Photoluminescence, depth profile, and lattice instability of hexagonal InN films

Article abstract of DOI:10.1063/1.1634691

The photoluminescence, structural properties, depth profile and lattice instability of hexagonal InN films were analyzed. The high quality InN films were grown on (0001) sapphire substrates by metalorganic molecular beam epitaxy. Depth profiling show a high, increasing oxygen concentration profile towards the volume of the film. The photoluminescence measurements revealed a low energy transition and a high energy feature. The results show that the spatially varying optical band gap was due to the formation of oxynitride.

Full text of DOI:10.1063/1.1634691

Photoluminescence, depth profile, and lattice instability of hexagonal InN films
O. K. Semchinova, J. Aderhold, J. Graul, A. Filimonov, and H. Neff
Citation: Applied Physics Letters 83, 5440 (2003); doi: 10.1063/1.1634691
View online: http://dx.doi.org/10.1063/1.1634691
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APPLIED PHYSICS LETTERS
VOLUME 83, NUMBER 26
29 DECEMBER 2003
Photoluminescence, depth profile, and lattice instability
of hexagonal InN films
a)
O. K. Semchinova, J. Aderhold, and J. Graul
Laboratorium f u¨ r Informationstechnologie (Lfl), Universit a¨ t Hannover, Schneiderberg 32,
D-30167 Hannover, Germany
A. Filimonov
Technical University St. Petersburg, Politeknichaskaya 29, St. Petersburg, Russia
H. Neff
VIR A/S, Kuldyssen 10, DK-2630 Taastrup, Denmark, and Federal University Campina Grande (UFCG),
Department of Electrical Engineering, 58207 Campina Grande, Pb, Brazil
͑
Received 10 April 2003; accepted 24 October 2003͒
High quality InN films have been grown on ͑0001͒ sapphire substrates by metalorganic molecular
beam epitaxy, characterized for crystal structure, chemical composition, and optical properties.
Depth profiling indicated a high, increasing oxygen concentration profile towards the volume of the
film. Photoluminescence revealed two different, coexisting features: a low energy transition around
0.8 eV, and a high energy feature, peaking near 2 eV. The former band originates from the direct
transition in the near surface range. The latter, low intensity band results from the bulk region.
Oxynitride formation has been accounted for a spatially varying optical band gap E , determined
G
to 0.61 eV for pure, defect free material. X-ray diffraction analysis supports defect mediated lattice
instability, as proposed recently by a molecular dynamic lattice theory. © 2003 American Institute
of Physics. ͓DOI: 10.1063/1.1634691͔
Indiumnitride has attracted interest as a promising mate-
rial for high power, high frequency applications, and for
gas as source material, and N gas bubbling of the high pu-
2
rity, electronic grade precursor. Residual gas analysis by
quadrupole mass spectrometry revealed O2 , H2O, hydrocar-
bons, and H2 as the dominant trace gases. The substrate was
covered with a predeposited AlN/GaN nucleation layer at 20
nm thickness, grown at 600 °C. Growth temperatures for InN
films were adjusted within the range 475–525 °C, with the
nominal film thickness set at 0.5 ␮m. The films revealed a
smooth, shiny and blackish appearance, but relatively poor
adhesion to the substrate, being characterized during growth
by reflection high-energy electron diffraction monitoring.
An extended sample analysis was performed and in-
cluded: carrier transport measurements in the Van der Pauw
configuration at ambient T, Raman spectroscopy ͑not
shown͒, x-ray diffraction, scanning electron, atomic force
microscopy ͑AFM͒, and Auger spectroscopy for surface
analysis. Secondary ion mass spectrometry ͑SIMS͒, employ-
ing a Cameca IMS4 secondary ion spectrometer, was used
for depths profiling of N, C, and O. For concentration deter-
mination, the following method was used: primary Cs ions
incident at an energy of 5.5 keV and current of 40 nA, with
1
,2
photovoltaics. Hexagonal InN films display unusual physi-
3
cal properties, such as type II superconductivity at 2.3 K,
and high carrier mobility in presence of high defect density.
At present, insufficient, partly controversial information is
available on optimum growth conditions, origin of electronic
defects, and optical and carrier transport properties.
The morphology of the thoroughly n-type hexagonal InN
films is characterized by columnar growth. Electrically ac-
tive defect concentration is Ͼ1018 cm , and varies with
growth conditions and substrate preparation. The exact value
of the optical band gap of InN still is controversial. Reported
experimental data are ranging from 0.7 to 2.2 eV, with the
Ϫ3
4
,5
most established gap value set at 1.89 eV. Reported mobil-
2
6
ity data are varying from 20 cm /V s for poor quality films,
2
to values in excess of 1000 cm /V s for molecular beam ep-
itaxy ͑MBE͒ grown epitaxial films.7
Compared to other direct group III–V semiconductors,
few data are available regarding photoluminescence.8,9 Here,
we report on the connection between photoluminescence
ϩ
detection of the secondary charged clusters CsX ͑with X
͑PL͒ and structural properties of high quality films, deposited
ϭC,N,O). Instrumental calibration for quantitative analysis
was achieved by GaN samples, implanted with carbon and
oxygen. PL spectroscopy at 77 K was performed, using the
onto ͑0001͒ oriented sapphire substrates. Details of the film
deposition methods and substrate preparation were provided
1
0
elsewhere.
4
88 nm laser line. Crystallographic characterization of the
films was achieved by ␪–2␪ scans and rocking curves at the
0002͒ reflex, with the full width at half maximum ͑FWHM͒
Briefly, samples were deposited in a RIBER 32 metalor-
ganic ͑MBE͒ system, employing triethylindium in absence of
a carrier gas as the group III precurser, and plasma excited
͑
varying from 50 to 1100 arc sec. Best values for the a and c
lattice constants at FWHM of 50 and 180 arc sec were de-
termined to: c: 5.7040 and a: 3.5414 Å. The films were
wurtzite phase, without traces of the cubic phase. The colum-
nar morphology is characterized by two different structures:
nitrogen. Attempts were made to maintain the O partial
2
pressure at a minimum level, using high purity ͑N 7.0͒ N2
a͒Electronic mail: cks@cynoron.de
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0
1

Appl. Phys. Lett., Vol. 83, No. 26, 29 December 2003
Semchinova et al.
5441
position. From the broken line, a band gap value of 0.61 eV
can be extrapolated for pure, defect free films. No correla-
tion between optical and structural film properties and indi-
vidual growth conditions has been made. High energy PL
spectra were recorded by a photomultiplier tube ͑PMT͒,
low energy spectra by a Ge photodetector ͑PD͒. While the
low energy PL data reveal little variation, the high energy
PL band is broadly varying from sample to sample in all,
peak intensity, position, and FWHM. From the difference
1
5
in detector sensitivities ͓PMT: D*Ϸ2ϫ10 ; Ge PD: Ϸ1
ϫ1011 (cm W Hz )] and signal noise levels, the high en-
ergy PL-quantum efficiency is estimated a factor 8ϫ104
lower. No further subband gap defect related PL-emission
features were detected.
Ϫ1
1/2
The SIMS spectra were taken, towards an analysis of the
chemical composition and depths distribution of oxygen, ni-
trogen, and carbon impurities. Due to the destructive and
extensive SIMS method, only a limited number ͑4͒ of films
was analyzed. Emphasis was given on influence of film
thickness on contamination levels, in connection with post-
growth film oxidation and degradation effects in ambient at-
mosphere. For both samples N489 (d:100 nm) and N292
(d:550 nm), apart from a slight increase in concentration
towards the outer surface, nitrogen and carbon contents re-
main relatively flat throughout the film. A substantially dif-
ferent behavior was observed for the oxygen distribution in
the films. In all four depths profiled films, independent of
film thickness, the oxygen concentration increases substan-
tially towards the volume of the films. Degradation and post-
growth volume film oxidation upon exposure to ambient at-
mosphere largely can be ruled out. Films, when directly
taken from the deposition chamber, revealed a very low outer
surface oxygen concentration Ͻ1%, as revealed by indepen-
dent Auger electron spectroscopy analysis. It is evident from
FIG. 1. Normalized photoluminescence spectra of different InN films. Indi-
vidual growth parameters: N2—flux: 2.4 sccm; In regul. pressure: 0.6 Torr:
Growth temperatures 500 °C: N292, N498, N506, N507; 490 °C: N503;
Low energy PL spectra 0.6 eVϽE Ͻ1.2 eV left scale, high energy PL spec-
G
4
tra right scale: scaling factor Ϸ8ϫ10 . Inset: PL-line broadening parameter
a
s
a
f
u
n
c
t
i
o
n
o
f
t
r
a
n
s
i
t
i
o
n
e
n
e
r
g
y
E
.
B
r
o
k
e
n
l
i
n
e
e
x
t
r
a
p
o
l
a
t
e
s
t
o
E
G
G
ϭ0.61 eV at ⌫ϭ0. High energy PL data absent for sample N292.
large columns in hexagonal symmetry with a diameter, rang-
ing from 1 to 2 ␮m and much smaller columns with diam-
eters around 0.1 to 0.3 ␮m. The size distribution varies from
sample to sample, typically revealing a higher concentration
of larger diameter columns.
Figure 1 outlines a set of normalized PL spectra of dif-
ferent samples, taken at 77 K and fabricated under similar
conditions, with growth conditions being indicated. Left side
spectra were recorded at 0.6 eVϽhvϽ1.0 eV, right set
data at 1.2 eVϽhvϽ2.6 eV. Apart from sample N489, at
thickness of 100 nm, all other films were approximately
11
recent work on oxynitride formation in sputtered films that
with changing chemical film composition and oxygen sto-
ichiometry, EG varies throughout the film thickness. The
500 nm thick. The low energy PL peak position is varying
lower the oxygen content, the lower the band gap E , and
G
from 0.75 to 0.9 eV, line broadening ⌫ from 80 to 115 meV.
The high energy PL peak is ranging from 1.6 eV to 2.2 eV,
with the ⌫ value ͑FWHM͒ increasing up to 450 meV. Inset
in Fig. 1 reveals a steadily increasing ⌫ value with PL peak
vice versa. Thus, in the bulk of the film, a substantially larger
E value is expected. The spatially varying optical transition
G
of an inhomogeneous composed material would account for
a spatially varying optical absorption coefficient throughout
the film.
Close inspection of the SIMS spectra in Fig. 2 reveals a
sharp increase of the oxygen concentration within a very
narrow range of few nanometers at the film surface. We at-
tribute this observation to the formation of a thin native ox-
ide upon aging, and exposure to ambient. Due to high a high
optical absorption coefficient in the film, exceeding
5
Ϫ1
1
0 cm
around a photon energy of 1 eV, only a small
fraction of exciting radiation at 488 nm reaches the interior
of the film. Hence, high energy PL signal remains low, partly
due to PL self-absorption. The large spatial variation of the
band gap energy, and associated oxygen concentration within
the light penetration depths, also would explain the observed
anomalously large FWHM of the blueshifted PL band, due to
alloy broadening effect.
The film specific inhomogeneous oxygen distribution
was observed in all four depths profiled films. Moreover, all
FIG. 2. SIMS depths profile of 0.1 ␮m-thin-film sample N489, and 0.5-␮m-
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thick sample N292.
films revealing low energy PL, when optically further ana-
1
36.165.238.131 On: Wed, 24 Dec 2014 01:57:30

5442
Appl. Phys. Lett., Vol. 83, No. 26, 29 December 2003
Semchinova et al.
FIG. 3. ͑a͒, ͑b͒, ͑c͒. Variation of a- and c-axis lattice
constants with XRD-line broadening parameter ͑3a͒,
͑
3b͒, and variation of the c-axis lattice constant vs low
energy PL-band gap position ͑3c͒. Solid lines serve as a
guide to the eye.
lyzed, also displayed the coexisting high energy PL band.
Therefore, it can be concluded that both physical phenomena
are correlated, considered an inherent film property. The con-
clusion is supported by the optical and structural analysis of
sample N489, where all features jointly appear.
likely, carbon results from, and is built into the lattice, upon
dissociation of the metalorganic precursors during growth.
The high concentration of oxygen within the InN films
most likely originates from two sources: ͑i͒ dissolved
oxygen/water from the precursor and impurities in the N2
gas, accounting for a constant background signal and ͑ii͒ by
outdiffusion of mobile oxygen at high temperature from the
sapphire substrate, accounting for the observed spatially
varying concentration profile. Outdiffusion of oxygen from
single crystalline sapphire substrates at temperature as low as
400 °C has been reported earlier,14 with the film morphology
supporting fast diffusion along columnar grain boundaries.
The large concentration of oxygen, and its chemical bonding
behavior, is expected to most likely being present as a non-
Apart from the earlier mentioned physical anomalies,
InN displays lattice instability, mediated by crystal defects.
This is illustrated in Figs. 3͑a͒ and 3͑b͒, correlating the a-
and c-axis lattice constants with the x-ray diffraction ͑XRD͒
line broadening parameter ͑FWHM͒, referring to the afore-
mentioned best lattice constants. For the c-axis lattice con-
stant taken from 30 samples, ten films displayed a slight
lattice expansion of approximately 0.0025 Å, fairly indepen-
dent of the XRD-broadening parameter. Twenty samples, and
majority of films, revealed lattice compression that, which
large scatter of data, increases with the XRD-broadening pa-
rameter. The a-axis lattice constant also reveals lattice com-
pression, nearly independent of the XRD broadening param-
eter. As illustrated in Fig. 3͑c͒, where the c-axis lattice
distortion is plotted against the low energy PL-band gap po-
sition, no clear correlation is observable, but samples with
lattice expansion showed weak, or even lack of PL emission.
Recent molecular dynamics calculations, provided in Ref.
stoichiometric oxynitride in the form InN1ϪxO with N va-
y
cancies being partly filled by oxygen, or a solid solution with
In O , accounting for the observed PL blueshift within the
2
3
8
volume of the film.
1
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1