Flow modulation epitaxy of indium gallium nitride

Journal of Electronic Materials, Oct 1997 by Keller, S, Mishra, U K, DenBaars, S P

The In-composition of the FME grown films was generally lower than the In-composition of layers grown under the same conditions in the conventional growth mode. Thus, the In-incorporation efficiency decreased with increasing ammonia flush time, decreasing ammonia flow during group-III injection and decreasing group-III injection time. All these results point toward the In-incorporation being desorption limited. The indium desorption can be suppressed by reducing the growth temperature, increasing the partial pressure of ammonia and "trapping", as in the case of long group-III injection times or higher growth rates. 7 Besides lowering the indium incorporation efficiency, low ammonia flows cause the formation of deep levels, in the same way as low growth temperatures due to the reduced ammonia decomposition efficiency. The nature of the deep levels is not well understood yet, but carbon related defects and/or nitrogen vacancies are likely recombination centers. All discussed factors prevent one from taking full advantage of the FME growth mode and make a careful process optimization necessary.

Consideringthe surface morphology, the FME grown samples were generally rougher than those grown by conventional MOCVD. Obviously, the high surface coverage of N-H species established during the ammonia flush time and the lower growth temperature reduce the surface mobility of Ga-(CH^sub 3^)^sub x^ and In(CH^sub 3^)^sub x^ species and cause the formation of a high number of small islands. This hypothesis is supported by the result, that, for instance, the increase of the group-III precursor injection time from 3 to 10 s and the increase of x^sub In^ from 0.26 to 0.3 (samples according to Fig. 3) did not show any remarkable effect on the island size. Just the surface roughness increased from a RMS value of 5.2 to 11 nm. Otherwise, higher surface mobilities as under the conditions of a low ammonia flow during group-III injection or higher growth temperatures as in the case of conventional MOCVD supported agglomeration of the small islands and formation of big islands, respectively. Further investigations are necessary, to determine the critical thickness of the island formation and to evaluate possibilities to grow smooth thin layers for quantum structures.

CONCLUSIONS

The indium incorporation efficiency for growth performed in the FME mode decreased with increasing ammonia flush time, decreasing ammonia flow during group-III injection, and decreasing group-III precursor injection time. Under optimized conditions, InGaN films up to an indium mole fraction of x^sub In^ = 0.29 showing intense band edge related luminescence at room temperature could be obtained. The surface roughness of the films grown by FME was generally higher than those of films of comparable thickness grown by MOCVD.

ACKNOWLEDGMENT

The authors wish to acknowledge the support of the Defense Advanced Research Projects Agency (Anis Husain) grant No. N00014-96-1-0738 and the U.S. Army Research Office (John Zavada) grant No. DAAH04-95-1-0329.