Mingkai Li

Single-phase quaternary MgxZn1−xO1−ySy alloy thin films grown by pulsed laser deposition

Quaternary-alloy MgxZn1−xO1−ySy thin films were grown quasi-epitaxially on c-plane sapphire substrates by pulsed laser deposition. Single-phase wurtzite MgxZn1−xO1−ySy films with compositions of 0.07 < x < 0.21 and y ≥ 0.8 were achieved using various ceramic targets. The S contents in the quaternary alloy films were far beyond the reported solid solubility limits of S in single-phase ternary alloy ZnO1−ySy films. The bandgap of MgxZn1−xO1−ySy, inferred from optical transmission measurements, was narrower than the bandgap of MgxZn1−xO while broadened compared with that of ZnO1−ySy. The broadening effect was enhanced with the increase of Mg content. The simultaneous substitution of cation (Zn2+) and anion (O2−) by isoelectronic elements (Mg2+ and S2−) offers further flexibility for the band-gap engineering and potentially facilitates the p-type doping of ZnO.

First-principles calculations of the phase equilibrium of BexZn1−xO alloys

Bandgap engineering of ZnO is crucial towards its practical applications. Due to their wide bandgap, BexZn1−xO alloys are promising materials for making optoelectronic devices that function in the solar-blind wavelength region. In this study, a theoretical investigation of the thermodynamics of these BexZn1−xO alloys is carried out using both first-principles calculations and the cluster expansion method. The cluster expansion method is used to describe the disordered alloys. It is revealed that, for both wurtzite (WZ) and zincblende (ZB) phase BexZn1−xO alloys, the formation enthalpies of all structures are positive for the whole range of composition. This implies the occurrence of miscibility gap when BeO and ZnO form alloys. A good comparison between the density functional theory used and the effective cluster interaction fitted formation enthalpies validates the cluster expansion method in the calculation of the formation enthalpies. The phase diagram of BexZn1−xO has been derived based on the theoret...

Theoretical investigation of the structural, electronic, and thermodynamic properties of CdS1-xSex alloys

In this work, the structural, electronic, and thermodynamic properties of wurtzite (WZ) and zincblende (ZB) CdS1-xSex alloys are investigated using the density functional theory (DFT) and the cluster expansion method. A special quasirandom structure containing 16 atoms is constructed to calculate the band structures of random alloys. The band gaps of CdS1-xSex alloys are direct and decrease as the Se content increases. The delta self-consistent-field method is applied to correct band gaps that are underestimated by DFT. The band offsets clearly reflect the variation in valence band maxima and conduction band minima, thus providing information useful to the design of relevant quantum well structures. The positive formation enthalpies of both phases imply that CdS1-xSex is an immiscible system and tends to phase separate. The influence of lattice vibrations on the phase diagram is investigated by calculating the phonon density of states. Lattice vibration effects can reduce the critical temperature Tc and i...

First-principles study of divalent IIA and transition IIB metals doping into Cu2O

Divalent IIA metals such as Be, Mg, Ca, Sr, Ba and transition IIB metals such as Zn, Cd were investigated as possible n-type dopants into the Cu2O theoretically by using the first-principles calculations based on density functional theory. By systematical analyses of the lattice parameters, the bond length, the electronic structure, the local density of states and the defect formation energy for various doping systems, it is revealed that Ca, Sr, Ba and Be are more suited for n-type doping into Cu2O as shallow donors, compared to Mg which introduces a relatively deep donor level in Cu2O. Meanwhile, Zn and Cd can hardly be doped into Cu2O due to the positive formation energy of relevant defects.

Optical properties of the nonpolar a-plane MgZnO films grown on a-GaN/r-sapphire templates by pulsed laser deposition

Nonpolar (112¯0)a-plane MgZnO films were grown on different a-GaN/r-sapphire templates by pulsed laser deposition (PLD), where the growth temperature of GaN buffer layers varied from 700 °C to 1000 °C. High-quality a-plane MgZnO epitaxial film was deposited on the optimized 1000 °C a-GaN/r-sapphire template. Temperature-dependent PL measurements of a-plane MgZnO films reveal an S-type temperature dependence of the excitonic recombination energy. It is resulted that the excitons are localized in alloy-induced potential fluctuations at low temperature and the room-temperature quantum efficiency is calculated to be 9.2%. An involvement of band-tail states in the radiative recombination was considered, and a quantitative description of the blue temperature-induced shift was obtained assuming a Gaussian shape of the band tail.

Strain dependent anisotropy in photoluminescence of heteroepitaxial nonpolar a-plane ZnO layers

Nonpolar a-plane ZnO layers with anisotropic in-plane strains were prepared on the three substrates of r-sapphire, a-GaN, and a-Al0.08GaN templates via a pulsed laser deposition system, to investigate the distinguishing anisotropic photoluminescence properties of a-ZnO grown on foreign substrates. The optical anisotropy of nonpolar ZnO grown on GaN and AlGaN templates was investigated via polarization-dependent photoluminescence (PL) measurement and polarization transmission spectra measurement. The 0.3 μm a-ZnO layer grown on the a-GaN template has significant anisotropic optical properties with a degree of polarization (DOP) of the photoluminescence (PL) spectrum of about 0.8907, larger than 0.8786 of ZnO on a-Al0.08GaN or 0.8408 of a-ZnO on r-sapphire, revealing that the a-GaN may be the best candidate for the fabrication of modulators and that the increase of the Al component x of p-AlxGa1-xN will attenuate the anisotropic properties of the heteroepitaxial a-ZnO layer, providing a valuable basis for the choice of appropriate substrate for nonpolar a-plane ZnO based polarized optoelectronic devices. Moreover, the relationship between crystal quality anisotropy and optical anisotropy was proposed.

Annealing and characterisation of CuInS2 thin films prepared on sapphire substrates by pulsed laser deposition

Abstract Quasi-epitaxial CuInS2 thin films were deposited on c-sapphire substrates at optimised temperature by the pulsed laser deposition method using a self-made ceramic target. Crystalline and stoichiometric CuInS2 film with a band gap energy of 1·43 eV is obtained by a post-annealing/sulphurisation process at a temperature of 500°C, which shows a highly (112) preferential orientation with a smallest full width at half maximum value of 0·19°. The as-deposited films show the coexistence of CuInS2 chalcopyrite and CuAu orderings measured by Raman spectroscopy, and a polymorphic transformation of metastable CuAu ordering into the equilibrium chalcopyrite structure takes place at the annealing temperature of 500°C with the presence of a surface segregated CuxS phase. The morphologies and compositions of the films before and after annealing are characterised by scanning electron microscopy and energy dispersive X-ray analysis.