I Porosity superlattices : a new class of Si I 1 heterostructures

Porosity superlattices have been investigated by transmission electron microscopy, photoluminescence and reflectance spectroscopy. The superlattices were formed on p-type doped Si using two different techniques. Firstly, for homogeneously doped substrates we have periodically varied the formation current density and thereby t h e porosity. Secondly, the current density was kept constant while etching was performed on periodically doped Si layers. For the first type of superlattices the layer thicknesses were determined by transmission electron microscopy. The results are in good agreement with the values calculated from the etching rate and time. For both types of superlattices, reflectance and photoluminescence spectra show strong modulation due to the periodicity of the superlattice. Since the middle of the fifties it has been well known how porous Si layers can be formed [l], but interest in porous Si-its microscopic structure and optical properties-has dramatically increased due to reports of photoluminescence (PL) [Z] and electroluminescence (EL) [3,4]. Quantum efficiencies of more than 10% for PL and 0.1% for EL stimulated again the idea of Si based optoelectronics and have been the driving force for a lot of research activities over the last two years. Typical luminescence spectra show a broad band with a peak maximum between 550 and 850 nm and a full width at half maximum (FWHM) of more than 100 nm [2]. For the EL it seems nearly impossible to tune the wavelength of the emission peak with the exception of using electrolytical front contacts [5,6]. In this paper we show for the first time that porosity superlattices can act as filters which will reduce the FWHM and shift the maximum of the emitted light in a reproducible way. Porosity superlattices are a new type of Si based heterostructures which exhibit a periodic variation in depth of the porosity. Despite the continuing debate on the mechanisms of the luminescences-how must the basic quantum size model be modified to explain the luminescences behaviours-porosity superlattices open a wide field of possible applications especially coloured flatscreens made of Si. In addition, porosity superlattices can lead to a better understanding of the reaction kinetics during the formation process of porous Si layers. Porosity superlattices can be formed in different ways. First, etch parameters, such as the current density 0022-3727/94/061333+04$19.50