Structural,electronic,and magnetic properties of transition-metal atom adsorbed two-dimensional GaAs nanosheet

By using first-principles calculations within the framework of density functional theory,the electronic and magnetic properties of 3d transitional metal(TM) atoms(from Sc to Zn) adsorbed monolayer Ga As nanosheets(Ga As NSs) are systematically investigated.Upon TM atom adsorption,Ga As NS,which is a nonmagnetic semiconductor,can be tuned into a magnetic semiconductor(Sc,V,and Fe adsorption),a half-metal(Mn adsorption),or a metal(Co and Cu adsorption).Our calculations show that the strong p–d hybridization between the 3d orbit of TM atoms and the 4p orbit of neighboring As atoms is responsible for the formation of chemical bonds and the origin of magnetism in the Ga As NSs with Sc,V,and Fe adsorption.However,the Mn 3d orbit with more unpaired electrons hybridizes not only with the As 4p orbit but also with the Ga 4p orbit,resulting in a stronger exchange interaction.Our results may be useful for electronic and magnetic applications of Ga As NS-based materials.     

A dual-blue light-emitting diode based on strain-compensated InGaN-AlGaN/GaN quantum wells

A strain-compensated InGaN quantum well(QW) active region employing a tensile AlGaN barrier is analyzed.Its spectral stability and efficiency droop for a dual-blue light-emitting diode(LED) are improved compared with those of the conventional InGaN/GaN QW dual-blue LEDs based on a stacking structure of two In0.18Ga0.82N/GaN QWs and two In0.12Ga0.88N/GaN QWs on the same sapphire substrate.It is found that the optimal performance is achieved when the Al composition of the strain-compensated AlGaN layer is 0.12 in blue QW and 0.21 in blue-violet QW.The improvement performance can be attributed to the strain-compensated InGaN-AlGaN/GaN QW,which can provide a better carrier confinement and effectively reduce leakage current.     

High-k gate dielectric GaAs MOS device with LaON as interlayer and NH_3-plasma surface pretreatment

High-k gate dielectric Hf Ti ON Ga As metal-oxide–semiconductor(MOS) capacitors with La ON as interfacial passivation layer(IPL) and NH3- or N2-plasma surface pretreatment are fabricated, and their interfacial and electrical properties are investigated and compared with their counterparts that have neither La ON IPL nor surface treatment. It is found that good interface quality and excellent electrical properties can be achieved for a NH3-plasma pretreated Ga As MOS device with a stacked gate dielectric of Hf Ti ON/La ON. These improvements should be ascribed to the fact that the NH3-plasma can provide H atoms and NH radicals that can effectively remove defective Ga/As oxides. In addition, La ON IPL can further block oxygen atoms from being in-diffused, and Ga and As atoms from being out-diffused from the substrate to the high-k dielectric. This greatly suppresses the formation of Ga/As native oxides and gives rise to an excellent high-k/Ga As interface.     

Strain drived band aligment transition of the ferromagnetic VS_2/C_3N van der Waals heterostructure

Exploring two-dimensional(2 D) magnetic heterostructures is essential for future spintronic and optoelectronic devices.Herein,using first-principle calculations,stable ferromagnetic ordering and colorful electronic properties are established by constructing the VS_2/C_3 N van der Waals(vdW) heterostructure.Unlike the semiconductive properties with indirect band gaps in both the VS_2 and C_3 N monolayers,our results indicate that a direct band gap with type-Ⅱ band alignment and p-doping characters are realized in the spin-up channel of the VS_2/C_3 N heterostructure,and a typical type-Ⅲband alignment with a broken-gap in the spin-down channel.Furthermore,the band alignments in the two spin channels can be effectively tuned by applying tensile strain.An interchangement between the type-Ⅱ and type-Ⅲ band alignments occurs in the two spin channels,as the tensile strain increases to 4%.The attractive magnetic properties and the unique band alignments could be useful for prospective applications in the next-generation tunneling devices and spintronic devices.     

Thermal stress reduction of GaAs epitaxial growth on V-groove patterned Si substrates

We investigate the thermal stresses for GaAs layers grown on V-groove patterned Si substrates by the finite-element method. The results show that the thermal stress distribution near the interface in a patterned substrate is nonuniform,which is far different from that in a planar substrate. Comparing with the planar substrate, the thermal stress is significantly reduced for the Ga As layer on the patterned substrate. The effects of the width of the V-groove, the thickness, and the width of the SiO₂ mask on the thermal stress are studied. It is found that the SiO₂ mask and V-groove play a crucial role in the stress of the Ga As layer on Si substrate. The results indicate that when the width of V-groove is 50 nm, the width and the thickness of the SiO₂ mask are both 100 nm, the Ga As layer is subjected to the minimum stress. Furthermore,Comparing with the planar substrate, the average stress of the Ga As epitaxial layer in the growth window region of the patterned substrate is reduced by 90%. These findings are useful in the optimal designing of growing high-quality Ga As films on patterned Si substrates.     

First-principles study of Ga7As7 ionic cluster and influence of multi-charge on its structure

This paper investigates the structures and stabilities of neutral Ga7As7 cluster and its ions in detail by using first-principles density functional theory. Many low energy structures of Ga7As7 cluster are found. It confirms that the ground state structure of neutral Ga7As7 cluster is a pentagonal prism with four face atoms like a basket structure, as reported by previous works. The ground state structures of positive Ga7As7 cluster ions are different from that of the neutral cluster. These investigations suggest that Ga atoms occupy the capping positions more easily than As atoms. Mulliken population analyses also show that Ga atoms can lose or obtain charge more easily than As atoms. It finds that the neutral Ga7As7 cluster can become more stable by gaining one or two additional electrons but further more electrons would cause the decrease of binding energy. The ionisation energy increases with the increase of the number of the removed electrons. These calculated results indicate that the net magnetic moment of the neutral Ga7As7 cluster is zero because all electrons are paired together in their respective molecular orbits. But for the ionic Ga7As7 cluster with odd number of electrons, the net magnetic moment is 1.0 μB due to an unpaired electron.     

Bianchi Type-Ⅱ Inflationary Models with Stiff Matter and Decaying Cosmological Term

We deal with Einstein＇s field equations with a time-decaying cosmological term of the forms （i） ∧=β（a/a） ＋ α/a62 and （ii）∧ = α/a^2, where a is the average scale factor of the universe, α and β are constants for a spatially homogeneous and anisotropic LRS Bianchi type-Ⅱ spacetime. Exact solutions of the field equations for stiff matter are obtained by applying a special law of variation for the Hubble parameter. Anisotropic cosmological models are presented with a constant negative deceleration parameter which corresponds to the accelerated phase of the present universe. The cosmological constant A is obtained as a decreasing function of time that is approaching a small positive value at the present epoch, which is corroborated by the consequences from recent supernovae Ia observations. The physical and kinematical behaviors of the models are also discussed.     

Optimization of a GaN-based irregular multiple quantum well structure for a dichromatic white LED

GaN-based irregular multiple quantum well(IMQW) structures assembled two different types of QWs emitting complementary wavelengths for dichromatic white light-emitting diodes(LEDs) are optimized in order to obtain near white light emissions.The hole distributions and spontaneous emission spectra of the IMQW structures are analysed in detail by fully considering the effects of strain,well-coupling,valence band-mixing and polarization effect through employing a newly developed theoretical model from the k · p theory.Several structure parameters such as well material component,well width,layout of the wells and the thickness of barrier between different types of QWs are employed to analyse how these parameters together with the polarization effect influence the electronic and the optical properties of IMQW structure.Numerical results show that uniform hole distributions in different types of QWs are obtained when the number of the QWs emitting blue light is two,the number of the QWs emitting yellow light is one and the barrier between different types of QWs is 8nm in thickness.The near white light emission is realized using GaN-based IMQW structure with appropriate design parameters and injection level.     

Surface Carbonization of GaN and the Related Structure Evolution during the Annealing Process

To explain the stabilization mechanism of the carbon-ion-implanted GaN under the diamond growth environment,the luminescence characteristics and structure evolution correlative with sites' carbon atoms located for highfluence carbon-ion-implanted Ga N are discussed. GaN is implanted with carbon ion using fluence of 2×10~(17) cm~(-2) and energy of 45 keV. Then the implanted samples are annealed at 800℃ for 20 min and 1 h under the N_2 atmosphere. The luminescence characteristics of carbon-ion-implanted GaN are evaluated by photoluminescence spectrum at wavelength 325 nm. The lattice damage of Ga N is characterized by Raman spectrum and the corresponding vacancy-defect evolution before and after annealing is measured by slow positron annihilation. The results show that most of the carbon atoms will be located at the interstitial sites after carbon ion implantation due to the weak mobility. As the implanted samples are annealed, strong yellow luminescence is emitted and the vacancies for Ga(V_(Ga)) are reduced resulting from the migration of interstitial carbon(C_i) and formation of complexes(CGaand/or C_(Ga)-C_i) between them. As the annealing time is prolonged, the carbon ions accommodated by the vacancies are saturated, vacancy clusters with carbon atoms appear and the concentration of C_(Ga) diminishes, which will have an adverse effect on the diamond film nucleation and growth.     

Metamorphic growth of 1.55 μm InGaAs/InGaAsP multiple quantum wells laser structures on GaAs substrates

We fabricate a Ga As-based In Ga As/In Ga As P multiple quantum wells(MQWs) laser at 1.55 μm. Using two-step growth method and thermal cyclic annealing, a thin low-temperature In P layer and a thick In P buffer layer are grown on Ga As substrates by low-pressure metal organic chemical vapor deposition technology. Then, highquality MQWs laser structures are grown on the In P buffer layer. Under quasi-continuous wave(QCW) condition, a threshold current of 476 m A and slope efficiency of 0.15 m W/m A are achieved for a broad area device with 50 μm wide strip and 500 μm long cavity at room-temperature. The peak wavelength of emission spectrum is1549.5 nm at 700 m A. The device is operating for more than 2000 h at room-temperature and 600 mA.     

BOUND POLARONS IN ASYMMETRIC QUANTUM WELLS IN AN ELECTRIC FIELD

We have proposed the Hamiltonian of a polaron bound to a donor impurity in semiconductor quantum wells (QWs) in the presence of an electric field. The couplings of an electron with the confined bulk like longitudinal optical (L₀) phonons, halfspace L₀ phonons and interface phonons are considered. In particular, the interaction of the impurity with the various phonon modes is also included. We have calculated the ionization energy of a bound polaron in Al_xl Ga_1-xl As/GaAs/Al_xr Ga_1-xr As asymmetric and symmetric QWs. Results are obtained as a function of the barrier height (or equivalently of Al concentration x),the well width, the electric field intensities and the position of impurity in the QWs. Our numerical calculations show clearly that the interaction between the impurity and the phonon field plays an important role in screening the Coulomb interaction. It is shown that for at hin well (＜12nm), the cumulative effects of the electronphonon coupling and the impurityphonon coupling can contribute appreciably to the donor ionization energy and polarizability.     

Manipulation of Dirac Fermions in Nanochain-Structured Graphene

Graphene has afforded an ideal 2D platform for investigating a rich and fascinating behavior of Dirac fermions.Here,we develop a theoretical mechanism for manipulating the Dirac fermions in graphene,such as from type-Ⅰ to type-Ⅱ and type-Ⅲ,by a top-down nanopatterning approach.We demonstrate that by selective chemical adsorption to pattern the 2D graphene into coupled 1D armchair chains(ACs),the intrinsic isotropic upright Dirac cone becomes anisotropic and strongly tilted.Based on model analyses and first-principles calculations,we show that both the shape and tilt of Dirac cone can be tuned by the species of chemisorption,e.g.,halogen vs hydrogen,which modifies the strength of inter-AC coupling.Furthermore,the topological edge states and transport properties of the engineered Dirac fermions are investigated.Our work sheds lights on understanding the Dirac fermions in a nanopatterned graphene platform,and provides guidance for designing nanostructures with novel functionality.     

Spacer layer thickness fluctuation scattering in a modulation-doped AlxGa1-xAs/GaAs/AlxGa1-xAs quantum well

We theoretically study the influence of spacer layer thickness fluctuation（SLTF） on the mobility of a twodimensional electron gas（2DEG） in the modulation-doped Al x Ga 1 x As/GaAs/Al x Ga 1 x As quantum well.The dependence of the mobility limited by SLTF scattering on spacer layer thickness and donor density are obtained.The results show that SLTF scattering is an important scattering mechanism for the quantum well structure with a thick well layer.     

Spacer layer thickness fluctuation scattering in a modulation-doped Al_xGa_1-xAs/GaAs/Al_xGa_1-xAs quantum well

We theoretically study the influence of spacer layer thickness fluctuation(SLTF) on the mobility of a twodimensional electron gas(2DEG) in the modulation-doped Al x Ga 1 x As/GaAs/Al x Ga 1 x As quantum well.The dependence of the mobility limited by SLTF scattering on spacer layer thickness and donor density are obtained.The results show that SLTF scattering is an important scattering mechanism for the quantum well structure with a thick well layer.     

Effects of Al and Ti/Cu on Synthesis of Type-IIa Diamond Crystals in Ni70Mn25Co5-C System at HPHT

High-quality type-Ⅱa gem diamond crystals are successfully synthesized in a NiToMn25Co5-C system by temperature gradient method （TGM） at about 5.5 GPa and 1560 K. Al and Ti/Cu are used as nitrogen getters respectively. While nitrogen getter Al or Ti/Cu is added into the synthesis system, some inclusions and caves tend to be introduced into the crystals. When Al is added into the solvent alloy, we would hardly gain high-quality type-Ⅱa diamond crystals with nitrogen concentration Nc 〈 1 ppm because of the reversible reaction of Al and N at high pressure and high temperature （HPHT）. Piowever, when Ti/Cu is added into the solvent alloy, high-quality type-Ⅱa diamond crystals with Nc 〈 1 ppm can be grown by decreasing the growth rate of diamonds.     

A Green＇s function model for ferromagnetism and spin excitations of （Ga,Mn）As diluted magnetic semiconductors

We study (Ga, Mn)As diluted magnetic semiconductors in terms of the Ruderman--Kittel--Kasuya--Yosida quantum spin model in Green's function approach. Random distributions of the magnetic atoms are treated by using an analytical average of magnetic configurations. Average magnetic moments and spin excitation spectra as functions of temperature can be obtained by solving self-consistent equations, and the Curie temperature T_C is given explicitly. T_C is proportional to magnetic atomic concentration, and there exists a maximum for T_C as a function of carrier concentration. Applied to (Ga, Mn)As, the theoretical results are consistent with experiment and the experimental T_C can be obtained with reasonable parameters. This modelling can also be applied to other diluted magnetic semiconductors.     

Improvement of TE-polarized emission in type-Ⅱ InAlN–AlGaN/AlGaN quantum well

The optical properties of the type-Ⅱ lineup In_xAl_(1-x)N–Al_(0.59)Ga_(0.41)N/Al_(0.74)Ga_(0.26)N quantum well(QW) structures with different In contents are investigated by using the six-by-six K–P method.The type-Ⅱ lineup structures exhibit the larger product of Fermi–Dirac distribution functions of electron f_c~n and hole(1-f_v~(Um)) and the approximately equal transverse electric(TE) polarization optical matrix elements(|M_x|~2) for the c1–v1 transition.As a result, the peak intensity in the TE polarization spontaneous emission spectrum is improved by 47.45%–53.84% as compared to that of the conventional AlGaN QW structure.In addition, the type-Ⅱ QW structure with x～0.17 has the largest TE mode peak intensity in the investigated In-content range of 0.13–0.23.It can be attributed to the combined effect of |M_x|~2 and f_c~n(1-f_v~( Um)) for the c1–v1, c1–v2, and c1–v3 transitions.     

Anisotropic Bianchi Type Ⅰ Cosmological Models with Generalized Chaplygin Gas and Dynamical Gravitational and Cosmological Constants

This paper deals with study of generalized Chaplygin gas model with dynamical gravitational and cosmological constants. In this paper a new set of exact solutions of Einstein field equations for spatially homogeneous and anisotropic Bianchi type Ⅰ space-time have been obtained. The solutions of the Einstein's field equations are obtained by considering(i) the power law relation between Hubble parameter H and scale factor R and(ii) scale factor of the form R =-1/t + t~2, t 1. The assumptions lead to constant and variable deceleration parameter respectively. The physical and dynamical behaviors of the models have been discussed with the help of graphical representations. Also we have discussed the stability and physical acceptability of solutions for solution type-Ⅰ and solution type-Ⅱ.     

Electric Field Control of Interface Related Spin Splitting in Step Quantum Wells

Spin splitting of the Aly Ga1-y As/GaAs/A1x Ga1-x As/A1-y Ga1-y As （x ≠ y） step quantum wells （ Q Ws） has been theoretically investigated with a model that includes both the interface and the external electric field contribution. The overall spin splitting is mainly determined by the interface contribution, which can be well manipulated by the external electric field. In the absence of the electric field, the Rashba effect exists due to the internal structure inversion asymmetry （SIA）. The electric field can strengthen or suppress the internal SIA, resulting in an increase or decrease of the spin splitting. The step QW, which results in large spin splitting, has advantages in applications to spintronic devices compared with symmetrical and asymmetrical QWs. Due to the special structure design, the spin splitting does not change with the external electric field.     

Optimum Indium Concentration for Growth of 1.3μmInAs/InxGa1-xAs Quantum Dots in a Well

Five InAs/InxGa1-xAs quantum dots in a well （DWELL） with different indium concentration x are grown by solid source molecular beam epitaxy. The high quantum dot density is observed in the InAs/In0.3Ga0.7As DWELL. The photoluminescence （PL） experiments indicate that the ground state peaks of InAs/In0.15 Ga0.85As and InAs/In0.22 Ga0.78As DWELLs shift to 1.31 and 1.33μm, respectively. The optical properties are investigated by using the PL and piezoreflectance spectroscope methods. An abnormal blue shift has been observed with the further increase of x from 0.22 to 0.30.