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We introduce a charge separation model in this work to explain the mechanism of enhanced photoconductivity of polycrystalline lead salt photoconductors. Our results show that this model could clarify the heuristic fabrication processes of such lead salt detectors that were not well understood and often considered mysterious for nearly a century. The improved lifetime and performance of the device, e.g., responsivity, are attributed to the spatial separation of holes and electrons, hence less possibility of carrier recombination. This model shows that in addition to crystal quality the size of crystallites, the depth of outer conversion layer, and doping concentration could all affect detector performance. The simulation results agree well with experimental results and thus offer a very useful tool for further improvement of lead salt detectors. The model was developed with lead salt family of photoconductors in mind, but may well be applicable to a wider class of semiconducting films. [ABSTRACT FROM AUTHOR]

n-CdS/p-PbSe heterojunction is investigated. A thin CdS film is deposited by chemical bath deposition on top of epitaxial PbSe film by molecular beam epitaxy on Silicon. Current-voltage measurements demonstrate very good junction characteristics with rectifying ratio of ~178 and ideality factor of 1.79 at 300 K. Detectors made with such structure exhibit mid-infrared spectral photoresponse at room temperature. The peak responsivity Rk and specific detectivity D* are 0.055 A/W and 5.482×108 cm.Hz1/2/W at λ=4.7 lm under zero-bias photovoltaic mode. Temperature-dependent photoresponse measurements show abnormal intensity variation below ~200 K. Possible reasons for this phenomenon are also discussed. [ABSTRACT FROM AUTHOR]

The CaF2 nano-structures grown by thermal vapor deposition are presented. Significant responsivity improvement (>200%) of mid-infrared PbSe detectors incorporating a 200 nm nano-structured CaF2 coating was observed. The detector provides a detectivity of 4.2×1010 cm ⋅ Hz1/2/W at 3.8 lm, which outperforms all the reported un-cooled PbSe detectors. Structural investigations show that the coating is constructed by tapered-shape nanostructures, which creates a gradient refractive-index profile. Analogy to moth-eye antireflective mechanism, the gradient refractive-index nanostructures play the major roles for this antireflection effect. Some other possible mechanisms that help enhance the device performance are also discussed in the work. [ABSTRACT FROM AUTHOR]

Flower-like bundles of ZnO nanosheets have been prepared by using preheating hydrothermal process without any surfactants. The flower-like bundles consist of many thin and uniform hexagonal-structured ZnO nanosheets, with a thickness of 50 nm. The selected area electronic diffraction (SAED) and high-resolution transmission electron microscope (HRTEM) images indicate that the ZnO nanosheets are single crystal in nature. The growth mechanism of the flower-like bundles of ZnO nanosheets is discussed based on the morphology evolution with growth times and reaction conditions. It is believed that the formation of flower-like bundles of ZnO nanosheets is related to the shielding effect of OH- ions and the self-assembly process, which is dominated by a preheating time. Room temperature photoluminescence spectra results show that the annealing atmosphere strongly affects the visible emission band, which is sensitive to intrinsic and surface defects, especially oxygen interstitials, in flower-like bundles of ZnO nanosheets. [ABSTRACT FROM AUTHOR]

An optimal design of two dimensional (2D) hexagonal photonic bandgap (PBG) resonating micro-optical defect cavity on IV-VI lead salt material has been carried out. The nature of both the transverse electric (TE) and transverse magnetic (TM) band structure for the electromagnetic waves in the periodic triangular lattice pattern is formulated by the well-established plane wave expansion (PWE) method. The defect cavity is engineered to resonate at ~4.17 µm in TM bandgap. The field distribution in the defect cavity has been analyzed based on two very efficient and popular schemes - perturbation correction finite difference (FD) method and finite difference time domain (FDTD) mechanism which is truncated by uniaxial perfectly matched layer (UPML) absorbing boundary condition (ABC). FD method efficiently solves Helmholtz equations to evaluate the field distribution in the semiconducting waveguide for any single spectral wavelength. The numerical results by FD method are re-established by the FDTD scheme that incorporates a precise numerical analysis within a specified wavelength range. [ABSTRACT FROM AUTHOR]

This paper presents recent developments in the field of lead-salt light-emitting devices, having enormous applications in mid-infrared (MIR) opto-electronics. For the first time, achievement of an electrically excited edge-emitting QW laser on PbSnSe substrate is described. Novel fabrications of freestanding microstructures in the form of rod, tube and pillar, having vast applications in MEMS and NEMS, are reported. [ABSTRACT FROM AUTHOR]

We present a detailed study on the surface morphologies and chemical composition of growth pits in lead selenide (PbSe) epilayers grown by molecular-beam epitaxy. Various growth pits were investigated by scanning electron microscopy and energy-dispersive x-ray analysis. The vast majority of growth pits within the PbSe epilayers contained either single or multiple PbSe microcrystals with a distinct cuboid shape. Based on the observed structures and morphologies of the PbSe microcrystals, the formation mechanism of the cuboid structures was considered to be (100) preferential crystal growth originating from PbSe polycrystalline seeds. [ABSTRACT FROM AUTHOR]

Theoretical simulations on a new [110]-oriented IV-VI lead-salt midinfrared (IR) edge-emitting laser are performed in comparison with the conventional [100] oriented laser. The modal gain of quantum-well (QW) structure on [110] orientation is significantly higher than that on the [100] orientation. Light versus current (L-I) characteristics at different operation temperatures are simulated by solving the coupled rate equations. The maximum operation temperature of [110] laser is 70 degree higher than that of conventional [100] laser. To further improve the laser performance a new Si3N4 clad laser structure is proposed to improve the heat dissipation. The effects of other parameters on the performance of the lasers, such as Auger recombination, Schockley-Read recombination, facet reflectivity and cavity length, are also investigated. The simulated results show that with optimized laser structure and modestly reduced Auger recombination [110] IV-VI lead-salt edge-emitting laser could produce output power of about 10 mW at room temperature. [ABSTRACT FROM AUTHOR]

A high-quality as-grown PbSe film with a record low threading dislocation density of 9×105 cm-2 on patterned Si(111) substrate has been obtained using molecular beam epitaxy. The mechanisms leading to the remarkable reduction in threading dislocation density are analyzed. Based on the analysis, further reduction in dislocation density is anticipated. Materials with such low dislocation density should significantly improve the Si-based IV-VI group device performance. [ABSTRACT FROM AUTHOR]