Reseacrh On Carrier Mobility Of Strained Si

With the decrease in Si-based device characteristic size and the increase in theintegrated degree and complexity for Ics, a series of problems about material, physics ofdevices, devices structure, process technology appear, and some parasitic effects (suchas higher leakage current, short-channel effect, hot carrier effect, degeneration of carriermobility and so on) are produced. As a result, the performance of devices and circuit isseriously degenerated. Si-based strain engineering can significantly improve carriermobility and device performance, and compatible with traditional silicon processing.Therefore, it obtained the domestic and foreign widespread attention.Mobility is one of important physical parameter for material and device. Theexisted empirical or semi-empirical formula identify fitting parameters based onexperimental data, and Monte Carlo method focus on dealing with stochastic processrelated to scattering based on microscopic description of motion of particle. Theenhancement mechanism of mobility is not obvious for empirical formula,semi-empirical formula or Monte Carlo method. The analytical models built in thisdissertation have clearer physical meanings and emphasize on enhancement mechanismof mobility with a view to the change of Si material due to stress.In this dissertation, the carrier scattering rates and mobilities are mainly researched,and the models proposed can be used to calculate Strained Si mobility with differentstress configurations(uniaxial or biaxial, tensile stress or compressive stress) atarbitrary orientation, which can offer some useful foundation for strained Si devices inelectrical characteristic. The main research work and the results are listed as follows:1.2-D threshold voltage model of strained Si device included the distribution oftransverse surface potential and short channel effect is proposed, which emphasizemodeling process. For simplifying calculation, the origin of y-axis is located at theboundary of depletion layer. According to calculation results, the dependence ofthreshold voltage on germanium component of relaxed Si1-xGexsubstrate, channellength，and voltage of drain are studied in detail, then the influence of Drain-InducedBarrier-Lowering effect on scaled strained silicon MOSFET is obtained. The resultsshow that the effect of stress on device performances attribute to the change of materialparameters, so the performace enhancement of channel material is very important forthat of device.2. Based on Fermi golden rules and collision term in Boltzmann equation, elastic/inelastic scattering rate is firstly solved using relaxation time approximation.Then the scattering rate models about ionized impurity scattering, acoustic phononscattering, intervalley scattering/non polar optical phonon scattering for electron/holeare established. Finally, the electron and hole scattering rates of biaxial tensile strainedSi are analyzed, which can lay a good foundation for the research of carrier mobility.3. Based on electron scattering rate models, electron mobility model about themean momentum relaxation time is established in which electron energy is considered.Take (001),(110), and (111) surface orientation as examples, the splitting energy atconduction band/valence band is obtained by calculating energy of valleys according todeformation potential theory, and on the basis of this, the electron valley occupanciesare gotten. Then, the dependences of electron mobilities in three orientations (oneorientation normal to the growth plane, the other two mutually perpendicular in theplane) with different germanium constituent on impurity concentration are studied.4. Hole mobility play an important role for PMOS and CMOS devices. The holemobility model presented in this dissertation considers the holes’ energy and holeenergy-band occupancies at valence band. Take (001),(110), and (111) surfaceorientation as examples, tensile strained Si and compressive strained Si hole mobilitiesare studied, which offers some useful foundation for strained Si devices from materialperformance.