Impurity Effects On The Electronic Structures Of GaN-based Low-dimensional Systems And α-Fe₂O₃

As the global demand for energy grows inexorably, the studies of new energy materials have attracted much attention. In this thesis, firstly, hydrogenic impurity states are investigated in wurtzite InGaN staggered quantum wells by using variational methods and the effective mass approximation. Then, based on density functional theory, the electronic structures of Mg-doped GaN nanosheets and anion-doped hematite α-Fe2O3 are investigated using first-principles methods. In the charpter 1, we describe the studies of present energy and new energy material, the properties and applications of GaN and α-Fe2O3. The effective mass approximation, variational methods and the first-principles methods based on density functiona theory are introduced in the charpter 2. Then, the influences of impurity on the GaN-based low-dimensional structures and hematite α-Fe2O3 are investigated from charpter 3 to charpter 6. The obtained main results are given as follows.Firstly, impurity states are studied in the GaN-based low-dimensional structures. For the studies of hydrogenic impurity states in wurtzite InGaN staggered quantum wells, numerical results show that impurity states are dependent highly on the built-in electric field, impurity positions and structural parameters in the Ga-polar and N-polar wurtzite GaN/InxGa1-xN/InyGa1-yN/GaN staggered quantum wells. The binding energies of hydrogenic impurities have a maxmimum with the variation of impurity position in the Ga-polar and N-polar quantum wells, but the corresponding impurity positions are different. In the Ga-polar wurtzite InGaN staggered quantum wells, when impurities are located at zi=-Lw、-Lw/2、0 and Lw/2, the donor binding energies decrease with increasing the well wide; when impurity is located at zi=Lw, the donor binding energy is insensible to the variation of well width. Moreover, the effects of In content y on the donor energy are not obvious when impurity is located at any position in the Ga-polar wurtzite InGaN/GaN staggered quantum wells with In content y>0.125.In the N-polar wurtzite GaN/InxGa1-xN/InyGa1-yN/GaN staggered quantum wells, the influences of In content y are insensitive (obvious) to the donor (acceptor) binding energy in the quantum wells for any impurity case. In particular, numerical results also show that the built-in electric field induces the donor (or acceptor) binding energy of impurity located at Zi= Lw (or-Lw) becomes insensitive to the variation of the well width when the well width Lw>2nm in the N-polar InGaN quantum wells.For the studies of electronic structures in the Mg-doped two-dimensional GaN nanosheets, numerical results show that the structural parameters, the formation energies and transition energy levels depend highly on Mg-doping concentration in the two-dimensional GaN nanosheets. With increasing Mg doping concentration, the formation energies increase while the transition energy levels reduce monotonously in the Mg-doped GaN nanosheets. In addition, the calculations of the formation energy indicate that Mg substituting Ga atom in the two-dimensional GaN nanosheets is relatively easier under N-rich growth experimental conditions. These results are interesting and indicate that p-type conductivity can be tuned effectively by Mg doping concentration in the Mg-doped GaN nanosheets.Secondly, the electronic structures are investigated in the anion-doped hematite α-Fe2O3. In order to understand the characteristics of n-type and p-type impurities in the α-Fe2O3, the electronic structures and formation energies of group V and VII atom-doped α-Fe2O3 are investigated theoretically. Numerical results show that group Ⅴ and Ⅶ impurities modify obviously the valence band edges of α-Fe2O3, while its conduction band edges are not modified. For group V atoms (N, P and As) substituting O atom in the hematite α-Fe2O3, the unoccupied empty states occur inside the forbidden band gap, which indicate p-type impurity states are formed. For group VII atoms (F, Cl and Br) substituting O atom in the α-Fe2O3, the occupied states occur inside the band gap of α-Fe2O3, which indicate group VII dopants are the n-type donor impurities. In addition, numerical results also show that for group V and VII impurities in α-Fe2O3, the formation energies increase with increasing atom number in the periodic table.In order to understand the possibilities that α-Fe2O3 can be used as new solar cell materials, the influences of S substituting O atom on the band structures and optical properties of α-Fe2O3 are investigated. Numerical results show that with increasing S concentrations, the band gap values of the α-Fe2O3-xSx alloys are decreased and visible optical absorption coefficents are enhanced. In particular, numerical results also show that for proper S concentrations, the α-Fe2O3-xSx alloys have the direct band structures. For this kind of transformation of the band structures, it means a significant advantage for the effective optical absorption.For the studies of N and S codoped α-Fe2O3, numerical results show that compared to N-doped α-Fe2O3 case, the unoccupied empty states are very close to the valence band maximum of N and S codoped α-Fe2O3. This is because S substituting O atom makes the valence band edge shift towards the higher level. Therefore, the transition energy level is reduced and p-type conductivity is improved in the N and S codoped α-Fe2O3 case.Moreover, the calculations of the formation energy show that these anion-doped hematite α-Fe2O3 systems are relatively easier under the Fe-rich experimental conditions.