Effect Of Spacer Thickness Research Articles

Low field transport calculation of 2-dimensional electron gas in β-(AlxGa1−x)2O3/Ga2O3 heterostructures

β -Gallium oxide (β-Ga2O3) is an emerging wide bandgap semiconductor with potential applications in power and RF electronic systems. Previous theoretical calculation on a two-dimensional electron gas (2DEG) in β-(AlxGa1−x)2O3/Ga2O3 heterostructures, taking only polar optical and remote impurity scattering into account, shows improved mobility compared to bulk β-Ga2O3. However, the experimental results in 2DEGs have not achieved the predicted mobility values. In this work, we perform more comprehensive calculations to study the low field 2DEG transport properties in the β-(AlxGa1−x)2O3/Ga2O3 heterostructures. A self-consistent Poisson–Schrödinger simulation of a heterostructure is used to obtain the sub-band energies and the wavefunctions in the quantum well. The phonon dispersion is calculated based on the ab initio methods under the density functional theory and density functional perturbation theory frameworks. The different scatterings that are included in the calculation are due to phonons (polar and non-polar), remote impurities, the alloy disorder, and interface roughness. We include a full dynamic screening of polar optical phonons. We report the temperature dependent low-field electron mobility as a function of 2DEG density. The overall mobility is found to be increasing with electron density with an exception at low density where the antiscreening of LO phonons reduces mobility. The effect of spacer thickness, aluminum fraction, and roughness parameters on mobility is shown to be critically important. The effect of the confinement direction on 2DEG mobility is found to be small and comparable to bulk. A comparison of calculated mobility values with experimentally reported data shows a good agreement.

Ultrafast and wide tunable VCSEL using graphene passive cavity

A novel approach for achieving ultrafast and wide tunable vertical-cavity surface-emitting lasers (VCSELs) is presented. The design is based on incorporating multiple graphene flakes separated by SiO2 dielectric in a λ/2 cavity and placing multiquantum wells in one of the quarter wave layers of the bottom mirrors. The device concept enhances both the wavelength tuning range and tuning speed which is attractive for many photonic applications. By applying a gate voltage on the graphene flakes, the effective refractive index of the cavity can be changed with ultrahigh speed. We have shown that 2 graphene flakes inside a 5 µm-diameter passive cavity provides tuning speed as high as 32 GHz and wavelength tuning range of 3.8 nm. The tuning speed increases to 52 GHz when the device diameter is reduced to 3 µm. The effect of spacer thickness and the number of graphene flakes on the performance characteristics of the device is investigated. Our analysis shows that tuning speed as high as 2 GHz and wavelength tuning range of ~ 12 nm is obtained for VCSEL that incorporates 6 graphene flakes separated by 10 nm SiO2 spacers and biased in parallel. The tuning speed can be increased to ~ 16 GHz when the flakes are biased in series at the expense of higher gate voltage. A tradeoff between the wavelength tuning range and the tuning speed is obtained.

Composite Metamaterials for THz Perfect Absorption

The design, characterization, and experimental demonstrations are presented for triple‐band, near‐perfect, composite metamaterials that absorb in the terahertz regime. Three absorption peaks are observed at 0.537 THz, 0.948 THz, and 1.59 THz with the respective absorption coefficients of 0.957, 0.988, and 0.96. The effect of spacer thickness on absorption is analyzed with interference theory, and the conditions for unity absorption are obtained via comparison of amplitude and phase. The effect of the length (l) of four outer parts of Jerusalem cross on the absorption is analyzed via transmission line theory. Furthermore, the absorption effects of capacitance and inductance caused by the changing l are examined. The calculated multi‐band transmission line model corresponded well to simulations. The effect of the outside length on the absorber performance is further explained by electric field distributions, which is experimentally confirmed. Overall, the consistent conclusions provide design guidance for metamaterial absorbers, or the realization of tunable absorbers, and provide a further understanding of the mechanisms.

Effect of tunneling barrier as spacer on exchange coupling of CoFeB/AlO x/Co trilayer structures

Magnetic tunneling junctions (MTJs) have a sandwiched structure, which comprises a top ferromagnetic (FM1) layer, an insulating tunneling layer (spacer), and a bottom ferromagnetic (FM2) layer. Exchange coupling in MTJs has been extensively widely examined because the effect of spacer thickness on the ferromagnetic spin-coupling can be exploited in read-head sensors, spin-valve structures, and magntoresistance random access memories (MRAMs). In this investigation, MTJs were deposited in the sequence, glass/CoFeB(50 Å)/AlO x ( d)/Co(100 Å), where the thickness of the AlO x layer d = 12, 17, 22, 26 or 30 Å. Saturation magnetization ( M s) results demonstrate that the exchange coupling strength and coercivity ( H c) can be varied considerably by varying the tunneling barrier AlO x spacer. The X-ray diffraction patterns (XRD) include a main peak from hexagonal close-packed (HCP) Co with a highly (0 0 2) textured structure at 2 θ = 44.7°, and AlO x and CoFeB are amorphous phases. The full width at half maximum (FWHM) of the Co (0 0 2) peak declines as the AlO x thickness increases, revealing that the Co layer becomes more crystalline. The magnetic results reveal that the magnetic characteristics are related to the Co crystallinity. The exchange coupling strength increases with AlO x thickness. The coercivity ( H c) also increases, because the Co crystallinity is eliminated.

Lateral alignment of InGaAs quantum dots as function of spacer thickness

The effects of spacer thickness on lateral alignment and density of InGaAs quantum dots on GaAs(311)B substrates is investigated. As the thickness of the spacer layers is increased, the two-dimensional lateral ordering previously demonstrated on GaAs(311)B is replaced by the one-dimensional dot chains normally observed on GaAs(100). Additionally, the dot density is found to increase proportionally with spacer thickness. The transition of lateral alignment regimes results from two processes competing to dominate the growth mechanism: the elastic anisotropy of the matrix and the characteristics of surface diffusion.

The Effect of Spacer Thicknesses on Si-Based Resonant Interband Tunneling Diode Performance and Their Application to Low-Power Tunneling Diode SRAM Circuits

Si-based resonant interband tunneling diodes (RITD) with spacer thicknesses varying from 1 to 16 nm were grown and fabricated. The effect of spacer thickness on the peak-to-valley current ratio (PVCR), peak current density Jp, and voltage swing was studied. By increasing the tunneling spacer thickness up to 16 nm, RITDs with a J p of as low as 20 mA/cm2 with an associated PVCR of 1.6 were obtained, which are suitable for low-power tunnel diode SRAM applications. With the previously reported highest RITD Jp of 218 kA/cm2, a Jp spanning nearly seven orders of magnitude can be obtained by engineering the tunneling spacer thickness and doping densities, thus demonstrating tremendous flexibility to optimize Jp for different circuit applications (logic, memory, and mixed-signal). Using a low-current-density RITD developed in this paper, a bread-boarded one-transistor tunneling-based SRAM (TSRAM) memory cell with low standby power consumption was demonstrated. This is the first report of a Si-based TSRAM memory circuit using Si-based RITDs. The result demonstrates the potential of Si-based tunnel diodes for low-power memory applications

Effects of Spacer Thickness on Noise Performance of Bipolar Transistors

The effects of spacer thickness on noise performance of a bipolar junction transistor with different emitter widths and operation frequencies are examined. The minimum noise figure (NF/sub min/) derived from the Y-parameters as well as the base (r/sub B/) and emitter resistance (r/sub E/) obtained from the device simulation is used as a measure of noise characteristics. Furthermore, the noise resistance (R/sub n/), optimum source admittance (Y/sub sop/), and the associated gain (G/sub A,assc/) are also given in this brief. To achieve the minimum value of NF/sub min/, the spacer thickness should be targeted to an optimal value, and its value is frequency and geometry dependent.

Effects of spacer thickness on quantum efficiency of the solar cells with embedded Ge islands in the intrinsic layer

We report on the effects of spacer thickness on the external quantum efficiency (EQE) of the solar cells with Ge islands embedded into the intrinsic region of the Si-based p-i-n diode. The EQE response of the solar cells in the near-infrared region is dependent on the spacer thickness that separates the layers of self-assembled Ge islands. It was found that the EQE response has an optimum value when the spacer thickness can sustain a good vertical ordering of islands. On the other hand, random nucleation of islands due to a thicker spacer layer exhibits an inferior EQE response. Furthermore, a drastic decrease of the EQE response of the solar cells for a thinner spacer layer was observed.

低減速炉稠密流路におけるスペーサ効果基礎実験 : 中性子ラジオグラフィ法によるフローパターンと沸騰熱伝達の同時測定(FR2 計測制御の最先端)

Fundamental thermal-hydraulic experiments have been conducted within a tube for flow boiling characteristics study in the Reduced-Moderation Water Reactor (RMWR) core. It is necessary to study geometrical influences of fuel rod spacer on critical power are necessary to study for thermal-hydraulic design development of RMWR core rod assemblies. In the present study, the effects of spacer thickness and spacer-rod clearance on high-void fraction two-phase flow and heat transfer are focused. Neutron radiography(NRG) technique was used to visualize high-void fraction two-phase flow with sufficient spatial resolution. Thermo-couples on the inside surface of a heated tube were used. An electrically heated tube was used with a plate blockage of various thickness under atmospheric pressure conditions. The results show that the boiling heat transfer inside the spacer was higher than downstream, far from the spacer. The forced flow deviation by the spacer was observed with the NRG visualization.

Effects of spacer thickness on optical properties of stacked Ge/Si quantum dots grown by chemical vapor deposition

Photoluminescence investigations on stacked Ge/Si dots with different spacer thicknesses are presented. According to the emission energy shift in the Ge dots, we found that a thinner spacer layer will lead to remarkable Ge–Si intermixing during the stacking of the Ge/Si dots. Such material intermixing not only shallows the dot potential depth, but also softens the sharpness of the dot/spacer interface. In addition, the temperature of photoluminescence quenching also varies with the spacer thickness. Finally, we point out some important factors that are relevant to the room-temperature luminescence efficiency of stacked Ge/Si quantum dots.

Influence of spacer thickness on permeate flux in spiral-wound seawater reverse osmosis systems

A primary reason for flux decline during the initial period of a membrane separation process is concentration polarization of the solute at the membrane surface. The objective of the present study was to analyze and model concentration polarization in spiral-wound seawater membrane elements. In particular, the influence of spacer thickness in the membrane units on permeate flow and its salinity was evaluated. Membrane parameters were also estimated using an analytical osmotic pressure model for high salinity applications. The effects of spacer thickness on permeate flux suggest that the observed flux decreases by up to 50% in going from a spacer thickness of 0.1168 to 0.0508 cm. The different geometry/configuration of the spacer may influence turbulence at the membrane surface that in turn affected concentration polarization. This suggests less turbulence with the smaller spacer thickness. The membrane module with an intermediate spacer thickness of 0.0711 cm was found to be the best economically since it gave the highest water production rate (L/h).

The Effects of Spacer Thickness and Temperature on the Transport Properties of Modulation-Doped In0.53Ga0.47As/In0.52Al0.48As Heterojunctions

The measurements of resistivity and low-field Hall effect made in the temperature range 3.3–295 K have been used to investigate the transport properties of modulation-doped, lattice-matched In0.53Ga0.47As/In0.52Al0.48As heterojunctions as a function of the spacer thickness in the range from 0 to 400 Å. It is found that the sheet carrier density determined at temperatures below about 80 K decreases rapidly with increasing spacer thickness. The low-temperature Hall mobility increases substantially when increasing the spacer thickness from 0 to 100 Å, and decreases gradually with further increase in spacer thickness. The results suggest that, in addition to alloy scattering, remote ionized-impurity scattering is a major scattering mechanism at low temperatures in the samples with thin spacer layer and that background impurity scattering prevails in the samples with spacer thickness larger than about 100 Å. The effect of modulation doping on the mobility decreases progressively with increasing temperature: the electron mobility becomes practically independent of spacer thickness in the temperature range above about 200 K. The variation of Hall mobility with temperature in the range above 90 K has been used to determine the energy of longitudinal optical phonons that limit the mobility of electrons at high temperatures.

Photoluminescence properties of dense InAs/AlInAs quantum wire arrays

High-density, nanometer-scale InAs quantum wires are observed on InP (100) grown by molecular beam epitaxy. The detailed temperature-dependent photoluminescence (PL) properties are studied. The fast redshift of PL peak energy and the monotonic decrease of PL linewidth are observed and explained in terms of carrier relaxation effect, resulting from the efficient carrier transfer from smaller wires into nearby larger ones in coupled system. For the superlattice samples, the effect of spacer thickness on the PL properties was also investigated. Device applications of these quantum structures are very promising.

Effects of Spacer Thickness on the Performance of InGaAs/GaAs Quantum Dot Lasers

ABSTRACTIt is found that the performance of self-assembled In0.5Ga0.5As/GaAs multi-stack quantum dot lasers is sensitive to the GaAs spacer thickness between the dots. Reducing the spacer thickness from 30 nm to 10 nm leads to narrow photoluminescence linewidth, low threshold current, high characteristic temperature and high internal quantum efficiency. This behavior is attributed to inhomogeneous broadening caused by dot size fluctuation related to spacer thickness.