Atomic Layer Deposited HfO2 Films Research Articles

Nucleation promotion for the atomic layer deposition of HfO2 onto carbon nanotubes for device applications

Abstract The potential of semiconducting single-walled carbon nanotubes (CNTs) as the next-generation semiconductor information material as a successor to silicon-based technology is promising. However, a significant challenge for CNT based electronics at device level lies in the obtainment of high-quality gate dielectrics such as HfO2 on the dangling bond-free and chemically inert surface of CNTs. To address this issue, this paper studies the atomic layer deposition (ALD) of HfO2 onto the surface of carbon-based materials, and focuses on how to promote the nucleation via two methods, i.e. low-temperature Nanofog method and high-temperature variable parameter method. Both methods are confirmed to be effective in improving the continuity and uniformity of ALD HfO2 films.

The Evaluation of Interface Quality in HfO2 Films Probed by Time-Dependent Second-Harmonic Generation.

Time-dependent second-harmonic generation (TD-SHG) is an emerging sensitive and fast method to qualitatively evaluate the interface quality of the oxide/Si heterostructures, which is closely related to the interfacial electric field. Here, the TD-SHG is used to explore the interface quality of atomic layer deposited HfO2 films on Si substrates. The critical SHG parameters, such as the initial SHG signal and characteristic time constant, are compared with the fixed charge density (Qox) and the interface state density (Dit) extracted from the conventional electrical characterization method. It reveals that the initial SHG signal linearly decreases with the increase in Qox, while Dit is linearly correlated to the characteristic time constant. It verifies that the TD-SHG is a sensitive and fast method, as well as simple and noncontact, for evaluating the interface quality of oxide/Si heterostructures, which may facilitate the in-line semiconductor test.

Reduced leakage current in atomic-layer-deposited HfO2 thin films deposited at low temperature by in-situ defect passivation

HfO2 films grown via atomic layer deposition (ALD) at low temperature generally exhibit high impurity content, reduced density, and increased oxygen defects, resulting in degraded electrical properties. To overcome this issue, this study applied an in-situ defect passivation technique at low temperature, modifying the ALD oxygen source feeding step to suit heat-sensitive substrates. The electrical properties of the ALD HfO2 film grown at 80 °C are improved when the oxygen source feeding step is repeated twice, and especially the leakage current density is significantly decreased, which is approximately 1/7 smaller at an electric field of 1 MV/cm compared to the film grown via the conventional ALD process at 80 °C. This improvement in the electrical properties can be attributed to decreased carbon impurities and oxygen defects and increased film density. A simple modification of the oxygen source feeding step during the ALD process can effectively reduce defects within the ALD HfO2 films at the atomic layer level, even when grown at low deposition temperatures.

Integration of perovskite Pb[Zr0.35Ti0.65]O3/HfO2 ferroelectric-dielectric composite film on Si substrate

PurposeThe purpose of this paper is to investigate the effect of high-k material HfO2 as a buffer layer for the fabrication of metal-ferroelectric-insulator-silicon (MFeIS) structures on Si (100) substrate.Design/methodology/approachRF-sputtered Pb[Zr0.35Ti0.65]O3 or (PZT) and plasma-enhanced atomic layer deposited HfO2 films were selected as the ferroelectric and high-k buffer layer, respectively, for the fabrication of metal-ferroelectric-insulator-silicon (MFeIS) structures on Si (100) substrate. Multiple angle ellipsometry and X-ray diffraction analysis was carried out to obtain the crystal orientation, refractive index and absorption coefficient parameters of the deposited/annealed films. In the different range of annealing temperature, the refractive index was observed in the range of 2.9 to 2 and 1.86 to 2.64 for the PZT and HfO2 films, respectivelyFindingsElectrical and ferroelectric properties of the dielectric and ferroelectric films and their stacks were obtained by fabricating the metal/ferroelectric/silicon (MFeS), metal/ferroelectric/metal, metal/insulator/silicon and MFeIS capacitor structures. A closed hysteresis loop with remnant polarization of 4.6 µC/cm2 and coercive voltage of 2.1 V was observed in the PZT film annealed at 5000 C. Introduction of HfO2 buffer layer (10 nm) improves the memory window from 5.12 V in MFeS to 6.4 V in MFeIS structure with one order reduction in the leakage current density. The same MFeS device was found having excellent fatigue resistance property for greater than 1010 read/write cycles and data retention time more than 3 h.Originality/valueThe MFeIS structure has been fabricated with constant PZT thickness and varied buffer layer (HfO2) thickness. Electrical characteristics shows the improved leakage current and memory window in the MFeIS structures as compared to the MFeS structures. Optimized MFeIS structure with 10-nm buffer layer shows the excellent ferroelectric properties with endurance greater than E10 read/write cycles and data retention time higher than 3 h. The above properties indicate the MFe(100 nm)I(10 nm)S gate stack as a potential candidate for the FeFET-based nonvolatile memory applications.

Hardness, elastic modulus, and wear resistance of hafnium oxide-based films grown by atomic layer deposition

The investigation of mechanical properties of atomic layer deposition HfO2 films is important for implementing these layers in microdevices. The mechanical properties of films change as a function of composition and structure, which accordingly vary with deposition temperature and post-annealing. This work describes elastic modulus, hardness, and wear resistance of as-grown and annealed HfO2. From nanoindentation measurements, the elastic modulus and hardness remained relatively stable in the range of 163–165 GPa and 8.3–9.7 GPa as a function of deposition temperature. The annealing of HfO2 caused significant increase in hardness up to 14.4 GPa due to film crystallization and densification. The structural change also caused increase in the elastic modulus up to 197 GPa. Wear resistance did not change as a function of deposition temperature, but improved upon annealing.

Protective coatings of hafnium dioxide by atomic layer deposition for microelectromechanical systems applications

This work presents the investigation of HfO2 deposited by atomic layer deposition (ALD) from either HfD-CO4 or TEMAHf and ozone for microelectromechanical systems (MEMS) applications, in particular, for environmental protection of aluminum micromirrors. This work shows that HfO2 films successfully protect aluminum in moist environment and at the same time retain good reflectance properties of underlying material. In our experimental work, the chemical composition, crystal structure, electronic density and roughness of HfO2 films remained the same after one week of humidity treatment (relative humidity of 85%, 85°C). The reflectance properties underwent only minor changes. The observed shift in reflectance was only from 80–90% to 76–85% in 400–800nm spectral range when coated with ALD HfO2 films grown with Hf(NMeEt)4 and no shift (remained in the range of 68–83%) for films grown from (CpMe)2Hf(OMe)Me.

Spectroscopic investigation of the electronic structure of thin atomic layer deposition HfO2 films

The electronic structure of HfO2 thin films is investigated employing resonant photoelectron spectroscopy (resPES). The detailed analysis of the O1s resonance profile enables the determination of the partial density of states for the valence and the conduction bands as well as the electronic band gap to be 6.2 eV. The position of the charge neutrality level is evaluated. Thereby, it is demonstrated that the resPES data are able to combine information both for the valence as well as for the conduction band states. In addition, evidences for intrinsic in-gap states attributed to polaronic and charge transfer states are given. Electronic charges within the atomic layer deposition-HfO2 films are identified, pointing out that the amount of charges is essential to determine the accurate position of the surface potentials.

Optical characteristics of H2O-based and O3-based HfO2 films deposited by ALD using spectroscopy ellipsometry

Optical properties of thin atomic layer-deposited HfO2 films grown by H2O and O3 are analyzed by variable angle spectroscopic ellipsometry. By investigating the dielectric constant, it is found that a higher real part of the dielectric constant (e 1) value is observed for H2O-based film due to less silicate component in the film. Careful examination of the log scale of imaginary part of the dielectric constant (e 2) leads to the conclusion that the absorption features in the energy range of 3.2–5.35 eV originate from the interface layer between the silicon substrate and the native oxide. In particular, O3-based gate stacks have less sub-band gap defect states besides the silicon’s critical features. Moreover, a larger high-frequency dielectric constant, direct and indirect band gap values are obtained for O3-based film. Meanwhile, suitable valence band offsets (3.38 and 3.55 eV) and conduction band offsets (1.58 and 1.47 eV) are obtained for H2O- and O3-based HfO2 gate stacks, respectively, indicating both type of dielectric films can provide sufficient tunneling barriers for both electrons and holes.

Differences between direct current and alternating current capacitance nonlinearities in high-k dielectrics and their relation to hopping conduction

Capacitance nonlinearities were studied in atomic layer deposited HfO2 films using two types of signals: a pure ac voltage of large magnitude (ac nonlinearities) and a small ac voltage superimposed to a large dc voltage (dc nonlinearities). In theory, ac and dc nonlinearities should be of the same order of magnitude. However, in practice, ac nonlinearities are found to be an order of magnitude higher than dc nonlinearities. Besides capacitance nonlinearities, hopping conduction is studied using low-frequency impedance measurements and is discussed through the correlated barrier hopping model. The link between hopping and nonlinearity is established. The ac nonlinearities are ascribed to the polarization of isolated defect pairs, while dc nonlinearities are attributed to electrode polarization which originates from defect percolation paths. Both the ac and dc capacitance nonlinearities display an exponential variation with voltage, which results from field-induced lowering of the hopping barrier energy.

Effect of Using Ethanol as the Oxygen Source on the Growth and Dielectric Behavior of Atomic Layer Deposited Hafnium Oxide

Hafnium oxide (HfO2) is widely identified as a suitable high-k material to replace conventionally used low-k silicon oxide for gate dielectric layers in complementary metal-oxide semiconductor (CMOS) devices. High dielectric constant (~ 25) and compatibility with silicon substrate makes HfO2 an ideal candidate for future scaling of transistors.1 HfO2 has been deposited using various methods. Of all, atomic layer deposition (ALD) seems to be the suitable method to deposit ultra-thin conformal layers on deep trenches and future 3D transistor structures. ALD is a vapor phase deposition technique, in which sequential exposure and purging of a surface to reactive elements in vapor phase is used to grow thin films in layer-by-layer fashion with excellent control on film thickness and composition down to Å level.2 There were numerous studies on ALD of HfO2 using different precursor and oxygen source combinations.3 The property of the resultant HfO2films depends on the precursor combination, deposition conditions, post deposition treatment conditions etc.ALD of HfO2 is traditionally performed using oxygen, ozone or water as the oxygen source. These oxidizers oxidize the underlying silicon substrates during the initial ALD cycles and form thick interfacial silicon oxide layer. This layer reduces the dielectric constant of the structure. To reduce the risk of forming such thick interfacial layer we used ethanol as the ALD oxygen source. Ethanol showed good reactivity towards the hafnium precursor, tetrakis(diethylamino)hafnium, and displayed clean ALD growth behavior with a wide ALD process temperature window of 200-280 °C (Fig. 1). In this paper, ALD growth behavior, ALD process temperature window and dielectric characterization of the ALD HfO2films will be discussed in detail along with film compositional analysis.

Effect of Using Ethanol as the Oxygen Source on the Growth and Dielectric Behavior of Atomic Layer Deposited Hafnium Oxide

Use of common atomic layer deposition (ALD) oxygen sources such as oxygen, ozone or water often causes uncontrolled oxidation of the silicon surface by diffusion through the growing HfO2 film during the ALD process. This results in thick interfacial layer (IL) that is detrimental to future transistor scaling if further thinning of the IL is desired. In order to minimize the uncontrolled oxidation at the interface, we explored the use of ethanol as the ALD oxygen source in the ALD HfO2 process. The effect of using ethanol on the growth, composition and electrical behavior of ALD HfO2 films was studied and compared against films prepared using water based ALD process. Ethanol showed good reactivity with the hafnium precursor tetrakis(diethylamino)hafnium and displayed typical ALD growth behavior of linear film growth within a wide ALD process temperature window (200-280 °C). The ethanol based ALD process exhibited HfO2 growth rate of 0.05 nm/cycle which was less than 0.15 nm/cycle obtained when water was used as the ALD oxygen source. X-ray photoelectron spectroscopy (XPS) showed the deposition of stoichiometric HfO2 with trace amount of carbon in the film. SiO2 interfacial layer was not observed in as-deposited samples, whereas annealed samples showed SiO2 XPS features. Films prepared using the ethanol based ALD process showed comparable dielectric performance to that of water based ALD process films and displayed well behaved C-V and I-V curves with as-deposited (ethanol) HfO2 samples showing the lowest leakage current density of 5 x 10-8 A/cm2 at 1 V gate bias.

Low temperature atomic layer deposited HfO2 film for high performance charge trapping flash memory application

The impact of key process parameters on the electrical characteristics of atomic layer deposited HfO2 films has been systematically studied with MHOS devices via capacitance–voltage (C–V) measurement. C–V hysteresis curves revealed that charge storage capacity is significantly enhanced with decreasing substrate temperature from 350 down to 150 °C and/or increasing purge time of the inert gas. The developed HfO2 trapping layer was also demonstrated by a MAHOS memory device. Improved memory window, fast program speed and good retention characteristics have been obtained. The study provides a reference for memory performance improvement of HfO2-based charge trap flash memory.

Impact of surface treatments on high-κ dielectric integration with Ga-polar and N-polar GaN

Gallium- and nitrogen-polar GaN surfaces are subjected to a variety of pretreatments, including oxidation, before the application of high-κ dielectrics by atomic layer deposition (ALD) in order to assess the “best” preparation of smooth, clean, and electrically high-performing dielectric semiconductor interfaces. In terms of topographical and chemical cleanliness, a pretreatment with a wet chemical piranha etch (H2SO4:H2O2) was found to be optimum for both surfaces, and additionally, (NH4)2S is effective for N-polar surfaces. Both thermal and plasma oxidations were employed for controlled growth of native oxides. For Ga-polar surfaces, all native oxides were as smooth as pretreated surfaces, while for N-polar surfaces, all native oxides are much rougher except for very short, high temperature oxidations. ALD Al2O3 films on Ga-polar surfaces are smoother for pretreated surfaces than for as-received surfaces, whereas for N-polar surfaces the opposite is true. In general, ALD HfO2 films on Ga-polar surfaces are rougher (0.8 nm rms) than Al2O3 films (0.1 nm rms), whereas for piranha treated N-polar surfaces, HfO2 films are smoother than Al2O3 films. For Ga-polar surfaces, capacitance–voltage measurements of simple Al2O3 (measured κ = 9) capacitors show the smallest hysteresis for unintentionally oxidized surfaces (0.37 V), whereas simple HfO2 (measured κ = 14) capacitors show the smallest hysteresis for a thermal GaOx at the interface (0.1 V). In both cases, the thicker the GaOx at the interface the larger the negative threshold voltage shift—suggesting an electron trap. Calculated total trapped charges associated with the dielectrics range from 3.2 × 1011 cm−2 (for HfO2 on thermally oxidized GaN) to 1 × 1012 cm−2 for Al2O3 on thermally oxidized GaN and HfO2 on plasma oxidized GaN. Finally, the leakage current density for nearly all capacitors is <10−5 A-cm−2 at +8 V bias.

Characterization of atomic layer deposition HfO2, Al2O3, and plasma-enhanced chemical vapor deposition Si3N4 as metal–insulator–metal capacitor dielectric for GaAs HBT technology

Characterization was performed on the application of atomic layer deposition (ALD) of hafnium dioxide (HfO2) and aluminum oxide (Al2O3), and plasma-enhanced chemical vapor deposition (PECVD) of silicon nitride (Si3N4) as metal–insulator–metal (MIM) capacitor dielectric for GaAs heterojunction bipolar transistor (HBT) technology. The results show that the MIM capacitor with 62 nm of ALD HfO2 resulted in the highest capacitance density (2.67 fF/μm2), followed by capacitor with 59 nm of ALD Al2O3 (1.55 fF/μm2) and 63 nm of PECVD Si3N4 (0.92 fF/μm2). The breakdown voltage of the PECVD Si3N4 was measured to be 73 V, as compared to 34 V for ALD HfO2 and 41 V for Al2O3. The capacitor with Si3N4 dielectric was observed to have lower leakage current than both with Al2O3 and HfO2. As the temperature was increased from 25 to 150 °C, the breakdown voltage decreased and the leakage current increased for all three films, while the capacitance increased for the Al2O3 and HfO2. Additionally, the capacitance of the ALD Al2O3 and HfO2 films was observed to change, when the applied voltage was varied from −5 to +5 V, while no significant change was observed on the capacitance of the PECVD Si3N4. Furhermore, no significant change in capacitance was seen for these silicon nitride, aluminum oxide, and hafnium dioxide films, as the frequency was increased from 1 kHz to 1 MHz. These results show that the ALD films of Al2O3 and HfO2 have good electrical characteristics and can be used to fabricate high density capacitor. As a result, these ALD Al2O3 and HfO2 films, in addition to PECVD Si3N4, are suitable as MIM capacitor dielectric for GaAs HBT technology, depending on the specific electrical characteristics requirements and application of the GaAs devices.

Improved interfacial and electrical properties of atomic layer deposition HfO2 films on Ge with La2O3 passivation

We report the characteristics of HfO2 films deposited on Ge substrates with and without La2O3 passivation at 250°C by atomic layer deposition (ALD) using La[N(SiMe3)2]3 and Hf[N(CH3)(C2H5)]4 as the precursors. The HfO2 is observed to form defective HfGeOx at its interface during 500°C postdeposition annealing. The insertion of an ultrathin La2O3 interfacial passivation layer effectively prevents the Ge outdiffusion and improves interfacial and electrical properties. Capacitance equivalent thickness (CET) of 1.35nm with leakage current density JA of 8.3×10−4A/cm2 at Vg=1V is achieved for the HfO2/La2O3 gate stacks on Ge substrates.

Mechanical properties of ultra-thin HfO2 films studied by nano scratches tests

10-nm-thick atomic layer deposited HfO2 films were characterized in terms of wear resistance and indentation hardness to investigate the thermal annealing induced impacts on mechanical properties. The wear resistance of ultra-thin films at low loads was characterized using nano-scratch tests with an atomic force microscope. The depth of the nano-scratches decreases with increasing annealing temperature, indicating that the hardness of the annealed films increases with the annealing temperatures. Surface nanoindentation was also performed to confirm the nanoscratch test results. The hardness variation of the annealed films is due to the generation of HfSixOy induced by the thermal annealing. X-ray photoelectron spectroscopy measurements proved that the hardness of formed HfSixOy with increasing annealing temperatures. The existence of HfSixOy broadens the interface, and causes the increase of the interfacial layer thickness. As a result, the surface hardness increases with the increasing HfSixOy induced by the thermal annealing.

The Impact of Carbon Concentration on the Crystalline Phase and Dielectric Constant of Atomic Layer Deposited HfO2 Films on Ge Substrate

The effect of the carbon concentration on the crystalline phase and dielectric constant (k) of atomic layer deposited HfO2 films on Ge substrate was investigated. After annealing, the HfO2 films grown at 200°C and 280°C were crystallized to the tetragonal (t) and monoclinic (m) phases, respectively, which was related to the carbon contents within the films and grain boundary energy. To clarify this, the energy difference between a t- and a m- phases (ΔEtetra) was calculated by first principles calculations. The higher k value of t-HfO2 compared to amorphous and monoclinic HfO2 was experimentally confirmed.

Properties of Atomic Layer Deposited HfO2 Films on Ge Substrates Depending on Process Temperatures

The effect of deposition temperature and post deposition annealing (PDA) on the electrical properties of HfO2 films grown on a Ge substrate by atomic layer deposition (ALD) was investigated. The HfO2 films deposited at 280°C (280°C-HfO2) showed a large capacitance-voltage (C-V) hysteresis, but it was significantly reduced at 200°C (200°C-HfO2). Further reduction of deposition temperature to 160°C resulted in the abrupt increase of C-V hysteresis. The reduction of C-V hysteresis was originated from the smaller degree of intermixing between the HfO2 and Ge substrate with decreasing temperature, while the increase of C-V hysteresis comes from the low density of films grown at low temperatures. The C-V hysteresis characteristics were also strongly influenced by PDA temperatures. PDA temperatures higher than 550°C induced a large C-V hysteresis regardless of deposition temperature. Interestingly, after the PDA process, the HfO2 grown at 200°C is crystallized to the tetragonal phase, while the HfO2 grown at 280°C is crystallized to the monoclinic phase. These different crystalline phases result in a lower equivalent oxide thickness in the 200°C-HfO2 compared to the 280°C-HfO2

Investigation of oxygen-related defects and the electrical properties of atomic layer deposited HfO2 films using electron energy-loss spectroscopy

The electrical properties of hafnium dioxide (HfO2) thin films are often attributed to the oxygen composition and oxygen-related defects; however, there have been few systematic studies on the electronic structures of such oxygen atoms. In this study, we used electron energy-loss spectroscopy to identify the influence of the electronic states of the oxygen atoms in HfO2 thin films by comparing HfO2 samples for different oxygen source pulse time during atomic-layer deposition (ALD). Although all samples by ALD have higher oxygen content in the film than that in the reference stoichiometric HfO2 sample, variations in the local symmetry of amorphous HfO2 thin films were significantly affected by oxygen source pulse time. Moreover, leakage currents of high-oxygen content HfO2 samples with longer O3 pulse time decreased considerably, compared with those of low-oxygen content HfO2 samples with shorter O3 pulse time, in which oxygen-related defects were observed by the Vfb shift under constant voltage stress. After postdeposition annealing (PDA), the electronic structure of oxygen atoms in HfO2 films was affected by the initial oxidation states in the amorphous HfO2 films. Furthermore, after PDA, polycrystalline HfO2 in high-oxygen content samples was mostly of the monoclinic phase, whereas the metastable tetragonal phase was readily formed in low-oxygen content HfO2.

Thermal Stabilities of ALD-HfO2 Films on HF- and (NH4)2S-Cleaned InP

The thermal stabilities of the atomic layer deposited HfO2 films were investigated on HF-cleaned InP substrates with and without additional (NH4)2S cleaning. An abrupt interface without any interfacial layer between the HfO2 film and InP substrate was maintained during annealing at up to 600°C for both samples. The S-passivation resulting from the (NH4)2S cleaning reduced the interface defect density, thereby decreasing the frequency dispersion and low-field leakage current. The increase of the capacitance equivalent SiO2 thickness after the thermal annealing was also delayed by the S-passivation, which was possibly attributed to the retardation of substrate materials diffusion into the HfO2 film.