Influence of a two-dimensional electron gas on current—voltage characteristics of Al0.3{Ga}0.7 N/GaN high electron mobility transistors

The J-V characteristics of AltGa1 tN/GaN high electron mobility transistors（HEMTs） are investigated and simulated using the self-consistent solution of the Schro dinger and Poisson equations for a two-dimensional electron gas（2DEG） in a triangular potential well with the Al mole fraction t = 0.3 as an example.Using a simple analytical model,the electronic drift velocity in a 2DEG channel is obtained.It is found that the current density through the 2DEG channel is on the order of 10^13 A/m^2 within a very narrow region（about 5 nm）.For a current density of 7 × 10^13 A/m62 passing through the 2DEG channel with a 2DEG density of above 1.2 × 10^17 m^-2 under a drain voltage Vds = 1.5 V at room temperature,the barrier thickness Lb should be more than 10 nm and the gate bias must be higher than 2 V.     

Significant performance enhancement in A1GaN/GaN high electron mobility transistor by high-κ organic dielectric

The electrical properties of A1GaN/GaN high electron mobility transistor （HEMT） with and without high-κ organic dielectrics are investigated. The maximum drain current ID max and the maximum transconductance gm max of the organic dielectric/A1CaN/GaN structure can be enhanced by 74.5%, and 73.7% compared with those of the bare A1GaN/GaN HEMT, respectively. Both the threshold voltage VT and gm max of the dielectric/AlGaN/GaN HEMT are strongly dielectric-constant-dependent. Our results suggest that it is promising to significantly improve the performance of the A1GaN/GaN HEMT by introducing the high-κ organic dielectric.     

Hydrostatic pressure effect on the metal-insulator transition in LaO0.7Ca0.3Mn1−x(Fe/Ge)xO3 perovskites

Abstract

The temperature dependences of the resistivity of La_0.7Ca_0.3Mn_1?xFe_xO₃ and La_0.7Ca_0.3Mn_1?xFe_xO₃ (0 < × < 0.04) mixed crystals were studied under hydrostatic pressures up to 15kbar. The substitution of Fe for Mn results in an increase of the resistivity and a continuous decrease of the metal-insulator transition temperature T_mi while the substitution of Ge for Mn leads to a more complicated T_mi(x)-curve. In all cases T_mi shifts under pressure with a rate between 1.6 and 2.9K/kbar and a correlation between T_mi and its pressure derivative dT_mi/dP is observed which is in accordance with the general trend of dT_mi/dP versus T_mi as derived for other manganites and is discussed in terns of a competition between superexchange and double exchange.     

Influence of the Jahn—Teller distortion on magnetic ordering in TbMn1-xFexO3

The Jahn-Teller distortion plays an important role in determining the exchange interaction in rare-earth manganites.In this work we study the influence of the Jahn-Teller distortion on the magnetic structures of TbMn1-xFexO3（x = 0,0.02,0.05,0.10,and 0.20） single crystals in the basal MnO2 plane.The decrease in the quadruple splitting with the increasing Fe doping indicates the reduction of the Jahn-Teller distortion,which makes the nearest neighboring（NN） FM interaction dominant over the next nearest neighbor（NNN） AFM interaction.This alteration is favorable for the development of A-type AFM ordering instead of the spiral magnetic ordering,which collapses when x ≥ 0.05.The analysis of dielectric data indicates that the ferroelectricity is arising from the peculiar spiral magnetic ordering.     

Consideration of the spin density of conduction electrons in the formation of a hyperfine interaction field on 57Fe cores in disordered FexCr1 ? x alloys

M?ssbauer spectra of quenching-disordered Fe _x Cr_1−x alloys were measured over a wide range of chromium concentrations (0.9–45 at %), and a detailed analysis of their hyperfine structure was performed. Considering the known data that chromium atoms have an all but negligible perturbation effect on the magnetic moments of matrix atoms, it was concluded that the RKKY polarization of conduction electrons plays a leading part in the formation of the hyperfine magnetic structure of FeCr alloys. It is necessary to take into account the Zener effect.     

Distribution of germanium in Si1 ? xGex (x < 0.1) layers grown on the Si(001) substrate as a function of layer thickness

The germanium distribution in Si(001)/Si_{1 − x} Ge _x layers as a function of the layer thickness at a low dopant concentration (x < 6%) has been investigated using high-resolution X-ray diffractometry and low-temperature photoluminescence. It has been shown that the germanium concentration increases with increasing thickness of the SiGe layer with the formation of lateral inhomogeneities at the boundary between this layer and a silicon cap layer for a layer thickness of 30 nm or more. These inhomogeneities have an oriented character and give rise to anisotropic diffuse scattering for the system of (113) and (224) asymmetric reflections from SiGe. The luminescence of these films at low temperatures and low excitation densities is an emission of localized and delocalized excitons, which is characteristic of systems with disorder. The revealed nonuniform germanium distribution in the lateral direction is associated with the accumulation of germanium in the near-surface SiGe layer and with the partial relaxation of elastic strains due to the development of surface roughness and the preferred incorporation of germanium atoms into one side of the surface ripples.     

Dependences of the dielectric and pyroelectric properties of (1 ? x)PbFe1/2Nb1/2O3?xPbTiO3 ferroelectric ceramics solid solutions on the PbTiO3 content in a compositional range of 0 ≤ x ≤ 0.08

(1 − x)PbFe_1/2Nb_1/2O_3−x PbTiO₃ (PFN-xPT) ferroelectric ceramics with low dielectric loss and pronounced anomalies of dielectric and pyroelectric properties near the ferroelectric-ferroelectric phase transition was obtained by doping with lithium. The temperature dependence of the position of the morphotropic boundary between the rhombohedral (monoclinic) and tetragonal phases in the x-T phase diagram of the PFN-xPT system was determined for the first time.     

Influence of relaxation effects on probabilities of the 2s2p35S2－2s22p2 3P1，2 intercombination transitions in NII

Transition probabilities of the 2s2p^3 {}^5S_2－2s^22p^2 {}^3P_{1,2} intercombination transitions in NII have been calculated by using a large-scale multiconfiguration Dirac－Fock method. In the calculation the most important effects of relativity, correlation, and relaxation are considered. From the calculated transition probabilities, the lifetime of the 2s2p^3 {}^5S_2 metastable state is derived. The result is in excellent agreement with the latest experimental result. In the meantime the influence of anomalously strong relaxation effects on probabilities of the 2s2p^3 {}^5S_2－2s^22p^2 {}^3P_{1,2} lines in NII have been found.