Strucure evolution in AlN/InN multilayer structures
during decomposition of InN layers by annealing
G.Z.Radnóczi, L.Kótis, and B. Pecz
Research Institute for Technical Physics and Materials Science (MFA)
H-1121 Budapest Konkoly-Thege M.u. 29-33.
Abstract
Structures of thin InN and AlN films were grown on c-plane sapphire substrate by reactive DC
magnetron sputtering and annealed above the decomposition temperature of InN. The as grown and
annealed structures were characterized by transmission electron microscopy, X-ray diffraction, and
Auger depth profiling. TEM investigations show substantial improvement in the morphology during
annealing, while revealing epitaxial orientation for both as grown and annealed structures. XRD pole
figures recorded for classification of the misoriented grains in the epitaxial structure reveal a few
degrees deviation from the epitaxial orientation. AES depth profiles distorted by the surface roughness
show a small amount of In left after annealing. According to XRD pole figures recorded for In{101}
epitaxial metallic indium is present in these structures.
Introduction
While the purpose of most research efforts related to III-nitride material were to achieve good quality
GaN or III-nitride alloy substrates or layers, some results were also published on exotic structures (a
few of them obtained unintentionally) grown from III-nitride material. [nanograss][airgap][spinodal
decompAlInN] In this study we attempt growing epitaxial layers that are very weakly bonded to the
substrate or to the neighboring layer. Such a structure was reported [airgap] earlier as a so called “airgap structure”. The air-gap structure is produced by the following steps: a.) deposition of an epitaxial
InN layer b.) deposition of an epitaxial AlN layer and finally c.) heat treating the films to decompose
the InN layer. The AlN layer should remain intact after loosing the connection (i.e the InN layer) to the
substrate. In contrast to experiments described in [airgap] we tried using magnetron sputtering which is
known to produce films with by far higher defect densities than CVD or MBE grown films. The films
obtained with our growth method have a relatively high defect density, and a columnar structure with
column widths ranging from 20nm to 100 nm, and low angle grain boundaries or even free surfaces
(pores) between the columns.
This means that diffusion of species originating from the InN layer decomposition will be much
quicker through the diffusion channels than it could be in the case of a good quality AlN film. As a
consequence In is expected to diffuse very quickly into the AlN layer and influence its recrystallization
during annealing. In multilayer structures additional questions arise about the relation of the separate
AlN layers after annealing. These questions were also addressed by growing several thin InN/AlN pairs
in a multilayer stack.
*break down of InN
*evaporating In
Experimental
The growth experiments were carried out in a custom built UHV sputtering chamber pumped by a 500
l/s turbomolecular pump achieving a background pressure less than 8×10-8mbar (measured by a cold
cathode Penning gauge). The sample holder is heated by the radiation of two 1,5 kW lamps, its
temperature is measured indirectly by a K-type thermocouple and regulated by a PID controller. The
substrates were mechanically clamped onto the sample holder. The difference between the sample
temperature and temperature readout is assumed to be less than 50˚C in steady state.
Two off normal 2” DC magnetron sources are installed in the growth chamber with elemental metal Al
(99,9995%) and In(99,995%) targets. The sputtering gas was 99,999% N2 at a constant pressure of
1×10-2mbar for all experiments. The source parameters were 1A & 300V and 0.009A & 400V in
current control mode for the Al and In source respectively. Substrate bias was floating, varying between
11..14V for AlN deposition and 0.7..0.9V for InN deposition. Deposition rates were 6.25 nm/min for
AlN and 1.5 nm/min for InN.
Two different structures were grown and characterized. Both structures were grown twice for
producing one as grown and one annealed film, the growth temperature was 300˚C. Annealing was
carried out directly after growth by heating up the sample to 800˚C at a rate of 20˚C/min, keeping it
there for 10 min than cooling it down at the same rate it was heated up. The short annealing time was
chosen to prevent In from fully evaporating from the surface of the sample. This way we could observe
where In was accommodated after the InN broke down.
The structures were built from InN and AlN layer pairs (InN is closer to the substrate), the first one
consisting of one pair of 50 nm layers and the second one consisting of eight pairs of 10 nm layers as
shown in figure 1. For convenience the structures will be identified as follows: 1p and 1pa for the one
pair as grown and annealed samples respectively and 8p and 8pa for multilayer structures following the
same scheme. The growth experiments are summarized in table 1.
Figure 1. InN and AlN layer sequences grown
in the experiments.
Sample
name
Layer
sequence
Annealing
1p
1pa
8p
AlN 50 nm
InN 50 nm
AlN 10 nm
InN 10 nm
substrate
substrate
substrate
no
yes
no
8pa
8×
yes
Table 1. Growth sequences and annealing of the samples.
TEM investigations were carried out using a Philips CM20 microscope operating at 200 kV. Bright
field and dark field images were taken with an objective apperture of 20 μm. Diffraction patterns were
recorded from areas measuring 3-4 times the film thickness in diameter.
TEM sample preparation was carried out using high energy (13 kV) Ar+ ion milling [20kV gun] and
500eV Ar+ cleaning to remove the surface amorphized layer from the sample.
XRD pole figures were taken using a Bruker D8 Discovery diffractometer equipped with a 2D detector.
Pole figures were recorded for the following reflections: {10-12} for AlN at 2ϴ=49.8˚ and {10-11} for
InN at 2ϴ=33.1˚. {101} reflections of metallic indium also appeared on the InN pole figures due to
their scattering angle of 2ϴ=32.96˚ being very close to that of InN{10-11}.
Auger electron spectra were recorded in counting mode and differentiated numerically. Samples were
irradiated with 5keV electrons from a microfocus (30 μm) electron gun, Auger electrons were collected
by a pre-retarded CMA. The atomic concentration of the elements were calculated from Auger electron
peak-to-peak amplitudes by method of relative sensitivity factors.
Results and discussion
First all structures were characterized by X-ray diffraction. ϴ-2ϴ scans confirmed that practically no
alloying of the AlN and InN phase occurred during growth or annealing, only (0002) reflections of
AlN and InN were observed and in addition (101) reflection of the tetragonal In phase in the case of the
annealed multilayer structure. This indicates that a small amount of In was still present in the structure
after annealing. Furthermore the (101) plane of the In phase proved to be parallel to the c plane of the
wurtzit structure suggesting that (most of) the In is epitaxial to the AlN layers. Luckily the scattering
angle of the (101)In planes is very close to that of (10-11)InN planes, hence pole figure data for In and
InN could be acquired in one run. Pole figures for InN and In show epitaxial InN in the structures
before annealing. After annealing no In or InN reflections were observed in the one pair structure in
contrast to the multilayer structure in which (101) In reflections were recorded. The reflections
appeared at χ=****** which is close to the angle between (101)In and (10-1)In.
Pole figures for AlN {10-12} reflections were recorded showing that all layers have a texture very
close to the epitaxial orientation. Comparing AlN pole figures of annealed and as grown samples we
observe a small improvement in quality indicated by the FWHM of the phi vs intensity peaks. *******
The morphology of the films was investigated with TEM bright field images. As grown films exhibit a
columnar structure as usual for sputtered films. The typical column width is 35 nm, 25 nm for films 1p,
and 8p respectively. Column boundaries are determined by the first InN layer nucleation, column
widths do not change significantly throughout the structures.
Figure 2. Overview TEM micrographs of the as grown structures: one pair (left) and 8 pairs (right).
Micrographs taken on annealed films (Figure 3.) show that InN decomposed during annealing while the
AlN layers remained intact. In the case of the one pair structure a gap is clearly visible between the AlN
film and the sapphire substrate. No traces of remaining InN or In can be seen on the micrograph. The
film itself has a better crystalline quality as revealed from both electron- and X-ray diffraction data and
also a smoother surface than in the case of the as grown film shown on figure 2.
Figure 3. Micrographs of one pair (left) and 8pair (right) annealed structures.
*AES results
Conclusion
*no alloying of InN and AlN during heat treatment
*improved AlN layer quality after annealing in the single layer structure, no quality improvement on
the multilayer structure in contrast to the expectations.
References
[nanograss] Radnoczi, GZ; Seppanen, T; Pecz, B; Hultman, L; Birch, J: Growth of highly curved Al1xInxN nanocrystals phys. stat. sol. (a) 202 (7):R76-R78 2005
[airgap] A. Yamamoto*, Y. Hamano, T. Tanikawa, Bablu K. Ghosh, and A. Hashimoto: Formation of
“air-gap” structure at a GaN epilayer/substrate interface by using an InN interlayer , phys. stat. sol. (c)
0, No. 7, 2826 – 2829 (2003) / DOI 10.1002/pssc.200303424
[spinodal decompAlInN] Lin Zhou, David J. Smith, and Martha R. McCartney, D. S. Katzer and D. F.
Storm: Observation of vertical honeycomb structure in InAlN/ GaN heterostructures due to lateral
phase separation , APPLIED PHYSICS LETTERS 90, 081917 (2007)
[20kV gun]