Overgrown Layer Research Articles

Crystal Growth, composition, structure, and Raman spectroscopy of novel Ga,Ge-rich topaz

Single crystals of Ga-, Ge-, and Ga,Ge-rich (up to 41.42 wt% GeO2 and 33.95 wt% Ga2O3 or 0.98 apfu (atom per formula unit) Ge and 0.90 apfu Ga) and undoped topazes were grown on natural topaz seeds as a newly overgrown layer up to 5 mm thick. The thermogradient hydrothermal method was used at a temperature range of 600 − 650 °C and a pressure of 100 MPa under the condition of reverse temperature solubility factor of silica and alumina in the fluoride solution. The chemical composition and crystal structure of the grown topazes, as well as the distribution of gallium and germanium in the overgrown layer, were investigated by electron microprobe analysis (EMPA), single crystal X-ray diffraction (SCXRD) and Raman spectroscopy. The overgrown layers of Ge-, and Ga,Ge-rich topazes have a zonal distribution of elements, two zones corresponding to topaz Al2SiO4(F,OH)2 and krieselite (Al,Ga)2GeO4(F,OH)2 compositions are distinguished. The structural and spectroscopic studies show linear dependences of unit cell parameters and Raman shift on germanium and gallium contents and confirm the existence of a complete series of topaz-krieselite solid solution.

Heteroepitaxial growth of GaN on sapphire substrates by high temperature vapor phase epitaxy

This paper presents recent progress in the development of the high temperature vapor phase epitaxy (HTVPE) for a direct deposition of GaN layers on sapphire substrates and the impact of the improved deposition technique on the quality of the HTVPE GaN layers. This technique uses an improved HTVPE reactor for a better control of the growth process. The reactor contains a newly designed Ga evaporation cell made of refractory metals, which significantly reduces the concentration of process-induced impurities. As a result, the impurity concentrations in the HTVPE layers were reduced to or below 1016 cm−3. For a direct deposition of GaN on (0001)-oriented sapphire, a multi-step process was developed that includes the growth of a nucleation layer and a seed layer, which can be used as a template for further overgrowth with an increased growth rate to produce several ten µm thick GaN layers. The properties of the HTVPE layers are characterized by optical and scanning electron microscopy, glow discharge mass spectrometry, Raman spectroscopy, X-ray diffraction, and photoluminescence. The residual stresses and dislocation densities are presented for seed and overgrown layers, and compared with the properties of layers grown by established methods of chemical vapor phase epitaxy. In this context, challenges and prospects of the HTVPE method are discussed.

Deterministic Arrays of Epitaxially Grown Diamond Nanopyramids with Embedded Silicon‐Vacancy Centers

AbstractThe negatively charged silicon‐vacancy center (SiV) in diamond is a potential high‐quality source of single‐indistinguishable photons for quantum information processing and quantum electrodynamics applications. However, when embedded in bulk diamond, this emitter suffers from both, relatively low extraction efficiency attributed to total internal reflection as well as nondeterministic location. On the other hand, its implementation in nanodiamonds is impeded by optical dephasing owing to their degraded surface quality. Here a robust and deterministic template‐assisted bottom‐up process for the creation of high‐quality diamond nanopyramids incorporating SiVs is reported. This method employs a predefinition of high‐precision nanopillars by e‐beam lithography and dry etching, and subsequent epitaxial overgrowth during which Si atoms are introduced into diamond. The overgrown layer, including the nanopyramid arrays, is epitaxial as confirmed by comprehensive analyses. This diamond nanopyramid, hosting the SiVs, can pave the way toward efficient localized single‐photon source arrays and potentially outshine nanodiamonds and nanosized structures prepared by a top‐down method. The lateral confinement of the SiVs inside the deterministic nanosized pyramids enables their positioning at the center of cavities and optical structures for enhanced quantum efficiency and light–matter manipulation, and their employment as a system to explore collective coherent interactions between ensembles and confined light.

Analysis of Recrystallization of Fine-Crystalline Corundum in a Supercritical Water Medium Using the Lognormal Particle Size Distribution Function

The mechanism of synthesis of fine-crystalline corundum from aluminum hydroxide under induced nucleation conditions in a supercritical water fluid at 400°C and initial pressure of 26.4 MPa followed by exposure in the synthesis medium was studied. The crystal size distribution was analyzed by electron microscopy. The use of the lognormal function to describe the crystal size distribution of the synthesized corundum particles was substantiated. The possibility of using the lognormal particle size distribution function and the time dependence of its parameters to reveal the routes of product formation was analyzed. The dimensional spectrum of microcrystals has four components, whose appearance is associated with different routes of product formation. The number of distribution components remains unchanged during prolonged exposure, but their average characteristics have different time dependences. The mass redistribution during recrystallization, generally leading to a decrease in the average crystal size, is explained by differences in the crystal structure mobility of different components. It was concluded that the states of corundum in the newly formed crystals and in the overgrown layer on the particles of the inducing additive are different.

Constituent substitution in hot wall deposition of Bi2S3 films by reaction with substrates

Replacement of a constituent in the overgrown layers with the materials provided by the substrates is investigated when Bi2S3 films are deposited on III-V compound semiconductors and transition metals using a hot wall epitaxy method. The substitution caused by the S atoms is stronger than that by the Se and Te atoms in deposition of Bi2Se3 and Bi2Te3 films. Mere exposure to Bi2S3 vapor of In-containing compounds (InP and InAs) and Ga-containing compounds (GaAs and GaSb) at epitaxial growth temperatures results in a capping of their surfaces by In2S3 and GaS, respectively. A temperature window is revealed to exist for the GaAs case where growth of Bi2S3 microcrystals occurs within the temperature range for the sulfide formation. The sulfurization by the material substitution is thus demonstrated to be not always identical with that using elemental sulphur. We examine furthermore the sulfides synthesized on the surfaces of Cu, Ag, Ni, Mo, and W. The sulfides produced on Cu and Ag undergo material alterations when they are irradiated by a laser light.

The dislocation structure of diamond crystals grown on seeds in the Mg-C system

The dislocation structure of diamond crystals grown in the Mg-C system at a pressure of 7GPa and temperatures of 1800–1900°C is studied by X-ray topography and selective etching. According to the selective etching data, diamond crystals have two types of dislocations whose outputs on the {100} faces of crystal are associated with two types of etch pits. It has been demonstrated that the etch pits with the side wall inclination angle of about 7° are formed at the outcrop points of full edge dislocations, while the etch pits with the inclination angle of about 4° are associated with 45° mixed dislocations. It has been found that the dislocation structure of diamonds grown at 1900°C is completely determined by the seed crystals structure and the dislocation density is 105cm−2. The dislocation density in the diamond crystals grown at 1800°C increases by two or three orders of magnitude due to nucleation of dislocations at the seed-overgrown layer interface and in the overgrown layer. The high dislocation density leads to the mosaic structure of crystals and misorientation of single blocks, up to 1°. Local ring clusters of edge dislocations were found to be the dominant source of growth layers on the {100} faces of diamond.

Diamond crystallization in a CO2-rich alkaline carbonate melt with a nitrogen additive

Diamond crystallization was experimentally studied in a CO2-bearing alkaline carbonate melt with an increased content of nitrogen at pressure of 6.3GPa and temperature of 1500°C. The growth rate, morphology, internal structure of overgrown layers, and defect-impurity composition of newly formed diamond were investigated. The type of growth patterns on faces, internal structure, and nitrogen content were found to be controlled by both the crystallographic orientation of the growth surfaces and the structure of the original faces of diamond seed crystals. An overgrown layer has a uniform structure on the {100} plane faces of synthetic diamond and a fibrillar (fibrous) structure on the faceted surfaces of a natural diamond cube. The {111} faces have a polycentric vicinal relief with numerous twin intergrowths and micro twin lamellae. The stable form of diamond growth under experimental conditions is a curved-face hexoctahedron with small cube faces. The nitrogen impurity concentration in overgrown layers varies depending on the growth direction and surface type, from 100 to 1100ppm.

Improvement of optical quality of semipolar (112¯2) GaN on m-plane sapphire by in-situ epitaxial lateral overgrowth

Among the major obstacles for development of non-polar and semipolar GaN structures on foreign substrates are stacking faults which deteriorate the structural and optical quality of the material. In this work, an in-situ SiNx nano-network has been employed to achieve high quality heteroepitaxial semipolar (112¯2) GaN on m-plane sapphire with reduced stacking fault density. This approach involves in-situ deposition of a porous SiNx interlayer on GaN that serves as a nano-mask for the subsequent growth, which starts in the nanometer-sized pores (window regions) and then progresses laterally as well, as in the case of conventional epitaxial lateral overgrowth (ELO). The inserted SiNx nano-mask effectively prevents the propagation of defects, such as dislocations and stacking faults, in the growth direction and thus reduces their density in the overgrown layers. The resulting semipolar (112¯2) GaN layers exhibit relatively smooth surface morphology and improved optical properties (PL intensity enhanced by a factor of 5 and carrier lifetimes by 35% to 85% compared to the reference semipolar (112¯2) GaN layer) which approach to those of the c-plane in-situ nano-ELO GaN reference and, therefore, holds promise for light emitting and detecting devices.

Fabrication of porous aluminium nitride films on silicon substrate for a better overgrown layer

In this work, properties of aluminium nitride films grown on Si substrates with two different thicknesses were investigated. From our observations, the aluminium nitride film with the thickness of ∼122 nm exhibited a better characteristic than the aluminium nitride film grown at ∼80 nm. Further, the sample with thicker aluminium nitride film was fabricated into porous structures using photoelectrochemical etching with a diluted sodium hydroxide electrolyte. As witnessed under field emission scanning electron microscopy, the formation of porous structures can be observed even though they are few and not uniformly distributed on the surface. Moreover, defect clusters and grain boundaries in the porous sample disappeared after the etching process, thus giving a narrower peak in X-ray diffraction rocking curve measurement. This work is a starting point towards establishing the technology of fabricating porous aluminium nitride to make it useful as a template for the subsequent growth.

Nondestructive Evaluation of Photoelectrical Properties of a PVT Grown Bulk 15R-SiC Crystal

Investigation of excess carrier dynamics in a 15R-SiC bulk layer grown by physical vapour transport (PVT) on 15R-SiC substrate has been carried out using pump-probe techniques: an interband carrier injection by a picosecond laser pulse and measuring the induced absorption and diffraction of a probe beam. For this task, differential transmittivity (DT) and light induced transient grating (LITG) techniques were used. Room temperature carrier lifetime varied in the 3 ns 8 ns range at excess carrier densities above ΔN0 = 7×1017 cm-3 and was ascribed to the recovery time of optically recharged carrier traps, and their activation energy of Ea = 75 meV was determined. The presence of recharged traps caused the injection-dependence of the diffusion coefficient D, whereby its value dropped below 0.1 cm2/s at ΔN0 < 1×1018 cm-3 and gradually increased up to 0.7 cm2/s at higher injections. At elevated temperatures (300 K < T < 700 K), when the traps are thermally activated, the diffusivity increased up to ~ 1.5 cm2/s and was independent on ΔN0. The overgrown layer parameters were comparable to those of the used 15R PVT seed.

Investigation of GaN layers grown by high temperature vapor phase epitaxy

AbstractFirst experimental results on the growth of GaN films by a modified high temperature vapor phase epitaxy method (HTVPE) are presented. The method bases on a novel growth reactor, which was developed to overcome the drawbacks of conventional HTVPE. GaN films were deposited either on (0001) sapphire substrates or on MOVPE‐grown GaN templates of 15×15 mm2 with growth rates in the range of 2–7 µm/h. The influence of the substrate temperature and ammonia flow on the structural properties of the deposited films was systematically investigated in experiments on MOVPE templates. It is shown that the structural quality of the templates can be well reproduced in the overgrown layers. The layer quality and the stress relations are found to depend very sensitively on the ammonia flow in the carrier gas. Growth experiments with a direct, high‐temperature deposition of GaN films on sapphire demonstrated a strong dependence of the nucleation process on the substrate temperature and the carrier gas composition. The potential of HTVPE for the heteroepitaxial growth of GaN films on sapphire is discussed. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Coalescence-induced planar defects in GaN layers grown on ordered arrays of nanorods by metal–organic vapour phase epitaxy

The planar defect structure of coalesced GaN layers fabricated on ordered arrays of nanorods and grown by metal–organic vapour phase epitaxy has been studied using conventional and high-resolution transmission electron microscopy. During the process of coalescence, a boundary was created between two pyramids, where I1-type basal plane stacking faults propagating through the overgrown layers are terminated by Frank-Shockley partial dislocations. According to multislice HRTEM simulations of experimental observed images in the [] zone axis, the step-and hairpin-shaped basal prismatic stacking faults with inclined {} plane are consistent with Drum’s structural model, which has a lower formation energy compared with the model proposed by Amelinckx. Based on the observation that there are no stacking faults in the overgrown layers prior to the nanopyramid merging, the mechanism of coalescence induced stacking faults is proposed. This research contributes to the understanding of planar defect formation in III-nitride semiconductor grown by a coalescence process.

Greatly improved crystal quality of non‐polar GaN grown on a‐plane GaN nano‐rod template obtained using self‐organised nano‐masks

AbstractWe have developed a simple and effective approach for growth of a‐plane GaN with massively improved crystal quality on a‐plane GaN nanorod template on sapphire. The a‐plane GaN nanorod template is fabricated using self‐organised nickel nano‐masks. X‐ray diffraction rocking curves, measured along both symmetrical and asymmetrical directions, show a massive reduction in full width at half maximum (FWHM), meaning a significant reduction in dislocation density. These are all among best reports although our overgrown layer is only 4‐5 μm, much less than those using any other overgrowth technique. The threading dislocation density has been reduced down to 2.5×108 cm‐2 from typical 9×109 cm‐2 for the a‐plane GaN template on sapphire. The scanning electron microscope measurements show that two kinds of voids have been observed in the cross‐sectional image, which form during the coalescence process of the overgrowth. Taking the advantage of the SiO2 remaining on the top of the nano‐rods, theoretically all dislocations could be effectively blocked. Photoluminescence (PL) measurements have been performed, showing a strong band‐edge emission, which cannot be observed in the a‐plane GaN template on sapphire (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

The impact of an intermediate temperature buffer on the growth of GaN on an AlN template by hydride vapor phase epitaxy

The present study focused on the effect of an intermediate-temperature (IT; ∼900 °C) buffer layer on GaN films, grown on an AlN/sapphire template by hydride vapor phase epitaxy (HVPE). In this paper, the surface morphology, structural quality, residual strain, and luminescence properties are discussed in terms of the effect of the buffer layer. The GaN film with an IT-buffer revealed a relatively lower screw-dislocation density (3.29×10 7 cm −2) and a higher edge-dislocation density (8.157×10 9 cm −2) than the GaN film without an IT-buffer. Moreover, the IT-buffer reduced the residual strain and improved the luminescence. We found that the IT-buffer played an important role in the reduction of residual strain and screw-dislocation density in the overgrown layer through the generation of edge-type dislocations and the spontaneous treatment of the threading dislocation by interrupting the growth and increasing the temperature.

Epitaxial lateral overgrowth on (2 1¯ 1¯ 0) a-plane GaN with [0 1¯ 1 1]-oriented stripes

Epitaxial lateral overgrowth was applied to a-plane GaN on r-plane sapphire using SiO 2 stripe masks oriented parallel to [0 1¯ 1 1]. Coalescence and defect distribution was studied using scanning electron microscopy and cathodoluminescence. Defects, i.e., threading dislocations and basal plane stacking faults from the template propagate into the overgrown layer through the mask openings. Stacking faults spread into the whole overgrown layer, whereas threading dislocations are laterally confined in the region above the mask where a part of them is terminated at the inclined coalescence boundary. Lateral overgrowth and dislocation termination at the coalescence boundary leads to an improvement in luminescence intensity and crystal quality, in comparison to the template. The measured XRD rocking curve FWHM were 453″ with incidence along the [0 0 0 1] c-direction and 280″ with incidence along the [0 1 1¯ 0] m-direction.

Structural defects in GaN crystals grown by HVPE on needle-shaped GaN seeds obtained under high N 2 pressure

A bulk crystal of GaN grown by double hydride vapor phase epitaxy (HVPE) on a needle-shaped GaN single-crystalline seed, obtained by the high nitrogen pressure solution (HNPS) method, was studied by transmission electron microscopy (TEM). Structural defects at characteristic places, such as the seed–HVPE GaN interface, the first HVPE/second HVPE interface, and at morphological imperfections of the seed were characterized. The TEM studies have shown that despite high structural quality of the pressure-grown seed, structural defects are present in the HVPE-grown GaN crystal. A possible reason for strain and structural defects generation could be small lattice mismatch between highly oxygen-doped seed crystal and the HVPE GaN layer as well as between particular sectors of the HVPE GaN layer growing in different crystallographic directions. It was also observed that morphological imperfections of the seed surface may lead to the introduction of the high number of defects extending into the overgrown layer.

High-Quality Thick GaN Overgrown on an Array of SiO2 Nanomasks by HVPE

We report nanoepitaxy of thick GaN films by hydride vapor-phase epitaxy (HVPE) using a uniform array of nanomasks that were prepared by electron-beam evaporation and anodic aluminum oxide membranes. The controllable size and density of the nanoisland mask were about and , respectively. Subsequent overgrowth takes place selectively on the exposed GaN surface without the masks to the formation of the continuous layer. High-resolution X-ray diffraction shows reduction of threading dislocations in the overgrown layers. Improved optical property and significant stress relaxation in the epilayer are demonstrated by photoluminescence and micro-Raman spectra. The deposition method is a more promising technique for obtaining freestanding GaN substrates with high-quality and uniform features.

Structural Characterization of CF-PVT Grown Bulk 3C-SiC

The aim of the present work is to study the structural properties of 3C-SiC which is grown on (0001) 6H-SiC and on (100) 3C-SiC (Hoya) seeds using the Continuous Feed Physical Vapor Transport (CF-PVT) method. Transmission Electron Microscopy (TEM) observations confirm that the overgrown layer is of the 3C-SiC polytype. In the case of the 6H-SiC substrate, microtwins (MTs), stacking faults (SFs) and dislocations (D) are observed at the substrate-overgrown interface with most of the dislocations annihilating within the first few µm from the interface. In the case of 3C-SiC crystals grown on 3C seeds, repeated SFs are formed locally and also coherent (111) twins of 3C-SiC are frequently observed near the surface. The SF density is reduced at the uppermost part of the grown material.

Transition Mechanism of InAs Quantum Dot to Quantum Ring Revealed by Photoluminescence Spectra

The transition mechanism of InAs quantum dot (QD) to quantum ring (QR) was investigated. After the growth of InAs QDs, a thin layer of GaAs was overgrown on the InAs QD and the sample was annealed at the same temperature for a period of time. It was found that the central part of the InAs islands started to out diffuse and formed ring shape only after a deposition of a critical thickness (1 ~ 2 nm) of GaAs capped layer depending on the size of InAs QDs. This phenomenon was revealed by photoluminescence measurement and atomic force microscopy image. It is suggested that the strain energy provided by the GaAs overgrown layer is responsible for the InAs to diffuse out of the island to form QR.

X-ray diffraction micro-imaging of strain in laterally overgrown GaAs layers. Part II: analysis of multi-stripe and fully overgrown layers

As a development of our previous analysis of a single GaAs stripe, in this work spatially resolved X-ray diffraction (SRXRD) was applied for micro-imaging of strain in partially and fully overgrown GaAs layers grown by epitaxial lateral overgrowth (ELO) on SiO2-masked GaAs substrates. We show that a standard X-ray diffraction providing information integrated over many stripes leads to overestimation of lattice misorientation in such samples. On the contrary, the SRXRD allows precise measurements of local wing tilts of individual ELO stripes with micrometer-scale spatial resolution. A complex strain field was found in fully overgrown GaAs ELO samples. With the use of the SRXRD technique, the distribution of this strain was measured that allowed reproducing the shape of deformed lattice planes both inside the epitaxial layer as well as in the substrate underneath.