III-V nanowire growth for quantum photonics and optoelectronics

III–V semiconductor nanowire (NW) materials possess a combination of fascinating properties, including their tunable direct bandgap, high carrier mobility, excellent mechanical flexibility, and extraordinarily large surface‐to‐volume ratio, making them superior candidates for next generation electronics, photonics, and sensors, even possibly on flexible substrates. Understanding the synthesis, property manipulation, and device integration of these III–V NW materials is therefore crucial for their practical implementations. In this review, we present a comprehensive overview of the recent development in III–V NWs with the focus on their cost‐effective synthesis, corresponding property control, and the relevant low‐operating‐power device applications. We will first introduce the synthesis methods and growth mechanisms of III–V NWs, emphasizing the low‐cost solid‐source chemical vapor deposition (SSCVD) technique, and then discuss the physical properties of III–V NWs with special attention on their dependences on several typical factors including the choice of catalysts, NW diameters, surface roughness, and surface decorations. After that, we present several different examples in the area of high‐performance photovoltaics and low‐power electronic circuit prototypes to further demonstrate the potential applications of these NW materials. Towards the end, we also make some remarks on the progress made and challenges remaining in the III–V NW research field.

Recent advancements in the growth and formation of semiconducting nanostructures, particularly group 14 metalloid semiconductors such as silicon and germanium have shown how quantum effects can be engineered and controlled (1,2) to modify thermal and optical properties (3). In particular, the nanostructuring of silicon materials mediates phonon scattering and confinement, including reduction of thermal conductivity (4,5) and Si-based nanowire heterostructures have been employed as solar cell materials and nanoelectronic power sources. Crystal sizes below the Bohr radius also results in light emission, and effective bandgap modification can influence absorbed thermopower in nanoscale silicon, and for photovoltaics. Reducing size to the confinement regime can also perturbate phonon transport, and this has recently been show to occur in nanocrystallites on rough surfaces of silicon nanowires.We report intense red luminescence from mesoporous n+-Si(100) nanowires (NWs) and nanocrystal-decorated p-Si NWs fabricated using electroless metal assisted chemical (MAC) etching (6-9), and also demonstrate that nanoscale confinement is a cause for limited photon or heat transport in silicon nanowires formed by MACE. n+-Si NWs are composed of a labyrinthine network of silicon nanocrystals in a random mesoporous structure (10). p-type Si(100) NWs exhibit solid core structure, with a surface roughness that contains surface-bound nanocrystals. Both mesoporous n+-Si NWs and rough, solid p-Si NWs exhibit red luminescence at ~1.7 and ~1.8 eV, respectively. The red PL from monolithic arrays of p-type NWs with nanocrystal-decorated rough surfaces is comparatively weak, but originates from the surface bound nanocrystals. Significant PL intensity increase is found during excitation for mesoporous NWs. When quantum confinement effects are introduced in crystalline materials such as the nanocrystalline mesoporous NWs, the electron and phonon transport can be significantly altered in tandem with quantum confinement that causes intense red light emission. The data confirm that phonon confinement and scattering for silicon nanowires is due to surface-bound and internal nanostructure, rather than simply a NW diameter reduction in NW materials. References (1) S. Armatas and M. G. Kanatzidis, Science, 313, 817 (2006).(2) Kiraly, S. Yang and T. J. Huang, Nanotechnology, 24, 245704 (2013)(3) T. Canham, Appl. Phys. Lett., 57, 1046 (1990).(4) I. Hochbaum, R. Chen, R. D. Delgado, W. Liang, E. C. Garnett, M. Najarian, A. Majumdar and P. Yang, Nature, 451, 163 (2008).(5) I. Boukai, Y. Bunimovich, J. Tahir-Kheli, J.-K. Yu, W. A. Goddard III and J. R. Heath, Nature, 451, 168 (2008).(6) O'Dwyer, W. McSweeney and G. Collins, ECS J. Solid State Sci. Technol., 5, R3059 (2016).(7) McSweeney, H. Geaney and C. O'Dwyer, Nano Res., 8, 1395 (2015).(8) McSweeney, C. Glynn, H. Geaney, G. Collins, J. D. Holmes and C. O'Dwyer, Semicon. Sci. Technol., 31, 014003 (2015).(9) G. Chadwick, V. Mogili, C. O’Dwyer, J. Moore, J. Fletcher, F. Laffir, G. Armstrong and D. A. Tanner, RSC Adv., 3, 19393 (2013).(10) C. Glynn, K.-M. Jones, V. Mogili, W. McSweeney, and C. O'Dwyer, ECS J. Solid State Sci. Technol. 6, N3029 (2017).

Silicon is today the main material used in electronics. It is a very advanced and mature technology. It is therefore clear that new technological concepts and materials should be introduced through the integration on the silicon platform. III-V semiconductors, such as GaAs and InAs, are high performance semiconductors, with direct bandgap and high carrier mobility, what makes them very prominent candidates for future electronics and optoelectronics. Lattice, thermal and polarity mismatch have been for decades limiting their integration on Si in the form of thin film technology. The nanowire geometry brings new prospects in this direction by reducing the contact area between mismatched materials,. Now, while growth of defect-free III-V structures on silicon is important, use in applications requires the nanostructures to be placed deterministically following a certain design/order. In this thesis we have studied different mechanisms to obtain ordered nanostructures on a silicon substrate, with the goal of increasing its functionality. The core part of this work was dedicated to the integration of III-V nanowires on Si in the formof ordered arrays. We have considered both GaAs and InAs nanowires. This process has several requirements: substrate fabrication and the growth process should be CMOS compatible, nanowires within the arrays should be grown vertically with a yield close to 100% and nanowires should be clones-looking and performing the same. An additional point to consider is that the fabrication process should be compatible with large scale techniques. The substrate requirements for obtaining ordered arrays of Ga-assisted GaAs nanowires on silicon were identified. In particular, we focused on the initial stages of growth, that turned to be key for achieving a high yield in the arrays. We found that the positioning of the Ga droplet within the predefined holes on the substrate determined whether the nanowire would grow perpendicularly to the substrate. Our HRTEMstudies on the titled nanowires show that the initial nanowire seeds seem to nucleate at the corner of the nanoscale holes. Instead, the crystal seeds of vertical nanowires occupy homogeneously the nanoscale holes. Achieving high yield in the growth of GaAs arrays on silicon also allowed us to study their evolution in time. We demonstrated that there is an incubation time for nanowires to start growing. This incubation time is different from NW to NWand leads to a relatively broad length distribution. We proposed a solution and showed how an increase in the supersaturation in the Ga droplet leads to the formation of homogeneous arrays. Growth of InAs nanowires in ordered arrays on silicon was based on the concept of guided growth. We used a SiO2 nanotube templates to promote vertical growth. Due to the directionality of MBE, achieving growth inside a nanotube was especially challenging. Our results provided new insights on the role of different pathways of the In and As4 adatoms during growth. In this part, large scale patterning by phase shift photolithography was demonstrated as an alternative to the conventionally used electron beam lithography. Stain etching method for producing porous silicon was applied on top-bottom fabricated Si micropillars. This led to geometrically driven electrochemical dissolution of silicon trough the center of the microstructure forming microtubes. The optical properties were also studied and used to produce functional 3D LED.

Motivation Cell signaling proteins have been widely regarded as some of the most powerful biomarkers for cancer diagnosis because they are real therapy related molecules. Extracellular vesicles, exosomes, transport proteins to promote tumor growth and are stable in biofluids at room temperature.[1] Hence, detection of exosomal proteins (e.g., cluster of differentiation (CD) 9, CD 63, CD 81) would be easily deployed at the point-of-care (POC) diagnostic. A successful electrochemical biosensor is generally determined by electrodes and their modification as well as specific bio-recognition elements (BREs).Graphene like MoS2 has emerged in biosensing applications owing to their intriguing properties, such as good catalytic properties, interesting metallic-to-semiconducting transition from 1T to 2H phase, and good biocompatibility.[2] However, a particularly vexing problem for MoS2 as the sensing materials is the structural instability that the exfoliated or ultrathin two dimensional (2D) MoS2 is hard to be retained in in their freestanding state. The surface modification technologies, such as growth of building blocks could transfer the 2D to relatively stable 3D nanoarchitectures, in which each component can also exert different function.Herein, we demonstrate a nanostructure consisting of zinc oxide (ZnO) nanowires (NWs) array grown on 2D MoS2forming a new nanoarchitecture (ZnO-MoS2) providing high specific surface area and good electron transport capability. Then, zeolitic imidazolate framework 90 (ZIF-90) thin films are in-situ deposited onto ZnO NWs by using ZnO as precursor to directly react with imidazole-2-carboxyaldehyde (ICA). The integrated hybrid nanostructure, denoted as ZIF-90-ZnO-MoS2, possesses not only effective mass transport and electron transduction capabilities but also covalent bio-conjugation of BREs through the aldehyde group on the bridging ICA ligand in ZIF-90. Of particular note is the ZnO NWs array is an indispensable part of the hybrid nanostructure, since the in-situ growth of ZIF-90 and effective integration with layered MoS2 is still a challenge. Through the cooperation of three different kinds of building blocks, from signal generation, amplification to transduction could be achieved in an integrated nanoarchecture. Results and Discussion Figure 1a displays the scanning electron microscope (SEM) image of obtained ZIF-90-ZnO-MoS2 nanohybrid, from which the ordered NWs array is uniformly grown onto 2D MoS2 material. The higher magnification SEM (Figure 1b) and transmission electron microscopy (TEM) (Figure 1c) images show that the length and diameter of vertical ZnO NWs are 1.5 μm and 50 nm, respectively. Further high resolution TEM (HR-TEM) (Figure 1d) characterization demonstrates that the NWs are consisted of highly crystal ZnO core with the diameter of about 25 nm and ZIF-90 sheath with the thickness of about 12 nm.A FITC-labeled goat anti-human immunoglobulin G (IgG) is employed to verify our hypothesis. The inset of Figure 1e shows the fluorescence microscopy image of microelectrode immobilized with FITC-labeled goat anti-human IgG. It can be seen that only the region deposited with ZIF-90-ZnO-MoS2 nanohybrid visualizes green fluorescence. This result illustrates ZIF-90-ZnO-MoS2 nanohybrid possesses the capability of direct covalent bio-conjugation of antibody. And the results shown Figure 1e further confirm the developed hybrid material could effectively detect signaling protein.We next fabricated a microfluidic electrochemical microsensor chip, and the configuration is shown in Figure 1f. The microsensor chip is composed of a polydimethylsiloxane (PDMS) layer sealed with screen-printed electrodes. The microsensor is consisted of five electrodes, including counter electrode (CE), reference electrode (RE) and three different working electrodes. Among the three working electrodes, electrode 1 and 2 are locally deposited with fabricated ZIF-90-ZnO-MoS2 nanohybrid material as the test electrodes, and electrode 3 is printed with carbon conductive ink for the background signal deduction. The micro-channels on the chip are employed for the simultaneous immobilization of bio-recognition antibodies as well as dual detection of exosome markers (CD63 and CD9). From Figure 1g, our fabricated microsensor chip shows obvious response towards NCI-H 1650, and almost no signal for other cells supernatant (NCI-H 1650: human non-small cell lung cancer cells, 16 HBE: human bronchial epithelial cells, HFL-1: human fetal lung fibroblast-1 and HUVEC: human umbilical vein endothelial cells). Conclusion In summary, the ZIF-90-ZnO-MoS2 nanohybrid integrated with different kinds of building blocks realizes the cooperation of multi-function, including covalent bio-conjugation of antibodies, signal generation and transduction. To further combine with microfluidic electrochemical microsensor chip, the simultaneous immobilization of bio-recognition antibodies and dual detection of exosome markers can also be acquired.

We present a simple method for generating vertically aligned (VA) ZnO nanowire (NW) arrays with tailored electrical top contacts, suitable for photodetector and photovoltaic applications. ZnO NWs were synthesized on Si substrate in house using a hydro‐thermal method, producing dense and highly organized vertical ZnO NW (∼50–100 nm) arrays. The vertical geometry of the NW arrays facilitated further processing of the NW structures and streamlined in situ characterization of individual NWs as well as entire NW ensembles. The top contact metallization involved depositing a thin film of low work function metals such as aluminum (5 nm) followed by a thin film of gold (15 nm) onto NW arrays, partially embedded in an insulating silsesquioxane‐based material (SOG‐400F). The I–V characteristics of individual ZnO NWs were measured using current sensing atomic force microscopy (CS‐AFM). Compared with arrays of as‐grown ZnO NWs, the metalized top contacts of NWs resulted in improved I–V characteristics with good Ohmic behavior at the metal‐semiconductor contact (MSC).

The bottom-up fabrication of regular nanowire (NW) arrays on a masked substrate is technologically relevant, but the growth dynamic is rather complex due to the superposition of severe shadowing effects that vary with array pitch, NW diameter, NW height, and growth duration. By inserting GaAsP marker layers at a regular time interval during the growth of a self-catalyzed GaP NW array, we are able to retrieve precisely the time evolution of the diameter and height of a single NW. We then propose a simple numerical scheme which fully computes shadowing effects at play in infinite arrays of NWs. By confronting the simulated and experimental results, we infer that re-emission of Ga from the mask is necessary to sustain the NW growth while Ga migration on the mask must be negligible. When compared to random cosine or random uniform re-emission from the mask, the simple case of specular reflection on the mask gives the most accurate account of the Ga balance during the growth.

Density dependent growth and optical properties of periodic arrays of GaAs nanowires (NWs) by fast selective area growth MOVPE are investigated. As the period of the arrays is decreased from 500 nm down to 100 nm, a volume growth enhancement by a factor of up to four compared with the growth of a planar layer is observed. This increase is explained as resulting from increased collection of precursors on the side walls of the nanowires due to the gas flow redistribution in the space between the NWs. Normal spectral reflectance of the arrays is strongly reduced compared with a flat substrate surface in all fabricated arrays. Electromagnetic modeling reveals that this reduction is caused by antireflective action of the nanowire arrays and nanowire-diameter dependent light absorption. Irrespective of the periodicity and diameter, Raman scattering and grazing angle X-ray diffraction show signal from zinc blende and wurtzite phases, the latter originating from stacking faults as observed by high resolution transmission electron microscopy. Raman spectra contain intense surface phonons peaks, whose intensity depends strongly on the nanowire diameters as a result of potential structural changes and as well as variations of optical field distribution in the nanowires.

We develop a facile approach for the one-step growth of ultralong aligned copper phthalocyanine (CuPc) nanowire (NW) arrays in a large area, through a grating-assisted physical vapor deposition method. The grating serves as the alignment template that guides the oriented growth of NWs, however, the morphology of the NWs is not restricted by the template but is remarkably determined by the growth conditions. The resulting NW arrays possess rectangular cross-sections, ultralong length, perfect alignment and large uniformity, which hold great potential as promising blocks for future integrated devices. The mechanism of one-step growth and alignment of NW arrays has been well discussed. Transparent and flexible photoconductive devices based on the aligned NW arrays can be directly constructed on the grating, which exhibit a fast response speed, high stability and reproducibility. Transfer of the NW arrays from the grating for further applications, has also been achieved by using polydimethylsiloxane (PDMS) as an elastic stamp. Flexible and electrically stretchable NW devices have then been fabricated on PDMS and demonstrate superior performances. The ability to make large area ultralong flexible organic NW arrays opens new possibilities for integrated device applications of organic nanostructures.

In this work, the controlled fabrication of highly ordered ZnO nanowire (NW) arrays on silicon substrates is reported. Si NWs fabricated by a combination of phase shift lithography and etching are used as a template and are subsequently substituted by ZnO NWs with a dry etching technique and atomic layer deposition. This fabrication technique allows the vertical ZnO NWs to be fabricated on 4 in Si wafers. Room temperature photoluminescence and micro-photoluminescence are used to observe the optical properties of the atomic layer deposition (ALD) based ZnO NWs. The sharp UV luminescence observed from the ALD ZnO NWs is unexpected for the polycrystalline nanostructure. Surprisingly, the defect related luminescence is much decreased compared to an ALD ZnO film deposited at the same time on a plane substrate. Electrical characterization was carried out by using nanomanipulators. With the p-type Si substrate and the n-type ZnO NWs the nanodevices represent p–n NW diodes. The nanowire diodes show a very high breakthrough potential which implies that the ALD ZnO NWs can be used for future electronic applications.

The role of substrate roughness in ZnO nanowire (NW) arrays hydrothermal growth has been systematically studied. Six silicon substrates with different roughness by chemical etching have been selected to grow ZnO NW arrays hydrothermally after sputtering 5-nm-thick ZnO seed layer as catalyst. The as-grown samples reveal that average diameters and number densities of ZnO NW arrays are inversely proportional to the increasing substrate roughness observed by atomic-force microscopy and scanning electron microscopy. Furthermore, the theoretically derived equations based on nucleation with the Gibbs free energy to describe relations of substrate roughness versus average NW diameter and NW number density match well with experimental results. Research results in this paper can be used to control the number density and the average diameter of ZnO NW arrays by alternating substrate roughness.

Performance of solar cells fabricated on black multicrystalline Si by nanowire decoration

Solar cells and hydrogen fuel cells have attracted much attention recently because they are the sources of clean energy. Using sun light to decompose water into H2 and O2 has the advantage that light energy is transferred directly into chemical energy and the energy is stored directly in the hydrogen fuel cell. In this thesis we report the study of using TiO2 /Cu2O nanowire array to transfer light energy into chemical energy and store the energy in the hydrogen fuel cell. In our study we found that Au layer plays an important role in the growth of the TiO2 nanowires. Good quality TiO2 nanowires can be grown when a thin layer of Au was sputtered on top of a Ti (99.7%) and with the nanowires grown at high temperature using Vapor-Liquid-Solid method. The electrochemical deposition of Cu2O films was conducted in an alkaline electrolyte solution with TiO2 and Cu as working electrode and Cu2O oxide can be deposited on TiO2 nanowires by applying suitable voltage. We found that when the TiO2/Cu2O nanowire array is exposed to light illumination, the photocurrent generated in the TiO2/ Cu2O core-shell nanowire array increased seven times as compared with the photocurrent generated by the bare TiO2 nanowirarray. This result is attributed to the enhanced light absorption of TiO2/ Cu2O nanowires and the electron-hole separation due to the type-II band alignment of the TiO2/ Cu2O heterostructure. Our results indicate that TiO2/ Cu2O nanowire array is a promising system in making hydrogen fuel cell.

This presentation will highlight our group’s efforts with various scalable methods for continuous manufacturing of nanowires powders and arrays with help of plasmas. These scalable methods can be categorized in to the following: (a) scalable production of nanowires of low-melting metal oxide nanowire powders; (b) scalable production of nanowire arrays and powders for titania and related oxides; and (c) low-temperature growth of nanowires of silicon and related materials. Scalable production method for nanowires of low-melting metal oxides: A novel atmospheric microwave plasma reactor is designed to process about a kilogram of nanowires per day and has already been demonstrated for the synthesis of tin oxide, zinc oxide and aluminum oxide nanowires.1 The resulting nanowires are 50-100 nm in diameter and several microns long. Scalable production of titania and related metal oxide nanowires: We report a new ultrafast (reaction time on the scale of a minute) gas-phase method for synthesizing highly crystalline TiO2 nanowires (NWs) using direct oxidation of either Ti metal (foils or powders) or spherical TiO2 powders using an atmospheric pressure microwave plasma reactor.2 The current state of the art methods involve long reaction time scales (~1 day) for synthesis of TiO2 NWs. Low-temperature synthesis of nanowire arrays of silicon and related materials: This presentation will also highlight our recent experiments on the synthesis of silicon nanowire arrays at temperatures lower than 300 C.This presentation will also highlight our efforts in utilizing the nanowire based materials toward various energy conversion and storage applications including dye sensitized solar cells, lithium ion batteries, and photoelectrochemical water splitting.

We review the recent progress in the growth, structural, optical and electrical properties of GaAs/GaAsSb, GaAs/GaSb, and InAs/InSb nanowires (NWs) grown by the vapor-liquid-solid (VLS) mechanism. The structural characterization of GaAsSb NWs reveals that they adopt zinc blende (ZB) crystal phase, whereas the same growth conditions (except the Sb flux) produce GaAs NWs with wurtzite (WZ) crystal phase. With increasing mole fraction of Sb, the ZB GaAsSb NW forms a lower density of twinning planes. Low-temperature photo-luminescence (PL) characterization of axially heterostructured GaAs/GaAsSb NWs shows that the linear polarization of the PL emission from the ZB GaAsSb inserts is opposite (parallel to NW axis) to the WZ GaAs segments (perpendicular to NW axis) due to a difference in the optical selection rules between ZB and WZ crystals. GaSb NWs have been grown on top of GaAs NWs by the Au-assisted VLS method. The two most striking differences between the GaSb and GaAs NWs are: 1) The diameter of the GaSb NWs is significantly larger than that of the GaAs NWs. 2) Whereas the GaAs NWs have WZ crystal phase with stacking faults, the GaSb NWs exhibit a defect-free ZB crystal phase. Undoped GaSb NWs have so far been determined to be p-type. Low-temperature PL measurements on GaSb NWs reveal a PL peak near 0.8 eV, with the energy position dependent on the V/III ratio. InSb NWs grown on top of InAs NWs show similar behavior in terms of diameter and crystal phase change as the GaAs/GaSb NWs.

III-V semiconductor compound based nanowires (NWs) are expected to impact the fields of nano-electronic, nano-photonic, and photovoltaic devices. Self-assembly of crystal-phase controlled and high optical quality III-V NWs has been demonstrated. However, important physical and technological issues, such as carrier transport properties and reproducible incorporation of high dopant concentrations in NW materials, remain to be addressed for enabling robust nano-devices fabrication. In this work, we show the use of a multi-probe scanning tunneling microscope for the rapid electrical characterization of free-standing GaAs NWs, without any need for post-growth sample processing and contact fabrication. In particular, 2-probe I-V measurements were performed along the axis of a single 60-nm diameter unpassivated GaAs NW, and its resistance profile determined, obtaining high (in the range of GΩ) resistance values. Due to its reduced radial dimension, the NW is expected to be completely depleted. Analysis of the NW resistance profile reveals instead, that carrier transport is mediated by the NW surface states. Finally, by using the substrate as a reference electrode and placing the other three STM-tips along the NWs, we demonstrate a 4-point probe geometry that can be used for the electrical characterization of highly doped NWs.