Search

Comparing the aperture size of a membrane to the kinetic diameters of gases to be separated has long been regarded as the rule of thumb for identifying potential membrane materials. This method based upon molecular sieving could however fail for CO2 separations using metal-organic frameworks (MOFs). While the challenge may be attributed to the breathing effect or linker rotation for some highly flexible MOFs, the adsorbent-adsorbate Coulombic interaction can also significantly influence the diffusion of CO2 in MOFs. This work is aimed to investigate how the Coulombic interaction can alter the free energy profile of CO2 in a MOF channel. MOFs with one-dimensional (1D) channels (98 structures) in the orthorhombic system are probed in this in silico study. Coulombic interactions are identified to cause a notable offset between the transport bottleneck (the spot where the free energy reaches maximum) and the topological bottleneck (the spot where the pore limiting diameter is located). As a result, the transport bottleneck of CO2 and that of H2 or CH4 are found to be at different locations for a decent portion of studied MOF structures. This makes the pore limiting diameter of a MOF fail to serve as an effective indicator for molecular sieving. This study also investigates the Coulombic effect on the permeability of CO2 in MOFs, and suggests that a highly CO2 permeable and selective MOF would have strong CO2 adsorption sites that create an energetically homogeneous channel for transport. Finally, we also discuss the Coulombic effect on the diffusion of CO2 in MOFs when their flexibility is considered.

We report on the direct measurement of two-dimensional sheet charge density dependence of electron transport in AlGaN/GaN high electron mobility transistors. Pulsed IV measurements established increasing electron velocities with decreasing sheet charge densities, resulting in saturation velocity of 1.9 x 10^7 cm/s at a low sheet charge density of 7.8 x 10^11 cm-2. A new optical phonon emission-based electron velocity model for GaN is also presented. It accommodates stimulated LO phonon emission which clamps the electron velocity with strong electron-phonon interaction and long LO phonon lifetime in GaN. A comparison with the measured density-dependent saturation velocity shows that it captures the dependence rather well. Finally, the experimental result is applied in TCAD-based device simulator to predict DC and small signal characteristics of a reported GaN HEMT. Good agreement between the simulated and reported experimental results validated the measurement presented in this report and established accurate modeling of GaN HEMTs.

We report on the potential of high electron mobility transistors (HEMTs) consisting of high composition AlGaN channel and barrier layers for power switching applications. Detailed two-dimensional (2D) simulations show that threshold voltages in excess of 3 V can be achieved through the use of AlGaN channel layers. We also calculate the 2D electron gas mobility in AlGaN channel HEMTs and evaluate their power figures of merit as a function of device operating temperature and Al mole fraction in the channel. Our models show that power switching transistors with AlGaN channels would have comparable on-resistance to GaN-channel based transistors for the same operation voltage. The modeling in this paper shows the potential of high composition AlGaN as a channel material for future high threshold enhancement mode transistors.

Investigation of the properties of Al2O3-on-GaN metal-oxide-semiconductor diodes is reported. A new method is shown to calculate the metal-oxide barrier height based on the onset of the Fowler-Nordheim tunnelling current regime in direct bias. The Ni/Al2O3 barrier was extracted, for the first time with this method, and it was found to match other reports in the literature. The dependence of the effectiveness of this method on the oxide thickness is discussed. The breakdown field for Al2O3 was also measured and found to be in agreement with previous reports.

We report on our investigation of the electrical properties of metal/Al2O3/GaN metal-insulator-semiconductor (MIS) capacitors. We determined the conduction band offset and interface charge density of the alumina/GaN interface by analyzing capacitance-voltage characteristics of atomic layer deposited Al2O3 films on GaN substrates. The conduction band offset at the Al2O3/GaN interface was calculated to be 2.13 eV, in agreement with theoretical predications. A non-zero field of 0.93 MV/cm in the oxide under flat-band conditions in the GaN was inferred, which we attribute to a fixed net positive charge density of magnitude 4.60x1012 cm-2 at the Al2O3/GaN interface. We provide hypotheses to explain the origin of this charge by analyzing the energy band line-up., 8 pages, 4 figures, Applied Physics Letters

The enhanced optical transmission through the annular aluminum metal with different number of concentric rings arranged in a long-range periodic structure on the silicon substrate is investigated. The measured transmission spectra demonstrate that the surface plasmon modes depend sensitively on the structural parameters of the metallic rings. The degenerate (1,0), (1,1), and (2,1) Al∕Si surface plasmon modes are determined by the long periodicity of the structure. The localized modes in cavities with different length can be observed in this structure as well. These phenomena suggest that both surface plasmons and localized cavity modes are excited within super unit cell.