Nondestructive Complete Mechanical Characterization of Zinc Blende and Wurtzite GaAs Nanowires Using Time-Resolved Pump-Probe Spectroscopy

Microscopic Understanding of the Growth and Structural Evolution of Narrow Bandgap III-V Nanostructures

III–V group nanomaterials with a narrow bandgap have been demonstrated to be promising building blocks in future electronic and optoelectronic devices. Thus, revealing the underlying structural evolutions under various external stimuli is quite necessary. To present a clear view about the structure–property relationship of III–V nanowires (NWs), this review mainly focuses on key procedures involved in the synthesis, fabrication, and application of III–V materials-based devices. We summarized the influence of synthesis methods on the nanostructures (NWs, nanodots and nanosheets) and presented the role of catalyst/droplet on their synthesis process through in situ techniques. To provide valuable guidance for device design, we further summarize the influence of structural parameters (phase, defects and orientation) on their electrical, optical, mechanical and electromechanical properties. Moreover, the dissolution and contact formation processes under heat, electric field and ionic water environments are further demonstrated at the atomic level for the evaluation of structural stability of III–V NWs. Finally, the promising applications of III–V materials in the energy-storage field are introduced.

Ultrafast carrier relaxation dynamics of photoexcited GaAs and GaAs/AlGaAs nanowire array

Femtosecond optical pump-probe spectroscopy is employed to elucidate the ultrafast carrier nonradiative relaxation dynamics of bare GaAs and a core-shell GaAs/AlGaAs semiconductor nanowire array. Different from the single nanowire conventionally used for the study of ultrafast dynamics, a simple spin coating and peeling off method was performed to prepare transparent organic films containing a vertical oriented nanowire array for transient absorption measurement. The transient experiment provides the direct observation of carrier thermalization, carrier cooling, thermal dissipation and band-gap energy evolutions along with the carrier relaxations. Carrier thermalization occurs within sub-0.5 ps and proceeds almost independently on the AlGaAs-coating, while the time constants of carrier cooling and thermal dissipation are increased by an order of magnitude due to the AlGaAs-coating effect. The concomitant band-gap evolutions in GaAs and GaAs/AlGaAs include an initial rapid red-shift in thermalization period, followed by a slow blue and/or red shift in carrier cooling, and then by an even slower blue shift in thermal dissipation. The evolution is explained by the competition of band-gap renormalization, plasma screening and band-filling. These findings are significant for understanding the basic physics of carrier scattering, and also for the development of flexible optoelectronic devices.

Crystal phase engineering of self-catalyzed GaAs nanowires using a RHEED diagram

It is well known that the crystalline structure of the III-V nanowires (NWs) is mainly controlled by the wetting contact angle of the catalyst droplet which can be tuned by the III and V flux. In this work we present a method to control the wurtzite (WZ) or zinc-blende (ZB) structure in self-catalyzed GaAs NWs grown by molecular beam epitaxy, using in situ reflection high energy electron diffraction (RHEED) diagram analysis. Since the diffraction patterns of the ZB and WZ structures differ according to the azimuth [11̄0], it is possible to follow the evolution of the intensity of specific ZB and WZ diffraction spots during NW growth as a function of the growth parameters such as the Ga flux. By analyzing the evolution of the WZ and ZB spot intensities during NW growth with specific changes of the Ga flux, it is then possible to control the crystal structure of the NWs. ZB GaAs NWs with a controlled WZ segment have thus been realized. Using a semi-empirical model for the NW growth and our in situ RHEED measurements, the critical wetting angle of the Ga catalyst droplet for the structural transition is deduced.

Effect of hydrogen chloride etching on carrier recombination processes of indium phosphide nanowires

Introduction of in situ HCl etching to an epitaxial growth process has been shown to suppress radial growth and improve the morphology and optical properties of nanowires. In this paper, we investigate the dynamics of photo-generated charge carriers in a series of indium phosphide nanowires grown with varied HCl fluxes. Time resolved photo-induced luminescence, transient absorption and time resolved terahertz spectroscopy were employed to investigate charge trapping and recombination processes in the nanowires. Since the excitation photons generate charges predominantly in less than a half length of the nanowires, we can selectively assess the charge carrier dynamics at their top and bottom. We found that the photoluminescence decay is dominated by the decay of the mobile hole population due to trapping, which is affected by the HCl etching. The hole trapping rate is in general faster at the top of the nanowires than at the bottom. In contrast, electrons remain highly mobile until they recombine non-radiatively with the trapped holes. The slowest hole trapping as well as the least efficient non-radiative recombination was recorded for etching using the HCl molar fraction of χ_HCl = 5.4 × 10^-5.

Gigahertz acoustic vibrations of Ga-doped ZnO nanoparticle array

In this work, we present an experimental study on the acoustic vibrations of ZnO nanoparticles array with different concentration of Ga dopings by using femtosecond pump-probe technique. The Ga-doped ZnO (GZO) nano-triangle particles with the sizes of 190, 232 and 348 nm are fabricated by nanosphere lithography and pulsed laser deposition method. The result indicates that the frequency of acoustic vibrations of GZO nanoparticles decrease as the Ga-concentration is increased. Importantly, the vibration period of the GZO nanoparticles at the same Ga doping concentration show a nonlinear increase as the nanoparticle size is increased, which is different from the common linear dependency in undoped ZnO nanoparticles. It may be attributed to the crystal structure distortion and elastic characteristics variation due to Ga doping, and the elastic modulus at 7.3% Ga doping is decreased by 30%-60% for GZO nanoparticles with different sizes. The study can be very helpful for evaluating the crystal structure distortion and elastic characteristics of doped nano-materials with optical methods. Besides, it can offer a complementary method of thermal management in ZnO based optoelectronic devices.

Evaluation of Effective Elastic Properties of Nitride NWs/Polymer Composite Materials Using Laser-Generated Surface Acoustic Waves

In this paper we demonstrate a high potential of transient grating method to study the behavior of surface acoustic waves in nanowires-based composite structures. The investigation of dispersion curves is done by adjusting the calculated dispersion curves to the experimental results. The wave propagation is simulated using the explicit integral and asymptotic representations for laser-generated surface acoustic waves in layered anisotropic waveguides. The analysis of the behavior permits to determine all elastic constants and effective elastic moduli of constituent materials, which is important both for technological applications of these materials and for basic scientific studies of their physical properties.

Length measurement and spatial orientation reconstruction of single nanowires

The accurate determination of the geometrical features of quasi one-dimensional nanostructures is mandatory for reducing errors and improving repeatability in the estimation of a number of geometry-dependent properties in nanotechnology. In this paper a method for the reconstruction of length and spatial orientation of single nanowires (NWs) is presented. Those quantities are calculated from a sequence of scanning electron microscope (SEM) images taken at different tilt angles using a simple 3D geometric model. The proposed method is evaluated on a collection of SEM images of single GaAs NWs. It is validated through the reconstruction of known geometric features of a standard reference calibration pattern. An overall uncertainty of about 1% in the estimated length of the NWs is achieved.

Coherent Phonon Transport Measurement and Controlled Acoustic Excitations Using Tunable Acoustic Phonon Source in GHz-sub THz Range with Variable Bandwidth

We experimentally demonstrated a narrowband acoustic phonon source with simultaneous tunabilities of the centre frequency and the spectral bandwidth in the GHz-sub THz frequency range based on photoacoustic excitation using intensity-modulated optical pulses. The centre frequency and bandwidth are tunable from 65 to 381 GHz and 17 to 73 GHz, respectively. The dispersion of the sound velocity and the attenuation of acoustic phonons in silicon dioxide (SiO₂) and indium tin oxide (ITO) thin films were investigated using the acoustic phonon source. The sound velocities of SiO₂ and ITO films were frequency-independent in the measured frequency range. On the other hand, the phonon attenuations of both of SiO₂ and ITO films showed quadratic frequency dependences, and polycrystalline ITO showed several times larger attenuation than those in amorphous SiO₂. In addition, the selective excitation of mechanical resonance modes was demonstrated in nanoscale tungsten (W) film using acoustic pulses with various centre frequencies and spectral widths.