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Stationary photoconductivity of mixed Tl1−xAgxInSe2 crystals
Materials Chemistry and Physics, 12 (1985)
297-301
297
STATIONARY PHOTOCONDUCTIVITY OF MIXED Tl,_$gxInSe2 CRYSTALS
G.D.GUSEINOV, V.A.ALIYEV, A.U.MALSAGOV and L.M.CHAPANOVA Institute of Physics of the Azerbaijan SSR Academy of Sciences, Baku, 370143, pr. Narimanova 33, (USSR) Received 30 July 1984; accepted 10 September 1984 ABSTRACT single crystals of solid soluTl,_pg,InSe2 (x = 0 to 0.03) tion were grown by Bridgeman-Stockbarger techniques. Photoelectric measurements were taken at 300 and 77 K. With an increase in the value of x, the equilibrium concentration is increased 10 to 100 times. At 77 K a region of impurity photosensitivity has been detected which is bordered b the fundamental absorbtion edge. The photosensitivity ( Rd/Rp s of the solid solution crystals at 77 K increases from 8.2 (x = 0) to 55.2 (x = 0.03) the conclusion is made that when Tl is replaced by Ag then the sensitisation of semiconductor.
INTRODUCTION In recent years growing interest has been shown in complex semiconductive compounds. The crystals of such compounds are anisotropic and find application in non-linear opticsCl1. The photoacoustic effect and high photosensitivity found in these crystals show their promise for optoelectronics [2,3]. Among the new ternary semiconductive crystals is the T11nSe2 compound possessing a TlSe-type tetragonal structure and described by the DiE space group of symmetry [4,53. This report deals with the results of studies on the photoconductivity of single crystals of Tll++gxInSe2 (x = 0 to 0.03) solid solutions. EXPERIMENTAL Single crystals of the Tll_xAgxInSe2 system were grown by the Bridgeman-Stockbargermethod. The single-phase state of the crystals obtained was checked thermographically and with the use of 0254-0584/85/$3.30
0 Elsevier Sequoia/Printed
in The Netherlands
298
an X-ray diffraction technique. The crystals had a p-type conductivity. Specimens were prepared by cleaving from an ingot along two mutually perpendicular natural cleavage planes. Contacts were applied to the specimens by fusing-in indium. The applied electric field was directed along [OOI], whereas the light was perpendicular to the cleavage plane. Measurements were conducted at 300 and 77 Ii.Among the specimens selected for measurements were only those whose contacts provided stable photoelectric characteristics at low temperatures. The electric field intensity correaponded to the linear section of the current-voltage characteristic. The electrode spacings of the produced structures varied in the range from 0.15 to 0.25 cm, whereas the specimen areas perpendicular to the steady leakage current varied from 3.6~10~?o 9.4~10'~ cm2. Upon measuring the spectral characteristics of photocurrents, the light fluxes were equalized with respect to the numbers of quanta by means of a calibrated germanium photodiode. As shown in Table 1, the equilibrium concentration of holes in T11-xAgxInSe2 increasea by a factor of 10 to 100 with increasing x. At 300 K the photosensitivity (Rd/Rp at 100 lx) of solid-solution crystals is lower than that of TlInSe2, but at 77 K it shows a regular growth. Pig.1 depicts typical spectra of etationary photoconductivities of Tll_&gxInSe2 crystals at 300 K. No variation of band gap determined from amax was observed with increasing x within the experimental accuracy. This circumstance ie in good agreement with the opinions stated earlier [61 concerning the decisive part played by the coordination of trivalent cationa (In3+), rather than by univalent cations (Tl'+) with ionic bonds, in the formation of a forbidden gap for ternary compounds with a TlInSe2 -type lattice. Rowever, the long-wave edges of the spectra (htic)are different. Table 1. Some Photoelectric Characterietics of Mixed T11_xAgxInSe2 Crystals. Composition x=0
x = 0.01
300 K po,cm-3 Rd/Rp l.73x10"z 10.3 1.56xlOj$,2.54 2.85 2.18
77 K NF,cm-3
DD,cm-3
1.1 1.72~10'3 1.83x19' 1.0 3.95~10'~ 5.0x10 ' 1.0 5.28~10'~ 1.66~10" 1.05 2.2~10'~ 4.17x10'0
Rd/Rp
1tg,cm-3
8.2 17.1 42.0 55.2
3.67~10'~ 1.56~10'2 -
299
&OiO/f
ENERGY
(d)
-
Fig. 1. Spectral dependences of stationary photoconductivity of Tl,_tigxInSe2 at 300 K.
Fig.2 gives the lux-ampere characteristics (LAC'S) of Tl,_xAgxInSe2 specimens. It is evident from this figure that sublinear LAC's are observed with all the Tl ,_xAgxInSe2 (x = 0 to 0.03) compositions. At low excitation levels, I -L which changes to the I -Loo5 dependence as the intrinsic light intensity continues t: increase. The dashed lines in Fig.2 show the Ip+Lot
Fig. 2. I&x-ampere characteristics of T11_xAgxInSe2 at 300 K.
300
dependencea at a= 1 and a= 0.5. The appearance of a sublinear section is accounted for by the square-law recombination of holes, controlled by r-centres. This is possible inasmuch as p>) Nz , where p is the concentration of photoholes, and Nz is the concentration of dark electrons at r-centres. According to Fig.1 p-10 '4 to 10'5 cm-3. The values determined from the LAC inflection points are presented in Table 1 and satisfy the above inequality. At x = 0.03 a photocurrent saturation region is observed; when the excitation level is so high that the.r-centres are largely empty, I,-La, where 640.5. The concentsation of r-cendetermined from this inflection point ~~~~:: ~~;9~~~8~I~~~~: Superlinear IX's are observed with two compositions (x = 0.02 and x = 0.03) as the temperature decreases to 77 K. According to [7], it bears urtness to the existence of thermal quenching in this temperature range. Fig. 3 exhibits photoconductivity spectra of Tl,_~gxInSe2 (X = 0 to 0.03) single crystals at 77 K. substitution of thallium by silver in T11nSe2 crystals brings about an impurity photosensitivity. The region of impurity photosensitivity adjoins the fundamental absorption edge. It is conceivable that the silver atoms form an impurity band at the edge of one of the main bands.
f!!Omv
ENERGV
(eV) -
Fig. 3.Spectral dependences of stationary photoconductivity of T1,_AInSe2 single crystals at 77 K.
301
In conclusion it should be noted that all the concentration values presented in the report have been determined to the accuracy of the factor (the mobility of majority carriers 1. r
1
F.Tsernike and J.Nidvinter, hpplied non-linear optics, The 'Illir' Publishing House, Moscow, 1976, p. 142. 2 G.B.Abdullayev, G.D.Guseinov and V.D.Rustamov, Dokl. AN Azerb. SSR, 32 (1976) 20. 3 V.I.Tagirov, A.B.Bakhyshov, M.A.Sobeikh, A.M.Akhmedov and V.M.Salmanov, IN. VUZ'ov, Fizika, 11 (1978) 131. 4 D.Mi_iller, G.Eulenberger and H.Hahn, Z. anorg. allg. Chem., m (1973) 207. 5 G.D.Guseinov, G.G.Guseinov, E.M.Kerimova, M.Z.Ismailov, V.D.Ruatamov and L.A.Rzaeva, Mater. Kes. Bull., 13 (1978) 975. 6 G.D.Guseinov, Abstract of Doctor's Thesie, Baku, 1969, p.65. 7 A.Rose, Fundamentals of the photoconductivity theory, The 'Mir' Publishing House, Moscow, 1962, p. 128.
297-301
297
STATIONARY PHOTOCONDUCTIVITY OF MIXED Tl,_$gxInSe2 CRYSTALS
G.D.GUSEINOV, V.A.ALIYEV, A.U.MALSAGOV and L.M.CHAPANOVA Institute of Physics of the Azerbaijan SSR Academy of Sciences, Baku, 370143, pr. Narimanova 33, (USSR) Received 30 July 1984; accepted 10 September 1984 ABSTRACT single crystals of solid soluTl,_pg,InSe2 (x = 0 to 0.03) tion were grown by Bridgeman-Stockbarger techniques. Photoelectric measurements were taken at 300 and 77 K. With an increase in the value of x, the equilibrium concentration is increased 10 to 100 times. At 77 K a region of impurity photosensitivity has been detected which is bordered b the fundamental absorbtion edge. The photosensitivity ( Rd/Rp s of the solid solution crystals at 77 K increases from 8.2 (x = 0) to 55.2 (x = 0.03) the conclusion is made that when Tl is replaced by Ag then the sensitisation of semiconductor.
INTRODUCTION In recent years growing interest has been shown in complex semiconductive compounds. The crystals of such compounds are anisotropic and find application in non-linear opticsCl1. The photoacoustic effect and high photosensitivity found in these crystals show their promise for optoelectronics [2,3]. Among the new ternary semiconductive crystals is the T11nSe2 compound possessing a TlSe-type tetragonal structure and described by the DiE space group of symmetry [4,53. This report deals with the results of studies on the photoconductivity of single crystals of Tll++gxInSe2 (x = 0 to 0.03) solid solutions. EXPERIMENTAL Single crystals of the Tll_xAgxInSe2 system were grown by the Bridgeman-Stockbargermethod. The single-phase state of the crystals obtained was checked thermographically and with the use of 0254-0584/85/$3.30
0 Elsevier Sequoia/Printed
in The Netherlands
298
an X-ray diffraction technique. The crystals had a p-type conductivity. Specimens were prepared by cleaving from an ingot along two mutually perpendicular natural cleavage planes. Contacts were applied to the specimens by fusing-in indium. The applied electric field was directed along [OOI], whereas the light was perpendicular to the cleavage plane. Measurements were conducted at 300 and 77 Ii.Among the specimens selected for measurements were only those whose contacts provided stable photoelectric characteristics at low temperatures. The electric field intensity correaponded to the linear section of the current-voltage characteristic. The electrode spacings of the produced structures varied in the range from 0.15 to 0.25 cm, whereas the specimen areas perpendicular to the steady leakage current varied from 3.6~10~?o 9.4~10'~ cm2. Upon measuring the spectral characteristics of photocurrents, the light fluxes were equalized with respect to the numbers of quanta by means of a calibrated germanium photodiode. As shown in Table 1, the equilibrium concentration of holes in T11-xAgxInSe2 increasea by a factor of 10 to 100 with increasing x. At 300 K the photosensitivity (Rd/Rp at 100 lx) of solid-solution crystals is lower than that of TlInSe2, but at 77 K it shows a regular growth. Pig.1 depicts typical spectra of etationary photoconductivities of Tll_&gxInSe2 crystals at 300 K. No variation of band gap determined from amax was observed with increasing x within the experimental accuracy. This circumstance ie in good agreement with the opinions stated earlier [61 concerning the decisive part played by the coordination of trivalent cationa (In3+), rather than by univalent cations (Tl'+) with ionic bonds, in the formation of a forbidden gap for ternary compounds with a TlInSe2 -type lattice. Rowever, the long-wave edges of the spectra (htic)are different. Table 1. Some Photoelectric Characterietics of Mixed T11_xAgxInSe2 Crystals. Composition x=0
x = 0.01
300 K po,cm-3 Rd/Rp l.73x10"z 10.3 1.56xlOj$,2.54 2.85 2.18
77 K NF,cm-3
DD,cm-3
1.1 1.72~10'3 1.83x19' 1.0 3.95~10'~ 5.0x10 ' 1.0 5.28~10'~ 1.66~10" 1.05 2.2~10'~ 4.17x10'0
Rd/Rp
1tg,cm-3
8.2 17.1 42.0 55.2
3.67~10'~ 1.56~10'2 -
299
&OiO/f
ENERGY
(d)
-
Fig. 1. Spectral dependences of stationary photoconductivity of Tl,_tigxInSe2 at 300 K.
Fig.2 gives the lux-ampere characteristics (LAC'S) of Tl,_xAgxInSe2 specimens. It is evident from this figure that sublinear LAC's are observed with all the Tl ,_xAgxInSe2 (x = 0 to 0.03) compositions. At low excitation levels, I -L which changes to the I -Loo5 dependence as the intrinsic light intensity continues t: increase. The dashed lines in Fig.2 show the Ip+Lot
Fig. 2. I&x-ampere characteristics of T11_xAgxInSe2 at 300 K.
300
dependencea at a= 1 and a= 0.5. The appearance of a sublinear section is accounted for by the square-law recombination of holes, controlled by r-centres. This is possible inasmuch as p>) Nz , where p is the concentration of photoholes, and Nz is the concentration of dark electrons at r-centres. According to Fig.1 p-10 '4 to 10'5 cm-3. The values determined from the LAC inflection points are presented in Table 1 and satisfy the above inequality. At x = 0.03 a photocurrent saturation region is observed; when the excitation level is so high that the.r-centres are largely empty, I,-La, where 640.5. The concentsation of r-cendetermined from this inflection point ~~~~:: ~~;9~~~8~I~~~~: Superlinear IX's are observed with two compositions (x = 0.02 and x = 0.03) as the temperature decreases to 77 K. According to [7], it bears urtness to the existence of thermal quenching in this temperature range. Fig. 3 exhibits photoconductivity spectra of Tl,_~gxInSe2 (X = 0 to 0.03) single crystals at 77 K. substitution of thallium by silver in T11nSe2 crystals brings about an impurity photosensitivity. The region of impurity photosensitivity adjoins the fundamental absorption edge. It is conceivable that the silver atoms form an impurity band at the edge of one of the main bands.
f!!Omv
ENERGV
(eV) -
Fig. 3.Spectral dependences of stationary photoconductivity of T1,_AInSe2 single crystals at 77 K.
301
In conclusion it should be noted that all the concentration values presented in the report have been determined to the accuracy of the factor (the mobility of majority carriers 1. r
1
F.Tsernike and J.Nidvinter, hpplied non-linear optics, The 'Illir' Publishing House, Moscow, 1976, p. 142. 2 G.B.Abdullayev, G.D.Guseinov and V.D.Rustamov, Dokl. AN Azerb. SSR, 32 (1976) 20. 3 V.I.Tagirov, A.B.Bakhyshov, M.A.Sobeikh, A.M.Akhmedov and V.M.Salmanov, IN. VUZ'ov, Fizika, 11 (1978) 131. 4 D.Mi_iller, G.Eulenberger and H.Hahn, Z. anorg. allg. Chem., m (1973) 207. 5 G.D.Guseinov, G.G.Guseinov, E.M.Kerimova, M.Z.Ismailov, V.D.Ruatamov and L.A.Rzaeva, Mater. Kes. Bull., 13 (1978) 975. 6 G.D.Guseinov, Abstract of Doctor's Thesie, Baku, 1969, p.65. 7 A.Rose, Fundamentals of the photoconductivity theory, The 'Mir' Publishing House, Moscow, 1962, p. 128.