Wurtzite Nanowires Research Articles

Exploring Al composition and diameter effect on AlxGa1-xAs nanowire photocathodes via first-principles calculations with DFT+U method

Utilizing first-principles calculations with DFT+U method, the structural, electronic and optical behaviors of AlxGa1-xAs nanowire photocathodes modulated by Al composition and diameter are systematically investigated. The findings reveal that the structural stability of AlxGa1-xAs nanowires is progressively enhanced as either the Al composition or the wire diameter increases. Additionally, the charge transfer in nanowires induced by altering the Al composition is significantly greater than that caused by varying wire diameter. Wurtzite AlxGa1-xAs nanowire is a direct bandgap material. Moreover, the continuously reduced bandgap resulting from an increase in diameter confirms the size effect of the nanowire. In addition, the introduction of DFT+U method presents an accurate theoretical bandgap with higher efficiency. The error of the bandgap between predication and experimental results is less than 4% in the case of bulk AlxGa1-xAs. The optical property of AlxGa1-xAs nanowire is more susceptible to the influence of wire diameter compared to that of Al composition. Furthermore, the optical absorption spectrums exhibit a slight blueshift and a redshift with increasing Al composition and wire diameter, respectively. This study is anticipated to provide theoretical support for the design of optoelectronic devices based on AlxGa1-xAs nanowires.

InP Crystal Phase Heterojunction Transistor with a Vertical Gate-All-Around Structure.

Crystal phase transitions can form a new type of heterojunction with different atomic arrangements in the same material: crystal phase heterojunction (CPHJ). The CPHJ has an inherently strong impact on band engineering without concerns over critical thicknesses with misfit dislocations and a semiconductor-metal transition. In-plane CPHJ was recently demonstrated in two-dimensional (2D) transition-metal dichalcogenide (TMD) materials and utilized for conventional planar field-effect transistor applications. However, scalability such as gate electrostatic control, miniaturization, and multigate structure have been limited because of the geometrical issue. Here, we demonstrated a transistor using the CPHJ with a vertical gate-all-around structure by forming a CPHJ in conventional III-V semiconductors. The CPHJ, composed of wurtzite InP nanowires with zincblende InP substrates, showed an atomically flat heterojunction without dislocations and indicated a Type-II band discontinuity across the junction. The CPHJ transistor had moderate to good gate electrostatic controllability with high on-state currents and transconductance. The CPHJ offer will provide a new switching mechanism and add a new junction and device design choice to the long history of transistors.

Wurtzite nanowires strain control by DC electrical stimulation

Nanomechanics is a highly developed area of research, given the significant reported changes in material properties at the nanometer scale, requiring the development of new theories to explain the underlying mechanisms. Such theories must be based on measurements that are as accurate as possible, but unfortunately, conventional experimental techniques do not apply to such small components. Here we present a unique new method to control electro-mechanical forces on quasi −1D nanostructures through static electric fields with multiple ways of control of GaAs nanowires’ strain directly on the growth substrate.

High-quality vertically aligned InAs nanowires grown by molecular-beam epitaxy using Ag–In alloy segregation

InAs nanowires show important potential applications in novel nanoelectronic devices, infrared optoelectronic devices and quantum devices, and all these applications require controllable growth of the InAs nanowires. However, the growth direction of metal-assisted InAs nanowires on Si substrates is often random. Here, we develop a new approach to grow vertically aligned InAs nanowires on Si (111) substrates by molecular-beam epitaxy using Ag as catalysts. The vertically aligned one-dimensional InAs nanowires are grown on the parasitic two-dimensional InAs film on the Si substrates by using the Ag nanoparticles segregated from Ag–In alloy catalysts. The diameters of the vertically aligned InAs nanowires obtained by this method are mainly distributed between 20 and 50 nm. Detailed transmission electron microscope data show that the nanowires with thinner diameters tend to have less stacking faults and twin defects and high crystal quality pure wurtzite nanowires can be obtained. Using these vertically aligned InAs nanowires as the channel material of field effect transistors, we have obtained a field-effect mobility of ∼2800 cm2 V−1 s−1 and an I on/I off ratio of ∼104 at room temperature. Our work provides a new method for the controlled growth of high-quality vertically aligned InAs nanowires on Si substrates.

Fröhlich electron–phonon interaction Hamiltonian and potential distribution of a polar optical phonon mode in wurtzite nitride triangular nanowires

Polar optical phonon modes of wurtzite triangular nanowires (NWs) with three different cross sections, including the hemi-equilateral triangle (HET), the isosceles right triangle (IRT), and the equilateral triangle (ET), are deduced and analyzed using the dielectric continuum model. The exact and analytical phonon states of exactly confined (EC) modes in nitride NWs with HET, IRT, and ET cross sections are derived. The characteristic frequency of EC phonon modes in the triangular nitride NW systems is specified. Fröhlich electron–phonon interaction Hamiltonians in wurtzite NWs with three types of triangular cross sections are obtained. It is found from the numerical results that, among the three types of GaN NWs, the electron–phonon coupling of EC modes in NWs with an HET cross section is the weakest one, that in NWs with an ET cross section is the strongest one, and that in NWs with an IRT cross section is in the middle. The electrostatic potentials of EC modes in HET NWs are neither symmetric nor antisymmetric. The potential functions of EC modes in the ET NW structures have one (three) symmetric axis (axes) as the quantum numbers p and q take fractions (integers). The potential functions of EC modes in IRT NWs behave either symmetrically or anti-symmetrically, which are closely dependent on the parities of the quantum numbers p and q. With the increase of order-number of EC modes, the electron–phonon coupling becomes weaker and weaker. This reveals that cross-sectional morphology of quantum structures has an important influence on the symmetries of phonon modes and electron–phonon coupling strengths in low-dimensional quantum systems.

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.

DFT analysis of crystal polarity on graphene surface

We report an ab-initio study of the preferred polarity for wurtzite GaN nanostructures on virtual graphene substrates. By means of the density functional theory analysis we show that N-polar nanostructures on graphene are energetically favorable in comparison to Ga-polar. These finding are in agreement with experimentally observed N-polarity of wurtzite GaN nanowires grown on graphene substrate. We believe that the revealed polarity preference is of importance for piezoelectric and optoelectronic device design.

Formation of wurtzite sections in self-catalyzed GaP nanowires by droplet consumption

Wurtzite GaP nanowires are interesting for the direct bandgap engineering and can be used as templates for further growth of hexagonal Si shells. Most wurtzite GaP nanowires have previously been obtained with Au catalysts. Here, we show that long (∼500 nm) wurtzite sections are formed in the top parts of self-catalyzed GaP nanowires grown by molecular beam epitaxy on Si(111) substrates in the droplet consumption stage, which is achieved by abruptly increasing the atomic V/III flux ratio from 2 to 3. We investigate the temperature dependence of the length of wurtzite sections and show that the longest sections are obtained at 610 °C. A supporting model explains the observed trends using a phase diagram of GaP nanowires, where the wurtzite phase is formed within a certain range of the droplet contact angles. The optimal growth temperature for growing wurtzite nanowires corresponds to the largest diffusion length of Ga adatoms, which helps to maintain the required contact angle for the longest time.

Tunable local and global piezopotential properties of graded InGaN nanowires

Owing to the continuously tunable physical properties, composition-graded wurtzite nanowires (NWs) recently become an attractive class of materials with potential applications in novel nanodevices. In this paper, the piezopotential properties of graded InGaN NWs are studied through multiscale modeling. Our atomic-scale simulations show that the structural and material parameters of graded InGaN NWs are dependent on the NW position, which are well described by the analytic expressions based on the Vegard’s law. The non-uniform distribution of structural and material parameters triggers the flexoelectric effect in graded InGaN NWs that can enhance the polarization components in NWs. The modified electromechanical theory incorporating the flexoelectric effect and analytic expressions of the position-dependent structural and material parameters is employed in the finite element calculations of piezopotential. The piezopotential in graded InGaN NWs is found to be asymmetric to NW center, which is different to the symmetric piezopotential distribution observed in binary wurtzite NWs. Both the local piezopotential and the global piezopotential difference of graded InGaN NWs are enhanced due to the flexoelectric effect, which becomes more significant as the concentration of donors grows. In addition, the piezopotential distribution in graded InGaN NWs can be significantly affected by the length of graded InGaN region, while it almost has no influence on the piezopotential difference. However, both the local piezopotential and the global piezopotential difference of graded InGaN NWs can be greatly enhanced by increasing the maximum In composition. • Piezopotential of graded InGaN NWs is studied by multiscale modeling technique. • Material properties of graded InGaN NWs depend on the NW position. • Flexoelectric effect can increase the piezopotential of graded InGaN NWs. • Graded InGaN NWs have tunable local and global piezopotential properties.

Simultaneous Selective Area Growth of Wurtzite and Zincblende Self-Catalyzed GaAs Nanowires on Silicon.

Selective area epitaxy constitutes a mainstream method to obtain reproducible nanomaterials. As a counterpart, self-assembly allows their growth without costly substrate preparation, with the drawback of uncontrolled positioning. We propose a mixed approach in which self-assembly is limited to reduced regions on a patterned silicon substrate. While nanowires grow with a wide distribution of diameters, we note a mostly binary occurrence of crystal phases. Self-catalyzed GaAs nanowires form in either a wurtzite or zincblende phase in the same growth run. Quite surprisingly, thicker nanowires are wurtzite and thinner nanowires are zincblende, while the common view predicts the reverse trend. We relate this phenomenon to the influx of Ga adatoms by surface diffusion, which results in different contact angles of Ga droplets. We demonstrate the wurtzite phase of thick GaAs NWs up to 200 nm in diameter in the Au-free approach, which has not been achieved so far to our knowledge.

The optical absorption of nanowires with hexagonal cross-sections

We study the optical absorption of semiconductor nanowires with hexagonal cross-sections, such as most wurtzite nanowires. The typical semiconductor nanowires have obvious quantum confinement in the cross-sections. We focus on the extent to which this kind of nanowires could be approximated as cylindrical ones. A numerical method is developed to calculate the optical absorption in such system. It is found that whether or not the approximation of the hexagonal cross-section as a circular one is valid depends on which energy levels are mainly considered. The analysis of the linear absorptions concludes that only those subbands corresponding to the energy levels in the cross sections having hexagonal symmetry can be optically excited to. The results are helpful for the optical applications of nanowires based on their fine energy structures.

XXV Международный симпозиум Нанофизика и наноэлектроника", Нижний Новгород, 9-12 марта 2021 г. Формирование гексагональной фазы германия на поверхности нитевидных нанокристаллов AlGaAs методом молекулярно-пучковой эпитаксии

Experimental results of studying the germanium deposition on the surface of AlGaAs nanowires are presented. The formation of both cubic and hexagonal Ge phases was revealed using Raman spectroscopy. The inheritance of the crystal structure during the growth of thin germanium layers on the lateral surfaces of wurtzite nanowires was shown.

Small-scale effects on the piezopotential properties of tapered gallium nitride nanowires: The synergy between surface and flexoelectric effects

Apart from the conventional wurtzite nanowires (NWs) with constant cross-sections, tapered wurtzite NWs with truncated conical shapes are also widely employed in the design of piezotronic nanodevices. In this paper, the piezopotential properties of tapered gallium nitride (GaN) NWs are comprehensively studied by using a multiscale modelling technique. Our atomic-scale simulations show that due to the surface effect, the material properties of tapered NWs strongly rely on the position in NWs. Analytic models based on the core-shell theory and the thermodynamic theory are developed for the position-dependent material properties of tapered NWs. Besides the surface effect, the flexoelectric effect is another factor significantly affecting the piezopotential properties of tapered NWs by inducing additional polarization. After employing the analytic models for position-dependent material properties and introducing a modified electromechanical theory, both the surface and flexoelectric effects are considered in the continuum mechanics modelling of the piezopotential of tapered NWs. Our results show that the small-scale effects can increase the potential difference generated in tapered NWs, which will become more significant as the tip radius decreases and the half cone angle increases. Moreover, in the tapered NW with a small half cone angle the small-scale effects are found to majorly originate from the surface effect, while the flexoelectric effect becomes the dominant factor when the tapered NW possesses a relatively large half cone angle.

Uniaxial Strain and Optical properties of GaP Nanowires

The optical properties for wurtzite (WZ) Bulk GaP and GaP nanowires (GaP-NWs) in hexagonal(circular), triangular and square cross sectional are investigated by using full potential linear augmented plane waves and local orbitals (FP-LAPW-LO) method based on density functional theory (DFT). The wurtzite nanowires are investigated with diameters limited to ~27 oA. The optical functions as a function of photon energy have been investigated for unstrained bulk WZ-GaP and WZ-GaP-NWs in the first part. Finally, the optical functions are investigated for WZ-GaP-NWs by applying uniaxial strain from -5% to +5%. The effect of cross sectional variation in the area and shape of the NWs on the optical functions is clear when compared with bulk GaP. GaP-NWs kept real dielectric constant positive and refractive index close to vacuum value for a wider range of energy on the contrary of bulk GaP. It is found that the dielectric constants are slightly increased from the unstrained GaP-NWs values with strain increases. A shift in the threshold energy for toward higher energy as the strain changes from tensile to compressive. The refractive index shows a small change when passing from compressive toward tensile strain. It is found that GaP-NWs could play a role in solar cells structure and high speed electronic circuits.

Uniaxial Strain and Optical properties of GaP Nanowires

The optical properties for wurtzite (WZ) Bulk GaP and GaP nanowires (GaP-NWs) in hexagonal(circular), triangular and square cross sectional are investigated by using full potential linear augmented plane waves and local orbitals (FP-LAPW-LO) method based on density functional theory (DFT). The wurtzite nanowires are investigated with diameters limited to ~27 oA. The optical functions as a function of photon energy have been investigated for unstrained bulk WZ-GaP and WZ-GaP-NWs in the first part. Finally, the optical functions are investigated for WZ-GaP-NWs by applying uniaxial strain from -5% to +5%. The effect of cross sectional variation in the area and shape of the NWs on the optical functions is clear when compared with bulk GaP. GaP-NWs kept real dielectric constant positive and refractive index close to vacuum value for a wider range of energy on the contrary of bulk GaP. It is found that the dielectric constants are slightly increased from the unstrained GaP-NWs values with strain increases. A shift in the threshold energy for toward higher energy as the strain changes from tensile to compressive. The refractive index shows a small change when passing from compressive toward tensile strain. It is found that GaP-NWs could play a role in solar cells structure and high speed electronic circuits.

Hydrogenic impurity-related optical properties in a piezoelectric core–shell nanowire

The impurity-associated optical transitions in a piezoelectric core–shell nanowire were studied using the density matrix formalism. In the calculations, four electron states were considered using a finite-difference algorithm combined with a variational approach. An obvious blueshift in the optical spectrum, especially for the transition from the free electron to impurity-bound ground states, was predicted to result from the intrinsic Stark effect induced by the piezoelectric field. The effects of the optical transition between impurity-bound states on the absorption coefficient and the refractive index of wurtzite nanowires were enhanced as the piezoelectric polarization becomes stronger. The findings are helpful for guiding further experiments on the linear and nonlinear optical properties of piezoelectric nanowires.

Polarization and magneto-optical properties of excitonic emission from wurtzite CdTe/(Cd,Mg)Te core/shell nanowires

Wurtzite CdTe and (Cd,Mn)Te nanowires embedded in (Cd,Mg)Te shells are grown by employing vapour–liquid–solid growth mechanism in a system for molecular beam epitaxy. A combined study involving cathodoluminescence, transmission electron microscopy and micro-photoluminescence is used to correlate optical and structural properties in these structures. Typical features of excitonic emission from individual wurtzite nanowires are highlighted including the emission energy of 1.65 eV, polarization properties and the appearance B-exciton related emission at high excitation densities. Angle dependent magneto-optical study performed on individual (Cd,Mn)Te nanowires reveals heavy-hole-like character of A-excitons typical for wurtzite structure and allows to determine the crystal field splitting, ΔCR. The impact of the strain originating from the lattice mismatched shell is discussed and supported by theoretical calculations.

Size-Dependent Ultrasonic and Thermophysical Properties of Indium Phosphide Nanowires

Abstract The present work explores the diameter- and temperature-dependent ultrasonic characterization of wurtzite indium phosphide nanowires (WZ-InP-NWs) using a theoretical model based on the ultrasonic non-destructive evaluation (NDE) technique. Initially, the second- and third-order elastic constants (SOECs and TOECs) were computed using the Lennard-Jones potential model, considering the interactions up to the second nearest neighbours. Simultaneously, the mechanical parameters (Young’s modulus, shear modulus, elastic anisotropy factor, bulk modulus, Pugh’s ratio and Poisson’s ratio) were also estimated. Finally, the thermophysical properties and ultrasonic parameters (velocity and attenuation) of the InP-NWs were determined using the computed quantities. The obtained elastic/mechnical properties of the InP-NWs were also analyzed to explore the mechanical behaviors. The correlations between temperature-/size-dependent ultrasonic attenuation and the thermophysical properties were established. The ultrasonic attenuation was observed to be the third-order polynomial function of the diameter/temperature for the InP nanowire.

Metalorganic vapor phase epitaxy of wurtzite InP nanowires on GaN

The metalorganic vapor phase epitaxy of wurtzite InP nanowires on GaN (0001) is demonstrated. The InP nanowires exhibit the same wurtzite structure as the underlying wurtzite GaN. The photoluminescence studies indicate that the InP nanowires are single-phase wurtzite with high crystalline quality which is supported by transmission and scanning electron microscopy images. The position of the second valence band or valence band splitting energy is also deduced from the photoluminescence data to be ΔAB=30 meV at room temperature. The InP/GaN heterojunction can enable exotic optoelectronic and spintronic experiments and applications. In addition, these results can enable traditional III–V growth on III-N materials for heterojunction devices.

Phase Selection in Self-catalyzed GaAs Nanowires.

Crystal phase switching between the zincblende and wurtzite structures in III-V nanowires is crucial from the fundamental viewpoint as well as for electronic and photonic applications of crystal phase heterostructures. Here, the results of in situ monitoring of self-catalyzed vapor-liquid-solid growth of GaAs nanowires by molecular beam epitaxy inside a transmission electron microscope are presented. It is demonstrated that the occurrence of the zincblende or wurtzite phase in self-catalyzed nanowires is determined by the sole parameter, the droplet contact angle, which can be finely tuned by changing the group III and V fluxes. The zincblende phase forms at small (<100°) and large (>125°) contact angles, whereas pure wurtzite phase is observed for intermediate contact angles. Wurtzite nanowires are restricted by vertical sidewalls, whereas zincblende nanowires taper or develop the truncated edge at their top. These findings are explained within a dedicated model for the surface energetics. These results give a clear route for the crystal phase control in Au-free III-V nanowires. On a more general note, in situ growth monitoring with atomic resolution and at the technological-relevant growth rates is shown to be a powerful tool for the fine-tuning of material properties at the nanoscale.