Chem. Mater. 1991,3,1053-1059
excess electrons to the substrate is not large enough, they will accumulate in the film and neutralize with holes, decreasing therefore the total collected photocurrent. In that case, back-side illumination will result in larger quantum efficiencies than will front-side illumination. On the contrary, for nonporous films the interface between the film and the solution is limited to the film surface. Front illumination would therefore generate more excess holes and electrons near the active interface than back illumination for which a filtering effect is expected if the exciton diffusion length is less than the film thickness. A larger quantum yield is then expected for front illumination of a nonporous film. The filtering effect is well illustrated in Figure 5B compared to Figure 5A for compound 2. It is not seen for a film thickness of 300 A, but it i well observed for a 1750A-thick film. Exciton diffusion length in material 2 is therefore comprised between 330 and 1750 A. As far as Figure 6 is concerned, both directions of illumination give nearly identical action spectra for porous films of compound 6. Some filtering effect appears for front illumination for thickner films but is much less effective than the back illumination of compound 2, although both materials are characterized by a similar thickness absorption spectrum and maximum absorption coefficient. Some filtering effect is expected for the front illumination of a porous film of high-resistance material because lower photocurrent would be collected from the nonsubstrate region than from the near substrate region of the film. The active interface is however always the film-solution interface and not the film-Sn02 interface. Lowering the resistance of a porous film by increasing the charge carrier density or mobility or the kinetics of charge transfer be-
1053
tween crystallites would result in the disappearance of the filtering effect for front illumination, and thus front and back illuminations would yield the same action spectrum regardless of the thickness of the film. This was indeed observed for PTCDA.g
Conclusion The present work about the photoelectrochemical properties of some perylene derivatives has been undertaken to find out the best material to use in heterojunctions with chloroaluminum phthalocyanine, a p-type molecular semiconductor already studied extensively in our group. All perylenes investigated show a dominant photoanodic behavior. However, they need not be n-type semiconductors. Among all perylene derivatives studied, material 2 is by far the most photoactive. It also forms compact nonporous films at the substrate temperature of 160 "C and the growth rate of 0.8 A 5-l used. The nonporous character of material 2 films is important because the highly photoactive chloroaluminum phthalocyanine films grown at 2000 A s-l are porous, and the junction of two porous films could lead to short circuits between the two collecting electrodes. Registry No. 1, 128-65-4; 2, 6424-77-7; 3, 22047-48-9; 4, 2379-77-3;5,6707537-0;6,136847-29-5;7 (cisisomer),55034-81-6; 7 (trans isomer),55034-79-2;SnOz,18282-10-5;chloroaluminum phthalocyanine, 14154-42-8; 3,4,9,10-perylenetetracarboxylic dianhydride, 128-69-8; aniline, 62-53-3; p-anisidine, 104-94-9; 2,4-dimethylaniline, 95-68-1; 4-chloroaniline, 106-47-8; phenethylamine, 64-04-0;4-aminopyridine, 504-24-5; 1,a-phenylenediamine, 95-54-5; KHP, 877-24-7; HzQ, 123-31-9; Fe(CN)63-, 13408-62-3;Fe(CN)64-,13408-63-4;I,, 14900-04-0;I-, 20461-54-5; benzoquinone, 106-51-4.
Volatile Barium @-DiketonatePolyether Adducts. Synthesis, Characterization, and Metalloorganic Chemical Vapor Deposition R. Gardiner,* D. W. Brown,* and P. S. Kirlin* Advanced Technology Materials, Inc., 7 Commerce Drive, Danbury, Connecticut 06810
A. L. Rheingold Department of Chemistry, University of Delaware, Newark, Delaware 19716 Received May 1, 1991. Revised Manuscript Received August 26, 1991
The 2,5,8,11,14-pentaoxopentadecane(tetraglyme) adducts of barium complexes of 1,1,1,5,5,5-hexafluoro-2,4-pentanedione (H(hfacac)), l,l,l-trifluoro-2,4-pentanedione(H[tfacac]), and 2,2,6,6-tetramethyl-3,5-heptanedione (H(thd))have been prepared and characterized by X-ray crystallography, 'H and 13C NMR spectroscopies, melting and sublimation points, and elemental analysis. The thermal stability of the complexes is related to their solid-state structure in which the coordination number around the barium atom is nine with a monocapped, twisted square prism structure. Metalloorganic chemical vapor deposition experiments were performed using the 1,1,1,5,5,5-hexafluoro-2,4-pentanedionato and l,l,l-trifluoro-2,4pentanedionato complexes; the 2,2,6,6-tetramethyl-3,5-heptanedionato complex dissociated prior to sublimation. The as-deposited barium fluoride films were characterized by scanning electron microscopy and energy-dispersive X-ray spectroscopy.
Introduction since the discovery of superconducti~tyabove30 K in 1986,' numerous research efforts have focused on t h i n - f h (1) Bednorz, J. G.;Muller, K.A. Z. Phys. B 1986, 64, 189.
preparation of these materials for microelectronic applications. Metalloorganic chemical vapor deposition (MOCW) has significant advantages relative to physical vapor deposition (PVD)methods in the growth of superconducting thin films; large wafers can be uniformly coated, and scale up to large volumes is easily accom-
0897-4756/91/2803-1053$02.50/00 1991 American Chemical Society
Gardiner et al.
1054 Chem. Mater., VoZ. 3, No. 6,1991
plished. MOCVD also allows for the exacting control of the film microstructure and stoichiometry which is essential for the manufacture of superconducting devices with optimal and reproducible properties. Uniform films of BiSrCaCuO have already been grown over large areas (45 cm2) by MOCVD,2 but these films required a high temperature (T > 850 "C) anneal to become superconducting. Plasma-enhanced MOCVD (PE-MOCVD) of high-temperature superconducting thin films has been reported by several groups3+ at temperatures (570 "C 5 T 5 700 "C) which are compatible with standard silicon integrated circuit process technology. The primary obstacle to the widespread application of MOCVD to the growth of high-T, superconducting oxide thin films (YBaCuO, BiCaSrCuO, TlBaCaCuO) is the absence of an effective alkali metal, in particular barium, source reagent. Barium prefers high coordination numbers, up to 12: and the lack of steric saturation of many barium organometallic complexes leads them to be strongly associated in the solid state; barium organometallics often undergo thermal decomposition before vaporization? For example, barium P-diketonates (Ba(RCOCH2COR')2where R, R' = CHJ have low solubility and thermal decomposition during sublimation is great. The use of large, sterically bulky groups (R, R' = (CH3)3C,and R = (CH3)3C, R' = C3F7)to block open barium coordination sites reduces intermolecular interactions, increases solubility even in nonpolar solvents, and enhances volatility of the comp l e ~ e s .Even ~ with bulky ligands the barium atom is still not coordinatively saturated. The current MOCVD precursors for barium (barium l,l,1,2,2,3,3-heptafluoro-7,7dimethyloctane-4,6-dionate( B a ( f ~ d ) ~and ) barium 2,2,6,6-tetramethyl-3,5-heptanedionate (Ba(thd),)) undergo partial decomposition at the temperatures (>200"C) required to sustain sufficient usable vapor pressure.1°-12 Furthermore, association in the gas phase13 leads to fluctuations in element delivery rates. Improved stability of barium transport has been accomplished by saturating the carrier gas stream going to the Ba(thd)2source with the 2,2,6,6-tetramethyl-3,5-heptanedione vapor.14 Similarly, saturating the carrier gas with oxygen or nitrogen Lewis bases can lower the required bubbler temperature. Tetrahydrofuran (THF) lowers the reagent bubbler temperature from 300 to 180 OC15 and transport of a number of barium P-diketonates at temperatures as low as 150 "C has been achieved by adding nitrogen Lewis bases (e.g., triethylamine) to the carrier (2) Hamaguchi, N.; Vigil, J.; Gardiner, R.; Kirlin, P. S. Jpn. J . Appl. Phys. 1990,29, L596. (3) Zhao, J.; Marcy, H. 0.;Tonge, L. M.; Wessels, B. W.; Marks, T. J.; Kannewurf, C. R. Solid State Commun. 1990, 74, 1091. (4) Chern, C. S.; Zhao, J.; Li, Y. Q.; Norris, P.; Kear, B.; Gallois, B. Appl. Phys. Lett. 1990, 57, 721. (5) Kobayashi, K.; Ichikawa, S.; Okada, G. Jpn. J . Appl. Phys. 1989, 28, L2165. (6) Bai, G. R.; Tao, W.; Wang, R.; Xie, L. M.; Zhang, X. K. Appl. Phys. Lett. 1989, 55, 194. (7) Wells, A. F. Structural Inorganic Chemistry, 5th ed.; Clarendon: Oxford, 1984; p 574. (8) Evans, W. J. Adu. Organomet. Chem. 1985,24,131. (9) Berg, E. W.; Herrera, N. M. Anal. Chim. Acta 1972, 60, 117. (10) Kirlin, P. S.; Binder, R.; Gardiner, R.; Brown, D. W. Proc. SPIE Processing of F i l m for High Tc Superconducting Electronics 1989,1187, 115. (11) Malandrino, G.; Richeson, D. S.; Marks, T. J.; DeGroot, D. C.; Schindler, J. L.; Kannerwurf, C. R. Appl. Phys. Lett. 1989, 66, 444. (12) Nakamori, T.; Abe, H.; Kanamori, T.; Shibata, S. Jpn. J . Appl. Phys. 1988,27, L1265. (13) Belcher, R.; Cranley, C. P.; Majer, J. R.; Stephen, W. I.; Uden, P. C. Anal. Chim. Acta 1972,60, 109. (14) Dickinson, P. H.; Geballe, T. H.; Sanjuro, A.; Hildenbrand, D.; Craig, G.; Zisk, M.; Collman, J. J . Appl. Phys. 1989, 66, 444. (15) Chaffee, C. D., Ed. Supercond. Week 1989,3 (39), 3.
nitrogen
T cold trap
oven
Figure 1. Diagram of apparatus used to measure vapor pressure
gas.16 Excess hydrocarbons compete for surface sites and can lead to the co-deposition of carbon, thus interfering with the in situ deposition of the high temperature superconductors. Consequently, a mononuclear barium source reagent with a vapor pressure of -1 Torr at 150 "C which is thermally stable at its vaporization temperature would simplify MOCVD process development and increase process reproducibility and control. Our investigations have concentrated on the factors affecting the volatility, stability, and deposition chemistry of the barium complexes which determine their potential as MOCVD source reagents. We report here the synthesis and characterization of three barium P-diketonateotetraglyme adducts and preliminary MOCVD film growth results.
Experimental Section General Information. Synthesis and handling of all organometallic complexes were performed under nitrogen atmosphere using standard Schlenk techniques or in a Vacuum Atmospheres inert-atmosphere glovebox. Tetraglyme was distilled over Na/K alloy and stored over calcium hydride. Barium hydride, 1,1,1,5,5,5-hexafluoro-2,Cpentanedione, l,l,l-trifluoro2,4-pentanedione (Strem Chemicals), and 2,2,6,6-tetramethyl3,5-heptanedione (Aldrich Chemical Co.) were used as received. 'H NMR and 13CNMR spectra were measured a t 200 MHz with an IBM WP2OOSY spectrometer. Chemical shifts are reported in ppm on a 6 scale with residual resonances from the NMR solvents serving as internal standards for the 'H (C6D66 = 7.15 ppm) and l3C (c& 6 = 128.0 ppm) spectra. Elemental analyses were obtained from Galbraith Laboratories Inc., Knoxville, TN. Syntheses. Preparation of Barium Bis( 1,1,1,5,5,5-hexafluoro-2,4-pentanedionate)*TetraglymeAdduct (Ba(hfaC ~ C ) ~ . ( C H ~ O ( C H ~ C H ~ O1.) ~1,1,1,5,5,5-Hexafluoro-2,4CH~)), pentanedione (14.74 g, 70 mmol) was added to a stirring suspension of barium hydride (11.36 g, 35.0 mmol) in tetraglyme. Gas was constantly evolved throughout the addition of the pdiketone. The resulting, yellow solution was decanted from a white crystalline solid (11.3 g, 42%). The solid was then washed with -50 mL of cold pentane and dried under vacuum (0.2 Torr). The final product (10.9 g, 40%, mp 121-125 "C) was purified by vacuum sublimation a t 150 "C and 0.2 Torr. Anal. Calcd (found) for BaC2f112H2409:Ba, 17.75 (17.65); C, 31.05 (31.44); F, 29.47 (28.09); H, 3.13 (3.32). 'H NMR (C6D6) 6 3.04 (m, 6 H), 3.15 (m, 12 H), 3.30 (m, 4 H), 6.28 (s, 2 H) ppm. 13C NMR 6 58.98 (CH), 70.37 (CH,), 70.73 (CH,), 71.51 (CH,), 87.66 (CF,) ppm.
Preparation of Barium Bis( l,l,l-trifluoro-2,4-pentanedionate)-Tetraglyme Adduct (Ba(tfacac),*(CH,O(CH2CH20)4CH3)), 2. l,l,l-Trifluoro-2,4-pentanedione (5.0 g, 32.4 mmol) was added dropwise via syringe to a stirring slurry of barium hydride (2.26 g, 16.1 mmol) in tetraglyme. Gas was constantly evolved throughout the addition of the p-diketone. The reaction mixture was then stirred overnight, and the resulting mixture was filtered to leave a white crystalline solid and an orange (16) Barron, A. R.; Buriak, J. M.; Cheatham, L.; Gordon, R. J . Electrochem. SOC.1990,137 (4), 225C.
Chem. Mater., Vol. 3, No. 6, 1991 1055
Volatile Barium p-Diketonate Polyether Adducts a
I
I
I
Mass flow controller
Reagent bubbler
R.T liq.N2 cold traps
Figure 3. Diagram of horizontal hot wall reactor used to deposit barium fluoride films. I
0.0022
01
0.0023
I
I
0.0024 1/T K-1
0.0025
\
0.0021
I
I
I
0.0022
0.0023 1/T K-1
0.0024
0.0026
Vapor Pressure Results. The vapor pressures of 1 and 2 were measured a t several temperatures from 120 to 180 "C. The data were fit by a simple linear least-squares analysis to eqs l a and l b (correlation coefficient, R2, 0.893 for eq l a and 0.938 for eq
1
In Pl(Torr) = 21.875 - 9.0273 X 103/T In P2(Torr) = 27.477 - 1.1989 X 104/T
0.0025
Figure 2. (a) Vapor pressure plot for Ba(CF3COCHCOCF3)2(CHO(CH2CH20)4CH3). (b) Vapor pressure plot for Ba(CF3COCHCOCH& (CHO(CH&H20)4CH3). filtrate. The solid was washed with cold pentane and then dried in a nitrogen purge (4.2g, 40%). The crude material (0.95g) was sublimed a t 150 "C and 0.1 Torr to yield a white solid (0.60g, 25%, mp 133-136 "C). Anal. Calcd (found) for BaC20F6H3009: Ba, 20.63 (20.52);C, 36.08 (36.55);F, 17.12 (16.94);H, 4.54 (4.54). 'H NMR (C6D6)6 1.88 (s,6H) 2.93 (s, 6 H), 3.16 (m, 4 H), 3.36 (br s, 12 H), 5.83 (5, 2 H) ppm. 13C NMR 6 29.85 (CH,), 59.17 (CH), 70.01 (CH,), 70.11 (CH,), 70.50 (CH,), 71.12 (CH,), 93.28 (CF,), 194.06 (C=O) ppm.
Preparation of Barium Bis(2,2,6,6-tetramethyl-3,5-heptanedionate)-Tetraglyme Adduct (Ba(thd)2*(CH303. 2,2,6,6-Tetramethyl-3,5-heptanedione (2.65 (CH2CH20)4CH3)), g, 14.0 m o l ) was added to a stirring suspension of barium hydride (1.0g, 70 mmol) in -20 mL of tetraglyme; gas evolution started immediately after the P-diketone was added and continued throughout the addition. The reaction mixture was stirred overnight to yield a cream-colored solid and a pale yellow solution. The solid was fiitered, washed with cold pentane, and then dried under vacuum (0.2Torr), yielding a white solid (1.04g, 27%, mp 93-97 "C). Anal. Calcd (found) for B B C ~ ~ I & , Ba, O ~ :19.74 (18.91); C, 52.93 (52.05);H, 8.33 (8.03);O,18.91 (19.85).'H NMR (C&3) 6 1.39 (s, 36 H) 3.10 (br s, 6 H), 3.29 (m, 4 H), 3.36 (br s, 12 H), 5.85 (5, 2 H) ppm. 13C NMR 6 29.78, 29.66 ((CH3),C), 41.10 ((CH,),C), 59.15 (CH), 70.41 (CH,), 70.59 (CH,), 71.66 (CH3), 197.50 (C=O) ppm. Vapor Pressure Measurements. The vapor pressures of 1 and 2 were measured with the apparatus illustrated in Figure 1. The sample cylinder was loaded with 1.0 g of sample in an inert atmosphere. The cylinder was then connected to the system which was subsequently evacuated and backfilled with nitrogen three times. The entire system was then evacuated, the oven temperature was set, and the system was allowed to equilibrate with valves 1,2,and 3 open to vacuum. Valve 3 was then closed and 4 opened, and a pressure reading was taken after equilibration (1-2h). After each measurement valve 4 was closed and 3 opened, and a background reading taken. The vapor pressure was calculated as the difference between the total system and background pressures. The system was calibrated using the vapor pressure curve for tetraethylene glyc01.'~
(la)
(1b)
lb) which are plotted in Figure 2. The values of AHsubo of 1.09 and 1.44 kJ mol-l are found for the heats of sublimation for 1 and 2, respectively.18 MOCVD Experiments. Reduced pressure MOCVD experiments were carried out in a standard horizontal hot wall reactor (Figure 3). The precursor (0.5g) was loaded into the reagent bubbler in an inert atmosphere and connected to the system without exposing the compound to air. The system was evacuated to 0.05 Torr and bubbler temperature was raised to 175 "C. The argon carrier gas flow, regulated by an MKS mass flow controller, was 10 cm3 min-', and the total system pressure was maintained at 4 Torr. The reagent was transported through heated lines to the quartz reactor tube which was maintained at 650 "C. A typical growth run lasted 2 h, and the film deposited on the inner wall of the quartz tube was characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). Single-Crystal X-ray Analysis. The data collection was performed on a Nicolet R3m diffractometer and the computer programs and the sources of the scattering factors which were taken from the SHEZXTL program library (5.1).l9 Colorless crystals of 1 were obtained by reduced pressure sublimation, those of 2 and 3 by slow evaporation of a toluene solution. The crystals were mounted on glass fibers with epoxy cement. The basic crystallographic parameters obtained are listed in Table I. The unit cell parameters were obtained from the least-squares fit of 25 reflections (20"I 8 I25"). The structure of 1 was solved by direct methods which located the barium atom. 2 and 3 were each solved using Patterson maps which located the barium atoms. The remaining non-hydrogen atoms were located from subsequent difference Fourier syntheses. All hydrogen atoms were included as idealized isotropic contributions (d(CH) = 0.960 A; U = 1.2U for attached C). For 1 and 2 all non-hydrogen atoms were refined with anisotropic thermal parameters. For 3 all atoms were refined anisotropically except in one tert-butyl group, which was disordered, where only the carbon atoms with the greater contribution to the occupancy (77% ) were refined anisotropically.
Results and Discussion Our search for barium source reagents with improved volatility and deposition chemistry has been based on a simple premise: mononuclear barium complexes require filled coordination spheres. Research into blocking the open coordination sites using multidentate Lewis bases to reduce intermolecular interactions and increase volatility has met with some success.Bo NJVJV'JV'-Tetramethylethylenediamine has been used to complex with group I1 alkoxides to promote solubility.21 (17) CRC Handbook of Chemistry and Physics, 62nd ed.; CRC Press: Boca Raton, FL, 1981; p D-183. were derived from the Clayperon equation (18) The values of mHsubo @/dT = ( m s U b o ) (p)/ (Rp). (19) Sheldrick, G., Nicolet Corp., Madison, WI. (20) Matsuno, S.; Uchikawa, F.; Yoshizaki, K. Jpn. J . Appl. Phys. 1990,29, L947.
Cardiner et al.
1056 Chem. Mater., Vol. 3, No. 6, 1991 Table I. Crystallographic Data for 1,2, and 3 2
1
cryst dimens, mm cryst color D(calc), g/cm3 ~ ( M Ka), o cm-' temp, OC T(max)/ T(min)
(a) Crystal Parameters CzoHzP1209Ba C20H33609Ba pi Pi triclinic triclinic 9.6477 (21) 9.5655 (26) 11.1547 (22) 10.4002 (29) 15.4645 (31) 15.230 (4) 94.151 (16) 93.010 (22) 90.730 (17) 91.702 (22) 114.458 (15) 109.631 (21) 1509.2 (5) 1423.3 (6) 2 2 0.38 X 0.40 X 0.42 0.38 X 0.38 X 0.40 colorless colorless 1.702 1.553 14.61 15.07 23 (1) 23 (1) 1.085 1.099
diffractometer monochromator scan technique radiation 20 scan range, deg data collected (h,k,l) scan speed, deg/min rflns collected indpt rflns R(merg), % indpt rflns obsd F, > 50(F0) var in stds, 70
(b) Data Collection Nicolet R3m graphite Wyckoff Mo K a (A = 0.71073 A) 4-50 4-57 f13, f14, +21 f12, f14, +19 var 5-20 var 5-20 5539 7476 5320 7216 1.95 2.49 3 stdl197 rflns 3 std/197 rflns
