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Solution deposition of thin solid compound films by a successive ionic-layer adsorption and reaction process
Applications of Surface Science 22/23 (1985) 1061-1074 North-Holland, Amsterdam
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SOLUTION DEPOSITION OF THIN SOLID C O M P O U N D FILMS BY A SUCCESSIVE IONIC-LAYER ADSORPTION AND REACTION PROCESS * Y.F. N I C O L A U Laboratoire d'Eleetronique et de Technologie de l'Informatique, Cristallog~nkse et Recherche sur les Mat~riaux, IRDI-CEA, CEN. G, 85X, F-38041, Grenoble Cedex, France
Received 27 August 1984; accepted for publication 10 December 1984
The process is intended to grow polycrystalline or epitaxial thin films of water-insoluble ionic or ionocovalent compounds of the C,, A, type by heterogeneous chemical reaction at the solid--solution interface between adsorbed C"+ cations and A"- anions. The process involves an alternate immersion of the substrate in a solution containing a soluble salt of the cation of the compound to be grown and then in a solution containing a soluble salt of the anion. The substrate supporting the growing film is rinsed in high-purity deionized water after each immersion. Polycrystalline and epitaxial thin films of ZnS and CdS have been deposited following this process at room temperature on different substrates.
I. Introduction W e shall m e n t i o n two processes of thin film d e p o s i t i o n related to the process described here, n a m e l y : chemical bath d e p o s i t i o n a n d atomic layer epitaxy. C h e m i c a l bath d e p o s i t i o n is well k n o w n as a d e p o s i t i o n process for polycrystalline or a m o r p h o u s thin films of some metals such as Cu, Ag, A u or some chalcogenides such as Z n , Cd, Hg, Pb sulphides a n d selenides [1-9], a n d recently of some spinel ferrites [10]. In growing the thin films, a h e t e r o g e n e o u s growth m e c h a n i s m c o m p e t e s with a h o m o g e n e o u s one. F o r instance, for CdS deposition, the h e t e r o g e n e o u s growth m e c h a n i s m proceeds by chemical c o m b i n a t i o n of a d s o r b e d ions at the solid solution interface ( " i o n - b y - i o n " m e c h a n i s m [6]). This m e c h a n i s m gives a d h e r e n t m i r r o r - l i k e c o m p a c t films. T h e h o m o g e n e o u s growth m e c h a n i s m proceeds by s e d i m e n t a t i o n of CdS clusters f o r m e d in solution ( " c l u s t e r - b y - c l u s t e r " m e c h a n i s m [6]). This second m e c h a n i s m gives poorly a d h e r e n t , p o w d e r - l i k e p o r o u s films. T h e process is very easy to use, i n e x p e n s i v e a n d very fast; films * This work was performed under the CNET ec~ntract GR 771 146/15.11.83. 0378-5963/85/$03.30 O E l s e v i e r Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
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of 0.3-1.2/*m per immersion have been deposited during 10-60 min. But it seems impossible to suppress entirely' the homogeneous mechanism. The chalcogenides had the stoichiometric composition, but the thickness control was difficult and the films were contaminated by the counterions and bx other organic compounds present in the bath. Atomic layer epitaxy is a version of molecular beam epitaxy used for thc epitaxial growth of I I I - V or I I - V I semiconductor compounds. Following this process the films grow at the solid-vacuum interface by successive adsorption and chemical reaction of the compound atoms emitted alternatively from two sources [11,12]. This process gives epitaxial films of good quality and of controllable thickness, but the growth rate is slow, of the order of 0.1 #m/h. The equipment, the maintenance and the process are very expensive.
2. Process description Our process is intended to grow polycrystalline or epitaxial thin films of water-insoluble ionic or ioncovalent compounds of the (',,,A,, type bv heterogeneous chemical reactions at the solid-solution interface between adsorbed cations, [CLp] "+, and anions, [AL~]'" , following the reaction: m [ C L p ] " * + n[AL'q]" --~ C.,A. ~ + m p L +
nqL'
.
Here, Lp and L'q should be different ligands, but this is not a mandatory condition. The process involves an alternate immersion of the substrate in a solution containing a soluble salt of the cation of the compound to be grown and then in a solution containing a soluble salt of the anion of the compound to be grown. The substrate supporting the growing film is rinsed in high-purity deionized water after each immersion. In order to grow ternary or quaternary compounds, or doped compounds it is possible either to mix in solution the cations and/or anions of the compound to be grown or to introduce programmed immersions in solutions containing alternatively the cations and/or thc anions of the compound to bc grown. Oxygen may be supplied by adsorption of hydroxyl anions as a result of immersions in alkaline solutions and subsequent deprotonation by atlachment on the growing interface [13]. The pH of the solutions can be modilied by adding an acid or an alkali, but the resulting pH must be kept inside the pH domain allowing the sole deposition of the compound to be grown.
3. The growth mechanism The growth mechanism involves three most important steps: (i) specific adsorption of the most strongly adsorbed ions of the compound to be grown
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by substrate immersion in a solution of one of its salts, (ii) water rinsing of the excess solution still adhering to the substrate, and (iii) chemical reaction between the most strongly specifically adsorbed ions and the less strongly adsorbed ones by the subsequent substrate immersion in a solution containing the latter, entailing the growth. In the following, in view of a better understanding but without limiting the generality of the process, we shall discuss the growth mechanism taking as an example the growth of CdS films that we have studied. Aqueous solutions of C d S O 4 and Na2S were used as immersion baths. Let us consider first the growth of a CdS film on a single crystal CdS slice limited by (0001)Cd and (0001)S faces. By immersing the slice in the CdSO 4 solution, a specific adsorption of C d 2+ and C d O H + proceeds on the ((100]) face [14]; but in the following C d O H + are neglected. Saturation is reached after a certain time, which can be determined experimentally. A Helmholtz electrical double layer is formed at the solid-solution interface having C d 2+ in the inner Helmholtz plane (IHP) and SO ]- in the outer. At saturation the surface density of the adsorbed Cd 2+ depends only on the CdSO4 concentration in solution [14,15]. The Cd 2+ are likely adsorbed on their own future sites on the CdS lattice. But only a fraction of lattice Cd sites are covered by Cd 2+. By emersion of the CdS slide from the C d S O 4 solution, the electric double layer is kept unchanged as the emersion is hydrophilic, the thickness of the adherent solution layer being about 25/zm. Then, the CdS slide is dipped in flowing high-purity water. The adherent C d S O 4 solution layer is quickly washed-out by convection up to a limiting thickness known as the diffusion layer. C d 2+ and SO] diffuse from the diffusion layer into the flowing water and are eliminated. In order to avoid the homogeneous precipitation of CdS microcrystals in the diffusion layer at the next immersion in the Na2S solution, the rinsing time must be experimentally determined or calculated so that the residual activity of the C d 2+ in the diffusion layer [Cd~ +] should be [Cd~ +] < K s o [ H + ] / K I [ H S - ] . Here K~o ~ 10 -2~ is the solubility product of CdS, K 2 ~- 10-15 is the dissociation constant of HS-, [S2-] can be neglected [16]. [HS-] and [H +] are the ionic activities of HS- and of H + in the Na2S solution. If the specific adsorption of Cd 2+ is strong enough, it will be possible to reach this [Cd~ +] activity without desorption of Cd 2+. Note with [Cd~Hp] 2+ the activity of adsorbed C d 2+ after rinsing. During the next immersion in the Na2S solution, the HS , S 2 , O H - and Na + ions diffuse from the solution in the diffusion layer towards the solidsolution interface until their concentrations in the diffusion layer equal those in the bath. The immersion time can be experimentally determined (and/or calculated). The HS- and S 2 enter the outer Helmholtz layer, react with the adsorbed Cd 2+ and form CdS molecules likely attached by the Cd atom on the S interface if [CdiHP] 2+ < Kso/[S 2 ]. Then, possibly by surface diffusion, bi-dimensional nuclei of the CdS deposited layer are formed. The SO 2 diffuse back from the outer Helmholtz layer through the diffusion layer
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into the Na2S solution bath. A new electrical double layer is now formed by non-specifically adsorbed Na +, HS , S > and O H ions. HS- or S2 cannot be specifically adsorbed on the (0001)CdS face [14]. The hydrophilic emersion of the slide from the NaeS solution keeps the non-specific adsorbed electrical double layer also unchanged. By dipping the slide again in flowing high-purity water the adherent Na2S solution layer is also quickly washed-out by convection up to its limiting thickness, that of the diffusion layer. Now, Na +, HS , S2 and OH diffuse from the diffusion layer into the flowing water. As above, in order to avoid the homogeneous precipitation of CdS microcrystals at the next immersion in the CdSO 4 solution, the rinsing time must be experimentally determined or calculated so that the residual activity of HS in the diffusion layer, [HSr] should be [HS;] < K~[H+I/K,[Cd2+ l, lCd>l and [H+I being the activities of Cd > and H + in the CdSO4 solution bath. By the next immersion of the slide in the CdSO 4 solution a complete growth cycle is carried out. Cd 2+ and SO~ ions diffuse through the diffusion layer until their concentration in the layer equal those in the bath. The immersion time can in its turn be experimentally determined or calculated. The Cd 2+ are now again specifically adsorbed in the inner layer of thc Helmholtz electrical double layer. Cd > are likely to be partly adsorbed on the remaining free S atoms of the ((1001)CdS slide and partly at the steps of the bi-dimensional CdS nuclei. Na + will diffuse back through the diffusion layer toward the C d S 0 4 solution. After a few cycles, depending on the concentration of the solutions and on the immersion and rinsing times, the CdS slide becomes entirely covered by a CdS layer as a result of grain coalescence. Then the growth mechanism is repeated. On the (0001)CdS face, HS and/or S2 are specifically adsorbed [ 14,16] and subsequently Cd 2* cations react with them following a mechanism similar to that described above for the (0001)CdS face. The c-axis in hexagonal CdS is a polar axis. Consequently, thc specific adsorption of Cd 2+ on the (0001) face and of HS on the (0001) face must bc the strongest with respect to the Cd 2+ and HS adsorption on other crystallographic faces. But Cd 2+ and HS are also specifically adsorbed on other crystallographic faces. As a result the growth of CdS layers on other crystallographic faces should involve a growth mechanism similar to that described above. Nevertheless, smooth single crystal films arc expected as usual only on F crystallographic faces. Heteroepitaxial CdS layers can be" grown using this process (see further). The availability of a clean and flat substrate surface in aqueous solutions is of prime importance. The mechanism of the first CdS layer deposition will be different, depending on the substrate. With respect to the nature of the interface, substrate-electrolyte, many classes of substrates can be distinguished: clean metals, clean non-oxidic semiconductors, clean ionic non-oxidic insulators,
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oxides including oxidized metals and semiconductors, organic polymers and molecular crystals. The type of nucleation of the first CdS layer on the substrate and the CdS microcrystal habit determine the size and the orientation of the grains. In order to find suitable substrates for the deposition of a given compound and to select favourable adhesion and growth conditions for the first and subsequent layers of the compound to be grown, very useful information is found in the literature: (i) concerning the nature of the metal-electrolyte [17-20] and semiconductor-electrolyte [21-24] interfaces from electrochemistry literature, (ii) concerning adsorption at the oxideelectrolyte interface from the colloidal and interracial chemistry literature [13,25-27], and (rio concerning the growth mechanism and atomic structure of adsorbed layers from the surface physical chemistry literature [28-32]. Hydrophobic emersion of the substrate and/or of the film (no diffusion layer) should considerably increase the growth rate. It has been recently found that hydrophobic emersion of conductive electrodes of SnO 2, In203 and to a lesser extent Au, from different electrolytes keeps the electric double layer unchanged even at zero potential [33,34]. In order to strengthen the specific adsorption of ions from which the compound is synthesized, the substrate may be suitably polarized during immersion and during rinsing, if the substrate and the films are conductors or semiconductors. The substrate potential should be kept under the electrolyte decomposition potential. The crystal growth mechanism outlined above must be considered only as a rough picture of the atomic mechanism actually involved. Special studies are necessary in order to obtain quantitative data and a more precise understanding. However, two mechanism characteristics are evident: (i) at least one ion must be specifically adsorbed at the compound-electrolyte interface and at the substrate-electrolyte interface if the substrate has a different nature; (ii) the maximum growth rate per immersion cannot exceed the interreticular distance of the compound to be grown along the growth direction.
4. Equipment The equipment consists of four polypropylene beakers (1), of 250 ml each, containing the solutions and four rinsing vessels (2) lying in a circle on the circular PVC tray (3) of the machine (see fig. 1). The beakers and rinsing vessels are intercalated, each rinsing vessel being placed between a beaker containing a solution of a salt of the cation and another containing a solution of a salt of the anion (both, cations and anions, yielding the compound to be deposited).
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T w o or four substrates (4) are attached vertically by means of p o l y p r o p y lene tweezers on two or four P V C arms (5) set out in line or at right angles and s u p p o r t e d on the spindle (6). T h e spindle, of stainless steel, can turn and slide tightly in a Teflon bearing. It is driven by two stepping m o t o r s (7, 8). The vertical m o v e m e n t of the spindle is 1 0 0 m m , its rotation speed 6 rpm; the translation speed is adjustable between 1 and 2 cm/s.
8 7 Fig. 1. T h i n film d e p o s i t i o n m a c h i n e : 1 - b e a k e r s , 5 - a r m s , 6 - spindle, 7, 8 - m o t o r s , 9 - bell.
2-rinsing
vessels 3 - t r a y ,
4-substrates,
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The rinsing vessels contain transparent PVC rinsing beakers. They have a rectangular section of 20 x 50 mm. High-purity water having a resistivity of 18 M ~ cm at 25°C supplied by a deionization station passes through four rotameters and flows upwards in a laminary state inside the rinsing beakers and finally pours into the rinsing vessels. The water that overflows the beaker walls, having a mean resistivity higher than 1 M~2 cm is trapped inside the rinsing vessels, collected, purified and recirculated. The whole spindle with arms and substrates, beakers and rinsing vessels are all tightly enclosed by a transparent PVC bell (9). An inert gas passing through a rotameter flows inside the bell. Conductivity cells are placed on the tubes bringing the water into and out of the rinsing vessels. Inside of each beaker containing solutions it is possible to immerse specific and reference electrodes. Sliding contacts are mounted on the spindle which allow electric contacts to be made to each substrate. The machine is driven by means of an electronic programmer.
5. Experimental results Till now we have performed only an exploratory investigation on the feasibleness of thin film deposition according to the above-described process. CdS and ZnS have been chosen as compounds to be grown; Johnson Matthey Grade 1 CdSO4 and ZnSO 4 and Carlo Erba RPE-ACS Na2S aqueous solutions as electrolytes to supply Cd 2+, Zn 2+ and HS- ions. The following substrates have been tested: (i) single crystal slices of (lll)Si, ( l l l ) G e , (100)GaAs, {lll}InP, (Ill)CaF2, (0112)Ai203, (01]0)LiNbO3; (ii) glass and conducting SnO 2 covered glass slides; and (iii) polycrystalline Mo, Ti and Ta sheets. The technological parameters to be determined, which play an important role in a successful thin film deposition and which determine the growth rates, are: the concentrations of the salt solutions and their pH, the immersion times, the rinsing times related to the water flow rates and the hydrodynamics of the rinsing. The nature of the counter ions (viz. SO]- and Na+), the nature of the solvent, the presence of impurities giving more insoluble compounds than that to be deposited, the added complexing agents, the flow rate of the inert gas, the substrate polishing and etching, the value of the sulphide and substrate solubility also play a role. The concentration of the salt solutions should be high in order to promote the specific adsorption, but at the same time the rinsing time must be sufficient to entail ion diffusion from the diffusion layer. Optimum magnitudes for the solution concentration-rinsing time parameters should be experimentally determined. The p H of solutions play an important role. O H - may interfere with the specific anion adsorption entailing the con-
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tamination of the sulphide with hydroxide, may change the substrate surface state especially for oxides, and change the sulphide and substrate solubility. The counter ion should be weakly adsorbed and have a large dilfusiol: coefficient. The presence of different solvents added to the aqueous solutiom may change the solution viscosity, density, surface tension and solutiotl dielectric constant, entailing changes in the thickness of the adherent diffusion layer, in diffusion coefficients of ions, in the thickness of the Helmholtz layel and in adsorption coverage. The presence of impurities giving sulphides more insoluble than that to be deposited and having, in particular, an isomorphous structure, promotes a h o m o g e n e o u s adhesion to the substrate, a homogeneous film thickness and an increase of growth rate, as has previously been observed [1,3]. Added complexing agents introduce a supplementary activated kinetic step of decomplexation, thus increasing the surface relaxation time lhat promotes a regular growth [ 1]. But the specific adsorption of the complex ions might be decreased. The Na2S solution releases minute quantities of HeS owing to sulphide hydrolysis. In order to avoid CdS precipitation at the surface of the CdSO 4 solution the inert gas flow must be increased. The order of the immersions and the hydrophilic emersion speeds do not seem to play a role. Substrate polarization might play a role for clean metals and for clean low resistivity semiconductors. Illumination might also play a role for high resistivity semiconductors. Polycrystalline cubic (sphalerite) ZnS films of optical quality (homogeneous, adherent, compact, specularly reflecting), up to 2000-3000 ,~ in thickness, have been grown on Ge, GaAs, InP, AleO > LiNbO~ and glass using: Z n S O 4 (0.005M, pH = 3.9) and Na,S (0.005M, pH = 11.6) solutions, rinsing times of 80 s for a water flow rate of 30 l/h, immersion times of 40 s and N 2 flOW rate of 50 l/h. Owing to the hydrophilic emersion the adherent water layer dilutes the solutions. They were changed once a day. A mean growth rate of about 0.65 * per immersion has been observed. The microcrystals show the known sphalerite habit having probably {111} faces as the most important ones. Taking into account the interreticular distance of the sphalerite along the O [111] direction of 3.123A, a mean Zn > coverage of about 40/o can be tentatively deduced. The dominant growth mechanism probably implies first the Zn > adsorption on the {111} faces and subsequent reaction with HS . The polycrystalline non-oriented cubic ZnS phase was identified from its five diffraction rings shown on the R H E E D pattern. The 1 : 1 stoichiometry of the ZnS films was proved using the RBS method. Epitaxial ZnS films having different structures could also be observed among the experiments carried out under the above-mentioned growth conditions. Such ZnS films deposited on glass work well as light guides. Epitaxial CdS films of optical quality, up to 3000,~ in thickness, have been grown on ( l l l ) I n P , ( l l l ) G e and (100)GaAs using CdSO4 (0.{)05M, p H = 2.8) and Na2S (0.005M, p H = 11.6) solutions, rinsing times of l(10 s for
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a water flow rate of 30 l/h, immersion times of 40 s and a N 2 flow rate of 50 l/h. (0001)(Greenockite)CdS epitaxial films have been grown on ( l l l ) I n P , ( l l l ) C d S (Hawleyite) films on ( l l l ) G e , and (100)CdS films on (100)GaAs. Figs. 2a and 2b show two R H E E D patterns of the same hexagonal CdS films grown on ( l l l ) I n P . Fig. 2c shows a pattern of a (100)CdS epitaxial film grown on (100)GaAs, d~00CdS = 5.80/~, dl00GaAs = 5.654 A. Fig. 2d shows a pattern of a (I_ll)CdS epitaxial film grown on ( l l l ) G e , dmCdS = 3.36A, dl11Ge = 3.267/~. The pattern 2a shows the known epitaxial relation (000i)CdS II (iiT)InP, d0002CdS= 3.357 A, dllllnP = 3.388 ~ with (1120)CdS- H(01i)InP, dlt~oCdS = 2.068 A, d22olnP = 2.074/~. The pattern 2b shows three families of symmetricall_y equivalent oriented CdS microcrystals grown on (llT)InP, namely (0001)CdSII{lll}InP with {~032}Cd5 -II ( l l l ) I n P , d~032CdS= 1.125/~, d333InP = 1.129 A. The pattern 2a shows also another oriented CdS growth characterized by the following relations: (ll20)CdS II (220)InP, dH~0CdS o 2.068 ,g,, dz20InP 2.074 A with (1012)CdS - II(ll2)InP, di012CdS -- 2.450 A, d~2InP = 2.396 A. Thus the CdS film grown on ( l l l ) I n P shows differently oriented grains. It was found that the thickness of the epitaxial hexagonal CdS layers grown concomitantly under the same above-mentioned growth conditions after 870 cycles, was 1850 .~ on ( l l l ) I n P and on ( l l l ) G e , while only 1000 ,~ on (1Ti)InP and 1650/~ on (100)GaAs. A stronger specific adsorption of HS on (0001)CdS than of Cd 2+ on (0001)CdS was reported by Minoura et al. [14]. Based on Minoura's results, our proposed growth mechanism predicts thicker CdS films on ( l l l ) I n P than on (111), in agreement with the experiment. Inverted polarization between the layer and the substrate is improbable. Fig. 3 shows SEM microphotographs of epitaxial CdS layers. Some polycrystalline clusters, originating at the substrate-film interface, strongly depending on the substrate cleaning procedure, give diffraction rings on the R H E E D diagrams. The growth rate associated with the above-mentioned growth conditions was about 0.6,~, per immersion for the (0001)CdS face corresponding to about 20% HS- and S2- coverage. Using the same experimental conditions, polycrystalline hexagonal CdS films of optical quality have been obtained on LiNbO 3, SnO2-covered glass, Mo, Ta and Ti. ZnS and CdS films of poor quality, inhomogeneous and irreproducible have been obtained on Si and on CaF 2, possibly as a consequence of the irregular SiO z covering of the Si surface and of the large CaF 2 solubility. As a result of our exploratory investigation, the growth rate giving optical quality films is about 0.002/zm/h. This means a period of 6 days in order to deposit 2500,~ of ZnS or CdS. As compared with the growth rate of the atomic layer epitaxy process, our process growth rate is 50 times lower. By
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Fig. 2. R H E E D patterns: (a) and (b) hexagonal CdS grown on (111)InP, (c) (100)CdS grown on (100)GaAs, (d) (lll)CdS grown on (lll)Ge.
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Fig. 3. SEM microphotographs of epitaxial CdS: (a) 1850 ,~ (I)01)T)CdS on (]'TT)InP, (b) 1650/~ (I(X))CdS on GaAs. The bar lines represent 1.0 ~m.
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i n c r e a s i n g t h e w a t e r flow r a t e a n i n c r e a s e of t h e m e a n g r o w t h r a t e u p to 0.01 ~ m / h m i g h t b e e x p e c t e d . B u t b y i n c r e a s i n g t h e s u b s t r a t e d i m e n s i o n s a n d t h e i r n u m b e r , w h i c h i n v o l v e s o n l y t h e i n c r e a s e of t h e v o l u m e s of t h e vessels, it s h o u l d b e p o s s i b l e to r e a c h g r o w t h r a t e s p e r u n i t a r e a of t h e s a m e o r d e r of m a g n i t u d e as with o t h e r thin film d e p o s i t i o n p r o c e s s e s - i n p a r t i cular vacuum deposition.
Acknowledgements T h e a u t h o r a c k n o w l e d g e s t h e C N E T for p e r m i s s i o n to p u b l i s h this p a p e r . T h e a u t h o r is i n d e b t e d to C. R o u l i n a n d J.C. R e b r e y e n d for t h e d e s i g n a n d c o n s t r u c t i o n of t h e e q u i p m e n t , to M. D u p u y for R H E E D p a t t e r n i n t e r p r e t a t i o n , to J.C. M e n a r d , for his t e c h n i c a l a s s i s t a n c e , a n d to J.C. P e u z i n for v a l u a b l e c o m m e n t s a n d for his h e l p in s t a r t i n g this r e s e a r c h p r o g r a m .
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[24] V.V. Milenin, V.E. Primachenko, N.A. Rastrenenko, O.V. Snitko and N.M. Torchun, lzv. Akad. Nauk SSSR, Neorg. Mater. 18 (1982) 192. [25] G.A. Parks and I.L. de Bruyn, J. Phys. Chem. 66 (1962) 967. [26] D.E. Yates, S. Levine and Th.W. Healy, J. Chem. Soc. Faraday Trans. 1, 70 (1974) 1807. [27] G.A. Parks, Chem. Rev. 65 (1965) 177. [28] S.R. Morrison, The Chemical Physics of Surfaces (Plenum, New York, 1977). [29] A.W. Adamson, Physical Chemistry of Surfaces, 4th ed. (Wiley, New York, 1982). [30] J.L. Domange, J. Oudar and J. Benard, in: Molecular Processes on Solid Surfaces, Eds. E. Drauglis, R.D. Gretz and R.I. Jaffe (McGraw-Hill, New York, 1969) p. 353. [31] J. Oudar, in: Proc. 4th Intern. Conf. on Solid Surfaces and 3rd European Conf. on Surface Science, Cannes, 1980, Vol. 1, Suppl. Le Vide, Les Couches Minces 201 (1980) p. 645. [32] L. Ter-Minnassian-Saraga, in: Contact Angle, Wettability and Adhesion, Advances in Chemistry Series, Vol. 43, Ed. R.F. Gould (Am. Chem. Soc.. Washington, DC, 1964). [33] W.N. Hansen, J. Electroanal. Chem. 150 (1983) 133. [34] G.J. Hansen and W.N. Hansen, J. Electroanal. Chem. 150 (1983) 193.
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SOLUTION DEPOSITION OF THIN SOLID C O M P O U N D FILMS BY A SUCCESSIVE IONIC-LAYER ADSORPTION AND REACTION PROCESS * Y.F. N I C O L A U Laboratoire d'Eleetronique et de Technologie de l'Informatique, Cristallog~nkse et Recherche sur les Mat~riaux, IRDI-CEA, CEN. G, 85X, F-38041, Grenoble Cedex, France
Received 27 August 1984; accepted for publication 10 December 1984
The process is intended to grow polycrystalline or epitaxial thin films of water-insoluble ionic or ionocovalent compounds of the C,, A, type by heterogeneous chemical reaction at the solid--solution interface between adsorbed C"+ cations and A"- anions. The process involves an alternate immersion of the substrate in a solution containing a soluble salt of the cation of the compound to be grown and then in a solution containing a soluble salt of the anion. The substrate supporting the growing film is rinsed in high-purity deionized water after each immersion. Polycrystalline and epitaxial thin films of ZnS and CdS have been deposited following this process at room temperature on different substrates.
I. Introduction W e shall m e n t i o n two processes of thin film d e p o s i t i o n related to the process described here, n a m e l y : chemical bath d e p o s i t i o n a n d atomic layer epitaxy. C h e m i c a l bath d e p o s i t i o n is well k n o w n as a d e p o s i t i o n process for polycrystalline or a m o r p h o u s thin films of some metals such as Cu, Ag, A u or some chalcogenides such as Z n , Cd, Hg, Pb sulphides a n d selenides [1-9], a n d recently of some spinel ferrites [10]. In growing the thin films, a h e t e r o g e n e o u s growth m e c h a n i s m c o m p e t e s with a h o m o g e n e o u s one. F o r instance, for CdS deposition, the h e t e r o g e n e o u s growth m e c h a n i s m proceeds by chemical c o m b i n a t i o n of a d s o r b e d ions at the solid solution interface ( " i o n - b y - i o n " m e c h a n i s m [6]). This m e c h a n i s m gives a d h e r e n t m i r r o r - l i k e c o m p a c t films. T h e h o m o g e n e o u s growth m e c h a n i s m proceeds by s e d i m e n t a t i o n of CdS clusters f o r m e d in solution ( " c l u s t e r - b y - c l u s t e r " m e c h a n i s m [6]). This second m e c h a n i s m gives poorly a d h e r e n t , p o w d e r - l i k e p o r o u s films. T h e process is very easy to use, i n e x p e n s i v e a n d very fast; films * This work was performed under the CNET ec~ntract GR 771 146/15.11.83. 0378-5963/85/$03.30 O E l s e v i e r Science Publishers B.V. ( N o r t h - H o l l a n d Physics P u b l i s h i n g Division)
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of 0.3-1.2/*m per immersion have been deposited during 10-60 min. But it seems impossible to suppress entirely' the homogeneous mechanism. The chalcogenides had the stoichiometric composition, but the thickness control was difficult and the films were contaminated by the counterions and bx other organic compounds present in the bath. Atomic layer epitaxy is a version of molecular beam epitaxy used for thc epitaxial growth of I I I - V or I I - V I semiconductor compounds. Following this process the films grow at the solid-vacuum interface by successive adsorption and chemical reaction of the compound atoms emitted alternatively from two sources [11,12]. This process gives epitaxial films of good quality and of controllable thickness, but the growth rate is slow, of the order of 0.1 #m/h. The equipment, the maintenance and the process are very expensive.
2. Process description Our process is intended to grow polycrystalline or epitaxial thin films of water-insoluble ionic or ioncovalent compounds of the (',,,A,, type bv heterogeneous chemical reactions at the solid-solution interface between adsorbed cations, [CLp] "+, and anions, [AL~]'" , following the reaction: m [ C L p ] " * + n[AL'q]" --~ C.,A. ~ + m p L +
nqL'
.
Here, Lp and L'q should be different ligands, but this is not a mandatory condition. The process involves an alternate immersion of the substrate in a solution containing a soluble salt of the cation of the compound to be grown and then in a solution containing a soluble salt of the anion of the compound to be grown. The substrate supporting the growing film is rinsed in high-purity deionized water after each immersion. In order to grow ternary or quaternary compounds, or doped compounds it is possible either to mix in solution the cations and/or anions of the compound to be grown or to introduce programmed immersions in solutions containing alternatively the cations and/or thc anions of the compound to bc grown. Oxygen may be supplied by adsorption of hydroxyl anions as a result of immersions in alkaline solutions and subsequent deprotonation by atlachment on the growing interface [13]. The pH of the solutions can be modilied by adding an acid or an alkali, but the resulting pH must be kept inside the pH domain allowing the sole deposition of the compound to be grown.
3. The growth mechanism The growth mechanism involves three most important steps: (i) specific adsorption of the most strongly adsorbed ions of the compound to be grown
Y.F. Nicolau / Solution deposition of thin solid compound films
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by substrate immersion in a solution of one of its salts, (ii) water rinsing of the excess solution still adhering to the substrate, and (iii) chemical reaction between the most strongly specifically adsorbed ions and the less strongly adsorbed ones by the subsequent substrate immersion in a solution containing the latter, entailing the growth. In the following, in view of a better understanding but without limiting the generality of the process, we shall discuss the growth mechanism taking as an example the growth of CdS films that we have studied. Aqueous solutions of C d S O 4 and Na2S were used as immersion baths. Let us consider first the growth of a CdS film on a single crystal CdS slice limited by (0001)Cd and (0001)S faces. By immersing the slice in the CdSO 4 solution, a specific adsorption of C d 2+ and C d O H + proceeds on the ((100]) face [14]; but in the following C d O H + are neglected. Saturation is reached after a certain time, which can be determined experimentally. A Helmholtz electrical double layer is formed at the solid-solution interface having C d 2+ in the inner Helmholtz plane (IHP) and SO ]- in the outer. At saturation the surface density of the adsorbed Cd 2+ depends only on the CdSO4 concentration in solution [14,15]. The Cd 2+ are likely adsorbed on their own future sites on the CdS lattice. But only a fraction of lattice Cd sites are covered by Cd 2+. By emersion of the CdS slide from the C d S O 4 solution, the electric double layer is kept unchanged as the emersion is hydrophilic, the thickness of the adherent solution layer being about 25/zm. Then, the CdS slide is dipped in flowing high-purity water. The adherent C d S O 4 solution layer is quickly washed-out by convection up to a limiting thickness known as the diffusion layer. C d 2+ and SO] diffuse from the diffusion layer into the flowing water and are eliminated. In order to avoid the homogeneous precipitation of CdS microcrystals in the diffusion layer at the next immersion in the Na2S solution, the rinsing time must be experimentally determined or calculated so that the residual activity of the C d 2+ in the diffusion layer [Cd~ +] should be [Cd~ +] < K s o [ H + ] / K I [ H S - ] . Here K~o ~ 10 -2~ is the solubility product of CdS, K 2 ~- 10-15 is the dissociation constant of HS-, [S2-] can be neglected [16]. [HS-] and [H +] are the ionic activities of HS- and of H + in the Na2S solution. If the specific adsorption of Cd 2+ is strong enough, it will be possible to reach this [Cd~ +] activity without desorption of Cd 2+. Note with [Cd~Hp] 2+ the activity of adsorbed C d 2+ after rinsing. During the next immersion in the Na2S solution, the HS , S 2 , O H - and Na + ions diffuse from the solution in the diffusion layer towards the solidsolution interface until their concentrations in the diffusion layer equal those in the bath. The immersion time can be experimentally determined (and/or calculated). The HS- and S 2 enter the outer Helmholtz layer, react with the adsorbed Cd 2+ and form CdS molecules likely attached by the Cd atom on the S interface if [CdiHP] 2+ < Kso/[S 2 ]. Then, possibly by surface diffusion, bi-dimensional nuclei of the CdS deposited layer are formed. The SO 2 diffuse back from the outer Helmholtz layer through the diffusion layer
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Y.F. Nicolau / Solution deposition of thin solid compound films
into the Na2S solution bath. A new electrical double layer is now formed by non-specifically adsorbed Na +, HS , S > and O H ions. HS- or S2 cannot be specifically adsorbed on the (0001)CdS face [14]. The hydrophilic emersion of the slide from the NaeS solution keeps the non-specific adsorbed electrical double layer also unchanged. By dipping the slide again in flowing high-purity water the adherent Na2S solution layer is also quickly washed-out by convection up to its limiting thickness, that of the diffusion layer. Now, Na +, HS , S2 and OH diffuse from the diffusion layer into the flowing water. As above, in order to avoid the homogeneous precipitation of CdS microcrystals at the next immersion in the CdSO 4 solution, the rinsing time must be experimentally determined or calculated so that the residual activity of HS in the diffusion layer, [HSr] should be [HS;] < K~[H+I/K,[Cd2+ l, lCd>l and [H+I being the activities of Cd > and H + in the CdSO4 solution bath. By the next immersion of the slide in the CdSO 4 solution a complete growth cycle is carried out. Cd 2+ and SO~ ions diffuse through the diffusion layer until their concentration in the layer equal those in the bath. The immersion time can in its turn be experimentally determined or calculated. The Cd 2+ are now again specifically adsorbed in the inner layer of thc Helmholtz electrical double layer. Cd > are likely to be partly adsorbed on the remaining free S atoms of the ((1001)CdS slide and partly at the steps of the bi-dimensional CdS nuclei. Na + will diffuse back through the diffusion layer toward the C d S 0 4 solution. After a few cycles, depending on the concentration of the solutions and on the immersion and rinsing times, the CdS slide becomes entirely covered by a CdS layer as a result of grain coalescence. Then the growth mechanism is repeated. On the (0001)CdS face, HS and/or S2 are specifically adsorbed [ 14,16] and subsequently Cd 2* cations react with them following a mechanism similar to that described above for the (0001)CdS face. The c-axis in hexagonal CdS is a polar axis. Consequently, thc specific adsorption of Cd 2+ on the (0001) face and of HS on the (0001) face must bc the strongest with respect to the Cd 2+ and HS adsorption on other crystallographic faces. But Cd 2+ and HS are also specifically adsorbed on other crystallographic faces. As a result the growth of CdS layers on other crystallographic faces should involve a growth mechanism similar to that described above. Nevertheless, smooth single crystal films arc expected as usual only on F crystallographic faces. Heteroepitaxial CdS layers can be" grown using this process (see further). The availability of a clean and flat substrate surface in aqueous solutions is of prime importance. The mechanism of the first CdS layer deposition will be different, depending on the substrate. With respect to the nature of the interface, substrate-electrolyte, many classes of substrates can be distinguished: clean metals, clean non-oxidic semiconductors, clean ionic non-oxidic insulators,
Y.F Nicolau / Solution deposition of thin solid compound films
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oxides including oxidized metals and semiconductors, organic polymers and molecular crystals. The type of nucleation of the first CdS layer on the substrate and the CdS microcrystal habit determine the size and the orientation of the grains. In order to find suitable substrates for the deposition of a given compound and to select favourable adhesion and growth conditions for the first and subsequent layers of the compound to be grown, very useful information is found in the literature: (i) concerning the nature of the metal-electrolyte [17-20] and semiconductor-electrolyte [21-24] interfaces from electrochemistry literature, (ii) concerning adsorption at the oxideelectrolyte interface from the colloidal and interracial chemistry literature [13,25-27], and (rio concerning the growth mechanism and atomic structure of adsorbed layers from the surface physical chemistry literature [28-32]. Hydrophobic emersion of the substrate and/or of the film (no diffusion layer) should considerably increase the growth rate. It has been recently found that hydrophobic emersion of conductive electrodes of SnO 2, In203 and to a lesser extent Au, from different electrolytes keeps the electric double layer unchanged even at zero potential [33,34]. In order to strengthen the specific adsorption of ions from which the compound is synthesized, the substrate may be suitably polarized during immersion and during rinsing, if the substrate and the films are conductors or semiconductors. The substrate potential should be kept under the electrolyte decomposition potential. The crystal growth mechanism outlined above must be considered only as a rough picture of the atomic mechanism actually involved. Special studies are necessary in order to obtain quantitative data and a more precise understanding. However, two mechanism characteristics are evident: (i) at least one ion must be specifically adsorbed at the compound-electrolyte interface and at the substrate-electrolyte interface if the substrate has a different nature; (ii) the maximum growth rate per immersion cannot exceed the interreticular distance of the compound to be grown along the growth direction.
4. Equipment The equipment consists of four polypropylene beakers (1), of 250 ml each, containing the solutions and four rinsing vessels (2) lying in a circle on the circular PVC tray (3) of the machine (see fig. 1). The beakers and rinsing vessels are intercalated, each rinsing vessel being placed between a beaker containing a solution of a salt of the cation and another containing a solution of a salt of the anion (both, cations and anions, yielding the compound to be deposited).
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Y.F. Nicolau / Solution deposition of thin solid compound [i/ms
T w o or four substrates (4) are attached vertically by means of p o l y p r o p y lene tweezers on two or four P V C arms (5) set out in line or at right angles and s u p p o r t e d on the spindle (6). T h e spindle, of stainless steel, can turn and slide tightly in a Teflon bearing. It is driven by two stepping m o t o r s (7, 8). The vertical m o v e m e n t of the spindle is 1 0 0 m m , its rotation speed 6 rpm; the translation speed is adjustable between 1 and 2 cm/s.
8 7 Fig. 1. T h i n film d e p o s i t i o n m a c h i n e : 1 - b e a k e r s , 5 - a r m s , 6 - spindle, 7, 8 - m o t o r s , 9 - bell.
2-rinsing
vessels 3 - t r a y ,
4-substrates,
Y.F. Nicolau / Solution deposition of thin solid compound films
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The rinsing vessels contain transparent PVC rinsing beakers. They have a rectangular section of 20 x 50 mm. High-purity water having a resistivity of 18 M ~ cm at 25°C supplied by a deionization station passes through four rotameters and flows upwards in a laminary state inside the rinsing beakers and finally pours into the rinsing vessels. The water that overflows the beaker walls, having a mean resistivity higher than 1 M~2 cm is trapped inside the rinsing vessels, collected, purified and recirculated. The whole spindle with arms and substrates, beakers and rinsing vessels are all tightly enclosed by a transparent PVC bell (9). An inert gas passing through a rotameter flows inside the bell. Conductivity cells are placed on the tubes bringing the water into and out of the rinsing vessels. Inside of each beaker containing solutions it is possible to immerse specific and reference electrodes. Sliding contacts are mounted on the spindle which allow electric contacts to be made to each substrate. The machine is driven by means of an electronic programmer.
5. Experimental results Till now we have performed only an exploratory investigation on the feasibleness of thin film deposition according to the above-described process. CdS and ZnS have been chosen as compounds to be grown; Johnson Matthey Grade 1 CdSO4 and ZnSO 4 and Carlo Erba RPE-ACS Na2S aqueous solutions as electrolytes to supply Cd 2+, Zn 2+ and HS- ions. The following substrates have been tested: (i) single crystal slices of (lll)Si, ( l l l ) G e , (100)GaAs, {lll}InP, (Ill)CaF2, (0112)Ai203, (01]0)LiNbO3; (ii) glass and conducting SnO 2 covered glass slides; and (iii) polycrystalline Mo, Ti and Ta sheets. The technological parameters to be determined, which play an important role in a successful thin film deposition and which determine the growth rates, are: the concentrations of the salt solutions and their pH, the immersion times, the rinsing times related to the water flow rates and the hydrodynamics of the rinsing. The nature of the counter ions (viz. SO]- and Na+), the nature of the solvent, the presence of impurities giving more insoluble compounds than that to be deposited, the added complexing agents, the flow rate of the inert gas, the substrate polishing and etching, the value of the sulphide and substrate solubility also play a role. The concentration of the salt solutions should be high in order to promote the specific adsorption, but at the same time the rinsing time must be sufficient to entail ion diffusion from the diffusion layer. Optimum magnitudes for the solution concentration-rinsing time parameters should be experimentally determined. The p H of solutions play an important role. O H - may interfere with the specific anion adsorption entailing the con-
1068
Y.F. Nicolau / Solution deposition of thin solid compound films
tamination of the sulphide with hydroxide, may change the substrate surface state especially for oxides, and change the sulphide and substrate solubility. The counter ion should be weakly adsorbed and have a large dilfusiol: coefficient. The presence of different solvents added to the aqueous solutiom may change the solution viscosity, density, surface tension and solutiotl dielectric constant, entailing changes in the thickness of the adherent diffusion layer, in diffusion coefficients of ions, in the thickness of the Helmholtz layel and in adsorption coverage. The presence of impurities giving sulphides more insoluble than that to be deposited and having, in particular, an isomorphous structure, promotes a h o m o g e n e o u s adhesion to the substrate, a homogeneous film thickness and an increase of growth rate, as has previously been observed [1,3]. Added complexing agents introduce a supplementary activated kinetic step of decomplexation, thus increasing the surface relaxation time lhat promotes a regular growth [ 1]. But the specific adsorption of the complex ions might be decreased. The Na2S solution releases minute quantities of HeS owing to sulphide hydrolysis. In order to avoid CdS precipitation at the surface of the CdSO 4 solution the inert gas flow must be increased. The order of the immersions and the hydrophilic emersion speeds do not seem to play a role. Substrate polarization might play a role for clean metals and for clean low resistivity semiconductors. Illumination might also play a role for high resistivity semiconductors. Polycrystalline cubic (sphalerite) ZnS films of optical quality (homogeneous, adherent, compact, specularly reflecting), up to 2000-3000 ,~ in thickness, have been grown on Ge, GaAs, InP, AleO > LiNbO~ and glass using: Z n S O 4 (0.005M, pH = 3.9) and Na,S (0.005M, pH = 11.6) solutions, rinsing times of 80 s for a water flow rate of 30 l/h, immersion times of 40 s and N 2 flOW rate of 50 l/h. Owing to the hydrophilic emersion the adherent water layer dilutes the solutions. They were changed once a day. A mean growth rate of about 0.65 * per immersion has been observed. The microcrystals show the known sphalerite habit having probably {111} faces as the most important ones. Taking into account the interreticular distance of the sphalerite along the O [111] direction of 3.123A, a mean Zn > coverage of about 40/o can be tentatively deduced. The dominant growth mechanism probably implies first the Zn > adsorption on the {111} faces and subsequent reaction with HS . The polycrystalline non-oriented cubic ZnS phase was identified from its five diffraction rings shown on the R H E E D pattern. The 1 : 1 stoichiometry of the ZnS films was proved using the RBS method. Epitaxial ZnS films having different structures could also be observed among the experiments carried out under the above-mentioned growth conditions. Such ZnS films deposited on glass work well as light guides. Epitaxial CdS films of optical quality, up to 3000,~ in thickness, have been grown on ( l l l ) I n P , ( l l l ) G e and (100)GaAs using CdSO4 (0.{)05M, p H = 2.8) and Na2S (0.005M, p H = 11.6) solutions, rinsing times of l(10 s for
Y.F. Nicolau / Solution deposition of thin solid compound films
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a water flow rate of 30 l/h, immersion times of 40 s and a N 2 flow rate of 50 l/h. (0001)(Greenockite)CdS epitaxial films have been grown on ( l l l ) I n P , ( l l l ) C d S (Hawleyite) films on ( l l l ) G e , and (100)CdS films on (100)GaAs. Figs. 2a and 2b show two R H E E D patterns of the same hexagonal CdS films grown on ( l l l ) I n P . Fig. 2c shows a pattern of a (100)CdS epitaxial film grown on (100)GaAs, d~00CdS = 5.80/~, dl00GaAs = 5.654 A. Fig. 2d shows a pattern of a (I_ll)CdS epitaxial film grown on ( l l l ) G e , dmCdS = 3.36A, dl11Ge = 3.267/~. The pattern 2a shows the known epitaxial relation (000i)CdS II (iiT)InP, d0002CdS= 3.357 A, dllllnP = 3.388 ~ with (1120)CdS- H(01i)InP, dlt~oCdS = 2.068 A, d22olnP = 2.074/~. The pattern 2b shows three families of symmetricall_y equivalent oriented CdS microcrystals grown on (llT)InP, namely (0001)CdSII{lll}InP with {~032}Cd5 -II ( l l l ) I n P , d~032CdS= 1.125/~, d333InP = 1.129 A. The pattern 2a shows also another oriented CdS growth characterized by the following relations: (ll20)CdS II (220)InP, dH~0CdS o 2.068 ,g,, dz20InP 2.074 A with (1012)CdS - II(ll2)InP, di012CdS -- 2.450 A, d~2InP = 2.396 A. Thus the CdS film grown on ( l l l ) I n P shows differently oriented grains. It was found that the thickness of the epitaxial hexagonal CdS layers grown concomitantly under the same above-mentioned growth conditions after 870 cycles, was 1850 .~ on ( l l l ) I n P and on ( l l l ) G e , while only 1000 ,~ on (1Ti)InP and 1650/~ on (100)GaAs. A stronger specific adsorption of HS on (0001)CdS than of Cd 2+ on (0001)CdS was reported by Minoura et al. [14]. Based on Minoura's results, our proposed growth mechanism predicts thicker CdS films on ( l l l ) I n P than on (111), in agreement with the experiment. Inverted polarization between the layer and the substrate is improbable. Fig. 3 shows SEM microphotographs of epitaxial CdS layers. Some polycrystalline clusters, originating at the substrate-film interface, strongly depending on the substrate cleaning procedure, give diffraction rings on the R H E E D diagrams. The growth rate associated with the above-mentioned growth conditions was about 0.6,~, per immersion for the (0001)CdS face corresponding to about 20% HS- and S2- coverage. Using the same experimental conditions, polycrystalline hexagonal CdS films of optical quality have been obtained on LiNbO 3, SnO2-covered glass, Mo, Ta and Ti. ZnS and CdS films of poor quality, inhomogeneous and irreproducible have been obtained on Si and on CaF 2, possibly as a consequence of the irregular SiO z covering of the Si surface and of the large CaF 2 solubility. As a result of our exploratory investigation, the growth rate giving optical quality films is about 0.002/zm/h. This means a period of 6 days in order to deposit 2500,~ of ZnS or CdS. As compared with the growth rate of the atomic layer epitaxy process, our process growth rate is 50 times lower. By
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Y.F. Nicolau / Solution deposition of thin solid compound films
Y.F. Nicolau / Solution deposition of thin solid compound films
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Fig. 2. R H E E D patterns: (a) and (b) hexagonal CdS grown on (111)InP, (c) (100)CdS grown on (100)GaAs, (d) (lll)CdS grown on (lll)Ge.
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Y.F. Nicolau / Solution deposition of thin solid compound films
Fig. 3. SEM microphotographs of epitaxial CdS: (a) 1850 ,~ (I)01)T)CdS on (]'TT)InP, (b) 1650/~ (I(X))CdS on GaAs. The bar lines represent 1.0 ~m.
Y.F. Nicolau / Solution deposition of thin solid compound films
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i n c r e a s i n g t h e w a t e r flow r a t e a n i n c r e a s e of t h e m e a n g r o w t h r a t e u p to 0.01 ~ m / h m i g h t b e e x p e c t e d . B u t b y i n c r e a s i n g t h e s u b s t r a t e d i m e n s i o n s a n d t h e i r n u m b e r , w h i c h i n v o l v e s o n l y t h e i n c r e a s e of t h e v o l u m e s of t h e vessels, it s h o u l d b e p o s s i b l e to r e a c h g r o w t h r a t e s p e r u n i t a r e a of t h e s a m e o r d e r of m a g n i t u d e as with o t h e r thin film d e p o s i t i o n p r o c e s s e s - i n p a r t i cular vacuum deposition.
Acknowledgements T h e a u t h o r a c k n o w l e d g e s t h e C N E T for p e r m i s s i o n to p u b l i s h this p a p e r . T h e a u t h o r is i n d e b t e d to C. R o u l i n a n d J.C. R e b r e y e n d for t h e d e s i g n a n d c o n s t r u c t i o n of t h e e q u i p m e n t , to M. D u p u y for R H E E D p a t t e r n i n t e r p r e t a t i o n , to J.C. M e n a r d , for his t e c h n i c a l a s s i s t a n c e , a n d to J.C. P e u z i n for v a l u a b l e c o m m e n t s a n d for his h e l p in s t a r t i n g this r e s e a r c h p r o g r a m .
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23]
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Y.F. Nicolau / Solution deposition of thin solid compound films
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