To enlarge the tensile strain in Ge light emission diodes (s-Ge LED) we applied a GeSn virtual substrate (VS) on Si (001) with a Sn content of 4.5 %, to produce s-Ge LEDs. The LED stack was grown by molecular beam epitaxy. Electroluminescence investigations of the s-Ge LED show a major direct Ge peak and a minor peak at lower energy, which is formed by the GeSn-VS and the s-Ge indirect transition. The main peak of a 100 nm thick s-Ge LED is red-shifted as compared to the Ge peak of an unstrained reference Ge LED grown on Ge-VS. At a temperature of T = 80 K the increased tensile strain, produced by the GeSn-VS, causes a redshift of the direct Ge peak from 0.809 eV to 0.745 and 0.769 eV, namely for the s-Ge LED with a 100 and 200 nm thick active layer. At T = 300 K the direct Ge peak is shifted from 0.777 eV of the reference Ge LED to 0.725 eV (for 100 nm) and 0.743 eV (for 200 nm). The peak positions do not differ much between the 50 and 100 nm thick s-Ge LEDs. The intensities of the direct Ge peak increase with the s-Ge layer thickness. Moreover, the intensity of the 50 nm thick s-Ge sample is found to be larger than that of the 100 nm thick reference Ge LED.
We analyzed Ge- and GeSn/Ge multiple quantum well (MQW) light emitting diodes (LEDs). The structures were grown
by molecular beam epitaxy (MBE) on Si. In the Ge LEDs the active layer was 300 nm thick. Sb doping was ranging
from 1×1018 to 1×1020 cm-3. An unintentionally doped Ge-layer served as reference. The LEDs with the MQWs consist
of ten alternating GeSn/Ge-layers. The Ge-layers were 10 nm thick and the GeSn-layers were grown with 6 % Sn and
thicknesses between 6 and 12 nm. The top contact of all LEDs was identical. Accordingly, the light extraction is
The electroluminescence (EL) analysis was performed under forward bias at different currents. Sample temperatures
between <300 K and 80 K were studied. For the reference LED the direct transition at 0.8 eV dominates. With increasing
current the peak is slightly redshifted due to Joule heating. Sb doping of the active Ge-layer affects the intensity and at
3×1019 cm-3 the strongest emission appears. It is ~4 times higher as compared to the reference. Moreover a redshift of the
peak position is caused by bandgap narrowing.
The LEDs with undoped GeSn/Ge-MQWs as active layer show a very broad luminescence band with a peak around
0.65 eV, pointing to a dominance of the GeSn-layers. The light emission intensity is at least 17 times stronger as
compared to the reference Ge-LED. Due to incorporation of Sn in the MQWs the active layer should approach to a direct
semiconductor. In indirect Si and Ge we observed an increase of intensity with increasing temperature, whereas the
intensity of GeSn/Ge-MQWs was much less affected. But a deconvolution of the spectra revealed that the energy of
indirect transition in the wells is still below the one of the direct transition.
GeSn LED’s with Sn contents up to 4% exhibit light emission from the direct band transition although GeSn of low Sn contents is an indirect semiconductor.. The emission wavelength is red shifted compared to Ge. The redshift of the direct band transition is confirmed by different optical characterization techniques as photoluminescence, electroluminescence, photodetection and reflectivity. The photon emission energy decreases from 0.81 eV to 0.65 eV for compressively strained GeSn of 0% to 4% Sn content. Growth of GeSn up to 12% Sn is performed for which preliminary characterization results are given.
Dislocations are native nanowires. The realization of well-defined dislocation networks allows the electrical
characterization of only a small number of dislocations. Different types of dislocations were analyzed by integration into
the channel of MOSFETs. A substantial increase of the drain current was proved if only a few dislocations are present in
the channel of nMOSFETs. Low-temperature measurements indicate single-electron tunneling on dislocation core
A MOS-LED and a p-n LED emitting based on the dislocation-related luminescence (DRL) at 1.5 micron were already
demonstrated by the authors. Here we report recent observation of the Stark effect for the DRL in Si. Namely, a red/blue-shift
of the DRL peak positions was observed in electro- and photo-luminescence when the electric field in the pn-LED
was increased/lowered. Fitting the experimental data yields a strong characteristic coefficient of 0.0186 meV/(kV/cm)2.
This effect may allow realization of a novel Si-based emitter and modulator combined in a single device.
Photoluminescence and electroluminescence of boron and phosphorus implanted silicon have been studied as a function of temperature. Phosphorus implantation is found to have a similar effect on light emission as boron implantation. An increase of the band-to-band luminescence intensity by one order of magnitude is observed upon rising the temperature from 80 K to 300 K. Defect luminescence arising from the implanted layer is found only at low temperatures. The remarkable band-to-band luminescence is attributed to a high Shockley-Read-Hall lifetime caused by the gettering action of implantation defects.