Band Gap Of HfO2 Research Articles

Electron emission from deep traps in HfO2 under thermal and optical excitation

The ratio between the energies of optical and thermal ionization depends on the defect nature and the strength of the interaction of trapped electrons with phonons. Knowing this ratio for a certain type of defect allows one to predict, e.g., thermal emission energies from the optically measured values. We present the results of direct empirical extraction of the ratio between optical and thermal electron emission energies for HfO2 bulk electron traps combined with theoretical analysis of the physical mechanism of electron transitions from the trapped state to the mobility edge. We show that, by applying different excitation mechanisms, we affect the same deep traps inside the HfO2 band gap; i.e., these traps are both optically and thermally active and are likely to have similar nature. The extracted empirical optical/thermal ionization energy ratio of 2.2±0.3 is in good agreement with the polaronic nature of the probed electron traps, as shown by the results of theoretical calculations. Our results provide experimental and theoretical methodologies for consistently linking thermal and optical ionization energies of electron traps and describing their distributions in the band gap of amorphous oxides, and can help improve modeling frameworks for reliability issues related to oxide traps. Published by the American Physical Society 2024

Structural, Optical and Electrical Properties of HfO2 Thin Films Deposited at Low-Temperature Using Plasma-Enhanced Atomic Layer Deposition.

HfO2 was deposited at 80–250 °C by plasma-enhanced atomic layer deposition (PEALD), and properties were compared with those obtained by using thermal atomic layer deposition (thermal ALD). The ALD window, i.e., the region where the growth per cycle (GPC) is constant, shifted from high temperatures (150–200 °C) to lower temperatures (80–150 °C) in PEALD. HfO2 deposited at 80 °C by PEALD showed higher density (8.1 g/cm3) than those deposited by thermal ALD (5.3 g/cm3) and a smooth surface (RMS Roughness: 0.2 nm). HfO2 deposited at a low temperature by PEALD showed decreased contaminants compared to thermal ALD deposited HfO2. Values of refractive indices and optical band gap of HfO2 deposited at 80 °C by PEALD (1.9, 5.6 eV) were higher than those obtained by using thermal ALD (1.7, 5.1 eV). Transparency of HfO2 deposited at 80 °C by PEALD on polyethylene terephthalate (PET) was high (> 84%). PET deposited above 80 °C was unable to withstand heat and showed deformation. HfO2 deposited at 80 °C by PEALD showed decreased leakage current from 1.4 × 10−2 to 2.5 × 10−5 A/cm2 and increased capacitance of approximately 21% compared to HfO2 using thermal ALD. Consequently, HfO2 deposited at a low temperature by PEALD showed improved properties compared to HfO2 deposited by thermal ALD.

Fabrication and characterization of Pr6O11-HfO2 ultra-high temperature infrared radiation coating

In this study, pure HfO2 and Pr6O11-HfO2 coatings were prepared by atmospheric plasma spraying. The chemical compositions, morphologies, infrared radiation performances and thermal resistances of the coatings were characterized. The results showed that doping Pr6O11 could effectively improve the infrared emittance of the HfO2 coating. The HfO2 coating doping with 10 wt. % Pr6O11 exhibited the highest infrared emittance, which was 0.859 at room temperature and 0.883 at 1600 °C, correspondingly. This was mainly attributed to the oxygen vacancies, which created by the substitution of Hf4+ by Pr3+, could introduce localized energy states within the HfO2 band gap and increase the lattice distortion, producing lower symmetry vibrations. In addition, the Pr6O11-HfO2 infrared radiation coating possessed high tensile adhesive strength and good thermal resistance, which could withstand a high temperature treatment at 1700 °C for at least 50 h without exfoliation, and there was only a slight reduction in emittance.

β-Ga2O3-based metal–oxide–semiconductor photodiodes with HfO2 as oxide

We proposed β-Ga2O3-based metal–oxide–semiconductor (MOS) photodiodes using HfO2 as the oxide. Because of the larger band gap of HfO2 than that of β-Ga2O3 and the type-II band alignment at the HfO2/β-Ga2O3 interface, the MOS photodiodes exhibited almost no reduction in photoresponsivity against 254 nm light near zero external bias compared with Schottky photodiodes. In contrast, reverse leakage currents of the MOS photodiodes were substantially decreased compared with Schottky photodiodes owing to the increase in the apparent barrier height between the metal and the semiconductor. These features of the MOS photodiodes are beneficial for the future development of β-Ga2O3-based avalanche photodiodes without p–n junctions.

Investigating the impact of the defect dynamic characteristics on the PBTI in the high-κ gate device

Recent device reliability studies on the metal/high-κ device observed the inter-convertible characteristics of the electron trap levels between the shallow and deep defect states under cyclic positive-bias temperature stressing. Although the oxygen vacancy and oxygen interstitial defect, being two typical types of defects in the high-κ oxide, have been criticized as the culprit for the device reliability issue and investigated in many simulations, all results have indicated that the defect levels induced by them were either too shallow or too deep and failed to explain the above experimental observation. Nevertheless, studies on the static characteristics of vacancy-interstitial (VO-Oi) model showed scattering distributed electron trap levels within the bandgap, making it as a promising defect type that can account for the above experimental observation. In this work, we investigated the dynamic characteristics of the VO-Oi defect pair under PBTI stress by tuning the relative position of VO and Oi. Our simulation results show multiple energy barriers for the structure transformation along the Oi migration path, and the charge trap level of the specific defect pair during the Oi migration is shown to be adjustable within the HfO2 band gap, depending on the Oi positions. These results depict an atomic picture to help us understand the defect electrical behavior under cyclic positive bias stress condition.

Electrical characteristics and interface properties of ALD-HfO2/AlGaN/GaN MIS-HEMTs fabricated with post-deposition annealing

HfO2/AlGaN/GaN metal–insulator–semiconductor (MIS)-type high electron mobility transistors (HEMTs) on Si substrates were fabricated by atomic layer deposition of HfO2 layers and post-deposition annealing (PDA). The current–voltage characteristics of the MIS-HEMTs with as-deposited HfO2 layers showed a low gate leakage current (Ig) despite the relatively low band gap of HfO2, and a dynamic threshold voltage shift (ΔVth) was observed. After PDA above 500 °C, ΔVth was reduced from 2.9 to 0.7 V with an increase in Ig from 2.2 × 10−7 to 4.8 × 10−2 mA mm−1. Effects of the PDA on the HfO2 layer and the HfO2/AlGaN interface were investigated by x-ray photoelectron spectroscopy (XPS) using synchrotron radiation. XPS data showed that oxygen vacancies exist in the as-deposited HfO2 layers and they disappeared with an increase in the PDA temperature. These results indicate that the deep electron traps that cause ΔVth are related to the oxygen vacancies in the HfO2 layers.

Structural, optical and dielectric characterization of Au/HfO2/(Pt,TiN) capacitors

Structural, optical and dielectric properties of atomic layer deposited HfO2 were studied in close relation with the bottom electrode nature. We used XRD, HRTEM and AFM to study the internal microstructure and the surface roughness of the films. The HfO2/Pt sample shows a mixture of monoclinic and orthorhombic phases whereas the HfO2/TiN sample presents few crystallites in orthorhombic structure dispersed in a badly crystallized or amorphous phase. Raman spectroscopy shows that between 100 and 800cm−1, the HfO2/Pt device displays 13 Raman-active modes whereas the HfO2/TiN device reveals 10 Raman-active modes. Using the UV–Vis reflectance spectra, we expect both direct and indirect electronic transitions with optical band-gap energies of 5.11 and 4.61eV, respectively. Photoluminescence spectroscopy allows the identification of localized states in the HfO2 band-gap. PL emission spectra consist of five sub-peaks, centered at around 1.64–1.92eV, 2.10eV and 2.33eV. These energies are close to the trap levels calculated for oxygen vacancies, involved during the film deposition. As the temperature is increased up to 380°C, an obvious dispersion of the real permittivity takes place in the low frequency domain. This behavior, highly enhanced by the oxygen vacancies mobility, is typical for the electrode polarization process.

A Vacancy-Interstitial Defect Pair Model for Positive-Bias Temperature Stress-Induced Electron Trapping Transformation in the High- $\kappa $ Gate n-MOSFET

Recent device reliability studies have obser-ved the shallow-to-deep transformation of electron-trap states under positive-bias temperature stressing. Being two typical types of defects in the high- $\sf \kappa $ oxide, the oxygen vacancy and oxygen interstitial have been investigated in many simulations, but results have indicated that the corresponding defect levels are either too shallow or too deep and fail to explain the experimental observation. Here, we propose a vacancy-interstitial ( ${V}_{O}$ - ${O}_{i}$ ) model. By tuning the relative positions of ${V}_{O}$ and ${O}_{i}$ , we show that the charge trap level of the defect pair can be adjusted continuously within the HfO2 bandgap. This allows us to depict a possible atomic picture for understanding the shallow-to-deep transformation of electron trapping.

Band alignment of HfO2/AlN heterojunction investigated by X-ray photoelectron spectroscopy

The band alignment between AlN and Atomic-Layer-Deposited (ALD) HfO2 was determined by X-ray photoelectron spectroscopy (XPS). The shift of Al 2p core-levels to lower binding energies with the decrease of take-off angles θ indicated upward band bending occurred at the AlN surface. Based on the angle-resolved XPS measurements combined with numerical calculations, valence band discontinuity ΔEV of 0.4 ± 0.2 eV at HfO2/AlN interface was determined by taking AlN surface band bending into account. By taking the band gap of HfO2 and AlN as 5.8 eV and 6.2 eV, respectively, a type-II band line-up was found between HfO2 and AlN.

Physico-Chemical Characterization of Anodic Oxides on Hf as a Function of the Anodizing Conditions

Anodic films were grown to 5 V (Ag/AgCl) on mechanically polished Hf in 0.1 M ammonium biborate and 0.1 M NaOH. Independent of the anodizing conditions, the photoelectrochemical characterization allowed the observation of optical transitions at 3.25 eV, i.e. at photon energy lower than the bandgap of HfO2. They are attributed to localized states inside the gap of the oxide induced by the presence of oxygen vacancies. From the cathodic photocurrent spectra, it was possible to estimate an energy threshold of ∼2.15 eV for internal electron photoemission phenomena. The impedance measurements proved the formation of insulating oxides with ɛ = 19. The anodizing occurs under a high field regime with an activation energy of 1.1 eV and an activation distance of 3.8 Å.

Microstructural, electrical, and optical properties of sol–gel derived HfMgZnO thin films

This paper reports the characterization of sol–gel derived HfxMg0.05Zn0.95−xO (x = 0, 0.025, 0.05, 0.1) thin films. The films show high transparency (∼90%) in the visible light wavelength region. The incorporation of Hf in the thin films increases the bandgap and decreases the crystallinity in the as-deposited films. This may be due to the high bandgap of HfO2 and the crystal frustrating effect caused by mixing atoms of different sizes. As the annealing temperature increases, the bandgap changes because of the alteration of the stress status, grain growth, and atomic rearrangement. The resistivity also decreases slightly with an increase in the annealing temperature.

Energy band alignment of HfO2 on p-type (100)InP

The band alignment of HfO2 film on p-type (100) InP substrate grown by magnetron sputtering was investigated. The chemical states and bonding characteristics of the system were characterized by X-ray photoelectron spectroscopy (XPS). The results show that there is no existence of Hf–P or Hf–In and there are interfacial In2O3 and InPO4 at the interface. Ultraviolet spectrophotometer (UVS) was employed to obtain the band gap value of HfO2. In 3d and Hf 4f core-level spectra and valence spectra were employed to obtain the valence band offset of HfO2/InP. Experimental results show that the (5.88 ± 0.05) eV band gap of HfO2 is aligned to the band gap of InP with a conduction band offset (∆E c) of (2.74 ± 0.05) eV and a valence band offset (∆E v) of (1.80 ± 0.05) eV. Compared with HfO2 on Si, HfO2 on InP exhibits a much larger conduction band offset (1.35 eV larger), which is beneficial to suppress the tunneling leakage current.

Energy band alignment of atomic layer deposited HfO2 oxide film on epitaxial (100)Ge, (110)Ge, and (111)Ge layers

Crystallographically oriented epitaxial Ge layers were grown on (100), (110), and (111)A GaAs substrates by in situ growth process using two separate molecular beam epitaxy chambers. The band alignment properties of atomic layer hafnium oxide (HfO2) film deposited on crystallographically oriented epitaxial Ge were investigated using x-ray photoelectron spectroscopy (XPS). Valence band offset, ΔEv values of HfO2 relative to (100)Ge, (110)Ge, and (111)Ge orientations were 2.8 eV, 2.28 eV, and 2.5 eV, respectively. Using XPS data, variation in valence band offset, ΔEV(100)Ge>ΔEV(111)Ge>ΔEV(110)Ge, was obtained related to Ge orientation. Also, the conduction band offset, ΔEc relation, ΔEc(110)Ge>ΔEc(111)Ge>ΔEc(100)Ge related to Ge orientations was obtained using the measured bandgap of HfO2 on each orientation and with the Ge bandgap of 0.67 eV. These band offset parameters for carrier confinement would offer an important guidance to design Ge-based p- and n-channel metal-oxide field-effect transistor for low-power application.

Chemical and structural properties of conducting nanofilaments in TiN/HfO2-based resistive switching structures

Structural, chemical and electronic properties of electroforming in the TiN/HfO2 system are investigated at the nanometre scale. Reversible resistive switching is achieved by biasing the metal oxide using conductive atomic force microscopy. An original method is implemented to localize and investigate the conductive region by combining focused ion beam, scanning spreading resistance microscopy and scanning transmission electron microscopy. Results clearly show the presence of a conductive filament extending over 20 nm. Its size and shape is mainly tuned by the corresponding HfO2 crystalline grain. Oxygen vacancies together with localized states in the HfO2 band gap are highlighted by electron energy loss spectroscopy. Oxygen depletion is seen mainly in the central part of the conductive filament along grain boundaries. This is associated with partial amorphization, in particular at both electrode/oxide interfaces. Our results are a direct confirmation of the filamentary conduction mechanism, showing that oxygen content modulation at the nanometre scale plays a major role in resistive switching.

Comparison of methods to determine bandgaps of ultrathin HfO2 films using spectroscopic ellipsometry

With the replacement of SiO2 by high-k Hf-based dielectrics in complementary metal–oxide–semiconductor technology, the measurement of the high-k oxide bandgap is a high priority. Spectroscopic ellipsometry (SE) is one of the methods to measure the bandgap, but it is prone to ambiguity because there are several methods that can be used to extract a bandgap value. This paper describes seven methods of determining the bandgap of HfO2 using SE. Five of these methods are based on direct data inversion (point-by-point fitting) combined with a linear extrapolation, while two of the methods involve a dispersion model-based bandgap extraction. The authors performed all of these methods on a single set of data from a 40 Å HfO2 film, as well as on data from 20 and 30 Å HfO2 films. It was observed that the bandgap values for the 40 Å film vary by 0.69 eV. In comparing these methods, the reasons for this variation are discussed. The authors also observed that, for each of these methods, there was a trend of increasing bandgap with decreasing film thickness, which is attributed to quantum confinement. Finally, the authors observed a greater variation in bandgap values among the methods for the 40 Å films than among the methods for the 30 and 20 Å films. This is attributed to the larger tail in the extinction coefficient k curve for the 40 Å film.

Nonvolatile memories using deep traps formed in HfO2 by Nb ion implantation

We report nonvolatile memories (NVMs) based on deep-energy trap levels formed in HfO2 by metal ion implantation. A comparison of Nb- and Ta-implanted samples shows that suitable charge-trapping centers are formed in Nb-implanted samples, but not in Ta-implanted samples. This is consistent with density-functional theory calculations which predict that only Nb will form deep-energy levels in the bandgap of HfO2. Photocurrent spectroscopy exhibits characteristics consistent with one of the trap levels predicted in these calculations. Nb-implanted samples showing memory windows in capacitance–voltage (V) curves always exhibit current (I) peaks in I–V curves, indicating that NVM effects result from deep traps in HfO2. In contrast, Ta-implanted samples show dielectric breakdowns during the I–V sweeps between 5 and 11 V, consistent with the fact that no trap levels are present. For a sample implanted with a fluence of 1013 Nb cm−2, the charge losses after 104 s are ∼9.8 and ∼25.5% at room temperature (RT) and 85°C, respectively, and the expected charge loss after 10 years is ∼34% at RT, very promising for commercial NVMs.

Inelastic electron tunneling study of crystallization effects and defect energies in hafnium oxide gate dielectrics

Changes in phonon spectra and point defect populations that accompany crystallization of HfO2 were investigated by inelastic tunneling across Al/HfO2/SiO2/Si. Spectral features from tetragonal- and monoclinic-HfO2 vibrational modes are observed in annealed films, while they are not detected in as-deposited samples, consistent with selected area electron diffraction analysis. In addition to features indexed as vibrational modes, peaks whose amplitude and energy vary with bias history were detected for p-type Si. We attribute these features to point defect-related states in the HfO2 band gap and find good agreement between their energies and those predicted theoretically for oxygen vacancy levels in HfO2.

Role of oxygen vacancies in HfO2-based gate stack breakdown

We study the influence of multiple oxygen vacancy traps in the percolated dielectric on the postbreakdown random telegraph noise (RTN) digital fluctuations in HfO2-based metal-oxide-semiconductor transistors. Our electrical characterization results indicate that these digital fluctuations are triggered only beyond a certain gate stress voltage. First-principles calculations suggest the oxygen vacancies to be responsible for the formation of a subband in the forbidden band gap region, which affects the triggering voltage (VTRIG) for the RTN fluctuations and leads to a shrinkage of the HfO2 band gap.

Energetics and electronic structure of aluminum point defects in HfO2: A first-principles study

Using the plane-wave pseudopotential method within the generalized gradient approximation, we studied the atomic structure, energetics, and electronic structure of the interstitial and substitutional point defect of dopant aluminum in monoclinic HfO2. Our results show that the doped Al atom energetically prefers to substitute for the Hf atom under the oxygen-rich condition. Substitution of Al for Hf creates a shallow acceptor level near the valence band maximum, whereas both substitution of Al for O and interstitial Al introduce deep levels in the band gap of HfO2. We also discussed the possible effect of Al doping on the electronic properties of HfO2.

In situ characterization of initial growth of HfO2

The initial growth of HfO2 on Si (111) is monitored in situ by ultrahigh vacuum (UHV) scanning probe microscopy. UHV scanning tunneling microscopy and UHV atomic force microscopy reveal the topography of HfO2 films in the initial stage. The chemical composition is further confirmed by x-ray photoelectron spectroscopy. Scanning tunneling spectroscopy is utilized to inspect the evolution of the bandgap. When the film thickness is less than 0.6 nm, the bandgap of HfO2 is not completely formed. A continuous usable HfO2 film with thickness of about 1.2 nm is presented in this work.