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Band structure investigation of two low temperature phases in Ag2Se
Thin Solid Films, 73 (1980) L7-L8 © Elsevier Sequoia S.A., Lausanne--Printed in the Netherlands
L7
Letter Band structure investigation of two low temperature phases in Ag2Se A. G. ABDULLAYEV, V. K. ALIYEV, T. G. FUFAYEVA AND L. I. SKUGAREVA Institute of Space Researchfor Natural Resources, Azerbaijan Academy of Sciences, Baku (U.S.S.R.) (Received July 1, 1980; accepted September 17, 1980)
Ag2Se is an n-type degenerate semiconductor of high (nearly 1019 cm- 3) carrier concentration, irregularly high carrier mobility and with a degree of ionic conduction. The Ag2Se crystals were obtained by direct melting of the components in evacuated quartz ampoules. The quartz ampoules were slowly heated to the melting point of the compound (about 897 °C) and maintained at that temperature for 8 h. Then annealing was carried out at 700 °C for 20 h followed by s~ow cooling. The specimens obtained were investigated using X-ray analysis and were found to consist of the low temperature rhombic phase of Ag2Se. The initial specimens had an electron concentration of 3 x 1019 cm -3 and a mobility of 900 cm 2 V- ~ s- ~. Investigation of the band structure in bulk specimens of Ag2Se with different carrier mobilities by simultaneous measurements of galvanic and thermomagnetic properties showed that in this material Kane's dispersion law holds 1'2. Electron diffraction analysis of Ag2Se thin films revealed two low temperature phases: rhombic and tetragonal 3'4. This suggested that a band structure investigation of both low temperature phases would be interesting. It was decided to analyse the band structure of Ag2Se thin films using tunnel spectroscopy. Deposition of semiconductor layers was carried out by the flash evaporation of Ag2Se powder under a vacuum of 10 -6 Torr onto a polished plate of 2MgO 22A1203-5SIO 2, which was used simultaneously as a substrate for measurements of the conductivity and the Hall effect, onto an NaC1 crystal for electron diffraction analysis and onto a previously prepared AI/A1203 structure for tunnel spectroscopy measurements. The AI/AI203 structure was obtained by oxidizing aluminium films in a 0.1~ aqueous solution of tartaric acid (C4.H606) under conditions of either constant current or constant voltage. Ag2Se films prepared at a substrate temperature of 30 °C were tetragonal. Subsequent heat treatment at 100 °C partly converted the Ag2Se into the rhombic phase. Electron diffraction analysis confirmed the presence of both phases of Ag2Se. Hall effect and conduction measurements were carried out in a d.c. magnetic field which was varied in the range (6.5-17)x 103 Oe. The Hall effect was independent of the magnetic field. The results are shown in Table I. For the analysis of the band structures of both phases, I, dI/dU and d2l/dU 2 were measured as functions of the potential U applied to a tunnel junction of structure A1/A12OJAg2Se/AI. To measure dI/dU and d2I/dU 2 against U, the technique of detecting the harmonics of an a.c. signal was used 5. Our measurements
L8
LETTERS
TABLE I PROPERTIES OF Ag2Se
Ag2Se phase
Tetragonal Rhombic
n
a
U
(cm- 3)
(fl- 1 cm- 1)
(cm 2 V
5.7 x 1019 3.8 × 1019
3460 3320
380.6 531
-
1 S-
1)
Ef
Eg
m,
(cV)
(cV)
m0
0.19 0.225
0.07 0.17
0.071 0.045
were carried out at a frequency of 427 Hz and with an amplitude of 3 mV. The potential drop was measured on a resistance R . . . . where R . . . . ,~ Rspe¢imen.As is well known (see, for example, ref. 6), the transport of charge at low temperature between two conductors separated by a thin film insulator takes place mainly by electron tunnelling. Hence all measurements of the tunnel current and its derivatives were carried out at the temperature of liquid nitrogen. Analysis of the current-voltage characteristics showed a slight dependence on temperature in the range 4.2-80 K. As the temperature was increased, temporary polarity-dependent changes in the current (switching) were observed. This seemed to be due to the penetration of silver ions into the insulator. Figure 1 shows plots of I, dI/dU and d2I/dU 2 against U for both phases of Ag2Se at low temperature. The sharp minima in the plots of conductivity against potential indicate the existence in the semiconductor of a forbidden zone of energy E~. According to Chang 7 the Fermi energy Ef of a degenerate semiconductor coincides with the minimum in the conduction.
05
(a)
I~tf 02
d~
U. vott5
(b) (b)
Fig. 1. The dependence of l, dl/dUandd2l/dU20n U forAg2Se:(a)tetragonalphase;(b)rhombicphase.
Proceeding from Kane's dispersion law for spherically symmetric bands and using the expression for the electron concentration derived by Askerov 8, we calculated the effective mass rn~ of an electron at the bottom of the conduction band. The values of Ef, Eg and m. for both phases of Ag2Se are given in Table I. 1 2 3 4 5 6 7 8
S.A. Aliev, L. L. Korenblit and S. S. Shalyt, Fiz. Tverd. Tela, 7 (6) (1965) 1974. S.A. Aliev and N. A. Berdieva, Uch. Zap. M V i SSO Az. SSR, 3 (1978). G.A. Efendiev, E. R. Nuriev and R. B. Shafizade, Kristallografiya, 14 (1969) 5. E.R. Nuriev and R. B. Shafizade, Izv. Akad. Nauk Az. SSR, Ser. Fiz.-Mat. Tekh. Nauk, 2 (1972). Dzh. Edler and Dzh. Dzhekson, Pribory dlya Nauchnykh Issledovanil, 8 (1966) 63. E.L. Murphy and R. H. Good, Phys. Rev., 102 (1956) 1464. L.L. Chang, J. Appl. Phys., 39 (1968) 1465. B.M. Askerov, Kineticheskie, Effekty v Polyprovodnikakh, Nauka, Moscow, 1970, p. 128.
L7
Letter Band structure investigation of two low temperature phases in Ag2Se A. G. ABDULLAYEV, V. K. ALIYEV, T. G. FUFAYEVA AND L. I. SKUGAREVA Institute of Space Researchfor Natural Resources, Azerbaijan Academy of Sciences, Baku (U.S.S.R.) (Received July 1, 1980; accepted September 17, 1980)
Ag2Se is an n-type degenerate semiconductor of high (nearly 1019 cm- 3) carrier concentration, irregularly high carrier mobility and with a degree of ionic conduction. The Ag2Se crystals were obtained by direct melting of the components in evacuated quartz ampoules. The quartz ampoules were slowly heated to the melting point of the compound (about 897 °C) and maintained at that temperature for 8 h. Then annealing was carried out at 700 °C for 20 h followed by s~ow cooling. The specimens obtained were investigated using X-ray analysis and were found to consist of the low temperature rhombic phase of Ag2Se. The initial specimens had an electron concentration of 3 x 1019 cm -3 and a mobility of 900 cm 2 V- ~ s- ~. Investigation of the band structure in bulk specimens of Ag2Se with different carrier mobilities by simultaneous measurements of galvanic and thermomagnetic properties showed that in this material Kane's dispersion law holds 1'2. Electron diffraction analysis of Ag2Se thin films revealed two low temperature phases: rhombic and tetragonal 3'4. This suggested that a band structure investigation of both low temperature phases would be interesting. It was decided to analyse the band structure of Ag2Se thin films using tunnel spectroscopy. Deposition of semiconductor layers was carried out by the flash evaporation of Ag2Se powder under a vacuum of 10 -6 Torr onto a polished plate of 2MgO 22A1203-5SIO 2, which was used simultaneously as a substrate for measurements of the conductivity and the Hall effect, onto an NaC1 crystal for electron diffraction analysis and onto a previously prepared AI/A1203 structure for tunnel spectroscopy measurements. The AI/AI203 structure was obtained by oxidizing aluminium films in a 0.1~ aqueous solution of tartaric acid (C4.H606) under conditions of either constant current or constant voltage. Ag2Se films prepared at a substrate temperature of 30 °C were tetragonal. Subsequent heat treatment at 100 °C partly converted the Ag2Se into the rhombic phase. Electron diffraction analysis confirmed the presence of both phases of Ag2Se. Hall effect and conduction measurements were carried out in a d.c. magnetic field which was varied in the range (6.5-17)x 103 Oe. The Hall effect was independent of the magnetic field. The results are shown in Table I. For the analysis of the band structures of both phases, I, dI/dU and d2l/dU 2 were measured as functions of the potential U applied to a tunnel junction of structure A1/A12OJAg2Se/AI. To measure dI/dU and d2I/dU 2 against U, the technique of detecting the harmonics of an a.c. signal was used 5. Our measurements
L8
LETTERS
TABLE I PROPERTIES OF Ag2Se
Ag2Se phase
Tetragonal Rhombic
n
a
U
(cm- 3)
(fl- 1 cm- 1)
(cm 2 V
5.7 x 1019 3.8 × 1019
3460 3320
380.6 531
-
1 S-
1)
Ef
Eg
m,
(cV)
(cV)
m0
0.19 0.225
0.07 0.17
0.071 0.045
were carried out at a frequency of 427 Hz and with an amplitude of 3 mV. The potential drop was measured on a resistance R . . . . where R . . . . ,~ Rspe¢imen.As is well known (see, for example, ref. 6), the transport of charge at low temperature between two conductors separated by a thin film insulator takes place mainly by electron tunnelling. Hence all measurements of the tunnel current and its derivatives were carried out at the temperature of liquid nitrogen. Analysis of the current-voltage characteristics showed a slight dependence on temperature in the range 4.2-80 K. As the temperature was increased, temporary polarity-dependent changes in the current (switching) were observed. This seemed to be due to the penetration of silver ions into the insulator. Figure 1 shows plots of I, dI/dU and d2I/dU 2 against U for both phases of Ag2Se at low temperature. The sharp minima in the plots of conductivity against potential indicate the existence in the semiconductor of a forbidden zone of energy E~. According to Chang 7 the Fermi energy Ef of a degenerate semiconductor coincides with the minimum in the conduction.
05
(a)
I~tf 02
d~
U. vott5
(b) (b)
Fig. 1. The dependence of l, dl/dUandd2l/dU20n U forAg2Se:(a)tetragonalphase;(b)rhombicphase.
Proceeding from Kane's dispersion law for spherically symmetric bands and using the expression for the electron concentration derived by Askerov 8, we calculated the effective mass rn~ of an electron at the bottom of the conduction band. The values of Ef, Eg and m. for both phases of Ag2Se are given in Table I. 1 2 3 4 5 6 7 8
S.A. Aliev, L. L. Korenblit and S. S. Shalyt, Fiz. Tverd. Tela, 7 (6) (1965) 1974. S.A. Aliev and N. A. Berdieva, Uch. Zap. M V i SSO Az. SSR, 3 (1978). G.A. Efendiev, E. R. Nuriev and R. B. Shafizade, Kristallografiya, 14 (1969) 5. E.R. Nuriev and R. B. Shafizade, Izv. Akad. Nauk Az. SSR, Ser. Fiz.-Mat. Tekh. Nauk, 2 (1972). Dzh. Edler and Dzh. Dzhekson, Pribory dlya Nauchnykh Issledovanil, 8 (1966) 63. E.L. Murphy and R. H. Good, Phys. Rev., 102 (1956) 1464. L.L. Chang, J. Appl. Phys., 39 (1968) 1465. B.M. Askerov, Kineticheskie, Effekty v Polyprovodnikakh, Nauka, Moscow, 1970, p. 128.