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Optical and photoelectric properties of single-crystal PbGa2 S4 N. N. Musaeva, R. B. Dzhabbarov, U. F. Kasumov, and Kh. B. Ganbarova Physics Institute, National Academy of Sciences of Azerbaijan, Baku, Azerbaijan

共Submitted December 15, 2002兲 Opticheski Zhurnal 70, 66 – 69 共September 2003兲

This paper presents the results of a study of the structural, optical, and photoelectric properties of single-crystal PbGa2 S4 . The theory of direct optical transitions is used to determine the optical band gap of the indicated compound as E g ⫽2.78 eV. The spectral dependences of the photocurrent at various temperatures and applied voltages show that local centers that arise during the production of the crystal are present inside the band gap. The position of the energy level of slow recombination centers (r centers兲 is found to be equal to 0.24 eV. © 2003 Optical Society of America

INTRODUCTION

Lead thiogallate PbGa2 S4 belongs to the II–III2 – VI4 group of crystals 共where II is Mn or Pb, III is Ga or In, and VI is S, Se, or Te兲, most of which are distinguished by their interesting electrophysical properties 共photosensitivity, magnetization, luminescence, etc.兲. The compound PbGa2 S4 was first synthesized by sintering a stoichiometric mix of PbS and Ga2 O3 in a flux of hydrogen sulfide.1 Small single crystals were grown by a chemical-transport reaction. Unlike other compounds of the II–III2 – VI4 group, PbGa2 S4 has been little studied. We were the first to show that PbGa2 S4 is a photosensitive semiconductor in the spectral region 400–1000 nm that can be used as the basis for photodetectors and photoconverters that function in a wide spectral range. This paper presents the results of a study of the structural, optical, and photoelectric properties of single-crystal PbGa2 S4 grown by the Bridgman–Stockbarger method.

PRODUCTION AND STRUCTURAL STUDIES

The compound PbGa2 S4 was synthesized in evacuated 共to 10⫺3 Pa) quartz ampules from the elementary components taken in stoichiometric ratios. Single-crystal PbGa2 S4 grown by the Bridgman–Stockbarger method changes color from yellow to yellow-brown as it increases in thickness. The symmetry and structure of the resulting crystals were studied by x-ray analysis. X-ray diffraction patterns of powdered PbGa2 S4 were recorded on a DRON-3M diffractometer, using Cu K␣ radiation at room temperature and in the angular interval of 10°⭐2 ␪ ⭐90° 共Fig. 1兲. The 2␪ values and interplane spacings (d,Å) 共see Table I兲 calculated from the diffraction recording made it possible to determine the lattice parameters: a⫽20.73 Å, b⫽20.4 Å, and c⫽12.24 Å. It has been established that PbGa2 S4 crystallizes in 24 (z⫽32). rhombic symmetry with space group Fddd – D 2h According to our x-ray diffraction results and the literature data,1 the MII atoms occupy square antiprismatic sides formed by eight sulfur atoms. The gallium atoms are tetrahedrally coordinated with respect to four sulfur atoms, form676

J. Opt. Technol. 70 (9), September 2003

ing a GaS4 cell, with the sulfur atoms being located at the centers of deformed Pb2 Ga2 quadrilaterals, forming SPb2 Ga2 cells. OPTICAL STUDIES

To determine the band gap and the types of optical transitions that form the optical absorption spectra in PbGa2 S4 single crystals, we studied the optical properties in a wide interval of temperatures and photon energies. The spectral dependences of the transmittance T of PbGa2 S4 single crystals are obtained in the wavelength range 400– 800 nm and the temperature range 100–300 K on an apparatus based on an MDR-12 monochromator. Singlecrystal samples 10–50 ␮m thick were used for the studies. The optical absorption coefficient ␣ is computed from the experimental values of T, using the formula2 T⫽ 共 1⫺R 兲 2 exp共 ⫺ ␣ d 兲 ,

共1兲

where R is the reflectance, and d is the sample thickness. The spectral dependence of the optical absorption coefficient ␣ (h ␯ ) for single-crystal samples of PbGa2 S4 is shown in Fig. 2. As can be seen from the figure, when the energy of the incident photons is h ␯ ⬇E g , absorption coefficient ␣ falls off sharply and reaches a value of 6⫻102 cm⫺1 . The theory of interband optical transitions2,3 shows that ␣ as a function of photon energy h ␯ varies according to

FIG. 1. X-ray diffraction pattern of the compound PbGa2 S4 .

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TABLE I. Results of x-ray analysis.

␣ 共 h ␯ 兲 ⫽A 共 h ␯ ⫺E g 兲 r ,

共2兲

where h is Planck’s constant, ␯ is frequency, A is a constant, and r is a quantity that depends on the nature of the optical transition, taking values of 2, 3, 1/2, and 3/2. These results are analyzed on the basis of the theory of direct transitions for the energy region 2.0–2.9 eV.3 The experimental data are represented in coordinates ␣ 2 ⬃h ␯ (r

FIG. 3. ␣ 2 ⬃h ␯ dependence at temperatures of T⫽104 共1兲 and 289 K 共2兲.

⫽1/2) 共Fig. 3兲. It can be seen that, at energies h ␯ ⭓2.65 eV, the ␣ values fall on a straight line in the coordinates ␣ 2 ⫽ f (h ␯ ). The fact that ␣ 2 depends linearly on h ␯ is evidence that the intrinsic absorption edge in single-crystal PbGa2 S4 is formed by direct allowed optical transitions. By extrapolating the straight lines ␣ 2 ⫽ f (h ␯ ) to ␣ ⫽0, we find the band gap of PbGa2 S4 for the direct allowed transition, which equals 2.78 eV. As the temperature increases, E g decreases, with a temperature coefficient of dE/dT ⫽⫺5.95⫻10⫺4 eV/K. STUDY OF THE PHOTOCONDUCTIVITY

FIG. 2. Spectral dependence of the optical absorption coefficient in singlecrystal PbGa2 S4 at temperatures of T⫽104 共1兲 and 289 K 共2兲. 677


To study the photoconductivity of single-crystal PbGa2 S4 , we used samples 50–150 ␮m thick with a dark resistivity of 109 – 1010 ⍀cm. Indium contacts, whose ohmic nature was closely monitored, were soldered onto natural cleavage planes of the crystal. To measure their photoconductivity, the samples were placed in a liquid-nitrogen cryostat with a quartz window, equipped with a thermalstabilization system. Figure 4 shows the dependence of the photocurrent of a single-crystal sample of PbGa2 S4 on the wavelength of the incident radiation at a temperature of 294 K and various applied voltages. The spectrum covers the visible and IR region 共400–900 nm兲. It can be seen that the spectrum moves toward higher photocurrents as the applied electric field increases. The spectrum consists of a single broad band 共600– 800 nm兲, with the maximum photocurrent at a wavelength of 700 nm. Musaeva et al.

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FIG. 6. Temperature dependence of the photocurrent for single-crystal PbGa2 S4 .

FIG. 4. Spectral dependence of the photocurrent of single-crystal PbGa2 S4 at 290 K and an applied voltage of U⫽50 共1兲, 100 共2兲, 150 共3兲, 200 共4兲, 250 共5兲, 350 共6兲, and 400 V 共7兲.

Experiments show that the energy position of this maximum is independent of the external electric field and the temperature 共Fig. 5兲. Moreover, the photosensitivity in the intrinsic absorption region is almost negligible. The observation of these effects is associated with the implementation in the base region of the structures of a com-

FIG. 5. Spectral dependence of the photocurrent of single-crystal PbGa2 S4 at temperatures of T⫽104 共1兲 and 289 K 共2兲. 678


plex 共multicenter兲 recombination model, including the s intense recombination channel, r photosensitivity centers, and t trapping centers of majority charge carriers.4 Since the r centers are sensitizing, they determine the slowest electronrecombination rate and consequently have the longest lifetime,

␶ n⫽

gr , C nr P r

共3兲

where g r ⬃C pr N r is implemented between regions of temperature quenching of photocurrent 共TQP兲 and temperature activation of photocurrent 共TAP兲共Fig. 6兲 and depends on the output of recombination flux to the r channel (g r ⭐1), C nr and C pr are the electron- and hole-capture coefficients of slow r recombination centers, and N r and P r are the electron and hole concentrations on the r levels during illumination. The intensity of the impurity band associated with the r centers 共Fig. 6兲 is proportional to the value of ␶ n given by Eq. 共3兲 when g r ⫽1. In all probability, they depend on the process regimes for producing crystals and structures. The temperature dependences for impurity excitation of a sample of single-crystal PbGa2 S4 display TAP 共100– 131 K兲 and TQP 共131–205 K兲 regions 共Fig. 6兲. We assume that, at the temperature corresponding to the maximum photocurrent 共131 K兲, the main recombination flux passes through the r centers and that a subsequent increase of temperature causes generation of minority charge carriers from r centers and promotes a transition of these charges to s centers, in connection with which the freecarrier lifetime decreases, and TQP is observed. Using the method described in Ref. 5, we determined from the experimental data the energy position of r recombination centers and found it equal to 0.24 eV. It can be pointed out that intrinsic defects, including sulfur-atom vacancies caused by the escape of sulfur atoms from the composition during annealing, can act as photoelectrically active centers in PbGa2 S4 . The number of these defects is correlated with the processing regimes under which the crystals are produced. Musaeva et al.

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T. E. Peters and J. A. Baglio, J. Electrochem. Soc. 119, 230 共1972兲. Yu. I. Ukhanov, Optical Properties of Semiconductors 共Nauka, Moscow, 1977兲. 3 N. S. Pankov, Optical Processes in Semiconductors 共Mir, Moscow, 1973兲. 1 2

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V. E. Lashkarev, A. V. Lyubchenko, and M. K. Shekman, Nonequilibrium Processes in Photoconductors 共Naukova Dumka, Kiev, 1981兲. 5 R. H. Bube, Photoconductivity of Solids 共Wiley, New York, 1960; Inostr. Lit., Moscow, 1962兲.

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