Precise Characterization of High-Efficiency Photodetectors

The EQE measurements were conducted by a commercially available QE‐R system from Enlitech (with greater than ≈99.5% repeatability). A Xenon lamp was installed in the system. The light was then directed into a monochromator for single wavelength beam output. The wavelength step was set to 5 nm to obtain accurate measurements. The single wavelength light beam was directed onto the active area of the device. The illumination was modulated via a chopper (165 Hz) which was connected to a lock‐in amplifier, thereby ensuring that the device output photovoltage signal frequency was synchronized for accurate signal recording. The temperature of the measurement state was set to 25 °C using a temperature controller. The averaging sampling rate during data acquisition was set to 15, while the allowed noise range was set to 5E‐4 for precise measurements. Prior to the EQE measurement, the system was calibrated against two reference photodetectors: Si photodetector from 300 to 1100 nm (Enlitech Model RC‐S103011‐E) and Ge photodetector from 1100 to 1200 nm (Enlitech Model RC‐G108018‐E). The signal‐to‐noise ratio of the two reference photodetectors is given in Figure S17 in the Supporting Information. The EQE repeatability of separate devices is shown in Figure S5 in the Supporting Information. The IQE was calculated by the following equation
$$\mathrm{IQE}=\mathrm{EQE}/\mathrm{Absorption}$$ where the absorption results in Figure 3b were utilized. The spectral responsivity R_λ was determined by
$$R_{\lambda }=\mathrm{EQE}\times \lambda \times \frac{e}{hc}$$ where λ is the wavelength of photons, e is the elementary charge, h is Plank's constant, and c is the vacuum light speed.
Current–voltage (I–V), capacitance–voltage (C–V), and temporal response with on/off illumination were measured using a Keithley 4200 semiconductor analyzer. Diode lasers (Thorlabs) of different wavelengths (515, 780, and 980 nm) were properly aligned and focused onto the device region. The measured photocurrents by using the Keithley 4200 semiconductor analyzer (0.1 fA rated noise measurement with two remote amplifiers (4225‐RPM) for ultra‐low current measurement) also confirmed the high EQE results. The incident laser beam was calibrated by a sensitive optical power meter (Thorlabs PM200, with a S120C photodiode power sensor for power levels down to nanowatts). The photocurrents (linear region) in Figure 4d and the correspondingly calculated responsivity were consistent with the measured high EQE results as per the relationship given in Equation (4).
The photocurrent I_ph was calculated by
$$I_{\mathrm{ph}}=I_{\mathrm{light}}-I_{\mathrm{dark}}$$ where I_light is the current under illumination and I_dark is the dark current.
The specific detectivity D* was calculated by
$$D^{\ast }=\frac{R_{\lambda }\sqrt{S}}{\sqrt{2e{I}_{\mathrm{dark}}}}$$ where S is the effective area of the photodiode.
For temporal response measurements, fiber‐coupled diode lasers at 515 and 1060 nm wavelengths (Thorlabs LP515‐SF3 and Qphotonics QFBGLD‐1060‐10BTF) were mounted into laser mounts, which were connected to a laser driver (Thorlabs LDC 210C) and a temperature controller (Thorlabs TED 200C). A function generator (Hewlett Packard 33120A) was connected to the laser driver, to impress a square signal to modulate the laser frequency at 1000 Hz. The photovoltage of the devices was recorded by a digital oscilloscope (Tektronix MDO3024).