Phonon Transport And Manipulation Study In Si_1-xGe_x Epitaxial Films

As the first generation semiconductors,silicon and germanium have continuously attracted attentions both in fundamental research and practical applications.Except for the broad employment in the microelectronics dominated semiconductor industry,the SiGe alloys play important roles in many other areas such as thermoelectric energy conversion,opto-electronic detection,and high speed logic devices.It worth mentioning that due to the superior thermoelectric performance,SiGe has been used in thermoelectric devices which power the state-of-the-art space rovers.Within all the material properties,the heat conduction,especially the thermal transport at the microscale,has been the hot research topic for years.However,the study on thin films of this alloy system has been limited to lower Ge content Si1-xGex(x<0.5),because the increasing lattice mismatch between the SiGe film and the Si substrate as the Ge content increases.This mismatch results in deteriorated crystalline quality,making the characterization and mechanism study even harder.This thesis focuses on the phonon transport and manipulation in SiGe alloy thin films.Using time-domain thermoreflectance(TDTR)method,we measured the cross-plane thermal conductivities of a series of high quality Si1-xGex thin films grown on Si(100)substrates.Systematic study including the dependences of composition,thickness,and temperature has been carried out.Theoretical analyses based on the Boltzmann equation and Callaway model are performed to explain the phonon scattering mechanisms in the Si1-xGex(0?x?1)thin films.The key research results are summarized as follows:(1)the thermal conductivities of Si1-xGex(0?x?1)films with a full-composition spectrum have been measured using TDTR and the composition dependence has been shown as a "U" shape curve,a typical alloy scattering behavior.The results show a rather flat curve in the region of x=0.18 to 0.77.When x=0.32,the thermal conductivity reaches a minimum of 2.05 W/mK.(2)Through the thickness dependent thermal conductivity study,ballistic transport has been observed in Si1-xGex(x = 0.67,1)thin films.In addition,as the thickness increases,phonon transport turns to diffusive transport.It indicates that the phonon mean free path in these alloys are greater than 1?m.(3)Through the temperature dependent thermal conductivity measurements,the alloy behavior of the Si1-xGex(0?x?1)films is further proved.We calculated the phonon relaxation time and analyzed the phonon scattering mechanism.Furthermore,we use the calculated spectral phonon distribution to explain the temperature dependence in the measure temperature region(50-300 K).(2)Finally,we measured the interfacial thermal conductance of Al/Si and Al/Ge,found and proposed methods to improve the interfacial thermal conductance of metal/semiconductor interfaces.