AlGaN Epitaxial Layers Research Articles

Optical and Electrical Properties of AlxGa1-xN/GaN Epilayers Modulated by Aluminum Content.

AlGaN-based LEDs are promising for many applications in deep ultraviolet fields, especially for water-purification projects, air sterilization, fluorescence sensing, etc. However, in order to realize these potentials, it is critical to understand the factors that influence the optical and electrical properties of the device. In this work, AlxGa1-xN (x = 0.24, 0.34, 0.47) epilayers grown on c-plane patterned sapphire substrate with GaN template by the metal organic chemical vapor deposition (MOCVD). It is demonstrated that the increase of the aluminum content leads to the deterioration of the surface morphology and crystal quality of the AlGaN epitaxial layer. The dislocation densities of AlxGa1-xN epilayers were determined from symmetric and asymmetric planes of the ω-scan rocking curve and the minimum value is 1.01 × 109 cm-2. The (101¯5) plane reciprocal space mapping was employed to measure the in-plane strain of the AlxGa1-xN layers grown on GaN. The surface barrier heights of the AlxGa1-xN samples derived from XPS are 1.57, 1.65, and 1.75 eV, respectively. The results of the bandgap obtained by PL spectroscopy are in good accordance with those of XRD. The Hall mobility and sheet electron concentration of the samples are successfully determined by preparing simple indium sphere electrodes.

Enhanced structural and optical properties of high Al composition non-polar a-plane AlGaN epitaxial layer by optimizing growth flow sequence

Enhanced structural and optical properties of high Al composition non-polar a-plane AlGaN epitaxial layer by optimizing growth flow sequence

Nanoindentation of AlGaN Films Formed on SiC/Si Substrates Grown by the Method of Coordinated Substitution of Atoms

In this article, the mechanical and deformation characteristics of epitaxial films of AlGaN solid solutions formed on silicon substrates of crystallographic orientations (001), (011) and (111) with a silicon carbide (SiC/Si) buffer layer synthesized by the method of the coordinated substitution of atoms were studied using the nanoindentation method. The growth of AlGaN epitaxial layers were carried out both directly on the SiC/Si hybrid substrate and on the SiC/Si substrate additionally coated with an AlN buffer layer. The morphology and structure of the surface of the layers was studied by atomic force microscopy. The structural characteristics of hybrid AlGaN substrates and films grown with and without an aluminum nitride buffer layer were measured. An unambiguous relationship has been established between the mechanical properties (elasticity modulus and hardness) and the surface structure of AlGaN films. It has been found that the AlN buffer layer has a significant effect on the mechanical and deformation properties of AlGaN films at the initial moment of indentation, when the layer is predominantly elastically deformed. The roughness was determined and the surface morphology of the AlGaN films was characterized. For the first time, experimentally, using the nanoindentation method, the hardness parameters and the reduced modulus of elasticity of epitaxial AlGaN grown on hybrid SiC/Si and AlN/SiC/Si substrates were measured.

Kinetic Monte Carlo simulations for AlN and AlGaN epitaxial growth on AlN

With increasing availability of AlN substrates, growth of AlN and AlGaN epitaxial layers on AlN(0001) has become a subject of extended experimental investigations. For a deeper understanding of the growth process, we developed a kinetic Monte Carlo (KMC) model for the adsorption, desorption, and diffusion on a stepped Al(001) surface under MOVPE conditions. Nitrogen is provided via ammonia and we assumed that the surface is covered by N in a 2 × 2 N reconstruction. This reconstruction could be obtained in our calculations for a flat surface. We performed several runs for a vicinal AlN(0001) surface with a miscut of 3° in m-direction [11¯00] direction. The results are analysed in detail both for AlN and AlGaN epitaxial layers. In addition, we investigate the influence of dislocations by a simple approach for bond and diffusion energies.

Structure and properties of composites based on aluminum and gallium nitrides grown on silicon of different orientations with a buffer layer of silicon carbide

The microstructure and pyroelectric properties of AlxGa1-xN composite epitaxial layers grown on SiC/Si(111) and SiC/Si(110) hybrid substrates by the chloride-hydride epitaxy have been studied. The phenomenon of spontaneous formation of a system of heterojunctions consisting of periodic AlxGa1-xN layers of different composition located perpendicular to the direction of growth, was discovered during the growth of layers. Measurements of the pyroelectric coefficients of these heterostructures have shown that regardless of the orientation of the initial Si substrate and their pyroelectric coefficients have close values of the order of γ~(0.7-1)·10-10 C/cm2K. It is shown that to increase the magnitude of the pyroresponse it is necessary to deposit an AlN layer with a thickness exceeding 1 μm on the AlxGa1-xN/SiC/Si surface. This leads to record values of the pyroelectric coefficient γ~18·10-10 C/cm2K for AlN crystals and films. Keywords: silicon carbide-on-silicon substrates, chloride-hydride epitaxy, AlGaN epitaxial layers, aluminum nitride, gallium nitride, pyroelectric properties.

Reduction of hexagonal defects in N-polar AlGaN epitaxial layers grown with reformed pulsed-flow technology

The N-polar AlGaN epi-layers were successfully grown on (0001) c-plane sapphire substrates by using the reformed metal-organic chemical vapor deposition technology featured with an optimized pulsed NH3-flow supplying scheme. In order to reduce the hexagonal defects in the N-polar AlGaN epi-layers, the conventional NH3 pulsed-flow technology was reformed. Particular attention should be paid on the impacts of the variation in the duty ratio and the NH3-purge term within each growth cycle on the surface morphology and crystalline quality of the grown N-polar AlGaN epi-layers. A smooth surface morphology with a root-mean-square value as small as 5.7 nm was obtained. On the other hand, the significantly decreased density of the hexagonal defects confirmed with microscope and X-ray diffraction measurements implies a remarkable improvement in crystalline quality of the N-polar AlGaN epi-layers. These results indicate that the reformed pulsed-flow technology with an optimized source supplying scheme plays a crucial role in improving the surface morphology and crystalline quality of the N-polar AlGaN epi-layers.

Ultraviolet Stimulated Emission in AlGaN Layers Grown on Sapphire Substrates Using Ammonia and Plasma‐Assisted Molecular Beam Epitaxy

Ammonia and plasma‐assisted (PA) molecular beam epitaxy modes are used to grow AlN and AlGaN epitaxial layers on sapphire substrates. It is determined that the increase in the thickness of the AlN buffer layer grown by ammonia molecular beam epitaxy (MBE) from 0.32 to 1.25 μm results in the narrowing of 101 X‐Ray rocking curves (XRCs), whereas no clear effect on the 002 XRC width is observed. It is shown that strong GaN decomposition during growth by ammonia MBE causes AlGaN surface roughening and compositional inhomogeneity, which leads to deterioration of its lasing properties. AlGaN layers grown by ammonia MBE at an optimized temperature demonstrate stimulated emission (SE) which peaks at λ = 330, 323, 303, and 297 nm, with SE threshold values of 0.7, 1.1, 1.4, and 1.4 MW cm−2, respectively. In comparison with these, AlGaN layers grown using PA MBE pulsed modes (migration‐enhanced epitaxy, metal‐modulated epitaxy, and droplet elimination by thermal annealing) show SE with a relatively low threshold (0.8 MW cm−2) at a considerably shorter wavelength of λ = 267 nm.

Electrochemical etching of AlGaN for the realization of thin-film devices

Heterogeneously integrated AlGaN epitaxial layers will be essential for future optical and electrical devices like thin-film flip-chip ultraviolet (UV) light-emitting diodes, UV vertical-cavity surface-emitting lasers, and high-electron mobility transistors on efficient heat sinks. Such AlGaN-membranes will also enable flexible and micromechanical devices. However, to develop a method to separate the AlGaN-device membranes from the substrate has proven to be challenging, in particular, for high-quality device materials, which require the use of a lattice-matched AlGaN sacrificial layer. We demonstrate an electrochemical etching method by which it is possible to achieve complete lateral etching of an AlGaN sacrificial layer with up to 50% Al-content. The influence of etching voltage and the Al-content of the sacrificial layer on the etching process is investigated. The etched N-polar surface shows the same macroscopic topography as that of the as-grown epitaxial structure, and the root-mean square roughness is 3.5 nm for 1 μm×1 μm scan areas. Separated device layers have a well-defined thickness and smooth etched surfaces. Transferred multi-quantum-well structures were fabricated and investigated by time-resolved photoluminescence measurements. The quantum wells showed no sign of degradation caused by the thin-film process.

Local Strain and Crystalline Defects in GaN/AlGaN/GaN(0001) Heterostructures Induced by Compositionally Graded AlGaN Buried Layers

Plastic strain relaxation in epitaxial layers is one of the crucial factors that limits the performance of III-nitride-based heterostructures. In this work, we report on strain relaxation and crystalline defects in heterostructures consisting of compositionally graded AlGaN epitaxial layers tensile-strained between a GaN-buffer and a GaN-cap. We demonstrate the effects of Al concentration and the shape of the concentration-depth profile in the buried graded layers on the accumulated elastic strain energy and how this influences the critical thickness for crack generation or fracture. It is shown that this fracture leads to the formation of partially relaxed regions with their degree of strain relaxation directly related to the density of cracks. Nevertheless, even though the in-plane coherency between the AlGaN layer and the GaN-buffer is broken, the in-plane coherency within the AlGaN layer is preserved for all regions. Furthermore, the tensile strain released in the buried graded AlGaN layers is consist...

(Invited) Interfacing Graphene with III-Nitrides for Power Device Applications

Full performance of GaN power devices is not attained yet due to their self-heating. Towards solving this issue Silicon Carbide is a common substrate for growth of GaN and AlGaN epitaxial layers. In spite of the superior thermal conductivity of SiC compared to e.g. Sapphire or Silicon, thermal management of GaN based devices is still a challenge. This talk will review recent research appraising the benefits of graphene integration with III-N for high power electronics. The merits of graphene as a 2D carbon made material are well demonstrated. We will present, from a material perspective, the recent progresses in the growth of high quality GaN layers on graphene templates. The latter are obtained via sublimation epitaxy on SiC and this process and the graphene quality will be also discussed in the view of template design and its role in the subsequent CaN and AlN deposition. Furthermore, the possibility and the advantage of templated growth of novel 2D Nitrides will be discussed, both from experimental and theoretical point of view of their formation mechanism. In order to gain full understanding of the new structure formation results from HRTEM and STEM characterization will be shown. Finally, strategies to use graphene for thermal management in high-power AlGaN/GaN transistors will be discussed. References A. Kovacs et al. Graphoepitaxy of highquality GaN layers on graphene/6H-SiC, Adv. Mater. Interfaces 2, 1400230 (2015).F. Giannazzo et al. Phys. Status Solidi A 214, No. 4 (2017)

Etched Al 0.32 Ga 0.68 N/GaN HEMTs with high output current and breakdown voltage (>600 V)

By etching the AlGaN layer close to the gate edge of the drain side, the novel AlGaN/GaN high-electron-mobility transistor with the etched AlGaN epitaxial layer (etched AlGaN/GaN high-electron-mobility transistors, HEMTs) is proposed in this work with the high-resistance GaN buffer layer. The relationship between the 2D electron gas (2DEG) concentration and the thickness of the AlGaN layer has been explored. The 2DEG density under the etched barrier is reduced to optimise the surface electric field and mobility of 2DEG. The electric field is reshaped by the etched AlGaN layer, featuring an extra electric field peak far away from the gate edge. The electric field peak near the gate is decreased effectively because of the extra electric field peak, which therefore improves the breakdown voltage (BV). The mobility of 2DEG is increased because of the low 2DEG concentration, which improves the output current (I DS). The experimental BV is improved from 152 V for the conventional structure to 620 V for etched AlGaN/GaN HEMTs because of the optimised surface electric field. The maximum output current (I max) is increased from 381 to 478 mA/mm. The trade-off between the BV and R on,sphas been improved to break the silicon limit. It is concluded that the gate-edge etching technique can significantly increase both BV and I DS for AlGaN/GaN HEMTs.

Nanocrack formation due to inverse piezoelectric effect in AlGaN/GaN HEMT

In AlGaN/GaN HEMTs, electrical degradation occurs due to high-voltage stress operation which can be described by a critical voltage operation by which degradation starts to take place which is irreversible. Our investigation is on how electrical degradation occurs due to physical degradation in the device. In this work using Griffith's Equation and inverse piezo electric effect we have shown how physical degradation affects electrical properties of the device and we have also shown how cracks are generated in AlGaN epitaxial layer of the device.

The effect of AlN/AlGaN superlattices on crystal and optical properties of AlGaN epitaxial layers * * Project supported in part by the National High Technology Program of China (No. 2014AA032608), the National Key R&D Program of China (Nos. 2016YFB0400800, 2016YFB0400083, 2016YFB0400082), in part by the National Natural Sciences Foundation of China (Nos. 6136047, 61206090, 61527814, 61674147, 61204053), the Beijing Municipal

We investigate the effect of AlN/AlGaN superlattices (SLs) on crystal and optical properties of AlGaN epitaxial layers. The result indicates that the crystal quality of AlGaN layers is consistent within a wide range of SLs thicknesses, while the optical properties are opposite. With SLs thickness decreasing from 20/44 to 17/36 and 15/29 nm, the full-width at half maximum of X-ray rocking curves for (0002)- and ( )-plane of n-AlGaN layers grown on SLs are consistent of around 250 arcsec and 700 arcsec, respectively. Meanwhile, the center of the low optical transmittance band decreases from 326 to 279 nm and less than 266 nm as the SLs thickness decreases. 280 nm deep ultraviolet light-emitting diodes (DUV-LEDs) structures are further regrown on the n-AlGaN layers. The electroluminescent intensities of samples are 30% higher than that of the sample whose low optical transmittance band appears around 279 nm. Optical simulations reveal that the SLs acts as distributed Bragg reflectors, thus less photons of the corresponding wavelength escape from the sapphire backside.

Phase separation-suppressed and strain-modulated improvement of crystalline quality of AlGaN epitaxial layer grown by MOCVD

High crystalline quality AlGaN films were grown on GaN templates by metalorganic chemical vapor deposition (MOCVD). Inhomogeneous distributions of Al compositions in both the vertical and lateral growth direction caused by the strong gas-phase pre-reaction occurring between Al precursors and NH3 were suppressed by optimizing the growth conditions. The residual strain in AlGaN/GaN induced by lattice mismatch, which results in the degradation of crystallinity, was modulated by varying thickness of a high temperature (HT) AlN interlayer (IL) inserted between AlGaN and GaN layers. Both the crystalline quality and Al-incorporation into the AlGaN are influenced by the residual strain related to the AlN IL thickness. Lastly, the understanding of the AlGaN growth and of the strain modification was employed to grow an AlGaN/GaN HEMT structure showing good electrical characteristics and uniformity.

Enhancing redshift phenomenon in time-resolved photoluminescence spectra of AlGaN epilayer**Project supported by the National Key Research and Development Program of China (Grant No. 2016YFB0400101) and Beijing Science and Technology Project, China (Grant No. Z151100003315024).

AlGaN epitaxial layer has been studied by means of temperature-dependent time-integrated photoluminescence (PL) and time-resolved photoluminescence (TRPL). An enhancing redshift phenomenon in TRPL spectra with increasing temperature was observed, and the localized excitons behaved like quasi two-dimensional excitons between 6 K and 90 K. We demonstrated that these behaviors are caused by a change in the carrier dynamics with increasing temperature due to the competition of carriers’ localization and delocalization in the AlGaN alloy.

Review—Group III-Nitride-Based Ultraviolet Light-Emitting Diodes: Ways of Increasing External Quantum Efficiency

There is a rapidly growing demand for highly efficient ultraviolet (UV) light sources for a wide variety of applications. In particular, state-of-the-art AlGaN deep UV light-emitting diodes (DUV LEDs) exhibit inadequately low external quantum efficiencies (EQEs). The low efficiencies are attributed to the inherent material properties of high-Al-content AlGaN including strained epitaxial layers, low carrier concentrations, and strong transverse magnetic (TM)-polarized light emission. Extensive efforts have been made to tackle these challenging issues and technological developments have been achieved and enabled the fabrication of reasonable EQE LEDs. In this review, recent advances in the growth of high-quality AlGaN epitaxial layers, transparent and reflective ohmic contacts, and light extraction for AlGaN-based UV LEDs are reviewed.

Statistical nanoscale study of localised radiative transitions in GaN/AlGaN quantum wells and AlGaN epitaxial layers

Compositional disorder has important consequences on the optical properties of III-nitride ternary alloys. In AlGaN epilayers and AlGaN-based quantum heterostructures, the potential fluctuations induced by such disorder lead to the localisation of carriers at low temperature, which affects their transition energies. Using the correlations between micro-photoluminescence, scanning transmission electron microscopy and atom probe tomography we have analysed the optical behaviour of Al0.25Ga0.75N epilayers and that of GaN/AlGaN quantum wells, and reconstructed in three dimensions the distribution of chemical species with sub-nanometre spatial resolution. These composition maps served as the basis for the effective mass calculation of electrons and holes involved in radiative transitions. Good statistical predictions were subsequently obtained for the above-mentioned transition and localisation energies by establishing a link with their microstructural properties.

Theoretical stress calculations in polar, semipolar and nonpolar AlGaN/GaN heterostructures of different compositions

The new comprehensive model of the process for matching epitaxial layers to substrates, in dependence of theirs crystallographic orientation, was developed to allow a theoretical prediction of the strain and stress in thin AlGaN epitaxial layers with different composition grown on GaN template. The elements of the continuous anisotropic materials strength theory was applied to develop the model. It was observed that in AlGaN/GaN heterostructures the stress was greater than the upper limit of acceptable tensile stress even for a small Al content and also that the stress could greatly vary, in a value and a direction, depending on substrate crystallographic orientation and an Al content in AlGaN layer. The obtained results theoretically explained the commonly observed technological problems occurring during the growth of AlGaN layers even with a small Al content.

Temperature-dependent efficiency droop in AlGaN epitaxial layers and quantum wells

Luminescence efficiency droop has been studied in AlGaN epitaxial layers and multiple quantum wells (MQWs) with different strength of carrier localization in a wide range of temperatures. It is shown that the dominant mechanism leading to droop, i.e., the efficiency reduction at high carrier densities, is determined by the carrier thermalization conditions and the ratio between carrier thermal energy and localization depth. The droop mechanisms, such as the occupation-enhanced redistribution of nonthermalized carriers, the enhancement of nonradiative recombination due to carrier delocalization, and excitation-enhanced carrier transport to extended defects or stimulated emission, are discussed.

Photomodification of carrier lifetime and diffusivity in AlGaN epitaxial layers

Nonradiative recombination rate and diffusivity of nonequilibrium carriers were modified by intense laser pulses in AlGaN epilayers with Al content ranging from 16 to 71%. The epilayers were examined before and after the photomodification using light-induced transient grating and photoluminescence spectroscopy techniques. The photomodification resulted in (i) enhancement of the nonradiative recombination rate and (ii) large changes of the diffusion coefficient of the nonequilibrium carriers, without imposing any macroscopic structural damage to the epilayers. The photomodification effect on the recombination rate was stronger in the layers with higher Al content indicating the involvement of the Al atoms in this process. The carrier diffusivity exhibited a rapid initial increase as a consequence of the photomodification followed by a slow decline, as the photomodification duration was increased. The enhancement of the diffusion coefficient of up to 2.4 times was accompanied by 13% decrease in the carrier lifetime.