Deuterium Out-Diffusion Kinetics in Magnesium-Doped GaN
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Deuterium Out-Diffusion Kinetics in Magnesium-Doped GaN Jacques Chevallier1, FranÁois Jomard1, Norbert H. Nickel2, Philippe de Mierry3, SÈbastien Chenot3, Yvon Cordier3, Marie-Antoinette di Forte-Poisson4, and Sylvain Delage4 1 Groupe d'Etude de la MatiËre CondensÈe, CNRS/UniversitÈ de Versailles, 1 place A.Briand, Meudon, 92190, France 2 Hahn-Meitner-Institut, KekulÈstrasse 5, Berlin, 12489, Germany 3 Centre de Recherche sur l'HÈtÈroÈpitaxie et Applications, CNRS, Parc de Sophia-Antipolis, Rue B.GrÈgory, Valbonne, 06560, France 4 III-V Laboratory, Alcatel-Thales, Marcoussis, 91460, France ABSTRACT A series of isothermal annealing experiments have been performed in the range of 790920∞C under N2 flow in order to study the deuterium out-diffusion kinetics of Mg-doped GaN grown on sapphire under deuterated ammonia. The deuterium concentration was measured by SIMS analysis before and after each annealing step. The kinetics closely follow a first-order law. The activation energy related to the deuterium out-diffusion process is 3.1 eV. In addition, deuterium effusion measurements were performed measuring the molecular HD flux while the specimens were annealed in ultra high vacuum with a linear heating rate. In contrast to SIMS, this method detects the species that migrated out of the sample. Peaks of the HD signal at 360 and 490∞C are attributed to the fragmentation of adsorbed CHxDy complexes. The molecular HD flux starts increasing at 800∞C which is the onset of the GaN decomposition and has its maximum at 920∞C. This HD flux is accompanied by the desorption of H and D containing radicals and molecules desorbing above 900∞C. INTRODUCTION During the metal-organic chemical vapor deposition (MOCVD) of Mg-doped GaN, hydrogen is incorporated and forms (Mg,H) complexes. As a result, an efficient passivation of the magnesium acceptors occurs. Activation of these acceptors by dissociation of the complexes is usually achieved by thermal annealing [1,2]. While the level of p-type conductivity is reasonable for light emitters, it is insufficient for the fabrication of high performance bipolar transistors or GaN-based dilute magnetic semiconductors. Since thermal annealing can generate compensating defects, an optimization of the annealing conditions is necessary to achieve the highest p-type conductivity. It has been established that the hydrogen desorption is limited by surface barrier effects and not by the (Mg,H) complex dissociation nor by the hydrogen diffusion [3]. The strong dependence of the hydrogen out-diffusion rate upon the nature of the ambient strongly supports this conclusion [4]. However, the surface reaction mechanisms limiting the hydrogen out-diffusion are not fully understood. In this work, we present the results of two complementary approaches. We follow the evolution of the H (D) concentration retained inside a D-passivated Mg-doped GaN as a function of the annealing temperature and we analyze the chemical nature of the species that form outside as a result of the D out-diffusion. These approac
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