Paul Drude Institute for Solid State Electronics

ABSTRACTRecent advances in molecular design have seen peripheral alkoxy groups replaced with alkylthienyl substituents in conjugated polymers for high‐performing organic photovoltaics. However, little is known about the mechanistic origins behind this improvement in performance. In this work, transient absorption spectroscopy is used in conjunction with resonance Raman spectroscopy to shed light on this question. Alkoxy‐substituted polymer PBDB is compared with alkylthienyl‐substituted PBDB‐T in blends with two different acceptors. Larger charge photogeneration yields are observed for PBDB‐T:ITIC‐Th compared to the PBDB:ITIC‐Th blend due to more efficient hole transfer. However, an active triplet formation mechanism via charge recombination leaves the two blends with similar polaron populations on longer timescales. Importantly, resonance Raman spectroscopy demonstrates that the alkylthienyl groups in PBDB‐T enable a stronger coupling of the S1 state to the benzodithiophene unit compared to the benzodithiophene‐dione unit. The observation that this localization of electron density on the benzodithiophene unit only occurs for PBDB‐T, and most prominently in the presence of the ITIC‐Th, suggests a strong interaction between the PBDB‐T and ITIC‐Th. This interaction may facilitate the more efficient hole transfer and greater charge photogeneration yields observed for PBDB‐T:ITIC‐Th. These results demonstrate the mechanistic origin of a valuable structure‐function relationship.

Abstract Highly regular porous armchair graphene nanoribbons having a width of 12 atoms were synthesized recently. A model for investigating the quantum transport properties in such nanoribbons is developed by using a tight-binding lattice. The mode propagation in the nanoribbons is examined by analyzing the equations of motion. The lattice-Green-function technique is employed in calculating the quantum transmission coefficients of the Bloch modes. The calculation method of the lattice Green functions is adjusted to be suitable for the porous geometry of the nanoribbons for the situations where the conventional method is inapplicable due to the presence of the pores. The quantum transport exhibits a Fabry–Pérot interference when the porous nanoribbon is sandwiched by pristine nanoribbons. The lower envelop of the interference oscillation agrees with the prediction derived from the scattering properties at a single porous-pristine junction in the single-mode transport regime. While the oscillation envelop cannot be predicted in general in the multi-mode transport regime due to the random superposition of the oscillation components associated with the individual modes, the transmission is found to become even less than the predicted lower bound occasionally. In addition, the modes are not always independent from each other in the superposition. The comparison of the envelop form is made also for the quantum interference properties in a porous-pristine-porous junction. The violation of the lower bound is extensive including the single-mode regime when the coexisting Fano resonance states appear to fuse with the Fabry–Pérot interference states.

• Demonstrated Li diffusion in Ga 2 O 3 epilayers relevant for pn heterojunctions. • Li-diffusion can induce a carrier system in otherwise insulating β-Ga 2 O 3. • Depth-dependent redistribution of deep level defects in Ga 2 O 3 upon Li-diffusion. • Additional Raman mode compatible with the presence of Li in the oxide matrix. • Suggested Li diffusion mechanism in β-Ga 2 O 3 through experiments and theory. The integration of lithium based compounds ( e.g. , Li:NiO/Ga 2 O 3 , LiGa 5 O 8 /Ga 2 O 3 ) in Ga 2 O 3 based pn -heterojunctions raises concerns about interface stability, i.e. , Li diffusion effects on Ga 2 O 3 properties. In this work the ex-situ diffusion of Li is investigated in three different Ga 2 O 3 epilayers systems [(001) κ-Ga 2 O 3 and (−201) β-Ga 2 O 3 heteroepitaxy on (001) α-Al 2 O 3 , and (010) β-Ga 2 O 3 homoepitaxy] at relevant temperatures for the synthesis / processing of Li-based epilayers. It is here experimentally demonstrated and quantified the Li diffusion in all the investigated Ga 2 O 3 epilayers systems and Li bulk ( D Li,bulk ) and 2D defects ( D Li,2D ) diffusion coefficients are provided. In the case of the (010) β-Ga 2 O 3 homoepitaxial layer (nominally free of structural defects), hybrid functional theory calculations foresee a diffusion mechanism mediated by Ga vacancies (V Ga ). Moreover, in the (010) β-Ga 2 O 3 homo-layer a significant effect on its functional properties ( e.g., additional Raman vibrational modes, induced conductivity in an otherwise insulating sample) upon the Li-diffusion process is experimentally highlighted and tentatively related to the passivation of acceptor defects ( i.e. , formation of V Ga - n Li complexes).

In this work, a studyof the structural and electronic propertiesof epitaxial GeTe-rich (GeTe)m(Sb2Te3)n alloys grownon Si substrate by molecular beam epitaxy is presented, with particularfocus on the effects of annealing at increasing temperatures. Thesamples, displaying a lamellar structure stabilized by epitaxy, wereinvestigated by X-ray diffraction and X-ray photoemission spectroscopyafter heating in ultra-high vacuum. The combined use of these techniques,supported by density functional theory calculations, reveals compositionaland structural changes induced by annealing, clarifying how the rearrangementof residual defects influences the stacking order of the epilayers.These results provide key insights into the vacancy ordering of GeTe-rich(GeTe)m(Sb2Te3)n, which are particularly relevant not onlyfor memory applications but also in light of the recent discoveryof (GeTe)m(Sb2Te3)n ferroelectricity.

The band gap of (In,Ga)N spans the entire visible spectral range. Thus, this material is very attractive for applications involving light emission or absorption. The nanowire morphology offers additional benefits such as efficient elastic strain relaxation. At the same time, bottom-up growth faces the challenge that changes in composition typically affect the morphology. Here, it is shown how this challenge can be overcome by combining the growth of thick (In,Ga)N layers by molecular beam epitaxy with the subsequent top-down fabrication of nanowires. Thus, composition and morphology are independently controlled during growth and structuring, respectively. The patterning is based on metal dewetting, which is a scalable approach that can be employed for large wafers. Furthermore, it is explained how such nanowires can serve as the basis for the fabrication of (In,Ga)N pseudo-substrates by coalescence overgrowth. These pseudo-substrates are promising for the epitaxial growth of red light-emitting diodes (LEDs) that require a high In content.

This manuscript presents a working model linking chemical disorder and transport properties in correlated‐electron perovskites with high‐entropy formulations and a framework to actively design them. This work demonstrates this new learning in epitaxial Srx(Ti,Cr,Nb,Mo,W)O3 thin films that exhibit exceptional crystalline fidelity despite a diverse chemical formulation where most B‐site species are highly misfit with respect to valence and radius. X‐ray diffraction, X‐ray photoelectron spectroscopy, and transmission electron microscopy confirm a unique combination of chemical disorder and structural perfection in thin and thick epitaxial layers. This combination produces an optical transparency window that surpasses that of the constituent end‐members in the UV and IR, while maintaining relatively low electrical resistivity. This work addresses the computational challenges of modeling such systems and investigate short‐range ordering using cluster expansion. These results showcase that unusual d‐metal combinations access an expanded property design space that is predictable using end‐member characteristics and their interactions – though unavailable to them – thus offering performance advances in optical, high‐frequency, spintronic, and quantum devices.

The monolithic integration of III-Sb materials on Si substrates enables the development of novel functionalities and promising device applications. However, due to the large lattice mismatch to Si, direct heteroepitaxy of III-Sb results in a very large and disadvantageous dislocation density. Here, we report on the effect of strained AlSb interlayers in GaSb as dislocation filters for the epitaxy on Si(001) substrates. The epitaxial stress introduced by the lattice mismatched AlSb layer leads to threading dislocation bending at the interfaces as a strain relieving mechanism, thereby facilitating the reaction and annihilation of dislocations during their lateral glide process. Using comprehensive electron channeling contrast imaging and scanning transmission electron microscopy, we measure the dislocation density at the surface and after filter and cap layers of different thicknesses and analyze the effect of plastic strain relaxation on dislocation reduction. The reduction mechanism is discussed in the context of misfit dislocation networks forming at the respective compressively and tensile strained interfaces. With the new findings, an effective double filter layer structure with adopted layer thicknesses could be realized, which has resulted in a very low threading dislocation density of 1.7×107cm−2.

Oxygen tracer diffusion experiments were used to characterize oxygen transport, and thus oxygen-vacancy behavior, in thin films of the perovskite-type oxide LaInO3, grown by molecular beam epitaxy (MBE) on DyScO3 substrates. 18O2/16O2 exchange anneals were performed at temperatures 620≤T/K≤920 at an oxygen activity of aO2=0.200, and also at varying oxygen activities of 0.018≤aO2≤0.500 at T=873 K; isotope penetration profiles in the films were measured subsequently by time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profiling. Complementary molecular dynamics simulations of oxygen tracer diffusion in undoped LaInO3, with three different sets of interatomic potentials, were also carried out as a function of temperature. From the analysis of both experimental and computational tracer diffusion data, we concluded that the oxygen vacancies present in the LaInO3 films form in order to charge compensate acceptor-type point defects (most probably, lanthanum vacancies introduced in the MBE growth process). ToF-SIMS profiles also yielded evidence of faster cation (Dy and Sc) diffusion (but not of faster anion diffusion) along dislocations in the LaInO3 thin films at T=1173 K, behavior consistent with positive dislocation cores surrounded by negative space-charge tubes.

An in situ transmission electron microscopy study of Bi quantum dot (QD) formation in an annealed GaAsBi/AlAs multiple quantum well (MQW) structure is presented in this work. The investigated structure, containing two GaAsBi QWs and embedded in an AlGaAs parabolic quantum barrier (PQB), was grown on semi-insulating GaAs (100) and was transferred onto an in situ heating holder (DENS solutions) and heated up to 650 °C. Sample evolution was continuously recorded in situ in bright-field STEM mode. The analysis revealed that QD formation occurs at lower annealing temperatures in case of in situ heating of lamella than in bulk. In addition, we find that the mechanism governing Bi QD formation is different in the in situ TEM experiment compared to bulk ex-situ annealing. Comparison of the ex-situ and in situ annealed structures, as well as in-depth postannealed structure TEM analysis, is presented.

Stacking of freestanding membranes enables the formation of interfaces beyond what can be obtained with classical heteroepitaxy. In particular, twisted interfaces provide unique physical properties not existent in the corresponding individual layers. An ideal twist grain boundary yields an in-plane screw-dislocation network, assuming sufficiently strong interactions across the interface, for example, via covalent or ionic bonding. Hereby, the distance between dislocation lines, that is the length scale of the Moiré pattern, is set by the twist angle between the adjacent crystalline surfaces and the lattice mismatch in case that different materials are placed together. The associated strain gradients of the periodic pattern are especially appealing for oxide-based perovskites due to the intricate connection between surface polarization and subtle structural deformations such as the oxygen octahedra tilt. Recently, freestanding oxide perovskites became available via the sacrificial layer approach, opening a pathway toward oxide-based Moiré materials. Here, we demonstrate efficient bonding of a freestanding SrTiO3 layer to a SrTiO3 single-crystal by initially conducting a wafer-bonding process at high temperature and only subsequently dissolving the sacrificial layer. We investigate the twisted SrTiO3/SrTiO3 interface with x-ray diffraction in grazing incidence geometry and observe clear signatures of a highly periodic lateral superlattice consistent with the formation of a screw-dislocation network. Our work demonstrates a robust route for the fabrication of twisted perovskites and their development into a functional material platform with designed strain gradients at the nanoscale.