Journal o f Electronic Materials, Vol. 8, No. 6, 1979
ELECTRONIC PROPERTIES OF CHALCOGENIDE GLASSES AND THEIR USE IN XEROGRAPHY
G. Pfister + Xerox Corporation Webster Research Center Webster, NY 14580
(Received April 19, 1979)
An overview of the xerographic process and the relevant physical phenomena involved in the formation of the latent image is presented. Emphasis is on the electronic properties of a-selenium but some properties of other disordered solids will be mentioned in order to amplify features general to the disordered solid state. Specifically the discussion includes a description of the Onsager mechanism of charge generation, transient dispersive transport, and recent models of defects in chalcogenide glasses. Keywords:
amorphous semiconductors, transport, photogeneration
xerography,
charge
Introduction The utilization of chalcogenide glasses such as selenium, or arsenic triselenlde as p h o t o r e c e p t o r materials in xerography constitutes the first large scale electronic application of amorphous semiconductors. Indeed, for many reasons amorphous semiconductors are ideally suited for this purpose. They can be produced in the required form of large area defect-free films that have (i) reproducible 789 0361-5235/79/IlOO-O789503.o0/I9 1979AIME
790
Pfister
physical properties, (ii) long term stability, (iii) chemical inertness to the hostile environment of a high voltage corona discharge, (iv) mechanical strength to withstand abrasion from toner particles, cleaning brushes and paper contact, and (v) may be rendered flexible which offers a multitude of machine configurations. Furthermore, the typically, low ca_r~ier~mobilities of amorphous semiconductors (i0 D - i0 ~ cm~/Vsec) do not normally limit the copying speed, and the generation efficiency of free carriers is sufficient that the electrostatic image can be formed without resorting to excessively intense illumination sources. While the earliest applications used amorphous selenium as photoreceptor (which therefore had to fulfill all these demanding requirements) it has now become possible to use functionalized multilayer structures and have generation and transport occur in different layers. For instance one may photogenerate positive charges in a thin sensitizing layer of a-selenium and inject the charge into a layer of poly(N-vinylcarbazole). Hence, generation and transport and to some degree the mechanical properties can be optimized independently. For this reason the present article will address some properties of organic disordered solids which are often more suited to illustrate phenomena typical of the disordered state. Before going into the main body of this review let me briefly describe the xerographic process as it is utilized in today's office copiers. This discussion also serves to outline the scope of this article. Fig. 1 schematically illustrates the five steps pertinent to the making of a plain paper copy: (I) sensitization of the xerographic plate by electrical charging (Fig. la), (2) imagewise exposure of the sensitized plate to produce a latent electrostatic image (Fig. Ib), (3) development of the latent image by fine toner particles (Fig. Ic), (4) transfer of the developed image to plain paper or other materials (Fig. id), and (5) fixation of the transferred image by fusing (Fig. l e ) . The remaining toner particles and electrical charges are then removed by cleaning with a brush and full frame light exposure (Fig. If). To illustrate a typical process and more clearly outline the pertinent physical phenomena, consider amorphous selenium (a-Se) as the photosensitive insulator, a-
Electronic Properties of Chalcogenide Glasses
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Basic steps in Xerography.
Se is deposited by vacuum evaporation onto an aluminum substrate to a thickness of several tens of microns. The free selenium surface is then charged to a positive surface potential V ~ ~ 700 volts by passing the xerographic plate under an appropriate corona charging device (Fig. la). Ideally the surface potential does not decay appreciably during the subsequent process steps except when illuminated with light. This requires that the intrinsic dark resistivity of the photoreceptor is sufficiently high
792
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(~ I012~ cm) and the injection of carriers from the free surface is negligible. The surface charge density for the ideal photoreceptor is o = CV where C is the geometrical s an~ V ~ the surface potential capacitance per unit area following the corona charging step. The charged photoreceptor is now exposed to light reflected from a document. In the illuminated areas the photoreceptor potential is discharged due to the photocurrent that flows perpendicular to the surface. Specifically, for positive surface potential, photogenerated electrons (negative charge carriers) will neutralize the positive surface charge while free holes (positive charge carriers) drift across the insulator layer to neutralize the induced negative charge on the aluminum substrate. The lateral conductivity of the photoreceptor has to be negligible since otherwise the sharpness of the latent image would diminish. The latent image is then developed by use of a toner which consists of toner particles ~10~m in diameter that are coated onto carrier beads ~100~m in diameter. The triboelectrically-charged developer particles are preferentially attracted to the surface potential gradient that exists at light-dark boundaries. The image is then transferred to a sheet of paper which has been charged by a corona to the opposite polarity to that of the toner and, finally, the image on the paper is rendered permanent by a heat or solvent fusing process. In this article the physical phenomena to be reviewed are associated with the formation of the latent image. This process involves the generation of free carriers by incident light and their transport across the thickness of the film. The quantities discussed in detail are the generation efficiency (~), i.e. the number of free carriers generated per absorbed photon, and the carrier mobility (~), i.e. the drift velocity per unit applied electric field. For chalcogenide glasses these quantities may exhibit field, wavelength and temperature dependences that are drastically different from those typical of wide band crystalline semiconductors. Indeed some of these properties are unique to the disordered state and require novel explanations. The next section gives an introductory quantitative description of idealized xerographic processes using a-Se
Electronic Properties of Chalcogenide Glasses as the example. This discussion clearly brings to attention the process-limiting physical mechanisms. The sample preparation and the techniques employed to measure charge generation and mobility are then described. A discussion of these measurements will then follow for charge generation and transport. Finally some recent ideas about defect states in amorphous chalcogenides shall be summarized.
Xerography The discharge of the surface potential, V(t), is a sensitive function of the photophysical parameters which characterize the photoreceptor, i.e. carrier generation efficiency ~, carrier transport ~ carrier lifetime Twith respect to deep traps and recombination. In addition, V(t) depends critically on the mode of illumination. Different discharge curves will be generated by pulsed or step illumination, and by light that is absorbed close to one surface or in the bulk of the photoreceptor. The analysis of the discharge curve is further complicated because the pertinent parameters ( ~ , ~ ) are often dependent upon the electric field E(t) = V(t)/L across the photoreceptor layer (which itself is a decaying function of time). As the discharge proceeds the electric field E(t) decreases and therefore the time t_ for the carriers T to transit the film, tT = L/ E(t) (transit time) increases. It is here that the carrier lifetime T and the concepts of emission-limited and space-charge limited discharge become important. To completely discharge the photoreceptor in a time that is reasonable in terms of device process steps, one requires that the carriers transit the film before they are deeply trapped, tTT for long times. In this limit the carriers become deeply trapped before they complete the transit and further decay of the surface potential is now determined by the release of the deeply trapped carriers which may be very slow. In addition to the loss of contrast potential (see below) the integration of these bulk trapped charges over succeeding imaging cycles can lead to significant build-up of a
793
794
Pfister
residual potential. For a-Se bulk trapping u s u a l l ~ i s not z a problem. For holes 9 % 10-50~sec and ~ %0.15 cm /Vsec. Hence, for a 50~m thick film and an initlal potential V ~ 7~0 volts one obtains^ for the initial transit time tT = L'/~V % (50 x 10--)z/(0.15 x 700)~sec = 0.23 psec i.e. o the surface voltage has to decay to less than % 0.02 V ~ before bulk trapping becomes important. For convenience, and as is appropriate for a-Se, the following discussion is restricted to the case where the incident light is absorbed close to the sample surface and bulk trapping is negligible. Two limiting idealized discharge characteristics can be distinguished depending upon the amount of charge in transit in comparison to the charge remaining on the photorecepto~ surface. The incident light F (= absorbed photons/cm sec) generates the charge density ~ = e~Ft_ in one transit time tT=L/uE. Hence, the ratio of ~he charge in transport ap a~d the s~rface charge density a = as E is a /a = e~FL~/(aa ~V~). Now if a
V . ) 9 mzn c i and flna~ly becomes overexposed (V < V . ). The exposure c ml~ range V > V . defines the exposure latztude. The field c in dependence
o~
~ broadens
the
exposure
latztude.
For a reciprocal xerographic system the shape of V(t) depends only upon the exposure X=Ft, It is seen from Eq. (3,4) that reciprocity holds only for intensity independent photogeneration efficiency, q=l,
798
Pfister
Space-Charse Limited Discharse The mathematical description of the space-charge limited discharge is much more complicated than that for the emission-limited discharge described before. Consider the case where the sample is illuminated by a light pulse of duration shorter than the transit tT (at the initial voltage V ) and of sufficient intensity that the injected charge density equals ~ = ee E = CV . The surface charge zs zmmedzately neutralized w~Ich causes the field at the surface to approach zero. However, the field on the front side of the charge is non-zero which causes an asymmetric evolution with time of the drifting charge since the front and trailing edges experience different (time dependent) fields. Indeed, in the mathematically ideal space-charge limited case it takes an infinitely long time for all the charge to leave the generation region because the field at the surface is exactly zero. It follows then that the discharge of the surface potential continues after the termination of the light excitation and after the leading edge of the drifting carrier packet reaches the substrate electrode. This behavior is obviously different from the emission-limited case where the discharge stops at most one transit time after the light excitation ceases9 9
9
.
8
.0
0
.
These ideas can be expressed in exact mathematical terms. For a field dependent mobility of the form
=
(Z)~
~o ~o
(6)
Detailed calculations of the space-charge limited and space-charge perturbed discharges were presented by Chen (2) for a variety of exposure modes. Chen (2) finds for flash illumination (charge generated in time E the quantum efficiency is constant and, in particular, ~xhibits no field or temperature dependence. This is not observed for a-Se. The photogeneration efficiency for a-Se exhibits strong field, temperature and photon energy dependencies. Figs. 6 and 7 show the essential features of the photogeneration efficiency of holes in a-Se (5). In Fig. 6 the dependence of the efficiency is plotted versus the energy of the incident light for various wavelengths. For reference, the absorption and fractional absorption are also
804
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plotted. In Fig. 7 the field dependence of the efficiency is shown for various photon energies. The outstanding feature of these results are that ~ remains strongly field (and temperature) dependent for photon energies substantially larger than the optical band gap energy E 9 The efficiency approaches a constant value at high ~ectric fields, photon energies or temperatures. Obviously an energy larger than the band gap is required to separate the excited electron-hole pair. I
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Fig. 6 Absorption coefficient e , fractional absorption and quantum efficiency ~ for 50~m thick amorphous selenium versus exciting photon energy. Applied field is the parameter for quantum efficiency curves (after Pal and Enck (5)).
Electronic Properties of Chalcogenide Glasses
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Fig. 7 Quantum efficiency of photoinjection of holes in aSe versus applied electric field with the wavelength of the exciting radiation as the parameter. Included are data on films of two different thicknesses. The solid lines are the theoretical Onsager dissociation efficiencies for ~o = I and for initial separations r~ indicated in the figure (after Pal and Enck (5)). These observations are not unique to a-Se or the disordered state. Field dependent generation efficiencies are observed for a number of organic polymeric photoconductors such as poly(N-vinylcarbazole) molecularly doped polymers or molecular crystals such as anthracene. For illustration Fig. 8 shows the generation efficiency for holes in a solid solution of triphenylamine (TPA) in a polycarbonate polymer matrix (6). The generation (6) and subsequent transport (7) of the holes is exclusively associated with the TPA molecule, i.e. in the absence of
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16 i o L=2.0/.z.m 9 L = 4,2p,.m A L=6.ZFm 9 L= 8.8/J.,.m T PA / POLYCARBONATE
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ELECTRIC FIELD(V/Cm)
Fig. 8 Hole photogeneration efficiency in the solid solutions of triphenylamine (TPA) in a polycarbonate host matrix, r is the Onsager radius, see Eqs. 12-14 (after o Borsenberger et al, (6)). TPA no photogeneration of significance is observed. Another interesting behavior of the photogeneration efficiency is displayed for holes in poly(N-vinylcarbazole) (PVK). Unlike in a-Se where the efficiency increases monotonically with increasing excitation energy, in PVK the generation efficiency increases stepwise with the steps occurring at the transitions into higher lying electronic states (8, 9) (Fig. 9). These unusual features of the photogeneration efficiency, in particular in a-Se, were the subject of much experimental work and discussion. But not until recently has a clear and complete set of experimental results been obtained which allows a detailed interpretation of the underlying generation process (5, 6). Indeed it appears that the same general interpreta-
Electronic Properties of Chalcogenide Glasses
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Fig. 9 Wavelength dependence of efficiency in poly(N-vinylcarbazole) (after Borsenberger et al. (9)).
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hole p h o t o ~ a t i o n at room temperature
tion is applicable to all systems mentioned, viz. a-Se, organic polymers and molecular crystals. It suffices therefore to discuss only a-Se in more detail.
It is proposed that the incident photon produces an electron-hole pair which is bound by their mutual Coulomb interaction. In the absence of an external field, the pair would recombine unless they are separated by a distance r o >r c = e /kTee o for which the Coulomb energy is of the order of the thermal energy (Fig. I0). The separation of the excited electron-hole pair is called the thermalization radius r . If r < r then recombination is likely o o c
808
Pf'tster
and therefore q r the carriers separate and .O C q ~ n where ~ is determzned by the efficiency of radia9 O O 9 tlonless transltlons or fluorescence which may dissipate the energy of the absorbed photon. If these are negligible, as seems to be the case for a-Se (but not for organic polymers) ~ ~ 1. It is reasonable that r increases with O 9 9 increasing excltatlon energy and that the~ barrier for carrier separation is reduced by the applied field, hence, ~(E,T,h~). The thermalization length increases with increasing photon energy and therefore the field dependence of n decreases. For high fields, the effect of the Coulomb barrier is negligible and the quantum efficiency approaches a field independent value. At low fields, the Coulomb energy dominates and q drops rapidly with decreasing excitation energy. Some of these expectations are clearly borne out in Figs. 6, 7. One notes, however, that at low fields where ~(E,T) should approach a field independent value, the experimental quantum efficiency drops significantly.
BAND EDGE
V(r ) § eEr
I
~-- V(r)
Fig. i0 Energetics involved in separation of photoexcited electron-hole pair subject to mutual Coulomb interaction and external field, r = critical radius where thermal and Coulomb energy are equal, r = "thermalization" radius. o
Electronic Properties of Chalcogenide Glasses
809
The drop of ~ at low fields has been shown to be due to recombination and trapping at the surface of selenium that is, at low fields the carriers diffuse back to the surface where they recombine. This mechanism competes with their separation in the bulk. The key to that explanation was an experiment utilizing two-photon absorption (I0) (Fig. ii). Instead of absorbing one photon of energy 2.34 eV close to the sample surface, the equivalent energy was provided by simultaneous absorption of two photons each of 1.17 eV energy. The 1.17 eV photons are absorbed uniformly in the bulk of the sample and therefore surface effects are negligible. The quantum efficiency measured using the two-photon technique did not show the drop off at low fields seen in the one photon experiment. As a mathematical framework to explain the generation of carriers in a-Se (and other materials with field dependent ~), the theory of Onsager (Ii) has been applied with considerable success. Onsager calculated the probability of an ion pair in an electrolyte diffusing apart under the action of an applied electrical field. The probability of separation is a function of E,T,r, 0. l
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Fig. Ii Hole photogeneration efficiency in a-Se as a function of applied field at room temperature. The resuits are shown for one-photon (2.34 eV) and two-photon (1.17 eV) generation. Also shown is the prediction of the Onsager theory using ~o = 1 and r ~ = 19~ (after Enck (I0) and Pai and Enck (5)).
Pf'tster
810
0 is the angle between the direction of the applied field and the ion pair and r the initial separation distance. Hence p = p(E,T,0,r). To simplify the analysis, one assumes that the distribution of the initial electronhole pairs g(r,0) is isotropic and the separation is fixed, i.e. g(r, e) = /1-~2--6(r-ro).~,,~ With this simplificaO
tion, the quantum efficiency for photogeneration reduces to the form (12)
n(E,T,r o) = ~of(E,T,ro )
f(ro,E) = 1
kT eEr O
(12)
~ Ig ( e2 eEr~ (13) g=O ekT-----~ E ) Ig (--k-~--) O
O
Ig(X) = Ig_l(X) - exp(-x)Xgg!
(14)
Hence, the functional dependence of n on E and T is determined by a single parameter r . The thermalization 9 O length r is itself expected to increase with increasing O photon energy as has been discussed. The solid lines in Figs. 7 and 8 were calculated from Eq. (12) for the values of r given in the Figures. The primary quantum yield for O . . . . a-Se zs n ~ I, at room temperature, whxch Indzcates that 9 9 O . . . . . . . xn thzs materlal the generatzon effzczency is izmzted only by pair recombination, i.e. the probability for an excited electron-hole pair to thermalize before recombination is approximately unity. For organic systems such as TPA in polycarbonate or poly(N-vinylcarbazole) the primary yield is ~ ~0.026 and ~ ~ 0.15, respectively (6, 8, 9). Hence, a substantial fraction of the excited electron-hole pairs dissipate their excess energy by radiative and nonradiative processes before thermalizing. The Onsager formulation fits the experimental results exceptionally well. This agreement is remarkable considering that only r is available for the fit of the field O dependence since ~_ Is only a scaling factor. Indeed for . O TPA zn polycarbonate Eq. (12 ) predicts correctly the field and temperature dependence using the values r ~ and ~o, determined from Fig. 8. Despite this apparent excellent
Electronic Properties of Chalcogenide Glasses
811
agreement we lack a fundamental interpretation of the thermalization length and future theoretical considerations will have to relate r to microscopic parameters. o For TPA in polycarbonate r is essentially temperao ture independent over the temperature range 220-330K (8). For a-Se r is weakly dependent on temperature, decreasing o on the average about five percent from 294K to 223K (5). There has been some discussion about the expected form of the temperature dependence of r when the Onsager theory o of dissociation is applied to explaln photogeneration in anthracene and carrier yield data in liquid hydrocarbons. In a crystalline material such as anthracene, the temperature dependence of the mean free path resulting from the temperature dependence of thermally-induced fluctuations in the lattice density causes the diffusion constant D to have an inverse temperature dependence. The thermalization distance which is proportional to ~ is expected to have a T -0.5 dependence (13), which would give an increase of 15% in r at 223K over 294K, rather than the decrease of 5% actually observed in a-Se. In an amorphous material, the mean free paths are very small and determined by the disorder of the material rather than by thermally-induced fluctuations in the atomic positions, so that the approximate temperature independence is not surprising. In liquid hydrocarbons, r is predicted to decrease slowly as T increases due to~ change in the amount of excess energy that must be lost by a hot carrier to achieve thermalization due to variation in kT. This also gives the wrong sign for the variation of r Q with temperature in a-Se. In the other materials mentloned, there are no accurate experimental determinations of r to compare with theory, o The most likely cause of the bulk of the observed variation of r with temperature in a-Se is the variation o of ~he band gap with temperature, reported to be - 7 x I0 -~ eV/K (14). As the temperature decreases, the band gap becomes wider and a carrier excited by a photon of a particular energy must lose a smaller amount of excess kinetic energy to thermalize, thus causing a decrease in r . Indeed, a qualitative estimate of the expected charge o~ r due to the 50meV increase in bandgap in going from 294K~ 223K predicts a change for r of 5-10% which is in satisfactory agreement with the observed result (5).
812
Pfister
The thrust of future studies of generation processes has to emphasize the connection of the Onsager parameters with microscopic processes. Such information may be obtained from time resolved studies of the initial thermalization step. Theoretical progress was recently made by Noolandi and Hong (15) who studied the time-dependent solutions of the Smoluchowski Equation with a Coulomb potential, the long tlme behavior (steady-state) of which underlies the original Onsager formalism.
Charge Transport;
Non-Gaussian Dispersion
Amorphous selenium was the first non-crystalline solid where the time-of-flight experiment was successfully applied to determine hole and electron drift mobilities (16). At room temperature the transient current pulse closely resembles the rectangular shape shown in Fig. 5, which indicates that the carrier sheet does not appreciably broaden as it propagates through the sample. From the transit time as a function of sample thickness ~nd field, one obtains ~h ~ 0.15 cm~/ysec and ~e ~ 0.007 cm /Vsec for the hole and electron drlft mobllity, respectlvely. As the temperature is lowered, the drift mobilities decrease and approach an exponential temperature dependence, exp(A/kT). The activation energy for holes is A h ~ 0.26 eV. For electrons it is somewhat larger, A ~ 0.3D eV. It is believed that the thermally activated ebehavior of charge transport reflects the interaction of the moving charge with traps and that A is a measure of the trap depth. In the presence of traps, the carriers spend only a fraction of their time in the transport states and only during that time do they contribute to the drift current. If one assumes for simplicity a single trap at a depth E t the transit time of a carrier can be expressed as
tT = to + nT R
(15)
where n is the n~mber of times the carrier falls into the trap and TR = ~ exp(~t/kT) is the release time into the transport states. In g_e~eral t 