Table of contentsSummaryIntroductionLiterature reviewFuture workReferencesSummaryGreen semiconductor emitters with good internal quantum efficiency are a major concern for obtaining white light by mixing colors. InGaN being a potential candidate as a green emitter was studied here. LEDs emitting shorter wavelengths, in the visible range, made of III nitrides, are already available on the market. Thus, bandgap tuning of GaN is carried out by incorporating InN in order to obtain a bandgap corresponding to λ ~ 550 nm (medium green). This report addresses the growth mechanism of InGaN nanowires, the problems of adjusting the bandgap, i.e. the difficulties of incorporating high concentrations of indium, necessary to reduce the bandgap of GaN, and how these problems are solved by optimizing growing conditions. Say no to plagiarism. Get Custom Essay on “Why Violent Video Games Should Not Be Banned”?Get Original EssayIntroductionInGaN finds application in many optoelectronic devices such as solar cells, LEDs, laser diodes, photodetectors, etc. The wide range of applications of InGaN is due to its tunable bandgap. The bandgap can vary from 0.69eV (λ=1.7µm for 𝐼𝐼𝐼𝐼𝑁𝑁) to 3.4eV (λ=365nm for 𝐺𝐺𝐺𝐺𝑁𝑁), which covers almost the entire solar spectrum and visible wavelength range. Therefore, it is very useful in heterojunction solar cells and visible light LEDs. Alloying is a common tool for tuning physical properties, such as lattice constant, as well as optical properties such as bandgap. Specifically for LEDs, III-nitrides are a promising material for high-efficiency solid-state lighting. To obtain white light, phosphors are used to partially convert the blue LED light into the yellow/green range. But this conversion comes with a high energy loss (~25%), known as Stokes loss, limiting the highest possible efficiency to well below 100%. While InGaN/GAN LEDs offer efficiencies above 80%. Thus, the conversion of phosphor to obtain white light can be eliminated by using LEDs emitting different wavelengths such as red, green/yellow and blue. Not only higher output, but solid state lighting offers a longer lifespan than phosphor based lighting. The problem when transitioning from phosphor-based lighting to LED lighting is that green LEDs have lower efficiency compared to red (III-nitride based) and blue (III). -based on phosphide). White LEDs without phosphor require at least one green emitter (λ ~ 550 nm). Thus, the efficiency of the white LEDs is limited by the maximum efficiency of the green emitter. Even green phosphor-based emitters have higher efficiency than solid-state emitters. So for white LEDs, the green emitter is very important. Red and blue LEDs are respectively made of III phosphides. When searching for a green emitter, phosphides and arsenides have also been explored, but the only material (other than nitrides) with a wide bandgap (necessary for the green region) is AlInP (green phosphide). aluminum and indium) for which no suitable substrate has been found and the characteristics of the material growth system are uncertain, such as the concentration of Al for green emission, etc. Therefore, the remaining option isIII nitrides for the green emitter. The efficiency obtained with InGaN/GaN layered LEDs is limited to 40%. The reasons behind this low efficiency could be higher concentration of defects due to unavailability of native substrates (defects contribute to non-radiative recombination sites in the film), lower concentration of In in InGaN due to the composition attraction effect, etc. But to achieve Green 2 emission from InGaN, a high concentration of In in the crystal is necessary to reduce the bandgap at the green wavelength. In a planar structure, the thickness is limited to a critical value after which the layer will break by plastic deformation, but for high In content in the layer we need thick layers. Planar layers are therefore useless. All these problems can be solved by moving to 3D growth of InGaN, e.g. nanowires. The nanowires are free of defects because the stress due to lattice mismatch is released in very few monolayers, due to the high surface area-to-volume ratio, and the rest of the wire grows unconstrained, so that the composition attraction is absent in nanowires. This review report focuses on growth. mechanisms of InGaN by understanding the growth of GaN nanomaterials (3D). The growth methods used to cultivate nitrides are CVD, VLS, MBE, etc. The vapor-liquid-solid (VLS) method is the most commonly used technique to grow semiconductor nanowires using a metal catalyst acting as nucleation sites. For spontaneous growth without the use of catalyst, MBE can be used by optimizing the different growth parameters such as temperature, Ga and N fluxes, as well as the different intermediate layers. In this article, we will study the problems associated with planar InGaN layers and how they are avoided using InGaN nanowires. InN, when alloyed with GaN, results in poor crystal quality and inhomogeneity due to a large difference in covalent radii between Ga (1.26 A°) and In (1.44 A°), which causes also significant internal constraints. Another issue that deserves attention is phase segregation in defect spaces. which reduces the In content in the crystals of the InGaN alloy and disrupts the stoichiometry affecting the bandgap, lattice constant and some other properties of interest. This problem arises due to high growth temperatures, high vapor pressure of InN compared to that of GaN, difference in formation enthalpies of InN and GaN, etc. Therefore, it can be minimized by optimizing growth parameters, such as using relatively small particles. growth temperatures, high V/III flux ratio (i.e. N/Ga flux), low growth rate and low growth pressure. High V/III ratios were found to suppress indium segregation during InGaN growth. All issues are discussed in detail in the later parts of this report.Literature ReviewThe literature review aims to study the growth mechanism of InGaN nanowires and how it is different from planar layers, as well as the effect compositional traction in InGaN. Most of the works studied focus on spontaneous (catalyst-free) MBE growth of InGaN on Si(111). Some groups have used buffer layers (AlN) to bridge the lattice imbalance between InGaN and Si. Ristic et al studied the effect of pre-deposited Ga droplets on the nucleation of GaN nanowires. Bare Si(111) wafers were used for modeling with Ga droplets to avoid the effects of the morphology of thebuffer layer, stress differences, crystal quality or thickness on the nucleation process. Ga droplet configuration was carried out at temperatures lower than Ga evaporation temperatures to prevent its desorption. The Ga flow used for the patterning and growth of the GaN nanocolumns was the same. Metallic Ga forms droplets on the surface of Si by coalescence 3 to reduce the surface energy. After patterning the Si substrates with Ga droplets, the substrate temperature was increased to the growth temperature. Ga desorption can occur at the growth temperature, but partial nitriding reduces the desorption rate. The substrates grown in nanocolumns were studied by SEM (scanning electron microscopy). What they observed was that the nanocolumns formed everywhere except for the Ga droplet sites. They concluded that the Ga droplets were hindering the growth of the nanocolumns. As the Ga droplet diameter decreased (substrates with varying droplet diameters were prepared for nanocolumn growth), nanocolumns were also observed at the droplet sites, but were slightly tilted relative to the substrate, as if they emerged from facets of partially nitrided Ga droplets. Nitriding was observed more in the case of smaller droplets. They reported that Ga droplets serve solely as Ga reservoirs, supplying Ga to nanowires when growth rates are faster and the V/III ratio is high. This is confirmed by two observations (1) high densities of nanowires around the Ga droplets and (2) the density of the nanocolumns was higher than that of the Ga droplets. These two observations confirm that the Ga droplets do not act as nucleation sites but Ga reservoirs. Both concerns addressed by Ristic et al. are the mechanisms responsible for the constant diameter of the nanocolumns throughout the length and nucleation process that determine the size and density of the nanocolumns. The growth of nanocolumns was expected to start from a random distribution of GaN islands. First, a 2D layer forms, then it transforms to 3D to minimize energy and stress buildup. When the stress reaches the threshold, nanocolumns form simultaneously across the entire substrate. Nucleation of the nanocolumns takes a long time to form over the entire surface of the substrate, as it occurs when the GaN islands reach a saturation density. These research groups did not report a change from 2D to 3D growth mode, but a direct nucleation of the islands when the saturation density was reached, i.e. following the Volmerweber (VW) growth mode. . This is supported by SEM and HRTEM analyzes which indicate no wetting layer formation in the GaN nanocolumns. A SixNy layer on Si has been reported, which may be due to the reaction of Si with an active nitrogen plasma. Nucleation can be explained by the theory of capillarity given by Volmer and Weber. According to this theory, there is a critical size of nuclei, which increases with increasing substrate temperature, above which a nucleus becomes stable and below which it decays by desorption or diffusion into other larger nuclei. Thus, all clusters with radii less than the critical size of nuclei disappear and all those with radii greater than critical radii survive and continue to grow into larger ones by scattering atoms. This determines the minimum size of nucleation sites at agiven growth temperature. Thus, the growth of large diameter nanocolumns can be predicted at higher temperatures. For this to occur, sufficient Ga flux is required to compensate for Ga desorption. The average diffusion length of the ad-Ga atom depends on the substrate temperature and the III/V flux ratio. The distance between two nuclei must be equal to twice the average diffusion length of Ga for saturation to occur. The diffusion length decreases with decreasing III/V ratio. This determines the diameter of the nanocolumn and its density. When the III/V ratio is very small (N-rich condition), the stable nucleation sites do not merge because the incoming Ga atoms will be incorporated at their apex. This behavior is explained by the tendency of hexagonal III nitrides to develop into columnar grains. When the III/V ratio is high, i.e., high Ga flux, vertical growth saturates and lateral growth continues, leading to an increase in diameter and ultimately coalescence to form a 2D layer. Since Ga has smaller diffusivity, lower flux rate (for vertical growth, higher N flux is required), hence the time required to reach nanocolumn density saturation is longer. Daruka and Barabasi studied that the growth mode depends on the total amount of material deposited and the network mismatch. They showed that growth mode and morphology can change from Frank-van der Merwe (FM) to SK to VW as network mismatch increases. They also concluded that the nucleation of GaN nanocolumns on Si(111) occurs via the VW growth mode due to strong lattice imbalance. The surface of the substrate is covered with GaN islands of different sizes during nucleation and gives rise to nanocolumns of variable diameter. Since nucleation does not begin at the same time at all points on the substrate, the height of the nanocolumns also varies. The height variation may be due to the diffusion of Ga along the side walls of the nanocolumn to its top. Thus, larger diameter nanocolumns grow more slowly. When nucleation is achieved, further growth of the nanocolumns depends on two Ga diffusion processes (1) Ga atoms directly hitting the top of the islands and (2) diffusion of Ga through the sidewalls to the top when the atoms collide with the surface of the substrate. This theory can be supported by the observations that Ga droplets act as reservoirs promoting the growth of neighboring islands, such that the density in the vicinity of Ga droplets increases, and also as the Ga flux increases.The diameter of the nanocolumn only increases at the top because more Ga atoms diffuse toward the top of the nanocolumn. The growth of the nanocolumn is explained by taking into account a high diffusion length of Ga along the side walls of the nanocolumn and a high adhesion coefficient of the nanocolumn tip, i.e. i.e. the lower plane (plane c). For a given surface, the atom ad and the temperature Qdes are fixed. Such a surface would have a high diffusion length, which would give a very short time for adsorption of ad atoms in the crystal. Thus, for vertical growth, i.e. adsorption of ad atoms on the c plane, the ad atoms of Ga on this plane must have a lower diffusion length due to the high concentration of sites of adsorption of atoms ad on this surface and a higher Qd because the plane c is the polar pole. surface with more dangling bonds. This explains vertical growth very well. Increasing the diameter at the top can bedue to the accumulation of ad Ga atoms on the top surface, resulting in an increase in residence time which ultimately leads to incorporation into the top sidewalls. When a buffer layer such as AlN is used, the network mismatch reduces, allowing for better wetting properties and thus favors the SK growth mode which causes the islands to coalesce and form larger islands between which nanocolumns can grow due to excess N. If the thickness of these islands increases, stress relaxation and increased surface roughness result. This compact and rough material coexists with the nanocolumns. The compact layer can still be avoided by further increasing the N flux even if an AlN buffer layer is used. It was concluded that the use of AlN increases the diffusion length of Ga, which can be reduced by increasing the flux of N. It is observed that excess N has two effects (1) it decreases the diffusion length of Ga and (2) it promotes vertical growth (c axis). Bertness et al. studied the spontaneous growth of GaN nanowires using MBE on Si(111) substrates using AlN as a buffer layer. The growth sequence begins with the growth of the AL pre-layer, then the AlN buffer layer followed by the raw GaN matrix layer for nanowire nucleation. Growth was carried out in the presence of a high N flux relative to the Ga or Al flux and the substrate temperature was maintained between 810 and 830 °C. FESEM and AFM confirmed the nucleation of the nanowires in the hexagonal wells formed in the GaN matrix layer. They observed that nanowire nucleation depends on nitrogen flow, substrate temperature, and the thickness of the AlN buffer layer. If the Al and AlN layers are ignored, no nucleation was observed and amorphous GaN was deposited even though a high nitrogen flux was maintained. At higher N flux rates, the growth rates of nanowires as well as matrix layers were found to be increased, which contrasts with the observation reported by J. Ristic´. The nucleation rate increases as the thickness of the AlN layer increases. They stated that the cause of these discrepancies could be the substrate degassing procedure and the growth system hardware that affect the formation of SiNx. Atomic N and excited molecular N2 were used to study nanowire growth and found that excited N2 has lower energy than atomic N and therefore reduces the diffusivity of Ga. Thus, atomic N is preferred for GaN growth. They also used NH3 as a nitrogen source and found that no nanowire growth occurred, but a good high hexagonal well density was observed. For the vertical growth of nanowires in hexagonal wells, they exhibited a higher C-plane adhesion coefficient than that of the sidewalls. The role of atomic N has been described as a stabilizer for the formation of the c-plane on the AlN surface. Better control of MBE growth parameters can result in better growth of nanowires. Goodman et al. reported growth of InGaN nanowires by plasma-assisted MBE. Using the technique reported by Kikuchi et al., GaN nanowires were first grown for a short time and then InGaN nanowires were grown by changing the growth conditions such as lower substrate temperature and a lower Ga flux to incorporate In into the crystal. Growth conditions such as low III/V ratio, etc. were maintained as in the case of GaN nanowires. The condition that must be changed is the substrate temperature. Indium has alow vaporization temperature, so at high growth temperatures it cannot be incorporated into nanowires. Since temperature plays an important role in the growth of InGaN, the effects of temperature variations have been studied by Goodman et al. At lower growth temperatures, no nanowire growth was observed. Thus, the temperature was increased but kept below the temperature required for the growth of GaN nanowires. This increase resulted in the incorporation of In into the crystals as well as the formation of nanowires. They also grew InGaN nanowires without using a GaN nanowire nucleation layer and found that they were the same as when the nucleation layer was used, but the densities were different in the two cases. Higher densities were obtained in the case of a nucleation layer that was grown on a Ga droplet patterned substrate (Ga droplet pattern results in higher densities of GaN nanowires). They observed nanowires of different diameters and heights. The higher height nanowires were thinner and the lower height ones were thicker. The possible cause of this phenomenon could be that at lower growth temperatures, the metal atoms do not receive enough energy to distribute between the wires. But after studying the variation in the diameter of GaN, they concluded that the variations in the case of InGaN are due to the incorporated In content. The main objective of the study by Goodman et al. consisted of growing InGaN nanowires with a constant In content along the entire length of the wire. But they observed that the content varies from 0 to 35% along the length of the nanowire. Variation in In content was also observed from wire to wire in the same growth sample. Kong et al. studied the above-mentioned problem of variation in concentration along the length of the wire. They found that as the InGaN epil-ayer grows on GaN nanowires, the strain increases due to the large lattice disparity between InN and GaN (~11%). This elastic deformation can cause a variation in the In content of the nanowires. It is also called compositional tensile effect or crystal tensile effect in InGaN/GaN nanowires. In this effect, the incorporation of In into the nanowires is hindered by the lattice imbalance, which causes the composition of the InGaN nanowires to grow toward that of the GaN substrate, i.e., the In concentration increases with the height and is minimal at the GaN/InGaN interface in order to match. the trellis. Pereira et al. also reported this effect. They studied this effect by depth-resolved cathodoluminescence (CL) and Rutherford backscatter spectrometry (RBS) and reported that the mole fraction increases for thicker (more relaxed) InGaN samples. To reduce the compositional tensile effect, the network relaxes elastically by increasing the diameter of the nanowire as well as plastically by introducing unsuitable dislocations at the interface. When the lattice is relaxed, the InGaN content increases, reducing the compositional pull effect and lattice mismatch. The incorporation of indium modifies the bandgap and therefore the emission wavelength. Keep in mind: this is just a sample. Get a personalized article from our expert writers now. Get a Custom Essay Future Work The aim of this study is to find an efficient growth method for InGaN nanowires. From the growth methods studied above, it can be concluded that MBE is a technique that can yield spontaneously growing nanowires free from impurities, 32:8, 737-745