Mixed crystals in the system Cu2MnGexSn1−xS4: Phase analytical investigations and inspection of tetrahedra volumes

Cu₂MnGeS₄ crystallizes orthorhombic in a wurtzite superstructure type while Cu₂MnSnS₄ crystallizes in a tetragonal sphalerite superstructure type. Lattice constants and thermal analyses of the solid solution series Cu₂MnGe_xSn_1−xS₄ are presented. A two-phase region is found from Cu₂MnGe_0.3Sn_0.7S₄ to Cu₂MnGe_0.5Sn_0.5S₄. The cell volume of the mixed crystals increases with increasing Sn content. The melting points increase smoothly with increasing Ge content to x=0.5 and then steeply for higher Ge contents. The single crystal X-ray structure analysis of Cu₂MnGe_0.55Sn_0.45S₄ is presented. The refinement converges to R=0.0270 and wR₂=0.0586, Z is 2. The volumes of the tetrahedra [MS₄] (M=Cu, Mn, Ge, Sn) are calculated. From these volumes the differences in size of the tetrahedra are derived and compared with the corresponding differences in the end members of the solid solution series. It turns out that the resulting structure type in these materials depends on the volume differences of the constituting tetrahedra [MS₄].     

First-principle study on the effect of high Li-2N co-doping on the conductivity of ZnO

Based on the density functional theory (DFT), using first-principles plane-wave ultrasoft pseudopotential method, the models of the unit cell of pure ZnO and two highly Li-2N co-doped supercells of Zn_0.9375Li_0.0625O_0.875N_0.125 and Zn_0.9167Li_0.0833O_0.8333N_0.1667 were constructed, and the geometry optimization for the three models was carried out. The total density of states (TDOS) and the band structures (BS) were also calculated. The calculation results showed that in the range of high doping concentration, when the co-doping concentration is more than a certain value, the conductivity decreased with the increase of co-doping concentration of Li-2N in ZnO, which agrees with the change trend of the experimental results.     

Surface and interface electronic properties of group III-nitride heterostructures

For group III-nitrides with wurtzite structure the presence of fixed polarization interface charges yields new challenges in order to understand and control Schottky barrier heights, band offsets and 2D confinement in heterostructure FETs. In this short review experimental results obtained by in situ photoemission spectroscopy on MBE AlGaN/GaN heterostructures grown on 6H–SiC are discussed, with emphasis on the presence and interplay of surface electronic states. Schrödinger–Poisson calculations are performed to get the complete band scheme at the selected heterojunctions. Results on the polarity dependence of Pt/GaN Schottky barrier values from the literature are also discussed.     

Effects of hydrogen annealing on the room temperature ferromagnetism and optical properties of Cr-doped ZnO nanoparticles

We explore the effects of hydrogen annealing on the room temperature ferromagnetism and optical properties of Cr-doped ZnO nanoparticles synthesized by the sol-gel method. X-ray diffraction and x-ray photoelectron spectroscopy data show evidence that Cr has been incorporated into the wurtzite ZnO lattice as Cr²⁺ ions substituting for Zn²⁺ ions without any detectable secondary phase in as-synthesized Zn_0.97Cr_0.03O nanopowders. The room temperature magnetization measurements reveal a large enhancement of saturation magnetization M_s as well as an increase of coercivity of H₂-annealed Zn_0.97Cr_0.03O:H samples. It is found that the field-cooled magnetization curves as a function of temperature from 40 to 400 K can be well fitted by a combination of a standard Bloch spin-wave model and Curie–Weiss law. The values of the fitted parameters of the ferromagnetic exchange interaction constant a and the Curie constant C of H₂-annealed Zn_0.97Cr_0.03O:H nanoparticles are almost doubled upon H₂-annealing. Photoluminescence measurements show evidence that the shallow donor defect or/and defect complexes such as hydrogen occupying an oxygen vacancy H_o may play an important role in the origin of H₂-annealing induced enhancement of ferromagnetism in Cr-H codoped ZnO nanoparticles.     

Influence of spin–orbit interactions on the polaron properties in wurtzite semiconductor quantum well

We have study the simultaneous effect of Rashba and Dresselhaus spin–orbit interactions on the polaron properties in wurtzite semiconductor quantum wells. The linear and cubic contributions of the bulk Dresselhaus spin–orbit coupling and the effects of phonon confinement on electron–optical-phonon interaction Hamiltonians are taken into account. We have found analytical solutions for the polaron energies as well as polaron effective mass within the range of validity of perturbation theory. It is shown that the polaron energy and effective mass correction are both significantly enhanced by the spin–orbit coupling. Wave number dependent phonon contribution on the electron energy has minima and varies differently of the spin-up and spin-down states. Polaron self-energy due to interface optical phonon modes has larger values than of the confined optical phonon modes ones. The polaron effective mass exhibits anisotropy and the contribution of the Dresselhaus spin–orbit coupling term on the polaron effective mass is dominated by Rashba one.     

Unambiguous evidence for wurtzite phase in capped CdS quantum dots

The problem of identification of the correct crystalline structure of CdS nanoclusters below 2.5 nm in size is outlined. Structure of thiophenol capped CdS nanoclusters in the size range 1.2–4.3 nm, synthesized using cadmium acetate solution in methanol, is discussed using powder XRD and electron diffraction data. Unambiguous confirmation of the wurtzite phase in CdS nanoclusters below 2.5 nm size is reported. The observation of 102 wurtzite peak in the XRD patterns of such nanoclusters indicates low stacking fault concentration.     

Facile synthesis of nanocrystalline wurtzite Cu-In-S by amine-assisted decomposition of precursors

Phase-pure ternary wurtzite Cu-In-S nanocrystals have been synthesized by a simple amine-assisted decomposition of mixed precursor complexes derived from S-methyl dithiocarbazate (SMDTC) at a relatively low temperature without using any external surfactant. The crystal phase, morphology, crystal lattice, and chemical composition of the as-prepared products were analyzed by using X-ray diffraction, transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and energy-dispersive X-ray spectroscopy (EDX). The optical properties show the pronounced quantum confinement effect in nanocrystals. A possible growth mechanism has been suggested for the formation of anisotropic wurtzite Cu-In-S It is believed that a combined effect of the chelating amine and precursors containing CH₃S unit plays a key role in the formation of the metastable phase of wurtzite Cu-In-S.     

Magnetism in Cr doped ZnO: Density-functional theory studies

We investigate the magnetism in wurtzite and zinc-blende Zn_0.875Cr_0.125O (wz- and zb-Zn_0.875Cr_0.125O) using the density-functional theory calculations. We demonstrate that the delta-doping structures are more energetically favorable than the homogeneous-doping ones. The delta-doping structure of zb-Zn_0.875Cr_0.125O shows robust ground state ferromagnetism (FM) with ferromagnetic stabilization energy of 575 meV/Cr-Cr pair, while delta-doping structure of wz-Zn_0.875Cr_0.125O shows weak ground state anti-FM. We discuss the magnetic coupling in Zn_1−xCr_xO, and the origin of FM in zb-Zn_1−xCr_xO using a crystal field model. Finally, we anticipate the potential spintronics applications of the zb-Zn_1−xCr_xO.     

Effect of electron–phonon interaction on surface states in wurtzite nitride semiconductors under pressure

A variational theory is proposed to study the electronic surface states in semi-infinite wurtzite nitride semiconductors under the hydrostatic pressure. The electronic surface state energy level is calculated, by taking the effects of the electron–Surface–Optical–phonon interaction, structural anisotropy and the hydrostatic pressure into account. The numerical computation has been performed for the electronic surface state energy levels, coupling constants and the average penetrating depths of the electronic surface state wave functions under the hydrostatic pressure for wurtzite GaN, AlN and InN, respectively. The results show that electron–Surface–Optical–phonon interaction lowers the electronic surface state energy levels. It is also found that the electronic surface state energy levels decrease with the hydrostatic pressure in wurtzite GaN and AlN. But for wurtzite InN, the case is contrary. It is shown that the hydrostatic pressure raised the influence of electron–phonon interaction on the electronic surface states obviously. The effect of electron–Surface–Optical–phonon interaction under the hydrostatic pressure on the electronic surface states cannot be neglected, in specially, for materials with strong electron–phonon coupling and wide band gap.