Low-Temperature Atomic Layer Deposition of Highly Conformal Tin Nitride Thin Films for Energy Storage Devices

Abstract

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 degrees C). This newly developed ALD scheme exhibits ideal ALD features such as self-limited film growth at 150 degrees C. The growth per cycle (GPC) was found to be similar to 0.21 nm/cycle at 70 degrees C, which decreased with increasing deposition temperature. Interestingly, when the deposition temperature was between 125 and 180 degrees C, the GPC remained almost constant at similar to 0.10 nm/cycle, which suggests an ALD temperature window, whereas upon further increasing the temperature to 200 degrees C, the GPC considerably decreased to similar 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: similar to 6.3) with similar to 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 degrees C), whereas cubic Sn3N4 was formed at a higher deposition temperature (175-200 degrees C). The stoichiometry of these thermally grown ALD-SnNx films (Sn-to-N ratio) deposited at 150 degrees C was determined to be similar to 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 similar to 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.