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Room-temperature oxygen sensitization in highly textured, nanocrystalline PbTe films: A mechanistic study
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Image of FIG. 1.
FIG. 1.

(Color online) (a) XRD spectra of thermally evaporated PbTe films on oxide-coated Si substrates. The film is a polycrystalline single fcc phase with strong (200) texture. Standard XRD data from PDF card # 03-065-0137 is also shown at the bottom for comparison. (b) Thickness dependence of peak intensity ratio, i.e., I(200)/I(220), showing the degree of texture increases sharply with decreasing film thickness. The ratio for the standard randomly oriented PbTe sample of PDF card # 03-065-0137 is 1.45.

Image of FIG. 2.
FIG. 2.

Schematic illustration of two-step nucleation and growth model for polycrystalline PbTe films on the SiO2/Si substrate. (a) Initial nucleation and growth of PbTe grains on the SiO2/Si substrate. The arrows are pointing to the [200] direction in PbTe. (b) Film growth stage. New grains with more random orientations are nucleated and grow on nucleation sites created in step (a).

Image of FIG. 3.
FIG. 3.

(Color online) AFM surface morphology images of thermally evaporated PbTe films show the nanocrystalline structure (a) 100 nm, (b) 200 nm, (c) 300 nm, (d) 500 nm, and (e) 1000 nm. Arrows in (c) and (d) indicate randomly oriented grains. (f) Thickness dependence of surface roughness and average grain size. Both surface roughness and average grain size decrease rapidly with decreasing film thickness.

Image of FIG. 4.
FIG. 4.

Cross-sectional TEM image shows the columnar structure of a 500 nm thick film. “Through-thickness” grain boundaries are indicated by the white arrows.

Image of FIG. 5.
FIG. 5.

(Color online) Oxygen concentration depth profiles obtained by SIMS for PbTe films with low and high oxygen concentrations (i.e., with and without the Ge23Sb7S70 capping layer). The two orders of magnitude concentration difference in the samples suggests that most oxygen in the “high oxygen concentration” sample comes from oxygen in-diffusion when the films are exposed to air.

Image of FIG. 6.
FIG. 6.

(Color online) (a) XPS spectra of lead 4f peaks; (b) XPS spectra of tellurium 3d peaks. The black solid curves in (a) and (b) correspond to XPS spectra collected on the surface of the films, and the red dotted curves are spectra taken on films after removal of the surface layer using Ar ion milling. The change of Pb-O and Te-O peak intensity after surface removal suggests the presence of a surface oxide layer in the sample. (c) XPS spectrum of oxygen 1s peaks for the PbTe film after removal of a surface oxidation layer by Ar ion milling. Two chemical binding states due to oxidation of Pb and Te are identified and the ratio of oxygen bonded to Pb and Te is calculated to be about 8:1, indicating oxygen preferentially bonds with Pb.

Image of FIG. 7.
FIG. 7.

(Color online) (a) Carrier concentration and (b) Hall mobility vs PbTe film thickness taken at room temperature at three different time intervals after deposition using a van der Pauw technique. No thickness or time dependence of either carrier concentration or Hall mobility has been observed. The carrier concentration has an average value of 1.2 × 1017 cm−3 and a standard deviation of 0.3 × 1017 cm−3. Hall mobility has an average value of 81 cm2V−1s−1 and a standard deviation of 13 cm2V−1s−1.

Image of FIG. 8.
FIG. 8.

(Color online) (a) Carrier concentration and (b) Hall mobility as a function of temperature. Temperature dependence of carrier concentration suggests thermally activated process in the PbTe film and Fermi level is 0.111 eV above the valence band. Temperature dependence of mobility shows ionized defect scattering dominates the electrical conduction in the PbTe film.

Image of FIG. 9.
FIG. 9.

(Color online) (a) Temperature dependence of resistivity of PbTe films with high and low oxygen concentrations from 80 K to 340 K. Both samples are 1000 nm in thickness. (b) Enlarged portion of interest in (a) from 240 K to 340 K with fitted data shows exponential temperature dependence of resistivity which is mainly due to the hole concentration dependence on temperature.

Image of FIG. 10.
FIG. 10.

Schematic band structure of PbTe films near grain boundary region: (a) “low oxygen concentration” sample and (b) “high oxygen concentration” sample. The relevant energy points are Ec conduction band edge, Ev valence band edge, EF Fermi level, ES bend banding on the surface of a grain, and Ea activation energy of electrical conduction according to the grain boundary conduction channel model. In the “high oxygen concentration” sample the Fermi level becomes pinned resulting in larger band bending and smaller activation energy.

Image of FIG. 11.
FIG. 11.

Responsivity spectrum of 100 nm thick nanocrystalline PbTe film under 0.1 mA bias current and at −60 °C cooled by a TEC. Film shows excellent photoconductive signal in the wavelength range of 0.8–5 μm, and responsivity as high as 25 V/W is measured at 3.5 μm wavelength.


Generic image for table
Table I.

Key properties of the PbTe films with low and high oxygen concentration (i.e., with and without the Ge23Sb7S70 capping layer).

Generic image for table
Table II.

Activation energy and power law’s exponent fitted from temperature dependence of carrier concentration and Hall mobility for polycrystalline PbTe films of different thicknesses.


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752b84549af89a08dbdd7fdb8b9568b5 journal.articlezxybnytfddd
Scitation: Room-temperature oxygen sensitization in highly textured, nanocrystalline PbTe films: A mechanistic study