1. Structural Properties of Boron-Doped Germanium-Tin Alloys
Grown by Molecular Beam Epitaxy
NUPUR BHARGAVA,1,3
JAY PRAKASH GUPTA,1
THOMAS ADAM,2
and JAMES KOLODZEY1
1.—Department of Electrical and Computer Engineering, University of Delaware, Newark,
DE 19716, USA. 2.—College of Nanoscale and Engineering, University of Albany, Albany,
NY 12203, USA. 3.—e-mail: nupur@udel.edu
Boron-doped Ge1ÀxSnx alloys with atomic fractions of tin up to x = 0.08 were
grown on n-Ge(001) substrates using solid-source molecular beam epitaxy, in
order to study their structural properties. The total boron concentration in the
alloys was $1018
cmÀ3
as measured by secondary-ion mass spectroscopy,
which also indicated low amounts of impurities such as carbon and oxygen.
More than 90% of the Sn atoms occupied substitutional lattice sites in the
alloy as determined by Rutherford backscattering spectrometry. High-reso-
lution x-ray diffraction showed that the boron-doped Ge1ÀxSnx alloys were
single crystals that were completely strained with low defect densities and
coherent interfaces for thickness up to 90 nm, and for Sn composition of 8%.
The boron-doped Ge1ÀxSnx/n-Ge formed p–n junctions with conventional rec-
tifying characteristics, indicating that the boron produced electrically active
acceptor states.
Key words: Germanium-tin alloys, boron doping, MBE
INTRODUCTION
Germanium-tin (Ge-Sn) alloys have gained tech-
nological importance due to the direct-bandgap
transition obtained by adding sufficient Sn to indi-
rect-bandgap Ge.1
Monolithic integration of Ge-Sn
optoelectronics with silicon-based integrated circuit
technology is cost effective and easier than the bon-
ded wafer approach used to integrate III/V-based
optoelectronics with Si wafers.2
The Ge-Sn alloy
system has a tunable bandgap1,3
and lattice con-
stant,4
which vary according to the amount of Sn
added to the Ge lattice. In addition, co-alloying with
silicon to form Ge1ÀxÀySnxSiy is expected to permit
bandgap tailoring,5
and lattice matching to Ge or to
virtual substrates such as relaxed SiGe. Addition of
Sn to Ge moves the absorption edge to longer wave-
lengths3
and produces infrared (IR) emitters at about
2 lm wavelength.6
The Ge-Sn alloy system has been
predicted to have much higher carrier mobility7
than
Ge for possibly higher-speed circuits.8
Recently,4
undoped Ge1ÀxSnx alloys were grown
with Sn composition up to 14.5%, and their lattice
constant increased with Sn composition. In this work,
boron-doped Ge1ÀxSnx alloys were investigated for
their structural properties for varying thickness and
composition, being grown over Ge substrates to
achieve high amounts of Sn (for the possible direct
bandgap) with less strain than Si substrates, but also
with limited thickness to reduce the defects and
relaxation in the layers. A defect-free structure is
important for possible optoelectronic applications.
The Ge substrates were n-type to produce hetero-
junction diodes with boron-doped Ge1ÀxSnx.
EXPERIMENTAL PROCEDURES
Boron-doped Ge1ÀxSnx (Ge-Sn:B) alloys were epi-
taxially grown on phosphorus-doped Ge buffer layers
on 3-inch n-type Ge(001) substrates with resistivities
of 0.005 X-cm to 0.02 X-cm. The layers were grown
using a solid-source two-chamber molecular beam
epitaxy (MBE) device (model 620; Veeco/EPI, Plain-
view, NY). A sample-loading chamber was sorption-
and cryopumped (CT-8F; Brooks/CTI Cryogenics,
(Received August 12, 2013; accepted February 18, 2014;
published online March 13, 2014)
Journal of ELECTRONIC MATERIALS, Vol. 43, No. 4, 2014
DOI: 10.1007/s11664-014-3088-3
Ó 2014 TMS
931

2. Chelmsford, MA) to a pressure of 10À6
Pa, while the
growth chamber was cryopumped and ion-pumped
(VIP 300; Duniway/Varian, Mountain View, CA) to a
base pressure of 4 9 10À8
Pa.
The Ge substrates were chemically cleaned to
remove organic contamination, particulate matter,
and surface oxide to provide a smooth surface for
growth.9,10
The cleaning treatment involved degre-
asing with solvents (trichloroethylene, acetone, and
methanol), detergent (FL-70) washing, etching
using dilute hydrofluoric acid (HF:10H2O), and final
dipping in hydrogen peroxide (H2O2:10H2O). To
reduce the exposure to air, the time between
chemical cleaning and loading of the substrate into
the MBE was less than 2 min. Additionally, the
substrates were degased in situ in ultrahigh vac-
uum at high temperatures to remove moisture,
carbon-based contamination, and surface oxide.
Evaporation of Ge and Sn was performed using
Knudsen effusion cells which had intrinsic Ge (tri-
ple zone refined polycrystalline) and metallic Sn
(6 N purity; United Mineral and Chemical Corpo-
ration, Lyndhurst, NJ), respectively, contained in
pyrolytic boron nitride (pBN) crucibles. p-Doping
was achieved by thermally evaporating elemental
boron from a high-temperature effusion cell.
n-Doping in the Ge buffer layer was achieved by
preferentially evaporating phosphorus from solid
GaP using a custom effusion cell with a perforated
pBN baffle. The n-Ge buffer layer was grown at a
substrate temperature of 420°C. Based on previ-
ously calibrated growth rates, the boron and phos-
phorus doping concentrations were maintained at
3 9 1018
cmÀ3
.
Ge1ÀxSnx:B layers were grown with three Sn
compositions (x = 0.025, 0.054, and 0.08). The series
of Ge0.975Sn0.025:B composition samples were grown
to thickness of $90 nm, 350 nm, and 950 nm, which
was controlled via growth conditions and measured
by Rutherford backscattering spectrometry (RBS),
stylus profilometry, and secondary-ion mass spec-
trometry (SIMS). The intended Sn composition was
achieved by varying the Ge growth rate as $19.1
A˚ /min, 8.7 A˚ /min, and 3.8 A˚ /min, respectively,
keeping the Sn growth rate constant at 0.8 A˚ /min.
The thickness variation for the samples with the
same Sn concentration was achieved by varying the
duration of growth as 1 h 1 min 44 s, 3 h 5 min 12 s,
and 9 h 15 min 36 s, respectively, keeping the Ge
and Sn growth rates constant at 19.1 A˚ /min and
0.8 A˚ /min, respectively.
For structural analysis of the alloys, high-resolu-
tion x-ray diffraction (XRD) measurements were
performed using a Philips X’Pert MRD system
(PANalytical, Westborough, MA). The incident
x-ray optics comprised a multilayer focusing mirror
and a four-bounce Bartels-type Ge(220) monochro-
mator in the path from the x-ray tube to the sample,
which provided monochromatic, well-collimated Cu
Ka1 radiation. The diffracted beam optics had a triple-
axis high-resolution goniometer with a three-bounce
Ge(220) analyzer in front of the detector. This system
was used to obtain triple-crystal x–2h rocking curves
and x–2h/x reciprocal-space maps (RSMs) for the
symmetric and asymmetric reflections.
The compositions of the Ge1ÀxSnx:B alloys were
measured by RBS. The RBS system used a 2-MeV
He++
ion, 10-nA to 20-nA probe beam with 2 mm
spot diameter incident on the sample in a vacuum
chamber maintained at $10À4
Pa, as described
previously.4
The backscattered ions were detected
using a Si surface barrier detector with 12 keV
energy resolution. To determine the substitutional-
ity of the Sn atoms in the Ge lattice, the RBS
channeling measurement was performed at the
angle of minimum yield, relative to the random
angle yield.4
To determine the doping concentration
in the Ge1ÀxSnx:B alloys, SIMS measurements were
performed.
RESULTS AND DISCUSSION
The structural properties were analyzed by mak-
ing three sets of comparisons: (a) doped versus
undoped alloys, (b) Ge0.975Sn0.025:B alloys with
varying thickness, and (c) 90-nm-thick Ge1ÀxSnx:B
alloys with varying composition of x = 0.025, 0.054,
and 0.08. As shown in Fig. 1, for the Ge0.975Sn0.025:B
sample, the Ge1ÀxSnx layer extended to about
950 nm in depth and had a boron concentration of
$5 9 1018
cmÀ3
, matching reasonably with the
doping concentration expected from the growth
conditions. The Sn composition and boron doping
were uniform through the thickness of the layer,
and the layer had low levels of impurities such as
carbon and oxygen, as shown by the low secondary-
ion counts on the right vertical axis. Although SIMS
measurements were not done for all samples, the
growth conditions were adjusted to keep the boron
doping the same.
Doped Versus Undoped Alloys
The XRD x–2h (004) rocking curves are shown in
Fig. 2 for boron-doped Ge0.975Sn0.025:B and undoped
Ge0.977Sn0.023, both with 350 nm thickness and
grown under the same conditions. For the rocking-
curve measurements, the samples were first aligned
to the substrate peak for the particular reflection
being measured [(004) in the case of Fig. 2], then the
Ge1ÀxSnx peaks were measured. In Fig. 2, there are
two major peaks: the substrate peaks at the Bragg
angle for n-Ge of 33.008°, and the layer peaks at a
lower angle, indicating an increase in perpendicular
lattice constant with addition of Sn, which expands
the Ge lattice since Sn is a larger atom than Ge. The
Bragg angle positions for the doped and undoped
Ge1ÀxSnx layer were around 0.03° apart. Although
the reason for this angular difference is not clear,
the RBS measurements showed a difference in
composition of about $0.2% between the two sam-
ples. As both the Bragg angle value from XRD and
the composition value from RBS are within the
Bhargava, Gupta, Adam, and Kolodzey932

3. measurement error of these measurements, the
difference in the Bragg angle and the composition
between the doped and undoped layer cannot be
attributed entirely to the addition of boron to the
layer, and may be due to systematic error such as
drift in the growth calibration. The small variations
in Bragg angle and composition could also be due to
inherent variation in composition across the wafer.
From Fig. 2, the loss of interference fringes in the
x–2h rocking curve for the doped sample, which
would appear as small peaks on the sides of the
alloy peak, suggests that addition of boron to the
layer led to a loss of layer coherency. Using Scher-
rer’s formula11
for the layer thickness of 350 nm,
the x-ray peak width (full-width at half-maximum)
was calculated to be 0.014°, which can be considered
to be an ideal minimum for a defect-free layer of this
thickness. The peak widths for the doped and
undoped layer were found to be 0.022° and 0.018°,
respectively. An increase in peak width indicates
degradation in crystal quality, perhaps due to the
presence of defects. The larger peak width of the
doped layer indicated that the undoped layer had
higher crystalline quality. The (224) RSMs for both
the undoped and doped layers (not shown here)
showed that both undoped and doped layers were
completely strained. The undoped and doped films
showed only slight differences in XRD measure-
ments, but it can be seen that, with addition of
doping, the Ge0.975Sn0.025 Bragg peak becomes
broader and loses interference fringes, indicating a
slight degradation in quality on doping.
RBS random and channeling measurements indi-
cated that the percentage of Sn atoms located on
substitutional lattice sites was $99% in the undoped
alloys, whereas it was $92% in the doped alloys. The
reason for the reduction in the amount of substitu-
tional Sn with doping is not clear, but it may be due to
an interaction of boron atoms with Sn, or to the boron
atoms preferentially occupying sites in the Ge lattice
instead of the Sn atoms.
Ge0.975Sn0.025:B Alloys with Varying Thickness
The XRD x–2h (004) rocking curves for
Ge0.975Sn0.025:B alloys with thickness of 90 nm,
350 nm, and 950 nm are shown in Fig. 3, with all
three showing the n-Ge substrate peak at $33.008°.
The Bragg angle for Ge0.975Sn0.025:B was around
32.78° for all three thicknesses. There was a slight
difference in the Bragg angle between the three
samples of $0.02°, which could be due to the mea-
surement position on the wafer. Interference fringes
were only visible for the sample with thickness
$90 nm, and as the thickness increased these
interference fringes disappeared, indicating that
the Ge0.975Sn0.025:B layer lost coherence with the
substrate for the thicker layers.
Fig. 1. SIMS measurements of 950-nm-thick Ge0.975Sn0.025:B alloy
showing the concentrations of Ge, Sn, B, P, C, and O versus depth.
The data indicated boron in the top Ge1ÀxSnx layer of thickness
$950 nm, with low concentrations of oxygen and carbon present in
the layer. The amounts of Ge, O, and C were not calibrated but are
given in terms of counts on the right ordinate axis.
Fig. 2. x–2h rocking curves in (004) symmetric reflections showing
peak intensity versus Bragg angle for p-Ge0.975Sn0.025 (boron doped)
and undoped Ge0.977Sn0.023, both with thickness of 350 nm. The
Bragg angles h [or x for the (004) reflection] at 33.008° correspond to
the Ge substrate.
Structural Properties of Boron-Doped Germanium-Tin Alloys Grown by Molecular Beam Epitaxy 933

4. Using Scherrer’s formula, the x-ray peak widths
were calculated to be 0.058°, 0.014°, and 0.0055°
using the finite layer thickness of 90 nm, 350 nm,
and 950 nm, respectively, whereas the peak widths
from the x-ray data in Fig. 3 were found to be 0.06°,
0.022°, and 0.009°, respectively. For lower thickness
of Ge0.975Sn0.025:B, the peak width from x-ray mea-
surement was in close agreement with the ideal
minimum peak width calculated from Scherrer’s
formula, indicating high crystal quality of the lay-
er.12
As the thickness of the Ge0.975Sn0.025:B layer
increased, however, the peak width increased more
rapidly than expected from Scherrer’s formula,
indicating additional degradation in the crystal
quality with increasing thickness, perhaps due to
presence of dislocations or mosaicity.
Figure 4 shows the asymmetrical (224) RSMs for
the Ge0.975Sn0.025:B alloy with varying thickness.
The x- and y-axes are the reciprocal lattice vectors
along [110] and [001] directions, Qk and Q?,
respectively, which represent the reciprocal of the
parallel and perpendicular lattice spacings, with a
plot of equal-intensity contours of the diffracted
beam. The lengths of the Qk and Q? vectors are
given in reciprocal lattice units (rlu) and are related
to the incident (x) and the diffracted (h) Bragg
angles by the following equations:4,13
Qk rluð Þ ¼ sinh  sin h À xð Þ; (1)
Q? rluð Þ ¼ sinh  cos h À xð Þ: (2)
There was a slight mismatch in substrate position
Qk and Q? values, between the three samples, could
be due to difference in the measurement region being
measured on the wafer and to variations in the initial
alignment of the n-Ge substrate peak. From Fig. 4,
an increase in peak width was observed with
increasing thickness. For all three samples, the
n-Ge substrate and the Ge0.975Sn0.025:B alloy peaks
were on the same Qk axis, indicating that the lattice
spacing was the same for the n-Ge substrate and for
Fig. 3. x–2h rocking curves in (004) symmetric reflections showing
peak intensity versus Bragg angle for p-Ge0.975Sn0.025 (boron doped)
with thickness of 90 nm, 350 nm, and 950 nm. The Bragg angle h [or
x for the (004) reflection] at 33.008° corresponds to the n-Ge sub-
strate.
Fig. 4. Reciprocal-space maps in the asymmetric (224) reflection, showing plots of equal-intensity contours versus the reciprocal lattice vectors
in the parallel (Qk, horizontal) and perpendicular (Q?, vertical) directions for p-Ge0.975Sn0.025 (boron-doped) alloy with thickness of (a) 90 nm,
(b) 350 nm, and (c) 950 nm grown on n-Ge substrates.
Bhargava, Gupta, Adam, and Kolodzey934

5. the Ge0.975Sn0.025:B alloy, which implies that the
Ge0.975Sn0.025:B alloy was completely strained for all
three thicknesses, within the measurement error.
The loss of interference fringes, as seen in Fig. 3, is
an indication of degraded crystal quality because of
higher strain with increasing thickness. However,
the strain energy due to lattice mismatch was con-
sidered to be insufficient to cause relaxation, since
the substrate and alloy peaks were on the same Qk
axis for thickness up to 950 nm. If defects were
present, they were not dense enough to cause sig-
nificant relaxation. Due to the good crystal quality
of the 90-nm-thick Ge0.975Sn0.025:B alloy, as deter-
mined by the presence of interference fringes and
the close correlation between the x-ray peak width
with Scherrer’s formula, a series of Ge1ÀxSnx:B
alloys were compared for varying Sn composition
with thickness of 90 nm.
Ge12xSnx:B Alloys with Thickness
of 90 nm and Varying Composition
Figure 5 shows the x–2h rocking curves for the
(004) reflection for three Ge1ÀxSnx:B alloys (all
90 nm thick) with Sn composition of x = 0.025,
0.054, and 0.08, each with two main peaks. The
x-ray peak with smaller peak width at higher Bragg
angle of $33.008° corresponds to the n-Ge substrate
(as in Fig. 3), whereas the peak at lower Bragg
angle corresponds to the Ge1ÀxSnx:B layer. As the
Sn composition increased, the Bragg angle of the
Ge1ÀxSnx:B layer peak moved to smaller angles,
indicating an increase in lattice constant. For all
three films, interference fringes near the alloy peaks
indicated that the alloy lattice was coherent with
that of the substrate. The x-ray peak widths of the
alloys were 0.06°, 0.056°, and 0.06° with increasing
Sn composition, being similar to the peak width for
an ideal layer of the same thickness (0.058°) calcu-
lated from Scherrer’s equation. The presence of
interference fringes along with the close correlation
between the layer thickness and the narrow x-ray
peak width indicated good quality of the Ge1ÀxSnx:B
Fig. 5. x–2h rocking curves in the (004) symmetric reflections
showing peak intensity versus Bragg angle for 90-nm-thick
p-Ge1ÀxSnx (boron-doped) layers with Sn composition of x = 0.025,
0.054, and 0.08. The Bragg angles h [or x for the (004) reflection] at
33.008° correspond to the n-Ge substrate.
Fig. 6. Reciprocal-space maps in the asymmetric (224) reflection, showing equal diffraction intensity contours versus the reciprocal lattice
vector, in the parallel (Qk, horizontal) and perpendicular (Q?, vertical) directions for the 90-nm-thick p-Ge1ÀxSnx (boron-doped) alloy with Sn
composition (x) of (a) 2.5%, (b) 5.4%, and (c) 8%.
Structural Properties of Boron-Doped Germanium-Tin Alloys Grown by Molecular Beam Epitaxy 935

6. alloys with varying compositions at thickness of
90 nm.
Figure 6 shows the (224) RSMs for the Ge1ÀxSnx:B
alloys with varying composition. The x-ray peak width
of the alloys was reasonably narrow (similar to the
ideal Scherrer’s value) for all three compositions, and
all peaks were on the same Qk (horizontal) axis as the
substrate, indicating the same in-plane spacing for the
n-Ge substrate and the Ge1ÀxSnx:B alloy. This lattice
matching implies that the 90-nm-thick Ge1ÀxSnx:B
alloy was completely strained for Sn compositions up
to the measured value of 8%. As the Sn composition
increased, the vertical Q? of the alloys decreased,
indicating an increase in the perpendicular lattice
constant, and also indicating compressive strain. For
allthesesamples,theQk remainedunchangedrelative
to the substrate, indicating that even the higher-
Sn-content layers were still strained.
The Sn compositions, both total and substitu-
tional, were found by RBS channeling in the ran-
dom and aligned directions.4
Figure 7 shows the
RBS random and aligned measurements for
Ge0.946Sn0.054:B. The small peak at channel number
$1300 corresponds to the Sn in the layer. The lar-
ger-scale inset portion in Fig. 7 shows a good
reduction in the Sn signal in the channeled lattice
direction as compared with the spectra in the ran-
dom lattice direction. The reduction in channeling
signal indicates low backscattering and therefore
good crystal quality, and was used to calculate the
percentage of substitutional Sn in the Ge lattice.4
For all the alloys used in this work, more than 90%
of the Sn occupied substitutional sites in the Ge
lattice. With increasing thickness, the channeling
spectra showed less of a reduction in channeling
signal, indicating deterioration in crystal quality
and that the percentage of Sn that was substitu-
tional was not as high.4
The x-ray diffraction measurements showed that
the 90-nm-thick boron-doped Ge1ÀxSnx/n-Ge struc-
tures had good structural properties such as no
relaxation, coherent interfaces, and peak width
corresponding to the minimum value expected from
the finite thickness of the layer, implying that the
Ge1ÀxSnx:B has minimum defects. However, the
alloys with higher thickness had worse crystal
properties.
The electronic properties of p–n heterojunction
diodes fabricated from the boron-doped Ge1ÀxSnx
alloys on n-type Ge were investigated for the 90-nm-
thickness samples with varying Sn composition.
Fig. 7. Rutherford backscattering spectrometry (RBS) measurements with intensity count on the y-axis and energy channel number on the x-axis
in the random (unaligned) and channeled (aligned) directions. Solid line is the simulation of the random spectra using the SIMNRA simulation
program resulting in a Sn composition of 5.4% for this alloy.
Fig. 8. Dark I–V measurements of p–n heterojunction diodes fabri-
cated from 90-nm-thick p-Ge1ÀxSnx alloys with different Sn contents
on n-type Ge substrates.
Bhargava, Gupta, Adam, and Kolodzey936

7. Current–voltage (I–V) measurements were per-
formed using a source meter (model 2400; Keithley,
Cleveland, OH) under no illumination. All hetero-
junction diodes showed conventional rectifying
characteristics at room temperature and below, as
depicted in Fig. 8.6,14
For all diodes, a ‘‘turn-on’’
voltage of about 0.4 V was obtained for significant
current flow. At a given forward voltage, the current
increased with increasing Sn content (x = 0.025 to
0.08), suggesting higher conductivity and/or greater
injection of charge carriers. The reverse leakage
current was higher than for a conventional Ge diode
and increased for higher Sn content (x = 0.025 to
0.08), which is consistent with a higher intrinsic
carrier concentration or a shorter recombination
lifetime with addition of Sn.14
CONCLUSIONS
Ge1ÀxSnx alloys were doped with boron to form
Ge1ÀxSnx:B alloys with varying thickness and with
Sn composition up to 8%. The Ge1ÀxSnx alloys
showed good structural characteristics. X-ray dif-
fraction showed interference fringes implying good
interfaces, and the narrow width of the intensity
peaks reasonably matched calculations of the ideal
peak width from Scherrer’s formula, implying few
crystalline defects. Reciprocal-space mapping
showed that the Ge1ÀxSnx:B alloys were completely
strained, in spite of the relatively high Sn contents
and thickness. As the thickness of the Ge0.975Sn0.025:B
alloys increased, the alloy peak lost the interference
fringes, indicating degradation of the alloys. The
Ge0.975Sn0.025:B was completely strained at thick-
ness of $950 nm. Rutherford backscattering showed
that the Ge-Sn:B alloys used in this study had more
that 90% Sn on substitutional lattice sites. Current–
voltage measurements showed that the fabricated
boron-doped Ge1ÀxSnx/n-Ge diodes exhibited good
rectifying characteristics.
ACKNOWLEDGEMENTS
The authors would like to thank D. Beatson, S.
DeVore, N. Faleev, K. Goossen, M. Kim, R. Martin, R.
Opila, M. Pikulin, G. Pomrenke, R. Soref, K. Unruh,
Y. K. Yeo, and S. Zollner for useful and meaningful
discussions. This work was financially supported by
the AFOSR under Grant Nos. FA9550-09-1-0688 and
FA9550-13-1-0022, by Voltaix Corporation under
Grant No. 12A01464, and by gifts from IBM Corpo-
ration, IR Labs, and Voltaix Corporation.
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Structural Properties of Boron-Doped Germanium-Tin Alloys Grown by Molecular Beam Epitaxy 937