Gallium nitride (GaN) and its allied binaries InN and AIN as well as their ternary compounds have gained an unprecedented attention due to their wide-ranging applications encompassing green, blue, violet, and ultraviolet (UV) emitters and detectors (in photon ranges inaccessible by other semiconductors) and high-power amplifiers. However, even the best of the three binaries, GaN, contains many structural and point defects caused to a large extent by lattice and stacking mismatch with substrates. These defects notably affect the electrical and optical properties of the host material and can seriously degrade the performance and reliability of devices made based on these nitride semiconductors. Even though GaN broke the long-standing paradigm that high density of dislocations precludes acceptable device performance, point defects have taken the center stage as they exacerbate efforts to increase the efficiency of emitters, increase laser operation lifetime, and lead to anomalies in electronic devices. The point defects include native isolated defects (vacancies, interstitial, and antisites), intentional or unintentional impurities, as well as complexes involving different combinations of the isolated defects. Further improvements in device performance and longevity hinge on an in-depth understanding of point defects and their reduction. In this review a comprehensive and critical analysis of point defects in GaN, particularly their manifestation in luminescence, is presented. In addition to a comprehensive analysis of native point defects, the signatures of intentionally and unintentionally introduced impurities are addressed. The review discusses in detail the characteristics and the origin of the major luminescence bands including the ultraviolet, blue, green, yellow, and red bands in undoped GaN. The effects of important group-II impurities, such as Zn and Mg on the photoluminescence of GaN, are treated in detail. Similarly, but to a lesser extent, the effects of other impurities, such as C, Si, H, O, Be, Mn, Cd, etc., on the luminescence properties of GaN are also reviewed. Further, atypical luminescence lines which are tentatively attributed to the surface and structural defects are discussed. The effect of surfaces and surface preparation, particularly wet and dry etching, exposure to UV light in vacuum or controlled gas ambient, annealing, and ion implantation on the characteristics of the defect-related emissions is described.

Photoluminescence (PL) spectroscopy is a powerful tool in studying semiconductor properties and identifying point defects. Gallium nitride (GaN) is a remarkable semiconductor material for its use in a new generation of bright white LEDs, blue lasers, and high-power electronics. In this Tutorial, we present details of PL experiments and discuss possible sources of mistakes. A brief analysis of near-band-edge emission includes basic characterization of GaN, essential findings about excitons in this material, and the explanation of less known details. We review modern approaches of quantitative analysis of PL from point defects in GaN. The updated classification of defects in undoped GaN and their latest identifications are presented. Typical mistakes in the interpretation of PL spectra from GaN are discussed, and myths about PL are refuted.

In high-purity GaN grown by hydride vapor phase epitaxy, the commonly observed yellow luminescence (YL) band gives way to a green luminescence (GL) band at high excitation intensity. We propose that the GL band with a maximum at 2.4 eV is caused by transitions of electrons from the conduction band to the 0/+ level of the isolated ${\mathrm{C}}_{\mathrm{N}}$ defect. The YL band, related to transitions via the −/0 level of the same defect, has a maximum at 2.1 eV and can be observed only for some high-purity samples. However, in less pure GaN samples, where no GL band is observed, another YL band with a maximum at 2.2 eV dominates the photoluminescence spectrum. The latter is attributed to the ${\mathrm{C}}_{\mathrm{N}}{\mathrm{O}}_{\mathrm{N}}$ complex.

A majority of the point defects in GaN that are responsible for broad photoluminescence (PL) bands remain unidentified. One of them is the green luminescence band (GL2) having a maximum at 2.35 eV which was observed previously in undoped GaN grown by molecular-beam epitaxy in Ga-rich conditions. The same PL band was observed in Mg-doped GaN, also grown in very Ga-rich conditions. The unique properties of the GL2 band allowed us to reliably identify it in different samples. The best candidate for the defect which causes the GL2 band is a nitrogen vacancy $({V}_{\mathrm{N}})$. We propose that transitions of electrons from the conduction band to the +/2+ transition level of the ${V}_{\mathrm{N}}$ defect are responsible for the GL2 band in high-resistivity undoped and Mg-doped GaN.

We have found a photoluminescence (PL) band with unusual properties in GaN. The blue band, termed as the ${\mathrm{BL}}_C$ band, has a maximum at about 2.9 eV and an extremely short lifetime (shorter than 1 ns for a free-electron concentration of about ${10}^{18}\mathrm{c}{\mathrm{m}}^{-3}$). The electron- and hole-capture coefficients for this defect-related band are estimated as ${10}^{-9}$ and ${10}^{-10}\mathrm{c}{\mathrm{m}}^3/\mathrm{s}$, respectively. The ${\mathrm{BL}}_C$ band is observed only in GaN samples with relatively high concentration of carbon impurity, where the yellow luminescence (the YL1 band) with a maximum at 2.2 eV is the dominant defect-related PL. Both the YL1 and ${\mathrm{BL}}_C$ bands likely originate from the ${\mathrm{C}}_{\mathrm{N}}$ defect, namely, from electron transitions via the $-/0$ and $0/+$ thermodynamic transition levels of the ${\mathrm{C}}_{\mathrm{N}}$. The ${\mathrm{BL}}_C$ band appears only at high excitation intensities in $n$-type GaN samples codoped with Si and C, and it can be found in a wide range of excitation intensities in semi-insulating (presumably $p$-type) GaN samples doped with C. The properties and behavior of the YL1 and ${\mathrm{BL}}_C$ bands can be explained using phenomenological models and first-principles calculations.

Tunable and abrupt thermal quenching of photoluminescence by increasing temperature has been observed for the blue band in high-resistivity Zn-doped GaN. The photoluminescence intensity dropped by several orders of magnitude within a few Kelvins, and the temperature at which that drop occurred could be tuned by changing the incident light intensity. Modeling the system with rate equations for competing electron-hole recombination flows through three defect species, one of which is a nonradiative deep donor, gives results consistent with these observations.