Porosity and oxidation of amorphous silicon films prepared by evaporation, sputtering and plasma-deposition

Solar Energy Materials 1 (1979) 471~,79 © North-Holland Publishing Company

POROSITY AND OXIDATION OF AMORPHOUS SILICON FILMS PREPARED BY EVAPORATION, SPUTTERING AND PLASMA-DEPOSITION* H. FRITZSCHE and C. C. TSAIt James Franck Institute and Department of Physics, University of Chicago, Chicaoo, IL 60637, USA Received 10 April 1979

The amount of water adsorbed by amorphous silicon films upon exposure to normal humidity levels has been measured with a quartz-crystal microbalance. Films electron-beam evaporated on room temperature substrates absorb about 11 molto H20 on the internal surfaces of an interconnected void structure. We estimate that nearly 15~o of the silicon atoms lietat internal void surfaces. The water causes internal oxidation and cannot be removed by drying. It produces an increase in resistance and a shift of the optical absorption edge to higher photon energies. Hydrogenated amorphous silicon prepared by sputtering at 200°C or by rf plasma deposition at 25 and 150°C, on the other hand, is not porous. Exposure to 60~o relative humidity at 300 K produces a 5 ~ thick oxide layer in about 20 h. Water adsorption strongly affects the conduction in the space charge layer near the surface of plasma-deposited films but has little or no effect on the conductivity of sputtered films. Long term relaxation effects are observed in the conductivity of freshly plasma-deposited amorphous silicon-hydrogen films prepared at room temperature. Based on the very low hydrogen content found in chemical-vapor-deposited (CVD) amorphous silicon we conclude that the CVD films are impervious to water.

1. Introduction It was recently reported [1] that the conductance of amorphous films of Si :H alloys prepared by glow discharge decomposition of Sill4 changes by many orders of magnitude at room temperature when dry films are exposed to small amounts of water, ammonia or other adsorbates. Since these materials are being extensively studied for possible solar energy applications [2-6], it is very important to ascertain whether the adsorption is limited to the outer surface or whether these gases penetrate the amorphous Si :H films through a connected network of microvoids. There is strong evidence that evaporatedlayers of amorphous germanium or silicon (a-Ge, a-Si) contain an internal network of microvoids. Their existence was first postulated [7] to explain that these amorphous layers are about 10-15~ less dense *supported in part by the Materials Research Laboratory Program of the National Science Foundation at the University of Chicago and NSF DM R 77-11683. tPresent address: Xerox PARC, 3333 Coyote Hill Road, Palo Alto, CA 94304, USA. 471

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H. Fritzsche, C. C. T~ai / Amorphous silicon films

than the corresponding crystals even though the nearest and next nearest neighbor distances are the same. Furthermore, the large internal surface area of the void network provided an explanation for the relatively large free spin concentration [-8] of order 1020 cm- 3 and for the internal stress [9] usually observed in these amorphous films prepared near room temperature. Direct evidence for the void structure was found in evaporated a-Ge by transmission electron microscopy [10, 11] and porosity measurements [12]. Recently proposed structural models [13, 14] for a-Si and a-Ge containing H or other additives provide additional motivation for direct measurements of the internal void structure surface. This paper reports measurements of the amount of H20 adsorbed or absorbed by a-Si films as they are exposed to humidity levels normally encountered in the laboratory [15]. The amounts of H20 taken up by the films are measured by using the standard quartz-crystal microbalance technique [16]. The total amount of H20 absorbed as well as its dependence on the film thickness reveals whether the absorption is a surface or a bulk process. We report measurements on a-Si prepared (1) by electron-beam evaporation in ultra-high vacuum, (2) by sputtering in an Ar-H2 gas mixture, and (3) by glow-discharge decomposition of Sill4. The last two methods yield hydrogenated a-Si which is of current interest for potential solar energy conversion devices. Preliminary results of in situ measurements of conductivity changes immediately after preparation of the a-Si films and upon exposure to air will be described briefly.

2. Experimental details Evaporated a-Si films were deposited at room temperature at a rate of 3 ~/s by electron beam evaporation in an oil-free ultra-high vacuum system which has a base pressure near 10-8 Torr. Sputtered films were prepared on substrates held at 200°C at a rate of 4 ~/s at 5 mTorr using a sputter gas mixture 4:1 of ultrapure Ar and H2. This preparation condition is similar to that used by W. Paul et al. [17]. The sputtering system was kept free of oil by employing a turbomolecular pump. Our plasma system for preparing a-Si :H by glow-discharge decomposition of Sill4 has been described earlier [ 18, 19]. Films were deposited onto the so-called anode plate at 25 and at 150° C, respectively, at a rate between 0.8 and 3 ~/s. In each case the a-Si or a-Si :H films were deposited on one side of an AT-cut quartz plate whose natural resonance frequency was 5 MHz. Since the commercial crystals* come with gold electrodes which may alloy with a-Si at high temperatures, we replaced them with Ni electrodes and contacted these to stainless steel wires by means of high-temperature silver paint. An AT-cut quartz oscillator shows a frequency shift [16, 20] A f = -(fo/doPo)Am/ A *International Crystal Corp., Oklahoma City, OK 73102.

(1)

H. Fritzsche, C. C. Tsai / Amorphous silicon films

473

when a mass increment Am is condensed on the area A of its electrodes. Here the symbols fo, do and Po stand for the frequency, thickness and the mass density of the quartz crystal. We used a 5 MHz AT-cut quartz in its fundamental shear mode with A =0.31 cm 2. The parameters in eq. (1) which are determined by the crystal yield

fo/doPo=4.83

x 107 Hz cm2/g.

(2)

By measuring the difference frequency between that of the test crystal and a thermally stabilized reference crystal we are able to determine Afwith an error of + 0.2 Hz and thus Am/A with an error of + 4 x 10-9 g/cm 2. This sensitivity corresponds to a fraction of a monolayer of H20.

3. Experimental results Fig. 1 shows the frequency decrease of quartz crystals, having on one of their electrodes an electron-beam evaporated a-Si film, after exposure to air which contained a relative humidity of 28 and 30~, respectively. The total weight increase due to the uptake of H20 is very large considering that a monolayer of H20 corresponds to a frequency decrease of only a couple of hertz. After an initial rapid rise the weight of the a-Si films increases slowly and saturates after 15 h. Within experimental error, the total weight increase is proportional to the thickness of the a-Si film. From this we conclude that the water penetrates the internal void structure of the evaporated a-Si. For both film thicknesses the water content at saturation corresponds to (11 +0.5) mol~. As shown in fig. 1 the water cannot be driven out by a subsequent heat treatment at 90°C in vacuum. One would expect that the internal silicon surface of the interconnected void structure oxidizes during this heat treatment. If the void structure remained otherwise intact and open, one should find an additional absorption of

<( Air in

A

600 50o 300 20o ioo 0

Evoporoted

o-Si

~T:28"(;

,'" t =0.63ml~__----~"

~ I

0

i

""., After 3h vocuum annea at 900 C

t = 0.16/~m H:28%

I

10

~

/

i

20 min

~"

I~llTh

Time Fig. 1. Mass increase according to eq. (1) of two evaporated a-Si films resulting from exposure to air having a relative humidity H. The mass increases in proportion to the film thickness t which indicates water absorption in the bulk.

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H. Fritzsche, C. C. Tsai / Amorphous silicon films

H20 on the oxidized void surfaces when the films are re-exposed to air. This is not observed. After heat treatment the frequency decrease amounts to only 5-6 Hz which corresponds to H20 adsorption on the outer surface only. Either the oxidation process or the heat treatment must have closed the access of the internal void network to the outside. We believe it is the former because in the case of evaporated a-Ge the void network remains accessible to repeated water absorption after annealing to above 150°C. If we assume that a monolayer of H20 is adsorbed on internal surfaces of the a-Si films then the results of fig. 1 yield an accessible internal surface area of 6 x 10 6 cm2/cm a. This means that in our particular a-Si films, which were evaporated on room temperature substrates, about 15~o of the silicon atoms lie at internal surfaces. In the case of freshly evaporated a-Ge similar water absorption studies revealed [12] an accessible internal surface area of only l05 cmE/cm 3. On the basis of internal strain measurements and the observation that the free spin density in a-Ge is not changed by water absorption [21], it was argued [9] that the total internal void surface area in a-Ge probably exceeds 10 6 cm2/cm 3. In contrast to a-Si, the water could be removed from a-Ge by drying [9] and the void structure coalesced or closed at considerably higher annealing temperatures than those reported here. The room temperature resistivity of the fresh, electron-beam deposited a-Si films was 2.5 x 104fl cm. It increased to 2 × l05 ~ cm within 10 min after exposure to air containing 30~o relative humidity. A slow increase to 3.5 x 105 f~ cm was observed in the subsequent 48 h. As emphasized earlier [22], the electrical transport properties of evaporated amorphous films should not be described by models which treat the material as homogeneous and thus neglect the internal voids and the associated space charge layers. It is not surprising to observe a considerable change in resistivity and also a blue shift of the fundamental optical absorption edge as the films take up 11 molto of water or oxygen. We now turn to the effect of moisture on amorphous silicon films prepared by the other two methods. Fig. 2 shows the frequency decrease or weight change of a quartz crystal carrying on one side a glow-discharge deposited a-Si :H film. The results are independent of film thickness and essentially the same for a-Si :H films prepared at 25 or 150°C by plasma decomposition or at 200°C by sputtering in an At-H2 gas mixture. The air admitted in this case had a relative humidity of 60~o. When air is admitted to the sample preparation chamber one observes a rapid weight increase followed by a slow rise which continues at an ever decreasing rate for many days. The full curve labelled (a) in fig. 2 follows the sequence of measurements. At certain intervals the moist air was pumped out and the sample immersed in a flow of dry high-purity argon at a pressure of 0.1 Torr, which also is the environment of the sample immediately after preparation. The dashed curve labelled (b) connects the points measured under this condition. Curve (b) lies lower than curve (a) by about 11 Hz. Part of this change, about 6 Hz, is caused by desorption of water from both sides of the quartz crystal, that is from the surface of the a-Si :H film as well as from the surface of the opposite Ni electrode. The remaining change of 5 Hz is caused by a combination of two effects, a change of the elastic moduli of the quartz due to the change of hydrostatic pressure and change of the aerodynamic loading of the quartz

H. Ftitzsche, C. C. Tsai / Amorphous silicon films

475

50 40 N

(o) (b) (C)

o-Si:H

l=O.23ym

p: 760 Tort 60% humidity p: 03 Tort dry Ar p: 0.1 Tort dry Ar ofter drying ol t40'C

A(25

O

~ 20 Po ~- I0 =o

(cl

0 I i i ] , i L I , I h l l l i l i l h l ' ? ~ - -

I0

20

Time (hours) Fig. 2. Mass increase [see eq. (1)] of an a-Si :H film which was plasma-deposited at 25~C on the anode plate of an rf plasma diode system. Water absorption and oxidation resulting from exposure to air of 60~o humidity occurs on the surface only.

[20]. For this reason we limit our interpretation in the following to curves (b) and (c) which were measured in the 0.1 Torr dry Ar gas environment which surrounded the sample before the air was admitted. Curve (c) was obtained after drying the sample at 140°C for one hour in this dry Ar gas flow at 0.1 Torr. It lies below curve (b) by 8 Hz because the last adsorbed water can be removed only at elevated temperatures. The 11 Hz frequency difference between curve (c) and the starting point represent oxidation of the a-Si :H surface and to a lesser degree of the opposite Ni electrode during the 20 h exposure of moist air. The slow rise of curve (b) indicates that oxidation at room temperature continues at a slowly decreasing rate. A quantitative determination of the oxide thickness is difficult at the present time because the actual surface area is larger than the geometric one on account of the surface roughness of the quartz crystal. With an estimated roughness factor of two we obtain an oxide thickness of about 5 ~. Our measurements are also unable to exclude the possibility that less than 0.2 molto H20 penetrates the bulk of these a-Si :H films. Fig. 3 shows the conductance change of a 1650 ~ thick a-Si :H film prepared by glow discharge decomposition at room temperature. During the conductance measurements a flow of dry Ar gas at a pressure of 0.1 Torr (curve b) was interrupted several times by admitting air of 1 atm pressure having a relative humidity of 61~ (curve a). The moist air produces a significant increase in conductance which essentially disappears after the flow of dry Ar is reestablished. Previous experiments have shown that adsorbed water produces a negative space charge layer and thus increases the conductance of these slightly n-type a-Si :H films. However, the observed large decrease in film conductance which continues for many hours in dry argon was not expected. There appears to be a long term structural relaxation in these films prepared at room temperature. In order to explore this effect further, we measured the time dependence of the

H. Fritzsche, C. C. Tsai ,; Amorphous silicon films

476

"7 gE

I 6508

o-Si:H

-I~ ~ . . . 2 =

(a}~ .

-14-

A (25)

(a) P=760Torr A,r ~ : 6 1 Y o

- _ ......... P= 0.1 T0rr Ar

~' -15 0 ....

I

.......

,,,,,IJ,,,I

IO

-

20 time (hours)

.... I .... J 3o

Fig. 3. Conductance change of a 1650 ~ thick a-Si :H film immediately after plasma-deposition at 25'C. Air of 61°~ humidity was admitted during the time periods of curve (a). The conductance was measured between co-planar electrodes of 1 cm width and 0.1 cm separation.

film conductance after depositing successive layers of a-Si :H on top of the same film without exposit to the atmosphere. Fig. 4 shows in the lower left corner the conductance of the same film 40 h after a second deposition which brought the film thickness to 2.950 ~. Additional layers varying in thickness between 86 and 340/~ produce an initial conductance increase of two orders of magnitude followed by a rapid and a very slow relaxation. The initial high conductance does not scale with the thickness of a new layer presumably because the first part of each layer goes already through the initially rapid relaxation process during the time of deposition. At 98 h the film was exposed for 7 min to a pure argon plasma. This is the time which produces a 300 ~ thick layer of a-Si :H in the presence of silane. Even though the conductance change is smaller than that following a deposition, the relaxation proceeds in a similar fashion. Since photoconductivity decays much faster we believe that in this case the conductance change is caused by the plasma bombardment of the upper surface layer. An additional layer of 9280 ,~ thickness was deposited at 140 h as shown in fig. 5 in order to determine whether the final conductance scales with film thickness. This

'S

-12

°g o o

~

+ °..,.

2 g

~

P: 0.1 T0rr Ar

.

-13

-14

,~2 95oi -15

80

90

100 time (hours)

I10

Fig. 4. Time dependence of the conductance of the a-Si :H film shown in fig. 3 45 h after depositing an additional 1300 J~ thick layer as well as further layers having thicknesses as indicated in the figure.

H. Fritzsche, C. C. Tsai / Amorphous silicon films

¥

-12

....

i , , , i .....

io . . . . . . . . .

-g

l ........ P : 0.I

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'1' Ar

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g,, -

-

-15

140

150

160

170

time (hours) Fig. 5. In situ conductance measurements of the a-Si :H film of the previous two figures after depositing an additional 9280 ~ thick layer.

experiment remained inconclusive because of the extraordinary long relaxation times at room temperature. A quartz crystal microbalance mounted adjacent to the conductance sample assured us that no impurities were adsorbed on the sample surface during the conductance measurements.

4. Conclusions

An open void structure revealing an internal surface area of about 6 x 106 cm2/cm a was observed in a-Si films which were deposited by electron-beam evaporation on room temperature substrates. When these films are exposed to the atmosphere they absorb a considerable amount of water which cannot be removed by drying. This results in an increase in resistivity and a shift of the absorption edge to higher energies. In contrast, hydrogenated a-Si films prepared by sputtering at 200°C in a gas mixture of Ar and H 2 or by rf plasma decomposition of silane at 25 and 150°C are impervious to moisture. The surfaces of these films oxidize in atmosphere. An oxide thickness of about 5/~ was observed after 20 h exposure to air having 60~o relative humidity. We conclude from these results that the large conductance changes which are observed when plasma-deposited a-Si :H films are exposed to various ambients are caused by changes in the surface potential and the adjacent space charge layer. The microbalance technique used in our experiments can only detect that part of a void structure which is interconnected and accessible to the outside. We therefore cannot exclude the presence of other closed void defects. It is moreover well known that the properties and the structure of amorphous tetrahedral films depend on various preparation parameters. It is therefore dangerous to generalize our findings to materials prepared under very different conditions. Our results agree however with the electron-micrograph studies of Barna et al. [23] who observed a coarse surface texture on evaporated films and a very smooth surface on plasma deposited films. Knights et al. [14] on the other hand found that plasma-deposited a-Si :H films prepared at room temperature on the anode plate (unbiased) show a columnar structure perpendicular to the substrate, which becomes more pronounced with

H. Fritzsche, C. C. Tsai Amorphous" silicon films

478

increasing rf power level. They reported that their films oxidize quite rapidly throughout the bulk when they are exposed to the atmosphere. Even though our plasma deposition apparatus is a copy of Knights' system, we did not observe these effects presumably because our films are deposited at smaller rf power. Conductivity measurements of a-Si:H films prepared at room temperature by plasma deposition show a rapid as well as a long term relaxation process. The top surface layer has a higher conductivity than the bulk and is strongly affected by ambients and surface treatments. Of great interest for photothermal solar converters is amorphous silicon prepared by chemical-vapor-deposition (CVD) of silane [24]. The question naturally arises whether CVD a-Si is porous or impervious to moisture. Since the pyrolytic decomposition of silane requires temperatures near 600°C one cannot use the quartz-crystal microbalance technique because these crystals fail to oscillate after being heated above the ot-fl phase transition of quartz at 573°C. However, recent SIMS analyses of CVD a-Si revealed [25] a hydrogen content of less than 0.4 at~o. These films were exposed to the atmosphere for extended periods of time before the analysis. It is very unlikely that this very low hydrogen content originates from water penetration. We agree with Seraphin et al. [25] that the hydrogen is incorporated during the chemicalvapor-deposition process and that the CVD films are impervious to water.

Acknowledgements We wish to thank David Dennison and Eugene Symbalisty for their help in carrying out this research.

References [11 [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]

[13] [14] [15]

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[16] G. Sauerbrey, Z. Phys. 155 (1959) 206,216. [17] W. Paul, A. J. Lewis, G. A. N. Connell and T. D. Moustakas, Solid State Commun. 20 (1976) 969; G. A. N. Connell and J. R. Pawlik, Phys. Rev. B13 (1976) 787. [18] H. Fritzsche, C. C. Tsai and P. Persans, Solid State Technol. 21 (1978) 55. [19] C. C. Tsai and H. Fritzsche, Solar Energy Mater. 1 (1979) 29. [20] C. D. Stockbridge, Vacuum Microbalance Techniques, ed. K. H. Behrndt (Plenum Press, New York, 1966) vol. 5, p. 147,179. [21] S. C. Agarwal, Phys. Rev. B7 (1973) 685. [22] H. Fritzsche, Amorphous and Liquid Semiconductors, ed. J. Tauc (Plenum Press, New York, 1974) p. 221. [23] A. Barna, P. B. Barna, G. Radnoczi, L. Toth and P. Thomas, Phys. Stat. Sol. (a) 41 (1977) 81. [24] D.C. Booth and B. O. Seraphin, SPIE vol. 161, Optics Applied to Solar Energy IV (1978) p. 72. [25] D. C. Booth, D. D. Allred and B. O. Seraphin, Proc. 8th Ann. Conf. on Amorphous and Liquid Semiconductors, Cambridge, MA, USA (27-31 August 1979) to be published.