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ISSN 1087-6596, Glass Physics and Chemistry, 2006, Vol. 32, No. 3, pp. 337–345. © Pleiades Publishing, Inc., 2006. Original Russian Text © N.V. Golubchenko, V.A. Moshnikov, D.B. Chesnokova, 2006, published in Fizika i Khimiya Stekla.

Investigation into the Microstructure and Phase Composition of Polycrystalline Lead Selenide Films in the Course of Thermal Oxidation N. V. Golubchenko, V. A. Moshnikov, and D. B. Chesnokova St. Petersburg State University of Electrical Engineering, ul. Professora Popova 5, St. Petersburg, 197376 Russia Received July 5, 2005

Abstract—The formation of oxide phases during thermal oxidation of doped polycrystalline lead selenide films is investigated as a function of the temperature–time conditions of annealing and introduced impurities. It is demonstrated that the microstructure and phase composition of the films doped with bismuth are characterized by evolutionary changes in the course of annealing. During heat treatment of lead selenide films doped with iodine, the microstructure of the film undergoes a drastic transformation of the “encapsulation” type and, simultaneously, the phase composition changes sharply. DOI: 10.1134/S108765960603014X

INTRODUCTION Processes of thermal oxidation of polycrystalline lead selenide films underlie the methods used for fabricating photodetectors and radiators that operate in the IR spectral range 2–5 µm at room temperature. An analysis of the data available in the literature (see, for example, [1–4]) demonstrates that annealing of films based on lead chalcogenides in an oxygen-containing atmosphere is accompanied by a number of physicochemical processes that bring about a change in the phase composition, as well as in the electrical and photoelectrical characteristics of films. In [5, 6], it was shown that one of the main factors responsible for the change in the electrical properties of semiconductor phases is oxygen diffusion over the film surface, along grain boundaries, and into the grain bulk. In our earlier work [5], we revealed that there is a correlation between the kinetics of change in the film properties and processes associated with the formation of oxide phases. However, the data on the change in the phase composition and microstructure of the films in the course of their thermal oxidation have rarely been discussed in the literature. It should be noted that these characteristics determine, to a considerable extent, the photoelectrical properties of annealed films and the mechanisms of photoconduction and photoluminescence in them. The purpose of this work was to study the evolution of the microstructure and phase composition of polycrystalline lead selenide films undoped and doped with iodine and bismuth during the formation of photosensitive film structures.

SAMPLE PREPARATION AND EXPERIMENTAL TECHNIQUE Lead selenide films were deposited onto glass substrates by thermal evaporation of polycrystalline lead selenide. The synthesis conditions of initial lead selenide provided a specified content of the basic components and doping impurities (bismuth, iodine) within the homogeneity region. In order to prepare PbSe : Bi films with n-type conductivity, bismuth atoms should occupy sites in the lead sublattice and play the role of donors. For this purpose, the PbSe composition was specified with a deviation from stoichiometry toward excess selenium. The films were grown in a quasiclosed volume by the hot-wall method under conditions such that the film composition virtually corresponded to the composition of the initial batch. The film thickness was equal to 0.6–0.8 µm depending on the preparation conditions. The elemental chemical and phase compositions, the structure, and the surface morphology of the initial and annealed polycrystalline films were investigated using a number of methods, such as scanning electron microscopy (SEM), electron probe microanalysis, optical microscopy, scanning tunneling microscopy (STM), and local tunneling spectroscopy. X-ray powder diffraction analysis was performed on a DRON-2 diffractometer (NiKα radiation, graphite monochromator). The phases were identified according to the data presented in [7]. The concentration of charge carriers in the initial samples was determined using the Hall effect. The concentration of charge carriers in the oxidized samples was evaluated by the local thermal probe method. The method is based on the correlation dependence of the thermopower measured at 300 K on the

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Fig. 2. X-ray diffraction diagrams of (a) initial PbSe : Bi films of the type A and (b) initial PbSe : I films of the type B.

EXPERIMENTAL RESULTS 20 kV

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Characterization of the Initial Films

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Fig. 1. SEM images of the surfaces of (a) initial PbSe : Bi films of the type A and (b) initial PbSe : I films of the type B.

Hall concentration of charge carriers [8]. The electrical characteristics (resistance, thermopower, charge carrier concentrations) were measured with the use of films having an identical area of 2 × 4 mm. In order to separate the properties of the annealed heterophase film from the properties of PbSe microcrystals, the electrical parameters were measured for all samples before and after chemical etching of oxide phases. A comparison of the experimental data for the films with and without oxide phases [5] allowed us to make the inference that the thermopower α measured by the local probe method (in the direction perpendicular to the PbSe film surface) predominantly characterizes the bulk properties of microcrystals. The resistance R measured along the film surface is equal to the sum of the series resistances of bulk grain regions, surface grain regions, grain boundaries, and dielectric phases on grain boundaries.

All the evaporated films under investigation had a polycrystalline column structure. The grain height corresponded to the film thickness (h = 600–1000 nm), and the mean transverse size amounted to a = 100–200 nm. The morphology and preferred orientation of microcrystals varied depending on the preparation conditions of films and chemical treatment of substrates. As an example, the SEM images and the X-ray diffraction diagrams of films of two types (A, B) with different morphologies and microstructures are presented in Figs. 1 and 2, respectively. Films of the type A had a pronounced texture along the (100) plane. The most intense (200) line in the X-ray diffraction patterns corresponded to the orientation of the (100) face with the lowest surface energy for structures of the NaCl type. Individual faceted microcrystals with angles of 90° between the faces are seen in the micrograph. Films of the type B are characteristic of films doped with iodine. These films contain microcrystals whose upper growth faces are aligned with the (110) planes. Unlike films of the type A (Fig. 2a), the X-ray diffraction patterns of films of the type B (Fig. 2b) contain (220) and (200) lines with close inten-

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sities. By varying the substrate surface morphology and evaporation conditions, it is possible to control the ratio between the numbers of microcrystals with the (100) and (110) orientations of upper growth faces. The width at half-maximum for the reflections in the X-ray diffraction patterns of all the films was equal to 0.1°–0.2°. This suggests that grains have a sufficiently perfect crystal structure.

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According to the STM and X-ray diffraction data (Fig. 2), all the films prepared from doped lead selenide had a single-phase composition. As follows from electron probe microanalysis, the film composition completely corresponds to the composition of the batch used during evaporation. In this work, we studied the properties of the films doped with bismuth and iodine. The bismuth and iodine contents in the films varied from 0.2 to 1.0 at % and from 0.5 to 1.0 at %, respectively. All the initial doped films had n-type conductivity. Moreover, we investigated the n-type and p-type undoped films that corresponded to the homogeneity region boundaries on the lead and selenium sides, respectively.

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Characterization of the Annealed Films Undoped and Doped with Bismuth The evolution of the microstructure and phase composition of the films was examined at different stages of their heat treatment in air at temperatures in the range 473–873 K. Annealing of the films leads to the formation of oxide phases whose composition and distribution over the film depend on the temperature and time of annealing. Two ranges, i.e., the low-temperature (T ≤ 673 K) and high-temperature (T ≥ 753 K) ranges, can be conventionally distinguished in the temperature range under investigation. Let us consider the specific features of the initial annealing stage at low temperatures.1 Under the given conditions, a yellow phase was visually observed on the surface of the doped films. This phase more clearly manifested itself on the surface of the undoped films that corresponded to the homogeneity region boundary on the lead side. In our opinion, this phase is formed by one of lead oxides. The band gap of lead oxides is wider (~2 eV) than that of lead selenide. The presence of wide-band-gap phases on the surface was confirmed by local tunneling spectroscopy. The analysis of the tunneling current–voltage characteristics of the naturally aged (at room temperature) films and those weakly oxidized at the initial annealing stage showed that the wide-band-gap phases (1.5–2 eV) are primarily formed on local surface regions in the vicinity of grain boundaries. No similar phases were found by any method for the films whose composition 1 Annealing

for 0.5 h at 473 K or for 5 min at 673 K can be considered the initial annealing stage. GLASS PHYSICS AND CHEMISTRY

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PbSeO3 (011) 29

PbSeO3 (–111) 28

27 2θ, deg

26

25

Fig. 3. (a) SEM image of the cleavage of the PbSe : Bi film annealed at T = 500 K and (b) a fragment of the X-ray diffraction pattern of this film.

corresponded to the homogeneity region boundary on the selenium side. An increase in the annealing time results in the appearance of a blue phase on the surface of the films with different initial compositions. This phase was identified as PbSeO3 lead selenite by X-ray powder diffraction. The nucleation, growth, and distribution of oxide phases; a change in their composition; and the microstructure of the films were examined in detail by X-ray diffraction analysis and scanning electron microscopy with the use of the films annealed under identical conditions. In the films annealed at T = 573 K, the column structure is retained, microcrystals are closely spaced, and islands of a new phase appear on the film surface. The SEM image of the cleavage of the bismuth-doped film (NBi = 0.5 at %) annealed for 2 h is displayed in Fig. 3a. In the X-ray diffraction patterns of these films, the ratios between the intensities of the reflections of the 2006

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(b) PbSeO3 (011)

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PbSeO3 (–111)

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2θ, deg

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PbSe (200)

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Fig. 5. SEM image of the surface of the PbSe : Bi film annealed at T = 673 K after etching of oxide phases.

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Fig. 4. (a) SEM image of the cleavage of the PbSe : Bi film annealed at T = 673 K and (b) a fragment of the X-ray diffraction pattern of this film.

PbSe phase for different planes are retained (Fig. 3b) and, hence, the orientation of microcrystals remains unchanged. Apart from the reflections of lead selenide, there arise weak broadened lines associated with lead selenite (Fig. 3b). An increase in the temperature by 100 K is attended by a considerable increase in the grain size to 300– 500 nm due to the recrystallization processes and by the more intensive formation of lead selenite (Figs. 4a, 4b). The faceting of microcrystals becomes less perfect; however, the close packing of grains and their preferred orientation are retained. The intensities of the reflections corresponding to the PbSeO3 phase increase (Fig. 4b). As the time of annealing at T ≤ 673 K increases, the X-ray powder diffraction patterns do not indicate the presence of other phases in addition to the PbSe and PbSeO3 phases. The number of the reflections attributed to the lead selenite phase becomes smaller and their intensity increases. This suggests that the lead selenite phase has a more perfect crystal structure and a

texture. Closely spaced microcrystals are covered by a rather thick oxide layer (at t = 30 min, the layer thickness is approximately equal to 300 nm). Etching with a selective etchant (Trilon B) was used for determining the properties of microcrystals and their distribution below the oxide layer. The analysis of the SEM images of the cleavages of the films after oxide dissolution (Fig. 5) demonstrates that the faceting of the outer surface of microcrystals disappears completely. These films have low photoconductivities ∆R/RT = 5–10% (where ∆R is the difference between the dark resistance RT and the resistance under exposure) and long photoresponse times τ ≈ 10–20 ms. Higher photosensitivities (S = 250–300 V/W) and short photoresponse times (τ ≈ 10–40 µs) were obtained after additional high-temperature annealing at T = 833–873 K, which resulted in a drastic transformation of the microstructure of the film and the formation of new oxide phases. The examination of the SEM images shows that two-stage heat treatment leads to a decrease in the grain height in the films and the formation of a homogeneous oxide layer on the film surface (Fig. 6a). Most likely, the oxide phases are formed primarily from small-sized grains. It is worth noting that there are pores uniformly distributed over the surface of the oxide layer. Furthermore, an oxide layer is formed at the substrate–film interface. Annealing results in an increase in the thickness of the oxide layers at the substrate and above the film (Fig. 6b). Therefore, multistage annealing leads to the formation of a multilayer structure that consists of two oxide layers and grains in between. In the X-ray diffraction patterns of the films doped with bismuth, the intensity of the lines assigned to the PbSeO3 phase decreases and there appear very weak broad lines corresponding to


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increase as a result a decrease in the lead selenite content (the intensity of the lines associated with the PbSeO3 phase decreases). After high-temperature annealing, grains below the oxide layer are linked together into chains and form a network (Fig. 8). Longterm annealings at these temperatures result in crystallization of oxide phases (Fig. 9). This leads to the deterioration of the film characteristics.

(‡)

Similar changes in the microstructure, properties, and phase composition were observed for films doped with cadmium and tin. 20 kV

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Characterization of the Annealed PbSe : I Films

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Films PbSe : I of the type B were annealed at temperatures of 690–770 K. The specific feature of thermal oxidation of the PbSe : I films is that even short-term annealing (t ≤ 5 min) leads to a drastic transformation of the microstructure of the film and a simultaneous change in the phase composition. The film structure is formed by capsules composed of rather large grains. These capsules are joined together into chains, which, in turn, form a network (Fig. 10a). The capsule shell is a heterophase layer, which, according to the X-ray diffraction data, consists of lead oxides, lead selenite, lead dioxyselenite, and lead tetraoxyselenite (Fig. 11). Investigation after etching of oxide shells demonstrates that capsules can contain several grains (Fig. 10b).

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Fig. 6. SEM images of the (a) surface and (b) cleavage of the photosensitive PbSe : Bi film.

new phases, i.e., lead dioxyselenite 2PbO · PbSeO3 and lead tetraoxyselenite 4PbO · PbSeO3 (Fig. 7). The dioxyselenite and tetraoxyselenite contents in the film

The distinguishing feature of initial films of the type B is the absence of preferred orientation of microcrystals (Fig. 2b). The intensities of the reflections from the (100) and (110) planes in the initial films are virtually identical. In the course of annealing, microcrystals with the (110) orientation disappear and microcrystals with the (100) orientation become dominant. This fact also indicates a structural transformation.

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PbSe (311) PbSe (222)

PbSeO3 (–312)

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Fig. 7. A fragment of the X-ray diffraction pattern of the photosensitive PbSe : Bi film. GLASS PHYSICS AND CHEMISTRY

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Fig. 8. SEM image of the photosensitive PbSe : Bi film after etching of oxide phases.

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Fig. 9. SEM image of the surface of the PbSe : Bi film annealed at T ≥ 870 K.

These films exhibit a photosensitivity (S = 350– 450 V/W) and photoluminescence at room temperature. The photoresponse time is τ ≈ 5–20 µs.

(‡)

DISCUSSION It is known [4, 10] that a large number of oxide phases (PbOx, SeO2, PbSeO3, 4PbO · PbSeO3, 2PbO · PbSeO3, PbSeO4, PbO · PbSeO4, 4PbO · PbSeO4, 2PbO · PbSeO4, etc.) can be formed in the Pb–Se–O system. The phase diagrams in the p O2 – p SeO2 coordinates at particular temperatures in the range 473–873 K were calculated using the partial pressure diagrams [10] in our previous work [9]. The analysis of the diagrams demonstrates that, among the aforementioned phases, the formation of the PbOx, PbSeO3, 2PbO · PbSeO3, and 4PbO · PbSeO3 phases is most probable at the pressure p O2 = 0.21 atm. A decrease in the selenium vapor pressure and an increase in the temperature result in an increase in the probability of forming oxyselenite phases of the nPbO · PbSeO3 type.

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The vapor pressure p SeO2 is not known in the course of annealing of the films in air. The pressure p O2 and the temperature can be treated as the controllable parameters of thermal oxidation of the PbSe : Bi films at the initial annealing stage (before the formation of a continuous layer of oxide phases). The temperature dependences of the equilibrium pressure p O2 calculated with the use of the data taken from [11] for the reactions ( gas )

PbSe(solid) + 1.5 O 2

( solid )

PbO(solid) + Se O 2

, (1)

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Fig. 10. Microstructure of the photosensitive PbSe : I film (a) before and (b) after etching of oxide phases.


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PbSe (111)

PbSe (222)

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Pb2O3 (001)

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PbSeO3 (011) 28 29

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PbSeO3 (–110) PbSeO3 (–111)

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40

4PbOPbSeO3 (413)

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PbO1.37 (110)

PbO1.57 (–352)

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29

28 2θ, deg

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Fig. 11. Fragments of the X-ray diffraction pattern of the photosensitive PbSe : I film.

( gas )

( solid )

PbSe(solid) + O 2

Pb(solid) + Se O 2 ( gas )

PbSe(solid) + 1.5 O 2

( solid )

PbSe O 3

,

(2) (3)

are plotted in Fig. 12. It can be seen from these dependences that, at atmospheric oxygen pressure, the formation of solid lead on the film surface according to reaction (2) is highly improbable at temperatures T ≥ 573 K and the other reactions are thermodynamically allowed. The formation of the PbSeO3 phase according to reaction (3), as well as through intermediate phases according to reaction (1) is most favorable. The formation of the PbOx and PbSeO3 phases is confirmed by the results of our experimental investigations of the films at the initial stage of thermal oxidation. The conclusion regarding the formation of lead and selenium oxides on the film surface was also drawn by Gautier et al. [3], who studied the kinetics of oxidation of lead selenide films at room temperature by X-ray photoelectron and Auger spectroscopy. At low annealing temperatures (T ≤ 573 K), lead selenite on the film surface nucleates in a nearly amorphous state, as can be judged from the low-intensity strongly broadened lines in the X-ray diffraction patterns (Fig. 3b). An increase in the temperature and time of annealing leads to the formation of a more perfect crystal structure of the PbSeO3 phase with a pronounced texture along the [–110] direction. The presence of lead selenite on the film surface was identified at all temperatures up to 873 K. In our earlier work [5], it was established that the kinetics of formation of the GLASS PHYSICS AND CHEMISTRY

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PbSeO3 phase on the PbSe film surface depends on the bismuth content in the initial films. In PbSe films undoped and doped with bismuth, lead oxyselenites are formed only at higher temperatures T ≥ 723 K. This is in good agreement with the data obtained by Zlomanov and Novoselova [4] for undoped lead selenide. The intensity of the lines associated with the PbSeO3 phase decreases with an increase in the content of oxyselenite phases. Most probably, experimentally observed dioxyselenite and tetraoxyselenite are formed according to the reactions ( solid )

3PbSe O 3

( solid )

2PbO · PbSe O 3

( gas )

+ 2Se O 2

,

5(2PbO · PbSeO3)(solid) ( gas )

3(4PbO · PbSeO3)(solid) + 2Se O 2

,

( gas )

3PbSe(solid) + 4.5 O 2 ( solid )

2PbO · PbSe O 3

( gas )

+ 2Se O 2

,

( gas )

5PbSe(solid) + 7.5 O 2 ( solid )

4PbO · PbSe O 3

( gas )

+ 4Se O 2

.

In all cases, these reactions yield gaseous selenium dioxide. The liberation of gaseous selenium dioxide can explain the appearance of numerous uniformly distributed pores on the film surface in the course of annealing at T ≥ 723 K. 2006

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oxides are intermediate phases responsible for the formation of lead selenite and lead oxyselenites.

PO2, Pa 1010 105

2 3

CONCLUSIONS

1

100 10–5

0.5

1.0

1.5

2.0 2.5 103/T, K–1

Fig. 12. Temperature dependences of the equilibrium oxygen pressure for reactions (1)–(3).

It should be noted that, as the annealing temperature increases, the microstructure of the undoped films and those doped with bismuth, cadmium, and tin undergoes an evolutionary change due to the sequential formation of the aforementioned oxide phases. The formation of oxide phases and the transformation of the microstructure in the films doped with iodine occur in a radically different way. With due regard for the foregoing, the structural transformation is caused by the following fast processes. Iodine diffuses from the microcrystal bulk toward the film surface and forms the PbI2 compound (with a rather low melting temperature Tm = 685 ± 1 K [12]) at the film surface and grain boundaries. The PbI2–PbSe system is characterized by an eutectic phase diagram, the eutectic temperature is equal to 670 K, and the eutectic point lies in the vicinity of the PbI2 compound (NPbSe = 12 at %) [13]. Therefore, at thermal-oxidation temperatures (Tm ≥ 693 K), a thin layer of a liquid phase is formed on the film surface and grain boundaries. This layer was observed in a number of SEM images of the film surface after annealing for a few minutes. Recrystallization of lead selenide grains most likely occurs through the liquid–crystal mechanism at a very high rate and ensures a high degree of purification, which is characteristic of materials prepared by the iodide technology [12]. Moreover, iodine serves as a catalyst and promotes the formation of the 2PbO · PbSeO3 and 4PbO · PbSeO3 phases. The temperature at which these phases are formed decreases by more than 100 K in the presence of iodine as compared to the films containing no iodine. This is explained by the fact that, at atmospheric oxygen pressure, the formation of lead oxides, for example, ( gas ) by the reaction PbI2 + x/2O2 PbOx + I 2 , is thermodynamically allowed. As was noted above, lead

Thus, it was revealed that thermal oxidation of lead selenide films occurs through two different mechanisms depending on the nature of the doping impurity. The microstructure and phase composition of undoped films and those doped with bismuth, tin, and cadmium undergo evolutionary changes with an increase in the annealing temperature. The microstructure of lead selenide films doped with iodine is characterized by a sharp transformation in the course of thermal oxidation. Furthermore, doping of films with iodine stimulates the formation of oxide and oxyselenite phases at sufficiently low temperatures of the order of 723 K. The data obtained in this work on the microstructure of photosensitive films, in particular, on the distribution of microcrystals of the PbSe narrow-band-gap semiconductor phase in wide-band-gap oxide phases can serve as the basis for the development of model concepts regarding the photoconduction mechanisms in these structures. ACKNOWLEDGMENTS We are grateful to V.M. Busov and S.I. Troshkov (Ioffe Physicotechnical Institute, Russian Academy of Sciences) for their assistance in performing investigations into the microstructure of the films. This work was supported by the Ministry of Education and Science of the Russian Federation in the framework of the Departmental Scientific Program “Development of the Scientific Potential of the Higher School—2005,” project no. 75414. REFERENCES 1. Petritz, R.L., Theory of an Experiment for Measuring the Mobility and Density of Carriers in the Space-Charge Region of a Semiconductor Surface, Phys. Rev., 1958, vol. 110, no. 6, pp. 1254–1262. 2. Torquemada, M.C., Radrigo, M.T., and Vergara, V., Role of Halogens in the Mechanism of Sensitization of Uncooled PbSe Infrared Photodetectors, J. Appl. Phys., 2003, vol. 93, no. 3, pp. 1778–1783. 3. Gautier, C., Combon-Muller, M., and Averous, M., Study of PbSe Layer Oxidation and Oxide Dissolution, Appl. Surf. Sci., 1999, vol. 141, pp. 157–163. 4. Zlomanov, V.P. and Novoselova, A.V., A Study of Interaction of Lead Selenide with Oxygen, Dokl. Akad. Nauk SSSR, 1961, vol. 247, no. 3, pp. 607–609. 5. Golubchenko, N.V., Moshnikov, V.A., and Chesnokova, D.B., Kinetics and Mechanisms of Oxidation of Polycrystalline Lead Selenide Films Doped with Bismuth, Izv. Vysch. Uchebn. Zaved., Mater. Elektron. Tekh., 2005, no. 1, pp. 23–28.


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Conference “High-Temperature Chemistry of Silicates and Oxides”), St. Petersburg: YaNUS, 2002, p. 92. Pashinkin, A.S. and Spivak, M.M., Diagrams of Partial Pressures in the Pb–Se–O and Sb–Se–O Systems, Neorg. Mater., 1988, vol. 24, no. 8, pp. 1332–1337. Termicheskie konstanty veshchestv. Spravochnik (Handbook on Thermal Constants of Materials), Glushko, V.P., Ed., Moscow: VINITI, 1965–1966, nos. 1–4. Rolsten, R.F., Iodide Metals and Metal Iodides, New York: Wiley, 1961. Translated under the title Iodidnye metally i iodidy metallov, Moscow: Metallurgiya, 1968. Shelimova, L.E., Tomashik, V.N., and Grytsiv, V.I., Diagrammy sostoyaniya v poluprovodnikovom materialovedenii (Phase Diagrams in Semiconductor Materials Science), Moscow: Nauka, 1991.