Novel Semiconductor Materials:Theoretical Design,Physical Properties And Energy Band Modulation

Since the first germanium semiconductor transistor had successfully invented in the Baer lab in 1950 s,the semiconductor material has experienced three generations.In solar cell industry,the current solar cell materials has some shortcomings,such as the indirect band gap structure,the high cost of raw materials,the materials inherent defects,the low carrier mobility,the short carrier diffusion length,the complex manufacturing process and so on.So,looking for direct band gap semiconductor materials or those materials that can turn into direct band gap semiconductor materials by band modulation to meet the requirement of solar cells or other optoelectronic industry is the key problem which needs to be solved in the materials science field.Based on the above,this thesis mainly explores novel materials using the first principles method,the main research system is the group IV element semiconductor materials,nitride semiconductor materials,carbide semiconductor materials,carbonitride semiconductor materials and the energy band modulation on the current CaO semiconductor material or new semiconductor materials designed in this thesis.Through the theoretical model,toghter with the simulation results,we analyze the formation enthalpy,elastic modulus,phonon spectra,band structure,density of states,elastic anisotropy and the Mulliken bond populations.Based on this,we study the stability,the mechanical properties,and the electrical properties.The thesis' major research contents are as follows: 1.We propose nine novel silicon phases,which has five monoclinic phases(space groups: Cm,C2/m,P21/m,P2/m),two orthorhombic phases(space groups: Amm2,P2221),a cubic phase(space group Pm-3m).Then,the stability,electrical and mechanical properties of these phases are investigated.We find that the enthalpy of formation of P2221 silicon with orthorhombic phase is lower compared with diamond phase silicon,which is the most stable in all new phases.The electronic properties of the new phases are analyzed by the optimize structures using the first principles calculations and electronic characteristics,we find that the monoclinic phase Cm silicon and P21/m silicon have a direct band gap,Amm2 silicon is a quasi-direct band gap semiconductor material.The compressibility of C2/m-20 silicon and P2/m silicon in monoclinic phase,and Amm2 silicon in orthorhombic phase along a-axis is better than that of diamond silicon,the compressibility of Cm silicon,P21/m silicon,C2/m-20 silicon,P2/m silicon,Amm2 silicon along the b-axis is better than that of diamond phase,while along the c-axis,only P21/m silicon and C2/m-20 silicon are better than that of diamond phase.Their bulk modulus is very close,in which P2221 silicon and C2/m-20 silicon are closest to diamond silicon,and cubic phase Pm-3m silicon is the furthest.Then,we study the orthogonal phase Cmcm germanium and hexagonal phase P63/mmc germanium.Through the formation enthalpy,elastic constants and phonon spectra,we prove the two allotropes of germanium are thermodynamically,mechanically and dynamically stable.Through the band structure simulations,we find that the band gap of P63/mmc germanium is very small,even close to 0,while Cmcm germanium is a direct band gap semiconductor with band gap of 0.93 eV.We also study the optical properties and thermal conductivity of Cmcm Ge and P63/mmc Ge,and we find that they are all better than diamond silicon.2.We explore novel binary compound semiconductors,which mainly include nitrides semiconductors and carbides semiconductors.Nitrides semiconductors mainly include two new phases of C3N4,a new phase of GaN.m-C3N4 and t-C3N4 has excellent chemical,mechanical and electronic properties,and they are cheap and easily to be synthesized.The elastic constants and phonon spectra show that the two new phases of C3N4 can stably exist on at least 100 GPa.The electron band structures show that the two C3N4 materials are both quasi-direct band gap semiconductor materials with a band gap of 4.2-4.5 eV.Then we study the physical properties of these two C3N4 materials under high pressure.We find that the band gaps increase with the increasing pressure,which makes it possible obtain direct band gap semiconductors and wider band gaps.Adopting a similar approach to study structural properties,mechanical properties,electronic structure and elastic anisotropy of Si3N4 are also systematic investigated in this thesis.The band gaps of Si3N4 also increase with the increasing pressure.In addition,we also design a new phase of GaN,the structure of GaN is consist of four-membered nitrogen-gallium rings and eight-membered nitrogen-gallium rings.The elastic constants,phonon spectra and formation enthalpy are to prove the stability of new phase of GaN.The GaN is also a direct band gap semiconductor with band gap of 1.85 eV.For ?-GaN,we mainly study the relations between mechanical properties and temperature.We also study three phases of carbides semiconductors,namely,P42/mnm BC,R3 B2 C,and I4 mm B3C.Only R3-B2 C is narrow and direct band gap semiconductor material with band gap of 0.17 eV.3.We study the BCN,a new ternary compound semiconductor material.,..Study shows that P3m1 BCN,Imm2 BCN and I-4m2 BCN these three novel phases BCN have excellent mechanical properties,especially the trigonal phase P3m1 BCN has a higher bulk modulus,higher shear modulus and higher Young's modulus than those of c-BN.The elastic modulus of orthorhombic Imm2 BCN and tetragonal I-4m2 BCN is very close to c-BN.Moreover,Imm2 BCN is a wide band gap direct band gap semiconductor with band gap of 2.45 eV,I-4m2 BCN and P3m1 BCN are indirect narrow band gap semiconductor materials and wide band gap semiconductor materials,respectively.In addition,we have systematically studied the structural,mechanical and anisotropic properties of Imm2 BCN,I-4m2 BCN and P3m1 BCN.4.Finally,we study the energy band modulation of semiconductor alloys.As we all know,CaO is an indirect band gap semiconductor.We add Mg atoms to the CaO,Ca1-x MgxO alloy has been successfully changed into a direct band gap semiconductor.Subsequently,we have also doped a novel GaN with Al atom,and study the change of Ga1-xAlxN alloys with the increasing of Al atom.Finally,we also study the C-Si and Si-Ge alloys,we successfully predict for silicon and germanium,respectively,with the same structure as those of carbon and silicon.We successfully find several kinds of direct band gap C-Si and Si-Ge alloys.The monoclinic phase C12Si8 is a narrow-band-gap semiconductor material with direct band gap(0.17 eV),and the Cmcm and P63/mmc phases of Si-Ge alloys are all direct band gap semiconductors.In addition,we also find that the optical properties and thermal conductivities of Cmcm and P63/mmc phases of Si-Ge alloys are both better than that of diamond silicon.And all of them meet the requirement of solar cells.