Surface Passivation Of Nanowires Research Articles

Highly Efficient Silicon Nanowire Surface Passivation by Bismuth Nano-Coating for Multifunctional Bi@SiNWs Heterostructures.

A key requirement for the development of highly efficient silicon nanowires (SiNWs) for use in various kinds of cutting-edge applications is the outstanding passivation of their surfaces. In this vein, we report on a superior passivation of a SiNWs surface by bismuth nano-coating (BiNC) for the first time. A metal-assisted chemical etching technique was used to produce the SiNW arrays, while the BiNCs were anchored on the NWs through thermal evaporation. The systematic studies by Scanning Electron Microscopy (SEM), energy dispersive X-ray spectra (EDX), and Fourier Transform Infra-Red (FTIR) spectroscopies highlight the successful decoration of SiNWs by BiNC. The photoluminescence (PL) emission properties of the samples were studied in the visible and near-infrared (NIR) spectral range. Interestingly, nine-fold visible PL enhancement and NIR broadband emission were recorded for the Bi-modified SiNWs. To our best knowledge, this is the first observation of NIR luminescence from Bi-coated SiNWs (Bi@SiNWs), and thus sheds light on a new family of Bi-doped materials operating in the NIR and covering the important telecommunication wavelengths. Excellent anti-reflectance abilities of ~10% and 8% are observed for pure SiNWs and Bi@SiNWs, respectively, as compared to the Si wafer (50–90%). A large decrease in the recombination activities is also obtained from Bi@SiNWs heterostructures. The reasons behind the superior improvement of the Bi@SiNWs performance are discussed in detail. The findings demonstrate the effectiveness of Bi as a novel surface passivation coating, where Bi@SiNWs heterostructures are very promising and multifunctional for photovoltaics, optoelectronics, and telecommunications.

ZnO nanowires/YAG:Ce functional heterostructure coatings with tunable optical properties

Luminescent ZnO nanowires (NWs)/YAG:Ce heterostructure coatings have been elaborated by impregnating cheap hydrothermally-grown ZnO NWs array with ground commercial YAG:Ce nanoparticles (NPs). TEM and photoluminescence quantum yield measurements have been used to make YAG:Ce NPs suitable for soaking within the ZnO NWs. Structural, morphological and optical studies of functional nanocomposite coatings have evidenced an optimal amount of YAG:Ce allowing the visible photoluminescence signal of ZnO NWs nearly to triple whereas slight red shift accompanied by an enlargement on the long wavelength side has been observed for YAG:Ce emission. These phenomena have been explained through the existence of structural defects on ZnO NWs surface and emission mechanisms implying either scattering effects or the passivation of NWs surface by YAG:Ce NPs. Furthermore, an energy diagram involving different structural defects of ZnO NWs as well as Ce3+ ions energy levels has been built from excitation and emission spectra and from decay curves recorded monitoring the orange-red emission of ZnO. The elaborated heterostructure functional coatings have proved to exhibit tunable optical properties in terms of spectral distribution playing on the excitation source. Hence, they appear as a smart way for developing LEDs-based devices with flexible photometric parameters for a broader range of optical applications.

Dimension- and Surface-Tailored ZnO Nanowires Enhance Charge Collection in Quantum Dot Photovoltaic Devices

The use of zinc oxide (ZnO) nanowires improves charge collection, and consequently power conversion efficiency, in quantum dot (QD) based photovoltaic devices. However, the role of the nanowire geometry (e.g., density, length, and morphology, etc.) relative to the QD properties remains unexplored, in part due to challenges with controlled nanowire synthesis. Here, we independently tailor nanowire length and the active device layer thickness to study charge collection in lead sulfide (PbS) QD photovoltaic devices. We then demonstrate consistently high internal quantum efficiency in these devices by applying quantum efficiency and total reflectance measurements. Our results show that significant losses originate from ZnO nanowire–QD interfacial recombination, which we then successfully overcome by using nanowire surface passivation. This geometry-tailored approach is generally applicable to other nanowire–QD systems, and the surface passivation schemes will play a significant role in future development of n...

Dense nanoimprinted silicon nanowire arrays with passivated axial p-i-n junctions for photovoltaic applications

We report on the fabrication and photovoltaic characteristics of vertical arrays of silicon axial p-i-n junction nanowire (NW) solar cells grown by vapor-liquid-solid (VLS) epitaxy. NW surface passivation with silicon dioxide shell is shown to enhance carrier recombination time, open-circuit voltage (VOC), short-circuit current density (JSC), and fill factor (FF). The photovoltaic performance of passivated individual NW and NW arrays was compared under 532 nm laser illumination with power density of ∼10 W/cm2. Higher values of VOC and FF in the NW arrays are explained by enhanced light trapping. In order to verify the effect of NW density on light absorption and hence on the photovoltaic performance of NW arrays, dense Si NW arrays were fabricated using nanoimprint lithography to periodically arrange the gold seed particles prior to epitaxial growth. Compared to sparse NW arrays fabricated using VLS growth from randomly distributed gold seeds, the nanoimprinted NW array solar cells show a greatly increased peak external quantum efficiency of ∼8% and internal quantum efficiency of ∼90% in the visible spectral range. Three-dimensional finite-difference time-domain simulations of Si NW periodic arrays with varying pitch (P) confirm the importance of high NW density. Specifically, due to diffractive scattering and light trapping, absorption efficiency close to 100% in the 400–650 nm spectral range is calculated for a Si NW array with P = 250 nm, significantly outperforming a blanket Si film of the same thickness.

Interface modification and trap-type transformation in Al-doped CdO/Si-nanowire arrays/p-type Si devices by nanowire surface passivation

The present work reports the fabrication and detailed electrical properties of Al-doped CdO/Si-nanowire (SiNW) arrays/p-type Si Schottky diodes with and without SiNW surface passivation. It is shown that the interfacial trap states influence the electronic conduction through the device. The experimental results demonstrate that the effects of the dangling bonds at the SiNW surface and Si vacancies at the SiOx/SiNW interface which can be changed by the Si–O bonding on the energy barrier lowering and the charge transport property. The induced dominance transformation from electron traps to hole traps in the SiNWs by controlling the passivation treatment time is found in this study.

Molecular beam epitaxial growth and optical properties of red-emitting (λ = 650 nm) InGaN/GaN disks-in-nanowires on silicon

We have investigated the radiative properties of InGaN disks in GaN nanowires grown by plasma enhanced molecular beam epitaxy on (001) silicon substrates. The growth of the nanowire heterostructures has been optimized to maximize the radiative efficiency, or internal quantum efficiency (IQE), for photoluminescence emission at λ = 650 nm. It is found that the IQE increases significantly (by ∼10%) to 52%, when post-growth passivation of nanowire surface with silicon nitride or parylene is applied. The increase in efficiency is supported by radiative- and nonradiative lifetimes derived from data obtained from temperature dependent- and time-resolved photoluminescence measurements. Light emitting diodes with p-i-n disk-in-nanowire heterostructures passivated with parylene have been fabricated and characterized.

Interface models and processing technologies for surface passivation and interface control in III–V semiconductor nanoelectronics

Interface models and processing technologies are reviewed for successful establishment of surface passivation, interface control and MIS gate stack formation in III–V nanoelectronics. First, basic considerations on successful surface passivation and interface control are given, including review of interface models for the band alignment at interfaces, and effects of interface states in nanoscale devices. Then, a brief review is given on currently available surface passivation technologies for III–V materials, including the Si interface control layer (ICL)-based passivation scheme by the authors’ group. The Si-ICL technique has been successfully applied to surface passivation of nanowires and to formation of a HfO 2 high-k dielectric/GaAs interfaces with low values of the interface state density.

Chemical Surface Passivation of Ge Nanowires

Surface oxidation and chemical passivation of single-crystal Ge nanowires with diameters ranging between 7 and 25 nm were studied. The surface chemistry differs significantly from that of well-studied monolithic atomically smooth single-crystal substrates. High-resolution Ge 3d XPS measurements reveal that Ge nanowires with chemically untreated surfaces exhibit greater susceptibility to oxidation than monolithic Ge substrates. Multiple solution-phase routes to Ge nanowire surface passivation were studied, including sulfidation, hydride and chloride termination, and organic monolayer passivation. Etching in HCl results in chloride-terminated surfaces, whereas HF etching leads to hydride termination with limited stability. Exposure to aqueous ammonium sulfide solutions leads to a thick glassy germanium sulfide layer. Thermally initiated hydrogermylation reactions with alkenes produce chemically stable, covalently bonded organic monolayer coatings that enable ohmic electrical contacts to be made to the nanowires.