Characterization of Be-Implanted GaN Annealed at High Temperatures
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Single crystalline (0001) gallium nitride layers were implanted with beryllium and subsequently annealed within the range of 300-1100 oC for 10-60 minutes under a flux of atomic nitrogen obtained using a rf plasma source. The nitrogen flux protected the GaN surface from decomposition in vacuum at high temperatures. SIMS measurements revealed that no long range diffusion of the implanted Be occurred at 900 or 1100 oC. XRD spectra showed defect-related peaks in the as-implanted samples; these peaks disappeared upon annealing at 900 oC and higher for 10 minutes. Photoluminescence (PL) measurements showed one new line at 3.35 eV which provided strong evidence for the presence of optically active Be acceptors. INTRODUCTION
Doping of semiconductors by ion implantation offers advantages in comparison to doping during film growth. (i) The concentration as well as the lateral and depth distributions of the dopants are precisely controllable, and (ii) almost all elements can be implanted with sufficiently high purity. However, this process is compromised by the radiation damage which has to be removed via annealing treatments. In the case of gallium nitride (GaN), this essential annealing procedure for dopant activation is very difficult due to the decomposition of the GaN surface for temperatures above 900 °C [1-3]. Annealing temperatures (TA) of around 1300 °C for >5 minutes are necessary for GaN to fulfil the rule of thumb claiming that implanted semiconductors should be annealed up to 2/3 of the melting point for satisfying electrical activation [3-5]. At this writing, three special annealing procedures for temperatures above 900 °C have been investigated with limited success: (i) Rapid thermal annealing (RTA). During this process GaN is heated up to high temperatures and cooled down within a few seconds. Decomposition is a time dependent diffusion process, and in this small time frame significant decomposition of the implanted layer starts only for temperatures over 1150 °C [6,7]. (ii) N2-overpressure. Annealing under N2-overpressure in the kbar range opposes the surface decomposition, but only a slightly higher temperature limit of 1250 °C can be realized due to the exponential rise of the N vapor pressure as a function of temperature [8]. (iii) Polycrystalline AlN cap. An AlN layer, sputter deposited after the implantation, on top of the GaN layer was used to protect decomposition during annealing. Depending on the crystalline quality of the AlN annealing temperatures of 1300 °C for 30 sec. and good results were reached but only for some selected samples [5,9]. In this article we introduce a new annealing technique which allows the annealing of GaN at a temperature of 1100 °C for at least 1 hour. We have applied this technique to Beryllium implanted GaN, because achievement and control of substantial activation of p-type dopants in GaN remains a critical issue vis a` vis improved performance of devices fabricated in this material. The most commonly used p-type dopant is magnesium (Mg) which substitutes on Ga sites and
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