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Schottky diodes on ZnIn2S4 single crystals
Solar Energy Materials 16 (1987) 315-318 North-Holland, Amsterdam
315
S C H O T r K Y D I O D E S O N Znln2S 4 SINGLE C R Y S T A L S O. V I G I L , S. LOPEZ, E. M O R R I S and O. C A L Z A D I L L A Department of Semiconductors, Faculty of Physics, University of La Habana, Cuba
F. L E C C A B U E MASPEC Institute of CNR, Via Chiavari 18/,4, 43100 Parma, Italy
Received 9 December 1986; in revised form 13 March 1987 Schottky diodes have been prepared on Znln2S4 single crystal ternary compounds using Au as the barrier contact. Compared with theoretical models, these diodes show majority carrier tunnel and interface state effects. The influence of chemical etching on the electrical properties is also reported.
During the past few years several devices, such as photosensitive heterojunctions [1,2] and electroluminescent diodes [3], have been developed using I I - I I I 2 - V I 4 compounds. In this type of device it is necessary to control the resistivity of the materials. The Z n l n z S 4 compound is a n-type semiconductor and presents a transmission of about 80% to radiation which has an energy below its band gap. We have recently obtained low resistivity samples and studied the possibility of using this compound as a window in heterojunction solar cells [4]. The present paper reports the preliminary experimental I - V and C - V characteristics of A u / Z n l n 2 S 4 Schottky diodes obtained by using a low resistivity substrate. The influence of the chemical etching on the electrical properties of the Schottky diodes is also reported. n-type Znln2S 4 single crystals were grown by chemical vapour transport in a closed quartz ampoule. In order to obtain single crystals with low resistivity (ranging from 0.8 to 10 f~ cm), annealing under a m a x i m u m Mn pressure at 900 K was performed [4]. The samples were chemically etched in two different ways for a fixed time of 20 s: (a) 3 H C 1 . 1 H N O 3 and (b) 10% HC1 solutions. Indium was chosen as the ohmic contact and was deposited by vacuum evaporation under a pressure of about 10 -6 Torr; the substrate was at room temperature (RT). Gold was selected as the Schottky contact. Schottky contacts about 100 thick were prepared under the same conditions as the ohmic ones. The devices were annealed under vacuum at 2 0 0 ° C and the device area was 7 × 10 -2 cm 2. A typical dark I - - V current at R T for the A u / Z n l n 2 S 4 Schottky diode as a function of the chemical etching and the photovoltaic characteristic of the etched diodes is shown in fig. 1. In the same figure a typical C - 2 - V plot is inserted; the diodes were etched with solution (a) and the capacitance measurements were made at 1 M H z and 0.2 V peak to peak. 0165-1633/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
316
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Fig. 1. I-Vcharacteristic of the Au/ZnIn,S, Schottky diode at RT. (a) 3 HCl.1 HNO, solution; (b) 10% HCl solutions; (c) non-etched substrate; and (d) under illumination with chemical etching (a) and 100 mW/cm2 light intensity. The insert shows the C-2-V characteristic of the Au/ZnIn,S, Schottky diode at RT and at 1 MHz.
0
L
-1
100
200
300 -d
fmV1
Fig. 2. Semilog plot of the I-I’characteristics of the Au/ZnIn,S, Schottky diode at low forward different temperatures: (a) 300, (b) 363 and (c) 403 K.
bias for
318
o. Vigil et al. / Schottky diodes on Z n l n e S 4 single c
j u n c t i o n solar cells [9]. In this case the o p e n circuit voltage is smaller than that of the diffusion voltage. T h e interface states are the c o n s e q u e n c e of the i n s u l a t i n g layer b e t w e e n the m e t a l a n d s e m i c o n d u c t o r contact. T h e etching of the Z n l n 2 S 4 surface influences the p r o p e r t i e s of the d i o d e s (see table 1). W i t h s o l u t i o n (a) it is p o s s i b l e to o b t a i n b e t t e r results; however, it is necessary to p e r f o r m a c o m p l e t e s t u d y on the etching effect of the Z n I n 2 $4 surface. A p r e l i m i n a r y study of A u / Z n I n z S 4 S c h o t t k y d i o d e s using a low Z n I n 2 S 4 resistivity single crystal s u b s t r a t e has p o i n t e d t o w a r d s the p o s s i b i l i t y of o b t a i n i n g a device with a I I - I I I 2 - V I 4 c o m p o u n d . W e have recently s t u d i e d the anti-reflecting c o a t i n g on Z n I n 2 S 4 single crystals to be used in p h o t o s e n s i t i v e devices [10]; therefore, it w o u l d be interesting to e v a l u a t e the Z n I n z S 4 ( a n d o t h e r I I - I I I 2 - V I 4 c o m p o u n d s ) as solar cell material.
References [1] S. Endo and T. Irie, Jpn. J. Phys. 19, Suppl. 19-3 (1980) 53. [2] F.G. Garcia and M.S. Tomar, Thin Solid Films 69 (1980) 137. [3] C. Paorici, C. Pelosi, N. Romeo, G, Sberveglieri and L. Tarricone, Phys. Status Solidi A 36 (1976) K33. [4] O. Vigil, O. Calzadilla, D. Seuret, J. Vidal and F. Leccabue, Solar Energy Mater. 10 (1984) 139. [5] S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1969). [6] A.M. Goodman, J. Appl. Phys. 34 (1963) 329. [7] P. Robinson and J. Wilson, Inst. Phys. Conf. Ser. 35 (1977) 229. [8] S. Ashok and K. Pande, Solar Cells 14 (1985) 61. [9] A. Rothwarf, Proc. 16th IEEE Photovoltaic Spec. Conf. (1982) 791. [10] O. Vigil, S. Lopez, E. Morris and F. Leccabue, to be published in J. Appl. Phys.
315
S C H O T r K Y D I O D E S O N Znln2S 4 SINGLE C R Y S T A L S O. V I G I L , S. LOPEZ, E. M O R R I S and O. C A L Z A D I L L A Department of Semiconductors, Faculty of Physics, University of La Habana, Cuba
F. L E C C A B U E MASPEC Institute of CNR, Via Chiavari 18/,4, 43100 Parma, Italy
Received 9 December 1986; in revised form 13 March 1987 Schottky diodes have been prepared on Znln2S4 single crystal ternary compounds using Au as the barrier contact. Compared with theoretical models, these diodes show majority carrier tunnel and interface state effects. The influence of chemical etching on the electrical properties is also reported.
During the past few years several devices, such as photosensitive heterojunctions [1,2] and electroluminescent diodes [3], have been developed using I I - I I I 2 - V I 4 compounds. In this type of device it is necessary to control the resistivity of the materials. The Z n l n z S 4 compound is a n-type semiconductor and presents a transmission of about 80% to radiation which has an energy below its band gap. We have recently obtained low resistivity samples and studied the possibility of using this compound as a window in heterojunction solar cells [4]. The present paper reports the preliminary experimental I - V and C - V characteristics of A u / Z n l n 2 S 4 Schottky diodes obtained by using a low resistivity substrate. The influence of the chemical etching on the electrical properties of the Schottky diodes is also reported. n-type Znln2S 4 single crystals were grown by chemical vapour transport in a closed quartz ampoule. In order to obtain single crystals with low resistivity (ranging from 0.8 to 10 f~ cm), annealing under a m a x i m u m Mn pressure at 900 K was performed [4]. The samples were chemically etched in two different ways for a fixed time of 20 s: (a) 3 H C 1 . 1 H N O 3 and (b) 10% HC1 solutions. Indium was chosen as the ohmic contact and was deposited by vacuum evaporation under a pressure of about 10 -6 Torr; the substrate was at room temperature (RT). Gold was selected as the Schottky contact. Schottky contacts about 100 thick were prepared under the same conditions as the ohmic ones. The devices were annealed under vacuum at 2 0 0 ° C and the device area was 7 × 10 -2 cm 2. A typical dark I - - V current at R T for the A u / Z n l n 2 S 4 Schottky diode as a function of the chemical etching and the photovoltaic characteristic of the etched diodes is shown in fig. 1. In the same figure a typical C - 2 - V plot is inserted; the diodes were etched with solution (a) and the capacitance measurements were made at 1 M H z and 0.2 V peak to peak. 0165-1633/87/$03.50 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
316
A f
0,6-
, /I
Vfvolts
I
0,4-
)
-1.0
-2,0
/#
,///.:d
.
40
I
-3.0
0.2. /
/
/’
i
Fig. 1. I-Vcharacteristic of the Au/ZnIn,S, Schottky diode at RT. (a) 3 HCl.1 HNO, solution; (b) 10% HCl solutions; (c) non-etched substrate; and (d) under illumination with chemical etching (a) and 100 mW/cm2 light intensity. The insert shows the C-2-V characteristic of the Au/ZnIn,S, Schottky diode at RT and at 1 MHz.
0
L
-1
100
200
300 -d
fmV1
Fig. 2. Semilog plot of the I-I’characteristics of the Au/ZnIn,S, Schottky diode at low forward different temperatures: (a) 300, (b) 363 and (c) 403 K.
bias for
318
o. Vigil et al. / Schottky diodes on Z n l n e S 4 single c
j u n c t i o n solar cells [9]. In this case the o p e n circuit voltage is smaller than that of the diffusion voltage. T h e interface states are the c o n s e q u e n c e of the i n s u l a t i n g layer b e t w e e n the m e t a l a n d s e m i c o n d u c t o r contact. T h e etching of the Z n l n 2 S 4 surface influences the p r o p e r t i e s of the d i o d e s (see table 1). W i t h s o l u t i o n (a) it is p o s s i b l e to o b t a i n b e t t e r results; however, it is necessary to p e r f o r m a c o m p l e t e s t u d y on the etching effect of the Z n I n 2 $4 surface. A p r e l i m i n a r y study of A u / Z n I n z S 4 S c h o t t k y d i o d e s using a low Z n I n 2 S 4 resistivity single crystal s u b s t r a t e has p o i n t e d t o w a r d s the p o s s i b i l i t y of o b t a i n i n g a device with a I I - I I I 2 - V I 4 c o m p o u n d . W e have recently s t u d i e d the anti-reflecting c o a t i n g on Z n I n 2 S 4 single crystals to be used in p h o t o s e n s i t i v e devices [10]; therefore, it w o u l d be interesting to e v a l u a t e the Z n I n z S 4 ( a n d o t h e r I I - I I I 2 - V I 4 c o m p o u n d s ) as solar cell material.
References [1] S. Endo and T. Irie, Jpn. J. Phys. 19, Suppl. 19-3 (1980) 53. [2] F.G. Garcia and M.S. Tomar, Thin Solid Films 69 (1980) 137. [3] C. Paorici, C. Pelosi, N. Romeo, G, Sberveglieri and L. Tarricone, Phys. Status Solidi A 36 (1976) K33. [4] O. Vigil, O. Calzadilla, D. Seuret, J. Vidal and F. Leccabue, Solar Energy Mater. 10 (1984) 139. [5] S.M. Sze, Physics of Semiconductor Devices (Wiley, New York, 1969). [6] A.M. Goodman, J. Appl. Phys. 34 (1963) 329. [7] P. Robinson and J. Wilson, Inst. Phys. Conf. Ser. 35 (1977) 229. [8] S. Ashok and K. Pande, Solar Cells 14 (1985) 61. [9] A. Rothwarf, Proc. 16th IEEE Photovoltaic Spec. Conf. (1982) 791. [10] O. Vigil, S. Lopez, E. Morris and F. Leccabue, to be published in J. Appl. Phys.