Epitaxial semimetallic HfxZr1-xB2 templates for optoelectronic integration on silicon

Article abstract of DOI:10.1063/1.2403189

High quality heteroepitaxial Hfx Zr1-x B2 (x=0-1) buffers were grown directly on Si(111). The compositional dependence of the film structure and ab initio elastic constants were used to show that hexagonal Hfx Zr1-x B2 possess tensile in-plane strain (0.5

Full text of DOI:10.1063/1.2403189

Epitaxial semimetallic Hf x Zr 1 − x B 2 templates for optoelectronic integration on
silicon
Radek Roucka, YuJin An, Andrew V. G. Chizmeshya, John Tolle, John Kouvetakis, Vijay R. D'Costa, José
Menéndez, and Peter Crozier
Citation: Applied Physics Letters 89, 242110 (2006); doi: 10.1063/1.2403189
View online: http://dx.doi.org/10.1063/1.2403189
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APPLIED PHYSICS LETTERS 89, 242110 ͑2006͒
Epitaxial semimetallic HfxZr1−xB2 templates for optoelectronic
integration on silicon
Radek Roucka, YuJin An, Andrew V. G. Chizmeshya, John Tolle, and John Kouvetakisa͒
Department of Chemistry and Biochemistry, Arizona State University, Tempe, Arizona 85287-1604
Vijay R. D’Costa, José Menéndez, and Peter Crozier
Department of Physics and Astronomy, Arizona State University, Tempe, Arizona 85287-1504
͑Received 10 August 2006; accepted 25 October 2006; published online 13 December 2006͒
High quality heteroepitaxial HfxZr1−xB2 ͑x=0–1͒ buffers were grown directly on Si͑111͒. The
compositional dependence of the film structure and ab initio elastic constants were used to show that
hexagonal HfxZr1−xB2 possess tensile in-plane strain ͑0.5%͒ as grown. High quality HfB2 films were
also grown on strain compensating ZrB2-buffered Si͑111͒. Initial reflectivity measurements of thick
ZrB2 films agree with first principles calculations which predict that the reflectivity of HfB2
increases by 20% relative to ZrB2 in the 2–8 eV range. These tunable structural, thermoelastic, and
optical properties suggest that HfxZr1−xB2 templates should be suitable for broad integration of III
nitrides with Si. © 2006 American Institute of Physics. ͓DOI: 10.1063/1.2403189͔
With increasing device integration and miniaturization, it
is highly desirable to grow Al–Ga–N optoelectronic devices
directly on Si wafers. These are inexpensive, widely avail-
able, and can be produced in large diameters suitable for
commercial scale integration. The main complication of this
approach is the difference in crystal structures ͑hexagonal
wurtzite versus diamond cubic͒. Even so, it is well known
that the hexagonal symmetry of the Si͑111͒ surface promotes
growth of hexagonal nitrides1 in spite of the significant mis-
match between the Si–Si distance ͑3.84 Å͒ and the hexago-
nal basal parameters of GaN ͑a0=3.19 Å͒ and AlN ͑a0
=3.099 Å͒. A large mismatch also exists between the thermal
expansion coefficients of Si ͑2.6ϫ10−6 K−1͒ and GaN ͑5.6
ϫ10−6 K−1͒. To circumvent these difficulties, we have re-
cently developed a hybrid substrate technology based on in-
termediate thin ZrB2 buffer layers.2,3 Perfectly epitaxial ZrB2
films with defect-free microstructures and atomically smooth
surfaces were deposited on Si͑111͒ via a coincidence-misfit
mechanism in which the strain is accommodated by edge
dislocations along the interface. This approach has several
important advantages: ͑a͒ the semimetallic character of ZrB2
provides technologically useful buffer layers that are reflec-
tive and possess low resistivity;4 ͑b͒ the thermal expansion
coefficient of ZrB2 ͑5.9ϫ10−6 K−1͒ is similar to that of GaN
͑5.6ϫ10−6 K−1͒, and its thermal conductivity is comparable
to single-crystal Si; and ͑c͒ the reported in-plane lattice pa-
rameter has a small mismatch with GaN ͑+0.7%͒ and AlN
͑−2.1%͒, accommodating close lattice matching.
Al0.2Ga0.8N, reducing strain and leading to high quality op-
tical properties.
While these studies showed that thin buffer layers are
suitable for subsequent heteroepitaxy, the desirable proper-
ties, including high conductivity, reflectivity, and thermal
matching, are only optimized in thicker layers ͑Ͼ100 nm͒.
An objective of this study is therefore to develop a practical
rapid growth process to obtain large-area, large thickness
films suitable for device development.
The thin ZrB2 films were typically grown by gas source
molecular beam epitaxy ͑GS-MBE͒ of Zr͑BH4͒4 at 900 °C
and 10−7 Torr. Our initial rapid growth rate studies were con-
ducted at higher pressures which led to poisoning of the
surface with B-rich fragments ͓due to gas phase reactions of
the Zr͑BH4͒4͔ and high surface roughness of ϳ20 nm. We
circumvented these problems by mixing Zr͑BH4͒4 ͑1%–2%
by volume͒ with high purity H2 to obtain thick films of up to
ϳ0.5 ␮m and surface roughness of ϳ2 nm. These films
were characterized by Rutherford backscattering ͑RBS͒, high
resolution cross sectional transmission electron microscopy
͑XTEM͒, Z-contrast imaging, and high resolution x-ray dif-
fraction ͑HR-XRD͒ as discussed below.
The thick ZrB2 layers synthesized here allowed us to
obtain reliable film reflectivities across the entire spectrum.
The dielectric functions in the 0.2–0.8 and 0.7–8.8 ranges
were determined at 22 °C using an infrared variable angle
spectroscopic ellipsometer and a variable angle ellipsometer,
respectively. The air reflectivity of pure ZrB2 grown on
Si͑111͒ is plotted as a function of incident photon energy in
Fig. 1 where it is compared with the reflectivity of bulk Si.
The sample exhibits metallic behavior with a reflectivity in-
creasing sharply from values near 0.5 to unity at low photon
energy ͑IR region͒. However, in the region from 2 to 6 eV
͑620–200 nm͒ relevant to AlGaN applications, the reflectiv-
ity of ZrB2 is lower than that of Si.
In our previous reports, we demonstrated the use of thin
͑20–30 nm͒ ZrB2 layers on Si͑111͒ to grow GaN by molecu-
lar beam epitaxy ͑MBE͒ and Al0.2Ga0.8N by metal organic
chemical vapor deposition3 ͓note that ZrB2 growth on
Si͑100͒ produced a two-domain surface structure precluding
subsequent nitride growth͔. We found that these samples ex-
hibit intense band edge photoluminescence at 10 K. In this
context ZrB2 virtually matches the lattice constant of
To elucidate these differences, we calculated the reflec-
tivities of ZrB2 and Si from first principles using density
functional theory ͑DFT͒. We also simulated the reflectivity of
the isostructural HfB2 analog to explore whether the
ZrB2–HfB2 system can be used to tune the optical properties
of buffer layers on Si. All calculations were performed using
a͒
Author to whom correspondence should be addressed; electronic mail:
jkouvetakis@asu.edu
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0003-6951/2006/89͑24͒/242110/3/$23.00 89, 242110-1 © 2006 American Institute of Physics
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242110-2
Roucka et al.
Appl. Phys. Lett. 89, 242110 ͑2006͒
FIG. 1. ͑Color online͒ Measured ͑symbols, 22 °C͒ and calculated ͑lines͒
reflectivities for ZrB2 ͑red͒, HfB2 ͑blue͒, and Si ͑gray͒ showing metallic
character ͑Rϳ1 in the IR region͒. A scissors correction of ⌬Escis=0.65 eV
was applied to the Silicon band structure.
FIG. 2. ͑Color online͒ ͑a͒ RBS spectra of 40 nm thick Zr0.70Hf0.30B2 film.
͑b͒ XRD ͑1គ13͒ reciprocal space map from Zr0.70Hf0.30B2 /Si͑111͒ yielding
a=3.176 Å and c=3.505 Å ͓reciprocal lattice units ͑rlu͒ are employed in
which Qx =␭a 3 ͑␭=1.54 Å͒ and Qy =3␭/2c͔. ͑c͒ Diffraction contrast mi-
crograph of the entire layer showing defect-free microstructure and smooth
surface. Inset: high resolution image of the perfectly epitaxial interface.
ͱ
the full-potential linearized augmented plane wave method
as implemented in the EXCITING DFT code5 ͑details will be
presented elsewhere͒. Due to the semimetallic character of
the compounds, the complex dielectric function was calcu-
lated as the sum of interband and intraband contributions.6
For the latter we use a Drude model ␧=1−␻2p/͑␻2+i␻⌫͒,
͑1 − x͒Zr͑BH4͒4 + xHf͑BH4͒4 → Zr1−xHfxB2 + B2H6
+ 5H2.
Figure 2͑a͒ shows the RBS spectrum of a Zr0.70Hf0.30B2
sample with a nominal thickness of 50 nm. The ratio of the
where ␻ is the free-electron plasma frequency ͑calculated
p
values are q␻p=4.56 and 4.81 eV for ZrB2 and HfB2, re-
spectively͒, and a lifetime broadening of ⌫=50 meV was
found to reproduce the observed low energy behavior. The
steep rise observed in IR reflectivities of both HfB2 and ZrB2
is due to the Drude term. The close agreement between the
observed and simulated spectra for Si and ZrB2 suggests that
the reflectivity of pure HfB2 films should be about 20%
larger than that of ZrB2 in the 2–8 eV range. Hf–Zr–B2
materials grown on Si may therefore offer tunable reflectivi-
ties across the entire spectral range. This behavior is ex-
pected to be of particular significance for the design of ni-
tride based interband ͑near-IR͒ and intersubband ͑IR͒
devices grown on Zr1−xHfxB2-buffered Si.
Alloys of ZrB2, with bulk hexagonal lattice constants
a0=3.169 Å and c0=3.525 Å, and HfB2 ͑a0=3.142 Å and
c0=3.48 Å͒ are expected to have lattice constants smaller
than those of ZrB2 and should therefore make it possible to
grow higher Al content AlxGa1−xN layers with less strain,
paving the way towards full integration of these materials
with Si. Accordingly, we synthesized Zr1−xHfxB2 materials
across the entire compositional range and explored their use-
fulness for simultaneous thermal and lattice matching appli-
cations. We develop a strain analysis for hexagonal films on
Si͑111͒ based on compositional interpolation of the elastic
constants of ZrB2 and HfB2.
aligned versus the random peak heights ͑␹ ͒ is 6% for both
min
Hf and Zr, indicating their full substitutionality in the alloy
and the high degree of epitaxial alignment of the film with
respect to the Si substrate. XTEM indicates that the layers
are perfectly monocrystalline, highly coherent, and atomi-
cally flat ͓Figs. 2͑b͒ and 2͑c͔͒. The lattice misfit is taken up
by pure edge-type dislocations via insertion of extra ͕11គ00͖
planes along the ͓112គ0͔ direction.2 Diffraction contrast mi-
crographs reveal that no threading dislocation cores propa-
gate to the surface within the field of view of ϳ3 ␮m. Elec-
tron energy loss spectroscopy ͑EELS͒ with a nanometer-
sized electron probe shows that the constituent Zr and Hf
elements appear together at every nanometer scale region
probed without any segregation of the individual alloy com-
ponents. Atomic force microscopy ͑AFM͒ revealed a smooth
surface with a roughness of ϳ2 nm for a 5ϫ5 ␮m2 area
which is highly suitable for buffer layer applications. The
surface morphology further improves by ex situ annealing of
the films at 900 °C and 10−9 Torr for 8 h. This yields a
roughness below 1.5 nm and a surface consisting of large
atomically flat areas connected by steps.
For all Zr1−xHfxB2 films, HR-XRD on-axis scans show
only the ͑001͒ and ͑002͒ peaks of the AlB2 structure, ori-
ented with ͑0001͒ parallel to the Si͑111͒. The ͑002͒ peaks,
which are more sensitive to the compositional changes of the
alloys, were significantly shifted from those of pure ZrB2.
The absence of splitting or broadening in these peaks cor-
roborates the full substitutionality of both Hf and Zr atoms in
the lattice. We have also observed thickness fringes in the
vicinity of the ͑001͒ and ͑002͒ reflections, confirming the
high quality of the interface and the uniformity and smooth-
ness of the layers. Accurate a and c lattice parameters were
extracted from the asymmetric ͑1គ13͒ reciprocal space maps
of the AlB2 structure and are provided in Table I for samples
with thicknesses of ϳ50 5 nm. Due to strain, these mea-
sured lattice parameters do not correspond to the relaxed
Films of Zr1−xHfxB2 solid solutions were grown by
GS-MBE using the Hf͑BH4͒4 and Zr͑BH4͒4 precursors
mixed with H2 at 2% by volume. The composition in the film
was tuned by varying the ratio of Hf͑BH4͒4/Zr͑BH4͒4 in the
stock mixture. The Si͑111͒ substrates were outgassed in the
MBE chamber at 650 °C and the native oxide was removed
by flashing at 1200 °C. The gases were then allow to react
on the substrate surface at 900 °C and 10−5−10−6 Torr ͑base
pressure of 10−10 Torr͒ for 30–120 min depending on film
thickness. Under these conditions both precursors thermally
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decomposed to form films according to
hexagonal lattice parameters a0 and c0.
129.120.242.61 On: Wed, 03 Dec 2014 09:57:35

242110-3
Roucka et al.
Appl. Phys. Lett. 89, 242110 ͑2006͒
TABLE I. Lattice parameters of Zr1−xHfxB2 films obtained from HR-XRD
analysis ͑bold font͒. The relaxed lattice parameters ͑a0 and c0͒ and calcu-
lated strains ͑␧ and ␧ ͒ are obtained from a compositional interpolation of
a
c
the elastic ratio ␰͑x͒ and bulk c/a ratio ␩͑x͒.
x
0
15
30
60
100
a
3.187
+0.52
3.521
−0.30
−0.584
1.114
3.170
3.531
3.183
+0.54
3.513
−0.32
−0.580
1.113
3.166
3.524
3.176
+0.50
3.505
−0.28
−0.574
1.112
3.160
3.515
3.167
+0.48
3.491
−0.27
−0.565
1.111
3.160
+0.66
3.467
−0.36
−0.552
1.108
3.140
3.480
␧
a
c
␧
c
␰͑x͒
␩͑x͒
a0
3.152
3.501
c0
FIG. 3. ͑Color online͒ ͑a͒ Z-contrast image of HfB2 /ZrB2 /Si͑111͒ hetero-
structure. ͑b͒ High resolution XTEM of the perfectly epitaxial HfB2 /ZrB2
interface and corresponding EELS composition profile of Hf ͑M edge͒ and
Zr ͑L edge͒.
For hexagonal films with the ͓0001͔ plane oriented nor-
mal to the substrate, the perpendicular ͑␧ ͒ and parallel ͑␧ ͒
c=3.48 Å of bulk HfB2, due to strain imposed by the Si
substrate. These results motivated us to pursue growth of
HfB2 films on isostructural ZrB2 buffers ͓rather than directly
on the Si͑111͒ surface͔ to promote formation of smooth films
suitable for nitride integration. These HfB2 films grow much
more readily ͑ϳ2 nm/min͒ and exhibit exceptional morpho-
logical and structural properties, including flat surfaces
͑AFM roughness ϳ2 nm͒, highly coherent interfaces, and
virtually defect-free microstructures. The XRD measure-
ments show that the layers are partially strained and the lat-
tice parameters are close to those of HfB2 films grown on Si
͑Table I͒. We note that growth of thin HfB2 layers has been
reported on oxidized Si via decomposition of Hf͑BH4͒4, but
no evidence of epitaxy was given.9 The XTEM data in Fig.
3͑a͒ indicate that our ZrB2 buffers bridge the strain differen-
tial between HfB2 and Si, allowing the formation of perfectly
epitaxial HfB2 films that cannot be obtained directly on Si.
XTEM Z-contrast images and EELS profiles of Zr and Hf
across the interface showed an abrupt transition of the ele-
ments from ZrB2 to HfB2 with no evidence of intermixing
between the two materials at the nanometer scale ͑Fig. 3͒.
In summary, we have shown that heteroepitaxial semi-
metallic HfxZr1−xB2 templates with tunable structural and op-
tical properties represent a potentially very useful practical
system for optoelectronic integration applications on Si.
c
a
strains are given by ␧c=−2C13␧a/C33, where ␧c=͑c−c0͒/c0
and ␧a=͑a−a0͒/a0. For bulk ZrB2 and HfB2 the known c/a
ratios ͑denoted by ␩ below͒ are only slightly different ͑1.114
and 1.108, respectively͒. Therefore, in order to determine the
strain state, we make the following approximations: ͑i͒ ␩ for
the relaxed epitaxial film is identical to that of the equilib-
rium bulk crystals and ͑ii͒ the c/a ratio ͑␩͒ and elastic ratio
␰=−2C13/C33 are both linear functions of composition;
HfB
ZrB
HfB
ZrB
2
2
2
2
␩͑x͒=x␩
+͑1−x͒␩
and ␰͑x͒=x␰
+͑1−x͒␰ , re-
spectively. Since the elastic constants of both ZrB2 and HfB2
are generally not well known, we used the VASP DFT code7
to calculated them using finite strain deformations of the
equilibrated systems.8 From inversion of the strain relation,
the relaxed lattice constants are then given by a0͑x͒
=͕c͑x͒/␩͑x͒−␰͑x͒a͑x͖͒/͕1−␰͑x͖͒ and c0͑x͒=␩͑x͒ a0͑x͒, and
these are listed in Table I. We note that the relaxed film
lattice constant of the end members match the known values
for the bulk phases. This provides additional justification for
the approximations discussed above. We also note that the
relaxed lattice constants for the alloys follow Vegard’s law
quite closely. Our analysis shows that ZrB2 and the alloy
films exhibit a slight tensile strain ͑␧aϳ +0.50%, ␧cϳ
−0.29%͒, while the HfB2 film is strained even more ͑␧aϳ
+0.66%, ␧cϳ−0.36%͒. The tensile state of these films ͑in-
cluding ZrB2͒ has significant implications for lattice engi-
neering, since this provides better matching with Ga-rich al-
loys. In particular, the measured value of a for the ZrB2 films
is essentially identical to that of GaN.
This work was supported by the NSF ͑EEC-0438400 and
DMR-0221993͒.
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