The process technologies of AlN growth by physical vapour transport are reviewed in this paper with a focus on the growth parameters, crucible materials, and the type of seeding/nucleation. In this context the three growth strategies for the first generation of AlN seeds, (i) grain selection, (ii) heteroepitaxially seeding on SiC, and (iii) spontaneous nucleation, are evaluated regarding their impact on the structural properties and the sizes of the grown AlN crystals. Major issues for subsequent homoepitaxial growth runs with controlled diameter enlargement, such as thermal field design and seed fixation, are addressed. Furthermore, the influences of the growth conditions on the main optical absorption bands in AlN are discussed.

Freestanding AlN single crystals are grown in a RF-heated furnace by physical vapor transport (PVT). Three different growth regimes with growth temperatures between 2080–2200°C result in different crystal habits and very high structural quality. The Rocking curves show FWHM < 21 arcsec in the 0002 and 101̄0 Reflection on the as-grown facets. Isometric AlN crystals with sizes up to 10 × 10 × 12mm3 show a zonar structure consisting of a yellowish core area which is grown on the N-polar (0001̄) facet and a nearly colorless edge region grown on prismatic {101̄0} facets. In the two growth zones nearly the same C concentrations but different amounts of O and Si are measured by secondary ion mass spectrometry (SIMS). The yellowish core area show a very low defect density (EPD ⩽ 100 cm−2) and a higher deep UV transparency compared to the edge region.

In this paper, the optimal growth conditions during the physical vapour transport of bulk AlN crystals are evaluated with regard to significantly increased deep UV transparency, while maintaining the high structural quality of the AlN crystals which are grown on N-polar c-facets. We show that carbon concentration [C], oxygen concentration [O], and the ratio between both concentrations [C]/[O] have a significant influence on the deep UV transparency. At 3[C] < [O] with [C] + [O] < 1019 cm−3, deep UV transparent AlN single crystals with absorption coefficients at around 265 nm (α265nm) smaller than 15 cm−1 can be prepared. These conditions can be achieved in the N-polar grown volume parts of the AlN crystals using growth temperatures in the range of TG = 2030–2050 °C and tungsten and tantalum carbide as getter materials for carbon and oxygen, respectively. Deep UV transparent AlN substrates (α265nm < 30 cm−1) ≥10 mm in diameter and of high crystalline perfection (rocking curve FWHM < 15 arcsec) are shown for the first time

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The defect related luminescence in the near UV region between 3 and 4 eV has been investigated in insulating, n-type and irradiation damaged AlN. A single luminescence band around 3.3 eV with a full width at half maximum of more than 500 meV, is attributed to a donor-acceptor pair transition at low temperatures. The substantial linewidth and a Stokes shift of around 1.3 eV result from strong electron-phonon coupling of the deep acceptor. While the chemical nature of the donor remains ambiguous, the acceptor species is attributed to the isolated Al vacancy, i.e. V 3ÀAl . This luminescence band might be interesting for studying n-type compensation mechanisms in AlN.

We report on the identification of a tri-carbon defect in AlN bulk crystals grown by physical vapor transport. The defect gives rise to a single infrared absorption line at 1769 cm−1 in unintentionally carbon doped crystals. This line splits into eight lines in crystals enriched with the carbon isotope 13C. The observed line patterns can unambiguously be assigned to a local vibrational mode of a defect that contains exactly three carbon atoms. The most probable arrangement of the three carbon atoms is on nearest-neighbor substitutional sites, replacing two nitrogen atoms and one aluminum atom, whereby one carbon-carbon bond is directed non-parallel and the other parallel to the crystal's c axis. It is suggested that the tri-carbon defect can exist in three different charge states (neutral, singly negative, and doubly negative) and hence possesses two transition levels within the band gap. This energy level scheme explains the appearance and disappearance of the local vibrational mode in dependence on the Fermi level position as well as a similar appearance-disappearance behavior of a strong ultraviolet absorption band at 4.7 eV that has been repeatedly reported in the literature. We propose that the singly negative charge state of the tri-carbon defect essentially contributes to that ultraviolet absorption.

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