Epitaxial Lateral Overgrown Research Articles

Radiative and Nonradiative Recombination Processes in AlGaN Quantum Wells on Epitaxially Laterally Overgrown AlN/Sapphire from 10 to 500 K

Radiative and nonradiative recombination processes are investigated in the temperature range from 10 to 500 K for AlGaN quantum wells on epitaxially laterally overgrown AlN/sapphire templates. Time‐integrated photoluminescence (PL) spectroscopy under selective excitation conditions demonstrates that the decrease in the radiative recombination efficiency with increasing temperature is one of the causes of the thermal droop in AlGaN‐based deep‐ultraviolet (DUV) light‐emitting diodes. Time‐resolved PL spectroscopy indicates that not only the decreasing nonradiative recombination lifetime but increasing radiative recombination lifetime with increasing temperature contributes to the thermal droop. The temperature dependence of the radiative recombination lifetime is discussed, revealing that luminescence linewidth is a valuable criterion for designing efficient AlGaN‐based DUV emitters.

Radiative Recombination and Carrier Injection Efficiencies in 265 nm Deep Ultraviolet Light‐Emitting Diodes Grown on AlN/Sapphire Templates with Different Defect Densities

The electro‐optical characteristics of deep ultraviolet light‐emitting diodes (DUV LEDs) emitting at 265 nm and grown on AlN/sapphire templates with different threading dislocation densities, i.e., high‐temperature annealed (HTA) AlN, epitaxially laterally overgrown (ELO) AlN, and HTA‐ELO AlN are analyzed. The external quantum efficiency of each individual device is separated into maximum radiative recombination efficiency, carrier injection efficiency, and light extraction efficiency. This is achieved by combining an ABC‐model‐based fit of the current‐dependent external quantum efficiency together with calibrated Monte Carlo ray‐tracing simulations. A maximum radiative recombination efficiency between 50% and 60% is estimated for DUV LEDs grown on ELO and HTA‐ELO AlN/sapphire, whereas the values for devices grown on HTA AlN/sapphire are around 45%. The extracted radiative recombination efficiency does not scale with the measured threading dislocation density (TDD), even when accounting for the inhomogeneous TDD in the AlN base layers. This discrepancy is attributed to the formation of dislocation half‐loops introduced by additional compressive strain caused by the HTA process and may result in the formation of additional nonradiative recombination centers in the AlGaN multi‐quantum well region. In addition, the carrier injection efficiency values ranging from 45% to 55% are determined for devices grown on all three templates.

Semipolar {202¯1} GaN Edge-Emitting Laser Diode on Epitaxial Lateral Overgrown Wing

Edge-emitting laser diodes (LDs) were fabricated on a reduced dislocation density epitaxial lateral overgrown (ELO) wing of a semipolar {202¯1} GaN substrate, termed an ELO wing LD. Two types of facet feasibility studies were conducted: (1) “handmade” facets, wherein lifted-off ELO wing LDs were cleaved manually, and (2) facets formed on wafers through reactive ion etching (RIE). Pulsed operation electrical and optical measurements confirmed the laser action in the RIE facet LDs with a threshold current of ~19 kAcm−2 and maximum light output power of 20 mW from a single uncoated facet. Handmade facet devices showed spontaneous, LED-like emission, confirming device layers remain intact after mechanical liftoff.

Optical and electrical characterizations of micro-LEDs grown on lower defect density epitaxial layers

We have fabricated μLEDs of mesa sizes 10 × 10 and 15 × 15 μm2 on native (2021¯) semipolar substrates and on epitaxial lateral overgrown (ELO) wings of the (2021¯) substrate. The ELO μLEDs exhibited very low leakage current (less than 10−10 A) under forward bias (V < 2 V) and at reverse bias voltages, which was a reduction in several orders of magnitude when compared with planar μLEDs under the same fabrication and sidewall passivation scheme. Electrical characterization revealed that the mesa sidewall is less damaged in plasma dry etching in the ELO μLEDs due to a lower material defect density than the planar μLEDs. Moreover, the ELO μLEDs showed improved optical performance over the planar μLEDs.

Reliability of UVC LEDs fabricated on AlN/sapphire templates with different threading dislocation densities

The impact of different AlN/sapphire template technologies [i.e., planar, epitaxial lateral overgrown (ELO), and high temperature annealed sputtered ELO] is studied with respect to the operation-induced degradation of 265 nm UVC LEDs. UVC LEDs with identical heterostructures were grown on templates providing different threading dislocation densities in the range of 0.8 × 109 cm−2 to 5.8 × 109 cm−2. A long-term stress experiment was performed on batches of LEDs, which were operated at a direct current of 200 mA corresponding to a current density of 60 A/cm2 and at a heat sink temperature of 20 °C. The UVC LEDs on templates with lower threading dislocation densities were found to provide a higher optical power and to degrade slower during 2000 h of operation. The experiment demonstrates an extrapolated L70 lifetime of more than 10 000 h for the high temperature annealed sputtered ELO technology. The results suggest that degradation is caused by operation-induced activation of defects whose density scales with the dislocation density.

Improved performance of UVC-LEDs by combination of high-temperature annealing and epitaxially laterally overgrown AlN/sapphire

We report on the performance of AlGaN-based deep ultraviolet light-emitting diodes (UV-LEDs) emitting at 265 nm grown on stripe-patterned high-temperature annealed (HTA) epitaxially laterally overgrown (ELO) aluminium nitride (AlN)/sapphire templates. For this purpose, the structural and electro-optical properties of ultraviolet-c light-emitting diodes (UVC-LEDs) on as-grown and on HTA planar AlN/sapphire as well as ELO AlN/sapphire with and without HTA are investigated and compared. Cathodoluminescence measurements reveal dark spot densities of 3.5 × 10 9 cm − 2 , 1.1 × 10 9 cm − 2 , 1.4 × 10 9 cm − 2 , and 0.9 × 10 9 cm − 2 in multiple quantum well samples on as-grown planar AlN/sapphire, HTA planar AlN/sapphire, ELO AlN/sapphire, and HTA ELO AlN/sapphire, respectively, and are consistent with the threading dislocation densities determined by transmission electron microscopy (TEM) and high-resolution X-ray diffraction rocking curve. The UVC-LED performance improves with the reduction of the threading dislocation densities (TDDs). The output powers (measured on-wafer in cw operation at 20 mA) of the UV-LEDs emitting at 265 nm were 0.03 mW (planar AlN/sapphire), 0.8 mW (planar HTA AlN/sapphire), 0.9 mW (ELO AlN/sapphire), and 1.1 mW (HTA ELO AlN/sapphire), respectively. Furthermore, Monte Carlo ray-tracing simulations showed a 15% increase in light-extraction efficiency due to the voids formed in the ELO process. These results demonstrate that HTA ELO AlN/sapphire templates provide a viable approach to increase the efficiency of UV-LEDs, improving both the internal quantum efficiency and the light-extraction efficiency.

Determination of Sapphire Off‐Cut and Its Influence on the Morphology and Local Defect Distribution in Epitaxially Laterally Overgrown AlN for Optically Pumped UVC Lasers

Determination of Sapphire Off‐Cut and Its Influence on the Morphology and Local Defect Distribution in Epitaxially Laterally Overgrown AlN for Optically Pumped UVC Lasers

Determination of Sapphire Off‐Cut and Its Influence on the Morphology and Local Defect Distribution in Epitaxially Laterally Overgrown AlN for Optically Pumped UVC Lasers

Herein, a systematic study of the morphology and local defect distribution in epitaxially laterally overgrown (ELO) AlN on c‐plane sapphire substrates with different off‐cut angles ranging from 0.08° to 0.23° is presented. Precise measurements of the off‐cut angle α, using a combination of optical alignment and X‐ray diffraction with an accuracy of ±5° for the off‐cut direction and ±0.015° for the off‐cut angle, are carried out. For ELO AlN growth, a transition from step flow growth at α < 0.14° with height undulations on the surface to step bunching with step heights up to 20 nm for α > 0.14° is observed. Furthermore, the terraces of the step‐bunched surface exhibit curved steps. An analysis of the local defect distribution by scanning transmission electron microscopy and a comparison with atomic force microscopy reveal a bunching of defects in line with the ELO pattern and a roughening of step edges in highly defective regions. In addition, a reduction in the threshold excitation power density for optically pumped ultraviolet‐C (UVC) lasers with smooth surface morphologies is observed.

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.

Influence of template properties and quantum well number on stimulated emission from Al0.7Ga0.3N/Al0.8Ga0.2N quantum wells

AlGaN multiple quantum well laser heterostructures for emission around 240 nm have been grown by metalorganic vapor phase epitaxy on epitaxially laterally overgrown (ELO) AlN/sapphire templates. The edge emitting laser structures showed optically pumped lasing with threshold power densities in the range of 2 MW cm−2. The offcut angle of the sapphire substrates as well as the number and the width of the quantum wells were varied while keeping the total thickness of the gain region constant. A larger offcut angle of 0.2° leads to step bunching on the surface as well as Ga accumulation at the steps, but also to an increased inclination of threading dislocations and coalescence boundaries resulting in a reduced dislocation density and thus a reduced laser threshold in comparison to lasers grown on ELO with an offcut of 0.1°. For low losses, samples with fewer QWs exhibited a lower lasing threshold due to a reduced transparency pump power density while for high losses, caused by a higher threading dislocation density, the quadruple quantum well was favorable due to its higher maximum gain.

AlGaN-based deep UV LEDs grown on sputtered and high temperature annealed AlN/sapphire

The performance characteristics of AlGaN-based deep ultraviolet light emitting diodes (UV-LEDs) grown by metalorganic vapor phase epitaxy on sputtered and high temperature annealed AlN/sapphire templates are investigated and compared with LEDs grown on epitaxially laterally overgrown (ELO) AlN/sapphire. The structural and electro-optical properties of the devices on 350 nm sputtered and high temperature annealed AlN/sapphire show similar defect densities and output power levels as LEDs grown on low defect density ELO AlN/sapphire templates. After high temperature annealing of the 350 nm sputtered AlN, the full widths at half maximum of the (0002) and (101¯2) reflections of the high resolution x-ray diffraction rocking curves decrease by one order of magnitude to 65 arc sec and 240 arc sec, respectively. The curvature of the sputtered and HTA AlN/sapphire templates after regrowth with 400 nm MOVPE AlN is with −80 km−1 much lower than the curvature of the ELO AlN/sapphire template of −160 km−1. The on-wafer measured output powers of 268 nm LEDs grown on 350 nm sputtered and high temperature annealed AlN/sapphire templates and ELO AlN/sapphire templates were 0.70 mW and 0.72 mW at 20 mA, respectively (corresponding to an external quantum efficiency of 0.75% and 0.78%). These results show that sputtered and high temperature annealed AlN/sapphire provide a viable approach for the fabrication of efficient UVC-LEDs with reduced complexity and thus reduced costs.

Comparative study of epitaxial lateral overgrowth on semipolar (11-22) GaN by using stripe and hexagon SiO2 mask patterns

We investigated defect reduction of epitaxial lateral overgrown (ELO) semipolar (11-22) GaN grown on m-plane sapphire substrate by using stripe and hexagon SiO2 patterns. We obtained the fully-coalescent semipolar ELO-GaN templates successfully regardless of pattern shape and size. However, ELO-GaN film grown on hexagonal patterns were more easily coalesced than ELO-GaN grown on stripe patterns during ELO process due to the lateral growths from six directions. Xray rocking curves from ELO-GaN films indicate that ELO-GaN had much better crystal quality than GaN template. However, the effect of the lateral overgrowth on hexagon patterns on crystal quality was limited because the large frictions from the six-directional growth on hexagon pattern resulted in crystallographic tilts. The photoluminescence emissions of donor-bound exciton (D0X) recombination were observed from both samples, but the D0X emission of stripe ELO-GaN film was considerably stronger than that of hexagon ELO-GaN. In addition, the defect-related emissions of stripe ELO-GaN were also weaker, indicating that stripe ELO-GaN has efficiently eliminated the non-radiative centers and suppressed the defects.

Spatial inhomogeneities in AlxGa1−xN quantum wells induced by the surface morphology of AlN/sapphire templates

The effects of the template on the optical and structural properties of Al0.75Ga0.25N/Al0.8Ga0.2N multiple quantum well (MQWs) laser active regions have been investigated. The laser structures for optical pumping were grown on planar c-plane AlN/sapphire as well as on thick epitaxially laterally overgrown (ELO) AlN layers on patterned AlN/sapphire. Two ELO AlN/sapphire templates were investigated, one with a miscut of the sapphire surface to the m-direction with an angle of 0.25°, the other with a miscut angle of 0.25° to the sapphire a-direction. The MQWs are studied by atomic force microscopy, plan-view cathodoluminescence (CL) at room temperature and 83 K as well as transmission electron microscopy using high-angle annular dark-field imaging and energy-dispersive x-ray spectroscopy. The results are compared to optical pumping measurements. It was found that the surface morphology of the templates determines the lateral wavelength distribution in the MQWs observed by spectral CL mappings. The lateral wavelength spread is largest for the laser structures grown on ELO AlN with miscut to sapphire a-direction caused by the local variation of the MQW thicknesses and the Ga incorporation at macrosteps on the ELO-AlN. A CL peak wavelength spread of up to 7 nm has been found. The MQWs grown on planar AlN/sapphire templates show a homogeneous wavelength distribution. However, due to the high threading dislocation density and the resulting strong nonradiative recombination, laser operation could not be achieved. The laser structures grown on ELO AlN/sapphire show optically pumped lasing with a record short wavelength of 237 nm.

Enhanced quantum efficiency of AlGaN photodetectors by patterned growth

Al0.45Ga0.55N metal–semiconductor–metal photodetectors were grown by MOVPE on planar and on stripe patterned epitaxial laterally overgrown (ELO) AlN/sapphire templates. By comparing devices on different template types, the influence of the average dislocation density, the distribution of dislocations, and composition modulations due to inhomogeneous gallium incorporation on the external quantum efficiency (EQE) were evaluated. The reduction of the average dislocation density from 6 × 109 cm−2 on the planar template to about 2 × 109 cm−2 by ELO increases the EQE. For electron transport perpendicular to the ELO stripes, this increase is about 70%. Due to the stripe‐like dislocation distribution in the absorber layers on ELO templates, the EQE becomes up to a factor of 3 higher than for planar templates, when the electrodes are perpendicular to the ELO stripes and electron transport is along the stripes. Photoconductive gain was found at elevated bias for absorber layers on standard ELO templates with electrodes perpendicular to the ELO stripes. The gain can be attributed to carrier transport in zig‐zag shaped Ga‐rich channels caused by facetted growth on ELO templates. The gain only appears, if the aluminum mole fraction differences Δx are higher than 0.07. At 30 V bias, the EQE of such detectors is more than 3 orders of magnitude higher compared to planar device.

Improvements in epitaxial lateral overgrowth of InP by MOVPE

Indium phosphide and silicon play important and complementary roles in communications wavelength photonic devices. Realizing high quality coalesced epitaxial lateral overgrown (ELO) InP films on Si could greatly reduce cost and encourage the proliferation of energy efficient photonic integrated circuits in consumer devices. By adjusting a parallel line ELO mask and metalorganic vapor phase epitaxial growth conditions, we have fully coalesced and partially coalesced epitaxial lateral overgrowth of InP on InP substrates and Si substrates having strain relaxed III/V buffer layers, respectively. Extended defects were investigated using transmission electron microscopy and were not found to originate at the coalescence of the nearest neighbor growth fronts for linear parallel growth windows oriented 60° off of [0−11] when using a high V/III ratio of 406. In addition, narrowly separated linear parallel growth windows having a large aspect ratio of 7.5 were seen to inhibit the upward propagation of stacking faults through several neighboring openings. Elimination of these two defect sources would leave primarily the challenge of optimizing the morphology of the overgrown InP as a substantial barrier to achieving coalesced ELO InP of sufficient quality for photonic device applications.

Performance Characteristics of UV-C AlGaN-Based Lasers Grown on Sapphire and Bulk AlN Substrates

The performance characteristics of optically pumped laser heterostructures emitting in the UV-C spectral range between 272 and 279 nm are investigated. The laser heterostructures were grown by metal-organic vapor phase epitaxy on (0001) planar AlN/sapphire, epitaxially laterally overgrown (ELO) AlN/sapphire, and bulk AlN substrates with threading dislocation densities ranging from 2×1010 to 104 cm-2. We found that the defect density strongly affects the laser performance. The lowest pulse threshold energy density of 50 mJ/cm2 under resonant optical pumping condition was obtained for an AlGaN multiple quantum well laser grown pseudomorphically on low defect density bulk AlN substrate. Lasing was also observed for AlGaN MQW heterostructures grown on ELO AlN/sapphire templates. The laser emission in all lasers was TE polarized. However, no lasing was observed for heterostructures grown on high defect density AlN/sapphire.

MOVPE growth of AlxGa1‐xN with x ∼ 0.5 on epitaxial laterally overgrown AlN/sapphire templates for UV‐LEDs

AbstractThe crystalline perfection of AlxGa1‐xN layers with Al content of x ∼ 0.5 on epitaxial laterally overgrown (ELO) AlN with low defect density was investigated by spatially and spectrally resolved cathodoluminescence, scanning electron and atomic force microscopy. Cathodoluminescence mapping reveals compositional variations caused by different Ga incorporation during growth over macro‐steps resulting from step bunching on the ELO‐AlN. Introducing AlN interlayers as marker the influence of the macrosteps as well as of the growth rate on the development of Ga‐rich regions was studied. With increasing growth rate from 0.2 to 1 µm/h the spatial extension of the Ga‐rich regions is strongly reduced. At the same time the difference in Ga content is increased. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Anisotropic Responsivity of AlGaN Metal–Semiconductor–Metal Photodetectors on Epitaxial Laterally Overgrown AlN/Sapphire Templates

Al0.4Ga0.6N metal–semiconductor–metal photodetectors on epitaxial laterally overgrown (ELO) AlN/sapphire templates show anisotropic device characteristics depending on the orientation of the electrode stripes with respect to the stripe pattern onto which the underlying ELO AlN buffer layers have been grown. With electrodes perpendicular to the stripes, a quantum efficiency (QE) of ∼140 was found for 20-V bias at room-temperature. This gain is explained by carrier transport along channels with increased Ga content resulting from faceted growth at the steps of the ELO template. The resulting potential barrier is confirmed by the activation energy found for the temperature dependence of the QE. In contrast, photodetectors with electrodes running parallel to these channels do not show gain but have an enhanced QE at elevated bias voltage compared to devices on planar AlN buffer layers. This effect is attributed to different densities of threading dislocations in the absorber layer.

Cathodoluminescence and TEM investigations of structural and optical properties of AlGaN on epitaxial laterally overgrown AlN/sapphire templates

Surface steps as high as 15 nm on up to 10 μm thick AlN layers grown on patterned AlN/sapphire templates play a major role for the structural and optical properties of AlxGa1−xN layers with x ≥ 0.5 grown subsequently by metalorganic vapour phase epitaxy. The higher the Ga content in these layers is, the stronger is the influence of the surface morphology on their properties. For x = 0.5 not only periodic inhomogeneities in the Al content due to growth of Ga-rich facets are observed by cathodoluminescence, but these facets give rise to additional dislocation formation as discovered by annular dark-field scanning transmission electron microscopy. For AlxGa1−xN layers with x = 0.8 the difference in Al content between facets and surrounding material is much smaller. Therefore, the threading dislocation density (TDD) is only defined by the TDD in the underlying epitaxially laterally overgrown (ELO) AlN layer. This way high quality Al0.8Ga0.2N with a thickness up to 1.5 μm and a TDD ≤ 5x108 cm−2 was obtained.

Study of crystallographic tilt and surface morphology on epitaxial lateral overgrown semipolar (11-22) GaN

The authors investigated surface morphologies and crystal properties of semipolar (11-22) epitaxial lateral overgrown (ELO) GaN films with the different SiO2 pattern thicknesses (100–240 nm) and widths (6–12 μm). As increasing SiO2 pattern width, the full width at half maximum (FWHM) of x-ray rocking curve (XRC) decreased, and then increased with the SiO2 pattern width (>10 μm). Regardless of SiO2 pattern width, XRC FWHMs of ELO-GaN films with 240 nm-thick SiO2 patterns were much higher than those with 100 nm-thick SiO2 patterns. Moreover, surface morphology of semipolar (11-22) ELO-GaN film was deteriorated with increasing SiO2 pattern width and thickness. From findings of this study, the authors suggested that surface morphology and crystal properties of semipolar (11-22) ELO-GaN film would be significantly affected by SiO2 pattern width and thickness due to the friction between GaN and SiO2 pattern.