We present an atomic layer deposition (ALD) process for the synthesis of tin nitride
(SnNx) thin films using tetrakis(dimethylamino) tin (TDMASn, Sn(NMe2)4) and ammonia
(NH3) as the precursors at low deposition temperatures (70-200 °C). This newly developed
ALD scheme exhibits ideal ALD features such as self-limited film growth at 150 °C.
The growth per cycle (GPC) was found to be ∼0.21 nm/cycle at 70 °C, which decreased
with increasing deposition temperature. Interestingly, when the deposition temperature
was between 125 and 180 °C, the GPC remained almost constant at ∼0.10 nm/cycle, which
suggests an ALD temperature window, whereas upon further increasing the temperature
to 200 °C, the GPC considerably decreased to ∼0.04 nm/cycle. Thermodynamic analysis
via density functional theory calculations showed that the self-saturation of TDMASn
would occur on an NH2-terminated surface. Moreover, it also suggests that the condensation
of a molecular precursor and the desorption of surface *NH2 moieties would occur at
lower and higher temperatures outside the ALD window, respectively. Thanks to the
characteristics of ALD, this process could be used to conformally and uniformly deposit
SnNx onto an ultranarrow dual-trench Si structure (minimum width: 15 nm; aspect ratio:
∼6.3) with ∼100% step coverage. Several analysis tools such as transmission electron
microscopy, X-ray diffraction (XRD), X-ray photoelectron spectroscopy, Rutherford
backscattering spectrometry, and secondary-ion mass spectrometry were used to characterize
the film properties under different deposition conditions. XRD showed that a hexagonal
SnN phase was obtained at a relatively low deposition temperature (100-150 °C), whereas
cubic Sn3N4 was formed at a higher deposition temperature (175-200 °C). The stoichiometry
of these thermally grown ALD-SnNx films (Sn-to-N ratio) deposited at 150 °C was determined
to be ∼1:0.93 with negligible impurities. The optoelectronic properties of the SnNx
films, such as the band gap, wavelength-dependent refractive index, extinction coefficient,
carrier concentration, and mobility, were further evaluated via spectroscopic ellipsometry
analysis. Finally, ALD-SnNx-coated Ni-foam (NF) and hollow carbon nanofibers were
successfully used as free-standing electrodes in electrochemical supercapacitors and
in Li-ion batteries, which showed a higher charge-storage time (about eight times
greater than that of the uncoated NF) and a specific capacity of ∼520 mAh/g after
100 cycles at 0.1 A/g, respectively. This enhanced performance might be due to the
uniform coverage of these substrates by ALD-SnNx, which ensures good electric contact
and mechanical stability during electrochemical reactions.