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Register a new userSynthesis of a new ternary nitride semiconductor -- Zn$_2$VN$_3$: A combinatorial exploration of the Zn-V-N phase space
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Computationally guided high-throughput synthesis is used to explore the Zn-V-N phase space, resulting in the synthesis of a novel ternary nitride Zn$_2$VN$_3$. Following a combinatorial PVD screening, we isolate the phase and synthesize polycrystalline Zn$_2$VN$_3$ thin films on conventional borosilicate glass substrates. In addition, we demonstrate that cation-disordered, but phase-pure (002)-textured, Zn$_2$VN$_3$ thin films can be grown using epitaxial stabilization on {alpha}-Al2O3 (0001) substrates at remarkably low growth temperatures well below 200 {deg}C. The structural properties and phase composition of the Zn$_2$VN$_3$ films are studied in detail using XRD and (S)TEM techniques. The composition as well as chemical state of the constituent elements are studied using RBS/ERDA as well as XPS/HAXPES methods. These analyses reveal a stoichiometric material with no oxygen contamination, besides a thin surface oxide. We find that orthorhombic Zn$_2$VN$_3$ is a weakly-doped p-type semiconductor demonstrating broadband room-temperature photoluminescence spanning the range between 2 eV and 3 eV, consistent with the bandgap of similar magnitude predicted by density functional theory-based calculations. In addition, the electronic properties can be tuned over a wide range via isostructural alloying on the cation site, making this a promising material for optoelectronic applications.
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Diluted ferromagnetic semiconductors (DMSs) that combine the properties of semiconductors with ferromagnetism have potential application in spin-sensitive electronics (spintronics) devices. The search for DMS materials exploded after the observation of ferromagnetic ordering in III-V (Ga,Mn)As films. Recently, a series of DMS compounds isostructural to iron-based superconductors have been reported. Among them, the highest Curie temperature $T_C$ of 230 K has been achieved in (Ba,K)(Zn,Mn)$_2$As$_2$. However, most DMSs, including (Ga,Mn)As, are p-type, i.e., the carriers that mediate ferromagnetism are holes. For practical applications, DMS with n-type carriers are also advantageous. Here we report the successful synthesis of a II-II-V diluted ferromagnetic semiconductor with n-type carriers, Ba(Zn,Co)$_2$As$_2$. Magnetization measurements show that the ferromagnetic transition occurs up to $T_{C} sim$ 45 K. Hall effect and Seebeck effect measurements jointly confirm that the dominant carriers are electrons. Through muon spin relaxation ($mu$SR), a volume sensitive magnetic probe, we have also confirmed that the ferromagnetism in Ba(Zn,Co)$_2$As$_2$ is intrinsic and the internal field is static.
We propose to replace Ga in (Ga,Mn)As with Li and Zn as a route to high Curie temperature, carrier mediated ferromagnetism in a dilute moment n-type semiconductor. Superior material characteristics, rendering Li(Zn,Mn)As a realistic candidate for such a system, include unlimited solubility of the isovalent substitutional Mn impurity and carrier concentration controlled independently of Mn doping by adjusting Li-(Zn,Mn) stoichiometry. Our predictions are anchored by detail ab initio calculations and comparisons with the familiar and directly related (Ga,Mn)As, by the microscopic physical picture we provide for the exchange interaction between Mn local moments and electrons in the conduction band, and by analysis of prospects for the controlled growth of Li(Zn,Mn)As materials.
Zinc-based nitride CaZn2N2 films grown by molecular beam epitaxy (MBE) with a plasma-assisted active nitrogen-radical source are promising candidates of next-generation semiconductors for light-emitting diodes and solar cells. This nitride compound has previously only been synthesized in a bulk form by ultrahigh-pressure synthesis at 5 GPa. Three key factors have been found to enable heteroepitaxial film growth: (i) precise tuning of the individual flux rates of Ca and Zn, (ii) the use of GaN template layers on sapphire c-plane as substrates, and (iii) the application of MBE with an active N-radical source. Because other attempts at physical vapor deposition and thermal annealing processes have not produced CaZn2N2 films of any phase, this rf-plasma-assisted MBE technique represents a promising way to stabilize CaZn2N2 epitaxial films. The estimated optical band gap is ~1.9 eV, which is consistent with the value obtained from bulk samples. By unintentional carrier doping, n- and p-type electronic conductions are attained with low carrier densities of the order of 1013 /cm3. These features represent clear advantages when compared with Zn-based oxide semiconductors, which usually have much higher carrier densities irrespective of their intentionally undoped state. The carrier mobilities at room temperature are 4.3 cm2/(Vs) for electrons and 0.3 cm2/(Vs) for hole carriers, which indicates that transport properties are limited by grain boundary scattering, mainly because of the low-temperature growth at 250 {deg}C, which realizes a high nitrogen chemical potential.
A new diluted ferromagnetic semiconductor (Sr,Na)(Zn,Mn)2As2 is reported, in which charge and spin doping are decoupled via Sr/Na and Zn/Mn substitutions, respectively, being distinguished from classic (Ga,Mn)As where charge & spin doping are simultaneously integrated. Different from the recently reported ferromagnetic (Ba,K)(Zn,Mn)2As2, this material crystallizes into the hexagonal CaAl2Si2-type structure. Ferromagnetism with a Curie temperature up to 20 K has been observed from magnetization. The muon spin relaxation measurements suggest that the exchange interaction between Mn moments of this new system could be different to the earlier DMS systems. This system provides an important means for studying ferromagnetism in diluted magnetic semiconductors.
Polycrystalline samples of A$_x$V$_2$Al$_{20}$ (A = Ce, Th, U, Np, Pu; 0.7 =< x =< 1.0) actinide intermetallics were synthesized using the arc-melting method. Crystal structure studies were performed by means of powder x-ray diffraction and the Rietveld refinement method. All studied compounds crystallize in the CeCr$_{2}$Al$_{20}$-type structure (space group Fd-3m, no. 227) with the actinide or Ce atom located in the oversized cage formed by Al atoms. Comparison of the crystallographic results with the reported data for LnV$_{2}$Al$_{20}$ (Ln = lanthanoids) counterparts reveals distinctly different behavior of the lanthanide- and actinide-bearing compounds. This difference is suggested to be caused by fairly localized character of the 4f electrons, whereas itinerant character of the 5f electrons is likely seen for U- and Np-containing phases. Magnetic susceptibility and specific heat measurements did not reveal any magnetic ordering in U$_{0.8}$V$_{2}$Al$_{20}$, Np$_{0.8}$V$_{2}$Al$_{20}$ and Pu$_{0.8}$V$_{2}$Al$_{20}$ down to 2.5 K. A small anomaly in low-temperature specific heat of Ce$_{0.8}$V$_{2}$Al$_{20}$, U$_{0.8}$V$_{2}$Al$_{20}$, and Np$_{0.8}$V$_{2}$Al$_{20}$ is observed, likely arising from a low-energy Einstein mode.
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