Roman Koerner

Electrically pumped lasing from Ge Fabry-Perot resonators on Si

Room temperature lasing from electrically pumped n-type doped Ge edge emitting devices has been observed. The edge emitter is formed by cleaving Si-Ge waveguide heterodiodes, providing optical feedback through a Fabry-Perot resonator. The electroluminescence spectra of the devices showed optical bleaching and intensity gain for wavelengths between 1660 nm and 1700 nm. This fits the theoretically predicted behavior for the n-type Ge material system. With further pulsed electrical injection of 500 kA/cm2 it was possible to reach the lasing threshold for such edge emitters. Different lengths and widths of devices have been investigated in order to maintain best gain-absorption ratios.

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.

Franz-Keldysh effect in GeSn pin photodetectors

The optical properties and the Franz-Keldysh effect at the direct band gap of GeSn alloys with Sn concentrations up to 4.2% at room temperature were investigated. The GeSn material was embedded in the intrinsic region of a Ge heterojunction photodetector on Si substrates. The layer structure was grown by means of ultra-low temperature molecular beam epitaxy. The absorption coefficient as function of photon energy and the direct bandgap energies were determined. In all investigated samples, the Franz-Keldysh effect can be observed. A maximum absorption ratio of 1.5 was determined for 2% Sn for a voltage swing of 3 V.

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.

Contact resistivities of antimony-doped n-type Ge1−x Sn x

As Ge1−x Sn x is being investigated for CMOS applications, obtaining contacts to n-type Ge1−x Sn x with low specific contact resistivity (ρ c) is a major concern. Here, we present results on specific contact resistivities of Sb doped n-type Ge1−x Sn x with 0 ≤ x ≤ 0.08 also with varying doping concentrations using Ni, Ag and Mn as contact metals. Our results show that Ni offers the lowest ρ c for all x values of Ge1−x Sn x . The lowest ρ c measured for Ni contacts on highly n-doped Ge0.92Sn0.08 is 2.29 × 10−6 Ω cm2. We find a strong dependence of the specific contact resistivity on doping, which we attribute to the fact that strong Fermi level pinning is present in metal/n-Ge1−x Sn x contacts.

The Zener-Emitter: Electron injection by direct-tunneling in Ge LEDs for the on-chip Si light source

While monolithically integrated light sources for Si photonics have been investigated using Ge and GeSn on Si substrates [1-3], the challenges in material quality and efficiency remain to be solved. Turning the Group-IV material into a direct semiconductor for CMOS compatible concepts [4] promises enhanced electrical to optical conversion efficiencies. However, the red-shift in emitting wavelength is challenging for the peripheral devices such as modulators and photodetectors in complex optoelectronic integrated circuits (OEICs) [5]. We investigated a new concept by utilizing a reverse biased Ge p+n Zener diode for injection of electrons into a forward biased light emitting Ge p+-i-n diode providing holes for the radiative transition. In Ge, the direct band-to-band tunneling (BTBT) dominates over the phonon assisted indirect BTBT, which is highly beneficial for the Zener-Emitter [6]. Moreover, possible low voltage operation due to highly conductive Ge tunnel diodes and avoidance of current crowding effects by the high-energetic electron filtering mechanism of Zener diodes are further increasing the electrical injection efficiency [7].

Zener tunnel-injection for Ge optical amplifiers, lasers and modulators (invited)

We present the Ge Zener-Emitter injection mechanism for synthesis of an indirect semiconductor optical amplifier (ISOA), featuring gain characteristics and electro-absorption modulation with extinction ratios > 14 dB by sufficient Moss-Burstein shift, for generic Ge-on-Si Photonics platform.

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.

Light sources for group IV photonics

Serving as the electrical to optical converter, the on-chip light source is the key component for monolithically integrated Group IV photonics. Here, we compare a variety of concepts for light generation on a Si chip, realized by Ge and GeSn heterostructures and quantum wells.

Ge and GeSn Light Emitters on Si

The heteroepitaxial growth of GeSn and Ge crystals on Si substrates are investigated for Si-based photonic applications. Light Emitting Diodes with emission wavelengths from 2,100 to 1,550 nm could be demonstrated with active intrinsic GeSn light emitting layers between Ge barriers. A clear shift of the direct band gap toward the infrared beyond 2 μm is measured. Emission intensity is increased compared to Ge Light Emitting Diodes. Room temperature lasing from electrically pumped n-type doped Ge edge emitting devices are demonstrated. The edge emitter is formed by cleaving Si-Ge waveguide heterodiodes, providing optical feedback through a Fabry-Pérot resonator. The electroluminescence spectra of the devices showed optical bleaching and intensity gain for wavelengths between 1,660 nm and 1,700 nm.