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Surface damage to InP substrates during r.f. sputtering
Thin SolidFilms,80(1981)
177-181
177
METALLURGICAL AND PROTECTIVE COATINGS
SURFACE DAMAGE TO InP SUBSTRATES SPUTTERING*
DURING
R.F.
N. M. PEARSALL, T. J. COUTTS AND R. HILL School of Physics, Newcastle upon Tyne Polytechnic
(Gt. Britain)
G. J. RUSSELL Department
of Applied Physics, University of Durham (Ct. Britain)
K. J. LAWSON Department
of Materials,
Cranfied Institute of Technology (Gt. Britain)
In connection with our work on InP-based solar cells using r.f. sputtering to deposit the n layer of the cell, we investigated the effect of the plasma on an InP substrate. Reflection electron diffraction studies indicated the presence of a polycrystalline InP surface layer for some samples held in the plasma for 2-3 min. This layer was also found for samples heated in flowing argon depending upon crystal orientation, which indicates that the effects seen in the substrates may be thermal in origin. Substrate samples were also compared with InP crystals which had been sputter etched.
1. INTRODUCTION
InP is a direct band gap semiconductor which is particularly well suited for solar cell applications owing to its high absorption coefficient in the visible wavelength range. It has accordingly been used in heterojunction devices in conjunction with a wide band gap semiconductor which is usually CdS or indium tin oxide (ITO)le3. The sunlight passes through the window layer (wide band gap semiconductor) and is absorbed in the narrow band gap material very close to the interface. Thus the conditions at the interface between the two materials are very important for the efficient operation of the device. In our work on InP-based solar cells we have used r.f. sputtering to deposit thin films of both CdS and IT0 on crystalline p-type InP substrates. This deposition technique is particularly applicable to the manufacture of solar cells owing to its low energy requirement and excellent materials usage. Workers at Stanford University3 have claimed that the sputtered ITO/p-InP solar cells which they have produced are, in fact, acting as InP homojunctions with the IT0 behaving only as a low resistance current-collecting layer. They suggest that this homojunction is formed by type conversion at the InP surface owing to bombardment during sputter deposition in the same way that sputter etching of a p-type crystal can produce an n layer at the surface. Therefore, in the course of attempting to characterize our devices, we felt *Paper presented at the 3rd International Conference on Ion and Plasma Assisted Techniques, Amsterdam, The Netherlands, June 3Oduly 2,1981. 0040-6090/8 1/OOOO-0000/$02.50
0 Elsevier Sequoia/Printed in The Netherlands
178
N. M. PEARSALL et al.
that the effect of the plasma on the surface of the InP during deposition should be investigated and this paper reports our preliminary results in this study. 2.
EXPERIMENTAL DETAILS
As-sawn p-type InP wafers were supplied by the Royal Signals and Radar Establishment, Malvern. These were zinc doped to a level of approximately 3 x 1016 crnm3 and could best be described as multicrystalline with a grain size of about 5 mm. The samples used in this study were generally about 0.25 cm2 in area and therefore were effectively single crystal although of unknown and random orientation. The experiments were later repeated using semi-insulating {lOO} InP. The as-sawn wafers were mechanically polished to a mirror finish and then etched in 1 vol.% 5 N bromine in methanol for a time necessary to produce a pattern indicative of a good single crystal when analysed using reflection electron diffraction (RED). The experiments were carried out in a Nordiko r.f. sputtering rig, although the depositon chamber was built to our own design. Argon was used as the sputtering gas and for all the experiments was kept at a pressure of 5 x 10T3 Torr. The system utilized a 1.25 kW 13.56 MHz r.f. generator, although the maximum power applied was 0.38 kW giving a power density of about 8 W cm-’ and a d.c. bias of about 2 kV on the target. This was achieved after a power wind-up taking approximately 1 min. The InP samples were secured to a water-cooled copper work table using silver conducting paint. Simulation of the deposition conditions required that the sample should be in the plasma and it was found that this could not be done without exposing the sample to material being sputtered from the target. In order to prevent a film being deposited on the sample the target was covered with aluminium foil since aluminium has a very low sputter yield. In fact, no aluminium could be detected on the sample after 5 min deposition using the covered target. The aluminium cover resulted in a reflected power from the target which was much higher than normal (40 W compared with less than 10 W) and therefore full power could not be reached. However, under usual cell fabrication conditions the InP would be protected by the depositing film before full power had been attained and it was therefore felt that the first minute of deposition could be adequately simulated. InP samples were also sputter etched by placing them on the target and striking the plasma for a few minutes. The structural effects of the experiments were investigated using a scanning electron microscope (SEM) and the surface properties were analysed by RED. Results were compared with those from samples which had undergone the heating step required for production of ohmic contacts to InP. This step is usually carried out before junction fabrication and involves annealing the wafers in flowing argon (2 1min- ‘) for 10 min at 350 “C. 3.
RESULTS
The samples which had been sputter etched showed dramatic effects which were visible without the aid of a microscope. After etching for 2 rnin at 8 W cm- 2 the surface of the sample appeared to be covered with a soft matt grey film. Closer inspection using the SPM showed this to consist of columns protruding from the surface of the crystal, some with spherical nodules at the top. The height of the
InP SUBSTRATE DAMAGE DURING R.F. SPUTTERING
179
columns was approximately 1 pm as can be seen in Figs. 1 and 2 which show two examples of sputter-etched samples. Figure 1 is taken from the (100) semi-insulating InP, whilst Fig. 2 shows a similar effect on a p-InP sample of unknown orientation. The {100) sample exhibits both a greater nodule density and a larger column apex angle. Energy-dispersive analysis by X-rays of the columns showed that they were indium rich with respect to the crystal base.
Fig. 1. Micrograph of {100) InP sputter etched for 2 min at 8 W cme2. Fig. 2. Micrograph of p-type InP sputter etched for 2 min at 8 W cmw2.
Figure 3 shows RED patterns of the sample whose surface is shown in Fig. 1. This is seen to contain an array of diffraction spots confirming the (lOO} orientation together with a number of polycrystalline rings. The more intense of these were identified as arising from metallic indium while the remainder could be attributed to InP. The RED patterns from p-type samples were predominantly InP. Those samples which had been held in the substrate position for up to 2 min in the plasma showed no topographical change of the surface. Figure 4 shows a typical substrate surface and it can be seen to be quite different from that of the sputter-
Fig. 3. RED pattern from a sputter-etched {100) sample. Fig. 4. Micrograph of a {100) InP substrate.
180
N. M. PEARSALL et a/.
etched sample shown in Fig. 3. Both micrographs were taken at the same magnilication. Compositional analysis of the surface using Rutherford backscattering is being undertaken. RED studies indicated the presence of polycrystalline InP at the surface (Fig. 5) in some cases. Polycrystalline rings can be clearly seen along with the single-crystal background. Some samples which had been heated in flowing argon ambient showed similar polycrystalline RED patterns depending upon surface orientation. Most of the ( 100) samples appeared to be unaffected by the heating. The differences may be partly attributed to the different sutiace topographies after etching.
Fig. 5. RED pattern from a p-type InP substrate.
4. DISCUSSION It is evident from the micrographs that the effect of the plasma on an InP substrate is very different from that of sputter etching. This is hardly surprising since a sample on the target has a potential of 2 kV and therefore will be bombarded by much higher energy ions than a sample which is in contact with the earthed work table will be. Severe effects can be seen on the sputter-etched sample (Fig. 1) which could be due to either preferential sputtering or loss of phosphorus due to the high temperature which the crystal surface reaches. Most likely it is a combination of these two mechanisms. RED confirms that the spherical nodules whichcan be seen at the tips of many of the columns are composed of metallic indium, indicating a substantial loss of phosphorus. The similarity of results obtained from substrate samples and from some of the samples heated in a furnace strongly suggests that the effects seen on the substrates are thermal in origin. The cause of this heating is as yet uncertain but could be electron and/or ion bombardment or simply r.f. heating of the crystal. The formation of a polycrystalline surface layer will modify the operation of any device formed by r.f. sputter depositing on a crystalline InP substrate and confirmation of the presence of this layer may lead to better understanding of any such devices. Since the effects seem to be very dependent on orientation it may also be possible to choose a substrate orientation to minimize the effect of the plasma. Although we have not investigated other semiconductors so far, it seems likely that similar effects may be found for compounds of similar stability.
IIlP SUBSTRATE DAMAGE DURING R.F. SPUTTERING
181
REFERENCES 1 J. L. Shay, S. Wagner, M. Bettini, K. J. Bachmann and E. Buehler, IEEE Trans. Electron Devices, 24 (4) (1977) 483-486. 2 A. Yoshikawa and Y. Sakai, Solid-State Electron., 20 (2) (1977) 133-139. 3 R. H. Bube., F. G. Courreges, A. L. Fahrenbruch and M. J. Tsai, Proc. 2nd European Communities Photovoltaic Solar Energy Co& Berlin, April 23-26, 1979, Reidel, Dordrecht, The Netherlands, 1979, pp. 432-439.
177-181
177
METALLURGICAL AND PROTECTIVE COATINGS
SURFACE DAMAGE TO InP SUBSTRATES SPUTTERING*
DURING
R.F.
N. M. PEARSALL, T. J. COUTTS AND R. HILL School of Physics, Newcastle upon Tyne Polytechnic
(Gt. Britain)
G. J. RUSSELL Department
of Applied Physics, University of Durham (Ct. Britain)
K. J. LAWSON Department
of Materials,
Cranfied Institute of Technology (Gt. Britain)
In connection with our work on InP-based solar cells using r.f. sputtering to deposit the n layer of the cell, we investigated the effect of the plasma on an InP substrate. Reflection electron diffraction studies indicated the presence of a polycrystalline InP surface layer for some samples held in the plasma for 2-3 min. This layer was also found for samples heated in flowing argon depending upon crystal orientation, which indicates that the effects seen in the substrates may be thermal in origin. Substrate samples were also compared with InP crystals which had been sputter etched.
1. INTRODUCTION
InP is a direct band gap semiconductor which is particularly well suited for solar cell applications owing to its high absorption coefficient in the visible wavelength range. It has accordingly been used in heterojunction devices in conjunction with a wide band gap semiconductor which is usually CdS or indium tin oxide (ITO)le3. The sunlight passes through the window layer (wide band gap semiconductor) and is absorbed in the narrow band gap material very close to the interface. Thus the conditions at the interface between the two materials are very important for the efficient operation of the device. In our work on InP-based solar cells we have used r.f. sputtering to deposit thin films of both CdS and IT0 on crystalline p-type InP substrates. This deposition technique is particularly applicable to the manufacture of solar cells owing to its low energy requirement and excellent materials usage. Workers at Stanford University3 have claimed that the sputtered ITO/p-InP solar cells which they have produced are, in fact, acting as InP homojunctions with the IT0 behaving only as a low resistance current-collecting layer. They suggest that this homojunction is formed by type conversion at the InP surface owing to bombardment during sputter deposition in the same way that sputter etching of a p-type crystal can produce an n layer at the surface. Therefore, in the course of attempting to characterize our devices, we felt *Paper presented at the 3rd International Conference on Ion and Plasma Assisted Techniques, Amsterdam, The Netherlands, June 3Oduly 2,1981. 0040-6090/8 1/OOOO-0000/$02.50
0 Elsevier Sequoia/Printed in The Netherlands
178
N. M. PEARSALL et al.
that the effect of the plasma on the surface of the InP during deposition should be investigated and this paper reports our preliminary results in this study. 2.
EXPERIMENTAL DETAILS
As-sawn p-type InP wafers were supplied by the Royal Signals and Radar Establishment, Malvern. These were zinc doped to a level of approximately 3 x 1016 crnm3 and could best be described as multicrystalline with a grain size of about 5 mm. The samples used in this study were generally about 0.25 cm2 in area and therefore were effectively single crystal although of unknown and random orientation. The experiments were later repeated using semi-insulating {lOO} InP. The as-sawn wafers were mechanically polished to a mirror finish and then etched in 1 vol.% 5 N bromine in methanol for a time necessary to produce a pattern indicative of a good single crystal when analysed using reflection electron diffraction (RED). The experiments were carried out in a Nordiko r.f. sputtering rig, although the depositon chamber was built to our own design. Argon was used as the sputtering gas and for all the experiments was kept at a pressure of 5 x 10T3 Torr. The system utilized a 1.25 kW 13.56 MHz r.f. generator, although the maximum power applied was 0.38 kW giving a power density of about 8 W cm-’ and a d.c. bias of about 2 kV on the target. This was achieved after a power wind-up taking approximately 1 min. The InP samples were secured to a water-cooled copper work table using silver conducting paint. Simulation of the deposition conditions required that the sample should be in the plasma and it was found that this could not be done without exposing the sample to material being sputtered from the target. In order to prevent a film being deposited on the sample the target was covered with aluminium foil since aluminium has a very low sputter yield. In fact, no aluminium could be detected on the sample after 5 min deposition using the covered target. The aluminium cover resulted in a reflected power from the target which was much higher than normal (40 W compared with less than 10 W) and therefore full power could not be reached. However, under usual cell fabrication conditions the InP would be protected by the depositing film before full power had been attained and it was therefore felt that the first minute of deposition could be adequately simulated. InP samples were also sputter etched by placing them on the target and striking the plasma for a few minutes. The structural effects of the experiments were investigated using a scanning electron microscope (SEM) and the surface properties were analysed by RED. Results were compared with those from samples which had undergone the heating step required for production of ohmic contacts to InP. This step is usually carried out before junction fabrication and involves annealing the wafers in flowing argon (2 1min- ‘) for 10 min at 350 “C. 3.
RESULTS
The samples which had been sputter etched showed dramatic effects which were visible without the aid of a microscope. After etching for 2 rnin at 8 W cm- 2 the surface of the sample appeared to be covered with a soft matt grey film. Closer inspection using the SPM showed this to consist of columns protruding from the surface of the crystal, some with spherical nodules at the top. The height of the
InP SUBSTRATE DAMAGE DURING R.F. SPUTTERING
179
columns was approximately 1 pm as can be seen in Figs. 1 and 2 which show two examples of sputter-etched samples. Figure 1 is taken from the (100) semi-insulating InP, whilst Fig. 2 shows a similar effect on a p-InP sample of unknown orientation. The {100) sample exhibits both a greater nodule density and a larger column apex angle. Energy-dispersive analysis by X-rays of the columns showed that they were indium rich with respect to the crystal base.
Fig. 1. Micrograph of {100) InP sputter etched for 2 min at 8 W cme2. Fig. 2. Micrograph of p-type InP sputter etched for 2 min at 8 W cmw2.
Figure 3 shows RED patterns of the sample whose surface is shown in Fig. 1. This is seen to contain an array of diffraction spots confirming the (lOO} orientation together with a number of polycrystalline rings. The more intense of these were identified as arising from metallic indium while the remainder could be attributed to InP. The RED patterns from p-type samples were predominantly InP. Those samples which had been held in the substrate position for up to 2 min in the plasma showed no topographical change of the surface. Figure 4 shows a typical substrate surface and it can be seen to be quite different from that of the sputter-
Fig. 3. RED pattern from a sputter-etched {100) sample. Fig. 4. Micrograph of a {100) InP substrate.
180
N. M. PEARSALL et a/.
etched sample shown in Fig. 3. Both micrographs were taken at the same magnilication. Compositional analysis of the surface using Rutherford backscattering is being undertaken. RED studies indicated the presence of polycrystalline InP at the surface (Fig. 5) in some cases. Polycrystalline rings can be clearly seen along with the single-crystal background. Some samples which had been heated in flowing argon ambient showed similar polycrystalline RED patterns depending upon surface orientation. Most of the ( 100) samples appeared to be unaffected by the heating. The differences may be partly attributed to the different sutiace topographies after etching.
Fig. 5. RED pattern from a p-type InP substrate.
4. DISCUSSION It is evident from the micrographs that the effect of the plasma on an InP substrate is very different from that of sputter etching. This is hardly surprising since a sample on the target has a potential of 2 kV and therefore will be bombarded by much higher energy ions than a sample which is in contact with the earthed work table will be. Severe effects can be seen on the sputter-etched sample (Fig. 1) which could be due to either preferential sputtering or loss of phosphorus due to the high temperature which the crystal surface reaches. Most likely it is a combination of these two mechanisms. RED confirms that the spherical nodules whichcan be seen at the tips of many of the columns are composed of metallic indium, indicating a substantial loss of phosphorus. The similarity of results obtained from substrate samples and from some of the samples heated in a furnace strongly suggests that the effects seen on the substrates are thermal in origin. The cause of this heating is as yet uncertain but could be electron and/or ion bombardment or simply r.f. heating of the crystal. The formation of a polycrystalline surface layer will modify the operation of any device formed by r.f. sputter depositing on a crystalline InP substrate and confirmation of the presence of this layer may lead to better understanding of any such devices. Since the effects seem to be very dependent on orientation it may also be possible to choose a substrate orientation to minimize the effect of the plasma. Although we have not investigated other semiconductors so far, it seems likely that similar effects may be found for compounds of similar stability.
IIlP SUBSTRATE DAMAGE DURING R.F. SPUTTERING
181
REFERENCES 1 J. L. Shay, S. Wagner, M. Bettini, K. J. Bachmann and E. Buehler, IEEE Trans. Electron Devices, 24 (4) (1977) 483-486. 2 A. Yoshikawa and Y. Sakai, Solid-State Electron., 20 (2) (1977) 133-139. 3 R. H. Bube., F. G. Courreges, A. L. Fahrenbruch and M. J. Tsai, Proc. 2nd European Communities Photovoltaic Solar Energy Co& Berlin, April 23-26, 1979, Reidel, Dordrecht, The Netherlands, 1979, pp. 432-439.