Progress of microscopic thermoelectric effects studied by micro- and nano-thermometric techniques

Heat dissipation is one of the most serious problems in modern integrated electronics with the continuously decreasing devices size. Large portion of the consumed power is inevitably dissipated inthe form of waste heat which not only restricts the device energy-efficiency performance itself, butalso leads to severe environment problems and energy crisis. Thermoelectric Seebeck effect is a greenenergy-recycling method, while thermoelectric Peltier effect can be employed for heat management byactively cooling overheated devices, where passive cooling by heat conduction is not sufficiently enough.However, the technological applications of thermoelectricity are limited so far by their very low conversion efficiencies and lack of deep understanding of thermoelectricity in microscopic levels. Probingand managing the thermoelectricity is therefore fundamentally important particularly in nanoscale. Inthis short review, we will first briefly introduce the microscopic techniques for studying nanoscale thermoelectricity, focusing mainly on scanning thermal microscopy (SThM). SThM is a powerful tool formapping the lattice heat with nanometer spatial resolution and hence detecting the nanoscale thermaltransport and dissipation processes. Then we will review recent experiments utilizing these techniques to investigate thermoelectricity in various nanomaterial systems including both (two-material)heterojunctions and (single-material) homojunctions with tailored Seebeck coefficients, and also spinSeebeck and Peltier effects in magnetic materials. Next, we will provide a perspective on the promisingapplications of our recently developed Scanning Noise Microscope (SNoiM) for directly probing thenon-equilibrium transporting hot charges (instead of lattice heat) in thermoelectric devices. SNoiMtogether with SThM are expected to be able to provide more complete and comprehensive understanding to the microscopic mechanisms in thermoelectrics. Finally, we make a conclusion and outlook onthe future development of microscopic studies in thermoelectrics.

Efficient thermal dissipation in wafer-scale heterogeneous integration of single-crystalline𝛽β-Ga_(2)O_(3)thin film on SiC

The semiconductor,β-Ga_(2)O_(3)is attractive for applications in high power electronic devices with low conduction loss due to its ultra-wide bandgap(∼4.9 eV)and large Baliga’s figure of merit.However,the thermal conductivity of𝛽β-Ga_(2)O_(3)is much lower than that of other wide/ultra-wide bandgap semiconductors,such as SiC and GaN,which results in the deterioration of𝛽β-Ga_(2)O_(3)-based device performance and reliability due to self-heating.To overcome this problem,a scalable thermal management strategy was proposed by heterogeneously integrating wafer-scale single-crystalline𝛽β-Ga_(2)O_(3)thin films on a highly thermally conductive SiC substrate.Characterization of the transferred𝛽β-Ga_(2)O_(3)thin film indicated a uniform thickness to within±2.01%,a smooth surface with a roughness of 0.2 nm,and good crystalline quality with an X-ray rocking curves(XRC)full width at half maximum of 80 arcsec.Transient thermoreflectance measurements were employed to investigate the thermal properties.The thermal performance of the fabricated𝛽β-Ga_(2)O_(3)/SiC heterostructure was effectively improved in comparison with that of the𝛽β-Ga_(2)O_(3)bulk wafer,and the effective thermal boundary resistance could be further reduced to 7.5 m 2 K/GW by a post-annealing process.Schottky barrier diodes(SBDs)were fabricated on both a𝛽β-Ga_(2)O_(3)/SiC heterostructured material and a𝛽β-Ga_(2)O_(3)bulk wafer.Infrared thermal imaging revealed the temperature increase of the SBDs on𝛽β-Ga_(2)O_(3)/SiC to be one quarter that on the𝛽β-Ga_(2)O_(3)bulk wafer with the same applied power,which suggests that the combination of the𝛽-Ga_(2)O_(3)thin film and SiC substrate with high thermal conductivity promotes heat dissipation in𝛽β-Ga_(2)O_(3)-based devices.