Thermal stress induced band gap variation of ZnO thin films

The growth temperature and post annealing-dependent optical and structural effect of RF magnetron sputtered ZnO thin films were examined. As the growth temperature increased, the lattice constant increased and approached the bulk value, suggesting a decrease in interfacial strain between the substrate and thin film. For the post annealed samples, the interfacial strain decreased further and was close to the bulk value regardless of the post annealing environments (in air and O₂). The optical properties of all ZnO thin films examined and revealed higher transparency (>90%). Furthermore, the optical band gap varied according to the growth temperature and post annealing environments due to a decrease in the interfacial strain effect.     

Band gap engineering of atomically thin two-dimensional semiconductors

Atomically thin two-dimensional(2D) layered materials have potential applications in nanoelectronics, nanophotonics, and integrated optoelectronics. Band gap engineering of these 2D semiconductors is critical for their broad applications in high-performance integrated devices, such as broad-band photodetectors, multi-color light emitting diodes(LEDs), and high-efficiency photovoltaic devices. In this review, we will summarize the recent progress on the controlled growth of composition modulated atomically thin 2D semiconductor alloys with band gaps tuned in a wide range, as well as their induced applications in broadly tunable optoelectronic components. The band gap engineered 2D semiconductors could open up an exciting opportunity for probing their fundamental physical properties in 2D systems and may find diverse applications in functional electronic/optoelectronic devices.     

立方氮化硼薄膜的光学带隙

用射频溅射法在p型Si(100)衬底上沉积立方氮化硼(c-BN)薄膜,薄膜的成分由傅里叶变换红外谱标识,用紫外-可见分光光度计测量了c-BN薄膜的反射光谱,利用K-K(Kramers-Kroning)关系从反射谱计算出c-BN薄膜的光吸收系数,进而确定c-BN薄膜的光学带隙.对于立方相含量为55.4%的c-BN薄膜,光学带隙为5.38eV. 关键词： 立方氮化硼薄膜 光学带隙 K-K关系     

Photonic band gap engineering in 2D photonic crystals

The polarization-dependent photonic band gaps (TM and TE polarizations) in two-dimensional photonic crystals with square lattices composed of air holes in dielectric and vice versa i.e., dielectric rods in air, using the plane-wave expansion method are investigated. We then study, how the photonic band gap size is affected by the changing ellipticity of the constituent air holes/dielectric rods. It is observed that the size of the photonic band gap changes with changing ellipticity of the constituent air holes/dielectric rods. Further, it is reported, how the photonic band gap size is affected by the change in the orientation of the constituent elliptical air holes/dielectric rods in 2D photonic crystals.     

Ultraviolet laser-induced photovoltaic effects in miscut ferroelectric LiNbO3 single crystals

This paper investigates the photovoltaic properties of miscut LiNbO 3 single crystal with different thicknesses under irradiation of a 248 nm ultraviolet laser pulse with 20 ns duration without an applied bias.Nanosecond photovoltaic response is observed and faster rise time is obtained in thinner samples.In accord with the 248 nm laser duration,the full width at half maximum of the photovoltaic signals keeps a constant of ～ 20 ns.With decrease of the crystal thickness,the photovoltaic sensitivity was improved rapidly at first and then decreased,and the maximum photovoltage occurred at 0.38 mm-thick single crystal.The present results demonstrate that decreasing the LiNbO3 single crystal thickness can obtain faster response time and improve the photovoltaic sensitivity.     

Diffraction effects in guided photonic band gap structure

The concept of the photonic band gap (PBG) structures stems from ideas of Yablonovitch. The idea is to design components so that they affect the properties of photons, in much the same way that ordinary semiconductor crystals affect the properties of electrons. In fact, the PBG structures forbid propagation of photons for a particular range of energy. They can be used to realise optical filters with large stop band and sharp transmission resonance. In the guided PBG structures, the existence of diffractive effects in the vertical dimension could limit the quality factor of such filters. In this paper, we have investigated the origin of diffraction losses in one-dimensional guided PGB structures using 2D and 3D numerical tools. We propose an analytical approach based on Bragg diffraction relation to explain these losses phenomenon. From this approach, the influence of some design parameters on the electromagnetic behaviour and the spectral response of PBG resonators will be explained.     

氮分压对氮化铜薄膜结构及光学带隙的影响

在不同的氮分压r(r=N₂/［N₂+Ar］)和射频功率P下，使用反应射频磁控溅射法，在玻璃基片上制备了氮化铜薄膜样品.用台阶仪测得了薄膜的厚度，用原子力显微镜、X射线衍射仪、紫外-可见光谱仪对薄膜的表面形貌、结构及光学性质进行了表征分析.结果表明，薄膜的沉积速率随P和r的增加而增大.薄膜表面致密均匀，晶粒尺寸为30nm左右.随着r的增加，薄膜颗粒增大，且薄膜由(111)晶面转向(100)晶面择优生长.薄膜的光学带隙E_g在1.47—1.82eV之间，随r的增加而增大. 关键词： 氮化铜薄膜 反应射频磁控溅射 晶体结构 光学带隙     

Effect of Nb doping on morphology,crystal structure,optical band gap energy of TiO2 thin films

Nb–TiO₂ nanofibers and thin films were prepared using a sol–gel derived electrospinning and spin coating, respectively, by varying the Nb/Ti molar ratios from 0 to 0.59 to investigate the effect of Nb doping on morphology, crystal structure, and optical band gap energy of Nb–TiO₂. XRD results indicated that Nb–TiO₂ is composed of anatase and rutile phases as a function of Nb/Ti molar ratio. As the Nb/Ti molar ratio rose, the anatase to rutile phase transformation and the reduction in crystallite size occurred. The band gap energy of Nb–TiO₂ was changed from 3.25 eV to 2.87 eV when the anatase phase was transformed to rutile phase with increasing the Nb doping. Experimental results indicated that the Nb doping was mainly attributed to the morphology, the crystal structure, the optical band gap energy of Nb–TiO₂, and the photocatalytic degradation of methylene blue.     

Origin of ultraviolet photovoltaic effect in Fe3O4 thin films

We have investigated the transport and ultraviolet photovoltaic properties of Fe$_{3}$O$_{4}$ thin films grown on glass substrates by facing-target sputtering technique. The nonlinear dependence of current-density on voltage suggests that the transport process is most likely the tunnelling process and grain boundaries act as barriers. Furthermore, nonequilibrium electron-hole pairs are excited in the grains and grain boundary regions for Fe$_{3}$O$_{4}$ film under ultraviolet laser, since the energy gap of Fe$_{3}$O$_{4}$ is smaller than the ultraviolet photon energy. And then the built-in electric field near the grain boundaries will separate carriers, leading to the appearance of an instant photovoltage.     

Magnetoelectric coupling effect of polarization regulation in BiFeO_3/LaTiO_3 heterostructures

An effective regulation of the magnetism and interface of ferromagnetic materials is not only of great scientific significance, but also has an urgent need in modern industry. In this work, by using the first-principles calculations, we demonstrate an effective approach to achieve non-volatile electrical control of ferromagnets, which proves this idea in multiferroic heterostructures of ferromagnetic La TiO_3 and ferroelectric Bi FeO_3. The results show that the magnetic properties and two-dimensional electron gas concentrations of La TiO_3 films can be controlled by changing the polarization directions of Bi FeO_3. The destroyed symmetry being introduced by ferroelectric polarization of the system leads to the transfer and reconstruction of the Ti-3 d electrons, which is the fundamental reason for the changing of magnetic properties.This multiferroic heterostructures will pave the way for non-volatile electrical control of ferromagnets and have potential applications.     

Effect of Er doping on optical band gap energy of TiO2 thin films prepared by spin coating

In order to evaluate the effect of Er doping in the range of 0–1.0 mol% on optical indirect band gap energy (E_g) of the film, the Er-doped TiO₂ (Er-TiO₂) thin films were spin-coated onto fluorine-doped SnO₂ coated (FTO) glasses. Glancing angle X-ray diffraction (GAXRD) results indicated that the films whose thickness was 550 nm consisted of pure anatase and FTO substrate. The anatase (101) TiO₂ peaks became broader and weaker with the rise in Er content. The apparent crystallite size decreased from 12 nm to 10 nm with increasing the amount of Er from 0 mol% to 1.0 mol%. UV–vis spectrophotometry showed that the values of E_g decreased from 3.25 eV to 2.81 eV with the increase of Er doping from 0 to 0.7 mol%, but changed to 2.89 eV when Er content was 1.0 mol%. The reduction in E_g might be attributed to electron and/or hole trapping at the donor and acceptor levels in the TiO₂ band structure.     

Energy band engineering of flexible gallium arsenide through substrate cracking with pre‐tensioned films

Flexible electronics based on the otherwise rigid conventional crystalline semiconductors is emerging as a new class of technology. However, the existing layer‐transfer approaches for implementing such technologies is mostly focused on maintaining the performance of the original device. Here we show that layer transfer through substrate cracking with a pre‐tensioned nickel film readily enables the manipulation of the electronic band structure in flexible gallium arsenide (GaAs) devices. We empirically and theoretically quantify the effect of ‘engineered' residual strain on the electronic band structure in these flexible GaAs devices. Photoluminescence and quantum efficiency measurements indicate the widening of the GaAs energy bandgap due to the residual compressive strain. The experimental results are in good agreement with our theoretical calculations. This study introduces a new way for strain engineering in flexible compound semiconductors with important implications for electronic and optoelectronic applications. (© 2016 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Low temperature synthesis wide optical band gap Al and (Al, Na) co-doped ZnO thin films

Al-doped ZnO (AZO) and (Al, Na) co-doped ZnO (ANZO) thin films were prepared via sol-gel technique with an annealing process at temperatures between 450 and 550 °C for 60 min in air ambient, and their structural and optical properties have been investigated. The deposited films exhibited hexagonal zinc oxide structure except annealing at 450 °C. For the 500 °C-annealed samples, the surface morphology was analyzed via scanning electron microscopy, Photoluminescence (PL) of different Na content ANZO thin films showed that there were very obvious violet and blue emission bands between 400 and 500 nm, and intensity of which were enhanced with Na content increasing. Transparency of the films was improved along with increasing Na content. The result of UV indicated the absorb bands appeared obviously red shift with Na doping into ZnO, the optical gaps of all films far beyond 3.37 eV of pure ZnO, and gradually decreased with Na content increasing, this is very virtual for improving photoelectricity performance of transparent conduct oxide (TCO) film. The possible origins responsible for structure and optical properties also had been discussed.     

Energy band and charge-carrier engineering in skutterudite thermoelectric materials

The binary CoSb₃ skutterudite thermoelectric material has high thermal conductivity due to the covalent bond between Co and Sb, and the thermoelectric figure of merit, ZT, is very low. The thermal conductivity of CoSb₃ materials can be significantly reduced through phonon engineering, such as low-dimensional structure, the introduction of nano second phases, nanointerfaces or nanopores, which greatly improves their ZT values. The phonon engineering can optimize significantly the thermal transport properties of CoSb₃-based materials. However, the improvement of the electronic transport properties is not obvious, or even worse. Energy band and charge-carrier engineering can significantly improve the electronic transport properties of CoSb₃-based materials while optimizing the thermal transport properties. Therefore, the decoupling of thermal and electronic transport properties of CoSb₃-based materials can be realized by energy band and charge-carrier engineering. This review summarizes some methods of optimizing synergistically the electronic and thermal transport properties of CoSb₃ materials through the energy band and charge-carrier engineering strategies. Energy band engineering strategies include band convergence or resonant energy levels caused by doping/filling. The charge-carrier engineering strategy includes the optimization of carrier concentration and mobility caused by doping/filling, forming modulation doped structures or introducing nano second phase. These strategies are effective means to improve performance of thermoelectric materials and provide new research ideas of development of high-efficiency thermoelectric materials.     

Optical band gap and optical constants in a-Se80Te20−xPbx thin films

The optical constants (absorption coefficient, refractive index, extention coefficient, real and imaginary part of dielectric constant) have been studied for a-Se₈₀Te_20−xPb_x (where x = 0, 2, 6, 10) thin films as a function of photon energy in the wave length range (500–1000 nm). It has been found that the optical band gap increases while the refractive index and the extinction coefficient (k) decreases on incorporation of lead in Se–Te system. The value of absorption coefficient (α) and the extinction coefficient (k) increases, while the value of refractive index (n) decreases with incident photon energy. The results are interpreted in terms of the change in concentration of localized states due to the shift in fermi level.     

Surface rumples and band gap reductions of cubic BaZrO3 (001) surface studied by means of first-principles calculations

Electronic properties of the (001) surface of cubic BaZrO$_{3}$ with BaO and ZrO$_{2}$ terminations have been studied using first-principles calculations. Surface structure, partial density of states, band structure and surface energy have been obtained. We find that the largest relaxation appears in the first layer of atoms, and the relaxation of the BaO-terminated surface is larger than that of the ZrO$_{2}$-terminated surface. The surface rumpling of the BaO-terminated surface is also larger than that of the ZrO$_{2}$-terminated surface. Results of surface energy calculations reveal that the BaZrO$_{3}$ surface is likely to be more stable than the PbZrO$_{3}$ surface.     

Multistep deposition of Cu2Si(S,Se)3 and Cu2ZnSiSe4high band gap absorber materials for thin film solar cells

Cu₂ZnSi(S,Se)₄ and Cu₂Si(S,Se)₃ are potential materials to obtain cost effective high band gap absorbers for tandem thin film solar cell devices. A method to synthesize Cu₂SiS₃, Cu₂SiSe₃and Cu₂ZnSiSe₄thin film absorbers is proposed. This method is based on a multistep process, using sequential deposition and annealing processes. X‐ray diffraction analysis performed on the final thin films have confirmed the presence of the Cu₂Si(S,Se)₃ and Cu₂ZnSiSe₄phases. Scanning electron microscopy images revealed the formation of polycrystalline layers with grains size up to 1 µm. The band gap of the ternary Cu₂SiSe₃ and Cu₂SiS₃, and quaternary Cu₂ZnSiSe₄ based thin films as determined from optical and photoluminescence measurements are found to be close to their theoretical values. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Surface rumples and band gap reductions of cubic BaZrO3 （001） surface studied by means of first-principles calculations

Electronic properties of the （001） surface of cubic BaZrO3 with BaO and ZrO2 terminations have been studied using first-principles calculations. Surface structure, partial density of states, band structure and surface energy have been obtained. We find that the largest relaxation appears in the first layer of atoms, and the relaxation of the BaO-terminated surface is larger than that of the ZrO2-terminated surface. The surface rumpling of the BaO-terminated surface is also larger than that of the ZrO2-terminated surface. Results of surface energy calculations reveal that the BaZrO3 surface is likely to be more stable than the PbZrO3 surface.     

LaAlO_3/SrTiO_3界面增强光伏效应

探索LaAlO_3/SrTiO_3(LAO/STO)界面产生的新奇物理特性对理解关联电子系统中多自由度耦合和设计功能材料器件具有重要的价值.本文通过脉冲激光沉积方法在SrTiO_3基底上制备了LAO/STO薄膜,研究了正面照射LAO/STO膜面和侧面照射LAO/STO界面时的光伏效应,探讨了LAO/STO界面对光伏效应的影响.研究结果表明,在同样光照能量下侧面照射LAO/STO界面产生的光电压远高于正面照射LAO/STO膜面产生的光电压,说明LAO/STO界面对光伏效应有明显的增强作用.通过偏压调控可以进一步增强照射LAO/STO界面产生的光电压,当偏压为60 V时, LAO/STO样品的位置探测灵敏度达到了36.8 mV/mm.这些研究结果为设计场调控位置敏感探测器等新型光电子器件提供了新的思路.     

Infrared laser-induced fast photovoltaic effect observed in orthorhombic tin oxide film

The SnO_2/SnO with an orthorhombic structure is a material known to be stable at high pressures and temperatures and expected to have new optical and electrical properties. The authors report a new finding of the infrared laser induced a fast photovoltaic effect arising from orthorhombic tin oxide film with an indirect band gap(~2.4 e V) which is deposited by pulsed laser deposition. The rising time of the photovoltaic signal is about 3 ns with a peak value of 4.48 mV under the pulsed laser beam with energy density 0.015 m J/mm~2. The relation between the photovoltages and laser positions along the line between two electrodes of the film is also exhibited. A possible mechanism is put forward to explain this phenomenon.All data and analyses demonstrate that the orthorhombic tin oxide with an indirect band gap could be used as a candidate for an infrared photodetector which can be operated at high pressures and temperatures.