Thermal conductivity of hetero-epitaxial ZnO thin films on c- and r-plane sapphire substrates: Thickness and grain size effect

Abstract

The thermal conductivities of c- and a-axis-oriented zinc oxide (ZnO) thin films with nominal thicknesses of 100, 200, and 300 nm are investigated. The c- and a-axis-oriented ZnO thin films were synthesized by radio frequency magnetron sputtering on the c- and r-plane sapphire substrates, respectively. The epitaxial relationship between the ZnO thin film and the c-plane sapphire substrate is (0001)[11¯00] || (0001)[112¯0], and that between the ZnO thin film and the r-plane sapphire substrate is (112¯0)[11¯00] || (011¯2)[112¯0]. The c-axis-oriented ZnO thin film has a columnar structure, whereas the a-axis-oriented ZnO thin film has a single domain-like structure and a significantly flat surface. The thermal conductivity of the c-axis-oriented ZnO thin film is in the range of 18–24 W m−1 K−1, whereas for the a-axis-oriented ZnO thin film, it is in the range of 24–29 W m−1 K−1. For the c-axis-oriented ZnO thin films, the phonon scattering on both the out-of-plane and in-plane grain boundaries affects the thermal conductivity. In contrast, the thermal conductivity of the a-axis-oriented ZnO thin films decreases with the decrease of the film thickness. The distribution of the normalized cumulative thermal conductivity of the a-axis-oriented ZnO thin films suggests that the heat transport carrier mostly consists of phonons with the mean free paths between 100 nm and 1 μm.

Dual Laser Beam Processing of Semiconducting Thin Films by Excited State Absorption

“…For polycrystalline ZnO-layers, thermal conductivities k th < 10 W/m·K are found in literature, which is well below k th ~ 30–100 W/m·K for epitaxially grown or bulk ZnO [ 45 , 46 , 47 , 48 , 49 ]. The thermal diffusion length is determined by µ = 2(α th ·Δt) 0.5 with α th = k th /c V where c V is the volumetric heat capacity with about 2.4–2.8·10 6 J/m³·K [ 47 , 49 ]. With d p = 62 nm, most of the heat is created by the pump beam at the ZnO–air interface.…”

“…Furthermore, the rapid D 2 -decrease in the delay range 20–50 ns ( Figure 5 ) can be attributed to heat dissipation and correlates with literature values [ 45 , 50 ]. As 2ω f is about 100 times larger than d L and µ for the time scale in consideration, one-dimensional heat conduction can be assumed [ 47 , 51 , 52 ]. Here, the heat should be mostly dissipated.…”

Characterization of Nanoscopic Cu/Diamond Interfaces Prepared by Surface-Activated Bonding: Implications for Thermal Management

“…The TBR of the Cu/diamond bonding interface was measured by the time domain pulsed-light-heating thermoreflectance technique . A thin-Cu/diamond bonding structure was fabricated to investigate the TBR of the bonding interface.…”

“…The TBR of the Cu/diamond bonding interface was measured by the time domain pulsed-light-heating thermoreflectance technique. 25 A thin-Cu/diamond bonding structure was fabricated to investigate the TBR of the bonding interface. First, a thin Cu film was deposited onto the photoresist layer coating the Si substrate by electron beam evaporation (EBE).…”

Polarization-Independent Large Third-Order-Nonlinearity of Orthogonal Nanoantennas Coupled to an Epsilon-Near-Zero Material

“…where T e (T l ) is the free-electron (lattice) temperature, g ep = 1.4 × 10 16 Wm −3 •K −1 is the electron-phonon coupling coefficient [41], C e = 3500 Jm −3 •K is the heat capacity of the electrons [41], C l = 2.83 × 10 6 Jm −3 •K is the heat capacity of the lattice, N is the nonthermal energy density stored in the excited electrons [42], τ ee (τ ep ) is the electron-electron (electronphonon) relaxation time [43,44], and P is the time-dependent absorbed power density:…”