Optical Properties Of InGaN/GaN Multiple Quantum Wells With A Gradually Changing In Content

With the development of society,the proportion of electricity used for lighting in the whole society is growing.Therefore,it is is urgent to develop energy-efficient and efficient new lighting.In recent years,light-emitting diodes(LEDs)have become the fourth-generation lighting method to replace traditional lighting because of their low energy consumption,long life and fast response.Among the LEDs prepared from various materials,gallium nitride(GaN)-based LEDs have attracted a lot of attention due to their advantages of direct band gap and the tuning ability of band gap,and have been widely used in general illumination,back lighting and display.GaN-based LEDs can tune band gap energy from 0.7 to 6.2 eV by changing the content of In,Ga,and A1 in the active region of indium gallium nitride(InGaN)or aluminum gallium nitride(AlGaN).The wavelength can range from near ultraviolet to near infrared.At present,the internal quantum efficiency(IQE)of InGaN/GaN multiple quantum well(MQWs)based LEDs has exceeded 90%in blue range.However,as the wavelength of light increases,the IQE of InGaN/GaN MQWs-based LEDs drops significantly,especially in the yellow-green range,is called"green gap"This problem mainly attributed to the deterioration of material quality and the increase of the built-in field perpendicular to the plane of the well in the MQWs when increasing indium content.The former is mainly caused by the large discrepancy in atomic size between indium and gallium,and a large lattice mismatch of 11%between InN and GaN,and thus leading to generation of phase separation or composition fluctuation and the resulting increase in nonradiative centers;the latter mainly results from the presence of the piezoelectric polarization induced by a large lattice and thermal mismatch between the InGaN well layer and the GaN barrier layer in the MQWs,and thus resulting in the spatial separation of electrons and holes wave function in the MQWs and the reduction of radiative recombination efficiency,which is known as quantum-confined Stark effect(QCSE).Therefore,further studying the light-emitting mechanism of InGaN/GaN MQWs,and improve electron and hole wave functions overlap,specific important to solve the problem of "green gap".In this paper,the optical properties of InGaN/GaN MQWs structures with gradually changing In composition were studied by photoluminescence(PL)spectra.The main research work is summarized as follows:(1)Photoluminescence properties of InGaN/GaN multiple quantum wells with a gradually changing indium content along the growth direction.Two different InGaN/GaN multiple quantum wells(MQWs)with a gradually increased and a gradually decreased In content in the InGaN well layer along the growth direction,were fabricated,and their photoluminescence(PL)properties are studied in the temperature range of 6-300 K and the excitation power range of 0.001-70 mW.The results show that compared with the MQWs with a gradually increased In content,the MQWs with a gradually decreased In content has a higher average In content in the InGaN well layer,due to the less significant volatilization of In incorporated therein.In addition,compared with the MQWs with increasing In content,the MQWs with a gradually decreased In content exhibits a stronger carrier localization effect at low excitation power,and exhibits a weaker carrier localization effect at high excitation power.(2)Luminous efficiency of three InGaN/GaN multi-quantum well structures with different growth modes of InGaN well layers.Three different InGaN/GaN multiple quantum wells(MQWs)with a gradually increased,a gradually decreased In content and a consistent In content in the InGaN well layer along the growth direction,were fabricated,the temperature dependences of PL spectra for three samples were studied,and the IQEs of three samples were compared.The results show that compared with the MQWs with a gradually changed In content,the MQWs with a unchanged In content has a higher average In content in the InGaN well layer,a stronger carrier localization effect and a smaller IQE It is indicated that the well layer structure with a gradually changed In content,may improve the wave function overlap of the electron holes and i the IQE.