Tunable the emission wavelength of InGaN/GaN multi-quantum 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 In(x)Ga(1-x)N (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).