Research On The Effects Of Polarization On The Current And Light Characteristics For UV-LEDs

Ultraviolet light emitting diodes (UV-LEDs) based on Ⅲ-nitride quantum wells (QWs) are in high demand for a variety of applications including medical, air and water purification, non-line-of-sight short range communications, analytical instrumentation, and biological agent detection.With the improvements of available crystal quality of Ⅲ-nitrides in recent years, QWs with high-Al-content AlGaN alloys can be achieved.However, high-Al-content deep ultraviolet (DUV) LEDs show very low external quantum efficiency (EQE).This low EQE can be partly attributed to the poor quality of AlGaN alloys,but it has also been supposed that the high polarization field in DUV-LEDs have strong impacts on their performances.Firstly, we studied the different I-Vbehaviors observed in the LEDs with different emitting wavelengths. We found that polarization field is responsible for this discrepancy of current transport mechanism based on the analyses of experiment data and theoretical simulation. At moderate bias, the direction of polarization-induced electric field is consistent with the build-in field of the p-n junction, and opposites to the bias field. According to the Poole-Frenkel effect, the Coulombic potential barrier can be lowered due to the electric field. Hence, the thermal activation of electrons and holes from deep centers can be increased by the polarization field.Tunneling effect can be enhanced by the high polarization in the DUV-LEDs. However, we can’t find this phenomenonin the UV-LED. Secondly, electroluminescence anisotropies are observed in high-Al-content AlGaN-based DUV-LED. High polarization field due to the characteristics of wurtzite material will change the position of splitting valence bands in the origin point of k space, which causes the differences of luminous intensity and peak position between the lights with electric field vibration of E∥c and E⊥c for the AlGaN-based LED and GaN-based LED.Lastly, we studied on the problem of the low luminous efficiency for DUV-LED with high-Al-content AlGaN QWs. Through simulation calculation, we find the internal quantum efficiency can be modulated significantly by the polarization in the active layer. The seperationof the wavefunctions for electron and hole can be enhanced by the polarization in the quantum well, and hence results in the decrease of radiative recombination probability. An optimization design is made by comparing the simulated EL luminous intensity for a group of devices with different Al-content in the barrier layer.