Large current density and anodization time needed for strong photoluminescence in porous silicon

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Abstract

We investigated the relation between anodization conditions and photoluminescent characteristics including spatial variations. The current density was between 7.9 and 39 mA cm−2, and the anodization time between 10 and 60 min. We clarified a generous tendency as follows: as the current density or the anodization time increases, the edge of the anodized area gives the stronger photoluminescence at first, and then the center also gives the stronger photoluminescence. In addition, we reported the difference between luminescing and non-luminescing porous silicon, from the aspects of photoluminescent, photoacoustic, and Raman spectra.

Introduction

Porous silicon attracts much attention as a light-emitting material formed from silicon [1]. Until now many experimental results have been reported which mainly suggested the mechanism of light emission or the relation between the porous structure and photoluminescent (PL) characteristics. Electroluminescence has been observed in porous silicon [2] and a light-emitting diode was fabricated from porous silicon [3].

The obstacle to the realization of light-emitting devices with porous silicon is partly related to poor reproducibility of the material. The electrical or optical properties of porous silicon are affected by such experimental conditions in forming porous silicon as anodization time, current density, composition of electrolyte, resistivity and the conduction type of silicon wafer, and irradiation of light during anodization. The post-anodization chemical etching is often performed to enhance the PL intensity.

Another problem is the difficulty in forming a uniform layer of porous silicon in the anodized area. Fujiwara et al. investigated the variation of PL structure depending on the position from the meniscus in laterally anodized porous silicon [4]. The spatial variation is not yet studied well.

We investigated how PL intensities depend on the current density and anodization time including the spatial variations. In addition, we compared Raman and photoacoustic (PA) spectra as well as PL spectra obtained from luminescing and non-luminescing porous silicon and discuss the differences between them.

Section snippets

Experimental details

We prepared porous silicon samples by anodization of silicon wafers in a solution (100 ml) of HF(50 wt.%):ethanol (1:1). The silicon wafer was (100) oriented p-type one whose conductivity was 1–10 Ω cm. The wafer was put at the bottom of the cylindrical container of an electrolyte in such a way that a hole at the center of the bottom was covered with the wafer from the outside. Current density was varied from 7.9 to 39 mA cm−2 and the anodization time from 10 to 60 min. The anodized area (2.54

Results

The dependence of photoluminescence on current density and anodization time are summarized in Table 1. It is difficult to show the results quantitatively because the data includes a spatial distribution based on a poor reproducibility. We then divided the results in four ranks as described below. Open and closed triangles denote that the edge of the anodized area provided weak and strong photoluminescence, respectively. We observed little photoluminescence at the center in these cases. Open and

Discussion

It is definitely important to make clear the correlation between anodization conditions and PL properties of porous silicon. In most cases, PL characteristics are not uniform over spatial positions. We indicated the rough results of PL observation in Table 1. PL results in Table 1 were classified in four groups, which was based on the fact as follows. As the anodization time becomes longer, the PL intensity increased in the edge area first, and then the center becomes luminescing after the PL

Conclusion

We investigated the dependence of photoluminescent characteristics on current density (7.9–39 mA cm−2) and anodization time (10–60 min.) during anodization. Larger current density or longer anodization time roughly results in stronger photoluminescence. The formation of porous silicon begins at the edge of the area at first, and then proceeds to the center. It is possibly because current density is not uniform over the formed area. The control of current distribution during anodization is

References (6)

  • M Ohmukai et al.

    Thin Solid Films

    (1997)
  • P.M Fauchet
  • F Namavar et al.

    Appl. Phys. Lett.

    (1992)
There are more references available in the full text version of this article.

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