Electron Mobility In Heterojunctions And Its Hydrostatic Pressure Effect

In this thesis,a memory function approach(MFA) is adopted to discuss theoretically the electron mobility parallel to the interfaces in heterojunctions consisting ofâ…¢-â…¤group semiconductors and in strained heterojunctions consisting ofâ…¡-â…¥group semiconductors,and the pressure effect on the mobility is also been investigated.Firstly,a variational method and the MFA are adopted to investigate the electron mobility in an Al_xGa_1-xAs/GaAs heterojunction composed ofâ…¢-â…¤group materials under pressure effect by considering the scattering from LO and IO phonons.The influence of a realistic interface potential in a heterojunction with a finite barrier and conduction band bending are taken into account.Meanwhile,the tunneling of electrons into the barrier is also considered.The properties of electron mobility versus temperature,Al concentration and electronic density are given and discussed, respectively.The results show that the electron mobility decreases obviously as the increase of temperature and pressure,whereas increases nonlinearly with A1 concentration and electronic density.The scatterings from the IO and LO phonons increase with pressure and the former becomes more obvious than the latter. Furthermore,the variation of electron mobility with A1 concentration and electronic density are dominant by the IO and LO phonons,respectively.Secondly,for the further discussion of the intersubband scattering of electrons in heterojunctions,the eigenvalues of an electron and its corresponding eigenfunctions in a ZnSe/Zn_1-xCd_xSe strained heterojunction with a finite barrier under hydrostatic pressure are obtained by solving the Schrodinger equation.A triangle potential approximation is adopted and the electronic penetrating into the barrier is also taken into account.The numerical results with and without strain effect are compared and analyzed.It is shown that the strain lowers the electronic eigen-energy levels and decreases the seperations between the energy-levels,and pressure effect strengthens this tendency of the drop.Meanwhile strain enhances the penetration probability of eigenfunctions.On the basis of above discussion,the theoretical model is improved to consider the effect of the free strain on both the barrier and the channel side,at the same time, a simplified coherent potential approximation(SCPA) is adopted to calculate the parameters of the ternary mixed crystals composed ofâ…¡-â…¥group materials.The Cd concentration and the electronic density influence,,;on the eigenstates of the electrons are discussed,respectively.The results indicate that the electronic eigen-energy levels and its seperations increase while the penetration probability of eigenfunctions decreases with increase of Cd concentration.Moreover,the eigen-energy levels and the penetration probability increase while the seperations between the energy-levels decrease with the electronic density.Lastly,based on the results of the ZnSe/Zn_1-xCd_xSe strained heterojunction composed ofâ…¡-â…¥group materials,the electron mobility parallel to the interface under hydrostatic pressure is investigated with the MFA by considering the intersubband and intrasubband scatterings from the optical phonons.A triangle potential approximation is adopted to simplify the potential of the conduction band bending and the electronic penetrating into the barrier is considered by a finite interface potential in our model.It is shown that the strain effect lowers the mobility of electrons while the hydrostatic pressure effect is more obvious to decrease the mobility.The contribution induced by the LO phonons in the channel side is dominant to decide the mobility.Compared with the intrasubband scattering,the effect of intersubband scattering is also important and is not negligible for the material studied.Thus,it indicates that the biaxial strain effect needs to be considered when one discusses the scattering problems of an electron in a strained heterojunction.