Facet-controlled Epitaxial Lateral Overgrowth Research Articles

Study of the morphology evolution of AlN grown on nano-patterned sapphire substrate

This study focused on the evolution of growth front about AlN growth on nano-patterned sapphire substrate by metal-organic chemical vapor deposition. The substrate with concave cones was fabricated by nano-imprint lithography and wet etching. Two samples with different epitaxy procedures were fabricated, manifesting as two-dimensional growth mode and three-dimensional growth mode, respectively. The results showed that growth temperature deeply influenced the growth modes and thus played a critical role in the coalescence of AlN. At a relatively high temperature, the AlN epilayer was progressively coalescence and the growth mode was two-dimensional. In this case, we found that the inclined semi-polar facets arising in the process of coalescence were type. But when decreasing the temperature, the semi-polar facets arose, leading to inverse pyramid morphology and obtaining the three-dimensional growth mode. The 3D inverse pyramid AlN structure could be used for realizing 3D semi-polar UV-LED or facet-controlled epitaxial lateral overgrowth of AlN.

High-Quality GaN Epilayers Achieved by Facet-Controlled Epitaxial Lateral Overgrowth on Sputtered AlN/PSS Templates.

It is widely believed that the lack of high-quality GaN wafers severely hinders the progress in GaN-based devices, especially for defect-sensitive devices. Here, low-cost AlN buffer layers were sputtered on cone-shaped patterned sapphire substrates (PSSs) to obtain high-quality GaN epilayers. Without any mask or regrowth, facet-controlled epitaxial lateral overgrowth was realized by metal-organic chemical vapor deposition. The uniform coating of the sputtered AlN buffer layer and the optimized multiple modulation guaranteed high growth selectivity and uniformity of the GaN epilayer. As a result, an extremely smooth surface was achieved with an average roughness of 0.17 nm over 3 × 3 μm2. It was found that the sputtered AlN buffer layer could significantly suppress dislocations on the cones. Moreover, the optimized three-dimensional growth process could effectively promote dislocation bending. Therefore, the threading dislocation density (TDD) of the GaN epilayer was reduced to 4.6 × 107 cm-2, which is about an order of magnitude lower than the case of two-step GaN on the PSS. In addition, contamination and crack in the light-emitting diode fabricated on the obtained GaN were also effectively suppressed by using the sputtered AlN buffer layer. All of these advantages led to a high output power of 116 mW at 500 mA with an emission wavelength of 375 nm. This simple, yet effective growth technique is believed to have great application prospects in high-performance TDD-sensitive optoelectronic and electronic devices.

Fabrication of InGaN/GaN stripe structure on (1 1 1)Si and stimulated emission under photo-excitation

InGaN/GaN QW laser structure was investigated on a GaN trapezoid grown on (1 1 1)Si substrate by selective MOVPE. The dislocation density in the active layer was reduced by a two-step growth method adopting facet controlled epitaxial lateral overgrowth (FACELO). A sample with 350 μm long cavity length showed narrowing of the spectral peak under optical excitation. The compositional non-uniformity originating from the ridge growth on the trapezoid is removed by adopting re-evaporation phenomenon under heat treatment during the growth process.

A 342-nm ultraviolet AlGaN multiple-quantum-well laser diode

The realization of semiconductor laser diodes and light-emitting diodes that emit short-wavelength ultraviolet light is of considerable interest for a number of applications including chemical/biochemical analysis, high-density data storage and material processing. Group III nitride materials are one of the most promising candidates for fabricating such devices. Here we describe an AlGaN multiple-quantum-well laser diode that emits light at 342 nm, the shortest wavelength ever reported for an electrically driven laser diode. To fabricate the laser, a low-dislocation-density AlGaN layer with an AlN mole fraction of 0.3 was grown on a sapphire substrate using a hetero facet-controlled epitaxial lateral overgrowth (hetero-FACELO) method1,2,3. An AlGaN multiple-quantum-well structure was then grown on the high-quality AlGaN layer. Lasing at a wavelength of 342.3 nm was observed under pulsed current mode at room temperature. Short-wavelength UV laser diodes are required for applications ranging from sensing, data storage and materials processing. Here, an electrically driven semiconductor laser that operates at 342.3 nm, the shortest wavelength so far, is reported. The device emits milliwatt-scale powers at room temperature when driven by pulsed current.

Fabrication of GaN‐based light emitting diodes using direct heteroepitaxial lateral overgrowth on patterned sapphire substrates with thick SiO2 masks

AbstractWe have successfully obtained high‐quality GaN films directly on thick SiO2 patterned sapphire substrates by utilizing modified facet‐controlled epitaxial lateral overgrowth (FACELO). GaN films were grown by metalorganic vapour phase epitaxy on the stripe patterned substrates with the SiO2 thickness of 1.2 μm. The threading dislocation density reached a value as low as 5 × 107 cm–2. We have also fabricated Light emitting diodes (LEDs). The output power and external quantum efficiency of the LED operated at 20 mA were 20.74 mW and 37.1%, respectively. This is due to not only the reduced dislocation density but also the improved light extraction efficiency because of the light scattering at the patterned GaN/thick‐SiO2‐masks interface. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Study of ELOG GaN for Application in the Fabrication of Micro-channels for Optoelectronic Devices

AbstractGallium Nitride (GaN) is a promising wide band gap semiconductor material for many optoelectronic applications, especially in the near UV range. Over the past several years, an extensive technical effort has been focused on improving the quality of GaN films through various overgrowth techniques such as epitaxial lateral overgrowth (ELOG), facet controlled epitaxial lateral overgrowth (FACELO), and Pendeoepitaxy. ELOG has been shown to reduce the density of threading dislocations by up to five orders of magnitude [1], however a complete physical model describing lateral overgrowth is needed in order to take full advantage of the process. A lateral overgrowth model will allow for the design and fabrication of three dimensional structures that can lead to novel devices and also to efficient biosensors by integrating micro and nano channels on the same chip as the optoelectronic components.A two-step process has been used to successfully control the geometry of overgrown GaN. Conditions have been identified which give a reduced lateral growth rate, in order to allow expansion of the {112n} plane to form vertical sidewalls and for the design of channel width. These geometries are being examined for possible application in laser diode and micro-channel fabrication for integrating bio-agent detection modules.

CBED study of grain misorientations in AlGaN epilayers

Large angle convergent beam electron diffraction (LACBED) has been used to examine AlGaN epilayers grown by facet-controlled epitaxial lateral overgrowth on GaN/(0 0 0 1) sapphire substrates in prototype UV laser structures. The substrates, defined by masks with seed openings along a 〈 1 0 - 1 0 〉 stripe direction, had GaN seed columns with { 1 1 - 2 2 } surfaces. Studies were carried out on cross-sectional samples cut perpendicular to the stripe axis. An LACBED analysis of the orientation of (0 0 0 2) planes, and of the ( 1 1 - 2 0 ) planes parallel to the stripe axis, revealed that the AlGaN wings were both rotated by angles of 1–2×10 −2 radians about the 〈 1 0 - 1 0 〉 stripe axis with respect to the underlying GaN, and distorted due to misfit strains. It is shown that the results are consistent with the observed structure of the AlGaN/GaN and the wing/wing boundaries, and with a new model for the generation of a-type misfit dislocations at the AlGaN/GaN interface.

The generation of misfit dislocations in facet-controlled growth of AlGaN∕GaN films

The relaxation of tensile stresses in AlGaN layers grown on GaN∕(0001)sapphire by facet-controlled epitaxial lateral overgrowth is reported. It is shown that a-type misfit dislocations are introduced at inclined {112¯2} AlGaN∕GaN interfaces, with strong evidence for a half-loop nucleation and glide mechanism driven by shear stresses present on the (0001) slip plane. In addition to relieving misfit stresses, these dislocations introduce grain rotations of up to 10−2rad across the AlGaN∕GaN boundaries, leading to tilt boundaries at the meeting front between laterally growing wings and between regions growing in the lateral and [0001] directions. The effects of these processes on the defect density in subsequent layers are examined.

Growth of high‐quality GaN on FACELO substrate by raised‐pressure HVPE

High-quality thick GaN was obtained by raised-pressure HVPE on GaN template grown by the facet-controlled epitaxial lateral overgrowth (FACELO) technique. The threading dislocation density of the thick GaN is 2 × 107 cm−2. The FWHM value of the (0004) X-ray rocking curve is 94.7 arcsec. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Epitaxial lateral overgrowth techniques used in group III nitrideepitaxy

Selective-area growth (SAG) and epitaxial lateral overgrowth (ELO)techniques used in group III nitride epitaxy are reviewed. Structurallycontrolled GaN in line patterns and dot patterns was obtained byusing the SAG technique. This structural control technique has beenapplied to field emitters, waveguides, facet lasers andlow-dimensional quantum structures. The ELO technique wasoriginally proposed as a way to reduce dislocation density inepitaxial GaN layers and various ELO techniques have been developedrecently. For example, facet-controlled ELO (FACELO) is one of theattractive techniques available for reducing dislocation density.The dislocation density is dramatically reduced to of the order of105-6 cm-2 with good reproducibility. In thisarticle, SAG and ELO techniques related to nitride epitaxy andapplications to optical and electronic devices are reviewed.Production of GaN hexagonal pyramids on dot-patterned GaN/sapphiresubstrates and the FACELO technique are also described as examplesof the SAG and ELO techniques.

Fabrication and characterization of low defect density GaN using facet-controlled epitaxial lateral overgrowth (FACELO)

Facet structures of GaN grown by epitaxial lateral overgrowth (ELO) via low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) are controlled by growth conditions such as reactor pressure and growth temperature, where this technique is called facet-controlled ELO (FACELO). The propagation mechanism of the threading dislocations for the different GaN facet structure is investigated. The distribution and density of the threading dislocations are observed by the growth pit density (GPD) method. Two typical models employing the FACELO are proposed; in one model, the dislocation concentrates only on the window area and, in the other model, only in the coalescence region in the center of the mask. In the latter model, the dislocation density is dramatically dropped to the order of 10 6 cm −2 with good reproducibility. The FACELO GaN shows no tilt of c-axis on the mask area and good optical properties.

Fabrication of GaN layer with Low Dislocation Density using Facet controlled ELO technique

ABSTRACTGaN layers with low dislocation density have been fabricated be means of facet-controlled epitaxial lateral overgrowth (FACELO) via low-pressure metalorganic vapor phase epitaxy (LP-MOVPE). The distribution of the dislocations in FACELO GaN was inspected by observation of InGaN growth pits. For FACELO with {11-20} facets as the first step, the dislocations concentrate only in the window region. For FACELO with {11-22} facets as the first step, the dislocations exist only in the coalescence region. The double FACELO, which was FACELO with {11-20} on FACELO with {11-22}, was demonstrated and dislocation density of less than 105 cm−2 was achieved.

Review of Facet Controlled Epitaxial Lateral Overgrowth (FACELO) of GaN via Low Pressure Vapor Phase Epitaxy

ABSTRACTFacet structures of GaN grown by epitaxial lateral overgrowth (ELO) via low pressure-metalorganic vapor phase epitaxy (LP-MOVPE) are controlled by growth conditions such as reactor pressure and growth temperature, where this technique is called FACELO (Facet Controlled ELO). The mechanism of the morphological change is discussed based on stability of the surface atoms. The propagation mechanism of the threading dislocations for the different GaN facet structure is also investigated. The distribution and density of the threading dislocations are observed by the growth pit density (GPD) method. Two typical models employing the FACELO are proposed; in one model, the dislocation concentrates only on the window area and, in the other model, only in the coalescence region in the center of the mask. In the latter model, the dislocation density is dramatically dropped to the order of 105−6 cm−2 with good reproducibility.