Broadband light emitting from multilayer-stacked InAs/GaAs quantum dots

We report the effect of the GaAs spacer layer thickness on the photoluminescence(PL) spectral bandwidth of InAs/GaAs self-assembled quantum dots(QDs).A PL spectral bandwidth of 158 nm is achieved with a five-layer stack of InAs QDs which has a 11-nm thick GaAs spacer layer.We investigate the optical and the structural properties of the multilayer-stacked InAs/GaAs QDs with different GaAs spacer layer thicknesses.The results show that the spacer thickness is a key parameter affecting the multi-stacked InAs/GaAs QDs for wide-spectrum emission.     

Growth and spectroscopic characteristics of Nd-doped PbWO₄ crystal

Nd-doped PbWO4 crystals are grown by using the modified Bridgman method.The spectroscopic properties of the crystals are investigated.The changes of the absorption band at 350 nm are discussed for samples annealed at 740℃ and 1040℃.The radiative lifetime of the 4 F 3/2 level is calculated by using the Judd-Ofelt theory according to the absorption spectrum of 0.5 at.% Nd-doped PbWO 4 crystal.The spontaneous Raman scattering properties of the crystals are analysed.     

Structural analysis and crystal-field calculations of Nd³⁺ in Gd_x Lu_1-x TaO₄（x = 0.85） polycrytalline

The crystal structural parameters of Nd 3+-doped rare earth orthotantalate Gd x Lu 1 x TaO 4(x = 0.85) are determined by applying the Rietveld refinement to its X-ray diffraction,and its emission and excitation spectra at 77 K are analysed.The relativistic model of ab initio self-consistent DV-Xα method,which is applied to the cluster NdO 8 in Gd x Lu 1 x TaO 4,and the effective Hamiltonian model are used to investigate its spin-orbit and crystal-field parameters.The free-ions and crystal-field parameters are fitted to the experimental energy levels at 77 K with a root-mean-square deviation of 14.92 cm 1.According to the crystal-field calculations,96 levels of Nd 3+ are assigned.Finally,the fitting results of free-ions and crystal-field parameters are compared with those already reported for Nd 3+:YAlO 3.The results indicate that the free-ion parameters are similar to those of the Nd3+ in Gdx Lu1-x TaO4 and YAlO 3 hosts,and the crystal-field interaction of Nd3+ in Gdx Lu1-x TaO4 is stronger than that in YAlO 3.     

Strain relaxation and optical properties of etched In_0.19Ga_0.81 N nanorod arrays on the GaN template

InGaN/GaN epilayers,which are grown on sapphire substrates by the metal-organic chemical-vapour deposition(MOCVD) method,are formed into nanorod arrays using inductively coupled plasma etching via self-assembled Ni nanomasks.The formation of nanorod arrays eliminates the tilt of the InGaN(0002) crystallographic plane with respect to its GaN bulk layer.Photoluminescence results show an apparent S-shaped dependence on temperature.The light extraction efficiency and intensity of photoluminescence emission at low temperature of less than 30 K for the nanorod arrays are enhanced by the large surface area,which increases the quenching effect because of the high density of surface states for the temperature above 30 K.Additionally,a red-shift for the InGaN/GaN nanorod arrays is observed due to the strain relaxation,which is confirmed by reciprocal space mapping measurements.     

Performance of La₂O₃/InAlN/GaN metal-oxide-semiconductor high electron mobility transistors

We report on the performance of La2O3/InAlN/GaN metal-oxide-semiconductor high electron mobility transistors(MOSHEMTs) and InAlN/GaN high electron mobility transistors(HEMTs).The MOSHEMT presents a maximum drain current of 961 mA/mm at Vgs = 4 V and a maximum transconductance of 130 mS/mm compared with 710 mA/mm at Vgs = 1 V and 131 mS/mm for the HEMT device,while the gate leakage current in the reverse direction could be reduced by four orders of magnitude.Compared with the HEMT device of a similar geometry,MOSHEMT presents a large gate voltage swing and negligible current collapse.     

Mechanical properties of silicon nanobeams with an undercut evaluated by combining the dynamic resonance test and finite element analysis

Mechanical properties of silicon nanobeams are of prime importance in nanoelectromechanical system applications.A numerical experimental method of determining resonant frequencies and Young’s modulus of nanobeams by combining finite element analysis and frequency response tests based on an electrostatic excitation and visual detection by using a laser Doppler vibrometer is presented in this paper.Silicon nanobeam test structures are fabricated from silicon-oninsulator wafers by using a standard lithography and anisotropic wet etching release process,which inevitably generates the undercut of the nanobeam clamping.In conjunction with three-dimensional finite element numerical simulations incorporating the geometric undercut,dynamic resonance tests reveal that the undercut significantly reduces resonant frequencies of nanobeams due to the fact that it effectively increases the nanobeam length by a correct value △L,which is a key parameter that is correlated with deviations in the resonant frequencies predicted from the ideal Euler-Bernoulli beam theory and experimentally measured data.By using a least-square fit expression including △L,we finally extract Young’s modulus from the measured resonance frequency versus effective length dependency and find that Young’s modulus of a silicon nanobeam with 200-nm thickness is close to that of bulk silicon.This result supports that the finite size effect due to the surface effect does not play a role in the mechanical elastic behaviour of silicon nanobeams with thickness larger than 200 nm.     

Density functional study of uranyl （VI） amidoxime complexes

Uranyl (Ⅵ) amidoxime complexes are investigated using relativistic density functional theory. The equilibrium structures, bond orders, and Mulliken populations of the complexes have been systematically investigated under a generalized gradient approximation (GGA). Comparison of (acet) uranyl amidoxime complexes ([UO 2 (AO) n ] 2 n , 1≤ n ≤4) with available experimental data shows an excellent agreement. In addition, the U-O(1), U-O(3), C(1)-N(2), and C(3)-N(4) bond lengths of [UO 2 (CH 3 AO) 4 ] 2 are longer than experimental data by about 0.088, 0.05, 0.1, and 0.056 A. The angles of N(3)-O(3)-U, O(2)-N(1)-C(1), N(3)-C(3)-N(4), N(4)-C(3)-C(4), and C(4)-C(3)-N(3) are different from each other, which is due to existing interaction between oxygen in uranyl and hydrogen in amino group. This interaction is found to be intra-molecular hydrogen bond. Studies on the bond orders, Mulliken charges, and Mulliken populations demonstrate that uranyl oxo group functions as hydrogen-bond acceptors and H atoms in ligands act as hydrogen-bond donors forming hydrogen bonds within the complex.     

A new generalized fractional Dirac soliton hierarchy and its fractional Hamiltonian structure

Based on the differential forms and exterior derivatives of fractional orders,Wu first presented the generalized Tu formula to construct the generalized Hamiltonian structure of the fractional soliton equation.We apply the generalized Tu formula to calculate the fractional Dirac soliton equation hierarchy and its Hamiltonian structure.The method can be generalized to the other fractional soliton hierarchy.     

In-situ multi-information measurement system for preparing gallium nitride photocathode

We introduce the first domestic in-situ multi-information measurement system for a gallium nitride (GaN) photo- cathode. This system can successfully fulfill heat cleaning and activation for GaN in an ultrahigh vacuum environment and produce a GaN photocathode with a negative electron affinity (NEA) status. Information including the heat clean- ing temperature, vacuum degree, photocurrent, electric current of cesium source, oxygen source, and the most important information about the spectral response, or equivalently, the quantum efficiency (QE) can be obtained during prepa- ration. The preparation of a GaN photocathode with this system indicates that the optimal heating temperature in a vacuum is about 700 C. We also develop a method of quickly evaluating the atomically clean surface with the vacuum degree versus wavelength curve to prevent possible secondary contamination when the atomic level cleaning surface is tested with X-ray photoelectron spectroscopy. The photocurrent shows a quick enhancement when the current ratio between the cesium source and oxygen source is 1.025. The spectral response of the GaN photocathode is flat in a wavelength range from 240 nm to 365 nm, and an abrupt decline is observed at 365 nm, which demonstrates that with the in-situ multi-information measurement system the NEA GaN photocathode can be successfully prepared.