Density of gap states in a-SiC:H films by means of photoconductive and photothermal spectroscopies

Physica B 205 (1995) 169-174

ELSEVIER

Density of gap states in a-SiC'H films by means of photoconductive and photothermal spectroscopies F. D e m i c h e l i s a'*, F. G i o r g i s a, C . F . P i r r i a, E. T r e s s o a, G . A m a t o b, U . C o s c i a c aDipartimento di Fisica del Politecnico, C.so Duca degli Abruzzi 24, 10129 Torino, Italy blEN Galileo Ferraris, Str. delle Cacce 91, 10135 Torino, Italy CDipartimento di Scienze Fisiche, Universita' di Napoli, Piazzale Tecchio 80, 80125 Napoli, Italy

Received 12 July 1994

Abstract

In the present work we report and discuss results on the gap density of states for a-SiC: H films, deposited by PECVD with different CH4 flow rates, through the deconvolution of CPM and PDS spectra. The occupied density of states in the gap is in first approximation the derivative of the CPM spectrum with respect to the photon energy. Below the Urbach edge the PDS and CPM techniques are sensitive to different electronic transitions. From the difference between PDS and CPM spectra the deep unoccupied density of defects above the Fermi level can be deduced. This procedure has been applied for the first time to the a-SiC:H binary alloy. We have obtained that the defect distribution ascribed to silicon dangling bonds can be fitted by gaussian curves with increasing correlation energy and halfwidths as the carbon content increases.

1. Introduction

High-quality films of a-SiC: H have a wide interest in many photovoltaic and optoelectronic applications [1-3]. The optical and electrical transport properties of the films are strongly dependent on the band tails and defect levels inside the pseudo-gap of the alloys. In high-quality a-Si:H films values of the Urbach edge as low as 42 meV [4] and 45 meV [5] have been obtained and in such films a density of defects lower than 1016cm -3 results. It is known that the addition of carbon to silicon gives rise to a widening of the pseudo-gap and a broadening of the Urbach edge due to the increasing compositional disorder and consequently * Corresponding author.

a larger localized state density. The Urbach edge is generally determined by calculating the slope of the experimental tail of the optical absorption spectrum at energies below the gap, usually measured by photothermal deflection spectroscopy (PDS) and constant photocurrent method (CPM), which allow the detection of an absorption coefficient < 103 cm-1. However, the two methods present some differences. The PDS technique is sensitive to all the electronic transitions, while the C P M one detects only transitions able to provide free carriers in the conduction band [6]. The knowledge of the defect density and of the energetic defect distribution in the pseudo-gap due to carbon incorporation in the a-Si: H network is of primary importance to understand the technological performance of a-SiC:H films and to try to

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F. Demichelis et al./ Physica B 205 (1995) 169-174

170

improve the optoelectronic properties of the alloy. Unfortunately due to the low electrical conductivity of a-SiC: H films (below 10 - 12 S cm - 1 with only some atomic percent of carbon) most of the standard methods for the evaluation of defect distribution, such as space charge limited current (SCLC), deep level transient spectroscopy (DLTS), time of flight (TOF), post transit time of flight (PTTOF), thermally stimulated current (TSC), are not applicable. In the present work we report and discuss PDS and CPM results on a-SiC:H films deposited by conventional PECVD. A derivative procedure [7] has been applied for the first time to sub-gap absorption spectra of a-SiC:H in order to get information on the energetic distribution of the occupied and empty electron density of states above and below the Fermi level. The knowledge of the DOS in the gap of a-SiC: H material is low due to the complexity of carbon chemistry. We deduced that in our samples deep occupied and empty defect distributions can be fitted by gaussian curves and that in films having a carbon-to-silicon ratio lower than 0.25 the defects are mainly due to silicon dangling bonds.

-~ .----,

D°\ +

Do\ .

.._. ,~

(z)

o~

\ E (arb.un.) Fig. 1. Models of electronic density of states and electronic transitions as seen by C P M (transitions 1 and 2) and PDS (transitions 1, 2 and 3) techniques.

distribution of gap states and of the optical transitions seen from CPM and PDS techniques. In the CPM case the deep empty states (transition 3) below the conduction edge need not be considered. Thus, the conduction band Nf(E -b hv)/hv can be approximated with a step function NcO(E + hv E~), where N~ is the DOS at the conduction edge E~ and O(E + hv - Eo) is the Heaviside function 0 -- 1 for E >>.Ec - hv, 0 = 0 for E < E~ - hr. So Eq. (1) becomes eCPM(hV) ~ A N t

Ni(E) f ( E ) O ( E + hv - E~)dE -oo

2. Theory

(2)

For amorphous semiconductors, the optical absorption coefficient ~(hv) can be written 1-8] as

where E~ is the conduction edge. From Eq. (2) we obtain

ct(hv) = hv

Ni(E ) f ( E ) = ~

N~(E)f(E){1 - f ( E

+ hv)}

-oo

x Nf(E + hv)dE

(1)

where A is a constant containing the optical matrix element, Ni(E) and Nf(E + hv) are the initial and final electronic states, h v is the photon energy and f ( E ) is the Fermi-Dirac distribution. Various methods have been proposed to deduce the DOS distribution from CPM absorption spectra by using expression (1). Recently Hata and Wagner 1-9] suggested a simple method to deconvolute the occupied DOS from CPM spectra, based on the fact that CPM detects only the electronic transition from occupied states to the conduction band, while PDS detects all the electronic transitions [6]. In Fig. 1 it is possible to see an example of the model

1 ~d~(hv)~ [ d(hv) Jhv = eo- e"

(3)

We deduce that the occupied states as a first approximation result to be the derivative of the CPM spectrum with respect to the photon energy. The described derivative procedure is reliable only when the conduction tail can be neglected. In order to verify this approximation, several absorption coefficient spectra have been obtained by imposing in Eq. (1) a DOS profile consisting in: parabolic bands for extended states separated by a gap of 1.9 eV, a gaussian with a halfwidth of 0.1 eV, and amplitude of Nc x 10- 5, lying at 0.9 eV above the valence tail with slope of 50meV, finally an exponential conduction tail slope ranging from 20 meV to 60 meV. As can be noted in Fig. 2, the

171

F. Demichelis et al./Physica B 205 (1995) 169 174

hv <~E¢ - E~ is given by A

A~(hv) ~ AN~

Nf(E + hv)(1 - f ( E

+ hv))

~s - oc

× O(E~. - E)dE

.'Z

o

where the valence band Ni(E)/hv has been approximated with another step function N ~ O ( E ~ - E). Thus, from Eq. (5) the deep empty defect distribution can be obtained as

"1 I ,'1 II ..'Z

o

0.5

1.0

1.5

2.0

Nr(E){1-f(E)}

!.5

P h o t o n e n e r g y (eV)

Fig. 2. Absorption coefficients obtained by convoluting DOS profiles consisting of parabolic extended bands (E, = 1.9eV), a gaussian with halfwidth 0.1 eV, and amplitude of N~ × 10 s, lying at 0.9eV above the valence tail of slope 50meV and variable conduction band slope: full line 20 meV, dashed line 40 meV and dotted line 60 meV (the straight line reports a slope of 50 meV).

slope of the Urbach tail reproduces that of the valence tail for a conduction tail slope smaller than 40meV. Otherwise, the Urbach tail is influenced also by the conduction tail, thus the derivative procedure should become questionable. In the case of the PDS technique, all the electronic transitions are detectable owing to the photothermal processes involved (see Fig. 1). In fact, it has been shown 1-8] that in PDS spectra, the absorption coefficient can be written as C~aos(hv) ~- A N t

+ hv

Ni(E) f ( E ) O ( E + hv - E~)dE

Ni(E)Nf(E + hv)f(E)

x {1 - f ( E

(5)

+ hv)}O(E~ - hv - E ) d E

(4) where the first integral is the same as in Eq. (2), while the second one considers the electronic transition between the valence band and the deep empty defects (not seen by CPM). If the transitions from the valence tail are neglected, the difference between the absorption coefficients obtained by means of PDS and CPM techniques, for

A N ~ [ d(hv) )h,.=~ ,:,'

(6)

where A~(hv) represents the difference between the PDS and the CPM absorption spectra and N~. is the DOS at valence edge E,..

3. Experimental a-SiC:H films were deposited by the conventional PECVD system described elsewhere 1-5] with deposition conditions reported in Table 1. In the following the samples will be named SiC followed by a number indicating the methane percentage in the CH4 + SiH4 plasma. CPM and PDS spectra, obtained by conventional equipment [6, 10], were calibrated by fitting the optical absorption coefficient deduced from transmission-reflection spectroscopy using a standard procedure [11].

4. Results and discussion Figs. 3(a) and (b) show the comparison between CPM and PDS absorption spectra for two aSiC: H films deposited at different CH4/(SiH4 + CH4) ratios. One can observe a good agreement between CPM and PDS data for the energy around the Urbach tail. For lower energy a remarkable difference occurs• The absorption spectra clearly indicate a widening of the Urbach edge and a higher contribution of the occupied density of states in the sub-gap absorption spectra when the carbon content increases. In the region c~> 104 cm-1 usually a-Si based alloys are described by Tauc's relation [12], which

172

F. Demichelis et al./Physica B 205 (1995) 169 174 i

10 5

i

i

1.0

1.5

2.0

I

i

I

a) STC40 I v

10 s o

o o

101

B

/

< 10 - t 0.5

10 6

7

2.5

b) SIC71

-~

0

l0 s

101

o

/

L

.~

10 -1 0.5

/ I

I

I

t .0

1.5

2.O

energy

(eV)

Photon

2.5

Fig. 3. Comparison between PDS (full line) and CPM (dashed line) spectra calibrated to the optical absorption coefficient for two a-SiC:H samples deposited by PECVD: (a) SIC40, (b) SIC71.

allows for the evaluation of the optical gap Eg reported in Table 2. Since it is known that the introduction of carbon makes this relation questionable [13], in Table 2 we report E, (mobility gap), which is the energy where the absorption coefficient shape changes from exponential (Urbach tail) to parabolic (fundamental absorption region). As reported in Ref. [12] E u is the distance between valence and conduction extended states. Taking into account the Tauc's gap Eg and the Urbach energy Eu, E, has been evaluated to be approximately Eg + 3 Eu. The Urbach edge parameter Eu, reported in Table 2, is generally a function of static disorder produced by changes in growth conditions [14].

It has to be pointed out that for low energies the PDS spectra show a higher absorption than CPM spectra. Such differences could be ascribed to the higher surface states sensitivity of the PDS technique. However, we have performed ESR measurements on samples grown by the same deposition conditions with a thickness in the range 2-4pm. The ESR technique, like PDS is influenced by surface states, but in our case, the amplitudes of the ESR signals show a linear behaviour with respect to the thickness. Thus, it can be concluded that for thickness higher than 2 ~tm bulk states dominate over surface states. This means that in our case the higher absorption detected by the PDS technique is only due to additional contributions from optical transitions which are not detected in CPM measurements [6, 10], as discussed in the previous section. Concerning the approximation about the conduction tail band, recently it has been reported [15] that a-SiC:H with a carbon-to-silicon atomic ratio below 0.25 shows a conduction band tail slope below 25meV. By Rutherford back-scattering measurements on similar a-SiC: H samples [20] we can conclude that in the present set of films the carbon-to-silicon atomic ratio is lower than 0.25, thus the aforesaid deconvolution can be applied. Fig. 4(a) and (b) show the result of the derivative procedures described above for the evaluation of DOS in the mobility gap for the a-SiC:H samples with optical spectra reported in Fig. 3. The occupied and unoccupied DOS have been normalized to the same constant, supposing that Nc (Eq. (3)) and N~ (Eq. (6)) were equal; the zero of energy has been taken for all the DOS at the valence band edge. Moreover, electrical measurements yielded the activation energy Ea by the Arrhenius plot (see Table 2), providing the Fermi levels EF = E~ - E a. Nevertheless the above deconvolution procedure is not affected by the limitation of other methods [6, 8], since no assumptions are made on the shape of the DOS gap distribution, we found for all the a-SiC:H samples two peaks that can be fitted by gaussians. Their parameters and the related errors, estimated calculating the covariance matrix, are reported in Table 2. We can observe that as the carbon is introduced into the alloy, the correlation energy Eeo r (the distance between the occupied and

F. Demichelis et al./Physica B 205 (1995) 169 174

173

r e m a i n p r e v a l e n t l y of a - S i : H type, even if c a r b o n clusters a n d c a r b o n defects are possible [18, 19].

I

a) SIC40 A

5. Conclusions

a v

•.

EF

~d

In conclusion, the present s t u d y p r o v i d e s an insight on the density of g a p states for a - S i C : H films with different c a r b o n content. W e have d e d u c e d t h a t the o c c u p i e d density of states, as a first a p p r o x i m a t i o n , is the derivative of the C P M s p e c t r u m with respect to the p h o t o n energy. T h e e m p t y defect d i s t r i b u t i o n n e a r the F e r m i level has been obt a i n e d by the difference between P D S a n d C P M a b s o r p t i o n s p e c t r a b e l o w the U r b a c h edge. B o t h the deep o c c u p i e d a n d e m p t y defects d i s t r i b u t i o n can be fitted by gaussian curves, a n d E S R m e a s u r e m e n t s s h o w t h a t in a - S i C : H films with low c a r b o n c o n t e n t such deep g a p defects are m a i n l y due to silicon d a n g l i n g bonds.

v

z

I

v

1

b) slc71

a v

v

z

0.0

I

I

I

0.5

1.0

1.5

2.0

References E -

v. v ( o r )

Fig. 4. DOS deconvolutions for two a-SiC:H samples (the dotted lines are the results of the derivative procedure, the continuous lines are the fitting by gaussian distributions which parameters are reported in Table 2). the u n o c c u p i e d peaks) increases w e a k l y from 0.38 eV for SIC20 to 0.66 eV for SIC71. T h e s a m e m o n o t o n i c a l b e h a v i o u r can be o b s e r v e d in the halfwidths of the defect peaks. F u r t h e r m o r e , carb o n i n c o r p o r a t i o n enlarges the g a p a n d decreases the valence b a n d tail slope. E l e c t r o n Spin R e s o n a n c e (ESR) results on all the a - S i C : H s a m p l e s of the present w o r k have s h o w n a g-shift in the range 2.0052-2.0055 a n d a spin density in the range 1016-1018cm -3, which are r e p o r t e d in T a b l e 2 a n d can be a s c r i b e d to silicon d a n g l i n g b o n d s [16] ( n a m e d D O/+ a n d D O/- b e l o w a n d a b o v e E v respectively). In fact, in the case of c a r b o n p r e d o m i n a n t defects, we c o u l d expect to have a 9-shift in the r a n g e 2.002-2.003 [17]. Thus, it is possible to a r g u e that, for a - S i C : H films d e p o s ited by P E C V D a n d h a v i n g a c a r b o n to silicon a t o m i c r a t i o lower t h a n 0.25, the defects in the g a p

[1] R.E. Holligsworth, P.K. Bath and A. Madam in: Proc. of the 19th IEEE Photov. Spec. Conf. (New Orleans, Louisiana, 1987) p. 684. [2] Y.M. Li, A. Catalano and B.F. Fieselmann, Amorphous Silicon Technology, Vol. 258 (Mat. Res. Soc., Pittsburg, 1992) p. 923. [3] M. Yoshimi, T. Ishiko, K. Hattori, H. Okamoto and Y. Hamakawa, J. Appl. Phys. 72 (1992) 3166. [4] G.D. Cody, J. Non-Cryst. Solids 141 (1992) 3. [5] F. Demichelis, G. Crovini, C.F. Pirri, E. Tresso, R. Galloni, R. Rizzoli, C. Summonte, G. Amato, P. Rava and A. Madan, in: Amorphous Silicon Technology 1993, eds. E.A. Schif, M.J. Thompson, A. Madam K. Tanaka and P.G. LeComber, Vol. 297 (Mat. Res. Soc., Pittsburg, 1993) p. 681. [6] J. Kocka, M. Vanecek and A. Triska, in: Amorphous Silicon and Related Materials, cd. H. Fritzsche (World Scientific, London, 1988) p. 297. [7] G. Amato, F. Giorgis and R. Spagnolo, J. Appl. Phys. 71 (1992) 3479. [8] M. Vanecek, A. Abraham, O. Stika, J. Stuchlik and J. Kocka, Phys. Stat. Sol. A 83 (1984) 617. [9] H. Hata and S. Wagner, Amorphous Silicon Technology 1991, Vol. 219 (Mat. Res. Soc., Pittsburg, 1991) p. 611. [10] H. Curtins and M. Favre, in: Amorphous Silicon and Related Materials, ed. H. Fritzsche (World Scientific, London, 1988) p. 329.

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F. Demichelis et al./Physica B 205 (1995) 169-174

[11] P. Sladek, Y. Bouizem, M.L. Theye and P. Roca i Cabarrocas, in: Proc. 1lth European Photovoltaic Solar Energy Conf. (Harwood, 1994) p. 71. [12] G.D. Cody, in: Semiconductors and Semimetals, Vol. 21B, ed. J. Pankove (Academic Press, New York, 1981) p. 11. [13] J. Bullot and M.P. Schmidt, Phys. Stat. Sol. B (1987) 345. [14] A. Skumanich, A. Frova and N.M. Amer, Solid State Commun. 54 (1985) 597. [15] S. Akita, Y. Nakayama, H. Yamano and T. Kawamura, Amorphous Silicon Technology, Vol. 149 (Mat. Res. Soc., Pittsburg, 1989) p. 167.

[16] H. Dersch, J. Stuke and J. Beichler, Phys. Stat. Sol. B 105 (1981) 265. [17] R.J. Gambino and J.A. Thompson, Solid State Commun. 34 (1980) 15. [18] J. Robertson, Phil. Mag. B 66 (1992) 615. [19] J.A. Reimer and M.A. Petrich, in: Amorphous and Related Materials, ed. H. Fritzsche (World Scientific, London, 1988) p. 3. [20] F. Demichelis et al., J. Non-Cryst. Solids 164-166 (1993) 1015.

Table 1 Deposition conditions (p is the deposition pressure, P is the RF power, T~ is the substrate temperature and t is the sample thickness) Sample

Sill4 [sccm]

CH4 [sccm]

p [Pa]

P [W]

Ts [°C]

t [p_m]

Si SIC20 SIC40 SIC50 SIC64 SIC71

40 40 30 40 40 28

10 20 40 70 70

78 78 78 78 78 78

3 3 3 3 3 3

200 200 200 200 200 200

2.6 2.3 2.7 2.2 2.6

Table 2 Optoelectronic properties of all the films and parameters of DOS deconvolution (E(D °\+) is the distance between the occupied D O`'+ defectpeak ofhalfwidth a(D°\+ ) and the conduction mobility edge and E(D°\ ) is the distance between the empty D °\- defect peak of halfwidth a(D °\- ) and the valence mobility edge)

Sample

Es I-eV]

El, I-eV]

E~ [eVl

El: [meV]

E(D °~'+) a(D °\+ ) [eV] [eV]

Si

1.75

1.83

0.75

55

0.95

SIC20

1.78

1.86

0.81

57

SIC40

1.85

1.95

0.85

68

SIC50

1.87

1.96

0.89

65

SIC64

1.95

2.06

0.92

70

SIC71

2.02

2.14

0.95

80

1.05 _+ 0.04 1.10 + 0.03 1.15 + 0.03 1.24 + 0.04 1.37 + 0.10

E(D °\ ) a(D °\+ ) [eV] [eV] 1.25

0.10 + 0.02 0.10 -6 0.04 0.12 + 0.02 0.13 + 0.02 0.17 + 0.03

1.19 _+ 0.05 1.25 + 0.02 1.21 + 0.06 1.34 + 0.05 1.43 ___0.05

0.08 _+ 0.05 0.06 __. 0.01 0.14 + 0.07 0.13 + 0.03 0.15 + 0.02

Ecor [eV] 0.35 +0.15 0.38 _+ 0.09 0.40 + 0.05 0.40 _ 0.09 0.53 + 0.09 0.66 + 0.15

Ns I-cm- 3]

g-shift -

5 x 1016 2.0055

6 × 1017 2.0055 2 x 1018 2.0052 6 x 1018 2.0052