Guillermo Santana

Optoelectronic properties of CdO/Si photodetectors

Abstract CdO/Si heterojunctions were fabricated by depositing CdO polycrystalline thin films on p-type single crystalline silicon wafers by CBD. The current–voltage characteristics under dark and illumination of CdO/Si devices resemble those of a light sensitive diode. From the CdO/Si diode spectral sensitivity curves (A/W), it has been implied that CdO films allow a high sensitivity response in the visible and the near infrared regions.

Spray pyrolysis deposition of cadmium–zinc oxide thin films

Mixed thin film oxides of cadmium and zinc with different compositions were deposited on glass substrates by spray pyrolysis. The effect of the various parameters on the growth and on the film properties is presented. The sample structure was studied by X-ray diffraction (XRD) having found the presence of a cubic phase resembling that of pure CdO XRD pattern for low Zn concentrations. For x=0.75 a low crystallinity of the ZnO structure was observed. Optical band-gap was also studied from optical transmittance measurements. As expected, the band-gap values change between those of pure CdO and those of ZnO.

Influence of post-thermal annealing on the properties of sprayed cadmium–zinc oxide thin films

Abstract We have prepared (ZnO)x(CdO)1−x thin films by spray pyrolysis. Mixed thin film oxides with different compositions of cadmium and zinc were deposited on glass substrates and exposed to different annealing treatments at a fixed temperature. The effects due to post-thermal annealing on the structural, optical and electrical properties of the variable composition thin films have been studied depending upon the x value. The crystalline structure was studied by X-ray diffraction (XRD). We found the presence of CdO XRD patterns for low Zn concentrations, and a mixing of CdO (cubic) and ZnO (hexagonal) phases for low Cd concentrations. In all cases, the crystallite sizes increased with the annealing. In addition, the band-gap and the resistivity vary between those found for pure CdO and ZnO. The changes of the latter parameters with the post-thermal annealing are studied also as a function of the x value.

Structural and optical properties of (ZnO)x(CdO)1−x thin films obtained by spray pyrolysis

Thin films of (ZnO)x(CdO)1−x oxides were deposited on glass substrates by spray pyrolysis and annealed in air at 450°C. The structural and optical properties of the as-grown and thermally annealed thin films are presented. The crystalline structure was studied by X-ray diffraction (XRD) having found the presence of the CdO cubic phase pattern for low Zn concentrations and a mixing of cubic-CdO and hexagonal-ZnO phases for low Cd concentrations. The crystallinity of all samples improves with the thermal annealing. The optical band-gap was also studied from the optical transmittance for the as-grown and annealed samples. As expected, the band-gap changes between that for pure CdO and that for ZnO.

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.

Thermal Oxidation of Silicon in Nitrous Oxide at High Pressures

Thermal oxidation of silicon in nitrous oxide (N 2 O) ambient at pressures from I to 4 atm has been studied. We show that the oxidation rate is different from the one predicted by the Deal-Grove model for normal oxidation in dry oxygen. In our case, the dependence observed for the oxide thickness as a function of the oxidation time is of the form x = x 0 + (γt) β , where x 0 is a native oxide thickness. For the temperature range between 900 and 1200°C, and 2 atm of pressure, the activation energy for γ is around 1.18 eV. In addition, the exponential factor β (at 1000°C) varies as the square root of the N 2 O pressure. These results indicate that thermal oxidation in N 2 O behaves in a completely different way than normal oxidation, very likely due to the influence of chemical reactions in the gas phase, to the catalytic influence of the N 2 O-SiO 2 interface, and to the incorporation of nitrogen into the oxide film itself. The results presented here establish the basis for the development of a more complete model for thermal oxidation of silicon in a N 2 O ambient.

Double anti-reflection layers for silicon solar cells obtained by spin-on

We have prepared double SiO/sub 2//TiO/sub 2/ anti-reflection layers to be applied on silicon solar cells by means of an inexpensive technique based on the spin-on deposition of sol-gel solutions. Experimentally we show that by using bi-layers with 647 /spl Aring/ of TiO/sub 2/ and 1064 /spl Aring/ of SiO/sub 2/ we can reduce the reflection losses below 4% for the whole AM 1.5 solar spectrum.

Electronic conduction in SiO2:N thin films grown by thermal oxidation of silicon in N2O

Abstract Silicon oxynitride [SiO 2 :N] thin films have been grown by oxidizing silicon in N 2 O at 900, 1000 and 1100 °C and at 760 and 1520 torr. It is shown that the dominant electrical conduction mechanism, for high electric fields, is the field assisted thermionic emission from the traps (Poole–Frenkel effect), and is not direct or Fowler–Nordheim tunneling, as typically occurs in thermal silicon oxide with similar thickness. Electrical conduction in these films occurs by field assisted electron emission from donor traps with energy levels varying in the range from 0.5 to 1 eV from the conduction band. The results shown here indicate that the best quality films are those grown at low temperature and pressure, since they give films with a higher critical electric field, a higher energy barrier depth at the traps and less donors compensated by acceptors than those grown at high temperatures and pressures.

Surface recombination velocity in silicon substrates determined from light beam induced current measurements

We show a simple method to determine the surface recombination velocity of silicon substrates from light beam induced current (LBIC) measurements. With this method we have studied the possible passivation effects due to SiN obtained by PECVD as compared to SiO/sub 2/ thermally grown and a combination of thermal SiO/sub 2/ with PECVD SiN. From LBIC measurements and by fitting the data to a realistic mathematical model we were able to determine both the diffusion length and the surface recombination velocity for silicon substrates covered with different passivating films. In this way we could determine that a combination of thin thermal SiO/sub 2/ with SiN is a better surface passivant than pure SiN films, but is not as good as a thick thermal SiO/sub 2/. However, it is expected that SiN or SiO/sub 2//SiN layers may be optimized in order to have similar passivation as a thick thermal SiO/sub 2/ film on top of silicon. In this case, a simple technique to determine the surface recombination velocity, such as the one presented here, should be very useful.