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Optics Express

Optics Express

  • Editor: Michael Duncan
  • Vol. 11, Iss. 10 — May. 19, 2003
  • pp: 1237–1242
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Aging of porous silicon and the origin of blue shift

Tevhit Karacali, Bulent Cakmak, and Hasan Efeoglu  »View Author Affiliations


Optics Express, Vol. 11, Issue 10, pp. 1237-1242 (2003)
http://dx.doi.org/10.1364/OE.11.001237


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Abstract

Aging effects of porous silicon (PS) and the origin of blue shift are investigated. Photoluminescence (PL) measurements of the PS prepared with HF-EtOH solution showed a 210 meV blue shift after 1.5 months. It is found from deconvolution of the PL spectra that this shift is not fully related to the quantum confinement (QC) effect. For stable PS formation, a HF-EtOH-H2O2 solution is used. A stable luminescence at 2.01 eV with a Gaussian distribution is obtained when the samples are kept in H2O2 for 45 min after the anodization.

© 2002 Optical Society of America

1. Introduction

In the past several decades, silicon-based process technology has advanced rapidly and has reached a mature level. Porous silicon (PS), first developed unintentionally by Uhlir while he performed surface cleaning and polishing of silicon (Si) [1

1. A. Uhlir, “Electrolytic shaping of germanium and silicon,” Bell Syst. Tech. J. 35, 333 (1956).

], has become an attractive material. Following this invention, formation of PS has attracted much attention and has been studied widely. PS is used commercially for thin layer transfer because of its advantageous physical structure [2

2. K. Yamagata and T. Yonehara, “Bonding, splitting and thinning by porous Si in ELTRAN ® SOI-Epi Wafer TM,” http://www.canon.co.jp/eltran

] and is used in IC technology because of its insulation properties [3

3. W.-K. Chen, ed., The VLSI Handbook, ISBN 0-8493-8593-8 (2000).

]. Thus PS has entered the arena of optoelectronics, particularly after an emission in the visible region from PS was reported in 1990 [4

4. L. T. Canham, “Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers,” Appl. Phys. Lett. 57, 1046 (1990). [CrossRef]

]. Following the 1990s, studies have focused on finding the source of this emission and in the meantime to find application fields in optoelectronics by use of this feature. Even today the reason for the emission has not been fully explained. The most spectacular proposal is the model of the quantum confinement (QC) effect [4

4. L. T. Canham, “Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers,” Appl. Phys. Lett. 57, 1046 (1990). [CrossRef]

5

5. V. Lehman and U. Gösele, “Porous silicon formation: a quantum wire effect,” Appl. Phys. Lett. 58, 856 (1991). [CrossRef]

]. According to this model, PS emits light in the visible region at room temperature, owing to crystal in nanometer dimensions, which is different from bulk Si crystal whose band gap is in the far-infrared region. The emission of PS at room temperature makes the QC model more valid than others [6

6. A. G. Cullis, L. T. Canham, and P. D. J. Calcott, “The structural and luminescence properties of porous silicon,” J. Appl. Phys. 82, 909 (1997). [CrossRef]

]. However, surface passivation effects also have to be considered together with the QC model because the model alone cannot explain the aging effect.

In general PS is formed with either electrochemical anodic etching in a HF-based solution or with chemical etching in a HF+oxidized solution. The formation of PS with electrochemical anodic etching in a HF-based solution is the technique most widely used by researchers. In this method, PS is generally formed by use water-diluted HF or HF+EtOH solution as an electrolyte. Then PS is left in ambient air and is then dried with one of the drying methods. While the PL peak intensity of the PS has been observed to increase with time in some studies, others have reported a decrease in this signal. However, most researchers have commonly observed that the position of the PL peak has shifted to the blue region with the passage of time [6

6. A. G. Cullis, L. T. Canham, and P. D. J. Calcott, “The structural and luminescence properties of porous silicon,” J. Appl. Phys. 82, 909 (1997). [CrossRef]

]. The change of the PL features with time is caused by the interaction of the PS with oxygen in the ambient [7

7. V. Mulloni and L. Pavesi, “Electrochemically oxidized porous silicon microcavities,” Mater. Sci. Eng. B 69, 59 (2000). [CrossRef]

]. It was also observed that the broadening of the PL spectra was caused by Si-O-Si vibration [8

8. H. Elhouichet and M. Oueslati, “The role of ambient ageing on porous silicon photoluminescence: evidence of phonon contribution,” Appl. Surf. Sci. 191, 1 (2002). [CrossRef]

]. Wolkin et al. showed that Si-O-Si bonds were formed from the contact of the PS surface with air even after a short exposure. They also demonstrated that these bonds caused more effective luminescence at 2.1 eV, which is dominant over the original luminescence at 3 eV. This situation has been reported as a red shift [9

9. M. V. Wolkinet al., “Electronic states and luminescence in porous silicon quantum dots: the role of oxygen,” Phys. Rev. Lett. 82, 197 (1999). [CrossRef]

]. As a result of weakening of these bonds by time and developing of SiO2 on the surface, the PL peak shifts from the red to the more blue region. To remove this drawback, the surface should be passivated. A simple technique for accomplishing this is to oxidize the PS chemically, anodically, or thermally [10

10. V. Lehman, Electrochemistry of Silicon ISBN:3-527-60027-2 (2002).

]. Yamani et al. reported that the red/orange emission became stable with the passage of time by use of electrochemical anodic etching in a H2O2-HF-based solution; however, the green/blue emission is unstable. It was observed that this was caused by a tiny crystallite formation [11

11. Z. Yamaniet al., “Red to green rainbow photoluminescence from unoxidized silicon nanocrystallites,” J. Appl. Phys. 83, 3929 (1998). [CrossRef]

].

2. Experiment

The samples used here were n-type Si oriented (100) with 7 Ω cm resistivity. Au/%2.5 Sb was evaporated to the rough back side of the Si, and then annealing at 400 °C for 5 min was carried out for ohmic contact formation. Two types of solution were prepared as electrolyte. The first solution was a mixture of HF (%40) and ethanol (%95) with a 1:1 ratio, and the second solution was a HF (%40):EtOH (%95):H2O2 mixture with a ratio of 6:4:1. The process of electrochemical anodic etching was carried out in a Teflon cell where the sample and platinum wire were selected to be anode and cathode, respectively. A schematic diagram of the cell is shown in Fig. 1.

Fig. 1. Simplified cross-sectional drawing of anodization cell. The designed cell provided practical wax-free sample mounting.

The samples were illuminated by a 15 mW He-Ne laser at 633 nm, the output of which was expanded to 1 cm diameter, since free holes required for the PS formation in n-type samples can be obtained by a means of illumination with an energy above bandgap. A constant current density of 10 mA/cm2 was applied to all samples for 10 min. Both the current and the illumination were stopped for 1 s after every 3 s to prevent accumulation of gas, which is released during the PS formation, where the sample was in contact with the cell. We aimed not only to prevent the gas accumulation but also to make the freshened electrolyte reach the surface of the sample by use of a mixer in the solution when the formation of the PS was stopped for 1 s. Following the rinsing of the PS sample prepared by the first solution in ethanol, it was left in ambient. Two groups of PS samples were prepared with the second solution. After these samples were rinsed in ethanol, the first group was left in ambient and the others were kept in H2O2 for 45 min. Photoluminescence (PL) was used in the optical characterization of the PS samples. The PL spectra were obtained at room temperature by use of the emission line of Hg at 365 nm. A PMT (Hamamatsu R955) was used as a detector. Luminescence from PS is dispersed with a 50 cm monochromator and a signal from a PMT detected with an SR850 amplifier. The PL measurements were repeated for different periods of times to investigate the aging effect.

3. Results and discussion

PL spectra of the PS samples are shown in Fig. 2 where the first type of solution has been used.

Fig. 2. Variation of aging of normalized PS luminescence with time, formed by HF:EtOH.

As the time passed, a shift to the blue region in the peak position and a decrease in the PL intensity were observed, which is in agreement with the results reported in the literature. After 1.5 months, the total shift was ~210 meV. To investigate the source of the shift, a Gaussian deconvolution of all the PL spectra in Fig. 2 is done, as illustrated in Fig. 3.

Fig. 3. Variation of aging of PS luminescence, formed by HF:EtOH, with time. (a) After 1 h, (b) after 1 day, (c) after 2 days, (d) after 2 wk, (e) after 1.5 months

Fig. 4. Variation of aging of PS luminescence with time, formed by 6HF:4EtOH:H2O2 (as measured).

As shown in Fig. 4, while the peak intensity decreases, the peak position shifts less to the blue region when compared with the PS samples prepared with the first type solution. Although this case is more stable, we achieved for the first time a much more stable luminescence by keeping the samples in H2O2 for 45 minutes following rinsing in ethanol. The PL spectra obtained with this process are illustrated in Fig. 5.

Fig. 5. Variation of aging of PS luminescence with time. PS is formed by 6HF:4EtOH:H2O2 solution and kept in H2O2 for 45 min.

Here not only are the three peaks observed in Fig. 3 removed, but also the Gaussian peak intensity at 2.01 eV remained stable even after 3 months. The possible reasons behind chemical impurity can be explained as follows: a surface area of 1000 m2/cm3 of PS is open to chemical impurities coming from the environment and the electrolyte [13

13. O. Bisi, S. Ossicini, and L. Pavesi, “Porous silicon: a quantum sponge structure for silicon based optoelectronics,” Surf. Sci. Rep. 38, 1 (2000). [CrossRef]

]. Whereas hydrogen and fluorine impurities arise from the electrolyte, carbon and oxygen impurities appear in response to the exposure of the sample to the environment after the anodization. The source of the carbon is not ethanol, because carbon impurities have been reported to occur on the PS samples formed in only a diluted HF solution [13

13. O. Bisi, S. Ossicini, and L. Pavesi, “Porous silicon: a quantum sponge structure for silicon based optoelectronics,” Surf. Sci. Rep. 38, 1 (2000). [CrossRef]

]. As environmental conditions vary from lab to lab, the effect of the amount of carbon on the optical and electrical properties of PS can also change. As a result, the chemical impurities can cause optical instabilities when surface passivation has not been performed immediately.

4. Summary and conclusions

To ensure the stability of the PL spectra, the surface of the PS samples was intentionally passivated. For this purpose, to the best of our knowledge, the PS samples were kept in H2O2 for 45 min. for the first time, by repeating of the same process as above in the second type solution (6HF:4EtOH:H2O2). Finally, the three peaks disappeared, and a very stable luminescence at 2.01 eV was achieved.

Acknowledgments

This work has been funded in part by the Ataturk University Research Fund (PN 2001/115) project.

References and links

1.

A. Uhlir, “Electrolytic shaping of germanium and silicon,” Bell Syst. Tech. J. 35, 333 (1956).

2.

K. Yamagata and T. Yonehara, “Bonding, splitting and thinning by porous Si in ELTRAN ® SOI-Epi Wafer TM,” http://www.canon.co.jp/eltran

3.

W.-K. Chen, ed., The VLSI Handbook, ISBN 0-8493-8593-8 (2000).

4.

L. T. Canham, “Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers,” Appl. Phys. Lett. 57, 1046 (1990). [CrossRef]

5.

V. Lehman and U. Gösele, “Porous silicon formation: a quantum wire effect,” Appl. Phys. Lett. 58, 856 (1991). [CrossRef]

6.

A. G. Cullis, L. T. Canham, and P. D. J. Calcott, “The structural and luminescence properties of porous silicon,” J. Appl. Phys. 82, 909 (1997). [CrossRef]

7.

V. Mulloni and L. Pavesi, “Electrochemically oxidized porous silicon microcavities,” Mater. Sci. Eng. B 69, 59 (2000). [CrossRef]

8.

H. Elhouichet and M. Oueslati, “The role of ambient ageing on porous silicon photoluminescence: evidence of phonon contribution,” Appl. Surf. Sci. 191, 1 (2002). [CrossRef]

9.

M. V. Wolkinet al., “Electronic states and luminescence in porous silicon quantum dots: the role of oxygen,” Phys. Rev. Lett. 82, 197 (1999). [CrossRef]

10.

V. Lehman, Electrochemistry of Silicon ISBN:3-527-60027-2 (2002).

11.

Z. Yamaniet al., “Red to green rainbow photoluminescence from unoxidized silicon nanocrystallites,” J. Appl. Phys. 83, 3929 (1998). [CrossRef]

12.

T. Yoshidaet al., “Near-IR LEDs fabricated with monodispersed nanocrystallite Si,” Solid State Technol. 45, 41 (2002).

13.

O. Bisi, S. Ossicini, and L. Pavesi, “Porous silicon: a quantum sponge structure for silicon based optoelectronics,” Surf. Sci. Rep. 38, 1 (2000). [CrossRef]

OCIS Codes
(100.1830) Image processing : Deconvolution
(130.5990) Integrated optics : Semiconductors
(160.4670) Materials : Optical materials
(250.0250) Optoelectronics : Optoelectronics
(250.5230) Optoelectronics : Photoluminescence

ToC Category:
Research Papers

History
Original Manuscript: December 12, 2002
Revised Manuscript: May 12, 2003
Published: May 19, 2003

Citation
Tevhit Karacali, Bulent Cakmak, and Hasan Efeoglu, "Aging of porous silicon and the origin of blue shift," Opt. Express 11, 1237-1242 (2003)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-11-10-1237


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References

  1. A. Uhlir, �??Electrolytic shaping of germanium and silicon,�?? Bell Syst. Tech. J. 35, 333 (1956).
  2. K. Yamagata and T. Yonehara, �??Bonding, splitting and thinning by porous Si in ELTRAN ® SOI-Epi Wafer TM,�?? <a href="http://www.canon.co.jp/eltran">http://www.canon.co.jp/eltran</a>
  3. W.-K. Chen, ed., The VLSI Handbook, ISBN 0-8493-8593-8 (2000).
  4. L. T. Canham, �??Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers,�?? Appl. Phys. Lett. 57, 1046 (1990). [CrossRef]
  5. V. Lehman and U. Gösele, �??Porous silicon formation: a quantum wire effect,�?? Appl. Phys. Lett. 58, 856 (1991). [CrossRef]
  6. A. G. Cullis, L. T. Canham, and P. D. J. Calcott, �??The structural and luminescence properties of porous silicon,�?? J. Appl. Phys. 82, 909 (1997). [CrossRef]
  7. V. Mulloni and L. Pavesi, �??Electrochemically oxidized porous silicon microcavities,�?? Mater. Sci. Eng. B 69, 59 (2000). [CrossRef]
  8. H. Elhouichet and M. Oueslati, �??The role of ambient ageing on porous silicon photoluminescence: evidence of phonon contribution,�?? Appl. Surf. Sci. 191, 1 (2002). [CrossRef]
  9. M. V. Wolkin et al., �??Electronic states and luminescence in porous silicon quantum dots: the role of oxygen,�?? Phys. Rev. Lett. 82, 197 (1999). [CrossRef]
  10. V. Lehman, Electrochemistry of Silicon ISBN:3-527-60027-2 (2002).
  11. Z. Yamani et al., �??Red to green rainbow photoluminescence from unoxidized silicon nanocrystallites,�?? J. Appl. Phys. 83, 3929 (1998). [CrossRef]
  12. T. Yoshida et al., �??Near-IR LEDs fabricated with monodispersed nanocrystallite Si,�?? Solid State Technol. 45, 41 (2002).
  13. O. Bisi, S. Ossicini, and L. Pavesi, �??Porous silicon: a quantum sponge structure for silicon based optoelectronics,�?? Surf. Sci. Rep. 38, 1 (2000). [CrossRef]

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