Improved Yellow Light Emission in the Achievement of Dichromatic White Light Emitting Diodes
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Improved Yellow Light Emission in the Achievement of Dichromatic White Light Emitting Diodes Zhao Si1, Tongbo Wei1, Jun Ma1, Ning Zhang1, Zhe Liu1, Xuecheng Wei1, Xiaodong Wang1, Hongxi Lu1, Junxi Wang1 and Jinmin Li1 1 Research and Development Center for Semiconductor Lighting, Institute of semiconductors, Chinese Academy of Sciences, P.O. Box 912, Beijing 100083, People’s Republic of China ABSTRACT A study about the achievement of dichromatic white light-emitting diodes (LEDs) was performed. A series of dual wavelength LEDs with different last quantum-well (LQW) structure were fabricated. The bottom seven blue light QWs (close to n-GaN layer) of the four samples were the same. The LQW of sample A was 3 nm, and that of sample B, C and D were 6 nm, a special high In content ultra-thin layer was inserted in the middle of the LQW of sample C and on top of that of sample D. XRD results showed In concentration fluctuation and good interface quality of the four samples. PL measurements showed dual wavelength emitting, the blue light peak position of the four samples were almost the same, sample A with a narrower LQW showed an emission wavelength much shorter than that of sample B, C, D. EL measurement was done at an injection current of 100 mA. Sample A only showed LQW emission due to holes distribution. Because of wider LQW, the emission wavelength of sample B, C and D was longer and peak intensity was weaker. Sample D with insert layer on top of LQW showed strongest yellow light emission with a blue peak. As the injection current increased, sample A showed highest output light power due to narrower LQW. Of the other three samples with wider LQW, sample D showed highest output power. Effective yellow light emission has always been an obstacle to the achievement of dichromatic white LED. Sample D with insert layer close to p-GaN can confine the hole distribution more effectively hence the recombination of holes and electrons was enhanced, the yellow light emission was improved and dichromatic white LED was achieved. INTRODUCTION Recently, tremendous progress has been made in InGaN/GaN based blue, green and shorter-wavelength light-emitting diodes (LEDs) [1-6]. It is of great interest to design an monolithic white LED without the need of a phosphor converter for long-wavelength light [712]. Although the band gap of InGaN can cover the wide spectral regime from near ultraviolet to near infrared principally [13-19], this material system still faces severe difficulties in attempting to produce LEDs emitting at wavelength longer than 530 nm. To achieve green and red emission, the In content in InxGa1-xN quantum-wells (QWs) should be higher than 20%, however, the internal quantum efficiency decrease significantly as the emitting wavelength becomes longer. With the increase of In content in InGaN QWs, the lattice mismatch between GaN and InGaN material becomes more serious. This leads to the In-phase separation and quantum-confined Stark effect (QCSE) [20]. Furthermore, the QCSE decrease effective band gap, which is beneficial
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