Nanostructured semiconductors have unique physical properties that can have many applications, including optoelectronics, nanoelectronics, etc. Functionalization of nanoelectronic devices often requires specific electrical properties in different regions of the nanowire to form a p-n junction or ohmic contact. Such locally doped regions can be created by selective ion implantation. In the present work, we investigated the microstructure and optical properties of GaAs/GaAs:Si/AlxGa1-xAs core-shell nanowires after ion implantation and postimplantation annealing. GaAs/GaAs:Si/AlxGa1-xAs core-shell nanowires were implanted with sulfur ions at a fluence of 2.3 x 10(15) cm(-2). After ion implantation, the nanowires were subjected to flash-lamp annealing (FLA) for 3.2 ms or to conventional rapid thermal annealing (RTA) for 90 s. Raman and microstructural analyses indicate that FLA treatment with an energy density of 102 J cm(-2) can almost fully restore the original crystalline quality of the nanowires. On the other hand, photoluminescence (PL) measurements show that nanowires subjected to RTA exhibit a stronger emission intensity; however, RTA at 550 degrees C leads to severe decomposition of the AlxGa1-xAs shell.

Nanostructured semiconductors have unique physical properties that can have many applications, including optoelectronics, nanoelectronics, etc. Functionalization of nanoelectronic devices often requires specific electrical properties in different regions of the nanowire to form a p‐n junction or ohmic contact. Such locally doped regions can be created by selective ion implantation. In the present work, we investigated the microstructure and optical properties of GaAs/GaAs:Si/Al x Ga 1−x As core–shell nanowires after ion implantation and postimplantation annealing. GaAs/GaAs:Si/Al x Ga 1−x As core–shell nanowires were implanted with sulfur ions at a fluence of 2.3 × 10 15 cm −2 . After ion implantation, the nanowires were subjected to flash‐lamp annealing (FLA) for 3.2 ms or to conventional rapid thermal annealing (RTA) for 90 s. Raman and microstructural analyses indicate that FLA treatment with an energy density of 102 J cm −2 can almost fully restore the original crystalline quality of the nanowires. On the other hand, photoluminescence (PL) measurements show that nanowires subjected to RTA exhibit a stronger emission intensity; however, RTA at 550°C leads to severe decomposition of the Al x Ga 1−x As shell.

Determining the concentration of electrically active dopants in III-V core-shell nanowires has long been a challenge due to the difficulty of developing ohmic contact with the nanowire core. In this paper, we have used a noncontact optical method to estimate the electron concentration in Si-doped GaAs:Si/Al0.25Ga0.75As core-shell nanowires. The temperature-dependent photoluminescence (PL) spectra of these GaAs:Si/Al0.25Ga0.75As nanowires indicate that at 15 K, the carrier concentration in the conductive core of the core-shell nanowires can be as high as 1.40 x 1018 cm-3. The band-filling effect was considered to accurately determine the concentrations of carriers in the nanowires using optical methods. The highest electron density is achieved for a nominal Si concentration of 1.89 x 1019 cm-3, and a further increase in Si concentration reduces the effective doping level due to the amphoteric behaviour of Si in GaAs. The dependence of the integrated PL intensity on the laser power at T = 15 K exhibits a typical two-thirds power dependence, indicating the presence of a nonradiative Auger recombination mechanism in the nanowire under high carrier concentration.

We investigate ultrafast electron dynamics in individual GaAs/InGaAs core-shell nanowires using near-infrared pump-mid-infrared probe nanospectroscopy based on a scattering-type scanning near-field technique. Our results reveal a distinct blue shift in plasmon resonance frequency induced by photodoping. By extracting time-dependent electron densities and scattering rates, we gain insights into the effects of chemical doping and nanowire surface states on recombination dynamics and carrier mobility. Varying the pump power over two orders of magnitude reveals carrier recombination times in the range from a few ps at high power to 100 ps at low power, dominated by bimolecular recombination. Our findings highlight the potential of time-resolved nanoscopy for contactless probing of free carrier mobility and recombination dynamics on a local scale in individual semiconductor nanostructures or nanodevices.

In the GaAs/Inx(Al,Ga)1-xAs core/shell nanowire (CSNW) geometry, narrow cores exhibit significant bandgap reduction and enhanced electron mobility because of their ability to sustain extreme tensile elastic strain. In such an elastic state, the coherency limits and the resulting physical properties of the nanowires are governed by the strain field distribution and plastic relaxation mechanisms. Using atomic-resolution transmission electron microscopy, we determined the three-dimensional strain field, critical misfit, and plastic relaxation relative to the indium content of the shell, while maintaining constant core-shell dimensions. The strain was mapped experimentally in both coherent and plastically relaxed nanowires with a core radius of 10 nm and thick shells and was compared to atomistic and continuum calculations. Our findings reveal that, while axial strains remain uniform, elastic relaxation induces radial and tangential strain gradients. This is attributed to the strain concentration at the sharp interfaces, which persisted even after plastic relaxation. For the pertinent growth conditions, the maximum sustained elastic strain in the cores was observed for the GaAs/In0.5Al0.5As nanowires. The plastic relaxation of nanowires with shells of high indium content involved Frank partials delimiting horizontal intrinsic stacking faults (SFs), misfit dislocations gliding on inclined close-packed planes, and stair-rod dislocations along SF junction lines attributed to nanowire bending.Ab initiocalculations showed that the heterojunction remained type I even for the highest elastic strain, despite the existence of strain gradients at the core-shell interface. Our results elucidate the elastoplastic behaviour of CSNWs with narrow cores, offering new perspectives on growth strategies to further push their coherency limits.

In material systems such as GaAs core/InxAl1-xAs shell nanowires, the elastic strain and the resulting electronic properties of the core can be continuously tailored via the selection of the ternary shell composition, extending the functionality range of the core material. Typically, the nanowire axis is oriented along [1(-)1(-)1(-)], and the epitaxial growth of the shell onto the {11(-)0} sidewalls of the core is performed at quite low temperatures, following common knowledge from thin film technology. Such conditions achieve smooth surfaces, though at the potential cost of the structural quality. Here, we examine the relation between the shell growth temperature and the electronic properties of the core, as the latter are dictated by the quality of the strained core/shell interface. We demonstrate lower rates of non-radiative recombination and scattering of electrons at the interface in nanowires with higher shell growth temperatures, reaching optimal values at 500 degrees C. Above this temperature, the electronic quality of the interface degrades in correlation with compositional and morphological deviations that occur in the shell. These findings manifest the key role of the shell growth temperature in the optimization of the electronic properties toward the realization of functional devices with low non-radiative carrier recombination and high electron mobility.

We report THz-pump / mid-infrared probe near-field studies on Si-doped GaAs-InGaAs core-shell nanowires utilizing THz radiation from the free-electron laser FELBE. Upon THz excitation of free carriers, we observe a red shift of the plasma resonance in both amplitude and phase spectra, which we attribute to the heating up of electrons in the conduction band. The simulation of heated electron distributions anticipates a significant electron population in both L- and X-valleys. The two-temperature model is utilized for a quantitative analysis of the dynamics of the electron gas temperature under THz pumping at various power levels.

Semiconductor nanowires have inspired plenty of novel nanotechnology device concepts in photonics, electronics, and sensing, owing to their unique functionalities and integrability in heterogeneous platforms. Lattice-mismatched core/shell heterostructures, in particular, open new avenues for strain engineering and material properties modification. A notable case is the widely tunable tensile strain in the core of GaAs/InxAl1-xAs core/shell nanowires, which can be used to tailor the GaAs bandgap for applications across near-infrared, like optical fiber telecommunication, imaging, photovoltaics, etc. As it is shown here, though, the bandgap narrowing under high tensile strain in the GaAs core is accompanied by fast non-radiative recombination, which is undesirable for any device application. The limiting role of the lattice-mismatched core/shell interface is revealed, and a novel core/dual-shell heterostructure that employs an intermediate AlyGa1-yAs shell (spacer) is proposed. This spacer decouples the GaAs/AlyGa1-yAs interface, which confines electrons and holes into GaAs, from the lattice-mismatched AlyGa1-yAs/InxAl1-xAs one, whereas the strain in GaAs is unaffected. Choosing the optimal spacer thickness, the photoluminescence yield increases significantly, with longer emission decay lifetimes and slower carrier cooling rates. Besides unlocking the potential of GaAs for photonic applications across near-infrared, the proposed heterostructure concept can also be adopted for other material systems. A novel core/dual-shell heterostructure with an AlGaAs spacer is proposed to separate the electronic and the strain interfaces, being beneficial for the optical performance in the strained core. Through time-resolved measurements, this solution enhances photoluminescence yield, extends emission decay lifetimes, and slows carrier cooling rates, broadening GaAs's applicability in photonics. image

As semiconductor devices approach dimensions at the atomic scale, controlling the compositional grading across heterointerfaces becomes paramount. Particularly in nanowire axial heterostructures, which are promising for a broad spectrum of nanotechnology applications, the achievement of sharp heterointerfaces has been challenging owing to peculiarities of the commonly used vapor-liquid-solid growth mode. Here, the grading of Al across GaAs/AlxGa1-xAs/GaAs heterostructures in self-catalyzed nanowires is studied, aiming at finding the limits of the interfacial sharpness for this technologically versatile material system. A pulsed growth mode ensures precise control of the growth mechanisms even at low temperatures, while a semiempirical thermodynamic model is derived to fit the experimental Al-content profiles and quantitatively describe the dependences of the interfacial sharpness on the growth temperature, the nanowire radius, and the Al content. Finally, symmetrical Al profiles with interfacial widths of 2-3 atomic planes, at the limit of the measurement accuracy, are obtained, outperforming even equivalent thin-film heterostructures. The proposed method enables the development of advanced heterostructure schemes for a more effective utilization of the nanowire platform; moreover, it is considered expandable to other material systems and nanostructure types.

We report on first THz-pump / MIR-probe SNOM studies on Si-doped GaAs-InGaAs core-shell NWs utilizing THz radiation from the free-electron laser FELBE. Upon intraband THz-pump we observe a red shift of the NW plasma resonance in both amplitude and phase spectra, while a controlled interband optical pumping induces a blue shift of the resonance. In both cases, the signal exponentially decays with a time constant of 4-5 ps. We attribute the blue shift to the contribution of photogenerated charge carriers, while the red shift is assigned to the heating of electrons in the conduction band accelerated by the THz electric field of the pump pulses and the subsequent increase of their effective mass due to the nonparabolic $\Gamma$-valley dispersion.

We utilize optical pump – THz probe spectroscopy to estimate electron mobility in strained GaAs/(In,Al)As core/shell nanowires. Our results demonstrate that strain-induced reduction of the effective electron mass leads to a remarkable increase of the mobility. The data analysis indicates an important role of the inhomogeneous plasmon broadening that may affect THz spectra of dense ensembles of nanowires.

InGaN/GaN quantum wells (QWs) with sub-nanometer thickness can be employed in short-period superlattices for bandgap engineering of efficient optoelectronic devices, as well as for exploiting topological insulator behavior in III-nitride semiconductors. However, it had been argued that the highest indium content in such ultra-thin QWs is kinetically limited to a maximum of 33%, narrowing down the potential range of applications. Here, it is demonstrated that quasi two-dimensional (quasi-2D) QWs with thickness of one atomic monolayer can be deposited with indium contents far exceeding this limit, under certain growth conditions. Multi-QW heterostructures were grown by plasma-assisted molecular beam epitaxy, and their composition and strain were determined with monolayer-scale spatial resolution using quantitative scanning transmission electron microscopy in combination with atomistic calculations. Key findings such as the self-limited QW thickness and the non-monotonic dependence of the QW composition on the growth temperature under metal-rich growth conditions suggest the existence of a substitutional synthesis mechanism, involving the exchange between indium and gallium atoms at surface sites. The highest indium content in this work approached 50%, in agreement with photoluminescence measurements, surpassing by far the previously regarded compositional limit. The proposed synthesis mechanism can guide growth efforts towards binary InN/GaN quasi-2D QWs.

Transistor concepts based on semiconductor nanowires promise high performance, lower energy consumption and better integrability in various platforms in nanoscale dimensions. Concerning the intrinsic transport properties of electrons in nanowires, relatively high mobility values that approach those in bulk crystals have been obtained only in core/shell heterostructures, where electrons are spatially confined inside the core. Here, it is demonstrated that the strain in lattice-mismatched core/shell nanowires can affect the effective mass of electrons in a way that boosts their mobility to distinct levels. Specifically, electrons inside the hydrostatically tensile-strained gallium arsenide core of nanowires with a thick indium aluminium arsenide shell exhibit mobility values 30-50 % higher than in equivalent unstrained nanowires or bulk crystals, as measured at room temperature. With such an enhancement of electron mobility, strained gallium arsenide nanowires emerge as a unique means for the advancement of transistor technology.

Charge transport in GaAs/InGaAs nanowires is studied using high-field terahertz pulses. With increasing terahertz field, the plasmon resonance redshifts and loses its spectral weight. The results provide evidence for inhomogeneous intervalley scattering across the nanowire.

We probe the electron transport properties in the shell of GaAs/In0.2Ga0.8As core/shell nanowires at high electric fields using optical pump/THz probe spectroscopy with broadband THz pulses and peak electric fields up to 0.6 MV/cm. The plasmon resonance of the photoexcited charge carriers exhibits a systematic redshift and a suppression of its spectral weight for THz driving fields exceeding 0.4 MV/cm. This behavior is attributed to the intervalley electron scattering that results in the doubling of the average electron effective mass. Correspondingly, the electron mobility at the highest fields drops to about half of the original value. We demonstrate that the increase of the effective mass is nonuniform along the nanowires and takes place mainly in their middle part, leading to a spatially inhomogeneous carrier response. Our results quantify the nonlinear transport regime in GaAs-based nanowires and show their high potential for development of nanodevices operating at THz frequencies.

We report nonlinear charge-carrier response in GaAs/InGaAs core/shell nanowires that are driven by intense THz pulses. In the first experiment, half-cycle THz pulses emitted from an organic DSTMS crystal lead to a red-shift of the plasmon peak indicating intervalley transfer of the electrons. In the second experiment, a single, highly electron doped nanowire is investigated by scattering near-field infrared microscopy using intense free-electron laser (FEL) pulses. Here the observed red shift of the mid-infrared plasma resonance depends on the pulse energy and can be explained by heating the electron system in the nonparabolic conduction band.

We show nonlinear plasmonic response in GaAs/In0.2Ga0.8As nanowires using high-field terahertz pulses. With increasing THz field, plasmon resonance redshifts and its spectral weight decreases indicating a spatially inhomogeneous intervalley electron scattering.

We report the observation of the intersubband AC-Stark effect in a single wide GaAs/AlGaAs quantum well. In a three-level configuration, the n = 2 to n = 3 intersubband transition is resonantly pumped at 3.5 THz using a free-electron laser. The induced spectral changes are probed using THz time-domain spectroscopy with a broadband pulse extending up to 4 THz. We observe an Autler-Townes splitting at the 1 - 2 intersubband transition as well as an indication of a Mollow triplet at the 2 - 3 transition, both evidencing the dressed states. For longer delay times, a relaxation of the hot-electron system with a time constant of around 420 ps is measured.

A multiple quantum-well semiconductor saturable absorber mirror (MQW-SESAM) structure has been investigated by femtosecond pump-probe laser spectroscopy at a central wavelength of around 1050 nm. Coherent acoustic phonons are generated and detected over a wide frequency range from ~15 GHz to ~800 GHz. In the optical absorption region, i.e., in the multiple quantum wells (In0.27Ga0.73As), acoustic frequency combs centered at ~365 GHz, with a comb spacing of ~33 GHz, are generated. Most importantly, in the transparent region, i.e., in the distributed Bragg reflector, which is formed by a non-doped long-period semiconductor GaAs/Al0.95Ga0.05As superlattice, the mini-Brillouin-zone center, as well as zone-edge acoustic modes, are observed. The mini-zone-center modes with a fundamental frequency of 32 GHz can be attributed to the spatial modulation of the pump optical interference field with a period very close to that of the distributed Bragg reflector, in combination with the periodic spatial modulation of the electrostriction coefficient in the distributed Bragg reflector. The excitation of mini-zone-edge modes is attributed to the stimulated subharmonic decay of the fundamental center modes. Their subsequent back-folding to the mini-Brillouin-zone center makes them Raman active for the probe light.