Optically selective films

This work consists in the development and the study of materials aimed at the upgrade of flat thermal solar panel performances. Transparent conductive oxides such as SnO₂:F are considered as they show an excellent transmittance in the visible and a good reflectance in the IR, with a transition located at 2 mm. Oxides absorbing in the visible such as CuO or CuS are considered as selective absorbers. The fabrication processes are the spray pyrolysis (in collaboration with the Federal University of Paraná) and the sol-gel deposition method.

 

Photovoltaic materials

This work consists in the development and the characterization of thin films of semi conducting materials for the fabrication of low-cost photovoltaic cells. The studied materials are basically CdTe and CuInSe₂. The fabrication process we are considering is electrodepositing. The characterization is made through various methods available at the Physics and Chemistry Institutes: UV-vis-IR spectroscopy, electrical measurements, Hall effect, X ray diffraction, X ray fluorescence, XPS, etc.

 

Measurements of the solar intensity

This work is aimed at the characterization of the solar radiation at ground level. For this, a black and white - type radiometer, giving a measurement of the sun intensity integrated in the whole solar spectrum, was built. Other detectors are also used such as Si photovoltaic cells whose sensitivity is more useful for photovoltaic systems, or UV TiO₂ sensors for measuring the solar intensity in the ultraviolet. Finally, we are developing a radio-spectrometer and a multi-spectral radiometer in order to determine the local atmosphere composition.

 

Investigation of optical and electronics properties of semiconductor systems

The  bandgap energies, dielectric functions, metal –nonmetal transition, conductivities of doped and undoped semiconductor materials,as for instance, GaAs,InP,GaN,AlN,InN and their alloys, SbBiI3, PbI2,PbX (X=Br,Cr,Te) CdXTe (X=Zn,Mn and Fe), SiC/Compounds, etc , spin polarization, porous materials and effect of optical transition due to impurity clusters in doped materials have been investigated experimentally and theoretically. Experimentally by transmission (TR),absorption (AB), photoluminescence (PL),Raman , spectroscopic ellipsometry (SE) and Hall measurements.

We use, as much as possible, the samples delineated for the measurements from our colaborators. Most of these materials  have very large band-gap energies and are therefore very promising materials for development as room temperature gamma-ray detectors and non-linear applications as well as recongnized as important semiconductors for optoelectronic and high-speed digital device developments.  The efficiency of these devices maybe highly dependent on the doping  and the variation of alloy compositions.  We  investigate such scenario theoretically and experimentally at low and room temperatures.

 

Porous Silicon

Owing to the wide-ranging of possible technological applications in opto-electronic devices and biocompatible materials as well, the interest in characterizing porous silicon  has recently increased very much. Porous silicon (PS) has been studied intensively since the discovery by Canham, that even at room temperature PS can emit very bright photoluminescence, in great contrast to crystalline silicon (c-Si). Usually, the  PS samples are produced by anodic etching of c-Si wafers in hydrofluoric (HF) solution.As reported by many authors  one of the main problems in the analysis of  porous silicon  samples is that there are no satisfactory models to explain the photoluminescence  phenomenum  and to predict their structural variations with the different formation parameters (doping level, HF concentration and current density). Focusing in this problem the objective of our study is to perform a comparison among different PS canonical structural patterns taking into account a possible surface structure correlation with PL and absorption  performances, a structural quantum phenomenum that we call active nanoporosity. In another hand, the roughness, fractal dimensions and phase disorder measures could be important parameters for a fine investigation of structural differences among samples with very high porosity of both the Scanning Force Microscopy (SFM) images. Also we use the  gradient pattern analysis technique as a reliable method to investigate, qualitatively and quantitatively, the morphology of PS active porosity.

Recently, Ferreira da Silva et al.,have performed a gradient pattern analysis of a canonical sample set (CSS) of scanning force microscopy (SFM) images of PS.  They applied the Gradient Pattern Analysis (GPA) to images of three typical PS samples distinguished by different absorption energy levels and aspect ratios (low, intermediate and high roughness). The GPA is an innovative technique, which characterizes the formation and evolution of extend patterns based on the spatio-temporal correlations between large and small amplitude fluctuations of the structure represented as a gradient field .

The structure formation GPA  models can be discrete or continuous where the discrete models demand to simulate automata universal aspects of the problem. The continuous models generally use differential equations, with stochastic character, where the main formation process obeys the general physical aspects of the problem. The continuous model used in this work is the (Kardar-Parisi-Zhang) KPZ equation. Below we show the PS image (left) and  a result  from KPZ 2D simulation (right), solved by means of the FTCS (forward time centered space) method, where the spatio-temporal scales (LxL, with 0.1 micrometer per pixel) are compatible to the real PS scales showing similarities between them.

 

Porous Diamond-Like-Carbon

Diamond-like-carbon (DLC) films have many attractive properties which can deal to potential applications. Besides mechanical and tribological characteristics, electrical and optical properties can, also, deserve special attention. More specifically, surface porosity in different kinds of materials exhibit very interesting optical effect related to porous density and size, suggesting new applications. In this work, porous diamond-like-carbon (PDLC)  has been investigated with transmission and reflection spectroscopies. Atomic Force Microscopy  (AFM) measurements has revealed an intriguing similarity between PDLC and porous silicon. The PDLC thin films were obtained on (100) silicon substrate by DC magnetron sputtering and stored in atmosphere environment. From transmission and reflection data we have observed broad spectra distributions, spanning the wavelength interval 900-350nm , corresponding to phonon energies from 1.46 to 3.50 eV, and with one broad maximum located around 825nm (1.6eV). The results are discussed in terms of surface-band oxidized-like absorber, which can be inferred as the source for the absorption from nanoporous. Porous diamond-like-carbon  has been investigated by photoluminescence, absorption and spectroscopic ellipsometry. We present the real and imaginary parts of the dielectric functions for the latest and compare them to the calculation done for diamond by a full-potential linearized augmented plane wave method within the local density approximation. We found a low real dielectric function of  about 1.5  at 0.8 eV whereas bulk diamond has e₁(0) a value of 5.5 .

 

Gallium and Alluminium Nitrides

The wide band gap group III-nitride semiconductors GaN, AlN, InN and their ternary and quaternary alloys have been extensively applied in optoelectronic and electronic device technology such as blue-green light emitting diodes, and high-temperature electronics.. Most of the work reported so far refers to the stable hexagonal (h-)(wurtzite) phase of the materials. However, the metastable cubic (c-)(zinc blende) modification arises as an advantageous  alternative for device applications. The fact that the cubic GaN-derived structures are free from modulation due to spontaneous and strain-induced piezoelectric effects makes the studies of their basic properties very important for the understanding of the device characteristic and improvement of their performance . Despite of its technological importance, so far there has been no reported detailed  investigation of the bandgap narrowing (BGN) of this material in the presence of high doping. The role of impurities is very important in fabricating devices. For high enough doping concentration the donor electrons are collected at the bottom of the lowest conduction-band. There are two band gaps of interest . The energy distance between the conduction and valence-band extrema, E_G,2, and the distance between the Fermi-level and the valence-band top, E_G,1. The former energy is called the reduced band-gap energy, which can be determined from emission measurements  like photoluminescence, whereas the later energy is called the optical band-gap energy. Photoluminescence (PL) measurements were carried out here to obtain E_G,1.

 

Carbides

The physical properties of the large bandgaps SiC, 3C, 15R, 6H and 4H-SiC ,make them as  prominent materials for high-power, high-temperature, and high-frequency devices. Devices like field effect transistors, bipolar storage capacitors, and ultraviolet detectors have been fabricated. We  investigate the optical and transport properties of undoped and n-type 4H-SiC, both experimentally and theoretically. The transmission and photoacoustic spectroscopy techniques have been used for the measurement of the optical bandgap energy. The total dielectric functions were determined by spectroscopic ellipsometry (SE), a powerful non-destructive technique for high accuracy measurements. The calculations of the total dielectric functions were performed by the full-potential linear muffin-tin-orbital method. The Generalized Drude Approach is used to the theoretical calculation.

 

Gallium Arsenide, Carbon p-type  (GaAs:C)

The GaAs semiconductors are well recognized as active materials in the design of band-gap engineered devices and as constituents of lattice matched heterostructure. The hole of impuritties in these materials is also very important in fabricating different types of devices. For instance, acceptor carbon in GaAs is for the high-concentration doping of a transistor base in epitaxial of heterobipolar structures and is used for the formation of buried p-type layers in field-effects transistors. We investigated  the resistivity of acceptor carbon-doped GaAs (GaAs:C) for temperature ranging from 1.7 to 300 K, with variation of the impurity concentration from the insulating to the metallic range, i.e., from about 10E17 cm-3 to 10E19 cm-3. The band conduction activation energy is obtained from the slope of the resistivity at low impurity concentration.The samples were prepared by ion implantation in Van der Pauw structures delineated in GaAs. The resistivities obtained experimentally are compared with resistivity values calculated from a generalized Drude approach (GDA) at similar temperatures and dopant concentrations. The impurity critical concentration Nc for the metal-nonmetal (MNM) transition is estimated from these results and calculated using three different computational methods,

 

Triad-Cluster Optical Transition

In a lightly n-type doped semiconductor, low-temperature spectroscopic measurements exhibit a series of atomic like lines which correspond to the optical transitions of the isolated impurity atoms. As the impurity concentration increases donor-clusters rapidly become important. As the clusters get more dense the absorption edge drops since one can expect that the clusters with larger numbers of donors will absorb at low energies below the ionization and transition levels of the isolated impurities. The first attempts to explain such low energies were corried out  by Nagasaka and Narita. Bajaj et al. have observed a peak on the low energy side of the 1s to 2p transition, denominated as line ''X'', in their spectroscopic investigations of the donors in three different III-V and II-VI semiconductor systems.   

The investigation of the shallow donors in GaN-based wide band gap semiconductors has recently attracted much interest as an important issue in the fabrication of optoelectronic and electronic devices. Work to date has concentrated on the search for the optical transitions from these donor impurities. In recent Fourier transformed infrared measurements of n-type doped wurtzite GaN by Moore et al. an unidentified sharp absorption line on the low energy side of the 1s to 2p transition was obtained at a certain impurity concentration. However, an explanation to the origin of this line was not provided. Since the position of the line is rather stringent, it cannot be explained in terms of transitions between isolated impurities. With these investigations in mind, we have directed the  work to investigate the ``X'' line in n-type GaAs as well as the low energy peak in n-type hexagonal GaN. The electronic structure of the donor-clusters is likely to be strongly affected by electron correlation, and self-consistent field calculations is inadequate even to capture the qualitative characteristics of the absorption for relatively separated molecular configurations. We have therefore pursued the investigation along the lines of a donor-triad-cluster model, but instead employed a multi-configurational self-consistent field approach to the electronic structure determination. We show for the first time that by the proper inclusion of electron correlation, the sharp absorption line ''X'' in GaAs as well as in GaN can be explained as electronic H3 to H3+ transitions in three-donor-molecules.

 

Energy Efficient  Windows  (Optical Selective Coatings)

Large-bandgap  undoped and heavily doped oxide semiconductors of, for instance,Zn, Cd, In, Ti and Sn,  and mixture of these, have many important aplications ascoating in optical and conducting  aplications. Bandgap widening , for instance, is an important effect in heavily doped  oxide semiconductors  used as transparent   heat-reflectors . They  are needed for creating  energy-efficient  windows with low thermal emittance, as well as for numerous other applications  related to energy efficiency. Energy efficient windows require coatings that combine high luminous  transmittance with high infrared reflectance or significant  electrical conductivity. As an example of oxide (as others as ZnO, TiO₂, SnO₂,PbO₂ ,Al₂O₃etc ) , In₂O₃ is ideal since it is transparent  in the visible range of the spectrum  and has a good chemical inertness  and strong adherence to glass. Heavily Sn doped  In₂O₃ , In₂O₃:Sn , it is a very flexible coating combiningmetal and insulator properties. Electrically, it is a conductor, and optically, it is still transparent  in the visible  but reflecting in the infrared. These flexible properties  can be utilized  in energy  effective windows.

 

Si_1-xGe_x alloys

Silicon Germanium (Si_1-xGe_x) alloy is good candidate as a substitute material for Si in low-power and high-speed semiconductor device technologies. Optoelectronic devices, such as heterojunction bipolar transistors, are already in industrial production. Si_1-xGe_x is also promising as alloying material for quantum well devices, infrared detectors, and modulation-doped field-effect transistors. Although much efforts have been paid on the growth of Si_1-xGe_x and Si_1-xGe_x /Si as well on electrical characterization, there is still a lack of information about the optical properties of Si_1-xGe_x . We are investigating the electronic structure of this alloy as a function of composition x.Extendind the investigation to the dielectric functions.

 

Lead Iodide

Lead iodide, PbI2, is a very important material with large technological applicability as room temperature radiation detector. It is a wide bandgap semiconductor E>2 eV with high environmental stability efficiency. The performance of the detector can not be fully understood unless the knowledge of the electronic and optical properties are available. Recently, its bandgap energy and thermal properties were determined by photoacoustic spectroscopy. The single crystal of PbI2 was grown by Bridgman's method with c-axis oriented perpendicular to the growth axis.The purpose of our work is to obtain the electronic structure of PbI2 , its dielectric functions epsilon1 and epsilon2 by ellipsometry method and theoretically by full-potential linear muffin-tin-orbital (FPLMTO) method and the temperature dependence of the measured bandgap energy by optical absorption. The obtained Eg(T) can be fitted by two different methods, leading to Eg(0K) and Eg(300K).

 

MOSFET (Metal-Oxide-Field-Effect-Transistor)

In contrast to three-dimensions, where a metal-nonmetal (MNM) transition is well understood to occur, experimental works carried out for two-dimensions before 1994 just confirmed the scaling theory of non-interacting electrons. According to this theory all two-dimensional (2D) electronic states are localized in the absence of a magnetic field and no metalic phase exist, i.e., MNM transition is therefore impossible. Recently experiments on metal-oxide-semiconductor field effect transistors (MOSFET) stucture have provided evidence of a true MNM transition in two-dimensions, indicating that electron correlations play an importante role in this transition. Similar effects have been found in other 2D systems. Since then many other works have appeared on MOSFET's. 

It is well know the occurrence of impurity band in n-type MOSFET structures. Such structures are nowadays excelent to investigate the 2D MNM transition. In our work we consider a more detailed investigation with the use of a Hubbard-type Hamiltonian representing the impurity electrons, where we consider the effects of external electric field, binding energy variation, screening, impurity location near and at semiconductor interface, disorder, correlation, and impurity concentration of experimental interest.

 

Spin Polarization

It is  worthwhile to figure out  that electron spin-filter is a basic spintronic device. However, despeite the fast growing research, and many proposals, there is still no definite experimental realization of such semiconductor device yet. A spin-filter device using nonmagnetic triple barriers III-V semiconductor structures has been recently proposed .It is based on the effect of electron spin polarization by ressonant tunneling, due in turn to the Rashba spin-orbit coupling , where the Kane –k.p method has also been  used. As a result of such coupling, it has been recently demonstraited the formation of spin-dependent minibands in the case of asymmetric superlattice, with asymmetric double-barrier unit cells. We  investigate the spin-dependent current for such superlattices with  results for instance to InGaAs based asymmetric structures with different pairs of lattice-matched barriers materials (InP, InAlAs and GaAsSb)and extend the model to spin dependent properties of magnetic dilute semiconductors by means of electronic structure calculation.This method can also been used for MOSFET (metal oxide field effect transition) as well.

 

Thermal lens and photoacoustic spectroscopy to determine the thermal and optical properties in bulk and thin film semiconductors

It is well recognized that the characteristic of the photothermal phenomena to determine the thermal and optical properties of materials via the energy transfer processes that results in heat generation is a particular advantage of these methods as compared to others conventional techniques, especially when the studied materials present high degree of: opacity, scattering and reflectance. The purpose of this work is to combine the particular abilities of the photoacoustic spectroscopy with those of the thermal lens spectrometry to study  different semiconductor samples. Three of them are in the thin film form, 4H-SiC, Ni₈₀Cr₂₀ and NiO, while the PbI₂ is a bulk single crystal. 4H-SiC was grown by hot wall chemical vapor deposition , Ni₈₀Cr₂₀ and NiO  were obtained by sputter deposition and the PbI₂ single crystal was grown by the Bridgman method with the c-axis oriented perpendicular to the growth axis .

 

Pressure induced phase transition in ErH3

The behaviour of hydrogen in the metallic environments governs the physical properties of a wide range of fundamental and technological interest. These include pure metallic hydrogen and metal hydrides, which are formed in the chemical reaction between metals and hydrogen. In particular, the yttrium and rare-earth hydrides display a dramatic changing in their optical properties depending on the hydrogen content. These systems can change reversibly from shiny metallic to transparent insulating films upon hydrogenation by varying the hydrogen gas pressure or by electrochemical means. Such behaviour is a consequence of the structure phase transformation accompanied by a metal-insulator tran-sition that occurs with increasing (decreasing)hydrogenconcentration.

The pressure induced phase transition in ErH3 is investigated by ab initio calculation. The electronic structure is using projected augmented wave (PAW) method and the properties for the phase transition were obtained by fitting the energy-volume set to the Murnaghan equation of states. We have found a structural phase transformation accompanied by an insulator-metal transition at 17Gpa in good agreement with the experimental finding. The similar phase transition is predicted to GdH3 as well.

 

Electronic and optical properties of rutile titanium oxide (TiO₂)

The compound rutile titanium dioxide (TiO₂) has been recongnized as very promising material with large technological applicabilities, as for instance coating and gas sensores.

Since the photocatalytic splitting of water by titanium dioxide (TiO₂) was first reported by Fujishima and Honda in 1972 , extensive investigation has been devoted to the study of this wide band-gap semiconductor. Titanium dioxide has been proven to be outstanding in several fields, including applications such as gas sensors , waveguides, and solar energy cells . Particularly, the strong oxidative power of photogenerated carriers on its surface has made TiO₂ one of the most practical photocatalytic materials for purification and treatment of polluted water and air .

The calculations for absorption, total real and imaginary parts of the dielectric function, as well as the optical band gap energy for the phase rutile were based on density function theory within the local density approximation (LDA), employing the full-potential linearized augmented plane wave (FPLAPW) method. We corrected the LDA band-gap energy self-consistently with an on-site Coulomb potential. The thin films for the measurements were prepared by DC magnetron sputtering and the transmission spectroscopy technique has been used to obtain the absorption spectrum. The theoretical result for the absorption compared qualitatively well with the experimental finding. The features of the dielectric functions are presented and the transition at the fundamental gap is discussed based on the electronic structure.

 

Optical properties of rhodamine 6G-doped TiO₂ sol-gel films

The optical properties of titania (TiO₂) thin films prepared by the sol-gel process and doped with rhodamine 6G were studied by Photoacoustic Spectroscopy. Rhodamine 6G-doping was achieved by adding 0.01%, 0.02%, 0.05% y 0.1% mol rhodamine to a solution that contained titanium isopropoxide as precursor. Two absorption regions were distinguished in the spectrum of a typical rhodamine 6G-doped TiO₂ film: an absorption band just over 2.2 eV, attributed to rhodamine 6G, and a band above 3.0 eV corresponding to TiO₂ absorption. A shift of these bands occured as a function of rhodamine 6G-doping. In addition, the optical absorption and band gap energy for the phase rutile of TiO₂ films were calculated with the local density approximation (LDA), employing the full-potential linearized augmented plane wave (FPLAPW) method. A comparison of these calculations with experimental data of TiO₂ films, prepared by sol-gel at room temperature is performed.

 

Optical and structural properties of indium nitride polycrystaline films

Indium nitride (InN) is a promising material for optoelectronic devices, such as low cost solar cells with high efficiency and various types of sensors. However, difficulties in the growth process do not permit yet a clear and complete characterization of the optical and structural properties .We will report some aspects of the optical and structural properties observed in several polycrystalline films of the InN on glass substrate, with thicknesses of the films in the 95-520 nm range. The InN films were deposited on glass using reactive DC magnetron. Raman scattering, transmission/absorption and X- ray diffraction measurements are used as base to discussion the mentioned properties. The present theoretical study of the electronic structure and optical properties of the hexagonal wz-InN are based on the local density approximation (LDA) within the density functional theory (DFT), employing the first-principles, full-potential linearized augmented plane wave (FPLAPW) method.

 

Electronic and optical properties of rutile tin oxide (SnO₂)

SnO2 is an important oxide for efficient dielectrics, catalysis, sensor devices, electrodes and transparent conducting coating oxide technologies. Tin oxide SnO₂ (TO) thin film is widely used in glass applications due to its low infra-red heat emissivity. In this work, the SnO₂ electronic band-edge structure and optical properties are studied employing a first-principle and fully relativistic full-potential linearized augmented plane wave (FPLAPW) method within the local density approximation (LDA).The optical band-edge absorption α(ω) of intrinsic SnO₂ is investigated experimentally by transmission spectroscopy measurements and their roughness in the light of the Atomic Force Microscopy (AFM) measurements. The sample films were prepared by spray pyrolisis deposition method onto glass substrate considering different thickness layers. We found for SnO₂ a very good qualitatively agreement of the calculated optical band-gap energy as well as the optical absorption with the experimental results.

 

Superconductivity in boron-doped homoepitaxial (001)-oriented diamond layers

The crystallinity and built-in strain of diamond epilayers grown by MPCVD with boron contents n_B varying from 2´10²⁰ to 2´10²¹cm^-3 as evaluated by SIMS are assessed by HR-XRDiffraction. Both ac transport and ac susceptibility measurements show that the critical concentration for the onset of superconductivity in (001)-oriented single crystalline films is about 5´10²⁰ cm^-3. This is also the critical hole concentration for the metal-non metal transition in this system, as evaluated by two types of theoretical calculations and by low temperature resistivity experiments. We report also preliminary results of point contact spectroscopy on an epilayer with a critical transition temperature T_c = 2.1K, which yield a superconducting gap around 0.3 meV compatible with a conventional BCS-type description of superconductivity in this system.