Venkateswarlu Gaddam

This work investigates the electronic band alignment of p-NiO/InAlN and p-Cr2O3/InAlN heterostructures for the application of achieving enhancement-mode (e-mode) InAlN-based high-electron-mobility transistors (HEMTs) using x-ray photoelectron spectroscopy (XPS), together with the Kraut method to extract the band alignment parameters from reference oxide films and InAlN HEMT structures. The extracted conduction and valence-band offsets of NiO/InAlN are 1.58 and 2.11 eV, respectively. For Cr2O3/InAlN, the extracted conduction and valence-band offsets are 2.07 and 3.13 eV, respectively. NiO and Cr2O3 thin films were deposited by reactive RF sputtering on metal-organic chemical vapor deposited In0.17Al0.83N HEMT structures grown on sapphire or SiC substrates. The energy bandgap of InAlN and Cr2O3 was determined from XPS energy-loss spectra to be 4.43 and 3.37 eV, respectively, while the energy bandgap of NiO was 3.9 eV, as reported in our previous work. Both heterostructures exhibited staggered type-II band alignment with large interfacial carrier barriers. The large valence-band offsets indicate strong hole confinement and suggest that both oxides are capable of electrostatically depleting the polarization-induced two-dimensional electron gas, thereby facilitating enhancement-mode (E-mode) HEMT operation through a positive threshold-voltage shift. Among the investigated oxides, Cr2O3 exhibited the larger valence- and conduction-band offsets, indicating stronger carrier-blocking capability and suggesting greater potential for suppressing gate leakage and achieving a more positive threshold voltage than the NiO/InAlN heterojunction.

Recently, hafnium zirconium oxide ( ${\mathrm {Hf}}_{x}{\mathrm {Zr}}_{{{1}}-{x}}{\mathrm {O}}_{2}$ , HZO) films gained considerable attention in sensors, displays, and memory devices. However, one of the main drawbacks of HZO material is that as the film thickness increases, the dielectric constant ( $\kappa$ ) can deteriorate. To address this issue, we investigated a study on the enhancement of dielectric constant for HZO films (>10 nm) at a low thermal budget. We experimentally demonstrated the existence of a morphotropic phase boundary (MPB) in the HZO films by employing postmetallization annealing (PMA) process at 350 °C. A high dielectric constant was achieved by optimizing process parameters, such as HZO compositions (1:1, 1:3, and 1:5), HZO film thicknesses (15, 30, and 45 nm), and different annealing conditions (PMA 350 °C and PDA 350 °C). The maximum dielectric constant of TiN electrodes was found to be around 38 near MPB for the 15-nm-thick HZO (1:5) films due to the formation of a high tetragonal phase (t-phase) in Zr-rich films. As the film thickness increases, the dielectric constant also decreases. On the other hand, the PDA devices exhibit exclusively ferroelectric behavior due to the higher orthorhombic phase (o-phase). Also, endurance measurements were performed on both PMA and PDA, revealing that these devices are endurable up to 1010 cycles. Furthermore, the dielectric constant was enhanced greatly to 43 at 15-nm-thick HZO and 39 at 30-nm-thick HZO by utilizing the Mo top electrode. This work provides a promising method for achieving a high dielectric constant in HZO films for smart electronic systems.

Recently, hafnia-based ferroelectrics are currently being investigated as next-generation memory devices due to their excellent CMOS process compatibility and functionality. However, some of the ferroelectric devices commonly exhibit an imprint effect due to charged defects around the interfacial layer, which has negative impacts on the ferroelectric memory devices. However, it can be applied to various applications as long as the imprint field is carefully adjusted. In this work, we introduced a strategy to control the imprint field in bilayer capacitors by utilizing tantalum oxide (TaO) interfacial layers and various Zr contents in Hf_0.83Zr_0.17O₂ (HZO) films. The TaO layer (1 nm) was inserted into the bilayer capacitors to alter the imprint field's (positive or negative) direction. Whereas to control the imprint fields, we adjusted the Zr doping content (17%–83%) in the ferroelectric HZO films (8 nm). As the Zr content increased, reduced imprint fields were observed in those bilayer capacitors. In addition, it was found that a high imprint field (+2.43 MV/cm) was observed in Hf-rich films (Hf_0.83Zr_0.17O₂) due to the higher amount of oxygen vacancies. In addition, we examined those oxygen vacancies through x-ray photoelectron spectroscopy depth profile analysis by considering sub-oxide fractions in the tantalum, which further confirms the root cause of the imprint field variations in the bilayer capacitors. Our study will contribute to a deeper understanding of imprinted hafnia-based ferroelectrics and will provide an insight into devices that utilize the imprint effect.

Hafnia-based ferroelectric materials are recently drawing a significant attention for future electronic devices; however, there is a need to further enhance their functionality for practical applications. Especially, an imprinting effect has been regarded as a defect to be reduced in ferroelectrics; yet, it can be positively applied to various electronic devices with the functionality of self-rectifying behavior and threshold voltage adjustment. For the first time, we report a high imprinting effect in bilayer stack capacitors TiN/Hf $_{\text{0.5}}$ Zr $_{\text{0.5}}$ O $_{\text{2}}$ (HZO)/tantalum oxide (TaO)/TiN by employing an optimal TaO insertion layer. Furthermore, the imprinting effect was enhanced by adopting high-pressure annealing (HPA) process. X-ray photoelectron spectroscopy (XPS) depth profile analysis reveals that the origin of the imprint field is the tantalum suboxides (Ta $_{\text{2}}$ O $_{\text{5{\kern-0.2pt}-\textit{x}}}$ ) with doubly positively charged oxygen vacancies $^{^{^{^{}}}}$ (V $_{\text{o}}^{{++}}$ ). The imprint field increases with increasing HPA temperatures and achieved a high imprint field of 1.65 MV/cm at 600 $^{\circ}$ C. Moreover, endurance was observed up to 10 $^{\text{8}}$ cycles without a breakdown while maintaining the original coercive fields up to 10 $^{\text{5}}$ cycles. We also investigated how the TaO insertion layer with an imprint field influences the switching, interfacial, and ferroelectric properties. These findings on the imprinting effect provide a new strategy to improve the functionality of hafnia-based ferroelectric devices in the near future.

The use of additional dielectric (DE) layers such as Al ₂ O ₃ , ZrO ₂ , HfO ₂ and Ta ₂ O ₅ with Hf _0.5 Zr _0.5 O ₂ (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 ₂ , ZrO ₂ and SiO ₂ (thickness of 10Å) while keeping FE HZO (100Å) and bottom HfO ₂ 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 ( P _r ~ 16.6 μC/cm ² ) was observed for the TiO ₂ 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.

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 _x Zr _1-x O ₂ (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 ( P _r ) of 31 μC/cm ² was achieved as compared to 19 μC/cm ² 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 ⁹ 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 P _r 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.

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/cm ³ 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/cm ³ , ~44%]. The best efficiency of ~88% is achieved [ESD ~26 J/cm ³ ] 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 10 ⁹ times of electric field cycling. Highest ESD of ~70 J/cm ³ 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.