Granular metals with SiNxdielectrics.

Understanding and controlling nanoscale interface phenomena, such as band bending and secondary phase formation, is crucial for electronic device optimization. In granular metal (GM) studies, where metal nanoparticles are embedded in an insulating matrix, the importance of interface phenomena is frequently neglected. We demonstrate that GMs can serve as an exemplar system for evaluating the role of secondary phases at interfaces through a combination of x-ray photoemission spectroscopy (XPS) and electrical transport studies. We investigated SiNxas an alternative to more commonly used oxide-insulators, as SiNx-based GMs may enable high temperature applications when paired with refractory metals. Comparing Co-SiNxand Mo-SiNxGMs, we found that, in the tunneling-dominated insulating regime, Mo-SiNxhad reduced metal-silicide formation and orders-of-magnitude lower conductivity. XPS measurements indicate that metal-silicide and metal-nitride formation are mitigatable concerns in Mo-SiNx. Given the metal-oxide formation seen in other GMs, SiNxis an appealing alternative for metals that readily oxidize. Furthermore, SiNxprovides a path to metal-nitride nanostructures, potentially useful for various applications in plasmonics, optics, and sensing.

Chiral photocurrent in a Quasi-1D TiS3(001) phototransistor.

The presence of in-plane chiral effects, hence spin-orbit coupling, is evident in the changes in the photocurrent produced in a TiS3(001) field-effect phototransistor with left versus right circularly polarized light. The direction of the photocurrent is protected by the presence of strong spin-orbit coupling and the anisotropy of the band structure as indicated in NanoARPES measurements. Dark electronic transport measurements indicate that TiS3is n-type and has an electron mobility in the range of 1-6 cm2V-1s-1.I-Vmeasurements under laser illumination indicate the photocurrent exhibits a bias directionality dependence, reminiscent of bipolar spin diode behavior. Because the TiS3contains no heavy elements, the presence of spin-orbit coupling must be attributed to the observed loss of inversion symmetry at the TiS3(001) surface.

The effect of metal-insulator interface interactions on electrical transport in granular metals.

We present an in-depth study of metal-insulator interfaces within granular metal (GM) films and correlate their interfacial interactions with structural and electrical transport properties. Nominally 100 nm thick GM films of Co and Mo dispersed within yttria-stabilized zirconia (YSZ), with volumetric metal fractions (φ) from 0.2-0.8, were grown by radio frequency co-sputtering from individual metal and YSZ targets. Scanning transmission electron microscopy and DC transport measurements find that the resulting metal islands are well-defined with 1.7-2.6 nm average diameters and percolation thresholds betweenφ= 0.4-0.5. The room temperature conductivities for theφ= 0.2 samples are several orders of magnitude larger than previously-reported for GMs. X-ray photoemission spectroscopy indicates both oxygen vacancy formation within the YSZ and band-bending at metal-insulator interfaces. The higher-than-predicted conductivity is largely attributed to these interface interactions. In agreement with recent theory, interactions that reduce the change in conductivity across the metal-insulator interface are seen to prevent sharp conductivity drops when the metal concentration decreases below the percolation threshold. These interface interactions help interpret the broad range of conductivities reported throughout the literature and can be used to tune the conductivities of future GMs.

Effect of Band Symmetry on Photocurrent Production in Quasi-One-Dimensional Transition-Metal Trichalcogenides.

Photocurrent production in quasi-one-dimensional (1D) transition-metal trichalcogenides, TiS3(001) and ZrS3(001), was examined using polarization-dependent scanning photocurrent microscopy. The photocurrent intensity was the strongest when the excitation source was polarized along the 1D chains with dichroic ratios of 4:1 and 1.2:1 for ZrS3 and TiS3, respectively. This behavior is explained by symmetry selection rules applicable to both valence and conduction band states. Symmetry selection rules are seen to be applicable to the experimental band structure, as is observed in polarization-dependent nanospot angle-resolved photoemission spectroscopy. Based on these band symmetry assignments, it is expected that the dichroic ratios for both materials will be maximized using excitation energies within 1 eV of their band gaps, providing versatile polarization sensitive photodetection across the visible spectrum and into the near-infrared.

Precursor-surface interactions revealed during plasma-enhanced atomic layer deposition of metal oxide thin films by in-situ spectroscopic ellipsometry.

We find that a five-phase (substrate, mixed native oxide and roughness interface layer, metal oxide thin film layer, surface ligand layer, ambient) model with two-dynamic (metal oxide thin film layer thickness and surface ligand layer void fraction) parameters (dynamic dual box model) is sufficient to explain in-situ spectroscopic ellipsometry data measured within and across multiple cycles during plasma-enhanced atomic layer deposition of metal oxide thin films. We demonstrate our dynamic dual box model for analysis of in-situ spectroscopic ellipsometry data in the photon energy range of 0.7-3.4 eV measured with time resolution of few seconds over large numbers of cycles during the growth of titanium oxide (TiO2) and tungsten oxide (WO3) thin films, as examples. We observe cyclic surface roughening with fast kinetics and subsequent roughness reduction with slow kinetics, upon cyclic exposure to precursor materials, leading to oscillations of the metal thin film thickness with small but positive growth per cycle. We explain the cyclic surface roughening by precursor-surface interactions leading to defect creation, and subsequent surface restructuring. Atomic force microscopic images before and after growth, x-ray photoelectron spectroscopy, and x-ray diffraction investigations confirm structural and chemical properties of our thin films. Our proposed dynamic dual box model may be generally applicable to monitor and control metal oxide growth in atomic layer deposition, and we include data for SiO2 and Al2O3 as further examples.

The electronic band structure of quasi-one-dimensional van der Waals semiconductors: the effective hole mass of ZrS3 compared to TiS3.

The band structure of the quasi-one-dimensional transition metal trichalcogenide ZrS3(001) was investigated using nanospot angle resolved photoemission spectroscopy (nanoARPES) and shown to have many similarities with the band structure of TiS3(001). We find that ZrS3, like TiS3, is strongly n-type with the top of the valence band ∼1.9 eV below the Fermi level, at the center of the surface Brillouin zone. The nanoARPES spectra indicate that the top of the valence band of the ZrS3(001) is located at [Formula: see text]. The band structure of both TiS3 and ZrS3 exhibit strong in-plane anisotropy, which results in a larger hole effective mass along the quasi-one-dimensional chains than perpendicular to them.