Electrical behaviour of fresh and stored porous silicon films
Introduction
The preparation of porous silicon (PS) films with efficient photoluminescence (PL) in the visible range at room temperature (RT) 1, 2has aroused a great interest in this material, because the main objective is to fabricate high quality optoelectronic devices that can be integrated with silicon microtechnology.
Photoluminescent PS films obtained by anodization consist of an interconnected network of small Si crystallites with sizes of few nm. The quantum efficiency of the PL of PS films is quite high (≅1%–10%) [3], while the efficiency of the electroluminescence (EL) is less than 0.01%. This low value reported on metal/PS [4], ITO/PS [5]and on p–n junction devices [6]has been previously attributed to a poor contact at the surface of PS. Later, the use of conductive polymers has not led to a significant improvement in the EL efficiency [7].
An already accepted model for the PL of PS films is that of quantum confinement of carriers inside crystallites that results in an increased band gap 1, 2. It has not been clear up to now whether this model is necessary to explain the electrical transport properties. Besides, PS films have a very large specific surface (≅1000 m2/cm3) and thus the surface contribution in electrical as well as optical properties must be taken into consideration 3, 8, 9, 10.
The electrical properties of PS films were investigated using sandwich configurations such as Schottky devices and p–n junctions. Different results, depending on the preparation conditions, were reported in the literature 11, 12, 13, 14, 15.
Thus the electrical conduction of a self-supporting PS film shows a hopping behaviour at low temperatures and a thermal-activation process at high temperatures [11]. In Ref. [12]the observed electric-field-enhanced conduction is explained by a Poole–Frenkel type mechanism. The analysis of the frequency dependence of the conductivity and of the dielectric constant strongly suggests that the low-frequency regime is governed by the fractal properties of PS and the high-frequency dispersion is due to a broad distribution of activation energies [13]. The reverse current, dependent on humidity in a M/PS structure, is discussed considering the generation-recombination process in the PS depletion region [14].
In the present paper, the electrical behaviour of fresh samples, as well as those stored under ambient conditions is investigated. The current–voltage (I–V) characteristics, the temperature dependence of the dark current, and thermally stimulated depolarisation currents (TSDC) are analysed. All data were taken on capacitance–voltage (C–V) characterised samples.
A simplified quantum confinement model is proposed. It provides a good explanation for the electrical properties of our fresh and stored PS films.
Section snippets
Experimental aspects
PS layers of approximately 25–35 μm thickness were prepared on (100) p-type wafers of crystalline Si (c-Si) with resistivity of 5–10 Ω cm. To improve the homogeneity of PS films, Al ohmic electrodes were prepared on the backside of the wafers by thermal evaporation followed by a sinterisation process at 450°C for 30 min. The wafers were electrochemically etched in dark, in an HF (49%)–C2H5OH electrolyte (with a volume ratio of 1:1) using a current density of 5–10 mA/cm2. After the etching, the
Fresh samples
At RT, our samples present a slow rectifying behaviour in the −30–+30 V range (Fig. 1), the I–V characteristics being practically linear in the −5–+5 V interval, in good agreement with the literature [12].
The I–T characteristic was recorded in the temperature range 150–300 K (Fig. 2) and exhibits only one activation energy Efdl whose value lies in the interval 0.49–0.55 eV. The value obtained for the activation energy is independent of the bias polarity at low voltage. At low bias, the injected
Analysis and discussion
PS films with high PL are formed by a network of small crystallites, wires and/or dots. The main crystallite size at the surface of our films, evaluated from transmission microscopy measurements [18]is of the order of 2.5–7 nm (Fig. 7). As we already mentioned in Section 1, the main contributions to the characteristics of the electrical transport properties in crystallites having the size in this range are determined by the quantum confinement and surface effects.
In the following, we will
Conclusions
In this paper we have shown that the main contribution to the electrical transport mechanism is given by the quantum confinement effects. The experimental data are well described by a 2D infinite quantum well model. This permits to explain the activation energies of dark and TSD currents by means of the discrete energy levels introduced into the longitudinal band gap. The surface effects are particularly observed in TSDC.
The stabilisation of the electrical properties of stored PS films is due
Acknowledgements
The authors wish to thank Prof. R. Grigorovici (Romanian Academy), Prof. A. Goldenblum (National Institute of Materials Physics, Bucharest) and Prof. A. Yelon (Ecole Polytechnique de Montreal) for helpful discussions. Prof. V. Teodorescu and Prof. L. Nistor from the National Institute of Materials Physics, Bucharest, are also acknowledged for measurements and discussions regarding TEM.
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