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Surfaces of (001) p-type silicon substrates were bombarded with either Ar or Kr ions prior to direct wafer bonding using an EVG© ComBond© high-vacuum wafer-bonding system.1 Previously, we showed that this ion bombardment treatment induces a thin ~nm amorphous region at the bonded interface.2 In this study, we extend upon this previous effort towards a fundamental understanding of this amorphous interfacial region by examining the dependence of the bonded interface’s properties with two different ion species. The ion energies used spanned from a factor of 0.5 to 2.5 times of a baseline energy E0. X-ray reflectivity measurements were performed on single unbonded wafers that were ion bombarded with either Ar or Kr to determine the thickness of the amorphous region as well as its mass density compared to bulk Si. Wafers treated with Ar showed no surface mass density dependence with ion energy, while wafers treated with Kr did show an ion energy dependence. While the Ar treatment reduces the surface density to ~80% of bulk Si density regardless of the energy used, the Kr treatment reduces the surface density to as low as ~70% of bulk Si density at 2.5⋅E0. Furthermore, for a given energy, the Ar treatment resulted in thicker layers than the Kr treatments, which is consistent with SRIM3 simulations because Kr is a heavier species than Ar. Current-voltage measurements were also used to probe the electrical properties of the bonded interfaces. Band structure modeling of the current-voltage measurements revealed that the amorphous region can be modeled with a step barrier that spans the thickness of the amorphous region. References Flötgen, et al., ECS Trans., 64(5), 103 (2014). E. Liao, et al., ECS Trans., 86(5), 55 (2018). F. Ziegler, et al., The Stopping Range of Ions in Solids vol. 1, (1985). Figure 1

Despite the exceptional thermal and mechanical functionalities of diamond, its superlative properties are highly subject to the presence of point defects, dislocations, and interfaces. In this study, polycrystalline diamond is ion implanted with C3+, N3+, and O3+ ions at an energy of 16.5 MeV, producing an amorphous layer at the projected range and a damaged crystalline region between the surface and amorphous layer. Using time-domain thermoreflectance in combination with thermal penetration depth calculations based upon the multilayer heat diffusion equation, it is determined that reductions in the thermal conductivity can span nearly two orders of magnitude while still maintaining a polycrystalline structure within the regions thermally probed. Dynamical diffraction simulations of high-resolution x-ray diffraction measurements demonstrate the formation of a strained layer localized at the end of range, with much lower levels of strain near the surface. Furthermore, within the polycrystalline region above the amorphous layer, the average number of displacements-per-atom from the ion irradiation is found to be 1 % , with mass impurity concentrations much less than 1%. These low defect concentrations within the thermally probed region demonstrate the remarkably large impact that dilute levels of defects from the ion implantation can have on the thermal conductivity of diamond.

Record DC power has been demonstrated in AlGaN/GaN high electron mobility transistors fabricated using a substrate replacement process in which a thick diamond substrate is grown by chemical vapor deposition following removal of the original Si substrate. Crucial to the process is a ~30 nm thick SiN interlayer that has been optimized for thermal resistance. The reductions obtained in self-heating have been quantified by transient thermoreflectance imaging and interpreted using 3D numerical simulation. With a DC power dissipation level of 56 W/mm, the measured average and maximum temperatures in the gate-drain access region were 176 °C and 205 °C, respectively.

The development of electronic devices, especially those that involve heterogeneous integration of materials, has led to increased challenges in addressing their thermal operational temperature demands. The heat flow in these systems is significantly influenced or even dominated by thermal boundary resistance at the interface between dissimilar materials. However, controlling and tuning heat transport across an interface and in the adjacent materials has so far drawn limited attention. In this work, we grow chemical vapor-deposited diamond on silicon substrates by graphoepitaxy and experimentally demonstrate tunable thermal transport across diamond membranes and diamond-silicon interfaces. We observed the highest diamond-silicon thermal boundary conductance (TBC) measured to date and increased diamond thermal conductivity due to strong grain texturing in the diamond near the interface. Additionally, nonequilibrium molecular dynamics simulations and a Landauer approach are used to understand the diamond-silicon TBC. These findings pave the way for tuning or increasing thermal conductance in heterogeneously integrated electronics that involve polycrystalline materials and will impact applications including electronics thermal management and diamond growth.

Post-implantation damage and defect characterization of multicycle rapid thermal annealing (MRTA) activated Mg+ implanted (0001) GaN on (110) sapphire substrate was investigated using high resolution X-ray scattering and transmission electron microscope techniques. The initial implant-induced isolated defects produced a pseudomorphic strain that matches the calculated recoil damage profile in the GaN - along the [0001] direction. Triple axis X-ray diffraction showed removal of the implant-induced elastic strain during two different MRTA annealing processes. An improvement in crystalline quality after the different MRTA processes was also noted, indicating that the MRTA processes can reduce crystalline strain and defects in as-grown material. For the different MRTA processes, an improvement in electrical conductivity was correlated with a lower (0004) triple axis rocking curve width and with a reduced presence of extended defects observed from electron microscopy measurements. The extended defects were identified to be circular defects with an observable screw component. The defect density was on the order of 1010 cm−2. While both MRTA processes led to a high activation efficiency, a higher residue defect density was correlated with a greater X-ray scattering rocking curve width of the symmetric (0004) reflection, which was further correlated with a lower hole mobility (20 cm2/V⋅s vs 40 cm2/V⋅s).

A unified understanding of interfacial thermal transport is missing due to the complicated nature of interfaces which involves complex factors such as interfacial bonding, interfacial mixing, surface chemistry, crystal orientation, roughness, contamination, and interfacial disorder. This is especially true for metal nonmetal interfaces which incorporate multiple fundamental heat transport mechanisms such as elastic and inelastic phonon scattering as well as electron phonon coupling in the metal and across the interface. All these factors jointly affect thermal boundary conductance (TBC). As a result, the experimentally measured interfaces may not be the same as the ideally modelled interfaces, thus obfuscating any conclusions drawn from experimental and modeling comparisons. This work provides a systematic study of interfacial thermal conductance across well controlled and ultraclean epitaxial (111) Al parallel (0001) sapphire interfaces, known as harmonic matched interface. A comparison with thermal models such as atomistic Green s function (AGF) and a nonequilibrium Landauer approach shows that elastic phonon scattering dominates the interfacial thermal transport of Al sapphire interface. By scaling the TBC with the Al heat capacity, a nearly constant transmission coefficient is observed, indicating that the phonons on the Al side limits the Al sapphire TBC. This nearly constant transmission coefficient validates the assumptions in AGF and nonequilibrium Landauer calculations. Our work not only provides a benchmark for interfacial thermal conductance across metal nonmetal interfaces and enables a quantitative study of TBC to validate theoretical thermal carrier transport mechanisms, but also acts as a reference when studying how other factors impact TBC.

Aluminum nitride (AlN) has garnered much attention due to its intrinsically high thermal conductivity. However, engineering thin films of AlN with these high thermal conductivities can be challenging due to vacancies and defects that can form during the synthesis. In this work, we report on the cross-plane thermal conductivity of ultra-high-purity single-crystal AlN films with different thicknesses (∼3-22 μm) via time-domain thermoreflectance (TDTR) and steady-state thermoreflectance (SSTR) from 80 to 500 K. At room temperature, we report a thermal conductivity of ∼320 ± 42 W m-1 K-1, surpassing the values of prior measurements on AlN thin films and one of the highest cross-plane thermal conductivities of any material for films with equivalent thicknesses, surpassed only by diamond. By conducting first-principles calculations, we show that the thermal conductivity measurements on our thin films in the 250-500 K temperature range agree well with the predicted values for the bulk thermal conductivity of pure single-crystal AlN. Thus, our results demonstrate the viability of high-quality AlN films as promising candidates for the high-thermal-conductivity layers in high-power microelectronic devices. Our results also provide insight into the intrinsic thermal conductivity of thin films and the nature of phonon-boundary scattering in single-crystal epitaxially grown AlN thin films. The measured thermal conductivities in high-quality AlN thin films are found to be constant and similar to bulk AlN, regardless of the thermal penetration depth, film thickness, or laser spot size, even when these characteristic length scales are less than the mean free paths of a considerable portion of thermal phonons. Collectively, our data suggest that the intrinsic thermal conductivity of thin films with thicknesses less than the thermal phonon mean free paths is the same as bulk so long as the thermal conductivity of the film is sampled independent of the film/substrate interface.

Diamond seeds were assessed for their role in the heterogeneous nucleation for diamond films deposited on silicon using chemical vapor deposition. Two diamond seed sizes – 4 nm and 20 nm – were studied. The study revealed that the larger seed size, even when with a smaller seed density, produces a larger grain size near the interface region, and led to a higher in-plane thermal conductivity as measured by Raman thermography. By fine control of the seed size and density, thermal conductivity near the nucleation region can therefore be improved. This demonstrates that the seeding condition is critical to diamond film growth for thermal applications in electronic devices.

As wide bandgap electronic devices have continued to advance in both size reduction and power handling capabilities, heat dissipation has become a significant concern. To mitigate this, chemical vapor deposited (CVD) diamond has been demonstrated as an effective solution for thermal management of these devices by directly growing onto the transistor substrate. A key aspect of power and radio frequency (RF) electronic devices involves transient switching behavior, which highlights the importance of understanding the temperature dependence of the heat capacity and thermal conductivity when modeling and predicting device electrothermal response. Due to the complicated microstructure near the interface between CVD diamond and electronics, it is difficult to measure both properties simultaneously. In this work, we use time domain thermoreflectance (TDTR) to simultaneously measure the in plane thermal conductivity and heat capacity of a 1 um thick CVD diamond film, and also use the pump as an effective heater to perform temperature dependent measurements. The results show that the in plane thermal conductivity varied slightly with an average of 103 W per meter per K over a temperature range of 302 to 327 K, while the specific heat capacity has a strong temperature dependence over the same range and matches with heat capacity data of natural diamond in literature.

Herein we report the wafer-scale synthesis of thin-film black arsenic–phosphorus (b-AsP) alloys via two-step solid-source molecular beam deposition (MBD) and subsequent hermetic thermal annealing. We characterize our thin films with a variety of compositional and structural metrology techniques. X-ray photoelectron spectroscopy and energy dispersive spectroscopy determine compositions of As0.78P0.22 for our thin films, while X-ray reflectivity measurements indicate film thicknesses of 6–9 nm. High-resolution transmission electron spectroscopy images reveal a nanocrystalline morphology with orthorhombic b-AsP grains on the order of ∼5 nm. Raman scattering spectroscopy is employed to characterize the vibrational spectra of our thin films, and the results obtained are in agreement with previously reported b-AsP spectra. Evidence of uniform wafer-scale growth is substantiated by Raman mapping. We simulate crystal structure, band gaps, and Raman spectra from first-principles DFT-based computations and find exc...

Ultra-wide bandgap semiconductors have attracted much interest over the last decade. These materials have strong chemical bonding that led to high temperature tolerance, high voltage and power handling capabilities as described by the Baliga figure of merit. Al-rich III-nitride materials are expected to have the best performance over the industry standard Si, and newer options such as SiC and GaN. Continuous improvements dictate the miniaturization of electronic devices, leading to increasingly higher power densities in smaller footprints, leading to heating that causes system failure. Thus, the thermal management is the limiting task for continuous performance scaling. In past we have reported on deep ultraviolet light emitting diodes (LEDs) and AlxGa1-xN (x>0.4) channel Heterojunction Field-Effect Transistors (HFETs) on 2-3 µm thick MOCVD grown high quality AlN/sapphire templates. For template thicknesses over 5 µm either sapphire or AlN patterning is needed to avoid stress related cracking. In this research, we for the first-time report on the direct MOCVD growth and characterization of crack-free, low-impurity AlN templates in excess of 16 μm on basal plane sapphire substrates without doing any external processing. Increasing the thickness improves thermal management and enables an easier sapphire substrate’s laser liftoff for increased DUV light extraction. It would also be used as a heat spreader in power electronics to minimize the operating temperature to maximize power output to industrial loads such as electric vehicles, and large turbines in manufacturing and transportation. All growths were carried out in custom MOCVD reactors with a fast-metalorganic switching manifold using a modified epitaxy procedure in which the Al- and N-precursors (Trimethyl Aluminum and NH3) are pulsed for increased surface mobility. At the initial pulsed growth stage, the growth-temperature and precursor-flow rates are adjusted to yield air-pocketed (voids) rough AlN layers. Most of these high defect materials are located near the AlN/sapphire interface. As the AlN thickness increases in the subsequent growths, the layers turned out smooth and the number of defects is significantly reduced due to dislocation bending/annihilation and cracking is avoided due to strain relief from the voids at the interface. All template growths were carried out at temperatures ~ 1200 - 1300 °C and 40 torr. From X-ray diffraction (XRD), Reciprocal space map (RSM), and Transmission Electron Microscopy (TEM) analysis of these thick AlN templates, we established that they were fully relaxed. Cross-section TEM data clearly shows the presence of voids at ~ 0.4 – 0.9 μm from the AlN/sapphire interface. There is a high density of dislocations present near the void region. Above this region, at least one order of magnitude lower dislocations is observed at the AlN smooth layer region. From the room-temperature Cathodoluminescence (CL) data we see an increase in the band edge emission and a decrease in the defect related long-wavelength signal with increasing AlN template thicknesses. The CL spectra were measured in the edge emission pump geometry on cleaved bars due to the transverse-magnetic polarized nature of the band-edge emission. The monochromatic CL imaging data also confirmed a highly defective region at the AlN/sapphire interface which is also the origin of the long wavelength (below bandgap) CL emission. The Atomic Force Microscopy (AFM) surface scans of the templates showed the surface roughness ~ 0.15 nm to 0.25 nm (for a 5 µm x 5 µm scan). The (102) off axis X-ray linewidths for all these samples ranged from 280 to 330 arc-secs. XRD and Raman experiments confirmed a residual compressive strain in AlN templates compared to the bulk sample. The measured stress ranges from -0.6 GPa to -1.2 GPa for all the templates, reasonable agreement with previous reports. We have found no obvious thickness dependence of the stresses measured from both XRD and Raman, an indication of preserving similar stresses in thick and thin templates. The thermal conductivity of our high-quality 16 µm thick template and the bulk AlN are measured at different temperatures by Time-domain Thermoreflectance (TDTR) technique. The thermal conductivity of the thick AlN at room temperature is 320 Wm-1K-1, or ~10% higher than that of the bulk AlN (~285 W/m-K). At low temperatures (120 K), it exceeds that of bulk AlN by more than 40%. In summary, these templates showed thermal conductivity as high or better than high-cost bulk AlN substrates, demonstrating for the first time that sapphire-based power electronics could be effectively managed thermally. Figure 1

In this study, thermal and structural characteristics of direct wafer bonded (0001) GaN on (001) Si were examined as a continuation of a previous effort1. EVG® ComBond® equipment was used for bonding under high vacuum (~10-8 mtorr) at room temperature by bombarding the GaN and Si surfaces with an Ar ion beam to remove unwanted native surface oxides.1,2 The samples are placed face-to-face and pressure is applied to initiate the bond. The [1010] GaN edge was aligned parallel to the Si [110] edge. An X-ray diffraction reciprocal space map of the (004) Si and (0004) GaN revealed that there is a ~0.2° tilt between the GaN and Si layers. Bombardment of the bonded surface causes an amorphous region at the interface that has been seen in many other bonded systems.3-5 High resolution transmission electron microscopy revealed a ~2 nm amorphous region at the bonded interface, which resides mostly in the Si as confirmed by energy dispersive x-ray spectroscopy. This is consistent with SRIM6 simulations that predict a shallower damage profile in GaN as compared to Si. The thermal boundary resistance of the GaN|Si interface was measured to be 3.9 m2K/GW,1 an interfacial thermal resistance lower than any of the current state-of-the-art GaN-based heterojunctions, including GaN|Si,3 GaN|SiC,4 and GaN|diamond.5 Subsequent annealing is expected to alter the thermal properties as the bonded interface is allowed to recrystallize10. The intermediate steps of the recrystallization at the bonded interface were examined using TEM to give insight on the interface recovery and reconfiguration mechanism of this ion bombarded GaN|Si interface. Dragoi, et al., ECS Trans., 86(5), 23 (2018) Flötgen, et al., ECS Trans., 64(5), 103 (2014) E. Liao, et al., ECS Trans., 86(5), 55 (2018) Xu, et al., Ceramics International, 45, 6552 (2019) Mu, et al., Appl. Surf. Sci., 416, 1007 (2017) F. Ziegler, et al., The Stopping and Range of Ions in Solids vol 1 (Oxford, Pergamon), (1985) Yates, et al., ASME InterPACK (2015) Cho, et al., Phys. Rev. B, 89, 115301 (2014) Sun, et al., APL, 106(11), 111906 (2015) Mu, F. et al., (2019). ACS applied materials & interfaces, 11(36), 33428-33434. Figure 1

In this study, geopolymers were prepared using a mixed precursor of coal fly ash (35 wt%) and metakaolin (65 wt%) in order to investigate the adsorption capacity for Pb2+and Cd2+ in solution. The specific surface area of the geopolymer was up to 82.8 mg/m2. The results show that the maximum adsorption capacity of the geopolymer for Pb2+ and Cd2+ removal were 164.1 and 78.2 mg/g, respectively. Mixing fly ash and metakaolin as raw materials for geopolymer synthesis not only yields an adsorbent material with excellent performance, but is also an effective way to utilize industrial waste resources.

Wide bandgap materials are suitable for high power and high temperature applications. However, these materials, and metal layers that are part of device structures must effectively transport heat from the active device regions. Structural defects, chemical defects, and interface properties can degrade thermal transport, but relatively little attention has been paid to the precise nature of these defects and how they impact thermal transport. Electron microscopy and x-ray scattering based techniques are employed to provide insight into the defects present in these materials. When these techniques are integrated with thermal transport measurements and theoretical insights gained from molecular dynamics simulations, a much better understanding of thermal transport in wide bandgap materials can be realized. Examples of how defects impact transport for diamond boundary interfaces, AlN layers, and GaN heterostructures will be presented.

Gallium oxide (Ga2O3) has attracted great attention for electronic device applications due to its ultra-wide bandgap, high breakdown electric field, and large-area affordable substrates grown from the melt. However, its thermal conductivity is significantly lower than that of other wide bandgap semiconductors such as SiC, AlN, and GaN, which will impact its ability to be used in high power density applications. Thermal management in Ga2O3 electronics will be the key for device reliability, especially for high power and high frequency devices. Similar to the method of cooling GaN-based high electron mobility transistors by integrating it with high thermal conductivity diamond substrates, this work studies the possibility of heterogeneous integration of Ga2O3 with diamond for the thermal management of Ga2O3 devices. In this work, Ga2O3 was deposited onto single crystal diamond substrates by atomic layer deposition (ALD), and the thermal properties of ALD-Ga2O3 thin films and Ga2O3–diamond interfaces with different interface pretreatments were measured by Time-domain Thermoreflectance. We observed a very low thermal conductivity of these Ga2O3 thin films (about 1.5 W/m K) due to the extensive phonon grain boundary scattering resulting from the nanocrystalline nature of the Ga2O3 film. However, the measured thermal boundary conductance (TBC) of the Ga2O3–diamond interfaces is about ten times larger than that of the van der Waals bonded Ga2O3–diamond interfaces, which indicates the significant impact of interface bonding on TBC. Furthermore, the TBC of the Ga-rich and O-rich Ga2O3–diamond interfaces is about 20% smaller than that of the clean interface, indicating that interface chemistry affects the interfacial thermal transport. Overall, this study shows that a high TBC can be obtained from strong interfacial bonds across Ga2O3–diamond interfaces, providing a promising route to improving the heat dissipation from Ga2O3 devices with lateral architectures.

Fundamental theories predict that reductions in thermal conductivity from point and extended defects can arise due to phonon scattering with localized strain fields. To experimentally determine how these strain fields impact phonon scattering mechanisms, we employ ion irradiation as a controlled means of introducing strain and assorted defects into the lattice. In particular, we observe the reduction in thermal conductivity of intrinsic natural silicon after self-irradiation with two different silicon isotopes, $^{28}\mathrm{Si}^{+}$ and $^{29}\mathrm{Si}^{+}$. Irradiating with an isotope with a nearly identical atomic mass as the majority of the host lattice produces a damage profile lacking mass impurities and allows us to assess the role of phonon scattering with local strain fields on the thermal conductivity. Our results demonstrate that point defects will decrease the thermal conductivity more so than spatially extended defect structures assuming the same volumetric defect concentrations due to the larger strain per defect that arises in spatially separated point defects. With thermal conductivity models using density functional theory, we show that for a given defect concentration, the type of defect (i.e., point vs extended) plays a negligible role in reducing the thermal conductivity compared to the strain per defect in a given volume.

The development of GaN-on-diamond devices holds much promise for the creation of high-power density electronics. Inherent to the growth of these devices, a dielectric layer is placed between the GaN and diamond, which can contribute significantly to the overall thermal resistance of the structure. In this work, we explore the role of different interfaces in contributing to the thermal resistance of the interface of GaN/diamond layers, specifically using 5 nm layers of AlN, SiN, or no interlayer at all. Using time-domain thermoreflectance along with electron energy loss spectroscopy, we were able to determine that a SiN interfacial layer provided the lowest thermal boundary resistance (

Understanding the impact of lattice imperfections on nanoscale thermal transport is crucial for diverse applications ranging from thermal management to energy conversion. Grain boundaries (GBs) are ubiquitous defects in polycrystalline materials, which scatter phonons and reduce thermal conductivity. Historically, their impact on heat conduction has been studied indirectly through spatially-averaged measurements, that provide little information about phonon transport near a single GB. Here, using spatially-resolved time-domain thermoreflectance (TDTR) measurements in combination with electron backscatter diffraction (EBSD), we make localized measurements of thermal conductivity within few \mu m of individual GBs in boron-doped polycrystalline diamond. We observe strongly suppressed thermal transport near GBs, a reduction in conductivity from ~1000 W/m-K at the center of large grains to ~400 W/m-K in the immediate vicinity of GBs. Furthermore, we show that this reduction in conductivity is measured up to ~10 \mu m away from a GB. A theoretical model is proposed that captures the local reduction in phonon mean-free-paths due to strongly diffuse phonon scattering at the disordered grain boundaries. Our results provide a new framework for understanding phonon-defect interactions in nanomaterials, with implications for the use of high thermal conductivity polycrystalline materials as heat sinks in electronics thermal management., Comment: This document is the unedited Author's version of a submitted work that was subsequently accepted for publication in Nano Letters, copyright American Chemical Society after peer review. To access the final edited and published work see this URL: https://pubs.acs.org/doi/10.1021/acs.nanolett.8b00534. Version 2: Updates Supplementary Figs S1 and S3. The journal published version is correct

The maximum output power of GaN-based high-electron mobility transistors is limited by high channel temperature induced by localized self-heating, which degrades device performance and reliability. Chemical vapor deposition (CVD) diamond is an attractive candidate to aid in the extraction of this heat and in minimizing the peak operating temperatures of high-power electronics. Owing to its inhomogeneous structure, the thermal conductivity of CVD diamond varies along the growth direction and can differ between the in-plane and out-of-plane directions, resulting in a complex three-dimensional (3D) distribution. Depending on the thickness of the diamond and size of the electronic device, this 3D distribution may impact the effectiveness of CVD diamond in device thermal management. In this work, time-domain thermoreflectance is used to measure the anisotropic thermal conductivity of an 11.8 μm-thick high-quality CVD diamond membrane from its nucleation side. Starting with a spot-size diameter larger than the thickness of the membrane, measurements are made at various modulation frequencies from 1.2 to 11.6 MHz to tune the heat penetration depth and sample the variation in thermal conductivity. We then analyze the data by creating a model with the membrane divided into ten sublayers and assume isotropic thermal conductivity in each sublayer. From this, we observe a two-dimensional gradient of the depth-dependent thermal conductivity for this membrane. The local thermal conductivity goes beyond 1000 W/(m K) when the distance from the nucleation interface only reaches 3 μm. Additionally, by measuring the same region with a smaller spot size at multiple frequencies, the in-plane and cross-plane thermal conductivities are extracted. Through this use of multiple spot sizes and modulation frequencies, the 3D anisotropic thermal conductivity of CVD diamond membrane is experimentally obtained by fitting the experimental data to a thermal model. This work provides an improved understanding of thermal conductivity inhomogeneity in high-quality CVD polycrystalline diamond that is important for applications in the thermal management of high-power electronics.

The scaling of package and circuit board dimensions is central to heterogeneous system integration. We describe our solderless direct metal-to-metal low pressure ( 20 MPa. The combined reduction of dielet interconnect pitch, dielet-to-dielet spacing and trace pitch will enable a Moore's law for packaging.

A study of the fundamental issues of materials integration for wafer bonding offers new pathways for achieving photonic materials with unique properties. First, the bonding of single crystal oxide crystals (YAG and yttria, for example) with different compositions and even different orientations is shown to critically depend upon several important parameters. A detectable level of subsurface damage usually accompanies the polishing of these crystals. We studied chemical mechanical polishing (CMP) of these oxide crystals to provide surfaces which exhibit sub-nm roughness, and the absence of subsurface damage. These surfaces are then activated and we use XPS and other surface sensitive techniques to assess the state of the surface after different processes. Next, we demonstrate that we can significantly reduce the bonding time and temperature to attach two single crystal pieces based on these improved polishing techniques. These techniques also apply to polycrystalline (ceramic) materials as well. Second, we demonstrate wafer bonding for polycrystalline laser gain media. Two issues are important here. First, the grain composition – and hence chemistry – may be different than the bulk composition / chemistry. Under ideal conditions, the grain boundaries may simply be regions of misorientation and be nearly atomically abrupt. The second issue is that the different grain orientations present on a surface may (i) exhibit different hardnesses and / or (ii) different chem-mech polishing rates. Therefore, the issues of chemistry are more important than is the case with single crystal materials. We determined that the best combination of solution chemistry and polishing parameters involves the use of the SUBA 500 (yellow) pad and the same applied force as for the single crystal YAG. For many combinations of force, chemistry, and pad material, a clear delineation of the grain boundaries was revealed. However, we were able to achieve a set of polishing conditions for which the boundaries are not preferentially etched. These surfaces also proved to be damage-free and were able to show a high fraction of bonded interface (>90%) on 50 mm diameter gain media disks.

This presentation highlights the use of x-ray scattering and electron microscopy to characterize selectively doped GaN layers for high power vertical diodes. Examples from these techniques will include assessment of residual impurities and crystalline damage in selectively doped regions resulting from dry etching or ion implantation processes and if such defects are removed by subsequent etching and / or annealing steps. X-ray reflectivity (XRR) combined with high angle reciprocal space mapping and dynamical diffraction simulations are effective means by which to assess the strain and defect distribution in the wafers. Advanced synchrotron techniques, through the use of hard x-ray nanoprobes and diffraction imaging, offer further insight into the development of these technologies. Transmission Electron Microscopy measurements, including electron energy loss spectroscopy, energy dispersive x-ray analysis, and selected area diffraction patterns, determine whether impurities were introduced by the selective doping process and whether the damage removal processes were able to remove growth and implantation-induced defects. A better understanding of the roles of defects on device performance is based on these techniques.

The development of compound semiconductor, wide bandgap electronics based on GaN is poised to impact future rf and advanced power electronic switches. The performance and reliability of these devices is impacted by heat dissipation, which is necessary in order to maintain reasonable device temperatures. Diamond is considered to be a suitable material to promote heat transfer away from the active regions of devices, and recently, several techniques have been developed to integrate diamond with semiconductor materials to achieve effective heat dissipation. We address the properties of polycrystalline diamond, both in thin film and bulk form, to better understand how the polycrystalline nature of diamond impacts thermal conductivity. The deposition of diamond onto, for example, GaN requires the inclusion of interfacial layers; the stability of these layers plays a large role in their effectiveness to protect the semiconductor from the deposition byproducts and to facilitate heat transfer from the GaN to the diamond layer. Novel surface engineering techniques also impact thermal transport across interfaces. We employed transmission electron microscopy imaging and electron energy loss spectroscopy, electron backscattered diffraction (SEM) and precession electron diffraction(TEM), high resolution x-ray scattering, combined with thermal transport measurements to address these issues.

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. We demonstrate 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 single crystal YAG-YAG and polycrystalline 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.

Au-Au thermocompression bonding is a versatile technique of high interest for a variety of applications. We have investigated Au-Au bonding using sputter deposited Au films under conditions of low temperature (150-250 °C) and low bonding pressure (∼3 MPa) for short times (15 min). The combination of low temperature and short times is important for applications involving both hermetic sealing and packaging scaling. The initial surface roughness of the Au film was in the 3-5 nm range with peak-to-valley heights of 20-30 nm and a lateral correlation length of ∼400 nm. For samples bonded at 150 °C, the void morphology at the bonded interface was related to the initial surface roughness. The void morphology was different when bonding at the higher temperatures: the void length (along the bonded interface) decreased significantly but the void height (perpendicular to the interface) increased. These results can be understood in terms of a combination of increased surface Au diffusivity and decreased yield stress and elastic modulus with increased bonding temperature. © 2018 The Japan Society of Applied Physics.

The stability of thin, flexible III–V layers / solar cells with metallization was addressed. Reduced cell thickness (reduced weight) places added constraints on metallization sequences, thicknesses, and thermal budgets for these cells. We subjected a structure of a thin (few μm) III–V layers (the focus here is on InP-based structures) with metallization backing (Au, Ni, and Cu-based metallizations, several μm thick) to annealing at 200 °C, 300 °C, or 400 °C for up to a twelve hours. X-ray diffraction is a particularly useful technique as measurements from the solar cell (semiconductor) side of the stack also reveals the metallization diffraction peaks and hence any reaction byproducts. This is due to the beam penetration through the few μm thick cell layers. After 200 °C for twelve hours, x-ray diffraction and transmission electron microscopy indicate reactions between the metal layers in the stack but the semiconductor is unchanged. After 300 °C, twelve hours, there is more extensive intermetallic compound formation. However, after annealing at 400 °C, twelve hours, the III–V layer is entirely consumed. In contrast, when much thicker cells / semiconductor layers are subject to similar annealing conditions, there may not be a significant impact on performance because only a few μm of the total cell thickness is consumed. When the total thickness is only a few μm, however, the reaction between the metal contact layer and the semiconductor leads to consumption of the entire device. These results demonstrate that (i) the total cell thickness is important when considering contact-semiconductor interactions at elevated operating or processing temperatures and (ii) x-ray diffraction through the thin cell provides a detailed assessment of reactions; it can be utilized for any combination of thin film solar cell and metallization.