Molecular Precursors for the Low-Temperature Deposition of Gallium Nitride, Boron Nitride, and Elemental Tin
Creators: Bykowski, Janelle F.
Publisher: name: University of Alberta Library
Year: 2023

<p>The deposition of bulk inorganic materials from aptly designed molecular precursors represents an attractive and exciting synthetic strategy to improve access to such materials and overcome current limitations with incorporating them into a broader array of consumer devices. In this Thesis, a series of novel, molecular and polymer-based synthetic routes were investigated with the overarching goal of depositing bulk films of gallium nitride (GaN), boron nitride (BN), and elemental tin (Sn).</p>

<p>First, the preparation of single-molecule precursors for GaN deposition was explored (Chapter 2). The N-heterocyclic carbene (NHC) supported amido-gallane system IMes•GaH2{NH(4-iPrC6H4)} (IMes = [(HCNMes)2C:], Mes = 2,4,6-Me3C6H2) was initially targeted with the goal of preparing a new polymeric precursor containing a Ga–N backbone. While multiple synthetic routes were tested for the preparation of this monomer, the reaction of purified Li[NH(4-iPrC6H4)] with IMes•GaH2Cl was found to be the most facile route to IMes•GaH2{NH(4-iPrC6H4)}. It is interesting to note that the corresponding chloro analogue IMes•GaCl2{NH(4-iPrC6H4)} could not be prepared, as the amide lacked sufficient steric protection to prevent the formation of the disubstituted product IMes•GaCl{NH(4-iPrC6H4)}2. Nevertheless a monosubstituted product could be obtained in high purity by increasing the steric profile of the amide, as demonstrated by the successful preparation and isolation of IMes•GaCl2(NHDipp) (Dipp = 2,6-iPr2C6H3). Through this work, a better understanding of the reactivities of such NHC-supported amido-Ga(III) precursors was achieved, which will support future efforts in further developing this promising, low-temperature method for GaN preparation.</p>

<p>Molecular-based synthetic approaches for BN deposition were also investigated in Chapter 2. First, ammonia borane (H3N•BH3) was reacted with 2,3-dimethyl-1,3-butadiene in an attempt to form a [2+4] cycloaddition product, which (upon H2 elimination) was postulated to afford a heterocycle containing a “H2N–BH2” unit. Unfortunately, no reaction was observed at room temperature, and multiple cyclic B–N-containing products formed upon heating. A second approach toward BN deposition involved reacting boc-butadienylamine (boc = tert- butyloxycarbonyl, CO2CMe3) with H3B•SMe2 in an attempt to form a hydroboration product featuring a six-membered B–N-containing heterocycle. Preliminary evidence suggests that more than one boron-containing species may have formed, although additional studies are required before the product(s) can be conclusively identified. Nevertheless, the proposed hydroboration strategy could eventually lead to a molecular precursor for BN deposition, especially if the amide side chain could be appropriately modified to prevent hydrogen bonding and improve the solubility of the starting monomer.</p>

<p>Next, the utility of molecular precursors for elemental Sn deposition was investigated (Chapter 3). A new NHC-supported bis-alkoxide Sn(II) precursor, ImMe2•Sn(OtBu)2 (ImMe2 = [(HCNMe)2C:]), was prepared. When treated with the hydride source pinacolborane (HBpin; Bpin = B(OCMe2)2), this species was found to undergo efficient OtBu/H metathesis to deposit Sn metal of remarkable crystallinity. The known Sn(II) species [Sn(OtBu)2]2 and Sn[N(SiMe3)2]2 were also amenable to similar processes, undergoing OtBu/H or N(SiMe3)2/H exchange, respectively, to form crystalline elemental Sn. The deposition of Sn on top of silicon wafers and copper plates was also investigated, and computational studies were performed to explore the plausibility of applying this method to obtain other Group 14 element films.</p>

<p>Overall, the work presented in this Thesis represents a contribution toward the development of new molecular/polymer-based synthetic routes for the low-temperature deposition of industrially-relevant inorganic materials. Such methods represent promising approaches toward expanding the utility of these materials in device fabrication within the semiconductor and electronics industries.</p>