Novel Materials and Fabrication Techniques for Enhanced Current Spreading and Light Extraction in High Efficiency Light-emitting Diodes
Although solid-state lighting based on III-nitride light-emitting diodes (LEDs) is on track to become the dominant technology for lighting our world, we have yet to reach the efficacy limit in these devices. Many challenges remain to be solved to achieve this goal. In this thesis, we discuss novel materials and fabrication techniques which can be used to enhance the efficiency of LEDs through improving lateral current spreading through p-type GaN and increasing light extraction.
Presented in the first part of this thesis is the growth and characterization of hydrothermal ZnO thin films. The effect of growth conditions on the morphology and optoelectronic properties of the films will be discussed. In particular, we studied the effects that group III dopants (Al, Ga, and In) have when introduced into the hydrothermal ZnO thin films. The Ga doped film showed the lowest resistivity, with a resistivity of 1.94 mΩ cm for films doped with 0.4 at.% Ga. Undoped ZnO films showed the lowest optical absorption coefficient of 441 cm-1 at 450 nm. This study represents the first time all three dopants have been systematically compared to one another.
The second part of the thesis focuses on an alternative approach to forming transparent contacts to p-GaN. This involved the regrowth of highly doped n-type GaN using ammonia molecular beam epitaxy (NH3 MBE) to form epitaxial tunnel junction contact. We discuss several aspects of the growth and performance of regrown TJ contacts on p-n diode structures as well as InGaN/GaN LEDs. Improved turn-on voltages and reduced series resistances have been realized by depositing highly doped Si-doped n-type GaN using MBE on polarization enhanced p-type InGaN contact layers grown using MOCVD. We compared the effects of different Si doping concentrations, and the addition of p-type InGaN on the forward voltages of p-n diodes and LEDs. It was found that increasing Si concentrations from 1.9x1020 to 4.6x1020 cm-3 and including a highly doped p-type InGaN at the junction both contribute to the narrowing of the depletion width, lowering series resistance from 4.2x10-3 to 3.4x10-3 Ω cm2 and decreasing turn-on voltages of the diodes.
The third, and last part, of this thesis, will focus device results utilizing the alternative current spreading contacts discussed in the previous chapters. LED devices fabricated using doped ZnO films will be analyzed as well as LEDs utilizing MBE n-GaN TJ contacts on blue InGaN flip-chip triangular LEDs grown on both free-standing GaN and patterned sapphire substrates. The performance of LEDs with Ga-doped ZnO (Ga:ZnO) and Sn-doped In2O3 (ITO) current-spreading layers (CSLs) has been evaluated at high injection current densities. LEDs with electron beam-hydrothermally deposited Ga:ZnO transparent CSLs showed improved performance compared to electron beam deposited ITO at all current densities. External quantum efficiency and wall plug efficiency were both higher for blue emitting LEDs with ZnO. Luminous efficacy increased greatly for the ZnO-based CSL with a peak value of 113 lm/W compared to 82 lm/W for the ITO-based CSL, a 37% improvement. Issues with metal organic chemical vapor deposition (MOCVD), device processing, and device performance are discussed as well.