Simultaneous Detection of Radiative and Non-Radiative Recombination in Porous Silicon
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ABSTRACT Radiative and non-radiative recombination in porous silicon (PSi) is measured under a wide injection range by detecting of photoluminescence (PL) and free-carrier absorption (FCA) decay. We used 2.34 eV and 140 ns light pulses for carrier excitation in PSi flakes. The excited carriers were probed by focused cw IR light and the carrier concentration was calculated by using the free-carrier absorption cross section of bulk (c-Si). The results demonstrate a scaling between the total PL yield in the S-band, integrated over wavelengths, and the free carrier concentration. At lower injections the observed free carrier and PL decay follow a similar, density-independent stretched-exponential law. At injections approaching 10"8 cm"3 , a fast recombination component appears reducing the lifetime by a factor of ten. This component is attributed to Auger recombination of separated e-h pairs on a set of Si subclusters. INTRODUCTION The strong PL emission from a visible (S-band) of PSi is characterized by a long radiative lifetime in the micro to millisecond range. The detailed recombination mechanism is not clear yet. An excitonic recombination model where exciton states, localized in a quantum-size structure, are split by strong exchange interaction into a spin singlet and a lower energy spin triplet can explain the low-temperature dependence of the radiative lifetime [1,2]. On the other hand, a surface-state mediated recombination, where excited carriers recombine by radiative tunneling between extended states can explain the typically observed stretched-exponential decay character of PL [3-6]. The carrier tunneling through the surface barriers to the effective non-radiative centers of the dangling bonds is believed to be responsible of PL yield and for the radiative lifetimes above 250 K [5]. So far, the intense experimental work on the PL has mainly been based only on PL studies. In this paper we continue our former study [7] of the simultaneous detection of PL dynamics in the S-band and free-carrier absorption. We focus on the injection dependence of PL and free carrier decays. Our results give insight into the nature of the radiative S-band and non-radiative states involved in PSi. EXPERIMENTAL PSi films have been anodically prepared from medium doped p- or n-type c-Si substrates in HF-ethanol-aqueous (1:1:1) solutions using illumination from a tungsten lamp for n-substrates. Free-standing flakes of 2 mm 2 typical size and 30 gim thickness were produced by abruptly increasing the current to 150 mA/cm 2 at the end of the etching period. The samples have been analyzed by scanning electron microscopy and with an optical microscope [7,8]. Samples of 543 Mat. Res. Soc. Symp. Proc. Vol. 358 01995 Materials Research Society
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