Papers by Zlatan Aksamija
Journal of Computational Electronics, Sep 25, 2023
Journal of computational electronics, Jun 27, 2024
Research Square (Research Square), Jul 12, 2023
To continue downscaling transistors, new materials must be explored. Two-dimensional (2D) materia... more To continue downscaling transistors, new materials must be explored. Two-dimensional (2D) materials are appealing due to their thinness and bandgap. The relatively weak van der Waals forces between layers in 2D materials allow easy exfoliation and device fabrication but also result in poor heat transfer to the substrate, which is the main path for heat removal. The impaired thermal coupling is exacerbated in few-layer devices where Joule heat dissipated in the layers further from the substrate encounters additional interlayer thermal resistance before reaching the substrate, which results in self-heating and thermal degradation of mobility. This study explores the electro-thermal properties of five popular 2D materials (MoS2, MoSe2, WS2, WSe2, and 2D black phosphorous). We simulate various devices with self-heating with a range of gate and drain biases and examine the effects on mobility and change in device temperature. The effects are compared to the isothermal case to ascertain the impact of self-heating. We observe that Joule heating has a significant effect on temperature rise, layer-wise drain current, and effective mobility. We show that black phosphorous performs the best thermally, owing to its relatively high thermal conductance to the substrate, while WSe2 performs the best electrically. This study will inform future thermally aware designs of nanoelectronic devices based on 2D materials.
Physical review applied, Jul 25, 2016
Efficient thermoelectric (TE) energy conversion requires materials with low thermal conductivity ... more Efficient thermoelectric (TE) energy conversion requires materials with low thermal conductivity and good electronic properties. Si-Ge alloys, and their nanostructures such as thin films and nanowires, have been extensively studied for TE applications; other group-IV alloys, including those containing Sn, have not been given as much attention as TEs, despite their increasing applications in other areas including optoelectronics. We study the lattice thermal conductivity of binary (Si-Sn and Ge-Sn) and ternary (Si-Ge-Sn) alloys and their thin films in the Boltzmann transport formalisms, including a full phonon dispersion and momentum-dependent boundary-roughness scattering. We show that Si-Sn alloys have the lowest conductivity (3 W=mK) of all the bulk alloys, more than 2 times lower than Si-Ge, attributed to the larger difference in mass between the two constituents. In addition, we demonstrate that thin films offer an additional reduction in thermal conductivity, reaching around 1 W=mK in 20-nm-thick Si-Sn, Ge-Sn, and ternary Si-Ge-Sn films, which is near the conductivity of amorphous SiO 2. We conclude that group-IV alloys containing Sn have the potential for high-efficiency TE energy conversion.
Meeting abstracts, May 1, 2020
Meeting abstracts, May 1, 2020
The persistent down-scaling of nanostructures, such as electronic devices, sensors, NEMS, and nan... more The persistent down-scaling of nanostructures, such as electronic devices, sensors, NEMS, and nanocomposites, increases the surface-to-volume ratio and introduces atomic-scale disorder at boundaries and interfaces. To avoid these issues, the nanoelectronics community has turned to intrinsically two-dimensional (2D) materials platforms. 2D materials have tremendous potential for next-generation nanoelectronics beyond the 5 nm technology node due to their atomic flatness and absence of dangling bonds, which prevents scattering from interface roughness. However, heat dissipation and its removal from hot spots in the monolayer remains a critical concern to the design of 2D-based devices [1]. Thermal currents flowing in an atomic layer can either dissipate through source/drain contacts in a transistor configuration, or through a supporting substrate via van der Waals (vdW) coupling to it. When a 2D material is supported by a substrate, the interfacial area formed between it and the substrate is often far larger than the lateral source/drain contact area. Thus, the majority of waste heat is removed across the 2D-substrate interface and then via the substrate. The thermal boundary conductance (TBC) between the 2D layer and substrate should be well characterized for reliable 2D device performance. Interfaces formed between 2D vdW materials and 3D substrates are fundamentally different than same-dimension 3D-3D and 2D-2D interfaces due to the presence of a vdW gap and the different dimensionalities of the phase spaces on either side of the interface. In this invited talk, I will review the progress in understanding lattice thermal transport, both in-plane and cross-plane, in 2D mono and few-layer materials from first principles. This work builds on vibrational properties of the 2D materials calculated from Density Functional Theory combined with Boltzmann transport equation for phonons. Then I will introduce our recent work aimed to tackle the question of selecting the best substrate for each 2D material from the point of view of heat dissipation and apply these results to 2D devices. Several recent papers measured the TBC between various monolayers and mostly the silicon dioxide (SiO2) substrate, reporting a wide range of values due to inconsistent sample quality. Therefore, it is imperative to build predictive methods for quantifying the TBC between MLs and various substrates. Here, we use a combination of phonon dispersions from first-principles density functional perturbation theory simulations and our 2D-3D TBC model [2, 3]. We investigate the TBC between combinations of six atomic layers (h-BN, graphene, MoS2, MoSe2, WS2, and WSe2) and six substrates (SiO2, AlN, GaN, 6H-SiC, diamond, and Al2O3). We show that TBC is higher for softer substrates with smaller speed of sound, but of the 6 substrates we compared, amorphous SiO2 consistently produced higher TBC than crystalline substrates. Our work helps build a roadmap for quantifying the TBC between various 2D monolayers and their substrates and provides a framework for other 2D-3D interfaces to be studied. Next, we employ our first-principles model to calculate the TBC of several beyond-graphene 2D materials, such as and blue and black phosphorene, on amorphous and crystalline substrates. A trend emerges that 2D materials with lower ZA branch frequencies have higher TBCs when placed on a-SiO2. Our results provide selection criteria for 2D materials that improve interfacial heat transport in 2D devices with amorphous and crystalline substrates. To further probe the role of interfacial phonon transport, we develop a coupled electro-thermal model for FL-WSe2 stacks where we simultaneously solve for the current and Joule heating by treating the stack a resistor network. The resulting rise in temperature is obtained from a FL-TBC model [4] to shed light on self-heating and heat dissipation in such devices. We find that the temperature rise in the top layers is significantly larger than the bottom layers because the bottom layers have higher TBC and conduct heat more efficiently to the substrate [4,5]. The higher temperature of top layers, in turn, significantly reduces their mobility, which is strongly temperature-dependent as they are shielded by the bottom layers from substrate impurity scattering. We also uncover that, unlike monolayer FETs, a significant amount of heat is dissipated laterally through the contacts in FL devices at high VDS due to relatively large thermal healing lengths of the top layers. References: [1] Yalon, E., et al., Nano Letters 17, 3429–3433 (2017). [2] Yasaei, P., et al., Advanced Materials Interfaces, 1700334 (2017). [3] Correa, G. C., Foss, C. J., and Aksamija, Z. Nanotechnology 28, 135402 (2017). [4] Behranginia et al., ACS Applied Materials & Interfaces 10, 24892-24898, 2018 [5] Yasaei et al., Advanced Materials 30, 1801629, 2018
Journal of Physics: Condensed Matter
2D materials have attracted broad attention from researchers for their unique electronic properti... more 2D materials have attracted broad attention from researchers for their unique electronic properties, which may be been further enhanced by combining 2D layers into vertically stacked van der Waals heterostructures (vdWHs). Among the superlative properties of 2D systems, thermoelectric (TE) energy conversion promises to enable targeted energy conversion, localized thermal management, and thermal sensing. However, TE conversion efficiency remains limited by the inherent tradeoff between conductivity and thermopower. In this paper, we use first-principles calculation to study graphene-based vdWHs composed of graphene layers and hexagonal boron nitride (h-BN). We compute the electronic band structures of heterostructured systems using Quantum Espresso and their TE properties using BoltzTrap2. Our results have shown that stacking layers of these 2D materials opens a bandgap, increasing it with the number of h-BN interlayers, which significantly improves the power factor (PF). We predict ...
Bulletin of the American Physical Society, Mar 16, 2021
2018 IEEE 18th International Conference on Nanotechnology (IEEE-NANO), 2018
Thermoelectric (TE) devices enable robust solid-state conversion of waste heat to electricity but... more Thermoelectric (TE) devices enable robust solid-state conversion of waste heat to electricity but their applications are still limited by relatively modest efficiency. Power factor controls the TE energy conversion efficiency of a material. A higher power factor also helps to increase the passive or electronic cooling ability. Single-layer (SL) 2-dimensional (2D) materials have been analytically shown to have higher power factors [1]. In this work, we extend our 3D model to simulate quantum transport and capture energy filtering in 2D SL $\text{MoS}_{2}$ that can improve power factor. Energy relaxation and quantum effects from periodic spatially varying potential barriers are modeled in the Wigner-Rode formalism. Our simulations show an increase in power factor in both cosine- and square-shaped barriers with the height of the potential barrier, resulting in over 30% power factor enhancement. This improvement in TE efficiency helps in the development of efficient waste-heat scavenging, body-heat-powered wearables, thermal sensors, and electronic cooling.
Bulletin of the American Physical Society, Mar 18, 2021
Bulletin of the American Physical Society, 2020
2018 76th Device Research Conference (DRC), 2018
Two-dimensional (2D) materials have tremendous potential for next-generation nano- and opto-elect... more Two-dimensional (2D) materials have tremendous potential for next-generation nano- and opto-electronics [1], [2]. However, heat dissipation and its removal from hot spots in the monolayer remains a critical concern to the design of 2D-based devices [2], [3]. Thermal currents flowing in a atomic layer can either dissipate through source/drain contacts, as in a transistor configuration, or through a supporting substrate via van der Waals (vdW) coupling to it. When a 2D mateiral is supported by a substrate, the interfacial area formed between it and the substrate is often far larger than the lateral source/drain contact area. Thus, it is suspected that the majority of waste heat is removed across the 2D-substrate interface and then via the substrate. Therefore, it is imperative that the thermal boundary conductance (TBC) between the 2D layer and substrate be well characterized for reliable 2D device performance. Herein we tackle the question of selecting the best substrate for each 2D ...
Journal of Computational Electronics, 2020
While two-dimensional (2D) materials have emerged as a new platform for nanoelectronic devices wi... more While two-dimensional (2D) materials have emerged as a new platform for nanoelectronic devices with improved electronic, optical, and thermal properties, and their heightened sensitivity to electrostatic and mechanical interactions with their environment has proved to be a bottleneck. Few-layer (FL) 2D devices retain the desirable thinness of their monolayer cousins while boosting carrier mobility. Here, we employ an electrothermal model to study FL field-effect devices made from transition metal dichalcogenides MoS 2 and WSe 2 and examine the effect of both electrical and thermal interlayer resistances, as well as the thermal boundary resistance to the substrate, on device performance. We show that overall conductance improves with increasing thickness (number of layers) at small gate voltages, but exhibits a peak for large gate voltages. Joule heating impacts performance due to relatively poor thermal conductance to the substrate and this impact, along with the location of the hot spot in the FL stack, varies with carrier screening length of the material. We conclude that coupled electrothermal simulation can be employed to design FL 2D devices with improved performance.
ACS Applied Materials & Interfaces, 2020
Physical Review Applied, 2016
Physical Review Letters, 2015
Journal of Computational Electronics, 2015
An increasing need for effective thermal sensors, together with dwindling energy resources, have ... more An increasing need for effective thermal sensors, together with dwindling energy resources, have created renewed interests in thermoelectric (TE), or solid-state, energy conversion and refrigeration using semiconductor based nanostructures. Effective control of electron and phonon transport due to confinement, interface, and quantum effects has made nanostructures a good way to achieve more efficient thermoelectric energy conversion. Theoretically, a narrow delta-function shaped transport distribution function (TDF) is believed to provide the highest Seebeck coefficient, but has proven difficult to achieve in practice. We propose a novel approach to achieving a narrow window-shaped TDF through a combination of a step-like 2-dimensional density-of-states (DOS) and inelastic optical phonon scattering. A shift in the onset of scattering with respect to the step-like DOS creates a TDF which peaks over a narrow band of energies. We perform a numerical simulation of carrier transport in silicon nanoribbons based on numerically solving the coupled Schrödinger-Poisson equations together with transport in the semi-classical Boltzmann formalism. Our calculations confirm that inelastic scattering of electrons, combined with the step-like DOS in 2-dimensional nanostructures leads to the formation of a narrow window-function shaped TDF and results in enhancement of Seebeck coefficient beyond what was already achieved through confinement alone. A further analysis on maximizing this enhancement by tuning the material properties is also presented.
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Papers by Zlatan Aksamija