Effect of annealing on the optical properties of plasma deposited amorphous hydrogenated silicon

Solar Energy Materials 1 (1979) 29~,2 ©North-H0lland Publishing Company

EFFECT OF ANNEALING ON THE OPTICAL PROPERTIES

OF

PLASMA DEPOSITED AMORPHOUS HYDROGENATED SILICON* C. C. T S A I and H. F R I T Z S C H E Department of Physics and The James Franck Institute, The University of Chicago, Chicago, Illinois 60637, USA Received 14 July 1978

Amorphous silicon films prepared by radiofrequency plasma decomposition of silane contain between 10 and 25 at ~o hydrogen depending on the potential and temperature of the substrate and plasma parameters. The way hydrogen is bonded in these films has been determined from the infrared stretching, bending, and rocking or wagging modes of the Si-H complexes. Monohydrides Sill, dihydrides SI'H2and short chains of polysilane (SiH2)n can be identified. The potential of the substrate with respect to the plasma and to a lesser degree the substrate temperature determine the prevalent Si-H complex. At substrate potentials close to that of the plasma (SiH2)n complexes are favored whereas isolated Sill and Sill2 groups are found in films prepared on strongly negative potential substrates. The films can be dehydrogenated by annealing. Substrate potential and temperature influence strongly the refractive index and the onset of interband optical transitions. The optical gap of these films lies between 1.6 and 1.85 eV which is appreciably higher than the values 1.2-1.5 eV of sputtered and evaporated amorphous Si films. As the hydrogen is driven out by annealing the optical gap decreases to 1.6-1.7 eV. At photon energies below the optical gap one observes a preparation sensitive absorption tail between 0¢=102 and 10 3 cm-t which cannot be removed by annealing. The films crystallize between 700 and 780°C with a crystallization energy of (2 + 0.3) kcal/mol.

I. Introduction The pioneering w o r k o f Spear [1, 2], Spear et al. [3] and L e C o m b e r et al. [4] showed that radio frequency plasma decomposition o f Sill4 and G e H 4 gas, respectively, yields a m o r p h o u s Si and Ge films whose properties are far superior to those o f sputtered or evaporated films. In particular, the films prepared by glow discharge d e c o m p o s i t i o n contain very few gap states a n d thus exhibit large excess carrier lifetimes [4, 5], large p h o t o c o n d u c t i v i t y [3, 6, 7] and a correspondingly large field effect [2, 8]. These films could be d o p e d [9-12] strongly n-type or p-type by adding either PH3( or AsH3) or B2H 6 to the Sill# or G e H 4 gas. In this m a n n e r one was able to p r o d u c e for the first time a m o r p h o u s semiconductor pn junctions [13] a n d Schottky barrier devices [14, 15]. D u r i n g the past two years plasma deposited films o f a m o r p h o u s silicon have been * Supported in part by NSF grant DM R77-11683 and the Materials Research Laboratory Program of the National Science Foundation at the University of Chicago. 29

30

C. C. Tsai and H. Fritzsche/ Amorphous hydroyenated silicon

studied extensively with the aim of relating the film properties with the various plasma deposition parameters [16--20]. Street et al. [21] studied in detail the photoluminescence and related it to the spin density in films prepared under various conditions. Zanzucchi et al. [22] compared the optical and photoconductive properties of amorphous silicon prepared in a dc and a rf plasma at different substrate temperatures. These and other studies revealed that plasma deposited silicon films are Si-H alloys whose structure, hydrogen content and hydrogen bonding depend in a complex manner on various parameters of the plasma-deposition process. Moreover, two quite different plasma deposition systems have been used. The groups at Dundee [1-4, 6-10], Marburg [19] and IBM [23, 24] used predominantly an inductively coupled plasma system whereas those at Xerox [12, 17, 21], RCA [14, 15, 22] and Chicago [20, 25] obtained their films in a capacitively coupled system. It is the aim of this paper to compare the optical properties of plasma deposited Si-H films prepared under various conditions in different laboratories and to report the effect of annealing on the infrared absorption and the fundamental absorption edge. For further characterization of our films we also report the temperature and heat of crystallization. Their density and'hydrogen content have been published earlier [26] and a preliminary report on field effect measurements [27] has been given.

2. Sample preparation The Si-H films were prepared in a diode plasma system which essentially duplicates the design of Knights [12, 17]. The plasma reactor is sketched in fig. 1. It consists of a 7.5 cm diameter stainless steel cross whose side arms serve as gas inlet and pumping connections. The 13.56 MHz power is applied to the top insert which is insulated from the grounded stainless steel cross and the grounded bottom insert, A, by a machinable ceramic flange, F. The top insert is surrounded by a grounded shield, S, in order to limit the plasma to the space between A and C which are separated by d = 1.3 or

13.56 M~i

Fig. 1. Schematic sketch of plasma reactor consisting of 7.5 cm diameter stainless steel cross. A = anode, C = c a t h o d e , F = c e r a m i c flange, H = h e a t e r , Q=insulating quartz cylinder, Sc=grounded screen, S = grounded shield.

C. C. Tsai and H. Fritzsche / Amorphous hydrogenated silicon

31

2.5 cm. The different mobilities of the ions and electrons in the plasma cause C to attain a negative potential which is nearly equal to the peak value of the rf voltage. A and C are thus termed anode and cathode, respectively. They can be heated to about 450°C. The diameter of C is 3.3 cm and that of A is 5.8 cm. The control and constancy of the potentials make Knight's diode system more convenient to handle than an inductively coupled one, although films of similar quality can be made in either system. The silane was diluted with Ar at a ratio of 1:60. A flow rate of 100 scc/min was maintained with a pressure of 0.15 Torr in the reactor. This high flow rate as well as the Ar dilution eliminated the nucleation of a-Si-H particles in the gas phase [28]. Fig. 2 shows that the deposition rate increases linearly with rf power. The rate is essentially independent of the Sill J A r ratio. This means that only a fraction of the Si in the plasma gets deposited on the substrate. The remaining Sill4 is thermally decomposed in a quartz tube held at 800°C behind the pump exhaust before the gas mixture is led into the atmosphere. For an electrode separation d=2.5 cm the deposition rate at the cathode is nearly twice that of the anode. The rates are nearly equal for d = 1.3 cm. For depositions at Ts=25°C the rates are between 2-3 times larger than those shown in fig. 2. Films having thicknesses up to 32 #m were deposited on aluminum, sapphire, germanium, silicon or 7059 glass substrates which were clamped to the anode and cathode plates. We usually used 15 W rf power which corresponds to a cathode bias of - 2 0 0 V. The thickness and the refractive index of the films were determined from the magnitude and the spectral dependence of the interference fringes at wavelengths between 1-2 #m.

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Ts =270 C d =2'5cm JO

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r

/ Cothode

/

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/

/i/

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o

20 40 R.E Power (W)

60

Fig. 2. Deposition rate on cathode and anode as a function o f rf power for an electrode separation d = 2 . 5 cm and temperature Ts =270°C.

3. Infrared absorption The infrared absorption of an anode film prepared at Ts = 25°C is shown in fig. 3. In agreement with other workers [29, 30] one observes two Si-H stretching modes at ~t = 2090 cm- t and v2 = 2000 cm- 1, respectively, two bending modes at ~4 = 895 cm- 1

('. C. Tsai and H Fritzsche ,, Amorphous hydro,qenated silicon

32

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Fig. 3. Infrared absorption spectrum of film prepared on the anode at 2 5 C while the cathode was heated to 2 7 0 C .

and ~s = 850 c m - 1 and a strong rocking or wagging mode absorption at 96 = 640 cm- L In addition we find a weak and quite broad absorption at 93 = 990 cm- 1. The most recent analysis [30] of hydrogen bonding in a-Si : H alloys suggests that one must consider the three structural units shown in table 1 to explain the absorption spectra. The distinguishing features of these structural units are the following: (i) isolated monohydride groups are identified by an absorption band v2= 2000 c m - ~ and by the absence of a bending mode ; (ii) isolated dihydride groups are identified by a single bending mode. They have two as yet unresolved stretching modes near 9~ = 2 1 0 0 cm-1 ; (iii) short chain polysilane units have two distinct bending modes near 850 and 890 cm-1. Their stretching modes also fall in the region of 2100 cm- 1 The characteristic doublet structure 850 cm- ~ and 890 c m - 1 was first attributed [29] Table 1 Vibrational frequencies of hydrogenated silicon Structure

Si

H

\/

Ii)

Si Si ~ Si

Si

/\ Si H

(iii)

2000

H

~2100

890

~640

~2100

850,890

~640

H H

Si~Si----Si--,Si

I

~640

\Si H

\/

(ii)

Vibrational frequencies ( c m - 1) stretching bending rocking or wagging

!

H

C. C. Tsai and H. Fritzsch/ Amorphous hydrogenated silicon

33

to SiHa groups but these were later shown to have a rocking mode close to 500 cm- 1 which is not observed in the infrared spectra. Knights et al. [30] concluded that in low Ts films the hydrogen is bonded predominantly in polysilane chain segments (Sill2), whereas films deposited at Ts~> 250°C contain isolated monohydride groups. Brodsky et al. [29] find a similar trend with substrate temperature but Brodsky [18, 23] emphasizes furthermore that low plasma pressures inhibit the formation of polysilane chains. Our results show that, in addition to the above mentioned trends, the substrate bias plays an essential role in determining the type of hydrogen bonding in these films. A comparison of figs. 4 and 5 shows that all anode films have the bending mode doublet which is characteristic of polysilane whereas the cathode films show only a single bending mode absorption which is attributed to isolated dihydride groups. An increase in substrate temperature decreases the absorption strength of both the polysilane (Sill2), and the dihydride in the range 850-890 cm-1 compared to the strength of the rocking or wagging mode absorption centered at 640 cm-1. This means that at high Ts an increasing fraction of hydrogen is bonded in isolated monohydride groups. Knights et al. [30] studied only anode films. Their failure to find polysilane in their Ts = 300° C film is probably due to the fact that the strong interference fringes made it difficult to detect the decreased absorption strength of the doublet structure. We nearly eliminated the presence of these fringes by using substrates of crystalline Si whose refractive index nearly matches that of our films. Figs. 6 and 7 show the corresponding stretching mode absorptions of these samples. The polysilane and dihydride structures cannot be distinguished here because both

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Fig. 4. Bending and rocking (or wagging) mode absorptions of hydride groups in anode films prepared at T5 =25, 270 and 450°C, respectively, on crystalline Si substrates. The film thickness and the transmission measured at ~ = 1000 c m - 1 are given for each curve. Fig. 5. Same as fig. 4 for cathode films deposited on crystalline Si substrates. Some of the low transmission values are caused by surface roughness.

C. C. Tsai and H. Fritzsche / Amorphous hydrogenated silicon

34 2x[O s

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2100 Wovenumber

25)

2000

1900

~ (cm-1)

Fig. 6. Stretching mode absorptions of hydride groups in anode films prepared at T~ = 25,270 and 450 ° C, respectively. Fig. 7. Same as fig. 6 for cathode films.

have modes near 2100 cm- x but the monohydride contributes solely to the 2000 cm- 1 absorption. Again one observes the dominant effect of the substrate potential relative to the plasma. The influence of substrate temperature on the relative abundances of the various hydride structures is most pronounced at low Ts. As Ts is increased from 270°C to 450°C one can see that, although the total hydrogen content decreases, the overall shape of the stretching mode absorption remains the same. Combining the information from the stretching and bending mode absorption we conclude that the cathode films contain predominantly monohydride at T~=25°C and, judging from the absorption strengths, equal proportions of isolated monohydride and dihydride units at T~>t 270° C. Anode films have a preponderance of polysilane chain segments at Ts = 25° C but at higher substrate temperatures most of their hydrogen is bonded as monohydride. The complexity of the plasma deposition process is appreciated by noting the difference in the Vl/V2 absorption ratio of the A(25) film shown in fig. 4 and of the A(25) film of fig. 3. Although both anodes were kept at Ts =25°C, the A(25) film of fig. 3 was prepared while the cathode was heated to T~= 270° C. This shows that the nature of the plasma near an electrode is influenced by the deposition conditions at the other electrode. From then on we kept both the anode and the cathode at the same T~. The early films of Knights [12, 17] were prepared with the anode heated while the cathode was kept at room temperature. We next discuss the effect of hydrogen effusion during 30 min anneals at successively higher temperatures on the infrared vibration spectra. The bending mode doublet of the A(270) sample shown in fig. 8 is asymmetric because the single mode of the dihydride is superimposed on the polysilane doublet. The annealing process decreases the polysilane content faster than the dihydride content. After an anneal at T, = 360° C the polysilane is no longer noticeable. At this temperature, the decrease of the

C. C. Tsai and H. Fritzsche / Amorphous hydrogenated silicon

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Fig. 8. Bending and rocking (or wagging) mode absorptions of a 6.29/~m thick anode film prepared on a

silicon substrate at 270°C after 30 min annealing at the temperatures indicated. The transmission values measured at 1000 cm- 1 are given for e~ach curve. They are low because of increased blistering (without annealing) of the film at higher annealing temperatures.

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Fig, 9. Effect of 30 min anneals on the stretching mode absorption of an anode film deposited at 7", = 270 ° C.

monohydride stretching mode absorption shown in fig. 9 has barely begun. Considerable loss of hydrogen at relatively low annealing temperatures is always seen [20, 25, 26] in anode film deposited at T, =25°C. Fig. 10 shows an example. Since these contain a large fraction of polysilane we conclude that these species release hydrogen most easily. At high T,, the two absorption bands at 1990 cm-x and 2090 ¢m-x decrease with equal strength. These are probably due to monohydride and dihydride units, respectively. The small absorption band at ~3---990 cm -~ is probably due to some oxygen contamination. The magnitude of this band shown in fig. 3 is the largest we have seen. In A(25) films it slowly grows over a period of weeks when the filmsare exposed to the atmosphere. Other films are stable after a minor surface oxidation. In the case of sample C(270) shown in fig. 11 the monohydride band near 2000 cmdecreases first whereas the dihydride absorption grows before both decline after

('. ('. Tvai and H. Fritzsche ,, AmotThous hydrogenated silicon

36

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Fig. 11

Fig. 10, Effect of 30 min anneals on the stretching mode absorption of an anode film deposited at T~ = 25:C. Fig. 11. Same as fig. 10 for a cathode film deposited at 270°C.

Ta =385°C. Such a transfer from the 9-2 (monohydride.) to the ~l (dihydride) mode may be obscured in sample A(270) shown in fig. 9 by the simultaneous change in polysilane concentration. The peak of the ~l mode shifts between 2100 cm- ~ and 2080 cm- 1. This is particularly noticeable in fig. 10. It may be that polymeric (Sill2), chains and isolated Sill2 sites have stretching modes at somewhat different energies. We believe, moreover, that in addition to the structures listed in table 1 there may exist others, such as nearest neighbor pairs of monohydrides Sill-Sill or of monoand dihydrides Sill-Sill2. Fig. 4 shows for instance, that the bending mode absorption of C(25) lies clearly at lower energy (V=875 cm -~) than the bending mode (~= 890 cm-~) of the cathode films deposited at higher temperatures. Fig. 12 relates the area under the stretching mode absorption with the hydrogen content as determined previously from hydrogen effusion curves. Following Levin

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•

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C(270) ,~ A(270) •

A(450) •

,

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,

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,

20 B0 Content ( a t % }

Fig. 12. Area under the stretching mode absorption bands for various films as a function of their total hydrogen content.

C. C. Tsai and H. Fritzsche / Amorphous hydrogenated silicon

37

and King [31], the integrated absorption coefficient is defined as F = I N f ~ d~/~,

(1)

where N (mol/cm 3) is the concentration of modes and ~ (cm-1) is the absorption coefficient. We chose as an example the film C(25) which contains the largest monohydride concentration. Its density [26] was found to be 2.10 g/cm 3. With the data shown in fig. 12 one obtains F =4.6 x 103 cm2/mol. This value compares well with F=4.8 × 10 3 cm2/mol which is the value 14.6 × 10 3 cm2/mol, measured by Levin and King [31] for Sill4 gas, divided by the mode degeneracy factor 3. We conclude that essentially all hydrogen measured by effusion is bonded and that the area under the infrared absorption bands can be taken as a measure of the hydrogen content. Difficulties with film cracking and peeling from the substrates limited the annealing studies to temperatures below 500-600°C. These problems were not caused by crystallization as will be seen in the following but by the mismatch of the thermal expansion of the substrate.

4. Crystallization Samples for differential thermal analysis (DTA) were obtained by etching away the AI foil onto which the films were deposited. All films investigated crystallized between 700°C and 780°C. Two typical DTA traces obtained with 11 and 12 mg of a-Si-H deposited on the anode at 25°C and 270°C respectively, are shown in fig. 13. More detailed studies and larger amounts of material are needed to see whether the structure at the lower temperatures has any significance, perhaps being associated with the hydrogen effusion process. The high temperature hydrogen effusion step [26] at T2 ~650°C is close to the crystallization temperature T~. However, the effusion step starts well below the crystallization temperature. Moreover, we never observed a burst of hydrogen which would be expected when a sudden crystallization occurs within a narrow temperature interval as in sample (b) of fig. 13. Nevertheless, the closeness of T2 and T¢ suggests that the bond reconstruction of the Si network

I

300

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400

r

d

r

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500 600 700 Tempereture (°C)

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800

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900

Fig. 13. Differential thermal analysis trace of anode films prepared (a) at 25°C and (b) at 270°C. The heating rate was 25°C/min. The exothermic heat of crystallization was 1.75 and 2.15 kcal/mol, respectively.

38

C.C. Tsai and H. Fritzsche / Amorphous hydroyenated silicon

associated with the release of hydrogen in the bulk film near T2 requires a degree of atom mobility similar to that needed for crystallization. The exothermic heat of crystallization varied between 1.7 and 2.2 kcal/mol [32], which is about a factor of two smaller than the value 4.33 kcal/mol calculated by Connel and Paul [33]. Unfortunately we were not able to find values for the heat of crystallization of amorphous Si prepared by sputtering or evaporation. Nagasima and Kubota [34] found, however, that a-Si films formed by chemical vapor deposition of Sill4 at 620°C crystallize between 665-700°C with a crystallization energy of 2.3 kcal/mol in agreement with our results. Studying the crystallization rate of a-Si films deposited by electron beam heating onto fused silica substrates, Blum and Feldman [35] observed that the crystallization temperature decreases appreciably when the films are held for longer times at a given temperature. The relatively high crystallization temperatures observed by us agree with their results if one considers our fast heating rate of 25° C/min.

5. Optical absorptionedge The optical absorption curve at(hv) of amorphous semiconductors consists typically of three parts [36]. (i) A high absorption region (ct~>104 cm -1) where the absorption coefficient is described by ~(hv) = B(hv - Eo)S/hv.

(2)

One usually finds s=2. In that case eq.(2) can be interpreted as resulting from optical transitions between a valence and a conduction band whose density of states depend on energy as N(E)ocE 1/2. An optical gap E 0 is then formally defined as the intercept of the (~thv) 1/2 vs. hv curve. (ii) An intermediate absorption range (1 < ct < 104 cm- 1) in which the absorption depends exponentially on photon energy : ot(hv) = cto exp (hv/E1)

(3)

with a slope parameter E1 which is usually between 0.05 and 0.08 eV. (iii) A weak absorption tail at low absorption constants (0t< 1 cm- 1) whose shape and magnitude depend on the purity, thermal history, and preparation conditions. Fig. 14 shows the absorption curves of our anode and cathode films, each prepared at Ts =25 and 270°C. Using eq.(2) with s = 2 the optical gap Eo for each film can be determined from the intercepts of plots (~thv) 1/2 v s . hv shown in fig. 15. We find B ~ 6 x 105 eV -1 cm -1 in close agreement with the values for other amorphous semiconductors [37]. Most striking and quite surprising is the presence of a large preparation-sensitive absorption tail which begins already at ~ 1 0 3 cm -1 for Ts--25°C films and at somewhat lower absorption coefficients for high T~ films. Zanzucchi et al. [22] find an even larger tail extending from ~ ~3 x 103 cm- 1 near Eo to 10a cm- 1 near midgap energies. Such large absorption tails below the gap energy are unexpected because

C. C. Tsai and H. Fritzsche / Amorphous hydrogenated silicon

....

~

I ....

I' r,,I' ' C(270) A(270)

105

39

I .... I .... i .... I .... I ''' Curve

1 A(25)

600 - 2 C(270) 3 A(270)

/

/ 500 -- ,5 c(25} sput?ered u-Si /

5

31

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.! ,(q4

300 ._':2 3 ~. I0

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102 _ ~ J 1.0

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. . . . cove~ona et ol. 1.5 2.0 2.5 Photon Energy hv (eV) Fig. 14

0.5

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Fig. 14. The optical absorption edge of anode and cathode films prepared at 25°C and 270°C, respectively. Also shown are the absorption curve of an A(25) film of Knights (ref. [ 12]) and of a film deposited at 25°C by Loveland et al. fief. [6]). Fig. 15. Plots of (o(hv)1/2 against photon energy of anode and cathode films prepared at 25°C and 270°C, respectively. Also shown are absorption curves of sputtered a-Si according to Brodsky et al., Phys. Rev. B1 (1970) 2632, and of crystalline Si.

these films have much fewer localized states than evaporated or sputtered a-Si near the central gap region. Moreover, the supernetwork of density fluctuations observed by electron microscopy in evaporated Si films is absent in these glow discharge deposited films [38]. Also, chalcogenide glasses, particularly alloy glasses, are believed to have many more gap states than glow discharge a-Si:H, yet they are very transparent (g ~<5 cm-1) at photon energies below the exponential absorption region [36, 37]. One is tempted to associate this absorption tail with the presence of Si-H and Sill2 units or polymeric (Sill2), chain segments. However, it will be shown below that this absorption does not disappear but instead it increases slightly after the hydrogen is driven out by annealing. This absorption tail begins at photon energies so close to the optical gap Eo that the exponential absorption region is obscured. It is not certain whether or not such a region exists in tetrahedral material since very sharp absorption edges have been reported for some evaporated amorphous Si and Ge films [39]. The optical gap Eo of glow discharge a-Si:H films falls between 1.6 and 1.85 eV which is considerably higher than the values 1.2 to 1.5 eV of sputtered or evaporated films. This is not surprising because of the substantial hydrogen content of these films and the fact that Si-H bonds are stronger than Si-Si bonds. Substrate bias has a strong effect on Eo, particularly at room temperature. Figs. 14 and 15 show that C(25) and A(25) lie at the two extremes. The difference between anode and cathode films disappears at higher substrate temperatures; films prepared at Ts = 450° C have

40

C. C. Tsai and H. Fritzsche /' Amorphous hydrogenated silicon

A(25] ~

18

~

3.5

0

2701

]

c-Si

200

400

600

Anneoling Temperature To ("C)

Fig. 16. The optical gap E o and the refractive index measured at ). = 2 # m as a function of 30 min anneals at various temperatures T~. The value of the refractive index of crystalline Si is marked

absorption curves between those of A(270) and C(270). There appear to be several competing factors which determine the optical gap, among these are the total hydrogen content, the dominant structure of hydrogen bonding, and defects which are affected by annealing at quite low temperatures. High values of Eo are observed when the films contain larger amounts of polysilane. We suspect that this is the case for the Ts=25°C anode film of Knights [12] for which he reported the very low density of 1.4 g/cm 3 and for the Ts =25°C film of Loveland et al. [6] whose absorption curves are shown for comparison in fig. 14. The high temperature films of Knights [12, 17] and of the Dundee group [6] agree well with ours. The changes of the refractive index measured at ). = 2#m and the optical gap with isochronal 30 rain annealing at successively higher temperatures is shown in fig. 16. The optical gaps of films prepared at room temperature shift first to higher energies, which is the normal trend for amorphous semiconductors whose structure stabilizes with annealing. This agrees with our observation that in this temperature range the spin concentration of Ts =25°C films decreases by about two orders of magnitude. At higher annealing temperatures E o decreases as the films lose their hydrogen. It is very surprising, however, that the structure sensitive absorption tail does not diminish with annealing. Its onset usually remains at or moves up to 0t ~ 103 cm- x. We intend to investigate whether this absorption tail is caused by absorption or by scattering of light on small a-Si dust particles which nucleate in the plasma and become incorporated as the film grows. The much lower absorption tail of some chalcogenide glasses was shown not to be a scattering-artifact [40].

6. Summary and conclusions The study of the infrared and band edge absorption of glow discharge deposited a-Si : H films reveals that the deposition conditions influence strongly the manner in which hydrogen is incorporated in these films. A new result of this study is the

C. C. Tsai and H. Fritzsche / Amorphous hydrogenated silicon

41

important effect of the electrical potential of the substrate with respect to the plasma on hydrogen incorporation. Whereas polymeric (Sill2), chains exist in anode films at all substrate temperatures, they are absent in cathode films even at Ts =25°C. In cathode films we observed for the first time a pure bending mode attributed to SiHz. It shifts from 875 cm-1 to 890 cm-1 as Ts is increased from 25°C to values above 250°C. This may be caused by complexing of mono- and dihydride groups in the C(25) films. The manner in which hydrogen is incorporated in anode films at T~= 25°C was found to depend on the temperature of the cathode. This illustrates the complexity of the plasma deposition process. The optical properties of the film deposited at 270°C on the cathode, on the other hand, were not noticeably influenced by the temperature of the anode. The room temperature anode films are most sensitive to preparation conditions. Knights [12] for example obtained low density ( ~ 1.4 g/cm 3) polymerized films which have an absorption edge near E0---2 eV and are very susceptible to oxidation. Our A(25) films are denser [26] (1.92 g/cm 3) contain only 26 at % hydrogen and only after 6 days exposure to the atmosphere did we notice a small growth of an absorption band near 990 cm- ~ which we attribute to an oxide. The films dehydrogenate upon annealing. Hydrogen from polysilane groups begins to effuse near 250°C whereas the monohydride and dihydride release their hydrogen at higher temperatures. The dehydrogenated a-Si has an optical gap between Eo = 1.6 and 1.7 eV. We annealed films near 400°C for several days without a noticeable change of their optical properties. The films crystallize near 700°C with an exothermic energy release of about (2+0.3) kcal/mol. We do not yet understand the relation between the manner hydrogen is incorporated and other film properties. The substrate potential which has a profound effect on the hydrogen incorporation does not seem to affect the density of unpaired spins [20] which is largely determined by the substrate temperature. Preliminary field effect measurements show, on the other hand, that A(270) films have lower gap state densities than C(270) films. We conclude, therefore, that the small concentration of polysilane always present in anode films does not have an adverse effect on the film quality. The cathode films are exposed to a stronger argon ion bombardment during growth than the cathode films. Yet, if this ion bombardment produces defects, and the higher gap state density, it does not increase the total spin density. Deposition at higher temperatures or dehydrogenation by annealing, on the other hand, increases both the gap state and the spin density [20].

Acknowledgements The help of D. Dennison, M. Tanidian, M. A. Vesaghi and P. Persans in many phases of this work and fruitful discussion with J. Knights are most gratefully acknowledged.

42

C. C. Tsai and H. Fritzsche / Amorphous hydro,4enated silicon

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