InP Core Shell Research Articles

Phosphorus-Controlled Nanoepitaxy of Asymmetric GaAs–InP Core–Shell Bent Nanowires: Implications for Bottom-Up Nanowire Transistors and Sensors

Phosphorus-Controlled Nanoepitaxy of Asymmetric GaAs–InP Core–Shell Bent Nanowires: Implications for Bottom-Up Nanowire Transistors and Sensors

Catalysis-Free Growth of III-V Core-Shell Nanowires on p-Si for Efficient Heterojunction Solar Cells with Optimized Window Layer

The growth of high-quality compound semiconductor materials on silicon substrates has long been studied to overcome the high price of compound semiconductor substrates. In this study, we successfully fabricated nanowire solar cells by utilizing high-quality hetero p-n junctions formed by growing n-type III-V nanowires on p-silicon substrates. The n-InAs0.75P0.25 nanowire array was grown by the Volmer–Weber mechanism, a three-dimensional island growth mode arising from a lattice mismatch between III-V and silicon. For the surface passivation of n-InAs0.75P0.25 core nanowires, a wide bandgap InP shell was formed. The nanowire solar cell was fabricated by benzocyclobutene (BCB) filling, exposure of nanowire tips by reactive-ion etching, electron-beam deposition of ITO window layer, and finally metal grid electrode process. In particular, the ITO window layer plays a key role in reducing light reflection as well as electrically connecting nanowires that are electrically separated from each other. The deposition angle was adjusted for conformal coating of ITO on the nanowire surface, and as a result, the lowest light reflectance and excellent electrical connectivity between the nanowires were confirmed at an oblique deposition angle of 40°. The solar cell based on the heterojunction between the n-InAs0.75P0.25/InP core-shell nanowire and p-Si exhibited a very high photoelectric conversion efficiency of 9.19% with a current density of 27.10 mA/cm2, an open-circuit voltage of 484 mV, and a fill factor of 70.1%.

Bending of GaAs–InP Core–Shell Nanowires by Asymmetric Shell Deposition: Implications for Sensors

Bending of GaAs–InP Core–Shell Nanowires by Asymmetric Shell Deposition: Implications for Sensors

Highly Quantum-Yield Enhancement of InP Quantum-Dot Emitting Red-Light Via Potassium Doping for Quantum-Dot Organic-Light-Emitting-Diode Display

Quantum dots (QD) have been researched intensively among several luminescent materials because they can simply adjust wavelength, high efficiency, high stability, and high color purity properties. In particular, eco-friendly InP based core-shell QDs without environmental regulations (i.e., restriction of hazardous substances in Europe) has been a great of attention.1 However, it is difficult to synthesize InP-based core-shell QDs, which requires a high temperature, a long reaction time, and a high reactivity precursor since the InP-based core-shell QDs need a strong covalent bonds, degrading quantum yield(QY)2. For this reason, many defects in QDs are generated during a InP core growth. Among the blue(B)-, green(G)-, and red(R)-light emitting QDs, especially R-light emitting InP based core-shell QDs contain numerous crystalline defects since the core size should be increased to reduce the energy band gap. In our study, we enhanced highly QY of R-light emitting InP based core-shell QDs(R-QDs) by doping potassium ions via injecting potassium ion precursor such as potassium iodide during a InP core growth. We characterized the mechanism why potassium ion doping highly enhanced QY by observing the crystallinity of core-shell QD and electron paramagnetic resonance(EPR) analysis.In general, the R-QDs contain vacancy defect in the InP core-QDs which emit the red-light with ~625-nm in wavelength, degrading QY by lattice scattering, as shown in Fig. 1(a). These vacancy defects induced lattice-scattering and cause a decrease of QY. Here, the vacancy defects in the InP core-QDs would be passivated by doping potassium ions via using potassium iodide as a dopant, as shown in Fig. 1(b). In the InP core-QDs, the mole fraction of potassium ions to indium ions linearly increased with the potassium iodide concentration, measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES), as shown in Fig. 1(c). Then, the InP core/ZnSe inter-shell/ZnSeS inter-shell/ZnS oueter-shell QDs (i.e.; InP core-shell QDs) were synthesized by using a precipitation method. The QY of the InP core-shell QDs peaked at a specific potassium iodide concentration; i.e., 91% at the potassium iodide concentration of 3%, as shown in Fig. 1(d). Otherwise, both full-width at half maximum (FWHM) and the wavelength emitting red-light gradually increased with the potassium iodide concentration.To understand the dependency of QY on the potassium iodide concentration, the crystallinity of InP core-shell QDs were investigated as a function of the potassium iodide concentration during growing the InP core-QDs, where in four different crystalline plane peaks were found; (unknown), (111), (220), and (311), as shown in Fig. 1(e). Particularly, the crystalline peak intensity of (unknown) rapidly increased with increasing QY, indicating the doping of potassium ions during growing the InP core-QDs passivated vacancy defects so that it suppressed an un-proper growth along unknown crystalline direction, as shown in Fig. 1(e). In addition, the exciton lifetime exponentially increased with QY, measured by time-resolved photoluminescence (TRPL), meaning that the passivation of vacancy defects with potassium ions enhanced the exciton lifetime of the InP core-shell QDs. It was found that there was a good correlation between QY and the crystalline peak intensity of (unknown) or the exciton lifetime, implying that the passivation of vacancy defects in the InP core-QDs by doping potassium iodide ions would enhance the QY of the InP core-shell QDs, as shown in Fig.1(f). Finally, the international commission on illumination (CIE) 1931 color spaces of a QD OLED display using R-, G-, and B-QD functional color filters, as shown in Fig.1(g), achieved 121.7% (NTSC) and 91.1% (Rec. 2020), as shown in Fig. 1(h) inset figure. Reference [1] Tamang, S.; Lincheneau, C.; Hermans, Y.; Jeong, S.; Reiss, P.; (2016). Chemistry of InP Nanocrystal Syntheses. Chem. Mater. 28, 2491−2506[2] Jang, E.; Kim, Y.; Won, Y.; Jang, H.; Choi, S.; (2020). Environmentally Friendly InP-Based Quantum Dots for Efficient Wide Color Gamut Displays. ACS Energy Lett. 5, 1316−1327 Figure 1

Measurements of Strain and Bandgap of Coherently Epitaxially Grown Wurtzite InAsP-InP Core-Shell Nanowires.

We report on experimental determination of the strain and bandgap of InAsP in epitaxially grown InAsP-InP core-shell nanowires. The core-shell nanowires are grown via metal-organic vapor phase epitaxy. The as-grown nanowires are characterized by transmission electron microscopy, X-ray diffraction, micro-photoluminescence (μPL) spectroscopy, and micro-Raman (μ-Raman) spectroscopy measurements. We observe that the core-shell nanowires are of wurtzite (WZ) crystal phase and are coherently strained with the core and the shell having the same number of atomic planes in each nanowire. We determine the predominantly uniaxial strains formed in the core-shell nanowires along the nanowire growth axis and demonstrate that the strains can be described using an analytical expression. The bandgap energies in the strained WZ InAsP core materials are extracted from the μPL measurements of individual core-shell nanowires. The coherently strained core-shell nanowires demonstrated in this work offer the potentials for use in constructing novel optoelectronic devices and for development of piezoelectric photovoltaic devices.

Vertical InGaAs Nanowire Array Photodiodes on Si

We demonstrated vertical InGaAs nanowire (NW) array photodiodes on Si that were optically responsive to visible light (635 nm) and near-infrared light (∼1.55 μm). The vertical NWs were directly grown on Si by selective-area growth. Implementation of a heavily Sn-doped contact layer in the InGaAs NWs improved the diode characteristic because of a lower series resistance, and as a result, a photoresponsivity of 0.25 A/W was obtained at 635 nm, which was twice that before the improvement. Moreover, the photocurrent density of InGaAs–InP core–shell NWs was about 20-fold higher under illumination with light in the 1.55 μm wavelength band as a result of suppression of surface recombination. These findings are expected to be useful for NW-based photovoltaic applications for optical interconnection and Si platforms.

Photoresponse and Field Effect Transport Studies in InAsP–InP Core–Shell Nanowires

A ternary InAsyP1−y alloy is suitable for an application to near-infrared (NIR) optical devices as their direct bandgap energy covers the entire NIR band. A nanowire (NW) system allows an epitaxial integration of InAsyP1−y alloy on any type of substrate since the lattice mismatch strain can be relieved through the NW sidewall. Nevertheless, the very large surface to volume ratio feature of the NWs leads to enormous surface states which are susceptible to surface recombination of free carriers. Here, ternary InAs0.75P0.25 NWs are grown with InP passivation layer (i.e., core–shell structure) to minimize the influence of the surface states, thus increasing their optical and electrical properties. A photoresponse study was achieved through the modeled band structure of the grown NWs. The model and experimental results suggest that 5-nm-thick InP shell efficiently passivates the surface states of the InAs0.75P0.25 NWs. The fabricated core–shell photodetectors and field-effect transistors exhibit improved photoresponse and transport properties compared to its counterpart core-only structure.

A Comparative Study of Absorption in Vertically and Laterally Oriented InP Core–Shell Nanowire Photovoltaic Devices

We have compared the absorption in InP core-shell nanowire p-i-n junctions in lateral and vertical orientation. Arrays of vertical core-shell nanowires with 400 nm pitch and 280 nm diameter, as well as corresponding lateral single core-shell nanowires, were configured as photovoltaic devices. The photovoltaic characteristics of the samples, measured under 1 sun illumination, showed a higher absorption in lateral single nanowires compared to that in individual vertical nanowires, arranged in arrays with 400 nm pitch. Electromagnetic modeling of the structures confirmed the experimental observations and showed that the absorption in a vertical nanowire in an array depends strongly on the array pitch. The modeling demonstrated that, depending on the array pitch, absorption in a vertical nanowire can be lower or higher than that in a lateral nanowire with equal absorption predicted at a pitch of 510 nm for our nanowire geometry. The technology described in this Letter facilitates quantitative comparison of absorption in laterally and vertically oriented core-shell nanowire p-i-n junctions and can aid in the design, optimization, and performance evaluation of nanowire-based core-shell photovoltaic devices.

Study of carrier concentration in single InP nanowires by luminescence and Hall measurements

The free electron carrier concentrations in single InP core–shell nanowires are determined by micro-photoluminescence, cathodoluminescence (CL) and Hall effect measurements. The results from luminescence measurements were obtained by solving the Fermi–Dirac integral, as well as by analyzing the peak full width at half maximum (FWHM). Furthermore, the platform used for Hall effect measurements, combined with spot mode CL spectroscopy, is used to determine the carrier concentrations at specific positions along single nanowires. The results obtained via luminescence measurements provide an accurate and rapid feedback technique for the epitaxial development of doping incorporation in nanowires. The technique has been employed on several series of samples in which growth parameters, such as V/III-ratio, temperature and dopant flows, were investigated in an optimization procedure. The correlation between the Hall effect and luminescence measurements for extracting the carrier concentration of different samples were in excellent agreement.

Synthesis of Doped InP Core–Shell Nanowires Evaluated Using Hall Effect Measurements

InP core-shell nanowire pn-junctions doped with Zn and Sn have been investigated in terms of growth morphology and shell carrier concentration. The carrier concentrations were evaluated using spatially resolved Hall effect measurements and show improved homogeneity compared to previous investigations, attributed to the use of Sn as the n-type dopant. Anisotropies in the growth rate of different facets are found for different doping levels that in turn affects the migration of Sn and In on the nanowire surface. A route for increasing the In migration length to obtain a more homogeneous shell thickness is presented.

Position-Controlled III–V Compound Semiconductor Nanowire Solar Cells by Selective-Area Metal–Organic Vapor Phase Epitaxy

We demonstrate position-controlled III-V semiconductor nanowires (NWs) by using selective-area metal-organic vapor phase epitaxy and their application to solar cells. Efficiency of 4.23% is achieved for InP core-shell NW solar cells. We form a 'flexible NW array' without a substrate, which has the advantage of saving natural resources over conventional thin film photovoltaic devices. Four junction NW solar cells with over 50% efficiency are proposed and discussed.

Facilitating growth of InAs–InP core–shell nanowires through the introduction of Al

InAs nanowires were grown on GaAs substrates by the Au-assisted vapour–liquid–solid (VLS) method in a gas source molecular beam epitaxy (GSMBE) system. Passivation of the InAs nanowires using InP shells proved difficult due to the tendency for the formation of axial rather than core–shell structures. To circumvent this issue, AlxIn1−xAs or AlxIn1−xP shells with nominal Al composition fraction of x=0.20, 0.36, or 0.53 were grown by direct vapour–solid deposition on the sidewalls of the InAs nanowires. Characterisation by transmission electron microscopy revealed that the addition of Al in the shell resulted in a remarkable transition from the VLS to the vapour–solid growth mode with uniform shell thickness along the nanowire length. Possible mechanisms for this transition include reduced adatom diffusion, a phase change of the Au seed particle, and surfactant effects. The InAs–AlInP core-shell nanowires exhibited misfit dislocations, while the InAs–AlInAs nanowires with lower strain appeared to be free of dislocations.

Growth of doped InAs yP 1− y nanowires with InP shells

We have studied the growth of doped InAs y P 1− y (InAsP) NWs and InAsP–InP core–shell NWs. Using hydrogen chloride (HCl) we were able to prevent radial growth on the InAsP NWs. Undoped and H 2S-doped InAsP NWs had almost pure wurtzite crystal structure, while DEZn-doped NWs showed wurtzite periodically separated by short sections of pure zincblende with rotational twins. Electrical measurements confirmed that the H 2S-doped and DEZn-doped NWs were highly n- and p-doped, respectively. The core crystal structure strongly influenced the subsequent radial growth of InP shells, and we observed perfectly smooth shells on H 2S-doped cores while the zincblende sections resulted in large bulges on the DEZn-doped cores. The average radial growth rate was more than twice as high on the DEZn-doped cores as on the H 2S-doped cores. We found that the radial shell growth decreased at increasing temperature, while the axial growth increased. These results demonstrate the importance of the NW core crystal structure for the growth of core–shell NWs.

Quantum-confinement effects in InAs–InP core–shell nanowires

We report the detection of quantum confinement in single InAs–InPcore–shell nanowires. The wires, having an InAs core with∼25 nm diameter, are characterized by emission spectra in which two peaks are identified underhigh excitation intensity conditions. The peaks are caused by emission from the ground andexcited quantized levels, due to quantum confinement in the plane perpendicularto the nanowire axis. We have identified different energy contributions in theemission spectra, related to the wurtzite structure of the wires, the strain betweenthe wurtzite core and the shell, and the confinement energy of the InAs core.Calculations based on six-band strain-dependent theory allow the theoretical estimation of the confined energy states in such materials,and we found these results to be in good agreement with those from the photoluminescencestudies.