Epitaxial Lateral Overgrowth GaN Research Articles

Microstructural and spectroscopic analysis of epitaxial lateral overgrowth GaN via the self-decomposing hexagonal graphene mask

The use of two-dimensional material like graphene to alleviate lattice mismatch has been an effective way to realize high-quality GaN on heterogeneous substrates. The lack of hanging bonds on the graphene surface provides a new attempt for epitaxial lateral overgrowth (ELOG). In this study, a hexagonal graphene mask was used for the growth of GaN, the graphene mask disappeared during the GaN growth process, but GaN still maintained the ELOG mode, and the threading dislocation density was significantly reduced. Raman and PL spectra demonstrated the stress relaxation in ELOG GaN and showed a stress relaxation of 0.157 GPa at the interface between the substrate and ELOG GaN. This study demonstrates the feasibility and advantages of graphene masks for nitrides and extends the research on stress relaxation of ELOG GaN using a graphene mask.

A self-disappear-mask for epitaxial lateral overgrowth of GaN films

Although GaN epitaxial lateral overgrowth (ELOG) technology has been developed for some years, this is the first effort to integrate it with two-dimensional (2D) materials owing to their superior qualities. The present study investigates a novel ELOG method for two-step GaN epitaxial lateral overgrowth (2S-ELOG) in metal organic chemical vapor deposition (MOCVD) employing a self-disappearing graphene mask. Photolithography is used to mask the graphene layers that are produced on the GaN through plasma accelerated chemical vapor deposition (PECVD). By adjusting the temperature to partly dissolve the substrate surface, the graphene vanished without interfering with the ELOG mode. Not only did the GaN generated using this approach retain the ELOG feature, but it also exhibited benefits in terms of stress relaxation and threading dislocation densities (TDDs) reduction. This study demonstrates that employing a graphene mask, self-disappearing-mask epitaxial lateral overgrowth (SDM-ELOG) technology may be used to apply a 2D nanoscale material mask to improve the crystal quality of a 3D material.

Structural characterization of ELO-GaN film on mask-stripe patterned GaAs (0 0 1) substrate grown by metalorganic vapor phase epitaxy

GaN films were epitaxial laterally overgrown on the [110]-stripe-patterned GaAs (001) substrate by MOVPE. AlGaN intermediate layers and low-temperature GaN buffer layer were inserted in order to prevent structural defects. The epitaxial lateral overgrowth of GaN results in a trapezoidal shape with flat facets. Structural phases and defects distribution were studied by TEM. Selective-area diffraction pattern indicates the mixed phase of GaN and a high density of stacking faults at nucleation. However, with the deposition of dielectric SiNx masks, a lateral overgrowth resulted in nearly free of defects, and the cubic phase transformed into hexagonal despite a cubic template. Moreover, dark-field TEM images were used to demonstrate the structural phase transformation.

The role of Ga supersaturation on facet formation in the epitaxial lateral overgrowth of GaN

In this paper, facet formation of (0001) {112¯0} {112¯2} facets during epitaxial lateral overgrowth (ELO) of GaN is investigated for different Ga vapor supersaturations. The ELO was conducted via metalorganic chemical vapor deposition on patterned GaN/sapphire templates with SiO2 masks aligned along the ⟨11¯00⟩ direction of GaN. Scanning electron microscopy was used to characterize the cross section shapes of the ELO GaN islands. A correlation of supersaturation, facet formation, and the shape of the ELO GaN islands is found. It is shown that {112¯2} facets are favored under high Ga vapor supersaturation, while {112¯0} facets are favored under low Ga vapor supersaturation. A qualitative model based on Wulff construction and density functional theory calculation is proposed to illustrate the mechanism of the facet formation of the ELO GaN islands.

Integration of GaN and Diamond Using Epitaxial Lateral Overgrowth.

Growth of single-crystalline GaN on polycrystalline diamond is reported for the first time. The structure was achieved using a combined process including selective diamond growth on GaN/Si wafers using hot filament chemical vapor deposition (CVD) and epitaxial lateral overgrowth of GaN on the window region between then above the diamond stripes via metal organic CVD. Optimization of the growth was performed by varying the ammonia to trimethylgallium mole ratio (V/III), chamber pressure, and temperature in the range of 8000-1330, 40-200 Torr, and 975-1030 °C, respectively. A lower pressure, higher V/III ratio, higher temperature, and GaN window mask openings along [11̅00] resulted in enhanced lateral growth of GaN. Complete lateral coverage and coalescence of GaN were achieved over a [11̅00]-oriented 5 μm-wide GaN window between 5 μm diamond stripes when using V/III = 7880, P = 100 Torr, and T = 1030 °C. The crystalline quality of overgrown GaN was confirmed using cross-sectional scanning electron microscopy, high-resolution X-ray diffraction, micro-Raman spectroscopy, transmission electron microscopy, and selective-area electron diffraction.

Optical properties evolution of GaN film grown via lateral epitaxial overgrowth

Spatially resolved Raman scattering mapping and room-temperature cathodoluminescence (CL) mapping of GaN via epitaxial lateral overgrowth (ELOG) on a SiNx-masked GaN template grown on a sapphire substrate were studied. Scanning electron microscope (SEM) measurements show that the ELOG layer is coalesced. Plane view Raman mapping shows that the E2 (high) phonon intensity is stronger in the wing regions than in the window regions. Transmission electron microscopy (TEM) of a cross-section indicates the dislocation annihilation process. Furthermore, the intensity of Raman mapping on a cross-section of A1 (TO) phonon in the coalesced area is more increased than the GaN template. Interestingly, the cross-sectional CL image shows a butterfly shaped pattern that provides a comprehensive understanding of the variations in the point defect distribution during the ELOG GaN regrowth process.

Novel patterned sapphire substrates for enhancing the efficiency of GaN-based light-emitting diodes.

In this study, a novel patterned sapphire substrate (PSS) was used to obtain mesa-type light-emitting diodes (LED), which can efficiently reduce the threading dislocation densities. Silicon nitride (Si3N4) was used as a barrier to form the PSS, replacing the commonly used silicon dioxide (SiO2). The refractive index of Si3N4 is 2.02, which falls between those of sapphire (1.78) and GaN (2.4), so it can be used as a gradient refractive index (GRI) material, enhancing the light extraction efficiency (LEE) of light-emitting diodes. The simulation and experimental results obtained indicate that the LEE is enhanced compared with the conventional PSS-LED. After re-growing, we observed that an air void exists on the top of the textured Si3N4 layer due to GaN epitaxial lateral overgrowth (ELOG). Temperature-dependent PL was used to estimate the internal quantum efficiency (IQE) of the PSS-LED and that of the PSS-LED with the Si3N4 embedded air void (PSA-LED). The IQE of the PSA-LED is 4.56 times higher than that of the PSS-LED. Then, a TracePro optical simulation was used to prove that the air voids will affect the final luminous efficiency. The luminous efficiency of the four different structures considered is ranked as Si3N4 (PSN-LED) > PSA-LED > PSS-LED with SiO2 (PSO-LED) > PSS-LED. Finally, we fabricated LED devices with different thickness of the Si3N4 barrier. The device shows the best luminance–current–voltage (LIV) performance when the Si3N4 thickness is 220 nm.

Epitaxial lateral overgrowth of GaN on nano-cavity patterned sapphire substrates

The epitaxial lateral overgrowth of GaN by metal-organic chemical vapor deposition using a nano-cavity patterned sapphire substrate (NCPSS) was investigated. The NCPSS, with a hexagonal non-close-packed nano-cavity pattern on the sapphire substrate, was fabricated by polystyrene sphere coating and size reduction by reactive ion etching, followed by deposition of alumina and thermal oxidation. The coalescence of GaN on the NCPSS was achieved by the formation of relatively large GaN islands and enhanced lateral overgrowth of the GaN islands over several nano-cavity pattern areas. The threading dislocation density (TDD) measured by cathodoluminescence measurement was significantly reduced from 2.4 × 108 cm−2 to 6.9 × 107 cm−2 by using the NCPSS. A dislocation behavior that contributes to the reduction of TDD of the GaN layer was observed by transmission electron microscopy. Raman spectroscopy revealed that the compressive stress in the GaN layer was reduced by 21% due to the embedded nano-cavities. In addition, the diffuse reflectance of GaN on the NCPSS was enhanced by 54% ∼ 62%, which is attributed to the increased probability of light extraction through effective light scattering by nano-cavities.

Evolution of threading dislocations in GaN epitaxial laterally overgrown on GaN templates using self-organized graphene as a nano-mask

Growth of high-quality GaN within a limited thickness is still a challenge, which is important both in improving device performance and in reducing the cost. In this work, a self-organized graphene is investigated as a nano-mask for two-step GaN epitaxial lateral overgrowth (2S-ELOG) in hydride vapor phase epitaxy. Efficient improvement of crystal quality was revealed by x-ray diffraction. The microstructural properties, especially the evolution of threading dislocations (TDs), were investigated by scanning electron microscopy and transmission electron microscopy. Stacking faults blocked the propagation of TDs, and fewer new TDs were subsequently generated by the coalescence of different orientational domains and lateral-overgrown GaN. This evolution mechanism of TDs was different from that of traditional ELOG technology or one-step ELOG (1S-ELOG) technology using a two-dimensional (2D) material as a mask.

Effect of curved graphene oxide in a GaN light-emitting-diode for improving heat dissipation with a patterned sapphire substrate

AbstactThis article reports on the development of a reduced graphene oxide (rGO)-embedded light-emitting diode (LED) on a patterned sapphire substrate (PSS), for improving heat dissipation and reducing threading dislocations. The prototype device fabrication involves the generation of scalable curved graphene oxide microscale patterns on a PSS, followed by thermal reduction and epitaxial lateral overgrowth of GaN in a metal–organic chemical vapor deposition system with a one-step process. Using the forward voltage method, the junction temperature Tj of the rGO-embedded LED was found to be reduced by about 17 °C from the ∼62 °C of the LED grown without the rGO buffer layer. Temperature-dependent light output power and chip surface temperature measurements were also performed and the results are discussed.

Spatially resolved and orientation dependent Raman mapping of epitaxial lateral overgrowth nonpolar a-plane GaN on r-plane sapphire.

Uncoalesced a-plane GaN epitaxial lateral overgrowth (ELO) structures have been synthesized along two mask stripe orientations on a-plane GaN template by MOCVD. The morphology of two ELO GaN structures is performed by Scanning electronic microscopy. The anisotropy of crystalline quality and stress are investigated by micro-Raman spectroscopy. According to the Raman mapping spectra, the variations on the intensity, peak shift and the full width at half maximum (FWHM) of GaN E2 (high) peak indicate that the crystalline quality improvement occurs in the window region of the GaN stripes along [0001], which is caused by the dislocations bending towards the sidewalls. Conversely, the wing regions have better quality with less stress as the dislocations propagated upwards when the GaN stripes are along []. Spatial cathodoluminescence mapping results further support the explanation for the different dislocation growth mechanisms in the ELO processes with two different mask stripe orientations.

GaN epitaxial lateral overgrowth on laser-textured sapphire

Laser-patterned (0001) sapphire has been evaluated as a substrate to perform GaN epitaxial lateral overgrowth (ELO) by metal-organic chemical vapor deposition (MOCVD). Several conditions of trench ablation were tested by varying scanning speed and laser fluence of a 266-nm picosecond laser. Growth of GaN epilayers was characterized by micro photoluminescence and cathodoluminescence (CL) spectroscopy. The spectra revealed a strain relaxation in the epitaxially overgrown GaN layer with improved quality expressed by enhanced intensity and redshift of the bandgap luminescence (from 3.44 to 3.40 eV). The bandgap against yellow luminescence (YL) intensity ratio also revealed that a defect-depleted area exists at the edge of the trench.

GaN Lateral Overgrowth by Hydride Vapor Phase Epitaxy through Nanometer-Size Channels Fabricated with Nanoimprint Lithography

Epitaxial lateral overgrowth (ELO) has been used for reducing the dislocation density to grow high-quality GaN crystals. In conventional ELO, micrometer-size channels formed on a mask material such as SiO2, where GaN growth starts, are generally used. In the present study, ELO through nanometer-size (50–80 nm) channels is investigated to improve the dislocation reduction ability. Channels are fabricated using nanoimprint lithography and dry etching. We demonstrate for the first time successful hydride vapor phase epitaxy (HVPE)-based GaN ELO growth through nanochannels. In the growth interface, distinct facet structures appear and coalescence between neighboring facets proceeds. The surface of a 20-µm-thick GaN layer becomes flat by the valleys between facet structures being buried. The dislocation density is shown to decrease to approximately 5×107 cm-2 for a 20-µm-thick GaN layer on sapphire. Photoluminescence measurements show a decay time of over 3 times longer than that of a conventional metalorganic chemical vapor deposition (MOCVD) template.

Improvement of quality and strain relaxation of GaN epilayer grown on SiC substrate by in situ SiNx interlayer

Improved structural quality and tensile stress releasing were realized in GaN thin films grown on 6H–SiC by metal organic chemical vapor deposition using an in situ porous SiNx interlayer. The SiNx was formed in situ in the growth chamber by simultaneous flow of diluted silane and ammonia, leading to the formation of a randomly distributed mask layer and induced lateral overgrowth similar to conventional epitaxial lateral overgrowth of GaN. The full width at half maximum (FWHM) of X-ray diffraction peaks decreases dramatically by the SiNx interlayer, indicating an improved crystalline quality. Also, it was found that the biaxial tensile stress in the GaN film was significantly reduced by in situ SiNx interlayer from Raman spectra. Low temperature photoluminescence spectra exhibited a narrower FWHM by the SiNx interlayer.

UGaN growth on laser patterned sapphire substrate: quality feature

AbstractLaser patterning technique as a flexible tool to grow high quality undoped gallium nitride (uGaN) on sapphire wafers is presented. A series of stripes were formed on sapphire wafer surface using Nd:YAG laser (100 kHz, 266 nm, 30 ps pulse) with variable pulse power (40‐670 mW), laser spot velocity over the wafer (1‐46 mm/s) and distance between the stripes (4‐15 μm). Stripes width was ∼2‐5 μm. After laser patterning sapphire wafers were cleaned using aqua regia and etched using sulphuric and phosphoric acid solution. SEM images after wet chemical etching show less surface roughness and open sapphire crystalline planes compared to surfaces treated with laser. Undoped GaN was grown by metal‐organic chemical vapour deposition (MOCVD). Used III/IV ratio: 2400, reactor pressure: 150 mBar, temperature: 1065 °C. SEM and AFM results show reduction of dislocation density in grown gallium nitride layer. The growing of this layer was performed using epitaxial lateral overgrowth (ELO) method on laser patterned sapphire substrates. Our observation revealed that the optimal laser power for sapphire substrate patterning for ELO GaN growth is 70‐300 mW. Moreover, we demonstrated intermediate GaN lateral growth states. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of Si doping on epitaxial lateral overgrowth of GaN: A spatially resolved micro-Raman scattering study

We report spatially resolved Raman scattering from Si-doped epitaxial laterally overgrown GaN structures to investigate spatial variations in stress and free electron concentration. The doping-induced increase in the free electron concentration is relatively higher in the laterally overgrown regions than in the coherently grown regions due to the increased contribution of the high-energy A1 longitudinal optical phonon–plasmon coupled mode. In addition, the E2(high) [E2(low)] phonon energy shifts downward (upward) more significantly in the laterally overgrown regions than in the coherently grown regions. The doping-induced Raman shifts of the E2(high) and E2(low) phonons in the laterally overgrown regions are approximately −0.6 and 0.11 cm−1, respectively, corresponding to the in-plane stress of ∼0.22 GPa.

Separation of laterally overgrown GaN template by using selective electrochemical etching

Selective lateral electrochemical etching was employed for detaching a GaN layer with lowered dislocation density produced by maskless epitaxial lateral overgrowth (ELO) process. The dislocation density in the ELO regions was more than an order of magnitude lower than in the reference regions. The separation of the ELO GaN template from sapphire substrate was achieved by selective lateral electrochemical etching in an oxalic acid. It was shown that such etching occurred preferentially along the GaN template/ELO interface by first creating small pores at the interface and then enlarging them. The etching rates as high as 150μm/min could be achieved, resulting in complete detachment of the entire ELO GaN film. The detached GaN films were transferred to glass substrates and bonded with conducting epoxy. Photoluminescence measurements showed a high luminescence efficiency and decreased strain in the detached ELO GaN template.

Nanoindentation of laterally overgrown epitaxial gallium nitride

Nanoindentation has been used to investigate and compare the mechanical properties of GaN grown by the lateral epitaxial overgrowth (LEO) method and the defective seed region prepared by metalorganic chemical vapour deposition. Common modulus of elasticity values (∼230 GPa) and hardness values (∼19 GPa) were found for both materials. The GaN response to nanoindentation was found to be purely elastic for low indentation loads with the onset of plasticity being marked by discontinuities or “pop-in” events in the indenter load-penetration curves. The maximum shear stress under the indenter at pop-in events for LEO GaN corresponds well with the critical shear stress necessary for homogeneous dislocation nucleation, indicating that the defects in this region are too sparse and do not aid in dislocation nucleation.

Recent progress of GaN growth on maskless chemical-etched grooved sapphire substrate

A new GaN lateral epitaxial overgrowth (LEO) technique using a chemical-etched grooved sapphire substrate is demonstrated here. The substrate fabrication, selective growth mechanism and dislocation reduction mechanism are discussed. By applying this technique, improvement in the quality of wing tilt free GaN LEO films and enhancement of GaN LEDs can be achieved.

Highly Efficient and Bright LEDs Overgrown on GaN Nanopillar Substrates

We presented a study of high-performance GaN-based light emitting diodes (LEDs) using a GaN nanopillars (NPs) structure grown on sapphire substrate by integrating RF-plasma molecular beam epitaxy (MBE) and metal-organic chemical vapor deposition (MOCVD). Nanoscale air voids were clearly observed at the interface between GaN NPs and the overgrown GaN layer by cross-sectional scanning electron microscopy. It can increase the light-extraction efficiency due to additional light scattering. The transmission electron microscopy images suggest the air voids between GaN NPs introduced during nanoscale epitaxial lateral overgrowth of GaN can suppress the threading dislocation density. Moreover, Raman spectrum demonstrated that the strain of the GaN layer grown on GaN NPs was effectively eliminated, resulting in the reduction of quantum-confined Stark effect in InGaN/GaN quantum wells. Consequently, the LEDs fabricated on the GaN NPs template exhibit smaller electroluminescent peak wavelength blue shift and great enhancement of the light output (70% at 20 mA) compared with the conventional LEDs.