Characterization of Optical/Electrical Properties and Application of High Crystalline Titanium Nitride Ultrathin Films

Recently, titanium nitride (TiN) attracts a great attention as a refractory plasmonic material owing to its superior optical properties in the visible and near-infrared spectral regions, high melting temperature, low extinction loss, and excellent mechanical and chemical stabilities. TiN also possesses the epsilon-near-zero (ENZ) point in the shorter end of visible spectrum than gold. One of the many exciting significances of ENZ materials is the strong enhancement of nonlinear optical processes and even higher nonlinear optical effect can be obtained for ultrathin ENZ films or metamaterials, in sharp contrast to conventional high nonlinear response for thick or bulk materials.<br/>In order to realize these excellent performances of TiN, the growth of high quality TiN particularly in the form of ultrathin film is requisite; however, most of previous works on TiN have been reported for the films grown via reactive sputtering, pulsed laser deposition, or atomic layer deposition techniques. Only recently, single-crystalline, stoichiometric TiN films could be realized by the molecular-beam epitaxy (MBE) under the ultrahigh vacuum (UHV) environment. In this work, we will present the growth of TiN ultrathin films with 2−10 nm in thickness and the characterization of optical and electrical properties. Owing to UHV growth, the influence of oxygen-related defects which causes critical degradation of performance is greatly reduced in our MBE-grown TiN epilayers. Formation of continuous and smooth ultrathin TiN films is realized by X-ray diffraction (XRD) and the surface morphology is measured by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The cross-section transmission electron microscope (TEM) image of the 2.4 nm TiN film shows a clear atomic alignment, confirming the growth of high crystalline quality epitaxial film with thickness even down to 2.4 nm.<br/>The optical and dielectric properties of ultrathin TiN epilayers are determined by the reflection/transmittance/absorption measurement and spectroscopic ellipsometry. As the film thickness decreases below 10 nm, the ENZ point of ultrathin TiN epilayers shifts from 480 nm to 550 nm, enabling the realization of tunable plasmonic material. The optical loss also decreases with reduction of film thickness and the optical transmittance increases up to &gt;75% for the 2.4 nm film over the whole visible regime. Electrical properties of the ultrathin TiN films are explored by employing the THz time-domain spectroscopy (THz-TDS) as well as the Hall effect measurement technique. THz-TDS is an optical and non-contact method to characterize the electrical properties of various materials including nanostructured metals, whereas the Hall effect technique is a direct electrical measurement method. THz spectroscopy reveals that the percolation thickness of the MBE-grown TiN epitaxial film is less than 2.4 nm, which is much thinner than the percolation thickness (6 – 10 nm) of noble metals. More importantly, the conductivities measured by THz-TDS are in excellent agreement with those measured by the Hall effect method. This is in sharp contrast to previous reports showing a large discrepancy between optically and electrically measured conductivities for ultrathin TiN films fabricated by other techniques in atmospheric environments. The excellent electrical properties of MBE-grown ultrathin TiN films are attributed to their high crystalline quality with the low density of oxygen-related and structural defects. Our results demonstrate the wide potential applications of ultrathin TiN epilayer films as refractory plasmonic material.<br/>Additionally, high optical transmittance and electrical conductivity of MBE-grown ultrathin films demonstrate that combined with its high mechanical, thermal, and chemical stabilities, they can serve as a high performance and robust transparent and conductive metal electrode compatible to complementary metal–oxide–semiconductor (CMOS) fabrication technology.