Optical, Electrochemical, and Structural Properties of Er-Doped

Article pubs.acs.org/JPCC

Optical, Electrochemical, and Structural Properties of Er-Doped Porous Silicon Guido Mula,*,† Susanna Setzu,† Gianluca Manunza,† Roberta Ruffilli,‡ and Andrea Falqui‡ †

Dipartimento di Fisica, Università di Cagliari, Cittadella Universitaria, S.P. 8 km 0.700, 09042 Monserrato (Ca), Italy Nanochemistry, Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy

‡

ABSTRACT: We present here a study of Er doping of n+-type porous silicon. The samples were characterized in situ by their electrochemical behavior and ex situ by optical reflectivity and scanning electron microscopy (SEM). A clear correlation between the optical properties and the Er content of the samples is demonstrated. Refractive index dependence on Er content has also been obtained through simulations of reflectivity spectra in the 350−2500 nm range.

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INTRODUCTION In the field of telecommunication technology, there is an active research effort for finding new and cheaper Si-based devices.1−3 As a matter of fact, the indirect band gap of silicon, the material of choice for micro- and nanoelectronics, forbids luminescence and electrooptic effects, so that one is forced to look for hybrid solutions involving necessarily complex and costly techniques.4 The development of silicon-based structures able to emit light would therefore greatly improve the integration of lightemitting and electronic devices by avoiding the need of using different materials.1−3 There have been several approaches to this field starting from Raman Si laser5 and Si nanocrystals laser.6,7 Rare-earth doping of Si and porous Si8−10 has been a relevant research field since the room-temperature 1.54 μm luminescence has been demonstrated. Er-doped silicon rich oxide (SRO) structures showed interesting light-emitting properties.11−14 Optical gain from Er-doped Si structures at 1.54 μm was also reported.15 Porous silicon16 has attracted a lot of interest after the discovery of its photoluminescence properties,17 and many papers were published about its possible applications in optoelectronics.18−20 However, the interest in light-emitting devices based only on porous Si faded even if interesting electroluminescence properties were reported.21 A renewed interest arose when researchers found the possibility to obtain light from rare-earth-doped Si structures.11−14 Er-doped porous Si showed interesting photoluminescence properties20−23 also from photonic band gap structures.24,25 The optical activation of Er atoms by thermal treatments has been studied also in porous silicon microcavities,25 and the effects of the temperature-induced structural densification have been explored. We present here an electrochemical, optical, and structural analysis of the Er-doping process in n+-type porous Si (PSi) single layers. The effects of the optical activation process of Er atoms will be the subject of a further publication. The Er© 2012 American Chemical Society

doping concentrations considered here are those useful for the realization of optically active devices (that is, with a ratio Er/Si of a few percent26,27). The aim of this study is to analyze the Erdoping process alone to better understand the physical and chemical processes involved and then to help achieve a much better control over the fabrication of the PSi:Er layers from the undoped PSi matrix to the final device. As a matter of fact, while several studies have been devoted to the luminescence properties of Er-doped PSi samples, very little attention has been dedicated to the characteristics of the doping process addressed in the present work in spite of their being a crucial step to obtain high-performance devices and of their needing a more detailed understanding than yet done.

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EXPERIMENTAL SECTION Porous Si (PSi) layers were prepared by an electrochemical etch in the dark of n+-doped (100) oriented crystalline Si wafer with a resistivity in the 3−7 mOhm/cm range. The etching solution was HF/H2O/ethanol in a 15/15/70 proportion, respectively. We used a constant-current approach, which allows for a better control over the structural properties of the porous layers. Moreover, in the constant-current experiment, the thickness of the porous layer is proportional to the etching time. The porosity of all samples is 55% (empty to full ratio). The Er doping of the PSi layers was also obtained electrochemically using a 0.1 M ethanolic solution of Er(NO3)3·5H2O in a constant-current process. All electrochemical processes were performed using a PARSTAT 2273 potentiostat by Princeton Applied Research. The electrochemical cell used for all experiments is shown in Figure 1, where the different parts are indicated. Received: February 24, 2012 Revised: April 27, 2012 Published: May 2, 2012 11256

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Figure 2. Time evolution of the applied voltage during a constantcurrent electrochemical Er insertion process. The measured potentials superpose almost perfectly for all samples. Each curve represents a different sample, and all samples have the same nominal thickness (10 μm). The dotted straight lines indicate the plateau voltage observed for all samples (horizontal line) and where the absolute voltage value begins to increase (vertical line).

Figure 1. Schematic of the cell used for the formation and Er doping of the PSi samples. The various components are indicated.

The reflectivity measurements were performed using a PerkinElmer Lambda 950 UV−vis−NIR spectrometer. 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 scanning electron microscope (SEM) Jeol JSM 6490-LA equipped with a W thermionic electron source and working at an acceleration voltage of 15 kV. To ensure a very limited environmental contamination, the samples were optically characterized shortly after the porous layer formation allowing just the time for the drying of the residual water from rinsing. After the optical measurements, the samples were reinserted in the electrolytic cell for doping. Again, after the doping process, the optical properties of the samples were measured allowing only a short waiting time for the drying. Studies of the PSi modifications due to aging have been described in the literature28 and indicate that environment-related modifications (as oxidation) become significant after time intervals markedly longer than the waiting times used in this work. In addition, the use of a methanolic doping solution without addition of water and the different charge sign of Er3+ and O2− ions, that will make them move in opposite directions during the electrochemical doping process, also help limiting unwanted PSi layer oxidation.

Er atom inserted into the PSi matrix. The values given in this report are those obtained within this hypothesis. We can see from Figure 2 that all curves are perfectly superposed and almost constant for about 400 s. For longer insertion times, the applied voltage decreases. The nominal Er doping reached for each process is indicated in the graph legend. At about 400 s, where the change in the measured voltage indicates that the behavior begins to modify, the nominal Er doping level is ∼4%. Above 4%, a surface deposit begins to be visible by naked eye on the samples. The same analysis on PSi layers with different thicknesses shows a dependence of the threshold time with respect to the sample thickness with thin PSi layers allowing proportionally more Er than thick layers. This effect will be discussed in more detail when discussing the electron microscopy analysis. The samples were characterized using energy-dispersive spectroscopy (EDS) by scanning electron microscopy (SEM). In Figure 3, an SEM image of a 29 μm thick sample (a) and chemical analysis images for Er (b) and Si (c) are shown with b and c obtained by EDS 2D Maps. The scale bar is reported in the lower part of each image. In the SEM image (Figure 3a), the PSi layer is the light gray top half of the sample, while the crystalline Si substrate is on the lower part and is of a darker shade of gray. In each part of Figure 3, the PSi layer and the bulk Si are indicated in the right side of each image. In the chemical analysis images, Er (Figure 3b) and Si (Figure 3c) are indicated by the gray dots. The figure indicates that the electrochemical insertion of Er allows for an effective Er impregnation of the PSi pores throughout the whole PSi thickness even for a very thick layer. The nominal Er doping of the layer is 2%. To characterize in more detail the presence of Er in the layer, we studied the variation of the Er/Si ratio along the formation direction. The results are shown in Figure 4. While Er is present in the whole porous layer, as shown in Figure 3, a linear decrease in the Er/Si signal ratio from the external surface toward the PSi/crystalline Si interface is observed. A similar behavior, but with a faster decrease, has been observed by Marstein et al.29 on p-type PSi. The reduction of Er/Si ratio with the increasing depth may be explained, given the columnar structure of n+-type PSi, with a reduction of the Er content within the doping solution because of a reduced exchange

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RESULTS AND DISCUSSION We prepared PSi samples with different thicknesses and different levels of Er doping. Our goal was to correlate the Er-doping process with the thickness of the samples and with their optical and structural properties. In Figure 2, we report the evolution of voltage versus time during the electrochemical insertion of Er in the PSi matrix in constant-current experiments. All curves are related to porous Si layers with the same thickness (d = 10 μm), and each curve is obtained on a single sample. The length of the curves is given by the duration of the doping process. For all experiments, we used an Er insertion current IEr = +1 mA. We chose the constant-current configuration since the most reasonable starting hypothesis was that the number of Er atoms inserted in the porous matrix was proportional to the charge transferred, that is, to the duration of the Er insertion process in a constantcurrent experiment. In this hypothesis, a rough estimate of the amount of Er inserted can be obtained by a first-order assumption that for each three electrons transferred there is an 11257

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doping process. In fact, the highest values of the Er/Si ratio are observed toward the external surface of thick samples, and thinner samples show higher values of the threshold time in the Er-doping process. Within this hypothesis, we estimate that the Er content of thinner samples is significantly more homogeneous. To study the effect of the presence of Er on the optical properties of PSi layers, we performed optical reflectivity on PSi samples before and after the Er-doping process. To ensure an optimal doping homogeneity, on the basis of the SEM-EDS results, this study was performed on 1.3 μm thick samples. In Figure 5, we report the results for a PSi sample prior to (solid

Figure 5. Comparison of the reflectivity spectra of the same sample before (solid line) and after (dashed line) the electrochemical insertion of Er. The effect of Er is a clear blueshift in the interference fringes (labeled Δλ) and a decrease in the absolute reflectivity of the Si-related peaks (labeled ΔR).

line) and after (dashed line) the Er insertion (8% in this case). The effect of Er is evidenced by the blueshift of the interference fringes. This shift is tightly related to the amount of Er as described more in detail in the following. A first correlation of the spectral data with the Er content can be obtained by plotting (Figure 6) the absolute reflectivity

Figure 3. SEM micrograph (a) and chemical analysis image for Er (b) and Si (c) of an Er-doped 29 μm thick PSi sample. The nominal amount of Er inserted within the pores is 2%. The PSi layer in the micrograph is on the higher half of the image (lighter gray), and the bulk crystalline Si is on the lower half (darker gray). The layers are indicated on the right side of each image. It is possible to see that Er, indicated by the gray dots in b, is present in the whole PSi layer thickness.

Figure 6. ΔR (right axis, triangles) and Δλ (left axis, circles) plotted as a function of the nominal Er content for a series of 1.3 μm thick PSi samples. The lines are intended only as a guide for eyes. For the definitions of ΔR and Δλ, see Figure 5.

change (ΔR in Figure 5) and the blueshift of the interference fringes (Δλ in Figure 5) as a function of the amount of Er. Both ΔR and Δλ exhibit a dependence on the Er-doping level. Below 8%, this dependence is almost linear, while above 8% the increase becomes clearly nonlinear. It is interesting to observe that 8% is the doping level above which a deposit is visible on the sample borders as indicated earlier. For very high doping levels (more than ∼15%), the whole surface is covered with the deposit. The presence of a jellylike Er(C2H5O)3 film on 5 μm

Figure 4. Er content as a function of the distance from the surface from SEM-EDS measurements in a 30 μm thick porous Si layer. A linear decrease of the Er content is observed.

efficiency from the solution within the pores with that outside the sample. This result is coherent with the dependence of the threshold time observed in the applied potential in the Er11258

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thick PSi using a 0.1 M ethanolic solution of Er(NO3)3·5H2O was previously reported by Petrovich et al.30 The reflectivity spectra of the samples have been fitted using a program whose parameters include the sample’s thickness and its roughness together with the absorption coefficient and refractive index dispersion curves defined over eight points at arbitrary wavelengths. The results of the fitting are shown in Figures 7 and 8 for a 1.3 μm thick sample before (Figure 7) and after (Figure 8) the

Figure 9. Refractive index dispersion curves for a 1.3 μm thick PSi sample before (solid line) and after (dashed line) the insertion of Er. The values are obtained from the fit of the reflectivity spectra shown in Figures 7 and 8.

Moreover, we showed that there is a threshold Er concentration above which there is the formation of a surface deposit that degrades the optical characteristics of the samples. The optical reflectivity spectra of the samples before and after the Erdoping process were fitted over a large wavelength range to derive the dispersion curves for the refractive index. The Erinduced modification of the layers’ optical properties were then evidenced. This information is essential for the design of Erdoped photonic band gap structures using porous Si. The issue of multilayer doping, in particular for thick structures, will be addressed in future publication. The present results, however, seem to suggest that the in-depth Er content homogeneity will be more strongly dependent on the doping process itself than on the porosity periodical variations of a photonic band gap structure.

Figure 7. Experimental reflectivity (solid line) and fit (dashed line) of a 1.3 μm thick PSi sample before the Er doping.

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AUTHOR INFORMATION

Corresponding Author

*Phone: +39 070 6754934/4787. Fax: +39 070 510171. E-mail: [email protected].

Figure 8. Experimental reflectivity (solid line) and fit (dashed line) of the 1.3 μm thick Er doped PSi sample of Figure 7 after Er doping. The nominal amount of Er concentration is 12%.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 useful discussions.

electrochemical Er doping to a nominal 12%. In both graphs, the experimental data are the solid line and the fit is the dashed line. There is a very good match of experimental and simulated spectra in the whole 450−2500 nm spectral range. From the fitting of the experimental data, we were able to calculate the refractive index dispersion curve for the samples before and after the Er doping. In Figure 9, we show the dispersion curves of the refractive index obtained from the fitting of the reflectivity spectra of Figures 7 and 8. It is clear how the Er doping causes an increase of the refractive index of the porous matrix. This increase is related to the amount of Er present in the layer as is evidenced by the progressive modification of the reflectivity spectra following the doping of the PSi layers with increasing amounts of Er.

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REFERENCES

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CONCLUSIONS In the field of new Si-based materials for optoelectronics, we investigated the Er-doping process of n+-type porous Si layers by several techniques. We were able to correlate the electrochemical behavior during the doping process with the optical reflectivity modification. We demonstrated that Er is present within the whole PSi layer even for very thick layers. 11259

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