Epitaxial Lateral Overgrown GaN Research Articles

Investigation of the Effect of Electron-Beam Irradiation on the Defect Structure of Laterally Overgrown GaN Films via the Induced-Current and Cathodoluminescence Methods

The low-energy electron irradiation effect on the defect structure on epitaxial laterally overgrown (ELOG) films and thick GaN crystals grown via the hydride epitaxy method is studied using a scanning electron microscope by the induced-current and cathodoluminescence methods. Electron-beam irradiation is carried out at room temperature. In this case, the electron-beam irradiation effect on dislocation segments located in the basal plane is studied in ELOG GaN films. At the same time, dislocations are introduced into GaN crystals by indentation at room temperature. It is found that the behavior of growth dislocations and those introduced by indentation is different under electron-beam irradiation.

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.

Epitaxial Lateral Overgrowth of GaN on Si (111) Substrates Using High‐Dose, N+ Ion Implantation

AbstractAn epitaxial, laterally‐overgrown (ELOG) GaN layer is deposited on a Si(111) substrate using high‐dose, N+ ion implantation. ELOG GaN is deposited on a Si(111) wafer with implantation stripes by metal‐organic (MO) CVD. The GaN layer on the N+ ion‐implanted region is polycrystalline and acts as a mask for the ELOG process. This is attributed to the growth rate of the polycrystalline GaN being much slower than that of epitaxial GaN. After 120 min, complete coalescence is achieved with a flat surface. Scanning cathodoluminescence (CL) microscopy and high resolution X‐ray diffraction (HRXRD) confirm the high optical and crystalline quality of the ELOG GaN layer.

Cathodoluminescence study of GaN-based film structures

GaN films grown on sapphire substrate with an emphasis on epitaxial lateral overgrown (ELOG) layers with an array of rhombic shaped mask area as well as InGaN/GaN MQW laser diode layer structures were investigated by cathodoluminescence (CL) spectroscopy and CL imaging at room and low temperatures. The microscopic imaging with a high-spatial resolution clearly reveals the distribution of threading dislocations and point defects in ELOG GaN films. The secondary electron and CL data measured on cleaved faces of laser diodes are analyzed in consideration with luminescence mechanisms in semiconductor heterostructures and around the p - n junction, providing important information on the defects and carrier dynamics in laser diode devices.

Neutron Radiation Effects in Epitaxially Laterally Overgrown GaN Films

Neutron radiation effects were studied in undoped n-GaN films grown by epitaxial lateral overgrowth (ELOG). The irradiation leads to carrier removal and introduces deep electron traps with activation energy 0.8 eV and 1 eV. After the application of doses exceeding 1017 cm−2, the material becomes semi-insulating n-type, with the Fermi level pinned near the level of the deeper electron trap. These features are similar to those previously observed for neutron irradiated undoped n-GaN prepared by standard metal–organic chemical vapor deposition (MOCVD). However, the average carrier removal rate and the deep center introduction rate in ELOG samples is about five-times lower than in MOCVD samples. Studies of electron beam induced current (EBIC) show that the changes in the concentration of charged centers are a minimum in the low-dislocation-density laterally overgrown regions and radiation-induced damage propagates inside these laterally overgrown areas from their boundary with the high-dislocation-density GaN in the windows of the ELOG mask.

Magnetotransport properties of AlxGa1−xN∕AlN∕GaN heterostructures grown on epitaxial lateral overgrown GaN templates

We studied the low-temperature magnetotransport properties of AlxGa1−xN∕AlN∕GaN heterostructures with a two-dimensional electron gas (2DEG). Structures with different Al compositions were grown by metal-organic vapor-phase epitaxy on three types of templates: conventional undoped GaN, in situ epitaxial lateral overgrown GaN using a SiNx nanomask layer, and ex situ epitaxial lateral overgrown GaN (ELO-GaN) using a stripe-patterned SiO2 mask. All of the samples display Shubnikov–de Haas (SdH) oscillations that confirm the existence of 2DEGs. Field-dependent magnetoresistance and Hall measurements further indicate that the overgrown heterostructures have a parallel conducting layer in addition to the 2DEG. To characterize the parallel channel, we repeated the measurements after the 2DEG was etched away. 2DEG carrier density values were then extracted from the SdH data, whereas the zero-field 2DEG conductivity was determined by subtracting the parallel channel conductivity from the total. The quantitative mobility spectrum analysis could not be applied in some cases, due to a large contact resistance between the parallel channels. The resulting 2DEG mobility is about a factor of 2 higher in the ELO-GaN and SiN–GaN samples as compared to the standard control samples. The mobility enhancement is attributed to a reduction of threading dislocations by the two ELO techniques employed.

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)

Prismatic stacking faults in epitaxially laterally overgrown GaN

We report on the presence of optically active stacking faults on basal and prismatic planes in epitaxially laterally overgrown GaN (ELOG) on {112¯2} facets. The structure of the faults has been analyzed using diffraction contrast electron microscopy. We show that stacking faults on {112¯0} prismatic planes involve a lattice displacement of 12⟨11¯01⟩, parallel to the fault plane. They appear as jogs connecting basal-plane stacking faults, the latter with a lattice displacement of 16⟨202¯3⟩. These faults are observed only in the laterally overgrown regions that grow on {112¯2} planes, which indicates that the stacking fault formation is closely related to the orientation of the growth surface. Possible formation mechanisms of these faults are discussed. Direct correlation between TEM and cathodoluminescence shows that these prismatic-plane and basal-plane stacking faults are optically active with light emission at 3.30 and 3.41eV, respectively.

Structural analysis of ELO-GaN grown on sapphire using the x-ray micro-beam of an 8-GeV storage ring

AbstractWe investigated the micrometer-scale structure of epitaxially laterally overgrown GaN (ELO-GaN) on a sapphire without a SiO2 mask using the 2×4 μm2 micro-beam x-ray of an 8-GeV storage ring. The GaN (0 0 0 12) rocking curve of a wing region had a sharp peak with a FWHM of 46 arcsec, while that of a seed region had several broad peaks. This peak-narrowing indicates that the density of threading dislocations (TD) is reduced and the grain size is extended in the wing region by the lateral growth. The observed grain size in the wing region was 10 μm × 5 μm. The mapping of the rocking curves shows that the wing regions hang down from the seed region at a tilting angle of 81.5 arcsec, although there is no SiO2 mask which is a cause of c-axis tilting in ELO-GaN. After comparing the results of ELO-GaN with and without coalescence, we conclude that the small c-axis tilting is caused by the compressive stress induced by the difference in the thermal expansion coefficient of the GaN and the sapphire substrate. Because a shrinkage of grain size was observed after the coalescence of the wing regions, it is thought the coalescence is one of the causes of the generation of the few threading dislocations in the wing regions. We believe that the reduction of the threading dislocations and the expansion of gain size in the wing regions are important for the further improvement of the device characteristics of GaN-based LDs grown on ELO-GaN.

Increase of free carrier lifetime in nonpolar a-plane GaN grown by epitaxial lateral overgrowth

Carrier recombination dynamics in epitaxial a-plane GaN and fully coalesced epitaxial laterally overgrown (ELOG) a-plane GaN films has been studied by means of time-resolved photoluminescence under high photoexcitation. The results were compared with conventional c-plane GaN films grown under similar conditions. In a-plane GaN epilayers, the total efficiency of electron-hole plasma spontaneous luminescence decreases 20 times, whereas the luminescence decay time reduces from τLU=42 to τLU⩽10ps in comparison with c-plane GaN films. Meanwhile, an essential increase in total emission efficiency (by more than two orders of magnitude) and an increase of the decay time up to τLU=430ps have been observed for an ELOG a-plane sample in comparison with a-plane GaN films. This confirms a significant reduction of the nonradiative recombination rate for nonequilibrium carriers. Assuming a saturation of the nonradiative deep-level transitions, the room-temperature free-carrier lifetime of τ=910ps for ELOG a-plane GaN sample was obtained, which indicates on an excellent quality of the a-plane ELOG GaN films.

Depth-resolved luminescence imaging of epitaxial lateral overgrown GaN using ionoluminescence

In this article, we utilize energetic mega-electron-volt (MeV) H + to excite light emission from epitaxial lateral overgrown GaN. Due to the high-penetration depth of MeV ions, this technique allows us to probe several microns below the surface, down to the interface region. Spatially and depth-resolved yellow luminescence imaging is carried out using a focused H + beam with energies of 0.5, 0.75 and 1 MeV. Results show an increase of the yellow luminescence with depth in the window regions with respect to the wing regions. Since the threading dislocations propagate to the surface, this suggests that the yellow luminescence is due to the point defects rather than threading dislocations.

Depth-resolved luminescence imaging of epitaxial lateral overgrown GaN using ionoluminescence

In this article, we utilize energetic mega-electron-volt (MeV) H+ to excite light emission from epitaxial lateral overgrown GaN. Due to the high-penetration depth of MeV ions, this technique allows us to probe several microns below the surface, down to the interface region. Spatially and depth-resolved yellow luminescence imaging is carried out using a focused H+ beam with energies of 0.5, 0.75 and 1 MeV. Results show an increase of the yellow luminescence with depth in the window regions with respect to the wing regions. Since the threading dislocations propagate to the surface, this suggests that the yellow luminescence is due to the point defects rather than threading dislocations.

Structural Defects related issues of GaN-based Laser Diodes

ABSTRACTStructural defects affecting the lifetime of GaN-based laser diodes (LDs) on epitaxial lateral overgrown (ELO) GaN layers have been investigated. Almost all of the threading dislocations that appeared in the wing regions have edge character, whereas the dislocations at the coalescence boundaries have both edge character and mixed character. The origins of the threading dislocations in the wing regions are the lateral extension of dislocations from the seed regions that contingently bend upwards to the epi-surface. Thus, edge dislocations are most considerable threading dislocations in GaN-based LDs on ELO GaN layers, since the laser stripes are fabricated in the wing regions. In the degraded LDs, neither dislocation multiplication from the threading dislocations nor any structural changes of the threading dislocations were observed. This indicates that degradation is not caused by dislocation multiplication at the active layers, which is usually observed in LDs featuring zincblende-based structures. Although the threading dislocations in the LD stripes do not multiply during device operation, our degradation experiments revealed that the lifetime of the GaN-based LDs depends on the dislocation density. The degradation rate was almost proportional to the square root of the aging time. Our results indicate that degradation is governed by a diffusion process, and a detailed degradation mechanism is proposed.

Slip systems and misfit dislocations in InGaN epilayers

We have studied the microstructure of InGaN layers grown on two different GaN substrates: a standard GaN film on sapphire and an epitaxial lateral overgrown GaN (ELOG) structure. These two materials exhibit two distinct mechanisms of strain relaxation. InGaN epilayers on GaN are typically pseudomorphic and undergo elastic relaxation by the opening of threading dislocations into pyramidal pits. A different behavior occurs in the case of epitaxy on ELOG where, in the absence of threading dislocations, slip occurs with the formation of periodic arrays of misfit dislocations. Potential slip systems responsible for this behavior have been analyzed using the Matthews-Blakeslee model and taking into account the Peierls forces. This letter presents a comprehensive analysis of slip systems in the wurtzite structure and considers the role of threading dislocations in strain relaxation in InGaN alloys.

Structure analysis of ELO‐GaN using a 2 × 4 μm2 micro‐beam X‐ray of an 8‐GeV storage ring

AbstractWe investigated the micrometer‐scale structure of epitaxially laterally overgrown GaN (ELO‐GaN) using a 2 × 4 μm2 micro‐beam X‐ray of an 8‐GeV storage ring. The GaN (0 0 0 12) rocking curve of the wing region had a sharp peak with a FWHM of 46 arcsec, but that of the seed region had several broad peaks. This peak‐narrowing corresponds to the reduction of the threading dislocation (TD) density and to the increase of grain size. The grain in the wing region was typically 5 μm × 5 μm and extended along the ELO stripe of GaN 〈100〉. On the other hand, a larger grain with a 10 μm × 10 μm was observed in the wing region of another ELO‐GaN sample which did not coalesce during the growth. We conclude that the homogeneity of the GaN c‐surface tilting can be declined by coalescence of the wing regions. We believe that this inhomogeneity of the GaN c‐surface tilting and the grain size strongly affect the performance of GaN‐based laser diodes grown on ELO‐GaN. Moreover, GaN c‐surface tilting of 81.5 arcsec was observed in the wing region of ELO‐GaN although no SiO2 mask was used. (© 2003 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Tilted-wing-induced stress distribution in epitaxial lateral overgrown GaN

Epitaxial lateral overgrowth (ELO) is one of the most extensively studied techniques used to improve the mismatched heteroepitaxy of GaN on sapphire (α-Al2O3) substrates. In this method, a mask is first deposited over the GaN seed layer and parallel windows are opened along a specific direction in the mask. GaN is grown vertically at the window position and then grows laterally along the mask surface (wing region). The adjacent GaN regions will coalesce to form a continuous film if enough growth time is used. The impact of the ELO of GaN on sapphire substrates using a SiO2 mask is measured with the white beam synchrotron X-ray topography technique. The topography results show that the crystal planes in the laterally overgrown regions (wings) are tilted. The maximum wing tilt is about 0.36° at a fill factor of 0.5 (fill factor measures the window width relative to the window width plus SiO2 mask width). High-resolution micro-Raman spectroscopy stress-mapping measurements, using Ar+ 488-nm laser excitation, indicate that the GaN epilayer is under compressive stress, as is expected from the growth conditions. The measured average compressive stress is about 460 MPa. Furthermore, a wave-like stress field is observed in the ELO GaN epilayer. The wave valley (low compressive stress region) is usually located at the coalesced region between two adjacent wings. In general, these coalesced regions exhibit about 60 MPa lower compressive stress than the average stress in the ELO epilayer. Voids formed by the tilted wings at the coalesced region are regarded as a possible reason for the lower compressive stress in the coalesced region.

Determination of the Structural and Luminescence Properties of Nitrides Using Electron Backscattered Diffraction and Photo‐ and Cathodoluminescence

In this paper we describe the use of electron backscattered diffraction (EBSD) for the characterisation of nitride thin films, and report its use in the study of the spatial variation of strain across an epitaxially laterally overgrown GaN (ELOG) thin film. We also discuss the combination of luminescence and EBSD measurements to enable luminescence properties of samples to be directly correlated with their crystallographic properties. We compare photoluminescence spectra and EBSD measurements from a set of GaN thin films grown on off-axis sapphire substrates, revealing the tilt of a GaN thin film grown on a 10° off-axis sapphire substrate to be responsible for the observation of luminescence defect bands in this film. We finally report on the use of EBSD to identify zinc blende regions in a predominantly wurtzite MBE film, with cathodoluminescence used to obtain correlated luminescence spectra.

Correlation of Surface Potential, Free Carrier Concentration and Light Emission in ELO GaN Growth Domains

We have carried out spatially resolved micro-Raman spectroscopy, cathodoluminescence microscopy and scanning capacitance microscopy in order to obtain a comprehensive understanding about the properties of different domains formed in epitaxial laterally-overgrown GaN. For this purpose a spherical pit was fabricated into the sample by mechanical grinding and polishing, penetrating through to the buffer layer at its center. We found areas showing sharp excitonic luminescence corresponding to local free-carrier concentrations n below 10 17 cm -3 as well as domains exhibiting broad luminescence originating from recombination of a doping plasma with n reaching 10 19 cm -3 . Simultaneously, we observed in the scanning microscopy investigations a substructure which could be explained by the existence of internal space charge regions.

Investigation of dislocations and defects in epitaxial lateral overgrown GaN by photoelectrochemical wet etching

We introduce a method based on photoelectrochemical (PEC) wet etching that can conveniently investigate the defects and dislocations in laterally overgrown GaN layers by scanning electron microscopy (SEM). The image of SEM illustrates that the dislocation density in homoepitaxial growth region is much higher than in the laterally overgrown region. Also from the SEM image, we can directly identify the region with high density of defects from the perfect region. It is confirmed that the quality of GaN is greatly improved by the epitaxial lateral overgrowth (ELO) technique. The concentration of KOH solution has an effect on etching; when KOH is below 0.02 M, the photo-current increases rapidly according to KOH. After that point, it tents to remain stable at the maximum value of 55 μA. In addition, we find that the photo-current gradually decreases if the etch time is extended. It is probably because more dislocations and defects are revealed.

Influences of mask width, fill factor, HCl addition and C doping on wing tilts in the epitaxial laterally overgrown GaN films by hydride vapor phase epitaxy

Crystal tilts in the epitaxial lateral overgrown (ELO) GaN region over SiO2 mask by hydride vapor phase epitaxial away from the opening region were investigated by scanning electron microscope and the four-circle x-ray diffraction method. The increased mask width and ratio of stripe opening width to stripe period (fill factor) lead to increased wing tilts. The introduction of additional HCl and C3H8/H2 can decrease wing tilts and improve the ELO GaN surface morphologies significantly. Moreover, it has been observed that a high growth rate will bring more defects and large wing tilts into the GaN materials.