Optical properties of exciton confinement in wurtzite ZnO/MgxZn1−xO coupled quantum dots

Based on the framework of effective-mass approximation and variational approach, optical properties of exciton are investigated theoretically in ZnO/Mg_xZn_1−xO vertically coupled quantum dots (QDs), with considering the three-dimensional confinement of electron and hole pair and the strong built-in electric field effects due to the piezoelectricity and spontaneous polarization. The exciton binding energy, the emission wavelength and the oscillator strength as functions of the different structural parameters (the dot height and the barrier thickness between the coupled wurtzite ZnO QDs) are calculated with the built-in electric field in detail. The results elucidate that structural parameters have a significant influence on the exciton state and optical properties of ZnO coupled QDs. These results show the optical and electronic properties of the quantum dot that can be controlled and also tuned through the nanoparticle size variation.     

Exciton in wurtzite GaN/AlxGa1−xN coupled quantum dots

Based on effective-mass approximation, we present a three-dimensional study of the exciton in GaN/Al_xGa_1−xN vertically coupled quantum dots (QDs) by a variational approach. The strong built-in electric field due to the piezoelectricity and spontaneous polarization is considered. The relationship between exciton states and structural parameters of wurtzite GaN/Al_xGa_1−xN coupled QDs is studied in detail. Our numerical results show that the strong built-in electric field in the GaN/Al_xGa_1−xN strained coupled QDs leads to a marked reduction of the effective band gap of GaN QDs. The exciton binding energy, the QD transition energy and the electron-hole recombination rate are reduced if barrier thickness L^AlGaN is increased. The sizes of QDs have a significant influence on the exciton state and interband optical transitions in coupled QDs.     

Size dependent effective band gap,optical absorption coefficients and refractive index changes in a wide ZnS/Zn1−xMgxS strained quantum well

Exciton binding energy of a confined heavy hole exciton is investigated in a Zn_1−xMg_xS/ZnS/Zn_1−xMg_xS single strained quantum well with the inclusion of size dependent dielectric function for various Mg content. The effects of interaction between the exciton and the longitudinal optical phonon are brought out. The effect of exciton is described by the effective potential between the electron and hole. The interband emission energy as a function of well width is calculated for various Mg concentration with and without the inclusion of dielectric confinement. Non-linear optical properties are carried out using the compact density matrix approach. The dependence of nonlinear optical processes on the well width is investigated for different Mg concentration. The linear, third order non-linear optical absorption coefficients values and the refractive index changes of the exciton are calculated for different concentration of magnesium content. The results show that the exciton binding energy is found to exceed LO phonon energy of ZnS for x>0.2 and the incorporation of magnesium ions and the effect of phonon have great influence on the optical properties of ZnS/Zn_1−xMg_xS quantum wells.     

Interband optical absorption in a strained CdxZn1−xO/ZnO quantum dot

Numerical calculations of the excitonic absorption spectra in a strained Cd_xZn_1−xO/ZnO quantum dot are investigated for various Cd contents. We calculate the quantized energies of the exciton as a function of dot radius for various confinement potentials and thereby the interband emission energy is computed considering the internal electric field induced by the spontaneous and piezoelectric polarizations. The optical absorption as a function of photon energy for different dot radii is discussed. Decrease of exciton binding energy and the corresponding optical band gap with the Cd concentration imply that the confinement of carriers decreases with composition x. The main results show that the confined energies and the transition energies between the excited levels are significant for smaller dots. Non-linearity band gap with the increase in Cd content is observed for smaller dots in the strong confinement region and the magnitude of the absorption spectra increases for the transitions between the higher excited levels.     

Electric-field-induced energy spectra and dispersions of ZnO/MgxZn1−xO MQWs

The energy spectra and dispersion relations of carriers in the presence of an electric field applied along the growth direction in ZnO/Mg_xZn_1−xO multiple quantum wells (MQW) are calculated using the asymptotic transfer method (ATM) on the basis of the quasistationary state approximation. The energy spectra of the carriers induce some quasi-bound levels under electric fields. The dispersion relations for the energy of the ground state and lower excitation states still have parabolic shapes for both the electrons and the heavy holes in the presence of a moderate electric field. Our results also reveal that the number of energy levels increases with increasing number of ZnO quantum wells and that the energies increase with both increasing Mg composition x and electric field strength.     

Structural and optical properties of ZnO nanorods grown on MgxZn1−xO buffer layers

ZnO nanorod arrays were synthesized by chemical-liquid deposition techniques on Mg_xZn_1−xO (x = 0, 0.07 and 0.15) buffer layers. It is found that varying the Mg concentration could control the diameter, vertical alignment, crystallization, and density of the ZnO nanorods. The X-ray diffraction (XRD), transmission electron microscopy (TEM), and selected area electron diffraction (SAED) data show the ZnO nanorods prefer to grow in the (0 0 2) c-axis direction better with a larger Mg concentration. The photoluminescence (PL) spectra of ZnO nanorods exhibit that the ultraviolet (UV) emission becomes stronger and the defect emission becomes weaker by increasing the Mg concentration in Mg_xZn_1−xO buffer layers.     

Optical properties of MgxZn1−xO nanowire photonic crystals

We investigate the optical properties of two-dimensional periodic arrays of well-aligned Mg_xZn_1−xO nanowires, i.e., Mg_xZn_1−xO nanowire photonic crystals. The nanowire photonic crystal can exhibit a photonic band gap in the visible range. As the mole fraction of Mg, x, increases, the edge frequencies of the band gap increase and the band gap size decreases. The characteristics of relative band gap and vacant point defect mode are also studied with varying x. From the finite-difference time-domain simulations, we show that the light extraction from nanowires can be controlled by varying the distance between optically excited nanowires and a waveguide, and the mole fraction of Mg. Controlling the light extraction from nanostructures can be useful in the implementation of nanoscale light emitting devices.     

Optical properties of MgxZn1−xO thin films deposited on silicon and sapphire substrate by rf magnetron sputtering

The index dispersion at UV–VIS range for polycrystalline Mg_xZn_1−xO films on silicon with different Mg concentration was obtained by spectroscopic ellipsometry (SE) method. It decreases with the increase of the Mg content. Above the relative peak wavelength, they are well fitted by the first-order Sellmeier relation. The band gap of films on sapphire of different Mg content was determined from transmission measurements. Photoluminescence (PL) illustrated that for Mg_xZn_1−xO films every PL peak corresponded to a special excitation wavelength. The wavelength of the PL peak was proportional to the special excitation wavelength. A strong peak was obtained in the blue band for the films due to the large amount of oxygen vacancies caused by excess Zn and Mg atoms, while weak peak at ultraviolet band.     

Growth of cubic MgxZn1−xO alloy films by electron beam evaporation

We report the growth of cubic Mg_xZn_1−xO alloy thin films on quartz by electron beam evaporation. It can be found that all the samples have sharp absorption edges by the absorption measurements. X-ray diffraction measurements indicate the Mg_xZn_1−xO films are cubic phase with preferred orientation along the (1 1 1) direction. Energy dispersive spectrometry (EDS) demonstrates that the Mg concentration in Mg_xZn_1−xO films is much higher than the ceramic target used, and the composition can be tuned in a small scope by varying the substrate temperature and the beam electric current. The reasons of this phenomenon are also discussed.     

Oxygen flux influence on the morphological, structural and optical properties of Zn1−xMgxO thin films grown by plasma-assisted molecular beam epitaxy

The Zn_1−xMg_xO thin films were grown on Al₂O₃ substrate with various O₂ flow rates by plasma-assisted molecular beam epitaxy (P-MBE). The growth conditions were optimized by the characterizations of morphology, structural and optical properties. The Mg content of the Zn_1−xMg_xO thin film increases monotonously with decreasing the oxygen flux. X-ray diffractometer (XRD) measurements show that all the thin films are preferred (0 0 2) orientated. By transmittance and absorption measurements, it was found that the band gap of the film decreases gradually with increasing oxygen flow rate. The surface morphology dependent on the oxygen flow rate was also studied by field emission scanning electron microscopy (FE-SEM). The surface roughness became significant with increasing oxygen flow rate, and the nanostructures were formed at the larger flow rate. The relationship between the morphology and the oxygen flow rate of Zn_1−xMg_xO films was discussed.     

The effect of growth ambient on the structural and optical properties of MgxZn1−xO thin films

Mg_xZn_1−xO (x = 0.0-0.20) thin films have been deposited by sol-gel technique on glass substrates and the effect of growth ambient (air and oxygen) on the structural, and optical properties have been investigated. The films synthesized in both ambient have hexagonal wurtzite structure. The c-axis lattice constant decreases linearly with the Mg content (x) up to x = 0.05, and 0.10 respectively for air- and oxygen-treated films, above which up to x = 0.20, the values vary irregularly with x. The change in the optical band gap values and the ultraviolet (UV) peak positions of Mg_xZn_1−xO films show the similar change with x. These results suggest that the formation of solid solution and thus the structural and optical properties of Mg_xZn_1−xO thin films are affected by the growth ambient.     

Cu(In,Ga)Se2 superstrate-type solar cells with Zn1−xMgxO buffer layers

Superstrate-type Cu(In,Ga)Se₂ (CIGS) thin film solar cells were fabricated using Zn_1−xMg_xO buffer layers. Due to the diffusion of Cd into CIGS during the growth of the CIGS layer, the conventional buffer material of CdS is not suitable. ZnO is a good candidate because of higher thermal tolerance but the conduction band offset (CBO) of ZnO/CIGS is not appropriate. In this study, the Zn_1−xMg_xO buffer layers were used to fulfill both the requirements. The superstrate-type solar cells with a soda-lime glass/In₂O₃:Sn/Zn_1−xMg_xO/CIGS/Au structure were fabricated with different band gap energies of the Zn_1−xMg_xO layer. The CIGS layers [Ga/(In + Ga)∼0.25] were deposited by co-evaporation method. The substrate temperature during the CIGS deposition of 450 °C did not cause the intermixing of the Zn_1−xMg_xO and CIGS layers. The conversion efficiency of the cell with Zn_1−xMg_xO was higher than that with ZnO due to the improvement of open-circuit voltage and shunt resistance. The results well corresponded to the behavior of the adjustment of CBO, demonstrating that the usefulness of the Zn_1−xMg_xO layer for the CBO control in the superstrate-type CIGS solar cells.     

Linear and nonlinear optical response of MgxZn1−xO: A density functional study

Total and partial density of states, frequency dependent complex refractive index including extinction coefficient, optical conductivity and transmission of Mg_xZn_1−xO (0≤x≤1) in rocksalt and wurtzite phases are calculated using full potential linearized augmented plane wave (FP-LAPW) method. The real part of refractive index decreases while the extinction coefficient, optical conductivity and transmission for rocksalt phase increases with the increase in Mg concentration. In wurtzite phase, ordinary and extraordinary indices decrease while extinction coefficient, optical conductivity and transmission increase in parallel as well as perpendicular to c-axis with the increase in the Mg concentration.     

Properties of MgxZn1−xO thin films sputtered in different gases

Mg_xZn_1−xO alloy films were prepared on sapphire substrates using Ar and N₂ as the sputtering gases. The effect of the sputtering gas on the structural, optical and electrical properties of the Mg_xZn_1−xO films was studied. By using N₂ as the sputtering gas, the Mg_xZn_1−xO film shows p-type conductivity and the band gap is larger than that employing Ar as the sputtering gas. The reason for this phenomenon is thought to be related to the reaction between N-O or N-Zn, and the N-doping.     

Electric field induced nonlinear optical properties of a confined exciton in a ZnO/Zn1−xMgxO strained quantum dot

Electric field induced exciton binding energy as a function of dot radius in a ZnO/Zn_1−xMg_xO quantum dot is investigated. The interband emission as a function of dot radius is obtained in the presence of electric field strength. The Stark effect on the exciton as a function of the dot radius is discussed. The effects of strain, including the hydrostatic and the biaxial strain and the internal electric field, induced by spontaneous and piezoelectric polarization are taken into consideration in all the calculations. Numerical calculations are performed using variational procedure within the single band effective mass approximation. Some nonlinear optical properties are investigated for various electric field strengths in a ZnO/Zn_1−xMg_xO quantum dot taking into account the strain-induced piezoelectric effects. Our results show that the nonlinear optical properties strongly depend on the effects of electric field strength and the geometrical confinement.     

Formation of ZnO and Zn1−xCdxO films on CdTe/CdZnTe single crystals

In this work we report on the properties of ZnO and Zn_1−xCd_xO films formed on top of CdTe and CdZnTe single crystals. The films have been obtained by thermal evaporation of Zn metal films and further oxidation in atmospheric conditions. The structural and compositional characteristics of the films have been analysed by means of scanning electron microscopy and energy-dispersive X-ray analysis. The chemical composition of the films as a function of growth parameters has been obtained. It has been possible to demonstrate by Raman spectroscopy the formation of both ZnO and Zn_1−xCd_xO films. The possible inter-diffusion effects between the films and the substrate, derived from the oxidation process, have been discussed. It has been possible to check by means of photoluminescence, the optical quality of the ZnO and Zn_1−xCd_xO films, also regarding to the presence of local changes. Differences between the optical spectra obtained from various ZnO films grown on top of CdTe and CdZnTe substrates enabled the determination of compositional differences introduced by the substrate when the deposition parameters are modified.     

Zn1−xMgxO/ZnO heterostructures studied by Kelvin probe force microscopy conjunction with probe characterizer

The surface potential of Zn_1−xMg_xO/ZnO heterostructure grown by radical source molecular beam epitaxy was measured by Kelvin force microscopy (KFM). A clear correlation was observed between the topographic image and the surface potential of Zn_0.56Mg_0.44O/ZnO heterostructure. The potential area around the surface pits was about 60 mV lower than that of the surrounding region, which suggests the effects of the pits on the electrical properties of the potential layer. In order to guarantee the accuracy of measurement, the probe shape was analyzed by probe characterizer and using Au thin films as a potential standard.     

Effect of substrate annealing on the quality of pulsed laser deposited Zn1−xMgxO thin films

The annealing effects of sapphire substrate before deposition on the quality of epitaxial Zn_1−xMg_xO thin films grown by pulsed laser deposition are reported. Our Experimental results indicate that the surface quality of Zn_1−xMg_xO thin films and hexagonal columnar growth is improved on the annealed sapphire substrate at high temperatures due to formation of atomic terraces on the substrate surface. The photoluminescence signals also increases with the increasing annealing temperature of the substrate.     

Effects of oxygen partial pressure and substrate temperature on the structure and optical properties of MgxZn1−xO thin films prepared by magnetron sputtering

Deposited with different oxygen partial pressures and substrate temperatures, Mg_xZn_1−xO thin films were prepared using a Mg_0.6Zn_0.4O ceramic target by magnetron sputtering. The structural and optical properties of the prepared thin films were investigated. The X-ray diffraction spectra reveal that all the films on quartz substrate are grown along (2 0 0) orientation with cubic structure. The lattice constant decreases and the crystallite size increases with the increase of substrate temperature. Both energy dispersive X-ray spectroscopy and calculated results suggest the ratio of Mg/Zn increases with increasing substrate temperature. The thin film deposited with T_s = 500 °C has a minimal rms roughness of 7.37 nm. The transmittance of all the films is higher than 85% in the visual region. The optical band gap is not sensitive to the oxygen partial pressure, while it increases from 5.63 eV for T_s = 100 °C to 5.95 eV for T_s = 700 °C. In addition, the refractive indices calculated from transmission spectra are sensitive to the substrate temperature. The photoluminescence spectra of Mg_xZn_1−xO thin films excited by 330 nm ultraviolet light indicate that the peak intensity of the spectra is influenced by the oxygen partial pressure and substrate temperature.     

Structural, optical and electrical properties of Zn1−xCdxO thin films prepared by PLD

Ternary polycrystalline Zn_1−xCd_xO semiconductor films with cadmium content x ranging from 0 to 0.23 were obtained on quartz substrate by pulse laser deposited (PLD) technique. X-ray diffraction measurement revealed that all the films were single phase of wurtzite structure grown on c-axis orientation with its c-axis lattice constant increasing as the Cd content x increasing. Atomic force microscopy observation revealed that the grain size of Zn_1−xCd_xO films decreases continuously as the Cd content x increases. Both photoluminescence and optical measurements showed that the band gap decreases from 3.27 to 2.78 eV with increasing the Cd content x. The increase in Cd content x also leads to the broadening of the emission peak. The resistivity of Zn_1−xCd_xO films decreases evidently for higher values of Cd content x. The shift of PL emission to visible light as well as the decrease of resistivity makes the Zn_1−xCd_xO films potential candidate for optoelectronic device.