Fluorescence Characteristics And Carrier Dynamics Of Nitrogen Related Compounds Epitaxial Structure

The material system of Ⅲ-Ⅴ group netrogen related compound includes gallium nitride（GaN）,indium nitride（InN）and aluminum nitride（AlN）,as well as ternary and quaternary alloy compounds,such as InGaN,InAlN,GaNAs,InGaNAs,AlGaInN,etc.This material system has excellent photoelectric properties and wide adjustment range of the band gap from near infrared to ultraviolet,which makes them widely used in light emitting device and solar cells.In this paper,we mainly use photoluminescence（PL）,time-resolved PL（TRPL）,pump-probe detection and other spectroscopic techniques to characterize InGaN thin films,InGaN Quantum Wells,GaNAs/InGaAs superlattices and other Ⅲ-Ⅴ nitride solar cell materials comprehensively.The electronic structure,luminescent properties and carrier dynamics of these materials are studied.The main results are listedas follows:1.The carrier transfer process of InGaN epitaxial films with high In component under stimulated emission is studied.A high-energy stimulated emission peak（SE）and two low-energy spontaneous emission peaks（SPE）were observed at room temperature.The temperature dependent PL spectrum shows that the peak energy of high-energy SE peak changes weakly,and its intensity decreases while the intensity of two low-energy SPE peaks increases abnormally with increasing temperature,which indicates that carrier transfer occurs between SE and SPE.Further analysis exhibits that energy transfer also exists between the two SPE.Moveover,TRPL shows that the lifetime of SE decreases faster than that of the SPE with increasing temperature.Combining the TRPL and transient differential reflectivity（（35）R/R）,we found that the hot carrier presents a complex multi-step relaxation process and a slow thermalization rate.These results are valuable for the research of hot carrier devices.2.The influence of built-in electric field on the carrier dynamics in InGaN/GaN p-i（MQW）-n structure is studied.The amplified spontaneous emission（ASE）is observed in PL spectrum,and the lifetime of ASE is longer than that of low energy localized states.TRPL measurements show that the decay of ASE becomes faster,and evolves from double exponential to multi-exponential decay process with the increasing of excitation power.The analysis shows that the Quantum Confinement Stark Effect（QCSE）caused by the built-in PN field plays a dominant role in the mechanism of carrier migration and recombination,while the influence of the electric field on the carriers in the defect state is obviously smaller because of the effect of the built-in electric field.The decay of QW emission shows a multi exponential decay process due to screening and dynamical de-screening effect under high excitation power.3.The band structure and the carrier transfer process between N-related deep level localized states are studied in short period superlattice（SPSL）GaNAs/InGaAs.One QW peak P_M（1.2 eV）and three N-related peaks P_A,P_A’,and P_B（0.77,0.83,0.92eV）are observed.It is found that carrier transfer exsits between QW and N-related defect states and also among different defect states.Based on the three-level rate equation,we have successfully simulated the carrier transfer process between the defect states.The QWs peak P_M presents a large blue shift（42 meV）with the increasing of excitation density,which indicates that the energy band in the GaNAs/InGaAs superlattice has a type-II structure.Based on the experimental results,we have established the band structure diagram of the GaNAs/InGaAs SPSL.4.The exciton recombination and its thermal excitation mechanism in CdTe/ZnTe quantum dots（QDs）and QWs are studied by using steady-state and time-resolved PL spectra.PL spectra show that the band gap of QWs follows the temperature shrinking effect,while the peak of QDs shows an anomalous blue shift platform of about 12 meV between 50-100 K.TRPL measurements shows that the lifetime of exciton fluorescence in CdTe QDs keeps stable at 4-25 K and slightly increases at 25-50 K.However,with increasing temperature from 50 K to 100 K,the lifetime of exciton fluorescence decreases sharply,and the fitting thermal activation energy is close to the energy shift of fluorescence peak.We think that the thermal activation of exciton corresponds to the thermal escape of hole from CdTe valence band to ZnTe barrier layer.