Research On The Structure And The Electrical And Optical Performances Of GaN-based Blue LEDs

Compared to the conventional semiconductor silicon, gallium nitride(GaN), a typical III-nitride semiconductor, is especially suitable for fabricating high-brightness and short-wavelength light-emitting devices, such as the blue light-emitting diode(LED), due to its favorable wide direct bandgap. So, GaN has great potential applications in the solid state illumination area. At a large forward current, however, the tradtional p-AlGaN electron blocking layer(EBL) will reduce the hole injection efficiency to the active layer, resulting in a sharp luminous efficiency decrease with the increasing current, which is usually called the efficiency droop behavior. Meanwhile, due to the large lattice and thermal expansion coefficient mismatches between(In)GaN film and common sapphire substrate, high density threading dislocations(TDs) are easy to form in the InGaN/GaN multiple quantum wells LEDs. The electrical TDs are not only the main transport path of leakage current, but also the effective non-radiative recombination centers of carriers, which severely limit the photoelectric efficiency of GaN-based LEDs. Therefore, in order to improve the electrical and optical properties of GaN-based LEDs, it is important to find the suitable EBL structure, improve the efficiency droop, reduce the internal dislocation density and improve the crystal quality.In this thesis, the influences of different composition-graded EBLs and the patterned sapphire substrate(PSS) on electrical and optical performances of InGaN/GaN LEDs are investigated. The main content and conclusions are summarized as follows.Firstly, the influence of Al composition-graded AlxGa1-xN EBL on the electrical and optical properties of InGaN/GaN blue LEDs is investigated by numerical simulation. Using the device simulation software Silvaco Atlas, the effect of different Al compositions in the p-type AlxGa1-xN EBL are studied. The results show that, among the uniform composition, stepped composition, graded composition and trapezoidal-shaped graded composition structure, the triangular-shaped graded Al composition EBL is the most effective to reduce the forward voltage. Using this structure, the electron leakage current is dramatically reduced and the hole injection efficiency is enhanced, leading to increased light output power and internal quantum efficiency(IQE). The designed EBL can improve the efficiency droop behavior under the high injection current by reducing the polarization effect.Secondly, the influences of conventional sapphire substrate(CSS) and PSS on electrical and optical performances of InGaN/GaN LEDs are investigated by comparison. The characterization and testing results of scanning electron microscopy, photoluminescence and cathodoluminescence indicate that, the crystal quality of epitaxial layer can be effectively improved by using the PSS, and the triangular pyramid PSS is especially effective to lower the dislocation density. The electrical and optical characteristics of the LEDs fabricated on different types of PSS are compared with those grown on the CSS. The results show that the PSS can effectively improve the electrical and optical properties of LED. Moreover, due to the smaller slanted angle, the triangular pyramid PSS can further enhance the light extraction efficiency owing to a wider light scattering range. Thus the output power and external quantum efficiency of LEDs on the triangular pyramid PSS are better.Finally, the physical mechanism of reverse leakage current in GaN-based LEDs is analyzed by temperature dependent current-voltage measurements. At low temperatures(< 220 K), the leakage current is attributed to variable-range-hopping conduction via localized defect states associated with TDs. The characteristic temperatures at the low and high biases are 1.2×109 K and 1.0×108 K, respectively. At high temperatures(> 220 K), there are two kinds of thermally activated mechanism. At low bias, the leakage current can be understood by a multi-step trap-assisted tunneling model. The electrons transition from the valence band of p-GaN to conduction band of the n-GaN by thermal activation and multi-step tunneling. At high bias, the leakage current is dominated by the Poole-Frenkel effect, composed of the electrons emitted from trap states to the conduction band. This suggests that the dislocation associated deep-level defects are the main reason of the large reverse leakage current.