Improvement mechanism of photoluminescence in iron-passivated porous silicon

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Abstract

Taking into account the H2 generation during porous silicon (PS) formation, a special hydrothermal etching process was designed and carried out in HF aqueous solution containing Fe3+ ions. Several chemical reactions were found to take place on the surface of Si nanocrystallites, iron ion termination of Si dangling bonds, and formation of an outer layer of α-Fe2O3, which result in a stronger and more stable photoluminescence (PL) band peaking at 670 nm in comparison with normal PS. The improved PL was interpreted in terms of the decrease of the density of Si dangling bonds due to iron ion termination, and outer α-Fe2O3 layer protecting the inner c-Si from being oxidized.

Introduction

Porous silicon (PS) emits efficiently visible light [1] and this has aroused many theoretical and experimental studies, investigating the physical and electronic properties and exploring optoelectronic device applications [2]. The quantum confinement effect due to a reduction of the crystal size has been taken as the most possible mechanism, because the blueshift of the optical band gap determined by the optical absorption spectrum and photoluminescence excitation (PLE) spectrum can be well explained by the quantum model [3], [4]. Theoretical calculations also show the band gap of silicon is likely to be shifted into the visible region when the size is decreased to a few nanometers [5], [6], [7]. However, the pure quantum model fails to account for some PL features in PS. For example, the blueshift of the PL peak energy is not sensitive to the decreased size in comparison with that of the optical band gap. It is considered that the interfacial region plays a role in the size-insensitive luminescence. The nonexponential behavior of the luminescence decay dynamics also suggests that the luminescence is sensitive to the surface structure [8], [9], [10], [11]. The above results make people believe that the photogeneration of carriers occurs in the c-Si core, whose band gap is modified by the quantum confinement effect, and the radiative recombination is controlled by the surface structure, which is sensitive to the sample preparation and drying conditions.

It is well established that the relatively clean fluorinated hydride surface of the freshly etched layers is slowly converted to a contaminated native oxide during exposure to ambient air [12]. Experiments also show the luminescent properties of PS dependent on the way in which the material is dried after fabrication because the composition and structure of the native oxide developed can also be highly variable [13], [14]. Both ambient air aged and thermally oxidized PS show blue and green luminescence, for example, but the luminescent properties of these samples are quite different from each other [15]. These results clearly show that the structure configuration of the silicon oxide influences the carrier recombination process, and should be responsible for the largely variable PL properties reported. To understand well the mechanisms of radiative transitions in PS and make use of this material in a practical device, it is necessary to modify its surface structure. Metal-passivation on the surface of Si crystallites has been reported by several groups [16], [17], [18]. However, normal passivation method such as sputtering of certain metals after erosion does not easily result in the bonding of Si dangling bonds with metal ions, and the formed metal layer, such as copper, accelerates rather than preventing the diffusion of excited carriers for nonradiative decay in nearby defects, which makes the luminescent process more difficult to understand. It is the purpose of this work to utilize a by-product of H2 in Si erosion reaction to help in situ passivate Si dangling bonds with iron ions and construct an iron oxide surface layer on the Si nanocrystallites to protect Si from oxidation.

Section snippets

Experimental

For the present experiments, P-type (heavily boron-doped, 0.05 Ω cm) (0 0 1) oriented silicon wafers were fixed in the bottom of a Teflon vessel. The solutions of 10 mol/l HF in 0.1 mol/l Fe(NO3)3 were added to the vessel until 70% of its volume was filled, then the vessel was placed in a stainless steel tank to perform hydrothermal treatment. Most of the hydrothermal erosion was performed at 140°C for 1 h. After the erosion process all the samples were dried in air. Microstructure of the porous

Results and discussion

Fig. 1 shows PL spectra from the as-prepared PS under 256 nm excitation at room temperature as a function of storing time in air. In the freshly prepared sample, a broad PL band peaks at 670 nm. No blueshift arises from simply storing luminescent PS in ambient air at room temperature, as shown in Fig. 1. Further, efficiencies are observed to increase. For comparison, normal PS was also prepared by the same technique and same condition just without the addition of Fe3+ ions in the erosion

Conclusions

Comparing with normal porous silicon, the iron-passivated PS exhibited PL stability both in intensity and peak position. Studies show the designed process can provide a way for the termination of Si dangling bonds, serving as nonradiative centers, with Fe2+ and Fe3+ ions. The elementary iron, formed on the surface of PS by the reduction of Fe3+ with H2 could be further oxidized to α-Fe2O3 during exposure to air, which can protect Si nanocrystallites to be oxidized. These reactions result in the

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