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 .

No abstract

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.

Gallium nitride (GaN) has become an indispensable compound semiconductor due to its numerous applications in opto- [1,2] and microelectronic [3,4] devices. GaN-based light emitting diodes and laser diodes show higher efficiencies than competing materials and, by alloying with Al or In, the bandgap can cover both the visible and UV spectral range. [5] In addition, AlGaN/GaN heterostructures lead to the formation of a 2D electron gas (2DEG) at the AlGaN/GaN interface, which enables high-frequency switching devices. [6,7] Furthermore, due to the large breakdown voltage and thermal stability, GaN is the perfect candidate for high-power applications. [8,9] As a prerequisite for GaN-based power electronic devices, highly insulating buffers are required. [10,11] Here, carbon doping plays a major role to compensate for donor impurities, which occur even in nominally undoped GaN films. However, the role of carbon-related defects in heterojunction field effect transistors is multiple. They were shown to play a major role in the buffer transport mechanisms generally [12] and to contribute to current collapse [13,14] as well as a dynamical shift of the threshold voltage [11,15] in the devices.Advancements in the field of GaN growth resulted in bulk material of improved structural quality, often with dislocation densities below 10 6 cm À2 . [16,17] As a consequence, the influence of point defects as a limiting factor of device performance is gaining importance. [18][19][20] Therefore, a better understanding of point defects in GaN is required to improve device performance further and to control unwanted side effects.To study optically active point defects in semiconductors, photoluminescence (PL) is one of the experimental techniques most frequently used. The impact of carbon on the luminescence properties is, without doubt, one of the most debated subjects related to point defects in GaN. Although those discussions reach back more than 40 years, [21] considerable progress in the understanding of carbon-related point defect levels was made in the last decade.Two main factors greatly improved the understanding of carbon-related defect levels in GaN. On one hand, the implementation of hybrid functionals in density functional theory (DFT) accounts for carrier localization at acceptor species. This led to vast changes in the predicted energy values of charge state

No abstract