Abstract
Analysis of transverse magnetic (TM) mode light extraction efficiency enhancement for AlGaN quantum wells (QWs) based deep ultraviolet (UV) light-emitting diodes (LEDs) with III-nitride micro-hemisphere and micro-dome structures on the p-type layer are studied and compared to that of the conventional deep-UV LEDs with flat surface. The transverse electric (TE) and TM components of the spontaneous emission of AlGaN QWs with AlN barriers were calculated by using a self-consistent 6-band k∙p method, which shows the TM component overtakes the TE component and becomes the dominant contribution of the spontaneous emission when the Al-content of the AlGaN QWs is larger than 0.66. The TM mode light extraction efficiency of the deep-UV LEDs emitting at 250 nm with AlGaN micro-domes as compared to the conventional LEDs with flat surface is calculated based on three dimensional finite difference time domain (3D-FDTD) method. The effects of the III-nitride micro-dome diameter and height as well as the p-type layer thickness on the light extraction efficiency were comprehensively studied. The results indicate optimized light extraction efficiency enhancement (>7.3 times) of the dominant TM polarized spontaneous emission for deep-UV LEDs with III-nitride micro-domes.
Highlights
Ultraviolet (UV) light-emitting diodes (LEDs) based on wide band gap AlGaN quantum wells (QWs) with AlN barriers have a wide range of applications in water/air purification, white light illumination, spectrometry and medical phototherapy [1,2,3,4]
LEDs is significantly attributed by the low light extraction efficiency, due to 1) total internal reflection from the high refractive index of the nitride semiconductors (n~2.4) in contrast to that of the free space (n = 1), and more importantly, 2) domination of the transverse magnetic (TM) component of the spontaneous emission from the high Al-content AlGaN QWs active region, where TM polarization is polarized along the direction normal to the surface, leading to extremely low light extraction efficiency
For high Al-content AlxGa1-xN QWs (x>0.66), the dominant transition is between the conduction band and crystal-field splitoff hole (CH) band, which is TM polarized spontaneous emission component [24]
Summary
Ultraviolet (UV) light-emitting diodes (LEDs) based on wide band gap AlGaN quantum wells (QWs) with AlN barriers have a wide range of applications in water/air purification, white light illumination, spectrometry and medical phototherapy [1,2,3,4]. Recent approaches for enhancing the light extraction efficiency of III-nitride LEDs mainly focus on the visible InGaN QWs based LEDs by using surface roughening [11,12], photonic crystals [13], SiO2/polystyrene colloidal microspheres [14,15,16,17], and graded refractive index materials [18]. We performed numerical calculations and analyses of dominant TM mode light extraction efficiency enhancement for AlGaN/AlN QWs based deep-UV LEDs emitting at 250nm by forming the III-nitride micro-domes on the p-type layer. We propose to enhance the light extraction efficiency of TM polarized spontaneous emission of AlGaN/AlN QWs based deep-UV LEDs emitting at 250nm by using micro-dome structures on the p-type layer. Our recent studies on light extraction efficiency for TFFC InGaN QWs LEDs by using GaN micro-domes have indicated significant enhancement of light extraction efficiency [19]
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