UCLA

Heterogeneous integration of semiconductors necessitates the understanding of their interfaces. This dissertation focuses on understanding thermal transport across wafer bonded interfaces by beginning from a fundamental understanding of wafer bonded Si|Ge interfaces, then by increasing the complexity by integrating Si|GaN. Finally, the possibilities of bonding GaN, AlN, and β-Ga2O3 are explored.First, we study the recovery of thermal conductivity in semiconductors that undergo amorphization due to ion implantation. Understanding this process is an important part of engineering wafer bonded interfaces that are subjected to ion bombardment to sputter off unwanted oxides prior to bonding. Depending on the implant energy and species, this ion bombardment treatment may lead to the amorphization of some semiconductor surfaces in the order of nanometers. Bonding these amorphized surfaces will alter the interfacial transport properties at the bonded interface so understanding the recrystallization process and thermal conductivity can be recovered is an important step to post bonding interface engineering. We find full recrystallization of amorphized silicon after annealing at 700 °C for 30 min, and full recovery of the thermal conductivity. Next, the Si-Ge system is used as a fundamental example of heterogeneous integration, which we can use to analyze the intricacies of interface engineering before introducing additional complexities when moving towards compound wide band gap semiconductors. Thermal boundary conductance (TBC) results show that the TBC of the as bonded sample is 47 ± 5 MW/(m2·K). The TBC for the sample annealed at 600 °C for 48 hours is 95 ± 5 MW/(m2·K), an improvement by a factor of two compared to the as-bonded interface. Recrystallization of the amorphous interface, interdiffusion, twist misorientation, and impurities at the interface all effect the thermal boundary conductance at bonded interfaces. Then, the evolution of structural and thermal interfacial properties of direct wafer bonded (0001) GaN to (100) Si with annealing is investigated. Direct wafer bonded GaN on Si with high thermal boundary conductance of 140 MW/(m2·K) is demonstrated in this work. Annealing at 450 °C for 7 hours and 700 °C for 24 hours was done to attempt to reconstruct the amorphous interface and to investigate the stability of the bonded interface at high temperatures. After annealing at 450 °C for 7 hours, a Ga-rich plane is observed across the interface near the surface of the Si in addition to SiNx formation at the original bonded interface. Further high temperature annealing (700 °C 24 hours) resulted in the formation of Ga-rich pyramidal features that form across the bonded interface in the silicon along (111) Si planes. While recrystallization was observed to have a beneficial impact in other bonded systems, the formation here of SiNx and Ga-rich pyramidal features in the Si have shown deleterious effects on thermal transport across the interface and a reduction in the measured TBC by a factor of two after annealing at 700 °C for 24 hours. Moving forward towards technologically relevant wide band gap materials like GaN, AlN, and β-Ga2O3, successful integration must be achieved before interface engineering is possible. GaN|AlN direct wafer bonding has been successfully achieved and preliminary characterization is reported. A ~1.5 nm interfacial region is observed, which is suspected to be caused by reconfiguration of the interface after a total anneal of 350 °C 22 hours, 600 °C 1 hour, and 800 °C 1 hour. No thicker amorphous or oxide interfacial layer commonly found in other bonding methods (surface activated bonding, plasma treatment, or other interfacial layers) are observed in this study. The thermal boundary conductance in the as-bonded case is 250 MW/(m2·K) Lastly, the chemical reaction between Al and various orientations of β-Ga2O3 are studied as a precursor to heterogeneous integration of β-Ga2O3. A mechanism for increased interdiffusion between Al and β-Ga2O3 along (010) β-Ga2O3 in contrast to (001) and (2 ̅01) oriented substrates is proposed. Theoretical studies of Al incorporation in (AlxGa1-x)2O3 alloys have predicted the preference of Al on octahedral sites in β-Ga2O3. A consecutive pathway of octahedral sites perpendicular to the interface presents itself in (010) β-Ga2O3 substrates that results in a thicker interfacial oxide layer. Under identical growth conditions, Al on (001) and (2 ̅01) β-Ga2O3 show thinner oxide layers that are sharper from the HRTEM. The rows of tetrahedral Ga sites act as barriers to interdiffusion of Al further into the β-Ga2O3 bulk.