Indirect photon interaction in PbS photodetectors

applied surface science ELSEVIER

Applied Surface Science 106 (1996) 498-501

Indirect photon interaction in PbS photodetectors E. Indrea *, Adriana Barbu Institute of Isotopic and Molecular Technology, P.O. Box 700, 3400 Cluj 5, Romania Received 17 September 1995; accepted 31 December 1995

Abstract A sensitization process by heat treatment in oxygen atmosphere was used in order to increase the photoconductivity of vacuum evaporated PbS thin films. The Fourier analysis of the (200) PbS X-ray diffraction profile has shown a decrease of the crystallites mean size and an increase of the sensitized PbS lattice distortion because of chemisorption of oxygen at the crystallites surfaces. The crystallites mean size distribution function indicates that the sensitized PbS film is a heterogeneous system of 320 A size crystallites separated by 160 A intercrystalline barriers. The optical photoacoustic spectra of the sensitized PbS film shows an absorption structure at low photon energies of the 0.47 eV absorption threshold. The broadening effect observed in the absorption spectrum suggests an indirect photon transitions in the fundamental absorption region.

1. Introduction A sensitization process by heat treatment in oxygen atmosphere was used to increase the photoconductivity of vacuum evaporated PbS thin films [1] which are good infrared detectors. The electrical and photoelectronic properties of the PbS semiconductor thin films differ from those of the bulk material because of the intercrystalline boundaries [2]. The electrical resistivity of the polycrystalline films was found to vary with the amount of adsorbed oxygen; the combined result of the resistivity and the Hall mobility measurements indicated that the resistance increases due to the accumulation of oxygen at the intercrystalline interfaces [3]. The chemisorption of oxygen on polycrystalline PbS occurs at conveniently measurable rates above 100°C and the oxy-

gen is retained at the crystallites surfaces [4]. The oxidation of PbS leads to formation of sulfates or oxysulfates, generating an intercrystalline region, and the PbS matrix becomes increasingly disordered. The effect of the heat treatment in oxygen atmosphere of the PbS thin films, has been examined by X-ray diffraction analysis and photoacoustic spectroscopy (PAS), presented in this paper. The average crystallite size, microstrains, the faults probability and crystallite size distribution function in PbS semiconductor thin films were determinate by single X-ray profile Fourier analysis [5]. The band-gap energy, an important parameter for electronic and optoelectronic devices, was analyzed using the photoacoustic technique [6].

2. Experimental measurements * Corresponding author. Tel.: + 40-64-185027/261 : fax: + 4064-185816; e-mail: itimc @ucluj.ro.

The PbS films were deposited by the vacuum evaporation technique, on 0.14 mm thick optical

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E. lndrea, A. Barbu /Applied Surface Science 106 (1996) 498-501

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glass substrate. Outgassing of the reactor was done at 10 6 Torr and 200°C for 4 h. During the sublimation the gold contact areas were shielded by a glass plate so that PbS film was formed between them in a zone of 0.5 × 0.5 cm 2. The substrate temperature was kept at 300°C, and a vacuum of 10 6 Torr was maintained during the deposition process. The vacuum evaporated film had to be heated in the presence of oxygen in order to increase its photoconductivity. The sensitization heat treatment was done at low temperature, only 100°C, for 2 h. The pressure of oxygen equivalent to the partial pressure in air was about 150 Torr. The photoconductivity measurements were performed using a standard set-up [7]. The radiation source was the Globar source of a Perkin Elmer 125 spectrophotometer, modulated at 18 Hz frequency and the PbS photoconductive signal was processed by a Unipan 232B lock-in nanovoltmeter. The conductivity type was determined by the 'hot point' method [7]. The X-ray diffraction patterns were obtained by means of a standard DRON-3M powder diffractometer, working at 45 kV and 30 mA, and equipped with scintillation counter with single channel pulse height discriminator associated counting circuitry. The Cr K~ radiation, V filtered, was collimated with Soller slits. The data of the (200) PbS profile were collected in a step-scanning mode with A 2 0 = 0.025 ° steps and then transferred to a PC for processing. Pure silicon powder standard sample was used to correct the data for instrumental broadening. The Fourier analysis of the (200) PbS peak profile was processed by a XRLINE [5] computer program. The so called 'open cell PA technique' [8,9] was used to record the PA spectra near the PbS fundamental threshold. The PA experimental set-up is described in Fig. l. The optical incident radiation of

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a 100 W halogen lamp was modulated at 8 Hz with a mechanical chopper and focused by means of a two lens convergent system on the monochromator entrance. The samples were mounted on the top of a 1/2 inch condenser microphone and the PA signal was recorded by means of a lock-in amplifier. A 50 meV resolution was characteristic for the entire investigated spectral energy range of 0.3 + 0.7 eV. The PA spectra were calculated and normalized by an adequate software.

3. Results During the above mentioned heat treatment the resistance of the PbS films increased from 100 ~ to about 10 000 {2. The initially n-type evaporated PbS films were converted to p-type by heat treatment in oxygen atmosphere. The spectral detectivity determined at room temperature, at 2.5 /xm wavelength, was 3.4 × 10 s c m H z 1/2 W -1 The single (200) PbS X-ray diffraction line was analyzed in order to determine the microstructural parameters of the semiconductor films. The X-ray diffraction line broadening is caused by the small size of the crystallites, the lattice strains and lattice faults, and the experimental diffraction geometry [10,11]. The structural information obtained by single X-ray profile Fourier analysis of polycrystalline PbS films were: the effective crystallite mean size, the root mean square (rms) of the microstrains averaged along [hkl] direction, and the stacking fault probability. Table 1 summarizes the microstructural parameters of PbS as grown (AG) film and for PbS sensitized (S) film, respectively. It can be observed in

Table 1 Structural parameters for the AG PbS films and for the S PbS films PbS

Deff (~)

( 2 2 \/)2 /0 z0 vA ,n3 IU

a

AG S

484 322

1.44 3.43

0.014 0.048

D~ff is the effective crystallite size along the perpendicular direction to the (200) planes, (e2)~/00 is the rms of the microstrain averaged along the [200] direction and o~ is the stacking fault probability.

E. Indrea, A. Barbu / Applied Surface Science 106 (1996)498-501

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Table 1 that the crystallite size value decreased due to the sensitization treatment. The microstrain value in the PbS matrix increases and the increase in the strain takes place with the formation of stacking faults. The crystallite size distribution function was determined from the second derivative of the straincorrected Fourier coefficients by a method developed by Aldea and Indrea [5]. Fig. 2 shows the crystallite size distribution function for the AG PbS film and S PbS film, respectively. The crystallite size distribution function gives lower dimensions for the crystallite sizes of S PbS films, meaning that the magnitude of the intercrystallite zones increases by heat treatment in oxygen atmosphere, even at temperature of 100°C. Fig. 3 presents the PA absorption spectra of the AG PbS film and for the S PbS film, respectively, with a signal/noise ratio higher than 5. The absorption curve shape near the absorption threshold allowed evaluation of the energies for the direct and, possibly, the indirect electron transitions. The PA spectra of the AG PbS films show a sharp absorption edge. An abrupt increase in absorption suggests the presence of only a direct band gap. The absorption edge was fitted with an exponential function obtaining an 0.47 eV absorption threshold photon energy which corresponds closely to that previously reported [12] for the direct transition in PbS compound, 0.45 eV, as determined by means of classical

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Crystaltite size {A) Fig. 2. The crystallite size distribution function o f the A G PbS ( * ) films and S PbS (C)) films.

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Fig. 3. The photoacoustic absorption spectra of the AG PbS ( O ) films and S PbS ( / x ) films.

IR spectroscopy. The absorption spectrum of S PbS films, Fig. 3, shows an increasing distortion of the absorption edge and an absorption feature on the lower energy side of the absorption threshold.

4. Discussions

The increase in the electrical resistance at 10000 f~ and the changes from n- to p-type conductivity of the infrared photoconductive S PbS films suggests structural changes in the sensitized films. The X-ray diffraction analysis reveals that the PbS crystallites of initially 484 A mean size decrease to 322 A (see Table 1 and Fig. 2) separated by intercrystallite zones of 162 A. By sensitization treatment of the PbS semiconductor, the adsorbed oxygen is allowed to diffuse to the grain boundaries. Chemisorbed oxygen is largely retained at the PbS crystallites surfaces [4] producing oxide-intercrystalline barriers. The scattering of the carriers from the microcrystals surfaces significantly reduces their mobility, increasing the electrical resistance. The tendency towards p-type behavior suggests that sulphur vacancies in the PbS matrix are filled by oxygen atoms [2]. Acceptor-type levels are associated with some imperfections of the lattice and with oxygen vacancies

E. lndrea, A. Barbu / Applied Surface Science 106 (1996) 498-501

in the p-type intercrystalline regions [2,13]. When e l e c t r o n - h o l e pairs are created by photoexcitation, the electrons are excited from the filled valence band of the intercrystalline regions to acceptor levels, creating in this region an excess of holes. This trapping of electrons leads to the hole concentration increase in the PbS matrix, enhancing the hole photoconductivity. PbS is an ionic semiconductor in which the maximum of the valence band and the minimum of the conduction band are located at the boundary of the Brillouin zone along [111] direction and is known to have a direct optical band gap of 0.45 eV [12]. The low optical absorption of S PbS films can be attributed to small crystallite size and low crystallinity [14]. The broadening effect, observed in the absorption at low photon energy, suggests the occurrence of indirect transitions in the fundamental absorption region. The transitions take place due to the excitation of free carriers within a band, which results from the simultaneous scattering of mobile carriers by phonons or impurities [15]. Such kind of behavior is attributed to the local lattice strains and distortions in the PbS lattice caused by the replacement of the sulphur atoms by oxygen. The obtained results provide an experimental support to Petritz photoconductivity model [16] which considers that photoconductive films of the lead salt family are composed of a system of crystallites separated by intercrystalline barriers.

5. Conclusions A heat treatment at only 100°C in the presence of oxygen, of the vacuum evaporated PbS thin films, enhances their photoconductive effect. The X-ray

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profile Fourier analysis reveals that the S PbS films are heterogeneous systems consisting of PbS crystallites separated by intercrystalline barriers providing an experimental support to Petritz model of photoconductivity. The broadening effect, observed in the PA absorption at low photon energy of the absorption threshold of the S PbS films, suggests the occurrence of indirect photon transition caused by the local lattice strains in the PbS lattice.

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