InGaN Growth Research Articles

Molecular Beam Epitaxy Growth of GaN/InGaN Heterostructures under In‐Bilayer Stabilized Conditions

Molecular beam epitaxy is applied for the GaN/InGaN growth under In‐bilayer stabilized conditions, which require substrate temperatures above 580 °C for sufficient indium desorption. Under these conditions the indium content is limited to , as determined by a novel reflection high energy electron diffraction technique, which is based on indium adsorption and subsequent consumption at varying gallium flux. As‐grown structures show degraded surfaces after indium desorption. This effect is eliminated by applying excess gallium before and after InGaN growth, which results in morphologies and dislocation densities comparable to GaN/AlGaN structures.

Nucleation mechanism of gas-phase InGaN nanoparticles

The nucleation mechanism of nanoparticles during the growth of InGaN by metal–organic chemical vapor deposition was investigated using density functional theory quantum chemical calculations to explore molecules that are most likely to become the cores of nanoparticles. On the basis of previous research on the nucleation mechanism of nanoparticles in the III-V system, several possible nucleation pathways are proposed for gas-phase InGaN nanoparticles, focusing on the cross-reaction between In and Ga. The possibility of forming different cross-reaction products is also discussed. Among the several reaction pathways examined, the possibilities of generating several cross-reaction products with similar configurations (i.e., [MMGaNHDMInNH2]2 and [MMInNHDMGaNH2]2, [MMGaNHDMInNH2]3 and [MMInNHDMGaNH2]3) were similar; the self-polymerization of [MMXNH]n was most likely to occur, and [MMInNH]4 and [MMInNH]6 were most likely to become the core of nanoparticles.

Incorporation of indium into GaN layers in the context of MOVPE thermodynamics and growth – Ab initio studies

Indium incorporation on the GaN surface (0001) has been analyzed in the context of InGaN growth by Metal Organic Vapor Phase Epitaxy (MOVPE). To understand phenomena at the atomic level, calculations based on Density Functional Theory (DFT) and thermodynamic analysis of the surface processes have been performed. The considerations are limited to the terrace on GaN (0001) surface, covered fully by NH3/NH2 species. The ab initio results indicate that the adsorption energy strongly depends on the electronic properties of the surface. The adsorption energy varies from EadsDFT=-10.0eV for fully NH2 covered to EadsDFT=-3.0eV for mixed 0.69 NH2 + 0.31 NH3 covered surface. This change is related to Fermi level position moved from within valence band, across the bandgap, up to within conduction band. The shift in Fermi energy impacts the energy gained during adsorption due to electron occupation of surface states. Furthermore, a comprehensive thermodynamic analysis was conducted, revealing an additional, albeit smaller, contribution from the vibrational degrees of freedom to free energy. The combined effect of these factors leads to an increase in the equilibrium pressures of indium related to the increase in hydrogen pressure, which correlates with high coverage by ammonia molecules. The results demonstrate that the significant reduction in indium incorporation into InGaN layers during MOVPE growth under conditions of low hydrogen concentration in the vapor can be attributed to the influence of hydrogen on In adsorption processes.

High crystallinity N-polar InGaN layers grown on cleaved ScAlMgO4 substrates

We have grown high-crystallinity InGaN layers on ScAlMgO4 (SAM) substrates using metalorganic vapor-phase epitaxy. We have prepared atomically flat SAM substrates by cleaving them along the c-plane and have utilized direct InGaN growth without any low-temperature buffer layer. The resulting InGaN layer has a distinct hexagonal hillock morphology and remarkable crystalline quality. The x-ray rocking curve measurements showed that (0002̄) and (10–1–2) peaks full widths at half-maximum are as good as 384 and 481 arcsec, respectively. The calculated threading dislocations densities are as low as 2.9 × 108 and 1.6 × 109 cm−2 in the case of screw-type and edge-type dislocations, respectively.

Plasma-Assisted Halide Vapor Phase Epitaxy for Low Temperature Growth of III-Nitrides

Developing growth techniques for the manufacture of wide band gap III-nitrides semiconductors is important for the further improvement of optoelectronic applications. A plasma-assisted halide phase vapor epitaxy (PA-HVPE) approach is demonstrated for the manufacture of undoped and In-doped GaN layers at ~600 °C. A dielectric barrier discharge (DBD) plasma source is utilized for the low-temperature activation of ammonia. The use of the plasma source at a growth temperature of ~600 °C increases the growth rate from ~1.2 to ~4–5 µm/h. Furthermore, the possibility for the growth of InGaN at ~600 °C has been studied. Precursors of GaCl and InCl/InCl3 are formed in situ in the reactor by flowing HCl gas over a melt of metallic Ga and In, respectively. The In concentration was low, in the order of a few percent, as the incorporation of In is reduced by plasma due to the activation of chlorine-containing species that etch the relatively poorly bonded In atoms. Nevertheless, the approach of using plasma for ammonia activation is a very promising approach to growing epitaxial III-nitrides at low temperatures.

The Introduction of Intermediate Layers for InGaN Growth on ScAlMgO4 through Trihalide Vapor‐Phase Epitaxy

Herein, the effect of intermediate layers for high‐speed InGaN growth using trihalide vapor‐phase epitaxy (THVPE) on ScAlMgO4 (SAM) is investigated. The coverage and thickness of the intermediate layer have a significant impact on both composition and crystalline quality. Herein, InGaN is successfully fabricated with 17% of indium on SAM in a lattice‐matched state employing THVPE, showing the X‐ray rocking curve full width at half maximum values less than 1000 arcsec, in a two‐step growth. THVPE is used to indicate the possibility of fabricating high‐quality InGaN quasi‐substrate with high indium composition.

Structure of V-defects in long wavelength GaN-based light emitting diodes

The V-defect is a naturally occurring inverted hexagonal pyramid structure that has been studied in GaN and InGaN growth since the 1990s. Strategic use of V-defects in pre-quantum well superlattices or equivalent preparation layers has enabled record breaking efficiencies for green, yellow, and red InGaN light emitting diodes (LEDs) utilizing lateral injection of holes through the semi-polar sidewalls of the V-defects. In this article, we use advanced characterization techniques such as scattering contrast transmission electron microscopy, high angle annular dark field scanning transmission electron microscopy, x-ray fluorescence maps, and atom probe tomography to study the active region compositions, V-defect formation, and V-defect structure in green and red LEDs grown on (0001) patterned sapphire and (111) Si substrates. We identify two distinct types of V-defects. The “large” V-defects are those that form in the pre-well superlattice and promote hole injection, usually nucleating on mixed (Burgers vector b=±a±c) character threading dislocations. In addition, “small” V-defects often form in the multi-quantum well region and are believed to be deleterious to high-efficiency LEDs by providing non-radiative pathways. The small V-defects are often associated with basal plane stacking faults or stacking fault boxes. Furthermore, we show through scattering contrast transmission electron microscopy that during V-defect filling, the threading dislocation, which runs up the center of the V-defect, will “bend” onto one of the six {101¯1} semi-polar planes. This result is essential to understanding non-radiative recombination in V-defect engineered LEDs.

Effect of InGaN well layer growth rate upon photoluminescence of InGaN/GaN multiple-quantum-well structures

The photoluminescence (PL) spectra of a range of InGaN/GaN multiple-quantum-well (MQW) structures dependent on temperature that have different well layer growth rates (WLGRs) were analyzed. The results show that the InGaN well layers of all structures are composed of two separate phases: In-rich clusters and an InGaN matrix. Also, with increasing WLGR, the In-rich clusters-related PL intensity increases significantly relative to its InGaN matrix-related counterpart at low temperatures, meanwhile the behavior of temperature-dependent relaxation followed by expansion of the carriers in both phases becomes less significant. These behaviors are interpreted thus: an increasing WLGR can shorten exposure of the upper layer face to the atmosphere during the growth of InGaN well layers, and this suppresses the migration of In atoms on the surface from the In-rich clusters to the surrounding InGaN matrix, thus resulting in an increasing of the density of states ratio of the In-rich clusters to the InGaN matrix and a weakening of the composition fluctuation in both phases.

Strain-induced formation of self-assembled InGaN/GaN superlattices in nominal InGaN films grown by plasma-assisted molecular beam epitaxy

In this work, we report the spontaneous formation of superlattice structures in nominal InGaN films grown by plasma-assisted molecular beam epitaxy. A 700-nm-thick self-assembled ${\mathrm{In}}_{0.2}{\mathrm{Ga}}_{0.8}\mathrm{N}/\mathrm{GaN}$ superlattice with excellent structural quality was achieved. Strain was studied as a possible driving force for the formation of self-assembled superlattice (SASL) structure by growth of InGaN on ZnO substrate using similar growth conditions. The SASL structures were optically characterized using photoluminescence spectroscopy. Structural characterization was conducted via transmission electron microscopy and atom probe tomography. High-resolution x-ray diffraction (XRD) and XRD reciprocal space map were utilized to determine the average composition and the degree of relaxation of InGaN films. We propose that the vertical phase separation observed in the SASL structure is caused by high-temperature growth and intensified by strain. This work provides a method for engineering strain and growth of thick InGaN films for a variety of applications including solar cells and photodetectors.

A synchrotron X-ray topography study of crystallographic defects in ScAlMgO4 single crystals

ScAlMgO4 is a promising material that can be used to fabricate lattice-matched substrates for the epitaxial growth of GaN and InGaN. To improve its crystal quality, we used synchrotron X-ray topography to study the dislocations in a 50.8-mm-diameter ScAlMgO4 single crystal grown via the Czochralski method. Dislocation character was analyzed in terms of Burgers vectors by comparing dislocation contrasts recorded using six g-vectors that represented the m+c, a+c, and c-type crystal planes. The central area of the substrate, which corresponded to the portion grown vertically under the seed crystal, was characterized by millimeter-sized domains bound by mixed-type dislocation arrays with both in-plane and c-axis Burgers vectors. Basal plane dislocations (BPDs) typically formed an arrow-like shape with three branches. Growth striations with alternating high- and low-dislocation-density areas were observed in the diameter-expanding area and high-dislocation-density areas were covered with tangled BPD networks. The mechanisms of dislocation generation, multiplication, and reaction are discussed based on their line directions and dislocation character.

Dislocation and indium droplet related emission inhomogeneities in InGaN LEDs

This report classifies emission inhomogeneities that manifest in InGaN quantum well blue light-emitting diodes grown by plasma-assisted molecular beam epitaxy on free-standing GaN substrates. By a combination of spatially resolved electroluminescence and cathodoluminescence measurements, atomic force microscopy, scanning electron microscopy and hot wet potassium hydroxide etching, the identified inhomogeneities are found to fall in four categories. Labeled here as type I through IV, they are distinguishable by their size, density, energy, intensity, radiative and electronic characteristics and chemical etch pits which correlates them with dislocations. Type I exhibits a blueshift of about 120 meV for the InGaN quantum well emission attributed to a perturbation of the active region, which is related to indium droplets that form on the surface in the metal-rich InGaN growth condition. Specifically, we attribute the blueshift to a decreased growth rate of and indium incorporation in the InGaN quantum wells underneath the droplet which is postulated to be the result of reduced incorporated N species due to increased N2 formation. The location of droplets are correlated with mixed type dislocations for type I defects. Types II through IV are due to screw dislocations, edge dislocations, and dislocation bunching, respectively, and form dark spots due to leakage current and nonradiative recombination.

Metal Organic Vapor Phase Epitaxy of Thick N-Polar InGaN Films

Hillock-free thick InGaN layers were grown on N-polar GaN on sapphire by metal organic vapor phase epitaxy using a digital growth scheme and H2 as surfactant. Introducing Mg to act as an additional surfactant and optimizing the H2 pulse time, In compositions up to 17% were obtained in 100 nm thick epilayers. Although Mg adversely affected the In incorporation, it enabled maintenance of a good surface morphology while decreasing the InGaN growth temperature, resulting in a net increase in In composition. The parameter space of growth temperature and Mg precursor flow to obtain hillock-free epilayers was mapped out.

Combined SEM-CL and STEM investigation of green InGaN quantum wells

The microstructure of green-emitting InGaN/GaN quantum well (QW) samples grown at different temperatures was studied using cross-section scanning transmission electron microscopy (STEM) and plan-view cathodoluminescence (CL). The sample with the lowest InGaN growth temperature exhibits microscale variations in the CL intensity across the sample surface. Using STEM analysis of such areas, the observed darker patches do not correspond to any observable extended defect. Instead, they are related to changes in the extent of gross-well width fluctuations in the QWs, with more brightly emitting regions exhibiting a high density of such fluctuations, whilst dimmer regions were seen to have InGaN QWs with a more uniform thickness.

Comprehensive model toward optimization of SAG In-rich InGaN nanorods by hydride vapor phase epitaxy

Controlled growth of In-rich InGaN nanowires/nanorods (NRs) has long been considered as a very challenging task. Here, we present the first attempt to fabricate InGaN NRs by selective area growth using hydride vapor phase epitaxy. It is shown that InGaN NRs with different indium contents up to 90% can be grown by varying the In/Ga flow ratio. Furthermore, nanowires are observed on the surface of the grown NRs with a density that is proportional to the Ga content. The impact of varying the NH3 partial pressure is investigated to suppress the growth of these nanowires. It is shown that the nanowire density is considerably reduced by increasing the NH3 content in the vapor phase. We attribute the emergence of the nanowires to the final step of growth occurring after stopping the NH3 flow and cooling down the substrate. This is supported by a theoretical model based on the calculation of the supersaturation of the ternary InGaN alloy in interaction with the vapor phase as a function of different parameters assessed at the end of growth. It is shown that the decomposition of the InGaN solid alloy indeed becomes favorable below a critical value of the NH3 partial pressure. The time needed to reach this value increases with increasing the input flow of NH3, and therefore the alloy decomposition leading to the formation of nanowires becomes less effective. These results should be useful for fundamental understanding of the growth of InGaN nanostructures and may help to control their morphology and chemical composition required for device applications.

MOCVD growth of thick V-pit-free InGaN films on semi-relaxed InGaN substrates

The MOCVD growth of InGaN:Si base layers on a semi-relaxed InGaN substrate, where growth is generally difficult due to the presence of V-pits, is examined. These V-pits can propagate through the crystal, causing severe morphological degradation and significantly reducing material quality for device use. Such V-pits may also be a source of leakage current if they extend from the substrate through p-n junction. A wide range of InGaN growth conditions and their impact on V-pit formation and density are investigated. The use of thin GaN interlayers, carrier gas selection, and V/III ratio are found play a critical role in managing V-pit quantity and size. Finally, high temperature GaN interlayers are implemented, fully eliminating V-pit formation in 1200 nm thick InGaN base layers grown coherently on semi-relaxed InGaN substrates.

Cathodoluminescence spatially resolves optical transitions in thick group-III and N-polar InGaN films

The growth of thick group-III polar InGaN films beyond the critical thickness remains a challenge due to the large miscibility gap and lattice mismatch between InN and GaN leading to phase separation and inhomogeneous distribution of indium that impacts the luminescence properties across the film. The growth of N-polar InGaN can circumvent these challenges due to the increased stability of InN, thus improving film quality. However, overall luminescence from N-polar InGaN films is consistently lower than that of group-III polar InGaN films grown under identical conditions. In this study, spatially resolved cathodoluminescence (CL) measurements are used to reveal the optical properties of group-III and N-polar thick InGaN films. In the case of group-III polar films, predominant CL luminescence arises from the periphery of V-pits which are regions with a high accumulation of indium, while negligible CL luminescence is observed at the apex of the V-pit, indicative of centers for non-radiative recombination. Large differences in the CL luminescence intensity between the group-III polar and N-polar InGaN are a result of significant differences in the carrier lifetimes of the respective polarities (∼50–200 ps: N-polar, ∼500–700 ps: group-III polar InGaN). Since the decay behavior of the transient in N-polar InGaN is bi-exponential, it is suspected that oxygen impurities play a dominant role in the overall luminescence quenching in N-polar InGaN films.

Experimental quantification of atomically-resolved HAADF-STEM images using EDX

Atomically-resolved mappings of the indium composition in InGaN/GaN multi-quantum well structures have been obtained by quantifying the contrast in HAADF-STEM. The quantification procedure presented here does not rely on computation-intensive simulations, but rather uses EDX measurements to calibrate the HAADF-STEM contrast. The histogram of indium compositions obtained from the mapping provides unique insights into the growth of InGaN: the transition from GaN to InGaN and vice versa occurs in discreet increments of composition; each increment corresponds to one monolayer of the interface, indicating that nucleation takes longer than the lateral growth of the step. Strain-state analysis is also performed by applying Peak-Pair Analysis to the positions of the atomic columns identified the quantification of the contrast. The strain mappings yield an estimate of the composition in good agreement with the one obtained from quantified HAADF-STEM, albeit with a lower precision. Possible improvements to increase the precision of the strain mappings are discussed, opening potential pathways for the quantification of arbitrary quaternary alloys at atomic scales.

Role of defect saturation in improving optical response from InGaN nanowires in higher wavelength regime

Growth of InGaN, having high Indium composition without compromising crystal quality has always been a great challenge to obtain efficient optical devices. In this work, we extensively study the impact of non-radiative defects on optical response of the plasma assisted molecular beam epitaxy (PA-MBE) grown InGaN nanowires, emitting in the higher wavelength regime ( nm). Our analysis focuses into the effect of defect saturation on the optical output, manifested by photoluminescence (PL) spectroscopy. Defect saturation has not so far been thoroughly investigated in InGaN based systems at such a high wavelength, where defects play a key role in restraining efficient optical performance. We argue that with saturation of defect states by photo-generated carriers, the advantages of carrier localization can be employed to enhance the optical output. Carrier localization arises because of Indium phase segregation, which is confirmed from wide PL spectrum and analysis from transmission electron microscopy (TEM). A theoretical model has been proposed and solved using coupled differential rate equations in steady state to undertake different phenomena, occurred during PL measurements. Analysis of the model helps us understand the impact of non-radiative defects on PL response and identifying the origin of enhanced radiative recombination.

A MOVPE method for improving InGaN growth quality by pre-introducing TMIn* *Project supported by the National Key Research and Development Program of China (Grant Nos. 2016YFB0400803 and 2016YFB0401801) and the National Natural Science Foundation of China (Grant Nos. 61674138, 61674139, 61604145, 61574135, and 61574134).

We propose a metal organic vapor phase epitaxy (MOVPE) method of pre-introducing TMIn during the growth of u-GaN to improve the subsequent growth of InGaN and discuss the impact of this method in detail. Monitoring the MOVPE by the interference curve generated by the laser incident on the film surface, we found that this method avoided the problem of the excessive InGaN growth rate. Further x-ray diffraction (XRD), photoluminescence (PL), and atomic force microscope (AFM) tests showed that the quality of InGaN is improved. It is inferred that by introducing TMIn in advance, the indium atom can replace the gallium atom in the reactor walls, delivery pipes, and other corners. Hence the auto-incorporation of gallium can be reduced when InGaN is grown, so as to improve the material quality.

InGaN islands and thin films grown on epitaxial graphene

In this work, the growth of InGaN on epitaxial graphene by molecular beam epitaxy is studied. The nucleation of the alloy follows a three-dimensional (3D) growth mode in the observed temperature range of 515 °C–765 °C, leading to the formation of dendrite-like islands. Careful Raman scattering experiments show that the graphene underneath is not degraded by the InGaN growth. Moreover, lateral displacement of the nuclei during an atomic force microscopy (AFM) scan demonstrates weak bonding interactions between the InGaN and the graphene. Finally, a longer growth time of the alloy gives rise to a compact thin film in a partial epitaxial relationship with the SiC underneath the graphene.