Study On The P-type Doping And Transport Properties Of AlGaN/GaN Superlattice

As a representative of the third generation of semiconductor material, GaN-based materials are famous for their large band gap (0.75-6.2eV), strong polarization effects (4～5times of GaAs), high critical field strength (2MV/cm), high electron saturation velocity (2×107cm/s) and thermal conductivity (1.3W/cmK) and other outstanding features. GaN materials and devices based on it have been researched systematically and widely in recent decades. GaN based microwave power devices have been present in the field of military radar and space applications. In the civilian field, many types of GaN LED and LD devices have been widely used in automotive electronics and high-end lighting equipment. However, the application and development of the GaN-based devices, particularly AlGaN/GaN multi-layer structure, still have many obstacles that worthy to be studied. The energy band shape of the devices can be modified by adjusting the Al content of the AlGaN/GaN superlattice. Thus the AlGaN/GaN superlattice structure has great application potential in the high-frequency oscillator, light-emitting devices and photo-detectors and many other fields. In addition, there are still lots of problems to be studied, especially in the aspect of p-type doping in GaN, this problem seriously limits the development of the HBT and LED devices which need high-quality p-type materials. This paper is performed under this background, focused on the application in the p typed doping of AlGaN/GaN superlattice and the steady-state nonlinear response of the AlGaN/GaN superlattice under THz electric field.At the first part, the limits of the available methods to enhance the p-type doping were analyzed, based on this analysis a new method was devised to enhance the p-type doping efficiency by utilizing the superlattice structure. We systematically studied the mechanism of superlattice doping, calculated the relationships of superlattice period and Al content with the doping efficiency, optimized the parameters of doping experiments. Based on these researches the following results are obtained:1. The valence miniband effect and the strain effect of the AlGaN/GaN superlattice are the key to enhace the p type doping. The formation of the valence miniband can lower the activation energy of the acceptors in AlGaN, which greatly enhanced the acceptor ionization ratio of the acceptors. Furthermore the tensile stress makes the doping of Mg to the AlGaN layer much easier.2. The formation of the valence miniband of AlGaN/GaN superlattice is mainly due to the polarization effect and the strain effect. The difference of the averaged hole concentrations with and without polar effect and strain effect can be about one magnitude. 3. The analysis on the roles of polarization effect and strain effect are verified by the following experiments. The superlattice period, Al content and chamber pressure are optimized by experiments. High chamber pressure, Al content and a length of period around10nm are benefit to the doping efficiency.4. A Mg doped AlGaN/GaN superlattice sample of p=4.36x1018/cm3and R=0.31Ω·cm respectively is obtained. This result is better than the available result of p-type bulk GaN. This result does prove the efficiency of superlattice structure in p type doping and find its application in the LED and HBT devices.At the second part, we carried out the study of the steady-state nonlinear response of the AlGaN/GaN superlattice under THz electric field. The work and results can be concluded as:First, the conduction band offset model of GaN including the strain effect and the polarization effect is established. Based on the ground electron wave function in a single AlGaN/GaN well, the first miniband energy dispersion is obtained by the tight binding approach method. Then the relationships of the energy dispersion with Al content, period length, strain effect and polarization effect were explored in detail.Second, the conventional simple cosine dispersion model is improved. Based on this improved dispersion relation, a transport model of the miniband electron of AlGaN/GaN superlattice under THz electric field is constructed within the semi-classical theory.Third, the steady-state nonlinear response of the AlGaN/GaN superlattice under THz electric field is studied with the Fourier transform analysis. Calculation and analysis are focused on the relationships of dynamic resistance, phase and relative efficiency of the third harmonic response with the strength and frequency of the fundamental and the third harmonic excitation field.