J. Phys. Chem. 1996, 100, 155-162
155
Gas Phase Ion-Molecule Reactions in Phosphine/Silane Mixtures Paola Antoniotti, Lorenza Operti, Roberto Rabezzana, Gian Angelo Vaglio,* and Paolo Volpe Dipartimento di Chimica Generale ed Organica Applicata, UniVersita` di Torino, Corso Massimo d’Azeglio 48, 10125 Torino, Italy
Jean-Franc¸ ois Gal, Renaud Grover, and Pierre-Charles Maria Laboratoire de Chimie Physique Organique, Groupe FT-ICR, UniVersite´ de Nice-Sophia Antipolis, Parc Valrose, 06108 Nice Cedex 2, France ReceiVed: May 24, 1995; In Final Form: July 31, 1995X
Phosphine/silane mixtures have been investigated by ion trap mass spectrometry, and reaction pathways together with rate constants of the main reactions are reported. Mechanisms of ion-molecule reactions have been elucidated by single and multiple isolation steps, and exact mass measurements have been performed by Fourier transform mass spectrometry. The SiHn+ (n ) 0-3) ions react with phosphine to give SiPHn+ (n ) 1-4) ions. These ions further react and yield SiP2Hn+ (n ) 2-5) and Si2PHn+ (n ) 3-7) ions, which, in turn, react following different pathways with silane or phosphine to give Si2P2Hn+ (n ) 5, 6, 8) and Si3PHn+ (n ) 5-7), respectively. Mixed SiPHn+ (n ) 1-4) ions also originate from the PHn+ (n ) 0, 1) phosphine primary ions, as well as from the P2Hn+ (n ) 0-3) secondary ions of phosphine. Protonation of phosphine from several ionic precursors is a very common process and yields the stable phosphonium ion, PH4+. Trends in total abundances of tertiary SiP2Hn+ (n ) 2-4) and Si2PHn+ (n ) 3-7) ions as a function of reaction time for different PH3/SiH4 pressure ratios show that excess of silane favors the nucleation of mixed Si-P ions. The mechanism and energetics of the reaction of Si+ with PH3 have been investigated by ab initio calculations, and the most stable structure of the SiPH+ product, with a hydrogen bridge between silicon and phosphorus, has been identified.
Introduction Gas phase reactions involving ions from monosilane and monogermane have attracted a great deal of interest in the past years from both experimental and theoretical points of view.1-14 These investigations were aimed at extending the fundamental knowledge of ion-molecule reactions and their mechanisms. Moreover, an understanding of the ion chemistry of these systems can be useful in the improvement of the deposition of amorphous semiconductor films by radiolytic methods.15-17 Recently, results have been reported for monosilane,3,4,8-13 monogermane,5,7,14 methylsilane,18-22 methylgermane,22,23 mixtures of the above compounds,24-26 and monosilane with ammonia.27,28 As part of a study on gas phase reactivity and determination of rate constants of ion-molecule reactions by ion trap mass spectrometry, we report here the results concerning the phosphine/silane system. The gas phase behavior and the reaction mechanisms are discussed, and the rate constants for the first nucleation reactions are determined and compared with rate constants calculated according to the Langevin and average dipole orientation (ADO) theories.29 Moreover, ab initio theoretical calculations have been performed on the reaction of Si+ with phosphine as the simplest model of the interaction of silicon-containing ions with PH3. Ion-molecule reactions of PH3 alone are considered30,31 as a preliminary step to the investigation of the PH3/SiH4 system described here. The reactions observed in the mixtures studied here can be used to identify the best experimental conditions for direct phosphorus doping during the deposition of amorphous silicon solid by radiolytic activation of appropriate gaseous mixtures. Studies on low-pressure chemical vapor deposition of phosphorusX
Abstract published in AdVance ACS Abstracts, December 1, 1995.
0022-3654/96/20100-0155$12.00/0
doped polycrystalline silicon32 and on gas pyrolysis of PH3 and PH3/Si2H633 have been reported recently. Experimental Section Experiments were run on a Finnigan Mat ITMS 70 ion trap mass spectrometer, and a FT-ICR spectrometer was used for exact mass measurements. The ITMS experiments were performed at 333 K to obtain data comparable with previous results.28 The gas inlet system was modified to introduce simultaneously reagents and buffer gases into the ion trap through four different lines. The pressures were measured by a Bayard Alpert ionization gauge. Phosphine and silane were admitted into the trap at pressures in the range (2-4) × 10-7 Torr, and helium or helium/argon (4:1) mixture buffer gases were added to a pressure of about 5.0 × 10-4 Torr. The two commercial reagent gases, phosphine and silane, were obtained at a high purity degree. Prior to use, each of them was introduced into a separate flask, containing anhydrous sodium sulfate as a drier, which was connected to the gas inlet system of the instrument. Helium and argon were obtained commercially in extrahigh purity and were used without further purification. Quite frequent bake-out of the manifold and lines was performed to reduce the water background in the trap. Four different kinds of scan modes for ion-molecule reaction experiments were used, each of which has been previously described in detail.22,24,26,28 In the first kind of experiment,22 all the ions were stored for times ranging from 0 to 500 ms to study the variations of ionic abundances as a function of reaction time. The other three experiments all consisted of the isolation of selected ionic species to determine ion-molecule reaction pathways and rate constants. Isolation was performed in a single step applying suitable rf voltages,34,35 the so-called resonance ejection isolation, or a superimposition of rf and dc voltages to © 1996 American Chemical Society
156 J. Phys. Chem., Vol. 100, No. 1, 1996
Figure 1. Scan function for resonance ejection isolation of a selected ion (timing event: A ) ionization, B ) reactions of all ions, C ) ejection of ions at m/z lower than the selected ion, D ) ejection of ions at m/z higher than the selected ion, E ) reactions of the isolated ion, F ) acquisition, G ) interscan).
the trap;24,26 this last procedure was used in multiple isolation steps28 for those ions having the same nominal mass but different composition. As an example, with the aim of investigating the reaction pathways of SiH3+, m/z 31, it is not possible to isolate solely SiH3+ ions at m/z 31, as P+ ions have the same m/z value. Therefore, it is necessary to isolate the ionic species at m/z 30, SiH2+, which reacts with silane to form SiH3+ as the only ionic species at m/z 31. Moreover, the presence of some ions, such as SiPHn+ (n ) 1-4), at the same nominal mass as Si2H4+ (m/z 60), Si2H5+ (m/z 61), P2+ (m/z 62), and P2H+ (m/z 63), respectively, has been confirmed by comparing their abundances in experiments in which the partial pressure of PH3 or SiH4 has been reduced to zero and increased again to the original value by a manual closing and opening of the entrance of the lines to the trap. In all experiments, ionization was effected by electron beam bombardment for ionization times in the range 1-10 ms. In the experiments without ion isolation following the ionization event, only reaction (time ranging from 0 to 500 ms) and acquisition in the 10-250 u range took place. When a single ion isolation step was performed either with or without application of a dc voltage, ionization was followed by a reaction time suitable to maximize the abundance of the ions to be stored. Isolation of the selected ions, their reactions with neutrals present in the trap for convenient reaction times, and acquisition were carried out successively. In multiple isolation steps experiments, reaction of the isolated ions with neutral gases was followed by isolation of one of the secondary ionic products, which, in turn, was reacted for suitable reaction times. Acquisition and interscan periods completed the experiment. Figure 1 reports the scan mode of resonance ejection isolation of ions,34,35 in which only rf voltages are applied. This method was used in some experiments as a check for thermalization of the isolated ionic species. In fact, no field is directly applied to the selected ions, which, therefore, should have a lower excitation energy. In period A, electrons enter the ion trap for a suitable period and ionization takes place. The rf field is set so that all the ions above m/z 10 experience stable trajectories. In period B an appropriate reaction time is given to increase the abundances of the ions to be stored and, then, the first stage of isolation is initiated. In period C, the rf voltage is slowly increased to such a level as to make unstable the trajectories of all ions with a m/z ratio lower than that of the ion to be isolated. The rf voltage is quickly decreased to the starting value, and period D follows to eliminate ions with a m/z ratio higher than
Antoniotti et al. that of the ion under examination. An additional ac voltage at a fixed radiofrequency is applied to the ring electrode during period D, which makes unstable the trajectories of ions having that frequency. At the same time, the rf voltage is decreased in such a way that all ions of m/z higher than that of the ion to be isolated sequentially come into resonance with the fixed radiofrequency, become unstable, and exit the trap. The rf voltage is set back to the initial value, and the stored ions are allowed to react with neutrals present in the trap at variable times (period E). Acquisition (period F) and interscan (period G) follow as usual. Every time an rf or a dc voltage is applied, a settling time is necessary to stabilize the trap contents. Therefore, even when the reaction time is set to zero, some delay has elapsed since the ionization event, and it cannot be ruled out that ionmolecule reactions have already occurred to some extent. Some complementary experiments were performed by using the FT-ICR spectrometer built at the University of Nice-Sophia Antipolis.36 In particular, ion composition was examined by exact mass measurements to confirm the results of the ITMS experiments and to identify ions with the same nominal mass and different formula, formed from a unique precursor, which cannot be identified in any ion trap mass spectrometer experiment. These experiments were done at both 30 and 70 eV ionization energy and were conducted in the broad-band mode in the mass range from m/z 14 to m/z 200 at a total pressure of 1.3 × 10-7 Torr with a PH3/SiH4 ratio equal to 1:1. Under these conditions, the resolution is about 1500 at m/z 60 and 3000 at m/z 30. Exact masses were obtained by calibrating the instrument with known ions (accuracy: 1-3 ppm). Then, the ion composition was deduced from its mass by using the Brucker CMS 47 software. The difference between the calculated and the measured mass is in the range 1-30 ppm. This accuracy allows us to differentiate easily ions of the same nominal mass containing either 28Si (27.97693 u) or 31P (30.97376 u). In particular, we have checked the composition of ions at m/z 62 (P2+, SiPH3+) and 63 (P2H+, SiPH4+) that can coexist during the same period. Some collision-induced dissociation reactions (Ar collision gas) have been performed to confirm some ion compositions. In experiments for the determination of rate constants by ion trap mass spectrometry, accurate pressure measurements were necessary. For this purpose, the ion gauge was calibrated and the indicated pressure was further corrected on the basis of the relative sensitivity of the ion gauge response with different gases.37 Rate constants were determined for reactions of SiHn+ (n ) 0-3), PHn+ (n ) 0-3), Si2Hn+ (n ) 2-5), SiPHn+ (n ) 1-3), and P2Hn+ (n ) 0-3) in a phosphine/silane mixture. The presence of helium or helium/argon mixture buffer gases at different pressures does not affect these measurements. Every value reported is the average of at least three different experiments, and the uncertainties of the measurements fall within 20%. In these experiments, a dynamically programmed scan26 functions within the acquisition software environment and operates by increasing incrementally the reaction time from 0 to 50 ms by 0.2 ms steps after the acquisition of each mass spectrum. In the determination of rate constants, it has been taken into account that ions are ejected from the trap at the rate of 5500 u/s during the acquisition event. Hence, the reaction time includes the variable time at the fixed value plus the time of the rf ramp until the ions under examination exit the trap. As both parallel and consecutive ion-molecule reactions have been observed, which in some cases give the same ionic product,
Gas Phase Ion-Molecule Reactions in Phosphine/Silane
J. Phys. Chem., Vol. 100, No. 1, 1996 157
TABLE 1: Rate Constants for Reactions of SiHn+ (n ) 0-3) Ions with PH3 in a PH3/SiH4 Mixturea reaction Si+
SiPH+
+ PH3 f + H2 SiH+ + PH3 f SiPH2+ + H2 SiH2+ + PH3 f PH4+ + SiH f SiH3+ + PH2 f SiPH3+ + H2 SiH3+ + PH3 f SiPH4+ + H2
kexp
∑kexp
kADOb
efficiencyc
4.6 6.1 0.8 6.4 2.1 2.6
4.6 6.1
15.06 14.91
0.31 0.41
9.3 2.6
14.78 14.65
0.63 0.18
a Rate constants are expressed as 10-10 cm3 molecule-1 s-1; experiments were run at 333 K; uncertainity is within 20%. b Rate constants have been calculated according to the ADO theory,29 taking dipole moment of phosphine from ref 48 and polarizability of phosphine from ref 49. c Efficiency has been calculated as the ratio ∑kexp/kADO.
TABLE 2: Rate Constants for Reactions of PHn+ (n ) 0-3) Ions with SiH4 in a PH3/SiH4 Mixturea reaction
kexp
∑kexp
kLb
efficiencyc
P+ + SiH4 f SiPH2+ + H2 PH+ + SiH4 f SiH3+ + PH2 f SiPH3+ + H2 PH2+ + SiH4 f SiH3+ + PH3 PH3+ + SiH4 f PH4+ + SiH3
2.0 7.1 0.7 9.0 6.5
2.0
12.26
0.16
7.8 9.0 6.5
12.17 12.07 11.98
0.64 0.75 0.54
a Rate constants are expressed as 10-10 cm3 molecule-1 s-1; experiments were run at 333 K; uncertainity is within 20%. b Rate constants have been calculated according to the Langevin theory,29 taking polarizability of silane from ref 50. c Efficiency has been calculated as the ratio ∑kexp/kL.
the determination of rate constants requires complex calculations which have been described previously.26,28 Further experiments were also performed with isolation of every product ion; subsequent reaction of each isolated ion species revealed its contribution to the formation of ionic species common to different precursors. Moreover, in some reactions, a unique ionic precursor yields a unique product ion by reaction with both neutral reactants. In such cases, as the reaction of PH3+ with both SiH4 and PH3 to give PH4+ or the reaction of SiH2+ with both neutral reagents to give SiH3+, the rate constants of formation of PH4+30 and SiH3+10 measured in self-condensation have been subtracted from the rate constants determined in the PH3/SiH4 mixture. The results are the rate constants for formation of SiH3+ from SiH2+ and PH3 (Table 1) and of PH4+ from PH3+ and SiH4 (Table 2). Thermalization of reactant ions was obtained in a short time by unreactive collisions with helium or helium/argon mixture buffer gases which are present in the trap at a pressure of 5.0 × 10-4 Torr. The efficiency of ion cooling was indicated by the single-exponential behavior of plots and by the similarity of results obtained in experiments involving isolation with or without application of a dc voltage. Ab initio quantum mechanical calculations have been performed using a IBM/VM-CMS and a RISC/6000 version of the Gaussian series of programs.38 The geometrical structures, corresponding to critical points identified on the potential energy hypersurface, have been determined by way of complete gradient39 optimization at the Mo¨ller-Plesset level of theory truncated at the second order (MP2),40 using a split-valence shell 6-31G(d)41 basis set, and also at the MP2(FULL) level of theory, including all the electrons in the correlation calculations, using a 6-31G(d,p)42 basis, which adds p functions to hydrogen atoms. The critical points have been characterized as minima or firstorder saddle points by diagonalization of the analytical Hessian (calculation of the vibrational frequencies). The MP2(FULL)/ 6-31G(d,p) geometries were used for recomputing the relative energies at a higher theoretical level. Single-point calculations were performed by quadratic configuration interaction theory,43
Figure 2. Variation of total abundances with time for SiHn+, PHn+, PH4+, Si2Hn+, P2Hn+, and SiPHn+ ion families in a PH3(2.5 × 10-7 Torr)/SiH4(2.5 × 10-7 Torr) mixture.
including the contribution from single, double, and triple excitations, QCISD(T), with the larger 6-31G(d,p) basis set. Results Figure 2 shows the variation of the total abundances of the SiHn+ (n ) 0-3), PHn+ (n ) 0-3), Si2Hn+ (n ) 2-5), P2Hn+ (n ) 0-3), and SiPHn+ (n ) 1-4) ion families and of PH4+ as a function of time for the PH3 (2.5 × 10-7 Torr)/SiH4 (2.5 × 10-7 Torr) mixture. The abundances of both primary ion families, PHn+ and SiHn+, decrease with reaction time, but the PHn+ ions show a sharper decrease than do the corresponding silicon-containing ions. Among the secondary ions, the Si2Hn+, P2Hn+, and SiPHn+ ions show a very similar behavior, their abundances increasing in the first 50-100 ms of reaction and changing very slightly up to 300 ms reaction time. On the contrary, PH4+ displays a sharp increase of abundance during the time range considered here, becoming the most abundant ion after about 100 ms. For the same mixture and in the same time range, Figure 3 reports the relative abundances of the primary ions of both reagents, PH3 and SiH4. The abundances of all ions decrease with reaction time, as expected, the only exception being the SiH3+ ionic species. In fact, its abundance increases, reaches a maximum, and decreases only after 100 ms of reaction. This trend indicates that the primary ion SiH3+ is also produced in an ion-molecule process and is responsible for the slower decrease of the SiHn+ (n ) 0-3) ionic family compared with the PHn+ (n ) 0-3) ions of Figure 2. In Figure 4, the abundances of the secondary SiPHn+ (n ) 1-4) ions are reported as a function of reaction time for mixtures containing PH3 and SiH4 at partial pressure ratios 1:1, 5:1, and 1:5, respectively. Even if the slopes of the three curves are initially different, after 300 ms of reaction they become closer and do not seem to be affected by the relative concentrations of the reagent gases. Figure 5 shows the variation of the combined abundances of tertiary Si2PHn+ (n ) 3-7) and SiP2Hn+ (n ) 2-4) ions with time for the same PH3/SiH4 mixtures as in Figure 4. In this case, it is evident that Si-P clustering proceeds to a larger extent with increasing SiH4 content in the reacting mixture. In the tables are reported the ion-molecule reactions occurring in PH3/SiH4 mixtures together with the experimental rate constants for formation of ionic products; the collisional rate
158 J. Phys. Chem., Vol. 100, No. 1, 1996
Figure 3. Variation of abundances with time for Si+, SiH+, SiH2+, SiH3+, P+, PH+, PH2+, and PH3+ ions in a PH3(2.5 × 10-7 Torr)/SiH4(2.5 × 10-7 Torr) mixture.
Antoniotti et al.
Figure 5. Variation of total abundance with time for the combined Si2PHn+ and SiP2Hn+ ion families in PH3/SiH4 mixtures with 1:1, 1:5, and 5:1 partial pressure ratios.
TABLE 3: Rate Constants for Reactions of Si2Hn+ (n ) 2-5) Ions with PH3 in a PH3/SiH4 Mixturea reaction
kexp
∑kexp
kADOb
efficiencyc
Si2H2+ + PH3 f Si2PH3+ + H2 Si2H3+ + PH3 f SiPH2+ + SiH4 f Si2PH4+ + H2 Si2H4+ + PH3 f SiPH3+ + SiH4 f Si2PH5+ + H2 f Si2PH7+ Si2H5+ + PH3 f PH4+ + Si2H4 f Si2PH6+ + H2
4.5 1.7 3.6 0.7 0.4 0.4 0.3 1.8
4.5
12.74
0.35
5.3
12.70
0.42
1.5
12.66
0.12
2.1
12.63
0.17
a
Rate constants are expressed as 10-10 cm3 molecule-1 s-1; experiments were run at 333 K; uncertainity is within 20%. b Rate constants have been calculated according to the ADO theory,29 taking dipole moment of phosphine from ref 48 and polarizability of phosphine from ref 49. c Efficiency has been calculated as the ratio ∑kexp/kADO.
Figure 4. Variation of total abundance with time for the SiPHn+ ion family in PH3/SiH4 mixtures with 1:1, 1:5, and 5:1 partial pressure ratios.
constants are calculated according to the Langevin or ADO theories,29 and the reaction efficiencies are reported as the ratio of experimental to collisional rate constants. The well-known self-condensation processes of both silane3,8,10,12 and phosphine30 occur as parallel reactions of the reported pathways, but are not shown for reasons of clarity. Tables 1 and 2 report the ion-molecule reactions of the primary ions of silane, SiHn+ (n ) 0-3) with PH3, and of the primary ions of phosphine, PHn+ (n ) 0-3) with SiH4, respectively. Tables 3 and 4 show the ion-molecule reactions of the secondary ions of silane, Si2Hn+ (n ) 2-5) with PH3, and of the secondary ions of phosphine, P2Hn+ (n ) 0-3) with silane, respectively. Finally, Table 5 reports the reaction pathways of the SiPHn+ (n ) 1-3) ion family with both SiH4 and PH3. The PH4+ and SiPH4+ ions are not shown in these tables, as they are unreactive under the experimental conditions here used. The most frequent reaction pathway displayed by the SiHn+ (n ) 0-3) primary ions reacting with PH3 (Table 1) involves elimination of a hydrogen molecule to form the SiPHn+ (n )
1-4) ion family. SiH2+ is also involved in a hydrogen transfer from PH3 to give SiH3+, characterized by a rather high rate constant, and in a slow protonation of PH3 to PH4+. The PHn+ (n ) 0-3) ions show a good reactivity with SiH4 (Table 2), but only PHn+ (n ) 0, 1) ions are precursors of mixed SiPHn+ (n ) 2, 3) ions with elimination of a hydrogen molecule. PH2+ reacts with silane to give SiH3+ by eliminating PH3, while PH3+ abstracts a H atom to give PH4+ as the sole product. As shown in Table 3, all the secondary Si2Hn+ (n ) 2-5) ions react with PH3 to form the Si2PHn+ (n ) 3-6) ion family with elimination of H2. The associated rate constants are rather high for n ) 2, 3 and lower for n ) 4, 5. Si2Hn+ (n ) 3, 4) ions also eliminate SiH4 to form the SiPHn+ (n ) 2, 3) ions containing a new Si-P bond. Also the secondary P2Hn+ (n ) 0-3) ions exhibit ionmolecule processes with silane to give ion products containing new Si-P bonds (Table 4). A common pathway is the elimination of PH3 and formation of the SiPHn+ (n ) 1-4) ions with high efficiency for P2H+ and low efficiency for P2+, P2H2+, and P2H3+. The SiPHn+ (n ) 1-3) species react with both SiH4 and PH3 and eliminate a hydrogen molecule to form Si2PHn+ (n ) 3-5) and SiP2Hn+ (n ) 2-4) ions, respectively (Table 5). In some cases, further steps of ion nucleation take place yielding ions containing three silicon and one phosphorus atoms or two silicon and two phosphorus atoms. However, all the
Gas Phase Ion-Molecule Reactions in Phosphine/Silane
J. Phys. Chem., Vol. 100, No. 1, 1996 159
Figure 6. Optimized geometrical parameters corresponding to the critical points on the hypersurface for the Si+ + PH3 reaction: (6a) ionmolecule complex; (6b) isomer of a; (6b′) transition structure of H migration; (6c) electrostatic complex. Bond distances are in angstroms, and bond angles are in degrees. MP2/6-31G(d) and MP2(FULL)/6-31G(d,p) (starred) values are shown.
TABLE 4: Rate Constants for Reactions of P2Hn+ (n ) 0-3) Ions with SiH4 in a PH3/SiH4 Mixturea reaction P2 + SiH4 f + PH3 f SiPH2+ + PH2 f SiP2H2+ + H2 P2H+ + SiH4 f SiPH2+ + PH3 f SiP2H3+ + H2 P2H2+ + SiH4 f SiPH3+ + PH3 f SiPH4+ + PH2 f P2H3+ + SiH3 P2H3+ + SiH4 f SiPH4+ + PH3 +
SiPH+
kexp 1.0 1.6 1.0 4.5 3.1 0.8 4.4 0.4 0.4
∑kexp
kLb
efficiencyc
3.6
10.59
0.34
7.6
10.56
0.72
5.6 0.4
10.53 10.51
0.53 0.038
a Rate constants are expressed as 10-10 cm3 molecule-1 s-1; experiments were run at 333 K; uncertainity is within 20%. b Rate constants have been calculated according to the Langevin theory,29 taking polarizability of silane from ref 50. c Efficiency has been calculated as the ratio ∑kexp/kL.
TABLE 5: Rate Constants for Reactions of SiPHn+ (n ) 1-3) Ions with SiH4 and PH3 in a PH3/SiH4 Mixturea reaction SiPH+ + SiH4 f Si2H2+ + PH3 f Si2PH3+ + H2 SiPH+ + PH3 f SiP2H2+ + H2 SiPH2+ + SiH4 f Si2PH4+ + H2 SiPH2+ + PH3 f PH4+ + SiPH f SiP2H3+ + H2 SiPH3+ + SiH4 f Si2H4+ + PH3 f Si2PH5+ + H2 SiPH3+ PH3 f SiP2H4+ + H2 SiPH3+ + SiH4 f SiPH4+ + SiH3 or SiPH3+ + PH3 f SiPH4+ + PH2
kexp ∑kexp kADO or kLb efficiencyc 1.9 3.8 6.5 2.3 1.0 2.2 4.3 0.2 0.6
5.7 6.5 2.3
10.65 12.66 10.62
0.53 0.51 0.22
3.2
12.63
0.25
4.5 0.6
10.59 12.59
0.43 0.048
Figure 7. Optimized geometrical parameters corresponding to the critical points on the hypersurface for the Si+ + Ph3 reaction: (7a) product; (7b) transition structure of H2 elimination. Bond distances are in angstroms, and bond angles are in degrees. MP2/6-31G(d) and MP2(FULL)/6-31G(d,p) (starred) values are shown.
5.4
Rate constants are expressed as 10-10 cm3 molecule-1 s-1; experiments were run at 333 K; uncertainity is within 20%. b Rate constants have been calculated according to the Langevin theory,29 taking polarizability of silane from ref 50, or to the ADO theory,29 taking dipole moment of phosphine from ref 48 and polarizability of phosphine from ref 49. c Efficiency has been calculated as the ratio ∑kexp/kL or ∑kexp/kADO. a
ions produced in these reactions were of such low abundances that their rate constants of formation could not be determined.
The theoretical investigation of the mechanism of the reaction between Si+ and PH3 has enabled the determination of the remarkable critical points, corresponding to minima or transition structures, on the potential energy hypersurface. The geometries are given in Figures 6 and 7. As many optimized structures at the MP2/6-31G(d) level of theory contain bridged hydrogen atoms, the geometries have been reoptimized using the more complete 6-31G(d,p) basis set. However, the introduction of p functions on the hydrogen atoms did not lead to remarkable changes in the description of the potential energy hypersurface
160 J. Phys. Chem., Vol. 100, No. 1, 1996
Antoniotti et al.
TABLE 6: Total and Relative Energies (Hartrees and Kcal/mol) for the Reaction Si+ + PH3 structure 6a 6b 6b′ 6c 7a 7b a
Si+ + PH3 SiPH3+ HSiPH2+ Si(H)PH2+ (H migration TS) HP(H2)Si+ SiHP+ + H2 SiPH(H2)+ (H2 elimination TS) PH2+ + SiH
MP2/6-31G(d) -631.137 434 -631.210 080 -631.199 098 -631.126 539 -631.157 897 -631.144 483 -631.119 676 -631.049 207
MP2(FULL)/6-31G(d,p)
0.0 -45.6 -38.7 +6.8 -12.8 -4.4 +11.1 +55.4
-631.175 872 -631.257 478 -631.246 004 -631.151 320 -631.210 002 -631.191 962 -631.171 984 -631.096 049
0.0 -51.21 -44.01 +15.41 -21.42 -10.10 +2.44 +50.09
QCISD(T)/6-31G(d,p)a -631.211 170 -631.281 116 -631.268 245 -631.204 495 -631.235 418 -631.218 781 -631.200 398 -631.126 689
0.0 -43.9 -35.8 +4.2 -15.2 -4.8 +6.8 +53.0
Calculated using the MP2(FULL)/6-31G(d,p) optimized geometries.
but only to slight variations of the bond lengths and angles. The study of the potential energy surface corresponding to the SiPH3+ ion has lead to the identification of some isomeric structures, the most important being SiPH3+ (6a), HSiPH2+ (6b), and HP(H2)Si+ (6c) (Figure 6). Structure 6a, the most stable one, is a minimum on the potential energy hypersurface and corresponds to an ion-molecule complex between the Si+ ion and the PH3 neutral molecule in which the silicon is directly bound to the phosphorus atom. Structure 6b is also a stable minimum and corresponds to the isomer formed from 6a by migration of a hydrogen atom from phosphorus to silicon, through transition structure 6b′ in which the hydrogen atom is bridged between silicon and phosphorus. The activation energy is 52.4 kcal/mol at the MP2 level of theory, which decreases to 48.1 kcal/mol at the QCISD(T) level of theory (Table 6). The energy of the transition structure lies 4.2 kcal/mol above that of the starting species Si+ and PH3 at the higher level of calculation. In the third structure, 6c, the Si+ ion interacts with the phosphine hydrogens, yielding an electrostatic complex in which the silicon atom is bridged between two hydrogen atoms and the phosphorus atom. This structure, which is a minimum on the potential energy surface, is the least stable one (Table 6). Therefore, the reaction mechanism is likely to proceed through formation of the most stable species, which is 6a. Its successive fragmentation can follow two different pathways, only one of them being energetically favored. When the fragmentation gives the products PH2+ and SiH, the reaction is strongly endothermic (Table 6), according to thermochemical data.44 When reaction proceeds from SiPH3+ 6a by loss of a hydrogen molecule, the SiHP+ ion is formed. The study of the potential energy surface of the SiHP+ ion has lead to the identification of a stable structure, corresponding to a minimum, in which the hydrogen atom is bridged between silicon and phosphorus (Figure 7a). Elimination of H2 from 6a species takes place through a transition structure, in which the H-H bond is partially formed with a length of 0.80 Å, and the two P-H bonds, 1.70 and 1.75 Å long, are partially broken (Figure 7b). Despite the rather high activation energy of the H2 elimination, 50.7 kcal/mol (Table 6), the difference of energy between the starting reagents and the dissociation transition structure 7b is only 6.8 kcal/mol at the QCISD(T) level of theory (Table 6). Discussion When the neutral reagent is silane, a common reaction pathway consists of the addition of SiH4, followed by loss of a hydrogen molecule. This reaction has also been observed in self-condensation of silane, as previously reported.3,8,10,12 The primary PHn+ (n ) 0, 1) and the secondary P2Hn+ (n ) 0, 1) ions of phosphine follow this path to form new Si-P bonds. Moreover, all the SiPHn+ (n ) 1-3) ions react according to the same path as well, yielding the Si2PHn+ (n ) 3-5) ionic
species, which react with silane according to reaction 1:
Si2PHn+ + SiH4 f Si3PHn+2+ + H2
(1)
Elimination of H2 also takes place from the reaction of SiP2H3+ with silane, as follows:
SiP2H3+ + SiH4 f Si2P2H5+ + H2
(2)
Enthalpies of self-condensation reactions which occur with elimination of H2 indicate that this kind of process is thermodynamically favored.28 Unfortunately, it has been impossible to calculate the ∆H° values for the reactions involving ions containing phosphorus and silicon atoms, as heats of formation of these ionic species are not available in the literature. Two other pathways are observed in reactions with SiH4:
PyHn+ + SiH4 f SiPy-1Hn+2+ + PH2
(3)
(y ) 1, n ) 1; y ) 2, n ) 0, 2) and
SixPyHn+ + SiH4 f Six+1Py-1Hn+1+ + PH3
(4)
Reaction 4 has been observed for x ) 0, y ) 1, n ) 2; x ) 0, y ) 2, n ) 0-3; and x ) 1, y ) 1, n ) 1, 3. When the neutral reagent is phosphine, the most frequent process consists of the addition of a PH group and elimination of a hydrogen molecule. This pathway occurs also in selfcondensation processes of phosphine,30 and it has been observed for both primary, SiHn+ (n ) 0-3), and secondary, Si2Hn+ (n ) 2-5), ions of silane to form new Si-P bonds in the SiPHn+ (n ) 1-4) and Si2PHn+ (n ) 3-6) ionic species. Also the ion family SiPHn+ (n ) 1-3) follows this path, giving SiP2Hn+ (n ) 2-4) ions. Moreover, the tertiary ions Si2PHn+ (n ) 4, 5) react as follows:
Si2PHn+ + PH3 f Si2P2Hn+1+ + H2
(5)
In self-condensation of phosphine, the reaction pathway eliminating H2 is always exothermic, as previously reported.30 Again, lack of thermodynamic data does not allow the calculation of the enthalpies of the reactions involving ionic species with Si-P bonds. Theoretical calculations of the mechanism of reaction of Si+ with PH3 are in agreement with these experimental results. In fact, a starting coordination of the electron pair of phosphorus to Si+ leads to the formation of the stable ion-molecule complex 6a. It evolves to the SiHP+ (7a) and H2 products, through the transition structure 7b that possesses an energy only slightly higher than the reagent species. Results of ab initio calculations on the Si+/SiH4 system show a different initial attack of Si+ on an hydrogen atom of silane.4 The surface energy of a similar
Gas Phase Ion-Molecule Reactions in Phosphine/Silane
J. Phys. Chem., Vol. 100, No. 1, 1996 161
ion-molecule complex, i.e. Si+ interacting with a hydrogen atom of PH3, has also been investigated. However, the resulting structure 6c is less stable than 6a by about 29 kcal/mol. Moreover, the starting complex of Si+/SiH4 eventually eliminates H2 by migration of a H atom and formation of a bridge of a second hydrogen between the two silicon atoms. On the contrary, the Si+/PH3 complex 6a directly eliminates H2 from the phosphorus atom. Other processes which occur when the neutral reagent is PH3 involve loss of a SiH group, SiH3, silane, Si2H4, PH2, or SiPH. In addition, condensation can occur without neutral loss from the reactants; for example, tertiary Si2PH5+ ions react with PH3 as follows:
Si2PH5+ + PH3 f Si2P2H8+
(6)
This process leads to the formation of adduct ions which are unreactive under the experimental conditions employed here. When a hydrogen transfer takes place, the neutral reagent may be either silane of phosphine, while the charged reagent and product species are the same, such as in reactions 7 and 8:
SiPH3+ + SiH4 f SiPH4+ + SiH3
(7)
SiPH3+ + PH3 f SiPH4+ + PH2
(8)
When the heats of formation of the neutrals are considered,44 reaction 8 is found to be more favored than reaction 7 by about 8 kcal/mol, which suggests that the main hydrogenating species is PH3. Enthalpies of phosphine protonation reactions could be calculated for processes starting from the SiH2+ and PH3+ ions, whose ∆Hf° are available in the literature.44,45 It turns out that all of these protonation enthalpies are negative. Therefore, this process is likely to be thermodynamically favored also for all the other ionic species which are involved in the protonation of phosphine, due to its high proton affinity. Si-P clustering reactions proceed faster with primary ions of silane than with primary ions of phosphine. In both ion families, the less hydrogenated ions, Si+, SiH+, P+, and PH+, show the highest reactivity or are the only species involved in the formation of new Si-P bonds. Among the secondary ions, the Si2Hn+ (n ) 2-5) and SiPHn+ (n ) 1-3) ion families react with comparable rates to give new species containing Si-P bonds, which are Si2PHn+ (n ) 3-7) and SiP2Hn+ (n ) 2-4), respectively. Again, ions with low hydrogen content (Si2H2+, Si2H3+, SiPH+, and SiPH2+) show the highest rate constants. All the secondary ions of phosphine react with high rate constants in self-condensation processes, protonation of phosphine being the fastest reaction.30 However, P2H+ and P2H2+ are also involved in reactions with SiH4 and give SiPH2+, SiP2H3+, and SiPH4+ at rather high rates. Comparison between the PH3/SiH4 system and the NH3/SiH4 system studied previously28 indicates some important differences. Primary ions of silane generally react faster with NH3 than with PH3, to give the corresponding SiNHn+ (n ) 2-4) and SiPHn+ (n ) 1-4) ions, respectively. On the contrary, formation of new Si-N or Si-P bonds from the silane secondary ions, Si2Hn+ (n ) 2-5), is almost equivalent in the two systems, the reactivity being higher toward PH3 when n ) 2, 3 and toward NH3 when n ) 4, 5. In the NH3/SiH4 system, N2Hn+ ions are never observed in the experimental conditions used, while in the corresponding PH3/SiH4 mixture, the phosphine secondary ions, P2+, P2H+, and P2H2+, give Si-P bonds with rather high efficiencies. Mixed secondary ions of the NH3/ SiH4 system, SiNHn+ (n ) 2, 3), react slowly with SiH4, while
they are good reagents for protonation of NH3 to NH4+, a thermodynamically favored process. On the other hand, the SiPHn+ (n ) 1-3) ions show a good reactivity with the two neutral reagents, PH3 and SiH4, yielding species containing new bonds. SiNH4+, as well as SiPH4+, is unreactive in the reaction time considered here. Therefore, even if in the NH3/SiH4 mixture the nucleation reactions start faster than in the PH3/ SiH4 system, clustering proceeds at a higher rate from SiPHn+ (n ) 1-3) ions than from the corresponding SiNHn+ (n ) 1-3) ionic species. Moreover, it is remarkable that in the NH3/SiH4 system a great number of processes take place with NH3 as neutral reagent. Substitution of a Si by a N atom occurs in several pathways, while the reverse process with breakdown of a Si-N bond has never been observed. On the contrary, in the PH3/SiH4 mixtures, displacement of a Si by a P atom takes place as well as substitution of a P by a Si atom. A factor affecting these trends is likely to be the energy of the Si-N bond,46 which is higher than that of Si-P47 by about 20 kcal/mol. In the PH3/SiH4 system, the observed values of rate constants indicate a higher contribution of silicon-containing ions to the formation of mixed Si-P clusters, which is in agreement with the increase of the ion current transported by mixed tertiary and higher order Si-P ions with increasing pressure of silane. Conclusion To prepare phosphorus-doped amorphous silicon, the phosphorus should be present in trace amounts, and a direct doping from a PH3/SiH4 system should be performed using a PH3/SiH4 system with a large excess of silane. Moreover, according to the results described above, the high reactivity of siliconcontaining ions toward phosphine leads to the formation of products with a higher P/Si ratio with respect to the PH3/SiH4 ratio in the reagents. Therefore, a radiolytic or plasma deposition of phosphorus-doped silicon amorphous solid should start from a PH3/SiH4 mixture containing PH3 at a lower percentage than that desired in the final solid product. Acknowledgment. The authors thank Italian Consiglio Nazionale delle Ricerche (C.N.R.) for financial support. References and Notes (1) Mayer, T. M.; Lampe, F. W. J. Phys. Chem. 1974, 78, 2433. (2) Mayer, T. M.; Lampe, F. W. J. Phys. Chem. 1974, 78, 2645. (3) Mandich, M. L.; Reents, W. D., Jr.; Jarrold, M. F. J. Chem. Phys. 1988, 88, 1703. (4) Raghavachari, K. J. Chem. Phys. 1988, 88, 1688. (5) Benzi, P.; Operti, L.; Vaglio, G. A.; Volpe, P.; Speranza, M.; Gabrielli, R. J. Organomet. Chem. 1988, 354, 39. (6) Wlodek, S.; Bohme, K. D. J. Am. Chem. Soc. 1989, 111, 61. (7) Benzi, P.; Operti, L.; Vaglio, G. A.; Volpe, P.; Speranza, M.; Gabrielli, R. J. Organomet. Chem. 1989, 373, 289. (8) Mandich, M. L.; Reents, W. D., Jr.; Kolenbrander, K. D. J. Chem. Phys. 1990, 92, 437. (9) Raghavachari, K. J. Chem. Phys. 1990, 92, 452. (10) Mandich, M. L.; Reents, W. D., Jr. J. Chem. Phys. 1991, 95, 7360. (11) Raghavachari, K. J. Chem. Phys. 1991, 95, 7373. (12) Reents, W. D., Jr.; Mandich, M. L. J. Chem. Phys. 1992, 96, 4429. (13) Raghavachari, K. J. Chem. Phys. 1992, 96, 4440. (14) Benzi, P.; Operti, L.; Vaglio, G. A.; Volpe, P.; Speranza, M.; Gabrielli, R. Int. J. Mass Spectrom. Ion Processes 1990, 100, 647. (15) Castiglioni, M.; Tuninetti, M.; Volpe, P. Gazz. Chim. Ital. 1983, 113, 457. (16) Belluati, R.; Castiglioni, M.; Volpe, P.; Gennaro, M. C. Polyhedron 1987, 6, 441. (17) Antoniotti, P.; Benzi, P.; Castiglioni, M.; Operti, L.; Volpe, P. Chem. Mater. 1992, 4, 717. (18) Mandich, M. L.; Reents, W. D., Jr.; Bondybey, V. E. J. Phys. Chem. 1986, 90, 2315. (19) Raghavachari, K. J. Phys. Chem. 1988, 92, 6284. (20) Kickel, B. L.; Fisher, E. R.; Armentrout, P. B. J. Phys. Chem. 1992, 96, 2603. (21) Nguyen, K. A.; Gordon, M. S.; Raghavachari, K. J. Phys. Chem. 1994, 98, 6704.
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