Comparative SEM and Cathodoluminescence Microanalysis of Porous GaP Structures
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Comparative SEM and Cathodoluminescence Microanalysis of Porous GaP Structures M.A. Stevens-Kalceff,1 S. Langa,2 I.M. Tiginyanu,2 J. Carstensen,3 M. Christophersen,3 and H. Föll3 1 Department of Applied Physics, University of Technology, Sydney, Australia 2 Laboratory of Low-Dimensional Semiconductor Structures, Technical University of Moldova, Chisinau, Moldova 3 Materials Science Department, Christian-Albrechts University, Kiel, Germany ABSTRACT Electron microscopy and cathodoluminescence (CL) microanalysis were used for a comparative study of porous layers fabricated by electrochemical etching of n-GaP substrates in a sulfuric acid solution. Both the CL and morphology of porous layers were found to depend upon the anodic current density. At high current density (100 mA/cm2) anodization leads to the formation of so-called current-line oriented pores and an increase in the CL intensity. We observed selfinduced voltage oscillations giving rise to a synchronous modulation of the diameter of pores and CL intensity. When the current density decreased to values as low as 1 mA/cm2 the pores began to grow along crystallographic directions and the CL intensity was observed to be lower than that of bulk GaP. INTRODUCTION Porosity is an effective tool for engineering basic parameters of semiconductor materials [1]. Compared with porous Si, III-V materials have a number of important advantages related to the possibility of changing the chemical composition and further extending the fields of applications of porous structures using properties specific to acentricity. In particular, porous III-V compounds were found to exhibit Fröhlich-type surface-related vibrations with porosity-tunable frequencies and efficient optical second harmonic generation [2-5]. The enhanced nonlinear optical response and intense luminescence reported for porous III-V compounds may enable the development of a fully integrated light source and frequency converter sub-system. So far, most experiments investigating emission characteristics of porous III-V materials have been restricted to photoluminescence (PL). The PL of porous GaP, GaAs and InP at energies above the band gap of the bulk material has been attributed to quantum size effects [610]. Furthermore, a porosity-induced intensification of the near-band-edge emission was observed in gallium phosphide [11]. However, no correlation between luminescence properties and morphology features of porous III-V material has been reported yet. In this work, we study the morphology and CL characteristics of porous layers obtained by electrochemical dissolution of n-GaP substrates. We report, for the first time, drastic modification in both morphology and CL intensity of porous gallium phosphide layers with changing anodization conditions. Self-induced voltage oscillations were observed during anodic etching at high current density and their impact on pore morphology and CL intensity was evidenced. The results of the CL microanalysis indicate the possibility of controlling the spatial distribution of emission
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