Transfer-printing-enabled GeSn flexible resonant-cavity-enhanced photodetectors with strain-amplified mid-infrared optical responses.

Mid-infrared (MIR) flexible photodetectors (FPDs) constitute an essential element for wearable applications, including health-care monitoring and biomedical detection. Compared with organic materials, inorganic semiconductors are promising candidates for FPDs owing to their superior performance as well as optoelectronic properties. Herein, for the first time, we present the use of transfer-printing techniques to enable a cost-effective, nontoxic GeSn MIR resonant-cavity-enhanced FPDs (RCE-FPDs) with strain-amplified optical responses. A narrow bandgap nontoxic GeSn nanomembrane was employed as the active layer, which was grown on a silicon-on-insulator substrate and then transfer-printed onto a polyethylene terephthalate (PET) substrate, eliminating the unwanted defects and residual compressive strain, to yield the MIR RCE-FPDs. In addition, a vertical cavity was created for the GeSn active layer to enhance the optical responsivity. Under bending conditions, significant tensile strain up to 0.274% was introduced into the GeSn active layer to effectively modulate the band structure, extend the photodetection in the MIR region, and substantially enhance the optical responsivity to 0.292 A W-1 at λ = 1770 nm, corresponding to an enhancement of 323% compared with the device under flat conditions. Moreover, theoretical simulations were performed to confirm the strain effect on the device performance. The results demonstrated high-performance, nontoxic MIR RCE-FPDs for applications in flexible photodetection.

Efficient Suppression of Charge Recombination in Self-Powered Photodetectors with Band-Aligned Transferred van der Waals Metal Electrodes.

Recombination of photogenerated electron-hole pairs dominates the photocarrier lifetime and then influences the performance of photodetectors and solar cells. In this work, we report the design and fabrication of band-aligned van der Waals-contacted photodetectors with atomically sharp and flat metal-semiconductor interfaces through transferred metal integration. A unity factor α is achieved, which is essentially independent of the wavelength of the light, from ultraviolet to near-infrared, indicating effective suppression of charge recombination by the device. The short-circuit current (0.16 µA) and open-circuit voltage (0.72 V) of the band-aligned van der Waals-contacted devices are at least 1 order of magnitude greater than those of band-aligned deposited devices and 2 orders of magnitude greater than those of non-band-aligned deposited devices. High responsivity, detectivity, and polarization sensitivity ratio of 283 mA/W, 6.89 × 1012 cm Hz1/2 W-1, and 3.05, respectively, are also obtained for the device at zero bias. Moreover, the efficient suppression of charge recombination in our air-stable self-powered photodetectors also results in a fast response speed and leads to polarization-sensitive performance.

Planar GeSn lateral p-i-n resonant-cavity-enhanced photodetectors for short-wave infrared integrated photonics.

We report normal-incidence planar GeSn resonant-cavity-enhanced photodetectors (RCE-PDs) with a lateral p-i-n homojunction configuration on a silicon-on-insulator (SOI) platform for short-wave infrared (SWIR) integrated photonics. The buried oxide of the SOI platform and the deposited SiO2 layer serve as the bottom and top reflectors, respectively, creating a vertical cavity for enhancing the optical responsivity. The planar p-i-n diode structure is favorable for complementary-metal-oxide-semiconductor-compatible, large-scale integration. With the bandgap reduction enabled by the 4.2% Sn incorporation into the GeSn active layer, the photodetection range extends to 1960 nm. The promising results demonstrate that the developed planar GeSn RCE-PDs are potential candidates for SWIR integrated photonics.

Raman scattering study of GeSn under 〈1 0 0〉 and 〈1 1 0〉 uniaxial stress.

The application of strain into GeSn alloys can effectively modulate the band structures, thus creating novel electronic and photonic devices. Raman spectroscopy is a powerful tool for characterizing strain; however, the lack of Raman coefficient makes it difficult for accurate determination of strain in GeSn alloys. Here, we have investigated the Raman-strain function of Ge1-xSnxalong 〈1 0 0〉 and 〈1 1 0〉 directions. GeSn nanomembranes (NMs) with different Sn compositions are transfer-printed on polyethylene terephthalate substrates. External strain is introduced by bending fixtures with different radii, leading to uniaxial tensile strain up to 0.44%. Strain analysis of flexible GeSn NMs bent along 〈1 0 0〉 and 〈1 1 0〉 directions are performed by Raman spectroscopy. The linear coefficients of Raman-strain for Ge0.96Sn0.04are measured to be -1.81 and -2.60 cm-1, while those of Ge0.94Sn0.06are decreased to be -2.69 and -3.82 cm-1along 〈1 0 0〉 and 〈1 1 0〉 directions, respectively. As a result, the experimental ratio of linear coefficient (ROLC) of Ge, Ge0.96Sn0.04and Ge0.94Sn0.06are 1.34, 1.44 and 1.42, which agree well with theoretical ROLC values calculated by elastic compliances and phonon deformation potentials (PDPs). In addition, the compositional dependence of PDPs is analyzed qualitatively. These fundamental parameters are important in designing high performance strained GeSn electronic and photonic devices.

GeSn lateral p-i-n waveguide photodetectors for mid-infrared integrated photonics.

In this Letter, we demonstrate mid-infrared (MIR) lateral p-i-n GeSn waveguide photodetectors (WGPDs) on silicon, to the best of our knowledge for the first time, as a key enabler of MIR electronic-photonic integrated circuits (EPICs). Narrow-bandgap GeSn alloys were employed as the active material to enable efficient photodetection in the MIR region. A lateral p-i-n homojunction diode was designed and fabricated to significantly enhance the optical confinement factor of the guided modes and thus enhance the optical responsivity. Thus, a photodetection range of up to 1950 nm and a good responsivity of 0.292 A/W at 1800 nm were achieved. These results demonstrate the feasibility of planar GeSn WGPDs for monolithic MIR EPICs on silicon.