Photoluminescence of porous silicon stain etched and doped with erbium and ytterbium

https://doi.org/10.1016/j.physe.2008.09.015Get rights and content

Abstract

A novel low cost process has been developed for application in porous silicon-based photodetectors and solar cells, where conventional doping processes are not affordable because of the high processing cost and technical difficulties. Ytterbium and erbium (Yb3+–Er3+) ions were introduced into luminescent porous silicon stain etched by thermal diffusion. Doping profiles were evaluated by energy-dispersive spectroscopy analysis. The visible and near-infrared photoluminescence of Yb3+–Er3+ co-doped stain-etched porous silicon layers is observed and evaluated under 980 nm pumping. Up-conversion processes that could improve the efficiency of silicon-based solar cells are detected.

Introduction

Rare-earth-doped materials have been studied during several years due to their promising applications in optoelectronic devices [1], [2], [3]. One of the most considered hosting material has been silicon because of its relevance in the microelectronic and telecommunications industries [4].

Also, the rare-earth-doped materials have been considered in the photovoltaic industry because of the optical down-conversion [5], up-conversion [6] and down-shifting processes [7] introduced and the potential increase of the photoconversion efficiency that these processes could represent applied to solar cells. However, it is quite challenging to obtain a cost effective rare-earth doping process in silicon.

One of the most-promising alternatives to the conventional silicon doping procedures is to use porous silicon layers (PSLs) [8] as rare-earth host, because of its large surface area, allowing easy penetration of the ions into the matrix and the oxidation of the porous layers, necessary for the ion activation [9].

Rare earths and, in particular, erbium can be introduced into PSL by ion implantation [10], electrochemical migration [11] or spin-on technique [12]. However, in order to implement rare earths doping processes in conventional solar cell production lines, it is necessary to avoid high processing costs (ion implantation), technical difficulties (epitaxy) or long-processing times due to specific chemical processes (sol–gel) or vacuum systems (CVD and sputtering).

The thermal diffusion process could be seen as the logical low-cost alternative to dope silicon-based solar cells with rare earths in solar cell production lines without the need of extra or large steps. Nevertheless, erbium [13], [14] and ytterbium [15] exhibit a very low diffusion rate in silicon and, consequently, very high temperatures and long diffusion times are required to achieve the desired concentrations profiles.

In the literature, several authors refer to Yb3+ co-doped porous silicon [16], [17], as an adequate partner for Er3+ in order to enhance the low absorption cross-section that shows the Er3+ ion at 980 nm. Nevertheless, the literature related to the possibilities of doped PSLs stain etched is scarce. In previous works, no transitions lines were found in the near-infrared when doping with Er3+ ions [18].

The objective of this work is to study the photoluminescence (PL) of stain-etched PSLs co-doped with erbium and ytterbium to detect up-conversion processes that could be applied in silicon-based solar cells to enhance the photoconversion efficiency.

Section snippets

Experimental

The PSL were formed by immersion of p-type Cz Si (1 0 0) substrates from Deutsche Solar, with resistivity between 0.8 and 1.2 Ω cm, in an aqueous HF/HNO3 solution. In order to assure the homogeneity of the porous surface, low concentrations of HNO3 in HF were used [19]. The resulting porous surface was characterized by means of Micromeritics ASAP 2020 surface area and a porosimetry system. The maximum pore size was determined by (Brunauer, Emmet and Teller) BET analysis [20] to properly adjust for

Results and discussion

To find the PSLs which would act as suitable doping host, low HNO3 concentrations in HF, varying the etching time from 15 to 60 s were evaluated [19]. Table 1 shows porosity and thickness of the layers calculated by nitrogen adsorption of the samples, while Table 2 shows the calculated BET parameter. It was found that the maximum pore size was obtained for etching times larger than 30 s. Some measurements were also taken over 60 s etching time (not presented), but the pore size did not show a

Conclusions

A simple low-cost method is used to dope with Er3+ and Yb3+ ions PSLs through immersion of these PSLs into a nitrate solution of erbium and ytterbium in ethanol (Er (NO3)3:Yb(NO3)3: C5 H5OH). The complete incorporation of Er3+ and Yb3+ is confirmed by EDS measurements. A systematic study shows that the pore size is optimal at 60 s etching time. After the thermal treatment the measured visible and near-infrared PL spectra of the PSLs show up-conversion processes which are clearly favoured by the

Acknowledgement

This work has been supported by the Ministerio de Educación y Ciencia (Project no. ENE2007-60720/ALT) and the Focus-Abengoa Foundation.

References (23)

  • S. Libertino et al.

    Mater. Sci. Semicond. Proc.

    (2000)
  • A.J. Kenyon

    Prog. Quantum Electron.

    (2002)
  • A. Irrera et al.

    Physica E

    (2007)
  • S. Tanabe

    J. Alloy Compd.

    (2006)
  • C. Strümpel et al.

    Sol. Energy Matter Sol. Cells

    (2007)
  • T. Trupke et al.

    Energy Mater. Sol. Cells

    (2006)
  • V. Svrcek et al.

    Thin Solid Films

    (2004)
  • N.V. Gaponenko et al.

    J. Lumin.

    (2006)
  • Stephen Roberts et al.

    Mater. Lett.

    (1995)
  • E. Snoeks et al.

    Opt. Mater.

    (1996)
  • B. González-Díaz et al.

    Mater. Sci. Eng.: B

    (2008)
  • Cited by (15)

    • Synthesis of gold nanoparticles chemically doped with porous silicon for organic vapor sensor by using photoluminescence

      2018, Optik
      Citation Excerpt :

      We can dope AuNPs with PS for two reasons. Open structures allows to doping and large surface area allows easy penetration into the PS [23]. Doping changed the optical properties of PS where PL is achieved with higher intensity where AuNPs will fill up pores of PS that lead to enhanced PL [24].

    • Decalin-assisted light emitting porous Si formation and its optical, surface and morphological properties

      2017, Applied Surface Science
      Citation Excerpt :

      Si in bulk form is a poor light emitting material due to its indirect band gap [1]. The discovery of the luminescent porous silicon [2,3] at room temperature has prompted many research groups [4–15] to investigate a variety methods to produce light emitting porous silicon (Lep-Si) specimens for various applications. Lep-Si samples have found numerous applications including LEDs [11], solar cells [16–19], sensors [20–22] and hydrogen production [23].

    • A new cost-effective polymeric film containing an Eu(III) complex acting as UV protector and down-converter for Si-based solar cells and modules

      2015, Solar Energy Materials and Solar Cells
      Citation Excerpt :

      This result suggest a potential application of down-converters in concentrated PV where light traps are used, collecting most of the isotropic down-converted photons to the solar cells and, thus, enhancing the EQE in the UV spectral range. On the other hand, most works considering the application of down-converters in solar cells normally do not estimate the increment in cost of this additional procedure [10,13], do not explain how these costs have been calculated [20] or are so much costly that they are not currently applicable in the solar cells industry [21]. We have calculated the cost of the luminescent film in order to evaluate its commercial viability.

    • Combined up conversion, down conversion and down shifting photo-luminescence of low cost erbium-ytterbium co-doped porous silicon produced by stain etching

      2011, Thin Solid Films
      Citation Excerpt :

      Several authors refer to ytterbium as the adequate partner for erbium in order to enhance the poor absorption cross-section that shows the erbium at 980 nm [20–22]. Furthermore, the observed quenching effect due to the auto absorption of the Er ions and the presence of OH− ions [23,24] has not been detected when co-doping with ytterbium. The implementation of the rare earth ions in silicon can be carried out by ion implantation [25], electrochemical migration [5], chemical vapor deposition [26], spark processing [27] and even sol–gel processes [28].

    View all citing articles on Scopus
    View full text