Degradation mechanisms of 275-nm-band AlGaN quantum well deep-ultraviolet light-emitting diodes fabricated on a (0001) sapphire substrate were investigated under hard operation conditions. The optical output power (Po) initially decreased by about 20 % within the operating time less than 102 h and then gradually decreased to about 60 % by 484 h. Because the degradation of the wells was less significant than the Po reduction, the initial degradation is attributed essentially to the decrease in carrier injection efficiency, most likely due to de-passivation of initially H-passivated preexisting nonradiative recombination centers (NRCs) in a Mg-doped p-type Al0.85Ga0.15N electron blocking layer. According to our database on the species of vacancy-type defects acting as NRCs in GaN and AlN, vacancy clusters comprised of a cation vacancy and nitrogen vacancies are the most suspicious origins of the NRCs.
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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