Fabrication and characterization of La-doped HfO2 gate dielectrics by metal-organic chemical vapor deposition

La-doped HfO₂ gate dielectric thin films have been deposited on Si substrates using La(acac)₃ and Hf(acac)₄ (acac = 2,4-pentanedionate) mixing sources by low-pressure metal-organic chemical vapor deposition (MOCVD). The structure, thermal stability, and electrical properties of La-doped HfO₂ films have been investigated. Inductive coupled plasma analyses confirm that the La content ranging from 1 to 5 mol% is involved in the films. The films show smaller roughness of ∼0.5 nm and improved thermal stability up to 750 °C. The La-doped HfO₂ films on Pt-coated Si and fused quartz substrates have an intrinsic dielectric constant of ∼28 at 1 MHz and a band gap of 5.6 eV, respectively. X-ray photoelectron spectroscopy analyses reveal that the interfacial layer is Hf-based silicate. The reliable value of equivalent oxide thickness (EOT) around 1.2 nm has been obtained, but with a large leakage current density of 3 A/cm² at V_g = 1V + V_fb. MOCVD-derived La-doped HfO₂ is demonstrated to be a potential high-k gate dielectric film for next generation metal oxide semiconductor field effect transistor applications.     

Interfaces analysis of the HfO2/SiO2/Si structure

The physical and chemical properties of the HfO₂/SiO₂/Si stack have been analyzed using cross-section HR TEM, XPS, IR-spectroscopy and ellipsometry. HfO₂ 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—Сp₂Hf(N(C₂H₅)₂)₂.The amorphous interface layer (IL) between HfO₂ 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 HfO₂ synthesis, and at the annealing of the HfO₂/SiO₂/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 HfO₂/SiO₂ interface, and its chemical structure depends on the thickness of the SiO₂ underlayer.     

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.     

Low-temperature deposition of stoichiometric HfO2 on silicon: Analysis and quantification of the HfO2/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 HfO₂/Si interface created by the low-temperature deposition ultrathin stoichiometric HfO₂ on Si substrates by reactive sputtering is presented. Analysis, quantification and calculation of layer thickness of an HfO₂/Hf-Si-O_x/SiO₂ 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.     

Effects of N2 and NH3 remote plasma nitridation on the structural and electrical characteristics of the HfO2 gate dielectrics

The remote plasma nitridation (RPN) of an HfO₂ film using N₂ and NH₃ has been investigated comparatively. X-ray photoelectron spectroscopy and Auger electron spectroscopy analyses after post-deposition annealing (PDA) at 700 °C show that a large amount of nitrogen is present in the bulk film as well as in the interfacial layer for the HfO₂ film nitrided with NH₃-RPN. It is also shown that the interfacial layer formed during RPN and PDA is a nitrogen-rich Hf-silicate. The C-V characteristics of an HfO_xN_y gate dielectric nitrided with NH₃-RPN have a smaller equivalent oxide thickness than that nitrided with N₂-RPN in spite of its thicker interfacial layer.     

Layer structure variations of ultra-thin HfO2 films induced by post-deposition annealing

To meet challenges for a smaller transistor feature size, ultra-thin HfO₂ high-k dielectric has been used to replace SiO₂ for the gate dielectric. In order to accurately analyze the ultra-thin HfO₂ films by grazing incidence X-ray reflectivity (GIXRR), an appropriate material model with a proper layer structure is required. However, the accurate model is difficult to obtain, since the interfaces between layers of the ultra-thin HfO₂ films are not easily identified, especially when post-deposition annealing process is applied. In this paper, 3.0 nm HfO₂ films were prepared by atomic layer deposition on p-type silicon wafer, and annealed in Ar environment with temperatures up to 1000 °C. The layer structures and the role of the interfacial layer of the films in the post-deposition annealing processes were evaluated by X-ray diffraction and X-ray photoelectron spectroscopy (XPS). The experimental results and analysis showed that layer thicknesses, crystal phases and chemical structures of the ultra-thin HfO₂ films were significantly dependent on annealing temperatures. The binding energy shifts of Hf 4f, O 1s, and Si 2p elements revealed the formation of Hf silicate (Hf-O-Si bonding) with increasing annealing temperatures. Due to the silicate formation and increasing silicon oxide formation, the interface broadening is highly expected. The structure analysis of the GIXRR spectra using the modified material structure model from the XPS analysis confirmed the interfacial broadening induced by the post-deposition annealing.     

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.     

Atomic layer deposited HfO2 based metal insulator semiconductor GaN ultraviolet photodetectors

A report on GaN based metal insulator semiconductor (MIS) ultraviolet (UV) photodetectors (PDs) with atomic layer deposited (ALD) 5-nm-thick HfO₂ insulating layer is presented. Very low dark current of 2.24 × 10⁻¹¹ A and increased photo to dark current contrast ratio was achieved at 10 V. It was found that the dark current was drastically reduced by seven orders of magnitude at 10 V compared to samples without HfO₂ insulating layer. The observed decrease in dark current is attributed to the large barrier height which is due to introduction of HfO₂ insulating layer and the calculated barrier height was obtained as 0.95 eV. The peak responsivity of HfO₂ inserted device was 0.44 mA/W at bias voltage of 15 V.     

Growth, microstructure and electrical properties of sputter-deposited hafnium oxide (HfO2) thin films grown using a HfO2 ceramic target

Hafnium oxide (HfO₂) thin films have been made by radio-frequency (rf) magnetron-sputtering onto Si(1 0 0) substrates under varying growth temperature (T_s). HfO₂ ceramic target has been employed for sputtering while varying the T_s from room temperature to 500 °C during deposition. The effect of T_s on the growth and microstructure of deposited HfO₂ films has been studied using grazing incidence X-ray diffraction (GIXRD), and high-resolution scanning electron microscopy (HR-SEM) coupled with energy dispersive X-ray spectrometry (EDS). The results indicate that the effect of T_s is significant on the growth, surface and interface structure, morphology and chemical composition of the HfO₂ films. Structural characterization indicates that the HfO₂ films grown at T_s < 200 °C are amorphous while films grown at T_s > 200 °C are nanocrystalline. An amorphous-to-crystalline transition occurs at T_s = 200 °C. Nanocrystalline HfO₂ films crystallized in a monoclinic structure with a (−1 1 1) orientation. An interface layer (IL) formation occurs due to reaction at the HfO₂-Si interface for HfO₂ films deposited at T_s > 200 °C. The thickness of IL increases with increasing T_s. EDS at the HfO₂-Si cross-section indicate that the IL is a (Hf, Si)-O compound. The electrical characterization using capacitance-voltage measurements indicate that the dielectric constant decreases from 25 to 16 with increasing T_s. The current-voltage characteristics indicate that the leakage current increases significantly with increasing T_s due to increased ILs.     

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.     

Influence of ZrO2 in HfO2 on reflectance of HfO2/SiO2 multilayer at 248 nm prepared by electron-beam evaporation

Influence of ZrO₂ in HfO₂ on the reflectance of HfO₂/SiO₂ multilayer at 248 nm was investigated. Two kinds of HfO₂ with different ZrO₂ content were chosen as high refractive index material and the same kind of SiO₂ as low refractive index material to prepare the mirrors by electron-beam evaporation. The impurities in two kinds of HfO₂ starting coating materials and in their corresponding single layer thin films were determined through glow discharge mass spectrum (GDMS) technology and secondary ion mass spectrometry (SIMS) equipment, respectively. It showed that between the two kinds of HfO₂, either the bulk materials or their corresponding films, the difference of ZrO₂ was much larger than that of the other impurities such as Ti and Fe. It is the Zr element that affects the property of thin films. Both in theoretical and in experimental, the mirror prepared with the HfO₂ starting material containing more Zr content has a lower reflectance. Because the extinction coefficient of zirconia is relatively high in UV region, it can be treated as one kind of absorbing defects to influence the optical property of the mirrors.     

Investigation of thermal annealing effects on microstructural and optical properties of HfO2 thin films

In the present paper, we investigate the effect of thermal annealing on optical and microstructural properties of HfO₂ thin films (from 20 to 190 nm) obtained by plasma ion assisted deposition (PIAD). After deposition, the HfO₂ films were annealed in N₂ ambient for 3 h at 300, 350, 450, 500 and 750 °C. Several characterisation techniques including X-ray reflectometry (XRR), X-ray diffraction (XRD), spectroscopic ellipsometry (SE), UV Raman and FTIR were used for the physical characterisation of the as-deposited and annealed HfO₂ thin films. The results indicate that as-deposited PIAD HfO₂ films are mainly amorphous and a transition to a crystalline phase occurs at a temperature higher than 450 °C depending on the layer thickness. The crystalline grains consist of cubic and monoclinic phases already classified in literature but this work provides the first evidence of amorphous-cubic phase transition at a temperature as low as 500 °C. According to SE, XRR and FTIR results, an increase in the interfacial layer thickness can be observed only for high temperature annealing. The SE results show that the amorphous phase of HfO₂ (in 20 nm thick samples) has an optical bandgap of 5.51 eV. Following its transition to a crystalline phase upon annealing at 750 °C, the optical bandgap increases to 5.85 eV.     

Thermal stability of HfO2 nanotube arrays

Thermal stability of highly ordered hafnium oxide (HfO₂) nanotube arrays prepared through an electrochemical anodization method in the presence of ammonium fluoride is investigated in a temperature range of room temperature to 900 °C in flowing argon atmosphere. The formation of the HfO₂ nanotube arrays was monitored by current density transient characteristics during anodization of hafnium metal foil. Morphologies of the as-grown and post-annealed HfO₂ nanotube arrays were analyzed by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Although monoclinic HfO₂ is thermally stable up to 2000 K in bulk, the morphology of HfO₂ nanotube arrays degraded at 900 °C. A detailed X-ray photoelectron spectroscopy (XPS) study revealed that the thermal treatment significantly impacted the composition and the chemical environment of the core elements (Hf and O), as well as F content coming from the electrolyte. Possible reasons for the degradation of the nanotube at high temperature were discussed based on XPS study and possible future improvements have also been suggested. Moreover, dielectric measurements were carried out on both the as-grown amorphous film and 500 °C post-annealed crystalline film. This study will help us to understand the temperature impact on the morphology of nanotube arrays, which is important to its further applications at elevated temperatures.     

The effects of plasma treatment on the thermal stability of HfO2 thin films

The thermal stability of pure HfO₂ thin films is not high enough to withstand thermal processes, such as S/D activation or post-metal annealing, in modern industrial CMOS production. In addition, plasma nitridation technology has been employed for increasing the dielectric constant of silicon dioxide and preventing boron penetration. In this experiment, atomic layer deposition (ALD) technology was used to deposit HfO₂ thin films and inductively coupled plasma (ICP) technology was used to perform plasma nitridation process. The C-V and J-V characteristics of the nitrided samples were observed to estimate the effect of the nitridation process. According to this study, plasma nitridation process would be an effective method to improve the thermal stability of HfO₂ thin films.     

具有HfN/HfO2栅结构的p型MOSFET中的负偏置-温度不稳定性研究

研究了HfN/HfO₂高K栅结构p型金属-氧化物-半导体(MOS)晶体管(MOSFET)中，负 偏置-温度应力引起的阈值电压不稳定性(NBTI)特征.HfN/HfO₂高K栅结构的等效 氧化层厚度(EOT)为1.3nm，内含原生缺陷密度较低.研究表明，由于所制备的HfN/HfO₂ 高K栅结构具有低的原生缺陷密度，因此在p-MOSFET器件中观察到的NBTI属HfN/HfO2高K栅结构的本征特征，而非工艺缺陷引起的；进一步研究表明，该HfN/HfO₂高K栅结构中观察到的NBTI与传统的SiO₂基栅介质p-MOSFET器件中观察 到的NBTI具有类似的特征，可以被所谓的反应-扩散(R-D)模型表征： HfN/HfO₂ 栅结构p-MOSFET器件的NBTI效应的起源可以归为衬底注入空穴诱导的界面反应机理，即在负 偏置和温度应力作用下，从Si衬底注入的空穴诱导了Si衬底界面Si-H键断裂这一化学反应的 发生，并由此产生了Si^＋陷阱在Si衬底界面的积累和H原子在介质层内部的扩散 ，这种Si^＋陷阱的界面积累和H原子的扩散导致了器件NBTI效应的发生. 关键词： 高K栅介质 负偏置-温度不稳定性(NBTI) 反应-扩散(R-D)模型     

Optical and electrical properties of plasma-oxidation derived HfO2 gate dielectric films

High-k gate dielectric HfO₂ thin films have been deposited on Si(1 0 0) by using plasma oxidation of sputtered metallic Hf thin films. The optical and electrical properties in relation to postdeposition annealing temperatures are investigated by spectroscopic ellipsometry (SE) and capacitance-voltage (C-V) characteristics in detail. X-ray diffraction (XRD) measurement shows that the as-deposited HfO₂ films are basically amorphous. Based on a parameterized Tauc-Lorentz dispersion mode, excellent agreement has been found between the experimental and the simulated spectra, and the optical constants of the as-deposited and annealed films related to the annealing temperature are systematically extracted. Increases in the refractive index n and extinction coefficient k, with increasing annealing temperature are observed due to the formation of more closely packed thin films and the enhancement of scattering effect in the targeted HfO₂ film. Change of the complex dielectric function and reduction of optical band gap with an increase in annealing temperature are discussed. The extracted direct band gap related to the structure varies from 5.77, 5.65, and 5.56 eV for the as-deposited and annealed thin films at 700 and 800 °C, respectively. It has been found from the C-V measurement the decrease of accumulation capacitance values upon annealing, which can be contributed to the growth of the interfacial layer with lower dielectric constant upon postannealing. The flat-band voltage shifts negatively due to positive charge generated during postannealing.     

Thickness-dependent tunable characteristics of (Ba0.5Sr0.5)0.925K0.075TiO3 thin films prepared by pulsed laser deposition

The thickness-dependent dielectric properties and tunability of pulsed laser deposited (Ba_0.5Sr_0.5)_0.925K_0.075TiO₃ (BSKT) thin films with different thickness ranging from 80 to 300 nm has been investigated. Dielectric properties of the BSKT thin films are substantially improved as the BSKT film thickness increases, which can be explained by the model of a low-permittivity dead layer that is connected in series with the bulk region of the film. The estimated values of thickness and the average dielectric constant for the dead layer are 2.4 nm and 23.5, respectively, in a Pt/BSKT/Pt capacitor structure. The tunability and figure of merit increased with increasing film thickness, which are attributed to the change in lattice parameter and the dead layer effect.     

Effect of modification of S-terminated Ge(1 0 0) surface on ALD HfO2 gate stack

When S-termination on a Ge(1 0 0) surface was desorbed at an elevated temperature and an atomic layer deposition (ALD) HfO₂ film was deposited, interfacial thickness was less than 1 nm. As a result, the equivalent oxide thickness (EOT) of the stack on the initially S-terminated surface was thinner than that deposited on the O₃-oxidized surface, while HfO₂ film thickness was almost identical on both surfaces. Nevertheless, the HfO₂ stack on the initially S-terminated surface exhibited improved leakage current characteristics due to an increase in barrier height. Its thinner but robust interface will contribute to the scaling down of gate oxide integrity.     

Gd-doping of HfO2

An increase in the density of states between the oxygen 2p bands and the Fermi level is seen with increasing Gd concentrations. In addition, for the Gd-doped HfO₂ films, the Gd 4f photoexcitation peak at 5.5 eV below the valence band maximum was identified using resonant photoemission. Electrical measurements show pronounced rectification properties for lightly-doped Gd:HfO₂ films on p-Si and for heavily-doped Gd:HfO₂ films on n-Si, suggesting a crossover from n-type to p-type behavior with increasing doping level. In addition, there is an increase in the reverse bias current with neutron irradiation.     

Characteristics improvement of HfO2/Ge gate stack structure by fluorine treatment of germanium surface

Chemical reactivity of fluorine molecule (F₂)-germanium (Ge) surface and dissociation of fluorine (F)-Ge bonding have been simulated by semi-empirical molecular orbital method theoretically, which shows that F on Ge surface is more stable compared to hydrogen. Ge MIS (metal insulator semiconductor) capacitor has been fabricated by using F₂-treated Ge(1 0 0) substrate and HfO₂ film deposited by photo-assisted MOCVD. Interface state density observed as a hump in the C-V curve of HfO₂/Ge gate stack and its C-V hysteresis were decreased by F₂-treatment of Ge surface. XPS (X-ray photoelectron spectroscopy) depth profiling reveals that interfacial layer between HfO₂ and Ge is sub-oxide layer (GeO_x or HfGeO_x), which is believed to be origin of interface state density.F was incorporated into interfacial layer easily by using F₂-treated Ge substrate. These results suggest that interface defect of HfO₂/Ge gate stack structure could be passivated by F effectively.