Towards single nanowire solar cell based on novel radial p-n junction

One of the most promising applications, the 1D semiconductor nanowires (NWs) offer, is the development of next generation solar cells at low-cost and higher efficiency. Group III–V semiconductor NWs, especially GaAs and InP, are the ideal semiconductor materials to build such devices on low-cost platforms. The aim of this work was to fabricate and study the fundamental working mechanisms of single nanowire solar cell and photodetector based on novel radial p-n junction in a core-shell geometry. The Au-assisted GaAs NWs were grown and doped in situ using metalorganic vapour phase epitaxy (MOVPE). In this study, a unique lithography-free technique was employed on an ensemble of NWs to isolate the core (p-type) from the shell (n-type) on the growth substrate. Electron beam lithography (EBL) was used to make metal contacts to the single core-shell NWs. In that respect, specific contact schemes were developed to realize ohmic contacts between the metal electrodes and the NWs. Electrical measurements of single nanowire device revealed perfect I-V behaviour confirming the formation of radial p-n junction diode. For characterization of NWs, scanning electron microscopy (SEM), transmission electron microscopy (TEM), photoluminescence (PL) and photocurrent spectroscopies were employed. Furthermore, surface passivation of NWs was identified as one of the key issues in functioning of the photovoltaic device as no photo response was detected without the same. The surface passivation of GaAs NWs with higher band gap AlGaAs layers resulted in dramatic enhancement of the PL intensity, and the photocurrent measurements of the device revealed a broad photo response in the visible spectrum range of the light, indicating a successful working of a single nanowire as a photodetector. However, although a functioning radial p-n junction was demonstrated, no solar response was detected from the single nanowire structure because of yet many unaddressed challenges such as high contact resistance, doping optimisation, absence of intrinsic region and unoptimised geometry. In summary, a single nanowire based photodetector was successfully demonstrated, and a full fabrication process was studied and developed for making progress towards realising the functional single nanowire based radial p-n junction solar cells in the future

Wireless Power Transfer system : Development and Implementation

The goal of the project was to develop a Wireless Power Transfer (WPT) System and implement it in innovative flooring. The project was done at NextFloor Oy, a company based in Helsinki, Finland. At the moment, Wireless Power Transmission is in the forefront of electronics research, which is why this study started as an initial attempt to investigate WPT in order to keep up with the fast growing industry. The main steps of the project were to study the physics behind wireless electricity transfer technologies, review the current industry situation and evaluate available solutions. Furthermore, the major objective included design of the printed circuit boards (PCBs), assembly and testing and development of original prototypes. As a result, the system consisting of a transmitter and receiver boards, was developed. It utilizes the inductive coupling method in order to transfer electricity wirelessly. The unique prototypes, a Qi standard compatible demo-table and non-standardized wireless power floor-demo, were designed to work together with the NextFloor ingenious flooring system. The goals were met and the 40 Watt Wireless Power Transfer System showed encouraging results. The primary part of this study was conducted during six months and further development and enhancements are carried out currently. The next essential phase includes the development of the 100-Watt WPT system and its laptop charging implementation and, finally, R&D of the magnetic resonance wireless energy transmission

III–V nanowires on black silicon and low-temperature growth of self-catalyzed rectangular InAs NWs

We report the use of black silicon (bSi) as a growth platform for III–V nanowires (NWs), which enables low reflectance over a broad wavelength range as well as fabrication of optoelectronic devices by metalorganic vapor phase epitaxy. In addition, a new isolated growth regime is reported for self-catalyzed InAs NWs at record-low temperatures of 280 °C–365 °C, where consistently rectangular [-211]-oriented NWs are obtained. The bSi substrate is shown to support the growth of additionally GaAs and InP NWs, as well as heterostructured NWs. As seed particles, both ex-situ deposited Au nanoparticles and in-situ deposited In droplets are shown feasible. Particularly the InAs NWs with low band gap energy are used to extend low-reflectivity wavelength region into infrared, where the bSi alone remains transparent. Finally, a fabricated prototype device confirms the potential of III–V NWs combined with bSi for optoelectronic devices. Our results highlight the promise of III–V NWs on bSi for enhancing optoelectronic device performance on the low-cost Si substrates, and we believe that the new low-temperature NW growth regime advances the understanding and capabilities of NW growth.Peer reviewe

Surface potential response from GaP nanowires synthesized with mixed crystal phases

In this work, we investigate variations of surface potentials along a single gallium phosphide (GaP) nanowire (NW) synthesized with a mixed crystal phase along the growth direction. GaP NWs synthesized with both wurtzite (WZ) and zincblende (ZB) phases were studied. The measurements were performed on a standard Atomic Force Microscopy (AFM) set-up equipped with Kelvin Probe Force Microscopy (KPFM) module in PeakForce Tapping Mode. KPFM Measurements from two structures were analyzed. Variations of surface poten-tials were observed in a single GaP NW with WZ/ZB segments. An average difference in surface potential was 55±11 mV. This is explained by different crystal structures along the NW. The work expands the understanding of crystal structure-dependent electrical transport properties of GaP NWs.Peer reviewe

Thermoelectric Characteristics of InAs Nanowire Networks Directly Grown on Flexible Plastic Substrates

Publisher Copyright: ©III-V semiconductor nanowires have shown promise for thermoelectric applications, but their use in practical devices has conventionally been hindered by complex fabrication processes and device integration. Here, we characterize the thermoelectric properties of InAs nanowire networks directly grown on flexible polyimide plastic. The n-type nanowire networks achieve a high room-temperature Seebeck coefficient of -110.8 mu V K-1 and electrical conductivity of 41 S cm(-1), resulting in a thermoelectric power factor of 50.4 mu W m(-1) K-2. Moreover, the nanowire networks show remarkable mechanical flexibility with a relative change in resistance below 0.01 at bending radii below 5.2 mm. We further establish the thermoelectric performance of InAs nanowire networks on plastic using a facile proof-of-concept thermoelectric generator producing a maximum power of 0.44 nW at a temperature gradient of 5 K. The findings indicate that direct growth of III-V nanowire networks on plastic substrates shows promise for the development of flexible thermoelectrics applications.Peer reviewe

Direct Growth of Light-Emitting III–V Nanowires on Flexible Plastic Substrates

Semiconductor nanowires are routinely grown on high-priced crystalline substrates as it is extremely challenging to grow directly on plastics and flexible substrates due to high-temperature requirements and substrate preparation. At the same time, plastic substrates can offer many advantages such as extremely low price, light weight, mechanical flexibility, shock and thermal resistance, and biocompatibility. We explore the direct growth of high-quality III–V nanowires on flexible plastic substrates by metal-organic vapor phase epitaxy (MOVPE). We synthesize InAs and InP nanowires on polyimide and show that the fabricated NWs are optically active with strong light emission in the mid-infrared range. We create a monolithic flexible nanowire-based p–n junction device on plastic in just two fabrication steps. Overall, we demonstrate that III–V nanowires can be synthesized directly on flexible plastic substrates inside a MOVPE reactor, and we believe that our results will further advance the development of the nanowire-based flexible electronic devices.Peer reviewe

Management of light and scattering in InP NWs by dielectric polymer shell

Understanding and management of light is of great importance for nanoscale devices. This report demonstrates enhanced absorption, photoluminescence and scattering in InP nanowires when coated with dielectric polymer shell. The shells increase absorption and emission by a factor of ~2 and photoluminescence by a factor of ~4. A thorough optical characterization is provided, including reflectance, transmission, luminescence and scattering to incident and transmitted directions. From this characterization, we derive the distribution of absorbed light within the structure (InP nanowires, Au seed particles and the substrate). Additionally, reflectance, transmission and emission are shown to become increasingly diffuse with the dielectric shells. The results are thought to provide better understanding in light-matter interaction in nanostructures, as well as to provide valuable tools for light and scattering management in nanoscale optoelectronics.Peer reviewe

Thermal conductivity suppression in GaAs–AlAs core–shell nanowire arrays

| openaire: EC/H2020/645241/EU//TransFlexTegSemiconductor nanowire heterostructures have been shown to provide appealing properties for optoelectronics and solid-state energy harvesting by thermoelectrics. Among these nanoarchitectures, coaxial core–shell nanowires have been of primary interest due to their electrical functionality, as well as intriguing phonon localization effects in the surface-dominated regime predicted via atomic simulations. However, experimental studies on the thermophysical properties of III–V semiconductor core–shell nanowires remain scarce regardless of the ubiquitous nature of these compounds in solid-state applications. Here, we present thermal conductivity measurements of the arrays of GaAs nanowires coated with AlAs shells. We unveil a strong suppression in thermal transport facilitated by the AlAs shells, up to ∼60%, producing a non-monotonous dependence of thermal conductivity on the shell thickness. Such translation of the novel heat transport phenomena to macroscopic nanowire arrays paves the way for rational thermal design in nanoscale applications.Peer reviewe

Growth of GaAs Nanowire – Graphite Nanoplatelet Hybrid Structures

We study the formation of GaAs nanowire - graphite nanoplatelet hybrid nanostructures. The quasi van-der-Waals epitaxy was used to grow the vapor-liquid-solid nanowires on a silicon substrate covered by graphite nanoplatelets. We have found that either horizontal or inclined nanowires can form depending on the relative positions of graphite nanoplatelets, as well as on the placement of catalyst nanoparticles. We present the model, which is capable of the description of the experimentally observed scenarios of planar and non-planar NW growth. Both theoretical and experimental studies show that the use of nanoplatelet substrate allows engineering the morphology of planar and inclined nanowires.Peer reviewe

Low‐Power Continuous‐Wave Second Harmonic Generation in Semiconductor Nanowires

| openaire: EC/H2020/631610/EU//GrabFastSemiconductor nanowires (NWs) are promising for realizing various on‐chip nonlinear optical devices, due to their nanoscale lateral confinement and strong light–matter interaction. However, high‐intensity pulsed pump lasers are typically needed to exploit their optical nonlinearity because light couples poorly with nanometric‐size wires. Here, microwatts continuous‐wave light pumped second harmonic generation (SHG) in AlGaAs NWs is demonstrated by integrating them with silicon planar photonic crystal cavities. Light–NW coupling is enhanced effectively by the extremely localized cavity mode at the subwavelength scale. Strong SHG is obtained even with a continuous‐wave laser excitation with a pump power down to urn:x-wiley:18638880:media:lpor201800126:lpor201800126-math-0001W, and the cavity‐enhancement factor is estimated around 150. Additionally, in the integrated device, the NW's SHG is more than two orders of magnitude stronger than third harmonic generations in the silicon slab, though the NW only couples with less than 1% of the cavity mode. This significantly reduced power requirement of NW's nonlinear frequency conversion would promote NW‐based building blocks for nonlinear optics, especially in chip‐integrated coherent light sources, entangled photon pairs and signal processing devices.Peer reviewe