1. Introduction
The high-temperature epitaxial
growth of gallium nitride (GaN) and related device structures directly on
[00.1]-oriented silicon carbide (SiC) substrates is significantly hampered by
the mismatches in the lattice parameters and the coefficients of thermal
expansion between these materials. In the use of metalorganic vapor phase
epitaxy (MOVPE) it is common practice to deposit an intermediate
buffer/nucleation layer of aluminum nitride (AlN) or aluminum gallium nitride
(AlGaN) to facilitate nucleation of the subsequently deposited GaN film. Buffer
layers on SiC(00.1) have not been as intensively investigated as those on
similarly oriented sapphire. In the latter case, a buffer layer deposited at
low temperatures promotes the nucleation and growth of GaN islands at high
temperatures. Tanaka et al. have shown that AlN deposited on 3.5° off-axis
6H-SiC(00.1) substrates at ∼1100°C by gas source
molecular beam epitaxy grows via the Stranski–Krastanov (S–K) mode. A coherent
AlN layer covers the SiC surface to a thickness of ∼4 nm at which point the total strain energy in the film
exceeds the surface energy associated with the formation of AlN islands which
nucleate and subsequently coalesce. Complete AlN films are commonly grown via
MOVPE at ∼1100°C using on-axis
SiC(00.1) substrates.
Buffer layers and subsequent GaN
layers contain a high density of threading dislocations when grown on the basal
planes of both sapphire and SiC. The source of threading dislocations was
originally postulated to result from the tilt and twist in low-angle boundaries
which form during coalescence of GaN islands in the initial stages of growth.
However, results from Mahajan and co-workers have indicated that this
hypothesis is not correct for GaN films deposited on sapphire. These
investigators assert that in this material assembly, the source of the
threading dislocations are either climb segments of dissociated Shockley
partials (pure edge dislocations) or coalescence of Frank faults (pure screw
dislocations), both of which are located in the highly faulted regions of the
low-temperature GaN nucleation layer. Stacking faults are rarely observed in
AlN(00.1) and GaN(00.1) films grown on 6H-SiC(00.1) and AlN/SiC(00.1)
substrates, respectively, via MOVPE; thus, the source(s) of threading
dislocations in these structures have not been definitively established and may
well be a function of island coalescence in the initial AlN and GaN layers.
Evidence to support this conjecture is presented herein.
A significant percentage of the
dislocations in the AlN deposited on the SiC(00.1) continues to thread into the
subsequently grown GaN. However, as GaN deposited via MOVPE at 1010°C has been
shown by Einfeldt et al. to also
grow on coalesced AlN films via the S–K mechanism, new threading dislocations
are likely formed during the coalescence of the islands of the GaN. Related
research by the same team of investigators has recently determined that Al0.15Ga0.85N films grown
on SiC(00.1) initially phase separates into GaN- and AlN-rich islands. The
former grows vertically and laterally more rapidly than the latter, and the Al-rich
islands are eventually enveloped by the GaN-rich phase. These defects are
predominantly edge in character (a-type), are oriented normal to the surface of
the film and have a density of approximately 1×109 cm−2 within
the GaN. They are now presenting significant barriers to the advancement and
commercialization of selected III-Nitride-based devices as they cause (1)
carrier scattering and recombination, (2) high leakage currents at device
junctions, (3) lower than expected breakdown fields and (4) decreased internal
efficiencies and higher threshold current densities in InGaN/GaN laser diodes.
A commonly quoted structural
property used to relate to the character of a particular thin film is the
full-width at half-maximum (FWHM) value of the X-ray rocking curve. Broadening
of a rocking curve is related to a marked increase in the dislocation content.
Multiple peaks in this curve are indicative of multiple crystal domains of
different orientations in the sampling area. On-axis rocking curves of
III-Nitride films grown on SiC(00.1) are typically acquired from the (00.2),
(00.4) or (00.6) reflections, and their FWHM are influenced by the density of
screw dislocations. Off-axis rocking curves are commonly acquired from several
peaks including the (10.3), (10.5) or (20.1) reflections, and their FWHM are
influenced by the densities of dislocations having screw, edge and mixed
characters. Direct measurement of the direction of pure edge dislocations is
not possible in on-axis (00.1) substrates, such as those used in this research,
as the incident beam direction would have to be 90°, i.e. from the side of the
wafer. However, techniques to estimate the density of pure edge dislocations
are described below and employed for that purpose in this research.
It is important to note that domains
and domain tilting in the SiC substrates can have a pronounced effect on the
measured FWHM of the rocking curves of subsequently deposited nitride films.
Typical 6H-SiC(00.1) substrates contain numerous domains with an approximate
average diameter of a millimeter that are misaligned to each other . In
GaN/AlN/6H-SiC(00.1) assemblies the substrate variability causes sufficiently
large changes in the FWHM of these curves to hide improvements in the films due
to a reduction in dislocation density. The research presented below is
concerned with (1) the characterization routes and the analysis necessary to
determine the domain structure and defect substructure in SiC wafers, (2) the
procedures and analyses needed to remove the influence of the domain structure
on the FWHM values of X-ray rocking curves of GaN, (3) the dislocation
densities in GaN layers as a function of thickness and (4) the efficacy of (a)
the surface microstructure of AlN buffer layers deposited at different
temperatures and (b) of Al0.20Ga0.80N buffer layers in reducing the FWHM of the
rocking curves in subsequently deposited GaN films.
2. Experimental procedure
The GaN films investigated in this
research were grown in situ on as-received, 2 in diameter, research grade,
on-axis, 6H-SiC(00.1) wafers on which a 100 nm AlN or a 100 nm Al0.2Ga0.8N buffer layer
had been previously deposited. The growths were conducted in a vertical flow,
pancake style, metalorganic vapor-phase epitaxy system at approximately 1020°C
and 20 Torr. High-resolution X-ray diffraction was conducted using a
Philips X’Pert Materials Research Diffractometer (MRD) with a copper X-ray
source at a power setting of 40 kV and 45 mA. Approximately 100(ω/2θ) scans were
conducted over the entire surface of each SiC wafer using a four-bounce Ge 220
crystal monochromater on the incident beam side and an open slit on the
detector side. This arrangement allowed both the acquisition of the GaN and SiC
rocking curve peaks in the same scan and the same spot illumination for both
peaks, if the slight spot size variation due to the 2θ angle
change between the peaks was neglected. It was therefore important to choose
film and substrate peaks close to each other in reciprocal space to minimize
geometry variations.
Synchrotron white-beam X-ray
topography (SWBXT) images were acquired at the Stony Brook Synchrotron
Topography Facility at the National Synchrotron Light Source located at the
Brookhaven National Laboratory. The double-sided polished 6H-SiC(00.1) wafers
provided maximum image clarity. Images were taken in back-reflection geometry with
the reciprocal wave vector as g=00.24, a wavelength of λ=1.24 Å,
and specimen to film distance of 20 cm.
3. Results and discussion
The domains present in typical
research grade 6H-SiC wafers are quite variable in size and in the degree of
tilt. As such, characterization of numerous substrates was conducted to
determine if trends existed among wafers. Fig. 1 shows
a histogram of the frequency of occurrence of FWHM over the full range of
values acquired from over 2000 X-ray rocking curve scans of the (00.6) peak of
26 2-in 6H-SiC wafers. The scans were arranged on the substrate in a grid with
a 5 mm2 square
spacing. Due to the irregular shape of the curves, peak fitting to obtain the
FWHM values was not possible. The values shown in Fig. 1 were obtained by taking the widest
half-height possible in the recorded data. Typical scans and FWHM values
obtained from both a single 6H-SiC wafer and the associated GaN film grown on
an AlN buffer layer are shown in Fig. 2These results illustrate the
variability in rocking curves that exist in both materials.
Fig. 1. Plot of
normalized frequency of occurrence of the FWHM values of X-ray rocking curves
determined for SiC(00.6) vs. selected values of these FWHM measured within the
total range found in 26 6H-SiC(00.1) wafers.
Fig. 2. Four composite
sets of X-ray rocking curves acquired from different areas of a GaN(00.2) film
and the associated 6H-SiC(00.1) substrate that show that the (00.2) spectra of
the GaN mimic the width and principal features of the underlying (00.6) spectra
of the SiC.
The variability of the FWHM is particularly obvious when the
values are arranged in a contour plot format such as the 2.5 mm resolution
map of a typical wafer in this study as shown in Fig. 3. SWBXT images such as that shown in Fig. 4 compare favorably with rocking curve
maps of SiC wafers with regard to the identification of domain structures and
highly defective areas of the substrate. Dark areas of the latter image are
caused by agglomerations of defects. The black or white bands are indicative of
tilted domain boundaries. The more defective (darker) areas of the image
correlate with the areas of large FWHM values in Fig. 3.
Fig. 3. Contour map of
the FWHM of X-ray rocking curves acquired from a typical 6H-SiC(00.1) wafer
illustrating the variability within a single wafer. The scale at right shows
the ranges of values of the FWHM used in the map and the gray-scale value associated
with each.
Fig. 4. Synchrotron
back reflection topography image of the same wafer shown in Fig. 3. Note
the correlation between the darker, more defective areas in the topography
image and the higher values of the FWHM shown in Fig. 3.
Investigations concerning whether or not the results
presented above were due to surface and/or subsurface damage caused by
polishing were also conducted. Fig. 5 shows
contour maps acquired from four wafers from the same boule. It can be seen
within each map that the same patterns of high- and low-domain tilting are
propagated along the c-axis
of the boule during growth. Therefore polishing or etching the surface,
including hydrogen etching at high temperatures, would not effect the domain
tilting observed in the substrates.
Fig. 5. Maps of X-ray
rocking curve FWHM for four SiC(00.1) wafers from a single boule. Defective
areas of the wafers propagate vertically through the entire boule resulting in
similar patterns in the four wafers. The scale at right shows the ranges of
values of the FWHM used in the maps and the gray-scale value associated with
each.
3.2.
GaN X-ray measurement technique
The tilt among the domains measured within the GaN thin films grown on
the AlN/6H-SiC(00.1) substrates mimics the tilt among the domains within the
underlying wafer, as shown in Fig. 2. In general, the shapes of the GaN
peaks matched the shapes of the SiC peaks in all of the measurements made in
this study, whether the substrate peak shapes were regular or irregular. The
ability to obtain and correlate the GaN and SiC peaks in specific geometries
allows the FWHM values of this film and substrate combination to be plotted
with respect to each other as shown in Figs. 6(a) and (b) for both on- and off-axis data,
respectively, from a 1 μm thick GaN(00.1) film. The unity line in the
figure delineates where the data would lie if the FWHM values for the GaN and
the SiC were equal. The upper curve shows the measured FWHM values for both the
GaN (ordinate) and the SiC (abscissa) for the selected angles of diffraction
for each material. Data shown in this manner were always above the unity line,
indicating that the GaN FWHM values were always higher than that of the
corresponding SiC. The values of the FWHM to the right of the knee of the curve
in each of these figures were determined both by dislocation broadening in the
GaN and the extent of domain tilting in the SiC substrate. The FWHM reached a minimum
when the domain tilting and dislocation density in the sampled area were
reduced relative to other areas on the substrate and indicated that the FWHM
values of the GaN were now dominated only by dislocation broadening. Therefore,
to quantify the results for the GaN alone, it was necessary to use data
acquired for this material from areas of the film having FWHM equal to the
minimum to avoid the negative influence of the SiC domain tilting.
Fig. 6. FWHM values of
X-ray rocking curves of GaN and SiC plotted with respect to each other. The
vertical dashed line marks the transition from where the FWHM is limited by GaN
dislocation broadening (left side of line) to where the FWHM measurement limited
by dislocation broadening and domain tilt (right side of line): (a) on-axis
measurements and (b) off-axis measurements.
The
FWHM values for the on-axis X-ray rocking curves of the [00.2] direction in GaN
that occur to the left of the knee in the upper curve in Fig. 6(a) are broadened only by screw
dislocations (c-type, b=[00.1]) and are therefore not useful in the
characterization of threading dislocations which are predominantly edge
(a-type, b=[11.0]) or mixed (c+a type, b=1/3[11.3])
in character .
Additionally, the density of pure screw dislocations in GaN thin films on
AlN/SiC assemblies are at least 2–3 orders of magnitude lower than the density
of mixed or pure edge dislocations; thus, analysis of the [00.2] direction
alone would underestimate the dislocation broadening of the overall film. The
results shown in Fig. 6 support
this conclusion where the portion of the data showing only the dislocation
broadening is located at values ⩽190 s−1 (Fig. 6a) and ⩽420 s−1 (Fig. 6b) for the on- and off-axis data,
respectively. Since the off-axis scans are sensitive to the density of edge and
screw dislocations, and the density of the former is of the order of 1×109 cm−2 in these films, the influence of
crystal domain tilt is less evident, and the effect of dislocation broadening
is greater. This is shown by the occurrence of the knee in the upper curve of Fig. 6(b) at a higher FWHM value than in Fig. 6(a).
3.3.
GaN thickness results
As described in the Introduction, direct X-ray measurement of pure edge
dislocation rocking curve broadening is not possible because of the sample
geometry. The X-ray inclination angle technique discussed by Srikant et al. and
others allows an estimation of the density of the pure edge dislocations in a
film by making X-ray measurements of the dislocation broadening starting with
planes oriented parallel to the normal surface (0°) and subsequently tilting
the sample towards the pure edge geometry at 90° from the normal surface. Using
this technique it is possible to quantify the dislocation populations in the
film, if the exact character and distributions of the dislocations in the film
are known. Defect populations and distributions of GaN films on AlN/SiC
substrates grown via MOVPE have not been established by prior TEM examinations,
so it is not possible to make quantitative calculations for the current
samples. However, it is possible to make a meaningful comparison between samples,
if one assumes that similar growth conditions will result in similar
dislocation character and distribution.
A good test case for this hypothesis is the comparison of GaN thin films
with different thicknesses. Increasing the thickness of these films is known to
decrease the density of the threading dislocations through dislocation
annihilation mechanisms ; thus, a 0.5 um film should have a greater density of threading
dislocations than a 2.5 um film. Fig. 7 shows
a plot of the FWHM for several GaN films of varying thicknesses as a function
of the inclination angle. The data for the figure were obtained by using the
average FWHM value (taken from the dislocation broadening portion of the data
only) for each film studied at inclination angles ranging from 0° to 75°. This
allows a trend to be established so that a line extrapolated to the 90° lattice
inclination is indicative of the density of the edge dislocations. It is
important to note that the X-ray measurements taken in this study are a bulk
average of the entire thin film, and not a measurement of the defects at the
surface of the film.
Fig. 7. Plots of the
FWHM of X-ray rocking curves obtained at six inclination angles in GaN films of
various thicknesses. All data were acquired from the portions of the curves
similar to those shown in Fig. 6 controlled only by
dislocation broadening. All films had AlN buffer layers. Line fits are a guide
to the eye.
A
point of note with regard to the X-ray measurement sampling volume is that each
Bragg peak will have a different spot size due to the projection of the beam
cross-section onto the tilted sample. Peaks acquired using larger spot sizes
will represent a larger number of substrate domains and thus appear to have
more scatter and higher FWHM values if a poor area of the film has been
interrogated. This effect is particularly noticeable in some higher-order
planes such as the (10.1) and the (20.1), which are analyzed using spot sizes
that are roughly double of that used for the (00.2) peak. Alternatively, the
(00.4) and (00.6) peaks have spot sizes that are respectively 50% and 33% of
the size of the (00.2) peak.
3.4.
Buffer layer results
SEM analyses of our AlN buffer layers grown to the same thickness on
6H-SiC(00.1) wafers at 1010°C (the growth temperatures for our GaN films),
1130°C and 1220°C reveal an extremely pitted, a less pitted and a very smooth
microstructure, respectively, as shown in Fig. 8. Using the techniques described in 3.2 and 3.3we obtained the results shown in Fig. 9(a) for GaN films grown on these layers.
These plots show that the GaN films grown on the 1130°C and the 1220°C buffer
layers had the lowest and highest edge dislocation densities (as read from the
graph at the 90° inclination), respectively. That is, the growth of GaN upon
the smoothest AlN template resulted in the highest edge dislocation density in
the former material. The reason(s) for this phenomenon are not known at this
writing.
Fig. 8. SEM images of
the surface microstructure of AlN films after 100 nm of growth on 6H-SiC
at 1010°C (left), 1120°C (center), and 1220°C (right).
Fig. 9. Plots of the FWHM of X-ray rocking curves obtained at
three inclination angles in GaN films deposited at 1010°C on (a) AlN buffer
layers grown at different temperatures and (b) buffer layers of different
chemical composition. All data were acquired from the portions of the curves
similar to those shown in Fig. 6 controlled only by dislocation
broadening. All buffer layers are 100 nm thick. Line fits are a guide to
the eye.
An
AlxGa1−xN buffer layer is employed to both reduce the lattice
misfit that is present at the GaN/buffer interface and to allow doping of this
alloy for vertical device structures. The roughness of the surface of the AlxGa1−xN layer grown on SiC(00.1) surfaces decreases with
increasing growth temperature and influences the initial nucleation and the
growth of the GaN film. Recent research has shown that GaN does
not form islands on an Al0.2Ga0.8N buffer layer but continues the terrace structure
of this latter layer via step-flow. The steps of the GaN also gradually become
straighter and more regularly spaced, which indicates that the number of
defects at the steps that pin growth fronts are decreasing. As shown in Fig. 9(b),
the use of an AlGaN layer having either a graded or a fixed composition reduces
the edge dislocation density in the GaN when compared with the use of the best
AlN buffer layer deposited at 1130°C (see above). The GaN films grown on both
AlGaN buffer layers resulted in statistically equal edge dislocation densities;
however, that grown on the non-graded buffer layer had lower on-axis FWHM
values indicating a relatively lower density of screw dislocations.
The difference in the edge dislocation densities in the GaN films grown
on the AlGaN and the AlN buffer layers indicates two possibilities that
influence the formation of edge dislocations in GaN. (1) Island coalescence
increases the edge dislocation density of the final GaN films. GaN/AlN/SiC and
GaN/AlGaN/SiC assemblies have two and one island nucleation and coalescence
processes, respectively; for as implied above, GaN does not re-nucleate on
AlGaN. (2) Misfit at the GaN/buffer layer interface may contribute to the
formation of edge dislocations. Growth of GaN on an Al0.2Ga0.8N buffer layer
reduces the lattice mismatch between these layers by approximately 80% relative
to growth on AlN.
4. Summary
Wafers of 6H-SiC possess a varied domain structure that strongly
influences the values of the FWHM of the rocking curves of this material. The
magnitude of this influence makes it necessary to account for this variation
when investigating via X-ray methods the structure and microstructure of, e.g.,
AlN and GaN films that may be grown on these wafers. In this research, numerous
FWHM data points were simultaneously acquired for GaN(00.1) films and the
underlying SiC(00.1) substrates using on- and several off-axis diffraction
directions. These data were used to obtain estimates of the values of the FWHM
of the GaN rocking curves derived only from GaN dislocation broadening while
avoiding the influence of the domain tilting in the SiC substrates. These
values, acquired at particular angles of lattice plane inclination, showed that
the influence of the edge dislocation component on the FWHM of the rocking
curves increased with increasing inclination. This method also showed, as
expected, that the density of edge dislocations decreases as a function of GaN
thickness due to dislocation annihilation when these films are grown on AlN/SiC
substrates. It was assumed that samples grown under similar conditions have
similar dislocation population distributions.
The influence of the buffer layer on dislocation density in GaN films
was investigated via growth of this material on both AlN and AlGaN buffer
layers. A GaN film deposited on an AlN buffer layer grown at 1130°C possessed
lower FWHM values in the rocking curves for the edge dislocation component of
the film than did similar films deposited on AlN grown to the same thickness at
1010°C or 1220°C. GaN films deposited on Al0.2Ga0.8N buffer layers grow via a step-flow mechanism
which measurably reduces the density of threading edge dislocations because of
the combination of reduced misfit between the GaN and the AlGaN layers and the
absence of boundaries of coalesced islands.
Source: Journal of Crystal Growth
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