Hongbo Qin

A Theoretical Review on Interfacial Thermal Transport at the Nanoscale

With the development of energy science and electronic technology, interfacial thermal transport has become a key issue for nanoelectronics, nanocomposites, energy transmission, and conservation, etc. The application of thermal interfacial materials and other physical methods can reliably improve the contact between joined surfaces and enhance interfacial thermal transport at the macroscale. With the growing importance of thermal management in micro/nanoscale devices, controlling and tuning the interfacial thermal resistance (ITR) at the nanoscale is an urgent task. This Review examines nanoscale interfacial thermal transport mainly from a theoretical perspective. Traditional theoretical models, multiscale models, and atomistic methodologies for predicting ITR are introduced. Based on the analysis and summary of the factors that influence ITR, new methods to control and reduce ITR at the nanoscale are described in detail. Furthermore, the challenges facing interfacial thermal management and the further progress required in this field are discussed.

Mechanical, thermodynamic and electronic properties of wurtzite and zinc-blende GaN crystals

For the limitation of experimental methods in crystal characterization, in this study, the mechanical, thermodynamic and electronic properties of wurtzite and zinc-blende GaN crystals were investigated by first-principles calculations based on density functional theory. Firstly, bulk moduli, shear moduli, elastic moduli and Poisson’s ratios of the two GaN polycrystals were calculated using Voigt and Hill approximations, and the results show wurtzite GaN has larger shear and elastic moduli and exhibits more obvious brittleness. Moreover, both wurtzite and zinc-blende GaN monocrystals present obvious mechanical anisotropic behavior. For wurtzite GaN monocrystal, the maximum and minimum elastic moduli are located at orientations [001] and <111>, respectively, while they are in the orientations <111> and <100> for zinc-blende GaN monocrystal, respectively. Compared to the elastic modulus, the shear moduli of the two GaN monocrystals have completely opposite direction dependences. However, different from elastic and shear moduli, the bulk moduli of the two monocrystals are nearly isotropic, especially for the zinc-blende GaN. Besides, in the wurtzite GaN, Poisson’s ratios at the planes containing [001] axis are anisotropic, and the maximum value is 0.31 which is located at the directions vertical to [001] axis. For zinc-blende GaN, Poisson’s ratios at planes (100) and (111) are isotropic, while the Poisson’s ratio at plane (110) exhibits dramatically anisotropic phenomenon. Additionally, the calculated Debye temperatures of wurtzite and zinc-blende GaN are 641.8 and 620.2 K, respectively. At 300 K, the calculated heat capacities of wurtzite and zinc-blende are 33.6 and 33.5 J mol−1 K−1, respectively. Finally, the band gap is located at the G point for the two crystals, and the band gaps of wurtzite and zinc-blende GaN are 3.62 eV and 3.06 eV, respectively. At the G point, the lowest energy of conduction band in the wurtzite GaN is larger, resulting in a wider band gap. Densities of states in the orbital hybridization between Ga and N atoms of wurtzite GaN are much higher, indicating more electrons participate in forming Ga-N ionic bonds in the wurtzite GaN.

A first-principle study of the adsorption behavior of NO gas molecules on pristine and Al-doped penta-graphene

Penta-graphene (PG) has a large ratio of surface to volume and an intrinsic quasi-direct band gap that may benefit for the application of semiconductor sensors. In order to investigate the adsorption behavior of nitric oxide (NO) molecule on the surface of PG, in this study, the adsorption energy, charge transfer and adsorption distance of NO at different locations of pristine and Al-doped PG are investigated by the means of first principle calculations. Results show that the orientation of NO gas molecule has obvious influence on the adsorption behavior in some way. Compared with the pristine PG system, the Al-doped PG system has much higher adsorption energy and shorter adsorption distance. Moreover, the simulation result of total electron density of NO gas on Al-doped PG shows that chemical bonds may exist in the process of NO molecular adsorption, while NO molecule is physically adsorbed on the pristine PG. Furthermore, the result of density of states (DOS) shows that orbital hybridization could be formed between NO and Al-doped PG, indicating that the chemical adsorption occurs.

The interfacial thermo-mechanical reliability of 3D memory-chip stacking with through silicon via array

Thermal stress in through silicon vias (TSVs) caused by the mismatch of coefficient of thermal expansion (CTE) between the filling material and silicon is a key challenge for three-dimensional electronic packaging due to the sophisticated inner structures. In most of the previous studies, the thermomechanical properties of TSV structured chips were analyzed in a unit cell so as to simplify the problem. In this simulation study, a seven-layered memory-chip stacking model is constructed, and finite element method is used to evaluate the influence of the inner structure on thermo-mechanical stresses and the interfacial reliability of the TSV structure package. The simulation results show that, due to the large CTE and low yield strength of copper compared to silicon, the stress concentrated zone locates at the chip and PCB interface on the copper side. The placement of TSVs is one of the main considerations in IC design because it has a significant influence on the thermal stress. Use of Pb-free solder such as SAC305 has almost no influence on the maximum stress in TSV interconnects. The radius of keep-away-zone, where transistors should not be arranged, is two times of that of TSVs. TSV parameters have some influence on the interfacial stress. The maximum interfacial stress increases with decreasing diameters of TSVs when the diameter ratio of TSV to Cu nail (i.e., DTSV/DCu) keeps unchanged.

First-principles investigation of adsorption behaviors of small molecules on penta-graphene

The gas-adsorption behaviors of small molecules CO, H2O, H2S, NH3, SO2, and NO on pristine penta-graphene (PG) were investigated using first-principles calculations to explore their potential for use as advanced gas-sensing materials. Results show that, except for CO, H2O, H2S, NH3, and SO2 are physically adsorbed on the surface of penta-graphene with considerable adsorption energy and moderate charge transfer, while NO is prone to be chemically adsorbed on the surface of penta-graphene. Moreover, the electronic properties of PG can be effectively modified after H2O, H2S, NH3, SO2, and NO are adsorbed, and penta-graphene has potential for using in gas sensors via the charge-transfer mechanism.

The electronic properties of zinc-blende GaN, wurtzite GaN and pnma-GaN crystals under pressure

In this study, electronic properties such as the band structure, total density of state (TDOS) and partial density of states (PDOS) of zinc-blende GaN, wurtzite GaN and pnma-GaN crystals are explored through first-principles calculations within the generalized gradient approximation (GGA), and the influence of hydrostatic pressures on the electronic properties are also researched. Results show that the three GaN compounds are all semiconductors with a direct band gap. Although the band gap increases monotonically with the increase of hydrostatic pressure, the hydrostatic pressure has limited effect on the hybridization of valence band and conduction band. In addition, under the same pressure, the band gap of wurtzite GaN is slightly larger than that of the others. Further, calculation results also show that TDOSs of the three compounds are obviously different, and the increase in pressure reduces the peaks of both TDOSs and PDOSs.

The study on elastic properties of Cu 3 Sn under pressure via first-principle calculations

In this study, elastic properties of intermetallic compound Cu3Sn were calculated by first-principle based on density functional theory (DFT). To exchange correlation energy, generalized gradient approximation (GGA) function was used. Shear modulus, bulk modulus, elastic modulus and lame modulus for Cu3Sn were calculated and applied to evaluate elastic properties of Cu3Sn under hydrostatic pressures ranging from 0 to 60 GPa. Calculation results show that, compared with experimental values, errors of computed lattice constants are within 3%. The elastic modulus, bulk modulus, shear modulus and lame modulus of Cu3Sn are all increased obviously with the rise of the hydrostatic pressure. Moreover, the anisotropy of Cu3Sn is decreased with increasing pressure according to the calculation results of directional dependences of elastic moduli. Furthermore, anisotropic indices calculated under various pressures also prove the reduction of anisotropy with increasing hydrostatic pressure.

Influence of microstructure inhomogeneity on the electromigration behavior of flip chip solder joints

With increasing miniaturization of electronic devices and systems, the dimension of solder joints and pitches has been continuously scaling down, while the current density carried by solder joints increasing significantly, consequently a critical issue, electromigration (EM), has become a key reliability concern. The EM behavior in the joint is mainly dependent on the magnitude and distribution of the current density and thermal gradient. In this study, thermo-electrical finite element analysis was employed to characterize the influence of microstructure inhomogeneity on the current density and thermal gradient in micro-scale eutectic SnPb (63Sn37Pb) solder joints. Results show that, both geometry factor and microstructure inhomogeneity have obvious influence on the distribution of current density and thermal gradient in the flip chip solder joint. The current density, current crowding ratio and thermal gradient in the Sn-rich phase are much larger than that in the Pb-rich phase, thus the Pb atoms in Sn-rich phase are more prone to migrate under current stressing.