Simulation of optical absorption in conical nanowires.

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 HE1n modes, the red-shift of the HE1n 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.

Improving the yield of GaAs nanowires on silicon by Ga pre-deposition.

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.

Genetic Algorithm Optimization of Core-Shell Nanowire Betavoltaic Generators.

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.

Modeling the dynamics of interface morphology and crystal phase change in self-catalyzed GaAs nanowires.

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.

Modeling selective-area growth of InAsSb nanowires.

An analytical growth model is presented to explain the influence of antimony fractional flux on the morphology evolution of catalyst-free InAs1-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.

Doping assessment in GaAs nanowires.

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.

Tuning the morphology of self-assisted GaP nanowires.

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.

GaAs quantum dots in a GaP nanowire photodetector.

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.

Three-fold Symmetric Doping Mechanism in GaAs Nanowires.

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.

Optical design of a mid-wavelength infrared InSb nanowire photodetector.

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.

Methods of Ga droplet consumption for improved GaAs nanowire solar cell efficiency.

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.

Photoluminescence and photocurrent from InP nanowires with InAsP quantum dots grown on Si by molecular beam epitaxy.

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.

Conditions for high yield of selective-area epitaxy InAs nanowires on SiO x /Si(111) substrates.

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.

Wavelength-selective absorptance in GaAs, InP and InAs nanowire arrays.

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.

Current matching and efficiency optimization in a two-junction nanowire-on-silicon solar cell.

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.

Unlocking doping and compositional profiles of nanowire ensembles using SIMS.

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.

Photoluminescence model of sulfur passivated p-InP nanowires.

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 10¹° 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.

ZnO nanowire co-growth on SiO2 and C by carbothermal reduction and vapour advection.

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.

Contact planarization of ensemble nanowires.

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.

Mechanisms of molecular beam epitaxy growth in InAs/InP nanowire heterostructures.

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.