Breakdown spots of ultra-thin (EOT < 1.5 nm) HfO2/SiO2 stacks observed with enhanced—CAFM

In this work, the (gate) current versus (gate) voltage (I–V) characteristics and the dielectric breakdown (BD) of an ultra-thin HfO₂/SiO₂ stack is studied by enhanced conductive atomic force microscopy (ECAFM). The ECAFM is a CAFM with extended electrical performance. Using this new set up, different conduction modes have been observed before BD. The study of the BD spots has revealed that, as for SiO₂, the BD of the stack leads to modifications in the topography images and high conductive spots in the current images. The height of the hillocks observed in the topography images has been considered an indicator of structural damage.     

Influences of bottom electrode TaN on electrical characteristics for metal–HfO2–metal capacitors

For TaN–HfO₂–TaN_x capacitors, the effects of bottom electrode TaN_x with different nitrogen contents on electrical characteristics are exhibited. An obvious oxygen deficiency can be found in HfO₂ film by AES analysis for the sample with bottom electrode TaN_x sputtered at a N₂/(N₂ + Ar) flow ratio of 10%. The bottom electrode TaN_x films with different nitrogen contents apparently affect HfO₂/TaN_x interfacial property and electrical characteristic of follow-up deposited HfO₂ film. Experimental results reveal that the more defective TaN_x structure can induce more oxygen vacancies in subsequently deposited HfO₂ film and result in a higher leakage current density and a worse breakdown electric field. Two factors affect these electrical characteristics including interfacial stress and oxygen vacancy. It indicates that interfacial stress due to various stochiometric TaN_x structure dominates the voltage linearity property of capacitance, but more leak paths and local field breakdown defects can be induced by interfacial stress and oxygen vacancy. Better electrical characteristics can be attained for the sample with bottom electrode TaN_x sputtered at a N₂/(N₂ + Ar) flow ratio of 20%.     

Organic thin film transistors with HfO2 high-k gate dielectric grown by anodic oxidation or deposited by sol–gel

We report here on pentacene based organic field effect transistors (OFETs) with a high-k HfO₂ gate oxide. HfO₂ layers were prepared by two different methods: anodic oxidation and sol–gel. A comparison of the two processes on the electrical properties of OFETs is given. Ultra thin nanoporous (20 nm) sol–gel deposited oxide films were obtained following an annealing at 450 °C. They lead to high mobility and stable devices (μ = 0.12 cm²/V s). On the other hand, devices with anodic HfO₂ revealed a little bit more leaky and show some hysteresis. Anodization, however, presents the advantage of being a fully room temperature process, compatible with plastic substrates. Stability and response to a bias stress are also reported.     

Electrical characterization of the hafnium oxide prepared by direct sputtering of Hf in oxygen with rapid thermal annealing

Electrical characterization of the hafnium oxide (HfO₂) gate dielectric films prepared by Hf sputtering in oxygen was conducted. By measuring the current–voltage (I–V) characteristics at temperature ranging from 300 to 500 K, several abnormalities in the I–V characteristics are recorded. For temperatures below 400 K, the current–voltage characteristics in high field region can be plotted with the Fowler–Nordheim law but a stronger temperature dependence was observed. Large flatband voltage shifts in the Al/HfO₂/Si capacitor were observed. The capacitance–voltage characteristics and flatband shifts are found to depend strongly on the post-deposition annealing temperature and duration. To study the reliability against high electric field, constant voltage stressing on the samples was conducted. We found that the trap energy levels are shallow and the oxide traps can be readily filled and detrapped at a low bias voltage.     

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.     

Hafnium oxide thin films deposited by high pressure reactive sputtering in atmosphere formed with different Ar/O2 ratios

The physical and electrical properties of hafnium oxide (HfO₂) thin films deposited by high pressure reactive sputtering (HPRS) have been studied as a function of the Ar/O₂ ratio in the sputtering gas mixture. Transmission electron microscopy shows that the HfO₂ films are polycrystalline, except the films deposited in pure Ar, which are amorphous. According to heavy ion elastic recoil detection analysis, the films deposited without using O₂ are stoichiometric, which means that the composition of the HfO₂ target is conserved in the deposition films. The use of O₂ for reactive sputtering results in slightly oxygen-rich films. Metal-Oxide-Semiconductor (MOS) devices were fabricated to determine the deposited HfO₂ dielectric constant and the trap density at the HfO₂/Si interface (D_it) using the high–low frequency capacitance method. Poor capacitance–voltage (CV) characteristics and high values of D_it are observed in the polycrystalline HfO₂ films. However, a great improvement of the electrical properties was observed in the amorphous HfO₂ films, showing dielectric constant values close to 17 and a minimum D_it of 2×10¹¹ eV⁻¹ cm⁻².     

Electrical characterization of hafnium oxide and hafnium-rich silicate films grown by atomic layer deposition

An electrical characterization comparative analysis between Al/HfO₂/n-Si and Al/Hf-Si-O/n-Si samples has been carried out. Hafnium-based dielectric films have been grown by means of atomic layer deposition (ALD). Interface quality have been determined by using capacitance–voltage (C–V), deep level transient spectroscopy (DLTS) and conductance transient (G-t) techniques. Our results show that silicate films exhibit less flat-band voltage shift and hysteresis effect, and so lower disordered induced gap states (DIGS) density than oxide films, but interfacial state density is greater in Hf–Si–O than in HfO₂. Moreover, a post-deposition annealing in vacuum under N₂ flow for 1 min, at temperatures between 600 and 730 °C diminishes interfacial state density of Hf–Si–O films to values measured in HfO₂ films, without degrade the interface quality in terms of DIGS.     

Extracting the relative dielectric constant for “high-κ layers” from CV measurements – Errors and error propagation

The paper pursues an investigation of the errors associated with the extraction of the dielectric constant (i.e., κ value) from capacitance–voltage measurements on metal oxide semiconductor capacitors. The existence of a transition layer between the high-κ dielectric and the silicon substrate is a factor that affects – in general – the assessment of the electrical data, as well as the extraction of κ. A methodology which accounts for this transition layer and the errors related to other parameters involved in the κ value extraction is presented; moreover, we apply this methodology to experimental CV results on HfO₂/SiO_x/Si structures produced in different conditions.     

Degradation kinetics of ultrathin HfO2 layers on Si(1 0 0) during vacuum annealing monitored with in situ XPS/LEIS and ex situ AFM

The evolution of HfO₂(3–5 nm)/SiO₂(0.5 nm)/Si(1 0 0) stacks during vacuum annealing was monitored in situ with the combination of X-ray photoelectron spectroscopy and low energy ion scattering techniques and supplemented with atomic force microscopy analysis to investigate the mechanism that triggers HfO₂ degradation with Hf silicide formation. The reduction of SiO₂ interfacial layer and the formation of local paths for SiO escape into vacuum are believed to be critical at vacuum annealing above T > 850 °C for the reaction between HfO₂ and Si to start and eventually lead to the degradation of the former.     

Low-temperature conductance measurements of surface states in HfO2–Si structures with different gate materials

Metal-oxide-semiconductor capacitors based on HfO₂ gate stack with different metal and metal compound gates (Al, TiN, NiSi and NiAlN) are compared to study the effect of the gate electrode material on the trap density at the insulator–semiconductor interface.C–V and G–ω measurements were made in the frequency range from 1 kHz to 1 MHz in the temperature range 180–300 K. From the maximum of the plot G/ω vs. ln(ω) the density of interface states was calculated, and from its position on the frequency axis the trap cross-section was found. Reducing temperature makes it possible to decrease leakage current through the dielectric and to investigate the states located closer to the band edge.The structures under study were shown to contain significant interface trap densities located near the valence band edge (around 2×10¹¹ cm⁻²eV⁻¹ for Al and up to (3.5–5.5)×10¹² cm⁻² eV⁻¹ for other gate materials). The peak in the surface state distribution is situated at 0.18 eV above the valence band edge for Al electrode. The capture cross-section is 5.8×10⁻¹⁷ cm² at 200 K for Al–HfO₂–Si structure.     

Optical and electrical characterization of hafnium oxide deposited by MOCVD

The paper reports on electrical and optical investigations performed on HfO₂ high-k films deposited by Metal-organic chemical vapor deposition (MOCVD). Spectroellipsometry investigations show the presence of a transition layer between HfO₂ and the silicon substrate, which can be optically modelled as a mixture of Si and SiO₂; this information is further used in the assessment of the electrical measurements. Hysteresis effects have been observed in the Capacitance–Voltage (C–V) measurements for the as-deposited sample as well as the annealed samples. For the samples with large hysteresis, Poole–Frenkel (PF) conduction is the most likely dominant conduction mechanism. The energy of dominant trap level was found to be 0.7 eV.     

An investigation of surface state capture cross-sections for metal–oxide–semiconductor field-effect transistors using HfO2 gate dielectrics

MOSFETs and MOSCs incorporating HfO₂ gate dielectrics were fabricated. The I_DS–V_DS, I_DS–V_GS, gated-diode and C–V characteristics were investigated. The subthreshold swing and the interface trap density were obtained. The surface recombination velocity and the minority carrier lifetime in the field-induced depletion region measured from the gated diodes were about 2.73 × 10³ cm/s and 1.63 × 10⁻⁶ s, respectively. The effective capture cross section of surface state was determined to be 1.6 × 10⁻¹⁵ cm² using the gated-diode technique in comparison with the subthreshold swing measurement. A comparison with conventional MOSFETs using SiO₂ gate oxide was also made.     

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.     

Optical and electrical characterization of hafnium oxide deposited by liquid injection atomic layer deposition

Optical and electrical properties of a set of high-k dielectric HfO₂ films, deposited by liquid injection atomic layer deposition (LI-ALD) and post deposition annealed (PDA) in nitrogen (N₂) ambient at various temperatures (400–600 °C), were investigated. The films were prepared using the cyclopentadienyl of hafnium precursor [Cp₂Hf(CH₃)₂] with water deposited at 340 °C. The spectroscopic ellipsometric (SE) results show that the characteristics of the dielectric functions of these films are strongly affected by annealing temperatures. I–V results show that N₂-based PDA enhances the average energy depth of the shallow trapping defects from Poole–Frenkel conduction fitting. This also correlated with the measured increase in MOS capacitance–voltage hysteresis.     

Reliability characteristics of high-k dielectrics

In this paper, recent results of Weibull slopes, area scaling factors, and breakdown behaviors observed for both soft breakdown and hard breakdown are discussed. These results would help to shed light on the breakdown mechanism of HfO₂ gate dielectrics. The Weibull slope β of the hard breakdown for both the area dependence and the time-to-dielectric-breakdown distribution was found to be β=2, whereas that of the soft breakdown was about 1.4 (EOT=14 Å). We also integrated the time-to-breakdown characteristics of HfO₂ under unipolar AC voltage stress on MOS capacitors. The results show that longer lifetime of HfO₂ has been observed when compared to constant voltage stress. Higher frequency and lower duty cycle in the AC stress resulted in longer lifetime. As thickness decreases, the amount of lifetime enhancement decreases. The enhancement of unipolar t_BD is attributed to less charge trapping during the “on time”, t_on and charge detrapping during the off time, t_off. It is proposed that time (τ_in) for charge to be trapped in HfO₂ is longer than t_on of unipolar stress under high frequency. In addition to experimental results, possible solutions are discussed.     

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.     

Electrical characterisation and reliability of HfO2 and Al2O3–HfO2 MIM capacitors

Current leakage and breakdown of MIM capacitors using HfO₂ and Al₂O₃–HfO₂ stacked layers were studied. Conduction in devices based upon HfO₂ layers thinner than 8 nm is probably dominated by tunnelling. Al₂O₃–HfO₂ stacked layers provide a limited benefit only in term of breakdown field. Constant-voltage wear-out of samples using insulating layer thicker than 6 nm is dominated by a very fast increase of the leakage current. A two step mechanism involving the generation of a conduction path followed by a destructive thermal effect is proposed to explain breakdown mechanism.     

Full-band tunneling in high-κ dielectric MOS structures

High-κ oxides such as ZrO₂ and HfO₂ have attracted great interest, due to their physical properties, suitable to replacement of SiO₂ as gate dielectric materials. In this work, we investigate the tunneling properties of ZrO₂ and HfO₂ high-κ oxides, by applying quantum mechanical methods that include the full-band structure of Si and oxide materials. Semiempirical sp³s*d tight-binding parameters have been determined to reproduce ab initio band dispersions. Transmission coefficients and tunneling current have been calculated for Si/ZrO₂/Si and Si/HfO₂/Si MOS structures, showing a very low gate leakage current in comparison to SiO₂-based structures with equivalent oxide thickness.     

Recrystallization and dielectric properties of CaHfOx films on Si

The recrystallization and dielectric behavior for amorphous CaHfO_x films on Si substrates has been investigated. Upon conventional annealing in air, the CaHfO_x films remain amorphous up to an annealing temperature of 800 °C for annealing times of 1 h. This recrystallization temperature is significantly higher than that reported for HfO₂ subjected to rapid thermal annealing. Metal–insulator–semiconductor structures with Pt gate electrodes were fabricated with various CaHfO_x film thickness for capacitance–voltage and leakage current measurements. From this, the permittivity of CaHfO_x was determined, along with interface layer capacitance for films on Si. The enhanced stability against polycrystalline grain growth, along with the thermodynamic stability of both CaO and HfO₂ in contact with Si, suggests that CaHfO_x may be an attractive gate dielectric for future generation metal–oxide–semiconductor field-effect transistor applications.     

Conductive filament formation at grain boundary locations in polycrystalline HfO2 -based MIM stacks: Computational and physical insight

Resistive switching in high-κ (HK) dielectric based metal-insulator-metal (MIM) devices occurs locally and is accompanied by dynamic changes in the structural and electrical properties of the HK dielectric. In polycrystalline HfO₂ HK dielectric based MIM devices, grain boundaries (GBs) play a significant role in the formation of a percolation path for the resistive switching as the GB regions contain a large number of defects and favor the formation of conductive/low resistive paths. In this work, we present a multi-physics based combined Kinetic Monte Carlo-Finite element model (KMC-FEM) 3D percolation framework to simulate the resistive switching (high resistive state (HRS) to low resistive state (LRS)) process in TiN/HfO₂ (5 nm)/Pt MIM stacks. The KMC-FEM model describes the effect of GBs on the formation of conductive path during the HRS to LRS resistive switching. In addition, this model is used to find the statistical distribution of conductive filament/path formation in amorphous and polycrystalline HfO₂ dielectrics. Conductive atomic force microscopy and transmission electron microscopy observations on the characteristics of the HfO₂ dielectrics at the nanometer scale complement the simulation results. The results clearly show that the HRS to LRS resistive switching occurs preferably at the GB regions in polycrystalline HfO₂ and at random locations in amorphous HfO₂ -based MIM stacks.