On the reactivity of trimethylgallium with H2O, CH3OH, CH3OCH3, and NH3 in a multiple pulsed nozzle environment

Chemical Physics Letters 388 (2004) 12–17

www.elsevier.com/locate/cplett

On the reactivity of trimethylgallium with H2O, CH3OH, CH3OCH3,and NH3 in a multiple pulsed nozzle environment

Michael Lynch, Alexander Demchuk 1, Steven Simpson, Brent Koplitz *

Department of Chemistry, Tulane University, New Orleans, LA 70118-5698, USA

Received 4 December 2003; in final form 10 February 2004

Published online: 16 March 2004

Abstract

This work reports on the cluster formation of trimethylgallium in the presence of a series of oxygen-containing compounds. Two

or three pulsed nozzles are used in combination with laser light (k ¼ 193 nm) as well as NH3. Differences in the reactivity of

trimethylgallium with H2O, when compared to CH3OH or CH3OCH3, are observed.

� 2004 Elsevier B.V. All rights reserved.

1. Introduction

In the formation of GaN and related materials viametalorganic chemical vapor deposition (MOCVD), the

Lewis acid–base properties of the various precursors are

a central element. At low temperature, the main gas-

phase reaction in the III–V growth process is directed by

strong Lewis acid–base interactions between metal alk-

yls such as trimethylgallium (TMGa) (the electron ac-

ceptor) and ammonia (the electron donor) to form the

Lewis acid–base adduct compound (CH3)3M:NH3,where M is Al or Ga [1–3]. However, in the growth of

GaN it is often oxygen contamination that is a primary

cause for unsatisfactory film quality [4]. Note that it is a

similar type of Lewis acid–base relationship that is the

basis for the bonding between many oxygen-containing

species and metal alkyls. Coordination of such oxygen-

containing compounds (e.g., H2O, CH3OH, and

CH3OCH3) to group III alkyls is facilitated through thedonor properties of oxygen and the acceptor properties

of the metal alkyl. This interaction can be very strong

and lead to the formation of monomers, dimers, trimers,

or tetramers when TMGa is reacted with H2O, CH3OH,

or CH3OCH3 [5–8].

* Correponding author. Fax: +1-504-865-5596.

E-mail address: [email protected] (B. Koplitz).1 Present address: APA Optics, Inc., 2950 N.E. 84th Lane, Blaine,

MN 55449, USA.

0009-2614/$ - see front matter � 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.cplett.2004.02.047

The current work involves an exploration of the re-

activity of H2O, CH3OH, and CH3OCH3 with TMGa

using experimental conditions similar to those employedpreviously for the investigation of the laser-initiated

growth of GaN and AlN clusters [9–13]. The resulting

mass spectra show interesting trends in reactivity with

regard to changing systematically the atoms bonded to

the oxygen donor atom. We also investigate the reac-

tivity of TMGa in an environment where ammonia and

water are both available for interaction. This particular

experiment was conducted by utilizing a novel deliverysystem that incorporates three pulsed nozzles.

2. Experimental

The experimental apparatus consists of a high vac-

uum chamber equipped with a quadrupole mass spec-

trometer (QMS) and a specialized pulsed nozzle source.The basic approach has been described elsewhere

[10,11]. For the bulk of the experiments, a dual pulsed

nozzle assembly is utilized. Two independently-

controlled pulsed flows combine in a mixing region prior

to their release into vacuum. TMGa and either H2O,

CH3OH, or CH3OCH3 are introduced into the high

vacuum chamber via the nozzle assembly. Note that

TMGa is used with Ar as the carrier gas, while the waterand the methanol are heated to 100 and 60 �C, respec-tively, and introduced into the mixing region under their

mail to: [email protected]

-5000

0

5000

1 104

1.5 104

2 104

2.5 104

3 104

3.5 104

120 160 200 240 280 320 360 400

TMGa/CH3OH

Inte

nsit

y (r

el. u

nits

)

(CH3)

5Ga

3O

3(CH

3)3

+

CH3GaOCH

3

+

115

117

245

249

247

229183199

169

laser off (subtraction)

381

379

377

375

(b)

-1 104

-5000

0

5000

1 104

1.5 104

2 104

2.5 104

3 104

100 150 200 250 300 350 400

TMGa/O(CH3)

2

Mass (amu)

Inte

nsit

y (r

el. u

nits

)


145

147115

117

114116

(CH3)

2GaO(CH

3)

2

+

(CH3)3Ga+

(CH3)GaO(CH

3)+

(a)

M. Lynch et al. / Chemical Physics Letters 388 (2004) 12–17 13

own vapor pressure. The TMGa backing pressure was

typically �200 Torr in �3600 Torr of carrier gas, while

the reactant gases were varied between 200 and 5000

Torr as needed in order to explore pressure effects on

cluster formation. The opening time of each pulsed valvewas adjusted to be �0.2–0.3 ms with repetition rates of

10 Hz. For photolysis, the 193 nm output from an ArF

excimer laser (Lambda Physik LEXtra 200, 23 ns pulse

duration) with 10 mJ/pulse energy is focused into the

mixing/reaction region of the nozzle assembly with a

lens having a 250 mm focal length. Delay times between

sample injection and the laser pulse were typically

�5 ms. Mass analysis of the products is accomplishedwith an Extrel MEXM2000 QMS employing a cross-

beam electron impact ionizer.

For simultaneous studies involving TMGa, H2O, and

NH3, three independently-controlled pulsed nozzles

were used. Within the nozzle assembly, three channels

merge into one at the point of mixing/reacting. Note

that the nozzle assembly can actually handle up to four

independently-controlled pulsed flows operating simul-taneously. Finally, deuterated ammonia (ND3; Cam-

bridge Isotope Labs) was used in order to investigate

different reaction pathways.

-2 104

-1 104

0

1 104

2 104

3 104

4 104

120 160 200 240 280 320 360 400

TMGa/H2O

Mass (amu)

Inte

nsit

y (r

el. u

nits

)

217

199

299

315

333(CH3)3Ga

2(OH)

2

+(CH

3)5Ga

3(OH)

3

+

(CH3)3Ga+

219221

116

114

283


(c)

Mass (amu)

Fig. 1. Mass spectra of expansions containing TMGa and (a) dimethyl

ether, (b) methanol, and (c) water each with no laser radiation. The

inset in (b) shows a comparison with the expected pattern for 3 Ga

atoms with appropriate isotope combinations. Contributions from

TMGa alone have been subtracted.

3. Results and discussion

This section is broken down into three parts. Initially,we examine TMGa interacting with oxygen-containing

compounds in a constrained expansion with no laser

light. The same systems are then examined in the pres-

ence of 193 nm radiation. Finally, we look at the com-

petition involving TMGa and H2O when NH3 is also

present. Note that one can conduct all of these experi-

ments at a variety of pressures. The results shown here

illustrate the general reactivity trends displayed by thesystems and are representative of the pressures under

which MOCVD studies tend to be conducted.

3.1. Reactivity in the absence of laser radiation

Fig. 1a depicts the mass spectrum resulting from the

reaction of TMGa with dimethylether. In order to show

only those species that arrive at the detector as a resultof this reactivity, the spectrum of TMGa alone has been

subtracted. This signal is seen as the negative peaks at

114/116 amu. The peaks at 145/147 amu are attributable

to the 1:1 adduct (CH3)3Ga:O(CH3)2, with the loss of

one methyl group attached to the Ga atom due to

fragmentation during electron impact ionization. The

peaks at 115/117 amu are due to further fragmentation

of the parent adduct. Note that the 60:40 ratio of thepeak heights arises from the natural abundance of the

Ga69:Ga71 isotopes [14]. No other signals at higher

masses were evident.

As shown in Fig. 1b, the mass spectrum resulting

from the reaction of TMGa with CH3OH indicates a

clear formation of trimers and dimers. The peaks at375–381 amu correspond to the trimeric species

(CH3)5Ga3O3(CH3)þ3 , which itself can be attributed to

the loss of one methyl group from (CH3)6Ga3O3(CH3)3upon ionization and subsequent fragmentation. Note

that the range of masses correlates nicely with the gal-

lium isotope combinations one predicts from bringing

three Ga atom together, each having the natural abun-

dance probability discussed above. The expected distri-bution is shown in the inset of Fig. 1b. From IR

14 M. Lynch et al. / Chemical Physics Letters 388 (2004) 12–17

evidence, the trimeric species is thought to exist as a six-

membered ring with alternating Ga–O bonds, as shown

in Fig. 2 [8].

The primary product in the mass spectrum is the di-

meric (CH3)3Ga2O2(CH3)þ2 ion that most likely results

from the ionization and subsequent loss of CH3 from

(CH3)4Ga2O2(CH3)þ2 . The fact that the reaction of

TMGa with methanol produces trimers and dimers as

opposed to simple coordination compounds, as is the

case with TMGa/CH3OCH3, is not surprising. It has

been shown that compounds such as methanol that

contain hydrogen bound to a donor atom (oxygen in

this case) can eliminate methane when reacted withTMGa thus forming more stable products [6]. In the

case of methanol, the following reaction is believed to

take place:

ðCH3Þ3Gaþ ðCH3ÞOH ! ðCH3Þ2GaOðCH3Þ þ CH4

ð1Þwith the suggestion that in the vapor phase the product

of this reaction exists as a dimeric, four-membered ring

structure involving alternate Ga–O bonds, as shown inFig. 2 [6].

Fig. 1c depicts the 120–400 amu mass spectrum that is

obtained from reacting TMGa with water in the con-

strained expansion. The spectrum is similar to that ob-

tained with TMGa/CH3OH in that it is clear that

dimeric or timeric species are predominant. The spec-

trum with water, however, shows a larger signal due to

the trimer. The reaction of TMGa with H2O undercontrolled conditions goes by the following route:

ðCH3Þ3GaþH2O ! ðCH3Þ2GaOHþ CH4 ð2Þ

Ga

O

GaO

Ga

OCH3H3C

H H

H

H3C CH3

H3C CH3

O

Ga

O

Ga

CH3

CH3

H3C

H3C

H

H

Fig. 2. Bonding patterns predicted for GaO-containing trimeric and

dimeric species.

with the product of this reaction reported to have a

trimeric structure ((CH3)2GaOH)3 in benzene solution

[7] and a tetrameric structure when the solid product is

analyzed by X-ray diffraction [8].

The predominant reaction product in the spectrum isfound at 333 amu. This feature corresponds to the tri-

meric species (CH3)6Ga3(OH)þ3 with the loss of CH3 due

to fragmentation from the electron impact ionization.

A strong signal at 315 amu arises from the loss of H2O

from (CH3)5Ga3(OH)þ3 . Subsequent fragments can be

seen as smaller peaks at 299 amu (–CH4) and 285 amu

(–CH2). This data is in good agreement with other gas-

phase mass spectroscopic experiments exploring TMGa/H2O interactions [15].

In comparing the spectra of the three different oxygen

precursors in Fig. 1, a trend can be seen towards the

formation of larger aggregates of atoms when the oxy-

gen atom in the precursor molecule is bound to at least

one hydrogen atom. As indicated previously, it appears

that with an available hydrogen atom the elimination of

methane followed by aggregation of the resulting speciesdrives the formation of clusters in the gas phase [6].

3.2. Reactivity with a 193 nm excimer laser pulse

Introduction of a laser pulse creates a very reactive

situation, even though the temperature is not high as in a

typical MOCVD environment. Figs. 3 and 4 show the

effect of the 193 nm laser light on the three oxygen/gal-lium systems. As in Fig. 1, in order to show only those

species that arrive at the detector as a result of this in-

duced reactivity, the spectrum of TMGa alone (with the

193 nm radiation) has been subtracted. The lower mass

(100–400 amu) spectrum from the reaction of TMGa

with dimethylether illustrates the profound effect that the

laser radiation has on the reactivity of these two species.

As discussed earlier, the spectrum obtained without laserradiation shows only evidence of the monomeric adduct

(CH3)3Ga:O(CH3)2. With the 193 nm laser pulse focused

on the reactants, however, there is significant evidence of

larger aggregations, i.e., we observe more than the 1:1

adduct. In the 100–400 amu mass range, peaks are ob-

served that we attribute to dimeric and trimeric species.

Note the similarities in the laser-induced mass spectral

patterns found in Figs. 3a and b for TMGa/O(CH3)2 andTMGa/CH3OH, respectively, when compared with the

same mixtures in the absence of laser radiation (Figs. 1a

and b). Clearly, the laser facilitates the formation of tri-

mers and/or higher-order clusters, as shown in Figs. 4a

and b.Moreover, the patterns are very similar for the two

mixtures. The main difference appears to be that the

clustering is more advanced in the case of the TMGa/

CH3OHmixture. In other words, there is simply a greateramount of higher-order clusters formed. Mechanisti-

cally, the similar behavior for the two mixtures suggests

that a common route for cluster formation is at work.

-5000

0

5000

1 104

1.5 104

2 104

2.5 104

3 104

3.5 104

300 350 400 450 500 550 600

TMGa/H2O

Inte

nsit

y (r

el. u

nits

)

Mass (amu)

laser on (subtraction)

348

333

(CH3)6Ga

3(OH)

3

+

(CH3)5Ga

3(OH)

3

+

(c)

-5000

0

5000

1 104

1.5 104

2 104

2.5 104

3 104

3.5 104

300 350 400 450 500 550 600

TMGa/CH3OH

Inte

nsit

y (r

el. u

nits

)

Mass (amu)


475375

345

360

330

315

415430

445

460

490

390 590575

560

(CH3GaO)

(+100)

[(CH3)7Ga

4O

4(CH

3)2]+

(b)

-5000

0

5000

1 104

1.5 104

2 104

2.5 104

3 104

300 350 400 450 500 550 600

TMGa/O(CH3)

2

Inte

nsit

y (r

el. u

nits

)

Mass (amu)


[(CH3)4Ga

3O

3(CH

3)3]+

360

345

330

315 475

460

445

430 490

[(CH3)7Ga

4O

4(CH

3)2]+

(+100)

(CH3GaO)

415

(a)


ether, (b) methanol, and (c) water with 193 nm laser radiation. The

larger clusters observed are shown here. Contributions from TMGa

alone have been subtracted.

-1 104

-5000

0

5000

1 104

1.5 104

2 104

2.5 104

3 104

120 160 200 240 280 320 360 400

TMGa/H2O

Mass (amu)

Inte

nsit

y (r

el. u

nits

)

199

315333

217

299231


(CH3)

3Ga

2(OH)

2

+

(CH3)

4Ga

3(OH)

2O+

317

201

185154169

114

116

Ga2O+

(c)

-5000

0

5000

1 104

1.5 104

2 104

2.5 104

3 104

3.5 104

120 160 200 240 280 320 360 400

TMGa/CH3OH

Mass (amu)

Inte

nsit

y (r

el. u

nits

)


(CH3)

3Ga

2O

2(CH

3)

2

+

(CH3)

5Ga

3O

3(CH

3)2

+

115

117

114116

169183

199

215

230

245247

249

260

300

375

360

345

315330

(b)

-1 104

-5000

0

5000

1 104

1.5 104

2 104

2.5 104

3 104

100 150 200 250 300 350 400

TMGa/O(CH3)2

Mass (amu)

Inte

nsit

y (r

el. u

nits

)


(CH3)3Ga

2O

2(CH

3)2

+

245

(CH3)

5Ga

3O

3(CH

3)2

+

360

247

249

114116

230

215

185

169

200

111113

145147

345330260

(a)


ether, (b) methanol, and (c) water with 193 nm laser radiation. The

smaller clusters observed are shown here. Contributions from TMGa

alone have been subtracted.


For both the ether and the methanol systems, there is

clear evidence of dimer formation as shown by the fea-

tures at 260 amu in Figs. 3a and b. Also, trimer formation

manifests itself in the spectra, primarily in the form of

daughter ion peaks. In fact, for the TMGa/CH3OH sys-

tem (Fig. 4b) the peaks at �390 amu correspond to theparent (CH3)6Ga3O3(CH3)

þ3 ion itself. Likewise, the

TMGa/H2O system shown in Fig. 3c clearly has trimeric

features. However, the differences among the three sys-

tems become dramatic at higher masses. Despite the

strong tendencies of the TMGa/H2O mixture to make

small complexes, Fig. 4c demonstrates that no trace of

higher-order clusters (i.e., complexes greater than trimer)

-1 104

-5000

0

5000

1 104

1.5 104

100 150 200 250 300 350 400

TMG/H2O/ND

3 -TMG/H

2O

Inte

nsit

y (r

el. u

nits

)In

tens

ity

(rel

. uni

ts)

Mass (amu)

laser off(subtraction)

114

116

117119

121

(CH3)

2Ga:ND

3

+

(CH3)

2GaND

2

+

(CH3)

3Ga+

218 316 334

(CH3)

3Ga

2(ND

2)(OH)+

(CH3)

5Ga

3(ND

2)(OH)

2

+

(-18)

199217

315 333

OH2

(b)

-5000

0

5000

1 104

1.5 104

TMGa/H2O/NH

3 -TMGa/H

2O


(CH3)2GaNH

3

+

116

118

114 (CH3)3Ga

2(OH)

2

+

(CH3)5Ga

3(OH)

3

+

(a)

Fig. 5. Mass spectra of expansions containing TMGa and water mixed

with (a) NH3 and (b) ND3. Contributions from the TMGa and water

mixture have been subtracted. No laser radiation was used.

16 M. Lynch et al. / Chemical Physics Letters 388 (2004) 12–17

is observed. In contrast, the TMGa/CH3OH and to a

lesser extent the TMGa/CH3OCH3 mixtures clearly

make species containing four or more Ga–O units.

As noted, the 193 nm laser radiation has a strong in-

fluence on these mixtures, particularly TMGa/CH3OHand TMGa/CH3OCH3. Mechanistically, TMGa has a

relatively large absorption cross-section at 193 nm

(r ¼ 2� 10�17 cm2) [16], thus a likely first step is pho-

tolysis of TMGa to make Ga(CH3)2 and CH3 (although

other photo-activated pathways are possible). Nonethe-

less, one cannot rule out photon absorption by the

complexes that are formed by virtue of the expansion

itself in the absence of laser radiation, i.e., Fig. 1. How-ever, arguments against this particular route include the

fact that the photoreactive TMGa/CH3OH and TMGa/

CH3OCH3 outcomes are very similar (Figs. 3a and b),

while the non-photolyzed mixtures are very different with

regard to complex formation. Consequently, one would

expect that the photolysis of complexes plays a minor

role at most. Finally, the nature of subsequent reactive

steps is currently a matter of speculation. It must benoted, however, that 193 nm photon absorption by

CH3OCH3, CH3OH, or H2OH is unlikely, since the

absorption cross-section for each is relatively small [17].

3.3. Competition between water and ammonia with TMGa

Fig. 5 shows the mass spectra that result from the

mixing of TMGa, H2O, and either NH3 or ND3. Itspurpose is to explore how ammonia affects TMGa/H2O

reactivity, or conversely, one can observe how H2O af-

fects TMGa/NH3 reactivity. No laser radiation was

used, but three separate pulsed nozzles were utilized.

For clarity, the TMGa/H2O spectrum without a laser

pulse was subtracted from each spectrum. In Fig. 5a for

the case TMGa/H2O/NH3, the only obvious peak left

after subtraction occurs at 116/118 amu and corre-sponds to the (CH3)2Ga:NHþ

3 daughter ion of the

(CH3)3Ga:NH3 adduct. The large subtraction peaks due

to dimeric and trimeric GaO-containing species are not

surprising given their existence as positive peaks in the

spectrum of TMGa with only H2O (see Fig. 1c). Such

results suggest that (CH3)3Ga:NH3 adduct formation is

the only reactive ammonia pathway.

Analysis of Fig. 5b suggests a somewhat differentstory, however. Here, the use of ND3 produces the ex-

pected adduct with TMGa, but it also lessens the degree

of �negative� TMGa:H2O features. If ND3 were to have

no effect on the chemistry other than the production of

the (CH3)3Ga:ND3 adduct, then one would expect

Figs. 5a and b to be nearly identical. Instead, it appears

as though the ND3 is competing more effectively than

NH3 as a complexing agent with TMGa in the presenceof H2O. Note, however, that positive spectroscopic as-

signments in this mass region are difficult due to the

similar mass nature of the methyl, ammonia, and water

ions along with their daughter fragments. While the

results are intriguing, more exhaustive experiments areneeded before any conclusions can be drawn.

In closing, we add that pressure-dependence studies,

although beyond the scope of the current Letter, may

prove useful in exploring reactivity in these systems. In

particular, the competition between oxygen-containing

compounds and ammonia in the presence of TMGa

would benefit from such studies. Preliminary work in

this area is already underway in our laboratory.

Acknowledgements

Financial support from the Department of Energy,

NASA, the State of Louisiana via the Louisiana Edu-

cation Quality Support Fund, and the National Science

Foundation through Tulane University�s Center for

Photoinduced Processes is very much appreciated.

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Documents

On the reactivity of trimethylgallium with H2O, CH3OH, CH3OCH3, and NH3 in a multiple pulsed nozzle environment