Characterization of Er in porous Si
© Mula et al.; licensee Springer. 2012
Received: 26 April 2012
Accepted: 29 June 2012
Published: 9 July 2012
The fabrication of porous Si-based Er-doped light-emitting devices is a very promising developing field for all-silicon light emitters. However, while luminescence of Er-doped porous silicon devices has been demonstrated, very little attention has been devoted to the doping process itself. We have undertaken a detailed study of this process, examining the porous silicon matrix from several points of view during and after the doping. In particular, we have found that the Er-doping process shows a threshold level which, as evidenced by the cross correlation of the various techniques used, does depend on the sample thickness and on the doping parameters.
KeywordsLight-emitting devices Er doping Porous silicon Refractive index 81.05.Rm 61.43.Gt 78.20.-e
Efficient and cost-effective Si-based optoelectronic devices are required for all-silicon telecommunication technology[1–3]. The indirect bandgap of silicon, the material of choice for micro- and nanoelectronics, unfortunately forbids luminescence and electro-optic effects, requiring the use of hybrid solutions implying complex and costly techniques. Intense research is then devoted to the study of ways leading to efficient Si-based light-emitting structures that would cancel the need of integrating different materials[1–3].
Several solutions have been explored, from Raman Si laser to Si nanocrystal laser[6, 7]. The use of rare earth elements, in particular Er and Yb, for the doping of Si and porous Si[8–10] has been a relevant research field due to the observed room temperature 1.54-μm luminescence. In particular, Er-doped silicon-rich oxide structures showed interesting light-emitting properties[11–14]. Optical gain from Er-doped Si structures at 1.54 μm was also reported.
After its discovery, porous silicon (PSi) has attracted the interest of researchers when its photoluminescence was observed, and many papers were published about its possible applications in optoelectronics[18–20]. However, even if worthy-of-note electroluminescent properties were reported, the interest on PSi light-emitting devices faded. When the possibility to obtain light from rare-earth-doped Si structures[11–14] was proposed, a renewed interest for Er-doped PSi aroused, and remarkable photoluminescent properties[20–23], also from photonic bandgap structures[24, 25], were demonstrated.
While the emission properties have been studied, together with the Er optical activation process by high-temperature treatments, very little attention has been devoted to the doping process itself. For this reason, we present here a study on electrochemical, optical and structural analysis of the Er-doping process in PSi layers. The Er-doping concentrations considered are those useful for the realization of optically active devices (that is, with an Er/Si ratio of a few percent[26, 27]).
Porous Si layers were prepared by electrochemical etching of n+-doped (100)-oriented crystalline Si wafer (Siltronix, Archamps, France) in the dark, with a resistivity in the range 3 to 7 mΩ/cm. The etching solution was HF/H2O/ethanol in a 15:15:70 proportion, respectively. For better control over the structural properties of the porous layers, we used a constant current approach. The chosen porosity for all samples is 55% (empty to full ratio).
The Er doping of the PSi layers was obtained electrochemically using a 0.1-M ethanolic solution of Er(NO3)3·5H2O in a constant current process using a mechanical stirrer. The current density during the Er-doping process was 0.11 mA/cm2.
High-resolution scanning electron microscopy (HRSEM) images were obtained using a Jeol JSM 7500FA (Japanese Electron Optics Laboratories, Tokyo, Japan) equipped with a cold field emission electron source. Spatially resolved energy-dispersive spectroscopy (EDS) measurements (Er and Si chemical maps) were carried out using a Jeol JED 2300 Si(Li) detector in a Jeol JSM 6490-LA SEM equipped with a W thermionic electron source and working at an acceleration voltage of 15 kV. In both cases, imaging was obtained using secondary electrons.
Results and discussion
After formation and reflectivity measurements, the PSi layers were reinserted in the same cell used for the formation process. The doping process was performed in the dark using a constant current configuration. As a precaution, before the beginning of the doping process, the samples were kept for 2 min in the cell filled with the doping solution with the stirrer on to allow the solution to fully penetrate the pores, even if no significant dependence on the duration of this step has been observed. To characterize the Er-doping process, PSi samples with different thicknesses (1.25, 10 and 30 μm) and different doping levels (from 0.4% to 16%) were prepared. The nominal Er-doping level used in this work is obtained by a first-order estimate: the total number of Er atoms moved to the PSi matrix is assumed equal to 1/3 of the number of electrons transferred during the electrochemical doping process. The doping level is then calculated as the ratio of this number to the number of Si atoms constituting the PSi matrix. The doping process is performed shortly after the formation of the porous layer. After formation, the samples were dried, and their optical reflectivity (discussed later in this work) was measured. After this measurement, the samples were reinserted in the electrochemical cell for the Er doping, then dried again for a new reflectivity measurement.
The most noticeable feature of the curves shown in Figure4a,b is the voltage remaining constant up to a given threshold time. However, this time does not correspond to the same doping level for 1.25-μm-thick layers (Figure4a) and 10-μm-thick layers (Figure4b). In particular, it appears that this threshold is reached at a higher doping level for thinner samples since it occurs at about 8% doping for 1.25-μm-thick samples and at about 4% for 10-μm-thick samples.
It is interesting to correlate this result with the dependence of the threshold doping level from the samples’ thickness discussed earlier. Given the gradient of the Er content within the samples’ thickness, the fact that the highest values of the threshold Er/Si ratio are observed for thinner samples may be explained by the fact that while the samples’ surface remains the same, the time needed to reach the same doping level for thick layers is higher than that required for thin samples. As a consequence, and given that the solution mobility within the pores decreases with increasing pore length, the formation of a surface deposit is easier for thicker samples, blocking then the doping process earlier with increasing layer thickness.
In the field of new Si-based materials for optoelectronics, we investigated the Er-doping process of n+-type PSi layers by several techniques. We were able to correlate the electrochemical behavior during the doping process with the optical reflectivity modification. The optical reflectivity spectra of the samples before and after the Er-doping process were fitted over a broad-wavelength range to derive the dispersion curves for the refractive index. We demonstrated that Er is present within the whole PSi layer even for very thick layers. The Er-induced modification of the layers’ optical properties was then evidenced. Moreover, we showed that there is a threshold Er concentration above which the formation of a surface deposit that degrades the optical properties of the samples occurs. This information is essential for the design of Er-doped photonic bandgap structures using PSi.
This work was financially supported by the 851/DSPAR/2003 project funded by the Italian Ministry of University and Research. Dr. Michele Saba of the University of Cagliari is gratefully acknowledged for the useful discussions.
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