Study On The Recombination Of Carriers In InGaN Quantum Wells

The emitting wavelength of nitride-based luminous devices which utilize InGaN material as active region covers the whole range of visible light,and the InGaN-based luminous devices are widely employed in vast application fields,e.g.,the light-emitting diodes(LED)are implemented in solid-state lighting and micro-LED display,and the blue or green laser diodes(LD),which act as the light sources,are substantial in laser projection,laser-based lighting and blu-ray system.InGaN quantum well(QW),which is commonly used in the active region of LED and LD,has a dominant impact on the radiative efficiency of the luminous devices.Basically,the radiative recombination of carriers in the QW generates photons,which leads to the macroscopic luminescence.Besides the radiative recombination,the carriers may recombine non-radiatively,and it is the competition between radiative process and nonradiative process that determines the internal quantum efficiency(IQE)and gain of the QW.To improve the IQE and gain,it is necessary to study the recombination process of carriers in the InGaN QW.This work will mainly focus on the following aspects,which are the measurement of steady-state lifetime of carriers,the decoupling of radiative lifetime and nonradiative lifetime,the radiative lifetime in InGaN QW,the nonradiative lifetime in InGaN QW and the application of research results on blue and green LD.Time-resolved photoluminescence(TRPL)is a powerful tool to study the recombi-nation process of carriers.This work proposes a novel technique to measure the lifetime of carriers under steady state.Instead of the usual pulsed laser,a modulated quasi-CW laser is used as the excitation source in the TRPL experiment.Under the quasi-CW laser excitation,the steady-state lifetime of excess carriers is directly obtained from the experiment.After the power-dependent or temperature-dependent TRPL experiment,the radiative lifetime and nonradiative lifetime could be separated.The strong polarization field in wurtzite c-plane InGaN QW leads to the inclination of the energy band,which results in the separation of wavefunction and the quantum-confined stark effect(QCSE),and both the transition energy of band-edge emission and the radiative coefficient could be altered significantly.This work utilizes the finite difference method(FDM)to perform the numerical calculation,and the Schrodinger-Poisson equation is solved self-consistently.The eigenenergy and the wavefunction in the QW are obtained,and it is found that,compared to the results of the peak PL energy and radiative coefficient obtained from the power-dependent experiment,the peak PL energy follows quite well with the theoretical value,while the experimental radiative coefficient doesn't drop as fast as the theoretical value calculated from the wavefunction overlap when the QW width increases.Shockley-Read-Hall(SRH)recombination reduces the IQE of the active region,and is hazardous for the devices.This work studies the QW-number dependent and QW-width dependent SRH recombination lifetime by performing a temperature-dependent experiment,and the results reveal that SRH lifetime increase with the total InGaN thick-ness.The phenomenon is attributed to the existence of indium atoms compensating the SRH recombination centers generated during the epitaxy and thereby prolonging the SRH lifetime,which is justified by the analysis on the activation energies.After the optimized QW structure and growth parameters are utilized in the active region of blue LD,the performance of LD is improved,and the lowest threshold cur-rent density is lower than 1 kA/cm2,which is comparable to the recent progress in this field.What's more,the nonradiative and radiative lifetime of green QW are analyzed experimentally and theoretically,by comparing green QW with blue QW,the main fac-tors that limit the efficiency of the green QW are obtained,which provides guidance on active-region design in high-performance green LD.