We report the effects of N2 and O2 on the electrical properties of PbSe quantum-dot (QD) films treated with 1,2-ethanedithiol (EDT) by measuring the changes in the current−voltage characteristics of QD field-effect transistors (FETs). EDT-treated PbSe QD films at a base pressure of ∼10−5 Torr exhibit ambipolar transport. Exposing these films to N2 shifts the transfer characteristics toward negative gate-voltage values and increases the electron mobility. These changes could be reversed entirely by removing the N2 gas over the FET and returning to base pressure. Oxygen exposure shifts the transfer characteristics in the opposite direction toward positive gate-voltage values. Moreover, oxygen exposure reduces charge mobility but increases film conductivity. For exposures up to ∼108 langmuir, these O2-induced changes could be reversed completely by removing the O2 gas over the sample and returning to base pressure. However, after ∼1010 langmuir of O2 exposure, the changes are irreversible. The QD films then ...

Influence of Atmospheric Gases on the Electrical Properties of PbSe Quantum-Dot Films

For nearly a century, oxygen has been widely accepted as the key element that triggers photo-response in polycrystalline PbSe photoconductive detectors. Our photoluminescence and responsivity studies on PbSe samples, however, suggest that oxygen only serves as an effective sensitization improver and it is iodine rather than oxygen that plays the key role in triggering the photo-response. These studies shed light on the sensitization process for detector applications and ways to passivate defects in IV–VI semiconductors. As a result, high peak detectivity of 2.8 × 1010 cm·Hz1/2·W−1 was achieved at room temperature.

Study of sensitization process on mid-infrared uncooled PbSe photoconductive detectors leads to high detectivity

Alternative material photovoltaics (PVs) have started gaining more attention recently. Although the field is not new, it just started growing a few years ago. The PV market has been dominated by various silicon technologies, besides a few other popular thin films, such as CdTe, copper–indium–galium–selenide varieties and some III–V materials. This has been reflected in research as well. Successful developments of efficient solar cells using alternative absorbers will significantly enrich the PV industry and reduce the market gap with other energy sources. Hence, in this review, recent advances and trends to develop PVs using alternative materials are presented and discussed. The focus will be mainly on binary as well as environmentally friendly compounds and thin film devices. Nonetheless, some other more complex materials and structures will be briefly addressed.

Recent advances in alternative material photovoltaics

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.

Understanding sensitization behavior of lead selenide photoconductive detectors by charge separation model

Impact of thickness on vacuum deposited PbSe thin films

A series of PbSe thin films grown on a (111)-oriented Si substrate by molecular beam epitaxy were passivated by high-purity oxygen at different annealing temperatures. The photoluminescence intensity increased by more than two orders of magnitude at 4.5μm after annealing the samples in an O2 atmosphere at 350°C. X-ray photoelectron spectroscopy revealed that PbO and SeO2 were formed during the oxidation process of PbSe, thus confirming the formation of the surface passivation layer which resulted in the observed significant increase in PL intensity.

Influence of oxygen passivation on optical properties of PbSe thin films

MBE growth of PbSe thin film with a 9 × 105 cm-2 etch-pits density on patterned (1 1 1 )-oriented Si substrate

A new model for the mechanism of photoconductivity in annealed polycrystalline PbSe fllm is presented. The combined mechanism with respect to double heterojunction due to oxidation layer, dopant segregation and carrier trapping at grain-boundary is proposed. This letter focuses on characterizing the potential proflle, which is extremely important from the viewpoint of carrier transport phenomena. A potential proflle adjacent to the boundaries was calculated, and the efiect of biased voltage was in detail discussed, which shows that the mech- anism of photoconductivity of annealed polycrystalline PbSe fllm depends on properties of the grain boundaries.

https://www.piers.org/piersonline/pdf/Vol5No1Page61to64.pdf

Modeling of the Potential Profile for the Annealed Polycrystalline PbSe Film

PbSe thin films grown on (111)-oriented Si substrates by molecular-beam epitaxy were annealed under an oxygen atmosphere with a variety of temperatures and times. The photoluminescence intensity from the sample annealed at 350°C for 2 h was increased by 400-fold with frontside pumping and increased by 40-fold with backside pumping, respectively, in comparison with the as-grown sample. Furthermore, the mobility was increased twofold after annealing. Such large photoluminescence increases and mobility improvements could be attributed to the surface passivation and the dislocation and defect passivation caused by oxygen diffusion and reaction with the PbSe film.

D. Li

Papers

Influence of oxygen passivation on optical properties of PbSe thin films

MBE growth of PbSe thin film with a 9 × 105 cm-2 etch-pits density on patterned (1 1 1 )-oriented Si substrate

Modeling of the Potential Profile for the Annealed Polycrystalline PbSe Film

Influence of Oxygen Post-Growth Annealing on Optical and Electrical Properties of PbSe Thin Films

A new in-situ surface treatment during MBE-grown PbSe on CaF2/Si(111) heterostructure