| 1. |
- Athle, Robin, et al.
(författare)
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Effects of TiN Top Electrode Texturing on Ferroelectricity in Hf1-xZrxO2
- 2021
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Ingår i: ACS applied materials & interfaces. - : American Chemical Society (ACS). - 1944-8244 .- 1944-8252. ; 13:9, s. 11089-11095
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Tidskriftsartikel (refereegranskat)abstract
- Ferroelectric memories based on hafnium oxide are an attractive alternative to conventional memory technologies due to their scalability and energy efficiency. However, there are still many open questions regarding the optimal material stack and processing conditions for reliable device performance. Here, we report on the impact of the sputtering process conditions of the commonly used TiN top electrode on the ferroelectric properties of Hf1-xZrxO2. By manipulating the deposition pressure and chemistry, we control the preferential orientation of the TiN grains between (111) and (002). We observe that (111) textured TiN is superior to (002) texturing for achieving high remanent polarization (Pr). Furthermore, we find that additional nitrogen supply during TiN deposition leads to >5× greater endurance, possibly by limiting the scavenging of oxygen from the Hf1-xZrxO2 film. These results help explain the large Pr variation reported in the literature for Hf1-xZrxO2/TiN and highlights the necessity of tuning the top electrode of the ferroelectric stack for successful device implementation.
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| 2. |
- Athle, Robin
(författare)
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Ferroelectric Memristors - Materials, Interfaces and Applications
- 2024
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The backbone of modern computing systems rely on two key things: logic and memory, and while computing power hasseen tremendous advancements through scaling of the fundamental building block – the transistor, memory access hasn’tevolved as rapidly, leading to significant memory-bound systems. Additionally, the rapid evolution of machine learningand deep neural network (DNN) applications, has exposed the fundamental limitations of the traditional von Neumanncomputing architecture, due to its heavy reliance on memory access. The physical separation between the computing unitand the memory in von Neumann architectures is limiting performance and energy efficiency. A promising solution toaddress these challenges is the development of emerging non-volatile memory technologies that provide significant scalingand integration possibilities, fast switching speeds, and highly energy-efficient operations. Additionally, by integrating“memory resistors” (memristors) in large crossbar arrays, the computation can take place in-memory which can resolve thebottleneck in traditional von Neumann architectures.This thesis investigates the implementation of ferroelectric HfO2 in ferroelectric tunnel junctions (FTJs) and ferroelectricfield effect transistors (FeFETs) as potential candidates for emerging non-volatile memories and memristors.Initially, the thesis focuses on the integration of ferroelectric HfO2 onto the high mobility III-V semiconductor InAs forthe fabrication of metal-oxide-semiconductor (MOS) capacitors. Moreover, optimization of the processing conditions on thecritical interface between the semiconductor and high-k oxide is extensively studied using both electrical characterization andsynchrotron radiation techniques. After optimization of the annealing treatment and top electrode texturing, the fabricationof vertical InAs nanowire FeFETs is successfully implemented. The FeFET shows encouraging initial results with limitationssolvable by further process engineering.The fabrication of metal-insulator-metal (MIM) capacitors with a tungsten (W) top electrode enables ferroelectricity inHfxZr1?xO2 films down to 3.2 nm thickness. However, achieving ferroelectric properties in ultra-thin films requires anannealing temperature above the thermal budget for back-end-of-line (BEOL) integration. To combat this, nanosecond laserannealing (NLA) is introduced, where an ultrafast laser pulse confines the annealing both spatially and depth-wise. UsingNLA, we crystallize 3.6 nm-thick HfxZr1?xO2 films while still being BEOL compatible.The ability to fabricate thin ferroelectric HfO2 films opens up for the fabrication of FTJs, however, being constrained to aW top electrode is severely limiting the device design. By introducing the concept of a crystallization electrode (CE) and ametal replacement process, tuning of the FTJ device characteristics is achieved. We also highlight the impact of the postmetallizationannealing (PMA) temperature on the tunneling electroresistance ratio (TER) of the FTJ. Despite giving similarferroelectric properties, the PMA temperature strongly affects the interface quality which is key for FTJ performance.Partial polarization switching is utilized to achieve multi-state conductance levels in the FTJs, demonstrating its memristivecapabilities. The stable state retention and low variability are promising for the realization of in-memory computing usingcrossbar arrays. Finally, the impact of random telegraph noise (RTN) in ultra-scaled FTJs and the scalability of FTJ crossbararrays is assessed. The low conductance of FTJ memristors reduces the IR drop, while the self-rectifying current-voltageproperty relaxes the need for an external selector, results that encourage the realization of FTJ-based in-memory computingaccelerators.
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| 3. |
- Athle, Robin, et al.
(författare)
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Ferroelectric Tunnel Junction Memristors for In-Memory Computing Accelerators
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Ingår i: Advanced Intelligent Systems. - 2640-4567.
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Tidskriftsartikel (refereegranskat)abstract
- Neuromorphic computing has seen great interest as leaps in artificial intelligence (AI) applications have exposed limitations due to heavy memory access, with the von Neumann computing architecture. The parallel in-memory computing provided by neuromorphic computing has the potential to significantly improve latency and power consumption. Key to analog neuromorphic computing hardware are memristors, providing non-volatile multistate conductance levels, high switching speed, and energy efficiency. Ferroelectric tunnel junction (FTJ) memristors are prime candidates for this purpose, but the impact of the particular characteristics for their performance upon integration into large crossbar arrays, the core compute element for both inference and training in deep neural networks, requires close investigation. In this work, a W/HfxZr1−xO2/TiN FTJ with 60 programmable conductance states, a dynamic range (DR) up to 10, current density >3 A m−2 at V read = 0.3 V and highly nonlinear current–voltage (I–V) characteristics (>1100) is experimentally demonstrated. Using a circuit macro-model, the system level performance of a true crossbar array is evaluated and a 92% classification accuracy of the modified nation institute of science and technology (MNIST) dataset is achieved. Finally, the low on conductance in combination with the highly nonlinear I–V characteristics enable the realization of large selector-free crossbar arrays for neuromorphic hardware accelerators.
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| 4. |
- Athle, Robin, et al.
(författare)
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Impact of Temperature-Induced Oxide Defects on HfxZr1−xO2 Ferroelectric Tunnel Junction Memristor Performance
- 2023
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Ingår i: IEEE Transactions on Electron Devices. - 0018-9383. ; 70:3, s. 1412-1416
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Tidskriftsartikel (refereegranskat)abstract
- In this work, we evaluate the importance of the postmetallization annealing (PMA) temperature on the performance of HfxZr1−xO2-based ferroelectric tunnel junctions (FTJs). Our results indicate a significant difference in tunneling electroresistance (TER) ratio and endurance, depending on the PMA temperature despite negligible variations in remanent polarization. We conclude that the minimization of conductive oxide defect states is central to achieve high performance. Through carefully optimized PMA conditions, we demonstrate FTJs with a TER = 3 and low mean cycle-to-cycle variation of < 1.5% combined with at least 16 separable conductance states providing a 4-bit resolution analog FTJ.
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| 5. |
- Athle, Robin, et al.
(författare)
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Improved Endurance of Ferroelectric HfxZr1–xO2 Integrated on InAs Using Millisecond Annealing
- 2022
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Ingår i: Advanced Materials Interfaces. - : Wiley. - 2196-7350. ; 9:27
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Tidskriftsartikel (refereegranskat)abstract
- Ferroelectric HfxZr1–xO2 (HZO) is typically achieved by crystallization of an amorphous thin film via rapid thermal processing (RTP) at time scales of seconds to minutes. For integration on III–V semiconductors, this approach can severely degrade the sensitive HZO/III–V interface. To evaluate whether a reduced thermal budget can improve the interface quality, millisecond duration thermal anneals are utilized using a flash lamp annealer (FLA) on HZO/InAs capacitors. Through thorough electrical characterization such as polarization hysteresis, endurance, and capacitance-voltage measurements, as well as synchrotron-based chemical interface characterization, the FLA and RTP treatments are compared and the FLA results are found in lower interface defect density and higher endurance, but also have generally lower remanent polarization (Pr) compared to RTP. Additionally, ways to achieve high Pr and low interface defect density using multiple lower energy flashes, as well as by pre-crystallization during the ALD growth step are investigated. Using FLA, Pr exceeding 20 µC cm−2 is achieved, with extended endurance properties compared to RTP treatment and a considerably decreased defect density, indicative of a higher quality HZO/InAs interface. This work presents valuable insight into the successful integration of ferroelectric HZO on low thermal budget III–V semiconductors.
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| 6. |
- Athle, Robin, et al.
(författare)
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Top Electrode Engineering for Freedom in Design and Implementation of Ferroelectric Tunnel Junctions Based on Hf1- xZrxO2
- 2022
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Ingår i: ACS Applied Electronic Materials. - : American Chemical Society (ACS). - 2637-6113. ; 4:3, s. 1002-1009
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Tidskriftsartikel (refereegranskat)abstract
- Ferroelectric tunnel junctions (FTJs) based on ultrathin HfO2 have great potential as a fast and energy-efficient memory technology compatible with complementary metal oxide semiconductors. FTJs consist of a ferroelectric film sandwiched between two distinct electrodes, the properties of which are intricately linked to the electrical properties of the FTJs. Here we utilize a W crystallization electrode (CE) to achieve a high and reproducible remanent polarization, combined with a metal replacement process in which the W is carefully removed and replaced by another top electrode (TE). In this way we separate the ferroelectric film properties from the device design and can thereby evaluate the effect of the TE work function (WF) and conduction band electron density (ne) on the tunneling electroresistance (TER) and device reliability. We compare FTJs designed with a TiN bottom electrode and W, Cr, or Ni TE and find that the use of high electron density metals such as Ni or Cr as TE allows for an improved TER, albeit at the cost of reliability due to a large built-in electric field. To bypass this effect, a bilayer Cr/Ni TE is implemented, which allows for a high TER and minimal built-in field, leading to excellent retention and endurance beyond 108 cycles. The results presented here thus highlight a process flow for reliable design and implementation of FTJs.
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| 7. |
- Dahlberg, Hannes, et al.
(författare)
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Ferroelectric-Antiferroelectric Transition of Hf1- xZrxO2on Indium Arsenide with Enhanced Ferroelectric Characteristics for Hf0.2Zr0.8O2
- 2022
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Ingår i: ACS Applied Electronic Materials. - : American Chemical Society (ACS). - 2637-6113. ; 4:12, s. 6357-6363
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Tidskriftsartikel (refereegranskat)abstract
- The ferroelectric (FE)-antiferroelectric (AFE) transition in Hf1-xZrxO2 (HZO) is for the first time shown in a metal-ferroelectric-semiconductor (MFS) stack based on the III-V material InAs. As InAs displays excellent electron mobility and a narrow band gap, the integration of ferroelectric thin films for nonvolatile operations is highly relevant for future electronic devices and motivates further research on ferroelectric integration. When increasing the Zr fraction x from 0.5 to 1, the stack permittivity increases as expected, and the transition from FE to AFE-like behavior is observed by polarization and current-voltage characteristics. At x = 0.8 the polarization of the InAs-based stacks shows a larger FE-contribution as a more open hysteresis compared to both literature and reference metal-ferroelectric-metal (MFM) devices. By field-cycling the devices, the switching domains are studied as a function of the cycle number, showing that the difference in FE-AFE contributions for MFM and MFS devices is stable over switching and not an effect of wake-up. The FE contribution of the switching can be accessed by successively lowering the bias voltage in a proposed pulse train. The possibility of enhanced nonvolatility in Zr-rich HZO is relevant for device stacks that would benefit from an increase in permittivity and a lower operating voltage. Additionally, an interfacial layer (IL) is introduced between the HZO film and the InAs substrate. The interfacial quality is investigated as frequency-dependent capacitive dispersion, showing little change for varying ZrO2 concentrations and with or without included IL. This suggests material processing that is independently limiting the interfacial quality. Improved endurance of the InAs-Hf1-xZrxO2 devices with x = 0.8 was also observed compared to x = 0.5, with further improvement with the additional IL for Zr-rich HZO on InAs.
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| 8. |
- Menon, Heera, et al.
(författare)
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Fabrication of Single-Crystalline InSb-on-Insulator by Rapid Melt Growth
- 2022
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Ingår i: Physica Status Solidi (A) Applications and Materials Science. - : Wiley. - 1862-6300. ; 219:4
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Tidskriftsartikel (refereegranskat)abstract
- InSb has the smallest bandgap and highest electron mobility among III-V semiconductors and is widely used for photodetectors and high-frequency electronic applications. Integration of InSb directly on Si would drastically reduce the fabrication cost and enable new applications, however, it is very challenging due to its 19% lattice mismatch with Si. Herein, the integration of single-crystalline InSb microstructures on insulator-covered Si through rapid melt growth (RMG) is reported and specifically provides details on the fabrication process. The importance of achieving high-quality conformal capping layers at low thermal budget to contain the InSb melt is assessed when the sample is annealed. The importance of ensuring a pristine Si seed area to achieve single-crystalline InSb is illustrated and demonstrated here for the first time.
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| 9. |
- Menon, Heera, et al.
(författare)
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Improved quality of InSb-on-insulator microstructures by flash annealing into melt
- 2021
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Ingår i: Nanotechnology. - : IOP Publishing. - 0957-4484 .- 1361-6528. ; 32:16
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Tidskriftsartikel (refereegranskat)abstract
- Monolithic integration of III-V semiconductors with Silicon technology has instigated a wide range of new possibilities in the semiconductor industry, such as combination of digital circuits with optical sensing and high-frequency communication. A promising CMOS compatible integration process is rapid melt growth (RMG) that can yield high quality single crystalline material at low cost. This paper represents the study on ultra-thin InSb-on-insulator microstructures integrated on a Si platform by a RMG-like process. We utilize flash lamp annealing (FLA) to melt and recrystallize the InSb material for an ultra-short duration (milliseconds), to reduce the thermal budget necessary for integration with Si technology. We compare the result from FLA to regular rapid thermal annealing (seconds). Recrystallized InSb was characterized using electron back scatter diffraction which indicate a transition from nanocrystalline structure to a crystal structure with grain sizes exceeding 1 μm after the process. We further see a 100× improvement in electrical resistivity by FLA annealed sample when compared to the as-deposited InSb with an average Hall mobility of 3100 cm2 V−1 s−1 making this a promising step towards realizing monolithic mid-infrared detectors and quantum devices based on InSb.
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| 10. |
- Persson, Anton E. O., et al.
(författare)
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Integration of Ferroelectric HfxZr1-xO2 on Vertical III-V Nanowire Gate-All-Around FETs on Silicon
- 2022
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Ingår i: IEEE Electron Device Letters. - 0741-3106. ; 43:6, s. 854-857
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Tidskriftsartikel (refereegranskat)abstract
- We demonstrate a successful process scheme for the integration of a CMOS-compatible ferroelectric gate stack on a scaled vertical InAs nanowire gate-all-around MOSFET on silicon. The devices show promising device characteristics with nanosecond write time and large memory window of >1.5 V. In the current implementation, the device performance is mainly limited by access resistance, which is attributed to the thermal sensitivity of InAs. The findings indicate that the ferroelectricity is not intrinsically preventing future improvements of scaled III-V FeFETs.
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| 11. |
- Persson, Anton E.O., et al.
(författare)
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Reduced annealing temperature for ferroelectric HZO on InAs with enhanced polarization
- 2020
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 116:6
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Tidskriftsartikel (refereegranskat)abstract
- Deposition, annealing, and integration of ferroelectric Hf x Zr 1 - x O 2 (HZO) thin films on the high-mobility semiconductor InAs using atomic layer deposition are investigated. Electrical characterization reveals that the HZO films on InAs exhibit an enhanced remanent polarization compared to films formed on a reference TiN substrate, exceeding 20 μ C / cm 2 even down to an annealing temperature of 370 °C. For device applications, the thermal processes required to form the ferroelectric HZO phase must not degrade the high-κ/InAs interface. We find by evaluation of the capacitance-voltage characteristics that the electrical properties of the high-κ/InAs are not significantly degraded by the annealing process, and high-resolution transmission electron microscopy verifies a maintained sharp high-κ/InAs interface.
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