Gary L. Eesley

Transient thermoreflectance from thin metal films

This report describes the first demonstration of thermal diffusivity measurements using picosecond transient thermoreflectance (TTR). Although previously reported methods of measuring thermal transport properties of thin films require precise knowledge of the thermal properties of the substrate, this technique allows measurements on films as thin as 100 nm without any evidence of substrate interaction. The TTR measurement is modeled with a one‐dimensional heat flow equation using a two‐parameter fitting routine to determine the thermal diffusivity. The validity of our approach is confirmed by the TTR measured thermal diffusivity of single crystal nickel. We have also measured the thermal diffusivity of sputtered and evaporated single element metal films. Preliminary results from TTR measurements on compositionally modulated structures are also presented.

Generation and detection of picosecond acoustic pulses in thin metal films

We report on the use of picosecond duration laser pulses to generate picosecond acoustic pulses in metal films on the order of 200 nm thick. The acoustic pulses are detected by means of transient piezoreflectance measurements. We demonstrate the use of a thin metal overlayer to enhance the piezoreflectance signal.

Transient thermoreflectance from metal films

The first reported demonstration of thermal diffusivity measurements using picosecond transient thermoreflectance is described. Although previously reported methods of measuring thermal transport properties of thin films require precise knowledge of the thermal properties of the substrate, this technique permits measurements on films as thin as 100 nm without any evidence of substrate interaction.

Measurement of intrinsic stresses during growth of aluminum nitride thin films by reactive sputter deposition

Real‐time measurements of intrinsic stresses during growth of polycrystalline and epitaxial aluminum nitride (AlN) thin films on Si(111) are reported. Our room‐temperature measurements on polycrystalline films corroborate previous post‐growth measurements. Our high‐temperature measurements provide evidence of large intrinsic stresses during epitaxial growth of AlN on Si(111) and insignificant stress relaxation during growth.

Optical fiber refractometer

We report on the use of an optical fiber to measure the refractive index of liquid materials. With a relatively simple detection system, indices can be determined with an average accuracy of ±0.2%.

Relationship between structural phase transitions and elastic anomalies in metallic superlattices

A detailed study of the structural and elastic properties of Fe/Cu superlattices has been performed. These superlattices exhibit a structural phase transition as a function of layer thickness in which bcc α‐Fe transforms into fcc γ‐Fe. This structural phase transition in which the Fe layers become coherent with the Cu layers is signaled by clear‐cut changes in elastic and magnetic properties. The elastic properties studied through this transition indicate that in‐plane coherency plays an important role in the elastic behavior of metallic superlattices.

Picosecond acoustic pulse reflection from a metal‐metal interface

Picosecond duration laser pulses are used to generate ultrashort acoustic pulses in a bilayer metal film. Time‐resolved transient piezoreflectance measurements permit the observation of acoustic reflections from a metal‐metal interface within the film, as well as reflections from the film‐substrate interface. We show that our measurements are well described by an abrupt interface model of acoustic mismatch.

Electron-phonon relaxation rate in single-crystal lanthanum cuprate

We report the results of phonon relaxation processes obtained from picosecond transient thermoreflectance measurements of single‐crystal La2CuO4+y (y∼0.06). Nonequilibrium heating was found to occur on a picosecond time scale. The time scale of the nonequilibrium heating is a sensitive measurement of the modest electron‐phonon interaction (λ≤0.1) in the nonsuperconducting La2CuO4+y.

Relationship Between Interfacial Strain and the Elastic Response of Multilayer Metal Film

We have used x-ray diffraction and picosecond transient piezoreflectance to investigate the structural and elastic properties of three distinct multilayer systems: Mo/Ni, Pt/Ni and Ti/Ni. We demonstrate that the commonly observed lattice expansion perpendicular to the film plane is not a bulk effect, but is localized at the interface between the contacting metals. Incorporating the measured interfacial expansion into a universal binding relation, we show that the measured elastic softening versus compositional repeat distance can result from the interfacial strain.

Electro-optic sampling of surface fields

We demonstrate the use of time-resolved electro-optic sampling to externally monitor the dynamics of carrier heating and transport in semiconductors. This technique can be used to follow the time evolution of surface electric fields which result from the ultrafast optical generation of excess electron-hole pairs.