Rodolfo Camacho-Aguilera

Ge-on-Si laser operating at room temperature

Monolithic lasers on Si are ideal for high-volume and large-scale electronic-photonic integration. Ge is an interesting candidate owing to its pseudodirect gap properties and compatibility with Si complementary metal oxide semiconductor technology. Recently we have demonstrated room-temperature photoluminescence, electroluminescence, and optical gain from the direct gap transition of band-engineered Ge-on-Si using tensile strain and n-type doping. Here we report what we believe to be the first experimental observation of lasing from the direct gap transition of Ge-on-Si at room temperature using an edge-emitting waveguide device. The emission exhibited a gain spectrum of 1590-1610 nm, line narrowing and polarization evolution from a mixed TE/TM to predominantly TE with increasing gain, and a clear threshold behavior.

An electrically pumped germanium laser

Electrically pumped lasing from Germanium-on-Silicon pnn heterojunction diode structures is demonstrated. Room temperature multimode laser with 1mW output power is measured. Phosphorous doping in Germanium at a concentration over 4x1019cm-3 is achieved. A Germanium gain spectrum of nearly 200nm is observed.

Direct band gap narrowing in highly doped Ge

Direct band gap narrowing in highly doped n-type Ge is observed through photoluminescence measurements by determining the spectrum peak shift. A linear relationship between the direct band gap emission and carrier concentration is observed. We propose a first order phenomenological model for band gap narrowing based on two parameters whose values for Ge are EBGN = 0.013 eV and ΔBGN = 10−21 eV/cm−3. The application of these results to non-invasive determination of the active carrier concentration in submicron areas in n-type Ge structures is demonstrated.

High active carrier concentration in n-type, thin film Ge using delta-doping

We demonstrate CVD in situ doping of Ge by utilizing phosphorus delta-doping for the creation of a high dopant diffusion source. Multiple monolayer delta doping creates source phosphorous concentrations above 1 × 1020cm−3, and uniform activated dopant concentrations above 4 × 1019cm−3 in a 600-800nm thick Ge layer after in-diffusion. By controlling dopant out-diffusion, near-complete incorporation of phosphorus diffusion source is shown.

An electrically pumped Ge-on-Si laser

We present the first CMOS compatible, electrically pumped Fabry-Perot Ge laser with larger than 1mW output power and a gain spectrum width of nearly 200nm in the range from 1520nm to 1700nm.

High phosphorous doped germanium: Dopant diffusion and modeling

The in situ n-type doping of Ge thin films epitaxial grown on Si substrates is limited to 1 × 1019 cm−3 by the phosphorous out-diffusion during growth at 600 °C. By studying the phosphorous diffusion in Ge with different background doping, we find that the diffusion coefficient is extrinsic and is enhanced 100 times in Ge doped at 1 × 1019 cm−3 compared to intrinsic diffusivity. To achieve higher phosphorous concentration, delta-doped layers are used as a dopant source for phosphorous in-diffusion. We show that the doping level is a result of the competition between in-diffusion and dopant loss. The high diffusivity at high n-type carrier concentration leads to a uniform distribution of phosphorous in Ge with the concentration above 3 × 1019 cm−3.

Analysis of Threshold Current Behavior for Bulk and Quantum-Well Germanium Laser Structures

We analyze the optical gain of tensile-strained, n-germanium (n-Ge) material taking bandgap narrowing (BGN) for heavily doped Ge into account. Both the direct bandgap and indirect bandgap are narrowed by 60 meV. Our new modeling explains the wide lasing spectrum of 1520-1700 nm in electrically pumped Ge lasers. The calculated materials gain can reach 1000 cm-1 when the injected carrier density is ~mid-1019 cm-3 with a combination of 0.25% tensile strain and 4.5×1019 cm-3 n-type doping. The threshold current density is estimated to be 0.53 kA/cm2 for an optimized edge-emitting double-heterojunction Ge device, comparable to bulk III-V lasers. We also review current progress on Ge quantum-well (QW) structures. The threshold current density of most Ge QW structures is similar to bulk Ge. Only tensile-strained QWs show reduced threshold currents.

Infrared absorption of n-type tensile-strained Ge-on-Si

We analyze the IR absorption of tensile-strained, n-type Ge for Si-compatible laser applications. A strong intervalley scattering from the indirect L valleys to the direct Γ valley in n+ Ge-on-Si is reported for the first time to our knowledge. The intervalley absorption edge is in good agreement with the theoretical value. On the other hand, we found that the classical λ2-dependent Drude model of intravalley free-carrier absorption (FCA) breaks down at λ<15 μm. A first-principle model has to be employed to reach a good agreement with the experimental data. The intravalley FCA loss is determined to be <20 cm(-1) for n=4×10(19) cm(-3) at λ=1.5-1.7 μm, an order lower than the results from Drude model. The strong L→Γ intervalley scattering favors electronic occupation of the direct Γ valley, thereby enhancing optical gain from the direct gap transition of Ge, while the low intravalley free-electron absorption at lasing wavelengths leads to low optical losses. These two factors explain why the first electrically pumped Ge-on-Si laser achieved a higher net gain than the theoretical prediction using λ2-dependent free-carrier losses of bulk Ge and indicate the great potential for further improvement of Ge-on-Si lasers.

Engineering broadband and anisotropic photoluminescence emission from rare earth doped tellurite thin film photonic crystals

Broadband and anisotropic light emission from rare-earth doped tellurite thin films is demonstrated using Er3+-TeO2 photonic crystals (PhCs). By adjusting the PhC parameters, photoluminescent light can be efficiently coupled into vertical surface emission or lateral waveguide propagation modes. Because of the flexibility of light projection direction, Er3+-TeO2 is a potential broadband light source for integration with three-dimensional photonic circuits and on-chip biochemical sensors.

Optical characterization of Ge-on-Si laser gain media

Tensile strained Ge films with P concentrations as high as 3.4 × 10¹⁹ cm⁻³ are grown using UHVCVD. Photoluminescence measurements reveal significant direct band gap narrowing, enhanced photoemission, and optical bleaching.