Anomalous electric-field dependence of excitonic Fano-resonance spectra in semiconductor quantum-wells

Optical absorption spectra due to Fano resonance (FR) of an exciton in a quantum well with an external electric field perpendicular to the layer plane are presented, based on multi-channel scattering calculations incorporating a hole-subband mixing effect. Peak values of the calculated FR spectra exhibit anomalous field-dependent changes. These cannot be accounted for by the commonly-known quantum-confined Stark effect (QCSE) that has been applied exclusively to bound state spectra. This behavior, ascribable to correlation between Fano couplings and the QCSE, is revealed just in high-resolution spectra, otherwise the field-dependence results in nothing but the same as that of the bound-state spectra.     

Angular and NiFe thickness dependence of exchange bias in IrMn/NiFe/IrMn thin film

Exchange biased IrMn/NiFe/IrMn thin films were studied as a function of NiFe thickness. In plane angular dependence of a resonance field distribution which is measured by FMR was analyzed as a combined effect of an unidirectional anisotropy and an uniaxial anisotropy. The unidirectional anisotropic field and the uniaxial anisotropic field were linearly varied with NiFe thickness while the films with a thicker NiFe layer do not follow the linear variation. Resonance field and linewidth variations were also analysed with NiFe thickness.     

Nonresonant two-photon generation of excitonic matter in a CuCl pellet

A pressed CuCl pellet is optically excited at 2 K using an excitation energy in the range from 1892 to 2843 meV, which is far below the bandgap. The steady-state population dynamics unambiguously indicates an unusual two-photon generation of ground-state excitons. At high-excitation levels, the observed spectra exhibit rich spectral features arising from electron-hole plasma and electron-hole droplets formation. This nonresonant two-photon excitation is presumably assisted by impurity bands due to grain boundaries and surfaces in this random semiconductor.     

ZnS thin film prepared through a self-assembled thin film precursor route

Poly(zinc 1,6-hexanedithiolate) thin film, a precursor to prepare ZnS thin film, was self-assembled on a quartz substrate. The UV-vis spectra monitored the annealing process of the poly(zinc 1,6-hexanedithiolate) film, which revealed that the ZnS thin film began to form at approximately 515 K. The result of XRD confirmed the crystallinity of ZnS. With increase of annealing temperature, a red shift of the emission spectra was observed.     

Light-induced ultrafast phase transitions in VO2 thin film

Vanadium dioxide shows a passive and reversible change from a monoclinic insulator phase to a metallic tetragonal rutile structure when the sample temperature is close to and over 68 °C. As a kind of functional material, VO₂ thin films deposited on fused quartz substrates were successfully prepared by the pulsed laser deposition (PLD) technique. With laser illumination at 400 nm on the obtained films, the phase transition (PT) occurred. The observed light-induced PT was as fast as the laser pulse duration of 100 fs. Using a femtosecond laser system, the relaxation processes in VO₂ were studied by optical pump-probe spectroscopy. Upon a laser excitation an instantaneous response in the transient reflectivity and transmission was observed followed by a relatively longer relaxation process. The alteration is dependent on pump power. The change in reflectance reached a maximum value at a pump pulse energy between 7 and 14 mJ/cm². The observed PT is associated with the optical interband transition in VO₂ thin film. It suggests that with a pump laser illuminating on the film, excitation from the d_θ,? - state of valence band to the unoccupied excited mixed d_θ,?-π* - state of the conduction band in the insulator phase occurs, followed by a resonant transition to an unoccupied excited mixed d_θ,?-π* - state of the metallic phase band.     

Polarized angular dependence of three-dimensional light-scattering for nanoparticles on thin film wafer

Bidirectional ellipsometry has been developed as a technique for distinguishing among various scattering features near surfaces. The polarized angular dependence of three-dimensional light-scattering by the nanoparticles on thin film wafer is calculated and measured. These calculations and measurements yield angular dependence of bidirectional ellipsometric parameters for out-of-plane light-scattering. The experimental data show good agreement with theoretical predictions for different nanoparticle diameters and thin film thicknesses when bidirectional ellipsometry was employed to measure nanoparticles (60 nm, 100 nm, and 200 nm) on Si wafers with different film thicknesses of 2 nm, 5 nm, and 10 nm. Not only are the diameters of the nanoparticles determined, but also the film thicknesses can be calculated and distinguished from the measurement results. Additionally, the results indicate that improved accuracy is possible for measurements of scattering features from nanoparticles and thin films.     

The microstructure investigation of GeTi thin film used for non-volatile memory

GeTi thin film has been found to have the reversible resistance switching property in our previous work. In this paper, the microstructure of this material with a given composition was investigated. The film was synthesized by magnetron sputtering and treated by the rapid temperature process. The results indicate a coexist status of amorphous and polycrystalline states in the as-deposited GeTi film, and the grains in the film are extremely fine. Furthermore, not until the film annealed at 600 °C, can the polycrystalline state be detected by X-ray diffraction. Based on the morphological analysis, the sputtered GeTi has the column growth tendency, and the column structure vanishes with the temperature increasing. The microstructure and thermal property analysis indicate that GeTi does not undergo evident phase change process during the annealing process, which makes the switching mechanism of GeTi different from that of chalcogenide memory material, the most widely used phase change memory material.     

Post-deposition atomic terraces growth of ZnO thin films deposited on epi-GaN templates

We demonstrate how growth processes affect on ZnO film properties, which are to be essential guides to prevent defect formation in order to synthesize reproducible high quality ZnO films. First, we reveal that deposition at a low temperature is indispensable to transfer underlying GaN atomic terraces to ZnO surface. As the film thickness is increased, however, the terraces disappear to develop island morphology. It is found that the thick film surface is smoothed to the extent that atomic terraces can be seen after an appropriate thermal treatment. Adverse effects associated with high annealing temperatures are then demonstrated as evidenced by cracks formation, increased yellow cathode-luminescence and intermixing at the interface.     

Amorphous film thickness dependence for epitaxy of perovskite oxide films under excimer laser irradiation

We have studied the epitaxial growth of perovskite manganite LaMnO₃ (LMO) on SrTiO₃(1 0 0) in the excimer laser assisted metal organic deposition process. The LMO was preferentially grown from the substrate surface by the KrF laser irradiation. The study of amorphous LMO film thickness dependence on epitaxial growth under the excimer laser irradiation revealed that the photo-thermal heating effect strongly depended on the amorphous film thickness due to a low thermal conductivity of amorphous LMO: the ion-migration for chemical bond-forming at the reaction interface would be strongly enhanced in the amorphous LMO film with the large film thickness about 210 nm. On the other hand, the photo-chemical effect occurred efficiently for the amorphous film thickness in the range of 35-210 nm. These results indicate that the epitaxial growing rate was dominated by the photo-thermal heating after the photo-chemical activation at the growth interface.     

In situ measurement of the surface stress evolution during magnetron sputter-deposition of Ag thin film

We measured the evolution of in situ surface stress of Ag thin film during the magnetron sputter deposition. The measurement of force per width of Ag thin film showed that both the surface state and surface stress of Ag layer can be controlled through the variation of the deposition conditions such as the deposition temperature and rate. At room temperature, the force per width curve of Ag film deposited to 1 Å/s showed a typical curve consisting of three stages of surface stress. A brief presence of initial compressive stage and broad tensile maximum resulting in a compressive state had a tendency to disappear with increasing the deposition temperature. Meanwhile, a development of final compressive stage was more at higher temperature. Similar effect was observed but less obvious on increasing the deposition rate.     

The influence of substrate temperature variation on tungsten oxide thin film growth in an HFCVD system

Tungsten oxide (WO₃) thin films were deposited by a modified hot filament chemical vapor deposition (HFCVD) technique using Si (1 0 0) substrates. The substrate temperature was varied from room temperature to 430 °C at an interval of 100 °C. The influence of the substrate temperature on the structural and optical properties of the WO₃ films was studied. X-ray diffraction and Raman spectra show that as substrate temperature increases the film tends to crystallize from the amorphous state and the surface roughness decreases sharply after 230 °C as confirmed from AFM image analysis. Also from the X-ray analysis it is evident that the substrate orientation plays a key role in growth. There is a sharp peak for samples on Si substrate due to texturing. The film thickness also decreases as substrate temperature increases. UV-vis spectra show that as substrate temperature increases the film property changes from metallic to insulating behavior due to changing stoichiometry, which was confirmed by XPS analysis.     

AlF3 film deposited by IAD with end-Hall ion source using SF6 as working gas

A novel and effective process to fabricate high quality fluoride thin films was presented. Aluminum fluoride films deposited by a conventional thermal evaporation with an ion-assisted deposition (IAD) using SF₆ as a working gas at around room temperature were investigated. In this study, the optimal voltage and current, 50 V and 0.25 A, were found according to the optical properties of the films: high refractive index (1.489 at 193 nm), low optical absorption and extinction coefficient (<10⁻⁴ at 193 nm) in the UV range. The physical properties of the film are high packing density and amorphous without columnar structure. It was proved that using SF₆ working gas in IAD process is a good choice and significantly improves the quality of AlF₃ films.     

A study of optical absorption in amorphous hydrogenated silicon thin films of varied thickness

We report results obtained from optical absorption studies carried out on amorphous silicon thin films deposited by plasma-enhanced chemical vapour deposition (PECVD) from silane plasma. The influence of the film thickness was studied on the two series of samples deposited from undiluted silane and under moderate hydrogen dilution of silane. Spectral refractive indices and absorption coefficients were determined from transmittance spectra. The spectral absorption coefficients were used to determine the Tauc optical band-gap energies E_g, the B factors of the Tauc plots, the iso-energy values E₀₄ (energy at which the absorption coefficient is equal to 10⁴ cm⁻¹). The results were correlated with volume fractions of the amorphous phase and voids and with the film thickness.     

Influence of quantum dot density on excitonic transport and recombination in CdZnTe/ZnTe QD structures

We report on dynamics of excitons in Cd_xZn_1−xTe/ZnTe quantum dots (QDs) and present information of excitonic transport and recombination. Due to different growth methods, samples with different QD's densities were obtained. Time-resolved measurements reveal three decay mechanisms: (i) radiative recombination of excitons in the individual QDs; (ii) thermally activated escape of excitons and (iii) escape due to tunneling (hopping). In the high QD-density samples the hopping (r_HB=2700 ns⁻¹) is two orders of magnitude more efficient than in the low QD-density samples (r_HB=33 ns⁻¹). Radiative recombination rates are similar in both types of samples, r_R=1-1.3 ns⁻¹. Due to the good radiative to nonradiative recombination ratio, the low-density QDs can be a potential source of entangled photon pairs.     

Microstructure of hydrogenated silicon thin films prepared from silane diluted with hydrogen

We report results obtained from FTIR and TEM measurements carried out on silicon thin films deposited by plasma-enhanced chemical vapor deposition (PECVD) from silane diluted with hydrogen. The hydrogen content, the microstructure factor, the mass density and the volume per Si-H vibrating dipoles were determined as a function of the hydrogen dilution. Hydrogen dilution of silane results in an inhomogeneous growth during which the material evolves from amorphous hydrogenated silicon (a-Si:H) to microcrystalline hydrogenated silicon (μc-Si:H). With increasing dilution the transition from amorphous to microcrystalline phase appears faster and the average mass density of the films decreases. The μc-Si:H films are mixed-phase void-rich materials with changing triphasic volume fractions of crystalline and amorphous phases and voids. Different bonding configurations of vibrating Si-H dipoles were observed in the a-Si:H and μc-Si:H. The bonding of hydrogen to silicon in the void- and vacancy-dominated mechanisms of network formation is discussed.     

Modifications in structural and optical properties of Mn-ion implanted CdS thin films

In this paper, we report on modifications in structural and optical properties of CdS thin films due to 190 keV Mn-ion implantation at 573 K. Mn-ion implantation induces disorder in the lattice, but does not lead to the formation of any secondary phase, either in the form of metallic clusters or impurity complexes. The optical band gap was found to decrease with increasing ion fluence. This is explained on the basis of band tailing due to the creation of localized energy states generated by structural disorder. Enhancement in the Raman scattering intensity has been attributed to the enhancement in the surface roughness due to increasing ion fluence. Mn-doped samples exhibit a new band in their photoluminescence spectra at 2.22 eV, which originates from the d-d (⁴T₁ → ⁶A₁) transition of tetrahedrally coordinated Mn²⁺ ions.     

Surface characterization and microstructure of ITO thin films at different annealing temperatures

In this study, the electron beam evaporation method is used to generate an indium tin oxide (ITO) thin film on a glass substrate at room temperature. The surface characteristics of this ITO thin film are then investigated by means of an AFM (atomic force microscopy) method. The influence of postgrowth thermal annealing on the microstructure and surface morphology of ITO thin films are also examined. The results demonstrate that the film annealed at higher annealing temperature (300 °C) has higher surface roughness, which is due to the aggregation of the native grains into larger clusters upon annealing. The fractal analysis reveals that the value of fractal dimension D_f falls within the range 2.16-2.20 depending upon the annealing temperatures and is calculated by the height-height correlation function.     

Proposal of structures possessing high exciton concentration

The possibility of achievement of high exciton concentrations is analyzed. It is shown that high concentrations can be achieved in a three-layer thin molecular film due to the autoreduction processes taking place in it. Shortly, the appearance of high concentrations is the consequence of boundary conditions in film and of the magnitude of matrix elements of dipol-dipol interactions. The autoreduction takes place in the cases when matrix elements characterizing exciton transfer are less than statistical matrix elements. Based on numerical analysis, it was found that optical quanta concentrations of a three-layer film can achieve values of about 5×10⁻². The structures possessing so high concentration do not exist in nature, thus they have to be synthesised. For the current state of nanotechnology, it is not a problem. Fortunately achieving high concentrations requires only certain ratios of relevant characteristics of the film with a two-level exciton scheme, but not their single values.     

Theory of spin precession monitored by laser pulse

We first predict the splitting of a spin degenerate impurity level when this impurity is irradiated by a circularly polarized laser beam tuned in the transparency region of a semiconductor. This splitting, which comes from different exchange processes between the impurity electron and the virtual pairs coupled to the pump beam, induces a spin precession around the laser beam axis, which lasts as long as the pump pulse. It can thus be used for ultrafast spin manipulation. This effect, which has similarities with the exciton optical Stark effect we studied long ago, is here derived using the concepts we developed very recently to treat many-body interactions between composite excitons and which make the physics of this type of effects quite transparent. They, in particular, allow to easily extend this work to other experimental situations in which a spin rotates under laser irradiation.