Effect Of Quantum Well Structures On Optical And Electrical Properties Of GaN-Based Green LED On Silicon Substrate

In recent years,the semiconductor lighting technology has made dramatic development.With the development of material epitaxial growth and device fabrication technology,GaN-based light-emitting diodes(LEDs)have made rapid progress.At present GaN-based LEDs have been widely used in solid-state lighting,traffic lights,signal indicator lights,large-area outdoor display and other fields.However,high quantum efficiency of GaN-based LEDs can only be achieved at low current density due to the ‘efficiency droop' at high current density,especially for green and yellow LEDs with high Indium component.If the quantum efficiency of green and yellow LEDs can be improved,it will not only alleviate the ‘green gap',but also promote the wide application of multi-color white LEDs.As core part of luminescence,InGaN/GaN quantum wells(QWs)usually have a very important impact on the optoelectric performance of LEDs.In this paper,the structural design of QWs of GaN-based green LEDs on Si substrate is studied.By designing the AlGaN insertion layer(IL)in the quantum barriers(QBs),optimizing the cap layer thickness and designing different combined structure of QWs,the optoelectric performance of GaN-based green LEDs is further improved.The following research results are obtained:1.The epitaxial structure was designed by inserting AlGaN insertion layer(IL)into four bottom quantum barriers,and the effect of the AlGaN IL on carrier transport and quantum efficiency were studied.The results show that LED with the AlGaN IL shows high emission efficiency at low current,low forward leakage current and operating voltage.The improved performance is closely related to the V-pit structure.Based on LED structure with V-pit,AlGaN IL plays a key role in screening dislocations.Futhermore,the high bandgap of AlGaN also affects the carrier transport.It was found that the LED with AlGaN IL shows two emission peaks at low temperature,which can be assigned to bottom QWs emission and top QWs emission,respectively.When the current increases to the ‘critical current',the bottom QWs begin to participate in emission.In addition,AlGaN IL also has effect on blocking electrons,which reduces the carrier overflow.2.The effect of cap layer thickness on the quality of QWs and the optoelectric properties of LED was studied.The results show that the cap layer can effectively protect the InGaN QWs.With the increase of the cap layer thickness,the crystal quality of QWs becomes better,the In component in QWs becomes more uniform,and QW/QB interface becomes more abrupt.At low current,EQE of LEDs decreases with the increase of cap layer thickness.However EQE of LEDs increases with the increase of cap layer thickness at high current.Because the sample with thinner cap layer has higher density of localized center,it shows higher emission efficiency and longer wavelength at low curret density.At high current density,the sample with thicker cap layer has better crystal quality of QWs and steep QW/QB interface,so it can effectively restrict carriers and show higher emission efficiency.In addition,the emission peak originating from InGaN/GaN superlattice layer at low temperature was detected,confirming the hole injection promoted by V-pit sidewalls.3.The effect of different combination of preparing QWs(close to n-GaN)and emission QWs(close to p-GaN)on photoelectric properties of green LEDs is studied.The difference between preparation QWs and emission QWs is the thickness of GaN barrier.The thickness of GaN barrier of preparation QWs and emission QWs is 130? and 100?,respectively.The results show that the emission efficiency can be effectively improved by using the structure with preparing QWs and emission QWs.For preparing QWs,the crystal quality of InGaN well is poor due to the low growth temperature,and the GaN barrier play a key role in repairing well and provide a better platform for the subsequent growth.The thicker GaN barrier in preparing QWs has better effect of repairing well and promotes crystal quality of emission QWs.For emission QWs,the emission region is mainly concentrated in the emission QWs due to the mismatch between holes and electrons,naturally the thinner GaN barrier promotes carrier transport and improves the carrier injection efficiency.