84 research outputs found
Photoassisted tunneling from free-standing GaAs thin films into metallic surfaces
The tunnel photocurrent between a gold surface and a free-standing
semiconducting thin film excited from the rear by above bandgap light has been
measured as a function of applied bias, tunnel distance and excitation light
power. The results are compared with the predictions of a model which includes
the bias dependence of the tunnel barrier height and the bias-induced decrease
of surface recombination velocity. It is found that i) the tunnel photocurrent
from the conduction band dominates that from surface states. ii) At large
tunnel distance the exponential bias dependence of the current is explained by
that of the tunnel barrier height, while at small distance the change of
surface recombination velocity is dominant
Absence of an intrinsic value for the surface recombination velocity in doped semiconductors
A self-consistent expression for the surface recombination velocity and
the surface Fermi level unpinning energy as a function of light excitation
power () is presented for n- and p-type semiconductors doped above the
10 cm range. Measurements of 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 is described by a
power law in whereas for a passivated surface varies
logarithmically with . Furthermore, the variation in 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 depends on throughout the experimentally
accessible range of excitation powers and therefore that no instrinsic value
can be determined. Previously reported values of on a range of
semiconducting materials are thus only valid for a specific excitation power.Comment: 10 pages, 7 figure
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%
Imaging charge and spin diffusion of minority carriers in GaAs
Room temperature electronic diffusion is studied in 3 mum thick epitaxial p+
GaAs lift-off films using a novel circularly polarized photoluminescence
microscope. The method is equivalent to using a standard optical microscope and
provides a contactless means to measure charge (L) and spin (L_s) diffusion
lengths. The measured values of L and L_s are in excellent agreement with the
spatially averaged polarization and a sharp reduction in these two quantities
(L from 21.3 mum to 1.2 mum and L_s from 1.3 mum to 0.8 mum) is measured with
increasing surface recombination. Outwards diffusion results in a factor of 10
increase in the polarization at the excitation spot.Comment: 13 pages, 3 figure
Imaging ambipolar diffusion of photocarriers in GaAs thin films
Images of the steady-state luminescence of passivated GaAs self-standing
films under excitation by a tightly-focussed laser are analyzed as a function
of light excitation power. While unipolar diffusion of photoelectrons is
dominant at very low light excitation power, an increased power results in a
decrease of the diffusion constant near the center of the image due to the
onset of ambipolar diffusion. The results are in agreement with a numerical
solution of the diffusion equations and with a physical analysis of the
luminescence intensity at the centre of the image, which permits the
determination of the ambipolar diffusion constant as a function of electron
concentration.Comment: 5 figure
Room temperature broadband coherent terahertz emission induced by dynamical photon drag in graphene
Nonlinear couplings between photons and electrons in new materials give rise
to a wealth of interesting nonlinear phenomena. This includes frequency mixing,
optical rectification or nonlinear current generation, which are of particular
interest for generating radiation in spectral regions that are difficult to
access, such as the terahertz gap. Owing to its specific linear dispersion and
high electron mobility at room temperature, graphene is particularly attractive
for realizing strong nonlinear effects. However, since graphene is a
centrosymmetric material, second-order nonlinearities a priori cancel, which
imposes to rely on less attractive third-order nonlinearities. It was
nevertheless recently demonstrated that dc-second-order nonlinear currents as
well as ultrafast ac-currents can be generated in graphene under optical
excitation. The asymmetry is introduced by the excitation at oblique incidence,
resulting in the transfer of photon momentum to the electron system, known as
the photon drag effect. Here, we show broadband coherent terahertz emission,
ranging from about 0.1-4 THz, in epitaxial graphene under femtosecond optical
excitation, induced by a dynamical photon drag current. We demonstrate that, in
contrast to most optical processes in graphene, the next-nearest-neighbor
couplings as well as the distinct electron-hole dynamics are of paramount
importance in this effect. Our results indicate that dynamical photon drag
effect can provide emission up to 60 THz opening new routes for the generation
of ultra-broadband terahertz pulses at room temperature.Comment: 17 pages, 3 figure
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