HfO2 Deposition Research Articles

Electron trapping in HfO2 layer deposited over a HF last treated silicon substrate

Electron trapping in HfO2-based MOS structures was studied through pulsed capacitance-voltage (C-V) technique. 10 nm HfO2 layer was deposited by atomic layer deposition over a HF last treated Si substrate. The C-V curves were observed to shift to positive voltages driven by the positive applied voltage along the pulses, consistent with electron trapping due to tunneling transitions between the substrate and pre-existing defects within the oxide and the subsequent lattice relaxation through electron-phonon interaction. The dependences of the voltage shift for a given capacitance value (ΔVC) with stress bias and time, allowed to distinguish two mechanisms. An initial trapping process occurs for times shorter than the microsecond, probably associated with a thin non-stoichiometric SiOx interfacial layer, which is followed by a trapping process that starts after tens of μs and progressively slowed down, associated with traps within the HfO2 layer. Numerical simulations yield for the HfO2 traps an energy of 1.3 eV below the conduction band edge, decreasing exponentially with the distance from the Si interface with a characteristic length of 1.7 nm; and phonon and relaxation energies of 50 meV and 1 eV, respectively. These physical parameters are consistent with previous reports of electron trapping in HfO2 layers deposited on a controlled interfacial layer, suggesting that trapping properties of defects inside the HfO2 layer are insensitive to the treatment of the Si surface before HfO2 deposition. On the other hand, the observed large initial trapping suggests that the non-controlled SiOx interfacial region is more defective than a controlled one.

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.

Growth mechanism of HfO2 film on H-terminated Si (100) surface from atomic layer deposition process using tetrakis(ethylmethylamino)hafnium

The deposition of HfO2 on H-terminated Si (100) surfaces presents both interest and challenges due to the complexity of the surface reaction mechanisms. Previous experimental studies reported conflicting results regarding the formation of interfacial layers and the presence of incubation periods during the initial cycles of the HfO2 atomic layer deposition (ALD) process. In this study, we provide a detailed analysis of the surface reaction mechanism of HfO2 ALD on H-terminated Si (100) surfaces using tetrakis(ethylmethylamino)hafnium (TEMAH) and water. Our findings demonstrate that TEMAH chemisorbs onto the H-terminated Si (100) surface through the formation of two N-Si interfacial bonds. Additionally, the remaining ligands of the adsorbed Hf precursor are efficiently removed through hydroxylation during the water pulse, with low energy barriers of +25.9 to +30.0 kJ/mol. Furthermore, the N-Si interfacial bonds can be broken, allowing for the formation of Hf-O-Si interfacial bonds during either the water pulse or post-deposition processes. This work provides valuable insights into the surface reactions of HfO2 ALD and explains the varied interfacial bonding and incubation periods observed in prior studies.

The Impact of ALD Precursor Choice on Nucleation and Growth of Dielectrics on 2D Materials

Recently, atomically thin two-dimensional (2D) transition metal dichalcogenides (TMDs) have received significant attention due to their unique optical and electrical properties, lending themselves to exciting applications such as microelectronics, sensing, catalysis, photonics, and more. The properties of TMDs can be acutely impacted by their environment due to their atomic thinness. For instance, in a TMD-based transistor with a high-k dielectric surrounding the TMD semiconductor, the dielectric could dope the channel via charge transfer; it can improve charge carrier mobility by screening the impact of charged impurities on scattering; or it can worsen charge carrier mobility by introducing defect states or interfacial roughness. As such, the quality and nature of the dielectric is critical, and ultimately those properties are determined by the means of dielectric deposition. Atomic layer deposition (ALD) is a common thin film deposition technique used for depositing high-k dielectrics for transistor devices. ALD relies on sequential self-limiting reactions between a precursor or co-reactant and a surface in order to deposit a material in a layer-by-layer manner with a high degree of control over thickness, composition, and uniformity. However, the ideal basal planes of 2D materials possess no dangling bonds or reactive functional groups, making their surfaces largely inert to ALD chemistry. Hence, when conventional ALD is performed on TMDs, nucleation and growth proceeds only at defect sites, grain boundaries, and the more reactive edge sites. This sparse nucleation leads to poor quality films of low density, high surface roughness, and pinholes.In this work, we employ physisorption-assisted ALD processes and study the nucleation and quality of the deposited dielectrics. We deposit AlOx on atomically thin monolayer MoS2 (itself on SiO2/Si) using a series of ALD precursors in order to investigate the impact of the ALD precursor on nucleation. Using scanning electron microscopy (SEM), we study film nucleation and continuity as a function of ALD precursor, cycle number, and temperature. After optimizing each process for each precursor in terms of temperature and purge time for maximized nucleation, we fabricate transistor devices in which the deposited AlOx serves as a seed layer for high-k HfO2 deposition to create the gate stack. Au contacts and a Pd top gate were used to complete the devices. Using x-ray photoelectron spectroscopy (XPS) and electrical testing, we investigate the character of the dielectric/MoS2 interface. We observe that the precursor used in the ALD process can lead to a wide range of coverages, with more conventional precursors resulting in sparse coverage, and novel precursors introduced in this study leading to nearly full coverage after just 3 nm of AlOx is deposited. We hypothesize that the improved coverage for the novel precursor is the result of enhanced interactions between the precursor and the 2D material due to its particular chemical structure. The reduced nucleation delay leads to a denser, smoother film with fewer defects at the 2D/3D material interface. The devices fabricated using the novel precursor and improved dielectric show excellent performance such as good on/off ratios (106), small device-to-device variation (ΔV T < 1 V), and low effective oxide thickness (~1 nm). This work provides useful insights into how ALD precursors could be designed to improve the quality of dielectrics on 2D materials, potentially improving the viability of 2D materials for wide ranging applications.

Sputtered thin film deposited laser induced graphene based novel micro-supercapacitor device for energy storage application

Pioneering flexible micro-supercapacitors, designed for exceptional energy and power density, transcend conventional storage limitations. Interdigitated electrodes (IDEs) based on laser-induced graphene (LIG), augmented with metal-oxide modifiers, harness synergies with layered graphene to achieve superior capacitance. This study presents a novel one-step process for sputtered plasma deposition of HfO2, resulting in enhanced supercapacitance performance. Introducing LIG-HfO2 micro-supercapacitor (MSC) devices with varied oxygen flow rates further boosts supercapacitance performance by introducing oxygen functional groups. FESEM investigations demonstrate uniform coating of HfO2 on LIG fibers through sputtering. Specific capacitance measurements reveal 6.4 mF/cm2 at 5 mV/s and 4.5 mF/cm2 at a current density of 0.04 mA/cm2. The LIG-HfO2 devices exhibit outstanding supercapacitor performance, boasting at least a fourfold increase over pristine LIG. Moreover, stability testing indicates a high retention rate of 97% over 5000 cycles, ensuring practical real-time applications.

Substrate temperature effects on PEALD HfAlO dielectric films for IGZO-TFT applications

Aluminum incorporated hafnium oxide (HfAlO) has garnered significant attention due to its high dielectric constant. The present study employs supercycle plasma-enhanced atomic layer deposition (PEALD) to prepare HfAlO films, switching between HfO2 and Al2O3 deposition. The effects of substate temperature on the HfAlO films are systematically investigated. The experimental results indicate that increased substrate temperatures favor Al2O3 growth on HfO2, but excessively high temperatures result in deviated PEALD growth and deteriorated film properties. The PEALD window can be reached at substrate temperatures of 300 to 350 °C. The Al content is substantially affected by the substrate temperature, despite a constant Al2O3 PEALD cycle ratio used. The film deposited at 350 °C exhibits the highest Al content, density, and (111) crystalline orientation ratio, leading to a high breakdown field of 6 MV/cm and a considerable dielectric constant of 27. The HfAlO films are applied to indium gallium zinc oxide transistor as the dielectric layer, and the 350 °C-deposited HfAlO leads to the best device performance with a remarkably high on/off current ratio of 3.5 × 109, low threshold voltage of −0.1 V, saturated mobility of 6.1 cm2 V−1 s−1, and a low subthreshold swing of 76 mV/dec.

Chemisorption and Surface Reaction of Hafnium Precursors on the Hydroxylated Si(100) Surface

Hafnium oxide (HfO2) is widely recognized as one of the most promising high-k dielectric materials due to its remarkable properties such as high permittivity, wide band gap, and excellent thermal and chemical stability. The atomic layer deposition (ALD) of HfO2 has attracted significant attention in recent decades since it enables uniform and conformal deposition of HfO2 thin films on various substrates. In this study, we examined the initial surface reactions of a series of homoleptic hafnium precursors on hydroxylated Si(100) surfaces using density functional theory calculations. Our theoretical findings align with previous experimental studies, indicating that hafnium amides exhibit higher reactivity compared to other precursors such as hafnium alkoxides and hafnium halides in surface reactions. Interestingly, we found that the chemisorption and reactivity of hafnium precursors are considerably affected by their thermal stability and size. For alkoxide precursors, which have similar thermal stabilities, the size of alkoxide ligands is an important factor in determining their reactivity. Conversely, the reactivity of hafnium halides, which have ligands of similar sizes, is primarily governed by their thermal stability. These insights are valuable for understanding the surface reaction mechanisms of precursors on hydroxylated Si(100) surfaces and for designing new materials, particularly heteroleptic precursors, in future research.

Computer Modeling of Plasma-Enhanced Atomic Layer Deposition of HfO2 and ZrO2

Computer Modeling of Plasma-Enhanced Atomic Layer Deposition of HfO2 and ZrO2

N-Polar GaN Deep Recess HEMT With Atomic Layer Deposition HfO2 as Gate Insulator

N-Polar GaN Deep Recess HEMT With Atomic Layer Deposition HfO₂ as Gate Insulator

Atomic Layer Deposition of HfO2 Films Using TDMAH and Water or Ammonia Water.

Atomic layer deposition of HfO2 from TDMAH and water or ammonia water at different temperatures below 400 °C is studied. Growth per cycle (GPC) has been recorded in the range of 1.2-1.6 Å. At low temperatures (≤100 °C), the films grew faster and are structurally more disordered, amorphous and/or polycrystalline with crystal sizes up to 29 nm, compared to the films grown at higher temperatures. At high temperatures of 240 °C, the films are better crystallized with crystal sizes of 38-40 nm but grew slower. GPC, dielectric constant, and crystalline structure are improved by depositing at temperatures above 300 °C. The dielectric constant value and the roughness of the films have been determined for monoclinic HfO2, a mixture of orthorhombic and monoclinic, as well as for amorphous HfO2. Moreover, the present study shows that the increase in the dielectric constant of the films can be achieved by using ammonia water as an oxygen precursor in the ALD growth. The detailed investigations of the relationship between HfO2 properties and growth parameters presented here have not been reported so far, and the possibilities of fine-tuning and controlling the structure and performance of these layers are still being sought.

Domain Wall Movement in Undoped Ferroelectric HfO2: A Rayleigh Analysis

The discovery of ferroelectricity in doped HfO2 in 2011 gave rise to quite a stir in the scientific world that persists up to this day. The complementary metal oxide semiconductor compatibility, as well as good scalability, enables versatile applications ranging from ferroelectric field effect transistors to ferroelectric tunnel junctions and neuromorphic devices. Stabilizing the metastable polar orthorhombic phase with space group Pca21-phase, which is responsible for the ferroelectricity in HfO2, is still challenging. We demonstrate for the first time a sputter deposition of undoped ferroelectric HfO2 on superconducting NbN electrodes, with a remanent polarization of 6.4 μC/cm2. Grazing incident X-ray diffraction on the layer structure, dynamic hysteresis measurements, and electron energy loss spectroscopy on fabricated devices indicate a HfO2 layer with low oxygen deficiency. Furthermore, no evidence of interdiffusion of oxygen or nitrogen at the interfaces is found. A sudden “wake-up” for the transition from the dielectric state to the ferroelectric state as well as no classical fatigue effect for the degradation of the ferroelectric performance are observed. These analyses are extended by an investigation of Rayleigh behavior using impedance spectroscopy. In that way, the domain wall flexibility is quantified and classified within different regimes of the various domain wall motions.

Dielectric-on-Dielectric Achieved on SiO2 in Preference to W by Water-free Chemical Vapor Depositions with Aniline Passivation.

Selective and smooth dielectric-on-dielectric was achieved by water-free single-precursor chemical vapor deposition (CVD) processes with the help of aniline passivation. Aniline selective passivation was demonstrated on W surfaces in preference to SiO2 at 250, 300, and 330 °C. After aniline passivation, selective HfO2, Al2O3, and TiO2 were deposited only on the HF-cleaned SiO2 surface by water-free single-precursor CVD using hafnium tert-butoxide Hf(OtBu)4, aluminum-tri-sec-butoxide (ATSB), and titanium isopropoxide Ti(OiPr)4 as the precursor reactants, respectively. Hf(OtBu)4 and Ti(OiPr)4 single-precursor CVD was carried out at 300 °C, while the ATSB CVD process was conducted at 330 °C. HfO2 and Al2O3 nanoselectivity tests were performed on W/SiO2 patterned samples. Transmission electron microscopy images of the W/SiO2 patterned samples after deposition demonstrated nanoselectivity and low surface roughness of HfO2 and Al2O3 deposition on the SiO2 regions only.

Time evolution of surface species during the ALD of high-k oxide on InAs

Understanding the reaction mechanisms involved during the early stage of atomic layer deposition (ALD) of HfO2 on InAs is a key requirement for improving interfaces in III-V semiconductor-based devices. InAs is an excellent candidate to outperform silicon regarding speed and power consumption, and combined with HfO2, it gives promise for a new generation of ultra-fast MOSFETs. However, an improved interface quality and in-depth understanding of the involved surface species are needed. Here, we use in situ and operando ambient pressure XPS to follow in real-time the reaction mechanisms which control the ALD chemistry. Besides the removal of all unwanted oxide from the III-V, the same oxygen atoms are found to form HfOx already from the first half-cycle. In contrast to the standard ALD model, no hydroxyl groups are needed on the InAs surface. Furthermore, we observe an insertion reaction forming unexpected surface species. The second ALD half-cycle allows the immediate removal of all organic species leaving behind a uniform HfO2 layer partially terminated by hydroxyl groups. We find that prolonged exposure times upon both half-cycles guarantee a sharp InAs/HfO2 interface. Such an improved interface is an important step towards fast and sustainable III-V semiconductor-based electronics.

Improved drain lag by reduced surface current in GaN HEMT via an ultrathin HfO2 blanket layer

This paper reports the influence of an ultrathin 1.5 nm atomic-layer-deposited HfO2 blanket layer as a gate dielectric on GaN high-electron-mobility transistors (HEMTs) grown on a 4H-SiC substrate. Transistors with a gate length of 250 nm and a source-to-drain distance of 3 µm were manufactured. The proposed technique involves HfO2 deposition at 250 ∘C prior to the gate metallization with no additional lithography steps. This approach reduced the drain lag by 83% compared to the conventional design with no gate dielectric. The HfO2 layer suppressed the parasitic lateral conduction from the gate, reduced surface trapping, and improved gate electrostatics. The manufactured devices exhibited nearly three orders of magnitude decreased surface leakage, better turn-on behavior, and improved cut-off frequency f T linearity by 16%. High quality metal-oxide interface formation was confirmed by the conductance method. Results demonstrate that the blanket HfO2 deposition is a promising approach to improve the current dispersion characteristics and gate electrostatics of GaN HEMTs without incurring major changes to the established fabrication techniques.

Impact of process anneals on high-k/β-Ga2O3 interfaces and capacitance

Gallium oxide (β-Ga2O3) is becoming a popular material for high power electronic devices due to its wide bandgap and ease of processing. In this work, β-Ga2O3 substrates received various annealing treatments before atomic layer deposition of HfO2 and subsequent fabrication of metal–oxide–semiconductor (MOS) capacitors. Annealing of β-Ga2O3 with forming gas or nitrogen produced degraded capacitance–voltage (C–V) behavior compared to a β-Ga2O3 control sample with no annealing. A sample annealed with pure oxygen had improved C–V characteristics relative to the control sample, with a higher maximum capacitance and smaller flat-band voltage shift, indicating that oxygen annealing improved the C–V behavior. X-ray photoelectron spectroscopy also suggested a reduction in the oxygen vacancy concentration after O2 annealing at 450 °C, which supports the improved C–V characteristics and indicates that O2 annealing of β-Ga2O3 may lead to better MOS device performance.

A theoretical study of the atomic layer deposition of HfO2 on Si(1 0 0) surfaces using tetrakis(ethylmethylamino) hafnium and water

The atomic layer deposition (ALD) of hafnium oxide (HfO2) thin film has attracted considerable attention owing to its crucial role in downscaling semiconductor devices. Herein, we theoretically investigated the reaction mechanisms during the ALD of HfO2 thin film (HfO2–ALD) on silicon (Si) surface using tetrakis(ethylmethylamino) hafnium (TEMAH) and water. By using two surface models, fully and partially hydroxylated Si(100) surfaces, we demonstrated that controlling the concentration of hydroxyl groups on the surface is important to enhance the activity for ligand-exchange reactions (LERs) of TEMAH and thereby effectively promote HfO2–ALD. During the precursor pulse, LERs on the fully hydroxylated Si(100) surface easily convert TEMAH into an unsaturated Hf atom with a low energy barrier of 25.8 kJ/mol, while mono(ethylmethylamino) hafnium (MEMAH) is obtained as the final product on the partially hydroxylated Si(100) surface with an energy barrier of 61.3 kJ/mol. The remaining ligand of the adsorbed MEMAH can be removed via LER with water during the water pulse. This work not only provides a better understanding of the conversion of TEMAH into a Hf atom on the hydroxylated Si(100) surface but also offers us more insights into the role of surface hydroxyls in the formation of HfO2 thin films.

Process Optimization to Reduce Power in HfO2-Based Rram Devices for in-Memory Computing

Recently, HfO2-based resistive random-access memory (RRAM) devices have shown promise as candidates for in-memory computing applications. By engineering the distribution of defects or oxygen vacancies, the switching dielectric can potentially enable low power switching and multi-conductance levels. A higher concentration of oxygen vacancies closer to the top electrode a two-terminal RRAM device with a HfO2/Al2O3 bilayer structure reduces switching energy (1). Introducing excess oxygen vacancies near the top electrode through a hydrogen plasma treatment with a Ru as top electrode reduces the switching power of the device (2).In this work we study the pulsed SET operation of different HfO2-based RRAM devices for their possible uses as multi-level cells. We have compared two different RRAM devices with HfO2-based dielectrics. Device-A is prepared with hydrogen plasma treatment at mid-point of HfO2 deposition (10nm Ti/50nmTiN/3nm HfO2(plasma treatment) 3nm HfO2/5nm Ru/5nm ALD TiN/ 50nm PVD TiN). Whereas Device-B constitutes a HfO2/Al2O3 bilayer structure (10nm Ti/50nmTiN/7nm HfO2/1nm Al2O3/5nm Ru/5nm ALD TiN/50nm PVD TiN). Both the devices have Ru as the top metal.Fig. 1 and Fig. 2 show the conductance modulation obtained by applying successive pulse sequences (80 pulses) with increasing the pulse width tp (from 4 \U0001d707s to 10 ms) every 20 pulses where the pulse height, Vp, remained fixed at 2V during the experiment. A read pulse with voltage Vr=0.1V and pulse width tr=1ms is applied immediately after each pulse. The forming compliance current in Device-A is seven time lower than Device-B. Both devices clearly show promising behavior of multi-level conductance. Figure 1

ALD and PEALD deposition of HfO2 and its effects on the nature of oxygen vacancies

The high-k materials are essential in thin-film transistors. A well-controlled thin film deposition that produces less electronic defects induced by charged vacancies is highly desirable for avoiding high leakage current and and providing an exceptional dielectric strength. This work addresses the use of atomic layer deposition (ALD), plasma-enhanced ALD (PEALD), and its variants to produce low-defective HfO2. The X-ray photoelectron spectroscopy (XPS) and Capacitance-Voltage (C-V) analysis revealed that the thermal ALD (TALD) samples produce electron emissions of ∼ 1 eV above the valence band maximum, negative flat band voltage shift of 1.51 V, and low dielectric breakdown strength (4.37 MV/cm). These properties confirm the high density of positive oxygen vacancies (1.2 × 1013 cm−2) acting as shallow traps, despite its O/Hf ratio (1.84) being higher than the Direct Plasma ALD (DPALD) sample. On the other hand, PEALD induces the formation of neutral vacancies promoted by the electric field of the plasma sheath. These defects are less detrimental to the capacitor performance as their flat band shifts are −0.25 and 1.01 V for remote plasma ALD (RPALD) and DPALD samples. Their dielectric breakdown strength increases by ∼ 1 MV/cm, and a reduced current density by 3 orders of magnitude less than TALD samples. The O/Hf ratio in DPALD samples is 1.80, confirming the benefits of using PEALD approaches.

Study of MIS structures based on CdHgTe and HfO2 applied by PEALD

We investigate the HfO2/Hg0.78Cd0.22Te interface fabricated by plasma-enhanced atomic layer deposition (PEALD) at 120 °C During the deposition of HfO2, no donor-like defects are introduced into mercury cadmium telluride. X-ray photoelectron spectroscopy and ellipsometry were used to establish the optimal process regime at 120 °C and to demonstrate how HfO2 layer composition and growth rate per cycle depend on post-plasma purge time; the optimum is achieved at 6 s. Increasing the post-plasma purge time decreases the carbon and nitrogen impurity concentration in the HfO2 layer. Measurements of the admittance of metal-insulator-semiconductor (MIS) structures over the surface of a sample show that the electro-physical properties are uniform. We discuss the method of measuring the admittance of MIS structures that allows us to minimize the contribution of slow states with trapped charge on shape and shift of the C–V curve. The results demonstrate that the densities of fixed charge, slow states, and fast interfacial traps at the HfO2/MCT interface are greater than that for Al2O3/MCT (also formed by PEALD). The interface trap density is estimated from a normalized parallel conductance map, and the HfO2 film adheres well.

Role of Temperature, Pressure, and Surface Oxygen Migration in the Initial Atomic Layer Deposition of HfO2on Anatase TiO2(101)

The atomic layer deposition of HfO2 on a TiO2(101) surface from tetrakis(dimethylamido)hafnium and water is investigated using a combination of in situ vacuum X-ray photoelectron spectroscopy (XPS) and time-resolved ambient pressure XPS. Precursor pressures and surface temperature are tuned as to map the space state of the deposition. In the initial stages of ALD, a reaction mechanism based on dissociative adsorption dominates over a classic ligand exchange mechanism, typically evoked when metal-amido complexes and water are used as the precursors for metal oxide ALD. Surface species, including a dimethyl ammonium ion and an imine, are identified. It is found that they can be formed only if the active role of the TiO2(101) surface is taken into consideration. The temperature of the surface enhances the formation of these species based on an insertion reaction of a hydrogen atom, which then assists the formation of more than the expected monolayer of HfO2. A HfO2 overlayer is produced already during the first half-cycle, enabled by a reduction of the TiO2 support. Dosing water at high pressure allows hydroxyl formation, which marks the transition toward a well-described ligand exchange reaction type. From the experiments performed, we find that the ALD of HfO2 at room temperature, performed at high pressure, is mainly based on dissociation and that no side reaction occurs. These insights into the ALD reaction mechanism highlight how in situ studies can help understand how deposition parameters affect the growth of HfO2 and how the ALD model for transition metal oxide formation from amido complexes and water can be extended.