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GaAs nanowire (NW) arrays were grown by molecular beam epitaxy using the self-assisted vapor-liquid-solid method with Ga droplets as seed particles. A Ga pre-deposition step is examined to control NW yield and diameter. The NW yield can be increased with suitable duration of a Ga pre-deposition step but is highly dependent on oxide hole diameter and surface conditions. The NW diameter was determined by vapor-solid growth on the NW sidewalls, rather than Ga pre-deposition. The maximum NW yield with a Ga pre-deposition step was very close to 100%, established at shorter Ga deposition durations and for larger holes. This trend was explained within a model where maximum yield is obtained when the Ga droplet volume approximately equals the hole volume., (© 2021 IOP Publishing Ltd.)

The optical absorptance from arrays of GaAs nanowires (NWs) was examined by the finite element method. Absorptance in cylindrical NWs, frustum nanocones (with base wider than the top) and inverted frustum nanocones (with top wider than the base) was compared. The introduction of higher order HE 1n modes, the red-shift of the HE 1n modes along the NW length due to NW tapering, and the red-shift of the modes due to increase of the overall NW diameter all contribute to a broadening of the absorption spectrum in conical NWs as compared to NWs with a constant diameter. The optical reflectance versus NW top diameter shows a minimum due to a balance between reflectance from the top of the NWs and reflectance from the substrate between NWs. The optimum geometry for photovoltaic energy conversion was determined from the total photocurrent. An optimum photocurrent of 26.5 mAcm -2 was obtained, corresponding to a conical NW morphology with base diameter of 200 nm, top diameter of 110 nm, and length of 2000 nm. An optimized inverse tapered conical morphology gave comparable performance.

The droplet contact angle and morphology of the growth interface (vertical, tapered or truncated facets) are known to affect the zincblende (ZB) or wurtzite (WZ) crystal phase of III-V nanowires (NWs) grown by the vapor-liquid-solid method. Here, we present a model which describes the dynamics of the morphological evolution in self-catalyzed III-V NWs in terms of the time-dependent (or length-dependent) contact angle or top nanowire radius under varying material fluxes. The model fits quite well the contact angle dynamics obtained by in situ growth monitoring of self-catalyzed GaAs NWs in a transmission electron microscope. These results can be used for modeling the interface dynamics and the related crystal phase switching and for obtaining ZB-WZ heterostructures in III-V.

Numerical optimization has been used to determine the optimum junction design for core-shell nanowires used in betavoltaic generators. A genetic algorithm has been used to calculate the relative thickness, height, and doping of each segment within silicon, gallium arsenide, and gallium phosphide nanowires. Using the simulated spectra and energy deposition of nickel-63, nickel citrate, tritium, and tritiated butyl, devices with power output and overall efficiency up to 8 µW.cm -2 and 12%, respectively, have been predicted. Compared to previously investigated axial nanowires, the core-shell structures simulated here have realized drastic improvements by reducing surface recombination for longer nanowires. In addition, core-shell nanowires are shown to be capable of nearly matching the ideal performance predicted for this device structure. A new approach for calculating the practical upper limit of betavoltaic performance is presented and additional methods for improvement are discussed.

An analytical growth model is presented to explain the influence of antimony fractional flux on the morphology evolution of catalyst-free InAs 1-x Sb x semiconductor nanowires grown by the selective-area vapor-solid mechanism on a Si (111) substrate by molecular beam epitaxy. Increasing Sb fractional flux promoted radial growth and suppressed axial growth, resulting in 'nano-disks'. This behavior is explained by a model of indium adatom diffusion along nanowire facets.

Semiconductor nanowires (NWs) are a candidate technology for future optoelectronic devices. One of the critical issues in NWs is the control of impurity doping for the formation of p-n junctions. In this study, beryllium (p-type dopant) and tellurium (n-type dopant) in self-assisted GaAs NWs was studied. The GaAs NWs were grown on (111) Si by molecular beam epitaxy using the self-assisted method. The dopant incorporation in the self-assisted GaAs NWs was investigated using Raman spectroscopy, photoluminescence, secondary ion mass spectrometry and electron holography. Be-doped NWs showed similar carrier concentration as compared to thin film (TF) standards. However, Te-doped NWs showed at least a one order of magnitude lower carrier concentration as compared to TF standards. Dopant incorporation mechanisms in NWs are discussed.

Patterned arrays of self-assisted GaP nanowires (NWs) were grown on a Si substrate by gas source molecular beam epitaxy using various V/III flux ratios from 1-6, and various pitches from 360-1000 nm. As the V/III flux ratio was increased from 1-6, the NWs showed a change in morphology from outward tapering to straight, and eventually to inward tapering. The morphologies of the self-assisted GaP NWs are well described by a simple kinetic equation for the NW radius versus the position along the NW axis. The most important growth parameter that governs the NW morphology is the V/III flux ratio. Sharpened NWs with a stable radius equal to only 12 nm at a V/III flux of 6 were achieved, demonstrating their suitability for the insertion of quantum dots.

We report the structural, optical and electrical properties of GaAs quantum dots (QDs) embedded along GaP nanowires. The GaP nanowires contained p-i-n junctions with 15 consecutively grown GaAs QDs within the intrinsic region. The nanowires were grown by molecular beam epitaxy using the self-assisted vapor-liquid-solid process. The crystal structure of the NWs alternated between twinned ZB and WZ as the composition along the NW alternated between the GaP barriers and the GaAs QDs, respectively, leading to a polytypic structure with a periodic modulation of the NW sidewall facets. Photodetector devices containing QDs showed absorption beyond the bandgap of GaP in comparison to nanowires without QDs. Voltage-dependent measurements suggested a field emission process of carriers from the QDs.

A new dopant incorporation mechanism in Ga-assisted GaAs nanowires grown by molecular beam epitaxy is reported. Off-axis electron holography revealed that p-type Be dopants introduced in situ during molecular beam epitaxy growth of the nanowires were distributed inhomogeneously in the nanowire cross-section, perpendicular to the growth direction. The active dopants showed a remarkable azimuthal distribution along the (111)B flat top of the nanowires, which is attributed to preferred incorporation along 3-fold symmetric truncated facets under the Ga droplet. A diffusion model is presented to explain the unique radial and azimuthal variation of the active dopants in the GaAs nanowires.

We describe methods of Ga droplet consumption in Ga-assisted GaAs nanowires, and their impact on the crystal structure at the tip of nanowires. Droplets are consumed under different group V flux conditions and the resulting tip crystal structure is examined by transmission electron microscopy. The use of GaAsP marker layers provides insight into the behavior of the Ga droplet during different droplet consumption conditions. Lower group V droplet supersaturations lead to a pure zincblende stacking-fault-free tip crystal structure, which improved the performance of a nanowire-based photovoltaic device.

A periodic array of vertical InSb nanowires (nws) was designed for photodetectors in the mid-wavelength infrared (MWIR) region (λ = 3-5 μm). Simulations, using the finite element method, were implemented to optimize the nw array geometrical parameters (diameter (D), period (P), and length (L)) for high optical absorptance, which exceeded that of a thin film of equal thickness. Our results showed HE1n resonances in InSb nw arrays can be tuned by adjusting D and P, thus enabling multispectral absorption throughout the near infrared to MWIR region. Optical absorptance was investigated for a practical photodetector consisting of a vertical InSb nw array embedded in bisbenzocyclobutene (BCB) as a support layer for an ultrathin Ni contact layer. Polarization sensitivity of the photodetector is examined.

Polarization-resolved magneto-photoluminescence is studied in InGaAs/InP single quantum wells. In the range of the filling factor ν ≥ 4 the number of populated Landau levels contributing to the photoluminescence is found to be equal to the corresponding filling factor, while at ν ≤ 3 the number of emitting Landau levels is larger than the filling factor, which implies an occupancy of the Landau levels above the Fermi level. Such partial occupancy of the Landau levels with energies higher than the Fermi energy is due to shake-up processes caused by electron-electron interaction. In accordance to the theory, at the filling factor around ν = 2 the shake-up process was found to manifest itself in the downshifted cusp of the energy of the σ- polarized emission from the excited Landau levels, while no change was observed in the energy of the σ+ polarized emission. The different energies of differently spin-polarized excited Landau levels cause the magnetic field induced polarization of the emission from excited Landau levels. In addition, the bound electron-hole excitonic complexes (trions) associated with different Landau levels were observed.

Experimental data and a model are presented which define the boundary values of V/III flux ratio and growth temperature for droplet-assisted nucleation of InAs semiconductor nanowires in selective-area epitaxy on SiO(x)/Si (111) substrates by molecular beam epitaxy. Within these boundaries, the substrate receives a balanced flux of group III and V materials allowing the growth of vertically oriented nanowires as compared to the formation of droplets or crystallites.

InP nanowires with InAsP quantum dots (QDs) were grown by molecular beam epitaxy on a Si (111) substrates. The structure of the InAsP QDs were studied using transmission electron microscopy, allowing the development of a model where QD growth occurs by group V desorption from the surrounding substrate surface. Micro-photoluminescence was performed at 10 K showing emission at 1.47-1.49 eV from the InP wurtzite structure, and various emission peaks between 0.93 and 1.33 eV attributed to the QDs. The emission was tuned by the QD composition. The effectiveness of an AlInP passivation shell was demonstrated via an improvement in the photoluminescence intensity. Spectrally-resolved photocurrent measurements at room temperature demonstrated infrared response due to absorption within the QDs. The absorption red-shifted with increasing As composition of the QD.

The absorptance in vertical nanowire (nw) arrays is a result of three optical phenomena: radial mode resonances, near-field evanescent wave coupling, and Fabry-Perot (F-P) modes. The contribution of these optical phenomena to GaAs, InP and InAs nw absorptance was simulated using the finite element method. The study compared the absorptance between finite and semi-infinite nw lengths with varying geometrical parameters, including the nw diameter, length and array period. Simulation results showed that the resonance peak wavelength of the HE11 and HE12 radial modes linearly red-shifted with increasing nw diameter. The absorptance and spectral width of the resonance peaks increased as the nw length increased, with an absorptance plateau for very long nws that depended on diameter and period. Near-field coupling between neighboring nws was observed to increase with decreasing period. The effect of F-P modes was more pronounced for shorter nws, with a significant enhancement of HE12 over HE11 absorptance. Engineering of nw arrays to take advantage of these optical phenomena for multi-spectral photodetector applications is discussed.

The characteristic energies, occupancies and polarizations of the minibands formed by the Γ-Γ and Γ-Xz interlayer electon tunnelings in the InGaAs/InP superlattices are studied in the regime of the integer quantum Hall effect by polarization resolved photoluminescence. Accordingly, the magnetic field induced shrinkage of the interminiband gap, predicted by the theory, and as a consequence, the redistribution of charge over the superlattice minibands and the depolarization of the quantum Hall electron states are observed at odd filling factors. The response of the electrons residing in the InGaAs/InP superlattice minibands to the magnetic field is found very similar to the corresponding response of the electrons confined in the symmetric and anti-symmetric two-dimensional minibands of GaAs/AlGaAs double quantum wells. The presented results are evidence of the formation of the correlated states in multi-component electron systems formed in semiconductor multiple layers at odd filling factors.

Numerical simulation of the photocurrent density is performed for a two-junction nanowire (NW)-on-silicon solar cell under AM1.5G illumination. The photocurrent density is determined for NW diameters from 100-250 nm, period (spacing) from 250-1000 nm, and length of 5 μm. The dependence of photocurrent density on NW bandgap is also determined. For each NW bandgap, the optimum diameter and period are determined to obtain current matching between the top NW cell and the bottom Si cell.

Dynamic and time-of-flight (TOF) secondary ion mass spectrometry (SIMS) was performed on vertically standing III-V nanowire ensembles embedded in Cyclotene polymer. By embedding the NWs in Cyclotene, the top surface of the sample was made planar, while the space between the NWs was filled to protect the background substrate from the ion beam, thus allowing for the NWs to be sputtered and analyzed evenly as a function of depth. Using thin film standards, SIMS analysis was used to calculate the impurity dopant concentration as a function of height in the NW ensemble. This marked the first use of conventional SIMS to accurately determine the doping density with excellent depth resolution. Additionally, this is the first presentation of SIMS as the only reported tool for characterizing the segment height uniformity of any arbitrary axial heterostructure NW ensemble.

The effect of ammonium polysulfide solution, (NH₄)₂S(x), on the surface passivation of p-doped InP nanowires (NWs) was investigated by micro-photoluminescence. An improvement in photoluminescence (PL) intensity from individual NWs upon passivation was used to optimize the passivation procedure using different solvents, sulfur concentrations and durations of passivation. The optimized passivation procedure gave an average of 24 times improvement in peak PL intensity. A numerical model is presented to explain the PL improvement upon passivation in terms of a reduction in surface trap density by two orders of magnitude from 10¹² to 10¹⁰ cm⁻², corresponding to a change in surface recombination velocity from 10⁶ to 10⁴ cm s⁻¹. The diameter dependence of the PL intensity is investigated and explained by the model. The PL intensity from passivated nanowires decreased to its initial (pre-passivation) value over a period of seven days in ambient air, indicating that the S passivation was unstable.

Vertically aligned ZnO nanowires (NWs) were grown on Au-nanocluster-seeded amorphous SiO(2) films by the advective transport and deposition of Zn vapours obtained from the carbothermal reaction of graphite and ZnO powders. Both the NW volume and visible-to-UV photoluminescence ratio were found to be strong functions of, and hence could be tailored by, the (ZnO+C) source-SiO(2) substrate distance. We observe C flakes on the ZnO NWs/SiO(2) substrates which exhibit short NWs that developed on both sides. The SiO(2) and C substrates/NW interfaces were studied in detail to determine growth mechanisms. NWs on Au-seeded SiO(2) were promoted by a rough ZnO seed layer whose formation was catalysed by the Au clusters. In contrast, NWs grew without any seed on C. A correlation comprising three orders of magnitude between the visible-to-UV photoluminescence intensity ratio and the NW volume is found, which results from a characteristic Zn partial pressure profile that fixes both O deficiency defect concentration and growth rate.

InAs/InP axial nanowire heterostructures were grown by the Au-assisted vapour-liquid-solid method in a gas source molecular beam epitaxy system. The nanowire crystal structure and morphology were investigated by transmission electron microscopy for various growth conditions (temperature, growth rate, V/III flux ratio). Growth mechanisms were inferred from the InAs and InP segment lengths as a function of the nanowire diameter. Short InAs segment lengths were found to grow by depletion of In from the Au particle as well as by direct impingement, while longer segments of InAs and InP grew by diffusive transport of adatoms from the nanowire sidewalls. The present study offers a way to control the lengths of InAs quantum dots embedded in InP barriers.

The viability of four organic polymers (S1808, SC200, SU8 and Cyclotene) as filling materials to achieve planarization of ensemble nanowire arrays is reported. Analysis of the porosity, surface roughness and thermal stability of each filling material was performed. Sonication was used as an effective method to remove the tops of the nanowires (NWs) to achieve complete planarization. Ensemble nanowire devices were fully fabricated and I-V measurements confirmed that Cyclotene effectively planarizes the NWs while still serving the role as an insulating layer between the top and bottom contacts. These processes and analysis can be easily implemented into future characterization and fabrication of ensemble NWs for optoelectronic device applications.

The effect of sulfur passivation on core-shell p-n junction GaAs nanowire (NW) solar cells has been investigated. Devices of two types were investigated, consisting of indium tin oxide contact dots or opaque Au finger electrodes. Lateral carrier transport from the NWs to the contact fingers was achieved via a p-doped GaAs surface conduction layer. NWs between the opaque contact fingers had sidewall surfaces exposed for passivation by sulfur. The relative cell efficiency increased by 19% upon passivation. The contribution of the thin film grown between the NWs to the total cell efficiency was estimated by removing the NWs using a sonication procedure. Mechanisms of carrier transport and photovoltaic effects are discussed on the basis of spatially resolved laser scanning measurements.

InGaP nanowires (NWs) were grown by the Au-assisted method in a gas source molecular beam epitaxy system. The dependence of InGaP composition, morphology and stacking fault density was studied with respect to group III and V impingement rate and size of the Au particle. Compositional analysis showed that the NWs had an In-rich core and a Ga-rich shell structure. The In incorporation within the NW became limited as the Au seed particle size diminished or the group III and V flux decreased. The NWs had wurtzite (WZ) crystal structure with zinc blende (ZB) segments (stacking faults). The density of the stacking faults decreased as the group III flux decreased and the group V flux increased.

GaAs nanowire (NW)-based p-n photovoltaic devices, with two distinct p and n spatial distributions and where Te was the n-dopant, have been studied by impedance spectroscopy in the 10(3)-10(7) Hz frequency range and the - 1.5-1.5 V bias range. For a large n-core/p-shell overlap region within NWs in a coaxial geometry, the p-n junction properties (DC rectification and p-n depletion capacitance) are found to prevail. The impedance data at low bias for both NW devices show large frequency dispersions with relaxation frequencies that are compatible with carrier re-emission times from traps due to GaAs surface states. An increasing conductance with increasing frequency for low bias is observed, suggesting hopping transport through localized states. For large bias the conductance increases exponentially with bias and is frequency independent, indicating conduction through extended states in this regime.

This paper reviews recent advances in engineering spin quantum bits (qubits) in semiconductor quantum dots and describes an approach based on top-gated semiconductor nanowire devices. Fast electrical single-spin manipulation is achievable, in principle, using the spin-orbit interaction intrinsic to III-V materials, such as InAs, in concert with AC electric fields. Combined with sub-nanosecond gate control of the nearest-neighbor exchange interaction and spin readout by spin-to-charge conversion, a fully electrical solid-state quantum processor is within reach. We outline strategies for spin manipulation, robust readout and mitigation of decoherence due to nuclear fields that, when combined in a single device, should give a viable multi-qubit testbed and a building block for larger scale quantum devices.

A detailed continuum model is presented for predicting the growth characteristics of GaAs nanowires during chemical beam epitaxy. The model describes the transport processes of Ga and As adatoms on the substrate and nanowire sidewalls, and through the nanoparticle and the nanowire-catalyst interface (NCI). The growth mechanisms of nanowires within the NCI are described using an extended step-flow kinetic model. The vapor-liquid-solid and vapor-solid-solid growth mechanisms are both described in the kinetic model. The growth rate of the nanowires, the surface and bulk concentrations of adatoms, and the role of transport processes of Ga and As adatoms during chemical beam epitaxy were investigated. The growth mechanisms of the nanowires were found to vary with increasing length of the nanowire.

Hybrid solar cells with an energy conversion efficiency of 1.04% were fabricated by spin-coating poly(3-hexylthiophene) (P3HT) polymer onto vertically aligned n-type GaAs nanowire arrays synthesized by molecular beam epitaxy. Experimental parameters such as the P3HT solvent mixing ratio of chlorobenzene to dichlorobenzene and nanowire surface etching were studied with a view to improving the energy conversion efficiency.

The influence of GaAs(100) substrate surface preparation on Au-catalysed GaAs nanowires was studied. Elongated pits of varying dimensions and orientation were formed on GaAs(100) substrates depending on the interaction with Au and surface oxides. The resulting surface topography is shown to influence the density and orientation of nanowires. [111] B-oriented nanowires nucleated from Au particles lying on the sidewall facets of the pits, while [011]-oriented nanowires nucleated from Au particles lying outside the pits.

The morphology and crystal structure of Au-seeded GaAs nanowires (NWs) grown by molecular beam epitaxy were investigated as a function of the temperature, V/III flux ratio, and Ga flux. Low and intermediate growth temperatures of 400 and 500 °C resulted in a strongly tapered morphology, with stacking faults occurring at an average rate of 0.1 nm(-1). NWs with uniform diameter and the occurrence of crystal defects reduced by more than an order of magnitude were achieved at 600 °C, a V/III flux ratio of 2.3, and a Ga impingement rate on the surface of 0.07 nm s(-1). Comparison of nanowire densities on the various post-growth surfaces suggests a possible incubation time between the moment the Ga shutter is opened and when nanowire growth is initiated. Increasing the flux ratio favored uniform sidewall growth, making the process suitable for the fabrication of core-shell structures.

Genetic diversity at the MHC and non-MHC loci was investigated in three commercial broiler chicken pure lines. The MHC class II and IV loci were evaluated in Southern hybridizations and molecular genotypes based on RFLP were interpreted from pedigreed families. Four MHC class II and eight class IV genotypes were identified in the broiler lines, and their frequencies differed among the lines. Line-specific MHC genotypes were identified. The observed heterozygosities (59 to 67%) suggest that the MHC loci are highly polymorphic in the broiler lines. At least 9% of the genetic variation at the MHC was due to line differences; the remainder reflected individual variations. To characterize non-MHC genes, 41 microsatellite loci located throughout the chicken genome were evaluated in the broiler lines. Genetic variation was also observed at the microsatellite loci for the broiler lines; the number of alleles at a single locus ranged from one to eight, and the average number of alleles per locus was 3.5, 2.8, and 3.1 for each of the lines, respectively. The observed heterozygosities for microsatellite loci ranged between 0 and 89% in the lines. Based on the fixation index (Fst), about 19% of the genetic variation at microsatellite loci was attributed to broiler line differences. Deviations from Hardy-Weinberg equilibrium were detected at both MHC and non-MHC loci. Possible explanations for these deviations include genetic selection by the primary broiler breeder or the presence of null alleles that were not identified by the typing procedures described in this report. This study contributes to our knowledge on the molecular characteristics and genetic structure of a commercial broiler chicken population. Analysis of MHC and non-MHC loci suggests that there is still sufficient genetic diversity in the broiler lines to continue the progress toward improved broiler chicken production.

To identify candidate genes, chicken lines with the most divergent phenotypes are usually crossed to generate resource mapping populations, for example, either backcrossed or F2 populations. Linkage between the genetic marker and the phenotypic trait locus is then tested in the mapping population. As an initial step in the development of a mapping population from commercial broilers, the goal of the current research was to evaluate the phenotypic variation among three pure lines for antibody response to SRBC and in resistance to two economically important poultry diseases, Marek's disease (MD) and coccidiosis (Eimeria acervulina). Chicks from each line were received and separated into three experimental studies to evaluate each of their responses. In summary, broiler Line 3 had significantly lower antibody responses to SRBC immunizations compared to the other two lines, and nonvaccinated birds from Line 3 were also more susceptible to MD. With coccidiosis, the response was complex, and ranking of the lines was dependent on the age of infection, and whether it was a first or second challenge. With the first challenge, Line 1 was most susceptible at the younger age (Day 30), whereas Line 3 was susceptible at the older age (Day 58). Upon the second challenge, broiler Line 1 remained susceptible at the younger age, but Line 2 was more susceptible at the older age. Line 3 was completely resistant to the second challenge at the older age. Thus, although the broiler lines have been intensively selected for productivity and general livability, this study also demonstrates that the lines differ for immune response and disease resistance. Based on the phenotypic differences between Lines 1 and 3, they were chosen to establish a mapping population for identifying candidate genes that affect MD and coccidiosis in commercial broiler chickens.