Research ArticleAPPLIED OPTICS

Single-crystalline germanium nanomembrane photodetectors on foreign nanocavities

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Science Advances  07 Jul 2017:
Vol. 3, no. 7, e1602783
DOI: 10.1126/sciadv.1602783
  • Fig. 1 Fabrication and characterization of ultrathin Ge nanomembranes on foreign substrates.

    (A) Schematic of the fabrication process flow of the Ge nanomembranes on foreign substrates. The laboratory-made GeOI was used as the source wafer, and the membrane transfer–printing method (figs. S5 and S6) was then used to transfer the Ge membrane onto a foreign substrate. Subsequent thin-down process was adopted to obtain the desired Ge thickness (10 to 60 nm). (B) Optical microscopy image of ultrathin Ge (10 to 60 nm), which is transfer-printed on Al2O3/Ag/Si substrates. Scale bars, 75 μm for all five subfigures. (C) Triple-axis HR-XRD scans of our transferred crystalline ultrathin Ge (red) and evaporated amorphous Ge (blue) films. The thicknesses are both 20 nm. (D) Raman scattering results of the two samples in (C). The Raman signal of the transferred Ge membrane shows the typical single peak (300.9 cm−1) of single-crystalline Ge materials.

  • Fig. 2 Absorption results of ultrathin Ge nanomembrane on foreign substrate and its comparison with a traditional structure.

    (A) Schematic of ultrathin Ge (20 nm) nanomembrane on the foreign substrate. (B and C) Absorption spectra (B) and the spatial distribution of the absorption (C) for the structure shown in (A). (D) Schematic of a traditional epitaxial Ge on Si0.75Ge0.25 buffer layer/Si substrate. (E and F) Absorption spectrum (E) and the spatial distribution of the absorption (F) for the structure shown in (D). The color bars in (C) and (F) use the same scale for direct comparison.

  • Fig. 3 I-V measurements of ultrathin Ge photodetector under both dark and illuminated conditions.

    (A) Schematic of an ultrathin Ge photodetector on a nanocavity. (B) Optical microscopy image of ultrathin Ge photodetector. Scale bar, 50 μm. (C) I-V curves of the p-type Ge photoconductor under dark (black curve) and illuminated (colored curves) conditions. Inset: The photocurrent shows a linear relationship with the illuminated power. (D) Calculated normalized photocurrent–to–dark current ratio (NPDR) with the illuminated power and its comparison with a MoS2 photodetector. (E) Dark current of Ge membranes with different thicknesses (that is, 360, 300, 250, 75, 50, and 20 nm) under the bias of 1 V. (F) NPDR comparison between the Ge photodetectors with and without nanocavity structure (GeOI-based).

  • Fig. 4 Measurements of ultrathin Ge phototransistor and its analysis.

    (A) Transfer curves (VGS-IDS) of the ultrathin Ge transistor. (B) Photoresponsivity and VDS-IDS curves of the ultrathin Ge phototransistor under the illumination power density of 140.8 mW/cm2. The peak photoresponsivity is 4.7 A/W. (C) Top: Ultrathin Ge transistor photocurrent (|ΔIDS|) as a function of VGS under various power conditions: 3.36 (purple), 20.8 (green), 140.8 (orange), and 343.2 mW/cm2 (red), respectively. Bottom: Transconductance (|Gm|) of ultrathin Ge transistors. (D) Comparison of threshold voltage change (ΔVTH) of the ultrathin Ge phototransistor with that of the epitaxial-based III-V phototransistor. The measured ΔVTH values are obtained by subtracting the threshold voltages extracted from the VGS-IDS curves under the illuminated and dark conditions (fig. S9). The solid curve is plotted using the empirical equation for ΔVTH.

  • Fig. 5 Absorption measurements of ultrathin Ge sample with varying thickness (from 6 to 40 nm) on the same chip and the spectral photocurrent response.

    (A) Absorption spectra measured by Fourier transform infrared spectroscopy of the ultrathin Ge sample. The peak wavelength becomes larger as the Ge nanomembrane thickness changes from 6 to 40 nm. (B) Left axis: Photoresponsivity (A/W) results of three different samples on the same 220-nm-thick Al2O3/Ag substrate. The thicknesses of Ge films are (from top to bottom) 12, 17, and 26 nm, respectively. Right axis: Simulated absorption spectra of these three samples, with the same structure parameters mentioned above. The simulation is based on full-wave Maxwell’s equations.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/7/e1602783/DC1

    Supplementary Materials

    note S1. Fabrication and characterization of the laboratory-made GeOI wafer.

    fig. S1. Fabrication process flow of 4-inch GeOI and its finished sample.

    fig. S2. Characterization of laboratory-made GeOI.

    fig. S3. The GeOI sample used for the van der Pauw measurement.

    fig. S4. Thickness measurement of the corresponding ultrathin Ge membranes in Fig. 1B by AFM (XE-70 Park System).

    fig. S5. Demonstration of the ultrathin Ge nanomembranes from laboratory-made GeOI to foreign substrate.

    fig. S6. Titled scanning electron microscopy images of the ultrathin Ge membranes transferred onto foreign substrate.

    fig. S7. Dark current measurements of different thicknesses of the Ge membrane sample.

    fig. S8. The dark current and the photoresponse of the ultrathin GeOI sample.

    fig. S9. VGS-IDS curve under the dark and different illumination conditions.

    fig. S10. Simulated absorption spectra of ultrathin Ge nanomembranes on a 220-nm-thick Al2O3/Ag nanocavity substrate.

    fig. S11. Optical microscopy images of the samples for multispectral response measurement.

    table S1. Electronic properties of p-type GeOI sample.

  • Supplementary Materials

    This PDF file includes:

    • Supplementary Materials
    • note S1. Fabrication and characterization of the laboratory-made GeOI wafer.
    • fig. S1. Fabrication process flow of 4-inch GeOI and its finished sample.
    • fig. S2. Characterization of laboratory-made GeOI.
    • fig. S3. The GeOI sample used for the van der Pauw measurement.
    • fig. S4. Thickness measurement of the corresponding ultrathin Ge membranes in Fig. 1B by AFM (XE-70 Park System).
    • fig. S5. Demonstration of the ultrathin Ge nanomembranes from laboratory-made GeOI to foreign substrate.
    • fig. S6. Titled scanning electron microscopy images of the ultrathin Ge membranes transferred onto foreign substrate.
    • fig. S7. Dark current measurements of different thicknesses of the Ge membrane sample.
    • fig. S8. The dark current and the photoresponse of the ultrathin GeOI sample.
    • fig. S9. VGS-IDS curve under the dark and different illumination conditions.
    • fig. S10. Simulated absorption spectra of ultrathin Ge nanomembranes on a 220-nm-thick Al2O3/Ag nanocavity substrate.
    • fig. S11. Optical microscopy images of the samples for multispectral response measurement.
    • table S1. Electronic properties of p-type GeOI sample.

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