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Layer-by-layer electrodeposition of cadmium telluride onto silicon
j ........
ELSEVIER
CRYSTAL (]IRPOWTH
Journal of Crystal Growth 159 (1996) 200-204
Layer-by-layer electrodeposition of cadmium telluride onto silicon F. Jackson, L.E.A. Berlouis *, P. Rocabois Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK
Abstract
The electrodeposition of thin films of CdTe on n-Si{111} has been carded out using the method of continuous cycling of the potential from the open circuit value to that just positive of the cadmium deposition potential. Layer-by-layer growth, verified by in situ ellipsometry, is effected, with each cycle contributing some 50-60/~ to the CdTe film on the Si substrate. A slight dissolution of the film at the end of each anodic sweep appears to ensure growth uniformity of the layers. X-ray diffraction shows a preferred orientation of the polycrystalline CdTe film along the {111} and {311} directions. Elemental analysis of the layers reveal an excess in Te (Cd:Te = 4:6) which could arise as a consequence of Te growth being favoured over that of CdTe at low overpotentials during cycling. I. Introduction
Cadmium telluride thin films are currently employed in various optoelectronic devices such as solar cells [1-7], infrared devices [8-10] and also for 3,- and X-ray detection [11]. The direct growth of CdTe on silicon [12-16] could lead to an important technological advance, for example in hybridisation with silicon based microcircuitry, particularly in the development of infrared focal plane arrays and solar cells [17]. In this respect, the method of electrochemical deposition [3,5,7,10,15,16,18,19] of CdTe thin films offers an alternative economic route to the high costs of epitaxial techniques of metalorganic vapour phase epitaxy (MOVPE) [20,21], liquid phase epitaxy (LPE) [22,23] and molecular beam epitaxy (MBE) [12-14,24,25] currently used for example, in producing lattice matched substrates for the infrared detector material, CdxHgt_xTe (CMT). Several difficulties are encountered when using * Corresponding author.
silicon as a substrate for CdTe thin film growth. Firstly, the lattice mismatch between CdTe and silicon is large at 19%, although with MBE growth, this has been partially overcome by varying the orientation of the Si substrate to reduce this to 3.4% [12-14]. Secondly, there is also a relatively large thermal mismatch between CdTe and silicon, which could cause cracking or peeling of the films due to the wide temperature changes involved in fabrication or device operation. Thirdly and, more pertinent to electrochemical growth, is the presence of the insulating surface oxide which must be removed by chemical etching with either HF or ammonium fluoride. The latter has been successfully employed by Sugimoto et al. [16] to yield an atomically smooth surface. Finally, the growth of CdTe from aqueous solutions under illumination has been observed to lead to the formation of an instantaneous tellurium layer on the silicon [15]. In the present work, the growth of cadmium telluride onto n-type Si{111} using a layer-by-layer growth method is described and the effect of illumination on the electrodeposi-
0022-0248/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD! 0 0 2 2 - 0 2 4 8 ( 9 5 ) 0 0 8 19-5
F. Jackson et at./Journal of Crystal Growth 159 (1996) 200-204
tion is examined. The electrodeposited films were characterised using scanning electron microscopy (SEM), energy dispersive analysis of X-rays (EDAX) and X-ray powder diffraction (XRD).
was used with a saturated (KCI) calomel reference electrode (SCE) and a platinum gauze as a secondary electrode. For the in situ ellipsometry experiments, a Gaertner L116B Ellipsometer was used with light of o wavelength 6328 A, incident on the sample surface at an angle of 70° in the cell previously described [26]. The as-deposited films were annealed in an inert atmosphere, at 400°C for 15 min. At the point of sample insertion into the solution, the potential was set at - 0 . 2 V versus SCE [16] to minimise the formation of the spontaneous Te layer. The main method of electrodeposition for the CdTe layer then examined was that of continuous cycling between the open circuit potential (about - 0 . 2 V) to a potential just positive of cadmium deposition (typically - 0 . 6 6 V) although the method of a single potential step to - 0 . 6 6 V was also used. The morphology of the films was assessed using a JEOL 840A scanning electron microscope with an attached Tracor 5400 EDX facility. The XRD was carried out using a Siemens powder diffractometer.
2. Experimental procedure The silicon samples were prepared from n-Si( 111 ) wafers (resistivity < 2 f / • cm) and mounted onto the electrode holder, exposing a set area (0.28 cm 2) for the electrodeposition. All samples were cleaned using a four step process: first with isopropanol, followed by a hydrogen peroxide/sulphuric acid etch (1:2 mixture at T < I I0°C for 15 rain), a distilled water rinse and finally, an HF etch (10% v / v H 2 0 for 1 min under N2). In order to examine the effect of visible radiation on the growth of the CdTe thin films, the sample surface was kept either uniformly illuminated or in total darkness during the growth. The solution consisted of 0.1M CdSO 4, 2 × 10-aM TeO 2 and 0.5M K2SO 4. The pH was adjusted by a d d i n g H 2 S O 4 and maintained at 1.5 +0.2. The solution temperature was kept within the range 7585°C to promote uniform and crystalline film growth. The surface preparation methods were assessed using ellipsometry. A conventional three-electrode set up
200
201
3. Results The delta-psi ( A - ~ ) curve corresponding to the growth of CdTe on silicon under no illumination is
~ / b a r e substrate value •. ~
start of CdTe growth
150 o k=
•~ 100 <~ """
50
. . . . . . . | =ll,• ,
0
P
•
m
~
•
I
I
I
I
10
20
30
40
~F [degrees] Fig. 1. A-~b plot for the growth of CdTe on Si{111} by a potential step from the open circuit potential to - 0.66 V. Sample surface was not illuminated.
F. Jackson et al./ Journal of Crystal Growth 159 (1996) 200-204
202
m ~.
50
.
.
.
.
.
70
~
20.0
U)
65
<
60
17.5 ~-
55
50
. . . . 0
~ . . . . 1000
~ , 2000
,
,
,
t . . . . 3000
~ . . . . 4000
15 5000
time (s) Fig. 2. Variations of the ellipsometry parameters A and ~ with time during the growth of CdTe on Si{l 11} by repeated cycling between the limits - 0 . 3 2 and - 0 . 6 6 V. Sweep rate = 0.001 V s - ] . Inset shows the corresponding changes in the A_~/, plot. ( t = 0 on time axis corresponds to 115 rain of cycling or 10 prior cycles.)
shown in Fig. 1. The A - ~ curve for the electrodeposition, carried out at a constant potential value of - 0 . 6 6 V versus SCE, started close to the bare substrate (Si) value and followed the trajectory expected for the growth of CdTe (n = 2.65, k = 0.35). Three loops, displaced with respect to each other, were observed in the figure at longer deposition times. More significantly, the third loop appeared to spiral towards a unique A - $ value. For electrodeposition by the method of repeated potential cycling between the limits of - 0 . 3 2 and - 0 . 6 6 V, the d - t and $ - t plots for the growth are given in Fig. 2. The cyclic voltammograms in this instance (not shown) were characterised by an almost ohmic-limited growth behaviour and an immediate decrease in the deposition current occurred as soon as the voltage sweep was reversed. It is evident from the ellipsometry data in Fig. 2 that the growth of the CdTe layer followed the periodic changes in the potential and that very little further growth occurred once the reverse, i.e. anodic sweep, was started. At the end of each anodic sweep, where a slight positive current was recorded, a corresponding change in the A and values was observed indicating a small, but significant alteration in the electrodeposited layer. The XRD spectra of the deposit as-grown and after annealing are given in Fig. 3. Some evidence of crys-
tallinity was found in the as-grown CdTe layer. Apart from the main peak at 20 = 28.5 °, corresponding to the Si{111} substrate, a small and broad signal at 2 0 = 23.8 ° and a more prominent peak at 20 = 39.3 ° were observed. The latter two match the CdTe{111} and {220} crystallographic orientations respectively. Upon annealing, the substrate peak decreased in intensity and the CdTe{ll 1} signal was enhanced, but no similar effect was observed for the
annealed
. . . . 20
,
. . . .
r
. . . .
,
. . . .
30
~
i 40
.
.
.
.
.
.
.
.
.
i 50
20 [degrees] Fig. 3. X R D spectra for the e l e c ~ o d e p o s i ~ d CdTe layer on Si{l 11} before and a f a r a 15 min a n n e ~ in an ine~ atmosphere at 400°C.
F. Jackson et al. / Journal o f Crystal Growth 159 (1996) 2 0 0 - 2 0 4
8
. . . . . . . . . . . . . .
'
~ ~:~
3
tion times, beyond a film thickness ~ 500 ~,. This apparent non-uniformity in the growing layer, as evidenced also by the three displaced loops could come about because of roughness in the layer with increasing thickness beyond 500 ,~, or a deviation in the deposit composition from stoichiometric CdTe. The latter could arise as a consequence of the increased resistance with thickness of the deposited layer, in effect reducing the overpotential applied to the solid/electrolyte interface and so favouring the formation of tellurium-rich CdTe. In both instances, the simple three-phase model employed above is inadequate to deal with these phenomena. The spiralling of the zl-~b towards a unique value at the end of the third loop in Fig. 1 is however expected since the growing layer is an absorbing one (k :~ 0) with respect to h = 6328 ,~ (1.96 eV). The growth method of repeated cycling, (Fig. 2) indicates the possibility of forming the CdTe film layer-by-layer, with each cycle contributing a thickness ~ 50-60 A, the latter being evaluated from both the charge under the curve in the cyclic voltammograms and also from the corresponding increment in the A - $ values. From the almost linear 1 - V growth curve, it could be deduced that the growth of the CdTe is virtually dominated by the resistance of the deposit. Furthermore, as soon as the driving force for the growth, i.e. the electric field d V / d x across the deposit, was reduced by the positive-going voltage scan, the current at nominally the same potential was lowered, showing partial control of the electrodeposition by the high field growth mechanism [28]. The dissolution current at the end of the anodic scan modifies the outermost surface of the deposit as on the return cathodic scan, the current trace was virtually identical to the previous cathodic scans. These cyclic growths and surface modifications at the end of the anodic scans are readily observed in Fig. 2. It would appear furthermore, from the inset to Fig. 2, which shows the same experimental data and model in terms of the A-~p plot, that this dissolution at the end of each scan helps to maintain a more uniform growth of CdTe layer, since the locus of the A - 0 signature at the end of cycle lies parallel to that of the model for ideal CdTe growth. The crystallinity of the as-deposited layer in the ellipsometry cell was expectedly poor (Fig. 3) since it has been reported [29] that films electrodeposited o
Fig. 4. SEM photographs at (a) × 500 and (b) × 2500 magnification of electrodeposited CdTe films on Si{lll} grown by the repeated cycling method showing the presence of the "hexagonal" overlayer. Marker in both photographs is 10 /zm.
{220} orientation. The CdTe{311} orientation also became more prominent. SEM photographs ( × 500 and × 2500 magnification) of the as-deposited layer (Fig. 4) show a patterned surface, consisting of ridges overlaying the nominally flat surface.
4. Discussion
It is clear that carrying out electrodeposition in the dark virtually eliminates the formation of the Te layer previously reported [15]. This enables the CdTe layer to be formed significantly more adjacent to the Si substrate than would have been otherwise possible with the presence of the spontaneous Te layer. The three-phase model (substrate-film-ambient) [27] used to characterise the growth of the layer in Fig. 1 deviates from the experimental data at longer deposi-
203
204
F. Jackson et al. /Journal of Crystal Growth 159 (1996) 200-204
below 60°C are generally amorphous. The reduction of the S i { l l l } signal upon annealing is surprising and may be associated with the increase in the crystallinity of the deposit. This is further borne out by the increased intensity in the C d T e { l l l } and {311} signals. That lattice mismatch was nevertheless present in all the deposits, pre- and post-annealing can be readily inferred from the SEM photographs (Fig. 4) which show a patterned surface, On closer examination, it was observed that the deposit was much more dense in the raised areas corresponding to the patterns indicating that these regions are electrochemically more reactive. E D A X analysis on both bulk and raised portions of the deposit indicate that the Cd: Te ratios lie close to 4: 6. The tellurium enrichment of our layers could result from the repeated cycling method employed here, where, at low cathodic overpotentials, the electrodeposition of Te is favoured over that of CdTe.
5. Conclusions The electrodeposition of thin films of CdTe on n - S i { l l l } has been carried out using the method of continuous cycling of the potential from the open circuit value to that just positive of the cadmium deposition potential. Layer-by-layer growth is effected, with each cycle contributing some 5 0 - 6 0 A. of the CdTe film onto the Si substrate. The slight anodic dissolution of the electrodeposit at the end of each cycle helps maintain growth uniformity in the layers formed. The present growth method though appears to yield tellurium-rich CdTe.
Acknowledgements One of us (F.J.) is grateful to EPSRC for funding. Our thanks also to the Nuffield Foundation for an equipment grant.
References [1] S.A. Ringel, A.W. Smith, M.H. Macdougal and A. Rohatgi, J. Appl. Phys. 70 (1981) 881. [2] K.W. Mitchell,C. Eberspacher, F. Chen, J. Avery, G. Duran and W. Bottenberg,Proc. 18th IEEE Photovoltaic Specialists
Conference, Las Vegas, NV (IEEE, New York, 1985) p. 1359. [3] B.M. Basol, Sol. Energy 23 (1988) 89. [4] J.M. Woodcock, A.K. Turner, M.E. Ozsan and J.G. Summers, Proc. 22rid IEEE Photovoltaic Specialists Conference, Las Vegas, NV (IEEE, New York, 1991) p. 842. [5] S.K. Das and G.C. Morris, Sol. Energy 30 (1993) 107. [6] M.Y. Simmons, H.M. Alallak, A.W. Brinkman and K. Durose, J. Crystal Growth 117 (1992) 959. [7] T.L. Chu and S.S. Chu, Solid State Electron. 38 (1995) 533. [8] C.K. Ard, in: Properties of Narrow Gap Cd-based Compounds, Ed. P. Capper (Inspec, Park Ridge, 1994) p. 598, and references therein. [9] G. Sarusi, G. Cinader, A. Zemel, D. Eger and Y. Shapira, J. Appl. Phys. 71 (1992) 5050. [10] P. LoVecchio, M.B. Reine and M.N. Grimbergen, J. Vac. Sci. Technol. A 3 (1985) 246. [11] C.L. Johnson, E.E. Eissler, S.E. Cameron, Y. Kong, S. Fan and K,G. Lynn, in: Semiconductorsfor Room Temperature Radiation Detector Applications, Eds. R.B. James, T.E. Schlesinger,P. Siffert and L. Franks, Mater. Res. Soc. (1993) p. 477. [12] H. Ebe and H. Takigawa, Mater. Sci. Eng. B 16 (1993) 57. [13] J.P. Fanrie, R. Sporken, Y.P. Chen, M.D. Lange and S. Sivernanthan,Mater. Sci. Eng. B 16 (1993) 51. [14] T.J. de Lyon, D. Rajavel, S,M. Johnson and C.A. Cockrum, Appl. Phys. Lett. 66 (1995) 2119. [15] J.M. Fisher, L.E.A. Berlouis, L.J.M. Sawers, S.M. McDonald, S. Affrossman, D.J. Diskett and M.G. Astles, J. Crystal Growth 138 (1994) 86. [16] Y. Sugimoto and L.M. Peter, J. Electroanal. Chem. 381 (1995) 251. [17] R. Triboulet and A. Tromson-Carli,J. Electron. Mater. 22 (1993) 827. [18] M.P.R. Panicker, M. Knaster and F.A. Krbger, J. Electrochem. Soc. 125 (1978) 566. [19] L. Ndiaye, P. Cowache, M. Cadene, D. Lincot and J. Vedel, Thin Solid Films 224 (1993) 227. [20] S.J.C. Irvine and J.B. Mullin, J. Crystal Growth 79 (1986) 371. [21] I.B. Bhat, N.R. Taskar and S.K. Ghandi, J. Electrochem. Soc. 134 (1987) 195. [22] M.G. Astles, N. Gordon, D. Bradley, P.J. Dean, D.R. Wright and G. Blackmore, J. Electron. Mater. 13 (1984) 167. [23] S.P.S. Sangha, J.D. Medland, J.A. Berry, L.M. Rinn, J. Crystal Growth 83 (1987) 127. [24] R.B. Benz, B.K. Wagner, D. Rajavel and C.J. Summers, J. Crystal Growth 111 (1991) 725. [25] R. Sporken, M.D. Lange, S. Sivananthan and J.P. Faurie, Appl. Phys. Lett. 59 (1991) 81. [26] J.M. Fisher, L.E.A. Berlouis, B.N. Rospendowski,P.J. Hall and M.G. Astles, Semicond. Sci. Technol. 8 (1993) 1459. [27] R.M.A. Azzam and M.M. Bashara, Ellipsometry and Polarized Light (North-Holland,Amsterdam, 1977). [28] N. Carbrera and N.F. Mott, Rep. Prog. Phys. 12 (1948) 12. [29] J. Touskovfi,D. Kindl and J. Tousek, Sol. Energy 18 (1989) 377.
ELSEVIER
CRYSTAL (]IRPOWTH
Journal of Crystal Growth 159 (1996) 200-204
Layer-by-layer electrodeposition of cadmium telluride onto silicon F. Jackson, L.E.A. Berlouis *, P. Rocabois Department of Pure and Applied Chemistry, University of Strathclyde, 295 Cathedral Street, Glasgow G1 1XL, UK
Abstract
The electrodeposition of thin films of CdTe on n-Si{111} has been carded out using the method of continuous cycling of the potential from the open circuit value to that just positive of the cadmium deposition potential. Layer-by-layer growth, verified by in situ ellipsometry, is effected, with each cycle contributing some 50-60/~ to the CdTe film on the Si substrate. A slight dissolution of the film at the end of each anodic sweep appears to ensure growth uniformity of the layers. X-ray diffraction shows a preferred orientation of the polycrystalline CdTe film along the {111} and {311} directions. Elemental analysis of the layers reveal an excess in Te (Cd:Te = 4:6) which could arise as a consequence of Te growth being favoured over that of CdTe at low overpotentials during cycling. I. Introduction
Cadmium telluride thin films are currently employed in various optoelectronic devices such as solar cells [1-7], infrared devices [8-10] and also for 3,- and X-ray detection [11]. The direct growth of CdTe on silicon [12-16] could lead to an important technological advance, for example in hybridisation with silicon based microcircuitry, particularly in the development of infrared focal plane arrays and solar cells [17]. In this respect, the method of electrochemical deposition [3,5,7,10,15,16,18,19] of CdTe thin films offers an alternative economic route to the high costs of epitaxial techniques of metalorganic vapour phase epitaxy (MOVPE) [20,21], liquid phase epitaxy (LPE) [22,23] and molecular beam epitaxy (MBE) [12-14,24,25] currently used for example, in producing lattice matched substrates for the infrared detector material, CdxHgt_xTe (CMT). Several difficulties are encountered when using * Corresponding author.
silicon as a substrate for CdTe thin film growth. Firstly, the lattice mismatch between CdTe and silicon is large at 19%, although with MBE growth, this has been partially overcome by varying the orientation of the Si substrate to reduce this to 3.4% [12-14]. Secondly, there is also a relatively large thermal mismatch between CdTe and silicon, which could cause cracking or peeling of the films due to the wide temperature changes involved in fabrication or device operation. Thirdly and, more pertinent to electrochemical growth, is the presence of the insulating surface oxide which must be removed by chemical etching with either HF or ammonium fluoride. The latter has been successfully employed by Sugimoto et al. [16] to yield an atomically smooth surface. Finally, the growth of CdTe from aqueous solutions under illumination has been observed to lead to the formation of an instantaneous tellurium layer on the silicon [15]. In the present work, the growth of cadmium telluride onto n-type Si{111} using a layer-by-layer growth method is described and the effect of illumination on the electrodeposi-
0022-0248/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD! 0 0 2 2 - 0 2 4 8 ( 9 5 ) 0 0 8 19-5
F. Jackson et at./Journal of Crystal Growth 159 (1996) 200-204
tion is examined. The electrodeposited films were characterised using scanning electron microscopy (SEM), energy dispersive analysis of X-rays (EDAX) and X-ray powder diffraction (XRD).
was used with a saturated (KCI) calomel reference electrode (SCE) and a platinum gauze as a secondary electrode. For the in situ ellipsometry experiments, a Gaertner L116B Ellipsometer was used with light of o wavelength 6328 A, incident on the sample surface at an angle of 70° in the cell previously described [26]. The as-deposited films were annealed in an inert atmosphere, at 400°C for 15 min. At the point of sample insertion into the solution, the potential was set at - 0 . 2 V versus SCE [16] to minimise the formation of the spontaneous Te layer. The main method of electrodeposition for the CdTe layer then examined was that of continuous cycling between the open circuit potential (about - 0 . 2 V) to a potential just positive of cadmium deposition (typically - 0 . 6 6 V) although the method of a single potential step to - 0 . 6 6 V was also used. The morphology of the films was assessed using a JEOL 840A scanning electron microscope with an attached Tracor 5400 EDX facility. The XRD was carried out using a Siemens powder diffractometer.
2. Experimental procedure The silicon samples were prepared from n-Si( 111 ) wafers (resistivity < 2 f / • cm) and mounted onto the electrode holder, exposing a set area (0.28 cm 2) for the electrodeposition. All samples were cleaned using a four step process: first with isopropanol, followed by a hydrogen peroxide/sulphuric acid etch (1:2 mixture at T < I I0°C for 15 rain), a distilled water rinse and finally, an HF etch (10% v / v H 2 0 for 1 min under N2). In order to examine the effect of visible radiation on the growth of the CdTe thin films, the sample surface was kept either uniformly illuminated or in total darkness during the growth. The solution consisted of 0.1M CdSO 4, 2 × 10-aM TeO 2 and 0.5M K2SO 4. The pH was adjusted by a d d i n g H 2 S O 4 and maintained at 1.5 +0.2. The solution temperature was kept within the range 7585°C to promote uniform and crystalline film growth. The surface preparation methods were assessed using ellipsometry. A conventional three-electrode set up
200
201
3. Results The delta-psi ( A - ~ ) curve corresponding to the growth of CdTe on silicon under no illumination is
~ / b a r e substrate value •. ~
start of CdTe growth
150 o k=
•~ 100 <~ """
50
. . . . . . . | =ll,• ,
0
P
•
m
~
•
I
I
I
I
10
20
30
40
~F [degrees] Fig. 1. A-~b plot for the growth of CdTe on Si{111} by a potential step from the open circuit potential to - 0.66 V. Sample surface was not illuminated.
F. Jackson et al./ Journal of Crystal Growth 159 (1996) 200-204
202
m ~.
50
.
.
.
.
.
70
~
20.0
U)
65
<
60
17.5 ~-
55
50
. . . . 0
~ . . . . 1000
~ , 2000
,
,
,
t . . . . 3000
~ . . . . 4000
15 5000
time (s) Fig. 2. Variations of the ellipsometry parameters A and ~ with time during the growth of CdTe on Si{l 11} by repeated cycling between the limits - 0 . 3 2 and - 0 . 6 6 V. Sweep rate = 0.001 V s - ] . Inset shows the corresponding changes in the A_~/, plot. ( t = 0 on time axis corresponds to 115 rain of cycling or 10 prior cycles.)
shown in Fig. 1. The A - ~ curve for the electrodeposition, carried out at a constant potential value of - 0 . 6 6 V versus SCE, started close to the bare substrate (Si) value and followed the trajectory expected for the growth of CdTe (n = 2.65, k = 0.35). Three loops, displaced with respect to each other, were observed in the figure at longer deposition times. More significantly, the third loop appeared to spiral towards a unique A - $ value. For electrodeposition by the method of repeated potential cycling between the limits of - 0 . 3 2 and - 0 . 6 6 V, the d - t and $ - t plots for the growth are given in Fig. 2. The cyclic voltammograms in this instance (not shown) were characterised by an almost ohmic-limited growth behaviour and an immediate decrease in the deposition current occurred as soon as the voltage sweep was reversed. It is evident from the ellipsometry data in Fig. 2 that the growth of the CdTe layer followed the periodic changes in the potential and that very little further growth occurred once the reverse, i.e. anodic sweep, was started. At the end of each anodic sweep, where a slight positive current was recorded, a corresponding change in the A and values was observed indicating a small, but significant alteration in the electrodeposited layer. The XRD spectra of the deposit as-grown and after annealing are given in Fig. 3. Some evidence of crys-
tallinity was found in the as-grown CdTe layer. Apart from the main peak at 20 = 28.5 °, corresponding to the Si{111} substrate, a small and broad signal at 2 0 = 23.8 ° and a more prominent peak at 20 = 39.3 ° were observed. The latter two match the CdTe{111} and {220} crystallographic orientations respectively. Upon annealing, the substrate peak decreased in intensity and the CdTe{ll 1} signal was enhanced, but no similar effect was observed for the
annealed
. . . . 20
,
. . . .
r
. . . .
,
. . . .
30
~
i 40
.
.
.
.
.
.
.
.
.
i 50
20 [degrees] Fig. 3. X R D spectra for the e l e c ~ o d e p o s i ~ d CdTe layer on Si{l 11} before and a f a r a 15 min a n n e ~ in an ine~ atmosphere at 400°C.
F. Jackson et al. / Journal o f Crystal Growth 159 (1996) 2 0 0 - 2 0 4
8
. . . . . . . . . . . . . .
'
~ ~:~
3
tion times, beyond a film thickness ~ 500 ~,. This apparent non-uniformity in the growing layer, as evidenced also by the three displaced loops could come about because of roughness in the layer with increasing thickness beyond 500 ,~, or a deviation in the deposit composition from stoichiometric CdTe. The latter could arise as a consequence of the increased resistance with thickness of the deposited layer, in effect reducing the overpotential applied to the solid/electrolyte interface and so favouring the formation of tellurium-rich CdTe. In both instances, the simple three-phase model employed above is inadequate to deal with these phenomena. The spiralling of the zl-~b towards a unique value at the end of the third loop in Fig. 1 is however expected since the growing layer is an absorbing one (k :~ 0) with respect to h = 6328 ,~ (1.96 eV). The growth method of repeated cycling, (Fig. 2) indicates the possibility of forming the CdTe film layer-by-layer, with each cycle contributing a thickness ~ 50-60 A, the latter being evaluated from both the charge under the curve in the cyclic voltammograms and also from the corresponding increment in the A - $ values. From the almost linear 1 - V growth curve, it could be deduced that the growth of the CdTe is virtually dominated by the resistance of the deposit. Furthermore, as soon as the driving force for the growth, i.e. the electric field d V / d x across the deposit, was reduced by the positive-going voltage scan, the current at nominally the same potential was lowered, showing partial control of the electrodeposition by the high field growth mechanism [28]. The dissolution current at the end of the anodic scan modifies the outermost surface of the deposit as on the return cathodic scan, the current trace was virtually identical to the previous cathodic scans. These cyclic growths and surface modifications at the end of the anodic scans are readily observed in Fig. 2. It would appear furthermore, from the inset to Fig. 2, which shows the same experimental data and model in terms of the A-~p plot, that this dissolution at the end of each scan helps to maintain a more uniform growth of CdTe layer, since the locus of the A - 0 signature at the end of cycle lies parallel to that of the model for ideal CdTe growth. The crystallinity of the as-deposited layer in the ellipsometry cell was expectedly poor (Fig. 3) since it has been reported [29] that films electrodeposited o
Fig. 4. SEM photographs at (a) × 500 and (b) × 2500 magnification of electrodeposited CdTe films on Si{lll} grown by the repeated cycling method showing the presence of the "hexagonal" overlayer. Marker in both photographs is 10 /zm.
{220} orientation. The CdTe{311} orientation also became more prominent. SEM photographs ( × 500 and × 2500 magnification) of the as-deposited layer (Fig. 4) show a patterned surface, consisting of ridges overlaying the nominally flat surface.
4. Discussion
It is clear that carrying out electrodeposition in the dark virtually eliminates the formation of the Te layer previously reported [15]. This enables the CdTe layer to be formed significantly more adjacent to the Si substrate than would have been otherwise possible with the presence of the spontaneous Te layer. The three-phase model (substrate-film-ambient) [27] used to characterise the growth of the layer in Fig. 1 deviates from the experimental data at longer deposi-
203
204
F. Jackson et al. /Journal of Crystal Growth 159 (1996) 200-204
below 60°C are generally amorphous. The reduction of the S i { l l l } signal upon annealing is surprising and may be associated with the increase in the crystallinity of the deposit. This is further borne out by the increased intensity in the C d T e { l l l } and {311} signals. That lattice mismatch was nevertheless present in all the deposits, pre- and post-annealing can be readily inferred from the SEM photographs (Fig. 4) which show a patterned surface, On closer examination, it was observed that the deposit was much more dense in the raised areas corresponding to the patterns indicating that these regions are electrochemically more reactive. E D A X analysis on both bulk and raised portions of the deposit indicate that the Cd: Te ratios lie close to 4: 6. The tellurium enrichment of our layers could result from the repeated cycling method employed here, where, at low cathodic overpotentials, the electrodeposition of Te is favoured over that of CdTe.
5. Conclusions The electrodeposition of thin films of CdTe on n - S i { l l l } has been carried out using the method of continuous cycling of the potential from the open circuit value to that just positive of the cadmium deposition potential. Layer-by-layer growth is effected, with each cycle contributing some 5 0 - 6 0 A. of the CdTe film onto the Si substrate. The slight anodic dissolution of the electrodeposit at the end of each cycle helps maintain growth uniformity in the layers formed. The present growth method though appears to yield tellurium-rich CdTe.
Acknowledgements One of us (F.J.) is grateful to EPSRC for funding. Our thanks also to the Nuffield Foundation for an equipment grant.
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