Carbon-doping of GaN layers with thickness in the mm-range is performed by hydride vapor phase epitaxy. Characterization by optical and electrical measurements reveals semi-insulating behavior with a maximum of specific resistivity of 2 × 10 10 cm at room temperature found for a carbon concentration of 8.8 × 10 18 cm −3 . For higher carbon levels up to 3.5 × 10 19 cm −3 , a slight increase of the conductivity is observed and related to self-compensation and passivation of the acceptor. The acceptor can be identified as C N with an electrical activation energy of 0.94 eV and partial passivation by interstitial hydrogen. In addition, two differently oriented tri-carbon defects, C N -a-C Ga -a-C N and C N -a-C Ga -c-C N , are identified which probably compensate about two-thirds of the carbon which is incorporated in excess of 2 × 10 18 cm −3 .
AlN bulk single crystals grown by the physical vapor transport method may be beneficially applied as substrates for deep ultraviolet light emitting devices or as a basic material for piezoelectric resonators operating at high temperatures. Identification of point defects which deteriorate the optical, electrical, and electromechanical properties of AlN crystals for such applications is the subject of the present work. Using Raman spectroscopy, two local vibrational modes (LVMs) were discovered at wave numbers of 1189 cm−1 and 1148 cm−1. By analyzing an AlN crystal intentionally enriched with the carbon isotope 13C, it is unambiguously shown that the two LVMs originate from two different, but in each case carbon-related defects. Furthermore, it is evidenced that the defect underlying the LVM at 1189 cm−1 contains exactly two carbon atoms. The tricarbon defect-related LVM reported earlier in an infrared absorption study is found to be Raman active at 1772 cm−1. The Raman scattering intensity of all three LVMs strongly depends on the photon energy of the exciting light what is interpreted as a resonance Raman effect. This allows linking the identified defects with their contribution to the strong, carbon-related ultraviolet absorption around 4.7 eV and proves that these defects introduce optically and electrically active deep levels in the bandgap of AlN.
A high seed temperature (2251 °C) reveals the highest deep UV transparency (α265nm = 27 cm−1), a high structural perfection (EPD = 9 × 103 cm−2) and a suitable growth rate (R = 200 μm h−1).
Carbon doped GaN crystals grown by hydride vapor phase epitaxy have been investigated using mid-infrared and near-ultraviolet absorption spectroscopy. Two local vibrational modes (LVMs) at 1679 cm−1 and 1718 cm−1 as well as an absorption shoulder in front of the band edge absorption of GaN are discovered, all of which increase in intensity with the carbon concentration. The LVMs are similar in wavenumber position to an LVM formerly observed in carbon-rich AlN crystals and unambiguously assigned to a tri-carbon defect. Together with the polarization dependence of the LVMs, we conclude that in GaN the underlying defects are two crystallographically inequivalent configurations of each three nearest neighbor carbon atoms. The strength of both the infrared and ultraviolet absorption features implies concentrations of the underlying defects of the same order as the total carbon concentration. Hence, the tri-carbon defects contribute to the UV absorption and possess deep energy levels in GaN.
Carbon doping is used to obtain semi-insulating GaN crystals. If the carbon doping concentration exceeds 5×10 17 cm -3 , the carbon atoms increasingly form triatomic clusters. The tri-carbon defect structure is unambiguously proven by the isotope effect on the defects' local vibrational modes (LVMs) originally found in samples containing carbon of natural isotopic composition (~99% 12 C, ~1% 13 C) at 1679 cm -1 and 1718 cm -1 . Number, spectral positions, and intensities of the LVMs for samples enriched with the 13 C isotope (~99% and ~50%) are consistently interpreted on the basis of the harmonic oscillator model taking into account the probability of possible isotope combinations. Including the polarization dependence of the LVM absorption, we show that the tri-carbon defects form a triatomic molecule-like structure in two crystallographically different configurations: a basal configuration with the carbon bonds near the basal plane and an axial configuration with one of the carbon bonds along the c-axis. Finally, the disappearance of the LVMs under additional below-bandgap illumination is interpreted as defect recharging, i.e. the tri-carbon defects possess at least one charge state transition level within the bandgap and contribute to optical absorption as well as to the electrical charge balance.
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