Strain dependence of thermal conductivity of [0001]-oriented GaN nanowires

Abstract

The thermomechanical behavior of ͓0001͔-oriented GaN nanowires with 2.26 and 3.55 nm in diameter under tensile loading is analyzed using molecular dynamics simulations with the Green-Kubo method and quantum correction. A phase transformation from wurtzite to a tetragonal structure is observed. The thermal conductivity is found to decrease as the wires undergo tensile deformation and phase transformation, except for the smallest diameter and temperatures above 1495 K at which it remains largely constant as the axial strain increases. The different trends appear to result from phonon behavior changes primarily associated with the surface structures of the nanowires at the different conditions. The coupling between the thermal and mechanical behaviors of nanostructures is important because it provides a mechanism for designing nanoscale devices and for enhancing the performance of devices. The variation with loading or deformation of thermal conductivity of nanowires and nanofilms of a range of materials is of particular interest and has been extensively studied. 1-6 For example, it has been reported that a structural transformation from wurtzite ͑WZ͒ to a graphitelike ͑HX͒ structure in ͓0110͔-oriented ZnO nanowires under tensile loading causes significant changes in the thermal conductivity of the nanowires. 2,7 The dominant effects that cause the conductivity change come from differences in density of the crystalline structures.In related studies, a tetragonal structure ͑TS͒ with fouratom rings is observed in ͓0001͔-oriented ZnO ͑Refs. 8 and 9͒ and GaN ͑Ref. 10͒ nanowires. This structure results from a phase transformation from WZ under uniaxial tensile loading. So far, thermal response changes of the nanowires associated with this structural transform have not been studied. Here, we report the results of molecular dynamics ͑MD͒ calculations that relate to the response changes. The focus is on the thermal conductivity of wires of different sizes at different temperatures. To obtain a more accurate quantification of the conductivity and temperature, the empirical approach of Lee et al. 11 for quantum correction of MD data and temperature is used.In this analysis, GaN nanowires with the WZ structure and a hexagonal cross section like that in Fig. 1 of Ref. 10 are considered. The axis of the nanowires is in the ͓0001͔ crystalline direction. Periodic boundary conditions are specified in the axial direction and the lateral surfaces of the wires are traction-free. The length of the wires is 14.45 nm and the diameters considered are 2.26 and 3.55 nm. The calculations are carried out using the LAMMPS code. 12 The Buckingham potential 13 is used to describe the short range interactions between atoms and the Wolf summation 14,15 is used to evalu-ate the long-range Coulombic forces. This potential reproduces the structural, elastic, and dielectric constants of the WZ, zinc-blende, and rocksalt structures. 13 Native-defect formation and surface relaxation are also effectively predicted. 13 The results obtained by using this p...

Thermal and mechanical response of [0001]-oriented GaN nanowires during tensile loading and unloading

“…The thermal conductivity of the TS-structured nanowires with a strain of 0.1 is 4.2 W/mK which is 27% lower than that of the unstressed WZ nanowires. The decrease in conductivity is attributed to the strain of the wire and a higher disorder in the surface of the TS-structured wires than that of the WZ-structured wire, 17 as illustrated in Fig. 3.…”

“…15,16 Thermal conductivity of the ZnO nanowires increases during the transformation from WZ to HX because the density of the HX structure is higher than that of the WZ structure. 16 In our previous work, 17 a phase transformation from WZ to TS structure is shown to occur in [0001]-oriented GaN nanowires under tensile loading. In contrast to what is observed in the ½01 10-oriented ZnO nanowires, the phase transformation from WZ to TS lowers the thermal conductivity of the GaN nanowires.…”

“…The density of the TS structure is almost the same as that of the WZ structure, 11 but a surface layer with a structure different from the TS structure in the interior of the transformed nanowires causes the thermal conductivity to be lower. 17 During the WZ-to-HX transformation, nanowires consist of both WZ regions and HX regions. Wires in the intermediate states have thermal conductivities that decrease due to the formation of the interfaces first and increase with the progress of transformation because the fraction of atoms in the HX phase increases.…”

“…16 Such intermediate states are not observed for GaN and, consequently, the change in thermal conductivity is not significant during the transformation from WZ to TS under tensile loading. 17 unloading using molecular dynamics (MD) simulations. The effects of strain, structural change, and size on thermal conductivity are delineated.…”

Phonon properties and thermal conductivity of GaN nanofilm under prestress and surface/interface stress

“…Some studies had been carried out to monitor the phonon thermal conductivity in stressed GaN nanostructures [9e11]. For example, Junk et al adopted the molecular dynamics method to simulate the strain-dependent thermal conductivity in GaN nanowires, and they revealed that the deformation along the axial direction reduces the thermal conductivity for GaN nanowires with different shape of cross section [9]. Zhu and Ruan also investigated the relationship between the stress fields and the phonon thermal conductivity of GaN nanofilms and GaN nanowires [11].…”