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1. Limits to the strain engineering of layered square-planar nickelate thin films

   https://doi.org/10.1038/s41467-023-37117-4  

   Ferenc Segedin, Dan; Goodge, Berit H.; Pan, Grace A.; Song, Qi; LaBollita, Harrison; Jung, Myung-Chul; El-Sherif, Hesham; Doyle, Spencer; Turkiewicz, Ari; Taylor, Nicole K.; et al (December 2023, Nature Communications)

   Abstract The layered square-planar nickelates, Nd n +1 Ni n O 2 n +2 , are an appealing system to tune the electronic properties of square-planar nickelates via dimensionality; indeed, superconductivity was recently observed in Nd 6 Ni 5 O 12 thin films. Here, we investigate the role of epitaxial strain in the competing requirements for the synthesis of the n  = 3 Ruddlesden-Popper compound, Nd 4 Ni 3 O 10 , and subsequent reduction to the square-planar phase, Nd 4 Ni 3 O 8 . We synthesize our highest quality Nd 4 Ni 3 O 10 films under compressive strain on LaAlO 3 (001), while Nd 4 Ni 3 O 10 on NdGaO 3 (110) exhibits tensile strain-induced rock salt faults but retains bulk-like transport properties. A high density of extended defects forms in Nd 4 Ni 3 O 10 on SrTiO 3 (001). Films reduced on LaAlO 3 become insulating and form compressive strain-induced c -axis canting defects, while Nd 4 Ni 3 O 8 films on NdGaO 3 are metallic. This work provides a pathway to the synthesis of Nd n +1 Ni n O 2 n +2 thin films and sets limits on the ability to strain engineer these compounds via epitaxy. 

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2. Phase transition of Al ₂ O ₃ -encapsulated MoTe ₂ via rapid thermal annealing

   https://doi.org/10.1063/5.0097844  

   Sengupta, Rohan; Dangi, Saroj; Krylyuk, Sergiy; Davydov, Albert V.; Pavlidis, Spyridon (July 2022, Applied Physics Letters)

   Among group VI transition metal dichalcogenides, MoTe 2 is predicted to have the smallest energy offset between semiconducting 2H and semimetallic 1T′ states. This makes it an attractive phase change material for both electronic and optoelectronic applications. Here, we report fast, nondestructive, and full phase change in Al 2 O 3 -encapsulated 2H-MoTe 2 thin films to 1T′-MoTe 2 using rapid thermal annealing at 900 °C. Phase change was confirmed using Raman spectroscopy after a short annealing duration of 10 s in both vacuum and nitrogen ambient. No thickness dependence of the transition temperatures was observed for flake thickness ranging from 1.5 to 8 nm. These results represent a major step forward in understanding the structural phase transition properties of MoTe 2 thin films using external heating and underline the importance of surface encapsulation for avoiding thin film degradation. 

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3. Bright mid-infrared photoluminescence from high dislocation density epitaxial PbSe films on GaAs

   https://doi.org/10.1063/5.0070555  

   Meyer, Jarod; Muhowski, Aaron_J; Nordin, Leland; Hughes, Eamonn; Haidet, Brian; Wasserman, Daniel; Mukherjee, Kunal (November 2021, APL Materials)

   We report on photoluminescence in the 3–7 µm mid-wave infrared (MWIR) range from sub-100 nm strained thin films of rocksalt PbSe(001) grown on GaAs(001) substrates by molecular beam epitaxy. These bare films, grown epitaxially at temperatures below 400 °C, luminesce brightly at room temperature and have minority carrier lifetimes as long as 172 ns. The relatively long lifetimes in PbSe thin films are achievable despite threading dislocation densities exceeding 109 cm−2 arising from island growth on the nearly 8% lattice- and crystal-structure-mismatched GaAs substrate. Using quasi-continuous-wave and time-resolved photoluminescence, we show that the Shockley–Read–Hall recombination is slow in our high dislocation density PbSe films at room temperature, a hallmark of defect tolerance. Power-dependent photoluminescence and high injection excess carrier lifetimes at room temperature suggest that degenerate Auger recombination limits the efficiency of our films, although the Auger recombination rates are significantly lower than equivalent III–V bulk materials and even a bit slower than expectations for bulk PbSe. Consequently, the combined effects of defect tolerance and low Auger recombination rates yield an estimated peak internal quantum efficiency of roughly 30% at room temperature, unparalleled in the MWIR for a severely lattice-mismatched thin film. We anticipate substantial opportunities for improving performance by optimizing crystal growth as well as understanding Auger processes in thin films. These results highlight the unique opportunity to harness the unusual chemical bonding in PbSe and related IV–VI semiconductors for heterogeneously integrated mid-infrared light sources constrained by tight thermal budgets in new device designs. 

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4. Hall Effect in Polycrystalline Organic Semiconductors: The Effect of Grain Boundaries

   https://doi.org/10.1002/adfm.201903617  

   Choi, Hyun_Ho; Paterson, Alexandra_F; Fusella, Michael_A; Panidi, Julianna; Solomeshch, Olga; Tessler, Nir; Heeney, Martin; Cho, Kilwon; Anthopoulos, Thomas_D; Rand, Barry_P; et al (July 2019, Advanced Functional Materials)

   Abstract Highly crystalline thin films in organic semiconductors are important for applications in high‐performance organic optoelectronics. Here, the effect of grain boundaries on the Hall effect and charge transport properties of organic transistors based on two exemplary benchmark systems is elucidated: (1) solution‐processed blends of 2,7‐dioctyl[1]benzothieno[3,2‐b][1]benzothiophene (C₈‐BTBT) small molecule and indacenodithiophene‐benzothiadiazole (C₁₆IDT‐BT) conjugated polymer, and (2) large‐area vacuum evaporated polycrystalline thin films of rubrene (C₄₂H₂₈). It is discovered that, despite the high field‐effect mobilities of up to 6 cm²V⁻¹s⁻¹and the evidence of a delocalized band‐like charge transport, the Hall effect in polycrystalline organic transistors is systematically and significantly underdeveloped, with the carrier coherence factor α < 1 (i.e., yields an underestimated Hall mobility and an overestimated carrier density). A model based on capacitively charged grain boundaries explaining this unusual behavior is described. This work significantly advances the understanding of magneto‐transport properties of organic semiconductor thin films. 

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5. Comparative Solution Synthesis of Mn Doped (Na,K)NbO3Thin Films

   Kovacova, Veronika; Yang, Jung In; Jacques, Leonard; Ko, Song Won; Zh, Wanlin; Trolier-McKinstry, Susan (July 2020, Chemistry)

   null (Ed.)

   (K0.5Na0.5)NbO3 (KNN) is a promising lead-free alternative for ferroelectric thin films such as Pb(Zr,Ti)O3. One main drawback is its high leakage current density at high electric fields, which has been previously linked to alkali non-stoichiometry. This paper compares three acetate-based chemical solution synthesis and deposition methods for 0.5 mol% Mn-doped KNN film fabrication, using lower crystallization temperature processes in comparison to the sintering temperatures necessary for fabrication of KNN ceramics. This paper shows the crucial role of the A site homogenization step during solution synthesis in preserving alkali chemical homogeneity of Mn doped KNN films. Chemically homogeneous films show a uniform grain size of 80 nm and a leakage current density under 2.8V10@8 Acm@2 up to electric fields as high as 600 kVcm@1, which is the highest breakdown strength reported for KNN thin films. Solution synthesis involving two-step pyrolysis resulted in films with dense, columnar microstructures, which are interesting for orientation control and enhancement of piezoelectric properties. This study reports detailed solution synthesis and deposition processes with good dielectric, ferroelectric and breakdown field properties. An optimized fabrication method that should couple low leakage current density with dense and oriented microstructures is proposed. 

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