Ⅲ-Nitride Semiconductor Deep-ultraviolet Emitters:A Theoretical Study

Ⅲ-Nitride semiconductor-based deep-ultraviolet(DUV)light-emitting diode(LED)and laser diode(LD)are presented.Commercially advanced semiconductor LEDs and LDs simulation software are used to execute the in-depth theoretical study of Ⅲ-nitride DUV LEDs and LDs.Critical factors and challenges limiting device performance are identified based on a detailed literature study.A comprehensive design parameter gap is investigated with the help of advanced scripting capabilities.Several design strategies and materials are proposed to eliminate the critical problems partially or completely.This work will focus on both DUV LEDs and LDs including Nanowire(NW)structure and planner thin film structure.Five different structures have been proposed in which three are AlGaN-based LDs and two are AlGaN-based LEDs.AlGaN-based Nanowire(NW)LDs grown on sapphire substrates have a strong polarization-induced electric field.Such electric fields degrade the optoelectronic performance of DUV NW LD.A graded AlN composition AlxGa1-xN waveguide(WG)layer is used for the enhancement of DUV NW LD performance.Grading of WG induces bulk polarization charges which compensate for the effect of the polarization-induced electric field.According to the calculated optoelectronic characteristics of NW LD,it is found that grading of n-type WG(n-WG)increases the optical confinement factor(OCF)by 82%.Fortunately,the proposed graded nWG structure suppresses the leakage of the optical field from the active region and enhances carrier injection efficiency.Furthermore,if both n-WG and p-WG graded layers are used,the improvement is not obvious because of the current leakage from graded p-WG.Thus,graded n-WG based NW LD gives the highest 33.5%OCF with the lowest 7 mA and 4.59 V threshold current and voltage respectively.In AlGaN-based DUV NW LD Al mole fraction and thickness of quantum wells(QWs)and quantum barriers(QBs)are critical parameters for device performance.In Multiple-Quantum-Wells(MQW)NW LD,QBs play an important role in carriers’flow(electrons,holes).The impact of composition and the thickness of AlGaN-based QB is being studied.The study suggests that by properly increasing the Al composition of AlxGa1-xN QB,the height of conduction band barrier decreases,which enhances electron leakage from MQW,while increasing the height of valence band barrier,which,in turn,suppresses hole injection efficiency toward MQW.Such variations in band height greatly affect optoelectronic characteristics and the performance of AlGaN NW.Furthermore,the study also suggests that QBs thickness has much influence on DUV NW performance.The decrease in thickness of QBs has little effect on confinement of optical field,but a significant effect on optical emitted power.It is found that by decreasing the thickness of QBs,optical emitted power along with stimulated recombination rate is reduced.Contrastingly,as the thickness of QBs increases,hole injection efficiency gets enhanced while electron leakage from MQW gets reduced.The above observations suggest that the higher the thickness of QBs,the better is the carriers’ flow toward MQW and hence,improved the performance.The electron blocking layer(EBL)plays a vital role in blocking the electron overflow from an active region in the AlGaN-based DUV LED.Besides the blocking of electron overflow,EBL reduces hole injection toward the active region.We proposed a DUV NW LED structure without EBL by replacing it with a compositionally continuous graded hole source layer(HSL).Our proposed graded HSL without EBL provides a better electron blocking effect and enhanced hole injection efficiency.As a result,optical power is improved by 48%and series resistance is reduced by 50%with 4.8 V threshold voltage.Moreover,graded HSL without EBL offer reduced electric field within the active region,which leads to a significant increment in radiative recombination rate and enhancement of spontaneous emission by 34%at 254 nm wavelength,as a result,52%maximum internal quantum efficiency(IQE)with 24%efficiency drop is reported.High threshold current density is a big hurdle in the realization of AlGaN-based DUV LD.This threshold current density is dependent on OCF,laser gain,emission power,and absorption loss.By using compositional grading of AlGaN cladding layers along with graded WGs and EBL OCF,carrier injection efficiency,gain,and emission power of DUV LD can be improved.The calculated results for AlGaN DUV LDs show that by compositional grading of WGs,EBL,and CLs the device characteristic can be improved.It was demonstrated that by grading p-WG,EBL,p-CL along with n-WG and n-CL electron blocking capability,hole injection efficiency increases,and absorption loss decreases owing to the highest OCF of 17.4%,emission power of 94.6 mW,and 1369 m-1 gain at 267 nm wavelength.Such high value of OCF,gain,and power assure to reduce threshold current density.Commonly,the Al-rich AlGaN layer is used as EBL to block the overflow of electrons from the active region in AlGaN DUV LED.However,the Al-rich AlGaN layer leads to the disadvantages of severe electron overflow and hole blocking effect.Herein,we proposed a new AlInN-based EBL to improve the optoelectronic characteristics of AlGaN-based DUV LED.The calculated results show that conventional(fixed Al composition)AlInN EBL has superior hole injection,reduced electron overflow,and lower electric field as compared to conventional AlGaN EBL.By using AlInN EBL,IQE drop is reduced by 20%,and light output power improved by 165.30%.It was noticed that conventional AlInN EBL has a 28%IQE drop which can further be minimized by employing AlInN/AlInN superlattice EBL structure.It is found that superlattice EBL improved carrier distribution and reduced electric field resulting in a higher electron-hole wave-function overlap of 55%within MQW which elevates radiative recombination rate.AlInN superlattice EBL LED has drop-free 93%IQE,twice light output power,and spontaneous emission rate compared to conventional AlInN EBL LED.