Degradation of Porous Si Layers Caused by Thermal Treatment
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DEGRADATION OF POROUS Si LAYERS CAUSED BY THERMAL TREATMENT H. MONDER', M.G. BERGER', S. FROHNHOFFS, H. LUTH-, U. ROSSOW*, U. FROTSCHER** and W. RICHTER* *Institut ffir Schicht- und Ionentechnik, Forschungszentrum Jiilich, 5170 Jiilich, Germany **Institut ffir Festkidrperphysik, TUBerlin, 1000 Berlin, Germany ABSTRACT Porous Si films formed on different p-doped substrates are studied by Raman spectroscopy, photoluminescence, and spectroscopic ellipsometry. Due to a thermal treatment the morphology is changed. A reduction in the number of nanocrystals with diameters below 30A is found. It is shown that the photoluminescence is caused by the formation of small nanocrystals and that the effect of amorphous Si as a basic mechanism can be ruled out. The strain which must be taken into account for the interpretation of the Parnan spectra decreases with increasing heating temperature. INTRODUCTION The main problem of using porous Si in optoelectronic device structures is its degradation behaviour. During photoluminescence (PL) and electroluminescence (EL) measurements a decrease in the signal intensities is found. The reason for the decreasing intensities is not yet understood. Heating up the samples, e.g. by the absorption of the probe laser light, could explain the degradation. Porous materials in general exhibit a lower thermal conductivity than bulk material. This is also the case for porous Si and is one of the main problems during performing micro Raman measurements. Laser foci of a few pm used for PL or Raman studies result in high laser power densities even though the laser power is kept at about lmW. The absorption of the laser light causes a dramatic increase of the sample temperature in the laser focus. During micro Raman measurements for example the sample temperature in the laser focus can rise by a few hundred Kelvin depending on the used laser power, on the topology, and on the porosity of the samples [1]. The fast degradation in the PL signals where the intensity finally decreases to a saturation level, may indicate that thermally induced changes take place. This might also be the case for EL studies where an increasing sample temperature might occur due to the current flow. Therefore, the thermal stability of porous Si samples was studied by using optical methodes like Raman spectroscopy, photoluminescence, and spectroscopic ellipsometry. Combining these different techniques the morphology has been analyzed and its influence on the dielectric function has been studied. From a detailed lineshape analysis of the phonon Raman peak nanocrystal diameter distribution functions, strain values, and scattering contributions from amorphous Si axe obtained. From these results the influence of the morphology on the PL behaviour is studied. EXPERIMENTAL The porous Si layers were formed on different p-doped wafers (0.010cm, 0.2Qcm). The orientation of the substrate material was (100). The porous Si layers exhibit a porosity of 48% for the p++-doped samples and 70% for the p+-doped. The thicknesses of the porous films w
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