M. Oehme

Ge-on-Si vertical incidence photodiodes with 39-GHz bandwidth

Vertical-incidence Germanium photodiodes grown on thin strain-relaxed buffers on Silicon substrates are reported. For a mesa-type detector with a diameter of 10 /spl mu/m, a resistance-capacitance-limited 3-dB bandwidth of 25.1 GHz at an incident wavelength of 1552 nm and zero external bias has been measured. At a reverse bias of 2 V, the bandwidth is 38.9 GHz. The detector comprises a 300-nm-thick intrinsic region, and thus, has the potential for easy integration with Si circuitry and exhibits zero bias external quantum efficiencies of 23%, 16%, and 2.8% at 850, 1298, and 1552 nm, respectively.

Germanium-tin p-i-n photodetectors integrated on silicon grown by molecular beam epitaxy

GeSn heterojunction p-i-n diodes with a Sn content of 0.5% are grown with a special low temperature molecular beam epitaxy. The Sn incorporation in Ge is facilitated by a very low temperature growth step in order to suppress Sn surface segregation. Diodes with sharp doping transitions are realized as double mesa structures with a diameter from 1.5 up to 80 μm. An optical responsivity of these GeSn diodes of 0.1 A/W at a wavelength of λ=1.55 μm is measured. In comparison with a pure Ge detector the optical responsivity is increased by factor of 3 as a result of Sn caused band gap reduction.

New virtual substrate concept for vertical MOS transistors

We propose a new concept of thin SiGe virtual substrates in which the interactions of point defects with dislocations play a key role. Being purposely introduced in the thin SiGe buffer layers during their metastable growth, point defects promote the relaxation of strain. Firstly, they cause dislocations to climb which helps to annihilate threading dislocation arms with opposite Burgers vectors. Secondly, condensation of point defects results in prismatic dislocation loops inside the layers which avoids nucleation from the surface sites. As a consequence, point defects reduce the density of existing threading dislocations and prevent the generation of new ones. This solution should allow the formation of virtual substrates with thin relaxed SiGe buffer layers and low threading dislocation density. In this paper, we explain how point defects can be injected using modified MBE process techniques. These techniques utilize either the injection of low energy Si + ions or supersaturation of point defects resulting from very low temperature growth.

GeSn p-i-n detectors integrated on Si with up to 4% Sn

GeSn heterojunction photodetectors on Si substrates were grown with Sn concentration up to 4%, fabricated for vertical light incidence, and characterized. The complete layer structure was grown by means of ultra low temperature (100 °C) molecular beam epitaxy. The Sn content shifts the responsivity into the infrared, about 310 nm for the 4% Sn sample. An increase of the optical responsivity for wavelengths higher than 1550 nm can be observed with increasing Sn content. At 1600 nm, the optical responsivity is increased by more than a factor of 10 for the GeSn diode with 4% Sn in comparison to the Ge reference diode.

High bandwidth Ge p-i-n photodetector integrated on Si

The authors present a germanium on silicon p-i-n photodiode for vertical light incidence. For a Ge p-i-n photodetector with a radius of 5μm a 3dB bandwidth of 25GHz is measured at an incident wavelength of 1.55μm and zero external bias. For a modest reverse bias of 2V, the 3dB bandwidth increases to 39GHz. The monolithically integrated devices are grown on Si with solid source molecular beam epitaxy. The complete detector structure consisting of a highly p-doped Ge buried layer, an intrinsic absorption region, and a highly n-doped top contact layer of Ge∕Si is grown in one continuous epitaxial run. A low growth temperature sequence was needed to obtain abrupt doping transitions between the highly doped regions surrounding the intrinsic layer. A theoretical consideration of the 3dB bandwidth of the Ge detector was used to optimize the layer structure. For a photodiode with 5μm mesa radius the maximum theoretical 3dB frequency is 62GHz with an intrinsic region thickness of 307nm.

Electrical spin injection and transport in germanium

We report the first experimental demonstration of electrical spin injection, transport, and detection in bulk germanium (Ge). The nonlocal magnetoresistance (MR) in n-type Ge is observable up to 225 K. Our results indicate that the spin relaxation rate in the n-type Ge is closely related to the momentum scattering rate, which is consistent with the predicted Elliot-Yafet spin relaxation mechanism for Ge. The bias dependence of the nonlocal MR and the spin lifetime in n-type Ge is also investigated.

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.

Room-Temperature Electroluminescence From GeSn Light-Emitting Pin Diodes on Si

In this letter, a GeSn light-emitting pin diode integrated on Si via a Ge buffer is demonstrated and it is compared with a light-emitting pin diode made from pure, unstrained Ge on Si. The diode layer structures are grown with a special low-temperature molecular beam epitaxy process. The pseudomorphic GeSn layers (1.1% Sn content) on the Ge buffer are compressively strained. Both light-emitting pin diodes clearly show direct bandgap electroluminescence emission at room temperature. The electroluminescence peak of the GeSn light-emitting pin diode is shifted by 20 meV into the infrared region compared to the electroluminescence peak of the unstrained Ge light-emitting pin diode. The shift is due to the lower bandgap of GeSn and the influence of strain.

Relaxed SiGe buffers with thicknesses below 0.1 μm

Abstract Virtual substrates with relaxed SiGe buffers on Si substrates are needed for strain adjusted heterodevices with high Ge content. We have investigated the degree of relaxation in thin (

Composition and strain in thin Si1−xGex virtual substrates measured by micro-Raman spectroscopy and x-ray diffraction

Micro-Raman spectroscopy was employed for the determination of the germanium content, x and strain, e, in ultrathin SiGe virtual substrates grown directly on Si by molecular beam epitaxy. The growth of highly relaxed SiGe layers was achieved by the introduction of point defects at a very low temperature during the initial stage of growth. SiGe virtual substrates with thicknesses in the range 40–200 nm with a high Ge content (up to 50%) and degree of relaxation, r, in the range 20%–100% were investigated using micro-Raman spectroscopy and x-ray diffraction (XRD) techniques. The Ge content, x, and strain, e, were estimated from equations describing Si–Si, Si–Ge, and Ge–Ge Raman vibrational modes, modified in this study for application to thin SiGe layers. The alteration of the experimentally derived equations from previous studies was performed using independent data for x and r obtained from XRD reciprocal space maps. A number of samples consisting of a strained-silicon (s-Si) layer deposited on a SiGe vir...