Emission properties of a-plane GaN grown by metal-organic chemical-vapor deposition

Abstract

We report on the emission properties of nonpolar a-plane GaN layers grown on r-plane sapphire. Temperature-, excitation-density-, and polarization-dependent photoluminescences and spatially resolved microphotoluminescence and cathodoluminescence are employed in order to clarify the nature of the different emission bands in the 3.0–3.5eV spectral range. In the near band-edge region the emission lines of the donor-bound excitons (3.472eV) and free excitons (3.478eV) are resolved in the polarized low-temperature spectra, indicating a good quality of the layers. At low energies two other emissions bands with intensity and shape varying with the excited area are observed. The 3.42eV emission commonly attributed to the excitons bound to basal plane stacking faults shows thermal quenching with two activation energies (7 and 30meV) and an S-shaped temperature dependence of the peak position. This behavior is analyzed in terms of hole localization in the vicinity of the stacking faults. The emission band that peaked at 3.29eV is found to blueshift and saturate with increasing excitation intensity. The spatially resolved cathodoluminesence measurements show that the emission is asymmetrically distributed around the triangular-shaped pits occurring at the surface. The 3.29eV emission is suggested to involve impurities, which decorate the partial dislocation terminating the basal stacking faults.

Exciton localization on basal stacking faults in a-plane epitaxial lateral overgrown GaN grown by hydride vapor phase epitaxy

“…21 There are consequently some localization effects for BSF-bound excitons inside the stacking faults themselves, explaining qualitatively the redshift with temperature from 8 to 25 K due to the ionization of excitons bound to the shallower potential fluctuation in favor of the deeper ones. This explanation is also consistent with the 3-4 meV blueshift and the vanishing of the low-energy tail of the BSF-related emission observed by Paskov et al 22 when increasing the excitation density. The blueshift part of the S-shaped temperature dependence is commonly observed in epitaxial type-I and type-II quantum wells 23,24 due to fluctuations of the well width by one or two monolayers.…”

“…However, it is unlikely that quantum wells formed by stacking faults present such fluctuations because the distance between two changes in the stacking sequence is precisely determined by the type of the BSF: for instance, the width of I 1 stacking faults is exactly equal to 1.5c 0 , where c 0 is the unstrained lattice parameter of wurtzite GaN along the ͑0001͒ direction. Paskov et al 22 attributed similar temperature dependence of the BSF-related line to the delocalization of the holes from the potential where they are maintained in the type-II quantum well by Coulomb attraction to the electron. In other words, the blueshift would be due to a transition from an exciton bound to the BSF to a free-to-bound recombination involving a bound electron and a free hole.…”

“…We measure 18 meV for this localization energy, which is the difference between the lowtemperature extrapolated PL energy of ϳ3.433 eV and the measured minimum PL energy of 3.415 meV. Paskov et al 22 reported ϳ20 meV, instead. One explanation for this difference could be in a difference of internal electric fields, which Paskov et al 22 did not consider when discussing the hole escape from the Coulomb binding.…”

Nature of low-energy optical emission in doped AlGaN∕GaN heterostructures

“…1 correspond to LO-phonon replicas with the energy of ប LO Ϸ 90 meV. Additionally, our results are in good agreement with the recently reported data 13 about the acceptor-type mechanism as the dominant DAP recombination in the PL spectra with the participation of deep-level defect-related complexes abundantly produced in masstransport-grown GaN structures.…”

“…In addition, the spectrum demonstrates a few weak lines of excitonic emission at 3.41-3.43 eV due to shallow impurity-related complexes. 13 It should be noted that the PL spectra recorded with illumination from the back face of AlGaN / GaN heterostructures ͑U and D͒ reproduce in detail the PL spectrum of the GaN film except for the I͑A 0 X͒ peak that disappeared in the spectra corresponding to both samples U and D. The relative intensity and spectral position of the dominant I͑D 0 X͒ peak for D and S are nearly the same, only the strength of the donor-bound exciton D 0 X band in the PL spectra of U is significantly reduced with respect to the D heterostructure. The FWHM of the I͑D 0 X͒ peak in the spectrum of S is smaller than in the U and D spectra due to the more perfect structure of the epitaxial GaN film in comparison with AlGaN / GaN heterostructures.…”