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Analysis of the interface of indium tin oxide with InP
Thin Solid Films, 138 (1986) 87-90
87
PREPARATION AND CHARACTERIZATION
ANALYSIS O F T H E I N T E R F A C E O F I N D I U M TIN O X I D E W I T H InP A. SWARTZLANDER AND T. J. COUTTS
Solar Energy Research Institute, Golden, CO 80401 (U.S.A.) S. NASEEM
Newcastle Polytechnic, Newcastle Upon Tyne NE1 8 S T (Gt. Britain) T. P. MASSOPUST
Rocky Mountain Analytical Research Laboratories, Golden, CO 80401 (U.S.A.) (Received May 5, 1985; accepted September 16, 1985)
This paper is concerned with the analysis of interfaces of thin films of indium tin oxide (ITO) deposited by r.f. sputtering onto single-crystal substrates of indium phosphide (InP). Scanning Auger microscopy and depth profiling have been used to examine the abruptness of the interface between the two materials. The depth over which the indium line shifts from an energy characteristic of ITO to one of InP has been used as the principal indicator in this work. For very low rates of deposition of the ITO, and without post-deposition heating, the depth over which this transition occurs has been found to be as small as 3.0 nm. Changes in the position of the phosphorus line indicate that P205 forms at the interface when the samples are heated to 400 °C following deposition.
1. INTRODUCTION Although single-crystal InP may be too expensive for consideration as the absorber in solar cells for terrestrial application, there are several reasons why it may have potential for space applications. Firstly, it has a higher theoretical efficiency than silicon; secondly, the decrease in efficiency with temperature should be less than that of silicon; thirdly, its radiation resistance has been demonstrated as being superior to that of either silicon or gallium arsenide (GaAs) cells 1. Although various models of operation of the I T O / I n P cell (where ITO is indium tin oxide) have been proposed 2'3, it is still not entirely clear whether these cells operate as true heterojunctions, as buried homojunctions or even as semiconductor/insulator/semiconductor (SIS) junctions. Despite this lack of a thorough understanding, efficiencies of nearly 16% total area have recently been achieved 4 but it is clear that further progress toward the ultimate realizable efficiency of about 23% will depend on removing this deficiency. The work described in this note is intended to clarify interface processes, and particularly those related to the influence of the ITO deposition process and the effects of post-deposition heating. 0040-6090/86/$3.50
© ElsevierSequoia/Printedin The Netherlands
88
2.
A. SWARTZLANDER e t
al.
EXPERIMENTAL PROCEDURE
The single-crystal material was supplied by Mining and Chemical Products (Electronic Materials) Ltd. It was (100) oriented, zinc doped (2 x 1015 cm -3) and polished on the B face. Cleaning of individual substrates prior to I T O deposition was carried out using standard procedures 5 and the I T O deposition schedule was as follows. (i) Pre-sputter the I T O target (supplied by Nordike Ltd.; purity, 99.99~o) for 30min at a power density of approximately 7 W c m -2 to remove surface contamination. (ii) Reduce the power level so that no deposition can be detected (about 0.4 W c m - 2). (iii) Open the shutter for 30 min without increasing the power 6. (iv) Increase the power density to 0.7 W c m - 2 corresponding to a deposition rate of approximately 0.03 nm s - 1. The thickness of the films used in this work was 50.0 nm although working devices utilize greater thicknesses than this. Six samples of I T O / I n P were prepared in this way. One was analyzed by scanning Auger microscopy (SAM) without further treatment but the other five were heated in flowing nitrogen for 5 min at 100-500 °C before SAM analysis. Auger depth profiling was performed using a P e r k i n - E l m e r model 595 scanning Auger microprobe. During profiling the In M N N , the P K L L and the Sn M N N lines were monitored. The rate of sample erosion was approximately 0.5-0.6 nm m i n - 1, as estimated from the time at which the tin profile began to decrease significantly and the knowledge that the I T O was 50.0 nm thick. A line scan of the In M N N and P K L L lines was taken at intervals of approximately 40-50 s; hence the resolution of the technique is of the order of 0.5 nm. 3.
RESULTS AND DISCUSSIONS
A profile through the interface of the sample which had been heated at 200 °C is shown in Fig. 1. Since the binding energy of indium in I T O is different from that of indium in I n P (397 and 401 eV and 404 and 411 eV respectively) we can estimate the depth over which the transition from pure I T O to pure I n P takes place. In Fig. 2 we see scans of the In M N N line taken at various time intervals, again for the sample heated at 200 °C. The first significant change in the line shape and energy at which the minimum occurs is evident at 89.26 min (cycle 53). After 94.71 min (cycle 55) the change is complete. On this basis, the extent of the mixing of I T O and I n P is about 3.0 nm. However, this should be regarded as an overestimate for several reasons. Firstly, the Auger electrons escape from depths below the actual surface; secondly, the bombarding ions have a specific range depending on the sample and the beam energy; thirdly, the output is not continuous but discrete; fourthly, the result is influenced by surface roughness. The correction for the emission depth plus the ion range is of the order of 1.5 nm for a 1 keV beam 7. In our case, the beam energy was 4.5 keV so the ion range may have been somewhat different. Hence, the mixed region near the interface probably does not extend over greater than 1.5 nm. Although this was the smallest value obtained for any of the samples studied, we believe that it was
ANALYSIS OF THE ITO
- I n P INTERFACE
89
cycl'esl~
i
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i
ITO
InMNN
i
i
,,a,. ..t,, "" InxOv';" InP '~
0
a.
i
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6
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88
Sputtering
PLMM
110
Time
'°'
I 387
132
(min)
I
1
J
I i "
I
I
393 399 405 411 K i n e t i c E n e r g y (eV)
Fig. 1. Elemental profile through the I T O - I n P interface of a sample which had been heated at 200 °C for 5 min in flowing nitrogen. Assuming that the m i n i m u m in the indium profile indicates the actual junction, then we can calculate the sputtering rate to be about 0.55 n m min - 1. Fig. 2. Line scans of In M N N through the I T O - I n P interface showing that the transition from one indium signal to another (i.e. I T O to InP) takes place over about 5.5 min. Hence the region of mixing must be of the order of 3.0 nm.
probably the most realistic estimate since all the other factors mentioned above would have led to an apparent broadening. It is difficult to suggest mechanisms by which too small an estimate would have been obtained. In Fig. 3 we see the change in the P K L L line through the interface (from 1859 to 1867 eV). This sample had been heated at 400 °C. The shift in energy of 8 eV is very 7
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1836
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Fig. 3. Line scans of P K L L through the I T O - I n P interface of a sample which had been heated at 400 °C for 5 min in flowing nitrogen. The shift in the binding energy of 8 eV is associated with the formation of interfacial P205.
90
A. SWARTZLANDER et
al.
indicative of the formation of P 2 0 5 at the interface 7. This is important from the perspective of device fabrication since such interfacial layers are usually undesirable. In Fig. 1 the profile of the Sn M N N line over a considerable distance is shown, from which it is apparent that this element penetrates considerably into the InP. The tin profile did not appear to be influenced by post-deposition heating; an observation for which we do not offer an explanation. However, this may be of central importance to the operation of these devices since tin in this concentration would certainly be expected to compensate the p-type surface of the InP and, therefore, to form a buried homojunction. 4. CONCLUSIONS The following conclusions can be drawn from this work. (i) The interfacial mixing of ITO and InP occurs over a distance of several atomic layers. (ii) The interfaces are chemically stable up to a temperature of approximately 400 °C, at which a thin layer of P2Os forms. (iii) In-diffusion of tin occurs even at low power deposition and without heating. In actual devices this could cause a heavily compensated surface layer of high resistivity which could mar device performance 4 on lightly doped substrates. REFERENCES 1 A. Yamamoto, M. Yamaguchi and C. Uemura, Appl. Phys. Lett., 44 (1984) 611. 2 M.J. Tsai, A. L. Fahrenbruch and R. H. Bube, J. Appl. Phys., 51 (1980) 2296. 3 J. Shewchun, J. Loferski, R. Singh, J. Dubow and M. Spitzer, Proc. 13th IEEE Photovoltaic Specialists' Conf., Washington, DC, 1978, IEEE, New York, 1978, p. 528. 4 T.J. Coutts and S. Naseem, Appl Phys. Lett., 46 (1985) 154. 5 T.J. Coutts and N. M. Pearsall, Proc. Int. Workshop on the Physics of Semiconductor Devices, New Delhi, 1981, p. 380. 6 T.J. Coutts, N. M. Pearsall and L. Tarricone, J. Vac. Sci. Technol., 2 (1984) 140. 7 C.W. W i l m s e n a n d R . W. Kee, J. Vac. Sci. Technol.,14 (1977) 953.
87
PREPARATION AND CHARACTERIZATION
ANALYSIS O F T H E I N T E R F A C E O F I N D I U M TIN O X I D E W I T H InP A. SWARTZLANDER AND T. J. COUTTS
Solar Energy Research Institute, Golden, CO 80401 (U.S.A.) S. NASEEM
Newcastle Polytechnic, Newcastle Upon Tyne NE1 8 S T (Gt. Britain) T. P. MASSOPUST
Rocky Mountain Analytical Research Laboratories, Golden, CO 80401 (U.S.A.) (Received May 5, 1985; accepted September 16, 1985)
This paper is concerned with the analysis of interfaces of thin films of indium tin oxide (ITO) deposited by r.f. sputtering onto single-crystal substrates of indium phosphide (InP). Scanning Auger microscopy and depth profiling have been used to examine the abruptness of the interface between the two materials. The depth over which the indium line shifts from an energy characteristic of ITO to one of InP has been used as the principal indicator in this work. For very low rates of deposition of the ITO, and without post-deposition heating, the depth over which this transition occurs has been found to be as small as 3.0 nm. Changes in the position of the phosphorus line indicate that P205 forms at the interface when the samples are heated to 400 °C following deposition.
1. INTRODUCTION Although single-crystal InP may be too expensive for consideration as the absorber in solar cells for terrestrial application, there are several reasons why it may have potential for space applications. Firstly, it has a higher theoretical efficiency than silicon; secondly, the decrease in efficiency with temperature should be less than that of silicon; thirdly, its radiation resistance has been demonstrated as being superior to that of either silicon or gallium arsenide (GaAs) cells 1. Although various models of operation of the I T O / I n P cell (where ITO is indium tin oxide) have been proposed 2'3, it is still not entirely clear whether these cells operate as true heterojunctions, as buried homojunctions or even as semiconductor/insulator/semiconductor (SIS) junctions. Despite this lack of a thorough understanding, efficiencies of nearly 16% total area have recently been achieved 4 but it is clear that further progress toward the ultimate realizable efficiency of about 23% will depend on removing this deficiency. The work described in this note is intended to clarify interface processes, and particularly those related to the influence of the ITO deposition process and the effects of post-deposition heating. 0040-6090/86/$3.50
© ElsevierSequoia/Printedin The Netherlands
88
2.
A. SWARTZLANDER e t
al.
EXPERIMENTAL PROCEDURE
The single-crystal material was supplied by Mining and Chemical Products (Electronic Materials) Ltd. It was (100) oriented, zinc doped (2 x 1015 cm -3) and polished on the B face. Cleaning of individual substrates prior to I T O deposition was carried out using standard procedures 5 and the I T O deposition schedule was as follows. (i) Pre-sputter the I T O target (supplied by Nordike Ltd.; purity, 99.99~o) for 30min at a power density of approximately 7 W c m -2 to remove surface contamination. (ii) Reduce the power level so that no deposition can be detected (about 0.4 W c m - 2). (iii) Open the shutter for 30 min without increasing the power 6. (iv) Increase the power density to 0.7 W c m - 2 corresponding to a deposition rate of approximately 0.03 nm s - 1. The thickness of the films used in this work was 50.0 nm although working devices utilize greater thicknesses than this. Six samples of I T O / I n P were prepared in this way. One was analyzed by scanning Auger microscopy (SAM) without further treatment but the other five were heated in flowing nitrogen for 5 min at 100-500 °C before SAM analysis. Auger depth profiling was performed using a P e r k i n - E l m e r model 595 scanning Auger microprobe. During profiling the In M N N , the P K L L and the Sn M N N lines were monitored. The rate of sample erosion was approximately 0.5-0.6 nm m i n - 1, as estimated from the time at which the tin profile began to decrease significantly and the knowledge that the I T O was 50.0 nm thick. A line scan of the In M N N and P K L L lines was taken at intervals of approximately 40-50 s; hence the resolution of the technique is of the order of 0.5 nm. 3.
RESULTS AND DISCUSSIONS
A profile through the interface of the sample which had been heated at 200 °C is shown in Fig. 1. Since the binding energy of indium in I T O is different from that of indium in I n P (397 and 401 eV and 404 and 411 eV respectively) we can estimate the depth over which the transition from pure I T O to pure I n P takes place. In Fig. 2 we see scans of the In M N N line taken at various time intervals, again for the sample heated at 200 °C. The first significant change in the line shape and energy at which the minimum occurs is evident at 89.26 min (cycle 53). After 94.71 min (cycle 55) the change is complete. On this basis, the extent of the mixing of I T O and I n P is about 3.0 nm. However, this should be regarded as an overestimate for several reasons. Firstly, the Auger electrons escape from depths below the actual surface; secondly, the bombarding ions have a specific range depending on the sample and the beam energy; thirdly, the output is not continuous but discrete; fourthly, the result is influenced by surface roughness. The correction for the emission depth plus the ion range is of the order of 1.5 nm for a 1 keV beam 7. In our case, the beam energy was 4.5 keV so the ion range may have been somewhat different. Hence, the mixed region near the interface probably does not extend over greater than 1.5 nm. Although this was the smallest value obtained for any of the samples studied, we believe that it was
ANALYSIS OF THE ITO
- I n P INTERFACE
89
cycl'esl~
i
[
i
ITO
InMNN
i
i
,,a,. ..t,, "" InxOv';" InP '~
0
a.
i
f
i
I
\ (~,2X,~s) /
6
i
.....
^
PKL~
m ID
ffl ul <
I
44
66
~./
88
Sputtering
PLMM
110
Time
'°'
I 387
132
(min)
I
1
J
I i "
I
I
393 399 405 411 K i n e t i c E n e r g y (eV)
Fig. 1. Elemental profile through the I T O - I n P interface of a sample which had been heated at 200 °C for 5 min in flowing nitrogen. Assuming that the m i n i m u m in the indium profile indicates the actual junction, then we can calculate the sputtering rate to be about 0.55 n m min - 1. Fig. 2. Line scans of In M N N through the I T O - I n P interface showing that the transition from one indium signal to another (i.e. I T O to InP) takes place over about 5.5 min. Hence the region of mixing must be of the order of 3.0 nm.
probably the most realistic estimate since all the other factors mentioned above would have led to an apparent broadening. It is difficult to suggest mechanisms by which too small an estimate would have been obtained. In Fig. 3 we see the change in the P K L L line through the interface (from 1859 to 1867 eV). This sample had been heated at 400 °C. The shift in energy of 8 eV is very 7
...
6 - ', .•° "", , , :
•
-., ".- "" / .f
,
\ .~
°
...: ,, ."
*,*, C y c l e 8 -
uJ
4
z "o
3
°,
,-,
..."/
2
1 0
1836
1844
1852 1860 Kinetic energy
1868
Fig. 3. Line scans of P K L L through the I T O - I n P interface of a sample which had been heated at 400 °C for 5 min in flowing nitrogen. The shift in the binding energy of 8 eV is associated with the formation of interfacial P205.
90
A. SWARTZLANDER et
al.
indicative of the formation of P 2 0 5 at the interface 7. This is important from the perspective of device fabrication since such interfacial layers are usually undesirable. In Fig. 1 the profile of the Sn M N N line over a considerable distance is shown, from which it is apparent that this element penetrates considerably into the InP. The tin profile did not appear to be influenced by post-deposition heating; an observation for which we do not offer an explanation. However, this may be of central importance to the operation of these devices since tin in this concentration would certainly be expected to compensate the p-type surface of the InP and, therefore, to form a buried homojunction. 4. CONCLUSIONS The following conclusions can be drawn from this work. (i) The interfacial mixing of ITO and InP occurs over a distance of several atomic layers. (ii) The interfaces are chemically stable up to a temperature of approximately 400 °C, at which a thin layer of P2Os forms. (iii) In-diffusion of tin occurs even at low power deposition and without heating. In actual devices this could cause a heavily compensated surface layer of high resistivity which could mar device performance 4 on lightly doped substrates. REFERENCES 1 A. Yamamoto, M. Yamaguchi and C. Uemura, Appl. Phys. Lett., 44 (1984) 611. 2 M.J. Tsai, A. L. Fahrenbruch and R. H. Bube, J. Appl. Phys., 51 (1980) 2296. 3 J. Shewchun, J. Loferski, R. Singh, J. Dubow and M. Spitzer, Proc. 13th IEEE Photovoltaic Specialists' Conf., Washington, DC, 1978, IEEE, New York, 1978, p. 528. 4 T.J. Coutts and S. Naseem, Appl Phys. Lett., 46 (1985) 154. 5 T.J. Coutts and N. M. Pearsall, Proc. Int. Workshop on the Physics of Semiconductor Devices, New Delhi, 1981, p. 380. 6 T.J. Coutts, N. M. Pearsall and L. Tarricone, J. Vac. Sci. Technol., 2 (1984) 140. 7 C.W. W i l m s e n a n d R . W. Kee, J. Vac. Sci. Technol.,14 (1977) 953.