Session Q10: Superlattices

Self-limiting surface oxidation of II-VI Organic-Inorganic Hybrid Nanostructures

Contrary to the common experience that organic-inorganic hybrid materials, such as hybrid halide perovskites, exhibit poor low-term stability, some II-VI based hybrids show no apparent degradation over 15 years unprotected in air [1,2].  We investigate the surface properties of aged and new ZnTe(en)_0.5, using Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) and Raman spectroscopy. We elucidate the surface layer composition and quantify the thickness of the oxidation layer. Notably, our investigation unveils the presence of a thin but dense oxidation layer on the surface of these hybrid materials. The remarkable finding is that this oxidation layer, while thin, effectively shields the underlying material from degradation, explaining the extraordinary long-term stability. Our research significantly enhances our understanding of the properties of II-VI organic-inorganic hybrid nanostructures, providing valuable insights into their surface chemistry and structural robustness. These insights carry implications for practical applications, underlining the potential for harnessing these materials in real-world electronic devices and beyond.

[1] Tang et al., ACS Nano 15, 10565 (2021); [2] Tang et al., Small, doi.org/10.1002/smll.202302935   

Anisotropic Electronic Transport of II-VI Organic-Inorganic Hybrid Materials

Organic-inorganic hybrid materials provide unique properties and have several advantages that are not available in either organic or inorganic materials, such as a wide range of tunable optical and electronic properties, lattice-matching flexibility, improved processability, and low-cost fabrication. However, they often suffer from poor long-term stability and crystal structure disorder. β-ZnTe(en)_0.5 is a member of II-VI-based organic-inorganic hybrid nanostructures, exhibiting a uniform and fully ordered short-period superlattice structure without physical and chemical fluctuations [1]. The thickness of the inorganic sheets is comparable to 2D materials, and the structure can be viewed as a periodically stacked 2D material. The exceptional long-term stability of β-ZnTe(en)_0.5 make it promising for (opto)electronic applications [1, 2]. We apply a Space-Charge-Limited Current method to determine the carrier mobility of β-ZnTe(en)_0.5 along different symmetry axes. Along the organic-inorganic stacking direction, the mobility is in the order of 10^-3 cm²/(Vs), and in the plane parallel to the inorganic sheets, the mobility is anisotropic, in the order of 10 - 100 cm²/(Vs).

[1] Tang et al., ACS Nano 15, 10565 (2021); [2] Tang et al., Small, doi.org/10.1002/smll.202302935   

Title: Super-lattice and Magnetic Order in Iron Intercalated Bilayer Tantalum Disulfide

Atomically thin, magnetically doped transition metal dichalcogenides (TMDCs) are promising 2D magnets, offering significant tunability over existing 2D magnets that derive their magnetism from the host lattice. However as synthetic routes have only been recently developed for thin film intercalated magnets, the phase space in these materials has not yet been studied computationally. We investigate iron intercalated bilayer 2H-TaS2 with density functional theory, results from which predict a relatively large magnetic anisotropy favoring c-axis orientation, magnetic behavior strongly coupled to the local crystal field symmetry and thus the van der waals gap, and a strong preference for octahedral site occupancy. The Ising-type spins, strong preference for octahedral site occupation, and low magnetic ordering temperature permit use of a simple decoupled treatment of magnetic and super-lattice order. The resultant lattice Hamiltonian is investigated with Monte Carlo to predict the magnetic and intercalant phase space of this material and how it can be manipulated via strain and doping.   

Minimization of dark currents through Type-II Superlattice infrared photodetectors in the MWIR region

Type-II Superlattice (T2SL) photodetectors are at the forefront of optoelectronic detector technology, primarily attributed to their excellent band gap tunability and outstanding optoelectronic characteristics. However, the presence of dark current necessitates the use of an extra cryogenic setup and limits the detector’s performance. To understand the minimization of dark currents, we analyze the dark current characteristics of a 10monolayer (ML) /10ML InAs/GaSb T2SL p-i-n photodetector, simulated in a reverse bias voltage range at 77K. The doping concentration for the active region and the value of the minority carrier lifetime is selected as 5.5 X 10¹⁵ cm^-3  and 40ns [1] respectively. With an increase in the thickness of the active region, we observe a decrease in the electric field within it. Consequently, we achieve a remarkable reduction in the band-to-band tunneling current density, decreasing from 8.5 X 10^-1 A/cm² to 2.3 X 10^-3 A/cm²  and we observe that the trap-assisted tunneling current density decreases from  8.25 X 10^-4   A / cm²   to 7.5 X 10^-5   A / cm²  as we increase the thickness from 300nm to 700nm. The detailed computational models developed here establish the roles of various components of the dark currents in conjunction with the details of confinement effects, thereby setting a stage for the analysis of current generation photodetector devices. 

References:

[1] Le Thi, Yen, et.al., Japanese Journal of Applied Physics 58.4 (2019): 044002.     

Quantum Geometric Oscillations in Two-Dimensional Flat-Band Solids

Two-dimensional van der Waals heterostructures can be engineered into artificial superlattices that host flat bands with significant Berry curvature and provide a favorable environment for the emergence of novel electron dynamics. In particular, the Berry curvature can induce an oscillating trajectory of an electron wave packet transverse to an applied static electric field. Though analogous to Bloch oscillations, this novel oscillatory behavior is driven entirely by quantum geometry in momentum space instead of band dispersion. While the orbits of Bloch oscillations can be localized by increasing field strength, the size of the geometric orbits saturates to a nonzero plateau in the strong-field limit. In non-magnetic materials, the geometric oscillations are even under inversion of the applied field, whereas the Bloch oscillations are odd, a property that can be used to distinguish these two co-existing effects.   

Modeling of Meta-Graphene and Valleytronics on a Semiconductor

Two-dimensional materials fabricated as periodic nanostructures on a semiconducting surface offer versatile tunability. Adjusting the parameters of the nanopatterning alters the electronic properties of the material, thus opening new possibilities for designing novel electronic devices and exploring quantum geometric phenomena. In this work we model the electronic properties of artificially-tailored semiconducting quantum materials (such as meta-graphene and related valley Hall topological insulators), including the topological properties of interface states. To this end we consider a two-dimensional electron gas system patterned with periodic potentials. Using parameters achievable experimentally, we demonstrate systems exhibiting nonzero valley Chern numbers that protect the existence of domain wall edge states. We perturb these domain wall edge states with both charge-puddle induced disorder and random imperfections in the periodic potential and confirm they are robust under realistic experimental conditions.   

Interfacial Study of SrIrO3-SrNbO3 Superlattices Grown by Hybrid Molecular Beam Epitaxy

In recent years, there has been a significant surge of interest in 4d and 5d transition metal oxides due to their strong spin-orbit interactions and the consequent exotic phases such as correlated topological electronic states and spin structures. The heterointerface of such oxides is an excellent platform to investigate the intricate interplay among different order parameters, charge transfer phenomena, and emergent interfacial effects. SrNbO₃ is highly promising as a donor material and could be expected to donate electrons to SrIrO₃ [1], filling the hole-like pockets in the SrIrO₃ Fermi surface. To probe the interfacial phenomena in such a system, we successfully synthesized (SrNbO₃)_m/(SrIrO₃)_n superlattices with different thicknesses by using a hybrid molecular beam epitaxy system. The growth and crystallinity of the superlattices were monitored utilizing in-situ reflected high-energy electron diffraction, followed by ex-situ high-resolution XRD. The structures were also examined using scanning transmission electron microscopy (STEM). Temperature-dependent electrical transport measurements were carried out to probe the electronic nature of the superlattices. To observe the chemical states of the constituent elements and probe the charge transfer, we used in-vacuo X-ray photoelectron spectroscopy (XPS) and X-ray Absorption Spectroscopy (XAS).   

Study of Interface-Roughness Scattering Effects on Nonlinear Electron Transport in a Superlattice by Using Generalized Boltzmann Transport Equation

We have proposed an effective scattering-potential approach is for treating interface-roughness scattering of drifting electrons within a superlattice. By using an effective scattering potentials, we have developed a quasi-1D generalized Boltzmann transport equation iincluding a self-consistent internal scattering force. We have presented the dependence of a steady-state current on interface-roughness parameters at various temperatures and DC electric-field strengths based on calculated nonequilibrium electron occupation function. Our study has reveled the microscopic mechanism behind non-ohmic transport behavior by analyzing numerically computed nonequilibrium electron occupation function and its dependence on interface roughness parameters as well as the DC electric field strength.