Steady state and transient photoemission into amorphous insulators

JOURNALOF NON-CRYSTALLINESOLIDS4 (1970) 117--131 © North-Holland Publishing Co., Amsterdam

STEADY STATE AND TRANSIENT PHOTOEMISSION INTO AMORPHOUS

INSULATORS

J. MORT and A. I. LAKATOS Xerox Corporation, Rochester, New York 14603, U.S.A.

Steady state and transient photoemission of carriers from metal electrodes has been used to study the electronic energy level structure and transport properties of two amorphous insulators, the polymer poly-n-vinylcarbazole (PVK) and amorphous selenium. The results for these two materials illustrate different types of information which these combined techniques can give. Fine structure on the steady state spectral response of the photoemission for PVK is identified with vibrational splitting of the valence band which is -< 0.1 eV wide. The time resolved transport of holes photoemitted into this band reveals an effective hole mobility which is thermally activated and field dependent, with a value of about 10 7 cmZ/V sec at 105 V/cm and room temperature. For amorphous Se the transient photoemission of holes indicates a transition of the carrier supply from predominantly internal photogeneration to photoemission. The dependence of the number of transported holes as a function of temperature and applied field shows a change in moving through this transition region. These results are significant with respect to the field dependent photogeneration of carriers in amorphous Se. The observed field dependent supply of photoemitted carriers is attributed to a general metal-insulator interface limitation commonly referred to as an unsaturated blocking contact. 1. Introduction

T h e p h o t o e m i s s i o n o f electrons a n d holes into an insulator f r o m an adj a c e n t m e t a l o r s e m i c o n d u c t o r is being increasingly used as a tool for determ i n i n g i n f o r m a t i o n a b o u t the electronic energy level structure a n d carrier t r a n s p o r t in the insulator. Extensive m e a s u r e m e n t s on the oxides o f silicon have been r e p o r t e d by W i l l i a m s x) a n d G o o d m a n 2) a n d on single crystals o f a n t h r a c e n e by W i l l i a m s a n d Dresner3). F o r the m o s t p a r t the p h o t o e m i s s i o n studies to d a t e have been restricted to s t e a d y state measurements. The purpose o f this p a p e r is to describe an extension o f the usual e x p e r i m e n t a l m e t h o d to a use o f b o t h steady state a n d transient p h o t o e m i s s i o n techniques. This a p p r o a c h will be seen to yield c o n s i d e r a b l y m o r e i n f o r m a t i o n t h a n can be o b t a i n e d f r o m either technique individually. M e a s u r e m e n t s were m a d e on two a m o r p h o u s insulators, the p o l y m e r poly-n-vinyl c a r b a z o l e ( P V K ) a n d a m o r p h o u s selenium. T h e results for these two materials will serve to illustrate the different types o f i n f o r m a t i o n which can be realized. Section 2 o f the p a p e r will describe the e x p e r i m e n t a l m e t h o d s a n d s a m p l e p r e p a r a t i o n . 117

J. MORTANDA. I. LAKATOS

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Section 3.1 will present a n d discuss the results for P V K while section 3.2 will deal with a m o r p h o u s selenium. A s u m m a r y is given in the final section.

2. Experimental methods The experimental arrangement for the steady state measurements is illustrated in fig. 1a. The insulator with a thickness of the order of several microns was sandwiched between a nesa coated slide and a semitransparent evaporated top electrode. By choosing the bias polarity of the top electrode either electrons or holes could be photoemitted into the insulator from the metal. The resultant current flow was measured by the electrometer. The transient method differs in the excitation and detection and is illustrated in fig. lb. A 10/~sec light pulse from a xenon flash tube was appropriately filtered using narrow band pass filters. With sufficiently long R C time constants the charge displacement could be integrated to measure directly the transit time of the

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STEADY STATE AND TRANSIENT PHOTOEMISSION

119

photoemitted carriers and the relative number of charges transported, as shown in the insert. The spatially narrow sheet of charge required in this type of measurement is well defined by the metal electrode thickness. The PVK films were prepared by coating nesa glass with a solution of the polymer in toluene and cyclohexanone. The films were heated to 100°C for several hours to remove any residual solvent and then a top semitransparent metal electrode was evaporated. The selenium samples were evaporated onto nesa glass substrates held at 50°C followed by the deposition of the metal electrode.

3. Experimental results and interpretation Fig. 2a is a schematic energy level diagram which illustrates the process of photoemission of electrons or holes from a metal into an insulator. Only by absorbing photons with an energy equal to or greater than the appropriate barrier height can the carriers surmount the barrier. For a given metal the sum of the two thresholds defines a "photoinjection" bandgap. The possible significance of such a definition will be discussed in section 3. 3.1. POLY-n-VINYL CARBAZOLE Fig. 2b shows the results of the steady state spectral response measurements for Au electrodes on PVK which have been published elsewhere4). The square root of the quantum yield has been plotted versus photon energy. The extrapolation of the linear portion of this plot yields the threshold photon energy. These curves have been corrected for absorption and reflectance of the gold film and spectral variation of the light intensity. The observed photocurrents were linear in electric field and light intensity over the range of values used. There are two very significant facts about these curves. The first is that photocurrents are observed in the red and near infra-red, a spectral region where there is essentially no absorption by the PVK, and secondly, the appearance of fine structure on the curves. These data were obtained with the gold electrode biased positively with respect to the nesa. No photocurrent could be observed below 1.7 eV, however, when the bias polarity was reversed. Since the first strong absorption in PVK, viz that due to the 0-0 singlet exciton, occurs at 3.57 eV, these low energy photoresponses cannot be explained in terms of bulk photogeneration of carriers in the PVK. For this reason and because of the dependence on the bias polarity, these currents are attributed to the photoinjection of holes from the gold and the extrapolated thresholds as a direct measure of the barrier height. Similar measurements 4) have been made for copper and aluminum on PVK. These data give an average value of ~bM+ ~h is 6.1 eV where, referring

120

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that after allowing for the difference in photoemission thresholds essentially coincidental structure is found for the two metals4). The average peak separation is 0.1 eV which is a typical value for molecular vibrational energies in organic molecules. Following Williams and Dresner3), who made a similar observation in anthracene, the observed structure is interpreted in terms of photoinjection into a narrow valence band in PVK, which is split by the molecular vibrations into a series of narrow bands. The fact that we can observe the structure gives us the very valuable information that the width of the valence band in PVK is significantly less than 0.1 eV. It was therefore of considerable interest to study the transport properties of holes photoemitted from a metal into this narrow band. This was most easily done by looking for the transit time of holes photoemitted into PVK from the metal by a 10/~sec flash of red light. The experimental arrangement was that given in fig. lb, except it was found more convenient to look at the time derivative of the charge displacement, i.e., the induced current. Fig. 4a shows oscilloscope pictures of the transient current pulses in PVK due to holes photoemitted from gold. It can be seen that the duration of the current pulse decreases with increasing field, indicating that the holes are traversing the sample. The transit times are very long, being in the millisecond range.

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J. MORT AND A. I. LAKATOS

These current pulses are only observed if the gold electrode is positively biased for a structure with one gold electrode. For a sample with gold electrodes on the top and bottom, signals are seen with either polarity of bias. This is an independent verification of the conclusion reached from the steady state measurements that holes are photoemitted from the metal. A characteristic feature of the current pulses is the very long tail which indicates that a large fraction of the total transported charge exhibits a dispersion in effective transit times. It seems unlikely that this dispersion can arise from either diffusion, straggling due to a statistical variation in trapping events or from thermal release from a distribution of traps. In these instances the relative distribution of transit times within the pulse would be field dependent. In fig. 4a it is seen that this is not the case, since with increasing field the total distribution translates essentially unchanged to shorter time intervals. Although a well defined transit is not evident, one can be operationally defined by the point at which the long tail begins. Fig. 4b shows a plot of the reciprocal of this time versus applied voltage which would be linear if the

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STEADY STATE AND TRANSIENT PHOTOEMISSION

123

mobility were invariant with field. For the reasons cited above, it seems difficult to account for this curve in any other way except in terms of a field dependent mobility. The mobility value at a field of 105 V/cm is ~ 10-7 cm2/ V sec. Transit times as long as 100 msec have been observed, indicating that holes have a bulk deep trapping lifetime of at least this duration. Temperature dependence studies show that the differential mobility is thermally activated with an activation energy of 0.55 eV. The measurements reported here are substantially in agreement with the measurements of Regensburger 7) who studied the transport of holes photogenerated in PVK by ultraviolet light. The transient photoemission studies therefore show that the transport of holes photoemitted into PVK is characterized by a very low, thermally activated mobility together with a very long lifetime. This is a characteristic but not unique feature of a molecular hopping mode of charge transportS). However, the additional evidence from the steady state measurements that the width of the band into which the holes are photoemitted is <0.1 eV would appear to rule out the only alternative mechanism, i.e., a transport controlled by traps in thermal equilibrium with a conventional wide valence band. The pronounced tail in the transient current pulse could arise from a distribution of hopping probabilities for the carriers because of the disordered nature of this amorphous organic polymer. The reason for the field dependent mobility, however, is less clear and any discussion at the present time would be quite speculative. 3.2.

AMORPHOUS

SELENIUM

Extensive studies have shown that this material possesses quite unusual properties. It is an ambipolar photoconductor for which the mobilities and lifetimes for both carriers have been measured g-a1). There is a considerable gap between the photogeneration quantum efficiency edge and the optical absorption edgela). It is known that down to 2.0eV, electrons and holes are created within 30~o with equal efficiency and for photon energies of 2.4-2.5 eV the quantum efficiencies at high fields can approach within a factor of two of unity1°). For all photon energies the number of photogenerated carriers transported across a film is found to be field dependent. Tabak and Warter 11) were able to establish that this field dependence was not a bulk limitation, i.e. a range limitation, but arose from a supply limitation in the excitation region. They ruled out a surface recombination mechanism unless in the latter instance the recombination involved a hole and electron created by one photon. This they termed field-controlled photogeneration. Pai and Ing 12) further showed that the photogeneration process was thermally activated with an activation energy which increased with decreasing exciting

124

J. MORT A N D A. I. LAKATOS

photon energy. The absorption coefficient in amorphous selenium reaches high values and extends for an appreciable energy range which is indicative of transitions involving at least one energy band of reasonable width. On the other hand, the slope of the rising absorption edge suggests that the optical transitions are not between sharply defined energy bands. Various models have been proposed in the past to account for these properties 10). However, the most recent measurements have narrowed the choice for the origin of the non-photoconducting absorption and field controlled photogeneration in amorphous selenium to either one electron tail states or localized correlated excitonic type transitions. It was with the hope of differentiating between these two models that a study was made of steady state and transient photoemission of carriers from metals into amorphous selenium. The experimental arrangement was identical to that shown in fig. la. The threshold energies for both electron and hole photoemission into amorphous selenium were determined from steady state measurements in an analogous manner to that described in the last section for PVK. A summary of the results is given in table 1, where it is seen that there is a dependence on the metal work function of the threshold such that for gold the threshold for hole photoemission is less than that for electrons, whereas for aluminum the converse is true. The sum of the hole threshold and metal work function should equal, referring to fig. 2a, the photoemission threshold into vacuum. From the results for the three metals an average value of 5.7 eV is found for amorphous Se. The sum of the thresholds for hole and electron photoemission given in the last column, which has an average value of 2.2+0.2 eV, can be referred to as a "photoinjection" band gap. This is somewhat less than the band gap of 2.47 eV determined from quantum efficiency measurements 10). This point will be considered again later in this section. In the case of amorphous selenium the transient measurements are complicated because the photoemission and internal photogeneration of carriers overlap. Nevertheless, it proved possible to separate and identify these two TABLE 1 Thresholds for photoemission of electrons and holes from various metals into amorphous Se Threshold for Metal Threshold for holes Threshold for elec- vacuum photoemis~bh (eV) trons ~e (eV) sion ~bM+ q~n (eV) Au Cu A1

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regimes and study the number of photoemitted carriers as a function of applied field and temperature to see if those states involved in the field dependent internal photogeneration in amorphous selenium also play a role in the photoemission process. Referring to fig. 2b, the ratio of the number of collected holes Qh to electrons Qe was measured as a function of photon energy at a constant applied field, chosen to ensure no range limitation for either carrier. The number of charges transported per pulse was always such that Q < C V so that the condition for the small signal case was always satisfied. The results are shown in fig. 5. For photon energies greater than about 2.2 eV, it is seen that this ratio is approximately unity, indicating that electrons and holes are created with essentially equal quantum efficiency at these energies. Below about 2.2 eV, it is seen that although the absolute quantum efficiency is falling, the ratio rapidly increases due to the higher efficiency for photoemission of holes. There is, therefore, a relatively sharp transition from a situation where the source of collected holes is due to internal photogeneration (i.e. for photon energies greater than 2.2 eV) to the source being photoemission from the Au electrode. In this small signal case Qh is a linear function of light intensity in both regimes. Fig. 6 shows the field dependence of Qh at different photon energies. For energies above 2.2 eV, the curves have a shape which is a characteristic of amorphous

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of the figure refer to the activation energies obtained from the slopes of the lines. The mechanisms which could lead to a field dependent supply of the number of collected photoemitted holes can be conveniently classified as arising from bulk or interface limitations. In the case of bulk limitations, there is only a range limitation and a space charge limitation to be considered. The range cannot be a controlling factor because the fields employed were always such that the schubweg was longer than the sample thickness. Space charge effects can be ruled out since the integrated signal was always much less than the applied voltage and in addition Qh was linear in light intensity. In the case of interface limitations, Schottky barrier lowering is inconsistent with a monotonic field dependence persisting down to fields of only 350 V/cm and, in addition, in the steady state spectral studies no field dependence of the spectral threshold was detected. Surface trapping is rejected because Qh would be expected to be temperature dependent. Another process to be considered is one where holes could be photoemitted into quasi-localized states lying below the conducting states in amorphous selenium. A field dependent capture or release rate might thereby

128

S. MORTAND A.I. LAKATOS

account for the field dependent supply. Qualitatively two different types of states have to be considered and these are indicated schematically in fig. 8. The so called one electron tail states, fig. 8a, have been postulated to arise from the disordered nature of an amorphous material 13) while the second model, fig. 8b, involves states arising from localized correlated transitions. It can be seen from this figure that in the one electron states model carriers can be photoemitted into any of these states. In the case of the correlated states model, however, no photoemission is possible since without the prior excitation indicated these states do not exist, while the act of absorption only creates occupied states. The conclusion is, therefore, that correlated states cannot play a role in limiting the supply of photoemitted carriers but that the one electron tail states could. This conclusion, coupled with the experimental observation that different mechanisms control the supply of photoemitted and internally photogenerated holes, suggests that it is localized correlated states rather than one electron tail states which determine the non photoconducting absorption and field controlled photogeneration in amorphous selenium. This is consistent with the fact that one electron tail states cannot explain several important experimental observations. (a) Essentially equal quantum efficiencies are found for the photogeneration of electrons and holes even for quantum efficiencies substantially less than unity. Tail states could account for this only if fortuitously the tail state density for both bands was symmetric with respect to the middle of the band gap. (b) The non-photoconducting absorption and field dependent photogeneration cannot be explained in terms of tail states without expecting them to

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STEADY STATE AND TRANSIENT PHOTOEMISSION

129

play a role in the bulk transport since this class of states is not confined to the excitation region. No anomaly such as a field dependent mobility has been observed experimentally. (c) Equally strong non-photoconducting absorption is observed in single crystals of orthorhombic sulfur 14) indicating that this feature of amorphous selenium is not a characteristic of the amorphous state. On the other hand, the correlated states model can account for essentially equal quantum efficiencies for electrons and holes approaching almost unity, limit the photogeneration and not, since they exist only in the excitation region, the bulk transport and can occur in the crystalline and amorphous states. It would appear therefore that the only role remaining in which one electron tail states could manifest themselves is if they controlled the thermally activated hole mobility via a trap controlled mechanism. However, if tail states do control the drift mobility, then even though holes can be photoemitted into them, this mechanism could not be a limitation on the supply of photoemitted holes. This is because carriers in traps controlling the drift mobility equilibrate in a time very much less than a transit time, so that no carriers could be lost and the number of collected carriers must be independent of field. However, it should be noted that photoemission into such tail states would result in the photoinjection bandgap being less than that deduced from quantum efficiency measurements. The field dependent supply of photoemitted carriers must therefore have another origin and we suggest that it derives from the fact that we are dealing with a "blocking" contact below its saturation field 15). A contact is blocking to an insulator when the current through the insulator is completely determined by the rate at which the contact supplies carriers. A necessary but insufficient condition for such a contact is that the barrier height be much larger than kT. The additional required condition is that the rate of flow of charge to the interface from the metal ¼ nv must equal the rate of flow away into the insulator neltEsat, where n is the free carrier density sufficiently hot to overcome the barrier, v is the thermal velocity in the metal,/~ is the mobility in the insulator in the interface region, and Esa t is the field above which the contact is blocking 1~). Esat which is the field at which the contact saturates is therefore given by v/4# V/cm. For a typical semiconductor this field is usually about 10+-10 5 V/cm which, with a barrier width of 1/~m and height of 1 eV, is readily achieved even with no applied field. In the case of amorphous Se, since the hole drift mobility is only 0.1 cm2/Vsec, a saturation field of approximately 10 7 V / c m is to be expected, while any Schottky barrier width might well be substantially longer than the sample length. It is important to recognize that below the saturation field the rate of flow of charge into the insulator does not in general decrease because of a lower

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J. MORTANDA.I.LAKATOS

carrier velocity, but because the number of carriers entering the insulator is now field dependent. This is the advantage of the transient photoemission technique in that it shows directly that it is the number of carriers that is field dependent. This is in complete accord with the earlier conclusion that the field dependent supply does not arise from a bulk limitation. Very similar results were obtained for photoemission of holes from gold into PVK. Again using transient photoemission it was possible to eliminate any bulk limitation such as range or space charge limitation. Qh was found to be ~:E 1"4 and for a fixed field was essentially temperature independent. Similar measurements have been made on the transient photoemission of electrons from copper into a highly insulating single crystal of CdS. In this case a linear dependence of Qh versus field was followed by a pronounced saturation. The saturation field was found to be independent of light intensity but the saturation value of Qh varied linearly with intensity. The saturation field was reasonably close to the value expected from the relation given above and using a value of 300 cmZ/Vsec for the electron mobility in CdS. It thus appears that the field dependent supply of photoemitted carriers observed for the Au-Se system is really a fundamental characteristic of the metal-insulator interface. The limitation appears to be neither a pure contact or bulk limitation but rather a hybrid of the two. In this sense a true blocking contact, i.e. one independent of the intrinsic transport properties of the insulator, is a special case of a more general interface limitation which is determined by the interplay of intrinsic properties of both the metal contact and the insulator. Such an interface limitation could account for a field dependent supply over the range of fields observed and a linear dependence on light intensity. The specific field dependence to be expected is not obvious and must depend on the detailed microscopic mechanisms at the interface. In addition, a detailed microscopic model for the Au-Se, A u - P V K systems would have to account for the supply being in essence temperature independent, at least over the limited temperature range studied. This appears inconsistent with the prediction of the simple model that Esat is directly proportional to the carrier mobility in the insulator. Since the hole mobility in both amorphous Se and P V K fall exponentially with decreasing temperature, it would be expected that the yield of photoemitted carriers at a given field would change in a similar fashion. Studies on these aspects of the problem are continuing.

4. Summary The steady state and transient photoemission of carriers from metals into P V K and amorphous Se have been studied. P V K was a case where the steady state measurements revealed the narrow nature of the valence band in this

STEADYSTATEANDTRANSIENTPHOTOEMISSION

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material. The transient m e t h o d was then employed to study time resolved transport o f carriers in this n a r r o w band. The information a b o u t the bandwidth given by the steady state measurements is crucial in choosing between possible transport models. The studies on a m o r p h o u s Se illustrated another facet of the usefulness of these techniques. In this case, a comparative study o f the field and temperature dependence of photoemitted and internally photogenerated carriers appeared consistent only with a model in which the n o n - p h o t o c o n d u c t i n g absorption and field controlled photogeneration is ascribed to correlated excitonic transitions. After considering all other possibilities, it is suggested that the field dependent supply of carriers photoemitted into an insulator is a manifestation of a fundamental characteristic o f the metal-insulator interface. It is believed that this interface limitation is intermediate between the well-known extremes viz. contact and bulklimited currents. The ability o f these techniques to probe these properties of the metal-insulator interface stems from two facts. The first is that photoemitted carriers are essentially indistinguishable from thermally emitted carriers and the second is the ease with which it is possible, using transient techniques, to independently study either the velocity or the n u m b e r o f transported carriers.

Acknowledgements The authors take pleasure in thanking Drs. G. Lucovsky and M. T a b a k for m a n y discussions and suggestions during the course of this work.

References 1) 2) 3) 4) 5) 6) 7) 8) 9) 10) 11) 12) 13) 14) 15)

R. Williams, Phys. Rev. 140 (1965) A569. A. M. Goodman, Phys. Rev. 144 (1966) 588. R. Williams and J. Dresner, J. Chem. Phys. 46 (1967) 2133. A. I. Lakatos and J. Mort, Phys. Rev. Letters 21 (1968) 1444. J. H. Sharp, J. Phys. Chem. 71 (1967) 2587. M. Lardon, E. Lell-Doller and J. Weigl, Mol. Cryst. 2 (1967) 241. P. J. Regensburger, Photochem. Photobiol. 8 (1968) 429. D. J. Gibbons and W. E. Spear, J. Phys. Chem. Solids 27 (1966) 1917. W. E. Spear, Proc. Phys. Soc. (London) B 70 (1957) 669; B 76 (1960) 826. J. L. Hartke and P. J. Regensburger, Phys. Rev. 139 (1965) A970. M. D. Tabak and P. J. Warter, Jr., Phys. Rev. 173 (1968) 899. D. Pai and S. W. Ing, Phys. Rev. 173 (1968) 729. N. F. Mott, Adv. Phys. 16 (1967) 49. W. E. Spear and A. R. Adams, J. Phys. Chem. Solids 27 (1966) 281. A. Rose, Concepts in Photoconductivity and Allied Problems (Wiley, New York, 1963) ch. 8.