Bernhard Schwartz

GeSn/Ge multiquantum well photodetectors on Si substrates

Vertical incidence GeSn/Ge multiquantum well (MQW) pin photodetectors on Si substrates were fabricated with a Sn concentration of 7%. The epitaxial structure was grown with a special low temperature molecular beam epitaxy process. The Ge barrier in the GeSn/Ge MQW was kept constant at 10 nm. The well width was varied between 6 and 12 nm. The GeSn/Ge MQW structures were grown pseudomorphically with the in-plane lattice constant of the Ge virtual substrate. The absorption edge shifts to longer wavelengths with thicker QWs in agreement with expectations from smaller quantization energies for the thicker QWs.

The Zener-Emitter: A novel superluminescent Ge optical waveguide-amplifier with 4.7 dB gain at 92 mA based on free-carrier modulation by direct Zener tunneling monolithically integrated on Si

We report on the first experimental demonstration of a monolithic integrated Group-IV Ge semiconductor optical amplifier (SOA) — the Ge Zener-Emitter (ZE). The ZE is a device featuring light amplification up to 4.7 dB (92 mA) at center wavelength of 1700 nm and gain-bandwidth of 98 nm on Si (100). Our novel direct Zener band-to-band tunneling (BTBT) injection method enables low-voltage electron emission beyond the Boltzmann-limit (38 mV/dec at 1.55 K, 88 mV/dec at 300 K), achieving population-inversion at 0.45 V (41 mA). The ZE possesses a Si-Ge-Si hetero-structure with excellent CMOS integration compatibility by planar device design (550 nm) and an ultra-thin (100 nm) Ge virtual substrate (VS) on Si (100). Moreover, the ZE shows superior light emission properties with pulsed lasing at 1667 nm and superluminescent LED characteristic (150 cm−1 max. gain at 270 K, 100 cm−1 max. gain at 300 k). The developed ZE device presents a promising feature to monolithic Si-photonics filling the gap for energy-efficient light emission and amplification in a small footprint (1 mm) integrated waveguide-amplifier.

Comparison of EL emitted by LEDs on Si substrates containing Ge and Ge/GeSn MQW as active layers

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 comparable. 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.

Luminescence of strained Ge on GeSn virtual substrate grown on Si (001)

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.