The Basic Study On GaN/AlGaN Heterojunction In N-polar GaN Based HEMT

The polarity of N-polar GaN/AlGaN heterojunction materials is opposite to that of traditional Ga-polar AlGaN/GaN heterojunction materials, which could introduce several advantages for N-polar GaN based materials and devices like the ability to achieve low contact resistance and high two-dimensional electron gas(2DEG) confinement, and it has certain potential applications in high frequency, high power microwave transistor, enhanced devices and sensors. But preparation of N-polar GaN based materials is difficult and their material properties are livelier, which limit the application of GaN materials to some extent. The research of N-polar High Electron Mobility Transistor(HEMT) is just at the starting stage, and there are some problems about the Ohmic contacts on N-polar GaN in recent reports such as Ohmic contact resistance and its mechanisms. Therefore, these basic problems, the relation of 2DEG sheet density in N-polar GaN/AlGaN heterojunction materials and the structure of heterojunction, the epitaxial growth of N-polar GaN/AlGaN heterojunction materials, the preparation technology of N-polar GaN based HEMT, the characteristics and mechanisms of Ohmic contacts, Direct Current characteristics simulation of N-polar HEMT, etc, are needed to be further studied. Based on these problems, this paper carried out the following researches:1. By the self-consistent solution of the Schr?dinger and Poisson equation, combined with the spontaneous and piezoelectric polarization effect in N-polar GaN based materials, the effects of GaN channel layer, AlGaN back barrier layer with and without Si doping and AlN interlayer on the two-dimensional electron gas in N-polar GaN/AlGa N heterojunction were systematically studied for the first time. The results could provide a theoretical reference for the epitaxial growth of N-polar GaN based materials.2. The increase of Al content in AlGaN natural back barrier could improve the 2DEG sheet density, but it will lower the heterojunction interface quality and affect the 2DEG mobility. In order to improve the 2DEG sheet density of N-polar GaN HEMT and to reduce the impact on the 2DEG mobility at the same time, a new structure of N-polar GaN based HEMT with Al content gradient and AlN interlayer was put forward based on the conventional N-polar GaN based HEMT(This option has been applied for Chinese inventor’s patent right and has been accepted). Through the self-consistent solution of one-dimensional Schrodinger and Poisson equation, the 2DEG behavioral characteristics of this new N-polar GaN-based HEMT structure were simulated. On the basis the quasi-two-dimensional model of N-polar GaN-based HEMT was derived and the DC characteristic was simulated.3. The mechanism of Ohmic contacts on N-polar GaN grown by Metel-organic Chemical Vapor Deposition(MOCVD) and the relationship between the contacts resistance and the Ohmic contact metals were deeply researched, and the effects of Oxygen in N-polar GaN on ohmic contacts were systemly studied for the first time. Results showed that the Oxygen, which was higher in N-polar GaN than that of Ga-polar GaN and acted as background donors, reacted with Al contacts metals to form polycrystalline AlOx during high temperature annealing. Polycrystalline AlOx, which could increase Ohmic contact resistance, combined with the presence of AlN which has been reported, maked the Ohmic contacts on N-polar GaN harder to form compared with Ga-polar GaN. Thus it could provide a new idea for the mechanism analysis of N-polar GaN ohmic contacts.4.N-polar GaN/AlGaN heterojunction materials were grown on SiC substrate by MOCVD, and N-polar GaN based HEMT devices with 1μm gate were prepared using the preparation technology that is compatible to the preparation of Ga-polar GaN based HEMT for the first time in china. At Vgs=0V, Vds=10V, the device shows a drain current about 127.8mA/mm, the peak transconductance(gm) of 88.9mS/mm is abtained at Vgs=-0.57 V, and a threshold voltage(VT) of -1.46 V was extracted at Vds=10V. This results lay a solid foundation for the development of domestic N-polar GaN based HEMT...