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ISSN 1027-4510, Journal of Surface Investigation. X-ray, Synchrotron and Neutron Techniques, 2007, Vol. 1, No. 1, pp. 35–39. © Pleiades Publishing, Ltd., 2007. Original Russian Text © R.G. Valeev, P.N. Krylov, E.A. Romanov, 2007, published in Poverkhnost’. Rentgenovskie, Sinkhrotronnye i Neitronnye Issledovaniya, No. 1, pp. 41–45.

Structure and Properties of ZnSe Nanocomposite Thin Films R. G. Valeeva, b, P. N. Krylovb, and E. A. Romanovb a

Physical-Technical Institute, Ural Division, Russian Academy of Sciences, Izhevsk, Russia e-mail: [email protected] b Udmurt State University, Izhevsk, Russia Received December 30, 2005

Abstract—The structure and electrophysical and optical properties of semiconductor ZnSe nanocomposite thin films are studied. These films are obtained by discrete thermal evaporation in an ultrahigh vacuum. ZnSe films are synthesized in various structural states in the condensation temperature range 2–200°C. The optical spectra of these films are studied in the visible region. DOI: 10.1134/S1027451007010065

INTRODUCTION

ics and information representation systems [1, 2]. Zinc selenide belongs to materials in which variable-composition phases can exist; this fact requires precision control of film synthesis conditions during film deposition [1].

Zinc selenide belongs to most promising wideband-gap II–VI materials. ZnSe films are widely used in devices of short-wavelength semiconductor electron2

3

Vacuum chamber

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5 12

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7 10 8 9

Fig. 1. Diagram of a laboratory experimental setup for film synthesis: 1, Vacuum chamber; 2, high-vacuum ionization detector; 3, gas analyzer; 4, high-vacuum hand-operated valves; 5, turbo-molecular pump; 6, manometer; 7, reflector; 8 and 10, pneumatic control valves; 9, fore pump; 11, magnetic-discharge pump; 12, heated block.

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VALEEV et al. d, nm 50

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(‡)

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200 T, °C

Fig. 3. Dependence of the average ZnSe grain size on the substrate temperature.

MATERIALS AND METHODS

(b)

Nanocrystalline ZnSe films were obtained by thermal discrete evaporation of ZnSe powder in a high-vacuum experimental setup, whose diagram is shown in Fig. 1. The powder was fed into a molybdenum crucible (heated to 1500°C) from a vibrating bin; then, it was evaporated and condensed on glass-ceramic and quartz substrates. The operating conditions were as follows: the pressure was 10–6 Pa; the distance from the evaporator to the substrate was 23 cm; the deposition time was 5 min; the condensation temperatures were 20, 100, 150, and 200°C; and the deposition rate was 6−10 Å/s. The microstructure of the samples was studied by the microdiffraction method using a UEMV-100K transmission electron microscope. The structural state of the samples was monitored by x-ray diffraction using a DRON-3M diffractometer. Cu Kα radiation was used. Optical transmission, absorption, and reflection spectra were obtained by standard methods using an SF-26 spectrometer and an attachment to study reflection spectra. RESULTS AND DISCUSSION

(c) Fig. 2. Patterns of electron microdiffraction by ZnSe films obtained at substrate temperatures of (a) 20, (b) 150, and (c) 200°C.

The condensation-temperature dependence of the average diameter of ZnSe film grains was obtained using transmission electron microscopy and microdiffraction data (Fig. 2) and the procedure described in [3]. For films 0.5-µm thick, the average size of grains ranges from 17 to 43 nm (Fig. 3) as the temperature increases from room temperature to 200°C. It is seen from the x-ray pattern (Fig. 4) that the amplitude of the first (111) peak in the x-ray pattern increases with temperature. Taking into account the contribution of the

JOURNAL OF SURFACE INVESTIGATION. X-RAY, SYNCHROTRON AND NEUTRON TECHNIQUES Vol. 1 No. 1 2007

STRUCTURE AND PROPERTIES OF ZnSe NANOCOMPOSITE THIN FILMS

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I, arb. units

200°C

150°C 111 220

311 100°C

Amorphous halo from the substrate

20°C 0

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100 2θ, deg

Fig. 4. X-ray diffraction patterns of ZnSe films.

substrate to the diffraction pattern, one can conclude that, with increasing temperature, the (111) peak not only increases in the amplitude but also becomes narrower. This result indicates that the size of coherent scattering regions increases. Figure 5 shows the condensation-temperature dependence of the film conductivity σ measured at room temperature. The conductivity σ increases until the substrate temperature reaches 100°C, and then it decreases. This behavior can be explained by the fact that grain boundaries are barriers for charge carriers; as the grain size increases, the number of grain boundaries decreases, which results in an increase in conductivity. For the condensation temperature exceeding 100°C, the conductivity decreases, although the grain dimensions increase (Fig. 3). This is possibly related to the fact that impurities segregate to grain boundaries. The activation energies of impurities were calculated based on the studies of the temperature depen-

σ, 103 Ω–1 cm–1 0.8

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200 T, °C

Fig. 5. Dependence of the film conductivity on the condensation temperature.


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VALEEV et al. Transmission, % Reflection, % 100

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Fig. 6. The transmission and reflection spectra of ZnSe films obtained at substrate temperatures of (a) 20, (b) 50, (c) 150, and (d) 200°C.

dence of the conductivity performed in this work. The calculated values are given in the table. The lowest activation energy is observed for a condensation temperature of 100°C, which is related to an increase in the impurity concentration. Dependence of the impurity activation energy on the condensation temperature Condensation temperature T, °C

Activation energy Ea , eV

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1.09

The transmission and reflection spectra of ZnSe films are shown in Fig. 6. The refraction coefficient as a function of the condensation temperature was determined in the transmission region from the interference pattern of the transmission spectra of ZnSe thin films (Fig. 7). The refraction coefficient at a film deposition temperature of 20°C is lower than that at 100, 150, and 200°C, which can be related to both the change in the semiconductor stoichiometry and to an increase in the concentration of impurities mainly accumulated at grain boundaries. At a condensation temperature of 20°C, the film absorption edge (Fig. 8) is located at 2.1 eV and shifts to a shorter wavelength region (2.25 eV) at condensation temperatures of 150 and 200°C, because the grain size increases with temperature.


STRUCTURE AND PROPERTIES OF ZnSe NANOCOMPOSITE THIN FILMS

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CONCLUSIONS

Refraction coefficien 1.8 1.7 1.6 1.5 1.4 20 40 60 80 100 120 140 160 180 200 Condensation temperature, °ë

Nanocrystalline ZnSe films with an average grain diameter of 17–43 nm were synthesized. It has been found that the conductivity and its temperature dependence are different for films with different average grain diameters. The maximum conductivity was observed for films obtained at a condensation temperature of 100°C. The values of conductivity activation energies were determined using the temperature dependence of the conductivity. The films obtained at 100°C had the maximum conductivity and the weakest temperature dependence σ(T). For ZnSe films, the bandgap energy was determined using the absorption spectra. The bandgap energy varied from 2.1 to 2.25 eV as the condensation temperature increased.

Fig. 7. Dependence of the film refraction coefficient on the wavelength.

ACKNOWLEDGMENTS The authors are grateful to G. N. Konygin (PhysicalTechnical Institute, Ural Division, Russian Academy of Sciences, Izhevsk) for help in experiments using an x-ray diffractometer.

Absorption coefficient 0.6

20°C 100°C

This work was supported by the Russian Foundation for Basic Research, project nos. 04-03-96024 and 03-03-32415.

150°C 200°C

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REFERENCES 0.2

0 1.0

1. G. A. Il’chuk, V. Yu. Rud’, Yu. V. Rud’, et al., Fiz. Tekh. Poluprovodn. 34 (7), 809 (2000) [Semiconductors 34, 781 (2000)]. 1.5

2.0

2.5 Energy, eV

Fig. 8. Dependence of the film dimensionless absorption coefficient on the energy.

2. A. V. Khomchenko, Zh. Tekh. Fiz. 67 (9), 60 (1997) [Tech. Phys. 42, 1038 (1997)]. 3. S. S. Gorelik, Yu. A. Skakov, and L. N. Rastorguev, Roentgenography and Electron-Optical Analyzis (MISIS, Moscow, 1994) [in Russian].
