Surface modification and its impact on luminescent properties of porous silicon

Abstract

Continuous device miniaturization on silicon chips has imposed limits on performance and has led to a desire for the integration of optoelectronic devices with Si microelectronics. However, silicon itself is an indirect bandgap semiconductor and does not emit light efficiently. In 1990, efficient visible photoluminescence was reported from porous Si fabricated through an electrochemical etching approach. The discovery has initiated extensive studies concerning the fundamental mechanisms of luminescence and applications based on this material. Increasing evidence suggests that the observed light emission is attributed to the presence of small silicon crystallites (1-5 nm) with the corresponding surface states playing a crucial role. One property which is important for technological applications of porous Si is its electroluminescence (EL) from light emitting diodes (LEDs). The key issues for these devices are stability, quantum efficiency and emitting color. Porous Si diodes have been fabricated on p-type, n-type and p-n junction substrates. The influence of surrounding ambient atmosphere on the stability of electroluminescent porous Si diodes has been examined. It is found that EL degrades in oxidizing environments while it remains stable in inert gases such as nitrogen and argon. The results suggest that a stable, passivating porous Si surface is needed to maintain the stability of electroluminescence. A mild chemical modification by aluminum isopropoxide has been used to modify the surface of porous Si. The modified porous Si has much more stable electroluminescence at low bias voltages and the rate of electroluminescence degradation is very sensitive to applied bias. Transmission electron microscopy (TEM) and infrared (IR) spectroscopic measurements demonstrate that the degradation is related to Si nanoparticle oxidation. The activation energy of porous Si has also been estimated in these studies. Multi-color emission from LEDs is critical for optical devices and display materials. By sonicating the orange emitting porous Si layers, red emitting porous Si can be obtained. Porous Si light emitting diodes with distinct red and orange EL regions have been fabricated. The overall EL efficiency is improved by a conductive polymer coating. The sensitive and reactive nature of porous Si allows for the fabrication of chemical sensing microstructures. Surface derivatization of porous Si with the specific goals of stable, long-term passivation and generation of a surface with unique chemical stability and selectivity have been pursued. Carboxylic acid derivatives of macrocyclic cavity-containing molecules known as calixarenes can form thin films on the surface of porous Si. This treatment yields a stable interface demonstrating selectivity with regard to photoluminescence quenching behavior by Cu(II) ions or amines. Electroluminescence studies have also been carried out in the calixarene-coated porous Si liquid junction cells. It is found that the stability and efficiency of EL is a function of calixarene film composition. The passivation of porous Si by carboxylic acids provides an approach to synthesize luminescent silica glass by incorporating Si nanocrystallites into sol-gel matrices. Si nanocrystallites are extracted from porous Si layers by sonication. By using fatty acids as stabilizers, the photoluminescence of Si nanoparticles within a sol-gel remains unchanged for months. A more uniform distribution of Si nanocrystallites in sol-gel has been achieved by employing amino-acids. Possible energy transfer between Si nanocrystallites and amino-acid molecules in the glass has also been investigated.