Longgui Dai

Temperature-dependent photoluminescence in light-emitting diodes.

Temperature-dependent photoluminescence (TDPL), one of the most effective and powerful optical characterisation methods, is widely used to investigate carrier transport and localized states in semiconductor materials. Resonant excitation and non-resonant excitation are the two primary methods of researching this issue. In this study, the application ranges of the different excitation modes are confirmed by analysing the TDPL characteristics of GaN-based light-emitting diodes. For resonant excitation, the carriers are generated only in the quantum wells, and the TDPL features effectively reflect the intrinsic photoluminescence characteristics within the wells and offer certain advantages in characterising localized states and the quality of the wells. For non-resonant excitation, both the wells and barriers are excited, and the carriers that drift from the barriers can contribute to the luminescence under the driving force of the built-in field, which causes the existing equations to become inapplicable. Thus, non-resonant excitation is more suitable than resonant excitation for studying carrier transport dynamics and evaluating the internal quantum efficiency. The experimental technique described herein provides fundamental new insights into the selection of the most appropriate excitation mode for the experimental analysis of carrier transport and localized states in p-n junction devices.

Realization of high-luminous-efficiency InGaN light-emitting diodes in the "green gap" range.

Light-emitting diodes (LEDs) in the wavelength region of 535–570 nm are still inefficient, which is known as the “green gap” problem. Light in this range causes maximum luminous sensation in the human eye and is therefore advantageous for many potential uses. Here, we demonstrate a high-brightness InGaN LED with a normal voltage in the “green gap” range based on hybrid multi-quantum wells (MQWs). A yellow-green LED device is successfully fabricated and has a dominant wavelength, light output power, luminous efficiency and forward voltage of 560 nm, 2.14 mW, 19.58 lm/W and 3.39 V, respectively. To investigate the light emitting mechanism, a comparative analysis of the hybrid MQW LED and a conventional LED is conducted. The results show a 2.4-fold enhancement of the 540-nm light output power at a 20-mA injection current by the new structure due to the stronger localization effect, and such enhancement becomes larger at longer wavelengths. Our experimental data suggest that the hybrid MQW structure can effectively push the efficient InGaN LED emission toward longer wavelengths, connecting to the lower limit of the AlGaInP LEDs’ spectral range, thus enabling completion of the LED product line covering the entire visible spectrum with sufficient luminous efficacy.

Fabrication of 2-inch nano patterned sapphire substrate with high uniformity by two-beam laser interference lithography

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.

Tunable emission wavelength of InGaN/GaN multiquantum wells employing strain-accommodative structures

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).

Light output improvement of InGaN/GaN leds by self-assembled Ag nano-particles and SiN x surface roughening

A method of surface roughening is introduced to the InGaN/GaN LED chip fabrication process by self-assembled Ag nano-particles. 10nm silver film is deposited by E-beam evaporation (EBV) on SiNx passivation layer and the silver nano-scale particles self-assemble after 400°C Rapid Thermal Annealing (RTA), which are used as the mask for SiNx Reactive Ion Etching (RIE), and the surface roughness varies with different duration of etching. Test results show that its easy and effective to achieve the light output promotion; after RTA for 3min and RIE for 3min, the light output (LOP) is increased by 77.6% under 20mA injection current.