Heterostructure Engineering of Quantum Dots-in-a-Well Infrared Photodetectors.

摘要

Three of the most important characteristics of third-generation imaging systems are, high-operating temperature, multispectral operation, and large format arrays. The quantum dot infrared photodetector technology, owing to the three-dimensional confinement of carriers, the richness of the electronic spectra in quantum dots, and the mature III-V based fabrication technology, satisfy these requirements. This work focuses on quantum dots-in-a-well (DWELL) detectors in which InAs quantum dots are embedded in a compressively strained InGaAs-GaAs quantum well. Barriers separating two stacks of quantum dots can be GaAs, AlGaAs or a combination of different materials, with 'smart barriers'.;Motivation for this work is to improve the understanding and the performance of DWELL detectors to achieve high temperature operation and high signal to noise ratio for these detectors for given wavelength requirements, at applied biases compatible with CMOS technology. This aim has been pursued on three fronts: barrier designs, device designs and material systems. Smart barriers, such as resonant tunneling barriers have been demonstrated to improve the signal to noise ratio of the detector by reducing the dark current significantly, while keeping the photocurrent constant. A systematic experimental study has been conducted for understanding the effect of different types of transitions on the properties of DWELL detectors, which showed that bound to quasibound (B-Q) type of transitions optimize the device performance at moderate bias levels. The performance of B-Q type of architectures has been substantially improved by the use of confinement enhancing (CE) barriers that combine the advantages of high energy barriers, such as low dark current and high signal to noise ratio, with those of low energy barriers, such as high responsivity and longer peak wavelengths at low bias operation.;A new type of detector, a quantum dot based quantum cascade detector, has been proposed and implemented. QD-QCD exhibits a strong photovoltaic action, leading to strong performance at zero bias, by the virtue of internal electric field generated by the quantum cascade action in the barrier. The zero bias operation, combined with record low photoconductive gains for any quantum dot detectors, makes QD QCD very attractive for focal plane array applications.;For improved understanding, theoretical modeling of quantum dot strain, based on atomistic valence force field method as well as transport simulations of general heterostructure detectors with drift-diffusion model have been developed. The transport simulation results indicate the presence of a strong space charge region forming between the highly n-doped contact regions and non-intentionally doped barrier regions, which makes the internal electric field highly nonlinear in space. This has been verified by systematic experiments, in which effects of this electric field nonlinearity on the device parameters have been studied.;This work would enable a device designer to choose different device parameters such as spectral response position and shape, photoconductive gain, response, signal to noise ratio, dark current levels, activation energies etc. This knowledge has been utilized in demonstrating highly sensitive FPAs, as well as high operating temperature imaging (at 140K) with DWELL detectors. State of the art performance has been obtained from different devices at different wavelengths, such as such as a detectivity of 4x1011 cm.Hz1/2W-1 at 77K in a bound to quasibound device with a cutoff wavelength of 8.5 microm, which is higher than that obtained from state of the art QWIPs. Although the dark current levels are substantially lower than standard QWIPs, and background limited photodetection is at much higher temperature, the focal plane array sensitivities are lower than those of the state of the art QWIPs, by around 10 mK, due to lower quantum efficiency (a factor of 2--3) and higher photoconductive gain. This difference can be eliminated by the use of gratings or shape engineering through the use of submonolayer quantum dots and with smaller photoconductive gains with DWELL detectors.