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Photoluminescence from GaN implanted with Cd or Hg ions was studied and compared with first-principles calculations. In Cd-implanted GaN, the blue band (BLCd) with a maximum at 2.7 eV is attributed to the CdGa acceptor with an ionization energy of 0.55 eV. In Hg-implanted GaN, the green band (GLHg) with a maximum at 2.44 eV is attributed to the HgGa acceptor with an ionization energy of 0.77 eV. The shapes of the BLCd and GLHg bands are asymmetric, with a similar Franck–Condon shift of about 0.28 eV. The electron- and hole-capture coefficients for the CdGa and HgGa acceptors are found. The experimentally found parameters agree reasonably well with first-principles calculations using HSE hybrid functional satisfying the generalized Koopmans' theorem. [ABSTRACT FROM AUTHOR]

GaN samples were implanted with Be and annealed in different conditions in order to activate the shallow BeGa acceptor. Low-temperature photoluminescence spectra were studied to find BeGa-related defects in the implanted samples. A yellow band with a maximum at about 2.2 eV (the YLBe band) was observed in nearly all samples protected with an AlN cap during the annealing and in samples annealed under ultrahigh N2 pressure. A green band with a maximum at 2.35 eV (the GL2 band), attributed to the nitrogen vacancy, was the dominant defect-related luminescence band in GaN samples annealed without a protective AlN layer. The ultraviolet luminescence (UVLBe) band with a maximum at 3.38 eV attributed to the shallow BeGa acceptor with the ionization energy of 0.113 eV appeared in implanted samples only after annealing at high temperatures and ultrahigh N2 pressure. This is the first observation of the UVLBe band in Be-implanted GaN, indicating successful activation of the BeGa acceptor. [ABSTRACT FROM AUTHOR]

Ammonothermal GaN samples with the concentration of free electrons of 1018 and 1019 cm−3 were annealed in a wide range of temperatures (Tann = 300–1400 °C) under atmospheric N2 pressure and under ultra-high N2 pressure conditions to avoid the GaN decomposition. Photoluminescence (PL) studies reveal the YL2 band with a maximum at 2.3 eV before annealing and two new PL bands after annealing at Tann > 600 °C: the OL3 band with a maximum at 2.1 eV and the RL4 band with a maximum at 1.6–1.7 eV. The ammonothermal GaN samples have high concentrations of complexes containing gallium vacancy (VGa), hydrogen, and oxygen. The first-principles calculations suggest that the VGa-3Hi complex is the origin of the YL2 band, while the VGa-3ON complex is responsible for the RL4 band. [ABSTRACT FROM AUTHOR]

Photoluminescence (PL) from GaN substrates fabricated by the ammonothermal growth method was studied in a wide range of temperatures and excitation intensities, both with steady-state and time-resolved PL techniques. Three defect-related PL bands were detected: the ultraviolet luminescence band with the zero-phonon line at 3.27 eV, the Zn-related BL1 band with a maximum at 2.9 eV, and the yellow luminescence band (labeled YL2) with a maximum at 2.3 eV. The YL2 band belongs to an unknown defect and is different from the CN-related YL1 band. Its maximum blueshifts by 0.06 eV with increasing excitation intensity and redshifts by more than 0.1 eV with a time delay after a laser pulse. The YL2 band is preliminarily attributed to a defect complex containing the gallium vacancy. [ABSTRACT FROM AUTHOR]

Hybrid density functional theory has become a standard method for calculations of defects in semiconductors. The majority of work in this field is done using hybrid functionals tuned to reproduce the experimental bandgap of the host material. This approach usually yields results in reasonable agreement with the experiment. Alternatively, hybrid functional can be tuned to fulfill the generalized Koopmans' condition for defect orbitals, which cancels self-interaction energy and restores the linear behavior of energy with respect to electron occupation. Here, we investigate the methods of hybrid functional tuning, which both satisfy the generalized Koopmans' condition and reproduce the experimental bandgap, using one of the most well-studied defects in GaN, carbon acceptor. We test different charged defect correction schemes, the influence of Ga3d-electrons, and compare the results with accurate photoluminescence measurements. We find that using different charged defect correction methods can lead to substantially different hybrid functional parametrizations. However, the calculated optical properties of the carbon acceptor are found to be weakly dependent on specific parameters. [ABSTRACT FROM AUTHOR]

We observed varying thermal quenching behavior of the yellow luminescence band near 2.2 eV in different GaN samples. In spite of the different behavior, the yellow band in all the samples is caused by the same defect—the YL1 center. In conductive n-type GaN, the YL1 band quenches with exponential law, and the Arrhenius plot reveals an ionization energy of ∼0.9 eV for the YL1 center. In semi-insulating GaN, an abrupt and tunable quenching of the YL1 band is observed, where the apparent activation energy in the Arrhenius plot is not related to the ionization energy of the defect. In this case, the ionization energy can be found by analyzing the shift of the characteristic temperature of PL quenching with excitation intensity. We conclude that only one defect, namely, the YL1 center, is responsible for the yellow band in undoped and doped GaN samples grown by different techniques. [ABSTRACT FROM AUTHOR]

The surface photovoltage (SPV) technique was used to study the surface and electrical properties of Mg-doped, p-type AlxGa1-xN (0.06

The blue luminescence band with a maximum at 3.0 eV and the zero-phonon line at 3.33 eV (labeled BL2) is observed in high-resistivity GaN. Under prolonged ultraviolet (UV) light exposure, the BL2 band transforms into the yellow luminescence (YL) band with a maximum at 2.2 eV. Our hybrid functional calculations suggest that the BL2 band is related to a hydrogen-carbon defect complex, either CNON-Hi or CN-Hi. The complex creates defect transition level close to the valence band, which is responsible for the BL2 band. Under UV illumination, the complex dissociates, leaving as byproduct the source of the YL band (CNON or CN) and interstitial hydrogen. [ABSTRACT FROM AUTHOR]