This document summarizes research on infrared electroluminescence from GeSn heterojunction diodes grown by molecular beam epitaxy. Specifically, it reports on p-n heterojunction diodes fabricated from boron-doped p-type GeSn layers containing 8% Sn grown on n-type Ge substrates. Electroluminescence was observed from these diodes with a peak emission at 0.57 eV (2.15 microns). The emission intensity increased with higher drive currents and lower device temperatures. Total emitted power from a single edge facet was measured to be 54 microwatts at an applied peak current of 100 mA at 100 K. These results suggest GeSn materials may be useful for practical light

1. Infrared electroluminescence from GeSn heterojunction diodes grown by molecular
beam epitaxy
Jay Prakash Gupta, Nupur Bhargava, Sangcheol Kim, Thomas Adam, and James Kolodzey
Citation: Applied Physics Letters 102, 251117 (2013); doi: 10.1063/1.4812747
View online: http://dx.doi.org/10.1063/1.4812747
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2. Infrared electroluminescence from GeSn heterojunction diodes grown by
molecular beam epitaxy
Jay Prakash Gupta,1
Nupur Bhargava,1
Sangcheol Kim,1
Thomas Adam,2
and James Kolodzey1
1
Department of Electrical and Computer Engineering, University of Delaware, Newark, Delaware 19716, USA
2
Nanofab, University of Albany, SUNY, Albany, New York 12203, USA
(Received 18 April 2013; accepted 16 June 2013; published online 27 June 2013)
Infrared electroluminescence was observed from GeSn/Ge p-n heterojunction diodes with 8% Sn,
grown by molecular beam epitaxy. The GeSn layers were boron doped, compressively strained,
and pseudomorphic on Ge substrates. Spectral measurements indicated an emission peak at
0.57 eV, about 50 meV wide, increasing in intensity with applied pulsed current, and with reducing
device temperatures. The total integrated emitted power from a single edge facet was 54 lW at an
applied peak current of 100 mA at 100 K. These results suggest that GeSn-based materials maybe
useful for practical light emitting diodes operating in the infrared wavelength range near 2 lm.
VC 2013 AIP Publishing LLC. [http://dx.doi.org/10.1063/1.4812747]
Over the past few decades, there has been growing inter-
est in the research and development of Group IV semicon-
ductor optoelectronic devices compatible with silicon-based
circuits. The limiting factor for Group IV light emitters,
however, has been the absence of a direct energy bandgap,
important for efficient radiative transitions.1
Recently, there
has been significant development and progress on devices
based on germanium-tin (GeSn), a non-equilibrium Group
IV alloy. The larger lattice constant of GeSn relative to Ge
provides strain control for stressors in Ge based CMOS/
FETS.2,3
GeSn alloys have been reported to have higher car-
rier mobility4
and to allow bandgap control by composition
and strain.5
The possibility of an energy gap that is direct in
k space for Sn contents as low as 6%6
may lead to very effi-
cient Group IV optical devices.7
The smaller band gap of
GeSn than Ge improves the optical absorption at 1.55 lm
and will push the spectral range to longer infrared
wavelengths.8–10
The compatibility of GeSn alloys with sili-
con technology may expand its use for low-cost and com-
mercial applications.2,3
Recently, there have been reports on electrolumines-
cence (EL) from Si/GeSn p-i-n diodes with 2.2% Sn11
and
photoluminescence from GeSn alloys with $4% Sn on
Silicon (Si) substrates.12,13
In this study, we report on the
electrical and EL measurements of p-n heterojunction diodes
fabricated on n-type Ge-substrates from layers of boron
doped p-type GeSn with 8% Sn, grown by molecular beam
epitaxy (MBE), which is a significantly higher Sn content
than for the GeSn diodes with previously reported EL.
For this experimental study, several boron-doped p-type
GeSn alloy samples with different Sn concentrations were
grown by MBE on (001) oriented n-type Ge substrates with
resistivities of 0.005–0.02 X cm. The MBE growth chamber
(Veeco/EPI 620) used a closed-loop liquid He cryopump
(CTI Cryogenics CT-8 F) in combination with a Varian
400 L/s ion pump and maintained base pressures below
4 Â 10À8
Pa (3 Â 10À10
Torr). The thermal evaporation of
triple zone-refined polycrystalline Ge contained in a pyro-
lytic boron nitride (pBN) crucible from a Knudsen effusion
cell served as the Ge source, and the thermal evaporation of
6 -N purity Sn (United Mineral and Chemical Corp.) using a
pBN crucible equipped Knudsen effusion cell provided the
Sn source. The Ge substrates were chemically cleaned by
degreasing and etching followed by oxidation to produce
atomically smooth surfaces free of contamination.14–16
Prior
to growth, the Ge oxide formed during the final step of sub-
strate preparation was desorbed in vacuum by heating the
substrate to 520
C. To obtain p-type doping, a high temper-
ature solid source effusion cell was used for the evaporation
of boron acceptors. For the n-type doping, a custom effusion
cell having a pBN baffle was used for the preferential
evaporation of phosphorus from a solid source of high
purity GaP.
Prior to the GeSn alloy growth, Ge buffer layers,
30–40 nm thick, were grown and n-doped to concentrations
of 3 Â 1018
cmÀ3
at substrate temperatures of 420
C. During
the growth of the GeSn alloy (sample SGC636), the substrate
temperature was reduced to 225
C, and the chamber back-
ground pressure was found to be less than 4 Â 10À7
Pa.
Secondary ion mass spectrometry (SIMS) of the p-GeSn
layer revealed a boron doping concentration of 5 Â 1018
atom/cmÀ3
. The doping depth profile through the layer was
uniform, and other impurity (Fe, O, and Cr) concentrations
were comparatively negligible. X-ray diffraction (XRD)
indicated that the epitaxial layers were coherent and com-
pressively strained due to matching of the horizontal lattice
constant to the Ge buffer. The XRD indicated a layer thick-
ness of 100–110 nm, confirmed by stylus profilometry, and
layer strain parameters of exx ¼ À0.01308 and ezz ¼ 0.009.
Rutherford backscattering spectrometry measurements
(RBS) showed that the atomic percentage of Sn was 8%.
Using standard photolithography, light emitting diodes
(LEDs) were fabricated by evaporating metals onto patterned
photoresist followed by lift-off. For the electrical contacts on
top of the p-type Ge0.92Sn0.08 layer, rectangular meshes of Al
metal (300 nm) with individual mesh stripe width of 80 lm
and an area filling factor of 50% were thermally evaporated.
Full wafer coverage of Ti/Pd/Ag/Au (10/30/300/30 nm) was
thermally evaporated for the bottom contact to the n-Ge
substrate (Fig. 1). Individual devices were cleaved using a
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APPLIED PHYSICS LETTERS 102, 251117 (2013)
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3. diamond scriber to dice surface areas of approximately
2 mm  2 mm and 1 mm  1 mm.
The electrical dark-current versus voltage (I–V) charac-
teristics were measured with a Keithley 2400 Source Meter.
Data acquisition was carried out through a data and signal
interface (RS-232) controlled by LABVIEW software. All
diodes showed conventional rectifying characteristics at and
below room temperature, as depicted in Fig. 2 for a typical
device.
EL spectral measurements were performed using a
Thermo Nicolet Nexus-870 Fourier Transform Infrared
Spectrometer (FTIR) in step-scan mode at a resolution of
16 cmÀ1
, using the GeSn LED device as the signal source.
Conducting silver paste was used to affix the GeSn devices
to a copper heat sink mount. Wire bonds of Au metal were
used to connect the p-side metal electrodes to Au pads de-
posited on ceramic carriers. The Au pads were connected to
the leads of a cryostat using 4 -N purity Indium as a low tem-
perature solder. The diodes were mounted in an edge-
emitting configuration and focused using a parabolic metal
mirror through the FTIR external port onto the InGaAs
detector with sensitivity range between 12 000 cmÀ1
and
3800 cmÀ1
, and with a Quartz beamsplitter. The devices
were cooled down to temperatures of 100 K in a closed-loop
liquid He cryostat (ARS, Inc., DE-204SF) equipped with a
thallium bromide-iodide (KRS-5) optical window.
The device drive current was supplied with an Agilent
8114 A pulse generator set to yield 3 ms sequences at 333 Hz
repetition rate, with each sequence consisting of 5 individual
30 ls sub pulses at a 3.3 KHz repetition rate. The currents
indicated in Figs. 2 and 3 were the peak values of the current
pulses measured using a current probe with an oscilloscope.
The measured FTIR detector output was amplified and fil-
tered using a Lock-in amplifier, and then returned to the
input of FTIR electronics to obtain the interferogram.
The EL spectral intensity versus peak current is plotted
in Fig. 3 for the device at 300 K, showing the peak of the
spectral emission envelope at 0.57 eV, corresponding to
2.15 lm wavelength. The inset of Fig. 3 shows a typical
FTIR interferogram obtained for the spectra. A comparison
of the EL spectra of the Ge-LED17
and our GeSn-LED with
8% Sn content shows a considerable infrared shift of about
$500 nm. The FWHM is about 200 nm, which is typical for
GeSn devices as was reported by Roucka and Oehme
et al.11,17
The EL intensity increases monotonically with
drive current, with little shift in peak wavelength, implying
the absence of device heating under these drive conditions.
The main peak is attributed to bandgap transitions, as dis-
cussed below.
A high-energy shoulder around 2.0 lm (0.62 eV) was
observed in most GeSn EL spectra and from p-type Ge sam-
ples, and may be accounted by the phonon-assisted transi-
tions involving the Ge substrate, which has a higher bandgap
(Eg) than the Ge0.92Sn0.08, and is a thicker region with more
volume for emission.12
The power level of the EL spectral density curve was
calibrated from the emission curve of a recessed-cone black-
body radiator, accounting for the geometry and alignment of
the sample, and the FTIR optics, yielding the results in
Fig. 4. The Ge0.92Sn0.08 diode total emission power increased
somewhat sub-linearly with peak current at room tempera-
ture. The power reported in Fig. 4 gives the total power from
all 4-edge facets, assuming equal emission from each of the
four device edge facets. A power of $1 lW at room temper-
ature at the relatively low drive current of 15 mA makes it
attractive for practical GeSn diode applications.
FIG. 1. Cross section schematic of fabricated p-Ge1ÀxSnx/n-Ge (8% Sn) het-
erojunction diode (SGC636) showing top and bottom electrical contacts.
FIG. 2. Current-Voltage characteristics of a Ge0.92Sn0.08 (SGC 636) p-n
junction LED at varying temperatures, showing good rectifying behavior
even at room temperature.
FIG. 3. Emission spectra obtained from edge configuration of a Ge0.92Sn0.08
LED at room temperature. With increasing current, there is no shift in the
peak emission wavelength. The inset shows the FTIR interferogram obtained
at 200 mA drive current. The integrated EL intensity of $17 lW was meas-
ured from the device at I ¼ 250 mA peak current.
251117-2 Gupta et al. Appl. Phys. Lett. 102, 251117 (2013)
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4. Studies of the spectral output at low temperatures were
performed to evaluate its effects on device performance.
Fig. 5 plots EL intensity as a function of photon energy for
different temperatures at 50 mA peak current.
Interestingly, the peak intensity observed at 100 K was
$8 times higher than at 300 K. In addition, there was a shift
to higher photon energies at lower temperatures with a slope
of $0.26 meV/K, which is consistent with the well-known
Varshni increase of Eg of Ge, which is 0.24 meV/K.
Attempts to measure GeSn diodes with less Sn, near 4%, did
not reveal EL, which was attributed to weak output below
the detection limit, perhaps due to the inefficient optical
emission. In comparison, this 8% Sn sample had significant
emission, which is consistent with the simulations of
D’Costa et al. that indicate a direct bandgap for Sn contents
below 11 at. %.6
Based on the Van Roosbroeck–Shockley expression for
emission rate versus photon energy, as used by Roucka
et al.,11
the optical absorption coefficient was calculated. The
optical absorption coefficient a was matched to a conven-
tional power law versus the difference between the photon
energy and the bandgap: ah ¼ A(h-Eg)n
. The extracted
bandgap parameter was 0.559 eV, but this includes possible
phonon absorption and emission. Excitons are not expected at
these temperatures and have energies of only a few meV.18,19
The exponent fit (n ¼ 0.5442) for our Ge0.92Sn0.08 sample at
room temperature, closely matches the expected square root
dependence for a direct band gap material.10
The extracted
emission peak at 0.559 eV for 8% Sn is in close agreement
with the average of the two peaks observed by Tseng et al.
for similar Sn content p-i-n Ge/Ge0.922Sn0.078/Ge double het-
erostructure diode.20
The extracted bandgap energy includes
the variations with strain. In addition, theoretical analysis
using deformation potential theory indicated that the bandgap
energy for Ge0.92Sn0.08 alloys pseudomorphically grown on
Ge (100) change by only a 10 meV because of strain.21
In conclusion, significant spectral electroluminescence
near the wavelength of 2 lm has been observed from hetero-
junction light emitting diodes of p-Ge0.92Sn0.08/n-Ge.
Spectral intensities of the emission have been characterized
with respect to both temperature and current. The power
emitted from the device was calibrated and found to be
54 lW from a single facet at 100 K with a peak current of
100 mA, and approximately 216 lW for all 4 edge facets.
The evidence presented here shows that GeSn alloys can be
used to fabricate light emitting diodes operating at mid-IR
wavelengths, and may become attractive for commercial
applications and perhaps lasers.
This work was supported by the AFOSR under Grant
No. FA9550-09-1-0688, by Voltaix Corporation under Grant
No. 12A01464, and by gifts from IBM Corporation, IR Labs,
and Voltaix Corporation. Special thanks to D. Beatson,
M. Coppinger, K. Goossen, M. Kim, R. Martin, G. Pomrenke,
M. Pikulin, R. Soref, Yung-Kee Yeo and S. Zollner for useful
advice and discussions.
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Ge0.92Sn0.08/Ge LED plotted versus peak forward drive current at
T ¼ 300 K. The total power shown gives the total power for all 4 edge facets
of the device.
FIG. 5. SGC636 EL spectral intensity of Ge0.92Sn0.08/Ge LED obtained at
several temperatures at fixed 50 mA peak current. With decreasing tempera-
ture, the emitted intensity increased considerably, and the photon energy of
peak emission increased.
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