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1. Thermal annealing of GaN implanted with Be

   https://doi.org/10.1063/5.0080060  

   Reshchikov, M. A.; Andrieiev, O.; Vorobiov, M.; Ye, D.; Demchenko, D. O.; Sierakowski, K.; Bockowski, M.; McEwen, B.; Meyers, V.; Shahedipour-Sandvik, F. (March 2022, Journal of Applied Physics)

   GaN samples were implanted with Be and annealed in different conditions in order to activate the shallow Be Ga acceptor. Low-temperature photoluminescence spectra were studied to find Be Ga -related defects in the implanted samples. A yellow band with a maximum at about 2.2 eV (the YL Be band) was observed in nearly all samples protected with an AlN cap during the annealing and in samples annealed under ultrahigh N 2 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 (UVL Be ) band with a maximum at 3.38 eV attributed to the shallow Be Ga acceptor with the ionization energy of 0.113 eV appeared in implanted samples only after annealing at high temperatures and ultrahigh N 2 pressure. This is the first observation of the UVL Be band in Be-implanted GaN, indicating successful activation of the Be Ga acceptor. 

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2. Photoluminescence from defects in GaN

   https://doi.org/10.1117/12.2645878  

   Reshchikov, Michael A. (March 2023, Photoluminescence from defects in GaN)

   Morkoç, Hadis; Fujioka, Hiroshi; Schwarz, Ulrich T. (Ed.)

   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. 

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3. Photoluminescence from Defects in Be‐Doped GaN

   https://doi.org/10.1002/pssb.202400656  

   Reshchikov, Michael Alexander; McEwen, Benjamin; Shahedipour‐Sandvik, Shadi (February 2025, physica status solidi (b))

   While GaN is a crucial semiconductor material for bright light‐emitting devices, fabrication of p‐type GaN remains challenging since the Mg acceptor commonly used for p‐type doping is not shallow enough. Doping of GaN with Be is a promising path, yet no reliable p‐type GaN has been achieved by Be doping so far. One of the reasons is a poor understanding of point defects in Be‐doped GaN that can be studied by photoluminescence (PL). The yellow (YL_Be) band at 2.15 eV is the dominant PL band in Be‐doped GaN. In this work, a blue PL band named the BL_Beband is discovered. It has a maximum at 2.6 eV and a lifetime of 0.8 μs at temperatures below 100 K. The BL_Beband is observed in GaN samples with relatively high concentrations of Be (>10¹⁸ cm⁻³). Both the YL_Beand BL_Bebands likely originate from the isolated Be_Gadefect, namely from electron transitions via the −/0 and 0/+ thermodynamic transition levels of the Be_Ga. The 0/+ transition level is located at 0.1–0.2 eV above the valence band. Other broad PL bands in Be‐doped GaN were also observed and preliminarily attributed to Be‐containing complexes. 

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4. On the Origin of the Yellow Luminescence Band in GaN

   https://doi.org/10.1002/pssb.202200488  

   Reshchikov, Michael A. (December 2022, physica status solidi (b))

   The yellow luminescence (YL) band with a maximum at 2.2 eV is the dominant defect-related luminescence in unintentionally doped GaN. The discovery of the mechanism responsible for this luminescence band and related defects in GaN took many years. Eventually, a consensus has been reached that the CN acceptor is the source of the YL band (the YL1 band) in GaN samples grown by several techniques. Previously suggested candidates, such as VGa, VGaON, CNON, and CNSiGa, should be discarded. At the same time, other defects (such as the VN, BeGa, and CaGa) may cause luminescence bands with positions and shapes not much different from the YL1 band. In GaN containing high concentrations of gallium vacancies, oxygen, and hydrogen, complexes containing these species may also contribute in the red-yellow part of the photoluminescence spectrum. The main controversies related to the YL band are resolved. 

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5. Identity of the shallowest acceptor in GaN

   https://doi.org/10.1103/PhysRevB.111.045202  

   Reshchikov, M A; Demchenko, D O; McEwen, B; Shahedipour-Sandvik, F (January 2025, Physical Review B)

   The Be-related ultraviolet luminescence (UVLBe) band with a maximum at about 3.38 eV in GaN:Be is caused by the shallowest acceptor in GaN with the /0 transition level at 0.113 eV above the valence band. The luminescent properties of this acceptor were studied in detail. First-principles calculations identify this acceptor as the BeGaONBeGa complex. These calculations also predict a deep level, at 0.34 eV above the valence band. However, we were not able to find evidence for this level in photoluminescence experiments. 

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