Elhadj Dogheche

Laser Doppler vibrometry for evaluating the piezoelectric coefficient d33 on thin film

The problem of electromechanical film characterization, and, in particular, the determination of the piezoelectric activities of thin films deposited on substrates, is of fundamental importance in the development of structures for microelectromechanical system (MEMS) applications. The design and the architecture of the piezoelectric MEMS are directly related to the mechanical and the piezoelectric performances of the material. In this article, we present and compare some results obtained on different experimental setup for the determination of the d33 coefficient. We have optimized the experimental conditions using a laser Doppler vibrometer. The main problem is the contribution of the bending effect of the substrates on the d33 coefficient, which is an intrinsic property of the film. We show that the d33 values are directly related to parameters such as the top electrode diameter and the substrate holder. The results are in agreement with those obtained with the conventional double beam interferometer us...

Optical waveguide loss minimized into gallium nitride based structures grown by metal organic vapor phase epitaxy

The waveguide properties are reported for wide bandgap gallium nitride (GaN) structures grown by metal organic vapor phase epitaxy on sapphire using a AlN/GaN short period-superlattice (SPS) buffer layer system. A detailed optical characterization of GaN structures has been performed using the prism coupling technique in order to evaluate its properties and, in particular, the refractive index dispersion and the propagation loss. In order to identify the structural defects in the samples, we performed transmission electron microscopy analysis. The results suggest that AlN/GaN SPS plays a role in acting as a barrier to the propagation of threading dislocations in the active GaN epilayer; above this defective region, the dislocations density is remarkably reduced. The waveguide losses were reduced to a value around 0.65dB/cm at 1.55 μm, corresponding to the best value reported so far for a GaN-based waveguide.

Thick LiNbO3 layers on diamond-coated silicon for surface acoustic wave filters

Surface acoustic wave (SAW) technology based on lithium niobate/diamond/silicon multilayered structures has been investigated due to the large demand of frequency filters for communication systems, particularly for cellular phones. Thick LiNbO3 layers (∼1 μm) with a good morphology, i.e., surface roughness lower than 10 nm have been grown at 490 °C on diamond-coated silicon substrates by multistep rf magnetron sputtering. A SAW devices operating at 300 MHz was designed and fabricated in order to validate the proposed structure; in particular, our main objective was to verify the high velocity in the layered structure. Preliminary filter responses were very promising: the SAW velocity was estimated to 8200 m/s and the coupling factor was K2∼0.93%. The film growth, the technology, and the electrical measurements are described in this letter.

Optical waveguiding in epitaxial PbTiO 3 thin films

Epitaxial lead titanate (PbTiO(3)) thin films on SrTiO(3) (100) substrate were grown in situ by radio-frequency sputtering for optical waveguiding applications. The crystalline quality of the PbTiO(3) films deposited at 550 degrees C has been investigated through x-ray diffraction analysis. It indicates that thin films are completely c-axis oriented (rocking curve FWHM of 0.2 degrees for the 001 reflection). The transmission spectrum method has been used to measure the dispersion of the refractive index. At 632.8 nm, the PbTiO(3) film with an (001) orientation exhibits a refractive index of 2.61, which represents 98% of the bulk material. The prism-coupling technique has been also employed to determine the optical attenuation in the planar waveguide. In this study, we report a low propagation loss of 2.2 ? 0.2 dB/cm obtained in a PbTiO(3) optical waveguide.

High-frequency surface acoustic wave devices based on LiNbO3∕diamond multilayered structure

This research is focused on the development of a new technology for the fabrication of high-frequency surface acoustic wave (SAW) filters based on diamond and lithium niobate (LiNbO3) materials. LiNbO3 films are in situ deposited at 490 °C on polycrystalline diamond substrates by radio-frequency magnetron sputtering; a multistep growth process is proposed to deposit oriented LiNbO3 films with a smooth surface. Conventional interdigital transducers are fabricated on top of the piezoelectric LiNbO3 layer by standard optical lithography. We report experimental results for a SAW bandpass filter operating at a frequency above 2.30 GHz. We have observed that Rayleigh SAW modes are excited within this structure, with extremely high phase velocities (up to 12000m∕s). This latter result illustrates the greatest advantage of using diamond as an acoustic substrate. It allows the fabrication of SAW devices operating in the gigahertz frequency range using standard optical lithography.

High responsivity GaNAsSb p-i-n photodetectors at 1.3µm grown by radio-frequency nitrogen plasma-assisted molecular beam epitaxy

GaNAsSb/GaAs p-i-n photo notdetectors with an intrinsic GaNAsSb photoabsorption layer grown at 350 degrees C, 400 degrees C, 440 degrees C and 480 degrees C, have been prepared using radio-frequency nitrogen plasma-assisted molecular beam epitaxy in conjunction with a valved antimony cracker source. The i-GaNAsSb photoabsorption layer contains 3.3% of nitrogen and 8% of antimony, resulting in DC photo-response up to wavelengths of 1350 nm. The device with i-GaNAsSb layer grown at 350 degrees C exhibits extremely high photoresponsivity of 12A/W at 1.3 microm. These photodetectors show characteristics which strongly suggest the presence of carrier avalanche process at reverse bias less than 5V.

Growth Process and Surface Acoustic Wave Characteristics of LiNbO3/Diamond/Silicon Multilayered Structures

Lithium niobate (LiNbO3) films have been grown at 490°C on diamond-coated silicon substrates by radio-frequency magnetron sputtering. A multi-step process was developed to produce thick layers on diamond with a good structure and a smooth morphology. To assess the structural quality of our films, we performed X-ray diffraction, transmission electron microscopy and atomic force microscopy of the films. The structure is conserved whatever the film thickness. A delay line structure of wavelength of 28 µm was realized and transducers were deposited on top of LiNbO3. The objective was to verify the high velocity in such a multi-layered structure. The first results indicated a SAW velocity estimated of 8200 m/s and a coupling factor was k2 around 1%. The film growth, the technology and the electrical measurements are described in this paper.

Effect of varying pore size of AAO films on refractive index and birefringence measured by prism coupling technique.

Anodic aluminum oxide (AAO) films with different pore sizes were prepared to modulate the effective refractive index and birefringence. To investigate the relationship between the refractive index and the pore size of the AAO film, optical constants were obtained using a prism coupler with various lasers. With experimental results, the dispersion curve of alumina itself without pores was extracted using a theoretical anisotropic model. We demonstrated that AAO films could offer a wide range of refractive index and birefringence values for optical device applications. Furthermore, index profiles as a function of the thickness of the AAO films were obtained by inverse Wentzel-Kramer-Brillouin approximation to examine the optical homogeneity.

Growth and optical waveguiding properties of rf sputtered lithium niobate thin films on sapphire substrates

Abstract Thin films of lithium niobate have been grown in-situ by radiofrequency magnetron sputtering on (0001) sapphire substrates. The epitaxial quality has been studied as a function of deposition parameters. The films prepared under optimized conditions, exhibited a high degree of c-axis oriented crystalline structure as shown by x-ray diffraction in the θ−2θ and φ-scans configurations. Stoichiometry has been obtained at the substrate temperature of 490°C and gas (Ar/O2) pressure of 30mTorr. The optimum conditions for growing epitaxial LiNbO3 layers are reported in this work. Mirorlike thin films with a small surface roughness of 30 A have been qualified optically using the prism coupling technique. Both ordinary and extraordinary refractive indices have been determined (n0=2.348 and nc=2.208 respectively) at 632.8nm from the transverse electric and transverse magnetic mode excitations. The optical propagation loss is evaluated to be around 2dB.cm−1 for a 150nm thick planar waveguide.

Influence of deposition parameters on physico-chemical and optical properties of sputtered PbTiO3 thin films.

Abstract Stoichiometric thin films of lead titanate (PbTiO3) have been grown “in situ” without postannealing on (0001) sapphire substrates by rf magnetron sputtering technique. X-ray diffraction scans have revealed that as-grown films consist of the perovskite phase and are polycrystalline with a high (111) orientation. The structure, the microstructure and the optical properties have been studied as a function of the process parameters i.e., substrate temperature, gas pressure and the target composition, in particular the lead content. We report the dependence of the deposition conditions on the optical constants. The optimum experimental conditions (100m Torr and 600°C) to produce thin films with high transparency and refractive indices (n = 2.61 at 633 nm), similar to values for bulk materials, are given in this paper.