The active region of conventional GaN-based LEDs mostly adopts the InGaN/GaN quantum well structure. However, due to the large size difference between In and Ga atoms, there is a large lattice mismatch between InN and GaN, which leads to the generation of polarized electric field and tilted energy band. On the one hand, some holes escape, which results in decreased radiation recombination efficiency, thus inducing the quantum-confined Stark effect. On the other hand, since the bond energy of In—N is smaller than that of Ga—N, it is easy to form in gap atoms, thereby introducing crystal defects and reducing the internal quantum efficiency. The In composition gradient InGaN/GaN quantum well structure can solve the LED luminous efficiency reduction caused by lattice mismatch. However, the effects of In composition and thickness of the gradient layer on polarization charge concentration, carrier concentration, and LED power spectral density are still unclear. It is particularly important to study the effects of the material and structure of the quantum well gradient layer on the performance of GaN-based LED for improving the efficiency of GaN-based LEDs.
The numerical calculation model of GaN-based LED with In component gradient quantum well structure is built by Silvaco TCAD software. Based on the composite model, carrier statistical model, carrier transport model, self-consistent Schrodinger Poisson equation, and spontaneous polarization and piezoelectric polarization model of the built-in electric field, the effects of In component in the gradient layer and thickness of the top layer of the gradient layer on the polarization charge concentration, carrier concentration, and power spectral density are simulated and calculated. Firstly, the thickness of the gradient layer keeps constant, and the In composition of the gradient layer is changed. The changes of polarization charge concentration, carrier concentration, and power spectral density with In composition are calculated and analyzed. Secondly, the influence of the In composition on the top layer of the gradient layer is analyzed by keeping the In composition of other layers unchanged. Finally, the better In component in the above results is selected to analyze and calculate the influence of the thickness of the top layer of the graded layer.
The thickness of In component in the gradient layer exerts a significant effect on the performance of GaN-based LED with In component gradient quantum well structure. With the increasing In component in the gradient layer, the peak power spectral density of LED decreases gradually with the increase in In component. The power spectral density first increases and then decreases with the rising thickness of the top layer of the gradient layer. The power spectral density for the not uniform thickness of the non-top layer of the gradient layer is smaller than that for the uniform thickness. Reasonable control of the In composition and thickness of the gradient layer can address LED luminous efficiency reduction caused by lattice mismatch. The results can provide guidance for the design and development of high-efficiency GaN-based LEDs.