Germanium-tin alloys were grown by molecular beam epitaxy and characterized. The tin content ranged from 4-20% atomic percent, lowering the bandgap energy by about 10 meV per percent tin. A sample with 12% tin exhibited a direct bandgap of 0.623 eV at 100K. Heterojunction diodes of p-GeSn/n-Ge showed nearly ideal rectifying characteristics with low turn-on voltage but increasing reverse dark current with higher tin content and temperature, attributed to the decreasing bandgap. The results indicate GeSn alloys have potential for future electronic and optoelectronic devices.

1. The Properties of Germanium-Tin Alloys for Infrared Device Applications
James Kolodzeya
, Matt Coppingera
, Sangcheol Kima
, Nupur Bhargavaa
, Jay Guptaa
, Chaoying Nia
,
and Yung Kee Yeob
a
Department of Electrical and Computer Engineering, University of Delaware, USA, kolodzey@ee.udel.edu,
b
Department of Engineering Physics, Air Force Institute of Technology, USA,
Germanium-tin alloys are attracting renewed interest for applications including the strain control
of CMOS active channels in integrated circuits, and mid-infrared optical devices for medical imaging,
chemical spectroscopy, and military counter-measures. With sufficient Sn content above about 10 %,
there is the particularly interesting possibility of an energy bandgap that is direct in reciprocal space,
which may lead to efficient light emitters and detectors.
For this work, samples of germanium-tin alloys were grown by solid source molecular beam
epitaxy on Ge (001) wafer substrates. Doping of the alloys was achieved by evaporating boron acceptors
from a high temperature effusion cell, and phosphorus donors from a custom GaP baffled source. The
GeSn samples were characterized by: atomic force microscopy (AFM) for the surface morphology; x-ray
diffraction (XRD) for the crystal structure and lattice constant; transmission electron microscopy (TEM)
for the crystal structure; Fourier transform infrared (FTIR) spectroscopy for absorption and emission
spectra; and photoluminescence (PL) for radiative efficiency. From an analysis of the lattice constants,
the compositions ranged from about 4 atomic percent of Sn in Ge, to over 20 % Sn. To achieve good
morphology with increasing Sn content, the substrate temperature during MBE growth was reduced to a
range near 200 ºC. From AFM, the surface roughness was found to depend on temperature but was about
a few nm. TEM microscopy of a sample with 10 % Sn showed relatively sharp interfaces and good
crystal structure, but with dislocations, indicating that the GeSn lattice was relaxed slightly compared to
the Ge substrate. For a sample with 7 % Sn, photoluminescence at low temperatures showed weak peaks
near 0.5 eV that increased in intensity with excitation laser power, but were attributed to impurities rather
than band-to-band transitions.
To determine the infrared response of undoped GeSn layers, the spectral photocurrent was
measured by FTIR spectroscopy in the step-scan mode, at temperatures from 50 K to 150 K. Using
standard photolithography, photoconductive samples were patterned with interdigitated fingers of
evaporated Ti/Au metal contacts. For all samples, the contacts were found to be ohmic, and the dark
current-voltage characteristics were linear. The absorption energy decreased with increasing Sn content
by about 10 meV per atomic percent of Sn. The bandgap energies were determined by fitting a power law
to the absorption coefficient versus photon energy. For the Sn percentage of 12%, a bandgap as low as
.623 eV was found at 100 K, which is about 100 meV lower than the bandgap of bulk Ge at this
temperature. This Ge0.88Sn0.12 sample was found to exhibit a dependence of absorption on photon energy
that was consistent with a direct bandgap in k-space.
Heterojunction diodes of p-GeSn/n-Ge were fabricated and analyzed to determine their
characteristics and limitations. Measurements of the current versus voltage and its temperature and
composition dependence were performed in order to extract the diode parameters of: reverse saturation
current, ideality factor, series resistance, shunt resistance, and breakdown voltage. At low forward bias,
the diodes showed nearly ideal rectifying characteristics with a low turn-on voltage of about 0.4 volts.
With increasing Sn concentration, and increasing temperature, the reverse dark current increased, which
was attributed to a decreasing bandgap.
These findings indicated that GeSn alloys and structures have interesting electrical and optical
characteristics and may become useful for future electronic and optoelectronic devices. The authors
gratefully acknowledge the support of the AFOSR under grant number: FA9550-09-1-0688.