Generally, nano-scale patterned sapphire substrate (NPSS) has better performance than micro-scale patterned sapphire substrate (MPSS) in improving the light extraction efficiency of LEDs. Laser interference lithography (LIL) is one of the powerful fabrication methods for periodic nanostructures without photo-masks for different designs. However, Lloyd’s mirror LIL system has the disadvantage that fabricated patterns are inevitably distorted, especially for large-area twodimensional (2D) periodic nanostructures. Herein, we introduce two-beam LIL system to fabricate consistent large-area NPSS. Quantitative analysis and characterization indicate that the high uniformity of the photoresist arrays is achieved. Through the combination of dry etching and wet etching techniques, the well-defined NPSS with period of 460 nm were prepared on the whole sapphire substrate. The deviation is 4.34% for the bottom width of the triangle truncated pyramid arrays on the whole 2-inch sapphire substrate, which is suitable for the application in industrial production of NPSS.
In this paper, we focused on tuning the emission wavelength of InGaN/GaN multi-quantum wells (MQW) employing
strain-accommodative structures. Generally, the adjustment of emitting wavelength is realized by controlling the
quantum well (QW) thickness and the QW growth temperature, which decides the indium concentration. It needs large
thickness and low temperature to emit long wavelength photons. However, the material quality, electrical and optical
properties will degrade with low growth temperature or wide QW. Meanwhile, the growth of long wavelength LEDs
based on the InGaN material still faces severe difficulties because of the large (11%) lattice mismatch between InN and
GaN and the strong piezoelectric field-induced quantum-confined Stark effect (QCSE) induced by the high strain due to
lattice mismatch. Compared to the conventional LEDs, LEDs with proper strain-accommodative structures not only
increase the emitting wavelength but also reduce the strain in InGaN well. It provides an alternative approach to tune the
wavelength.
Two types of strain-accommodative structures are inserted between n-GaN and the multi-quantum wells: one is short
period super lattices (SPSL) consisted of 15 period of the 1-nm-thick InGaN well and the 2-nm-thick GaN barrier , and
the other is 45nm InxGa1-xN (x=0.07-0.09). The samples with strain-accommodative structures demonstrate that: firstly
the two structures would efficiently increase the wavelength, which should be attributed to the relief of strain in the
InGaN/GaN MQWs. The wavelengths of the two structures in the electroluminescence measurement were 561.6nm and
531nm, respectively. It is longer than that of the control sample (511.8nm). Secondly; the structures can weaken the
QSCE. When the current increased from 3mA to 20mA during the electroluminescence measurement, the peak
wavelength blue-shift were 5.1nm and 3.1nm, respectively. It is smaller than that of the control sample (7.4nm).
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