Low-hydrogen-concentration a-Si:H deposited by direct photo-CVD

Solar Energy Materials 23 (1991) 256-264 North-Holland

Solar Energy Materials

Low-hydrogen-concentration a-Si: H deposited by direct photo-CVD T. Shirafuji, M. Y o s h i m o t o , T. F u y u k i a n d H. M a t s u n a m i Department of Electrical Engineering, Kyoto UnirersiO', Yoshida-Hommachi, Sakyo-Ku, Kyoto 606, Japan a-Si:H films are dcposited by a direct VUV (147 nm) photo-CVD method. The films deposited at 200-400 °C have a low hydrogen concentration of less than 9 at% and hydrogen bonding configuration of mainly S i - H Thc film deposited at 3 0 0 ° C has a low defect-state density of 2.2× 10 I~ cm ~ and Urbach energy of 52 meV, which are estimated from below-gala absorption coefficients. The mobility gap detcrmined by an internal photoemission method is close to the optical gap, which suggests that the width of localized states at band edges is narrow.

1. Introduction

Hydrogenated amorphous silicon (a-Si: H) films have been widely used for solar cells, The films are mainly prepared by glow discharge (GD) plasma chemical vapor deposition (CVD) methods. This method has some problems such as damage to the films and impurity incorporation by ion-bombardment and sputtering of electrodes. These effects on the films are excluded by using photo-CVD methods. A low-pressure lamp (185 nm; 6.70 eV) [1] or an A,~F excimer laser (195 nm; 6.36 eV) [2] is used as a light source in photo-CVD. The energies of photons emitted from these conventional light sources are not enough to excite and deeomoose source gas Si2H 6 efficiently, since these photon energies are smaller than energies to excite to low-lying excited states of Si2H 6 [3]. Decomposition rate increases indeed up to 20-30 nm m i x - i as the photon flux or the source gas concentration increases even with these low-energy photons [4]. The decomposition path, however, does not change. Thus film properties are not expected to be changed as far as the photon energy does not change. We developed a novel direct photo-CVD method using vacuum ultraviolet (VUV) light (147 nm; 8.43 eV) emitted from a Xe resonance lamp. The optical cross section of Si2H 6 at 147 nm ,.'s larger than that at 185 or 195 nm, and the photon energy is enough to excite Si2H 6 to higher energy states and decompose it more efficiently. Films deposited by the photo-CVD method at substrate temperatures /~ub in the range 200-3013, °C showed a low dark-conductivity of ~ 10- ~ S cm-~ and an excellent photo- to dark-conductivity ratio of 105-106 under AM1 (100 mW cm-2~ illumination. With increasing T,ub from 200 to 300°C, the photo-conductivity degradation after lhe illumination for 3 h decreased from 50% to 10% [5]. 0165-1633/91/$03.50 ,,~ 1991 - Elsevier Science Publishers B.V. All rights reserved

7; Shirafuji et aL / a-Si : H deposited by direct photo-Cf/D

257

In this study, we investigate hydrogen concentration, hydrogen bonding configuration and the defect-state density of the photo-CVD films. Band edge features are also discussed with the aid of measurement for mobility gap.

2. Experimental Undoped a - S i : H films were deposited by direct decomposition of Si2H 6 using VUV light (147 nm) emitted from a microwave-excited Xe resonance lamp. A schematic diagram of the deposition chamber and the lamp was shown in elsewhere [5]. The flow rate of Si2H 6 was 2.5 cm 3 min -~. A window made of MgF 2 was used between the reaction chamber and the lamp. The window was blown with N 2 at a flow rate of 250 cm 2 min-] to prevent the deposition of films on it. The total pressure of the reaction chamber was 2.0 Torr. The s~,bstrate temperature was varied in the range of 200-400 o C. The typical deposition rate of the films was 0.6 nm min-]. The thickness of the films was about 300 nm. Bonded hydrogen concentration and hydrogen bonding configuration in the films were determined by Fourier transform infrared fiR) spectroscopy. The total hydrogen concentration in the films was also measured using a hydrogen effusion experiment [6]. Mid-gap states and Urbach tails were characterized from below-gap absorption coefficients measured using a constant photocurrent method (CPM) [7]. Mobility gap E , was determined using all internal photoemission method [8].

3. Hydrogen incorporation scheme Integrated IR absorption coefficients 12000 and 12090 are shown in fig. 1. These integrated absorption coefficients were obtained by deconvoluting IR absorption

|

E 0

b2 b_ W 0 0

40

Z

c- 30

I _

"~ _...

'

i

'

I

-- I0

I2ooo

r-

13_ er 0

o~ 20

_

--5

flD

a

t~_, 10 I2090

n,-

F.- 0

---

200

0

250 500 550 400 SUBSTRATE TEr.J~PE~ATURE (*C)

Fig. 1. I n t e g r a t e d absorption coefficieiii d e d u c e d from IR stretching absorption spectra of a-Si : H films deposited at substrate t e m p e r a t u r e s of 200-400 ° C.

258

T. Shirafuji et al. / a-Si: H deposited hv direct photo-CVD

spectra around 2000 c m - l into two components centered at 2000 cm-~ and 2090 c m - t , respectively. The IR absorption spectra of the photo-CVD films mainly consist of the component centered at 2000 cm-~ which has been attributed to Si-H stretching mode. The component at 2090 cm -t, which arises from S i - H 2, S i - H it. voids [9] and S i - H X (where X is contamination such as oxygen, nitrogen, halogen or carbon [10]) is much smaller than the component at 2000 cm -1. The small photo-conductivity degradation of the photo-CVD films, which we reported before [5], may be due to small 12o~o, because the origins of the photo-conductivity degradation are suggested to be S i - H 2 [11] a n d / o r the contaminations [12]. Si-H concentration N(Si-H) was calculated from the integrated absorption strength I2~,~1 for S i - H stretching mode according to N(Si-H)=Asl2ot~ o, with a proportionality constant A s of 1.4 × l02~ cm -2 [13]. The right side scale of the figure indicates the concentration normalized to at% using the atomic density of crystalline Si (5 × l0 z2 cm-3). The concentration of Si-Iq in the photo-CVD films decreases from 9 to 5 at% with increasing T,uh from 200 to 300 ° C. For T,ub > 400 o C, the concentration of S i - H is kept at around 5 at%. The total hydrogen concentration of the films deposited at T,ub of 2 0 0 - 3 0 0 ° C was determined by hydrogen effusion experiments, and the results showed a similar tendency, though the value was 2-3 at% higher than the S i - H concentration in the films. The inclusion of hydrogen molecules in these films may be a cause of the difference between the total hydrogen concentration and the S i - H concentration. The concentration of S i - H 2 is too small to explain the difference of 2-3 at%. From the results of Si-H and total hydrogen concentration measurements, the photo-CVD films are found to have a small amount of hydrogen. Recently it is suggested that the hydrogen concentration has a tendency to become low as the deposition rate decreases [I.,13j. Although the low deposition rate of 0.6 nm min-~ for our photo-CVD may be a cause o~ the low hydrogen concentration, the hydrogen concentration in the films deposited using a Xe resoimnce lamp is lower by 5 at% than that using a D 2 lamp (161 rim) [5] even for almost the same deposition rate of 1.25 and 1.40 nm min-J, respectively. From this result, the low hydrogen concentration is related with natures of precursors pioduced by high-energy excitation a n d / o r by photolysis of Si2H 6 with the Xe resonance lamp. In fact, the photon energy (8.4 eV) of the Xe resonance lamp is higher than energies to excite Si2H 6 to some low-lying states llAau (transition energy; 7.95 eV) and l~Eu (transition energy; 8.39 eV) [3], while the photon energy (7.7 eV) of th,:- D 2 lamp is lower than these transition energies. This also may suggest that t:-~e precursors produced with different excitation energy have different natures and different effects on a growing film. One might be afraid that the suppression of the hydrogen concentration causes the increase of dangling bonds. High photo- and dark-conductivity ratio~ however, shows that the dangling bonds are kept at a ~ow density in spite of the low hydrogen concentration in the films. Results of below-gap absorption measu~cnlents also showed that the dangling-bond density was rather decreased. Details are described in the following section.

T. Shirafi¢ji et aL / a-Si : H deposited by direct photo-CVD

259

4. Below-gap absorption coefficient Below-gap (hu < 1.8 eV) absorption coefficients were measured using a CPM in a DC mode and fitted with the absorption coefficients determined by transmission and reflection measurements in the range h u > 1.8 eV. The results are given in fig. 2. The closed symbols represent experimental points from the CPM measurements and the open symbols are those from the optical transmission and reflection measurements of the films. The region of exponential absorption tails (straight line in the range hu = 1.4-1.8 eV) is corresponding to the transition from localized states in the valence-band tail to extended states in the conduction band. The absorption is expressed as follows,

a(h.) =a,, exp(hu/E,),

(1)

where E u is the reciprocal gradient of the straight line, namely Urbach energy. The Urbach energy indicates the spread of the exponential band tail of the valence band, and the tail is suggested to be led by structural disorder [16]. The value of E u obtained is shown in fig. 3, which decreases from 60 to 52 meV as T,ub increases from 200 to 300°C. This result shows that the structural disorder is reduced as T.,.b increases from 200 to 300 ° C. For T.,,b > 400 ° C, E~ increases to 56.4 meV, which is suspected to be due to the inclusion of contaminations described later. 10e,

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. . . . . . . . . ,,, ENERGY ( eV ) Fig. 2. Below-gap optical absorption coelficients versus photon energy for a - S i : H films deposited at substrate temperatures o f 2 1 i i ) - 4 0 0 ° C . Open symbols and closed symbols are data mea,:ured using transmission and reflection measurement and constant photocurrent method, respectively. Solid lines are fitted results.

7". Shira.h~ji et al. / a-Si: H deposited by direct photo.CVD

260

~I00

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Fig. 3. Urbach energy deduced from data of constant photocurrent measurement for a-Si: H deposited at substrate temperatures of 200-400" C.

The shoulder of the absorption coefficient below 1.4 eV is corresponding to the transition from the defect states in the mid-gap to the extended states in the conduction band. We used a m e t h o d proposed by Van~f:ek et al. [7] to obtain the defect-state density by fitting the measured absorption coefficients and calculated ones. The defect-stat,z density ,%~(e~. in the mid-gap is supposed to have a broad Gaussian shape characterized by center level E I below the conduction-band edge, dispersion W and height N G ( - E l) = (27r)-1/X(A/W), namely,

NG(e) =

(2zr)-l/2(A/W)

exp[-(e +

E1)'-/(2W2)].

(2)

The density of the extended states in the conduction band is supposed to have a parabolic shape corresponding to a free electron as follows,

g¢(e) =

6.7 x 1021 •1/2 (E > 0).

(3)

In eqs. (2) and (3), E is an energy separation measured from the conduction-band edge, i.e. • = E - E¢. The optical absorption is calculat~,d as follows, C

=

x fNo(•-hu)g¢(e)

de,

(4)

where the constant C is corresponding to the t~ansitiol, matrix element. We obtained the value of C = 3.8 x 10 -3s cm ~ eV 2 from a = 1.5 × l0 s cm -~ at hu = 2.5 eV for the photo-CVD films which have Eop t = 1.75 eV. The integration of eq. (4) was carried out from 0 to 2.5 eV as a reasonable limit, because the integration over 2.5 eV made no significant changes on the result. T h e fitting parameters A, W and E, were d e t e r m i n e d almost uniquely, because the effects of changing parameters A, W and E~ on the shoulder of ab~orption coefficients were independent as follows: a parallel shift of the shoulder, a change of the shoulder curvature and a vertical shift of the shoulder, respectively. The fitted results are shown in fig. 2 by solid lines. The dcfcct-state density A shown in fig. 4 decreases from 9.0 × 10 ~5 to 2.2 x 10 ~5 cm -~ as T~,b increases from 200 to 300°C, but it increases to 1.0 X 1016 cm -3 as T,ub increases to 4 0 0 ° C . The lowest defect-state density of the p h o t o - C V D films is comparable or low compared with that of films prepared by advanced techniques reported recently [17,i8]. The

T. Shirafuji et al. / a-Si: H deposited by direct photo-CVD I¢)

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/

-

-

0 l.i.I i.d 13

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i

I_

l

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Fig. 4. Defect density deduced from data of constant photocurrent measurement for a-Si:H films deposited at substrate temperatures of 200--400 ° C.

average value of E~ is 1.0 eV which agrees with the energy level of dangling bonds suggested by other workers [7,19]. From these results, we can conclude that the dangling-bond density in the photo-CVD films is kept low in spite of the small amount of hydrogen in the films. With the change in the defect-statc density caused by increasing T,,,h, the degradation of photo-conductivity after AM1 100 mW cm -2 3 h illumination decreases from 50% to 10% [5] for T,ub of 200-300 ° C and increases to 20% for T,ub = 400 o C. Initial dark-conductivity and photo- to dark-conductivity ratio, ho,.:ever, are not so much altered and kept at the excellent values of ~ 10-ll S cm-I and 105-10 ~', respectively. Within the deposition conditions we have studied, the change in the defect-state density in the range 1015-10 i~' cm -3 may correlate with the change of the photo-conductivity degradation, but it may not correlate with the initial dark- and photo-conductivity. The defect-state density decreases with increasing T,~b in the range 200-300 o C, but it increases for T,uh--400 ° C. As a cause for the increase of the defect-state density, the contamination of impurities is suspected to arise on the growing film due to out-gas from chamber walls and substrate holder. Improvements of the chamber must be considered in case of high T~ub deposition even for the photo-CVD method. 5. M o b i l i t y g a p m e a s u r e m e n t

Mobility gap E u was determined using an i,ternal photoemission method. Sample structures of semitransparent m e t a l / i / p + / I T O and semitransparent m e t a l / i / n + / I T O were used to determine the separations @Bp and q'Bn of the valence- and conduction-band edges from the metal Fermi level. The intrinsic layer was deposited on the p +- and n +-layers to prepare the structure rejecting the flow of electrons and holes for the measurements, respectively. Ni and AI were used as semitransparent metal contacts on the i-layer. The energy separations ~Bp av.d q'B, are determined from the photon energy dependence of photocurrent at room temperature using the Fowler's relation for photoemission as follows, .,,}

r ( h v ) ~ ( /:~, - ,t,~)',

(5)

262

T. Shirofi~jiet al. / a-Si: H deposited by direc: 7hr,ta- C V D I

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Fig. 5. Examples of Y(hv) t/a at hv - @n plot for the film deposited at 300 o C. The points extrapolated onto the photon energy axis in (a) and (b) indicate ~-o~ and ~no, respectively. where yield Y ( h v ) is defined as pbntocurrent per incident photon, and (/)B is corresponding to @Bp or ~Bn" T h e p h o t o n energy was varied in the range 1.0-1.3 eV. The ene~'gy separations ~Bo and ~ B , are d e t e r m i n e d by extrapolation on Y ( h v ) ~/z versus h v plot. Mobility gap E~, is obtained from the sum o f @Bp and ~B." Figs. 5a and 5b show the examples of the plot to obtain (/)an and qbBp for the film deposited at T~,h = 300 o C. The results obtained using Ni as the s e m i t r a n s p a r e n t metal contact are shown in fig. 6. The results for AI are consistent with the difference b e t w e e n the work functions of Ni (4.9 eV) and AI (4.3 eV) [26], because ~ a , for Ni is larger than that for AI, and ~ p for Ni is smaller than that for Ai. T h e sums of ~ , and ~'~)Bn i.e. E~, obtained for Ni and AI almost agree with each other. The difference between the mobility gap and the optical gap, i.e. E , - E o o t , is attributed to the energy width of the localized states at the edge of the conduction or the valence bands. For the films deposited at T,,b of 200-300 ° C, o u r p h o t o - C V D films show a gap difference of E u ( 1 . 8 0 ) - E o p t ( 1 . 7 5 ) = 0.05 eV~ while G D films reported recently [8] show E~,( 1.89) - Eo~t(1.73) = 0. J 6 eV. This result suggests that the energy width of the localized states n e a r the bat;~ edges of our p h o t o - C V D films deposited at T~ h of 200-300 o C is n a r r o w e r than those of conventional G D

2.0

1

'

I

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>.

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>(.9

fl:

~z m

LO -- _- ~~ - - - - - - - - O ~ C~an {g:

--

~Bp 200 250 300 SUBSTRATE TEMPERATURE (~::;I

Fig. 6. Mohility gap determined by inlernal photoemission method for a-Si:H films deposited at ~t;h~tr,'.'tc icmperatures of 200-300 ° C. The a-Si : H films have E,,0t = 1.75 eV.

T. Shirafuji et al. / a- Si : H deposited by direct photo-CVD

263

films. This undoped a-Si:H is useful as an i-layer in p-i-n solar cell structures, since the carriers in the i-layer may have a long life time because of the low localized-state density at band edges. The correlation t~etween E u and E u which are both related with structural disorder and localization of electronic states, is not found in this study, since E u does not show any explicit dependence on T~b" as in fig, 6, whereas E~ decreases with increasing ~ub in the range 200-300 °C as in fig. 3.

6. Conclusion In conclusion, we have cleared hydrogen bonding configuration, hydrogen concentration, defect-state density and mobility gap of a - S i : H films deposited by direct photo-CVD using VUV light 1147 nm). The hydrogen bonding configuration is mainly S i - H and the hydrogen ~_-oncentration reaches as low as 5 at%, although the cause of the low hydrogen concentration is not clear. In spite of the low hydrogen concentration, the photo-CVD films have low defect-state density, which suggests that the dangling-bond density is suppressed enough by a small amount of hydrogen. The mobility gap of the films is closer to the optical gap than that of GD films, which means that the energy width of localized states near the band edges is narrower than that ol GD films. For the film deposited at high T,ub (401)o C), the S i - H concentration is almost the same value of 5 at% a~ that for T,ub = 300 ° C. The defect-state density and the Urbach energy, however, bccomes larger than those for T,ub = 300 °C. These changes may correlate with the chai~ge of photo-conductivity degradation rate, but may not correlate with the photo-conductivity itself.

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T. Shirafi~ji et al. / a-Si : H deposited by direct photo-CVD

[14] Y. ltishikawa, M. Ohnishi and Y. Kuwano, Mater. Res. Soc. 199{) Spring Meeting, San Francisco, USA, 1990, Mater. Res. Soc. Symp. Proc. 192 (1990) 3. [15] J. Perrin, 1. Solomon, B. Bourdon, J. Fontenille and E. Ligeon, Thin Solid Films 62 (1979) 327. [16] M.H. Cohen, M.Y. Chou, E.N. Economou, S. John and C.M. Soukoulis, IBM J. Res. Develop. 32 (1988) 82. [17] K. Shepard, Z.E. Smith, S. Aliishi and S. Wagner, Amorphous Silicon Technol. 1988, Nevada, USA, 1988, Mater. Res. Soc. Syrup. Proc. 118 (1988) 147. [18] H. Curtins, M. Favrc, Y. Ziegler, N. Wyrsch and A.V. Shah, Amorphous Silicion Technol. 1988, Nevada, USA, Mater. Res. Soc. Syrup. Proc. 118 (1988) 159. [19] T. Shimizu, X. Xu, H. Kidoh, A. Morimoto and M. Kumeda, J. Appi. Phys. 64 (1988) 5045. [241] A.M. Cowley and S.M. Sze, J. Appl. Phys. 36 ~,1965) 3212.