Solar Enersy Materials 17 (1988) 237-245 237 l~,Iorth-Holland, Amsterdam

A STUDY OF THE OPTOELECrRONIC A ~ STRUCTURAL PROPERTIES OF GLOW-DISCHARGE.DEPOSrrF~ FLUORINATED, HYDROGENATED AMORPHOUS SILICON THIN FILMS

Gautam GANGULY, S.C. DE, Swati RAY and A.K. BARUA Ener~ Research Unit, Indian Association for the Cultivation of $cienc~ Jadavpur, Calcutta 700 032, India

Received 16 February 1988

The optoelectronic and structural properties of a-Si:F:H film.~ prepared by the RF glow discharge decomposition of mixtures of silicon tetrafluoride and hydrogen have been studied as a function of the deposition parame,~ers, viz. the hydrogen concentratiun in the gas mixture, the RF power density and the substrate temperature. It has been found that the dep~ition parameters can be optimised to prepare photosensitive fluorinatvA material having a band gap of - 1.65 eV. The ohotoinduced changes in the properties are quite small, Under suitable deposition conditions hig~y conducting (o D mI0 -3 S ~n -1) films can also be produced.

1. Introduction

Hydrogenated amorphous silicon thin films have attracted widespread attention due to their poteuti~d as an inexpensive electronic mate~al. _The application of this material r~nge~ ~r~?m large area photovoltaic devices to thin fiLm transistors, imaging and light sensing devices, charge coupled devices and switching devices. This wide range demands a diversity of properties which has prompted the investigation of different amorphous silicon alloys.

One of the first alloys developed was fluorinated, hydrogenated amorphous silicon [1]. A detailed study of this material a -S i :F :H, was reported by the ECD group [2]. They used the glow discharge technique which is generally held to be a commercially viable process and yields high quality thin films. Other groups [3,4] have used modified gas composition to obtain higher deposition rates for the films. A systematic correlation of the film properties with the deposition parameters has, however, been lacking [5]. Here we report the results of a systematic study of the properties, both optoelectronic and structural, of a-Si: F : H films prepared by glow discharge decomposition of silicon tetrafluoride-hydrogen mixtures as a function of the deposition parameters.

2. Experimental details

The samples were prepared in a capacitatively coupled, radio frequency (RF), glow discharge plasma system described previously [6]. A mixture of silicon tetraflu-

0165-1633/88/$03.50 c~ Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

238 G. Ganguly et al. / Optoelectronic and structural properties of a-$i : F: H

oride and hydrogen gases was used with the proportion of the latter varying from 2 to 70~. The RF power density was kept at 150, 500 or 800 mW cm -2. The temperature of the substrates during deposition was kept fixed, in the range 150-300°C. The substrates used were glass (Coming 7059) for optoelectronic measurements, single crystal silicon wafers for infrared transmission measurements and thin stainless steel sheets for electron spin resonance measurements.

All samples were annealed at 150°C in vacuum (< 10 -s Torr) prior to opto- electronic characterization. The dark conductivity (oD), its activation energy (Eo) and the photoconducfivity (Oph) under white light of intensity - 20 mW cm-2 from a tungsten lamp were measured. The band gap (Eg) as well as the refractive index (n) were determined from the optical transmission data. The absorption coefficient in the range 1 to 2.5 eV was calculated by combining the spectral photoresponse data obtained in the constant photocurrent mode (CPM) with optical transmission data. The absorption coefficient in the infrared region of the spectrum was calcu- lated from the transmission data obtained on a double beam spectrophotometer. Illumination of the samples was carded out using a tungsten halogen lamp gener- ating an intensity of - 1 0 0 mW cm -2 at the sample surface. Electron spin resonance absorption spectra were obtained at liquid nitrogen temperatures using the peeled-off flakes collected in a quartz tube.

3. Results and discussion

3.1. Hydrogen dilution

The variation of the darkconductivity, photoconductivity and deposition rate of the films deposited at 250°C, at a power density of 500 mW cm -2 with the hydrogen concentration in the gas mixture is shown in fig. 1. As the hydrogen concentration increases from ~ 2% to - 6~, o D and Oph decrease from - 10 -8 to 10 -l° S cm -I and from 10 -5 to 10 -6 S cm -1, respectively, while the deposition rate increases gradually. With further increase in hydrogen concentration the deposition rate peaks sharply and the values of o D and Oph tend to increase. At a hydrogen concentration greater than 30~ the deposition rate falls below 10 A rain -1, while both o D and ap~ increase rapidly. The corresponding variation of the refractive index, the band gap (plotted as Eg/'2) and the activation energy with hydrogen concentration is shown in fig. 2. The variations of Eg and n are complementary. The band gap tends to a maximum of - 1.65 eV in the range 5~-15~ hydrogen concentration. In this region the activation energy is almost half the band gap, i.e. the films are intrinsic. As we have seen in fig. 1 the photogain and deposition rates of these samples are the largest.

The infrared absorption spectra in the range 500-1100 cxri -1 of the films pt'epared with 2~, 10~ and 50~ hydrogen concentration are shown in fig. 3. The absorption b~,nd for the film prepared with 2~ hydrogen concentration is limited to the 800-1100 cm -1 range. This band may have a significant contribution from the stretching mode of the SiF4 configuration at 1010 cm -1 in addition to the symmetric

G. Ganguly et al. / Optoelectronic and structural properties of a-Si : F: H 239

lo "4 20

E

10-o "~ 0

7 s 15 N - 1~? ¢

~.~ - 2 .~_

a

10 "~ 10

I I I I 4 10 2O 4O 100 HydroQen concentration (%)

Fig. 1. Darkconductivity (e), photoconductivity (o) and deposition rate (D) of a-Si: F: H fi!ms deposited at 250 o C at a power density of 500 mW cm-2 versus the hydrogen concentration in the gas mixture.

and asymmetric modes of the SiF2 configuration at 980 and 920 cm-1 [7], There are no observable hydrogen associated modes. The film prepared with 105 hydrogen concentration has a broad band in the 800-1100 cm -1 region which is probably due to the SiF 2 configuration. There is also a 650 cm- ~ band generally associated with the Si-H wagging vibration. The hydrogen content for this film has been calculated from the stretching mode to be - 5 at$. The absorption bands for the film prepared with 505 hydrogen concentration are sharp. A doublet appears, resolved

o.g

gp LU

¢ ILl

0.8

0.7

&5

3.0

2.5

I I I I I I I 2 4 ~ 10 20 40 80100

Hydrogen Concentration

Fig. 2. Activation energy (e), band gap plotted as Es/2 (o) a~d the refractive index (O) of the a-Si: H: F f i~s in fig. 1 versus the hydrogen concentra~on it, the gas mixture.

240 G. Gan~s'u~ et al. / Optoelectronic and structural properties of a-$i : F: H

°oo I 150C

'E ¢J

!00C

/ I

llll\ x

;lll , ' I

/ l l l l ", .,,"J ',

• / ,~"~' '--......., ,

II00 900 700 500 Wave number (cn~ t)

Fig. 3. Infrared absorption bands of a-Si: F: H films deposited at 250 o C at a power density of 500 mW cm-2 with 2.5% ( . . . . . ), 10% (-- -- --) and 50% ( ) hydrogen concentration in the gas mixture.

at 920 and 980 cm-1, and may be attribute0, t o the two types of stretching vibration of the SiF 2 configuration. Another, much wezker, band is resolved at - 800 cm- ~ and could arise from the SiF configuration. 'l-he peak at 650 cm- 1 is very strong and narrow. This suggests a reduction of disorder in these films.

The variations in the properties of the films prepared with different hydrogen concentrations reflect changes in the growth mechanism. The process of deposition is dominated by reactions between hydrogen and fluorine atoms. At low hydrogen concentration in the plasma, the breaking of SiF4 molecules takes place mainly by direct electronic impact. There is little possibility of etching and the Si-network is likely to be poor as indicated by the large value of o D and the significant proportion of trapped SiF 4 molecules. An increase in hydrogen concentration enhances the reactive dissociation of SiF~ radicals and the deposition rate increases. Hydrogen atoms become available at the growth surface so that dangling bond saturation and etching of weak bonds can take place. Thus, the optoelectronic properties improve. The band gap increases slightly due to the larger hydrogen and fluorine incorpora- tion. At hydrogen concentrations greater than 10% the excess hydrogen serves to dilute the plasma ~nd the etching reactions are enhanced. This favours the forma- tion of microcrystalline films as indicated by the increasing values of o o and eph as well as the sharp infrared absorption peaks observed in samples prepared with more than 30% hydrogen concentration.

G. Gangu~ et a L / Optoelectronic and structural properties of a-$i : F: H

3. 2. Power density

24~

The variation of the darkconducfivity and photoconductivity of two .~e~s of films prepared at 250°C with power densities of 150 and 800 mW cm -2 has been plotted against the hydrogen concentration in fig. 4. The changes in oD and Oph a~'~ qualitatively similar to that shown in fig. 1 for films prepared at 502 mW c m -2"

power density. The minima in OD are seen to shift to larger hydrogen concentration as the power density increases. While the minimum is fairly broad at medium power density they are quite sharp at both low and high power density. As the power density is increased, the dissociation of SiF, through electron J~Fact increases. Therefore, the requirement of hydrogen for reactive dissociation increases. This explains the shift in the o D minima. The absorption edges of Lhe films with the minimum o v and largest photogain at the three different power densities are shown in fig. 5. It can be seen that the sharpest exponential edge, as measured by the parameter E 0 [8] occurs at" medium power density (E0 = 69 meV). The disorder appears to be greater both at low (E o = 113 meV) and high (E o = 129 meV) power density. The lower portion of the absorption tail attributed to gap states [8] is also a minimum at intermediate power density which appears to be optimum for deposi- tion of a-Si: F: H films.

ld 3

to-S

ld

1(~ I°

10 -II I I I I I I I

2 4 8 10 20 4 0 8O 100

Hydrogen Concentration (,Olo)

Fig. 4. Darkconductivity and photoconductivity resp,~ctively of a -S i :F :H films deposited at 250 °C at power densities of 1S0 mW cm -2 (o, o ) and 800 mW ¢m -2 '(11, n) versus the hydrogen concentration in

the gas mixture.

'2d2 G, Ganguly et al, / Opwdectmnic and ~trucmral propertier of a.$i : F: t1

10 s

~0 4

~E I0 ~ "U

10 2

/ J

/

d

/ ~ . E , ( m e V )

1. 129 2. 69 3. 113

101 t t t I I I I I ] 1.2 1.4 t.6 1.B 2.0 2.2 2.4 2.6 2.~

Ecev) Fig. 5. Absorption coefficient of a -S i :F :H films prepared at 250°C with power density of 150 mW cm -2 (1), 500 mW cm -2 (2), 800 mW em -2 (3) and hydrogen concentration of 8~ (1), 10% (2), 30% (3)

respectively versus the photo~ energy.

~00

I

2OO

I E

• \ . \

/' ' I X ~ \ \ . / , / I I I "-.~ t900 2000 2100 2ZO(

Wavenumber (cm -1) Fig. 6. ][nfnu~,d absorption bands for a-Sh F: H films prepared at 250 o C with (1) 150 mW cm -2 power density, 8~ hydrogen concentration; (2) 500 mW cm -2, 10~; (3) 800 mW cm -2, 30~ and (4) 150 mW

cm -2, 20~.

O. C, anguly et aL / Optodectronic and structural proper~ies of a-$i : F: H 243

The infrared absorption bands in the 1900-2200 cm-z range of the spectrum for the above three films ~,nd a film prepared with 20~ hydrogen concentration at 150 mW era-2 power density are shown in fig. 6. We fred that the absorption band associated with the Si-H stretching mode is centred at 2000 cm -z and that for the films prepared under maximum photogain/minimum o D conditions at 150 and 500 mW cm-2 power density the absorption bands are of approximately equal intensity. At higher power density the band is considerably broadened and c~)ntains 2000 and 2100 cm-Z components which overlap. Thus, high power densities possibly result in a SiH2/(SiH2)~v configuration which is associated with inferior electronic proper- ties. At low power density, with high hydrogen concentration ( - 20~) in the gas mixture, the film again shows a doublet which is smaller, sharper and resolved. At this hydrogen concentration, with low power density, the film has high OD and Oph values. Transmission electron microscope studies have shown that the film has a microcrystalline structure. A detailed study of these films will be reported sep- arately.

3.3. Substrate temperature

The dark conductivity, photoconductivity and band gap of the films deposited with 10% hydrogen concentration and 5._00 mW cm -2 power density have been plotted versus the substrate temperature in fig. 7. The value of OVh varies little in the temperature range 150-250 °C but tends to decrease at higher temperatures. The band gap is steady in the temperature range 200-250°C and decreases at higher

J~ 16'

b 1o-,S

" , ~ - - o o

1.7 ..-%

>

1.6 tU

1.5

10-9 " ~ 10-1o _ ,

I I , I 156 200 250 300 Ts ( "C)

Fig. 7. Darkconductivity (@), photoconductivity (o) and bandgap (D) of a-Si: F: H films prepared w=~th a power density of 500 mW cm- 2 and hydrogen concentration of 10% versus the substrate temperat~e.

244 G. Ganguly et al. / Optoelectronic and strucmml properties of a-$i : F: H

2 0 0 0

o .m%

I E

o 1OOO

~ o o 2000 21oo 22oo Wave number (cn~ 1)

Fig. 8. Infrared absorption bands of a-Si :F:H films prepared with 500 mW era-2 power density and 10~$ hydrogen concentration at a substrate temperature of (1) 200 o C aad (2) 150 o C.

temperatures. The decrease of Oph and E s above 250 o C could be due to insufficient hydrogen incorporation. A~ the substrate temperature increases from 150 to 200 o C, E s and o v decrease rapidly. The decrease of the gap may be identified with an decrease in the hydrogen content as can be seen from the infrared spectra of the films deposited at 150°C and 200°C (fig. 8). The band is centred at 2100 cm -1 when the film is prepared at 150°C. This indicates the formation of S i H 2 / ( S i H 2 ) ~

configurations at low substrate temperature. The higher defect density may be the cause of the increase of o D as the substrate temperature decreases below 200 o C. At 200 o C, OD has a mimimum and at higher substrate temperatures it increases again. The increase of o n in the range 200-300°C could be associated with increasing midgap defect density. The CPM spectra show that the subband gap absorption is smaller at the lower substrate temperatures.

3.4. Photoinduced changes

The change in o D and Oph upon illumination could be observed only in those film~ which are photosensitive and have a low value of oD. These films probably possess a low density of states at midgap so that light-induced states affect the properties. The value of o n decreases by a factor of ~ 2 upon illumination in all cases. This is small compared with the one-order change observed in hydrogenated amorphous silicon films, It has been reported [4,9] that the spin density in a-Si: F: H film.~ is - 1017 cm-3. We have measured the spin density in one of our best samples and found a value of - 4 × 1017 cm- 3. The higher density of dangling bonds in this material may be the reason for the smaller light-indu~A changes.

G. Gansmly et al. / Optoelectronic and structural properties of a.Si : F: H 24 . ~

The change in o D upon illumination has been found to depend on the position of the Fermi level. The changes are quite small in 'all cases. The detailed results have been reported elsewhere [10].

4. Conclusions

The properties of a-Si :F:H films are very sensitive to the concentration of hydrogen in the gas mixture. We attribute this to the delicate balance in the concentration of hydrogen atoms with the SiF, radicals generated by electron impact. In a particular range of concentration the films are photosensitive, *,he gain is - 10 s, with a reasonably large photoconductivity - 10 -s S cm -1 and sharp band tail with E0 -- 69 meV. These films have a hydrogen content of - 5 at% bonded in the monohydfide configuration~ The fluorine is bonded mainly in the dihydride configuration. The power density needs to be optimised; both low and high power density is found to increase the disorder in the films. The substrate temperature range 200-250°C is most suitable. The decrease of the fight-induced changes as compared to a-Si: H is important for device stability.

Acknowledgement

Tiffs work has been carried out under a project funded by the Department of Non-Conventional Energy Sources, Government of India.

References

[1] S.R. Ovshinsky and A. Madan, Nature 276 (1978) 482. [2] A. Madan, S.R. Ovshk~sky and E. Bonn, Phil. Mag. B 40 (1979) 259. [3] H. Matsumura and S. Fundmwa J. Non-Cryst. solids 59/60 (1963) 739. [4] M. Janai, R. Weft and B. Pratt, J. Non-Cryst. Solids 59/60 (1983) 743. [5] R. Weil, M. Janai and B. Pratt, J. Phys. (Paris) 42 (1981) 643. [6] G. Ganguly, S. Ray arid A.K. Bama, Phil. Mag. B 54 (1986) 301. [7] P.K. Banerjec, J.M.T. Pcreira, S.S. Mitra and R. Dutta, J. Non-Cryst. Solids 87 (1986) 1. [8] O.D. Cody, in: Semiconductors and Semimctals, Vol. 21, Part B: Hydrogenated Amorphous Silicon,

Ed. J.I. Pankove (Academic Press, New York, 1984) ch. 2. [9] S. Ueda, M. Kumeda and T. Shimizu, J. Phys. (Pans) 42 0981) 729.

[10] S.C. De, G. Ganguly, S. Ray and A.K. Barua, in: Proc. 7Lh Intern. Conf. on Thin films, New Delhi (1988), Thin Solid Films, to be published.