1.
Introduction
As a wide-bandgap semiconductor, ZnO has unique
potentialities in the fields of optoelectronics, spintronics or nanostructures.
In addition, the small lattice-mismatch between ZnO and GaN motivates workers
to study growth of GaN on ZnO bulk or film substrate. Nowadays, several vendors
can offer ZnO substrates of polar face, non-polar face as well as semi-polar
face, grown by hydrothermal or melt growth methods. However, the price of ZnO
bulk substrate is still high. By comparison, heteroepitaxy growth is still an
alternative way to fabricate wide bandgap material-based devices. Earlier, we
described a method to grow thick ZnO film of ∼10 μm by metal vapor phase epitaxy (MVPE) . The advantage is that
MVPE is a kind of epitaxy growth method with growth rate as high as
120 μm/h. Therefore, it is easy to make large size of ZnO bulk substrate
with low cost and high efficiency, considering the ready availability of
sapphire substrate with large diameter (in the range of 50–200 mm).
However, crack is a crucial problem for thick film growth. Cracks appear when
ZnO film is a little thicker than 10 μm, not to mention the fact that the
thickness of a free-standing substrate is required to be larger than
100 μm.
Although research has been extended to investigate the
stress in fine structures such as quantum wells, there is still meaning to
inquire into the problems caused by stress in large lattice-mismatch system,
such as ZnO films grown on sapphire substrates. Unlike thin films (with
thickness less than 10 μm), thick films form cracks motivated by the large
residual tensile stress, composed by tensile growth stress, thermal mismatch
stress and lattice mismatch stress. We found that ZnO films, grown on sapphire
substrates with GaN templates, can be either tensile or compressive. A simple
way to investigate the residual stress in film is to divide it into two groups,
one thermal stress and another non-thermal stress, including lattice mismatch
stress and growth stress. In this letter, the value of non-thermal stress was deduced
from the crack density in ZnO film, while the data of the thermal stress was
evaluated. Annealing was used to decrease the non-thermal stress. And annealed
ZnO films were used as pseudo-substrates to grow crack-free ZnO films.
2.
Experiment
In
our experiment, ZnO samples were grown on c-plane Rubicon® sapphire substrates by a home-made
MVPE. Zinc vapor and water vapor were used as Zn and O sources, respectively.
Nitrogen was used as the carrier gas at a flow rate of 6 L/min. Other
growth parameters like total reactor flow (9.4 L/min), Zn carrier gas flow
(0.4 L/min) and H2O carrier gas
flow (3 L/min) were kept identical to allow a direct comparison. The
growth temperature is 1073 K. All the as-grown ZnO samples studied were
grown using a two-step growth method.
In the first step, a layer of ZnO film of about 2 μm is deposited on
sapphire directly, and then etched off by HNO3 dilution.
It is found that there are residual Al2ZnO4 crystal
nuclei or Al2ZnO4 buffer layer left on the etched
sapphire substrate. The second step is to grow another layer of ZnO film at
almost the same conditions. The goal of the two-step growth method is to obtain
a stable high quality Zn-polar ZnO film on sapphire substrate. Normally
polycrystalline or mixed polar ZnO films were obtained after the first growth,
while only Zn-polar ZnO films after the second growth. Two ZnO films were grown
by MVPE. One film with cracks has a thickness of 15 μm, labeled as sample
A. The other film without cracks has a thickness of 12 μm, labeled as
sample B. Sample A was used to evaluate the values of the non-thermal stress
and the thermal stress. Sample B was used as a pseudo-substrate to re-grow ZnO
films. Fig. 1 is the schematic drawing showing the growth of sample A
and B, the annealing of sample B, and the re-growth of ZnO films on annealed
sample B. The morphologies of the ZnO samples were observed by Hitachi S-4800
scanning electron microscope (SEM) and Digital optical microscopy. The residual
stress was evaluated through the curvatures of the ZnO samples using the
Dektak-8 surface profiler of Veeco.
Fig. 1. A schematic
drawing of the growth of sample A and B, the annealing of sample B, and the
re-growth of ZnO films on annealed sample B.
3.
Results and discussions
Fig. 2(a) is the surface morphologies of
as-grown sample A. Few cracks along 〈1 1 0〉 directions are observed on the surface. The regular
hexagonal mesas on the surface have a diameter of about 25–100 μm,
indicating the columnar growth mechanism. This means that cracks may form and
then be buried or healed during the subsequent film growth. A wet-etching
method was employed to dredge out the buried cracks. Sample A was dipped in
diluted HCl (1:100) at room temperature and then picked out every 30 s to
observe the changes of the morphologies by optical microscopy. To clarify the
etching process, the surface morphologies are recorded at almost the same
locations at different times (Fig. 2(a)–(f)).
The etching rate is ∼0.3 μm/min. After
30 s etching, new cracks were observed and etching pits appeared. With
time increasing, more and more cracks emerge and the widths of the cracks were
widened. This phenomenon affirms that many of the cracks had been buried at
different depth during the growth, and were dredged up again during the
etching. The more there are cracks, the larger the stress is. It is found that,
however, the crack density becomes saturated after 240 s etching. The
corresponding average crack space at saturation, determined to be ∼20 μm by dividing the observation area by the
whole length of the cracks, can be used to evaluate the maximum stress during
film growth. The tensile residual strain in film with cracks can be deduced by
the following expression.
(1
where Γ is the fracture
resistance, γ is Poisson's ratio, Y is the Young's module, Lsat is the crack space at
saturation for an array of cracks at 120°. Set Γ as 2.32 J/m2. The Young's modulus and Poisson's ratio are 154 GPa and 0.30
for ZnO. Therefore, the maximum residual tensile strain is about 2.88×10−3, corresponding to a stress of about 630 MPa.
Fig. 2. Surface
morphologies of sample A, etched in diluted HCl (1:100) at different times at
room temperature; (a) 0 s; (b) 30 s; (c) 90 s; (d) 120 s;
(e)180 s; (f) 240 s.
The value of thermal stress can be computed according to
Reeber's data in the growth temperature range. The thermal stress is
tensile for ZnO/sapphire grown at temperatures above 700 K rather than
compressive below 700 K. In our experiment, ZnO samples were grown at
1073 K. Thus, the corresponding maximum thermal strain is 1.1×10−4 emerging at 500 K. It should be noted
that strain instead of stress is used for comparison because strain is a
non-unit quantity irrelative with film properties.
We estimated the non-thermal strain by subtracting the
thermal strain from the residual strain. The maximum non-thermal strain should
be about 2.77×10−3, indicating that non-thermal
strain is the predominant component causing cracks during film growth.
Therefore, it is essential to reduce the non-thermal strain in the as-grown ZnO
films.
Annealing is one effective method to decrease non-thermal
stress. To evaluate the effect of annealing, sample B without cracks was grown.
The first annealing temperature was 900 °C for 10 h. The residual
stress in ZnO was determined directly using wafer curvature method, a direct
way to measure the change of radian. The strain in ZnO epilayer can be
calculated by Stoney formula
(2)
where Y is the Young's module, t is the thickness, Q is the curvature, s and f are subscripts denoting substrate and film, respectively. The Young's
modulus for sapphire is 400 GPa. The thickness is 12 μm for the ZnO
film, and 430 μm for the sapphire substrate. The wafer curvatures are
evaluated by subtracting the reciprocal of the radius of the ZnO film from that
of the sapphire substrate. The substrate curvature is 1.78×10−2 m−1. The modified
curvatures (Qf–Qs) of the as-grown sample B and the annealed films are
listed in Table 1. It is shown that
there is little change after the first time annealing. Hence, another annealing
was operated at 1000 °C for 10 h. It is shown in Table 1 that the strain was reduced to 56.2% after the second
time annealing. Almost half of the non-thermal stress was relaxed after
annealing.
Table 1.
Curvatures and the corresponding strain and stress in
as-grown sample B, and the 1st and 2nd annealed sample B, as well as the
deduced critical thickness hc.
Samples
|
Modified curvature (Qf–Qs) (m−1)
|
Strain σ
|
Stress (MPa)
|
hc(μm)
|
As-grown sample B
|
5.55×10−2
|
3.55×10−4
|
77.59
|
16.51
|
Sample B after the 1st annealing
|
4.30×10−2
|
2.75×10−4
|
60.11
|
27.50
|
Sample B after the 2nd annealing
|
2.77×10−2
|
1.77×10−4
|
38.69
|
66.39
|
Then the 2nd annealed ZnO film was used as a pseudo-substrate
on which to re-grow ZnO film. Fig. 3 shows
the microscopy images of samples grown on the 2nd annealed sample B and
as-grown sample B. First, another layer of ZnO was grown on as-grown and annealed
sample B in the same batch. The growth time is 10 min., and the
accumulated thickness is about 28 μm. It is shown in Fig. 3(a) that serious cracks
appear on B0,
grown on as-grown sample B, with part of the film flaking off the substrate.
Instead of cracks, however, isolated islands with flat tops are found on sample
B1 (Fig. 3(b)), grown on the 2nd annealed sample B. This
can be explained by the selective growth model, caused by the redistribution of
the stresses and the defects after annealing. Then, sample B1 was
reloaded into the growth chamber to re-grow 10 min. This time the total
thickness is 45 μm for the sample, labeled as sample B2.
There is obvious coalescence of the isolated islands in sample B2 (Fig. 3(c)). After that, another 10 min. growth of
ZnO was grown on sample B2 and the sample was called sample B3 in order. The accumulated thickness reaches ∼65 μm. It is shown in Fig. 3(d) that a flat surface occurs, indicating the
typical Zn-polar morphology. The cross-section image is shown inFig. 3(e),
exhibiting an even interface without cavies. Finally, another 3 min.
growth of ZnO was performed on sample B3.
The sample is called sample B4. Few cracks
in an interval of 3–4 mm were observed on sample B4 (Fig. 3(f)). The total thickness of sample B4 is
about 70 μm.
Fig. 3. Surface
morphologies of samples: (a) B0; (b) B1; (c) B2; (d) B3; (e) the cross section image of Sample B3; (f) B4. Sample B0 is grown on as-grown sample B, regarded as the
comparative counterpart of sample B1. Sample B1 is grown on the 2nd annealed sample B, and
samples B2, B3 and B4 are the consecutive growth based on the
predecessor with the beginner of sample B1. That is, sample
B2 is grown on B1, B3 on B2, and B4 on B3, respectively.
Cracks
appear when the film thickness is above its critical thickness. And the
beginning formation of cracks can be evaluated as the experimental critical
thickness. Therefore, the critical thickness is about 70 μm (sample B4) for ZnO film grown on annealed ZnO
pseudo-substrates, much higher than the value of about 15 μm (sample A)
for as-grown ZnO samples.
The theoretical critical thicknesses
for ZnO pseudo-substrate can be evaluated using the following expression:
(3
where εf is the co-axial
strain in ZnO pseudo-substrate obtained from Eq. (2),
and Z is a
dimensionless parameter set at 3.951 for small, penny-shaped cracks at the film
surface. The theoretical critical thickness for the as-grown sample B and the
1st and 2nd annealed sample B used for pseudo-substrates are listed in Table 1.
The results show that the computed data are close to the experimental values,
and that the critical thickness increases much after annealing, which can be
explained by re-crystallization during annealing.
Table 2 lists the full-width at half-maximum (FWHM) of DCXRD spectra
for (0 0 2) and (1 0 2) planes in as-grown sample B and the
re-growth samples including sample B1, B2 and B3. The FWHMs of (0 0 2) and
(1 0 2) planes decrease when the ZnO films grow thicker, indicating a
great improvement in the crystal quality. The FWHM of (0 0 2) and
(1 0 2) in sample B3 is almost half of those in the
as-grown sample B. The decreases of the FWHMs support that there exists a
process of re-crystallization during annealing.
Table 2.
FWHMs
of DCXRD curves for (0 0 2) and (1 0 2) planes in as-grown
sample B and the re-growth samples (B1, B2 and B3).
Samples
|
(0 0 2) (arcsec)
|
(1 0 2) (arcsec)
|
As-grown
sample B
|
467
|
824
|
Sample
B1
|
367
|
554
|
Sample
B2
|
305
|
484
|
Sample
B3
|
270
|
465
|
Crack-free
ZnO films with thickness beyond 65 μm should be obtained if optimized
annealing conditions are to be used, such as temperature, pressure and period.
To get a crack-free ZnO film of about 130 μm, the tensile strain in ZnO
should be reduced to 1.25×10−4, a little
larger than that of the thermal strain. Therefore, though more efforts should
be paid to reduce the non-thermal stress, it is feasible to get crack-free ZnO
film with thickness enough to serve as free-standing substrate. GaN template,
if added, is expected to speed up the process, as there have been many reports
about quality improvement in ZnO films after using GaN templates.
4.
Conclusions
In summary, we have obtained
crack-free ZnO film with thickness of ∼65 μm,
grown directly on sapphire substrate without GaN template. The measurements
using wafer curvature method show that non-thermal strain is larger than
thermal strain in ZnO/sapphire growth system. Annealing was used to reduce the
tensile stress in ZnO/sapphire films. Crack-free ZnO film was obtained by
re-growing ZnO on the annealed ZnO pseudo-substrate. To further improve the
critical thickness of ZnO film on sapphire substrate, a combination of using
annealed ZnO pseudo-substrate and exploiting GaN template is provided.
Source:Materials Science in Semiconductor Processing
If you need more information about Crack free ZnO thick film on sapphire substrate without GaN template grown by MVPE, please visit: http://www.powerwaywafer.com or send us email at gan@powerwaywafer.com.
If you need more information about Crack free ZnO thick film on sapphire substrate without GaN template grown by MVPE, please visit: http://www.powerwaywafer.com or send us email at gan@powerwaywafer.com.
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