Mariuca Gartner

Optical properties of LPCVD silicon oxynitride

Low pressure chemical vapour deposition (LPCVD) silicon oxynitride films of various compositions (from pure SiO2 to pure Si3N4) were deposited by changing the relative gas flow ratio. The effects of oxygen on the physical properties of the films were studied by spectroellipsometry (using Bruggeman approximation and Wemple Di Domenico model) and infrared spectroscopy. Refractive index measured by spectroellipsometry method is studied as a function of some deposition parameters: temperature of deposition, gases fluxes ratio. The high value of deposition temperature means low values in refractive index. More oxygen into films decreases the refractive index. The refractive index dispersion is studied by single-oscillator Wemple Di Domenico model. The optical band gap varies monotonically from 5 eV for silicon nitride, to 9eV for HTO LPCVD silicon dioxide and for the studied silicon oxynitride was found to be between 5 and 6 eV.

Optical and structural characterization of AlInN layers for optoelectronic applications

Al1−xInxN layers with an indium content between x=10.5% and x=24% were grown by metal-organic vapor-phase epitaxy and characterized concerning their optical, structural and morphological properties with regard to the realization of optoelectronic devices. The indium content and the strain of these layers were measured by high resolution x-ray diffraction. Ellipsometric measurements were used to determine the optical constants [refractive index n(λ) and extinction coefficient κ(λ)] in dependence of wavelength and indium content. The values determined for the electronic bandgaps are in good agreement with theoretical predictions and previous publications on this topic but are more focused on AlInN layers which are pseudomorphically grown on GaN. A bowing parameter of b=10.3±0.1 was determined for fully strained layers with an indium content between 13% and 24%. In order to investigate the suitability of these layers for use in distributed Bragg reflectors, the surface morphology is characterized with respec...

Band gap bowing of binary alloys: Experimental results compared to theoretical tight-binding supercell calculations for CdxZn1-xSe

Compound semiconductor alloys of the type ABC find widespread applications as their electronic bulk band gap varies continuously with x, and therefore a tayloring of the energy gap is possible by variation of the concentration. We model the electronic properties of such semiconductor alloys by a multiband tight-binding model on a finite ensemble of supercells and determine the band gap of the alloy. This treatment allows for an intrinsic reproduction of band bowing effects as a function of the concentration x and is exact in the alloy-induced disorder. In the present paper, we concentrate on bulk CdZnSe as a well-defined model system and give a careful analysis on the proper choice of the basis set and supercell size, as well as on the necessary number of realizations. The results are compared to experimental results obtained from ellipsometric measurements of CdZnSe layers prepared by molecular beam epitaxy (MBE) and photoluminescence (PL) measurements on catalytically grown CdZnSe nanowires reported in the literature.

TiO2-based vitreous coatings obtained by sol-gel method

Abstract TiO 2 films and TiO 2 films incorporating cobalt and copper ions by the sol-gel method were investigated. Undoped TiO 2 films crystallize to anatase above 300°C. Cu 2+ and Co 2+ inhibit the crystallization of anatase with Cu having a stronger influence then Co. Cu 2+ introduced in the lattice has a square planar coordination in TiO 2 matrix while Co 2+ has a octahedral coordination. These films are coloured thus the optical properties of TiO 2 films can be strongly modulated by the incorporation of a transition metal ion.

Microstructural and optical properties of as-deposited LPCVD silicon films

Abstract Silicon thin films were deposited by low-pressure chemical vapor deposition (LPCVD) on oxidized silicon substrates, from silane decomposition. The deposition temperatures used in our experiment have been 500, 530, 550, 590 and 615°C and the pressure values were 20, 53 and 100 Pa. The microstructure and the surface roughness of as-deposited films were investigated by X-ray diffraction (XRD), spectroelipsometry (using the Bruggemann-Effective Medium Approximation with a multilayer model) and AFM techniques. Three different models, Tauc, Cody and Wemple–Di Domenico were used to estimate the values of the optical gap. The results show the influence of the microstructure on the physical and optical properties of as-deposited LPCVD silicon films. It is pointed out that polycrystalline silicon thin films can be obtained by the LPCVD technique below 550°C.

Investigation on optical and microstructural properties of photoluminescent LPCVD SiOxNy thin films

Abstract Amorphous silicon oxynitride (a-SiOxNy) films of various compositions were deposited by low-pressure chemical vapour deposition (LPCVD) at 860°C and 400 mTorr, using a mixture of SiCl2H2–NH3–N2O. The dependence of the film properties on the deposition temperature and the gas flow ratio was studied by spectroellipsometry (SE) and infrared spectroscopy (IR). To calculate the optical and microstructural parameters of the films from SE data, three different approaches (Bruggeman-EMA, Cauchy and Sellmeier) were used. The comparison of the results shows a good agreement between all three models. Photoluminescence (PL) phenomena in as-deposited LPCVD silicon oxynitride are presented and compared to results reported in literature.

Spectroellipsometric Characterization of Multilayer Sol-Gel Fe2O3 Films

Multilayer Fe2O3 films were deposited by the sol-gel method on glass substrates using three successive deposition procedures. The films were thermally treated for 1 h at 300°C.The optical and microstructural properties of these films were investigated by spectroscopic ellipsometry (SE) in the 500–1000 nm range. The optical gap was found by fitting the dispersion of the film refractive index (n) with the Wemple-DiDomenico (WDD) formula.The ellipsometric measurements showed also that the Fe2O3 films are anisotropic. The birefringence values (Δn) of the sol-gel films (0.05–0.08) are smaller than the large values of the Fe2O3 (which are around 0.28) but increase with the crystalization of the films. AFM mesurements showed that the films treated at 300°C start to crystallize.

Investigation on preparation and physical properties of LPCVD SixOyNz thin films and nanocrystalline Si/SixOyNz superlattices for Si-based light emitting devices

Abstract In this paper, the deposition process and optical properties of nanocrystalline silicon (nc-Si) and Si x O y N z LPCVD thin films and nc-Si/Si x O y N z superlattices are studied. The possibility of a well-controlled LPCVD deposition process of nanocrystalline silicon films is analysed. Atomic force microscopy (AFM), X-ray diffraction (XRD) and spectroscopic ellipsometry (SE) were employed for the structural and optical characterization of these films. In order to obtain a superlattice for emitting devices, the nc-Si thin film preparation was optimised to provide a good reproducibility and average grain size around 10 nm.

In situ observation of Zn-induced etching during CdSe quantum dot formation using time-resolved ellipsometry

A combined segregation and desorption process has been observed in situ by ellipsometry in real-time during overgrowth of a CdSe layer by a ZnSe cap layer using migration enhanced epitaxy. This segregation enhanced etching of CdSe during Zn deposition is known to play an important role in the formation process of CdSe quantum dots. The time-resolved ellipsometry data can be fitted assuming a rapid thickness reduction of about 68% of the CdSe layer, consistent with results obtained by high-resolution x-ray diffraction after growth. Furthermore, a significant change in growth rate during deposition of CdSe has been observed.

Nanostructured SnO2–ZnO composite gas sensors for selective detection of carbon monoxide

A series of SnO2–ZnO composite nanostructured (thin) films with different amounts of SnO2 (from 0 to 50 wt %) was prepared and deposited on a miniaturized porous alumina transducer using the sol–gel and dip coating method. The transducer, developed by our research group, contains Au interdigital electrodes on one side and a Pt heater on the other side. The sensing films were characterized using SEM and AFM techniques. Highly toxic and flammable gases (CO, CO2, CH4, and C3H8) were tested under lab conditions (carrier gas was dry air) using a special gas sensing cell developed by our research group. The gas concentrations varied between 5 and 2000 ppm and the optimum working temperatures were in the range of 210–300 °C. It was found that the sensing performance was influenced by the amount of oxide components present in the composite material. Improved sensing performance was achieved for the ZnO (98 wt %)–SnO2 (2 wt %) composite as compared to the sensors containing only the pristine oxides. The sensor response, cross-response and recovery characteristics of the analyzed materials are reported. The high sensitivity (R S = 1.21) to low amounts of CO (5 ppm) was reported for the sensor containing a composite sensitive film with ZnO (98 wt %)–SnO2 (2 wt %). This sensor response to CO was five times higher as compared to its response to CO2, CH4, and C3H8, thus the sensor is considered to be selective for CO under these test conditions.