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.

Nanocarriers that localize a therapeutic to a disease site and release it “on-demand” via the clinician’s control will mitigate the adverse effects that reduce a patient’s quality of life while undergoing oncology treatment. Moreover, magnetically actuated drug delivery carriers are appealing platforms in next-generation targeted medicine, yet these carriers must be compatible with scalable fabrication techniques to realize their clinical translation. In this dissertation, a magnetically capped porous silicon nanocomposite (APTESPSi@Fe3O4), that responds to physiologically relevant temperatures, was developed using cost-effective, highly scalable methods such as electrochemical etching. Fourier transform infrared spectroscopy (FTIR), CHNS elemental analysis, and zeta potential confirmed that accelerated hydrolysis at 45 �C altered the porous silicon surface chemistry. This hydrolysis-mediated electrostatic degradation between the porous silicon and Fe3O4 caps translated to a thermoresponsive release behavior in dissolution studies with sorafenib (SFN), where minimal drug was released at room temperature and 37 �C, while an enhanced release occurred at 45 �C and 50 �C. The magnetic heat dissipation capabilities with application of an alternating magnetic field (AMF) was calculated by the specific absorption rate (SAR) through calorimetry and magnetic susceptibility measurements. Comparing these two methods revealed that the electrostatic interactions between the porous silicon and Fe3O4 do not hinder the Brownian relaxation and heat dissipation. The nanocomposite and its components demonstrated high cytocompatibility after 24 hours with RAW 246.7, MDA-MB-231, and HepG2 cells, but not with MCF-7. High cytocompatibility was also observed when the cells incubated with particles were heated to 45 �C for 15 min followed by 37 �C for the remaining 6 hour incubation period. Porous silicon and its nanocomposite improved the SFN solubility in in vitro studies with MDA-MB-231 and HepG2, resulting in increased anticancer activity in comparison to the free drug. Moreover, the anticancer activity was readily controlled from the magnetic nanocomposite by modulating the amount of SFN released with temperature. Confocal microscopy and flow cytometry showed a higher uptake of the amine-modified porous silicon in comparison to the magnetic nanocomposite in MDA-MB-231 cells. The temperature increase to 45 �C showed a reduced particle uptake, yet future studies monitoring the fluorescence from the free drug rather than the nanocarrier will prove useful. This novel system has laid the groundwork for a promising tool for clinicians to lessen the burden that millions of cancer patients face as they receive treatment.

Current limitations in both single crystal and polycrystalline (ceramic) solid state laser technologies for high power applications stem from thermal effects that cause degradation in both lasing efficiency and beam quality. YAG and Y2O3 have favorable material properties for producing these high power lasers. The objective of this dissertation was to formulate chemical mechanical polishing (CMP) processes for YAG and Y2O3 resulting in smooth surfaces, (< 1 nm RMS roughness), defect free, and subsequently suitable for direct bonding. The CMP process has been used in conjunction with surface activation to form YAG-YAG and Y2O3-Y2O3 bonded elements showing a proof of concept for the fabrication of composite laser elements. YAG single crystals were successfully polished with a 70-nm colloidal silica solution containing NaOH (pH 9.9). X-ray photoelectron spectroscopy measurements provided insights to the chemical impact of the NaOH. After NaOH exposure, YAG showed complete removal of Al from tetrahedral sites and a change in the Y3/2 and Y5/2 peak area ratios, indicating that the surface of YAG was sufficiently modified to allow the silica particles to abrade the byproduct surface layer. The final surface roughness after polishing was measured at 0.1 nm RMS roughness with no scratches deeper than 0.3 nm. YAG single crystals were also polished with a 70-nm Al2O3 slurry containing NaOCl (pH 11.4), however AFM measurements showed that the surface produced, 0.5 nm RMS roughness with no scratches deeper than 6.0 nm, was not as good as polishing with the colloidal silica slurry which was attributed to the effect of the chemical action of NaOH with the YAG surface. Polishing polycrystalline Y2O3 required a slurry with less aggressive chemical reactions, so the NaOCl/Al2O3 slurry was used to successfully polish the substrates to RMS roughness values of 0.5 nm and scratches no deeper than 1.0 nm. Triple axis diffraction rocking curves and double crystal x-ray diffraction imaging were used to demonstrate that the CMP process also removed subsurface damage that was present in the as-supplied material. The triple axis technique was also successfully employed for the first time to demonstrate that the polycrystalline Y2O3 subsurface damage was also significant reduced after the CMP process.

After CMP and surface treatment, YAG-YAG and Y2O3-Y2O3 were bonded together and annealed at 1425 ?C with no applied pressure. Cross section and plan view high resolution transmission electron images of the YAG-YAG bonded interface revealed a defect free bonding interface. Light transmission measurements showed that 90% of the interface area was bonded at room temperature contact and was further strengthened with annealing at 1425 ?C for 72 hours. These conditions possess a much lower thermal budget than those previously shown to be required to bond YAG without a CMP step. The CMP and surface treatment processes that have been developed are applicable towards the creation of solid state laser composite elements and enable further progress and development for high power laser applications.

Cleave Engineered Layer Transfer (CELT) is a technique which enables transferring large scale high quality epitaxial layers and devices onto alternative substrates through mechanically engineered substrates. The objective of this dissertation was to formulate a methodology to understand whether the formation of a porous III-V layer, with focus here on indium phosphide, can be feasibly employed to transfer III-V device layers with preserved crystallinity from the host substrate. The porous structures were prepared on (001) oriented InP substrates by electrochemical etching in diluted hydrochloride acid (HCl). The porosity of the layers was reproducibly controlled by etching parameters, such as current density and electrolyte concentration, and a numerical relationship between layer porosity, p and its Young's modulus, E as E = 87(1-p)⁴ was determined. This relationship was important for relating the etching parameters to the mechanical strength of the buried layer. Annealing (temperature range 450 °C-650 °C and time range 10 min- 8 hours) modified the porous layer morphology and was found to be beneficial for subsequent epitaxial growth and layer fracture. The morphology of the porous layers was monitored by scanning electron microscopy (SEM) and the epitaxial layers were studied by high resolution X-ray diffraction (XRD) and transmission electron microscopy (TEM). Nano-indentation was used to measure the Young's modulus of the porous InP layers. In some cases, dual porous layers were produced by changing electrolyte concentration and current density during the porous formation process. The top layer is less porous than the buried layer, with a surface pore fraction of less than 30%. The deeper, more porous buried layer was shown to facilitate fracture in the deeper layer and not through the denser, near surface layer, thus controlling the layer fracture formation through the buried layer only.

Epitaxial deposition of InP on the dual layer porous structure was achieved. The overlying epitaxial InP layer was found to possess a low threading dislocation density and demonstrated complete coalescence of the InP lattice. Specifically, TEM analysis showed the epitaxial layers were single crystalline and lattice registered to the porous layer, and neither grain boundaries nor threading locations were detected over a 35 μm² sampling areas, indicating a threading dislocation density lower than 3�10⁶ cm^-2. In addition, the porous structure was sufficiently weakened after growth and /or annealing to promote facile transfer of the top layer to alternative substrates. Over 1 cm² InP layers (the entire porous region formed) were successfully transferred onto both flexible PDMS substrates and glass substrates. Finally, fractured porous materials on both sides was shown to be readily removed by chemical mechanical polishing, indicating a pathway for employing the CELT process in a reusable process to mitigate the high costs of InP and other III-V substrates.

Limitations in single-crystal growth technology have led to the development of transparent

Polycrystalline Laser Materials (PLMs) as viable alternatives towards fabricating large

dimension laser gain media having engineered dopant profiles to improve the thermo-optic

properties and continue power scaling of solid-state laser systems operating in the 1 - 5 μm

wavelength range. Bulk scattering due to non-uniform refractive index distribution is the

primary loss mechanism in the PLMs, resulting in the degradation of laser performance and

lower output power. The objective of this dissertation was to formulate a methodology for fast,

reliable and accurate identification and characterization of bulk scatter in Erbium (Er) and

Neodymium (Nd) doped YAG (Yttrium Aluminum Garnet - Y₃Al₅O₁₂) and Yttria (Yttrium

Oxide - Y₂O₃) PLMs. Scanning Electron Microscopy, Transmission Electron Microscopy, and Confocal Laser Scanning Microscopy were used for material characterization and investigation

of PLM microstructures at various stages of fabrication, thus identifying the source of refractive index inhomogeneities, which result in bulk scattering. Three optical characterization

techniques, Transmitted Beam Wavefront Profiling (TBWP), Angle Resolved Scatter (ARS)

measurements, and Schlieren Imaging, to identify bulk scatter in PLMs, are developed. TBWP

was able to directly and quickly image the distortions introduced to a propagating laser beam

caused by the presence of bulk scattering in the PLMs. ARS was able to map the distribution of

the scattered intensity in space, and was found to be very sensitive for comparing samples. A

modified white light Schlieren imaging setup utilizing variable focusing capability, demonstrated

high sensitivity for directly imaging local spatial variations in refractive index across and

through the entire PLM regardless of dimensions. Finally, by comparing laser performance of

0.9% Nd:YAG single crystal, high quality and low quality PLMs, with results from TBWP and

Schlieren Images, the utility of characterization techniques towards identifying bulk scatter and

assessing the optical quality are validated. The methods developed are applicable towards the

characterization of any transparent material exhibiting bulk scatter.

Heat management problem limits the potential of further development of high-power devices. Diamond, with the highest thermal conductivity among all the materials, can be integrated near the hot spot region as a heat management component. The goal of this dissertation is to study grain morphology and thermal properties in polycrystalline diamond across boundaries using advanced characterization techniques. This work is anticipated to lead to an understanding of how to control and improve thermal transport in diamond. Diamond in device integration is achieved by chemical vapor deposition (CVD) at 750˚C with methane as the carbon source. A hydrogen plasma is used in the CVD process to remove any non-diamond carbon. In this research, transmission electron microscopy (TEM) and electron backscatter diffraction (EBSD) were used for grain morphology analysis. Both of the techniques can be used to separate the individual grains, which leads to an accurate size determination. In-plane thermal conductivity was measured with Raman thermography. Cross-plane thermal properties, including thermal boundary resistance (TBR), were measured with Time-domain thermoreflectance (TDTR). The combination of the advanced techniques has been done in several aspects to improve the diamond integration for better thermal transport.

The challenging issue for polycrystalline CVD diamond is the relatively lower thermal conductivity compared with single crystal diamond due to phonon scattering at grain boundaries, and the thermal conductivity continues to decrease closer to the nucleation region. At the interface, where the grain starts to nucleate and grow, the diamond structure is composed of small size, randomly oriented diamond grains. In this research, a larger diamond seed size is shown to increase the average diamond grain size near the nucleation region, which leads to a higher in-plane thermal transport for 1 μm diamond films. Also, with the combination of EBSD and spatially resolved TDTR, ~ 60% of thermal conductivity reduction is directly observed at the grain boundaries. While in the traditional model, the grain boundaries act as discrete thermal boundary resistances, this research has pointed out the impact is not restricted to the grain boundaries and can be observed in up to ~ 10 μm nearby region. Another challenge exists when integrating diamond on GaN-based high-power devices. The presence of hydrogen plasma at a high temperature can etch into the GaN substrate and cause a rough Diamond/GaN interface, which could increase the TBR value. In this research, an ultra-thin SiN is shown to be an effective protection layer, and a low TBR value of 9.5 m2K/GW is achieved.

GaAs-based compositionally graded buffer layers were investigated to determine the strain relaxation as a function of growth as well as whether ordering plays a role in the change in the strain relaxation mechanisms. X-ray reciprocal space mapping of (004) and (224) lattice points was employed to characterize the composition, tilt, lattice parameter and strain state of the step-grade layers. No CuPt ordering was observed in either x-ray diffraction or TEM, and the lack of observed ordering shows that the changes in the strain relaxation mechanisms do not necessarily depend on ordering.

Broadband multilayer antireflection coatings (ARCs) are keys to improving solar cell efficiencies. The goal of this dissertation is to optimize the multilayer Al2O3/TiO2/Al2O3 ARC designed for a III-V space multi-junction solar cell with understanding influences of post-annealing and varying deposition parameters on the optical properties. Accurately measuring optical properties is important in accessing optical performances of ARCs. The multilayer Al2O3/TiO2/Al2O3 ARC and individual Al2O3 and TiO2 layers were characterized by a novel X-ray reflectivity (XRR) method and a combined method of grazing-incidence small angle X-ray scattering (GISAXS), atomic force microscopy (AFM), and XRR developed in this study. The novel XRR method combining an enhanced Fourier analysis with specular XRR simulation effectively determines layer thicknesses and surface and interface roughnesses and/or grading with sub-nanometer precision, and densities less than three percent uncertainty. Also, the combined method of GISAXS, AFM, and XRR characterizes the distribution of pore size with one-nanometer uncertainty. Unique to this method, the diffuse scattering from surface and interface roughnesses is estimated with surface parameters (root mean square roughness σ, lateral correlation length ξ, and Hurst parameter h) obtained from AFM, and layer densities, surface grading and interface roughness/grading obtained from specular XRR. It is then separated from pore scattering. These X-ray scattering techniques obtained consistent results and were validated by other techniques including optical reflectance, spectroscopic ellipsometry (SE), glancing incidence X-ray diffraction, transmission electron microscopy and energy dispersive X-ray spectroscopy.

The ARCs were deposited by atomic layer deposition with standard parameters at 200 �C. The as-deposited individual Al2O3 layer on Si is porous and amorphous as indicated by the combined methods of GISAXS, AFM, and XRR. Both post-annealing at 400 �C for 40 min in air and varying ALD parameters can eliminate pores, and lead to consistent increases in density and refractive index determined by the XRR method, SE, and optical reflectance measurements. After annealing, the layer remains amorphous. On the other hand, the as-deposited TiO2 layer is non-porous and amorphous. It is densified and crystallized after annealing at 400 �C for 10 min in air. The multilayer Al2O3/TiO2/Al2O3 ARC deposited on Si has surface and interface roughnesses and/or grading on the order of one nanometer. Annealing at 400 �C for 10 min in air induces densification and crystallization of the amorphous TiO2 layer as well as possible chemical reactions between TiO2 and Si diffusing from the substrate. On the other hand, Al2O3 layers remain amorphous after annealing. The thickness of the top Al2O3 layer decreases – likely due to interdiffusion between the top two layers and loss of hydrogen from hydroxyl groups initially present in the ALD layers. The thickness of the bottom Al2O3 layer increases, probably due to the diffusion of Si atoms into the bottom layer. In addition, the multilayer Al2O3/TiO2/Al2O3 ARC was deposited on AlInP (30nm) / GaInP (100nm) / GaAs that includes the topmost layers of III-V multi-junction solar cells. Reflectance below 5 % is achieved within nearly the whole wavelength range of the current-limiting sub-cell. Also, internal scattering occurs in the TiO2 layer possibly associated with the initiated crystallization in the TiO2 layer while absent in the amorphous Al2O3 layers.

Aluminium Nitride (AlN) thin-films have been long used as a buffer layer on substrates to grow Gallium Nitride (GaN) for high power transistors. Researchers recently have started studying using AlN itself as an electronic material due to its higher band gap compared to GaN and applications in UV light source for photolithography systems. One of the main challenges of growing AlN on traditional substrates like Silicon (Si) is the large lattice mismatch and difference in crystal structures. Both these effects lead to strained and lower quality thin-films.This study discusses the procedure for performing high-resolution X-ray diffraction (HRXRD) measurements, discussing equipment setup and choosing the diffraction peaks that are most suitable for this analysis. Symmetric and asymmetric scans were performed to calculate out-of-plane and in-plane strains respectively. Analysis of the full-width at half-maximum (FWHM) was done to qualitatively compare the film-quality for different samples. This study analyzes the strain and film quality for AlN thin-films grown on Si. Two sets of samples have been analyzed – one with AlN thin-films grown on bare Si and second with a GaN nanowire interlayer between AlN and Si. The GaN nanowire layer serves to reduce the strain present in AlN by lowering the lattice mismatch compared to Si. All measurements have been performed using (HRXRD). Results show that samples with GaN interlayer have an almost completely relaxed AlN layer with better film quality compared to those grown on bare Si. The strain ranges from 0% to 0.3% for samples with GaN interlayer while AlN on bare Si sample has a strain of about 1%. For samples with the GaN layer, different thicknesses have been analyzed. Results indicate that the thicker layers are more strained and have a lower film quality. AlN films grown on bare Si were characterized for surface roughness and thickness using X-ray reflectivity (XRR). Data shows that the surface has a roughness of about 3 nm with the films being 200 nm thick. The presence of a GaN nanowire interlayer has shown to improve the AlN thin-films grown on Si, further paving the way for this material to be used in electronic applications.

This study investigates the mechanisms of helium implantation induced blistering andexfoliation in lithium niobate (LiNbO3). Wafers of LiNbO3 were implanted to a dose of 2.3×1016 He+ /cm2 and an accelerating voltage of 250 keV. The wafer was cooled such that it remained at room temperature during the implantation to limit helium diffusion or bubble growth due to the heating caused by the ion implantation process. The implant profile and exfoliation behavior of the samples for various annealing recipes were investigated using high resolution X-Ray diffraction (HRXRD), optical microscopy and contact profilometry. It was found that a low temperature “defect nucleation” anneal at 120 ˚C followed by a high temperature “bubble growth” anneal at 200 ˚C caused millimeter-scale large-area exfoliation of thin-film lithium niobate with a thickness of 800 nm, which is the projected range of the helium ion implantation done at 250 keV. In comparison, a single anneal at 200 ˚C caused µm-scale small-area exfoliation also referred to as blistering [1]. Large-area exfoliation is preferred over blistering, as it facilitates the next processing step of chemical mechanical polishing (CMP). Also, the long low temperature annealing improves the interfacial bond strength of the bonded wafers in the “Smart Cut” technique.