InGaN Buffer Layer Research Articles

The dissociation of (a+c) misfit dislocations at the InGaN/GaN interface.

In hexagonal materials, (a+c) dislocations are typically observed to dissociate into partial dislocations. Edge (a+c) dislocations are introduced into (0001) nitride semiconductor layers by the process of plastic relaxation. As there is an increasing interest in obtaining relaxed InGaN buffer layers for the deposition of high In content structures, the study of the dissociation mechanism of misfit (a+c) dislocations laying at the InGaN/GaN interface is then crucial for understanding their nucleation and glide mechanisms. In the case of the presented plastically relaxed InGaN layers deposited on GaN substrates, we observe a trigonal network of (a+c) dislocations extending at the interface with a rotation of 3° from <1 00> directions. High-resolution microscopy studies show that these dislocations are dissociated into two Frank-Shockley 1/6<2 03> partial dislocations with the I1 BSF spreading between them. Atomistic simulations of a dissociated edge (a+c) dislocation revealed a 3/5-atom ring structure for the cores of both partial dislocations. The observed separation between two partial dislocations must result from the climb of at least one of the dislocations during the dissociation process, possibly induced by the mismatch stress in the InGaN layer.

Biaxial, Hybrid Plastic-Elastic Relaxation in Micropatterned Semipolar InGaN Buffer Layers Improves Quality of Templated Quantum Well Emitters

Despite significant progress and multiple creative approaches taken toward the realization of long-wavelength III-nitride based light emitters over the past decade, high double-digit external quantum efficiencies remain elusive for GaN/InGaN based devices for wavelengths longer than 525 nm. This stems from the large mismatch between the GaN and InGaN lattice that manifests in deleterious crystal strain gradients, degrades surface growth morphology, and introduces diverse and potent nonradiative defects.A promising route to mitigation of strain-based effects is templating epitaxial growth on relaxed InGaN epilayers; in particular, a known and controllable platform for uniaxial strain relaxation is the creation of 1-D misfit dislocation arrays at semipolar GaN/InGaN heterointerfaces. Leveraging work by Hardy et al. on achieving >200 nm thick, partially relaxed semipolar InGaN layers [1] and precise determination of InGaN epilayer strain state and composition by Young et al. [2], we explore the potential for 2-D buffer layer relaxation in (11-22) InGaN layers via micron- and sub-micron patterning. Cathodoluminescence microscopy and x-ray diffraction indicated significant increases in biaxial epilayer relaxation with increased pillar aspect ratio; quantum wells deposited atop these relaxed InGaN buffers showed dramatic threading dislocation reduction, luminescence and morphology improvements, and evidence of improved indium incorporation relative to planar samples at nominally identical growth conditions.[1] Hardy, M.T., Young, E.C., Hsu, P.S., Haeger, D.A., Koslow, I.L., Nakamura, S., DenBaars, S.P., Speck, J.S. Suppression of m-plane and c-plane slip through Si and Mg doping in partially relaxed (20-21) InGaN/GaN heterostructures. Appl. Phys. Lett. 101, 132102 (2012).[2] Young, E. C., Romanov, A.E., Speck, J.S., Determination of composition and lattice relaxation in semipolar ternary (In,Al,Ga)N strained layers from symmetric x-ray diffraction measurements. Appl. Phys. Express. 4, 061001 (2011).

Формирование методом молекулярно-пучковой эпитаксии с плазменной активацией азота гетероструктур с множественными квантовыми ямами InN/InGaN на сапфире

The features of the process of growth of multilayer heterostructures with InN/InGaN quantum wells (QWs) by the method of molecular-beam epitaxy with nitrogen plasma activation in the mode of modulated metal fluxes are studied. To compensate for elastic stresses in the structure, the active region was formed in the form of an InN/InGaN superlattice matched over the average lattice parameter with the underlying InGaN buffer layer. It has been shown that during the growth of relatively narrow InN QWs up to 3 nm wide, there is no relaxation of elastic stresses in the active region of the structure, and the dislocation density remains at the level ND ∼ (3−4) · 10^10 cm−2, which corresponds to the dislocation density in InGaN buffer. Such structures demonstrate the most intense PL in the wavelength range of 1.3−1.5 µm. With the growth of wider QWs, the imperfection of the structures sharply increases (ND &gt; 10^11 cm−2), which is accompanied by a decrease in the emission intensity. The structures grown with InN/InGaN QWs demonstrated an order of magnitude better temperature stability of PL compared to bulk InN layers (PL quenching by ∼ 3 and ∼ 25 times, respectively, in the temperature range of 77−300 K). Nevertheless, at a low temperature (T = 77 K), the PL intensity of the studied structures with InN/InGaN QWs is noticeably inferior to that for the bulk InN layer, which apparently indicates a significant role of nonradiative recombination by the Shockley−Reed−Hall mechanism in structures with QWs (as opposed to Auger recombination in bulk InN).

Formation of heterostructures with multiple quantum wells InN/InGaN by the PA-MBE method on sapphire

The features of the process of growth of multilayer heterostructures with InN/InGaN quantum wells (QWs) by the method of molecular-beam epitaxy with nitrogen plasma activation in the mode of modulated metal fluxes are studied. To compensate for elastic stresses in the structure, the active region was formed in the form of an InN/InGaN superlattice matched over the average lattice parameter with the underlying InGaN buffer layer. It has been shown that during the growth of relatively narrow InN QWs up to 3 nm wide, there is no relaxation of elastic stresses in the active region of the structure, and the dislocation density remains at the level N_D~(3-4)·1010 cm-2, which corresponds to the dislocation density in InGaN-buffer. Such structures demonstrate the most intense PL in the wavelength range of 1.3-1.5 μm. With the growth of wider QWs, the imperfection of the structures sharply increases (N_D&amp;gt;1011 cm-2), which is accompanied by a decrease in the emission intensity. The structures grown with InN/InGaN QWs demonstrated an order of magnitude better temperature stability of PL compared to bulk InN layers (PL quenching by ~3 and ~25 times, respectively, in the temperature range of 77-300 K). Nevertheless, at a low temperature (T=77 K), the PL intensity of the studied structures with InN/InGaN QWs is noticeably inferior to that for the bulk InN layer, which apparently indicates a significant role of nonradiative recombination by the Shockley--Reed--Hall mechanism in structures with QWs (as opposed to Auger recombination in bulk InN). Keywords: indium and gallium nitride, molecular beam epitaxy, quantum well, photoluminescence, dislocations.

Growth of highly relaxed InGaN pseudo-substrates over full 2-in. wafers

A highly relaxed InGaN buffer layer was demonstrated over a full two-inch c-plane sapphire substrate by metalorganic chemical vapor deposition. The InGaN buffer layer was grown on a 100 nm GaN decomposition stop layer with a 3 nm thick high indium composition InGaN underlayer. After thermal decomposition of the underlayer at 1000 °C, a 200 nm thick In0.04Ga0.96N buffer showed 85% biaxial relaxation measured by a high resolution x-ray diffraction reciprocal space map. When used as a pseudo-substrate for the regrowth of InGaN/InGaN multi-quantum wells, the sample showed a 75 nm red-shift in room temperature photoluminescence when compared to a co-loaded GaN template reference. The longer emission wavelength is associated with higher indium incorporation in the InGaN layers from the lessening of the compositional pulling effect caused by compressive strain. Using this technique, a simple red light emitting diode was demonstrated with an active layer growth temperature of 825 °C and a peak wavelength of 622 nm at a current density of 20 A cm−2. This work represents a unique method to relax a III-nitride based layer over a full substrate.

Fabrication of relaxed InGaN pseudo-substrates composed of micron-sized pattern arrays with high fill factors using porous GaN

Fully or partially relaxed micron-sized InGaN patterns with fill factors up to 69% were demonstrated via porosification of the underlying GaN:Si layer. The impact of the porosification etch conditions and the pattern geometry on the degree of InGaN relaxation were studied and monitored via high resolution x-ray diffraction reciprocal space maps. Additionally, a 45 nm redshift in the photoluminescence emission from InxGa1−xN/ InyGa1−yN multi-quantum wells (MQWs) regrown on bi-axially relaxed InGaN buffer layers was observed when compared to a co-loaded reference sample grown on GaN. The longer emission wavelength was associated with higher indium incorporation into the InGaN layers deposited on the InGaN base layers with a lattice constant larger than GaN, due to the reduced lattice mismatch between MQW and InGaN base layer, also called compositional pulling effect.

Near-Infrared InGaN Alloys Grown on High-In-Composition InGaN Buffer Layer

Near-infrared InGaN alloys were grown on a strain-relaxed high-In-composition InGaN buffer layer based on GaN/sapphire substrate by plasma assisted molecular beam epitaxy. The In compositions of the InGaN light emitting layer and buffer layer determined by X-ray diffraction analysis are 56.7 and 61.1%, respectively. Transmission electron microscopy result shows that no dislocations newly generated at the interface of the InGaN structure layer and the underlying InGaN buffer layer can be observed obviously. The cathodoluminescence spectra exhibit two strong emission peaks at the near-infrared wavelengths of 1090 and 1200 nm, which can be attributed to intrinsic transition and recombination processes of the fabricated InGaN alloys according to the calculation using the energy versus composition equation with a reasonable bowing parameter.

InGaN/InGaN multiple-quantum-well grown on InGaN/GaN semi-bulk buffer for blue to cyan emission with improved optical emission and efficiency droop

In0.16Ga0.84N/In0.05Ga0.95N Multiple Quantum Well (MQW) structure grown on a 70 nm thick high quality semi-bulk InGaN buffer layer is reported. Temperature dependent photoluminescence (PL) reveals 67.5% of room temperature Internal Quantum Efficiency (IQE) at an emission peak of ∼455 nm with FWHM of 20 nm. Low temperature PL study shows clear improvement in emission intensity when conventional GaN buffer and GaN barrier are replaced by semi-bulk InGaN buffer in addition with InGaN barrier. Simulation confirms improved IQE and reduced efficiency droop when using semi-bulk as buffer which is attributed to the improved overlapping of electron-hole wave functions due to the reduced internal electric field from counteraction by surface polarization field. This efficiency improvement is very beneficial for high In content green LEDs where the efficiency is limited by polarization induced Quantum Confined Stark Effect (QCSE) for excess indium content.

Controlling optical polarization of {11–22} semipolar multiple quantum wells using relaxed underlying InGaN buffer layers

We successfully fabricated {11–22} multiple quantum wells (MQWs) having different emission peak wavelengths on partially or completely relaxed thick InGaN buffer layers with different In contents formed on a semipolar {11–22} GaN layer, which was grown on a patterned r-plane sapphire substrate. The polarization properties changed significantly with changing in In content and thickness for InGaN buffer layer. For the same In content of the InGaN buffer layer, the optical polarization changed with an increase in the thickness of the underlying InGaN buffer layer, indicating a change in the relaxation ratio of the InGaN buffer layer. Similarly, for the same thickness of the InGaN buffer layer, the optical polarization changed by changing In content of the InGaN buffer layer. Thus, the degree of optical polarization could be controlled by varying the In content of the underlying InGaN buffer layer.

High-quality InN films on GaN using graded InGaN buffers by MBE

The growth of high-quality thick InN films is challenging because of the lack of native substrates. In this work, we demonstrate the use of a linearly graded InGaN buffer layer for the growth of InN films on GaN substrates. A 500 nm InN film with <0.1 nm RMS roughness is obtained with a peak mobility of 1410 cm2/(V·s) at 300 K. A strong room temperature photoluminescence showing a bandgap of 0.65 eV with 79 meV linewidth is observed. A graded InGaN buffer is found to lead to extremely smooth and high-quality InN films.

Microstructure and magnetic properties of In1−xCrxN thin films

Microstructure and magnetic properties of In1−xCrxN thin films grown on GaN-on-sapphire templates by molecular beam epitaxy are investigated. Optimized growth conditions are identified for the In1−xCrxN thin films at reduced growth temperature. The In1−xCrxN thin films on the top of the InGaN buffer layers exhibit high crystalline-quality. The magnetic properties of In1−xCrxN thin films show a ferromagnetic behavior even at room temperature.

Composition and interface analysis of InGaN/GaN multiquantum-wells on GaN substrates using atom probe tomography

In0.20Ga0.80N/GaN multiquantum wells (MQWs) grown on [0001]-oriented GaN substrates with and without an InGaN buffer layer were characterized using three-dimensional atom probe tomography. In all samples, the upper interfaces of the QWs were slightly more diffuse than the lower interfaces. The buffer layers did not affect the roughness of the interfaces within the quantum well structure, a result attributed to planarization of the surface of the first GaN barrier layer, which had an average root-mean-square roughness of 0.18 nm. The In and Ga distributions within the MQWs followed the expected distributions for a random alloy with no indications of In clustering. High resolution Rutherford backscattering characterizations showed the ability to resolve the MQWs, and the resulting compositions and widths corroborated those determined from the atom probe analyses.

Metalorganic vapor phase epitaxy of GaN and lattice-matched InGaN on ScAlMgO4(0001) substrates

ScAlMgO4 (SCAM) (0001) can be used for metalorganic vapor phase epitaxy (MOVPE) of GaN and lattice-matched In0.17Ga0.83N. GaN grown on SCAM(0001) via a low-temperature GaN buffer layer shows excellent structural quality, indicating that the GaN-SCAM interface is stable during MOVPE. For lattice-matched InGaN on SCAM(0001), a lattice-matched InGaN buffer layer grown at a lower temperature effectively improves the surface and luminescence uniformity. The grown InGaN is nearly unstrained and exhibits photoluminescence peaking at 505 nm at room temperature. These achievements indicate that In0.17Ga0.83N/SCAM lattice-matched templates may pave the way toward longer-wavelength light-emitting and -detecting devices using InGaN with higher In contents.

Polarization characteristics of semipolar (112̄2) InGaN/GaN quantum well structures grown on relaxed InGaN buffer layers and comparison with experiment.

Partial strain relaxation effects on polarization ratio of semipolar (112̄2) InxGa1−xN/GaN quantum well (QW) structures grown on relaxed InGaN buffers were investigated using the multiband effective-mass theory. The absolute value of the polarization ratio gradually decreases with increasing In composition in InGaN buffer layer when the strain relaxation ratio (ε0y′y′−εy′y′)/ε0y′y′ along y′-axis is assumed to be linearly proportional to the difference of lattice constants between the well and the buffer layer. Also, it changes its sign for the QW structure grown on InGaN buffer layer with a relatively larger In composition (x > 0.07). These results are in good agreement with the experiment. This can be explained by the fact that, with increasing In composition in the InGaN subsrate, the spontaneous emission rate for the y′-polarization gradually increases while that for x′-polarization decreases due to the decrease in a matrix element at the band-edge (k‖ = 0).

Wavelength limits for InGaN quantum wells on GaN

The emission wavelength of coherently strained InGaN quantum wells (QW) is limited by the maximum thickness before relaxation starts. For high indium contents x&amp;gt;40% the resulting wavelength decreases because quantum confinement dominates. For low indium content x&amp;lt;40% the electron hole wave function overlap (and hence radiative emission) is strongly reduced with increasing QW thickness due to the quantum confined Stark effect and imposes another limit. This results in a maximum usable emission wavelength at around 600 nm for QWs with 40%-50% indium content. Relaxed InGaN buffer layers could help to push this further, especially on non- and semi-polar orientations.

Thick InGaN Growth by Metal Organic Vapor Phase Epitaxy with Sputtered InGaN Buffer Layer

A thick InGaN film was grown on a sapphire substrate by metal organic vapor phase epitaxy using a thin buffer layer of sputter-deposited InGaN. A thick film of highly luminescent In0.2Ga0.8N can be successfully grown at a rate as high as 2 µm/h. The crystal quality of InGaN grown on the sputter-deposited InGaN buffer layer was better than that of InGaN grown directly on the sapphire substrate. The sample showed n-type conduction with a carrier concentration and mobility of 5×1018 cm-3 and 3 cm2/(V·s), respectively.

Characteristics of an Indium-Rich InGaN p—n Junction Grown on a Strain-Relaxed InGaN Buffer Layer

An indium-rich InGaN p—n junction is grown on a strain-relaxed InGaN buffer layer. The results show that the n-InGaN is grown coherently on the buffer layer but the p-InGaN layer exhibits a partial strain relaxation. The fabricated InGaN p—n junction has a low reverse leakage current density on the order of 10−8 A/cm2 within the measured voltage range, and exhibits a wide spectral response due to the presence of band tail states or deep level states which origin from indium composition fluctuation or various defects. The measured peak responsivity at 438 nm is 31 mA/W at zero bias and reaches 118 mA/W at 3V reverse bias. In addition, the Raman spectra of the p- and n-type InGaN alloys are also analyzed.

Performance and polarization effects in (112¯2) long wavelength light emitting diodes grown on stress relaxed InGaN buffer layers

Long wavelength (525–575 nm) (112¯2) light emitting diodes were grown pseudomorphically on stress relaxed InGaN buffer layers. Basal plane dislocation glide led to the formation of misfit dislocations confined to the bottom of the InGaN buffer layer. This provided one-dimensional plastic relaxation in the film interior, including the device active region. The change of the stress state of the quantum well due to one-dimensional plastic relaxation altered the valence band structure, which produced a significant shift in polarization of emitted light. Devices grown on relaxed buffers demonstrated equivalent output power compared to those for control samples without relaxation.

Rapid growth and characterization of InN nanocolumns on InGaN buffer layers at a low ratio of N/In

c-Axis-aligned InN nanocolumn arrays were vertically grown on 3 μm GaN epilayers with InGaN buffer layers by radio-frequency molecular beam epitaxy without any catalysts. X-ray diffraction, transmission electron microscopy, and field-emission scanning electron microscope were used to study the structural properties of the nanocolumns. It has been found that without InGaN buffers, InN films, rather than nanocolumns, were grown even at the same N/In ratio. In addition, high-quality InN nanocolumns can grow faster on InGaN buffers. The growth mechanism was discussed and the joint actions of the gas–solid and Volmer–Weber modes promote the nucleation and the growth of InN nanocolumns.

MOVPE growth and properties of non-polar InGaN/GaN multiple quantum wells on γ-LiAlO 2 substrates

We report on growth as well as structural and optical properties of m-plane InGaN/GaN multiple quantum wells (MQWs) grown on LiAlO 2 substrate by metal–organic vapor phase epitaxy. During the growth, surface nitridations of LiAlO 2 substrate and Mg-doped InGaN buffer layers were employed. Pure m-plane and two-orientation (mixture of c- and m-plane) samples were prepared and verified by X-ray diffraction and Raman scattering. Atomic force microscopy (AFM) image of our m-plane surface morphology reveals a “fabric”-like pattern, i.e., stripes running perpendicular to each other. Photoluminescence from the m-plane MQW has polarization anisotropy, which can be attributed to the anisotropic in-plane strain. In two-orientation sample, we found that this optical polarization property survives; yet it is weakened. This indicates that polarized emission is a robust optical property in our samples. Therefore InGaN/GaN MQWs on LiAlO 2 have potential for application in efficient polarized light emitters.