Metal Coverage Research Articles

Fully screen-printed bifacial large area 22.6% N-type Si solar cell with lightly doped ion-implanted boron emitter and tunnel oxide passivated rear contact

We report on the fabrication of fully screen-printed bifacial large area (239 cm2) high-efficiency n-type Si solar cells with ion-implanted homogeneous boron emitter on the front side and carrier-selective tunnel oxide passivated contact (TOPCon) on the rear. Our phosphorus-doped poly-Si/SiOx passivated contact with SiNx capping layer gave excellent surface passivation with a very low recombination current density (J0_TOPCon) of ~1 fA/cm2 after a simulated firing treatment at ~770 °C without metallization. After screen-printed and fire-through metallization on n-TOPCon with ~13% metal coverage, metallized J0_TOPCon value increased to only ~5 fA/cm2. In addition, homogenous boron implanted emitter (~180 Ω/) passivated with ALD Al2O3 layer capped with PECVD SiNx/SiOx double-layer antireflection (DLAR) coating gave excellent passivation with emitter recombination current density of ~12 fA/cm2 prior to metallization. The industrial screen-printed, fire-through contacts with floating busbars on this boron emitter gave metallized emitter recombination current density of ~31 fA/cm2. This resulted in a low-cost industrial screen-printed n-type bifacial Si solar cell with 22.6% efficiency and 702 mV open-circuit voltage (Voc).

Aluminum Nitride Combined Overtone Resonators for the 5G High Frequency Bands

This work presents a new acoustic MEMS resonator technology, dubbed Aluminum Nitride (AlN) Combined Overtone Resonator (COR), capable of addressing the filter requirements for the 5G high frequency bands in the 6-40GHz range. The COR exploits the multimodal excitation of two higher-order Lamb waves ( $2^{\mathbf {nd}}$ and $3^{\mathbf {rd}}$ order Asymmetrical Lamb Waves) in a suspended thin-film AlN plate to transduce a 2-dimensional vibration mode with high electromechanical coupling coefficient $k_{t}^{2}$ (up to 1.9%) and quality factor $Q>1100 $ at twice the frequency of a fundamental thickness-extensional mode in the same structure. Analytical and finite-element method (FEM) models are developed to describe the working principle of the COR technology and predict the achievable $k_{t}^{2}$ , Q and lithographic frequency tunability. An 8.8 GHz COR prototype was fabricated showing a high $k_{t}^{2}~\sim ~0.3$ % (using a simple top-electrode-only configuration with a 2-mask process) and a groundbreaking $Q\sim {1100}$ which is the highest ever achieved among piezoelectric resonators above 6 GHz. The $f - Q$ product $\sim 1\times 10 ^{\mathbf {13}}$ is the highest among all demonstrated piezoelectric resonators with metallic coverage >50%. Additionally, the capability of the COR technology to deliver contiguous filters with bandwidths between 355 and 592 MHz (aggregated BW >2GHz) in the mmWave spectrum, with relaxed lithographic requirements, is demonstrated by FEM. [2019-0229]

NGC 6397: The metallicity trend along the isochrone revisited

Context. Recent work has used spectra of ∼5000 stars in NGC 6397 that were extracted from a MUSE mosaic to determine the atmospheric parameters for these stars by fitting the spectra against the Göttingen Spectral Library. A significant change in metallicity between the turn off and the red giant branch was found and was discussed as a possible manifestation of predicted effects of atomic diffusion. However, the small amplitude of the effect and inconsistency with earlier measurements call for more attention before this result is interpreted. Systematic effects due to the interpolation or to the synthetic spectra cannot be ruled out at this level of precision. Aims. We reanalyze the data with : the ELODIE and MILES reference libraries in order to assess the robustness of the result. These empirical libraries have a finer metallicity coverage down to approximately the cluster metalicity turn-off. Methods. We used the ULySS full-spectrum fitting package, together with the library interpolators to remeasure the three atmospheric parameters effective temperature, surface gravity, and [Fe/H] metallicity. Results. We find a very low [Fe/H] dispersion along the isochrone (0.07 dex), consistent with our error estimate (0.05 dex). However, the [Fe/H] trend is not reproducible. This shows that the data have the potential to reveal patterns of the magnitude of the expected physical effects, but the analysis methods need to be refined to cull systematic effects that currently dominate the patterns.

LVIS Blue as a Stand-alone “Flow Diverter”

Flow diversion fundamentally changes the treatment approach towards intracranial aneurysms. Most currently available devices established efficacy and safety data in the proximal anterior circulation; the distal and posterior circulations remain areas of active research. LVIS Blue is a stent with 28% metal coverage approved for use as a coil adjuvant. Some studies indicate potential “flow diverter” properties. We sought to evaluate the LVIS Blue as a stand-alone “flow diverter” for the treatment of intracranial aneurysms. We performed an observational single-center study to evaluate initial occlusion and occlusion at six months follow-up for patients with distal or posterior circulation aneurysms treated with the LVIS Blue as a “flow diverter” at our institution. Ten aneurysms were treated over the course of two years with six-month angiographic follow-up. Seven lesions were in the distal anterior circulation and were unruptured (five anterior communicating artery, one M2 middle cerebral artery, one pericallosal). Three were posterior circulation (two basilar tip aneurysms, one P2 posterior cerebral artery aneurysm). Follow up demonstrated treatment effect in nine of ten aneurysms (four complete aneurysm occlusions, five lesions with decreased size and flow). One lesion showed no treatment response. No ischemic or hemorrhagic complications were noted during placement or clinical follow-up. LVIS Blue can function safely as a “flow diverter” in the distal and posterior circulations. Further data regarding long-term efficacy is needed.

Preliminary Experience of Lvis Blue in the Internal Carotid Artery for The Treatment Of Wide-Necked Intracranial Aneurysms

Objective A new Low-profile Visualized Intraluminal Support device (LVIS Blue) is a braided stent that provides a higher degree of metal coverage and increasingly used for the stent-assisted coiling of intracranial aneurysms. In the present study, our preliminary experience of using LVIS Blue in the internal carotid artery (ICA) for the treatment of wide-necked intracranial aneurysms was reported. Methods This retrospective review included patients with ICA aneurysms prospectively collected from our database who were treated using LVIS Blue at our institution from September 2017 to December 2018. Angiographic results, clinical outcomes, and technical issues were evaluated. Results A total of 22 patients with unruptured wide-necked ICA aneurysms were treated using LVIS Blue. Seven aneurysms were located in the superior hypophyseal artery, 4 in posterior communicating artery, 4 in ICA dorsal wall, 3 in ICA bifurcation, 2 in ophthalmic artery, 1 in ICA ventral wall, and 1 in ICA lateral wall. During the procedure, suboptimal positioning and foreshortening of the stent occurred in 1 patient each. Immediate postembolization angiography showed complete obliteration in 13 patients (59.1%), residual neck in 4 (18.2%), and residual aneurysm in 5 patients (22.7%). Follow-up angiographic results showed complete obliteration in 19 patients (86.4%). In-stent stenosis (> 50% stenosis) was not observed on follow-up angiography. In addition, morbidity or mortality did not occur. Conclusion The LVIS Blue may be feasible and safe for stent-assisted coiling in ICA aneurysms. Foreshortening, suboptimal deployment, and difficulty to open proximal ends may be considerable technical issues in the tortuous ICA. Keywords: Intracranial aneurysm; Wide-necked aneurysm; LVIS Blue; Stent-assisted coiling.

P‐6.4: Failure Analysis of Al Corrosion Issue at Complementary Metal Oxide Semiconductor Backplane Solder Process for 1980PPI Micro Light Emitting Diode Display

Micro light emitting diode (LED) display fabricated on complementary metal oxide semiconductor (CMOS) backplane provide smaller size, higher contrast ratio and energy efficiency, can be accommodated to market applications such as augmented reality (AR)/virtual reality (VR). Hybridization of compound semiconductor Micro‐LED arrays with CMOS active matrix driver integrated circuit (IC) using flip‐chip technology is major solutions proposed, in which under bump metallurgy (UBM) and solder process is fabricated on CMOS backplane and followed by flip‐chip process with LED chips. In our experiment, AL corrosion issue at CMOS top metal electrode in UBM and solder process occurred during 1980PPI micro display fabrication. After scanning electron microscope (SEM) & focused ion beam (FIB) analysis, it's suspected that uneven passivation sidewall profile and poor bottom coverage of UBM metal at passivation sidewall induced developer liquid in following PR remove step penetrating to AL electrode surface and corrode it. Passivation etching recipe modification and UBM metal bottom coverage improvement could solve this issue

(Invited) Pd Shape-Controlled Nanoparticles Decorated with Promoter Metals for Electrochemical Nitrate Reduction

Nitrate (NO3 -) is the world’s most widespread surface and ground water contaminant that causes adverse effects on human health such as methemoglobinemia (“blue baby syndrome”) and cancer. While most nitrate removal strategies occur through the use of ion exchange resins, these approaches are not sustainable as waste disposal of the brines remains a critical challenge. Electrocatalytic NO3 - remediation; however, is one emerging approach for nitrate removal which does not produce waste as NO3 - is converted directly to inert dinitrogen (N2) gas. The main challenge with electrocatalytic NO3 - reduction is the low activity and selectivity of the NO3 - to N2 because this conversion is a rate-determining step and results in undesirable products such as nitrite (NO2 -) and ammonium (NH4 +). To achieve an efficient activity and selectivity for the denitrification, we develop Pd shape-controlled nanoparticles and introduce secondary metals by using the underpotential deposition (UPD) method. The electrochemically deposited metals mainly promote the reduction of NO3 - to NO2 - and Pd facets catalyze the reduction of NO2 - to N2. Under the metal desorption area, we can control the surface coverage of metals by sweeping stopped at selected potentials. Conducting RRDE tests enabled the measurement of improved activity and selectivity of NO3 - to N2 using shape-controlled Pd that contain surface modifications (metal atoms). This work demonstrates that electrocatalytic nitrate reduction is an important approach to reduce environmental impacts associated with removing NO3 - from water.

3D plasmonic design approach for efficient transmissive Huygens metasurfaces.

In this paper we present a design concept for 3D plasmonic scatterers as high- efficiency transmissive metasurface (MS) building blocks. A genetic algorithm (GA) routine partitions the faces of the walls inside an open cavity into a M x N grid of voxels which can be either covered with metal or left bare, and optimizes the distribution of metal coverage needed to generate electric and magnetic modes of equal strength with a targeted phase delay (Φt) at the design wavelength. Even though the electric and magnetic modes can be more complicated than typical low order modes, with their spectral overlap and equal strengths, they act as a Huygens source, with the accompanying low reflection magnitude. Square/hexagonal voxels inside square/rectangular cavities are thoroughly analyzed for operation at 8 µm, although the technique can be applied to different cavity geometries for operation across the electromagnetic spectrum. Results from full-wave simulations show the GA routine can repeatedly pinpoint scatterer geometries emitting at any Φt value across 2π phase space with transmittances of at least 60%, making these MS building blocks an attractive plasmonic alternative for practical optical applications. Full-scale metasurface devices are calculated from near-fields of the individual elements to validate the optical functionality.

Optimization of electroless plating of gold during MACE for through etching of silicon wafer

Deep etching of silicon (Si) is very much desirable for wide variety of applications. Under the context, a cost effective and reproducible through etching of ∼375 μm thick Si wafer is demonstrated through long hour metal assisted chemical etching (MACE) followed by short duration KOH etching. During MACE, apart from pH and temperature, metal catalyst size and coverage density during electroless plating plays an important role. Optimization of gold deposition in terms of plating solution concentration and deposition time during MACE is studied for effective through etching. HAuCl4 concentration of ∼5 mM for 30 s is found to be best suited for MACE and produces deep and highly dense pores in Si with threshold pore radius ∼250 nm and above. Following the MACE, KOH etching effectively scoops out porous Si to realize through etching.

Segment Occlusion vs. Reconstruction-A Single Center Experience With Endovascular Strategies for Ruptured Vertebrobasilar Dissecting Aneurysms.

Objective: Ruptured dissecting aneurysms of the intracranial vertebral arteries exhibit an extraordinarily high risk for morbidity and mortality and are prone to re-rupture. Therefore, early treatment is mandatory to induce stagnation of the critical dynamic mural process. Appropriate endovascular approaches are segment sacrifice and reconstruction, however, both carry specific risks and benefits. To date most studies discuss only one of these approaches and focus on one specific device or technique. Therefore, our study aimed to present our experiences with both techniques, providing a considered approach on when to perform endovascular reconstruction or sacrifice.Materials and Methods: We retrospectively reviewed patients with subarachnoid hemorrhage in our database, suffering from dissecting aneurysms of the intradural vertebral arteries and treated endovascularly in the acute setting. A total of 16 cases were included. Clinical history, radiologic findings and outcomes were analyzed.Results: In 7 patients a reconstructive approach was chosen with 4 of them receiving stent-assisted coiling as primary strategy. One of the 7 patients suffered early re-bleeding due to progression of the dissection and therefore treatment was augmented with implantation of 2 flow diverters. The remaining 2 patients were primarily treated with flow diverters in telescoping technique. In 9 patients a deconstructive approach was followed: 6 patients were treated with proximal coil-occlusion of the V4 segment, 3 patients received distal coiling of the V4 segment. Two patients died (GOS 1) in the subacute stage due to sequelae of recurrent episodes of raised intracranial pressure and parenchymal hemorrhage. Two patients kept severe disability (GOS 3), six patients had moderate disability (GOS 4) and seven patients showed full recovery (GOS 5). None of the patients suffered from a procedural or postprocedural ischemic stroke.Conclusions: In patients with good collateral vascularization, proximal, or distal partial segment sacrifice via with endovascular coil occlusion seems to yield the best risk-benefit ratio for treatment of ruptured dissecting V4 aneurysms, especially since no continued anticoagulation is required and possibly essential surgery remains feasible in this scenario. If possible, PICA occlusion should be avoided—although even proximal PICA occlusion can become necessary, when weighing against the risk of an otherwise untreated ruptured V4 dissecting aneurysm. Contrarily, if the dominant V4 segment is affected, the hemodynamic asymmetry prohibits occlusion and necessitates reconstruction of the respective segment. For this, implants with high metal coverage treating the entire affected segment appear to be the most promising approach.

Li and Na Adsorption on Graphene and Graphene Oxide Examined by Density Functional Theory, Quantum Theory of Atoms in Molecules, and Electron Localization Function.

Adsorption of Li and Na on pristine and defective graphene and graphene oxide (GO) is studied using density functional theory (DFT) structural and electronic calculations, quantum theory of atoms in molecules (QTAIM), and electron localization function (ELF) analyses. DFT calculations show that Li and Na adsorptions on pristine graphene are not stable at all metal coverages examined here. However, the presence of defects on graphene support stabilizes both Li and Na adsorptions. Increased Li and Na coverages cause metal nucleation and weaken adsorption. Defective graphene is associated with the presence of band gaps and, thus, Li and Na adsorptions can be used to tune these gaps. Electronic calculations show that Li– and Na–graphene interactions are Coulombic: as Li and Na coverages increase, the metal valences partially hybridize with the graphene bands and weaken metal–graphene support interactions. However, for Li adsorption on single vacancy graphene, QTAIM, ELF, and overlap populations calculations show that the Li-C bond has some covalent character. The Li and Na adsorptions on GO are significantly stronger than on graphene and strengthen upon increased coverages. This is due to Li and Na forming bonds with both carbon and oxygen GO atoms. QTAIM and ELF are used to analyze the metal–C and metal–metal bonds (when metal nucleation is present). The Li and Na clusters may contain both covalent and metallic intra metal–metal bonds: This effect is related to the adsorption support selection. ELF bifurcation diagrams show individual metal–C and metal–metal interactions, as Li and Na are adsorbed on graphene and GO, at the metal coverages examined here.

Ostium Ratio and Neck Ratio Could Predict the Outcome of Sidewall Intracranial Aneurysms Treated with Flow Diverters.

Incompletely occluded flow diverter treated aneurysms remain at risk of rupture and thromboembolic complications. Our aim was to identify the potential for incomplete occlusion of intracranial aneurysms treated by flow diverters. We investigated whether aneurysm ostium size in relation to parent artery size affects angiographic outcomes of flow diverter-treated sidewall aneurysms. Flow diverter-treated sidewall aneurysms were divided into "occluded" and "residual" (incomplete occlusion) groups based on 6-month angiographic follow-up. We calculated the ostium ratio, a new parameter defined as the aneurysm ostium surface area versus the circumferential surface area of the parent artery. We also calculated the neck ratio, defined as clinical aneurysm neck diameter versus parent artery diameter from pretreatment 2D DSA, as a 2D surrogate. We compared the performance of these ratios with existing aneurysm morphometrics (size, neck diameter, volume, aspect ratio, size ratio, undulation index, nonsphericity index, ellipticity index, bottleneck factor, aneurysm angle, and parent vessel angle) and flow diverter-related parameters (metal coverage rate and pore density). Statistical tests and receiver operating characteristic analyses were performed to identify significantly different parameters between the 2 groups and test their predictive performances. We included 63 flow diverter-treated aneurysms, 46 occluded and 17 residual. The ostium ratio and neck ratio were significantly higher in the residual group than in the occluded group (P < .001 and P = .02, respectively), whereas all other parameters showed no statistical difference. As discriminating parameters for occlusion, ostium ratio and neck ratio achieved areas under the curve of 0.912 (95% CI, 0.838-0.985) and 0.707 (95% CI, 0.558-0.856), respectively. High ostium ratios and neck ratios could predict incomplete occlusion of flow diverter-treated sidewall aneurysms. Neck ratio can be easily calculated by interventionists to predict flow-diverter treatment outcomes.

Tailoring Intercalant Assemblies at the Graphene–Metal Interface

The influence of graphene on the assembly of intercalated material is studied using low-temperature scanning tunneling microscopy. Intercalation of Pt under monolayer graphene on Pt(111) induces a substrate reconstruction that is qualitatively different from the lattice rearrangement induced by metal deposition on Pt(111) and, specifically, the homoepitaxy of Pt. Alkali metals Cs and Li are used as intercalants for monolayer and bilayer graphene on Ru(0001). Atomically resolved topographic data reveal that at elevated alkali metal coverage (2 × 2)Cs and (1 × 1)Li intercalant structures form with respect to the graphene lattice.

Electronic Structure and Bond Relaxation at Na/Ta(110) Interfaces and 1D‐Chain and 2D‐Ring Ta Metal Structures on Na(110)

Combining bond–band–barrier (BBB), density‐functional theory (DFT) and zone‐selective electron spectroscopy (ZES) correlations, we investigate the bonding mechanisms at Na/Ta(110) and Ta/Na(110) interfaces. When the Ta metal coverage on the Na(110) surface is increased from 7/9 ML to 8/9 ML, the Ta surface distribution changed from one‐dimensional chains to two‐dimensional ring structures. Moreover, the Ta‐induced shifts in the surface binding energy (BE) on the Na(110) surface were dominated by quantum entrapment, whereas the Na‐induced shifts in the BE on the Ta(100) surface are dominated by polarization. DFT calculations reveal that the Ta atomic chain is more stable than the structure of the Ta atomic ring. The adsorption of Ta atoms on the Na(110) surface is more stable than the structure of the Na atoms adsorbing on the Ta(110) surface. The 1D chain formations and 2D ring structures on the Na(110) surface are identified and explained in this paper.

Screen printed, large area bifacial N-type back junction silicon solar cells with selective phosphorus front surface field and boron doped poly-Si/SiOx passivated rear emitter

This paper reports on the effect of screen printed metallization on the passivation quality of a boron doped poly-Si/SiOx passivated contact (PC) structure composed of a very thin Si oxide (∼15 Å) capped with boron doped poly-Si. Our boron doped poly-Si/SiOx passivated contact (p-Poly Si/SiOx PC) with a SiNx capping layer gave excellent surface passivation with a very low saturation current density of ∼5 fA/cm2. After screen printed metallization on poly-Si with a metal coverage of ∼10%, this value increased to ∼17 fA/cm2. This paper also demonstrates the fabrication of screen printed, large area (239 cm2), high efficiency (∼21%) n-base bifacial back junction Si solar cells with p-Poly-Si/SiOx PC on the rear and a phosphorus implanted n++-n+ selective front surface field. Detailed analysis is performed to quantify recombination and extract the saturation current density contributions (J0) from each layer of the cell including the metallized front surface field and the tunnel oxide passivated contact. Finally, 2D device modeling of this back junction cell is performed by implementing a simple approach which replaces the p-Poly-Si/SiOx PC by an equivalent p-n junction with the same J0 and gives a good match between the measured and simulated cell parameters using the extracted J0 and recombination velocity values.

Surface Metallization Influence on Equivalence of Laser Simulation of Dose-Rate Effects

Pulsed laser simulation is a powerful method in studying dose-rate effects of semiconductor devices. However, blocking of the incident laser from the device layer by the surface metallization layers has always been an influencing factor affecting their equivalent relationship because the penetrations of laser light and gamma rays are different. For the quantitative study of the equivalent relationship, the conversion factor (CF) is defined as a comparison between the laser intensity and dose rate at the same equivalent basis [carrier generation rate (CGR), peak photocurrent]. The conventional models usually use CGR, and they suggest that the CF varies linearly with metallization coverage, but few of them considered the metallization distribution. In this paper, we propose a numerical simulation method for CF using the peak of transient response as the equivalent basis, and unambiguously taking into account the effects of metallization distribution. Backed up with the finite-element analysis, we find that the distribution of the metallization can lead to a significant change in the linear relation between CF and metallization coverage. Experimental data on discrete devices conform to these simulation results. In addition, the experimental verification in the classical diode structure further indicates the validity and the accuracy of the approach.

Toward Better Understanding of Flow Diversion in Bifurcation Aneurysms.

Flow diversion is being increasingly used to treat bifurcation aneurysms. Empiric approaches have generally led to encouraging results, and a growing body of animal and ex vivo literature addresses the fate of target aneurysms and covered branches. Our prior investigations highlighted the dynamic nature of metal coverage provided by the Pipeline Embolization Device and suggested strategies for creating optimal single and multidevice constructs. We now address the geometric and hemodynamic aspects of jailing branch vessels and neighboring target aneurysms. Fundamental electric and fluid dynamics principles were applied to generate equations describing the relationships between changes in flow and the degree of vessel coverage in settings of variable collateral support to the jailed territory. Given the high complexity of baseline and posttreatment fluid dynamics, in vivo, we studied a simplified hypothetic system with minimum assumptions to generate the most conservative outcomes. In the acute setting, Pipeline Embolization Devices modify flow in covered branches, principally dependent on the amount of coverage, the efficiency of collateral support, and intrinsic resistance of the covered parenchymal territory. Up to 30% metal coverage of any branch territory is very likely to be well-tolerated regardless of device or artery size or the availability of immediate collateral support, provided, however, that no acute thrombus forms to further reduce jailed territory perfusion. Basic hemodynamic principles support the safety of branch coverage during aneurysm treatment with the Pipeline Embolization Device. Rational strategies to build bifurcation constructs are feasible.

Pipeline Diameter Significantly Impacts the Long-Term Fate of Jailed Side Branches during Treatment of Intracranial Aneurysms.

Although covered side branches typically remain patent acutely following Pipeline Embolization Device embolization of intracranial aneurysms, the long-term fate of these vessels remains uncertain. We therefore elected to investigate factors that may influence the long-term patency of these covered side branches. We retrospectively evaluated the long-term patency of side branches covered by the Pipeline Embolization Device at our institution during treatment of intracranial aneurysms with at least 6 months of conventional angiography follow-up. Procedural and anatomic factors that might influence the fate of covered side branches were explored. One hundred forty-eight Pipeline Embolization Device treatments in 137 patients met the inclusion criteria. In 217 covered side branches, 29 (13.4%) were occluded on follow-up, and 40 (18.4%) were stenotic. All stenoses and occlusions were asymptomatic. In the entire cohort and in the largest subset of ophthalmic arteries, a smaller Pipeline Embolization Device diameter was associated with branch vessel occlusion (P = .001, P = .013). When we considered stenotic and occluded side branches together, smaller Pipeline Embolization Device size (P = .029) and administration of intraprocedural abciximab (P = .03) predicted side branch stenosis/occlusion, while anterior choroidal branch type (P = .003) was a predictor of gross side branch patency. A smaller Pipeline Embolization Device diameter is associated with delayed side branch stenosis/occlusion following Pipeline Embolization Device treatment, likely due to the higher metal density of smaller caliber devices. Although hemodynamic factors, including the potential for collateral flow, are still paramount in determining the fate of covered side branches, the amount of metal coverage at the side branch orifice also plays an important role.

A versatile low-resistance ohmic contact process with ohmic recess and low-temperature annealing for GaN HEMTs

Deeply recessed ohmic contacts for GaN-based high electron mobility transistors (HEMTs) are demonstrated. It is shown that low-resistance ohmic contacts can be achieved with recessing beyond the AlGaN Schottky barrier where the ohmic contacts are formed on the sidewall of the recess. This makes the process versatile and relatively insensitive to the exact recess depth. The ohmic contact is based on a gold-free metallization scheme consisting of a Ta/Al/Ta metal stack requiring a low-temperature annealing. Important parameters for this type of ohmic contact process include the metal coverage, slope of the etched sidewall, bottom Ta-layer thickness, as well as annealing temperature and duration. The optimized contact resistance is as low as 0.24 Ω mm after annealing at 575 °C. Moreover, this sidewall contact approach was successfully implemented on different epitaxial heterostructures with different AlGaN barrier thickness as well as with and without AlN exclusion layer. All the samples exhibited excellent contact resistances in a wide range of recess depths. The Ta-based, sidewall ohmic contact process is a promising method for forming an ohmic contact on a wide range of GaN HEMT epitaxial designs.

Understanding and Engineering Phonon-Mediated Tunneling into Graphene on Metal Surfaces

Metal-intercalated graphene on Ir(111) exhibits phonon signatures in inelastic electron tunneling spectroscopy with strengths that depend on the intercalant. Extraordinarily strong graphene phonon signals are observed for Cs intercalation. Li intercalation likewise induces clearly discriminable phonon signatures, albeit less pronounced than observed for Cs. The signal can be finely tuned by the alkali metal coverage and gradually disappears upon increasing the junction conductance from tunneling to contact ranges. In contrast to Cs and Li, for Ni-intercalated graphene the phonon signals stay below the detection limit in all transport ranges. Going beyond the conventional two-terminal approach, transport calculations provide a comprehensive understanding of the subtle interplay between the graphene-electrode coupling and the observation of graphene phonon spectroscopic signatures.