We investigated electrical characteristics of the Mg-doped AlGaN contact layers for DUV LEDs. We found that the contact resistances were exponentially increased from 0.14 to 15.1 Ω∙cm2 with an increase of AlN mole fraction from 0.37 to 0.58. Also, the offset voltages were linearly increased from 0.22 to 3.62 V.
While the LED with the AlGaN contact showed higher light output power, its operating voltage at 0.5 mA was 0.8 V higher than that with the GaN contact, showing a reasonable agreement with the abovementioned offset voltage. Further decreases of the offset voltage are important to obtain high-efficiency DUV LEDs.
To reduce the operating voltage, we analyzed the p–n junction of an aluminum gallium nitride (AlGaN) homojunction Tunnel Junction (TJ) deep-ultraviolet light-emitting diode using phase-shifting electron holography. We obtained a phase image reflecting the band alignment of the p–n homojunction and derived a depletion layer width of approximately 10 nm. We found the AlGaN homojunction TJ forms a p-n junction. Furthermore, the operating voltage reached 8.8 V at 63 A cm-2 by optimizing the structural characteristics of the AlGaN TJ, such as the thickness and impurity concentration, where the thickness of the TJ was 23 nm. We found that the TJ thickness should be at least the same as the depletion layer width at the AlGaN TJ.
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