Characterization of Be-Implanted GaN Annealed at High Temperatures
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this material. The most commonly used p-type dopant is magnesium (Mg) which substitutes on Ga sites and has an ionization energy of - 0.25 eV. One-to-two orders of magnitude higher atomic concentration of Mg must be incorporated into GaN to achieve the desired hole concentration at room temperature [10]. This incorporation reduces the hole mobility due to the enhanced carrier-impurity scattering processes [11]. Beryllium (Be) is a more promising candidate for p-type doping given its measured lower ionization energy of - 0.15 eV [12,13]. EXPERIMENTAL One-to-two ýtm thick epitaxial, monocrystalline and undoped GaN films were grown on on-axis n-type, Si-face cx(6H)-SiC(0001) substrates at 1000 "C and 45 Torr using a vertical, coldwall, RF inductively heated MOVPE deposition system [11 ]. A 0.1 jim high-temperature monocrystalline (1100 "C) AIN-buffer layer was deposited prior to the GaN growth. Deposition was performed using triethylaluminum (TEA) and triethylgallium (TEG) in combination with 1.5 SLM of ammonia (NH3) and 3 SLM of H, diluent. Beryllium was twice implanted at 100 keV and 200 keV to create a broad depth distribution of this element. The dose ratio between the two implantation energies was 2/3 to adjust the maximum impurity concentration of both implantations. TRIM simulations gave a mean ion range of 276 nm (FWHM = 175 nm) and 472 nm (FWHM = 1060 nm), respectively, for the two energies [14]. The total implantation dose ranged between 10'" cm 2 and 2.5x 101 cm-2. All implanted samples were first sequentially annealed in a tube furnace under vacuum at 300, 600 and 900 "C for 10 minutes. The samples were transferred into a MBE chamber for annealing at higher temperatures. Heating of the samples was performed under a flux of atomic nitrogen obtained using a rf plasma source (300 W, Model RF 4.5, SVT Associates). The atomic nitrogen flux protected the GaN surface at 1100 'C for at least 1 hour using a distance between sample and rf source of around 40 cm. Secondary ion mass spectroscopy (SIMS) was performed by the Analytical Instrumentation Facility at NC State University using a Cameca IMS-6f. Samples were analyzed with a 200 nA, 10 keV 0-primary beam. Two dimensional co-20 X-ray diffraction (XRD) pattern were measured using a Philips X'Pert-MRD system with a resolution of 0.001'. Photoluminescence measurements (PL) were performed after each annealing step at 14 K by exciting the GaN 0 " samples with a He-Cd laser (3.81 eV). Hall as implanted 900 C 17T measurements were done after the deposition of Ni(50 nm)-Au(100nm) contacts.
RESULTS AND DISCUSSION
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Figure 1 shows SIMS-profiles of implanted Be with a dose of 10" cm-2 before
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and after annealing at 900 'C and 1100 'C for
visible and in good agreement with TRIM
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i with 100and 200 key Dose:w10"cm
15 minutes. Both implantation profiles of the 100 keV and 200 keV implantation are clearly
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calculations [14]. Within the experimental resolution, no long range diffusion was Depth (jm
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