Study On Intersubband Transition In Al_xGa_1-xN/GaN Double Quantum Wells

Al_xGa_1-xN/GaN quantum wells(QWs) are of the large conduction band offset, the strong spontaneous and piezoelectric polarization effects, the large LO-phonon energy and the large electron effective masses. The large conduction band offset leads to the short intersubband transition (ISBT) wavelength within the optical communication wavelength range. The large LO-phonon energy and the large electron effective masses result in the ultrafast carrier relaxation dynamics. Thus,the ISBTs in Al_xGa_1-xN/GaN QWs are of promising application to optical communication, and have attracted significant attention. In this thesis, the ISBTs in Al_xGa_1-xN/GaN double QWs(DQWs) have been investigated by solving the one-electron Schr?dinger and Poisson equations self-consistently. The exchange-correlation potential is considered in the calculation, and the polarization field discontinuity is included as a fitting parameter. The main results are introduced as follows:(1) The strong polarization effects make the conduction band profile, the wavefunction and the electron distribution in Al_xGa_1-xN/GaN DQWs asymmetric. There are a large potential drop between the two wells and a large Stark shift between the corresponding subband pairs in the DQWs, and thus there is no resonance between the corresponding subband pairs. Therefore, the wavefunction of each subband is mainly located in one well of the DQWs.(2) Influence of the polarization field discontinuity and the structural parameters on the wavelength and the absorption coefficient of the ISBT in Al_xGa_1-xN/GaN DQWs has been studied. The wavelength and the absorption coefficient of the ISBT between the first odd order and the second even order subbands(the 1_odd-2_even ISBT) decrease with increasing the polarization field discontinuity. With increasing the Al composition of the central barrier, the wavelength of the 1_odd-2_even ISBT is almost unchanged, the absorption coefficient of the 1_odd-2_even ISBT firstly increases, and then changes to decreases. Thus, the absorption coefficient is of a maximum when the Al composition of the central barrier is 0.45. With increasing the thickness of the central barrier, the wavelength of the 1_odd-2_even ISBT increases, the absorption coefficient of the 1_odd-2_even ISBT decreases.(3) Influence of the polarization field discontinuity and the structural parameters on the anticrossing gap of the subband pairs in Al_xGa_1-xN/GaN DQWs has been investigated. The polarization field discontinuity has no effect on the anticrossing gap of the subband pairs. The anticrossing gap decreases with increasing the Al composition and the thickness of the central barrier. The anticrossing gap becomes 0 when the central barrier is thicker than 2 nm, indicating that the two wells have a significant coupling when the central barrier is thinner than 2 nm.(4) Influence of the right well width of Al_xGa_1-xN/GaN DQWs on the absorption coefficient of the ISBT in the DQWs has been studied, in order to design the structure of the three- and four-energy-level Al_xGa_1-xN/GaN DQWs. With increasing the right well width, the 1_odd-2_odd ISBT absorption coefficient decreases,but the 1_even-2_odd one increases. Meanwhile, with increasing the right well width, the 1_odd-2_even one firstly increases, and then changes to decrease when the right well becomes wider than 2.28 nm. Thus, with increasing the right well width, there are two intersecting points between the absorption coefficients of three different ISBTs in the DQWs. The three-energy-level system can be realized when the 1_odd-2_odd ISBT absorption coefficient intersects the 1_odd-2_even one. The four-energy-level system can be realized when the 1_odd-2_even ISBT absorption coefficient intersects the 1_even-2_odd one. In order to make the growth of Al_xGa_1-xN/GaN DQWs easier, GaN QWs of the right well is suggested to be replaced by Al_xGa_1-xN QWs. The three-energy-level system can be realized when the excited state in the left well reasonates with the based state in the right well. In the Al_xGa_1-xN/GaN DQWs designed above, the wavelengthes of the ISBTs are possible to be located around 1.3μm and 1.55μm in the same DQWs, which gives the possible way to realize the ultrafast two-colour optoelectric devices operating whin the optical communication wavelength range.