Deep-level traps in as-grown and electron-irradiated homo-epitaxial n-GaN layers grown by MOVPE

The results of junction spectroscopy measurements on deep-level defects in MOVPE (Metal-Organic Vapor-Phase Epitaxy) n-GaN samples grown on highly doped Ammono-GaN and subjected to 1.5 MeV electron irradiation are compared with the published results for epi-GaN materials grown by other techniques. It is found that in addition to the commonly observed deep-level traps in n-type GaN, such as E1 (0.25 eV) and E3 (0.59 eV), 1.5-MeV electron irradiation introduces two other electron traps, EE1 and EE2, with electronic levels at about 0.14 and 0.98 eV below the conduction band edge (EC), respectively. In the case of the EE1 level, a strong influence of the electric field (E) on the electron emission rate (eem) is observed. This suggests a donor type character of this trap level. Further, we have observed that strong electric field, as high as 2 x 105 V/cm, results in lowering the activation energy of electron emission from the EE1 level down to the value of 0.095 eV. The strong eem(E) dependence for the EE1 trap can explain the wide variation in electronic signatures of this trap reported in previous publications. The analysis of the EE1 trap concentrations in the electron irradiated samples allowed us to estimate the average production rate of this trap by 1.5 MeV electrons as 0.125 cm -1 for n-GaN material grown on Ammono-GaN substrate. A series of DLTS measurements with different filling pulse lengths revealed a complexity of the EE1 trap level structure, since at least three emission signals with different capture and emission rates have been separated within the broad EE1 emission signal. Possible origins of the detected deep -level traps are discussed.