USTC and NIMTE advance AlGaN UV-LEDs
Solid state deep ultraviolet (DUV) light source is the key technology to replace the conventional mercury-based lamp because of their environmentally friendly, long time and compact characteristics. Aluminum gallium nitride (AlGaN), as wide bandgap semiconductor, has been demonstrated as one of the ideal candidates in the pursuit of such clean DUV source. However, the development of AlGaN-based DUV light emitting diodes (DUV-LED) is still inefficient compared to the mature technology of the InGaN-based blue LEDs as the external quantum efficiency (EQE) of DUV-LEDs (emitting ~ 280 nm) reaches to a maximum of 20% only (in most cases are below 10%), whereas EQE ³ 80% obtained from InGaN based blue-LEDs. Due to the existence of indium segregation in InGaN films, compositional inhomogeneities are created, resulting in carrier localization effects in the InGaN multiple quantum wells (MQWs). This is one of the key advantages in the development of hUSTC and NIMTE team successfully synthesized, first-of-its-kind, AlGaN/AlGaN wavy multiple-quantum-wells on large misoriented sapphire substrates highly efficient InGaN-based blue LED. However, as the Al atoms have similar sizes but with much lower mobility compared to that of Ga atoms, compositional inhomogenities or thickness variation are difficult to form in AlxGa1-xN(0
Recently, researchers from University of Science and Technology of China (USTC) led by Prof. Haiding Sun and Prof. Shibing Long, and the Ningbo Institute of Materials Technology and Engineering (NIMTE), Chinese Academy of Sciences, led by Prof. Wei Guo had successfully synthesized a new kind of nanoscale structures so called “wavy quantum wells” and utilized in the application of high efficiency UV emitters. They demonstrated an effective solution to artificially create similar “phase separation” and thus generate compositional inhomoneneities in the AlGaN alloys (in analogy to that observed in InGaN/GaN-based blue LEDs) to achieve better carrier localization. They proposed to grow high Al composition AlGaN-based DUVLED emitting at ~280 nm on misoriented sapphire substrates which have a miscut angle as large as 4°, as shown in Figure 1 below:
Figure 1, Cross-sectional STEM image of AlGaN MQWs grown on (a-c) 0.2° and (d-f) 4° misoriented sapphire substrate.
By taking advantage of semi-polar template that results in a reduction in the polarization field as well as the benefit of misoriented substrate that leads to carrier localization, such large misoriented substrate can significantly boost the radiative recombination and the IQE via increased electron wavefunction overlap and enhanced carrier localization, in comparison with the devices grown on flat sapphire, here the team achieved:
1.A significantly enhanced photoluminescence intensity (at least 10 times higher), as shown in Figure 2(a).
2.Improved internal quantum efficiency (6 times higher at low excitation laser power) with a record high over 90%; as shown in Figure 2(b,c,d).
3.A much longer carrier lifetime was achieved for QWs grown on 4° misoriented sapphire substrate (1.60 ns) when compared with that grown on 0.2° misoriented sapphire (0.06 ns);
4. The wafer-level output power of the ultraviolet light emitting diodes on 4° misoriented sapphire substrate were 2-3 times higher on 0.2° misoriented sapphire, as shown in Figure 3.
Figure 2. (a) room temperature PL spectra of AlGaN MQWs grown on 0.2° and 4° sapphire substrates. Temperature-dependent PL spectra of AlGaN MQWs grown on (b) 0.2° and (c) 4° misoriented substrates. (d) IQE as a function pumping power for both MQWs grown on 0.2° (red dots) and 4° (blue dots) misoriented substrates.