J . Phys. Chem. 1989, 93, 1145-1 148
1145
Gain and Kinetics Characteristics of '*CO, and "CO, Pulsed Lasers K.-M. Jeong,* H. P. Chou, G. A. Theophanis, and V. Hasson Avco Research Laboratory, Inc., Everett, Massachusetts 02149 (Received: March 15, 1988)
Vibrational fluorescence measurements of the C 0 2 (v3) band have been made by using a single-pulse closed volume discharge in standard carbon dioxide laser mixtures for two different isotopes of carbon. These data are reduced to provide gain amplification propertiesof I2CO2and I3COz,which are then compared. The faster postpump decay rate of I3CO2is attributed to a significantly faster upper level deactivation rate constant. The C 0 2 laser kinetics are modeled and compared with experimental results.
Introduction The use of isotopically substituted carbon dioxide molecules in the active media of molecular carbon dioxide lasers was first demonstrated by Wieder and McCurdy,' Jacobs and Bowers: and S i d ~ w a y .Since ~ this work, many applications have emerged that take advantage of differences in emission line intensities and frequency positions relative to the spectral bands of 12C02. The dipole matrix elements of transitions to the mixed lower laser states [100,020]1and [100,020]11strongly depend on the amount of state mixing, which varies with isotopic substitution. Isotopic substitution can also contribute to significant variations in the collisional deactivation of the upper laser level (001). Since the basis of operation of the C 0 2 molecular laser is the transfer of vibrational energy from N 2 to the C02(v3) state, changes in deexcitation rates of this level are reflected in the gain coefficient that is a measure of the amplification properties of the gas media. Measured values of the small signal gain coefficient depend on several experimental parameters, including the excitation current, gas pressure, mixing ratio, discharge uniformity, and pulse length. This dependence is evident in the spread among previous gain measurements made under low-pressure continuous wave (CW) pumping conditions for '2C02and 13C02.4-6 The C W results also differ from recent work at high pressures and for pulsed operation.' Measurements are presented here of the small signal gain of I2CO2and 13C02laser mixtures under identical pulsed pumping conditions, and measurements of the vibrational relaxation of the (001) level of 12C02and I3CO2are also discussed. These measurements are then compared with predictions of a kinetics model of the excitation and deactivation processes in a C 0 2 laser. Experimental Section A single-pulse closed volume discharge in the isotopic laser mixture was used to determine the amplification and kinetics characteristics of the gain media. The UV preionized discharge amplifier has a 4.5 X 5.0 X 60.0 cm3 volume and a gain length of 3 X 60.0 cm since three passes of the probe beam were made. A variable-discharge-pulse generating network was used to produce a rectangular pump pulse of either 5.5, 13, or 24 ps. The energy loading was typically between 65 and 112 J/(L atm). The operating pressure was 1 atm, and the C02mole fraction was varied from 0.0396 to 0.0935. The following high-purity research grade (1) Wieder, I.; McCurdy, G. B. Phys. Reo. Lett. 1966,16, 565. McCurdy, G. B.; Wieder, I. IEEE J . Quantum Electron. 1966, QE-2, 385. (2) Jacobs, G. B.; Bowers, H. C. J . Appl. Phys. 1967, 38, 2692. (3) Sidoway, J. C. J. Appl. Phys. 1968, 39,4854. (4) Reid, J.; Siemsen, K. J . Appl. Phys. 1977, 48, 2712. (5) Brimacornbe, R. K.; Reid, J. IEEE J. Quantum Electron. 1983, QE-19, 1674. (6) Freed, L. E.; Freed, C.; ODonnell, R. G. IEEE J. Quantum Electron. 1982, QE-18, 1229. (7) (a) Kroeker, D.; Defaccio, M.; Pindroh, A.; Guyer, D. Topical Meeting on Coherent Loser Radar: Technology and Applications, Technical Digest Series; Optical Society of America: Washington, D.C., 1987; Vol. 16, pp 9-12. (b) Fisher, C. H.; Goldsmith, A. D.; Yoder, M. J.; Buczek, C. J.; DeFaccio, M. A,; Guyer, D.; Hellfled, H.; Kim, K. J.; Kroeker, K.; Pindroh, A.; Uchizono, S. A,; Wang, J. J.; Youman, D. G. CO, Loser Radar Amplifier Measurements Program Final Report; MIT Lincoln Laboratory, July, 1987.
0022-3654/89/2093-1145$01.50/0
gases were used: He (Liquid Carbonics, 99.9999%), N2 (Liquid Carbonics, 99.998%), and C02 (Cardox, 99.9+%). The isotopic content of the I3CO2was better than 99.0% (Isotec Inc.), and the chemical composition was the following: C 0 2 L 99.5 mol %, CO < 0.1 mol %, Ar < 0.2 mol %, N 2 < 0.1 mol %, and O2 < 0.1 mol %. The experimental setup used to probe the discharge amplifier is shown in Figure 1. The unamplified probe signal Io is determined by chopping a grating tunable C W laser beam (Ultra Lasertech, Inc. Model No. 8822/0G1). The laser output power (TEMw mode pattern) on a selected C 0 2 line was 6 W. The beam had a diameter of 3.7 mm and a divergence of 3.7 mrad. The beam is focused onto the chopper blade to achieve a fast signal rise time (about 3 ps). Following the opening of the chopper and stabilizing of the probe laser signal, the amplifier discharge is fired. Three passes of the probe beam through the discharge volume are necessary to achieve sufficient signal for tracking the gain as a function of time. The amplified probe signal I is detected by using a gold-doped Ge detector and recorded on an oscilloscope (Tektronix Model 7104). The small signal gain coefficient g is calculated by using the expression exp(gL) = I/Io,where L is the effective length of the discharge region. The measured gain was found to vary by about 10% from shot to shot and by about 20% for different gas fills.
Results and Discussion The gain and vibrational relaxation measurements were performed in the small signal limit for which the ratio of the laser intensity to the saturation intensity is much less than 1. The small signal gain was measured for the IP(18), IP(20), and IP(22) lines of 12C02and I3CO2. The decay rate was determined from linear least-squares fits to plots of In (gain) versus time (see Figure 2). The decay rates for several partial pressures and laser pulse lengths are summarized in Tables I and I1 for I2CO2 and I3CO2, respectively. The decay rates are corrected by the factor xC02/(xC02
+ Ke&N2)
where Xco2and XN2are the mole fractions in the He-N2-C02 - V N ~ ( ' ) ,and mixture. K = exp[hAv/kT], where Av = is 1.09 for % C 0 2 and 0.86 for l3CO2. This correction factor accounts for the fraction of the total vibrational energy residing in the C 0 2 molecule. The rate of vibrational energy transfer between CO2(O01) and N2(u=l)is very rapid, and the combined populations remain strongly coupled while other relaxation processes occur. The peak of the small signal gain coefficient was found to be approximately 40% greater for 13C02than for I2CO2for corresponding partial pressures and slightly decreased with pulse length. For small energy extraction, the inversion density rate equation is given by
d",
dt and the gain equation is
n
I
wo - - - --on T hv
0 1989 American Chemical Society
(1)
1146 The Journal of Physical Chemistry, Vol. 93, No. 3, 1989 TABLE I: Variation in Peak Gain and Decay Rate with
13C02 Pressure and
pulse length" peak gain, decay rate, % cm-' 105 s-I 5.5-ps
transition
He:N2:I3CO2
IP(18)
7.7:2.0:0.4 7.7:2.0:0.6 7.7:2.0:0.8 7.7:2.0:1.O 7.7:2.0:0.4 7.7:2.0:0.6 7.7:2.0:0.8 7.7:2.0:1.0 7.7:2.0:0.4 7.7:2.0:0.6 7.7:2.0:0.8 7.7:2.0:1.0
IP(20)
IP(22)
0.56 0.6 1 0.76 0.88 0.63 0.67 0.8 1 0.93 0.56 0.68 0.78 0.89
2.0 1.9 2.2 2.4 1.5 2.1 1.9 2.0 2.1 2.1 1.8 1.7
Jeong et al. Pulse Length 13-ps pulse lengthb
peak gain, % cm-'
decay rate,
0.62 0.68 0.76 0.81 0.63 0.70 0.74 0.82 0.55 0.65 0.72 0.82
24-ps pulse length'
decay rate,
105 s - ~
peak gain, 5% cm-'
2.3 2.1 2.1 2.0 1.9 2.0 2.0 2.2 1.7 2.0 1.9 2.2
0.61 0.7 1 0.72 0.75 0.70 0.74 0.78 0.82 0.68 0.71 0.84 0.94
2.1 2.1 2.2 2.3 2.4 2.1 2.3 2.5 1.6 2.1 2.2 2.4
105 s-1
"Energy loading: 67 J / ( L atm). bEnergy loading: 72 J / ( L atm). CEnergyloading: 112 J / ( L atm). TABLE 11: Variation in Peak Gain and Decay Rate with ' ~ O Pressure Z and Pulse Length 5.5-ps
transition IP( 18)
IP(20)
IP(22)
peak gain, % cm-l
He:N2:I2CO2 7.7:2.0:0.4 7.7:2.0:0.6 7.7:2.0:0.8 7.7:2.0:1.0 7.7:2.0:0.4 7.7:2.0:0.6 7.7:2.0:0.8 7.7:2.0:1.0 7.7:2.0:0.4 7.7:2.0:0.6 7.7:2.0:0.8 7.7:2.0:1.O
0.21 0.43 0.56 0.83 0.26 0.43 0.49 0.58 0.26 0.44 0.55 0.58
pulse length" decay rate, 105 s-1
1.2 1.2 1.2 1.4 1.2 1.5 1.3 1.3 0.74 1.1 1.1 1.2
13-ps pulse lengthb
peak gain, % cm-'
24-ps pulse lengthe
decay rate,
peak gain,
decay rate,
105 s-l
5% cm-I
105 s-1
0.29
1.2 1.2 1.2 1.4 1.2 1.5 1.3 1.3 0.72 1.1 1.1 1.2
0.55 0.31 0.45 0.46 0.59
0.66 0.78 0.49 0.56 0.6 1 0.77 0.12 0.46 0.56 0.66
0.78 1.1 1 .o 1.1
"Energy loading: 65 J/(L atm). bEnergy loading: 75 J/(L atm). CEnergyloading: 108 J / ( L atm). U.V. PREIONIZED CO 2 OISCHARCE
-i__i _ c _ -
I'
f
'if I
I
--l OSCILLOSCOPE
CAMODE
10
20
30
40
50
TIME $us)
(3)
The stimulated emission cross section is given by .(A) = (A2/ 87r)S(v)A,. Assuming a Lorentzian line broadening function S(v), .(A) is calculated for the four different gas mixtures to be about 1.1 X lo-'* cm2 for a 13C02line centered at 11.16 M r n and 0.6 X lo-'* cm2 for the 10.6 pm line of 12C02,a difference of 80%. The I3CO2observed peak gain enhancement is smaller than this factor and appears to reflect a much faster postpump gain decay and, as discussed below, also a smaller upper level population due to more rapid relaxation. As shown in Tables I and 11, the I3CO2 decay rates are generally 50% larger than those of 12C02. Two possible explanations of the difference in gain decay rates are a reduction in Nu (i.e., a faster population relaxation from
Figure 2. Comparison of (001) fluorescence and IP(20) gain decay for I2CO2and I3CO2: (a) I2CO2fluoresence; (b) "C02 fluorescence; (c)
I2CO2gain decay; (d) W02gain decay. the upper level) or an increase in Nl (Le., a population bottleneck in the lower level). As a test of the first possibility, the 4-hm fluorescence of the two laser mixtures was observed with the same experimental apparatus except that an InSb detector (SBRC Model E679) was substituted for the gold-doped Ge detector to optimize S/N. This fluorescence originates from molecules radiating in the asymmetric stretch vibrational mode and is, therefore, a good probe of the population in the upper laser level. In Figure 2 it is seen that the fluorescence decay times exhibit the same behavior between I2CO2and l3COZthat was observed
The Journal of Physical Chemistry, Vol. 93, No. 3, 1989 1147
'zCOz and 13C02Pulsed Lasers TABLE 111: Summary of (001) Vibrational Relaxation Rate Constants for Q = He,Nb lzCOg and I3CO2 k,, Torr-l s-'
Q He
'2C02
WO2
ref
90 107 w02
106 350
350 440
344
I3C0,
transition '2C0, IP(20)
8 7 8 7 this work 8 this work 7
85
N2
TABLE IV: Comparison of Experimental and Calculated Decay Rates
1340 1003
He:N2:C02 7.7:2.0:0.4 7.7:2.0:0.6
decay rate, lo5 s-I measd corr calcd 0.12 0.77 0.78
7.7:2.0:0.8
0.23 0.27
1.1 1.o
0.83 0.86
1.1 1.9
0.90
2.0
2.1 2.2 2.2
W02
7.7:2.0:1.0 7.7:2.0:0.4
IP(20)
7.7:2.0:0.6
0.33 0.36 0.51
7.7:2.0:0.8
0.62
2.0
7.7:2.0:1.0
0.83
2.3
2.0
this work
in the gain curves. These results indicate that the faster gain decay is due to a faster rate of deactivation from the ' T O 2 upper laser level and not bottlenecking. The fluorescence decay rate k' of the (001) laser level is given by
k' = PCOlkCOl + PNzkN2 + PHckHc
(4)
The experimental decay rates for different gas compositions were used in a linear least-squares routine to derive kc02 and kN2. The H e quenching rate constant kHewas taken to be 85 Torr-' s-I for deactivation of I2COzand 90 Torr-' s-' for 13C02.798This analysis yields the results summarized in Table 111. The uncertainties associated with these values are roughly f30% and are principally due to shot-to-shot variabilities and uncertainties in the Z = 0 base line. The quenching rate constants for Q = 12COzand N2 are in excellent agreement with measurements by Moore et aL8 The I3CO2deactivation rate constants agree within the stated uncertainties with a pulsed laser study' and are also qualitatively consistent with a pulsed electric discharge study of the vibrational and translational temperatures of mixtures containing isotopically substituted carbon d i ~ x i d e .Since ~ the rate of relaxation by I2CO2 is relatively slow, the effect of possible impurities was examined. First, particular care was taken to remove H 2 0 due to its deleterious effect on lasing action. A sodium aluminosilicate 4A molecular sieve (Matheson Purifier Model 462) was used downstream of the gas mixing tank to remove water in addition to careful purging of the inlet ports prior to an experiment. This molecular sieve cartridge reduced the water content to lo4% or, equivalently, to a few parts per million. Although quenching of 12C02(v3)by H 2 0 is more rapid,8,10,11k 27 000 Torr-' s-l, a concentration of 3 X mol % would give a first-order loss of -55 s-I versus -2.5 X lo4 s-' for relaxation of the v3 mode by lZCO2.The absence of a significant H 2 0concentration is also supported by the good agreement of the 12C02data with the previous literature measurement (the concentration of impurities other than H 2 0 in the He, Nz, and 12C02gases ranged from 0.1 to 15 ppm and would not effect these measurements given quenching rate constants"9l2 of -20-27 000 Torr-' 8).Second, analysis of the chemical composition of I3CO2showed four minor impurities of which C O is the fastest relaxer. However, a maximum C O impurity in of 0.1 mol % corresponds to a first-order loss of -200 s-I, which is a factor of -125 lower than that of relaxation by I2CO2. Thus, the faster relaxation of u3 by ' T O 2 cannot be attributed to impurities, and its dynamics is presently not understood. However, it provides an explanation for the -60% smaller 13C02peak gain coefficient reported by Freed et aL6 for a low-pressure C W laser. In the C W limit, t >> T , 2 gives g ( 1 3 c o z ) / g ( 1 2 c 0 2= ) ~ ( ' ~ C 0 ~ ) / r ( ' ~ CSince 0 ~ )the .
0.5
1
& A L 10
100
T
1000
(Ps)
Figure 3. Experimental and calculated peak gain ratios as a function of pulse length: (-) calculation of R for P = 760 Torr; (- - -) calculation of R for P = 260 Torr; a denotes He:N2:C02= 7.7:2.0:1.0; b denotes He:N2:C02= 7.7:2.0:1.0;c denotes He:N2:C02 = 3:2:1; (0) experimental data this work; (A) CW experimental data of Freed et a1.6
1967, 46, 4222.
measured decay rate for l3COZis larger, g(13C02)/g('2C02)is predicted to be less than 1.0. The experimental decay rates are compared in Table IV with the predictions of a kinetics model. The model was originally developed by Douglas-Hamilton and is described in ref 13. The model includes the dominant routes by which energy is transferred from one vibrational mode to another, as well as those by which energy is introduced into and extracted from the system. The activation and deactivation rate constants were recently updated by Lewis.I4 The energy conservation differential equations for the system are sequentially integrated, and the detailed time history of the C 0 2 mode population is determined. The intensity of stimulated emission required to satisfy external constraints yields the laser flux density output as a function of time. Application of this model to the COS amplifier requires as inputs (1) the gas total pressure, temperature, and composition and (2) the configuration of the device and pumping conditions, which include the pumping cross sections for vibrational levels of COz and N2, E/N (electric field divided by the gas density) and the electron velocity and temperature. For both '2C02 and l3COZ,the calculated decay rates agree well with the experimental values. The good agreement found for '2COzindicates that no major energy-transfer processes are omitted and, consequently, that a fast l3CO2relaxation rate is required to reproduce the experimental gain curves. This is shown in Figure 3, where the ratio of the ' T O 2 and I2CO2peak gains is plotted as a function of pulse length. The kinetics calculation also demonstrates that for pulse lengths greater than 60 bs, the l3CO2peak gain drops below that of I2CO2,consistent with the
(9) Petukhov, V. 0.;Sazhina, N. N.; Seregin, A. M.; Solodukhin, A. S.; Starovoltov, V. S.; Trushin, S. A.; Cheburkin, N. V.; Churakov, V. V. Sou. J . Quantum Electron. 1986, 16, 1135. (10) Rosser, W. A. Jr.; Gerry, E. T. J . Chem. Phys. 1969, 51, 2286. (11) Heller, D. F.; Moore, C. B. J . Chem. Phys. 1970, 52, 1005. (12) Lewis, P. F.; Trainor, D. W. AERL Technical Report, Amp 422. November, 1974.
(13) Douglas-Hamilton, D. H.; Lowder, R. S. AERL Kinetics Handbook; Prepared under Contract No. F29601-73-C-0116for the Air Force Weapons Laboratory, July, 1974. (14) Lewis, P. F. Final Interim Technical Report, PSI-161/TR-401, AVCO Research Laboratory, Inc., Everett, MA, 1986.
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-
(8) Moore, C. B.; Wood, R. E.; Hu, B.-L.; Yardley, J. T.J . Chem. Phys.
1148
J . Phys. Chem. 1989, 93, 1148-1 158
observations of Freed et a1.6 for the C W case. For identical gas compositions and pumping conditions, this crossover point is found to move to longer pulse widths as the total pressure of the gas media is decreased. It was also determined that the crossover point is reached at shorter pulse widths for helium free laser mixtures.
Conclusions A direct comparison has been made of the small signal gains of I2CO2and 13C02lasing gas mixtures in a pulsed pumping mode. Under these conditions, the 13C02peak gain was determined to be larger than that of 12C02due to a larger stimulated emission cross section. In the C W limit, g(13C02)C g(12C02)is predicted on the basis of the experimental decay times, 7(I3CO2) 7(I2CO2). From the measured postpump decays for different gas compositions, the rate constants for deactivation of the upper lasing level by I2CO2, I3CO2, and N2 have been determined. The I3CO2
self-relaxation rate constant is nearly a factor of 3 larger than that of '2C02and poses an interesting theoretical energy-transfer problem. These gain measurements have been confirmed by a kinetics model using the presently obtained rate constants. Application of the model to still longer pulse lengths yields peak gains in good agreement with C W measurements, indicating that a crossover occurs from g(13c02) > g(12C02)to g(I3CO2) c g(12C02)at pulse lengths near 60 pus. Acknowledgment. This work was supported by the AVCO Research Laboratory Independent Research and Development Program (Project No. 87009412). We thank Dan Huu Pham for performing the gain measurements. We also appreciate the loan of an InSb detector from the Infrared Technology Division, Air Force Geophysics Laboratory. Registry No. C02, 124-38-9; I3CO2, 1111-72-4.
Laser-I nduced Decomposition of Triethylgallium and Trimet hylgalllum Adsorbed on GaAs( 100) J. A. McCaulley, V. R. McCrary, and V. M. Donnelly* A T & T Bell Laboratories, Murray Hill, New Jersey 07974 (Received: April 4, 1988)
We report X-ray photoelectron spectroscopy (XPS) studies of excimer-laser-stimulated decomposition of triethylgallium (TEGa) and trimethylgallium (TMGa) adsorbed on GaAs( 100) in ultrahigh vacuum. Both TEGa and TMGa dissociatively chemisorb on GaAs at room temperature, whereupon irradiation by a pulsed ArF excimer laser at 193 nm leads to further decomposition and desorption of carbon-containing species, detected as a decrease in the intensity of the C(ls) XPS peak. The Ga in the adlayer remains on the surface. The apparent carbon removal rate coefficient (per laser pulse) decreases as carbon is removed, suggesting multiple reaction sites or second-order reactions. For TeGa, the carbon removal rate increases by 2 orders of magnitude between 10 and 100 mJ/cm2, and 4 orders of magnitude between 100 and 200 mJ/cm2. Adsorbed TMGa behaves similarly, but with decay rates about a factor of 20 slower. At -200 mJ/cm2 the rate of carbon removal for TEGa-dosed GaAs is independent of irradiation wavelength (193-nm ArF vs 351-nm XeF excimer laser pulses), while at -20 mJ/cm2, 193-nm light is much more effective in promoting the reaction than is light at 351 nm. At high fluence, thermal decompositiondue to transient laser-induced heating is the predominant decompositionmechanism. For low fluences, the wavelength dependence indicates direct electronic excitation of the adsorbate, while the supralinear fluence dependence precludes single-photon dissociation and supports a two-photon dissociation mechanism. These studies show that, in our previous work, laser-enhanced epitaxial deposition of GaAs from TEGa and As, molecular beams was due to laser-induced pyrolysis of adsorbed TEGa. Reaction mechanisms of laser-induced pyrolysis are related to carbon incorporation in GaAs grown by conventional techniques, which involve heterogeneous pyrolysis of gallium alkyls.
1. Introduction Deposition of thin epitaxial Alms is required for the fabrication of many microelectronic devices. Often, these layers must be patterned by photolithographic processes, whereupon further layers are deposited to build up complex structures. Selected-area growth of thin films would greatly simplify this procedure. Consequently, there has been considerable interest in using external energy sources such as lasers and ion beams to stimulate growth. Several studies of laser-enhanced metal-organic chemical vapor deposition (MOCVD) of GaAs have been reported.'-I0 More recently," (1) Donnelly, V. M.; Geva, M.; Long, J.; Karlicek, R. F. Appl. Phys. Lett. ..
we have shown that ArF excimer laser irradiation can enhance the rate of growth of GaAs in a metal-organic molecular beam epitaxy (MOMBE) system. Laser-enhanced growth in high vacuum underscores the importance of laser-initiated surface chemistry, which may also contribute at the higher pressures employed in the MOCVD process. Control of the growth rate by laser-initiated surface (rather than gas phase) chemistry is necessary to achieve high resolution, selected-area growth. From a more fundamental perspective, additional studies of the photodecomposition of molecules adsorbed on surfaces are needed to refine our understanding of mechanisms of surface photochemical reactions. Recently, numerous such studies have
1984, 44, 951.-
(2) Donnelly, V. M.; Brasen, D.; Appelbaum, A.; Geva, M. J. Appl. Phys. 1985, 58, 2022.
(3) Donnelly, V. M.; McCrary, V. R.; Appelbaum, A.; Brasen, D.; Lowe, W. P. J. Appl. Phys. 1987, 61, 1410. (4) McCrary, V. R.; Donnelly, V. M.; Brasen, D.; Appelbaum, A.; Farrow, R. C. In Photon, Beam, and Plasma Stimulated Chemical Processes at Surfaces; Donnelly, V. M., Herman, I. P., Hirose, M., Eds.;Materials Research Society: Pittsburgh, PA, 1987; Vol. 75, pp 223-231. (5) Balk, P.; Heinecke, H.; Plass, C.; Piitz, N.; Loth, H . J . Vac. Sci. Technol. 1986, A4, 71 1. ( 6 ) Haigfl. J. Vac. Sci. Technol. 1985, A4, 1456. (7) Nishizawa, J.; Abe, H.; Kurabayashi, T.; Sakurai, N. J . Vac. Sci. Technol. 1986, A4, 706.
0022-3654/89/2093-1148$01.50/0
(8) Doi, A.; Aoyagi, Y.; Namba, S. In Photon, Beam, and Plasma Stimulated Chemical Processes at Surfaces, Donnelly, V. M., Herman, I. P., Hirose, M., Eds.;Materials Research Society: Pittsburgh, PA, 1987; Vol. 75, pp 217-222. (9) Karam, N. H.; Bedair, S. M.; El-Masry, N. A.; Griffs, D. In Photon, Beam, and Plasma Stimulated Chemical Processes at Surfaces; Donnelly, V. M., Herman, I. P., Hirose, M., Eds.; Materials Research Society: Pittsburgh, PA, 1987; Vol. 75, pp 241-248. (10) Imine, S. J. C.; Mullin, J. B.; Tunnicliffe, J. J. Crystal Growth 1984, 68, 188.
(1 1) Donnelly, V. M.; Tu, C. W.; Beggy, J. C.; McCrary, V. R.; Lamont, M. G.; Harris, T. D.; Baiocchi, F. A,; Farrow, R. C. Appl. Phys. Lett. 1988, 52, 1065.
0 1989 American Chemical Society
