Microstructure and dielectric properties of La2O3 doped amorphous SiO2 films as gate dielectric material

As a potential gate dielectric material, the La₂O₃ doped SiO₂ (LSO, the mole ratio is about 1:5) films were fabricated on n-Si (0 0 1) substrates by using pulsed laser deposition technique. By virtue of several measurements, the microstructure and electrical properties of the LSO films were characterized. The LSO films keep the amorphous state up to a high annealing temperature of 800 °C. From HRTEM and XPS results, these La atoms of the LSO films do not react with silicon substrate to form any La-compound at interfacial layer. However, these O atoms of the LSO films diffuse from the film toward the silicon substrate so as to form a SiO₂ interfacial layer. The thickness of SiO₂ layer is only about two atomic layers. A possible explanation for interfacial reaction has been proposed. The scanning electron microscope image shows the surface of the amorphous LSO film very flat. The LSO film shows a dielectric constant of 12.8 at 1 MHz. For the LSO film with thickness of 3 nm, a small equivalent oxide thickness of 1.2 nm is obtained. The leakage current density of the LSO film is 1.54 × 10⁻⁴ A/cm² at a gate bias voltage of 1 V.     

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.     

Infrared optical properties of Ba(Zr0.20Ti0.80)O3 and Ba(Zr0.30Ti0.70)O3 thin films prepared by sol-gel method

Ba(Zr_xTi_1−x)O₃ (BZT) (x = 0.20 and 0.30) thin films are deposited on Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrate by sol-gel method. X-ray diffraction patterns show that the thin films have a good crystallinity. Optical properties of the films in the wavelength range of 2.5-12 μm are studied by infrared spectroscopic ellipsometry (IRSE). The optical constants of the BZT thin films are determined by fitting the IRSE data using a classical dispersion formula. As the wavelength increases, the refractive index decreases, while the extinction coefficients increase. The effective static ionic charges are derived, which are smaller than that in a purely ionic material for the BZT thin films.     

Band bending measurement of HfO2/SiO2/Si capacitor with ultra-thin La2O3 insertion by XPS

The flat band voltage shifts of HfO₂/SiO₂/nSi capacitors with ultra-thin La₂O₃ insertion at HfO₂/SiO₂ interface have been confirmed using hard X-ray photoelectron spectroscopy (HX-PES). By increasing the amount of La₂O₃ insertion, the binding energy of Si 1s core spectra increases, which means that the surface potential of Si substrate also increases. A voltage drop difference of HfO₂ and La₂O₃ at SiO₂ interface can be estimated to be 0.40 V.     

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.     

Chemical vapor deposition of ZrxTi1−xO2 and HfxTi1−xO2 thin films using the composite anhydrous nitrate precursors

Zr-Ti and Hf-Ti composite nitrates were successfully developed as single-source precursors for the chemical vapor deposition (CVD) of Zr_xTi_1−xO₂ and Hf_xTi_1−xO₂ thin films. The Zr-Ti nitrate can be assumed as a solid solution of the individual Zr and Ti nitrates, and the Zr/Ti molar ratio in the deposited Zr_xTi_1−xO₂ films is consistent with that in the precursor. The Hf-Ti nitrate appears to be a mixture of the Hf and Ti nitrates and the composition of the deposited Hf_xTi_1−xO₂ films depends remarkably on the heating time of precursor. Both Zr_xTi_1−xO₂ and Hf_xTi_1−xO₂ films exhibit trade-off properties between band gap and dielectric constant. The obtained results suggest that Zr_xTi_1−xO₂ and Hf_xTi_1−xO₂ films are promising candidates for gate dielectric application to improve the scalability and reduce the leakage current of the future complementary metal-oxide-semiconductor (CMOS) devices.     

Microstructural and optical investigations of sol-gel derived ferroelectric BaTiO3 nanocrystalline films determined by spectroscopic ellipsometry

Ferroelectric BaTiO₃ nanocrystalline films (BTNFs) with the crystalline sizes of about 30 nm were grown on Pt/Ti/SiO₂/Si substrates by a modified sol-gel method. Spectroscopic ellipsometry (SE) was used to characterize the films in the photon energy range of 1.5-5.0 eV with a five-phase layered model (air/surface rough layer/BaTiO₃/interface layer/Pt). The optical properties in the transparent and absorption regions have been investigated with the Forouhi-Bloomer dispersion relation. With the aid of the structural and dielectric function models, the microstructure and electronic structure of the BTNFs can be readily obtained. It was found that the refractive index reaches the value of 2.20 in the transparent region. Based on the Sellmeier dispersion analysis, the single-oscillator energy is about 4.7 eV for the BTNFs. The long wavelength refractive index n(0) can be estimated to about 2.00 at zero point. The direct optical band gap energy approaches approximately 4.2 eV and Urbach band tail energy is 262±2 and 268±1 meV respectively with increasing crystalline size. A higher band gap observed can be owing to the known quantum confinement effect in the nanocrystalline formation and different fraction of amorphous and crystalline components. The theoretical analysis based on the effective mass approximation theory is well used to explain these experimental data.     

Reduced dielectric loss and leakage current in CaCu3Ti4O12/SiO2/CaCu3Ti4O12 multilayered films

The CaCu₃Ti₄O₁₂/SiO₂/CaCu₃Ti₄O₁₂ (CCTO/SiO₂/CCTO) multilayered films were prepared on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition method. It has been demonstrated that the dielectric loss and the leakage current density were significantly reduced with the increase of the SiO₂ layer thickness, accompanied with a decrease of the dielectric constant. The CCTO film with a 20 nm SiO₂ layer showed a dielectric loss of 0.065 at 100 kHz and the leakage current density of 6×10⁻⁷ A/cm² at 100 kV/cm, which were much lower than those of the single layer CCTO films. The improvement of the electric properties is ascribed to two reasons: one is the improved crystallinity; the other is the reduced free carriers in the multilayered films.     

Structural, optical properties and band gap alignments of ZrOxNy thin films on Si (1 0 0) by radio frequency sputtering at different deposition temperatures

ZrO_xN_y thin films have been prepared by radio frequency magnetron sputtering at various substrate temperatures. The effect of substrate temperature on structural, optical properties and energy-band alignments of as-deposited ZrO_xN_y thin films are investigated. Atomic force microscopy results indicate the decreased root-mean-square (rms) values with substrate temperature. Fourier transform infrared spectroscopy spectra indicate that an interfacial layer has been formed between Si substrate and ZrO_xN_y thin films during deposition. X-ray photoelectron spectroscopy and spectroscopy ellipsometry (SE) results indicate the increased nitrogen incorporation in ZrO_xN_y thin films and therefore, the decreased optical band gap (E_g) values as a result of the increased valence-band maximum and lowered conduction-band minimum.     

Trap-controlled behavior in ultrathin Lu2O3 high-k gate dielectrics

Amorphous Lu₂O₃ high-k gate dielectrics were grown directly on n-type (100) Si substrates by the pulsed laser deposition (PLD) technique. High-resolution transmission electron microscope (HRTEM) observation illustrated that the Lu₂O₃ film has amorphous structure and the interface with Si substrate is free from amorphous SiO₂. An equivalent oxide thickness (EOT) of 1.1 nm with a leakage current density of 2.6×10⁻⁵ A/cm² at 1 V accumulation bias was obtained for 4.5 nm thick Lu₂O₃ thin film deposited at room temperature followed by post-deposition anneal (PDA) at 600 °C in oxygen ambient. The effects of PDA process and light illumination were studied by capacitance-voltage (C-V) and current density-voltage (J-V) measurements. It was proposed that the net fixed charge density and leakage current density could be altered significantly depending on the post-annealing conditions and the capability of traps to trap and release charges.     

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).     

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.     

Modelling and Optimization for Deposition of SiOxNy Films by Radio-Frequency Reactive Sputtering

SiOxNy films are deposited by reactive sputtering from a Si target in Ar/O2/N2 atmospheres. In order to achieve the control of film composition and to keep a high deposition rate at the same time, a new sputtering model based on Berg＇s work is provided for the condition of double reactive gases. Analysis based on this model shows that the deposition process can easily enter the target-poisoning mode when the preset gas flow （N2 in this work） is too high, and the film composition will change from nitrogen-rich to SiO2-like with the increase of oxygen supply while keeping the N2 supply constant. The modelling results are confirmed in the deposition process of SiOxNy. Target self-bias voltages during sputtering are measured to characterize the different sputtering modes. FTIR-spectra and dielectric measurements are used to testify the model prediction of composition. Finally, an optimized sputtering condition is selected with the O2/N2 flow ratio varying from 0 to I and N2 supply fixed at I sccm. Average deposition rate of 17nm/min is obtained under this selected condition, which has suggested the model validity and potential for industry applications.     

Field electron emission from HfNxOy thin films deposited by direct current sputtering

HfN_xO_y thin films were deposited on Si substrates by direct current sputtering at room temperature. The samples were characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). SEM indicates that the film is composed of nanoparticles. AFM indicates that there are no sharp protrusions on the surface of the film. XRD pattern shows that the films are amorphous. The field electron emission properties of the film were also characterized. The turn-on electric field is about 14 V/μm at the current density of 10 μA/cm², and at the electric field of 24 V/μm, the current density is up to 1 mA/cm². The field electron emission mechanism of the HfN_xO_y thin film is also discussed.     

Praseodymium silicide formation at the Pr2O3/Si interface

The interface and layer structure of praseodymium (Pr) oxide layers grown on Si(0 0 1) from a high-temperature effusion cell are studied using grazing incidence X-ray diffraction. Due to the interdiffusion of praseodymium and silicon atoms, Pr silicide forms in the layers. We find that Pr silicide is the favorable structure under oxygen deficient growth conditions in the Pr oxide layer. To avoid the silicidation, additional oxygen must be supplied. The formation of Pr silicide is suppressed for layers grown with an oxygen partial pressure of 10⁻⁷ mbar at a substrate temperature of 700 °C.     

Electrical behavior of BaZr0.1Ti0.9O3 and BaZr0.2Ti0.8O3 thin films

BaZr_0.1Ti_0.9O₃ and BaZr_0.2Ti_0.8O₃ (BZT) thin films were deposited on Pt/Ti/LaAlO₃ (1 0 0) substrates by radio-frequency magnetron sputtering, respectively. The films were further annealed at 800 °C for 30 min in oxygen. X-ray diffraction θ-2θ and Φ-scans showed that BaZr_0.1Ti_0.9O₃ films displayed a highly (h 0 0) preferred orientation and a good cube-on-cube epitaxial growth on the LaAlO₃ (1 0 0) substrate, while there are no obvious preferential orientation in BaZr_0.2Ti_0.8O₃ thin films. The BaZr_0.1Ti_0.9O₃ films possess larger grain size, higher dielectric constant, larger tunability, larger remanent polarization and coercive electric field than that of BaZr_0.2Ti_0.8O₃ films. Whereas, BaZr_0.1Ti_0.9O₃ films have larger dielectric losses and leakage current density. The results suggest that Zr⁴⁺ ion can decrease dielectric constant and restrain non-linearity. Moreover, the enhancement in dielectric properties of BaZr_0.1Ti_0.9O₃ films may be attributed to (1 0 0) preferred orientation.     

Characterization of HfOxNy gate dielectrics using a hafnium oxide as target

Hafnium oxynitride (HfO_xN_y) gate dielectric has been deposited on Si (1 0 0) by means of radio frequency (rf) reactive sputtering using directly a HfO₂ target in N₂/Ar ambient. The thermal stability and microstructural characteristics for the HfO_xN_y films have been investigated. XPS results confirmed that nitrogen was successfully incorporated into the HfO₂ films. XRD analyses showed that the HfO_xN_y films remain amorphous after 800 °C annealing in N₂ ambient. Meanwhile the HfO_xN_y films can also effectively suppress oxygen diffusion during high temperature annealing and prevent interface layer from forming between HfO_xN_y films and Si substrates. AFM measurements demonstrated that surface roughness of the HfO_xN_y films increase slightly as compared to those pure HfO₂ films after post deposition annealing. By virtue of building reasonable model structure, the optical properties of the HfO_xN_y films have been discussed in detail.     

Analysis of codeposited Gd2O3/SiO2 composite thin films by phase modulated spectroscopic ellipsometric technique

Tailoring of the refractive index of optical thin films has been a very fascinating as well as challenging topic for developing new generation optical coatings. In the present work a novel Gd₂O₃/SiO₂ composite system has been experimented and probed for its superior optical properties through phase modulated spectroscopic ellipsometry, spectrophotometry and atomic force microscopy. The optical parameters of the composite films have been evaluated using Tauc-Lorentz (TL) formulations. In order to derive the growth dependent refractive index profiles, each sample film has been modeled as an appropriate multilayer structure where each sub-layer was treated with the above TL parameterizations. All codeposited films demonstrated superiority with respect to the band gap and morphological measurements. At lower silica mixing compositions such as in 10-20% level, the composite films depicted superior spectral refractive index profile, band gap as well as the morphology. This aspect highlighted the fact that microstructural densifications in composite films can override the chemical compositions while deciding the refractive index and optical properties in such thin films.     

Influence of initial surface reconstruction on the interface structure of HfO2/GaAs

We show that the bonding structures and electrical properties of the HfO₂/GaAs interface can be controlled by a choice of the reconstruction on the initial GaAs surface. Electron-beam evaporation of HfO₂ onto the c(4 × 4) surface yielded As-O bonds at the interface, while Ga-O bonds were dominant at the interfaces formed on the (2 × 4) and (4 × 6) surfaces. Influences of the initial surface reconstruction on the interface structure persisted even after annealing at 673 K. Electrical characterization of Ir/HfO₂/GaAs capacitors indicated that the interfacial As-O bonds cause weak Fermi level pinning. It was also suggested that the interfaces dominated by the Ga-O bonds have trapping states in the upper half of the GaAs bandgap.     

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.