DIPJYOTI DAS
@kaist.ac.kr
Korea Advanced Institute of Science and Technology
Dipjyoti Das received his B.E. degree in Electronics and Telecommunication Engineering from Assam Engineering College, Gauhati University, Assam, India, and the Ph.D. degree in Organic Optoelectronic Devices from Indian Institute of Technology Guwahati, Assam, India in 2018. He worked as a postdoctoral fellow in Electrical Engineering Department, Korea Advanced Institute of Science and Technology (KAIST) from August 1, 2018 to August 31, 2020. He will be joining as a post doctoral fellow in the School of Electrical and Computer Engineering (ECE) at the Georgia Institute of Technology soon. His current research interests include high-k ferroelectric HZO capacitors, energy storage capacitors, FEFETs and neuromorphic engineering.
EDUCATION
PhD, Indian Institute of Technology Guwahati
RESEARCH INTERESTS
Organic Optoelectronic Devices, CMOS compatible hafnia based memory devices
Scopus Publications
Scholar Citations
Scholar h-index
Scholar i10-index
Scopus Publications
Matthias Passlack, Nujhat Tasneem, Chinsung Park, Prasanna Venkat Ravindran, Hang Chen, Dipjyoti Das, Shimeng Yu, Edward Chen, Jer-Fu Wang, Chih-Sheng Chang,et al.
AIP Publishing
A comprehensive quantitative root cause study of defect evolution leading to memory window closure from a charge balance and charge trapping perspective throughout all phases of a Si channel Hf0.5Zr0.5O2 (HZO) ferroelectric field-effect-transistor (FEFET) is reported. Starting with the first write pulse, an excessive SiO2 interlayer field is revealed that triggers the creation of defect levels Dit in excess of 1015 cm−2 eV−1 at the HZO–SiO2 interface screening ferroelectric (FE) polarization while enabling FE switching. Under subsequent early bipolar fatigue cycling (up to 104 cycles), defect creation commences at the SiO2–Si interface due to the high injected hole fluence (0.39 C/m2) during each stress pulse causing negative bias instability (NBI), which shifts the threshold voltage of the erase state VT,ERS by −0.3 V with accrual of permanently captured charge Nit of up to +5 × 10−3 C/m2 (3 × 1012 cm−2). Subsequently, Nit NBI generation at the SiO2–Si interface accelerates reaching levels of +7 × 10−2 C/m2, locking both FEFET program and erase drain current vs gate–source-voltage (ID–VGS) characteristics in the FEFET on-state inducing memory window closure at 105 cycles while FE switching (switched polarization Psw = 0.34 C/m2) remains essentially intact. These findings guide the down-selection toward suitable semiconductor/FE systems for charge balanced, reliable, and high endurance FEFETs.
Chinsung Park, Harshil Kashyap, Dipjyoti Das, Jae Hur, Nujhat Tasneem, Sarah Lombardo, Nashrah Afroze, Winston Chern, Andrew C. Kummel, Shimeng Yu,et al.
Institute of Electrical and Electronics Engineers (IEEE)
Strategies to reduce the interfacial oxide layer thickness in ferroelectric (FE) Hf0.5Zr0.5O2 (HZO) metal-oxide-semiconductor capacitor (FE-MOS) structures on Ge and Si substrates were investigated by electrode engineering, as means to reduce the write voltage in FE field-effect transistors (FEFETs). When the gate metal in Ge FE-MOS capacitors is changed from W (control) to Pt/Ti, the coercive voltage is reduced from ~2.5 to ~0.9 V (a 66% reduction) along with a 64% increase in the capacitance consistent with an interfacial layer (IL) thinning. High-resolution scanning transmission electron microscopy (HR-STEM) reveals no visible IL with Pt/Ti electrodes in Ge FE-MOS, suggesting the scavenging of oxygen from the GeOx IL by the Pt/Ti electrode. However, a much smaller reduction of the coercive voltage was observed on Si FE-MOS structures with Pt/Ti electrodes. In this study, it is demonstrated that IL thinning might provide a pathway to reduce the write voltage in FEFETs based on conventional semiconductor channel materials down to a logic-compatible level.
Dipjyoti Das, Prasanna Venkatesan Ravindran, Chinsung Park, Nujhat Tasneem, Zheng Wang, Hang Chen, Winston Chern, Shimeng Yu, Suman Datta, and Asif Khan
Institute of Electrical and Electronics Engineers (IEEE)
Dipjyoti Das, Hyeonwoo Park, Zekai Wang, Chengyang Zhang, Prasanna Venkatesan Ravindran, Chinsung Park, Nashrah Afroze, Po-Kai Hsu, Mengkun Tian, Hang Chen,et al.
IEEE
Dipjyoti Das, Peddaboodi Gopikrishna, Debasish Barman, Ramesh Babu Yathirajula, and Parameswar Krishnan Iyer
Elsevier BV
Dipjyoti Das, Batzorig Buyantogtokh, Venkateswarlu Gaddam, and Sanghun Jeon
Institute of Electrical and Electronics Engineers (IEEE)
Achieving low equivalent oxide thickness (EOT) with CMOS-compatible materials is of prime importance for further miniaturization of dynamic random access memory (DRAM) technology. Despite several efforts made in this regard, especially by using high dielectric constant (<inline-formula> <tex-math notation="LaTeX">$\\kappa $ </tex-math></inline-formula>) materials, utilization of novel electrodes to counterfeit the high leakage current limits its immediate implementation. Herein, we experimentally demonstrate CMOS-compatible ~4.8 Å-EOT ferroelectric (FE) Zr-rich Hf<sub><italic>x</italic></sub>Zr<sub>1–<italic>x</italic></sub>O<sub>2</sub> (HZO) with morphotropic phase boundary (MPB) formed by high-pressure annealing (HPA) and rapid cooling (RC) processes for next-generation DRAM device applications. Such low EOT was achieved by precise process optimization such as HZO composition, annealing condition, and thickness in the vicinity of MPB. The utilization of HPA reduces the physical thickness for emergence of MPB in Zr-rich HZO films, whereas the RC of the HPA samples reduces the leakage current considerably. A maximum dielectric constant (<inline-formula> <tex-math notation="LaTeX">$\\kappa $ </tex-math></inline-formula>) of ~49 was achieved for 6.0-nm HZO [1:2] films for 500 °C HPA-RC resulting in an EOT as low as ~4.8 Å. This is the lowest among all reported EOT value using CMOS-compatible HZO films with leakage current <10<sup>−7</sup> A/cm<sup>2</sup> using TiN as top and bottom electrodes and can be of significant importance for the future DRAM technology.
Dipjyoti Das and Asif Islam Khan
Institute of Electrical and Electronics Engineers (IEEE)
A Ferroelectric Field-Effect Transistor (FEFET) is a promising candidate for next-generation memory devices because it offers numerous advantages, such as its high speed, low energy profile, and nondestructive readout process. FEFETs merge logic and memory functionality in a single device, allowing efficient data transfer and a high packing density, which could make them vital for use in future in-memory computing architectures. Although issues related to the integration of perovskite materials in the state-of-the art semiconductor industry limited their commercial success, the discovery of ferroelectricity in CMOS-compatible doped hafnium oxides led to a re-emergence of FEFETs in advanced microelectronics. The demonstration of ferroelectricity in doped hafnia at extremely thin film thicknesses plus its low permittivity, high coercive field (Ec), environment-friendly composition, and excellent CMOS compatibility are expected to unleash the promise of FEFETs, and significant progress has been made in this regard.
Venkateswarlu Gaddam, Dipjyoti Das, Taeseung Jung, and Sanghun Jeon
Institute of Electrical and Electronics Engineers (IEEE)
The use of additional dielectric (DE) layers such as Al<sub>2</sub>O<sub>3</sub>, ZrO<sub>2</sub>, HfO<sub>2</sub> and Ta<sub>2</sub>O<sub>5</sub> with Hf<sub>0.5</sub>Zr<sub>0.5</sub>O<sub>2</sub> (HZO) ferroelectric (FE) layer, namely bi-layer systems are drawing much attention in the recent past due to enhanced FE properties and their applications in memory technology. However, HZO based tri-layer capacitors with various DE layers are not yet reported. The present paper demonstrates the enhanced FE properties of MFM tri- layer capacitors were found at low temperature (350 °C) using rapid thermal annealing (RTA) process. Various tri-layer capacitors were fabricated by changing top dielectric layers such as TiO<sub>2</sub>, ZrO<sub>2</sub> and SiO<sub>2</sub> (thickness of 10Å) while keeping FE HZO (100Å) and bottom HfO<sub>2</sub> DE layer (10Å) as constant and traditional P-E curves for all those tri-layer (DE/FE/DE) capacitors were observed. Among all the tri-layer capacitors, highest remanent polarization (<inline-formula> <tex-math notation="LaTeX">$\\text{P}_{\\text {r}} \\sim ~16.6~\\mu \\text{C}$ </tex-math></inline-formula>/cm<sup>2</sup>) was observed for the TiO<sub>2</sub> top DE at 350 °C. The addition of top DE layers induced strain in the FE film due to their different thermal expansion coefficients and as a resulting in enhancing o-phase. The demonstration of excellent ferroelectric property by the as fabricated metal-ferroelectric-metal (MFM) capacitors with crystallization temperature as low as 350 °C can be of significant importance in sensor and display applications.
Dipjyoti Das, Batzorig Buyantogtokh, Venkateswarlu Gaddam, and Sanghun Jeon
Institute of Electrical and Electronics Engineers (IEEE)
In this study, we have carried out an in-depth analysis on the role of high-pressure annealing (HPA) conditions on ferroelectricity as well as the interfacial property of Hf<sub><italic>x</italic></sub>Zr<sub>1–<italic>x</italic></sub>O<sub>2</sub> (HZO) capacitors in metal–ferroelectric–metal (MFM) structure. Unlike conventional 1:1 HZO films, HZO demonstrates the highest ferroelectricity at 1:3 Hf:Zr ratio in HPA and a significantly higher maximum remanent polarization (<inline-formula> <tex-math notation="LaTeX">${P}_{\\text {r}}$ </tex-math></inline-formula>) of <inline-formula> <tex-math notation="LaTeX">$31~\\mu \\text{C}$ </tex-math></inline-formula>/cm<sup>2</sup> was achieved as compared to <inline-formula> <tex-math notation="LaTeX">$19~\\mu \\text{C}$ </tex-math></inline-formula>/cm<sup>2</sup> obtained in rapid thermal annealing (RTA) (HZO [1:1]). To understand the influence of HPA conditions, HZO [1:3] capacitors are annealed at various temperatures (300 °C, 400 °C, 500 °C, and 600 °C) and pressures (1, 50, and 200 atm). Ferroelectricity in HZO is found to enhance with increasing HPA temperature or pressure due to higher ferroelectric phase formation as revealed by grazing incidence X-ray diffractometer (GIXRD) analysis. Transient pulse switching measurement suggests the reduction in the effective thickness of the interfacial nonferroelectric film at a high HPA temperature or pressure which was also validated by impedance analysis. Further, impedance analysis reveals a reduction in the number of oxygen vacancy defects with increasing annealing temperature or pressure implying a phase transition from the tetragonal phase of nonferroelectric layer to the ferroelectric orthorhombic phase. In addition, excellent endurance property till 10<sup>9</sup> cycles with reduced wake-up effect was observed in HZO [1:3] capacitors with increasing the HPA temperature, while higher annealing pressure is found to increase <inline-formula> <tex-math notation="LaTeX">${P}_{r}$ </tex-math></inline-formula> without affecting its endurance property. The results obtained herein can be of significant scientific importance, especially to achieve enhanced ferroelectric property with reduced wake-up effect in Zr-rich HZO ferroelectrics.
Hyunjin Joh, Gopinathan Anoop, Won-June Lee, Dipjyoti Das, Jun Young Lee, Tae Yeon Kim, Hoon Kim, WooJun Seol, Jiwon Yeom, Sanghun Jeon,et al.
American Chemical Society (ACS)
Low-temperature deposition of inorganic ferroelectric (FE) thin films is highly demanded for lowering the environmental impact through lesser energy consumption. Doped HfO2-based FE thin films comm...
Dipjyoti Das, Venkateswarlu Gaddam, and Sanghun Jeon
Institute of Electrical and Electronics Engineers (IEEE)
In this letter, we demonstrate an effective way to enrich the performance of HfxZr1–x O2 (HZO) energy storage capacitors (ESCs) by inserting Al2O3 dielectric interlayer (DIL) in the middle of HZO in TiN/HZO/TiN structure. The impact of DIL (1 Å, 5 Å and 10 Å) is investigated in three different HZO compositions [1:1, 1:2 and 1:3]. Irrespective of HZO composition, insertion of DIL at critical thickness enhances the energy storage density (ESD) and efficiency of the ESCs. Grazing incident X-ray diffractometer (GIXRD) analysis reveals that Al2O3 DIL controls the grain size of the HZO films resulting in lower energy dissipation by controlling the linearity of the hysteresis loop. At 4.5 MV/cm, the best ESD ~55 J/cm3 with ~68% efficiency is achieved for HZO [1:2] capacitors at a critical Al2O3 thickness of 1 Å which is ~35% and ~55% enhanced as compared to that without DIL [~41 J/cm3, ~44%]. The best efficiency of ~88% is achieved [ESD ~26 J/cm3] for HZO [1:3] capacitors at a critical DIL thickness of 10 Å. The HZO [1:2] ESC with 1 Å DIL displays robust thermal stability in 25 °C to 200 °C temperature range. Moreover, the aforesaid ESCs demonstrate excellent electrical stability even after 109 times of electric field cycling. Highest ESD of ~70 J/cm3 at ~50% efficiency is achieved for the same ESC when operated at 6.0 MV/cm. The results obtained herein provide a new strategy to achieve high performance ESCs and can be of significant scientific importance.
Yongsun Lee, Youngin Goh, Junghyeon Hwang, Dipjyoti Das, and Sanghun Jeon
Institute of Electrical and Electronics Engineers (IEEE)
In recent years, several experimental approaches have been adopted to study and understand the mechanism and improve the ferroelectricity of fluorite-type hafnia-based ferroelectric materials. In this regard, significant efforts have been made to elucidate the role of top electrode and bottom electrode (TE and BE) materials in defining the ferroelectricity in such systems, especially in terms of induced mechanical tensile stress by these materials during the process of annealing. However, the effect of the electrode material was not investigated both at TE and BE, and despite numerous efforts, there is still a lack of accurate and systematic understanding. In this report, we have carried out a systematic investigation on the effect of TE and BE materials having different coefficient of thermal expansion (CTE), by changing the electrode material one at a time, both at the top and bottom. The influence of the TE was confirmed using [TE/Hf0.5Zr0.5O2(HZO)/TiN] structure in which the BE was fixed as TiN, and the influence of the BE was confirmed using [TiN/HZO/BE] structure by fixing TiN as the TE. As revealed by polarization versus electric field and residual stress analysis, smaller CTE of the electrode was found to result in higher tensile stress in the HZO films during the annealing process, facilitating the formation of higher ferroelectric o-phase and thereby resulting in greater ferroelectricity. Although the influence of TE and BE on the ferroelectric property of HZO films was found to show similar trends according to the CTE value of the electrodes, the influence of TE on the ferroelectric property of the HZO capacitors is found to be mainly due to the variation in the induced mechanical tensile stress; pulse switching measurement and X-ray photoelectron spectrometer (XPS) analysis suggest that in case of BE, both the induced mechanical tensile stress and the interfacial dead layer were found to play a significant part. As a result, BE was found to have a greater influence on ferroelectricity of the HZO capacitors when compared with that of TE. The highest remnant polarization of 48.2 and $58.7~\\mu \\text{C}$ /cm2 was obtained for W with the lowest of CTE of $4.5\\times 10^{-6}/^{\\circ }\\text{C}$ in both the configurations. The results obtained in this article are expected to provide a new way out to optimize the interface quality and ferroelectricity in HZO-based capacitors.
Dipjyoti Das, Venkateswarlu Gaddam, and Sanghun Jeon
The Institute of Electronics Engineers of Korea
Dipjyoti Das, Taeho Kim, Venkateswarlu Gaddam, Changhwan Shin, and Sanghun Jeon
Elsevier BV
Abstract Recently, negative capacitance (NC) effect in the dielectric/ferroelectric (DE/FE) bilayer system has received significant attention due to its potential in achieving sub- 60 mV/decade subthreshold swing in FETs as well as extremely large capacitance density in dynamic random-access memory (DRAM). However, such reports, to date, are primarily based on conventional perovskite FE materials which are not compatible with the present CMOS technology. Herein, we study the interfacial charge density ( σ i ) and negative capacitance (NC) effect in CMOS compatible Hf-Zr-Al-O (DE) /Hf0.5Zr0.5O2 (FE) bilayer system. The DE layer of various thicknesses (5–20 A) was deposited on the top of FE layer (100 A) and the DE layer thickness was found to play a crucial role in determining σ i . The NC effect in the aforesaid DE/FE system was suppressed due to the contribution of σ i . The σ i at the interface of the DE layer and FE layer was found to be in the range of −0.57 Cm−2 to −0.18 Cm−2 for the DE thickness range of 5–20 A.
Dipjyoti Das and Sanghun Jeon
Institute of Electrical and Electronics Engineers (IEEE)
In this article, we report the fabrication of Zr-rich high-<inline-formula> <tex-math notation="LaTeX">${k}$ </tex-math></inline-formula> ferroelectric hafnium zirconium oxide (HZO) capacitor with TiN as the top and bottom electrodes demonstrating an equivalent oxide thickness (EOT) of 5.7 Å and remanent polarization (<inline-formula> <tex-math notation="LaTeX">${P}_{r}$ </tex-math></inline-formula>) of <inline-formula> <tex-math notation="LaTeX">$\\sim 16~\\mu \\text{C}$ </tex-math></inline-formula>/cm<sup>2</sup>. High-<inline-formula> <tex-math notation="LaTeX">${k}$ </tex-math></inline-formula> value and low EOT was achieved by utilizing multiphase region of HZO as well as high pressure post metallization annealing (HPPMA). Despite the high-<inline-formula> <tex-math notation="LaTeX">${k}$ </tex-math></inline-formula> value of Zr-rich HZO films, the emergence of multiphase region at higher physical thickness when annealed using rapid thermal annealing (RTA) limits its EOT value. On the contrary, multiphase emerges at a smaller physical thickness in HPPMA due to the formation of more o-phase as revealed by grazing incidence X-ray diffractometer (GIXRD). The smaller physical thickness of HPPMA together with the demonstration of significantly higher dielectric constant (>50) by HZO in the vicinity of multiphase, was therefore, found to be very effective in reducing the EOT.
Murali Gedda, Dipjoyti Das, Parameswar Krishnan Iyer, and Giridhar U Kulkarni
IOP Publishing
In order to realize low-cost and efficient organic light-emitting diodes (OLEDs), the transparent anode should have excellent optical and electrical properties, among other factors. Typically, transparent conductive oxides have been widely used for transparent top electrodes, but they suffer from several drawbacks. We herein report the fabrication of efficient indium-free transparent OLEDs using metal-mesh based top electrodes, made of any metal of choice, Au, Ag or Cu. The fabricated devices on inch square substrates exhibited superior emission characteristics without any color shift. In terms of workfunction matching, Cu did the best. With a Cu-TCE of low sheet resistance (~7 Ω sq−1), uniform emission characteristics were achieved with relatively high current efficiency and luminance, comparable to those from the ITO based devices.
Venkateswarlu Gaddam, Dipjyoti Das, and Sanghun Jeon
IEEE
Recently, dielectric/ferroelectric (DE/FE) bilayer systems have been extensively investigated for achieving high remanent polarization in Hf<inf>0.5</inf>Zr<inf>0.5</inf>O<inf>2</inf>(HZO) based MFM capacitors. Herein, we report significant enhancement in the ferroelectric property of HZO capacitors by incorporating Ta<inf>2</inf>O<inf>5</inf>as the dielectric seed layer. Thickness of the Ta<inf>2</inf>O<inf>5</inf>layer was incorporated at both top and bottom of the HZO films and the thickness of the seed layer was varied from 10 to 50 Å. When the Ta<inf>2</inf>O<inf>5</inf>dielectric films were inserted at the top, the highest remanent polarization ~ <tex>$\\mathbf{16.83}\\ \\mu\\mathbf{C}/\\mathbf{cm}^{\\mathbf{2}}$</tex> was observed in case of 20 Å films as compared to that of ~ <tex>$\\mathbf{13.21}\\ \\mu\\mathbf{C}/\\mathbf{cm}^{\\mathbf{2}}$</tex> of the reference HZO device. Similarly, for bottom Ta<inf>2</inf>O<inf>5</inf>dielectric films, the highest remanent polarization ~ <tex>$\\mathbf{15.24}\\ \\mu\\mathbf{C}/\\mathbf{cm}^{\\mathbf{2}}$</tex> was observed in case of 20 Å films. When we compared both the stacks, the best result was observed in case of top Ta<inf>2</inf>O<inf>5</inf>. The coercive field <tex>$(\\mathbf{E}_{\\mathbf{c}})$</tex> was also found to be nearly same with the HZO based device despite the incorporation of the dielectric layer. The enhanced ferroelectricity of these devices can be used in memory devices, FeFETs, FTJ and sensors applications.
Sanghun Jeon, Dipjyoti Das, and Venkateswarlu Gaddam
IEEE
In recent years, hafnium zirconium oxide (HZO) has received significant attention of the research community due to the demontsrattion of ferroelectricity at sub 10 nm thickness and good CMOS-compatibility. However, ferroelectricity in HZO is dependent on various factors such as the HZO composition, materials used as top or bottom electrode, post metallization technique, annealing pressure and temperature etc. Herein, we have systematically investigated the effect of high pressure post metallization annealing (HPPMA) temperature on the ferroelectric properties of TiN/Hf0.25Zr0.75O2/TiN capacitors. Hf0.25Zr0.75O2, that show antiferroelectric behavior in rapid thermal annealing, demonstrate antiferroelectric to ferroelectric phase transition depending on the HPPMA temperature. Maximum remanent polarization (Pr) of ~29μC/cm2 is achieved for HPPMA temperature 550 °C whereas the maximum dielectric constant (k) of ~46 is achieved for HPPMA temperature 350 °C. The interfacial capacitance of the MFM capacitors increased with increasing the HPPMA annealing temperature. The results obtained in this study open up the possibility of achieving high-k ferroelectric capacitors using Zr rich HZO films that can be applicable to various electronic devices.
Vekateswarlu Gaddam, Dipjyoti Das, and Sanghun Jeon
Institute of Electrical and Electronics Engineers (IEEE)
This article highlights the role of HfO<sub>2</sub> seed/dielectric insertion layers on the ferroelectric properties of hafnium zirconium oxide (HZO)-based metal-ferroelectric-metal (MFM) capacitors. Maximum remanent polarization (<inline-formula> <tex-math notation="LaTeX">${P}_{r}$ </tex-math></inline-formula>) of <inline-formula> <tex-math notation="LaTeX">$22.1~\\mu \\text{C}$ </tex-math></inline-formula>/cm<sup>2</sup> was achieved when HfO<sub>2</sub> seed layer of critical thickness (10 Å) was inserted at the bottom of HZO films. However, as the bottom HfO<sub>2</sub> seed layer thickness increases from 10 to 200 Å, the <inline-formula> <tex-math notation="LaTeX">${P}_{r}$ </tex-math></inline-formula> was found to decrease. The same critical thickness of HfO<sub>2</sub> seed layer when inserted at top of the HZO films was found to give a maximum <inline-formula> <tex-math notation="LaTeX">${P}_{r}$ </tex-math></inline-formula> of <inline-formula> <tex-math notation="LaTeX">$19.6~\\mu \\text{C}$ </tex-math></inline-formula>/cm<sup>2</sup>. The obtained <inline-formula> <tex-math notation="LaTeX">${P}_{r}$ </tex-math></inline-formula> in both the cases (bottom and top HfO<sub>2</sub> seed layer) was found to be better than the reference HZO device (<inline-formula> <tex-math notation="LaTeX">$17.7~\\mu \\text{C}$ </tex-math></inline-formula>/cm<sup>2</sup>). Moreover, increased coercive field <inline-formula> <tex-math notation="LaTeX">$({E}_{c})$ </tex-math></inline-formula> was noticed due to the HfO<sub>2</sub> seed layer insertion (1.11 MV/cm for bottom HfO<sub>2</sub> and 1.09 MV/cm for top HfO<sub>2</sub>) when compared with the reference device (1.04 MV/cm). Short pulse switching measurements were carried out on the as-fabricated capacitors, and an enhanced <inline-formula> <tex-math notation="LaTeX">${E}_{c}$ </tex-math></inline-formula> and interfacial capacitance was observed in case of devices with HfO<sub>2</sub> seed layer. The crystal structures, and interfacial layer effect was keenly observed and analyzed for the HZO films with and without the seed layer using grazing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS), respectively. The HfO<sub>2</sub> seed layer was found to result in higher o-phase formation, and facilitates the diffusion/migration of oxygen atoms from seed layer to the substrate or vice versa in the HZO capacitors giving rise to enhanced ferroelectricity.
Dipjyoti Das, Venkateswarlu Gaddam, and Sanghun Jeon
Institute of Electrical and Electronics Engineers (IEEE)
In this letter, we report excellent ferroelectricity with high remanent polarization (Pr) in Zr rich hafnium zirconium oxide (HZO) films using high pressure post metallization annealing (HPPMA). HZO films annealed using rapid thermal annealing (RTA) show highest ferroelectricity when the Hf:Zr ratio is 1:1 and exhibit antiferroelectric property for Zr rich films. However, under HPPMA, Zr rich films demonstrate enhanced ferroelectric property as compared to 1:1 HZO films and the best result was observed for 1:3 HZO films. The HZO (1:3) films by HPPMA show $\\text{P}_{\\text {r}}$ as high as $\\sim ~29~\\mu \\text{C}$ /cm2, which is almost 50% higher than that achieved for the best condition of RTA ( $\\text{P}_{\\text {r}}~\\sim ~19~\\mu \\text{C}$ /cm2). Besides, due to the presence of high Zr, the HZO (1:3) films by HPPMA exhibit considerably higher dielectric constant (k ~ 43) and low coercive field ( $\\text{E}_{c}\\sim 1.21$ MV/cm) as compared to that of HZO (1:1) films by RTA (k ~ 37, $\\text{E}_{\\text {c}}\\sim ~1.35$ MV/cm). We believe the enhanced ferroelectricity in Zr rich HZO films can be due to more o/t-phase formation in ZrO2 under HPPMA as compared to that of RTA. On the other hand, more m-phase formation was observed in HfO2 under HPPMA, resulting in a significant drop of $\\text{P}_{\\text {r}}$ in case of Hf rich HZO. The demonstration of enhanced ferroelectric properties by Zr rich HZO films can be helpful for high functional semiconductor device applications.
Dipjyoti Das, Junghak Park, Minho Ahn, Sungho Park, Jihyun Hur, and Sanghun Jeon
IOP Publishing
An amorphous InZnO/MoS2 heterojunction-based phototransistor with excellent photoconductive gain and responsivity over the entire visible range has been demonstrated. The photogenerated current of the InZnO phototransistor at long light wavelength (>600 nm) was significantly improved by utilizing narrow bandgap MoS2 as the capping layer (1.3 eV). At lower wavelength, photocarriers are generated due to the optical absorption of both InZnO and MoS2 layers, whereas the latter ensures significant photocarrier generation even at the higher wavelength region of the visible spectrum. The photogenerated carriers subsequently transfer to the underlying InZnO layer of superior carrier mobility that has a high channel conduction of additional electrons from the optically-induced doubly positively charged oxygen vacancies (Vo++) where the gate field is screening, thereby leading to the higher photoconductive gain of the InZnO/MoS2 phototransistors. The dynamic photosensitivity behaviour of the aforesaid phototransistor reveals the presence of persistent photoconductivity (PPC) due to the oxygen vacancy associated with InZnO which can be removed by applying a reset gate pulse from -15 to +5 V. The optical properties of these phototransistors were further enhanced by replacing the opaque Ti/Au electrode by an ultrathin transparent Ti/Au electrode. Utilization of the transparent electrode results in enhanced electron injection from source to channel due to a reduced barrier height under illumination giving rise to a ten-fold improvement in the photocurrent and responsivity of the phototransistors. A position-dependent study of the photocurrent w.r.t beam position also reveals that the enhancement in photocurrent is strongly dependent on the position and is at its maximum when the beam is placed near the source region.
Dipjyoti Das, Peddaboodi Gopikrishna, Debasish Barman, Ramesh Babu Yathirajula, and Parameswar Krishnan Iyer
Springer Science and Business Media LLC
Abstract White organic/polymer light emitting diode (WOLED/WPLED) processed from solution has attracted significant research interest in recent years due to their low device production cost, device flexibility, easy fabrication over large area including roll to roll and ability to print in various designs and shapes providing enormous design possibilities. Although WOLEDs fabricated using solution process lack their thermally evaporated counterparts in terms of device efficiency, remarkable progress has been made in this regard in recent years by utilizing new materials and device structures. In the present review, we have summarized and extrapolated an excellent association of old and modern concept of cost-effective materials and device structure for realization of white light. In particular, this article demonstrated and focused on design, and development of novel synthesis strategy, mechanistic insights and device engineering for solution process low cost WOLEDs device. Herein, an overview of the prevailing routes towards white light emitting devices (WLEDs) and corresponding materials used, including polymer based WLED, small molecules emitters based thermally activated delayed fluorescence (TADF), perovskite light-emitting diodes (PeLEDs) and hybrid materials based LEDs, color down-converting coatings with corresponding best efficiencies ever realized. We presume that this exhaustive review on WLEDs will offer a broad overview of the latest developments on white SSL and stonework the approach en route for innovations in the immediate future.
Junghak Park, Dipjyoti Das, Minho Ahn, Sungho Park, Jihyun Hur, and Sanghun Jeon
Springer Science and Business Media LLC
Abstract In recent years, MoS2 has emerged as a prime material for photodetector as well as phototransistor applications. Usually, the higher density of state and relatively narrow bandgap of multi-layer MoS2 give it an edge over monolayer MoS2 for phototransistor applications. However, MoS2 demonstrates thickness-dependent energy bandgap properties, with multi-layer MoS2 having indirect bandgap characteristics and therefore possess inferior optical properties. Herein, we investigate the electrical as well as optical properties of single-layer and multi-layer MoS2-based phototransistors and demonstrate improved optical properties of multi-layer MoS2 phototransistor through the use of see-through metal electrode instead of the traditional global bottom gate or patterned local bottom gate structures. The see-through metal electrode utilized in this study shows transmittance of more than 70% under 532 nm visible light, thereby allowing the incident light to reach the entire active area below the source and drain electrodes. The effect of contact electrodes on the MoS2 phototransistors was investigated further by comparing the proposed electrode with conventional opaque electrodes and transparent IZO electrodes. A position-dependent photocurrent measurement was also carried out by locally illuminating the MoS2 channel at different positions in order to gain better insight into the behavior of the photocurrent mechanism of the multi-layer MoS2 phototransistor with the transparent metal. It was observed that more electrons are injected from the source when the beam is placed on the source side due to the reduced barrier height, giving rise to a significant enhancement of the photocurrent.
A.G. Odeshi, A.A. Tiamiyu, A.K. Khan, N. Katwal, D. Das, and I.N.A. Oguocha
Elsevier BV
A.G. Odeshi, A.A. Tiamiyu, D. Das, N. Katwal, I.N.A. Oguocha, and A.K. Khan
Elsevier BV