Hemispherical Patterned Sapphire Substrate Research Articles

Catalyst-free growth of single crystalline β-Ga2O3 microbelts on patterned sapphire substrates

The high density and large quantity β-Ga2O3 microbelts were synthesized by changing the growth temperature on patterned sapphire substrates (PSS) by chemical vapor deposition equipment. The width of the microbelts was about 1–1.5 μm, and the length was about 15 μm. The results of different growth temperature influenced for surface morphology and crystal structure of β-Ga2O3 indicated that the optimal growth temperature of β-Ga2O3 microbelts was at 900 °C. Furthermore, the growth mechanism of microbelts was also studied. It was found that the hemispherical PSS played an important role for β-Ga2O3 microbelts formation. The optical absorption spectrum indicated that optical band gap energy of microbelts was about 4.78 eV.

Characteristics of GaN-based 500 nm light-emitting diodes with embedded hemispherical air-cavity structure

GaN-based 500 nm light-emitting diodes (LEDs) with an air-cavity formed on a laser-drilled hemispherical patterned sapphire substrate (HPSS) were investigated. The cross-section transmission electron microscopy image of the HPSS-LED epilayer indicated that most of the threading dislocations were bent towards the lateral directions. It was found that in InGaN/GaN multiple quantum wells (MQWs) of HPSS-LEDs, there were fewer V-pits and lower surface roughness than those of conventional LEDs which were grown on flat sapphire substrates (FSSs). The high-resolution x-ray diffraction showed that the LED grown on a HPSS has better crystal quality than that grown on a FSS. Compared to FSS-LEDs, the photoluminescence (PL) intensity, the light output power, and the external quantum efficiency at an injected current of 20 mA for the HPSS-LED were enhanced by 81%, 65%, and 62%, respectively, such enhancements can be attributed to better GaN epitaxial quality and higher light extraction. The slightly peak wavelength blueshift of electroluminescence for the HPSS-LED indicated that the quantum confined Stark effect in the InGaN/GaN MQWs has been reduced. Furthermore, it was found that the far-field radiation patterns of the HPSS-LED have smaller view angles than that of the FSS-LED. In addition, the scanning near field optical microscope results revealed that the area above the air-cavity has a larger PL intensity than that without an air-cavity, and the closer to the middle of the air-cavity the stronger the PL intensity. These nano-light distribution findings were in good agreement with the simulation results obtained by the finite difference time domain method.

Optimized double-sided pattern design on a patterned sapphire substrate for flip-chip GaN-based light-emitting diodes

This study reports on the development and testing of a cost- and time-effective means to optimize a double-sided hemispherical patterned sapphire substrate (PSS) for highly efficient flip-chip GaN-based light-emitting diodes (LEDs). A simulation is conducted to study how light extraction efficiency (LEE) changed as a function of alteration in the parameters of the unit hemisphere for LEDs that are fabricated on a hemispherical PSS. Results show that the LEE of LED flip chip could be enhanced with the optimized hemispherical PSS by over 0.508 and is ∼115.3% higher than that of flip-chip LEDs with non-PSS. This study confirms the high efficiency and excellent capability of the optimized hemispherical PSS pattern to improve LED efficacy.

Study of defects in LED epitaxial layers grown on the optimized hemispherical patterned sapphire substrates

In this work, we focus on the study of defects in GaN grown on an optimized hemispherical patterned sapphire substrate (PSS). It is demonstrated that the proposed patterns can on the one hand induce the formation of stacking faults, and on the other hand, reduce the strain caused by thermal misfit and lattice misfit. Consequently, the optimized hemispherical patterns work successfully for both the reduction in the number of dislocations spreading to multiple quantum wells and the improvement in surface morphology. The dominant mechanism of defect multiplication and the effects of optimized hemispherical patterns in terms of materials science and device technology are elucidated.

Enhance Light Emitting Diode Light Extraction Efficiency by an Optimized Spherical Cap-Shaped Patterned Sapphire Substrate

This work has proposed a new way to optimize the spherical cap-shaped patterned sapphire substrate (PSS) for highly efficient GaN-based light emitting diodes (LEDs), which has been compared with the hemisphere patterned one. This pattern is achieved by changing the height of hemispherical units on the basis of hemispherical PSS. The height, the distance and the radius of the spherical cap-shaped unit are subsequently optimised by optical simulation. It is revealed that this optimised spherical cap-shaped PSS can enhance light extraction yield of LEDs by over 10% compared with LEDs grown on the optimal hemispherical PSS. The effectiveness of this spherical cap-shaped PSS has been demonstrated by subsequent crystal growth and characterization of LED wafers, and therefore sheds light on a further improvement on LED efficacy by the design of novel patterns for the application of PSS technology.

Improvement of crystal quality and optical property in (11−22) semipolar InGaN/GaN LEDs grown on patterned m-plane sapphire substrate

Semipolar GaN layers were grown on the m-plane hemispherical patterned sapphire substrates (HPSS) using metal organic chemical vapor deposition in order to reduce the defect density and enhance the extraction efficiency of light. The roughness values of the GaN surface grown on the planar sapphire and the HPSS were 30 and 23nm root-mean-square roughness for a 20×20-μm2 area, respectively. The reduction of basal stacking fault density was demonstrated by x-ray rocking curve of off-axis planes and cross-sectional transmission electron microscopy. The low-temperature photoluminescence measurement showed that the near band-edge emission from HPSS semipolar GaN was approximately one order of magnitude stronger than that from planar semipolar GaN layer.The InGaN light emitting diode grown on the HPSS showed an output power approximately 1.5 times that on the planar m-sapphire.

Pattern Design of and Epitaxial Growth on Patterned Sapphire Substrates for Highly Efficient GaN-Based LEDs

This work represents a cost and time effective approach for pattern design of patterned sapphire substrates (PSS) for highly efficient GaN-based light emitting diodes (LEDs). Simulation is used to study how external quantum efficiency changes with the change in parameters of the unit hemisphere for LEDs fabricated on hemispherical PSS. Through a series of comparisons on simulation results, the most effective pattern to improve external quantum efficiency of LEDs on hemispherical PSS is revealed. The subsequent crystal growth of LED wafers demonstrates that both photoluminescence and electroluminescence intensities dramatically increase by about a half for LEDs grown on this pattern-optimized PSS compared to that of LEDs on non-PSS, which straight-forwardly proves the high efficiency of the optimized hemispherical patterns of PSS for improving LED efficacy.

Enhancement of InGaN-Based Vertical LED With Concavely Patterned Surface Using Patterned Sapphire Substrate

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> To improve the external quantum efficiency, we have proposed a new method utilizing surface roughening of vertical-type light-emitting diodes (VT-LEDs) fabricated on hemispherical patterned sapphire substrate by using a laser lift-off technique. The advantages of this method are simple and reproducible in transferring the well-defined patterns on sapphire into GaN layer. The VT-LED with concavely patterned surface showed a nearly twofold increase in the output power compared to the normal planar surface. This improvement in the VT-LED performances is attributed to the increase in the escaping probability of photons from the LED surface. </para>