M. Shamsa

Micro-Raman investigation of optical phonons in ZnO nanocrystals

We have measured nonresonant and resonant Raman-scattering spectra from ZnO nanocrystals with an average diameter of 20nm. Based on our experimental data and comparison with the recently developed theory, we show that the observed shifts of the polar optical-phonon peaks in the resonant Raman spectra are not related to the spatial phonon confinement. The very weak dispersion of the polar optical phonons in ZnO nanocrystals does not lead to any noticeable redshift of the phonon peaks for 20-nm nanocrystals. The observed phonon shifts have been attributed to the local heating effects. We have demonstrated that even the low-power ultraviolet laser excitation, required for the resonant Raman spectroscopy, can lead to the strong local heating of ZnO nanocrystals. The latter causes significant (up to 14cm−1) redshift of the optical-phonon peaks compared to their position in bulk crystals. Nonresonant Raman excitation does not produce noticeable local heating. The obtained results can be used for identification ...

Thermal conductivity of diamond-like carbon films

The authors report the thermal conductivity (K) of a variety of carbon films ranging from polymeric hydrogenated amorphous carbons (a-C:H) to tetrahedral amorphous carbon (ta-C). The measurements are performed using the 3ω method. They show that thermal conduction is governed by the amount and structural disorder of the sp3 phase. If the sp3 phase is amorphous, K scales linearly with the C–C sp3 content, density, and elastic constants. Polymeric and graphitic films have the lowest K (0.2–0.3W∕mK), hydrogenated ta-C:H has K∼1W∕mK, and ta-C has the highest K (3.5W∕mK). If the sp3 phase orders, even in small grains such as in micro- or nanodiamond, a strong K increase occurs for a given density, Young’s modulus, and sp3 content.

Thermal conduction in nanocrystalline diamond films: Effects of the grain boundary scattering and nitrogen doping

The authors investigated thermal conductivity (K) in nanocrystalline diamond (NCD) films on silicon using the 3ω and laser flash techniques. The K temperature dependence has been studied for the undoped and nitrogen-doped NCD films for T=80–400K and compared with that in microcrystalline diamond (MCD) films. The effects of phonon scattering from the grain boundaries and film interfaces on thermal conduction have been studied using three different models. For NCD the room temperature K is 0.1–0.16W∕cmK and decreases with nitrogen doping. The K temperature dependence in NCD is different from that in MCD films and can be adequately described by the phonon-hopping model.

Thermal conductivity of nitrogenated ultrananocrystalline diamond films on silicon

The authors report on the experimental investigation of the thermal conductivity of nitrogenated ultrananocrystalline diamond (UNCD) films on silicon. For better accuracy, the thermal conductivity was measured by using two different approaches: the 3ω method and transient “hot disk” technique. The temperature dependence of the thermal conductivity of the nitrogenated UNCD films was compared to that of undoped UNCD films and microcrystalline diamond (MCD) films on silicon. It was shown that the temperature dependence of the thermal conductivity of UNCD films, which is substantially different from that for MCD films, can be adequately described by the phonon-hopping model. The room-temperature thermal conductivity of UNCD is 8.6–16.6 W/m K and decreases with the addition of nitrogen. The obtained results shed light on the nature of thermal conduction in partially disordered nanostructured materials and can be used for estimating the thermal resistance of doped UNCD films.

Thermal conductivity of ultrathin tetrahedral amorphous carbon films

We investigate the thermal conductivity of ultrathin tetrahedral amorphous carbon (ta-C) films on silicon, down to subnanometer thickness. For films with an initial sp3 content of 60%, the thermal conductivity reduces from 1.42to0.09W∕mK near room temperature as the thickness decreases from 18.5to∼1nm. The variation in ta-C film thickness is accompanied by changes in Young’s modulus, density, and sp3 content. The thermal resistance of the finite-thickness interface layer, which forms between ta-C and silicon, is ∼10−8m2K∕W near room temperature, thus producing a noticeable effect on thermal transport in ultrathin ta-C films.

Electrical and Thermal Conductivity of Ge ∕ Si Quantum Dot Superlattices

Recently proposed thermoelectric applications of quantum dot superlattices made of different material systems depend crucially on the values of the electrical and thermal conductivities in these nanostructures. We report results of the measurements of Hall mobility and thermal conductivity in a set of Ge0.5Si0.5/Si quantum dot superlattices. The average measured in-plane Hall mobility for the undoped Ge/Si quantum dot superlattices on a p-type substrate is 233.5 cm 2

Phonon-hopping thermal conduction in quantum dot superlattices

We investigated the thermal conductivity in Ge∕Si quantum dot superlattices both theoretically and experimentally. It is proposed that thermal conduction through the quantum dot superlattices can be described by the phonon-hopping model with the interface transparency obtained from the experiment. Thermal conductivity has been measured as a function of temperature T from 10K to 400K. We have observed an order of magnitude decrease in thermal conductivity value compared to bulk and shift of its peak position to higher temperatures. The thermal conductivity manifests T0.7-T0.9 dependence for T⩽200K. The phonon-hopping model describes the measured thermal conductivity surprisingly well over the wide range of T from ∼40Kto400K. The model allows one to include the specifics of thermal conduction in quantum dot superlattices, such as the dot size, disorder, and interface quality. Our results suggest that the examined quantum dot superlattices are closer to the disordered or polycrystalline materials in terms of...

Micro-Raman spectroscopic characterization of ZnO quantum dots, nanocrystals and nanowires

Nanostructures, such as quantum dots, nanocrystals and nanowires, made of wurtzite ZnO have recently attracted attention due to their proposed applications in optoelectronic devices. Raman spectroscopy has been widely used to study the optical phonon spectrum modification in ZnO nanostructures as compared to bulk crystals. Understanding the phonon spectrum change in wurtzite nanostructures is important because the optical phonons affect the light emission and absorption. The interpretation of the phonon peaks in the Raman spectrum from ZnO nanostructures continues to be the subject of debates. Here we present a comparative study of micro-Raman spectra from ZnO quantum dots, nanocrystals and nanowires. Several possible mechanisms for the peak position shifts, i.e., optical phonon confinement, phonon localization on defects and laser-induced heating, are discussed in details. We show that the shifts of ~2 cm-1 in non-Resonant spectra are likely due to the optical phonon confinement in ZnO quantum dots with the average diameter of 4 nm. The small shifts in the non-Resonant spectra from ZnO nanowires with the diameter ~20 nm - 50 nm can be attributed to either defects or large size dispersion, which results in a substantial contribution from nanowires with smaller diameters. The large red-shifts of ~10 cm-1 in the resonant Raman spectrum from nanocrystals were proved to be due to local laser heating.

Investigation of Thermal Crosstalk Between SOI FETs by the Subthreshold Sensing Technique

Experimental-modeling investigation of the transient thermal crosstalk between the field-effect transistors implemented on a silicon-on-insulator substrate is reported. The measurements were performed using a high-speed electrical pulse-probe sampling technique, which allowed detection of thermally modulated subthreshold currents. The technique achieved a temperature resolution of ~50 mK, a time resolution of 5 ns, and a temperature sensitivity of ~0.6 muA/K. The finite-element method was used to solve the heat diffusion equation and to obtain the temperature profiles for the given device structures. The combined high-resolution experimental-simulation approach allowed the study of the thermal crosstalk between two adjacent devices and probe the local temperature at different locations of the structure. The effects of the interface quality, layer thickness, material selection, and interdevice spacing on the heat diffusion and device performance were investigated in detail.

Measurements of Inter-and-Intra Device Transient Thermal Transport on SOI FETs

In this paper, we report the first resolving detailed thermal transients for CMOS devices. Furthermore we investigate different heat paths between and inside devices to reveal the importance of the thermal conductivity of the gate. This work is extended to study thermal transport within a sub-micrometer CMOS FET where we are able to detect the delayed heat pulse at the source due to heat generation in the drain. We show both by measurements and simulations that oxide does not afford good isolation and that the main cooling mechanism of SOI devices is to the gate, with transfer resistance playing an important role.