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Effects of In2O3 deposition conditions on the SiO2-Si interface
Solar Cells, 23 (1988) 269 - 271
269
Short Communication
EFFECTS OF In203 DEPOSITION CONDITIONS ON THE SiO2-Si INTERFACE M. SULEMAN and S. NASEEM
Centre for Solid State Physics, University o f the Punjab (New Campus), Lahore 20 (Pakistan) (Received September 9, 1987; accepted in revised form December 23, 1987)
The SiO2-Si interface plays a vital role in the performance of semiconductor/insulator/semiconductor (SIS) type silicon-based solar cells. We have recently studied the aging effects in In2OJSiO:/Si solar cells using Auger electron spectroscopy [1]. It was found that the SiO2-Si interface width of the as-grown sample was 34 A. However, this increased after aging and after In203 deposition. It was interesting to note that free silicon existed throughout the SiO2 layer when In203 was deposited at 200 °C on top of the SiO2. It was suggested that the existence of free silicon was due to a solid state reduction of the SiO2 layer. The cause of this reduction was assumed to be the deposition process of In2Oa. A simple experiment was performed to show that this reduction was due to the In2Oa deposition, and not due to the substrate heating alone. The sample for this study was prepared in a similar way to that reported previously [1]. The In203 layer was not deposited in this case. However, the sample was heated at 200 °C in vacuum for about 30 min; this was the time necessary to deposit 6500 A of In203 film, as recorded during the previous investigation. Auger analysis of this sample was performed using a scanning Auger microprobe system (JEOL JAMP-10S). The data were obtained using a primary energy of 10 keV. The depth profiling was carried out using a Varian ion gun (Model 981-2043) with scanning facility. The ion gun was operated in scanning mode, using an ion energy of 3 keV and emission current of 30 mA, at an argon pressure of 5 × 10 -s Torr. The sputter rate of the oxide was calibrated from the time of sputtering and the thickness of the SiO2 layer. The sputter rate, thus calculated, was 24 A min- 1. The silicon (LVV) and oxygen (KVV) Auger lines were monitored during AES analysis. Figure 1 shows the Auger depth profiling of a sample, which was heated at 200 °C for 30 min. This figure shows an abrupt interface. The corrected width of this interface was calculated according to the procedure of Johannessen et al. [2], and was found to be 32 A. The interface width of this sample compares well with that of the freshly prepared 0379-6787/88/$3.50
O Elsevier Sequoia/Printed in The Netherlands
270 r
7
Si(LW)
I 1
T
T
[
]
-[ .
I 3
4
5
6
7
.
.
.
.
.
r
.
.
.
.
.
.
i
78 eV
I 2
I
SpuKer
8
9
Time (Mins.)
Fig. 1. Auger depth profile of an SiO2/Si sample which was heated at 200 °C. t
I
/
o (K\%')
tn
o
SiIL\~)
0
•
I
1 17
l
78 eV
314
I Sputter
511
\ G8
85
Time (Mins.)
Fig. 2. Auger depth profile of an as-prepared SiO2/Si sample (reproduced from [1]). sample (34 A as o b t a i n e d from Fig. 2). Also, there was n o evidence o f free silicon in the SiO 2 layer as observed previously after the d e p o s i t i o n of In203. This comparison leads t o the c o n c l u s i o n that the elevated substrate temperature used for the In20~ d e p o s i t i o n does n o t affect the S i O 2 - S i
--
271
interface. Rather, it is the deposition of In203 which is responsible for the presence of free silicon throughout the SiO2 layer, as suggested earlier [ 1 ]. It can be concluded that the as-grown interface of SiO2-Si is abrupt, whereas it widens with the deposition of a thermally evaporated In203 overlayer. A solid state reduction of the SiO2 layer also takes place owing to the In203 layer deposition. This reduction can have serious effects on controlling the thickness of the insulating layer in SIS solar cells, and hence on the overall performance of these cells.
1 S. Naseem, I. Ahmad, N. A. Malik and M. Suleman, Sol. Cells, 20 (1987) 289. 2 J. S. Johannessen, W. E. Spicer and Y. E. Strausser, J. Appl. Phys., 47 (1976) 1.
269
Short Communication
EFFECTS OF In203 DEPOSITION CONDITIONS ON THE SiO2-Si INTERFACE M. SULEMAN and S. NASEEM
Centre for Solid State Physics, University o f the Punjab (New Campus), Lahore 20 (Pakistan) (Received September 9, 1987; accepted in revised form December 23, 1987)
The SiO2-Si interface plays a vital role in the performance of semiconductor/insulator/semiconductor (SIS) type silicon-based solar cells. We have recently studied the aging effects in In2OJSiO:/Si solar cells using Auger electron spectroscopy [1]. It was found that the SiO2-Si interface width of the as-grown sample was 34 A. However, this increased after aging and after In203 deposition. It was interesting to note that free silicon existed throughout the SiO2 layer when In203 was deposited at 200 °C on top of the SiO2. It was suggested that the existence of free silicon was due to a solid state reduction of the SiO2 layer. The cause of this reduction was assumed to be the deposition process of In2Oa. A simple experiment was performed to show that this reduction was due to the In2Oa deposition, and not due to the substrate heating alone. The sample for this study was prepared in a similar way to that reported previously [1]. The In203 layer was not deposited in this case. However, the sample was heated at 200 °C in vacuum for about 30 min; this was the time necessary to deposit 6500 A of In203 film, as recorded during the previous investigation. Auger analysis of this sample was performed using a scanning Auger microprobe system (JEOL JAMP-10S). The data were obtained using a primary energy of 10 keV. The depth profiling was carried out using a Varian ion gun (Model 981-2043) with scanning facility. The ion gun was operated in scanning mode, using an ion energy of 3 keV and emission current of 30 mA, at an argon pressure of 5 × 10 -s Torr. The sputter rate of the oxide was calibrated from the time of sputtering and the thickness of the SiO2 layer. The sputter rate, thus calculated, was 24 A min- 1. The silicon (LVV) and oxygen (KVV) Auger lines were monitored during AES analysis. Figure 1 shows the Auger depth profiling of a sample, which was heated at 200 °C for 30 min. This figure shows an abrupt interface. The corrected width of this interface was calculated according to the procedure of Johannessen et al. [2], and was found to be 32 A. The interface width of this sample compares well with that of the freshly prepared 0379-6787/88/$3.50
O Elsevier Sequoia/Printed in The Netherlands
270 r
7
Si(LW)
I 1
T
T
[
]
-[ .
I 3
4
5
6
7
.
.
.
.
.
r
.
.
.
.
.
.
i
78 eV
I 2
I
SpuKer
8
9
Time (Mins.)
Fig. 1. Auger depth profile of an SiO2/Si sample which was heated at 200 °C. t
I
/
o (K\%')
tn
o
SiIL\~)
0
•
I
1 17
l
78 eV
314
I Sputter
511
\ G8
85
Time (Mins.)
Fig. 2. Auger depth profile of an as-prepared SiO2/Si sample (reproduced from [1]). sample (34 A as o b t a i n e d from Fig. 2). Also, there was n o evidence o f free silicon in the SiO 2 layer as observed previously after the d e p o s i t i o n of In203. This comparison leads t o the c o n c l u s i o n that the elevated substrate temperature used for the In20~ d e p o s i t i o n does n o t affect the S i O 2 - S i
--
271
interface. Rather, it is the deposition of In203 which is responsible for the presence of free silicon throughout the SiO2 layer, as suggested earlier [ 1 ]. It can be concluded that the as-grown interface of SiO2-Si is abrupt, whereas it widens with the deposition of a thermally evaporated In203 overlayer. A solid state reduction of the SiO2 layer also takes place owing to the In203 layer deposition. This reduction can have serious effects on controlling the thickness of the insulating layer in SIS solar cells, and hence on the overall performance of these cells.
1 S. Naseem, I. Ahmad, N. A. Malik and M. Suleman, Sol. Cells, 20 (1987) 289. 2 J. S. Johannessen, W. E. Spicer and Y. E. Strausser, J. Appl. Phys., 47 (1976) 1.