Electroless deposition on copper substrates and characterization of thin films of copper (I) selenide

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Mat. R e s . B u l l . , Vol. 27, p p . 1185-1191, 1992. P r i n t e d i n t h e USA. 0025-5408/92 $5.00 + .00 C o p y r i g h t (c) 1992 P e r g a m o n P r e s s L t d .

ELECTROLESS DEPOSITION ON COPPER SUBSTRATES AND CHARACTERIZATION OF THIN FILMS OF COPPER (I) SELENIDE

Santosh K.Haram and K.S.V.Santhanam Chemical Physics Group, Tata Institute of Fundamental Research Homi Bhabha Road, Colaba, Bombay 400005, India and M.Neumann-Spallart and C.L~vy-Cl~ment Laboratoire de Physique des Solides, C.N.R.S., I, place Aristide Briand, F-92195 Meudon, France

( R e c e i v e d J u n e 19, 1992; C o m m u n i c a t e d b y A. Wold)

ABSTRACT:

Cuprous selenide films are prepared by reactin~ oxide free copper metal with selenous acid at room temperature (22~C). The formation of Cu(1) oxide is avoided by degassing the bath with argon. From the X-ray diffractograms (XHD) and electron probe microanalysis (EPMA) of such films it can be concluded that copper deficient, orthorhombic Cu(1) selenide is formed. MATERIALS INDEX: copper, selenides

Introduction Optoelectronic devices with high conversion efficiency using a minimal amount of raw materials are becoming increasingly important. Such devices are made of thin films of materials selected because of their absorption and electronic properties. Copper chalcogenides and copper indium selenide are amongst the most promising materials being used in solar cells (1,2). Copper (1) selenide, a typically p-type semiconductor with a bandgap of 1.1-1.29 eV (3) is of particular interest because it can be used as the window layer in heterojunction solar cells (I) and as a starting material for the synthesis of thin films of copper indium selenide (4). There are several methods available for the formation of thin films of copper (1) selenide of different crystallographic modification, stoichiometry, and chemical inertness. Copper (1) selenide of varying stoichiometry, Cu2-xSe, (0 < x < 0.25) prepared by flash evaporation (5) has been shown to consist of the cubic and the tetragonal phases. Other preparation methods include reaction of Cu with Se dissolved in hot benzene (5), chemical bath deposition on glass plates at pH I0 leading to cubic Cul.gSe (3), and selenisation of a Cu film with Se vapour at elevated temperatures leading to a mixture of cubic Cu2Se and Cu2-xSe (x=0.85) (4). Besides cubic (6) and tetragonal (7) forms there are also two other modifications of copper (1) selenide that have been

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reported only for bulk material: orthorhombic (8,9,10) and monoclinic (6) copper selenide. In this paper a new method of preparation is described which involves the spontaneous formation of a black film of Cuz-xSe upon immersion of copper plates into aqueous selenous acid solutions. The copper substrate itself is one o£ the reactants. The deposition will shown to be electroless, i.e. the reaction leading to film formation is the sum of individual half cell reactions, namely anodic dissolution of the base material and reduction of solution species, shunted together by internal cell currents. Films prepared in such a way are expected to have an intimate physical and electrical contact to the conducting substrate. Moreover, we wanted to see if crystalline modifications other than yet reported for thin films can be made in such a way, which might be important in the search for chemically more stable layers of improved electronic properties.

Experimental All chemicals were AIR grade. Argon gas of high purity (max. 2 ppm of oxygen) was used. The substrates, i.e. Cu plates, were first polished to mirror finish and then etched with a solution of i0% nitric acid in ethanol, rinsed with triply distilled water, and dried in a dessicator under vacuum. Depositions were carried out by placing copper plates vertically in the plating solution for typically half an hour. The plating bath was composed of selenous acid in 0.3% sulfuric acid at a pH of about I. After film formation, the samples were immediately rinsed with At-saturated triply distilled water and dried in a vacuum dessicator where they were kept for storage between subsequent analyses. In some cases Ar gas was bubbled through the solution before and during film growth. X ray diffraction analysis (XRD) was carried out with a JEOL-8030 X-ray diffractometer using Cu-Ka radiation. The synthesised films were either used as such or transferred onto glass plates (using double Scotch 3M adhesive tape) prior to XRD. For electron probe microanalysis (EPMA) of films on Cu and of detached flakes a Camebax apparatus was used. Scanning electron microscopy was carried out using a JEOL JSM-80 apparatus. Results

and Discussion

By exposing the copper plates to the plating bath, black coloured thin films were obtained. The concentration of selenous acid in the plating solution was found to be a critical parameter which was optimized to obtain films at controlled growth rate. An optimal concentration of selenous acid was i0 mH. At higher concentrations the films tended to peel off during growth. At lower concentrations the rate of film growth was very slow leading to unregular deposits of copper selenide. Thin films (up to 500 ~) were strongly adhering to the substrate and could not be lifted off. Thicker films that could be peeled off were made for XRD analysis in order to avoid interference of the support (Cu). The reaction of the Cu plates with H SeO may proceed as: 2 3 2 Cu ( 2 Cu + H SeO + 4 H÷ + 4 e" 2 3

> 2 Cu 2÷ + 4 e ) Cu Se + 3 H 0 2 2

(I) (2)

where the reactions 1 and 2 correspond to localised anodic and cathodic reactions, respectively, shorted together by i n t e r n a l cell currents. The overall process leads to the electroless synthesis of copper (1) selenide:

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4 Cu + H S e O + 4 H÷ 2 3

SELENIDES

> 2 Cu 2÷ + Cu Se + 3 H 0 2 2

Along w i t h the p r e v i o u s reactions, through r e a c t i o n s 4 and 5: H SeO 2

3

+ 6 H ÷ + 6 e-

2 H Se + H S e O 2

2

3

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the

formation

of

(3) Se

is

also

possible

> H Se + 3 H 0

(4)

) 3 Se + 3 H 0

(5)

2

2

2

However, reactions 4 and 5 are considered to be kinetically hindered at extreme pH v a l u e s and low c o n c e n t r a t i o n s of selenous acid (11). As we c o n d u c t e d our e x p e r i m e n t s at p H I using a low c o n c e n t r a t i o n of selenous acid, the above reactions do not occur, This reaction m e c h a n i s m has also been c o n f i r m e d by laser interferometry (12). EPMA was used for d e t e r m i n i n g the composition of the films. For very thin films (I0 to i00 nm) the p r o g r a m "STRATA" (SAMX company, France) was used. From the experimental results (EPMA) it is seen that Cu2-xSe w i t h x around 0.22 for a 30 min. d e p o s i t i o n (average of typically I0 points analyzed on films on Cu and d e t a c h e d flakes) is formed. Thicker films were not h o m o g e n e o u s in composition, showing a higher copper content near the substrate. This could be a c o n s e q u e n c e of increasing d i f f i c u l t y e n c o u n t e r e d by the e l e c t r o l y t e to reach the g r o w t h zone of the film. Figure 1 shows an e l e c t r o n m i c r o g r a p h for a Cu2-xSe film. The presence of small r o s e - l i k e arranged crystallites being oriented perpendicular to the surface is noticed.

FIG, 1 Scanning Electron Micrograph Deposition t i m e 30 m i n . ,

of Copper Selenide Film. magnification 4000 x

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I I I I I l i l l l l l l l i l l l l l l i l l l l i l J l l l t l l l l l l l l l l l l l i l | l l l l l l l l l l l l i l ; l l i l .

-r4

P~

I I I I I I I I I I |

I0

20

30

25

E6

27

2 theta /degree

28

I I I I I I I I

40

2 theta

50

60

70

/degree

FIG.2 XRD s p e c t r a o f c o p p e r s e l e n i d e f o r m e d by e l e c t r o l e s s deposition. lower spectrum: sample obtained without degassing the plating solution, Cu20 p e a k s a r e marked w i t h • u p p e r s p e c t r u m : s a m p l e o b t a i n e d by Ar d e g a s s i n g o f t h e p l a t i n g s o l u t i o n . inset: detail of upper curve;

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Figure 2 shows the XRD of copper selenide films formed in oxygen containing and oxygen free plating baths. The XRD spectrum which is obtained in the presence of oxygen can be attributed to the presence of a mixture of copper selenide and Cu20. The formation of the latter compound which was eliminated by continuously bubbling Ar was most probably caused by the oxidation of Cu by dissolved oxygen in the presence of sulfuric acid (5). Hence for the synthesis of pure copper selenide the removal of dissolved oxygen from the medium is essential. A large number of samples was prepared with and without Ar bubbling. Whereas samples prepared in the presence of oxygen yielded XRD diagrams with varying peak intensities due to varying content of Cu20, all samples produced by using oxygen-free baths gave identical diagrams. The XRD of such films compares well to the diffraction pattern of the orthorhombic phase of Cu (I) selenide as formed by hydrothermal growth (8,9) in contrast to copper selenide formed by most other methods (13-19). Minor contributions corresponding to additional peaks or shoulders on some main peaks were noticed. None of them corresponded to copper from the support or to copper oxides. TABLE 1 X - R a y Parameters of the Synthesised Films. hkl

d

/,$ observed

200 030 250 410 (060) 211 071 441 090 620-511 371 541-461 322-750 701 402 422 910 920-282 940 732-492 0112 2150 003-592 852 7130-9100 d

calculated

6.894 6.799 3.516 3.403 3.367 2.332 2,314 2.267 2.244 2,084 2,060 1,777 1,762 1.704 1.681 1.530 1.518 1.467 1.362 1.347 1.337 1.307 1.235 1.227

I/I

d ~' 0

3.9 17.3 13.4 21.1 40.0 5.2 3.1 15.6 6.2 3.1 I00.0 6.7 15.1 0.6 0.6 0.9 1.8 0.7 0.3 1.9 1.4 1,3 0, I 0,5

/A

ca I cu I at ed

6.904 6,798 3,512 3.403 3.364 2.339 2.310 2.266 2.245-2.244 2.085 2.065-2.061 1.777-1.776 1.762 1.705 1.682 1.530 1.517 1.469 1.363 1.347 1.334 1.308 1.235 1.228-1.226

was obtained by using a = 13.807 •, b = 2 0 . 3 9 3

A,

c

=

3.923

]k

Bragg peak positions were measured accurately with the program package "Diffrac-AT" (Socabim-Siemens, France) which also allowed to deconvolute merged peaks and shoulders and to obtain FWHM values and peak intensities. The deconvoluted XRD spectrum is formed of 24 single peaks which could be indexed with an orthorhombic cell. Using initial values of the lattice parameters

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reported in the JCPDS file (8) (a = 14.103 A, b = 20.402 A, and c = 4.133 A), a least square refinement for the XRD of a film with x = 0.22 gave the following parameters: a= 13.807 ± 0.005 A, b = 20.393 ± 0.005 A, and c = 3.923 ±0.005 A. The recalculated d-values differ from the measured ones only by few tenths of a percent and are listed in Table I. The intensities of the main Bragg peaks are somewhat different from the reported intensities (8). This may be due to the observed preferential orientation of the crystallltes. Without the doubling of the first peak an indexatlon with a= 6.995 ~, b= 20.417 ~, c= 4.033 A would be possible, similar to that reported by Stevels and Jellinek

(I0).

Conclusion A new electroless deposition technique for preparing thin films of copper (I) selenide on copper substrates at room temperature is presented. The constituting elements of the material are provided by the substrate (Cu) and by the electrolyte (selenous acid). The obtained copper deficient polycrystalline thin films (Cuz-×Se) show an orthorhombic structure. We shall compare the physical properties of these films with those of recently reported (I) thin films of cubic Cuz-xSe prepared by chemical bath deposition at 90°-95°C and study the influence of various annealing procedures on the thin films. The ease with which orthorhombic Cu (I) selenide is formed should be pointed out; the method does not require high vacuum nor heating or electricity. Contiguous films of good crystallinity are obtained. Preliminary results show that these thin films are promising precursors for the synthesis of copper indium selenide layers in photovoltaic cells. Acknowledgement We are grateful to M.Rommeluere for her assistance in performing the microprobe analysis, to C.Godart for providing the parameter refinement program, and to the French Foreign Office for funding travel expenses. References i. 2. 3. 4. 5. 6. 7. 8. 9.

I0. 11. 12. 13. 14. 15. 16.

H.Okimura, T.Matsumae, and R.Makabe, Thin Solid Films 71,53(1980) J.J.Loferski, 3.Appl. Phys. 27,777(1956) G.K.Padam, Thin Solid Films 150,L89(1987). T.L.Chu, S.S.Chu, S.C.Lin and J.Yue, J.Electrochem. Soc. 131,2182(1984) S.G.Ellis, J.Appl. Phys. 38,2906(1967), A. Tonejc, J.Mat. Science 15,3090(1980) W.Borchert, Z.Kristallogr. 106,5(1945) O.C.Kopp and O.B.Cavin, J.Crystal Growth 67,391(1984) File No. 37-1187, "Powder Diffraction File Alphabetical Index, Inorganic Phases", JCPDS International Center for Diffraction Data, Swarthmore, PA (1990) A.L.N.Stevels and F.Jellinek, Rec. Trav. Chim. Pays-Bas 90,273(1971) M.S.Kazacos and B.Miller, J.Electrochem. Soc. 127,869(1980) R.N.O'Brien and K.S.V.Santhanam, J.Electroanal. Chem. 260,213(1989) S.Kashida and 3.Akai, J.Phys. C: Solid State Phys. 21,5329(1988) R.M.Murray and R.D.Heyding, Can. J.Chem. 53,878(1975) Z.Vricik, O.Milat, V.Horvatic and Z.Orgorelec, Phys. Rev. B 24,5398(1981) B.Venglis, K.Valatska, N.Shiktor and V.Yasutis, Soy. Phys. Solid State 28,1499(1986)

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SELENIDES

17. N.A.Krushatina, A.N.Titor and Y.E.Turkhan, Real Structura i Sovistva Tved Tel Sverdlosk 89(1983) 18. M. Oliveria, R.K. McMullan and B.J. Wuenseh, Solid State Ionics 28-30(part 2),1332(1987) 19. O.Milat, Z.Vieie and B.Ruscic, Solid State Ionics 23,37(1987)

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