Recently, Integrated Microwave Photonics (IMWP) on Silicon-on-Insulator (SOI) platform has attracted much attention. SOI as a platform for passive devices has been studied for operation at a wavelength of 1.55 μm. It has been acknowledged that Si suffers from Two-Photon Absorption (TPA) at this wavelength, which potentially limits the dynamic range of microwave photonic links. The TPA effect diminishes at longer wavelengths, and completely vanishes at a wavelength of 2.2 μm. So far, the detailed effects of TPA on the performance of the SOI platform in the context of IMWP have not been well-explored. In this work, a systemic simulation has been performed in order to investigate the effects of nonlinear TPA on the performance of SOI microwave photonic links at 1.55 μm, particularly the dynamic range of the system was studied in-depth. Furthermore, system performance at wavelengths from 1.55 to 2.2 μm was investigated by scaling the device design, where the parasitic effects of TPA are avoided. Based on theoretical analysis, the result showing microwave photonic links at 2.2 μm outperform that at 1.55 μm was obtained as expected. Strip and rib waveguides at 1.55 μm show P1dB points of 3.75 mW and 69.54 mW, respectively, and at 2.2 μm, these waveguides performed linearly throughout the simulated range (up to 1 W of input power) which is due to the dramatically reduced TPA effect. It is noted that the wavelengths at which SOI avoids parasitic TPA match well with the working wavelengths of emerging GeSn lasers and photodetectors.
Silicon-based optoelectronic devices have long been desired owing to the possibility of monolithic integration of photonics with high-speed Si electronics and the aspiration of broadening the reach of Si technology by expanding its functionalities well beyond electronics. To overcome the intrinsic problem of bandgap indirectness in the group-IV semiconductors such as Si and Ge, a new group-IV based material, GeSn alloy, has attracted increasing interests. The group-IV GeSn alloy has been demonstrated to become direct bandgap material with more than 8% Sn incorporation, which opens a new opportunity towards a Si-based light source with fully complementary metal-oxide-semiconductor (CMOS) compatibility. The GeSn laser contributes strongly to the progress of optoelectronic integration towards next-generation photonic integrated circuit on the Si platform, as it fills the deficiency of the efficient group-IV band-to-band lasers. Moreover, due to the tunable bandgap of GeSn, the lasing operation wavelength covers broad near- and mid-infrared range. Recently, the GeSn optically pumped lasers based on direct bandgap GeSn alloys have been demonstrated.
In this work, the following aspects have been investigated: i) the novel growth strategy to obtain high Sn compositions based on spontaneous-relaxation-enhanced (SRE) Sn incorporation and the GeSn virtual substrate (VS) approaches. The maximum Sn composition of 22.3% was achieved; ii) the demonstration of GeSn optically pumped heterostructure lasers. The operation wavelength covers from 2 to 3 µm and the maximum lasing temperature is 265 K; iii) the demonstration of GeSn quantum well laser. The significantly reduced lasing threshold compared to heterostructure laser was achieved.
This research investigates the fundamental limits and trade-space of quantum semiconductor photodetectors using the Schrödinger equation and the laws of thermodynamics.We envision that, to optimize the metrics of single photon detection, it is critical to maximize the optical absorption in the minimal volume and minimize the carrier transit process simultaneously. Integration of photon management with quantum charge transport/redistribution upon optical excitation can be engineered to maximize the quantum efficiency (QE) and data rate and minimize timing jitter at the same time. Due to the ultra-low capacitance of these quantum devices, even a single photoelectron transfer can induce a notable change in the voltage, enabling non-avalanche single photon detection at room temperature as has been recently demonstrated in Si quanta image sensors (QIS). In this research, uniform III-V quantum dots (QDs) and Si QIS are used as model systems to test the theory experimentally. Based on the fundamental understanding, we also propose proof-of-concept, photon-managed quantum capacitance photodetectors. Built upon the concepts of QIS and single electron transistor (SET), this novel device structure provides a model system to synergistically test the fundamental limits and tradespace predicted by the theory for semiconductor detectors.
This project is sponsored under DARPA/ARO's DETECT Program: Fundamental Limits of Quantum Semiconductor Photodetectors.
To harness the advanced fabrication capabilities and high yields of the electronics industry for photonics, monolithic growth and CMOS compatibility are required. One promising candidate which fulfils these conditions is GeSn. Introducing Sn lowers the energy of the direct Γ valley relative to the indirect L valley. The movement of the conduction band valleys with Sn concentration is critical for the design of efficient devices; however, a large discrepancy exists in the literature for the Sn concentration at which GeSn becomes a direct band gap. We investigate the bandgap character of GeSn using hydrostatic pressure which reversibility modifies the bandstructure. In this work we determine the movement of the band-edge under pressure using photocurrent measurements. For a pure Ge sample, the movement of the band-edge is dominated by the indirect L valley with a measured pressure coefficient of 4.26±0.05 meV/kbar. With increasing Sn concentration there is evidence of band mixing effects, with values of 9.4±0.3 meV/kbar and 11.1±0.2 meV/kbar measured for 6% and 8% Sn samples. For a 10% Sn sample the pressure coefficient of 13±0.5 meV/kbar is close to the movement of the direct bandgap of Ge, indicating predominately direct Γ-like character for this GeSn alloy. This further suggests a gradual transition from indirect to direct like behaviour in the alloy as also evidenced from theoretical calculations. The implications of this in terms of optimising device performance will be discussed in further detail at the conference.
The GeSn alloy with Sn composition of 11% has been grown using an industry standard reduced-pressure chemical vapor deposition reactor in a single run epitaxy. Low-cost commercially available GeH4 and SnCl4 were used as Ge and Sn precursors, respectively. The material characterization showed that the threading dislocations were trapped in the Ge/GeSn interface and do not propagate to the GeSn layer, resulting in high quality material. The temperature-dependent photoluminescence study revealed that the direct bandgap GeSn alloy was achieved, as the emission intensity significantly increased at low temperature. The sample was than fabricated into photoconductive detectors and waveguide lasers. For the photodetector, the spectral response wavelength cutoff at 3.0 μm was observed. The specific detectivity of 3.5×1010 cm•Hz1/2W-1 was achieved, which is close to that of market dominating InGaAs photodetectors that are operating in the same wavelength range; For the waveguide laser, the lasing threshold pumping power density of 86.5 kW/cm2 at 10 K and the highest operating temperature of 110 K were obtained. Furthermore, the characteristic temperature was evaluated as 65 K.
In this work, high performance GeSn photoconductor and light emitting diodes (LED) have been demonstrated. For the photoconductor, the high responsivity was achieved due to high photoconductive gain, which is attributed to the novel optical and electrical design. The longwave cutoff at 2.4 μm was also observed at room temperature. For LED, temperature-dependent study was conducted. The electroluminescence (EL) spectra at different temperatures were obtained and EL peak shift was observed. Moreover, the emission power at different temperatures was measured. High power emission at 2.1 μm was achieved.
Ge1-xSnx/Ge thin films and Ge/Ge1-xSnx/Ge n-i-p double heterostructure (DHS) have been grown using commercially available reduced pressure chemical vapor deposition (RPCVD) reactor. The Sn compositional material and optical characteristics have been investigated. A direct bandgap GeSn material has been identified with Sn composition of 10%. The GeSn DHS samples were fabricated into LED devices. Room temperature electroluminescence spectra were studied. A maximum emission power of 28mW was obtained with 10% Sn LED under the injection current density of 800 A/cm2.
Si based Ge1-xSnx photoconductors, with Sn incorporation of 0.9, 3.2, and 7%, were fabricated using a CMOS-compatible process. Temperature dependent study was conducted from 300 to 77 K. The first generation device (standard photoconductor, PD) shows long wavelength cut-off beyond 2.1 μm for 7%-Sn devices at room temperature. The peak responsivity and D* of the 7% Sn device at 1.55 μm were obtained at 77K as 0.08 A/W and 1×109 cm*Hz1/2*W-1, respectively. Improved responsivity and specific detectivity (D*) were observed on second generation devices by a newly designed electrode structure (photoconductor with interdigitated electrodes, IEPD). The enhancement factor of responsivity was up to 15 at 77 K.
The injection and temperature dependence of the spontaneous emission quantum efficiency of molecular beam epitaxy
grown InGaAs/GaAs quantum wells is determined using excitation dependent photoluminescence (PL) measurements.
The PL measurements were performed at temperatures from 50 to 300 K using a HeNe pump laser with powers ranging
from 0.6 to 35 mW. The quantum efficiency is inferred from the power law predicted by the rate equations that links
pump power and integrated PL signal. The peak spontaneous emission quantum efficiency of molecular beam epitaxy
(MBE) grown InGaAs/GaAs triple quantum wells is determined to be 0.941 at 300K with an overall best value of 0.992
at 100 K.
The fundamental mechanisms of electroluminescence (EL) refrigeration in heterostructure light emitting diodes, is
examined via carrier energy loss (and gain) during transport, relaxation, and recombination, where the contribution of
electrons and holes are treated separately. This analysis shows that the EL refrigeration process is a combination of
thermoelectric cooling that mainly occurs near the metal/semiconductor contacts and radiative recombination which
mainly occurs in the active region. In semiconductors such as GaAs, electrons and holes make different contributions
to the refrigeration processes as a result of their different densities of states.
A theoretical study of photoluminescence refrigeration in semiconductors has been carried out using a model that takes into account photon recycling and includes the rate equations for both carriers and photons. General expressions for cooling efficiency, cooling power density, and the cooling condition are derived. The investigation of the photoluminescence refrigeration in an intrinsic GaAs slab shows that net cooling is accessible when quantum efficiency and luminescence extraction are high, and that photon recycling contributes strongly to photoluminescence refrigeration when the luminescence extraction efficiency is small.