Epitaxial Lateral Overgrown Research Articles

Microstructures of two-step facet-controlled ELO-GaN grown by MOVPE method—effect of mask geometry

Microstructures in epitaxial, lateral overgrown (ELO)-GaN were analyzed by cross-sectional transmission electron microscopy with recent to the behavior of threading dislocations and generation of horizontal dislocations (HDs). GaN was grown by two-step ELO technique controlling the facet planes. The side facets of the ELO-wing were controlled to have vertical { 2 1 1 ̄ 0 } planes or slanting { 2 1 1 ̄ 2 } planes. It was clarified that the mask size had an effect on microstructures in ELO-GaN. In the case of narrow mask (window/terrace) of (3/3) μm, the c-axis of GaN was not tilted, and the area with low dislocation density was over the mask-terrace. In the case of the wide mask of (3/7) μm, HDs are generated due to the internal stress accumulated during the lateral growth and the location of HDs depended upon the direction of side facets.

Finite element analysis of epitaxial lateral overgrown GaN: Voids at the coalescence boundary

We report on the finite element analysis of stress distribution at the coalescence boundary in epitaxial lateral overgrown (ELO) GaN related to voids. Different void geometries were considered in our model to investigate the influence of their size/shape on the stress distribution. Large compressive stress is localized in the vicinity of the voids, also an increased tensile stress is present at the corners of the SiN mask. Confocal micro-Raman mapping experiments confirm the presence of increased stress at the coalescence boundary of ELO GaN.

Impurity incorporation in epitaxially laterally overgrown GaN detected by cryogenic photoluminescence microscope with sub-micron spatial resolution

Impurity incorporation from a SiO 2 mask in epitaxially laterally overgrown (ELO) GaN was depicted using a newly developed cryogenic scanning photoluminescence (PL) microscope. Both plan-view and cross-sectional PL images with a spatial resolution of 0.3 μm were obtained for emissions ascribed to free excitons (E XA) and excitons bound to donors (D 0X) at 15 K. In the cross-sectional μ-PL images, a bright emission of D 0X was observed in the area adjacent to the SiO 2 mask within a distance of approximately 8 μm from the mask, indicating an enhanced incorporation of donors such as Si atoms during the lateral growth. In the area beyond 8 μm from the mask, the D 0X emission was suppressed, showing a decrease in the donor incorporation after coalescence of ELO GaN on the SiO 2 mask.

Stress at the Coalescence Boundary of Epitaxial Lateral Overgrown GaN

We have investigated stress fields in epitaxial lateral overgrown (ELO) GaN fabricated by metalorganic vapor phase epitaxy. An increased compressive stress was found at the coalescence boundary of two adjacent wings of ELO GaN using confocal micro-Raman spectroscopy. Voids present at the coalescence boundary were identified as major source for this stress concentration. Stress concentration in the vicinity of voids was illustrated using finite element analysis. Differences in stress and crystalline quality between wing and window areas of ELO GaN were also investigated.

Dislocations in GaN-Based Laser Diodes on Epitaxial Lateral Overgrown GaN Layers

We have investigated dislocations in GaN-based laser diodes (LDs) on epitaxial lateral overgrown (ELO) GaN layers using transmission electron microscopy and cathodoluminescence microscopy and found a correlation between dislocations and device reliability. Dislocation density in the seed regions of ELO GaN layers is of the order of 108 cm—2, while that in the wing regions is less than mid-106 cm—2. The origin of dislocations in the wing regions is the extension of defects in highly defective regions near the GaN layer/substrate interface in the seed regions. The lifetime of LDs has a strong correlation with consumption power. However, some LDs have a shorter lifetime although their consumption power is almost the same. In the LDs with short lifetimes, dislocations lying in the c-plane were formed below the active regions, bent towards the c-axis and threaded upwards to active regions. These newly created dislocations can become detrimental to the device lifetime.

Dislocations in GaN-Based Laser Diodes on Epitaxial Lateral Overgrown GaN Layers

We have investigated dislocations in GaN-based laser diodes (LDs) on epitaxial lateral overgrown (ELO) GaN layers using transmission electron microscopy and cathodoluminescence microscopy and found a correlation between dislocations and device reliability. Dislocation density in the seed regions of ELO GaN layers is of the order of 10 8 cm -2 , while that in the wing regions is less than mid-10 6 cm -2 . The origin of dislocations in the wing regions is the extension of defects in highly defective regions near the GaN layer/substrate interface in the seed regions. The lifetime of LDs has a strong correlation with consumption power. However, some LDs have a shorter lifetime although their consumption power is almost the same. In the LDs with short lifetimes, dislocations lying in the c-plane were formed below the active regions, bent towards the c-axis and threaded upwards to active regions. These newly created dislocations can become detrimental to the device lifetime.

Raman mapping of epitaxial lateral overgrown GaN: Stress at the coalescence boundary

Using micro-Raman scattering spectroscopy we have investigated stress fields in epitaxial lateral overgrown (ELO) GaN fabricated by metalorganic vapor phase epitaxy using a two-step growth method. The presence of an increased compressive stress at the coalescence boundary of two adjacent wings of ELO GaN was identified. From changes in the E2 (high) phonon frequency we estimate the magnitude of the stress concentration at the coalescence boundary to be on the order of ≈0.07 GPa with respect to the ELO GaN wing. Mechanisms for the stress concentration at the coalescence boundary were studied. Differences in stress and crystalline quality between wing and window regions of ELO GaN were also investigated.

Reduction in crystallographic tilting of lateral epitaxial overgrown GaN by removal of oxide mask

The lateral overgrowth of GaN was carried out by low-pressure metalorganic chemical vapor deposition. SiO2 mask was removed just before coalescence and a subsequent lateral overgrowth was carried out to complete the fabrication of a SiO2-removed lateral epitaxial overgrown (LEO) GaN layer. The crystallographic tilting of (0002) plane, that was apparent in our standard LEO GaN layers, was absent in SiO2-removed LEO layer and x-ray diffraction measurement indicated a superior crystallinity for the SiO2-removed LEO layer. These results are attributed to the elimination of the interface between oxide mask and laterally grown GaN layer. The reduced crystallographic tilting in SiO2-removed LEO GaN layer also enhanced the quality of the coalesced fronts, as determined from cathodoluminescence images.

Channeling contrast microscopy on lateral epitaxial overgrown GaN

High resolution (<1 μm) channeling contrast microscopy (CCM) is employed to map variations of crystallographic orientation across micron-sized regions of lateral epitaxial overgrown (LEO) GaN thin film structures. The sample consists of 6 μm thick GaN stripes, aligned along the [1 1 ̄ 0 0] GaN direction, grown by LEO from 3 μm wide Si 3N 4 windows spaced 13 μm apart. Axial CCM using 2 MeV He + is employed to investigate the crystalline structure of the LEO GaN layer. A low χ min of ∼2.8% and a critical angle ψ 1/2 of ∼0.85° is found, comparable with data obtained from broad beam channeling [J. Portmann, C. Huag, R. Benn, T. Frey, B. Schottker, D.J. As, Nucl. Instr. and Meth. B 155 (1999) 489]. 5 μm wide bands of high and low yield at a periodicity of 13 μm are observed in the CCM maps. This contrast is due to the opposing tilts of the [0 0 0 1] axis in the overgrown regions (the so-called wings) on either side of the window regions. From angular scan curves this tilt is found to be ±0.45° in the direction perpendicular to the GaN stripes and no measurable wing tilt is found along the stripe direction.

Microstructure of laterally overgrown GaN layers

Transmission electron microscopy study of plan-view and cross-section samples of epitaxial laterally overgrown (ELOG) GaN samples is described. Two types of dislocation with the same type of Burgers vector but different line direction have been observed. It is shown that threading edge dislocations bend to form dislocation segments in the c plane as a result of shear stresses developed in the wing material along the stripe direction. It is shown that migration of these dislocations involves both glide and climb. Propagation of threading parts over the wing area is an indication of high density of point defects present in the wing areas on the ELOG samples. This finding might shed light on the optical properties of such samples.

Crystal Tilts in Epitaxially Laterally Overgrown GaN FilmsDetermined by Four-Circle X-Ray Diffraction

Crystal tilts in epitaxially laterally overgrown (ELO) GaN films via hydridevapour phase epitaxy (HVPE) on sapphire substrates have been investigated byusing the four-circle x-ray diffraction method. Three diffraction peakscorresponding to the (0002) reflection of vertically epitaxial andtilted GaN domains are observable in the x-ray rocking curve. The angleseparations Δω between the main peak and two lobes changewith the azimuth angle ϕ. The dependence of Δω onϕ and the crystal tilt angle θ has beencalculated based on the standard kinetic x-ray diffraction model. Thecrystal tilt angle of a typical HVPE ELO GaN sample has been determined tobe 2.379°.

The coalescence of ELO layers ofInxGa1-xAs grown on patterned (111) GaAs byliquid phase epitaxy

InxGa1-xAs (x = 0.06) epitaxial lateralovergrown (ELO) layers were grown on (111)B GaAs patternedsubstrates covered with SiNx masks by liquid phase epitaxy.The thickness of the ELO layer with a { 111} A growth front was foundto decrease as the growth progressed. On the other hand, for theELO layer with a { 111} B growth front, this kind of decrease ofthickness was found to be very small. When the layers withdifferent growth fronts ({ 111} A and { 111} B) coalesced,dislocations were found to be generated. In the case of thecoalesced ELO layers which were in constant contact with thebasal SiNx mask, the surface became concave due to the decreaseof thickness of the layers. This problem of decrease ofthickness was not observed when a large enough void structure waspresent in the grown ELO layers of InGaAs.

AlGaInN high-power lasers grown on an ELO-GaN layer

Epitaxially laterally overgrown (ELO)-GaN substrates with dislocation-free regions can be prepared with a relatively small thickness using atmospheric pressure metal-organic chemical vapor deposition (MOCVD). In order to determine the effect of ELO-GaN layer on the reliability of blue–violet laser diodes (BV-LDs), we fabricated BV-LDs on the lateral-growth region of the ELO-GaN layer. The thickness of the ELO-GaN layer was maintained at 5 μm to inhibit wafer bending, so the total thickness of the BV-LDs was approximately 7 μm. The lifetime of the BV-LDs with a constant output power of 20 mW under CW operation at 25°C was more than 500 h. This lifetime is several times that of our conventional BV-LDs on sapphire substrates. The ELO-GaN layer was very effective in raising the lifetime of BV-LDs.

The structure and optoelectronic properties of dislocations inGaN

This paper reviews the structure of threading dislocations,nanopipes and inversion domains in (0001) GaN films grown by metal-organicchemical vapour deposition. A new mechanism for generation of misfitdislocations in epitaxial laterally overgrown (ELOG) GaN and in AlGaN/GaNinterfaces is described. Recent cathodoluminescence studies in the scanningelectron microscope are described, showing that individual threading defectsact as non-radiative recombination centres in ELOG GaN and in InGaN quantumwells in GaN.

High spatial resolution thermal conductivity and Raman spectroscopy investigation of hydride vapor phase epitaxy grown n-GaN/sapphire (0001): Doping dependence

We have measured high resolution thermal conductivity (κ) and Raman spectra {E2 mode [high frequency], A1 mode [longitudinal optical (LO)], and high frequency LO-plasmon coupled mode [LPP+]} at 300 K of three series of n-GaN/sapphire (0001) samples fabricated by hydride vapor phase epitaxy (HVPE). The former was determined with a scanning thermal microscope while the latter was obtained using a micro-Raman system, both having a spatial resolution of ≈2–3 μm. For all three sets of samples the thermal conductivity decreased linearly with log n, about a factor of two decrease in κ for every decade increase in n. Also, we found a correlation between film thickness and improved thermal conductivity. Furthermore, κ≈1.95 W/cm K for one of the most lightly doped samples (≈6.9×1016 cm−3), higher than previously reported κ≈1.7–1.8 W/cm K on lateral epitaxial overgrown (LEO) material with n≈(1–2)×1017 cm−3 [V. M. Asnin et al., Appl. Phys. Lett. 75, 1240 (1999)], κ=1.55 W/cm K on LEO samples using a third-harmonic technique [C. Y. Luo et al., Appl. Phys. Lett. 75, 4151 (1999)], and κ≈1.3 W/cm K on a HVPE sample [E. K. Sichel and J. I. Pankove, J. Phys. Chem. Solids 38, 330 (1977)]. The carrier concentration dependence of κ is similar to that of other semiconductors in a comparable temperature range. On a log–log scale the linewidth of the observed E2 Raman mode remained constant up to n≈1×1018 cm−3 and then increased linearly. The carrier concentration obtained from the LPP+ mode is less than the Hall effect determination. This is probably due to the fact that the latter measures n in both the epilayer and GaN/sapphire interfacial region [D. C. Look and R. J. Molnar, Appl. Phys. Lett. 70, 3377 (1997); W. Götz et al., Appl. Phys. Lett. 72, 1214 (1998)] while the Raman signal is primarily from the epilayer.

Reduction mechanisms for defect densities in GaN using one- or two-step epitaxial lateral overgrowth methods

A transmission electron microscopy study of the reduction mechanisms for defect densities in epitaxial lateral overgrown (ELO) GaN films is presented. In the standard one step ELO, the propagation of defects under the mask is blocked, whereas the defects in the window regions thread up to the surface. We propose an alternative two step ELO method. In a first step, dislocations close to the edge of the (0001) top facet bend at 90°, thereby producing a drastic reduction in the density of defects above the window. After the coalescence, induced by lateral growth in a second step, dislocations are mainly observed in the coalescence boundaries. The density of defects is decreased to 2×10−7 cm−2 over the entire surface and areas nearly 5 μm wide with 5×106 cm−2 dislocations between the center of the windows and the coalescence boundaries are obtained.

Role of Dislocations in InGaN-Based LEDs and Laser Diodes

UV InGaN and GaN single-quantum-well structure light-emitting diodes (LEDs) were grown on epitaxially laterally overgrown (ELOG) and sapphire substrates. When the emission wavelength of UV InGaN LEDs was shorter than 380 nm, the external quantum efficiency (EQE) of the LED on ELOG was much higher than that on sapphire only at high-current operation. At low-current operation, both LEDs had the same EQE. When the active layer was GaN, EQE of the LED on sapphire was much lower than that on ELOG at both low- and high-current operation due to the lack of localized energy states formed by In composition fluctuations. In order to improve the lifetime of laser diodes (LDs), ELOG had to be used because the operating current density of the LDs is much higher than that of LEDs. A violet InGaN multi-quantum-well GaN/AlGaN separate-confinement-heterostructure LD was grown on ELOG on sapphire. The LDs with cleaved mirror facets shows an output power as high as 40 mW under room-temperature continuous-wave (CW) operation. The stable fundamental transverse mode was observed at an output power of up to 40 mW. The estimated lifetime of the LDs at a constant output power of 10 mW was more than 2,000 hours under CW operation at an ambient temperature of 60°C.

ROLE OF DISLOCATIONS IN InGaN-BASED LEDs AND LASER DIODES

UV InGaN and GaN single-quantum-well structure light-emitting diodes (LEDs) were grown on epitaxially laterally overgrown (ELOG) and sapphire substrates. When the emission wavelength of UV InGaN LEDs was shorter than 380 nm, the external quantum efficiency (EQE) of the LED on ELOG was much higher than that on sapphire only at high-current operation. At low-current operation, both LEDs had the same EQE. When the active layer was GaN, EQE of the LED on sapphire was much lower than that on ELOG at both low- and high-current operation due to the lack of localized energy states formed by In composition fluctuations. In order to improve the lifetime of laser diodes (LDs), ELOG had to be used because the operating current density of the LDs is much higher than that of LEDs. A violet InGaN multi-quantum-well GaN/AlGaN separate-confinement-heterostructure LD was grown on ELOG on sapphire. The LDs with cleaved mirror facets shows an output power as high as 40 mW under room-temperature continuous-wave (CW) operation. The stable fundamental transverse mode was observed at an output power of up to 40 mW. The estimated lifetime of the LDs at a constant output power of 10 mW was more than 2,000 hours under CW operation at an ambient temperature of 60°C.

Improved characteristics of InGaN multiple-quantum-well laser diodes grown on laterally epitaxially overgrown GaN on sapphire

InGaN multiple-quantum-well laser diodes have been fabricated on fully coalesced laterally epitaxially overgrown (LEO) GaN on sapphire. The laterally overgrown “wing” regions as well as the coalescence fronts contained few or no threading dislocations. Laser diodes fabricated on these low-dislocation-density regions showed a reduction in threshold current density from 10 to 4.8 kA/cm2 compared to those on conventional planar GaN on sapphire. The internal quantum efficiency also improved from 3% for laser diodes on conventional GaN on sapphire to 22% for laser diodes on LEO GaN on sapphire.

Structural Properties of Laterally Overgrown GaN

Structural properties of epitaxially laterally overgrown (ELO) GaN on patterned GaN ‘substrates’ by hydride vapor phase epitaxy (HVPE) have been investigated. The epitaxially lateral overgrowth of GaN on SiO2 areas is realized and a planar ELO GaN film is obtained. Scanning electron microscope, transmission electron microscope (TEM) and atomic force microscope (AFM) are used to study the structure and surface morphology of the ELO GaN materials. AFM images indicate that no observable step termination is detected over a 4 μm2 area in the ELO region. TEM observations indicate that the dislocation density is very low in the ELO region. No void at the coalescence interface is observed. Lattice bending as high as 3.3° is observed and attributed to pileup of threading dislocations coming from the underlying GaN “seeding layer” and tilting horizontally and quenching at the coalescence interface.