Deposition and characterisation of a-Si: H films prepared by reactive ion beam sputtering

Journal of Non-Crystalline Solids 101 (1988) 111-116 North-Holland, Amsterdam

111

DEPOSITION AND CHARACTERISATION OF a - S i : H FILMS PREPARED BY REACTIVE ION BEAM S P U I T E R I N G Mohan Krishan BHAN, Subhash C. K A S H Y A P and L.K. M A L H O T R A Thin Film Laboratory, Department of Physics and Centre for Materials Science and Technology, Indian Institute of Technoh~gy, New Delhi-110016, India

Received 9 July 1987 Revised manuscript received 28 October 1987 Physical properties of hydrogenated amorphous silicon (a-Si:H) films prepared by reactive ion beam sputtering under varying H2:Ar flow ratios and various substrate temperatures are reported. Ellipsometricstudies reveal that both the increase in H 2 :Ar flow ratio and decrease in substrate temperature lead to density-deficit films. The films are poorly photoconducting due to the presence of defect centres created possibly by Sill2, (Sill2) ~ and SiO species in the midgap.

1. Introduction The incorporation of hydrogen in the amorphous silicon (a-Si) network is now an accepted means for reducing the density of defect states in the midgap [1-4] and making it a useful optoelectronic material [5-7]. Though the glow discharge decomposition of silane is the most widely experimented and, at present, the only technique used in large scale production of hydrogenated amorphous silicon ( a - S i : H ) films, a number of techniques [8-10] have been used on a laboratory scale for the deposition of these films. Reactive ion beam sputtering (RIBS) is one such technique which has been reported by a few workers [11,12] for the preparation of a-Si : H films. In an earlier paper [11] from our laboratory, some results on the preparation and characterisation of a - S i : H films using RIBS technique were reported. The present work is a continuation of that study to understand the RIBS prepared aSi : H films, with emphasis on the effect of hydrogen on the growth and microstructure of the films.

2. Experimental The experimental details for the preparation of a-Si : H films by RIBS have been reported earlier from our laboratory [11]. 0022-3093/88/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

For the present work, amorphous silicon films were grown on various substrates including C o m ing 7059, quartz and polished Si wafers, under the following two different conditions: (1) Substrate temperature (Ts) fixed at 3 0 0 ° C and H 2 : A r flow ratios varying from 0 : 1 to 16:1. (2) H 2 : Ar flow ratio fixed at 10 : 1 and substrate temperatures (Ts) varying from 30 to 300 ° C. The b e a m - v o l t a g e and -current were set at 1500 V and 20 mA, respectively, for a pure argon flow of 0.2 sccm. The addition of H 2 to Ar resuited in lowering of the beam current. Under an extreme flow ratio of 16:1 ( H 2 : A r ) , the b e a m current reduced to 10 mA, thereby reducing the deposition rate from 2.5 A s -1 (in the case of pure Ar) to 0.5 A s - l . The film thickness was measured with a T a y l o r - H o b s o n Talyserf with an accuracy of +4%. The temperature dependence of electrical conductivity, OtD(~-~-I cm-1), was measured with a gap cell structure, using A1 as electrodes. The hydrogen content, C n (at.%), in the films was estimated from the infrared (IR) spectra recorded on a Perkin-Elmer double-beam I R spectrometer (Model 683), in the 200-4000 cm -1 range. The optical transmission ( T ) and reflectivity ( R ) were measured with a Hitachi double-beam spectrometer (Model 330), in the 0.35-2.5 # m range. The refractive index (n), extinction coefficient ( k ) and

112

M.K. Bhan et al. / Deposition and characterisation of a-Si ."H films

absorption coefficient (a) were calculated using the R and T data thus obtained. In order to estimate the imaginary part of the dielectric constant (%) of these films, spectroscopic ellipsometry measurements were performed using an automatic rotating polarizer ellipsometer (SOPRA, ES2G) in the energy range 2.0-5.0 eV. The a-Si : H films deposited under both conditions were confirmed to be amorphous by X-ray diffraction.

S,

H2. Ar I) O:1 if) 4 : 1 II;) S : 1 iv ) 10:1 v) ~ : I vl) u,: 1 vii) 16:1

n(0.6eV)

|)

1

_

/ J

T S • 300'C

3.9e 3 79

3.70 3.6t. 3.~, 3.~.,; 3.32

,,¢

| /

~ ~

J

/

")

A iii} 14) /o

vi)

u < IZ

3. Results and discussion

3.1. Effect of varying H 2 : Ar flow ratio In the case of films grown under condition (1), the deposition rate was found to depend strongly on H 2 : Ar flow ratios (table 1). By increasing the H 2 : Ar flow ratio from 0 : 1 to 16 : 1, the deposition rate showed a decrease from 2.5 to 0.5 A, s-1. The lowering in the growth rate is understandable since the addition of hydrogen to argon reduces the ionization efficiency of Ar gas, thereby reducing both the number of Ar ÷ ions in the discharge and the sputtering rate. Figure 1 shows the variation of refractive index (n) with photon energy (hp) for the films prepared under condition (1). Since n of a-Si : H film varies quite slowly with hu, the variation of n for the films has been compared only at a single value of photon energy, 0.6 eV. The refractive index shows a decrease from 3.98 for the unhydrogenated (i.e. pure a-Si) films to 3.32 for the hydrogenated (a-Si:H) films prepared at a 16:1

PHOTON

ENERGY(eV)

Fig. 1. Variation of refractive index ( n ) with photon energy for the films deposited under condition (1).

flow ratio, suggesting a decrease in the film density with increasing H 2 : Ar flow ratios. The optical band gap (E~) of a-Si : H films was obtained by extrapolating the plot of (ahv) 1/2 vs hr. With the increase of H 2 :Ar flow ratio from 0 : 1 to 16 : 1, E~ shows an increase from 1.32 (for pure a-Si) to 1.95 eV (for a - S i : H films) as summarized in table 1. The result is qualitatively similar to that obtained for glow discharge deposited a - S i : H films [13]; in this case too the increase in hydrogen content of the films has led to an increase in Eg. The increase in Eg with the enhancement of H 2 : Ar flow ratio can be attributed to the decrease of density of states in the midgap due to

Table 1 Optical, structural and electrical parameters of a-Si : H films prepared at fixed T~ ( = 300 ° C) and varying H z : Ar H 2 : Ar flow

Deposition rate

ratio

(,~ s - 1)

0 :1 4 :1 8 :1 10 : 1 12:1 14 : 1 16 : 1

2.50 1.66 1,32 1.11 0.92 0.60 0.50

n (0.6 e g )

Eg (eV)

( 2max/h P (eV)

Ci4

(at.%)

O"D(RT) ( I2 - a cm - 1)

AE (eV)

Pre-

exponent factor (D -1 cm -1)

3.98 3.79 3.70 3.64 3.54 3.44 3.32

1.32 1.70 1.83 1.85 1.87 1.90 1.95

23.0/3.40 22.0/3.65 20.0/3.65 19.5/3.65 17.5/3.65 17.2/3.65 16.0/3.65

0 11.2 16.2 17.4 20.1 22.8 30.3

7.2 X 10- 6 3.6 x 10- 7 2.5 x 10- 8 2.9 x 10- 9 7.9X10 -9 4.6 × 10-7 5.4 x 10 -6

0.56 0.60 0.63 0.68 0.64 0.56 0.54

90 750 480 1100 500 210 320

M.K. Bhan et a L / Deposition and characterisation of a-Si : H films

i)

Ts = 300°C

ii I

2C

lO

5

I

2.5

I

I

I

~,

3 3.5 /-, L.5 PHOTON ENERGY(eV)

Fig. 2. Variation of i~2 spectra with photon energy for samples deposited at Ts = 300 °C but different H2:Ar flow ratios: (i) 0 : 1, (ii) 4:1, (iii) 8 : 1, (iv) 10:1, (v) 12 : 1, (vi) 14: 1, (vii) 16 : 1.

increased incorporation of hydrogen atoms in the films. Spectroscopic ellipsometry was performed in the energy range 2.0-5.0 eV to calculate the imaginary part (c 2 = 2 n k ) of the complex dielectric function (~') - a parameter which determines the quality of a film in terms of its (c2) peak position and the maximum peak value, (E2max [14,15]. Figure 2 shows the variation of E2 with photon energy for all the samples deposited under condition (1). It is observed that with the increase of H 2 : Ar flow ratio from 4 : 1 to 16 : 1, the IE2maxdecreases from 22 to 16 but the peak position of c 2 remains fixed at 3.65 eV. The increase in the density of voids is responsible for the decrease in ~2max value. The effect is consistent with the one established for glow discharge [15,16] and seen in ion-beamsputtered a - S i : H films [14,17,18]. The highest value of IE2max= 22 obtained in the present study, however, is smaller than the corresponding value of 26 for the films prepared by glow discharge technique. This implies that the films obtained by ion b e a m sputtering technique are m o r e density-deficit than the films grown by glow discharge technique. The I R spectra of the films prepared under condition (1) showed the three stretching, bending and wagging modes peaking at 2060-2100, 840 and 640 cm - l , respectively [19]. These films also

113

showed the S i - O absorption band near 1000 c m - t. Figure 3 shows the absorption coefficient (a), as calculated [20] from the I R data for stretching mode (1860-2300 cm-1), as a function of wavenumber for these films. The integrated intensity under the stretching mode has been used to estimate the hydrogen content (CH) of these films. It is evident from the figure that the hydrogen content in the material increases with the increase of H 2 : Ar flow ratios. This is because at higher flow ratios there is an increase in hydrogen concentration in the chamber which in turn leads to incorporation of more hydrogen in the films. The peak position of a vs. wave number curve for the films deposited at H 2 : A r flow ratio of 4 : 1 shows the stretching vibration peaking at 2060 cm 1. This suggests the presence of Sill (monohydride) and Sill 2 (dihydride) type of bonding. The same film gives the C2rnax close to 22, with C H equal to 11.2 at.% (fig. 4). The peak position shifts towards higher wave numbers when the H 2 : Ar flow ratio is increased and the peak generally becomes quite broad. It is likely that the films deposited at high H z : A r flow ratios (typically 16:1) may contain Sill, Sill 2 and (Sill2) . bonding configurations. The ~2max decreases and in the films deposited at H 2 : Ar flow ratio of 16 : 1, is only 16. The C H as expected increases to 30.3 at.% in these films.

1500 t i 7E

~2 :~'r '~) 8 : 1 ii;) 10 1 v;l~

TS = 300"c

~

L. LU U

I

19{30

I

2000

2 200

2100 WAVE

NUMBER(cm

2300

"1 )

Fig. 3. Variation of absorption coefficient (a) with wavenumber for the samples deposited at Ts = 300 °C with varying H2:Ar flow ratios.

114

M.K. Bhan et aL / Deposition and characterisation of a-Si : H films H2:Ar flow ratio w'lr¥1ng

~,

2S' -

e--

o

•

•

•

:~

~2c

3

HYDROGEN CONCENTRATION(ot.%)

temperature of 300 ° C give the best and a clean band gap material having an activation energy, AE, of 0.68 eV, and room temperature dark conductivity, o D (RT), of 10 - 9 ~2 - 1 c m - 1 . The sampies, however, showed only one order of change on exposure to a white light source (1012 photons cm -2 s - ] ) in their conductivity. The poor photoconductivity may primarily be due to the defect centres in the midgap, created by Sill 2 and possibly (SiHz) . and SiO, which act as recombination centres for the charge carriers.

Fi8. 4. Variation of (Zmax and n with hydrogen content (CH) for the films deposited at Ts = 300 ° C and at different H 2 : A r flow ratios.

In order to have an idea of the electronic quality of the films prepared under condition (1), the temperature dependence of electrical conductivity has been investigated in detail. Figure 5 shows the temperature variation of dark conductivity (OD) in the range 30-250 ° C for these films. It is evident from the figure that the films deposited at a 10:1 flow ratio and at a substrate

,,'[

H2 :Ar

?S = 300"C

IJt) 8 +v) 10: I v)T2:l vi) lt, : I vii) 16; 1

~ 16'

i

)

3.2. Effect of varying substrate temperature (Ts) To understand the influence of substrate temperature on the film properties, the a - S i : H films were grown at different Ts ranging from 3 0 - 3 0 0 ° C and at a fixed H z : A r flow ratio of 10:1 (condition (2)). It was observed that the deposition rate of a - S i : H films decreased from 1.65 to 1.11 ~, s -1 with the increase of Ts from 30 to 300 ° C (table 2). Figure 6 shows the variation of refractive index (n) with photon energy (hv) for different films prepared under these conditions. The value of n (at 0.6 eV) increases from 3.32 to 3.64 when Ts is increased from 30 to 300 o C. The effect is due to the decreased density of films deposited at lower temperatures, and is similar in nature to that of increasing the H 2 :Ar flow ratios in the films prepared at 300°C substrate temperature under condition (1). On increasing the substrate temperature from 30 to 300°C, the optical band gap (Eg) decreases from 1.95 to 1.85 eV. A detailed variation of Eg with varying Ts is shown in table 2.

+6~

u

Table 2 Optical and structural parameters of a-Si : H films prepared at fixed 10 : 1 flow ratio of H 2 : Ar and varying Ts Ts ( o C) 10l

'~a'

~

i

2'.o

]

i

2!++

)

'

3.0

Deposition rate

n (0.6 eV)

Es (eV)

(2max/hv (eV)

CH (at.%)

3.32 3.39 3.52 3.64

1.95 1.92 1.90 1.85

17.0/3.5 17.8/3.5 19.0/3.5 19.5/3.65

28.2 25.2 22.1 17.4

(X s -1) 3.5

10001T( ~'t}

Fig. 5. Temperature variation of dark conductivity for samples deposited at Ts = 300 o C with varying H z : Ar flow ratios.

30 100 200 300

1.65 1.51 1.32 1.11

115

M.K. Bhan et aL / Deposition and characterisation of a-Si : H films H2:Ar

flow rutio

=10:1

TSC'C) r,(0.6~V) i) 30 ii) 100 iii) ~00 iv) 300

3 -32

3.39 3.52 3.6/~

~7 iv) .

~ i i i )

~;ooo~ z

i

!

a

I

I

1900

I

2000 WAVE

1 I

_

1 1.5

I 2

PHOTON ENERGY(eV)

Fig. 6. Variation of refractive index with photon energy for the films deposited under condition (2).

The variation of ( 2 vs. hu for the films deposited under condition (2) is shown in fig. 7. On increasing the substrate temperature from 30 to 3 0 0 ° C , the (2ma~ increased from 17 to 19.5 and the peak position showed a shift from 3.5 to 3.65 eV. Thus at lower Ts, higher void fraction exists, i.e. a-Si : H films are more density-deficit. Like the films prepared under condition (1), the films prepared at a fixed 10 : 1 flow ratio of H 2 : Ar and at varying substrate temperatures (30-300 ° C)

H2:Ar

flow

rQtlo:tO:l

I

2100

2200

NUMBERCc~

2300

I)

Fig. 8. Variation of absorption coefficient (a) with wave number for the films deposited at H2:Ar flow ratio of 10:1 and at different substrate temperatures.

also showed the three stretching (2100-2080 c m - l ) , bending (840 cm -1) and wagging (640 cm -1) modes and S i - O (1000 cm -~) absorption band. The variation of absorption coefficient, a, with wavenumber for the films prepared under condition (2) is shown in fig. 8. On changing Ts from 30 to 300 o C, the hydrogen content decreases from 28.2 to 17.4 at.%. This suggests the likely dissociation of S i - H bonds at higher substrate temperatures. This could also account for the decrease of deposition rate and optical band gap at higher substrate temperatures. The peak position of the a vs wavenumber curve shifts from 2100 to 2080 c m - l , when the temperature increases from

! iv)

20

TS

,¢ o r y i n g

15 10

x

o

E17.E

<

Ii

2.5

I

1

1

I

3

3.5

4

4,5

PH010N ENERGY(eV ) Fig. 7. Variation of (z spectra with photon energy for samples deposited at H2:Ar flow ratio of 10:1 and at different substrate temperatures: (i) 30°C, (ii) 100°C, (iii) 200°C, (iv)

300° C.

~] - -

L

~

25

__

,

3i0

13

HYDROGEN CONCENTRATION(or.%)

Fig. 9. Variation of (2max and n with hydrogen content (CH) for the films deposited at H2:Ar flow ratio of 10:1 and at different substrate temperatures.

116

M.K. Bhan et al. / Deposition and characterisation of a-Si : H films

30 to 300 ° C, indicating the change from a mixture of Sill 2 and ( S i l l : ) , type of bonding to Sill z bonding in the films. The corresponding decrease in (2max value with decreasing substrate temperature, i.e. increasing CH, is shown in fig. 9.

4. Conclusions

Infrared and spectroscopic ellipsometric investigations of a-Si : H films prepared by RIBS technique reveal that both the increase in H 2 : Ar flow ratios and decrease in substrate temperatures, increase the hydrogen content, which effects the growth process considerably and makes the a-Si : H films density-deficit (by increasing the void fraction) and inhomogeneous as revealed by lowering of E2max"

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W.E. Spear and P.G. LeComber, Phil. Mag. 35 (1976) 935. H. Fritzsche, Solar Energy Mater. 3 (1980) 447. D.E. Carlson, J. Vac. Sci. Technol. 20 (1982) 290. W. Paul and D.A. Anderson, Solar Energy Mater. 5 (1981) 229. [5] D.E. Carlson and C.R. Wronsky, Appl. Phys. Lett. 28 (1976) 671. [6] Japan Annual Reviews in Electronics, Computers and Telecommunications, Vol. 6, Amorphous Semiconductor

Technologies and Devices, ed. Y. Hamakawa (OHM, North-Holland, 1983) ch. 5, 6. [7] Japan Annual Reviews in Electronics, Computers and Telecommunications, Amorphous Semiconductor Technologies and Devices, Ed. Y. Hamakawa (OHM, NorthHolland, 1982) ch. 4. [8] R.C. Chittick, J.H. Alexander and H.F. Sterling, J. Electrochem. Soc. 116 (1967) 77. [9] T.D. Moustakas, D.A. Anderson and W. Paul, Sol. St. Commun. 23 (1977) 155. [10] L.K. Malhotra, Proc. Int. Workshop Thin Film Technology and Applications, New Delhi, 1984, eds. K.L. Chopra and L.K. Malhotra (Tata McGraw-Hill, New Delhi, 1984) p. 306. [111 J. Singh, R.C. Budhani and K.L. Chopra, J. Appl. Phys. 56 (1984) 1097. [12] J. Saraie, M. Kobayashi, Y. Fujii and H. Matsunami, Thin Solid Films 80 (1981) 169. [13] K. Tanaka, K. Nakagawa, A. Matsuda, M. Matsumura, H. Yamamoto, S. Yamasaki, H. Okushi and S. lizima, Jpn. J. Appl. Phys. 20, Suppl. 20-1 (1981) 267. [14] R.W. Collins, W.J. Biter, A.H. Clark and H. Windischmann, Thin Solid Films 129 (1985) 127. [15] B. Drevillon, C. Senemaud, C. Cardinaud, M.D. Khodja and C. Codet, Phil. Mag. B54 (1986) 335. [16] D. Ewald, M. Milleville and G. Weiser, Phil. Mag. B40 (1979) 291. [17] R.W. Collins, H. Windischmann and J.M. Cavese, J. Appl. Phys. 58 (1985) 954. [18] R.W. Collins, B.G. Yocobi, K.M. Jones and Y.S. Tsuo, J. Vac. Sci. Technol. 4 (1986) 153. [19] M.H. Brodsky, Amorphous Semiconductors (Springer, Berlin, 1979) p. 35. [20] M.H. Brodsky, M. Cardona and J.J. Cuomo, Phys. Rev. B16 (1977) 3556.