ULTRAHIGH-VACUUM ARCJET NITROGEN-SOURCE FOR SELECTED ENERGY EPITAXY OF GROUP-III NITRIDES BY MOLECULAR-BEAM EPITAXY

The key technical challenge in the molecular beam epitaxial (MBE) grow th of group III nitrides is the lack of a suitable source of incorpora table nitrogen. In contrast with the growth of the other III-V compoun d semiconductors by MBE, direct reaction of N-2 with excess group III metal is not feasible, because of the high bond strength of dinitrogen . An incorporatable MBE nitrogen source must excite N-2 forming a beam of atomic nitrogen, active nitrogen (N-2()), or nitrogen ions. rf an d electron cyclotron resonance sources use electron impact excitation to obtain atomic nitrogen and in the process generate a wide variety o f excited ions and neutrals. Experiments have shown that ionic species in the beam degrade the morphology of the epitaxial layer and generat e electrically active defects. Recent theoretical studies have predict ed that ground state atomic nitrogen will successfully incorporate int o the growing GaN surface, while atomic nitrogen in either of the exci ted doublet states will lead to etching. In this article, we report on the development of an ultrahigh vacuum-compatible arcjet source which uses an electric are to thermally dissociate N-2. The thermal excitat ion mechanism offers selective excitation of nitrogen and control of k inetic energy of the active species. This source has been fabricated f rom refractory materials and uses two stages of differential pumping t o minimize the pressure in the growth chamber. The arcjet has been rel iably operated at power levels of 10-300 W, with no visible degradatio n of the thoriated tungsten cathode after 300 h. No metal contaminant lines can be found in the optical emission spectrum. Using an Ar-seede d beam for calibration of the optical spectrum, we find that the arcje t plasma is far from local thermodynamic equilibrium, and show that th e fraction of atomic nitrogen in the beam ranges from 0.3% to 9%. This corresponds to a flux of 0.1-4 monolayers per second at the MBE sampl e location. With an articulated Langmuir probe sampling the beam at th e MBE growth position, we find a positive ion flux of less than 4X10(- 9) A/cm(2), a maximum ion kinetic energy of 3.5 eV, a median electron energy of 1 eV, and a maximum electron energy of less than 4 eV. With increasing arcjet power, the ion and electron fluxes increase and the ion energy distribution shifts to lower energies. No change in the ele ctron spectrum is observed. Quadrupole mass spectra of the ion flux me asured on the arcjet axis show that the N+/N-2(+) ratio has a maximum at an arcjet power of about 35 W. (C) 1998 American Vacuum Society.