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Nickel-doped β-Ga2O3 crystals were investigated by optical absorption and photoconductivity, revealing Ni-related deep levels. The photoconductivity spectra were fitted using the phenomenological Kopylov and Pikhtin model to identify the energy of the zero-phonon transition (thermal ionization), Franck–Condon shift, and effective phonon energy. The resulting values are compared with the predicted ones by first-principle calculations based on the density functional theory (DFT). An acceptor level (0/−) of 1.9 eV and a donor level (+/0) of 1.1 eV above the valence band minimum are consistently determined for NiGa, which preferentially incorporates on the octahedrally coordinated Ga site. Temperature-dependent resistivity measurements yield a thermal activation energy of ∼2.0 eV that agrees well with the determined Ni acceptor level. Conclusively, Ni is an eminently suitable candidate for compensation doping for producing semi-insulating β-Ga2O3 substrates due to the position of the acceptor level (below and close to the mid-bandgap). [ABSTRACT FROM AUTHOR]

In this work, we explored the growth regime of (100) β-Ga2O3 homoepitaxial films on substrates with different miscut angles (1°, 2°, and 4°) in the MOVPE system. Under a low O2/Ga ratio growth condition, step-flow growth of (100) β-Ga2O3 homoepitaxial films can be maintained up to 3 μm on substrates with different miscut angles. Moreover, the results reveal that the growth rate decreases slightly with decreasing miscut angles, which matches estimation of the Burton–Cabrera–Frank theory and can be explained by the model of adsorption–desorption. By comparing the miscut-dependent growth rates, we give experimental evidence on the fundamental difference between the growth of (100) β-Ga2O3 films by MOVPE and MBE. In addition, a transport model is proposed to explain the desorption process in terms of the boundary layer and the kinetic resistance. [ABSTRACT FROM AUTHOR]

In this work, we shed light on the mechanism behind the complexity of the anomalous Hall effect (AHE) in the rare earth-transition metal (RE-TM) compound. To achieve a comprehensive picture, we first theoretically investigate the pure elemental metals and observe that the AHE of RE Gd is negative while that of TM Co is positive. We then investigate magnetic properties and AHE behavior for two typical cases of TM-rich and RE-rich compositions that correspond to GdCo5 and GdCo3 alloys, respectively. The observation that net magnetization is not a decisive factor in assessing the AHE value is consistent with recent claims. Besides, our investigations reveal a significant role of the Gd sublattice to the AHE. The reduction AHE signal under a high-applied field is attributed to the counteraction between the positive and negative AHC contributions when the TM and RE sub-moments are aligned parallel. On a bright note, our new findings can be successfully employed to unravel two types of aberrant humps of AHE signals near compensation temperature. [ABSTRACT FROM AUTHOR]

We have systematically studied the growth, by the Czochralski method, and basic physical properties of a 2 cm and 2 in. diameter bulk β-(AlxGa1−x)2O3 single crystal with [Al] = 0–35 mol. % in the melt in 5 mol. % steps. The segregation coefficient of Al in the Ga2O3 melt of 1.1–1.2 results in a higher Al content in the crystals than in the melt. The crystals were also co-doped with Si or Mg. [Al] = 30 mol. % in the melt (33–36 mol. % in the crystals) seems to be a limit for obtaining bulk single crystals of high structural quality suitable for homoepitaxy. The crystals were either semiconducting (no intentional co-dopants with [Al] = 0–30 mol. % and Si-doped with [Al] = 15–20 mol. %), degenerately semiconducting (Si-doped with [Al] ≤ 15 mol. %), or semi-insulating ([Al] ≥ 25 mol. % and/or Mg-doped). The full width at half maximum of the rocking curve was 30–50 arcsec. The crystals showed a linear but anisotropic decrease in all lattice constants and a linear increase in the optical bandgap (5.6 eV for [Al] = 30 mol. %). The room temperature electron mobility at similar free electron concentrations gradually decreases with [Al], presumably due to enhanced scattering at phonons as the result of a larger lattice distortion. In Si co-doped crystals, the scattering is enhanced by ionized impurities. Measured electron mobilities and bandgaps enabled to estimate the Baliga figure of merit for electronic devices. [ABSTRACT FROM AUTHOR]

Probing and controlling the valley degree of freedom in graphene systems by transport measurements has been a major challenge to fully exploit the unique properties of this two-dimensional material. In this theoretical work, we show that this goal can be achieved by a quantum-wire geometry made of gapped graphene that acts as a valley filter with the following favorable features: (i) all electrical gate control, (ii) electrically switchable valley polarity, (iii) robustness against configuration fluctuation, and (iv) potential for room temperature operation. This valley filtering is accomplished by a combination of gap opening in either bilayer graphene with a vertical electrical field or single layer graphene on h-BN, valley splitting with a horizontal electric field, and intervalley mixing by defect scattering. In addition to functioning as a building block for valleytronics, the proposed configuration makes it possible to convert signals between electrical and valleytronic forms, thus allowing for the integration of electronic and valleytronic components for the realization of electro-valleytronics. [ABSTRACT FROM AUTHOR]

We explored a Mg-alloyed ZnO material (or MgZnO alloy) on a lattice-matched scandium aluminum magnesium oxide substrate for obtaining most effective photodetectors with highest (∼10 A/W) and fastest (up to 3 GHz) responses and two orders of magnitude UV/VIS rejection ratio. Device operation was related to its material electronic properties (carrier lifetime, diffusivity, and diffusion length) via photoluminescence and light-induced pump–probe techniques. Defect-related response is found suitable for sensitive logarithmic detectors, while exciton-related response is favorable to ultrafast linear UV solar blind detectors. Two-contact planar detector design allows their application in low-cost environmental detection systems. [ABSTRACT FROM AUTHOR]

In this study, a 300 °C-annealed 3 × 4 V/ZnO multilayer structure demonstrates the lowest resistivity (3.82 × 10−3 Ω cm) and the highest mobility (18 cm2/V s) among the studied V/ZnO multilayer structures. By measuring the energy bandgap (Eg), work function (Φ), and electron affinity (χ) by ultraviolet photoelectron spectroscopy and photoluminescence analysis, the corresponding energy band diagram at the ZnVxOy/ZnO interface can be constructed. A potential is observed at the ZnVxOy/ZnO interface, which induces the two-dimensional electron gas (2DEG) effect, and this is attributed to the high-mobility conduction path. The potential well directly relates to the Φ difference between the ZnO and ZnVxOy layers, which is determined to be 0.22, 0.46, and −0.1 eV for the as-deposited, 300 °C-annealed, and 500 °C-annealed V/ZnO multilayer structures, respectively. The 300 °C-annealed V/ZnO multilayer structure could possibly have the largest depth in the potential well. This supports the 2DEG mechanism for the high mobility of the 300 °C-annealed V/ZnO multilayer structure. [ABSTRACT FROM AUTHOR]

We investigate the current gain of quantum-well heterojunction bipolar transistors (QW-HBTs) under different operation temperatures. It is experimentally shown that the introduction of a QW into the base region of HBTs results in an increasing current gain with the temperature, which is a behavior opposite to that of typical HBTs. An enhancement of current gain around 226% is observed as the temperature of QW-HBT is raised from 35 to 95 °C. This unusual trend is mainly attributed to the shorter escape time of electrons in the QW at the higher temperature. A modified charge-control model based on thermionic emissions of electrons in the QW region is constructed to model this temperature-dependent phenomenon, and the result agrees well with our experimental observations. [ABSTRACT FROM AUTHOR]

We investigate the effects of surface tunneling on electrostatics and transport properties of two-dimensional electron gases (2DEGs) in undoped Si/SiGe heterostructures with different 2DEG depths. By varying the gate voltage, four stages of density-mobility dependence are identified with two density saturation regimes observed, which confirms that the system transitions between equilibrium and nonequilibrium. Mobility is enhanced with an increasing density at low biases and, counterintuitively, with a decreasing density at high biases as well. The density saturation and mobility enhancement can be semiquantitatively explained by a surface tunneling model in combination with a bilayer screening theory. [ABSTRACT FROM AUTHOR]

Recently, ultralarge (>10%) strain with fully reversible elastic deformation has been experimentally achieved in silicon nanowires [H. Zhang et al., Sci. Adv. 2, e1501382 (2016)]. With this breakthrough, here in this work, based on the first principles calculation, the structural and electric properties of silicon under ultralarge strain are comparatively investigated. Unlike previous theoretical/simulation investigations on silicon nanowires with only a few nanometers, bulk silicon models are employed here to provide more realistic and comparable results to our experimentally tested samples (∼100 nm diameter). Strong anisotropic effects are induced by loading strain along all different orientations. Simultaneously, the band structures evolution demonstrates electronic anisotropy with the loading strain on three orientations. Silicon keeps an indirect bandgap under increased strain along the ⟨ 100 ⟩ orientation while transforming to a direct bandgap with strain along ⟨ 110 ⟩ and ⟨ 111 ⟩ orientations. Furthermore, ultralarge strain on these two orientations would diminish the bandgap and result into metallization. These results provide insights into understanding "elastic strain engineering" of silicon nanowire applications and demonstrate the possibility of tuning the electronic and optical properties through pure mechanical straining of functional materials. [ABSTRACT FROM AUTHOR]

In this work, we report a simple method for solution-processed organic light emitting devices (OLEDs), where single-layer graphene acts as the anode and the hexa-peri-hexabenzocoronene exfoliating agent (HBC-6ImBr) provides surface modification. In SEM images, the PEDOT:PSS solution fully covered the graphene electrode after coating with HBC-6ImBr. The fabricated solution-processed OLEDs with a single-layer graphene anode showed outstanding brightness at 3182 cd/m² and current efficiency up to 6 cd/A which is comparable to that of indium tin oxide films, and the OLED device brightness performance increases six times compared to tri-layer graphene treated with UV-Ozone at the same driving voltage. This method can be used in a wide variety of solution-processed organic optoelectronics on surface-modified graphene anodes. [ABSTRACT FROM AUTHOR]

Reflectance spectroscopy was utilized to examine the light hole and heavy hole exciton splitting of ZnSe1-xSx epilayers that were grown on misoriented gallium arsenide (001) substrates. The composition of the layers were used to determine both exciton energies. Results showed that an increase in the incorporation of sulphur was noted due to misorientation of the substrate in both thin and thick epilayers.

Research was conducted to examine quantized transitions in a Zn(sub 0.79)Cd(sub 0.21)Se/ZnSe double quantum well using the contactless electroreflectance (CER) technique. This molecular beam epitaxy technique was used to develop the quantum well. Results show the existence of several intersub-band transitions in the CER spectra. Excitonic transitions were also seen between the heavy-hole band state and the conduction band state.

Raman scattering is used for examining the longitudinal-optical-phonon and plasmon coupling (LOPC) as a function of applied pressure for n-type ZnSe:Cl layers. The pressure-induced weakening of the longitudinal-optical-phonon-plasmon coupling efficiency has shown that pressure has tended to degrade the n-type characteristic of ZnSe:Cl due to the emergence of the new deep donorlike state.

Atomic force microscopy, photoluminescence (PL), and photoluminescence excitation (PLE) measurements were used to investigate the InGaN quantum dots (QDs) deposited on Si[N.sub.x] nanomasks. It was found that the size of QDs could be controlled by Si[N.sub.x] nanomasks, enabling the manipulation of quantum confinement effect.

The morphology of basal plane dislocations (BPDs) in 4H-SiC homoepitaxial layers was investigated using plane-view transmission x-ray topography and molten KOH etching. Based on the morphologies three types of BPDs are distinguished which include interfacial dislocations, curved dislocations and circular loop dislocations around micropipes.

Picosecond ultrasonic experiments were performed to generate and detect acoustic pulse in piezoelectric (111) epilayers of [In.sub.x][Ga.sub.1-x]As. It was found that the sound velocity exhibits an obvious bowing with different alloy compositions and supports a nonlinear compositional dependence of the elastic constants of [In.sub.x][Ga.sub.1-x]As.

The microstructural evolution occurring within the eutectic SnPb solder during electromigration is investigated utilizing Blech specimens. Three-dimensional simulation is performed to examine the change in resistance and current-density distribution during electromigration.

A feasible "lightning-shaped" zigzag graphene nanoribbon (ZGNR) structure for planar interconnects is proposed. Based on the density functional theory and non-equilibrium Green's function, the electron transport properties are evaluated. The lightning-shaped structure increases significantly the conductance of the graphene interconnect with an odd number of zigzag chains. This proposed technique can effectively utilize the linear I-V characteristic of asymmetric ZGNRs for interconnect application. Variability study accounting for width/length variation and the edge effect is also included. The transmission spectra, transmission eigenstates, and transmission pathways are analyzed to gain the physical insights. This lightning-shaped ZGNR enables all 2D material-based devices and circuits on flexible and transparent substrates. [ABSTRACT FROM AUTHOR]

The quest of obtaining sustainable, clean energy is an ongoing challenge. While silicon-based solar cells have widespread acceptance in practical commercialization, continuous research is important to expand applicability beyond fixed-point generation to other environments while also improving power conversion efficiency (PCE), stability, and cost. In this work, graphene-on-silicon Schottky junction and graphene-insulator-silicon (GIS) solar cells are demonstrated on flexible, thin foils, which utilize the electrical conductivity and optical transparency of graphene as the top transparent contact. Multi-layer graphene was grown by chemical vapor deposition on Cu-Ni foils, followed by p-type doping with Au nanoparticles and encapsulated in poly(methyl methacrylate), which showed high stability with minimal performance degradation over more than one month under ambient conditions. Bendable silicon film substrates were fabricated by a kerf-less exfoliation process based on spalling, where the silicon film thickness could be controlled from 8 to 35 lm based on the process recipe. This method allows for re-exfoliation from the parent Si wafer and incorporates the process for forming the backside metal contact of the solar cell. GIS cells were made with a thin insulating Al2O3 atomic layer deposited film, where the thin Al2O3 film acts as a tunneling barrier for holes, while simultaneously passivating the silicon surface, increasing the minority carrier lifetime from 2 to 27 μs. By controlling the Al2O3 thickness, an optimized cell with 7.4% power conversion efficiency (PCE) on a 35 lm thick silicon absorber was fabricated. [ABSTRACT FROM AUTHOR]

Large magnetoresistance (MR) has been widely reported in the A2B3 (A=Sb or Bi; B=Se or Te) family of topological insulators (TIs). Sb2Se3is not a TI that was confirmed by the extracted zero Berry phase and the X-ray diffraction. An extremely large MR was observed in the Sb2Se3 crystals. This large MR increased quadratically with the magnetic field applied. The observed MR ratio was 830% at 10K and 9 T, which was larger than that previously reported for all A2B3 family TIs. This large MR originated from two carriers with high mobility. The inversely square root of the MR ratio was proportional to the resistance. [ABSTRACT FROM AUTHOR]

A study investigates the influence of operation voltages and doping densities on superlattice infrared photodetectors (SLIPs) with a single current blocking layer. The results show that the measurement data and the simulation results are identical for devices applied under different voltages and with different quantum-well doping densities.

Research describing dielectric relaxation in strontium bismuth tantalate vanadate layered perovskite is presented. In particular the dielectric constants and dc conductivity of the compound are investigated.

Cd1-x-yZnxMnyTe alloys were studied using capacitance and dissipation factor measurements, to determine dielectric properties.

Research describing the local oxidation of silicon nitride films is presented. Scanning auger microscopy is used to investigate the chemical changes induced by a conductive probe atomic force microscope.

A new study compares the fatigue properties of (Pb0.6Sr0.4)TiO3 thin films deposited with a buffer layer of (La0.5Sr0.5)MnO3 materials with those of films grown using (La0.5Sr0.5)CoO3 materials as a buffer layer.

Magnetic characterization is performed on the cubic phases Lambda-Li0.07Mn2O4, Li0.98M2O4, and the tetragonal phase Li1.82Mn2O4, prepared by electrochemical cycling of a single starting LiMn2O4 spinel. The results show a spin-glass behavior observed in all three materials in nearly stoichiometric LiMn2O4 and a short-range antiferromagnetic correlation is observed in Li2Mn2O4 at temperatures below the paramagnetic temperature regime, which is ascribed to collective Jahn-Teller distortion causing a 2D nearest-Mn-neighbors interaction.

A new study uses a liquid phase method to investigate the effect of crystal orientation and doping on the activation energy for GaAs oxide growth.

Results indicate that strong lateral composition modulation induces regions with different In content in the short-period-superlattice structures. Data further show that modification of this lattice thermal expansion leads to abnormal band-gap energies.

Capacitance and dissipation factor measurements of Zn1xMnxSe epilayers were taken at temperatures from 5 to 475 K and from 20 Hz to 100 kHz frequencies. A thermally activated relaxation contribution to the dielectric response, which requires the presence of charge redistribution, was found. Consistency in the process of activation achieved from the dissipation factor and capacitance was also observed.

Energy-dispersive x-ray diffraction studies were performed to analyze the structure of phase transitions under high pressure. It was observed that the zinc blende to rock salt phase transition pressures of Zn0.93Mn0.07Se and Zn0.76Mn0.24Se bulk crystals are 11.8 +/- 1.5 and 9.9 +/- 0.5 GPa, respectively. The respective bulk moduli are 61.8 +/- 0.8 and 60.5 +/- 0.8 GPa. The pressure-induced zinc blende to rock salt structure phase transition was seen as an indication of the semiconductor to metal transition for Zn(sub 1-x)Mn(sub x)Se.

Research was conducted to examine the effects of thermal annealing on photoluminescence and structural properties of (ZnSe)2(CdSe)n short-period-superlattices multiple quantum wells (SPS MQW). Molecular beam epitaxy using high-resolution transmission electron microscopy and photoluminescence was used for the development of SPS MQW. Results indicate that SPS structures remain in the quantum well regions of as-grown SPS MQW but intermix and disappear in the well regions of annealed (ZnSe)2(CdSe)n SPS MQW.

Electromigration behavior in the eutectic SnAg3.8Cu0.7 solder stripes was investigated in the vicinity of the device operation temperature of 100 °C by using the edge displacement technique. Measurements were made for relevant parameters for electromigration of the solder, such as drift velocity, threshold current density, activation energy, as well as the product of diffusivity and effective charge number (DZ*). The threshold current densities were estimated to be 4.3×104 A/cm2 at 80 °C, 3.2×104 A/cm2 at 100 °C, and 1.4×104 A/cm2 at 120 °C. These values represent the maximum current densities that the SnAg3.8Cu0.7 solder can carry without electromigration damage at the three stressing temperatures. The electromigration activation energy was determined to be 0.45 eV in the temperature range of 80–120 °C. The measured products of diffusivity and the effective charge number, DZ*, were -1.8×10-10 cm2/s at 80 °C, -5.0×10-10 cm2/s at 100 °C, and -7.2×10-10 cm2/s at 120 °C. [ABSTRACT FROM AUTHOR]

The photoluminescence characteristics of Zn(sub 1-x)Mg(sub x)Se epilayers grown on GaAs substrates with various misorientation angles by molecular beam epitaxy were studied. Photoluminescence data and electron-probe microanalysis showed that the Mg incorporation in the Zn(sub 1-x)Mg(sub x)Se epilayer decreases with increasing misorientation angle. Also, the photoluminescence spectrum indicated that the full width at half maxima of the band-edge excitonic emission and the intensity of the defect-related donro-acceptor emission in Zn(sub 1-x)Mg(sub x)Se epilayers substantially decreased when a substrate with misorientation angle of 15 deg was applied.

The manner by which thermal annealing affects the interdiffusion of Zn1-xCdxSe/ZnSe quantum wells and the outdiffusion of atoms from the gallium arsenide substrate into the ZnSe-based II-VI heterostructures was analyzed. The methods used in the study were photoluminescence, contactless electroreflectance and secondary ion mass spectroscopy. It was observed that there was degradation of the quantum well after a high-temperature anneal caused by the cadmium interdiffusion in the well regions and a potent gallium outdiffusion from the substrate to the epitaxial side.

A study was conducted to show that the emission wavelength of (InP)2/(GaP)2 short-period superlattice quantum wells can be changed by altering the tilt angle of the substrate or by adjusting the interruption time during switching. Variable temperature photoluminescence spectroscopy was carried out to characterize the samples. Results indicated that the growth mechanism can be determined through the bonding preference between the group III adatoms on the surface.

Spectral analysis of optically excited currents in single-crystal (100)InAs/amorphous (a-)Al2O3/metal structures allows one to separate contributions stemming from the internal photoemission (IPE) of electrons into alumina and from the trapping-related displacement currents. IPE spectra suggest that the out-diffusion of In and, possibly, its incorporation in a-Al2O3 lead to the development of ≈0.4eV wide conduction band (CB) tail states. The top of the InAs valence band is found at 3.45±0.10eV below the alumina CB bottom, i.e., at the same energy as at the GaAs/a-Al2O3 interface. This corresponds to the CB and the valence band offsets at the InAs/a-Al2O3 interface of 3.1±0.1eV and 2.5±0.1eV, respectively. However, atomic-layer deposition of alumina on InAs results in additional low-energy electron transitions with spectral thresholds in the range of 2.0-2.2eV, which is close to the bandgap of AlAs. The latter suggests the interaction of As with Al, leading to an interlayer containing Al-As bonds providing a lower barrier for electron injection. [ABSTRACT FROM AUTHOR]

Spectral analysis of optically excited currents in single-crystal (100)InAs/amorphous (a-)Al2O3/metal structures allows one to separate contributions stemming from the internal photoemission (IPE) of electrons into alumina and from the trapping-related displacement currents. IPE spectra suggest that the out-diffusion of In and, possibly, its incorporation in a-Al2O3 lead to the development of ≈0.4eV wide conduction band (CB) tail states. The top of the InAs valence band is found at 3.45±0.10eV below the alumina CB bottom, i.e., at the same energy as at the GaAs/a-Al2O3 interface. This corresponds to the CB and the valence band offsets at the InAs/a-Al2O3 interface of 3.1±0.1eV and 2.5±0.1eV, respectively. However, atomic-layer deposition of alumina on InAs results in additional low-energy electron transitions with spectral thresholds in the range of 2.0-2.2eV, which is close to the bandgap of AlAs. The latter suggests the interaction of As with Al, leading to an interlayer containing Al-As bonds providing a lower barrier for electron injection. [ABSTRACT FROM AUTHOR]

Tunable surface plasmon resonance (SPR) and dipole cavity plasmon modes of the scattering cross section (SCS) spectra on the single solid-gold/gold-shell nanorod have been numerically investigated by using the finite element method. Various effects, such as the influence of SCS spectra under x- and y-polarizations on the surface of the single solid-gold/gold-shell nanorod, are discussed in detail. With the single gold-shell nanorod, one can independently tune the relative SCS spectrum width by controlling the rod length and rod diameter, and the surface scattering by varying the shell thickness and polarization direction, as well as the dipole peak energy. These behaviors are consistent with the properties of localized SPRs and offer a way to optically control and produce selected emission wavelengths from the single solid-gold/gold-shell nanorod. The electric field and magnetic distributions provide us a qualitative idea of the geometrical properties of the single solid-gold/gold-shell nanorod on plasmon resonance. [ABSTRACT FROM AUTHOR]

Although the diffusion control and dopant activation of Ge p-type junctions are straightforward when using B+ implantation, the use of the heavier BF2+ ions or even BF+ is still favored in terms of shallow junction formation and throughput—because implants can be done at higher energies, which can give higher beam currents and beam stability—and thus the understanding of the effect of F co-doping becomes important. In this work, we have investigated diffusion and end-of-range (EOR) defect formation for B+, BF+, and BF2+ implants in crystalline and pre-amorphized Ge, employing rapid thermal annealing at 600 °C and 800 °C for 10 s. It is demonstrated that the diffusion of B is strongly influenced by the temperature, the presence of F, and the depth of amorphous/crystalline interface. The B and F diffusion profiles suggest the formation of B-F complexes and enhanced diffusion by interaction with point defects. In addition, the strong chemical effect of F is found only for B in Ge, while such an effect is vanishingly small for samples implanted with F alone, or co-implanted with P and F, as evidenced by the high residual F concentration in the B-doped samples after annealing. After 600 °C annealing for 10 s, interstitial-induced compressive strain was still observed in the EOR region for the sample implanted with BF+, as measured by X-ray diffraction. Further analysis by cross-sectional transmission electron microscopy showed that the {311} interstitial clusters are the majority type of EOR defects. The impact of these {311} defects on the electrical performance of Ge p+/n junctions formed by BF+ implantation was evaluated. [ABSTRACT FROM AUTHOR]