Photoionization Spectroscopy of Traps in Doped and Undoped AlGaN/GaN HEMTs

Abstract

PACS: 71.55.Eq; 73.20.Hb; 85.30.De Deep-level defects and surface states are supposed to be responsible for the limitation of AlGaN/GaN high electron mobility transistor (HEMT) performance. In order to investigate the influence of these traps, photoionization spectroscopy was used to study doped and undoped HEMTs grown on sapphire in different metalorganic vapour-phase epitaxy reactors. This measurement technique is based on the optical reversion of the current collapse and it allows one to determine photoionization cross-sections of the participating traps. For doped and undoped HEMTs nearly the same two defect levels with excitation energies of 3.2 eV and 2.9 eV were determined. By varying the source-gate voltage it was found that the photoionization cross-section is reduced for positive gate bias, i.e. the virtual gate on the gate-drain access region is partially neutralized due to the removal of trapped electrons from surface states.

Photoionization spectroscopy of deep defects responsible for current collapse in nitride-based field effect transistors

“…In the region hν > 2.5 eV, Wolter et al [50,51] observed photoionization spectra with thresholds quite similar to those described above at 2.85 and 3.2 eV. Both of these transitions were interpreted as photoionization from deep trap levels.…”

“…Wolter et al [51] have also reported an interesting investigation of the effect on the photoionization spectra of gate bias and of the spatial separation between the 2DEG and the surface [50]. They observe that: (i) for devices where the 2DEG is closer to the AlGaN surface, the drain current is lower and the response function S(hν) in the photoionization spectrum is raised, suggesting a higher trap concentration, and (ii) as the gate bias is reduced (i.e.…”

“…As discussed above, the 2.85 eV absorption threshold (Trap2) has been associated with the HR-GaN layer in several ways: Figure 13. Photoionization spectrum of a doped AlGaN/GaN HEMT structure as a function of applied gate bias (after [51]).…”

“…Photoionization spectroscopy studies have recently been carried out by Wolter et al [49][50][51] on several MOCVD-grown AlGaN/GaN HEMT devices, most of which employed doped AlGaN barrier layers. Spectra obtained in these studies exhibit features similar to those in figures 9 and 10(a) for photon energies hν > 2.5 eV, as shown in figure 12.…”

Investigation of traps in AlGaN/GaN HEMTs on silicon substrate

“…1). For HEMTs grown on sapphire we could determine two different traps at the activation energies around 3.2 eV and 2.9 eV, These two energy levels could be attributed to carbon-related centers in the GaN buffer [1,7] and were measured independently whether the heterostructure contained a doped AlGaN carrier supply layer or not [8]. Beside these two excitation levels we found one additional trap level at an energy of 2.1 eV for HEMTs on silicon substrate (Fig.…”