J. Cuffe

Reduction of the thermal conductivity in free-standing silicon nano-membranes investigated by non-invasive Raman thermometry

We report on the reduction of the thermal conductivity in ultra-thin suspended Si membranes with high crystalline quality. A series of membranes with thicknesses ranging from 9 nm to 1.5 μm was investigated using Raman thermometry, a novel contactless technique for thermal conductivity determination. A systematic decrease in the thermal conductivity was observed as reducing the thickness, which is explained using the Fuchs-Sondheimer model through the influence of phonon boundary scattering at the surfaces. The thermal conductivity of the thinnest membrane with d = 9 nm resulted in (9 ± 2) W/mK, thus approaching the amorphous limit but still maintaining a high crystalline quality.

Lifetimes of Confined Acoustic Phonons in Ultrathin Silicon Membranes

We study the relaxation of coherent acoustic phonon modes with frequencies up to 500 GHz in ultrathin free-standing silicon membranes. Using an ultrafast pump-probe technique of asynchronous optical sampling, we observe that the decay time of the first-order dilatational mode decreases significantly from ~4.7 ns to 5 ps with decreasing membrane thickness from ~194 to 8 nm. The experimental results are compared with theories considering both intrinsic phonon-phonon interactions and extrinsic surface roughness scattering including a wavelength-dependent specularity. Our results provide insight to understand some of the limits of nanomechanical resonators and thermal transport in nanostructures.

Phonons in slow motion: dispersion relations in ultrathin Si membranes.

We report the changes in dispersion relations of hypersonic acoustic phonons in free-standing silicon membranes as thin as ∼8 nm. We observe a reduction of the phase and group velocities of the fundamental flexural mode by more than 1 order of magnitude compared to bulk values. The modification of the dispersion relation in nanostructures has important consequences for noise control in nano- and microelectromechanical systems (MEMS/NEMS) as well as opto-mechanical devices.

Ultra-thin free-standing single crystalline silicon membranes with strain control

We report on fabrication and characterization of ultra-thin suspended single crystalline flat silicon membranes with thickness down to 6 nm. We have developed a method to control the strain in the membranes by adding a strain compensating frame on the silicon membrane perimeter to avoid buckling after the release. We show that by changing the properties of the frame the strain of the membrane can be tuned in controlled manner. Consequently, both the mechanical properties and the band structure can be engineered, and the resulting membranes provide a unique laboratory to study low-dimensional electronic, photonic, and phononic phenomena.

Examining thermal transport through a frequency-domain representation of time-domain thermoreflectance data

Laser-based time-domain thermoreflectance (TDTR) and frequency-domain thermoreflectance (FDTR) techniques are widely used for investigating thermal transport at micro- and nano-scales. We demonstrate that data obtained in TDTR measurements can be represented in a frequency-domain form equivalent to FDTR, i.e., in the form of a surface temperature amplitude and phase response to time-harmonic heating. Such a representation is made possible by using a large TDTR delay time window covering the entire pulse repetition interval. We demonstrate the extraction of frequency-domain data up to 1 GHz from TDTR measurements on a sapphire sample coated with a thin layer of aluminum, and show that the frequency dependencies of both the amplitude and phase responses agree well with theory. The proposed method not only allows a direct comparison of TDTR and FDTR data, but also enables measurements at high frequencies currently not accessible to FDTR. The frequency-domain representation helps uncover aspects of the measurement physics which remain obscured in a traditional TDTR measurement, such as the importance of modeling the details of the heat transport in the metal transducer film for analyzing high frequency responses.

Nanostructured p-type Cr/V2O5 thin films with boosted thermoelectric properties

The urgent need for non-toxic and abundant thermoelectric materials has become a significant motivation to improve the figures of merit of metal oxides in order to remove the barrier towards their widespread use for thermoelectric applications. Here we show the influence of a Cr layer in boosting the thermoelectric properties of vanadium pentoxide (V2O5) thin films, deposited by thermal evaporation and annealed at 500 °C. The Cr to V2O5 thickness ratio controls the morphological and thermoelectric properties of the thin films produced. The optimized Seebeck coefficient and power factor values at room temperature are +50 μV K−1 and 7.9 × 10−4 W m−1 K−2, respectively. The nanograin structure of the films is responsible for an improvement in the electrical conductivity up to 3 × 105 (Ω m)−1 with a typical thermal conductivity of 1.5 W m−1 K−1. These results combine to yield promising p-type thermoeletric CrV2O5 thin films with a ZT of 0.16 at room temperature.

Thermal transport in suspended silicon membranes measured by laser-induced transient gratings

Studying thermal transport at the nanoscale poses formidable experimental challenges due both to the physics of the measurement process and to the issues of accuracy and reproducibility. The laser-induced transient thermal grating (TTG) technique permits non-contact measurements on nanostructured samples without a need for metal heaters or any other extraneous structures, offering the advantage of inherently high absolute accuracy. We present a review of recent studies of thermal transport in nanoscale silicon membranes using the TTG technique. An overview of the methodology, including an analysis of measurements errors, is followed by a discussion of new findings obtained from measurements on both “solid” and nanopatterned membranes. The most important results have been a direct observation of non-diffusive phonon-mediated transport at room temperature and measurements of thickness-dependent thermal conductivity of suspended membranes across a wide thickness range, showing good agreement with first-princ...

Enhanced light extraction in ITO-free OLEDs using double-sided printed electrodes

We show how nanoimprint lithographic techniques are particularly suited for the realization of OLED device structures. We tested them to realize nanopatterned metallic electrodes containing photonic crystals to couple the light out and plasmonic crystals showing extraordinary transmission. At similar current densities, a two-fold electroluminescence is achieved with devices having double-sided structured metallic electrodes as compared to a control OLED with an ITO anode. The use of combined nanoimprint lithography processes has the potential to expand the performance range of various organic optoelectronic devices.

Lifetime of high-order thickness resonances of thin silicon membranes

Femtosecond laser pulses are used to excite and probe high-order longitudinal thickness resonances at a frequency of ∼270 GHz in suspended Si membranes with thickness ranging from 0.4 to 15 μm. The measured acoustic lifetime scales linearly with the membrane thickness and is shown to be controlled by the surface specularity which correlates with roughness characterized by atomic force microscopy. Observed Q-factor values up to 2400 at room temperature result from the existence of a local maximum of the material Q in the sub-THz range. However, surface specularity would need to be improved over measured values of ∼0.5 in order to achieve high Q values in nanoscale devices. The results support the validity of the diffuse boundary scattering model in analyzing thermal transport in thin Si membranes.

Calculation of the specific heat in ultra-thin free-standing silicon membranes

The specific heat of ultra-thin free-standing membranes is calculated using the elastic continuum model. We first obtain the dispersion relations of the discrete set of acoustic modes in the system. The specific heat is then calculated by summing over the discrete out-of-plane wavevector component and integrating over the continuous in-plane wavevector of these waves. In the low-temperature regime (T < 4 K), the flexural polarization is seen to have the highest contribution to the total specific heat. This leads to a linear dependence with temperature, resulting in a larger specific heat for the membrane compared to that of the bulk counterpart.