A. C. E. Chia

Analytical model of surface depletion in GaAs nanowires

Poissons equation is solved to provide a comprehensive model of nanowire (NW) surface depletion as a function of interface state density, NW radius, and doping density. This model improves upon established theory by giving distinct solutions to the cases of full and partial NW depletion while implementing the charge neutrality level and accurate Fermi-Dirac statistics. To explain the underlying physics, key parameters were plotted as a function of both interface state density and NW radius, showing interesting features such as the lowering of the Fermi level in fully depleted NWs and marked increase in surface depletion width and built-in surface potential (relative to a planar film equivalent) in partially depleted NWs. Finally, examination of NW conductivity found that for NWs of radius acrit, the minimum NW radius before which the entire NW is depleted, conductivity can be reduced by up to 95% relative to bulk. Additionally, majority carrier inversion is predicted to occur in thin NWs.

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.

Highly ordered vertical GaAs nanowire arrays with dry etching and their optical properties

We report fabrication methods, including metal masks and dry etching, and demonstrate highly ordered vertical gallium arsenide nanowire arrays. The etching process created high aspect ratio, vertical nanowires with insignificant undercutting from the mask, allowing us to vary the diameter from 30 nm to 400 nm with a pitch from 250 nm to 1100 nm and length up to 2.2 μm. A diameter to pitch ratio of ∼68% was achieved. We also measured the reflectance from the nanowire arrays and show experimentally diameter-dependent strong absorption peaks resulting from resonant optical mode excitations within these nanowires. The reflectance curves match very well with simulations. The work done here paves the way towards achieving high efficiency solar cells and tunable photodetectors using III-V nanowires.

Electrical transport and optical model of GaAs-AlInP core-shell nanowires

GaAs nanowires were passivated by AlInP shells grown by the Au-assisted vapor-liquid-solid method in a gas source molecular beam epitaxy system. Transmission electron microscopy confirmed a core-shell GaAs-AlInP structure. Current-voltage measurements on ensemble nanowires indicated improved carrier transport properties in the passivated nanowires as compared to their unpassivated counterpart. Similarly, individual nanowires showed improved photoluminescence intensity upon passivation. A detailed model is presented to quantify the observed improvements in nanowire conduction and luminescence in terms of a reduction in surface charge trap density and surface recombination velocity upon passivation. The model includes the effects of high-level injection, bulk recombination, and surface recombination. The model can be used as a tool for assessing various passivation methods.

Improved conductivity and long-term stability of sulfur-passivated n-GaAs nanowires

The surface passivation of n-type GaAs nanowires (NWs) by ammonium polysulfide solution, (NH4)2Sx, is described. The passivation resulted in a two order of magnitude increase in current density in an ensemble NW device. A depletion and recombination model is used to explain the results in terms of a reduction in surface trap density upon passivation. The results are comparable to a previous passivation method using AlInP shells. The S passivation was found to be unstable according to the degradation in the ensemble NW conductivity after a 7 months exposure to air.

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.

Surface depletion and electrical transport model of AlInP-passivated GaAs nanowires

Fabrication, current?voltage characterization and analytical modeling of an AlInP-passivated GaAs nanowire (NW) ensemble device are presented. During fabrication, sonication was used as a novel and crucial step to ensure effective contacting of the NWs. Current?voltage characteristics of the passivated NW devices were fitted using an analytical surface depletion and transport model which improves upon established models by implementing a non-uniform density of GaAs surface states and including a NW diameter distribution. Scanning electron microscopy, capacitance?voltage characterization and secondary ion mass spectrometry were used to fix key parameters in the model. A 55% decrease in surface state density was achieved upon passivation, corresponding to an impressive four order of magnitude increase in the effective carrier concentration of the NWs. Moreover, the thickest NWs in the ensemble were found to dictate the device characteristics, which is a behavior that should be common to all ensemble NW devices with a distribution in radius. As final confirmation of effective passivation, time-resolved photoluminescence measurements showed a 25??improvement in carrier lifetime upon passivation. The fabrication and passivation methods can be easily implemented into future optoelectronic applications.

Nanowire dopant measurement using secondary ion mass spectrometry

A method is presented to improve the quantitative determination of dopant concentration in semiconductor nanowire (NW) arrays using secondary ion mass spectrometry (SIMS). SIMS measurements were used to determine Be dopant concentrations in a Be-doped GaAs thin film and NW arrays of various pitches that were dry-etched from the same film. A comparison of these measurements revealed a factor of 3 to 12 difference, depending on the NW array pitch, between the secondary Be ion yields of the film and the NW arrays, despite being identically doped. This was due to matrix effects and ion beam mixing of Be from the NWs into the surrounding benzocyclobutene that was used to fill the space between the NWs. This indicates the need for etched NWs to be used as doping standards instead of 2D films when evaluating NWs of unknown doping by SIMS. Using the etched NWs as doping standards, NW arrays of various pitches grown by the vapour-liquid-solid mechanism were characterized by SIMS to yield valuable insights into doping mechanisms.

Low resistance indium tin oxide contact to n-GaAs nanowires

Indium tin oxide (ITO) was deposited by RF sputtering on n-GaAs nanowires grown by the Au-assisted vapor?liquid?solid process in a molecular beam epitaxy (MBE) system. The ITO formed an Ohmic contact with n-doped (n = 8???1018?cm?3) GaAs nanowires with a specific contact resistance of <1.41???cm2. Insertion of a 25?nm thick indium layer between 500?nm thick ITO and the GaAs nanowires resulted in a reduction of specific contact resistance to <0.13???cm2?after annealing at 400??C for 30?s. The In/ITO film had an average transmittance of 89% from 400 to 900?nm and a sheet resistance of 13??/?, which is well suited for nanowire-based optoelectronic applications.

Multi-spectral optical absorption in substrate-free nanowire arrays

A method is presented of fabricating gallium arsenide (GaAs) nanowire arrays of controlled diameter and period by reactive ion etching of a GaAs substrate containing an indium gallium arsenide (InGaP) etch stop layer, allowing the precise nanowire length to be controlled. The substrate is subsequently removed by selective etching, using the same InGaP etch stop layer, to create a substrate-free GaAs nanowire array. The optical absorptance of the nanowire array was then directly measured without absorption from a substrate. We directly observe absorptance spectra that can be tuned by the nanowire diameter, as explained with rigorous coupled wave analysis. These results illustrate strong optical absorption suitable for nanowire-based solar cells and multi-spectral absorption for wavelength discriminating photodetectors. The solar-weighted absorptance above the bandgap of GaAs was 94% for a nanowire surface coverage of only 15%.