Hydrogen in GaN

Determination of the local vibrational modes (LVM) of the Mg-H complex in GaN provides satisfying confirmation of the significance of hydrogen in GaN and useful information, when compared with theory, on the structure of the complex. At the present time the stretch frequency is in fact the only reliably established physical parameter available from experiment for the Mg-H complex in GaN. 

Jorg Neugebauer and Chris G. Van de Walle, Theory of Hydrogen in GaN... 

R. J. Molmr, Hydride Vapor Phase Epitaxial Growth of TTI-V Nitrides T. D. Moustakas, Growth of III-V Nitrides by Molecular Beam Epitaxy Z. Liliental-Weber, Defects in Bulk GaN and Homoepitaxial Layers C G. Van tie Walk and N. M. Johnson, Hydrogen in III-V Nitrides... 

A8.7 Yellow luminescence in GaN A8.8 Hydrogen and acceptor compensation in GaN A8.9 3d transition metals in GaN and related compounds A8.10 Er-doped GaN and AIN... 

The electron effective mass in GaN is now reasonably well established by cyclotron resonance measurements [14-16] asm, = (0.22 0.0 l)m, and the low frequency dielectric constant (appropriately averaged spatially) e(0) = 9.5 0.2, from infrared refractive index and optic phonon energy measurements [17]. We can therefore derive a reliable value for the hydrogenic donor ionisation energy of EDH = (33.0 1.5) meV which compares well with IR absorption measurements, giving Ed = (35 1) meV [18] (see below). The discrepancy is readily explained in terms of a small chemical shift. 

FIGURE 1 Plot of measured acceptor energies EA versus (AX)2 in GaN, from Podor [15], AX is the difference between the acceptor electronegativity and that of the Ga atom which it replaces. The extrapolation to AX = 0 yields a value of approximately 100 meV for the hydrogenic value Eah. Note that, on this basis, Be is expected to show an acceptor energy of about 100 meV, a value which has been observed experimentally [19]. 

FIGURE 1 also shows the formation energy of H2 molecules in GaN. It is clear that H2 is unstable with respect to dissociation into monatomic hydrogen. 

The behaviour of isolated interstitial hydrogen, as discussed in Section B, provides crucial information about interaction with impurities. Since both the solubility and the diffusivity of hydrogen in n-type GaN are low, hydrogen-donor complexes will rarely form, and we focus on complexes with acceptors. 

FIGURE 2 Schematic illustration of the structure of an Mg-H complex in GaN. Hydrogen resides in an antibonding position behind a nitrogen atom. 

FIGURE 3 Formation energy as a function of Fermi level for Mg, nitrogen vacancies, interstitial H and hydrogen-vacancy complexes in GaN [1,14],... 

Hydrogen plays an important role in III-V semiconductors especially in the nitrides, passivating the electrical activity of shallow and deep level impurities. This is more important in GaN grown by metal-organic vapour phase epitaxy (MOVPE) than by molecular beam epitaxy (MBE). Using SIMS... 

Lee et al reported the implantation of calcium in GaN to produce p-type material [4,14], A Ca acceptor level with an activation energy of 170 meV was measured. Hydrogen passivation of the Ca acceptors, forming electrically inactive Ca-H complexes, was similar to that obtained in MOVPE grown Mg-doped GaN [14], The implanted Ca did not show measurable redistribution for RTA at 1125°C, and an upper limit of diffiisivity of 3 x 10 13 cm2/s was deduced in GaN [4],... 

---