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In this section the potential of group 13 silylpnictides as precursors to thin films and bulk materials is explored. The decomposition pathways of some o f the precursors, whose preparation was described in section 2 .2, were explored by

Thermal Gravimetric Analysis (TGA). The compounds [Cl2Ga{NH(SiMe3 ) } ] 2 (32), [Cl3Ga(NH(SiMe3)2}] (39), [Cl2Ga(HN(SiMe2Ph)}]2 (83), [Et2Al{N(SiMe2H)2}]n

(89) and [Cl2Ga{As(SiMe2Ph)2}]n (102) were pyrolysed under a number of

conditions to ascertain their suitability as single-source precursors to bulk group 13 pnictides. Vapour-phase deposition studies of 39 and 89 were carried out in order to assess the ability of silylamides to produce thin films. The films produced were analysed in an attempt to determine the phase of the material produced and the amount of contamination present.

2.3.1 Thermal Gravimetric Analysis

Initial investigations into the suitability of using the compounds prepared as precursors to group 13 pnictides involved obtaining TGA. Typically, a knovm amount (approximately 10 mg) of the sample was heated from 20 to 500 °C in a sealed aluminium pan under N2 at a uniform rate of 10 °C per minute. Before the

heating is carried out a small pinhole is made in the lid of the pan to allow the gaseous decomposition products to escape. During the experiment, N2 is flowed

across the pan to avoid oxidation of the sample and to remove any gaseous byproducts. By measuring the weight of the pan during the experiment a graph of weight against temperature may be plotted. Any weight losses and the temperature they occur at are obtained, which in turn gives information on the decomposition pathway. Therefore, the temperature required for decomposition of the precursor to occur is also identified (if <500 °C). This is important data if the precursor is to be considered for use in CVD. A compound should not decompose at too high a temperature or the range of substrates, which may be deposited upon, is limited. Decomposition at too low a temperature is also a problem as the compound must be stable at the temperatures required to transport it in the vapour phase to avoid decomposition before the substrate.

2.3.2 Decomposition properties of [X2Ga{NHR}]n

The synthesis of the potential precursors [X2Ga{NHR}]n (where X = Cl, R =

SiMeg (32), SiMe2Ph (83), ^Bu (84); X = Br, R = SiMeg (33)) was discussed in

section 2.2. The decomposition properties of these compounds were measured using TGA and compared. The decomposition of [X2Ga{NH(SiMe3 ) } ] 2 (where X = Cl

(32), Br (33) (shown in Fig. 2.20)) are clean and show total weight losses o f 52% and

67% respectively. This does not indicate that decomposition to GaN has occurred by 500 °C (where the total weight loss would be 63% and 74% respectively). However, these results suggest that decomposition to form GaN may occur at slightly above 500 °C as shown by the fact that both samples are still losing weight at 500 °C (shown by the downward slope in the TGA). The TGA of 32 and 33 indicate that a two step decomposition process is occurring. The first step occurs in the temperature range 175 - 300 °C for 32 and 160 - 220 °C for 33 and involves a weight loss of 47%

for both compounds. This suggests that the complexes are decomposing with loss of XSiMc3 to form a material of the type [XGaNHJn (calculated weight losses X = Cl,

47%; X = Br, 48%), as shown in Eq. 2.36.

[X2Ga{NH(SiMe3 ) } ] 2--- ► 2/n [XGaNH]„ + 2 XSiMe^ (2.36)

The loss of HX to form GaN, which is the second step in the decomposition pathway, is calculated to be 16% (X = Cl) and 25% (X = Br), as shown in Eq. 2.37. The overall weight losses therefore show that not all the HX is lost and that distinct steps are not observed in the temperature ranges, therefore the steps occur in overlap.

[XGaNHJn--- ► n GaN + 2 HX (2.38)

Recently, the imido species [ClGaNHJn (n = 1-4, 6) has been computationally

investigated as an intermediate in the decomposition of the adduct [ClsGalNHs}] to give GaN.^^^ In this investigation the authors concluded that HCl would be a poor leaving group when compared with Hz or CH4, due to the strong Ga-Cl bond

TGA %

100

0

100

Figure 2.20 TGA of [Br2Ga{NH(SlMe3 ) } ] 2 (33)

The TGA of [Cl2Ga(NH(SiMe2Ph)2 } ] 2 (83) shows a total weight loss of 70%

which is close to the calculated total weight loss of 71%. If a similar decomposition pathway to that suggested for 32 was followed then such complete decomposition would not be expected as the first step would form a material of composition [ClGaNHJn. The two observed weight losses (44% and 26%) do not correspond with those expected (59% and 13%). This suggests that an alternative decomposition pathway is taking place or that more than one decomposition process may occur.

The TGA of [C^Ga(NH(^Bu)} ]] (84) shows two distinct weight losses between 160 - 280 °C (55%) and 310 - 450 °C (12%), which suggests that complete decomposition may have occurred. The total weight loss is therefore 67%, which is slightly greater than the calculated total weight loss to GaN (61%). This may be due to minor quantities of the sample subliming during the TGA or to a small error in recording the initial mass. The decomposition of 84 does not appear to follow a similar route to the decomposition of 32 because [ClGaNHJn would be formed as the result of the first step and this species would not be expected to decompose below 500 °C. However, the ^Bu group has the ability to leave via a p-hydride elimination pathway and therefore a different decomposition route may be followed. Groups which undergo p-hydride elimination (e.g. Et, ^Bu) are often used in single-source precursors due to their ability to undergo clean decomposition at low temperature.

The decomposition properties of [Cbln(NH(SiMe3)}]n were not determined

in this study. This was because it would not be expected that the precursor would decompose below 500 °C (compared with 32) and InN is thermally unstable at 500 °C which may lead to misinterpretation of the results.

2.3.3 Decomposition properties of [Et] AI {N(SiMeiH):} ] n

The decomposition properties of [Et2Al(N(SiMe2H)2}]n (89) were

determined by TGA and the results are shown in Figure 2.21. The decomposition of

89 is clean and shows a total weight loss of 79% starting at 40 °C and finishing at