Theodosia Gougousi

Microcontact patterning of ruthenium gate electrodes by selective area atomic layer deposition

Patterned octadecyltrichlorosilane monolayers are used to inhibit film nucleation, enabling selective area atomic layer deposition (ALD) of ruthenium on SiO2 and HfO2 surfaces using bis-(cyclopentadienyl)ruthenium and oxygen. X-ray photoelectron spectroscopy indicated that OTS could deactivate film growth on thermal silicon oxide and hafnium oxide surfaces. The growth rate of ALD Ru is similar on various starting surfaces, but the growth initiation differed substantially. Metal-oxide-semiconductor capacitors were fabricated directly using the selective-area process. Capacitance measurements indicate the effective work function of ALD Ru is 4.84±0.1eV on SiO2, and the effective work function is reduced on HfO2∕SiO2 layers.

Properties of La-silicate high-K dielectric films formed by oxidation of La on silicon

In this article, we present data on the properties of La-based high-k dielectric films prepared by oxidation of La deposited by physical vapor deposition on silicon. Films are characterized by x-ray photoelectron spectroscopy, infrared absorption, and capacitance versus voltage analysis. We find that when we oxidize La metal sputter deposited on Si substrates, it reacts with the silicon substrate to form La silicate. La films as thick as 300 A will react completely with Si under moderate oxidation conditions (900 °C for 10 min) suggesting a very rapid silicidation reaction between La and Si. Under some processing conditions the as-deposited films contain a small La2O3 component that reduces to La silicate upon anneal at high temperatures. La-silicate films do not phase separate into La2O3 and SiO2 upon annealing at 1050 °C, and their resistance to H2O incorporation depends critically on the oxidation temperature. Electrical measurements show a high concentration of positive fixed charge.

Carbonate formation during post-deposition ambient exposure of high-k dielectrics

When thick films of group-III (La, Y)- and group-IV (Hf, Zr)-based high-k dielectrics are exposed to ambient for several months, Fourier transform infrared spectroscopy shows formation of carbonate species in the film bulk, likely due to reaction with atmospheric CO2. Group-III-based films show signs of carbonate feature growth within 10 min of air exposure, especially in films processed at relatively low temperatures (<600 °C). Carbonate formation is verified also for group-IV-based films, but at a significantly reduced concentration. Post-exposure annealing can reduce the carbonate observed in the IR spectra. However, post-exposure annealing likely does not remove carbon contamination, and it results in interface silicon oxide growth. The observed reactions of high-k films with the ambient may impose significant constraints on the post-deposition handling of high-k films.

Interface of atomic layer deposited HfO2 films on GaAs (100) surfaces

HfO2 films have been deposited by using a tetrakis(dimethylamino)hafnium/H2O atomic layer deposition (ALD) process on GaAs. X-ray photoelectron spectroscopy measurements show that the HF and NH4OH predeposition surface treatment results in efficient removal of the Ga and As native oxides. No interface oxidation is detected after 15cycles of HfO2 ALD implying effective passivation of the GaAs surface. Spectroscopic ellipsometry confirms linear growth at 1.0A∕cycle on both starting surfaces, while Rutherford backscattering spectrometry indicates steady-state coverage after about 10 ALD cycles. For films grown on native oxide GaAs, complete removal of the As oxide is observed after 20 ALD cycles.

Electron-ion recombination rate coefficient measurements in a flowing afterglow plasma

Abstract The flowing-afterglow technique in conjunction with computer modelling of the flowing plasma has been used to determine accurate dissociative-recombination rate coefficients α for the ions O2+, HCO+, CH5+, C2H5+, H3O+, CO2+, HCO2+, HN2O+, and N2O+ at 295 K. We find that the simple form of data analysis that was employed in earlier experiments was adequate and we largely confim earlier results. In the case of HCO+ ions, published coefficients range from 1.1 × 10−7 to 2.8 × 10−7 cm3/s, while our measurements give a value of 1.9 × 10−7 cm3/s.

Nucleation of HfO2 atomic layer deposition films on chemical oxide and H-terminated Si

HfO2 thin films have been deposited by an atomic layer deposition (ALD) process using alternating pulses of tetrakis-ethylmethylamino hafnium and H2O precursors at 250 °C. The as-deposited films are mainly amorphous and nearly stoichiometric HfO2 (O/Hf ratio ∼1.9) with low bonded carbon content (∼3 at. %). A comparison of the nucleation stage of the films on OH- and H-terminated Si(100) surfaces has been performed using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy (XPS), and spectroscopic ellipsometry (SE). We find for the initial 5–7 process cycles that the film nucleates more efficiently on the OH-terminated surface. However, after the 7th cycle both surfaces exhibit similar surface coverage, which takes about 40 cycles to reach a steady growth rate per cycle. Angle resolved XPS measurements reveal the formation of a ∼6 A interfacial layer after four ALD cycles on the H-terminated surface and the thickness of the interfacial layer does not change substantially between the 4th...

Measurement of the absolute yield of CO(a 3Π)+O products in the dissociative recombination of CO2+ ions with electrons

A flowing-afterglow technique is described for measuring the absolute yield of a radiative product state from ion–electron recombination. The technique is applied to CO2++e− dissociative recombination. The measured yield of CO(a 3Π)+O(3P) is 0.29±0.10. This includes cascade from higher triplet states of CO. The vibrational distribution in CO(a 3Π,v=0–3) is approximately Boltzmann, with an effective temperature of 4200±300 K. The measured rate constant for quenching of CO(a) by CO2 is (1.0±0.2)×10−11 cm3 s−1, somewhat lower than previous measurements.

High Electron Velocity Submicrometer AlN/GaN MOS-HEMTs on Freestanding GaN Substrates

AlN/GaN heterostructures with 1700-cm²/V·s Hall mobility have been grown by molecular beam epitaxy on freestanding GaN substrates. Submicrometer gate-length (L_G) metal-oxide-semiconductor (MOS) high-electron-mobility transistors (HEMTs) fabricated from this material show excellent dc and RF performance. L_G = 100 nm devices exhibited a drain current density of 1.5 A/mm, current gain cutoff frequency f_T of 165 GHz, a maximum frequency of oscillation f_max of 171 GHz, and intrinsic average electron velocity v_e of 1.5 ×10⁷ cm/s. The 40-GHz load-pull measurements of L_G = 140 nm devices showed 1-W/mm output power, with a 4.6-dB gain and 17% power-added efficiency. GaN substrates provide a way of achieving high mobility, high v_e, and high RF performance in AlN/GaN transistors.

The role of the OH species in high-k/polycrystalline silicon gate electrode interface reactions

In this letter, reactions occurring at the interface between polycrystalline silicon (poly-Si) and LaSiOx high-dielectric-constant (high-k) insulating layers are characterized using x-ray photoelectron spectroscopy. Dielectrics were formed by sputter deposition of metal on silicon, followed by oxidation at 900 °C. Amorphous silicon was deposited on top by plasma-enhanced chemical vapor deposition from silane, followed by anneal at 650–1050 °C. We show that if the dielectric layer is exposed to sufficient water vapor before polysilicon deposition, annealing at 1050 °C for 10 s is sufficient to completely oxidize ∼25 A of deposited silicon. Minimal reaction is observed without deliberate water exposure. This demonstrates the importance of the dielectric surface condition in determining reactivity of high-k/polysilicon interfaces.

Growth and Interface Evolution of HfO2 Films on GaAs(100) Surfaces

The initial film growth 2–100 cycles and the interface evolution of HfO2 thin films on GaAs surfaces were investigated for an atomic layer deposition chemistry that utilizes tetrakis ethylmethyl amino hafnium and H2O at 250°C. Starting surfaces include native oxide and HF or NH4OH-etched substrates. X-ray photoelectron spectroscopy shows that deposition on native oxide GaAs surfaces results in the gradual consumption of the arsenic and gallium oxides. Arsenic oxides are easier to remove, leaving some metallic arsenic–arsenic suboxide at the interface. The removal of the gallium oxides is slower, and some residual Ga2O3 and Ga2O are detected after 100 process cycles. High resolution transmission electron microscopy confirms the presence of an almost sharp interface for the 100 cycle 12 nm film and indicates that the as-deposited film is polycrystalline. The depositions on either HF or NH4OH-etched substrates result in a sharp interface with very little residual gallium oxide and arsenic suboxide present. Rutherford backscattering spectroscopy shows that steady-state growth comparable to that achieved on SiO2 is reached after 20 ALD cycles for all GaAs surfaces; however, high initial surface activity is detected for the etched surfaces.