137 research outputs found
Photocurrent and Photoconductance Properties of a GaAs Nanowire
We report on photocurrent and photoconductance processes in a freely
suspended p-doped single GaAs nanowire. The nanowires are grown by molecular
beam epitaxy (MBE), and they are electrically contacted by a focused ion beam
(FIB) deposition technique. The observed photocurrent is generated at the
Schottky contacts between the nanowire and metal source-drain electrodes, while
the observed photoconductance signal can be explained by a photogating effect
induced by optically generated charge carriers located at the surface of the
nanowire. Both optoelectronic effects are sensitive to the polarization of the
exciting laser field, enabling polarization dependent photodetectors
Size dependence of the bulk and surface phonon modes of gallium arsenide nanowires as measured by Raman Spectroscopy
Gallium arsenide nanowires were synthesized by gallium-assisted molecular
beam epitaxy. By varying the growth time, nanowires with diameters ranging from
30 to 160 nm were obtained. Raman spectra of the nanowires ensembles were
measured. The small line width of the optical phonon modes agree with an
excellent crystalline quality. A surface phonon mode was also revealed, as a
shoulder at lower frequencies of the longitudinal optical mode. In agreement
with the theory, the surface mode shifts to lower wave numbers when the
diameter of the nanowires is decreased or the environment dielectric constant
increased.Comment: 4 pages, 3 figure
GaAs nanowires and related prismatic heterostructures
The growth of GaAs nanowires by the gallium-assisted method with molecular beam epitaxy (MBE) is presented in this review article. The structure of the grown nanowires was investigated by means of scanning and transmission electron microscopy as well as Raman spectroscopy. Their optical properties were revealed by performing photoluminescence measurements at the single nanowire level. Furthermore, by tuning the MBE conditions to planar growth, quantum heterostructures on the side facets of the nanowires were achieved. High-resolution transmission electron microscopy proved that the grown heterostructures have epitaxial precision, while photoluminescence measurements showed that they possess excellent optical quality. These quantum heterostructures constitute templates for developing novel nanowire based devices, such as a high electron mobility one-dimensional transistor or third generation solar cells
Free standing modulation doped core-shell GaAs/AlGaAs hetero-nanowires
Modulation doped AlGaAs/GaAs core-shell nanowire structures were grown by molecular beam epitaxy. A Si delta-doping was introduced in the AlGaAs shell around the {110} facets of the GaAs core. The wires are typically highly resistive at low temperatures. However, they show a pronounced persistent photoconductivity effect indicating activation of free carriers from the delta-doped shell to the GaAs core. The n-type character of the channel is demonstrated by applying a back-gate voltage. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinhei
Unintentional high density p-type modulation doping of a GaAs/AlAs core-multi-shell nanowire
Achieving significant doping in GaAs/AlAs core/shell nanowires (NWs) is of
considerable technological importance but remains a challenge due to the
amphoteric behavior of the dopant atoms. Here we show that placing a narrow
GaAs quantum well in the AlAs shell effectively getters residual carbon
acceptors leading to an \emph{unintentional} p-type doping. Magneto-optical
studies of such a GaAs/AlAs core multi-shell NW reveal quantum confined
emission. Theoretical calculations of NW electronic structure confirm quantum
confinement of carriers at the core/shell interface due to the presence of
ionized carbon acceptors in the 1~nm GaAs layer in the shell.
Micro-photoluminescence in high magnetic field shows a clear signature of
avoided crossings of the Landau level emission line with the Landau
level TO phonon replica. The coupling is caused by the resonant hole-phonon
interaction, which points to a large 2D hole density in the structure.Comment: just published in Nano Letters
(http://pubs.acs.org/doi/full/10.1021/nl500818k
Valence band structure of polytypic zinc-blende/wurtzite GaAs nanowires probed by polarization-dependent photoluminescence
We conducted temperature-dependent measurements of the photoluminescence (PL) polarization on GaAs nanowires (NWs) with polytypic zinc-blende/wurtzite structure in order to probe the symmetry and energy structure of the valence band in the wurtzite segments of the NWs. The low-temperature measurements revealed that in most of the investigated cases, the ground level of the interface excitons responsible for the PL is formed by the heavy hole. To describe the observed temperature dependence of the degree of PL polarization, we developed a theoretical model that allows an estimation of the splitting between the heavy hole and light hole exciton subbands in these NWs. This splitting is smaller than expected in pure wurtzite on the basis of recent first-principles calculations, which may be attributed to the multiple twinned nature of the wurtzite sections, which effectively behave as a polytype of lower hexagonality
Effect of the GaAsP shell on optical properties of self-catalyzed GaAs nanowires grown on silicon
We realize growth of self-catalyzed core-shell GaAs/GaAsP nanowires (NWs) on
Si substrates using molecular-beam epitaxy. Transmission electron microscopy
(TEM) of single GaAs/GaAsP NWs confirms their high crystal quality and shows
domination of the zinc-blende phase. This is further confirmed in optics of
single NWs, studied using cw and time-resolved photoluminescence (PL). A
detailed comparison with uncapped GaAs NWs emphasizes the effect of the GaAsP
capping in suppressing the non-radiative surface states: significant PL
enhancement in the core-shell structures exceeding 2000 times at 10K is
observed; in uncapped NWs PL is quenched at 60K whereas single core-shell
GaAs/GaAsP NWs exhibit bright emission even at room temperature. From analysis
of the PL temperature dependence in both types of NW we are able to determine
the main carrier escape mechanisms leading to the PL quench
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