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.     

Interfacial transition regions at germanium/Hf oxide based dielectric interfaces: Qualitative differences between non-crystalline Hf Si oxynitride and nanocrystalline HfO2 gate stacks

The contribution from a relatively low-K SiON (K ∼ 6) interfacial transition region (ITR) between Si and transition metal high-K gate dielectrics such as nanocrystalline HfO₂ (K ∼ 20), and non-crystalline Hf Si oxynitride (K ∼ 10-12) places a significant limitation on equivalent oxide thickness (EOT) scaling. This limitation is equally significant for metal-oxide-semiconductor capacitors and field effect transistors, MOSCAPs and MOSFETs, respectively, fabricated on Ge substrates. This article uses a novel remote plasma processing approach to remove native Ge ITRs and bond transition metal gate dielectrics directly onto crystalline Ge substrates. Proceeding in this way we identify (i) the source of significant electron trapping at interfaces between Ge and Ge native oxide, nitride and oxynitride ITRs, and (ii) a methodology for eliminating native oxide, or nitride IRTs on Ge, and achieving direct contact between nanocrystalline HfO₂ and non-crystalline high Si₃N₄ content Hf Si oxynitride alloys, and crystalline Ge substrates. We then combine spectroscopic studies, theory and modeling with electrical measurements to demonstrate the relative performance of qualitatively different nanocrystalline and non-crystalline gate dielectrics for MOS Ge test devices.     

Work function control at metal high-dielectric-constant gate oxide interfaces

There has been difficulty in finding metals of sufficiently large or small effective work function to act as metal electrodes on high-dielectric-constant gate oxides. To understand the factors affecting the effective work function, we have calculated the Schottky barrier heights (SBH) of a range of metals at different configurations on (1 0 0) and (1 1 0) HfO₂ surfaces. On (1 0 0) surfaces, different O-terminations are found to be able to shift the SBH by up to 1 eV. Metals of different work function from Y to Ni are found to be able to shift the SBH by over 1 eV. This is a key conclusion which contrasts with the ‘Fermi level pinning’ on HfO₂ found by some groups. On the non-polar (1 1 0) surface, the low work function metals like Al and Hf are found to bond to O sites, whereas high work function metals like Ni can bond to both Hf and O sites. Thus, many factors such a termination and stoichiometry control the SBH and the effective work function of a metal.     

Epitaxial strontium oxide layers on silicon for gate-first and gate-last TiN/HfO2 gate stack scaling

We show that a thin epitaxial strontium oxide (SrO) interfacial layer enables scaling of titanium nitride/hafnium oxide high-permittivity (high-k) gate stacks for field-effect transistors on silicon. In a low-temperature gate-last process, SrO passivates Si against SiO₂ formation and silicidation and equivalent oxide thickness (EOT) of 5 Å is achieved, with competitive leakage current and interface trap density. In a gate-first process, Sr triggers HfO₂-SiO₂ intermixing, forming interfacial high-k silicate containing both Sr and Hf. Combined with oxygen control techniques, we demonstrate an EOT of 6 Å with further scaling potential. In both cases, Sr incorporation results in an effective workfunction that is suitable for n-channel transistors.     

Impact of the TiN electrode deposition on the HfO2 band gap for advanced MOSFET gate stacks

The impact of the deposition of a TiN electrode on the high-k oxide HfO₂ has been investigated, focussing on the dielectric band gap. After the gate elaboration, a non-destructive approach combining Spectroscopic Ellipsometry (SE), Reflection Electron Energy Loss Spectroscopy (REELS) and X-ray Photoelectron Spectroscopy (XPS) was developed to probe the buried metal/high-k interface. The overall optical band gap is 5.9 ± 0.1 eV with no change after the metal gate deposition. A local reduction of 1 eV is measured near the TiN layer, due to N diffusion at the interface creating N 2p states at the top of the HfO₂ valence band. Increased disorder and defects are identified in the high-k after gate elaboration by XPS, REELS and SE.     

Structural and electrical characteristics of RF-sputtered HfO2 high-k based MOS capacitors

The HfO₂ high-k thin films have been deposited on p-type (1 0 0) silicon wafer using RF magnetron sputtering technique. The XRD, AFM and Ellipsometric characterizations have been performed for crystal structure, surface morphology and thickness measurements respectively. The monoclinic structured, smooth surface HfO₂ thin films with 9.45 nm thickness have been used for Al/HfO₂/p-Si metal-oxide-semiconductor (MOS) structures fabrication. The fabricated Al/HfO2/Si structure have been used for extracting electrical properties viz dielectric constant, EOT, barrier height, doping concentration and interface trap density through capacitance voltage and current-voltage measurements. The dielectric constant, EOT, barrier height, effective charge carriers, interface trap density and leakage current density are determined are 22.47, 1.64 nm, 1.28 eV, 0.93 × 10¹⁰, 9.25 × 10¹¹ cm⁻² eV⁻¹ and 9.12 × 10⁻⁶ A/cm² respectively for annealed HfO₂ thin films.     

Investigation of atomic vapour deposited TiN/HfO2/SiO2 gate stacks for MOSFET devices

HfO₂ films were grown by atomic vapour deposition (AVD) on SiO₂/Si (1 0 0) substrates. The positive shift of the flat band voltage of the HfO₂ based metal-oxide-silicon (MOS) devices indicates the presence of negative fixed charges with a density of 5 × 10¹² cm⁻². The interface trap charge density of HfO₂/SiO₂ stacks can be reduced to 3 × 10¹¹ eV⁻¹ cm⁻² near mid gap, by forming gas annealing. The extracted work function of 4.7 eV preferred the use of TiN as metal gate for PMOS transistors. TiN/HfO₂/SiO₂ gate stacks were integrated into gate-last-formed MOSFET structures. The extracted maximum effective mobility of HfO₂ based PMOS transistors is 56 cm²/Vs.     

Application of non-linear optical second harmonic generation and X-ray absorption and spectroscopies to defect related properties of Hf silicate and Hf Si oxynitride gate dielectrics

Three different Hf oxide based dielectrics have emerged as viable candidates for applications in advanced ULSI devices. This article focuses on two of these: (i) phase separated Hf silicates with (i) 70–85% nano-crystalline HfO₂ with a nano-grain size <2 nm, and 15–30% ～2 nm non-crystalline SiO₂ inclusions, and (ii) Hf Si oxynitride alloys, the most promising of which has a composition, (HfO₂)_0.3(SiO₂)_0.3(Si₃N₄)_0.4 designated as 3/3/4 Hf SiON. X-ray absorption spectroscopy has been applied to identification of defect associated with vacancy structures in phase separated silicates, and network disruption defects in the Hf Si oxynitrides. Optical second harmonic generation is introduced in this article for the first time as a non-invasive approach for detecting macroscopic strain, that is shown to be absent in these low defect density dielectrics, the phase separated Hf silicates, and Hf Si oxynitrides, but present in HfO₂ films, and Hf silicates with lower HfO₂ content, e.g., the 40% HfO₂ film of this article.     

Annealing effect on physical and electrical characteristics of thin HfO2, HfSixOy and HfOyNz films on Si

We report the effect of annealing on electrical and physical characteristics of HfO₂, HfSi_xO_y and HfO_yN_z gate oxide films on Si. Having the largest thickness change of 0.3 nm after post deposition annealing (PDA), HfO_yN_z shows the lowest leakage current. It was found for both as-grown and annealed structures that Poole-Frenkel conduction is dominant at low field while Fowler-Nordheim tunneling in high field. Spectroscopic ellipsometry measurement revealed that the PDA process decreases the bandgap of the dielectric layers. We found that a decreasing of peak intensity in the middle HfO_yN_z layer as measured by Tof-SIMS may suggest the movement of N toward the interface region between the HfO_yN_z layer and the Si substrate during the annealing process.     

Characterization of hafnium oxide grown on silicon by atomic layer deposition: Interface structure

Ultra-thin films of hafnium oxide deposited on Si(1 0 0) substrates by means of atomic layer deposition using tetrakis(diethylamino)hafnium as the hafnium precursor are characterized. These films and interface structures are probed using Fourier transform infrared spectroscopy along with Z-contrast imaging and electron energy loss spectroscopy (EELS) of a scanning transmission electron microscope. The interface structure of HfO₂/Si(1 0 0) is further investigated using angle resolved X-ray photoelectron spectroscopy to probe the core level orbitals (Hf 4f, Si 2p, O 1s) at high resolution. The interfacial differences are also examined by probing the Hf 4f bonding with normal incidence XPS in thin and thick films. The XPS studies show that the binding energies remain unchanged with film depth and that there is no apparent signature of silicate structure in the as-deposited films. EELS spectra taken at the interface and XPS measurements suggest the interface is mainly silicon oxide. Two different cleaning methods used show difference only in the thickness of the silicon oxide interlayer.     

ALD of LaHfOx nano-laminates for high-κ gate dielectric applications

Electrical properties and thermal stability of LaHfO_x nano-laminate films deposited on Si substrates by atomic layer deposition (ALD) have been investigated for future high-κ gate dielectric applications. A novel La precursor, tris(N,N′-diisopropylformamidinato) lanthanum [La(ⁱPrfAMD)₃], was employed in conjunction with conventional tetrakis-(ethylmethyl)amido Hf (TEMA Hf) and water (H₂O). The capacitance-voltage curves of the metal oxide semiconductor capacitors (MOSCAPs) showed negligible hysteresis and frequency dispersion, indicating minimal deterioration of the interface and bulk properties. A systematic shift in the flat-band voltage (V_fb) was observed with respect to the change in structure of nano-laminate stacks as well as La₂O₃ to HfO₂ content in the films. The EOTs obtained were in the range of ∼1.23-1.5 nm with leakage current densities of ∼1.3 × 10⁻⁸ A/cm² to 1.3 × 10⁻⁵ A/cm² at V_fb − 1 V. In addition, the films with a higher content of La₂O₃ remained amorphous up to 950 °C indicating very good thermal stability, whereas the HfO₂ rich films crystallized at lower temperatures.     

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。     

Electrical and structural properties of hafnium silicate thin films

Thin (4 nm) hafnium silicate (HfO₂)_x(SiO₂)_1−x/SiO₂ gate stacks (0 < x < 1) grown by metal organic chemical vapour deposition (MOCVD) are investigated in this study. The focus is on extracting the optical constants, and hence bandgaps as well as dielectric constants. The VUV (vacuum ultraviolet) spectroscopic ellipsometry (VUV-SE) technique in the spectral range 140–1700 nm, together with current–voltage and capacitance–voltage techniques were used for studying the optical and electrical properties of the layers, respectively. The bandgap was found to increase from 5.24 eV for HfO₂ to 6 eV for Hf-silicate with 30% Hf. The permittivity was reduced from 21 for HfO₂ layers to 8 for Hf-silicate with x = 0.3. The results suggest that the optimal Hf content is above 0.6, for which the permittivity higher than 10 can be achieved.     

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.     

Al passivation effect at the HfO2/GaAs interface: A first-principles study

III–V semiconductor interfacing with high-k dielectrics is critical for the high mobility metal oxide semiconductor field transistor (MOSFET) device. In this work, we utilize first-principles method to explore the electronic structures of the interface GaAs/HfO₂ with the presence of Al interfacial defects. The simulation results indicate that Al substitutions & interstitials tend to increase the thermal stability of the interface. Meanwhile, this substitution removes the lower-half gap states of GaAs, partially passivating the interface and consequently suppressing the gap states. Also, we find that the band offset displays a dependence on the point defects of Al replacements of interfacial Hf and Ga.     

Electrical properties of metal–HfO2–silicon system measured from metal–insulator–semiconductor capacitors and metal–insulator–semiconductor field–effect transistors using HfO2 gate dielectric

Metal–insulator–semiconductor (MIS) capacitors and metal–insulator–semiconductor field effect transistors (MISFETs) incorporating HfO₂ gate dielectrics were fabricated using RF magnetron sputtering. In this work, the essential structures and electrical properties of HfO₂ thin film were examined. The leakage current measured from MIS capacitors depends on the sputtering gas mixture and the annealing temperature. The best condition to achieve the lowest leakage current is to perform the annealing at 500 °C with a mixture of 50% N₂ and 50% O₂ gas ratio. Aluminum is used as the top electrode. The Al/HfO₂ and the HfO₂/Si barrier heights extracted from Schottky emission are 1.02 eV and 0.94 eV, respectively. An Al/HfO₂/Si energy band diagram is proposed based on these results.     

Mechanisms of reduction in hole concentration in Al-doped 4H-SiC by electron irradiation

From the temperature dependence of the hole concentration in unirradiated lightly Al-doped 4H-SiC epilayers, an Al acceptor with E_V + 0.2 eV, which is an Al atom (Al_Si) at a Si sublattice site, and an unknown deep acceptor with E_V + 0.35 eV are found, where E_V is the top of the valence band. Both the densities are similar. With irradiation of 0.2 MeV electrons the Al acceptor density is reduced, while the unknown deep acceptor density is increased. Judging from the minimum electron energy required to displace a substitutional C atom (C_s) or the Al_Si, the bond between the Al_Si and its nearest neighbor C_s is broken due to the displacement of the C_s by this irradiation. Moreover, the displacement of the C_s results in the creation of a complex (Al_Si-V_C) of Al_Si and a carbon vacancy (V_C), indicating that the possible origin of the deep acceptor with E_V + 0.35 eV is Al_Si-V_C.     

A comparative study of depth profiling of interface states using charge pumping and low frequency noise measurement in SiO2/HfO2 gate stack nMOSFETs

Charge pumping and low frequency noise measurements for depth profiling have been studied systematically using a set of gate stacks with various combinations of IL and HfO₂ thicknesses. The distribution of generated traps after HCI and PBTI stress was also investigated. The drain-current power spectral density made up all of the traps of IL in 0 < z < T_IL and the traps of HfO₂ in T_IL < z < T_HK. The traps near the Si/SiO₂ interface dominated the 1/f noise at higher frequencies, which is common in SiO₂ dielectrics. For the HfO₂/SiO₂ gate stack, however, the magnitude of the 1/f noise did not significantly change after HCI and PBTI because of more traps in the bulk HfO₂ film than at the bottom of the interface.     

Band offsets at interfaces of (1 0 0)InxGa1−xAs (0 ⩽ x ⩽ 0.53) with Al2O3 and HfO2

The electron energy band alignment at interfaces of In_xGa_1?xAs (0 ? x ? 0.53) with atomic-layer deposited insulators Al₂O₃ and HfO₂ is characterized using combined measurements of internal photoemission of electrons and photoconductivity. The measured energy of the In_xGa_1?xAs valence band top is found to be only marginally influenced by the semiconductor composition. This result suggests that the observed bandgap narrowing from 1.42 to 0.75 eV when the In content increases from 0 to 0.53 occurs mostly through downshift of the semiconductor conduction band bottom. Electron states originating from the interfacial oxidation of In_xGa_1?xAs lead to reduction of the electron barrier at the semiconductor/oxide interface.     

A low gate leakage current and small equivalent oxide thickness MOSFET with Ti/HfO2 high-k gate dielectric

Annealing effects on electrical characteristics and reliability of MOS device with HfO₂ or Ti/HfO₂ high-k dielectric are studied in this work. For the sample with Ti/HfO₂ higher-k dielectric after a post-metallization annealing (PMA) at 600 °C, its equivalent oxide thickness value is 7.6 Å and the leakage density is about 4.5 × 10⁻² A/cm². As the PMA is above 700 °C, the electrical characteristics of MOS device would be severely degraded.