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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_Gaacceptor. 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_Beband) was observed in nearly all samples protected with an AlN cap during the annealing and in samples annealed under ultrahigh N₂pressure. 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_Gaacceptor with the ionization energy of 0.113 eV appeared in implanted samples only after annealing at high temperatures and ultrahigh N₂pressure. This is the first observation of the UVL_Beband in Be-implanted GaN, indicating successful activation of the Be_Gaacceptor. 

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2. 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 C_Nacceptor is the source of the YL band (the YL1 band) in GaN samples grown by several techniques. Previously suggested candidates, such as V_Ga, V_GaO_N, C_NO_N, and C_NSi_Ga, should be discarded. At the same time, other defects (such as the V_N, Be_Ga, and Ca_Ga) 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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3. Photoluminescence from GaN Implanted with Be and F

   https://doi.org/10.1002/pssb.202300131  

   Reshchikov, Michael Alexander; Andrieiev, Oleksandr; Vorobiov, Mykhailo; Demchenko, Denis O.; McEwen, Benjamin; Shahedipour-Sandvik, Shadi (May 2023, physica status solidi (b))

   GaN samples are implanted with Be and F and annealed in different conditions to activate the Be_Gaacceptors. Photoluminescence spectra are studied to recognize the defects. The UVL_Beband with a maximum at 3.38 eV and the YL_Beband with a maximum at 2.15 eV are observed and associated with Be. The sequential implantation of Be and F ions into GaN at 600 °C reduces the concentration of nitrogen vacancies (V_N), as evidenced by the lack of the green luminescence band associated with the isolated nitrogen vacancy. First‐principles calculations are employed to find parameters of defects that can form after implantation. 

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4. 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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5. Passivation of acceptors in GaN by hydrogen and their activation

   https://doi.org/10.1088/1361-6528/ada298  

   Reshchikov, M_A; Andrieiev, O.; Vorobiov, M.; Ye, D.; Demchenko, D_O; McEwen, B.; Shahedipour-Sandvik, F. (January 2025, Nanotechnology)

   Abstract GaN is an important semiconductor for energy-efficient light-emitting devices. Hydrogen plays a crucial role in gallium nitride (GaN) growth and processing. It can form electrically neutral complexes with acceptors during growth, which significantly increases the acceptor incorporation. Post-growth annealing dissociates these complexes and is widely utilized for activating Mg acceptors and achieving conductive p-type GaN. In this work, we demonstrate that other acceptors, such as C and Be, also form complexes with hydrogen similar to Mg. The effect of thermal annealing of GaN on photoluminescence (PL) was investigated. In samples moderately doped with Be, the Be_Ga-related yellow luminescence (YL_Be) band intensity decreased by up to an order of magnitude after annealing in N₂ambient at temperaturesT_ann= 400 °C–900 °C. This was explained by the release of hydrogen from unknown traps and the passivation of the Be_Gaacceptors. A similar drop of PL intensity atT_ann= 350 °C–900 °C was observed for the C_N-related YL1 band in unintentionally C-doped GaN and also attributed to passivation of the C_Nacceptors by hydrogen released from unknown defects. In this case, the formation of the C_NH_icomplexes was confirmed by the observation of the rising BL2 band associated with these complexes. AtT_ann> 900 °C, both the YL_Beand YL1 intensities were restored, which was explained by the removal of hydrogen from the samples. Experimental results were compared to the first principles calculations of complex dissociation and hydrogen diffusion paths in GaN. 

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