J.L. Peña

Comparison of properties of CdS thin films grown by two techniques

Abstract Polycrystalline cadmium sulfide (CdS) thin films were deposited on glass substrates by chemical bath deposition (CBD) and close-spaced sublimation (CSS) techniques. The typical deposition temperatures between these techniques are quite different. The CdS thin films deposited by CBD were prepared using two methods of bath agitation: magnetic and ultrasonic agitation. We found that films deposited with ultrasonic agitation presented a cleaner surface with minor presence of contaminants, similar to the obtained for CSS films, as demonstrated by Auger analysis. Properties of the CdS films such as morphology, optical transmission, crystallinity and band gap energy, are discussed in order to compare them in both techniques. We obtain that crystallinity of CdS films depends strongly on the temperature used for deposition. In agreement with several works, films prepared by CBD technique presented a cubic structure, while films grown by CSS technique exhibited an hexagonal symmetry.

Electromigration in gold thin films

Abstract We study the modification of gold thin film surface by Scanning Tunnelling Microscopy (STM) and grazing incidence X-ray to understand the process of electromigration. As a result of the applied current we have observed by STM a large movement of matter in the film surface that results in a strong rearrangement and modification of all the surface structures in a matter of minutes. We obtained that the 〈111〉 peak position shift towards the gold standards with elapsed time indicating that the surface strains formed during film growth disappears. The conclusion of the study is that the electrical current induces a rearrangement of the gold film surface.

Evidence of scattering effects on the thermal transport in indium-doped CdTe films

Photoacoustic (PA) heat transmission measurements were used to study thermal transport in n-type polycrystalline CdTe films doped with metallic indium. Thermal diffusivity (TD) at room temperature was determined using an open cell PA technique. The TD obtained for each sample was correlated with x-ray diffraction measurements. It can be shown that the TD of the undoped films under investigation is significantly reduced compared with corresponding doped films. This reduction is a result of the high concentration of indium atoms and defects depending on the deposition parameters. It is shown that the effects of CdTe lattice shrink produced by indium doping, induces an increase in phonon scattering reflected in the large TD decrease even with moderated indium concentration. We have calculated the relation between the TD and the lattice parameter as a polynomial function (cm2 s-1) = -35225.2+14259a-1532.7a2-27.9a3+8.4a4.

Structural transition from CdTe to CdIn2Te4 in films grown by close paced vapor transport combined with free evaporation

In order to obtain CdIn2Te4 thin films, (CdTe)12x(In2Te3)x thin films were grown on glass substrates by the close spaced vapor transport combined with free evaporation technique (CSVT-FE) using coevaporation of CdTe and In2Te3. When the saturation limit of In in the CdTe structure is achieved, a CdIn2Te4 thin film is obtained. Indium incorporation is controlled by the temperature of the In2Te3 source. The composition of the films was investigated by Auger electron spectroscopy and X-ray diffraction was used to evaluate the structural transition of the films from CdTe (zincblend) to CdIn2Te4 (tetragonal). Optical characterization by Raman, transmission and photoreflectance spectroscopies gave us further evidence of the success in the production of CdIn2Te4 thin films. q 2000 Elsevier Science S.A. All rights reserved.

Sn-doped CdTe as promising intermediate-band photovoltaic material

The formation energies, charge transition levels and quasiparticle defect states of several tin-related impurities are investigated within the DFT + GW formalism. The optical spectrum obtained from the solution of the Bethe–Salpeter equation shows that the absorption strongly increases in the sub-bandgap region after doping, suggesting a two-step photoexcitation process that facilitates transitions from photons with insufficient energy to cause direct transitions from the valence to the conduction band via an intermediate-band. We propose Sn-doped CdTe as a promising candidate for the development of high-efficiency solar cells, which could potentially overcome the Shockley–Queisser limit.

A comparative study of CdS:F and CdS:O thin films deposited by reactive RF-sputtering technique for window layer application in solar cells

In this work, we report our results on the study of CdS thin films doped with fluorine (CdS:F) and oxygen (CdS:O) prepared by the RF-sputtering technique with CHF3 and O2 as reactive gases, respectively. XPS and XRD measurements showed that CdS:F is composed of CdS and CdF2 compounds, while CdS:O is composed of CdS, CdSO4, CdSO3 and CdO2. Both CdS:F and CdS:O samples are highly oriented along the (0 0 2) plane, but with less crystallinity than CdS film. Scanning electron microscopy images showed that the morphology is changed due to the presence of reactive gases. The optical transmittance in the short wavelength range (λ < 500 nm) is improved in doped films, as higher values were achieved in CdS:O samples. By increasing the reactive gas concentration during the sputtering, the bandgap of CdS:O films is increased, while for CdS:F films it slightly decreases; in both kinds of samples, the refractive index is decreased and all doped films have higher electrical resistivity than CdS.

Thermal behavior of ITO/TiO2/CdS/CdTe solar cells

In this work, a TiO2 thin film was deposited as a buffer layer onto ITO film at front contact of CdS/CdTe solar cell. The TiO2 thin film was prepared by reactive rf-sputtering from a Ti target. The CdTe film was grown by conventional CSS technique. The cells were activated by annealing at 400 °C in Argon-Oxygen-CHClF2 atmosphere. From DRX analysis of TiO2 thin film was determinate that it has Rutile phase. The solar cell was characterized by J-V curve and Efficiency vs Temperature, the latter was nonlinear. The solar cell efficiency was around 9.5% at room temperature.

Refractive index properties of oxygenated amorphous CdS thin film deposited by rf-sputtering

The refractive index of 1.21 to 2.38 of oxygenated amorphous CdS thin films can be fabricated by using reactive RF-Sputtering from CdS target of 4N of purity. The oxygenated amorphous CdS with 2.3 of refractive index has a band gap of 2.5 e V. These properties are optically appropriate for matching the ITO and CdTe thin films in amorphous-CdS/CdTe solar cells application. For 10-20% of Oxygen/Argon and 50-70 Watts of the RF-Sputtering power, all thin film shows high transmittance, low refractive index of 1.21 and 3.7 eV of band gap. XRD spectra shows amorphous evidence. This thin film could be used as an antireflective layer on glass substrate.

The ZnO-reflectance effect on the heterojunction ITO/ZnO/CdS/CdTe

The reported structure in this work was fabricated using layers ITO/ZnO/CdS/CdTe grown onto borosilicate commercial glass. The thin films of ITO and CdS were deposited using rf-sputtering in argon atmosphere from ITO and CdS targets. The ZnO layer also was deposited by rf-sputtering from metallic Zn target. All targets were of 5N of purity. The CdTe film was about 8 μm of thickness, was deposited onto CdS using a conventional closed space sublimation technique. By using basic calculus for optical antireflective coatings, was demonstrated that film ZnO has the nearest refraction index as needing for optical matching for ITO and CdS layers. The reflectance of the structure glass/ITO/ZnO/CdS/CdTe when the ZnO thickness is varied from 100-400 nm it is reported. In this work also the reflectance of the intermediate structures such as glass/ITO, glass/ITO/ZnO and glass/ITO/ZnO/CdS are reported. We found a minimum reflectance of 9% of the complete structure between 500-850 nm of wavelength. By taking the optimal thickness, a solar cell was fabricated, was activated by using CHClF2 gas mixed with Ar at 400 °C for 5 minutes. As a back contact 15 nm of Cu and 500 nm of Mo were deposited by rf-sputtering in Ar atmosphere. The efficiency was about 10.1% for a cell of 1.01 cm2. After two months, the solar cell preserves its characteristics.

The CdS/CdTe solar cells with reactively sputtered a-MoOx/Mo back contact

In this work amorphous MoOx thin film was used as buffer layer at back contact of CdS/CdTe solar cell. The a-MoOx thin film was made by reactive rf-sputtering from a Mo target. From DRX analysis of MoOx/glass thin film, both α-MoO3 and β-MoO3 phases were identified. The CdTe film was grown by conventional CSS technique. The cells were activated by annealing at 400 °C in Argon-Oxygen-CHClF2 atmosphere. The curves of J-V and its efficiency vs. temperature of the solar cells were obtained. The solar cell efficiency at room temperature was around 11.2%. The temperature coefficient of the efficiency was 0.014%/°C in range of 40-65 °C.