Electronic and thermal properties of wide bandgap materials from density functional theory
Date
2019
Authors
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Publisher
University of Delaware
Abstract
Wide 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.