Ferroelectronics Lab

Understanding and utilizing non-volatile properties of materials

New Publication! Sub-100 Ω/□ sheet resistance of GaN HEMT with ScAlN barrier

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Abstract: A low sheet resistance of 95.5 Ω/□ at room temperature has been demonstrated in an MBE-grown Sc0.15Al0.85N/AlN/GaN epitaxial HEMT structure. Owing to the strong spontaneous and piezoelectric polarization of ScAlN, a large two-dimensional electron gas density of 7.8 × 1013 cm−2 and a relatively high mobility of 836 cm2/V·s were demonstrated with a 15 nm Sc0.15Al0.85N barrier. Further investigation under low temperature on this structure reveals a reduced sheet resistance to 33.3 Ω/□ and mobility increased to 4223 cm2/V·s at 10 K. The dependence of sheet carrier density, mobility, and the associated sheet resistance on ScAlN thickness was further studied. The compelling electron transport properties demonstrated in the structure position ScAlN as a strong contender as the barrier layer in future GaN HEMT devices.

Read more at Applied Physics Letters

New Publication! “Endotaxial Stabilization of 2D 1T-TaS2 Charge Density Waves via In-Situ Electrical Current Biasing”

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Abstract: 1T-TaS2 is a layered, two-dimensional material which is host to several charge density wave (CDW) states with three distinct phases: an insulating commensurate (C) phase and the metallic nearly-commensurate (NC) and incommensurate (IC) phases [1-3]. CDW phase selection can be achieved via biasing, making 1T-TaS2 an attractive candidate for device applications [4-6]. The insulating C phase, however, only forms below ∼180 K [1, 7] for bulk 1T-TaS2 and even lower for thin flakes [5], leaving the metal-insulator transition unreachable for room temperature devices.

Recent work has shown endotaxial heterostructures of 2H-TaS2/1T-TaS2 can stabilize 2D C-CDW states in the twinned commensurate (tC) phase at room temperature with a single metal-insulator transition at ∼350 K [3, 8], paving the way for devices operable at room temperature. Previously, this phase has been realized by directly heating 1T-TaS2 past its polytype transition for a few minutes and then cooling it back to room temperature [3, 8].

Here, we show that the tC-CDW state can be synthesized electronically via current. Using an in-house built transmission electron microscopy (TEM) biasing holder, we can source current through exfoliated 1T-TaS2 flakes allowing us to drive and observe the polytype conversion in both real and reciprocal space in-situ. For sufficiently thin flakes, a current of around 210 µA/µm2 is enough to switch from the NC phase to the IC phase and back again. Upon sourcing higher currents of around 750 µA/µm2 the normal NC to IC transition is observed before seeing polytype conversion occur. Holding at this current for around 30 seconds longer is enough to stabilize the tC-CDW phase at room temperature. Similarly to the NC-IC transition, we can switch between the tC and IC phases of this new endotaxial structure by sourcing current through the sample. Using in-situ TEM we can correlate a polytype transition and the associated tC-CDW formation through electrical signatures. Further, this conversion is more localized compared to heating the sample in bulk.

In summary, we report current driven stabilization of 2D CDWs in 1T-TaS2 in and characterize the electronic switching of the NC to IC transition via in-situ TEM.

Read more at Microscopy and Microanalysis

New Publication! “Geometric effects in the measurement of the remanent ferroelectric polarization at the nanoscale” 

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Abstract: A resurgence of research on ferroelectric materials has recently occurred due to their potential to enhance the performance of memory and logic. For the design and commercialization of such technologies, it is important to understand the physical behavior of ferroelectrics and the interplay with device size, geometry, and fabrication processes. Here, we report a study of geometric factors that can influence the measurement of the remanent ferroelectric polarization, an important measurement for understanding wakeup, retention, and endurance in ferroelectric technologies. The areal size scaling of W/Hf0.5Zr0.5O2/W capacitors is compared in two typical structures: an island top electrode with a continuous ferroelectric layer and an island top electrode/ferroelectric layer (etched ferroelectric layer). Error in the evaluation of the switched area leads to anomalous scaling trends and increasing apparent remanent polarization as capacitor sizes decrease, most strongly in continuous ferroelectric layer capacitors. Using TEM and electric field simulations, this is attributed to two effects: a processing artifact from ion milling that creates a foot on the top electrode and a fringe electric field penetrating outside of the capacitor area. With the correction of the switching area, the 2Pr for both samples agree (∼32 μC cm−2) and is invariant in the capacitor sizes used (down to 400 nm diameter). Our work demonstrates that the determination of the actual capacitor structure and local electric field is needed to evaluate the intrinsic ferroelectric behavior at the nanoscale.

Read more on Applied Physics Letters

New Publication! “Conductive filament formation in the failure of Hf0.5Zr0.5O2 ferroelectric capacitors” 

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Abstract: Ferroelectric materials provide pathways to higher performance logic and memory technologies, with Hf0.5Zr0.5O2 being the most popular among them. However, critical challenges exist in understanding the material’s failure mechanisms to design long endurance lifetimes. In this work, dielectric failure due to repeated switching cycles, occurring through oxygen vacancy motion and leading to the formation of a conductive filament, is demonstrated. A field modified hopping barrier of ∼150–400 meV is observed, indicating a vacancy charge of 0.4–0.6e markedly different from the charge states predicted in the literature. After failure, the capacitor leakage current is high (∼25 mA) and constant with capacitor area, consistent with filament formation. Conductive atomic force microscopy measurements and field distribution simulations suggest a local failure mechanism consistent with filament formation along the boundary of the island capacitor due to an enhanced electric field.

Full text available at APL Materials

New Publication! “Thermodynamic Origins of Nonvolatility in Resistive Memory”

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Abstract: Electronic switches based on the migration of high-density point defects, or memristors, are poised to revolutionize post-digital electronics. Despite significant research, key mechanisms for filament formation and oxygen transport remain unresolved, hindering our ability to predict and design device properties. For example, experiments have achieved 10 orders of magnitude longer retention times than predicted by current models. Here, using electrical measurements, scanning probe microscopy, and first-principles calculations on tantalum oxide memristors, we reveal that the formation and stability of conductive filaments crucially depend on the thermodynamic stability of the amorphous oxygen-rich and oxygen-poor compounds, which undergo composition phase separation. Including the previously neglected effects of this amorphous phase separation reconciles unexplained discrepancies in retention and enables predictive design of key performance indicators such as retention stability. This result emphasizes non-ideal thermodynamic interactions as key design criteria in post-digital devices with defect densities substantially exceeding those of today’s covalent semiconductors.

Full text available from Matter

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Our research is at the intersection of multiple disciplines, drawing on principles and methodologies from materials science, chemistry, physics, and electrical engineering. Our mission is to pioneer … Read More

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Ferroelectronics Lab
Address: 2030 H.H. Dow

T: (734) 763-6914
E: jtheron@umich.edu