Wurtzite GaAs Quantum Wires: One-Dimensional Subband Formation

Unveiling Variations in Electronic and Atomic Structures Due to Nanoscale Wurtzite and Zinc Blende Phase Separation in GaAs Nanowires

Phase separation is an intriguing phenomenon often found in III-V nanostructures, but its effect on the atomic and electronic structures of III-V nanomaterials is still not fully understood. Here we study the variations in atomic arrangement and band structure due to the coexistence of wurtzite (WZ) and zinc blende (ZB) phases in single GaAs nanowires by using scanning transmission electron microscopy and monochromated electron energy loss spectroscopy. The WZ lattice distances are found to be larger (by ∼1%), along both the nanowire length direction and the perpendicular direction, than the ZB lattice. The band gap of the WZ phase is ∼20 meV smaller than that of the ZB phase. A shift of ∼70 meV in the conduction band edge between the two phases is also found. The direct and local measurements in single GaAs nanowires reveal important effects of phase separation on the properties of individual III-V nanostructures.

Epitaxial growth of crystal phase quantum dots in III-V semiconductor nanowires

Crystal phase quantum dots (QDs) are formed during the axial growth of III-V semiconductor nanowires (NWs) by stacking different crystal phases of the same material. In III-V semiconductor NWs, both zinc blende (ZB) and wurtzite (WZ) crystal phases can coexist. The band structure difference between both crystal phases can lead to quantum confinement. Thanks to the precise control in III-V semiconductor NW growth conditions and the deep knowledge on the epitaxial growth mechanisms, it is nowadays possible to control, down to the atomic level, the switching between crystal phases in NWs forming the so-called crystal phase NW-based QDs (NWQDs). The shape and size of the NW bridge the gap between QDs and the macroscopic world. This review is focused on crystal phase NWQDs based on III-V NWs obtained by the bottom-up vapor-liquid-solid (VLS) method and their optical and electronic properties. Crystal phase switching can be achieved in the axial direction. In contrast, in the core/shell growth, the difference in surface energies between different polytypes can enable selective shell growth. One reason for the very intense research in this field is motivated by their excellent optical and electronic properties both appealing for applications in nanophotonics and quantum technologies.

Correlated and in-situ electrical transmission electron microscopy studies and related membrane-chip fabrication

Understanding the interplay between the structure, composition and opto-electronic properties of semiconductor nano-objects requires combining transmission electron microscopy (TEM) based techniques with electrical and optical measurements on the very same specimen. Recent developments in TEM technologies allow not only the identification and in-situ electrical characterization of a particular object, but also the direct visualization of its modification in-situ by techniques such as Joule heating. Over the past years, we have carried out a number of studies in these fields that are reviewed in this contribution. In particular, we discuss here i) correlated studies where the same unique object is characterized electro-optically and by TEM, ii) in-situ Joule heating studies where a solid-state metal-semiconductor reaction is monitored in the TEM, and iii) in-situ biasing studies to better understand the electrical properties of contacted single nanowires. In addition, we provide detailed fabrication steps for the silicon nitride membrane-chips crucial to these correlated and in-situ measurements.

Thermal decomposition of GaAs nanowires

The realization of GaAs nanowire (NW) high-performance quantum devices operated at room temperatures requires that their diameters have to be less than 10 nm. It is shown, that the GaAs NWs with sub 10 nanometers diameters can be fabricated using the thermal decomposition technique. It is demonstrated, that depending on annealing conditions, the NW lengths, as well as shapes, can be modified significantly. The GaAs NWs with bottle-like and diameter-modulated shapes can be obtained. At the first stage of the thermal annealing in the presence of As flux, an increase in NW length was found.

Control of Conductivity of InxGa1-xAs Nanowires by Applied Tension and Surface States

The electronic properties of semiconductor AIIIBV nanowires (NWs) due to their high surface/volume ratio can be effectively controlled by NW strain and surface electronic states. We study the effect of applied tension on the conductivity of wurtzite In_xGa_1-xAs (x ∼ 0.8) NWs. Experimentally, conductive atomic force microscopy is used to measure the I-V curves of vertically standing NWs covered by native oxide. To apply tension, the microscope probe touching the NW side is shifted laterally to produce a tensile strain in the NW. The NW strain significantly increases the forward current in the measured I-V curves. When the strain reaches 4%, the I-V curve becomes almost linear, and the forward current increases by 3 orders of magnitude. In the latter case, the tensile strain is supposed to shift the conduction band minima below the Fermi level, whose position, in turn, is fixed by surface states. Consequently, the surface conductivity channel appears. The observed effects confirm that the excess surface arsenic is responsible for the Fermi level pinning at oxidized surfaces of III-As NWs.

Self-Formed Quantum Wires and Dots in GaAsP-GaAsP Core-Shell Nanowires

Quantum structures designed using nanowires as a basis are excellent candidates to achieve novel design architectures. Here, triplets of quantum wires (QWRs) that form at the core-shell interface of GaAsP-GaAsP nanowires are reported. Their formation, on only three of the six vertices of the hexagonal nanowire, is governed by the three-fold symmetry of the cubic crystal on the (111) plane. In twinned nanowires, the QWRs are segmented, to alternating vertices, forming quantum dots (QDs). Simulations confirm the possibility of QWR and QD-like behavior from the respective regions. Optical measurements confirm the presence of two different types of quantum emitters in the twinned individual nanowires. The possibility to control the relative formation of QWRs or QDs, and resulting emission wavelengths of the QDs, by controlling the twinning of the nanowire core, opens up new possibilities for designing nanowire devices.

Unexpected benefits of stacking faults on the electronic structure and optical emission in wurtzite GaAs/GaInP core/shell nanowires

Wurtzite (WZ) GaAs nanowires (NWs) are of considerable interest for novel optoelectronic applications, yet high quality NWs are still under development. Understanding of their polytypic crystal structure and band structure is the key to improving their emission characteristics. In this work we report that the Ga1-xInxP shell provides ideal passivation on polytypic WZ GaAs NWs, producing high quantum efficiency (up to 80%). From optical measurements, we find that the polytypic nature of the NWs which presents itself as planar defects does not reduce the emission efficiency. A hole transferring approach from the valence band of the zinc blende segments to the light hole (LH) band of the WZ phase is proposed to explain the emission enhancement from the conduction band to LH band. The emission intensity does not correlate to the minority carrier lifetime which is usually used to quantify the optical emission quality. Theoretical calculation predicted type-II band transition in polytypic WZ GaAs NWs is confirmed and presents efficient emission at low temperatures. We further demonstrate the performance of single NW photodetectors with a high photocurrent responsivity up to 65 A W-1 operating over the wavelength range from visible to near infrared.

Twofold origin of strain-induced bending in core-shell nanowires: the GaP/InGaP case

Nanowires have emerged as a promising platform for the development of novel and high-quality heterostructures at large lattice misfit, inaccessible in a thin film configuration. However, despite core-shell nanowires allowing a very efficient elastic release of the misfit strain, the growth of highly uniform arrays of nanowire heterostructures still represents a challenge, for example due to a strain-induced bending morphology. Here we investigate the bending of wurtzite GaP/In _x Ga_1-x P core-shell nanowires using transmission electron microscopy and energy dispersive x-ray spectroscopy, both in terms of geometric and compositional asymmetry with respect to the longitudinal axis. We compare the experimental data with finite element method simulations in three dimensions, showing that both asymmetries are responsible for the actual bending. Such findings are valid for all lattice-mismatched core-shell nanowire heterostructures based on ternary alloys. Our work provides a quantitative understanding of the bending effect in general while also suggesting a strategy to minimise it.

Temperature dependent electronic band structure of wurtzite GaAs nanowires

It has recently become possible to grow GaAs in the wurtzite crystal phase. This ability allows interesting tests of band-structure theory. Wurtzite GaAs has two closely spaced direct conduction bands as well as three nondegenerate valence bands. The energies of the band edges are not well known, in particular not as a function of temperature. In order to improve the accuracy we have studied the temperature dependence of the conduction band minimum as well as of the second valence band maximum using resonant Raman scattering (of up to 3LO Raman lines). We find that the temperature dependence of the bandgap in wurtzite GaAs is very similar to that in zinc blende GaAs. Our results show that they have the same band gaps not only at 7 K but also at room temperature to within 5 meV. This is in some discrepancy with previous work. We find that the energy difference between the first two Γ_9V and Γ_7V valence bands is constant, around 100 meV, over the investigated temperature range, 7 K to 300 K. Due to a fortuitous spacing of the energy bands we find a very unexpected and strong quadruple resonance in the resonant Raman scattering.

Crystal-Phase Quantum Wires: One-Dimensional Heterostructures with Atomically Flat Interfaces

In semiconductor quantum-wire heterostructures, interface roughness leads to exciton localization and to a radiative decay rate much smaller than that expected for structures with flat interfaces. Here, we uncover the electronic and optical properties of the one-dimensional extended defects that form at the intersection between stacking faults and inversion domain boundaries in GaN nanowires. We show that they act as crystal-phase quantum wires, a novel one-dimensional quantum system with atomically flat interfaces. These quantum wires efficiently capture excitons whose radiative decay gives rise to an optical doublet at 3.36 eV at 4.2 K. The binding energy of excitons confined in crystal-phase quantum wires is measured to be more than twice larger than that of the bulk. As a result of their unprecedented interface quality, these crystal-phase quantum wires constitute a model system for the study of one-dimensional excitons.

Addressing the Fundamental Electronic Properties of Wurtzite GaAs Nanowires by High-Field Magneto-Photoluminescence Spectroscopy

At ambient conditions, GaAs forms in the zincblende (ZB) phase with the notable exception of nanowires (NWs) where the wurtzite (WZ) lattice is also found. The WZ formation is both a complication to be dealt with and a potential feature to be exploited, for example, in NW homostructures wherein ZB and WZ phases alternate controllably and thus band gap engineering is achieved. Despite intense studies, some of the fundamental electronic properties of WZ GaAs NWs are not fully assessed yet. In this work, by using photoluminescence (PL) under high magnetic fields (B = 0-28 T), we measure the diamagnetic shift, ΔE_d, and the Zeeman splitting of the band gap free exciton in WZ GaAs formed in core-shell InGaAs-GaAs NWs. The quantitative analysis of ΔE_d at different temperatures (T = 4.2 and 77 K) and for different directions of B⃗ allows the determination of the exciton reduced mass, μ_exc, in planes perpendicular (μ_exc = 0.052 m₀, where m₀ is the electron mass in vacuum) and parallel (μ_exc = 0.057 m₀) to the ĉ axis of the WZ lattice. The value and anisotropy of the exciton reduced mass are compatible with the electron lowest-energy state having Γ_7C instead of Γ_8C symmetry. This finding answers a long discussed issue about the correct ordering of the conduction band states in WZ GaAs. As for the Zeeman splitting, it varies considerably with the field direction, resulting in an exciton gyromagnetic factor equal to 5.4 and ∼0 for B⃗//ĉ and B⃗⊥ĉ, respectively. This latter result provides fundamental insight into the band structure of wurtzite GaAs.

Bandgap Energy of Wurtzite InAs Nanowires

InAs nanowires (NWs) have been grown on semi-insulating InAs (111)B substrates by metal-organic chemical vapor deposition catalyzed by 50, 100, and 150 nm-sized Au particles. The pure wurtzite (WZ) phase of these NWs has been attested by high-resolution transmission electron microscopy and selected area diffraction pattern measurements. Low temperature photoluminescence measurements have provided unambiguous and robust evidence of a well resolved, isolated peak at 0.477 eV, namely 59 meV higher than the band gap of ZB InAs. The WZ nature of this energy band has been demonstrated by high values of the polarization degree, measured in ensembles of NWs both as-grown and mechanically transferred onto Si and GaAs substrates, in agreement with the polarization selection rules for WZ crystals. The value of 0.477 eV found here for the bandgap energy of WZ InAs agrees well with theoretical calculations.