Local modification of GaAs nanowires induced by laser heating

Raman characterization of single-crystalline Ga0.96Mn0.04As:Zn nanowires realized by ion-implantation

Recent progress in the realization of magnetic GaAs nanowires (NWs) doped with Mn has attracted a lot of attention due to their potential application in spintronics. In this work, we present a detailed Raman investigation of the structural properties of Zn doped GaAs (GaAs:Zn) and Mn-implanted GaAs:Zn (Ga_0.96Mn_0.04As:Zn) NWs. A significant broadening and redshift of the optical TO and LO phonon modes are observed for these NWs compared to as-grown undoped wires, which is attributed to strain induced by the Zn/Mn doping and to the presence of implantation-related defects. Moreover, the LO phonon modes are strongly damped, which is interpreted in terms of a strong LO phonon-plasmon coupling, induced by the free hole concentration. Moreover, we report on two new interesting Raman phonon modes (191 and 252 cm^-1) observed in Mn ion-implanted NWs, which we attribute to E_g (TO) and A_1g (LO) vibrational modes in a sheet layer of crystalline arsenic present on the surface of the NWs. This conclusion is supported by fitting the observed Raman shifts for the SO phonon modes to a theoretical dispersion function for a GaAs NW capped with a dielectric shell.

Photodegradation of Si-doped GaAs nanowire

Researching optical effects in nanowires may require a high pump intensity which under ambient conditions can degrade nanowires due to thermal oxidation. In this work we investigated the photodegradation of a single Si-doped GaAs nanowire by laser heating in air. To understand the changes that occurred on the nanowire we carried out Raman spectroscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, and photoluminescence spectroscopy in laser damaged regions as well as in non-affected ones. From Raman Stokes and anti-Stokes measurements we estimated the local temperature that the oxidation process of the nanowire (NW) surface starts at as 661 K, resulting in two new Raman modes at 200 cm⁻¹ and 259 cm⁻¹. Scanning electron microscopy and energy dispersive X-ray spectroscopy measurements showed a significant loss of arsenic in the oxidized regions, but no erosion of the nanowire. Micro-photoluminescence measurements showed the near-band-edge emission of GaAs along the nanowire, as well as a new emission band at 755 nm corresponding to polycrystalline β-Ga₂O₃ formation. Our results also indicate that neither amorphous As nor crystalline As were deposited on the surface of the nanowire. Combining different experimental techniques, this study showed the formation of polycrystalline β-Ga₂O₃ by oxidation of the nanowire surface and the limits for performing spectroscopic investigations on individual GaAs NWs under ambient air conditions.

In order to comprehend the photodegradation of GaAs NWs, we investigated their thermal oxidation process in air induced by laser heating in a broad local temperature range.

Understanding the effect of nanowire orientation on time evolution of Raman spectra from laser irradiated InAs nanowire surface

We investigate differences observed in the time evolution of Raman spectra for differently oriented (in plane) InAs nanowires (NWs), using polarized Raman spectroscopy. Specially designed polarized Raman spectroscopy experiments elucidate that laser irradiation leads to the formation of an oriented crystalline oxide film on the InAs NW surface. Both the formation of oriented crystalline oxides and Raman selection rules leading to the presence/absence of oxide peaks in the unpolarized Raman spectra are uncommon occurrences and can lead to incorrect interpretations of the oxidation process, if not looked into carefully. Further, the specially designed heating and cooling experiments for a mixed phase (wurtzite + zinc blende) InAs NW revealed the formation of specific allotropes of elemental As, i.e. gray-As (rhombohedral) and black-As (orthorhombic: metastable) at low (700-950 K) and high simulated temperatures (1000-1300 K) on the InAs NW surface, respectively. Both have high electrical conductivity due to a layered structure and control over the growth of only a few layers using laser irradiation envisages properties similar to graphene. This kind of surface of InAs NWs has the potential for novel device applications, where a semiconductor-insulator-metal heterostructure is required.

Unified mechanism of the surface Fermi level pinning in III-As nanowires

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 _x Ga_1-x As (0 ≤ x ≤ 0.45) and Ga _x In_1-x 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 _x Ga_1-x As and Ga _x In_1-x 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.

Synthesis and optoelectronic properties of quaternary GaInAsSb alloy nanosheets

Quasi-one-dimensional (1D) nanostructures have been extensively explored for electronic and optoelectronic devices on account of their unique morphologies and versatile physical properties. Here, we report the successful synthesis of GaInAsSb alloy nanosheets by a simple chemical vapor deposition method. The grown GaInAsSb alloy nanosheets are pure zinc-blende single crystals, which show nanosize-induced extraordinary optoelectronic properties as compared with bulk materials. μ-Raman spectra exhibit a multi-mode phonon vibration behavior with clear frequency shifts under varied laser power. Photoluminescence measurements reveal a strong light emission in the near-infrared region (1985 nm), and the obtained Varshni thermal coefficients α and β are smaller than those of the bulk counterparts due to the size confinement effect. In addition, photodetectors (PDs) based on these single-alloy nanosheets were constructed for the first time. The PDs show a strong response in the near-infrared region with the external quantum efficiency of 8.05 × 10⁴%, and the responsivity of 0.675 × 10³ A W^-1. These novel nanostructures would make contributions to the study of fundamental physical phenomena in quasi-1D nanomaterial systems and can be potential building blocks for optoelectronic and quantum devices.

Morphology and composition of oxidized InAs nanowires studied by combined Raman spectroscopy and transmission electron microscopy

Any device exposed to ambient conditions will be prone to oxidation. This may be of particular importance for semiconductor nanowires because of the high surface-to-volume ratio and only little is known about the consequences of oxidation for these systems. Here, we study the properties of indium arsenide nanowires which were locally oxidized using a focused laser beam. Polarization dependent micro-Raman measurements confirmed the presence of crystalline arsenic, and transmission electron microscopy diffraction showed the presence of indium oxide. The surface dependence of the oxidation was investigated in branched nanowires grown along the [Formula: see text] and [Formula: see text] wurtzite crystal directions exhibiting different surface facets. The oxidation did not occur at the [Formula: see text] direction. The origin of this selectivity is discussed in terms transition state kinetics of the free surfaces of the different crystal families of the facets and numerical simulations of the laser induced heating.

Resonant photo-thermal modification of vertical gallium arsenide nanowires studied using Raman spectroscopy

Gallium arsenide nanowires have shown considerable promise for use in applications in which the absorption of light is required. When the nanowires are oriented vertically, a considerable amount of light can be absorbed, leading to significant heating effects. Thus, it is important to understand the threshold power densities that vertical GaAs nanowires can support, and how the nanowire morphology is altered under these conditions. Here, resonant photo-thermal modification of vertical GaAs nanowires was studied using both Raman spectroscopy and electron microscopy techniques. Resonant waveguiding, and subsequent absorption of the excited optical mode reduces the irradiance vertical GaAs nanowires can support relative to horizontal ones, by three orders of magnitude before the onset of structural changes occur. A power density of only 20 W mm(-2) was sufficient to induce local heating in the nanowires, resulting in the formation of arsenic species. Upon further increasing the power, a hollow nanowire morphology was realized. These findings are pertinent to all optical applications and spectroscopic measurements involving vertically oriented GaAs nanowires. Understanding the optical absorption limitations, and the effects of exceeding these limitations will help improve the development of all III-V nanowire devices.

Enhanced photothermal conversion in vertically oriented gallium arsenide nanowire arrays

The photothermal properties of vertically etched gallium arsenide nanowire arrays are examined using Raman spectroscopy. The nanowires are arranged in square lattices with a constant pitch of 400 nm and diameters ranging from 50 to 155 nm. The arrays were illuminated using a 532 nm laser with an incident energy density of 10 W/mm(2). Nanowire temperatures were highly dependent on the nanowire diameter and were determined by measuring the spectral red-shift for both TO and LO phonons. The highest temperatures were observed for 95 nm diameter nanowires, whose top facets and sidewalls heated up to 600 and 440 K, respectively, and decreased significantly for the smaller or larger diameters studied. The diameter-dependent heating is explained by resonant coupling of the incident laser light into optical modes of the nanowires, resulting in increased absorption. Photothermal activity in a given nanowire diameter can be optimized by proper wavelength selection, as confirmed using computer simulations. This demonstrates that the photothermal properties of GaAs nanowires can be enhanced and tuned by using a photonic lattice structure and that smaller nanowire diameters are not necessarily better to achieve efficient photothermal conversion. The diameter and wavelength dependence of the optical coupling could allow for localized temperature gradients by creating arrays which consist of different diameters.

Raman spectroscopy of self-catalyzed GaAs(1-x)Sb(x) nanowires grown on silicon

Thanks to their wide band structure tunability, GaAs(1-x)Sb(x) nanowires provide exciting perspectives in optoelectronic and energy harvesting applications. The control of composition and strain of these ternary alloys is crucial in the determination of their optical and electronic properties. Raman scattering provides information on the vibrational properties of materials, which can be related to the composition and strain. We present a systematic study of the vibrational properties of GaAs(1-x)Sb(x) nanowires for Sb contents from 0 to 44%, as determined by energy-dispersive x-ray analyses. We find that optical phonons red-shift with increasing Sb content. We explain the shift by alloying effects, including mass disorder, dielectric changes and ionic plasmon coupling. The influence of Sb on the surface optical modes is addressed. Finally, we compare the luminescence yield between GaAs and GaAs(1-x)Sb(x), which can be related to a lower surface recombination rate. This work provides a reference for the study of ternary alloys in the form of nanowires, and demonstrates the tunability and high material quality of gold-free ternary antimonide nanowires directly grown on silicon.

E(1)(A) electronic band gap in wurtzite InAs nanowires studied by resonant Raman scattering

We report on resonant Raman experiments carried out on wurtzite InAs nanowires. Resonant conditions have been obtained by tuning either the excitation energy or the band gap through external high pressure at fixed excitation energy. A complete azimuthal study of the Raman spectra with two laser excitation lines (2.41 and 1.92 eV) has also been performed on a single wire. The measured E2(H) mode resonance indicates that the E1(A) gap is about 2.4 eV, which is considerably reduced with respect to the zinc-blende InAs E1 gap. These findings confirm recent theoretical calculations of crystal phase induced bandstructure modifications.

Raman study on zinc-blende single InAs nanowire grown on Si (111) substrate

We report polarized Raman scattering studies on single InAs nanowires (NWs). The NWs were grown by metalorganic chemical vapor deposition on Si (111) substrates without external catalyst and showed a zinc-blende crystal structure. The single NWs were studied for different polarization excitation of the incident laser beam relative to the NW axis. The transverse optical (TO) mode exhibits maximum intensity when both the incident and analyzed light polarizations are parallel to the NW axis. The TO mode of InAs NWs is found to act like a nearly perfect dipole antenna, which can be attributed to the one-dimensional NW geometry and Raman selection rules.

Nanowire heating by optical electromagnetic irradiation

The dissipative absorption of electromagnetic energy by 1D nanoscale structures at optical frequencies is applicable to several important phenomena, including biomedical photothermal theranostics, nanoscale photovoltaic materials, atmospheric aerosols, and integrated photonic devices. Closed-form analytical calculations are presented for the temperature rise within infinite circular cylinders with nanometer-scale diameters (nanowires) that are irradiated at right angles by a continuous-wave laser source polarized along the nanowire's axis. Solutions for the heat source are compared to both numerical finite-difference time domain (FDTD) simulations and well-known Mie scattering cross sections for infinite cylinders. The analysis predicts that the maximum temperature increase is affected not only by the cylinder's composition and porosity but also by morphology-dependent resonances (MDRs) that lead to significant spikes in the local temperature at particular diameters. Furthermore, silicon nanowires with high thermal conductivities are observed to exhibit extremely uniform internal temperatures during electromagnetic heating to 1 part in 10(6), including cases where there are substantial fluctuations of the internal electric-field source term that generates the Joule heating. For a highly absorbing material such as carbon, much higher temperatures are predicted, the internal temperature distribution is nonuniform, and MDRs are not encountered.

Pressure tuning of the optical properties of GaAs nanowires

The tuning of the optical and electronic properties of semiconductor nanowires can be achieved by crystal phase engineering. Zinc-blende and diamond semiconductors exhibit pressure-induced structural transitions as well as a strong pressure dependence of the band gaps. When reduced to nanoscale dimensions, new phenomena may appear. We demonstrate the tuning of the optical properties of GaAs nanowires and the induction of a phase transition by applying an external pressure. The dependence of the E(0) gap on the applied pressure was measured, and a direct-to-indirect transition was found. Resonant Raman scattering was obtained by pressure tuning of the E(0) and the E(0) + Δ(SO) gaps with respect to the excitation energy. The resonances of the longitudinal optical modes LO and 2LO indicate the presence of electron-phonon Fröhlich interactions. These measurements show for the first time a variation of ionicity in GaAs when in nanowire form. Furthermore, the dependence of the lattice constant on applied pressure was estimated. Finally, we found a clear indication of a structural transition above 16 GPa.