Ga Droplet Formation Research Articles

Plasma-free anisotropic selective-area etching of β-Ga2O3 using forming gas under atmospheric pressure

ABSTRACT We demonstrate a facile and safe anisotropic gas etching technique for β-Ga2O3 under atmospheric pressure using forming gas, a H2/N2 gas mixture containing 3.96 vol% H2. This etching gas, being neither explosive nor toxic, can be safely exhausted into the atmosphere, simplifying the etching system setup. Thermodynamic calculations confirm the viability of gas-phase etching above 676°C without the formation of Ga droplets. Experimental verification was achieved by etching ( 1 ˉ 02) β-Ga2O3 substrates within a temperature range of 700–950°C. Moreover, selective-area etching using this method yielded trenches and fins with vertical and flat sidewalls, defined by (100) facets with the lowest surface energy density, demonstrating significant anisotropic etching capability.

Simulation of GaAs nanowire annealing

The GaAs nanowire (NW) morphology transformation during a high temperature annealing of densely packed wires was analyzed by Monte Carlo simulation. For long densely spaced NWs, the arsenic evaporation with its removal from the system occurs only in the upper NW part at such distance that As2 molecules can overcome due to the channeling between adjacent NWs before reaching vacuum. In the lower NW part, two oppositely directed processes proceed: destruction and restoration of the NW side wall due to the readsorbed material. Thus, the lower part of wire evaporation is close to a quasi-equilibrium process, and the upper part is close to free evaporation into vacuum. An excess of gallium primarily occurs at the top part of the crystal, since it is in the conditions close to a free arsenic evaporation. An excess gallium gathering leads to the Ga droplet formation. Droplets in the middle part of the NW side walls are formed later. A droplet-free region is formed on the sidewall of sufficiently long NWs with a length>345 nm and the distance of 28 nm between the wires after the annealing at T = 890 K. That means that the congruent evaporation temperature varies along the NW axis due to the variation of arsenic pressure, resulting in the congruent evaporation from the lower NW region and incongruent evaporation from the upper wire region. Simulation results predict the dendrite (thin nanowire branching off original wires) growth during a prolonged annealing under incongruent conditions.

Modeling of Masked Droplet Deposition for Site-Controlled Ga Droplets.

Site-controlled Ga droplets on AlGaAs substrates are fabricated using area-selective deposition of Ga through apertures in a mask during molecular beam epitaxy (MBE). The Ga droplets can be crystallized into GaAs quantum dots using a crystallization step under As flux. In order to model the complex process, including the masked deposition of the droplets and a reduction of their number during a thermal annealing step, a multiscale kinetic Monte Carlo (mkMC) simulation of self-assembled Ga droplet formation on AlGaAs is expanded for area-selective deposition. The simulation has only two free model parameters: the activation energy for surface diffusion and the activation energy for thermal escape of adatoms from a droplet. Simulated droplet numbers within the opening of the aperture agree quantitatively with the experimental results down to the perfect site-control, with one droplet per aperture. However, the model parameters are different compared to those of the self-assembled droplet growth. We attribute this to the presence of the mask in close proximity to the surface, which modifies the local process temperature and the As background. This approach also explains the dependence of the model parameters on the size of the aperture.

Arsenic‐Induced Growth of Dodecagonal GaN Microrods with Stable a‐Plane Walls

AbstractNano‐ and micro‐rods of GaN offer many functionalities that are not present in regular flat nanostructures. Therefore, development of new growth methods of such structures is a hot topic. In this work the arsenic‐induced growth of GaN microrods under Ga‐rich conditions in the molecular beam epitaxy is presented. It is a self‐catalyst vapor–liquid–solid process with native Ga droplets. The formation of Ga droplets is induced by antisurfactant properties of arsenic. The presence of As during the epitaxial process promotes the growth of dodecagonal microrods with 12 walls: six m‐planes and six a‐planes. It is possible since As changes the growth rates for the different GaN planes comparing to arsenic‐free conditions, where hexagonal microrods are usually formed. The growth parameters and their influence on the sample morphology are carefully studied in this work. Microrods with an average height and diameter of 3 and 0.7 µm, respectively, and the density of 2.3 × 107 cm−2, are obtained under optimal growth conditions. The observed mechanism of growth of microrods can also be present in other material systems by introducing atoms with antisurfactant properties under metal‐rich conditions, where the surface is covered by a metal monolayer.

Langmuir evaporation of GaAs(1 1 1)A and GaAs(1 1 1)B: Monte Carlo simulation

The Langmuir evaporation (LE) of GaAs vicinal surfaces with various crystallographic orientations is analyzed at the atomistic level by Monte Carlo simulation. The evolution of the surface morphology and LE characteristics (gallium and arsenic evaporation rates, congruent evaporation temperature Tc, effective activation energy of Ga desorption) are studied under the increasing annealing temperature for GaAs(111)A and GaAs(111)B singular, and vicinal surfaces with various atomic step density. At low temperatures T < Tc, the step-terraced morphology of a vicinal surface is preserved in the LE process. At high temperatures T > Tc, the characteristics and crystallographic orientation specificity of surface morphology are determined by the formation of liquid Ga droplets. It is shown by simulation that, as compared to the (111)B surface, on the (111)A surface, the congruent evaporation temperature is substantially higher and is more sensitive to the atomic step density, as well as to the concentration of intentionally introduced defects on the terraces of the initial step-terraced surface. These observations are explained by two morphological peculiarities of LE on (111)A and B surfaces: (i) on the (111)A surface, Ga droplets penetrate deep into the substrate; (ii) on the (111)B surface, vacancy islands are more readily formed on terraces.

Preferred growth direction of III–V nanowires on differently oriented Si substrates

One of the nanowire (NW) characteristics is its preferred elongation direction. Here, we investigated the impact of Si substrate crystal orientation on the growth direction of GaAs NWs. We first studied the self-catalyzed GaAs NW growth on Si (111) and Si (001) substrates. SEM observations show GaAs NWs on Si (001) are grown along four <111> directions without preference on one or some of them. This non-preferential NW growth on Si (001) is morphologically in contrast to the extensively reported vertical <111> preferred GaAs NW growth on Si (111) substrates. We propose a model based on the initial condition of an ideal Ga droplet formation on Si substrates and the surface free energy calculation which takes into account the dangling bond surface density for different facets. This model provides further understanding of the different preferences in the growth of GaAs NWs along selected <111> directions depending on the Si substrate orientation. To verify the prevalence of the model, NWs were grown on Si (311) substrates. The results are in good agreement with the three-dimensional mapping of surface free energy by our model. This general model can also be applied to predictions of NW preferred growth directions by the vapor-liquid-solid growth mode on other group IV and III–V substrates.

Analysis of GaAsBi growth regimes in high resolution with respect to As/Ga ratio using stationary MBE growth

The control of Bi incorporation and material properties in III-V-Bi alloys has proved challenging due to their high sensitivity to the epitaxial growth parameters. Here, we present a methodology for determining the variation in the Ga, As, and Bi fluxes and the temperature across a stationary substrate in molecular beam epitaxy. By correlating the flux distributions with material properties, we identify distinct regimes for epitaxy of GaAsBi. In particular, we devise a detailed image of the interplay between Bi incorporation and structural properties of a bulk GaAs0.96Bi0.04 layer grown on GaAs(1 0 0) with respect to the As/Ga ratio. The influence of As/Ga is analyzed with high resolution over the important stoichiometric range (i.e. As/Ga = 0.6–1.6). Growth outside the near-stoichiometric As/Ga regime leads to decreased Bi incorporation, decreased structural quality and the formation of Ga, Ga/Bi or Bi droplets. On the other hand, growth at As/Ga = 1.00–1.17 leads to maximized material quality. For this regime, the surface roughness is further optimized by fine-tuning the As/Ga ratio to suppress surface mounding to a value of 0.5 nm. The results reveal the extreme sensitivity of GaAsBi growth to small variations in the As/Ga ratio, and demonstrate the applicability of stationary growth in studying these effects.

X-ray Diffraction Analysis of the Angular Stability of Self-Catalyzed GaAs Nanowires for Future Applications in Solar-Light-Harvesting and Light-Emitting Devices

Semiconductor nanowires are a class of materials that recently have gained increasing interest in solar-cell applications and light-emitting devices. Finding reproducible processing conditions is fundamental for their future mass production. In this work, the stability of individual epitaxial GaAs nanowires (NWs) under molecular beam epitaxy (MBE) processing conditions is studied by means of a time-resolved in situ micro X-ray diffraction (μXRD) method and scanning electron microscopy. Our proposed μXRD method is a nondestructive characterization technique where individual nanoobjects of different dimensions, crystal orientations, and structures are detectable under MBE processing conditions. NWs were grown by self-catalyzed MBE onto prepatterned Si(111) substrate. When exposed to MBE processing conditions at 610 °C without supply of a source material, or with only an arsenic (As) supply, we observe evaporation from the facets with no indication of gallium (Ga) droplet formation. Furthermore, the NWs, whi...

Droplet Epitaxy for III-V Compound Semiconductor Quantum Nanostructures on Lattice Matched Systems

During the Ga-molecules suppling on to the lattice-matched AlGaAs surface, we discussed the Ga-droplet formation dynamics based on Ga migration and diffusion behavior on GaAs (100), (111)A and (111)B surfaces. We discussed the formation mechanism of GaAs quantum dots (QDs), quantum dot molecules (QDMs; coupled QDs), single quantum rings (QRs), and double quantum rings (d-QRs; coupled QRs) complexes. Since the shape of GaAs quantum nanostructures (QNs) on (100) surface is affected by the out-migration of Ga-molecules due to the specific surface As coverage condition during crystallization, we can also control the shape of QNs by varying the conditions of droplet epitaxy process. In addition, by using the nature of the Ga-migration on the (111)A and (111)B surfaces, we fabricated ultra-high density (1.6 × 1011 /cm2) GaAs QDs and ultra-low density (6.5 × 105 /cm2) GaAs QNs on the AlGaAs (111)A and (111)B surfaces surface, respectively.

Crystal Structures of GaN Nanodots by Nitrogen Plasma Treatment on Ga Metal Droplets

Gallium nitride (GaN) is one of important functional materials for optoelectronics and electronics. GaN exists both in equilibrium wurtzite and metastable zinc-blende structural phases. The zinc-blende GaN has superior electronic and optical properties over wurtzite one. In this report, GaN nanodots can be fabricated by Ga metal droplets in ultra-high vacuum and then nitridation by nitrogen plasma. The size, shape, density, and crystal structure of GaN nanodots can be characterized by transmission electron microscopy. The growth parameters, such as pre-nitridation treatment on Si surface, substrate temperature, and plasma nitridation time, affect the crystal structure of GaN nanodots. Higher thermal energy could provide the driving force for the phase transformation of GaN nanodots from zinc-blende to wurtzite structures. Metastable zinc-blende GaN nanodots can be synthesized by the surface modification of Si (111) by nitrogen plasma, i.e., the pre-nitridation treatment is done at a lower growth temperature. This is because the pre-nitridation process can provide a nitrogen-terminal surface for the following Ga droplet formation and a nitrogen-rich condition for the formation of GaN nanodots during droplet epitaxy. The pre-nitridation of Si substrates, the formation of a thin SiNx layer, could inhibit the phase transformation of GaN nanodots from zinc-blende to wurtzite phases. The pre-nitridation treatment also affects the dot size, density, and surface roughness of samples.

Focused ion beam implantation for the nucleation of self-catalyzed III-V nanowires

Focused ion beam implantation was used for the formation of Ga droplets which act as nucleation points for self-catalyzed, molecular beam epitaxy-grown nanowires on Si(111). In order to further optimize the growth, the substrate after implantation was analyzed via atomic force microscopy, which indicated that sputtering depth had a strong influence on the subsequent nanowire growth. Lower beam voltage accelerations led to beam defocusing and thereafter better growth, while focused, high-acceleration beams led to a larger sputtering depth, worse growth and more tilted nanowire growth. The optical quality of the nanowires was evaluated using spatially resolved photoluminesence measurements at room temperature.

Ga predeposition for the Ga-assisted growth of GaAs nanowire ensembles with low number density and homogeneous length

We present a novel two-step growth approach for the Ga-assisted growth of GaAs nanowires (NWs) by molecular beam epitaxy on Si. In the first step only Ga is deposited for the controlled formation of Ga droplets and in the second step NWs are grown from these droplets at lower Ga flux. This variation of the Ga flux leads to a decoupling of the formation of Ga droplets and the formation of NWs. Thus, the total density of crystal objects can be varied by only changing the parameters of the first step. Also, the NW length distribution of such an ensemble is more homogeneous. Annealing of the droplets can improve the homogeneity even further. The resulting GaAs NW ensembles are ideally suited for the subsequent growth of shell structures. Finally, our new approach enabled us to explore the nucleation of different crystalline objects and analyze the impact of the droplet size on the vertical yield of NWs in detail.

Impact of the Ga Droplet Wetting, Morphology, and Pinholes on the Orientation of GaAs Nanowires

Ga-catalyzed growth of GaAs nanowires on Si is a candidate process for achieving seamless III/V integration on IV. In this framework, the nature of silicon’s surface oxide is known to have a strong influence on nanowire growth and orientation and therefore important for GaAs nanowire technologies. We show that the chemistry and morphology of the silicon oxide film controls liquid Ga nucleation position and shape; these determine GaAs nanowire growth morphology. We calculate the energies of formation of Ga droplets as a function of their volume and the oxide composition in several nucleation configurations. The lowest energy Ga droplet shapes are then correlated to the orientation of nanowires with respect to the substrate. This work provides the understanding and the tools to control nanowire morphology in self-assembly and pattern growth.

Characterization and density control of GaN nanodots on Si (111) by droplet epitaxy using plasma-assisted molecular beam epitaxy.

In this report, self-organized GaN nanodots have been grown on Si (111) by droplet epitaxy method, and their density can be controlled from 1.1 × 1010 to 1.1 × 1011 cm-2 by various growth parameters, such as substrate temperatures for Ga droplet formation, the pre-nitridation treatment of Si substrate, the nitridation duration for GaN crystallization, and in situ annealing after GaN formation. Based on the characterization of in situ RHEED, we can observe the surface condition of Si and the formation of GaN nanodots on Si. The surface nitridaiton treatment at 600°C provides a-SiNx layer which makes higher density of GaN nanodots. Crystal GaN nanodots can be observed by the HRTEM. The surface composition of GaN nanodots can be analyzed by SPEM and μ-XPS with a synchrotron x-ray source. We can find GaN nanodots form by droplet epitaxy and then in situ annealing make higher-degree nitridation of GaN nanodots.

Self-catalyzed ternary core-shell GaAsP nanowire arrays grown on patterned Si substrates by molecular beam epitaxy.

The growth of self-catalyzed ternary core-shell GaAsP nanowire (NW) arrays on SiO2 patterned Si(111) substrates has been demonstrated by using solid-source molecular beam epitaxy. A high-temperature deoxidization step up to ∼ 900 °C prior to NW growth was used to remove the native oxide and/or SiO2 residue from the patterned holes. To initiate the growth of GaAsP NW arrays, the Ga predeposition used for assisting the formation of Ga droplets in the patterned holes, was shown to be another essential step. The effects of the patterned-hole size on the NW morphology were also studied and explained using a simple growth model. A lattice-matched radial GaAsP core-shell NW structure has subsequently been developed with room-temperature photoluminescence emission around 740 nm. These results open up new perspectives for integrating position-controlled III-V NW photonic and electronic structures on a Si platform.

Fundamental role of arsenic flux in nanohole formation by Ga droplet etching on GaAs(001).

Nanoholes with a depth in the range of tens of nanometers can be formed on GaAs(001) surfaces at a temperature of 500°C by local etching after Ga droplet formation. In this work, we demonstrate that the local etching or nanodrilling process starts when the Ga droplets are exposed to arsenic. The essential role of arsenic in nanohole formation is demonstrated sequentially, from the initial Ga droplets to the final stage consisting of nanoholes surrounded by ringlike structures at the surface and Ga droplets consumed. The kinetics of local etching depends on the arsenic flux intensity, while the ringlike structures are basically the same as those formed underneath the droplets in the absence of arsenic. These structures show motifs with well-defined crystalline facets that correspond to those expected from surface energy minimization. These experimental results are qualitatively analyzed for a better understanding of the nanohole formation underlying processes.

Self-assembly of Ga droplets attached to GaAs quantum dots

We report the site-selective formation of Ga droplets on a surface where self-assembled GaAs quantum dots (QDs) were formed by droplet epitaxy. On the surface with the GaAs QDs, all the Ga droplets form adjacent to the QDs and are attached to the (111)B side facets of the QDs, which was confirmed by selective etching. While some of the QDs have two small droplets on both sides, most of the QDs have one large droplet on one side. We attribute the mechanism of the site-selectivity of the droplets to the morphology of the QDs and the high surface energies of the (111)B side facet.

Formation of Ga droplets on patterned GaAs (100) by molecular beam epitaxy

In this paper, the formation of Ga droplets on photo-lithographically patterned GaAs (100) and the control of the size and density of Ga droplets by droplet epitaxy using molecular beam epitaxy are demonstrated. In extension of our previous result from the journal Physical Status Solidi A, volume 209 in 2012, the sharp contrast of the size and density of Ga droplets is clearly observed by high-resolution scanning electron microscope, atomic force microscope, and energy dispersive X-ray spectrometry. Also, additional monolayer (ML) coverage is added to strength the result. The density of droplets is an order of magnitude higher on the trench area (etched area), while the size of droplets is much larger on the strip top area (un-etched area). A systematic variation of ML coverage results in an establishment of the control of size and density of Ga droplets. The cross-sectional line profile analysis and root mean square roughness analysis show that the trench area (etched area) is approximately six times rougher. The atomic surface roughness is suggested to be the main cause of the sharp contrast of the size and density of Ga droplets and is discussed in terms of surface diffusion.

MBE growth of high electron mobility 2DEGs in AlGaN/GaN heterostructures controlled by RHEED

We have grown 2DEG AlGaN/GaN heterostructures by molecular beam epitaxy (MBE) with electron mobilities up to 21500 cm2V−1s−1 at 2 K. In-situ RHEED was applied to optimize different aspects of Ga-rich growth. This paper gives a compact overview of the experimental key aspects that significantly affect the low temperature electron mobility in AlGaN/GaN heterostructures. Growth at the transition towards Ga droplet formation produced the best results. A quantitative analysis of the magnetoresistance confirmes scattering at dislocations as the dominant scattering process at low temperature.

X-ray Photoemission Spectroscopy Study of Low-Temperature Nitridation of GaAs(001) Surface Using RF Radical Source

The nitridation mechanism of the GaAs(001) surface using an RF-radical source at a low temperature of 350 °C was systematically studied by changing the As supply and nitridation time to obtain a smooth nitrided layer without the formation of Ga droplets. Atomic force microscopy (AFM) measurements indicated that supplying As is useful in suppressing the re-evaporation of As atoms and in maintaining a smooth surface. However, the degree of nitridation was decreased with increasing As pressure in the samples nitrided for 30 min. In contrast, the time dependence of nitridation indicated that the degree of nitridation increases with nitridation time. After optimizing the conditions, a two-monolayer-thick GaN layer was successfully obtained by nitridation for 120 min, regardless of the supply of the As molecular beam. The structure of the nitrided layers was also investigated using angle-resolved X-ray photoemission spectroscopy, and a thin layer that contains As and N atoms was found to cover the nitrided GaN layer.