Study Of Electroluminescence/Photoluminescence Properties On InGaN/GaN Multiple Quantum Wells Based LED

.In the past three decades,GaN-based light-emitting diodes(LEDs)have made great progress.Compared with traditional lighting methods,GaN-based LEDs have been widely used in liquid-crystal display(LCD)backlight,large-screen display and general lighting because of their advantages of low energy consumption,long service life,fast response speed and small size,which greatly improves people's daily life.In addition,because of its fast modulation speed,good stability under atmospheric environment and impact conditions,and linear behavior under continuous current reduction and pulse width dimming,GaN-based LEDs are also considered as excellent electronic devices.However,it is very difficult to prepare high quality and high performance GaN single crystals and their alloy compounds and to control their photoelectric properties.With the development of epitaxial growth technology,especially Metal-Organic Chemical Vapour Deposition(MOCVD),it is possible to grow high quality GaN and GaN-based alloys.In 1989,high quality P-type GaN was prepared and the conductivity of n-type GaN was successfully regulated.Inspired by the above technological breakthroughs,the world's first commercial high-brightness blue GaN-based LEDs and long-life GaN-based ultraviolet laser diodes(LDs)were introduced in 1993 and 1997,respectively,through inexhaustible efforts by researchers around the world.To this end,the 2014 Nobel Prize in Physics was awarded to three physicists,Isamu Akasaki,Hiroshi Amano and Shuji Nakamura,for their outstanding contributions to GaN-based LEDs and LDs.At the same time,GaN-based electronic devices,such as high electron mobility transistors(HEMT),have also developed rapidly.InAlGaN,as a GaN-based alloy compound,has attracted great attention and research enthusiasm from researchers because of its direct band gap and adjustable band gap.For example.by changing the In and AI contents,the band gap can be increased from 0.7 eV of indium nitride(InN)to 6.2 eV of aluminum nitride(AIN),that is,the emission wavelength can cover the wide spectrum range from near infrared to deep ultraviolet including visible light.Among them,InGaN,which is one of GaN-based alloy compounds,has been widely used as luminescent materials in the active region of LEDs,such as InGaN/GaN multiple quantum wells(MQWs)based LEDs because its luminescent wavelength can cover the whole visible light range by adjusting the content of In.Recently,it has been reported that the extermal quantum efficiency(EQE)of InGaN/GaN MQWs-based blue LEDs with low In content can be as high as 80%.However,as the In content increases(corresponding to an increase in the emission wavelength),the quantum efficiency of InGaN/GaN MQWs-based LEDs drops sharply,that is,the so-called "green gap".This is because,due to the difference in size of In atoms and Ga atoms and the large lattice mismatch between GaN and InN(11%),slight composition fluctuation or phase separation often occurs in the InGaN well layer.Although component fluctuations and phase separation can enhance the carrier localization effect and improve their quantum efficiency under certain conditions,the non-radiative recombination centers induced by the component fluctuation and phase separation can also lead to a decrease in the quantum efficiency,that is,the resulting quantum efficiency should be the result of the competition between the two mechanisms.At the same time,there is also a large lattice mismatch between the GaN epitaxial layer and the substrate(e.g.,sapphire and silicon)and thus the formation of structural defects(e.g.threading dislocations),which leads to the formation of structural defects,and this also reduces the quantum efficiency.On the other hand,the polarization field induced by the large lattice mismatch between GaN barrier layer and InGaN well layer will cause the energy band to tilt and cause the space separation of electrons and holes in InGaN well layer,which leads to the red shift of the emission peak energy and reduces the recombination efficiency of carriers,this is,the so-called quantum confinement stark effect(QCSE).Therefore,in-depth study of the effects of growth process and structure parameters on the optical properties of InGaN/GaN MQWs-based LED and clarification of its carrier dynamics mechanism are of great significance for enriching the condensed state theory,preparing high-performance InGaN/GaN MQWs-based LED and developing new materials/structuresIn this dissertation,the influence of temperature,excitation power and injection current on the optical properties of blue or green InGaN/GaN MQW structures with different growth processes and structure parameters were systematically investigated by means of electroluminescence(EL),photoluminescence(PL),XRD and TEM methods,and the dynamic mechanism of the carriers inside these InGaN/GaN MQW structures clarified.The main conclusions of the dissertation are listed below:(1)EL properties of InGaN/GaN MQW-based LEDs with different indium contents and different well widths.Two InGaN/GaN MQW-based blue LEDs emitting photons at approximately the same wavelength,with different indium contents and well widths,were grown on a(0001)-oriented sapphire substrate by using MOCVD,and temperature dependences of their EL spectra at different fixed injection currents(0.01-200 mA)were investigated.The results showed that,compared to sample B with a lower indium content(12.8%)and larger well width(3.4 nm),sample A with a higher indium content(15%)and smaller well width(2.6 nm),had a stronger carrier localization effect and higher EQE at the lower fixed currents,however,upon increasing the injection current,both the localization effect and EQE decreased at a faster rate.This behavior could be explained by energy band models constructed by author:the former was mainly attributed to the high-In-content-induced larger potential fluctuation in the sample A,which resulted in the deeper localizaed states,and to the stronger quantum confinement effect(QCE)caused by the smaller well width;the latter was mainly attributed to the the larger lattice mismatch between the InGaN well layer and GaN barrier due to the higher In content in the InGaN well layer,which resulted in the strong polarization field and significantly growing in the electron leakage and/or electron overflow,and to the smaller high-energy localized state density due to the smaller well width.(2) "Double-W-shaped,temperature dependence of emission linewidth in an InGaN/GaN MQWs structure with intense phase separation.The blue InGaN/GaN MQW-based LED chip was grown on a(0001)sapphire substrate using MOCVD.The temperature dependences of the PL spectra from a blue InGaN/GaN MQW structure had been investigated at lower excitation power.Two peaks(PM and PD)related to InGaN were observed in the complete PL spectrum,and these were derived from the transitions associated with the InGaN matrix,and In-rich quasi-quantum dots(QDs),respectively,on account of the intense phase separation.Upon increasing the temperature(6-300 K),both the PM and P o linewidths showed a "double-W-shaped"(narrowing-broadening-narrowing-broadening-narrowing-broadening)temperature dependence,while that of IM was "M-shaped"(increase-decrease-increase-decrease)and accompanied by an inverted "V-shaped"(increase-decrease)relationship for both ID and their total emission IT(IT=IM+ID).These behaviors were attributed to the relaxation and thermalization of carriers inside their respective phase structures and the transfer of carriers between the two-phase structures.These processes were due to the strong phase separation and significant component fluctuation in the InGaN well layers.(3)Wave-shaped temperature dependence characteristics of the electroluminescence peak energy in a green InGaN-based LED grown on silicon substrate.The green InGaN/GaN MQWs-based LED was grown on a(111)-oriented Si substrate using MOCVD.Temperature dependences at different injection currents of the EL spectra from the green InGaN/GaN MQW-based LED,were investigated over a wide range of injection currents(0.001-350 mA)and temperatures(6-350 K).The results showed that with increasing injection current,the temperature behavior of the EL peak energy gradually evolved from an "approximately V-shaped"(slight decrease-significant decrease-decrease-increase)temperature dependence into a"wave-shaped,(increase-decrease-increase-decrease-increase,i.e.three-step blueshift)relationship which became an "approximately inverted V-shaped"(marked increase-slight increase-decrease)temperature dependence.This behavior indicated that there were three zones with different average In contents in the InGaN well layer of the green LED,and this leaded to a stronger carrier localization effect.However,the significant In composition fluctuation caused by the high In content also causes more structural defects.Therefore,in the lower injection current range and when the temperature was gradually lowered(350-6 K),the temperature behavior of the EQE value was dominated by deactivation of non-radiative centers in both the higher and lower temperature ranges.Moreover,the high-In-content-induced structural defects in the InGaN well layers had also been confirmed by the injection current dependence of the integrated EL intensity.(4)PL properties of blue and green InGaN/GaN MQWs grown on a single sapphire substrate.An InGaN/GaN MQW sample containing both blue MQWs(BMQWs)and green MQWs(GMQWs)active regions(the latter deposited on the former)was grown by MOCVD on the(0001)-oriented sapphire substrate,and PL characteristics of the sample were investigated.The results showed that the emission(PG)from BMQWs demonstrated more significant "S-shaped,dependences on temperature of peak energy than emission(PB)from the BMQW,and the excitation power-dependent carrier-scattering effect was observed only in the PG emission;when the excitation power increased to 100 from 0.005 mW at both 6 and 300 K,the GMQWs showed a more significant blueshift(narrowing)of peak positions(linewidth)and a lower internal quantum efficiency(IQE)than the BMQWs.All of the above behaviors could be attributed to the fact that compared with the BMQW.the higher indium content in the GMQW resulting from the lower growth temperature and late growth of the GMQW compared with the BMQW.induced a more significant composition fluctuation,a stronger quantum confined Stark effect,and more non-radiative centers.