J. Phys. Chem. 1993,97, 4505-4508
4505
Enhanced Photoemission from Short-Wavelength Photochemically Etched Porous Silicon Vincent V. Doan, Reginald M. Penner, and Michael J. Sailor' Department of Chemistry, University of California at San Diego, La Jolla, California 92093-0506, and Institute for Surface and Interface Science, Department of Chemistry, University of California at Itvine, Itvine. California 9271 7 Received: November 17, 1992; In Final Form: February 1, 1993
The visible photoluminescence (PL) from p-type porous Si is up to 100 times brighter if the p-Si substrate is irradiated with blue (450 nm) light during electrochemical preparation. The PL spectrum is also shifted to the blue by ca. 80 nm relative to a sample etched under comparable conditions in the dark. Red (700 nm) illumination during the etch has no effect on the subsequent PL spectrum. The surfaces of all these samples are much smoother and the PL is brighter than luminescent porous Si prepared via an initial electrochemical etch and subsequent chemical etch. Atomic force microscope images indicate that the surface of the shortwavelength photochemically etched porous Si is significantly stronger than Si etched in the dark. The increased PL intensity is attributed to photocorrosion of electrically isolated Si regions close to the surface of the porous Si layer.
Introduction Recent studies have shown that certain electrochemical and/ or chemical etches of Si produce a microporous material ("porous Si") that photoluminesces in the visible region of the electromagnetic spectrum.'-3 The remarkable luminescence of this material has generated interest because of the unique role that Si plays in modern electronics technology. It has been shown that photoluminescent porous Si can also be produced by a photoelectrochemicaletch4 The photoelectrochemicaletch takes advantage of the rectifying nature of the semiconductor/liquid junction; illumination reduces the net etch rate (hole current) at ptype Si, while it increases the etch rate at n-Si. In this work, we report that if p-Si is irradiated with low levels of shortwavelength light (450 nm) during the electrochemical etch, the photoluminescence of the resulting porous Si is 1-2 orders of magnitude more intense than p-Si etched in the dark. Similarly, others have reported an increase in photoluminescence intensity from ptype Si if anodization is performed under white light illumination.'" This is a surprising result, because illumination of p s i with high light intensity can totally inhibit electrochemical corrosion. Here we interpret these results in terms of two competing processes; the first is the electrochemical etch, which is driven by the potential applied to the Si electrode and can occur only at regions within the Si that are in good electrical contact with the substrate. The second process is a purely photochemical corrosion that can happen even at electrically isolated regions within or on the surface of the porous Si material. It is concluded that this photochemical process is more effective at generating highly luminescent porous Si.
Experimental Section Single-crystal polished test wafers of ptype (boron-doped) Si of 9.54 S2 cm resistivity, 0.40 mm thick, and (100) orientation were obtained from International Wafer Service and cut into rectangles with areas of ca. 0.15 cm2. These were ohmically contacted on the back by scratching with Ga/In eutectic and affixing a Cu wire with conductive Ag paint. The entire contact and edge were coated with epoxy, and the resulting device was used as the working electrode in a two-electrode electrochemical cell. A Pt flag with a press-contacted Pt wire attached was used as the counter electrode. The etching bath was a 5050 (by To whom correspondence should be addressed at the University of California at San Diego.
0022-3654/93/2097-4505~04.00/0
volume) solutionof aqueous 49%HF (Fisher Scientific, Electronic Grade) and 95% ethanol (Quantum Chemical Company). Photoelectrochemical etching was carried out in optical quality polystyrene cuvettes (Baxter Scientific Products). A 300-W ELH-type (tungsten) lamp fitted with 20-nm-bandpass interference filters (either 700- or 450-nm wavelength) was used as the etching light source. The samples were etched galvanostatically, using a Princeton Applied Research Model 363 potentiostat/galvanostat. Luminescentporous Si resulted after etching at a current density of 5 mA/cm2 for 30 min. The samples were then removed from the bath, rinsed with ethanol, and dried under a stream of nitrogen. PL spectra were collected with a Princeton Instruments Model LN/CCD-576T photodetector/Acton research 0.25-m monochromator, and the samples were always measured (in air) immediately after drying. Scanning electron microscope (SEM) images were obtained with 10-keV electrons in secondary electron imaging mode, using a Cambridge Model 360electron microscope. Atomic forcemicroscope(AFM) images and area statistics were obtained with a Park Scientific Instruments SFM-BD2-210 scanning force microscope. Transmission infrared measurements were made with a Mattson Instruments 2020 Galaxy series Fourier transform infrared spectrometer. Results
Irradiation during Electrochemical Etch. All electrodes were galvanostatically etched at the same nominal current density of 5 mA/cm2. Cross-sectionalSEM images revealed that the porous layer produced under these conditions is approximately 5 pm thick. Figure 1 presents the PL spectrum of three different electrodes. Sample A was electrochemicallyetched in the dark, samples B and C were illuminated with 1.75 X 1015 photon s-I cm-2 of 450- and 700-nm light, respectively, during electrochemical etch. For the electrodes etched under illumination, the photon flux was measured at the electrode surface, using a calibrated photocell and corrected for the absorbance and reflectance of the cuvette and HF/ethanol solution. The results of Figure 1werereproduced three times. In all cam,the electrode etched in blue-light photoluminesced at least 10times moreintense than the electrodes etched in the dark. PL on all electrodes was measured using the 442-nm line of a He/Cd laser, which was expanded to a power density of 0.5 mW/cm2 to avoid irradiationinduced changesin the samples during data accumulation. Figure 2 shows a plot of photoetch light intensity vs PL enhancement ratio (relative to a dark etch) for samples etched as described 0 1993 American Chemical Society
Doan et al.
4506 The Journal of Physical Chemistry, Vol. 97, No. 17,1993
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Figure 1. Emission spectra of luminescent porous Si samples etched in the dark (A), with 1.75 X 10’5 photon s-I cm-2 of 450-nm light (B),and 1.75 X lOI5 photon s-I cm-2 of 700-nm light (C). Excitation source for the PL spectra was the 442-nm line of a He/Cd laser (0.5 mW/cm2).
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0 20 40 60 80 100 Pholm Flux (photons-s”-cm”x Id’) Figure 2. Correlation PL enhancement to photoetch light intensity. Samples were uniformly illuminated with 442-nm light during the electrochemical etch. All samples were etched for 30 min and at a constant current density of 5 mA/cm2. The abscissa (Z/Zo)was measured as the ratio of integrated PL intensity of the photoelectrochemically etched samples ( I ) to integrated PL intensity of a sample etched in the dark (lo). The PL excitation source was a He/Cd laser operating at 442 nm and held at a constant power output (0.5 mW/cm2).
above, using blue (442 nm) irradiation. The PL intensity reached a maximum for electrodes etched under 4 X 1015 photon s-1 cm-2 irradiation. All of these electrodes had a mirror finish with dark brown surface coloration. At higher photoelectrochemical etch irradiation intensities,the PL intensity dropped off and the surface of the electrodes appeared cracked. AFM images (Figure 3) revealed that the samples etched under blue irradiation are rougher than those etched in the dark (area statistical rms roughness for samples A-C are 10,43, and 15 A, respectively). The photoetched samplescontain irregular surface features with dimensions in the size range of hundreds of nanometers. In contrast, dark-etched Si samples display pores with diameters on the order of nanometers,characteristicof porous Si that has been etched at low current densities.9JO Irradiation after Electrochemical Etch. To try to separate a purely photochemical etch process from one involving electric current flow (a photoelectrochemical etch), electrodes that were originally electrochemically etched in the dark were subjected to subsequent chemical etches (soaking) in the presence or absence of external illumination. Figure 4 displays the PL results. Initially,the p-type electrodeswere etched in the dark at a current density of 5 mA/cm2 for 30 min. The galvanostat was then disconnected and the electrodes were allowed to sit in the HF/ ethanol solution for an additional 30 min. During that time, the electrodes were either left in the dark (sample A in Figure 4) or illuminated with 450-nm light (sample B, Figure 4) or with 700-nm light (sample C, Figure 4). Irradiation intensity for both samples soaked in the light was 1.75 X 1015photon s-1 cm-2. As has previously been observed by Canham et al.,I the subsequent chemical etch leads to enhanced PL intensity and a blue shift in A., However, the PL spectra of samples irradiated with
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C. Figure 3. Atomic force microscopic images of the porous Si samples from Figure 1, etched in the dark (A), with blue 450-nm light (B), and 700-nm light (C). The z-axis (vertical) scale is the same in all three images; the black bar at the bottom-left corner of (A) represents 10 A.
blue light (Figure 4B) show an approximately 2-fold increase in intensity over those etched in the dark (Figure 4A) or with red illumination (Figure 4C). The PL spectra for samples illuminated with red light during thechemical etch (Figure 4C) do not change significantly from Si chemically etched in the dark for the same period of time (Figure 4A). These results are consistent with those obtained above for the samples irradiated during electro-
The Journal of Physical Chemistry, Vol. 97,No. 17,1993 4507
Photochemically Etched Porous Si
SCHEME I I Blue Red Light Light
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Figure 4. Emission spectra of luminescent porous Si samples etched in the dark and subsequently soaked in HF/ethanol solutions for 30 min. (A) Sample soaked in the dark. (B)Sample soaked while being illuminated with 1.75 X l o i 5photon s-I cm-2 of 450-nm light. (C) Sample soaked while being illuminated with 1.75 X lOI5 photon s-I cm-* of 700-nm light. Excitation source for the PL spectra was the 442 nm line of a He/Cd laser (0.5 mW/cm2).
Figure 5. Scanning electron microscope image of the surface of a porous Si electrode etched in the dark, and then soaked in HF/ethanol for 30 min, under blue irradiation. The image shows stress cracks induced during the chemical soak step. Images of electrodes before soaking were featureless at a comparable magnification.
chemical etch; only short-wavelengthirradiation has a significant effect on the PL spectrum of porous Si. The chemical etch process tends to produce cracked films (Figure 5 ) , while the electrochemical etch gives smooth, homogeneous films. Thus all the electrodes that had been soaked in HF/ethanol solutions after the electrochemical etch displayed cracks that were visible in an optical microscope. The cracks were only apparent on samples that had been soaked under open circuit conditions for at least 30 min. Cracks were not observed on any of the samples that had been etched exclusively under galvanostatic control, under any illumination conditions. Apparently, the chemical etch produces more strained porous Si films than the electrochemical etch, and the strain induces the formation of cracks.
Discussion Mechanism of Photocorrosion. It has previously been observed that a slow chemical etch of porous Si renders the material more visibly photoluminescent. The fluorescent material can be generated by immersion of porous Si in HF/ethanol solution for several hours, and the A, of PL shifts to higher energies with longer immersion times.’ The blue shift of the PL spectrum has been attributed to the thinning of “quantum wires” within the porous Si material, although other interpretations for the origin of the visible luminescence in porous Si have been proposed.11 Regardless of the mechanism of PL, the correlationof the emission spectrum to the extent of electrochemical or chemical etch has been confirmed by several workers.IJ2 In particular, the emission
HF/Ethanol Etching Solution
maximum of porous Si produced by a photoelectrochemical etch has been shown to correlate with the light intensity used during the e t ~ h . ~ - * J ~ Single-crystal Si electrodes immersed in H F etching solutions display rectification properties analogous to Schottky junctions.1416 The anodiccurrent necessary for the corrosion reactant that forms porous Si corresponds to the forward-bias current for p-Si, and the reverse bias current for n-Si. As in Schottky solar cells,17 illumination of a p-Si/solution interface generates a photocurrent that is opposite in sign to the forward-bias current, and thus illumination of p-Si electrodes inhibits the corrosion process (the forward-bias current correspondsto the corrosion current in p-Si). By contrast, illumination of n-Si/electrolyte systems causes an enhancement of corrosion in the illuminated areas because in that case the photocurrent corresponds to a corrosion ~ u r r e n t . ~ All of the Si samples used in the present work were p-type, and thus illumination is expected to inhibit corrosion. The electrochemical etch was always performed in a constant-current mode (galvanostatic), and so the net anodic (corrosion) current was the same on all samples, regardless of illumination intensity. Thus it is somewhat surprising that illumination with short wavelength light causes an enhancement in PL intensity (Figures 1 and 2) and a roughening of the surface (Figure 3), suggestive of a greater extent of corrosion. As will be discussed below, we interpret these results in terms of a separate photochemical corrosion process, that modifies the local corrosion rate as a function of depth within the porous Si layer. Reason for Enhancement with Blue Photocorrosion. Porous Si typically forms in a columnar structureon (100)-oriented Si.15J6J3 The electrochemical etch that produces porous Si can occur only in regions where the Si columns are in good electrical contact with the substrate. Thus, as the fibers within the porous Si layer become thinner, the holes required to etch the material are less likely to make it from the substrate into these thinner regions. In contrast, a photoetching process can be much more effective at etching the thinner regions. The short penetration depth of blue (450 nm) light in Si (0.43 pm)I9 ensures that electron-hole pairs are generated close to the fiber/solution interface (Scheme I). It is known that in bulk Si crystals, electronic holes generated close enough to the surface are capable of overcoming the builtin potential of the junction and crossing the interface.20 In the case of porous Si, this effect should become even more pronounced as the size of the Si fibers becomes smaller than the width of the space-charge region. Thus the close proximity of holes to the surface and the small dimensions of the Si fibers combine to enhance the photochemical oxidative etching process for short illumination wavelengths. Alternatively, red (700 nm) light has a penetration depth (8.4 pm)19greater than the thickness of the porous Si layer (0-5 pm), and so red illumination is expected to result primarily in excitation of the substrate (Scheme I). This is the more normal condition encountered for semiconductor/liquid junctions and can result in passivation of the oxidative corrosion process.17 Since the
4508 The Journal of Physical Chemistry, Vol. 97, No. 17, 1993
experiments in this work were performed under constant current conditions, red-light illumination has no effect on the net etch current density. The data for illumination of Si either during or after the electrochemical etch (Figures 1 and 4) support this interpretation. The dependence of PL intensity on photoetch intensity for 442 nm illumination (Figure 2) is interpreted as a balance between the rate of production of new luminescent material (via electrochemical or photochemical etch) and the rate of destruction of this material (via either electrochemical or photochemical dissolution). The initial rise of the curve in Figure 2 corresponds to an increase in production of highly photoluminescent material, driven by the blue photocorrosion process. At some point, the rate of photoetching will exceed the rate of production of new porous Si from the electrochemical process. When this happens, the total amount of luminescent material will decrease, and this corresponds to the dropoff in PL intensityat higher photocorrosion light intensities (Figure 2). A similar interpretation has been proposed for p s i etched with various intensities of white (tungstenhalogen) light.* Nature of the Luminescent Species. It should be pointed out that the data presented heredo not indicatea specific luminescence mechanism for porous Si." The enhanced corrosion with short wavelength light may produce a greater number of smaller quantum structures,or it may induce a differenttype of amorphous Si or other chemical species formation in the film. Raman data from Asano et al.s have shown that photoelectrochemically etched p-type porous Si is still crystalline, and infrared measurements on the samples in the present study showed no detectable surface oxide species (and the same ratio of Si mono- and dihydride stretching modes) on either light- or dark-etched samples. However, XPS and infrared data from Tsai et a1.8 indicate that the concentration of surface SiH2 species and silicon oxides correlates with the light intensity used during white-light photoelectrochemical etches. The data are consistent with either the quantum-confined Si model, with specific surface species acting as nonradiative traps, or with the luminescent chemical species model. Further studies into the chemical and structural nature of porous Si are required to illucidate the mechanism of luminescence.
Conclusions Short-wavelength illumination during electrochemical preparation of porous Si leads to as much as a 100-fold intensity
Doan et al. enhancement and a blue shift of the visible PL spectrum. The enhancement is interpreted as arising from a local photochemical corrosion process. Isolated, small Si structuresare more efficiently etched via this photochemical process than the electrochemical one. The short wavelength is important because of its smaller penetration depth. Soaking (chemical etching) induces cracks in the porous Si film, while photoelectrochemically etched electrodes are uniform to below 50 A. This implies that the chemical etch produces more strained porous Si films than the electrochemical etch.
Acknowledgment. Theauthors wish to thank Daniel F. Harvey and Julie L. Heinrich for assistance in obtaining IR data. The work at UCSD was supported in part by the U.S. Office of Naval Research, through Grant N00014-92-5-1810, and at UCI by the NSF Young Investigator Award through Grant DMR-9257000. References and Notes (1) Canham, L. T. Appl. Phys. Lett. 1990.57, 10461048. (2) Cullis, A. G.; Canham, L. T. Nature 1991,353, 335-338. (3) Lehmann, V . ; Gosele, U. Appl. Phys. Lett. 1990,58,856-858. (4) Doan, V . V.;Sailor, M. J. Appl. Phys. Lett. 1992, 60, 619-620. (5) Asano, T.; Higa, K.; Aoki, S.; Tonouchi, M.; Miyasato, T. Jpn. J . Appl. Phys. 1992, 31, L373-L375. (6) Suemune, I.; Noguchi, N.; Yamanishi, M. Jpn. J . Appl. Phys. 1992, 31, L233-L236. (7) Tsai, C.; Li, K.-H.; Kinosky, D. S.; Qian, R.-Z.; Hsu, T.-C.; Irby, J. T.; Banerjee, S.K.; Tasch, A. F.;Campbell, J. C.; Hance, B. K.; White, J. M. Appl. Phys. Lett. 1992,60, 1700-1702. (8) Tsai, C.; Li, K.-H.; Campbell, J. C.; Hance, B. K.; Arendt, M. F.; White, J. M.; Yau, S.-L.; Bard, A. J. J . Electron. Mater. 1992,21,995-1OOO. (9) Gomez-Rodriguez, J. M.; Baro, A. M.; Parkhutik, V . P. Appl. Surf. Sci. 1990, 44, 185-192. (10) George, T.; Anderson, M. S.;Pike, W. T.; Lin, T. L.; Fathauer, R. W.; Jung, K. H.; Kwong, D.L. Appl. Phys. Lett. 1992,60, 2359-2361. (11) Sailor, M. J.; Kavanagh, K. L. Ado. Mater. 1992, 4, 432-434. (12) Koshida, N.; Koyama, H. Jpn. J . Appl. Phys. 1991, 30, L1221L1223. (13) Doan, V. V.;Sailor, M. J. Science 1992, 265, 1791-1792. (14) Gaspard, F.; Bsiesy, A.; Ligeon, M.; Muller, F.; Herino, R. J . Electrochem. SOC.1989, 136, 3043-3046. (15) Smith, R. L.; Chuang, S.-F.;Collins, S.D. J . Electron. Mater. 1988, 17, 533-541. (16) Smith, R. L.; Chuang, S.-F.;Collins, S.D. Sens. Actuat. 1990, ,421A23, 825-829. (17) Lewis, N. S.J . Electrochem. SOC.1984, 131, 2496-2503. (18) Chuang, S.-F.; Collins, S.D.; Smith, R. L. Appl. Phys. Lett. 1989, 57,675-677. (19) Aspnes, D. E.; Studna, A. A. Phys. Reu. B 1983,27, 985. (20) Kumar, A.; Lewis, N. S.J. Phys. Chem. 1990, 94, 6002-6009.
