Germanium based photonic components toward a full silicon/germanium photonic platform

Strained germanium nanowires have recently become an important material of choice for silicon-compatible optoelectronic devices. While the indirect bandgap nature of germanium had long been problematic both in light absorption and emission, recent successful demonstrations of bandstructure engineering by elastic strain have opened up the possibility of achieving direct bandgap in germanium, paving the way towards the realization of various high-performance optical devices integrated on a silicon platform. In particular, the latest demonstration of a low-threshold optically pumped laser in a highly strained germanium nanowire is expected to vitalize the field of silicon photonics further. Here, we review recent advances and challenges in strained germanium nanowires for optoelectronic applications such as photodetectors and lasers. We firstly introduce the theoretical foundation behind strained germanium nanowire optoelectronics. And several practical approaches that have been proposed to apply tensile strain in germanium nanowires are further discussed. Then we address the latest progress in the developments of strained germanium nanowire optoelectronic devices. Finally, we discuss the implications of these experimental achievements and the future outlook in this promising research field.

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Strained germanium nanowire optoelectronic devices for photonic-integrated circuits.

For several decades, an astonishing progress has been seen in the development of integrated circuits used in electronic devices. This progress was based on the miniaturization of electronic components fabricated in complementary metal-oxide-semiconductor (CMOS) technology. However, at current and future length scales, the metal interconnects constitute a bottleneck for a further increase in computing power. This bottleneck can be resolved by replacing the electrical interconnects by optical data transmission. To do so, an efficient monolithic laser source is required. Direct band gap III-V lasers offer excellent efficiencies, but it is a great challenge to integrate these on the CMOS platform. Many teams worldwide are working on overcoming the serious material incompatibilities between III-V materials and the group IV material silicon (Si). A much more attractive solution is to employ a material that intrinsically complies with Si technology such as germanium (Ge), which is nowadays already in use in silicon foundries. However, at first glance it seems unpractical to make a laser from Ge due to the unfortunate alignment of the electronic bands which form an indirect band gap. Nonetheless, it has been known since a long time that this band alignment can be modified by increasing the unit cell of the crystal or by alloying Ge with Sn. Doing either way, it is expected that the radiative recombination efficiency increases and the bands align in a direct configuration, just like the bands for III-V laser materials. In this thesis, the optical properties of these two classes of so-called direct band gap group IV materials are investigated. To achieve the necessary unit cell deformation, tensile-strained Ge microbridges are used. These systems have been developed for this very purpose at the Paul Scherrer Institute over the last few years. A part of this development is reported in the thesis work of Dr. Martin J. Süess. For the second approach, GeSn alloys are investigated at the Paul Scherrer Institute in close collaboration with the Forschungszentrum Jülich, where the epitaxy of the alloys was developed. This collaborative work led to the demonstration of an optically pumped group IV laser. Together with the material development, this achievement has already been summarized in the thesis of Dr. Stephan Wirths. The aim of this work is to gain insight into the physics of such direct band gap group IV semiconductors on the route towards an efficient CMOS light source and, thus, covers both systems. In the first part of this dissertation, model calculations are presented to quantify the advantageous effects of uniaxial tensile stress on the properties of Ge in terms of electron distribution and optical gain. The experimental investigation of these advantageous effects is given in the second part of this thesis, which deals with the fabrication and

Direct Band Gap Germanium for Si-compatible Lasing

In this paper we investigate the structural and optical properties of few strain-unbalanced multiple Ge/GeSi quantum wells pseudomorphically grown on GeSi reverse-graded substrates. The obtained high epitaxial quality demonstrates that strain symmetrization is not a mandatory requirement for few quantum-well repetitions. Photoluminescence data, supported by a thorough theoretical modeling, allow us to unambiguously disentangle the spectral features of the quantum wells from those originating in the virtual substrate and to evaluate the impact on the optical properties of key parameters, such as quantum confinement, layer compositions, excess carrier density, and lattice strain. This detailed understanding of the radiative recombination processes is of paramount importance for the development of Ge/GeSi-based optical devices.

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Photoluminescence study of interband transitions in few-layer, pseudomorphic, and strain-unbalanced Ge/GeSi multiple quantum wells

A 2.7% tensile strained Ge/SiGe heterostructure nanowire (NW) is in-situ fabricated by a three-dimensional Ge condensation method. The NW metal-semiconductor-metal (MSM) photodetector demonstrates an ultra-broadband detection wavelength of 400-2400 nm, showing a high responsivity of >3.46×102 A/W with a photocurrent gain of >4.32×102 at 1550 nm under −2 V. A high normalized photocurrent to dark current ratio (NPDR) of 1.88×1011 W−1 at 1550 nm under −1 V is achieved. The fully complementary metal-oxide-semiconductor (CMOS) compatible, simple and scalable process suggest that the Ge heterostructure NW is promising for low cost, high performance near-infrared or short wavelength infrared focal plane array applications.

Broadband 400-2400 nm Ge heterostructure nanowire photodetector fabricated by three-dimensional Ge condensation technique

We present germanium microdisk optical resonators under a large biaxial tensile strain using a CMOS-compatible fabrication process. Biaxial tensile strain of ~0.7% is achieved by means of a stress concentration technique that allows the strain level to be customized by carefully selecting certain lithographic dimensions. The partial strain relaxation at the edges of a patterned germanium microdisk is compensated by depositing compressively stressed silicon nitride layer. Two-dimensional Raman spectroscopy measurements along with finite-element method simulations confirm a relatively homogeneous strain distribution within the final microdisk structure. Photoluminescence results show clear optical resonances due to whispering gallery modes which are in good agreement with finite-difference time-domain optical simulations. Our bandgap-customizable microdisks present a new route towards an efficient germanium light source for on-chip optical interconnects.

Ge Microdisk with Lithographically-Tunable Strain using CMOS-Compatible Process

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Sungdae Woo

Papers

Ge Microdisk with Lithographically-Tunable Strain using CMOS-Compatible Process

Ge microdisk with lithographically-tunable strain using CMOS-compatible process.