In-situ investigation of the growing a-Si:D surface by infrared reflection absorption spectroscopy

Journal of Non-CrystallineSolids 164-166 (1993) 103-106 North-Holland

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Section 3. In-situ characterisation

In-situ investigation of the growing a-Si:D surface by infrared reflection absorption

spectroscopy Yasutake TOYOSHIMA, Akihisa MATSUDA and Kazuo ARAI Electrotechnical Laboratory, Umezono, Tsukuba 305 JAPAN The bonded deuterium atoms on the growing surface of a-Si:D films are investigated by use of polarization modulated infrared absorption reflection spectroscopy (PM-IR-RAS), which is capable of monolayer detection for surface deuterides. Similar to the hydride coverages on the growing a-Si:H surfaces, a shift in bonding configuration from higher to lower (SiD3--->SiD2--->SiD) is observed with an increase of the growth temperature. A clear splitting (1545 and 1570cm-1) in the stretching mode absorption is observed at the 25°C growth and assigned to the SiD 3 species on the top surface. At higher temperatures, the deuteride coverage is, when compared to the hydride counterpart, slightly more resistive to the thermal desorption judging from the remainder of the coverages observed after the film growth at 370°C. 1. INTRODUCTION Deuterated amorphous silicon (a-Si:D) is an interesting material from the view point of managing the photodegradation because its behaviors on light soaking are different from those of hydrogenated ones (a-Si:H) [1-3]. It has been suggested in detailed studies [1,2] that the difference originates from the structures of the Si network rather than from the simple mass difference between H and D. Since the network structures are primarily determined in the growth processes, it is thus important to study the growth mechanism in a-Si:D. An outstanding feature already reported in a-Si:D growth is a shift in the onset of the steep increase in growth rate to the higher temperature side, which ispossibly explained in the difference of the thermal desorption rates, namely the heavier the atom, the slower the rate, of the surface coverages by hydrides or deuterides [3]. We have investigated the surface coverage by means of polarization modulated (PM) infrared absorption reflection spectroscopy [4-7], and been successful in identifying the monolayer hydride coverage of growing a-Si:H surfaces with the aid of the isotope substitution technique [6]. In this report, we investigate the surface coverages of a-Si:D films after the growth as well as during the growth for further understanding of the growth kinetics in these materials, with a focus on the high temperature difference in H and D on the growing surfaces, 2. EXPERIMENTAL The experimental setup for this work is basically identical to the previous ones [4-7]. The film growth

(0.3A/s) is performed with a SiD4 (Tri-Chemical Inc.) flow rate of 5 cm3/min, total pressure of 20mTorr and rf power of IW in a conventional diode reactor as employed before [6,7]. However for IR observations, since the stretching band of SiDx species completely overlaps the annoying water bands, it is absolutely necessary to avoid this interference. For this purpose, Mo substrates are used instead of previous AI substrates. When AI substrates are used, the surface oxides and water molecules bonded therein will react with the growing a-Si:H (or a-Si:D) film and, as a result, induce significant distortion in spectral baselines [7]. On the other hand, the surface oxides of Mo substrates are deoxidized with Sill4 exposure before the film growth and the water bands almost disappear with this deoxidation [7]. Several other factors, such as an intensive dry N2 purging of the transfer optics and a good stability of ambient temperature, are also critical for SiDx observations to minimize the water interference. The procedure to obtain the PM spectra should be described here because of its importance from the view point of time resolution. Every PM spectrum described below is always composed of a set of P and S polarization spectra [7]. In order to extract the film absorptions from the P spectrum, the normalization of the P spectrum by the S spectrum is necessary because the P spectrum carries information about the gas phase ambient in addition to the film absorptions while the S spectrum only the ambient, due to the surface selection rules [6]. Although it is ideal to take both P and S spectra at the same time, we employ sequential acquisition of the couple of spectra, say first P and then S spectrum, because of

0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All fights reserved.

104

Y. Toyoshima et al. / In-situ investigation of the growing a-Si:D surface

the modulation consistency in the use of a Fourier transform spectrophotometer. It takes almost 2 minutes to obtain a single spectrum that is an average of 256 scans, so there is as much time difference between each P and S spectrum. The changes under film growth during this interval are usually small enough to be neglected, but not for the surface monolayer detection because the surface signal is also small. Thus, unless observations during the film growth are absolutely essential, we have interrupted the film growth so that the couple of P and S spectra are obtained, in a practical sense, at the same time. An additional advantage in growth interruption is that a quasi-time resolution of a few seconds can be accomplished as shown in the following section, 3. RESULTS AND DISCUSSION 3.1. Surface deuterides at 25°C growth The detection scheme of surface coverage is illustrated in Fig.1. First, a-Si:D film is deposited from SiD4 for 20 min on the Me substrate. At the last stage of this growth, the during-growth spectrum, Fig.l(a), is obtained. Then, after the ideal growth of an a-Si:H monolayer, the surface deuteride coverage is totally replaced by the hydride coverage, as illustrated in Fig.l(b). Because of the enhancement in absorption signal intensity specific to the top surface, which originates from the low refractive index thereof [6], the hydride coverage absorptions are predominant in the hydride band regions of PM spectra, until the overlayer (a-Si:H) thickness is a few tens of ,~. Further deposition of overlayer brings the spectra of Fig.l(c) where the bulk hydride absorptions start to appear. To make

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these changes in PM spectra more clear, each PM spectrum under exchange is normalized by the initial PM spectra obtained during the a-Si:D growth, namely (d)=(b)/(a) and (e)=(c)/(a). Note that in Fig.l(d) and (e) spectra (let us call these the PM difference spectra), the exchanged (lost) part absorptions appear as the upward features while newly-deposited (emerged) part appear as the downward in PM difference spectra because they are displayed in transmittance here in our case. Figure 2 shows the quasi-time resolved changes obtained in the intermittent exchange operations. At around 1600cm-1, the SiD x species appear as the upward features and the hydride counterparts downward at around 2100cm-1. A prominent feature in this observation is that there is a clear doublet appearing in the deuteride mode region. Judging from the dominance in trihydride at this temperature [6] and from the reported SiD 3 splitting on the crystalline Si surface [8,9], the doublet feature (peaks at 1545 and 1570cm-1) is assigned to the stretching mode vibration of the SiD3 species on the growing a-Si:D surfaces. A presence of the bending doublet at 655 and 670cm-1 observed here (see Fig.3) supports this assignment. According to our scheme shown in Fig.l, only a monolayer growth will be sufficient for a total isotope exchange of the surface coverage. Our ~ 6m~ 3m 5iH3 I.tJ t..) Z < r-

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105

Y. Toyoshima et al. I ln-situ investigation of the growing a-Si.'D surface

observation shown in Fig.2 is compatible with this view since the intensity of deuteride mode (upward) almost saturates at 6s exchange, which corresponds to 2A, a thickness comparable to the Si monolayer, The hydride counterpart, which shows similar saturation in the downward intensity shift at an earlier stage of exchange, afterwards shows the additional increase in the band tail of lower frequency side, as seen in the 6min spectrum in Fig.2. This should originate from the bulk hydride contribution to this absorption band. In a prolonged exchange, we found an additional change in the deuteride band due to the further substitution of bulk D by H especially at elevated temperatures. We set the exchange time at lmin in the surface monitoring shown in Fig.3 because of a practical compromise between these exchange behaviors and experimental convenience.

Table 1 Vibrational frequencies (cm-1) of surface SiDx species Crystalline Si, stretching mode [8,9] Isolated Clustered SiD 1512 1510,1515 SiD2 1534 1525,1545 SiD 3 1550 1525,1560 Amorphous (present work) Stretching SiD 1515 SiD2 1520,1540 SiD~ 1545,1570

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3.2. Temperature dependence of bonding configurations In Fig.3, the absorption bands of surface SiDx species and their hydride counterparts are shown for various temperatures. Because of the limitation in the acquisition time mentioned earlier, only the deuteride features are obtained during the growth. So the hydride absorptions above 350°C in Fig.3 are not correct due to the thermal desorption [6]. We have assigned the 100°C bands mainly originating from the SiD2 species because there is a bending doublet whose frequencies are slightly smaller than those observed in 25°C spectrum. At 240°C, almost no features are observed in the SiDx bending region, so this can be ascribed to the SiD dominance with possible contribution of isolated SiD2. The SiD species should be dominant with further increase of temperature. Table 1 summarizes these assignments with reported ones of the crystalline surface. When one considers the broad band nature in amorphous materials, their agreement looks acceptable. We found no prominent difference in the preferred bonding configuration on temperature between H and D. 3.3. Thermal desorption at high temperature Although we tried to track the presumed difference in the thermal desorption rate between H and D by monitoring the coverages during the film growth, we failed to see a meaningful difference in PM spectra probably because the thermally desorbed site can be cured by the arriving precursors (mainly Sill 3 or SiD3) during the film growth and thus the coverage loss would be less than a few percent of full coverage, which is well below the detection limit of our instrument. We then have to investigate the coverages

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106

Y. Toyoshima et al. / In-situ investigation of the growing a-Si:D surface

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FREQUENCY (cm-1) Figure 5. Observations of the thermal desorption of surface coverages after growth for H (left) and D (righ0 at 370"C. The during-growth coverages (iii) [Fig.4(f)] are divided intO the thermally-desorbed (i) and the remained (ii) fractions, which correspond to Fig. 4(d) and (e), respectively.

We have investigated the surface deuteride coverages of the growing a-Si:D films by PM-IRRAS with an isotope substitution technique. Similar tO the hydride case, a shift in bonding configuration from higher to lower is observed with increasing growth temperature. A tentative assignment of the vibrational frequencies is given for surface SiDx species on a-Si:D films. Evidence is shown to prove that, although the difference is rather delicate, the deuteride coverages are more resistive to thermal desorption than the hydride ones.

ACKNOWLEDGMENTS after the film growth in a scheme described in Fig.4. First, PM spectrum (a) is observed during the film growth. Then, after this growth and thus the reactor is in a high vacuum, the post-growth spectrum (b) is obtained. The start of spectrum (b) acquisition is exactly 30s after the discharge has turned off. At this moment, a certain fraction of surface coverage is already thermally desorbed, but there still exists the remainder of surface deuterides. Finally, the remainder coverage is totally substituted by the isotope overlayer, which results in Fig.4(c). By taking the changes in these spectra, we can get the PM difference spectra of the thermally desorbed fraction (d) by (a)/(b), as well as the remainder fraction (e) by (b)/(c) after the growth, out of the during-growth coverage (f) by (a)/(c). The results are shown in Fig.5 at a substrate temperature of 370°C. It can be seen from these spectra that about 1/3 of during-growth coverage(iii) remains after growth in the D case[(ii) left], while this is about 1/4 in the H case[(ii) right]. Thus, based on this observation at 370°C, we conclude here that the thermal desorption rate in surface SiD coverage is strictly slower than that of Sill. Our preliminary interpretation of this normal isotope effect on desorption rate is related to the preexponential factor, rather than to the activation energy in Arrhenius type expression, namely, simply the heavier the atom the slower the rate, as is observed on the crystal Si surfaces [10]. Our detection scheme of Fig.5 has not worked well at

The authors are grateful to Drs. G. Ganguly, H. Okushi and K. Tanaka for their interest and continuous encouragement throughout this work. REFERENCES 1. G. Ganguly, A. Suzuki, S. Yamasaki, K. Nomoto and A. Matsuda, Appl. Phys. Lett., 68 (1990) 3738. 2. G. Ganguly, S. Yamasaki and A. Matsuda, Phil. Mag. B, 63 (1991) 281. 3. G. Ganguly and A. Matsuda, Jpn. J. Appl. Phys., 31 (1992) L1269. 4. Y.Toyoshima, K.Arai, A.Matsuda and K.Tanaka, Appl. Phys. Lett., 56 (1990) 1540. 5. Y.Toyoshima, K.Arai, A.Matsuda and K.Tanaka, Appl. Phys. Lett., 57 (1990) 1028. 6. Y.Toyoshima, K.Arai, A.Matsuda and K.Tanaka, J. Non-Cryst. Solids, 137/138 (1991) 765. 7. Y. Toyoshima, Thin Solid Films, in print. 8. V. A. Burrows, Y.J. Chabal, G. S. Higashi, K. Ragha~,achari and S. B. Christman, Appl. Phys. LeU., 53 (1988) 998. 9. Y.J. Chabal, G. S. Higashi, K. Raghavachari and V.A. Burrows, J. Vac. Sci. Technol., A7 (1989) 2104. 10. B.G. Koehler, C.H. Mak, D.A. Arthur, P.A.Coon and S.M.George, J. Chem. Phys., 89 (1988) 1709.