bluuebunny/crossref_metadata_2025_split huggingface dataset:
Electronic and Optical Properties of Short-Period Si/Sige Superlattices: Effects of Interfacial Atomic-Scale Roughness
Authors: Fettu Gabriel , Attiaoui Anis , Samik Mukherjee , Bauer Matthias , Moutanabbir Oussama 
Published: May 2020

SiGe/Si superlattice (SL) are currently a central building block of nanosheet transistors proposed for the 7 nm technology nodes and beyond [1], where elective wet-etching of the SiGe layers has been used to release the Si layers and form a vertically stacked channels architecture [2]. Consequently, the interfacial abruptness and uniformity in heterostructures are critical to control their electronic and optical properties. Recently, we demonstrated a 3-D atomistic-level mapping of the roughness and uniformity of buried epitaxial interfaces in Si/SiGe SLs with a layer thickness in the 1.5-7.5 nm range [3]. This direct quantification of the abruptness of buried interfaces enabled a direct evaluation of interfacial effects on electronic and optical properties of epitaxial heterostructures. For instance, spectroscopic ellipsometry indicated the first observation of a new superlattice-related optical transition between 2.2 and 2.7 eV. However, the interpretation of this new transition can only be achieved using a theoretical framework considering atomic-level details of the interfacial roughness. To that end, we have been carrying out theoretical investigations to build a correct quantum mechanical model that incorporates the effect of the interface, directly measured from atom probe tomography (APT), to interpret the possible SL-related optical transition. Thus, an 8-band k⋅p formalism was developed and validated for group IV semiconductors [4], where a quantum mechanical incorporation of the interface width is highly coveted because the microscopic interface asymmetry (MIA) effect can greatly influence the electronic and optical properties of short-period SLs, induce strong interactions between different SL subbands, and enhance the absorption strength considerably [5]. It has been shown that the nature of the SL interfaces can have a significative impact on the optical confinement properties of other group IV heterostructures [6]. To test this model, experimental optical characterization of four different SLs (the mean Ge concentration of the layers within the SLs is in the 25 to 30 at. % range and is the periodicity of the SLs) will be presented and discussed. Then, by simulating the SL optical properties with the developed 8-band k⋅p method, the effect of the interface will be investigated to try and explain the observed spectroscopic transition.

[1] G. Hellings, et al, in 2018 IEEE Symp. VLSI Technol. (IEEE, 2018), pp. 85–86.
[2] K. Komori, et al, Solid State Phenom. 282, 107 (2018).
[3] S. Mukherjee, et al, ArXiv: Cond-Mat 1908.00874, 1 (2019).
[4] T. B. Bahder, Phys. Rev. B 41, 11992 (1990).
[5] H. M. Dong, et al, Thin Solid Films 589, 388 (2015).
[6] F. Szmulowicz, et al, Phys. Rev. B - Condens. Matter Mater. Phys. 69, 155321 (2004).
Static Negative Capacitance Response in Ferroelectric/Paraelectric Superlattices
Authors: Gharbi Mariem , Sylvestre Alain , Dellis Jean-Luc , Le Marrec Françoise , Karkut Mickael , Lemee Nathalie 
Published: May 2020

Recently Zubko et al.(1) reported a negative capacitance effect in PbTiO3/SrTiO3 superlattices. The negative capacitance of ferroelectrics is proposed as a solution to overcome the “Boltzmann tyranny” that holds back the process of miniaturization. Nowadays, the transistors are becoming too small, which increased the energy dissipation hence the self-heating of the electronic chips. The phenomenon of negative capacitance is gaining a great interest since it has been proposed as a way to reduce the power consumption of field-effect transistors(2), by replacing the conventional SiO2 gate insulator by an insulator that provides a negative capacitance effect.
Actually, in ferroelectrics, “the negative capacitance state” takes place at the frontier between the two stable polarization states, during the transient processes of switching, and therefore is intrinsically thermodynamically unstable
Hence, negative capacitance cannot be evidenced for single layer ferroelectrics but should be stabilized in a series capacitance system. The ferroelectric capacitance can be stabilized in the negative regime, when a paraelectric capacitor and a ferroelectric capacitor are connected in series. Hence, the paraelectric/ferroelectric superlattices provide a model system to investigate the negative capacitance effect.
The negative capacitance was shown to be strongly related to the existence of a 180° stripe domain structure. In fact, the depolarization field inside such structure is very strong and induces 180° stripe nanodomains formation in order to screen this field and to maintain the ferroelectric state. This nanodomains, in particularly, the domain wall motion greatly influences the dielectric properties of the heterostructure.
Here we report an experimental study on a series of Pb(Zr0.2Ti0.8)O3 /SrTiO3 superlattices in which we evidenced 180° stripe nanodomains. The investigation of such system is motivated by the fact that leakage currents are reduced in PZT compared to the ferroelectric PbTiO3, investigated by Zubko et al.,and also by the possibility to vary the misfit strain related to PbTiO3 /SrTiO3 superlattices . For this study the thickness of the ferroelectric layers was fixed to 4 unit cells and those of the paraelectric layer was varied from 3 to 10 unit cells. The domain’s electrostatic effect in the superlattice is confirmed by a noticeable reduction of the Curie temperature. We will present an in-depth analysis of the dielectric response of these superlattices by impedance spectroscopy. The temperature dependent dielectric measurements will be presented and will emphasize a negative capacitance effect in these superlattices.

References

- Zubko et al., Nature, 2016, 534, 524–528.
- S. Salahuddin and S. Datta, Nano Lett.8, 405 (2008)
Electrical Characterization of Sputter Deposited AlxSc1-XN Thin Films
Authors: Tsai SungLin , Kusafuka Kazuki , Hoshii Takuya , Wakabayashi Hitoshi , Tsutsui Kazuo , Kakushima Kuniyuki 
Published: May 2020

Ferroelectric materials have been utilized for non-volatile memory applications [1]. Beside extensive works on ferroelectric HfO2 thin film [2, 3], recent works on nitride-based materials have proven that AlxSc1-xN (ASN) also shows ferroelectricity with a box-like characteristic [4]. Commonly Pt electrodes with a large Schottky barrier are utilized to overcome the small bandgap of ASN. However, a high remnant polarization of over 100 µC/cm2 is attractive for future non-volatile memory applications. The purpose of this study is to measure the basic electrical properties of ASN films deposited by sputtering with W electrodes.
50-, 75-, and 100-nm-thick ASN films were deposited by DC reactive sputtering with an Al0.57Sc0.43 target in Ar and N2 ambient at a substrate temperature of 400oC. Both top and bottom electrodes were deposited at room temperature.
Leakage current characteristics of the fabricated metal-insulator-metal (MIM) capacitors indicate a breakdown field of 3 MV/cm. Temperature-dependent leakage current measurement has indicated a Schottky barrier height of 1.0 eV at W/ASN interface, which is rather small for memory application and needs to be engineered. Hysteresis in the polarization-voltage measurements can be hardly observed due to excess leakage current. However, ferroelectric hysteresis curves were obtained from capacitance-voltage (CV) measurements at 100 kHz for all the samples with different thicknesses. The coercive field taken from the hump in the CV curves was 0.9 MV/cm for 50-nm-thick sample and 1.4 MV/cm for 75 and 100 nm-thick samples, which are lower than the reported thick ASN samples [4], implying thickness-dependent properties of ASN films.
In conclusion, JV and CV characteristics of sputter-deposited ASN films on W electrode were performed. Relatively low energy offset at metal/ASN interface was extracted. Ferroelectric hysteresis was obtained from CV measurement even for a film as thin as 50 nm.

References

[1] T. S. Böscke, et al., Appl. Phys. Lett., 99, 102903 (2011). [2] S. Kundu, et al., Nature Scientific Reports, 5, 8494 (2015). [3] M. Kobayashi, et al., IEEE J. Electron Dev. Soc., 7, 134 (2018). [4] S. Fichtner, et al., J. Appl. Phys., 125, 114103 (2019).

Figure 1
(Invited) Atomic-Layer-Deposition of in-Situ Crystallized Ge2Sb2Te5 Alloy and Gete/Sb2Te3 Superlattice, and Their Phase-Change Performances
Authors: Hwang Cheol Seong 
Published: May 2020

This paper introduces a new atomic layer deposition process for highly conformal, nanocrystalline-as-deposited GeTe-Sb2Te3 pseudo-binary film growth at a deposition temperature of 130 °C. The process utilizes Ge(II)-guanidinate, Te(Si(CH3)3)2, and Sb(OC2H5)3 with the NH3 co-reagent. The alternative GeTe and Sb2Te3 subcycles produced various film compositions, all consistent with the GeTe-Sb2Te3 tie lines, owing to the stoichiometric reactions between the precursors without involvement of undesirable side reactions. The density of the nanocrystalline Ge2Sb2Te5 (GST225) films was 6.2 g/cm3, similar to the density of the bulk crystalline material. The crystallization behaviors indicated that the distribution of the constituent elements of the GST225 films was highly uniform at the atomic level, as opposed to the case of the low-temperature (100 °C)-deposited films. The cubic to hexagonal transition at 350 °C upon post-annealing produced (0001) hexagonal planes highly aligned along the substrate. The demonstration of the phase change memory device achieved high cycling endurance (>107).
The same ALD process was further exploited to grow the GeTe/Sb2Te3 superlattice structure, which might be an even more energy-efficient phase change material. It was found that the Sb2Te3 layer could be grown in highly c-axis oriented manner on SiO2 surface, which then can be used as the template layer for the subsequent growth of also highly c-axis oriented GeTe layer. Direct growth of the GeTe layer on SiO2 did not result in the similar optimal results. However, a certain intermixing occurs during the alternating growth of the Sb2Te3 and GeTe layers, of which details are dependent on the thickness ratio of the two layers. Interestingly, the intermixed layers still showed a certain type of layered structure, which appeared to be related to the van der Waals nature of the Sb2Te3 layer. However, such a layered structure could hardly be achieved on metallic surface such as TiN. This might be related with the highly interacting chemical nature of the TiN surface as well as the relatively rough surface topography. Adopting graphene or ultra-thin SiO2 layer on the TiN recovered the highly c-axis aligned growth behavior.
Understanding the Formation of Soluble Bases in Nickel-Rich Layered Materials and Their Reactivity Towards Electrolyte
Authors: Martinez Ana Cristina , Grugeon Sylvie , Cailleu Dominique , Courty Matthieu , Tran-Van Pierre , Delobel Bruno , Laruelle Stephane 
Published: May 2020

In the last years, LiNixMnyCozO2 (x+y+z = 1) layered materials have been widely used as positive electrode materials in lithium-ion batteries for electric car applications, because of their high energy density given by the nickel content. Unfortunately, their stability towards H2O and CO2 is compromised when x>0.5, favoring the formation of surface species commonly called soluble bases (SB). The presence of SB has shown to increase the cell’s impedance, decrease the capacity retention and evolve gas during functioning1-4. Therefore, in this work, accurate SB quantification methods were implemented on four high-nickel layered materials with the aim of further deciphering their formation mechanisms and reactivity towards electrolyte.
The optimization of a procedure that allows the quantification of SB in the as-received and after H2O/CO2storage of the NMC532, 622, 811 and LiNi0.9Co0.1O2, will be shown through the complementary use of acid-base titration and inductively coupled plasma (ICP) techniques. We unveiled the presence and quantified different lithium and sodium salts. Moreover, thanks to additional TGA/MS analysis, we propose a novel NMC surface degradation mechanism upon H2O/CO2 storage that involves the presence of lithium and nickel-based surface species. Finally, through nuclear magnetic resonance technique (19F and 31P NMR), the chemical reactivity of these surface species towards a commercial LiPF6-based liquid electrolyte was studied at room and at high temperature, evidencing the presence of lithium fluorophosphate as major decomposition product.
1. Liu, H. S., et al. (2006). "Investigation and improvement on the storage property of LiNi0.8Co0.2O2 as a cathode material for lithium-ion batteries." Journal of Power Sources 162(1): 644-650.
2. Bi, Y. J., et al. (2016). "Stability of Li2CO3 in cathode of lithium ion battery and its influence on electrochemical performance." Rsc Advances 6(23): 19233-19237.
3. Renfrew, S. E. and B. D. McCloskey (2017). "Residual Lithium Carbonate Predominantly Accounts for First Cycle CO2 and CO Outgassing of Li-Stoichiometric and Li-Rich Layered Transition-Metal Oxides." Journal of the American Chemical Society 139(49): 17853-17860.
4. Mahne, N., et al. (2018). "Electrochemical Oxidation of Lithium Carbonate Generates Singlet Oxygen." Angewandte Chemie-International Edition 57(19): 5529-5533.
Origins of Capacity Fade and Material Degradation in Ni-Rich NMC Li-Ion Batteries
Authors: Dose Wesley M , Mahadevegowda Amoghavarsha , Morzy Jędrzej K , Ducati Caterina , Grey Clare P. , De Volder Michael F. L. 
Published: May 2020

Nickel-rich layered oxide cathode materials are promising candidates for automotive lithium-ion batteries due to their low cost and high reversible capacity.1 However, paired with graphite anodes these materials exhibit faster voltage and capacity fading compared to isostructural LiNixMnyCozO2 (NMC) cathodes with lower nickel content.2 In this work we will explore the origins of capacity fade and material degradation in graphite/LiNi0.8Mn0.1Co0.1O2 (NMC811) full cells. With a focus on strategically designed electrochemical protocols (see Fig. 1), the role of several key cycling parameters (e.g. upper cut-off voltage, time at high voltage, phase transitions) on the degradation are decoupled. The ageing processes are quantified in terms of the full cell capacity retention, impedance rise, electrode slippage, and NMC cathode deterioration. We find that most full cell capacity loss and impedance rise is caused by increasing the upper cut-off voltage to >4.2 V, with the impedance increase largely driven by concomitant full state-of-charge (SOC) cycling. Time spent at high voltage (voltage holds) has less impact on these parameters. These findings are supported by electron microscopy and spectroscopy experiments (SEM, high-resolution TEM, and STEM-EELS) on cycled electrodes, which show clear evidence for cycle history dependent degradation including particle cracking, shearing, and surface reconstruction.
References
1. Schipper, E. M. Erickson, C. Erk, J.-Y. Shin, F. F. Chesneau, and D. Aurbach, J. Electrochem. Soc., 164, A6220, 2017.
2. Jung, M. Metzger, F. Maglia, C. Stinner, H. A. Gasteiger, J. Electrochem. Soc., 164, A1361, 2017.

Figure 1
Ultra-Stable Performance of Ni-Rich Layered Oxide Cathodes for Lithium-Ion Batteries Using Ionic Liquid Electrolyte
Authors: Wu Fanglin , Kuenzel Matthias , Kim Guk-Tae , Passerini Stefano 
Published: May 2020

High energy density lithium-ion batteries (LIBs) have continually been of great research interest amplified by the exponentially growing demand for electric vehicles.[1] Among the investigated systems, nickel-rich transition metal oxides appear very promising owing to their high specific capacity,[2] and low cobalt content, according to necessity of reducing the cobalt proportion in LIB cathodes due to its severe shortage and strong environmental and ecological concerns.[3] However, the increased nickel content also introduces previously overcome challenges of poor capacity retention and thermal stability due to the vulnerability of the nickel rich oxide towards interfacial side reactions.[4] Herein, a low volatility and non-flammable ionic liquid-based (IL) electrolyte[5] consisting of 0.8Pyr14FSI-0.2LiTFSI is employed to overcome those challenges faced by a Ni-rich LiNi0.8Co0.1Mn0.1O2 positive electrode. The cathode exhibits remarkable electrochemical performance in half-cell configuration, delivering a specific capacity of approximately 200 mAh g-1 and practically no capacity fading after 100 cycles at 0.1C at room temperature. After 600 cycles at 0.5C the cathode has an outstanding capacity retention of 97%, which is substantially higher than with the conventional organic carbonate-based electrolyte used for reference, attributed to a much more stable electrode-electrolyte interfacial layer. Even at elevated temperature (40 °C) the LiNi0.8Co0.1Mn0.1O2 in combination with the IL electrolyte offers a specific capacity of more than 200 mAh g-1 as well as a capacity retention around 99% after 100 cycles at 0.5C. To the best of our knowledge these results exceed the state-of-the-art performance reported for any Ni-rich positive electrode.

Reference

[1] W. Liu, P. Oh, X. Liu, M. J. Lee, W. Cho, S. Chae, Y. Kim, J. Cho, Angewandte
Chemie
2015, 54, 4440.
[2] S. H. A. Heist, and S. H. Lee, J
. Electrochem
. Soc
2019, 166 (6) A873-A879.
[3] C.Vaalma, D. Buchholz, M. Weil, S. Passerini, Nature Review Materials
2018, 3, 18013.
[4] U.-H. Kim, L.-Y. Kuo, P. Kaghazchi, C. S. Yoon, Y.-K. Sun, ACS Energy Letters
2019, 4, 576.
[5] G. A. Elia, U. Ulissi, S. Jeong, S. Passerini, J. Hassoun, Energy & Environmental Science
2016, 9, 3210.
Are Protons Involve in the Fading Capacity in Li Ion Aqueous Batteries? Shedding Light By Means of First Principle Computations
Authors: Posada Pérez Sergio , Rignanese Gian-Marco , Hautier Geoffroy 
Published: May 2020

Commercial lithium ion batteries are composed of an anode based on carbon, a transition metal oxide cathode separated by an organic liquid electrolyte containing a lithium salt such as LiPF6. Nevertheless, operating with an organic electrolyte presents many drawbacks. The organic electrolyte is flammable, expensive and need to be kept away from any water contamination. On the other hand, working in aqueous electrolyte would therefore be extremely beneficial especially to lower production cost and obtain higher safety and lower toxicity. The performance of aqueous based LIBs has been demonstrated since the foundational work from Dahn et al. who combined an anode of VO2 and a cathode of LiMn2O4 in an aqueous electrolyte.
In contrast to organic electrolytes, where only the inserted metal ions can (de)intercalate in the electrode material, the use of aqueous electrolytes opens the possibility of proton co-insertion. Proton intercalation has been one of the common assumptions to explain performance issues in aqueous electrolytes such as capacity fading and the increase in lithium diffusion barriers. Several (electro)chemical studies combined with first-principles computations have confirmed the easy protonation of layered electrodes at different ranges of pH values and this fact has been directly related with the detrimental in cathode materials. Nevertheless, several studies reported the excellent performance of layered LiCoO2 and LiCo1/3Ni1/3Mn1/3O2 as positive electrode -without coating- using neutral or basic salt in water electrolytes. Thus, it remains an open question: Why these layered cathode materials so prone to proton intercalation could perform as well in aqueous environment?
In this work, we use density functional computations to investigate the lithium/proton exchange in some well-known cathode materials. The analysis of the proton insertion mechanism has been carried out in order to demonstrated which specific chemical and structural features lead to electrochemically stable materials for aqueous batteries. To analyze the Li+/H+ exchange, several Li+/H+/vacancy orderings and arrangements were computed to study the phase stability at different pH values.
Given that the importance of proton insertion for aqueous batteries, this study provides fundamental investigations about the role of H-intercalation in the performance of aqueous battery operations.
(Invited) Layered Sodium Transition Metal Oxides As Cathode Materials for Na-Ion Batteries
Authors: Lee Kyu Tae 
Published: May 2020

Thanks to their excellent electrochemical performance, Li-ion batteries have been successfully utilized as a power source in electric vehicles and large-scale energy storage systems. However, there are some concerns that the current technology of Li-ion batteries is not able to meet the demands for power sources in future energy storage applications. For example, lithium resources are expensive and too geographically constrained, leading to a substantial increase in the cost of Li-ion batteries. In this connection, new rechargeable battery systems such as sodium ion, sodium metal, magnesium, metal-air, and metal-sulfur batteries are being considered as alternatives to replace Li-ion batteries. In particular, Na-ion batteries have attracted rapidly increasing attention owing to the low cost of Na-ion batteries.1-3

Because of their low cost, the main potential application of Na-ion batteries is considered to be large-scale energy storage systems. However, it should be stressed that it is not easy to reduce the cost of Na-ion batteries relative to that of Li-ion batteries, in spite of the inexpensive Na precursors. The current Na-ion batteries have slightly lower energy densities than Li-ion batteries. This implies that the cost per energy ($/Wh) of Na-ion batteries cannot be easily reduced because the energy density limitations offset the cost savings due to the use of Na. Therefore, it is necessary to develop high capacity electrode materials to improve the energy density of Na-ion batteries.
In this presentation, we will focus on our recent works involving the development of high capacity layered-structured sodium transition metal oxide cathode materials to improve the energy density of Na-ion batteries. We have examined various layered Na1-xMO2 (M = Mn, Fe, Ni, and their solid solution analogues) materials, along with understanding the role of dopants in the reaction mechanisms of those electrode materials. We also discuss the failure mechanisms of those cathode materials and suggest a strategy for improving electrochemical performance.
[1] Y. Kim, K.-H. Ha, S. M. Oh, K. T. Lee, Chem. Eur. J., 20 (2014) 11980.
[2] S. Y. Hong, Y. Kim, Y. Park, A. Choi, N.-S. Choi, K. T. Lee, Energy Environ. Sci., 6, (2013) 2067.
[3] M.-S. Kwon, S. G. Lim, Y. Park, S.-M. Lee, K. Y. Chung, T. J. Shin, K. T. Lee, ACS Appl. Mater. Interfaces, 9 (2017) 14758.