Switchable hidden spin polarization and negative Poisson's ratio in two-dimensional antiferroelectric wurtzite crystals

Two-dimensional(2D)antiferroelectric materials have raised great research interest over the last decade.Here,we reveal a type of 2D antiferroelectric(AFE)crystal where the AFE polarization direction can be switched by a certain degree in the 2D plane.Such 2D functional materials are realized by stacking the exfoliated wurtzite(wz)monolayers with“self-healable”nature,which host strongly coupled ferroelasticity/antiferroelectricity and benign stability.The AFE candidates,i.e.,Zn X and Cd X(X=S,Se,Te),are all semiconductors with direct bandgap atΓpoint,which harbors switchable antiferroelectricity and ferroelasticity with low transition barriers,hidden spin polarization,as well as giant in-plane negative Poisson's ratio(NPR),enabling the co-tunability of hidden spin characteristics and auxetic magnitudes via AFE switching.The 2D AFE wz crystals provide a platform to probe the interplay of 2D antiferroelectricity,ferroelasticity,NPR,and spin effects,shedding new light on the rich physics and device design in wz semiconductors.

Built-in electric field effect on cyclotron mass of magnetopolarons in a wurtzite In_xGa_(1-x)N/GaN quantum well

The cyclotron mass of magnetopolarons in wurtzite InxGa1-xN/GaN quantum well is studied in the presence of an external magnetic field by using the Larsen perturbation method. The effects of the built-in electric field and different phonon modes including interface, confined and half-space phonon modes are considered in our calculation. The results for a zinc-blende quantum well are also given for comparison. It is found that the main contribution to the transition energy comes from half-space and interface phonon modes when the well width is very small while the confined modes play a more important role in a wider well due to the location of the electron wave function. As the well width increases, the cyclotron mass of magnetopolarons first increases to a maximum and then decreases either with or without the built-in electric field in the wurtzite structure and the built-in electric field slightly reduces the cyclotron mass. The variation of cyclotron mass in a zinc-blende structure is similar to that in a wurtzite structure. With the increase of external magnetic field, the cyclotron mass of polarons almost linearly increases. The cyclotron frequency of magnetopolarons is also discussed.

Screening influence on the Stark effect of impurity states in strained wurtzite GaN/Al_xGa_(1-x)N heterojunctions under pressure

The screening effect of the random-phase-approximation on the states of shallow donor impurities in free strained wurtzite GaN/AlxGa1-xN heterojunctions under hydrostatic pressure and an external electric field is investigated by using a variational method and a simplified coherent potential approximation. The variations of Stark energy shift with electric field, impurity position, A1 component and areal electron density are discussed. Our results show that the screening dramatically reduces both the blue and red shifts as well as the binding energies of impurity states. For a given impurity position, the change in binding energy is more sensitive to the increase in hydrostatic pressure in the presence of the screening effect than that in the absence of the screening effect. The weakening of the blue and red shifts, induced by the screening effect, strengthens gradually with the increase of electric field. Furthermore, the screening effect weakens the mixture crystal effect, thereby influencing the Stark effect. The screening effect strengthens the influence of energy band bending on binding energy due to the areal electron density.

Nonlinear dynamics in wurtzite InN diodes under terahertz radiation

We carry out a theoretical study of nonlinear dynamics in terahertz-driven n＋nn＋ wurtzite InN diodes by using time-dependent drift diffusion equations. A cooperative nonlinear oscillatory mode appears due to the negative differential mobility effect, which is the unique feature of wurtzite InN aroused by its strong nonparabolicity of the I＂1 valley. The appearance of different nonlinear oscillatory modes, including periodic and chaotic states, is attributed to the competition between the self-sustained oscillation and the external driving oscillation. The transitions between the periodic and chaotic states are carefully investigated using chaos-detecting methods, such as the bifurcation diagram, the Fourier spectrum and the first return map. The resulting bifurcation diagram displays an interesting and complex transition picture with the driving amplitude as the control parameter.

Polar interface and surface optical vibration spectra in multi-layer wurtzite quantum wires： transfer matrix method

The polar interface optical （IO） and surface optical （SO） phonon modes and the corresponding Froehlich electron phonon-interaction Hamiltonian in a freestanding multi-layer wurtzite cylindrical quantum wire （QWR） are derived and studied by employing the transfer matrix method in the dielectric continuum approximation and Loudon＇s uniaxial crystal model. A numerical calculation of a freestanding wurtzite GaN/AlN QWR is performed. The results reveal that for a relatively large azimuthal quantum number m or wave-number kz in the free z-direction, there exist two branches of IO phonon modes localized at the interface, and only one branch of SO mode localized at the surface in the system. The degenerating behaviours of the IO and SO phonon modes in the wurtzite QWR have also been clearly observed for a small kz or m. The limiting frequency properties of the IO and SO modes for large kz and m have been explained reasonably from the mathematical and physical viewpoints. The calculations of electron-phonon coupling functions show that the high-frequency IO phonon branch and SO mode play a more important role in the electron phonon interaction.

Phonon-assisted intersubband transitions in wurtzite GaN/In_x Ga_(1-x) N quantum wells

A detailed numerical calculation on the phonon-assisted intersubband transition rates of electrons in wurtzite CaN/InxGal-xN quantum wells is presented. The quantum-confined Stark effect, induced by the built-in electric field, and the ternary mixed crystal effect are considered. The electron states are obtained by iteratively solving the coupled SchrSdinger and Poisson equations. The dispersion properties of each type of phonon modes are considered in the derivation of Fermi＇s golden rule to evaluate the transition rates. It is indicated that the interface and half- space phonon scattering play an important role in the process of 1 2 radiative transition. The transition rate is also greatly reduced by the built-in electric field. This work can be helpful for the structural design and simulation of new semiconductor lasers.

Effects of electron–optical phonon interactions on the polaron energy in a wurtzite ZnO/Mg_xZn_(1-x)O quantum well

We investigated the properties of polarons in a wurtzite ZnO/MgxZn1-xO quantum well by adopting a modified Lee–Low–Pines variational method, giving the ground state energy, transition energy, and phonon contributions from various optical-phonon modes to the ground state energy as functions of the well width and Mg composition. In our calculations, we considered the effects of confined optical phonon modes, interface-optical phonon modes, and half-space phonon modes, as well as the anisotropy of the electron effective band mass, phonon frequency, and dielectric constant. Our numerical results indicate that the electron–optical phonon interactions importantly affect the polaronic energies in the ZnO/MgxZn1-xO quantum well. The electron–optical phonon interactions decrease the polaron energies. For quantum wells with narrower wells, the interface optical phonon and half-space phonon modes contribute more to the polaronic energies than the confined phonon modes. However, for wider quantum wells, the total contribution to the polaronic energy mainly comes from the confined modes. The contributions of the various phonon modes to the transition energy change differently with increasing well width. The contribution of the half-space phonons decreases slowly as the QW width increases, whereas the contributions of the confined and interface phonons reach a maximum at d ≈ 5.0 nm and then decrease slowly. However,the total contribution of phonon modes to the transition energy is negative and increases gradually with the QW width of d.As the composition x increases, the total contribution of phonons to the ground state energies increases slowly, but the total contributions of phonons to the transition energies decrease gradually. We analyze the physical reasons for these behaviors in detail.

A COMPREHENSIVE INVESTIGATION ON A PECULIAR DEFECT IN WURTZITE GaN FILM

Apeculiar crystal defect was observed by experiment of high resolution electron microscopy( HREM) .It wasidentifiedtoformed bytwothreading dislocationsand asegmentof(11 20) domain boundary. An atomicstructure model was proposed for the anomalous defect. Theresultofthe weak beam experimentofelectron microscopeaffirmedthesuggested model.Thestructuralpropertiesofseveral GaN(11 2 0) domain boundaries werestudied by Molec ular Dynamicscalculations. Thetheoreticalresultsshowedthatthelike atom bonding domain boundary, whichcomposesthe main partofthe peculiar defect, has much higher formationenergy than itcounterpartof unlike atom bonding domain boundary. Theoutcome providesanotherevidenceforthesuggested model.

Electronic structure and optical properties of the scintillation material wurtzite ZnS(Ag)

In order to investigate the effect of Ag doping(ZnS(Ag)) and Zn vacancy(V_(Zn)) on the alpha particle detection performance of wurtzite(WZ) ZnS as a scintillation cell component, the electronic structure and optical properties of ZnS, ZnS(Ag), and V_(Zn)were studied by firstprinciple calculation based on the density functional theory. The results show that the band gaps of ZnS, ZnS(Ag),and V_(Zn)are 2.17, 1.79, and 2.37 eV, respectively. Both ZnS(Ag) and V_(Zn)enhance the absorption and reflection of the low energy photons. A specific energy, about 2.9 eV,leading to decrease of detection efficiency is observed. The results indicate that Ag doping has a complex effect on the detection performance. It is beneficial to produce more visible light photons than pure WZ ZnS when exposed to the same amount of radiation, while the increase of the absorption to visible light photons weakens the detection performance. Zn vacancy has negative effect on the detection performance. If we want to improve the detection performance of WZ ZnS, Ag doping will be a good way,but we should reduce the absorption to visible light photons and control the number of Zn vacancy rigorously.

Surface saturation control on the formation of wurtzite polytypes in zinc blende SiC nanofilms grown on Si-(100) substrates

We investigate the formations of wurtzite （WZ） SiC nano polytypes in zinc blende （ZB） SiC nanofilms hetero-grown on Si-（100） substrates via low pressure chemical vapor deposition （LPCVD） by adjusting the Si/C ratio of the introduced precursors. Through SEM, TEM, and Raman characterizations, we find that the nanofilms consist of discrete WZ SiC nano polytypes and ZB SiC polytypes composed of WZ polytypes （WZ ＋ ZB） and disordered ZB SiC polytypes, respectively, according to Si/C ratios of 0.5, 1.5, and 3. We attribute the WZ polytype formation to being due to a kinetic mechanism based on the Si/C surface saturation control.

Binding energies of impurity states in strained wurtzite GaN/Al_xGa_(1-x)N heterojunctions with finitely thick potential barriers

Ground state binding energies of donor impurities in a strained wurtzite GaN/AlxGal_xN heterojunction with a po- tential barrier of finite thickness are investigated using a variational approach combined with a numerical computation. The built-in electric field due to the spontaneous and piezoelectric polarization, the strain modification due to the lattice mismatch near the interfaces, and the effects of ternary mixed crystals are all taken into account. It is found that the binding energies by using numerical wave functions are obviously greater than those by using variational wave functions when impurities are located in the channel near the interface of a heterojunction. Nevertheless, the binding energies using the former functions are obviously less than using the later functions when impurities are located in the channel far from an interface. The difference between our numerical method and the previous variational method is huge, showing that the former should be adopted in further work for the relevant problems. The binding energies each as a function of hydrostatic pressure are also calculated. But the change is unobvious in comparison with that obtained by the variational method.

A Molecular Dynamics Study on the (1120) Domain Boundary Structures in Epitaxial Wurtzite GaN

A computer program has been developed for the moIlcular dynamics calculation of ionic orstrong-ionic covalent systems. Ewald summation algorithm and Keating potentiaI model areadopted to calculate the long-range Coulomb interaction and the short-range bonding forces,respectively. A theoretical study on the domain boundary structures in epitaxial wurtzite GaN film is accomplished with the program. The calculation result is used in the structure formationexplanation of an interesting defect observed by HREM experiment.

Ab Initio Calculation of Dielectric Function in Wurtzite GaN Based on Walter's Model

The wavelength-dependent and frequency-dependent dielectric function of wurtzite-GaN is cMculated totally from fundamental parameters such as the lattice constant using Waiter＇s ab initio model. The errors occurring in the cMculation are carefully reduced by/inear interpolation of energy data. The Kramers-Kronig transform of the real part of greater range is obtained by extrapolation of the reM part. The calculation is time-consuming but meaningful The long-wave results are similar to the experimental data of the photon and are useful for related investigation of properties of wide-gap semiconductors such as electron scattering like the Auger recombination and impact ionization.

Bandgap Engineering in Wurtzite GaAs Nanowires by Hydrostatic Pressure

Band structure of wurtzite （WZ） GaAs nanowires （NWs） is investigated by using photoluminescenee measurements under hydrostatic pressure at 6 K. We demonstrate that WZ GaAs NWs have a direct bandgap transition with an emission energy of 1.53eV, corresponding to the optical transition between conduction band Г7c and valence band Г9v in WZ GaAs. The direct-to-pseudodirect bandgap transition can be observed by applying a pressure approximately above 2.5 GPa.

First-principles study of the electronic structures and optical properties of C-F-Be doped wurtzite ZnO

The electronic structure and optical properties of pure, C-doped, C~ codoped and C-F-Be cluster- doped ZnO with a wurtzite structure were calculated by using the density functional theory with the plane-wave ultrasoft pseudopotentials method. The results indicate that p-type ZnO can be obtained by C incorporation, and the energy level of Co above the valence band maximum is 0.36 eV. The ionization energy of the complex Zn16O14CF and ZnlsBeO14CF can be reduced to 0.23 and 0.21 eV, individually. These results suggest that the defect complex of ZnlsBeO14CF is a better candidate for p-type ZnO. To make the optical properties clear, we investigated the imaginary part of the complex dielectric function ofundoped and C-F-Be doped ZnO. We found that there is strong absorption in the energy region lower than 2.7 eV for the C-F-Be doped system compared to pure ZnO.

Electronic properties of wurtzite GaAs： A correlated structural, optical, and theoretical analysis of the same polytypic GaAs nanowire

III-V compound semiconductor nanowires are generally characterized by the coexistence of zincblende and wurtzite structures. So far, this polytypism has impeded the determination of the electronic properties of the metastable wurtzite phase of GaAs, which thus remain highly controversial. In an effort to obtain new insights into this topic, we cross-correlate nanoscale spectral imaging by near-field scanning optical microscopy with a transmission electron microscopy analysis of the very same polytypic GaAs nanowire dispersed onto a Si wafer. Thus, spatially resolved photoluminescence spectra could be unambiguously assigned to nanowire segments whose structure is known with lattice-resolved accuracy. An emission energy of 1.528 eV was observed from extended zincblende segments, revealing that the dispersed nanowire was under uniaxial strain presumably due to interaction with its supporting substrate. These crucial information and the emission energy obtained for extended pure wurtzite segments were used to perform envelope function calculations of zincblende quantum disks in a wurtzite matrix as well as the inverse structure. In these calculations, we varied the fundamental bandgap, the electron mass, and the band offset between zincblende and wurtzite GaAs. From this multi-parameter comparison with the experimental data, we deduced that the bandgap between the F8 conduction and A valence band ranges from 1.532 to 1.539 eV in strain-free wurtzite GaAs, and estimated values of 1.507 to 1.514 eV for the F7-A bandgap.

Au-catalysed free-standing wurtzite structured InAs nanosheets grown by molecular beam epitaxy

In this study,we report the growth of free-standing InAs nanosheets using Au catalysts in molecular beam epitaxy.Detailed structural characterizations suggest that wurtzite structured InAs nanosheets,with features of extensive{1120}surfaces,grown along the<1102>direction and adopted{0001}nanosheet/catalyst interfaces,are initiated from wurtzite structured[0001]nanowires as the inclined epitaxial growth due to relatively higher In concentrations in Au catalysts,and grown from these inclined nanostructures through catalyst-induced axial growth and their enhanced lateral growth under the high growth temperature.Based on the facts that the nanosheets contain large low energy{1120}surfaces and{0001}nanosheet/catalyst interfaces,the growth of our nanosheets is a thermodynamically driven process.This study provides new insights into fabricating free-standing Ⅲ-Ⅴ nanosheets for their applications in future nanoscale devices.

The electronic and magnetic properties of wurtzite Mn：CdS, Cr：CdS and Mn：Cr：CdS： first principles calculations

In this article, density functional theory（DFT） based on generalized gradient approximation（GGA）and GGACU, U is Hubbard term, is used to study the electronic properties of CdS doped with different dopants（Cr, Mn）. The calculations are carried out for Mn-doped CdS, Cr-doped CdS, and co-doping of Mn/Cr in CdS simultaneously. It is found that hopping of electrons is possible with Cr：CdS and Mn：Cr：CdS while Mn：CdS does not allow the hopping of electrons. Moreover, double exchange interactions are observed in Cr：CdS and d-d superexchange interactions are observed in Mn：CdS. Now the problem becomes interesting when one magnetic ion（Cr） supporting double exchange interactions and another ion（Mn） supporting d-d super-exchange interactions are doped simultaneously in the same system（CdS）. The co-doped CdS is more stable even at high Curie temperature due to p-d double exchange interactions and d-d super exchange interactions. Furthermore, the Cr-3d and Mn-3d states present in-between the band gap are responsible for inner shell transitions and hence for optical properties.Therefore, the co-doped system is taken into account to enhance its applications in the field of spintronic and magneto-optical devices.

Pressure influence on bound polarons in a strained wurtzite GaN/Al_xGa_(1_x)N heterojunction under an electric field

The binding energies of bound polarons near the interface of a strained wurtzite GaN/Al_xGa_（1-x）N het-erojunction are studied by using a modified LLP variational method and a simplified coherent potential approximation under hydrostatic pressure and an external electric field.Considering the biaxial strain due to lattice mismatch or epitaxial growth,the uniaxial strain effects and the influences of the electron-phonon interaction as well as impurity-phonon interaction including the effects of interface-optical phonon modes and half-space phonon modes,the binding energies as functions of pressure,the impurity position,areal electron density and the phonon effect on the Stark energy shift are investigated.The numerical result shows that the contributions from the interface optical phonon mode with higher frequency and the LO-like half space mode to the binding energy and the Stark energy shift are important and obviously increase with increasing hydrostatic pressure,whereas the interface optical phonon mode with lower frequency and the TO-like half space mode are extremely small and are insensitive to the impurity position and hydrostatic pressure.It is also shown that the conductive band bending should not be neglected.

Exploring the band structure of Wurtzite InAs nanowires using photocurrent spectroscopy

Nano Research volume We use polarized photocurrent spectroscopy in a nanowire device to investigate the band structure of hexagonal Wurtzite InAs.Signatures of optical transitions between four valence bands and two conduction bands are observed which are consistent with the symmetries expected from group theory.The ground state transition energy identified from photocurrent spectra is seen to be consistent with photoluminescence emitted from a cluster of nanowires from the same growth substrate.From the energies of the observed bands we determine the spin orbit and crystal field energies in Wurtzite InAs.This information is vital to the development of crystal phase engineering of this important III-V semiconductor.