Photoconductive mechanism of IR-sensitive iodized PbSe thin films via strong hole–phonon interaction and minority carrier diffusion

Abstract

Photoconductive PbSe thin films are highly important for mid-infrared imaging applications. However, the photoconductive mechanism is not well understood so far. Here we provide additional insight on the photoconductivity mechanism using transmission electron microscopy, x-ray photoelectron microscopy, and electrical characterizations. Polycrystalline PbSe thin films were deposited by a chemical bath deposition method. Potassium iodide (KI) was added during the deposition process to improve the photoresponse. Oxidation and iodization were performed to sensitize the thin films. The temperature-dependence Hall effect results show that a strong hole–phonon interaction occurs in oxidized PbSe with KI. It indicates that about half the holes are trapped by KI-induced self-trapped hole centers ( V k center), which results in increasing dark resistance. The photo Hall effect results show that the hole concentration increases significantly under light exposure in sensitized PbSe, which indicates the photogenerated electrons are compensated by trapped holes. The presence of KI in the PbSe grains was confirmed by I 3 d 5 / 2 core-level x-ray photoelectron spectra. The energy dispersive x-ray spectra obtained in the scanning transmission electron microscope show the incorporation of iodine during the iodization process on the top of PbSe grains, which can create an iodine-incorporated PbSe outer shell. The iodine-incorporated PbSe releases electrons to recombine with holes in the PbSe layer so that the resistance of sensitized PbSe is about 800 times higher than that of PbSe without the iodine-incorporated layer. In addition, oxygen found in the outer shell of PbSe can act as an electron trap. Therefore, the photoresponse of sensitized PbSe is from the difference between the high dark resistance (by KI addition and iodine incorporation) and the low resistance after IR exposure due to electron compensation (by electron traps at grain boundary and electron–hole recombination in KI hole traps).

Photoconductive PbSe thin films for infrared imaging

“…The majority of progress in this area has been accomplished using the CBD method, which is also the preferred deposition method for achieving the highest IR responsivity. [72][73][74][75] The CBD process is low-cost, and the setup is straightforward, requiring commonly available chemical lab facilities and equipment. Various precursor chemistries shown in Table 3 are used as sources of lead and selenium.…”

“…Many groups reported that both oxidation and iodization must occur to achieve any appreciable IR photoresponse in thin-films of PbSe. [72][73][74][75]79,80 Until recently, the understanding is that oxidation of the top B50% of the polycrystalline PbSe film, as well as along grain boundaries, produces forms of PbSeO 3 or Pb 3 O 2 (SeO 3 ), which acts as a large bandgap, potential barrier, selectively trapping electrons and extending hole carrier lifetime to allow transport to electrodes. The role of iodine was accepted by most as aiding in the recrystallization and assisting in the controlled incorporation of oxygen into the PbSe lattice.…”

“…Golubchenko et al reported that the presence of iodine reduces the formation temperature of the oxyselenites such as PbSeO 3 by more than 100 K. 81,82 While this may be one of its roles, recent work by Jang et al and Suh et al show that the symbiotic relationship between oxygen and iodine is multipurpose. 73,75,83 Regardless of the order and role of oxygen and iodine, both thermal processes reduce selenium content in the upper part of the PbSe film to form PbI 2 and various PbSe x I y O z , and PbSe x O 1Àx complexes. The net results are reduced selenium content, the formation of lead oxide/PbI 2 layers, and the diffusion of iodine and oxygen into the PbSe film and/or along PbSe grain boundaries.…”

“…Both Jang et al and Suh et al show that iodine incorporation in the actual CBD deposition bath is necessary to incorporate iodine into the PbSe lattice during the film deposition, in addition to a subsequent thermal iodization process to form a PbI 2 layer. 73,75,83 Jang et al reported the chemical reactions during oxidations and iodization, respectively. 60…”

“…Chemical bath deposition is the most common and currently produces the highest IR responsivity of all the deposition methods. [72][73][74][75] CBD films are generally deposited on silicon, glass, or GaAs substrates of thicknesses ranging from 0.2-2 mm. The typical randomly oriented polycrystalline morphology produced by CBD seems to lend well to the subsequent sensitization processing, which requires diffusion and incorporation of both oxygen and iodine in and around the grain boundaries.…”