Browsing by Author "Wang, Tianshi"
Now showing 1 - 3 of 3
Results Per Page
Sort Options
Item Band alignment and p-type doping of ZnSnN2(American Physical Society, 2017-05-31) Wang, Tianshi; Ni, Chaoying; Janotti, Anderson; Tianshi Wang, Chaoying Ni, and Anderson Janotti; Wang, Tianshi; Ni, Chaoying; Janotti, AndersonComposed of earth-abundant elements, ZnSnN2 is a promising semiconductor for photovoltaic and photoelectrochemical applications. However, basic properties such as the precise value of the band gap and the band alignment to other semiconductors are still unresolved. For instance, reported values for the band gap vary from 1.4 to 2.0 eV. In addition, doping in ZnSnN2 remains largely unexplored. Using density functional theory with the Heyd-Scuseria-Ernzerhof hybrid functional, we investigate the electronic structure of ZnSnN2, its band alignment to GaN and ZnO, and the possibility of p-type doping. We find that the position of the valence-band maximum of ZnSnN2 is 0.39 eV higher than that in GaN, yet the conduction-band minimum is close to that in ZnO, which suggests that achieving p-type conductivity is likely as in GaN, yet it may be difficult to control unintentional n-type conductivity as in ZnO. Among possible p-type dopants, we explore Li, Na, and K substituting on the Zn site. We show that while LiZn is a shallow acceptor, NaZn and KZn are deep acceptors, which we trace back to large local relaxations around the Na and K impurities due to the atomic size mismatches.Item Electronic and thermal properties of wide bandgap materials from density functional theory(University of Delaware, 2019) Wang, TianshiWide bandgap materials, e.g. SiC, GaN, ZnO, Ga2O3, and diamond, find many applications in microelectronics such as high electron mobility transistors (HEMT), field effect transistors, and light-emitting diodes (LED). Great efforts are being made on the wide bandgap materials as mandated by next generation device design and fabrications. This work focuses on three topics: heat transport in SiC/diamond/Si systems, the electronic properties of (AlxGa1-x)2O3 alloys, and phonon-limited electron transport in ZnO and GaN. ☐ The computational simulation is based upon the density functional theory (DFT) which provides efficient means to investigate a system from a quantum mechanics description and has become a powerful tool in computational materials science. This work uses a combined approach of DFT, density functional perturbation theory (DFPT), classical heat diffusion models, and special quasirandom structures (SQS) models. ☐ Heat transport is critical in diamond/Si/SiC system for advanced applications in high energy laser (HEL) mirrors and semiconductor devices. We calculated the relations between the effective thermal conductivity and grain size in polycrystalline diamond. We also derived thermal barrier resistances in Si/diamond, Si/SiC, SiC/diamond, and GaN/diamond heterostructures which are important parameters in developing relevent composites and simulating device performances. ☐ (AlxGa1-x)2O3 alloys are promising materials for solar-blind UV photodetectors and high-power transistors. From hybrid-functional calculations, we computed formation enthalpies, band gaps, and band edge positions of (AlxGa1-x)2O3 alloys. We found the formation enthalpies of (AlxGa1-x)2O3 alloys are relatively low and that (AlxGa1-x)2O3 with x=0.5 can be considered as an ordered compound AlGaO3 in the monoclinic phase, with Al occupying the octahedral sites and Ga occupying the tetrahedral sites. In addition, most of the band offset of the (AlxGa1-x)2O3 alloys arises from the discontinuity in the conduction band. Our results can explain the available experimental data and consequences for designing modulation-doped field effect transistors (MODFETs) based on (AlxGa1-x)2O3/Ga2O3. ☐ Electron mobility in oxides are well known to be much lower than that in nitrides; however, the reason remains unclear. For example, the measured room-temperature (RM) electron Hall mobilities of intrinsic GaN and ZnO are significantly different, i.e. 1350 and 440 cm2/Vs, respectively. From first-principles calculations, we find the difference is from the much stronger electron-phonon (e-ph) scattering in ZnO, which is dominated by piezoelectric and polar-optical-phonon (POP) interactions. Furthermore, the stronger e-ph coupling strength in ZnO is the origin of the stronger piezoelectric interaction, while the higher longitude optical (LO) phonon frequency results in the stronger POP interaction in ZnO. This work also highlights the importance of piezoelectric interaction in strongly ionic materials.Item Thermal transport across metal silicide-silicon interfaces: An experimental comparison between epitaxial and nonepitaxial interfaces(American Physical Society, 2017-02-22) Ye, Ning; Feser, Joseph P.; Sadasivam, Sridhar; Fisher, Timothy S.; Wang, Tianshi; Ni, Chaoying; Janotti, Anderson; Ning Ye, Joseph P. Feser, Sridhar Sadasivam, Timothy S. Fisher, Tianshi Wang, Chaoying Ni, and Anderson Janotti; Ye, Ning; Feser, Joseph P.; Wang, Tianshi; Ni, Chaoying; Janotti, AndersonSilicides are used extensively in nano- and microdevices due to their low electrical resistivity, low contact resistance to silicon, and their process compatibility. In this work, the thermal interface conductance of TiSi2, CoSi2, NiSi, and PtSi are studied using time-domain thermoreflectance. Exploiting the fact that most silicides formed on Si(111) substrates grow epitaxially, while most silicides on Si(100) do not, we study the effect of epitaxy, and show that for a wide variety of interfaces there is no dependence of interface conductance on the detailed structure of the interface. In particular, there is no difference in the thermal interface conductance between epitaxial and nonepitaxial silicide/silicon interfaces, nor between epitaxial interfaces with different interface orientations.While these silicide-based interfaces yield the highest reported interface conductances of any known interface with silicon, none of the interfaces studied are found to operate close to the phonon radiation limit, indicating that phonon transmission coefficients are nonunity in all cases and yet remain insensitive to interfacial structure. In the case of CoSi2, a comparison ismade with detailed computational models using (1) full-dispersion diffuse mismatch modeling (DMM) including the effect of near-interfacial strain, and (2) an atomistic Green’ function (AGF) approach that integrates near-interface changes in the interatomic force constants obtained through density functional perturbation theory. Above 100 K, the AGF approach significantly underpredicts interface conductance suggesting that energy transport does not occur purely by coherent transmission of phonons, even for epitaxial interfaces. The full-dispersion DMM closely predicts the experimentally observed interface conductances for CoSi2, NiSi, and TiSi2 interfaces, while it remains an open question whether inelastic scattering, cross-interfacial electron-phonon coupling, or other mechanisms could also account for the high-temperature behavior. The effect of degenerate semiconductor dopant concentration onmetal-semiconductor thermal interface conductance was also investigated with the result that we have found no dependencies of the thermal interface conductances up to (n or p type) ≈1 × 1019 cm−3, indicating that there is no significant direct electronic transport and no transport effects that depend on long-range metal-semiconductor band alignment.