Amorphous Superlattices Research Articles

Record High Power Factor and Low Thermal Conductivity in Amorphous/PbTe/Amorphous Multiple Quantum Wells

AbstractQuantum well (QW) superlattice is one of the proposals to improve the thermoelectric properties and provide a rich platform for the next generation of thermoelectric device. Previous QW have two main challenges that need to be addressed: i) decrease the electron tunneling across the layers in the semiconductor‐based multiple QWs (MQW), and ii) decrease the thermal conductivity in the oxide‐based MQW. Herein, the study demonstrates amorphous based PbTe/amorphous‐STO MQWs with ultrahigh power factor of 40.9 µW cm−1 K−2and record low thermal conductivity of ≈0.49 W m−1 K−1at room temperature. The high performance of PbTe/amorphous‐STO MQWs is attributed to strong quantum confine effect and its intrinsic low thermal conductivity of amorphous superlattice structure. The results open up a new avenue toward modulating thermoelectric properties beyond traditional MQWs of thermoelectric materials.

Quantum confinement effects in amorphous In–Ga–Zn–O thin-film transistors with quantum well channel

We confirmed that quantum confinement effects are clearly observed even in a disordered material. Employing amorphous In–Ga–Zn–O (a-IGZO) thin-film transistors (TFTs) with quantum well channels, it was observed that field-effect mobility–gate voltage relations exhibited plateaus when the well thickness was 5 nm or less. Device simulation incorporating the quantum confinement effects reproduced the well-thickness dependence of the plateau onset voltage, verifying that the quantized electronic levels in the a-IGZO well dominate the TFT characteristics and, thus, the electron transport is free electron like with a coherent length of ∼5 nm. The field-effect mobility did not exceed the drift mobility of bulk a-IGZO, which is different from amorphous silicon superlattice TFT.

Pseudocapacitance-dominated zinc storage enabled by nitrogen-doped carbon stabilized amorphous vanadyl phosphate

Rechargeable zinc-based batteries are attracting considerable attention for practical energy storage owing to their abundant reserves, low cost, and high safety. However, it remains a significant challenge to develop cathode materials with robust structure and high energy density. Layered VOPO4·xH2O was proposed as a cathode candidate for aqueous Zinc-ion batteries (ZIBs), but its structural stability and conductivity need to be further optimized. Herein, a novel amorphous superlattice of Vanadyl phosphate intercalated with Nitrogen-doped carbon (VOP/NC) is formed by organic molecule interlayer engineering and in-situ carbonization. The nitrogen-doped carbon as interlayer guest not only acts as a structure stabilizer to inhibit the dissolution of active materials, but also serves as capacity contributor to boost the pseudocapacitive capabilities. Based on the characteristics of stable structure and abundant active sites, the modified VOP/NC performs well in ZIBs in terms of long-term durability (1,000 cycles at 500 mA g−1) and high reversible capacity (187.9 mA h g−1 at 50 mA g−1). Furthermore, an intercalation pseudocapacitive Zn2+ storage mechanism accompanied by reversible cationic (Vn+/V(n−1)+) and anionic (N3−/N2−) redox reactions is proposed. This strategy is promising to intercalative layered materials for advanced zinc-based energy storage applications.

Heat conduction below diffusive limit in amorphous superlattice structures

Advanced means to further reduce the thermal conductivity of dense amorphous materials by nanostructuring is important for thermal management in future electronic and optical devices. While the works so far have realized the reduction by scattering propagons, here, we demonstrate that nanostructures can in addition inhibit transport of diffusons, by measuring the thermal conductivity of amorphous-silicon/amorphous-silica superlattices with a periodicity ranging from 4.5 nm to 29.3 nm at room temperature. The measurements show that thermal conductivity of the superlattice with periodicity below 9.5 nm is significantly below the amorphous diffusive limit (~1 W/m K). The measured thermal conductivity can be reproduced using a combination of the phonon-gas kinetics model and Allen-Feldmann theory without any fitting parameters, with the interfacial transmittance of propagons and diffusons obtained by atomistic Green’s function. In addition to the known reduction of propagon transport, the effective mean free paths of diffusons are reduced from that of the bulk (1–3 nm) to several angstroms or even atomistic by extreme boundary scattering at the interfaces in the superlattice, giving rise to a significant tunability of the amorphous thermal conductivity using truly nanoscale structures. The influence of interface scattering on the diffusion transport can be effectively described by the Boltzmann transport equation, and in this sense, propagons and diffusons do not fundamentally differ. All data needed to evaluate the conclusions in the paper are present in the paper and/or the Supplementary Materials. Additional data related to this paper may be requested from the authors.

Cross-plane thermal conductivity in amorphous Si/SiO2 superlattices

Heat conduction in superlattices demonstrates various atomic-scale effects, one of which is the ultra-low thermal conductivity. Remarkably, theoretical works even promise sub-amorphous thermal conductivity in superlattices made of amorphous materials. Yet, these predictions were not tested experimentally. Here, we experimentally study the cross-plane thermal transport in amorphous Si/SiO2 superlattices at room temperature. Using the micro time-domain thermoreflectance technique, we measured the thermal conductivity of superlattices with periods of 6.6, 11.8, and 25.7 nm. The thermal conductivity values are in the range of 1.1–1.5 W m−1 K−1 and generally agree with the values reported for amorphous Si and SiO2. However, the superlattice with the highest density of interfaces seems to have the thermal conductivity slightly below the amorphous limit. These data suggest that heat conduction below the amorphous limit might be possible in amorphous superlattices with a periodicity shorter than 6.6 nm.

Observation of two-level defect system in amorphous Se superlattices

Amorphous Se is a well-known photoconductor from its early applications in xerography and ultra-sensitive photodetectors like the High-gain Avalanche Rushing Photoconductor (HARP) device. The established way of fabricating the photoconductor for the HARP is rotational thermal evaporation using multilayers of Se and As2Se3. However, the electronic effects of multilayering have not yet been clarified. In this report, we investigated the multilayer structure as a superlattice of Se and As2Se3 fabricated using rotational evaporation and show that the superlattice structure results in the uniformization of the defect levels in the base materials. We found four energy levels associated with defects in As2Se3 and three levels in amorphous Se. In comparison, the superlattice structure of the two materials shows two clear energy levels at EC,Se − 0.533 eV and EV,Se + 0.269 eV. The resulting two occupied energy levels explain the photoelectronic and transport properties observed in multilayer amorphous Se. This result “reinvents” the multilayer structure as a material with observed quantum effects, which significantly improves the material performance in photodetection.

Synthesis of carbon planar structures with specified properties

A new method is proposed that allows an integrated technology to be used for the synthesis of carbon planar structures with predetermined properties. The method is based on the avalanche annealing of amorphous short-period superlattices. The synthesized samples are investigated by Raman spectroscopy and photoluminescence. The possibility of synthesizing carbon layers with diamond-like, graphite-like, graphene, or other structures is demonstrated experimentally using the example of a C/SiC superlattice.

Kapitza resistance and the thermal conductivity of amorphous superlattices

We report on the thermal conductivities of amorphous Stillinger-Weber and Lennard-Jones superlattices as determined by non-equilibrium molecular dynamics simulations. Thermal conductivities decrease with increasing interface density, demonstrating that interfaces contribute a non-negligible thermal resistance. Interestingly, Kapitza resistances at interfaces between amorphous materials are lower than those at interfaces between the corresponding crystalline materials. We find that Kapitza resistances within the Stillinger-Webber based Si/Ge amorphous superlattices are not a function of interface density, counter to what has been observed in crystalline superlattices. Furthermore, the widely used thermal circuit model is able to correctly predict the interfacial resistance within the Stillinger-Weber based amorphous superlattices. However, we show that the applicability of this widely used thermal circuit model is invalid for Lennard-Jones based amorphous superlattices, suggesting that the assumptions made in the model do not hold for these systems.

Amorphous sub-nanometre Tb-doped SiOxNy/SiO2 superlattices for optoelectronics

Amorphous sub-nanometre Tb-doped SiOxNy/SiO2 superlattices were fabricated by means of alternating deposition of 0.7 nm thick Tb-doped SiOxNy layers and of 0.9 nm thick SiO2 barrier layers in an electron-cyclotron-resonance plasma enhanced chemical vapour deposition system with in situ Tb-doping capability. High resolution transmission electron microscopy images showed a well-preserved superlattice morphology after annealing at a high temperature of 1000 °C. In addition, transparent indium tin oxide (ITO) electrodes were deposited by electron beam evaporation using a shadow mask approach to allow for the optoelectronic characterization of superlattices. Tb3+ luminescent spectral features were obtained using three different excitation sources: UV laser excitation (photoluminescence (PL)), under a bias voltage (electroluminescence (EL)) and under a highly energetic electron beam (cathodoluminescence (CL)). All techniques displayed Tb3+ inner transitions belonging to 5D4 levels except for the CL spectrum, in which 5D3 transition levels were also observed. Two competing mechanisms were proposed to explain the spectral differences observed between PL (or EL) and CL excitation: the population rate of the 5D3 state and the non-radiative relaxation rate of the 5D3–5D4 transition due to a resonant OH-mode. Moreover, the large number of interfaces (trapping sites) that electrons have to get through was identified as the main reason for observing a bulk-limited charge transport mechanism governed by Poole–Frenkel conduction in the J–V characteristic. Finally, a linear EL–J dependence was measured, with independent spectral shape and an EL onset voltage as low as 6.7 V. These amorphous sub-nanometre superlattices are meant to provide low-cost solutions in different areas including sensing, photovoltaics or photonics.

Thin Films and Superlattices in the Bi-Fe-O System Prepared by ALD

Bismuth ferrite (BiFeO3) is one of the most technologically promising multiferroics with a simple perovskite structure and high ferroelectric and antiferromagnetic ordering temperatures. High-quality heteroepitaxial growth of BiFeO3 thin films has been demonstrated mainly by PLD, MBE, RF sputtering and metal-organic chemical vapor deposition (MOCVD). While the first three techniques listed above require ultrahigh vacuum and thus are rather expensive, particularly when used on an industrial scale, MOCVD may serve as an appropriate deposition method for the device fabrication. However, as nanoelectronic device sizes shrink, the demand for a high-precision deposition of thin layers on high-aspect-ratio structures grows stronger. ALD simply does not encounter this obstacle and can produce atomically smooth coverage on structures with extremely high aspect ratio. We will discuss the growth of BiFeO3 thin films and (Bi-O)x(Fe-O)y superlattices by ALD and the influence of the growth parameters on the phase composition of the resulting films. In short, the preparation of the films involved two stages: the deposition of the amorphous films and superlattices on Si wafers, SrTiO3(001), Nb:SrTiO3(001) at 250 C and a subsequent annealing of as-deposited films at various temperatures. Using XRD analysis and TEM imaging the annealed BiFeO3 films were demonstrated to be highly crystalline; piezoresponse force microscopy showed an expected response of the ferroelectric phase. The ALD growth of BiFeO3 will provide a new way of convenient, technologically-oriented and commercially realizable deposition of a promising multiferroic for nanoelectronic devices.This work was supported by the NSF DMR under 1124696 and by the ARO under W911.

Fabrication and Physical Properties of Organic Tricolor Superlattice

SUMMARYBy combining the two concepts of superlattices and organic amorphous materials, we attempted to fabricate a new material that exhibits photo‐switching piezoelectric properties. In order to utilize the merits of amorphous materials that can be deposited quickly, we started by building equipment specialized for the fabrication of organic amorphous superlattices. Using this equipment, we fabricated a p‐type semiconductor/piezoelectric polymer/n‐type semiconductor superlattice that responded to photoirradiation both electrically and mechanically. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(12): 32–36, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11556

Structure of sputter-deposited films of β-tantalum-aluminum alloys

Ion-plasma sputtering and the codeposition of ultradisperse particles of Ta and Al have been used to prepare solid solutions in the entire range of concentrations of the binary system in the form of alloy coatings. The formation of the solid solution of these alloys directly in the process of codeposition confirms the theory of the thermofluctuation melting of small particles and coalescence of quasi-liquid clusters of subcritical size. Upon the formation of coatings via the deposition of nanolayers of tantalum of less than 0.8 nm for β-Ta and 1.1 nm for Al, the spontaneous mutual dissolution of the components occurs with the formation of solid solutions of one metal in the other. Beginning with a concentration of ∼85 at % Al in the alloy, Al atoms control the type of symmetry of the arising lattice. An increase in the characteristic dimensions (thickness of sublayers) of tantalum and aluminum leads to the appearance of solid solutions of these metals in these coatings in addition to the β tantalum and aluminum phases, as well as of amorphous regions and superlattices formed by nanoclusters of one metal in the matrix of the other. It has been established that the formation of these superlattices is controlled by the size factor.

Optical evidence for quantization in transparent amorphous oxide semiconductor superlattice

Optical evidence for quantization in transparent amorphous oxide semiconductor superlattice

Fabrication of a new type of organic-inorganic hybrid superlattice films combined with titanium oxide and polydiacetylene

We fabricated a new organic-inorganic hybrid superlattice film using molecular layer deposition [MLD] combined with atomic layer deposition [ALD]. In the molecular layer deposition process, polydiacetylene [PDA] layers were grown by repeated sequential adsorption of titanium tetrachloride and 2,4-hexadiyne-1,6-diol with ultraviolet polymerization under a substrate temperature of 100°C. Titanium oxide [TiO2] inorganic layers were deposited at the same temperatures with alternating surface-saturating reactions of titanium tetrachloride and water. Ellipsometry analysis showed a self-limiting surface reaction process and linear growth of the nanohybrid films. The transmission electron microscopy analysis of the titanium oxide cross-linked polydiacetylene [TiOPDA]-TiO2 thin films confirmed the MLD growth rate and showed that the films are amorphous superlattices. Composition and polymerization of the films were confirmed by infrared spectroscopy. The TiOPDA-TiO2 nanohybrid superlattice films exhibited good thermal and mechanical stabilities.PACS: 81.07.Pr, organic-inorganic hybrid nanostructures; 82.35.-x, polymerization; 81.15.-z, film deposition; 81.15.Gh, chemical vapor deposition (including plasma enhanced CVD, MOCVD, ALD, etc.).

Fabrication and Physical Properties of Organic Tricolor Superlattice

SUMMARY By combining the two concepts of superlattices and organic amorphous materials, we attempted to fabricate a new material that exhibits photo-switching piezoelectric properties. In order to utilize the merits of amorphous materials that can be deposited quickly, we started by building equipment specialized for the fabrication of organic amorphous superlattices. Using this equipment, we fabricated a p-type semiconductor/piezoelectric polymer/n-type semiconductor superlattice that responded to photoirradiation both electrically and mechanically. © 2013 Wiley Periodicals, Inc. Electron Comm Jpn, 96(12): 32–36, 2013; Published online in Wiley Online Library (wileyonlinelibrary.com). DOI 10.1002/ecj.11556

Thermal conductivity of GeTe/Sb2Te3 superlattices measured by coherent phonon spectroscopy

We report on evaluation of lattice thermal conductivity of GeTe/Sb2Te3 superlattice (SL) by using femtosecond coherent phonon spectroscopy at various lattice temperatures. The time-resolved transient reflectivity obtained in amorphous and crystalline GeTe/Sb2Te3 SL films exhibits the coherent A1 optical modes at terahertz (THz) frequencies with picoseconds dephasing time. Based on the Debye theory, we calculate the lattice thermal conductivity, including scattering by grain boundary and point defect, umklapp process, and phonon resonant scattering. The results indicate that the thermal conductivity in amorphous SL is less temperature dependent, being attributed to dominant phonon-defect scattering.

Ultrafast dephasing of coherent optical phonons in atomically controlledGeTe/Sb2Te3superlattices

Femtosecond dynamics of coherent optical phonons in GeTe/Sb$_{2}$Te$_{3}$ superlattices (SLs), a new class of semiconductor SLs with three different states, have been investigated by using a reflection-type pump-probe technique at various lattice temperatures. The time-resolved transient reflectivity (TR) obtained in as-grown SLs exhibits the coherent A$_{1}$ optical modes at 5.10 THz and 3.78 THz, while only the single A$_{1}$ mode at 3.68 THz is observed in annealed SLs. The decay rate of the A$_{1}$ mode in annealed SLs is strongly temperature dependent, while that in as-grown SLs is not temperature dependent. This result indicates that the damping of the coherent A$_{1}$ phonons in amorphous SLs is governed by the phonon-defect (vacancy) scattering rather than the anharmonic phonon-phonon coupling.

Crystalline Amorphous Semiconductor Superlattice

A new class of superlattice, crystalline amorphous superlattice (CASL), by alternatively depositing two semiconductor materials, is proposed. CASL displays three states depending on the component materials' phase: both polycrystalline phases, both amorphous phases, and one polycrystalline phase while another amorphous phase. Using materials capable of reversible phase transition, CASL can demonstrate reversibility among three states. GeTe/Sb(2)Te(3) CASL has been synthesized and proved by x-ray reflectometry and TEM results. The reversible transition among three states induced by electrical and laser pulse was observed. The changes in the optical absorption edge, electrical resistivity, thermal conductivity, and crystallization temperature as a function of layer thickness are interpreted as quantum or nanoeffects. The unique properties of CASL enable the design of materials with specific properties.

Large enhancement of the thermoelectric Seebeck coefficient for amorphous oxide semiconductor superlattices with extremely thin conductive layers

AbstractHerein we demonstrate that amorphous oxide semiconductor (AOS) superlattices composed of a‐In–Zn–O (well) and a‐In–Ga–Zn–O (barrier) layers, fabricated on SiO2 glass substrate by pulsed laser deposition at room temperature, exhibited an enhanced Seebeck coefficient |S |. The |S | value increases drastically with decreasing a‐In–Zn–O thickness (dIZO) when dIZO < ∼5 nm, and reached 73 µV K–1 (dIZO = 0.3 nm), which is ∼4 times larger than that of bulk |S |3D (19 µV K–1), while it kept its high electrical conductivity, clearly demonstrating that the quantum size effect can be utilized in AOS superlattices. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Quantum size effects in amorphous Si superlattice solar cells

Amorphous silicon/alloy superlattices provide advantages in solar cell design, such as (a) effective band gap widening (b) effective mass separation (c) increased open-circuit voltage. The latter increases via Fermi level control, due to p-doping of potential barriers, pushing EF towards the valence bands, with simultaneous widening of the effective band gap, thus leading to potentially higher collection incident wavelengths. The density of gap states in the heavily doped layer is modeled as an exponential whose parameter kT* can be varied by the doping concentrations, while its activation energy saturates at some value. This communication provides (i) a general formulation of the problem at finite temperatures as well as numerical results for specific realizable contacts (ii) detailed treatment of gap states (iii) the neutrality condition (iv) a relation between Fermi level position and open-circuit voltage in the nitride region (superlattice p-region). For a p-(a-SiN: H/a-Si: H)-i (a-Si: H)-n (a-Si: H) sample, we compute the Fermi level position relative to the a-Si: H valence band edge. For low and wide gap thin layers of the order of 2.5–3.5 nm, open-circuit voltage values are predicted in excess of 1.05 V, and efficiencies are predicted in excess of 12%.