Effect of Mg, Zn, Si, and O on the Lattice Constant of Gallium Nitride Thin Films
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"Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA W. Kriegseis and B. K. Meyer
Justus-Liebig-UniversittGiessen, Heinrich-Buff-Ring 16, D-35392 Giessen, Germany ABSTRACT This study analyzes the impact of most common impurities and dopants on the c lattice parameter 3for thin films of Gallium 3 Nitride (GaN) deposited on basal plane sapphire. Both Mg (_1017 cm- ) and Zn (-3 x 1020 cm ) doping were found to expand the c lattice parameter as much as +0.38% and +0.62%, respectively. On the contrary, Oxygen up to concentrations 9 1021 cm 3 is shown to replace N in GaN thin films reducing the c parameter only by a small amount. Incorporation of Si leads to a large decrease of the c parameter which can not be attributed to the different size of Ga and Si atom. It is suggested that doping alters the film stoichiometry by a predicted Fermi level dependence of defect formation energies. The impact of stoichiometry on c lattice parameter and the effect of hydrostatic strain on resistivity in undoped and doped GaN is discussed. INTRODUCTION Gallium nitride (GaN) and related Ill-nitride compounds have generated widespread interest in materials science because of the realization of high brightness blue, green light emitting diodes and blue lasers [1-2]. GaN thin films are typically grown on sapphire substrates using molecular beam epitaxy (MBE), metal organic chemical vapor deposition (MOCVD), or pulsed laser deposition (PLD) [ 1-5]. Buffer layers of AIN and GaN have been introduced to partially take up the lattice mismatch and differences in thermal expansion coefficient to improve the film quality [6-7]. Usually, undoped as-grown GaN thin films are n-type. Controlled n-doping is achieved using Si as a dopant [7]. Mg-doped GaN grown by MOCVD only exhibit p-type conductivity when Mg is activated by low-energy electron-beam irradiation or by thermal annealing under nitrogen atmosphere [8-9]. In case of MBE, Mg-doped GaN is p-type in as-grown layers and does not require further processing [4]. Although pn-junction based electronic and optoelectronic devices are already commercially available, controlled p-doping of GaN is still difficult [1-3, 1012]. This is in part due to a lack of understanding of the mechanisms of deep acceptor incorporation in GaN. Large stress up to 1.2 GPa may be present in GaN thin films because of the large lattice and thermal expansion mismatch between GaN and sapphire [13]. The design of the buffer layers as well as of stoichiometry affect the stress in the thin films. Additionally, the low MBE growth temperature (0.4 Tm where Tm = -2800 K [14]) leads to non-stoichiometric growth of GaN that results in large fluctuations of the lattice parameters [13]. From Table 1 one can see that there is a large covalent radii variation among matrix elements, common impurities, and dopants in GaN [1516]. A similar argument holds for the ionic radii of GaN. Table 1: Covalent radii of Ga, N, Mg, and other impurities. I~ementla4 NiIi i C ll:]M Gel 0 1 Be P1,. Cal All In I As I Sb I' 3•i1 r
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