Electron Mobility In Wurtizite Nitride Quantum Wells And The Modulation By Strain And Pressure

Due to their large bandgap, high breakdown field, large heat conductivity, high electron saturation velocity and well stability, wurtzite nitrides are important candidates for devices with high frequency and high power. At present, many theoretical and experimental works focus on the GaN based high electron mobility transistors (HEMTs). Wurtzite nitride quantum wells (QWs) are the basic structures in GaN-HEMTs. The effect of strong spontaneous and piezoelectric polarization induces a two dimensional electron gas (2DEG) with high sheet density near the interfaces in the structures. The electric properties of a HEMT can be controlled by the mobility and sheet density of a 2DEG. In this thesis, the electron mobility in wurtzite nitride QWs in the temperature range, in which the scattering from optical phonons has a main effect, is discussed based on the dielectric continuous model, uniaxial model and the force balance equation of Lei and Ding. Main contents and results obtained are generalized as follows:(1) Size effect on electron mobility in wurtzite AIN/GaN/AIN QWs. The results show that under normal pressure, the main contribution to electron mobility at room temperature (RT) is from the scattering of interface optical phonons for cases of narrow and wide wells, whereas it is from that of confined optical phonons for cases of intermediate well width. As well width increases, electron mobility first decreases from a small value to a valley, then increases and reaches a saturated value. For a given well width, electron mobility in QWs with finite thick barriers is greater than that with infinite thick barriers. The decrease of built-in electric field (BIF) in AIN/GaN/AIN QWs enhances electron mobility. The size of QWs affects the BIF and distribution of 2DEG. The optimization of the size can enhance electron mobility and accordingly improve the properties of HEMTs.(2) Pressure effect on electron mobility in AIN/GaN/AIN QWs. It is found that, on one hand, the electron effective mass, phonon frequency, BIF, band offset and high-frequency dielectric constant increase with increase of hydrostatic pressure, on the other hand, the number of phonons in the system reduces. As a result, the electron mobility increases slightly as hydrostatic pressure increases.(3) Effect of ternary mixed crystals (TMC) of optical phonon modes in AlxGa1-xN/GaN/AlxGa1-xN QWs and its influence on electron mobility. It is found that optical phonon modes in QWs vary with the components of TMC. Phonon modes such as localized modes, interface modes, half-space modes, and propagating modes exist in certain regions of the component and frequency due to the anisotropy of phonon dispersion in wurtzite nitrides. At the same time, the dispersion relation of the same phonon modes also varies with the component. The results show that the influence on electrons from anti-symmetric phonons becomes relatively stronger while that from symmetric phonons becomes weaker with increase of Al component. Electron mobility decreases with increase of Al component and temperature, whereas it increases obviously with increase of the sheet density of electrons.(4) Influence of the introduction of an InxGa1-xN nanogroove in a strained wurtzite AIN/GaN/AIN QWs on electron mobility. The results show that the optical phonon modes can be changed by the introduction of InGaN/GaN interfaces and the TMC effect. It can also be found that electron wave function will shift to InGaN layer as long as the conductor band energy at GaN/InGaN interface is lower than that at AIN/GaN interface. When electrons mainly distribute in the well region, electron mobility is greater than the case without an InGaN nanogroove and it first increases and then decreases as In component increases. After the 2DEG transfers to the InGaN layer, electron mobility drops sharply and then increases with the increase of In component.The above conclusions may direct the related experiments and design of HEMTs.