Carbon doping of GaN: Proof of the formation of electrically active tri-carbon defects

Abstract

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.

Investigation of carbon-related complexes in highly C-doped GaN grown by metalorganic vapor phase epitaxy

“…In particular, two absorption peaks at 1679 and 1718 cm −1 have been reported as observed in highly C-doped GaN and AlN. 28,29) Since the peak positions were consistent with the reports, they could be considered to originate from the tricarbon complexes of C N -C Ga -C N (basal), which consist only of C N -C Ga bonds nearly perpendicular to the c-axis, and C N -C Ga -C N (axial), which consist of the C N -C Ga bond parallel to the c-axis, respectively. 30) Figure 3 shows the detailed atomic configurations for tri-carbon complexes.…”

“…29) On the other hand, the C Ga -C N -C Ga complexes vibrate at frequencies very close to the LVMs at 1679 and 1718 cm −1 , and are formed close to the C N -C Ga -C N complexes. 28) So far, the tri-carbon defects C N -C Ga -C N are abundant due to the strong reflection intensity in highly C-doped GaN. However, as shown in Fig.…”

Current Status of Carbon‐Related Defect Luminescence in GaN

“…[43,119] A detailed analysis of the isotope effect on the LVMs in crystals with 13 C-enriched doping clearly proved the assignment of those modes to complexes involving three C atoms. [120] Based on their polarization behavior, those LVMs were proposed to be related to crystallographically inequivalent configurations of C N À C Ga,off À C N (C Ga,off denotes an off-center Ga site). [120] Furthermore, the intensity of these complex-related LVMs was sensitive to additional illumination with light of 385 nm (3.22 eV), indicating the presence of related charge state transition levels in the GaN bandgap.…”

“…[120] Based on their polarization behavior, those LVMs were proposed to be related to crystallographically inequivalent configurations of C N À C Ga,off À C N (C Ga,off denotes an off-center Ga site). [120] Furthermore, the intensity of these complex-related LVMs was sensitive to additional illumination with light of 385 nm (3.22 eV), indicating the presence of related charge state transition levels in the GaN bandgap. This is further supported by the observation of an absorption onset at 2.5-2.6 eV resulting in a bulk photovoltaic effect in surface photovoltage (SPV) measurements [121] and a PL band around 1.62 eV in those samples, [93] for clarity further referred to as RL C .…”

The movement of the Fermi level in heavily C doped GaN

“…Especially, Matsubara et al, 30) demonstrated that the concentration of V N + is comparable to that of C N in heavily C doped GaN when the concentrations of O, Si, and H are low. Additionally, when C concentration exceeds its solid solubility during growth, C atoms would tend to exist in the form of C N + (rather than C N -), or substitute Ga site to form C Ga instead, or form C related complexes such as tricarbon C N -C Ga -C N , [31][32][33] which could act as donors and have been theoretically investigated in literatures. 8,31,34) Therefore, combining with the main influence of the above two aspects, high concentration of compensation donors leads to the reverse shift of the Fermi level upward to the conduction band.…”