Gallium Nitride Based Heterostructure Interband Tunnel Junctions.

摘要

This thesis describes the design, molecular beam epitaxy growth, fabrication and characterization of Gallium Nitride (GaN)-based interband tunnel junctions (TJs) surpassing the state-of-the-art device performance. GaN and AlGaN-based TJs are attractive for hole injection in visible, and ultraviolet light emitters, respectively. Tunnel junctions enable monolithic integration of multiple active regions for multi-color light emitters and multi-junction solar cells. A major outstanding issue with the visible emitters is the efficiency droop problem, which refers to the reduction in efficiency of a light emitting diode at higher operating current density. Efficient TJs are attractive to overcome this issue as it enables epitaxial cascading of identical active regions, and a low current-high voltage operation of the cascaded structure. In such a structure, the carriers are regenerated at the tunnel junction sites and each of the individual active regions can be operated at its peak efficiency. In this work, two nanoscale heterostructure band engineering approaches, namely, polarization engineering and midgap states assisted tunneling, are used to demonstrate low resistance Gallium Nitride tunnel junctions. In the case of the polarization-based approach, the high spontaneous and piezoelectric polarization sheet charge at the GaN/InGaN heterointerface is utilized to create large band bending over a few nanometers, thereby reducing the tunneling barrier width. Such polarization engineered GaN/InGaN/GaN tunnel junctions are used to demonstrate record reverse current and tunneling under forward bias, leading to the first observation of interband tunneling-related negative differential resistance in III-nitrides. Tunnel hole injection in a GaN PN junction with a tunneling specific resistivity as low as 10-4 Ohm cm2 is achieved using this approach. The second approach involves the use of embedded Gadolinium Nitride (GdN) nano-islands for inter-band tunneling in GaN. By creating midgap states for tunneling, the tunnel barrier is effectively halved, leading to an increase in the tunneling current. Using this approach, a tunneling specific resistivity of 10-3 Ohm cm2 is obtained. GdN-based and InGaN-based TJs are then regrown on a blue light emitting diode (LED) wafer for non-equilibrium tunnel injection of holes. The regrowth interface depletion is found to increase the forward voltage of the tunnel junction LEDs. Finally, tunnel injection of holes is extended to wider band gap Al0.3Ga0.7N, which is attractive for incorporation in ultra-violet and deep ultra-violet emitters. The low tunneling resistance achieved in this work is promising for efficient hole injection in UV/deep UV light emitters and epitaxial cascaded light emitters to eliminate efficiency droop.