In this paper, an ultra-thin buffer technology for the epitaxial growth of SixGe1-x-ySny structures on Si or Si-on-Insulator substrates by using molecular beam epitaxy is presented. This technology builds the basis for integrated photonic devices as detectors, modulators and light sources. The paper discusses different device families with different material compositions, which all use a relaxed Ge virtual substrate with high quality. These are pseudomorphic Ge/Ge1-ySny structures, SixGe1-x-ySny structures lattice matched to Ge and (partially) relaxed Ge1-ySny virtual substrates. The photonic devices consist of heterojunction diodes with vertical pin doping structures. As an example, Ge/Ge1-ySny multi quantum well photodetectors which active regions made from Nx(Ge0.93Sn0.07/Ge) multi-quantum well structures are presented. Optical measurements at high frequencies are successfully performed on these photodetectors. A 3-dB bandwidth above 40 GHz is measured at the optical telecommunication wavelength of 1550 nm.
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.
Nanohole array surface plasmon resonance (SPR) sensors offer a promising platform for high-throughput label-free biosensing. Integrating nanohole arrays with group-IV semiconductor photodetectors could enable low-cost and disposable biosensors compatible to Si-based complementary metal oxide semiconductor (CMOS) technology that can be combined with integrated circuitry for continuous monitoring of biosamples and fast sensor data processing. Such an integrated biosensor could be realized by structuring a nanohole array in the contact metal layer of a photodetector. We used Fouriertransform infrared spectroscopy to investigate nanohole arrays in a 100 nm Al film deposited on top of a vertical Si-Ge photodiode structure grown by molecular beam epitaxy (MBE). We find that the presence of a protein bilayer, constitute of protein AG and Immunoglobulin G (IgG), leads to a wavelength-dependent absorptance enhancement of ~ 8 %.
The aim of integrating plasmonic functionality with photonic devices is twofold: on the one hand, plasmonic
nanoantennas can enhance the functionality of photonic devices and enable their miniaturization. On the other hand,
photonic devices can be a part of plasmonic transmission lines and act e.g. as plasmon detectors. Here, we present results
on both aspects in a CMOS-compatible device setup using Ge PIN-photodetectors and Al nanostructures. Plasmonic
nanoantennas are metallic nanostructures that enable the control and manipulation of optical energy in the visible and
near-infrared spectrum and have been proposed as a means to enhance absorption and quantum yields for photovoltaics,
to increase spatial resolution for optical microscopes and to enhance the energy efficiency of light-emitting devices. We
present experimental results on the enhancement of Ge PIN-photodetector efficiency by Al nanoantennas. In order to
investigate plasmon waveguiding and detection, metal grating structures and metal-insulator-metal slot waveguides were
fabricated by electron beam lithography in the Al metallization layer of Ge PIN-photodetectors. Photocurrent maps of
the devices under local illumination show that plasmons can be optically excited at the grating and are then guided by the
slot waveguide towards the Ge PIN-photodetector where they are detected as photocurrent. Using Ge PIN-photodetectors
and Al nanostructures as a CMOS-compatible device setup, we show how plasmonic nanostructures can be used for
efficiency enhancement of photonic devices and discuss plasmon detection with Ge PIN-photodetectors with possible
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.
This work concentrates on the device characteristics and performance of Ge-on-Si p-i-n diodes for the use as absorption modulators. At first, the impact of temperature on electrical and on optical characteristics of these p-i-n diodes is investigated. Secondly, the feasibility of optical modulation using the Franz-Keldysh effect is demonstrated for temperatures up to 359 K. The Ge-on-Si p-i-n diodes are grown using a molecular beam epitaxy system. The layer structure includes a double Si/Ge-heterojunction and an intrinsic zone with a thickness of 500 nm. During the growth process several annealing steps are performed to reduce the dislocation density and incorporate tensile strain in the intrinsic zone. The dark current is proportional to the diode area and amounts to 40 mA/cm2 at a reverse voltage of 1 V. An analysis of the temperature dependence of the dark current shows that it is dominated by generation/recombination of carriers probably at threading dislocations. The optical absorption spectra recorded show a shrinkage of the infrared cut off wavelength of about 0.6 nm/K. In addition the change of absorption at the direct bandedge with different applied biases, i.e. the Franz- Keldysh effect, is demonstrated for temperatures from 300 K to 359 K. With regard to modulation of an optical signal the on/off ratio is evaluated as function of the voltage swing. With a moderate voltage swing of 2 V the maximal absorption change is 300 cm-1 and the optimal working regime shifts from 1625 nm at 300 K to 1665 nm at 337 K.
This work presents the limiting factors of fast Germanium p-i-n photodetectors for optical on-chip communication. The photodetectors are grown by molecular beam epitaxy on Silicon and Silicon on insulator substrates. On-wafer RF and optical RF measurements up to 40 GHz are performed at a wavelength of 1.55 μm. Different de-embedding procedures are used to obtain the amplitude and phase of the device impedance and the equivalent circuit description. An analysis of the reflection coefficient compared to the equivalent circuit explains the frequency characteristic and it is used to determine background doping of the intrinsic layer and the expansion of the space charge width. The optical bandwidth is measured for different bias voltages and background doping. The RC limitation of the detectors is shown and analyzed leading to adjusted parameters for high speed detectors at zero-bas.