IR Diagnostics of MOCVD Reaction Chemistry
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between TMA and NH 3 can be converted to a stable trimer by elimination of methane. They have also shown that the trimer can be used for deposition of AIN at a significantly lower temperature than that required for TMA and NH 3 alone.4 The IR spectroscopy 8of the adduct between TMA and NH 3 has been examined in the solid state at low temperatures and recently in matrix isolation, 9 but apparently not in the gas phase. Recently, it was reported that AIN can be deposited at - 500 °C using NP 3 1. o This is especially intriguing given that AIF3 is more stable than AIN and is an indication that kinetics plays an important role in the formation of AIN from TMA and NF 3 . The gas-phase reactions of NF 3 with R3Ga (R = Me, Et) have been examined, and no reactions were observed to occur at temperatures up to 150 oC whereas reaction with NH3 gives an adduct. 1 From a chemical perspective, NH 3 and NF 3 are very different molecules. NH 3 has strong N-H bonds (108 kcal/mol) 12 and a strong dipole moment (1.5 D)13 and acts as a strong Lewis base to form adducts with acceptor molecules like TMA. NF 3 , on the other hand, has weak N-F bonds (60 kcal/mol) 12 and a negligible dipole moment (0.2 D)1 3 and is not known to form adducts. Given these significant differences and the indication of a kinetically controlled deposition of AIN, we embarked on an investigation to compare and contrast the gas-phase chemistry of TMA with NH 3 and NF 3 . For the work with TMA and NH 3 and NF 3 , a static gas cell made of pyrex with a 15 cm path length and equipped with KBr windows was mounted in the sample compartment of an FTIR spectrometer (Nicolet 800). The cell was connected to a stainless-steel vacuum line used for sample preparation. An MCT detector was used and the resolution was 2 cm- 1. As a starting point in this study, we examined the reaction of TMA with NH3 to establish the reactivity of TMA. Figure 1 shows the spectra obtained when TMA (10 torr) and excess NH 3 are mixed in the cell at 25 0(. The reaction is very rapid, occurring as soon as the stopcock was opened. All of the major bands are in good agreement with the spectra reported for the adduct between TMA and NH 3 in the solid state.8 As with the solid-state study, we do find some significant differences with the matrix isolation spectra, especially the intensity of the band at - 1600 cm-1. Given that most of the bands are in good agreement with the solid-state spectra, we assign the spectrum to the adduct. Confirmation of this assignment will have to wait for further spectroscopic investigations. The formation of a significant amount of involatile solid material on the walls of the cell was also 4 observed and may be the trimer or higher polymeric material, as reported by Interrante et al. The same experiment was repeated with NF 3 at room temperature, and no reaction was observed. This is0 consistent with the poor base properties of NF 3 . However, when the cell was heated to 58 C, a new product was observed as shown in Figure 2. The new bands in Figure 2 can be assig
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