Electronic Structures And Optical Properties Of Semiconductor Materials GaN,SiGe And ZrS₂-based Low-Dimensional Systems

In recent years,as the global demand for energy grows inexorably,new energy-efficient and environm ent-friendly optoelectronic materials have received much attention.Among these studied new materials GaN and related heterostructures,two-dimensional（2D）graphene-like materials with suitable gap and high performance optoelectronic properties have been used intensively in light-emitting diodes,laser diodes and nanoelectronic fields.In this thesis,based on the variational methods,we studied exciton states and optical properties of wurtzite InGaN staggered quantum wells（QWs）considering polaried electric field and quantum size effects.Moreover,the effects of laser field on the exciton states and optical properties of direct band gap Ge/SiGe QWs are also calculated by using the variational methods.In adddition,based on density functional theory,we also study the doping effects on electronic structures of 2D ZrS₂ nanosheet.Chapter one gives a overview of the present studies of semiconductor materials and the properties and applications of GaN,SiGe and ZrS₂ monolayer.The effective mass approximation,variational methods and density functional theory are introduced in chapter two.And from the chapters three to six,we study exciton states and optical properties of wurtzite InGaN QWs and direct-gap GeSi QWs,the electronic structures of ZrS₂ monolayer by using the different atoms substituting S atom.The obtained main results are given as follows.1)The exciton states and optical properties of wurtzite InGaN/GaN staggered QWs are studied using variational method.The numerical results show that the quantum size,indium content and polarized electric field have obvious influence on exciton state and optical properties in the InGaN QWs.For the symmetric InGaN staggered QWs,the ground-state exciton binding energy and the oscillator strength decrease while the inter-band emission wavelength increases when the well width and indium content increase,and the Stark effects are also significant.The polarized electric field has great influences on the linear optical susceptibility.For the asymmetric InGaN staggered QWs,the ground-state exciton binding energy and the oscillator strength have maximum values with different indium contents,and the ground-state exciton binding energy and the Stark effect of intensity has a minimum value,while the band-to-band emission wavelength changes monotonically.The theoretical results show that the optical properties of wurtzite InGaN staggered QWs can be controlled by varying the staggered barrier height,indium content and quantum size,which provides a theoretical basis for the further studies of InGaN blue-green light-emitting diodes and other nano-optoelectronic devices.2)Based on the effective mass approximation,we study the effects of laser field and quantum size on the exciton states and optical properties of the direct bandgap Ge/SiGe QWs by using the variational method.The numerical results show that the effects of laser field and quantum size on the inter-band optical transition energy and ground-state exciton binding energy of the direct bandgap Ge/SiGe QWs are obvious.Under different laser field intensities,when the well width increases,the inter-band energy transition energy is reduced.While there is a maximum value in the change of ground-state exciton binding energy with the well width.Under a given well width,the inter-band optical transition energy increases monotonically with the increasing of the laser field intensity;however,the ground-state exciton binding energy decreases monotonously.It has also been found that in the Ge/SiGe QWs,the band-gap optical transitions energy and the ground-state exciton binding energy are very sensitive to the laser field due to quantum size effect when the well width is very small.3)Based on density functional theory,we studied the electronic structures,formation energy and transition level of group V and VII atoms-doped 2D ZrS₂ monolayer.The results show that the lattice constant and bond length dZr-S of ZrS₂ nanosheets are 3.68 ? and 2.57 ?,respectively.The ZrS₂ monolayer belongs to indirect band gap semiconductor material with the band gap of 1.127 eV.The spin-up and spin-down density of states are symmetric in the N-doped ZrS₂ monolayer,in other words,the N-doped ZrS₂ monolayer shows a non-magnetic ground state.For P and As-doped ZrS₂ monolayer,the spin-up and spin-down states are asymmetric,in other words,the P and As-doped ZrS₂ monolayer systems exhibit a magnetic ground state.Moreover,group VII doping does not induce the magnetic ground-states.For group V atoms,the atomic radius has a very important effect on the transition energy level and the formation energy of ZrS₂ nanosheets doping system.Using group V atoms（N,P and As）to substitute the S atoms of ZrS₂ monolayer,the state of the acceptor impurity can be induced,while group VII atoms（F,Cl and Br）substituting S can induce the n-type impurity states.In particular,N-doping has the lowest formation energy and shallow transition energy in the group V-doped ZrS₂ monolayer,and the doping of N in the ZrS₂ monolayer can provide effective p-type carriers and be easily realized under Zr-rich experimental conditions.These results may provide clues to further experiments on the impurity states of the two-dimensional ZrS₂ nanosheet.4)Based on first-principles calculations,we studied the electronic structures and optical properties of 2D ZrS₂ monolayer by using Se substituting S atom.The calculated results indicate that when S is substituted by Se atom,the band gap value of the 2D ZrS₂ is reduced.Moreover,when Se doping concentration increases,the threshold values of optical absorption shift towards lower energy region.Thus,we can conclude from these results that Se substituting S can modulate effectively the gap value and optical properties of 2D ZrS₂ nanosheets,which is helpful to the related experiment in the future.In this thesis,the influences of optical properties of wurtzite staggered InGaN staggered QWs and direct-gap GeSe QWs have been investigated.In addition,based on first-principles methods,we have also studied the electronic structure of the group V and VII atoms-doped ZrS₂ monolayer,and modulation of band gap and optical absorption of 2D ZrS₂ monolayer by Se substituting S atom.We expect that the above theoretical studies can be helpful to the related physical experiments.