Papers by Keyword: Heterostructure

Abstract: Multifunctional Ag₂S-Ag heterogeneous porous nanorods were synthesised by the irradiation of microwave in 210s .The nanorods were characterized by scanning electron microscopic (SEM) and X-ray diffraction (XRD). The photocatalytic activity for degradation of crystal violet and the lowest detectable limit of crystal violet are tested. Results show that the degradation rate of crystal violet can reach almost 100% within 50 min, the detection limit of crystal violet as low as 1×10^-6 M. They indicate that the Ag₂S-Ag heterogeneous porous nanorods would be a promising material for environmental protection.

Abstract: TiO₂/SnO₂ thin films heterostructures are grown by the sol-gel-dip-coating technique. It was found that the crystalline structure of TiO₂ depends on the annealing temperature and the substrate type. TiO₂ films deposited on glass substrate, submitted to thermal annealing until 550°C, present anatase structure, whereas films deposited on quartz substrate transform to rutile structure when thermally annealed at 1100°C. When structured as rutile, this oxide semiconductor has very close lattice parameters to those of SnO₂, making easier the heterostructure assembling. Electrical properties of TiO₂/SnO₂ heterostructure were evaluated as function of temperature and excitation with different light sources. The temperature dependence of conductivity is dominated by a deep level with energy coincident with the second ionization level of oxygen vacancies in SnO₂, suggesting the dominant role of the most external layer material (SnO₂) to the electrical transport properties. The fourth harmonic of a Nd:YAG laser line (4.65eV) seems to excite the most external layer whereas a InGaN LED (2.75eV) seems to excite electrons from the ground state of a quantized interfacial channel as well as intrabandgap states of the TiO₂ layer.

Abstract: In the present study, the heterostructures of ZnO Nanoparticle (NP)/CdS nanowire (NW), SnO₂NP/CdS NW, NiS NP/CdS NW, FeS NP/CdS NW, Ag₂S NP/CdS NW, and Au NP/CdS NW have been successfully fabricated via the two-stage solvothermal process. Field-emission scan electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS) were adopted to characterize the as-prepared products. The optical properties of the as-obtained heterostructures were separately investigated. New insights into understanding and controlling the synthesis of different NW heterostructures are demonstrated in the reliable solvothermal route. We demonstrate that CdS NWs synthesized for 2h are the bifunctional mediator acting as catalyst or active spot for the growth of NW heterostructures Furthermore, understanding and controlling this phenomenon is a great asset for the realization of the formation mechanism of the NW heterostructures and opens up new ways toward for construction of other semiconductor heterostructures with novel properties.

Abstract: Heterostructures consisting of ZnO and diamond appear to have an elusive nature. A rectifying behaviour was previously observed only for heterojunctions with very lightly doped p-type diamond using residual boron gas during the chemical vapour deposition process or type IIb diamond. Other studies, however, claimed to obtain a rectifying behaviour for heterojunctions with p-type diamond with higher carrier densities between 10¹⁸ 10¹⁹ cm^-3. In this work we investigate the behaviour of n-type ZnO on heavily boron-doped p-type diamond. This heterostructure that is sensitive to UV light has been fabricated using ZnO nanorods grown on heavily boron-doped chemical vapour deposition diamond substrates. The I - V measurements show a rectifying characteristic. The threshold voltages under dark and UV conditions are 3.66 and 2.52 V, respectively. The UV illumination also results in an increased current flow. The electrical behaviour due to the UV illumination will be discussed.

Abstract: Resistance switching properties of nanostructured In2_{Subscript text}O₃ films grown on Pt and LaNiO₃ (LNO) bottom electrodes have been investigated. High quality In₂O₃/LNO/SiO₂ and In₂O₃/Pt/Ti/SiO₂/Si heterostructures were grown by pulsed laser deposition. High purity Ag was thermally evaporated on In₂O₃ active layer to form top electrode. The Ag/In₂O₃/M (M = LNO, Pt) structure was characterized by grazing incidence XRD, AFM and cross sectional SEM. Pollycrystalline growth of oxides LNO and In₂O₃ was confirmed by GIXRD, where as AFM show nanostructured growth with smooth surface morphology. Two terminal I-V characteristics showed reproducible hysteresis suggesting two distinct resistance states in the film. Typical resistance switching ratio (Ron/Roff) of the order of 113 % and 72% have been estimated for In₂O₃ device grown on LNO and Pt substrates, respectively. The observed resistance switching characteristics offers lot of promise for new class of binary oxide materials with oxide (LNO) as bottom electrode leading to better suitability for nanoelectronics RRAM devices.

Abstract: Al0.35Ga0.65N/GaN- and Al0.2Ga0.8N/AlN/GaN-heterostructures high electron mobility transistors (HEMTs) with a gate length (LG) varying from 1.2 to 0.08 µm were fabricated on silicon Si(111) substrates using a 3C-SiC transition layer. Metal organic chemical vapour deposition (MOCVD) was used to growth the AlGaN-heterostructures and a low pressure chemical vapour deposition (LPCVD) to create the 3C-SiC(111) transition layer preventing Ga-induced melt back etching and Si-out diffusion. Reduced Al content and an AlN interlayer improved the device performance. The HEMTs with LG=0.08µm had a maximum drain current density of 1.25 A/mm and a peak extrinsic transconductance of 400 mS/mm. A unity current gain cut-off frequency (ƒT) of 180 GHz and maximum frequency (ƒmax) of 70 GHz were measured on these devices.

Abstract: Microstructure engineering of thermoelectric materials can resolve the conflicts of electrical and thermal transports. Especially, one-dimensional structure can obviously improve the thermoelectric figure of merit because of its crystal anisotropy and strong quantum confinement effect. In this paper, the Te nanowires, one-dimensional core-shell heterostructure of Te/Bi and Te/Bi₂Te₃ were controlled synthesized by microwave assisted chemical synthesis. The effect of PVP concentration and reductant dropping rate on the microstructure of the Te nanowires were investigated. The experimental results showed that with increasing the amount of PVP, the Te nanowires got less crystallinity and its surface become more rough due to its steric hindrance effect. With decreasing reductant dropping rate, the longer and thiner Te nanowires were obtained. Epitaxial growth can describe the relation of core Te and shell Bi (or Bi₂Te₃). It has been found that Bi shell uniformly surrounded around Te nanowires core, but Bi₂Te₃ sheets were perpendicular to the c-axis of Te nanowires. The different core-shell heterostructure structure can be obtained by adjusting reaction conditions and controlling diffusion kinetics of Te and Bi.

Abstract: Reduction in threshold current density is the major challenge in the field of semiconductor laser design. The threshold current density can be minimized by introducing low dimensional material system with narrow band gap. InN has a narrow band gap of 0.7 eV and quantum dot provides three dimensional confinement factor. In this paper, we propose then InN quantum dot as the active layer material that will serve both the purpose of narrow band gap and three dimensional confinement. The simulation results show that the current density reduces drastically with the cavity length.

Abstract: In the last years oxide materials for electronics show significant progress. However, many details regarding technology control of the properties have to be solved. For electronics, thin films and heterestructures are important taking advantage of integration and synergetic concepts leading to new types of devices and functionalities. It is notable that, while fabrication of new devices and materials showing new phenomena are booming, the growth principles and concepts are somehow developing slowly within this general trend. This is because in many cases, growth of materials is very personalized. Understanding of the bi-directional relationship between the general and particular principles deserves attention. The immediate benefit is that knowledge on growth for one material can be transferred to another one. In our work we have analyzed such relationships for some oxide multicomponent perovskites. Materials used in our examples are Bi-Sr-Ca-Cu-O and YBa₂Cu₃O₇, (Ca, Sr)CuO₂, (Ca, Ba)CuO₂ and Bi₄Ti₃O₁₂. Presented thin films or heterostructures are with c-axis and non-c-axis orientations and based on these examples we discuss some of the growth principles.

Abstract: Growth of high quality GaN/AlN heterostructures by plasma assisted molecular beam epitaxy (PAMBE) is possible with excess of Ga on the surface. During growth of AlN this additional Ga acts as surfactant and improves mobility of the Al adatoms on the growing surface, at the possible cost of Ga segregation and creation of mixed AlGaN interlayer. Scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS) were used to determine chemical composition of high crystallographic quality GaN-AlN multilayer structure. It was shown that segregation occurs at AlN-GaN heterointerfaces, while GaN-AlN interfaces have abrupt stepwise change of the chemical composition. HRTEM results show creation of trench defects at the periphery of growing AlN islands in the case of nonoptimized growth.