Chunwang Zhao

Full-Field Strain Mapping at a Ge/Si Heterostructure Interface

The misfit dislocations and strain fields at a Ge/Si heterostructure interface were investigated experimentally using a combination of high-resolution transmission electron microscopy and quantitative electron micrograph analysis methods. The type of misfit dislocation at the interface was determined to be 60° dislocation and 90° full-edge dislocation. The full-field strains at the Ge/Si heterostructure interface were mapped by using the geometric phase analysis (GPA) and peak pairs analysis (PPA), respectively. The effect of the mask size on the GPA and PPA results was analyzed in detail. For comparison, the theoretical strain fields of the misfit dislocations were also calculated by the Peierls-Nabarro and Foreman dislocation models. The results showed that the optimal mask sizes in GPA and PPA were approximately three tenths and one-tenth of the reciprocal lattice vector, respectively. The Foreman dislocation model with an alterable factor a = 4 can best describe the strain field of the misfit dislocation at the Ge/Si heterostructure interface.

Strain Field Mapping of Dislocations in a Ge/Si Heterostructure

Ge/Si heterostructure with fully strain-relaxed Ge film was grown on a Si (001) substrate by using a two-step process by ultra-high vacuum chemical vapor deposition. The dislocations in the Ge/Si heterostructure were experimentally investigated by high-resolution transmission electron microscopy (HRTEM). The dislocations at the Ge/Si interface were identified to be 90° full-edge dislocations, which are the most efficient way for obtaining a fully relaxed Ge film. The only defect found in the Ge epitaxial film was a 60° dislocation. The nanoscale strain field of the dislocations was mapped by geometric phase analysis technique from the HRTEM image. The strain field around the edge component of the 60° dislocation core was compared with those of the Peierls–Nabarro and Foreman dislocation models. Comparison results show that the Foreman model with a = 1.5 can describe appropriately the strain field around the edge component of a 60° dislocation core in a relaxed Ge film on a Si substrate.

Effects of V heavy doping on the magnetic and optical properties in anatase TiO2

A half-metal diluted magnetic semiconductor (DMS) can be formed in heavy V-doped TiO2. Contradictory experimental results in the literature have reported about the absorption spectra blueshift and redshift results in heavy V-doped TiO2. This study aims to reveal the mechanism of half-metal DMS in heavy V-doped TiO2 and solve the problem of absorption spectra blueshift and redshift in the doping system. In this study, models of the unit cells of pure anatase TiO2 and two V heavy-doped supercells of Ti0.96875V0.03125O2 and Ti0.9375V0.0625O2 were constructed based on density functional theory, which uses the first-principles plane-wave ultrasoft pseudopotential method. All models were obtained through geometry optimization. Local density approximation (LDA) + U was used to calculate the band structure, density of states (DOS), orbital charge and absorption spectrum of the doping system. The calculated results under the condition of electron spin showed that in the heavy doping concentration range, the volume of supercells increases, the total energy and formation energy decrease and the stability of the supercells increases as V doping concentration increases. Furthermore, the interaction of p–p states is weaker than that of p–d states, which results in the valence band maximum shifting toward the low-energy region, and also the optical bandgap becomes narrower as well as the redshift and intensity of the absorption spectrum become more notable. Noticeably, the hybrid coupling effect of Ti-3d and V-3d states becomes stronger, and the magnetic moment increases. The Fermi levels of spin-up band structure within the conduction band, which form the n-type degenerate semiconductors, and the Fermi levels of spin-down band structure within the bandgap indicate that the doping system has semiconductor features. Therefore, V-doped anatase TiO2 is an extremely promising DMS because of its high electron polarizability of nearly 100%. The calculation results are consistent with the experimental data; these results explain the problems reasonably and adequately. Therefore, the research findings can help solve the contradiction of the redshift and blueshift in the preparation of photocatalysts and half-metal diluted magnetic semiconductors of V heavy-doped anatase TiO2.

Study on the effect of high V doping on the conductivity of anatase TiO2

Based on density functional theory (DFT) and local density approximation (LDA), the band structure and the density of states of all models of pure and V-doped anatase TiO2 systems were calculated using plane-wave ultra-soft pseudo potential method after geometry optimization. The calculation results show that, with the increase of the doping concentration of V in anatase TiO2, the free electron concentration and electron effective mass increase; the electronic mobility and electronic conductivity decrease, which is consistent with the change trend of the experimental results.

Effect of heavy Ag doping on the physical properties of ZnO

The band structure, density of state and absorption spectrum of Zn1−xAgxO (x = 0.02778, 0.04167) were calculated. Results indicated that a higher doping content of Ag led to a higher total energy, lower stability, higher formation energy, narrower bandgap, more significant red shift of the absorption spectrum, higher relative concentration of free hole, smaller hole effective mass, lower mobility and better conductivity. Furthermore, four types of model with the same doping content of double Ag-doped Zn1−xAgxO (x = 0.125) but different manners of doping were established. Two types of models with different doping contents of double Ag-doped Zn1−xAgxO (x = 0.0626, 0.0833) but the same manner of doping, were also established. Under the same doping content and different ordering occupations in Ag double doping, the doped system almost caused magnetic quenching upon the nearest neighbor –Ag–O–Ag– bonding at the direction partial to the a- or b-axis. Upon the next-nearest neighbor of –Ag–O–Zn–O–Ag– bonding at t...

Study on the effects of Ga-2N high co-doping and preferred orientation on the stability, bandgap and absorption spectrum of ZnO

Currently, the stability and visible light properties of Ga-2N co-doped ZnO systems have been studied extensively by experimental analysis and theoretical calculations. However, previous theoretical calculations arbitrarily assigned Ga- and 2N-doped sites in ZnO. In addition, the most stable and possible doping orientations of doped systems have not been fully and systematically considered. Therefore, in this paper, the electron structure and absorption spectra of the unit cells of doped and pure systems were calculated by first-principles plane-wave ultrasoft pseudopotential with the GGA+U method. Calculations were performed for pure ZnO, Ga-2N supercells heavily co-doped with Zn1−xGaxO1−yNy (x = 0.03125 − 0.0625, y = 0.0625 − 0.125) under different co-doping orientations and conditions, and the Zn16GaN2O14 interstitial model. The results indicated that under different orientations and constant Ga-2N co-doping concentrations, the systems co-doped with Ga-N atoms vertically oriented to the c-axis and with...