Nanocalorimetry

The group is largely involved in nanocalorimetric analysis of low dimensional systems aiming to analyze thermodynamics and kinetics playing a role in phase transitions occurring in thin and ultrathin films or nanostructured materials.

We use membrane-based nanocalorimetry at very fast heating rates (104 - 105 K/s), a technique developed by Les Allen at the University of Illinois.

Phase formation

Formation of Pd2Si on single-crystalline Si (100) at ultrafast heating rates: An in-situ analysis by nanocalorimetry

The fabrication of nm-size transistors in complementary metal-oxide-semiconductor (CMOS) technology highly depends on the formation of low-ohmic contacts by the reaction between silicon and a thin metallic layer. As the dimensions of the devices are reduced to the nm scale, the thermally induced solid-state formation of the silicide phase becomes extremely dependent on process parameters and material characteristics, such as metal thickness, substrate orientation, or surface roughness.

As an example, the kinetics of intermediate phase formation between ultrathin films of Pd (12 nm) and single-crystalline Si (100) is monitored by in-situ nanocalorimetry at ultrafast heating rates. The heat capacity curves and the extracted reaction enthalpy are consistent with values reported at slower heating rates for the formation of Pd2Si, as also confirmed by XTEM and XRD. A kinetic model which goes beyond the conventional linear-parabolic growth to consider independent nucleation and lateral growth of Pd2Si along the interface and vertical growth mechanisms is developed to fit the calorimetric curves. The model is used to extract the effective interfacial nucleation/growth and diffusion coefficients at the unusually high temperatures of silicide formation achieved at very fast heating rates.

APL2013-fig2

(a) Specific heat capacity difference between 1st and 2nd scan vs. Temperature for 12 nm Pd samples measured at three different rates. The continuous lines correspond to the calculated data using the nucleation and growth model. (b) Comparison between linear-parabolic growth (LP) and nucleation and growth models (NG) (see supplementary material for further details Ref. 18). The dotted lines within the NG peak are the interface and vertical contribution to the calorimetric peak. (c) T-HR-T diagram for the Pd2Si formation compared with DSC data from Ref. 10 . See Appl. Phys. Lett. 102, 143111 (2013)

Kinetics of silicide formation over a wide range of heating rates spanning six orders of magnitude

Kinetic processes involving intermediate phase formation are often assumed to follow an Arrhenius temperature dependence. This behavior is usually inferred from limited data over narrow temperature intervals, where the exponential dependence is generally fully satisfied. However, direct evidence over wide temperature intervals is experimentally challenging and data are scarce. Here, we report a study of silicide formation between a 12 nm film of palladium and 15 nm of amorphous silicon in a wide range of heating rates, spanning six orders of magnitude, from 0.1 to 105 K/s, or equivalently more than 300K of variation in reaction temperature. The calorimetric traces exhibit several distinct exothermic events related to interdiffusion, nucleation of Pd2Si, crystallization of amorphous silicon, and vertical growth of Pd2Si. Interestingly, the thickness of the initial nucleation layer depends on the heating rate revealing enhanced mass diffusion at the fastest heating rates during the initial stages of the reaction. In spite of this, the formation of the silicide strictly follows an Arrhenius temperature dependence over the whole temperature interval explored. A kinetic model is used to fit the calorimetric data over the complete heating rate range.

Calorimetry is complemented by structural analysis through transmission electron microscopy and both standard and in-situ synchrotron X-ray diffraction.

APL2014-fig3

(a) Thickness of the initial Pd2Si that forms at the a-Si/Pd interface as a function of the heating rate. (b) Schematics of the initial growth mode at the interface a-Si/Pd for slow (top) and fast (bottom) heating. See Appl. Phys. Lett. 105, 013113 (2014)

Magnetic transitions

Evidence of finite-size effect on the Néel temperature in ultrathin layers of CoO nanograins

Using highly sensitive specific heat measurements, we have shown that it is possible to measure the thermodynamic signatures of an antiferromagnetic second-order phase transition in a system that is close to the 0D limit. A significant reduction in the Néel temperature of CoO ultrathin films (down to 1.5 nm) is revealed by highly sensitive specific heat measurement. The scaling of the Néel temperature with the nanograin diameter is characteristic of phase transition in finite 3D systems. The presence of loosely coupled Co spins at the grain surface was derived from the analysis of the magnetic entropy, which decreases very significantly as the size of the nanograins is reduced. This observation has very special importance for exchange-bias nanosystems where antiferromagnetic CoO layers are often studied in conjunction with ferromagnetic ones.

PRB-2011fig4

Specific heat extracted from the raw heat capacity measured on the different CoO thin films: from 1.5 nm to 20 nm. Inset: a photograph of the nanocalorimeter, in the center the platinum heater/thermometer can be distinguished.

PRB-2011fig2

Log-log plot of the variation of the Néel temperature as a function of the mean diameter of the CoO nanograins. The solid line in the best fitting obtained with the parameters set to ξ0=1 nm and ν=0.48. The inset shows the grain diameter as function of the TN in normal scale. See Phys. Rev. B 83, 140407(R) (2011)

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