1. IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 10, OCTOBER 2013 1217
Current–Voltage Characteristics of GeSn/Ge
Heterojunction Diodes Grown by
Molecular Beam Epitaxy
Sangcheol Kim, Jay Gupta, Nupur Bhargava, Matthew Coppinger, and James Kolodzey
Abstract—Heterojunction diodes of p-GeSn/n-Ge were fab-
ricated by solid-source molecular beam epitaxy on Ge sub-
strates to investigate their electrical properties. Measurements
of the current–voltage characteristics and their temperature
and composition dependence were performed to extract the
diode parameters of reverse saturation current, ideality factor,
series resistance, and shunt resistance. The diodes showed good
rectifying behavior with low turn-ON voltages in forward bias.
The reverse saturation current increased with increasing Sn
content and increasing temperature, and the magnitude of the
breakdown voltage decreased with increasing temperature. These
results suggest that Ge-Sn diodes may be useful for Ge-based
circuits and optoelectronics.
Index Terms—Germanium (Ge), germanium-tin (Ge-Sn),
heterojunction diodes, tin (Sn).
I. INTRODUCTION
RECENTLY there has been interest in the development
of Ge1−xSnx alloys due to their predicted properties
of indirect-to-direct bandgap crossover at Sn contents near
6% [1], and their compatibility with silicon technology [1], [2].
Nonequilibrium growth methods such as molecular beam
epitaxy (MBE) and chemical vapor deposition are required to
obtain Sn contents in excess of the thermodynamic equilibrium
values, due to the low solubility of Sn in Ge [3] and the
nearly 15% lattice mismatch between α-Sn and Ge [4]. Recent
interest in GeSn has motivated studies of electrical and optical
devices. The properties of p-i-n photodetectors as well as the
infrared emission from GeSn diodes have been reported with
up to 4% Sn content [9]–[12]. The electrical characteristics
of GeSn diodes need further investigation, however, with
increasing Sn content. In this letter, we report the electrical
measurements of heterojunction diodes fabricated from epi-
taxial layers of boron doped p-type Ge1−xSnx grown by MBE
on n-type Ge-substrates with up to 9% Sn, and some of their
characteristics and limitations.
Manuscript received May 25, 2013; revised July 30, 2013; accepted
August 3, 2013. Date of publication September 4, 2013; date of current version
September 23, 2013. This work was supported in part by the Air Force Office
of Scientific Research under Grant FA9550-09-1-0688, Voltaix Corporation
under Grant 12A01464, and gifts from IBM Corporation, IR Labs, and Voltaix
Corporation. Sangcheol Kim and Jay Gupta contributed equally to this work.
The review of this letter was arranged by Editor J. Cai.
The authors are with the Department of Electrical and Computer Engineer-
ing, University of Delaware, Newark, DE 19716 USA (e-mail: guptajay@
udel.edu).
Color versions of one or more of the figures in this letter are available
online at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/LED.2013.2278371
TABLE I
GROWTH PARAMETERS OF GeSn SAMPLES
II. EXPERIMENTAL DETAILS
The p-doped GeSn alloys were grown by solid source MBE
on n-type Ge (001) substrates (resistivity = 0.005–0.02 -cm),
chemically cleaned, and prepared as previously reported [5].
Ultrahigh purity sources of Ge (triple zone refined) and Sn
(6N purity) were evaporated from thermal effusion cells with
pyrolytic boron nitride (pBN) crucibles. For p-type doping,
a high temperature solid source effusion cell was used for
elemental boron evaporation. For n-type doping, a custom
effusion cell having a perforated baffle of pBN was used for
the preferential evaporation of phosphorus from solid GaP.
Before the GeSn alloy growth, n-doped Ge buffer lay-
ers were grown at substrate temperatures of 420 °C with
phosphorus doping to concentrations of 3 × 1018 cm−3,
except for the Ge device as explained below. A p-doped
Ge layer (∼800 nm) on n-Ge substrate was fabricated as
a Sn-free reference (Sample D). The growth parameters for
these samples are summarized in Table I. Secondary ion
mass spectrometry of the p-Ge1−xSnx layers revealed boron
doping concentrations, as listed in Table I. The doping depth
profile through the layer was uniform, and other impurity
(Fe, O, and Cr) concentrations were negligible (the measured
raw O Poisson corrected intensity count in the layer was
∼10–100 cm−3). All Ge1−xSnx alloy samples were grown
using identical Sn deposition rates of 0.8 Å/min, while varying
the Ge deposition rate by adjusting the Ge effusion cell
temperatures. For fixed growth duration, the decrease in Ge
deposition rate for higher Sn contents produced thinner layers,
as indicated in Table I. The composition of Sn was confirmed
by Rutherford back scattering spectrometry (RBS). The RBS
channeling indicated that > 95% of the Sn atoms were located
on substitutional lattice sites. X-ray diffraction (XRD) showed
good, coherent GeSn crystals, and the XRD fringe pattern
yielded the thickness of the GeSn layers given in Table I.
The GeSn layer thickness from stylus profilometry agreed
reasonably with the XRD results.
0741-3106 © 2013 IEEE

2. 1218 IEEE ELECTRON DEVICE LETTERS, VOL. 34, NO. 10, OCTOBER 2013
Fig. 1. Dark current measurements of p-GeSn/n-Ge heterojunction diodes
at room temperature. Inset: I–V measurements of samples with different Sn
contents (A[9%], B[5.4%], C[2.4%], and D[0 %]).
Standard photolithography and metal evaporation were
used to fabricate the p-side electrical contacts of rectangular
meshes with an area filling factor of 50% of Al (300 nm)
on the p-doped GeSn, and full wafer coverage of Ti/Ag
(30/300 nm) for the substrate bottom contact. The width of
individual metal mesh stripes was 80 μm. Metalized samples
were diced into chips with top surface areas of 2 mm × 2 mm
and 1 mm × 1 mm before electrical measurements.
The electrical current versus voltage (I–V) characteristics
were measured with a Keithley 2400 Source Meter under
no illumination. As shown in Fig. 1, the diodes showed
rectifying I–V characteristics, with a turn-ON voltage of
∼0.4 V for significant current flow. These I–V characteristics
were similar to those reported in [9], but with a lower dark
current density, ∼1 A/cm2 at −0.6 V for 9% Sn. This
dark current density is similar to that reported in [10], but
higher than reported in [11]. A low dark current typically
indicates high material quality. At a given forward voltage,
the current increased with increasing Sn content, suggesting
higher conductivity and/or greater injection of charge carriers,
as discussed below.
The I–V characteristics were evaluated as follows to extract
the diode parameters including reverse saturation current I0,
ideality factor η, series resistance Rs, and shunt conduc-
tance G. The diode current equation can be written as
I = I0exp[q(Va − RsIcorr)/ηkT] + GVa (1)
where reverse saturation current I0 = qA(n2
i Dn,p
(ND,A Ln,p), q is the electronic charge, ni is the intrinsic
carrier concentration, A is the cross-sectional area, Dn,p are
the diffusion constants of electrons and holes, ND,A are the
donor and acceptor concentrations on the n and p side, Ln,p
is the diffusion lengths of electrons and holes, Va is the
externally applied voltage, and Icorr is the corrected diode
current as explained below.
Equation (1) was manipulated [6], [7] to extract the diode
parameters, as shown for Sample A in Fig. 2, based on the
device I–V data. Fig. 2(a) shows the derivative dI/dVa against
Fig. 2. Manipulated dark I–V characteristics for the p-GeSn/n-Ge hetero-
junction diodes (Sample A). (a) Plot of the derivative dI/dVa against Va to
obtain G. (b) Plot of dVa/dIcorr against 1/Icorr to obtain η and Rs.
TABLE II
EXTRACTED DIODES PARAMETERS AS DERIVED USING (1)
Va to obtain G ≈ dI/dVa near 0 V where it dominated other
terms. The extracted GVa product was subtracted from the
current I in (1) to yield the corrected current Icorr = I−GVa,
which was then used to derive the remaining parameters.
For Rs, the reciprocal derivative dVa/dIcorr = ηkT/q Icorr + Rs,
was plotted versus (Icorr)−1 in Fig. 2(b). The y-intercept
of Fig. 2(b) shows Rs, and η was obtained from the slope
(ηkT/q). The effect of variation in B doping (>1 × 1018cm−3)
in GeSn layers of different samples is accounted for in
the extracted series resistance and saturation current density.
The I0 was evaluated from the y-axis intercept by the extrap-
olation on a semilogarithmic plot (not shown) of Icorr versus
Va − Rs Icorr.
Table II summarizes the extracted diodes parameters. The
reverse saturation current increased and the series resistance
decreased with increasing Sn content, but no clear trend was
found for the ideality factors and the conductance. An ideality
factor between 1 and 2 is expected for current contributions
from diffusion current and from defect-mediated recombina-
tion. For samples with our resistivity and size, a series resis-
tance of ∼0.28 is expected, in reasonable agreement with
the observed values. Whereas the diode incremental resistance
is ∼1 , the parallel shunt resistance (1/G) ranges from
∼13 to 22 and may be due to surface leakage. With (1), the
increase in I0 with Sn content can be caused by increases in
the intrinsic concentration ni and/or the diffusivities Dn,p, and
decreases in the doping ND,A and/or to the diffusion length
Ln,p, and also can be influenced by point defects, extended
defects, and precipitations. The relative contributions of these
parameters to I0 can be explained by the following analysis.
During MBE growth, the higher Sn samples were achieved
by reducing the Ge growth rate, producing a thinner sample
with higher doping since the boron acceptor cell tempera-
ture was held constant. Since higher doping decreases I0,
it is unlikely that the higher doping was responsible for the

3. KIM et al.: I–V CHARACTERISTICS OF GeSn/Ge HETEROJUNCTION DIODES 1219
Fig. 3. I–V characteristics of the n-Ge/p-GeSn heterojunction at different
temperatures for Sample A. Inset: Arrhenius plot for the calculation of
activation energy of the reverse saturation current (extracted from the diode
equation).
increased I0 with Sn. The diffusivity and diffusion length are
proportional to the carrier lifetime Ln,p2 = Dn,pτ, and usually
all three parameters are affected by the material quality. It
is unlikely that one would increase and the others decrease,
which would be necessary if these parameters caused I0 to
change. The most likely contributor is the increased intrinsic
concentration, which varies exponentially with the bandgap
Eg: n2
i ∼ T3exp (−Eg/kT). An increase in ni with Sn
is consistent with a decreasing bandgap and the observed
higher I0. The required change in Eg to support the observed
change in I0 is ∼140 meV for 9% Sn, which is reasonable
based on published calculations of Eg versus Sn [8].
From the temperature dependence of the reverse satura-
tion current of Sample A, the Arrhenius activation energy
EA = 0.29eV was found from a fit to I0 = IConst
exp (−EA/kT), with constant preexponential factor IConst, as
shown in the inset of Fig. 3. This activation energy is slightly
< Eg/2 of Ge (Eg = 0.67 eV), and may suggest a process
of generation/recombination current. If the observed change
in Io with temperature were fully attributed to the change in
bandgap based on the influence of n2
i , then the change would
be ∼1.8 meV/K, which is larger than the 0.39 meV/K of the
Ge bandgap in this temperature range (300–370 K) [13]. If
the change in bandgap of GeSn with temperature is assumed
to be same as for Ge, then the temperature dependence of
parameters such as diffusion coefficient and diffusion length
may be responsible for the additional temperature changes. It
is not possible, however, to extract the relative contributions
from the available data.
The reverse breakdown voltage was extracted from the
intersecting tangent lines of the two current branches (below
and above the breakdown knee) as shown in Fig. 4. The
breakdown voltage decreased with increasing temperature,
with a temperature coefficient of −0.011 V K−1, consistent
with a mechanism of Zener tunneling, which is enhanced in
lower bandgap materials.
III. CONCLUSION
The electrical properties of GeSn heterojunction diodes
were investigated under various conditions, along with the
extraction of diode parameters. The magnitude of the reverse
saturation current increased with increasing Sn content,
suggesting that the addition of Sn reduced the bandgap.
These findings indicated that GeSn/Ge p-n diodes have
Fig. 4. Dark reverse biased I–V characteristics at various temperatures for
Sample A. The breakdown voltage was ∼1.75 V at 297 K, as indicated by
the straight lines along the current branches.
reasonable characteristics and could be useful for the
fabrication of future Ge-Sn heterojunction electronic and
optoelectronic devices.
ACKNOWLEDGMENT
The authors would like to thank T. Adam, D. Beatson,
S. De Vore, K. Goossen, S. Hegedus, R. Martin, R. Opila,
M. Pikulin, G. Pomrenke, R. Soref, and Y. K. Yeo for useful
discussions.
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