The effect of oxygen on epitaxial PbTe, PbSe and PbS films

Thin Solid Films - Elsevier Sequoia S.A., Lausanne - Printed in The Netherlands

T H E E F F E C T O F O X Y G E N ON EPITAXIAL PbTe, PbSe A N D PbS FILMS R. F. EGERTON* AND C. JUHASZ** Materials Section, Electrical Engineering Department, Imperial College, London, S. I41.7. (Gt. Britain) (Received August 29, 1969)

SUMMARY

Epitaxial films of the semiconductors lead telluride, lead selenide and lead sulphide were prepared by vacuum sublimation on to heated mica, and Hall measurements made in vacuum and during subsequent exposure of the films to air. The results are discussed in terms of adsorption and diffusion of oxygen; in particular, rapid initial changes in the Hall coefficient and resistivity of n-type PbTe samples imply that large band bending ( > 20 kT) occurs at the surface or that the room temperature diffusion coefficient of oxygen is high ( > 10-10 cm2sec - 1). Long-term changes in the electrical properties were also studied.

1. INTRODUCTION Although the effect of oxygen on the lead chalcogenides has been the subject of many previous investigations, the exact processes involved are still in dispute. Most of the early experiments were performed on polycrystalline layers, in an attempt to discover the mechanism of infrared photoconductivity. When prepared by vacuum evaporation, these films were normally n-type, with carrier concentrations of the order of 1018 cm -3. Upon exposure to oxygen, their resistivity and photoconductivity increased (typically by a factor of 10 to 100 at room temperature or 104 to 106 if the experiment was carried o u t a t 77 °K); in many cases, these quantities then went through a maximum, at which point the Seebeck and Hall coefficients changed from negative to positive values 1-6. Simultaneously with this increase in resistivity, the Hall mobility was found to decrease by a factor between 2 and 5 (ref. 7), probably due to an increase in the height of potential * Present address: Zenith Radio Research Corporation, 6 Dalston Gardens, Stanmore, MiddleSeX. ** Present address: The Moore School of Electrical Engineering, University of Pennsylvania, Philadelphia, Pa. 19104, U.S.A. Thin Solid Films, 4 (1969) 239-253

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R. F. EGERTON,C. JUHASZ

barriers between adjacent crystallites of the film a. These effects were attributed to the creation of acceptor states by adsorption of oxygen at crystallite boundaries or by diffusion into the crystaUites themselves. Measurements have been made of the kinetics of oxygen uptake by single crystals crushed in vacuum 9-1 ~, the results being variously interpreted in terms of oxygen adsorption, diffusion and oxide formation. Experiments have also been conducted 12 on epitaxial films of lead selenide on rocksalt, which are not appreciably photoconductive and have electrical mobilities almost as high as bulk PbSe. These results were discussed entirely in terms of an adsorption model, which gave unexpectedly large values of surface charge density and implied that carrier scattering at the semiconductor surface is entirely specular. The present study was carried out on epitaxial (non-photoconductive) films of PbTe, PbSe and PbS grown by vacuum deposition on to heated mica. X-ray diffraction and electron microscopy of these films t 3 shows that the semiconductor is oriented with {111} planes parallel to the substrate surface, but contains twin (double-positioning) boundaries. The latter do not have a serious effect on the electrical properties, since the Hall mobility increases with decreasing temperature and may be as high as 70 % of bulk single-crystal values down to 77 ° K l ' t ' ~ 5

2. EXPERIMENTAL Films of PbTe, PbSe and PbS were prepared by subliming the respective compounds at a pressure of about 2 x 10-6 torr. The substrates were pieces of ruby muscovite mica, cleaved in air just before mounting in the evaporation chamber, where they were clamped to a rotatable disk. The semiconductor was deposited at a substrate temperature of between 200 and 300 °C, the evaporation source consisting of a small silica tube containing lumps of the material (of 5 N purity), heated by a surrounding spiral of tantalum wire. The evaporation rate was 5 A sec- ~ and final film thickness normally about 3000 A. In certain cases, the semiconductor film was coated immediately afterwards with a 1000 A layer of insulator (silicon oxide or magnesium fluoride), evaporated from an open boat source. A molybdenum mask just in front of the substrate was used to define a sample shape suitable for Hall measurements, which were performed in s i t u by rotating the substrate into the gap of an electromagnet giving a d.c. field of 1.6 kGauss. A d.c. current (typically 100/~A) was passed through the sample and the room temperature conductivity and Hall coefficient were measured in vacuum and during exposure of the film to various ambient gases. After removing from the evaporation chamber, the sample was mounted in air in a cryostat and Hall measurements (with a magnetic field of 2.0 kGauss) made between 300 and 77 °K. Thin Solid Films, 4 (1969) 239-253

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3. RESULTS

3.1. Long-term effects After preparation, films were exposed to the atmosphere and their electrical properties measured (as a function of temperature) over a period of several months during storage (at room temperature) in air. Changes in Hall coefficient were observed, which are summarised in Table I. TABLE I C H A N G E S I N T H E H A L L C O E F F I C I E N T OF FILMS~ M E A S U R E D O V E R A 1 2 - M O N T H P E R I O D

Initial material

Ageing characteristics

n-PbTe n-PbSe

Rn becomes positive in the temp, range 77-300 °K Rn at 77 °K becomes more negative Ra at 300 °K becomes positive I Rn ] increases, but Rn remains negative for all temperatures Rn increases slightly R . decreases a small amount Little change in R~; remains negative

n-PbS p-PbTe p-PbSe n-type PbTe, PbSe and PbS with SiO overlayer

The Hall coefficients of particular samples, measured up to 1 year after preparation, are shown in Fig. 1. The largest changes were observed for PbTe films which were initially n-type. Here, RH first of all increased in magnitude but after a few months changed from negative to positive values and subsequently decreased slightly. In the case of n-type PbS, the change was much slower and corresponded only to the first stage in PbTe. P-type samples showed little variation with time, the Hall coefficient remaining positive. RR (crn s C-~) 100I

~

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Fig. 1. Hall coefficient of PbTe, PbSe and PbS films (measured at 77 °K) as a function of time. Thin Solid Films, 4 (1969) 239-253

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R . F . EGERTON, C. JUHASZ

Unusual effects were observed in n-type PbSe films, as shown in Fig. 2. When measured soon after deposition, RH was negative and roughly constant over the temperature range 77-300 °K, as expected for a uniform doping of about 4 x 10 is cm -3. U p o n storing in air for 3 days, RH became positive at room temperature and passed through zero at 210 °K. Four days after deposition, the temperature at which Rr~ - 0 had moved lower, but after 8 weeks this temperature increased to about 290 °K. After 40 weeks, RH was negative up to room temperature. RH (cm 3 C-1) T

,300

200

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2

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-8-

Time after deposition: o 1 hour a 3 days A 4 days

8 weeks x

40 weeks

Fig. 2. The Hall coefficient as a function of temperature for PbSe sample 53.

To test whether water vapour had any effect on the ageing process, certain samples were stored in a desiccator and others were kept in a chamber connected to a rotary vacuum pump. In neither case did this have any significant effect on the changes taking place. However, PbTe and PbSe films coated with an evaporated layer of insulator remained n-type during 2 years storage in air, though there was some increase (20-100 ~ ) in I RH I.

3.2. Short-term effects The Hall coefficient and resistivity of lead telluride films were measured inside the vacuum chamber as soon as the samples had cooled to room temperature, within a few minutes of deposition. Measurements were also made during admission of different ambient gases. Nitrogen, hydrogen and water vapour were found to have little effect on the films. The only gas which caused a rapid change in Thin Solid

Films, 4 (1969) 239-253

EFFECT OF OXYGEN ON EPITAXIALPbTe, PbSe AND PbS FILMS

243

electrical properties was oxygen, and no difference was observed between the effect of pure oxygen and air. Figure 3 shows the effect of air on a 3000 A n-type film of lead teUuride. When measured in vacuum soon after deposition, the Hall mobility (#a = RN/p) was 1200 cm2volt - Isec- 1, and the Hall coefficient - 1.4 cm3coulomb - ~, corresponding to an apparent carrier concentration (n* = I 1/qRrt l) of 4 x 10 ~s c m - 3 .

/

RH (crn ~ C")

After reaching atmospheric - - - ~ / -

Change in

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0

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, ~ - - - Initial conditions in high vacuum 0.602

0.604

0.006

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Fig. 3. The Hall coefficient and resistivity of an n-type PbTe film (sample A24) during exposure to air. Fig. 4. Change in resistivity and Hall coefficient of an n-type PbTe film upon sudden exposure to air. After storage at a pressure below 4 x 10- 6 torr for 2 hours, the Hall coefficient and resistivity had both increased by a factor of two, the Hall mobility remalning unaltered. U p o n admitting air to the deposition chamber, RrI and p increased further, with only a slight decrease in #a. Admitting a pulse of air caused the resistivity to rise for a few seconds and then decay, as shown in Fig. 4. Some of the experimental points in Fig. 3 were obtained while Rri and p were still changing; the values attained after a few minutes at atmospheric pressure are therefore less than the m a x i m u m values recorded. The relaxation effect sometimes continued over a period of a few hours when samples were kept at atmospheric pressure, Rrt and p decreasing a further 10 or 20%. I f the deposition chamber was re-evacuated after exposing a film to air, the resistivity and Hall coefficient decreased, #n again remaining constant. In this way, RM and p could be reduced to about half their values at atmospheric pressure, i.e. roughly three times the initial values in vacuum. Thin Solid Films, 4 (1969) 239-253

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R. F. EGERTON, C. JUHASZ

Figure 3 is typical of the results obtained from six n-type PbTe samples (all 3000 A films) which had Hall coefficients between - 1 and - 2 cm 3 coulomb-1 in vacuum and between - 4 and - 7 cmacoulomb - 1 after exposure to air, with #~ --~ 1000-1300 cm2volt-lsec -~. In each case, the graph of RH against p was approximately a straight line, which could be extrapolated to pass close to the origin. Exposure of a 1000/~ n-type PbTe film to air gave results similar to Fig. 3, both the initial and final values of Rn and p being similar to those of the 3000 A films. A 3000 A film of PbSe also showed the same kind of behaviour, though the change in Hall coefficient upon contact with oxygen was slightly less than for PbTe. The same kind of measurements were made on PbTe films which were lightly doped before exposure to air. Figure 5(a) shows results obtained from a sample which was high-resistivity p-type in vacuum. In this case, admission of oxygen caused a net fall in resistivity and Hall coefficient, although both these quantities initially increased and went through a maximum before decreasing. The points obtained by re-evacuating from atmospheric pressure do not lie along the original curve; instead, R H is higher for the same resistivity. It was found possible to control the initial carrier concentration by heating samples in vacuum to about 410 °C. This probably caused preferential evaporation (a)

RH

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Fig. 5. The Hall coefficient (in cma coulomb- ') and resistivity (ohm cm) of PbTe films during exposure to air; (a) sample A38a, (b) sample A38b, (c) sample A39, (d) sample A40. Points labelled v and a represent the conditions in vacuum and at atmospheric pressure respectively. The symbols A, B, C and D refer to Fig. 7. Points obtained by re-evacuation are represented by crosses. Thin Solid Films, 4 (1969) 239-253

EFFECT OF OXYGEN ON EPITAXIAL

PbTe, P b S e AND P b S FILMS

245

of tellurium from the film, since the effect was always in the direction of n-type doping. The samples were cooled slowly to room temperature; because of the fairly high self-diffusion coefficients at elevated temperatures, the doping in the resulting film was expected to be uniform. Figures 5(b), (c) and (d) show results for samples which had positive Hall coefficients directly after preparation, but which were made lightly n-type (RH negative) by the annealing treatment before exposure to air. In each case, the effect of oxygen was sufficient to cause RI~ to change sign, so that the films appearedp-type at atmospheric pressure. In Fig. 5(c), I RH I and p both go through a maximum before decreasing; in Fig. (5b) only p goes through a maximum, and in Fig. 5(d) I RHI and p decrease monotonically. Again, the values are not repeated upon re-evacuation.

4. DISCUSSION

4.1. Lona-term effects The Hall coefficient changes shown in Fig. 1 are explained qualitatively by assuming that a p-type impurity is being slowly added to the film. The most likely source of this is atmospheric oxygen, since oxygen is known to produce p-type conduction in the lead chalcogenides in the form of polycrystalline films t'2'4 and single crystals 16. One possibility is that oxygen enters the film by diffusion from the external surface. The diffusion coefficients for oxygen have apparently not been measured, but the room temperature coefficients of self-diffusion of lead increase from PbS through PbSe to PbTe 17, consistent with the relative rates of change of RH in the three types of film. To a first approximation, the number of oxygen atoms (N' per unit area) which have entered the semiconductor at a time t should be given is by N' = (2Ns/x/r0 (Dt) ~ where Ns is the surface concentration of oxygen. If N~ is taken as the concentration of oxygen in the gas phase, the rate of penetration would be proportional to the ambient pressure, whereas storing the samples at 0.1 torr instead of 1 atmosphere was found to have little effect on the ageing rate. A possible explanation is that oxygen is present at the surface as an oxide or an adsorbed layer which is at least a monolayer thick even at low pressures; N~ and dN'/dt would then be independent of pressure. A layer of insulator on the surface will presumably help to prevent oxygen entering the semiconductor, but this protection might not be perfect because of pinholes. A diffusion process would involve an impurity gradient within the film, perpendicular to the substrate surface. There is some experimental evidence for this: Thin Solid Films, 4 (1969) 239-253

246

R . F . EGERTON, C. JUHASZ

(1) Field-effect measurements, made by applying an electric field through the mica substrate, showed n-type conductivity modulation in certain PbTe films, even though the sample gave a positive Hall coefficient. As there is not likely to be a strong inversion layer at the mica interface 19, this suggests that the semiconductor adjacent to the substrate was n-type, with the rest of the film p-type. (2) The sign reversal (with temperature) of the Hall coefficient of initially n-type PbSe films (Fig. 2) has been found previously in bulk crystals of PbTe and PbSe 20,21 and appears to be associated with an impurity gradient within the sample. The effect was also observed in epitaxial films of PbSe on rocksalt by Zemel 17, who put forward an explanation based on the assumption that the film consists of an n-type layer and a p-type layer of higher doping. This argument relies on the fact that the Hall mobility in the lead chalcogenides increases less rapidly with decreasing temperature when the carrier concentration is high, due to the energy dependence of the effective mass 22'23. To be effective, this requires a very substantial doping level (1019 cm -3 or more) in the p-type layer. Zemel assumed that the non-uniform doping was produced during film growth; however, this cannot be true in the present case, because RH did not exhibit the sign reversal until samples had been exposed to the atmosphere for a day or two. An alternative explanation might be that oxygen creates deep acceptor levels at or near the surface, which only become ionised at higher temperatures. However, it is difficult to see why the Hall-reversal temperature eventually increases (Fig. 2), using either of these models. The present results are in contrast with some of those previously reported for epitaxial PbTe and PbSe on NaCI, where long-term ageing effects were not observed~2,24. However, samples grown on rocksalt are in {100) orientation (which precludes double-positioning boundaries, though the films do contain low-angle grain boundaries and dislocations). Another difference is that the previous films were often several microns thick, compared with the present thickness of 20004000 A. 4.2. Short-term effects

The comparatively rapid changes which occurred when PbTe films were initially exposed to oxygen are conveniently classified according to the doping level in the semiconductor. 4.2.1. One-carrier films

These are samples which were heavily n-type when measured in vacuum and remained n-type during exposure to air. The present results may be compared with those of Brodsky and Zeme112 for epitaxial films of PbSe on rocksalt. In the latter case, the films were first exposed to air, and the resistivity and Hall coefficient measured at room temperature Thin Solid Films, 4 (1969) 239-253

EFFECT OF OXYGEN ON EPITAXIAL

PbTe, P b S e AND P b S FILMS

247

as the pressure was reduced below atmospheric. The change in electrical properties was attributed to the removal of adsorbed oxygen from the film surface. By removing electrons from inside the semiconductor, oxygen was believed to create a surface space-charge region, which would be an accumulation layer in the case of p-type films. Reducing the ambient pressure would therefore decrease the a m o u n t of band bending near the surface. The change in surface conductance (film thickness x change in conductivity) for p-type films was quite large (3 x 10 -3 o h m - l ) . Assuming flat bands at low ambient pressures, the surface band bending was estimated to be at least 9 kT at atmospheric pressure. Based on this interpretation and the fact that the Hall mobility was nearly independent of ambient pressure, the authors concluded that surface scattering in PbSe is completely specular. Applying this adsorption model to the present results for n-type PbTe films immediately leads to difficulties. The change in surface conductance in this case is even higher (2 x 10 -2 o h m - l ) , which implies very large band bending. I f the bands are fiat in vacuum, the donor concentration can be taken as typically 3.5 x 1018 cm -3. The effect of oxygen must now be to create a depletion layer at the surface; the maximum amount of depletion (before inversion occurs) can be calculated from degenerate space-charge theory 2s, which gives 1.6 x 10-3 o h m - 1 for the change in surface conductance. To account for the magnitude of the experimental change, it is therefore necessary to assume that there is an accumulation layer at the surface in vacuum, with a downward band bending of at least 20 kT. This would make the semiconductor highly degenerate, with a surface charge density of 1014 cm -2 and an internal electric field (5 x l0 s volt cm -1 at the surface) approaching the breakdown strength of most materials.

4.2.2. Two-carrier films These are samples which had low carrier concentrations in vacuum and which became p-type upon exposure to air. I f the effect of oxygen is one of adsorption with charge transfer at the external surface, the interpretation is helped by re-plotting the experimental curves (Fig. 5) in terms of at and Ra2t (where a is the conductivity, R is the Hall coefficient and t the thickness of the film), as in Fig. 6. This is because the surface quantities Aa and ARa 2, which represent excess carriers in a surface space-charge region, are additive with respect to the bulk parameters at and Ra2t (refo 26). Moreover, theoretical values of Aa and ARa 2 can be calculated from surface space-charge theory 2s,27. F o r this calculation, values of the electron concentration nb and mobility #n within the film are required, pn can be initially estimated from the gradient of the experimental curve (Fig. 6) away from the conductivity minimum, and values of nb and #n consistent with the experimental values of at and Ra2t (extrapolated to the fiat-band point) obtained by successive approximations. Figure 6 shows the "best fit" obtained for sample A38b with a calculated curve assuming nb = 1.5 X 1017 c m - 3 and p, = 350 cm 2 v o l t - lsec- 1. Comparison w i t h Thin Solid Films, 4 (1969) 239-253

-a

248

R . F . EGERTON, C. JUHASZ

the experimental curve implies that the surface potential vs is about - 7 in vacuum and - 1 2 in air. The charge transferred by oxygen to the space-charge region would be about 1013 (units are positive electronic charges per cm2). However, there is a difference in shape between the two curves in Fig. 6. The experimental curve has a shallower minimum, which is also true for other samples which were 0"t (P.'~x10 "~) 4

_::/

Z~O"

(~"x I0-4)

-11/ Theoreticol

curve

for adsorption model

2.

,<-2J ""~-3

,/

,~ ,/

-10~

"~

-,~/

1.

AR0"2 O.~)5

o.~5

0.1(cm=OC V -2 sec-2)

-o.~5

--2

o.b5 R~,

6

Fig. 6. Exposure of PbTe sample A38b to air; comparison with adsorption model. Points v, and a represent the conditions in vacuum and at atmospheric pressure respectively. External l,/surface

A

IEc

B

Ei

Ev

C

D

Fig. 7. Band bending due to oxygen in an n-type PbTe film. Thin Solid Films, 4 (1969) 239-253

EFFECT OF OXYGEN ON EP1TAXIAL PbTe, P b S e AND P b S FILMS

249

measured. This suggests that the mechanism is not purely one of charge transfer, but that diffusion may be occurring. Nevertheless, the qualitative features of the R . versus p curves (Fig. 5) can still be interpreted in terms of band bending, whether this is due to adsorption or diffusion or both. Figure 7 shows different stages of band bending for an n-type semiconductor; the corresponding regions are labelled on the experimental curves (Fig. 5). Figure 7A shows a depletion layer at the external surface, which probably represents the condition of samples A38b and A39 in vacuum. A reduction in the number of electrons in the film (due to the presence of oxygen) causes R . and p to increase initially. But when the hole concentration at the surface becomes approximately equal to the electron concentration in the interior of film, the resistivity reaches a maximum (Fig. 7B). If the process is just adsorption, the surface potential at this point is given by vS - -2Ub (ref. 27). With further band bending, conduction becomes dominated by holes near the surface and if oxidation proceeds far enough, R , reaches a (positive) maximum and then decreases (Fig. 7C). If the sample is kept at atmospheric pressure for a few hours, p starts to increase and R . becomes more positive, so that the points do not lie on the original curve (Fig. 5). It does not seem possible to account for this non-reversible behaviour in terms of an adsorption model, but if diffusion is assumed to take place, this can result in the gradual conversion of the greater part of the semiconductor from n-type to p-type due to the removal or compensation of the original n-type impurity centres. This would cause an increase in both Hall coefficient and resistivity. Eventually, the whole of the film would become p-type (Fig. 7D) and if the ambient pressure were reduced, the sample would behave as a p-type semiconductor, R . and p both increasing along a straight line passing through the origin with gradient equal to the hole mobility. The observed behaviour upon re-evacuation is in reasonable agreement with this prediction. In Fig. 5(b) the points obtained lie on a line of gradient 200 cm2volt - lsec-1, which is the same as the estimated hole mobility in this sample. The fact that sample A38a (Fig. 5a) also shows this non-reversible behaviour would indicate that part of the film was originally n-type.

4.3. Comparison of adsorption and diffusion models One possibility of distinguishing experimentally between adsorption and diffusion is to examine the kinetics of the process. In the case of a lightly-doped n-type PbTe film which was kept at a constant pressure of 1.5 x 10- 6 torr for several hours directly after preparation, the Hall coefficient was observed to change uniformly with time, suggesting that the arrival of oxygen at the surface is the rate-limiting process at these low pressures. When a similar sample was suddenly exposed to air at atmospheric pressure, the changes in Hall coefficient and conductivity were not linear with time. But when the conThin Solid Films, 4 (1969) 239-253

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R. F. EGERTON, C. JUHASZ

0" {3"~_cm-*)

6

4.

2-

0

5

10

t

15

Y't (sec) ~

Fig. 8. Change in conductivity with time for PbTe sample A47b, following exposure to the atmosphere.

ductivity is plotted against t ~, the graph is linear over parts of the range (Fig. 8), as expected for a diffusion process. This curve is similar in shape to those obtained from the effect of oxygen on polycrystalline PbSe films 4, except that the times involved in the latter case were much longer. The higher rate for PbTe could be due to a higher diffusion coefficient. Though none of the present results are conclusive, the evidence suggests that diffusion plays a role in the effect of oxygen on epitaxial films of PbTe. A plausible explanation would be the following. (a) Diffusion within the semiconductor takes place over a period of hours and causes the non-reversible effects noted in Section 4.2.2. (b) This process continues over a period of months and is responsible for the long-term ageing effects described in Sections 3.1 and 4.1. (c) The rapid effect observed when oxygen is admitted to clean PbTe surfaces is mainly due to adsorption and charge transfer at the surface. However, there are several remaining objections to the adsorption model. (1) To account for the observed change in surface conductance for the highly-doped n-type films in terms of a surface space-charge region, the band bending would have to be extremely high, with a large surface potential in vacuum (Section 4.2.1). This conclusion comes from calculations based on existing spacecharge theory, including degeneracy but neglecting surface quantisation and impurity-band effects. Quantisation may be significant 28 for surface charge densities greater than 1012 cm -2 in PbTe, whereas the charge densities required for the adsorption mechanism are more than an order of magnitude higher than this. However, the effect of quantisation would be to make the required surface potentials even larger. Moreover, the large change in surface potential should result in Thin Solid Films, 4 (1969) 239-253

EFFECT OF OXYGEN ON EPITAXIAL

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251

an observable change ( ~ 50 %) in carrier mobility, due to the non-parabolicity of the energy bands in lead teUuride. Experimentally, the Hall mobility was found to be nearly constant. (2) PbTe and PbSe films measured in air have sometimes shown high Hall coefficients, which would be impossible in the presence of a degenerate spacecharge layer at the surface. (3) The change in surface conductance upon initial exposure of PbTe films to oxygen was much higher for n-type than for p-type films, and appeared to increase with electron concentration and film thickness. These facts are more easily explained in terms of diffusion than adsorption. Recently, Zemel and Kaplit 29 have proposed that a surface-space-charge region might arise from the formation of a defected oxide layer at the surface of the lead chalcogenides, rather than ionisation of oxygen at surface states. This does not account for the present experimental results because the changes in electrical properties are still assumed to occur within a space-charge region inside the semiconductor. Consequently, it is necessary to consider whether the rapid changes in RH and p could be due to diffusion.

4.4. Diffusion mechanisms Assuming that in an n-type PbTe film the impurity centres are tellurium vacancies (Brebrick and Gubner 3°) it is reasonable to postulate that oxygen atoms could diffuse into the film to fill vacant tellurium sites, thereby reducing the electron concentration in the semiconductor. However, it is possible that oxygen does not actually enter the lattice, but removes ionised tellurium vacancies from the vicinity of the surface, thereby setting up a vacancy concentration gradient. Diffusion of vacancies towards the surface would then result eventually in an almost intrinsic semiconductor. This corresponds to self-diffusion of tellurium in lead telluride, assuming that the latter does occur via a vacancy mechanism 31, and the speed of this process can therefore be estimated from the measured diffusion coefficient (D) of tellurium 3t. The conductivity change per square of surface is approximately A[a] = (2/~/Tz)ql~nnb(Dt)~ where n b is the carrier concentration initially present in the sample (assuming total ionisation of vacancies). Taking nb = 3 X 101 a c m - a, #n = 1000 cm2volt - 1see- x and D = 10 -18 cm2sec -1 at 300 °K, A[a] N 1.8x 10 -6 ohm -1 after 10 seconds, and 9 × 10 -4 ohm -~ after 1 month. Since the conductivity change for the highly n-type films was about 2 x 10 -2 ohm-1 and took place in less than a minute, this process cannot account for the high rate observed. On the other hand, if oxygen diffuses into the semiconductor, this may take place by a vacancy or an interstitial mechanism. In the latter case, the diffusion Thin Solid Films, 4 (1969) 239-253

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R . F . EGERTON, C. JUHASZ

coefficient could be quite high. Bloem and Kr6ger 32 report that copper diffuses interstitially in lead sulphide (in the temperature range 1(X)--350 °C) with a very low activation energy (0.31 eV), implying D ,~ 4 x 10 - s cm2sec -1 at room temperature. The speed of a similar process in lead teUuride would depend on the interstitial solubility (nsat) of oxygen. But if the diffusion coefficient of oxygen in PbTe were comparable with that of copper in PbS, the experimental rate of change would be explained if nsat ,~ 1017 cm -3, which is less than the value (about 3 x 10 is cm -3) for Cu in PbS. The possibility of interstitial diffusion depends on whether the diffusing species is small enough to fit into the interstices of the lattice. Bloem and Kr6ger a2 argue that this is the case for copper in lead sulphide, at least for the ionised (Cu ÷) form. Since an oxygen atom has a smaller radius than the Cu ÷ ion (0.6 A compared with 0.96 A) and PbTe has a slightly larger interatomic distance than PbS, it is not unreasonable to expect interstitial diffusion of atomic oxygen. A further possibility is that diffusion may take place via defects in the film such as dislocations or double-positioning boundaries, leading to enhanced diffusion rates compared with bulk single crystals. In support of this, Zemel 17 has suggested that the diffusion coefficient of lead in an epitaxial film of PbSe on rocksalt is a factor of 104 higher than in bulk material, the dislocation density in similar films of PbS having been estimated to be as high as 2 x 101° cm -2 (ref. 33). But if oxygen does diffuse down double-positioning boundaries in PbTe, this must not create space-charge barriers in the path of the current carriers, otherwise there would be a substantial reduction in Hall mobility, which was not observed in monopolar films.

5. CONCLUSIONS

Long-term changes in the Hall coefficient and resistivity of PbTe, PbSe and PbS films on mica can be accounted for by the gradual penetration of oxygen by diffusion into the semiconductor over a period of months after preparation. More rapid changes which occur when PbTe films are initially exposed to air could be partly due to adsorption of oxygen at the surface or to the formation of a surface oxide layer, but the experimental results are most easily explained in terms of the diffusion of oxygen, provided that the diffusion coefficient is sufficiently large.

ACKNOWLEDGEMENTS

The authors would like to thank Professor J. C. Anderson, Dr. A. J. Crocker, Dr. M. Green, Mr. M. J. Lee and Professor J. N. Zemel for their helpful advice, and the Science Research Council for financial support. Thin Solid Films, 4 (1969) 239-253

EFFECT OF OXYGEN ON EPITAXIAL

PbTe, P b S e

AND P b S FILMS

253

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