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1. Tuning Electrical and Thermal Transport in AlGaN/GaN Heterostructures via Buffer Layer Engineering

   https://doi.org/10.1002/adfm.201705823  

   Yalamarthy, Ananth Saran ; So, Hongyun ; Muñoz Rojo, Miguel ; Suria, Ateeq J. ; Xu, Xiaoqing ; Pop, Eric ; Senesky, Debbie G. ( March 2018 , Advanced Functional Materials)

   Progress in wide bandgap, III–V material systems based on gallium nitride (GaN) has enabled the realization of high‐power and high‐frequency electronics. Since the highly conductive, 2D electron gas (2DEG) at the aluminum gallium nitride (AlGaN)/GaN interface is based on built‐in polarization fields and is confined to nanoscale thicknesses, its charge carriers exhibit much higher mobilities compared to their doped counterparts. This study shows that such 2DEGs also offer the unique ability to manipulate electrical transport separately from thermal transport, through the examination of fully suspended AlGaN/GaN diaphragms of varied GaN buffer layer thickness. Notably, ≈100 nm thin GaN layers can considerably impede heat flow without electrical transport degradation. These achieve 4× improvement in the thermoelectric figure of merit (zT) over externally doped GaN, with state‐of‐the‐art power factors of 4–7 mW m^‐1K^‐2. The remarkable tuning behavior and thermoelectric enhancement, elucidated here for the first time in a polarization‐based heterostructure, are achieved because electrons are at the heterostructured interface, while phonons are within the material system. These results highlight the potential for using 2DEGs in III–V materials for on‐chip thermal sensing and energy harvesting.

    

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2. A Review of Scalable Hexagonal Boron Nitride ( h ‐BN) Synthesis for Present and Future Applications

   https://doi.org/10.1002/adma.202207374  

   Naclerio, Andrew E. ; Kidambi, Piran R. ( December 2022 , Advanced Materials)

   Hexagonal boron nitride (h‐BN) is a layered inorganic synthetic crystal exhibiting high temperature stability and high thermal conductivity. As a ceramic material it has been widely used for thermal management, heat shielding, lubrication, and as a filler material for structural composites. Recent scientific advances in isolating atomically thin monolayers from layered van der Waals crystals to study their unique properties has propelled research interest in mono/few layeredh‐BN as a wide bandgap insulating support for nanoscale electronics, tunnel barriers, communications, neutron detectors, optics, sensing, novel separations, quantum emission from defects, among others. Realizing these futuristic applications hinges on scalable cost‐effective high‐qualityh‐BN synthesis. Here, the authors review scalable approaches of high‐quality mono/multilayerh‐BN synthesis, discuss the challenges and opportunities for each method, and contextualize their relevance to emerging applications. Maintaining a stoichiometric balance B:N = 1 as the atoms incorporate into the growing layered crystal and maintaining stacking order between layers during multi‐layer synthesis emerge as some of the main challenges forh‐BN synthesis and the development of processes to address these aspects can inform and guide the synthesis of other layered materials with more than one constituent element. Finally, the authors contextualizeh‐BN synthesis efforts along with quality requirements for emerging applications via a technological roadmap.

    

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3. Transparent InGaZnO-Based Resistive Random Access Memory

   Qin, Fei ; Lee, Sunghwan ( May 2022 , Materials Research Society)

   The major focus of artificial intelligence (AI) research is made on biomimetic synaptic processes that are mimicked by functional memory devices in the computer industry [1]. It is urgent to find a memory technology for suiting with Brain-Inspired Computing to break the von Neumann bottleneck which limits the efficiency of conventional computer architectures [2]. Silicon-based flash memory, which currently dominates the market for data storage devices, is facing challenging issues to meet the needs of future data storage device development due to the limitations, such as high-power consumption, high operation voltage, and low retention capacity [1]. The emerging resistive random-access memory (RRAM) has elicited intense research as its simple sandwiched structure, including top electrode (TE) layer, bottom electrode (BE) layer, and an intermediate resistive switching (RS) layer, can store data using RS phenomenon between the high resistance state (HRS) and the low resistance state (LRS). This class of emerging devices is expected to outperform conventional memory devices [3]. Specifically, the advantages of RRAM include low-voltage operation, short programming time, great cyclic stability, and good scalability [4]. Among the materials for RS layer, indium gallium zinc oxide (IGZO) has attracted attention because of its abundance and high atomic diffusion property of oxygen atoms, transparency, and its easily modulated electrical properties by controlling the stoichiometric ratio of indium and gallium as well as oxygen potential in the sputter gas [5, 6]. Moreover, since the IGZO can be applied to both the thin-film transistor (TFT) channel and RS layer, the IGZO-based fully integrated transparent electronics are very promising [5]. In this work, we proposed transparent IGZO-based RRAMs. First, we chose ITO to serve as both TE and BE to achieve high transmittance in the visible regime of light. All three layers (TE, RS, BE layers) were deposited using a multi-target magnetron sputtering system on glass substrates to demonstrate fully transparent oxide-based devices. I-V characteristics were evaluated by a semiconductor parameter analyzer, and our devices showed typical butterfly curves indicating the bipolar RS property. And the IGZO-based RRAM can survive more than 50 continuous sweeping cycles. The optical transmission analysis was carried out via an UV-Vis spectrometer and the average transmittance around 80% out of entire devices in the visible-light wavelength range, implying high transparency. To investigate the thickness dependence on the properties of RS layer, 50nm, 100nm and 150nm RS layer of IGZO RRAM were fabricated. Also, the oxygen partial pressure during the sputtering of IGZO was varied to optimize the property because the oxygen vacancy concentration governs the RS and RRAM performance. Electrode selection is crucial and can impact the performance of the whole device [7]. Thus, Cu TE was chosen for our second type of device because the diffusion of Cu ions can be beneficial for the formation the conductive filament (CF). Finally, a ~5 nm SiO2 barrier layer was employed between TE and RS layers to confine the diffusion of Cu into the RS layer. At the same time, this SiO2 inserting layer can provide an additional interfacial series resistance in the device to lower the off current, consequently, improve the on/off ratio and whole performance. In conclusion, the transparent IGZO-based RRAMs were established. To tune the property of RS layer, the thickness layer and sputtering conditions of RS were adjusted. In order to engineer the diffusion capability of the TE material to the RS layer and the BE, a set of TE materials and a barrier layer were integrated in IGZO-based RRAM and the performance was compared. Our encouraging results clearly demonstrate that IGZO is a promising material in RRAM applications and overcoming the bottleneck of current memory technologies. 

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4. Ultra‐High Interfacial Thermal Conductance via Double hBN Encapsulation for Efficient Thermal Management of 2D Electronics

   https://doi.org/10.1002/smll.202205726  

   Ye, Fan ; Liu, Qingchang ; Xu, Baoxing ; Feng, Philip X. ‐L. ; Zhang, Xian ( February 2023 , Small)

   Heat dissipation is a major limitation of high‐performance electronics. This is especially important in emerging nanoelectronic devices consisting of ultra‐thin layers, heterostructures, and interfaces, where enhancement in thermal transport is highly desired. Here, ultra‐high interfacial thermal conductance in encapsulated van der Waals (vdW) heterostructures with single‐layer transition metal dichalcogenides MX₂(MoS₂, WSe₂, WS₂) sandwiched between two hexagonal boron nitride (hBN) layers is reported. Through Raman spectroscopic measurements of suspended and substrate‐supported hBN/MX₂/hBN heterostructures with varying laser power and temperature, the out‐of‐plane interfacial thermal conductance in the vertical stack is calibrated. The measured interfacial thermal conductance between MX₂and hBN reaches 74 ± 25 MW m⁻²K⁻¹, which is at least ten times higher than the interfacial thermal conductance of MX₂in non‐encapsulation structures. Molecular dynamics (MD) calculations verify and explain the experimental results, suggesting a full encapsulation by hBN layers is accounting for the high interfacial conductance. This ultra‐high interfacial thermal conductance is attributed to the double heat transfer pathways and the clean and tight vdW interface between two crystalline 2D materials. The findings in this study reveal new thermal transport mechanisms in hBN/MX₂/hBN structures and shed light on building novel hBN‐encapsulated nanoelectronic devices with enhanced thermal management.

    

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5. Differences in water and vapor transport through angstrom-scale pores in atomically thin membranes

   https://doi.org/10.1038/s41467-022-34172-1  

   Cheng, Peifu ; Fornasiero, Francesco ; Jue, Melinda L. ; Ko, Wonhee ; Li, An-Ping ; Idrobo, Juan Carlos ; Boutilier, Michael S. ; Kidambi, Piran R. ( December 2022 , Nature Communications)

   Abstract The transport of water through nanoscale capillaries/pores plays a prominent role in biology, ionic/molecular separations, water treatment and protective applications. However, the mechanisms of water and vapor transport through nanoscale confinements remain to be fully understood. Angstrom-scale pores (~2.8–6.6 Å) introduced into the atomically thin graphene lattice represent ideal model systems to probe water transport at the molecular-length scale with short pores (aspect ratio ~1–1.9) i.e., pore diameters approach the pore length (~3.4 Å) at the theoretical limit of material thickness. Here, we report on orders of magnitude differences (~80×) between transport of water vapor (~44.2–52.4 g m −2 day −1 Pa −1 ) and liquid water (0.6–2 g m −2 day −1 Pa −1 ) through nanopores (~2.8–6.6 Å in diameter) in monolayer graphene and rationalize this difference via a flow resistance model in which liquid water permeation occurs near the continuum regime whereas water vapor transport occurs in the free molecular flow regime. We demonstrate centimeter-scale atomically thin graphene membranes with up to an order of magnitude higher water vapor transport rate (~5.4–6.1 × 10 4  g m −2 day −1 ) than most commercially available ultra-breathable protective materials while effectively blocking even sub-nanometer (>0.66 nm) model ions/molecules. 

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