Influence of sputtering conditions on H content and SiH bonding in aSi: H alloys

Journal of Non-Crystalline Solids 45 ( 1981) 15- 27 North-Holland Publishing Company

15

INFLUENCE OF SPU'ITERING CONDITIONS ON H CONTENT AND Si-H BONDING IN a-Si:H ALLOYS *

P.M. M A R T I N A N D W.T. P A W L E W I C Z Pacific Northwest Laborato~ **, Riehland, Washington 99352, USA Received 18 August 1980 Revised manuscript received 24 February 1981

The influence of sputtering conditions on H concentration and Si-H bonding has been determined for the case of diode reactive sputtering of Si in Ar-H mixtures, and a simple model for Si-H reaction kinetics has been developed. H content and Si-H bonding can be varied and controlled over wide ranges by appropriate selection of sputtering conditions. Si-H reaction kinetics are found to be governed primarily by deposition rate (reaction time), H partial pressure (H availability) and possibly substrate temperature (mobility of Si and H on the surface of the growing film). H concentration increases with decreasing deposition rate and increasing H partial pressure. Sill bonding increases with increasing deposition rate and decreasing H partial pressure. Polymerization of H and Si (Sill 2, Sill3) occurs for lowest deposition rates and highest H partial pressures. Polymerization reactions are enhanced in films deposited at very high target power densities. The high power density causes substrate temperature to be higher than expected, and the enhanced polymerization may be due to increased Si and H surface mobility at the elevated temperatures.

1. Introduction A n u m b e r of recent publications have demonstrated that the bonding and composition of sputtered a-Si:H could vary considerably, resulting in wide ranges in electrical and optical properties [1-4]. The controlled and reproducible deposition of films with the properties desired for photovoltaic applications requires a thorough understanding of the influence of sputtering conditions on network organization and incorporation of hydrogen. Pawlewicz [1] f o u n d that argon pressure (PAr), substrate temperature (Ts) and s u b s t r a t e - t a r g e t spacing played significant roles in determining film properties for n o n h y d r o g e n a t e d a-Si. N e t w o r k defects were minimized by sputtering at high PAr ("~ 20 pa) and large s u b s t r a t e - t a r g e t spacings ( ~ 12 cm), the conditions at which thermalization of plasma species occurred. T r a p p e d Ar was f o u n d in films deposited with low PAr" It is to be expected that * Work supported by the Material Sciences Division of the Department of Energy--Office of Basic Energy Sciences. ** Operated by Battelle Memorial Institute for the US Department of Energy. 0022-3093/8 l / 0 0 0 0 - 0 0 0 0 / $ 0 2 . 5 0 © 1981 N o r t h - H o l l a n d

16

P.M. Martin, W.T. Pawlewicz / Influence of sputtering conditions

these sputtering conditions along with hydrogen partial pressure (PH) and target power density (PT) must also influence H concentration and bonding in a-Si:H. In addition to determining property-composition relationships in films deposited with a wide range of sputtering conditions, Martin and Pawlewicz [2] determined that the dominant Si-H bond type changed with increasing H content. Sill was dominant in films with H content < 8 at.%, mixtures of Sill + Sill 2 were found in films with H content between 8 and 20 at.%, and Sill 2, Sill 3 or mixtures of Sill 2 + Sill 3 were found in films with H content between 20 and 35 at.%. Jeffrey et al. [3] found that Sill 2 bond density decreased from 1022/cm3 to an undetectable level with an increase in target power density from 1.0 to 3.2 W / c m 2. They suggested that increased ion temperatre in the plasma due to higher target powers was responsible for the observed decrease in Sill 2. The influence of other deposition conditions on Sill 2 bonding and the influence of target power density on other types of Si-H bonding was not reported. Freeman and Paul [4] determined Si-H composition of films deposited with a limited range of p H, PAr and Ts. They found a maximum H concentration of ~ 25 at.% for PH ~ 0.26 Pa and Pat = 0.67 Pa with a deposition rate (RD) of 0.10 nm/s. Maximum H incorporation occurred for Ts = 200°C. Hydrogen partial pressure was found to influence Si-H bond-type in the following ways--(1) Sill 3 infrared stretching vibration at 2140 cm -~ was predominantly formed at PH ~> 0.04 Pa; (2) Sill 2 was formed when PH > 0.027 Pa and this was the dominant bond-type for pn > 0.067 Pa; (3) (Sill2) . or possibly Sill 3 was formed at the highest PH; and (4) Sill was present at all PH but occurred most frequently at low PH and was never the dominant bond-type. Si-H bonding mechanisms have been discussed by Mell and Brodsky [5], Brodsky [6] and Freeman and Paul [4]. The first two papers suggest that polymeric forms of a-Si:H (Sill 2 and Sill 3) occur for plasma conditions which promote ion-ion collisions (high silane pressures) and that Sill results from surface reactions at low silane pressures. Freeman and Paul do not acknowledge a Sih 3 bond as such but prefer to interpret the bond responsible for the infrared stretching mode at 2140 cm-~ as H bonded at several possible sites. One is caused by trihydride (Sill3) bonding on the surface of voids [7], a second site is along the axis joining a vacancy site and a nearest neighbor and a third is a tetrahedral interstitial site. The second bond type occurs more frequently at high H concentrations [8]. Anderson et al. [9] determined that argon pressure strongly influenced film network organization, in agreement with earlier work b y Pawlewicz [1]. HOwever, Anderson et al. proposed that films deposited with high PAr ( > 4.0 Pa) were of lower quality due to insufficiently energetic ion bombardment during film growthl Post-deposition oxygen and nitrogen surface contamination occurred and was attributed to a high density of voids. The results presented here provide a more detailed and cohesive picture of the influence of pn, PAt, PT and rf-induced dc bias applied to the substrate (Vs) on H concentration and Si-H bonding for films prepared over a wider

P.M. Martin, W. 7". Pawlewicz / Influence of sputtering conditions

range of sputtering conditions. Based on these results, a simple model for Si-H reaction kinetics is constructed from which criteria for obtaining a desired H concentration and Si-H bond type can be deduced.

2. Experimental 2.1. Sputtering hardware Except for the substrate biasing network, the rf diode sputtering system used to deposit a-Si:H has been described previously [1]. A if-induced dc bias of 0-300 V could be applied to the electrode on which the substrate was held. A resistive heater was embedded in this electrode to provide control over substrate temperature. To minimize parasitic losses due to the heater and thermocouple circuitry, rf choking networks were incorporated. Both target and substrate rf networks were tuned by minimizing reflected power. Depositions were made on either fused silica substrates for ultraviolet-visible spectrophotometric measurements, or germanium mirrors for spectrophotometric measurements in the near infrared. Sputtering targets were n-type electronic grade single crystal silicon wafers with a resistivity of 2.7~2cm ( n ~ 10 ~5

cm-3). Depositions were made with premixed Ar + 3% H 2 (99.9995% minimum purity) or Ar (99.9999% minimum purity) + 25 to 50% H 2 (99.9999% minimum purity). All sputtering gases were passed through a gas purifier to further reduce H20, 02 and trace amount of organics before introduction to the sputtering chamber. To decrease contamination in the chamber due to residual H20, 02 and N 2, a variable orifice was placed between the liquid nitrogen cold trap and diffusion pump. With the variable orifice, the diffusion pump could be throttled to obtain the desired gas pressure in the chamber, while still maintaining unthrottled cryopumping of contaminants by the cold trap. R D ranged from 0.04 to 0.8 n m / s depending primarily on Pv. For the same PT, Rt) also varied with total sputtering gas pressure [1] and the substrate bias. PT was either 2.2, 6.6 or 13.1 W / c m 2. Substrate-target spacing was fixed at 3.18 cm, and all depositions were made at Ts = 225°C. Total gas pressures (PAr "~-PH)of 2.67, 10.0 and 20.0 Pa were used with hydrogen concentrations of 3, 25 and 50%. Vs was fixed at ,~ 0 (rf grounded, dc floating), - 1 5 0 or - 300 V with typical substrate power densities of 0, 1.4 and 3.8 W / c m 2.

2.2. Characterization of deposits Film thicknesses measured spectrophotometrically [10] ranged from 0.4 to 2 #m on chemically inert, optically-polished silica substrates and 2 to 7/z on optically-polished Ge mirrors. Transmissive spectrophotometry in the region 2500 to 25 000 nm was used to obtain complete vibrational absorption spectra of films deposited on Ge substrates. Reflectance measurements in the region 2500 to 7500 nm were performed on films deposited on silica substrates to

17

18

P.M. Martin, W.T. Pawlewicz / Influence of sputtering conditions

determine the location of dominant Si-H bond-stretching vibrational modes. Because total H concentration could not be measured by infrared techniques for films on silica substrates, replica were made on Ge substrates and H concentrations were assumed to be the same. The amount of hydrogen incorporated into the films was determined from IR vibrational absorption spectra using a method similar to that of Brodsky et al. [11] and Freeman and Paul [4]. Details of this calculation have been presented elsewhere [2]. Identification of the Si-H bond structure was based on the work of Brodsky et al. [11] and Knights et al. [ 12]. The sputtered films showed no post-deposition contamination. Infrared vibrational spectra taken immediately after deposition and after a period of several months showed no difference in Si-H absorption strength or peak location, no evidence of Si-O absorption peaks at either 2250 or 1100 cm-t, and no diffcrence in Si-N absorption strength near 830 c m - t . X-ray diffraction was performed on many films, especially those deposited with Px = 13.1 W / c m 2, to eliminate spurious results due to crystallization. SEM was used to determine surface features of the films. SEM results are not presented because the surface roughness of the amorphous films, regardless of deposition conditions, was less than 5 nm so that features, columnar or otherwise, could not be resolved with the available instrumentation. All amorphous films were extraordinary smooth, mirror-like and featureless. Gas evolution analyses were performed on many of the a-Si:H films on Ge mirrors to determine hydrogen concentration for comparison with infrared spectroscopic results and to estimate the concentration of impurity gases such as Ar, 02 and N 2. The total H content determined by this technique agreed reasonably well with that calculated from the equations of Freeman and Paul [4]. Evolved gases were identified by a He ionization detector gas chromatograph. Gas evolution analysis showed no detectable 02, and average N 2 and Ar concentrations were near 1 at.%.

2.3. Experimental design A 3-level, 4-parameter Box-Behnken [13] experimental design consisting of 27 prespecified sets of sputtering conditions was used to simultaneously investigate the influence of PT, (PAr + P n ) , PH and VB at low, intermediate and high values of each parameter. This efficient design allowed determination of the principal effect of each deposition condition as well as the effects of interaction between conditions. It thus helped to avoid misleading functional dependences that sometimes appear in experimental designs which vary each parameter independently.

3. Results

Various mixtures of Sill, Sill2, (SiH2) n and Sill 3 and a wide range of hydrogen concentrations were observed in the sputtered a-Si:H films. This

P.M. Martin, W.T. Pawlewicz / Influence of sputtering conditions

PT(W/cm2)

40

o_30\~ -% v~

0 2.2 &/x 6.6 O 13.1

20 Z

8 1o

"~-

0

I

I

I

l

I

1

1

I

0.1 0.2 0.3 0.4 0.5 0.60.l 0.8 DEPOSITIONRATE,nm/sec

Fig. 1. Influence of target power and deposition rate on H concentration of sputtered a-Si:H films. Dark symbols indicate gas evolution results.

broad range of results was obtained by varying sputtering conditions over wide ranges as specified by the Box-Behnken design. The IR absorption spectra of the films displayed peaks at 2000, 2090 or 2150 cm-1. These peaks have been attributed to the bond stretching modes of Sill, Sill 2 and Sill3, respectively [11,12]. In most films one peak was dominant, but a triplet absorption with three roughly equal peak amplitudes was observed for some films. The triplet stretching mode was found for films deposited at PT (13.1 W/cm2), PAr q-PH ~> 10 Pa, and PH ~> 2.4 Pa. Evidence of the three stretching modes, sometimes barely distinguishable, was present in most films in the form of asymmetric absorption peaks. Only the stretching mode at 2000 cm-l is usually observed for a-Si:H produced by plasma-assisted decomposition of silane at low pressures ( ~ 67 Pa) and Ts between 200 and

PT(W/cm2) 002.2 & A 6.6 SIH3 -CDO O

~

Sill2 t Sill

oeL ,'-,A

I

N 0

J 0.1

I

I

J

I

I

J

I

0.2 0.3 0.4 0.5 0.6 0.7 0.8 DEPOSITIONRATE,nrnlsec

Fig. 2. Influence of deposition rate on dominant Si-H bond type for PT = 2.2 and 6.6 W / c m 2. Dark symbols indicate trace amounts of (Sill2) ..

19

20

P.M, Martin, W.T. Pawlewicz / Influence of sputtering conditions

300°C [6,11], or for plasma-treated evaporated a-Si:H [14]. The bond stretching modes at 2090 and 2150 cm -I are usually found for a-Si:H produced by plasma-assisted decomposition of silane at high pressures ( ~ 133 Pa) and Ts less than 150°C, or for a-Si with ion implanted hydrogen [15]. The bond-bending absorption doublets of Sill 3 and (Sill2) n at 905, 860 cm -~ and 890, 845 cm-~, respectively, were sometimes difficult to discern because an absorption due to SiN ( ~ 830 cm- i) masked much of the detail. In most cases, however, SiH2, (Sill2) n and Sill3 could be identified by locating these doublets. Fig. 1 demonstrates that R D and PT significantly affect hydrogen concentration. For all Pv, H content decreases with increasing R D. The curves for PT = 2.2 and 6.6 W / c m 2 appear to be similar while the curve for PT = 13.1 W,/cm 2 is distinct. H concentration decreases most rapidly with R D when PT = 2.2 and 6.6 W / c m 2. This decrease is less pronounced for Px = 13.1 W / c m 2, although H concentrations were higher .for the same R D. This difference and the general decrease of H content with increasing R D both can be related to S i - H reaction kinetics and will be discussed later. Fig. 2 displays dominant S i - H bond type for PT = 2.2 and 6.6 W / c m 2. Results for PT = 13.1 W / c m 2 are shown in table 1. The most significant result presented here is that dominant S i - H bonding changes from Sill 3 to Sill 2 to Sill with increasing R D. Specifically, for R D ~<0.10 n m / s , Sill 3 is the dominant bond type. (Sill 2 + Sill3) or Sill 2 is formed for R D between 0.1 and 0.3 n m / s , with Sill 2 usually the dominant bond type. For R D > 0.3 rim/s, Sill appears. Most films deposited with PT = 13.1 W / c m 2 fail to show a single dominant bond type. Being composed of relatively equal amounts of Sill, [SiH2 + (Sill2),] and Sill 3. In most cases, [Sill 2 +(SiH2),] or Sill is marginally dominant. Note that in these films (SiH2) . is also evident, but cannot be Table I Hydrogen composition of a-Si:H films deposited with PT = 13.1 W / c m 2 Sample No.

Deposition rate (nm/s)

at.% H

Sill bond type "

P H (Pa)

PAr (Pa)

0.33

23.2

5.1

15.0

6a ~ lib b

0.44 0.59

23.1

0.8 2.5

2.3 7.4

17 a 19b b

0.68 0.61

19.7

2.5 5.0

10.0 5.0

26

0.73

16.9

Sill 2 , Sill, Sill 3, (SiHz)n Sill 2, Sill 3, Sill Sill 2, Sill, Sill 3, (SiH2)n Sill 2, Sill, SiH~ Sill 2 , Sill, Sill 3 , (Sill2), SiH2, Sill, Sill3, (SiH~),

0.3

9.7

2

Bond type obtained from reflectance, presence of (Sill2) . undeterminable, b Partially polycrystalline. Listed in order of relative strength.

P.M. Martin, W.T. Pawlewicz/ Influence of sputtering conditions

21: t,q

,/

Y

~

2

Pjlw/cm2) 0 2.2 /x 6.6

1

[] 13.1

3

212

1

~ J

0

0

I

I

l

0.1 0.2 0.3 0.4

I

I

0.5

I

21

I

0.6 0.7 0.8

DEPOSIIIONRATE,nm/sec Fig. 3. Change in H bonded as Sill with deposition rate and target power.

quantitatively separated from Sill 2 due to an absorption near 830 cm attributed to SiN. The results for PT = 13.1 W / c m 2 are segregated from those for PT = 2.2 and 6.6 W / c m 2 because X-ray diffraction and SEM analyses show that some of the PT = 13.1 W / c m 2 films were partially polycrystalline with a grain size of ~ 15 nm. The crystallization is probably due to an elevated surface temperature caused by the high PT" The increase in Sill bonding with increasing RD is shown more clearly in fig. 3. The concentration of hydrogen bonded as Sill was calculated from the area under the 2000 cm l peak using a least squares fit of the absorption data to a gaussian distribution. Sill bonding was found to increase rapidly with R D up to about 0.3 n , / s and then increase slowly to values above 5.0 at.% H at higher R DFig. 4 displays the influence of p n on hydrogen concentration. For PT = 2.2, 6.6 and 13.1 W / c m 2 H concentration increases rapidly with PH for PH < 1.0

4O z 0

z o

,z g

30

/'zf'~

'

0

20 ~

~--

© 2.2

i0 Ol

:; Iw/¢mz PT

m •

& A 6.6 •

•

[] 13.1

1

i

I

I

I

I

2

3

4

5

PHIPa) Fig. 4. Influence of H partial pressure on H concentration in sputtered a-Si:H films for PT 6.6 and 13.1 W / c m 2. Dark symbols indicate gas evolution results.

2.2,

P.M. Martin, W.T. Pawlewicz / Influence of sputtering conditions

22

PTlWlcm2) O 2.2

>.-

~SiH 3

o

(Z)

&

6.6

•

6.6, TRACE(Sill2)n O

O

0

:a Sill2 ' ~ 6 Zx ,,:( z

.~ Sill ..~ A R ,

0

I

I

i

L

I

I

I

I

I

I

1

2

3

4

5

6

7

8

9

10

PHIPa) Fig. 5. Influence of H partial pressure on dominant S i - H bond type for PT = 2.2 and 6.6 W / c m 2. Dark symbols indicate trace amounts of (Sill2) ..

Pa. Highest H Concentrations are obtained for PH > 1.0 Pa, independent of PT. The dependence of H content on PT is again evident. For PH > 1.0 Pa, H content saturates at 35, 21 and 23 at.% for PT = 2.2, 6.6 and 13.1 W / c m 2 respectively. H concentration may decrease at high Pn and PT = 2.2 and 13.1 W / c m 2, according to gas evolution results. Sill bonding is obtained only at low p n and intermediate PT, as shown in fig. 5. An increase in PH results in Sill 2 becoming the dominant bond type for intermediate PT and Sill 3 for low PT- That is, the simplest bond type (Sill) occurs at low Pr~ and higher RD, and increasingly more complex bonding occurs as Pia is raised and PT lowered. At almost all PH, Sill, [Sill2 + (Sill2),] and Sill 3 occur in relatively equal amounts for PT = 13.1 W / c m 2.

7

PT(Wlcm2) O 2.2

5 -r

ZX 6.6

3\ 4

:m,

o

2

013 0

[] 13.1

o

I I

i 2

3

4

o 5

i 6

I

I

I

7

8

9

10

PH(Pa) Fig. 6. Change in H bonded as S i - H in sputtered a-Si:H films with H partial pressure for PT = 2.2, 6.6 and 13.1 W / c m 2.

P.M. Martin, W.T. Pawlewicz/ Influence of sputtering conditions

23

4O ~ 0

20

121

ix

§

ix

PT(wlcm21

02.2

10

/x

&/x

~=

6.6

• [] 13.1 I I

I

I

I

I

I

I

2

4

6

8

I0

12 14 16

PAr(Pa) Fig. 7. Influence of argon partial pressure and target power on H concentration in sputtered a-Si:H films. Dark symbols indicate gas evolution results.

The influence of PH on the concentration of hydrogen bonded as Sill is displayed in fig. 6. The increase in Sill bonding with PT is again evident. Also note that for each PT there exists a PH where Sill bonding is a maximum. The decrease in Sill bonding for high PH is seen most clearly for the PT = 2.2 W / c m 2 curve. Hydrogen concentration decreases sharply from 3.1 at.% to --~0.1 at.% with an increase in PH from 0.67 to 4.9 Pa. The increase in Sill bonding at low PH is most evident from the other two curves. For PT = 6.6 W / c m 2, hydrogen concentration increasing initially to --~ 5.5 at.% at PH "~ 1.33 Pa, and decreases slightly at about 5.0 at.% at higher PH. A maximum hydrogen concentration of 6.5 at.% is attained at Pn = 0.26 Pa for PT = 13.1 W / c m 2. Clearly, for any value of PT, H bonded as Sill must approach zero as PH approaches 0.

PT(Wlcmz) 002.2 A / ' , 6.6

Sill3 ° m

Sill 2

o zxlP.

o

oO

.~

ixix

z~,

ix

Sill

I

2

J

4

J

6

~

8

ll0

I

12

~

4

I

16

118

PAr(Pal

Fig. 8. Influence of argon partial pressure on dominant S i - H bonding in sputtered a-Si:H films. Dark symbols indicate trace amounts of (Sill2) ,.

P.M. Martin, IV,T. Pawlewicz / Influence of sputtering conditions

24 7

ra

6

o --~

[3

5

D

~

4

,x

PT(W/cm2)

3 2

::=

o

1 0

2

4

6

0

2.2

~

6.6

[] 13.1 I

L

I

I

I

8

I0

12

14

16

18

20

PAr(Pa) Fig, 9. Change in H bonded as Sill with argon pressure and target power.

The influence of PAr on H concentration is shown in fig. 7. For PT = 2.2 W / c m 2 H content increases with PAr from 28 at.% for 2 Pa to 36 at.% for 7.5 Pa. For PT = 6.6 and 13.1 W / c m 2 the data are too scattered to deduce a dependence but are not inconsistent with the conclusions reached for PT = 2.2 W / c m 2 only Sill 3 was observed in 5 out of 6 cases. The influence of PAr on dominant S i - H bonding is a-Si:H 3 films is shown in fig. 8. Fig. 9 shows the influence of PAr on H bonded as Sill. Scatter in the data prevents unambiguous interpretation. For PT = 2.2 W / c m 2, Sill bonding appears to be independent of PAr" The increase in Sill content with increasing PT is, however, very clear in the figure. Hydrogen composition and S i - H bonding displayed no discernible dependence on rf-induced dc bias applied to the substrate.

4. Discussion

The two most salient results in figs. 1-5 are that: (1) total concentration of H incorporated into a film decreases with RD; and (2) polymerization occurs at low R D while predominantly Sill bonding results at high R D. These two results suggest a remarkably simple model for the reaction kinetics of the S i - H system which appear to be governed primarily by the amount of H available to the growing film and the time permitted for reaction by the R D. In cases where is varied, the mobility of Si and H atoms on the surface of the growing films is probably also important. The influence of R D (reaction time) can be seen most clearly in figs. 1, 2 and 3. For any set of deposition parameters (PT, PH, PAr, Vs) plotted in fig. 1, H content decreases with increased R D. The rate of decrease is highest at the lowest R D. Fig. 2 shows that polymerization occurs at lower R o and that Sill 2 and Sill 3 bonding gives way to Sill as R D is increased. The quantitative increase in Sill bonding with increased R D is shown in fig. 3. The high H concentrations found for low values of RD thus may be attributed to the prolonged time period that Si had to react with H in

P.M. Martin, W.T. Pawlewic: / Influence of sputtering conditions

25

the plasma before condensing on the film. Additionally, H may be incorporated by chemisorption of H on the surface of the film [4], which is exposed to the plasma for longer time periods before subsequent Si:H layers are deposited. The influence of pH (H available for reaction) on composition and bonding can clearly be seen in figs. 4, 5 and 6. Fig. 4 shows the total H incorporated in the film increases with the amount available during film growth. Fig. 5 shows that polymerization occurs when H is abundant and that simple Sill bonds result when H is scarce. Fig. 6 shows that Sill bonding first increases as H is added to the sputtering gas, and then falls off when polymerization sets in at high PH. This figure also shows formation of Sill is favored by high R D because in this case reaction time is too short for polymerization. Additional support for the reaction kinetics model is provided by figs. 7, 8 and 9 which show high H contents for lowest R D (lowest PT), polymerization at lowest RD (lowest Px) and Sill bonding at highest R D (highest PT)The data in table 1 obtained for P'r --- 13.1 W / c m 2 require special consideration. Here polymerization occurred extensively even though the deposition rate was the highest employed, in apparent contradiction to the results discussed above. Crystallization of some of the films deposited for Px = 13.1 W / c m 2, mentioned earlier, strongly suggests that Ts was significantly higher than 225°C for these films due to plasma heating. These films thus cannot be compared with those deposited at P'r = 2.2 and 6.1 W / c m 2. By themselves, however, they may indicate that the mobility of Si and H atoms on the surface of the growing film plays an important role in polymerization. Higher Ts appears to result in higher mobility and thus'increased likelihood of polymerization. The higher mobility at elevated Ts may also explain why films f o r Px ~= 13.1 W / c m 2 had higher H content as sown in fig. l, although physical trapping of H at the high rates may also play a role. Findings similar to the reaction kinetics model presented here have been reported elsewhere. Brodsky [6] Mell and Brodsky [5] suggest that Sill 3 is formed by a polymerization reaction of Si with H at high p n and PAy Freeman and Paul, referring to Pandey et al. [7] propose that the bond responsible for the stretch mode at 2140 cm -1 (commonly referred to as Sill3) may be Sill3 bonding on the surface of voids in the film or, referring Singh et al. [8] that H bonds along an axis joining a vacancy to a nearest neighbor Si atom at a tetrahedral interstitial site. The second mechanism is though to occur at high H concentrations. The explanations of both authors are consistent with the results presented here. Brodsky further predicts a decrease in Sill bonding with the onset of polymerization. Figs. 3, 5, 6 and 9 show that Sill 3 content decreases with increasing Sill content. The results that Sill 3 dominates in films having the highest H concentrations, and that Sill 3 forms when H exposure to the film's surface is maximum, both support the explanation of Freeman and Paul. It seems likely that both mechanisms contribute to the formation of Sill 3. The nearly-constant H concentration for PAl > 5.0 Pa may be explained by

26

P.M. Martin, W.T. Pawlewicz / Influence of sputtering conditions

considering the collisions of Si atoms with Ar ions. Above a certain PAr, Si atoms, as well as Ar ions, are thermalized by collisions in the plasma. Consequently the reaction time of Si with H atoms either remains constant or is diminished, with H concentration similarly affected. Anderson et al. [9] estimated the substrate-target distance required for thermalization of Si atoms as a function of PT and PA~. For the substrate-target distance used here, most Si atoms would be thermalized for PA~ > 3.0 Pa, increasing for larger PT. Above this PA~, the energy related Si-H reaction probability should remain essentially constant. Because of the wide range of deposition conditions, it is inappropriate to compare all the results presented here to the results of Freeman and Paul [4]. Comparisons for low PT, lOW PH and low PA~ were attempted. Similar to their results, Sill was never the dominant bond type and Sill content increased with decreasing pn. Freeman and Paul found Sill 2 to dominate for PH in the range 0.067-0.67 Pa, while the presented results show that Sill 2 formed when PH ~>0.67 Pa. Freeman and Paul deposited films at a deposition rate of 0.10 n m / s and, according to fig. 3, either Sill 3 or Sill 2 could be dominant. This discrepancy in results is explainable if the S i - H bond type having the stretch mode at 2140 cm-~ can indeed be attributed to H bonded at several possible sites (which depend on the exact deposition conditions). It is interesting that an if-induced dc bias applied to the substrate had no detectable influence on H composition or S i - H bond type. However, the effects of bias sputtering are not entirely predictable in that it improves the properties of some materials and has either no or detrimental effects on the properties of other materials [16].

5. Summary and conclusions H incorporation and Si-H bonding in diode-sputtered a-Si:H can be explained using a simple model for Si-H reaction kinetics both in the plasma and on the surface of the growing film. Hydrogen content and Si-H bonding are influenced by the amount of H present in the plasma (PH), the reaction time (R~ 1) and the mobility of H and Si on the surface of the growing film. Total H content decreases with increasing R D and increases with increasing Pn, eventually saturating at high PH" Sill bonding increases with increasing R D and decreasingpH, two factors which reduce the Si-H reaction probability. As exposure of H to Si increases, i.e. decreased R D and increased PH, polymerization occurs and Sill 3 forms. An increased mobility of H and Si near the a-Si crystallization temperature is believed to be responsible for polymerization when high target power densities are used. The slightly higher H content of these films, compared to those deposited at similar R D and lower PT, may result from increased mobility due to substrate heating and H trapping.

P.M. Martin, W.T. PawlewicT. / blfluence of sputtering conditions

The authors gratefully acknowledge the technical assistance of I.B. Mann for performing the depositions, D.D. Hays for computer analysis of data, H.K. Kjarmo for SEM photography, and P.T. Raney for performing vacuum fusion analysis.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16]

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