Figure 5

Photocurrent spectra of samples under different temperatures and zero bias. The PC measurements were conducted using Bruker Equinox 55 FTIR spectrometer under step-scan mode with a resolution of 16 cm^-1. The IR beam was chopped before it arrived at the sample, and the signal from the sample was fed through a high-speed pre-amp and then input to a lock-in amplifier, which was locked into the chopper frequency. The inset shows the calculated conduction band diagram of one period of 30-stage QDCL active core under zero bias.