The effect of Si surface nitridation on the interfacial structure and electrical properties of (La2O3)0.5(SiO2)0.5 high-k gate dielectric films

Thermal stability, interfacial structures and electrical properties of amorphous (La₂O₃)_0.5(SiO₂)_0.5 (LSO) films deposited by using pulsed laser deposition (PLD) on Si (1 0 0) and NH₃ nitrided Si (1 0 0) substrates were comparatively investigated. The LSO films keep the amorphous state up to a high annealing temperature of 900 °C. HRTEM observations and XPS analyses showed that the surface nitridation of silicon wafer using NH₃ can result in the formation of the passivation layer, which effectively suppresses the excessive growth of the interfacial layer between LSO film and silicon wafer after high-temperature annealing process. The Pt/LSO/nitrided Si capacitors annealed at high temperature exhibit smaller CET and EOT, a less flatband voltage shift, a negligible hysteresis loop, a smaller equivalent dielectric charge density, and a much lower gate leakage current density as compared with that of the Pt/LSO/Si capacitors without Si surface nitridation.     

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.     

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.     

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.     

Composition of Ta2O5 stacked films on N2O- and NH3-nitrided Si

The composition and microstructure of rf sputtered 20 nm Ta₂O₅ on N₂O or NH₃ Rapid Thermal Nitrided (RTN) Si substrates have been investigated by X-ray photoelectron spectroscopy. RTN at 800 and 850 °C is effective to suppress active oxidation of Si. There is no evidence for the presence of SiO₂ at Si interface. A lightly nitrided surface is established in both cases without a formation of detectable oxynitride layer at Si. A layered nature of the films is observed, with stoichiometric tantalum pentoxide at and close to the films’ surface. In the depth, the films are mixed ones whose composition depends on the nitridation ambient. N₂O treatment stimulates oxidation processes during the film deposition while NH₃ nitridation results to a less effective oxidation and produces Ta-silicate like film. The correlation between the composition of the interfacial regions and the nitridation gas is also discussed. The results suggest that hydrogen, as a component of nitridation ambient, plays significant role in the reactions controlling the exact composition of the deposited Ta₂O₅, activating reactions with nitrogen. Nitrogen related reactions likely occur with NH₃ processing but do not with N₂O one. The presence of nitrogen feature is not detected in N₂O-samples spectra at all. In the integration perspective, preliminary RTN of Si in N₂O or NH₃ could be a suitable way to produce layered Ta₂O₅-based films with more or less presence of tantalum silicate with a trace of nitrogen, either only at the interface with Si (N₂O-process) or in the whole film (NH₃-process).     

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.     

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.     

Evidence of GeO volatilization and its effect on the characteristics of HfO2 grown on Ge substrate

HfO₂ films are deposited by atomic layer deposition (ALD) using tetrakis ethylmethylamino hafnium (TEMAH) as the hafnium precursor, while O₃ or H₂O is used as the oxygen precursor. After annealing at 500℃ in nitrogen, the thickness of Ge oxide's interfacial layer decreases, and the presence of GeO is observed at the H₂O-based HfO₂ interface due to GeO volatilization, while it is not observed for the O₃-based HfO₂. The difference is attributed to the residue hydroxyl groups or H₂O molecules in H₂O-based HfO₂ hydrolyzing GeO₂ and forming GeO, whereas GeO is only formed by the typical reaction mechanism between GeO₂ and the Ge substrate for O₃-based HfO₂ after annealing. The volatilization of GeO deteriorates the characteristics of the high-κ films after annealing, which has effects on the variation of valence band offset and the C–V characteristics of HfO₂/Ge after annealing. The results are confirmed by X-ray photoelectron spectroscopy (XPS) and electrical measurements.     

Structural and optical properties of nitrogen-incorporated HfO2 gate dielectrics deposited by reactive sputtering

High-k HfO_xN_y thin films with different nitrogen-incorporation content have been fabricated on Si (1 0 0) substrate by means of radio-frequency reactive sputtering method. Analyses from X-ray diffraction (XRD) and atomic force microscopic have indicated that the increase of the crystallization temperature of HfO₂ thin films and the decrease of the roughness root-mean-square value of HfO₂ thin films due to the incorporation of nitrogen. Based on a parameterized Tauc-Lorentz (TL) dispersion model, the optical properties of the HfO_xN_y thin films related to different nitrogen-incorporation content are systematically investigated by spectroscopic ellipsometer. Increase in the refractive index and the extinction coefficient and reduction in band gap with increase of nitrogen-incorporation content are discussed in detail.     

Influence of postdeposition annealing on structural properties and electrical characteristics of thin Tm2O3 and Tm2Ti2O7 dielectrics

We describe the structural properties and electrical characteristics of thin thulium oxide (Tm₂O₃) and thulium titanium oxide (Tm₂Ti₂O₇) as gate dielectrics deposited on silicon substrates through reactive sputtering. The structural and morphological features of these films were explored by X-ray diffraction, X-ray photoelectron spectroscopy, secondary ion mass spectrometry, and atomic force microscopy, measurements. It is found that the Tm₂Ti₂O₇ film annealed at 800 °C exhibited a thinner capacitance equivalent thickness of 19.8 Å, a lower interface trap density of 8.37 × 10¹¹ eV⁻¹ cm⁻², and a smaller hysteresis voltage of ∼4 mV than the other conditions. We attribute this behavior to the Ti incorporated into the Tm₂O₃ film improving the interfacial layer and the surface roughness. This film also shows negligible degrees of charge trapping at high electric field stress.     

Stabilization of a very high-k crystalline ZrO2 phase by post deposition annealing of atomic layer deposited ZrO2/La2O3 dielectrics on germanium

The impact of the ZrO₂/La₂O₃ film thickness ratio and the post deposition annealing in the temperature range between 400 °C and 600 °C on the electrical properties of ultrathin ZrO₂/La₂O₃ high-k dielectrics grown by atomic layer deposition on (1 0 0) germanium is investigated. As-deposited stacks have a relative dielectric constant of 24 which is increased to a value of 35 after annealing at 500 °C due to the stabilization of tetragonal/cubic ZrO₂ phases. This effect depends on the absolute thickness of ZrO₂ within the dielectric stack and is limited due to possible interfacial reactions at the oxide/Ge interface. We show that adequate processing leads to very high-k dielectrics with EOT values below 1 nm, leakage current densities in the range of 0.01 A/cm², and interface trap densities in the range of 2-5 × 10¹² eV⁻¹ cm⁻².     

The effect of PVA (Bi2O3-doped) interfacial layer and series resistance on electrical characteristics of Au/n-Si (110) Schottky barrier diodes (SBDs)

Two types of Schottky Barrier Diodes (SBDs) with and without PVA (Bi₂O₃-doped) polymeric interfacial layer, were fabricated and measured at room temperature in order to investigate the effects of the PVA (Bi₂O₃-doped) interfacial layer on the main electrical parameters such as the ideality factor (n), zero-bias barrier height (Φ_B0), series resistance (R_s) and interface-state density (N_ss). Electrical parameters of these two diodes were calculated from the current-voltage (I-V) characteristics and compared with each other. The values of Φ_B0, n and R_s for SBDs without polymeric interfacial layer are 0.71 eV, 1.44 and 4775 Ω, respectively. The values of Φ_B0, n and R_s for SBDs with PVA (Bi₂O₃-doped) polymeric interfacial layer are 0.74 eV, 3.49 and 10,030 Ω, respectively. For two SBDs, the energy density distribution profiles of interface states (N_ss) were obtained from forward-bias I-V measurements by taking the bias dependence of R_s of these devices into account. The values of N_ss obtained for the SBD with PVA (Bi₂O₃-doped) polymeric interfacial layer are smaller than those of the SBD without polymeric 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.     

Structural and dielectric properties of Ru-based gate/Hf-doped Ta2O5 stacks

Hf-doped Ta₂O₅ thin films are studied with respect to their composition, dielectric and electrical properties. The incorporation of Hf is performed by sputtering of a 0.7 nm thick Hf layer on top of Ta₂O₅ and subsequent annealing to stimulate diffusion of Hf into Ta₂O₅ and their intermixing. The elemental in-depth distribution of the films is investigated by the time of flight secondary ion mass spectroscopy (ToF-SIMS), which has revealed that Hf and Ta₂O₅ are intermixed throughout the whole thickness. Two sub-layers exist in all the samples - an upper homogeneous Hf-doped Ta₂O₅ sub-layer and a near interfacial region which is a mixture of Ta- and Si-oxides. The X-ray reflectivity (XRR) analysis shows existence of interfacial layer with a thickness of about 1.9-2 nm, irrespectively of the total thickness of the stacks. Metal-oxide-Si structures with Ru and RuO₂ metal electrodes have been prepared and investigated in terms of dielectric constant, effective work function (EWF) and interfacial layer parameters. The influence of post-metallization annealing steps on these parameters was also studied.     

The improvement of Al2O3/AlGaN/GaN MISHEMT performance by N2 plasma pretreatment

This paper discusses the effect of N 2 plasma treatment before dielectric deposition on the electrical performance of a Al2O3 /AlGaN/GaN metal-insulator-semiconductor high electron mobility transistor(MISHEMT),with Al2O3 deposited by atomic layer deposition.The results indicated that the gate leakage was decreased two orders of magnitude after the Al2O3 /AlGaN interface was pretreated by N 2 plasma.Furthermore,effects of N 2 plasma pretreatment on the electrical properties of the AlGaN/Al2O3 interface were investigated by x-ray photoelectron spectroscopy measurements and the interface quality between Al2O3 and AlGaN film was improved.     

In situ X-ray photoelectron spectroscopy characterization of Al2O3/GaSb interface evolution

GaSb(0 0 1) was treated with (NH₄)₂S_x and the evolution of the interfacial chemistry was investigated, in situ, with monochromatic X-ray photoelectron spectroscopy (XPS), following heat treatment and exposure to trimethylaluminum (TMA) and deionized water (DIW) in an atomic layer deposition reactor. Elemental Sb (Sb-Sb bonding) as well as Sb³⁺ and Sb⁵⁺ chemical states were initially observed at the native oxide/GaSb interface, yet these diminished below the XPS detection limit after heating to 300 °C. No evidence of Ga-Ga bonding was observed whereas the Ga¹⁺/Ga-S chemical state was robust and persisted after heat treatment and exposure to TMA/DIW at 300 °C.     

Characterization of Y2O3 gate dielectric on n-GaAs substrates

Physical and electrical properties of sputtered deposited Y₂O₃ films on NH₄OH treated n-GaAs substrate are investigated. The as-deposited films and interfacial layer formation have been analyzed by using X-ray photoelectron spectroscopy (XPS) and secondary ion mass spectroscopy (SIMS). It is found that directly deposited Y₂O₃ on n-GaAs exhibits excellent electrical properties with low frequency dispersion (<5%), hysteresis voltage (0.24 V), and interface trap density (3 × 10¹² eV⁻¹ cm⁻²). The results show that the deposition of Y₂O₃ on n-GaAs can be an effective way to improve the interface quality by the suppression on native oxides formation, especially arsenic oxide which causes Fermi level pinning at high-k/GaAs interface. The Al/Y₂O₃/n-GaAs stack with an equivalent oxide thickness (EOT) of 2.1 nm shows a leakage current density of 3.6 × 10⁻⁶ A cm⁻² at a V_FB of 1 V. While the low-field leakage current conduction mechanism has been found to be dominated by the Schottky emission, Poole-Frenkel emission takes over at high electric fields. The energy band alignment of Y₂O₃ films on n-GaAs substrate is extracted from detailed XPS measurements. The valence and conduction band offsets at Y₂O₃/n-GaAs interfaces are found to be 2.14 and 2.21 eV, respectively.     

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.     

Metal carbide-induced negative flatband voltage shift in TaCx and HfCx/HfO2 gate stacks

We systematically investigated the role of the top interface for TaC_x and HfC_x/HfO₂ gate stacks on the effective work function (Φ_m,eff) shift by inserting a SiN layer at the gate/HfO₂ top interface or HfO₂/SiO₂ bottom interface. We found that Φ_m,eff of the TaN gate electrode on HfO₂ was larger than that on SiO₂ because of the HfO₂/SiO₂-bottom-interface dipole. On the other hand, we found that Φ_m,eff values of the TaC_x and HfC_x gate electrodes on HfO₂ agree with Φ_m,eff on SiO₂. This is because the potential offset of the opposite direction with respect to the bottom interface dipole appears at the metal carbide/HfO₂ interface. It is thus concluded that the top interface in the metal carbide/HfO₂ gate stacks causes the negative Φ_m,eff shift.     

Stability of La2O3 and GeO2 passivated Ge surfaces during ALD of ZrO2 high-k dielectric

La₂O₃ grown by atomic layer deposition (ALD) and thermally grown GeO₂ are used to establish effective electrical surface passivations on n-type (1 0 0)-Ge substrates for high-k ZrO₂ dielectrics, grown by ALD at 250 °C substrate temperature. The electrical characterization of MOS capacitors indicates an impact of the Ge-surface passivation on the interfacial trap density and the frequency dependent capacitance in the inversion regime. Lower interface trap densities can be obtained for GeO₂ based passivation even though a chemical decomposition of the oxidation states occur during the ALD of ZrO₂. As a consequence the formation of a ZrGeO_x compound inside the ZrO₂ matrix and a decline of the interfacial GeO₂ are observed. The La₂O₃ passivation provides a stable amorphous lanthanum germanate phase at the Ge interface but also traces of Zr germanate are indicated by X-ray-Photoelectron-Spectroscopy and Transmission-Electron-Microscopy.