Expanded Stability of Layered SnSe-PbSe Alloys and Evidence of Displacive Phase Transformation from Rocksalt in Heteroepitaxial Thin Films.

Bulk PbSnSe has a two-phase region, or miscibility gap, as the crystal changes from a van der Waals-bonded orthorhombic 2D layered structure in SnSe-rich compositions to the related 3D-bonded rocksalt structure in PbSe-rich compositions. This structural transition drives a large contrast in the electrical, optical, and thermal properties. We realize low temperature direct growth of epitaxial PbSnSe thin films on GaAs via molecular beam epitaxy using an in situ PbSe surface treatment and show a significantly reduced two-phase region by stabilizing the Pnma layered structure out to Pb0.45Sn0.55Se, beyond the bulk limit around Pb0.25Sn0.75Se at low temperatures. Pushing further, we directly access metastable two-phase films of layered and rocksalt grains that are nearly identical in composition around Pb0.50Sn0.50Se and entirely circumvent the miscibility gap. We present microstructural and compositional evidence for an incomplete displacive transformation from a rocksalt to layered structure in these films, which we speculate occurs during the sample cooling to room temperature after synthesis. In situ temperature-cycling experiments on a Pb0.58Sn0.42Se rocksalt film reproduce characteristic attributes of a displacive transition and show a modulation in electronic properties. We find well-defined orientation relationships between the phases formed and reveal unconventional strain relief mechanisms involved in the crystal structure transformation using transmission electron microscopy. Overall, our work adds a scalable thin film integration route to harness the dramatic contrast in material properties in PbSnSe across a potentially ultrafast crystalline-crystalline structural transition.

Gradual degradation in InAs quantum dot lasers on Si and GaAs.

Reliable quantum dot lasers on silicon are a key remaining challenge to successful integrated silicon photonics. In this work, quantum dot (QD) lasers on silicon with and without misfit dislocation trapping layers are aged for 12 000 hours and are compared to QD lasers on native GaAs aged for 8400 hours. The non-trapping-layer (TL) laser on silicon degrades heavily during this time, but much more modest gradual degradation is observed for the other two devices. Electroluminescence imaging reveals relatively uniform gradual dimming for the aged TL laser on silicon. At the same time, we find nanoscale dislocation loop defects throughout the quantum dot-based active region of all three aged lasers via electron microscopy. The Burgers vector of these loops is consistent with . We suggest that the primary source of degradation, however, is the generation and migration of point defects that substantially enhance non-radiative recombination in the active region, the visible symptom of which is the formation of dislocation loops. To prevent this, we propose that laser fabrication should be switched from deeply etched to shallow etch ridges where the active region remains intact near the mesa. Additionally, post-growth annealing and altered growth conditions in the active region should be explored to minimize the grown-in point defect density.

Impact of Transfer for Surgical Management of Preterm Necrotising Enterocolitis or Focal Intestinal Perforation.

OBJECTIVE: To compare outcomes after surgically managed necrotising enterocolitis (NEC) and focal intestinal perforation (FIP) in infants <32 weeks requiring transfer to or presenting in a single surgical centre. DESIGN: Retrospective review of transferred and inborn NEC or FIP, from January 2013 to December 2020. PATIENTS: 107 transfers with possible NEC or FIP contributed 92 cases (final diagnoses NEC (75) and FIP (17)); 113 inborn cases: NEC (84) and FIP (29). RESULTS: In infants with a final diagnosis of NEC, medical management after transfer was as common as when inborn (41% TC vs 54% p = 0.12). Unadjusted all-cause mortality was lower in inborn NEC (19% vs 27%) and FIP (10% vs 29%). In infants undergoing surgery unadjusted mortality attributable to NEC or FIP was lower if inborn (21% vs 41% NEC, 7% vs 24% FIP). In regression analysis of surgically treated infants, being transferred was associated with increased all-cause mortality (OR 2.55 (1.03-6.79)) and mortality attributable to NEC or FIP (OR 4.89 (1.80-14.97)). CONCLUSIONS: These data require replication, but if confirmed, suggest that focusing care for infants at highest risk of developing NEC or FIP in a NICU with on-site surgical expertise may improve outcomes.

III/V-on-Si MQW lasers by using a novel photonic integration method of regrowth on a bonding template.

Silicon photonics is becoming a mainstream data-transmission solution for next-generation data centers, high-performance computers, and many emerging applications. The inefficiency of light emission in silicon still requires the integration of a III/V laser chip or optical gain materials onto a silicon substrate. A number of integration approaches, including flip-chip bonding, molecule or polymer wafer bonding, and monolithic III/V epitaxy, have been extensively explored in the past decade. Here, we demonstrate a novel photonic integration method of epitaxial regrowth of III/V on a III/V-on-SOI bonding template to realize heterogeneous lasers on silicon. This method decouples the correlated root causes, i.e., lattice, thermal, and domain mismatches, which are all responsible for a large number of detrimental dislocations in the heteroepitaxy process. The grown multi-quantum well vertical p-i-n diode laser structure shows a significantly low dislocation density of 9.5 × 104 cm-2, two orders of magnitude lower than the state-of-the-art conventional monolithic growth on Si. This low dislocation density would eliminate defect-induced laser lifetime concerns for practical applications. The fabricated lasers show room-temperature pulsed and continuous-wave lasing at 1.31 µm, with a minimal threshold current density of 813 A/cm2. This generic concept can be applied to other material systems to provide higher integration density, more functionalities and lower total cost for photonics as well as microelectronics, MEMS, and many other applications.

Phase Stability and Diffusion in Lateral Heterostructures of Methyl Ammonium Lead Halide Perovskites.

Mixed halide hybrid organic-inorganic perovskites have band gaps that span the visible spectrum making them candidates for optoelectronic devices. Transport of the halide atoms in methyl ammonium lead iodide (MAPbI3) and its alloys with bromine has been observed in both dark and under illumination. While halide transport upon application of electric fields has received much attention, less is known regarding bromide and iodide interdiffusion down concentration gradients. This work provides an upper bound on the bromide-iodide interdiffusion coefficient Di in thin films of MAPb(BrxI1-x)3 using a diffusion couples of lateral heterostructures. The upper bound of Di was extracted from changes in the interface profiles of the heterostructures upon exposure to heat. The stability of thoroughly heated interfacial profiles suggests that the miscibility gap extends to higher temperatures and to a higher fractional composition of bromine than predicted by theory. The results of this work provide guidance for compositions of thermally stable heterostructures of hybrid halide perovskites.

Fast Diffusion and Segregation along Threading Dislocations in Semiconductor Heterostructures.

Heterogeneous integration of semiconductors combines the functionality of different materials, enabling technologies such as III-V lasers and solar cells on silicon and GaN LEDs on sapphire. However, threading dislocations generated during the epitaxy of these dissimilar materials remain a key obstacle to the success of this approach due to reduced device efficiencies and reliability. Strategies to alleviate this and understand charge carrier recombination at threading dislocations now need an accurate description of the structure of threading dislocations in semiconductor heterostructures. We show that the composition around threading dislocations in technologically important InGaAs/GaAs/Ge/Si heterostructures are indeed different from that of the matrix. Site-specific atom probe tomography enabled by electron channeling contrast imaging reveals this at individual dislocations. We present evidence for the simultaneous fast diffusion of germanium and indium up and down a dislocation, respectively, leading to unique compositional profiles. We also detect the formation of clusters of metastable composition at the interface between Ge and GaAs, driven by intermixing in these two nearly immiscible materials. Together, our results have important implications for the properties of dislocations and interfaces in semiconductors and provide new tools for their study.

Praseodymium Cuprate Thin Film Cathodes for Intermediate Temperature Solid Oxide Fuel Cells: Roles of Doping, Orientation, and Crystal Structure.

Highly textured thin films of undoped, Ce-doped, and Sr-doped Pr2CuO4 were synthesized on single crystal YSZ substrates using pulsed laser deposition to investigate their area-specific resistance (ASR) as cathodes in solid-oxide fuel cells (SOFCs). The effects of T' and T* crystal structures, donor and acceptor doping, and a-axis and c-axis orientation on ASR were systematically studied using electrochemical impedance spectroscopy on half cells. The addition of both Ce and Sr dopants resulted in improvements in ASR in c-axis oriented films, as did the T* crystal structure with the a-axis orientation. Pr1.6Sr0.4CuO4 is identified as a potential cathode material with nearly an order of magnitude faster oxygen reduction reaction kinetics at 600 °C compared to thin films of the commonly studied cathode material La0.6Sr0.4Co0.8Fe0.2O3-δ. Orientation control of the cuprate films on YSZ was achieved using seed layers, and the anisotropy in the ASR was found to be less than an order of magnitude. The rare-earth doped cuprate was found to be a versatile system for study of relationships between bulk properties and the oxygen reduction reaction, critical for improving SOFC performance.

An evaluation of operative management of displaced volar Barton's fractures using volar locking plate.

Volar Barton's fractures are common but more convincing treatment methods are still controversial. Malunion can cause serious disability. Twenty-six consecutive patients were treated with open reduction Internal fixation using volar locking plate. Records of 21 cases were available for follow-up for a mean period of 18 months (range 12-34 months). All 21 fractures healed within 8 to 14 weeks (mean 9.3 weeks). The wrist function was satisfactory in 90.4% cases (19 out of 21 patients). This surgical technique can provide high success rate. Locking plate has better efficacy in terms of immediate stability, maintaining anatomic reduction and early mobilisation.