J . Phys. Chem. 1992, 96, 3007-3014
3007
Stability of Substituted Lewis Adducts of Substituted Gallanes and Arsines Charles W. Bock,* Department of Chemistry, Philadelphia College of Textiles & Sciences, Philadelphia, Pennsylvania I9144, and American Research Institute, Marcus Hook, Pennsylvania I9061
Mendel Trachtman, Department of Chemistry, Philadelphia College of Textiles & Sciences, Philadelphia, Pennsylvania I9144
and Gilbert J. Mains* Department of Chemistry, Oklahoma State University, Stillwater, Oklahoma 74078 (Received: June 14, 1991)
The Lewis acid/base adducts formed by the reaction of gallane, chlorogallane, trichlorogallane, fluorogallane, trifluorogallane, methylgallane, and dimethylgallane with arsine have been studied using molecular orbital methods using basis sets derived from those published by Huzinaga. A similar study was performed which employed methyl-substituted arsines. The calculated dissociation energies of these adducts were structurally dependent and varied between 10 and 20 kcal mol-' at the MP4(SDTQ) level (HF geometries). For the adducts of chlorogallane and fluorogallanewith arsine, evidence is found for bonding contributions between the halogen atom and the arsenic atom. Except for these cases, Ga-qAs bond lengths and symmetric stretching frequencies were simply related to the acid/base characteristics of the constituent gallanes and arsines. Since it is known that adducts of substituted gallanes and arsines lead to epitaxial growth (MBE) and to thin films (MOCVD) with desirable properties, the adducts reported here may also find practical applications.
Introduction There is intense interest in GaAs compounds. The application of group 111-V semiconductors in high-speed computers, lasers, light emitting diodes, solar cells, and other solid-state devices is the driving force for this research.I Although mostly focused on GaAs crystalline material, research on atoms adsorbed on various crystal faces using scanning tunneling microscopy2v3and on photoelectron spectroscopy4 and on thin GaAs films with desirable properties produced by both metal-organic chemical vapor deposition (MOCVD) and conventional molecular beam epitaxy (MBE) have been reported. These latter methods have generated significant interest in the chemistry of precursor m~lecules.~The following simplified mechanism was suggested GaAs organometallic precursors Lewis adduct polymeric deposit (1)
-
-
-
in which the Lewis acid/base adduct may either dissociate to the original organometallic precursors or undergo some internal rearrangements, decompose, or polymerize. The latter pathway is undesirable since it does not contribute to GaAs formation. Relevant studies of GaAs clusters6-* and GaAs liquid9 have also recently appeared. A recent calculationlo of the structure and vibrational spectrum of the Lewis acid/base adduct, H,Ga-.AsH,, which could be involved as a precursor in the CVD of GaAs, served to focus our interest in related adducts. We have suggested" that p-H bridged (1) Christou, A,, Rupprecht, H. S. Eds. Gallium Arsenide and Related Compounds 1987; Fourteenth International Symposium held in Heraklion, Cred, 28 Sept.-1 Oct. 1987; IOP Conference Series No. 91; Institute of Physics: Philadelphia, PA, 1988. (2) Griffith, J. E.; Kochanski, G. P. Annu. Reu. Mater. Sci. 1990, 20, 219. (3) Stroscio, J. A,; Feenstra, R. M.; Fein, A. P. Phys. Reu. B 1987, 36, 7718. (4) Epp, J. M.; Dillard, J. G. Chem. Mater. 1990, 2, 449. (5) Omstead, T. R.; Jensen, K. F. Chem. Mater. 1990, 2, 39. (6) (a) Balasubramanian, K. Chem. Phys. Leu. 1988, 150, 71. (b) Balasubramanian, K. Chem. Phys. Lett. 1990, 171, 58. (7) Zhang, Q.L.; Liu, Y.; Curl, R. F.; Tittel, F. K.; Smalley, R. E. J . Chem. Phys. 1988,88, 1670.
(8) Wang, L.; Chibante, L. P. F.; Tittel, F. K.; Curl, R. F.; Smalley, R.
E. Chem. Phys. Lett. 1990, 172, 335.
(9) Zhang, Q. M.; Chiarotti, G.; Selloni, A,; Car, R.; Parrinello, M. Phys. Reu. B 1990. 42. 5071. (10) Dobbs, K. D.; Trachtman, M.; Bock, C. W.; Cowley, A. J . Phys. Chem. 1990, 94, 5210. (11) Bock, C. W.; Trachtman, M.; Murphy, C.; Muschert, B.; Mains, G. J. J . Phys. Chem. 1991, 95, 2339.
0022-36S4/92/2096-3007$03.00/0
dimers of BH3, AlH3, and GaH, can be thought of as Lewis acid/base adducts in which the empty p orbital on an XH, molecule interacts with a bonding pair of electrons in a companion H-YH2 molecule, as well as in the more popular terms of three-center two-electron bonds. The role of group IIIA hydrides as atom traps has intrigued us since we found evidence for an H,B...Si adduct in a study of silicon boron hydridesI2 and for a H3B-.A1 adduct in a study of aluminum boron hydrides.13 We recently reported a study of GaH,, ASH,, x = 1-3, GaAs, (GaAs),, and Ga2As2H4isomers.I4 We report here a study of the Lewis acid/base adducts of substituted gallanes and arsines which is focused on the strength of the dative bonds formed and the ease with which these adducts may be distinguished from mixtures of precursors.
Computational Methods All calculations were carried out using the Gaussian 8815aof Gaussian series of programs on the Cray Y-MP/832 computer at the Pittsburgh Supercomputing Center. Geometries were optimized using the Huzinaga (43321/4321/4*) basis seti6 which includes a split valence shell and d-type polarization functions on the Ga and As atoms, and the 6-31G* basis set15 for the C, C1, F, and H atoms. This basis set will be referred to as HUZSP*. Correlation effects were included by performing single point calculations at the MP2, MP3, MP4SDQ, and MP4SDTQ/ HUZSP**//RHF/HUZSP* levels, where HUZSP** is the same basis set as HUZSP* but augmented by p-type polarization (12) Mains, G. J.; Bock, C.; Trachtman, M. J . Phys. Chem. 1989, 93, 1745. (13) Mains, G. J.; Bock, C.; Trachtman, M.; Finley, J.; McNamara, K.; Fisher, M.; Wociki, L. J . Phys. Chem. 1990, 94, 6996. (14) Bock, C. W.; Dobbs, K. D.; Mains, G. J. J . Phys. Chem. 1991, 95, 7668. (15) (a) GAUSSIAN 88, Frisch, M. J.; Head-Gordon, M.; Schlegel, H. B.; Raghavarchari, K.; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whitesides, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Fluder, E. M.; Topiol, S.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA. (b) GAUSSIAN 90, Revision F, Frish, M. J.; Head-
Gordon, M.; Trucks, G. W.; Foresman, J. B.; Schlegel, H. B.; Raghavarchari, K.; Robb, M.; Binkley, J. S.; Gonzalez, C.; Defrees, D. J.; Fox, D. J.; Whitesides, R. A.; Seeger, R.; Melius, C. F.; Baker, J.; Martin, R. L.; Kahn, L. R.; Stewart, J. J. P.; Topiol, S.; Pople, J. A. Gaussian, Inc., Pittsburgh, PA, 1990.
(16) Huzinaga, S.; Andzelm, J.; Klobukowski, M.; Radzio-Andzelm, E.; Sakai, Y.; Tatewaki, H. Gaussian Basis Sets for Molecular Calculations; Elsevier: Amsterdam, 1984.
0 1992 American Chemical Society
Bock et al.
3008 The Journal of Physical Chemistry, Vol. 96, No. 7, 1992 TABLE I: Energies" for Substituted Gallanes level
GaH3
RHF/HUZSP*//RHF/HUZSP* RHF/HUZSP**//RHF/HUZSP*
-1923.12002 -1923.125 53 RMP2/HUZSP**//RHF/HUZSP* -1923.215 45 MP3/HUZSP**//RHF/HUZSP* -1923.229 67 MP4(SDQ)/HUZSP**//RHF/ -1923.23440 HUZSP* MP4(SDTQ)/HUZSP**//RHF/ -1923.236 55 HUZSP* MP4( FULL)/HUZSP*// -1923.203 53 MP2(FULL)/ zero point energy* 12.1 figure
"In hartrees.
GaH2CI -2382.09984 -2382.10368 -2382.315 78 -2382.337 28 -2382.341 38
GaCI,
GaFH,
GaF3
-3300.04031 -2022.03621 -3300.04031 -2022.03994 -3300.497 43 -2022.301 38 -3300.532 21 -3300.53491
9.31
-1962.166 -1962.175 -1962.413 -1962.438 -1962.444
10 -2001.211 51 17 -2001.224 17 90 -2001.61021 83 73
-2220.47 1 24 -1 962.45 1 47
-2382.346 91 -3300.547 92 -2382.31996
-2219.85226 -2219.85226 -2220.453 01 -2220.438 19 -2220.456 44
GaHz(CH3) GaH(CH3)2
-2022.297 51 2.5
9.8
4.2
31.9 la
51.7 lb
kcal mol-'.
TABLE 11: Energies'' for Substituted Arsines level RHF/HUZSP*//RHF/HUZSP* RHF)HUZSP**)/RHF/HUZSP*
RMP2/HUZSP**//RHF/HUZSP* MP3/HUZSP**//RHF/HUZSP*
MP4(SDQ)/HUZSP**//RHF/HUZSP* MP4(SDTQ)/HUZSP**//RHF/HUZSP* MP2(FULL)/HUZSPS//MP2(FULL)/HUZSP* zero point energyb figure
ASH, -2233.878 34 -2233.88499 -2234.01692 -2234.036 29 -2234.040 23 -2234.042 94 -2233.999 26 14.7
AsH2(CH3) -2272.91567 -2272.952 48 -2273.20248 -2273.231 66 -2273.236 99 -2273.24433
AsH(CHd2 -2311.954 14 -231 1.96704 -2312.39020
35.2 2a
55.1 2b
" In hartrees. In kcal mol-'. functions on all the H atoms, with the exponent taken directly from the 6-31G** basis set.I5 In specific cases MPZ=FULL/ HUZSP* and MP2=FULL/HUZSP** optimizations were also performed. In the case of the three dimethyl adducts, H(CH,),Ga...AsH,, H2(CH3)Ga...AsH2(CH3), and H,Ga...AsH(CH3)2, correlation effects could only be included up to the MP2/ HUZSP* *//RHF/HUZSP* level. Vibrational frequencies were obtained for all the precursors and adducts using analytical second derivatives calculated at the RHF//HUZSP*//RHF/ HUZSP* level to assure that a stable state was obtained in all cases.
H
1.
H'TH K U
H1
U
H1
(a)
(b) Figure 1. Structural data for gallanes given at the RHF/HUZSP*// RHF/HUZSP* level: (a) GaH2(CH3)bond lengths Ga-H = 1.5864 A, Ga-C = 1,9909 A, C-H = 1.0873 A, C-HI = 1.0840; bond angles H-Ga-H = 118.75', H - C G a = 112.35'. H-C-H = 108.49', H-C-HI = 106.96'; (b) GaH(CH3), bond lengths Ga-H = 1.5918 A, G a C = 1.9937 A, C-H = 1.0877 A, C-HI = 1.0847; bond angles H-Ga-C = 118.55', H C - G a = 112.60°, H C - H = 108.30', H-C-HI = 106.90'.
Results and Discussion The Reactants. The geometries of the gallane and arsine precursors are given in Figures 1 and 2, respectively, and the corresponding energies at various computational levels are listed in Tables I and 11. GaH,. Gallane and digallane have been the subject of several recent publication~."~'~J~ GaH, exhibits D3hsymmetry in the isolated state, typical of sp2hybridization. The energies obtained are given in Table I. We find the Ga-H bond length to be 1.582, 1.583, and 1.569 A at the RHF/HUZSP*, MP2(FULL)/ HUZSP*, and MP2(FULL)/HUZSP** levels, respectively, which compares well with the CASSCF relativistic core potential values of 1.58 and 1.557 A reported by Balasubramanian,Is before and after he employed second order configuration interaction (SOCI). At the MP2( FULL)/HUZSP* * //MP2( FULL)/HUZSP* * level, the dissociation energy is found to be 75.8 kcal mol-', compared with a value of 80.9 kcal mol-' found by Balas~bramanian.~~ Thus, as noted previously,I8 GaH2 is much less stable than might be expected as a consequence of the closed shell electronic structures of both GaH and GaH,. CaH2Cl. Chlorogallane exhibits C , symmetry as expected. The Ga-H and Ga-C1 bond lengths at the MP2(FULL)/ HUZSP*//MP2(FULL)/HUZSP* level were 1.5697 and 2.1569 A, respectively. The H-Ga-CI bond angle is 115.3' at the same level. Energies are given in Table I. Chlorination does not destroy the planarity of the molecule, but the H-Ga-Cl bond angle reduction suggests repulsion between the positive H atoms and/or attraction between the positive H atoms and the negative halogen
atom. The energy of the empty p orbital (LUMO) on Ga in monochlorogallane is significantly lower than that in unsubstituted gallane. As in the case of gallane, GaHzCl probably exists as the dimer in nature.19 Indeed, based upon the evidence for me-
(17) Lammertsma, K.; Leszczynski, J. J . Phys. Chem. 1990. 94, 2806. (18) Balasubramanian, K. Chem. Phys. Leu. 1989, 164, 231.
(19) Sheka, L. A.; Chaw, I. S.; Mityureva, T. T. The Chemistry of Gallium;Elsevier: New York, 1966; pp 71-78.
(a) (b) Figure 2. Structural data for arsines given at the RHF/HUZSP*//
RHF/HUZSP* level: (a) AsH2(CH3)bond lengths As-H = 1.5188 A, As-C = 1.9779 A, C-H = 1.0828 A, C-HI = 1.0806 A;bond angles H-AS-H = 93.49', H-AS-C = 96.83', As-C-H = 108.66', As GaCl, > GaH2F > GaH, > GaH,Cl > GaH,CH, > GaH(CH,),. The order of the first two is particularly noteworthy, as it is the reverse of that reported for the corresponding boron compounds, where the central atom is considerably smaller. Similarly, with gallane as the common Lewis acid and again using the dissociation energies of the adducts as a measure of base strength, we find the order to be AsH(CH,), > ASH,(CH,) > ASH,, which is the expected order. As can be seen from Tables VI and VII, the adducts are readily identified from the parent hydrides and, because of the different physical properties, should lead to readily purified adducts. Since purity is of primary importance, the advantage of using an adduct in the MOCVD process for the preparation of thin film 111-V semiconductors is obvious. In fact, adducts are also useful for epitaxial growth of GaAs, where it is found that the adduct between diethylchlorogallane and triethylarsine decomposes between 450 and 700 'C to give epitaxial layers of GaAs. Using these calculations as a guide, we hope the adducts calculated here can be prepared to determine if they are practical candidates for MOCVD deposition of GaAs. Acknowledgment. M.T. acknowledges support from the Pittsburgh Supercomputer Center, Grant No. C H R S J P , for computer time on the GRAY Y-MP/832. G.J.M. is grateful for an NCSA Grant CHE890003N which permitted access to the CRAY-2 system and to Oklahoma State University for access to the IBM 3090 which contributed to the understanding of these systems. Registry No. GaH,, 13572-93-5; GaH2C1, 43311-11-1; GaCI,, 13450-90-3; GaFH,, 139276-45-2; GaF,, 7783-51-9; GaH2(CH,), 122631-94-1; GaH(CH,),, 103680-44-0; AsHzCH,, 593-52-2; ASH,, 7784-42-1; AsH(CH,),, 593-57-7; H,GP*.AsH~,139276-46-3; H3Gp.e AsH2(CH,), 139276-53-2; H,G~.-AsH(CH,)~,139276-54-3; C13GaASH,, 139276-48-5; H,CIGa.-AsH,, 139276-47-4; F,Ga-AsH,, 139276-50-9; FH2Ga-.AsH3, 139276-49-6; (CH,)H2Ga-AsH3, 139276-51-0; (CH,)2HGa.-AsH,, 139276-52-1; H2(CH3)Ga-AsH2(CH,), 139276-55-4.
