Zirconium Mediated Hydrogen Outdiffusion from p-GaN
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Institute of Electron Technology, Al.Lotnikow 46, Warsaw, Poland, [email protected]. Institute of Experimental Physics, Warsaw University, Warsaw, Poland 3 Institute of Physics PAS, Warsaw, Poland, 4 Department of Material Science and Eng., NCSU, Raleigh, NC 27695-7907, USA 5 SMPCE, Macquarie University, Sydney NSW, Australia 2
ABSTRACT We have shown that Zr-based metallization can effectively remove hydrogen from the p-type GaN subsurface, which eventually leads to the formation of an ohmic contact. As the release of hydrogen starts at ~9000C, the thermal stability of the contact system is of particular importance. The remarkable thermal behavior of the ZrN/ZrB2 metallization is associated to the microstructure of each individual Zr-based compound, as well as to the interfacial crystalline accommodation. INTRODUCTION The role of hydrogen in the passivation and activation of dopants in MOCVD grown GaN is commonly recognized, however, experimental knowledge on its behavior remains still incomplete [1]. Recent interest in GaN-based devices has stimulated studies on p-type doping issues [2, 3] as well as the search for novel ohmic contacts. In particular, Murakami [4] recommended metals with high binding energies to hydrogen for making ohmic contacts to p-type GaN, and this idea has been experimentally verified with the use of Ta/Ti contacts [5]. These contacts provided the lowest reported contact resistivity values (3x10-5Ωcm2 for hole concentration of 7x1017cm-3), but they quickly degraded. The accumulation of hydrogen in areas of high defect density has been considered [6], it is therefore very likely that hydrogen could be gettered in the superficial layer of p-GaN. In consequence, the subsurface volume may be characterized by a free carrier concentration lower than the bulk. The present study explores the problem of the accumulation of hydrogen in the near-surface region of p-GaN and the possibility of its removal by using ZrN/ZrB2 metallization and annealing. Zirconium possesses one of the highest absorptive capabilities for hydrogen of metal hydride systems, while ZrN and ZrB2 are distinguished for their exceptional low resistivities and high melting points. The content of hydrogen was quantified using secondary ion mass spectrometry (SIMS), while the stability of Zr-based caps on GaN under annealing has been examined using transmission electron microscopy (TEM) together with SIMS. EXPERIMENTAL PROCEDURE The samples used in this study were (0001) oriented, Mg-doped GaN epilayers, 0.5-2 µm thick, grown via MOCVD on undoped GaN, with AlN buffer films predeposited on 6H-SiC or sapphire substrates. The hole concentration was
F99W10.9
p = 1-5∗1017cm-3. Prior to insertion into the deposition chamber, the surface of GaN was etched in buffered HF, and dipped in NH4OH:H2O (1:10). Thin films of Zr-N and Zr-B were deposited by DC magnetron sputtering in Ar discharge, from ZrN(99.5%) and ZrB2 (99.5%) targets, respectively. The process parameters were first optimized with regard to the stoichiometric composition of depo
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