arXiv subjects

A. C. H. Rowe

Publications and source records attributed to A. C. H. Rowe.

At least 19 recordsLinked to original sources

arXiv2026

Nanoscale imaging of reduced forward bias at V-defects in green-emitting nitride LEDs

Record wall-plug efficiencies in long-wavelength, III-nitride light-emitting diodes (LEDs) have recently been achieved through improvements in electrical efficiency in devices containing V-defects. Numerical modeling suggests this may be due to reduced barrier heights for charge injection in thinned, low-Indium quantum wells parallel to semi-polar V-defect facets. To test this proposition, a novel approach in which the tip of a scanning tunneling luminescence microscope as a local hole injector, is used to map the optoelectronic properties of commercial, green-emitting LED heterostructures around V-defects with nanoscale spatial resolution. A 1 V reduction in the forward bias necessary for current injection at V-defect rims is observed. This, combined with the observation of small (~10 meV) blue shifts in the locally emitted electroluminescence, unambiguously confirms the charge injection mechanism.

physics.app-ph↗

arXiv2023

Deep-level structure of the spin-active recombination center in dilute nitrides

A Gallium interstitial defect (Ga$_{\textrm{i}}$) is thought to be responsible for the spectacular spin-dependent recombination (SDR) in GaAs$_{1-x}$N$_x$ dilute nitride semiconductors. Current understanding associates this defect with two in-gap levels corresponding to the (+/0) and (++/+) charge-state transitions. Using a spin-sensitive photo-induced current transient spectroscopy, the in-gap electronic structure of a $x$ = 0.021 alloy is revealed. The (+/0) state lies $\approx$ 0.27 eV below the conduction band edge, and an anomalous, negative activation energy reveals the presence of not one but \textit{two} other states in the gap. The observations are consistent with a (++/+) state $\approx$ 0.19 eV above the valence band edge, and a hitherto ignored, (+++/++) state $\approx$ 25 meV above the valence band edge. These observations can inform efforts to better model the SDR and the Ga$_{\textrm{i}}$ defect's local chemical environment.

cond-mat.mtrl-sci↗

arXiv2022

A systematic study of spin-dependent recombination in GaAs$_{1-x}$N$_x$ as a function of nitrogen content

A systematic study of spin-dependent recombination (SDR) under steady-state optical pumping conditions in dilute nitride semiconductors as a function of nitrogen content is reported. The alloy content is determined by a fit of the photoluminescence (PL) intensity using a Roosbroeck-Shockley relation and verified by a study of the GaN-like LO$_2$ phonon peak in a Raman spectroscopy map. PL spectra taken from alloys of the form GaAs$_{1-x}$N$_x$ where $0.022 < x < 0.036$ exhibit PL intensity increases when switching from a linearly- to a circularly-polarized pump up to a factor of 5 for $x = 0.022$. This work used a 1.39 eV laser with a radius of 0.6 $μ$m. The observed SDR ratio monotonically decreases with increasing $x$, reaching 1.5 for $x = 0.036$. Moreover, the excitation power required to obtain maximum SDR systematically increases with increasing $x$, varying from 0.6 mW for $x = 0.022$ to 15 mW for $x = 0.036$. These observations are consistent with an increase in the density of electronically active defects with increasing nitrogen content, both those responsible for the SDR as well as other, standard Shockley-Read-Hall (SRH) centers.

cond-mat.mtrl-sci↗

arXiv2021

Piezoresistance in defect-engineered silicon

The steady-state, space-charge-limited piezoresistance (PZR) of defect-engineered, silicon-on-insulator device layers containing silicon divacancy defects changes sign as a function of applied bias. Above a punch-through voltage ($V_t$) corresponding to the onset of a space-charge-limited hole current, the longitudinal $\langle 110 \rangle$ PZR $π$-coefficient is $π\approx 65 \times 10^{-11}$~Pa$^{-1}$, similar to the value obtained in charge-neutral, p-type silicon. Below $V_t$, the mechanical stress dependence of the Shockley-Read-Hall (SRH) recombination parameters, specifically the divacancy trap energy $E_T$ which is estimated to vary by $\approx 30$~$μ$V/MPa, yields $π\approx -25 \times 10^{-11}$~Pa$^{-1}$. The combination of space-charge-limited transport and defect engineering which significantly reduces SRH recombination lifetimes makes this work directly relevant to discussions of giant or anomalous PZR at small strains in nano-silicon whose characteristic dimension is larger than a few nanometers. In this limit the reduced electrostatic dimensionality lowers $V_t$ and amplifies space-charge-limited currents and efficient SRH recombination occurs via surface defects. The results reinforce the growing evidence that in steady state, electro-mechanically active defects can result in anomalous, but not giant, PZR.

physics.app-ph↗

arXiv2019

Mechanical stress dependence of the Fermi level pinning on an oxidized silicon surface

A combination of micro-Raman spectroscopy and micro-XPS (X-ray photo-electron spectroscopy) mapping on statically deflected p-type silicon cantilevers is used to study the mechanical stress dependence of the Fermi level pinning at an oxidized silicon (001) surface. With uniaxial compressive and tensile stress applied parallel to the $\langle$110$\rangle$ crystal direction, the observations are relevant to the electronic properties of strain-silicon nano-devices with large surface-to-volume ratios such as nanowires and nanomembranes. The surface Fermi level pinning is found to be even in applied stress, a fact that may be related to the symmetry of the Pb$_0$ silicon/oxide interface defects. For stresses up to 160 MPa, an increase in the pinning energy of 0.16 meV/MPa is observed for compressive stress, while for tensile stress it increases by 0.11 meV/MPa. Using the bulk, valence band deformation potentials the reduction in surface band bending in compression (0.09 meV/MPa) and in tension (0.13 meV/MPa) can be estimated.

cond-mat.mes-hall↗

arXiv2018

Exciton diffusion in WSe2 monolayers embedded in a van der Waals heterostructure

We have combined spatially-resolved steady-state micro-photoluminescence ($μ$PL) with time-resolved photoluminescence (TRPL) to investigate the exciton diffusion in a WSe$_2$ monolayer encapsulated with hexagonal boron nitride (hBN). At 300 K, we extract an exciton diffusion length $L_X= 0.36\pm 0.02 \; μ$m and an exciton diffusion coefficient of $D_X=14.5 \pm 2\;\mbox{cm}^2$/s. This represents a nearly 10-fold increase in the effective mobility of excitons with respect to several previously reported values on nonencapsulated samples. At cryogenic temperatures, the high optical quality of these samples has allowed us to discriminate the diffusion of the different exciton species : bright and dark neutral excitons, as well as charged excitons. The longer lifetime of dark neutral excitons yields a larger diffusion length of $L_{X^D}=1.5\pm 0.02 \;μ$m.

cond-mat.mes-hall↗

arXiv2016

Spin and recombination dynamics of excitons and free electrons in p-type GaAs : effect of carrier density

Carrier and spin recombination are investigated in p-type GaAs of acceptor concentration NA = 1.5 x 10^(17) cm^(-3) using time-resolved photoluminescence spectroscopy at 15 K. At low pho- tocarrier concentration, acceptors are mostly neutral and photoelectrons can either recombine with holes bound to acceptors (e-A0 line) or form excitons which are mostly trapped on neutral acceptors forming the (A0X) complex. It is found that the spin lifetime is shorter for electrons that recombine through the e-A0 transition due to spin relaxation generated by the exchange scattering of free electrons with either trapped or free holes, whereas spin flip processes are less likely to occur once the electron forms with a free hole an exciton bound to a neutral acceptor. An increase of exci- tation power induces a cross-over to a regime where the bimolecular band-to-band (b-b) emission becomes more favorable due to screening of the electron-hole Coulomb interaction and ionization of excitonic complexes and free excitons. Then, the formation of excitons is no longer possible, the carrier recombination lifetime increases and the spin lifetime is found to decrease dramatically with concentration due to fast spin relaxation with free photoholes. In this high density regime, both the electrons that recombine through the e-A0 transition and through the b-b transition have the same spin relaxation time.

cond-mat.mtrl-sci↗

arXiv2016

Luminescence imaging of photoelectron spin precession during drift in p-type GaAs

Using a microfabricated, p-type GaAs Hall bar, is it shown that the combined application of co-planar electric and magnetic fields enables the observation at 50 K of spatial oscillations of the photoluminescence circular polarization due to the precession of drifting spin-polarized photoelec- trons. Observation of these oscillations as a function of electric field E gives a direct measurement of the minority carrier drift mobility and reveals that, for E = 800 V/cm, spin coherence is preserved over a length as large as 25μm.

cond-mat.mes-hall↗

arXiv2016

Geometric and chemical components of the giant piezoresistance in silicon nanowires

A wide variety of apparently contradictory piezoresistance (PZR) behaviors have been reported in p-type silicon nanowires (SiNW), from the usual positive bulk effect to anomalous (negative) PZR and giant PZR. The origin of such a range of diverse phenomena is unclear, and consequently so too is the importance of a number of parameters including SiNW type (top down or bottom up), stress concentration, electrostatic field effects, or surface chemistry. Here we observe all these PZR behaviors in a single set of nominally p-type, $\langle 110 \rangle$ oriented, top-down SiNWs at uniaxial tensile stresses up to 0.5 MPa. Longitudinal $π$-coefficients varying from $-800\times10^{-11}$ Pa$^{-1}$ to $3000\times10^{-11}$ Pa$^{-1}$ are measured. Micro-Raman spectroscopy on chemically treated nanowires reveals that stress concentration is the principal source of giant PZR. The sign and an excess PZR similar in magnitude to the bulk effect are related to the chemical treatment of the SiNW.

cond-mat.mes-hall↗

arXiv2015

Ambipolar spin-spin coupling in p$^+$-GaAs

A novel spin-spin coupling mechanism that occurs during the transport of spin-polarized minority electrons in semiconductors is described. Unlike the Coulomb spin drag, this coupling arises from the ambipolar electric field which is created by the differential movement of the photoelectrons and the photoholes. Like the Coulomb spin drag, it is a pure spin coupling that does not affect charge diffusion. Experimentally, the coupling is studied in $p^+$ GaAs using polarized microluminescence. The coupling manifests itself as an excitation power dependent reduction in the spin polarization at the excitation spot \textit{without} any change of the spatially averaged spin polarization.

cond-mat.mes-hall↗

arXiv2015

Effect of the Pauli principle on photoelectron spin transport in $p^+$ GaAs

In p+ GaAs thin films, the effect of photoelectron degeneracy on spin transport is investigated theoretically and experimentally by imaging the spin polarization profile as a function of distance from a tightly-focussed light excitation spot. Under degeneracy of the electron gas (high concentration, low temperature), a dip at the center of the polarization profile appears with a polarization maximum at a distance of about $2 \; μm$ from the center. This counterintuitive result reveals that photoelectron diffusion depends on spin, as a direct consequence of the Pauli principle. This causes a concentration dependence of the spin stiffness while the spin dependence of the mobility is found to be weak in doped material. The various effects which can modify spin transport in a degenerate electron gas under local laser excitation are considered. A comparison of the data with a numerical solution of the coupled diffusion equations reveals that ambipolar coupling with holes increases the steady-state photo-electron density at the excitation spot and therefore the amplitude of the degeneracy-induced polarization dip. Thermoelectric currrents are predicted to depend on spin under degeneracy (spin Soret currents), but these currents are negligible except at very high excitation power where they play a relatively small role. Coulomb spin drag and bandgap renormalization are negligible due to electrostatic screening by the hole gas.

cond-mat.mes-hall↗

arXiv2015

Anomalous temperature dependence of photoelectron charge and spin mobilities in p+-GaAs

The effect of an electric field on the spatial charge and spin profiles of photoelectrons in p+-GaAs is studied as a function of lattice and electron temperature. The charge and spin mobilities of photoelectrons are equal in all conditions and exhibit the well known increase as the temperature is lowered. It is shown that this is related mainly to the electron statistics rather than the majority hole statistics. This finding suggests that current theoretical models based on degeneracy of majority carriers cannot fully explain the observed temperature dependence of minority carrier mobility.

cond-mat.mes-hall↗

arXiv2014

Piezoresistance in Silicon and its nanostructures

Piezoresistance is the change in the electrical resistance, or more specifically the resistivity, of a solid induced by an applied mechanical stress. The origin of this effect in bulk, crystalline materials like Silicon, is principally a change in the electronic structure which leads to a modification of the charge carriers effective mass. The last few years have seen a rising interest in the piezoresistive properties of semiconductor nanostructures, motivated in large part by claims of a giant piezoresistance effect in Silicon nanowires that is more than two orders of magnitude bigger than the known bulk effect. This review aims to present the controversy surrounding claims and counter-claims of giant piezoresistance in Silicon nanostructures by presenting a summary of the major works carried out over the last 10 years. The main conclusions that can be drawn from the literature are that i) reproducible evidence for a giant piezoresistance effect in un-gated Silicon nanowires is limited, ii) in gated nanowires a giant effect has been reproduced by several authors, iii) the giant effect is fundamentally different from either the bulk Silicon piezoresistance or that due to quantum confinement in accumulation layers and heterostructures, the evidence pointing to an electrostatic origin for the piezoresistance, iv) released nanowires tend to have slightly larger piezoresistance coefficients than un-released nanowires, and v) insufficient work has been performed on bottom-up grown nanowires to be able to rule out a fundamental difference in their properties when compared with top-down nanowires. On the basis of this, future possible research directions are suggested.

cond-mat.mes-hall↗

arXiv2013

The effect of Pauli blockade on spin-dependent diffusion in a degenerate electron gas

Spin-polarized transport of photo-electrons in bulk, p-type GaAs is investigated in the Pauli blockade regime. In contrast to usual spin diffusion processes in which the spin polarization decreases with distance traveled due to spin relaxation, images of the polarized photo-luminescence reveal a spin-filter effect in which the spin polarization increases during transport over the first 2 microns from 26 % to 38 %. This is shown to be a direct consequence of the Pauli Principle and the associated quantum degeneracy pressure which results in a spin-dependent increase in the minority carrier diffusion constants and mobilities. The central role played by the quantum degeneracy pressure is confirmed via the observation of a spin-dependent increase in the photo-electron volume and a spin-charge coupling description of this is presented.

cond-mat.mes-hall↗

arXiv2013

Absence of an intrinsic value for the surface recombination velocity in doped semiconductors

A self-consistent expression for the surface recombination velocity $S$ and the surface Fermi level unpinning energy as a function of light excitation power ($P$) is presented for n- and p-type semiconductors doped above the 10$^{16}$ cm$^{-3}$ range. Measurements of $S$ on p-type GaAs films using a novel polarized microluminescence technique are used to illustrate two limiting cases of the model. For a naturally oxidized surface $S$ is described by a power law in $P$ whereas for a passivated surface $S^{-1}$ varies logarithmically with $P$. Furthermore, the variation in $S$ with surface state density and bulk doping level is found to be the result of Fermi level unpinning rather than a change in the intrinsic surface recombination velocity. It is concluded that $S$ depends on $P$ throughout the experimentally accessible range of excitation powers and therefore that no instrinsic value can be determined. Previously reported values of $S$ on a range of semiconducting materials are thus only valid for a specific excitation power.

cond-mat.mtrl-sci↗

arXiv2012

Piezoresistance in silicon at uniaxial compressive stresses up to 3 GPa

The room-temperature longitudinal piezoresistance of n-type and p-type crystalline silicon along selected crystal axes is investigated under uniaxial compressive stresses up to 3 GPa. While the conductance ($G$) of n-type silicon eventually saturates at $\approx 45%$ of its zero-stress value ($G_0$) in accordance with the charge transfer model, in p-type material $G/G_0$ increases above a predicted limit of $\approx 4.5$ without any significant saturation, even at 3 GPa. Calculation of $G/G_0$ using \textit{ab-initio} density functional theory reveals that neither $G$ nor the mobility, when properly averaged over the hole distribution, saturate at stresses lower than 3 GPa. The lack of saturation has important consequences for strained silicon technologies.

cond-mat.mtrl-sci↗

arXiv2010

On giant piezoresistance effects in silicon nanowires and microwires

The giant piezoresistance (PZR) previously reported in silicon nanowires is experimentally investigated in a large number of surface depleted silicon nano- and micro-structures. The resistance is shown to vary strongly with time due to electron and hole trapping at the sample surfaces. Importantly, this time varying resistance manifests itself as an apparent giant PZR identical to that reported elsewhere. By modulating the applied stress in time, the true PZR of the structures is found to be comparable with that of bulk silicon.

cond-mat.mes-hall↗

arXiv2010

Spin dependent photoelectron tunnelling from GaAs into magnetic Cobalt

The spin dependence of the photoelectron tunnel current from free standing GaAs films into out-of- plane magnetized Cobalt films is demonstrated. The measured spin asymmetry (A) resulting from a change in light helicity, reaches +/- 6% around zero applied tunnel bias and drops to +/- 2% at a bias of -1.6 V applied to the GaAs. This decrease is a result of the drop in the photoelectron spin polarization that results from a reduction in the GaAs surface recombination velocity. The sign of A changes with that of the Cobalt magnetization direction. In contrast, on a (nonmagnetic) Gold film A ~ 0%.

cond-mat.mes-hall↗