Photo-induced changes in the coefficient of the temperature dependence of the Fermi level in discharge-produced amorphous silicon

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Photo-induced changes in the coefficient of the temperature dependence of the Fermi level in

discharge-produced amorphous silicon

J. Bullot, M. Galin, Mélanie Gauthier, B. Bourdon, Y. Catherine

To cite this version:

J. Bullot, M. Galin, Mélanie Gauthier, B. Bourdon, Y. Catherine. Photo-induced changes in the coefficient of the temperature dependence of the Fermi level in discharge-produced amorphous silicon.

Journal de Physique, 1982, 43 (9), pp.1419-1424. �10.1051/jphys:019820043090141900�. �jpa-00209523�

Photo-induced changes ⁱⁿ the coefficient of the temperature dependence

of the Fermi level in discharge-produced amorphous ^silicon

J. Bullot (*), ^{M. Galin} (*), M. Gauthier (**), ^{B. Bourdon} (***) and Y. Catherine (+) (*) Laboratoire des Matériaux Amorphes, (LA 75), Université de Paris-Sud, ⁹¹⁴⁰⁰ Orsay, ^France (**) ^ERA 718, ^Bât. 350, Université de Paris-Sud, ⁹¹⁴⁰⁰ Orsay, ^France

(***) Laboratoire de Marcoussis, CRCGE, Route de Nozay, ⁹¹⁴⁶⁰ Marcoussis, ^France

(+) Laboratoire de Physique Corpusculaire, ^ERA 924, 2, ^{rue de la} Houssinière, 44072 Nantes Cedex, ^France

(Reçu ^{le I S mars} 1982, révisé le 6 mai, accepté ^{le 17 mai} 1982)

Résumé.

Nous décrivons quelques ^{nouveaux résultats} concernant les variations de la conductivité _03C3D et de la

photoconductivité 03C3p (effet Staebler-Wronski) de a-Si : H. Nous montrons que certains films ^sont beaucoup ^moins photosensibles que d’autres; pour ^ces échantillons la variation de l’énergie d’activation 0394E03C3 ^de 03C3D quand ^on passe de l’état recuit A à l’état irradié B est petite ^{et dans certains cas} 0394E03C3 ~ ^{0. Nous} interprétons ^{ces résultats à l’aide}

de la théorie de la conductivité métallique ^{minimum et nous} calculons la variation (03B2A - 03B2B) ^du coefficient de

température du niveau de Fermi quand ^on passe de A à B ^{et en} déduisons les valeurs limites de 03B2A ^et 03B2B. ^Les

films très sensibles à l’effet Staebler-Wronski sont tels que 03B2B > 03B2A’ 03B2B étant de l’ordre de 4-10 ^x 10-4 eV K-1.

Au contraire pour les films peu sensibles 03B2A ^et 03B2B ^sont négatifs ^et la variation de 03B2, quand ^on passe de l’état A à l’état B, ^{est faible.}

Abstract.

New data on photo-induced changes ^{in the} photoconductivity 03C3p and ^dark conductivity 03C3D (Staebler- Wronski effect) ^{of a-Si : H are} described. Some films are shown to be much less sensitive to light exposure than others : the change ^{in the} activation _energy 0394E03C3 ^of 03C3D between the annealed state A and the fully-irradiated ^{state B}

is small and in some cases 0394E03C3 ~ ^{0. The results are} interpreted ^{in terms of} minimum metallic conductivity theory

and the change ⁱⁿ the coefficient of the temperature dependence of the Fermi level (03B2A - 03B2B) ^{between states A and B}

is calculated. Upper ^{and lower bounds of} 03B2A ^and 03B2B ^{are deduced.} It is found that for films exhibiting large photo-

induced effects 03B2B > 03B2A. ^The coefficient 03B2B ^is positive ^and typically ~ ^{4-10 x 10-4 eV K-1. In contrast for most}

films exhibiting ^small changes upon illumination both 03B2A ^and 03B2B ^are negative ^{and the} change ^when passing ^from

state A to state B is small.

Classification

Physics ^Abstracts

72.80N - 72.40

73.60

1. Introduction.

Since the discovery ^of photo-

induced changes ^{in the} photoconductivity up ^and dark conductivity ^{CD of} hydrogenated amorphous

silicon by Staebler and Wronski [1, 2] (SW) ^a great deal of work has been devoted to this problem [3-11].

We recall that SW showed that when intrinsic or

doped ^{a-Si : H films are submitted to} long ^{and intense} light exposures both up ^{and 6p} decrease, ^{the latter} by nearly four orders of magnitude. Annealing ^at

~

150°C reverses the process. When passing ^rever- sibly from the annealed state (state A) ^{to the} fully-

irradiated state (state B) SW showed that changes ⁱⁿ

the activation energy Ea ^of (JD and the pre-exponen- tial factor _o-o in :

are such that :

Typical ^{values are :} AEa ⁼ E: - EB N - ^{0.3 eV and} 0/ UB -0.04.

Such an effect is of major importance ^not only ^for

solar cells applications but also for comparing ^older conductivity ^and photoconductivity measurements carried out on films in an intermediate state i.e. having

received a variable amount of room-light irradiation between preparation ^and measurement.

Whether these light-induced conductivity changes

are a bulk or a surface effect is still ^a matter of debate.

Following Staebler and Wronski [1, 2] light-induced

effects are bulk effects associated with changes ^{in the} deep gap ^states. Dersch et al. [12] ^{have observed the}

enhancement of the dark ESR signal ^{after a} long ^and

intense exposure ^to light. Following ^{these authors}

weak Si-Si bonds break by photoexcitation ^and

metastable dangling ^{bonds are formed.}

Alternatively Solomon et al. [13] ^have proposed

that the SW effect ^can be explained by the existence of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphys:019820043090141900

1420

a variable charge density ^at the film-substrate interface

leading ^to downwards band bending ^{in state A.} They

demonstrated that by applying ^{to an} annealed film a

d.c. electric field ^across the sample ^{held at a} high temperature they ^{could obtain a state similar to}

that obtained by light irradiation.

In the following paper ^we present ^{some new data}

on glow discharge (GD) ^{a-Si : H films} having ^weak light-induced changes. These data altogether ^with

former results in the literature are analysed ^{in terms}

of minimum metallic conductivity theory.

2. Experimental.

^{Films 1 to 5 were} prepared by

GD decomposition ^{of silane} by ^{one of us} (B. B.) ^{at the}

Centre de Recherches CGE/Marcoussis. ^{Film 6 was} prepared by ^{one of us} (Y. C.) ^at the Universite de Nantes using ^{the same} technique but a.different _expe- rimental set-up. ^{The main} deposition parameters ^are shown in table I. Conductivity ^and photoconductivity

measurements were carried out in a coplanar geometry

with vacuum-evaporated aluminum electrodes 2 mm

apart. ^The sample ^{holder was} kept ^under a $ 10-5

torr vacuum during measurements. SW type ^irradia- tion was performed by illuminating the film with a

800 watts quartz-iodine lamp through ^{a 600-900 nm} band-pass ^filter (MTO ^{A 600} b’), the incident _power

being - ^{15 mW cm-2.}

3. Results.

All samples ^show photo-induced

effects but depending upon their origin ^{two kinds of}

films are to be distinguished. ^Whereas film 5 and 6 have

typical ^SW behaviour with large changes ⁱⁿ E(1 ^and cho when passing ^{from state A to B} (see ^Table II), ^{films 1}

to 4 were found to be much less sensitive to light

irradiation (Fig. 1). ^For the latter the results may be summarized as follows :

a) starting from the annealed state (state A) ^{at room} temperature long exposure ^to light results in a (J D

change by ^{about one order of} magnitude only.

Table I.

Deposition parameters of GD films.

Table II.

Photo induced changes ⁱⁿ E, ^and 6p in annealed (A) and fully-irradiated (B) ^{states and} corresponding

changes ⁱⁿ p.

Fig. ^1.

Effect of illumination with 600-900 nm light,

power - 15 mVVcm2. (o) : ^film 1; (0) : ^{film 3.}

b) ^{In the} high temperature range the dark conduc-

tivity activation _energy suffers minor changes ^when passing ^{from state A to B. The} largest ^{value of} AE,,

is

0.08 eV (film 3). ^{For films 1 and 2} AE,, - ⁰ (see Fig. 2 and Table II).

Fig. ^2.

Temperature dependence ^of CrD for film 1 in the annealed (A), intermediate (I) ^and fully-irradiated (B)

states.

c) ^The photo-induced changes ⁱⁿ do either follow the SW behaviour : Jf > a’o ^{(films 3 and 4) or the} opposite behaviour : Jf o o (films ^{1 and} 2).

d) ^{As far as the} photoconductivity ^{changes during}

illumination ^are concerned ^films l.to 4 suffer a smaller decrease than typical ^Sw’s samples. This is shown in

figure 3 where the photoconductivity efficiency qpT is plotted ^{as a} function of the photon energy. q is the

Fig. ^3.

^qpt efficiency product ^{as a} function of photon

energy for film 1 in the annealed state (A) ^{and after a 240 min.}

exposure (B). ^Dashed curves, from reference [2].

quantum efficiency for electron-hole pair production,

p the electron mobility and r the electron lifetime. At each photon energy ^{n/ZT is} given by

where ip ^{is the} photocurrent ^{at an} applied voltage V,

L the inter-electrode spacing, No ^the light flux, ^{R the} reflectivity, ^{a the} absorption coefficient and d the film thickness.

It is seen that for film 1 at hv ⁼ 2 eV _ri,uc is changed by - 20, whereas SW’s data (Ref. [2] Fig. 4) ^{show a} change by - ^120.

4. Discussion.

Clearly experimental ^conditions during ^{the GD} preparation of a-Si : H films are of

major importance concerning ^the _Op and aD light sensitivity. Recently Guha et al. [7] have shown that for films ^{prepared in a GD of a silane (10 %1)-hydro-}

gen (90 0’(’ ) ) mixture the reduction in _6p for an exposure of 2 h at 150 m W em - 2 is only ^{a factor of 7.} Following

these authors this behaviour might ^{be due to the}

presence of ^excess hydrogen ^{in the} plasma. ^{On the}

other hand Hack and Milne [6] ^have interpreted ^their

data ^on RF-sputtered ^{a-Si in terms of} band-bending

1422

at the film-substrate interface and conclude that

photo-induced ^{effects are} dependent ^{on the substrate}

material. In the light ^{of our} data and of previous

results in the literature it ^seems quite ^{difficult to}

draw _any definite conclusion concerning ^the experi-

mental origin ^{of such} large discrepancies ^{from one}

film to another. Only ^a systematic study ^{of the} depen-

dence of the SW effect _upon the deposition parameters could possibly improve ^our understanding. Among

these parameters the substrate material, ^the gas

composition ^{and the} deposition temperature ^{should be} first taken into account.

At the moment we are faced with quite ^different

behaviours of a : Si-H films and some puzzling ^data

like the aAo/ a’ ratio which varies between 0.16 and 7.8 for films prepared ^{under the same} conditions but on

different substrates. It seems highly ^{desirable to}

rationalize the experimental observations in terms of

a significant physical parameter.

In intrinsic material minimum metallic conductivity theory [14, 15] predicts ^that conductivity ^{is due to} charge transport ^{near the} mobility edge Ec ^and Ev.

For electrons in a-Si : H :

(TD Qmin exp[ - (Ec - EF)/kT] ⁽⁴⁾

where (Jmin ’" 103 C) cm is the minimum metallic

conductivity.

The position ^{of the Fermi} energy EF ^is temperature dependent ^{and on account of the} temperature depen-

dence of the optical gap [17] ^{it is} generally ^assumed

that to first order :

so that

(Ee - EF)o = Ea ^{is the} experimental activation energy, and

Following ^Mott [14] :

where a is the radius of the localized state at Ec ^{and z}

the coordination number.

In a typical ^SW experiment ^both aD and E(1 change

upon exposure ^to light ^and subsequent annealing.

Let #A ^and #B be the coefficients of the temperature dependence of the Fermi level in states A and B

respectively. It follows from (6) ^{that :}

with (Ef - E)) = AE,, equation (9) ^{can be written}

as

Plots of In (a’la’) ^vs. 10’IT ^{are shown in} figure ⁴

for films 1 to 6 together ^{with SW’s} original ^{data taken}

from references [1] ^and [2]. ^{From the} intercept ^we

calculate the change ^{in the} temperature coefficient of the Fermi level (PA - #B) ^when passing ^{from A to B.}

Fig. ^4.

^{Plots of} equation (10). ^The figures along ^{the lines}

refer to the film number in table II. Data for films 7 and 8 from references [1] ^and [2] respectively.

The method for determining (#A - #B) ^involves

calculation of the best lines in figure ^{4. In all cases}

the correlation coefficients are a 0.997 and the accuracy is estimated ±5%. Large variations of

(PA - Po) ^are observed from - 6.7 x 10-4 eV K - ’

to 1.7 x 10-4 eV K-’ (see ^Table II). ^{From this} simple analysis ^{the studied films} may be divided into

two categories. ^{The first one includes the films} having

a high sensitivity ^to light exposure (films ^{5 to} 8). ^For

these åP = PA - PB ^{has a} large negative ^{value and}

#B > #A. ^To get ^{an order} of magnitude ^{of the} respective

values of these coefficients we calculate #A ^{from :}

taking simultaneously for the minimum metallic con-

ductivity ^{the extreme values} (Jmin ===1 000 f2-’ cm-’

and 100 fl - I cm-1 as suggested by ^Mott [14]. Recently

Fritzsche [16] ^{has discussed the} possible ^{values of this}

parameter. ^With Umin - 1000 fl - ’ cm-1 he showed

that the calculated density ^{at the} mobility edge Ec

is too high; by adopting u.,i. _ 102 fl- I cm -1 ¹ he concludes that either ^a larger ^electron mobility ^at Ec

than usually accepted ^must be considered or that the

mobility ^is thermally ^activated.

The calculated 13 A and ^are reported ^{in table II.}

Large variations from one sample ^{to another are}

observed but it is seen that j8p ^is substantially larger

than #A, ^the PBjPA ^ratio varying ^{between 2.7 and 8.5.}

In two cases we note that 13 changes sign.

Let us now consider low sensitivity ^films (samples ¹

to 4) ^{for which} (13 A - PB) ^{is small} positive ^or negative.

Taking ^{the same values of} (JmiD ^as before we report ⁱⁿ table II the calculated values of PA ^and #B. ^{For this}

set of samples ^both 13 A ^and PB ^{are small and} negative except for films 3 and 4 with _{amin -} 100 fl - , cm - 1 .

On this ground ^the studied films may be divided into two classes : i) ^films exhibiting large photo-

induced effects : in this case for most samples fl ^is positive in the annealed and in the fully-irradiated

states and suffers an appreciable increase when

passing from the first to the second state ; ii) ⁱⁿ

contrast films exhibiting ^{moderate or small} photo-

induced changes ^{have a small and} negative P ^{in both}

states [23].

Now a question ^arises concerning ^the magnitude

of the coefficients resulting from the above analysis.

The coefficient of the temperature dependence ^{of the}

Fermi level is often related to the coefficient of the temperature dependence ^{of the} optical gap Eg :

y ⁼ aEg/aT; this is the basis for writing equation (5).

It is assumed that either ⁼ y/2 [17,18] or P ⁼ y [19].

The experimental determinations of _y by ^several

authors [ 19, 20] ^{lead to} y ⁼ 3.5 - 3.7 ^x 10-4 eV K -1.

In view of the PB ^values calculated above which for

highly light ^{sensitive films all exceed y it seems hard to} correlate 13 ^and y. Tardy ^{and Meaudre} [20] ^reached

the same conclusion in a recent work.

Finally ^{it is} interesting ^{to link the} preceding ^dis-

cussion to the Meyer-Neldel [21] ^rule relating ^{6o and} Ea:

several authors [2, 16, 18, 22] ^have reported ^such

variations and found 15 A’ -- 35 eV - 1 for _many intrinsic and doped samples. Considering ^{the rever-}

sible SW effect we have :

and from (7) :

In figure ^{5 we} plot (#A - #B) ^{for the} samples ^{1 to 8}

as a function of AE,. The best line : (13 A - #B) =

5.9 x 10- 5 + 1.95 x 10- 3 i1.E(1 yields ^{A’ = 22.6 eV-’}

and Jgo ⁼ UB

It has to be noted that in figure ^{5 we} included data on

sputtered samples from reference [6] (films ^{9 and} 10).

What is noticeable in these results is that the reversible SW effect was not obtained by ^{the usual} annealing-

illumination cycle ^{but instead} by applying ^a

2.5 x 10’ V cm-’ D.C. field at 180°C following ^a

method due to Solomon et al. [12]. Nevertheless states A and B are reached. As seen in figure ^{5 the two} points

calculated from equation (10) ^fit reasonably ^the regres- sion line.

Fig. ^{5. - Plot of} equation (14). ^The figures along ^each point

refer to the film number in table II. For samples ^{9 and}

10 see text Note that oEa

^{0 was taken} for films 1 and 2.

References

[1] STAEBLER, ^{D. L. and} WRONSKI, ^C. R., Appl. Phys. ^Lett.

31 (1977) ^292.

[2] STAEBLER, ^{D. L. and WRONSKI, C.} R., ^J. Appl. Phys. ⁵¹ (1980) ^3262.

[3] FUHS, W., MILLEVILLE, ^{M. and} STUKE, J., Phys. ^Status

Solidi B 89 (1978) ^495.

[4] TANIELIAN, M., FRITZSCHE, H., TSAI, C. C. and SYMA- BLISTI, E., Appl. Phys. ^{Lett. 33} (1978) ^353.

[5] JOUSSE, D., ^BASSET, R., DELIONIBUS, ^{S. and} BOURDON, B., Appl. Phys. ^{Lett. 37} (1980) ^208.

[6] HACK, ^{M. G. and} MILNE, ^W. I., Thin Solid Films 76

(1981) ^195.

[7] GUHA, S., NARASIMHAN, ^{K. L. and} PIETRUSZKO, ^S. M.,

J. Appl. Phys. ⁵² (1981) ^859.

[8] TANIELIAN, ^M. H., GOODMAN, ^{N. B. and} FRITZSCHE, H.,

9th Int. Conf. ^Am. Liq. Semiconductors, ^Grenoble 1981, Ed. B. K. Chakraverty ^{and D.} Kaplan,

p. 375.

[9] POWELL, ^M. J., EASTON, ^{B. C. and} NICHOLLS, ^D. H.,

9th Int. Conf. ^Am. Liq. Semiconductors, ^Grenoble 1981, p. 379.

[10] ^{ELIOTT, S.} R., ^Phil. Mag. ^{B 39} (1979) ^349.

[11] COHEN, ^J. D., LANG, ^D. V., HARBISON, J. P. and

SERGENT, ^A. M., ^{9th Int.} Conf. ^Am. Liq. ^Semi-

conductors, ^Grenoble 1981, p. 371.

1424

[12] DERSCH, H., STUKE, ^{J. and} BEICHLER, J., Appl. Phys.

Lett. 38 (1981) ^456.

[13] SOLOMON, I., DIETL, ^{T. and} KAPLAN, D., ^J. Physique 39 (1978) 1241.

[14] MOTT, ^{N. F. and} DAVIES, ^E. A., Electronic Processes in

Non-Crystalline ^Materials (Oxford, ^Clarendon Press) ^1979.

[15] MOTT, ^N. F., J. Phys. ^{C 13} (1980) ^5433.

[16] FRITZSCHE, H., ^Sol. Energy ^{Mater. 3} (1980) ^477.

[17] NAGELS, P., in Amorphous Semiconductors, Topics ⁱⁿ Applied Physics, ed. M. H. Brodsky (Springer N. Y.) 1979, ^vol. 36, p.113.

[18] SPEAR, ^W. E., ALLAN, D., ^LE COMBER, ^{P. and} GHAITH, A., Phil. Mag. ^{B 41} (1980) ^419.

[19] PERRIN, ^{J. and} SOLOMON, I., ^J. Non-Cryst. ^{Solids 37} (1980) 407.

[20] TARDY, ^{J. and} MEAUDRE, R., Solid State Commun.

39 (1981) 1031.

[21] ^{MEYER, W. and} NELDEL, H., ^{Z. Techn.} Physik ¹⁸ (1937) 588.

[22] ^{CARLSON, D. E. and WRONSKI, C. R., in} Amorphous semiconductors, Topics ⁱⁿ Applied Physics, ^ed.

M. H. Brodsky (Springer ^N. Y.) 1979, ^vol. 36, p. 287.

[23] ^{A further comment can} be made when comparing ^the

behaviour of films 1, 2 and 5 which were all depo-

sited on the same substrate but at different tem-

peratures. Whereas film 5 shows a typical ^SW

behaviour : 0394E03C3 ^{= - 0.16 eV,} 03C3A0/03C3B0

0.03,

films 1 and 2 show an opposite trend with 0394E03C3

0,

03C3A0/03C3B0 = 7.8 and 2 respectively. ^This might ^indicate

that the nature of the substrate is not the main

factor governing photo-induced changes.