Oxygen Vacancies In HfO2 Research Articles

Impact of lanthanum doping on the electronic structure of oxygen vacancies in hafnium oxide

The electronic structure of oxygen vacancies in lanthanum-doped hafnium oxide (HfO2:La) is studied using the density functional theory. The simulation is carried out for the optimal structure of HfO2:La in three crystalline phases for an oxygen vacancy involved in the impurity charge compensation that is structural element, as well as for a second purposely added oxygen vacancy. It is shown that the HfO2 doping with La increases the oxygen vacancy content in such a way that the twice positively charged vacancy concentration increases and the neutral one decreases. The doping with La facilitates new oxygen vacancies formation in HfO2. Oxygen vacancies in HfO2:La and HfO2 have a similar electronic structure and are traps for electrons. However, the deeper trap and the electrostatic repulsion caused by the impurity in HfO2:La suppress the vacancies participation in the charge transport. Based on the obtained results, the mechanisms of HfO2-based resistive and ferroelectric memory improvements, when HfO2 doped with lanthanum, are discussed.

Effect of Ce doping on ferroelectric HfO2 from first-principles: Implications for ferroelectric thin films and phase regulation

In this work, the effect of Ce doping on the structure and oxygen defects of ferroelectric HfO2 is studied by first-principles calculation. According to the formation energy of Ce doping and the influence of Ce doping on the formation enthalpy of each phase, it is concluded that the concentration of Ce doping is 8–9.375 formula unit% which is most beneficial to the formation of ferroelectric orthorhombic phase. The phase transition from ferroelectric orthorhombic phase to cubic phase will occur when the doping concentration is greater than 12.5 formula unit%. The mechanism of Ce doping was also studied. From the view of electric balance, it is concluded that the existence of Ce3+ will promote the formation of oxygen vacancies in HfO2. These findings are helpful for the preparation of Ce-doped HfO2 ferroelectric thin films and deepen the understanding of the Ce doping mechanism.

The Effects of Ag Nanoislands on the Volatile Threshold-Switching Behaviors of Au/Ag/HfO2/Ag Nanoislands/Au Devices

Volatile threshold-switching (TS) devices have been used as selectors and to simulate neurons in neural networks. It is necessary to find new ways to improve their performance. The randomness of conductive filament (CF) growth and the endurance of the devices are urgent issues at present. Here, we explored embedded Ag nanoislands (NIs) in HfO2-based TS devices to limit the position of the CF and facilitate its growth at the same time. The Au/Ag(2 nm)/HfO2(4 nm)/Ag NIs/Au volatile TS devices exhibited forming-free characteristics with improved endurance compared with the devices without Ag NIs, which was ascribed to the enhanced localization of the electrical field and increased oxygen vacancies in HfO2 induced by the Ag NIs. A mechanism was proposed to explain the volatile TS behaviors of the devices. The Ag NIs and the thickness of the HfO2 layers played key roles in whether the devices required forming. This work shows that the use of metal NIs is an effective and convenient way to improve the performance of TS devices.

Damage effect of hafnium oxide gate dielectric based metal–oxide–semiconductor structure under gamma-ray irradiation

The radiation response of HfO2 films on a silicon substrate under gamma rays is studied in this article. HfO2 films with the thickness of 12.8 and 4.3 nm are prepared on a p-type silicon substrate by using the atomic layer deposition method, and the HfO2/Si MOS structure is irradiated under gamma rays with the total dose of 1.2/2.5/4 Mrad. The generation, transportation, and trapping characteristics of radiation induced charges are studied by using electronic, physical, and chemical methods. First, radiation induced oxide and interface trapped charge densities are found to be up to 1012 cm−2, and oxygen vacancies in HfO2 and Hf–Si metallic bonds at the HfO2/Si interface are dominant defects in the HfO2/Si system. Second, the leakage current through HfO2 increases with the increase in the radiation total dose and the crystallinity also increases after a large total dose of irradiation. Third, the valence band offset between HfO2 and Si is found to decrease slightly after irradiation. From the results, we can see that HfO2 is radiation resistant from the aspect of charge trapping even under a very large total dose of radiation, but the radiation induced leakage current and crystal structure change in the HfO2 film cannot be ignored. This provides a reference for microelectronic devices working in the space environment.

Modulation of electron transfer in Si/SiO2/HfO2/Graphene by the HfO2 thickness

We studied by Raman spectroscopy the effect of the variation of the thickness of the HfO2 layer in Si/SiO2/HfO2/Graphene heterostructures on the density of carrier transferred to graphene. Three thicknesses of the HfO2 layer equal to 30, 20, and 5 nm were used. Analysis of the behavior of peaks G and 2D as well as the Raman intensity ratios ID/IG and I2D/IG shows that the density of carriers transferred to graphene increases when the thickness of the HfO2 layer decreases from 30 to 5 nm. Analysis of the photoluminescence (PL) spectra of Si/SiO2/HfO2 structures reveals the presence of bands associated with defects. These are essentially associated with oxygen vacancies in HfO2 and are responsible for the transfer of carriers to the graphene layer. The measured refractive index of HfO2 by ellipsometry shows an increase in the density of the defects in HfO2 with a decrease in its thickness in agreement with Raman results.

Total-Ionizing-Dose Effects and Low-Frequency Noise in 16-nm InGaAs FinFETs With HfO2/Al2O3 Dielectrics

Total-ionizing-dose mechanisms are investigated in 16-nm InGaAs FinFETs with an HfO2/Al2O3 gate-stack. Transistors are irradiated up to 500 krad(SiO2) and annealed at high temperatures. Irradiated devices show negative threshold-voltage Vth shifts, subthreshold stretch-out, and leakage current increases. These result from positive charge trapping in the gate oxide and shallow trench insulators, and increases in the interface and border-trap charge densities. Low-frequency noise measurements at different temperatures indicate a significant increase of noise magnitude in irradiated devices at an activation energy of ~0.4 eV. Density functional theory (DFT) calculations strongly suggest that transistor Vth shifts are due primarily to hole trapping at oxygen vacancies in HfO2, and the increased noise is due primarily to O vacancies in Al2O3. Additional contributions to the noise from defects in the GaAs buffer layer are also likely, primarily at low temperatures.

The research of data retention of HfO2 as trapping layer: First-principles study

The influence of the Charge Trapping Memory storage feature was researched by doping the substitutional impurity Al and introducing oxygen vacancy within HfO2. Materials studio and Vienna Ab-initio Simulation Package were used to investigate the influence of doping Al on the formation of the oxygen vacancy in HfO2 as trapping layer. At the same time, the interaction energy of two defects under different distances was calculated. The calculation results show that doping the substitutional impurity Al reduces the formation energy of oxygen vacancies in the HfO2, and the reduced formation energy of the three-fold-coordinated O vacancy is larger than that of the four-fold-coordinated O vacancy. After having studied three different defect distances between substitutional impurity Al and three-fold-coordinated O vacancy, the results indicate that the system acquires the largest charge trapping energy, the most quantum states when the distance of the defects is 2.107Å. As a conclusion, the data retention in the trapping layer of monoclinic HfO2 can be improved by doping the substitutional impurity Al. This work will provide a theoretical guidance for the performance improvement with respect to the data retention of Charge Trapping Memory.

A Study on Organic Thin-Film Transistors Using Hf–La Oxides With Different La Contents as Gate Dielectrics

The effects of La content in HfLaO gate dielectric on the performance of pentacene organic thin-film transistor (OTFT) fabricated on Si have been studied. The OTFT with Hf0.103La0.897O y gate dielectric shows high performance such as high carrier mobility of 3.45 cm $^{\textsf {2}}\cdot \textsf {V}^{-\textsf {1}}\cdot \textsf {s}^{-\textsf {1}}$ (132 times and 40 times higher than those of devices using Hf oxide and La oxide, respectively), small threshold voltage of −2.09 V, and negligible hysteresis of −0.029 V. Binding-energy shift of Hf 4f peak in the X-ray photoelectron spectroscopy spectrum indicates that La incorporation can passivate the oxygen vacancies in HfO2. Atomic force microscope reveals that the La incorporation can reduce the surface roughness of the gate dielectric by suppressing the crystallization of HfO2. Therefore, by using Hf0.103La0.897O y as gate dielectric, OTFT with high carrier mobility and small threshold voltage can be obtained.

The Impact of Different TiN Capping Metal Thicknesses on High-k Oxygen Vacancies in n-MOSFETs

This letter investigates the impact of two different TiN capping metal thicknesses on high-k oxygen vacancies in n-MOSFETs. Traditionally, oxygen vacancies in HfO2 can be repaired by nitrogen in the TiN gate, and these repairs become more pronounced as thickness increases. Nevertheless, in this letter, where the thicknesses of the TiN capping metal was less than 30 A, more oxygen vacancies were found in thick-capped devices than in the thin-capped devices because the repair of oxygen vacancies is dominated by the diffusion of oxygen rather than by the diffusion of nitrogen.

Effective work-function control technique applicable to p-type FinFET high-k/metal gate devices

In this work, we investigated the effect of so-called WF (Work Function) setting anneal (high temperature annealing on TiN/HfO2 stack) on gate stack properties. It was found that intermixed layer created in-between TiN and HfO2 during WF setting anneal has negative fixed charge and reduces pFET Vt (positive Vt shift). In addition, higher anneal temperature further reduces pFET Vt while keeping nFET Vt almost unchanged. This could be explained by passivation of oxygen vacancies in HfO2 with diffused oxygen from TiN layer. By combining these effects, one can further push effective work function towards valence band edge which enables wider coverage of transistor Vt option.

The Origin of Traps Responsible for Localization and Charge Transport in Memory Devices

High-κ dielectrics are used now in ReRAM, FeRAM and charge trap (TANOS) flash devises. In present report the nature of deep traps responsible for the charge transport in the most important high-κ dielectrics of flash devices is considered [K.A. Nasyrov and V.A. Gritsenko, J. Appl. Phys. 109, 097705 (2011); V.A. Gritsenko, T.V. Perevalov and D.R. Islamov, Physics Reports 613, 1-20 (2016)]. The charge transport in high-κ dielectrics is controlled by electron and hole traps. In HfO2 charge transport is controlled by trap assisted tunneling between traps [D.R. Islamov, V.A. Gritsenko, C.H. Cheng and A. Chin, Appl. Phys. Lett. 105, 222901 (2014)] (Fig.1). Thermal trap energy in HfO2 is equal to 1.25 eV. The trap nature in HfO2 was studied with luminescence experiments. Blue luminescence band near 2.7 eV has excitation energy at 5.2 eV (Fig.2). Absorption peak of oxygen vacancy in HfO2 has equal energy as blue band excitation. Hence, the 5.2 eV excitation peak and 2.7 eV luminescence band are related to oxygen vacancies [T.V. Perevalov, V.Sh. Aliev, V.A. Gritsenko et al., Appl. Phys. Lett. 104, 071904 (2014)]. The photoluminescence Stokes shift (5.2-2.7)/2=1.25 eV is equal to thermal trap energy (Fig.3). So, the oxygen vacancy in HfO2 is the electron trap [V.A. Gritsenko, T.V. Perevalov and D.R. Islamov, Physics Reports 613, 1-20 (2016)]. Similar experiments were made for Al2O3, ZrO2, Hf0.5Zr0.5O2, Si3N4. It was concluded that in HfO2, Al2O3, ZrO2, HfZrO2 the oxygen vacancies are electron and hole traps. In Si3N4 the Si-Si bond is responsible for electron and hole localization and the charge transport [V.A. Gritsenko, Electronic structure of traps in silicon nitride for flash memory devices, chapter in book“Thin Films on Si: Electronic and Photonic Applications”, accepted for publication by “World Scientific Press”, 2016]. Figure 1

Interfacial Interaction between HfO2 and MoS2: From Thin Films to Monolayer

The interfacial interaction between high-κ dielectrics and two-dimensional channel materials plays an important role in determining electronic properties of high-performance electronic devices. In this study, via first-principles calculations, we show that the interaction between oxygen-terminated HfO2 (111) and the MoS2 monolayer is weak and dominated by the van der Waals force. This weak interaction results in symmetric band offsets which are larger than 1 eV. The presence of oxygen vacancies in HfO2 enhances the interfacial interaction significantly, leading to electron–hole puddles, larger effective masses, and localized midgap states in the MoS2. In contrast, strong interaction is found at the interface of hafnium-terminated HfO2 (111) and the MoS2 monolayer, but it results in inferior electronic properties. In addition, when the thickness of MoS2 increases, the formation of an oxygen-terminated HfO2 thin film on MoS2 becomes endothermic accompanied by asymmetrically increased band offsets due to red...

(Invited) The Influence of Defects on the Electronic Properties of Hafnia

Currently, high-κ dielectrics, such as hafnium oxide (hafnia) HfO2, zirconia, alumina etc. are replacing SiO2 - traditional dielectric in silicon devices. Among high-κ dielectrics the hafnia causes the greatest interest because it combines a wide band gap of E g=5.6-5.8eV, high permittivity κ=16-40, high thermal stability (melting point ≈2780°С), high thermodynamic stability in contact with silicon, and large energy barriers for electrons (2.0eV) and holes (2.5eV) with respect to the silicon [Robertson J, Wallace R M, Mater. Sci. Eng.: R: Reports 88, 1 (2015)]. Now, HfO2 is under intensively studied mainly due to its prospective application as high-κ gate dielectric in metal-oxide-semiconductor (MOS) field-effect transistors [Lee S J et al., J. Appl. Phys. 92, 2807 (2002)].In 2007, Intel Corporation implemented high-κ HfO2 as the gate dielectric in silicon devices of Intel Core family instead of SiO2. This phenomenon was called the revolution in microelectronics. Currently, hafnia replaces SiO2 in flash memory devices. Hafnia (pure, doped, and oxygen-deficient) attracts additional interest as a perspective candidate for nonvolatile resistive random access memory (ReRAM) and ferroelectric random access memory (FeRAM). Hafnia is widely used in optical and protective coatings in the production of special types of glass for fiber optic products.Hafnia has defects. The most thermodynamically favorable defects are oxygen vacancies (VO). The defects act as electron and hole traps and play a dual role in silicon devices. If HfO2 is used as a gate dielectric layer and the blocking elements in a flash memory, then the electrons capture on the oxygen vacancies (traps) and conductivity via traps will play negative roles, devices will degrade, reliability will reduce. At the same time, using the hafnia as the storage medium in the flash memory cell requires localization of electrons (and holes) at the traps to store information. Hence, understanding the nature of traps responsible for the electrons (and holes) localization and the charge transport is required to control the properties of HfO2-based electronic devices.According to quantum-chemical simulations, there are energy levels in the band gap for neutral and charged oxygen vacancies in HfO2 as shown in Fig.1: (a) [Xiong K et al., Appl. Phys. Lett. 87, 18350 (2005)], (b) [Gavartin J L et al., Appl. Phys. Lett. 89, 36 (082908)], and (c) [Broqvist P, Pasquarello A, Appl. Phys. Lett. 89, 262904 (2006)]. Indeed, XPS experiments and ab initio simulations show electron states at 3.3eV above the valence band ceiling (Fig.2) [Perevalov T et al., Microelectron. Eng. 109, 21 (2013)]. Optical transitions on the VO accord to blue photoluminescence (PL) band at 2.7eV [Ito T et al., J. Appl. Phys. 97, 054104 (2005)], which is excited by photons with energy near 5.2eV (PLE) [Perevalov T V et al., Appl. Phys. Lett. 104, 071904 (2014)] (Fig.3). The configuration coordination energy diagrams of optical transition on a neutral oxygen vacancy in hafnia is shown in Fig.4.The current through HfO2 in hafnia-based structures grows exponentially with increasing of the gate voltage and temperature. So, Fowler-Nordheim emission does not limit the current in HfO2. Many studies of the charge transport in HfO2 described experiment results by Poole-Frenkel (PF) mechanism. However, the fitting parameters, obtained by experimental data analysis in terms of PF model, are far from realistic values. E.g., either the frequency factor in 5-7 order lower than expected, or trap density in 13-16 orders lower than values obtained from CV measurements. It was shown [Islamov D R et al., Appl. Phys. Lett. 105, 222901 (2014)] that the charge transport in HfO2 is described by phonon-assisted tunneling between traps (PATT) in the bulk of HfO2 [Nasyrov K A, Gritsenko V A, J. App. Phys. 109, 093705 (2011)]. Found thermal trap energy of 1.25eV and optical trap energy of 2.5eV, obtained from analysis of experiment data with PATT model, do not depend on technique of HfO2 synthesis. Fig.5 shows experiment I-V characteristics with results of PATT calculations. Fig.6 shows configuration coordination energy diagram of trap ionization process on negative charged oxygen vacancy in hafnia. Despite the fact that obtained trap density is ~1019-1020cm-3, CV measurements shows that trapped charge density is ~5×1018cm-3 [Islamov D R et al., Appl. Phys. Lett. 105, 222901 (2014); Lu W-T et al., Appl. Phys. Lett. 85, 3525 (2004)]. This phenomenon can be explained by the Coulomb repulsion of charge carriers, leading to the Wigner crystallization of trapped electrons (holes) to form a hexagonal glassy structure [Shaimeev S S, Gritsenko V A, Wong H, Appl. Phys. Lett. 96, 263510 (2010)].

Resistive switching and synaptic properties of fully atomic layer deposition grown TiN/HfO2/TiN devices

Recently proposed novel neural network hardware designs imply the use of memristors as electronic synapses in 3D cross-bar architecture. Atomic layer deposition (ALD) is the most feasible technique to fabricate such arrays. In this work, we present the results of the detailed investigation of the gradual resistive switching (memristive) effect in nanometer thick fully ALD grown TiN/HfO2/TiN stacks. The modelling of the I-V curves confirms interface limited trap-assisted-tunneling mechanism along the oxygen vacancies in HfO2 in all conduction states. The resistivity of the stack is found to critically depend upon the distance from the interface to the first trap in HfO2. The memristive properties of ALD grown TiN/HfO2/TiN devices are correlated with the demonstrated neuromorphic functionalities, such as long-term potentiation/depression and spike-timing dependent plasticity, thus indicating their potential as electronic synapses in neuromorphic hardware.

Research of data retention for charge trapping memory by first-principles

In this paper, the influence of charge trapping memory storage feature is studied by doping the substitutional impurity Al and introducing oxygen vacancy within HfO2. HfO2 is widely used in trapping layer of charge trapping memory, for it belongs to high dielectric constant materials with the abilities to shrink the device size and improve the device performance. Materials studio and Vienna Ab-initio Simulation Package are used to investigate the influence of doping Al on the formation of the oxygen vacancy in HfO2 as a trapping layer. At the same time, the interaction energy of two defects at different distances is calculated. Results show that doping the substitutional impurity Al reduces the formation energy of oxygen vacancies in HfO2, and the reduced formation energy of the three-fold-coordinated O vacancy is larger than that of the four-fold-coordinated O vacancy. After having studied three different defect distances between the substitutional impurity Al and the three-fold-coordinated O vacancy, the results indicate that the system acquires the largest charge trapping energy, the most of quantum states, the smallest population number, and the longest Al–O bond length when the distance between the defects is 2.107 Å. Studying the bond length changes of the three systems after writing a hole, we obtain a result that the change of Al–O bond length is the smallest when the distance between defects is 2.107 Å. In conclusion, the data retention in the trapping layer of monoclinic HfO2 can be improved by doping the substitutional impurity Al. This work will provide a theoretical guidance for the performance improvement in the data retention of charge trapping memory.

Multi-scale quantum point contact model for filamentary conduction in resistive random access memories devices

We depart from first-principle simulations of electron transport along paths of oxygen vacancies in HfO2 to reformulate the Quantum Point Contact (QPC) model in terms of a bundle of such vacancy paths. By doing this, the number of model parameters is reduced and a much clearer link between the microscopic structure of the conductive filament (CF) and its electrical properties can be provided. The new multi-scale QPC model is applied to two different HfO2-based devices operated in the unipolar and bipolar resistive switching (RS) modes. Extraction of the QPC model parameters from a statistically significant number of CFs allows revealing significant structural differences in the CF of these two types of devices and RS modes.

Au doping effects in HfO2-based resistive switching memory

In this paper, first principles calculation is performed to investigate the impact of Au doping on the properties of oxygen vacancy in HfO2, including defect energy level and formation energy (Ef). The Ef is significantly reduced in Au-doped HfO2 film. In the experiments, the Cu/HfO2: Au/Pt memory devices are fabricated and exhibit high resistance ratio, reliable date retention and good endurance. This performance improvement is clarified to the suppressed randomicity of oxygen vacancy filaments’ formation by trivalent Au-doping effect. The achieved results also demonstrate the validity of implantation doping approach for the fabrication of RRAM devices.

Interplay between gadolinium dopants and oxygen vacancies in HfO2: A density functional theory plus Hubbard U investigation

The influence of gadolinium (Gd) doping on the oxygen vacancy (VO) in monoclinic HfO2 have been studied by the first-principles calculations within the spin-polarized generalized gradient approximation plus Hubbard U approach. It is found that the Gd dopant and VO show strong attractive interaction, resulting in a cooperative effect that the substitution of Gd for Hf (GdHf) would increase the probability of oxygen vacancies generation and vice versa. The GdHf is more energetically favorable to be next to the vacancy site of a three-coordinated oxygen (O3), forming a complex defect GdHf + VO. A single GdHf acts a hole donor and passivates the defect states of VO. Our results suggest that the decrease of the VO-related defect states observed in the photoluminescence spectra of Gd-HfO2 is because Gd doping passivates the defect states of VO, rather than caused by decrease of VO concentration. Our findings would clarify the debate about the influence of Gd doping on the oxygen vacancies in HfO2.

First principles study on influence of oxygen vacancy in HfO2 on charge trapping memory

With the further scaling down of device dimensions, charge trapping memory with high k materials HfO2 serving as capture layer shows good endurance and high storage capacity. Its relatively simple process and complete compatibility with the conventional semiconductor process furthermore make it widely studied. The oxygen vacancies in HfO2 are studied using the first-principles calculation in order to learn their influence on the storage properties of charge trapping memory. Write and erase operations of memory devices are simulated via changing the number of electrons in the super cell with defects. The results show that basically the distance between oxygen vacancies has no effect on charge trapping, but the number of oxygen vacancies does affect it. The more the number of oxygen vacancies, the stronger the electron capture ability is. Moreover, four-fold coordinated oxygen vacancy (Vo4) has lager capability for trapping charge than three-fold coordinated oxygen vacancy (Vo3). The analysis of density of states shows that Vo4 induces a large number of quantum states with deep energy levels which is little affected by distance and has large possibility of trapping charges. The results show that oxygen vacancy defects in HfO2 tetravalent coordination are conducive to improving the storage characteristics of charge trapping memory.

HfO2-assisted SiO2 reduction in HfO2/SiO2/Si stacks

The ultra-thin SiO2 interface layer (SiO2-IL) in the HfO2/SiO2/Si stack was found to be reduced during the ultra-high vacuum (UHV) annealing. Along with thin SiO2-IL reduction in UHV, SiO desorption and silicide formation also took place. The thermal desorption spectroscopy and X-ray photoelectron spectroscopy results showed that the SiO2-IL reduction occurred at temperatures lower than the temperature showing SiO desorption and silicidation by accurately controlling the annealing conditions. The mechanism of SiO2-IL reduction can be explained by the oxygen diffusion through oxygen vacancies in HfO2 generated in the gate stack formation and thermal treatment. It is also understandable that the silicidation process is associated with the inhomogeneous SiO desorption.