Hafnium Silicate Research Articles

Low-Voltage P3HT Field-Effect Transistors Fabricated Using High-k Gate Insulators

Hafnium silicate (HfSiO5) thin film (ε r = 7.7) and strontium titanate (SrTiO3) thin film (ε r = 12.1) were prepared on a heavily doped n-type silicon wafer by sputtering. Wet-processed P3HT field-effect transistors (FETs) fabricated using the self-assembled monolayer (SAM)-treated HfSiO5 or SAM-free SrTiO3 as a gate insulator showed saturated output characteristics at a driving voltage as low as −10 V for HfSiO5 and −3 V for SrTiO3. Hole mobilities of P3HT-FETs fabricated on the HfSiO5 and the SrTiO3 were 1.1 × 10−3 and 5.7 × 10−3 cm2/Vs, respectively.

Extension of Far UV spectroscopic ellipsometry studies of High-κ dielectric films to 130 nm

Next generation CMOS devices use a high-κ dielectric layer (HfO2, HfSiO, HfSiON and La2O3) grown on thin interfacial silicon dioxide as the gate dielectric. The higher dielectric constant of the Hf oxide based film stack allows a decrease in equivalent oxide thickness (EOT). Because the high-κ film stack has a greater physical thickness than an electrically equivalent SiO2 film, the tunneling current decreases. It is a critical metrology requirement to measure the thickness of silicon dioxide and high-κ film stacks. Spectroscopic ellipsometry (SE) in the far UV wavelength region can be used to differentiate the high-κ films from silicon dioxide. This is due to the non-zero nature of the imaginary part of the dielectric function (beyond 6eV) in the far UV region for high-κ films. There has been some conjecture that optical studies should be extended beyond 150nm further into the VUV. This study addresses these concerns through determination of the dielectric function down to 130nm. We show the fitted dielectric function of hafnium silicates and lanthanum oxide down to 130nm. X-ray reflectivity (XRR) measurements were also performed on the high-κ films to complement the thickness measurements performed with SE.

Study of reactions between HfO2 and Si in thin films with precise identification of chemical states by XPS

Reactions between HfO2 and Si in HfSiO films during deposition and post-annealing have been studied. Intermixing of HfO2 and Si is achieved by radio frequency sputtering with HfO2/Si compound targets, and post-annealing is used to promote the reaction at different temperatures. The structural characteristics of the mixture, HfSiO films, are analyzed by X-ray photoelectron spectroscopy and X-ray diffraction, and a careful assessment of chemical states is performed for precise identification. XPS results show that with ratios of Si:Hf ranging from 0 to 0.3 in HfSiO films, Si fully reacts with HfO2 to form silicate during deposition. However, SiO2 appears when the ratio of Si:Hf rises to 1.2. When the annealing temperature reaches 600°C, decomposition of hafnium silicate is observed and hafnium silicide forms in the bulk of the films. XRD results reveal that HfSiO films remain amorphous with the annealing temperature below 600°C but crystallize at 800°C.

Effect of Annealing on the Interfacial and Structural Properties of Amorphous Silicon-Hafnia Films

Chemical and physical properties of the interface formed between amorphous silicon (a-Si) and hafnium oxide (HfO2) were investigated using X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy (XPS). XRR showed that the interface layer formed between the a-Si and HfO2 layer had a thickness of 0.7 ± 0.3 nm and a density of 4.4 ± 0.2 g/cm3. Based on the XPS results, the interface comprised HfO2, silicon oxide, and hafnium silicate phases. High-temperature annealing led to the conversion of elemental silicon and nonstoichiometric silicate into silicon dioxide phases. This reaction was also accompanied by the chemical reduction of the HfO2 phase. Annealed films crystallized into the monoclinic phase of HfO2 with a preferred orientation along the (111) axis.

Comparison of HfSiOx Thin Films Deposited by ALD with Moisture Using Different Silicon Sources

The scaling of semiconductor devices using various high-k materials has been investigated, and replacing the standard SiO2 gate dielectric by high-k material was a major step. Hafnium-based material is currently one of the preferred candidates. To improve integration of Hafnium into the gate dielectric, HfSiOx has been chosen for low power devices while HfZrOx is more common for high-performance applications. Using widely available processes, HfO2 can be deposited using amine or alkoxide based compounds and different reducing agents such as ozone, oxygen or moisture. However, depositing HfSiOx by Atomic Layer Deposition (ALD) with moisture is more challenging. Few or no available Silicon precursors are reactive with H2O alone to deposit SiO2 by ALD. This work is intended to present precursor solutions to deposit hafnium silicate using moisture as oxidant.

Low-temperature deposition of stoichiometric HfO 2 on silicon: Analysis and quantification of the HfO 2/Si interface from electrical and XPS measurements

An understanding of the exact structural makeup of dielectric interface is crucial for development of novel gate materials. In this paper a study of the HfO 2/Si interface created by the low-temperature deposition ultrathin stoichiometric HfO 2 on Si substrates by reactive sputtering is presented. Analysis, quantification and calculation of layer thickness of an HfO 2/Hf–Si–O x /SiO 2 gate stack dielectrics have been performed, using X-ray photoelectron spectroscopy (XPS) depth profile method, angle resolved XPS and interface modeling by XPS data processing software. The results obtained were found to be in good agreement with the high frequency capacitance–voltage (C–V) measurements. The results suggest a development of a complex three layer dielectric stack, including hafnium dioxide layer, a narrow interface of hafnium silicate and broad region of oxygen diffusion into silicon wafer. The diffusion of oxygen was found particularly detrimental to the electrical properties of the stack, as this oxygen concentration gradient leads to the formation of suboxides of silicon with a lower permittivity, κ.

First principles calculations of electronic and dielectric properties of hafnium silicates

Ab initio calculations are used to investigate the electronic structure and dielectric properties of crystalline monoclinic hafnium silicates (Hf1-xSixO2) as a function of silicon content x (x = 0, 0.25, 0.50, and 0.75). Densities of states (DOS), electronic band gaps, dielectric constants of Hf1-xSixO2 are calculated through a screened exchange (sX) method within the local density approximation (LDA). The sX method has been found to give good band gap for HfO2 dielectric. Band gaps of Hf1-xSixO2 are shown to increase with increasing concentration x of silicon. Static dielectric constants of Hf1-xSixO2 are observed to decrease with increasing the Si content. Results suggest that the optimal Si concentration should be below 25%, for which the permittivity higher than 13 can be achieved. Our calculations are in very good agreement with recent experimental findings.

Power-law scaling of proton conductivity in amorphous silicate thin films

Amorphous hafnium silicate, a-Hf 0.1Si 0.9O x , thin film with thickness of 32, 41, 55, 80, 110, 120, 180 and 320 nm was prepared by multiple spin-cast process and the proton conductivity across the films was measured at intermediate temperatures (100–400 °C) in dry atmosphere. The morphologically- and compositionally-uniform films were prepared on a substrate as confirmed by SEM, RBS and XPS measurements. a-Hf 0.1Si 0.9O x thin film clearly revealed the H/D isotope effect on ionic conductivity, indicating that protonic conduction is dominant in the measured temperature range. The films did not reveal thickness-dependent proton conductivity in dry air and the σ at given temperatures is almost constant at any thickness. No increment of σ in a-Hf 0.1Si 0.9O x thin films by reduction of thickness might be related to the absence of the highly-conductive acid network with mesoscopically-sized length because of the relatively low concentration of Brønsted acid sites inside films.

Evaluation of the Coulomb-limited mobility in high-κ dielectric metal oxide semiconductor field effect transistors

An efficient numerical method for the evaluation of the Green’s function used in the calculation of the Coulomb-limited electron mobility in high-κ metal oxide semiconductor field effect transistors is presented. This simple method is applicable to gate stacks with an arbitrary number of layers of varying dielectric permittivity. A charge profile with varying dielectric profile is demonstrated to show an increase in Coulomb-limited mobility of 16% in comparison to a point charge located at the interface. A metal gate reduces the scattering potential due to its infinite dielectric constant which leads to lesser impact of charge in comparison to a polysilicon gate. The Coulomb-limited mobility for devices having identical equivalent oxide thickness of 0.5–0.8 nm with (a) a hafnium silicate interfacial layer (IL) and (b) zero IL is presented.

Interfaces analysis of the HfO 2/SiO 2/Si structure

The physical and chemical properties of the HfO 2/SiO 2/Si stack have been analyzed using cross-section HR TEM, XPS, IR-spectroscopy and ellipsometry. HfO 2 films were deposited by the MO CVD method using as precursors the tetrakis 2,2,6,6 tetramethyl-3,5 heptanedionate hafnium—Hf(dpm) 4 and dicyclopentadienil-hafnium-bis-diethylamide—Сp 2Hf(N(C 2H 5) 2) 2. The amorphous interface layer (IL) between HfO 2 and silicon native oxide has been observed by the HRTEM method. The interface layer comprises hafnium silicate with a smooth varying of chemical composition through the IL thickness. The interface layer formation occurs both during HfO 2 synthesis, and at the annealing of the HfO 2/SiO 2/Si stack. It was concluded from the XPS, and the IR-spectroscopy that the hafnium silicate formation occurs via a solid-state reaction at the HfO 2/SiO 2 interface, and its chemical structure depends on the thickness of the SiO 2 underlayer.

Atomic Layer Deposition of Hafnium Silicate Film and Its Application to OTFTs

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Flicker-Noise Improvement in 100-nm $L_{g}\ \hbox{Si}_{0.50}\hbox{Ge}_{0.50}$ Strained Quantum-Well Transistors Using Ultrathin Si Cap Layer

This letter presents a record low flicker-noise spectral density in biaxial compressively strained p-channel 100-nm LgSi0.50Ge0.50 quantum-well FETs (QWFETs) with ultrathin Si (~2 nm) barrier layer and 1-nm EOT hafnium silicate gate dielectric. The normalized power spectral density of Id fluctuations (SId/Id 2) in Si0.50Ge0.50 QWFETs exhibits significant improvement by ten times over surface channel unstrained Si pMOSFETs at high Vg due to strong confinement of holes within the high-mobility QW and strong quantization in the ultrathin Si barrier layer enabled by low-thermal-budget device processing. The noise behavior in strained QW devices is found to evolve from being correlated mobility fluctuation dominated across most of Vg range to being Hooge mobility fluctuation dominated at very high Vg.

HfO2-High-k Dielectric for Nanoelectronics

Thermodynamic consideration of Hf-HfO2-SiO2-Si system, cross-sectional HR-TEM, XPS, IR-spectroscopy and Null ellipsometry technique were used to determine the chemical composition and structure of the HfO2 thin films deposited on Si. The interface layer consists of the hafnium silicate with smoothly varying chemical composition along the film thickness. Formation of the IL occurs as during the HfO2 film deposition so at annealing of the HfO2/SiO2/Si structure. Kinetics of the hafnium silicate formation was studied by IR-spectroscopy.

DLTS of ALD HfO2 on s-Si/SiGe/Si: Effects of s-Si Thickness and Surface Nitridation

MOS capacitors have been used as depth-dependent probing tools for MOS-DLTS studies of ALD HfO2 on s-Si/SiGe/Si composite samples. Surface nitridation prior to HfO¬2 deposition was carried out to improve the HfO2/s-Si interface by suppressing hafnium silicate interlayer formation. The overall effects of nitridation on the HfO2/s-Si interface and the s-Si/SiGe bulk region with different strained Si layer thicknesses were studied. The relationship between DLTS peak amplitude and pulsing time indicates the emission source in bulk region is related with Ge decorated dislocations and in the interface region it originates from the HfO2/s-Si interface. In the s-Si/SiGe bulk region, nitridation reduces the trap concentration and shifts trap energy level toward mid-gap. For the interface, nitridation shifts trap energy level toward valence band and reduces hole capture cross section significantly.

Structure of Hafnium Silicate Films Formed by Atomic Layer Deposition

Hf distributions in as-grown and annealed (HfO2)0.25(SiO2)0.75 films with thicknesses in the range 4-13 nm were investigated by high resolution transmission electron microscopy (HRTEM), angle resolved X-ray photoelectron spectroscopy (ARXPS) and medium energy ion scattering (MEIS). HRTEM images show a layered structure in films thinner than 8 nm. ARXPS data also show a non-uniform distribution of Hf throughout the film depth. A diffusion of SiO2 to the film surface after a longer time anneal was observed by MEIS. All these observations provide evidence for surface-directed spinodal decomposition in the pseudobinary (HfO2)x(SiO2)1-x alloy system.

Comparison oF Electrical Measurements with Structural Analysis oF Thin High-k Hafnium-based Films

We present a detailed characterization of thin high-k Hf-based dielectric/SiO2 stacks on Si by electrical measurements (capacitance - voltage (C-V) and current-voltage (I-V)), structural analysis using transmission electron microscopy (TEM) and compositional analysis using medium energy ion scattering (MEIS). We show that the electrical results are consistent with and can be explained by the structural and compositional results. Specifically, the structural results show that the effect of nitridation of the films using a decoupled plasma nitridation method is an increase in SiO2 thickness, which results in a capacitance and leakage current decrease, revealed by the electrical measurements. Nitridation also reduces the density of interface traps for these samples. The reduction is more effective in the case of nonstoichiometric hafnium silicate films (HfSiOx) than in the case of HfO2.

Surface-directed spinodal decomposition in hafnium silicate thin films

Surface-directed spinodal decomposition in hafnium silicate thin films

High-resolution Rutherford backscattering spectrometry study on process dependent elemental depth profile change of hafnium silicate on silicon

Process characterization of HfSiO(N)/SiO(N) films was performed by nondestructive elemental depth profile analysis using high-resolution Rutherford backscattering spectrometry. As-grown atomic layer deposition HfSiO film (2 nm) was found not to be uniform but to have a gradient near the top surface with Si concentration higher than the bulk. Regardless of native oxide removal, HfSiO grown on HF-last Si results in an almost identical interfacial SiOx thickness with that of HfSiO grown on chemical oxide. Dependence of interfacial layer on oxygen transport behavior during high-temperature annealing was also investigated. Whereas only the upward oxygen outdiffusion from the HfSiO layer was observed for the films deposited on HF-last Si and SiON, downward oxygen transport to the bottom interfacial layer was also observed when HfSiO film was deposited on chemical oxide. This result indicates that the bonding properties of chemical oxide are different from those of SiO2 and that chemical oxide can accommodate oxygen ions. Finally, thermal stability of nitrogen in HfSiO introduced by NH3 annealing and plasma nitridation was compared. Nitrogen desorption by spike anneal was only observed for HfSiON film with nitrogen introduced by plasma nitridation process.

Atomic Layer Deposition of Hafnium Silicate Thin Films Using Tetrakis(diethylamido)hafnium and Tris(2-methyl-2-butoxy)silanol

Hafnium silicate films were grown by atomic layer deposition using the liquid precursors tetrakis(diethylamido)hafnium (TDEAH) and tris(2-methyl-2-butoxy)silanol, (TMBS). Using in situ ellipsometry, ex situ high resolution transmission electron microscopy (HRTEM), medium energy ion scattering (MEIS), and X-ray photoelectron spectroscopy (XPS), the details of the film thickness and composition were examined as functions of both the substrate temperature and silanol pulse time. Both HRTEM and MEIS measurements revealed that the films comprised two layers, with the surface layer containing more Hf than the layer in contact with the substrate. A self-limiting growth with a rate was observed only after several initial cycles, a behavior that is ascribed to the chemistry of the initial Si substrate surface. Hf 4f XPS confirmed that the films were stoichiometric throughout despite the nonconstant Hf concentration with depth. A reaction mechanism between TDEAH and TMBS is proposed.

Estimate of dielectric density using spectroscopic ellipsometry

The optical dielectric functions for hafnium oxide and hafnium silicate films were extracted from spectroscopic ellipsometry measurements and the density then calculated using a previously proposed method. The values obtained were then compared to those obtained using X-ray reflectometry. The optical dielectric functions for gadolinium oxide films deposited under various conditions were also extracted from spectroscopic ellipsometry measurements. It was found using medium energy ion scattering that gadolinium oxide films deposited using Gd[N(SiCH 3) 2] 3 and H 2O as precursors, contained significant levels of silicon and the silicon concentration was directly proportional to the wafer deposition temperature. This effect was also observed in the density measurements extracted from spectroscopic ellipsometry data.