Understanding the relationship between the grain size and mechanical properties of nanocrystalline magnesium aluminate (MgAl2O4) spinels is important due to their strong potential for transparent structural applications. In this study, nanocrystalline MgAl2O4 with grain sizes ranging from 3.7 to 80 nm have been synthesized by environmentally-controlled pressure assisted sintering, and the effect of grain size on plasticity mechanisms under nanoindentation was investigated for grains sizes within the Hall-Petch regime (80 nm), the inverse Hall-Petch regime (3.7 nm), and near the transition grain size (10.5 nm). Transmission electron microscopy (TEM) and electron diffraction of a sample with an 80 nm grain size revealed significant residual lattice distortion and grain boundary decohesion induced by dislocation plasticity. In contrast, a sample with a 3.7 nm grain size did not show any dislocations or residual lattice distortion even within the severely deformed region right below the indent. Instead, shear bands formed, and atomic scale grain boundary decohesion was observed only within shear bands. Large-scale atomistic simulations of MgAl2O4 with a 3.7 nm grain size show that plastic strain is developed mostly at grain boundaries without dislocation nucleation or grain growth, suggesting that grain boundary sliding is the dominant mechanism by which shear bands develop for the inverse Hall-Petch regime. Our results provide an important insight into the plasticity mechanisms for nanocrystalline ceramics with different grain sizes.

Fabrication of bulk nanocomposite solids with macroscopic form factor and nanoscopic structural control is a central challenge in nanomaterials engineering. This work introduces an approach to this challenge wherein precise nanostructural control is provided by ultrahigh-aspect-ratio atomic layer deposition and complete densification with nanostructure preservation is enabled by environmentally controlled pressure-assisted sintering. To demonstrate the capability to tune functional properties via nanocomposite design, this approach is used to produce millimeter-scale, fully dense, bicontinuous SiO2/ZnO:Al nanocomposites: atomic layer deposition is used to infiltrate SiO2 nanoparticle compacts with conductive ZnO:Al, and then environmentally controlled pressure-assisted sintering is used to remove the residual porosity. The nanocomposites' optical and electrical properties are dictated by the ZnO:Al channel width and volume fraction, which are precisely controllable via the number of atomic layer deposition cycles. Increasing the ZnO:Al channel width from similar to 6.5 to similar to 13 nm (increasing the volume fraction from 16% to 27%) intensifies visible reflectance (producing a bright blue color) and increases the electrical conductivity by nearly an order of magnitude (from 0.69 to 6.5 Omega-1 cm-1). Temperature-dependent conductivity measurements indicate Efros-Shklovskii variable range hopping while also suggesting proximity to a metal-insulator transition. Due to their tunable optical and electronic properties and robust mechanical properties, these SiO2/ZnO:Al nanocomposites are promising candidates for conductive window applications; more broadly, the fabrication strategy demonstrated here is amenable to a wide range of material combinations, and it enables nanoscale architectures with tailorable properties for application areas including photonics, sensing, energy generation and storage, thermal management, and structural materials.

Boron nitride remains a critical material for myriad applications requiring high-temperature stability and thermal management. This work explores the utilization of Al2O3 atomic layer deposition (ALD) on boron nitride nanotube (BNNT) fabrics to push the stability limit of boron nitride above 900 °C. Long precursor exposures during 50 cycle ALD runs were used to completely infiltrate high-aspect-ratio BNNT fabrics, resulting in uniform, conformal coatings on the interior surfaces of the fabrics. Brunauer–Emmett–Teller method surface area measurements found that the BNNT fiber mat surface area was reduced from 434 ± 3 to 117 ± 1 m2 g−1 after ALD and scanning electron microscopy confirmed that the ALD coating was uniform throughout the thickness of the fabric. We report the first thermal conductivity measurements for Al2O3-coated BNNT fabrics where thermal conductivity increased by approximately 15% in the in-plane direction and 192% in the through-plane direction after ALD compared to uncoated BNNT fabrics. Fabric exposures in pure oxygen at 1000 °C demonstrated that coated BNNT fabrics had superior oxidation resistance over uncoated fabrics. Interestingly, the presence of alumina led to the formation of Al4B2O9 instead of liquid B2O3, resulting in the shape retention of the coated BNNT fabrics after the 2 h oxygen exposure. The work supports ALD as a promising method for improving the high-temperature performance of BNNT fabrics or BNNT-based composites.

Industrial application of superhard materials (Vickers hardness, HV > 40 GPa) such as diamond and cubic boron nitride is limited by high costs and complex routes of synthesis. Tungsten carbide (WC) is a common industrial material valued for its hardness, but falls well short of qualification as a superhard material even in its less common but harder binderless form (HV ∼ 26 GPa). Importantly, recent efforts have demonstrated the potential for alternative materials, such as WC, to achieve similar hardness to diamond and cubic boron nitride via microstructural refinement. However, despite recent advances in sintering technology, even the smallest grained binderless WC (< 100 nm) has failed to achieve HV values above 33 GPa. In this work, multiple hardening mechanisms are exploited through a unique sintering approach proving WC as a candidate superhard material. Environmentally Controlled – Pressure Assisted Sintering (EC-PAS) is utilized to produce > 99% dense, binderless nanocrystalline WC ceramics with hardness as high as 39 GPa. The unprecedented WC hardness is attributed to the combined effects of small average crystallite size and, importantly, deformation-induced nanoscale intragranular defects including stacking faults. The demonstration of the superposition of multiple hardening mechanisms provides a new avenue to improve hardness of ceramics beyond traditional Hall-Petch hardening, yielding new classes of superhard materials.

Conformal atomic layer deposition (ALD) inside macroscopic nanoporous solids with aspect ratios greater than 103 can require ALD reactant exposures on the order of 103 Torr-s or greater. For some ALD chemistries, such large exposures raise the concern of non-self-limiting deposition. In the case of ZnO ALD from diethylzinc (DEZ) and H2O, exposures in the 10–103 Torr-s range have resulted in metallic Zn deposition at typical temperatures used for ZnO ALD on wafers (e.g., ∼180 °C). This Zn deposition can be suppressed by lowering the deposition temperature, but this slows H2O desorption and, thus, can necessitate impractically long purge times. In this work, we use static-dose ALD with DEZ and H2O exposures >104 Torr-s to deposit ZnO inside Al2O3 nanoparticle compacts (NPCs) with 50.5 ± 0.3% porosity, 100 nm NP diameter, 1.55 ± 0.05 mm thickness, and an aspect ratio of 7800 ± 200 (based on the half-thickness), and we explore a novel approach to the deposition temperature, T: T is cycled between 160 °C (for H2O purges) and 120 °C (for all other steps). For comparison, we also deposit ZnO with T held constant at 120 or 160 °C. Whereas the T = 160 °C process results in Zn metal deposition and nonuniform infiltration, the temperature-cycled process yields apparently self-limiting ZnO deposition at a growth per cycle (GPC) of ∼2.1 Å/cyc, forming an electrically conductive ZnO network that is uniform throughout the thickness of the NPC, with the exception of some ZnO depletion near the NPC surfaces, possibly due to the (unoptimized) long DEZ purge time. The T = 120 °C process produces similar results, although the GPC is slightly elevated, indicating diminished removal of H2O and/or OH during purges. We employ scanning electron microscopy with energy-dispersive x-ray spectroscopy, x-ray diffractometry, electrical resistivity measurements, and ALD chamber pressure analysis in our comparison of the three ALD processes.

As gallium oxide-based heterojunction devices gain prominence, low-resistance contacts to aluminum gallium oxide material are of increasing importance for high performance and access to modulation doped layers. Here, the activation of ion-implanted silicon donors is investigated as a function of donor density from 5 x 10(18) cm(-3) to 1 x 10(20) cm(-3), activation anneal duration from 6 s to 600 s, and activation temperature from 900 degrees C to 1140 degrees C. Importantly, ohmic behavior was achievable across a reasonably wide process window at moderate to high doping concentrations. Specific contact resistance of 1 x 10(-3) Omega cm(2) and sheet resistance of 2.8 k Omega/square were achieved for a 60 nm-deep 1 x 10(20) cm(-3) box implant after activation at 1000 degrees C for 6 s with standard Ti/Au contacts. Under these conditions, an activation efficiency of 7% was observed with Hall mobility of similar to 32 cm(2)/Vs. Furthermore, we demonstrate a Schottky diode formed of implanted material with a rectification ratio > 10(6) and further confirm the Hall carrier density results using capacitance-voltage profiling analysis. Finally, we show the significant impact of anneal duration and the potential for deleterious over-annealing which reduces the active carrier density, mobility, and resultant material conductivity.

Activation of ion‐implanted p‐type dopants in gallium nitride has demonstrated great progress utilizing high pressures to enable novel and traditional device architectures; however, such conditions consistently exhibit anomalously enhanced diffusion up to several microns in very short periods of time for device relevant concentrations. Here, this diffusion is shown to be modulated by unintentional hydrogen content within the anneal ambient and thus controllable by inclusion of a high‐temperature hydrogen getter. Furthermore, diffusion is also shown to be greatly suppressed using co‐implanted oxygen at low concentrations while simultaneously maintaining characteristics of p‐type material in photoluminescence. Subsequently, after annealing at 1300 °C for 30 min in 3.8 kbar of nitrogen pressure, the magnesium concentration in the diffusion tail is suppressed by 28% at 1–1.5 μm in depth using a hydrogen getter alone, which reduces hydrogen uptake by 45% and fully suppressed at >1 μm in depth using co‐implantation alone and further reduced with concurrent use of a hydrogen getter. Co‐implantation alone reduces the in‐diffused magnesium dose by 60% compared to reference samples.

The low thermal conductivity of β-Ga2O3 is a significant concern for maximizing the potential of this ultra-wide bandgap semiconductor as a power switching device technology. Here, we report on the use of nanocrystalline diamond (NCD) deposited via microwave plasma enhanced chemical vapor deposition (MP-CVD) as a top-side, device-level thermal management solution on a lateral β-Ga2O3 transistor. NCD was grown via MP-CVD on β-(AlxGa1−x)2O3/β-Ga2O3 heterostructures prior to the gate formation of the field-effect transistor. A reduced growth temperature of 400 °C and a SiNx barrier layer were used to protect the oxide semiconductors from etching in the MP-CVD H2 plasma environment. Raman spectroscopy showed a highly sp3-bonded NCD film was obtained at 400 °C, with grain size of about 50–100 nm imaged via atomic force microscopy. The incorporation of the NCD heat-spreading layer resulted in a β-(AlxGa1−x)2O3/β-Ga2O3 heterostructure field-effect transistor showing a decrease in the total thermal resistance at the gate by 42%. The fabrication process, including the NCD etch in the gate region, will need to be improved to minimize the impact of these processes on important device characteristics (i.e., drain current, threshold voltage, and leakage current).

Selective area doping via ion implantation is crucial to the implementation of most modern devices and the provision of reasonable device design latitude for optimization. Herein, we report highly effective silicon ion implant activation in GaN via Symmetrical Multicycle Rapid Thermal Annealing (SMRTA) at peak temperatures of 1450 to 1530 °C, producing a mobility of up to 137 cm2/Vs at 300K with a 57% activation efficiency for a 300 nm thick 1 × 1019 cm−3 box implant profile. Doping activation efficiency and mobility improved alongside peak annealing temperature, while the deleterious degradation of the as-grown material electrical properties was only evident at the highest temperatures. This demonstrates efficient dopant activation while simultaneously maintaining low levels of unintentional doping and thus a high blocking voltage potential of the drift layers for high-voltage, high-power devices. Furthermore, efficient activation with high mobility has been achieved with GaN on sapphire, which is known for having relatively high defect densities but also for offering significant commercial potential due to the availability of cheap, large-area, and robust substrates for devices.

Codoping of gallium nitride for improved acceptor ionization has long been theorized; however, reduction to practice proves difficult via growth. Herein, implementation of codoping via ion implantation and symmetric multicycle rapid thermal annealing utilizing magnesium codoped with silicon or oxygen is demonstrated. Results show enhanced photoluminescence with both donor species but with an order of magnitude greater increase with concurrent p‐type hall for codoping with oxygen. Furthermore, the addition of nitrogen to balance stoichiometry suppresses defect photoluminescence signals. The incorporation of the donor and nitrogen demonstrates defect reduction beyond magnesium, only implants despite the additional implant dose and resultant damage with coimplantation. The enhanced hole concentrations evident with oxygen incorporation reveal important considerations for device design given unintentional doping during growth and future incorporation of ion implantation capabilities.

A multi-scale, computational model is developed to describe the growth characteristics of single-crystal diamond in the High-Pressure, High-Temperature (HPHT) process. This model is the first to connect phase-change kinetics governing crystal growth to the continuum transport of carbon through the growth cell. Results show the importance of convective transport driven by buoyant flow in the metallic solvent, which increases the growth rate by nearly an order of magnitude over that obtained under diffusion alone. Parametric studies show how crystal growth may be kinetically-limited or transport-limited, depending on the value of the macroscopic kinetic coefficient. Estimating this kinetic coefficient from growth experiments yields a phase-change Damköhler number of unity, indicating a mixed regime where phase-change kinetics and transport are comparable and strongly coupled in this system. Mechanisms responsible for slowing growth as the crystal size increases are explained. Supersaturation inhomogeneities along the facets of larger crystals are predicted, which may be relevant to solvent inclusion formation during growth.

To develop transparent materials with superior mechanical properties, nanocrystalline magnesium aluminate (MgAl2O4) spinel with grain sizes ranging from 3.7 to 80 nm has been synthesized by environmentally controlled pressure assisted sintering (EC-PAS). In this study, we investigated the microstructure and grain size dependence of the mechanical properties of nanocrystalline MgAl2O4 by performing transmission electron microscopy, nanoindentation, uniaxial micropillar compression, and micro-cantilever bending. Electron microscopy confirmed that the EC-PAS synthesis technique produces a nearly fully dense grain structure with a porosity of less than 1% in larger grain-sized ceramics and observably pore-free grain structures in the smaller grain-sized ceramics. Mechanical characterization revealed that nanoindentation hardness, compressive fracture strength, and fracture toughness each exhibit distinct grain size dependence. Our experimental results and numerical analyses point to a change in dominant strain accommodating mechanisms from dislocation-based plasticity to shear banding as the grain size is reduced, as previously suggested by the literature. Practical implications of the change in strain accommodation mechanisms manifest as the emergence of indentation size effect, weak grain size dependence of hardness and strength, and a ~2-fold increase in apparent fracture toughness for the smaller grain-sized ceramics.

In light of the importance of selective area doping in GaN to enable planar process technology, and to avoid the complications from the etch/regrowth process, ion implantation is the recognizable alternative. Annealing to activate dopant species and repair the damage to a crystal poses a challenge for GaN since the material will decompose to Ga + N-2 at atmospheric pressure and relevant temperatures. In this research, in situ high- and low-temperature epitaxial and ex situ sputtered AlN caps were examined in different stacking arrangements to study the optimum conditions for Mg ion implantation and activation. Concurrently, a matrix of different implantation doses was also investigated to better understand the dose-dependent activation. Each sample has a unique cap stack and four different implant doses, including an unimplanted reference quadrant. The results show that poorer quality cap films enable nitrogen to leave the crystal during annealing and leave nitrogen vacancies behind. Furthermore, a high dose is needed at the surface to facilitate ohmic contact formation. The results suggest that in situ epitaxial-grown AlN caps are more suitable for GaN activation annealing, and high-temperature thin caps provide the best barrier to prevent crystal disintegration. We reveal a timely strategy for preserving the quality of GaN crystal structure during the electrical activation of the ion-implanted Mg atoms. This work provides valuable information that bridges the gap between device processing and electrical characterization of GaN devices, presenting a clear path towards achieving an electrical activation of implanted Mg while maintaining the integrity of the crystalline structure of GaN.

Single crystal (−201) β-Ga 2 O 3 substrates doped with Si and Sn have been thermally annealed in N 2 and O 2 atmospheres. Structural and electrical properties evaluation was performed via a number of experimental methods in order to quantify the effects of the doping and annealing ambient on the properties of these samples. All samples annealed in O 2 exhibited significantly lower carrier concentration, as determined by capacitance–voltage measurements. Schottky barrier diodes exhibited excellent rectification when the Ga 2 O 3 was annealed in N 2 , and significantly lower forward current using O 2 -annealed Ga 2 O 3 substrates. Deep level transient spectroscopy revealed four deep trap levels with activation energies in the range of 0.40–1.07 eV. Electron spin resonance showed a decrease in shallow donor concentration, and cathodoluminescence spectroscopy revealed nearly two orders of magnitude lower emission intensity in O 2 -annealed Ga 2 O 3 samples. Raman spectroscopy revealed a carrier concentration dependent Raman mode around 254 cm −1 observed only when the final anneal of (−201) β-Ga 2 O 3 was not done in O 2 . Secondary ion mass spectroscopy measurements revealed diffusion of unintentional Fe towards the surface of the (−201) Ga 2 O 3 samples after annealing in O 2 . Depth resolved positron annihilation spectroscopy showed an increased density of vacancy defects in the bulk region of O 2 -annealed Ga 2 O 3 substrates.

Valence and conduction band offsets of atomic layer deposition (ALD) Al2O3 deposited on bulk AlN crystals were determined using x-ray photoelectron spectroscopy to be ΔEV = 0.75 eV and ΔEC = −1.45 eV, with a measured energy gap of the Al2O3 film of 6.9 eV. In addition, crystalline AlN deposited by atomic layer epitaxy on sapphire was evaluated, resulting in a valence band offset of ΔEV = −0.75 eV and a conduction band offset of ΔEC = 3.25 eV due to the wider bandgap of the crystalline Al2O3 substrate compared to amorphous ALD Al2O3. Both heterojunctions exhibited type-II behavior and similar valence band offsets.

Many atomic layer deposition (ALD) reactions are highly exothermic, with some likely releasing hundreds of kJ/mol per cycle. In ALD on conventional substrates (e.g., wafers), this exothermicity is typically ignored, because the deposited mass is small compared to the substrate mass. However, in the case of high-surface-area substrates such as metal-organic frameworks (MOFs) or nanopowders, the mass deposited per cycle can be a substantial fraction of the substrate mass, raising the possibility of nonnegligible reaction heat. To understand the potential impact of this heat on nanostructured substrates and ALD processes, we measure via pyrometry the surface temperature of a ceramic nanopowder bed undergoing particle ALD (pALD). Depositing Al2O3, from trimethylaluminum (TMA) and water on Y2O3-stabilized ZrO2 (YSZ) nano powder with an average nanoparticle (NP) diameter of 8 nm, we observe temperature gains, Delta T, around 20 degrees C above the baseline. Under certain conditions, the maximum local Delta T may be significantly greater than 20 degrees C, according to an estimate of the temperature of a hypothetical isolated NP based on standard formation enthalpies and sticking coefficients from the literature. Examining the dependence of T(t) curves on the precursor, cycle number, and NP size, we find that the pyrometry dataset also provides insights into pALD kinetics and the evolution of deposition chemistry and substrate morphology.

Here we present a direct comparison between the activation of implanted Mg ions in N-polar and Ga-polar substrates to produce p-type GaN via symmetric multicycle rapid thermal annealing (SMRTA). Physical dopant activation was achieved by annealing in moderate nitrogen pressures (3.3 MPa) in conjunction with a bi-layer cap. Photoluminescence shows activation was more readily achieved for N-polar films with measured UV luminescence up to similar to 15x as intense as yellow/green luminescence compared to similar to 2.4x for Ga-polar films. The greater activation of N-polar material was primarily due to a higher thermal stability compared to Ga-polar films. This demonstration of implanted Mg activation by SMRTA enables a facile route toward next generation vertical devices. (C) 2019 The Japan Society of Applied Physics

GaN-based power switching devices are of significant interest for high efficiency power conversion circuits in medium voltage applications. However, there are still significant challenges preventing mass production and widespread adoption. Recently, GaN-based vertical and lateral power devices have attracted significant interest due to promising device results coupled with progress in native substrate, epitaxial growth, and processing technology developments. However, there are two significant limiting factors for vertical GaN power devices at present. The first issue is substrate and epitaxial layer uniformity and reliability, and the second is the lack of a planar selective-area doping process. While p-type epitaxial growth capability and the ability to stack layers are quite mature from the LED industry, power devices require planar selective-area doping, which is a routine step by ion implantation in the Si and SiC device industry. The ability to implant and activate dopants, particularly p-type dopants, in GaN still remains a challenge as implant activation typically requires annealing at temperatures ~ 2/3 of the melting point, which is a region where the GaN crystal is unstable and readily decomposes to Ga + N2 at atmospheric pressure. The NRL-developed symmetric multicycle rapid thermal annealing (SMTRA) technique has been the most successful, demonstrating electrical activation of up to ~10% of the implanted Mg dopant atoms using a combination of a temporary, thermally stable capping layer, annealing in a moderate nitrogen overpressure, and performing a well-optimized annealing temperature profile including multiple spike anneals. Advances in the SMRTA process development, including optimization of capping processes, implantation profile, and processing steps such as metallization and surface treatments will be reviewed here. In addition, the evaluation of components of the SMRTA process, such as the cap and overpressure anneal, have been applied to Si implantation for n-type doping and will be presented here. This is a critical step to reduce contact resistance in lateral GaN structures as well as form the source regions of vertical devices that is typically accomplished by selective area regrowth. The use of ion implantation and activation utilizes fewer process steps and avoids impurity incorporation at sidewalls compared to the selective area regrowth process, and has been shown to yield lower contact resistance as well. This presentation will review advances in both Mg and Si ion implantation and assess the viability of this process for practical device fabrication.