Weili Liu

Thermal conduction in AlxGa1−xN alloys and thin films

We report on experimental and theoretical investigation of thermal conduction in AlxGa1−xN alloys. A focus of this study is on understanding the effect of the Al mass fraction x and temperature on thermal conductivity in AlxGa1−xN thin films. The thermal conductivity of a set of AlxGa1−xN thin films as well as a pure GaN sample was measured using the differential 3ω technique in the temperature range from 80 to 400 K. Application of the virtual-crystal model allowed us to elucidate the strength of the mass-difference and strain-field-difference phonon scattering in AlxGa1−xN alloy system. The obtained thermal-conductivity temperature dependence indicates the high degree of disorder in the system. The measured variation of the thermal conductivity with the Al fraction x is in good agreement with the theory predictions. The measured data and calculation procedure are useful for evaluating the self-heating effect in AlxGa1−xN/GaN heterostructure field-effect transistors and for the device structure optimization.

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.

Temperature dependence of thermal conductivity of AlxGa1−xN thin films measured by the differential 3ω technique

Reliable values of thermal conductivity of thin films made of GaN and its alloys are important for further development of nitride technology due to the problem of self-heating in GaN-based power transistors and optical devices. Using the differential 3ω technique we measured the thermal conductivity of AlxGa1−xN thin films (x=0 and 0.4) grown by the hydride vapor phase epitaxy. Thermal conductivity of the examined Al0.4Ga0.6N alloy, which is about 25W∕mK at 300K, displays a rather unusual temperature dependence. A noticeable growth of the thermal conductivity with temperature up to 350K is more characteristic for amorphous or completely disordered materials. The measured high-temperature thermal conductivity data are in good agreement with predictions based on the virtual crystal model. Obtained results are important for modeling the self-heating effects in GaN transistors and can be used for the device structure optimization.

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 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.

Thermoelectric effects in wurtzite GaN and AlxGa1−xN alloys

We have investigated theoretically the thermoelectric effects in wurtzite GaN crystals and AlxGa1−xN alloys. The electron-transport model includes all dominant energy-dependent electron-scattering mechanisms, such as charged dislocation and ionized impurity scattering, polar optical phonon, deformation potential, and piezoelectric acoustic-phonon scattering. The results of the calculation show that GaN-based alloys may have some potential as thermoelectric materials at high temperature. It was found that the thermoelectric figure-of-merit for bulk GaN at T=300K is about 0.0017 while it can reach 0.2 in the thermally resistive Al0.4Ga0.6N alloy at T=1000K. The obtained results agree well with available experimental data. The developed calculation procedure can be used for the optimization of the thermoelectric properties of GaN alloys. The proposed integration of the GaN high-power microwave and optoelectronic devices with the active thermoelectric cooling implemented on the same material system can improv...

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...

Assembly and characterization of hybrid virus-inorganic nanotubes

Recently, rod-shaped viruses have attracted attention as biological templates for assembly of nanostructures. Tobamoviruses such as the type strain of Tobacco mosaic virus (TMV-U1, or -common) have a cylindrical shape and dimensions suitable for nanoelectronic applications: 300nm long and 18nm in diameter with a 4nm axial channel. TMV particles can be coated with metals, silica, or semiconductor materials and may also form end-to-end assemblies to be used as interconnects or device channels. In this letter, we report the preparation of TMV-U1 templated organic-metal nanotubes, and their structural characterization using transmission electron microscopy and micro-Raman spectroscopy. Reproducible phonon signatures different from that of native TMV-U1 were observed from the metal-coated TMVs. Our results indicate that Raman spectroscopy can be used for monitoring of the bio-assisted nanostructure assembly and for analyzing the vibrational modes of the resulting bio-inorganic junctions.

Evidence for possible flexoelectricity in tobacco mosaic viruses used as nanotemplates

Electromechanical coupling in individual tobacco mosaic viruses has been studied using piezoresponse force microscopy. Possible origins of the observed high resolution contrast, including the topographic crosstalk, difference in the elastic properties, and the intrinsic electromechanical coupling due to the piezoelectric and flexoelectric effects are discussed. Using simple estimates, we argue that, due in part to the small size and high symmetry of this particular material system, flexoelectric coupling can dominate the observed electromechanical behavior. The electrical manipulation of the virus particles, essential for nanoelectronic applications for which they are proposed, has also been demonstrated.

The Ambient Temperature Effect on Current-Voltage Characteristics of Surface-Passivated GaN-Based Field-Effect Transistors

We have studied experimentally the effect of ambient temperature on performance of the surface-passivated Al 0.2 Ga 0.8 N/GaN heterostructure field-effect transistors in the temperature range from 25°C to 250°C. The measured data have been compared with physics-based modeling of the GaN transistor characteristics under different ambient temperatures. The experimental data, showing about 33% degradation in the saturation current with a temperature increase from 25°C to 250°C, agrees well with the results of simulations performed using ISE DESSIS software. Obtained results and analytical extrapolations can be used for predicting device performance in changing environments, as well as for optimization of the device structure.