Effect Of Growth Temperature Of Well Layers On Optical Properties Of InGaN/GaN MQW-based LED

With the development of society,the consumption of electric energy is still increasing.The total electricity consumption of lighting accounts for a proportion of 12%-15%in the total electricity consumption.As the fourth-generation advanced light source,light-emitting diodes(LEDs)is attracting more and more attention due to its small size,high brightness,high luminescence efficiency,long lifetime,environmental protection,and is gradually replacing incandescent lamp and fluorescent lamp,becoming the leading role in the field of white lighting.As a representative of the third generation of compound semiconductor materials,GaN material has excellent physical and chemical properties such as high melting point,high thermal conductivity,high hardness,high chemical stability and high breakdown electric field intensity,making it the preferred material for semiconductor optoelectronic devices.In addition,GaN and its related alloys(AIGaN,InGaN,etc.)are direct bandgap materials.The bandgap can range from 0.7 eV of InN to 6.2 eV of AIN by regulating In and A1 components,and hence the luminescence wavelength can cover from Near Infrared to Deep Ultraviolet.InGaN MQW-based structure LED can cover the UV to the visible spectrum,which has been widely used in solid-state white lighting devices.However,the efficiency of InGaN/GaN MQWs based LED significantly decreased with the increasing of indium components,which is known as the green gap.The reasons are attributed to two points:(1)InN and GaN have different atomic size and severe lattice mismatch,which cause the well layer crystalline quality deteriorates and a large number of non-radiative composite centers are generated with the increasing of In component.(2)there is a strong piezoelectric polarization electric field induced by lattice mismatch between the InGaN well layer and the GaN barrier layer in the active region,which leads to the the well layer energy band tilting,and hence the spatial separation of electrons and holes,and reduces the luminescence efficiency,which is known as quantum-confined Stark effect(QCSE).QCSE became more severe with the increasing of indium component,and hence further decrease the luminescence efficiency.Spite of the high density of defects and the strong QCSE,the the luminescence efficiency of InGaN/GaN MQWs is still surprisingly high.The reason is attributed to that the deep localized states in InGaN/GaN MQWs structure dominante radiative recombination of carrier.The deep localized states can localize the carriers and hinder them being captured by the non-radiative recombination centers,and hence reduce the influence of the non-radiative recombination at the dislocation.The formation mechanism of localization centers was considered to be related to In-rich clusters.These mechanisms were effect on the luminescence efficiency of LED,but the polarization electric field of LED and the luminescence mechanism of localization states are still not clear,which requires further research.The growth temperature of well layer as a key growth parameter can affect the homogeneity of In content,crystal quality,and hence affect the local effect of QWs and LED luminescence performance.Therefore,it is important to investigate effect of growth temperature of well layers on the photoelectric properties of InGaN/GaN MQW-based LED.In this paper,two InGaN/GaN MQW-based LEDs with different growth temperature of well layers were grown on c-plane sapphire substrates,and their photoluminescence properties and electroluminescence spectra were studiedThe main research work is summarized as follows(1)Photoluminescence properties of InGaN/GaN multiple quantum wells with different growth temperature of well layersThe photoluminescence properties of two InGaN/GaN MQW-based LEDs with different growth temperature of well layers were studied in the temperature range of 10-300 K and the excitation power range of 0.001-50 mW.The results show that,sample B has higher Indium component in the InGaN well layer,lower IQE,stronger carrier localization effect,stronger QCSE,and more non-radiative recombination centers which means Poorer crystal quality compared with sample A which has higher growth temperatures in quantum well layers.(2)Electroluminescence properties of InGaN/GaN multiple quantum wells with different growth temperature of well layersThe electroluminescence properties of two InGaN/GaN MQW-based LEDs with different growth temperature of well layers were studied in the temperature range of 10-300 K and the excitation power range of 0.0001-20 mA.We found that two samples showed abnormal "M-shape"(increase-decrease-increase-decrease)temperature dependence of EL peak energy,and sample B showed more significant"M-shape" behavior.Through the hypothetical energy band structure model construction,the temperature dependence of different "M-shape" EL peak energy shown in sample A and sample B was explained that there were two in-rich regions(zones)or two phase structures with different composition Indium components in the well layer of sample A and sample B.moreover,the two phase structures in the well layer of sample B with lower growth temperature have higher Indium content,more significant Indium component fluctuation,stronger carrier localized effection,stronger QCSE and more non-radiative recombination centers.In addition,the average Indium content difference between two phase structures in sample B is larger,which caused more obvious carriers relaxation and thermal expansion process in their respective phase structures or between two phase structures compared with sample A.