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

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

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

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 $n=0$ Landau level emission line with the $n=2$ 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

Structural and optical properties of high quality zinc-blende/wurtzite GaAs hetero-nanowires

The structural and optical properties of 3 different kinds of GaAs nanowires with 100% zinc-blende structure and with an average of 30% and 70% wurtzite are presented. A variety of shorter and longer segments of zinc-blende or wurtzite crystal phases are observed by transmission electron microscopy in the nanowires. Sharp photoluminescence lines are observed with emission energies tuned from 1.515 eV down to 1.43 eV when the percentage of wurtzite is increased. The downward shift of the emission peaks can be understood by carrier confinement at the interfaces, in quantum wells and in random short period superlattices existent in these nanowires, assuming a staggered band-offset between wurtzite and zinc-blende GaAs. The latter is confirmed also by time resolved measurements. The extremely local nature of these optical transitions is evidenced also by cathodoluminescence measurements. Raman spectroscopy on single wires shows different strain conditions, depending on the wurtzite content which affects also the band alignments. Finally, the occurrence of the two crystallographic phases is discussed in thermodynamic terms.Comment: 24 page