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An ab initio and density functional theory study of the trimethylgallium-hydrogen selenide adduct: (CH3)3Ga:SeH2
THEO CHEM Journal of Molecular Structure (Theochem) 432 (1998)129- 137
ELSEVIER
An ab initio and density functional theory study of the trimethylgallium-hydrogen selenide adduct: (CH&Ga:SeH* N. Maung Advanced Materials Research Laboratory.
North East Wales Institute. Plas Coch, Mold Road, Wrexham LLII ?AW, UK
Received 7 November 1997; revised 10 December 1997; accepted 19 December 1997
Abstract The molecular structure (equilibrium geometry) and binding energy of the trimethylgallium (TMGa)-hydrogen selenide (H$e) adduct, (CHj)XGa:SeHz, have been computed using ab initio molecular orbital and density functional theory (DFT) methods, and where possible, compared with experimental results. The structure of the precursors TMGa and HzSe are perturbed to only a small extent upon adduct formation. (CH3)3Ga:SeH, was found to be = 16.6 kcal mol-’ less stable than the precursors at the B3LYP/6-31 l+G(2d,p)//B3LYP/6-31 l+G(2d,p) level of computation indicating that the (CH3)3Ga:SeH2 adduct is unlikely to be a stable gas phase species under MOCVD growth conditions. However, the H3Ga:SeH2 adduct was found to be = 6.5 kcal mol-’ more stable than the precursors at the B3LYP/6-311 + G(2d,p)//B3LYP/6-31 l+G(2d,p) level indicating that H,Ga:SeH? should be a stable Ga-Se bonded species. 0 1998 Elsevier Science B.V. Keywords: MOCVD; Trimethylgallium-hydrogen Density functional theory; Binding energy
selenide
1. Introduction Recent
developments
in thin film growth
techni-
ques, particularly metal-organic chemical vapour deposition (MOCVD) have led to dramatic improvements in the purity and structural perfection of semiconductor thin films. Epitaxial films of the cubic wide band gap III-VI (13-16) semiconductor gallium selenide, GazSej, have potential applications in short-wavelength optoelectronic devices, e.g. greenblue light-emitting diodes [ 1,2]. The growth of GazSej has been carried out using trimethylgallium (TMGa) with hydrogen selenide (H2Se) [3]. Although Ga2Se3 with adequate structural properties could be obtained [4-61, a severe parasitic gas phase reaction occurs with these reagents, 0166-1280/98/$19.00
0 1998 Elsevier
PII SO 166- 1280(98)00044-X
adduct; Gallane-hydrogen
selenide
adduct; Molecular
orbital;
compromising the compositional uniformity and surface morphology of the epilayers. More recent attempts to improve GazSej materials quality have resulted from the use of alternative organoselenium reagents [7,8] and this has been shown to enhance the structural quality of the Ga2Se3 films [6]. Although the quality of epitaxial films in thin film technologies is of critical importance there is little or no concensus regarding the details of the growth process. Other than the overall stoichiometry of the reaction, in the case of Ga2Se3 growth from TMGa and H2Se 2(CH,),Ga+
3H2Se -
GafSe3 + 6CH4
little is known about the participation of any intermediate species that may be created or destroyed in
Science B.V. All rights reserved.
130
N. Muung/Journal
of Molecular Structure (Throchem) 432 (1998) 129-137
the growth process [SJ. The ability of individual (CHj)@a molecules to form complexes with Lewis bases is well documented [9] and Piocos and Ault have investigated the formation of molecular complexes of trimethylgallium with group V hydrides [lo-121 using matrix isolation and cryogenic thin film techniques. Ab initio molecular orbital calculations describing the geometries, harmonic frequencies and binding energies of the gallane and TMGa adducts with AsH3 have also been published [ 13,141. These results all support the MOCVD observations that at or near room temperature there are very rapid reactions when mixing TMGa and HzSe [3]. In this paper, we report the results of allelectron ab initio molecular orbital and density functional theory (DFT) calculations, both local and non-local gradient corrected methods, of the l:] trimethylgallium-hydrogen selenide complex and the analogous Lewis acid/base adduct formed between gallane, GaH3, and H#e, and discuss their properties with reference to the related experimental data of Piocos and Ault [lo- 121 and recent theoretical data [13,14].
density functional theory (DFT) methods [18]. Local density functional (LDF) theory using the SVWN (Slater LSD exchange functional with local correlation provided by the Vosko, Wilk and Nusair expression), and non-local hybrid-density functional theory B3LYP (Becke’s three-parameter exchange functional with non-local correlation provided by the Lee-Yang-Parr (LYP) expression) and non-local BLYP methods were used with the 6-31 l+G(2d,p) basis set. This was the largest basis set that we could reasonably handle given our computational resources. The standard GAUSSIAN basis sets [19] and default convergence criteria were used. Since, scale factors for the 6-31 l+G(2d,p) basis set have not yet been published the scale factors for the appropriate 6-3lG(d) basis set were used for scaling the zero-point and thermal energies at the various computational levels [20]: RHF/3-21G* 0.9404, RHF/6-3 lG(d) 0.9135, B3LYP/6-3 lG(d) 0.9804, BLYP/6-3lG(d) 1.0119 and SVWN/6-3lG(d) 1.0079. A scale factor of 0.9613 at the B3LYP/6-3 lG(d) level was used for adjusting the computed harmonic vibrational frequencies at the B3LYP/631 l+G(2d,p)//B3LYP/6-31 l+G(2d,p) level [20].
2. Procedure 3. Results and discussion All calculations were performed using the GAUSSIAN 94 [ 151 program package on a Silicon Graphics RlOOOO workstation. The SPARTAN 4.1.2 [ 161 program was used to generate starting structures and to animate vibrational frequencies. The geometries of TMGa, H2Se and the (CH3)3Ga:SeH2 adduct were initially optimized at the restricted HartreeFock (RHF), RHF/3-2lG* level. The RHF/3-2lG* model has previously demonstrated its effectiveness in providing satisfactory equilibrium geometries for molecules incorporating third- and fourth-row, main group elements. However, previous calculations on donor-acceptor complexes [ 17 J have indicated that a larger basis set is probably needed to better describe the Lewis linkage, here Ga-Se, and so we also carried out geometry optimizations at the RHF/ 6-3 1 l+G(2d,p) level. Practical considerations prevented us from optimizing the structures at the second-order Moller-Plesset (MP2) level of theory and so the effect of electron correlation has been studied by performing geometry optimizations using
The growth of high quality Ga$e3 at low temperatures using TMGa and H2Se taken together with the observation of the extensive room temperature parasitic gas phase reaction, clearly indicates that different mechanisms control the deposition of GazSej from this precursor combination compared with Se metalorganic sources. The detailed mechanism by which this reaction proceeds is unknown. A possible mechanism, consistent with the little experimental data, invokes the formation of a (CHj)8Ga:SeH2 adduct between TMGa and H+e followed by successive methane elimination reactions as in the following sequence: Adduct formation (CH,)3Ga + HzSe * (CH&Ga
: SeH,
(1)
Elimination HzSe : Ga(CHs), -
HSe-Ga(CH3)2
+ CH,
(2)
N. Maung/Journal of Molecular Structure (Theochem) 432 (1998) 129-137
131
average Ga-C-H bond angle
1.464 (1.453) (1.470) ,a?’ H
92.1 (93.2) (91.3)
111.0 (111.3) (111.1) [lll.O]
H
2.010
120
(112.9)
H
PO.81
90.6
H
Fig. I. RHF/3-21G*, RHF/6-31 l+G(2d,p) (in parentheses), B3LYP/6- 3 1 l+G(2d,p) (curly brackets), BLYP/6-31 I+G(2d,p) (square brackets) optimized geometries with experimental values in italics, for (CHT)&a [14], HzSe 1171 and (CH,),Ga:SeH*. The bond lengths are in .& and the bond angles in degrees ((u and /3 refer to the a(Ga-C-H) values in Table 1).
HSe-Ga(CH3)2
-
CHsGa=Se
+ CH4
(3)
Specifically, we are interested in the stability of the (CH3)3Ga:SeH2 complex and so this species was investigated initially in order to assess the likelihood of the above scheme. Optimized geometrical parameters of the source gas molecules are given in Fig. 1 and Table 1 (SVWN LDF method) along with the available experimental data. Even the RHF/3-21G* level does a reasonable job of describing the structures of the source precursors. The geometries of H$e, TMGa and (CH3)3Ga:SeH2 computed at the B3LYP/
6-3 11+G(2d,p) level are taken to be the most representative. For H$e, the computed Se-H bond is 0.01 A longer, and the HSeH bond angle is 0.7” larger than the corresponding experimental values [ 171. For TMGa the computed Ga-C bond length of 1.990 A, is some 0.022 A longer than the experimental value [14]. The average computed C-H bond length, 1.093 A, and the GaCH bond angle, 111. l”, are very similar to the experimental results. At the RHF/3-2 lG* level the Ga-Se bond for the (CH3)sGa:SeH2 complex is 2.809 A, but one would expect the long Ga-Se bond length to be basis set dependent and it is -0.45 A longer with the larger
132
N. Maung/Journal
Table I Optimized
geometries
and the electronic
properties
of Molecular Structure (Theochem) 432 (1998) 129-137
of the source precursors
at the SVWN/6-3 I I+G(Zd,p) level”
calculated
TMGa (Djh symmetry) SVWN/6-3 I l+G(2d,p)//SVWN/6-3 r(Ga-C)
I l+G(Zd,p)
Expt. h
1.946
1.968 I .087 120.0 I I I.2
I.101 120.0 ol(l13.3)and/3(110.0)(111.1)
r(C-H) a(C-Ga-C) a(Ga-C-H) HzSe (Cl, symmetry)
SVWN/6-31 I+G(2d,p)NSVWN/6-3 r(Se-H) cl(H-Se-H) GaH
I I+G(2d,p)
Expt.’
I .46 90.6
I .474 90.3
i (Dih symmetry) SVWN/6-31 l+G(2d,p)//SVWN/6-3
r(Ga-H) a(H-Ga-H)
I l+G(2d,p)
Exp1.d
I.558 120.0
I .560 120.0
‘r and a indicate the bond length in A and the bond angles in degrees, respectively. and p correspond to those displayed in Fig. I .) hRef. 1141. ‘Ref. [ 171. dRef. [2l].
RHF/6-3 1 l+G(2d,p) basis set. The calculated Ga-Se bond distance of 3.258 A at the RHF/6-31 l+G(2d,p) level is probably too long with this truncated basis set. The Ga-Se bond distance found here is in good agreement with a value of 3.079 A for the Ga-As bond length computed by Bock and Trachtman [ 141, using a modified Huzinaga basis set for Ga and As, for
(In the case of u(Ga-C-H)
the trimethylgallium-arsine adduct, (CHs)sGa:AsHs. Although longer donor-acceptor linkages compared to normal single bonds are not unusual, we would expect some shortening of the Ga-Se bond if electron correlation were included in the geometry optimization. This is indeed the case, and at the B3LYPi 6-31 l+G(2d,p) level the calculated Ga-Se bond
Table 2 Calculated total energies (Hartrees), zero-point energies (ZPE, kcal mol.‘), thermal corrections at 0 K (kcal mol-‘) and at 298 K (kcal mol-‘) for the (CH1)?Ga:SeH2 adduct Method
(CH ,) ,Ga:SeH,
for TMGa the bond angles a
to 298 K (TC, Hartrees) and binding energies
H,Se
(CHx)qGa
Binding
energy
at 0 K” RHF/3-21G*//RHF/3-2lG* ZPEIRHFI3.2lG*
- 4422.066723
- 2032.2 I2732
82.7
71.2
0. I35678
TC RHF/6-31
I+G(Zd,p)//RHF/6-31
ZPE/RHF/6-3
l+G(Zd,p)
I I+G(Sd,p)
TC
- 2042.08475
103.9
69.7
I I+G(Zd,p)NB3LYP/6-31
ZPE/B3LYP/6-3
I+C(Zd,p)
-4447.412015 97.7
I I + G(2d.p)
BLYP/6-31
I+G(2p,d)//BLYP/6-3
ZPE/BLYP/6-31
I I+G(Zd,p)
I + G(2d.p)
- 2044.582778
95.3
64.6
0. I m4YO
TC SVWNlh-3
I l+G(2d,p)//SVWN/6-31
ZPE/SVWN/6-31
IcG(2d.p)
lffi(2d.p)
- 444 I .639744 95.3
0.1 I1315 - 2041.585738 64.6
0.159568
TC
“Numbers in parentheses
are binding energy values without correction
- 20. I (2.8)
-
IX.2
-
18.6 (3.9)
-
16.6
-
19.8 (2.9)
-
17.8
0.016077 I
0.110648
- 4447.334608
- 2400.95Y470 9.1
66.3
0.159473
TC
8.9
0.016537 I
II.108617 - 2044.65438
10.1 (12.4)
9.1
0. I I3656
- 4443.048736
0.158152
B3LYP/6-3
- 2389.834299
Binding at 298 K
0. I 10856
for zero-point
- 2402.751419 x.5 0.016130 - 2402.747152 8.2 0.016146 - 2400.0329
I7
8.4 0.016303
energy differences.
- 9.2 (13.2)
- 7.1
energy
133
N. Maung/Joumal of Molecular Structure (Theochem) 432 (I 998) 129-137
Table 3 Optimized
geometries
of the (CH2),Ga:SeH2
and HsGa:SeH:,
adducts calculated
SVWN/6-31 l+G(2d,p)//SVWN/6-31 (CHj),Ga:SeH2
at the SVWN/6-31 l+G(2d,p)
l+G(2d,p) HsGa:SeH2
(C, symmetry)
-
I.562
1.101 118.9 I 11.4 -
r(C-H) a(C-Ga-C) a(Ga-C-H) a(H-Ga-H) a(H-Se-Ga) a(C-Ga-Se) r(Se-H) a(H-Se-H) r(Ga-Se)
(C, symmetry)
-
1.961
r(Ga-C) r(Ga-H)
levela
118.9 97.9 99.9 1.478 90.5 2.596
95.9 95.2 1.478 90.4 2.682
‘r and a indicate the bond length in .k and the bond angles in degrees, respectively.
length is 2.998 A, although at the pure DFf BLYP/631 l+G(2d,p) level the calculated Ga-Se bond length is 3.060 A (see Fig. 1). Table 2 gives the total energies and binding energies for the reaction (CH&Ga
: SeH2 -
(CH&Ga
+ H2Se
At the highest levels of computation, B3LYP/ 6-31 l+G(2d,p)//B3LYP/6-31 l+G(2d,p) and BLYP/ 6-31 l+G(2d,p)//BLYP/6-31 l+G(2d,p), the (CHJ)3 Ga:SeH* complex is predicted to be -16.6 and -17.8 kcal mol-’ less stable, respectively, than the precursors at 298 K. For the (CHj)3Ga:AsH3 adduct Bock and Trachtman [ 141 found the adduct to be more stable than the precursors by -6.1 kcal mol-’ at the MP2/HUZSP*//RHF/HUZSP* level. The instability of the (CH3)sGa:SeH2 complex compared with the (CH3)sGa:AsH3 adduct is in agreement with the known ordering of AsH3 and H#e base strengths [9]. As is usually found with LDF theory, an inadequate description of the metal-ligand bond with the SVWN model (Table 1) leads to a very short dative Ga-Se bond of only 2.682 A (Table 3). This is -0.3 and -0.4 A shorter than the Ga-Se bond lengths computed using the B3LYP and BLYP models, respectively (see Fig. 1). The importance of nonlocal corrections is further illustrated by the enhanced stability of the SVWN adduct compared to the precursors, where the computed binding energy (Table 2) indicates that (CH3)sGa:SeH2 is -7.1 kcal mol-’ less stable than the precursors.
We were interested in determining the effect of Ga alkylation on the binding energy of the adduct and so also carried out computations on the HjGa:SeH* complex using the same theoretical models. Although monomeric GaH3 has not yet been isolated high level ab initio calculations have been performed [2 l-231. Balasubramanian [2 l] using multiconfiguration SCF complete active space calculations (CASSCF) followed by full second-order configuration interaction (SOCI) found GaH3 to have Dxhsymmetry with a computed Ga-H bond length of 1.56 A. Optimized geometrical parameters for GaH3 are given in Fig. 2 and Table 1 (SVWN LDF method) and for the H3Ga:SeHz adduct in Fig. 2 and Table 3 (SVWN LDF method). FOJ GaH3, the computed Ga-H bond is just 0.006 A longer than the high level ab initio estimate [21]. This is better than the level of agreement (-0.02 A longer) obtained by Dobbs et al. [ 131 using a modified Huzinaga basis set, HUZSP*. For the adduct dissociation reaction involving H3Ga:SeH2 (see Table 4) H,Ga : SeH2 -
H1Ga+H2Se
the H3Ga:SeH2 complex is predicted to be -6.5 kcal mol-’ more stable than the precursors at the B3LYP/ 6-31 l+G(2d,p)//B3LYP/6-31 l+G(2d,p) level and -5.9 kcal mol-’ more stable than the precursors at the BLYP/6-3 1 l+G(2d,p)//BLYP/6-31 l+G(2d,p) level. Once again, the local SVWN/6-3 1 l+G(2d,p)// SVWN/6-31 l+G(2d,p) level estimate of the binding
N. Maung/Journal
134 1.464 (1.453) (1.470) [1.482]
H \
I20 c (93.2) (91.3) 190.81
of Molecular Structure (Theochem) 432 (1998) 129-137
1s93 (1.573) (1.566) [ 1.5751 1.560
Ga -H
/
H
90.6
Da
1.601 (1.578) (1.571) [ I.5801
97.8
97.6 (98.3) (97.8) [97.8]
^^ 118.8 (119.4) (119.2) I 1wzl ;’
1.462 (1.453) (1.472) [ 1.4851 .J ,...”+ 92.3 (93.5) (91.4) w.91
H
A.
H
[2.838]
Cl Fig. 2. RHF/3-21G*, RHF/6-31 l+G(Zd,p) (in parentheses), B3LYP/ 6-31 l+G(2d,p) (curly brackets), BLYP/6-31 I+G(2d,p) (square brackets) optimized geometries with experimental values in italics, for H2Se [17], GaHl [21] and H1Ga:SeH2. The bond lengths are in A and the bond angles in degrees.
energy at 17.4 kcal mall’, is substantially larger than all the other estimates and some 5 kcal mol-’ more stable than the binding energy calculated at the RHF/3-21G* level. The H3Ga:SeH2 adduct is thus substantially more stable than the (CHJ3Ga:SeHz adduct and this is in agreement with simple chemical arguments which would predict that methyl substitution donates electrons into the empty 4p orbitals on the gallium, thus making trimethylgalhum a weaker Lewis acid than GaH3. Furthermore, the lower binding energy of the TMGa adduct is consistent with its longer SF--Se bond length (2.998 A compared with 2.803 A for H3Ga:SeH2) at the B3LYP/6-31 l+G(2d,p)NB3LYP/
6-31 l+G(2d,p) level. The distortion from planarity of the GaC3 backbone in (CH3)sGa:SeH2 (i.e. LCGaSe = 94.2”) is less than the distortion from planarity of GaH3 in H3Ga:SeH2 (i.e. LHGaSe = 98.1”). Furthermore, in both adducts the H2Se moiety distorts towards planarity, and the HSeGa angle is virtually identical for both H3Ga:SeH2 and (CH3),Ga:SeH2. Thus, the precursors of (CH3)RGa:SeH2 are distorted less than the precursors of H3Ga:SeH2, again suggesting that the Lewis interaction is weaker in the methylated adduct. The Ga-Se bond is the result of a donor-acceptor interaction between a lone pair on H2Se with the empty 41, orbital on the Ga atom of either TMGa or GaH3. The weak interactions in this case are illustrated by the Mulliken charge distribution (Table 5) at the RHF/6-31 l+G(2d,p) level which shows little or no charge transfer from the Se atom on the H$e group to the Ga atom on the TMGa group or the GaH3 group. In the case of the TMGa adduct the Ga atom acquires further positive charge (+O.O9e) on adduct formation while the Se atom acquires excess negative charge (-0.0%). In the case of the GaHs adduct the Ga atom acquires excess positive charge (+O.O5e) and the Se atom again acquires excess negative charge (-0.05e). The changes are due to the methyl carbons acquiring (-0.03e) from the Ga atom in the case of the TMGa adduct while the hydrogen atoms acquire (-0.03s) from the Ga atom in the case of the GaH3 adduct. The computed harmonic frequencies for the HiGa:SeHz adduct and parent hydrides are listed in Table 6. Also included in the table are the experimental frequencies for H$e [24] and recent ab initio data for GaH7 [ 131. The computed frequencies have been scaled using the B3LYP/6-3 1G(d) scaling factor 1201. In the case of H2Se very good agreement is obtained with experiment. For GaH3 the level of agreement is not as good but still adequate to allow comment on the H3Ga:SeH2 adduct. All the hydride fundamentals are red shifted in the adduct with respect to the individual hydrides. This is not inconsistent with the binding energy estimate which would imply a weakly bound H3Ga:SeH2 adduct. Both the binding energy estimate and the pattern (direction and magnitude) of frequencies are comparable to those found by Bock and Trachtman [ 141 for the trimethylgallium-arsine adduct.
135
N. Maung/Joumal of Molecular Structure (Theochem) 432 (1998) 129-137
Table 4 Calculated total energies (Hartrees), zero-point energies (ZPE, kcal mol-‘), thermal corrections 0 K (kcal mol-‘) and at 298 K (kcal mol-‘) for the HJGa:SeH2 adduct H ,Ga:SeH z
Method
RHF/3-21G*lIRHF/3-2IG* ZPElRHFI3.2IG* TC RHF/6-31 I+G(2d,p)//RHF/6-31 I+G(Zd.p) ZPEIRHFI6-3 I l+G(Zd.p) TC B3LYP/6-31 I+G(2d,p)//B3LYP/6-31 l+G(Zd,p) ZPuB3LYP/6-3 I I+G(Zd.p) TC BLYP/6-31 I+G(2d,p)//BLYP/6-31 I+G(2d,p) ZPEIBLYPb3 I l+G(Zd,p) TC SVWN/6-31 I+G(2d,p)llSVWN/6-31 I+G(Zd.p) ZPEISVWNb31 l+G(Zd.p) TC
“Numbers in parentheses
- 4305.563360 23.8 0.041488 - 4325.886509 23.4 0.040357 - 4329.403750 22.3 0.040999 - 4329.405069 21.7 0.04 1034 - 4324.295163 22.4 0.041713
to 298 K (TC, Hartrees) and binding energies at
Binding energy atOK”
GaH,
- 2389.834299 9. I 0.016537 - 2400.959470
- 1915.705194 Il.9 0.020925 - 1924.918094 12.1 0.020783 - 1926.638361 Il.6 0.02 I334 - 1926.645027 II.3 0.02 I342 - 1924.230898 I I.6 0.021742
are binding energy values without correction
4. Conclusion The suggestion that the gas phase interaction between TMGa and H2Se during MOCVD growth of GazSe3 may result in the formation of an intermediate (CH3)3Ga:SeH2 adduct has been investigated theoretically. Ab initio calculations at the RHF/6-31 l+G(2d,p) level and DFT calculations at the B3LYP/6-31 l+G(2d,p), BLYP/6-31 l+G(2d,p) and SVWN/6-3 1 l+G(2d,p) levels indicate that such a complex is unstable with respect to its components (binding energy - - 17 kcal malli) at 298 K in general agreement with the trends observed in the published matrix isolation data for trimethylgallium
9.1 0.016077 - 2402.751419 8.5 0.016130 - 2402.747 I52 8.2 0.016146 - 2400.032917 x.4 0.016303
for zero-point
Bindmg energy at 298 K
12.3 (15.0)
12.5
3.6 (5.6)
3.4
6.6 (8.8)
6.5
5.9 (8 II
5.‘)
17.3 (19.7)
17.4
energy differences.
with group V bases. Although more extensive allelectron methods with more complete basis sets, namely 6-31 l+G(2df,2p) or better, are required in order to obtain more accurate values for the binding energies, it is probably the case that they will not change the binding energy sufficiently to change the conclusion that the molecular complex is extremely unstable. By contrast, the analogous H3Ga:SeHz adduct is predicted to be a stable Ga-Se bonded species at all levels of computation examined here, with a binding energy at the B3LYP/6-31 l+G(2d,p)//B3LYP/6-31 l+G(2d,p) level of -6.5 kcal mall’ at 298 K. In conclusion, while the (CH3)3Ga:SeH2 adduct formed by mixing TMGa
Table 5 Mulliken net charges, qi for the atoms i in the source molecules, the (CH3)3Ga:SeH2 and H1Ga:SeH2 adducts and their dipole moments, CL,in D computed at the RHF/6-31 I+G(2d,p)//RHF/6-31 l+G(2d,p) level. The values in parentheses refer to either experimental values or high level ab initio results Electronic
properties
qGa
9c qu, %, 4 Se % fi aRef. [ 141. bRef. [ 171. ‘Ref. [21].
(CH&Ga
HzSe
1.62 -0.92 0.13 N/A N/A N/A 0.01 (0)”
N/A N/A N/A N/A -0.14 0.07 0.84 (0.40)b
GaH, 0.49 N/A N/A -0.16 N/A N/A 0 (0)’
(CH&Ga:SeH,
H iGa:SeH,
1.71 -0.95 0.12 N/A -0.19 0.11 I .75
0.54 N/A N/A -0.19 -0.19 0.1 I 2.12
136
N. Maung/Journal of Molecular Structure (Theochem) 432 (1998) 129-137
Table 6 Comparison of fundamental frequencies (cm-‘) computed at the B3LYP/6-31 l+G(2d,p)NB3LYP/6-31 H,Ga:SeH* adduct and the parent hydrides Parent hydride experimental”
751 825 1034 2006 202 I 2345 2358
Parent hydride computed h
693 722 1039 I896 1898 2329 2344
HIGa:SeHZ
28 107 273 298 454 499 684 708,711 I022 1859, 1862 1869 2312 2326
l+G(Zd,p) level and assignments
Freq. shift
for the
Assignment
az, twist about Ga-Se a ,, Ga-Se sym st
-9 (-14,-l I) -17 (-37,-34) -29 -17 -18
a?, GaHl e2, GaH3 a i, HzSe eZ, GaH1 a,, GaHl a i, HzSe b2? HzSe
sym def assym def bend assym st sym st sym st assym st
‘Ref. [24]. hRef. [21].
and H2Se during MOCVD growth is most unlikely to be a stable complex, it could exist as a transient species giving rise to the subsequent methane elimination reactions and deposition of Ga2Se3 amorphous/ polymeric deposits.
Acknowledgements We would like to thank the Higher Education Funding Council for Wales (HEFCW) for provision of a Silicon Graphics RlOOOO workstation.
References [I ] N. Teraguchi, F. Kato, M. Konagai, K. Takahashi, Jpn. J. Appl. Phys. 28 (1989) L2134. [2] J.O. Williams, N. Maung, AC. Wright, Metal-organic chemical vapour deposition (MOCVD) for the preparation of new materials and devices, IUPAC Monographs on Chemistry for the 21st Century, Interfacial Science, Blackwell, Oxford, 1997. [3] N. Maung, G.H. Fan, T.L. Ng, J.O. Williams, AC. Wright. J. Crystal Growth 158 (1996) 68-78. [4] D. Morley, M. von der Emde, D.R.T. Zahn, V. Offerman, T.L. Ng, N. Maung, A.C. Wright, G.H. Fan, I.B. Poole, J.O. Williams, J. Appl. Phys. 79 (6) (1996) 3 196. [5] M. von der Emde, D.R.T. Zahn, T.L. Ng, N. Maung, G.H. Fan, LB. Poole, J.O. Williams, A.C. Wright, Appl. Surf. Sci. 104 (1996) 575-579.
[6] T.L. Ng, N. Maung, G.H. Fan, I.B. Poole, J.O. Williams, A.C. Wright, Advanced Materials 2 (5) (I 996) 185- 189. [7] G.H. Fan, N. Maung, T.L. Ng, P.F. Heelis, J.O. Williams, AC. Wright, D.F. Foster, D.J. Cole-Hamilton, J. Crystal Growth 170 (1997) 485. [8] A.C. Jones, P. O’Brien, The Chemistry of Compound Semiconductor CVD - Preparation and Uses, VCH, Weinheim, 1997. [9] G.E. Coates, M.L.H. Green, K. Wade, Organometallic Compounds, Vol. I, 3rd edn, Methuen, London, 1967. [lo] E.A. Piocos, B.S. Ault, J. Am. Chem. Sot. 111 (1989) 8978. [I 11 E.A. Piocos, B.S. Ault, J. Phys. Chem. 95 (1991) 6827. [I21 E.A. Piocos, B.S. Ault, J. Phys. Chem. 96 (1992) 7589. [ 131 K.D. Dobbs, M. Trachtman, C.W. Bock, A.H. Cowley, J. Phys. Chem. 94 (1990) 5210. [ 141 C.W. Bock, M. Trachtman, Structural Chemistry 4 (I) (1993) 15. 1151 M.J. Frisch, G.W. Trucks, H.B. Schlegel, P.M.W. Gill, B.C. Johnson, M.A. Robb, J.R. Cheeseman, T.A. Keith, G.A. Petersson, J.A. Montgomery, K. Raghavachari, M.A. Al-Laham, V.G. Zakrzewski, J.V. Ortiz, J.B. Foresman, J. Cioslowski, B.B. Strefanov, A. Nanayakkara, M. Challacombe, C.Y. Peng, P.Y. Ayala, W. Chen, M.W. Wong, J.L. Andres, E.S. Replogle, R. Gomperts, R.L. Martin, D.J. Fox, J.S. Binkley, D.J. Defrees, J. Baker, J.P. Stewart, M. Head-Gordon, C. Gonzalez, J.A. Pople, GAUSSIAN 94 (Revision E. I), Gaussian Inc., Pittsburgh, PA, 1995. [ 16) SPARTAN version 4.1.2, Wavefunction Inc., I8401 Von Karman Ave., #370. Irvine, CA 92715, U.S.A., 01995 Wavefunction Inc. [I71 W.J. Hehre, L. Radom, P.R. v. Schleyer, J.A. Pople, Ab Initio Molecular Orbital Theory, John Wiley, New York, 1986. [I81 R.G. Parr, W.R. Kenan Jr., Y. Weitao, Density-Functional
N. h4aung/Journal
of Molecular
Strucrure
Theory of Atoms and Molecules, Oxford University Press, Oxford, 1995. [19] Gaussian 94 (Revision D.1 , higher) User’s Reference, Gaussian Inc., Pittsburgh, PA, 1994- 1996. [20] J.B. Foresman, A.E. Frisch, Exploring Chemistry with Electronic Structure Methods, 2nd edn, Gaussian Inc., Pittsburgh, PA, 1995-1996.
(Theochem)
432 (1998) 129-137
131
[2 I] K. Balasubramanian, Chem. Phys. Lett. 164 (1989) 23 I [22] C. Liang, R.D. Davy, H.F. Schaefer III, Chem. Phys. Lett. 159 (1989) 393. [23] K. Lammertsma, J. Leszczynski. J. Phys. Chem. 94 (1990) 2806. [24] K. Nakamoto, Infrared Spectra of Inorganic and Co-ordination Compounds, 2nd edn, John Wiley, New York. 1970.
ELSEVIER
An ab initio and density functional theory study of the trimethylgallium-hydrogen selenide adduct: (CH&Ga:SeH* N. Maung Advanced Materials Research Laboratory.
North East Wales Institute. Plas Coch, Mold Road, Wrexham LLII ?AW, UK
Received 7 November 1997; revised 10 December 1997; accepted 19 December 1997
Abstract The molecular structure (equilibrium geometry) and binding energy of the trimethylgallium (TMGa)-hydrogen selenide (H$e) adduct, (CHj)XGa:SeHz, have been computed using ab initio molecular orbital and density functional theory (DFT) methods, and where possible, compared with experimental results. The structure of the precursors TMGa and HzSe are perturbed to only a small extent upon adduct formation. (CH3)3Ga:SeH, was found to be = 16.6 kcal mol-’ less stable than the precursors at the B3LYP/6-31 l+G(2d,p)//B3LYP/6-31 l+G(2d,p) level of computation indicating that the (CH3)3Ga:SeH2 adduct is unlikely to be a stable gas phase species under MOCVD growth conditions. However, the H3Ga:SeH2 adduct was found to be = 6.5 kcal mol-’ more stable than the precursors at the B3LYP/6-311 + G(2d,p)//B3LYP/6-31 l+G(2d,p) level indicating that H,Ga:SeH? should be a stable Ga-Se bonded species. 0 1998 Elsevier Science B.V. Keywords: MOCVD; Trimethylgallium-hydrogen Density functional theory; Binding energy
selenide
1. Introduction Recent
developments
in thin film growth
techni-
ques, particularly metal-organic chemical vapour deposition (MOCVD) have led to dramatic improvements in the purity and structural perfection of semiconductor thin films. Epitaxial films of the cubic wide band gap III-VI (13-16) semiconductor gallium selenide, GazSej, have potential applications in short-wavelength optoelectronic devices, e.g. greenblue light-emitting diodes [ 1,2]. The growth of GazSej has been carried out using trimethylgallium (TMGa) with hydrogen selenide (H2Se) [3]. Although Ga2Se3 with adequate structural properties could be obtained [4-61, a severe parasitic gas phase reaction occurs with these reagents, 0166-1280/98/$19.00
0 1998 Elsevier
PII SO 166- 1280(98)00044-X
adduct; Gallane-hydrogen
selenide
adduct; Molecular
orbital;
compromising the compositional uniformity and surface morphology of the epilayers. More recent attempts to improve GazSej materials quality have resulted from the use of alternative organoselenium reagents [7,8] and this has been shown to enhance the structural quality of the Ga2Se3 films [6]. Although the quality of epitaxial films in thin film technologies is of critical importance there is little or no concensus regarding the details of the growth process. Other than the overall stoichiometry of the reaction, in the case of Ga2Se3 growth from TMGa and H2Se 2(CH,),Ga+
3H2Se -
GafSe3 + 6CH4
little is known about the participation of any intermediate species that may be created or destroyed in
Science B.V. All rights reserved.
130
N. Muung/Journal
of Molecular Structure (Throchem) 432 (1998) 129-137
the growth process [SJ. The ability of individual (CHj)@a molecules to form complexes with Lewis bases is well documented [9] and Piocos and Ault have investigated the formation of molecular complexes of trimethylgallium with group V hydrides [lo-121 using matrix isolation and cryogenic thin film techniques. Ab initio molecular orbital calculations describing the geometries, harmonic frequencies and binding energies of the gallane and TMGa adducts with AsH3 have also been published [ 13,141. These results all support the MOCVD observations that at or near room temperature there are very rapid reactions when mixing TMGa and HzSe [3]. In this paper, we report the results of allelectron ab initio molecular orbital and density functional theory (DFT) calculations, both local and non-local gradient corrected methods, of the l:] trimethylgallium-hydrogen selenide complex and the analogous Lewis acid/base adduct formed between gallane, GaH3, and H#e, and discuss their properties with reference to the related experimental data of Piocos and Ault [lo- 121 and recent theoretical data [13,14].
density functional theory (DFT) methods [18]. Local density functional (LDF) theory using the SVWN (Slater LSD exchange functional with local correlation provided by the Vosko, Wilk and Nusair expression), and non-local hybrid-density functional theory B3LYP (Becke’s three-parameter exchange functional with non-local correlation provided by the Lee-Yang-Parr (LYP) expression) and non-local BLYP methods were used with the 6-31 l+G(2d,p) basis set. This was the largest basis set that we could reasonably handle given our computational resources. The standard GAUSSIAN basis sets [19] and default convergence criteria were used. Since, scale factors for the 6-31 l+G(2d,p) basis set have not yet been published the scale factors for the appropriate 6-3lG(d) basis set were used for scaling the zero-point and thermal energies at the various computational levels [20]: RHF/3-21G* 0.9404, RHF/6-3 lG(d) 0.9135, B3LYP/6-3 lG(d) 0.9804, BLYP/6-3lG(d) 1.0119 and SVWN/6-3lG(d) 1.0079. A scale factor of 0.9613 at the B3LYP/6-3 lG(d) level was used for adjusting the computed harmonic vibrational frequencies at the B3LYP/631 l+G(2d,p)//B3LYP/6-31 l+G(2d,p) level [20].
2. Procedure 3. Results and discussion All calculations were performed using the GAUSSIAN 94 [ 151 program package on a Silicon Graphics RlOOOO workstation. The SPARTAN 4.1.2 [ 161 program was used to generate starting structures and to animate vibrational frequencies. The geometries of TMGa, H2Se and the (CH3)3Ga:SeH2 adduct were initially optimized at the restricted HartreeFock (RHF), RHF/3-2lG* level. The RHF/3-2lG* model has previously demonstrated its effectiveness in providing satisfactory equilibrium geometries for molecules incorporating third- and fourth-row, main group elements. However, previous calculations on donor-acceptor complexes [ 17 J have indicated that a larger basis set is probably needed to better describe the Lewis linkage, here Ga-Se, and so we also carried out geometry optimizations at the RHF/ 6-3 1 l+G(2d,p) level. Practical considerations prevented us from optimizing the structures at the second-order Moller-Plesset (MP2) level of theory and so the effect of electron correlation has been studied by performing geometry optimizations using
The growth of high quality Ga$e3 at low temperatures using TMGa and H2Se taken together with the observation of the extensive room temperature parasitic gas phase reaction, clearly indicates that different mechanisms control the deposition of GazSej from this precursor combination compared with Se metalorganic sources. The detailed mechanism by which this reaction proceeds is unknown. A possible mechanism, consistent with the little experimental data, invokes the formation of a (CHj)8Ga:SeH2 adduct between TMGa and H+e followed by successive methane elimination reactions as in the following sequence: Adduct formation (CH,)3Ga + HzSe * (CH&Ga
: SeH,
(1)
Elimination HzSe : Ga(CHs), -
HSe-Ga(CH3)2
+ CH,
(2)
N. Maung/Journal of Molecular Structure (Theochem) 432 (1998) 129-137
131
average Ga-C-H bond angle
1.464 (1.453) (1.470) ,a?’ H
92.1 (93.2) (91.3)
111.0 (111.3) (111.1) [lll.O]
H
2.010
120
(112.9)
H
PO.81
90.6
H
Fig. I. RHF/3-21G*, RHF/6-31 l+G(2d,p) (in parentheses), B3LYP/6- 3 1 l+G(2d,p) (curly brackets), BLYP/6-31 I+G(2d,p) (square brackets) optimized geometries with experimental values in italics, for (CHT)&a [14], HzSe 1171 and (CH,),Ga:SeH*. The bond lengths are in .& and the bond angles in degrees ((u and /3 refer to the a(Ga-C-H) values in Table 1).
HSe-Ga(CH3)2
-
CHsGa=Se
+ CH4
(3)
Specifically, we are interested in the stability of the (CH3)3Ga:SeH2 complex and so this species was investigated initially in order to assess the likelihood of the above scheme. Optimized geometrical parameters of the source gas molecules are given in Fig. 1 and Table 1 (SVWN LDF method) along with the available experimental data. Even the RHF/3-21G* level does a reasonable job of describing the structures of the source precursors. The geometries of H$e, TMGa and (CH3)3Ga:SeH2 computed at the B3LYP/
6-3 11+G(2d,p) level are taken to be the most representative. For H$e, the computed Se-H bond is 0.01 A longer, and the HSeH bond angle is 0.7” larger than the corresponding experimental values [ 171. For TMGa the computed Ga-C bond length of 1.990 A, is some 0.022 A longer than the experimental value [14]. The average computed C-H bond length, 1.093 A, and the GaCH bond angle, 111. l”, are very similar to the experimental results. At the RHF/3-2 lG* level the Ga-Se bond for the (CH3)sGa:SeH2 complex is 2.809 A, but one would expect the long Ga-Se bond length to be basis set dependent and it is -0.45 A longer with the larger
132
N. Maung/Journal
Table I Optimized
geometries
and the electronic
properties
of Molecular Structure (Theochem) 432 (1998) 129-137
of the source precursors
at the SVWN/6-3 I I+G(Zd,p) level”
calculated
TMGa (Djh symmetry) SVWN/6-3 I l+G(2d,p)//SVWN/6-3 r(Ga-C)
I l+G(Zd,p)
Expt. h
1.946
1.968 I .087 120.0 I I I.2
I.101 120.0 ol(l13.3)and/3(110.0)(111.1)
r(C-H) a(C-Ga-C) a(Ga-C-H) HzSe (Cl, symmetry)
SVWN/6-31 I+G(2d,p)NSVWN/6-3 r(Se-H) cl(H-Se-H) GaH
I I+G(2d,p)
Expt.’
I .46 90.6
I .474 90.3
i (Dih symmetry) SVWN/6-31 l+G(2d,p)//SVWN/6-3
r(Ga-H) a(H-Ga-H)
I l+G(2d,p)
Exp1.d
I.558 120.0
I .560 120.0
‘r and a indicate the bond length in A and the bond angles in degrees, respectively. and p correspond to those displayed in Fig. I .) hRef. 1141. ‘Ref. [ 171. dRef. [2l].
RHF/6-3 1 l+G(2d,p) basis set. The calculated Ga-Se bond distance of 3.258 A at the RHF/6-31 l+G(2d,p) level is probably too long with this truncated basis set. The Ga-Se bond distance found here is in good agreement with a value of 3.079 A for the Ga-As bond length computed by Bock and Trachtman [ 141, using a modified Huzinaga basis set for Ga and As, for
(In the case of u(Ga-C-H)
the trimethylgallium-arsine adduct, (CHs)sGa:AsHs. Although longer donor-acceptor linkages compared to normal single bonds are not unusual, we would expect some shortening of the Ga-Se bond if electron correlation were included in the geometry optimization. This is indeed the case, and at the B3LYPi 6-31 l+G(2d,p) level the calculated Ga-Se bond
Table 2 Calculated total energies (Hartrees), zero-point energies (ZPE, kcal mol.‘), thermal corrections at 0 K (kcal mol-‘) and at 298 K (kcal mol-‘) for the (CH1)?Ga:SeH2 adduct Method
(CH ,) ,Ga:SeH,
for TMGa the bond angles a
to 298 K (TC, Hartrees) and binding energies
H,Se
(CHx)qGa
Binding
energy
at 0 K” RHF/3-21G*//RHF/3-2lG* ZPEIRHFI3.2lG*
- 4422.066723
- 2032.2 I2732
82.7
71.2
0. I35678
TC RHF/6-31
I+G(Zd,p)//RHF/6-31
ZPE/RHF/6-3
l+G(Zd,p)
I I+G(Sd,p)
TC
- 2042.08475
103.9
69.7
I I+G(Zd,p)NB3LYP/6-31
ZPE/B3LYP/6-3
I+C(Zd,p)
-4447.412015 97.7
I I + G(2d.p)
BLYP/6-31
I+G(2p,d)//BLYP/6-3
ZPE/BLYP/6-31
I I+G(Zd,p)
I + G(2d.p)
- 2044.582778
95.3
64.6
0. I m4YO
TC SVWNlh-3
I l+G(2d,p)//SVWN/6-31
ZPE/SVWN/6-31
IcG(2d.p)
lffi(2d.p)
- 444 I .639744 95.3
0.1 I1315 - 2041.585738 64.6
0.159568
TC
“Numbers in parentheses
are binding energy values without correction
- 20. I (2.8)
-
IX.2
-
18.6 (3.9)
-
16.6
-
19.8 (2.9)
-
17.8
0.016077 I
0.110648
- 4447.334608
- 2400.95Y470 9.1
66.3
0.159473
TC
8.9
0.016537 I
II.108617 - 2044.65438
10.1 (12.4)
9.1
0. I I3656
- 4443.048736
0.158152
B3LYP/6-3
- 2389.834299
Binding at 298 K
0. I 10856
for zero-point
- 2402.751419 x.5 0.016130 - 2402.747152 8.2 0.016146 - 2400.0329
I7
8.4 0.016303
energy differences.
- 9.2 (13.2)
- 7.1
energy
133
N. Maung/Joumal of Molecular Structure (Theochem) 432 (I 998) 129-137
Table 3 Optimized
geometries
of the (CH2),Ga:SeH2
and HsGa:SeH:,
adducts calculated
SVWN/6-31 l+G(2d,p)//SVWN/6-31 (CHj),Ga:SeH2
at the SVWN/6-31 l+G(2d,p)
l+G(2d,p) HsGa:SeH2
(C, symmetry)
-
I.562
1.101 118.9 I 11.4 -
r(C-H) a(C-Ga-C) a(Ga-C-H) a(H-Ga-H) a(H-Se-Ga) a(C-Ga-Se) r(Se-H) a(H-Se-H) r(Ga-Se)
(C, symmetry)
-
1.961
r(Ga-C) r(Ga-H)
levela
118.9 97.9 99.9 1.478 90.5 2.596
95.9 95.2 1.478 90.4 2.682
‘r and a indicate the bond length in .k and the bond angles in degrees, respectively.
length is 2.998 A, although at the pure DFf BLYP/631 l+G(2d,p) level the calculated Ga-Se bond length is 3.060 A (see Fig. 1). Table 2 gives the total energies and binding energies for the reaction (CH&Ga
: SeH2 -
(CH&Ga
+ H2Se
At the highest levels of computation, B3LYP/ 6-31 l+G(2d,p)//B3LYP/6-31 l+G(2d,p) and BLYP/ 6-31 l+G(2d,p)//BLYP/6-31 l+G(2d,p), the (CHJ)3 Ga:SeH* complex is predicted to be -16.6 and -17.8 kcal mol-’ less stable, respectively, than the precursors at 298 K. For the (CHj)3Ga:AsH3 adduct Bock and Trachtman [ 141 found the adduct to be more stable than the precursors by -6.1 kcal mol-’ at the MP2/HUZSP*//RHF/HUZSP* level. The instability of the (CH3)sGa:SeH2 complex compared with the (CH3)sGa:AsH3 adduct is in agreement with the known ordering of AsH3 and H#e base strengths [9]. As is usually found with LDF theory, an inadequate description of the metal-ligand bond with the SVWN model (Table 1) leads to a very short dative Ga-Se bond of only 2.682 A (Table 3). This is -0.3 and -0.4 A shorter than the Ga-Se bond lengths computed using the B3LYP and BLYP models, respectively (see Fig. 1). The importance of nonlocal corrections is further illustrated by the enhanced stability of the SVWN adduct compared to the precursors, where the computed binding energy (Table 2) indicates that (CH3)sGa:SeH2 is -7.1 kcal mol-’ less stable than the precursors.
We were interested in determining the effect of Ga alkylation on the binding energy of the adduct and so also carried out computations on the HjGa:SeH* complex using the same theoretical models. Although monomeric GaH3 has not yet been isolated high level ab initio calculations have been performed [2 l-231. Balasubramanian [2 l] using multiconfiguration SCF complete active space calculations (CASSCF) followed by full second-order configuration interaction (SOCI) found GaH3 to have Dxhsymmetry with a computed Ga-H bond length of 1.56 A. Optimized geometrical parameters for GaH3 are given in Fig. 2 and Table 1 (SVWN LDF method) and for the H3Ga:SeHz adduct in Fig. 2 and Table 3 (SVWN LDF method). FOJ GaH3, the computed Ga-H bond is just 0.006 A longer than the high level ab initio estimate [21]. This is better than the level of agreement (-0.02 A longer) obtained by Dobbs et al. [ 131 using a modified Huzinaga basis set, HUZSP*. For the adduct dissociation reaction involving H3Ga:SeH2 (see Table 4) H,Ga : SeH2 -
H1Ga+H2Se
the H3Ga:SeH2 complex is predicted to be -6.5 kcal mol-’ more stable than the precursors at the B3LYP/ 6-31 l+G(2d,p)//B3LYP/6-31 l+G(2d,p) level and -5.9 kcal mol-’ more stable than the precursors at the BLYP/6-3 1 l+G(2d,p)//BLYP/6-31 l+G(2d,p) level. Once again, the local SVWN/6-3 1 l+G(2d,p)// SVWN/6-31 l+G(2d,p) level estimate of the binding
N. Maung/Journal
134 1.464 (1.453) (1.470) [1.482]
H \
I20 c (93.2) (91.3) 190.81
of Molecular Structure (Theochem) 432 (1998) 129-137
1s93 (1.573) (1.566) [ 1.5751 1.560
Ga -H
/
H
90.6
Da
1.601 (1.578) (1.571) [ I.5801
97.8
97.6 (98.3) (97.8) [97.8]
^^ 118.8 (119.4) (119.2) I 1wzl ;’
1.462 (1.453) (1.472) [ 1.4851 .J ,...”+ 92.3 (93.5) (91.4) w.91
H
A.
H
[2.838]
Cl Fig. 2. RHF/3-21G*, RHF/6-31 l+G(Zd,p) (in parentheses), B3LYP/ 6-31 l+G(2d,p) (curly brackets), BLYP/6-31 I+G(2d,p) (square brackets) optimized geometries with experimental values in italics, for H2Se [17], GaHl [21] and H1Ga:SeH2. The bond lengths are in A and the bond angles in degrees.
energy at 17.4 kcal mall’, is substantially larger than all the other estimates and some 5 kcal mol-’ more stable than the binding energy calculated at the RHF/3-21G* level. The H3Ga:SeH2 adduct is thus substantially more stable than the (CHJ3Ga:SeHz adduct and this is in agreement with simple chemical arguments which would predict that methyl substitution donates electrons into the empty 4p orbitals on the gallium, thus making trimethylgalhum a weaker Lewis acid than GaH3. Furthermore, the lower binding energy of the TMGa adduct is consistent with its longer SF--Se bond length (2.998 A compared with 2.803 A for H3Ga:SeH2) at the B3LYP/6-31 l+G(2d,p)NB3LYP/
6-31 l+G(2d,p) level. The distortion from planarity of the GaC3 backbone in (CH3)sGa:SeH2 (i.e. LCGaSe = 94.2”) is less than the distortion from planarity of GaH3 in H3Ga:SeH2 (i.e. LHGaSe = 98.1”). Furthermore, in both adducts the H2Se moiety distorts towards planarity, and the HSeGa angle is virtually identical for both H3Ga:SeH2 and (CH3),Ga:SeH2. Thus, the precursors of (CH3)RGa:SeH2 are distorted less than the precursors of H3Ga:SeH2, again suggesting that the Lewis interaction is weaker in the methylated adduct. The Ga-Se bond is the result of a donor-acceptor interaction between a lone pair on H2Se with the empty 41, orbital on the Ga atom of either TMGa or GaH3. The weak interactions in this case are illustrated by the Mulliken charge distribution (Table 5) at the RHF/6-31 l+G(2d,p) level which shows little or no charge transfer from the Se atom on the H$e group to the Ga atom on the TMGa group or the GaH3 group. In the case of the TMGa adduct the Ga atom acquires further positive charge (+O.O9e) on adduct formation while the Se atom acquires excess negative charge (-0.0%). In the case of the GaHs adduct the Ga atom acquires excess positive charge (+O.O5e) and the Se atom again acquires excess negative charge (-0.05e). The changes are due to the methyl carbons acquiring (-0.03e) from the Ga atom in the case of the TMGa adduct while the hydrogen atoms acquire (-0.03s) from the Ga atom in the case of the GaH3 adduct. The computed harmonic frequencies for the HiGa:SeHz adduct and parent hydrides are listed in Table 6. Also included in the table are the experimental frequencies for H$e [24] and recent ab initio data for GaH7 [ 131. The computed frequencies have been scaled using the B3LYP/6-3 1G(d) scaling factor 1201. In the case of H2Se very good agreement is obtained with experiment. For GaH3 the level of agreement is not as good but still adequate to allow comment on the H3Ga:SeH2 adduct. All the hydride fundamentals are red shifted in the adduct with respect to the individual hydrides. This is not inconsistent with the binding energy estimate which would imply a weakly bound H3Ga:SeH2 adduct. Both the binding energy estimate and the pattern (direction and magnitude) of frequencies are comparable to those found by Bock and Trachtman [ 141 for the trimethylgallium-arsine adduct.
135
N. Maung/Joumal of Molecular Structure (Theochem) 432 (1998) 129-137
Table 4 Calculated total energies (Hartrees), zero-point energies (ZPE, kcal mol-‘), thermal corrections 0 K (kcal mol-‘) and at 298 K (kcal mol-‘) for the HJGa:SeH2 adduct H ,Ga:SeH z
Method
RHF/3-21G*lIRHF/3-2IG* ZPElRHFI3.2IG* TC RHF/6-31 I+G(2d,p)//RHF/6-31 I+G(Zd.p) ZPEIRHFI6-3 I l+G(Zd.p) TC B3LYP/6-31 I+G(2d,p)//B3LYP/6-31 l+G(Zd,p) ZPuB3LYP/6-3 I I+G(Zd.p) TC BLYP/6-31 I+G(2d,p)//BLYP/6-31 I+G(2d,p) ZPEIBLYPb3 I l+G(Zd,p) TC SVWN/6-31 I+G(2d,p)llSVWN/6-31 I+G(Zd.p) ZPEISVWNb31 l+G(Zd.p) TC
“Numbers in parentheses
- 4305.563360 23.8 0.041488 - 4325.886509 23.4 0.040357 - 4329.403750 22.3 0.040999 - 4329.405069 21.7 0.04 1034 - 4324.295163 22.4 0.041713
to 298 K (TC, Hartrees) and binding energies at
Binding energy atOK”
GaH,
- 2389.834299 9. I 0.016537 - 2400.959470
- 1915.705194 Il.9 0.020925 - 1924.918094 12.1 0.020783 - 1926.638361 Il.6 0.02 I334 - 1926.645027 II.3 0.02 I342 - 1924.230898 I I.6 0.021742
are binding energy values without correction
4. Conclusion The suggestion that the gas phase interaction between TMGa and H2Se during MOCVD growth of GazSe3 may result in the formation of an intermediate (CH3)3Ga:SeH2 adduct has been investigated theoretically. Ab initio calculations at the RHF/6-31 l+G(2d,p) level and DFT calculations at the B3LYP/6-31 l+G(2d,p), BLYP/6-31 l+G(2d,p) and SVWN/6-3 1 l+G(2d,p) levels indicate that such a complex is unstable with respect to its components (binding energy - - 17 kcal malli) at 298 K in general agreement with the trends observed in the published matrix isolation data for trimethylgallium
9.1 0.016077 - 2402.751419 8.5 0.016130 - 2402.747 I52 8.2 0.016146 - 2400.032917 x.4 0.016303
for zero-point
Bindmg energy at 298 K
12.3 (15.0)
12.5
3.6 (5.6)
3.4
6.6 (8.8)
6.5
5.9 (8 II
5.‘)
17.3 (19.7)
17.4
energy differences.
with group V bases. Although more extensive allelectron methods with more complete basis sets, namely 6-31 l+G(2df,2p) or better, are required in order to obtain more accurate values for the binding energies, it is probably the case that they will not change the binding energy sufficiently to change the conclusion that the molecular complex is extremely unstable. By contrast, the analogous H3Ga:SeHz adduct is predicted to be a stable Ga-Se bonded species at all levels of computation examined here, with a binding energy at the B3LYP/6-31 l+G(2d,p)//B3LYP/6-31 l+G(2d,p) level of -6.5 kcal mall’ at 298 K. In conclusion, while the (CH3)3Ga:SeH2 adduct formed by mixing TMGa
Table 5 Mulliken net charges, qi for the atoms i in the source molecules, the (CH3)3Ga:SeH2 and H1Ga:SeH2 adducts and their dipole moments, CL,in D computed at the RHF/6-31 I+G(2d,p)//RHF/6-31 l+G(2d,p) level. The values in parentheses refer to either experimental values or high level ab initio results Electronic
properties
qGa
9c qu, %, 4 Se % fi aRef. [ 141. bRef. [ 171. ‘Ref. [21].
(CH&Ga
HzSe
1.62 -0.92 0.13 N/A N/A N/A 0.01 (0)”
N/A N/A N/A N/A -0.14 0.07 0.84 (0.40)b
GaH, 0.49 N/A N/A -0.16 N/A N/A 0 (0)’
(CH&Ga:SeH,
H iGa:SeH,
1.71 -0.95 0.12 N/A -0.19 0.11 I .75
0.54 N/A N/A -0.19 -0.19 0.1 I 2.12
136
N. Maung/Journal of Molecular Structure (Theochem) 432 (1998) 129-137
Table 6 Comparison of fundamental frequencies (cm-‘) computed at the B3LYP/6-31 l+G(2d,p)NB3LYP/6-31 H,Ga:SeH* adduct and the parent hydrides Parent hydride experimental”
751 825 1034 2006 202 I 2345 2358
Parent hydride computed h
693 722 1039 I896 1898 2329 2344
HIGa:SeHZ
28 107 273 298 454 499 684 708,711 I022 1859, 1862 1869 2312 2326
l+G(Zd,p) level and assignments
Freq. shift
for the
Assignment
az, twist about Ga-Se a ,, Ga-Se sym st
-9 (-14,-l I) -17 (-37,-34) -29 -17 -18
a?, GaHl e2, GaH3 a i, HzSe eZ, GaH1 a,, GaHl a i, HzSe b2? HzSe
sym def assym def bend assym st sym st sym st assym st
‘Ref. [24]. hRef. [21].
and H2Se during MOCVD growth is most unlikely to be a stable complex, it could exist as a transient species giving rise to the subsequent methane elimination reactions and deposition of Ga2Se3 amorphous/ polymeric deposits.
Acknowledgements We would like to thank the Higher Education Funding Council for Wales (HEFCW) for provision of a Silicon Graphics RlOOOO workstation.
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