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.     

Electron states at the (1 0 0)Ge/HfO2 Interface

The energy distribution of extended and localized electron states at the Ge/HfO₂ interface is determined by combining the internal photoemission of electrons and holes from Ge into the Hf oxide and AC capacitance/conductance measurements. The inferred offsets of the conduction and valence band at the interface, i.e., 2.0 ± 0.1 and 3.0 ± 0.1 eV, respectively, suggest the possibility to apply the deposited HfO₂ layer as a suitable insulator on Ge. The post-deposition annealing of the Ge/HfO₂ structures in oxygen results in 1 eV reduction of the valence band offset, which is attributed to the growth of a GeO₂ interlayer. However, this treatment enables one to substantially reduce the density of Ge/HfO₂ interface traps, approaching ≈1×10¹² cm⁻² eV⁻¹ near the Ge midgap.     

掺氮氧化铪的光学特性研究

掺氮氧化铪是半导体工业非常重要的材料。在本论文中,我们利用Hf[N(C₂H₅)(CH₃)]₄ 和 H₂O₂作为原子层淀积的前驱体,制备了二氧化铪材料。然后,我们使用快速热退火的办法,在不同温度下,对二氧化铪进行了氮掺杂工艺。我们对掺氮二氧化铪的组分,跟硅界面的稳定性以及薄膜材料的光学特性随退火温度的变化进行了细致的研究。研究发现,随着退火温度的提高,二氧化铪薄膜材料的氮组分从1.41% 上升至 7.45%,相应的,薄膜材料的禁带宽度从5.82 eV 降低为 4.94 eV。     

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.     

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.     

Charge trapping and interface states in hydrogen annealed HfO2–Si structures

Metal–oxide–semiconductor (MOS) capacitors based on HfO₂ gate stacks with Al and TiN gates are compared to study the effect of the gate electrode material to the properties of insulator–semiconductor interface. The structures under study were shown to contain interface trap densities of around 2 × 10¹¹ cm⁻² eV⁻¹ for Al gate and up to 5.5 × 10¹² cm⁻² eV⁻¹ for TiN gate. The peak in the surface state distribution was found at 0.19 eV above the valence band edge for Al electrode. The respective capture cross-section is 6 × 10⁻¹⁷ cm² at 200 K.The charge injection experiments have revealed the presence of hole traps inside the dielectric layer. The Al-gate structure contains traps with effective capture cross-section of 1 × 10⁻²⁰ cm², and there are two types of traps in the TiN-gate structure with cross-sections of 3.5 × 10⁻¹⁹ and 1 × 10⁻²⁰ cm². Trap concentration in the structure with Al electrode was considerably lower than in the structure with TiN electrode.     

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.     

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.     

Effects of La2O3 incorporation in HfO2 gated nMOSFETs on low-frequency noise

The effect of La₂O₃ incorporation on the spatial trap distribution in HfO₂ gate dielectrics is investigated. The incorporation of La₂O₃ in HfO₂ dielectric has been found to improve the effective mobility in addition to reduced interface-state density. The trap distribution analysis in the HfO₂ layer extracted by combining the charge pumping (CP) method and the low-frequency noise (LFN) method has revealed significant reduction in the amount of traps at HfO₂/SiO₂-interlayer interface and in the HfO₂ layer by La₂O₃ incorporation.     

Fluorine Passivation in Gate Stacks of Poly-Si/TaN/HfO2 (and HfSiON/HfO2)/Si Through Gate Ion Implantation

Fluorine passivation in poly-Si/TaN/HfO₂/p-Si and poly-Si/TaN/HfSiON/HfO₂/p-Si gate stacks with varying TaN thickness through gate ion implantation has been studied. It has been found that when TaN thickness was less than 15 nm, mobility and subthreshold swing improved significantly in HfO₂ nMOSFETs; while there was little performance improvement in HfSiON/HfO₂ nMOSFETs due to the blocking of F atoms by the HfSiON layer in gate dielectrics, as has been proved by the electron energy loss spectroscopy mapping     

Ferroelectricity-modulated resistive switching in Pt/Si:HfO2/HfO2-x/Pt memory

It is investigated for the effect of a ferroelectric Si:HfO₂ thin film on the resistive switching in a stacked Pt/Si:HfO₂/highly-oxygen-deficient HfO_2-x/Pt structure. Improved resistance performance was observed. It was concluded that the observed resistive switching behavior was related to the modulation of the width and height of a depletion barrier in the HfO_2-x layer, which was caused by the Si:HfO₂ ferroelectric polarization field effect. Reliable switching reproducibility and long data retention were observed in these memory cells, suggesting their great potential in non-volatile memories applications with full compatibility and simplicity.     

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.     

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.     

SiSiO2 interface states based on optically activated conductance technique

Experimental technique recently developed by Poon and Card has been used to determine the energy distribution of the interface states at the SiSiO₂ interface using AlSiO₂nSi structure with an oxide thickness $\text{?38}\text{A}\text{?}$. The distribution obtained in the band gap of silicon was distorted U-shaped. The distortion in the lower half of the band gap was more pronounced. Surface state density in the structure studied was of the order of 10¹²/cm² eV.     

Effective formation of interface controlled Y2O3 thin film on Si(1 0 0) in a metal-(ferroelectric)-insulator-semiconductor structure

Yttrium was deposited on the chemical oxide of Si and annealed under vacuum to control the interface for the formation of Y₂O₃ as an insulating barrier to construct a metal-ferroelectric-insulator-semiconductor structure. Two different pre-annealing temperatures of 600 and 700 °C were chosen to investigate the effect of the interface state formed after the pre-annealing step on the successive formation of Y₂O₃ insulator and Nd₂Ti₂O₇ (NTO) ferroelectric layer through annealing under an oxygen atmosphere at 800 °C. Pre-anneal treatments of Y-metal/chemical-SiO₂/Si at 600 and 700 °C induced a formation of Y₂O₃ and Y-silicate, respectively. The difference in the pre-anneal temperature induced almost no change in the electrical properties of the Y₂O₃/interface/Si system, but degraded properties were observed in the NTO/Y₂O₃/interface/Si system pre-annealed at 600 °C when compared with the sample pre-annealed at 700 °C. C-V characteristics of the NTO/Y₂O₃/Si structured system showed a clockwise direction of hysteresis, and this gap could be used as a memory window for a ferroelectric-gate. A smaller hysteric gap and electrical breakdown values were observed in the NTO/Y₂O₃/Si system pre-annealed at 600 °C, and this was due to an unintentional distribution of the applied field from the presence of an interfacial layer containing Y-silicate and SiO₂ phases.     

Effects of nitrogen annealing on electron scatterings in SiSiO2 interface

The effective mobility of electrons at Si (100) surfaces was measured as a function of electron density N_s = 5 × 10¹¹?1 × 10¹³ cm^?2 at 4.2K for samples with and without annealing (10 min–2 hr) in nitrogen gas at 1000°C after wet thermal oxidation. A great part of the scattering by Coulomb and short-range potentials was reduced by a short (～10 min) anneal time, although the subsequent annealing resulted in a slight increase in the number of the scatterers. On the other hand, scattering by a surface roughness potential was reduced with increase in the anneal time. These scattering effects associated with N₂ annealing are discussed.     

Electronic structure of Pt/HfO2 interface with oxygen vacancy

Using first-principles calculations, we study the electronic structures of Pt/HfO₂ interface in the presence of oxygen vacancy. The energetics and charge transfer are examined when the oxygen vacancy is at various distances from the interface. It is found that the oxygen vacancy is strongly attracted to the interface and the charge transfer decreases monotonically as the vacancy moves away from the interface, albeit the amount of charge transfer is small. The charge transfer results in the decrease of the effective work function of Pt, consistent with the vacancy mechanism to explain the shift in the flat-band voltage.     

The doped Si/SiO2 interface

A study is reported of the influence of dopant atoms on the SiSiO₂ interface states of thermally oxidized silicon. It was found that acceptor or donor atoms induce interface states and oxide charges. The effect is largest in the case of acceptor dopants and is independent of the doping process. The influence of the dopant atoms on oxide charge is probably related to the different segregation coefficients of acceptors and donors.     

Influence of passivating interlayer on Ge/HfO2 and Ge/Al2O3 interface band diagrams

The energy band alignment between Ge, HfO₂ and Al₂O₃ was analyzed as influenced by passivating interlayers (ILs) of different composition (GeO₂, Ge₃N₄, Si/SiO_x). From internal photoemission and photoconductivity experiments we found no IL-sensitive dipoles at the Ge/HfO₂ interfaces, the latter being universally characterized by conduction and valence band offsets of 2.1 and 3.0 eV, respectively. However, in the case of HfO₂ growth using H₂O-based atomic layer deposition, the Ge oxide IL appears to have a narrower bandgap, 4.3 eV, than the 5.4–5.9 eV gap of bulk germania. Accordingly, formation of this IL yields significantly reduced barriers for hole and, particularly, electron injection from Ge into the insulator. Changing to a H-free process for HfO₂ and Al₂O₃ deposition suppresses the formation of the narrow-gap Ge oxide.     

Characterization of the annealing impact on La2O3/HfO2 and HfO2/La2O3 stacks for MOS applications

The impact of various rapid thermal annealing used during the integration on the La₂O₃/HfO₂ and HfO₂/La₂O₃ stacks deposited by Atomic Layer deposition was analyzed. The consequences of lanthanum localization in such stacks on the evolution of the films during the rapid thermal annealing are investigated in term of morphology, crystalline structure, silicate formation and film homogeneity as a function of the depth. It appeared that the La₂O₃ location has an impact on the temperature of the quadratic phase formation which could be linked to the formation of SiOHfLa silicate and the resistance of the films to dissolution in HF 0.05 wt%.