Synthesis of N-Heterocyclic Stannylene (Sn(II)) and Germylene (Ge(II)) and a Sn(II) Amidinate and Their Application as Precursors for Atomic Layer Deposition

Thin films containing germanium or tin have a great variety of current and potential applications, particularly their oxides or chalcogenides. Chemical vapor deposition (CVD) and atomic layer deposition (ALD) are popular ways to make these thin films conformally even on challenging nanostructured substrates. The success of these processes depends on having precursors that are sufficiently stable, volatile, and reactive. In this paper we optimize the syntheses of the following three precursors: 1 and 2 are racemic Ge(II) or Sn(II) cyclic amides made from N-2,N-3-di-tert-butylbutane-2,3-diamine, and 3 is bis(N,N'-diisopropylacetamidinato)tin-(II). All three compounds are demonstrated to be effective precursors for ALD of their monosulfides, GeS or SnS, by reaction with H2S. 2 has also been reported previously to make polycrystalline SnO2 by ALD with oxidizing agents such as H2O2. The cyclic amides 1 and 2 are more volatile than the amidinate 3, vaporizing sufficiently for ALD even at precursor temperatures below 40 degrees C, whereas 3 vaporizes at temperatures over 90 degrees C. 1 and 2 can thus be used at lower substrate temperatures than 3. GeS or SnS can be deposited on substrates even at temperatures below 50 degrees C, while ALD of SnS from 3 becomes slow below substrate temperatures of 100 degrees C because of insufficient vapor pressure. The amount of growth per ALD cycle is higher for the cyclic amide 2 than for the amidinate 3. The GeS films are smooth and amorphous, while the SnS films are polycrystalline and granular. All of these films are uniformly thick inside holes with aspect ratios (depth/diameter) greater than 40:1.