Characterisation and passivation of interface defects in (1 0 0)-Si/SiO2/HfO2/TiN gate stacks

The density and energy distribution of electrically active interface defects in the (1 0 0)Si/SiO₂/HfO₂ system are presented. Experimental results are analysed for HfO₂ thin films deposited by atomic layer deposition and metal-organic chemical vapour deposition on (1 0 0)Si substrates. The paper discusses the origin of the interface states, and their passivation in hydrogen over the temperature range 350–550 °C.     

Growth of epitaxial Pr2O3 layers on Si(1 1 1)

The growth of Pr₂O₃ layers on Si(1 1 1) has been studied by X-ray diffraction, Low-energy electron diffraction (LEED) and atomic force microscopy (AFM). Pr₂O₃ starts to grow as a 0.6-nm thick layer corresponding to one unit cell of the hexagonal phase (1 ML). The X-ray results indicate that layers thicker than 0.6 nm do not grow with the hexagonal phase. Growth takes place at a sample temperature of 500–550 °C. Annealing of the monolayer in UHV at a temperature above 700 °C leads to the formation of Pr₂O₃ and PrSi₂ islands. Silicide islands are found only at annealing in UHV and do not occur at annealing in oxygen atmosphere of 10⁻⁸ mbar. The LEED pattern after heating to 730 °C shows a (2×2) and (√3×√3) superstructure and after heating to 1000 °C a (1×5) superstructure occurs. The superstructures seen in the LEED pattern arise from silicide structures in the area between the islands. The silicide remains on the surface and cannot be removed with flashing to 1100 °C. Further deposition of Pr₂O₃ on the surface covered with silicide phases does not lead to growth of ordered layers.     

Detection of nitrogen incorporation in nm-thin HfO2 layers on (1 0 0)Si by electron spin resonance

We report on a low-temperature electron spin resonance (ESR) study of (1 0 0)Si/HfO₂ entities with ultrathin layers of amorphous (a)- HfO₂ deposited by distinct chemical vapor deposition (CVD) techniques using chemically different precursors. The incorporation of N is revealed in (1 0 0)Si/HfO₂ structures with ultrathin a-HfO₂ films deposited by CVD using Hf(NO₃)₄ as precursor: Upon ⁶⁰Co γ-irradiation, a prominent ESR powder pattern is observed, which via ESR measurements at two observational frequencies has been incontrovertibly identified as originating from NO₂ radicals (density 55 at ppm). The molecules are found to be stabilized and likely homogeneously distributed in the a-HfO₂ network. Based on symmetry considerations, it is suggested that during deposition, N is incorporated in the HfO₂ network as neutral N≡O₃ precursors, which are transformed into ESR-active NO₂ radicals upon γ-irradiation. The N incorporation appears inherent to the particular nitrado CVD process, an aspect that may bear on the electrical properties of the insulator, such as, e.g., introducing charge traps.     

Impact of nitrogen incorporation on interface states in (1 0 0)Si/HfO2

The energy distribution of (1 0 0)Si/HfO₂ interface states and their passivation by hydrogen are studied for different levels of nitrogen incorporation using different technological methods. The results are compared to those of N-free samples. The nitrogen in the (1 0 0)Si/HfO₂ entity is found to increase the trap density in the upper part of the Si band gap and to hinder the passivation of traps in molecular hydrogen in this energy range. At the same time, the passivation of fast interface traps in the lower part of the band gap proceeds efficiently, provided the thickness of the grown Si₃N₄ interlayer is kept minimal. However, the lowest achievable interface trap density below midgap is set by the presence of slow N-related states, likely related to traps in the insulator.     

Admittance spectroscopy of traps at the interfaces of (1 0 0)Si with Al2O3, ZrO2, and HfO2

Admittance (ac) measurements were carried out to determine the interface trap density (D_it) as a function of energy E in the Si bandgap at interfaces of Si with different insulating oxides (Al₂O₃, ZrO₂, HfO₂). The results are compared to those of the conventional thermal SiO₂/Si interface. The results show that a significant portion of the interface trap density in the as-deposited and de-hydrogenated samples is related to the amphoteric Si dangling bond defects (P_b0 -centers). The D_it is much enhanced for the Al-containing insulators as compared to Si/SiO₂ but can be reduced by annealing in O₂. As to annealing in H₂, efficient passivation of P_b0 centers by hydrogen is achieved for Si/ZrO₂ and Si/HfO₂ interfaces, yet it fails for Si/Al-containing oxide entities. Among the insulators studied, the results suggest HfO₂ to be the best choice of an alternative insulator.     

Observation and characterization of defects in HfO2 high-K gate dielectric layers

A deeper understanding of Hf-based high-K materials in terms of their structural and electrical defects is important for device implementation. We have studied the occurrence of such defects using wet-etch defect delineation, electron microscopy, depth-profiling and conventional electrical measurements. It is evident that defects are present in HfO₂ films that are related to the microstructure and stoichiometry of the film, which in turn depend on the deposition temperature, starting surface and post-deposition treatments. These results appear to be independent of the deposition technique. Two types of defects were observed, those that are physically visible and cause immediate failures especially on large-area structures, and those that cause high leakage but not immediate failures. The existence of defects affects not only leakage or performance but will also affect the reliability through trapping of charge at the defect sites. As films continue to be scaled thinner, the requirements on defect reduction to minimize electrical impact may become more stringent.     

Photoemission (XPS and XPD) study of epitaxial LaAlO3 film grown on SrTiO3(0 0 1)

This study investigates the tensile-strained growth of LaAlO₃ on SrTiO₃(0 0 1) substrate by molecular beam epitaxy (MBE). Growth was controlled in situ by reflection high energy electron diffraction (RHEED). The characterization was carried out ex situ by photoemission and atomic force microscopy (AFM). Photoelectron spectroscopy (XPS) reveals the development of a TiO_x-rich interface. Photoelectron diffraction (XPD) confirms that a 1.2-nm-thick pseudomorphic LaAlO₃ film has been grown on SrTiO₃(0 0 1) substrate with a perpendicular lattice parameter of 0.372±0.02 nm.     

Epitaxial growth of LaAlO3 on Si(0 0 1) using interface engineering

In this work, the potentiality of molecular beam epitaxy techniques to prepare epitaxial lanthanum aluminate (LaAlO₃) films on Si(0 0 1) is explored. We first demonstrate that the direct growth of LaAlO₃ on Si(0 0 1) is impossible : amorphous layers are obtained at temperatures below 600 °C whereas crystalline layers can be grown at higher temperatures but interfacial reactions leading to silicate formation occur. An interface engineering strategy is then developed to avoid these reactions. SrO and SrTiO₃ have been studied as buffer for the subsequent growth of LaAlO₃. Only partial LaAlO₃ epitaxy is obtained on SrO whereas high quality layers are achieved on SrTiO₃. However both SrO and SrTiO₃ appear to be unstable with respect of Si at the growth temperature of LaAlO₃ (700 °C). This leads to the formation of relatively thick amorphous interfacial layers. Despite their instability at high temperature, these processes could be used for the fabrication of twins-free LaAlO₃ templates on Si, and for the fabrication of complex oxide/Si heterostructures for various applications.     

Density functional theory study of first-layer adsorption of ZrO2 and HfO2 on Ge(1 0 0)

Density functional theory was used to performed a survey of transition metal oxide (MO₂ = ZrO₂, HfO₂) ordered molecular adsorbate bonding configurations on the Ge(1 0 0)-4 × 2 surface. Surface binding geometries of metal-down (O-M-Ge) and oxygen-down (M-O-Ge) were considered, including both adsorbate and displacement geometries of M-O-Ge. Calculated enthalpies of adsorption show that bonding geometries with metal-Ge bonds (O-M-Ge) are essentially degenerate with oxygen-Ge bonding (M-O-Ge). Calculated electronic structures indicate that adsorbate surface bonding geometries of the form O-M-Ge tend to create a metallic interfaces, while M-O-Ge geometries produce, in general, much more favorable electronic structures. Hydrogen passivation of both oxygen and metal dangling bonds was found to improve the electronic structure of both types of MO₂ adsorbate systems, and induced the opening of true semiconducting band gaps for the adsorbate-type M-O-Ge geometries. Shifts observed in the DOS minima for both O-M-Ge and M-O-Ge adsorbate geometries are consistent with surface band bending induced by the adsorbate films, where such band bending extends much further into the Ge substrate than can be modeled by the Ge slabs used in this work.     

Evidence that N2O is a stronger oxidizing agent than O2 for both Ta2O5 and bare Si below 1000 °C and temperature for minimum low-K interfacial oxide for high-K dielectric on Si

N₂O is known to be the stronger oxidizing agent than O₂ for the post-deposition annealing of Ta₂O₅·N₂O should also be stronger than O₂ for Si oxidation. However, NO released from N₂O is also a nitridation agent which can produce silicon oxynitride at a temperature above 1000 °C and silicon oxynitride can be a diffusion barrier for oxygen. Below 1000 °C, SiO sublimation can make the comparison of N₂O oxidation and O₂ oxidation of Si difficult. Below 750 °C, N₂O is obviously the faster oxidizing agent than O₂ for bare Si. Furthermore, our results show that minimum interfacial SiO_x, which has a low dielectric constant, occurs at about 800 °C or 950 °C for high-K metallic oxide gate insulator for future generations of CMOS because rapid thermal oxidation at these two temperatures can help to reduce leakage current or charge trapping by suppressing oxygen vacancies without too much low-K interfacial SiO_x formation.     

Structural and electrical properties of Er-doped HfO2 and of its interface with Ge (0 0 1)

Er-doped HfO₂ thin films with Er content ranging from 0% to 15% are deposited by atomic layer deposition on native oxide free Ge(001). The crystallographic phase is investigated by X-ray diffraction and is found to depend on the Er%. The cubic fluorite structure develops on Ge for Er% as low as 4% and is stable after annealing at 400 °C in N₂. Microstrain increases with increasing the Er content within the fluorite structure. Time of flight secondary ion mass and electron energy loss spectroscopy evidence a Ge diffusion from the substrate that results in the formation of a Ge-rich interfacial region which does not present a structural discontinuity with the oxide. The diffusion of Ge is enhanced by the annealing and causes a reordering of the crystal lattice. In annealed films the interface defect density measured by low temperature conductance measurements is found to decrease with decreasing the Er content.     

Growth of highly (1 1 1) oriented CuIn0.75Al0.25Se2 thin films

CuIn_0.75Al_0.25Se₂ thin films prepared onto glass substrates at T_S=573 K were single phase, nearly stoichiometric and polycrystalline with a strong (1 1 1) preferred orientation showing sphalerite structure. The results of X-ray diffraction and electron diffraction studies are compared, interpreted and correlated with micro-Raman spectra. The optical absorption studies indicated a direct band gap of 1.16 eV with high absorption coefficient (>10⁴ cm^?1) near the fundamental absorption edge.     

Impact of Al-, Ni-, TiN-, and Mo-metal gates on MOCVD-grown HfO2 and ZrO2 high-κ dielectrics

In this work we compare the impacts of nickel (Ni), titanium-nitride (TiN), molybdenum (Mo), and aluminium (Al), gates on MOS capacitors incorporating HfO₂- or ZrO₂-dielectrics. The primary focus lies on interface trapping, oxide charging, and thermodynamical stability during different annealing steps of these gate stacks. Whereas Ni, Mo, and especially TiN are investigated as most promising candidates for future CMOS devices, Al acted as reference gate material to benchmark the parameters. Post-metallization annealing of both, TiN- and Mo-stacks, resulted in very promising electrical characteristics. However, gate stacks annealed at temperatures of 800 °C or 950 °C show thermodynamic instability and related undesirable high leakage currents.     

Control of the formation of ultrathin CoSi<Subscript>2</Subscript> layers during the rapid thermal annealing of Ti/Co/Ti/Si(100) structures

The initial Ti(8 nm)/Co(10 nm)/Ti(5 nm) structures formed on the Si(100) substrate by magnetron sputtering were subjected to two-stage rapid thermal annealing (RTA) in the nitrogen ambient. The samples of the structures were controlled using the time-of-flight SIMS, the Auger spectroscopy, scanning electron microscopy, X-ray dispersion microprobe analysis, and measurements of the layer resistance at each stage of annealing. At the RTA-1 stage (550°C, 45 s), a sacrificial layer formed on the surface. This layer consisted of the titanium (oxy)nitride coating, into which the residual impurities (O, C, and N) were forced out, and the transient Co-Si-Ti(TiO,TiN) layer with a high cobalt content and a low (trace) titanium content. After the selective removal of this sacrificial layer, the surface composition corresponded to monosilicide CoSi, which transformed into the highly conductive CoSi₂ phase at the RTA-2 stage (830°C, 25 s).     

Fabrication and electrical characteristics of ultrathin (HfO2)x(SiO2)1−x films by surface sol-gel method and reaction-anneal treatment

Hafnium oxide (HfO₂) films were deposited on Si substrates with a pre-grown oxide layer using hafnium chloride (HfCl₄) source by surface sol-gel process, then ultrathin (HfO₂)_x(SiO₂)_1−x films were fabricated due to the reaction of SiO₂ layer with HfO₂ under the appropriate reaction-anneal treatment. The observation of high-resolution transmission electron microscopy indicates that the ultrathin films show amorphous nature. X-ray photoelectron spectroscopy analyses reveal that surface sol-gel derived ultrathin films are Hf-Si-O alloy instead of HfO₂ and pre-grown SiO₂ layer, and the composition was Hf_0.52Si_0.48O₂ under 500 °C reaction-anneal. The lowest equivalent oxide thickness (EOT) value of 0.9 nm of film annealed at 500 °C has been obtained with small flatband voltage of −0.31 V. The experimental results indicate that a simple and feasible solution route to fabricate (HfO₂)_x(SiO₂)_1−x composite films has been developed by means of combination of surface sol-gel and reaction-anneal treatment.     

In situ atomic layer deposition and synchrotron-radiation photoemission study of Al2O3 on pristine n-GaAs(0 0 1)-4 × 6 surface

This work presents the in situ reflection high-energy electron diffraction (RHEED), scanning tunneling microscopy (STM) and synchrotron-radiation photoemission studies for the morphological and interfacial chemical characterization of in situ atomic layer deposited (ALD) Al₂O₃ on pristine molecular beam epitaxy (MBE) grown Ga-rich n-GaAs (1 0 0)-4 × 6 surface. Both the RHEED pattern and STM image demonstrated that the first cycle of ALD-Al₂O₃ process reacted immediately with the GaAs surface. As revealed by in situ synchrotron-radiation photoemission studies, two types of surface As atoms that have excess in charge in the clean surface served as reaction sites with TMA. Two oxidized states were then induced in the As 3d core-level spectra with chemical shifts of +660 meV and +1.03 eV, respectively.     

Growth and structural properties of crystalline LaAlO3 on Si (0 0 1)

Crystalline LaAlO₃ was grown by oxide molecular beam epitaxy (MBE) on Si (0 0 1) surfaces utilizing a 2 ML SrTiO₃ buffer layer. This SrTiO₃ buffer layer, also grown by oxide MBE, formed an abrupt interface with the silicon. No SiO₂ layer was detectable at the oxide-silicon interface when studied by cross-sectional transmission electron microscopy. The crystalline quality of the LaAlO₃ was assessed during and after growth by reflection high energy electron diffraction, indicating epitaxial growth with the LaAlO₃ unit cell rotated 45° relative to the silicon unit cell. X-ray diffraction indicates a (0 0 1) oriented single-crystalline LaAlO₃ film with a rocking curve of 0.15° and no secondary phases. The use of SrTiO₃ buffer layers on silicon allows perovskite oxides which otherwise would be incompatible with silicon to be integrated onto a silicon platform.     

Epitaxy of BaTiO3 thin film on Si(0 0 1) using a SrTiO3 buffer layer for non-volatile memory application

In this study we report the epitaxial growth of BaTiO₃ films on Si(0 0 1) substrate buffered by 5 nm-thick SrTiO₃ layer using both MBE and PLD techniques. The BaTiO₃ films demonstrate single crystalline, (0 0 1)-oriented texture and atomically flat surface on SrTiO₃/Si template. The electrical characterizations of the BaTiO₃ films using MFIS structures show that samples grown by MBE with limited oxygen pressure during the growth exhibit typical dielectric behavior despite post deposition annealing process employed. A ferroelectric BaTiO₃ layer is obtained using PLD method, which permits much higher oxygen pressure. The C-V curve shows a memory window of 0.75 V which thus enable BaTiO₃ possibly being applied to the non-volatile memory application.     

Photoemission study of the Si(1 1 1)-native SiO2/copper phthalocyanine (CuPc) ultra-thin film interface

The Ultraviolet and X-ray Photoemission Spectroscopy (UPS, XPS) investigation was done to examine the interface formation between deposited copper phthalocyanine (CuPc) thin films and covered with native oxide n- and p-type silicon Si(1 1 1) substrates. The UPS results indicated the existence of small interface dipole effect for very first layer of CuPc deposited on both types of substrates. The dipoles were oriented differently depending on silicon conductivity type. In this paper we present that near the inorganic/organic interface the phthalocyanine’s molecular orbital levels shift downwards 0.20 ± 0.05 eV in the case of n-Si substrate and upwards 0.25 ± 0.05 eV for p-Si indicating the different displacement of the negative charge within the interface region. This tendency was also confirmed by conducted XPS study of the core levels. It is highly probable that band bending-like shift is provoked by the continuous change of CuPc molecule orientation induced by interface polarization layer.     

Au/CaF2/nSi(1 1 1) tunnel emitter phototransistor

Owing to the improved growth technology of the 1-2 nm calcium fluoride films on silicon, tunnel metal-insulator-semiconductor transistors have been fabricated. Measured output characteristics show both saturation and active mode of operation. Estimated value of current gain exceeds 1000 approaching the theoretically estimated value in this system. This result supports the candidacy of calcium fluoride for being a vital dielectric in silicon-based functional electronics.