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.
In this presentation, seeded growths of large diameter GaN crystals using the low-pressure acidic ammonothermal (LPAAT) method operated at around 100 MPa will be demonstrated. Nearly bowing- and mosaic-free GaN crystals exhibiting the full-width at half-maximum values for the 0002 X-ray rocking curves below 20 arcsec were achieved on high lattice coherency c-plane AAT seeds with gross dislocation densities in the order of 104 cm−2. The photoluminescence spectra of the grown crystals exhibited a predominant near-band-edge emission at 295 K, of which intensity was one order of magnitude higher than the characteristic deep-state emission called "yellow luminescence band". A nearly bowing-free large diameter c-plane GaN crystal was eventually obtained.
The results of complimentary time-resolved photoluminescence and positron annihilation measurements on Mg-implanted GaN on GaN fabricated using various I/I sequences will be shown to identify the species and quantify the concentrations and minority carrier capture coefficients of major midgap recombination centers (MGRCs) created by the I/I processes. Because vacancy clusters comprised of Ga vacancies (VGa) and N vacancies (VN) such as (VGaVN)3 were assigned as major vacancy-type defects and the room-temperature photoluminescence lifetime for the NBE emission increased with decreasing their concentration, (VGaVN)3 are assigned as major nonradiative recombination centers with electron capture coefficient of 5×10-6 cm3s-1, which is an order of magnitude larger than the hole capture coefficient of VGaVN in n-GaN (6×10-7 cm3s-1).
Financial supports: CSTI-SIP, MEXT (JPJ005357, JPJ009777, JP16H06427, JP21H01826), PNCRD TECHMATSTRATEG-III/0003/2019-00 and PNSC 2018/29/B/ST5/00338.
With respect to (Al,In,Ga)N epilayers and quantum wells, threading dislocations (TDs) have long been believed to as the principal limiting factor for the internal quantum efficiency of the near-band-edge emission. However, the realization of low TD density GaN and AlN substrates and (Al,In,Ga)N layers enabled investigating the roles of point defects and impurities without interferences by TDs, and vacancy-complexes have been revealed to act as origins of major Shockley- Read-Hall (SRH)-type nonradiative recombination centers (NRCs) in GaN. Accordingly, the concentration of NRCs (NNRC) must be decreased in both optical devices and power-switching electronic devices. Here we show the results of positron annihilation and time-resolved luminescence measurements on n- and p-type GaN, AlN, and Al0.6Ga0.4N alloys to reveal the origins of major intrinsic SRH-NRCs and to obtain their capture coefficients for minority carriers. For unintentionally doped and doped n-type GaN, divacancies comprising of a Ga-vacancy (VGa) and a N-vacancy (VN), namely VGaVN, are assigned as major SRH-NRCs with a hole capture-coefficient (Cp) of 6×10-7 cm3s-1. For Mg-doped ptype GaN epilayers grown by metalorganic vapor phase epitaxy (MOVPE), VGa(VN)2 are assigned as major NRCs with electron capture-coefficient (Cn) of 8×10-6 cm3s-1. For Mg-implanted GaN, VGaVN are the dominant NRCs right after implantation, and they agglomerate into (VGaVN)3 clusters with Cn of 5×10-6 cm3s-1 after high-temperature annealing. Since AlN films grown by MOVPE usually contain vacancy-clusters comprising of an Al-vacancy (VAl) such as VAl(VN)2-3, complexes of a cation-vacancy and a few VNs may be the major NRCs in AlN and Al0.6Ga0.4N alloys.
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