Theoretical Research On Anisotropic Mobility In Non-Polar AlGaN/GaN Heterojunction

GaN material has the advantages of wide band gap, high breakdown field, high electron mobility, high electron saturation velocity, anti-corrosion, anti-radiation, etc., so it is very good for the application of high frequency, high temperature and high power devices. High electron mobility transistor(HEMT) devices based on GaN heterojunction and the conductance of two dimensional electron gas(2DEG) have been extensively studied in recent years. Theoretically, the nonpolar GaN heterojunction is a better choice than the polar GaN heterojunction to fabricate the enhancement- mode HEMTs. However, the reported experimental results show that the 2DEG mobility in polar GaN HEMTs is isotropic, and it can reach almost 2000 cm2/(Vs). Meanwhile the 2DEG mobility in nonpolar GaN HEMTs is anisotropic, and the maximum value is less than 200 cm2/(Vs). The special 2DEG transport properties in nonpolar GaN heterojunction have a negative effect on the development of nonpolar GaN HEMTs. The preliminary study shows that the transport behaviour may be related to the anisotropy of the interface morphology of the nonpolar GaN heterojunct io n and the defect of basal stacking fault(BSF), but the mechanism is still not clear.Based on the general theory of anisotropic transport of 2DEG, the models of several anisotropic scattering mechanisms at the interface of nonpolar AlGaN/GaN heterojunct io n are established. The model of the influence of the stacking faults on 2DEG transport properties in nonpolar AlGaN/GaN heterojunction is also established, and the calculated results are consistent with the experimental data. The physics of these anisotropy scattering mechanisms and the influence of the charged BSFs are described as follows.In the nonpolar m-plane AlGaN/GaN heterojunction, the stripe-shaped morphology of the heterojunction interface can lead to the anisotropic interface roughness scattering and the linear charge dipole scattering, while there also exists misfit dislocation scattering. We first establish the scattering model of an infinite and uniform stripe/line–shaped scatter, and find that it only changes the component of the incoming two-dimensional wave vector which is perpendicular to the scatter, and it is the principle that causes the anisotropy of the scattering mechanism and the resulting mobility. Subsequently, the model is used to analyze these specific scattering mechanisms, and the mathematical expressions for the variation of the mobilities limited by them with the electric field are obtained. The calculation results demonstrate that the mobilities limited by these scattering mechanisms show differe nt anisotropic characteristics. The analysis shows that the difference of the anisotropy is caused by the different properties of the bare scattering potential generated by the different scatters in combination with the dielectric screening effect which is determined by 2DEG.In the nonpolar GaN heteroepitaxial films, BSFs feature a density of about 105cm-1. The experimental results show that BSFs have a dominant effect on the anisotropic transport of bulk carriers in nonpolar GaN films. Therefore, the transport of 2DEG should have been affected in a similar way, but there is no quantitative theoretical analysis. Konar proposed a theoretical model of the effect of charged BSFs on the transport of bulk carriers in nitride semiconductors(carriers tunneling through a BSF and diffusively transporting between BSFs). Based on the model, we take into account the quantum properties of 2DEG, and establish a model for the tunneling probability of two dimensional electrons through a BSF and how it changes with the direction of electric field, then quantitatively analyze the impact of BSFs. The results show that charged BSFs lower the 2DEG mobility to about one tenth or less, and the effect is dependent on the direction of the external field. The analysis shows that the anisotropy of the 2DEG mobility is dominated by the anisotropic electron tunneling process, but the 2DEG mobility nearly parallel to BSFs can be decreased obviously by the enhanced interface roughness scattering at relatively higher 2DEG density.