High detectivity quantum well infrared photodetectors

This dissertation describes the characterization, optimization and physics of the high-detectivity Quantum Well Infrared Photodetector (QWIP) and Infrared Hot-Electron Transistor (IHET).; In order to characterize the detectors for high-detectivity operation at low temperatures with very little incident infrared radiation, we have to measure very low levels of current and current noise under cryogenic conditions. We developed a low-noise cryogenic probing system capable of measuring currents at the femtoAmpere (fA) level with integration time ≤1sec and current noise spectra at the 1fA/√Hz level with the sample cooled to cryogenic temperatures. The amplification is 0.1V/1pA with f_3-dB = 70Hz, 1.1V/pA with f _3-dB = 20Hz, and 10.1V/pA with f_3-dB = 1.4Hz. The input current noise power density is 0.8fA/√Hz r.m.s. and the input voltage noise power density is ≈0.3μV/√Hz r.m.s. at 1Hz frequency.; We performed high-sensitivity measurements on the dark current and the noise current of IHETs and their constituent QWIPs at 4.2K. We found that the dominant noise of the QWIPs in this regime is not from the expected shot noise but from the 1/f noise and a bias-independent noise. By filtering out the tunneling dark currents, the IHETs reduce the dark current and the 1/f noise associated with the impurity-assisted tunneling current, and improve the detector sensitivity and uniformity.; We optimized a 10μm-cutoff infrared hot-electron transistor (IHET) to achieve a detectivity of D*(QWIP) = 1.2 × 1013 cm√Hz/W and D*(IHET) > 7.5 × 1012 cm√Hz/W at 4.2K. This large D* is accomplished by using InGaAs in the quantum wells and a low filter barrier at the collector to achieve large photocurrent transfer ratio. The noise of the QWIP at the emitter of the optimized IHET is dominated by the bias-independent noise. The 1/f noise is reduced. The 1/f current noise is due to the conductance fluctuation in impurity-assisted tunneling via DX centers in the quantum well barriers. The filter barrier of the IHET blocks the tunneling current and hence its noise at the collector and thus improves the detector sensitivity.