Elsevier

Thin Solid Films

Volume 325, Issues 1–2, 18 July 1998, Pages 271-277
Thin Solid Films

Electrical behaviour of fresh and stored porous silicon films

https://doi.org/10.1016/S0040-6090(98)00429-5Get rights and content

Abstract

We have measured I–V and C–V characteristics, the temperature dependence of dark currents, and thermally stimulated depolarisation currents on fresh and stored samples of photoluminescent porous silicon. By storage in ambient, the low rectifying I–V curves become strong rectifying, and C–V curves become MIS-like. I–T characteristics for fresh samples have only one activation energy, in the 0.49–0.55 eV range. After storage, a slightly modified value, of about 0.50–0.60 eV is observed at low temperatures only. At about 280 K, the activation energy suddenly changes to 1.20–1.80 eV. Also, both the number and the positions of maxima in thermally stimulated depolarisation currents change by storage. The annealing at about 50°C induces small reversible changes in I–T characteristics and strong irreversible ones in thermally stimulated depolarisation currents, both for fresh and stored samples. A simplified quantum confinement model is proposed to explain the main aspects of the electrical behaviour of porous silicon films. The surface and/or interface contributions are observed especially in thermally stimulated depolarisation currents. The changes induced by storage are attributed to the oxidation process of the internal surface of 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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