Patterned Sapphire Substrates Light-emitting Diodes Research Articles

In-situ mapping of electroluminescent enhancement of light-emitting diodes grown on patterned sapphire substrates

The mechanism for enhancing extraction efficiency of light emitting diodes (LEDs) grown on patterned sapphire substrates (PSSs) was observed by the in-situ lateral electroluminescence (EL) mapping using optical microscopy equipped with a laser energy profiler. The observed spatial intensity distribution of epilayers, varying from epilayer to epilayer on the lateral surface of the PSS LED chip, revealed that the perimeter scattering on the convex facets of PSSs converges the propagation of emitted light with random directionality into a spot near the top surface of the buffer layer. Moreover, this in-situ sidewall mapping implied that the enhancement of light extraction of the PSS LED is due to reducing the total internal reflection effect, resulting from the spot located closer to the LED/air interface. Simulated results and EL images of convex patterns on the PSS surface were consistent with sidewall surface-based observations.

Influence of the microstructure geometry of patterned sapphire substrates on the light extraction efficiency of GaN LEDs.

The influence of the microstructure geometry of patterned sapphire substrates (PSS) on the light extraction efficiency (LEE) of GaN light-emitting diodes (LEDs) is numerically analyzed. Cone structures of various dimensions are studied, along with dome and mixed microstructures. LEE is found to mainly depend on the microstructure surface slope. LEE rises quickly with slope and flattens out when the slope exceeds 0.6. Scaling down the microstructure has little effect on LEE. Light rays are found to travel longer distances in PSS LEDs, as compared with LEDs grown on a flat substrate. Keeping GaN absorption loss low is important for LEE optimization.

Electrical Characteristics of GaN-Based Light-Emitting Diodes on Patterned Sapphire Substrates

Patterned sapphire substrate light-emitting diodes display obvious negative capacitance (NC) at large forward biases. This is measured using a method based on a small signal alternating current together with direct I–V plots. The NC in patterned sapphire substrate LEDs grows exponentially with the forward applied voltage. This observation is unexpected and in contrast with Shockley’s p–n junction theory, which only includes an increasing diffusion capacitance and not a NC. However, this result is in good agreement with conventional sapphire substrate LEDs. Furthermore, the negative terminal capacitance confirmed the prediction of Laux and Hess’ theory. The ideal factor of a patterned sapphire substrate LED is about 5, greatly exceeding the traditional theoretical value. The capacitance increased to a maximum and then gradually decreased, which was similar to the results for a p–n junction. Patterned sapphire substrate LEDs can withstand higher voltages than conventional sapphire substrate LEDs. This work could further confirm the existence of NC.

Efficiency Improvement of Blue LEDs Using a GaN Burried Air Void Photonic Crystal With High Air Filling Fraction

In this paper, we investigate the efficiency enhancement of blue InGaN/GaN light-emitting diodes (LEDs) by incorporating a burried air void photonic crystal (BAVPC) layer within the epitaxial structure. As compared with the conventional patterned sapphire substrate (C-PSS) LEDs and flat sapphire substrate LEDs with BAVPC, the fabricated patterned sapphire substrate (PSS) LEDs with BAVPC exhibit the lowest full-width at half-maximum of (002) and (102) diffraction peaks, the highest light output power of 20.6 mW, and the highest external quantum efficiency of 37.4%. Remarkable performance improvement in the PSS LED with BAVPC is attributed to the better epitaxial quality with threading dislocations terminated by the BAVPC and the higher scattering at interface between GaN and air-void. By positioning the BAVPC directly below the multiple quantum wells (MQWs), it would cause the reduction in the number of trapped optical modes. The methodology optically isolates the MQWs from the underlying substrate and increases the optical output power. Moreover, threading dislocations are significantly suppressed using the BAVPC with high air filling fraction of ~ 50%. It is well proposed that this methodology provides a promising alternative to C-PSS LEDs.

Growth and Fabrication of GaN Light Emitting Diode on Patterned-Sapphire Substrate

In this study, the growth of GaN-films-based light emitting diodes(LEDs), have been clearly demonstrated on wet-etching patterned sapphire substrate (PSS) and conventional LEDs. The wet-etching PSS LED device showed a significantly higher performance than the conventional LED device. The wet-etching PSS LED device exhibited a leakage current of 9 mA under a bias of 8 V, an external quantum efficiency and output powers of 35.09 % and19.23 mW under 20 mA injection current as compared to 12 mA, 27.23%, and 15.02 mW of the conventional LED device.

Degradation mechanism of light‐emitting diodes on patterned sapphire substrate

AbstractWe have investigated possible degradation mechanism for patterned sapphire substrate (PSS)‐based blue light‐emitting diodes (LEDs) using an acceleration burn‐in test under high current stress. Normal LEDs without a PSS had also been compared. Measurements showed that the PSS‐LED has lower series resistance and higher output power compare to those of the normal LED. The acceleration test (@ 490 mA/cm2) results showed that the normal LED suffers from current crowding irrespective of acceleration time while the PSS LED endures up to 100 s. In addition, the estimated lifetime of the PSS‐LED is 1.6 times as high as that of the normal LED at high electrical stress of 490 A/cm2, indicating that the PSS‐LED shows excellent reliability characteristics. Based on electrical/optical measurement results, acceleration test data, and Auger depth profile results, possible degradation mechanism of the PSS‐LED will be discussed in terms of junction temperature (originating from Joule heating), carrier crowding, and p‐contact failure.

Improvements in Optical Power and Emission Angle of Blue Light Emitting Diodes Using Patterned Sapphire Substrates with Low Threading Dislocation Densities

GaN blue light-emitting diodes (LEDs) with a peak emission wavelength of approximately 461 nm were fabricated on a c-face lens patterned sapphire substrate (PSS) and an unpatterened sapphire substrate (UPSS) by metal organic chemical vapor deposition (MOCVD). The crystal structure of an epitaxial GaN film was improved by using the PSS. The peak wavelengths with electroluminescence intensities of 462 and 464 nm were measured for PSS and UPSS LEDs using a 20 mA injection current. It was found that the electroluminescence intensity of the LEDs grown on the lens PSS was about 61% higher than that of the LEDs prepared on the UPSS. The emission angle of the PSS LED increased by 1.5 times compared with that of the UPSS LED. The luminous intensities of both LEDs increased similarly on both circular and square reflectors. The output power of the PSS LED was 33% greater than that of the LED UPSS. In addition, the reduction in full width at half maximum (FWHM) in the ω-scan rocking curves of GaN on the PSS suggested an improved crystal quality. These significant improvements in output power and emission angle resulted from the enhanced light extraction and the reduced threading dislocation (TD) density using the PSS method.

The Output Power Enhancements of GaN-Based Blue Light-Emitting Diodes with Highly Reflective Ag/Cr/Au Trilayer Omnidirectional Reflective Electrode Pads

In this study, we demonstrated the fabrication of GaN-based light-emitting diodes (LEDs) on a patterned sapphire substrate (PSS) or non-PSS with nonalloy Ag/Cr/Au trilayer metal electrode pads on the exposed n+-GaN and indium tin oxide layers to be the n-type electrode and p-type electrode, respectively. The forward voltages of all LED samples at 20 mA were in the range of 3.1–3.2 V and were sufficiently low compared with those of commercial GaN-based LEDs. In the present experiment, the 20 mA output power of GaN-based LEDs with Ag/Cr/Au trilayer electrode bonding pads can be increased by a magnitude of 17% compared with those of the conventional LEDs with nonalloy Cr/Au bonding pads. Moreover, the 20 mA output power of GaN-based LEDs will be increased by more than 40 and 11% by introducing PSS and Ag/Cr/Au trilayer electrode bonding pads compared with those of the conventional LEDs and PSS LEDs, respectively, with nonalloy Cr/Au bonding pads.

Optical and Structural Properties of InGaN–AlGaN Ultraviolet Light-Emitting Diodes

We fabricated and investigated the performances of InGaN-AlGaN ultraviolet (UV) light-emitting diodes (LEDs) emitting at 380 nm. The output power of a conventional LED, a patterned sapphire substrate LED (PSS LED), and a PSS flip-chip LED (PSS FCLED) were about 0.94, 1.86, and 5.18 mW, respectively, at a forward injection current of 20 mA. These results indicate that the light output-powers of the PSS LED and PSS FCLED were enhanced as much as 97% and 451% compared to the conventional LED. Subsequent optical simulations confirm the remarkable enhancements in optical power of the PSS FCLED at UV wavelengths.

GaN-Based Light-Emitting Diodes Grown on Photonic Crystal-Patterned Sapphire Substrates by Nanosphere Lithography

GaN-based light-emitting diodes (LEDs) grown on photonic crystal-patterned sapphire substrates (PCPSS) have been demonstrated. PCPSS was fabricated by nanosphere lithography, and the photonic crystal structure was the hexagonal-lattice pattern. The forward voltages of PCPSS and patterned sapphire substrates (PSS) LEDs were smaller than that of conventional sapphire substrates (CSS) LED, and it infers the epitaxial film quality of PCPSS and PSS LEDs has been slightly improved. The luminance intensity of PCPSS LED was 1.63 and 1.51 times higher than those of CSS and PSS LED at 20 mA injection current. The enhancement in the luminance intensity of PCPSS LED is attributed to the photonic crystal structure.

Growth and characterization of InGaN-based light-emitting diodes on patterned sapphire substrates

The microstructure and electrical properties of the InGaN-based blue light-emitting diodes (LEDs) fabricated on patterned sapphire substrates (PSSs) with parallel grooves have been investigated. The PSS with parallel grooves (ridge: 2 μm; trench: 3 μm) along the (11–20) direction were etched using an inductively coupled plasma etcher with different etching depths. From the transmission electron microscopy observation, the PSS was confirmed to be an efficient way to reduce the threading dislocation density in the GaN epilayer. The output power of the PSS LED (461 nm) showed 33% higher than that of the conventional LED. For a typical lamp-form PSS LED operating at an injection current of 20 mA, the output power and external quantum efficiency were measured to be 7.1 mW and 10.1%, respectively. The achieved improvement of the output power is not only due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, but also due to the enhancement of the extraction efficiency using the PSS. Finally, better reliability of the PSS LED performance was also observed.

Growth and characterization of 380-nm InGaN/AlGaN LEDs grown on patterned sapphire substrates

The 380-nm InGaN/AlGaN light-emitting diodes (LEDs) were fabricated on a conventional and patterned sapphire substrates (PSSs) by metalorganic vapor phase epitaxy (MOVPE). Micro-photoluminescence (PL) measurements showed superior near-band-edge luminescence intensity from the overhang area as compared to the layer directly on the flat sapphire region. This was accompanied by a small redshift of the PL peak wavelength that could be attributed to a relief of compressive stress in the epitaxial lateral overgrowth region. From the temperature-dependent PL measurements, we obtain an integrated PL intensity ratio at 300–10 K of ∼13.7% and thermal activation energy of 94 meV from the InGaN/AlGaN MQW PSS LED sample. Under a 20-mA current injection, the output power increased from 3.75 to 5.06 mW, corresponding to about 35% increase in external quantum efficiency. It is evident that the increase in output power depends on both the defect reduction of epitaxial lateral overgrowth of GaN and the scattering of emitted light at the GaN/sapphire interface.

InGaN‐based light‐emitting diodes grown on grooved sapphire substrates

AbstractWe investigate the microstructure and electrical properties of the InGaN‐based light‐emitting diodes (LEDs) fabricated onto patterned sapphire substrates (PSSs) with parallel grooves. The PSS with parallel grooves (ridge: 3 μm; trench: 2 μm) along the (11‐20) direction were etched using an inductively‐coupled‐plasma etcher with different etching depths. It was found that the threading dislocation density in the GaN epilayer can be effectively reduced by the present PSS technique as indicated from the transmission‐electron‐microscopy result. For a typical lamp‐form PSS LED operating at a forward current of 20 mA, the output power and external quantum efficiency were measured to be 7.1 mW and 10.1%, respectively. The achieved improvement of the output power is not only due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, but also due to the enhancement of the extraction efficiency using the PSS. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Enhancing the output power of GaN-based LEDs grown on wet-etched patterned sapphire substrates

GaN-based light-emitting diodes (LEDs) with emitting wavelength of 450 nm were grown on patterned sapphire substrates (PSSs) fabricated by chemical wet etching. The crystallography-etched facet was {1-102} R-plane with a 57deg against {0001} C-axis and had superior capability for enhancing light extraction efficiency. The light output power of the PSS LED was 1.15 times higher than that of the conventional LED at an injection current of 20 mA. The output power and external quantum efficiency were estimated to be 9 mW and 16.4%, respectively. The improvement was attributed not only to geometrical shapes of {1-102} crystallography-etched facets that efficiently scatter the guided light to find escape cones, but also to dislocation density reduction by adopting the PSS growth scheme

Efficiency improvement of near-ultraviolet InGaN LEDs using patterned sapphire substrates

The use of conventional and patterned sapphire substrates (PSSs) to fabricate InGaN-based near-ultraviolet (410 nm) light-emitting diodes (LEDs) was demonstrated. The PSS was prepared using a periodic hole pattern (diameter: 3 /spl mu/m; spacing: 3 /spl mu/m) on the (0001) sapphire with different etching depths. From transmission-electron-microscopy and etch-pit-density studies, the PSS with an optimum pattern depth (D/sub h/=1.5 /spl mu/m) was confirmed to be an efficient way to reduce the thread dislocations in the GaN microstructure. It was found that the output power increased from 8.6 to 10.4 mW, corresponding to about 29% increases in the external quantum efficiency. However, the internal quantum efficiency (@ 20 mA) was about 36% and 38% for the conventional and PSS LEDs, respectively. The achieved improvement of the output power is not only due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, but also due to the enhancement of the extraction efficiency using the PSS. Finally, better long-time reliability of the PSS LED performance was observed.

Improvement in light-output efficiency of near-ultraviolet InGaN–GaN LEDs fabricated on stripe patterned sapphire substrates

InGaN/GaN multi-quantum wells near ultraviolet light-emitting diodes (LEDs) were fabricated on a patterned sapphire substrate (PSS) with parallel stripe along the 〈 1 1 ¯ 0 0 〉 sapphire direction by using low-pressure metal-organic chemical vapor deposition (MOCVD). The forward- and reverse-bias electrical characteristics of the stripe PSS LEDs are, respectively, similar and better than those of conventional LEDs on sapphire substrate. The output power of the epoxy package of stripe PSS LED was 20% higher than that of the conventional LEDs. The enhancement of output power is due not only to the reduction of dislocation density but also to the release of the guided light in LEDs by the geometric shape of the stripe PSS, according to the ray-tracing analysis.

Near-Ultraviolet InGaN/GaN Light-Emitting Diodes Grown on Patterned Sapphire Substrates

We describe the microstructure and optical properties of near-ultraviolet InGaN-GaN light-emitting diodes (LEDs) fabricated onto conventional and patterned sapphire substrates (PSSs) using metalorganic chemical vapor deposition. The PSS LED with an optimized hole depth (1.5 µm) shows an improvement of the room-temperature photoluminescence intensity by one order of magnitude compared with that of the conventional LED. As much as a 63% increased light emission intensity of the PSS LED was obtained at a forward current of 20 mA. For a typical lamp-form PSS LED (at 20 mA), the output power and external quantum efficiency were estimated to be 10.4 mW and 14.1%, respectively. The increase of the output power could be partly due to the improvement of the internal quantum efficiency upon decreasing the dislocation density, which was further confirmed by the transmission-electron-microscopy and etch-pit-density studies for the GaN-on-PSS samples. Moreover, the emitted light scattering at the GaN/PSS interface could also contribute to the enhancement of light extraction efficiency.

Enhanced output power of near-ultraviolet InGaN-GaN LEDs grown on patterned sapphire substrates

Near-ultraviolet nitride-based light-emitting diodes (LEDs) with peak emission wavelengths around 410 nm were fabricated onto c-face patterned sapphire substrates (PSS). It was found that the electroluminescence intensity of the PSS LED shown 63% larger than that of the conventional LED. For a typical lamp-form PSS LED operating at a forward current of 20 mA, the output power and external quantum efficiency were estimated to be 10.4 mW and 14.1%, respectively. The improvement in the light intensity could be attributed to the decrease of threading dislocations and the increase of light extraction efficiency in the horizontal direction using a PSS.