Fig. 1. Schematic of the graded HgCdTe photodetector and energy band structure (a) schematic diagram of HgCdTe photodetector, (b) cross-section view of the graded HgCdTe photodetector. Energy band profiles of p-type MCT material, (c) without grading, (d) with grading, (e) cross-sectional TEM image of the graded HgCdTe materials, and (f) EDS mappings of the structure

Fig. 2. Device fabrication and the measurement in detail (a) Fabrication process of HgCdTe photodetector, (b) The normalized spectral response spectrum of MCT photodetector based on graded LPE and VPE materials at 300 K, the absorption cut-off wavelength is broadened to 4.6um, (c) I-V curves of LPE MCT photodetector for incident laser powers from 0 to 177.8 µW. The inset describes the photovoltaic mechanism for detecting mid-infrared radiation, (d) Linear plots (bule) and logarithmic plots (red) of dark current in LPE MCT photodetector as a function of bias voltage, (e) The schematic model description of the composition profile and the band diagram associated with the photodiode, and (f) FTIR spectra recorded after continuous etching of the HgCdTe epilayer (d = thickness of epilayer from the CdZnTe substrate after wet etching)

Fig. 3. Optoelectronic properties and frequency response of graded HgCdTe photodetectors under laser illumination at room temperature (a) impulse response measurement setup schematics for characterization of ultafast photodetectors, (b) lightwave Component Analyzer (LCA) and network analyzer setup for bandwidth characterization of high-speed photodetecto. (c) relative response versus switching frequency of LPE n-on-p photodetector, showing the 3 dB cutoff frequency of 115 MHz, (d) the relative response as a function of light intensity modulation frequency of VPE n-on-p photodetector, showing the 3 dB cutoff frequency of 750 MHz, (e) time response of the LPE photodetector under 1550 nm femtosecond pulse laser with the pulse width of l00 femtosecond. The repetition frequency of pulse is 80MHz and the analog output signal of LPE photodetector under pulse laser is measured, the curve is extracted from a highspeed oscilloscope, (f) time response of the VPE photodetector under 1550 nm femtosecond pulse laser, (g) time response curve of the LPE photodetector under 2000 nm switching conditions at U_bias= 0 V, (h) the time-resolved photoresponse curves of the VPE photodetector at room temperature under conditions of U_bias= 0 V.