Non-constant anodization current effects on spectra of porous silicon LEDs G. Barillaro a, *, A. Diligenti a , M. Piotto b , M. Allegrini c , F. Fuso c , L. Pardi c a Dipartimento di Ingegneria dell’Informazione, Universita ` di Pisa, Via Diotisalvi 2, Pisa 56126, Italy b Centro di Studio per Metodi e Dispositivi per Radiotrasmissioni, Via Diotisalvi 2, Pisa 56126, Italy c INFM, Dipartimento di Fisica ‘‘Enrico Fermi’’, Universita ` di Pisa, Via Buonarroti 2, Pisa 56127, Italy Received 6 November 2002; accepted 23 November 2002 Abstract Porous silicon (PoSi) LEDs are today under investigation for integration of optoelectronic silicon devices with standard microelectronic circuits. The electrical and optical properties of these devices depend on the anodization parameters (current density and time) of the PoSi formation process. However, a constant anodization current is generally used to fabricate the active PoSi layer of LEDs, and only few works exist in which a non-constant anodization current is reported. In this work, a study of the anodization current effects on the electroluminescence (EL) spectra of PoSi LEDs having around 0.1% of external quantum efficiency is presented. Several anodization current waveforms (constant, linear, triangular, and trapezoidal) were used to fabricate layers with different mechanical and optical properties. EL spectra of fabricated devices are reported and discussed. # 2003 Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Light-emitting diode; Electroluminescence spectra 1. Introduction Silicon-based optoelectronic devices represent today an active field of research due to the possibility of on- chip integration with standard microelectronic circuits. Porous silicon (PoSi) and other light-emitting silicon- based materials contain nanostructures or crystallites in the nanometer range, that modify the optical properties of crystalline silicon by quantum confinement of elec- trons and holes in the small volume of nanocrystals. As a matter of fact, PoSi layers can efficiently emit light in the visible part of electromagnetic spectrum at room temperature when excited with photons (photolumines- cence) or electrons (electroluminescence (EL)). PoSi light-emitting diodes (LEDs) are probably the most attractive application of PoSi (for a review, see [1]), although several other optical devices, like detectors, wave guides, and switches are under investigation [2,3]. In less than 10 years, significant progress has been achieved: LEDs emitting throughout the visible spec- trum have been fabricated and the best measured external quantum efficiency (EQE) at room temperature has risen from 10 5 % for early Schottky-like devices [4] to 0.1% for the next generation p  /n junction devices [5]. Electrical and optical characteristics of these devices basically depend on the fabrication process as well as the properties (morphological, electrical, and optical) of PoSi, which in turn depend on the anodization process parameters (current density, HF concentration, tem- perature, etc.). Although the fabrication processes of LEDs can be quite different, the PoSi layer is generally produced by means of anodization in HF solution using a constant current density with a proper value in order to obtain a higher porosity and then a better EQE. In this work, several different anodization current waveforms (constant, linear, triangular, and trapezoi- dal) were used to fabricate PoSi layers with different optical properties in an LED structure fully compatible with an industrial process. The effects of constant and non-constant current anodization on the EL spectra of LEDs are presented and discussed. * Corresponding author. Tel.:  /39-050-568657; fax:  /39-050- 568522. E-mail address: g.barillaro@ing.unipi.it (G. Barillaro). Materials Science and Engineering B101 (2003) 266  /269 www.elsevier.com/locate/mseb 0921-5107/03/$ - see front matter # 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-5107(02)00694-3