P- and N-Type Implantation Doping of GaN with Ca and O
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Mat. Res. Soc. Symp. Proc. Vol. 423 * 1996 Materials Research Society
A second goal of this work is to examine alternative n-type dopants in GaN. N-type doping of GaN during growth [10,11] or by ion implantation [12] has primarily been done using Si which is reported to have a donor ionization level between -25 and -60 meV [ 11,13]. O is of particular interest as a possible alternative n-type dopant due to its position next to N in the periodic table and its suspected role as a background impurity in as-grown GaN [14]. In fact, studies of 0 introduction in GaN during growth have shown 0 to act as a donor [14,15]; however, we are not aware of any reports prior to this work on the electrical characterization of O-implanted GaN or on the redistribution properties of 0 at high temperatures. We report here on the ion implantation and electrical activation of Ca as an acceptor and 0 as a donor in GaN. Variable temperature Hall measurements were used to estimate the ionization levels for both dopants. The thermal stability of both species is assessed using secondary ion mass spectrometry (SIMS) profiles of as-implanted and annealed samples. We also reported the first III-N device fabricated with ion implantation doping - a GaN junction field effect transistor. EXPERIMENTAL The GaN layers used in the implant doping experiments were 1.5 to 2.0 jim thick grown on c-plane sapphire substrates by metalorganic chemical vapor deposition (MOCVD) in a multiwafer rotating disk reactor at 1040 'C with a -20 nm GaN buffer layer grown at 530 'C [16]. The GaN layers were unintentionally doped, with background n-type carrier concentrations < 5x10 1 cm 3 . 4"Ca or 160 ions were implanted at a dose of 5x1014 cm-2 at energies of 180 or 70 keV, respectively, to place the ion peak roughly 100 nm from the surface. One Ca-sample was also implanted with 31p (130 keV, 5xl0' cm-) to study the effect of co-implantation, which has been shown to be required to achieve p-type conduction for Mg-implantation in GaN [12]. SIMS samples were prepared with the same implant conditions except `80 isotopes were used to reduced interference problems with background 160 and improve the sensitivity of the SIMS analysis for 0. Samples were annealed for 10 to 15 s in flowing N2 in a SiC coated graphite susceptor between 900 and 1150 'C to study the electrical activation and redistribution of the dopant species. RESULTS AND DISCUSSION Figure 1 is the electrical activation data for Ca-implanted GaN with and without the P coimplantation [17]. An unimplanted and annealed sample is included for comparison. Both the Ca-only and the Ca+P samples convert from n-type to p-type after a 1100 'C anneal. This is slightly higher than the temperature required to achieve p-type conduction in Mg+P implanted GaN [12] and may be the result of more implantation induced damage associated with the heavier Ca-ion. The acceptor activity is seen to continue to increase after annealing at 1150 'C with the P co-implanted sample having a 43% lower sheet resistance and a correspondingly
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