Laser-induced decomposition of dimethyl cadmium on a quartz substrate

applied

surface science ELSEVIER

Applied SurfaceScience 84 (1995)31-43

Laser-induced decomposition of dimethyl cadmium on a quartz substrate S.-P. Lee, M.C. Lin * Department of Chemistry, Emory Uniuersily Atlanta, GA 30322, USA

Received24 April 1994;acceptedfor publication 31 August 1994

Abstract The photochemistry of adsorbed dimethyl cadmium at submonolayer levels on a fused quartz surface has been investigated at 193 and 248 nm using a rare-gas fluoride excimer laser. The desorbed gaseous products, which include CH,, CH,, CH,, C,H,, C,H,, C,H,, Cd, CH,Cd and (CH,),Cd, have been detected by time-of-flight mass spectrometry with either electron impact or resonance-enhanced multiphoton ionization. The translational energies of these desorption products could be characterized in terms of Maxwell-Boltzmann temperatures, ThlB’s, which depend strongly on sample dosage (surface coverage), laser fluence and photon energy. The T,’ s of the C, and C, hydrocarbon species were found to be much lower than those of Cd, CH,Cd and (CH,),Cd, suggesting that the electronic excitation/relaxation mechanism may be involved in the desorption of Cd and Cd-containing species. The rotational temperature of the CH, radical, determined by analysis of REMPI spectra, was found to be much colder than its translational temperature. A realistic mechanism is proposed for the W-photochemistry of adsorbed (CH,),Cd.

1. Introduction Dimethyl cadmium (DMCd) is an important organometallic source molecule for the deposition of cadmium sulfide, telluride and other compound II/VI semiconductors by LCVD (laser chemical vapor deposition) or OMCVD (organometallic CVD> [l-6]. The photochemistry of DMCd in the gas phase has also been a subject of considerable current interest. Spectroscopic studies of the excited states of DMCd in the UV [5-111 and VW [9,10] regions of the spectrum have been made recently by many groups. The absorption in the UV region begins near 270 nm, appearing as a broad, featureless band which

* Corresponding

author.

0169-4332/95/$09.50 8 1995 Elsevier Science B.V. All rights reserved SSDI 0169-4332(94)00360-2

increases sharply below 230 nm with regular structures [5-111. The broad, continuous absorption in the lower-energy regime has been attributed to excitation to the first excited, triplet state (which is bent) according to Amirav et al. [8], instead of a singletsinglet transition, as was proposed by Chen and Osgood [51. The stronger absorption peaking near 215 nm is now commonly accepted as due to the X ‘Zl-B ‘7~” transition [5,8]. The photodissociation of DMCd in the B state has been shown to produce CH, and CH,Cd (A ‘21, which fluoresces in the 440-445 nm region [8,9,11]. The CH, radical was measured to carry an average translational energy of 24 kJ/mol when DMCd was photolyzed at 193 nm [ll]. Interestingly, a similar study made at 248 nm resulted in no detectable fragmentation, although the absorption cross section

32

S.-P. Lee, M.C. Lin /Applied

at this wavelength is half as strong as that at 193 nm 15-101 and the photon energy is more than sufficient to break both C-Cd bonds. Our present interest lies in the elucidation of the mechanism for the photodissociation of DMCd adsorbed on solid surfaces in relation to the LCVD of Cd. The first attempt to deposit Cd films on solid surfaces was made by Deutsch, Ehrlich, and Osgood [2,3,12], who used a cw UV laser at 257 nm to irradiate the surface directly in a static cell containing l-10 Torr of DMCd. Under their conditions, the excitation of DMCd in both gaseous and adsorbed phases can promote photochemical decomposition reactions. Accordingly, the photochemistry is far more complex than the laser-induced dissociation of adsorbed DMCd in vacuum, in which secondary reactions of desorbed, gas-phase species with the surface and the complications by other physicochemical processes, including surface heating by cw radiation, can be avoided. As a first step toward the understanding of the complex deposition process for practical applications, we have carried out a study of the photodissociation of adsorbed DMCd in vacuum under sub-monolayer conditions, using a time-offlight mass spectrometric (TOF-MS) method. Both the conventional electron impact ionization (EI) and the resonance-enhanced multiphoton ionization (REMPI) techniques were employed for molecular as well as transient (atomic and radical) species detection. We have examined the differences in surface photochemistry induced by different photon energies, laser powers, and DMCd coverages. These results are reported herein.

2. Experimental details The experimental set-up is essentially the same as the one employed before [13-151. The laser photolysis/ time-of-flight measurement system consists of two chambers separated by a 3 mm skimmer. The surface photolysis chamber was pumped by a 1000 f/s Diffstak (Edwards) diffusion pump, which was separated from the chamber with a gate valve. The analysis chamber, which housed an Extrel C-50 quadrupole mass spectrometer (QMS), was evacuated with a 1000 e/s Leybold turbomolecular pump. After a proper bakeout, the background pressures

Surface Science 84 (1995)

31-43

were typically 1 X lo-’ and 5 X 10F9 Torr in the photolysis and mass spectrometer chambers, respectively. The solid sample (25 mm diameter fused quartz) was mounted in the photolysis chamber using a brass ring welded onto a copper block, which was directly attached to a liquid-N, reservoir. The copper block could also be heated up to 400 K before each photolytic experiment to remove possible volatile impurities from the solid surface. The substrate was typically aligned normal to the skimmer-QMS analysis axis and interacted with the photolysis laser beam at a 10” grazing angle. The distance between the substrate and the center of the ionization region of the QMS was measured to be 13 cm. The photolysis laser (Lambda Physik EMG-102 MSC) was operated at either 193 nm (ArF) or 248 nm (KrF). The energy of the laser could be varied from several mJ/pulse to 30 mJ/pulse, with a typical photolysis energy of about 20 mJ/pulse covering a rectangular area of about 1 cm2 across the center of the surface. The dimethyl cadmium sample (10% DMCd in He) was dosed onto the cold substrate (whose temperature could be varied from 140 to 400 K) through a stainless steel capillary tube held 2 cm away from the surface at a 45” angle. The flow rate of the gas mixture was controlled by a needle valve in conjunction with a mass-flow meter (MKS type 258 B). During an experimental run, the pressure in the source chamber was typically maintained at < 10m6 Torr and the repetition rate of the photolysis laser kept at 3 Hz which allowed us to have < 0.1 L (langmuir = lo- 6 Torr * s) dosage between laser shots. The detection of the desorbed photolysis products by QMS could be achieved by either El or REMPI (as mentioned before). For REMPI/TOF detection, an excimer-laser-pumped dye laser (Lambda Physik EMG 201MSC and FL3002) was used by focusing the beam at the center of the ionization region through a 15 cm focal-length lens. Typical outputs of the dye laser were 3 mJ/pulse for CH, and 8 mJ/pulse for Cd detection using p-terphenyl dye. Ions were collected and amplified through a channeltron ion multiplier. The outputs from the channeltron were further amplified by using a fast response pulse-amplifier (Thorn EMI Type C632) and moni-

S.-P. Lee, M.C. Lin /Applied Surface Science 84 (1995) 31-43

tored by using either a 100 MHz digital storage oscilloscope (Nicolet Model 450 Waveform Acquisition System) for EI/TOF detection or a gated integrater and boxcar signal averaging system (Stanford Research Model 250 and 245) controlled by computer (IBMPC-AT) for REMPI/TOF detection and REMPI spectral scan. A multichannel digital delay/pulse generator (Stanford Research Model DG535) was used to synchronize the photolysis extimer laser, the dye laser, and the data acquisition system (boxcar, etc.). The delayed trigger-pulse to the dye laser could be varied to scan TOF profiles for REMPI/TOF or fixed at the peaks of TOF profiles for REMPI spectral scan The natural abundances of Cd isotopes are 12.25% of Cdlro, 24.17% of Cd112, and 28.73% of Cd ‘I4 . In our experiment, we were able to observe those ratios but our results and discussion were based only on the most abundant isotope (Cd114). Both CH, and Cd can be readily detected by REMPI in the 330-340 nm range of the spectrum. The CH, radical was ionized by (2 + 1) REMPI at 333.5 nm via the resonance state 3p2XZ [16]. For spectral scans of the desorbed CH, fragment, the probing dye laser was delayed variably between 160 and 225 ps after photodissociation. The boxcar gate was set at 28 ps after the dye laser pulse. Cd could also detected by (2 + 1) REMPI via 5d ‘D, at 337.7 nm, 5d’D, (J = 0, 1, 2) at 336.0336.2 nm as well as 6p1Pl at 333.8 nm [17]. For its spectral scans, the dye laser was delayed by about 360 ps after the photolysis pulse, with a boxcar gate opened at 74 /_LSfollowing the dye laser pulse.

3. Results and discussion The overall patterns of desorption/fragmentation of DMCd adsorbed on SiO,, photolyzed at both 193 and 248 nm, are shown in Fig. 1 in terms of the TOF profiles of various species. Dots shown in the figure represent the experimental data and the fitted Maxwell-Boltzmann distributions are superimposed as solid curves. All of these profiles have been corrected for the transit time through the quadrupole mass spectrometer between the ionizer and the detector and they represent the time-of-arrival of those species at the ionizer. Accordingly, if any of them

33

193nm

0

100

200 Time

300

400

Of Flight

500

600

( pet

)

700

800

Fig. 1. Overall view of TOF profiles of observed species from the photolysis of adsorbed DMCd on SiO, by 193-and 248 nm irradiation. The fluence for both wavelengths was 20 mJ/cm2. pulse and the surface exposure was maintained at 0.03 L; T, = 143 K. The superimposed solid lines are fitted Maxwell-Boltzmann distributions.

derived from the cracking of heavier molecules by electron ionization, their TOF profiles should resemble those of their parent ions. It is easy to see from these profiles that almost all the observed species carry slightly different translational energies, suggesting that a lot of photofragmentation processes occur at the surface. The fitted Maxwell-Boltzmann translational temperatures (TMB) of various species from the photolysis on the quartz substrate are summarized in Tables 1 and 2. The extent of photofragmentation depends strongly on the laser photon energies as shown in Fig. 2, in which the total ion count of each fragment normalized with respect to the parent count is compared for both 193 and 248 nm. The results clearly illustrate the significant effect of photon energy on the photofragmentation of DMCd. The effect of pho-

S. -P. Lee, M. C. Lin /Applied Surface Science 84 (I 995) 31-43

34

Table 1 Photofragments and desorption products from the 193 nm photolysis of adsorbed dimethyl cadmium on quartz at 143 K a Laser energy

m/z (amu)

Ion counts

rmax ( /.Ls)

11.0

14 15 28 29 30 114 129 144 14 15 28 29 30 114 129 144 14 15 28 29 30 114 129 144 14 15 28 29 30 114 129 144

19150 133637 68663 13458 20934 7578 17400 9093 40110 191204 169657 39100 55419 27536 82103 54329 71783 344294 270080 79513 73576 87010 188519 114779 105368 390769 312612 84346 89078 134430 227615 117981

148 151 199 199 205 326 335 336 122 131 167 173 162 251 252 225 104 111 145 142 149 208 216 201 98 108 140 143 142 197 207 199

12.8

16.8

20.4

:&a N/Ah 355 C N/A

376 N/A N/A 666 508 470 N/i 598 584 590 1485 698 665

helpful for the characterization of photodissociation processes at the surface. In the following sections, the effects of these varying conditions on fragmentation and desorption dynamics will be discussed.

Table 2 Photofragments and desorbed products from the 248 nm photolysis of adsorbed dimethvl cadmium on auartz at 143 K a Laser energy 20.6

22.7

C

144 711 1170 965 1909 775 697

24.0

C

201 777 1605 1328 1920

26.7

a The DMCd exposure was typically 0.03 L. b Not available. ’ C,H: derives entirely from C,Hi.

ton energy may be attributed to the difference in both absorptivity and fragmentation path-ways. At 193 nm, it is possible to excite both the first and the second excited state of DMCd, while at 248 mn only the first excited state can be reached. Both excited states are repulsive, giving rise to dissociation products with slightly different internal energy distributions as will be discussed later. The information on how the adsorbed species react under different conditions, such as surface coverage, excitation energy and fluence, etc. should be

28.9

m/z (amu)

Ion counts

14 15 28 29 30 114 129 144 14 15 28 29 30 114 129 144 14 15 28 29 30 114 129 144 14 15 28 29 30 114 129 144 14 15 28 29 30 114 129 144

15687 59200 31527 14321 10405 46931 117766 71145 30521 119023 60074 28617 17356 94042 217966 144352 34733 141075

260 261 114 124 167 183 168 231 241 236 107 118

N/A 30982 17429 109618 232003 160370 54741 195945 74086 46271 22289 158853 337517 168090 42626 148667 72921 37481 23322 128960 286776 172731

183 161 218 229 225 99 110 151 177 153 209 216 219 107 118 161 166 161 219 228 226

r!n,, ( /.Ls) 121 133 172 188 171

a The DMCd exposure was typically 0.03 L. b Not available. ’ C,H: derives entirely from C,Hi. ” DMCd+ fragment.

T 2; 522 465 N/Ah 248 531 1001 d 1115 594 527 C

276 566 1190 d 1360 684 584 N/A 293 610 1360 d 1508 808 677 C

298 684 1560 146 1591 664 580 C

272 612 1360 241 1483

35

S.-P. Lee, M.C. Lin / Applied Surface Science 84 (1995) 31-43

3.1. Photodesorption

of DMCd

3.1.X. Coverage dependence The dependence of the photodesorption of DMCd

on its coverage over the quartz surface has been examined by changing the repetition rate of the laser pulse. The repetition rate was varied from 0.1 to 20 Hz at a constant pressure (lower than 10m6 Torr of the DMCd mixture) with a steady flow rate of 0.2 mcc/s, so that the exposure is given in units of langmuir. Unless specified otherwise, all experiments were carried out with the exposure of 0.030 L. The representative TOF profiles of DMCd at different exposures are compared in Fig. 3, showing typical changes of most probable energies and yields in time and the fitted Tm’s summarized in the inset of the figure. Both the translational temperature and total desorption yield were found to increase quite linearly with increasing exposures up to 20 mL, above which deviation from the linearity occurs. The results summarized in Fig. 3 unambiguously show that not only the desorption yield but also the translational temperature of DMCd was strongly affected by exposure or surface coverage. Similar observations have been reported in many laser-desorption studies [19-241. -

m/z

(

AMU )

Fig. 2. Comparison of total ion-count distributions of observed species from the photolysis of adsorbed DMCd on SiO, at 193 and 248 nm, using the same fluence (20 mJ/cm2.pulse), with the ion-fragmentation pattern of DMCd detected in the gas phase. The ion counts were normalized with respect to the parent (DMCd) counts.

260

460

660

Time Of Flight ( pet

lciO0

)

Fig. 3. Representative TOF profiles and translational temperatures of the photodesorbed DMCd from the SiO, surface at different exposures by 248 nm irradiation with 29 mJ/cm’. pulse fluence; T, = 143 K.

Polanyi and coworkers have studied in detail the angular dependence of desorption yields on surface coverage [19-211. For the OCS desorption from LIF(OOl), for example, they observed a sharp variation in the angular distribution from cos 8 with - lop3 L exposure to cos”8 with 21 L exposure [21]. The enhancement of the desorption yields toward the surface normal at higher coverages is believed to result from the reduction in “the thermal liberation of the absorbate-surface bond and the permitted cone-of-escape angles for photodesorption product” [21]. The sharp increase in TM, and desorption yield with surface coverage would suggest that a considerable energy and momentum transfer occurs at or near the surface during the desorption process. 3.1.2. Dependence on laser wavelength and ji’uence The dependence of the DMCd desorption yield on photon energy and laser fluence is shown in Fig. 4. These experiments were carried out with the available intensity ranges of the 193 and 248 nm radiations. At the same fluence, the photon yield with the 248 nm radiation was substantially smaller than that with 193 nm, presumably because of the lower absorptivity of DMCd at the former wavelength as discussed earlier. For that reason, the experiment was carried out at higher fluences with the 248 nm radiation.

S.-P. Lee, M.C. Lin /Applied

36

Surface Science 84 (1995) 31-43

on surface coverage dependence. The leveling-off of the TMr, value at high fluences indicates the absence of a two-photon excitation process.

0

200

400

600

Time

Of Flight

( psec

600 )

Fig. 4. Comparison of changes in TOF profiles of photodesorbed DMCd with laser fluence between 193 and 248 nm radiations. The surface exposure was 0.03 L, laser fluence 20 ml/cm’. pulse; T, = 143 K.

The results obtained with both wavelengths presented in Fig. 4 show changes in total desorption counts and translational energy distributions (as shown in the insets) with the laser fluence. Both quantities increase rapidly with fluence and reach plateau values at fluences higher than 17 mJ/cm* * pulse at 193 nm and 24 mJ/cm’ . pulse at 248 nm. Similar results were found for other desorbed fragments as presented in Tables 1 and 2. The effect of laser fluence on the translational energy of desorbed species has been studied before [19-241. For desorption from LiF surfaces, such an enhancement effect has been attributed to the increase in the intensity of the photoacoustic wave induced by F-center excitation at higher fluences [19-211. With optically opaque substrates, surface heating becomes important. For the desorption of DMCd from the quartz surface, the initial rapid increase in TMB with fluence may be due to the enhanced surface heating and desorption yield, which may contribute to the narrowing of the cone of desorption as described above

3.1.3. Dependence on surface temperature Since the adsorption of molecules on solid surfaces decreases with surface temperature CT,), therefore, one expects a lower desorption yield and a corresponding slower translational energy distribution at a higher surface temperature. Indeed, the result of photodesorption of DMCd at temperatures ranging from 145 to 160 K using a 248 nm laser shows a very sharp decrease in total desorption yields with increasing surface temperature. At T, higher than 166 K, the signal was almost not detectable. At T, = 150 K, the fitted TMB drops by more than 1000 K while the surface was heated by less than 25 K. This result is consistent with the large TMn change observed, from 2300 to 1500 K when the exposure was reduced from 48 to 5 mL as shown in Fig. 3. 3.2. Product measurements The desorbed species resulting from photolysis at the surface were detected 13 cm from the surface in the quadrupole mass spectrometer using either EI or REMPI. The neutral species detected are H, CH,, CH,, CH,, C,H,, C,H,, C,H,, Cd, CH,Cd (MMCd), and (CH,),Cd (DMCd). With the exception of H, which cannot be reliably measured with our present QMS setting, the TOF distributions of these species have been measured in detail. Under higher laser fluence conditions, ions were detected to desorb from the surface with much higher translational energies than those of neutral species. A similar observation was made in our earlier experiment with trimethyl gallium, in which Ga+ ions were detected abundantly [13]. In the present experiment, only neutral products were measured under lowfluence conditions.

3.2.1. Cd and CH,Cd formation As shown in Fig. 2, both Cd and CH,Cd are major ion fragments of DMCd. As a group, Cd, MMCd and DMCd appear with similar TOF distributions (see Figs. 1 and 5) and are also associated with high translational temperatures. Their TMB’sare typi-

37

S.-P. Lee, MC. Lin /Applied Surface Science 84 (1995) 31-43

tally twice as high as those of C, and C, products, suggesting that substantial electronic excitation/ relaxation might have contributed to their desorption processes. This is perhaps not unexpected because ions have been generated from the photolysis with a high laser fluence. In our previous study of the photodissociation of adsorbed trimethyl gallium on quartz and LiF(OO1) substrates, the desorbed Ga atom was also measured to have a much higher translational temperature than the CH, radical [13]. Both species were detected by REMPI/MS. In this case, Ga+ ions were also detected to desorb from the deposited Ga film as alluded to above. The deconvoluted TOF profiles of MMCd+ with both wavelengths are compared in Fig. 5. Under these experimental conditions, the observed MMCd+ signal with 193 nm was estimated to have 32% contribution from MMCd while the result with 248 nm was almost entirely derived from DMCd. The estimated TMu of MMCd with 193 nm was much lower than that of DMCd. The slow desorption

I

A = 193nm

DMCd( 1920K,

68%)

MMCd( 1328K,

32%)

h = 248nm

0

200

Time

400

Of Flight

600

( ~sec

: ::

A = 240nm

DMCd(479K. 54%)

i

Time

Of Flight

( pet

)

Fig. 6. TOF profiles of photofragmented Cd from SiO, detected by two different methods under the same conditions. Upper: selectively detected by (2+ 1) REMPI at 337.7 nm. Lower: detected by electron impact ionization including contribution from DMCd. Photofragmented at 248 nm with 26 mJ/cm*. pulse fluence and surface exposure, 0.005 L, T, = 143 K.

CH,Cd+

.E.

I

600

1000

)

Fig. 5. TOF profiles of MMCd+ from photolysis at 193 and 248 nm on the SiO, substrate, including contributions from DMCd. DMCd dosage 0.03 L, laser fluence 20 mJ/cm* pulse; T, = 143 K.

velocity of MMCd may suggest a stronger binding of the CH,Cd radical with the surface. In Fig. 6, the TOF distributions of Cd+ (m/z = 114) were obtained from the photolysis of 0.005 L DMCd with 248 nm radiation; both EI and REMPI ionizations were utilized in the mass analysis. The result of a REMPI scan is shown in the inset of this figure. The distribution obtained by (2 + 1) REMPI at 337.7 nm can be quantitatively characterized by a Maxwell-Boltzmann distribution with TMB = 458 K. The distribution obtained by EI, on the other hand, contains two components: one from the desorbed Cd atom (46%, TMB = 458 K) and the other from the desorbed DMCd (54%, TM, = 479 K). The deconvoluted results for the neutral Cd are summarized in Tables 1 and 2 for the photolysis of 0.03 L DMCd with both 193 and 248 nm, respectively, under different fluence conditions.

38

S. -P. Lee, M. C. Lin /Applied

Surface Science 84 (I 995) 31-43

3.2.2. C, formation As shown in Fig. 1, &HI,

C,Hz and C,Hl have distinctly different TOF profiles from those of DMCd+. Accordingly, they are primarily the products of surface photochemical reactions, with very minor contributions from the fragmentation of DMCd+. Among the three C, products, C,H: has the highest ion signal, but its TOF profile tracks closely that of C,Hz and the relative ratio of C,H:/C,Hl remains constant under different fluence and coverage conditions and is close to the known C,Hi fragmentation pattern. Thus, C, H: derives mainly from C,Hl. The desorption profiles of C,H, are distinctly different from those of DMCd, the only heavier photodesorption product which can possibly generate a C,Hz ion by ion fragmentation. The parent DMCd+ ion, however, produces very little m/z = 30 ion fragment as indicated in Fig. 2. Similar to the photodesorption of DMCd, the observed translational temperature of C,H, is much higher with 193 nm than that with 248 run. The profiles presented in Fig. 7, with 20 mJ/cm’ * pulse fluence, for example, have TMB= 531 K with 248 nm and 777 K with 193 nm. C,H, is most certainly the product of the CH, recombination reaction on the surface: 2 CH,(a) + C,H,(g). Similar to the C, products, the TOF profiles of C,H, also contain a fast component (75%-85%) desorbed directly from the region of the substrate aligned with the QMS analysis axis and a much slower component, possibly desorbed after migration into the line-of-sight of the QMS. This component is also too slow to be accounted for by DMCd, the only heavy species which may contribute to the m/z = 30 signal. The translational temperature of C,H, is comparable to those of the C, products as summarized in Tables 1 and 2. The C,H: profiles differ somewhat from those of C,Hi. This is illustrated in Fig. 8. The photolysis with the 193 nm laser generates a higher C,Hl ion signal, with the major contribution from C, H,. At 248 nm, the C,Hl ion signal is much lower, probably because the surface concentrations of the precursors of C, H, are comparatively lower.

CA+ A = 193nm

fast (777K, 85%) slow (lOZK, 15%)

:.

s .:;

A = 248nm

*.7x. :

fast (531K, 78%) slow ( 57K, 22%)

,“‘.““‘,.“““‘,I”“““‘I

200

400

Time Of Flight Fig. 7. TOF profiles

of C,Hl

600

800

( psec from SO,

1000

) surface

photolysis.

Solid lines show the fitting with two Maxwell-Boltzmann distributions. DMCd dosage 0.03 L, laser fluence 20 ml/cm*. pulse; T, = 143 K.

C,H, cess:

may derive from the recombination pro-

CH,(a) + CH,(a) + C,H,(a)

-, C,H,(g).

After subtracting the contribution from C,H,, the deconvoluted profiles differ distinctly from those of C,H,, which can be relatively cleanly characterized as indicated above. The translational temperature of C,H, is lower than that of C,H,, reflecting the possibility that the rate of C,H, formation on the surface is much slower, due to the lower concentration of CH,(a). C,H, is also expected to interact more strongly with the surface than C,H, does. 3.2.3. CH, and CH, radical formation

The CH, radical is a primary product of the gas-phase photolysis of DMCd. It can be formed by photodissociation both at 248 nm via A-state excitation and at 193 nm via A- as well as B-state excita-

S.-P. Lee, MC. Lin /Applied Surface Science 84 (1995) 31-43

tion [6,8]. The possibility of CH, production in the gas phase has been ruled out by Bersohn and coworkers [ll]. The photolysis of adsorbed DMCd at these wavelengths appears to have generated both CH, and CH, radicals according to the differences in TOF appearance times and relative counts of the m/z = 14 and m/z = 15 ions under different fluence conditions. Although the relative ion yields are not quantitatively meaningful due to possible cracking contributions from larger fragments and the parent molecule, they are qualitatively useful for us to ascertain their primary origins. From the measured signal intensities of ions as functions of laser fluence, we have noted that the relative yields of CH; and CH: behave somewhat differently as the fluence of the photolysis laser is varied. This observation, together with the resolvable appearance and peak (t_> times summarized in Tables 1 and 2,

;

CH,’

* 7.0

h = 193nm A

h = 248nm

slow

Time h = 193nm

C,H,(201K,

Of Flight

( psec

( psec

22%)

)

21%)

h = 248nm

Time

Of Flight

(57K,

Fig. 9. Comparison of EI/TOF profiles of CH: produced by 193 and 248 nm photolyses. DMCd dosage = 0.03 L, laser fluence = 20 mJ/cm’.pulse; T, = 143 K.

: .;+%, . .. .

C,H.(250K,

39

69%)

)

Fig. 8. TOF profiles of C,H, from SO, surface photolysis including contributions from C,H,. DMCd dosage = 0.03 L, laser fluence = 20 mJ/cm’.pulse; T, = 143 K.

allows us to conclude that CH, radicals were produced in the surface photolysis of DMCd at both wavelengths. In Fig. 9 we have shown the deconvoluted TOF profiles obtained from the photolysis at 248 and 193 nm. These profiles consist of a fast, high-T,, peak, a smaller component from CH: (which can be reliably measured by REMPI as shown below) and a very slow component. The slowest peak is too slow to be accounted for by the fragmentation of a larger ion such as DMCd+. It is believed to be resulted from the diffusion-desorption process similar to that noted for C, H,. The detection of CH, radicals is straightforward because CH,-REMPI spectroscopy is well-established [ 181.Multiphoton ionization was made at 333.5 nm using the (2 + 1) REMPI scheme via the 3p2A’2 Rydberg state of the CH, [16,18]. A typical wavelength scan of the CH, radical is shown in the inset of Fig. 10, which presents the TOF profiles of the

40

S.-P. Lee, h4.C. Lin/Applied

m/z = 15 ion produced by both REMPI and EI methods. Although both EI and REMPI TOF profiles obtained from the photolysis on the SiO, surface are very similar, the EI profiles are more convoluted, containing contributions from C, H 6 and, possibly, CH,. These signals also show small contributions from very slow desorption processes as alluded to above. Additionally, the peaks in the EI signals, which are not present in the REMPI profile, apparently derived from a small CH,-containing species. One likely candidate is CH,, which might be formed by the surface reaction, H(a) + CH,(a) + CH,(g). The CH, REMPI spectrum shown in Fig. 10 contains the rotational energy information which will be analyzed and discussed later with reference to the dynamics of photofragmentation of DMCd.

Surface Science 84 (1995) 31-43

‘1 ;;

h = 246nm

0:

3.3. Photodissociation mechanism The photodissociation mechanism of the adsorbed DMCd, based on the data presented in the preceding sections, is summarized in Fig. 11. This mechanism is very different from that of the gas phase which consists of only CH,, CH,Cd and/or Cd as primary fragmentation products when DMCd is excited at the A or the B state [5-8,111. The photolysis of DMCd on solid surfaces leads not only to additional fragmentation of CH, and CH,Cd, but also to the formation of recombination products. This unambiguous conclusion was made possible by the additional REMPI detection of Cd and CH, as illustrated in Figs. 6 and 10, respectively. The quantification of CH, and Cd TOF distributions allows us to deconvolute the TOF distributions of CH,, CH, and Cd obtained by EI/MS quite reliably. The deconvoluted relative contributions to various species are tabulated in Tables 3 and 4 for photolysis on quartz at 193 and 248 nm, respectively, under the same fluence and dosage conditions (20 mJ/cm* . pulse and 0.03 L). Our definitive determination of the TOF distributions of CH, by REMPI/MS also allows us to rule out the possibility of the direct production of the radical without surface mediation, such as DMCd(a) + hv+ CH,(g) + CH,Cd(a) + 2 CH,(g) + Cd(a).

500 Time

Of Flight

1500

1000

( pet

)

Fig. 10. TOF profiles of photodesorbed CH, radicals from the SiO, surface (T, = 143 K) detected by two different methods under the same conditions (photofragmented by 248 nm at 26 mJ/cm’.pulse and the surface exposed at 0.009 L). Inset: CH, (2 + 1) REMPI wavelength scan. The traditional temperature of CH, determined by REMPI is This = 276 K under the low-dosage conditions.

The average translational energies, ((E,) = m( u>*/ 2) of the CH, radical measured in the 193 nm photolysis on quartz, with a typical dosage of 0.03 L, range from 3.8 to 7.4 kJ/mol, when laser fluence is varied from 11 to 20 mJ/cm2 (see the data presented in Table 1). This range of (E,) is significantly lower than that reported by Bersohn and coworkers [ll], 24.3 kJ/mol, in their gas-phase photolysis of DMCd at 193 nm using a comparable fluence of 2 MW/cm* (which is approximately 20 mJ/cm* for a typical ArF-excimer laser with a pulse-width of about 10 ns). Since the adsorption energy of DMCd on quartz is expected to be significantly smaller than the photon energy (i.e., the amount of available energy for DMCd fragmentation on the surface and in the gas phase should be about the same), the low values of

r

41

S.-P. Lee, MC. Lin/Applied Surface Science 84 (1995) 31-43

(CH3)2a

(a)

hv

(CH3)2a

(9)

CH3a

CH3 CH3a

(9)

(a)

(a) cd

(a)

-

cd

(9)

CH2 (a) + H (a) + Cd (a)

-E

CH3

(cd

CH2

(a)

CH4

(g)

- H(3)

I-

CH3

(a)

-

CH2

(9)

+ H(a)

+ W(3)

I

C2b

(9)

C2H5

(a)

+ W(a) -

Cd-h

(9)

Fig. 11. Mechanism of the photodesorption and photofragmentation of DMCd adsorbed on solid substrates by UV laser irradiation.

Table 3 Bohzmann temperatures of all observed species and contributions from other heavier ones from the 193 nm photolysis of DMCd adsorbed on a cold quartz surface (20 mJ/cm’ pulse, 0.03 L and T, = 143 K) Observed ions T,,

(K)

CHz CH3

m/z 14

15

775

697

0.88 0.12

0.95

CA C,H, C,% Cd MMCd DMCd

0.05

28

1

29

30

114

129

144

201

777

1605

1328

1920

0.21 0.79

1 0.68 0.32

1

0.73 0.27

Table 4 Boltzmann temperatures of all observed species and contributions from other heavier ones from the 248 nm photolysis of DMCd adsorbed on a cold quartz surface (20 ml/cm’ pulse, 0.03 L and T, = 143 K) Observed ions

m/c 14

15

TMn (K)

522

465

C%

0.62 0.37

0.93

CH3

28

CA C,Hs C2H6

Cd MMCd DMCd

0.07

1

29

30

114

248

531

1001

0.69 0.31

1

129

144 1115

0.46 0.54

1

S.-P. Lee, M.C. Lin /Applied Surface Science 84 (1995) 31-43

42

(A)

0

100

200

300

400

F.” ( cm-’

332

500

600

)

3.4. Extent of film deposition

333

Wavelength

with both 193 and 248 nm photolysis supports the conclusion that the production of this and other photofragments are surface-mediated. It is also interesting to note that the rotational temperature of CH, (155 K), which is close to the substrate temperature of 143 K, is about a factor of two lower than its translational temperature (276 K) under these experimental conditions (0.009 L and 20 mJ/cm2 . pulse). DMCd, the parent molecule, possesses the highest translational temperature measured. This temperature was found to be affected strongly by photon energy, surface coverage and laser fluence as discussed in detail before. The high translational energy of the desorbed DMCd observed at both photolysis wavelengths may due in large part to the strong coupling of electronic excitation/ surface relaxation processes which are unavailable to small alkyl fragments.

( nm

)

Fig. 12. Analysis of rotational temperature from (2 + 1) REMPI spectra of CH, radicals formed by 193 and 248 nm photolysis on SiO, at 143 K: (A) Boltzmann plots of P- and O-branch lines. Ckj represents the intensity divided by the product of rotational line strength (Akj), rotational energy (v) in wave number and statistical weight factor (grj). (B) Comparison of a representative spectrum taken by 193 mn photolysis with the simulated spectrum at the evaluated temperature (1.55 K). Exposure: 0.009 L and fluence: 20 mJ/cm’.

(E,) measured in the present study, suggest clearly that the production of CH,, similar to all other desorbed species, is surface-mediated. The production of C,H, from surface photolysis, totally absent in the analogous study in the gas phase, is the strongest indication of the surface-mediation effect. The REMPI spectrum shown in Fig. 10 represents the well-known 00, band of the 3p 2A’+ X ‘H Rydberg transition which we first reported in 1983 [16]. The analysis of this spectrum allows us to obtain the rotational temperature of the desorbed CH, radical to be 155 f 15 K, independent of photon energy, as indicated in Fig. 12A. The simulated spectrum with this rotational temperature, shown in Fig. 12B, agrees well with the measured one. The fact that the rotational temperature of the CH, radical is the same

After the completion of this series of surface photochemical experiments, the SiO, surface was analyzed with X-ray photoelectron spectroscopy. The result of this analysis indicated that very little Cd metal was deposited on the substrate. This finding suggests that under the fluence of N 20 mJ/cm2 pulse (10 ns), Cd could be very efficiently removed from these solid substrates. We hope to study the effect of Group VI elements on the formation of II/VI semiconductive films under the influence of similar laser intensities.

4. Conclusions The photolysis of (CH,),Cd, adsorbed on a quartz substrate at submonolayer levels, with ArF (193 nm) and KrF (248 nm) lasers was found to desorb as well as to fragment extensively. Products detected by time-of-flight mass spectrometry, using either electron impact ionization (for all species) or resonanceenhanced multiphoton ionization (for Cd and CH,), are H, CH,, CHs, CH,, C,H,, CzH5, C,&, CH,Cd and (CH,),Cd. Both measured yields and fitted Maxwell-Boltzmann temperatures of these products (except H) were found to increase with surface coverage, laser

S. -P. Lee, MC. Lin /Applied

fluence and photon energy. The desorbed hydrocarbon products, C, and C, species, typically have about the same translational energies, which are significantly lower than those of the Cd-containing ones; i.e., Cd, CH,Cd and (CH,),Cd. This observation was attributed to the coupling of the electronic excitation/ surface relaxation process to desorption, giving rise to higher translational temperatures for the metal-atom-containing species. The electronic excitation of the adsorbed organometallic species and the deposited metal film can occur quite readily. In fact, ions were detected at higher fluences, under which the yields of neutral products were found to drop slightly. The analysis of the photolyzed surfaces under these high-fluence conditions revealed very little Cd deposition on the SiO, substrate. A detailed mechanism for the photodesorption/photodecomposition of (CH,),Cd adsorbed on solid surfaces is proposed.

Acknowledgments The authors gratefully acknowledge the support received from the Office of Naval Research under contract No. NOOO14-89-J-1235. We also thank Dr. Y. He for carrying out a preliminary experiment of this study.

Note added in proof The photochemistry of DMCd adsorbed on a silicon substrate with native oxide has been investigated with varying KrF (222 nm> laser fluences by V.N. Varakin and A.P. Simonov [Chem. Phys. L&t. 190 (1992) 481. Time-of-flight profiles of DMCd, DMCd+ and their neutral and ionic fragments were measured without deconvoluting overlapping signals. Interestingly, no report of C, species was made. Other references on related studies by the authors can be found in this article.

Surface Science 84 (1995) 31-43

43

References [l] D. B&rerle, Chemical Processing with Lasers (Springer, Berlin, 1986). [2] R.M. Osgood, Jr., Annu. Rev. Phys. Chem. 34 (1983) 77. [3] D.J. Erlich, R.M. Osgood and T.F. Deutsch, IEEE J. Quantum Electron. QE-16 (1980) 1233. [4] CD. Strinespring and A. Freedman, Chem. Phys. Len. 143 (1988) 584. [5] C.J. Chen and R.M. Osgood, Jr., J. Chem. Phys. 81 (1984) 318, 327. [6] J.O. Chu, G.W. Flynn, C.J. Chen and R.M. Osgood, Chem. Phys. Lett. 119 (1985) 206. [7] C. Jonah, P. Chandra and R. Bersohn, J. Chem. Phys. 55 (1971) 1903. [8] A. Amirav, A. Penner and R. Bersohn, J. Chem. Phys. 90 (1989) 5232. [9] T. Ibuki, A. Hiraya and K. Shobatake, J. Chem. Phys. 92 (1990) 2797. M. Suto, C. Ye and L.C. Lee, J. Chem. Phys. 89 (1988) 160. illI C.F. Yu, F. Youngs, K. Tsukiya, R. Bersohn and J. Presses, J. Chem. Phys. 85 (1986) 1382. WI T.F. Deutsch, D.J. Erhlich and R.M. Osgood, Jr., Appl. Phys. Len. 35 (1979) 175. [131 E. Villa, J.A. Dagata and M.C. Lin, in: Advances in Laser Science IV, AJP Conf. Proc. 191 (1989) p. 379. b41 E. Villa, J.A. Dagata and M.C. Lin, J. Chem. Phys. 92 (1990) 1407. I151 J.A. Dagata, E. Villa and M.C. Lin, Appl. Phys. B 51 (1990) 443. [161J.W. Hudgens, T.G. DiGiuseppe and M.C. Lin, J. Chem. Phys. 79 (1983) 571. [171 C.E. Moore, Atomic Energy Levels, Vol. IV, NSRDSNBS35 (National Bureau of Standards, US Department of Commerce, 1971) p. 55. k31 M.C. Lin and W.A. Sanders, in: Advances in Multiphoton Processes and Spectroscopy, Vol. 2, Ed. S.H. Lin (World Scientific, Singapore, 1986) p. 333. D91 I. Harrison, J.C. Polanyi and P.A. Young, J. Chem. Phys. 89 (1988) 1475, 1498. K. Leggett, M.S. Matyiaszczyk, J.C. PO1 St.J. Dixon-Warren, Polanyi and P.A. Young, J. Chem. Phys. 93 (1990) 3659. [211 J.C. Polanyi and P.A. Young, J. Chem. Phys. 93 (1990) 3673. La K. Domen and T.J. Chuang, J. Chem. Phys. 90 (1989) 3318, 3332. 1231F.L. Tabares, E.P. Marsh, G.A. Back and J.P. Cowin, J. Chem. Phys. 86 (1987) 738. [241 Y. Bu, S.-P. Lee and MC. Lin, Photodesorption of adsorbed benzene from LiF(OO1) by selective excitation at 308, 248 and 193 nm, unpublished work.

m