Atomic layer deposition of GaN using GaCl3 and NH3

926 Kim, Kim, and Anderson: Atomic layer deposition of GaN using GaCl 3 and NH 3 926
FIG. 5. Color online GIXD patterns for a ALD-GaN and b CVD-GaN
films grown on Si100 at 650 °C.
dependency of film thickness on the number of cycles was
then examined by XRR. The ALD growth conditions were
7 s of GaCl 3 exposure time at 650 °C with the other steps at
the base conditions, while the cycle number was varied in the
range 5–200.
The results are shown in Fig. 3 and indicate that the
growth rate of ALD-GaN gradually increased toward a constant
value of 2.1 Å/cycle below 15 cycles, and this transition
period is termed the transient stage. The growth rate
becomes constant 2.10.2 Å/cycle above 15 cycles, exhibiting
a linear dependency of film thickness on the cycle
number. This is expected to occur because the adsorption
characteristics of GaCl 3 on bare Si or SiO x /Si are different
than those on N terminated GaN. During the first few cycles,
self-limiting adsorption likely does not occur. However, once
the surface is fully covered and presumably a few layers
thick, relaxed GaN becomes the growth surface, which allows
self-limiting adsorption.
During the transient stage, it is possible that the adsorption
of reactants and preferential growth on as-deposited film
compete, leading to three dimensional 3D growth. For example,
studies on ALD of high- materials have reported 3D
growth during the transient stage. 14 To probe this possibility
for ALD-GaN, AFM images of the surface of films deposited
in the transient stage were taken. Figure 4 depicts the surface
morphology of ALD-GaN films grown for 5, 10, and
FIG. 6.Color online Surface morphology of the 60-nm-thick GaN films
deposited at 650 °C under a ALD growth condition and b CVD growth
condition.
15 cycles. Figures 4a and 4b show that the surface roughness,
R q root-mean-square roughness is relatively high for
films grown with 5 and 10 cycles R q =0.84 nm for 5 cycles,
R q =0.72 nm for 10 cycles, consistent with the 3D growth.
The surface of the sample grown for 15 cycles Fig. 4c,
however, is substantially smoother R q =0.55 nm. Note that
the linear region begins at 15 cycles and is considered the
point at which complete coverage of GaN on Si substrate is
obtained. This conclusion is supported by noting that the
surface roughness of the sample grown for 150 cycles Fig.
4d is close to that of 15 cycles R q =0.58 nm.
D. Film microstructure
Given the differences in reaction time and delivery sequence,
differences in microstructure between ALD and
CVD films might be expected. To explore this possibility,
GIXD patterns of GaN film deposited in ALD mode were
compared to those deposited outside the ALD process window
at higher deposition rate and those deposited in CVD
mode. GIXD patterns for all ALD GaN films are presented in
Fig. 5a that reveals the change in the crystalline orientation
for two different GaCl 3 exposure times. The lower GIXD
pattern in Fig. 5a shows the result for the film grown in
J. Vac. Sci. Technol. A, Vol. 27, No. 4, Jul/Aug 2009