Volume 115, Issue 1, 07 January 2014
Index of content:
This study investigates the onset of islanding (Stranski-Krastanow transition) in strained pure germanium (Ge) and dilute silicon-germanium (SiGe) alloy layers grown by chemical vapour deposition on Si(001) substrates. Integration of compositional profiles is compared to a novel method for quantification of X-ray maps acquired in cross-sectional scanning transmission electron microscopy, together with simulations of surface segregation of Ge. We show that Si1−x Ge x alloys for germanium concentrations x ≤ 0.27 grow two-dimensionally and stay flat up to considerable layer thicknesses, while layers with concentrations in the range 0.28 < x ≤ 1 form islands after deposition of ∼3.0/x monolayers (=quarter unit cells in the diamond lattice, ML). The uncertainty in the amount of deposited material for pure Ge is ±(0.2–0.3) ML. Modelling shows that of the amount of germanium deposited, 0.7 ML segregate towards the free surface so that only ∼2.3/x ML are directly incorporated in the layer within a few nanometres, in good agreement with our measurements. For pure Ge (x = 1), this thickness is smaller than most values quoted in the literature, which we attribute to the high sensitivity of our method to fractional monolayer changes in the effective chemical width of such thin layers.
- Lasers, Optics, and Optoelectronics
115(2014); http://dx.doi.org/10.1063/1.4861144View Description Hide Description
193-nm excimer laser interaction with ZnO single-crystal at 0.05–500 KW/cm2 intensities is investigated under ultra high vacuum conditions by time resolved photoluminescence (PL) spectroscopy. A dominant 3.18 eV PL emission band at 295 K is observed. This band shows unusually long 0.52 ± 0.01 μs lifetime, indicating a defect mediated emission mechanism. The demonstrated negative thermal quenching for this band confirms its free electron to acceptor type transition. The involved acceptor is attributed to zinc vacancy with ∼100 meV shallow acceptor state. This study finds that 193-nm interactions produce Zn vacancies in transient states at 0.05–50 KW/cm2 excitation intensities and in stable state at 500 KW/cm2 or above intensities. The transient zinc vacancy production at such low intensities further validates the Frenkel pair creation as mechanism for creating these defects in ZnO single-crystal.
Characterization of 2.3 μm GaInAsSb-based vertical-cavity surface-emitting laser structures using photo-modulated reflectance115(2014); http://dx.doi.org/10.1063/1.4861146View Description Hide Description
We report angle dependent and temperature dependent (9 K–300 K) photo-modulated reflectance (PR) studies on vertical-cavity surface-emitting laser (VCSEL) structures, designed for 2.3 μm mid-infrared gas sensing applications. Changing the temperature allows us to tune the energies of the quantum well (QW) transitions relative to the VCSEL cavity mode (CM) energy. These studies show that this VCSEL structure has a QW-CM offset of 21 meV at room temperature. Consequently the QW ground-state transition comes into resonance with the CM at 220 ± 2 K. The results from these PR studies are closely compared with those obtained in a separate study of actual operating devices and show how the PR technique may be useful for device optimisation without the necessity of having first to process the wafers into working devices.
Study of gain and photoresponse characteristics for back-illuminated separate absorption and multiplication GaN avalanche photodiodes115(2014); http://dx.doi.org/10.1063/1.4861148View Description Hide Description
The gain and photoresponse characteristics have been numerically studied for back-illuminated separate absorption and multiplication (SAM) GaN avalanche photodiodes (APDs). The parameters of fundamental models are calibrated by simultaneously comparing the simulated dark and light current characteristics with the experimental results. Effects of environmental temperatures and device dimensions on gain characteristics have been investigated, and a method to achieve the optimum thickness of charge layer is obtained. The dependence of gain characteristics and breakdown voltage on the doping concentration of the charge layer is also studied in detail to get the optimal charge layer. The bias-dependent spectral responsivity and quantum efficiency are then presented to study the photoresponse mechanisms inside SAM GaN APDs. It is found the responsivity peak red-shifts at first due to the Franz-Keldysh effect and then blue-shifts due to the reach-through effect of the absorption layer. Finally, a new SAM GaN/AlGaN heterojunction APD structure is proposed for optimizing SAM GaN APDs.
An effective medium study of surface plasmon polaritons in nanostructured gratings using attenuated total reflection115(2014); http://dx.doi.org/10.1063/1.4856255View Description Hide Description
Recent work studied surface plasmon resonances in structured materials by the method of attenuated total reflection using a prism on top of a metallic grating. That calculation considered Transverse Magnetic polarized radiation, involved an expansion in 121 Fourier modes, and found a number of interesting features. Many of these features were attributed to localized plasmons or other factors, which arise from a discrete structure. We use a simple effective medium theory to address the same problem, and find many of the same reflection features observed in the more complex calculation, indicating that localization is not an important factor. We also evaluate the possibility of using some of the new features in the reflection spectrum for bio-sensing and find that the sensitivity of the system to small changes in relative permittivity is increased compared to some standard methods.
- Plasmas and Electrical Discharges
Experimental investigations of electron density and ion energy distributions in dual-frequency capacitively coupled plasmas for Ar/CF 4 and Ar/O 2/CF 4 discharges115(2014); http://dx.doi.org/10.1063/1.4859595View Description Hide Description
The electron density and ion energy distribution (IED) are investigated in low-pressure dual-frequency capacitively coupled Ar/CF 4 (90%/10%) and Ar/O 2/CF 4 (80%/10%/10%) plasmas. The relations between controllable parameters, such as high-frequency (HF) power, low-frequency (LF) power and gas pressure, and plasma parameters, such as electron density and IEDs, are studied in detail by utilizing a floating hairpin probe and an energy resolved quadrupole mass spectrometer, respectively. In our experiment, the electron density is mainly determined by the HF power and slightly influenced by the LF power. With increasing gas pressure, the electron density first goes up rapidly to a maximum value and then decreases at various HF and LF powers. The HF power also plays a considerable role in affecting the IEDs under certain conditions and the ion energy independently controlled by the LF source is discussed here. For clarity, some numerical results obtained from a two-dimensional fluid model are presented.
- Structural, Mechanical, Thermodynamic, and Optical Properties of Condensed Matter
115(2014); http://dx.doi.org/10.1063/1.4859455View Description Hide Description
Two doses (1013 and 1015 cm−2) of tungsten (W) atoms were implanted in different Si(001) wafers in order to study W diffusion in Si. The samples were annealed or oxidized at temperatures between 776 and 960 °C. The diffusion profiles were measured by secondary ion mass spectrometry, and defect formation was studied by transmission electron microscopy and atom probe tomography. W is shown to reduce Si recrystallization after implantation and to exhibit, in the temperature range investigated, a solubility limit close to 0.15%–0.2%, which is higher than the solubility limit of usual metallic impurities in Si. W diffusion exhibits unusual linear diffusion profiles with a maximum concentration always located at the Si surface, slower kinetics than other metals in Si, and promotes vacancy accumulation close to the Si surface, with the formation of hollow cavities in the case of the higher W dose. In addition, Si self-interstitial injection during oxidation is shown to promote W-Si clustering. Taking into account these observations, a diffusion model based on the simultaneous diffusion of interstitial W atoms and W-Si atomic pairs is proposed since usual models used to model diffusion of metallic impurities and dopants in Si cannot reproduce experimental observations.
115(2014); http://dx.doi.org/10.1063/1.4859599View Description Hide Description
Strained Ge islands have been grown on fully relaxed Si0.5 Ge 0.5 substrate by pulsed laser ablation technique. The formation of strained Ge islands has been found for film with higher thickness following Stranski–Krastanov growth mechanism. The variation of strain with changing Ge layer thickness has been analyzed using Raman spectroscopy and high-resolution X-ray diffraction techniques. X-ray photoelectron spectra have shown the absence of any Si-Ge intermixing and oxidation of Ge films. A strong no-phonon photoluminescence emission from Ge islands has been observed, showing the superior optical characteristics of the islands grown on relaxed substrate.
115(2014); http://dx.doi.org/10.1063/1.4861153View Description Hide Description
The ignition of aluminum particles under high pressure and temperature conditions is considered. The laser ablation method is used to generate oxide-free aluminum particles exposed to pressures ranging between 0.35 and 2.2 GPa. A continuous wave CO2 laser radiation heats the surface of the aluminum target until ignition is observed. We confirm ignition by a spectroscopic analysis of AlO vibronic band of 484 nm wavelength, and the radiant temperature is measured with respect to various pressures for estimating the heating energy for ignition. The ignition characteristics of the oxide-free aluminum particles exposed to extremely high pressures are reported.
On shock response of nano-void closed/open cell copper material: Non-equilibrium molecular dynamic simulations115(2014); http://dx.doi.org/10.1063/1.4861029View Description Hide Description
Non-equilibrium molecular-dynamic simulations were carried out on model three-dimensional nano-void copper material with different idealised pore structure and porosity to highlight differences in response behaviour between them when subjected to various piston velocities simulating planar shock loading of different intensities. This article demonstrates and explains from a mechanistic perspective the differences in response observed with respect to Hugoniot elastic limits, dislocation line and jet formation, void collapse mechanism and hot spot generation, specific volume, partial recrystallisation and temperature evolution in void collapsed regions, shock and particle velocity curves.
An analysis of lead-free (Bi0.5Na0.5)0.915-(Bi0.5K0.5)0.05Ba0.02Sr0.015TiO3 ceramic for efficient refrigeration and thermal energy harvesting115(2014); http://dx.doi.org/10.1063/1.4861031View Description Hide Description
This article demonstrates the colossal energy harvesting capability of a lead-free (Bi0.5 Na 0.5)0.915-(Bi0.5K0.5)0.05Ba0.02Sr0.015TiO3 ceramic using the Olsen cycle. The maximum harvestable energy density estimated for this system is found to be 1523 J/L (1523 kJ/m3) where the results are presented for extreme ambient conditions of 20–160 °C and electric fields of 0.1–4 MV/m. This estimated energy density is 1.7 times higher than the maximum reported to date for the lanthanum-doped lead zirconate titanate (thin film) system. Moreover, this study introduces a generalized and effective solid state refrigeration cycle in contrast to the ferroelectric Ericson refrigeration cycle. The cycle is based on a temperature induced polarization change on application of an unipolar electric field to ferroelectric ceramics.
115(2014); http://dx.doi.org/10.1063/1.4861034View Description Hide Description
Optical phonons are measured to probe the origins of the reported anomalously high piezoelectric response in aluminum scandium nitride (Al 1− x Sc x N). Epitaxial layers with 0 ≤ x ≤ 0.16 deposited on sapphire(0001) exhibit a refractive index below the band gap, which increases from 2.03 for x = 0 to 2.16 for x = 0.16, corresponding to a dielectric constant ε∞ = 4.15 + 3.2x. Raman scattering shows that zone-center E2(H) and A1(TO) phonon modes shift to lower frequencies with increasing x, following linear relationships: ω(E2(H)) = 658–233x (cm−1) and ω(A1(TO)) = 612–159x (cm−1). Similarly, zone-center E1(TO) and A1(LO) phonon mode frequencies obtained from specular polarized infrared reflectance measurements red-shift to ω(E1(TO)) = 681–209x (cm−1) and ω(A1(LO)) = 868–306x (cm−1). The measured bond angle decreases linearly from 108.2° to 106.0°, while the length of the two metal-nitrogen bonds increase by 3.2% and 2.6%, as x increases from 0 to 0.16. This is associated with a 3%–8% increase in the Born effective charge and a simultaneous 6% decrease in the covalent metal-N bond strength, as determined from the measured vibrational frequencies described with a Valence-Coulomb-Force-Field model. The overall results indicate that bonding in Al-rich Al 1− x Sc x N qualitatively follows the trends expected from mixing wurtzite AlN with metastable hexagonal ScN. However, extrapolation suggests non-linear composition dependencies in bond angle, length, and character for x ≥ 0.2, leading to a structural instability that may be responsible for the reported steep increase in the piezoelectric response.
115(2014); http://dx.doi.org/10.1063/1.4861147View Description Hide Description
The effects of dislocation climb on plastic deformation during loading and unloading are studied using a two-dimensional discrete dislocation dynamics model. Simulations are performed for polycrystalline thin films passivated on both surfaces. Dislocation climb lowers the overall level of the stress inside thin films and reduces the work hardening rate. Climb decreases the density of dislocations in pile-ups and reduces back stresses. These factors result in a smaller Bauschinger effect on unloading compared to simulations without climb. As dislocations continue to climb at the onset of unloading and the dislocation density continues to increase, the initial unloading slope increases with decreasing unloading rate. Because climb disperses dislocations, fewer dislocations are annihilated during unloading, leading to a higher dislocation density at the end of the unloading step.
- Electronic Structure and Transport
Charge transport mechanisms of graphene/semiconductor Schottky barriers: A theoretical and experimental study115(2014); http://dx.doi.org/10.1063/1.4859500View Description Hide Description
Graphene has been proposed as a material for semiconductor electronic and optoelectronic devices. Understanding the charge transport mechanisms of graphene/semiconductor Schottky barriers will be crucial for future applications. Here, we report a theoretical model to describe the transport mechanisms at the interface of graphene and semiconductors based on conventional semiconductor Schottky theory and a floating Fermi level of graphene. The contact barrier heights can be estimated through this model and be close to the values obtained from the experiments, which are lower than those of the metal/semiconductor contacts. A detailed analysis reveals that the barrier heights are as the function of the interface separations and dielectric constants, and are influenced by the interfacial states of semiconductors. Our calculations show how this behavior of lowering barrier heights arises from the Fermi level shift of graphene induced by the charge transfer owing to the unique linear electronic structure.
115(2014); http://dx.doi.org/10.1063/1.4859755View Description Hide Description
First-principles calculations are performed to inspect the electronic and transport properties of a Fe-thiacrown molecular device, namely, a Au-Fe(9S3)2-Au junction. It is found that the junction has a low-spin (LS) ground state and a high-spin (HS) metastable state. Further study shows that the HS state is a conducting state while the LS state is a nearly insulating one, which means that a switch between these two spin configurations results in a good electrical switching behavior and can serve as an ON/OFF state for a logic unit. Thus, it may find applications as switches or memories in molecular electronic circuits.
115(2014); http://dx.doi.org/10.1063/1.4861130View Description Hide Description
Experimental measurements of optical and electronic properties and local-density approximation (LDA) calculations on polycrystalline Ga3−xIn5+xSn2O16—the so-called “T-phase” in the Ga2O3-In2O3-SnO2 ternary system—have revealed it to be a good candidate for n-type transparent conducting oxide applications, particularly in the replacement of tin-doped indium oxide as a transparent electrode in organic photovoltaics. Room temperature conductivity of over 1000 S cm−1 was measured in polycrystalline bulk samples. Band structure calculations reveal a highly dispersed conduction band, corresponding to an electron effective mass of about 0.2 me . Normalized carrier mobility and concentration trends indicate that conductivity changes in T-phase are attributable to changes in carrier concentration, with mobility remaining relatively constant through the range of processing conditions and sample composition. Screened exchange LDA calculations yield a fundamental band gap of about 2.60 eV. A relatively constant optical band gap in the range of 2.9–3.0 eV along the range of T-phase composition was measured by diffuse reflectance of bulk samples, whereas ab-initio simulations predict a decreasing fundamental band gap with increasing In-to-Ga ratio. This is attributed to an increasing Burstein-Moss shift—corresponding to increasing free electron concentration—with increasing In-to-Ga ratio.
The dynamics of photoinduced defect creation in amorphous chalcogenides: The origin of the stretched exponential function115(2014); http://dx.doi.org/10.1063/1.4861143View Description Hide Description
The article discusses the dynamics of photoinduced defect creations (PDC) in amorphous chalcogenides, which is described by the stretched exponential function (SEF), while the well known photodarkening (PD) and photoinduced volume expansion (PVE) are governed only by the exponential function. It is shown that the exponential distribution of the thermal activation barrier produces the SEF in PDC, suggesting that thermal energy, as well as photon energy, is incorporated in PDC mechanisms. The differences in dynamics among three major photoinduced effects (PD, PVE, and PDC) in amorphous chalcogenides are now well understood.
115(2014); http://dx.doi.org/10.1063/1.4861176View Description Hide Description
Based on nonequilibrium Green's functions in combination with density-functions theory, the transport properties of armchair graphene nanoribbon (AGNR) devices were investigated, in which one lead is undoped armchair graphene nanoribbons, and the other is phosphorus (P)-doped armchair graphene nanoribbons. The results manifest that there is the rectification behavior with large rectifying ratio in the AGNR devices and the rectification characteristics can be modulated by changing the width of the graphene nanoribbons. On the contrary, for the same width of the graphene nanoribbons, the position of P dopant has little or no effect on changing I-V characteristics.
- Magnetism and Superconductivity
Effect of superconducting spacer layer thickness on magneto-transport and magnetic properties of La0.7Sr0.3MnO3/YBa2Cu3O7/La0.7Sr0.3MnO3 heterostructures115(2014); http://dx.doi.org/10.1063/1.4861139View Description Hide Description
We have studied the magneto-transport and magnetic properties of LSMO/YBCO/LSMO trilayers on LaAlO3 (001) substrate, deposited using pulsed laser deposition technique. From x-ray diffraction measurements, it is confirmed that the grown trilayer films are single phase natured. The temperature dependent resistivity shows a metallic behavior below 350 K. At low temperature from resistivity fitted data, we observe that electron-electron, electron-phonon, and electron-magnon interactions are the main factors for scattering of carriers. The ferromagnetic LSMO layers suppress the critical temperature of YBCO spacer layer. We observe maximum magnetoresistance value ∼49% at 250 K for LSMO(200 nm)/YBCO(50 nm)/LSMO(200 nm) trilayer. Magnetization measurements reveal that at room temperature the YBCO spacer layer is allowing the LSMO layers to interact antiferromagnetically.
Superconducting and magneto-transport properties of BiS2 based superconductor PrO1-xFxBiS2 (x = 0 to 0.9)115(2014); http://dx.doi.org/10.1063/1.4859535View Description Hide Description
We report superconducting properties of PrO1-xFxBiS2 compounds, synthesized by the vacuum encapsulation technique. The synthesized PrO1-xFxBiS2 (x = 0.1, 0.3, 0.5, 0.7, and 0.9) samples are crystallized in a tetragonal P4/nmm space group. Both transport and DC magnetic susceptibility measurements showed bulk superconductivity below 4 K. The maximum Tc is obtained for x = 0.7 sample. Under applied magnetic field, both Tc onset and Tc (ρ = 0) decrease to lower temperatures. We estimated highest upper critical field [Hc 2 (0)] for PrO0.3F0.7BiS2 sample to be above 4 T (Tesla). The thermally activated flux flow activation energy (U 0) is estimated 54.63 meV in 0.05 T field for PrO0.3F0.7BiS2 sample. Hall measurement results showed that electron charge carriers are the dominating ones in these compounds. Thermoelectric effects (Thermal conductivity and Seebeck coefficient) data suggest strong electron-electron correlations in this material.
115(2014); http://dx.doi.org/10.1063/1.4859597View Description Hide Description
The magnetic susceptibility of solid oxygen had long been observed only in the restricted pressure region below 0.8 GPa. We succeeded in extending the pressure region up to 3.3 GPa by clamping condensed oxygen in the sample chamber of a miniature diamond anvil cell and measuring the dc magnetic susceptibility using a superconducting quantum interference device magnetometer. In this experiment, the well-known α–β and β–γ transitions are observed in the phase diagram, suggesting consistency with the previous results of X-ray and Raman studies. In addition, a new magnetic anomaly is observed in the β phase.