Traps in InAlN/GaN and AlGaN/GaN high electron mobility transistors (HEMTs) are identified and compared using constant drain-current deep level transient spectroscopy (CID-DLTS). For both structures with different barrier materials, the same drain-access electron trap at EC−0.57 eV dominates the drain-controlled CID-DLTS trap spectrum. This suggests that the physical source of this trap, previously associated with drain-lag, is not present in the barrier but instead is likely to reside in the GaN-buffer. Gate-controlled CID-DLS measurements, which are preferentially sensitive to the barrier under the gate, reveal different trap spectra for the two HEMTs, showing that choice of barrier materials can influence under-gate trap signatures.

The intrinsic anisotropic optical properties of wurtzite AlN are investigated in absorption and emission. Full access to the anisotropy of the optical response of the hexagonal material is obtained by investigating the (1100) plane of a high-quality bulk crystal allowing electric field E polarization perpendicular (E ⊥ c) and parallel (E c) to the optical axis c. Spectroscopic ellipsometry yields the ordinary (ε ⊥ ) and extraordinary (ε ) dielectric functions (DFs) from 0.58 up to 20 eV. The comparison of the experimental data with recently calculated DFs demonstrates that Coulomb interaction has a strong impact not only on the spectral dependence around the fundamental absorption edge but also on the high-energy features usually discussed in terms of van Hove singularities. The fits of the second-order derivatives of ε and ε ⊥ provide the transition energies for the main features in this range. The DFs close to the fundamental absorption edge, dominated by free excitons, exciton-phonon complexes, and the exciton continuum, are independently confirmed by reflectivity and synchrotron-based photoluminescence excitation studies. Values for the band gaps, the crystal field ( cf = −221 ± 2 meV), and spin-orbit splittings ( so = 13 ± 2 meV) are obtained. Furthermore, we obtain accurate values for the static dielectric constants of ε S⊥ = 7.65 and ε S = 9.21, entering, e.g., the calculations of exciton binding energies. Photoluminescence is used to investigate the emission properties of the same sample.

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The impact of in situ low-power N 2 plasma pretreatment, prior to silicon-nitride (SiN) deposition, was investigated in AlGaN/GaN high-electron mobility transistors (HEMTs). These studies reveal that the use of N 2 plasma in HEMT passivation reduces current-collapse and gate-lag effects. Such treatment is also beneficial to improve gate leakage, and from RF measurements, no degradation of f max was observed. These beneficial effects of the N 2 plasma pretreatment seem to be due to a significant reduction in interface charge density, as shown in this letter using GaN MIS devices, where a decrease of 60% was observed.

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