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
Phonon transport across an interface is of fundamental importance to applications ranging from electronic and optical devices to thermoelectric materials.The phonon scattering by an interface can dramatically suppress...Phonon transport across an interface is of fundamental importance to applications ranging from electronic and optical devices to thermoelectric materials.The phonon scattering by an interface can dramatically suppress the thermal transport,which can benefit thermoelectric applications but create problems for the thermal management of electronic/optical devices.In this aspect,existing molecular dynamics simulations on phonon transport across various interfaces are often based on estimates of atomic structures and are seldom compared with measurements on real interfaces.In this work,planar Si/Ge heterojunctions formed by film-wafer bonding are measured for the interfacial thermal resistance (R_(K)) that is further compared with predictions from existing simulations and analytical models.The twist angle between a 70-nm-thick Si film and a Ge wafer is varied to check the influence of the crystal misorientation.Detailed transmission electron microscopy studies are carried out to better understand the interfacial atomic structure.It is found that the alloyed interfacial layer with mixed Si and Ge atoms dominates the measured thermal resistance(R_(K)).Some oxygen impurities may also help to increase RK due to the formation of glassy structures.Following this,RK reduction should be focused on how to minimize the interdiffusion of Si and Ge atoms during the formation of a Si/Ge heterojunction.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.
基金the support from National Science Foundation CAREER Award(grant number CBET-1651840)TEM and SEM analyses were performed at the Kuiper Materials Imaging and Characterization facility+2 种基金NASA(grants#NNX12AL47G and#NNX15AJ22G)NSF(grant#1531243)for funding of the instrumentation in the Kuiper Materials Imaging and Characterization Facility at the University of ArizonaU.S.Department of Commerce,National Institute of Standards and Technology as part of the Center for Hierarchical Materials Design(CHiMaD)grant 70NANB19H005.
文摘Phonon transport across an interface is of fundamental importance to applications ranging from electronic and optical devices to thermoelectric materials.The phonon scattering by an interface can dramatically suppress the thermal transport,which can benefit thermoelectric applications but create problems for the thermal management of electronic/optical devices.In this aspect,existing molecular dynamics simulations on phonon transport across various interfaces are often based on estimates of atomic structures and are seldom compared with measurements on real interfaces.In this work,planar Si/Ge heterojunctions formed by film-wafer bonding are measured for the interfacial thermal resistance (R_(K)) that is further compared with predictions from existing simulations and analytical models.The twist angle between a 70-nm-thick Si film and a Ge wafer is varied to check the influence of the crystal misorientation.Detailed transmission electron microscopy studies are carried out to better understand the interfacial atomic structure.It is found that the alloyed interfacial layer with mixed Si and Ge atoms dominates the measured thermal resistance(R_(K)).Some oxygen impurities may also help to increase RK due to the formation of glassy structures.Following this,RK reduction should be focused on how to minimize the interdiffusion of Si and Ge atoms during the formation of a Si/Ge heterojunction.
