A.M. Andrade

Evidence of quantum size effects in a-Si:H/a-SiCx:H superlattices. Observation of negative resistance in double barrier structures

Abstract Variations in the optical gap, in the parallel conductivity and in the conductivity activation energy were observed in A-Si:H/a-SiCx:H multilayers. These results are consistent with the existence of bounded states in the a-Si wells. Also, non linearities in the current vs. voltage curves of multiple a-SiCx:H barriers embedded in the intrinsic layer of n-i-n a-Si:H structures were studied. Several cases of negative resistence, even at room temperature, were found. These results are consistent with a “sequential tunneling” phenomenon, although a bulk effect cannot be ruled out.

Effect of the substrate on the structural properties of low temperature microcrystalline silicon films – a Raman spectroscopy and atomic force microscopy investigation

Abstract Raman spectroscopy and atomic force microscopy (AFM) techniques were used to determine the properties of microcrystalline silicon (μc-Si:H) films deposited at low temperature (100°C) by a conventional plasma enhanced chemical vapor deposition (PECVD), 13.56 MHz RF, reactor from PH3/SiH4/H2 gas mixtures in a triode coupling configuration. In this work, the correlation between surface morphology and crystallinity was investigated for μc-Si:H films deposited on single crystal silicon, polyethylene teraphtalate (PET) and Corning 7059 glass substrates. The growth process of μc-Si:H was investigated as a function of the deposition time in the range of 15–260 min. We observed from atomic force microscopy analysis and Raman spectroscopy that the growing surface roughness affects the crystallization process and determines the average grain size of the deposited material. We also observed that the substrate properties affected the orientation of the initial layers, acting as a seed for the formation of a crystalline-like material.

Study of heterojunction diodes using POMA/PANI and amorphous/microcrystalline silicon structures

Photovoltaic diode based on a heterojunction structure combine on amorphous and microcrystalline silicon, deposited by PECVD process, with polyaniline- (PANI) and poly(o-methoxyaniline)-(POMA), using PIN structures. We report IxV characteristic curves of glass/ITO/a-Si:H(p)/POMA or PANI/μc-Si:H(n)/Al structures. Preliminary results show a dependence of the IxV curves on the polymer doping level. This structure shows an insulating behavior for undoping or very low doping, photovoltaic effect at medium doping and a typical resistance curve at high doping level. The device is a promising tool to understand transport and electronic properties of the polymers.

Improved backside hydrogenation on semicrystalline silicon solar cells by RF plasma

The results obtained from the optimization of the backside hydrogenation on semicrystalline silicon solar cells by RF plasma process are described. Process variables were the hydrogen pressure and the treatment time. Solar cells, photocurrent, and photovoltage, were increased after treatment without generating front side defects, which reduce the fill factor, and consequently the conversion efficiency was improved. The experiments show that backside hydrogenation for semicrystalline silicon solar cells is very effective. The short-circuit current density shows relative increases of over 40% for cells with initial photocurrent values smaller than 18 mA/cm/sup 2/ after the hydrogen treatment. For solar cells fabricated on solar grade substrates, photocurrent values increased even more, reaching +95%.<<ETX>>

Reator de UV-Ozônio com lâmpada a vapor de mercúrio a alta pressão modificada para tratamento superficial de óxidos transparentes condutivos utilizados em dispositivos poliméricos eletroluminescentes

An UV-Ozone reactor was developed with an ignition tube extracted into HID mercury lamp used to irradiation on zinc oxide (ZnO) and fluorinated tin oxide (FTO) films for PLEDs devices. Different exposures times were used. In contact angle measurements revealed better results for ZnO and FTO by 15 and 5 min, respectively. In Diffuse Reflectance Infra-red Fourier Transformed (DRIFT) spectroscopy allowed the observation of water, hydrocarbon and carbon dioxide adsorbed on the untreated TCO surfaces. After the UV-Ozone treatment the contaminants were significantly reduced or eliminated and the PLEDs devices decreased threshold voltages in comparison with respectively untreated TCOs.

Photovoltaic effect on μc-Si:H(n+)/poly(o-methoxyaniline)/a-Si:H(p) structures

Abstract Heterostructures, having poly(o-methoxyaniline) (POMA) deposited by spin coating process as active layer, were built with n -type microcrystalline silicon and p -type amorphous silicon deposited by CVD process near room temperature. J x F device characteristic curves showed rectification properties whose current enhances under visible light illumination. This photomechanism may act directly in the bulk polymer, generating photocarriers, or in the p -Si/POMA interface injecting positive carriers from Si into the POMA bulk. Photovoltage phenomenon was also observed, indicating the possibility of fabricating photovoltaic solar cells with POMA films.

Influence of Hydrogen Plasma in the Properties of a-Si: H/a-Sic:H Interfaces

In this work we study the a-Si:/a-Sic:H in interface multilayer structures deposited by glow discharge technique. In order to improve the quality of the interfaces we introduce an extra hydrogen plasma step between the deposition of consecutive layers without interrupting the RF power. We compare this method with the traditional one, just changing the gas mixture without interrupting the discharge. To evaluate this method, identical multilayer structures with and without the extra hydrogen plasma were fabricated. The interfaces were characterized by small angle x-ray diffraction using reflection geometry and by opto-electronic measurements. The influence of the structure parameters on the experimental diffractograms will be discussed and compared with theoretical results.

Perfume Fragrance Discrimination Using Resistance And Capacitance Responses Of Polymer Sensors

This work shows a comparison between electrical resistance and capacitance responses of ethanol and five different fragrances using an electronic nose based on conducting polymers. Gas chromatography—mass spectrometry (GC‐MS) measurements were performed to evaluate the main differences between the analytes. It is shown that although the fragrances are quite similar in their compositions the sensors are able to discriminate them through PCA (Principal Component Analysis) and ANNs (Artificial Neural Network) analysis.

Effect of the deposition parameters on the electro optical properties and morphology of microcrystalline hydrogenated silicon alloys

Microcrystalline phosphorus-doped hydrogenated silicon alloy films were deposited in a remote plasma CVD (chemical vapor deposition) system. The film properties were studied as a function of RF power density and hydrogen concentration in the reaction gas mixture. The properties of the deposited films are extremely sensitive to the RF power density, in the studied range of 250 mW/cm/sup 2/ to 625 mW/cm/sup 2/. Very low values of electrical resistivity were obtained. For an RF power density of 500 mW/cm/sup 2/, rho =3*10/sup -2/ Omega -cm, while rho =1.9*10/sup 3/ Omega -cm for 625 mW/cm/sup 2/, indicating the predominance of the amorphous tissue over the microcrystalline phase. High doping efficiencies which can be correlated to large grain size are indicated by the very low values of the activation energy as low as 30 meV for 500 mW/cm/sup 2/, that were obtained.<<ETX>>

Influence of a DC Electric Field on Hydrogen Incorporation and Doping Efficiency of a-Si:H Films

Doped and undoped a-Si:H and a-SiC:H films were deposited by RF decomposition of silane and silane/methane mixtures. DC electric and magnetic fields were applied during deposition and their influence on the electro-optical properties of the films was investigated. It is found that the doping efficiency depends on the intensity and sign of the electric field. It is observed that the stability under light stressing and the transport properties of undoped films depend on the intensity of the electric field.