Origin of the Infrared Band From Porous Silicon
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ABSTRACT
The photoluminescence (PL) infrared (IR)-band of p-doped porous Si (PS) films is studied by steady-state and time-resolved PL and by photoluminescence excitation (PLE) in detail. In analogy to the S-band in the visible the IR-band shifts to higher energies with reduced average nanocrystal size. The IR- and S-bands are very different in their decay behavior and in their recombination lifetimes. The temperature-dependent PL intensity shows non-exponential decay with lifetime distributions in the nsec-)Asec range in contrast to the stretched exponential decay shape of the S-band corresponding to lifetime distributions in the jusec -msec range. The origin of the IR-band is likely related to radiative recombination at deep defects in Si nanocrystals with quantum-upshifted band gaps. INTRODUCTION
The discovery of efficient PL in Si nanostructures has stimulated enormous interest towards the fundamental understanding of its origin and the possible application in optoelectronics [1]. Intense research in recent years has concentrated on the strong PL S-band at 1.1 eV - 2.0 eV [25] of oxidized Si nanoclusters and PS. Today it is generally accepted that the bulk of experimental and theoretical data is only consistent with models based on quantum confinement in surface-passivated silicon nanocrystals. At low temperatures PS samples of different doping types, doping concentrations and porosities show a broad low-energy band clearly separated from the broad high-energy S-band. In the following the low-energy band will be labeled the Infrared (IR)-band. Although Pickering et al. [6] in 1984 reported about a broad band between 0.8 eV and 1 eV for the first time, only few papers on this IR-band of PS appeared in the literature [7-9]. Pickering et al. attributed the broad bands at 0.8 eV, 0.95 eV and 1.0 eV - 1.1 eV to defects deep in the band gap, to dangling bond defects, and to defects associated with oxygen, respectively. In 1993 Koch [8] suggested the radiative capture of free electrons by a positively charged dangling bond state as the origin of the IR-band. In this paper we present a detailed study of IR-band properties. Investigated are the dependence of the IR-band on excitation energy, on the average nanocrystal size, on temperature and on oxidation treatment; the focus is on the time-dependence of this infrared recombination radiation. EXPERIMENTAL
PS films were prepared by anodic etching of boron-doped p-(100) Si wafers of 0.01 0cm and 10 Qcm resistivity in ethanoic HF solutions under dark conditions. The current density (5..200 mA/cm 2) and HF concentrations (15..45 %) were adjusted so as to yield films of 45..80 % porosity. After etching the samples were rinsed in ethanol and dried slowly in an air/ethanol atmosphere. For PL experiments from 4.2 K to room temperature we placed the samples in a LHe flow type cryostat where the porous layers were excited by the blue 457.9 nm line of an argon ion laser. A Xe arc lamp mounted to a 1/4 m monochromator served as the excitation source in the wavelength range from
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