Electrical And Electroluminescence Characteristics Of GaN-based Light-Emitting Diodes

Ⅲ-nitride semiconductors, consisting of InN, GaN, AlN and their alloys, have attracted extensive attentions in recent years for their promising future for many important applications, especially in opto-electronic fields. This primarily attributes to their direct bandgaps, which range from0.7eV to6.2eV, suitable for fabrication high-efficiency green, blue, violet and ultra violet light emitting devices. Till now, tremendous success have been achieved in these fields, for example, the InGaN-based blue light emitting diodes (LEDs) have already been commercialized. However, as to LEDs with other emission wavelength, such as green LEDs and deep ultraviolet LEDs, there is still very limited report on their characteristics and physical mechanisms. So intensive researches are still needed both in theory and applications.The phenomenon that external quantum efficiency (EQE) significantly degrades under higher injection current or the so-called efficiency droop problem is a critical restriction for the usage of LEDs in high power application. GaN-based green LEDs suffer from serious efficiency droop and their efficiency could drop more rapidly than that of blue LEDs as the injection current increases. Here, the efficiency droop behaviors of GaN-based green light emitting diodes (LEDs) are studied as a function of temperature from300to480K, in contrast with control GaN based blue LED and violet LED. The efficiency of the green LEDs is found to degrade as temperature increases, which is mainly caused by activation of new non-radiative recombination centers within the LED active layer. Meanwhile, the EQE of the green LEDs starts to decrease at low injection current level with a temperature-insensitive peak-efficiency-current. Around the onset point of efficiency droop, the electroluminescence spectra of the green LEDs also exhibit a monotonic blue-shift of peak energy and a reduction of full-width-at-half-maximum as injection current increases. Carrier delocalization is believed to play an important role for causing efficiency droop in GaN-based green LEDs.A detailed analysis of the electrical and spectral features of deep-UV LEDs can offer an important tool for the understanding of carrier recombination dynamics in the active region as well as the evaluation of the quality of the devices. This paper presents a detailed study of the electrical and electroluminescence (EL) behavior of deep-UV LEDs with peak emission at290nm. In particular, we have studied the temperature-dependent reverse leakage current mechanism, temperature-dependent EL and efficiency droop behavior and the effect of self-heating on the diodes’optical characteristics.At first, the reverse leakage characteristics of AlGaN-based ultraviolet light-emitting diodes fabricated on sapphire substrate are studied by temperature-variable current-voltage (I-V) measurement from300to450K. At low reverse bias range (0-0.5V), the reverse leakage current exhibits tunneling characteristics. Meanwhile, under a more negative reverse bias range (0.5V-TV) the log(I)-log(V) plots is in good agreement with the transport mechanism of space-charge limited current. Under high negative reverse bias (>7V), the I-V relationship can be explained by Poole-Frenkel effect. A phenomenological leakage current model focusing on electron transmission primarily through continuous defect band formed by linear defects like dislocations is suggested to explain the reverse current-voltage characteristics.Secondly, through the analysis of the temperature-dependent forward I-V characteristics, as well as the spectrum of the UV LED, we find that a carrier leakage mechanism caused by insufficient hole injection should be the mechanism, as well as the two effects caused by the increasing temperature, the ionization of Mg acceptors and the activation of nonradiating recombination center, should be the most important origin of the temperature-dependent EL and efficiency droop behavior. Self-heating effect also have much influnce on the EL and efficiency droop behavior of the diode, which will improve the optical efficiency at low current level, while make it drop more rapidly at high current level.