UCLA

With bandgaps > 2 eV, the appealing electronic and optical properties of wide bandgap semiconductor materials such as β-Ga2O3, GaN, 4H-SiC, AlN, and diamond are promising candidates for next generation high power electronic devices. While these materials have been demonstrated to surpass silicon’s fundamental efficiency limits at high power and/or frequencies, their fullest potential has yet to be realized. This is primarily due to a lack of fundamental understanding of material processing and thermal management of heat generated during device performance. The focus of this dissertation will be addressing fundamental processing of heterogenous integration with focus on β-Ga2O3 as well as the origin of surface roughening for homoepitaxial GaN growth for ~kV device layers. Chemical mechanical polishing of β-Ga2O3 is a key issue for this emerging material. Smooth surfaces (< 0.5 nm rms) and subsurface damage-free (010) β-Ga2O3 were achieved with low-pressure (1 kPa) chemical mechanical polishing. Material removal rates ranged from ~200 μm/hr to 0.4 μm/hr depending on the lapping/polishing slurry used. With 3 orders of magnitude of control over the material removal rate, β-Ga2O3 can be efficiently lapped and polished to achieve damage-free substrates suitable for epitaxy, wafer bonding and layer transfer. Next, an important step in the successful transfer of β-Ga2O3 layers along a non-cleavage plane (010) is demonstrated through exfoliation via helium ion implantation for the first time. Helium implanted substrates were annealed at 200 �C followed by 500 �C to initiate helium bubble nucleation and promote bubble growth at the implanted projected range, respectively. Micron-sized surface blistering covering the entire implanted area was observed, confirming exfoliation. These observations match early reports of silicon blistering which, when combined with established direct wafer bonding practices, leads to large-scale transfer of controlled thickness β-Ga2O3� layers along non-cleavage-plane orientations. Thermal strain at elevated temperatures due to differences in coefficients of thermal expansion between materials is an important consideration for heterogeneous integration processes. The coefficients of thermal expansion of β-Ga2O3 were measured from single crystalline substrates because: (1) the high anisotropy of β-Ga2O3 and (2) technological relevance to heterogenous integration. All CTE values reported here are linear under the temperature regime relevant for epitaxial growth – between room temperature to 1000 �C. A technique to fabricate novel heterojunction interfaces referred to as surface activated bonding involves bombarding wafer surfaces with noble gas ions in ultrahigh vacuum prior to bonding. The resulting bonded interface typically consists of a few ~nm thick amorphous or damaged region. However, there is a lack of fundamental understanding of these interfaces. Si bonded to Si structures were fabricated as a model system to fundamentally understand bonded interfaces fabricated using this ion bombardment method. These thin amorphous interfaces are highly electrically resistive and impede electron transport across the bonded interfaces. However, post-bond annealing is demonstrated to recrystallize the bonded interface and form conductive interfaces at temperatures of 450 �C compared to ~1000 �C when utilizing other wafer bonding methods. Another model system used in this work were InP|InP bonded structures to study the impact of relative crystallographic orientation between the two wafers (twist misalignment) on the electronic transport across bonded interfaces. Twist misalignment between bonded wafers is found to impede electronic transport across the interface. The findings presented here suggest that misorientation plays an important role in the transport properties of interfaces. This is especially important for heterogenous integration of materials that may not have the same crystal structure where minimizing mismatch between orientations corresponds to minimizing tilt and twist misorientation. With the development of polishing and exfoliation for β-Ga2O3, a thin film of (201) β-Ga2O3 was exfoliated and transferred to (0001) 4H-SiC. The (201) orientation of β-Ga2O3 best matches the basal plane of 4H-SiC to minimize tilt misorientation; and the in-plane directions were aligned such that [010] β-Ga2O3 ∥ [1120] 4H-SiC to minimize twist misorientation. The surface activated bonding technique was utilized for bonding, which induced a thin ~nm amorphous interfacial region at the bonded interface. Annealing the bonded structure at 800 �C for 1 hour: (1) removed residual strain in the exfoliated β-Ga2O3 layer due to the ion implant, (2) reduced lattice mosaicity in the β-Ga2O3 film, and (3) recrystallized the amorphous bonded interface. The thermal transport across the bonded interface increased with the change in structural characteristics. The thermal conductivity of the transferred β-Ga2O3 layer doubled and the thermal boundary conductance improved by ~20% after the anneal. GaN is more technologically matured than β-Ga2O3, but one of the major challenges with GaN is maintaining smooth surfaces during epitaxial growth (~tens of microns) to fabricate high power device layers and to facilitate bonding and layer transfer. It is found that localized lattice distortions in GaN substrates serve as nucleation sites for macro-steps and macro-terraces. After nucleating, these macro-features grow laterally along the surface and coalesce, leading to significant roughening of the wafer surface. While previous studies focused on substrate miscut as a means to control macro-feature formation, localized lattice tilt from defects is another important contributor to macro-feature formation. Hence, near zero-defect GaN substrates will be necessary for achieving thick GaN device layers on the order of tens to hundreds of microns for ~kV to ~20 kV applications.