GLOW-DISCHARGE a-Si : F PREPARED FROM SiF2 GAS

HAL Id: jpa-00220762

https://hal.archives-ouvertes.fr/jpa-00220762

Submitted on 1 Jan 1981

HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.

L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

GLOW-DISCHARGE a-Si : F PREPARED FROM SiF2 GAS

R. Weil, M. Janai, B. Pratt, K. Levin, F. Moser

To cite this version:

R. Weil, M. Janai, B. Pratt, K. Levin, F. Moser. GLOW-DISCHARGE a-Si : F PRE- PARED FROM SiF2 GAS. Journal de Physique Colloques, 1981, 42 (C4), pp.C4-643-C4-646.

�10.1051/jphyscol:19814141�. �jpa-00220762�

page

GLOW-DISCHARGE a - S i : F PREPARED FROM S i F 2 GAS

R. Weil, M. Janai, B. Pratt, K. Levin and F. Moser

Dept. o f Physics and Solid S t a t e I n s t i t u t e , 'Pechnion-Israel I n s t i t u t e of Technology, Z2000 f l a i f a , I s r a e l

Abstract.- A system for the deposition of fluorinated amorphous silicon by DC glow-discharge in a plasma of SiF2

+

SiF4 is described. The a-Si:F films contain from 0.5% to 15% fluorine with no hydrogen present.

Fluorine content was determined by nuclear reaction, ESCA and IR spectro- scopy. The fluorine content of the films was found not to depend on the sub- strate temperature. Electrical and optical as well as ESR measurements, were made on the films as a function of fluorine content. No influence of the

fluorine was evident on the conductivity, photoconductivity or ESR signal, showing that the fluorine in the films did not satisfy dangling bonds.

Introduction.- There is an active interest in the effect of fluorine on the pro- perties of amorphous based on the observations that fluorinated amor- phous silicon (a-Si:F) has better thermal stability than a - ~ i : ~ ~ , . and that fluorine should also act as a dangling bond terminator. Much of the experimental work has been done on samples prepared by glow discharge in a gas mixture containing SiFx and H2 or SiH4. Consequently, these samples contained both hydrogen and fluorine. Other w ~ r k e r s ~ ' ~ * ~ used samples prepared by sputtering in an atmo- sphere containing Ar and SiF4. In our grou we have succeeded in producing a-Si:P by chemical vapor deposrtion of SiF?

'' ^\

^si. report is on the a-Si:E thin films we have produced by glow dischargz in an atmosphere of SiF2 and SiF4.

Thus we introduce in our films no elements besides silicon and fluorine.

FCUC P W

F I G I S C H Y A T I C DIAGRAM OF GLOW OlSCHPlROE APPARATUS

U

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19814141

C4-644 JOURNAL DE PHYSIQUE

Thin Film Preparation.- Fig. 1 illustrates the apparatus used to produce the thin films. Proceeding from right to left, the starting material is SiF4 gas obtained from Matheson, and further purified by distillation. This was achieved by freezing the gas at 80 K and pumping out the gases that are volatile at that temperature.

The SiF4 is then transferred t o a cooled clean cylinder by warming up the original container. A needle valve provides a pressure gradient between the high pressure cylinder and the low pressure part of the system. A flow-meter is in the circuit to ascertain that gas is being transferred to the reaction chamber at a constant rate.

The actual flow rate of SiF4 is calculated from the difference of pressure in the storage cylinder before and after the run, and the known volume of the cylinder

(420 cc). The SiF4 then flows through furnace TI where it comes in contact with a charge of small silicon cubes, of approximately 2 mm on the side. These cubes are cut from undoped single crystal silicon, since we want to keep the system free of unknown elements. In the furnace, reaction (1) takes place:

lloo0c SiF4

+

^{S i r2 SiF2}

I N i h A R t D R E f L t C T A h L E

SAJAPLE F 755 (G D l The exit of the furnace was

STAIN.ESS STEEL S ~ ~ S T H A I E designed to allow quick expansion of

the gas in order to cool it as rapidly as possible. The reason is that reac- tion (2) occurs between approximately

90

L 600'~ and 900°c:

2SiF2 600°C - 900°C siF4 ⁺ Si

L I 5

(2)

rn ,., ZERO OFFSET ON

6 0 /

It is thus desirable to bring

5 0 - the gas to room temperature as rapidly

as possible, in order to maximize the

4 0 -

amount of SiF2 gas in the reaction

30- chamber. From our previous work we

expect that at the low pressure

20

-

( ~ 1 Torr) prevailing in the high tem-

perature reactor most of the SiF4 is

10 - converted to SiFZ. We know, however,

that a small portlon of Sir2 is re-

1403 1303 IMO 1100 1000 430 800 700 converted to SiF4 because at the

FREOUEWY ICI'I exit of furnace T1 a CVD film of Si

is deposited. We also know that a large proportion of the gas is SiF

2 Fig. 2 because this gas polymerizes at liquid

Nitrogen temperature lo

.

When we heat the cold trap located after the reaction chamber the polymer, which is meta-stable at room temperature, stays behind while the SiF4 evaporates. Pending the instal- lation of in-situ analytical facilities we do not know the relative proportion of SiF2/SiF4 in the gas fed to the reaction chamber.

Another valve located between the furnace and the reaction chamber allows the removal of freshly deposited samples without admitting air to the rest of the system.

The reaction chamber is a 45 mm ID Quartz tube terminated by O-ring sealed stainless steel flanges with feed-throughs for the gas inlet and outlet and for the glow discharge electrodes. Heating of the substrate is achieved by the same system described in ref. 6.

The glow discharge was achieved by establishing a DC high voltage potential between an anode grid and a cathode grid; the stainless steel grid dimensions were

over the whole grid area. Typical opera tin^ conditions are illustrated in Fig. 1.

The gas pressure in the reaction chamber was of the order of 0.3 Torr. The power density over the substrate was approximately 0.1 w/cm2. positions were made on fused silica, single crystal silicon and stainless steel substrates. Occasionally runs were made with the three types of substrates simultaneously present in the de- position chamber. Deposition rates were of the order of d / s e c . Samples on fused silica substrates are convenient for ESR measurements and for coplanar electrode electrical measurements. Samples made on silicon single crystal substrates and on stainless steel substrates were used for IR optical measurements and for Auger spectroscopy.

The samples we have produced so far are not uniform in thickness. Even though the plasma appeared uniform, the electrode shape was shadowed on the sample: this probably reflects the effect of the ionic current at each particular location.

The diffusion and fore pumps were used to evacuate thessystem and purge it be- fore the run was started. During the run, the SiF2 and SiF4 were trapped cryo- genically in order to protect the pumps. The option of recovering the used SiF4 after a run was available. However, we preferred to release the gas to the atmo- sphere through a bubbler (not shown) in order to avoid possible contamination in sub- sequent runs.

Determination.of Fluorine Content of the Sample.- We have used two main techniques to measure the fluorine content of the elow discharge films. On the one hand. we have used the nuclear reaction 19~(p,a)y60*, the de;ails of which are described in Ref. '5. Calibration of the system was done with single crystal silicon samples in- to which known quantities of fluorine had previously been implanted. A further calibration check was made using a CdF2 crystal as a stoichiometric fluorine com- pound. CdF2 was lightly Indium doped and heated in metal vapor, in order to in- crease its conductivity and thus avoid the build up of charge and consequent re- pulsion of the proton probing beam.

The other technique used to measure the fluorine content of our samples was Electron Spectroscopy for Chemical Analysis (ESCA). For this work a model 590 A-555 PHI system was used. On some samples, grown on conducting substrates, Auger Elec- tron Spectroscopy (AES) was also performed. The surface analysis measurements agree with the nuclear reaction measurements except for one sample on which the nuclear reaction yielded of the order of 50% F while the surface analysis showed only a couple of percent fluorine. This sample also flaked off the substrate. Possibly the F in this sample was in the form of occluded, volatile, SiF4 which could not be detected by surface probes.

Fluorine Content as a Function of Substrate Temperature.- Samples were grown on sub- strates held at 200°C to 400°c aid showed variations in F content from 0.5

- - - .- - - -

atomic percent to 15%,but no correlaticnbetween the two variables. Furthermore, measurements made at different locations, within and outside the electrode area, of a sample grown at 200°C show a variation of F content from 2.5% to 8 %. This suggests that other factors, such as the ion current in the immediate vicinity of the deposition area override the effect of substrate temperature.

Electrical and Optical Measurements as a Function of Fluorine Content.- All the elec- trical measurements were made using coplanar A1 or Au electrodes. The gaps were of the order of 200 um wide and the contacts were fully ohmic. The film thickness was of the order of 0.5 um on fused silica substrates. The measurements were made in an atmosphere of dry nitrogen. The value of conductivity fluctuated from sample to sample in the range of 10 (~.~m)-'. The electrical activation energy and optical band gap were obtained by standard methods

'.

The optical gap was found to be between 1.5-1.6 eV while the electrical activation energy was of the order of.45 eV.

Photoconductivity measurements were made at 5770 using a filtered mercury arc

C4-646 JOURNAL DE PHYSIQUE

lamp. The UXT product was calculated from the linear slope of the plot of photo- conductivity vs. light intensity, yielding a value of the order of lo-'' cm2/volt;

this low value testifyes to a large number of traps for the photo-excited carriers.

No variations, beyond statistical fluctuations, were observed in the electric photoconductive and gap properties as a function of F content which varied from 1% to 15%. Furthermore, the properties of our samples grown by glow discharge are the same as those of sample we previously made by CVIj 5 .

Electron Spin Resonance (ESR) Measurements.- The room temperature spin density of the samples was measured byESR as a function of F content. The samples were on fused silica substrates and had a film volume of the order of 1 0 - ~ c m ~ . It was found that the fluorine does not rgduce the ESR signal which is at the same level

(3 x 1019 spins/cm3) as found in our CVD samples. The large ESR signal seems to be a bulk rather than a surface effect, since an almost uniform value of spin density was obtained on various films with volume/surface ratios varying by a factor of five.

Infrared Spectrum.- Fig. 2 is the infrared reflectance spectrum taken with a Beckman Model IR 42 50 Spectrophotometer. While the instrument was used in the reflectance mode with an angle of incidence of 30° to the normal, the sample was deposited on a polished stainless steel surface. Thus, the measurement is, in effect, the result of a double pass absorption. The assignment of the lines to particular Si-F vibra- tions was done in accordance with the conclusion of ~~rawal'. An estimate ofnthe quantity of fluorine present based on the absorption strength of the pertinent lines and on the data given by Ley et als

.

is consistent with the measurements we made by nuclear reaction and by ESCA.

Conclusions.- We have demonstrated the possibility of producing a-Si:F by glow dis- charge in the absence of hydrogen, with gases containing only silicon and fluorine.

- The presence of fluorine in our samples was demonstrated by Nuclear Reaction, by ESCA and by IR spectroscopy. Quantitatively,all methods are consistent with one another.

-

Tn our glow discharge samples, the content of fluorine does not depend on substrate temperature, but seems to depend on the plasma conditions at the deposition site.

- The Fluorine, as incorporated in our samples, does not appear to compensate dangling bonds. This conclusion is based on the invariance of the ESR signal as a function of fluorine content and on a consistently low mobility life time product independent of the fluorine content. The inactivity of the fluorine in our samples is in contrast with the results reported by Matsumura et alZy9, on a-Si:F obtained by sputtering in an Argon atmosphere. We are not in a position to establish at this time whether the Ar or the sputtering process is instrumental in activating F in the sputtered films or whether further research in our laboratory will allow us to find glow discharge conditions under which F will be active in compensating dang-

ling bonds and reducing the states in the gap.

Acknowledgements.- The help of R. Kalish with the Nuclear Reaction Measurements, of R. Brener for ESCA measurements, of Amos Kessel and Ruth Shaviv for technical assis- tance are gratefully acknowledged. This research was supported in part by a grant from the United States

-

Israel Binational Science Foundation (BSF), Jerusalem, Israel, and by the Israel National Council for Research and Development.

References.

1. A. MADAN and S.R. OVSHINSKY, J. Non Cryst. Solids 35/36 (1980) 171.

2. G.H. MATSUMURA, Y. NAKAGOME, and S. EURUKAWA, Appl. Phys. Lett.

36

(1980) 439.

3. R.C. FISH and D.C. LICCIARDELLO, Phys. Rev. Lett.

41

(1978) 889.

4. B.K. AGRAWAL, Phys. Rev. Lett.

5

(1981) 774.

5. M. JANAI et al., J. Appl. Phys.

52

(1981) 3622.

6. M. JANAI, S. AFTERGOOD, R.B. WEIL, and B. PRATT, J. Electrochem. Soc., in press.

7. T. SHIMADA, Y. KATAYAMA, and S. HORIGONE, Jpn. J. Appl. Phys.

19

(1980) L 265.

8. L.L. LFI, H.R. SHANKS, C.K. FANG, K. J. GRUNTS, and M. CARDONA, Phys. Rev.

(1980) 6140.

9. H. MATSUMIIRA, Y. NAKAGOME, and S. FURUKAWA, J. Appl. Phys. 52 (1981) 291.

10. W.M. INGLE et al.,Reports, ERDA JPL Contract No. 954442 1976-78.