Gas Phase Adduct Reactions in MOCVD Growth of GaN
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3((CH3)3Ga:NH3)• [(CH3)2Ga:NH2]3 + 3CH4.
This overall reaction reaction has to proceed steps. The The first step is thestep hydrogenelimination withbeen thesuggested subsequent release inoftwo a CH4[12]. second is the oligimerization of this specie into the three member ring. (3) (CH3)3Ga:NH3 • (CH3)2Ga:NH2 + CH4 (4) 3((CH3)2Ga:NH2)• [(CH3)2Ga:NH2]3 The estimated energy required for the elimination process is 49 kcal/mol [ 12]. The gas phase reaction between (CH3)3Ga and NH3 at elevated temperatures, characteristic of MOVPE growth environments (~200°C-1100°C) is still not understood. The actual significance of adduct formation at these higher process temperatures has not been determined, despite the potential impact it may have on reactor modeling and optimization and on process design. EXPERIMENT We studied the high temperature gas phase reactions between TMG and NH3 by means of in situ mass spectrometry within a flow tube isothermal reactor [15], shown in fig. 1. The gas phase decomposition of TMG, NH3, and the combined TMG-NH3 systems in Hz and D2 were studied over the temperature range of 200-800°C. A two temperature zone reaction furnace was used. This reactor circumvents any premature decomposition of the TMG prior to mixing and reaction with the NH3, and avoids any unintentional adduct formation at room temperature. The two primary reactants, TMG and NH3, were allowed to mix in the first hot zone of the reactor, that was kept in 150°C. This adduct and its initial reaction products are transported to the second Gases
Gas Sampling System Furnace
Liquid Sources
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Figure 1: Schematic of the experimental system temperature zone were they undergo decomposition. The reacted stream was sampled from within the hot isothermal regions of the reactor through -75 l.tm diameter quartz nozzle. The gas was further expanded in molecular flow and sampled by a residual gas analyzer (RGA). The inlet gas stream always contained 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. The TMG mole fraction in the inlet of the reactor was 0.015 - 0.05. Measurements of the co-pyrolysis of TMG and ammonia utilized both NH3, as well as deuterated ammonia (ND3), in order to label the products and to distinguish between possible reaction pathways. All experiments were done at a pressure of 76 Torr. Peaks at m/e
values of 16, 17, 20, 30, 69, 84 and 99 were monitored corresponding to molecular fragments CH 4, CH 3D, ND 3, C2H6 , Ga, CH 3Ga and (CH 3)2Ga respectively. RESULTS The gas phase homolysis of TMG in H2 and D 2 was carried out and compared to previous FTIR measurements of TMG decomposition in H2 [7] as a reference, and served as a base line for understanding the decomposition of TMG in H2/NH 3. Since the electron impact in the RGA immediately breaks the adduct Ga-N bond[14], the peaks at nme = 69, 84 and 99, characteristic of TMG decomposition, are also indicat
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