Aile Tamm

Advanced cyclopentadienyl precursors for atomic layer deposition of ZrO2 thin films

ZrO2 thin films were grown onto silicon (100) substrates by atomic layer deposition (ALD) using novel cyclopentadienyl-type precursors, namely (CpMe)2ZrMe2 and (CpMe)2Zr(OMe)Me (Cp = cyclopentadienyl, C5H5) together with ozone as the oxygen source. Growth characteristics were studied in the temperature range of 250 to 500 °C. An ALD-type self-limiting growth mode was verified for both processes at 350 °C where highly conformal films were deposited onto high aspect ratio trenches. Signs of thermal decomposition were not observed at or below 400 °C, a temperature considerably exceeding the thermal decomposition temperature of the Zr-alkylamides. Processing parameters were optimised at 350 °C, where deposition rates of 0.55 and 0.65 A cycle−1 were obtained for (CpMe)2ZrMe2/O3 and (CpMe)2Zr(OMe)Me/O3, respectively. The films grown from both precursors were stoichiometric and polycrystalline with an increasing contribution from the metastable cubic phase with decreasing film thickness. In the films grown from (CpMe)2ZrMe2, the breakdown field did not essentially depend on the film thickness, whereas in the films grown from (CpMe)2Zr(OMe)Me the structural homogeneity and breakdown field increased with decreasing film thickness. The films exhibited good capacitive properties that were characteristic of insulating oxides and did not essentially depend on the precursor chemistry.

Modification of Hematite Electronic Properties with Trimethyl Aluminum to Enhance the Efficiency of Photoelectrodes

The electronic properties of hematite were investigated by means of synchrotron radiation photoemission (SR-PES) and X-ray absorption spectroscopy (XAS). Hematite samples were exposed to trimethyl aluminum (TMA) pulses, a widely used Al-precursor for the atomic layer deposition (ALD) of Al2O3. SR-PES and XAS showed that the electronic properties of hematite were modified by the interaction with TMA. In particular, the hybridization of O 2p states with Fe 3d and Fe 4s4p changed upon TMA pulses due to electron inclusion as polarons. The change of hybridization correlates with an enhancement of the photocurrent density due to water oxidation for the hematite electrodes. Such an enhancement has been associated with an improvement in charge carrier transport. Our findings open new perspectives for the understanding and utilization of electrode modifications by very thin ALD films and show that the interactions between metal precursors and substrates seem to be important factors in defining their electronic and photoelectrocatalytic properties.

Investigation of ZrO2 – Gd2O3 Based High-k Materials as Capacitor Dielectrics

Atomic layer deposition (ALD) of ZrO 2 ―Gd 2 O 3 nanolaminates and mixtures was investigated for the preparation of a high permittivity dielectric material. Variation in the relative number of ALD cycles for constituent oxides allowed one to obtain films with controlled composition. Pure ZrO 2 films possessed monoclinic and higher permittivity cubic or tetragonal phases, whereas the inclusion of Gd 2 O 3 resulted in the disappearance of the monoclinic phase. Changes in phase composition were accompanied with increased permittivity of mixtures and laminates with low Gd content. Further increase in the lower permittivity Gd 2 O 3 content above 3.4 cat. % resulted in the decreased permittivity of the mixtures. Leakage currents generally decreased with increasing Gd content, whereby laminated structures demonstrated smaller leakage currents than mixed films at a comparable Gd content. Concerning the bottom electrode materials, the best results in terms of permittivity and leakage currents were achieved with Ru, allowing a capacitance equivalent oxide thickness of ∼ 1 nm and a current density of 3 × 10 ―8 A/cm 2 at 1 V. Charge storage values up to 60 nC/mm 2 were obtained for mixtures and laminates with thickness below 30 nm. In general, at electric fields below 2-3 MV/cm, normal and trap-compensated Poole-Frenkel conduction mechanisms were competing, whereas at higher fields, Fowler-Nordheim and/or trap-assisted tunneling started to dominate.

Behavior of zirconium oxide films processed from novel monocyclopentadienyl precursors by atomic layer deposition

ZrO2 thin films were grown by atomic layer deposition from new cyclopentadienyl precursors on planar TiN and SiO2∕Si substrates, as well as on silicon having deep trenches with aspect ratio of 1:60. The films demonstrated conformal growth in trenches achieving step coverage of 80%–90%. ZrO2 films crystallized either after exceeding threshold thickness or upon annealing dominantly in the cubic or tetragonal polymorph. Leakage current densities of (7–9)×10−8A∕cm2 at capacitance equivalent oxide thicknesses of 0.76–0.82nm were reached.

Atomic Layer Deposition and Characterization of Erbium Oxide-Doped Zirconium Oxide Thin Films

ZrO 2 films doped with Er 2 O 3 were grown by atomic layer deposition from tris(2,2,6,6-tetramethyl-3,5-heptanedionato)erbium, bis(methylcyclopentadienyl)methoxymethylzirconium, and ozone as precursors at 350°C. The erbium content was 1―5 cation %. The films were uniform in thickness. The ZrO 2 :Er 2 O 3 films were crystallized already in the as-deposited state. Upon annealing at 650°C, they were stabilized in the form of cubic or tetragonal polymorph of ZrO 2 . Enhancement in capacitance required intense crystallization that was observed when the film thickness exceeded 4.4 nm. The permittivity of the ZrO 2 :Er 2 O 3 films could reach 31. The capacitors based on the doped ZrO 2 possessed lower capacitance equivalent oxide thickness compared to the nondoped ZrO 2 and also comparable leakage current densities.

Dysprosium oxide and dysprosium-oxide-doped titanium oxide thin films grown by atomic layer deposition

Dysprosium oxide and dysprosium-oxide-doped titanium oxide thin films were grown by atomic layer deposition on silicon substrates. For depositing dysprosium and titanium oxides Dy(thd)3-O3 and TiCl4-O3 were used as precursors combinations. Appropriate parameters for Dy(thd)3-O3 growth process were obtained by using a quartz crystal microbalance system. The Dy2O3 films were deposited on planar substrates and on three-dimensional substrates with aspect ratio 1:20. The Dy/Ti ratio of Dy2O3-doped TiO2 films deposited on a planar silicon substrate ranged from 0.04 to 0.06. Magnetometry studies revealed that saturation of magnetization could not be observed in planar Dy2O3 films, but it was observable in Dy2O3 films on 3D substrates and in doped TiO2 films with a Dy/Ti atomic ratio of 0.06. The latter films exhibited saturation magnetization 10−6 A cm2 and coercivity 11 kA/m at room temperature.

Atomic layer deposition and properties of ZrO2/Fe2O3 thin films

Thin solid films consisting of ZrO2 and Fe2O3 were grown by atomic layer deposition (ALD) at 400 °C. Metastable phases of ZrO2 were stabilized by Fe2O3 doping. The number of alternating ZrO2 and Fe2O3 deposition cycles were varied in order to achieve films with different cation ratios. The influence of annealing on the composition and structure of the thin films was investigated. Additionally, the influence of composition and structure on electrical and magnetic properties was studied. Several samples exhibited a measurable saturation magnetization and most of the samples exhibited a charge polarization. Both phenomena were observed in the sample with a Zr/Fe atomic ratio of 2.0.

Magnetic and Electrical Performance of Atomic Layer Deposited Iron Erbium Oxide Thin Films

Mixed films of a high-permittivity oxide, Er2O3, and a magnetic material, Fe2O3, were grown by atomic layer deposition on silicon and titanium nitride at 375 °C using erbium diketonate, ferrocene, and ozone as precursors. Crystalline phases of erbium and iron oxides were formed. Growth into three-dimensional trenched structures was demonstrated. A structure deposited using tens to hundreds subsequent cycles for both constituent metal oxide layers promoted both charge polarization and saturative magnetization compared to those in the more homogeneously mixed films.

Properties of HfO2 and HfO2: Y films grown by atomic layer deposition in an advanced monocyclopentadienyl-based process

Uniform HfO2 and HfO2:Y (7–8 cat.% Y) thin films were grown from a novel cyclopentadienyl-alkylamido precursor CpHf(NMe2)3, (CpMe)3Y and O3 at 300 °C. HfO2:Y films possessed somewhat higher roughness, and crystallized upon annealing at 500 °C in the form of cubic or tetragonal polymorph, compared to monoclinic HfO2 films. HfO2:Y demonstrated lower capacitance equivalent oxide thickness compared to HfO2, although HfO2films occurred slightly better insulating. Higher capacitance required intense crystallization achieved in the films grown to thicknesses exceeding 6 nm.

RRAM Memories with ALD High-K Dielectrics: Electrical Characterization and Analytical Modeling

Resistive switching phenomena with adequate repetitiveness on Ta 2 O 5 -TiO 2 -Ta 2 O 5 and TiO 2 -Ta 2 O 5 -TiO 2 stacks are reported. In particular, 5–nm-thick TiO 2 films embedding a monolayer of Ta 2 O 5 show the best behavior in terms of bipolar cycles loop width, with separate low and high resistive states up to two orders of magnitude. Tantalum oxide layer increases the defect density in titania that becomes less leaky, and thus, resistive switching effects appear. Small signal ac parameters measured at low and medium frequencies, namely capacitance and conductance, also show hysteretic behavior during a whole bipolar switching cycle. This means that the memory state can be read at 0 V, without any power consumption. High-frequency measurements provide information about dipole relaxation frequency values in the dielectric bulk, and this can be connected with resistive switching behavior. Finally, a double tunneling barrier model fits I-V curves at the low-resistance state even at the bias range where reset occurs and a sharp fall takes place.