Talk abstract

Controlled p-type doping is crucial for the development of electronic and optoelectronic devices based on the group III-V nitrides. Up to now Mg has been the commonly used p-type dopant in molecular beam epitaxy (MBE) of both hexagonal and cubic phase GaN and is the only acceptor in GaN that reproducibly generates useful p conduction. Despite this breakthrough the hole concentration in GaN:Mg has remained low (p_max = 6x10^17 cm-3) due to the large Mg- acceptor ionization energy and compensation effects [1]. MBE-growth further showed that Mg is very volatile, and requires low substrate temperatures and N-rich growth conditions. Both conditions are disadvantageous for high quality epilayers, especially for the cubic phase, where N-rich conditions deteriorate the phase purity. In this contribution we studied in detail the incorporation of C in cubic phase GaN and demonstrate the practicability of C as an alternative acceptor for p-type doping. Cubic GaN (c-GaN) were grown by rf-plasma assisted molecular beam epitaxy (MBE) on semi-insulating GaAs (001) substrates [2]. C-doping of the c-GaN was achieved by e-beam evaporation of a graphite rode through adjusting the power between 0 and 400 W. The C-flux was externally calibrated through the growth of C-doped GaAs epilayers assuming a sticking coefficient of one for both GaAs and c-GaN and measuring the hole concentration by Hall-effect measurements at room temperature. In this way a maximum C-concentration of about 10^20 cm-3 was achievable in c-GaN, as confirmed by additional secondary ion mass spectroscopy (SIMS) measurements. At room temperature Hall-effect measurements showed a hole concentration of the c-GaN epilayer of 6x10^17 cm-3 with a mobility of 200 cm^2/Vs. With increasing e-beam evaporation power a new photoluminescence line at 3.08 eV appeared at 2 K. Temperature dependent measurements of the 3.08 eV line showed a thermalization above 100 K, which is typical for a donor acceptor (D0A0) transition. From the spectral energy the binding energy of the C acceptor is estimated to be about E_C = 0.210 eV. Our experiments demonstrate that C indeed introduces a shallow acceptor in c-GaN and that the binding energy of C is about 15 meV lower than that observed for the Mg acceptor in c-GaN [3].

[1] U. Kaufmann, P. Schlotter, H. Obloh, K. Koehler, and M. Maier, Phys. Rev. B 62 (2000) 10867 [2] D. Schikora, M. Hankeln, D.J. As, K. Lischka, T. Litz, A. Waag, T. Buhrow, F. Henneberger, Phys. Rev. B 54 (1996) R8381 [3] D.J. As, T. Simonsmeier, B. Schoettker, T. Frey, D. Schikora, W. Kriegseis, W. Burkhart, and B.K. Meyer, Appl. Phys. Lett. 73 (1998) 1835