Yi-Chiau Huang

Demonstration of a Ge/GeSn/Ge Quantum-Well Microdisk Resonator on Silicon: Enabling High-Quality Ge(Sn) Materials for Micro- and Nanophotonics

We theoretically study and experimentally demonstrate a pseudomorphic Ge/Ge0.92Sn0.08/Ge quantum-well microdisk resonator on Ge/Si (001) as a route toward a compact GeSn-based laser on silicon. The structure theoretically exhibits many electronic and optical advantages in laser design, and microdisk resonators using these structures can be precisely fabricated away from highly defective regions in the Ge buffer using a novel etch-stop process. Photoluminescence measurements on 2.7 μm diameter microdisks reveal sharp whispering-gallery-mode resonances (Q > 340) with strong luminescence.

Hole Mobility Enhancement in Compressively Strained

Germanium tin (GeSn) pMOSFETs with channel Sn composition of 7% are fabricated using a low thermal budget process. GeSn pMOSFETs show enhancement in hole mobility over control Ge devices by 85% in high inversion charge density regime. Hole mobility improvement observed in GeSn channel pMOSFETs compared with Ge control is due to the biaxial compressive strain in GeSn resulting from epitaxial growth of GeSn thin films on relaxed Ge buffer layers.

{\rm Ge}_{0.93}{\rm Sn}_{0.07}

We present a new etch chemistry that enables highly selective dry etching of germanium over its alloy with tin (Ge(1-x)Sn(x)). We address the challenges in synthesis of high-quality, defect-free Ge(1-x)Sn(x) thin films by using Ge virtual substrates as a template for Ge(1-x)Sn(x) epitaxy. The etch process is applied to selectively remove the stress-inducing Ge virtual substrate and achieve strain-free, direct band gap Ge0.92Sn0.08. The semiconductor processing technology presented in this work provides a robust method for fabrication of innovative Ge(1-x)Sn(x) nanostructures whose realization can prove to be challenging, if not impossible, otherwise.

pMOSFETs

We propose ZnO as an attractive interfacial layer (IL) option for n-type metal-IL-semiconductor (MIS) contacts because of (i) good conduction band alignment between ZnO and Si/Ge/SiC, (ii) high n-type doping possible in ZnO, and, (iii) low Fermi-level pinning factor for metal/ZnO contacts. Device simulations suggest better scalability for MIS contacts versus silicides/germanides for future FinFET technologies. Contact diode measurements on Ti/n⁺-ZnO/n-Ge and Ti/n⁺-ZnO/n-Si devices show nearly 1000X increase in current densities due to the presence of an n⁺-ZnO IL. In comparison to alternate IL options such as Al₂O₃ and TiO₂, n⁺-ZnO gives significantly higher current densities on n-Ge as demonstrated through device simulations and experimental data. Specific contact resistivity of (0.8-1.5) × 10^-6 Ω cm² is demonstrated through four-probe measurements on circular TLM devices fabricated on n⁺-Ge (1 × 10¹⁹ cm^-3) epi layers using n⁺-ZnO IL.

Highly Selective Dry Etching of Germanium over Germanium–Tin (Ge1–xSnx): A Novel Route for Ge1–xSnx Nanostructure Fabrication

We study the effect of surface passivation on pseudomorphic multiple-quantum-well Ge0.97Sn0.03/Ge p-i-n photodetectors. A combination of ozone oxidation to form GeOx and GeSnOx on the surface of the diodes followed by atomic layer deposition of Al2O3 for protection of these native oxides provides reduced dark current. With a temperature-dependent investigation of dark current, we calculate the activation energy to be 0.26 eV at a bias of −0.1 V and 0.05 eV at −1 V for the sample passivated by this ozone method. Based on these activation energy results, we find that the current is less dominated by bulk tunneling at lower reverse bias values; hence, the effect of surface passivation is more noticeable with nearly an order-of-magnitude improvement in dark current for the ozone-passivated sample compared to control devices without the ozone treatment at a voltage of −0.1 V. Passivation also results in a significant enhancement of the responsivity, particularly for shorter wavelengths, with 26% higher respons...

ZnO: an attractive option for n-type metal-interfacial layer-semiconductor (Si, Ge, SiC) contacts

The development of a precise micromachining process for Ge1–xSnx has the potential to enable both the fabrication and optimization of Ge1−xSnx-based devices in photonics and microelectromechanical systems. We demonstrate a digital etching scheme for Ge0.922Sn0.078 based on a two-stage, highly selective CF4 plasma dry etch and HCl wet etch. Using X-Ray Reflectivity, we show consistent etch control as low as 1.5 nm per cycle, which is defined as one dry etch step followed by one wet etch step. The etch rate increases to 3.2 nm per cycle for a longer dry etch time due to physical sputtering contributions, accompanied by an increase in RMS surface roughness. By operating within a regime with minimal sputtering, we demonstrate that good digital etch depth control and surface quality can be achieved using this technique.

Passivation of multiple-quantum-well Ge0.97Sn0.03/Ge p-i-n photodetectors

Although the development of a monolithically-integrated, silicon-compatible light source has been traditionally limited by the indirect band gaps of Group IV materials, germanium-tin (Ge1-xSnx) is predicted to exhibit direct band gap behavior. In pseudomorphic conditions with materials of smaller lattice constant, the accumulation of compressive strain in Ge1-xSnx counteracts this behavior to prevent the direct band gap transition. One possible approach to compensate for this compressive strain is to introduce tensile strain into the system, which can be achieved by applying an external stressing agent to post-fabricated devices. We describe a suspended Ge0:922Sn0:078 multiple quantum well microdisk resonator cavity strained by 140 nm of highly compressively stressed silicon nitride. Raman shifts and photoluminescence redshifts indicate that an additional 0.23-0.30% strain can be induced in these microdisks with this approach. The ability to tune the optical performance of these resonator structures by strain engineering has the potential to enable the development of low threshold Ge1-xSnx-based lasers on Si.

Dry-wet digital etching of Ge1−xSnx

This paper presented the selective epitaxial germanium growth on silicon by trench filling and doping. The process operated in a reaction rate limited regime at a low temperature (350°C to 400°C) to ensure reasonable growth rate, decent surface morphology, and high dopant incorporation in Ge.

Strained germanium-tin multiple quantum well microdisk resonators towards a light source on silicon

Ge/strained GeSn/Ge quantum wells are grown on a 300 mm Si substrate by chemical vapor deposition. The direct bandgap emission from strained GeSn is observed in the photoluminescence spectra and is enhanced by Al2O3/SiO2 passivation due to the field effect. The electroluminescence of the direct bandgap emission of strained GeSn is also observed from the Ni/Al2O3/GeSn metal-insulator-semiconductor tunneling diodes. Electroluminescence is a good indicator of GeSn material quality, since defects in GeSn layers degrade the electroluminescence intensity significantly. At the accumulation bias, the holes in the Ni gate electrode tunnel to the strained n-type GeSn layer through the ultrathin Al2O3 and recombine radiatively with electrons. The emission wavelength of photoluminescence and electroluminescence can be tuned by the Sn content.

Selective Epitaxial Germanium Growth on Silicon - Trench Fill and In Situ Doping

The worlds first GeSn p-FinFETs formed on a novel GeSn-on-insulator (GeSnOI) substrate is reported, with channel lengths L<inf>ch</inf> down to 50 nm and fin width W<inf>Fin</inf> down to 20 nm. In comparison with other reported GeSn p-FETs, record low S of 79 mV/decade, record high G<inf>m, int</inf>, of 807 μS/um (VDs of −0.5 V), and the highest G<inf>m, int</inf>/S<inf>sat</inf>, were achieved. The highest high-field hole mobility of 208 cm2/Vs (at inversion carrier density of 8×10⁻² cm⁻²) for GeSn p-FETs with CVD grown GeSn channel was also obtained.