Photoluminescence from defects in GaN

Abstract

We present the most recent results of photoluminescence (PL) studies, classification of defects in GaN and their properties. In particular, the yellow luminescence band (labeled YL1) with a maximum at 2.17 eV in undoped GaN grown by most common techniques is unambiguously attributed to the isolated CN acceptor. From the Zero-Phonon Line (ZPL) at 2.59 eV, the −/0 level of this acceptor is found at 0.916 eV above the valence band. The PL also reveals the 0/+ level of the CN at 0.33 eV above the valence band, which is responsible for the blue band (BLC), with the ZPL at 3.17 eV. Another yellow band (YL2) with a maximum at 2.3 eV, observed only in GaN grown by the ammonothermal method, is attributed to the VGa3H complex. The nitrogen vacancy (VN) causes the green luminescence (GL2) band. The VN also forms complexes with acceptors such as Mg, Be, and Ca. These complexes are responsible for the red luminescence bands (the RL2 family) in high-resistivity GaN. The results from PL studies are compared with theoretical predictions. Uncertainties in the parameters of defects are discussed.

The Origin of the Yellow Luminescence Band in Be-Doped Bulk GaN

“…At room temperature (when we expect only the YL X band), the PLE spectrum also shows an onset at about 2.7-2.8 eV [29]. We assume that the YL X band is caused by electron transitions from the conduction band (or from shallow donors) to a deep defect level, as it occurs for almost all defects in GaN [7,17]. Then, the results of the current work and those of previous studies indicate that the related defect level is located no closer than 0.8 eV to the VBM.…”

“…However, unlike the PL quenching in conductive n-type GaN, the parameter EA does not have a physical meaning [17]. The quenching is very abrupt, yet the slope of the quenching could be smoothened by potential fluctuations [42], or other reasons [38].…”

“…Nonexponential and slow PL decay after a laser pulse agrees with the assumed type of transition. This PL cannot be caused by transitions from an excited state to the ground state of some defect because such transitions are characterized by exponential PL decay, even at very low temperatures, and positions of such PL bands are not affected by local electric fields [7,17]. At this point, transitions from a deep donor to an acceptor (or a deeper donor) cannot be excluded since such transitions are possible in heavily doped GaN and are characterized by slow, nonexponential decay.…”

“…A different (as will be shown below) yellow band is observed in GaN doped or implanted with beryllium (Be) [9,[17][18][19]. A renewed interest in doping III-nitrides with Be is fueled by recent findings that shallow Be-related acceptors can potentially be used for conductive p-type GaN and AlN [20][21][22].…”

“…The YL1 band can also be recognized in PL spectra from undoped GaN grown by other techniques such as the High Nitrogen Pressure Solution (HNPS) method [15] and the hydride vapor phase epitaxy (HVPE) method [12]. Very low concentrations of carbon (10 15 -10 16 cm −3 ) in HVPE-grown undoped GaN lead to the quantum efficiency of the YL1 band close to unity thanks to a relatively high holecapture cross-section of the C N defect [16,17]. This explains why the C N -related YL1 band is omnipresent in undoped GaN and GaN doped with various impurities.…”