In the Laboratory
Understanding the Origin of Luminescence in Porous Silicon
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An Undergraduate Solid-State Chemistry Experiment D. P. Lasher, B. A. DeGraff, and B. H. Augustine* Department of Chemistry, James Madison University, Harrisonburg, VA 22807; *
[email protected] Since the first report of efficient visible, room-temperature light emission from etched crystalline silicon by Canham in 1990 (1), many research groups have published on the development of porous silicon (PSi) technology (2). Porous silicon has been used to demonstrate key concepts of semiconductor chemistry in the laboratory (3). One of the driving forces behind PSi research has been the desire to develop an all-silicon-based optoelectronics technology. However, in addition to its technological interest, PSi has proved to be fascinating from a fundamental materials science perspective. Since PSi layers are readily fabricated using common experimental apparatus available in most undergraduate chemistry or physics laboratories, the material is ideal for introducing the concepts of solid-state luminescence to upper-division physical chemistry students. Successful implementation of this experiment can range from a simple qualitative demonstration of light emission from silicon (3) requiring only crystalline silicon wafers, acid etching solution, and a hand-held UV lamp to more sophisticated fabrication and characterization experiments that we report here. PSi layers can be prepared with either chemical (4) or electrochemical (1) etching. Students characterized the luminescence using a fluorimeter, and emission lifetime using a laser-based lifetime instrument (5). Additionally, in this experiment we try to establish a link between surface microstructure and materials properties through the use of atomic force microscopy (AFM). C AUTION : The etching reported here requires HF-based etchants, which are extremely hazardous if not properly handled. Safety and disposal procedures must be rigorously followed for this experiment (6 ).
Theoretical Background Bulk crystalline silicon has a band gap of 1.1 eV, which should result in luminescence in the near infrared (1100 nm), but silicon is an inefficient optical material because it possesses an indirect band gap. Practically, this means that an electron–hole pair generated in silicon does not have the proper momentum to radiatively recombine, and instead returns to the ground state via nonradiative recombination at defect states in the crystal lattice or the surface of the semiconductor. For this reason, solid-state light-emitting diodes (LEDs) are generally fabricated from direct band gap semiconductors such as GaAs. The first report of efficient photoluminescence (PL) in the visible (1) from electrochemically etched PSi was surprising for two reasons: (i) something must have happened to increase the band gap from 1.1 eV to >1.8 eV (680 nm) during etching, and (ii) significant changes must have occurred to increase the luminescence efficiency by several orders of magnitude, overcoming the indirect nature of the silicon band gap. Addressing these fundamental issues has
resulted in publication of literally thousands of papers in the past decade. The mechanism that best explains these results is known as the “quantum-confinement model” (QCM). According to the QCM, as the chemical or electrochemical etching proceeds, a complicated network of pores (voids) and crystalline silicon features remains with typical feature sizes on the order of nanometers. When an electron–hole pair is generated, it is trapped in a potential energy box, the barriers being the voids at the surface of the nanoparticles. The QCM would suggest that as the particle size is decreased, the energy of the PL peak should increase as expected from the quantum mechanical confinement of particles in a smaller box. This would result in a larger energy band gap in the silicon material. The change in band gap, ∆E, is given to first order by the simple effective mass approximation developed by Brus (7) for semiconductor nanocrystals as
∆E =
2 h π2
2R 2
1 + 1 – 1.8e εR m e* m h*
2
where m e* and m h* are the effective mass of the electron and hole in silicon, respectively, R is the radius of a spherical nanocluster or feature, e is the charge on an electron, ε is the dielectric constant, and h⁄ is Plank’s constant, h/2π. The effective masses have values of me* = 0.19me and m h* = 0.52me, where me is the mass of an electron (8). The sharp increase in luminescence efficiency is explained by the passivating effect of H from the HF on the silicon surface (9), lowering the dangling bond density, which is a primary nonradiative pathway. Although the details of the model are considerably more complicated and several experimental results do not completely fit a simple quantum confinement of carriers explanation (10–12), PSi represents an attractive model system for introducing undergraduate students to the concepts of semiconductor band gap engineering. Overview of Procedures and Concepts Learned
Experimental Procedures For many chemistry students, this is the first exposure to semiconductor experimental techniques. For this reason, it is suggested that they first clean the silicon wafers. This prepares students for handling the wafers with tweezers after the etching is completed and teaches them about the necessity to keep semiconductor surfaces free of contamination. Students are then required to chemically and electrochemically etch silicon samples using a HF-based etchant. The chemical etch is a HF (49% in H2O)–HNO3–H2O (1:3:5) solution, and the electrochemical etch is HF (49% in H2O)–EtOH (1:1). Typical etch times for the chemical etch are 5–10 min; the electrochemical etch times are under 2 min.
JChemEd.chem.wisc.edu • Vol. 77 No. 9 September 2000 • Journal of Chemical Education
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In the Laboratory
Summary of Evaluation Studies We have used this experiment in the quantum mechanics semester of physical chemistry. Immediately before this experiment, students were introduced to an experimental demonstration of the particle-in-a-box concept in a molecular system (15–17 ). This background enables students to make the connection between HOMO–LUMO absorption in a molecule and band-to-band absorption in a solid. It is good preparation for discussing the theoretical concept of quantum confinement and why a decrease in semiconductor nanocrystal size would result in a blue shift in photoluminescence. The experiment is also aesthetically appealing because one can clearly see that the silicon has been taken from a nonluminescent state to a highly luminescent state. The figures show representative emission and microscopy data from the chemical and electrochemical etching of silicon. In Figure 1, lines A and B shows the PL data from chemically etched samples and line C shows an electrochemically etched sample. One observes a broad PL peak centered between 680 nm (1.8 eV) to 730 nm (1.7 eV), depending on sample preparation conditions. The broad peak has been attributed to the large size distribution of nanocrystalline regions present after etching. Lines A and B also illustrate the dependence of the emission wavelength on excitation wavelength, again suggesting a large size distribution of luminescent species. Figures 2A and 2B are atomic force microscopy (AFM) images of a silicon wafer as received (Fig. 2A) and after electrochemical etching for 2 min to produce a uniform PSi layer (Fig. 2B). The RMS roughness found from the sample 1202
1.2
1.0
Relative emission intensity
Photophysical measurements included emission spectra taken at two excitation wavelengths (typically 250 and 300 nm) and a determination of the excited-state lifetimes. The former can be obtained with any good fluorimeter for which a solidstate sample holder is available. Because typical sample emission is in the orange/red, it is useful to obtain corrected (for PMT response) emission spectra to avoid significant distortion of the spectrum. Further, the more diverse feature sizes resulting from the chemical etch give rise to an emission spectrum that is dependent on excitation wavelength. Lifetime data were obtained using an apparatus based on a small nitrogen laser as the excitation source (5). The decay data are collected using a digital oscilloscope, transferred to a PC, then reduced and fitted to a sum of exponentials using locally written software. Typically, two or three lifetimes are required to fit the decay data, τ1 < 1 µs and τ2 > 10 µs. The necessity of at least two lifetime components differing by an order of magnitude suggests either (i) a very broad distribution of lifetimes or (ii) more than one mode of emission production. The long radiative lifetime component of porous silicon suggests that the silicon remains an indirect band gap material even at such small nanocluster sizes as predicted by theory (13), and this was confirmed by a careful experiment (14 ). Atomic force microscopy was done on a Digital Instruments Multimode AFM operating in contact mode in air. Typical lateral scan sizes ranged from 1 to 30 µm at a scan rate of 2 Hz. Samples were cleaved, cleaned with compressed nitrogen, and mounted on magnetic pucks using double-sided tape. The cantilever and sample were mounted and aligned before the laboratory began in order to speed up the AFM experiment.
B
C A
0.8
0.6
0.4
0.2 575
625
675
725
775
825
Wavelength / nm Figure 1. Photoluminescence intensity vs wavelength from (A) 7-min chemical etch at λexcite = 360 nm, (B) 7-min chemical etch at λexcite = 300 nm, and (C) 2-min electrochemical etch at λexcite = 300 nm.
Figure 2. Atomic force microscopy of silicon (A) as-received and (B) after 2-min electrochemical etch. Both images were taken in contact mode at a scan rate of 2 Hz. Bar indicates feature height. Lighter features are higher in the image.
as received is 1.6 nm, whereas the roughness in the porous silicon sample is 59 nm. One can observe an approximately circular geometry to the large 1–2-µm pores in the surface of Figure 2B, which is approximately the shape predicted by Canham (1) in his simple geometrical model to explain the generation of nanometer-sized regions in porous silicon. It should be noted that the data generated from the AFM are not essential for successful implementation of this experiment, but they illustrate to students the connection between the surface structure and optical properties of PSi. While the AFM data are helpful to show the changes in surface morphology with etching, one must be careful to note that the luminescence is being generated from nanoparticles on the order of 1–5 nm, whereas the smallest particles observed with AFM were on the order of 20 nm. This is a function of the resolution of the AFM, the geometry of the AFM probe, and the complicated microstructure that results after PSi fabrication.
Journal of Chemical Education • Vol. 77 No. 9 September 2000 • JChemEd.chem.wisc.edu
In the Laboratory
List of Unusual Chemicals and Instruments eutectic1
•
GaIn liquid or Al deposited on the backside of the silicon wafers (18)
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p-Type Si wafers (2–4 Ω-cm resistivity)2
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Custom-fabricated electrochemical etch vessel (see instructors notesW)
3.
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Atomic force microscope
4.
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Luminescence lifetime instrument
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Concentrated hydrofluoric acid (49% HF in water)
5.
WARNING: One must use extreme caution with HF (6 ). 6.
Conclusions Porous silicon is an attractive model system to introduce undergraduate physical chemistry students to solid state chemistry, optical properties of semiconductors and the connection between the microstructure of a material and concomitant properties. W
Supplemental Material
Supplemental material for this article is available in this issue of JCE Online.
7. 8. 9. 10.
11. 12. 13. 14.
Notes 1. GaIn eutectic can be purchased from Alfa-Aesar (Stock #12478). 2. Research quantities of silicon wafers can be purchased from Virginia Semiconductor, Fredricksburg, VA.
Literature Cited 1. Canham, L. T. Appl. Phys. Lett. 1990, 57, 1046. 2. There are many reviews of porous silicon fabrication and
15. 16. 17. 18.
characterization. The following are particularly useful. Collins, R. T.; Fauchet, P. M.; Tishler, M. A. Physics Today 1997, 50 (1), 24. Jung, K. H.; Shih S.; Kwong, D. L. J. Electrochem. Soc. 1993, 140, 3046. Ludwig, M. H. Crit. Rev. Solid State Mater. Sci. 1996, 21 (4), 265–351. Swisher, R.; Richmond, G. L.; Sercel, P. C. J. Chem. Educ. 1996, 73, 738. Bsiesy, A.; Vial, J. C.; Gaspard, F.; Herino, R.; Ligeon, M.; Muller, F.; Romestein, R.; Wasiela, A.; Haimaoui, A.; Bomchil, G. Surf. Sci. 1991, 254, 195. DeGraff, B. A.; Horner, D. A. J. Chem. Educ. 1996, 73, 279. Armour, M. A. Hazardous Laboratory Chemicals Disposal Guide; CRC Press: Boca Raton, FL, 1991. Also see instructors notes.W Brus, L. E. J. Chem. Phys. 1984, 80, 4403. Pankove, J. I. Optical Properties of Semiconductors, 2nd ed.; Dover: New York, 1985. Higashi, G. S.; Chabal, Y. J.; Trucks, G. W.; Ragharachari, K. Appl. Phys. Lett. 1991, 58, 1656. Xie, Y.-H.; Wilson, W. L.; Ross, F. M.; Mucha, J. A.; Fitzgerald, E. A.; Macualey, J. M.; Harris, T. D. J. Appl. Phys. 1992, 71, 2403. Brandt, M. S.; Fuchs, H. D.; Stutzmann, M.; Weber, J.; Cardona, M. Solid State Commun. 1992, 81, 307. Koch, F.; Petrova-Koch, V.; Muschik, T.; Nikolov, A.; Gavrilenko, V. Mater. Res. Soc. Symp. Proc. 1993, 283, 197. Hyberson, M. S. Phys. Rev. Lett. 1994, 72, 1514. Schuppler, S.; Friedman, S. L.; Marcus, M. A.; Aldler, D. L.; Xie, Y.-H.; Ross, F. M.; Harris, T. D.; Brown, W. L.; Chabal, Y. J.; Brus, L. E.; Citrin, P. H. Phys. Rev. Lett. 1994, 72, 2648. Anderson, B. D. J. Chem. Educ. 1997, 80, 985. Farrell, J. J. J. Chem. Educ. 1985, 62, 351. Gerkin, R. E. J. Chem. Educ. 1965, 42, 490. To make electrical contact to the silicon wafer, one needs to form an ohmic contact to the silicon. See for example: Hummel, R. E. Electronic Properties of Materials, 2nd ed.; Springer: Berlin, 1993; p 113.
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