Strain engineering project significantly increases light output of green LEDs

A report recently released by the Chinese Academy of Sciences stated that Chinese researchers have used strain engineering to inject 150mA of current into a 530nm light-emitting diode ( LED ), increasing the light output power by 28.9%. This research is a collaborative project between the Institute of Semiconductors of the Chinese Academy of Sciences and the University of Hong Kong.

　　Schematic diagram of epitaxial materials for conventional LEDs and shallow quantum well (SQW) LEDs (Figure 1, left). Light intensity-current-voltage (LIV) characteristics of conventional LEDs and SQW LEDs (Figure 2, top right). Comparison of EQE and current characteristics of conventional LEDs and SQW LEDs (Figure 3, bottom right).

　　Green-emitting nitrogen semiconductor LED structures tend to have low light output due to the difficulty in producing the high indium content indium gallium nitride required for longer-wavelength light emission. In addition to material quality challenges, the strain induced by the lattice mismatch with pure gallium nitride (GaN) also causes a large piezoelectric effect, which generates an electric field that causes electrons and holes to separate, reducing the recombination rate of light quanta (i.e., the quantum trapped Stark effect, QCSE).

　　The researchers solved this problem by inserting a layer of low-indium-content indium gallium nitride (InGaN) before the high-indium-content light-emitting layer. Through simulation experiments, they found that the low-indium-content indium gallium nitride (InGaN) layer can weaken the strain-induced electric field in the active light-emitting multiple quantum well (MQW) structure.

　　The epitaxial material of the low-indium-content indium gallium nitride (InGaN) shallow quantum well (SQW) step is realized by using metal organic vapor deposition ( MOCVD ) technology on the control plane (0001) sapphire (Figure 2). The traditional multiple quantum well (MQW) active area consists of 12 3nm In0.3Ga0.7N well periods between 12nm gallium nitride (GaN) grids, while the shallow quantum well (SQW) structure consists of 12 2nm In0.1Ga0.9N shallow wells + 3nm In0.3Ga0.7N deep well periods between 12nm gallium nitride (GaN) grids. These materials are then made into 256μm x 300μm mesa structure wafers.

　　At low temperature (85K) and room temperature (298K), a 325nm cadmium nitrogen laser was used to stimulate the photoluminescence spectrum of the material. One of the effects of the shallow quantum well (SQW) is to reduce the half-maximum width (FWHM) of the spectrum peak of the traditional LED material from 16.7nm to 13.1nm for the SQW material at 85K, and from 20.1nm to 15.7nm at 298K.

　　The peak intensity was also increased using a shallow quantum well (SQW) structure in this study. These results provide evidence for the improvement in the crystal quality of the SQW material. In particular, the narrow FWHM means "more consistent indium distribution and less carrier localization" due to reduced strain in the active region. The peak height of the shallow quantum well (SQW) material at 298K is 55.1% of that at 85K, while the corresponding proportion of the traditional structure is 24.1%. This shows that the smaller the quantum confined Stark effect (QCSE), the higher the recombination rate and internal quantum effect (IQE) of the luminescent material that the shallow quantum well (SQW) material can provide.

　　Electroluminescence was measured in an integrating sphere as light intensity-current-voltage (LIV) (Figure 2). The voltage performance of the SQW and conventional devices was about the same, while at 150mA the light output of the SQW LED (49.3mW) was 28.9% higher than the conventional device (38.4mW).

　　The researchers attribute this enhanced property to an improved coincidence of electron and hole wave functions, which increases the recombination rate of photons. For photoluminescence, this performance was not improved to the same extent because the bias in electroluminescence enhances the polarization electric field. The external quantum effect (EQE) is 10.2-13.3% higher than that of conventional LED devices (Figure 3).