Epitaxial growth of inn films by molecular-beam epitaxy using hydrazoic acid (HN3) as an efficient nitrogen source

Article abstract of DOI:10.1021/jp068949g

Epitaxial InN films have been successfully grown on c-plane GaN template by gas-source molecular-beam epitaxy with hydrazoic acid (HN₃) as an efficient nitrogen source. Results in residual-gas analyzer show that the HN₃ is highly dissociated to produce nitrogen radicals and can be controlled in the amounts of active nitrogen species by tuning HN₃ pressure. A flat and high-purity InN epifilm has been realized at the temperature near 550 °C, and a growth rate of 200 nm/hr is also achieved. Moreover, the epitaxial relationship of the InN(002) on the GaN(002) is reflected in the X-ray diffraction, and the full-width at half-maximum of the InN(002) peak as narrow as 0.05° is related to a high-quality crystallinity. An infrared photoluminescence (PL) emission peak at 0.705 eV and the integrated intensity increasing linearly with excitation power suggest that the observed PL can be attributed to a free-to-bound recombination.

Full text of DOI:10.1021/jp068949g

J. Phys. Chem. A 2007, 111, 6755-6759
6755
Epitaxial Growth of InN Films by Molecular-Beam Epitaxy Using Hydrazoic Acid (HN3) as
an Efficient Nitrogen Source†
‡
§
§
|
⊥
#
§
J. T. Chen, C. L. Hsiao, H. C. Hsu, C. T. Wu, C. L. Yeh, P. C. Wei, L. C. Chen, and
K. H. Chen*,
§,+
Graduate Institute in Electro-Optical Engineering, Tatung UniVersity, Taipei104, Taiwan, Center for
Condensed Matter Sciences, National Taiwan UniVersity, Taipei 106, Taiwan, Materials Science and
Engineering, National Taiwan UniVersity, Taipei106, Taiwan, Graduate Institute of Materials Science and
Technology, National Taiwan UniVersity of Science and Technology, Taipei 106, Taiwan, Department of
Materials Science and Engineering, National Tsing Hua UniVersity, Hsinchu 300, Taiwan, and Institute of
Atomic and Molecular Sciences, Academia Sinica, Taipei 106, Taiwan
ReceiVed: December 27, 2006; In Final Form: March 26, 2007
Epitaxial InN films have been successfully grown on c-plane GaN template by gas-source molecular-beam
epitaxy with hydrazoic acid (HN
the HN is highly dissociated to produce nitrogen radicals and can be controlled in the amounts of active
nitrogen species by tuning HN pressure. A flat and high-purity InN epifilm has been realized at the temperature
3
) as an efficient nitrogen source. Results in residual-gas analyzer show that
3
3
near 550 °C, and a growth rate of 200 nm/hr is also achieved. Moreover, the epitaxial relationship of the
InN(002) on the GaN(002) is reflected in the X-ray diffraction, and the full-width at half-maximum of the
InN(002) peak as narrow as 0.05° is related to a high-quality crystallinity. An infrared photoluminescence
(PL) emission peak at 0.705 eV and the integrated intensity increasing linearly with excitation power suggest
that the observed PL can be attributed to a free-to-bound recombination.
Introduction
grown by other techniques.2,3 Generally, N2-plasma source and
ammonia are two common nitrogen sources used in the MBE
Indium nitride (InN) is considered as a very attractive material
for future photonic and electronic devices owing to its outstand-
ing properties such as smallest electron effective mass, largest
mobility, highest peak and saturation velocities, and smallest
13,14
technique.
However, the N2-plasma source needs a high cost
plasma generator system, and ammonia shows a poor efficiency
of producing atomic nitrogen. To meet the efficient cost and to
obtain a large amount of atomic nitrogen, a high-performance
nitrogen source, hydrozoic acid (HN3), is considered to be the
nitrogen source for growing high-quality InN by gas-source
molecular-beam epitaxy (GSMBE). Although the HN3 source
has been used to grow nitride semiconductors by CVD growth
1
-7
direct band gap among the nitride semiconductors.
These
excellent properties suggest that there may be distinct advantages
offered by using InN in high-frequency centimeter and mil-
limeter wave devices. Besides, the InGaN quantum wells are
indispensable for light emitting devices because incorporation
of small concentrations of indium in the active GaN layer
increases luminescence efficiency considerably. Hence, the use
of InN and its alloys with GaN and AlN makes it possible to
extend the emission spectrum of nitride-based light-emitting
diodes (LEDs) and laser diodes (LDs) from ultraviolet to near-
15,16
technique and shows good crystalline quality,
there are rare
reports on the use of HN3 to grow GaN thin film in MBE
1
7
technique. Unfortunately, the growth of epitaxial InN film is
much more difficult than that of other nitride semiconductors
because of its extremely low decomposition temperature of 550
3
infrared region.8
-13
°C. Therefore, the successful demonstration in the growth of
InN at low temperature is important because this source can be
possible to replace the use of ammonia (NH3) in the GSMBE
technique. In addition, a comparable growth rate of 200 nm/hr
to an N2-plasma source shows a potential to be a useful nitrogen
source for the growth of nitride semiconductors in general. In
this work, we present the growth of InN films at different
temperatures by MBE technique using hydrazoic acid (HN3)
as an efficiency nitrogen source. No extra thermal cracker was
used even when the growth temperature was as low as 250 °C.
The chemical properties of HN3 were characterized by
temperature-dependent and pressure-dependent mass spec-
troscopy. Crystalline structure of InN films was investigated
by in-situ reflection high-energy electron diffraction (RHEED)
as well as ex-situ field-emission scanning electron microscopy
However, obtaining a high quality InN is difficult due to
thermal instability of InN and large lattice mismatch between
InN and common substrates.1-4 Many fabrication methods,
which including molecular-beam epitaxy (MBE), chemical vapor
deposition (CVD), sputtering, and so on, have been used to
obtain high quality InN epitaxial films.1
-4,10-12
Among them,
thin films grown by the MBE technique show better crystalline
quality, higher mobility, and lower carrier density than that
†
Part of the special issue “M. C. Lin Festschrift”.
*
To whom correspondence should be addressed. P.O. Box 23-166, Taipei
1
2
0617, Taiwan; E-mail: chenkh@pub.iams.sinica.edu.tw, Phone: 886-2-
366-8232, Fax: 886-2-2362-0200.
‡
§
|
Tatung University.
Center for Condensed Matter Sciences, National Taiwan University.
Materials Science and Engineering, National Taiwan University.
National Taiwan University of Science and Technology.
National Tsing Hua University.
(
FESEM) and X-ray diffraction (XRD) analyses. Power-
⊥
#
+
dependent photoluminescence (PL) spectroscopy was used to
investigate the optical property of InN films.
Academia Sinica.
1
0.1021/jp068949g CCC: $37.00 © 2007 American Chemical Society
Published on Web 05/15/2007

6756 J. Phys. Chem. A, Vol. 111, No. 29, 2007
Chen et al.
Figure 2. (A) Cross sectional FESEM image with tiled 15° of InN
epitaxial film grown on the GaN template at 550 °C. (B) EDX analysis
of the InN layer.
of H2O from the reactor. The prepurified HN3 gas was further
dried by two U-shaped traps maintained at dry ice temperature
before being condensed at a liquid-nitrogen U-trap. The system
was subjected to continuing diffusion pumping to avoid the
buildup of non-condensable gases such as N2. The solid HN3
collected at the liquid N2-trap, after sealing off with two high-
vacuum stopcocks, was allowed to slowly heat up to room
temperature and liquefy overnight. The clear liquid HN3 (which
is safe to handle) was then stored at dry ice temperature for a
long term use. The vapor from the liquid at dry ice temperature,
about 0.7 Torr, is sufficient for chemical-beam epitaxy (CBE)
or MBE applications. It should be noted that the entire HN3
generator is placed inside a hood with a protective plastic sliding
door. Solid HN3 should be handled with extreme care as it is
known to explode during the melting process. Therefore, despite
its advantages over other nitrogen sources, the inexperienced
user of HN3 is advised to contact the authors for precautions.
Figure 1. (A) RGA result of the gaseous products of thermal
dissociation of hydrazoic acid at room temperature. (B) Dependence
of amounts of related fragments and substrate temperature. (C)
Dependence of amounts of HN
pressure. The amounts of activated nitrogen species of HN
a linearly controllable behavior with HN
temperature.
3
-related fragments and HN
exhibited
pressure at a given substrate
3
partial
3
3
Experimental Procedures
InN thin films were grown on the GaN template using a
homemade GSMBE system, which consists of loading and
growth chambers. Residual-gas analyzer (RGA), reflection high-
energy electron diffraction (RHEED), and beam-flux monitor
Prior to InN growth, the GaN template was cleaned by
acetone, methanol, and D.I. water with ultrasonic bath to remove
residual organic contaminations on substrate surface, and further
chemically cleaned by dipping a dilute HCl solution for 5 min
to remove surface native oxides. After the chemical cleaning
process, the GaN template was annealed at 800 °C for 30 min
in the growth chamber before the growth of InN. After cooling
down to 200 °C, appearance of GaN surface reconstruction
RHEED patterns ensured a perfect surface condition.18 After-
ward, various growth conditions were applied to obtain an
epitaxial InN film on the GaN template, which included tuning
V/III ratio and substrate temperature. Usually, InN films were
deposited for 1-10 h and the growth pressure was controlled
(
BFM) were equipped in the growth chamber. The base pressure
-
9
of the growth chamber was pumped to ∼1 × 10 Torr using
a combination of a mechanical pump and a turbomolecular
pump. The indium source was evaporated and ejected from a
conventional Knudsen cell (K-cell) while the nitrogen source
was supplied from highly volatile HN3 precursor during the
GSMBE process.1 HN3 was prepared in vacuum by acidifica-
tion of an appropriate amount of NaN3 at room temperature in
a pre-evacuated round-bottom Pyrex flask by slowly dripping
5,16
5
0-60% H3PO4 solution using a funnel attached to a stopcock.
-
7
-5
The HN3 gas generated from the reaction was first passing
through a large, dry ice cooled trap to remove a large amount
in the range of 10 -10 Torr. Several techniques were used
to characterize the as-grown InN films, including in-situ RGA

Epitaxial Growth of InN Films
J. Phys. Chem. A, Vol. 111, No. 29, 2007 6757
Figure 3. RHEED patterns taken at different growth temperature. (A) 250, (B) 350, (C) 450, (D) 500, (E) 550, and (F) 600 °C.
and RHEED, as well as ex-situ FESEM, XRD analysis, and
PL spectroscopy.
species are in a balanced status. From above results, the data
indicates that the decomposed HN3-related fragments form either
NH3-related fragments or N2 and H2 species. Active nitrogen
Results and Discussion
species, such as HN and N, still follow the trend of HN -related
3
fragments. Hence, the V/III ratio at a stoichiometric condition,
Figure 1A shows a mass spectrum of the thermal dissociation
of HN3 using the RGA at substrate temperature of room
a very important parameter in MBE growth, can still be
controlled by tuning the HN3 partial pressure at different
substrate temperature from the trend of HN3-related fragments,
which is shown in the temperature-dependent mass spectrum.
-6
temperature under the pressure of 4.0 × 10 Torr. A substantial
+
size of N signals (m/z ) 14) from atomic nitrogen and related
fragments such as HN, HN2, and NH2 were detected, which
reveals HN3 has low dissociation energy and can be easily
decomposed at such low temperature. The observation of these
fragments is consistent with the result of infrared reflection
A further study on the dependence of HN3 partial pressure
and amounts of main fragments is shown in Figure 1C. The
amounts of these fragments show nearly linear increase as
increasing the HN3 partial pressure. Compared with other
nitrogen source generated by RF plasma, in which amount of
atomic nitrogen is controlled nonlinearly by plasma power,20
while the amounts of reactive nitrogen species of HN3 exhibited
a linearly controllable behavior with HN3 pressure at a given
substrate temperature. This result can be an advantage in
manipulation of growth process. Therefore, through the use of
HN3, it is possible to decrease the typical V/III ratio as low as
100, whereas other growth techniques utilizing NH3 as the
nitrogen source needs a very high V/III ratio of 40 000, and a
high growth temperature (>700 °C) is necessary to decompose
1
9
absorption spectroscopy (IRRAS) studied by Russell et al.
Temperature-dependent mass spectrum is used to study the
thermal decomposition of HN3 in the temperature range of 25-
800 °C, as shown in Figure 1B. All of the HN3-related fragments
decrease and shows the same trend as increasing the substrate
temperature. When temperature is lower than 300 °C, the H2
does not increase as increasing temperature but NH3, NH2, and
N2 fragments increase, which indicates that the decomposed
HN3-related fragments form ammonia (NH3) related fragments
not H2. When temperature is higher than 400 °C, a large amount
of H2 was produced and the NH3-related fragments decrease
dramatically. Until the temperature higher than 600 °C, all the
1
4,21
the NH3 reagent.
Both conditions of high V/III ratio and

6758 J. Phys. Chem. A, Vol. 111, No. 29, 2007
Chen et al.
Figure 5. Power dependent PL spectra of InN epitaxial film measured
at room temperature. The inset shows the plot of integrated PL intensity
vs excitation power.
growth temperature. On the contrary, a blurred pattern appears
while raising the growth temperature to 600 °C as a result of to
the formation of indium metal on the substrate surface. It is
consistent with the result of thermal decomposition of InN when
2
,3
the growth temperature is higher than 550 °C.
The crystalline quality of InN is investigated by XRD
analysis, and a semilogarithmic diagram of θ/2θ scan XRD is
plotted to recognize some weak signals from the off-axis crystal
plane, as shown in Figure 4A. At lower growth temperature of
2
50 °C, besides a prominent on-axis InN(002) peak at 31.3° in
the pattern, there are two small peaks appeared at 29.1 and 33.2°
are corresponding to the off-axis crystal planes of InN(100) and
InN(101) of wurtzite InN (Joint Committee on Powder Dif-
fraction Standards (JCPDS) 02-1450), respectively.12 Although
the low-temperature-grown InN film shows a bit polycrystalline
phase, no In(101) signal at 32.9° is shown in the XRD pattern.
This significant result implies that the indium and nitrogen,
which is provided by HN3 source, can be completely reacted
even at the growth temperature as low as 250 °C. When the
growth temperature is in the range of 450-550 °C, the XRD
signals show only InN(002), GaN(002) at 34.6°, and Al2O3-
Figure 4. (A) XRD θ/2θ scan of the InN films grown on GaN
templates at different growth temperature. (B) A plot of the fwhm of
InN(002) vs growth temperature.
high growth temperature limit the growth of a high-quality InN
film using the MBE technique.1-3 Besides, comparing with NH3,
the lower atomic ratio of hydrogen/nitrogen in HN3 and the use
of the lower V/III flux ratio during growth with HN3, the excess
hydrogen by product in the growth atmosphere is vastly reduced.
This may possibly attributed to suppression of the hydrogen
(006) at 41.4° without any extra off-axis InN crystal plane,
indicating that InN films are wurtzite structure and are grown
along the c-axis direction with a high crystallinity. As the growth
temperature increasing to 600 °C, a dominated In(101) diffrac-
tion peak appearance indicates that the InN cannot be grown at
temperature higher than its decomposition temperature. This
result is in agreement with the pervious observation from
RHEED. To further investigate the crystalline quality, the full-
width at half-maximum (fwhm) of InN(002) peak versus growth
temperature is plotted in Figure 4B. As the thermal dynamic
prediction, the crystalline quality becomes better as increasing
growth temperature to its decomposition temperature. A narrow
fwhm of 0.06° is obtained when InN film was grown at
passivation of acceptors, which is believed to be one of the
possible reasons of intrinsic n-type conductivity.22 Hence, the
use of HN3 as an efficient nitrogen source can be an excellent
candidate to grow high quality nitride semiconductors.
Figure 2A shows a typical cross sectional SEM image of a
flat InN layer grown on the GaN template at 550 °C. The as-
grown InN film on the GaN template shows a continuous growth
without voids appeared at interface, which indicates a well
coalescence in grain boundaries at initial growth. In addition,
the composition of InN film was determined by energy
dispersive X-ray spectroscopy (EDX) as shown in Figure 2B.
Only In and N elements appear in the spectrum and no any
noticeable contaminants such as oxygen, carbon, or other
impurities are detected, which indicates that high-purity InN
film has been grown by HN3-source MBE. A similar observation
5
50 °C. Furthermore, the X-ray rocking curve analysis was used
to examine the mosaicity of the InN film. A narrow fwhm of
.31° is also obtained on a 350 nm thick InN film, indicating a
good crystalline quality with a low misorientation subgrains in
0
2
3
the film. In addition, owing to the high dissociation of HN3,
the efficiency of the InN growth is much increased. While the
fluxes of indium and HN3 were increased simultaneously, the
growth rate up to about 200 nm per hour was also achieved.
16
was reported previously by Bu et al. In order to further study
the growth behavior of InN using HN3 source, InN films were
grown at different substrate temperatures. From RHEED pat-
terns, as shown in Figure 3, the InN film grown at low
The PL measurement was taken from a 0.5 m monochromator
equipped with a 300 gr/mm grating and an extend-InGaAs
detector (detecting range: 0.5-1.1 eV) using a frequency-
o
temperature of 250 C reveals a ring-like pattern and becomes
a streaky-like pattern as increasing substrate temperature to
+
4
50 °C, implying the surface changes from rough with poor
doubled Nd -YAG laser at 532 nm as the excitation source.
crystalline quality to smooth with better quality at increasing
Figure 5 shows power-dependent PL spectra measured at room

Epitaxial Growth of InN Films
J. Phys. Chem. A, Vol. 111, No. 29, 2007 6759
temperature from an InN film grown at 550 °C. The emission
peak is at 0.705 eV with a fwhm of 118 meV, which is
consistent with the emission of most reports in the range of
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.6-0.8 eV in recent for InN epifilms as well as one-
4
2, 2549.
2,3,10-12
dimensional InN nanostructures
The inset of the Figure
(
4) Tansley, T. L.; Foley, C. P. J. Appl. Phys. 1986, 59, 3241.
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shows the dependence of excitation power and PL integrated
(
intensity of the InN film. The integrated PL intensity increases
linearly with the excitation power near 2 orders of magnitude
and shows no saturation even at the highest excitation power
of 82 mW. The emission originates from the free electrons above
the conduction band to holes localized in the valance band tail
20, 361.
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Phys. Lett. 2003, 82, 1601.
(
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We have demonstrated the use of HN3 reagent as an efficient
nitrogen source for growing high-quality InN film in GSMBE.
Both the crystalline quality and the growth rate are comparable
with the radio frequency MBE technique. FESEM image shows
that a flat and smooth InN thin film has been realized. The XRD
analyses demonstrate that epitaxial InN thin film with wurtzite
structure grown along c-plane direction and epitaxial relationship
between InN and GaN is InN(002)//GaN(002) and the fwhm
of the InN(002) peak is as narrow as 0.06°. Furthermore, the
grown InN film shows the IR-PL emission peak of 0.705 eV at
room temperature, which can be attributed to a free-to-bound
emission. These findings reveal that HN3 has a great potential
to be an alternative nitrogen source in the growth of high-quality
nitride semiconductors. Therefore, we believe that the HN3 could
be a high-performance nitrogen source, which can be applied
to other growth techniques, such as MOVPE, HVPE, etc.
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Acknowledgment. The authors acknowledge the financial
support from the National Science Council(NSC), Ministry of
Education, Academia Sinica, Taiwan, and Asian Office of
Aerospace Research and Development under AFOSR. We are
grateful to Prof. M. C. Lin for the construction of the HN3
generator and the detailed procedure employed for preparation
and purification of the HN3.
6
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