Ge
1-xSn
x alloys are recently applied in infrared photodetector [
38,
39]. By using a high quality 820 nm thick Ge
0.97Sn
0.03 alloy film grown on Si(001) by molecular beam epitaxy, we have successfully fabricated GeSn p-i-n photodetectors [
23]. The schematic cross-section view of the detector is shown in Fig. 12. Circular mesas with diameters ranging from 20 to 200 μm were formed by etching the patterned films down to the Si substrate using an inductively coupled plasma etcher. For a 50 μm diameter device, the diode currents were 35.3, 45.2, and 534 μA at -1, 0, and 1 V, respectively. The measured dark current density was 1.8 A/cm
2 at -1 V. A disadvantage of Ge detector is the drastic decrease of the photoresponse of Ge detector beyond 1550 nm, which makes the device difficult to be applied in infrared region beyond 1550 nm. Comparing with the Ge detector, the responsivity of the GeSn detector decreases much slower and is higher beyond 1550 nm. Even at a wavelength as long as 1640 nm, the responsivity is still relatively high. At 1310, 1540, and 1640 nm, the responsivities are 0.52, 0.23, and 0.12 A/W, respectively, corresponding to external quantum efficiencies of 49.2%, 18.5%, and 9.1%. The responsivities spectra of the Ge
0.97Sn
0.03 detector under -1 V bias are shown in Fig. 13. The photocurrent spectrum measured at 0 V using a Fourier transform infrared spectrometer is also shown. As can be seen, the detectors exhibite relatively high responsivities in all the measured wavelengths (1310-1640 nm) and are shown to have a photoresponse up to 1800 nm, covering the whole telecommunication windows. The high performance of the Ge
0.97Sn
0.03 detector is guaranteed by the high quality Ge buffer layer, which enables the promising application for all telecommunication wavebands detection.