Growth Monitoring With Submonolayer Sensitivity Via Real-Time Thermal-Conductance Measurements

Abstract

Growth monitoring during the early stages of film formation is of prime importance to understand the growth process, the microstructure and thus the overall layer properties.In this work, we demonstrate that phonons can be used as sensitive probes to monitor real time evolution of film microstructure during growth, from incipient clustering to continuous film formation. For that purpose, a silicon nitride membrane-based sensor has been fabricated to measure in-plane thermal conductivity of thin film samples. Operating with the 3 -Völklein method at low frequencies, the sensor shows an exceptional resolution down to ( · ) = 0.065 · · , enabling accurate measurements.Validation of the sensor performance is done with organic and metallic thin films. In both cases, at early stages of growth, we observe an initial reduction of the effective thermal conductance of the supporting amorphous membrane, , related with the surface phonon scattering enhanced by the incipient nanoclusters formation. As clusters develop, reaches a minimum at the percolation threshold. Subsequent island percolation produces a sharp increase of the conductance and once the surface coverage is completed increases linearly with thickness The thermal conductivity of the deposited films is obtained from the variation of with thickness.

Evidence of thermal transport anisotropy in stable glasses of vapor deposited organic molecules

“…The QCM true rate was previously calibrated by measuring a reference sample at the profilometer. The samples were deposited directly onto the silicon nitride membrane-based sensor and subsequently measured in differential mode using the 3ω-Vöklein [44][45][46][47] method. The sensors, a sample and a reference, were placed on a substrate with a heating element and a Pt100 and the temperature was controlled during the deposition and the measurement of the films.…”

“…In-plane measurements: 3ω-Völklein. We use a modification of the 3ω-Völklein method [46] previously developed by Volklein et al [47]. In this setup, we use a silicon nitride membrane with the same symmetry as a metal wire that is used for heating/sensing.…”

“…The in-plane thermal conductivity, 𝑘 ∥ , of the vapour-deposited films was evaluated by the modified 3ω-Volklein technique [44][45][46][47], while through-plane, 𝑘 ⊥ , measurements were carried out using the 3ω method developed by Cahill and coworkers [53]. The experimentally measured in-plane thermal conductivity of the TPD layers is presented as a function of the deposition temperature in Figure 2a.…”

“…This methodology, frequently used for inorganic nanowires of low thermal conductance, requires lengthy or sophisticated approaches to precisely place the sample bridging the heater/sensor platforms. We have recently shown that a modification of the 3ω-Volklein technique [44][45][46][47] can be used to monitor in real-time the growth of organic layers and to measure their in-plane thermal conductance [46]. The high sensitivity of the technique and its versatility makes it an ideal tool to explore the in-plane thermal transport characteristics of organic thin films.…”