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.
GaInN/GaN multiple quantum shells (MQS) nanowires and p-GaN shells were embedded with n-GaN layers through tunnel junction (TJ) shells using metalorganic chemical vapor deposition (MOCVD) method. The MQS nanowires were selectively grown on n-GaN/sapphire or GaN substrates. The fabrication process of laser structures with different resonators of 600500, 750, 1000 μm, and cavity widths of 7, 12, and 17 μm were investigated with insulating layer on the sidewalls of the ridge. The structures of the fabricated devices were characterized by scanning electron microscope (SEM) and current-voltage-light output characteristics were evaluated. Two different methods for mirror formation, etching and cleavage, were developed for the laser devices. During the investigation, a superior mirror formation suffered from the difference in etching rate between GaInN and GaN, generating concaves in the MQS region. Bluegreen light emission was observed from the entire ridge surface of the MQS index-guided laser structures. A maximum current density of emission at 17.9 kA/cm2 has been confirmed in the devices. The electroluminescence and cathodoluminescence measurements demonstrated that the r-plane and c-plane at the top of the MQS are dominant at low current densities, and the m-plane emission becomes stronger as the current density increases.
A tunnel junction and a n-GaN cap layer grown on the multi-quantum shells (MQS) /nanowires are introduced to decrease the resistivity and optical loss. The selective-area growth of the MQS/nanowire core-shell structures on the template was performed by metalorganic vapour phase epitaxy (MOVPE). Further, the MQS structure was covered with the tunnel junction and the n-GaN cap layer. Here, the growth conditions of the n-GaN cap layer were systemically investigated. The effect of p-GaN shape on the morphology of grown n-GaN cap layer was also assessed.
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