Polarization fields in GaN/AlN nanowire heterostructures studied by Off axis holography

In this work, we present an off-axis holography study of GaN/AlN heterostructured nanowires grown by plasma-assisted molecular-beam epitaxy. We discuss the sample preparation of nanowire samples for electron holography and combine potential profiles obtained using holography with theoretical calculations of the projected potential in order to gain understanding of the potential distribution in these nanostructures. The effects of surface states are discusse

Cathodoluminescence of stacking fault bound excitons for local probing of the exciton diffusion length in single GaN nanowires

We perform correlated studies of individual GaN nanowires in scanning electron microscopy combined to low temperature cathodoluminescence, microphotoluminescence, and scanning transmission electron microscopy. We show that some nanowires exhibit well localized regions emitting light at the energy of a stacking fault bound exciton (3.42 eV) and are able to observe the presence of a single stacking fault in these regions. Precise measurements of the cathodoluminescence signal in the vicinity of the stacking fault give access to the exciton diffusion length near this location

UV Photosensing Characteristics of Nanowire-Based GaN/AlN Superlattices

We have characterized the photodetection capabilities of single GaN nanowires incorporating 20 periods of AlN/GaN:Ge axial heterostructures enveloped in an AlN shell. Transmission electron microscopy confirms the absence of an additional GaN shell around the heterostructures. In the absence of a surface conduction channel, the incorporation of the heterostructure leads to a decrease of the dark current and an increase of the photosensitivity. A significant dispersion in the magnitude of dark currents for different single nanowires is attributed to the coalescence of nanowires with displaced nanodisks, reducing the effective length of the heterostructure. A larger number of active nanodisks and AlN barriers in the current path results in lower dark current and higher photosensitivity, and improves the sensitivity of the nanowire to variations in the illumination intensity (improved linearity). Additionally, we observe a persistence of the photocurrent, which is attributed to a change of the resistance of the overall structure, particularly the GaN stem and cap sections. In consequence, the time response is rather independent of the dark current.Comment: This document is the unedited Author's version of a Submitted Work that was subsequently accepted for publication in Nano Letters (2016), copyright (C) American Chemical Society after peer review. To access the final edited and published work see http://dx.doi.org/10.1021/acs.nanolett.6b0080

Assessment of active dopants and p-n junction abruptness using in-situ biased 4D-STEM

A key issue in the development of high-performance semiconductor devices is the ability to properly measure active dopants at the nanometer scale. 4D scanning transmission electron microscopy and off-axis electron holography have opened up the possibility of studying the electrostatic properties of a p-n junction with nm-scale spatial resolution. The complete description of a p-n junction must take into account the precise evolution of the concentration of dopants around the junction, since the sharpness of the dopant transition directly influences the built-in potential and the maximum electric field. Here, a contacted silicon p-n junction is studied through in-situ biased 4D-STEM. Measurements of electric field, built-in voltage, depletion region width and charge density in the space charge region are combined with analytical equations as well as finite-element simulations in order to evaluate the quality of the junction interface. The nominally-symmetric, highly doped ($N_A = N_D = 9\space x \space10^{18} cm^{-3}$) junction presents an electric field and built-in voltage much lower than expected for an abrupt junction. These experimental results are consistent with electron holography data. All measured junction parameters are compatible with the presence of an intermediate region with a graded profile of the dopants at the p-n interface. This hypothesis is also consistent with the evolution of the electric field with bias. These results demonstrate that in-situ biased 4D-STEM enables a better understanding of the electrical properties of semiconductor p-n junctions with nm-scale resolution.Comment: 13 pages, 5 figure

Optical properties of single ZnTe nanowires grown at low temperature

Optically active gold-catalyzed ZnTe nanowires have been grown by molecular beam epitaxy, on a ZnTe(111) buffer layer, at low temperature 350\degree under Te rich conditions, and at ultra-low density (from 1 to 5 nanowires per micrometer^{2}. The crystalline structure is zinc blende as identified by transmission electron microscopy. All nanowires are tapered and the majority of them are oriented. Low temperature micro-photoluminescence and cathodoluminescence experiments have been performed on single nanowires. We observe a narrow emission line with a blue-shift of 2 or 3 meV with respect to the exciton energy in bulk ZnTe. This shift is attributed to the strain induced by a 5 nm-thick oxide layer covering the nanowires, and this assumption is supported by a quantitative estimation of the strain in the nanowires

Residual strain and piezoelectric effects in passivated GaAs/AlGaAs core-shell nanowires

International audienceWe observe a systematic red shift of the band-edge of passivated GaAs/Al0.35Ga0.65As core-shell nanowires with increasing shell thickness up to 100 nm. The shift is detected both in emission and absorption experiments, reaching values up to 14 meV for the thickest shell nanowires. Part of this red shift is accounted for by the small tensile strain imposed to the GaAs core by the AlGaAs shell, in line with theoretical calculations. An additional contribution to this red shift arises from axial piezoelectric fields which develop inside the nanowire core due to Al fluctuations in the shell

Extraction of the homogeneous linewidth of the spectrally diffusing line of a CdSe/ZnSe quantum dot embedded in a nanowire

International audienceWe present a simple method to extract the homogeneous linewidth of a single photon emitter line exhibiting fast (down to 1 ns) spectral diffusion (SD). It is based on a recently developed technique using photon correlation measurements on half of the line. Here we show that the SD induced bunching depends on the ratio between the width of the homogeneous line and the spectral diffusion amplitude. Using this technique on a CdSe/ZnSe quantum dot, we investigate the temperature dependence of its fast SD amplitude and its homogeneous excitonic linewidt