Gallium nitride(GaN),as a typical representative of new semiconductor materials,is widely applied in optoelectronic and microelectronic devices owing to its eminent physical and chemical characteristics,and has been a research hotspot in recent years.It has been known that the previous researches mostly focused on the influence of size effect,quantum confinement effect,surface/interface scattering effect and stress field on the phonon properties and thermal performance of nanostructures,respectively.However,the surface/interface coupling effect,such as quantum confinement and surface/interface scattering coupling effect,surface/interface scattering and stress field coupling effect,or quantum confinement and surface charge coupling effect is lacking in the existing literatures.In this paper,the impact of three coupling effect mentioned above on the phonon properties and thermal performance of GaN nanofilms are separately studied in quantity by applying the elastic model and Boltzmann transport equation(BTE).This work would contribute to the control of the thermal conductivity of GaN nanostructures through surface engineering.Firstly,the elastic theory based on the continuous mechanics and finite difference method are introduced to describe the acoustic phonon properties of spatially confined GaN nanofilm,including the phonon dispersion relations,phonon average group velocity and phonon density of state(DOS).Meanwhile,the surface/interface scattering effect is involved based on BTE to establish the cross-plane phonon transport model.Then the quantum confinement effect and surface scattering effect are taken into account in estimating the thermal conductivity of GaN nanofilm.The numerical results show that compared with the quantum confinement effect,the surface scattering could result in the order-of-magnitude reduction of the cross-plane thermal conductivity,leading to the remarkable change of size effect on the conductivity in GaN nanofilm.Secondly,the acoustoelastic effect theory is addressed to describe the influence of stress filed on the phonon properties of nanofilm.On the basis of BTE,the in-plane phonon transport model is also established to study quantitatively the impact of surface/interface scattering effect on the in-plane thermal conductivity of stressed GaN nanofilm.The theoretical results indicate that the value of in-plane phonon thermal conductivity is less than the cross-plane counterpart.Besides,the applied prestress field can significantly change the phonon dispersion relations.The phonon thermal conductivity would be improved if prestress is negative and vice versa.Similarly,the surface scattering and prestress coupling effect can also generate the significant change of size effect on the conductivity in GaN nanofilm.Finally,the effect of surface charge attached to the nanofilm on the thermal conductivity is also investigated.Our numerical results predict that surface charge can also alter the phonon properties,and the negative charge increases the thermal conductivity and vice versa.In addition,the existence of the surface charges can significant modify the dependence of temperature and geometrical size on the thermal conductivity of GaN nanofilm.For example,the positive surface charges weaken the temperature-sensitivity of thermal conductivity but reinforce the size effect of conductivity.In conclusion,some innovative and effective ways,such as surface/interface scattering mechanism,the introduction of applied prestress field and surface charge are demonstrated in this work to achieve the target on controlling the phonon properties and thermal performance of GaN nanofilms.We believe that the present work could be helpful for the precise design of the reliable and preeminent GaN-based nanoelectric devices. |