Results and Discussions The polarization charge concentration at the non-top layer of the gradient layer increases with the rising In composition of the gradient layer, and the polarization charge concentration at the top layer of the gradient layer gradually decreases with the increase in the In composition (Fig. 2). This is mainly because the lattice mismatch in the non-top layer increases, the polarization electric field improves, and the polarization charge concentration increases with the rising In composition. However, the larger In composition at the top layer of the gradient layer leads to a smaller lattice mismatch with the In_0.38Ga_0.62N layer, weaker polarization, and lower polarization charge concentration. The carrier concentration and power spectral density increase with the rising In composition in the gradient layer (Figs. 3 and 4), which is mainly because the increase in In composition increases the depth of InGaN/GaN quantum well and the carrier concentration, and enhances recombination efficiency. The interface charge between the top layer of the gradient layer and In_0.38Ga_0.62N decreases with the increasing thickness of the top layer of the gradient layer (Fig. 8). This is mainly because the increase in the thickness of the top layer of the sub5 gradient layer can fully release the stress generated by the lattice mismatch, weaken the polarization, and reduce the polarization charge density. The carrier concentration also decreases with the increase in the thickness of the top layer of the gradient layer (Fig. 9), and the peak power spectral density increases first and then decreases with the rising thickness of the top layer of the gradient layer (Fig. 10). This is because when the thickness of the top layer of the gradient layer is too small, electron tunneling may occur to reduce the carrier concentration and the recombination efficiency. On the contrary, when the thickness of the top layer of the gradient layer is too large, it will lead to the electron leakage, hole concentration and recombination rate will be reduced, and the power spectral density decreases. The peak power spectral density of the non-uniform thickness structure of the gradient layer is smaller than that of the non-uniform thickness structure of the top layer (Fig. 13). This is mainly because the dislocation and defect of the non-uniform thickness structure of the gradient layer are relatively small, which can reduce non-radiative recombination and increase the power spectral density.

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.