Tunable photonic strength in porous GaP

Abstract

The light-scattering properties of porous gallium phosphide, prepared by electrochemical etching, are investigated. We show that the photonic strength of the porous semiconductor can be tuned from weak to extremely strong. This tunability is related to the density and size of the pores, which are controlled by the dopant density of the GaP crystals, and the etching potential. Moreover, electrochemical etching does not introduce any significant optical absorption, which makes porous GaP suitable for many photonic applications. © 2002 American Institute of Physics. ͓DOI: 10.1063/1.1485316͔Photonic materials interact strongly with light as a result of a variation of the refractive index n on length scales of the order of the optical wavelength . The field of photonic materials has undergone a spectacular growth 1 due to the wide range of applications that they have: efficient light-emitting diodes, low-threshold lasers, microcavities, waveguides, and fast optical switches. In addition, photonic systems exhibit fascinating fundamental phenomena, such as inhibition of spontaneous emission and light localization.Pore formation by means of electrochemical etching has emerged as a very promising technique for tailoring the photonic properties of semiconductors. Most electrochemical studies have focused on silicon ͑Si͒ ͑Ref. 2͒ rather than on GaP. Si is transparent only for infrared radiation Ͼ1.1 m. In contrast, gallium phosphide ͑GaP͒ is transparent for light in the yellow and red part of the visible spectrum Ͼ0.55 m. This absence of optical absorption, together with its very high refractive index (nϭ3.3), 3 makes GaP a fascinating material for optical applications in the important wavelength range, 0.55 mϽϽ1.1 m, a range where Si shows strong absorption. In spite of this advantage, up until now, relatively little work has been performed on porous GaP. 4 Only recently, a first study of the photonic strength of porous GaP was reported:5 strong scattering of visible light without optical absorption was measured.In this letter, we show that it is easy to tune the photonic strength of porous GaP in a wide range by controlling the density and size of the pores. This tailoring can be achieved by using n-type GaP crystals with different dopant densities and by varying the etching potential. Moreover, since no optical absorption could be detected in any of the samples, porous GaP is suitable for photonic applications in the visible spectrum: these include diffuse reflectors, Bragg reflectors, and random lasers.The photonic strength is defined in terms of the transport mean free path l , which is the average length required to randomize the direction of propagation of the light by scattering. A small value of l corresponds to efficient scattering or high photonic strength. The transport mean-free path is given bywhere is the density of scatterers and is the transport cross section. 6 In our samples the pores are the scatterers, so the density of scatterers is determined by the pore density. For a given refractive-index constras...

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