| InGaN based white light emitting diode (LED) is considered as the next generation lighting instrument. It has been realized and commercialized, mainly applied in display backlighting, automotive system, solid-state lighting, etc. The most common white LEDs use an InGaN blue LED chip combined with a phosphor wavelength converter. The use of phosphor brings some problems for the LEDs, including decrease of efficiency caused by energy conversion, and the degradation of phosphor itself. Therefore, phosphor-free white LEDs still have much more potential. Apart from phosphor brought problems, the electroluminescence (EL) efficiency of nitride LEDs should be increased, especially under high current injection density which is required by many applications of nitride LEDs.In this paper, we measured the temperature dependent electroluminescence (EL) and current-voltage characteristics of one kind of phosphor-free white nitride LED and a blue nitride LED with the same structure which is for comparison. The white LED was achieved by certain doping in the multi-quantum wells (MQWs). The localized recombination characteristics of the white LED were analyzed. We also measured the electroluminescence of a series of commercial LEDs, including AlGalnP red and yellow LEDs and InGaN green, blue, cold white and warm white LEDs, with different output power, in order to investigate the EL efficiency droop phenomenon. The main results are presented as follow:(1) The electroluminescence of the phosphor-free white LED has two emission peaks, and their temperature dependency is different, which indicate that the emission mechanisms are different as well. The 430nm emission peak is attributed to band to band radiative recombination, and the 495nm emission peak is attributed to the localized recombination related to deep levels in the active region. The intensity of the deep-level related emission peak decreases with the increase of temperature. According to the temperature dependency of emission intensity, we obtained the average activation energy of the deep levels, which is about 199meV.(2) According to the I-V characteristics of the phosphor-free white LED, when the current through the device is zero, the bias on it is not zero, and value of the bias is changing with temperature. We believe that deep-level states in the active region of the white LED became extra source of carriers and contribute to the unique I-V characteristics of the white LED.(3) The electroluminescence of AlGaInP based LEDs shows red shift as the injection current increases, while the InGaP based LEDs show blue shift at the early stage and then red shift. The blue shift of InGaN LEDs is attributed to polarization in nitride semiconductors, and red shift is probably caused by heating of the device.(4) All the measured commercial LEDs suffer from efficiency attenuation. But the efficiency attenuation behaviors are different between the AlGalnP based LEDs and InGaN based LEDs. The efficiency droop of InGaN based LEDs has two stages, and at the early stage it decreases more intensively, while the efficiency of AlGaInP based red LED decreases slower under relative low injection current. We simulate the carrier concentration dependency of the EL efficiency considering the Auger recombination process, and compared it with the measured InGaN efficiency variation behavior and believe that the efficiency droop of InGaN LEDs are related to Auger recombination. (5) The red shift of the emission peak of InGaN phosphor white LED under high injection current is larger than that of the InGaN blue LED, which indicates that phosphor used in the device aggravates heating, and depress the device performance. The efficiency of warm white LED decreases more intensively than the cold white LEDs, and this also shows that using phosphor will depress the device performance. Study of phosphor-free white LED is still of great importance. |
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