Growth and Characterisation of InP Nanowires and Nanowire-Based Heterostructures for Future Optoelectronic Device Applications

Indium Phosphide (InP) forms a cornerstone amongst direct band-gap III-V compound semiconductors with the possibility for a wide range of other III-V alloys to be lattice matched with it. It is commonly used in optical communications related device applications, high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs). The very low surface recombination velocity of InP has made its nanowire counterpart a standout amongst nanowires of other III-V materials with successful demonstrations in nanowire solar cells, lasers and single photon sources. Considerable progress has been made in terms of InP nanowire growth in the past decade. Defect-free wurtzite (WZ) phase nanowires with good optical quality have been achieved on InP (111)B substrates. However, there are unexplored areas related to nanowire heterostructures that may hold promise for future device applications. Furthermore, InP nanowires aimed for future integrated devices need to be grown on the Si (111) substrates, and preferably on Si (100) substrates, in order to be integrated with microelectronics and other planar devices on a single chip. This dissertation presents a progressive advancement of Au seeded InP nanowire growth by MOVPE, from heterostructures grown on InP (111)B substrates to nanowire growth on Si (111) substrates and [100] oriented InP substrates. A number of diverse techniques have been employed to understand the growth process and characterise the samples. Scanning and transmission electron microscopy, atomic force microscopy, X-ray diffraction (XRD) and energy dispersive X-ray spectroscopy (EDX) have been used for structural and compositional analysis, while room and low temperature photoluminescence (PL) and PL mapping have been used for optical characterisation. InP-InxGa1-xAs nanowire quantum wells (QWs) emitting in the 1.3 μm optical communications wavelength region are grown on InP (111)B substrates. Detailed structural and optical analysis carried out using cross-sectional TEM (X-TEM) and PL mapping reveal asymmetric diffusion at the two interfaces of the QW, and broad, yet bright and homogenous PL emission along the complete length of the nanowire, with no emission visible from the InP nanowire core or outer barrier. The emission wavelength of the QW is tuned in the 1.3 μm range by varying the QW thickness as well as composition. The WZ phase QWs are optically modelled using the kp method. Multiple QWs comprised of three QWs and showing strong emission is also demonstrated. InP nanowire growth on Si (111) substrates has been carried out using an intermediate buffer layer. A two-step approach is used for the growth of the buffer layer and the growth parameters are optimised for both steps in order to achieve a smooth layer that covers the underlying Si substrate. It is seen that the layer fully relaxes by forming dislocations at the interface and is of (111)B polarity. Over 97% vertical nanowire yield is achieved on the buffer layers, and these nanowires are found to be similar in morphology and optical properties to those grown homoepitaxially on InP (111)B substrates under the same growth conditions. InP nanowires grown on the industry standard [100] orientated substrates are examined by studying the growth directions, facets and crystal structure of the different types, namely, vertical, non-vertical and planar nanowires grown on InP (100) substrates. The seemingly random growth directions of the non-vertical nanowires are actually found to be and directions that acquire complex orientations with respect to the substrate due to the consecutive three dimensional twinning that takes place at the initial stages of growth. These directions are mathematically calculated and verified by the measurements carried out on individual nanowires. It is shown that 99% of the nanowires grown on InP (100) substrate are either , or oriented with growth facets of either {100} or {111}. The relative yields of each type of nanowire grown on InP (100) substrates are controlled by optimising the pre-growth annealing and growth conditions. A maximum of 87%, 100% and 67% yield is achieved for vertical, planar and non-vertical nanowires, respectively. The novel families of side facets of nanowires are engineered to obtain cross-sectional shapes ranging from square to octagonal while maintaining a high vertical yield. Growth parameters and post-growth in-situ annealing conditions are tuned in order to achieve this. Finally, InGaAs QWs are grown on a novel and asymmetric facet combination of [100] nanowires, demonstrating the intended non-uniform complex growth that results in different thicknesses and compositions on the different types of nanowire facets. Overall, this work explores new avenues of InP nanowire and heterostructure growth aimed for future optoelectronic devices that are directly integrable with planar devices and Si technology. The findings presented, especially those on growth on [100] oriented substrates, bring many unforeseen opportunities for nanowire device development to light

Engineering the side facets of vertical [100] oriented InP nanowires for novel radial heterostructures

In addition to being grown on industry-standard orientation, vertical [100] oriented nanowires present novel families of facets and related cross-sectional shapes. These nanowires are engineered to achieve a number of facet combinations and cross-sectional shapes, by varying their growth parameters within ranges that facilitate vertical growth. In situ post-growth annealing technique is used to realise other combinations that are unattainable solely using growth parameters. Two examples of possible novel radial heterostructures grown on these vertical [100] oriented nanowire facets are presented, demonstrating their potential in future applications

Growth of InP nanowires on silicon using a thin buffer layer

InP nanowires (NWs) are grown on Si substrate using a thin inter-mediate buffer layer. The buffer layer is grown in two steps. An initial nucleation layer is crucial to accommodate the lattice mismatch between InP and Si. A high quality 2nd layer is grown on this initial layer with smooth morphology suitable for the NW growth. More than 97% vertical yield is achieved on the buffer layer and the morphology and photoluminescence of the NWs are similar to those grown on InP(111)B substrate

Droplet manipulation and horizontal growth of high-quality self-catalysed GaAsP nanowires

Self-catalyzed horizontal nanowires (NWs) can greatly simplify the CMOS integration processing compared with the regular vertical counterparts. However, self-catalyzed growth mode poses challenges in manipulating the droplets to produce single-crystalline horizontal NWs with a uniform diameter. Here, we demonstrated a novel method to manipulate the droplet through altering the droplet surface energy. Ga-droplet was successfully moved from top to sidewalls in GaAsP NWs by introducing Be and lowering the surface energy, and pinned at the tip despite the absence of planar defects. This can switch the growth direction, with a successful rate of 100 %, from vertical to horizontal through the assistance of few sparse twins. The produced NWs tend to be bounded by low energy facets, which leads to the self-catalysed growth of horizontal NWs with a greatly improved diameter uniformity along the axis. Besides, the lowered surface energy can effectively suppress the wurtzite nucleation, producing pure zinc blende single-crystalline horizontal NWs. This study establishes an essential step toward the efficient integration of NWs into CMOS compatible devices

Multiple radial phosphorus segregations in GaAsP core-shell nanowires

Highly faceted geometries such as nanowires are prone to form self-formed features, especially those that are driven by segregation. Understanding these features is important in preventing their formation, understanding their effects on nanowire properties, or engineering them for applications. Single elemental segregation lines that run along the radii of the hexagonal cross-section have been a common observation in alloy semiconductor nanowires. Here, in GaAsP nanowires, two additional P rich bands are formed on either side of the primary band, resulting in a total of three segregation bands in the vicinity of three of the alternating radii. These bands are less intense than the primary band and their formation can be attributed to the inclined nanofacets that form in the vicinity of the vertices. The formation of the secondary bands requires a higher composition of P in the shell, and to be grown under conditions that increase the diffusivity difference between As and P. Furthermore, it is observed that the primary band can split into two narrow and parallel bands. This can take place in all six radii, making the cross sections to have up to a maximum of 18 radial segregation bands. With controlled growth, these features could be exploited to assemble multiple different quantum structures in a new dimension (circumferential direction) within nanowires

Growth of Pure Zinc-Blende GaAs(P) Core-Shell Nanowires with Highly Regular Morphology

The growth of self-catalyzed core–shell nanowires (NWs) is investigated systematically using GaAs(P) NWs. The defects in the core NW are found to be detrimental for the shell growth. These defects are effectively eliminated by introducing beryllium (Be) doping during the NW core growth and hence forming Be–Ga alloy droplets that can effectively suppress the WZ nucleation and facilitate the droplet consumption. Shells with pure zinc-blende crystal quality and highly regular morphology are successfully grown on the defect-free NW cores and demonstrated an enhancement of one order of magnitude for room-temperature emission compared to that of the defective shells. These results provide useful information on guiding the growth of high-quality shell, which can greatly enhance the NW device performance

Long-Term Stability and Optoelectronic Performance Enhancement of InAsP Nanowires with an Ultrathin InP Passivation Layer

The influence of nanowire (NW) surface states increases rapidly with the reduction of diameter and hence severely degrades the optoelectronic performance of narrow-diameter NWs. Surface passivation is therefore critical, but it is challenging to achieve long-term effective passivation without significantly affecting other qualities. Here, we demonstrate that an ultrathin InP passivation layer of 2-3 nm can effectively solve these challenges. For InAsP nanowires with small diameters of 30-40 nm, the ultrathin passivation layer reduces the surface recombination velocity by at least 70% and increases the charge carrier lifetime by a factor of 3. These improvements are maintained even after storing the samples in ambient atmosphere for over 3 years. This passivation also greatly improves the performance thermal tolerance of these thin NWs and extends their operating temperature from <150 K to room temperature. This study provides a new route toward high-performance room-temperature narrow-diameter NW devices with long-term stability

Self-Catalyzed AlGaAs Nanowires and AlGaAs/GaAs Nanowire-Quantum Dots on Si Substrates

[Image: see text] Self-catalyzed AlGaAs nanowires (NWs) and NWs with a GaAs quantum dot (QD) were monolithically grown on Si(111) substrates via solid-source molecular beam epitaxy. This growth technique is advantageous in comparison to the previously employed Au-catalyzed approach, as it removes Au contamination issues and renders the structures compatible with complementary metal–oxide–semiconductor (CMOS) technology applications. Structural studies reveal the self-formation of an Al-rich AlGaAs shell, thicker at the NW base and thinning towards the tip, with the opposite behavior observed for the NW core. Wide alloy fluctuations in the shell region are also noticed. AlGaAs NW structures with nominal Al contents of 10, 20, and 30% have strong room temperature photoluminescence, with emission in the range of 1.50–1.72 eV. Individual NWs with an embedded 4.9 nm-thick GaAs region exhibit clear QD behavior, with spatially localized emission, both exciton and biexciton recombination lines, and an exciton line width of 490 μeV at low temperature. Our results demonstrate the properties and behavior of the AlGaAs NWs and AlGaAs/GaAs NWQDs grown via the self-catalyzed approach for the first time and exhibit their potential for a range of novel applications, including nanolasers and single-photon sources

Hole and electron effective masses in single InP nanowires with a Wurtzite-Zincblende homojunction

The formation of wurtzite (WZ) phase in III–V nanowires (NWs) such as GaAs and InP is a complication hindering the growth of pure-phase NWs, but it can also be exploited to form NW homostructures consisting of alternate zincblende (ZB) and WZ segments. This leads to different forms of nanostructures, such as crystal-phase superlattices and quantum dots. Here, we investigate the electronic properties of the simplest, yet challenging, of such homostructures: InP NWs with a single homojunction between pure ZB and WZ segments. Polarization-resolved microphotoluminescence (μ-PL) measurements on single NWs provide a tool to gain insights into the interplay between NW geometry and crystal phase. We also exploit this homostructure to simultaneously measure effective masses of charge carriers and excitons in ZB and WZ InP NWs, reliably. Magneto-μ-PL measurements carried out on individual NWs up to 29 T at 77 K allow us to determine the free exciton reduced masses of the ZB and WZ crystal phases, showing the heavier character of the WZ phase, and to deduce the effective mass of electrons in ZB InP NWs (me= 0.080 m0). Finally, we obtain the reduced mass of light-hole excitons in WZ InP by probing the second optically permitted transition Γ7C ↔ Γ7uV with magneto-μ-PL measurements carried out at room temperature. This information is used to extract the experimental light-hole effective mass in WZ InP, which is found to be mlh = 0.26 m0, a value much smaller than the one of the heavy hole mass. Besides being a valuable test for band structure calculations, the knowledge of carrier masses in WZ and ZB InP is important in view of the optimization of the efficiency of solar cells, which is one of the main applications of InP NWs