InGaN Layers Research Articles

Atomic Diffusion of Indium through Threading Dislocations in InGaN Quantum Wells.

The compositional and structural investigations of threading dislocations (TDs) in InGaN/GaN multiple quantum wells were carried out using correlative transmission electron microscopy (TEM) and atom probe tomography (APT). The correlative TEM/APT analysis on the same TD reveals that the indium atoms are diffused along the TD and its concentration decreases with distance from the InGaN layer. On the basis of the results, we directly observed that the indium atoms originating from the InGaN layer diffuse toward the epitaxial GaN surface through the TD, and it is considered to have occurred via the pipe diffusion mechanism induced by strain energy relaxation.

Anisotropic Strain Relaxation in Semipolar (112¯2) InGaN/GaN Superlattice Relaxed Templates.

Semipolar (112) InGaN/GaN superlattice templates with different periodical InGaN layer thicknesses were grown on m-plane sapphire substrates using metal-organic chemical vapor deposition (MOCVD). The strain in the superlattice layers, the relaxation mechanism and the influence of the strain relaxation on the semipolar superlattice template were explored. The results demonstrated that the strain in the (112) InGaN/GaN superlattice templates was anisotropic and increased with increasing InGaN thickness. The strain relaxation in the InGaN/GaN superlattice templates was related to the formation of one-dimension misfit dislocation arrays in the superlattice structure, which caused tilts in the superlattice layer. Whereas, the rate of increase of the strain became slower with increasing InGaN thickness and new misfit dislocations emerged, which damaged the quality of the superlattice relaxed templates. The strain relaxation in the superlattice structure improved the surface microtopography and increased the incorporation of indium in the InGaN epitaxial layers.

Impact of Generation and Relocation of Defects on Optical Degradation of Multi-Quantum-Well InGaN/GaN-Based Light-Emitting Diode.

The defectiveness of InGaN-based quantum wells increases with low indium contents, due to the compressive strain induced by the lattice mismatch between the InGaN and GaN layers, and to the stronger incorporation of defects favored by the presence of indium. Such defects can limit the performance and the reliability of LEDs, since they can act as non-radiative recombination centers, and favor the degradation of neighboring semiconductor layers. To investigate the location of the layers mostly subjected to degradation, we designed a color-coded structure with two quantum wells having different indium contents. By leveraging on numerical simulations, we explained the experimental results in respect of the ratio between the emissions of the two main peaks as a function of current. In addition, to evaluate the mechanisms that limit the reliability of this type of LED, we performed a constant-current stress test at high temperature, during which we monitored the variation in the optical characteristics induced by degradation. By comparing experimental and simulated results, we found that degradation can be ascribed to an increment of traps in the active region. This process occurs in two different phases, with different rates for the two quantum wells. The first phase mainly occurs in the quantum well closer to the p-contact, due to an increment of defectiveness. Degradation follows an exponential trend, and saturates during the second phase, while the quantum well close to the n-side is still degrading, supporting the hypothesis of the presence of a diffusive front that is moving from the p-side towards the n-side. The stronger degradation could be related to a lowering of the injection efficiency, or an increment of SRH recombination driven by a recombination-enhanced defect generation process.

Analysis of phonon transport through heterointerfaces of InGaN/GaN via Raman imaging using double-laser system: The effect of crystal defects at heterointerface

The pump and probe technique in Raman spectroscopy is used to demonstrate the phonon transport properties of an In0.05Ga0.95N/GaN heterostructure. The pump laser generates phonons via the energy relaxation of the generated carriers in the electronic energy bands of the InGaN layer, and the Raman signal is obtained using the probe laser. In the present study, 532-nm and 325-nm lasers are utilized. The phonon transport from the InGaN layer to the GaN layer across the heterointerface is blocked near the crystal defects inherited from the GaN layer. This phenomenon is consistent with our previous report demonstrating the blocking of phonon transport across the In0.16Ga0.84N/GaN heterointerface near the misfit dislocations in the In0.16Ga0.84N layer. Lateral phonon transport over a distance of 20 μm is observed, which is dominated by diffusive phonon transport. This method has the advantage of enabling the study of phonon transport processes inside and at interfaces of films with crystal defects by visualizing the shift of phonon-mode energies due to local heating.

Influence of well position on the electroluminescence characteristics of InGaN/GaN single quantum well red light-emitting diodes

In this paper, the influence of the position of InGaN layer well on the electroluminescence (EL) properties of red InGaN/GaN single quantum well (SQW) light-emitting diodes (LEDs) is investigated numerically. It is found that as the InGaN well shifts from the P-region to the N-region, the LED's EL intensity decreases, and the spectrum red-shifts. By analyzing the EL spectra, energy band structures and carrier distribution, it is considered that, when the well position moves away from the P-region, the injection barrier width of holes increases, while that of electrons decreases. Thus, the injection efficiency of electrons and holes is enhanced and weakened, respectively. Compared with electrons, the injection efficiency of holes with larger effective mass can be significantly reduced by a thicker barrier. As a result, the hole concentration in InGaN QW is reduced more severely, leading to a reduction in the total amount of carriers. The decreased concentration of injected carriers may weaken the carrier screening effect to the polarization electrical field, thereby the polarization-induced quantum confined-Stark effect is enhanced. As a consequence, for the SQW red LED whose InGaN well is far away from the P-region, the EL intensity of the SQW red LED is reduced and the peak wavelength becomes longer.

C-Plane Blue Micro-LED With 1.53 GHz Bandwidth for High-Speed Visible Light Communication

A high-speed c-plane InGaN/GaN blue micro-scale light-emitting diodes (Micro-LEDs) with a pixel diameter of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$60~\mu \text{m}$ </tex-math></inline-formula> , and a high −3 dB modulation bandwidth of 1.53 GHz at the current density of 5996 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> were demonstrated. The ultra-thin quantum well (QW) structure with InGaN layer of 1 nm was designed, and optimized variable temperature growth processes were adopted to increase In incorporation and localization effect in the quantum wells, which greatly reduce the polarization field and increase the overlap of electron-hole wave functions. The differential carrier lifetime was effectively shortened, increasing the modulation bandwidth of the Micro-LED. And a data rate of 5.27 Gbps with a bit error rate (BER) of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$3.1\times 10^{-{3}}$ </tex-math></inline-formula> was demonstrated with orthogonal frequency-division multiplexing (OFDM) signal for a 1 m free space real-time visible light communication (VLC) based on such high-speed micro-LED. Promising results in this work may support a great development of high-speed VLC applications.

Microstructural analysis of N-polar InGaN directly grown on a ScAlMgO4(0001) substrate

We report the characterization of a N-polar InGaN layer deposited by metalorganic vapor-phase epitaxy on a ScAlMgO4(0001) (SAM) substrate without a low-temperature buffer layer. The InGaN layer was tensile-strained, and its stoichiometry corresponded to In0.13Ga0.87N. We also present the microstructural observation of the InGaN/SAM interface via integrated differential phase contrast-scanning transmission electron microscopy. The results show that the interface between N-polar InGaN and SAM occurs between the O atoms of the O–Sc SAM surface and the (Ga,In) atoms of InGaN.

Angle-dependence of ADF-STEM intensities for chemical analysis of InGaN/GaN

In this paper we perform angular resolved annular-dark field (ADF) scanning-transmission electron microscopy (STEM) to study the scattered intensity in an InGaN layer buried in GaN as a function of the scattering angle. We achieved angular resolution with a motorized iris aperture in front of the ADF detector. Using this setup, we investigated how the intensities measured in various angular ranges agree with multislice simulations in the frozen-lattice approximation. We observed a strong influence of relaxation induced surface-strain fields on the ADF intensity, measured its angular characteristics and compared the result with simulations.To assess the agreement of the measured intensity with simulations, we evaluated the specimen thickness in GaN and the indium concentration in InGaN for each angular interval by comparing the measured intensities with simulations. The thickness was strongly overestimated for scattering angles below 40mrad and also the evaluated indium concentration varies with the considered angular range. Using simulations, we investigated which angular ranges show a high sensitivity to variations of the thickness and which intervals strongly depend on the indium concentration. By combining two angular intervals, the indium concentration and the specimen thickness were determined simultaneously, which has potential advantages over the usual quantification method. It is shown that inelastic scattering, surface contamination and mistilt can have an influence on the measured intensity, especially at lower scattering angles below 30–50mrad, which might explain the observed difference between the frozen lattice simulation and the experiment.

Effect of spiral-like islands on structural quality, optical and electrical performance of InGaN/GaN heterostructures grown by metal organic chemical vapour deposition

InGaN layers were grown by metal organic chemical vapour deposition (MOCVD) technique on GaN/sapphire substrates by varying the growth time. The formation of spiral-like growth domains was observed using atomic force microscopy, revealed that the InGaN layer is atomically flat. In addition, the surface roughness was found to be dependent on the thickness of InGaN layer. The spiral-like islands was correlated with screw type threading dislocation density of InGaN. The thickness-dependent threading dislocation density was investigated using high-resolution x-ray diffraction. The Indium composition and thickness of InGaN were found to be 15–16% and 20–50 nm using the smooth fit software. The structural characteristics obtained using reciprocal space mapping indicate that the InGaN/GaN heterostructures are coherently strained. Photoluminescence (PL) spectra exhibit variations in the band-edge emissions between 437.0 and 443.5 nm peaks with varying temperature, showcasing a slight shift in all the InGaN samples. The low-temperature PL spectra revealed dominant emission mechanism. From the hall effect data it was observed that InGaN layer attained high mobility value close to that of the theoretical limit of GaN and/or InN at 300 K. It is worth to note that the compressive strain present in InGaN layer led to high sheet concentration when compared to that of the tensile strained InGaN layer. Hence, the spiral like islands entrenched InGaN layers which can be effectively utilized for optoelectronic applications.

PA MBE growth of intermediate-composition InGaN layers for red and near-IR laser sources

This paper presents the results of studying the growth of InGaN layers with a high (50-80%) indium content by molecular beam epitaxy with nitrogen plasma activation on sapphire substrates with GaN/AlN buffer layers. It is shown that the processes of dissociation and phase separation of the growing InGaN layer, which occur in structures with an indium fraction of about 50%, cannot be suppressed due to the transition to a lower temperature growth (470oC-&amp;gt;390oC) without significant degradation of the crystalline quality of the formed structures and a sharp decrease in their emissivity. As an alternative approach to suppressing diffusion processes on the growth surface and, as a result, obtaining homogeneous InGaN layers with an [In] content ~50%, high-temperature (470oC) growth under highly nitrogen-enriched conditions (flux ratio III/V ~0.6) was tested. The InGaN layers grown in this way show intense photoluminescence, while at the same time showing no signs of phase separation according to X-ray diffraction data. This is critically important for the possibility of implementing optical amplification and laser generation in such structures in the red region of the spectrum and in the immediately adjacent part of the near infrared region. Keywords: indium and gallium nitride, molecular beam epitaxy, spinodal decomposition, photoluminescence.

Формирование слоев InGaN средних составов методом МПЭ ПА для лазерных источников красного и ИК диапазона

This paper presents the results of studying the growth of InGaN layers with a high (50-80%) indium content by molecular beam epitaxy with nitrogen plasma activation on sapphire substrates with GaN/AlN buffer layers. It is shown that the processes of dissociation and phase separation of the growing InGaN layer, which occur in structures with an indium fraction of about 50%, cannot be suppressed due to the transition to a lower temperature growth (470°C - 390°C) without significant degradation of the crystalline quality of the formed structures and a sharp decrease in their emissivity. As an alternative approach to suppressing diffusion processes on the growth surface and, as a result, obtaining homogeneous InGaN layers with an [In] content of ~ 50%, high-temperature (470°C) growth under highly nitrogen-enriched conditions (flux ratio III/V ~ 0.6) was tested. The InGaN layers grown in this way show intense photoluminescence, while at the same time showing no signs of phase separation according to X-ray diffraction data. This is critically important for the possibility of implementing optical amplification and laser generation in such structures in the red region of the spectrum and in the immediately adjacent part of the near infrared region.

Plasma-Assisted Molecular Beam Epitaxy of In-Rich InGaN: Growth Optimization for Near-IR Lasing

Near-infrared stimulated emission (SE) from InGaN layers grown by plasma-assisted molecular beam epitaxy has been studied, and the influence of the growth temperature (T gr) on the SE threshold has been revealed. The obtained experimental data strongly suggest a two-layer model for the grown InGaN structure with a thin defect-rich interface layer and a relatively pure InGaN bulk responsible for light emission. For the latter, the crystalline quality appears to be unaffected by the growth temperature, at least in terms of free electron concentration, which is supported by the similar spontaneous luminescence intensities measured throughout the entire T gr range of 430 °C–510 °C. However, the quality of the interface layer improves with increasing T gr, leading to a decrease in the SE threshold down to ∼10 kW cm−2 at T = 77 K for the samples grown at T gr = 470 °C–480 °C. For the higher growth temperatures (T gr ≥ 490 °C), the SE threshold increases rapidly with T gr, apparently related to the strong waveguide losses due to the increasing surface roughness of the InGaN layer, and SE vanishes completely at T gr = 510 °C, further suppressed by the partial phase separation of the InGaN alloy.

Peculiarities of nucleation and growth of InGaN nanowires on SiC/Si substrates by HVPE

The growth of InGaN layers on hybrid SiC/Si substrates with orientations (100), (110), and (111) by the HVPE method was studied at temperatures that wittingly exceed the temperature of InN decomposition onto nitrogen atoms and metallic In (1000oC). On substrates with orientations (110) and (111), the formation of InGaN whisker nanocrystals was observed. The shape and growth mechanisms of nanocrystals were investigated. It is shown that nanocrystals nucleate on the (111) surface only inside V-defects formed at the points where screw dislocations exit onto the surface. On the (110) surface, nanocrystals are formed only on pedestals that arise during the film growth. An explanation is given for the difference in the growth mechanisms of nanocrystals on substrates of different orientations. Keywords: InGaN, heterostructures, SiC on Si, silicon, whisker nanocrystals, nanostructures, atomic substitution method

Особенности зарождения и роста нитевидных нанокристаллов InGaN на подложках SiC/Si методом хлорид-гидридной эпитаксии

The growth of InGaN layers on hybrid SiC/Si substrates with orientations (100), (110), and (111) by the HVPE method was studied at temperatures that wittingly exceed the temperature of InN decomposition onto nitrogen atoms and metallic In (1000C). On substrates with orientations (110) and (111), the formation of InGaN nanocrystals was observed. The shape and growth mechanisms of nanocrystals were investigated. It is shown that nanocrystals nucleate on the (111) surface only inside V-defects formed at the points where screw dislocations exit onto the surface. On the (110) surface, nanocrystals are formed only on pedestals that arise during the film growth. An explanation is given for the difference in the growth mechanisms of nanocrystals on substrates of different orientations.

Ultra-weak emission and microcurrents instabilities in blue GaN LEDs at different stages of degradation

It is found that the ultra-weak luminescence observed in microcurrents mode in blue GaN LEDs with multiple quantum wells is due to tunnel-recombination processes with the participation of defect states and local potential wells of various depths, which arise as a result of &#x0D; planar fluctuations of indium in the InGaN layers of the active region. Digital photographs were obtained and patterns of ultra-weak luminescence of the surface of LED crystals were analyzed. It is shown that the patterns of luminescence, along with the current-voltage characteristic, demonstrate significant changes after testing even at the initial stages of degradation, which indicates a high sensitivity of these parameters to degradation processes and the possibility of their use in diagnostic and non-destructive testing methods.

Suppression of V-pits formation in InGaN layer by stepped growth with annealing interval

An epitaxial method of stepped growth with additional annealing interval is proposed to suppress the V-pit formation in InGaN layer. Detailed investigation on the influences of annealing condition indicates that more cycles in an annealing interval, relatively higher annealing temperature or longer annealing time can better reduce the density and size of V-pits in InGaN, and finally obtain a InGaN layer with much less V-pits. Micro-photoluminescence mapping measurements present a better spatial distribution and a smaller full width half maximum of luminescence peaks, providing more information about the advantage of stepped growth with annealing interval. Moreover, an experiment for thick InGaN layer growth with a relatively higher indium content also proves that such stepped growth could suppress the V-pits effectively.

Normally-off GaN HEMTs with InGaN p-gate cap layer formed by polarization doping

Narrow gate margin has been the critical limiting factor for the p-gate normally-off GaN HEMTs, imposing significant challenges in both gate drive design and gate reliability. In this work, by developing dopant-free p-type polarization doping technique in composition-graded InGaN layer, high-quality Schottky contact between the gate metal and cap layer was demonstrated, achieving excellent gate current blocking performance (10–6 mA mm−1) after the turning-on of the gate heterojunction structure. Resultantly, normally-off GaN HEMTs with enhanced gate breakdown voltage up to 15.2 V was realized, being especially beneficial for the simplification of gate drive design and the safe operation of gate terminal.

Understanding the Luminescence Characteristics of Ultraviolet InGaN/AlGaN Multiple Quantum Wells with Different In Gradients

The electroluminescence (EL) properties of InGaN/AlGaN ultraviolet light-emitting multiple quantum wells (MQWs) with identical average In content but different In gradients (In content increases linearly, along the growth direction) are investigated numerically. It is found that the luminescence efficiency is improved, and the EL spectral peak wavelength becomes longer for the MQW sample with a larger In gradient. Since the influence of In gradient is different for the conduction and valence bands in InGaN layers, the distribution of electrons and holes in QWs may be changed, leading to a redshift of EL spectra. In particular, when the In gradient increases, the overlap integral of electron-hole wavefunction in InGaN QWs increases, resulting in a higher radiative recombination rate and an enhanced EL intensity.

Polarization-induced hole doping for long-wavelength In-rich InGaN solar cells

The lack of high-quality In-rich p-type InGaN restricts the development of high-efficiency InGaN solar cells toward the whole solar spectrum. In this work, we report a polarization-induced (PI) hole doping method for In-rich InGaN by using the In composition graded p-type InGaN layer with lightly Mg doping. The hole mobility as high as ∼40 cm2/V s is obtained, which is more than ten time higher than that of the single-layer p-type InGaN with the same average In mole fraction. The In-rich InxGa1−xN (x ∼ 0.4) solar cell with the PI doped p-type InGaN exhibits a peak absorption at the wavelength as long as 594 nm. As a result, the conversion efficiency of InGaN solar cell is improved by over 1.5 times compared to that with single-layer p-type InGaN.

Thermal boundary resistance enhancement through interfacial polarization electric field induced in GaN/InxGa1-xN superlattice

In this paper, we have theoretically investigated the effect of interfacial polarization electric (IPE) field due to interfaces on thermal boundary resistance (TBR) of GaN/InGaN superlattices (SLs). The distribution of temperature in the active region of GaN/InGaN devices determines the TBR including and excluding IPE field induced by the strain of the SL compared with experiments and thermal simulations. The calculated TBR was found to be 9.3 × 10 −9 W −1 m 2 K for devices at room temperature which plays a key role in thermal management of the devices. The determined effective TBR was found to decrease with temperature; but was enhanced by polarization mechanism developed at the interface of GaN/InGaN SL. The contribution of very low thermal conductivity of InGaN layer of the SLs for improved thermoelectric efficiency requires comparatively a large value of TBR and its temperature dependences are also discussed. • Interfacial electric field is exploited to enhance thermal boundary resistance of GaN/In x Ga 1-x N/GaN SLs. • Electric field modifies thermal parameters and phonon properties of SL via inverse piezoelectric effect. • TBR is enhanced from 2.10 to 5. 30 × 10 −9 m 2 KW −1 closer to reported value of InGaAs SL. • TBR is enhanced in presence of field and shows more than 15% ehancement due to enhanced TBR. • k cp in presence (absence) of field for x = 0.1 and 0.9 are 4.652 (5.710) and 6.083 (7.327) Wm −1 K −1 .