Zinc-blende InAs Research Articles

Strong coupling between a microwave photon and a singlet-triplet qubit

Combining superconducting resonators and quantum dots has triggered tremendous progress in quantum information, however, attempts at coupling a resonator to even charge parity spin qubits have resulted only in weak spin-photon coupling. Here, we integrate a zincblende InAs nanowire double quantum dot with strong spin-orbit interaction in a magnetic-field resilient, high-quality resonator. The quantum confinement in the nanowire is achieved using deterministically grown wurtzite tunnel barriers. Our experiments on even charge parity states and at large magnetic fields, allow us to identify the relevant spin states and to measure the spin decoherence rates and spin-photon coupling strengths. We find an anti-crossing between the resonator mode in the single photon limit and a singlet-triplet qubit with a spin-photon coupling strength of g/2π = 139 ± 4 MHz. This coherent coupling exceeds the resonator decay rate κ/2π = 19.8 ± 0.2 MHz and the qubit dephasing rate γ/2π = 116 ± 7 MHz, putting our system in the strong coupling regime.

Temperature effects on thermodynamic and mechanical properties of the InP, InAs and InSb compounds

The thermodynamic and mechanical properties of the zinc-blende indium pnictides InP, InAs and InSb compounds have been investigated thanks to the statistical moment method in statistical mechanics. We have derived the analytical expressions of thermal induced atomic displacement, lattice constant, elastic moduli (Young’s modulus, bulk modulus and shear modulus) and elastic constants of the zinc-blende compounds. The difference of temperature effects on mechanical properties of InSb comparing to InP and InAs compounds has been pointed out. We show that InSb is less affected by temperature while InP changes its mechanical properties from hardness to softness quickly when the temperature increases. The advancement of this method is that it has included the anharmonic effects of thermal lattice vibrations by taking into account the higher-order atomic displacement terms.

Phase Control Growth of InAs Nanowires by Using Bi Surfactant

To realize practical applications of nanowire-based devices, it is critical, yet challenging, to control crystal structure growth of III-V semiconductor nanowires. Here, we demonstrate that controlled wurtzite and zincblende phases of InAs nanowires can be fabricated using bismuth (Bi) as a surfactant. For this purpose, catalyst free selective area epitaxial growth of InAs nanowires was performed using molecular beam epitaxy (MBE). During the growth, Bi was used which may act as a wetting agent influencing the surface energy at growth plane ends, promoting wurtzite crystal phase growth. For a demonstration, wurtzite and zincblende InAs nanowires were obtained with and without using Bi-flux. Photoluminescence spectroscopy (PL) analysis of the nanowires indicates a strong correlation between wurtzite phase and the Bi-flux. It is observed that the bandgap energy of wurtzite and zincblende nanowires are ∼0.50 eV and ∼0.42 eV, respectively, and agree well with theoretical estimated bandgap of corresponding InAs crystal phases. A blue shift in PL emission peak energy was found with decreasing nanowire diameter. The controlled wurtzite and zincblende crystal phase and its associated heterostructure growth of InAs nanowires on Si may open up new opportunities in bandgap engineering and related device applications integrated on Si. Furthermore, this work also illustrates that Bi as a surfactant could play a dynamic role in the growth mechanism of III-V compound semiconductors.

Electronic structures and optical properties of sulfur-passivated InAs nanowires by first-principles study

Electronic structures and optical properties of S-passivated InAs nanowires are studied by first-principles. The computation is carried out on the InAs (1‾10) surface, which has a low surface energy and is a common side facet of <111>-oriented zinc blende InAs nanowires. Through comparing different geometrical possibilities, it is found that S atoms prefer to insert the interchain bridge site, and occupy both the interchain bridge site and bridge site, for a non-diffused half-monolayer coverage and full non-diffused monolayer coverage, respectively. The electronic energy bands shift towards lower energy region and the band gap narrows after S adsorption. The density of states near Fermi level increases after S adsorption, indicating that S adsorption can enhance the conductivity and leads to a metallic-like nanowire surface. In addition, the adsorption of S significantly improves the absorption and optical conductivity of InAs nanowires in the low frequency range.

First principles calculations on theoretical band gap improvement of IIIA-VA zinc-blende semiconductor InAs

The structural, electronic, dielectric and vibrational properties of zinc-blende (ZB) InAs were studied within the framework of density functional theory (DFT) by employing local density approximation and norm-conserving pseudopotentials. The optimal lattice parameter, direct band gap, static dielectric constant, phonon frequencies and Born effective charges calculated by treating In-4d electrons as valence states are in satisfactory agreement with other reported theoretical and experimental findings. The calculated band gap is reasonably accurate and improved in comparison to other findings. This work will be useful for more computational studies related to semiconductor devices.

Photoluminescence Characteristics of Zinc Blende InAs Nanowires

A detailed understanding of the optical properties of self-catalysed (SC), zinc blende (ZB) dominant, nanowires (NWs) is crucial for the development of functional and impurity-free nanodevices. Despite the fact that SC InAs NWs mostly crystallize in the WZ/ZB phase, there are very limited reports on the photoluminescence (PL) properties of ZB InAs NWs. Here, we report on the PL properties of Molecular Beam Epitaxy grown, SC InAs NWs. The as-grown NWs exhibit a dominant band to band (BtB) peak associated with ZB, InAs with an emission energy of ~0.41 eV in good agreement with the band gap energy of ZB InAs and significantly lower than that of the wurtzite phase (~0.48 eV). The strong BtB peak persists to near room temperature with a distinct temperature-dependent red-shift and very narrow spectral linewidth of ~20 meV (10 K) which is much smaller than previously reported values. A narrowing in PL linewidth with increasing NWs diameter is correlated with a decline in the influence of surface defects resulting from an enlargement in NWs diameter. This study demonstrates the high optical property of SC InAs NWs which is compatible with the Si-complementary metal-oxide-semiconductor technology and paves the way for the monolithic integration of InAs NWs with Si in novel nanodevices.

First principle characterization of structural, electronic, mechanical, thermodynamic, linear and nonlinear optical properties of zinc blende InAs, InSb and their InAsxSb1-x ternary alloys

The structural and electronic properties of InAsxSb1-x ternary alloys are studied using plane-wave pseudopotential method based on the density functional theory. The Born effective charge, elastic constants, phonon dispersion curves, linear and nonlinear optical properties of these compounds are calculated using density functional perturbation theory. Based on the calculated phonon frequency, the specific heat at constant volume and entropy are obtained within the harmonic approximation. Moreover, due to the density functional theory band gap problem, the band gap correction is performed within the one-shot GW approximations. The distinguished role of In 4d states on the electronic energy band gap and nonlinear properties of these alloys are extensively explored by the comparison between the calculated results of Hartwigsen-Goedecker-Hutter and Trouiller-Martins-type pseudopotentials. The calculated results show that InAsxSb1-x alloys are mechanically stable and can be a good candidate for high-performance nonlinear optical material. The calculated results of InAs and InSb compounds are in acceptable agreement with available theoretical and experimental results, so the calculated results of InAsxSb1-x alloys will be helpful for future experimental and theoretical investigations.

Engineering hybrid epitaxial InAsSb/Al nanowires for stronger topological protection

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 interfaces allows for a hard induced superconducting gap and 2$e$ transport in Coulomb charging experiments, similar to experiments on InAs/Al and InSb/Al materials, and find measurements consistent with topological phase transitions at low magnetic fields due to large effective $g$-factors. Finally we present a method to grow pure wurtzite InAsSb nanowires which are predicted to exhibit even stronger spin-orbit coupling than the zincblende structure.

Spatial Control of Multiphoton Electron Excitations in InAs Nanowires by Varying Crystal Phase and Light Polarization.

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 ab initio GW calculations of first to third order multiphoton excitations and using finite-difference time-domain simulations, we explain the experimental findings: While the electric-field enhancement due to the semiconductor/vacuum interface has a similar effect for all NW segments, the second and third order multiphoton transitions in the band structure of WZ InAs are highly anisotropic in contrast to ZB InAs. As the crystal phase of NWs can be precisely and reliably tailored, our findings open up for new semiconductor optoelectronics with controllable nanoscale emission of electrons through vacuum or dielectric barriers.

Electron-hole interactions in coupled InAs-GaSb quantum dots based on nanowire crystal phase templates

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 broken gap of InAs-GaSb heterojunctions. The Coulomb blockade in the electron and hole transport is studied, and periodic interactions of electrons and holes are observed and can be reproduced by modeling. Distorted Coulomb diamonds indicate voltage-induced ground-state transitions, possibly a result of changes in the spatial distribution of holes in the thin GaSb shell.

Single-electron transport in InAs nanowire quantum dots formed by crystal phase engineering

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 with support from transmission electron microscope images of typical nanowires. From experiments on controlled filling of nearly empty dots with electrons, up to the point where Coulomb oscillations can no longer be resolved, we estimate a lower bound for the ZB-WZ conduction-band offset of 95 meV.

Crystal Phase- and Orientation-Dependent Electrical Transport Properties of InAs Nanowires.

We report a systematic study on the correlation of the electrical transport properties with the crystal phase and orientation of single-crystal InAs nanowires (NWs) grown by molecular-beam epitaxy. A new method is developed to allow the same InAs NW to be used for both the electrical measurements and transmission electron microscopy characterization. We find both the crystal phase, wurtzite (WZ) or zinc-blende (ZB), and the orientation of the InAs NWs remarkably affect the electronic properties of the field-effect transistors based on these NWs, such as the threshold voltage (VT), ON-OFF ratio, subthreshold swing (SS) and effective barrier height at the off-state (ΦOFF). The SS increases while VT, ON-OFF ratio, and ΦOFF decrease one by one in the sequence of WZ ⟨0001⟩, ZB ⟨131⟩, ZB ⟨332⟩, ZB ⟨121⟩, and ZB ⟨011⟩. The WZ InAs NWs have obvious smaller field-effect mobility, conductivities, and electron concentration at VBG = 0 V than the ZB InAs NWs, while these parameters are not sensitive to the orientation of the ZB InAs NWs. We also find the diameter ranging from 12 to 33 nm shows much less effect than the crystal phase and orientation on the electrical transport properties of the InAs NWs. The good ohmic contact between InAs NWs and metal remains regardless of the variation of the crystal phase and orientation through temperature-dependent measurements. Our work deepens the understanding of the structure-dependent electrical transport properties of InAs NWs and provides a potential way to tailor the device properties by controlling the crystal phase and orientation of the NWs.

Electronic structures of [1 1 1]-oriented free-standing InAs and InP nanowires

We report on a theoretical study of the electronic structures of the [1 1 1]-oriented, free-standing, zincblende InAs and InP nanowires with hexagonal cross sections by means of an atomistic , 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 C3v double point group. It is shown that all bands of these nanowires are doubly degenerate at the -point and some of these bands will split into non-degenerate bands when the wave vector k moves away from the -point as a manifestation of spin-splitting due to spin–orbit interaction. It is also shown that the lower conduction bands of these nanowires all show simple parabolic dispersion relations, while the top valence bands show complex dispersion relations and band crossings. The band state wave functions are presented by the spatial probability distributions and it is found that all the band states show -rotation symmetric probability distributions. The effects of quantum confinement on the band structures of the [1 1 1]-oriented InAs and InP nanowires are also examined and an empirical formula for the description of quantization energies of the lowest conduction band and the highest valence band is presented. The formula can simply be used to estimate the enhancement of the band gaps of the nanowires at different sizes as a result of quantum confinement.

The size-dependent electronic and optical properties of InAs quantum dots

Energy levels, direct energy band gap, refractive index and high-frequency dielectric constant of zinc-blende InAs spherical quantum dots and their size dependence have been investigated. The calculations are based on a pseudopotential approach. As a result of the quantum confinement effect, the predicted direct energy band gap is found to be significantly augmented relative to the bulk value, whereas the obtained high-frequency dielectric constant is shown to be significantly reduced.

Growth of InAs nanowires with the morphology and crystal structure controlled by carrier gas flow rate

We report on the growth of single crystal InAs nanowires on Si/SiOx substrates by chemical vapor deposition (CVD). Both pure zincblende InAs nanowires and twin superlattice InAs nanowires are produced. We demonstrate that the morphology and crystal structure of these nanowires can be controlled by tuning the H2 carrier gas flow rate in a CVD furnace and show that highly selected growth of twin superlattice nanowires are achieved at high carrier gas flow rates. Our work provides a new route to grow and phase-engineer single crystal InAs nanowires for a wide range of potential applications.

Numerical guidelines for setting up a k.p simulator with applications to quantum dot heterostructures and topological insulators

The k.p perturbation method for determination of electronic structure first pioneered by Kohn and Luttinger continues to provide valuable insight to several band structure features. This method has been adapted to heterostructures confined in up to three directions. In this paper, numerical details of setting up a k.p Hamiltonian using the finite difference approximation for such confined nanostructures is explicitly demonstrated. Nanostructures belonging to two different symmetry classes, namely, the cubic zincblende and rhombohedral crystals are considered. Rhombohedral crystals, of late, have gained prominence as candidates for the recently discovered topological insulator (TI) class of materials. Lastly, the incorporation of a strain field to the k.p Hamiltonian and the corresponding matrix equations for computing the intrinsic and an externally applied strain in heterostructures within a continuum approximation is shown. Two applications are considered (1) Computation of the eigen states of a multi-million zincblende InAs quantum dot with a stress-reducing InGaAs layer of varying Indium composition embedded in a GaAs matrix and (2) Dispersion of a rhombohedral TI $$\hbox {Bi}_{2}\hbox {Se}_{3}$$Bi2Se3 film and the necessary alteration in presence of proximity-induced superconductivity.

Crystalline and band alignment properties of InAs/Ge (111) heterostructure

In the present work, important information on crystalline structure and band offset of InAs/Ge heterojunction epitaxially grown directly on Ge (111) substrate is presented. As revealed by high resolution x-ray diffraction (HRXRD) and Raman measurements the InAs nanostructures are highly crystalline and oriented with the substrate having predominantly zinc-blende (ZB) structure. The relaxation ratio of grown layer is of 95%. The lateral and vertical coherence lengths are estimated to be ∼65 nm and ∼14 nm, respectively, which closely match with atomic force microscopy results. As revealed from HRXRD experiments, two domains/sub-lattices of ZB InAs (111) structures having different stacking configurations were found to coexist. A band diagram is constructed for InO/InAs/Ge system. Valence band offset of 300 meV and conduction band offset of 20 meV for InAs/Ge have been determined. Hence, InAs/Ge (111) system can have potential application in low power devices.

Pressure-Induced Phase Transitions in InAs Studied by Angular-Dispersive X-ray Diffraction and Raman Spectroscopy

Pressure-induced phase transitions in indium arsenide (InAs) at ambient temperature were investigated by using angular-dispersive X-ray diffraction (ADXRD) and Raman scattering under high pressure up to around 9.8(1) and 9.9(2) GPa, respectively, with a 4:1 (in volume ratio) methanol-ethanol mixture as the pressure transmitting medium (PTM). In situ ADXRD measurements revealed that InAs starts to transform from zinc blende (ZB) structure to rock-salt (RS) phase at 6.7(1) GPa, consistent with most previously reported studies. The phase transition proceeds steadily over a wide range of pressures and reaches completion at the highest applied pressure of 9.8 GPa with significant metallization, as suggested by the disappearance of Raman modes. In contrast to previous studies, our data did not show evidence of structural transition to Cmcm phase, presumably due to the different PTMs used and/or insufficient pressure applied. The order of appearance of the major diffraction peaks of RS phase InAs with increasing pressure and intensity are explained by the binding energy density of the corresponding plane families. The zero-pressure isothermal bulk moduli (B 0 ) and its first-pressure derivatives (B � 0 ) for the ZB InAs are 64(4) GPa and 3.8(2), respectively. The intensity of both the LO and TO phonon modes of ZB phase InAs exhibits an abrupt decrease at 7.7(1) GPa, where equal weight fractions of ZB and RS phases are identified, suggesting the metallic nature of the RS phase.

Controlling the crystal phase and structural quality of epitaxial InAs nanowires by tuning V/III ratio in molecular beam epitaxy

In this study, we demonstrated the control of crystal phase and structural quality of Au-catalyzed InAs nanowires grown on the GaAs {111}B substrates by tuning the V/III ratio in molecular beam epitaxy. It has been found that InAs nanowires can only be grown in a relatively narrow window of the V/III ratio. It is also demonstrated that the V/III ratio can be used to control the structural quality of wurtzite structured and zinc-blende structured InAs nanowires under low V/III ratios, and defect-free wurtzite structured and zinc-blende structured InAs nanowires were successfully achieved. This study provides an insight into the controlled growth of high-quality wurtzite structured and zinc-blende structured InAs nanowires through the V/III ratio engineering.

Atomic scale surface structure and morphology of InAs nanowire crystal superlattices: the effect of epitaxial overgrowth.

While shell growth engineering to the atomic scale is important for tailoring semiconductor nanowires with superior properties, a precise knowledge of the surface structure and morphology at different stages of this type of overgrowth has been lacking. We present a systematic scanning tunneling microscopy (STM) study of homoepitaxial shell growth of twinned superlattices in zinc blende InAs nanowires that transforms {111}A/B-type facets to the nonpolar {110}-type. STM imaging along the nanowires provides information on different stages of the shell growth revealing distinct differences in growth dynamics of the crystal facets and surface structures not found in the bulk. While growth of a new surface layer is initiated simultaneously (at the twin plane interface) on the {111}A and {111}B nanofacets, the step flow growth proceeds much faster on {111}A compared to {111}B leading to significant differences in roughness. Further, we observe that the atomic scale structures on the {111}B facet is different from its bulk counterpart and that shell growth on this facet occurs via steps perpendicular to the ⟨112⟩B-type directions.