Effect of cobalt impurities on the electrical and photoelectrical properties of GaS single crystals

Solid State Communications, Vol. 53, No. 9, pp. 811-815, 1985. Printed in Great Britain.

0038-1098/85 $3.00 + .00 Pergamon Press Ltd.

EFFECT OF COBALT IMPURITIES ON THE ELECTRICAL AND PHOTOELECTRICAL PROPERTIES OF GaS SINGLE CRYSTALS B.G. Tagiyev, G.M. Niftiyev, S.M. Bashirov and D.S. Azhdarova Institute of Physics, The Azerbaijan SSR Academy of Sciences, Baku*

(Received 13 August 1984 by C. W. McCombie) Static dark current-voltage characteristics (CVC's), the temperature dependence of electric conductivity [a(T)], the currents of thermostimulated depolarization (TSD), the spectral distribution of photoconductivity (SDPC) and photoluminescence (PL) have been studied in GaS (0.1 at % Co) single crystals. The results of complex investigations of CVC's, a(T) dependences, TSD, the SDPC and PL show that the forbidden gap of GaS (0.1 at % Co) single crystals exhibits acceptor levels (Ev + 0.26 and E v + 0.63 eV). DOPING OF GaSe SINGLE crystals with transition elements (Fe, Ni, Co, Cr) has a profound effect on their photoelectrical properties [ 1 - 4 ] . As far as we know, the effect of an impurity in the form of such a transition element as Co on the electrical and photoelectrical properties of GaS single crystals is not clearly understood. The present report deals with an investigation of the static dark current-voltage characteristics (CVC), the temperature dependence of electric conductivity [a(T)], the currents of thermostimulated depolarization (TSD), the spectral distribution of photoconductivity (SDPC), and the photoluminescence (PL) in GaS (Co) single crystals. GaS (0.1 at % Co) single crystals were grown by the Bridgeman method and had a p-type conductivity. The specimen contacts were made by fusing-in indium at the opposite mirror surfaces perpendicular to the C-axis of the crystal. Figure 1 gives the static dark CVC's for In-GaS (0.1 at % Co)-In in the temperature range from 293 to 400 K. The following sections are revealed in the CVC's: an ohmic section (1 ~ U), a quadratic section (1~ U 2 ) and a region of sharp current growth (1~ U", where n = 3 to 12). The existence of a quadratic section after a linear section and the fulffdment of the c o n d i t i o n s / ~ L -a , V u u ~ L 2 and 0 =Po/Nt "~ 1 (where/' is the current density, L is the electrode spacing, Put~ is the voltage corresponding to the ultimate trap filling, 0 is the trapping factor, P0 is the concentration of free carriers, and N t is the concentration of traps) show that the transfer of charge carriers is due to monopolar injection. The trapping factor (0) was determined from the quadratic region of the CVC with the formula/" = ~ eeolaO V2/L 3 *USSR, 370143, Baku, Prospekt Narimanova, 33.

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EFFECT OF COBALT IMPURITIES ON GaS SINGLE CRYSTALS

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A shift of the TSD maximum with growing degree of trap filling gives us good reason to believe that a severe retrapping is typical of these traps and they are distributed quasi-linearly in energy. A decrease of the TSD current in the lowtemperature region in strong electric fields, the existence of residual conductivity (RC) and a prolonged relaxation (PR) in these single crystals give us good reason to believe that the impurities and layer packing defects produce potential barriers in which a thermofield reduction occurs in strong electric fields. It should be noted that the sign of the TSD current was always opposite to that of the external polarizing field, independent of the polarization conditions (polarization temperature, polarization time, and polarising-field intensity). The trap location depth corresponding to the hightemperature peak was determined from the temperature dependence of the section of the initial current-growth [6], with the formula depending on the shapes of the TSD peaks [7-9] and by the method of approximate evaluation [10-12] and amounted to 0.63 +-0.03 eV. The concentration (Nt) and the trapping cross-section (St) were determined according to [7] and amounted t o N t = 4 x 1014 cm -3 andS t = 1.2 x 10 -20 cm 2, respectively. The trap location depth corresponding to the

Vol. 53, No. 9

EFFECT O F COBALT IMPUR/TIES ON GaS SINGLE CRYSTALS

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values ofp, Nv and e at 293 K were/a = 20 cm 2 V -~ sec [ 1 3 ] , N v = 1021 cm -3 [13], and e = 5.9 [ 1 4 ] , r e s p e c tively. The same levels (Et = 0.63 eV) obtained b y three methods independent o f one another (0 (T), a ( T ) and TSD) are indicative o f the fact that the transfer o f charge carriers in GaS (0.1 at % Co) single crystals is due to monopolar hole injection. Figure 4 shows the SDPC a t F = 4 x 1 0 3 V c m -1 and at a temperature o f 293 K for pure GaS (Curve 1), Gas (0.5 at % Co) (Curve 2) and Gas (0.1 at % Co) (Curve 3). It can be seen that in the SDPC of pure GaS single crystals and GaS single crystals doped with 0.05 at % Co a peak is revealed at a wavelength o f 0.504/~m and at a wavelength o f 0.496/~m, respectively. An addition o f 0.1 at % Co to GaS contributes to the appearance of supplementary maximum in the long-wave region. Figure 5 gives the SDPC of GaS C0.1 at % Co) single crystals at 293 K in different electric fields. It is seen that two maxima at wavelengths o f 0.484 and 0.550/am are revealed in the PC spectrum, and the values o f b o t h o f these maxima increase with the electric field. Figure 6 illustrates the SDPC o f GaS (0.1 at % Co) single

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EFFECT OF COBALT IMPURITIES ON GaS SINGLE CRYSTALS 20

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Fig. 7. PL of GaS (solid line) and GaS <0.1 at % Co> (dashed line) single crystals at 77 K. crystals at F = 4 x 10 a V cm-* and at different temperatures. As the temperature grows, the energy position of the short-wave maximum shifts towards long waves and the photosensitivity decreases. Starting with the temperature of 330 K the long-wave maximum disappears. The disappearance of the long-wave maximum of PC gives us good reason to conclude that in the longwave region the PC is of an impurity nature, whereas the shifting of the short-wave maximum with growing temperature bears witness to the fact that the short-wave maximum is associated with the intrinsic PC corresponding to the non-straight edge of a GaS single crystal. The band gap determined from the maximum of the intrinsic photoconductivity for GaS <0.1 at % Co> is E e 2.559 eV. The energy difference between the band gap and the position of impurity PC amounts to 0.26 eV. A decrease of PC below 293 K is typical of all the crystals under study. Figure 7 depicts PL spectra of pure GaS single crystals (solid line) and GaS <0.1 at % Co> crystals (dashed line) at 77 K. It can be seen that with pure GaS narrow peaks are observed at wavelengths of 0.483 and 0.486/am in the region of 0.480 to 0.492/am, and wide bands with maxima at 0.595 and 1.067/am are observed in the regions from 0.50 to 0.64/am and from 0.9 to 1.3/am, respectively. Introduction of 0.1 at % Co into GaS contributes to the appearance of an arm at a wavelength of 0.493 #m and a wide-band radiation with a maximum of 0.532/am. As the per-cent content of cobalt in GaS is increased the intensity of wide-band radiation in the region of 0.5 to 0.64/am increases, whereas the intensity of infrared radiation in the 1.0/am region decreases. The energy difference between the indirect band of GaS and the maximum of the PL band in the 0.56 to 0.62/am region amounts to 0.63 eV. A

level with an energy of 0.63 eV has also revealed from the dependences 0 (T), or(T) and TSD. We believe that the radiation band in the 0.56 to 0.62/am region is associated with the transition of electrons from the conduction band to the acceptor level (Ev + 0.63 eV). In the 1.0/am region, donor and acceptor levels are responsible for the wideband radiation. This band is associated with radiative trapping of electrons from donor levels (Ec -- 0.57 eV) by acceptor levels (Ev + 0.9 eV) [15]. A comparison between the PL spectra of pure GaS single crystals and GaS single crystals doped with cobalt shows that cobalt impurities contribute to the appearance of a radiation band in the green region (Xmax = 0.532/am). The'narrow bands in the 0.48 to 0.493 m wavelength region are associated with indirect, free and bound excitons [ 1 6 - 1 8 ] . Levels with an energy of 0.26 eV are revealed both in the PC spectrum and PL spectrum at the same time. It is supposed that the impurity PC is due to the transition of thermo-optically filled electrons from acceptor levels (E v + 0.26 eV) to the conduction band, whereas the impurity PL with the maximum ~kma x = 0.532 m is due to the recombination of excited electrons from the condution band with acceptor level holes (Ev + 0.26 eV). Thus, the results of complex investigations of CVC's, o(T) dependences, TSD, the SDPC and PL show that the forbidden gap of Gas <0.1 at % Co> single crystals exhibits acceptor levels (Ev + 0.26 and E v + 0.63 eV). REFERENCES 1. 2.

B.G. Tagiyev, G.M. Nigtiyev & S.M. Bashirov FTP, 17, 1320 (1983). Yu.P. Gnatenko, Z.D. Kovalyuk & P.A. Skubenko, Phys. Star. Sol. (b), 106,621 (1981).

Vol. 53, No. 9 3. 4. 5. 6. 7. 8. 9. 10.

EFFECT OF COBALT IMPURITIES ON GaS SINGLE CRYSTALS

Yu.P. Gnatenko, Z.D. Kovalyuk & P.A. Skubenko UFZh, 27,838 (1982). Yu_P.Gnatenko, Z.D. Kovalyuk, P.A. Skubenko & Yu.I. Zhirko, FTT, 2, 25,445 (1983). R.H. Bube,J. Appl. Phys. 33, 1733 (1962). G.F.I. Gurlic & A.F. Gibson Proc. Phys. Soc. (London), 60, 574 (1948). G.A. Bordovskiy, In a collection of reportes on "Photoconductive lead oxides". Leningrad, 9, 87 (1976). 1.G.Simmons & G.W. Taylor, Phys. R ev. B: Solid State, 5, 1619 (1972). Ch.V. Lushik & A.N. Doklady SSSR, 101,641 (1955). K.W.Boer, S. Oberl~mder & I. Voigt, Ann. Phys., 7,130 (1958).

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R.H. Bube, 1. Chem. Phys. 23, 18 (1955). R.H. Bube, Phys. Rev. 99, 1105 (1955). A.H.Kipperman & A.M. Sliepenbeek, Nuovo Cimento, 63B, 36 (1969). 14. K.R. AUakhverdiev,S.S. Babaev E.Yu. Salaev & M.M. Tagiev Phys. Stat. Sol. (b), 96, 177 (1979). 15. R.M.A.Gieti & F. Van der Maesen, Phys. Star. Sol (a), 10, 73 (1972). 16. I.M.Catalono, A. Cingolani & A. Minafra, Sol. Stat. Com., 22,225 (1977). 17. B.S.Razbirin, V.P. Mushinskiy, M.I. Karaman, A.N. Starukhin & Ye.M. Gamarts, FTT, 17, 2124(1975). 18. G.D.Belenkiy, M.O. Gojayev, & E.Yu. Salayev, A letter to ZLTF, 26 385 (1977). 11. 12. 13.