GaAs Nanostructures Research Articles

Nucleation-Limited Kinetics of GaAs Nanostructures Grown by Selective Area Epitaxy: Implications for Shape Engineering in Optoelectronics Devices.

The growth kinetics of vertical III-V nanowires (NWs) were clarified long ago. The increasing aspect ratio of NWs results in an increase in the surface area, which, in turn, enhances the material collection. The group III adatom diffusion from the NW sidewalls to the top sustains a superlinear growth regime. In this work, we report on the growth of different GaAs nanostructures by selective area MOVPE on GaAs (111)B substrates. We show that the opening dimensions and geometry qualitatively alter the morphology and height evolution of the structures. We compare the time evolution of vertical GaAs NWs stemming from circular holes and horizontal GaAs nanomembranes (NMs) growing from one-dimensional (1D) rectangular slits on the same substrate. While NW heights grow exponentially with time, NMs surprisingly exhibit sublinear kinetics. The absence of visible atomic steps on the top facets of both NWs and NMs suggests layer-by-layer growth in the mononuclear mode. We interpret these observations within a self-consistent growth model, which links the diffusion flux of Ga adatoms to the position- and shape-dependent nucleation rate on top of NWs and NMs. Specifically, the island nucleation rate is lower on top of the NMs than that on the NWs, resulting in the total diffusion flux being directed from the top facet to the sidewalls. This gives a sublinear height evolution for the NMs. These results open innovative perspectives for shape engineering of III-V nanostructures and new avenues for the design of optoelectronics and photonic devices.

Electron levels of defects in In(Ga)As/(In)GaAs nanostructures: A review

The data on electron levels induced by defects in In(Ga)As/(In)GaAs nanostructures, their localization, activation energy and identification have been systematically reviewed. Point defects inherent to GaAs and found in the (In)GaAs-based nanostructures have been listed, and their classification has been clarified, including EB3, EL2, EL3, EL4 (M4), EL5, EL6 (M3), EL7, EL8, EL9 (M2), EL10 (M1), EL11 (M0) and M00. The effect of the interfaces on the formation of different types of extended defects has been described. All the levels of electron traps found in heterostructures with quantum wells, wires and dots by deep level spectroscopies have been collected in a table with indication of the detection technique, object, location in the structure and their origin assumed. This overview can be useful as a reference material for researchers who study these nanostructures.

Enhanced surface passivation of GaAs nanostructures via an optimized SiO2 sol-gel shell growth

Gallium arsenide (GaAs) is an important compound semiconductor for optoelectronic devices. However, the fast nonradiative recombination velocity of GaAs requires surface passivation for the nano-optoelectronic and nanophotonic applications. Despite strides have been made in GaAs surface passivation, chemical treatments like sulfidation and nitridation suffer from reproducibility and stability issues, thus requiring protective layer encapsulation. Traditional vacuum based film passivation exposes samples to high temperature plasma, causing surface damage, and is expensive. Here, we demonstrate an effective passivation method via sol-gel growth of SiO2 shell on both dry etched GaAs nanopillar and bottom-up grown GaAs nanowires at room temperature with uniform coating and controlled thickness from sub-10 nm to around 100 nm, showing advantages in low cost as well as suitability for different types of nano morphology. X-ray photoelectron spectroscopy analysis confirms that the SiO2 sol-gel method with the addition of (NH4)2S can significantly remove the native oxides on the surface. Under the optimized SiO2 coating procedure, the emission intensity of dry etched GaAs nanopillar with a diameter of 0.5 μm is enhanced about 29.6 times and the decay lifetime of the minority carriers with a diameter of 1.8 μm increased from 43.6 to 477.5 ps. Thus, the easy fabrication, broad suitability, as well as effective passivation effect turn sol-gel growth of SiO2 into a cost-effective surface passivation approach for III–V nanostructures.

Metallic Nanoalloys on Vertical GaAs Nanowires: Growth Mechanisms and Shape Control of Ni-GaAs Compounds.

GaAs nanowires are promising candidates for emerging devices in a broad field of applications (e.g., nanoelectronics, photodetection, or photoconversion). These nanostructures benefit greatly from a vertical integration, as it allows for the exhibition of the entire nanowire surface. However, one of the main challenges related to vertical integration is the conception of an efficient method to create low resistive contacts at nanoscale without degrading the device performance. In this article, we propose a complementary metal-oxide-semiconductor (CMOS)-compatible approach to form alloyed contacts at the extremities of vertical GaAs nanowires. Ni-based and Pd-based alloys on different vertical GaAs nanostructures have been characterized by structural and chemical analyses to identify the phase and to study the growth mechanisms involved at the nanoscale. It is shown that the formation of the Ni3GaAs alloy on top of nanowires following the epitaxial relation Ni3GaAs(0001)∥GaAs(111) leads to a pyramidal shape with four faces. Finally, guidelines are presented to tune the shape of this alloy by varying the initial metal thickness and nanowire diameters. It will facilitate the fabrication of a nanoalloy structure with tailored shape characteristics to precisely align with a designated application.

Comparative optical analysis of GaAs nanostructures for photovoltaic applications using finite element method

In this manuscript, a thorough comparative analysis of six GaAs based nanostructures (hollow and solid) is performed on the basis of their optical performance. These nanostructures are known to exhibit excellent anti-reflection properties, owing to their ability to generate a broadband absorption spectrum through efficient photon harvesting. Using the Finite Element Method (FEM) of the commercially available COMSOL Multiphysics package, the absorption characteristics, optical short circuit current density (JSC), electric field and photogeneration rates of six different nanostructures namely concentric nanocylinder (CNCy), hollow concentric nanocylinder (HCNCy), inverted nanopencil (INPe), hollow nanopencil (HNPe), nanorod + nanohemisphere (NR + NHe), and hollow nanorod + hollow nanohemisphere (HNR + HNHe) are computed. The optical performance of these nanostructures is largely dependent on their geometrical parameters such as filling ratio (FR = Diameter/Period), spacing and structural dimensions. The optimized values of these parameters can play a vital role in capturing the optical resonance modes by the nanostructures to produce absorption enhancement. It has been observed that the nanostructures with base diameter of 240 nm, period in the range of 300–350 nm and FR of 0.8 exhibit better optical characteristics. Optical JSC and optical efficiency of 29.45 mA cm−2 and 42.26%, respectively for CNCy nanostructure with FR of 0.8 and diameter of 240 nm is the highest among all the nanostructures. The effect of the angle of incidence of the photons striking the nanostructures on the average absorptance in both Transverse Electric (TE) and Transverse Magnetic (TM) modes are also investigated. In addition to this, we have also computed the effective refractive index for all the nanostructures using Maxwell Garnett formula in order to estimate the surface anti-reflection characteristics of these nanostructures.

THz Response of Charge Carriers in Nanoparticles: Microscopic Master Equations Reveal an Unexplored Equilibration Current and Nonlinear Mobility Regimes

Herein, the THz mobility of charge carriers in low‐dimensional semiconductors based on a density matrix approach involving master equations for population and polarization dynamics is modeled. Pulsed THz fields induce intraband transitions between quantized subband states, creating polarization and subsequent charge transport that governs the electron mobility. It is shown that an equilibration current emerges—a purely quantum mechanical contribution understood via the Ehrenfest theorem in 1D—reshaping the low‐frequency mobility. Apart from thermal population, the results further demonstrate that the frequency‐dependent mobility becomes THz field strength and spectrum, as well as pulse width and chirp dependent, already at moderate THz probe fields of 1 kV cm−1, e.g., for 1D CdSe or GaAs nanostructures. The parametric nature of the underlying master differential equations for polarization and population, driving the intraband conductivity, results in a nonlinear, third‐order mobility and susceptibility, causing a nontrivial field dependence as well as power broadening even at moderate field strength. The obtained results are in good agreement with experiments. The observed high nonlinearities strongly impact the design and interpretation of THz charge carrier mobility experiments and further allow applications like coherent control, frequency mixing, or synthesis through a field‐controlled nonlinearity or high harmonics generation, especially interesting for future 6G telecommunication.

Nitrogen plasma passivation of GaAs nanowires resolved by temperature dependent photoluminescence

We demonstrate a significant improvement in the optical performance of GaAs nanowires achieved using a mixed nitrogen-hydrogen plasma which passivates surface states and reduces the rate of nonradiative recombination. This has been confirmed by time-resolved photoluminescence measurements. At room temperature, the intensity and lifetime of radiative recombination in the plasma-treated nanowires was several times greater than that of the as-grown GaAs nanowires. Low-temperature measurements corroborated these findings, revealing a dramatic increase in photoluminescence by two orders of magnitude. Photoelectron spectroscopy of plasma passivated nanowires demonstrated a yearlong stability achieved through the replacement of surface oxygen with nitrogen. Furthermore, the process removed the As0 defects observed on non-passivated nanowires which are known to impair devices. The results validate plasma as a nitridation technique suitable for nanoscale GaAs crystals. As a simple ex situ procedure with modest temperature and vacuum requirements, it represents an easy method for incorporating GaAs nanostructures into optoelectronic devices.

Investigation of the Optoelectronic Performance of GaAs Nanostructures for Solar Cell Applications

In this article, we have performed a comprehensive comparative analysis of the optoelectronic performance of five different GaAs based nanostructures namely cylindrical nanowire (CNW), truncated nanocone (TNC), hexagonal nanowire (HNW), truncated nanopyramid (TNP) and rectangular nanowire (RNW). These nanostructures can improve the photon harvesting capability and the absorption characteristics of the devices. The optical absorptance, optical Jsc, electric field and photogeneration rates of GaAs based nanostructures are computed using Wave Optics Module of the commercially available COMSOL Multiphysics package. The electrical performance of these nanostructures obtained in the form of J-V characteristics is analysed using the Semiconductor Module. The important geometrical parameters of the nanostructures such as period (P), diameter (D), Width (W), bottom W and D (Wbot/Dbot), top W and D (Wtop/Dtop), and filling ratio (FR) are varied in order to have a deep level understanding of the effect of the geometry of the nanostructures on their optical performance. The surface recombination velocity (SRV) of the p and n regions of the active material is varied to find out the impact on the electrical performance of the investigated nanostructures. We observed that the optical characteristics of the nanostructures are dependent on the geometrical dimensions, whereas the electrical characteristics depend substantially on the recombination mechanisms.

Comparative study of GaAs nanostructures synthesized in air and distilled water by picosecond pulsed laser ablation and application in hazardous molecules detection

This work explored the fundamental differences/mechanisms between the GaAs substrates ablated in two different media of air and distilled water (DW). A scan area of 5 × 5 mm2 was ablated by a picosecond laser with a pulse duration of 30 ps, a repetition rate of 10 Hz, a wavelength of 1064 nm, and a pulse energy of 2 mJ. The spacing between raster scan lines was varied (0.05–0.35 mm), keeping the scan speed (0.15 mm/s) constant. The obtained GaAs nanostructures (NSs) were thoroughly analyzed using microscopy techniques. A clear increase in separation between the raster scan lines was observed with an increase in the scan spacing for the GaAs NSs fabricated in air, whereas the same result was not observed in DW. Moreover, structures with debris were formed in air irrespective of the spacing, unlike the formation of uniform quasiperiodic GaAs NSs throughout the sample in the case of DW ablation. To the best of our knowledge, there are no reports on the detailed studies involving DW in the fabrication of quasiperiodic NSs of GaAs. Further, these quasiperiodic GaAs NSs formed in DW were coated with a thin layer of gold using the thermal evaporation method, annealed at 400 °C for 1 h in an ambient atmosphere. As a consequence of annealing, Au NPs were uniformly decorated on the quasiperiodic NSs of GaAs imparting plasmonic nature to the whole structures. Subsequently, the Au NPs decorated GaAs NSs were utilized as surface enhanced Raman scattering substrates for the detection of methylene blue (dye molecule) and Thiram (pesticide molecule) at low concentrations.

Multi-functional gallium arsenide nanoparticles and nanostructures fabricated using picosecond laser ablation

GaAs based nanomaterials have attracted significant attention owing to their applications in electronic, photonic, and sensing devices. In the present communication, we report on the simultaneous fabrication of GaAs nanoparticles (NPs) and nanostructures (NSs) in a single experiment achieved using picosecond pulsed laser ablation in liquids along with the demonstration of their versatile multifunctional applications. GaAs substrates are ablated in three different liquids of distilled water (DW), ethanol, and polyvinyl alcohol (PVA). The morphology and compositions of the synthesized NPs and NSs are analysed using various characterization techniques. Optical and nonlinear optical (NLO) studies are carried out for these NPs in the colloidal form which demonstrated the presence of two-photon absorption. Further, the ablated GaAs substrates exhibited quasi-periodic surface structures with wide angle anti-reflection property and hydrophobic nature. These NSs with a thin layer of gold acted as surface-enhanced Raman scattering (SERS) substrates for detecting trace level hazardous materials such as thiram (a pesticide molecule) and malachite green (a dye molecule). Furthermore, these NSs provided superior Raman enhancements with excellent reproducibility and reusability thereby evidencing their versatility. Collectively, this work reveals quite a few possible multifunctional applications of the laser ablation products of GaAs (NPs and NSs).

Study of the effect of ultra-low arsenic flux on the formation of In(As)/GaAs nanostructures by droplet epitaxy

In this paper, we present the results of studying the influence of arsenic pressure in the range of ultra-low values (10-7-10-6 Pa) on the processes of modification of In/GaAs(001) droplets with various initial sizes obtained by droplet epitaxy. We experimentally demonstrate that exposure of droplets to the ultralow arsenic flux makes it possible to reduce the droplet size to subcritical sizes while maintaining the initially specified surface density. The exposure of droplet nanostructures in the arsenic flux can be accompanied only by a decrease in their size, which is more typical for droplets obtained at large amounts of indium deposited material. For droplets with a smaller initial size, the formation of rings along the perimeter of the initial droplets and holes inside the rings is typical along with the droplet reduction. We also reveal that the dependence of the relative volume of droplets subjected to diffusion decay in the arsenic flux becomes more significant with a decrease in their initial size. Keywords: droplet epitaxy, nanostructures, In(As)/GaAs, arsenic flux.

Geometrical Optimization of Gallium Arsenide (GaAs) nanostructure based Solar Cells

In order to meet the demand of rising global power, the research on nanostructured solar cell is gaining momentum at a massive rate. Planar solar cells are less efficient compared to nanostructured solar cells, since nanostructured solar cells have large surface to volume ratio. We have considered GaAs based nanostructures like nanowire (NW), nanocone (NC), and truncated nanocone (TNC), which are designed with a substrate height of 200 nm, nanostructure height of 2000 nm, and diameter ranging from 60 nm to 220 nm with four different filling ratios (FR) of 0.3, 0.5, 0.7, and 0.9. We have investigated the absorption spectra, electric field profiles, and photogeneration profile of the proposed nanostructures for the wavelength range of 300–860 nm considering the Finite Element Method (FEM) as implemented in COMSOL Multiphysics. Considering the optical absorption spectra, we have calculated the optical short circuit density (Jsc) analytically. We have observed that the optical current density for the geometrically optimized nanowire, nanocone, and truncated nanocone are 27.85 mA/cm2, 30.78 mA/cm2 and 27.27 mA/cm2 respectively. This variation in optical current is due to the different light reflection, and absorption capability of the nanostructures, which is confirmed from the absorption, transmittance, and reflectance profile of the nanostructures.

Study of the initial stage of GaAs growth on FIB-modified silicon substrates

In this work, we studied the effect of the deposition thickness, growth rate, arsenic flux, and implantation dose on the morphology of the GaAs nanostructures grown on modified Si areas. It is shown that an increase in the growth rate at the initial stages of the growth process leads to the transition of the growth regime from layered-like to one-dimensional with the formation of nanowires. Studies of the effect of As4 pressure have shown that a change in the equivalent As4 flux in the range of 3.7 - 5.0 ML/s does not lead to any significant change in the structure of the GaAs layer in the modified areas. An increase in the implantation dose during processing with a focused ion beam led to disordering of the directions of the grown nanowires due to the degradation of the substrate crystal structure.

Quantitative Assessment of Carrier Density by Cathodoluminescence. I. GaAs Thin Films and Modeling

Doping is a fundamental property of semiconductors and constitutes the basis of modern microelectronic and optoelectronic devices. Their miniaturization requires contactless characterization of doping with nanometer scale resolution. Here, we use low- and room-temperature cathodoluminescence (CL) measurements to analyze p-type and n-type GaAs thin films over a wide range of carrier densities ($2\times 10^{17}$ to $1\times 10^{19}$ cm$^{-3}$). The spectral shift and broadening of CL spectra induced by shallow dopant states and band filling are the signature doping. We fit the whole spectral lineshapes with the generalized Planck's law and refined absorption models to extract the bandgap narrowing (BGN) and the band tail for both doping types, and the electron Fermi level for n doping. This work provides a rigorous method for the quantitative assessment of p-type and n-type carrier density using CL. Taking advantage of the high spatial resolution of CL, it can be used to map the doping in GaAs nanostructures, and it could be extended to other semiconductor materials.

Theoretical study on the optoelectronic properties of GaAs nanostructures with Al component gradient change

In order to improve the photoemission characteristics of the GaAs photocathode, GaAs nanostructures (nanoholes and nanowires) with three sublayers and Al component of gradient distribution are designed in this article. Finite element method was used to study the effects of Al component gradient interval, component range, height, diameter, and other parameters on light absorption of nanostructures. And the quantum efficiency of these nanostructures was calculated. The results show that the wide band absorption can be achieved with small gradient interval and component variation range, and the nanostructure can enhance the light capture ability with certain angle inclination. In the gradient component nanostructures, the thickness of the top most layer has a great influence on the quantum efficiency. Highlights The nanohole and nanowire structures with three sublayers and Al component gradient distribution were designed. The effects of gradient interval, range of Al components, and geometric parameters on the optical properties of nanostructures are discussed.

Comparison and analysis of optical properties between vertical and inclined GaAs nanostructures

In this paper, COMSOL multi-physics field commercial software was used to design the simulation model of GaAs nanostructures array (vertical nanoholes, vertical nanowires, inclined nanoholes and inclined nanowires), and the changes of light absorption of these structures in the wavelength range of 200–840 nm were studied. The electric field distribution, carrier distribution and quantum efficiency of nanostructures are calculated and analyzed under certain structural parameters. The results show that the light absorption performance of nanowire structure in the short-wave region is better than that of the nanoholes structure. With a certain inclined angle, the inclined nanowire structure has a stronger light capture ability than the vertical nanowire structure, and the light absorption of nanowire structure has a minimum value at the 550 nm wavelength.

Theoretical study on optoelectronic properties of GaAs nanostructures

In this paper, GaAs frustum of a cone, cone, frustum of a hexagonal pyramid and hexagonal pyramid array models were designed (frustum is a portion of a cone or pyramid after the top is removed), and the influence of geometric parameters on the photoelectric emission characteristics was studied. And we used MATLAB to calculate and analyze the influence of geometric parameters on the carrier distribution and quantum efficiency of nanostructures. It is found that increasing the bottom diameter of the pyramid can effectively improve the optoelectronic characteristics of the photocathode. Compared with the cone structure, the frustum structure array has better light absorption characteristics in the short wave region, and has better quantum efficiency.

Effect of doping Si on the structural, electronic and optical properties of GaAs nanostructures

In the recent years, with advances in material growth, there has been a considerable interest in the compound semiconductors of group III-V, in particular GaAs. Silicon (Si) is the most suitable substance for the n-gallium arsenide type [1]. In this study, the structural and electron properties of Ga6As4H10 and Ga6As3SiH10 nanocrystals are investigated using the quasi-potential and density functional formulation (DFT) method and with the approximation of LDA in the quantum espresso package. The results of the calculations show that the larger the size of the nanocrystal, the more the decrease of band gap. By replacing the Si atomic impurity by the As atom in the Ga6As4H10 nanocrystal, the energy gap becomes smaller than the non-degenerate state, and the fermi level approaches the edge of the conduction band, in which the Ga6As3SiH10 nanocrystal is a n-type semiconductor. The charge density of the charge around the atoms shows an ion-covalent bond between Si and Ga atoms. In this study, the optical properties of gallium arsenide nanocrystals have been investigated; calculations are performed with single-particle approximation. Gusin software is also used to obtain the optical spectrum of the nanocrystal. The optical spectrometry for gallium arsenide nanocrystals shows the transition to blue.

Synthesis of gallium arsenide nanostructures for solar cell applications

Gallium arsenide (GaAs) nanostructures are fabricated on gold coated silicon substrate with the help of GaAs ions generated by high density, high temperature and extremely non-equilibrium argon plasma in a modified dense plasma focus device. The nanostructures were found to have average dimension in range of 17 nm to 21 nm. It is found that the gold acts as a nucleating agent for growth of GaAs nanostructures. Emission peak in photoluminescence spectra confirms good quality of nanostructures related to its purity. These nanostructures are found to possess the high absorption in a wide optical range which will have applications in solar cell. Hall measurements reveal high carrier concentration of ~2.4 × 1019 cm−3. The electrical properties show open circuit voltage of 0.50 V, short circuit current density of 32.7 mA/cm2, fill factor of 80% and efficiency ~13.12%. The obtained optical and electrical transport properties indicate that the GaAs nanostructures so obtained are the potential candidate for solar cell applications.

Effect of initial crystallization temperature and surface diffusion on formation of GaAs multiple concentric nanoring structures by droplet epitaxy**Project supported by the National Natural Science Foundation of China (Grant Nos. 61564002 and 11664005), the Science and Technology Foundation of Guizhou Province, China (Grant No. QKH-[2017]1055), and Guizhou University Talent Foundation (Grant No. GDJHZ-[2015]23).

GaAs multiple concentric nano-ring structures (CNRs) are prepared with multistep crystallization procedures by droplets epitaxy on GaAs (001) to explore the influence of different initial crystallization temperatures on CNRs morphology. Atomic force microscope (AFM) images show that GaAs nanostructures are more likely to form elliptical rings due to diffusion anisotropy. Meanwhile, with the increase of initial crystallization temperature, the inner ring height and density of CNRs are increased, and outer rings are harder to form. In addition, the mechanism of formation of CNRs is discussed by classical nucleation theory and diffusion theory. The method can be used to calculate the diffusion activation energy of gallium atoms (0.7±0.1 eV) on the GaAs (001) surface conveniently.