Jonathan Boulanger

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.

Opportunities and pitfalls in patterned self-catalyzed GaAs nanowire growth on silicon

Periodic arrays of self-catalyzed GaAs nanowires (NWs) were grown on Si substrates by gas source molecular beam epitaxy (GS-MBE) using patterned oxide templates. The various challenges of the patterning process that result in undesired outcomes are described, such as pattern transfer by wet/dry etching, oxide thickness variations, and native oxide re-growth. Transmission electron microscopy (TEM) results are used to illustrate each case. In particular, we show that a linearly increasing length–radius distribution, analogous to that observed for unpatterned self-catalyzed growth on substrates with thin oxides, may be obtained even when using patterned oxide masks due to an unintended residual layer of oxide, as confirmed by TEM analysis. We explain how a linear length–radius dependence can result from the individual NWs beginning their growth at different times, accompanied by significant radial growth. The spread in obtained NW dimensions was decreased by improving the patterning method.

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.

Patterned gold-assisted growth of GaP nanowires on Si

The patterning of metal seed particles for the vapor–liquid–solid (VLS) growth of semiconductor nanowires is examined. We report the challenges of obtaining GaP nanowires grown by molecular beam epitaxy (MBE) from a patterned array of Au seed particles created by electron beam lithography. Transmission electron microscopy studies revealed the presence of a thin silicon oxide layer over the patterned Au seed particles. Au acted as a catalyst for silicon oxide growth at even moderate temperatures. In particular, the annealing of Au-patterned Si (1 1 1) substrates at low temperatures required for indium mounting (200–300 °C) was responsible for the formation of an oxide layer that was detrimental to Au-assisted growth. Removal of the oxide layer by a buffered HF etch prior to MBE sample loading enabled patterned VLS growth.

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%.

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.

Resonant photo-thermal modification of vertical gallium arsenide nanowires studied using Raman spectroscopy

Gallium arsenide nanowires have shown considerable promise for use in applications in which the absorption of light is required. When the nanowires are oriented vertically, a considerable amount of light can be absorbed, leading to significant heating effects. Thus, it is important to understand the threshold power densities that vertical GaAs nanowires can support, and how the nanowire morphology is altered under these conditions. Here, resonant photo-thermal modification of vertical GaAs nanowires was studied using both Raman spectroscopy and electron microscopy techniques. Resonant waveguiding, and subsequent absorption of the excited optical mode reduces the irradiance vertical GaAs nanowires can support relative to horizontal ones, by three orders of magnitude before the onset of structural changes occur. A power density of only 20 W mm(-2) was sufficient to induce local heating in the nanowires, resulting in the formation of arsenic species. Upon further increasing the power, a hollow nanowire morphology was realized. These findings are pertinent to all optical applications and spectroscopic measurements involving vertically oriented GaAs nanowires. Understanding the optical absorption limitations, and the effects of exceeding these limitations will help improve the development of all III-V nanowire devices.

Gallium loading of gold seed for high yield of patterned GaAs nanowires

A method is presented for maximizing the yield and crystal phase purity of vertically aligned Au-assisted GaAs nanowires grown with an SiOx selective area epitaxy mask on GaAs (111)B substrates. The nanowires were grown by the vapor-liquid-solid (VLS) method in a gas source molecular beam epitaxy system. During annealing, Au VLS seeds will alloy with the underlying GaAs substrate and collect beneath the SiOx mask layer. This behavior is detrimental to obtaining vertically aligned, epitaxial nanowire growth. To circumvent this issue, Au droplets were pre-filled with Ga assuring vertical yields in excess of 99%.