The Anisotropic Transport Property Of Two-dimensional Electron Gas In Non-polar AlGaN/GaN Heterojunctipn

The non-polar AlGaN/GaN hetero-junction attracts the researcher’s attentionbeause that it is easier to fabricate enhancement-mode HEMT device as well as itshigher emission efficiency. It was shown that there is a strong anisotropy in thenon-polar GaN material resulting in the anisotropy of the carrier transport properties.The anisotropic carreir transport behaviour in bulk non-polar GaN material wasexplained in terms of scattering from charged stacking fault （SF），but little study wasever made for the two-dimensional electron gas （2DEG） in non-polar AlGaN/GaNhetero-junctions. The transport properties of2DEG are the fundamental to theresearches on HEMT. In this thesis, the anisotropic transport properties of the2DEG ina-plane （or [11-20] plane） AlGaN/GaN heterojunction are focused on. The mainresearch work and conclusions are as follows:1．A momentum relaxation rate model of2DEG from charged stacking faultscattering is proposed. In this model, the transport of carriers is dealt with as a currentdrift between two SFs and then tunnel through the barrier and quantum well caused bya SF. Based on the work of A. Konar et al[18],the momentum relaxation rate model ofthe interface roughness scattering and the interface dipole scattering are given by usingthe Fang-Howard variational wave function. A low-field mobility model of2DEG inthe non-polar AlGaN/GaN heterostructure is established. In addition to the abovescattering mechanisms, polarized optical phonon scattering, piezoelectric scattering,deformation potential scattering, alloy disorder scattering, dislocation scattering, theremote ionized impurity scattering by modulation doping and background ionizedimpurity scattering are also considered. In all the scattering mechanisms, chargedstacking fault scattering, interface roughness scattering, interface dipole scattering anddislocation scattering are anisotropic.2．According to the above low-field mobility model, the mobility of2DEG in thenon-polar GaN heterostructures is analyzed as a function of2DEG density, structureparameters and temperature.When the2DEG density increases, the mobility of2DEGalong c-axis and m-axis for a-plane AlGaN/GaN heterojunction increases first and thendecreases, and the mobility of these two directions shows significant anisotropy.When2DEG concentration is5×1012cm-2, the mobility along the [1-100] direction is4timeslarger than that of [0001] direction.However, when the2DEG concentration is 5×10¹³cm^-2, the mobility along the direction of [0001] is40%larger than that of [1-100]direction. This is primarily caused by the different major scattering mechanisms. Whilethe2DEG mobility of m-plane GaN heterostructures is smaller in [0001] direction andthe anisotropy is weakened with increasing2DEG concentration. The above model isembeded into the semiconductor device simulator ATLAS, and its effect on thecharacteristics of the non-polar GaN HEMT devices is simulated.