Study On Opto-electronic Properties Of Semipolar GaN And LEDs Grown By Selective Area Epitaxy

The bandgap of Ⅲ-nitride semiconductors changes continuously from 0.7 eV to 6.2 eV and covers a extremely broad spectrum range between near-infrared to ultraviolet. Ⅲ-nitride semiconductors also have excellent optical and electrical properties, and higher luminous efficiency than other materials with a larger application range. These advantages make nitride LEDs the most successful opto-electronic devices by far whose emitting wavelength range from near ultraviolet to blue-green. However, nitride hetero-structures have a strong built-in electric field due to its strong polarization and large strain, causing the separation of electron and hole wave functions in quantum well structures and the reduction of the luminous efficiency.Semi-polar and non-polar GaN-based LEDs gradually attract a lot of attention, for the reduced polarization effect will bring more excellent physical properties and greater potential applications. Epitaxy of semi-polar and non-polar GaN-based materials lacks of proper substrates since the GaN substrates have the disadvantages such as high cost and small size, while it is difficult to achieve high luminous efficiency due to the inevitable poor crystal quality when using other substrates. Selective Area Epitaxy(SAE) is a promising method for grow semi-polar GaN material and devices on c-face sapphire, which can reduce the dislocation density effectively and achieve semi-polar GaN material and devices with different kinds of facets.In this study, the research is focused on the optical and electrical properties of the semi-polar GaN material and devices. We study deeply the influence factors in the SAE growth process of semi-polar GaN materials, study the dislocation evolution and optical properties of semi-polar GaN by SAE growth method. We study the optical properties of semi-polar InGaN/GaN multiple quantum wells, prove their excellent luminescent properties by comparing their polarization properties with polar InGaN/GaN MQWs. We also fabricate the semi-polar GaN-based LEDs, discuss the the key technology issue in the fabricating process, and explore their optical and electrical properties.The main content and results are listed as follows:1. The formation mechanism of the semipolar GaN facets on stripe and cross masks with different directions and sizes by SAE is studied. It is suggested that the formation mechanism is strongly related to the surface energy and stability. SAE with stripe masks oriented along the direction of [11-20] and [1-100] form{1-101} and{11-22} semipolar plane, separately. We find the thermal stability of{1-101} semi-polar facet is better than{11-22} facet. SAE with cross masks form three kinds of semipolar plane:{1-101}、{21-33} and{11-22}, while the{21-33} facet will disappear when growth temperature elevates. We study the influence of growth temperature and fill factors of mask on semi-polar microstructures and growth rate of different facets. We also find the increase of the fill factors and growth temperature can increase the surface migration of the atoms to enhance the lateral epitaxial growth and form the low-surface-energy (0001) facet. It is shown that the growth rate is controlled by the mass transport of reactant.2. We study the mechanism of dislocation evolution in SAE process and find that the dislocation density can be reduced by SAE. Furthermore, the increase of fill factors and mask dimension can reduce the dislocation density and improve the quality of crystal more effectively. We study the optical properties of semipolar GaN specifically. Low-temperature PL spectra show the photoluminescence peak near 3.41 eV and 3.29 eV come from the base stacking fault (BSF) and prismatic stacking fault (PSF), respectively. We prove the BSF of semi-polar GaN forms mainly in lateral epitaxial region. The PL peak energy of NBE of semi-polar GaN decreases as temperature increases, while the PL peak energy of BSF presents so-called S-shaped temperature dependence for the discontinuity of the conduction and valence band caused by BSF will lead to the carrier localization.3. We study the optical properties of semi-polar InGaN/GaN MQWs. It is found that the luminescence peak positions of {11-22},{1-101} and (0001) are 412 nm, 436 nm and 518 nm with the same growth condition, respectively. And the luminescence peak of the top area at the same {1-101} facet red shifts 23 nm compared with the bottom area. The reason is that the difference of growth rates causes the different well width and In incorporation efficiency. Moreover, the stronger migration ability of In atoms than Ga in SAE causes the different In composition. The wavelength of different surfaces of InGaN/GaN MQWs grown with cross masks by SAE is ordered as {1-101}>{21-33}>{11-22}, which is consistent with the order of growth rate. We found the uneven distribution of the clusters-glowing phenomenon of (0001) surface MQWs, proved that (0001) surface MQWs has a larger dislocation density than semipolar facets and cause the InGaN phase segregation.4. PL study show that as for the semipolar InGaN/GaN MQWs, the blue shift of peak position with the increase of laser power reduces by 5 times comparing to the (0001) surface MQWs, prove that the reduced polarization electric field and QCSE effect of semipolar InGaN/GaN MQWs. The internal quantum efficiency of{1-101} and{11-22} InGaN/GaN MQWs are 65.6% and 55.7%, respectively, which are about 4 times higher than the (0001) MQWs(15.9%). This confirms the reduced QCSE effect with reduced polarization electric field can greatly improve the luminous performance of InGaN/GaN multiple quantum well.We discover that PL peak energy of semi-polar InGaN/GaN MQWs decreases as temperature increases, not like the so-called S-shaped temperature dependence of PL peak energy of c-plane InGaN/GaN MQWs, which is caused by the reduced polarization electric field and deep well of semi-polar MQWsmaking the carrier localization effect weakened. We also calculate the PE and total polarization based on model of strain inducing polarization, proves the reduced polarization effect of semi-polar InGaN/GaN MQWs, causing the flatten energy band, the blue-shifted peak position, reduced QCSE effect, improved radiative recombination efficiency.5. We have fabricated the semipolar GaN-based LED devices. It is found that Mg doping can enhance the migration ability of atoms and promote the lateral growth causing the thickness of pGaN on semipolar plane is larger than c-polar plane. We study the optical properties of semi-polar LED, and prove the reduced polarization electric field and the consequent reduced QCSE effect. Electrical I-V test results show that the electrical performance of semi-polar LED is worse than that of conventional c-surface LED for the forward voltage is 6.3 V@20 mA and the reverse leakage current is 2 mA@-5 V, due to the pGaN growth process of SAE and the metal deposition process of uneven device process, indicating the material growth and chip manufactuing process of semi-polar LED still remains to be optimized.