Al2O3 Al2O3 Atomic Layer Deposition Research Articles

Effect of the number and distribution of Al2O3 atomic layer deposition cycles within HfO2 layer on ferroelectric characteristics

We comprehensively analyze the effects of the number and distribution of Al2O3 atomic layer deposition (ALD) cycles into a 10-nm-thick HfO2 matrix on the ferroelectric switching behavior. An ALD cycle containing one pulse for Hf (or Al) precursor and one pulse of water as reactant is repeated 150 times for the given thickness of 10 nm. Spontaneous remnant polarization (Pr) is enabled through the formation of crystalline Al-doped HfO2 (Al:HfO2) by incorporating at least two Al2O3 ALD cycles evenly into the HfO2 film under annealing at 600 °C for 3 min following W top electrode (TE) deposition. When more than four Al2O3 cycles are used, the Al elements function as leakage sources rather than stressors, resulting in an open hysteresis loop and a weak endurance of 105 cycles. Notably, an improved 2 Pr of ∼9 μC/cm2 is achieved when the Al2O3 layers are concentrated near the lower region of the HfO2. On the other hand, as the Al2O3 layers are intensively located in the upper region of the HfO2, a dielectric response is observed in the polarization–voltage and current–voltage measurements. Our results indicate that the two mechanical stresses induced by the Al dopant with a size smaller than Hf and the difference in the thermal expansion coefficient between TE and Al:HfO2 effectively activate both the lower and upper sites. Therefore, many dipoles are observed to participate in the polarization owing to the stresses that are applied evenly throughout the Al:HfO2 layer to form the orthorhombic phase.

High Efficiency Kesterite Solar Cells Through a Dual Treatment Approach: Improving the Quality of both Absorber Bulk and Heterojunction Interface

AbstractCu2ZnSn(S,Se)4 (CZTSSe) solar cells, composed of earth‐abundant materials, face challenges in achieving high power conversion efficiency (PCE) due to non‐radiative recombination. The limitations primarily stem from the prevalence of CuZn‐related and SnZn‐related defects in the bulk of the absorber and at the heterojunction interface region. To overcome these challenges, a dual treatment approach is proposed that involves Ag‐alloying in the bulk and Al2O3 atomic layer deposition (ALD) process at the p–n interface. Comprehensive characterizations of the fabricated devices reveal that Ag‐alloying leads to a substantial reduction in deep defects within the bulk, while Al2O3 ALD process facilitates the formation of a well‐defined p–n interface region, along with the defect passivation. This synergetic effect enables PCE enhancement accompanied by the improvements of all device parameters. Notably, devices subjected to the additional Al2O3 ALD process exhibit a substantial improvement in fill factor, which is found to be consistent with a reduced element intermixing width at the p–n heterojunction region. Due to improvements in both the bulk absorber and the heterojunction, the highest PCE of 13.33% is achieved without an anti‐reflection coating.

Recovery of Degraded Ni-Rich NMC811 Particles for Lithium-Ion Batteries

Nickel-rich cathode materials attract a lot of attention due to their high energy density. However, their sensitivity to air and moisture hinders their practical applications. Exposing Ni-rich NMC in the air will produce by-products that are not conducive to lithium-ion transmission. Water washing can be used to remove by-products on the surface, but it can also introduce new problems, such as the generation of NiO on the particle surface, which will reduce the electrochemical performance of the material. In this study, Al2O3 atomic layer deposition (ALD) was applied on degraded NMC811 particles to recover the electrochemical performance of the degraded NMC811 particles, which had been exposed to moisture. XPS and XRD analysis verified that the Al2O3 ALD coating removed the contaminant components on the particle surface, such as LiOH and Li2CO3, as well as reactivation of inactive Ni due to reaction with moisture. Furthermore, the coating film can also inhibit side reactions during the charge and discharge process and enhance the cycling stability.

First-Principles Study of Amorphous Al2O3 ALD Coating in Li-S Battery Electrode Design

The Li-S battery is exceptionally appealing as an alternative candidate beyond Li-ion battery technology due to its promising high specific energy capacity. However, several obstacles (e.g., polysulfides’ dissolution, shuttle effect, high volume expansion of cathode, etc.) remain and thus hinder the commercialization of the Li-S battery. To overcome these challenges, a fundamental study based on atomistic simulation could be very useful. In this work, a comprehensive investigation of the adsorption of electrolyte (solvent and salt) molecules, lithium sulfide, and polysulfide (Li2Sx with 2 ≤x≤ 8) molecules on the amorphous Al2O3 atomic layer deposition (ALD) surface was performed using first-principles density functional theory (DFT) calculations. The DFT results indicate that the amorphous Al2O3 ALD surface is selective in chemical adsorption towards lithium sulfide and polysulfide molecules compared to electrolytes. Based on this work, it suggests that the Al2O3 ALD is a promising coating material for Li-S battery electrodes to mitigate the shuttling problem of soluble polysulfides.

Deposit and etchback approach for ultrathin Al2O3 films with low pinhole density using atomic layer deposition and atomic layer etching

­Ultrathin Al2O3 atomic layer deposition (ALD) films with low pinhole density were fabricated using a deposit and etchback approach. This strategy was able to avoid the pinholes that originated during nonuniform nucleation of Al2O3 ALD films. In this method, an Al2O3 ALD film was deposited to a thickness greater than the desired thickness to reduce the number of pinholes and form a more continuous Al2O3 ALD film. Subsequently, the Al2O3 ALD film was etched back to a smaller thickness using thermal Al2O3 atomic layer etching (ALE). The number of pinholes in the resulting Al2O3 ALD film was measured by the percentage yield of metal-insulator-metal (MIM) capacitors based on an Ag/Al2O3/Al structure that did not have an electrical short. The device yield was improved using the deposition and etchback approach. For example, using device areas of 0.01 mm2, Al2O3 ALD films that were grown to 5 nm in the MIM capacitor gave a yield of 30%–40%. In contrast, Al2O3 ALD films that were grown to 24 nm and then etched back to 5 nm to form the MIM capacitor provided a yield of 65%–75%. This increase in yield of approximately 100% indicates that the deposit and etchback approach can improve the yield of MIM devices based on ultrathin Al2O3 ALD films. Although this method has been previously suggested to improve the quality of ultrathin films, this report is believed to be the first demonstrated application of using the deposit and etchback approach for device fabrication. Additional experiments revealed that a portion of the yield improvement can be attributed to the fluorination of the Al2O3 ALD films that produced a volume expansion when forming AlF3. This expansion may produce a compressive stress that helps to close the pinholes. The dielectric constant was also measured for Al2O3 ALD films versus Al2O3 film thickness. The dielectric constant was the same for as-deposited Al2O3 ALD films and Al2O3 ALD films that were first grown to 24 nm and then etched back to smaller thicknesses. This agreement indicates that the dielectric constant can be understood in terms of a series capacitor model and that Al2O3 ALE does not affect the electrical properties of the Al2O3 films.

Smoothing surface roughness using Al2O3 atomic layer deposition

Al2O3 atomic layer deposition (ALD) was used to smooth the roughness on silicon wafers obtained prior to chemical mechanical polishing (CMP). The initial silicon wafers had an average RMS surface roughness of 3.3 nm as determined by atomic force microscopy (AFM) measurements. AFM line scans also measured an average lateral spacing of ≈490 nm between the surface asperities. The RMS roughness decreased and the average lateral spacing increased progressively with number of Al2O3 ALD cycles. After 3000 Al2O3 ALD cycles that deposit an Al2O3 film thickness of 370 nm, the RMS roughness reduced to 1.5 nm and the average lateral spacing between the surface asperities increased to ≈890 nm. Additional Al2O3 ALD cycles produced little change in the RMS roughness or average lateral spacing. The efficiency of the smoothing decreased when the lateral distance between the surface asperities was much larger than the Al2O3 ALD film thickness. Power spectral density (PSD) analysis revealed that the ALD smoothing was most effective for surface topographical features with lateral spacings in the range of 10s to 100s of nanometers. Reflectivity studies of silver films deposited on the silicon wafers also demonstrated that Al2O3 ALD smoothing improved the optical performance of reflective mirrors.

SF4 as the Fluorination Reactant for Al2O3 and VO2 Thermal Atomic Layer Etching

Thermal atomic layer etching (ALE) is an important technique for the precise isotropic etching of nanostructures. Thermal ALE of many materials can be achieved using a two-step fluorination and ligand-exchange reaction mechanism. Most previous thermal ALE processes have used HF as the fluorination reactant. Alternative fluorination reactants may be needed because HF is a weak nucleophilic fluorination reactant. Stronger fluorination agents may be required for the fluorination of some materials. To demonstrate the usefulness of SF4 as an alternative to HF, thermal Al2O3 ALE was compared using SF4 or HF together with Sn(acac)2 as the metal precursor for ligand exchange. SF4 and HF were observed to behave similarly as fluorination reactants during Al2O3 ALE. The mass gains during the initial SF4 and HF exposures on Al2O3 atomic layer deposition (ALD) films at 200 °C were comparable at 35 and 38 ng/cm2, respectively, using quartz crystal microbalance measurements. In addition, the etch rates were similar at 0.20 and 0.28 Å/cycle for Al2O3 ALE using SF4 and HF, respectively, at 200 °C. Thermal VO2 ALE was also performed for the first time using SF4 or HF and Sn(acac)2 as the reactants. There was evidence that SF4 is a stronger fluorination reactant than HF for VO2 fluorination. The mass gains during the initial SF4 and HF exposures on VO2 ALD films were 38 and 20 ng/cm2, respectively, at 200 °C. Thermal VO2 ALE also had a higher etch rate when fluorinating with SF4 compared with HF. Etch rates of 0.30 and 0.11 Å/cycle were measured for VO2 ALE using SF4 and HF, respectively, together with Sn(acac)2 at 200 °C. Fourier transform infrared experiments were also used to monitor fluorination of the Al2O3 and VO2 ALD films by SF4 or HF. FTIR difference spectroscopy was used to observe the increase of Al–F and V–F stretching vibrations and the loss of the Al–O and V—O/V═O stretching vibrations for Al2O3 and VO2, respectively, versus SF4 or HF exposure at 200 °C. Additional absorbance features after fluorination of the Al2O3 ALD films by SF4 were consistent with SFx surface species. SF4 is a useful fluorination agent for thermal ALE processes and can be used as an alternative to HF. In addition, SF4 may be necessary when fluorination requires a stronger fluorination reactant than HF.

Doped Si nanoparticles with conformal carbon coating and cyclized-polyacrylonitrile network as high-capacity and high-rate lithium-ion battery anodes

Doped Si nanoparticles (SiNPs) with conformal carbon coating and cyclized-polyacrylonitrile (PAN) network displayed capacities of 3500 and 3000 mAh g−1 at C/20 and C/10, respectively. At 1 C, the electrode preserves a specific discharge capacity of ∼1500 mAh g−1 for at least 60 cycles without decay. Al2O3 atomic layer deposition (ALD) helps improve the initial Coulombic efficiency (CE) to 85%. The dual coating of conformal carbon and cyclized-PAN help alleviate volume change and facilitate charge transfer. Ultra-thin Al2O3 ALD layers help form a stable solid electrolyte interphase interface.

Atomic layer etching of Al2O3 using sequential, self-limiting thermal reactions with Sn(acac)2 and hydrogen fluoride.

The atomic layer etching (ALE) of Al2O3 was demonstrated using sequential, self-limiting thermal reactions with tin(II) acetylacetonate (Sn(acac)2) and hydrogen fluoride (HF) as the reactants. The Al2O3 samples were Al2O3 atomic layer deposition (ALD) films grown using trimethylaluminum and H2O. The HF source was HF-pyridine. Al2O3 was etched linearly with atomic level precision versus number of reactant cycles. The Al2O3 ALE was monitored at temperatures from 150 to 250 °C. Quartz crystal microbalance (QCM) studies revealed that the sequential Sn(acac)2 and HF reactions were self-limiting versus reactant exposure. QCM measurements also determined that the mass change per cycle (MCPC) increased with temperature from -4.1 ng/(cm(2) cycle) at 150 °C to -18.3 ng/(cm(2) cycle) at 250 °C. These MCPC values correspond to etch rates from 0.14 Å/cycle at 150 °C to 0.61 Å/cycle at 250 °C based on the Al2O3 ALD film density of 3.0 g/cm(3). X-ray reflectivity (XRR) analysis confirmed the linear removal of Al2O3 and measured an Al2O3 ALE etch rate of 0.27 Å/cycle at 200 °C. The XRR measurements also indicated that the Al2O3 films were smoothed by Al2O3 ALE. The overall etching reaction is believed to follow the reaction Al2O3 + 6Sn(acac)2 + 6HF → 2Al(acac)3 + 6SnF(acac) + 3H2O. In the proposed reaction mechanism, the Sn(acac)2 reactant donates acac to the substrate to produce Al(acac)3. The HF reactant allows SnF(acac) and H2O to leave as reaction products. The thermal ALE of many other metal oxides using Sn(acac)2 or other metal β-diketonates, together with HF, should be possible by a similar mechanism. This thermal ALE mechanism may also be applicable to other materials such as metal nitrides, metal phosphides, metal sulfides and metal arsenides.

Utilization of Al2O3 Atomic Layer Deposition for Li Ion Pathways in Solid State Li Batteries

Limited interfacial contact area between active material and solid state electrolyte (SSE) in solid state lithium batteries (SSLBs) can be addressed by utilizing a thermally treated Al2O3 atomic layer deposition (ALD) coating as a lithium ion transport pathway. SSLBs with a LiCoO2/Li3.15Ge0.15P0.85S4/77.5Li2S-22.5P2S5/Li configuration were built and tested using charge-discharge cycles between 3.3 ∼ 4.3 V (vs. Li/Li+) at a current density of 45 μA cm−2. An increase of more than 10% in the inital discharge capacity as well as good cycling stability are achieved from SSLBs using LiCoO2 particles coated with 2 and 4 cycles of Al2O3 ALD. The dQ/dV analysis, electrochemical impedance spectroscopy (EIS), and the overpotential study with galvanostatic intermittent titration technique (GITT) were conducted to elucidate the enhancement of Li+ transport through Al2O3 ALD layers surrounding LiCoO2.

Quantitative in situ infrared analysis of reactions between trimethylaluminum and polymers during Al2O3 atomic layer deposition

The reactions of trimethylaluminum (TMA) toward substrates during the Al2O3 atomic layer deposition (ALD) on a variety of polymers were studied by in situ Fourier transform infrared spectroscopy (FTIR). The experiments demonstrate that TMA reacts with certain nucleophilic functional groups on the polymer surface during the first several ALD cycles. For some polymer substrates, TMA vapor penetrates into the polymer and reacts in the polymer bulk. In both cases, the initial reaction plays an important role in the nucleation and growth of Al2O3. For chemically inert polymers, such as polypropylene, nucleation of Al2O3 ALD is relative slow at the initial stage due to the lack of reactive groups on the substrate. However, polyester, polyamide and polyether are more reactive, and in situ FTIR spectra showed a larger extent of reaction with TMA, facilitating the nucleation of ALD film on these polymers. By comparing FTIR spectra, we quantitatively estimate the extent of TMA reaction towards different polymers, and confirmed the results using X-ray photoelectron spectroscopy and scanning electron microscopy. Results give insight into the importance of the polymer structure in determining the nature and extent of the reaction during ALD film processing on polymer substrates.

(Invited) Aluminium Oxide Atomic Layer Deposition on Semiconductor Substrates

Al2O3 Atomic Layer Deposition (ALD) on semiconductor substrates finds potential applications in CMOS devices, memory devices, solar cells, etc. In this paper, we study the TMA/H2O ALD of Al2O3 on Si, Ge and GaAs substrates. Total reflection X-Ray Fluorescence (TXRF) is used to investigate the growth-per-cycle evolution during the early ALD reaction cycles. TXRF demonstrates growth inhibition from the second reaction cycle on many substrates, including conventionally used fully hydroxylated SiO2. Theoretical calculations, based on Density Functional Theory, indicate the formation of electron deficient bonds at the Al2O3/semiconductor interface, by interaction between neighboring monomethyl aluminium surface species. Both experiment and theory thus indicate that the surface chemistry of Al2O3 ALD of is more complex than previously described.

Critical tensile and compressive strains for cracking of Al2O3 films grown by atomic layer deposition

Al2O3 atomic layer deposition (ALD) is a model ALD system and Al2O3 ALD films are excellent gas diffusion barrier on polymers. However, little is known about the response of Al2O3 ALD films to strain and the potential film cracking that would restrict the utility of gas diffusion barrier films. To understand the mechanical limitations of Al2O3 ALD films, the critical strains at which the Al2O3 ALD films will crack were determined for both tensile and compressive strains. The tensile strain measurements were obtained using a fluorescent tagging technique to image the cracks. The results showed that the critical tensile strain is higher for thinner thicknesses of the Al2O3 ALD film on heat-stabilized polyethylene naphthalate (HSPEN) substrates. A low critical tensile strain of 0.52% was measured for a film thickness of 80 nm. The critical tensile strain increased to 2.4% at a film thickness of 5 nm. In accordance with fracture mechanics modeling, the critical tensile strains and the saturation crack densities scaled as (1/h)1/2 where h is the Al2O3 ALD film thickness. The fracture toughness for cracking, KIC, of the Al2O3 ALD film was also determined to be KIC = 2.30 MPa m1/2. Thinner Al2O3 ALD film thicknesses also had higher critical strains for cracking from compressive strains. Field-emission scanning electron microscopy (FE-SEM) images revealed that Al2O3 ALD films with thicknesses of 30–50 nm on Teflon fluorinated ethylene propylene (FEP) substrates cracked at a critical compressive strain of ∼1.0%. The critical compressive strain increased to ∼2.0% at a film thickness of ∼20 nm. A comparison of the critical tensile strains on HSPEN substrates and critical compressive strains on Teflon FEP substrates revealed some similarities. The critical strain was ∼1.0% for film thicknesses of 30–50 nm for both tensile and compressive strains. The critical compressive strain then increased more rapidly than the critical tensile strain for thinner films with thicknesses < 30 nm. The high critical tensile and compressive strains for thin Al2O3 ALD films should be very useful for flexible gas diffusion barriers on polymers.

Oxidant prepulsing of Ge (100) prior to atomic layer deposition of Al2O3: In situ surface characterization

We investigate the effects of H2O oxidant pulsing of Ge (100) substrates prior to Al2O3 atomic layer deposition (ALD) to synthesize metal-oxide-semiconductor devices. The prepulsing reduces the hysteresis in capacitance-voltage measurements from 490 to 30 mV at the flatband voltage and appears to reduce the density of fast interface states while causing a small increase in the midgap interface state density. Real-time x-ray photoelectron spectroscopy during prepulsing revealed the dynamic evolution of Ge–O bonding states and the formation of a surface layer with ∼70% hydroxyl termination which appears to produce a less defective interfacial region after subsequent Al2O3 ALD.

Gas diffusion ultrabarriers on polymer substrates using Al2O3 atomic layer deposition and SiN plasma-enhanced chemical vapor deposition

Thin films grown by Al2O3 atomic layer deposition (ALD) and SiN plasma-enhanced chemical vapor deposition (PECVD) have been tested as gas diffusion barriers either individually or as bilayers on polymer substrates. Single films of Al2O3 ALD with thicknesses of ≥10 nm had a water vapor transmission rate (WVTR) of ≤5×10−5 g/m2 day at 38 °C/85% relative humidity (RH), as measured by the Ca test. This WVTR value was limited by H2O permeability through the epoxy seal, as determined by the Ca test for the glass lid control. In comparison, SiN PECVD films with a thickness of 100 nm had a WVTR of ∼7×10−3 g/m2 day at 38 °C/85% RH. Significant improvements resulted when the SiN PECVD film was coated with an Al2O3 ALD film. An Al2O3 ALD film with a thickness of only 5 nm on a SiN PECVD film with a thickness of 100 nm reduced the WVTR from ∼7×10−3 to ≤5×10−5 g/m2 day at 38 °C/85% RH. The reduction in the permeability for Al2O3 ALD on the SiN PECVD films was attributed to either Al2O3 ALD sealing defects in the SiN PECVD film or improved nucleation of Al2O3 ALD on SiN.

Gas Diffusion Barriers on Polymers Using Multilayers Fabricated by Al2O3 and Rapid SiO2 Atomic Layer Deposition

Single Al2O3 atomic layer deposition (ALD) films on polymers have demonstrated excellent gas diffusion barrier properties. Further improvements can be achieved using multilayers of Al2O3 ALD layers with other inorganic layers. In this study, multilayers of Al2O3 ALD and rapid SiO2 ALD were grown on Kapton and heat-stabilized polyethylene naphthalate substrates. Transmission rates for tritium through the films were measured using the radioactive HTO tracer method. Comparison with previous Ca tests and tritium exchange experiments with alcohols indicated that the tritium in HTO may diffuse as either molecular HTO or atomic tritium. Assuming that the tritium diffuses only as HTO, single Al2O3 ALD films reduced the effective water vapor transmission rate (WVTR) to ∼1 × 10-3 g/m2/day. When the Al2O3 ALD film was directly exposed to the saturated H2O/HTO vapor pressure, the barrier properties deteriorated markedly after 4−5 days. Al2O3 ALD barriers that were not subjected to direct H2O/HTO exposure indefinitely...

Nucleation and Growth during Al2O3 Atomic Layer Deposition on Polymers

Nucleation and growth during Al2O3 atomic layer deposition (ALD) were explored on a variety of polymer films at 85 °C. Al2O3 ALD was performed using sequential exposures of Al(CH3)3 [trimethylalumi...

Atomic Layer Deposition of Al2O3 Films on Polyethylene Particles

Sequential exposures of Al(CH3)3 and H2O at 77 °C were used to encapsulate low-density polyethylene (LDPE) particles with an ultrathin Al2O3 film. FTIR studies revealed that the nucleation of Al2O3 atomic layer deposition (ALD) on the LDPE particles occurred primarily via adsorption of Al(CH3)3 onto the LDPE surface or absorption of Al(CH3)3 into the LDPE particle followed by the reaction with H2O. The FTIR spectra then revealed the progressive switching between AlCH3* and AlOH* species with alternating exposure to Al(CH3)3 and H2O. This nucleation of Al2O3 ALD did not require the existence of specific chemical functional groups on the polymer. The FTIR spectra also demonstrated that the sequential exposures of Al(CH3)3 and H2O led to an increase in Al2O3 bulk vibrational modes. The increase of the absorbance for the Al2O3 bulk vibrational modes was linear with the number of AB cycles. The presence of an Al2O3 film on the LDPE particles was confirmed using transmission electron microscopy (TEM). The TEM i...