CuAlSe2 Thin Films Obtained by Chalcogenization

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CuAlSe2 Thin Films Obtained by Chalcogenization

S. Marsillac, K. Benchouk, C. El Moctar, J. Bernède, J. Pouzet, A. Khellil, M.

Jamali

To cite this version:

S. Marsillac, K. Benchouk, C. El Moctar, J. Bernède, J. Pouzet, et al.. CuAlSe2 Thin Films Ob- tained by Chalcogenization. Journal de Physique III, EDP Sciences, 1997, 7 (11), pp.2165-2169.

�10.1051/jp3:1997249�. �jpa-00249708�

CuAlse~ Thin Films Obtained by Chalcogenization _(*)

S. Marsillac (~), K. Benchouk (~), C. El Moctar (~), J-C- BernAde (~'**), J. Pouzet (~), ^A. Khellil (~) ^{and M.} Jamali (~)

(~) #quipe Couches _{Minces et} Mat6riaux Nouveaux, GPSE, FSTN, BP 92208, 2

de la HoussimAre~ 44322 Nantes Cedex 03, France

(~) LPMCE, Oran Es S6ma, Algdrie

(Received 4 February1997, revised 23 June 1997, accepted 11 August 1997)

PACS.61.lo.i X-ray diffraction and scattering PACS.68.55.a Thin film _structure and morphology

Abstract. CuAlSe2 thin films have been synthesized by chalcogenization of thin Cu and

Al layers sequentially deposited by evaporation under

It is shown that CuAlSe2 films

are

obtained with

Cu2-rise and Se phases present at the surface These surface phases

are

suppressed by annealing under

and by chemical etching in

KCN solution. At the

end of the _process, the XRD spectrum demonstrates that textured CuAlSe2 films have been

obtained with preferential orientation of the crystallites along the (l12) direction. The gap of the films is 2.7 eV

expected. The films

nearly stoichiometric~ but their surface is quite rough The XPS spectra show that

Na diffuses from the substrate toward the surface

during the annealing _process. However, this Na is etched by KCN.

1. Introduction

Ternary I-III-VI2 chalcopyrite semiconductors have received considerable attention in recent years because of their applications in photovoltaic devices.

If the CuInSe2 is the most extensively studied compound of the family, _some other ternary chalcopyrites have attracted much attention _on recent years because of large potential appli-

cations. Among them, CuAlSe2, which is _a large band gap material Eg

=

2.67eV [ij _can

be used _{as an} emitting layer of _a blue light photoelectroluminescent devices _{or as a} window in heterostructures photovoltaic devices. If for the former application heteroepitaxial films _are

needed, in the _case of the latter application polycrystalline films _can be used.

In this paper, we describe _{a very} simple and cheap technique which allows to obtain well crystallized CuAlSe2 Polycrystalline thin films _some attempts have been done also with CuAlTe2.

(*) This paper

presented at the 'Uourudes Maghrdbmes

les Sciences des Matdriaux" held _at Harnmamet the 8, 9 and 10 November 1996

(**) Author for correspondence

© Les #ditions _de Physique 1997

2166 JOURNAL DE PHYSIQUE III N°11

2. Experimental Techniques

CuAlSe2 films _were obtained by chalcogenization (annealing und(r selenium atmosphere of

Cu/Al/Cu...Al/Cu thin layers sequentially deposited).

The substrates

polished soda lime glass

silicon. chemically cleaned. Then the sub- strates were out gased "in situ" prior to deposition by heating to 400 K for 2 h. The Cu and Al films _were deposited in _vacuum (pressure 5 _x 10~~ Pa) by evap)ration _{from two tungsten}

crucibles. The purity of Al _was 99.99%, that of Cu _was 99.999% aid that of Se _was 99.999%.

Layers of Cu and Al _were deposited sequentially _on the heated substjates ^{(400K < Ts < 450K).}

During the deposition,

rotating substrate holder _was positionned in front of the Cu evaporation

and

front of the Al evaporation source to the _sequence. The evaporation rates and film thicknesses _were ineasured

situ by vibrating _quartz method,

the quartz head being joined to the substrate holder. The number for each constituent varied from six _{to ten} in order to deposit Cu /Al /Cu -Al /Cu The relative thicknesses of the layers _were calculated to achieve the desired atomic ratio

1.1. The thickness of the Cu layers

about 10

and its e,>aporation rate _was 2 nms~~, in the _case of

aluminium they _were 13 _nm and 0.2 nms~~ respectively. A Cu is used because it has

been shown, in the _case of CuInSe2. that such

_excess improve crystalline quality of the

films [2j. ^After deposition the multilayer structures were with _a small amount of

chalcogen in _a Pyrex tube sealed under _vacuum (10~~ Pa). Then samples were annealed

under chalcogen atmosphere. Chalcogens have _a high _vapour therefore during the

annealing they _were in the _vapour state. The annealing varied from 823 K to 853

K while the annealing time varied between 24 h and 48 h. The of the films after

reaction with chalcogen was measured by

mechanical finger.

The films have been characterized by X-ray diffraction (XRD), Photoelectron Spec-

troscopy (XPS) and optical absorption.

3. Experimental Results

The XRD of thin films _on glass substrate _are reported

Figure 1. _can be _seen in Figure la

that just ^after annealing, CuAlSe2 but also Cu2-&Se and Se _are in the films The

surface of these films _appears to be quite inhomogeneous, small and large grains of about 1 to 5 ~Jm size _appear, randomly distributed at the surface the films (not shown).

It has been shown earlier that annealing under selenium in

Pyrex tube induces

some selenium condensation at the surface of the films during the of the tube. This

selenium _excess at the surface of the films _was sublimated by the samples under

dynamic _vacuum for 6 h at 570 K After this annealing the Se has disappeared but those related to Cu2-&Se _are still there (Fig. lb) _as the large residing _on the surface.

Therefore, _as usually done in the _case of CuInSe2 (3j, ^the binary present in the

powder have been etched in _a 0.iM KCN solution for _some

The films _were etched for periods of i min and XRD _were recorded after each

treatment until the Cu2-&Se peaks disappear (Fig ic). Then the of the surface

of the films shows that the large grains have disappeared. As by the X-ray diffraction spectrum (Fig. ic). at the end of the _process, only ^the 1112) peak is clearly visible, which shows that the majority of the large crystallites _are preferentially oriented) _{along the} (i12) direction.

The samples have also been studied by XPS, however, quantitative information _are difficult to obtain because the binding energies ^{for Cu} and Al _are adjacent _to each other (Cu3s and A12s; Cu3p and A12p; etc.). XPS lines _are shown in Figure 2 for ~uAlSe2 thin films before

and after KCN treatment.

u w

I

j 26 -

u

26 -

a)

5000

j

~

l f

I

U S

$ 1

Z Z

O O

~ ~

Z Z

o

lo 30

26 - 26 -

b) c)

Fig 1. XRD phtterns of

multilayer Cu/Al/Cu..Al/Cu thin film

glass substrate a) after

annealing under Se atmosphere. b) after l~~ post-treatment (annealing under dynamic vacuum).

c) after 2~~ post-treatment (etching by

KCN solution)

1(cps>

Cv3#

Cu3s A12p

A12s

b

a

120

90 80

60

-E(eV)

Fig 2. XPS spectra of- a) CuAlSe2 thin film

glass substrate before KCN treatment,

b) CuAlSe2 thin films

glass substrate after KCN treatment.

2168 JOURNAL DE PHYSIQUE III N°11

1(CPS)

285 280 275

~E(eV)

Fig 3 XPS spectrum of the Cls peak after 0, 1, 3 minutes of etching.

For the discussion below the carbon peak of contamination at of the samples has been taken _as reference: Cls

=

284.8 eV. The Se3d peak is situated 55 eV while the binding

energy of the Cu2p5/2 ^{is 932.3 eV.} The Al peaks

adjacent to peaks (A12p ^and Cu3p;

A12s and Cu3s), therefore _we have proceeded to _a decomposition. peak situated at 74.5 eV _can be attributed to A12p while the other situated at around eV corresponds to Cu3p.

The binding _energy of the different elements Se3d, A12p and _are respectively 55.3 eV, 72.65 eV and 932.4 eV [4j. ^The electronegativity of Se being the of the three elements,

its binding _energy in the CuAlSe2 compound should be that of the element alone, while that of the _anions should be higher This is the _case of A12p peak. In the _case of Cu2p5 _/2, it is well known that its binding _energy is poorly to its oxidation state.

Moreover, the binding energies ^obtained for Se and Cu _are in good with the values measured _on other compounds of the _same ternary chalcopyrite such _as CuInSe2 (3j.

The films have also been studied by XPS before KCN chemical The result depends

on the substrate. When _a soda lime glass is used _{a new} peak at 66 eV (Fig. 2a). In

fact, simultaneously to this peak, another _one situated at 1072 eV also detected. This last peak has been already discussed in the _case of CuInSe2 (5j: ^{it to Na diffusion from} the soda lime substrate towards the surface of the films. Therefore peak situated at 66 eV

can be attributed _to Na2s. The intensity of these Na peaks when the etching time increases which shows that Na is accumulated mainly at the of the films. Fortunately

Na is soluble in the KCN solution and the corresponding peaks after chemical etching

(Fig. 2b). The carbon contamination of the thin films has been by the evolution of the Cis peak after etching (Fig. 3). It _can be _seen that this peak after i or 2 min of etching. Therefore, _{we can} conclude that there is _no strong pollution

_our thin films.

CuAlSe2 is _a direct _gap semiconductor [ii. Therefore, the direct [band ^{gap can be deduced}

from the plot (ohu)~ _vs. hu. The _room temperature ^indicate a direct _gap of 2.7 eV which is in accordance with the crystal [lj.

The _same process has bien used to obtain CuAlTe2 However in that _case only large CuAlTe2

grains ^{embeded id}

disordered "CuAl" matrix

obtained

4. Discussion, Conclusion

Oriented CuAlSe2 thin films have been obtained by thermal annealing under selenium atmo-

sphere of Cu/Al/Cu. Al/Cu thin sequentially deposited metallic layers. At the end of the

process (annealing under selenium atmosphere, annealing under dynamic _vacuum, chemical

etching) the CuAlSe2 films show properties expected. The optimum temperature for the _an-

nealing under selenium atmosphere _appears to be around 820 K. The annealing duration is optimum at about 24 h. When the films _are obtained _on soda lime glass, there is Na at the surface of the films. The good crystallization and texturation of the films _can be related to this Na diffusion, since it has been shown, in the _case of CuInSe2 (5], ^{that the} crystalline quality

is strongly enhanced by Na diffusion through the films. This is corroborated by the fact that the CuAlTe2 thin films obtained _on Si substrates present the _same XRD spectra than those obtained _on glass substrate but with _very small peaks intensity which is the consequence of _a bad crystallization. So _we have not studied these films _more.

References

iii Shirakata S., Chichibu S., Matsumoto S. and Isomura S., Low-pressure metalorganic chem- ical vapor deposition of CuAlSe2, Jpn J- Appl- Phys. 32 (1993) L 167-

[2] Sheer R-, Diesner K. and Lewerenz H.J., Experiments

the microstructure of evaporated CuInSe2 thin films, Thin Solid Films 268 (1995) 130-

[3] Scheer R. and Lewerenz H-J-, CuInSe2 thin films surface stoeichiometry and phase _segre- gation _source, J. Vac- So. Technol. A12 (1994) 56.

[4] Wagner C-D-, Riggs W-M-, Davis M-E-, ^Moulder S-F- and Muilenberg G E., Hand-Book of

X-ray photoelectron spectroscopy ^CD, (Perkin elmer / Eden prairie (M.N.), 1979).

[5] Heske C., Fink R., Jacob D- and Umbach E., Electronic structure, composition, and local surface photovoltage effects of polycristalline Cu(In Ga)Se2 thin films after Na deposition, Proceeding of the 13th European Photovoltaic Solar Energy Conference, Nice, France. (Ed.

H-S- Stephens and Associates, Bedford, U-K. 23-27 October 1995) _p. 2003.