Effects of adatom mobility and Ehrlich–Schwoebel barrier on heteroepitaxial growth of scandium nitride (ScN) thin films

Abstract

Scandium nitride (ScN) is an emerging rock salt indirect bandgap semiconductor and has attracted significant interest in recent years for thermoelectric energy conversion, as a substrate for defect-free GaN growth, as a semiconducting component in single-crystalline metal/semiconductor superlattices for thermionic energy conversion, as well as for Al1−xScxN-based bulk and surface acoustic devices for 5G technologies. Most ScN film growth traditionally utilizes physical vapor deposition techniques such as magnetron sputtering and molecular beam epitaxy, which results in stoichiometric films but with varying crystal quality, orientations, microstructures, and physical properties. As epitaxial single-crystalline ScN films with smooth surfaces are essential for device applications, it is important to understand the ScN growth modes and parameters that impact and control their microstructure. In this Letter, we demonstrate that large adatom mobility is essential to overcome the Ehrlich–Schwoebel (E–S) and grain boundary migration barriers and achieve defect (voids, dislocations, stacking faults, etc.)-free single-crystalline ScN films. Using the substrate temperature to tune adatom mobility, we show that nominally single-crystalline ScN films are achieved when the homologous temperature is higher than ∼0.3. For homologous temperatures ranging from 0.23 to 0.30, ScN films are found to exhibit significant structural voids in between pyramidal growth regions with multiple in-plane orientations resulting from additional lateral growth off the facets of the pyramids and broken epitaxy after ∼80 nm of growth. The in-depth discussion of the growth modes of ScN presented here explains its varying electrical and optical properties and will help achieve high-quality ScN for device applications.

Liquid-Metal-Assisted Growth of Vertical GaSe/MoS2 p–n Heterojunctions for Sensitive Self-Driven Photodetectors

“…As shown in Figure c,d and Figure S6, Ga, Ga 2 Se, and GaSe 2 encounter an energy barrier (>0.6 eV), which is 0.4–0.8 eV higher than their surface diffusion barriers and makes crossing over the edge more difficult. The additional barrier is known as the ES barrier, − due to decreasing the nearest neighbor coordination number of the atom and periodicity breaking of the potential field (Figure S7), which dictates the epitaxial growth modes of thin films. − The large molecules tend to stay at the edge to nucleate and grow epitaxially, forming a vertical stack. By contrast, GaSe molecules only need to overcome a small energy barrier (<0.11 eV), thereby flipping down the side of MoS 2 more easily and contributing to the lateral growth.…”

Multifunctional Irradiation-Induced Defects for Enhancing Thermoelectric Properties of Scandium Nitride Thin Films

“…A few voids are observed in the low-magnification image (Figure 2a) that originate from the lack of adatom mobility during growth and the resultant Ehrlich−Schwoebel barrier, as was discussed in detail previously for ScN films. 51,52 STEM-EDS mapping reveals the uniform distribution of scandium and nitrogen atoms throughout the film and unintentional oxygen incorporation mostly in the voids (Figure 2b−d). Voids are also depleted of scandium and nitrogen atoms.…”

Synthesis and study of ScN thin films

“…Howbeit, nitrogen vacancies (V N ) are known to form a defect energy level at ≈1.26 eV above the valence band maxima at Γ point of the Brillouin zone [21]. In this context, it is to be mentioned here that significance of high T s depositions in supression of defects were found to be conflicting in literature [13,17,22,23], yet have not been highlighted so far.…”

“…[13][14][15][16], and few of them aided with process parameters are tabulated in Table I. As can be seen from Table I, conventional use of high substrate temperature (T s ≥ 823 K) has been an integral part during the synthesis of ScN thin film samples, possibly due to higher adatom mobility promoting enhanced crystalline defect free ScN growth [17]. In addition, intensive research attention have also been dedicated to get an insight on explicit defect contributions and microstructural growth behavior (e.g.…”

“…In terms of defects, even though it is well known that during the growth of ScN itself, finite incorporation of substitutional (O N ) and/or interstitial oxygen (O i ) is inherent regardless of T s [16,18,20], but combined study of first principles density functional theory (DFT) with site occupancy disorder technique reveals that the electronic band structure of ScN remains unaltered despite of a shift of the Fermi energy level to the bottom of the conduction band [4]. Nonetheless, only recently, the primary contribution of oxygen incorporation has also been attributed to the surface oxidation [13,17]. Howbeit, nitrogen vacancies (V N ) are known to form a defect energy level at ≈1.26 eV above the valence band maxima at Γ point of the Brillouin zone [21].…”