Quantum emitters comprised in nanowire structures represent one of the most promising technologies for quantum photonics applications. The most common approach of nanowire-based quantum dot formation consists in changing the chemical composition along the nanowire axis in a very short length scale. Other methods consist in the self-assembly of small InAs islands on GaAs nanowire facets following the Stranski-Krastanov mechanism [1]. Recently it has been shown that ternary alloys obtained on the nanowire facets exhibit segregation phenomena, leading to the formation of nanoscale islands [2].
We present a quantum-dot-in-nanowire system formed in a self-assembly manner in the AlGaAs shells grown around GaAs nanowires. The quantum dots form at the apex of a GaAs/AlGaAs interface. They are highly stable, and exhibit extremely bright emission and small linewidth. We present a complete study on the growth parameters leading to the formation of bright quantum emitters, substantiated by high resolution transmission electron microscopy and luminescence spectroscopy studies.
Finally, we show how these quantum dots in nanowires can be cut in the shape of a nanoscale optical and mechanical resonators [3,4]. The excellent optical properties survive the fabrication process, thereby opening a new way of embedding quantum dots in high quality cavities.
References
[1] Uccelli, E. et al. 2010 ACS Nano 9 2
[2] Heiss M. et al. 2013 Nat. Mater.12 439-444
[3] Martinez, R.V. et al. 2014 Nano Lett. 14 524
[4] Montinaro, M. et al, 2014 Nano Lett asap

GaAs nanosheets are grown using selected area growth (SAG) metal organic chemical vapor deposition (MOCVD). These GaAs nanostructures show enhanced optical and electronic properties compared to GaAs nanowires even in the non-passivated form. Photoluminescent intensity enhancement by over 6X, 2-fold enhancement in carrier&’s lifetime and carrier&’s diffusion length of 170 nm are handful of optoelectric improvements that GaAs nanosheets provide over their counterparts, GaAs nanowires. GaAs nanosheets are in form of triangle with average base size of 6 µm and height of 2-3 µm. Here, we demonstrate the effects of AlGaAs passivation on these nanostructures. The AlGaAs layer is grown in the MOCVD chamber with the thickness of 20 nm. The Photoluminescent spectra of the AlGaAs passivated GaAs nanosheets are measured using a micro-PL setup using a 100X objective lens over the range of 750 nm to 1000 nm; samples are excited using a 532 nm CW laser at relatively low powers. The formation and uniformity of AlGaAs layer is confirmed by Raman measurement which is also performed in the micro-PL setup. The Time-resolved PL data is collected at 10K using a Ti-Sapphire laser, where the samples are excited at 558nm using a 50X objective lens (NA of 0.6). The measured carrier&’s lifetime for passivated nanosheets is 4.2 ns which shows significant increase in comparison to the non-passivated nanosheet. AlGaAs passivation of GaAs nanosheets also improves the photoluminescence efficiency by 42X and decrease the surface recombination velocity.

III-V semiconductor nanowire (NW) heterostructures are known to hold large potential for various important device applications, such as for photo-detectors, solar cells, lasers, light emitting diodes and transistors. For example, we recently demonstrated that single-mode optically pumped NW lasers can be fabricated from individual radial core-shell GaAs-AlGaAs NWs with high optical efficiency that exploit the many benefits of the core-shell geometry, such as effective surface passivation, carrier confinement and optical waveguiding [1,2]. In most radial core-shell GaAs-AlGaAs NWs studied so far, the electronic properties are mostly described by the 3D bulk-like properties of the NW core, despite the 1D-like aspect ratio of the NWs. Thus, only very limited studies exist on exploring the 1D-like electronic nature of GaAs-based NWs, although major performance enhancements are expected from exploiting these electronically strongly confined systems.
Here, we report on the growth, structural and optical properties of autocatalytic GaAs-Al_0.3Ga_0.7As core-shell NWs with ultrathin GaAs core diameters (~5-20 nm). All NW growths were performed on SiO₂-mask templated Si (111) substrates via solid-source molecular beam epitaxy (MBE). The GaAs NW cores were grown in an autocatalytic, Ga-droplet mediated vapor-liquid-solid (VLS) growth mode, while the Al_0.3Ga_0.7As shell was overgrown radially on the {110} sidewall facets in a non-VLS growth mode [2]. After growth we performed systematic micro-photoluminescence spectroscopy (µ-PL), photoluminescence excitation (PLE) spectroscopy as well as transmission electron microscopy (TEM) on exactly the same NWs for direct correlation studies.
The ultrathin NWs exhibit a characteristic wurtzite (WZ) region along one half and a zincblende (ZB) region along the other half of the NW with occasional stacking defects. Low-temperature µ-PL data evidences very strong blue-shifts (up to > 100 meV above the free exciton energy of GaAs) in the PL energy for both the WZ and ZB parts, when the NW core diameter is decreased from ~40 nm down to ~5 nm [3]. This diameter dependence of the PL peak energy is in good agreement with simulations of electrons and holes confined in 1D quantum wires. Interestingly, the strongly blue-shifted PL exhibits also some sharp PL features with line widths below 100 ueV [3]. This indicates that also axial confinement of the excitons is present, likely stemming from stacking defects along the NW axis. PLE experiments reveal additional transitions appearing at 3 meV higher energies for distinct energy ranges of the excitation laser which could be attributed to the different charge states of these localized excitons. These investigations present a promising first attempt to explore the rich potentials of truly 1D-like electronic nature of ultrathin GaAs NWs.
¹ B. Mayer, et al., Nature Comm. 4, 2931 (2013)
² D. Rudolph, et al., Nano Lett. 13, 1522 (2013)
³ B. Loitsch, et al., in preparation (2014)

Growing of high quality InP nanowires for optoelectronic and photovoltaic device applications has been the subject of intense research effort in recent years, in particular in the context of nanowire solar cells, which have shown promising high efficiency (of 13.8% with only 12% of surface coverage). Nanowires can be fabricated by a variety of crystal growth techniques. However, so far no report has shown diameter-independent pure crystal phase InP nanowires, with proven high quantum efficiency, long room temperature minority carrier lifetime, low surface recombination velocity and taper-free nanowire arrays.
We report successful growth of stacking-fault-free and taper-free wurtzite InP nanowires with a wide range of diameters using selective-area metal-organic vapor-phase epitaxy. We have experimentally determined a quantum efficiency of ~50%, on par with InP epilayers. The direct link between growth conditions such as growth temperature and precursor flow rates with nanowire morphology, crystal structure and optical properties will be presented in detail. Their excellent structural and optical quality lead to further demonstration of room temperature photonic mode lasing from these nanowires, opening up new possibilities for both fundamental quantum optics, as well as optoelectronic/photovoltaic device applications.

Low temperature photoluminescence (PL), PL imaging and photoluminescence excitation (PLE) measurements are conducted on single nanowires (NWs) to explore the optical and electronic properties of GaAs/AlGaAs quantum well tubes (QWTs). These QWTs are radial heterostructure NWs with thin GaAs layers varying from 8nm to 1.5nm thickness, sandwiched by AlGaAs barriers on both sides wrapping around a central GaAs core. PL emissions of high quantum efficiency are detected from the QWTs as well as GaAs core. As QWT well width decreases from 8nm to 2nm, a large number of sharp emission lines from the QW begin to be observed: we attribute this response to exciton localization due to non-uniform well widths and alloy fluctuations. PLE spectra indicate that transitions from electron and hole excited states only appear for well widths larger than 5nm. Based on the structural information obtained from HAADF-STEM images of NW cross-sections, we carried out a cylindrically symmetric theoretical model to calculate the energy levels. The transitions of the electron and hole ground states as well as the first excited states from the QW observed in the experiments are in good agreement with our theoretical calculations.
In order to understand the narrow lines, we implemented a detailed PL imaging spectroscopy with enhanced spatial resolution and collection efficiency. High resolution spatially-resolved PL images are taken at 10K for QWTs with different well widths. The PL images reveal many localized states distributed at different spatial positions along the 2nm and 4nm QWT wires. No such localized states are observed in the 6nm and 8nm QWT wires.
We acknowledge the NSF through DMR-1105362, 1105121 and ECCS-1100489. We also acknowledge the support of the Australian Research Council (ARC) and the Australian National Fabrication Facility.

III-V group nanowires (NWs) are considered as versatile building blocks for future optoelectronic applications due to their efficient strain relaxation, superior optical and electrical properties. Ternary alloy NWs offer tunable bandgap and the possibility of different band alignment with various heterostructures. In particular, GaAs_1-xSb_x has great potential for applications of devices operating in the near infrared wavelength region. In addition, the band alignment of GaAs_1-xSb_x with other semiconductors can be tailored to form a type I, type II (staggered) or type III (broken) structure for different applications. Interestingly, staggered band alignment can be used to favor the separation of charge carriers for photo-detection and photovoltaic applications.
Despite these key advantages, GaAs_1-xSb_x NWs have not been studied much. Comprehensive studies of the synthesis of heterostructures or their optical properties are missing. From the growth perspective, polarity has recently been demonstrated to cause 3D compositional inhomogeneity in ternary NWs and core-shell heterostructures. However, very few studies have considered metal-catalyzed NWs grown on substrates with different polarities, such as (111)A. Consequently, the poor understanding of NWs grown on (111)A substrate restricts the development of polarity driven NW heterostructures and their applications.
Here we present growth studies on Au-seeded GaAs_1-xSb_x vertical NWs grown on GaAs(111) substrate with both A and B polarity. We demonstrate the crucial role of the substrate and nanowire polarity in improving the material quality of GaAs_1-xSb_x NWs in terms of tapering and crystal quality. Next, growth of GaAs_1-xSb_x/GaAs and GaAs_1-xSb_x/InP core shell heterostructures was realized. Optimized shell morphologies were found to depend on the growth condition and Sb content in the core. With the passivation effect of shell, the optical properties of the nanowires improved significantly. Both type I and II band alignment photoluminescence emission were observed in temperature and power dependent optical spectroscopy studies. Strain engineering using either core/shell thickness ratio or composition of the core was also investigated, offering emission tunability over a large range of wavelength in the infra-red region. Finally, potential benefits from the original type II staggered band alignment in photodetectors application were investigated.

Band gaps are one of the most important parameters of semiconductor materials for optoelectronic applications since they determine the spectral features of absorption and emission processes. Semiconductor nanowires have been proposed and demonstrated as building blocks for future optical and electronic devices. An important task in the research of nanowire photonics and optoelectronic is to achieve wires with engineered gaps ^[1]. In this talk, we will report our recent research progress on bandgap engineering based on single semiconductor nanowires. In details, we will show how to realize bandgap tenability along the length of single nanowires, through in-situ composition control during the growth ^[2]. Using the engineered nanowire structures, some interesting nanophotonics applications have been realized, like on-nanowire white lighting ^[3], asymmetric waveguiding ^[4], wavelength splitters ^[5], low-threshold nanowire lasers ^[6] and room temperature near-infrared photodetectors ^[7].
References:
[1] X. J. Zhuang, C. Z. Ning and A. L. Pan*, Adv. Mater., 2012, 24, 13. (Invited Review)
[2] F. X. Gu, Z. Y. Yang, H. Yu, J. Y. Xu, P. Wang, L. M. Tong, A. L. Pan*, J. Am. Chem. Soc., 2011, 133, 2037.
[3] Z. Y. Yang, J. Y. Xu, P. Wang, X. J. Zhuang, A. L. Pan*, L. M. Tong, Nano Lett., 2011, 11, 5085.
[4] J. Y. Xu, X. J. Zhuang, A. L. Pan*, et al. Scientific Reports., 2012, 2, 820.
[5] J. Y. Xu, X. J. Zhuang, A. L. Pan*, et al. Nano. Lett., 2012, 12, 5003.
[6] P. F. Guo, X. J. Zhuang, J. Y. Xu, Q. L. Zhang, W. Hu, A. L. Pan*, et al. Nano Lett., 2013, 13, 1251.
[7] L. Ma, W. Hu, Q. Zhang, P. Ren, X. Zhuang, H. Zhou, A. L. Pan*, et al. Nano Lett., 2014, 14, 694.

Semiconductor nanowire (NW) lasers provide the potential to generate highly localised, intense monochromatic optical fields in a geometry that is very well suited to an efficient coupshy;shy;shy;ling to highly integrated nanophotonic elements and optical circuits. In this sense, employshy;shy;ing III-V NW lasers is most attractive since their emission wavelength can be tuned to the important telecommunications band and they can be directly grown on CMOS-compatible silicon (Si) platform, which could enable the III-V lasers to emit unishy;directionally into the underlying Si photonic hardware. Fabrication of III-V NW lasers, and in particular of GaAs-based materials, is however a challenging task since the extenshy;dshy;shy;ed surface-to-volume ratio and associated surface states as well as issues with low refractive index contrast between GaAs and Si pose deleterious impacts on optical efficiency and effective resonant laser cavity function.
Here, we report recent progress in achieving efficient suppressshy;ion of surface-state mediated non-radiative recombination using surface-passivated GaAs-AlGaAs core-shell NWs. These sophisticated NW heterostructures are further demonstrated to be functional as nanolasers up to room-temperature when subjected to optical excitation, both in pulsed and continuous-wave (cw) mode. In this talk, we will address in logical order the following steps: (i) design criteria for fabrication of Fabry-Perot resonators with single-mode transverse optical confinement for the GaAs-NW system [1], (ii) the growth of GaAs-AlGaAs core-shell NW laser structures with suppressed surface recombination [2], (iii) different optical excitation schemes (pulsed mode and cw mode) and the respective input-output characteristics as a function of temperature, and (iv) estimation of carrier density close to threshold. Most importantly, we will highlight that clear lasing operation via “s-like” dependence of the luminescence peak intensity with excitatshy;ion power density is observed that persists even up to room-temperature with threshold power densities as low as 0.76 kW/cm² [1].
[1] B. Mayer, et al., Nature Communications 4, 2931 (2013).
[2] D. Rudolph, et al., Nano Letters 13, 1522 (2013).

We investigate the optical and transport properties of single GaAs/AlGaAs core-multishell Quantum Well Tube (QWT) nanowire heterostructures. The QWT is defined by a thin 4 nm or 8 nm GaAs layer embedded inside a thick 40% AlGaAs shell which surrounds a 50nm diameter GaAs NW core. Single nanowire devices were fabricated by standard photolithography followed by deposition of Ti (20nm)/Al (500nm) metal contacts on both ends of the nanowire. The QWT nanowire devices exhibit very low (sub pA) dark current and are extremely photosensitive (10nA) under modest illumination. These QWT devices exhibit peaks in the photocurrent (PC) when excitons are resonantly excited in the GaAs core or the ground and excited states of electrons and holes confined to the quantum well tube. Spectroscopic signatures are also seen of excitons excited into the continuum above the AlGaAs barriers. The identification of these spectroscopic peaks in the photocurrent are confirmed through simultaneous measurements of the photoluminescence (PL) of the ground state excitons in the core and QWT, as well as photoluminescence excitation (PLE) measurements of the excited states on the same devices. The observed resonant peaks in the PL, PC, and PLE spectra agree very well with each other and also with calculated values of confined energies of electrons and holes in the quantum well tube where the hexagonal symmetry of the nanowires is approximated by cylindrical symmetry. The identified optical transitions in photocurrent measurement of 8nm QWT nanowire device are the core (GaAs), and the transition between the electron and hole quantum confined states hh1→e1, hh2→e2, lh2→e2 and the onset of the shell (AlGaAs). For the 4nm QWT in addition to the core (GaAs) absorption, the transitions VB (AlGaAs)→e1, VB→e1, hh1→CB (AlGaAs)and hh2→CB are evident. We observe a strong quenching of the PL intensity from the quantum well tubes as we increased the source- drain bias, consistent with field-dependent ionization of the excitons which produces the photocurrent.
We acknowledge the NSF through DMR-1105362, 1105121 and ECCS-1100489. We also acknowledge the support of the Australian Research Council (ARC) and the Australian National Fabrication Facility.

Composition/band gaps modulation is particularly important for the development of semiconductor based multifunctional and high-performance optoelectronic components, such as tunable lasers and broadband-response photodetectors. Quasi one dimensional (1D) semiconductor nanoribbons, with depth from tens to hundreds of nanometers, have aroused great interest for their high crystal quality and potential applications in integrated photonics and electronics ^[1]. Recently we have developed a source-exchanging chemical vapor deposition route and successfully realized the growth of semiconductor nanostructures with engineered composition/band gaps ^[2]. In this talk, we will summarize our new achievements in band gap engineered nanoribbons, with band gap/composition gradually or abruptly changed along the width direction of the ribbons ^[3-5]. The controlled growth of these nanoribbon heterostructures, involved in-situ ions exchange and lateral epitaxial growth, will be discussed in details. Some interesting applications using these novel nanoribbon structures, including dual-color nanolasers ^[3] and high-performance broadband photodetectors ^[4], will also be presented in the talk.
References:
[1] Z. W. Pan, Z. R. Dai, Z. L. Wang, Science, 2001, 291, 1947-1949.
[2] X. J. Zhuang, C. Z. Ning, A. L. Pan*, Adv. Mater., 2012, 24, 13. (Invited Review)
[3] J. Y. Xu, L. Ma, P. F. Guo, X. J. Zhuang, X. L. Zhu, W. Hu, X. F. Duan, A. L. Pan*, J. Am. Chem. Soc., 2012, 134, 12394-12397.
[4] P. F. Guo, X. J. Zhuang, J. Y. Xu, X. Zhu, W. Hu, H. Zhou, X. Wang, X. F. Duan, A. L. Pan*, Adv. Mater., 2014, 26, 2844-2849.
[5] P. F. Guo, D. Li, X. J. Zhuang, A. L. Pan*, 2014, submitted.

Using a bimetallic catalyst, the crystallographic orientation of GaN nanowires can be directed either along the a- or m-axis. The resulting a- and m-axis wires have triangular cross-sections displaying unique semipolar side facets of either the {1-101} or {11-22} family, respectively, and a common (0001) facet. These surfaces exhibit distinct differences in their optical and chemical properties. Cathodoluminescence results suggest enhanced non-radiative charge-carrier recombination caused by the m-axis nanowire surface. The differences in their surface structures affect the growth rate and indium incorporation during overgrowth. The resulting heterostructures, with GaN nanowire cores and InGaN multiple quantum well shells, show dramatic differences in their light emission depending on the growth axis.

Group III-nitride light-emitting diodes (LEDs) are promising candidates for next-generation lighting sources because of their high efficiency, long life, and environmental friendliness. To successfully replace conventional light sources, the quantum efficiency is determined using the internal quantum efficiency (IQE) and the light extraction efficiency (LEE). Although an IQE of nearly 80% has been achieved, much room remains for LEE enhancement because most of the generated photons from the active layer remain inside the LEDs because of the total internal reflection (TIR) at the semiconductor-air interface. To circumvent this problem, a number of solutions based on geometrical optics have been developed.
To eliminate the TIR and to extract more light from the device surface, several approaches have been proposed and demonstrated, including growth of nano wire such as zinc oxide (ZnO), and integration of two-dimensional (2D) photonic crystal (PC) patterns on the device surface. Vertically aligned ZnO nanorods have been grown on the transparent electrode of GaN LEDs to improve the light output. Although the methods did not include a complex lithography, significant thermal damage on the transparent electrodes is occurred during the growth. Several groups have applied PC on the top p-GaN layer of GaN-based LED devices, increasing the emission efficiency, but elaborate grating patterns on those PC-LED devices had to be written using electron beam lithography, which is intrinsically expensive, low-throughput and small-area processing.
Here, we present the fabrication of PC structure with controllable period of the pattern to maximize the light extraction efficiency by using laser interference lithography (LIL). LIL method allows the production of very precise nanometer sized structures, uniformly and over large area. To quantitatively demonstrate the effect of PC structure on light extraction efficiency in LEDs, we used the 3-D finite-difference time-domain (FDTD) method. This PC structure by laser interference lithography could be used in other optical devices such as organic LEDs, solar cells and laser diodes, due to simple and easy fabrication with large area capability.

Here we use the composition of a bimetallic catalyst to control the crystallographic growth-axis of GaN nanowires. Using the vapor-liquid-solid mechanism, the gold/nickel ratio of a catalyst particle can be tuned to selectively grow nanowires either along the a-axis or m-axis. We demonstrate this growth strategy is independent of the epitaxial substrate. Moreover, by carefully choosing the catalyst composition and growth condition, the nanowires spontaneously switch growth direction from a- to m-axis allowing direct comparison of their optical properties, which we explore using Cathodoluminescence.

Despite huge recent advances in research on the growth of semiconductor nanowires, several questions have yet to be addressed regarding nanowire growth dynamics for a full understanding of the problem. The mechanical stability of the catalyst nanoparticle during vapor-liquid-solid growth is one of these questions, which has been related to the formation of kinks or structural deformations along the nanowires. In this work, we analysed InGaP nanowires using scanning and transmission electron microscopy. Our InGaP nanowires present spontaneous core-shell formation, with an In-rich core, despite the growth environment with similar In and Ga precursor flows and the larger temperature (480C) used. This is not totally unexpected since the growth was carried out using Au nanoparticles which show higher affinity to In than Ga. Some of the grown nanowires are <211> oriented and present twins and stacking faults parallel to the nanowire axis. The observed configuration of twins is associated with a change in the polarity of the growing interface. However, different growth rates can be expected for different facet polarities for III-V compounds. Therefore, changes in polarity could lead to a mechanical instability of the nanoparticle for nanowires growing with <211> orientation. We observe several stacking faults only for interfaces with a determined polarity, possibly due to an increase of the growth rate of the corresponding segment of the metal/semiconductor interface. In addition, the nanoparticle migrates away from the top of our nanowires when the growth is terminated, indicating stability for some specific condition on a <111> oriented sidewall facet. These results suggest a self-regulated stacking fault generation mechanism, in order to equalize the growth rate of the facets forming the nanoparticle/nanowire interface.

The exotic properties of topological insulators (TIs) like suppression of back scattering, spin-momentum locking and linear E-K relation are very promising for applications in various fields like electronics, spintronics, optoelectronics and thermoelectrics. For future applications of the topological surface states, however, a major bottleneck remains the lack of high quality bulk materials devoid of unintentional doping. One of the effective means to address this issue is to increase the surface-to-bulk ratio by synthesizing thin film or nanostructures. Though a lot of work has been done to study the growth of topological insulators by various state-of-the-art techniques, the growth of high quality nanostructures is still a major challenge. In this work, we report substrate-independent catalyst-free growth of nanowires of high purity antimony telluride (Sb₂Te₃) by the pulsed laser deposition (PLD) technique; Sb₂Te₃ is a 3D topological insulator material that is promising for information storage and thermoelectric applications [1].
We contrived an unconventional PLD setup for the nanowire growth that affords precise control on the horizontal growth direction. Structural, morphological, compositional and electrical characterization of the samples was done to evaluate the quality of the nanowires. We observed insensitivity of the nanowire properties to the substrate by growing on silicon (cubic and used most commonly), SiO₂ (amorphous dielectric), glass (amorphous) and mica (hexagonal symmetry). We have also investigated the effect of growth time on the areal number density of nanowires. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) were used to study the morphology and topography of the nanowires. Transmission electron microscopy (TEM) confirmed the single crystalline growth and hexagonal symmetry of the nanowires; this was further corroborated by Raman spectroscopy. Finally, energy dispersive X-ray spectroscopy (EDS) revealed that the Sb₂Te₃ nanowires are precisely stoichiometric.
Reference:
[1] Shahil et al., J. Appl. Phys.111 (2012), 054305

The growth of III-V semiconductor nanowires using the vapor-liquid-solid mechanism by metal-organic chemical vapor deposition (MOCVD) has emerged as a leading approach to bottom-up nanowire synthesis. Precise control of the growth parameters is essential for understanding the growth process and for synthesizing materials of high electronic and optical quality for use in future applications and fundamental studies of electronic behavior in this material system. We have used a close coupled shower head MOCVD to grow gold-catalyzed GaAs/Al_0.35Ga_0.65As core/shell nanowires. By using an in-situ optical pyrometer, we are able to improve our control of the surface temperature during the core nanowire growth. We use liquid helium temperature photoluminescence characterization to evaluate the optical quality of the GaAs/AlGaAs nanowires at each deposition temperature. We find a narrow region of optimal temperatures for the GaAs core nanowire growth around 430 °C that results in sharp band edge luminescence at a wavelength of 818 nm. Furthermore, we see that the defect peak at 830 nm in the photoluminescence spectra is dependent on the growth surface orientation, i.e., (100) vs. (111)B. This result indicates the important role of precise temperature control in synthesizing high quality semiconductor nanowire heterostructures. This research is supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering under Award No. DE-SC0001304.

III-V nanowires (NWs) are considered key elements for the fabrication of future nano-photonic devices. GaAs is a prime candidate for such applications. Recently, optically-pumped lasing has been reported for single GaAs-AlGaAs core-shell NWs [1,2]. Indeed, the overgrowth of a wider bandgap AlGaAs shell around GaAs NWs leads to effective passivation of GaAs surface states and to improved minority carrier diffusion lenghts, and recombination lifetimes with respect to bare GaAs NWs. However, growth conditions (temperature, V:III precursor ratios in the vapor, etc.) and relevant geometrical parameters (namely, the shell-thickness to core-radius ratio) of GaAs-AlGaAs core-shell NWs affects the nanostructure carrier lifetime [3], as well as built-in lattice strain and core radiative emission [4].
GaAs-AlGaAs core-shell NWs were grown by low pressure MOVPE in an Aixtron reactor, using trimethylgallium, trimethylaluminum and tertiarybuthylarsine as Ga, Al and As precursors. GaAs core NWs were grown at 400°C on either semi-insulating (111)B-GaAs or (111)Si substrates [5] by the Au-catalyzed method, after which an Al_0.33Ga_0.67As shell was overgrown at 650°C by conventional MOVPE. In some cases a few-nm thin GaAs cap layer was deposited around the shell to protect it from oxidation. During NW core and shell+cap growth the V:III precursors ratio in the vapor was changed between 5:1 and 30:1.
We demonstrate that, under conventional MOVPE growth conditions, the initial diameter, height and density of core NWs strongly impact on the actual shell growth rate. We further explain such findings by validating a detailed MOVPE growth model of AlGaAs shell based on the mass-transport of III-group species. Besides ensuring strict control over shell thickness in core-shell and core-multishell NW structures, the model allows to calculate effective (around the NWs) vapor stoichiometry during shell growth.
7K photoluminescence of GaAs-AlGaAs core-shell NWs is thus reported as function of the NW relevant geometrical parameter, and values of V:III effective ratios during AlGaAs shell growth obtained from the model. After deconvoluting the strain-dependent red-shift of GaAs excitonic emission from the shell-dependent localization effect in present core-shell NWs [4], the latter is reported as function of as-calculated V:III effective ratios and discussed.
References
[1] D. Saxena, S. Mokkapati, P. Parkinson, N, Jiang, Q, Gao, H,H, Tan, C, Jagadish, Nature Photon. 7 (2013) 963.
[2] B. Mayer, D. Rudolph, J. Schnell, S. Morkötter, J. Winnerl, J. Treu, K. Müller, G. Bracher, G. Abstreiter, G. Koblmüller, J.J. Finley, Nature Comm. 4 (2013) 2931.
[3] N Jiang, Q. Gao, P. Parkinson, J. Wong-Leung, S. Mokkapati, S. Breuer, H.H. Tan, C.L. Zheng, J. Etheridge, C. Jagadish, Nano Lett. 13 (2013) 5135.
[4] P. Prete, I. Miccoli, F. Marzo, N. Lovergine, Phys. Status Sol. - RRL 7 (2013) 874.
[5] I. Miccoli, P. Prete, F. Marzo, D. Cannoletta, N. Lovergine, Cryst. Res. Technol. 46 (2011) 795.

One key advantage of ZnO is its ability to grow with the nanowire (NW) shape by low-cost and surface scalable deposition techniques such as the low-temperature chemical bath deposition (CBD) process. Although widely investigated by determining chemicals and seed- layer structural morphology effects, the nucleation and growth mechanisms of ZnO NWs by CBD are still not completely understood. This results in an insufficient control of their structural properties and uniformity, which is detrimental for integrating ZnO NWs in advanced electronic, optoelectronic and photovoltaic devices. In this work, ZnO NWs are grown by CBD either on various ZnO seed layers (with different diameter, roughness, texture) grown on Si substrates or on as-grown and electron-beam patterned ZnO single crystals with distinct crystal orientation and polarity [1-3]. It is shown that the growth of ZnO NWs is limited by the mass transport of chemical precursors in solution [1]. Additionally, ZnO NWs only develop homoepitaxially on polar c-p