Amorphous Silicon-Carbon-Fluorine Alloy Films

A new amorphous semiconductor alloy system Si„C„F~ has been prepared by rf sputtering of polycrystalline SiC in an Ar+SiF4 atmosphere. Dark conductivity and optical absorption of thin films are measured as functions of F concentration. Infrared spectra indicate a preferential attachment of fluorine to carbon over silicon. The bonded fluorine concentration is estimated to be as high as 40 at. %. The principal reststrahlen band shifts to higher frequencies and appears to sharpen with the increase of fluorine concentration. Fluorinated films are observed to be resistant to high-temperature annealing.


I. INTRODUCTION
Preparation and properties of amorphous silicon carbide films were first reported' in 1974 followed by a detailed investigation of electrical properties and the annealing behavior of sputtered films.The effect of hydrogen on these properties was also in-  vestigated.A better coordination of the binary random network on incorporation of hydrogen was observed.Recently, Auorine has attracted a great deal of attention as a terminator of dangling bonds.
A higher thermal stability of amorphous materials has been achieved by terminating unsatis- fied bonds with Auorine atoms.%e report the preparation and properties of a new amorphous semiconductor alloy system, Si"C"F&.Infrared spectra of stretching and bending fundamentals of Si -F and C -F bonds were recorded by absorption spectroscopy.Electrical conductivity and optical- absorption edge as functions of Auorine content were investigated.The dark conductivity decreases by a few orders of magnitude and the band gap in- creases with increasing fluorine content.The amount of fluorine in the alloy was determined from the integrated intensities of Si-F and C-F stretching-mode bands.Films with as high as 40 at. % of fluorine have been prepared.Infrared-  absorption spectra indicate a preferential attachment of fluorine to carbon over silicon.The effects of an- nealing on incorporated Auorine content and dark conductivity of this ternary alloy system were also studied.

II. EXPERIMENTAL
Amorphous Si-C-F alloy films were prepared in our laboratory by rf sputtering at a constant power of 200 %' on water-cooled substrates.An intrinsic poiycrystaliine silicon carbide target (99.99%SiC), 100 mm in diameter and 10-mm thick, was used in an oil-pumped vacuum system evacuated to a base vacuum of 0.66 mPa.The sputtering was done in a pure (99.999go)Ar + SiF4 atmosphere at a fixed ar- gon pressure of 1.33 Pa.Partial pressures (0 -0.08 Pa) of SiF4 were chosen as a deposition parameter.
The distance between the target and the substrate was 5 cm.The rate of sputtering varied between 0.07 and 0.11 nms '.Silicon to carbon ratio in amorphous Si-C-F alloys was determined by elect- ron probe microanaiysis (EPMA) measurments and was within 5% of the stoichiometric target material composition at all fluorine levels.
Infrared transmission and reflection were measured in the re- gion 200 -3500 cm ' with a Perkin-Elmer model 5808 (ratiometric) spectrophotometer.For infrared measurements the samples were deposited on pol- ished single-crystal silicon substrates, slightly wedged (-0.7') so as to avoid interference fringes.Qc1983 The American Physical Society The samples investigated were between 1.0and 1.5-pm thick.Samples appear smooth and shiny when viewed under a microscope.Samples were found to react mildly to atmospheric moisture when exposed over a period of 1 month.Scanning electron microscopy did not exhibit any surface structure down to 1000 A. Molybdenum electrodes were sput- tered on the films, the separation between the electrodes being 1.0 mm.The dark conductivity was measured in the temperature range of 100 -400 K in situ under high vacuum (-O.l mPa).A Chromel-Alumel thermocouple monitored the tern- perature of the film.The voltage source was made up of a 5.4-V mercury cell.No field dependence of the conductivity was observed in the range of the field employed.The absorption-edge measurements were done using a Spex double-beam spectropho- tometer.The uncertainty in the measurements of the absorption coefficient is estimated to be within +2.0% in the energy range availed.Annealing was carried out in a quartz tube in a variable tempera- ture furnace under inert argon atmosphere.spectroscopy.Figure 1 shows the infrared spectra of the amorphous Si-C-F alloys at different fluorine composition.

III. RESULTS AND
One significant feature is that the principal reststrahlen band of SiC at 790 cm ' is shifted continuously to higher frequencies and it ap- pears to sharpen with increasing fluorine content.
As the partial pressure of SiF4 in the sputtering at- mosphere is increased to 6%, the fluorine-induced reststrahlen frequency shifts to higher energy by 12% and the halfwidth of the band decreases by 38%.The band at 960 cm ' is attributed to Si-F   stretching in C2SiF2 and C3SiF configurations by comparison with the Si-F stretching-band frequen- cy (948 cm ') in C12SiF2 and C13SiF.The presence of two fluorine atoms on a common silicon atom and their relative motion is confirmed by the The parameter pF represents the partial pressure of SiF4 in the sputter- ing atmosphere.As calculated later, the at.%%u oof in- corporated fluorine is found to increase with in- creasing p"(see Fig. 3).At temperatures higher FIG. 4. Room-temperature absorption edge of amor- phous Si"C"F"alloys at different fluorine levels.deformation-mode frequency at 350 cm '.Like- wise the band at 1120 cm ' is the stretching-band frequency of C -F bonds in Si2CF2 and Si3CF con- figurations.
This agrees well with the C-F stretching-mode frequency' of 1110cm ' in IzCF2.A normal coordinate analysis yields a frequency" of 1100 for C-F stretching mode in +~C-F.The band at 430 cm ' is assigned to a F-C-F bending mode.
The assignments are summarized in Table I.The infrared spectra showed no detectable presence of oxygen or nitrogen in the films.Sputtered films of a-Si:H and other alloys are, however, known to con- tain -0.1 at.%%u oof oxygen .Thi samoun t of oxygen, to a certain extent, may influence the electrical properties of the films but in the face of -40 at.% of fluorine such effects are expected to be insignifi- cant.
than room temperature, the conductivity curves could be approximated by the general expression cr =croexp( E, -/kT).Owing to a slight departure from linearity of the curves even at high tempera- tures, a least-squares fit of the first six data points at high temperatures are used to calculate activation energy.The activation energy gap E, is found to in- crease with increasing fluorine concentration as shown in Fig. 2. The preexponential factor lies be- tween 4.5&10 and 7.0&(10 for all fluorinated samples.We consider E, and 00 as experimental parameters relating samples with different F con- tent, since their significance as real material parame- ters is uncertain.The difference between the mea- sured conductivity at low temperatures and the ex- trapolated straight-line fit of the high-temperature region could be attributed to the hopping between localized states.The continuous curvature of the data points might, however, also suggest that a hopping between localized states is playing an important role at all temperatures.The room temperature con- ductivity tends to saturate at higher fluorine levels.

B. Absorption edge
The transmission and reflection measurements were made near the principal absorption edge and the absorption coefficient was calculated within an error bar of +2.0%.The optical pseudogap E~i s obtained from the straight-line fit of (ahv)'~p lot- ted against hv (Fig. 4} assuming the relation ahv =80(hv -Eg ) .The straight-line fit yields Eg --1.35 eV for the unfluorinated sample and it in- creases to 1.80 eV for the sample with the highest fluorine concentration.
At lower photon energies, the absorption spectrum tails off, indicating the presence of localized gap states.The optical pseudo- gap widens on incorporating fluorine, possibly due to the removal of localized states from the gap.

C. Fluorine estimation
The absorption coefficient for the Si-C reststrahlen band is calculated from the infrared transmission data as shown in Fig.  absorption goes down by -52% at the highest fluorine concentration.It is worthwhile to note that the Si-C stretching frequency suffers a blue shift when compared to the long-wavelength TO-mode frequency (790 cm '} of crystalline SiC.The in- crease in the number of Si -F and C -F bonds in the sputtered films with the increase of the partial pres- sure of SiF4 in the sputtering atmosphere is evident from Figs. 3(b) and 3(c}.In order to obtain the total number of Si -F and C -F bonds in the alloys, we have calculated the absolute absorption intensity of a single bond from the absorption spectrum of the corresponding gaseous fluorides (SiF4 or CF4).The absolute absorption intensity A of a single Si -F or a C -F stretching bond in SiF4 or CF4, expressed in cm '/cm atm, is given by '  '2   5p 4c' ~Q t, where p is the vector dipole moment of the mole- cule, Q is the normal coordinate for the particular vibration, and c is the velocity of light.The value of the bond dipole derivate (5/L/5Q) for the stretching vibration in SiF4 and CF4 is assumed to be +167.4 and +219.8 esu, respectively.' However, since the local field at any point inside a solid is different from the applied field, the magnitude of A for a bond inside an amorphous solid will be lower corn- pared to the gaseous phase.A local field correction factor L is introduced in the equation '2 L represents the enhancement of the macroscopic field within a medium of homogeneous dielectric constant e. L is calculated in terms of the dielectric constant as given by ' ' The value of the dielectric constant is assumed to be 10.2 in the present work.'   The number of bonds N per unit volume of an al- loy is related to the integrated band intensity as N =, a(co)Cko,   were carried out in argon atmosphere for l h each.
The bonded fluorine concentration changes by -7% when annealed up to 800 K. Fluorine is observed to evolve mainly from silicon.The atomic percentage of fluorine bonded to carbon, in fact, increases at higher annealing temperatures.This may be due to the trapping of fluorine atoms evolved from silicon by carbon atoms.Results are summarized in Table III.We speculate that the appetite of silicon and carbon for fluorine atoms goes through a maxima at different bonded flourine concentrations and at dif- ferent pF.Hence the preferentiality factor at dif- ferent fluorine levels and at different annealing tem- peratures will be different.Even at 800 K the fluorinated amorphous silicon carbide holds on to -93%%uo of its bonded fluorine atoms and thus satis- fies most of its dangling bonds.The inference is also confirmed by the variation of dark conductivity at room temperature with annealing as shown in Fig. 6.No deterioration of the electrical conductivi- ty of the film due to evolution of fluorine is ob- served even at highest annealing temperature.
where co i is the band-center frequency.The integral is extended over the entire band and is calculated by evaluating the area under the absorption curve, the error in the evaluation being less than 5%.The den- sity of fluorine atoms in films is calculated on the assumption that fluorine atoms which are not bond- ed to silicon atoms are bonded to carbon atoms.
The results are summarized in Table II.Fluorine is found to attach preferentially to carbon over silicon.The quantity representing the preferential fluorine attachment to carbon is defined as (concentration of C -F bonds) X (Si content) (concentration of Si -F bonds) X(C content) and is found to have a value between 1.75 and 2.48, the latter being the value at the highest fluorine con- centration.Likewise in a-Sii "Ge":H alloys, Paul et al. ' have found that incorporated hydrogen at- tached preferentially to Si over Ge atoms.

D. Annealing effects
Figures 5(a) and 5(b) show the infrared-absorption bands of Si-F and C-F stretching modes, respective- ly, in a-SiC:F (pF =0.06) at three different annealing temperatures.The annealing at 400, 600, and 800 K IV.CONCLUSIONS An unexpectedly high total fluorine atomic con- centration, much more than needed to satisfy broken bonds, is found in the amorphous films.As the par- tial pressure of SiF4 in the sputtering atmosphere is changed from 1. 0 to 6.0%, the fluorine concentra- tion varied from 8 to 43 at.%%uo .It isworthwhil e to point out that SiC:H samples sputtered in 45% partial pressure of hydrogen have shown' to contain atomic density of hydrogen as high as 100% relative to (Si+ C).The density of amorphous alloys de- creased with increasing fluorine composition.The fluorinated samples did not show any deterioration up to the annealing temperature of 800 K.The bulk of fluorine that evolved on annealing was from sil- icon.

ACKNOWLEDGMENTS
Our thanks are due to Professor J. Tauc and Mr.
T. Kirst of Brown University for making available their infrared spectrophotometer.
We would also like to thank Mr. C. Nielson of Japan Electron- Optics Labs Peabody, MA, for performing the EPMA of the samples.
. 1. Infrared spectra of amorphous Si C"F"alloys at different fluorine compositions.pF represents partial pressure of SiF4 in the sputtering atmosphere. 5032 FIG. 2. Dark conductivity as a function of tempera-ture for samples with different fluorine contents.
FIG.3.Infrared-absorption coefficient of various vibrational modes at different fluorine concentrations: (a) Si-C stretching mode, (b) Si-F stretching mode, (c) C-F stretching mode.

Figure 2
Figure2shows the variation of the dark conduc- tivity with temperature for various samples with dif- ferent fluorine contents.The parameter pF FIG.5.Infrared-absorption coefficient at different annealing temperatures showing: (a) Si-F stretching mode, (b) C-F stretching mode.
FIG.6.Room-temperature dark conductivity of sam- ples with different fluorine contents at different annealing temperature.

TABLE I .
Assignments of the infrared-absorption bands observed in Si-C-F alloys at dif-

TABLE II .
3(a}.The peak Material Atomic concentration of different constituents in amorphous Si"C"F»alloys. AmorphousSi:C:F

TABLE III .
Effect of annealing on fluorine content of amorphous fluorinated silicon carbide prepared at 6% partial pressure of SiF4.