavior of the luminescence spectra near the excitonic resonance, by using an excitation at 4.026 eV from an excimer laser. At low intensities of excitation, spectra show a saturation of the I2 line due to the finite donor density in the sample. Higher intensities of excitation induce collision processes between photo-created quasi-particles.Using a dye laser as a pump beam, we measure the induced variation of absorption of a probe beam as a function of the intensity and of the wavelength of the excitation. With increasing intensities of the pump beam, transmission spectra show a red-shift of the absorption edge and of the excitonic resonance.Pulsed degenerate four-wave mixing experiments were performed using the third harmonic of a picosecond Nd-YAG laser at 3.492 eV. A characteristic time constant of 16 ps has been measured, which is independent of the temperature, spacing of the interference fringe and of the intensity of the pump beams.

AlGaN is an important material in the design of nitride devices. However, little is known concerning its growth with high Al contents. We have studied the growth of AlxGa1-xN epilayers on c-face sapphire by low pressure MOVPE (76 Torr), using triethylgallium, trimethylaluminum and ammonia as precursors. The solid versus gas phase composition relationship was determined experimentally and was fitted using a kinetic model. Then the structural properties of the layers (x=0–1) were studied, using X-ray diffraction, scanning electron microscopy and transmission electron microscopy. We demonstrate that at high Al content, the buffer layer defects are replicated into the AlGaN layer.

International audience; We report here on the study of the optical properties of ZnCdSe-ZnSe quantum structures elaborated by metal-organic chemical vapour deposition on GaAs substrates. The band structure of these samples is experimentally investigated by means of photoluminescence and photoreflectance spectroscopy. This makes clear the relationship between strain, carrier confinement and excitonic effects. We have computed for the first time heavy- and light-hole excitons associated with ground and excited states sub-bands, in the context of a self-consistent two-parameter trial function.

International audience; CdS thin films were grown on different substrates (glass, GaAs, Si, Al2O3) by CBD. The rms roughness of the CdS layers, measured on AFM pictures, ranges from 11 to 95 nm depending on the chemical nature of the substrate Thermal annealing of the samples leads to an improvement of the CdS crystalline quality with a conversion from cubic to hexagonal phase, whatever substrate is used, as assessed by XRD. Thermal annealing also improves the optical properties. While as-grown CdS layers photoluminescence is due to defect related emission around 1.7eV, annealed samples show dominant near band edge photoluminescence emission around 2.4 e V.

The optical properties of GaN epilayers grown by metal‐organic vapor‐phase epitaxy on (0001)‐oriented sapphire are investigated by means of photoluminescence, reflectance, and differential spectroscopy. We obtain quantitative information about the intrinsic or extrinsic nature of the 2 and 300 K photoluminescence features. From detailed investigations of the reflectance properties of these layers we can quantify the residual strain field in these layers and determine the GaN deformation potentials. Comparison of these values with quantities measured on other semiconductors with wurtzite symmetry is also addressed. Last we utilize photoreflectance spectroscopy to measure exciton binding energies.

We report on a quantitative analysis of the band gap of hexagonal GaN epilayers in terms of the joint contributions of the actual wurtzite symmetry on the one hand and of residual strain fields on the other hand. This investigation leads to revision of the previous modelings based on quasicubic descriptions of the valence-band physics and gives ${\mathrm{\ensuremath{\Delta}}}_{1}$=10\ifmmode\pm\else\textpm\fi{}0.1 meV, ${\mathrm{\ensuremath{\Delta}}}_{2}$=6.2\ifmmode\pm\else\textpm\fi{}0.1 meV, and ${\mathrm{\ensuremath{\Delta}}}_{3}$=5.5\ifmmode\pm\else\textpm\fi{}0.1 meV. Last we propose a set of deformation potentials for the hexagonal GaN semiconductor.

GaN is a wide gap semiconductor which is now used to produce blue and green light emitting diodes, blue laser diodes and which has numerous other potential applications, like high frequency HEMT transistors, UV sensors, etc. A complicated two step process, using a low temperature buffer layer, subsequently annealed and followed by the deposition of the monocrystalline semiconductor was developed, and due to the excellent results obtained following this method, a rush towards applications resulted. There is now a need to investigate in more detail the growth mechanisms, and the influence of the growth parameters, in order to ensure a better reproducibility of the results. In this paper, we report an investigation of the growth mechanisms and the influence of the growth parameters using in-situ reflectance experiments. The reflectance measurements allow us to follow the growth rates, and the changes in the surface morphology (transitions between islands growth and 2D growth). Additional exsitu measurements (AFM ) were performed at different stages of the growth process to ensure additional information. As a result, we demonstrate that the recrystallization of the low temperature buffer layer is a critical step, which is drastically influenced by the composition of the annealing atmosphere (amount of ammonia present in the gas phase), while the deposition temperature and buffer thickness have a moderate effect. We will discuss here the growth mechanisms which may be involved to explain such a behavior.

Up to date, different groups have reported ordering phenomena in nitride alloys like InGaN or AlGaN. These evidences of ordering during and after growth lead us to study the atomic arrangement of Al and Ga on site III in the AlGaN alloy using a thermodynamical approch. We have defined the enthalpy and the entropy of our system using the Keating model and the cluster variation method limited to the first coordination shell. This study gives us a good way to understand ordering, and moreover to see why at three special compositions (25%, 50% and 75%), the alloys have good chances to present a better crystalline quality than in other cases.We have grown AlGaN samples increasing the Al composition by 10% steps, in the full range of composition from GaN to AlN. All the growth parameters were kept constant except for the alloy composition. X-Ray diffraction measurements performed on these samples indicate that FWHM increases linearly with Al composition, except for three compositions, where according to our thermodynamical study, we observe an improved crystalline quality.

International audience; Long-wavelength optical phonons of Ga1-xAlxN solid solutions have been identified in a wide compositional range by Raman spectroscopy. The Al and El polar phonon frequencies evolve continuously with x from one-member crystal to the other. The same behavior seems to hold true for the silent B-1 mode, which manifests itself by an interference with an unidentified continuum. Coupling of the longitudinal-optic (LO) modes associated with the two types of bonds, via the macroscopic electric field, is treated by a generalization of the dielectric model [D. T.Hon and W. L,. Faust, J. Appl. Phys. 1, 241 (1973)]. This approach accounts for the observed frequencies and supports the apparent one-mode behavior of the polar LO phonons. (C) 1998 American Institute of Physics.

Long wavelength optical phonons of AlxGa1−xN solid solutions have been identified in the whole compositional range by Raman spectroscopy. The frequencies of A1 and E1 polar phonons increase continuously with x from one-member crystal to the other. A generalization of the dielectric model of Hon and Faust is used to treat the coupling of the longitudinal optic (LO) mode. This approach accounts for the observed frequencies and confirms the so-called one-mode behaviour of polar LO phonons. Moreover,a signature of the coupling of a discrete mode (tentatively associated to silent q=0 B1 mode) with an unidentified continuum has been obtained.

Raman spectroscopy is used to study the effect of the built-in biaxial stress on the E2 and A1 (LO) q = 0 phonon modes of wurtzite GaN layers deposited by metal organic vapor phase epitaxy on (0001) sapphire substrate. By means of phonon frequency shifts, the biaxial pressure coefficients of the mode frequencies are determined and used to calculate the corresponding deformation potentials. Stress calibration has been performed using reflectance data.

For more than twenty years hydrostatic pressure has been used as a variable in experimental measurements in semiconductors. The investigation of its effect on the properties of 3D systems is clearly important for a basic understanding of band structure, conduction processes, etc. As early as 1968, W. PAUL (1) gave an extensive plenary paper on this subject, and since then many authors have reviewed further developments R.A. STRADLING (2) published one of the latest papers on the subject in the Advances in Solid State Physics (1985).