Control Of The Optical Characteristics Of In GaN/GaN Multiple Quantum Wells By Interface Modification

In GaN/GaN multiple quantum well?MQW?structures,acting as active region,are widely employed in optoelectronic devices such as blue and green light emitting diodes?LEDs?,and laser diodes?LDs?.Nevertheless,the growth of high quality In GaN/GaN MQWs still faces great challenges,owing to the lack of native substrates and the large differences in physical and chemical properties between InN and GaN,especially for the green LEDs with high indium composition.For LEDs or LDs,the injected carriers recombine in the MQW region to achieve electro-optical energy conversion,and the energy conversion efficiency is directly related to the quality of the MQW region.Therefore,the MQW region is the core of the device.The high quality InGaN/GaN MQW structures are the basis and prerequisite for obtaining LED devices with high luminous efficiency.The main features of the high quality InGaN/GaN MQW structures include low defect density and abrupt quantum well/barrier interface.In order to acquire high quality In GaN/GaN MQW structures,the influence of low-temperature GaN cap layer thickness and adding small flow H₂ during the growth of low-temperature GaN cap layer on the optical and microstructure properities of MQWs was investigated.On the basis of these results,the two-step growth method was proposed.To improve interface quality,H₂treatment at quantum well/barrier interfaces was studied,and the effect of H₂treatment temperature on the optical and microstructure properties of MQWs was deeply discussed.Finally,the influence of periodical interruption during the growth of the InGaN quantum well layer,and the indium treatment during the periodical interruption,and the indium pretreatment before the InGaN well layer growth on the properties of MQWs were studied.The details are as follows:1.The influence of low-temperature GaN cap layer thickness on the optical properties and surface morphology of blue light emitting MQWs was studied.It was found that GaN cap layer with 1-nm-thick can effectively protect indium atom from evaporation during the temperature ramp-up and high temperature barrier growth process.A thinner cap layer cannot inhibit indium loss effectively,while a thicker one can decrease the photoluminescence?PL?intensity by degrading the quality of MQWs.Further,it was found that by introducing small flow H₂ during the low-temperature cap layer growth,the internal quantum efficiency of InGaN/GaN MQW structure was increased by 3.8times.However,the PL peak energy blue shifted by 73 me V,and the line width also broadened 23 meV.The large improvement in internal quantum efficiency can be attributed to the fact that H₂ significantly reduces the number of impurities and defects formed by indium segregation at quantum well/barrier interfaces.The blue-shift in peak energy and broadening of line width may be related to the over-etching of InGaN by H₂.On the basis of these results,a two-step method was proposed to grow the low-temperature GaN cap layer.The first step of the cap layer was grown in N₂ atmosphere,and the second step was grown by introducing a small H₂ flow.The results show that the two-step method can greatly decrease the blue-shift in peak energy and the broadening of line width,and effectively improve the luminous efficiency of InGaN/GaN MQW structure.2.The effects of H₂ treatment at quantum well/barrier interfaces on the optical and structural characteristics of InGaN/GaN MQWs were investigated.The results showed that the small flow H₂ treatment do not change the quantum well thickness and average indium content,yet significantly improves PL intensity of MQW structure.H₂ treatment temperature also has a great effect on the characteristics of MQW structure.At higher treatment temperature,more active H atoms were generated from the cracking of H₂ and NH₃,resulting in that the impurities,indium-rich clusters and other defects at well/barrier interfaces can be removed more effectively,thereby contributing to the improvement in internal quantum efficiency and surface/interface quality of MQW structure.At lower treatment temperature,the influence of H₂ treatment on the interface properities of MQW structure is very weak.3.The influence of periodical interruption during the growth of InGaN quantum well layer on the optical and structural properties of green light emitting MQW structures was investigated.The results show that as the interruption time increases,the number of defects in MQW structure decreases gradually,and the PL intensity increases gradually.However,the reduction of indium composition in MQWs lead to the blue-shift in PL peak energy.A large red-shift in PL peak energy?117 meV?was obtained by indium treatment on the growing surface during the periodical interruption of InGaN growth,yet PL intensity was decreased by 21%.Indium treatment prior to the InGaN growth makes 91 meV red-shifts in PL peak position and a little enhancement in PL intensity.Therefore,the indium treatment process is an effective way to make the wavelength red-shift and obtain long-wavelength emission LEDs.