Massimo Tallarida

Ellipsometry and XPS comparative studies of thermal and plasma enhanced atomic layer deposited Al2O3-films

Summary We report on results on the preparation of thin (<100 nm) aluminum oxide (Al2O3) films on silicon substrates using thermal atomic layer deposition (T-ALD) and plasma enhanced atomic layer deposition (PE-ALD) in the SENTECH SI ALD LL system. The T-ALD Al2O3 layers were deposited at 200 °C, for the PE-ALD films we varied the substrate temperature range between room temperature (rt) and 200 °C. We show data from spectroscopic ellipsometry (thickness, refractive index, growth rate) over 4” wafers and correlate them to X-ray photoelectron spectroscopy (XPS) results. The 200 °C T-ALD and PE-ALD processes yield films with similar refractive indices and with oxygen to aluminum elemental ratios very close to the stoichiometric value of 1.5. However, in both also fragments of the precursor are integrated into the film. The PE-ALD films show an increased growth rate and lower carbon contaminations. Reducing the deposition temperature down to rt leads to a higher content of carbon and CH-species. We also find a decrease of the refractive index and of the oxygen to aluminum elemental ratio as well as an increase of the growth rate whereas the homogeneity of the film growth is not influenced significantly. Initial state energy shifts in all PE-ALD samples are observed which we attribute to a net negative charge within the films.

Atomic Layer Deposition of Ruthenium Films from (Ethylcyclopentadienyl)(pyrrolyl)ruthenium and Oxygen

Ru films were grown by atomic layer deposition in the temperature range of 275―350°C using (ethylcyclopentadienyl)(pyrrolyl)ruthenium and air or oxygen as precursors on HF-etched Si, SiO 2 , ZrO 2 , and TiN substrates. Conformal growth was examined on three-dimensional silicon substrates with 20:1 aspect ratio. ZrO 2 promoted the nucleation of Ru most efficiently compared to other substrates, but the films roughened quickly on ZrO 2 with increasing film thickness. The minimum number of cycles required to form continuous and conductive metal layers could be decreased by increasing the length of the oxygen pulse. In order to obtain well-conducting Ru films growth to thicknesses of at least 8―10 nm on any surface was necessary. Resistivities in the ranges of 30―60 and 14―16 μΩ · cm were achieved for 4―6 and 10―15 nm thick films, respectively. Delamination became an issue in the Ru films grown to thicknesses about 10 nm and higher.

Surface chemistry and Fermi level movement during the self-cleaning of GaAs by trimethyl-aluminum

The removal of the native oxides from NH4OH-cleaned p-GaAs (100) by exposure to trimethyl-aluminum (TMA) was studied by in situ photoelectron spectroscopy using synchrotron radiation. The reduction of high-valence As- and Ga-oxides occurred through different routes: while As3+ was reduced to As(1±Δ)+ suboxides (with 0 ≤ Δ ≤ 1), Ga3+ was directly removed. The surface Fermi level was shifted by about 100 meV towards the valence band edge upon TMA exposure. This indicates that removing the native oxide of GaAs by TMA is insufficient to create interfaces between GaAs and Al2O3 with defects densities below the 1012 cm−2 range.

The initial atomic layer deposition of HfO2∕Si(001) as followed in situ by synchrotron radiation photoelectron spectroscopy

We have grown HfO2 on Si(001) by atomic layer deposition (ALD) using HfCl4 and H2O as precursors. The early stages of the ALD were investigated with high-resolution photoelectron spectroscopy and x-ray absorption spectroscopy. We observed the changes occurring in the Si2p, O1s, Hf4f, Hf4d, and Cl2p core level lines after each ALD cycle up to the complete formation of two layers of HfO2. From the analysis of those variations, we deduced the growth properties of HfO2. The first layer consists of a sparse and Cl-contaminated oxide because of the incomplete oxidation, and the second layer is denser than the first one and with an almost stoichiometric O∕Hf ratio. At the completion of the second layer, the x-ray absorption spectra revealed the change of the Hf-oxide chemical state due to the transition from the thin Hf-oxide to the bulklike HfO2.

A facile corrosion approach to the synthesis of highly active CoOx water oxidation catalysts

Ultra-small rock salt cobalt monoxide (CoO) nanoparticles were synthesized and subjected to partial oxidation (‘corrosion’) with ceric ammonium nitrate (CAN) to form mixed-valence CoOx (1 < x < 2) water oxidation catalysts. Spectroscopic, microscopic and analytical methods evidenced a structural reformation of cubic CoO to active CoOx with a spinel structure. The superior water oxidation activity of CoOx has been established in electrochemical water oxidation under alkaline conditions. Electrochemical water oxidation with CoOx was recorded at a considerably low overpotential of merely 325 mV at a current density of 10 mA cm−2 in comparison to 370 mV for CoO. Transformation of both octahedral CoII and CoIII sites into amorphous Co(OH)2–CoOOH is the key to high electrochemical activity while the presence of a higher amount of octahedral CoIII sites in CoOx is imperative for an efficient oxygen evolution process.

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.

Capacitance and conductance versus voltage characterization of Al2O3 layers prepared by plasma enhanced atomic layer deposition at 25 °C≤ T ≤ 200 °C

In this work, plasma enhanced atomic layer deposited (PE-ALD) samples were prepared at substrate temperatures in the range between room temperature (RT) and 200 °C and investigated by capacitance–voltage and conductance–voltage recordings. The measurements are compared to standard thermal atomic layer deposition (T-ALD) at 200 °C. Very low interface state density (Dit) ∼1011 eV−1 cm−2 could be achieved for the PE-ALD process at 200 °C substrate temperature after postdeposition anneal (PDA) in forming gas at 450 °C. The PDA works very effectively for both the PE-ALD and T-ALD at 200 °C substrate temperature delivering also similar values of negative fixed charge density (Nfix) around −2.5 × 1012 cm−2. At the substrate temperature of 150 °C, highest Nfix (−2.9 × 1012 cm−2) and moderate Dit (2.7 × 1011 eV−1 cm−2) values were observed. The as deposited PE-ALD layer at RT shows both low Dit in the range of (1 to 3) × 1011 eV−1 cm−2 and low Nfix (−4.4 × 1011 cm−2) at the same time. The dependencies of Nfix, Dit, and relative permittivity on the substrate temperatures and its adjustability are discussed.In this work, plasma enhanced atomic layer deposited (PE-ALD) samples were prepared at substrate temperatures in the range between room temperature (RT) and 200 °C and investigated by capacitance–voltage and conductance–voltage recordings. The measurements are compared to standard thermal atomic layer deposition (T-ALD) at 200 °C. Very low interface state density (Dit) ∼1011 eV−1 cm−2 could be achieved for the PE-ALD process at 200 °C substrate temperature after postdeposition anneal (PDA) in forming gas at 450 °C. The PDA works very effectively for both the PE-ALD and T-ALD at 200 °C substrate temperature delivering also similar values of negative fixed charge density (Nfix) around −2.5 × 1012 cm−2. At the substrate temperature of 150 °C, highest Nfix (−2.9 × 1012 cm−2) and moderate Dit (2.7 × 1011 eV−1 cm−2) values were observed. The as deposited PE-ALD layer at RT shows both low Dit in the range of (1 to 3) × 1011 eV−1 cm−2 and low Nfix (−4.4 × 1011 cm−2) at the same time. The dependencies of Nfix, Dit...

In situ ALD experiments with synchrotron radiation photoelectron spectroscopy

In this contribution, we describe some features of atomic layer deposition (ALD) investigated by means of synchrotron radiation photoelemission spectroscopy (SR-PES). In particular, we show how the surface sensitivity of SR-PES combined with the in situ nature of our investigations can point out interactions between the substrate and ALD precursors. We observed changes on all substrates investigated, included Si, GaAs, Ru and their surface oxides. These interactions are extremely important during the first ALD cycles and induce modifications in the substrate, which might lead to its functionality enhancement.

Thermal and plasma enhanced atomic layer deposition of TiO2: Comparison of spectroscopic and electric properties

Titanium oxide (TiO2) deposited by atomic layer deposition (ALD) is used as a protective layer in photocatalytic water splitting system as well as a dielectric in resistive memory switching. The way ALD is performed (thermally or plasma-assisted) may change the growth rate as well as the electronic properties of the deposited films. In the present work, the authors verify the influence of the ALD mode on functional parameters, by comparing the growth rate and electronic properties of TiO2 films deposited by thermal (T-) and plasma-enhanced (PE-) ALD. The authors complete the study with the electrical characterization of selected samples by means of capacitance–voltage and current–voltage measurements. In all samples, the authors found a significant presence of Ti3+ states, with the lowest content in the PE-ALD grown TiO2 films. The observation of Ti3+ states was accompanied by the presence of in-gap states above the valence band maximum. For films thinner than 10 nm, the authors found also a strong leakag...

HfO2∕Si interface formation in atomic layer deposition films: An in situ investigation

The authors have studied the initial stages of the atomic layer deposition (ALD) of HfO2 onto Si by means of x-ray photoelectron spectroscopy using synchrotron radiation. The ALD was obtained using HfCl4 and H2O as precursors. The investigation was carried out in situ giving the possibility to determine the properties of the grown film after each ALD cycle. The Si 2p, O 1s, and Hf 4d+Cl 2p spectra show the growth of HfO2 in a smooth way until the complete formation of two oxide layers. The averaged growth rate is found to be 0.33 (one layer after three cycles) in accordance with previous works but, within the formation of one oxide layer, each ALD cycle behaves in a distinct way: the oxidation step in the various cycles shows a different efficiency leading to the inclusion of Cl impurities into the Hf oxide. In relation to the experimental results we discuss the origin of the Cl contamination proposing a mechanism based on the adsorption geometry of HfCl4 onto the–OH terminated substrate.