Original
Paper
phys. stat. sol. (a) 205, No. 12 (2008)
2887
tions. The appearance of rather less dominant transitions of input precursors is required to achieve droplet-free InN
(0.81 eV, 0.87 eV, 1.15 eV) and the structural and electri- film. Similar findings have also been reported by others [8,
cal quality of the InN films have been discussed.
23]. Although, the successful InN growths under compara-
tively low V/III ratios have also been reported [11, 12, 24],
the droplet-free growth of InN under low V/III ratios is not
common to all MOVPE reactor geometries. The use of
pulsed growth mode, however, allowed the growth of me-
tallic droplet free InN films without requiring high V/III
ratio of input precursors. On the other hand, continuous
supply of indium and nitrogen precursors into the reactor
chamber resulted in the In droplets on the surface for the
same temperature range and V/III ratio of input precursors.
In a pulse mode, and for an optimum V/III ratio of 12460,
the effects of growth temperature (510–575 °C) on the PL
transitions have been investigated.
2 Experiments The epitaxy of InN on GaN/sapphire
templates was performed by using the Veeco P-75
MOCVD reactor. TMIn (12.5 µmol/min) and NH
3
(3.5 slm) were used as the group III and V precursors,
respectively, and N was used as the ambient and carrier
2
gase. All the growths of InN layers were conducted at a
growth pressure of 200 Torr. The investigated input ratio
(V/III ratio) of group V and III precursors was 12460, and
the growth temperatures were varied from 510 °C up to
575 °C. The GaN templates (2.5 µm thick) on sapphire
were grown using standard low temperature GaN buffer
layer followed by annealing and high temperature GaN
film. Thin InN films were grown on GaN templates in a
The PL analysis performed both at room temperature
and at 77 K show that the observed luminescence spectra
have strong dependence on the growth temperature of
the films. Figure 2(a), (b), and (c) show low temperature
(77 K) PL spectra of the films grown at 575 °C, 550 °C,
and 510 °C, respectively. It is observed that the appearance
and intensity of different transition change with tem-
perature. The InN films grown at 575 °C have higher lumi-
nescence intensity and narrower FWHM values than
the ones grown at 510 °C. The appearance of other-
wise less dominant transitions are more pronounced at the
room temperature PL spectra of the films as shown in
Fig. 3(a)–(c).
pulse growth mode [10], where NH was constantly flow-
3
ing, while the TMIn was sent into the reactor chamber for
a 36-seconds pulse and then it bypassed the reactor cham-
ber for an 18 s pulse for a total cycle time of
54-seconds. The schematic of the pulsed growth mode is
shown in Fig. 1. This pulsing cycle was repeated for
80 times resulting in a film thickness of ~220 nm.
The optical quality and band gap of InN were deter-
mined from the photoluminescence studies carried out
at room temperature (T = 300 K) and low temperature
(T = 77 K). The electrical properties of the grown films
were obtained from Hall measurements in a Van Der Pauw
method. The atomic force microscopy (AFM) measure-
ments in tapping mode were used to monitor the surface
morphology of the films. The X-ray diffraction (XRD)
measurements, using a PANalytical MRD instrument with
parallel beam geometry and CuKa radiation, were per-
formed to evaluate the presence of secondary phases and
crystallinity of the InN films.
The room temperature PL spectrum of the film grown
at 510 °C exhibits dominant peak transition at ~0.81 eV
(Fig. 3c). Also, the films grown at relatively low growth
temperature are observed to have luminescence features at
~1.15 eV. It is important to note that this transition at
1.15 eV and the weak shoulder appearing at 0.87 eV be-
come almost diminished and weak when the film is grown
at higher temperature (Fig. 3a). Particularly, the PL spectra
for InN films grown at higher temperatures (550–575 °C)
exhibited dominant peak luminescence at 0.77–0.78 eV.
Recently, the PL transition at 0.76 eV has been associated
to Mg related acceptor level [21]. Figure 2(c), and Fig. 3(b)
and (c) also show luminescence shoulders at ~0.75–
0.76 eV to the low energy side of the dominant transitions,
despite the fact no intentional Mg doping of InN films was
carried out in this study. This finding indicates that the ap-
pearance of these less dominant transitions might have
their origin in defects which are incorporated in the films
grown at relatively low temperatures. Further study will be
required to fully understand the nature of optically active
defects and their correlation with the observed less domi-
nant luminescence features.
3 Results and discussion In an earlier publication,
we reported the results of the experimental investigation of
a wide range of temperature and V/III ratio of input pre-
5
cursors [22]. It was found that a high V/III ratio (>3 × 10 )
36 s
18 s
TMIn
NH3
The use of higher growth temperature also improves
the electrical quality of the InN films, as shown is Fig. 4.
The background n-type carrier concentrations of InN films
19
range from 1.39 × 10 cm (n = 2.78 × 10 cm ) up to
–3
14
–2
2D
Time (seconds)
19
–3
14
–2
3.9 × 10 cm (n = 7.8 × 10 cm ), with the lowest
2D
background doping materials obtained at growth tem-
pulsed MOVPE growth of InN alloy.
perature of 575 °C. The electron mobility values for the
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