Laser Fabrication of Lead Selenide Thin Films for Infrared Focal Plane Array Applications

The demand for integrating infrared (IR) detectors and imaging systems in consumer products and defense applications is rapidly growing. The requirement for IR uncooled, highly responsive, and spectrally tailored devices are driving a resurgence of interest in lead selenide (PbSe) thin films for detectors and focal plane array (FPA). PbSe has been used for MWIR (3 – 5 µm) detectors for over 70 years, however, the photoconductivity mechanism is still not well understood, and higher device performance is desired. The large Bohr exciton radius of PbSe (~46 nm) facilitates strong quantum confinement properties and spectral tailoring as far down as 690 nm. New thin-film deposition methods and processing approaches are needed to better control and expand PbSe thin film properties and to understand the photoconductivity mechanism.
The primary goals of this thesis work are (1) to demonstrate a new method for PbSe thin film fabrication using laser sintering deposition (LSD) of micro/nano particles of PbSe (2) to improve the understanding of the photoconductive mechanism in PbSe thin films through optical, electrical, morphological, and structural characterization (3) to fabricate a photodetector sensor and focal plane array device using the LSD method combined with laser patterning, avoiding some of the difficulties of using photolithography liftoff.
The new method introduced in this work is a 3-step process called Laser Sinter Deposition (LSD). The first step produces colloidal nanocrystals (NC) by planetary ball milling high-purity PbSe in a methanol solution. The second step is the rapid centrifugal deposition of the PbSe NCs on silicon substrates. Finally, laser sintering is accomplished by rastering a pulsed or continuous-wave laser (UV, visible, or IR) to sinter and form a continuous polycrystalline thin film with superior substrate adhesion and highly tailorable properties. Laser processing has the advantage of allowing spatially defined sintering, ultra-rapid sintering (millisecond to nanosecond) to activate doping, selectively ablation of material with depth control, reorganizing crystalline structures including defect reduction, and the potential for trapping of non-equilibrium structures. Another advantage of LSD is the ability for selective depth sintering (SDS) by exploiting the absorption depth of different laser wavelengths.
This work shows that LSD can be tailored to produce a thin film of PbSe with nearly identical properties to films prepared by Chemical Bath Deposition (CBD), in terms of electrical, compositional, and physical properties. In contrast to CBD, LSD produces superior substrate adhesion, unique morphological control (columnar versus equiaxed grains), and provides spatially tailorable grain modification (lateral and transverse)., In contrast to all other deposition methods, the LSD process also allows in-situ doping of the nanocrystals prior to laser sintering which allows precise concentration of dopant potassium iodide, resulting in a more dense film, an inter-grain coating of PbI2, and a 300% increase in photoresponse compared to films without KI. PbSe thin films were oxidized and iodized (sensitization process) to enhance the photoresponse. We report a process agnostic model of the complex interactions between oxidation and iodization to identify the narrow process window that exists in the sensitization process. In doing this, we identify a previously unreported compound (Pb3Se2O6I2) that exists in the transition layer between the base layer of PbSe and the top layer of PbI2. This compound is only observed in highly photoresponsive films. The inter-grain PbSeO3 coating is confirmed through Transmission Electron Microscopy (TEM) and helps in regulating the diffusion of iodine along grain boundaries which assists to densify the film and enhancing the photoresponse. The presence of iodine decreases dark current, while simultaneously assisting in recrystallizing PbSe during sensitization.
One interesting discovery is that p-type PbSe film with a carrier concentration of 4.8 x 1018 cm-3 can be converted to n-type with a carrier concentration of 9.2 x 1018 cm-3 with a simple laser treatment using a 355 nm (UV) wavelength laser by selectively reducing the selenium concentration. The UV treatment also improved carrier mobility by nearly 500%, from 6.5 to 37.5 cm2/V·s. Laser processing also simplifies device fabrication by allowing selective in-situ patterning of PbSe, the metal conductor (Au), and substrate, in one system to eliminate the need for photolithography. Unlike other forms of rastered patterning, such as e-beam lithography, laser patterning is accomplished at raster speeds of 10 meters per second in one step without photoresist or chemicals.
This work successfully demonstrates the complete laser fabrication of both discrete 3.5 cm2 photodetectors with a Detectivity of 1.01 x 109 cm·Hz-1/2/W and the capability of laser patterning focal plane array (FPA) with vertical contact pixels as small as 60 x 70 µm at a 100 µm pitch to fabricate a 37 x 68 pixel FPA on a 1 cm x 2 cm substrate in < 30 seconds. This work demonstrates the functional operation of a laser patterned 120 pixel FPA prototype of lateral contact pixels (240 µm x 260 µm). This work successfully demonstrates lasers can be used successfully to fabricate both thin films of a few microns in thickness as well as complex integrated devices such as focal plane arrays (FPAs).
In summary, we have successfully demonstrated a new method of thin-film deposition using laser sintering of nanoparticles, provided an enhanced understanding of the photoconductive mechanism in PbSe, and IR devices like photodetectors and focal plane arrays can be fabricated by direct laser patterning.