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a useful nonlinear optical material. More extensive exploratory research will be needed to meet these goals. Acknowledgment. We thank the Office of Naval Research and E. 1. du Pont de Nemours & Co. for their support, Bill Farneth (CR&DD, E. I . du Pont) and Ray Gorte (University of Penn-
sylvania) for helpful discussions and advice, and Professor Hellmut Eckert (UC Santa Barbara) for assistance in obtaining 'HMAS N M R spectra. Registry No. MA, 74-99-7; MA (homopolymer), 28391-48-2; NA', 7440-23-5; NHS, 7664-41-7; I,, 7553-56-2.
H Atom Generatlon from the 193-nm Photolysis of Trlethylgalllum Subhash Deshmukh, Jeffrey L. Brum, and Brent Koplitz* Department of Chemistry, Tulane University, New Orleans, Louisiana 70118 (Received: April 9, 1990; In Final Form: July 19, 1990)
Under collisionless conditions, the 193-nm photolysis of triethylgallium (TEG) readily produces H atoms. The H atoms are probed by using two-photon (121.6 + 364.7 nm) ionization, and the H atom Doppler profile at Lyman-a demonstrates that only a small amount of product energy appears as H atom translational motion, the mean kinetic energy being 0.3 f 0.1 eV. Despite the relatively small H atom kinetic energy release, photolysis/probe delay studies on TEG suggest that H atom formation occurs on a time scale of < I 0 0 ns. These experimental observations are discussed in terms of a one-photon dissociation mechanism involving an ethyl radical intermediate.
Understanding the gas-phase photochemistry of metallorganic compounds is of considerable interest for many microelectronics applications.Is2 This interest is fueled, in part, by the widespread use of lasers as tools to control/enhance the deposition of thin films on substrates. The fundamental photochemistry of metal alkyls such as triethylgallium (TEG) or triethylaluminum (TEA) is complex, and much remains to be learned. However, compounds such as these are frequently used as the metal atom carriers in the growth of GaAs, AIGaAs, etc.'" Moreover, lasers are often utilized to affect the chemistry of these compounds in growth environments. Consequently, it is imperative that the basic photochemistry of these molecules be understood if one hopes to optimally influence the reactions that occur in laser-enhanced deposition processes, at least where photochemical mechanisms are concerned. The identification of atomic hydrogen resulting from the photodissociation of metal alkyls can be useful in understanding the elementary mechanism(s) of photodissociation. In addition, generating H atoms in semiconductor growth environments has significant practical importance in terms of enhancing actual growth processes!" In the fabrication of GaAs, AIGaAs, and related 111-V semiconductor materials, TEG has become increasingly popular as a Ga atom carrier. Since TEG has a large absorption cross section (-9 X cm2) at 193 nm,' photochemistry can be easily initiated with an excimer laser. In this article, we report on our findings that the 193-nm photolysis of TEG under collisionless conditions results in substantial H atom production. Detection of the H atoms is accomplished via two-photon (121.6 + 364.7 nm) ionization with subsequent time-of-flight (TOF) mass (1) Bauerlc, D.Springer Series in Chemical Physics on Laser Processing and Diagnostics; Springer: New York, 1984; Vol. 39. (2) Donnelly, V. M.;Herman, I. P.; Hirose, M.Photon, Beam, and Plasma Stimulated Chemical Processes at Surfaces; Materials Research Society: Pittsburgh, 1987; Vol. 75. (3) Manasevit, H. M. J . Crysr. Growrh 1981, 55, 1. (4) M a , S.;Kawase, R.; Sato, T.; Shimizu, I.; Kokado, H. Appl. Phys. l a t i . 1986, 48, 33. (5) Sheng, T. Y.; Yu, Z. Q.;Collins, G. J. Appl. Phys. Lett. 1988,52, 576. (6) Kiely. C. J.; Tavitian, V.;Jones, C.; Eden, J. G. Appl. Phys. Lett. 1989, 55, 64. (7) McCrary, V. R.; Donnelly, V. M. J . Crysr. Growth 1987, 84, 253.
spectrometric measurement.!' For TEG photolysis, the H atom Doppler profile at Lyman-a is presented and discussed with respect to NH3 photolysis under similar conditions. Delay studies, Le., H atom production as a function of delay between the photolysis and probe lasers, are also presented. Our results are discussed in terms of possible photofragmentation routes, and they suggest that the dominant pathway for H atom formation is a one-photon process involving an intermediate, most likely the ethyl radical. The experimental arrangement used for this work has been described previo~sly.~Briefly, photolysis and probe laser beams counterpropagate through the ionization region of a TOF mass spectrometer (TOFMS). Commercially available TEG (99.999% purity, Air Products) and NH3 (Matheson) were used without further purification. Introduction of the sample into the TOFMS was accomplished via a leak valve, and operating pressures were kept constant at 3 X lo4 Torr for TEG and 2 X lo4 Torr for NH3. Experiments are conducted as follows. The output from an excimer laser (Questek 2220, ArF, 193 nm) is mildly focused into the ionization region of the TOFMS with a lens (focal length (fl) = 1 m). The photon intensity in the ionization region is estimated to be 1G5photonscm%-l, but it is difficult to achieve a more accurate determination of this value since the exact dimensions of the laser beam focal region are not easy to ascertain. After a variable time delay, the output of an excimer-pumped dye laser (Lambda Physik LPX 105, FL 3002) counterpropagates through the TOFMS. Prior to entering the TOFMS chamber, a small fraction (- 10") of the fundamental laser radiation (365 nm)is frequency-tripled by focusing it into a cell containing 100 Torr of Kr. The resulting vacuum-ultraviolet radiation (frequency bandwidth 1 cm-l at 121.6 nm) is tuned through the Lyman-a transition of atomic hydrogen. Both the tripled output and the remaining fundamental radiation are refocused by a LiF lens (fl = 64 mm) and overlapped with the photolysis beam inside the TOFMS. Sequential absorption of 121.6- and 364.7-nm photons by the H atom photoproduct results in H+ions. These ions are accelerated, pass through a field-free drift tube, and ultimately impact on a microchannel plate detector. After amplification,
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(8) Zacharias, H.; Rottke, H.; Danon. J.; Welge, K. H. Opt. Commun. 1981, 37, 15. (9) Brum, J. L.; Deshmukh, S.;Koplitz, B. Chem. Phys. Letr. 1990, 165, 413.
0022-3654/91/2095-0720%02.50/0 0 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 2, 1991 721
193-nm Photolysis of Triethylgallium
A
h
2 5
-3
vo
=L 300
+3
0
WV Probe Frequency (cm-1) Figure 1. H atom Doppler profile at Lyman-a (82259.1 cm-I) resulting from the 193-nm photolysis of TEG. The fwhm value for the Doppler profile is 2.8 cm-I.
the signal is captured by a 100-MHz digitizer (DSP Co.) and sent to a small computer for averaging, storage, and processing. The H atom Doppler profile a t Lyman-a resulting from the 193-nm photolysis of TEG is shown in Figure 1. The full width a t half-maximum (fwhm) value for the Doppler profile is -2.8 cm-l, and from this value, one can calculate an H atom kinetic energy. However, that energy is not the mean H atom kinetic energy. This situation occurs because the faster moving H atoms can display little or no Doppler shift as well as a large Doppler shift. Also, a larger observed Doppler shift must be weighted more heavily than a smaller one due to a Jacobian factor. In addition, when compared with a 6 function frequency bandwidth for the probe, the actual probe bandwidth (-I cm-’ at 121.6 nm) results in about a 10% broadening of the spectrum. When all factors are taken into account, a mean H atom kinetic energy of 0.3 f 0.1 eV is extracted, assuming an isotropic spatial distribution of H atoms. To reduce the chance of systematic error, photolysis studies were performed on N H 3 under similar experimental conditions, and good agreement with previous H atom Doppler studies on NH3 was obtained.I0 In order to gauge the absolute intensity of the H atom signal, NH3 was used as a reference, since its quantum yield for H atom formation is 1.O.” The H atom signal from NH3 was larger than the H atom signal from TEG by approximately a factor of 15. However, the absorption cross section for NH3 (( 1.2) X 1O-I’ cm2 at 193 nm)” is slightly larger than that for TEG at the same wavelength, -9 X lo-’* cm2.’ Also, we suspect that due to differences in ionization potentials, the ionization gauge has a greater efficiency for TEG detection. The net effect of these factors places our estimate of the quantum yield for H atom formation from TEG at 10-20%, although the uncertainty is rather large. There are several plausible photodissociation pathways that could give rise to H atom generation in the 193-nm photolysis of TEG. Of course, the initial step is absorption of a 193-nm photon (6.4 eV) by the TEG parent molecule. It is unlikely that TEG absorbs a second photon, since continued uppumping would have to compete with dissociation, a process most likely occurring on a subnanosecond time scale. At our rather modest photon intensities (photons.cm-2.s-’), the cross section for absorption of a second photon would have to be cm2 to be competitive with dissociation. However, there are at least two possible dissociative channels available to an electronically excited TEG molecule, denoted as TEG*. One alternative is for TEG* to undergo a @-hydrogenelimination reaction, Le., Ga(C2H5)3 HGa(C2HJ2 + C2H4. There is evidence that this may be an important channel in thermal semiconductor chemistry involving TEG,’* but to our knowledge it has not been implicated as a photolytic route. Nonetheless, it remains a possibility. The other logical route for TEG* dissociation is straightforward cleavage of the Ga-C bond leading to Ga(C2H5)2and C2H5. From bond N
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(10) Koplitz, B.; Xu, Z.; Wittig, C. Chem. Phys. Lerr. 1987, 137, 505. ( I 1 ) Okabe, H.Phorochemisrry of Small Molecules; Wiley-lnterscience: New York, 1978. (12) Putz, N.; Heinicke, H.; Heyen, M.; Balk, P.; Weyers, M.; Luth, H. J . Crysf. Growrh 1986, 77, 223.
200
100
400
Delay (nsec)
Figure 2. H atom signal as a function of delay between the photolysis and probe lasers. The H atom signal from TEG is referenced to the H atom signal arising from NH3,which should be prompt on the time scale of the experiment. This procedure reduces any effects due to nonideal spatial overlap between the two laser beams.
energy measurements, cleavage of the initial Ga-C bond in TEG requires -2.3 eV.” Consequently, -4.1 eV of energy (plus parent thermal energy) must be partitioned between kinetic and internal degrees of freedom in the two photofragments, Ga(C2H5)* and C2H5. Since the energy needed to break the C-H bond in C2H5 to form C2H4 and H is only 1-1.5 eV,I4 an overall one-photon absorption/disscciation route to H atom formation is energetically possible. It is likely that a good deal of the 4-5 eV of energy available to the products is deposited in Ga(C2H5), internal motion and/or translational energy of the Ga(C2H5)2and C2HSphotofragments. As a result, one would expect that only a limited amount of the available energy would appear as C2H5 internal excitation. If indeed the C2H5 fragment goes on to react and considering the 1-1.5 eV needed for C2H5to become C2H4and H, there is probably not much energy remaining for H atom kinetic motion. Also, internal excitation of the C2H4fragment constitutes an additional energy ‘‘sink’’. Consequently, a one-photon dissociation mechanism is consistent with the narrow Doppler profile in Figure 1, from which we extract a mean H atom kinetic energy of 0.3 f 0.1 eV. Although a one-photon mechanism involving an ethyl radical intermediate seems likely, the ethyl radical itself can also absorb a 193-nm photon that would lead to H atom production. However, we suspect that this pathway would result in a significantly broader Doppler profile, since 5-5.5 eV of energy would be available for photoproduct excitation plus the amount contained in the ethyl radical resulting from the initial dissociation process. Although not impossible, it would be surprising if only 0.3 eV of this available energy were channeled into translational motion. In addition, studies involving the 193-nm photolysis of triethylaluminum produced a similarly narrow H atom Doppler profile? and modeling studies indicate that a one-photon mechanism is in operation in this case.I5 In the case of TEG photolysis power studies, thin-film deposition on the chamber windows severely hampers H atom detection over extended periods of time because the deposited material readily attenuates the 121.6-nm probe radiation. Nonetheless, crude power studies (not shown) point toward a one-photon power dependence. Finally, we emphasize once again that one can make arguments for H atom production involving the HGa(C2H5),intermediate discussed above instead of a C2H5 intermediate. Experiments are currently under way in our laboratory to distinguish between these two routes. In fact, investigations into haloethane photolysis in our laboratory have identified signficant H atom production.I6 Here, the ethyl radical is clearly the logical intermediate. Although not conclusive, these results certainly suggest that the ethyl radical plays an important role in H atom production subsequent to TEG photolysis.
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(13) Kol’yakova, G. M.; Rabinovich, 1. B.; Zorina, E. N. Dokl. Akud. Nauk SSSR 1973, 209,616. (14) Ruscic, B.; Berkowitz, J.; Curtiss, L. A.; Pople, J. A. J . Chem. Phys. 1989, 91, 114. (15) Brum, J . L.; Deshmukh, S.;Koplitz, B. J . Chem. Phys., in prus. (16) Deshmukh, S.;Brum, J.; Koplitz, B. Chem. Phys. Left., in prus.
722 The Journal of Physical Chemistry, Vol. 95, No. 2, 1991
Deshmukh et al.
Clearly, the photolysis of TEG produces H atoms with only a small amount of kinetic energy. From the perspective of dynamics, one can ask whether this small kinetic energy release is indicative of a process occurring on a relatively long time scale. This possibility prompted us to conduct photolysis/probe delay studies. Such studies have been reported previously by Tsukiyama and Bersohn on H atom formation subsequent to the photolysis of molecules such as cycloheptatriene and toluene.17 For certain systems (e.g., toluene dissociated at 193 nm), there is a definite “risetime” for H atom formation, on the order of a microsecond.I7 In several of these cases, simple statistical theories do a reasonably good job of modeling the observed behavior. Figure 2 shows the results for H atom production as a function of probe delay for the 193-nm photolysis of TEG. In order to reduce any experimental artifacts, for example nonideal overlap of the photolysis and probe laser beams, the H atom signal from TEG is referenced to the H atom signal from NH, photodissociation. In terms of absolute intensity, the 193-nm photolysis of both NH, and TEG produces an H atom signal that decreases sharply with probe delay. It is known that the 193-nm photodissociation of NH, takes place on a subnanosecond time scale,IE Le., fast on the time scale of our laser pulse duration (- 15-20 ns). Assuming an isotropic spatial distribution of H atoms in both cases,our studies show that the growth/decay behavior of H atoms from both sources is basically the same, at least on the time scale that was investigated. One would expect the TEG H atom signal to fall off slightly less rapidly than the NH3 H atom signal simply because the H atoms from TEG are traveling more slowly. Indeed, Figure 2 suggests that this behavior exists at rather long probe delay times, but our focus is on the relative rise of the TEG H atom signal. In the case of TEG photolysis with a probe delay >IO0 ns, we see no discernible evidence for an H atom risetime. At delay times less than 50 ns, the pulse duration of each laser (15-20 ns) tends to obscure any observation of an H atom risetime. There may, in fact, be a risetime for TEG H atom formation occurring on the order of tens of nanoseconds. Experiments using slightly shorter pulses will address this issue. We also note the fact that no discernible H atom risetime is significant. Since a small H atom kinetic energy release means that it is possible that dissociation is occurring not far above the energy activation barrier, it is logical to suspect that a slow H atom formation rate on the order of IO6 s-I might be in operation. As noted above, such behavior has been seen in other photolysis reactions involving H atom prod~ction.’~In several of these reactions, the H atom kinetic energy release was even greater than that observed for 193-nm TEG photolysis. However, our measurements place the H atom production rate for TEG photolysis at >lo7 s-I, too fast to be measured with our current experimental arrangement. This finding is not inconsistent with previous RRKM calculations by Lin and Laidler involving ethyl radical decomposition.19 Although a statistical calculation may not be totally applicable in this case, it does give an indication of the magnitude of the ethyl
radical decomposition rate as a function of energy above the dissociation barrier. Using a value of 1.7 eV for the barrier height19 and 1.5 eV for the bond energy,I4 a minimal passage over the barrier would leave 0.2 eV of energy available to the products. Since only 0.3 eV is deposited into translational motion, it is conceivablethat a slow rate of H atom formation might occur. However, the RRKM calculations of Lin and Laidler predict a rapid increase in the rate for even modest energies above the barrier.19 As little as 0.1 eV above the barrier should produce a rate of loEs-l, clearly faster than we can measure with our current experimental arrangement. Since one would expect that internal modes of the ethylene photoproduct would also be populated, it is likely that somewhat more than 0.3 eV is available to the products. Although the small H atom kinetic energy release prompted us to measure the rate of H atom formation, we are not surprised that this rate is too fast for us to determine. In pump/probe experiments, it is valid to ask whether or not the “probe“ laser itself is initiating any photochemistry. Although this involvement is a potential problem, we assert that any “probe” photochemistry is minimal, at least in terms of H atom production. The reasoning for this assertion has to do with the speed of an H atom photoproduct relative to a heavier intermediate photoproduct, for example, CIHS. Given the same amount of kinetic energy release for an H atom versus C2Hs,the H atom moves 5.4 times as fast because its mass is smaller by a factor of 29. Consequently, H atoms should leave the photolysis/probe focal region much more quickly than ethyl radicals. If ”probe” photochemistry were playing a significant role, we expect that the H atom signal would not fall off as quickly as it does, since ethyl radicals should remain in the focal region a relatively long time. Thus, H atom production resulting from CzHS“probe photolysis” would not be very sensitive to short probe delays. Since the delay behavior of H atom production from NH, and TEG photolysis is similar, and the falloff in absolute H atom signal is, in fact, quite rapid in both cases, we conclude that probe-induced H atom generation is minimal. Of course, one could argue that TEG photolysis produces a CzHSradical that receives a huge amount of kinetic energy, causing it to travel at speeds usually associated with the motion of an H atom photoproduct. Although possible, we consider this event unlikely because of the large number of internal degrees of freedom present in a TEG molecule. In summary, significant H atom generation is observed following the 193-nm photodissociation of TEG. The kinetic energy associated with the H atoms is very small, as measured by Doppler techniques. Our studies suggest a one-photon dissociation pathway to H atom production, but we are currently working on identifying and probing the intermediate, presumably the ethyl radical, in order to better understand the photochemistry involved. Positive identification of the relevant intermediate and/or measuring the rate of H atom formation will provide an additional handle so that a more meaningful comparison with simple statistical theories can be made.
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(17) Tsukiyama. K.; Bersohn, R. J . Chem. Phys. 1987, 86, 745. (18) Ashfold, M. N. R.; Bennett, C. L.; Dixon, R. N. Chem. Phys. 1985, 93, 293. (19) Lin, M. C.; Laidler, K. J. Trans. Faraday SOC.1968, 64, 79.
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Acknowledgment is made to the donors of the Petroleum Research Fund, administered by the American Chemical Society, for partial support of this work. We also acknowledge the National Science Foundation for support via its EPSCoR program. The comments of a reviewer are greatly appreciated.
