Katherine L. Saenger

Pulsed laser deposition of diamond‐like carbon films

Carbon thin films have been prepared by 248 nm excimer laser vaporization of graphite targets. The effect of a variety of process parameters on the film properties is investigated. Deposition at or below room temperature yields diamond‐like films with low hydrogen content, high optical transmission, and high resistivity. Electron energy loss spectra indicate sp3 bond fractions of 70–85%. Detailed analyses of the pseudodielectric functions, measured using spectroscopic ellipsometry, show the films to have normal dispersion and an index of refraction of 2.5 in the visible wavelength region. The effects of a low pressure hydrogen background and the use of auxiliary pulsed and dc plasma enhancements are also examined.

Vapor deposition processes for amorphous carbon films with sp3 fractions approaching diamond

Trends in recently reported data on high sp3 fraction (up to 85%), nonhydrogenated amorphous diamond‐like carbon films deposited by ion beam sputtering and laser vaporization are examined. The degree of diamondlike film character is found to depend upon the deposition technique as well as the substrate temperature and thermal diffusivity. The data suggest that the combination of incident particle kinetic energy and surface accommodation determine the physical properties of the resultant film. A model is proposed for the condensation of energetic carbon atoms into diamondlike films in which a quench‐type surface accommodation mechanism is operative.

(Ba,Sr)TiO 3 dielectrics for future stacked- capacitor DRAM

Thin films of barium-strontium titanate (Ba,Sr)TiO3 (BSTO) have been investigated for use as a capacitor dielectric for future generations of dynamic random-access memory (DRAM). This paper describes progress made in the preparation of BSTO films by liquid-source metal-organic chemical vapor deposition (LS-MOCVD) and the issues related to integrating films of BSTO into a DRAM capacitor. Films of BSTO deposited on planar Pt electrodes meet the electrical requirements needed for future DRAM. The specific capacitance and charge loss are found to be strongly dependent on the details of the BSTO deposition, the choice of the lower electrode structure, the microstructure of the BSTO, the post-electrode thermal treatments, BSTO dopants, and thin-film stress. Films of BSTO deposited on patterned Pt electrodes with a feature size of 0.2 µm are found to have degraded properties compared to films on large planar structures, but functional bits have been achieved on a DRAM test site at 0.20-µm ground rules. Mechanisms influencing specific capacitance and charge loss of BSTO films are described, as are the requirements for the electrode and barrier materials used in stacked-capacitor structures, with emphasis given to the properties of the Pt/TaSi(N) electrode/barrier system. Major problems requiring additional investigation are outlined.

Extremely high electron mobility in Si/SiGe modulation‐doped heterostructures

We report record high electron mobility in modulation‐doped Si/SiGe. Samples grown by ultrahigh vacuum chemical vapor deposition (UHV‐CVD) with mobility values in the range of 3.2–5.2×105 cm2/V s have been measured at 0.4 K. The current and temperature dependence of the magnetoresistance in those samples have been examined and the scattering times are deduced from these measurements. At high magnetic field (≳10 T), fractional quantum Hall filling factors have been observed, and the corresponding activation energies have been calculated. These are significantly larger than previously reported values in Si/SiGe, and are comparable to those in GaAs/AlGaAs modulation‐doped heterostructures with mobility higher than 1×106 cm2/V s.

Picosecond optical studies of amorphous diamond and diamondlike carbon: Thermal conductivity and longitudinal sound velocity

A picosecond pump‐probe technique is used to measure the room‐temperature thermal conductivity κ and longitudinal sound velocity cl of amorphous diamond (a‐D) and diamondlike carbon (DLC) thin films. Both κ and cl were found to decrease with film hydrogen content. Depending on the film deposition technique, κ is in the range 5–10×10−2 W cm−1 K−1 for a‐D, and 3–10×10−3 W cm−1 K−1 for DLC. Values of cl were found to be in the range 14–18×105 cm s−1 for a‐D, and 6–9×105 cm s−1 for DLC.

Studies of atomic oxygen in O2+CF4 rf discharges by two‐photon laser‐induced fluorescence and optical emission spectroscopy

We have used two‐photon laser‐induced fluorescence to obtain quantitative measurements of the concentration of ground state O atoms in O2+CF4 rf discharges. Absolute calibration was achieved by generating a known concentration of atomic oxygen by UV laser photolysis of O2. Trace amounts of Ar were added to serve as an inert reference gas for concurrent optical emission measurements, in which the plasma‐induced optical emission intensities from O* and Ar* lines were recorded. Emission line shapes were measured using a Fabry–Perot interfermoter to gain information on the mechanisms for formation of excited oxygen atoms in the plasma. Two excitation mechanisms were found to be important: (1) electron impact excitation of ground state atoms, e+O → O*+e, and (2) dissociative excitation of O2, e+O2 → O*+O+e. Evidence for both excitation mechanisms was obtained for O* (8446 A) emission, with atomic excitation being dominant, whereas dissociative excitation appeared to be the dominant mechanism for O* (7774 A) em...

Characterization of laser vaporization plasmas generated for the deposition of diamond-like carbon

Pulsed laser vaporization of graphite is rapidly emerging as an effective technique for the preparation of high quality diamond‐like carbon films. However, the dynamics of the process and mechanisms by which diamond‐like properties are obtained have not been well understood. The characteristics of the vapor plume generated by 248 nm KrF excimer laser irradiation of a graphite target are investigated using laser induced fluorescence and a Langmuir probe. It is found that the kinetic energy of the C2 molecule increases with laser fluence, reaching a value in excess of 12 eV in the moderate fluence range (3–5 J/cm2) employed for deposition. The Cn+ ions are 5–10 times more energetic and comprise ∼10% of the vapor flux. A notable finding is that irradiation of the surface at an angle of 70° with respect to the target normal increases the ion velocity when compared with 0° laser incidence at the same surface fluence. Analysis of the films prepared under such conditions supports the theory that diamond‐like fil...

Time‐resolved optical emission during laser ablation of Cu, CuO, and high‐Tc superconductors: Bi1.7Sr1.3Ca2Cu3Ox and Y1Ba1.7Cu2.7Oy

Time‐resolved measurements of optical emission from the ablation plume of ArF excimer‐laser‐irradiated Cu, CuO, Bi1.7Sr1.3Ca2Cu3Ox, and Y1Ba1.7Cu2.7Oy are reported. Data were collected on several emission lines for a variety of neutral and ionic species, including Cu+, Cu, Sr+, Sr, Ca+, Ca, and Y. The time dependence of the emission intensity is analyzed to obtain effective particle velocities. The results indicate most probable velocities of (3–10)×105 cm/s for the neutral species and (1–2)×106 cm/s for the ions, in qualitative agreement with previous work. However, the time dependence of the optical emission intensity can be influenced by radiation trapping and the detailed mechanisms of species excitation. These effects are shown to complicate the quantitative relationship between the emission intensity versus time and the particle velocity.

On the origin of spatial nonuniformities in the composition of pulsed-laser-deposited films

A model is proposed for the origin of spatial nonuniformities in the composition of pulsed laser‐deposited films derived from multicomponent targets. It is based on the idea that the forward peaking of each species in the plume depends on the species mass via the ratio of two mass‐dependent velocities: the flow velocity, which characterizes the directed forward motion, and the mean random thermal velocity, which characterizes lateral motion. The on‐axis enrichment of light‐mass species observed in films deposited at moderate laser fluences (e.g., Cu in depositions of YBa2Cu3O7) is attributed to the dual effects of a mass‐dependent collision rate and collision effectiveness. In weak expansions, these effects leave the lower mass species with a lower temperature, a higher flow velocity, and a relatively more forward‐peaked distribution than their high‐mass counterparts. The improved compositional uniformity observed for depositions at higher laser fluences is attributed to an incomplete transition to a stro...

Modulation‐doped n‐type Si/SiGe with inverted interface

We report the growth of n‐type modulation‐doped Si/SiGe with the doped SiGe supply layer underneath the strained Si channel. The mobility and charge density are measured in samples with 2‐ and 3‐nm‐thick spacers using gated Hall measurements. A peak room temperature mobility of 2200 cm2/V s is measured at a sheet density of 2.5×1012 cm−2. The measurements indicate a clear mobility modulation especially near threshold. Our layer design allows the gate to induce a sheet charge density of up to 3.2×1012 cm−2, before any significant reduction in the mobility is observed.