Papers by Keyword: Heterojunction

Abstract: In the last decades, silicon carbide (SiC) based heterostructures have gained a remarkable place in research field due to their exceptional properties. These properties make SiC highly suitable for high temperature, high frequency, and high power electronics applications. The most prominent polytypes (among 200 types) of SiC like 3C-SiC, 4H-SiC and 6H-SiC, have distinctive electrical and physical attributes that make them promising candidates for high performance optoelectronic applications. Silicon (Si) also has been accepted as a promising material for wide range of electronic, optical and optoelectronic applications. Heterostructures fabricated by the direct bonding of SiC polytype and Si may have interesting physical and electrical attributes. In this paper, micro and nano-scale simulations of the nn-heterostructures of Si/4H-SiC and Si/3C-SiC have been done with Silvaco TCAD and QuantumWise Atomistix Toolkit (ATK) softwares respectively. Voltage-current density characteristics of the nanoscale and microscale simulated devices are computed and discussed. In nanoscale devices, the effects of defects due to lattice misplacements (axial displacement of bonded wafers) are also studied. These simulations are the preparation for our future experiments, which are targeted to produce either a high electron mobility diode or a light emitting diode, by direct bonding (diffusion welding) of SiC polytypes.

Abstract: In recent years, graphene has sparked the interest of researchers due to its promising electrical and physical attributes. These attributes make it highly suitable to develop electronic devices with ultra-high mobility of charge carriers. Meanwhile silicon carbide (SiC), a wide bandgap semiconductor material, is being used for high temperature optoelectronic applications. SiC has more than 250 different crystalline forms, these are called polytypes. Some of these polytypes (such as 4H-SiC, 6H-SiC and 3C-SiC) have exceptional physical and electrical properties. Electronic devices which have SiC and graphene as their constituent materials may combine the outstanding attributes of both materials. This article attempts to simulate electronic devices having SiC and graphene as their constituent materials. For this purpose, simulations of a novel nn-heterojunction 4H-6H/SiC diodes with the inclusion of an armchair nanoribbon layer have been carried out. All of the simulations have been run using QuantumWise Atomistix Toolkit (ATK) software, which is an atomic scale electronic device simulator. The density of the states, charge carrier densities and current-voltage curves of the simulated devices have been computed. The simulation results showed a significant improvement in the electrical conduction properties of nn-heterojunction 4H-6H/SiC diodes after the inclusion of the armchair graphene nanoribbons. These simulations provide the groundwork for our future experiments, which will be targeted on fabricating high mobility diodes and/or field effect transistors.

Abstract: In this study, acid-doped polyaniline-zinc oxide composites were prepared and used to investigate their effect on the removal of methyl orange dye under visible light exposure. The composites were prepared by mixing different volume fractions of polyaniline and ZnO powders. Their effect on the removal of methyl orange was determined through transmittance spectroscopy. Polyaniline-zinc oxide composites of 75% volume fractions showed the highest percent removal of methyl orange. This was attributed to the presence of p-n heterojunctions and adsorption capability of the polyaniline molecules.

Abstract: Ge p-i-n photodetectors with and without graphene on active area fabricated and investigated the graphene effects on opto-electrical properties of photodetectors. The photodetectors were characterized with respect to their dark, photocurrents and responsivities in the wavelength range between 1530-1630 nm. For a 250 um-diameter device at room temperature, it was found that dark current of p-i-n photodetector with graphene were reduced significantly compared with photodetector without graphene. This improvement is attributed to the passivation of the graphene layers that leads to the efficient light detection. Therefore, it is noted that the uniform coverage of graphene onto the Ge surface plays a significant role in advancing their opto-electrical performance of photodetector.

Abstract: The catalytic performance strongly relies on the composition, structure, and property of the material used. Earth-abundant tungsten oxides family (WO_x≤3) has received considerable attention in photocatalysis, electrochemistry and catalytic hydrogenation due to its highly tunable structure and unique physicochemical properties. Substantial efforts have been made by us to improve the photocatalytic activity of WO_x≤3 by enhancing light harvesting, charge transfer and separation, including defect engineering, morphology control, and hetero-junction construction. Additionally, the semiconductor-to-metal transition of WO_x≤3 has been found with the increase of defect concentration, suggesting H₂ can be activated on them in a similar way of a metal catalyst. As a result, WO_2.72 also can function as a versatile and efficient catalyst for the saturation of olefins and selective transform of nitroarenes to anilines.

Abstract: Organic-inorganic hybrid perovskite materials offer the potential for realization of low-cost and flexible next-generation solar cells fabricated by low-temperature solution processing. Although efficiencies of perovskite solar cells have dramatically improved up to 22.1% within the past 6 years, there is still considerable room for further improvement in device efficiency through development of smooth and large grain size perovskite films. Commonly, the smooth perovskite layers and large grains are alternative to CH₃NH₃PbI_3-xCl_x. However, the previous one-step fabrication of CH₃NH₃PbI_3-xCl_x using PbCl₂ needs long annealing time. Herein, the active layer, a noncontinuous CH₃NH₃PbI_3-xCl_x film with micrometre-level crystalline grain, was fabricated using a one-step spin-coating process followed by high-temperature annealing and exhibited an increased efficiency of 9.93%. This finding enables the formation of larger grain size perovskite films and the demonstration of efficient perovskite heterojunction solar cells.

Abstract: This paper describes a simple two-step hydrothermal method for the design of heterojunction structure combined with TiO₂ nanorods coated with ZnO nanoparticles. TiO₂ nanorods on F-doped tin oxide (FTO) conducting glass are synthesized by hydrothermal method and ZnO coating is prepared through a two-step procedure. The XRD patterns confirm that the nanostructure consists of rutile TiO₂ and wurtzite ZnO without any other impure phase. The FE-SEM results show that ZnO is uniformly distributing on the surface of TiO₂ nanorods, forming a well-connecting heterojunction. PL studies and UV-vis absorption spectra present the enhanced optical performance of ZnO/TiO₂ heterojunction. The heterojunction structure ZnO/TiO₂ nanorods show excellent photoelectrochemical performance. The performance and the preparation parameters are also optimized accordingly.

Abstract: Key steps in the fabrication of high-efficiency a-Si:H/c-Si heterojunction solar cells are the controlled pyramid texturing of the c-Si substrates to minimize reflection losses and the subsequent passivation by deposition of a high-quality a-Si:H layer to reduce recombination losses. This contribution reviews our recent results on the optimization of the wet-chemical texturing of crystalline Si wafers for the preparation of heterojunction solar cells with respect to low reflection losses, low recombination losses and long minority carrier lifetimes. It is demonstrated, that by joint optimization of both saw damage etch and texture etch the optical and electronic properties of the resulting pyramid morphology can be controlled. Effective surface passivation and thus long minority charge carrier lifetimes are achieved by deposition of intrinsic amorphous Si ((i) a-Si:H) layers. It is shown, that optimized (i) a-Si:H deposition parameters for planar Si (111) wafers can be transferred to a-Si:H layer deposition on random pyramid textured Si (100) wafers. Statistical analysis of the pyramid size distribution revealed that a low fraction of small pyramids leads to longer minority charge carrier lifetimes and, thus, a higher V_oc potential for solar cells.

Abstract: A novel infrared photodiode based on a graphene/n-type silicon heterojunction is explored. The heterojunction photodiode of interest has a large Schottky barrier that results in a low dark current. Graphene serves as the absorbing medium at a wavelength for which silicon is transparent. Under infrared illumination, photo-excited electrons in the graphene gain energy and thus have a greater probability to overcome the barrier and contribute to the photocurrent. We have demonstrated photodiode operation of a graphene/n-Si heterojunction at 1.3 and 1.55 μm wavelength, with 14% internal quantum efficiency and 1.5 pW/Hz^1/2 noise-equivalent power, for potential use in silicon photonics.

Abstract: In this work, the ultraviolet (UV) sensors based on heterojunction between layer of zinc oxide nanoparticles (ZnO NPs) and poly (3,4-ethylenedioxythiophene):poly (styrenesulfonic acid) (PEDOT:PSS) were fabricated, characterized and studied in the electrical response to UV 365 nm. The ZnO NPs layer was solution-based coated on the top of PEDOT:PSS film on the patterned indium tin oxide (ITO) coated on glass. Aluminum was deposited as the top electrode of the device. The current-voltage (I-V) characteristic shows the rectifying behavior in the dark field. With the UV irradiation, the reverse bias current can be found and the forward bias current also highly increases. The current-voltage data fitting with the thermionic emission model shows that the potential barrier height at the heterojunction decreases with illuminating by UV light. Relative high photoresponse of the device exhibits the potential to UV detector application.