Native defects and their effects on properties of sputtered InN films

Article abstract of DOI:10.1063/1.3003865

The concept of defect chemistry is applied to investigate the native defects in the InN films prepared by radio frequency magnetron sputtering. Growth temperature and pressure ranged from 150 to 300 °C and from 0.005 to 0.07 torr, respectively, for the pu

Full text of DOI:10.1063/1.3003865

Native defects and their effects on properties of sputtered InN films
Dong-Hau Kuo and Chun-Hung Shih
Citation: Applied Physics Letters 93, 164105 (2008); doi: 10.1063/1.3003865
View online: http://dx.doi.org/10.1063/1.3003865
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APPLIED PHYSICS LETTERS 93, 164105 ͑2008͒
Native defects and their effects on properties of sputtered InN films
Dong-Hau Kuo1,a͒ and Chun-Hung Shih2
1Department of Polymer Engineering and Institute of Materials Science and Technology, National Taiwan
University of Science and Technology, Taipei, 106 Taiwan, Republic of China
2Department of Materials Science and Engineering, National Dong Hwa University, Shoufeng, Hualien, 974
Taiwan, Republic of China
͑Received 22 August 2008; accepted 30 September 2008; published online 24 October 2008͒
The concept of defect chemistry is applied to investigate the native defects in the InN films prepared
by radio frequency magnetron sputtering. Growth temperature and pressure ranged from 150 to
300 °C and from 0.005 to 0.07 torr, respectively, for the purpose of changing the defects and the
related properties. InN is expected to form Frenkel defects, indium vacancies, and interstitials. Other
major defects for the nitrogen-rich InN films include nitrogen-on-indium antisites and nitrogen
interstitials at higher nitrogen pressure, as supported by the results of x-ray photoelectron
spectroscopy. Structure, composition, and electrical properties coincide with defect types and
density. © 2008 American Institute of Physics. ͓DOI: 10.1063/1.3003865͔
Indium nitride ͑InN͒ with a direct band gap from ϳ0.9
to ϳ0.7 eV has enormous applications in optoelectric de-
vices such as solar cells, light-emitting diodes, and laser di-
odes after combining with GaN to form InGaN alloys, hav-
ing band gaps spanning the spectrum from infrared to
ultraviolet.1,2 The major debates now for InN have been the
origin of its invariably n-type conductivity and its defect
type.
InN films were grown by rf magnetron sputtering in an
ambient of nitrogen, with an indium target. The substrate was
a single crystalline sapphire ͑0001͒ plate. The growth tem-
perature was controlled at 150–300 °C and the growth pres-
sure ranged from 0.005 to 0.07 torr. During the deposition,
the rf sputtering power was kept constant at 40 W. The struc-
ture and the crystallinity of the films were evaluated by XRD
͑Rigaku D/Max-2500, Japan͒. A field-emission scanning
electron microscope ͑SEM͒ ͑JEOL JSM 6500F, Japan͒
equipped with energy-dispersive spectroscopy was used to
observe growth morphology and to analyze chemical com-
position. A Hall effect measurement system ͑HL 5500, Na-
nometrics Inc., USA͒ was used to measure the resistivity and
carrier concentration. XPS ͑VG Scientific, UK͒ with an
Al K␣ radiation source was used to determine the chemical
bonding after a sputter etch using argon ions for 10 min.
Figure 1 shows the XRD spectra of sputtered InN films
The Kröger–Vink notation has been developed to de-
scribe the defects in an ionic compound.3 There are three
parts in this notation: the main body representing a vacancy
or ion, the subscript part for host lattice site or interstitial,
and the superscript part for the relative charge. The uninten-
tional n-type conductivity of InN has long been assumed to
be due to nitrogen vacancies ͑V¯N ͒, oxygen ͑ON· ͒ and silicon
͑Si·In͒ impurities, or interstitial hydrogen ͑H·i͒.4–6 In this re-
port, the relative charges on the superscript part are added to
the above defects. Positron annihilation measurements have
been conducted on epitaxial InN films to identify In
vacancies.7 The major defects had been attributed to intersti-
tial nitrogen or the combination of nitrogen-on-indium anti-
site defects and indium vacancies, based on the analyses of
lattice dimension by x-ray diffraction ͑XRD͒ and photoelec-
tron peaks by x-ray photoelectron spectroscopy ͑XPS͒.8,9
The growth of InN has been difficult because of the low
dissociation temperature and the lack of a lattice-matched
substrate. High quality single-crystalline epitaxial InN films
have been grown by metalorganic vapor phase epitaxy and
molecular beam epitaxy ͑MBE͒.1,10 InN films display differ-
ent band-gap values between 1.8 and 0.9 eV, which were
related to the growth-dependent stoichiometry or the In/N
ratio in films.11,12 rf sputtering produces polycrystalline InN
films with high N/In ratios.9
(a)
300 o
C
250 oC
200 oC
150 o
C
(b)
3x10-3 torr
8x10-3 torr
3x10-2 torr
7x10-2 torr
To investigate the native defects and their effects on film
performance, we deposited InN films by rf magnetron sput-
tering with an indium target at 150–300 °C and rf power of
40 W. Growth at different pressures of 0.005–0.07 torr was
also conducted for the purpose of creating defects.
20
30
40
50
60
70
2 theta (degree)
FIG. 1. ͑Color online͒ XRD spectra of sputtered InN films deposited at ͑a͒
different substrate temperatures with a constant pressure of 0.03 torr and at
͑b͒ different growth pressures with a constant substrate temperature of
300 °C.
a͒
Electronic mail: dhkuo@mail.ntust.edu.tw.
0003-6951/2008/93͑16͒/164105/3/$23.00
93, 164105-1
© 2008 American Institute of Physics
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164105-2
D.-H. Kuo and C.-H. Shih
Appl. Phys. Lett. 93, 164105 ͑2008͒
80
60
40
(a)
(b)
(a)
(b)
(c)
(d)
N
N
In
In
20
1.04
(e)
200 nm
c/c0
1.02
1
c/c0
cubic
FIG. 2. SEM morphologies of sputtered InN thin films deposited at different
substrate temperatures of ͑a͒ 150 °C and ͑b͒ 300 °C under a constant pres-
sure of 0.03 torr.
a/a0
a/a0
0.98
10-1
1022
(f)
Resistivity
e- concentration
1021
10-2
10-3
10-4
deposited at ͑a͒ different substrate temperatures with a con-
stant pressure of 0.03 torr and at ͑b͒ different growth pres-
sures with a constant substrate temperature of 300 °C. For
the temperature effect ͓Fig. 1͑a͔͒, the InN films had a wurtz-
ite structure. The growth orientation of InN films changed
from a random orientation, the c-axis orientation, to a pre-
ferred ͓101គ1͔ orientation, as the growth temperature in-
creased from 150 to 300 °C. The reason for obtaining ran-
domly oriented grains at 150 °C is attributed to the low
surface kinetic energy. As the growth temperature increased
to 200 °C, InN adatoms were arranged into the orientation
of the ͓0001͔ substrate. At a higher growth temperature of
300 °C, the growth of polycrystalline InN films had a pre-
ferred thermodynamically stable orientation of ͓101គ1͔. For
the pressure effect ͓Fig. 1͑b͔͒, the c-axis dominated orienta-
tion of the wurtzite phase occurred at low growth pressures.
The preferred ͓101គ1͔ orientation for the 300 °C-grown InN
in Fig. 1͑a͒ changed to the c-axis substrate orientation at low
pressure due to the larger mean free path. As the growth
pressure increased, InN films had poor crystallinity. At a high
pressure of 0.07 torr, a cubic phase of InN was produced.13
Figure 2 displays SEM morphologies of sputtered InN
thin films deposited at different substrate temperatures of ͑a͒
150 °C and ͑b͒ 300 °C under a constant pressure of 0.03
torr. The InN films were polycrystalline. The grain character-
istics of InN films are related to their growth orientation
revealed by XRD analysis. The randomly oriented grains had
a round shape. The preferentially oriented ͓101គ1͔ grains had
a shape of elongated pyramid.
e- concentration
1020
0.01
0.1
100 150 200 250 300 350
o
Working pressure (torr)
Deposition temperature ( C)
FIG. 3. ͑Color online͒ The variations in ͓͑a͒ and ͑d͔͒ atomic percentage, ͓͑b͒
and ͑e͔͒ lattice dimension, and ͓͑c͒ and ͑f͔͒ resistivity and carrier concen-
tration of sputtered InN films deposited at ͓͑a͒–͑c͔͒ different substrate tem-
peratures with a constant pressure of 0.03 torr and at ͓͑d͒–͑f͔͒ different
growth pressures with a constant substrate temperature of 300 °C.
state of hydrostatic strain, as shown in Figs. 3͑b͒ and 3͑e͒.
The p¯ossib¯le positively charged intrinsic defects in InN
··
In
include VN , Ini , and N . The possible negatively charged
intrinsic defects include Niٞ and VIٞn. The indium-on-nitrogen
defects cannot occur because the nitrogen site is too small for
indium to occupy. Based on the facts of ͑i͒ nitrogen-rich
··
composition, ͑ii͒ the existing defects of N antisites and Niٞ
In
interstitials proposed by Butcher et al.,8,9 ͑iii͒ larger lattice
expansion at higher pressure of 0.07 torr, and ͑iv͒ the higher
resistivity at higher growth pressure, the defect-formation
equations are formulated as given below.
Case 1. Under low nitrogen pressure,
InIn = In¯i + VIٞn,
͑1͒
͑2͒
͑3͒
Vٞ + 1/2N = N·· + 5e ,
Ј
2
In
In
InIn + 1/2N2 = In¯ + N·· + 5e .
Ј
i
In
Figure 3 shows the variations in ͓͑a͒ and ͑d͔͒ atomic
percentage, ͓͑b͒ and ͑e͔͒ lattice dimension, and ͓͑c͒ and ͑f͔͒
resistivity and carrier concentration of sputtered InN films
deposited at ͓͑a͒–͑c͔͒ different substrate temperatures with a
constant pressure of 0.03 torr and at ͓͑d͒–͑f͔͒ different
growth pressures with a constant substrate temperature of
300 °C. From the composition analysis in Figs. 3͑a͒ and
3͑d͒, InN films were nitrogen rich and had a N/In ratio of
ϳ1.9. At a higher pressure of 0.07 torr, the ratio increased to
2.35. Lattice dimension changed with process condition, as
shown in Figs. 3͑b͒ and 3͑e͒. The lattice constants a and c
changed in the opposite way at different growth temperatures
in Fig. 3͑b͒. The lattice dimension expanded a lot at the
pressure of 0.07 torr due to a phase change ͓Fig. 3͑e͔͒. The
resistivity decreased with the growth temperature but in-
creased with growth pressure ͓Figs. 3͑c͒ and 3͑f͔͒. The car-
rier concentration did not have an apparent change with tem-
perature ͓Fig. 3͑c͔͒, but it slightly decreased with growth
pressure ͓Fig. 3͑f͔͒. Hydrostatic strain, based on the slope of
lattice constants a and c, was observed in the sputtered InN
films.8 Our sputtered InN films were not totally under the
Case 2. Under high nitrogen pressure,
InIn = In¯i + VIٞn,
͑4͒
͑5͒
͑6͒
Vٞ + N = N·· + Nٞ + 2e ,
Ј
2
In
In
i
InIn + N2 = In¯ + N·· + Nٞ + 2e .
Ј
i
In
i
The above equations need to satisfy the balances of mass,
lattice site, and electrical charge. To realize the n-type con-
duction for InN, the overall defect equations need to have net
electrons. The major assumption in formulating the defect
equation is the formation of intrinsic Frenkel defects ͓Eqs.
͑1͒ and ͑4͔͒. As a good comparison to InN, the wurtzite ZnO
also prefers to form Frenkel defects of V·O· and OiЉ from the
larger oxygen anions.3 The consideration of larger In3+ cat-
ions in InN to form Frenkel defects of In¯i and VIٞn is rea-
sonable. The in situ formed indium vacancies ͓Eqs. ͑1͒ and
͑4͔͒ are occupied by nitrogen coming from the growth atmo-
sphere, and hence N·· antisites defects are generated ͓Eqs.
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164105-3
D.-H. Kuo and C.-H. Shih
Appl. Phys. Lett. 93, 164105 ͑2008͒
smaller c and larger a lattice values ͓Fig. 3͑b͔͒ as a result of
the stress compensation between tension and compression
arising from the vacancies and interstitials. For the effect of
growth pressure, the InN films deposited at 300 °C and
0.008 torr did not show Nٞi ͓Fig. 4͑a͔͒, which should lead to
higher electron concentration ͓Eq. ͑3͔͒. At pressures lower
than 0.008 torr, InN films did not have Nٞi and resistivity
remained low. At a higher pressure of 0.07 torr, many Ni
ٞ
defects were produced and the resistivity was higher ͓Fig.
3͑f͔͒. However, the InN film produced at 0.07 torr had a high
N/In ratio of 2.35 ͓Fig. 3͑d͔͒ and favored the formation of a
new cubic phase for the purpose of stress release ͓Fig. 1͑b͔͒.
In summary, InN films were fabricated at 150–300 °C
with growth pressures of 0.005–0.07 torr by rf magnetron
sputtering. InN films were nitrogen rich. At a higher growth
pressure of 0.03 torr, the defect of nitrogen interstitials was
observed by XPS. At pressures below 0.03 torr, the nitrogen-
on-indium antisites were the major defects. Supported by the
observed defects, the defect-formation equations were for-
mulated.¯The existing defects in nitrogen-rich InN films in-
N-In bonding
NIn
Ni
··
In
··
In
clude Ini , VIٞn, N , and Niٞ with N and Niٞ as the major
FIG. 4. ͑Color online͒ The variations in the N 1s XPS spectra with the
growth pressures of ͑a͒ 0.008 torr and ͑b͒ 0.03 torr for sputtered InN films.
defects. Electrical resistivity, which is higher at higher defect
concentration of nitrogen interstitials, can be predicted from
the defect-formation equations.
͑2͒ and ͑5͔͒. Once many vacancies are occupied, nitrogen
from the higher growth pressure can enter interstitial sites to
form Niٞ defects ͓Eq. ͑6͔͒. Under the higher nitrogen pres-
sure, it is expected that the resistivity of InN films will in-
crease ͓Fig. 3͑f͔͒ due to the decrease in electron concentra-
tion ͓Eqs. ͑3͒–͑6͔͒. The reason for the in situ defect
formation is expectedly related to the low defect-formation
energy ͑2.5 eV for ZnO versus 1.1 eV for AgCl͒.
This work was supported by the National Science Coun-
cil of the Republic of China under Grant No. NSC 96-2628-
E-011-118-MY3.
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