Barrier Optimization of HgCdTe nBn Infrared Detectors

This thesis investigates the nBn infrared detecting device structure, specifically those made from HgCdTe grown on CdTe/Si substrates by Molecular Beam Epitaxy (MBE), and the improvement to performance that replacing the alloy barrier layer with a properly designed superlattice can yield. The main impacts that the superlattice barrier imparts on the devices comes from the electron and hole mobilities and the band alignment, specifically through the valence band offset (VBO) and conduction band offset (CBO) between barrier and absorber layers. The research explores the device designs both theoretically, through the use of numerical simulations, and experimentally, though the fabrication and characterization of many device designs. Ultimately it is shown that the biggest improvement given by the superlattice barrier to the devices comes from its impact on the electron effective mass in the barrier (blocking unwanted, electron-mediated dark currents), while maintaining a reasonable VBO and CBO. The study begins by detailing the relevance of infrared technology. Material selection is crucial for optimizing detector performance, and in the case of the nBn structure, the selection of the barrier layer material is of particular importance. This thesis discusses the development of a superlattice for the barrier as a means to adjust the band alignment and carrier mobilities, potentially overcoming some intrinsic limitations of nBn devices. The experimental section of the research outlines the processes involved in the growth and fabrication of the devices. Techniques such as Molecular Beam Epitaxy (MBE) are detailed, noting their importance in achieving the desired material properties and device structures. The thesis also discusses the challenges and solutions associated with doping and metallization, critical steps that influence the final device performance. Characterization methods play a vital role in understanding the operational capabilities of the fabricated devices. Techniques like Fourier Transform Infrared (FTIR) spectroscopy are used to evaluate the spectral response of the detectors, demonstrating their capability as an infrared detector. Other techniques like photoconductive lifetime and dark current measurement were used to help optimize the device fabrication process and diagnose troublesome processes like passivation. In conclusion, this thesis presents a comprehensive examination of the HgCdTe nBn structure for infrared detection. Several alloy barrier and superlattice barrier devices were fabricated, characterized, and compared. Higher than expected dark currents were seen, but a self consistent explanation, derived from theoretical modeling, of increased Hg in both the alloy barrier and the CdTe layers of the superlattice barrier closely matches the experimental results. The overall higher performance of the superlattice barrier devices also agrees with the theoretical results, indicating that the improved performance was likely due to the increased effective electron mass in the barrier, even with a band alignment similar to the alloy barrier design. This shows the improvement to device performance afforded by the superlattice barrier while retaining the robustness and simplified device fabrication that the nBn device design affords.