High Temperature Gas Phases Reactions of Trimethylgallium with Ammonia and Trimethylamine
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445 Mat. Res. Soc. Symp. Proc. Vol. 423 ©1996Materials Research Society
CarrierGas HDExhaust
Ammonia
NH,, ND3
TMN
Zone I
Zone 2 TC
Furnace
Calibration Gases CH 4 , C2H6 , N2 , Ar
z PC
RGA Control
Figure 1: Schematic of the experimental system At low temperatures, a Lewis acid - Lewis base interactions between TMG (electron acceptor) and NH 3 (electron donor) lead to the intermediate adduct compound, Trimethylgallium monoamine [TMG:NH 3][6,10,11,12]: (CH 3 )3 Ga + NH3 4-- (CH3 )3 Ga:NH3
(1)
The structure and thermodynamic of this adduct, at low temperatures, have been intensely studied[10,1 1,12]. Recent studies[1 1] show that above 90 0C one methane molecule per one Ga atom is eliminated, and a six member ring Cyclo(triammido-hexamethyltrigallium) [(CH 3)2Ga:NH 2]3 is formed in the reaction: 3((CH3 )3 Ga:NH3 ) = [(CH3 )2 Ga:NH2]b + 3CH4 .
(2)
This overall reaction has been suggested to proceed in two steps. The first step is the hydrogenelimination reaction with the subsequent release of a CH4 . The second step is the oligimerization of this specie into the three member ring. (3) (4)
(CH3 )3 Ga:NH3 => (CH 3 )2 Ga:NH2 + CH4 3((CH3 )2 Ga:NH2 )=* [(CH3 )2 Ga:NH2] 3
These reactions were recently studied in the high temperature range (200 - 800'C) [5]. A stable species, probably the trimeric compound, decomposes above a temperature of 500 TC with an apparent activation energy of 50.5 kcal/mole [5]. Similar information for the adduct formation between TMG and TMN and the interaction of such an adduct with ammonia is not yet available and is the scope of this study EXPERIMENT We have studied the high temperature gas phase reactions between TMG, TMN, and, NH 3 by means of in situ mass spectrometry within a flow tube isothermal reactor suitable for kinetic studies [8,13]. A schematic of the experimental system is shown in Figure 1. A two temperature 446
zone furnace is used. TMG and TMN were allowed to mix in the first hot zone of is that reactor, the maintained at a temperature OC. This -150 of temperature is low enough thermal prevent to decomposition of TMG or the TMG:TMN adduct but is high enough to suppress adduct condensation. This adduct and its initial
TMN CH 3D
9 I0 8 *7
.w6 W a64 -CH6
• 2 1 0
700 800 500 600 300 400 200 0 100 are products reaction transported to the second Temperature (°C) temperature zone were they are mixed with NH 3. The Figure 2: Plot of partial pressure of gas phase species over a range of temperature during homolysis of TMN. temperature of the second zone is varied from 200 - I 800'C. The gases are sampled from within the hot isothermal regions of the second zone of the reactor through -150 ýtm diameter quartz nozzle. The gas was further expanded in molecular flow and monitored by a residual gas analyzer (RGA). The inlet gas stream contained a small amount (0.5%) of Ar during all experiments, to allow for data normalization and to avoid gas expansion artifacts. Data were obtained as function of temperature through the continuous ramping of the reactor temperature in
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