HfO2 Surface Research Articles

Effect of Microwave Annealing on the Sensing Characteristics of HfO2 Thin Film for High Sensitive pH-EGFET Sensor.

Recently, certain challenges have persisted in PH sensor applications, especially when employing hafnium oxide (HfO2) thin films as sensing layers, where issues related to sensitivity, hysteresis, and long-term stability hamper performance. Microwave annealing (MWA) technology, as a promising solution for addressing these challenges, has gained significant attraction due to its unique advantages. In this article, the effects of microwave annealing (MWA) treatment on the sensing behaviors of Extended-Gate Field-Effect Transistors (EGFETs) utilizing HfO2 as a sensing film have been investigated for the first time. Various power levels of MWA treatment (1750 W/2100 W/2450 W) were selected to explore the optimal processing conditions. A thorough physical analysis was conducted to characterize the surface of the MWA-treated HfO2 sensing thin film using techniques such as X-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Our findings reveal that MWA treatment effectively increased the surface sites (Ns) in the HfO2 sensing thin film, consequently leading to an increase in the pH sensitivity of EGFETs to 59.6 mV/pH, as well as a reduction in hysteresis and an enhancement in long-term stability. These results suggest that MWA offers a straightforward, energy-efficient method to enhance overall HfO2 sensing film performance in EGFETs, offering insights for HfO2 applications and broader microelectronics challenges.

Au Nanoparticles/HfO₂/Fully Depleted Silicon-on-Insulator MOSFET Enabled Rapid Detection of Zeptomole COVID-19 Gene With Electrostatic Enrichment Process

In this work, a novel sensing structure based on Au nanoparticles/HfO2/fully depleted silicon-on-insulator (AuNPs/HfO2/FDSOI) MOSFET is fabricated. Using such a planar double gate MOSFET, the electrostatic enrichment (ESE) process is proposed for the ultrasensitive and rapid detection of the coronavirus disease 2019 (COVID-19) ORF1ab gene. The back-gate (BG) bias can induce the required electric field that enables the ESE process in the testing liquid analyte with indirect contact with the top-Si layer. It is revealed that the ESE process can rapidly and effectively accumulate ORF1ab genes close to the HfO2 surface, which can significantly change the MOSFET threshold voltage ([Formula: see text]). The proposed MOSFET successfully demonstrates the detection of zeptomole (zM) COVID-19 ORF1ab gene with an ultralow detection limit down to 67 zM (~0.04 copy/[Formula: see text]) for a test time of less than 15 min even in a high ionic-strength solution. Besides, the quantitative dependence of [Formula: see text] variation on COVID-19 ORF1ab gene concentration from 200 zM to 100 femtomole is also revealed, which is further confirmed by TCAD simulation.

Effect of Deposition Temperature on the Surface, Structural, and Mechanical Properties of HfO2 Using Chemical Vapor Deposition (CVD)

A high-temperature-resistant and high-performance hafnium oxide (HfO2) coating for aerospace engines was developed. HfO2 thin film coatings were prepared by chemical vapor deposition (CVD) at 1200, 1250, 1300, and 1350 °C, respectively. The crystal structure and surface morphology of the HfO2 thin films at different deposition temperatures were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM), and atomic force microscopy (AFM). The effect of deposition temperature on the mechanical properties of the HfO2 thin films was studied by means of the drainage method and nanoindentation. The results showed that the HfO2 coatings deposited at different deposition temperatures are monoclinic crystal structures, and the surface morphology changes from cellular particles to columnar particles, and finally, to a loose and porous layered structure. When the deposition temperature is 1300 °C, the density of the HfO2 film is the highest, 9.62 ± 0.03 g/cm3, and the hardness is the highest, 7.33 ± 0.04 GPa. A HfO2 coating with the best comprehensive mechanical properties was prepared by changing the deposition temperature, which provided an experimental basis for the application of a HfO2 coating in the aerospace field.

(Invited) Transport and Reaction Kinetics of Thermal ALE in High Aspect Ratio Hafnium Oxide Structures

Thermal atomic layer etching is rapidly becoming an important complementary processing technology in manufacturing 5 and 3 nm devices in the semiconductor industry. Critically, stacked chip architectures such as 3D NAND and 3D DRAM require conformal isotropic etching to remove material such as HfO2 in hard-to-reach locations with aspect ratios that can be >50. To achieve repeatable device performance throughout a 3D stack, the removal rate (etch per cycle) of the etched material during an etch process need to be controlled such that the overall etch is the same from top to bottom of the device stack. In this work, we have modelled the reaction kinetics and transport processes of reactants and by-products during a cyclical ligand exchange-based ALE process. This ALE process consists of two steps: a fluorination step, followed by a fluorine-to-chlorine ligand exchange-based removal step. Experimental data revealed that the fluorine dosing during the fluorination step was predominantly responsible for controlling the etch rate of the ALE process but had only a minimal impact on the etch profile inside these holes. The ligand exchange dosing, on the other hand, predominantly controlled the etch profile (depth loading) with equal etch rates top-to-bottom obtained when the step was operated close to saturation. Our model predicts, in agreement with the experiment, that adsorption and reaction rates during fluorination on HfO2 surfaces are significantly slower than transport times inside these deep holes leading to essentially flat fluorination profiles even if the fluorination step is not operated in saturation mode. In contrast, transport rates with the ligand exchange molecule are slow in comparison but adsorption and ligand exchange rates with the fluorinated hafnium appear to be significantly higher than for fluorine during the fluorination step. Slow transport in combination with high surface reaction rates for the ligand exchange step led to an etch rate that was dependent on aspect ratio (feature depth) in processes that used sub-saturation exposures.

Antibiotic drugs removal by visible light-driven photocatalysis using Pt/Ru nanoparticle-decorated hafnium oxide nanohybrids

Hafnium oxide (HfO2) nanohybrids with metal nanoparticle incorporation were fabricated to develop a prominent photocatalyst for environmental water treatment. The synthesis of ruthenium/platinum nanoparticles-incorporated HfO2 (Ru/HfO2 and Pt/HfO2) was done using a simple hydrothermal technique. The as-synthesized nanohybrids were well characterized; HfO2 had a Nanoleaf morphology, whereas Ru/HfO2 and Pt/HfO2 both had evenly attached Ru and Pt NPs on the surface of HfO2, respectively. These nanohybrids were utilized as a photocatalyst for the degradation of antibiotics nitrofurantoin (NFT) and ciprofloxacin (CIP) in an aqueous solution by leveraging the inherent benefits of metal oxides and metal nanoparticles. Both nanohybrids displayed splendid photocatalytic activity, for instance, Ru/HfO2 exhibited ∼100% degradation of NFT and CIP in 15 and 25 min with rate constants of 0.3280 and 0.1919 min−1, respectively. The photocatalytic degradation mechanism was validated by using reactive species scavenging studies and mass spectrum analysis; the achieved degradation was predominantly governed by the hydroxyl radicals through an oxidation pathway. Moreover, metal nanoparticles play a vital role in trapping photo-excited electrons, which improves catalytic activity. The sustainable nature of nanohybrids was validated by investigating their reusability and practical applicability in real samples analysis. This HfO2-based nanohybrid is anticipated to generate a wide platform for developing imminent materials for use in a variety of environmental remediation applications.

Formation and crystallization of TiO2 nanostructures on various surfaces

A comparative study of the synthesis of TiO2 nanorods on fluorine-doped tin oxide (FTO) glass, Si, SiO2, Si/Ta, Si/TiN, Si/TiN/Ti and Si/HFO2 substrates by hydrothermal reaction is presented. Detailed analysis on the growth of TiO2 on pre-annealed Si/TiN/Ti and HfO2 (HFO) surfaces is also given. For Si/TiN/Ti, a pre-annealing procedure led to the transformation of Ti to a TiO2 layer which acts as a seed for aligned growth of TiO2 nanorods. In contrast, Si/HFO does not provide a nucleation site for the formation of aligned nanorods. Various samples were prepared by varying the synthesis conditions, i.e. pre- and post-annealing temperatures and hydrothermal reaction time to figure out the optimum conditions which lead to unidirectional and highly aligned nanorods. X-ray diffraction, scanning electron microscopy, ultraviolet–visible spectroscopy and Raman spectroscopy were used to study structural, morphological and optical properties of synthesized samples. It is found that TiO2 nanorods exhibit a rutile phase on the Si/Ti/TiN and Si/HFO substrates, but highly oriented vertical growth of nanorods has been observed only on pre-annealed Si/TiN/Ti substrates. On the other hand, TiO2 nanorods form dandelion-like structures on Si/HFO substrates. Growth of vertically oriented TiO2 nanorods on Si/TiN/Ti is attributed to the TiO2 seed layer which forms after the process of pre-annealing of substrates at a suitable temperature. Variation in hydrothermal reaction time and post-annealing temperature brings further improvement in crystallinity and morphology of nanorods. This work shows that the pre-annealed Si/TiN/Ti substrate is the optimal choice to achieve vertically oriented, highly aligned TiO2 nanorods.

The effect of bulk conversion into surface on physical properties of HfO2: First principle study

HfO2 is among the most widely used ceramic material at high temperatures and has a direct energy band gap, but its value is above the solar spectrum region. The conversion from bulk to a surface may bring the energy band gap down. Therefore, this article seeks to address the important structural, electronic and thermodynamics behavior of HfO2 through density functional theory. The surface (001) of bulk HfO2 for different thicknesses are analyzed through the FP-LAPW + lo scheme and performed in the WIEN2k package for the first time. The optimized lattice parameters and band gaps of bulk HfO2 concur well with the previously available results. Additionally, the thermodynamic behaviors of HfO2 (i.e. bulk modulus, coefficient of thermal expansion, Debye temperature, heat capacity and entropy) are calculated through a Quasi-harmonic Debye model fixed in the Gibbs2 code. The computed thermodynamic behaviors of bulk HfO2 of (001) surface with different thicknesses are well correlated with corresponding published results.

HfO2/SiO2 nanolaminate-based composites prepared by ion beam sputtering for low-loss optics

HfO2 prepared by ion beam sputtering (IBS) is widely used as a high-refractive index material for making low-loss laser films. The challenge of a high-performance HfO2 film is to simultaneously obtain an amorphous morphology with low scattering and a stoichiometric structure with low absorption. Furthermore, nanometer-sized voids are commonly present in IBS films due to excessive oxygen and argon adsorption during deposition, which is the primary barrier to achieve a smooth surface and low optical loss of HfO2. Thin amorphous SiO2 layers were added periodically into HfO2 coatings using the IBS process to synthesize amorphous HfO2 / SiO2 nanolaminate-based composites. The resulting composites exhibited excellent comprehensive performance with a dense amorphous microstructure and a void-free smooth surface. High-temperature annealing was performed to ensure superior stoichiometry and lower absorption. However, the crystalline states and microstructure of some composites evolved during the gradual annealing. We present a detailed study of the crystallization, surface topography, and absorption evolution in HfO2 / SiO2 nanolaminates as a function of HfO2 sublayer thickness and thermal annealing temperature. Moreover, the interplay between crystallization, surface topography, and absorption is elucidated. The HfO2 / SiO2 nanolaminate with 19 thin layers maintained a dense amorphous structure with low absorption after annealing. Finally, a 1064-nm HfO2 / SiO2 nanolaminate-SiO2 high-performance reflector was prepared and achieved lower absorption with a smooth surface after annealing, which demonstrated the great potential of the HfO2 / SiO2 nanolaminates for considerably improving optical loss.

A Mechanistic Study of the HF Pulse in the Thermal Atomic Layer Etch of Crystalline and Amorphous HfO2

HfO2 is a high-k material that is crucial in semiconductor devices. Atomic-level control of material processing is required for the fabrication of thin films of this material at nanoscale device sizes. Atomic layer deposition (ALD) and thermal atomic layer etching (ALE) allow the fabrication of ultra-thin films for semiconductor device processing. ALD is a well-known metal oxide thin film deposition technique that enables a high level of control over film thickness. Thermal ALE, which is growing in importance, uses self-limiting halogenation (e.g. HF exposure) producing a non-volatile modified layer. Subsequent ligand exchange reactions remove up to a monolayer of the metal oxide. This modern approach for controlled etching is the reverse of ALD. In gate oxides, the use of amorphous materials is better than crystalline films as the lack of grain boundaries allow for a high breakdown voltage. Amorphous materials have a lower density than crystalline materials.Experimental studies have shown that HfO2 amorphous films have higher etch rates per cycle than crystalline HfO2 films regardless of the fluorination reactant. Given that it is difficult to investigate ALE reactions directly using experimental techniques, first-principles-based atomic-level simulations using density functional theory (DFT) can give deep insights into the precursor chemistry and the reactions that drive the etch of different materials. This contribution presents first-principles density functional theory modelling to examine the etch chemistry of crystalline and amorphous films of HfO2. HF molecules adsorb on the surface of HfO2 by forming hydrogen bonds and may remain intact or dissociate to form Hf-F and O-H bonds. The adsorption of one HF molecule at the bare surface of both the crystalline and amorphous models results in dissociative adsorption at all binding sites. For multiple HF adsorption, we find mixed molecular and dissociative adsorption of HF molecules at the bare surfaces of the crystalline and amorphous models of HfO2. We also present a thermodynamic analysis approach to compare reaction models representing the self-limiting (SL) and continuous spontaneous etch (SE) processes taking place during an ALE pulse.

Nanoscale-Thick Thin Films of High-Density HfO2 for Bulk-like Optical Responses

The high refractive index, broadband transparency, and low spectral absorption of HfO2 (hafnia) make it a suitable choice for thermally stable nanoscale optical filters, UV mirrors, and antireflection coatings. However, achieving dense thin films of HfO2 with bulk-like optical properties has been a challenge due to differences in film stoichiometric and spatial uniformity at nanoscale thicknesses. Here, we assess HfO2 thin films (i.e., <200 nm thicknesses) prepared using pulsed laser deposition (PLD) at different substrate temperatures (20–675 °C) and the associated chemical, structural, and optical properties are reported. X-ray diffraction analysis reveals that nanoscale-thick thin films of HfO2 deposited at an ambient substrate temperature (20 °C) using PLD are amorphous with embedded nanocrystallites, whereas films deposited on heated substrates are polycrystalline monoclinic HfO2. For the films deposited on heated substrates, further analysis shows that the nanocrystalline phase does not change with increasing substrate temperatures; however, the texture of the crystalline orientation changes to favor (111) at 300–675 °C from the initial (002) orientation at ambient substrate temperature. Such differences in nanoscale-thick thin-film HfO2 PLD process-dependent amorphousness, crystallinity, and surface textures discussed here exhibit minimal influence on the resulting broadband optical properties (250 nm–30 μm). Furthermore, we show that the complex refractive index of high-density nanoscale-thick thin-film HfO2 prepared at an ambient substrate temperature using PLD resembles that of bulk HfO2 responses. The achievement of fully stoichiometric, high-density, thin-film HfO2 from PLD is expected to further enable thermally robust nanoscale photonic device integration involving low-dimensional high-performance optical and optoelectronic applications.

Kinetics of HfO2 etching and impurity elimination with atomic hydrogen beams

Modification of the surface composition, stoichiometry, morphology, and structure of HfO2 ultra-thin layers upon exposure to atomic hydrogen beams has been investigated by a combination of time-of-flight secondary ion mass spectrometry (ToF-SIMS) depth profile analysis, angle-resolved x-ray photoelectron spectroscopy (ARXPS), atomic force microscopy (AFM) imaging, and transmission electron microscopy (TEM). The etching reaction has been found to be thermally activated (Ea = 96 kJ/mol) and governed by the expression Retch(Å/h) = 3.03 × 108e-1.15x104T within the 350–400 °C temperature range. The rate law determined experimentally, i.e., Retch(Å/h) = 1.07 × 10−1[H2]1/2, is consistent with a reaction mechanism that involves the formation of a volatile hafnium dihydroxide, Hf(OH)2 in two successive elementary steps. Precise control of the etching conditions allows to reach atomically smooth HfO2 surfaces having r.m.s. roughness below 0.2 nm, with no evidence of Hf metal formation derived from the reaction with atomic H. Elimination of C, Cl, F, and S contaminants from the HfO2 surface through the formation of the corresponding volatile hydrides has proved to be generally most effective at high temperatures and H2 flow rates.

Effect of HfO2 Passivation Layer on Light Extraction Efficiency of AlInN Nanowire Ultraviolet Light-Emitting Diodes

One of the probable reasons behind the limitation of light extraction efficiency (LEE) in III-nitride nanowire (NW) deep ultraviolet (UV) light-emitting diodes (LEDs) is the presence of the high surface density states that significantly contribute to the non-radiative recombination near the surface. Herein, we investigate the LEE of a single AlInN NW UV LED in the entire UV wavelength regime using finite-difference time-domain simulations. It is found that these LEDs favor transverse-magnetic (TM) polarized LEE over transverse-electric (TE) polarized emission. Further, we examine the role of the HfO2 surface passivation layer in the improvement of LEE of AlInN NW LED at ~282 nm wavelength. Our results show that the TM-polarized LEE of such LED without any passivation is only ~25.2%, whereas the maximum recorded LEE is ~40.6% with the utilization of 40 nm HfO2 passivation layer. This study provides a promising approach for enhancing the LEE of NW UV LEDs.

Surface reaction of the hafnium precursor with a linked amido-cyclopentadienyl ligand: A density functional theory study

We studied heteroleptic Hf precursors with a linked amido-cyclopentadienyl ligand by density functional theory (DFT) calculation to enable high-temperature atomic layer deposition processes. The thermolysis and hydrolysis of Hf precursors were simulated to expect thermal stability and reactivity with hydroxyl groups. The effects of alkyl groups in the precursors were also investigated. We constructed the hydroxylated HfO2 surface and then simulated the surface reactions of the precursors. The precursors with the linked ligand showed higher activation energies for thermolysis and lower activation energies for hydrolysis as compared with CpHf(NMe2)3. The precursors with the linked ligand also showed low activation energies for the serial ligand exchange reactions on the HfO2 surface, significantly lower than those of CpHf(NMe2)3. Therefore, the DFT calculation suggests that the Hf precursors with the linked ligand are promising due to their thermal stability and reactivity better than CpHf(NMe2)3.

Selective atomic layer etching of HfO2 over silicon by precursor and substrate-dependent selective deposition

The early work of John Coburn and Harold Winters revealed a synergistic effect in ion-enhanced Si etching by the concurrent irradiation of Ar+ and XeF2. This finding provided an important foundation for the development of plasma dry etching processes. The experimental results of Coburn and Winters also found effects that are useful for the development of atomic layer etching (ALE) processes characterized by a self-limited etch rate. ALE approaches are widely established and can be utilized in either directional or isotropic etching by employing proper surface modification and product removal steps. Nevertheless, the development of material selective ALE processes is still limited. An approach that combines substrate-selective deposition with etching opens a new processing window for selective ALE. The authors studied the deposition behavior of mixtures of methane (CH4) with trifluoromethane (CHF3) and mixtures of methane with octafluorocyclobutane (C4F8) on HfO2 and Si surfaces. The experimental results show that a CH4/C4F8 mixture produces a comparable fluorocarbon (FC) deposition thickness on both HfO2 and Si during the deposition step. In contrast, a CH4/CHF3 mixture deposits an FC film on Si, whereas it fluorinates the HfO2 surface with negligible FC deposition. Utilizing these behaviors allows for an ALE process based on CH4/CHF3 for selective removal of HfO2 over Si. Surface characterization data that provide mechanistic insights into these processes are also provided and discussed.

O2 plasma treated biosensor for enhancing detection sensitivity of sulfadiazine in a high-к HfO2 coated silicon nanowire array

In this paper, highly uniform silicon nanowire (SiNW) arrays (∼30 nm in diameter) used as FET biosensors are fabricated by using a CMOS-compatible spacer image transfer (SIT) technology with high efficiency compared with conventional electron beam (E-beam) lithography. An effective biochemical sensing approach that treated with optimization of O2 plasma on the SiNW biosensors coated with a 10 nm HfO2 high dielectric constant (high-к) film is demonstrated to enhance immobilization of antibodies. Furthermore, water contact angle (WCA) measurements verify that the increased number of hydroxyl groups (OH) on the HfO2 surfaces treated with O2 plasma reaches or surpasses that on a conventional SiO2 surface. Therefore, it is observed that the IDS/I0 is enlarged by 3.15 times compared with that of the HfO2 surface without O2 plasma treatment. In addition, real-time and label-free detection of sulfadiazine with high sensitivity and specific recognition is successfully achieved using the SiNW FET biosensor with the optimized process. And a 10 pg⋅mL−1 limit of detection (LOD) is achieved for the SiNW FET biosensor using O2 plasma treatment on an ultrathin HfO2 mentioned above. The proposed O2 plasma treatment on HfO2 provides an effective solution for future FET biosensors coated with high-к dielectrics.

Repeatable and reproducible formation/rupture of oxygen vacancy filaments in the vicinity of a polycrystalline HfO2 surface

We theoretically investigated the dynamics of oxygen vacancies (Vo’s) and the effect of changing their charge states by performing first-principles molecular dynamics simulations at a temperature of 1000 K. Calculations were performed for two structures of HfO2: a slab with (110) surfaces and a bulk single crystal. Our studies revealed that when the charge state of the Vo’s changed from neutral (VO0) to divalent (VO2+), Vo’s repelled each other and dispersed, in both the slab and bulk structures. In contrast, when the charge state of the Vo’s changed from VO2+ to VO0, the Vo’s attracted each other and resumed their original positions only in the slab structure. Therefore, the repeatable and reproducible formation/rupture of a VO filament can occur near the crystal surface, where the symmetry of the bulk crystal is broken. This result is consistent with the experimental results demonstrating that the resistive switching of the resistive random access memory develops in polycrystalline metal oxides rather than in single crystalline metal oxides.

Multi-Vt Performance Dependence on Capping Layer Position by NPT for PMOS Device Applications

In this paper, we present the comprehensive investigation of multi-Vt performance dependence on ALD-TiN capping layer position by NPT (nitrogen plasma treatment) process. The basic performance of fabricated PMOSCAPs are strongly dependent on the capping layer position by NPT process. 1) Flat-band voltage (VFB) negative shift is suppressed by increasing capping layer thickness before NPT process and EWF is toward band edge for the low power PMOS application, compared to the direct NPT at high-k surface. 2) Without capping layer shielding, the direct NPT process at HfO2 (high-k) surface brings out the oxygen vacancy generation and metal gate fermi level pinning and EWF toward band center for high performance PMOS application, while the capping layer can shield the nitrogen plasma with improvement of equivalent oxide thickness (EOT) and bulk trap density (Not). 3) The interface trap density (Dit) by conductance method is found to be significantly reduced by 35% compared to NPT process applied at capping layer top position. 4) The unique charge neutrality and oxygen vacancy model incorporating with capping layer shielding is reasonably proposed and successfully explains the above multi-Vt performance dependence on NPT position. The investigation of multi-Vt performance on capping layer position dependence by NPT will encourage the process improvement and advance HKMG technology for PMOS device multi-applications.

Effect of HfO2 nitridation on structural, optical and electrical properties of GaN films grown on HfO2/Si(100) by laser molecular beam epitaxy

Growth of high quality III-nitride films on high-k oxide/Si substrates is crucial for the fabrication of nitride based metal-oxide-semiconductor devices. Here, we report the effect of nitridation of HfO2 surface on the physical properties of gallium nitride (GaN) films grown on HfO2 (5 nm)/p-Si(100) substrates using laser molecular beam epitaxy (LMBE) technique. The grazing incidence x-ray diffraction (GIXRD) measurements showed the prominent GaN growth along (0002) direction. It is observed that the HfO2 nitridation significantly improves the surface coverage of GaN film with larger grains of size in the range 200 ∼ 250 nm. Room temperature photoluminescence (PL) spectroscopy exhibits a sharp, intense near band-edge emission line at 3.4 eV with negligible defect-related deep bands for the GaN film grown with HfO2 nitridation. Also, the current-voltage characteristics of Au/GaN Schottky contacts show a reduced leakage current for HfO2 nitridation. Thus, the surface nitridation of HfO2 is found vital to improve the quality of GaN films for the development of GaN-on-Si technology.

Electronic structure of strongly reduced [formula omitted] surface of monoclinic [formula omitted

Material surface is playing an increasingly important role in electronic devices as their size down to nanoscale. Here, by first-principles calculations we studied the surface oxygen-vacancies (Vos) induced electronic-structure variation of HfO2, in order to explore its potential applications in surface-controlled electronic devices. Firstly, it is found that single Vo tends to segregate onto the surface and attracts each other as they form pairs, making the formation of vacancies-contained functional surface possible. Then extensive Vo-chains whose formation/rupture can represent the high/low conductivity state are constructed. The electronic states induced by the Vos remain localized in the band-gap region for most of the Vo-chains studied here. A transition to a metallic conductance is found in metastable Vo-chain with formation energy increased by 0.25 eV per Vo. Moreover, we highlight the significance of the Hubbard U correction for density functional theory when studying the electronic-structure based conductance in the oxides. By comprehensive calculations, we find a conductivity-stability dilemma of the Vo-chains, providing guideline for understanding and designing the electronic devices based on HfO2 surface.

The Effect of Oxygen Adsorption for Vacancy-Induced d0 Magnetism in HfO2 (110) Surface

Based on the density functional theory, we investigated the electronic structures and magnetic properties of the oxygen adsorption on the defective cubic HfO2 (110) surface. The adsorption capacities of the perfect and defective surfaces follow in the sequence as: oxygen-deficient model >perfect surface >hafnium-deficient model. When the oxygen molecules adsorb on the nonmagnetic perfect HfO2 surface, the system has a local magnetic moment and the antiferromagnetic coupling is more stable. For the defective cubic HfO2 (110) surface, it is found that the oxygen vacancy is more easy to form than the hafnium vacancy by comparing the vacancy formation energy. And the hafnium vacancy could induce a large magnetic moment while the oxygen vacancy alone could not. After adsorption of the oxygen molecule, the nonmagnetic oxygen-deficient HfO2 model is transformed into the magnetic surface. On the contrary, the introduction of adsorbed oxygen molecule causes the magnetic moment of hafnium-deficient HfO2 surface to reduce. Additionally, if the oxygen molecules adsorb on the defective HfO2 (110) surface, the ferromagnetic coupling is energetically favorable.