Design and growth of high-power gallium nitride light-emitting diodes

• Main Author: Zhang, Zihui
• Other Authors: Hilmi Volkan Demir
• Format: Theses and Dissertations
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Electrical and electronic engineering::Microelectronics; DRNTU::Engineering::Electrical and electronic engineering::Optics, optoelectronics, photonics; DRNTU::Engineering::Electrical and electronic engineering::Semiconductors
• Online Access: https://hdl.handle.net/10356/61870
• Institution: Nanyang Technological University

In this dissertation, the InGaN/GaN multiple quantum well (MQW) light-emitting diodes (LEDs) have been studied from multiple aspects including improvement of material quality, suppression of quantum confined Stark effect (QCSE), promotion of carrier transport, enhancement of current spreading, reduction of electron overflow as well as increase in hole concentration of p-GaN through the generation of three-dimensional hole gas by dopant-free methods, with the aim to improve the optical performance of the devices. The InGaN/GaN LED epitaxial wafers are grown by metal-organic chemical vapor deposition (MOCVD) system on c-plane sapphire substrates. As it is well-known that the crystalline quality is crucial for high-efficiency InGaN/GaN LEDs, we have discussed and demonstrated the epitaxial films with optimized crystalline quality in this thesis. Besides, InGaN/GaN LEDs grown along polar-orientations (i.e., c+/c- orientations) suffer from the QCSE in the MQWs, which significantly reduces the spatial overlap of the electron-hole wave functions,and thus decreases the radiative recombination rates in the device active region. We have designed and demonstrated that the QCSE can be effectively suppressed by Si step-doping the quantum barriers,which enhances the optical output power and external quantum efficiency (EQE). In addition, by further Mg doping the Si step-doped quantum barriers with a proper Mg doping level and doing position, we have obtained the InGaN/GaN LEDs with PN-type quantum barriers. With this quantum barrier architecture, on one hand, the QCSE has been effectively screened, and on the other hand, the hole transport has also been promoted significantly.Hence, an even more enhancement in the optical output power and EQE has been obtained. The current crowding in the InGaN/GaN LEDs due to the low p-type conductivity is another challenge for achieving high-efficiency LEDs.In this dissertation, we have proposed a solution for current spreading enhancement by embedding a thin weakly doped n-GaN layer into the p-GaN layer.Such that, the p-GaN/n-GaN/p-GaN (PNP-GaN) current spreading layer is formed. The advantage of this design is to achieve the current spreading layer directly in the MOCVD growth, which saves the post-growth treatment in the fabrication process for modulating the current distribution. Besides incorporating a resistive layer (i.e., weakly doped n-GaN) in the p-GaN layer to improve the current spreading, another way for a better current spreading is to increase the electrical conductivity of the top contact layer, i.e., p+-GaN layer for the conventional InGaN/GaN LEDs. Therefore, we have grown the heavily doped n+-GaN layer on top of the p+-GaN layer to form a tunnel junction as the top contact layer. It is proved that the p+-GaN/n+-GaN tunnel junction has significantly improved the current spreading and thus the device performance. The additional voltage drop on the p+-GaN/n+-GaN tunnel junction can be relieved by inserting an InGaN layer between the p+-GaN and n+-GaN, i.e., a polarization tunnel junction which can also further improve the optical performance of the devices. We have also suggested InGaN/GaN LED devices with the electron overflow reduction through incorporating n-type InGaN layer and n-type AlGaN layer into the n-GaN layer, respectively, accompanied with which a mean-free-path model has been proposed and demonstrated to explain the mechanism of the electron overflow reduction through decreasing the electron mean free path. Lastly, as it has been mentioned previously, the low p-type conductivity in the InGaN/GaN LEDs has substantially limited the LED performance. The Mg dopants in the p-type GaN layers are only ionized by less than 1% at room temperature due to the large binding energy of Mg acceptors.In this dissertation, we have increased the hole concentration by inducing the three-dimensional (3D) hole gas without additional Mg doping. Our experiment shows that the generation of the 3D hole gas has nothing to do with any external dopants. More importantly, the 3D hole gas can be injected into the MQWs for the radiative recombination. Hence, the 3D hole gas can be another hole source besides the one donated by the ionized Mg dopants.