First Principles Study on Electronic Structure and Optical Properties of Lu Doped GaN at Different Doping Concentration
Objective As a third-generation novel semiconductor material emerging alongside SiC,GaN has become a hot topic in the fields of high-temperature and high-power microwave devices,laser devices,and optoelectronic devices due to its excellent characteristics.It has been widely used in microwave communication,lasers,detectors,and ultraviolet light-emitting diodes.Doping,as a new paradigm for material modification,can directly and effectively control and improve the thermoelectric,photoelectric,and magnetic properties of materials,giving them new characteristics and extending their applications.Materials based on rare earth elements have excellent optical,electrical,magnetic,and catalytic properties,and are the foundation for building various new functional materials.It is expected that rare earth element doping can improve GaN's visible light absorption.We study the electronic structures and optical properties of GaN doped with different concentrations(atomic number fraction)of Lu using the first-principles plane wave ultrasoft pseudopotential method.The calculation results provide theoretical support for the development of device applications of GaN semiconductor photoelectric materials doped with rare earth element Lu.Methods We adopt the CASTEP software package using the first-principles calculation method based on the density functional theory.We utilize the projected augmented wave method as the pseudo potential and apply the generalized gradient approximation function proposed by Perdew-Burke-Ernzerhof to express the exchange correlation interaction.We adopt the plane wave expansion with a cut-off energy of 450 eV and leverage the conjugate gradient method to optimize the lattice constants and atom positions of the diverse models.The K-point grid in the Monkhorst-Pack form is set as 4× 4×2 for bulk models.As the GGA method underestimates the band gap value of materials,we use the GGA+U plane wave pseudopotential method to correct the band gap.A supercell model with 2×2×2 is built,including 16 Ga atoms and 16 N atoms,with a total of 32 atoms.To make the calculation results more accurate,we conduct a truncation energy convergence test on the supercell systems.Considering the symmetry of GaN crystal,we study the stability of different spatial ordered configurations with the same doping amount at concentrations of 12.5％and 18.75％.Results and Discussions From the formation energy(Table 1),it can be seen that except for the Ga0.9375Lu0.0625N system,which has a higher formation energy,the values of the formation energy and binding energy of other doping systems are all negative,indicating that doping enhances the structural stability of intrinsic GaN.The formation energy of the Ga0.9375Lu0.0625N system has a positive value,making it more challenging to achieve doping compared to other doping concentrations.Under different concentrations,the Lu-doped GaN systems show direct band gap P-type semiconductor characteristics(Fig.4),and the band gaps are all narrowed.The reduction of band gap is beneficial for electron transition,thereby improving the optical properties of the GaN system.The absorption edges of Lu-doped GaN under four concentrations show a red shift phenomenon(Fig.7),indicating an improvement in light response capability.The intrinsic GaN has a small absorption coefficient in the visible light range and has low utilization of visible light.When the doping concentration of Lu is 25％,the Ga0.75Lu0.25N system forms a wider visible light absorption region.Conclusions We calculate the electronic structures and optical properties of intrinsic GaN and Lu-doped Ga1-xLuxN(x=0.0625,0.125,0.1875,0.25)at different doping concentrations using the first-principles plane wave ultrasoft pseudopotential method under density functional theory.In addition,we study the stability of the same doping and different spatially ordered occupancy architectures when the Lu doping concentration is 12.5％and 18.75％.The calculation results show that the values of lattice parameters of the Lu-doped GaN are increased,and the band gap values of the doped GaN are reduced compared to the intrinsic band gap(3.40 eV)due to the shallow energy level impurities induced by the doping of Lu.Compared with the intrinsic GaN,the static dielectric constants of the Lu-doped GaN increase and even reach 5.42 when the doping concentration of Lu is 25％.The imaginary parts of the dielectric function and the absorption spectrum of the Lu-doped GaN shift in the low-energy direction.The red-shift phenomenon occurs which extends the absorption spectral range and enhances the photocatalytic performance of GaN.
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