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The electronic properties of cadmium sulphide thin films following low energy helium ion bombardment
Thin Solid Films
Elsevier Sequoia
S.A., Lausanne
THE ELECTRONIC PROPERTIES THIN FILMS FOLLOWING LOW ION
- Printed
255
in Switzerland
OF CADMIUM SULPHIDE ENERGY HELIUM
BOMBARDMENT
THOMAS
AMBRIDGE”
Department
(Received
AND
of Electrical
November
GEORGE
Engineering,
CARTER
University
of Salford,
Salford
A45 4 WT, Lanes.
(Gt. Britain)
20. 1971)
SUMMARY
Cadmium sulphide thin films, held at - 180°C within an ultra-highvacuum chamber, were bombarded with helium ions, whose energy was in the range (18-125 eV) within which the threshold for the displacement of cadmium and sulphur atoms was expected to lie. Electrical conductivity changes were observed at the temperature of bombardment, as a function of ion energy and dose, and in addition the conductivity was continuously monitored during subsequent annealing. A thermally stimulated conductivity peak was noted, and tentatively ascribed to the release of electrons, trapped at isolated sulphur vacancies created near the film surface by the ion bombardment; also, an irreversible change in conductivity indicated annealing of these defects below room temperature. Because the effects being investigated were confined to the surface of the films, the influence of contaminant gases, even in the ultra-highvacuum range, could not be neglected; in particular the possibility of interactions with adsorbed
hydrogen
has been considered.
1. INTRODUCTION
The electronic properties of intrinsic point defects are of great interest in the characterisation of semi-conductor materials. A convenient means of introducing such defects into bulk material is high energy electron irradiation’. A technique applicable to the production of surface defects is low energy inert ion bombardment; in this case, if the ion energies used are in the region of the threshold values required to effect atomic displacements, the effects will be confined to within one or two atom layers of the surface. * Present
address:
Post Office Research
Thin Solid Films, 10 (1972) 255-264
Department,
Dollis
Hill, London
NW2 7DT (Gt. Britain).
256
T. AMBRIDGE.
G. CARTER
Hughes and Carter’ have reported pronounced changes in the electrical resistivity of CdS thin films subjected to 250 eV argon ion bombardment, at room temperature. The present study also examines changes in the electrical resistivity of CdS films, although, in this case, helium ions in the energy range 18 eV to 125 eV were employed and the target maintained at a temperature of - 180 “C during bombardment. Thermally stimulated plotted during subsequent heating, to study electronic and their annealing characteristics. 2.
conductivity curves were properties of the defects,
EXPERIMENTAL
2.1. Apparatus
and materials
Polycrystalline cadmium sulphide films were produced in situ in the ion bombardment target chamber by evaporation onto glass substrates previously provided with chromium contact pads separated by a 1 mm gap. A partiallyenclosed inner deposition chamber system was used, with walls heated to 150 “C, substrate heated to 250 “C, and high purity source material held at 750 “C within a quartz crucible. 9 cm below the substrate. The residual pressure during deposition was in the low lo-’ torr region; following deposition the pressure was reduced to below lop9 torr and maintained in the ultra-high-vacuum range throughout the course of the subsequent series of experiments, except when helium was admitted for ion bombardment. Details of the apparatus are given elsewhere3. Three different samples were used in the experiments described below: these were tilm “ M “, which had a thickness of 1400 A, and a room temperature dark resistivity of 1.5 x 1O4 Q cm, film “ N ” (900 A, 3 X lo3 52 cm), and film “ 0 ” (800 A, 3 x10’
Q cm). All films had dark
resistivities
greater
than
10”
52 cm
at - 180 ‘C. 2.2. Electrical conductance measurements Exploratory experiments with CdS film “M”
showed
that, when the target
was maintained at - 180 “C, and bombarded with helium ions (He+) with energies between 25 and 125 eV, the electrical conductivity (under weak illumination from the ion gun filament) increased sharply initially and continued to increase steadily with increasing ion dose, reaching a value two to four orders of magnitude greater than the pre-bombardment value, for doses of the order 1015 ions cm-‘. Following each bombardment. the target was heated to 350°C and the conductivity at - 180°C invariably returned to the pre-bombardment value (within *20”,,). At first it appeared that the dependence of the magnitude of the conductance change on the ion energy was random and irreproducible, although, after a large number of experimental runs, a pattern of behaviour was revealed and subsequently confirmed by a more systematic investigation of films “N” and “ 0 “. Thin Solid Films. 10 (1972)
255-264
ELECTRONIC
PROPERTIES
The following to study
OF
2.57
CdS FILMS
series of bombardments
the anomalous
ion energy
was carried
dependence:
out on film “ N ” in order
Each
morning
the film was
heated to 350 “C, after standing in a vacuum of about 5 X lo- lo torr overnight, cooled to - 180 “C, and bombarded with 5 x 1014 monoenergetic He+ ions per cm2. Immediately afterwards the cycle was repeated at least once, ions of the same energy as in the first bombardment being used for the second bombardment. Following the last bombardment each day, heating to 350 “C was carried out and the sample cooled to room temperature. The ion energy was increased daily in steps of 10 eV, from 25 eV to 125 eV. In each case the increase in conductivity resulting from the first bombardment each day was notably greater than that resulting from the second bombardment at the same energy (typically by a factor of four). Figure 1 shows the increase in
-
Pre-
bombardment
corductance
I 1
I
I
I
I
2
3
4
5
He+ dose (ions cm-’
x 10-14)
Fig. 1. Conductance changes of CdS film “N” function of ion dose. -w 1st 25 eV bombardment, -n2nd 25 eV bombardment, -t_ 1st 45 eV bombardment, + 2nd 45 eV bombardment.
at - 180°C
during
He+
ion bombardment,
as a
conductance with ion dose for first and second bombardments using ion energies of 25 eV and 45 eV. It is clear that the rate of change of conductance was characThin Solid Films, 10 (1972) 255-264
258
T. AMBRIDGE.
G. CARTER
teristic of the energy although the total change was subject to some other influence. The final value of conductance following each first and second bombardment in the series is shown in Fig. 2. If all the first bombardments
He+
,on energy
as one set
(eV)
Fig. 2. Conductance change at - 180 ‘C following of ion energy: CdS “N “. @--I st bombardments. + 2nd bombardments, -_i--2nd bombardments. repeat series.
and the second
bombardments
definite
dependence
energy
are considered
He’ ion dose of 5 x 10’; ions cm-‘.
as a separate
set, as indicated
of the conductance
change
as a function
by the solid lines, a
is indicated.
Additional
confirmation of the energy dependence was obtained by carrying out third and sometimes fourth bombardments at the energy used on the same or previous day: these invariably produced changes similar to those resulting from second bombardments previously carried out at the same energies. Further, five more first and second bombardments were carried out, with daily decreasing ion energies: although the absolute magnitude of the effects had diminished, the relationship between first and second bombardments, and the energy dependence, was consistent with that earlier observed. (For clarity, only points relating to the second bombardments in this series are shown in Fig. 2.) It should be noted that bombardment with 18 eV He+ ions produced a change in conductance, although no detailed investigations were carried out since the maximum possible ion dose rate Thin Solid Films, 10 (1972) 255-264
ELECTRONIC
was a factor
PROPERTIES
OF
259
CdS FILMS
of three below the rate of about
10”
ions cm- 2 see- ’ used in the
energy range 25 eV to 125 eV. A similar series of bombardments was carried out on CdS film “O”, although the pre- and post-bombardment heating was limited to a maximum temperature of only 200 “C. It was felt that this would eliminate possible effects due to the desorption of hydrogen (and other contaminant gases) from the stainless steel cryostat, since partial pressures up to 10d8 torr had previously been noted during heating to 350 “C. The conductance changes obtained for doses of 5 x 1014 He+ ions cm- 2 are shown in Fig. 3, as a function of energy for first and second bom-
I----
He’ ionenergy
(ev)
Fig. 3. Conductance change at - 180 “C following of ion energy: CdS film “0”. -O-1st bombardments, __O_ 2nd bombardments.
He+ ion dose of 5 x lOI ions cm-2,
as a function
bardments. It is seen that, although the order of magnitude of the changes was similar to that for film “N”, the relationship between the two sets ofbombardments for film “0” was less consistent and the energy dependence less certain. Because of the lack of self-consistency, the latter results were probably less representative than the previous ones; however, the sharp initial increase in effect with increasing energy was still apparent and, in the case of the first bombardments, a maximum effect for an ion energy of 45 eV was in fair agreement with the previous result. Thin Solid Films, IO (1972) 255-264
260 2.3.
T. AMBRIDGE.
Thermally stimulated conductivity
Throughout - 180°C observing
the dark thermally
the heating conductance stimulated
runs
G. CARTER
measurements
which
followed
the ion bombardments
at
of the films was monitored with the object of conductivity (TSC) peaks, corresponding to the
emptying of electron traps created by the bombardment. Such traps would have become populated by electrons excited into the conduction band by the illumination from the ion gun filament; usually no additional source of illumination was employed. Invariably, a well defined peak was observed, at a temperature in the region of - 160” to - 140 “C, which appeared to be directly associated with the increased conductance following the bombardment. For the cases in which this increased conductance was least, no further peaks were observed and the minimum conductance following the peak was rarely more than a factor of three below the peak value; beyond the conductance valley a continuous rise in conductance, similar to that obtained from non-bombarded films. was observed. For those cases in which the increased current was largest, a second peak appeared above the background level, for all three CdS films investigated, in the range - 35 ’ to - 25 “C. This peak was seen following He+ ion doses of 5 x 1014 ions cm-’ with energies in the range 25 to 125 eV for film “ M ” , 25 to 55 eV for film “N” and 25 to 45 eV for film “0”. Greatly improved definition of the latter peak was obtained for 45 eV He’s bombardment of film “0” when the dose was increased to 1.7 x 1015 ions cm-‘. Two minor peaks, merging into this peak, were always apparent after such a dose. A typical TSC spectrum for this case is shown in Fig. 4. Also apparent in this spectrum is a peak at about +45 “C. This peak was never observed following the 5 x lOi cm-’ ion doses but was reproduced accurately in three TSC runs which followed successive 45 eV He+ ion doses of 1.7 x lOi ions cmP2, for film “ 0 *‘. None of the peaks described could be made to re-appear by illumination of the films at - 180 “C after cooling from the maximum temperature (at least 200 “C) of the previous TSC run. Tests were carried out to ascertain the nature of the major prominences in the TSC spectra: The peak at - 140 “C behaved as a genuine TSC peak, corresponding to electron trap emptying, in that it was fully reproducible after terminating the post-bombardment heating at - 55 “C, cooling the sample to - 180 “C, illuminating to fill the trap, and re-heating. In addition, the use of more intense illumination. from a filtered 500 watt tungsten lamp, increased the magnitude of the peak. indicating more efficient trap filling. The other peaks were unaffected by additional illumination and could not be made to re-appear using the above procedure. which suggests that this part of the so-called TSC spectrum did not relate to electron trapping phenomena but represented conductance changes corresponding to irreversible physical changes within the sample. In particular, it appeared that the peak at - 30 “C was associated with the elimination of the defects responsible for the - 140 “C peak, since it was not possible to observe the latter after heating Thin Solid Film-, 10 (1972) 255-264
ELECTRONIC
PROPERTIES
Tempwat
4 ’
0 1
’
’
5
’
’
OF
we
’
261
CdS FILMS
(‘0
8 ’
10
’
’
1 1 15
Timdmid
Fig. 4. Thermally stimulated conductance of CdS 1.7 X1015 ions cm-* at - 180°C. NB. Heater switched on at time t = 0 mins.
film “0”
following
45 eV He+
ion dose
of
the sample to - 15 “C, and subsequently cooling and illuminating as before. The - 140 “C peak was analysed, using a particularly well-defined example obtained
following intense illumination of film “ 0 “, after a dose of 1.7 x lOi of Luschik’s4 formula yielded an estimated trap He+ ions cm- 2. Application depth of 0.15 eV which was used as a trial value in attempting to fit a theoretical TSC curve, assuming the slow re-trapping approximation derived by Cowell and Woods’. The closest fit subsequently obtained suggested a trap depth of 0.17 eV. An electron capture cross-section of 6 x 10e2’ cm2 was calculated for the trap, which is consistent with the weak re-trapping assumption. Unfortunately, no estimate of the trap density could reasonably be made, since electron mobility and lifetime were unknown and further, it would have been difficult to judge the extent to which the trap was initially filled, in view of the isothermal decay at - 180 “C which appears to have been responsible for the observed enhanced conductance at this temperature. 2.4.
The influence of contaminant gases It was thought possible that the reason for the difference between the effects of the first and second bombardments at fixed energy was associated with a Thin Solid Films, 10 (1972) 255-264
262
T. AMBRIDGE.
G. CARTER
contaminant gas, adsorbed on the film surface, which was not desorbed by heating to 350°C and did not reveal any influence on electrical properties until ion bombardment was performed. In particular, hydrogen was suspected; in a subsidiary experiment6 it was shown that the conductance of a CdS film at - 180 “C was insensitive to the presence of molecular hydrogen (at 3 x 10d6 torr), although adsorbed atomic hydrogen caused an increase in conductance by many orders of magnitude. In the present case it is possible that molecular hydrogen was adsorbed on the films overnight (e.g. one monolayer, assuming unity sticking probability, at the measured partial pressure of 1 x lo- ‘O torr); ion bombardment could have dissociated the molecules and the atomic hydrogen would then have contributed to the increased conductance. Although this process has not been proved conclusively, further evidence was obtained for the influence of adsorbed hydrogen on the results. First, when the overnight partial pressure of hydrogen was deliberately increased to IO-* torr and then pumped away prior to a typical heating, cooling and 25 eV He+ bombardment schedule (two bombardments), the conductance changes were a factor of four above those observed on the previous day for 25 eV bombardments, although the ratio of the effects of the first and the second bombardments remained the same. Secondly, the irreversible conductance change noted at +45 “C, following higher dose bombardments, corresponds very closely to a similar feature we have seen when heating CdS films previously exposed to atomic hydrogen at - 180 “C and attributed to hydrogen desorption6. 3.
DISCUSSION
AND
CONCLUSION
The interpretation of the effects described is made difficult by the fact that contaminant gases appear to have added a significant contribution, in spite of the low background pressures maintained in the target chamber. This aspect in itself should be of some interest, especially in view of the likelihood that the ion beam was instrumental in activating a normally electrically inactive contaminant. However. it is interesting to attempt an interpretation of the results on the assumption that the effects of contaminants were secondary. The conductance increases observed at - 180 “C almost certainly represented isothermal release of electrons from donor type levels, about 0.17 eV below the conduction band, created by the ion bombardment. It is tempting to ascribe these to isolated sulphur vacancies; Woods and Nicholas7 have attributed a level at about 0.14 eV in CdS single crystals to the same source. A tentative explanation for the energy dependence of the magnitude of the conductance increases may be put forward in terms of the minimum ion energies necessary to create point defects by atomic displacement. On the basis of their high energy electron irradiation experiments, Kulp and Kelley’ and Kulp’ have deduced threshold displacement energies of 8.7 eV for sulphur atoms and 7.3 eV Thin Solid Films, 10 (1972) 255-264
ELECTRONIC
PROPERTIES OF
CdS FILMS
263
for cadmium
atoms in bulk CdS. Taking the case of maximum energy transfer (i.e. assuming head-on collisions), the He + ion energies sufficient to transfer these amounts are 22 eV and 55 eV respectively. Thus, the initial increase in effect with increasing energy should be associated with an increase in the number of sulphur displacements, as suggested above. Furthermore, the observed reversal for energies above 45 eV to 55 eV might be explained in terms of cadmium displacements, implying that the new defects introduce a compensation mechanism to the existing 0.17 eV level; this point will be returned to shortly. In view of the inconsistencies between the samples investigated, it would be premature to attempt to explain the behaviour relating to the higher energy bombardments; note that multiple defects would be possible in this range. It will be recalled that some increase in film conductance had been observed for He+ ion energies as low as 18 eV; this suggests that the threshold energy for sulphur displacements is lower in the surface region, than in bulk material. A similar reduction might be expected in the case of cadmium displacements; this would require the He + ion energy of 55 eV, corresponding to the maximum conductance change in film “ N “, to represent somewhat greater than the minimum value to effect cadmium displacements at the surface. This is feasible since the contribution to electrical properties would probably be too small to distinguish, very close to the threshold point. The conductance tierSUS temperature relationship following bombardment appears to contain information on defect annealing as well as indicating the existence of the 0.17 eV donor-type trapping level. It should be pointed out that the fact that the detailed structure of the higher temperature peaks was only observed in those cases when the initial peak was greatest does not imply that the features responsible were absent at other times, but rather that the general rise in conductance with temperature (not a result of bombardment) was sufficient to obscure their effects. Since the peak at - 30 “C was associated with the disappearance of the level responsible for the low temperature peak, it is proposed that this represents the onset of annealing of sulphur vacancy-interstitial pairs. This is in contrast to the results of Kulp and Kelley’ who were able to produce defects in CdS by electron bombardment at room temperature and who reported that the fluorescence attributed to sulphur interstitials was sometimes still present after heating to 400 “C. However, a significant difference in annealing behaviour would be expected from the fact that, whereas the electron-induced damage was distributed throughout the thickness of a CdS platelet, the ion-induced damage in the present study was confined to the surface layer, with a very high defect density resulting from at least 0.5 bombarding ions per surface atom. Little can be said about the two additional prominences superimposed upon the -30°C peak following the heaviest doses, since these could not be studied in isolation; it is possible that these were associated with the defect annealing process to which the major peak was attributed. Thin Solid Films, 10 (1972) 255-264
264
T. AMBRIDGE.
No new characteristic of heating,
following
features
were observed
the higher energy
in the conductance
bombardments,
G. CARTER
as a function
which could be assigned
to cadmium vacancy or interstitial defects, apart from the decrease in magnitude of the previously noted peaks. This implies that any electron traps produced were at levels close to the centre region of the band-gap; these would capture electrons at the expense of the shallow level at 0.17 eV and subsequent thermal release would occur at temperatures where the “ background” conductance was too high to permit observation. Thus, no detailed conclusions can be reached regarding the trapping or annealing characteristics of the defects. The consistent results on film “N” suggest that complete annealing was achieved during the heating to 350 “C, whereas the less consistent results obtained from film “ 0 “, when heating was limited to 200 “C, may have arisen from incomplete annealing. It has been shown that the surface nature of the interactions studied presents considerable experimental and interpretative difficulties; in particular, it must be emphasised that the influence of contaminant gases, even in the ultra-high vacuum range, is sufficiently important to make it necessary to regard most of the interpretation of results as extremely speculative. Further studies along the lines described here, including extending the ion energy range downwards, should be valuable, if combined with other techniques, such as Auger spectroscopy to detect surface contamination, and spectral luminescence measurements to determine the characteristics of defect levels at the temperature of bombardment. ACKNOWLEDGMENTS
One of us (T.A.) wishes to thank the Science Research Council financial support, in the form of a Ph.D. studentship grant.
for providing
REFERENCES
1 2 3 4 5 6 7 8 9
F. L. Voo~ (ed.), Radiation Effects in Semiconductors, Plenum Press, New York, papers therein. D. M. HUGHES AND G. CARTER, P&s. Status Solidi, 25 (1968), 449. T. AMBRIDGE, J. Sci. Znstr., (1972) in press. Ch. B. LUSCHIK, Dok. Akad. Nauk. SSR, 101 (1955) 641. T. A. T. COWELL AND J. WOODS, Brit. J. Appl. Phys., 18 (1967) 1045. T. AMBRIDGE AND G. CARTER, J. Phys. D: Appl. Phys., 4 (1971) 1630. J. WOODS AND K. H. NICHOLAS, Brit. J. Appl. Phys., 15 (1964) 1361. B. A. KULP AND R. H. KELL!ZY, J. Appl. Phys., 31 (1960) 1057. B. A. KULP, Phys. Rev., 125 (1962) 1865.
Thin Solid Films, 10 (1972) 255-264
1968; numerous
Elsevier Sequoia
S.A., Lausanne
THE ELECTRONIC PROPERTIES THIN FILMS FOLLOWING LOW ION
- Printed
255
in Switzerland
OF CADMIUM SULPHIDE ENERGY HELIUM
BOMBARDMENT
THOMAS
AMBRIDGE”
Department
(Received
AND
of Electrical
November
GEORGE
Engineering,
CARTER
University
of Salford,
Salford
A45 4 WT, Lanes.
(Gt. Britain)
20. 1971)
SUMMARY
Cadmium sulphide thin films, held at - 180°C within an ultra-highvacuum chamber, were bombarded with helium ions, whose energy was in the range (18-125 eV) within which the threshold for the displacement of cadmium and sulphur atoms was expected to lie. Electrical conductivity changes were observed at the temperature of bombardment, as a function of ion energy and dose, and in addition the conductivity was continuously monitored during subsequent annealing. A thermally stimulated conductivity peak was noted, and tentatively ascribed to the release of electrons, trapped at isolated sulphur vacancies created near the film surface by the ion bombardment; also, an irreversible change in conductivity indicated annealing of these defects below room temperature. Because the effects being investigated were confined to the surface of the films, the influence of contaminant gases, even in the ultra-highvacuum range, could not be neglected; in particular the possibility of interactions with adsorbed
hydrogen
has been considered.
1. INTRODUCTION
The electronic properties of intrinsic point defects are of great interest in the characterisation of semi-conductor materials. A convenient means of introducing such defects into bulk material is high energy electron irradiation’. A technique applicable to the production of surface defects is low energy inert ion bombardment; in this case, if the ion energies used are in the region of the threshold values required to effect atomic displacements, the effects will be confined to within one or two atom layers of the surface. * Present
address:
Post Office Research
Thin Solid Films, 10 (1972) 255-264
Department,
Dollis
Hill, London
NW2 7DT (Gt. Britain).
256
T. AMBRIDGE.
G. CARTER
Hughes and Carter’ have reported pronounced changes in the electrical resistivity of CdS thin films subjected to 250 eV argon ion bombardment, at room temperature. The present study also examines changes in the electrical resistivity of CdS films, although, in this case, helium ions in the energy range 18 eV to 125 eV were employed and the target maintained at a temperature of - 180 “C during bombardment. Thermally stimulated plotted during subsequent heating, to study electronic and their annealing characteristics. 2.
conductivity curves were properties of the defects,
EXPERIMENTAL
2.1. Apparatus
and materials
Polycrystalline cadmium sulphide films were produced in situ in the ion bombardment target chamber by evaporation onto glass substrates previously provided with chromium contact pads separated by a 1 mm gap. A partiallyenclosed inner deposition chamber system was used, with walls heated to 150 “C, substrate heated to 250 “C, and high purity source material held at 750 “C within a quartz crucible. 9 cm below the substrate. The residual pressure during deposition was in the low lo-’ torr region; following deposition the pressure was reduced to below lop9 torr and maintained in the ultra-high-vacuum range throughout the course of the subsequent series of experiments, except when helium was admitted for ion bombardment. Details of the apparatus are given elsewhere3. Three different samples were used in the experiments described below: these were tilm “ M “, which had a thickness of 1400 A, and a room temperature dark resistivity of 1.5 x 1O4 Q cm, film “ N ” (900 A, 3 X lo3 52 cm), and film “ 0 ” (800 A, 3 x10’
Q cm). All films had dark
resistivities
greater
than
10”
52 cm
at - 180 ‘C. 2.2. Electrical conductance measurements Exploratory experiments with CdS film “M”
showed
that, when the target
was maintained at - 180 “C, and bombarded with helium ions (He+) with energies between 25 and 125 eV, the electrical conductivity (under weak illumination from the ion gun filament) increased sharply initially and continued to increase steadily with increasing ion dose, reaching a value two to four orders of magnitude greater than the pre-bombardment value, for doses of the order 1015 ions cm-‘. Following each bombardment. the target was heated to 350°C and the conductivity at - 180°C invariably returned to the pre-bombardment value (within *20”,,). At first it appeared that the dependence of the magnitude of the conductance change on the ion energy was random and irreproducible, although, after a large number of experimental runs, a pattern of behaviour was revealed and subsequently confirmed by a more systematic investigation of films “N” and “ 0 “. Thin Solid Films. 10 (1972)
255-264
ELECTRONIC
PROPERTIES
The following to study
OF
2.57
CdS FILMS
series of bombardments
the anomalous
ion energy
was carried
dependence:
out on film “ N ” in order
Each
morning
the film was
heated to 350 “C, after standing in a vacuum of about 5 X lo- lo torr overnight, cooled to - 180 “C, and bombarded with 5 x 1014 monoenergetic He+ ions per cm2. Immediately afterwards the cycle was repeated at least once, ions of the same energy as in the first bombardment being used for the second bombardment. Following the last bombardment each day, heating to 350 “C was carried out and the sample cooled to room temperature. The ion energy was increased daily in steps of 10 eV, from 25 eV to 125 eV. In each case the increase in conductivity resulting from the first bombardment each day was notably greater than that resulting from the second bombardment at the same energy (typically by a factor of four). Figure 1 shows the increase in
-
Pre-
bombardment
corductance
I 1
I
I
I
I
2
3
4
5
He+ dose (ions cm-’
x 10-14)
Fig. 1. Conductance changes of CdS film “N” function of ion dose. -w 1st 25 eV bombardment, -n2nd 25 eV bombardment, -t_ 1st 45 eV bombardment, + 2nd 45 eV bombardment.
at - 180°C
during
He+
ion bombardment,
as a
conductance with ion dose for first and second bombardments using ion energies of 25 eV and 45 eV. It is clear that the rate of change of conductance was characThin Solid Films, 10 (1972) 255-264
258
T. AMBRIDGE.
G. CARTER
teristic of the energy although the total change was subject to some other influence. The final value of conductance following each first and second bombardment in the series is shown in Fig. 2. If all the first bombardments
He+
,on energy
as one set
(eV)
Fig. 2. Conductance change at - 180 ‘C following of ion energy: CdS “N “. @--I st bombardments. + 2nd bombardments, -_i--2nd bombardments. repeat series.
and the second
bombardments
definite
dependence
energy
are considered
He’ ion dose of 5 x 10’; ions cm-‘.
as a separate
set, as indicated
of the conductance
change
as a function
by the solid lines, a
is indicated.
Additional
confirmation of the energy dependence was obtained by carrying out third and sometimes fourth bombardments at the energy used on the same or previous day: these invariably produced changes similar to those resulting from second bombardments previously carried out at the same energies. Further, five more first and second bombardments were carried out, with daily decreasing ion energies: although the absolute magnitude of the effects had diminished, the relationship between first and second bombardments, and the energy dependence, was consistent with that earlier observed. (For clarity, only points relating to the second bombardments in this series are shown in Fig. 2.) It should be noted that bombardment with 18 eV He+ ions produced a change in conductance, although no detailed investigations were carried out since the maximum possible ion dose rate Thin Solid Films, 10 (1972) 255-264
ELECTRONIC
was a factor
PROPERTIES
OF
259
CdS FILMS
of three below the rate of about
10”
ions cm- 2 see- ’ used in the
energy range 25 eV to 125 eV. A similar series of bombardments was carried out on CdS film “O”, although the pre- and post-bombardment heating was limited to a maximum temperature of only 200 “C. It was felt that this would eliminate possible effects due to the desorption of hydrogen (and other contaminant gases) from the stainless steel cryostat, since partial pressures up to 10d8 torr had previously been noted during heating to 350 “C. The conductance changes obtained for doses of 5 x 1014 He+ ions cm- 2 are shown in Fig. 3, as a function of energy for first and second bom-
I----
He’ ionenergy
(ev)
Fig. 3. Conductance change at - 180 “C following of ion energy: CdS film “0”. -O-1st bombardments, __O_ 2nd bombardments.
He+ ion dose of 5 x lOI ions cm-2,
as a function
bardments. It is seen that, although the order of magnitude of the changes was similar to that for film “N”, the relationship between the two sets ofbombardments for film “0” was less consistent and the energy dependence less certain. Because of the lack of self-consistency, the latter results were probably less representative than the previous ones; however, the sharp initial increase in effect with increasing energy was still apparent and, in the case of the first bombardments, a maximum effect for an ion energy of 45 eV was in fair agreement with the previous result. Thin Solid Films, IO (1972) 255-264
260 2.3.
T. AMBRIDGE.
Thermally stimulated conductivity
Throughout - 180°C observing
the dark thermally
the heating conductance stimulated
runs
G. CARTER
measurements
which
followed
the ion bombardments
at
of the films was monitored with the object of conductivity (TSC) peaks, corresponding to the
emptying of electron traps created by the bombardment. Such traps would have become populated by electrons excited into the conduction band by the illumination from the ion gun filament; usually no additional source of illumination was employed. Invariably, a well defined peak was observed, at a temperature in the region of - 160” to - 140 “C, which appeared to be directly associated with the increased conductance following the bombardment. For the cases in which this increased conductance was least, no further peaks were observed and the minimum conductance following the peak was rarely more than a factor of three below the peak value; beyond the conductance valley a continuous rise in conductance, similar to that obtained from non-bombarded films. was observed. For those cases in which the increased current was largest, a second peak appeared above the background level, for all three CdS films investigated, in the range - 35 ’ to - 25 “C. This peak was seen following He+ ion doses of 5 x 1014 ions cm-’ with energies in the range 25 to 125 eV for film “ M ” , 25 to 55 eV for film “N” and 25 to 45 eV for film “0”. Greatly improved definition of the latter peak was obtained for 45 eV He’s bombardment of film “0” when the dose was increased to 1.7 x 1015 ions cm-‘. Two minor peaks, merging into this peak, were always apparent after such a dose. A typical TSC spectrum for this case is shown in Fig. 4. Also apparent in this spectrum is a peak at about +45 “C. This peak was never observed following the 5 x lOi cm-’ ion doses but was reproduced accurately in three TSC runs which followed successive 45 eV He+ ion doses of 1.7 x lOi ions cmP2, for film “ 0 *‘. None of the peaks described could be made to re-appear by illumination of the films at - 180 “C after cooling from the maximum temperature (at least 200 “C) of the previous TSC run. Tests were carried out to ascertain the nature of the major prominences in the TSC spectra: The peak at - 140 “C behaved as a genuine TSC peak, corresponding to electron trap emptying, in that it was fully reproducible after terminating the post-bombardment heating at - 55 “C, cooling the sample to - 180 “C, illuminating to fill the trap, and re-heating. In addition, the use of more intense illumination. from a filtered 500 watt tungsten lamp, increased the magnitude of the peak. indicating more efficient trap filling. The other peaks were unaffected by additional illumination and could not be made to re-appear using the above procedure. which suggests that this part of the so-called TSC spectrum did not relate to electron trapping phenomena but represented conductance changes corresponding to irreversible physical changes within the sample. In particular, it appeared that the peak at - 30 “C was associated with the elimination of the defects responsible for the - 140 “C peak, since it was not possible to observe the latter after heating Thin Solid Film-, 10 (1972) 255-264
ELECTRONIC
PROPERTIES
Tempwat
4 ’
0 1
’
’
5
’
’
OF
we
’
261
CdS FILMS
(‘0
8 ’
10
’
’
1 1 15
Timdmid
Fig. 4. Thermally stimulated conductance of CdS 1.7 X1015 ions cm-* at - 180°C. NB. Heater switched on at time t = 0 mins.
film “0”
following
45 eV He+
ion dose
of
the sample to - 15 “C, and subsequently cooling and illuminating as before. The - 140 “C peak was analysed, using a particularly well-defined example obtained
following intense illumination of film “ 0 “, after a dose of 1.7 x lOi of Luschik’s4 formula yielded an estimated trap He+ ions cm- 2. Application depth of 0.15 eV which was used as a trial value in attempting to fit a theoretical TSC curve, assuming the slow re-trapping approximation derived by Cowell and Woods’. The closest fit subsequently obtained suggested a trap depth of 0.17 eV. An electron capture cross-section of 6 x 10e2’ cm2 was calculated for the trap, which is consistent with the weak re-trapping assumption. Unfortunately, no estimate of the trap density could reasonably be made, since electron mobility and lifetime were unknown and further, it would have been difficult to judge the extent to which the trap was initially filled, in view of the isothermal decay at - 180 “C which appears to have been responsible for the observed enhanced conductance at this temperature. 2.4.
The influence of contaminant gases It was thought possible that the reason for the difference between the effects of the first and second bombardments at fixed energy was associated with a Thin Solid Films, 10 (1972) 255-264
262
T. AMBRIDGE.
G. CARTER
contaminant gas, adsorbed on the film surface, which was not desorbed by heating to 350°C and did not reveal any influence on electrical properties until ion bombardment was performed. In particular, hydrogen was suspected; in a subsidiary experiment6 it was shown that the conductance of a CdS film at - 180 “C was insensitive to the presence of molecular hydrogen (at 3 x 10d6 torr), although adsorbed atomic hydrogen caused an increase in conductance by many orders of magnitude. In the present case it is possible that molecular hydrogen was adsorbed on the films overnight (e.g. one monolayer, assuming unity sticking probability, at the measured partial pressure of 1 x lo- ‘O torr); ion bombardment could have dissociated the molecules and the atomic hydrogen would then have contributed to the increased conductance. Although this process has not been proved conclusively, further evidence was obtained for the influence of adsorbed hydrogen on the results. First, when the overnight partial pressure of hydrogen was deliberately increased to IO-* torr and then pumped away prior to a typical heating, cooling and 25 eV He+ bombardment schedule (two bombardments), the conductance changes were a factor of four above those observed on the previous day for 25 eV bombardments, although the ratio of the effects of the first and the second bombardments remained the same. Secondly, the irreversible conductance change noted at +45 “C, following higher dose bombardments, corresponds very closely to a similar feature we have seen when heating CdS films previously exposed to atomic hydrogen at - 180 “C and attributed to hydrogen desorption6. 3.
DISCUSSION
AND
CONCLUSION
The interpretation of the effects described is made difficult by the fact that contaminant gases appear to have added a significant contribution, in spite of the low background pressures maintained in the target chamber. This aspect in itself should be of some interest, especially in view of the likelihood that the ion beam was instrumental in activating a normally electrically inactive contaminant. However. it is interesting to attempt an interpretation of the results on the assumption that the effects of contaminants were secondary. The conductance increases observed at - 180 “C almost certainly represented isothermal release of electrons from donor type levels, about 0.17 eV below the conduction band, created by the ion bombardment. It is tempting to ascribe these to isolated sulphur vacancies; Woods and Nicholas7 have attributed a level at about 0.14 eV in CdS single crystals to the same source. A tentative explanation for the energy dependence of the magnitude of the conductance increases may be put forward in terms of the minimum ion energies necessary to create point defects by atomic displacement. On the basis of their high energy electron irradiation experiments, Kulp and Kelley’ and Kulp’ have deduced threshold displacement energies of 8.7 eV for sulphur atoms and 7.3 eV Thin Solid Films, 10 (1972) 255-264
ELECTRONIC
PROPERTIES OF
CdS FILMS
263
for cadmium
atoms in bulk CdS. Taking the case of maximum energy transfer (i.e. assuming head-on collisions), the He + ion energies sufficient to transfer these amounts are 22 eV and 55 eV respectively. Thus, the initial increase in effect with increasing energy should be associated with an increase in the number of sulphur displacements, as suggested above. Furthermore, the observed reversal for energies above 45 eV to 55 eV might be explained in terms of cadmium displacements, implying that the new defects introduce a compensation mechanism to the existing 0.17 eV level; this point will be returned to shortly. In view of the inconsistencies between the samples investigated, it would be premature to attempt to explain the behaviour relating to the higher energy bombardments; note that multiple defects would be possible in this range. It will be recalled that some increase in film conductance had been observed for He+ ion energies as low as 18 eV; this suggests that the threshold energy for sulphur displacements is lower in the surface region, than in bulk material. A similar reduction might be expected in the case of cadmium displacements; this would require the He + ion energy of 55 eV, corresponding to the maximum conductance change in film “ N “, to represent somewhat greater than the minimum value to effect cadmium displacements at the surface. This is feasible since the contribution to electrical properties would probably be too small to distinguish, very close to the threshold point. The conductance tierSUS temperature relationship following bombardment appears to contain information on defect annealing as well as indicating the existence of the 0.17 eV donor-type trapping level. It should be pointed out that the fact that the detailed structure of the higher temperature peaks was only observed in those cases when the initial peak was greatest does not imply that the features responsible were absent at other times, but rather that the general rise in conductance with temperature (not a result of bombardment) was sufficient to obscure their effects. Since the peak at - 30 “C was associated with the disappearance of the level responsible for the low temperature peak, it is proposed that this represents the onset of annealing of sulphur vacancy-interstitial pairs. This is in contrast to the results of Kulp and Kelley’ who were able to produce defects in CdS by electron bombardment at room temperature and who reported that the fluorescence attributed to sulphur interstitials was sometimes still present after heating to 400 “C. However, a significant difference in annealing behaviour would be expected from the fact that, whereas the electron-induced damage was distributed throughout the thickness of a CdS platelet, the ion-induced damage in the present study was confined to the surface layer, with a very high defect density resulting from at least 0.5 bombarding ions per surface atom. Little can be said about the two additional prominences superimposed upon the -30°C peak following the heaviest doses, since these could not be studied in isolation; it is possible that these were associated with the defect annealing process to which the major peak was attributed. Thin Solid Films, 10 (1972) 255-264
264
T. AMBRIDGE.
No new characteristic of heating,
following
features
were observed
the higher energy
in the conductance
bombardments,
G. CARTER
as a function
which could be assigned
to cadmium vacancy or interstitial defects, apart from the decrease in magnitude of the previously noted peaks. This implies that any electron traps produced were at levels close to the centre region of the band-gap; these would capture electrons at the expense of the shallow level at 0.17 eV and subsequent thermal release would occur at temperatures where the “ background” conductance was too high to permit observation. Thus, no detailed conclusions can be reached regarding the trapping or annealing characteristics of the defects. The consistent results on film “N” suggest that complete annealing was achieved during the heating to 350 “C, whereas the less consistent results obtained from film “ 0 “, when heating was limited to 200 “C, may have arisen from incomplete annealing. It has been shown that the surface nature of the interactions studied presents considerable experimental and interpretative difficulties; in particular, it must be emphasised that the influence of contaminant gases, even in the ultra-high vacuum range, is sufficiently important to make it necessary to regard most of the interpretation of results as extremely speculative. Further studies along the lines described here, including extending the ion energy range downwards, should be valuable, if combined with other techniques, such as Auger spectroscopy to detect surface contamination, and spectral luminescence measurements to determine the characteristics of defect levels at the temperature of bombardment. ACKNOWLEDGMENTS
One of us (T.A.) wishes to thank the Science Research Council financial support, in the form of a Ph.D. studentship grant.
for providing
REFERENCES
1 2 3 4 5 6 7 8 9
F. L. Voo~ (ed.), Radiation Effects in Semiconductors, Plenum Press, New York, papers therein. D. M. HUGHES AND G. CARTER, P&s. Status Solidi, 25 (1968), 449. T. AMBRIDGE, J. Sci. Znstr., (1972) in press. Ch. B. LUSCHIK, Dok. Akad. Nauk. SSR, 101 (1955) 641. T. A. T. COWELL AND J. WOODS, Brit. J. Appl. Phys., 18 (1967) 1045. T. AMBRIDGE AND G. CARTER, J. Phys. D: Appl. Phys., 4 (1971) 1630. J. WOODS AND K. H. NICHOLAS, Brit. J. Appl. Phys., 15 (1964) 1361. B. A. KULP AND R. H. KELL!ZY, J. Appl. Phys., 31 (1960) 1057. B. A. KULP, Phys. Rev., 125 (1962) 1865.
Thin Solid Films, 10 (1972) 255-264
1968; numerous