Studies of the density of states at the band edges and in the pseudo-gap in a-SiGe:H alloys by combined photothermal deflection spectroscopy and X-ray spectroscopy

Journal of Non-Crystalline Solids 114 (1989) 471-473 North-Holland

471

STUDIES OF THE DENSITY OF STATES AT THE BAND EDGES AND IN THE PSEUDO-GAP IN a-SiGe:H ALLOYS BY COMBINED PHOTOTHERMAL DEFLECTION SPECTROSCOPY AND X-RAY SPECTROSCOPY. L. CHAHED(1), A. GHEORGHIU(1), M.L. THEYE(1), I. ARDELEAN(2), C. SENEMAUD(2) AND C. GODET(3) (I) Laboratoire d'Optique des Solides, URA CNRS 781, Universit~ P. et M. Curie, 4 place Jussieu, 75252 Paris Cedex 05, France. (2) Laboratoire de Chimie Physique, URA CNRS 176, Universit6 P. et M. Curie, 11, rue P. et M. Curie, 75231 Paris Cedex 05, France. (3) Laboratoire de Physique des Interfaces et des Couches Minces, URA CNRS 258, Ecole Polytechnique, 91128 Palaiseau, France.

The effects of alloying on the density of states near the band edges and in the pseudo-gap in a-Sil_xGex:H alloys with O { x ~ 1 prepared under identical conditions by glow discharge decomposition of SiH4/(GeH 4 + H2) mixtures, have been investigated by combining photothermal deflection spectroscopy and soft X-ray spectroscopy. A tentative model for defect-related states is deduced from the data.

150°C. The Ge concentration x, measured by RBS

1. INTRODUCTION Most of the studies on a-Sil_xGex:H alloys show

and electron microprobe, was varied by modifying

a rapid deterioration of their optoelectronic properties

the

with increasing x. This has been related both to

flow rate being kept constant at 40 sccm. The total

an increase of the defect state density and to the

hydrogen

development

effusion experiments,

of

a

microheterogeneity.

However,

the distribution of states in the pseudo-gap as a

(SiH4)/(GeH4+H2) f l o w rate content,

ratio,

the total

as determined by ERDA and remained comprised between

9 and 11% for all samples.

is still speculative. The

The PDS experiments were performed down to

aim of this work is to investigate the modifications

0.6 eV on thick (d >/3 ~m) films deposited on 7059

upon alloying of the density of states at the band

Corning glass substrates. The PDS spectra were

edges and in the pseudo-gap in well-characterized

calibrated

alloys prepared under identical conditions over the

calculated from the optical constants and thickness

function of composition

by

fitting

film

values

at high efiergies (o¢-NI to 5.103 cm-l). The optical

spectroscopy

coefficient

transmission

absorptance

techniques,

deflection

from

the

whole concentration range, by two complementary photothermal

deduced

to

o¢.

measurements

(PDS) and soft X-ray spectroscopy (SXS). A tentative

absorption

was then obtained by

model for defect-related states is deduced from

a fringe-averaging procedure using appropriate thin

these data.

film expressions. The SXS experiments were performed with a Johann-type bent crystal vacuum spectrometer, on 1 ~ m thick films deposited on A1 and Be foils

2. EXPERIMENT with 0x
for emission and absorption respectively. A (020)

prepared in the oven-like multiplasma monochamber

gypsum crytal was used as a monochromator, giving

ARCAM reactor I by r.f. gow-discharge decomposition

an energy resolution of 2.10-4 .

The a-Sil_xGex:H samples,

of mixtures of pure Sill 4 and GeH4 diluted in H2 (10% GeH4, 90% H2). Optimized values were used for the r.f. power density (0.05 Wcm-2) and the total

pressure

maintained at

(90 a

mTorr).

The substrates were

well-controlled temperature

0022-3093/89/$03.50 © Elsevier Science Publishers B.V. (North-Holland)

of

3. RESULTS AND DISCUSSION Figure 1 shows the modifications of the optical absorption

edge

composition range.

u p o n alloying

over

the

whole

L. Chahed et al./Studies of the density of states

472

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FIGURE 1 Optical absorption edges for a-Sil_xGex:H alloys with increasing Ge content : x = 0(I), 0.05(2), 0.16(3), 0.25(4), 0.36(5), 0.45(6), 0.60(7), 0.81(8), 1(9).

- the decrease of the optical gap Eo4 with increasing x is approximately linear, at least up to x N 0.60

LU 5 0

e.~/.o...a I~

5g X

ig0

(a'b. Y-)

FIGURE 2 Variations with Ge content x of the optical gap Eo4 (o) and of the energy Emax of the AO6 maximum (+) (a); of the exponential edge inverse slope Ee (o) (b).

(figure 2a) : Eo4(eV)= 1.92 - 0.67 x. This variation is slower than the ones reported recently for other

defined maximum which shifts towards low energies

G.D. a-Sil_xGex:H(F) alloys2-4, but very close to

in the same way as the edge ; the (Eo4-Emax)

that found for non-hydrogenated sputtered a-SiGe

difference remains of the order of 0.45 eV in all

alloys5. It is taken as representative of Ge alloying

cases (figure 2a). This suggests the emergence of

effects only. The deviations at the highest x values

a defect-related structure in the density of states,

could be due to differences in hydrogen bonding

which must remain at the same distance from one

or/and changes in microstructure.

of the band edges as the gap closes up upon alloying ;

the inverse slope of the exponential edge Ee

distance

could only be

determined

by an

appropriate deconvolution of the data. These states

an

are ascribed to Ge dangling bonds, which is consistent

increase

of disorder upon alloying. However,

the Ee values are certainly overestimated for x >I0.36

with the correlated increase of the spin density

because of the partial overlap of the defect and

associated with such defects for x >~0.306.

tail states contributions. -

this

increases with increasing x (figure 2b), which suggests

Figure 3 shows a comparison of the Si K~ emission

while for x ~<0.25 the alloy absorption edges remain

and Si K absorption spectra, which give respectively

very similar to the a-Si:H one, especially at low

the distributions of occupied (valence) and unoccupied

energies where the spectra cannot be distinguished

(conduction) S i p states, for two alloys with x = 0.16

within experimental uncertainties,

and 0.60. Since the involved core level is the same

for x >~0.36 the

situation is strikingly different. The sub-band gap

(Si ls) for both emission and absorption, the two

absorption starts to increase with increasing x, and

spectra can be put on a unique energy scale for

a shoulder clearly develops below the exponential

a given alloy. Moreover, since the energy of that

edge. If that edge is subtracted in the usual way,

core level is practically

the

the

resulting

extra-absorption

AO~.,

the

integral

insensitive to alloying7,

spectra of different alloys can be directly

of which increases roughly exponentially with x

compared on the same energy scale. The binding

in agreement with previous results 3, exhibits a well-

energy scale referred to the Fermi level taken as

L. Chahed et al. / Studies of the density of states

~inding ~nergy (eV) -5 0 * 5 ] I I

-10 i

Si K~ c~mlsslorl

to be detected in our experiments. It may be noticed

÷10 i

that it is precisely for x ~ 0.30 that the sample

Si K obsorption

microheterogeneity becomes by hydrogen evolution

important, as shown

experiments. The creation

of a high density of Ge dangling bonds seems therefore

/

related to

the development

of a

microstructure

in these non-optimized alloys. ACKNOWLEDGEMENTS

i

1830

473

We would like to thank J.P. Stoquert for the RBS

I

18/,0

1~50

and ERDA measurements, M. Cuniot for the electron

Photon Energy (aV)

microprobe

experiments,

and

G.

Sardin for

the

hydrogen effusion experiments. This work has been

FIGURE 3 Si KI~ emission and Si K absorption spectra for a-SilLxGex:H alloys with x= 0.16 (dashed line) and 0.60 (continuous line).

supported

by the AFME and the PIRSEM-CNRS,

and by the CCE under contract EN3S-0062-F.

REFERENCES the origin has also been indicated on the figure 7. The essential result for our purpose is that the upper edge of the valence state distribution moves upwards with increasing x, while the bottom edge of the conduction

state

distribution remains roughly

at

the same energy. A more quantitative analysis shows that the narrowing of the gap upon alloying can essentially be ascribed to a shift of the valence band edge towards low binding energies. We thus propose that the shoulder which develops at low energies in the absorption spectra for x increasing from 0.36 to I can be related to additional states introduced by Ge danglihg bonds, which remain at the same distance from the valence band edge for all concentrations, and therefore move upwards with it upon alloying. This is at variance with models which tend to locate the neutral Ge dangling bond states near mid-gap throughout the whole composition range3,8. Such defect states can hardly be detected on the emission spectra of figure 3, but they appeared as a well-resolved structure in previous data obtained for more defective a-SiGe:H alloys 9. They could co]'ncide with the states evidenced above the valence band tail by time of flight experiments in similar alloys 10.

For

x,< 0.25, the

density of additional

defect states due to alloying is probably too small

I. P. Roca i Cabarrocas, B. Equer, J. Huc, A. Lloret and J.P.M. Schmitt, i n : Proc. 7th E.C. Photovoltaic Solar Energy Conf. (Sevilla), eds. A. Goetzberger, W. Palz and G. Willeke (Reidel, 1987) p. 533. 2. A. Skumanich, A. Frova and N.M. Amer, Solid State Commun. 54 (1985) 597. 3. S. Aljishi, Z . E . Smith and S. Wagner, i n : Amorphous silicon and related materials, Vol. B, ed. H. Fritzsche (World Scientific, Singapore, 1989) p. 887. 4. K.D. Mackenzie, J.H. Burnett, J.R. Eggert, Y.M. Li. and W. Paul, Phys. Rev. B38 (1988) 6120. 5. J. Dixmier, P. Elka~m, M. Cuniot, H. Labidi, L. Chahed A. Gheorghiu and M.L. Th~ye, in : Proc. ~th E.C. Photovoltaic Solar Energy Conf. "(Florence), Vol. I, eds. I. Solomon, B. Equer and P. Helm (Kluwer Ac. PubL, Dordrecht, 1988) p. 976. 6. L. Chahed, M.L. Th~ye, S. Basrour, J.C. Bruy~re, C. Godet and C. Lloret, in : Proc. 8th E.C. Photovoltaic Solar Energy Conf. (Florence), Vol. I, eds I. Solomon, B. Equer and P. Helm (Kluwer Ac. Publ., Dordrecht, 1988) p. 846. 7. C. S~n~maud, I. Ardelean, L. Chahed, A. Gheorghiu and M.L. Th~ye, i n : Proc. 8th E.C. Photovoltaic Solar Energy Conf. (Florence), Vol. I, eds. I. Solomon, B. Equer and P. Helm (Kluwer Ac. Publ., Dordrecht, 1988) p. 959. 8. K.D. Mackenzie, J.R. Eggert, D.J. Leopold, Y.M. Li, S. Lin and W. Paul, Phys. Rev. B31 (1985) 2198. 9. C. S¢~n~maud, C. Cardinaud and G. Villela, Solid State Commun. 50 (1984) 643. I0. R. Vanderhaghen and C. Longeaud, this volume.