Based on the high-angle annular dark-field scanning transmission electron microscopy and energy dispersive X-ray spectroscopy studies, we unravel the origin of spontaneous core–shell AlGaAs nanowires grown by gold-assisted molecular beam epitaxy. Our AlGaAs nanowires have a cylindrical core and a tapered shell. The composition of the shell is close to nominal, while the aluminum content in the core is systematically smaller than nominal. After switching off the group III fluxes, the aluminum content in the droplet and in the topmost part of the nanowire rapidly tends to zero, while gallium remains there at a high percentage. We present a quantitative model to explain these findings. Lower aluminum composition in the core is attributed to its lower surface diffusivity, with the aluminum collection length of 250 nm against 780 nm for gallium at the substrate temperature 510 °C and under the nominal aluminum content of 0.2. These values decrease to 8 and 160 nm when the nominal aluminum content is raised to 0.6. On the other hand, aluminum leaves the droplet at least 100 times faster than gallium, with a typical bonding rate with arsenic on the order of 1000 nm/s.
Quantum dots tuned to atomic resonances represent an emerging field of hybrid quantum systems where the advantages of quantum dots and natural atoms can be combined. Embedding quantum dots in nanowires boosts these systems with a set of powerful possibilities, such as precise positioning of the emitters, excellent photon extraction efficiency and direct electrical contacting of quantum dots. Notably, nanowire structures can be grown on silicon substrates, allowing for a straightforward integration with silicon-based photonic devices. In this work we show controlled growth of nanowire-quantumdot structures on silicon, frequency tuned to atomic transitions. We grow GaAs quantum dots in AlGaAs nanowires with a nearly pure crystal structure and excellent optical properties. We precisely control the dimensions of quantum dots and their position inside nanowires and demonstrate that the emission wavelength can be engineered over the range of at least 30 nm around 765 nm. By applying an external magnetic field, we are able to fine-tune the emission frequency of our nanowire quantum dots to the D 2 transition of 87 Rb. We use the Rb transitions to precisely measure the actual spectral line width of the photons emitted from a nanowire quantum dot to be 9.4 ± 0.7 μeV, under nonresonant excitation. Our work brings highly desirable functionalities to quantum technologies, enabling, for instance, a realization of a quantum network, based on an arbitrary number of nanowire single-photon sources, all operating at the same frequency of an atomic transition.
GaN nanowires were grown using selective area plasma-assisted molecular beam epitaxy on SiOx/Si(111) substrates patterned with microsphere lithography. For the first time, the temperature–Ga/N2 flux ratio map was established for selective area epitaxy of GaN nanowires. It is shown that the growth selectivity for GaN nanowires without any parasitic growth on a silica mask can be obtained in a relatively narrow range of substrate temperatures and Ga/N2 flux ratios. A model was developed that explains the selective growth range, which appeared to be highly sensitive to the growth temperature and Ga flux, as well as to the radius and pitch of the patterned pinholes. High crystal quality in the GaN nanowires was confirmed through low-temperature photoluminescence measurements.
Semiconducting nanowires, unlike bulk, can be grown in both wurtzite and zincblende crystal phases. this unique feature allows for growth and investigation of technologically important and previously unexplored materials, such as wurtzite AlGaAs. Here we grow a series of wurtzite AlGaAs nanowires with Al content varying from 0.1 to 0.6, on silicon substrates and through a comparative structural and optical analysis we experimentally derive, for the first time, the formula for the bandgap of wurtzite AlGaAs. Moreover, bright emission and short lifetime of our nanowires suggest that wurtzite AlGaAs is a direct bandgap material.Polytypism 1 is an exceptional property of nanowires and a new degree of freedom which enables the engineering of the electronic structure without change of material. For example, today's atomically-precise control over the crystal-phase switching in nanowires 2,3 allows to grow strain-free polytypic formations along the growth axis 4,5 , even small enough to form quantum dots 6,7 . The wurtzite phase is not observable at ambient conditions in bulk of any A III B V materials except for nitrides, while it can be obtained in nanowires. For this property and its technological implications, a great deal of attention has been drawn, in recent years, to nanowires system from scientific community 8-10 . However, for designing of novel structures and devices, knowledge of bandgaps and band alignments of the different crystal phases of new materials is crucial.In particular, Al X Ga 1-X As nanowires provide a promising platform for fabrication of advanced devices. For example, adding the Al component to the widely studied GaAs 11,12 allows to tune the emission in a wide range of wavelengths while, AlGaAs, having higher energy than GaAs, allows the combination of these two materials to fabricate strain-free quantum devices 13 .However, the knowledge about wurtzite AlGaAs is limited in the literature [14][15][16][17] , and is mainly grown as a shell around wurtzite GaAs core 15,16 . Importantly, the bandgap of wurtzite AlGaAs was neither predicted theoretically nor measured experimentally.In this work, we grow wurtzite AlGaAs nanowires, in a wide range of Al content x, and we present a comparative optical and structural study, empirically revealing the trend for the bandgap of wurtzite Al X Ga 1-X As. We grow our samples by Au-catalyzed vapor-liquid-solid technique in a molecular beam epitaxy (MBE) reactor (see methods section for details) obtaining high crystalline quality structures with any chosen Al content.
Generation of electric current is observed when GaAs nanowires with wurtzite crystal structure are bent by the probe of an atomic force microscope. The current originates from a piezo active phase in the nanowires due to the piezoelectric effect. Increasing of the piezo-potential in bent nanowires enhances tunneling through the probe-nanowire Schottky barrier due to the thermionic field emission. Laser illumination amplifies short-circuit current pulses by two orders of magnitude from 9 pA to 1 nA due to the piezo-phototronic effect. Utilization of such piezo-phototronic effect in GaAs nanowires is a solution to accelerate the efficiency of hybrid energy sources "piezoelectric nanogenerator À solar cell" comprised of III-V nanowires.
The data on growth peculiarities and physical properties of GaAs insertions embedded in AlGaAs nanowires grown on different (1 1 1) substrates by Au-assisted molecular beam epitaxy are presented. The influence of nanowires growth conditions on structural and optical properties is studied in detail. It is shown that by varying the growth parameters it is possible to form structures like quantum dots that emit in a wide wavelengths range. These quantum dots show sharp and intense emission lines when an optical signal is collected from a single nanowire. The technology proposed opens new possibilities for integration of direct-band A III B V materials on silicon platform.
Fermi level pinning at the oxidized (110) surfaces of III-As nanowires (GaAs, InAs, InGaAs, AlGaAs) is studied. Using scanning gradient Kelvin probe microscopy, we show that the Fermi level at oxidized cleavage surfaces of ternary Al Ga As (0 ≤ x ≤ 0.45) and Ga In As (0 ≤ x ≤ 1) alloys is pinned at the same position of 4.8 ± 0.1 eV with regard to the vacuum level. The finding implies a unified mechanism of the Fermi level pinning for such surfaces. Further investigation, performed by Raman scattering and photoluminescence spectroscopy, shows that photooxidation of the Al Ga As and Ga In As nanowires leads to the accumulation of an excess of arsenic on their crystal surfaces which is accompanied by a strong decrease of the band-edge photoluminescence intensity. We conclude that the surface excess arsenic in crystalline or amorphous forms is responsible for the Fermi level pinning at oxidized (110) surfaces of III-As nanowires.
Harvesting hybrid mechanical and solar ambient energy with one small device remains a challenge. Here, we report on producing electric current using a Schottky type metal-oxide-semiconductor structure formed by an n-InP layer covered with native oxide and an atomic force microscope (AFM) probe with a conductive coating. The tip’s sliding reciprocating motion during AFM scanning in contact mode produces a direct current signal in the probe-sample circuit. Two electric power generation mechanisms exist. A strong current was detected under sample illumination because of a photovoltaic effect with efficiency of 7% at the Si/InP heterojunction. Having the sample set in complete darkness, we observed current pulses of the opposite polarity, which suggests the existence of another mechanism not connected to photogeneration. This dark current originates from the tunneling of triboelectrically induced charge redistribution on the metal/oxide interface. The current polarity corresponds to electronic quantum mechanical tunneling through the oxide layer from the metal tip into InP. The current density exceeded 15 kA/m2. This is 2 and more than 4 orders greater than that in silicon- and polymer-based triboelectric nanogenerators, respectively. The open-circuit voltage value was 15 mV, and output electric power reached 110 W/m2. Understanding of triboelectric phenomena in photovoltaic semiconductor materials will allow creation of a new type of high-current hybrid energy devices that combine triboelectric nanogenerators and solar cells.
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