In recent year, nanoporous Si thin films have been widely studied for their potential applications in thermoelectrics, in which high thermoelectric performance can be obtained by combining both the dramatically reduce...In recent year, nanoporous Si thin films have been widely studied for their potential applications in thermoelectrics, in which high thermoelectric performance can be obtained by combining both the dramatically reduced lattice thermal conductivity and bulk-like elec- trical properties. Along this line, a high thermoelectric figure of merit （ZT） is also anticipated for other nanoporous thin films, whose bulk counterparts possess superior electrical properties but also high lattice thermal conductivities. Numerous thermoelectric studies have been carried out on Si-based nanoporous thin fills, whereas cost-effective nitrides and oxides are not systematically studied for similar thermoelectric benefits. In this work, the cross-plane thermal conductivities of nanoporous Ino.lGao.9N thin films with varied porous patterns were measured with the time-domain thermoreflectance techni- que. These alloys are suggested to have better electrical properties than conventional SixGel x alloys; however, a high ZT is hindered by their intrinsically high lattice thermal conductivity, which can be addressed by introdu- cing nanopores to scatter phonons. In contrast to previous studies using dry-etched nanopores with amorphous poreedges, the measured nanoporous thin films of this work are directly grown on a patterned sapphire substrate to minimize the structural damage by dry etching. This removes the uncertainty in the phonon transport analysis due to amorphous pore edges. Based on the measurement results, remarkable phonon size effects can be found for a thin film with periodic 300-nm-diameter pores of different patterns. This indicates that a significant amount of heat inside these alloys is still carried by phonons with -300 nm or longer mean flee paths. Our studies provide important guidance for ZT enhancement in alloys of nitrides and similar oxides.展开更多
文摘In recent year, nanoporous Si thin films have been widely studied for their potential applications in thermoelectrics, in which high thermoelectric performance can be obtained by combining both the dramatically reduced lattice thermal conductivity and bulk-like elec- trical properties. Along this line, a high thermoelectric figure of merit （ZT） is also anticipated for other nanoporous thin films, whose bulk counterparts possess superior electrical properties but also high lattice thermal conductivities. Numerous thermoelectric studies have been carried out on Si-based nanoporous thin fills, whereas cost-effective nitrides and oxides are not systematically studied for similar thermoelectric benefits. In this work, the cross-plane thermal conductivities of nanoporous Ino.lGao.9N thin films with varied porous patterns were measured with the time-domain thermoreflectance techni- que. These alloys are suggested to have better electrical properties than conventional SixGel x alloys; however, a high ZT is hindered by their intrinsically high lattice thermal conductivity, which can be addressed by introdu- cing nanopores to scatter phonons. In contrast to previous studies using dry-etched nanopores with amorphous poreedges, the measured nanoporous thin films of this work are directly grown on a patterned sapphire substrate to minimize the structural damage by dry etching. This removes the uncertainty in the phonon transport analysis due to amorphous pore edges. Based on the measurement results, remarkable phonon size effects can be found for a thin film with periodic 300-nm-diameter pores of different patterns. This indicates that a significant amount of heat inside these alloys is still carried by phonons with -300 nm or longer mean flee paths. Our studies provide important guidance for ZT enhancement in alloys of nitrides and similar oxides.
