Investigation and Improvement of Performance in GaN-based Optoelectronic and Microelectronic Devices
III-Nitride materials has been intensively studied over the past few decades. It has remarkable material properties not only for optoelectronic devices but also application in microelectronic devices. The III-Nitride materials have excellent properties such as wide direct bandgap, high breakage voltage, high electron mobility, and high operation frequency. These properties make III-Nitride materials very attractive for application in blue/ultraviolet light-emitting diodes (LEDs), blue vertical-cavity surface-emitting lasers (VCSELs), and high electron mobility transistors (HEMTs). Although these devices have been development by different techniques, enhanced performance is still essential. The motivation of this work is to figure out the method that can be used to improve the device performance sufficiently. First part, a tapered AlGaN electron blocking layer (EBL) with step-graded aluminum composition is analyzed in blue LED numerically and experimentally. The simulation results demonstrated that such tapered structure can effectively enhance the hole injection efficiency as well as the electron confinement. Consequently, the LED with a tapered EBL grown by metal-organic chemical vapor deposition (MOCVD) exhibits reduced efficiency droop behavior of 29% as compared with 44% for original LED, which reflects the improvement in hole injection and electron overflow in our design. In the second part, blue LEDs with graded-composition AlGaN/GaN superlattice (SL) EBL were designed and grown by metal-organic chemical vapor deposition. The simulation results demonstrated that the LED with a graded-composition AlGaN/GaN SL EBL have superior hole injection efficiency and lower electron leakage over the LED with a conventional AlGaN EBL or with a normal AlGaN/GaN SL EBL. Consequently, the efficiency droop can be alleviated to be about 20% from maximum at injection current of 15 to 120 mA, which is smaller than that for conventional AlGaN EBL (30%). The corresponding experimental results also confirm that the use of a graded-composition AlGaN/GaN SL EBL can markedly enhance the light output power by 60%. In the third part, flip-chip ultraviolet light-emitting diodes (FCUV-LEDs) on patterned sapphire substrate (PSS) at 375 nm were grown by an atmospheric pressure MOCVD. A specialized reactive plasma deposited (RPD) AlN nucleation layer was utilized on the PSS to enhance the quality of the epitaxial layer. By using high-resolution X-ray diffraction, the full-width at half-maximum of the rocking curve shows that the FCUV-LEDs with RPD AlN nucleation layer had better crystalline quality when compared to conventional GaN nucleation samples. As a result, a much higher light output power was achieved. The improvement of light output power at an injection current of 20 mA was enhanced by 30%. Further photoluminescence measurement and numerical simulation confirm such increase of output power can be attributed to the improvement of material quality and light extraction In the fourth part, the design and fabrication of GaN-based VCSELs with a composition-graded electron blocking layer (GEBL) are revealed experimentally and theoretically. It has been demonstrated that the laser output performance is improved by using a GEBL when compared the typical VCSEL structure with rectangular EBL. The output power obtained at 20 kA/cm2 is enhanced by a factor of 3.8 by the successful reduction of threshold current density from 12.6 kA/cm2 to 9.2 kA/cm2 and the enlarged slope efficiency. Numerical simulation results also suggest that the improved laser output performances are due mainly to the reduction of electron leakage current and the enhanced hole injection efficiency in the multiple-quantum-well (MQW) active region. In the fifth, the carbon-doped AlN/GaN superlattice (AlN/GaN SL) structure was introduced into the epitaxial growth of AlGaN/GaN HEMTs on Si (111) substrates, which could suppress the leakage from channel to substrate. Compared with conventional AlN/AlGaN double under layer (DUL) structure for AlGaN/GaN HEMTs, the results of electric properties imply that the vertical leakage can be dramatically decreased as two and half times as the carbon-doped AlN/GaN SL structure was introduced. Therefore, a threshold voltage of 2.0 V and maximum drain current of 175 mA/mm at the VGS of 2 V could be achieved. The output of this dissertation provide a great help on enhancing performance in GaN-based optoelectronic and microelectronic devices.
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