Luminescence dynamics for the near-band-edge (NBE) emission peak at around 250 nm of c-plane Si-doped Al0.6Ga0.4N films grown on AlN templates by low-pressure metalorganic vapor phase epitaxy were studied using deep ultraviolet time-resolved photoluminescence and time-resolved cathodoluminescence spectroscopies. For the films with the Si-doping concentration, [Si], lower than 1.9 × 1017 cm–3, the doping lessened the concentration of cation vacancies, [VIII], through the surfactant effect or the aid of the reactant doping in a form of H3SiNH2. However, the room-temperature nonradiative lifetime, and, consequently, the equivalent value of internal quantum efficiency in the weak excitation regime steeply decreased when [Si] exceeded 1018 cm−3. Simultaneously, the intensity ratio of the deep-state emission band to the NBE emission abruptly increased. Because the increase in [Si] essentially gives rise to the increase in [VIII] (for [Si]>1.9×1017 cm−3) and the overcompensation of Si is eventually observed for the film with [Si] = 4.0 × 1018 cm−3, the formation of acceptor-type native-defect complexes containing Si such as VIII-SiIII is suggested.

Time-resolved photoluminescence (TRPL) and positron annihilation measurements, as well as Al0.23Ga0.77N/GaN heterostructure growth by metalorganic vapor phase epitaxy were carried out on very low defect density, polar c-plane and nonpolar m-plane freestanding GaN (FS-GaN) substrates grown by hydride vapor phase epitaxy. Room-temperature photoluminescence (PL) lifetime for the near-band-edge (NBE) excitonic emission of the FS-GaN substrates increases with increasing positron diffusion length (L+); i.e., decreasing gross concentration of charged and neutral point defects and complexes. The best undoped c-plane FS-GaN exhibits record-long L+ being 116 nm. The fast component of the PL lifetime for its NBE emission increases with temperature rise up to 100 K and levels off at approximately 1.1 ns. The result implies a saturation in thermal activation of nonradiative recombination centers. The surface and interface roughnesses for a Si-doped Al0.23Ga0.77N/GaN/Al0.18Ga0.82N/GaN heterostructure are improved by the use of FS-GaN substrates, in comparison with the structure fabricated on a standard GaN template. The emission signals related to the recombination of a two-dimensional electron gas and excited holes are recognized for an Al0.23Ga0.77N/GaN single heterostructure grown on the c-plane FS-GaN substrate.

Articles you may be interested inHomoepitaxial AlN thin films deposited on m-plane ( 1 1 ¯ 00 ) AlN substrates by metalorganic chemical vapor deposition

NH 4 F is demonstrated to be a promising mineralizer for the acidic ammonothermal crystal growth of GaN. In comparison with other acidic mineralizers such as NH 4 Cl, NH 4 Br, and NH 4 I, NH 4 F behaves distinctively different. First, NH 4 F affords a negative temperature gradient for crystal growth of GaN in supercritical NH 3 at a temperature range from 550 to 650 °C. Second, it enables GaN crystal growth in polar (c plane), semipolar, and nonpolar directions (a plane and m plane). Third, NH 4 F remarkably increases both the growth rate and quality of the GaN crystal. With the aid of NH 4 F, self-nucleation of GaN and bulk growth of hexagonal GaN crystals from the self-nucleated seed have been realized.

Impacts of point defects and impurities on the carrier recombination dynamics in AlN are revealed by time-resolved spectroscopy and positron annihilation measurements. Intrinsically short low-temperature excitonic radiative lifetime (τR∼10 ps) was elongated with the increase in Al-vacancy concentration up to 530 ps, irrespective of threading dislocation density. A continuous decrease in τR with temperature rise up to 200 K for heavily doped samples revealed the carrier release from the band-tail formed due to impurities and point defects. Because room-temperature nonradiative lifetime was equally short for all samples, high temperature growth with appropriate defect management is necessary in extracting radiative nature of AlN.