Growth Of GaN Layers Research Articles

GaN thickness dependence of GaN/AlN superlattices on face-to-face-annealed sputter-deposited AlN templates

Recently, deep-ultraviolet (DUV) light-emitting devices have attracted attention for various applications. GaN/AlN superlattices have emerged as a promising alternative for achieving high-efficiency DUV emission. To fabricate superlattices with high crystal quality and abrupt interfaces, we have utilized face-to-face-annealed sputter-deposited AlN template substrates characterized by a flat surface and low dislocation density. Furthermore, radio-frequency plasma-assisted molecular beam epitaxy with in situ reflection high-energy electron diffraction monitoring was employed for the growth process. The growth of the superlattices follows a specific sequence. Step 1: AlN growth, Step 2: conversion of Al droplets to AlN, Step 3: GaN growth, and Step 4: evaporation of Ga droplets. This study explored the impact of GaN thickness on the GaN/AlN superlattice. The GaN thickness was linearly controlled by changing the duration of Step 3. This approach allowed for the growth of a flat GaN layer up to 1 monolayer (ML) and achieved superlattices with abrupt interfaces. Single-peak cathodoluminescence (CL) emission at 240–245 nm was observed from the superlattices, with the peak shift toward longer wavelengths as the GaN thickness increased. In contrast, quantum dot-like GaN islands were generated with a thickness of over 1 ML, induced by compressive strain. Superlattices with thicker GaN exhibited broad CL emission with multiple peaks. However, the AlN barrier layer reduced the surface roughness and maintained abrupt interfaces within the superlattices. Therefore, to obtain sharp single-peak UV emission from GaN/AlN superlattices, the growth sequence should be controlled to obtain flat GaN layers without dots.

Suppression of Mixing of Metastable Zincblende Phase in GaN Crystal Grown on ScAlMgO4 Substrates by Radio‐Frequency Plasma‐Assisted Molecular Beam Epitaxy

ScAlMgO4 (SAM) substrates have attracted significant attention as template substrates for fabricating bulk GaN crystals. Radio‐frequency plasma‐assisted molecular beam epitaxy (RF‐MBE) is preferred for the growth of the first GaN layer. However, the metastable zincblende (ZB) phase is mixed on the SAM substrate with a smaller terrace width, coupled with lower growth temperatures in MBE. Herein, the time evolution of GaN growth on SAM substrates is investigated to elucidate the mechanism underlying ZB mixing. ZB‐GaN is generated as stacking faults during the coalescence of the initially grown wurtzite‐ (WZ‐) GaN islands on adjacent SAM terraces, owing to the extraordinarily large step height close to three bilayers of WZ‐GaN. Therefore, high‐temperature growth on a SAM substrate with wider terraces suppresses the generation of the ZB phase in the initial growth stage. ZB‐GaN also decreases with an increase in the growth time. Based on these results, a 940 nm‐thick GaN layer is grown at 750 °C on the SAM substrate with a terrace width of 820 nm. A pure WZ‐GaN layer from the GaN/SAM interface with a flat surface with a root mean square value of below 1 nm and a dislocation density of 7.3 × 109 cm−2 is obtained.

Semi-polar GaN(11-22) on nano-structured Si(113): a structure for reducing thermal stresses

The article reports the growth of semi-polar GaN(11-22) layers using epitaxy from metal organic compounds on a nanostructured NP-Si(113) substrate. It was shown that upon the emergence of an island layer, elastic deformed structures of GaN(11-22)/NP-Si(113) form a nano-meter compliant silicon layer on a substrate while elastic stresses conditioned by the difference of temperature coefficients of GaN and Si in such a structure decrease

SiC/Si Hybrid Substrate Synthesized by the Method of Coordinated Substitution of Atoms: A New Type of Substrate for LEDs

This paper proposes a new type of substrate for manufacturing LEDs based on AlInGaN heterostructures. Instead of depositing SiC layers on the surface of Si using the conventional method, a new method involving the coordinated substitution of atoms (MCSA) to form the SiC layer is proposed. This new approach enables the growth of epitaxial GaN layers with low defect content and facilitates transfer to any surface. The paper details the technology of manufacturing LEDs on SiC/Si substrates obtained by the MCSA and elaborates on the benefits of using these substrates in LED production. Additionally, the advantages of the growth interface between SiC and Si materials are discussed. Moreover, it is found that thinner SiC layers (<200 nm) contribute to the scattering of the LED’s own radiation in the heterostructure waveguide, which decreases its absorption by silicon. For flip-chip LEDs with the substrate removed, substrates with thicker SiC layers (~400 nm) and a growth porous layer of several microns at the SiC-Si interface is utilized to simplify Si substrate removal and enhance the manufacturing process’s cost-effectiveness.

First-principles study for self-limiting growth of GaN layers on AlN(0001) surface

The GaN thickness dependence of surface structural stability and adsorption behavior of Ga adatom in GaN layers on a AlN(0001) surface are investigated on the basis of first-principles calculations to clarify the self-limiting growth on AlN(0001) surface during metal-organic vapor phase epitaxy. The calculations demonstrate that the stability of reconstructed GaN layers on a AlN(0001) surface is similar to that of a GaN(0001) surface irrespective of the GaN film thickness. Furthermore, we find that the adsorption of a Ga adatom on the AlN(0001) surface easily occurs compared with that on AlN(0001) surface with GaN layers. The difference in the adsorption behavior implies that the growth of GaN layers on a AlN(0001) surface is suppressed. The calculated results provide theoretical guidance for understanding the self-limiting growth of GaN layers, resulting in the formation mechanism of GaN quantum wells.

(001) and (11n)n = 1,3 GaAs substrate orientations for growth of GaN layers by AP-MOVPE: impact of GaN buffer layer thickness

(001) and (11n)n = 1,3 GaAs substrate orientations for growth of GaN layers by AP-MOVPE: impact of GaN buffer layer thickness

The influence of residual GaN on two-step-grown GaN on sapphire

In this paper, it is found that the residual GaN has a great impact on the two-step-grown GaN on sapphire substrates. The samples which are grown with a conventional system baking process present a higher dislocation density than those without the baking process. It is found that, without baking, residual GaN accumulate in the MOCVD reactor, decompose, form the nucleation site on the sapphire substrate during the process of high-temperature surface treatment, and thus influence the initial nucleation process of GaN layer growth. Based on this understanding, a novel growth method to improve the quality and stability of two-step-grown GaN on sapphire is proposed, in which the TMGa and NH3 were added in the high-temperature surface treatment process. With this method, as low as 3.71 × 107 cm−3 and 1.94 × 108 cm−3 for screw and edge dislocation density of GaN epitaxial films were achieved on sapphire.

Dislocation structure of the AlGaN/GaN/α-Al2O3 heterostructures epitaxial layers at doping GaN with С and Fe

The aim of this work was to study the influence of the iron and carbon doping of the epitaxial GaN layer on sapphire on the growth features of epitaxial films and their dislocation structure. Investigation methods used were: mass spectroscopy of secondary ions, selective chemical etching on spherical sections, and also single-crystal diffractometry.It is shown that doping during growth of an epitaxial GaN layer with carbon can lead to a significant decrease in the dislocation density in epitaxial layers.It has been shown that for samples doped with iron, a decrease in the number of short dislocations located in the bulk of the structure is characteristic, but a large number of extended dislocations are generated contributing to the diffusion of iron into the working regions of heterostructures, which can adverselyinfluence on the electrical parameters of the structures. In the course of the work, a technique for determining the density of dislocations in epitaxial films was proposed using two schemes of selective etching of spherical thin sections, which makes it possible to determine the dislocation density distribution over the depth of epitaxial films.

Образование полуполярных III-нитридных слоев на поверхности Si(100), структурированной с помощью самоформирующейся наномаски

The epitaxial growth of AlN and GaN layers was studied by Metalorganic Vapor Phase Epitaxy, on a Si(100) substrate, on the surface of which a V-shaped nanostructure with sub-100 nm element size (NP-Si(100)) was formed. It is shown that a corrugated surface is formed from semipolar AlN(10-11) planes with opposite "c"axes during the formation of a semipolar AlN layer at the initial stage of epitaxy. Then, during the growth of the GaN layer, the transition from the symmetric state of two semipolar AlN planes to an asymmetric state with a single orientation of the "c"-axis of the semipolar GaN(10-11) layer occurs, and the "c" direction in the growing semipolar layer coincides with the direction of the flow of N2+ ions to the silicon surface during the formation of a nanomask.

The Influence of Reactor Pressure on the Properties of GaN Layers Grown by MOVPE

We have studied the growth of GaN layers by the metalorganic vapor phase epitaxy (MOVPE) on sapphire substrates at various reactor pressures, including ones above the atmospheric level. It is established that the epitaxial growth at higher pressures does not affect the crystalline perfection of epilayers, their electron mobility, and background impurity level, but leads to the formation of GaN surface with lower lateral scale of inhomogeneities. In addition, the reactor pressure influences the ratio of edge and impurity lines in the photoluminescence spectra and leakage current level in reversely biased Schottky barriers.

Influence of a Nanoporous Silicon Layer on the Practical Implementation and Specific Features of the Epitaxial Growth of GaN Layers on SiC/por-Si/c-Si Templates

A set of structural and spectroscopic methods of diagnostics is used to study the influence of a nanoporous silicon (por-Si) transition layer on the practical implementation and specific features of growth of GaN layers on SiC/por-Si/c-Si templates by molecular-beam epitaxy with the plasma activation of nitrogen. It is shown that a por-Si transition layer introduced into a template, in which a 3C-SiC layer is created by the method of atom substitution, offers unquestionable advantages over standard silicon substrates. Specifically, such an approach makes it possible to lower the level of stresses in the crystal lattice of the epitaxial GaN layer by about 90% and to reduce the fraction of vertical dislocations in the GaN layer. The GaN layer is grown on the surface of the SiC layer, which in turn is on the surface of the SiC/por-Si/c-Si template. It is found for the first time that the use of the SiC/por-Si/c-Si template brings about the formation of a qualitatively more uniform GaN layer free of visible extended defects.

The Origin and Influence of Compensatory Current in AlGaN/GaN Type High Electron Mobility Transistor Heterostructures with Two Conducting Channels on the Hall Measurements

The second conducting channel is created in AlGaN/GaN type high electron mobility transistor (HEMT) heterostructures deposited by the metal organic vapor phase epitaxy (MOVPE) technique, in which the pressure changes during the growth of the buffer GaN layer to ensure its high resistivity. It is stated that a second, parasitic, conducting channel is induced as a result of nonintentional doping that occurs at the GaN–GaN interface. The Hall measurements, in the wide range of temperatures, from 77 to 420 K, are used to obtain sheet resistivity, sheet carrier concentration, and electron mobility of the heterostructures. The theoretical model of the multilayer transport in AlGaN/GaN type HEMT heterostructures, based on an equivalent circuit, allows for estimation of compensatory current. Based on the theoretical model, the correction map for the Hall measurement of the samples with two conducting channels is evaluated. The measured electron mobility μmeas obtained from Hall measurement is applied for the determination of the 2D electron gas (2DEG) mobility μ1 of the samples with two conducing channels using the equation μ1 = αμmeas. It is observed that the appropriate correction coefficient α depends on second channel parameters, i.e. the sheet resistance and mobility of the second conducting channel.

Large-area, thermally-sulfurized WS2 thin films: control of growth direction and use as a substrate for GaN epitaxy

We present a detailed study of the influence of metal seed thickness, amount of sulfur, sulfurization time and temperature on the morphology of large-area WS2 thin films prepared by sulfurization of electron-beam evaporated tungsten layers. We show how preferably horizontally or vertically oriented films can be achieved. The WS2 films with horizontal morphology were used as nearly lattice-matched substrates for the growth of GaN layers by MOVPE. While epitaxial films of GaN could be demonstrated, the thin-film WS2 layers show a enhanced propensity for degradation in the strongly reducing and high-temperature MOVPE growth environment.

Growth of GaN layers using Ga2O vapor synthesized from Ga2O3 and carbon

Abstract In this study, we grew GaN layers using Ga2O vapor synthesized from Ga2O3 powder and carbon powder. To precisely regulate the growth rate of the GaN layers, we controlled the supply rate of Ga2O vapor by controlling the reaction rate between Ga2O3 and carbon. The results showed that the growth rate increased linearly with partial pressure of Ga2O vapor and that GaN layers with the smooth (0 0 0 1) surface could be grown at 10–49 μm/h. X-ray diffraction (XRD) measurements revealed that the epitaxial layer was single-crystalline GaN and the FWHM of the GaN epitaxial layers ranged from 51 to 105 arcsec, confirming that the epitaxial layers inherit the high crystallinity of the seed substrates. Secondary ion mass spectrometry (SIMS) analysis showed that the oxygen concentrations in the epitaxial layers at the growth rates of 15 and 49 μm/h were 2.0 × 1017 and 2.5 × 1019 atoms/cm3, respectively. The carbon concentration in the epitaxial layers was below the detection limit. These values were the lowest seen in crystals obtained by the conventional technique using Ga2O vapor. These results show that this method, using Ga2O vapor produced by the reaction of Ga2O3 with carbon, has the potential to grow GaN crystals with high crystallinity and high purity by promoting the smoothness of the (0 0 0 1) surface.

Using a Multi-Layer Stacked AlGaN/GaN Structure to Improve the Current Spreading Performance of Ultraviolet Light-Emitting Diodes.

To obtain excellent current spreading performance of ultraviolet light-emitting diodes (UVLEDs), a 60-period stacked Si modulation-doped n-AlGaN/u-GaN structure is proposed to replace the traditional n-AlGaN structure. The high-resolution X-ray diffraction ω-scan rocking curves show that the periodic growth of AlGaN and GaN layers plays a positive role in reducing dislocation density. Compared with the conventional UV light-emitting diodes (LEDs), light emission micrographs of devices with a multi-layer stacked n-AlGaN/u-GaN structure reveal higher brightness and a more uniform distribution. In addition, the output power and external quantum efficiency under a 20-mA injection current are increased by 22% and 26.5%, respectively. Experimental and simulation results indicate that a multi-layer stacking structure can alleviate the current crowding effect in four ways: (1) a reduction in dislocation density; (2) replacement of quasi-two-dimensional electron transport with electronic bulk transport to enhance electron mobility; (3) an increase in electron concentration without improving the impurity concentration; and (4) a weakening of the electron scattering effect by reducing the impurity concentration.

HRXRD study of the effect of a nanoporous silicon layer on the epitaxial growth quality of GaN layer on the templates of SiC/por-Si/c-Si

Using High Resolution X-ray Diffraction (HRXRD) diagnostic techniques the influence of the transition layer of nanoporous silicon on the practical implementation and certain features of the epitaxial growth of GaN layers with the use of molecular beam epitaxy were investigated by means of plasma activation of nitrogen (MBE PA) on the templates of SiC/por-Si/c-Si. For the first time it was shown that introducing of the transition layer of nanoporous silicon in the template of SiC/por-Si/c-Si where the layer of 3C-SiC was obtained by substitution of the atoms had a number of indisputable advantages as compared with conventional silicon substrates. Particularly, such an approach, in fact, enabled a 90% reduction in the level of stresses in the crystalline lattice of the epitaxial GaN layer which was synthesized on SiC surface of SiC/por-Si/c-Si template by means of MBE PA technique as well as to decrease some of vertical dislocations within GaN layer.

Effect of the transition porous silicon layer on the properties of hybrid GaN/SiC/por-Si/Si(1 1 1) heterostructures

Using the complexes of structural and spectroscopic diagnostic techniques, we investigated the influence of the layer of nanoporous silicon on the practical implementation and certain features of the epitaxial growth of GaN layers with the use of MBE PA on the templates of SiC/por-Si/c-Si.For the first time, the layer of nanoporous silicon has been introduced in the template of SiC/por-Si/c-Si where the layer of 3C-SiC was obtained by the substitution of the atoms; this had a number of indisputable advantages as compared to conventional silicon substrates. In particular, the abovementioned approach enabled a 90% reduction in the stress levels in the crystalline lattice of GaN layer that was synthesised on the SiC surface of the SiC/por-Si/c-Si template; it also decreased some of the vertical dislocations within the GaN layer. We also found that the use of the SiC/por-Si/c-Si template resulted in the formation of a more uniform (by its quality) GaN layer without any visible extensive defects. As a result, this enabled us to attain unique optical and electro-physical characteristics for the hybrid heteroepitaxial GaN/SiC structures.The obtained data will be very important for understanding the basic physics of nanoheterostructures of GaN/SiC/por-Si, thus promoting their potential applications in optoelectronics.

Влияние давления при эпитаксии на свойства слоев GaN

Growth of GaN layers by MOVPE on sapphire substrates at various pressures, including above atmospheric, was studied. It is shown that epitaxy at higher pressures does not change the crystal perfection of the layers, the electron mobility, and the impurities incorporation, but leads to the formation of a surface with a smaller lateral scale of inhomogeneities. The epitaxy pressure also affects the ratio of the intensity of the band-edge and impurity-related lines in the photoluminescence spectra and the leakage currents in the reverse-biased Schottky barrier.

Влияние слоя нанопористого кремния на практическую реализацию и особенности эпитаксиального роста слоев GaN на темплейтах SiC/por-Si/c-Si

Abstract A set of structural and spectroscopic methods of diagnostics is used to study the influence of a nanoporous silicon ( por -Si) transition layer on the practical implementation and specific features of growth of GaN layers on SiC/ por -Si/ c -Si templates by molecular-beam epitaxy with the plasma activation of nitrogen. It is shown that a por -Si transition layer introduced into a template, in which a 3 C -SiC layer is created by the method of atom substitution, offers unquestionable advantages over standard silicon substrates. Specifically, such an approach makes it possible to lower the level of stresses in the crystal lattice of the epitaxial GaN layer by about 90% and to reduce the fraction of vertical dislocations in the GaN layer. The GaN layer is grown on the surface of the SiC layer, which in turn is on the surface of the SiC/ por -Si/ c -Si template. It is found for the first time that the use of the SiC/ por -Si/ c -Si template brings about the formation of a qualitatively more uniform GaN layer free of visible extended defects.

Recent progress of high temperature vapor phase epitaxy for the growth of GaN layers – Controlled coalescence of nucleation layers

Abstract This paper describes recent developments of the high temperature vapor phase epitaxy (HTVPE) for the growth of GaN seed layers on sapphire substrates. The concept of a controlled coalescence of nucleation layers adopted from metal organic vapor phase epitaxy is used for the first time in HTVPE to reduce the density of threading dislocations of the seed layers. This concept implies a delay of coalescence by reducing the ammonia flow and increasing the admixture of hydrogen in the carrier gas. A series of nucleation and seed layers grown under different ammonia flows and hydrogen concentrations is presented. The layers are analyzed by X-ray diffraction as well as by optical and scanning electron microscopy. Microscopic investigations reveal that nucleation layers with large facetted growth islands are formed during the delayed coalescence. The measured rocking curves and estimated dislocation densities from the mosaicity model indicate a significant improvement of the crystal quality of the corresponding seed layers. The lowest dislocation densities are found to be in the range of 4–5 × 108 cm−2, compared with a dislocation density of about 1 × 109 cm−2 in a reference layer grown without an intentional delay of coalescence. A possible explanation is that the existence of large growth islands with extended pyramidal facets causes a preferred bunching and annihilation of dislocations.