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Pressure has an important effect on chemical bonds and their chemical properties. The atypical compounds NaCl3 and CsF3 are predicted to be stable at high pressure and show unique physical and chemical properties. By using ab initio random structure searching and density functional theory calculations, we predicted multiple thermodynamically stable atypical compounds, which are RbF2, RbF3, RbF4, and RbF5 in the pressure range of 0–300 GPa. In these stable compounds, homonuclear bondings of F3, F4, and F5 species are easily formed. The electron structure calculation showed that except for Fd-3m phase of RbF2, these stable compounds are insulators and F 5p orbitals play an important role in the Fermi level. It is interesting that the compounds RbF5 could be stable at nearly ambient pressure and 0 K which will stimulate experimental studies in the future.

In this paper, tin oxidation (SnO x )/tin-sulfide (SnS) heterostructures are synthesized by the post-oxidation of liquid-phase exfoliated SnS nanosheets in air. We comparatively analyzed the NO2 gas response of samples with different oxidation levels to study the gas sensing mechanisms. The results show that the samples oxidized at 325 °C are the most sensitive to NO2 gas molecules, followed by the samples oxidated at 350 °C, 400 °C and 450 °C. The repeatabilities of 350 °C samples are better than that of 325 °C, and there is almost no shift in the baseline. Thus this work systematically analyzed the gas sensing performance of SnO x /SnS-based sensor oxidized at 350 °C. It exhibits a high response of 171% towards 1 ppb NO2, a wide detecting range (from 1 ppb to 1 ppm), and an ultra-low theoretical detection limit of 5 ppt, and excellent repeatability at room temperature. The sensor also shows superior gas selectivity to NO2 in comparison to several other gas molecules, such as NO, H2, SO2, CO, NH3, and H2O. After x-ray diffraction, x-ray photoelectron spectroscopy, scanning electron microscopy, transmission electron microscope, and electron paramagnetic resonance characterizations combining first principle analysis, it is found that the outstanding NO2 sensing behavior may be attributed to three factors: the Schottky contact between electrodes and SnO x /SnS; active charge transfer in the surface and the interface layer of SnO x /SnS heterostructures; and numerous oxygen vacancies generated during the post-oxidation process, which provides more adsorption sites and superior bandgap modulation. Such a heterostructure-based room-temperature sensor can be fabricated in miniaturized size with low cost, making it possible for large-scale applications.

With technology computer-aided design (TCAD) simulation software, we design a new structure of gallium oxide on gallium–nitride Schottky barrier diode (SBD). The parameters of gallium oxide are defined as new material parameters in the material library, and the SBD turn-on and breakdown behavior are simulated. The simulation results reveal that this new structure has a larger turn-on current than Ga2O3 SBD and a larger breakdown voltage than GaN SBD. Also, to solve the lattice mismatch problem in the real epitaxy, we add a ZnO layer as a transition layer. The simulations show that the device still has good properties after adding this layer.

A novel O2 plasma-based digital etching technology for p-GaN/AlGaN structures without any etch-stop layer was investigated using an inductively coupled plasma (ICP) etcher, with 100 W ICP power and 40 W rf bias power. Under 40 sccm O2 flow and 3 min oxidation time, the p-GaN etch depth was 3.62 nm per circle. The surface roughness improved from 0.499 to 0.452 nm after digital etching, meaning that no observable damages were caused by this process. Compared to the dry etch only methods with Cl2/Ar/O2 or BCl3/SF6 plasma, this technique smoothed the surface and could efficiently control the etch depth due to its self-limiting characteristic. Furthermore, compared to other digital etching processes with an etch-stop layer, this approach was performed using ICP etcher and less demanding on the epitaxial growth. It was proved to be effective in precisely controlling p-GaN etch depth and surface damages required for high performance p-GaN gate high electron mobility transistors.

In this work, AlGaN/GaN HEMTs with dual-layer SiNx stressors (composed of a low-stress layer and a high-stress layer) were investigated. The low-stress padding layer solved the surface damage problem which was caused during the deposition of the high-stress SiNx and provided a good passivated interface. The HEMTs with the dual-layer stressors showed a 1 V increase in the threshold voltage (Vth) with comparable DC and RF amplification performance to the baseline devices. Moreover, the off-current (Ioff) was shown to be reduced by one to three orders of magnitude in the strained devices. The reduction in the off-currents was a result of the lower electric field in AlGaN, which suppressed the gate injection current. These improvements using the dual-layer stressor scheme supports strain engineering as an effective approach in the pursuit of the normally-off operation of AlGaN/GaN HEMTs.

Based on density functional theory calculations, it is found that for substitutional N in diamond the C 3v symmetry structure is more stable, while C 3v and D 2d symmetry patterns for the substitutional P in diamond have comparable energies. Moreover, the substitutional N is a deep donor for diamond, while P is a shallow substitutional n-type dopant. This is attributed to the different doping positions of dopant (the N atom is seriously deviated from the substitutional position, while the P atom nearly locates in the substitutional site), which are determined by the atomic radius.

Based on ab initio calculations, it is found that the donor center of substitutional sulfur (S) in diamond with C 2 v symmetry is more stable than that with C 3 v symmetry, which is different from previous reports in literature. The energy difference of C 2 v and C 3 v structures is qualitatively affected by the supercell size, and the 216-atom supercell could be proposed as the minimum to obtain stable configuration of substitutional S in diamond. Using supercells of up to 512 atoms, the donor level of substitutional S with C 2 v symmetry is deep.

In this work, the field plate termination is studied for Ga2O3 Schottky barrier diodes (SBDs) by simulation. The influence of field plate overlap, dielectric material and thickness on the termination electric field distribution are demonstrated. It is found that the optimal thickness increases with reverse bias increasing for all the three dielectrics of SiO2, Al2O3, and HfO2. As the thickness increases, the maximum electric field intensity decreases in SiO2 and Al2O3, but increases in HfO2. Furthermore, it is found that SiO2 and HfO2 are suitable for the 600 V rate Ga2O3 SBD, and Al2O3 is suitable for both 600 V and 1200 V rate Ga2O3 SBD. In addition, the comparison of Ga2O3 SBDs between the SiC and GaN counterpart reveals that for Ga2O3, the breakdown voltage bottleneck is the dielectric. While, for SiC and GaN, the bottleneck is mainly the semiconductor itself.

The energy band offsets between indium–gallium–zinc oxide (IGZO) and β-Ga2O3 were examined by angle-resolved X-ray photoelectron spectroscopy (AR-XPS). The Ga 2p spectra from the heterojunction contributed by the upper IGZO film and the β-Ga2O3 substrate were deconvoluted into two sub-peaks with the binding energy difference of 0.3 eV, in good agreement with the theoretical model. Meanwhile, the bandgaps of IGZO and β-Ga2O3 were measured to be 3.44 ± 0.1 and 4.64 ± 0.1 eV from the ultraviolet–visible (UV–vis) transmittance spectra. The valence and conduction band offsets between the IGZO and β-Ga2O3 were consequently determined to be 0.49 ± 0.05 and 0.71 ± 0.1 eV, respectively. These findings reveal that IGZO is an attractive intermediate semiconductor layer (ISL) for reducing the electron barrier height at metal/Ga2O3 interfaces.

This paper describes a technique to eject liquid droplets in almost any direction with a nozzleless self-focusing acoustic transducer (SFAT) built on a ZnO thin film as well as on a thick PZT substrate. Sectoring of the SFAT annular rings of half-wave-band sources to create a piezoelectrically inactive area causes the droplet ejections to be directed non-perpendicular (i.e., oblique) to the liquid surface. The direction of the droplet ejections depends on the size of the piezoelectrically inactive area within the area of the half-wave-band sources. Droplets are ejected from the center part of the annular rings toward the open inactive area. Various openings up to 90° of pie shape have been made and tested to show that the ejection direction becomes less vertical as the piezoelectrically inactive area in the transducer increases. Additionally, a multi-directional ejector built on ZnO film has been demonstrated to eject micron-sized liquid droplets (several microns in diameter) in any of eight predetermined directions on demand. Larger size liquid droplets (about a hundred microns in diameter) have also been directionally ejected from a sectored SFAT built on a PZT substrate.

A novel enhancement-mode AlGaN/GaN high electron mobility transistor (HEMT) is proposed and studied. Specifically, several split floating gates (FGs) with negative charges are inserted to the conventional MIS structure. The simulation results revealed that the decreases with the increase of polarization sheet charge density and the tunnel dielectric (between FGs and AlGaN) thickness, while it increases with the increase of FGs sheet charge density and blocking dielectric (between FGs and control gate) thickness. In the case of the same gate length, the will left shift with decreasing FG length. More interestingly, the split FGs could significantly reduce the device failure probability in comparison with the single large area FG structure.

Periodic hexagonal unit cell silicon nanohole (SiNH) structures with different structural dimensions have been fabricated using polystyrene spheres and investigated in terms of their microstructures and optical reflectance. Compared with a planar Si wafer, such structures exhibit much lower reflectance due to the enhanced scattering introduced by the SiNHs. Optical simulation using the finite element method has also been carried out in order to optimize the structural periodicity and hole diameter for maximum light absorption. The optimum structure is found to have a periodicity of 693 nm and hole diameter of 560 nm, and it yields the highest ultimate efficiency of 28.8%.

The finite element method is used to simulate light absorption in periodic hybrid Si nanohole (SiNH)/PEDOT:PSS arrays. The structural periodicity (P) and hole diameter (D) of the hybrid SiNH structure are varied to maximize light absorption. In terms of the solar cell performance under the AM1.5G spectrum, the highest ultimate efficiency achieved is 30.5%, when the D/P ratio is 0.8 and P is 600 nm. We have successfully fabricated the SiNH structure based on a low cost electroless chemical etching approach using a silver catalyst. The SiNH diameters formed vary from ∼200 to 300 nm, with periodicities from ∼300 to 1000 nm. The SiNH structure reveals a low average reflectance of 4% for incident light in the range 300 to 1100 nm.

In this paper, we have demonstrated the crystallization and simultaneous formation of the nanocone structure on the 1.6 µm thick amorphous silicon (a-Si) film using a one-step femtosecond laser annealing approach. The a-Si layer is crystallized after the annealing process, as confirmed from their Raman spectra. Meanwhile, the conical structures with diameters ranging from 160 nm to 1.4 µm are formed upon laser beam irradiation at different fluences, as deduced from scanning electron microscope and atomic force microscope characterization. The highest absorption is achieved when the sample is irradiated with a laser fluence of 588 mJ cm−2. The developed process has potential applications in the fabrication of high efficiency and low cost Si thin-film solar cells. Moreover, the physics and mechanism involved in the laser annealing process are also discussed.

We report on a novel TaO x -based nonpolar resistive random access memory (RRAM) cell design with a Cr/TaO x /Al (top-to-bottom) structure. Extensive studies of the switching mechanism of the nonpolar RRAM were performed. The thermal coefficient of resistance of the RRAM in the low-resistance state is observed to adopt different polarities depending on how the preceding set operation was performed. On the basis of this observation, the coexistence of metallic ions and oxygen vacancies in the dominant filament is deduced, and a hybrid filament hypothesis is proposed for the first time to explain the observations. A switching model is provided to explain the microscopic changes in the nonpolar TaO x -based RRAM.

Numerical simulation of optical absorption characteristics of gallium arsenide (GaAs) thin-film solar cells by the three-dimensional finite element method is presented, with emphasis on optimizing geometric parameters for nanowire and nanocone structures to maximize the ultimate photocurrent under AM1.5G illumination. The nanostructure-based GaAs thin-film solar cells have demonstrated a much higher photocurrent than the planar thin films owing to their much suppressed reflection and high light trapping capability. The nanowire structure achieves its highest ultimate photocurrent of 29.43 mA/cm2 with a periodicity (P) of 300 nm and a wire diameter of 180 nm. In contrast, the nanocone array structure offers the best performance with an ultimate photocurrent of 32.14 mA/cm2. The results obtained in this work provide useful guidelines for the design of high-efficiency nanostructure-based GaAs solar cells.

Investigation of solar energy harvesting in hexagonally arranged Si nanowire (NW) arrays is performed through optimizing the structural parameters, such as array periodicity (P), Si NW diameter (D) and length (L). The results demonstrate that there exist wide P and D/P 'windows' for the Si NW arrays, locating around 600 nm and 0.833 (i.e., D = 500 nm), respectively, for achieving enhanced light absorption compared to their thin film counterparts with the same thickness, but with much less materials consumption. Calculation of the ultimate efficiency (UE) indicates that the light trapping capability is not monotonically increased with L, and that UE vibration is found when L is >1000 nm. Comparison of the light absorption spectra for hexagonally and squarely arranged Si NW arrays demonstrates that these two most widely employed array symmetries in practice have little impact on the light trapping capability.

A microscopic mechanism for the unipolar resistive switching phenomenon in nickel oxides is proposed based on the thermal decomposition of oxygen ions from oxygen-rich clusters and their recombination with electron-depleted vacancies induced by local electric field in conductive filaments. The proposed physical feature is confirmed by x-ray photoelectron spectroscopy, transmission electron microscopy and electrical measurements in the as-deposited NiO x samples. The deduced formulae under reasonable approximations directly demonstrate the relationships of switching parameters that were widely observed and questioned in different material systems, indicating the universal validity of the proposed mechanism.

This paper presents a novel micromachined probe with a miniature film-bulk-acoustic-resonator (FBAR) mass sensor integrated at the tip. The detailed fabrication processes for a silicon microprobe and an SU-8 microprobe are described and discussed. The electrical performances of FBARs integrated on both the microprobes were characterized and compared. The semiconducting silicon was found to degrade the FBAR's quality factor (Q) significantly, due to the GHz electromagnetic wave's energy loss in the low-resistivity silicon. Yet, the FBAR on the SU-8 probe was measured to have a higher Q than that on the silicon probe, as the SU-8 polymer material is highly resistive and electrically insulating. As an application demonstration, the fabricated SU-8 probe was successfully used in detecting Hg2+ concentration in liquids. The integration of a resonant mass sensor with a micromachined probe offers easy access to sensing environment with minimal disturbance to the environment.

A one-dimensional Mason model is employed to investigate the second harmonic mode response of a multilayer composite film bulk acoustic resonator (FBAR), particularly the dependence of the effective coupling coefficient (K2t, eff) on material properties and relative position in the acoustic stack. The simulation results for AlN-based FBAR with electrode layer having relatively low acoustic impedance and additional temperature compensation layer reveals that the maximum values of K2t, eff are obtained with a thickness ratio (between the non-piezoelectric layer and piezoelectric layer) that is close to its acoustic velocity ratio. The fundamental mode and second harmonic mode operation of an FBAR are compared. The maximum achievable K2t, eff is comparable (5.39% versus 5.16%) for the temperature compensated FBAR (with Mo as electrodes) operating at fundamental and second harmonic modes. However, the trimming-mass and crossover temperature sensitivities of the second harmonic mode are lower, indicating its potential advantage over the fundamental mode for high frequency applications above 2 GHz (such as filters, low phase noise temperature stable oscillator applications). Experimental results on a ZnO-based FBAR (Al/ZnO/Al/SixNy) operating around 5 GHz with various thicknesses of ZnO and SixNy show good agreement with numerical modeling.

A study on the unipolar resistive switching of a CoO thin film shows that the compliance current (CC) has strong effects on the local structure of conducting filaments (CFs). Lower CCs produce CFs with simple connectivity and good controllability, resulting in a narrow distribution of switching voltages (SVs) and a high ratio of high-to-low resistance states. In contrast, the stronger net-like CFs are formed at higher CCs, and their complete rupture is difficult. Thus, the lower high-resistance states and a wide distribution of SVs appear in the reversible switching processes. Our results suggest that the generation of CFs with simple local structures is as important as the rupture process.

In this paper, the effects of structural parameters on the optical characteristics of crystalline Si thin films with the surface decorated by Si nanocone (SiNC) arrays are investigated by simulation. It is found that the SiNC base diameter should be equal to the array periodicity for efficient solar energy harvesting, and the optimized light absorption could already be realized when the SiNC height reaches ∼400nm. An ultimate efficiency of ∼31.5% can be achieved when the periodicity is ∼600nm for an 800nm thick Si film with a 400nm high nanocone array although the total thickness is only 1200nm. And the ultimate efficiency could be further increased to some extent due to the enhanced light absorption in the low energy region by appropriately increasing the Si film thickness. The underlying physics is also discussed in this work. (Some figures in this article are in colour only in the electronic version)

This paper describes a novel bonding technique using reactive multilayer Ni/Al foils as local heat sources to bond Parylene-C layers to another Parylene-C coating on a silicon wafer. Exothermic reactions in Ni/Al reactive multilayer foils were investigated by x-ray diffraction (XRD) and differential scanning calorimetry. XRD measurements showed that the dominant product after exothermic reaction was ordered B2 AlNi compound. The heat of reaction was calculated to be ?57.9?kJ?mol?1. A numerical model was developed to predict the temperature evolution in the parylene layers and silicon wafers during the bonding process. The simulation results revealed that localized heating occurred during the reactive foil joining process. Our experimental observation showed that the parylene layer was torn when the bond was forcefully broken, indicating a strong bond was achieved. Moreover, leakage test in isopropanol alcohol showed that reactive foil bonds can withstand liquid exposure. This study demonstrated the feasibility of reactive foil joining for broad applications in bio-microelectromechanical systems and microfluidic systems.