Interpreting picosecond acoustics in the case of low interface stiffness

Hohensee, Gregory T.; Hsieh, Wen‐Pin; Losego, Mark D.; Cahill, David G.

doi:10.1063/1.4766957

Cited by 66 publications

(53 citation statements)

References 14 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…On Si, the Ta film was deposited slightly below 600°C to prevent the formation of an extensive Ta-Si silicide layer at the interface. Picosecond acoustics 16,35 and Rutherford backscattering spectroscopy confirmed that TaSi 2 or Ta 2 O 5 layers of appreciable thickness were not formed. For control experiments, Al and Ta films were also deposited on Si wafers that had 105 nm layers of thermal oxide on the surface, and Al films were deposited on single crystal Al, Cu, and Ni substrates, see Supplementary Fig.…”

Section: Methodsmentioning

confidence: 77%

Anisotropic failure of Fourier theory in time-domain thermoreflectance experiments

2014

Self Cite

View full text Add to dashboard Cite

The applicability of Fourier's law to heat transfer problems relies on the assumption that heat carriers have mean free paths smaller than important length scales of the temperature profile. This assumption is not generally valid in nanoscale thermal transport problems where spacing between boundaries is small (o1 mm), and temperature gradients vary rapidly in space. Here we study the limits to Fourier theory for analysing three-dimensional heat transfer problems in systems with an interface. We characterize the relationship between the failure of Fourier theory, phonon mean free paths, important length scales of the temperature profile and interfacial-phonon scattering by time-domain thermoreflectance experiments on Si, Si 0.99 Ge 0.01 , boron-doped Si and MgO crystals. The failure of Fourier theory causes anisotropic thermal transport. In situations where Fourier theory fails, a simple radiative boundary condition on the heat diffusion equation cannot adequately describe interfacial thermal transport.

show abstract

Section: Methodsmentioning

confidence: 77%

Anisotropic failure of Fourier theory in time-domain thermoreflectance experiments

2014

Self Cite

View full text Add to dashboard Cite

show abstract

“…The native oxide of Si, SiOx, generally has a weak effect on picosecond acoustics. [21] The echo at 50 ps is the reflection at the BP/substrate interface. Although the acoustic impedance of BP is smaller than Si, we do not expect an intimately bonded interface between BP and Si, as there is finite 6 roughness for both the exfoliated BP surface and Si substrate.…”

mentioning

confidence: 99%

“…If the medium underneath has a smaller Z, then the acoustic echo undergoes a π-phase shift. [21] Picosecond acoustics data for a 55 nm thick BP sample are compared to those for the substrate in Figure 3A. The first upward echo at 28 ps that repeats periodically is a combination of the reflections from the NbV/AlOx and AlOx/BP interfaces having a similarly large contrast in Z (see Table S1).…”

mentioning

confidence: 99%

Anisotropic Thermal Conductivity of Exfoliated Black Phosphorus

et al. 2015

Self Cite

View full text Add to dashboard Cite

We ascertain the anisotropic thermal conductivity of passivated black phosphorus (BP), a reactive 2D nanomaterial with strong in-plane anisotropy. We measure the room temperature thermal conductivity by time-domain thermoreflectance for three crystalline axes of exfoliated BP. The thermal conductivity along the zigzag direction (86 ± 8 W m −1 K −1 ) is ~2.5 times higher than that of the armchair direction (34 ± 4 W m −1 K −1 ). TOC Figure 2Black phosphorus (BP), a stable phosphorus allotrope at ambient temperature and pressure, [1] is a two-dimensional electronic material with desirable properties for transistor, [2, 3] thermoelectric, [4] and optical sensing [5] applications. Few-layer BP flakes can be exfoliated from bulk crystals due to weak interlayer bonding. [2, 3,6] In contrast to the planar character of graphite and transition metal dichalcogenides, BP has a puckered, honeycomb structure, leading to heightened chemical reactivity [7] and pronounced in-plane anisotropy.Experimental and theoretical examinations of the electrical, [3, 4,6] optical, [3,8] mechanical, [9] and thermal [4,10,11,12] properties reveal distinct anisotropy along BP's two high-symmetry, inplane directions. These symmetry axes are commonly referred to as the zigzag and armchair directions, with lattice constants of a = 3.314 Å and c = 4.376 Å, respectively. [1] Understanding an electronic material's thermal conductivity is critical for the thermal management of small-scale devices and for exploring potential thermoelectric applications.Despite extensive electrical characterization of exfoliated BP, experimental measurements of BP's thermal properties are few. [12,13] First-principles calculations of the anisotropic thermal conductivity of monolayer BP, that is, phosphorene, predict that the thermal conductivity along the zigzag direction is two-or three-fold higher [10,11] than along the armchair direction; for example, ref. 11 finds 110 and 36 W m −1 K −1 , respectively, in the two directions. Of this, the electronic contribution to the thermal conductivity is markedly small, less than 3 W m −1 K −1 , even at a high carrier concentration of ~10 12 cm −2 . [11] Experimentally, Slack found the thermal conductivity of bulk, polycrystalline BP to be 10 W m −1 K −1 at room temperature, [13] but no anisotropic effects were examined. Only mechanically exfoliated BP flakes, with defined symmetry axes, allow an assessment of anisotropic thermal properties in all three high symmetry directions of the crystal. A recent preprint [12] reports the in-plane, anisotropic thermal transport for exfoliated, few-layer BP using micro-Raman spectroscopy; still, the extracted values were much smaller than theoretically predicted for phosphorene. For thinner (<15 nm) BP samples, the measured BP thermal conductivity is modified by phonon scattering from oxidized regions, substrates, and surface imperfections. By contrast, thermal measurements on thicker (>100 nm) BP flakes, especially those protected against ambient oxidation, provide an intrinsic ...

show abstract

“…In the case of the Al film, we were also able to confirm the film thicknesses of most films with picosecond acoustics. 28,29 We report an uncertainty in transducer film thickness of 3 nm. An illustration of sample geometries studied in this work is shown in Fig.…”

Section: Gallium Oxide Fabrication and Chemical Characterizationmentioning

confidence: 81%

Size effects in the thermal conductivity of gallium oxide (β-Ga2O3) films grown via open-atmosphere annealing of gallium nitride

Szwejkowski

Creange

Sun

et al. 2015

Journal of Applied Physics

View full text Add to dashboard Cite

Gallium nitride (GaN) is a widely used semiconductor for high frequency and high power devices due to of its unique electrical properties: a wide band gap, high breakdown field, and high electron mobility. However, thermal management has become a limiting factor regarding efficiency, lifetime, and advancement of GaN devices and GaN-based applications. In this work, we study the thermal conductivity of beta-phase gallium oxide (β-Ga2O3) thin films, a component of typical gate oxides used in such devices. We use time domain thermoreflectance to measure the thermal conductivity of a variety of polycrystalline β-Ga2O3 films of different thicknesses grown via open atmosphere annealing of the surfaces of GaN films on sapphire substrates. We show that the measured effective thermal conductivity of these β-Ga2O3 films can span 1.5 orders of magnitude, increasing with an increased film thickness, which is indicative of the relatively large intrinsic thermal conductivity of the β-Ga2O3 grown via this technique (8.8 ± 3.4 W m−1 K−1) and large mean free paths compared to typical gate dielectrics commonly used in GaN device contacts. By conducting time domain thermoreflectance (TDTR) measurements with different metal transducers (Al, Au, and Au with a Ti wetting layer), we attribute this variation in effective thermal conductivity to a combination of size effects in the β-Ga2O3 film resulting from phonon scattering at the β-Ga2O3/GaN interface and thermal transport across the β-Ga2O3/GaN interface. The measured thermal properties of open atmosphere-grown β-Ga2O3 and its interface with GaN set the stage for thermal engineering of gate contacts in high frequency GaN-based devices.

show abstract

Interpreting picosecond acoustics in the case of low interface stiffness

Cited by 66 publications

References 14 publications

Anisotropic failure of Fourier theory in time-domain thermoreflectance experiments

Anisotropic failure of Fourier theory in time-domain thermoreflectance experiments

Anisotropic Thermal Conductivity of Exfoliated Black Phosphorus

Size effects in the thermal conductivity of gallium oxide (β-Ga2O3) films grown via open-atmosphere annealing of gallium nitride

Contact Info

Product

Resources

About