Ray Lapierre

Ray Lapierre
McMaster University | McMaster · Department of Engineering Physics

Doctor of Philosophy

About

148
Publications
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3,773
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Publications

Publications (148)
Article
The recombination dynamics of photoexcited holes is studied in an In0.53Ga0.47As/InP quantum well in a quantizing magnetic field. The experimental data are analyzed in a framework of the four level system formed in the valence band by Landau levels. As a result, the characteristic times including the inter-Landau level transfer time, the spin relax...
Article
Polarization-resolved photoluminescense was used to study spin relaxation of photoexcited holes in In0.53Ga0.47As/InP quantum wells in a quantizing magnetic field as a function of temperature. At a temperature below 10K, the circular polarization of the photoluminescence due to the spin-split valence band Landau levels was found temperature-indepen...
Article
A model used to simulate and optimize the performance of nanowire-based betavoltaic generators is developed. The optimum nanowire array geometry is established for devices made of silicon, gallium arsenide, and gallium phosphide for both nickel-63 and tritium sources by computing the energy capture efficiency for each case. The captured power for n...
Article
Full-text available
The commonly employed methyl-ammonium containing perovskites are unstable at high temperature or under moisture attack, thus limiting their commercial applications. To overcome this barrier, a new perovskite possessing both thermal stability and water resistance has been constructed, involving pyrrolidinium rings (CH2)4NH2PbI3 (PyPbI3), through a s...
Article
The geometry of a gallium phosphide nanowire (NW) array has been optimized for maximum current generation in a betavoltaic (BV) device. The energy capture efficiency for various device geometries with different radioisotope source compounds was calculated in GEANT4. A validation of GEANT4 for BV device simulation was performed by comparing a model...
Article
Nonradiative processes are studied in InGaAs/InP quantum wells (QWs) as a function of the pump power and the temperature, using time-resolved photoluminescence. Intravalley disorder induced direct and intervalley phonon-assisted indirect Auger processes are found to be responsible for nonradiative recombination in low mobility and high mobility qua...
Article
Semiconductor nanowires (NWs) represent a new class of materials and a shift from conventional two-dimensional bulk thin films to three-dimensional devices. Unlike thin film technology, lattice mismatch strain in NWs can be relaxed elastically at the NW free surface without dislocations. This capability can be used to grow unique heterostructures a...
Article
A betavoltaic device is reported that directly converts beta energy from a 63Ni radioisotope into electrical energy by impact ionization in a GaP nanowire array. The GaP nanowires are grown in a periodic array by molecular beam epitaxy on silicon using the self-assisted vapor-liquid-solid method. By growing GaP nanowires with large packing fraction...
Article
A new epitaxial thin film transfer method is introduced which utilizes film/substrate systems with sufficiently strong interaction for heteroepitaxial alignment, but with weak adhesion interaction such that inherent stress caused by heteroepitaxy combined with thermomechanical stress causes the deposited structures to separate at the interface. The...
Article
Full-text available
The dynamics of differently spin-polarized carriers photoexcited in a system of Landau levels is investigated in an InGaAs/InP quantum well. Shake-up emission from Landau levels above the Fermi level is observed, and it is shown to significantly affect the recombination dynamics of Landau levels. The shake-up effect is found to occur due to the int...
Preprint
Full-text available
Formation of multiple twin boundaries during the growth of semiconductor nanowires is very common. However, the effects of such planar defects on the electronic and optical properties of nanowires are not very well understood. Here, we use a combination of ab initio simulation and experimental techniques to study these effects. Twin boundaries in G...
Article
Here, a new promising perovskite structure of KMnF3 has been fabricated and characterized, which yields bandgap of 1.6 eV with fascinating moisture-resistance and phase stability. Investigation of structural, optical, stability and transport properties have done by XRD, SEM, UV–vis–NIR spectroscopy, photoluminescence and electrical conductivity tes...
Article
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 molec...
Article
Patterned arrays of self-assisted GaP nanowires were grown on a Si substrate by gas source molecular beam epitaxy using various V/III flux ratios from 1 to 6, and various pitches from 360 to 1000 nm. As the V/III flux ratio was increased from 1 to 6, the nanowires showed a change in morphology from outward tapering to straight, and eventually to in...
Article
In an effort to control aggregation and sintering of phosphor nanoparticles at elevated annealing temperatures, glycothermally synthesized cerium-doped yttrium aluminum garnet (Ce:YAG) nanoparticles were annealed in a matrix of aluminum oxide between 1000 °C and 1200 °C. Scanning electron microscopy images showed that glycothermal synthesis yields...
Article
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 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 NW alternat...
Article
GaAsP self-assisted core-shell p-i-n nanowires were grown on Si solar cells. The resulting tandem cell exhibited an enhanced Voc of 1.16 V, increasing from 0.54 V for the Si cell alone. Nevertheless, the efficiency of the tandem cell was only 3.51% as compared with 9.33% for the Si cell due to a current-limiting short-circuit current density from t...
Article
Vertical III-V nanowires are capable of resonant absorption at specific wavelengths by tuning the nanowire diameter, thereby exceeding the absorption of equivalent thin films. These properties may be exploited to fabricate multispectral infrared (IR) photodetectors, directly integrated with Si, without the need for spectral filters or vertical stac...
Article
Full-text available
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. T...
Article
A review of III-V nanowire-based infrared photodetectors is provided including single nanowires, ensemble nanowires, and heterostructured nanowires. The performance metrics of reported nanowire photodetectors are compared. The potential advantages of nanowire photodetectors, including enhanced absorption, fast carrier collection, multispectral dete...
Article
GaP nanowires (NWs) were grown on a Si substrate by gas source molecular beam epitaxy using self-assisted growth. Selective-area growth was achieved using a patterned oxide mask. Periodic GaAsP marker layers were introduced during growth to study the growth progression using transmission electron microscopy. We demonstrate control of the NW morphol...
Chapter
The development of III−V materials on Si platforms, with the aim of reducing production costs while achieving high conversion efficiency, has been a continuing area of photovoltaic research in the last decades [1,2]. This process is challenging due to large lattice mismatches, the polar non-polar interfaces and the differences in thermal expansion...
Article
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 th...
Article
Full-text available
The superconducting proximity effect is probed experimentally in Josephson junctions fabricated with InAs nanowires contacted by Nb leads. Contact transparencies $t \sim 0.7$ are observed. The electronic phase coherence length at low temperatures exceeds the channel length. However, the elastic scattering length is a few times shorter than the chan...
Article
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...
Article
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 nano...
Article
Full-text available
The native oxide at the surface of III-V nanowires, such as InAs, can be a major source of charge noise and scattering in nanowire-based electronics, particularly for quantum devices operated at low temperatures. Surface passivation provides a means to remove the native oxide and prevent its regrowth. Here, we study the effects of surface passivati...
Article
The efficiency of GaAs nanowire solar cells can be significantly improved without any new processing steps or material requirements. We report coupled optoelectronic simulations of a GaAs nanowire (NW) solar cell with vertical p-i-n junction and high band gap AlInP passivating shell. Our frequency-dependent model facilitates calculation of quantum...
Article
A single-junction core-shell GaAs nanowire (NW) solar cell on Si (1 1 1) substrates is presented. A Ga-assisted vapor-liquid-solid growth mechanism was used for the formation of a patterned array of radial p-i-n GaAs NWs encapsulated in AlInP passivation. Novel device fabrication utilizing facet-dependent properties to minimize passivation layer re...
Article
Polarization-resolved magneto-photoluminescence is studied in InGaAs/InP single quantum wells. In the range of the filling factor [Formula: see text] the number of populated Landau levels contributing to the photoluminescence is found to be equal to the corresponding filling factor, while at [Formula: see text] the number of emitting Landau levels...
Article
Full-text available
The electron transport and recombination processes of photoexcited electron-hole pairs were studied in InGaAs/InP single quantum wells. Comprehensive transport data analysis reveals a asymmetric shape of the quantum well potential where the electron mobility was found to be dominated by interface-roughness scattering. The low-temperature time-resol...
Article
The effects of temperature on the optical properties of lattice matched InGaAs/InP superlattices are reported. We found that the PL spectra are dominated by the recombination of carriers in the lowest energy minibands formed by - and -X. Effects of temperature on the contribution of each of these minibands are analyzed using two distinct models, pr...
Article
Full-text available
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 SiOx /Si (111) substrates by molecular beam epitaxy. Within these boundaries, the substrate receives a balanced flux of group III and V m...
Article
Full-text available
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 compari...
Article
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...
Article
Full-text available
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...
Article
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, pred...
Article
The photo-thermal properties of vertically etched gallium arsenide nanowire arrays are examined using Raman spectroscopy. The nanowires are arranged in square lattices with a constant pitch of 400 nm, and diameters ranging from 50 to 155 nm. The arrays were illuminated using a 532 nm laser with an incident energy density of 10 W/mm2. Nanowire tempe...
Article
The optoelectronic properties of single crystal CdTe thin films were investigated by photoluminescence spectroscopy, photoreflectance spectroscopy and variable angle spectroscopic ellipsometry. The room temperature bandgap was measured to be 1.51 eV and was consistent between spectroscopic measurements and previously reported values. Breadth of ban...
Article
Full-text available
Vertically oriented and ordered GaAs nanowire arrays have been grown by the self-assisted mechanism using substrates prepared with nano-patterned oxide templates. Patterned Ga-assisted GaAs nanowire growth on (111) silicon by molecular beam epitaxy showed that the axial and radial growth rates increased with increasing interhole spacing. A model is...
Article
Full-text available
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 In...
Article
The photo-thermal properties of vertically etched gallium arsenide nanowire arrays are examined using Raman spectroscopy. The nanowires are arranged in square lattices with a constant pitch of 400 nm, and diameters ranging from 50 to 155 nm. The arrays were illuminated using a 532 nm laser with an incident energy density of 10 W/mm2. Nanowire tempe...
Article
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 wi...
Article
Full-text available
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...
Article
Full-text available
We report on the surface passivation of Au-assisted Te-doped GaAs nanowires (NWs) grown by metalorganic vapor phase epitaxy. The electrical properties of individual free standing NWs were assessed using a tungsten nano-probe inside a scanning electron microscope. The diameter independent apparent resistivity of both strained and relaxed passivated...
Article
Full-text available
The Josephson supercurrent in a Nb-InAs nanowire-Nb junction was studied experimentally. The nanowire goes superconducting due to the proximity effect, and can sustain a phase coherent supercurrent as large as ~55 nA. An unexpected modulation of the junction critical current in an axial magnetic field is observed, which we attribute to a novel form...
Article
Full-text available
Recent work on the photoluminescence (PL) properties of InAsP/InP nanowires are reported. InP nanowires were grown on < 111 > Si substrates by the Au-assisted vapor-liquid-solid process in a gas source molecular beam epitaxy system. InAsyP1-y segments were grown within the InP nanowires, creating single or multiple quantum dots structures. The quan...
Article
Full-text available
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 Ω cm2. Insertion of a 25 nm thick indium layer between 500...
Article
The control of the doping in nanowires (NWs) is of fundamental importance for the implementation of NW-based devices. A method is presented to obtain local information about doping by monitoring the Fermi-energy position within the bandgap at the surface along single NWs through spatially resolved x-ray photoemission spectroscopy. The experimental...
Article
Ordered arrays of vertically aligned self-catalyzed GaAs nanowires have been grown by gas source molecular beam epitaxy (GS-MBE) on silicon substrates using nano-patterned oxide templates. Several growth processes of different duration were performed under identical conditions and with identical sample preparation. To determine the influence of pat...
Article
This article reviews recent developments in nanowire-based photovoltaics (PV) with an emphasis on III-V semiconductors including growth mechanisms, device fabrication and performance results. We first review the available nanowire growth methods followed by control of nanowire growth direction and crystal structure. Important device issues are revi...
Article
High efficiency III–V nanowire solar cell: III–V nanowire-based photovoltaics (PV) are a promising replacement for thin-film solar cells, offering the potential for less material utilization, excellent anti-reflection properties, lattice-mismatched growth on silicon, and high power conversion efficiency. In the Review@RRL article by LaPierre et al....
Conference Paper
A concept for a nanowire-based photovoltaic (PV) device is presented along with the requirements for achieving high photoconversion efficiency including nanowire morphology, crystalline structure, nanowire dimensions (diameter, period (spacing) and length), avoidance of misfit dislocations, low resistance contacts, controlled doping for p-n junctio...
Article
Full-text available
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 sur...
Article
Full-text available
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...
Article
Poisson's equation is solved for a radial pn junction nanowire (NW) with surface depletion. This resulted in a model capable of giving radial energy band and electric field profiles for any arbitrary core/shell doping density, core/shell dimensions, and surface state density. Specific cases were analyzed to extract pertinent underlying physics, whi...
Article
Full-text available
It is shown for a gated semiconductor nanowire device at low temperature that variations in electrical conductance with magnetic field and gate voltage reveal patterns that can be unambiguously assigned to specific transverse electronic subbands in the nanowire. This method applies to the ballistic and quasiballistic regimes, and is a useful probe...
Article
Structural and optical properties of heterostructured GaAs/AlGaAs/GaAs core/inner shell/outer shell nanowires are studied. Transmission electron microscopy and Raman scattering measurements unambiguously identify the presence of segments crystallized in zincblende and wurtzite phases, which spread to the shells. Four observed photoluminescence line...
Conference Paper
We have modeled the growth of InGaP wires using Au-assisted MBE growth. Our model takes the effective diffusivities of the group III species as fitting parameters and produces excellent match with a wide range of experimental data. We show that the chemical potential for different species over the 2D surface may be predicted for a given set of proc...
Article
Full-text available
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 e...
Article
We study random telegraph noise in the conductance of InAs nanowire field-effect transistors due to single electron trapping in defects. The electron capture and emission times are measured as functions of temperature and gate voltage for individual traps, and are consistent with traps residing in the few-nanometer-thick native oxide, with a Coulom...
Article
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...
Article
Fabrication, electrical characterization and analytical modeling of an AllnP-passivated GaAs NW ensemble is presented. Novel processing steps were used to fabricate NW ensemble devices which were subsequently characterized electrically and fit with an analytical model, showing a 48% reduction in interface state density and an impressive four order...
Article
Be-doped InP nanowires were grown by the gold-assisted vapour-liquid-solid mechanism in a gas source molecular beam epitaxy system. The InP nanowire length versus diameter [L(D)] dependence revealed an unexpected transition with increasing Be dopant concentration. At Be dopant concentration below {approx}10{sup 18} cm{sup -3}, nanowires exhibited t...
Article
InAs nanowires with AlxIn1−xP or AlxIn1−xAs shells were grown on GaAs substrates by the Au-assisted vapour-liquid-solid method in a gas source molecular beam epitaxy system. Core diameters and shell thicknesses were measured by transmission electron microscopy (TEM). These measurements were then related to selected area diffraction patterns to veri...
Article
Full-text available
The reflectance, transmittance, and absorptance of GaAs nanowire (NW) arrays are calculated by solving Maxwell's equations using the finite element method. The model is compared with measurement results from well-ordered periodic GaAs NW arrays fabricated by dry etching. The model results are also compared with the reflectance measured from NWs gro...
Article
Direct determination of carrier concentration and doping in a single nanowire is achieved by placing four electrical contacts along its sidewall.