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Improved GaN MBE-Growth using Bismuth as a Surfactant


E.O. Lawrence Berkeley National Laboratory




For the first time, using flexible MBE technology, Mike Rubin, Eicke Weber and colleagues at Berkeley Lab has grown large, two-dimensional GaN and other crystals of group III-N compounds at low temperatures. The lower temperature of film formation using MBE versus MOCVD allows higher concentrations of dopant to be incorporated into the final structure, as well as making it easier to manipulate the concentration of component parts of the final film.

Usually, group III-N films are grown at temperatures that are low compared with the melting point to prevent chemical decomposition during the growth process. MOCVD, one of the two primary methods used to grow GaN films, results in larger grain sizes, which is desirable and exploited by industry. However, because of the high temperatures at which MOCVD films are formed, suitable metal organic precursors are limited to create and dope the desired film. In addition, formation temperatures are close enough to thermal equilibrium to limit variation in the amounts of various materials that make up the final film. This Berkeley Lab innovation using MBE provides an alternative to these MOCVD-related shortcomings.

Prior to these advances, a drawback to MBE films had been that they frequently exhibited a three-dimensional, instead of two-dimensional, crystal growth and resulted in a thin film composed of small but oriented grains. Eicke Weber and colleagues have addressed this issue through a process designed to stimulate two-dimensional GaN film growth at low temperatures by MBE. These new, low-temperature films are grown using bismuth as a surfactant. This results in crystalline films containing a plurality of two-dimensional GaN crystal grains, of which each crystal lattice structure coalesces between the grains to form a continuous crystal, with a concentration of p-type carriers that exceeds that currently possible by MOCVD. Using this new method, the surface diffusion coefficient can be varied by more than 4 orders of magnitude at a growth temperature as low as 725K.




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