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    High-Temperature Annealing of AlGaN
    (Weinheim : Wiley-VCH, 2020) Hagedorn, Sylvia; Khan, Taimoor; Netzel, Carsten; Hartmann, Carsten; Walde, Sebastian; Weyers, Markus
    In the past few years, high-temperature annealing of AlN has become a proven method for providing AlN layers with low dislocation densities. Herein, the example of Al0.77Ga0.23N is used to investigate whether annealing can also improve the material quality of the ternary alloy. A detailed analysis of the influence of annealing temperature on structural and optical material properties is presented. It is found that with increasing annealing temperature, the threading dislocation density can be lowered from an initial value of 6.0 × 109 down to 2.6 × 109 cm−2. Ga depletion at the AlGaN surface and Ga diffusion into the AlN buffer layer are observed. After annealing, the defect luminescence between 3 and 4 eV is increased, accompanied by an increase in the oxygen concentration by about two orders of magnitude. Furthermore, due to annealing optical absorption at 325 nm (3.8 eV) occurs, which increases with increasing annealing temperature. It is assumed that the reason for this decrease in ultraviolet (UV) transmittance is the increasing number of vacancies caused by the removal of group-III and N atoms from the AlGaN lattice during annealing.
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    Substrate Developments for the Chemical Vapor Deposition Synthesis of Graphene
    (Weinheim : Wiley-VCH Verlag, 2020) Shi, Q.; Tokarska, K.; Ta, H.Q.; Yang, X.; Liu, Y.; Ullah, S.; Liu, L.; Trzebicka, B.; Bachmatiuk, A.; Sun, J.; Fu, L.; Liu, Z.; Rümmeli, M.H.
    Since the isolation of graphene and numerous demonstrations of its unique properties, the expectations for this material to be implemented in many future commercial applications have been enormous. However, to date, challenges still remain. One of the key challenges is the fabrication of graphene in a manner that satisfies processing requirements. While transfer of graphene can be used, this tends to damage or contaminate it, which degrades its performance. Hence, there is an important drive to grow graphene directly over a number of technologically important materials, viz., different substrate materials, so as to avoid the need for transfer. One of the more successful approaches to synthesis graphene is chemical vapor deposition (CVD), which is well established. Historically, transition metal substrates are used due to their catalytic properties. However, in recent years this has developed to include many nonmetal substrate systems. Moreover, both solid and molten substrate forms have also been demonstrated. In addition, the current trend to progress flexible devices has spurred interest in graphene growth directly over flexible materials surfaces. All these aspects are presented in this review which presents the developments in available substrates for graphene fabrication by CVD, with a focus primarily on large area graphene.