We compare the electronic characteristics of nanowire field-effect transistors made using single pure wurtzite and pure zincblende InAs nanowires with nominally identical diameter. We compare the transfer characteristics and field-effect mobility versus temperature for these devices to better understand how differences in InAs phase govern the electronic properties of nanowire transistors.
Using transient Rayleigh scattering (TRS) measurements, we obtain photoexcited carrier thermalization dynamics for both zincblende (ZB) and wurtzite (WZ) InP single nanowires (NW) with picosecond resolution. A phenomenological fitting model based on direct band to band transition theory is developed to extract the electron-hole-plasma density and temperature as a function of time from TRS measurements of single nanowires which have complex valence band structures. We find that the thermalization dynamics of hot carriers depends strongly on material (GaAs NW vs. InP NW) and less strongly on cry…
Using NMTO-downfolding technique we have revisited the issue of relative stability between zincblende (ZB) and wurtzite (WZ) symmetries in case of CdS, CdSe and CdTe semiconductors. Our computed ionicity factors using accurate NMTO-downfolding successfully brings out the right trend within the CdX series - CdS being most ionic stabilizes in WZ symmetry while CdSe and CdTe being more covalent stabilizes in ZB symmetry. Our NMTO constructed Wannier functions corresponding to only valence bands provide a nice demonstration of this fact. The tendency towards ZB stability is governed by the covalen…
Strain distribution in zincblende and wurtzite GaAs nanowires bent by a one-sided (In, Al)As stressor shell: consequences for torsion, chirality, and piezoelectricity
GaAs nanowires (NWs) grown by molecular-beam epitaxy may contain segments of both the zincblende (ZB) and wurtzite (WZ) phases. Depending on the growth conditions, we find that optical emission of such NWs occurs either predominantly above or below the band gap energy of ZB GaAs [E(g,ZB)]. This result is consistent with the assumption that the band gap energy of wurtzite GaAs [E(g,WZ)] is larger than E(g,ZB) and that GaAs NWs with alternating ZB and WZ segments along the wire axis establish a type II band alignment, where electrons captured within the ZB segments recombine with holes of the ne…
While the properties of wurtzite GaAs have been extensively studied during the past decade, little is known about the influence of the crystal polytype on ternary (In,Ga)As quantum well structures. We address this question with a unique combination of correlated, spatially-resolved measurement techniques on core-shell nanowires that contain extended segments of both the zincblende and wurtzite polytypes. Cathodoluminescence hyperspectral imaging reveals a blueshift of the quantum well emission energy by $75\pm15$ meV in the wurtzite polytype segment. Nanoprobe x-ray diffraction and atom probe …
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 ($μ$PL) 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 determi…
We study the electronic properties of GaAs nanowires composed of both the zincblende and wurtzite modifications using a ten-band k.p model. In the wurtzite phase, two energetically close conduction bands are of importance for the confinement and the energy levels of the electron ground state. These bands form two intersecting potential landscapes for electrons in zincblende/wurtzite nanostructures. The energy difference between the two bands depends sensitively on strain, such that even small strains can reverse the energy ordering of the two bands. This reversal may already be induced by the …
A systematic numerical investigation of spin-orbit fields in the conduction bands of III-V semiconductor nanowires is performed. Zinc-blende InSb nanowires are considered along [001], [011], and [111] directions, while wurtzite InAs nanowires are studied along [0001] and [10$\overline{1}$0] or [11$\overline{2}$0] directions. Realistic multiband $\vec{k} \cdot \vec{p}\,$ Hamiltonians are solved by using plane-wave expansions of real-space parameters. In all cases the linear and cubic spin-orbit coupling parameters are extracted for nanowire widths from 30 to 100 nm. Typical spin-orbit energies …
Emission dynamics in zincblende InAsxP1-x quantum dots in InP nanowires: influence of quantum dot size, composition and nanowire geometry
Wrocław University of Science and Technology（弗罗茨瓦夫科技大学）；NEST Istituto Nanoscienze CNR and Scuola Normale Superiore（意大利国家研究委员会纳米科学研究所和比萨高等师范学院）；Universidad Carlos III de Madrid（马德里卡洛斯三世大学）
We report a combined electron transmission and Raman spectroscopy study of InAs nanowires. We demonstrate that the temperature dependent behavior of optical phonon energies can be used to determine the relative wurtzite fraction in the InAs nanowires. Furthermore, we propose that the interfacial strain between zincblende and wurtzite phases along the length of the wires manifests in the temperature-evolution of the phonon linewidths. From these studies, temperature-dependent Raman measurements emerge has a non-invasive method to study polytypism in such nanowires.
We use polarized photocurrent spectroscopy in a nanowire device to investigate the band structure of hexagonal Wurtzite InAs. Signatures of optical transitions between four valence bands and two conduction bands are observed which are consistent with the symmetries expected from group theory. The ground state transition energy identified from photocurrent spectra is seen to be consistent with photoluminescence emitted from a cluster of nanowires from the same growth substrate. From the energies of the observed bands we determine the spin orbit and crystal field energies in Wurtzite InAs. This …
The combination of strong spin-orbit coupling, large $g$-factors, and the coupling to a superconductor can be used to create a topologically protected state in a semiconductor nanowire. Here we report on growth and characterization of hybrid epitaxial InAsSb/Al nanowires, with varying composition and crystal structure. We find the strongest spin-orbit interaction at intermediate compositions in zincblende InAs$_{1-x}$Sb$_{x}$ nanowires, exceeding that of both InAs and InSb materials, confirming recent theoretical studies \cite{winkler2016topological}. We show that the epitaxial InAsSb/Al inter…
We employ first-principles techniques tailored to properly describe semiconductors (modified Becke-Johnson potential added to the exchange-correlation functional), to obtain the electronic band structures of both the zinc-blende and wurtzite phases of GaAs, GaSb, InAs, and InSb. We extract the spin-orbit fields for the relevant valence and conduction bands at zone center, by fitting the spin-splittings resulting from the lack of space inversion symmetry of these bulk crystal structures, to known functional forms---third-order polynomials. We also determine the orientations of the spin-orbit ve…
We study the effect of the Dresselhaus spin-orbit interaction on the electronic states and spin relaxation rates of cylindrical quantum dots defined on quantum wires having wurtzite lattice structure. The linear and cubic contributions of the bulk Dresselhaus spin-orbit coupling are taken into account, along with the influence of a weak external magnetic field. The previously found analytic solution for the electronic states of cylindrical quantum dots with zincblende lattice structures with Rashba interaction is extended to the case of quantum dots with wurtzite lattices. For the electronic s…
Most III-V semiconductors, which acquire the zinc-blende phase as bulk materials, adopt the metastable wurtzite phase when grown in the form of nanowires. These are new semiconductors with new optical properties, in particular, a different electronic band gap when compared with that grown in the zinc-blende phase. The electronic gap of wurtzite InAs at the Gamma-point of the Brillouin zone (E0 gap) has been recently measured, E0 = 0.46 eV at low temperature. The electronic gap at the A point of the Brillouin zone (equivalent to the L point in the zinc-blende structure, E1) has also been obtain…
The response of semiconductor materials to external magnetic fields is a reliable approach to probe intrinsic electronic and spin-dependent properties. In this study, we investigate the common Zeeman splitting features of novel wurtzite materials, namely InP, InAs, and GaAs. We present values for the effective g-factors of different energy bands and show that spin-orbit coupling effects, responsible for the spin splittings, also have noticeable contributions to the g-factors. Within the Landau level picture, we show that the nonlinear Zeeman splitting recently explained in magneto photolumines…
We present here an atomistic theory of the electronic and optical properties of hexagonal InAsP quantum dots in InP nanowires in the wurtzite phase. These self-assembled quantum dots are unique in that their heights, shapes, and diameters are well known. Using a combined valence-force-field, tight-binding, and configuration-interaction approach we perform atomistic calculations of single-particle states and excitonic, biexcitonic and trion complexes as well as emission spectra as a function of the quantum dot height, diameter and As versus P concentration. The atomistic tight-binding parameter…
We have developed an 8-band Effective Mass Approximation model that describes the zero field spin splitting in the band structure of zincblende heterostructures due to bulk inversion asymmetry (BIA). We have verified that our finite difference Hamiltonian transforms in almost all situations according to the true $D_{2d}$ or $C_{2v}$ symmetry of [001] heterostructures. This makes it a computationally efficient tool for the accurate description of the band structure of heterostructures for spintronics. We first compute the band structure for an AlSb/GaSb/AlSb quantum well (QW), which presents on…
In this study, we present a complete experimental and theoretical investigation of the fundamental exciton Zeeman splitting in wurtzite InP nanowires. We determined the exciton gyromagnetic factor, $g_{exc}$, by magneto-photoluminescence spectroscopy using magnetic fields up to 29 T. We found that $g_{exc}$ is strongly anisotropic with values differing in excess of 50\% between the magnetic field oriented parallel and perpendicular to the nanowire long axis. Furthermore, for magnetic fields oriented along the nanowire axis, $g_{exc}$ is nearly three times larger than in bulk zincblende InP and…
Uncapped InN nanostructures undergo a deleterious natural aging process at ambient conditions by oxygen incorporation. The phases involved in this process and their localization is mapped by Transmission Electron Microscopy (TEM) related techniques. The parent wurtzite InN (InN-w) phase disappears from the surface and gradually forms a highly textured cubic layer that completely wraps up a InN-w nucleus which still remains from original single-crystalline quantum dots. The good reticular relationships between the different crystals generate low misfit strains and explain the apparent easiness …
While non-nitride III-V semiconductors typically have a zincblende structure, they may also form wurtzite crystals under pressure or when grown as nanowhiskers. This makes electronic structure calculation difficult since the band structures of wurtzite III-V semiconductors are poorly characterized. We have calculated the electronic band structure for nine III-V semiconductors in the wurtzite phase using transferable empirical pseudopotentials including spin-orbit coupling. We find that all the materials have direct gaps. Our results differ significantly from earlier {\it ab initio} calculation…
We report growth and characterization of a coupled quantum dot structure that utilizes nanowire templates for selective epitaxy of radial heterostructures. The starting point is a zinc blende InAs nanowire with thin segments of wurtzite structure. These segments have dual roles: they act as tunnel barriers for electron transport in the InAs core, and they also locally suppress growth of a GaSb shell, resulting in coaxial InAs-GaSb quantum dots with integrated electrical probes. The parallel quantum dot structure hosts spatially separated electrons and holes that interact due to the type-II bro…
The pressure dependent phonon modes of predominant wurtzite InAs nanowires has been investigated in a diamond anvil cell under hydrostatic pressure up to 58 GPa. The TO and LO at Gamma point and other optical phonon frequencies increase linearly while the LO TO splitting decreases with pressure. The recorded Raman modes have been used to determine the mode Gruneisen parameters and also the value of Borns transverse effective charge. The calculated Borns transverse effective charge exhibits a linear reduction with increasing pressure implying an increase in covalency of nanowires under compress…
The spatial distribution of the local density of states (LDOS) at Mn acceptors near the (110) surface of p-doped InAs is investigated by Scanning Tunneling Microscopy (STM). The shapes of the acceptor contrasts for different dopant depths under the surface are analyzed. Acceptors located within the first ten subsurface layers of the semiconductor show a lower symmetry than expected from theoretical predictions of the bulk acceptor wave function. They exhibit a (001) mirror asymmetry. The degree of asymmetry depends on the acceptor atoms' depths. The measured contrasts for acceptors buried belo…
We theoretically investigate the D'yakonov-Perel' spin relaxation properties in diffusive wurtzite semiconductor nanowires and their impact on the quantum correction to the conductivity. Although the lifetime of the long-lived spin states is limited by the dominant $k$-linear spin-orbit contributions in the bulk, these terms show almost no effect in the finite-size nanowires. Here, the spin lifetime is essentially determined by the small $k$-cubic spin-orbit terms and nearly independent of the wire radius. At the same time, these states possess in general a complex helical structure in real sp…
We report on a low-temperature transport study of a single-gate, planar field-effect device made from a free-standing, wurtzite-crystalline InAs nanosheet. The nanosheet is grown via molecular beam epitaxy and the field-effect device is characterized by gate transfer characteristic measurements and by magnetic field orientation dependent transport measurements. The measurements show that the device exhibits excellent electrical properties and the electron transport in the nanosheet is of the two-dimensional nature. Low-field magnetoconductance measurements are performed for the device at diffe…
Optical properties of semiconductors can exhibit strong polarization dependence due to crystalline anisotropy. A number of recent experiments have shown that the photoluminescence intensity in free standing nanowires is polarization dependent. One contribution to this effect is the anisotropy of the dielectric function due to the fact that most nanowires crystalize in the wurtzite form. While little is known experimentally about the band structures wurtzite phase III-V semiconductors, we have previously predicted the bulk band structure of nine III-V semiconductors in wurtzite phase.Here, we p…
We report tunneling spectroscopy measurements of the Zeeman spin splitting in InAs few-electron quantum dots. The dots are formed between two InP barriers in InAs nanowires with a wurtzite crystal structure grown by chemical beam epitaxy. The values of the electron g-factors of the first few electrons entering the dot are found to strongly depend on dot size and range from close to the InAs bulk value in large dots |g^*|=13 down to |g^*|=2.3 for the smallest dots. These findings are discussed in view of a simple model.
Abstract Characterizing the crystalline disorder properties of the heterovalent ternary semiconductors continues to challenge solid-state theory. Here, a Landau theory is developed for the wurtzite-based ternary semiconductor ZnSnN2. It is shown that the symmetry properties of two nearly co-stable phases, with space groups Pmc21 and Pbn21, infer that a reconstructive phase transition is the source of crystal structure disorder via a mixture of the phases. The site exchange defect, which consists of two adjacent antisite defects, is identified as the nucleation mechanism of the transition. A La…
We report on growth of InP-InAs core-shell nanowires and demonstration of the formation of single quantum structures, which show Coulomb blockade effect, over entire lengths of the nanowires. The core-shell nanowires are grown by a selective area growth technique via metal-organic vapor phase epitaxy. The as-grown core-shell nanowires are found to be of wurtzite crystals. The InP cores have a hexagonal cross section, while the InAs shell are grown preferentially on specific {1$\bar{1}$00} facets, leading to the formation of the core-shell nanowires with an overall triangular cross section. The…
We report on a theoretical study of the electronic structures of the [111]-oriented, free-standing, zincblende InAs and InP nanowires with hexagonal cross sections by means of an atomistic $sp^{3}s^{*} $, spin-orbit interaction included, nearest-neighbor, tight-binding method. The band structures and the band state wave functions of these nanowires are calculated and the symmetry properties of the bands and band states are analyzed based on the $C_{3v}$ double point group. It is shown that all bands of these nanowires are doubly degenerate at the $Γ$-point and some of these bands will split in…
We demonstrate the control of multiphoton electron excitations in InAs nanowires (NWs) by altering the crystal structure and the light polarization. Using few-cycle, near-infrared laser pulses from an optical parametric chirped-pulse amplification system, we induce multiphoton electron excitations in InAs nanowires with controlled wurtzite (WZ) and zincblende (ZB) segments. With a photoemission electron microscope, we show that we can selectively induce multiphoton electron emission from WZ or ZB segments of the same wire by varying the light polarization. Developing \textit{ab-initio GW} calc…
We report electrical characterization of quantum dots formed by introducing pairs of thin wurtzite (WZ) segments in zinc blende (ZB) InAs nanowires. Regular Coulomb oscillations are observed over a wide gate voltage span, indicating that WZ segments create significant barriers for electron transport. We find a direct correlation of transport properties with quantum dot length and corresponding growth time of the enclosed ZB segment. The correlation is made possible by using a method to extract lengths of nanowire crystal phase segments directly from scanning electron microscopy images, and wit…
We report in-situ synthesis of crystalline indium islands on InAs nanowires grown by molecular beam epitaxy. Structural analysis by transmission electron microscopy showed that In crystals grew in a tetragonal body-centred crystal structure within two families of orientations relative to wurtzite InAs. The crystalline islands had lengths < 500 nm and low-energy surfaces, suggesting that growth was driven mainly by surface energy minimization. Electrical transport through In/InAs devices exhibited Cooper pair charging, evidencing charge parity preservation and a pristine In/InAs interface, with…
Built-in electrostatic fields in Zincblende quantum dots originate mainly from - (1) the fundamental crystal atomicity and the interfaces between two dissimilar materials, (2) the strain relaxation, and (3) the piezoelectric polarization. In this paper, using the atomistic NEMO 3-D simulator, we study the origin and nature of the internal fields in InAs/GaAs quantum dots with three different geometries, namely, box, dome, and pyramid. We then calculate and delineate the impact of the internal fields in the one-particle electronic states in terms of shift in the conduction band energy states, a…
The electronic band structure of complex nanostructured semiconductors has a considerable effect on the final electronic and optical properties of the material and, ultimately, on the functionality of the devices incorporating them. Valence electron energy-loss spectroscopy (VEELS) in the transmission electron microscope (TEM) provides the possibility of measuring this property of semiconductors with high spatial resolution. However, it still represents a challenge for narrow-bandgap semiconductors, since an electron beam with low energy spread is required. Here we demonstrate that by means of…
In this paper we show how to link the electronic structures of two III-V systems, one a direct gap material, GaAs, and the other an indirect gap material, GaP, from their bulks right down to the shape of thin nanowires. GaAs and GaP bulk and nanowire systems are studied in the zincblende and wurtzite structures both free of strain and subjected to biaxial strains perpendicular to the [111]/[0001] direction. We provide an interpretation of the band structure of nanowires, grown along the [111] (zinc-blende structure) and the [0001] (wurtzite structure) directions, in terms of the bulk band stru…
Crystal-phase low-dimensional structures offer great potential for the implementation of photonic devices of interest for quantum information processing. In this context, unveiling the fundamental parameters of the crystal phase structure is of much relevance for several applications. Here, we report on the anisotropy of the g-factor tensor and diamagnetic coefficient in wurtzite/zincblende (WZ/ZB) crystal-phase quantum dots (QDs) realized in single InP nanowires. The WZ and ZB alternating axial sections in the NWs are identified by high-angle annular dark-field scanning transmission electron …
Semiconductor Rashba nanowires are quasi-one dimensional systems that have large spin-orbit (SO) coupling arising from a broken inversion symmetry due to an external electric field. There exist parametrized multiband models that can describe accurately this effect. However, simplified single band models are highly desirable to study geometries of recent experimental interest, since they may allow to incorporate the effects of the low dimensionality and the nanowire electrostatic environment at a reduced computational cost. Commonly used conduction band approximations, valid for bulk materials,…
