Study on cavity engineered sapphire substrate for highly efficient GaN-based light-emitting diodes

Abstract

The GaN-based white light-emitting diodes (LEDs) have attracted much attention as a substitute for conventional illumination such as incandescent light bulbs and fluorescence lamps because of its high efficiency and long life. However, rapid penetration of the LEDs into lighting market has been limited due to its high cost. A major drawback of epitaxial growth of the GaN layer is that native substrates are not yet available in large scale, so heteroepitaxy using sapphire substrate have been typical method for the epitaxial growth. The large differences in the lattice constant and thermal expansion coefficient between GaN and sapphire substrate cause high density of threading dislocations and severe wafer bow. In addition, total internal reflection of the emitted light due to the large difference in the refractive index between the GaN epitaxial layer and outside (air) is one of the factors that reduce light extraction efficiency. To achieve the high efficiency and productivity for cost reduction of the GaN-based LEDs, these technical issues need to be resolved. In this research, in order to overcome the problems, we proposed a growth scheme using cavity engineered sapphire substrate (CES) in which a two-dimensionally patterned cavities are arrayed on a sapphire substrate. Amorphous alumina film was deposited by atomic layer deposition on a photoresist patterned sapphire substrate, and subsequent high temperature annealing resulted in the formation of a cavity array surrounded by a crystallized sapphire shell by solid-phase epitaxy (SPE). It was confirmed that well-defined air-cavity array was successfully incorporated into the sapphire substrate. Also, the amorphous alumina layer was fully crystallized into single crystalline α-phase from the sapphire substrate, indicating that the CES can act as a substrate for the epitaxial growth of GaN. In the growth scheme using the CES, the GaN layer is grown on the SPE α-Al2O3 layer and resultantly crystalline quality of the GaN layer could be dependent on the characteristics of the SPE α-Al2O3, which arouses the importance of the fundamental understanding on the SPE mechanism. Accordingly, we investigated the SPE of stripe-shaped cavity amorphous Al2O3 membrane structure on a sapphire substrate. TEM analysis revealed that the SPE process occurred through 2 stages of the phase transformation from amorphous to γ-Al2O3 and subsequently to α-Al2O3. During the phase transformation to γ-Al2O3, beside SPE at the interface between the amorphous alumina layer and sapphire substrate, nanocrystalline γ-Al2O3 was formed in the upper part of the membrane. However, during the SPE from γ- to α-phase, random nucleation was not observed in our investigation condition, resulting that the whole alumina membrane was transformed into α-Al2O3 by SPE. During the phase transformations, volume of the alumina membrane was contracted by the density increase, which induces stresses and deflections in the Al2O3 membrane structure. Furthermore, the activation energies of the SPE procedure from amorphous to γ-phase and that from γ- to α-phase were obtained as 3.1 eV and 3.9 eV, respectively, by precise measurement of the SPE rate using TEM analysis. In addition, SPE mechanism of the amorphous alumina into the intermediate γ-phase was investigated in detail by phase/orientation mapping using a scanning nanobeam diffraction technique of TEM. This evidently revealed presence of the two stacking-mismatched domains in the epitaxial γ-Al2O3 layer, which can be distinguishable only at the specific projecting direction. More importantly, distribution of the stacking-mismatched domains in the SPE γ-Al2O3 layer gives significant information for understanding the formation mechanism of the γ-Al2O3 domains. The growth behavior of GaN on the CES was investigated. The GaN film was observed to fill the spaces between the cavities at the initial stage of growth and then grow laterally over the cavities, leading to a completely coalesced pit-free smooth surface. CL dark spot density was reduced from 1.9 × 108 cm-2 to 1.4 × 108 cm-2, demonstrating that the threading dislocation density was reduced using the CES. Also, the incorporation of cavities was observed to significantly reduce the stress in the GaN film by ~30%. The output power of LED on CES at an input current of 20 mA was measured to be 2.2 times higher than that on the planar sapphire substrate, indicating that the cavity pattern at the interface significantly enhanced the light extraction. To suppress the undesired growth of GaN on the cavity pattern, we suggested growth of GaN layer on a partially crystallized CES (PCCES) in which only the planar region between the patterns was crystallized into single crystalline (0001) α-Al2O3 while the alumina shell surrounding the cavities consisted of nanocrystalline γ-Al2O3. Due to limited growth rate of nanocrystalline GaN islands on the nanocrystalline alumina shell, c-plane GaN from the planar region laterally overgrows the nanocrystalline GaN islands on cavity patterns without interrupting by them. By using the PCCES, threading dislocations in the GaN region above the cavity patterns was significantly reduced compared to that on the existing CES. As a result, reverse leakage current for the GaN Schottky diode on PCCES was reduced by one order of magnitude compared to that on the existing CES.