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The holy grail of achieving efficient operation of infrared (IR) rectennas continues to be the realization of a high performance rectifier. In this paper, we have fabricated metal–insulator–metal (MIM) diodes based on TiO2, ZnO, and NiO thin films using shadow mask evaporation, photolithography, and sputtering. The electron affinities of oxides have been measured by a combination of variable angle spectroscopic ellipsometry and x-ray photoelectron spectroscopy, as well as deduction from the extraction of metal/oxide barrier heights of Fowler–Nordheim tunneling plots. Our results confirm a low value for the electron affinity of NiOx of ∼2.1–2.5 eV, which correlates with the high zero-bias dynamic resistance (RD0) of ∼500 kΩ of an associated MIM diode. These values render NiOx to be unsuitable for use in a rectenna device. Better performance has been observed from diodes based on TiO2 and ZnOx films. The best rectification performance was achieved for a Au/2.6 nm ZnOx/Cr diode, scaled down to 1 μm2 device area, showing a zero-bias dynamic resistance of RD0 = 71 kΩ, zero-bias responsivity β0 = 0.28 A/W, and a coupling efficiency of ηc = 2.4 × 10−5% for rectification at 28.3 THz. The main significance of this study is that it employs a methodology whereby key parameters of the MIM stack are derived from physical measurements, which are then used to assist in the fitting of electrical current–voltage data to produce a reliable appraisal of diode performance in an IR rectenna. [ABSTRACT FROM AUTHOR]

We study the interaction between dibromobianthryl (DBBA) and the Ag(100) surface using scanning tunneling microscopy and density functional theory. DBBA is prochiral on adsorption and forms racemic domains with molecular rows aligned with the substrate nearest-neighbor [011] and [0-11] directions. Deposition at elevated temperatures leads to the formation of disordered meandering graphene nanowires of constant width. [ABSTRACT FROM AUTHOR]

The Al/HfOx/AlOy/indium tin oxide (ITO)/polyethylene terephthalate (PET) flexible resistive random access memory (ReRAM) device was fabricated at room temperature to study the effect of AlOy interfacial layer on resistive switching (RS). Incorporating the interfacial layer AlOy improves the cycle-to-cycle, cell-to-cell uniformity and switching resistance ratio (>103). It introduces the oxygen vacancies in HfOx which controls the formation and rupture of filament and plays an important role in improving RS characteristics. The device also maintains the stable switching operation under the flexible condition and at elevated temperature up to 100 °C. The device area and temperature-dependent switching characteristics confirm the Ohmic (hopping) conduction in low-resistance state (LRS) and the space charge-limited conduction (SCLC) in high-resistance state (HRS). [ABSTRACT FROM AUTHOR]

A comprehensive study of atomic-layer deposited thulium oxide (Tm2O3) on germanium has been conducted using x-ray photoelectron spectroscopy (XPS), vacuum ultra-violet variable angle spectroscopic ellipsometry, high-resolution transmission electron microscopy (HRTEM), and electron energy-loss spectroscopy. The valence band offset is found to be 3.0±60.2 eV for Tm2O3/p-Ge from the Tm 4d centroid and Ge 3p3/2 charge-corrected XPS core-level spectra taken at different sputtering times of a single bulk thulium oxide sample. A negligible downward band bending of ~0.12 eV is observed during progressive differential charging of Tm 4d peaks. The optical band gap is estimated from the absorption edge and found to be 5.77 eV with an apparent Urbach tail signifying band gap tailing at ~5.3 eV. The latter has been correlated to HRTEM and electron diffraction results corroborating the polycrystalline nature of the Tm2O3 films. The Tm2O3/Ge interface is found to be rather atomically abrupt with sub-nanometer thickness. In addition, the band line-up of reference GeO2/n-Ge stacks obtained by thermal oxidation has been discussed and derived. The observed low reactivity of thulium oxide on germanium as well as the high effective barriers for holes (~3 eV) and electrons (~2 eV) identify Tm2O3 as a strong contender for interfacial layer engineering in future generations of scaled high-κ gate stacks on Ge. [ABSTRACT FROM AUTHOR]

The Al-doped HfOx flexible resistive random access memory (ReRAM) device with Ag top electrode (TE) is fabricated on indium tin oxide (ITO) coated polyethylene terephthalate (PET) with low thermal budget process. The oxygen vacancies created by Al doping may assist Ag inclusion to create/rupture the filament at lower operating voltages (VSET ≈ 0.46 V and VRESET ≈ −0.93 V) and SET/RESET currents (ISET ≈ 2 × 10−5 A and IRESET ≈ 8 × 10−5 A). The Ag/Al:HfOx/ITO/PET ReRAM exhibits highly stable resistive switching (RS) behaviour with lower switching power (PSET ≈ 9.2 µW and PRESET ≈ 74.4 µW). The stable switching parameters like SET/RESET voltages, resistances in high resistance states (HRS) and low resistance states (LRS) are observed even at higher temperature (100 °C) and in flexible condition (i.e. 5 mm dia). The current conduction mechanism in HRS is dominated by space charge limited conduction whereas LRS is not completely Ohmic in nature. The RS mechanism has been explained by the formation of the combined effect of Ag atoms and oxygen vacancies. Considering the improved performance of the ReRAM device fabricated at low-temperature process, it may provide a promising candidate for the low power flexible electronics applications. [ABSTRACT FROM AUTHOR]

Crystalline and quasicrystalline allotropes of Pb are formed by evaporation on the fivefold surface of the icosahedral (i) Ag-In-Yb quasicrystal under ultra-high vacuum. Lead grows in three dimensional quasicrystalline order and subsequently forms fivefold-twinned islands with the fcc(111) surface orientation atop of the quasicrystalline Pb. The islands exhibit specific heights (magic heights), possibly due to the confinement of electrons in the islands. We also study the adsorption behavior of C60 on the two allotropes of Pb. Scanning tunneling microcopy reveals that a high corrugation of the quasicrystalline Pb limits the diffusion of the C60 molecules and thus produces a disordered film, similar to adsorption behavior of the same molecules on the clean substrate surface. However, the sticking coefficient of C60 molecules atop the Pb islands approaches zero, regardless of the overall C60 coverage. [ABSTRACT FROM AUTHOR]

A study into the optimal deposition temperature for ultra-thin La2O3 and Y2O3/Ge gate stacks has been conducted in this paper with the aim to tailor the interfacial layer for effective passivation of the Ge interface. A detailed comparison between the two lanthanide oxides ( La2O3 and Y2O3) in terms of band line-up, interfacial features, and reactivity to Ge using medium energy ion scattering, vacuum ultra-violet variable angle spectroscopic ellipsometry (VUV-VASE), X-ray photoelectron spectroscopy, and X-ray diffraction is shown. La2O3 has been found to be more reactive to Ge than Y2O3, forming LaGeOx and a Ge sub-oxide at the interface for all deposition temperature studied, in the range from 44 °C to 400°C. In contrast, Y2O3/Ge deposited at 400 °C allows for an ultra-thin GeO2 layer at the interface, which can be eliminated during annealing at temperatures higher than 525 °C leaving a pristine YGeOx/Ge interface. The Y2O3/Ge gate stack deposited at lower temperature shows a sub-band gap absorption feature fitted to an Urbach tail of energy 1.1 eV. The latter correlates to a sub-stoichiometric germanium oxide layer at the interface. The optical band gap for the Y2O3/Ge stacks has been estimated to be 5.7±0.1 eV from Tauc-Lorentz modelling of VUV-VASE experimental data. For the optimal deposition temperature (400 °C), the Y2O3/Ge stack exhibits a higher conduction band offset (∼2.3 eV) than the La2O3/Ge °2 eV), has a larger band gap (by about 0.3 eV), a germanium sub-oxide free interface, and leakage current (°10°7 A/cm2 at 1V) five orders of magnitude lower than the respective La2O3/Ge stack. Our study strongly points to the superiority of the Y2O3/Ge system for germanium interface engineering to achieve high performance Ge Complementary Metal Oxide Semiconductor technology. [ABSTRACT FROM AUTHOR]

Alkyl-modified silicon nanocrystallites are efficient fluorophores which are of interest for fundamental spectroscopic studies and as luminescent probes in biology because of their stability in aqueous media. In this work we have investigated these particles using scanning tunneling microscopy, synchrotron-radiation excited photoemission, and x-ray excited optical luminescence (XEOL). During the course of illumination with 145-eV photons we have monitored the evolution of the Si2p core level and, in samples which have suffered prolonged atmospheric exposure, observed in real time the growth of an extra Si2p component attributed to in situ photoinduced oxidation of the Si nanocrystallites. XEOL reveals that two emission bands are active upon soft-x-ray photon excitation and that photoluminescence intensity decreases with photon exposure, which is attributed to charge trapping within the film. [ABSTRACT FROM AUTHOR]

The surface electronic properties of bulk-grown β-Ga2O3 ( 2 ¯ 01) single crystals are investigated. The band gap is found using optical transmission to be 4.68 eV. High-resolution x-ray photoemission coupled with hybrid density functional theory calculation of the valence band density of states provides insights into the surface band bending. Importantly, the standard linear extrapolation method for determining the surface valence band maximum (VBM) binding energy is found to underestimate the separation from the Fermi level by ∼0.5 eV. According to our interpretation, most reports of surface electron depletion and upward band bending based on photoemission spectroscopy actually provide evidence of surface electron accumulation. For uncleaned surfaces, the surface VBM to Fermi level separation is found to be 4.95 ± 0.10 eV, corresponding to downward band bending of ∼0.24 eV and an electron accumulation layer with a sheet density of ∼5 × 1012 cm−2. Uncleaned surfaces possess hydrogen termination which acts as surface donors, creating electron accumulation and downward band bending at the surface. In situ cleaning by thermal annealing removes H from the surface, resulting in a ∼0.5 eV shift of the surface VBM and formation of a surface electron depletion layer with upward band bending of ∼0.26 eV due to native acceptor surface states. These results are discussed in the context of the charge neutrality level, calculated bulk interstitial hydrogen transition levels, and related previous experimental findings. [ABSTRACT FROM AUTHOR]

Amorphous Ga2O3 thin films were deposited on p-type (111) and (100) surfaces of silicon and (100) germanium by atomic layer deposition (ALD). X-ray photoelectron spectroscopy (XPS) was used to investigate the band alignments at the interfaces using the Kraut Method. The valence band offsets were determined to be 3.49± 0.08 eV and 3.47± 0.08 eV with Si(111) and Si(100) respectively and 3.51eV± 0.08 eV with Ge(100). Inverse photoemission spectroscopy (IPES) was used to investigate the conduction band of a thick Ga2O3 film and the band gap of the film was determined to be 4.63±0.14 eV. The conduction band offsets were found to be 0.03 eV and 0.05eV with Si(111) and Si(100) respectively, and 0.45eV with Ge(100). The results indicate that the heterojunctions of Ga2O3 with Si(100), Si(111) and Ge(100) are all type I heterojunctions. [ABSTRACT FROM AUTHOR]

The effect of fluorine doping on the switching stability of Ta2O5 resistive random access memory devices is investigated. It shows that the dopant serves to increase the memory window and improve the stability of the resistive states due to the neutralization of oxygen vacancies. The ability to alter the current in the low resistance state with set current compliance coupled with large memory window makes multilevel cell switching more favorable. The devices have set and reset voltages of <1V with improved stability due to the fluorine doping. Density functional modeling shows that the incorporation of fluorine dopant atoms at the two-fold O vacancy site in the oxide network removes the defect state in the mid bandgap, lowering the overall density of defects capable of forming conductive filaments. This reduces the probability of forming alternative conducting paths and hence improves the current stability in the low resistance states. The doped devices exhibit more stable resistive states in both dc and pulsed set and reset cycles. The retention failure time is estimated to be a minimum of 2 years for F-doped devices measured by temperature accelerated and stress voltage accelerated retention failure methods. [ABSTRACT FROM AUTHOR]

The role of nitrogen doping on the stability and memory window of resistive state switching in N-doped Ta2O5 deposited by atomic layer deposition is elucidated. Nitrogen incorporation increases the stability of resistive memory states which is attributed to neutralization of electronic defect levels associated with oxygen vacancies. The density functional simulations with the screened exchange hybrid functional approximation show that the incorporation of nitrogen dopant atoms in the oxide network removes the O vacancy midgap defect states, thus nullifying excess defects and eliminating alternative conductive paths. By effectively reducing the density of vacancy-induced defect states through N doping, 3-bit multilevel cell switching is demonstrated, consisting of eight distinctive resistive memory states achieved by either controlling the set current compliance or the maximum voltage during reset. Nitrogen doping has a threefold effect: widening the switching memory window to accommodate the more intermediate states, improving the stability of states, and providing a gradual reset for multi-level cell switching during reset. The N-doped Ta2O5 devices have relatively small set and reset voltages (<1V) with reduced variability due to doping. [ABSTRACT FROM AUTHOR]