J. Aguilar-Hernández

Mechanism of photoluminescence of silicon oxide films enriched by Si or Ge

Photoluminescence peculiarities of silicon oxide films enriched by Si or Ge have been investigated. Photoluminescence (PL) and Raman spectra were measured before and after thermal annealing at 800 °C. The dependences of PL peculiarities on the concentration of Si and Ge, as well as on the existence (or absence) of Si (Ge) quantum dots (QDs) in silicon oxide films are analyzed for the photoluminescence mechanism study in the above-mentioned systems.

Analysis of the 1.55 eV PL band of CdTe polycrystalline films

Abstract Photoluminescence (PL) spectra of CdTe at low temperatures usually show a narrow band at 1.55 eV, sometimes together with phonon replicas. It has been shown that this band is associated with a transition involving an acceptor level due to a cadmium vacancy; however, there is discrepancy about the high-energy level which causes this transition. It is assumed that this high-energy level comes from the bottom of the conduction band or arises from non-excited level due to a superficial donor. In order to clarify the origin of this level, we have carried out selective pair luminescence (SPL) studies at 10 K and also analyzed the temperature dependence of PL in CdTe polycrystalline samples. Our results allowed us to conclude that the 1.55 eV PL band arises from the overlap of two bands peaking at 1.550 and 1.556 eV, respectively. The first one is associated with a donor–acceptor pair transition and the second one (1.556 eV) is probably due to a transition involving excitons bound to a superficial acceptor of the type Cu i + –V Cd − .

Photoluminescence Studies of Semiconducting Polycrystalline CdTe Films

We report the first systematic measurements of the temperature dependence of the photoluminescence (PL) in the range of 10–300 K in CdTe. The experiments were carried out on semiconducting CdTe films of high quality grown by a modified close-spaced vapour transport (CSVT) technique. Several luminescence bands were observed, one around 1.4 eV showing a temperature-independent behaviour, and another band located around 1.52 eV, at 300 K, showing a strong temperature dependence. The excitonic origin of this band at low temperatures has been confirmed by the dependence of the PL intensity on the excitation intensity. A band-to-band luminescent recombination component at high temperatures is also observed. This study allows us to elucidate the nature and basic physical properties of the bound exciton as well as the temperature dependence of the band gap.

Influence of the growth conditions in the properties of the CdTe thin films deposited by CSVT

We present in the results of this work, the influence of oxygen and the gradient of temperature in the processing of CdTe thin films, by using the close space vapor transport (CSVT) technique. In general the kinetic process is strongly influenced by these parameters; where a reduction in the growth rate, grain size; an increase in the intergrain barrier height; and structural changes are present.

Passivation properties of CdS thin films grown by chemical bath deposition on GaSb: the influence of the S/Cd ratio in the solution and of the CdS layer thickness on the surface recombination velocity

Evidences of the passivation effect are given when thin films of CdS are deposited on GaSb crystalline substrates, using a bath chemical deposition method. The passivation process is studied through photoacoustic and photoluminescence experiments. The surface recombination velocity calculated from photoacoustic measurements decreases and the radiative recombination rate as measured from photoluminescence spectra increases when the nominal S/Cd ratio in the layer deposition solution increases. The influence of the CdS layer thickness on the surface passivation of GaSb is also studied.

Influence of the growth conditions on the photoluminescence spectrum of CdTe polycrystalline films deposited by the close space vapor transport technique

A study of the influence of some growth parameters, the gradient of temperature between source and substrate and introduction of oxygen in the growth chamber, on the optical properties of CdTe thin films grown by Close Space Vapor Transport (CSVT) is presented. This study was carried out by means of photoluminescence (PL). The combined effect of the temperature gradient and oxygen in the growth chamber on the crystalline quality of CdTe thin films were analyzed. Important changes in the photoluminescence spectra were observed. A decrease of the intensity of the excitonic PL band is observed, as oxygen concentration increases. This accounts for an increase in the defect density due to the presence of oxygen.

Photoluminescence Study of Gallium Nitride Thin Films Obtained by Infrared Close Space Vapor Transport

Photoluminescence (PL) studies in GaN thin films grown by infrared close space vapor transport (CSVT-IR) in vacuum are presented in this work. The growth of GaN thin films was done on a variety of substrates like silicon, sapphire and fused silica. Room temperature PL spectra of all the GaN films show near band-edge emission (NBE) and a broad blue and green luminescence (BL, GL), which can be seen with the naked eye in a bright room. The sample grown by infrared CSVT on the silicon substrate shows several emission peaks from 2.4 to 3.22 eV with a pronounced red shift with respect to the band gap energy. The sample grown on sapphire shows strong and broad ultraviolet emission peaks (UVL) centered at 3.19 eV and it exhibits a red shift of NBE. The PL spectrum of GaN films deposited on fused silica exhibited a unique and strong blue-green emission peak centered at 2.38 eV. The presence of yellow and green luminescence in all samples is related to native defects in the structure such as dislocations in GaN and/or the presence of amorphous phases. We analyze the material quality that can be obtained by CSVT-IR in vacuum, which is a high yield technique with simple equipment set-up, in terms of the PL results obtained in each case.

Photoluminescence of silicon oxide films enriched by Si or Ge

Photoluminescence (PL) and Raman spectra of silicon oxide films enriched by Si or Ge have been investigated before and after thermal annealing at 1150°C and 800°C, respectively. The dependences of PL peculiarities on the concentration of Si and Ge, as well as on the existence (or absence) of Si (Ge) quantum dots in silicon oxide films are analyzed. It is concluded that the PL spectrum of the oxide films enriched with Ge and at least the high-energy part of the spectrum of the films enriched with Si are due to defects in the silicon oxide films.

Electropolymerization of Polypyrrole Films in Aqueous Solution with Side-Coupler Agent with Side-Coupler Agent

A preliminary study of the electrochemical synthesis and optical characterizacion of the conducting polymer polypyrrole (PPy) was carried out, in order to understand the in-situ electropoliymerization of PPy. Electropolymerization was performed in a three electrode cell by using freshly prepared monomer solutions in presence of a side-coupler agent to hydrophobic groups: sodium dodecylsulfate (DDS), in order to improve the adherence polymer film to the surface of the working electrode. The adherence of the polymer film promotes either the ionic or electronic transport at the interface solution-working electrode. A polar chemical reactive, sodium tetrafluoroborate, TFB, was also used. In order to taylor and optimize the physical properties of the PPy films we varied some parameters during the electropolymerization: the monomer concentration, the electrolyte concentration, pH of the solution and the cell potential. This allowed us to control the oxidation level (impurification or doping) of the polymer. The obtained PPy films were characterized by using UV-Vis and IR spectroscopy. Moreover through scanning electronic microscopy we were able to observe well developed helical structures of polypyrrole.

Structural and Optical Characterization of CdSe Films Grown by Chemical Bath Deposition

We present in this work experimental results and their analysis concerning the structural and optical characterization of cadmium selenide (CdSe) films grown by the Chemical Bath Deposition (CBD) technique, at different bath temperatures Td : 0≤Td≤80 °C. Structural characterization was carried out by using X-ray diffraction, whereas optical characterization was done by using optical absorption and photoluminescence spectroscopies. X-ray results showed a change of the crystalline structure as a function of Td , from the zincblende to wurzite one when Td increases from 0 to 80 °C. The respective band-gap, Eg , was calculated from the absorption spectrum giving rise to values in the range 1.85 to 2.30 eV, as a function also of Td . Thses values are higher than the well accepted value of 1.75 eV for the CdSe bulk material. The radiative emission was analyzed throughout the photoluminescence (PL) spectra. All samples showed radiative emission above the band-gap, at room temperature, this radiative emission having a dependence on the pressure inside the cryostat. A quenching of the PL signal is observed when the pressure is decreased at values as low as 60 mTorr.