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Fabrication of Single Crystal
Gallium Phosphide Thin Films on
Glass
Hal
Emmer
1
, Christopher T.
Chen
1
, Rebecca
Saive
1
, Dennis Friedrich
2,3
, Yu
Horie
1
, Amir
Arbabi
1
, Andrei
Faraon
1
& Harry A.
Atwater
1
Due to its high refractive index and low absorption coefficient, gallium phosphide is an ideal material
for photonic structures targeted at the visible wavelengths. However, these properties are only
realized with high quality epitaxial growth, which limits substrate choice and thus possible photonic
applications. In this work, we report the fabrication of single crystal gallium phosphide thin films
on transparent glass substrates via transfer bonding. GaP thin films on Si (001) and (112) grown by
MOCVD are bonded to glass, and then the growth substrate is removed with a XeF
2
vapor etch. The
resulting GaP films have surface roughnesses below 1
nm RMS and exhibit room temperature band edge
photoluminescence. Magnesium doping yielded p-type films with a carrier density of 1.6
×
10
17
cm
−
3
that exhibited mobilities as high as 16 cm
2
V
−
1
s
−
1
. Due to their unique optical properties, these films
hold much promise for use in advanced optical devices.
The realization of high quality gallium phosphide films on arbitrary substrates via transfer fabrication is an impor
-
tant step towards creating novel advanced optical devices. Gallium phosphide is an important photonic material
due to its high refractive index (n
>
3 throughout the visible range
1
) and high transparency across much of the
visible range. Absorption is very low below the indirect bandgap at 2.26
eV and still weak (
α
<
3500 cm
−
1
) below
the L-transition at 2.5–2.6 eV
1
–
3
. In fact, GaP has the highest bandgap of all commonly explored high refractive
index semiconductor materials with n
>
3 throughout the visible range
4
. These properties make GaP well suited
for many advanced visible and infrared photonics applications requiring high refractive index contrast, such as
high contrast gratings
5
and nanophotonic metasurfaces
6
.
High index contrast enables strong light confinement due to the change in amplitude of the electric field at an
interface between materials of different index
7
. This confinement allows the use of subwavelength structures to
guide and otherwise interact with light, necessary for the fabrication of highly integrated photonic devices
8
. The
confinement within a slot waveguide, for example, is given by
n
n
/
HS
22
, where
n
n
an
d
HS
are the refractive indices of
the high index material and the substrate, respectively. The structures explored by Almeida at el. used the Si-air
and Si-SiO
2
materials systems, useful with low losses only beyond the Si band gap; in this case, the telecom wave-
length 1.55
μ
m was discussed. Use of gallium phosphide in similar structures would extend the useful wavelength
for this type of device well into the visible range.
Beyond waveguides, a diverse array of photonic devices exploit high index contrast. Metasurfaces created with
high index contrast materials can enable control over wave amplitude, phase and polarization
9
in subwavelength
antennas, yielding flat filters
10
and lenses
11
created via systematic variation of the dimensions of subwavelength
resonators. GaP also has a high second-order non-linear optical coefficient
12
, useful in photonic devices for par
-
ametric down-conversion, second harmonic generation
13
and sum/difference frequency generation
14
. Gallium
phosphide resonant cavities used to measure non-linear properties have previously been fabricated on free stand-
ing GaP membranes, grown on a sacrificial AlGaP layer
15
. The transfer of thin GaP films from GaP/Si allows the
fabrication of GaP films on large-area, low-cost substrates with a simpler and less expensive fabrication scheme.
The growth of GaP on Si is well understood; and while limitations exist in the achievable surface roughness
16
and
density of lattice defects in the form of antiphase domains, sufficient material quality for optical devices has been
1
Applied Physics and Materials Science, California Institute of Technology, Pasadena, 91125, USA.
2
Joint Center for
Artificial Photosynthesis, California Institute of Technology, Pasadena, CA, 91125, USA.
3
Institute for Solar Fuels,
Helmholtz-Zentrum Berlin, für Materialien und Energie GmbH, Hahn-Meitner-Platz 1, 14109, Berlin, Germany.
Correspondence and requests for materials should be addressed to H.A.A. (email:
haa@caltech.edu
)
Received: 8 February 2017
Accepted: 30 May 2017
Published: xx xx xxxx
OPEN
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achieved
13
. The substrate removal process draws from the well-established epitaxial lift-off technique
17
,
18
; here,
substrate reuse is not critical, because the substrate is silicon as opposed to expensive GaAs.
Epitaxial growth of GaP was obtained on both (112) and (001)
−
6° offcut silicon substrates, as verified by X-ray
diffraction. High resolution x-ray diffraction spectra with reciprocal space maps are shown in Fig.
1
. Reciprocal
space maps of films ~ 50
nm thick, below the critical thickness for GaP grown on silicon, show Laue oscillations
and film peak widths (36.2 arcseconds) matching the width of the substrate peak (36 arcseconds). This indicates
that thin films are not relaxed and have low crystalline defect density. Reciprocal space maps of films above the
critical thickness show that films were fully relaxed when grown on (112) substrates and 96% relaxed when grown
on the offcut (001) substrates. At thicknesses higher than the critical thickness, such as those demonstrated in this
paper, threading dislocations are expected and observed for both substrate orientations
19
.
A schematic of the substrate removal process is shown in Fig.
2
. First, a mesa etch was performed on the GaP
films, defining a 0.2 inch square appropriate for Hall effect measurements. The mesa etch also created a sharp edge
which allowed thickness measurements to be performed using a stylus contact profilometer. Measured growth
rates varied very little; all thickness measurements were within 10% of the target thickness. Following the mesa
etch, contacts were formed at sample corners. The metallization step not only contacts the film for electronic
measurements, but also acts as a spacer to define the thickness of the adhesive layer that holds the GaP film to
the glass handle. Samples were then attached to glass handle substrates with an adhesive interlayer. Two different
interlayers were explored, which had different properties. Crystalbond 509 wax was fast and easy to apply – the
sample was simply pressed onto the substrate on a hotplate. However, this interlayer resulted in buckled films that,
while acceptable for electronic measurements, had relatively poor optical quality. Optically smooth films were
achieved by using an SU-8 2002 interlayer between GaP and glass and transfer with a wafer bonding tool
20
. Silicon
wafers were then removed with a pulsed XeF
2
vapor etcher. This dry silicon etch was chosen for the substrate
removal process due to its high selectivity for silicon over glass, most organics, and most metals
21
and generally
gentle etching action, which ensures good adhesion between the film and the glass handle substrate.
An optical microscope image showing a pinhole defect in the film is shown in Fig.
3a
, and two SEM cross
sections of GaP films of different thicknesses bonded with thin and thick interlayers are shown in Fig.
3b
and
c
.
The pinhole defects were present in the original GaP film as grown on silicon, likely resulting from particulate
contamination occurring prior to or during growth. The thickness of the interlayer can be increased by adding
metal contacts to the film corners; films with our typical Hall measurement contacts had an interlayer thickness
of approximately 1
μ
m.
Figure 1.
High resolution x-ray diffractometry of GaP films grown on Si. (
a
), (
b
) x-ray diffraction spectra
of ~200
nm thick GaP films grown on (112) and (001) offcut 6° towards [111] substrates, respectively. The
amorphous background is due to the use of a glass slide onto which the 1
×
1 cm sample is mounted. (
c
), (
d
)
reciprocal space maps of ~50
nm thick films. (
e
), (
f
) reciprocal space maps of ~200
nm thick films.
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Optical transmission measurements were performed on the GaP/wax/glass structure in a spectrophotometer
equipped with an integrating sphere as described elsewhere
22
. Transmission measurements were normalized to a
reference spectrum consisting of wax and glass. A Tauc plot (
α
h
ν
1/2
vs photon energy, where
α
is the absorption
coefficient, h
ν
is photon energy, and ½ is used for an indirect gap
23
) was generated from the normal incidence
transmission data and used to extract the bandgap, as shown in Fig.
3d
. The Tauc plot is linear near the expected
band edge, with a good linear fit, indicating an indirect bandgap
23
. The extents of the fit were selected to maintain
an R
2
value of 0.999. We can extrapolate this curve to extract a bandgap of 2.24
eV, in good agreement with the
accepted value of 2.26 eV
2
. The features in the transmission data at photon energies below the bandgap result
from Fresnel reflections between the GaP film, wax, and glass.
Atomic force microscopy measurements of both surfaces, before and after the substrate removal process, are
shown in Fig.
4(a) and (b)
. The top surface RMS roughness varied slightly based on the thickness and growth
conditions of the film, but roughness as low as 1
nm were measured. The bottom surface, which became the top
surface following substrate removal, was reliably smooth with RMS roughness below 1
nm.
The crystallinity of GaP films following substrate removal was confirmed using X-ray diffraction. Samples
were measured before and after the substrate removal process, and as expected, the crystalline, oriented films
maintain their crystallinity following substrate removal, as shown in Fig.
4c
. Similarly, samples from poor growths
which resulted in non-oriented GaP films maintained their orientation following substrate removal.
Hall measurements were performed on transferred films in order to measure the effectiveness of doping using
cyclopentadienyl-magnesium during MOCVD growth. It was necessary to perform these measurements on
transferred samples to avoid the influence of the silicon substrate on the measurement
24
. Hole concentrations in
the mid 10
18
cm
−
3
range were achieved with a mobility of approximately 10 cm
2
V
−
1
s
−
1
, resulting in a champion
resistivity of 0.12 Ω-cm. A champion mobility of 16 cm
2
V
−
1
s
−
1
was measured in a sample with a carrier den-
sity of 1.6
×
10
17
cm
−
3
. The carrier densities and mobilities achieved are both slightly below, but within an order
of magnitude of, the best results reported for Mg doping of MOCVD-grown GaP films, achieved on AlGaInP
substrates
25
.
Photoluminescence measurements were performed on transferred GaP films. We found that substrate removal
was necessary to extract enough signal to perform measurements. This was expected, since gallium phosphide
has a high index of refraction (n ~ 4 near its emission peaks), well matched to the silicon substrate, which absorbs
strongly at these wavelengths. Therefore, emitted photons which reach the back surface will be transmitted to the
silicon substrate and absorbed with a high probability. We note that considering the solid angle of the front escape
cone, one expects only 1.6% of the total luminescence emission to escape from a GaP/Si structure.
Figure 2.
Schematic of the substrate removal process. (
a
) The GaP film is first etched to form a square 0.2
inches on a side. Metal contacts are then deposited and annealed on the corners, extending off the edge. (
b
) The
sample is then bonded to a glass slide and (
c
) the silicon substrate etched away with XeF
2
, leaving the GaP film
and metal contacts. (
d
) photograph of a typical contacted, substrate removed GaP film.
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Photoluminescence spectra for an undoped sample are shown in Fig.
5a
. At low temperatures, the primary
emission peak for both doped and undoped samples was located at approximately 2.59
eV. As the temperature
increases towards room temperature, the undoped sample develops a second peak at approximately 2.26
eV,
which corresponds to the primary indirect energy gap
2
. As shown in Fig.
4(b)
, in a heavily doped sample, good
fits can be generated by considering the indirect gap peak either broadening from a full width half max of 0.34
eV
to 0.38
eV, or shifting to 2.15
eV. Regardless of the fitting procedure used, this change was found to correspond to
emission from dopant states close to the band edge.
In conclusion, single-crystal GaP (001) films bound to glass with a transparent interlayer were fabricated by
a transfer/wafer bond/Si substrate removal process. These films had excellent optical, mechanical, and electronic
properties. Due to the distinctive properties of GaP, these films open the door for fabrication of a wide variety of
thin film photonic devices.
Methods
Gallium Phosphide Growth.
Gallium phosphide was grown on silicon (001)-6º and (112) substrates using
a 1
×
3-inch Thomas Swan Epitor II MOCVD system with a close-coupled showerhead. A two step growth pro-
cess was used, consisting of a low temperature (450
°C) atomic layer deposition-like nucleation layer and high
temperature (575
°C) growth
26
. The metalorganic precursors used were triethylgallium (63
μ
mol min
−
1
) and ter
-
tiarybutylphosphine (3205
μ
mol min
−
1
), and the surface temperature of the substrate was monitored with a
pyrometer.
Figure 3.
Micrographs and transmission spectrum of substrate removed GaP film. (
a
) Optical micrograph
showing some typical defects in the films. The scale bar is 200
μ
m in the large image and 20
μ
m in the inset. (
b
)
Cross sectional scanning electron microscope image of a thin (200
nm) GaP film bonded to glass with a thin
interlayer. This particular film did not have metal contacts, and as a result, the interlayer was only 100
nm thick.
The scale bar is 1
μ
m. (
c
) Cross sectional scanning electron microscope image of a thick (400
nm) GaP film
bonded to glass with a thick interlayer (1300
nm), determined by the thickness of the metal contacts. The scale
bar is 1
μ
m. (
d
) Tauc plot generated from normal incidence transmission measurements of a substrate removed
GaP film. A linear fit was generated from the red data points with bounds selected to maintain R
=
0.999, and
the fit indicates a bandgap of 2.24
eV.
Figure 4.
Atomic force micrographs and x-ray diffraction spectra of GaP films. Atomic force microscopy
images of the GaP film top surface following growth on silicon (
a
), and following substrate removal (
b
). Note
that the top surface following substrate removal is the surface which had previously been in contact with silicon
in the as-grown film on substrate. (
c
) x-ray diffraction spectra of the films following substrate removal show that
excellent crystallinity is maintained.
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Processing for Liftoff.
Gallium phosphide films on silicon were patterned by photolithography and the
GaP was etched with a 5:1:1 mixture of deionized water, 97% sulfuric acid, and 30% hydrogen peroxide, similar
to approaches previously used for GaAs etching
27
. Etch completion was judged by eye and confirmed by dipping
in buffered hydrofluoric acid, owing to the hydrophobic nature of clean Si, whereas Si covered by GaP remained
hydrophilic.
To form contacts, ohmic Ni/Zn/Au alloy
28
contacts of 100
nm total thickness were evaporated through a
shadow mask onto the p-type GaP film corners. After evaporation, the contacts were annealed and an additional
500
nm of silver was evaporated to protect the ohmic contact and define the thickness of the interlayer.
Bonding Process.
The sample and glass slide (Eagle XG 20/10, Coresix) were thoroughly cleaned with sol
-
vents and dried with N
2
, and baked at 180
°C for 5
minutes to dehydrate. SU-8 2002 was spun on both the sample
and the glass slide at 3000 RPM for 30
seconds. If any particles are present distorting the resist, the surface was
re-cleaned and dehydrated. Both the glass and sample were then soft baked for 2
minutes at 95
°C. The sample was
brought into contact with the glass, then baked for 30
minutes in a convection oven at 95
°C. The sample was then
placed in a Suss wafer bonder with 500 mbar of pressure at 95
°C. Finally, the SU-8 was hardened with a 1
minute
flood exposure using 365
nm light through the glass and a 20
minute hot plate bake at 180
°C.
Photoluminescence Measurements.
Photoluminescence measurements were performed using a
Coherent Libra Ti:sapphire laser, fed into an OPerA solo OPA, equipped with nonlinear optics capable of a variety
of outputs. A 360
nm excitation pulse was chosen with an intensity of 20 nJ/pulse. A 364
nm long pass filter was
used to block the primary beam and a streak camera was used as a detector to acquire spectral data.
X-ray diffraction Measurements.
X-ray diffraction measurements were performed using a Panalytical
X’Pert Pro system equipped with a Cu X-ray anode paired to a hybrid monochromator (18 arcseconds resolu
-
tion). All measurements were done after aligning to the Si substrate peak. 2
θ
-
ω
measurements were performed
with a receiving slit before the detector, while rocking curves and reciprocal space mapping were done with a
triple bounce Ge (220) analyzer (12 arcseconds resolution).
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Acknowledgements
The information, data, or work presented herein was funded in part by the U.S. Department of Energy, Energy
Efficiency and Renewable Energy Program, under Award Number DE-EE0006335. Work at the Molecular
Foundry was supported by the Office of Science, Office of Basic Energy Sciences, of the U.S. Department of
Energy under Contract No. DE-AC02–05CH11231.We gratefully acknowledge critical support and infrastructure
provided for this work by the Kavli Nanoscience Institute at Caltech. The authors thank Shaul Aloni of the
Molecular Foundry for his assistance with GaP MOCVD growth and Phillip Jahelka for helpful discussion.
Author Contributions
H.E. and H.A. conceived the experiment following discussions with A.F. H.E. performed bonding and etch
processes, scanning electron microscopy, electrical and optical characterization and prepared the manuscript.
C.C. and R.S. performed GaP growths and contributed to manuscript text. R.S. performed atomic force
microscopy measurements. C.C. performed X.R.D. measurements and prepared figures. D.F. performed
photoluminescence measurements and discussed results. Y.H. and A.A. assisted in bonding process development.
All authors discussed results and edited the manuscript.
Additional Information
Competing Interests
:
The authors declare that they have no competing interests.
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