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Porous silicon interfaces: properties and applications to optoelectronic devices.

J.M. Martínez-Duart, R.J. Martín-Palma.
Departamento de Física Aplicada C-12, Universidad Autónoma de Madrid, 28049 Cantoblanco, Madrid, España.
L.Vázquez, F. Agulló-Rueda
Instituto de Ciencia de Materiales de Madrid (ICMM, CSIC). 28049 Cantoblanco, Madrid, España.
R. Guerrero-Lemus
Departamento de Física Básica, Universidad de La Laguna, 38204 Tenerife, Spain.

In this talk we would like to give an insight of some of the potential advantages that porous silicon (PS) surfaces and interfaces present in optical and optoelectronic devices. The results on the properties of the PS interfaces and devices will be mainly based in those obtained in our laboratory, in the OPTIC program of the European Community, in which we have participated as partners and, in a smaller proportion, in results obtained in other laboratories. The potential advantages of the use of PS in optoelectronic devices are mainly due to: a) the surface roughness of the material that makes it to behave as an antireflection coating; b) the PS photoluminescent properties which permit to convert part of the ultraviolet solar spectrum in visible photons with higher quantum efficiency; c) the PS band gap can be adjusted by varying the formation parameters; d) the incident radiation is dispersed by the porous structure and therefore diffused light reaches the junction; e) easiness of fabrication of superlattices and PS graded coatings with optimal optical properties; f) the relaxation of the momentum-conserving rule which makes it PS to behave somewhat as a direct band-gap semiconductor.

A very important previous step while dealing with PS consists in being able to produce samples with reproducible properties. For this object the surface of the films have been characterized by surface and thin film analytical techniques such as XPS, FTIR, TEM, SEM, AFM and micro-Raman spectroscopies. Obviously, it is also important the stabilization of the surface of PS carried out by techniques like rapid thermal processing in oxygen and nitrogen atmospheres and also by hydrogen passivation.

One of the problems that first arises while trying to develop an optoelectronic device based on PS is due to the contact resistance between the metal and the porous silicon. Results are presented for PS contacts to gold, aluminum and titanium, and also to conducting polymers. High values of the ideality coefficient (n) for these devices arise when the high concentration of interface states in equilibrium with the semiconductor is large, as a result of the broken bonds at the surface of the nanostructured layer.

We have determined the optoelectronic properties (responsivity, quantum efficiency, etc.) of porous silicon photodiodes based in PS/metal interfaces of the Schottky type and relate their electrical response to the fabrication parameters. We also described the behavior of solar cells based on PS under standard illumination conditions and found that the angular dependence of their electrical response agrees with a cosine type law. Special interest has been placed in the development of antireflection coatings with very low values of reflectivity in the whole solar spectrum. In addition we describe solar cell structures based on multicrystalline silicon with efficiencies higher than 13 % when PS is prepared electrochemically and around 12 % for stain-eched porous silicon.

Finally, we have developed porous silicon superlattices by periodically varying the electrochemical formation parameters. In this way we control the porosity of the individual coatings which is directly related to the refractive index. TEM cross sectional images have been obtained to determine the thickness and porosity of the layers. Also, by means of micro-Raman spectroscopy we have obtained the crystallite size, stresses and photoluminescent properties as a function of depth. We also prove that interference filters with excellent optical properties can be developed from these superlattices.

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