Controlling the Er content of porous silicon using the doping current intensity
© Mula et al.; licensee Springer. 2014
Received: 12 May 2014
Accepted: 20 June 2014
Published: 4 July 2014
The results of an investigation on the Er doping of porous silicon are presented. Electrochemical impedance spectroscopy, optical reflectivity, and spatially resolved energy dispersive spectroscopy (EDS) coupled to scanning electron microscopy measurements were used to investigate on the transient during the first stages of constant current Er doping. Depending on the applied current intensity, the voltage transient displays two very different behaviors, signature of two different chemical processes. The measurements show that, for equal transferred charge and identical porous silicon (PSi) layers, the applied current intensity also influences the final Er content. An interpretative model is proposed in order to describe the two distinct chemical processes. The results can be useful for a better control over the doping process.
KeywordsPorous Silicon Er doping Electrochemical impedance spectroscopy Reflectivity Scanning electron microscopy
The rare earth doping of Si as a means to obtain efficient light emission 1.5 μm has attracted a lot of interest [1–7] since, given its indirect bandgap, Si photoluminescence can be obtained only through strong quantum confinement . Porous silicon (PSi) studies already reported interesting Er-related photoluminescence [2, 9–11] or electroluminescence . Unfortunately, this research activity did not lead, till now, to market-valuable devices, basically because almost no research has been devoted to the understanding of the doping process itself. Most studies, even very recent ones , use only optical properties as a means to optimize the Er doping process on bulk Si  or PSi [3, 9]. However, given the large internal surface of the material, the electrochemical doping of PSi is a quite complex process that we are just beginning to understand: all we have are just a few studies on the cyclic voltammetry of the Er deposition process , on the effect of doping duration , and on the evolution of the doping process as a function of several parameters [14, 15].
The luminescence in itself being not an issue, we focused our study on the control of the electrochemical doping process of PSi. We will show that gaining detailed information about the early stages of the process is instrumental for understanding the final results of the doping process and the key for its optimization. In our study, significant information in the understanding of these early stages is obtained by electrochemical impedance spectroscopy (EIS). This is a very valuable technique for porous materials [16–20] and has already been successfully applied to PSi for the study of cyclic oxidation [21, 22].
PSi layers were prepared by electrochemical etching in the dark of n+-doped (100)-oriented crystalline Si wafers having 3 to 7 mΩ/cm resistivity from Siltronix (Archamps, France). The etched bulk Si surface area is about 0.9 cm2. The etching solution was HF/H2O/ethanol in a 15/15/70 proportion, respectively, and the etching current density was 50 mA/cm2 in all cases. HF being an extremely hazardous material (e.g., see ), all precautions have been taken to ensure the safety of the persons involved in the porous samples preparation.
The Er doping was performed in constant current configuration with current densities in the 0.01 to 2.2 mA/cm2 range using a 0.11 M solution of in EtOH. EIS measurements and Er doping processes were always performed with the same electrochemical cell used for the PSi formation. The Er solution used was also the same in both cases. The EIS measurements were made in the galvanostatic regime (GEIS) using a constant bias current in the 0.01 to 1 mA range, a frequency range from 100 kHz to 100 mHz, and an AC amplitude of 2 to 10 μA, depending on the bias current intensity.
All electrochemical processes were performed using a PARSTAT 2273 potentiostat by Princeton Applied Research (Oak Ridge, TN, USA). A schematic of the cell used for the experiments can be found in .
Spatially resolved energy dispersive spectroscopy (EDS) measurements for quantitative Er content determination were carried out using a JEOL JED 2300 Si(Li) detector in a scanning electron microscope (SEM) JEOL JSM 6490-LA (JEOL Ltd., Akishima, Japan) equipped with a W thermionic electron source and working at an acceleration voltage of 15 kV.
The fitting of the reflectivity spectra was performed using the SCOUT software from W. Theiss Hard- and Software (Aachen, Germany).
Results and discussion
If the doping process were independent on the doping current, the data should follow a horizontal line, since no evolution would be expected. However, our results, even with the large spread, indicate that there is a clear trend, although a fully quantitative determination cannot be obtained. It must be noted that a spread in the data is expected because there are several small parameters that can affect the results. For instance, the minute differences in the surface/bulk properties of the starting Si wafer will affect the shape of the pore openings and, in turn, the diffusion of the Er solution within the pores. This effect is also expected for samples coming from different parts of the starting Si wafer (32 samples are obtained for each 4-in. wafer). The line fit is shown as a guide for the eyes to evidence the trend. Given the correlation of the samples optical properties with their Er content [14, 15], based on the data of Figure 1, we can get a first hint that this evolution indicates a current intensity-dependent Er content.
To gain further insight in the differences between ST and DT regimes, we studied the evolution of the first stages of the doping process by means of GEIS. GEIS spectroscopy is a very useful technique with high sensitivity to surface changes and well suitable to the characterization of porous materials: it allows analyzing the response of the samples under a wide frequency window. Moreover, the equivalent circuit approach was used to interpret the mechanism of the process. Parallel–series combinations of circuital electrical elements are used to simulate the response. Resistors (R) and capacitors (C) are mainly adopted but also constant phase element (CPE) is often used, instead of C, to take account for possible non-ideality of the capacitor behavior: their admittance is expressed by Y = Q (jω) n , the value of n being 1 for perfect capacitors .
Analogous discussion may be done on data obtained during high current doping (Figure 4b): in this case, the final part of the spectrum is better resolved and a further semicircle clearly appears. As shown in the inset of Figure 4b, a further circuital element was needed in the equivalent circuit to fit the related experimental data: a Warburg element W, corresponding to a CPE with n = 0.5 .
Different processes can be evocated to interpret this behavior, also considering the high values of cell potential which establish at high current. As reported in the literature [13–15], adsorption of Er ions, as well as redox processes involving molecular hydrogen and Er+3 itself, could occur in this condition. However, in the present work, no evidence of Er reductive peaks was found in the cyclic voltammetries carried out on pristine PSi layers in the same range of potentials (data not shown). Moreover, a jelly-like phase, constituted by Er ethanolate, has been observed following Er doping with similar parameters . The presence of this jelly-like phase within the pores and the proportionality of the rate of the deposit formation to the current density have also been reported .
On the basis of these results, a possible interpretative model of the observed behavior can be proposed: the applied electric field induces a migration of the Er3+ ions present in the electrochemical solution towards the inner pores surface, so generating a distribution of charges inside the pores, as well as a charge transfer of the ions inside of the solid structure. These two processes originate two resistive/capacitive responses in the GEIS spectra (second and third circles in Figure 4a,b).
According to the interpretation derived by the equivalent circuits, the first semicircle (from the left, higher frequencies) is attributed to the bulk Si. It does not evolve with time in each series of measurements, since bulk Si is not affected by the doping process. A variation of the diameters of the other semicircles is measured in time, at a variable extent, especially in data at highest current. The appearance/disappearance of the responses is connected with the time constants related to the different processes. From the fitting described earlier, values in the order of microseconds are obtained for the first RC element, so confirming a rapid process of charge adjustment in the bulk solid phase. Slower processes, represented by the other semicircles, are observed at lower current doping (time constants of order of 10-1 s), while an acceleration of them is observed at higher current (time constants in the order of ms). The presence of the DT can tentatively be associated to the large and rapid variation observed in the third semicircle in the higher current time evolution, not visible in the lower current measurements.
EDS-SEM measurements of Er content
Er (At%) at I = +0.5 mA
Er (At%) at I = +0.05 mA
The measured Er% for the sample doped using the lower current intensity is lower at all depths with respect to the other sample. Even if the Er% for this sample is below the quantitative threshold, the SEM-EDS measurements demonstrate that the total amount of Er deposited is significantly different for lower and higher current intensities despite the transferred charge and the PSi parameters being identical: lower currents lead to lower doping levels. It is not possible, at present, to correlate directly the Er distribution with our model and the GEIS measurements since the considered thicknesses are too different: 2.5 μm for GEIS and 22 μm for the EDS-SEM.
The SEM-EDS data give then further support to the already consistent interpretation of the optical and electrochemical measurements we described earlier, adding a direct measurement of the significant difference in the Er content for samples having as sole difference the doping current intensity. These results also strongly suggest that the doping current is a very good candidate to control and optimize the Er doping process of porous silicon.
We demonstrate that the voltage transitory of constant-current Er doping of PSi samples is tightly related to the final doping level. From the shape of the transitory, it is possible to anticipate the effectiveness of the doping process: a qualitative correlation of the final Er content with the transitory shape has been evidenced. This work therefore shows that a good understanding and control of the initial steps of the Er doping process is a key to the optimization of the whole process itself. Although it is presently too early to determine which are the best Er-doping conditions for porous silicon, we demonstrate that the result of the doping process depends on the parameter settings and that the current intensity is a relevant doping factor.
energy dispersive spectroscopy by scanning electron microscopy
electrochemical impedance spectroscopy
galvanostatic electrochemical impedance spectroscopy
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