Overgrown GaN Research Articles

In-Situ Preparation of GaN Sacrificial Layers on Sapphire Substrate in MOVPE Reactor for Self-Separation of the Overgrown GaN Crystal

GaN layers on sapphire substrates were prepared by using metal organic vapor phase epitaxy (MOVPE) combined with an in-situ H2 etching process for the purpose of later self-separation of thick GaN crystals produced by hydride vapor phase epitaxy (HVPE) on such substrates. The etching process results in deep pits and long voids that formed on the surface and along the lower interface between GaN and sapphire, respectively. The pits, which were investigated by SEM analysis, can be modified in their aspect ratio and density by controlling the etching parameters. Using a proper set of in-situ etching parameters, a seed layer with internal voids can be prepared, which is suitable for HVPE overgrowth and the self-separation process. The quality of the in-situ-etched seed GaN layer and overgrown GaN crystal were characterized by X-ray diffraction (XRD) and defect selective etching (DSE). With the aid of atomic force microscopy (AFM) in tapping mode, the interface morphology of the separated GaN crystal was analyzed. The crystal quality of the separated HVPE-GaN crystal is comparable to the crystal grown on untreated GaN MOVPE-seed, which did not separate from the sapphire substrate. The introduced technique to promote the crystal separation during the HVPE process has no obvious drawback on the quality of the grown GaN crystals. Using this technique, the self-separation occurs more gently due to a weakened interface between GaN/sapphire. The conventional separation from an untreated seed by pure thermomechanical action results in higher mechanical forces on the crystal and consequently much higher risk of crystal breakage.

V-Defect and Dislocation Analysis in InGaN Multiple Quantum Wells on Patterned Sapphire Substrate

InGaN/GaN multiquantum well (MQW) structures have been grown on cone-shaped patterned sapphire substrates (CPSS) by metalorganic chemical vapor deposition (MOCVD). From the transmission electron microscopy (TEM) results, we found that most of the threading dislocations (TDs) in the trench region of the CPSS were bent by lateral growth mode. Also the staircase-like TDs were observed near the slant region of the cone pattern, they converged at the slope of the cone patterned region by staircase-upward propagation, which seems to effectively prevent TDs from vertical propagation in the trench region. The associated dislocation runs up into the overgrown GaN layer and MQW, and some (a+c) dislocations were shown to decompose inside the multi-quantum well, giving rise to a misfit segment in the c-plane and a V-shape defect. From cross-sectional TEM, we found that all V defects are not always connected with TDs at their bottom, some V defects are generated from the stacking mismatch boundaries induced by stacking faults which are formed within the MQW due to the strain relaxation.

Semi-polar (11-22) AlGaN on overgrown GaN on micro-rod templates: Simultaneous management of crystal quality improvement and cracking issue

Thick and crack-free semi-polar (11-22) AlGaN layers with various high Al compositions have been achieved by means of growth on the top of nearly but not yet fully coalesced GaN overgrown on micro-rod templates. The range of the Al composition of up to 55.7% was achieved, corresponding to an emission wavelength of up to 270 nm characterised by photoluminescence at room temperature. X-ray diffraction (XRD) measurements show greatly improved crystal quality as a result of lateral overgrowth compared to the AlGaN counterparts on standard planar substrates. The full width at half maximums of the XRD rocking curves measured along the [1-100]/[11-2-3] directions (the two typical orientations for characterizing the crystal quality of (11-22) AlGaN) are 0.2923°/0.2006° for 37.8% Al and 0.3825°/0.2064° for 55.7% Al, respectively, which have never been achieved previously. Our calculation based on reciprocal space mapping measurements has demonstrated significant strain relaxation in the AlGaN as a result of utilising the non-coalesced GaN underneath, contributing to the elimination of any cracks. The results presented have demonstrated that our overgrowth technique can effectively manage strain and improve crystal quality simultaneously.

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.

Improved Crystal Quality of (112̄2) Semi-Polar GaN Grown on A Nanorod Template

(112̄2) semi-polar GaN with significantly improved crystal quality has been achieved by means of overgrowth on a (112̄2) semi-polar nanorod template. The nanorod template was fabricated on a standard (112̄2) semi-polar GaN layer on m-plane sapphire using a self-organized nickel nano-mask technique. In comparison with any conventional overgrowth technique, the nano-mask approach has demonstrated a very quick coalescence with a thickness of less than 1 µm. X-ray rocking curve measurements as a function of azimuth angle has shown a massive reduction in linewidth for our overgrown GaN. Transmission electron microscope measurements have confirmed a significant reduction in the dislocation density. Dramatic improvement in optical properties has been exhibited by photoluminescence (PL) measurements performed at room temperature, showing that the PL intensity from the band edge emission is 30 times higher than that of the standard semi-polar GaN on sapphire.

Modification of dislocation behavior in GaN overgrown on engineered AlN film-on-bulk Si substrate

The changes that the AlN buffer and Si substrate undergo at each stage of our substrate engineering process, previously shown to lead to a simultaneous and substantial reduction in film crack density and dislocation density in overgrown GaN, are presented. Evidence of ion-implantation assisted grain reorientation for AlN islands coupled with physical isolation from the bulk Si substrate prove to be the dominating driving forces. This is further emphasized with x-ray diffraction analysis that demonstrates a reduction in the in-plane lattice constant of AlN from 3.148 Å to 3.113 Å and a relative change in rotation of AlN islands by 0.135° with regard to the Si substrate after substrate engineering. Misfit dislocations at the AlN-Si interface and disorder that is normally associated with formation of amorphous SiNx at this interface are considered to be two of the major contributors to dislocation nucleation within overgrown GaN. Following our technique, the disappearance of disorder at the AlN-Si interface is observed. Extensive ellipsometry and transmission electron microscopy suggests that larger AlN islands with a smoother surface morphology could further reduce the dislocation density below that previously reported. A 1.2 μm GaN layer deposited on an AlN buffer with larger islands and smoother morphology exhibits a 14× reduction in surface pit density after undergoing the ion-implantation assisted substrate modification technique.

Efficient reduction of defects in (1120) non-polar and (1122) semi-polar GaN grown on nanorod templates

(1120) non-polar and (1122) semi-polar GaNs with a low defect density have been achieved by means of an overgrowth on nanorod templates, where a quick coalescence with a thickness even below 1 μm occurs. On-axis and off-axis X-ray rocking curve measurements have shown a massive reduction in the linewidth for our overgrown GaN in comparison with standard GaN films grown on sapphire substrates. Transmission electron microscope observation demonstrates that the overgrowth on the nanorod templates takes advantage of an omni-directional growth around the sidewalls of the nanostructures. The dislocations redirect in basal planes during the overgrowth, leading to their annihilation and termination at voids formed due to a large lateral growth rate. In the non-polar GaN, the priority 〈0001〉 lateral growth from vertical sidewalls of nanorods allows basal plane stacking faults (BSFs) to be blocked in the nanorod gaps; while for semi-polar GaN, the propagation of BSFs starts to be impeded when the growth front is changed to be along inclined 〈0001〉 direction above the nanorods.

Overgrowth of GaN on GaN nanowires produced by mask-less etching

We report on the generation of GaN nanowires (NWs) using mask-less reactive ion etching (RIE). The NWs are believed to be the result of a high etching rate in regions where a high dislocation density is present in the GaN films grown on sapphire substrates. We have studied the effect of defect densities in the original GaN films and its relation to the generation of these NWs. We show that defect reduction in the overgrown GaN is related to the presence of a network of embedded voids generated between these nanowires during the regrowth on the etched nanowires. We show that further reduction in dislocation density can be achieved by repeating the process of nanowire generation and overgrowth. Also we report on the residual strain and curvature in GaN after the first and second embedded voids approach (EVA).

X‐ray diffraction study of GaN grown on patterned substrates

AbstractOwing to its great potential in optoelectronic devices, a lot of effort has recently been put into improving the crystal quality of heteroepitaxial GaN. In this study, GaN layers grown on patterned GaN/sapphire substrates are non‐destructively investigated by X‐ray diffraction techniques. The effect of the patterning is characterized by determining the strain and lattice tilt and twist in the overgrown GaN layers.High resolution X‐ray diffraction (HRXRD) scans and reciprocal space maps (RSMs) were utilized for this purpose. Synchrotron radiation X‐ray topography (SR‐XRT) was used to examine the microstructure of the samples. The patterning was found to significantly improve the crystal quality of the GaN layers, especially by reducing the amount of edge dislocations. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Epitaxial lateral overgrowth of non-polar GaN(1 1̄ 0 0) on Si(1 1 2) patterned substrates by MOCVD

m-Plane GaN was grown selectively by metal–organic chemical vapor deposition (MOCVD) on patterned Si(1 1 2) substrates, where grooves aligned parallel to the Si〈1 1 0〉 direction were formed by anisotropic wet etching to expose the vertical Si{1 1 1} facets for growth initiation. The effect of growth conditions (substrate temperature, chamber pressure, and ammonia and trimethylgallium flow rates) on the growth habits of GaN was studied with the aim of achieving coalesced m-plane GaN films. The epitaxial relationship was found to be GaN(1 1̄ 0 0) || Si(1 1 2), GaN[0 0 0 1] || Si[1 1 –1], GaN[1̄ 1̄ 2 0] || Si[1 1̄ 0]. Among all growth parameters, the ammonia flow rate was revealed to be the critical factor determining the growth habits of GaN. The distribution of extended defects, such as stacking faults and dislocations, in the selectively grown GaN were studied by transmission electron microscopy in combination with spatially resolved cathodoluminescence and scanning electron microscopy. Basal-plane stacking faults were found in the nitrogen-wing regions of the laterally overgrown GaN, while gallium-wings were almost free of extended defects, except for the regions near the GaN/Si{1 1 1} vertical sidewall interface, where high dislocation density was observed.

InGaN/GaN White Light-Emitting Diodes Embedded With Europium Silicate Thin Film Phosphor

This article describes the successful fabrication of europium-silicate thin film phosphor and its application to InGaN/GaN white light-emitting diodes (LEDs) in order to improve the photometric properties of the LEDs, including their correlated color temperatures (CCT) and color rendering index (CRI). The europium-silicate compounds are deposited on GaN templates grown on sapphire substrates by RF-sputtering and then annealed at 1000°C in an N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> ambient to form a thin film phosphor that produces yellow or red emissions. The thin film phosphor is then patterned with stripes to grow a GaN buffer layer by epitaxially laterally overgrown GaN (ELOG) techniques, on which LED structures are grown by metal organic chemical vapor deposition. The ELOG sample shows no pits on the surface, and the full widths at half maximum (FWHMs) of its X-ray rocking curve for the (002) and (102) planes are as low as 249 and 416 arcsec, respectively. The optical spectrum from the embedded thin film phosphor is adjusted to have a maximum intensity at 560-600 nm and a FWHM as wide as 90 nm to make up for the low efficiency at these wavelengths of conventional YAG-based yellow phosphor. Finally, we observed a tristimulus coordinate (x, y) = (0.33, 0.39), CCT = 5607 K, and CRI = 77.6 from the white LEDs with thin film phosphor as compared with (x, y) = (0.30, 0.28), CCT = 8467 K, and CRI = 66.52 for the white LEDs without thin film phosphor.

Structural perfection of laterally overgrown GaN layers grown in polar- and non-polar directions

The structural quality of GaN overgrown layers was evaluated using Transmission Electron Microscopy methods. Growth on polar and non-polar substrates was compared. Independent from growth polarity much better structural quality of the overgrown areas compared to the seed areas was obtained, but overgrowth on non-polar substrates is more difficult. For the latest samples, two wings on the opposite sites of the seed area grow in two different polar directions with different growth rates. Wings grown with Ga polarity are much wider than wings grown with N-polarity making coalescence of these layers difficult. Defects formed in the overgrown wings were characterized and their density was compared. It is shown that two-step growth (using two different temperatures) lead to much smaller misorientation between the wings than one step growth

Freestanding 2-in GaN layers using lateral overgrowth with HVPE

Hydride vapor phase epitaxy (HVPE) was used for the lateral overgrowth of thick GaN layers over masks of tungsten silicide nitride (WSiN). This mask material reduces sticking between the overgrown GaN and the mask and promotes the self-separation of the overgrown layers. Evaluation of the reflectance transient of the growing GaN layer indicates that the separation already occurs during growth and is caused by the intrinsic tensile strain of the growing GaN layer. This strain causes a strong concave bowing which is preserved even after cooling down. To reduce this residual bow of the freestanding GaN layers different approaches have been evaluated. It was found that the moment of separation is influenced by the mask geometry and that the temperature at this moment has an effect on the residual bow. The strain state of the template and the resulting bow at growth temperature are indicated to affect the residual bow as shown from the comparison of SiC- and sapphire-based GaN templates.

Structural perfection of laterally overgrown GaN layers grown in polar- and non-polar directions

Structural perfection of laterally overgrown GaN layers grown in polar- and non-polar directions

Spatial variations of doping and lifetime in epitaxial laterally overgrown GaN

Spatial variations of donor concentration and diffusion length/lifetime were studied for epitaxial laterally overgrown undoped n-GaN samples by electron beam induced current (EBIC) and microcathodoluminescence (MCL). The dependence of the EBIC signal collection efficiency on the probing beam accelerating voltage shows that the local electron concentration is three times lower and the local lifetime about twice as high in the laterally overgrown regions compared to the regions grown in the SiO2 mask windows. Band edge MCL profiling shows that the lifetime difference could be an order of magnitude higher. EBIC scans along the SiO2 stripes in the low dislocation density overgrown regions show long propagation distances of holes in the quasineutral part of the structure, explained by the existence of a potential profile forming a trough for transport of holes while spatially separating nonequilibrium carriers.

AlGaN/GaN multiple quantum wells grown on facet-controlled epitaxial lateral overgrownGaN/sapphire templates

Epitaxial lateral overgrowth of GaN with () facets was realized by metal–organic chemical vapour deposition on GaN/sapphire (0001) substrateswith an SiO2 stripe mask. After wet etching of the mask, periodic AlGaN/GaN multiple quantum wells(MQWs) were grown on the whole surface. Cross section transmission electron microscopyshowed that the average growth rate on the () facet is lower than on the (0001) plane. The concentration of Al of AlGaN/GaN MQWswas higher on the (0001) facet than on the () surface, as measured by secondary ion mass spectrometry and high resolution x-raydiffraction (HR-XRD). Micro-Raman scattering spectroscopy revealed a significantrelaxation of compressive stress in the laterally overgrown GaN. Micro-photoluminescencespectra confirmed quantum confinement of electrons in MQWs. The achieved opticalquality of MQWs on the () facet is comparable with that on the (0001) plane.

Structural analysis of ELO‐GaN grown on a sapphire substrate as the underlying layer of a GaN‐based laser diode

AbstractUsing micro‐beam X‐ray diffraction (XRD), small c‐axis tilting was observed in maskless epitaxially laterally overgrown GaN (ELO‐GaN), which is a suitable underlying layer for realizing a reliable GaN‐based blue‐violet laser diode. By analyzing the micro‐scale structure of ELO‐GaN without coalescence and the ambient‐temperature dependence of the tilting angle, we conclude that the small c‐axis tilting is caused by compressive stress in the seed region of ELO‐GaN. (© 2007 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Characterisation of nitride thin films by electron backscatter diffraction and electron channelling contrast imaging

In the present paper the authors describe the use of electron backscatter diffraction (EBSD) mapping and electron channelling contrast imaging (in the scanning electron microscope) to study tilt, strain, atomic steps and dislocations in epitaxial GaN thin films. Results from epitaxial GaN thin films and from a just coalesced epitaxial laterally overgrown GaN thin film are shown. From the results it is deduced that EBSD may be used to measure orientation changes of the order of 0·02° and strain changes of order 2 × 10−4 in GaN thin films. It is also demonstrated that channelling contrast in electron channelling contrast images may be used to image tilt, atomic steps and threading dislocations in GaN thin films. In addition the authors will consider the results of the first many-beam dynamical simulations of EBSD patterns from GaN thin films, in which the intensity distributions in the experimental patterns are accurately reproduced.

Defect reduction and efficiency improvement of near-ultraviolet emitters via laterally overgrown GaN on a GaN/patterned sapphire template

An approach to improve the defect density and internal quantum efficiency of near-ultraviolet emitters was proposed using a combination of epitaxial lateral overgrowth (ELOG) and patterned sapphire substrate (PSS) techniques. Especially, a complementary dot array pattern corresponding to the underlying PSS was used for the ELOG-SiO2 mask design. Based on the transmission-electron-microscopy and etch-pit-density results, the ELOG∕SiO2∕GaN∕PSS structure can reduce the defect density to a level of 105cm−2. The internal quantum efficiency of the InGaN-based ELOG-PSS light-emitting diode (LED) sample showed three times in magnitude as compared with that of the conventional GaN/sapphire one. Under a 20mA injection current, the output powers of ELOG-PSS, PSS, and conventional LED samples were measured to be 3.3, 2.9, and 2.5mW, respectively. The enhanced output power could be due to a combination of the reduction in dislocation density (by ELOG) and improved light extraction efficiency (by PSS). Unlike the previ...

GaN epilayers on nanopatterned GaN/Si(1 1 1) templates: Structural and optical characterization

Template-based nanoscale epitaxy has been explored to realize high-quality GaN on Si(1 1 1) substrates. We have employed polystyrene-based nanosphere lithography to form the nano-hole array patterns on GaN/Si(1 1 1) template and then, subsequent regrowth of GaN is carried out by metalorganic chemical vapor deposition (MOCVD). During the initial growth stage of GaN on such nanopatterned substrates, we have observed formation of nanoislands with hexagonal pyramid shape due to selective area epitaxy. With further epitaxial regrowth, these nanoislands coalesce and form continuous GaN film. The overgrown GaN on patterned and non-patterned regions is characterized by high-resolution X-ray diffraction (HRXRD) and high-spatial resolution optical spectroscopic methods. Micro-photoluminescence (PL), micro-Raman scattering and scanning electron microscopy (SEM) have been used to assess the microstructural and optical properties of GaN. Combined PL and Raman data analyses show improved optical quality when compared to GaN simultaneously grown on non-patterned bulk Si(1 1 1). Such thicker GaN templates would be useful to achieve III-nitride-based opto- and electronic devices integrated on Si substrates.