Potential causes for current-induced nitride laser degradation
A team from Aalto University, Finland, suggests that current induced point defect activation is a possible cause for the degradation of GaN-based laser diodes. The researchers performed experiments on MOCVD-grown nitride device and GaN samples. They used a tightly focused low energy electron beam irradiation to generate local current densities up to 130 kA/cm2on the sample surface, about one order of magnitude higher than in GaN-based laser diodes during operation.
The team discovered that irradiation by 5 – 20 keV electron beam reduced the integrated band-to-band luminescence of the nitride samples by as much as 75 %, in correlation with the electron beam energy dissipation density and total irradiation dose. The findings were unexpected, since the threshold energies for vacancy generation in GaN, for example, are much higher, about 150 keV for nitrogen and 500 keV for gallium vacancies. Photoluminescence measurements revealed that the electron beam energy dissipation density was a key factor in luminescence reduction. The strongest decrease in luminescence intensity was observed with the lowest energy electron beam, which has very tight energy dissipation confinement. With the same irradiation energy the decrease in luminescence intensity showed an exponential decay as a function of irradiation dose. The degradation of optical quality was associated with the apparition of point defects, detected by depth-resolved positron annihilation spectroscopy. The point defects were identified as in-grown gallium vacancies rather than isolated gallium vacancies typically introduced by high electron energy irradiation. The apparent concentration of the in-grown gallium vacancy defects was found to increase up to low - 1017cm-3with increasing dose of the low energy electron beam. In comparison, gallium vacancy related defects in as-grown films are typically found with concentrations at or below the detection limit of around 1016cm-3.
These observations led the team to the interpretation that the low energy electron beam irradiation strips hydrogen from pre-existing inactive gallium vacancy – hydrogen complexes, causing their activation. A similar effect has been observed earlier in MOCVDgrown magnesium-doped p-GaN (Gelhausen et al., Appl. Phys. Lett. 83, 3293 (2003)), where magnesiumacceptors are passivated by hydrogen during MOCVDgrowth. These magnesium-hydrogencomplexes can be broken by low energy electron beam irradiation to activate the acceptors.
The inactive in-grown galliumvacancies in MOCVDGaN are most likely passivated with more than two hydrogen atoms, since complexes with only one and two hydrogen atoms are readily detectable with positrons. According to earlier theoretical calculations (Wright, J. Appl. Phys. 90, 1164 (2001)) the gallium vacancy complexed with three hydrogen atoms is neutral, and the energy required to remove one hydrogen atom from the complex to activate it is relatively low, about 1 eV. Hence gallium vacancies passivated by three hydrogen atoms could account for the present observations. The team suggests that the presence of a rather large number of strongly hydrogen-passivated in-grown gallium vacancies with a low activation energy would lead to current induced degradation of MOCVD grown GaN. This degradation mechanism could be behind the limited lifetime of GaN-based laser diodes.
Further experiments, especially with current densities generated by device-like operation, are necessary to shed more light on this issue.
More details of this work have been described in the paper, "Low energy electron beam induced vacancy activation in GaN", by H. Nykänen et al, Applied Physics Letters, 10, 122105, (2012). http://dx.doi.org/10.1063/1.3696047