NH 3 as Nitrogen Source in MBE growth of GaN
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highly reproducible, unsophisticated and does not require an expensive plasma source. EXPERIMENT AND RESULTS For our investigations we use an almost standard MBE system (Riber 32) adapted to group V gas sources. The system is turbo pumped, the attached gas control and handling system is home made. NH3 is introduced into the system through a standard high temperature injector (Riber HTI 432). Effusion cells are used to supply the group IlI species Ga and Al. Unless otherwise mentioned GaN layers are grown at growth rates of 700 nm/h to a thickness of approximately 2 pm. By increasing the growth rate to 1.2 pm/h comparable crystal and optical properties are achieved. Characterization of the grown structures is carried out by optical and scanning electron microscopy (SEM), atomic force microscopy (AFM), X-ray diffraction (XRD), photoluminescence (PL) and cathodoluminescence (CL). Electrical data are obtained from Hall and CV measurements. A quadrupole mass analyzer (QMA) is used for detection of background gas species and for the NH 3 cracking studies.
135 Mat. Res. Soc. Symp. Proc. Vol. 395 in 19 9 6 Materials Research Society
To evaluate the suitability of NH 3 as nitrogen source for MBE we first investigate the thermal cracking using a high temperature gas injector. Figure 1. reveals that the 4.8 eV binding energy of NH 3 despite being significant lower than that of N2 still requires cracking temperatures above 6001C for thermal cracking (pyrolysis).
350 .1
~
250
0
111
150 5o 500 7WO Injector temp. [pC] 9 0e
1100
Figure 1: Concentration of NH 3 , N2 and NH 2 at the QMA as a function of injector temperatui A more detailed study of the ammonia cracking was recently published by our group [4]. Therein thermodynamic equilibrium data are presented and possible dissociation processes on the surfaces are discussed. In quintessence, we assume that the dissociation steps on the surface can be described according to [5] : NH3 , T N#H.d
N*H 36 d # NH 2.d + H.d NH 2,d + H 4 #- NHd + 2Hd NH6 d + 2H.4 * Ng + 3H.d
2H64d # H2, 2N.d # N 2, (g -A gaseous , ad - adsorpt)
(1) (2) (3)
(4) (5) (6)
with a decomposition rate (for T > 5500C) given by
K1K2[NH3] K-1 +
K2
Clearly, cracking is described by the overall reaction 3 1 NH 3 -- + -N 2 + -H 2 s2 which is verified by our QMA data (fig.1).
136
(7)
We propose that applying OSC of ammonia on a GaN surface the NH 3 dissociation undergoes reactions similar to steps (1) to (4), but that at least one nitrogen species can chemically react with Ga atoms on the surface before being desorbed as N2 . Therefore OSC of NH 3 succeeds whereas cracking in the injector fails. Applying OSC for GaN growth we found that growth can be accomplished at temperatures as low as approx. 5501C. This is in reasonable agreement with the temperatures we found to be necessary for ammonia cracking (fig.1). However, the best GaN crystal properties were achieved at substrate temperatures around 8000 C using a beam equivalent NH 3 pressure of approx. 10-r torr and a V/IlI ratio of approx. 20. To pr
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