Preparation and Characterization of Freestanding Porous Silicon Films with High Porosities

Abstract

We have prepared noncollapsed freestanding porous silicon (ps) films with various porosities higher than 90% using electrochemical etching-electropolishing, chemical dissolving, and supercritical drying methods. For the first time, the optical absorption of ps films with porosities higher than 80% have been obtained in transmission measurements. A blue shift of the transmission curve and a sharp increase of the photoluminescence (PL) intensity with enhanced porosity have been observed. An experimental result for decreasing the average diameter of the Si crystallites in the ps film with increasing porosity has been obtained by Raman scattering measurement. No notable dependence of the PL peak energy of the ps film upon decreasing the sizes of the Si crystallites or upon increasing the porosity is contrary to the prediction of the quantum confinement model for the PL of ps.The strong visible light emission from porous silicon (ps) has received a great deal of attention due to its potential applications in optoelectronics. 1 Up to now, the ps samples with photoluminescence (PL) covering the entire visible region and extending to the near IR and near UV region have been obtained. [2][3][4] Recently, ps-based light emitting diodes (LEDs) with efficiencies greater than 0.1% and LEDs integrated into a microelectronic circuit have been reported. 5 However, the origin of the PL is still controversial. 1,6-11 Freestanding ps films allow direct measurement of the characteristics, especially optical absorption 12,13 of porous layers and transport properties, 14,15 and allow a comparison PL with optical absorption, 16 without any influence from the Si substrates. Thus, a study of freestanding ps films may help us in understanding the microstructure of ps and the origin of the PL. In addition, the freestanding ps films have characteristics such as large surface-to-volume ratio, highly porous structure, and low index of refraction. These properties suggest other potential applications such as filter, catalyst supports, chemical sensors, and antireflection coatings in solar energy cells for increased photovoltaic efficiency. 17,18 Only freestanding ps films with porosities below 80% have been reported, 12,13 probably because the skeletons of the ps films with high porosities are partially destroyed by capillary forces under natural drying. The supercritical drying process can avoid the liquid-vapor phase boundary and thereby eliminate the structural collapse caused by capillary forces. This technique has been used successfully to prepare a variety of low-density aerogels 19 and ps with high porosity. [20][21][22] However, this process has never been used in freestanding ps films. In this paper, we demonstrate noncollapsed freestanding ps films with porosities higher than 90% using the anodization-electropolishing and supercritical drying processes. The structural and optical characterization of these films is presented.Experimental The substrates used were (111)-oriented p-type silicon wafers with resistivity of 0.01-0.015 Ω cm....

Optical absorption property of oxidized free-standing porous silicon films

“…Meanwhile, we also observed that the transmission at the low energy (the plateau region), T 0 , increases with porosity. The blue-shifts of the transmission are correspondent to the results given elsewhere [20][21][22][23][24][25]. Figure 2 shows four Raman spectra of the free-standing p PS films with porosities of 58, 68, 79, and 87%.…”

“…Free-standing PS films with porosities higher than 80% were prepared by the electrochemical etching-electropolishing, chemical dissolving, and supercritical drying method. A detailed description of the experimental procedure and apparatus are given in previous papers [24,25]. All of the free-standing PS films samples were oxidized in air at 200 °C for 40 or 200 h accumulatively.…”

“…Behren et al [23] demonstrated unambiguously and quantitatively the role of quantum confinement in the optical properties of PS with porosities up to 92%. Recently, we have compared the PL spectra with the absorption spectra of PS samples with porosities in a wide range [24][25][26]. A continuous blue-shift of the transmission curves of PS samples with increasing porosity from 41 to 94% were also observed, but there was no correlation between the PL with optical absorption spectra of PS [25].…”

Influence of Etching Current Density on Visible Electroluminescence from Porous n‐Si under Cathodic Bias

“…13 A study of the EL of porous Si as a function of time at a constant potential shows a redshift, 11 even though oxidation of the hydride surface of porous Si during the EL process reduces the dimension of porous Si skeleton. A more recent paper 14 shows that no notable dependence of the PL peak energy of the porous silicon has been observed upon decreasing the size of the silicon crystallites from 5.0 to 2.6 nm. To understand these recent studies, mechanisms related to the surface chemical species such as silicon hydrides have been proposed.…”

“…The procedure was described in detail elsewhere. 14 All n-Si wafers were photoelectrochemically etched at different current densities under 80 mW/cm 2 illumination for 10 min. The porous n-Si electrode was immersed in a three-electrode, onecompartment quartz cell with a Pt coil and an Ag|AgCl reference electrode and its EL measured in 0.15 M Na 2 S 2 O 8 , 0.20 M Na 2 SO 4 , and 3.5 M C 2 H 5 OH using an Aminco Bowman series 2 luminescence spectrometer and an EG&G Princeton Applied Research Corporation model 273 potentiostat/galvanostat unit.…”