Lin and co-workers2 have observed enhancement of the
rate of deposition of InN produced by CVD at 700 K with
HN3 and In͑C2H5)3 when the surface is irradiated with the
output of a pulsed XeCl laser at 308 nm, and attribute this
result to the creation of free N atoms in the surface layer.13
John and co-workers14 have reported the photochemically
enhanced CVD of GaN on sapphire when mixtures of
Ga͑CH3)3 and NH3 were irradiated with either an ArF laser
͑193 nm͒ or a low pressure Xe lamp, at a substrate tempera-
ture of 700 K. The mechanism is unknown in this case, but
appears to involve fragmentation of both precursors by ab-
sorption at wavelengths below 250 nm. Both of these mecha-
nisms appear to be quite different from that found in the
present experiments.
atmosphere. As shown, the absorbance of the complexed N2
declines sharply, indicating that it is removed by the gentle
heating. The major species produced has the sharp absor-
bance shown at 552 cmϪ1. This feature is in excellent agree-
ment with the frequency of the E1 ͑TO͒ phonon mode in
GaN, and compares well with spectra of GaN films pub-
lished in the literature.4
A number of experiments were performed in which sev-
eral HN3 /Ga͑CH3)3 mixtures were irradiated in succession
over fused silica substrates, with the UV radiation directed
through the substrate rather than through the cell wall. This
treatment produced very uniform, strongly adherent green
films. Measurement of UV/vis absorption spectra indicated
an absorption onset near 400 nm, and film thicknesses of
200–400 nm were determined from oscillations in the UV
spectra and measurements with a stylus scanner. Analysis
with a scanning electron microscopy ͑SEM͒/energy disper-
sive x-ray analysis ͑EDAX͒ indicated very uniform topogra-
phy ͑no granularity evident at magnifications up to
100 000x͒ and the presence of Ga, as expected.
This work was supported by the Air Force Office of
Scientific Research under Grant No. F49620-97-1-0036. We
are grateful to Thomas Ely of the University of Denver and
to Dr. Richard Matson of the National Renewable Energy
laboratory for help with the electron microscopy.
It is clear that the thermal reaction between Ga͑CH3)3
and HN3 is very slow at room temperature, and occurs only
with Ga͑CH3)x species on the walls of the reaction vessel.
The data suggest that this thermal process proceeds in a man-
ner analogous to the reactions of HN3 with BCl3 and
Al͑CH3)3 to generate azide substituents on the metal, with
the release of methane. The large acceleration of the reaction
under the influence of UV radiation does not appear to in-
volve the production of azides. The NH(a1⌬) produced by
photodissociation of HN3 is isoelectronic to O(1D) and
1 R. Ishihara, O. Sigiura, and M. Matsumura, Appl. Phys. Lett. 60, 3244
͑1992͒.
2 Y. Bu, L. Ma, and M. C. Lin, J. Vac. Sci. Technol. A 11, 2931 ͑1993͒.
3 Y. Bu, M. C. Lin, L. P. Fu, D. G. Chtchekine, G. D. Gilliland, Y. Chen, S.
E. Ralph, and S. R. Stock, Appl. Phys. Lett. 66, 2433 ͑1995͒.
4 D. G. Chtchekine, L. P. Fu, G. D. Gilliland, Y. Chen, S. E. Ralph, K. K.
Bajaj, Y. Bu, M. C. Lin, F. T. Bacalzo, and S. R. Stock, J. Appl. Phys. 81,
2197 ͑1997͒.
5 R. L. Mulinax, G. S. Okin, and R. D. Coombe, J. Phys. Chem. 99, 6294
͑1995͒.
6 C. J. Linnen and R. D. Coombe, J. Phys. Chem. B101, 1602 ͑1997͒.
7 A. P. Baronavski, R. G. Miller, and J. R. McDonald, Chem. Phys. 30, 119
͑1978͒.
CH (1A ), and is well known11 to participate in electrophilic
Ј
2
8 U. Mazur and A. Cleary, J. Phys. Chem. 94, 189 ͑1990͒; X. D. Wang, K.
W. Hipps, and U. Mazur, J. Chem. Phys. 96, 8485 ͑1992͒.
9 J. M. Frisch, G. W. Trucks, M. Head-Gordon, P. M. Gill, M. W. Wong, J.
B. Foresman, B. G. Johnson, H. B. Schlegel, M. A. Robb, E. S. Repogle,
R. Gomperts, J. L. Andres, K. Rachavachari, J. S. Binkley, C. Gonzalez,
R. L. Martin, D. J. Fox, D. J. DeFrees, J. Baker, J. J. Stewart, and J. A.
Pople, GAUSSIAN 94, Gaussian, Inc., Pittsburgh, PA, 1994.
10 T. Ibuki, A. Hiraya, K. Shobatake, Y. Matsumi, and M. Kawasaki, Chem.
Phys. Lett. 160, 152 ͑1989͒.
insertion reactions as do these species. In the present case,
we believe that NH(a1⌬) inserts into the Ga–CH3 bond of
the surface-bound Ga͑CH3)x .The transient Ga–NH–CH3 in-
termediate thus formed would rapidly eliminate CH4 to leave
surface-bound GaN, as indicated by the IR spectra ͑Fig. 4͒.
In the gas phase, the NH(a1⌬) produced by photodissocia-
tion can react rapidly12 with the parent HN3 to produce NH2
and N2. The NH2 thus produced can also react with HN3 to
generate NH3. The broad IR feature centered near 3500
cmϪ1 in the spectrum of the film before warming ͑Fig. 3͒ is
attributable to the incorporation of NH3 produced in this
manner into the film.
11 See, for example, S. M. Singleton and R. D. Coombe, J. Phys. Chem. 99,
16296 ͑1995͒.
12 J. R. McDonald, R. G. Miller, and A. P. Baronavski, Chem. Phys. 30, 113
͑1978͒.
13 Y. Bu and M. C. Lin, Surf. Sci. 301, 118 ͑1994͒.
14 P. C. John, J. J. Alwan, and J. G. Eden, Thin Solid Films 218, 75 ͑1992͒.
90 Appl. Phys. Lett., Vol. 72, No. 1, 5 January 1998 C. J. Linnen and R. D. Coombe
On: Mon, 22 Dec 2014 20:34:54