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Correlation of Raman and photoluminescence spectraof porous siliconR. TsuUniversity of North Carolina at Charlotte, Charlotte, North Carolina 28223H Shena) and M DuttaU.S. Army Electronics Technology and Devices Laboratory, Fort Monmouth, New Jersey 077034601(Received 13 September 199 1; accepted for publication 6 November 199 1)The discovery of luminescence in electrochemically etched porous silicon is an extremelyimportant scientific breakthrough with enormous technological implications. It opens the doorfor silicon, the most important microelectronic material, as a possible material foroptoelectronics applications. Our result, a correlation of Raman and photoluminescencespectra, shows that the observed luminescence is originated from extremely smallmicrostructures. As the luminescent peak increases in photon energy, the Raman featureshifts to lower energy, remaining sharp, and eventually splits, developing into TO and LOmodes. No peak at 480 cm- is observed, which indicates no substantial contributionfrom an amorphous region. These data provide strong evidence of the role of microstructuresin porous silicon.

A year ago in March, Lehmann and Gosele submittedtheir work to Applied Physics Letters on the optical ab-sorption of microporous silicon by electrochemical etchingin an electrolyte consisting of a 1:l ratio of ethanol andHF. They observed the upshift of the apparent absorptionedge from 1.1 eV, the indirect gap of crystalline silicon, toas high as 1.76 eV. This was attributed to the dramatictwo-dimensional quantum-confinement effect in quantumwires. Independently and shortly afterwards Canham re-ported the striking room-temperature luminescence spectraof microporous silicon having a peak position of 1.58 eVwith a luminescence efficiency far greater than the usualphonon-assisted transition. The observation of lumines-cence in microporous silicon is also attributed to quantumconfinement of needlelike structures having a diameter inthe order of 30 A. Very recently electroluminescence3 inthe visible range was observed during the anodic oxidationof porous silicon. This is an extremely important scientificbreakthrough with enormous technological implications,since it opens the door for silicon, the most importantmicroelectronic material, being useful for optoelectronicsapplications. There appears to be an important fundamen-tal issue, which is the role of momentum conservation inoptical transitions in indirect band-gap semiconductors.Crystalline silicon has a conduction-band minimum at theA point of the Brillouin zone (BZ), while the valence-bandmaximum is at the zone center of the BZ. With an indirectenergy gap of 1.1 eV, crystalline silicon is not expected toemit light in the visible. Using the Heisenberg uncertaintyprinciple, a microstructure of 30 A is not small enough tosignificantly relax the momentum conservation allowingtransition without the assistance of phonon. Therefore, onepossible origin of the 1.7 eV is from the pseudo-direct gapof amorphous silicon (a-Si). With a large surface-to-vol-ume ratio in the highly porous structure, it seems likelythat the significant fraction of the surface tissue regionLGEO-Centers, Inc., NJ. Operations, Lake Hopatcong, NJ 07849.

being amorphous is responsible for the observed lumines-cence. Although the transmission experiment of Lehmannand GBsele does support the bulk behavior, a careful Ra-man study of the phonon structure should provide addedsupport to the bulk phenomena.In this letter we present a correlated Raman and pho-toluminescence (PL) study of microporous silicon. Thedownshift of the Raman peaks correlated well with theupshift of the photoluminescence peak; a splitting of theLO and TO phonon was also observed in a sample with thehighest observed luminescence peak located at 1.86 eV.The average size from the splitting data was deduced to bein the order of 20 A. The present experimental resultsdiscard the possibility of the luminescence from the a-Sisurface tissue region, since it is difficult to understandwhy such a correlation would exist for a-Si.Single-crystalline silicon wafers (5 a cm, p type) wereetched using the procedures described in Ref. 1. The bestresults (judged by the intensity of photoluminescence)were found to be similar to the condition given, i .e., 55mA/cm2 electrochemically etch in an electrolyte of one toone 98% ethanol and 48% HF. All samples were etchedfor 20 min at room temperature. The surface of the etchedsample looks brown. Although reasonable care was takento insure a uniform etch, such as keeping the sample par-allel with the platinum screen, however, due to the pres-ence of a meniscus and possible bubbles, some nonunifor-mity was noticeable near the top of the meniscus.All optical measurements were made at room temper-ature, using an Ar + laser as an excitation with a wave-length at 4579 A. The power used was 50 mW. Sampleswere mounted on an X-Y stage. Both Raman and photo-luminescence spectra were taken at the same spot and dur-ing the same run. At each spot, three spectra, parallel,crossed polarized4 Raman, and unpolarized PL, weretaken. In contrast to the reference silicon, Raman scatter-ing from the porous silicon was unpolarized, indicating acomplete breakdown of the polarization selection rules for

112 Appl. Phys. Lett. 60 (l), 6 January 1992 0003-6951/92/010112-03$03.00 @I 1992 American Institute of Physics 112Downloaded 15 Oct 2008 to 192.167.72.93. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp

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s1I.-BzHa

A B C

I.8 2 2.2 2.4Photon Energy (ev)

FIG. 1. Room-temperature photoluminescence spectra from porous sili-con: A, B, C are the spectra taken at different spots of one sample.

(100) surface. In fact, the walls of porous silicon haveorientation other than (loo), which definitely voids the( 100) polarization selection rules for Rama n signal. There-fore, only the crossed polarized Raman spectra were dis-cussed below. Spectra (both Raman and PL) from differ-ent spots were different, especially near the top of themeniscus. Luminescence from regions on the sample closerto the meniscus usually peaked at higher energy, indicatinga smaller average size of the microstructure.Shown in Fig. 1 is the PL spectra from three differentspots of one sample. Spectrum A with a peak at 1.65 eVwas taken at a point farther away from the meniscus, whilespectrum C with a peak at 1.86 eV was taken from a spotclose to the edge of meniscus. Spectrum B was taken inbetween. During the electrochemical etch, the electric fieldlines made an angle to the (100) surface of the waferwithin the meniscus, and then presumably resulting insmaller microstructures. The detailed mechanism of thisphenomena is not fully understood.Figure 2 gives the Raman spectra of the same sampleat spots A and C. Also plotted in Fig. 2 is the Raman

490 500 510 520 530 5

7;c-Si

i\~

I' x10I \II

Raman Shift (cm-)

FIG. 2. Room-temperature Raman spectra from porous silicon (solidlines) and crystalline silicon (dashed line): A, C were taken at differentspots of one porous silicon sample.113 Appl. Phys. Lett., Vol. 60, No. 1, 6 January 1992

spectrum from reference crystalline silicon, which ispeaked at 522 cm ~ 2 and has a full width at half maxima(FWHM) of 3.0 cm - I. Raman intensity is 10 times stron-ger from porous sili con than from crystall ine silicon, per-haps due to the surface enhancement5 or resonance effect,6which is under further investigation. The peak position ofPL from sample A shifts downward by more than 3 cm -. .At spot C the Raman peak further shifts to 5 18 cm - andshows an additional peak at 510 cm r. All the Ramanpeaks from porous silicon remain reasonabl y sharp(FWH M = 5 cm - ) . The reason that Raman peaks shiftto lower energy for smaller microstructure is due to theconfinement of optical phonons. A finite particle size I cor-responds to a momentum q = f2~/1. Therefore, the posi-tion of the Raman peak reflects the position of the phonondispersion at q = =l=2r/Z. For large structure, q = 0, henceonly the zone center phonons are allowed in Raman scat-tering. Since the optical phonons (LO and TO) of siliconare degenerate at q = 0, only one peak at 522 cm - is seenin the Raman spectrum for crystalline silicon. If q#O isallowed, then it should be possible to observe both LO andTO provided that the linewidth is narrower than the sep-aration of the LO and TO phonons. With silicon, for LO= 5 18 cm - and TO = 510 cm - , phonon dispersion7gives I= 20 A. However, simple momentum relaxationmodelsY9 oes not predict this behavior. For example, in aPC-sil icon of characteristic size 20 A, the broadeningparameter8y9 s FWH M = 13 cm - , which is too large toobserve the separated LO and TO phonons in Raman spec-tra. In addition, we did not observe a broad peak at 480cm-, indicating that there is no substantial contributionto the Raman intensity from possible a-Si in the tissueregion near the surface. These facts indicate that the mi-crostructure is surprisingly free of disorder. Our conclu-sion supports the basic model promoted by Refs. 1 and 2,that the source of the luminescence is due to the quantumconfinement of microstructure having a characteristic di-mension of 20-30 A. Original ly the origin of the 1.7-eVpseudo-direct gap of a-Si was attributed to the I-L gap ofc-S& and the luminescence in the porous silicon was attrib-uted to the same origin. We want to state with emphasisthat our present experiments have dispelled this model.Simple application of the Heisenberg principle does notallow significant optical transitions even for a 20-W micro-structure. However , in forming quantum confinement,enough mixing of states occurs for different k, making theoptical transition possible.In conclusion, we have presented a correlated Ramanand photoluminescence study of porous silicon made byelectrochemical etching. As the luminescent peak increasesin photon energy, the Raman feature shifts to lower en-ergy, remaining sharp, and eventually splits, developinginto TO and LO modes. No peak at 480 cm ~ is observed,which i ndicates no substantial contribution from an amor-phous region. These data provide strong evidence of theexistence of microstructures in the order of 20-30 A.The work of R. Tsu was performed at the U.S. ArmyETDL , Fort Monmouth, under the sponsorship of the Sci-entific Service Program of GEO-Centers Inc. His work is

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also partially supported by AR0 and NOR. We are grate-ful to U. G6sele for helpful discussions, and to R. Zeto andR. Piekarz for their assistance in electrochemical etching.V. Lehmann and U. Gijsele, Appl. Phys. Lett. 58, 856 (1991).L. T. Canham, Appl. Phys. Lett. 57, 1046 (1990).sA. Halimaoui, C Oules, G. Bomchil, A. Bsiesy, F. Gaspard, R. Herino,M. Ligeon, and F. Muller, Appl. Phys. Lett. 59, 304 (1991).See, for example, E. Anastassakis, in Dynamica l Properties of Solid,edited by G. K. Horton and A. A. Maradudin (North-Holland, Am-sterdam, 1982), Chap. 3.

114 Appl. Phys. Lett., Vol. 60, No. 1, 6 January 1992

5See, or example, A. Otto, in Light Scattering in Solid Iv, edited by M.Cardona and G. Guntherodt (Springer, New York, 1984), p. 289.6See, or example, M. Cardona, in Light Scattering in Solid II, edited byM. Cardona and G. Guntherodt (Springer, New York, 1982), p. 19.G. Nolsson and G. Nelin, Phys. Rev. B 6, 3777 ( 1972).*H. Richter, Z. P. Wang, and L. Ley, Solid State Commun. 39, 625(1981).9R. Tsu, S. S. Chao, M. Ize, S. R. Ovshin sky, G. J. Jan, and F. H. Pollak,J. Phys. (Paris) 42, 269 (1981).J E. Smith, Jr., M. H. Brodsky, and D. Weaire, Phys. Rev. Lett. 30,I.141 (1973). R. Tsu, P. Menna, and A. H. Maban, Solar Cells 21, 189 (1987).

Tsu, Shen, and Dutta 114Downloaded 15 Oct 2008 to 192.167.72.93. Redistribution subject to AIP license or copyright; see http://apl.aip.org/apl/copyright.jsp