Nature of chemical states of sulfur embedded in atomic‐layer‐deposited HfO2 film on Ge substrate for interface passivation

Sulfur was embedded in atomic‐layer‐deposited (ALD) HfO₂ films grown on Ge substrate by annealing under H₂S gas before and after HfO₂ ALD. The chemical states of sulfur in the film were examined by S K‐edge X‐ray absorption spectroscopy. It was revealed that the valences of S‐ions were mostly –2 at Ge/HfO₂ interface (GeS_x or HfO_2–yS_y to passivate the interface), while they were mostly +6 in HfO₂ layers (sulfates; HfO_2–z(SO₄)_z). The leakage current density in post‐deposi‐tion‐treated film was lower than that in pre‐deposition‐treated one. This suggests that the passivation of defects in oxide layer by sulfate ions is more effective to lower the leakage current rather than the interface defect passivation by S^2– ions. (© 2015 WILEY‐VCH Verlag GmbH &Co. KGaA, Weinheim)     

Latent Fingerprint Imaging Using Dy and Sm Codoped HfO2 Nanophosphors: Structure and Luminescence Properties

The development of latent fingerprints (LFPs) detected at the site of crime is considered as an imperative physical evidence in forensic investigations. Herein, a rapid and cost‐effective approach using nonhazardous Dy and Sm codoped HfO₂ nanophosphors is demonstrated to be utilized for LFP imaging. Sol–gel method can produce Dy and Sm codoped HfO₂ of monoclinic phase with crystallite size ranging from ≈10 to 25 nm. While selected area electron diffraction and lattice spacing calculated from high‐resolution transmission electron microscopy confirm monoclinic phase, Le‐Bail profile refinement of X‐ray diffraction (XRD) patterns demonstrate an exponential increase in lattice volume with incorporating various concentrations of Dy and Sm in HfO₂ lattice. Exciting Dy and Sm codoped HfO₂ with 393 nm photoluminescence spectra show emissions in blue, yellow, and near red regions emerging primarily due to energy transfer from Dy³⁺ to Sm³⁺ through multipolar interaction suggested by time resolved decay spectra. Combining excitation and emission spectra, an energy band diagram is proposed. Owing to excellent emissions, Dy and Sm codoped HfO₂ are explored for LFP imaging with good selectivity and resolution over multivariate surfaces like float glass and aluminum foil. The third‐level details like enclosure and termination–bifurcation are extracted due to nanosized nature of particles.     

Reduction of charge trapping in HfO2 film on a Ge substrate by trimethylaluminum pretreatment

Trimethylaluminum pretreatment prior to HfO₂ deposition is introduced for native oxide reduction. It is identified that the trimethylaluminum pretreatment could effectively reduce native oxide, which is transformed to an aluminum oxide interfacial layer. Formation of the thin aluminum oxide layer suppresses Ge diffusion into HfO₂, reducing hysteresis in the ca‐ pacitance–voltage curve. Moreover, the device reliability of the trimethylaluminum pretreated sample is improved in a constant current stress test. This work indicates that trimethylaluminum pretreatment is an effective in‐situ method for the gate dielectric stack formation to reduce charge trapping in the HfO₂ film on a Ge substrate. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Monoclinic HfO2:Eu X-ray phosphor

HfO₂ and HfO₂:Eu powders were synthesized with Pechini method at temperatures in the range of 600–1500 °C. Structural, radioluminescence and photoluminescence properties of the powders were investigated. The highest light output of about 20% of the efficiency of commercial GOS:Eu was found for materials containing 0.5% of Eu and prepared at 1500 °C. The data shows, that further improvement of light output could be obtained if materials are processed at yet higher temperatures. Emission spectra indicate that Eu³⁺ experiences variety of symmetries of its surroundings. Undoped materials produce broad band emission peaking at 480 nm and showing a significant afterglow.     

Enhanced bipolar resistive switching of HfO2 with a Ti interlayer

The characteristics of resistive switching of TiN/HfO₂/Ti/HfO₂/Pt/Ti stacks on SiO₂/Si substrates were investigated and compared to TiN/HfO₂/Pt/Ti stacks in order to study Ti interlayer effects on resistive switching. The Ti interlayers were deposited in situ during the reactive sputtering of HfO₂ films. The current–voltage measurements showed that the Ti interlayers enhanced the memory window but reduced the endurance of SET/RESET operations. The energy filtered images by TEM showed asymmetric oxygen accumulation at the Ti/HfO _x interfaces. Subsequent heat treatment improved the endurance of SET/RESET operation of TiN/HfO₂/Ti/HfO₂/Pt/Ti stacks.     

Band bending and band alignment at HfO2/HfSixOy/Si interfaces

Band bending and band alignment at HfO₂/SiO₂/Si and HfO₂/Hf/SiO₂/Si interfaces were investigated using X-ray photoelectron spectroscopy. After Hf-metal pre-deposition, a 0.55 eV band bending in Si and a 1.80 eV binding energy decrease for Hf 4f and O 1s of HfO₂ were observed. This was attributed to the introduction of negative space charges at interface by Hf pre-deposition. Band bending decrease and synchronous binding energy increases of O 1s and Hf 4f for HfO₂ were observed during initial Ar⁺ sputtering of the Hf pre-deposited sample. This was interpreted through the neutralization of negative space charges by sputtering-induced oxygen vacancies.     

Effect of thermal annealing sequence on the crystal phase of HfO2 and charge trap property of Al2O3/HfO2/SiO2 stacks

In this study, we investigated the effect of a post annealing sequence on the HfO₂ crystal phase and the memory window of charge trap devices with TiN-Al₂O₃-HfO₂-SiO₂-Si stacks. The charge trap dielectrics of HfO₂ were deposited by atomic layer deposition and were annealed in an oxygen environment with or without Al₂O₃ blocking oxides. X-ray diffraction analysis showed that, after thermal annealing, the predominant crystal phase of HfO₂ is divided into tetragonal and monoclinic phase depending on the presence or absence of Al₂O₃ blocking oxide. In addition, deconvolution of X-ray diffraction spectra showed that, with increasing annealing temperature, the fraction of the tetragonal phase in the HfO₂ film was enhanced with the Al₂O₃ blocking oxide, while it was reduced without the Al₂O₃ blocking oxide. Finally, measurements of program/erase and increase-step-pulse programming showed that the charge trap efficiency and the memory window of the charge trap devices increased with decreasing fraction of tetragonal HfO₂.     

Effects of heavy-ion irradiation on the electrical properties of rf-sputtered HfO2 thin films for advanced CMOS devices

HfO₂-based metal-oxide semiconductor (MOS) capacitors were irradiated with high-energy ion beam to study the irradiation effects in these films. HfO₂ thin films deposited by radio frequency (rf)-sputtering were irradiated with 80 MeV O⁶⁺ ions. The samples were irradiated and characterized at room temperature. Devices were characterized via 1 MHz capacitance–voltage (C?V) measurements using the midgap method. The irradiation induced dispersion in accumulation and depletion regions with increasing fluence is observed. After irradiation, the midgap voltage shift (Δ V _mg) of?0.61 to?1.92 V, flat band voltage shift (Δ V _fb) of?0.48 to?2.88 V and threshold voltage shift (Δ V _th) of?0.966 to?1.96 V were observed. The change in interface trap charge and oxide trap charge densities after 80 MeV O⁶⁺ ions irradiation with fluences were determined from the midgap to flat band stretch out of C?V curves. The results are reported and explained in terms of changes in microstructure and dielectric properties of the HfO₂ thin films after irradiation.     

Dielectric mismatch and shallow donor impurities in GaN/HfO2 quantum wells

In this work we investigate electron–impurity binding energy in GaN/HfO₂ quantum wells. The calculation considers simultaneously all energy contributions caused by the dielectric mismatch: (i) image self-energy (i.e., interaction between electron and its image charge), (ii) the direct Coulomb interaction between the electron–impurity and (iii) the interactions among electron and impurity image charges. The theoretical model account for the solution of the time-dependent Schrödinger equation and the results shows how the magnitude of the electron–impurity binding energy depends on the position of impurity in the well-barrier system. The role of the large dielectric constant in the barrier region is exposed with the comparison of the results for GaN/HfO₂ with those of a more typical GaN/AlN system, for two different confinement regimes: narrow and wide quantum wells.     

Atomic layer deposition of HfO2 on graphene from HfCl4 and H2O

Atomic layer deposition of HfO₂ on unmodified graphene from HfCl₄ and H₂O was investigated. Surface RMS roughness down to 0.5 nm was obtained for amorphous, 30 nm thick hafnia film grown at 180°C. HfO₂ was also deposited in a two-step temperature process where the initial growth of about 1 nm at 170°C was continued up to 10–30 nm at 300°C. This process yielded uniform, monoclinic HfO₂ films with RMS roughness of 1.7 nm for 10–12 nm thick films and 2.5 nm for 30 nm thick films. Raman spectroscopy studies revealed that the deposition process caused compressive biaxial strain in graphene, whereas no extra defects were generated. An 11 nm thick HfO₂ film deposited onto bilayer graphene reduced the electron mobility by less than 10% at the Dirac point and by 30–40% far away from it.     

Electrical and dielectric properties of Al/HfO2/p-Si MOS device at high temperatures

The temperature dependence of capacitance–voltage (C–V) and conductance–voltage (G/w–V) characteristics of Al/HfO₂/p-Si metal-oxide-semiconductor (MOS) device has been investigated by considering the effect of series resistance (R_s) and interface state density (N_ss) over the temperature range of 300–400 K. The C–V and G/w–V characteristics confirm that the N_ss and R_s of the diode are important parameters that strongly influence the electric parameters in MOS device. It is found that in the presence of series resistance, the forward bias C–V plots exhibits a peak, and its position shifts towards lower voltages with increasing temperature. The density of N_ss, depending on the temperature, was determined from the (C–V) and (G/w–V) data using the Hill–Coleman Method. Also, the temperature dependence of dielectric properties at different fixed frequencies over the temperature range of 300–400 K was investigated. In addition, the electric modulus formalisms were employed to understand the relaxation mechanism of the Al/HfO₂/p-Si structure.     

Comparative study of pulsed laser deposited HfO2 and Hf–aluminate films for high-k gate dielectric applications

The thermal stability and the electrical properties of HfO₂ and Hf–aluminate films prepared by the pulsed laser deposition technique have been investigated by X-ray diffraction, differential thermal analysis, capacitance–voltage correlation, leakage-current measurements and high-resolution transmission electron microscopy observation, respectively. A crystallization transformation from HfO₂ amorphous phase to polycrystalline monoclinic structure occurs at about 500 °C. In contrast, the amorphous structure of Hf–aluminate films remains stable at higher temperatures up to 900 °C. Rapid thermal annealing at 1000 °C for 3 min leads to a phase separation in Hf–aluminate films. Tetragonal HfO₂(111) is predominant, and Al₂O₃ separates from Hf–aluminate and is still in the amorphous state. The dielectric constant of amorphous HfO₂ and Hf–aluminate films was determined to be about 26 and 16.6, respectively, by measuring a Pt/dielectric film/Pt capacitor structure. A very small equivalent oxide thickness (EOT) value of 0.74 nm for a 3-nm physical thickness Hf–aluminate film on a n-Si substrate with a leakage current of 0.17 A/cm² at 1-V gate voltage was obtained. The interface at Hf–aluminate/Si is atomically sharp, while a thick interface layer exists between the HfO₂ film and the Si substrate, which makes it difficult to obtain an EOT of less than 1 nm. PACS 77.55.+f; 81.15.Fg; 73.40.Qv     

Influence of substoichiometer on the laser-induced damage characters of HfO2 thin films

HfO₂ is one of the most important high refractive index materials for depositing high power optical mirrors. In this research, HfO₂ thin films were prepared by dual-ion beam reactive sputtering method, and the laser-induced damage thresholds (LIDT) of the sample were measured in 1-on-1 mode for laser with 1064 nm wavelength. The results indicate that the LIDT of the as-grown sample is only 3.96 J/cm², but it is increased to 8.98 J/cm² after annealing under temperature of 200 °C in atmosphere. By measuring the laser weak absorption and SIMS of the samples, we deduced that substoichiometer is the main reason for the low LIDT of the as-grown sample, and the experiment results were well explained with the theory of electronic-avalanche ionization.     

Microstructure and interfacial properties of HfO2-Al2O3 nanolaminate films

High-k HfO₂-Al₂O₃ composite gate dielectric thin films on Si(1 0 0) have been deposited by means of magnetron sputtering. The microstructure and interfacial characteristics of the HfO₂-Al₂O₃ films have been investigated by using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR) and spectroscopic ellipsometry (SE). Analysis by XRD has confirmed that an amorphous structure of the HfO₂-Al₂O₃ composite films is maintained up to an annealing temperature of 800 °C, which is much higher than that of pure HfO₂ thin films. FTIR characterization indicates that the growth of the interfacial SiO₂ layer is effectively suppressed when the annealing temperature is as low as 800 °C, which is also confirmed by spectroscopy ellipsometry measurement. These results clearly show that the crystallization temperature of the nanolaminate HfO₂-Al₂O₃ composite films has been increased compared to pure HfO₂ films. Al₂O₃ as a passivation barrier for HfO₂ high-k dielectrics prevents oxygen diffusion and the interfacial layer growth effectively.     

Thermal instability of HfO2 on InP structure with ultrathin Al2O3 interface passivation layer

We studied the thermal stability of HfO₂ on an InP structure when an Al₂O₃ interface passivation layer (PL) was introduced. In contrast to the thick (～4 nm) Al₂O₃‐PL, an almost complete disappearance of the thin (～1 nm) Al₂O₃‐PL was observed after a post‐deposition anneal at 600 °C. Based on various chemical and electrical analyses, this was attributed to the intermixing of the thin Al₂O₃‐PL with HfO₂, which might have been accompanied by the out‐diffusion of a substantial amount of substrate elements. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Temperature dependence of electric quadrupole interaction of181Ta in HfO2

High-resolution TDPAC determination of EQI of¹⁸¹Ta in HfO₂ is performed. Room temperature parameters of ω_Q = 111 (1) MHz and η=0.36(1) are obtained. The temperature dependence of ω_Q and η in the range 78–1100 K is established. An explanation of the observed phenomena in a dielectric lattice is discussed.     

Electrical property of HfOxNy-HfO2-HfOxNy sandwich-stack films

The effects of HfO_xN_y on the electrical property of HfO_xN_y-HfO₂-HfO_xN_y sandwich-stack (signed as SS) films were investigated. Excellent electrical performances were achieved in SS films, with a high dielectric constant of 16 and a low leakage current of ∼2 × 10⁻⁸ A/cm² at 1 MV/cm. Schottky (SK) emission and Frenkel-Poole (PF) emission are found to be the dominant mechanisms for the current conduction behavior. After a long time stress, the flat-band voltage shift in the SS film is much smaller than that in a pure HfO_xN_y film indicating fewer charge traps existed in the SS film. Based on the experiments, the new SS structure is more favorable for the improvement of electrical performances than a pure HfO_xN_y or HfO₂ structure.     

Broad excitation of Er luminescence in Er-doped HfO2 films

We investigated the broad and sensitized luminescence properties of Er-doped HfO₂ films synthesized by pulsed laser deposition (PLD) and ion implantation techniques. In the investigation we focused on the mechanism of energy transfer in the host matrix. Based on the comparison of photoluminescence (PL), photoluminescence excitation (PLE), and cathode-luminescence (CL), as well as on microstructure measurements, an excitation transfer process resulting in the broad excitation for Er, luminescence at 1540 nm, is identified. In this process, the oxygen vacancies and Hf in the host HfO₂ serve mainly as effective sensitizers for neighboring Er ions in the nonresonant excitation process. Furthermore, the direct Er³⁺ intra-4f transitions and full spectral emission of Er ions in the HfO₂ matrix are clearly observed under the wide-spectrum excitation in the CL measurement. This reveals more detailed features for the energy transfer and transition processes.     

Characteristics of sandwich-structured Al2O3/HfO2/Al2O3 gate dielectric films on ultra-thin silicon-on-insulator substrates

Sandwich-structure Al₂O₃/HfO₂/Al₂O₃ gate dielectric films were grown on ultra-thin silicon-on-insulator (SOI) substrates by vacuum electron beam evaporation (EB-PVD) method. AFM and TEM observations showed that the films remained amorphous even after post-annealing treatment at 950 °C with smooth surface and clean silicon interface. EDX- and XPS-analysis results revealed no silicate or silicide at the silicon interface. The equivalent oxide thickness was 3 nm and the dielectric constant was around 7.2, as determined by electrical measurements. A fixed charge density of 3 × 10¹⁰ cm⁻² and a leakage current of 5 × 10⁻⁷A/cm² at 2 V gate bias were achieved for Au/gate stack /Si/SiO₂/Si/Au MIS capacitors. Post-annealing treatment was found to effectively reduce trap density, but increase in annealing temperature did not made any significant difference in the electrical performance.     

Effect of SiO2 protective layer on the femtosecond laser-induced damage of HfO2/SiO2 multilayer high-reflective coatings

Two kinds of HfO₂/SiO₂ 800 nm high-reflective (HR) coatings, with and without SiO₂ protective layer were deposited by electron beam evaporation. Laser-induced damage thresholds (LIDT) were measured for all samples with femtosecond laser pulses. The surface morphologies and the depth information of all samples were observed by Leica optical microscopy and WYKO surface profiler, respectively. It is found that SiO₂ protective layer had no positive effect on improving the LIDT of HR coating. A simple model including the conduction band electron production via multiphoton ionization and impact ionization is used to explain this phenomenon. Theoretical calculations show that the damage occurs first in the SiO₂ protective layer for HfO₂/SiO₂ HR coating with SiO₂ protective layer. The relation of LIDT for two kinds of HfO₂/SiO₂ HR coatings in calculation agrees with the experiment result.