Characterization of Er in porous Si

Abstract

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.

Figure 1

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

Figure 2

HRSEM cross-section image of a typical empty PSi sample. The columnar nature of the pores is visible. The inset shows a Ã×150,000 magnification of the indicated area. The structure of the pores’ walls is clearly visible.

Figure 3

HRSEM plane view of the surface of a typical PSi sample. The pore openings are clearly visible as the darker areas in the image.

Figure 4

Time evolution of the applied voltage during a constant current electrochemical Er insertion process. The behavior recorded for two different thicknesses is shown: 1.25 (a) and 10 μm (b). In both cases, the curves from different samples are well superposed. The vertical dotted lines indicate the approximate position where the behavioral change begins. The horizontal dotted lines indicate the plateau voltage common to all samples.

Figure 5

Optical microscopy image of a sample with Er accumulation onto the surface. It is possible to see that the formation of the surface deposit is position dependent, decreasing from the sample border towards the sample center.

Figure 6

SEM image and EDS map of a 29-μm-thick sample. SEM image (a) and corresponding EDS chemical map (b) for Er (blue) and Si (red) of an Er-doped 29-μm-thick PSi sample. In both cases, two arrows (one on each side of the image) indicate the interface between PSi and crystalline Si, and a green line indicates the surface of the sample. The nominal amount of Er inserted within the pores is 2%. The PSi layer in the micrograph is on the top half of the image (a) (lighter gray), and the bulk crystalline Si is on the bottom half (darker gray). The EDS analysis (b) confirms that Er atoms are present in the whole PSi layer thickness.

Figure 7

Variation of the Er/Si ratio along the formation direction. Er content as a function of the distance from the PSi surface, as obtained from the SEM-EDS map shown in Figure 6 and performed on a 29-μm-thick porous Si layer. A linear decrease of the Er content is observed. The red line is a linear fit of the data that indicate a decrease of about 3%/μm.

Figure 8

Optical reflectivity of a PSi sample before and after the Er insertion. Comparison of the reflectivity spectra of the same sample before (red solid line) and after (green dashed line) the electrochemical insertion of Er (8% in this case). 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).

Figure 9

Correlation of the shifts on the Er-doping level. ΔR (right axis, blue empty triangles) and Δλ (left axis, red full triangles) plotted as a function of the nominal Er content for a series of 1.25-μm-thick PSi samples. The lines are intended only as guide for the eye. For the definition of ΔR and Δλ, see Figure 8.

Figure 10

Ratios of layer refractive index after and before the Er doping. For 1.25- (a) and 10-μm-thick (b) PSi samples. The ratios (large blue dots with error bars) are obtained by dividing the 2 nL values obtained after and before the Er doping from the reflectivity spectra as described in the text. The observed behavior for high and low doping is coherent with the doping threshold discussed earlier.

Figure 11

Example of simulation of the optical reflectivity spectra of PSi samples. The experimental data are those shown in Figure 8. The curves in (a) are the experimental (solid red line) and simulated (dashed blue line) reflectivity before the Er doping. The curves in (b) are the experimental (solid green line) and simulated (dashed black line) reflectivity after the Er doping. The good agreement of the experimental data and their simulation in the whole spectral range considered ensures the reliability of the fitting results.

Figure 12

Refractive index dispersion curves for a 1.3-μm-thick PSi sample. Before (solid line) and after (dashed line) the insertion of Er.