LPE (Italy)

This paper reports on the structural and electrical properties of AlGaN/GaN heterostructures grown onto 2°-off-axis silicon carbide (4H–SiC) epitaxial layers. The quality of the grown heterostructures was assessed by a combination of different analytical techniques, revealing a density of conductive dislocations in the order of 1 × 109 cm−2. Electrical characterizations of test patterns and transistors provided insights into the transport properties in the two-dimensional electron gas channel, demonstrating a high current density and a field effect mobility (μFE) of about 1800 cm2/Vs. The temperature dependence of μFE allowed us to identify optical phonon scattering as the dominant transport mechanism in the system at high temperature. In addition to demonstrating the suitability of 2°-off-axis 4H–SiC epilayers for the growth of functional AlGaN/GaN high electron mobility transistors, these results provide useful insights for device manufacturers aiming at the monolithic integration of novel GaN devices onto SiC epitaxial layers.

Perinatal opioid use disorder is increasing. Integrated obstetric/addiction care models likely optimize parent-infant dyad outcomes, but the ideal combination of services is unknown. This study (1) describes pregnancy-to-postpartum service utilization by people receiving buprenorphine at an integrated Obstetric/Addiction Clinic and (2) explores the association between service utilization and postpartum buprenorphine continuation. This retrospective medical record review study uses research registry data from an outpatient Obstetric/Addiction Clinic. All patients are invited to participate in the research registry. For patients who consent, monthly medical record abstractions are conducted beginning with their first clinic visit to collect demographic, obstetric, and substance use disorder treatment variables. Present analyses included patients who delivered an infant between June 2019 and June 2021, started buprenorphine during pregnancy, and were receiving buprenorphine at delivery. Overall service utilization was the number of services (range 0-12) used between 28-weeks gestation and 12-weeks postpartum. Bivariate analyses and multivariable logistic regression assessed associations between service utilization and buprenorphine continuation. Participants (n = 42) were primarily non-Latinx White (67%) with comorbid psychiatric diagnoses (95%). On average, participants used 6 services; prenatal care, mental health care, and postpartum contraception were most utilized. Overall, 69% of participants continued buprenorphine at 6 months postpartum. This did not differ by level of service utilization (bivariate [ P = 0.07], multivariable [ P = 0.16]). Integrated care with a harm reduction focus supports pregnancy-to-postpartum service utilization and buprenorphine continuation in a patient sample at high risk for medication for opioid use disorder discontinuation. Further work is needed to identify evidence-based methods to individualize integrated obstetric/addiction care.

This work provides a comprehensive investigation of nitrogen and aluminum doping and its consequences for the physical properties of 3C-SiC. Free-standing 3C-SiC heteroepitaxial layers, intentionally doped with nitrogen or aluminum, were grown on Si (100) substrate with different 4° off-axis in a horizontal hot-wall chemical vapor deposition (CVD) reactor. The Si substrate was melted inside the CVD chamber, followed by the growth process. Micro-Raman, photoluminescence (PL) and stacking fault evaluation through molten KOH etching were performed on different doped samples. Then, the role of the doping and of the cut angle on the quality, density and length distribution of the stacking faults was studied, in order to estimate the influence of N and Al incorporation on the morphological and optical properties of the material. In particular, for both types of doping, it was observed that as the dopant concentration increased, the average length of the stacking faults (SFs) increased and their density decreased.

Free standing 3C-SiC wafers with a dimeter of 50 mm and a thickness of ca. 0.8 mm have been grown on a regular base using 3C-SiC CVD seed transfer from Si wafers to a poly-SiC-carrier and a sublimation epitaxy configuration. Up to the thickness of almost 1 mm, stable growth conditions of the cubic polytype have been achieved. The high supersaturation was kept stable by the proper design of the hot zone that enables a high axial temperature gradient at the growth interface. The Sirich gas phase was realized by the application of a Tantalum getter that was integrated into the graphitebased growth cell. Furthermore, an adaption of the growth setup allowed the growth of 3C material with a diameter of 95 mm and bulk material up to 3 mm on 25 mm diameter. Computer simulations were used to determine the supersaturation of the growth setup for different source-to-seed distances. The minimum supersaturation necessary for stable growth of cubic SiC was found to be higher 0.1 for seed already containing the required 3C polytype.

We report the study of the effect of the growth rate and of the doping on the stress and the defect density of a Cubic Silicon Carbide (3C-SiC) bulk layer grown at low temperature on a silicon substrate. After the growth process, the silicon substrate was melt inside the CVD reactor used for the deposition and then the intrinsic stress was measured by the curvature of the wafer without influence of the thermal stress between silicon and 3C-SiC. A considerable increase of the curvature was observed increasing the doping of the layer. The average stress is compressive and then produces a convex bow. At the same time, the average quality of the grown material deteriorates increasing the doping concentration. Using μ-Raman measurement in cross-section of the 3C-SiC grown samples, it was possible to observe the dependence of the stress and of the quality of the material as a function of the thickness and of the growth rate, due to the variation of the growth rate during the process. In particular, the increase of the growth rate produced both an increase of the stress and a decrease of the material quality. Furthermore, the increase of the doping concentration produced both an increase of the stress and a further deterioration of the crystal quality.

In this work, results related to the temperature influence on the homo-epitaxial growth process of 3C-SiC is presented. The seed for the epitaxial layer was obtained by an innovative technique based on silicon melting: after the first step of the hetero-epitaxial growth process of 3C-SiC on a Si substrate, Si melts, and the remaining freestanding SiC layer was used as a seed layer for the homo-epitaxial growth. Different morphological analyses indicate that the growth temperature and the growth rate play a fundamental role in the stacking faults density. In details, X-ray diffraction and micro-Raman analysis show the strict relationship between growth temperature, crystal quality, and doping incorporation in the homo-epitaxial chemical vapor deposition CVD growth process of a 3C-SiC wafer. Furthermore, photoluminescence spectra show a considerable reduction of point defects during homo-epitaxy at high temperatures.

3C-SiC is a promising material for low-voltage power electronic devices but its growth is still challenging. Heteroepitaxy of 3C-SiC on Si micrometer-sized pillars is regarded as a viable method to achieve high crystalline quality, minimizing the effects of lattice and thermal expansion mismatch. Three-dimensional micro-crystals with sharply-faceted profiles are obtained, eventually touching with each other to form a continuous layer, suspended on the underlying pillars. By comparing experimental data and simulation results obtained by a phase-field growth model, here we demonstrate that the evolution of the crystal morphology occurs in a kinetic regime, dominated by the different incorporation times on the crystal facets. These microscopic parameters, effective to characterize the out-of-equilibrium growth process, are estimated by a best-fitting procedure, matching simulation profiles to the experimental one at different deposition stages. Then, simulations are exploited to inspect the role of a different pillar geometry and template effects are recognized. Finally, coalescence of closely spaced crystals ordered into an hexagonal array is investigated. Two possible alignments of the pattern are compared and the most convenient arrangement is evaluated.

The cubic polytype of SiC (3C-SiC) is the only one that can be grown on silicon substrate with the thickness required for targeted applications. Possibility to grow such layers has remained for a long period a real advantage in terms of scalability. Even the relatively narrow band-gap of 3C-SiC (2.3eV), which is often regarded as detrimental in comparison with other polytypes, can in fact be an advantage. However, the crystalline quality of 3C-SiC on silicon has to be improved in order to benefit from the intrinsic 3C-SiC properties. In this project new approaches for the reduction of defects will be used and new compliance substrates that can help to reduce the stress and the defect density at the same time will be explored. Numerical simulations will be applied to optimize growth conditions and reduce stress in the material. The structure of the final devices will be simulated using the appropriated numerical tools where new numerical model will be introduced to take into account the properties of the new material. Thanks to these simulations tools and the new material with low defect density, several devices that can work at high power and with low power consumption will be realized within the project.

Smooth pursuit eye movements in primates unveil the temporal dynamics of 2D motion integration. Tracking of single, tilted bars is always initiated in a direction close to the velocity orthogonal to the bar orientation. This initial bias is gradually reduced: 300ms after visual motion onset both target and eye movement directions are perfectly aligned. This time course is believed to reflect the temporal dynamics of the neural solution for the aperture problem in macaque area MT (Pack et al., 2001). We investigated how high-level cues can influence this temporal dynamics. Eye movements were recorded in macaque monkeys using the scleral search coil technique. Long (20°) bars were drifted along the horizontal direction (speed 10°/s) with different bar orientations (−45°, 0 and +45° relative to the vertical axis). We found no effect of a static presentation (500ms) prior setting target into motion with unpredictable directions. In a second set on experiments, we blocked both bar orientation and direction conditions so that 2D motion was fully predictable. We found strong anticipatory responses along the 2D motion trajectory. However, 100ms after target visual motion onset, strong pursuit biases were again observed suggesting that predicting target motion had no influence upon solving the aperture problem. In a final set of experiment, we briefly (200ms) blanked target motion during steady-state tracking (i.e. when pursuit and target directions were aligned). Blanking the image drastically reduced eye velocity and in many cases pursuit was stopped. At target reappearance, monkeys reinitiated pursuit responses along the orthogonal direction to bar orientation, albeit with a weaker direction bias. These results suggest that high-level prediction about incoming target motion cannot be used to solve the aperture problem. This demonstrates that low-level motion computation remains impenetrable to cognitive factors to preserve the ability to quickly react to new visual motion input.