P. JOHN e t al. : Infrared Vibrational Modes in Amorphous Silicon 607

phys. stat,. sol. (b) 104, 607 (1981)

Subject classification: 2 and 20.1; 6; 22.1.2

Department of Chemistry (a) and Department of Physics (b), Heriot- Watt Unioeraity, Riccarton, Currie, Edinburgh1)

Studies of the Oscillator Strengths of Infrared Vibrational Modes in Glow-Discharge Hydrogenated Amorphous Silicon BY P. JOHN (a), I. M. ODEH (a), M. J. K. THOMAS (a), M. J. TRICKER^) (a), and J. I. B. WILSON (b)

The infrared oscillator strengths of the stretching and wagginglrocking modes in glow-discharge hydrogenated amorphous silicon, a-Si, is compared to similar data reported recently for sputtered material. For both materials the integrated band intensity of the wagging/rocking mode a t 640 cm-l is proportional to the total hydrogen concentration, NH, and is independent of preparation condi- tions. The derived oscillator strength for the latter mode is identical in glow-discharge and sputter- ed a-Si films. I n contrast, the integrated band intensity of the composite silicon-hydrogen stretch- ing mode is film dependent; the derived average oscillator strength, F, (2000 + 2100), is signifi- cantly smaller in glow-discharge, compared to sputtered a-Si. A reanalysis of previous isothermal dehydrogenation data is presented in which I', (2000 + 2100) is shown to be a function of N , and increases markedly a t hydrogen contents < 10 atyo. No conclusions can be drawn, however, con- cerning the oscillator strengths of the deconvoluted bands a t 2000 and 2100 em-l occurring within the stretching region. A previously proposed model for the mechanism of the dehydrogenation of a-Si is unaffected by the present results.

Die Infrarot-Oszillatorstarken der ,,Stretching"- und ,,Wagging/Rocking-Moden" in hydrogenisier- tem amorphem Silizium, a-Si, das durch Glimmentladung hergestellt wurde, werden mit ahnlicheii Werten verglichen, die kurzlich fur gesputtertes Material berichtet wurden. Fur beide Materialien ist die integrierte Bandenintensitat der ,,Wagging/Rocking"-Mode bei 640 cm-I proportional zur gesamten Wasserstoffkonzentration, NH, und unabhangig von den Praparationsbedingungen. Die abgeleitete Oszillatorstarke fur letztere Mode ist identisch in a-Si-Schichten, die sowohl durch Sputtern als auch durch Glimmentladung hergestellt werden. Im Gegensatz dazu ist die integrierte Bandenintensitat der kombinierten Silizium-Wasserstoff ,,Stretching"-Mode schichtabhangig; die abgeleitete mittlere Oszillatorstlirke, Fs (2000 + 2100) ist betrachtlich geringer in Glimnientladungs-a-Si verglichen mit gesputtertem a-Si. Eine erneute Analyse fruherer isothermer Dehydrogenisierungswerte wird durchgefuhrt, in der gezeigt wird, daI3 F, (2000 + 2100) eine Funktion von N H ist und bei Wasserstoffkonzentrationen <I0 Atyo merklich ansteigt. Jedoch lassen sich keine Schlusse beziiglich der Oszillatorstarken in den entfalteten Banden bei 2000 und 2100 em-l ziehen, die innerhalb des ,,Stretching"-Bereichs auftreten. Ein fruher vorgeschla- genes Modell fur den Mechanismus der Dehydrogenisierung von a-Si wird von den vorgelegten Ergebnissen nicht beeinflullt.

1. Introduction Recently Shanks et al. [I] have reported an extensive study of the relationship be- tween the integrated intensity of the infrared bands and the hydrogen content of sputtered amorphous silicon, a-Si, films. I n this study the total hydrogen concen- tration of the films was measured by the resonant lH(15N, q) 12C nuclear reaction. The individual oscillator strengths of the modes exhibiting absorption maxima

l) Edinburgh EH14 4AS, Great Britain. z, Present address: B.P. Research Centre, Sunbury-on-Thames, Great Britain.

39'

608 P. JOHN, I. M. ODEH, M. J. K. THOMAS, M. J. TRICKER, and J. I. B. WILSON

a t ca. 2100, 2000, and 640 cm-1 were calculated on the basis of the nuclear technique for the calibration of total hydrogen. Whereas there is common agreement that the 640 cm-I band is associated with the wagging and rocking modes of pre- dominantly SiH and SiH, groupings the detailed assignment of the silicon-hydrogen stretching bands at 2000 and 2100 remains unresolved [l to 51. With regard to the latter controversy Shanks et al. [1] concluded, first, that the oscillator strength, r,, of the wagging mode is independent of the total hydrogen content, N,, of sput- tered a-Si films. Secondly, the apparent oscillator strength for the composite stretching mode, r,(2000 + ZlOO) , is a function of the hydrogen concentration and the conditions of sample preparation. The dependence of r , ( Z O O O + 2100) on the hydrogen concen- tration was stated to arise mainly from the marked change of the oscillator strength of the 2000 cm-I mode, r,(2000), especially a t low hydrogen content (< 10 atyo) ; rs(2lO0) being apparently less sensitive to N,.

I n a previous publication [6] we described a similar study of a-Xi films, prepared by glow discharge of SiH,, employing a non-destructive 25 MeV a-particle scattering technique in conjunction with an infrared spectroscopic investigation. The former technique has the advantage that both the silicon and hydrogen atomic concentrations (plus other elements, e.g. 0) may be accurately determined thus permitting calculation of the atomic percentages without) resort to density data. In agreement with the work of Shanks et al. [ l ] we observed [6] that, for as-prepared glow-discharge as well as sputtered a-Xi films, 7, was independent of the atomic percentage of hydrogen whereas r,(2000 + 2100) varied with film preparation conditions.

In the present paper we report a further study of the dependence of r,(2000 + 2100) on the hydrogen content of glow-discharge prepared a-Xi films. This data is compared to that obtained previously for both sputtered [ l ] and glow-discharge [B] films. Whilst we confirm that r,( 2000 + 2100) decreases monotonically with increasing N,, in the range 2 to 15 atyo i t is demonstrated that the interpretation by Shanks et al. [l], of their data in terms of changes in the relative oscillator strengths of the individual stretching modes, is erroneous.

We have previously [7 , 81 studied the thermal dehydrogenation of glow-discharge a-Si films by infrared spectroscopy. The hydrogen content, within particular silicon- hydrogen moieties, of the sample during isothernial heating was derived from analysis of the stretching modes on the assumption that the oscillator strengths of those modes were identical and, furthermore, remained constant. These were necessary assumptions since, a t that time, there was no direct evidence to the contrary. In view of the new information, relating to the dependence of the oscillator strengths of l',(2000 + 2100) and I', on N,, we have re-analysed our data. This re-analysis includes previously unreported data on the behaviour of the integrated band intensity of the wagging mode during isothernial heat treatment.

2. Experimental

The details of the preparation, in-situ thermal dehydrogenation studies and decon- volution of the infrared absorption bands of a-Si films, have been described elsewhere [6 to 81. The wagging mode intensity was corrected for an absorption a t ca. 640 cni-l, due to the crystalline Si substrate, for samples at elevated temperatures. Isothernial dehydrogenation of a glow-discharge a-Xi film (212C-1), 2 pm thick, was carried out atJ 350 "C. The film was deposited on a single crystal Si substrate a t a substrate temperature of 100 "C, silane pressure of 0.1 Torr employing 20 W rf power.

Infrared Vibrational Modes in Glow-Discharge Hydrogenated Amorphous Silicon 609

3. Results and Discussion The interference free absorption coefficients, &(a), a t a wave number w were deter- mined, as previously described [6] for the composite stretching envelope in the wave number range 1860 to 2200 em-l. The integrated band intensity, I,(2000 + 2100), defined by

Is = s w dm we

was numerically evaluated, using the trapezoidal rule, from infrared spectra of an a-Si film 212C-1 obtained at various times during thermal dehydrogenation at 350 "C. The time dependences of the integrated band intensity, I,, of the wagging and rocking modes, defined similarly, and I,(2000 + 2100) are shown in Fig. 1.

I n order to interpret the data in Fig. 1 it is necessary t o determine the oscillator strengths of the various modes via an analysis in which N , is independently deter- mined. Experiments of this type have been carried out for sputtered [l] and glow- discharge [ 61 a-Si films using nuclear and v.-particle scattering techniques Fespectively. Both investigations have shown that N , is directly proportional to I,, i.e. N , = A,I, for differing film preparation conditions and thus provides a reliable internal cali- brant for the total hydrogen content of such films.

The dependence of I, on N , is compared in Fig. 2 with the data of Shanks et al. [1] obtained for as-prepared sputtered a-Si films. The proportionality constant A, = = 1.6 x 1019 was determined empirically [GI. For comparative purposes, we have adopted the previous procedure [l] of dividing N , by the atom density of crystalline Xi (5 x cm+) to provide an approximate measure of the atomic percentage of hydrogen. For glow-discharge a-Si films the silicon atom density lies in the range (3.0 to 4.0) x loz2 em-3 [B]. Remarkably good agreement is observed be- tween the two sets of data despite the alternative methods of material preparation. Shanks et al. [l] measured = 37.6 cm2 mmol-l whereas the present data, perhaps fortuitously, gives T, = (37.6 & 4.3) em2 mniol-l with the exclusion of one data point included in [GI. Similarly the dependence of I,(2000 + 2100) on hydrogen concen-

0 10

0 50 100 150 200" 1060 1080 I I 1 I ,, I

t fmml -- Fig. 1

Fig. 1. Time dependence of I, (2000 + 2100) (0 ) and I, (0 ) on the isothermal dehydrogenation of a glow-discharge a-Si film (212'2-1) at 350 "C

Fig. 2. Correlation of I, with the hydrogen concentration, NII, determined by 25 MeV a-particle scattering and nuclear techniques. Glow-discharge films, 0 [S]; sputtered Elms, o [l], o [9]; fluorinated samples, a [l]. A, = 1.6 x lOI8 cm+ [I, 6, 91

610 P. JOHN, I. M. ODEH, M. J. K. THOMAS, M. J. TRICKER, and J. I. B. WILSON

Fig. 3 Fig. 4

Fig. 3. Correlation of I, (2000 + 2100) with the hydrogen concentration, NH, determined by 25 MeV a-particle scattering and nuclear techniques. Glow-discharge films, 0 [6]; sputtered films, o [l], 0 [9]; fluorinated samples, A [l]. A, = 2.2 X lozo cm-2 [GI for glow-discharge a-Si

Fig. 4. Changes in the integrated band intensities during the isothermal dehydrogenation of an a-Si film (212C-1) a t 350 "C. Variation of I, (2000 + 2100)/Iw (O) , 1,(2000)/1, (o), and I , (2100)/1, (0) versus hydrogen content, N,, empirically derived from I , a t 640 cm-l. One measurement is included for heat treatment a t 530 "C as indicated. The bracketed number8 refer to the number of superimposed data points

tration is presented in Fig. 3 and the combined data exemplify the conclusion drawn by Shanks et al. [ l ] that Ts(2000 + 2100), in contrast, is film dependent.

Films prepared a t low substrate temperatures < 50 "C exhibited oscillator strengths I',(2000 + 2100) in the range 2.8 to 3.2 om2 mmol-l whereas Ts(2000 + 2100) of films prepared at higher substrate temperatures were significantly lower [6].

In Fig. 4 is shown the variation of IJIW and, also, the deconvoluted integrated band intensities, 1,(2000)/1, and 1,(2100)/1w with respect of I , for film 212C-1. These data were derived from the analysis of infrared spectra obtained a t differing times during thermal dehydrogenation at 350 "C. The form of each of these curves is similar to those published previously [ l ] for as-prepared and annealed (< 260 "C) sputtered films. On the assumption that r, remains constant throughout dehydrogen- ation, the oscillator strength of the combined stretching mode, e.g. 2000 cm-1 plus 2100 cm-l, is a function of the hydrogen content. The change in r,(SOOO + 2100) is apparently more pronounced a t values of N , < 10 atyo although the paucity of the present data in this region is noted. Investigations a t lower hydrogen concentrations were precluded by the slow rate of H, evolution at the 'thermostationary state' ulti- mately attained [7]. Furthermore, the individual values of I,(2000)/1, and 1,(2100)/1w also exhibit, approximately, the same trend with N , as observed by Shanks et al. [I]. Although we agree that there is a variation of Ts(2000 + 2100) with N , we regard the conclusions of Shanks et al. relating to the variations in the individual oscillator strengths, i.e. T,(2000) and rs(2100) as erroneous. It is a necessary prerequisite that, in order to determine T,( 2000) and r,(2100) individually, the absolute concentrations of the silicon-hydrogen moieties be independently estimated. These concentrations are not independently known as the nuclear calibration technique measures only the total hydrogen atomic concentration. I n their interpretation of their data Shanks et al. implicitly assume, without justification, that the concentrations of the species giving rise to absorption a t 2000 and 2100 cm-l are equal in all the films studied and,

Infrared Vibrational Modes in Glow-Discharge Hydrogenated Amorphous Silicon 611

IZO t Id7-

-12 $

-16 $j

-14 a

- 10 0.8

j 126 I I I I 04

-

r_

0 50 100 150 200 250 t imin) -

Fig. 5. The change in hydrogen concentration, N,, upon isothermal dehydrogenation of film 212C-1 as a function of time, o I', = 2.3 cm2 mmol-1 [B]; 0 data from [7] (right-hand scale)

04 0 50 100 150 200 250

t imin) - Fig. 5. The change in hydrogen concentration, N,, upon isothermal dehydrogenation of film 212C-1 as a function of time, o I', = 2.3 cm2 mmol-1 [B]; 0 data from [7] (right-hand scale)

moreover, equal to the total hydrogen content. The latter, clearly incorrect, con- sequence of their conclusions can be readily derived from the equations given in the paper [l]. Thus the variation of the individual values of r , ( S O O O ) and r,(2100) with N , for sputtered and glow-discharge films has not been determined. The dependence of 1,(2000)/1, and 1~(2100)/1, upon I," may equally well arise from unknown varia- tions in the relative concentrations of groupings in the a-Si network giving rise to these absorption bands.

We include one further datum point in Fig. 4 which is derived from thermal de- hydrogenation a t 530 "C for 1200 min. Interestingly, in agreement with Shanks et al. [l] the value of r,(2000 + 2100) is markedly diminished with respect to the low- temperature value.

Fig. 5 presents a plot of the variation of N,, derived from I,, during thermal dehydrogenation of a-Xi (221-C) a t 350 "C as a function of time superimposed on a previously published plot [7]. Although the sigmoidal shape of the curves are similar i t can be seen that the absolute initial hydrogen concentration is smaller, and the rate of dehydrogenation faster, than previously reported [7]. The revision in the data is derived from both measurement of rw and rs [B] and the information presented in this paper. A thermostationary state is, however, still evident. E'urther the conclusion that one type of silicon hydrogen grouping is not exclusively removed on low-temper- ature dehydrogenation remains valid. No additional comment can be made regarding the individual rates of decay of the silicon-hydrogen groupings giving rise to the absorption at 2000 and 2100 cm-l; the assumption that the oscillator strengths of the individual modes are equal does not seem unreasonable until definitive exper- imental evidence proves otherwise, The suggestion, first made in previous publications [7,8], that hydrogen is evolved from clustered sites of silicon-hydrogen groupings, predominantly SiH and SiH,, orientated such that the terminal hydrogen atoms are in close proximity, is also unaffected.

Acknowledgement

We thank the S.R C. for financial support.

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612 P. JOHN et al.: Infrared Vibrational Modes in Amorphous Silicon

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(Received October 20, 1980)