UNCOOLED HIGH-PERFORMANCE MICRO AND NANOSTRUCTURED LEAD SELENIDE-BASED MWIR PHOTODETECTORS

Abstract

Lead selenide (PbSe) is a highly promising semiconductor for mid-wavelength infrared (MWIR) photodetectors, offering a favorable combination of narrow bandgap energy, strong optical absorption, and compatibility with low-cost manufacturing processes. Despite challenges such as identifying suitable substrates for single-crystalline growth and mitigating defect-induced performance degradation, sensitized PbSe has emerged as a cost-effective, high-performance solution for uncooled sensing applications. This dissertation develops a comprehensive suite of PbSe-based heterostructure and nanostructured photodetectors, demonstrating significant advancements in device architecture, material quality, and photodetector performance.As a foundational step, the integration of monocrystalline PbSe with vicinal germanium (Ge) substrates was realized, establishing a new pathway for forming high-quality heterogeneous interfaces despite the significant lattice constant and thermal expansion coefficient mismatch between PbSe and Ge. This was achieved through an in-situ surface treatment of the Ge substrate that enhanced adatom surface kinetics prior to PbSe deposition, resulting in crack-free, single-crystalline n-type PbSe films on p-type Ge (100) substrates. The resulting heterojunction exhibited strong diode-like rectifying behavior with a high rectification factor and low reverse-bias current density. X-ray diffraction (XRD) measurements revealed narrow full width at half maximum (FWHM), and strong photoluminescence (PL) confirmed the formation of high-quality PbSe (100) epitaxial films on industry-standard vicinal Ge substrates. This integration not only demonstrated robust epitaxial alignment but also opened possibilities for dual-band detector architectures and other PbSe-based heterojunction devices. Building on this foundation, a novel epitaxial CdSe-on-PbSe type-II heterojunction photovoltaic detector was demonstrated via molecular beam epitaxy (MBE). For the first time, high-quality, single-phase cubic CdSe was epitaxially grown on p-type single-crystalline PbSe films, as confirmed by in-situ reflection high-energy electron diffraction (RHEED). The resulting p–n junction exhibited rectifying behavior with a rectification factor exceeding 50 at room temperature. Under zero-bias photovoltaic operation, a 30 μm × 30 μm pixel achieved a responsivity of 0.06 A/W and a specific detectivity (D*) of 6.5 × 10⁸ Jones at room temperature. Device performance improved significantly with thermoelectric cooling, reaching a responsivity of 0.441 A/W and a D* of 4.4 × 10⁹ Jones at 230 K, underscoring the effectiveness of epitaxial interface engineering in enhancing carrier transport and suppressing recombination losses in PbSe-based MWIR detectors. The research further advanced toward scalable, low-cost device architectures through the development of nanocrystalline PbSe/CdSe heterostructure photoconductors, fabricated via vapor phase deposition (VPD) on commercially available SiO₂/Si substrates. Optimization of post-deposition oxygen annealing improved film quality and defect passivation, resulting in a room-temperature D* of 8.57 × 10⁸ Jones and a peak detectivity of 2.49 × 10⁹ Jones, with an interband cutoff wavelength of 4 μm. Crucially, this approach demonstrated a clear pathway toward low-cost, large-area MWIR detection systems while maintaining compatibility with silicon-based readout integrated circuits (ROICs) for monolithic integration. Further improvements were achieved by transitioning to PbSe/CdSe nanostructured photoconductors with small pixel dimensions, building directly upon insights gained from the larger-area heterostructure devices. Aggressive pixel scaling and precise control of the annealing process enabled the formation of highly interconnected nanostructures that facilitated efficient photogenerated carrier transport and further defect suppression. The reduction in pixel size not only enhanced device performance but also revealed critical insights into the underlying gain mechanisms and the role of traps, which had been obscured in earlier large-area devices. These small-pixel devices, defined with active areas of 17.5 × 20 μm² via reactive ion etching, exhibited tunable absorption edges at 3.75 μm and 4.0 μm, with room-temperature specific detectivities of 2.17 × 10¹⁰ Jones and 1.61 × 10¹⁰ Jones, respectively. External quantum efficiencies exceeding 100% confirmed strong photoconductive gain. Noise analysis indicated 1/f noise as the dominant mechanism, with behavior consistent with Hooge’s empirical relation for homogeneous semiconductors, validating the high electronic quality achieved through process optimization. Collectively, the advancements presented in this dissertation chart a progressive trajectory in PbSe-based MWIR photodetector development — from epitaxial, high-performance heterojunction diodes to scalable heterostructure photoconductors, and finally to optimized nanostructured small-pixel devices with record performance at room temperature. Through deliberate control of material growth, interface engineering, and device scaling, this work significantly advances the state of PbSe-based infrared detection. These developments establish a robust framework for future PbSe MWIR sensing technologies, offering a pathway toward compact, low-cost, high-performance detectors suitable for a wide range of commercial, industrial, and defense applications.