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  • Applied Research on Si CMOS compatible Ge Lasers

Strain engineered Ge micro- and nanostructures

Tensile strain engineered Ge microbridges on Si(001)

 

Silicon photonics (SP) enables the merging of electricity and light in one chip. Owing to worldwide, ever increasing, research successes in SP, the ability to process, store, and transmit information has been enormously increased. The major remaining outstanding hurdle to realizing a low-cost, fully functional SP, is the realization of an integrated laser light source since it is extremely difficult to obtain efficient light emission from silicon, owing to its indirect band gap and thus the realization of a Si-integrated light source represents today the “Holy Grail” of SP. An approach based on slightly tensile strained (2.5×10-3) Ge/Si heterostructures has led to the demonstration of both optically [12] and electrically [13] pumped laser. This achievement has been welcomed by the scientific community as a leap toward a monolithically integrated silicon-based photonic platform.

Fig. 2: Left top: Scanning electron microscope image of a Ge microstripe surmounted by a SiN stressor layer. The stressor induces a lateral expansion of the stripe resulting in a tensile strain field (Left bottom panel). Right: µ-PL spectra of microstripe of differend sizes featuring increasing value of the tensile strain. Notice the achieved ~70× enhancement of the PL intensity.

Unfortunately, the device suffered of extremely high lasing threshold current density, which limited its operating lifetime to few minutes. Band engineering of Ge through application of higher tensile strain appears to be a promising way to increase the optical gain [14]. Different strategies based on heteroepitaxy or micromachining have been recently proposed, and relatively high values of biaxial and uniaxial strain obtained (see Ref.[15] for a review). Nevertheless, the proposed methods rely either on non CMOS-qualified materials (such as Ge on III-V substrates [16] or Ge on GeSn alloys [17), or on fabrication schemes leading to microstructures having shape and/or sizes which prevents their embedding within standard fabrication processes, at least on the short-medium term [18,19].

 

At IHP we are pursuing a CMOS-based fabrication approach to obtain Ge microstripes on SOI substrates. The tensile strain in Ge is induced by the elastic relaxation, occurring after the lithographic definition of the microstructure, of a compressively stressed SiN layer deposited on top of a Ge/Si heterostructure. These structures show a spatially homogeneous strain distribution over any investigated length ([10 x 150] µm) and feature uniaxial strain values up to ~1.5 %, or an equivalent biaxial tensile strain value up to ~0.9 % [20]. Owing to this strain field the photoluminescence intensity is enhanced by almost two order of magnitudes respect to bulk Ge [21].

 

Supported by a thorough theoretical modeling of the gain mechanism in such structure [22,23], we have designed new tensile Ge diode structures included in optical cavity that are predicted to achieve lasing at threshold sizably lower than what reported in literature so far.

 

A further important element to achieve stable lasing of an integrated Ge laser device is given by the need of a low resistance ohmic metallic contact. Due to Si CMOS compatibility, Co- and Ni-germanide contacts are investigated worldwide and Fig.3 shows the results of an STM study in our labs on room temperature deposited Ni on Ge(001) where the evolution of the surface morphology as a function of annealing temperature is studied by our teams [24].

Fig.3: Scanning tunneling topographic images (400 x 400 nm2, STM insets 20 x 20 nm2, LEED insets taken at 120 eV). (a) clean Ge(001) surface (Vs = – 2 V, I = 70 pA, Z-Range 1.8 nm, inset Vs = – 1.3 V, I = 70 pA, Z-Range 0.32 nm); (b) sample surface after Ni deposition (Vs = – 2 V, I = 80 pA, Z-Range 1.4 nm, inset Vs = 2 V, I = 80 pA, Z-Range 0.55 nm); (c) sample surface after annealing at 100°C (Vs = – 2 V, I = 120 pA, Z-Range 1.5 nm, inset Vs = – 2 V, I = 120 pA, Z-Range 0.55 nm);  (d) sample surface after annealing at 200°C (Vs = – 2 V, I = 80 pA, Z-Range 1.4 nm, inset Vs = – 1.2 V, I = 80 pA, Z-Range 0.63 nm).

The achievement of an integrated laser will allow IHP to complete its photonic “toolkit”. This technological platform would enable the addition of adding photonic-based detection capability to IHP´s wireless sensor network boards, leading to the cost effective fabrication of portable sensing devices capable of detecting hazardous materials, biomolecules, and gases, in wireless sensor network.

 

Das Gebäude und die Infrastruktur des IHP wurden finanziert vom Europäischen Fonds für regionale Entwicklung, von der Bundesregierung und vom Land Brandenburg.