Correlation between the photoluminescence and chemical bonding in porous silicon

Thin Solid Films 255 (1995)

Correlation

191-195

between the photoluminescence porous silicon

and chemical bonding in

D. Dimova-Malinovska”%*, M. Sendova-Vassileva”, Ts. Marinovab, V. Krastevb, M. Kamenovaa, N. Tzenov” -‘Central Luboratory for Solar Energy and New Energy Sources, Bulgarian Academy of Sciences, 72 Txrigratlsko Chausser Blvd.. 1784 Sofia, Bulguria hlnstiturr of Gunerul and Inorganic Chemistry, Bulgarian Academy of Scirn~es. Acad. G. Bontchev Str.. BI. I I. 1I I3 So/iu. Bulguria

Abstract The changes in the visible photoluminescence and Raman spectra of n-type porous silicon as a result of rapid thermal annealing and chemical treatment have been studied. Modification of the chemical bonds due to these treatments has been observed by X-ray photoemission spectroscopy. Our results show that the change in the luminescence spectra of porous silicon is related to the modification of the chemical bonds and that complexes such as the terminations in siloxene as well as silicate molecules play a role in determining the optical properties of this new photonic material. The surface containing siloxene-like bonding exhibits

very intensive photoluminescence. The presence of silicates leads to a decrease in luminescence intensity and to the appearance of a weak high energy band in the photoluminescence spectra. Keytvord.s: Etching;

Luminescence;

Raman

scattering;

X-ray photoelectron

1. Introduction Porous silicon (PS), which has been studied for the last 15 years as a material for microelectronics [ 11, has stimulated considerable interest in the scientific community because of its visible photoluminescence (PL) reported first by Pickering et al. [2] and more recently by Canham [3]. This remarkable property of PS offers the possibility that the material will have uses in optoelectronic applications. Unfortunately, the mechanism responsible for the PL is not well understood and various models have been proposed for it. Apart from the physical quantum confinement model [3], several alternative mechanisms such as chemical quantum confinement in a siloxene molecule [4], amorphous Si formation [ 51 and a surface state mechanism [6] are being considered. Although the explosion in the study of the optical, vibrational and structural properties is a reality, very few papers have been reported on the relation between PL and chemical bonding by using X-ray photoemission spectroscopy (XPS) [7-91. On the

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author.

0040-6090/95/$9.50 (“ 1995 ~ SSDI 0040-6090(94)05652-8

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spectroscopy

other hand, various chemical [lo] and thermal [ 1 l] treatments of PS result in an energy shift of the PL peak and several suggestions have been given to explain this behaviour. In this connection it is important to relate the PL changes of PS to the chemical bonding as a result of the different treatments. The aim of this work is to look for a correlation between visible PL and chemical bonding on the PS surface as well as to study the changes in the chemical bonds as a result of chemical and thermal treatments of PS.

2. Experimental

details

The porous silicon layers were formed on n-type Si = ( 100) wafers (resistivity about 0.004 R cm). The samples were anodized in 49%HF:H,O:C,H,OH = 2: 1:l electrolyte at 80 mA cm-* for 6 min under 250 W halogen lamp illumination from 9 cm distance. Some of the samples were treated by rapid thermal annealing (RTA) in a vacuum of 5 x lo-’ Pa for 1 min at 800 and 1000 “C. As-anodized and annealed PS layers were subjected to chemical treatment (CT) in

192

D. Dimova-Malinovska

et al.

I Thin Solid Films 255 (1995) 191- 195

49O/HF:H,O:HNO, = 50:50:1 solution for 1 and 4 min and then rinsed in deionized H,O. The XPS studies were carried out in an Escalab MK II (VG Scientific) electron spectrometer with base pressures in the preparation and analysis chambers of 2 x lops and 1 x lo-* Pa respectively. The photoelectrons were excited using an X-ray source (Mg Kor, 1256.3 eV). At an analyser pass energy of 20 eV the instrumental resolution measured as the full width at half-maximum (FWHM) of the Ag 3d,,, photoelectron peak was 1.2 eV. The surface sensitivity was estimated as 10 ML (monolayers). Binding energies (BEs) of the C Is, 0 1s and Si 2p peaks were measured with an accuracy of fO.1 eV. The normalized XPS intensities (Z/a), which are proportional to the effective concentrations of the corresponding elements in the surface layer [ 121, were determined as the integrated peak areas I divided by their corresponding photo-ionization crosssections c taken from Ref. [ 131. The background in the peak area computation was assumed to be linear. Photoluminescence and Raman measurements were performed on a double-monochromator Spex 1403 with a photomultiplier in photon-counting mode. The excitation was made by 488 nm Ar+ laser radiation. A 15 mW incident power was employed. The spectral resultion was 2.5 cm-‘.

3. Results and discussion The elemental abundances in as-deposited, CT and RTA samples (before and after CT) are listed in Table 1, The elemental content in the near-surface region was measured to be 37”/0 Si, 52% 0 and 11% C for asdeposited PS. These data are very close to those reported in Ref. [8]. A very small concentration of F ( < 0.5 at.%) was detected within the limits of detection. It should be noted that the C concentration increases in the depth of the PS layer. The origin of the C could be related mainly to the C,HSOH in the electrolyte, though some absorption from the air is possible. Scanning (SEM) and transmission (TEM) electron microscopy analyses show that the surface of PS has an Table 1 Elemental concentrations PS samples Time of CT (min)

0

I 4

(from

Unannealed

XPS) in as-prepared,

samples

CT and RTA

RTA ( 1000 “C) samples

C (at.%)

0 (at.%)

Si (at.%)

C (at.%)

0 (at.%)

Si (at.%)

11 26 45

52 13 20

37 61 35

37 19 20

39 21 24

24 60 56

4 3 2

1 280 (a)

285 290 binding energy (eV)

95

100 105 Binding energy (eV)

(b)

Fig. 1. XP spectra of C Is (a) and Si 2p (b) peaks of PS samples as prepared (curves 1) and after CT for 1 min (curves 2) and 4 min (curves 3). Curve 4 corresponds to the sample subjected to 4 min of CT followed by 10 min of Ar ’ sputtering.

amorphous structure and the pores are larger at the c-Si-PS interface. The electrolyte located in these pores stays there even after drying of the sample, because the existing amorphous layer on top of the PS layer slows down the evaporation of the electrolyte containing C,H,OH. Thus a higher concentration of C could be expected at the interface. XP spectra show that the distribution of the C concentration becomes more homogeneous in the depth of the PS layer after RTA owing to diffusion at the annealing temperature. Fig. 1 shows the C 1s and Si 2p peaks from anoditally oxidized Si. The carbon in the near-surface region is present with one maximum at 284.5 eV (Fig. l(a), curve 1). The CT in 49’%HF:H,O:HNO, = 50:50:1 solution for 1 min followed by water rinsing results in the appearance of an additional higher energy shoulder at 286.6 eV (curve 2) which is related to COH bonding. After CT for 4 min an asymmetry of the peak appears in the lower energy region at 283.1 eV (curve 3) owing to amorphous Sic. The higher C content (45 at.%) in the depth of the PS layer than on the surface of PS probably stimulates the creation of amorphous SIC. As can be seen in Fig. l(b) a large Si 2p peak centred at 101.4 eV, which corresponds to the + 2 state of Si, is present in the near-surface region (curve 1). This band is probably a superposition of several bands which Pearsall et al. [8] associate with hydride and hydroxyl terminations such as are present in siloxene. Our recent results [ 141 of an XPS study of as-prepared and annealed powder siloxene show similar Si 2p spectra. A shoulder at 98.7 eV, which is typical of amorphous silicon, is also observed in Fig. l(b) (curve 1). The presence of a-Si on the surface of the as-prepared PS layer is confirmed by TEM and SEM. The Si 2p peak becomes sharper and its position shifts to lower binding energy, 100.5 eV after 1 min of CT (curve 2). According to Watanabe et al. [ 151, this 1 eV shift is probably related to the presence of Si-Si, and Si-SiO, configu-

D. Dimova-Malinovska

280 (a)

’ 265 290 Binding energy (eV)

96

et rd. 1 Thin Solid Films 255 (1995) 191- 195

101 106 Binding energy (eV)

(b)

I’)3

have been subjected to 10 min of Ar sputtering with 3 keV energy in the XPS equipment. A change in the high binding energy region after ion etching is observed: the shoulder due to OH and silicate bonds disappears. No significant changes are observed in the C 1s peak after Ar+ sputtering. Fig. 3 illustrates the PL spectra of PS samples as prepared (curves 1) and after 1 min (curves 2) and 4 min (curves 3) of CT and RTA. The non-treated sample shows strong visible luminescence with a maximum at about 710 nm. After chemical treatment the PL intensity decreases sharply, the peak shifts to the “blue” and a shoulder at about 560 nm appears. The PL

Fig. 2 XP spectra of C 1s (a) and Si 2p (b) peaks of RTA samples at 1000 C before (curves I) and after CT for I min (curves 2) and 4 min (curves 3). Curves 4 corresponds to the sample subjected to 4 min of CT followed by 10 min of Ar sputtering.

rations. Additionally, a weak wide peak at about 103.2 eV is observed. It probably consists of two different peaks, one due to the Si-OH groups [ 161 and the other due to the silicates [ 151. After a longer CT of 4 min the shape of the Si 2p peak changes (curve 3): the maximum is observed at 100.7 eV, which is due to Si-Si, and Si - SiO, bonding and to Si-C bonds in a-Sic, as has already been mentioned. A very well expressed shoulder in the higher binding energy region is observed which is attributed to an increase in the quantity of Si bonded to OH groups and of Si bonds in silicates [ 15, 161. This oxidation is a result of the chemical treatment in the etching solution followed by rinsing in water, as has been observed for oxidation of c-Si [ 161. Fig. 2 displays the XP spectra of C 1s and Si 2p peaks of PS samples after RTA at 1000 “C. A sharp peak in the C Is spectrum, centred at 284.5 eV, is observed in the near-surface region (Fig. 2(a), curve 1). A peak at 287.5 eV with lower intensity and larger FWHM is present too and is probably related to a fourfold-coordinated carbon (C-S&) which is created during the thermal treatment. After 1 min of CT of the RTA sample a shoulder at about 286.6 eV appears in the C 1s spectrum owing to the presence of C-OH bonding (curve 2). The CT for 4min results in a broadening of the C 1s peak and better expression of the shoulder at 286.6 eV. In the near-surface region of the RTA sample the Si 2p peak has a maximum at 103.1 eV (Fig. 2b, curve 1) which is typical for Si in silicates [8, 171. A low intensity shoulder at 99.4 eV due to amorphous Si is observed. This shoulder becomes a very intense peak at 99.5 eV after 1 min of CT and a weak wide peak at 103.0 eV due to Si in Si-OH and SiO, is observed too. A very intense peak at 100.3 eV due to Si-Si, and Si-SiO, is seen after 4 min of CT. Curves 4 in Fig. 1(b) and 2(b) are the Si 2p peaks of as-prepared and RTA samples after 4 min of CT which

I

(a)

I

600 (b)

I

600 Wavelength

700 (nm)

700 Wavelength

800

860 (nm)

Fig. 3. Luminescence spectra of PS samples as prepared (curves 1). after CT (a) for 1 min (curve 2) and 4 min (curve 3) and after RTA (b) at 800 ‘C (curve 2) and 1000 -C (curve C). Curves 4 correspond to samples subjected to 4 min of CT (a) and 1000 C RTA (b) followed by 10 min of Ar ’ sputtering.

D. Dimova-Malinovska et al. 1 Thin Solid Films 255 (1995) 191L195

194

intensity decreases with the temperature of RTA as can be seen in Fig. 3(b). The higher energy band at about 560 nm appears after RTA at 1000 “C while the “red” one almost disappears. The PL spectra of the 4 min CT samples (unannealed and after RTA) measured after 10 min of ion etching by Ar+ are shown in Fig. 3 (curves 4) too. The Ar+ etching results in an increase in the PL intensity of the “red” band and the disappearance of the 560 nm band. The Raman spectra of as-formed and RTA samples before and after CT are shown in Fig. 4. The spectra in the range 450-550 nm were fitted with gaussians and

Shift

400 (cm-‘)

600

Raman

Shift

400 (cm-‘)

600

Raman

260

200

Fig. 4. Raman spectra of PS samples prepared (curves 1) after CT (a) for 1 min (curve 2) and 4 min (curve 3) and after RT (b) at 800 “C (curve 2) and 1000 “C (curve C). Curves 4 correspond to samples subjected to 4 min of CT (a) and 1000 “C RTA (b) followed by 10 min of Ar+ sputtering. The peak of S-5 transverse optical vibrations is centred at 521 cm-‘.

lorentzians. The spectrum of the as-prepared sample consisits of two bands: one very intense peak centred at 522 cm-’ and the other less intense and broader at 511 cm-‘. These peaks could be related to the Si-Si vibrations from the crystalline substrate and the very large crystallites in the PS layer and from the nanocrystallites in the PS layer respectively according to Kozlowski and Lang [ 181.After RTA the FWHM values of these peaks decrease, which could be related to the increase in the crystallite size in the skeleton of PS as our SEM study shows. An additional broad band at 490 cm-’ is observed as well. In the Raman spectra of the CT samples an additional band at about 430 cm-’ appears (Fig. 4(a), curves 2 and 3). The bands at about 490 and 430 cm-’ are probably due to vibrations in silicates as has been reported in Ref. [ 193.Although Fuchs et al. [ 201have attributed the broad Raman band from 430 to 490 cm-’ to the vibrations of the Si, rings in siloxene showing a very intense PL signal, our XPS study confirms that this Raman band in PS is present in the spectra of samples containing silicate bonds on their surface. These bonds are destroyed during the Ar+ sputtering and are not present either in Raman (Fig. 4, curves 4) or in XP spectra of Ar+ sputtered samples (Figs. l(b) and 2(b), curves 4). Our results show that strong visible PL is observed from the as-anodized sample in which XPS detects the presence of hydride or hydroxyl terminations such as in siloxene as well as an amorphous film on the surface of PS. After CT and RTA the chemical bonding changes: silicate, Si-Si,, Si-Si03, Si-C and C-OH bonds appear, the PL intensity decreases and the position of the PL peaks changes as well. Similarly, a decrease in the “red” PL and the appearance of a higher energy band around 580-620 nm after etching of PS in dilute KOH have been observed by Li et al. [IO]. The authors have related this with two effects: etching of PS and release of hydrogen from the surfaces. In our case the chemicals used for treatment have two effects: etching and oxidizing. It is shown by Fujinami and Chilton[ 161 that the chemical structure of the c-Si surface after oxidation in pure water contains oxidized Si-OH bonding. In our case the chemical treatment in 49%HF:H,O:HNO, = 50:50:1 solution followed by water rinsing as well as by RTA results in the surface oxidation of nanocrystallites in PS samples, a decrease in the PL intensity and the appearance of a high energy band in the PL spectra (560 nm) and the SiO, band in the Raman spectra. This oxidized film seems to be thin and Ar+ sputtering destroys it, as a consequence of which an intense “red” PL signal appears again.

4. Conclusions Our results show that the change in the luminescence spectra of porous silicon is related to the modification

D. Dimouu-Malinooska

et (11. I Thin Solid Films 255 (1995)

of the chemical bonds and that complexes such as the terminations in siloxene as well as silicate molecules play a role in determining the optical properties of this new photonic material. The surface containing -H and -OH bonding exhibits very intense photoluminescence. The presence of silicates leads to lower luminescence intensity and to the appearance of a high energy band in the PL spectra.

Acknowledgements

The authors thank Dr. G. Beshkov for his help with the RTA experiments. This work has been supported by the Bulgarian National Scientfic Foundation (contracts F234 and X426) and the Commission of the European Communities (PECO Project # 7839).

References [I]

Y. Watanabe. Y. Arita. Y. Yokoyama and Y. Igarashi, .I. Electrochetn. Sot.. 122 (1975) 1351. [2] C. Pickering. M. J. J. Beale, D. J. Robbins, P. J. Pearson and R. Grecf. J. P~JY. C: Solid Srate Ph,ys., 17 (1984) 6535. [3] L. T. Canham, Appl. P/y. Lert., 37 (1990) 1046. [4] P. Deak, M. Rosenbauer. M. Stutzmann, Y. Weber and M. Brandt. Phy. Rec. Lerf.. 69 (1992) 2531.

191~ 195

195

[5] M. Voos. Ph. Uzan, C. Delalande. G. Bastard and A. Halimaoui. Appl. Phys. Lett., 61 (1991) 1213. [6] F. Koch, V. Petrova-Koch. T. Muschik. A. Nikolov and V. Gavrilenko. MRS Synp. Proc,., 238 ( 1993 I 197. [7] M. Jeske, K. G. Jung, J. W. Schultze, M. Thonissen and H. Miinder. Ah.r/r. 5111 Con/. ECASIA‘93. (‘tr/crtlitr. Ocrohrr 1993 p. 127. Surf Intw/frrcc And.. in Press. [8] T. P. Pearsall. J. C. Adams. J. N. Kidder, P. S. Williams, S. A. Chambers. J. Lath. D. T. Schwartz and R. Z. Nosho. Thin Solid F;/nzs. 22-7 ( 1992) 200. [9] J. R. Dahn. B. M. Way, E. W. Fuller. W. J. Wcidanz. J. S. Tee. D. D. Klug. T. Van Burren and T. Ticdjc. J. ,3pp1. P/I~x., 7.~ (1994) 1946. [lo] K.-H. Li, C. Tsai. J. Sarathy and J. C. <‘ampbell. .4pp/. Phv.~. Le/l.. 62 (1993) 3192. [I I] R. Kumar, Y. Kitoh and K. Ham. .4pp/. Ph~.v. Lee/.. 63 ( 1993) 3032. [I?] M. P. Seah and M. T. Anthony. Srrr/. /nt~,r:/ircx,, .4ntrl., 6 (1984) 730. [ 131J. H. Scofield. J. Hectron Spec~rrosc~.Rcklr. Phwwm.. 8 ( 1976) 129. [ 141 D. Dimova-Malinovska. M. Sendova-Vassileva. Ts. Marinova and V. Krastev. unpublished. [ 151 H. Watanabe, K. Hagd and T. Lohner, J. .Yon-Cr>,.sr. Solids. 164 166 (1993) 1085. [ 161 M. Fujinami and N. B. Chilton, Appi. Phys. Le/r.. 63 ( 1993) 3458. [ 171 B. Herreros. H. He. T. L. Rarr and J. Klinowsi. J. Pi1y.s. C&m.. 98 ( 1994) 1303. [ 181 F. Kozlowski and W. Lang, J. Appl. Plr~,.c.. 72 (1992) 5401. [ 191 F. L. Gallener. A. J. Leadbetter and M. W. Stringfellow. Ph~r. RN. B. -17 (1983) 1052. [20] H. Fuchs. M. Stitzmann, M. Brandt, M. Rosenbauer, J. Weber. A. Breitschwerdt, P. Deak and M. Cardona. Plz>s. Rw. B. 4X (1993) x171.